Integrated Pollution Prevention and Control (IPPC)

Reference Document on the application of Best Available

Techniques to Industrial Cooling Systems

December 2001

EUROPEAN COMMISSION

Executive Summary

Industrial Cooling Systems i

EXECUTIVE SUMMARY

This reference document on the application of best available techniques to industrial cooling

systems (BREF) reflects the information exchange carried out according to Article 16 (2) of

Council Directive 96/61/EC on IPPC. The document has to be seen in the light of the preface

that describes the objective of the document and its use.

In the framework of IPPC, industrial cooling has been identified as a horizontal issue. It means

that “Best Available Techniques”(BAT) in this document is assessed without an in-depth

assessment of the industrial process to be cooled. Notwithstanding, BAT for a cooling system

is considered within the cooling requirements of the industrial process. It is acknowledged that

BAT for cooling a process is a complex matter balancing the cooling requirements of the

process, the site-specific factors and the environmental requirements, which allows

implementation under economically and technically viable conditions.

The term “industrial cooling systems” refers to systems to remove excess heat from any

medium, using heat exchange with water and/or air to bring down the temperature of that

medium towards ambient levels.

In this document, BAT is described for cooling systems that are considered to work as auxiliary

systems for the normal operation of an industrial process. It is acknowledged that reliable

operation of a cooling system will positively affect the reliability of the industrial process.

However, the operation of a cooling system in relation to process safety is not covered in this

BREF.

This document presents an integrated approach to arrive at BAT for industrial cooling systems

acknowledging that the final BAT solution is mainly a site-specific matter. With respect to the

selection of a cooling system, this approach can only discuss which elements are linked to the

environmental performance of the cooling system, rather than select and (dis) qualify any of the

applied cooling systems. Where reduction measures are applied, the BAT approach attempts to

highlight the associated cross-media effects thus emphasising that reduction of the different

emissions of cooling systems needs balancing.

The five chapters of the main document describe the BAT approach, its key issues and

principles, the cooling systems and their environmental aspects, the key BAT findings and the

conclusions and recommendations for further work. Eleven annexes give background

information addressing specific aspects of designing and operating cooling systems and

examples to illustrate the BAT approach.

1. Integrated approach

The integrated BAT approach considers the environmental performance of the cooling system

in the context of the overall environmental performance of an industrial process. It aims at

minimisation of both the indirect and direct impacts of the operation of a cooling system. It is

based on the experience that the environmental performance of cooling of a process largely

depends on selection and design of the cooling system. Therefore, for new installations the

approach focuses on prevention of emissions by selection of an adequate cooling configuration

and by proper design and construction of the cooling system. Additionally, reduced emissions

are achieved by optimization of daily operation.

For existing cooling systems there is on a short term less potential for prevention by

technological measures and emphasis is on emission reduction by optimized operation and

systems control. For existing systems a large number of parameters, such as space, availability

of operating resources and existing legislative restrictions, may be fixed and leave few degrees
Executive Summary

ii Industrial Cooling Systems

of freedom to change. However, the general BAT approach in this document can be considered

as a long-term goal, which fits with equipment replacements cycles for existing installations.

The BAT approach acknowledges that cooling is an essential part of many industrial processes

and should be seen as an important element in the overall energy management system. The

efficient use of energy in industrial processes is very important from the environmental and

cost-efficiency points of view. First of all, BAT means that attention must be paid to the overall

energy efficiency of the industrial or manufacturing process, before measures are taken to

optimize the cooling system. To increase overall energy efficiency, industry aims to reduce the

amount of non-recoverable heat by applying proper management of energy and by adopting a

range of integrated energy-saving programmes. This includes energy exchange between

different units inside the cooled industrial or manufacturing process as well as links outside this

process with adjacent processes. There is a tendency towards a concept of heat recovery for

industrial regions when industrial sites are interlinked or are linked with district heating or

greenhouse farming. Where no further recovery and re-use of this heat is possible, it may have

to be released into the environment.

Distinction is made between low level (10-25oC), medium level (25-60oC) and high level (60oC)

non-recoverable heat. In general, wet cooling systems are applied for low level heat and dry

cooling systems for high level heat. For the medium level no single cooling principle is

preferred and different configurations can be found.

After optimization of the overall energy efficiency of the industrial or manufacturing process a

given amount and level of non-recoverable heat remains and a first selection for a cooling

configuration to dissipate this heat can be made balancing:

- the cooling requirements of the process,

- the site limitations (including local legislation) and

- the environmental requirements.

The cooling requirements of the industrial or manufacturing process must always be met to

ensure reliable process conditions, including start-up and shut down. The required minimum

process temperature and the required cooling capacity must be guaranteed at all times so as to

enhance the efficiency of the industrial or manufacturing process and reduce the loss of product

and emissions to the environment. The more temperature sensitive these processes, the more

important this will be.

Site conditions limit the design options and the possible ways a cooling system can be operated.

They are defined by the local climate, by the availability of water for cooling and discharge, by

the available space for constructions and by the sensitivity of the surrounding area to emissions.

Depending on the process demands on cooling and the required cooling capacity, site selection

for a new installation can be very important (e.g. large cold water source). Where the choice for

a site is driven by other criteria or in the case of existing cooling systems, the cooling

requirements of the process and the site characteristics are fixed.

For cooling, the local climate is important, as it affects the temperature of the ultimate coolant

water and air. The local climate is characterised by the pattern of wet and dry bulb

temperatures. In general, cooling systems are designed to fulfil the cooling requirements under

the least favourable climatic conditions that can occur locally, i.e. with highest wet and dry bulb

temperatures.

The next step in the selection and design of the cooling system aims to meet the BAT

requirements, within the requirements of the process to be cooled and the site limits. This

means that emphasis here is on the selection of adequate material and equipment to reduce

maintenance requirements, to facilitate operation of the cooling system and the realisation of

environmental requirements. Besides the release of heat into the environments other

environmental effects can occur such as the emission of additives used for conditioning of
Executive Summary

Industrial Cooling Systems iii

cooling systems. It is emphasized that where the amount and level of heat to be dissipated can

be reduced, the resulting environmental impact of the industrial cooling system will be lower.

The principles of the BAT approach can also be applied to existing cooling systems.

Technological options may be available, such as a change of cooling technology, or a change or

modification of existing equipment or chemicals used, but they can only be applied to a limited

extent.

2. Applied cooling systems

Cooling systems are based on thermodynamic principles and are designed to promote the heat

exchange between process and coolant and to facilitate the release of non-recoverable heat into

the environment. Industrial cooling systems can be categorized by their design and by the main

cooling principle: using water or air, or a combination of water and air as coolants.

The exchange of heat between process medium and coolant is enhanced by heat exchangers.

From the heat exchangers the coolant transports the heat into the environment. In open systems

the coolant is in contact with the environment. In closed systems the coolant or process

medium circulates inside tubes or coils and is not in open contact with the environment.

Once-through systems are commonly applied to large capacity installations in locations where

sufficient cooling water and receiving surface water are available. If a reliable water source is

not available, recirculating systems (cooling towers) are used.

In open recirculating towers, cooling water is cooled down by contact with a airstream. Towers

are equipped with devices to enhance the air/water contact. The airflow can be created by

mechanical draught using fans or by natural draught. The mechanical draught towers are used

widely for small and large capacities. Natural draught towers mostly are applied for large

capacities (e.g. power industry).

In closed circuit systems the tubes or coils in which the coolant or process medium circulates

are cooled, in turn cooling the substance they contain. In wet systems an airflow cools by

evaporation the tubes or coils which are sprayed with water. In dry systems only an airflow

passes the tubes/coils. In both designs coils can be equipped with fins, enlarging the cooling

surface and thus the cooling effect. Closed circuit wet systems are widely used in industry for

smaller capacities. The principle of dry air-cooling can be found in smaller industrial as well as

in large power plant applications in those situations where sufficient water is not available or

water is very expensive.

Open and closed hybrid cooling systems are special mechanical tower designs, which allows

wet and dry operation to reduce visible plume formation. With the option of operating the

systems (in particular small cell-type units) as dry systems during periods of low ambient air

temperatures, a reduction in annual water consumption and visible plume formation can be

achieved.
Executive Summary

iv Industrial Cooling Systems

Table 1: Example of technical and thermodynamic characteristics of the different cooling systems

for industrial (non-power plant) applications

Cooling system Cooling

medium

Main cooling

principle

Minimum

approaches

(K)4)

Minimum achievable

end temperature of

the process medium5)

(°C)

Capacity of

industrial process

(MWth)

Open once-through

system - direct Water Conduction/Convection 3 – 5 18 – 20 <0.01 - > 2000

Open once-through

system - indirect

Water Conduction/

Convection 6 – 10 21 – 25 <0.01 - > 1000

Open recirculating

cooling system -

direct

Water1)

Air2) Evaporation3) 6 – 10 27 – 31 < 0.1 – >2000

Open recirculating

cooling system -

indirect

Water1)

Air2) Evaporation3) 9 – 15 30 – 36 < 0.1 - > 200

Closed circuit wet

cooling system

Water1)

Air2)

Evaporation

+ convection 7 – 147) 28 – 35 0.2 – 10

Closed circuit dry air

cooling system Air Convection 10 – 15 40 – 45 < 0.1 – 100

Open hybrid cooling Water1)

Air2)

Evaporation

+ convection 7 – 14 28 – 35 0.15 - 2.5 6)

Closed hybrid

cooling

Water1)

Air2)

Evaporation

+ convection 7 – 14 28 – 35 0.15 - 2.5 6)

Notes:

1) Water is the secondary cooling medium and is mostly recirculated. Evaporating water transfers the heat to the air

2) Air is the cooling medium in which the heat is transferred to the environment.

3) Evaporation is the main cooling principle. Heat is also transferred by conduction/convection but in a smaller ratio.

4) Approaches relative to wet or dry bulb temperatures

Approaches of heat exchanger and cooling tower must be added

5) End temperatures depend on the site’s climate (data are valid for average middle European climate conditions

30°/21°C dry / wet bulb temperature and 15°C max. water temperature

6) Capacity of small units – with a combination of several units or specially built cooling, systems higher capacities

can be achieved.

7) Where an indirect system applies or convection is also involved the approach in this example increases with 3-5K leading to 

an increased process temperature

The table shows the characteristics of the applied cooling systems for a given climatic situation.

The end temperature of the process medium leaving the heat exchanger after cooling depends

on the coolant temperature and on the design of the cooling system. Water has a higher specific

heat capacity than air and therefore is the better coolant. The temperature of the coolant air and

water depend on the local dry and wet bulb temperatures. The higher the bulb temperatures the

more difficult it is to cool down to low end temperatures of the process.

The end temperature of the process is the sum of the lowest ambient (coolant) temperature and

the minimum required temperature difference between coolant (entering the cooling system)

and process medium (leaving the cooling system) over the heat exchanger, which is also called

the (thermal) approach. Technically the approach can be very small by design, but costs are

inversely proportional to the size The smaller the approach the lower the process end

temperature can be. Each heat exchanger will have its approach and, in the case of additional

heat exchangers, in series, all approaches are added to the temperature of the coolant (entering

the cooling system) to calculate the achievable end temperature of the process. Additional heat

exchangers are used in indirect cooling systems, where an extra cooling circuit is applied. This

secondary circuit and the primary cooling circuit are linked by a heat exchanger. Indirect

cooling systems are applied where leakage of process substances into the environment must be

strictly avoided.
Executive Summary

Industrial Cooling Systems v

For the cooling systems commonly applied in the power industry, minimum approaches and

cooling capacities are somewhat different from non-power plant applications because of the

special requirements of the steam condensation process. The different approaches and relevant

power generating capacities are summarized below.

Table 2: Examples of capacity and thermodynamic characteristics of different cooling systems for

applications in power industry

Cooling system Applied approaches (K) Capacity of power generating

process (MWth)

Open once-through systems 13-20 (terminal difference 3-5) < 2700

Open wet cooling tower 7-15 < 2700

Open hybrid cooling tower 15-20 < 2500

Dry air-cooled condenser 15-25 < 900

3. Environmental aspects of the applied cooling systems

The environmental aspects of cooling systems vary with the applied cooling configuration, but

the focus is predominantly on increasing the overall energy efficiency and reduction of

emissions to the aquatic environment. The consumption and emission levels are very sitespecific and where it is possible to quantify them they show large variation. In the philosophy

of an integrated BAT approach, cross-media effects must be taken into account in the

assessment of each environmental aspect and the associated reduction measures.

• Energy consumption

The specific direct and indirect consumption of energy is an important environmental aspect

relevant for all cooling systems. The specific indirect energy consumption is the energy

consumption of the process to be cooled. This indirect energy consumption can increase due to

a sub-optimal cooling performance of the applied cooling configuration, which may result in a

temperature rise of the process (∆K) and is expressed in kWe/MWth/K.

The specific direct energy consumption of a cooling system is expressed in kWe/MWth and

refers to the amount of energy consumed by all energy consuming equipment (pumps, fans) of

the cooling system for each MWth it dissipates.

Measures to reduce the specific indirect energy consumption are:

- to select the cooling configuration with the lowest specific indirect energy consumption (in

general once through systems),

- to apply a design with small approaches and

- to reduce the resistance to heat exchange by proper maintenance of the cooling system.

For example, in case of power industry a change from once through to recirculating cooling

means an increase in energy consumption for auxiliaries, as well as a decrease of efficiency in

the thermal cycle.

To reduce the specific direct energy consumption, pumps and fans with higher efficiencies are

available. Resistance and pressure drops in the process can be reduced by design of the cooling

system and by application of low resistance drift eliminators and tower fill. Proper mechanical

or chemical cleaning of surfaces will maintain low resistance in the process during operation.
Executive Summary

vi Industrial Cooling Systems

• Water

Water is important for wet cooling systems as the predominant coolant, but also as the receiving

environment for cooling water discharge. Impingement and entrainment of fish and other

aquatic organisms occur with large water intakes. Discharge of large amounts of warm water

can also influence the aquatic environment, but the impact can be controlled by suitable location

of intake and outfall and assessment of tidal or estuarine flows to insure adequate mixing and

advective dispersion of the warm water.

Consumption of water varies between 0.5 m3

/h/MWth for an open hybrid tower and up to 86

m3

/h/MWth for an open once-through system. Reduction of large water intakes by once-through

systems requires a change towards recirculating cooling at the same time it will reduce the

discharge of large amounts of warm cooling water and may also reduce emissions of chemicals

and waste. The water consumption of recirculating systems can be reduced by increasing the

number of cycles, by improving the water make up quality or by optimizing the use of waste

water sources available on or off site. Both options require a complex cooling water treatment

programme. Hybrid cooling, which allows dry cooling during some periods of the year, with a

lower cooling demand or with low air temperatures and so can reduce water consumption in

particular for small cell-type units.

Design and positioning of the intake and various devices (screens, barriers, light, sound) are

applied to reduce the entrainment and impingement of aquatic organisms. The effect of the

devices depends on the species. Costs are high and measures are preferably applied in a

greenfield situation. Lowering the required cooling capacity if possible by increasing the reuse

of heat may reduce emissions of warm cooling water to the receiving surface water.

• Emissions of heat into the surface water

As mentioned before the emissions of heat into surface water can have environmental impact on

the receiving surface water. Factors of influence are e.g. the available cooling capacity of the

receiving surface water, the actual temperature and the ecological status of the surface water.

Emissions of heat can result in the exceeding the EQS for temperature during warm summer

periods as a consequence of heat discharges into the surface water resulting from cooling water.

For two ecological systems (Salmonid waters and Cyprinid waters) thermal requirements have

been taken up in Directive 78/659/EEC. Relevant for the environmental impact of heat

emissions is not only the actual temperature in the water, but also the temperature rise at the

boundary of the mixing zone as a consequence of the heat discharge into the water. The amount

and level of the heat discharged into the surface water related to the dimensions of the receiving

surface water are relevant to the extent of the environmental impact. In situations where heat

discharges at relatively small surface waters and the hot water plume reaches the opposite side

of the river or canal this can lead to barriers for the migration of Salmonides.

Besides these effects high temperature as a consequence of heat emissions can lead to increased

respiration and biological production (eutrophication) resulting in a lower concentration of

oxygen in the water.

When designing a cooling system the above aspects and the possibilities to reduce the heat

dissipated into the surface water have to be taken into account.

• Emissions of substances into surface water

Emissions into the surface water from cooling systems caused by:

- applied cooling water additives and their reactants,

- airborne substances entering through a cooling tower,

- corrosion products caused by corrosion of the cooling systems’ equipment, and

- leakage of process chemicals (product) and their reaction products.
Executive Summary

Industrial Cooling Systems vii

Proper functioning of cooling systems may require the treatment of cooling water against

corrosion of the equipment, scaling and micro- and macrofouling. Treatments are different for

open once-through and recirculating cooling systems. For the latter systems, cooling water

treatment programmes can be highly complex and the range of chemicals used can be large. As

a consequence, emission levels in the blowdown of these systems also show large variation and

representative emission levels are difficult to report. Sometimes the blowdown is treated before

discharge.

Emissions of oxidizing biocides in open once-through systems, measured as free oxidant at the

outlet, vary between 0.1 [mg FO/l] and 0.5 [mg FO/l] depending on the pattern and frequency of

dosage.

Table 3: Chemical components of cooling water treatments used in open and recirculating wet

cooling systems

Water quality problems

Examples of chemical

treatment* Corrosion Scaling (Bio-)fouling

Oncethrough

systems

Recirculating

systems

Oncethrough

systems

Recirculating

systems

Oncethrough

systems

Recirculating

systems

Zinc X

Molybdates X

Silicates X

Phosphonates X X

Polyphosphonates X X

Polyol esters X

Natural organics X

Polymers (X) (X) X

Non-oxidizing biocides X

Oxidyzing biocides X X

* chromate is not widely used anymore due to its high environmental effect

Selecting and applying cooling equipment that is constructed of material suitable for the

environment in which it will operate can reduce leakage and corrosion. This environment is

described by:

- the process conditions, such as temperature, pressure, flow speed,

- the media cooled, and

- the chemical characteristics of the cooling water.

Materials commonly used for heat exchangers, conduits, pumps and casing are carbon steel,

copper-nickel and various qualities of stainless steel, but titanium (Ti) is increasingly used.

Coatings and paints are also applied to protect the surface.

• Use of biocides

Open once-through systems are predominantly treated with oxidizing biocides against

macrofouling. The amount applied can be expressed in the yearly used oxidative additive

expressed as chlorine-equivalent per MWth in connection with the level of fouling in or close to

the heat exchanger. The use of halogens as oxidative additives in once-through systems will

lead to environmental loads primarily by producing halogenated by-products.
Executive Summary

viii Industrial Cooling Systems

In open recirculating systems, pretreatment of water is applied against scaling, corrosion and

micro-fouling. With the relatively smaller volumes of recirculating wet systems alternative

treatments are successfully applied, such as ozone and UV light, but they require specific

process conditions and can be quite costly.

Operational measures reducing harmful effects of cooling water discharge are the closing of the

purge during shock treatment and the treatment of the blowdown before discharge into the

receiving surface water. For treatment of blowdown in a wastewater treatment facility the

remaining biocidal activity must be monitored as it may affect the microbial population.

To reduce the emissions in the discharge and to reduce the impact on the aquatic environment,

biocides are selected which aim to match the requirements of the cooling systems with the

sensitivity of the receiving aquatic environment.

• Emissions to air

The discharged air from dry circuit cooling towers is usually not considered as the most

important aspect of cooling. Contamination may occur if there is a leak of product, but proper

maintenance can prevent this.

The droplets in the discharge of wet cooling towers can be contaminated with water treatment

chemicals, with microbes or with corrosion products. The application of drift eliminators and

an optimized water treatment programme reduce potential risks.

Plume formation is considered where the horizon-marring effect occurs or where risk exists of

the plume reaching ground level.

• Noise

The emission of noise is a local issue for large natural draught cooling towers and all

mechanical cooling systems. Unattenuated sound power levels vary between 70 for natural

draught and about 120 [dB(A)] for mechanical towers. Variation is due to differences in

equipment and to place of measurement as it differs between air inlet and air outlet. Fans,

pumps and falling water are the major sources.

• Risk aspects

Risk aspects of cooling systems refer to leakage from heat exchangers, to storage of chemicals

and to microbiological contamination (such as legionnaire’s disease) of wet cooling systems.

Preventive maintenance and monitoring are applied measures to prevent leakage as well as

microbiological contamination. Where leakage could lead to discharges of large amounts of

substances harmful to the aquatic environment, indirect cooling systems or special preventive

measures are considered.

For prevention of the development of Legionellae pneumophila (Lp) an adequate water

treatment programme is advised. No upper concentration limits for Lp, measured in colony

forming units [CFU per liter], could be established below which no risk is to be expected. This

risk has to be particularly addressed during maintenance operations.

• Residues from cooling systems operation

Little has been reported on residues or wastes. Sludges from cooling water pretreatment or

from the basin of cooling towers have to be regarded as waste. They are treated and disposed of
Executive Summary

Industrial Cooling Systems ix

in different ways depending on the mechanical properties and chemical composition.

Concentration levels vary with the cooling water treatment programme.

Environmental emissions are further reduced by applying less harmful conservation methods for

equipment and by selecting material that can be recycled after decommissioning or replacement

of cooling systems’ equipment.

4. Key BAT conclusions

BAT or the primary BAT approach for new and existing systems are presented in Chapter 4.

The findings can be summarised as follows.

It is acknowledged that the final BAT solution will be a site-specific solution, but for some

issues techniques could be identified as general BAT. In all situations the available and

applicable options for reuse of heat must have been examined and used to reduce the amount

and level of non-recoverable heat, before the dissipation of heat from an industrial process into

the environment is considered.

For all installations BAT is a technology, method or procedure and the result of an integrated

approach to reduce the environmental impact of industrial cooling systems, maintaining the

balance between both the direct and indirect impacts. Reduction measures should be considered

maintaining at minimum the efficiency of the cooling system or with a loss of efficiency, which

is negligible, compared with the positive effects on the environmental impact.

For a number of environmental aspects, techniques have been identified that can be considered

BAT within the BAT approach. No clear BAT could be identified on the reduction of waste or

on techniques to handle waste while avoiding environmental problems such as contamination of

soil and water or, with incineration, of air.

• Process and site requirements

Selection between wet, dry and wet/dry cooling to meet process and site requirements should

aim at the highest overall energy efficiency. To achieve a high overall energy efficiency when

handling large amounts of low level heat (10-25oC) it is BAT to cool by open once-through

systems. In a greenfield situation this may justify selection of a (coastal) site with reliable large

amounts of cooling water available and with surface water with sufficient capacity to receive

large amounts of discharged cooling water.

Where hazardous substances are cooled that (emitted via the cooling system) involve a high risk

to the environment, it is BAT to apply indirect cooling systems using a secondary cooling

circuit.

In principle, the use of groundwater for cooling has to be minimized, for instance where

depletion of groundwater resources cannot be ruled out.

• Reduction of direct energy consumption

Low direct energy consumption by the cooling system is achieved by reducing resistance to

water and/or air in the cooling system, by applying low energy equipment. Where the process

to be cooled demands variable operation, modulation of air and water flow has been

successfully applied and can be considered BAT.
Executive Summary

x Industrial Cooling Systems

• Reduction of water consumption and reduction of heat emissions to water

The reduction of water consumption and the reduction of heat emissions to water are closely

linked and the same technological options apply.

The amount of water needed for cooling is linked to the amount of heat to be dissipated. The

higher the level of reuse of cooling water, the lower the amounts of cooling water needed.

Recirculation of cooling water, using an open or closed recirculating wet system, is BAT where

the availability of water is low or unreliable.

In recirculating systems an increase of the number of cycles can be BAT, but demands on

cooling water treatment may be a limiting factor.

It is BAT to apply drift eliminators to reduce drift to less than 0.01% of the total recirculating

flow.

• Reduction of entrainment

Many different techniques have been developed to prevent entrainment or to reduce the damage

in case of entrainment. Success has been variable and site-specific. No clear BAT have been

identified, but emphasis is put on an analysis of the biotope, as success and failure much depend

on behavioural aspects of the species, and on proper design and positioning of the intake.

• Reduction of emissions of chemical substances to water

In line with the BAT approach, the application of the potential techniques to reduce emissions

to the aquatic environment should be considered in the following order:

1. selection of cooling configuration with lower emission level to surface water,

2. use of more corrosion resistant material for cooling equipment,

3. prevention and reduction of leakage of process substances into the cooling circuit,

4. application of alternative (non-chemical) cooling water treatment,

5. selection of cooling water additives with the aim of reducing impact on the environment,

and

6. optimized application (monitoring and dosage) of cooling water additives.

BAT is reducing the need for cooling water conditioning by reducing the occurrence of fouling

and corrosion through proper design. In once-through systems, proper design is to avoid

stagnant zones and turbulence and to maintain a minimum water velocity (0.8 [m/s] for heat

exchangers, 1.5 [m/s] for condensers).

It is BAT to select material for once-through systems in a highly corrosive environment

involving Ti or high quality stainless steel or other materials with similar performance, where a

reducing environment would limit the use of Ti.

In recirculating systems, in addition to design measures, it is BAT to identify the applied cycles

of concentration and the corrosiveness of the process substance to enable selection of material

with adequate corrosion resistance.

It is BAT for cooling towers to apply suitable fill types under consideration of water quality

(content of solids), expected fouling, temperatures and erosion resistance, and to select

construction material which does not need chemical conservation.

The VCI concept applied by chemical industry aims at minimizing the risks for the aquatic

environment in case of leakage of process substances. The concept links the level of

environmental impact of a process substance with the required cooling configuration and

monitoring requirements. With higher potential risks for the environment in case of leakage the
Executive Summary

Industrial Cooling Systems xi

concept leads to improved anti-corrosiveness, indirect cooling design and an increasing level of

monitoring of the cooling water.

• Reduction of emissions by optimized cooling water treatment

Optimization of the application of oxidizing biocides in once-through systems is based on

timing and frequency of biocide dosing. It is considered BAT to reduce the input of biocides by

targeted dosing in combination with monitoring of the behavior of macrofouling species (e.g.

valve movement of mussels) and using the residence time of the cooling water in the system.

For systems where different cooling streams are mixed in the outlet, pulse-alternating

chlorination is BAT and can reduce even further free oxidant concentrations in the discharge.

In general, discontinuous treatment of once-through systems is sufficient to prevent fouling.

Depending on species and water temperature (above 10-12°C) continuous treatment at low

levels may be necessary.

For seawater, BAT-levels of free residual oxidant (FRO) in the discharge, associated with these

practices, vary with applied dosage regime (continuous and discontinuous) and dosage

concentration level and with the cooling system configuration. They range from ≤ 0.1 [mg/l] to

0.5 [mg/l], with a value of 0.2 [mg/l] as 24h-average.

An important element in introducing a BAT-based approach to water treatment, in particular for

recirculating systems using non-oxidizing biocides, is the making of informed decisions about

what water treatment regime is applied, and how it should be controlled and monitored.

Selection of an appropriate treatment regime is a complex exercise, which must take into

account a number of local and site-specific factors, and relate these to the characteristics of the

treatment additives themselves, and the quantities and combinations in which they are used.

In order to assist the process of BAT decision making on cooling water additives at a local

level, the BREF seeks to provide the local authorities responsible for issuing an IPPC permit

with an outline for an assessment.

The Biocidal Products Directive 98/8/EC regulates the placing of biocidal products on the

European market and considers as a specific category the biocides used in cooling systems. The

information exchange shows that in some Member States specific assessment regimes are in

place for the application of cooling water additives.

The discussion as part of the information exchange on industrial cooling systems resulted in two

proposed concepts for cooling water additives, which can be used as a complementary tool by

the permitting authorities:

1. A screening assessment tool based on the existing concepts, which allows a simple relative

comparison of cooling water additives in terms of their potential aquatic impact (the

Benchmarking Assessment, Annex VIII.1).

2. A site specific assessment of the expected impact of biocides discharged in the receiving

water, following the outcome of the Biocidal Products Directive and using the methodology

to establish Environmental Quality Standards (EQSs) of the future Water Framework

Directive as key elements (the Local Assessment for Biocides, Annex VIII.2).

The Benchmarking Assessment can be seen as a method to compare the environmental impact

of several alternative cooling water additives while the Local Assessment for Biocides provides

a yard stick for the determination of a BAT compatible approach for biocides in particular

(PEC/PNEC <1). The use of local assessment methodologies as a tool in controlling industrial

emissions is already common practice.
Executive Summary

xii Industrial Cooling Systems

• Reduction of emissions to air

The reduction of the impact of emissions to air from cooling tower operation is linked to the

optimization of cooling water conditioning to reduce concentrations in the droplets. Where drift

is the main transporting mechanism, the application of drift eliminators, resulting in less than

0.01% of the recirculating flow being lost as drift, is considered BAT.

• Reduction of noise

Primary measures are applications of low noise equipment. The associated reduction levels are

up to 5 [dB(A)].

Secondary measures at inlet and outlet of mechanical cooling towers have associated reduction

levels of a minimum of 15 [dB(A)] or more. It must be noted that noise reduction, in particular

by secondary measures, can lead to pressure drop, which needs extra energy input to

compensate.

• Reduction of leakage and microbiological risk

BAT are: preventing leakage by design; by operating within the design limits and by regular

inspection of the cooling system.

For the chemical industry in particular, it is considered BAT to apply the safety concept of VCI

as has been mentioned before for reduction of emissions to water.

The occurrence in a cooling system of Legionella pneumophila cannot be fully prevented. It is

considered BAT to apply the following measures:

- avoid stagnant zones and keep sufficient water velocity,

- optimize cooling water treatment to reduce fouling, algae and amoeba growth and

proliferation,

- apply periodic cleaning of the cooling tower basin and

- reduce respiratory vulnerability of operators by supplying noise and mouth protection when

entering an operating unit or when high-pressure cleaning the tower.

5. Distinction between new and existing systems

All key BAT conclusions can be applied to new systems. Where it involves technological

changes, the application may be limited for existing cooling systems. For small cooling towers

produced in series, a change in technology is considered to be technically and economically

feasible. Technological changes for large systems are generally cost intensive requiring a

complex technical and economic assessment involving a large number of factors. Relatively

small adaptations to these large systems, changing part of the equipment, may be feasible in

some cases. For more extensive changes of technology a detailed consideration and assessment

of the environmental effect and the costs may be necessary.

In general, BAT for new and existing systems are similar, where the focus is on reducing

environmental impact by improvement of the systems’ operation. This refers to:

- optimization of cooling water treatment by controlled dosage and selection of cooling water

additives aiming at reduction of the impact on the environment,

- regular maintenance of the equipment, and

- monitoring of operating parameters, such as the corrosion rate of the heat exchanger

surface, chemistry of the cooling water and degree of fouling and leakage.
Executive Summary

Industrial Cooling Systems xiii

Examples of techniques considered BAT for existing cooling systems are:

- application of suitable fill to counteract fouling,

- replacement of rotating equipment by low noise devices,

- prevention of leakage by monitoring heat exchanger tubes,

- side stream biofiltration,

- improvement of the quality of the make up water, and

- targeted dosage in once-through systems.

6. Conclusions and recommendations for future work

This BREF has met a high level of support from the Technical Working Group (TWG). To

assess and identify BAT for the process of industrial cooling is generally considered as complex

and very site- and process-specific, involving many technical and cost aspects. Still, there is

clear support for the concept of general BAT for cooling systems based on the general BREFPreface and the introduction on BAT in Chapter 4.

The process of information exchange revealed a number of issues where further work is needed

when this BREF is reviewed. The local assessment of cooling water treatment will require

further investigation on how to take into account all relevant factors and chemical

characteristics related to the site, but at the same time clear guidance and a workable procedure

are necessary. Other fields of interest where additional efforts would be needed concern

alternative cooling water treatment techniques, the minimization of microbiological risk and the

relevance of emissions to air.

Industrial Cooling Systems i

Draft Reference Document on the application of Best Available Techniques to

Industrial Cooling Systems

Executive summary .................................................................................................................................... i

Preface .........................................................................................................................................................1

Scope............................................................................................................................................................5

Glossary.......................................................................................................................................................7

Thermodynamic definitions......................................................................................................................7

Other definitions.......................................................................................................................................8

Abbreviations and acronyms ..................................................................................................................13

1 General BAT concept for industrial cooling systems....................................................................15

1.1 Sources of heat, heat levels and application ranges ...................................................................19

1.2 Level of cooling system and influence on process efficiency....................................................20

1.2.1 Temperature sensitive applications ......................................................................................20

1.2.2 Non-sensitive applications ...................................................................................................22

1.3 Optimising the primary process and reuse of heat.....................................................................22

1.3.1 Optimising the primary process ...........................................................................................22

1.3.2 Use of waste heat off-site.....................................................................................................23

1.4 Selecting of a cooling system in order to meet the process requirements and site conditions...23

1.4.1 Process requirements............................................................................................................23

1.4.2 Site selection ........................................................................................................................25

1.4.3 Climatic conditions ..............................................................................................................28

1.4.4 Mathematical modelling, simulations on models and tests on pilot loops ...........................30

1.5 Selecting a cooling technique in order to meet environmental requirements ............................30

1.5.1 General comparison between air and water cooled systems ................................................30

1.5.2 Design factors and choice of materials.................................................................................31

1.5.3 Options for a technological change of existing systems.......................................................32

1.5.3.1 Retrofit – reasons and considerations.............................................................................33

1.5.3.2 Change of heat transfer technology ................................................................................34

1.5.3.3 Replacement of outdated heat transfer technology by modern one................................35

1.5.3.4 Upgrading existing heat transfer technology..................................................................36

1.6 Economic considerations ...........................................................................................................37

2 Technological aspects of applied cooling systems..........................................................................39

2.1 Introduction ...............................................................................................................................39

2.2 Heat exchangers.........................................................................................................................41

2.2.1 Shell and tube heat exchangers ............................................................................................41

2.2.2 Plate and frame heat exchangers ..........................................................................................41

2.2.3 Environmental issues of heat exchangers.............................................................................42

2.3 Once-through cooling systems...................................................................................................42

2.3.1 Direct once-through cooling systems...................................................................................42

2.3.2 Once-through cooling systems with cooling tower..............................................................43

2.3.3 Indirect once-through cooling systems.................................................................................44

2.4 Open recirculating cooling systems...........................................................................................45

2.4.1 Natural draught wet cooling towers .....................................................................................46

2.4.2 Mechanical draught wet cooling towers...............................................................................48

2.4.2.1 Forced draught wet cooling towers ................................................................................49

2.4.2.2 Induced draught wet cooling towers...............................................................................50

2.5 Closed circuit cooling systems ..................................................................................................51

2.5.1 Air-cooled cooling systems..................................................................................................51

2.5.1.1 Natural draught dry cooling tower..................................................................................52

2.5.1.2 Air-cooled liquid cooling systems..................................................................................54

2.5.1.3 Air-cooled steam condensers..........................................................................................55

2.5.2 Closed circuit wet cooling systems ......................................................................................56

2.5.2.1 Mechanical draught wet closed circuit cooling systems.................................................57

2.5.2.2 Evaporative steam condensers........................................................................................57

2.6 Combined wet/dry cooling systems...........................................................................................58

2.6.1 Open wet/dry (hybrid) cooling towers .................................................................................58

2.6.2 Closed circuit hybrid cooling systems..................................................................................59

2.6.2.1 Sprayed (finned) coils.....................................................................................................60

2.6.2.2 Adiabatic coolers, wetting and pre-cooling the air that cools the coils ..........................60

2.6.2.3 Combined technology.....................................................................................................61
ii Industrial Cooling Systems

2.6.2.4 Costs of hybrid systems ................................................................................................. 61

2.7 Recirculating cooling systems................................................................................................... 62

2.7.1 Direct recirculating cooling systems.................................................................................... 62

2.7.2 Indirect recirculating cooling systems ................................................................................. 62

2.8 Costs of cooling systems........................................................................................................... 62

3 Environmental aspects of industrial cooling systems and applied prevention and reduction

techniques......................................................................................................................................... 65

3.1 Introduction............................................................................................................................... 65

3.2 Consumption of energy ............................................................................................................. 67

3.2.1 Direct consumption of energy.............................................................................................. 67

3.2.2 Indirect consumption of energy ........................................................................................... 67

3.2.3 Reduction of required energy for cooling............................................................................ 70

3.3 Consumption and emission of cooling water ............................................................................ 71

3.3.1 Consumption of water ......................................................................................................... 71

3.3.1.1 Intake of water and water requirements ......................................................................... 71

3.3.1.2 Applied techniques to reduce water consumption.......................................................... 73

3.3.2 Fish entrainment .................................................................................................................. 74

3.3.2.1 Level of entrainment ...................................................................................................... 74

3.3.2.2 Applied reduction techniques......................................................................................... 75

3.3.2.3 Costs of sound devices and light systems ...................................................................... 77

3.3.3 Heat emission to surface water ............................................................................................ 77

3.3.3.1 Levels of heat emission.................................................................................................. 77

3.3.3.2 Legislative requirements of heat emissions ................................................................... 78

3.3.3.3 Applied reduction techniques......................................................................................... 79

3.4 Emissions from cooling water treatment................................................................................... 80

3.4.1 Application of cooling water treatment ............................................................................... 80

3.4.2 Emissions of chemicals into the surface water .................................................................... 83

3.4.2.1 Oxidising biocides ......................................................................................................... 83

3.4.2.2 Non-oxidising biocides .................................................................................................. 84

3.4.2.3 Factors influencing emissions of biocides ..................................................................... 84

3.4.2.4 Emission levels .............................................................................................................. 85

3.4.2.5 Legislation ..................................................................................................................... 85

3.4.3 Reduction of emissions to the surface water........................................................................ 86

3.4.3.1 General approach ........................................................................................................... 86

3.4.3.2 Reduction by selection of material and systems design ................................................. 91

3.4.4 Reduction by application of additional and alternative cooling water treatment................. 91

3.4.5 Reduction of emissions by assessment and selection of cooling water additives................ 92

3.4.6 Optimising the use of cooling water additives..................................................................... 95

3.4.6.1 Dosage of cooling water additives ................................................................................. 96

3.4.6.1.1 Dosage regimes ........................................................................................................ 96

3.4.6.1.2 Dosage systems ........................................................................................................ 97

3.4.6.2 Monitoring of cooling water .......................................................................................... 98

3.4.6.2.1 Monitoring of scale inhibitors, corrosion inhibitors and dispersants........................ 98

3.4.6.2.2 Monitoring of biofouling.......................................................................................... 99

3.5 Cooling air use and air emissions............................................................................................ 101

3.5.1 Air requirements................................................................................................................ 101

3.5.2 Direct and indirect emissions............................................................................................. 101

3.5.3 Cooling tower plumes........................................................................................................ 103

3.5.3.1 Plume formation........................................................................................................... 103

3.5.3.2 Plume abatement.......................................................................................................... 103

3.6 Noise emissions....................................................................................................................... 104

3.6.1 Sources of noise and noise levels ...................................................................................... 104

3.6.2 Noise abatement ................................................................................................................ 107

3.6.2.1 Noise control of cascading water (wet cooling towers) ............................................... 107

3.6.2.1.1 Primary measures ................................................................................................... 107

3.6.2.1.2 Secondary measures ............................................................................................... 107

3.6.2.1.3 Dry-cooling towers................................................................................................. 108

3.6.2.2 Noise control of mechanical equipment (mechanical draught cooling towers) ........... 108

3.6.2.2.1 Primary measures ................................................................................................... 108

3.6.2.2.2 Secondary measures ............................................................................................... 109

3.6.2.3 Costs of noise reduction............................................................................................... 109
Industrial Cooling Systems iii

3.7 Risk aspects associated with industrial cooling systems..........................................................110

3.7.1 Risk of leakage...................................................................................................................110

3.7.1.1 Occurrence and consequences......................................................................................110

3.7.1.2 Reduction of leakage ....................................................................................................111

3.7.1.3 Reduction by preventive maintenance..........................................................................112

3.7.2 Storage and handling of chemicals.....................................................................................113

3.7.3 Microbiological risk...........................................................................................................113

3.7.3.1 Occurrence of microbes................................................................................................113

3.7.3.2 Measuring of bacteriae .................................................................................................114

3.7.3.3 Techniques to reduce microbiological risks .................................................................114

3.8 Waste from cooling system operation......................................................................................117

3.8.1 Formation of sludges..........................................................................................................117

3.8.2 Residues from cooling water treatment and cleaning operations .......................................117

3.8.3 Residues as a result of retrofitting, replacing and decommissioning of the installation.....117

3.8.3.1 Use of plastics ..............................................................................................................118

3.8.3.2 Treatment of timber used for wet cooling tower construction......................................118

3.8.3.3 Wet cooling tower fill...................................................................................................118

4 Best available techniques for industrial cooling systems ............................................................119

4.1 Introduction .............................................................................................................................119

4.2 A horizontal approach to defining BAT for cooling systems ..................................................120

4.2.1 Integrated heat management...............................................................................................121

4.2.1.1 Industrial cooling = Heat management.........................................................................121

4.2.1.2 Reduction of the level of heat discharge by optimization of internal/external heat reuse

......................................................................................................................................121

4.2.1.3 Cooling system and process requirements....................................................................121

4.2.1.4 Cooling system and site requirements ..........................................................................123

4.2.2 Application of BAT in industrial cooling systems.............................................................124

4.3 Reduction of energy consumption ...........................................................................................125

4.3.1 General...............................................................................................................................125

4.3.2 Identified reduction techniques within the BAT-approach ................................................126

4.4 Reduction of water requirements.............................................................................................127

4.4.1 General...............................................................................................................................127

4.4.2 Identified reduction techniques within the BAT-approach ................................................127

4.5 Reduction of entrainment of organisms...................................................................................128

4.5.1 General...............................................................................................................................128

4.5.2 Identified reduction techniques within the BAT-approach ................................................128

4.6 Reduction of emissions to water..............................................................................................128

4.6.1 General BAT approach to reduce heat emissions...............................................................128

4.6.2 General BAT approach to reduce chemical emissions to water.........................................129

4.6.3 Identified reduction techniques within the BAT-approach ................................................131

4.6.3.1 Prevention by design and maintenance.........................................................................131

4.6.3.2 Control by optimised cooling water treatment .............................................................133

4.7 Reduction of emissions to air...................................................................................................134

4.7.1 General approach ...............................................................................................................134

4.7.2 Identified reduction techniques within the BAT-approach ................................................135

4.8 Reduction of noise emissions ..................................................................................................135

4.8.1 General...............................................................................................................................135

4.8.2 Identified reduction techniques within the BAT-approach ................................................136

4.9 Reduction of risk of leakage ....................................................................................................136

4.9.1 General approach ...............................................................................................................136

4.9.2 Identified reduction techniques within the BAT-approach ................................................137

4.10 Reduction of biological risk.....................................................................................................137

4.10.1 General approach ...............................................................................................................137

4.10.2 Identified reduction techniques within the BAT-approach ................................................138

5 Concluding remarks.......................................................................................................................139

5.1 Timing of the work ..................................................................................................................139

5.2 Sources of information ............................................................................................................139

5.3 Recommendations for future work ..........................................................................................139

References ...............................................................................................................................................141

Annexes....................................................................................................................................................149
iv Industrial Cooling Systems

List of tables

Table 1.1: Heat temperature levels and application range.......................................................................... 20

Table 1.2: Emissions of an average Western European power plant due to an efficiency loss of 3% ....... 21

Table 1.3: Relative effect on the delivery of electrical power due to the application of wet, wet/dry or dry

cooling towers to units of 1300 MWe..................................................................................... 21

Table 1.4: Relative effect on the delivery of electrical power due to the application of wet, wet/dry or dry

cooling towers to a 290 MWth combined cycle unit ............................................................... 22

Table 1.5: Large cooling demand-related criteria for site selection ........................................................... 27

Table 1.6: Climatic conditions in Europe................................................................................................... 29

Table 1.7: Comparison of different cooling systems with a required maximum sound power level ......... 32

Table 1.8: Technological upgrading options for existing systems ............................................................. 34

Table 1.9: Example for conversion of a once-through system into a recirculating system ........................ 35

Table 1.10: Example for conversion of an outdated mechanical draught wet cooling tower into modern

design ..................................................................................................................................... 36

Table 1.11: Example for replacement of outdated fill of a mechanical draught wet cooling tower with

modern high efficiency fill ..................................................................................................... 36

Table 1.12: Example for the improvement of acoustic performance by addition of sound attenuation..... 37

Table 2.1: Example of technical and thermodynamic characteristics of the different cooling systems for

industrial (non-power plant) applications............................................................................... 40

Table 2.2: Examples of capacity and thermodynamic characteristics of different cooling systems for

applications in power industry................................................................................................ 41

Table 2.3: Cost elements for water and air cooling systems ...................................................................... 63

Table 3.1: Environmental issues of the different industrial cooling systems ............................................. 66

Table 3.2: Example of a comparison of yearly specific direct and indirect energy demand of different

cooling systems and the consequences for CO2-emissions per MWth .................................... 69

Table 3.3: Water requirement of the different cooling systems ................................................................. 72

Table 3.4: Fish impingement rates (FIR) at power stations. Annual catches normalised to the cooling

water flow............................................................................................................................... 75

Table 3.5: Available fish protection technologies for cooling water intake devices.................................. 76

Table 3.6: Thermal requirements of water temperatures for two ecological systems (European Directive

78/659/EEC)........................................................................................................................... 78

Table 3.7: Chemical components of cooling water treatments used in open and recirculating wet cooling

systems ................................................................................................................................... 82

Table 3.8: Consumption of hypochlorite in wet cooling systems in The Netherlands............................... 83

Table 3.9: Average required airflow for the different cooling systems.................................................... 101

Table 3.10: Examples of capacity and associated unattenuated sound power levels of cooling systems

equipment of a large refinery................................................................................................ 106

Table 3.11: Comparison of unattenuated sound power levels at air inlet and air outlet measured at various

types of wet cooling towers of conventional construction ................................................... 106

Table 3.12: Noise emissions of different cooling systems without noise attenuation.............................. 106

Table 3.13: Example of cost increase with different fan design for reduced sound power level ............. 109

Table 3.14: Effects of temperature and biocide treatment on CFU-levels in cooling towers................... 116

Table 4.1: Examples of process requirements and BAT .......................................................................... 122

Table 4.2: Examples of site characteristics and BAT............................................................................... 123

Table 4.3: BAT for increasing overall energy efficiency......................................................................... 126

Table 4.4: BAT for reduction of water requirements ............................................................................... 127

Table 4.5: BAT for reduction of entrainment........................................................................................... 128

Table 4.6: BAT for reduction of emissions to water by design and maintenance techniques.................. 131

Table 4.7: BAT for reduction of emissions to water by optimised cooling water treatment.................... 133

Table 4.8: BAT for reduction of emissions to air..................................................................................... 135

Table 4.9: BAT for the reduction of noise emissions............................................................................... 136

Table 4.10: BAT to reduce the risk of leakage......................................................................................... 137

Table 4.11: BAT to reduce biological growth.......................................................................................... 138
Industrial Cooling Systems v

List of figures

Figure 1.1: Breakdown structure showing the factors involved in the determination of BAT for waste heat

discharge systems ...................................................................................................................17

Figure 1.2: Tower plan area as a function of the percentage of time the design wet bulb temperature is

exceeded .................................................................................................................................24

Figure 2.1: Schematic representation of a direct once-through cooling system .........................................42

Figure 2.2: Schematic representation of a direct once-through cooling system with a cooling tower

applied in power industry........................................................................................................43

Figure 2.3: Schematic representation of an indirect once-through cooling system ....................................44

Figure 2.4: Schematic representation of an open recirculating system.......................................................45

Figure 2.5: Natural draught wet cooling tower counter flow......................................................................47

Figure 2.6: Natural draught wet cooling tower cross flow..........................................................................48

Figure 2.7: Fan assisted natural draught cooling tower ..............................................................................49

Figure 2.8: Schematic representation of a forced draught tower with counter flow design........................50

Figure 2.9: Schematic presentation of a cell type induced draught cooling tower, cross flow design........51

Figure 2.10: Schematic presentation of principle of a dry natural draught cooling tower..........................53

Figure 2.11: Example of natural draught dry cooling tower for a power plant application........................53

Figure 2.12: Schematic presentation of the principle of a dry air-cooled cooling system ..........................54

Figure 2.13: Example of a dry air-cooled liquid cooler in a chemical process...........................................54

Figure 2.14: Schematic presentation of the principle of a direct air-cooled condenser ..............................55

Figure 2.15: Example of an air-cooled condenser for condensation of turbine exhaust steam...................56

Figure 2.16: Schematic presentation of the principle of a closed recirculating wet cooling tower with

induced draught.......................................................................................................................57

Figure 2.17: Schematic representation of hybrid cooling tower principle (example applied in power

industry)..................................................................................................................................58

Figure 2.18: Schematic presentation of the principle of a closed circuit hybrid cooling tower..................60

Figure 2.19: Combined dry/wet operation of a hybrid cooling system.......................................................61

Figure 3.1: Graphic representation of interrelation between the different water quality problems ............81

Figure 3.2: Design scheme for cooling water systems aiming at reduction of biocide application ............87

Figure 3.3: “approach” for reduction of biocide use in industrial cooling water systems ..........................88

Figure 3.4: Additive concentration pattern resulting of improperly designed monitoring and dosage

regime .....................................................................................................................................95

Figure 3.5: Additive concentration pattern resulting of properly designed monitoring and dosage regime

................................................................................................................................................96

Preface

Industrial Cooling Systems 1

PREFACE

1. Status of this document

Unless otherwise stated, references to “the Directive” in this document means the Council

Directive 96/61/EC on integrated pollution prevention and control. This document forms part of

a series presenting the results of an exchange of information between EU Member States and

industries concerned on best available techniques (BAT), associated monitoring, and

development in them. It is published by the European Commission pursuant to Article 16(2) of

the Directive, and must therefore be taken into account in accordance with Annex IV of the

Directive when determining “best available techniques”.

2. Relevant legal obligations of the IPPC Directive and the definition of BAT

In order to help the reader understand the legal context in which this document has been drafted,

some of the most relevant provisions of the IPPC Directive, including the definition of the term

“best available techniques”, are described in this preface. This description is inevitably

incomplete and is given for information only. It has no legal value and does not in any way alter

or prejudice the actual provisions of the Directive.

The purpose of the Directive is to achieve integrated prevention and control of pollution arising

from the activities listed in its Annex I, leading to a high level of protection of the environment

as a whole. The legal basis of the Directive relates to environmental protection. Its

implementation should also take account of other Community objectives such as the

competitiveness of the Community’s industry thereby contributing to sustainable development.

More specifically, it provides for a permitting system for certain categories of industrial

installations requiring both operators and regulators to take an integrated, overall look at the

polluting and consuming potential of the installation. The overall aim of such an integrated

approach must be to improve the management and control of industrial processes so as to ensure

a high level of protection for the environment as a whole. Central to this approach is the general

principle given in Article 3 that operators should take all appropriate preventative measures

against pollution, in particular through the application of best available techniques enabling

them to improve their environmental performance.

The term “best available techniques” is defined in Article 2(11) of the Directive as “the most

effective and advanced stage in the development of activities and their methods of operation

which indicate the practical suitability of particular techniques for providing in principle the

basis for emission limit values designed to prevent and, where that is not practicable, generally

to reduce emissions and the impact on the environment as a whole.” Article 2(11) goes on to

clarify further this definition as follows:

“techniques” includes both the technology used and the way in which the installation is

designed, built, maintained, operated and decommissioned;

“available” techniques are those developed on a scale which allows implementation in the

relevant industrial sector, under economically and technically viable conditions, taking into

consideration the costs and advantages, whether or not the techniques are used or produced

inside the Member State in question, as long as they are reasonably accessible to the operator;

“best” means most effective in achieving a high general level of protection of the environment

as a whole.
Preface

2 Industrial Cooling Systems

Furthermore, Annex IV of the Directive contains a list of “considerations to be taken into

account generally or in specific cases when determining best available techniques ... bearing in

mind the likely costs and benefits of a measure and the principles of precaution and prevention”.

These considerations include the information published by the Commission pursuant to

Article 16(2).

Competent authorities responsible for issuing permits are required to take account of the general

principles set out in Article 3 when determining the conditions of the permit. These conditions

must include emission limit values, supplemented or replaced where appropriate by equivalent

parameters or technical measures. According to Article 9(4) of the Directive, these emission

limit values, equivalent parameters and technical measures must, without prejudice to

compliance with environmental quality standards, be based on the best available techniques,

without prescribing the use of any technique or specific technology, but taking into account the

technical characteristics of the installation concerned, its geographical location and the local

environmental conditions. In all circumstances, the conditions of the permit must include

provisions on the minimisation of long-distance or transboundary pollution and must ensure a

high level of protection for the environment as a whole.

Member States have the obligation, according to Article 11 of the Directive, to ensure that

competent authorities follow or are informed of developments in best available techniques.

3. Objective of this Document

Article 16(2) of the Directive requires the Commission to organise “an exchange of information

between Member States and the industries concerned on best available techniques, associated

monitoring and developments in them”, and to publish the results of the exchange.

The purpose of the information exchange is given in recital 25 of the Directive, which states that

“the development and exchange of information at Community level about best available

techniques will help to redress the technological imbalances in the Community, will promote the

world-wide dissemination of limit values and techniques used in the Community and will help

the Member States in the efficient implementation of this Directive.”

The Commission (Environment DG) established an information exchange forum (IEF) to assist

the work under Article 16(2) and a number of technical working groups have been established

under the umbrella of the IEF. Both IEF and the technical working groups include

representation from Member States and industry as required in Article 16(2).

The aim of this series of documents is to reflect accurately the exchange of information which

has taken place as required by Article 16(2) and to provide reference information for the

permitting authority to take into account when determining permit conditions. By providing

relevant information concerning best available techniques, these documents should act as

valuable tools to drive environmental performance.

4. Information Sources

This document represents a summary of information collected from a number of sources,

including in particular the expertise of the groups established to assist the Commission in its

work, and verified by the Commission services. All contributions are gratefully acknowledged.

5. How to understand and use this document

The information provided in this document is intended to be used as an input to the

determination of BAT in specific cases. When determining BAT and setting BAT-based permit

conditions, account should always be taken of the overall goal to achieve a high level of

protection for the environment as a whole.
Preface

Industrial Cooling Systems 3

The rest of this section describes the type of information that is provided in each section of the

document.

Chapters 1 provides information on the issue of cooling industrial processes and on the

horizontal approach taken to present BAT applied to industrial cooling systems.

Chapter 2 describes the cooling systems and their configurations commonly used by industry. In

this chapter some associated performance data are presented as well as an overview of the

relevant environmental issues.

Chapter 3 describes more extensively the environmental issues with reference to the emission

reduction and other techniques that are considered to be most relevant for determining BAT

both generally and as a basis for setting permit conditions. As far as applicable, this information

includes the consumption and emission levels considered achievable that relate to the cooling

systems applied. Techniques that are generally seen as obsolete are not included.

Chapter 4 concludes on general BAT within the primary BAT-approach acknowledging that

BAT for cooling systems finally is a site-specific solution.

Chapter 5 gives a general conclusion on the result of the information exchange process with

respect to industrial cooling systems and describes elements for future work.

In twelve annexes additional information is given on thermodynamics, energy, operating

factors, as well as information on techniques and practices to be considered in the application of

BAT when operating an industrial cooling system.

The purpose is thus to provide general indications regarding the emission and consumption

levels that can be considered as an appropriate reference point to assist in the determination of

BAT-based permit conditions or for the establishment of general binding rules under Article

9(8). It should be stressed, however, that this document does not propose emission limit values.

The determination of appropriate permit conditions will involve taking account of local, sitespecific factors such as the technical characteristics of the installation concerned, its

geographical location and the local environmental conditions. In the case of existing

installations, the economic and technical viability of upgrading them also needs to be taken into

account. Even the single objective of ensuring a high level of protection for the environment as

a whole will often involve making trade-off judgements between different types of

environmental impact, and these judgements will often be influenced by local considerations.

Although an attempt is made to address some of these issues, it is not possible for them to be

considered fully in this document. The techniques and levels presented in chapters 3 and 4 will

therefore not necessarily be appropriate for all installations. On the other hand, the obligation to

ensure a high level of environmental protection including the minimisation of long-distance or

transboundary pollution implies that permit-conditions cannot be set on the basis of purely local

considerations. It is therefore of the utmost importance that the information contained in this

document is fully taken into account by permitting authorities.

Since the best available techniques change over time, this document will be reviewed and

updated as appropriate. All comments and suggestions should be made to the European IPPC

Bureau at the Institute for Prospective Technological Studies at the following address:

Edificio Expo-WTC; Inca Garcilaso s/n, E-41092 Seville – Spain

Telephone: +34 95 4488 284 Fax: +34 95 4488 426

e-mail eippcb@jrc.es

Internet: http://eippcb.jrc.es

Scope

Industrial Cooling Systems 5

SCOPE

This reference document on BAT for industrial cooling systems is a horizontal document that

focuses on the cooling systems commonly used within the industrial activities of Annex 1 to the

IPPC Directive. The industry sectors with high relevance are the chemicals, food, glass, iron and

steel, refineries, pulp and paper and the incinerators. In the power industry, an incomparable

amount of information and experience has been gained with respect to cooling. Also, the power

industry relatively has the largest direct and indirect impacts on the environment with suboptimal cooling. In a separate annex, special attention is paid to this sector and any disparities

between power stations and other industrial activities have been assessed. Although installations

for the production of nuclear power are not part of the scope of Annex I of the IPPC Directive,

the applied environmental techniques are considered in this document where they relate to the

cooling systems of the conventional section of these installations. Cooling systems of small

combustion plants and air conditioning systems for both industrial and domestic use are

excluded.

The scope of the term “cooling systems” in this reference document is confined to systems to

remove waste heat from any medium, using heat exchange with water and/or air to bring down

the temperature of that medium towards ambient levels. This includes only part of refrigeration

systems, but excludes the issue of refrigerants such as ammonia and CFCs. Also, direct contact

cooling and barometric condensers are not assessed as they are considered to be too processspecific. The following industrial cooling systems or configurations are covered in this

document:

• Once-through cooling systems (with or without cooling tower)

• Open recirculating cooling systems (wet cooling towers)

• Closed circuit cooling systems

- air-cooled cooling systems

- closed circuit wet cooling systems

• Combined wet/dry (hybrid) cooling systems

- open hybrid cooling towers

- closed circuit hybrid tower

In this document, BAT is described for cooling systems that are considered to work as auxiliary

systems for the normal operation of an industrial process. It is acknowledged that reliable

operation of a cooling system will positively affect the reliability of the industrial process.

However, the function of a cooling system in relation to process safety is not within the scope of

this BREF.

Within the horizontal “approach”, integration means addressing all relevant environmental

aspects and the way in which they are interrelated, whilst acknowledging that balancing the

various aspects requires expert judgement. Where appropriate, the relevance of the

environmental performance of a cooling system within the performance of the entire industrial

process is indicated.

The document addresses the following environmental aspects and the methods and techniques

for reduction of emissions:

• effects of process and equipment design, and of material and maintenance;

• resource consumption (water, air, energy, chemical substances);

• emissions of chemicals and heat both to water and air,

• emissions of noise and plumes;

• waste generation and emissions to soil and terrestrial habitats;

• risk aspects;

• pollution arising from specific events (starts/stops) or incidents and

• decommissioning of installations.
Scope

6 Industrial Cooling Systems

This document will give a review of available techniques for industrial cooling systems, but will

not give solutions on what the best cooling system is and does not intend to disqualify any of the

existing systems applied. Nor will it give guidelines on whether a process needs a cooling

system at all. This means that the document will not go into the detail of the production

processes themselves that require cooling, where overall energy efficiency measures would be

addressed. A general “approach” is followed, which leads to a balanced choice of a new system

or of measures to optimise an existing cooling system aiming at prevention of environmental

emissions related to the operation of cooling systems.
Glossary

Industrial Cooling Systems 7

Glossary

Terminology used for the different aspects of industrial cooling systems varies widely and

various terms are often used for the same element. To avoid confusion and to avoid explanatory

repetition in this document this section gives a number of definitions of terms and abbreviations.

Thermodynamic definitions

Approach (1) in a conduction heat exchanger device, the

temperature difference between the temperature of the

process medium leaving the heat exchanger and the

temperature of the cooling medium entering the heat

exchanger.

(2) in an evaporative system (e.g. wet cooling tower),

the difference between the temperature of the process

medium leaving the cooling system and the wet bulb

temperature of the air entering the cooling tower or the

evaporative cooling system.

(3) in a condenser see terminal difference.

Design Dry Bulb Temperature the temperature of the ambient air for which the heat

exchanger is designed. Usually 95 % values are used –

the design temperature will not be exceeded in 95 % of

time. Dry bulb temperature is the relevant temperature

for sensible heat transfer.

Design Wet Bulb Temperature the lowest temperature, to which air can be cooled

down by adiabatic evaporation. It is the relevant

temperature for latent heat transfer. Design wet bulb

temperature is the temperature of saturated air, which

will be used for the design of the evaporative waste

heat exchanger. Usually 95 % values are used – the

design temperature will not be exceeded in 95 % of

time. Wet bulb temperature is always below dry bulb

temperature.

Heat Rejection Capacity the amount of heat, which can be rejected by a cooling

system measured in kWth (or MWth).

Latent Heat Transfer Heat transfer through the evaporation of water into the

air. The heat transfer capacity of evaporating water is

much higher than the heat transfer capacity of air.

Level of Waste Heat the temperature level at which the heat has to be

transferred. Depending on the process, the waste heat is

generated at a specific temperature level.

LMTD Logarithmic mean temperature difference is a measure

of the driving force of the heat exchange depending on

the temperature of the cold stream (coolant) and of the

process stream to be cooled.
Glossary

8 Industrial Cooling Systems

Range Range is the difference between the inlet and the outlet

temperature at a heat exchanger.

Sensible Heat Transfer Heat transfer through conduction and convection is

called sensible heat transfer.

Terminal difference is the temperature difference in a condenser. It

corresponds to the temperature difference between the

temperature of the steam entering the condenser (or

steam condensed leaving the condenser) and the

temperature of the cooling medium (water) leaving the

condenser. The values of “terminal difference” vary

between 3 and 5 K.

Waste Heat Waste heat is the inherent, yet unwanted, not

recuperative heat, that must be removed from industrial

or manufacturing processes and has to be transferred to

the environment.

Other definitions

BAT approach methodology presented in this document to arrive at a a

definition of BAT for industrial cooling systems and to

identify techniques within this definition

Bioconcentration factor the capacity of a substance to bioaccumulate defined as

the ration between the concentration of a substance in

an organism and its concentration in the water (in a

state of equilibrium). The bioconcentration is always

determined by experiment.

Blow down (BD, kg/s) the intentional draining of a cooling system to balance

the increasing concentration of solids in the cooling

system and in practice it is the water that has to be

withdrawn from an evaporative cooling system in order

to control the cycle of concentration. It is calculated as

BD = E*1/(x-1), where E is the evaporation loss and x

is the concentration factor. Calculating the blowdown

usually includes the non-evaporative losses such as

windage, drift and leakage.

Biocide chemical that kills or slows down the growth of

undesirable organisms. In cooling water systems a

biocide kills or slows down the growth of macro- and

micro-fouling organisms, thereby minimising organic

pollution in the cooling system. The most important

biocides are: chlorine, sodium hypochlorite, ozone,

quaternary ammonium and organic bromide.

Biocide demand the quantity of biocide that is reduced or converted to

inert or less active forms of the biocide by substances

in the water or the quantity of biocide that give

complete reaction with all biocide-reactable materials.
Glossary

Industrial Cooling Systems 9

Biochemical oxygen demand (BOD) a measure of the oxygen required to break down

(also named Biological oxygen demand) organic materials in water. Higher organic loads

require larger amounts of oxygen and may reduce the

amount of oxygen available for fish and aquatic life

below acceptable levels. It can be assessed using a

standard 5-day (BOD5) or 7-day (BOD7) test.

Bio-slime or bio-slime is defined as the bacterial film that

develops on any substrate immersed in water. It

consists of algae and sessile microbial population,

comprising slime producing bacteria and anaerobic

sulphate reducing bacteria. Microfouling promotes the

deposition of macrofouling.

Breakpoint the demand of oxidyzing biocide by water impurities

which must be overcome before a viable biocideconcentration in the cooling water is available.

Chemical oxygen demand (COD) a measure of the oxygen-consuming capacity of

inorganic and organic matter present in water or waste

(discharged cooling) water; the amount of oxygen

consumed from a chemical oxidant in a specific test.

(normally referring to analysis with dichromate

oxidation)

Coatings materials applied to surfaces to form either a low

friction surface to reduce pump losses, or a protective

layer to reduce erosion, corrosion and fouling.

Concentration factor (CR) concentration factor or cycles of concentration is the

ratio of concentration of any particular solute in the

recirculating cooling water to that in the makeup water.

Calculated as CR = MU/BD, where MU is the make up

water and BD the blow down.

Condenser cooler used for condensation of a gas flow (or steam).

Condensation places extra demands on the heat

exchanger: there must be space for the vapour volume.

Condensers of power stations are therefore extremely

large and specifically designed.

Coolant synonym for cooling medium. In many cases the

coolant is water or air, but can also be water mixed

with an antifreeze substance or a medium such as oil or

a gas.

Corrosion can be defined as the destruction of a metal by

(electro-) chemical reaction with its environment.

Corrosion inhibitors are chemical substances that are able to slow down the

corrosion process in water. They are de-aeration

substances, passivating inhibitors (eg chromate, nitrite,

molybdate, and orthophosphate), precipitating

inhibitors (zinc phosphate, calcium carbonate and

calcium orthophoshate), and adsorption inhibitors
Glossary

10 Industrial Cooling Systems

(glycine derivates, aliphatic sulfonates and sodium

silicate).

Counter flow is the principle where the air flows in the opposite

direction within the heat exchanger. In counter flow

towers air moves upward opposed to the downward

flow of the cooling water. This design provides good

heat exchange because the coolest air contacts the

coolest water. Headers and spray nozzles are used to

distribute the water.

Cross flow is the principle where air flows perpendicular to the

process fluid within the heat exchanger. In cross flow

towers air flows horizontally across the downward

stream of the cooling water.

Cycles of concentration (or “cycles”) are a comparison of the dissolved solids

level of the blowdown with that of the makeup water.

Thus, it is defined as quotient of salt concentration in

the blow-down and the salt concentration in the makeup.

Dispersion substances or dispersants are chemicals that prevent the growth

and deposit of particles present in water by increasing

the electric charge resulting from absorption. As a

result the particles repel each other and remain

suspended.

Drift eliminators devices that change the direction of airflow, imparting

centrifugal force to separate water droplets from the

air.

Drift loss the loss of water due to small droplets, which are

emitted into the draught air, leaving the top of a

cooling tower.

Evaporative loss (E, kg/s) the mass of cooling water, which is evaporated per

time unit during the operation of an evaporative

cooling-system.

Free oxidant (FO)/ applied measure of free oxidants in the discharge of

Total residual oxidants (TRO) cooling water systems. Also referred to as TRO or total

chlorine (TC) or free chlorine (FC).

Free available chlorine (FAC) free chlorine represents an equilibrium mixture of

or free residual chlorine hypochlorous acid and hypochlorite ion OCl− in the

cooling water system. Both are oxidants, but OCl−

 is far

less effective than HOCl.

Hardness stabilisers are chemical substances, which, added to water, are

able to prevent the deposit of hardness salts by

hindering the crystallisation process through absorption

of the nucleation nuclei of the crystals. In this way the

growth of amorphous crystals, which are relatively

easy to keep in suspension and give less cause to

deposits, is encouraged.
Glossary

Industrial Cooling Systems 11

Hazardous substances substances or groups of substances, that have one or

several dangerous properties, such as toxic, persistent,

bioaccumulative, or are classified as dangerous for the

human or environment according to the Directive

67/548 (Dangerous substances directive).

Macrofouling undesirable organisms in cooling water circuits, which

are visible with the naked eye. Macrofouling is

represented mainly by mussels, barnacles, and serpulid

polychaetes that encrust the walls of the circuits with

their calcareous walls, filamentous organisms such as

hydroids, and other organisms such as sponges,

bryozoans, and tunucates.

Make up (M, kg/s) is defined as all water mass per time unit, which is

added to the system to compensate the loss of water

due to evaporation and blow down.

Maximum allowable risk-level the concentration of a substance in surface water where

95% of the species are protected. The toxicity and

degradability are important aspects.

Mechanical draught tower cooling towers equipped with fans to push cooling air

through the tower (forced draught) or to pull cooling

air through the tower (induced draught).

Micro-fouling or bio-slime is defined as the bacterial film that

develops on any substrate immersed in water. It

consists of algae and sessile microbial population,

comprising slime producing bacteria and anaerobic

sulphate reducing bacteria. Microfouling promotes the

deposition of macrofouling.

Natural draught tower large size cooling towers without fans, but designed to

take advantage of the density difference between air

entering the tower and the warmer air inside the tower

to create a flow of cooling air.

Non-oxidising biocides mostly organic substances used for cooling water

treatment particularly in recirculating cooling systems.

Their working is more specific than that of oxidising

biocides oxidising some species more effectively then

others. They exert their effects on micro-organisms by

reaction with specific cell components or reaction

pathways in the cell.

Oxidising biocides mostly inorganic substances particularly applied in

open once-through systems against fouling. They

attack organisms via a non-specific mechanism. The

biocide oxidises the cell wall or enters the cell and

oxidises the cell components. These biocides are fast

working and because of their non-specificity have a

broader spectrum than the non-oxidising biocides.
Glossary

12 Industrial Cooling Systems

Plume is the visible re-condensation of evaporated water in

the discharged air of a cooling tower.

Precipitation softening this process is used to reduce water-hardness,

alkalinity, silica, and other constituents. The water is

treated with lime or a combination of lime and soda ash

(carbonate ion). Water with moderate to high hardness

(150-500 ppm as CaCO3) is often treated in this

fashion.

Process medium the process medium will always refer to the medium to

be cooled.

Scaling process of precipitation in CWS that occurs when the

concentration of salts in the water film near the heat

exchanger exceeds the solubility.

Sound Pressure Level (Lp) the measure for the immission of sound – the amount

of sound at a defined direction and distance from the

sound source. It is measured in dB per frequency band

or A weighted as dB(A). The measure is logarithmic,

this means that doubling the sound pressure level is

equal to an increase 6 dB(A).

Sound Power Level (Lw) the measure for the amount of sound-energy, which is

radiated (emitted) from a sound source. It measured in

dB per frequency band or A weighted as dB(A). The

measure is logarithmic, which means that doubling the

sound power level is equal to an increase of 3 dB(A).

Hydraulic half time is defined as the time needed to reduce the initial

concentration of a non-degradable compound to 50%

of its initial concentration.

Total available chlorine (TAC)/ the sum of free chlorine and combined chlorine,

total residual chlorine (TRC) with combined chlorine as the available chlorine in

chloramines or other compounds having a N-C link, in

cooling water system.

Total residual oxidants (TRO) oxidant capacity measured in cooling water system via

stoichiometric method (iodide.iodine). TRO is

numerical and operational equivalent to TRC and TAC.

Variable speed drive a way of controlling the speed of a motor, usually

electronically using an inverter. The speed can be

varied manually, but is more often controlled via a

signal from the process, e.g. pressure, flow, level, etc.
Glossary

Industrial Cooling Systems 13

Abbreviations and acronyms

Abbrev./acronym Explanations Pagenumber

ACC Air-cooled condenser 304

AOX Adsorbable organic halogens (X = Cl, Br) 98

ATP Adenosine triphosphate 103

BAT Best available technique 1

BCDMH Bromo-chloro-dimethyl hydantoin 212

BCF Bioconcentration factor 245

BNPD Broomnitropropaandiol 130

BNS ß-brom-ß-nitrostyrene 98

BOD Biochemical oxygen demand (also named Biological oxygen demand) 23

BPM Best practical means 108

BREF BAT reference document 5

BTM Best technical means 108

CCA Copper sulphate, potassium dichromate, arsenic pentoxide 132

CFU Colony forming units 154

COD Chemical oxygen demand 23

CWS Cooling water system 26

DBNPA Dibromo-nitrilopropionamide 95

DPD N-N-diethyl-p-phenylenediamine 103

EDF Electricité de France 36

EIPPCB European Integrated Pollution Prevention and Control Bureau 3

EOX Extractable organic halogens (X= Cl, Br) 278

EQS Environmental quality standard 10

EUR or € Unit of european currency 48

FAC Free available chlorine 24

FO Free oxidant 11

FRO Free residual oxidant 15

IEF Information exchange forum 2

€ or EUR European currency unit 10

kWth or kWe 1000 Watt (thermal or electric) 9

LD Legionnaire’s disease 128

Lp Legionella pneumpophila 128

mg/l Milligram per litre 15

MBT Methylene(bis)thiocyanate 98

MIC Microbiologically influenced corrosion 205

Mt or Mt Metric tonne 188

MWth or MWe 1000000 Watt (thermal or electric) 8

mwg Metre water gorge 82

Nf Naegleria fowleri 128

NOEC No observed effect level 82

PEC Predicted environmental concentration 15

PHMB Polyhexamethylenebiguanidechloride (QAC) 130

PNEC Predicted no effect concentration 15

Pow Partition coefficient over the phases n-octanol and water 108

ppm Parts per million 26

RIZA Dutch water management institute for inland water management and

waste water treatment

155

QAC Quarternary Ammonium Compounds 130

QSARs Quantitative structure activity relationship 245

TBTO Tributyltinoxide 105

TDS Total dissolved solids 263

TEMA Tubular Exchange Manufacturers Association 194

THM Trihalomethanes 211

TOC Total organic carbon 221
Glossary

14 Industrial Cooling Systems

TRO Total residual oxidant 24

TWG Technical working group 17

UV Ultra violet (light) 95

VCI Association of chemical industry in Germany 14

VDI Association of German Engineers (Verein Deutscher Ingenieure) 118

VFD Variable frequency drive 290

WFD Water Framework Directive (to be adopted) 144
Chapter 1

Industrial Cooling Systems 15

1 GENERAL BAT CONCEPT FOR INDUSTRIAL COOLING

SYSTEMS

In numerous industrial processes, heat has to be removed by what is called a waste heat removal

system or cooling system. Operating these cooling systems has certain environmental

consequences. The level and character of the environmental impact varies depending on the

cooling principle and the way these systems are operated. To minimise this impact an

“approach” can be followed which aims at prevention of emissions by proper design and

selection of techniques.

Within the framework of IPPC, cooling should be considered as an integrated part of the overall

energy management of an industrial process. The intention should be to reuse superfluous heat

of one process in other parts of the same process or in different processes on site in order to

minimise the need for discharge of waste heat into the environment. This will affect the overall

energy efficiency of a process and reduce the demand for cooling, for the required capacity of

the system and for its operational demands. The optimisation of energy efficiency, however, is a

complex exercise and regarded as highly process-specific and as such beyond the scope of this

horizontal document. If there are no options for reuse on-site, this does not have to lead

automatically to discharge of heat into the environment, but options for reuse off-site in

industrial or civil applications may be considered. In the end, if options for reuse of heat cannot

be exploited any further, discharge of superfluous heat into the environment is to be considered.

Once the level of heat to be removed has been assessed, a first selection of the appropriate

system for cooling can be decided upon. Much of the environmental performance due to the

operation of a cooling system can be influenced by proper design and by selection of the right

material taking into account the process requirements and local aspects. It is reported that 80%

of cooling system performance has already been determined at the design table and 20% by the

way the cooling system is operated (so-called 80/20 rule). Many different factors need balancing

in assessing what is BAT (best available techniques) for the reduction of the environmental

impact of cooling. Right from the start it is important to realise that a cooling system is an

auxiliary, but generally crucial and integrated system for an industrial process and that every

change applied to the process of cooling may potentially affect the performance of the industrial

or manufacturing process to be cooled.

Therefore, the integrated assessment of the consumption and emissions of cooling systems and

the decision on the application of a cooling technique both should be made in the light of the

total environmental performance of the plant and within the requirements of the process to be

cooled, ultimately balanced with costs. The required level of cooling must be guaranteed, with

minimal consequences for the environment. The required level of cooling is process-specific.

Where some processes can tolerate a certain temporary rise in process temperature, other more

temperature sensitive processes might not, as this will have a large impact on the environmental

performance of the whole plant.

According to IPPC, the environmental performance of the cooling systems discussed in this

BREF must be improved by applying BAT. The question is if and how BAT for cooling

systems can be determined in a general sense, where the final determination on what is best is

certainly a local matter answering the specific requirements of process, environment and

economics. To structure and in some way simplify the complex process of determination of

BAT, this document follows the “approach” described above and presented in Figure 1.1. This

“approach” should lead to a balanced decision on the application of a system for cooling and on

its optimisation based on BAT for both new and existing situations.

The BAT concept consists of the following steps aiming at reduction of emissions and

minimisation of the environmental impact:
Chapter 1

16 Industrial Cooling Systems

• reduce the final level of waste heat produced, considering options for reuse;

• define process requirements;

• consider general site conditions;

• assess environmental requirements:

- options for minimisation of resource consumption

- options for reduction of emissions

• develop system operation (maintenance, monitoring and risk prevention)

• apply economic requirements

In Figure 1.1, the BAT “approach” is presented in a schematic way showing the most relevant

factors involved in the determination of BAT for industrial cooling systems. For the sake of

clarity not all links that can possibly be made between different aspects of cooling have been

added in this scheme. For example, there is a link between sound attenuation measures and the

reduction of specific direct energy consumption; and the achievable minimum end temperature

of a cooling system is limited by the local climatic conditions.

In the following sections the BAT ”approach” will be further discussed in the light of common

principles of operating industrial cooling systems and, where possible, indicating what the

application of BAT means in the spirit of the IPPC-Directive. By its nature, this optimisation

cannot be an exact mathematical comparison of various solutions. The optimisation process

includes a similar challenge for all environmental balances, as it requires a comparison of

different environmental impacts and a decision about which ones are the least severe or most

acceptable. Nevertheless, the suggested BAT “approach” aims at providing significant

information on the implications of various solutions for the environment, on costs and risks as

well as the influencing factors. Based on this information, a decision can be made which is

much more justified than just concentrating on optimising one single factor (e.g. water intake,

energy consumption, plume or noise emission etc.).

Examples will be given to indicate the direction of the changes, rather than to specify particular

emissions or reductions. Where appropriate, data are shown or reference is made to the annexes,

but for most of the factors involved, data on resource use and on emissions of cooling systems

are either limited or they are too specific to be generally applicable.

Summarising, the assessment of a cooling system, balancing the different factors, is founded on

the following points:

• the requirements of the process to be cooled take precedence over the measures for

reduction of the environmental impact of a cooling system;

• applying the BAT “approach” is not aiming at a disqualification of any of the configurations

described in Chapter 2;

• the BAT “approach” has more freedom for optimisation and prevention of emissions in the

design phase in case of new installations, but for existing installations design options should

also be considered;

• consequently, for existing plants it is expected that the BAT “approach” will start further

down in the consecutive assessment steps;

• a further distinction can be made between large custom-made cooling systems and smaller

systems (series product) with respect to the level of environmental impact;

• optimisation should be seen as the application of design options, of reduction techniques

and of good operator practice;

• the level of reduction of emissions resulting from the BAT “approach” is not predictable,

but depends on the demands placed on the cooling system;

• the BAT “approach” aims at cooling system operation balancing the requirements by the

process to be cooled and by the local environmental objectives;

• selection schemes are useful in making a balanced choice and

• finally each balanced result will have a certain environmental impact.
Industrial Cooling Systems 

high

temperature

medium

temperature

required

cooling capacity

required end

temperature

cooling of (potentially) harmful

substances

less

temperature

sensitive

processes

temperature sensitive

processes temperature

sensitivity of the

process

level of waste

 heat (§1.2)

water-cooled

systems (Ch. 2)

air-cooled systems (Ch. 2)

all types of cooling

systems (Ch. 2)

environmental focus

on cooling system

environmental focus

on process

indirect circuit

cooling systems (§ 3.7)

achievable minimumend temperature with

the cooling system

achievable capacity

with the cooling

system

2. Process

Requirements (§1.3)

1. Preventive

Approach (§1.1)

low

temperature

Requirements 

Order of Approach

decision crossing point without junction junction (one way) key to the symbols: 

Selection ofTechnology

Operatingthe Syste

Selection

of the Design

Figure 1.1: Breakdown structure showing the factors involved in the determination of BAT for waste heat dischar[tm134, Eurovent, 1998]
Chapter 1

18 

4. Minimisationof environmentalimpact accordingto the BAT

Climate

conditions

Space restrictions

/site location

Water availability

and quality

Re-circulation

cooling tower

Thermal or

physical

treatment

Hybrid systems

Minimization of

emissions of water

treatment additives (§ 3.4)

Design, material

and lay-out

Optimal dosage

of additives

Minimization of

noise emissions (§ 3.6)

Sound

attenuation -

active and

passive

Minimization ofheat emissionsto water (§ 3.3)Minimization of

plume generation (§ 3.5)

Closed circuit

(re-circulation)

systems

Once-through

system

Investment

cost

Cost for energy,water and water

treatment

Maintenance

cost

e.g. once-through

systems, open

cooling towers

e.g. hybrid

cooling systems

e.g. durable

materials(Annex IV)

Minimization

of resource

consumption

Regular

maintenance

Achievable

approach with

the cooling

system

 3. Site

Requirements

3.2EnvironmentalRequirements (§ 1.5 + Ch. 3)3.3EconomicRequirements (§ 2.7 + Annex VIII)

3.1General SiteConditions (§ 1.4)

with

Dry air-cooled

systems

Minimization of

energy use (§ 3.2)

Biological risks

(§ 3.7) Selection of less

hazardous additives

Mininization of

waste (§ 3.8)

Figure 1.1: (continued): Breakdown structure showing the factors involved in the determination of BAT for waste [tm134, Eurovent, 1998]
Chapter 1

Industrial Cooling Systems 19

1.1 Sources of heat, heat levels and application ranges

All industrial and manufacturing processes which use energy transform different forms of

energy (mechanical, chemical, electrical etc.) into heat and noise. Depending on the process,

this heat cannot always be fully recovered and/or re-used, but has to be removed from the

process by cooling. The amount of non-recoverable heat can be called waste heat, and it needs

to be transferred to the environment, as this serves as a heat sink. A number of processes with a

specifically high production of waste heat and a high demand for cooling are mentioned below.

In many processes different sources of waste heat exist and at different levels: high (above

60 °C), medium (25-60°C), and low (10-25°C). Also, different processes with their specific

demand can be found within the same production site. Large cooling systems are used for large

combustion plants (power industry), in the chemical industry, refineries, the iron and steel

industry, the food industry, the pulp and integrated paper industry, incinerators and in the glass

industry.

Within a similar process cooling is applied for different purposes such as cooling of process

substances in a heat exchanger, of pumps and compressors, of vacuum systems and of steam

turbine condensers. The following major sources of waste heat can be distinguished with their

related levels of waste heat.

• Friction - by definition the transformation of mechanical energy into heat. Cooling systems

for these processes are usually indirect systems with oil as a primary coolant. Because oil is

used as a cooling medium, the cooling system is sensitive to high temperatures. Therefore,

the average temperature of the waste heat is at medium level.

• Combustion - the transformation of chemical energy by oxidation into heat. The waste heat

level of combustion processes is variable.

• Exothermic Processes (chemical) - Many chemical processes are exothermic: chemical

energy is transformed into heat without any combustion. Exothermic processes are often

very sensitive to the efficiency of the removal of waste heat. The temperature level of the

waste heat is medium to high, depending on the process.

• Compression - compressing a gas leads to the generation of heat. This heat usually has to be

removed as waste heat at a medium to high temperature level.

• Condensation (thermodynamic cycles) - Many processes work on the principles of

thermodynamic cycles. A liquid medium is evaporated, taking up energy, and is

consequently condensed, transforming rejected energy as heat. Thermodynamic systems are

very sensitive to temperature and the temperature level is medium to low.

The waste heat level is an important factor to be taken into account when selecting an industrial

cooling system. Table 1.1 shows the temperature ranges of the medium to be cooled and the

most suitable cooling systems. The lower the waste heat level, the more difficult it is to cool

with dry air-cooled systems. In practice, air-cooling is often used for process temperatures

above about 60oC. Heat levels above 100oC are generally pre-cooled with air-coolers if no

options for reuse are available. Evaporative cooling is, in principle, often used to cool process

flows with medium and low temperatures. For low temperatures also once-through systems are

used, especially where large capacities are needed.

The ranges should not be taken as fixed when selecting a cooling system. For the high

temperature range 50oC as well as the above-mentioned 60oC is being used. Also, temperatures

depend largely on the local situation (climate and temperature of the coolant) and the potential

application of a system will vary accordingly. So, once-through systems are also applied at

higher temperature levels, provided that admissible discharge temperatures at the outlet into the

receiving water will not be exceeded. For processes to be operated throughout the year under

varying climatic conditions it can also be necessary to use a combination of different cooling

systems.
Chapter 1

20 Industrial Cooling Systems

Table 1.1: Heat temperature levels and application range

[tm139, Eurovent, 1998]

Temperature range Suitable Cooling System Typical Applications

Low temperature

(10 – 25 °C)

• once-through systems

(direct/indirect)

• wet cooling towers

(mechanical/natural draught)

• hybrid cooling towers

• combined cooling systems

• power generation

• (petro-) chemical processes

Medium temperature

(25 – 60 °C)

• once-through systems

(direct/indirect)

• wet cooling towers

(mechanical/natural draught)

• closed circuit cooling towers

• evaporative condensers

• air-cooled fluid coolers

• air-cooled condensers

• hybrid cooling towers/ condensers

• hybrid closed circuit cooling tower

• refrigeration cycles

• compressor

• cooling of machines

• autoclave cooling

• cooling of rotary kilns

• steel plants

• cement plants

• power generation in warmer regions

(Mediterranean)

High Temperature

(above 60 °C)

• once-through systems

(direct/indirect) in special cases

• wet cooling towers

(mechanical/natural draught)

• air-cooled fluid cooler/ condensers

• waste incineration plants

• engine cooling

• cooling of exhaust fumes

• chemical processes

1.2 Level of cooling system and influence on process

efficiency

1.2.1 Temperature sensitive applications

Many chemical and industrial processes are temperature critical applications. The efficiency of

the process is sensitive to temperature and/or pressure and therefore correlated with the

efficiency of the removal of waste heat. For these processes, the horizontal “approach” of best

available cooling technology is connected with the vertical “approach” of best available process

technology. Examples for temperature critical applications are:

• power generation,

• thermodynamic cycles,

• exothermic processes.

Integrated pollution prevention means that selection of best available cooling technology and

application of techniques, of treatments or ways of operating should consider not only the direct

environmental impacts of the different cooling systems, but also the indirect environmental

impacts due to varying efficiencies of the different processes. It has to be decided at local level

whether should be pursued by focussing on the cooling system rather than on the production

process. The increase of the indirect impacts can be considerably higher than the decrease of

direct impacts of the selected cooling system.

Power plants (see Annex XII) are the most important source of waste heat. The transformation

of fossil energy into electrical energy is connected with many of the waste heat generating

processes mentioned in Section 1.1. Waste heat is generated during combustion, friction of the

turbine, condensation of the steam and transformation of the electricity. A separate cooling

water system, for the auxiliary systems using oil or gas for smooth operation of equipment, also

generates a small amount of waste heat. If the cooling requirement of the power generating

system cannot be met, it immediately shows in a decrease of the overall efficiency and in an

increase of air emissions.
Chapter 1

Industrial Cooling Systems 21

This correlation is illustrated by the following example for a power plant, in which an

alternative, presumably less effective, cooling system leads to a loss in efficiency of the power

plant of about 3% (Table 1.2). As a result the resource input of the power plant and its emissions

to air will also increase with about 3%. As the emissions also depend on the fuel used, they

could easily be different in another situation, but no data were available to evaluate this point

further.

Table 1.2: Emissions of an average Western European power plant due to an efficiency loss of 3%

[tm139, Eurovent, 1998]

Emissions to air Emissions / energy

input in [g/kWh]

Additional emission due to a loss

of 3% efficiency [g/kWh]

CO2 485 14.6

SO2 2.4 0.072

NOx 1.0 0.031

Dust 0.2 0.006

Primary energy input: 2.65 kW and additional energy input 0.08 kW

How the selection of a cooling system can affect the performance is well illustrated by the

following examples taken from Caudron [tm056, Caudron, 1991]. These figures highlight the

effects of the selection of a cooling system under the given climate conditions. So care must be

taken, as the loss of efficiency that can occur depends on the choice of the cooling system, the

climatic conditions and the design of the turbine. Condenser vacuums (condenser pressures) will

vary accordingly as illustrated in the following tables. In areas where higher ambient

temperatures occur, the vacuum levels are higher with dry systems and can reach up to 425

mbar. But many other factors, such as fouling, scaling, corrosion and sub-optimal design, may

lead to similar losses of efficiency.

Table 1.3: Relative effect on the delivery of electrical power due to the application of wet, wet/dry

or dry cooling towers to units of 1300 MWe.

[tm056, Caudron, 1991]

Wet cooling tower

Wet/dry

cooling

tower

Dry cooling tower Type of

refrigeration

system Oncethrough

Wet

natural

draught

Natural

draught

Induced

draught

Induced

draught

Natural

draught

Induced

draught

Approach K (dry air

11[°C] / wet air

9[°C])

- 12 12.5 12.5 13.5 16 17

Nominal

condensation

pressure (mbar)

44 68 63 63 66 82 80

Thermal power

(MWth) 1810 1823 2458 - - - -

Electrical power

Delivered (MWe) 955 937 1285 1275 1275 1260 1240

Difference of

electrical power

delivered (%)

+ 1.9 0 0 - 0.8 - 0.8 - 2 - 3.5

In the table the once-through system is taken as starting point to benchmark the other systems.

The approaches of the recirculating cooling systems are additional to the approach of the heat

exchanger (condenser) which is assumed to be equal for all systems.
Chapter 1

22 Industrial Cooling Systems

From this table it is clear that the choice of cooling system, such as for a dry instead of a wet

system, needs careful consideration. The table also shows why many power plants are located

preferably on the coast or on large rivers. From the production point of view, once-through

systems are more efficient than the reference system (wet natural draught).

For combined cycles, the condenser pressure and electrical power delivered vary similarly with

the type of the cooling system and the relative loss of power becomes even more clear.

Table 1.4: Relative effect on the delivery of electrical power due to the application of wet, wet/dry

or dry cooling towers to a 290 MWth combined cycle unit

[EDF, pers. comm., 1999]

Wet cooling tower

Type of cooling system Once-through Natural

draught

Induced

draught

Air-cooled

condenser

Approach K (dry air

11[°C]/wet air 9[°C]) / ≈8 ≈8 ≈29

Nominal condensation

pressure (mbar) 34 44 44 74

Thermal power (MWth) 290 290 290 290

Difference of electrical

power delivered

(MWe)

+ 0.65 0 -1.05 -5.65

1.2.2 Non-sensitive applications

Other applications are less sensitive to temperature. The efficiency of these processes is less

correlated with temperature or pressure. For these processes the focus should be on the

economically and ecologically most efficient cooling system to dissipate waste heat that remains

after all the possible options for reuse have been exploited.

1.3 Optimising the primary process and reuse of heat

Optimisation of the overall energy efficiency of the primary process will not be dealt with in

depth in this reference document. However, in the preventive “approach” of IPPC this

optimisation should be done first before removal of waste heat is contemplated. In other words,

the need for heat discharge has to be minimised, which will simultaneously affect the

configuration and size of the required cooling system. Furthermore, the cooling system does not

necessary mean discharge into the environment, as successful attempts have been made to use

this energy as well.

1.3.1 Optimising the primary process

The optimisation of the primary process can significantly reduce the overall environmental

effects. In many Member States the majority of the non-recoverable heat to be disposed of by

cooling systems is due to power generation. Depending on the overall efficiency, up to 60 % of

the fuel energy is transferred into waste heat. If the efficiency of the power generating process is

increased, environmental effects can be reduced and the cooling system plays a crucial role

here. For other industrial sectors this principle can be applied as well, at the same time lowering

the energy costs, the amount of heat discharged to the environment, as well as the emissions to

air (CO2). Generally, the higher the heat level the more easily it can be recovered.
Chapter 1

Industrial Cooling Systems 23

A few examples of currently applied techniques are:

• preheating fuel or raw materials (metals);

• pinch-technology;

• external applications (e.g. heating greenhouses/ residential areas);

• co-generation in power industry.

Instead of using cooling water or air only, it is customary at refineries to preheat fuels by using

a cold incoming hydrocarbon stream to cool down a hot refined stream leaving the unit.

Consequently, there is a reduced need to preheat the (cold) crude fuel and a lower demand for

cooling water. Depending on the process the number of cold streams can be limited and a

certain need for cooling water or air will remain.

Co-generation, or combined heat and power generation, is used in the power industry and in

other industrial sectors (e.g. paper industry, (petro-) chemical industry). Where both forms of

energy are needed, their generation can be combined. This saves energy, reduces CO2 and SO2

emissions and requires hardly any cooling, thereby avoiding the need for (large) cooling

systems.

1.3.2 Use of waste heat off-site

If optimising the waste heat generating process does not lead to any further waste heat

reduction, the BAT “approach” would be to assess, whether any option of reuse of waste heat

can be found. This issue is beyond the scope of IPPC as it also relates to generic good

environmental energy management. It can be done on an existing site as well as an integral part

of site selection (see next chapter). Finding adequate consumers is, however, not a trivial task.

Often requirements of consumers are not reconcilable with the cooling demands. In some cases,

heat consumers require a higher temperature level than planned. If it is technically possible to

operate the primary process on a higher temperature level, the overall energy balance has to be

carefully observed. Often the loss of energy efficiency in the primary process outweighs the

savings through the “waste“ energy consumption. Also, care should be taken in creating a

situation, in which there develops a dependency on the availability of “waste” heat.

A number of examples can be found of the external application of “waste” heat of power

stations for district heating of homes and offices during winter periods or heating of

greenhouses by applying co-generation or operating a combined cycle. The applications can

raise fuel utilisation efficiency from around 40% up to more than 70% and thus decrease the

cooling demand of the installation. In the examples reported, a hybrid cooling tower with

variable fan speed is used to be able to adapt to the varying need for district heating. In another

case the tower only needed to be operated in dry mode at about 10% of its total capacity as soon

as outdoor air temperature had dropped to 5oC, simply because maximum external use of heat

was reached at that point. This raises the question of the extent to which the potential options for

reuse can influence the choice of a cooling system where flexibility of operation is required.

Currently no examples are known that show how the options for reuse are reflected in the

selection of a cooling system.

1.4 Selecting of a cooling system in order to meet the process

requirements and site conditions

1.4.1 Process requirements

Once the level of heat (high, medium, and low) has been assessed, a first rough selection could

be done by applying Table 1.1. In addition to the heat level, many more factors are also
Chapter 1

24 Industrial Cooling Systems

involved in the selection of a cooling system in order to meet the process requirements and

general site conditions, such as:

• the required minimum end temperature of the substance to be cooled,;

• the required cooling capacity;

• the requirement for an indirect circuit, which increases the approach;

• climatic conditions, water availability and space requirements.

In view of the indirect effects of sub-optimal process cooling, the required minimum end

temperature of the process to be cooled is crucial. This means that the cooling system(s) used or

to be chosen will have to achieve this end temperature and at the same time meet other (processrelated) requirements. Cooling system performance should preferably be optimised, taking into

account the annual temperature range of the coolant. For wet cooling, the wet bulb temperature

is important and there is some flexibility to select a design temperature, which in its turn will

affect the size of the cooling system and its power requirements. Reduction of the size of

cooling systems has to be carefully evaluated and accepted only case by case. Some plants have

to be operated the whole year round with acceptable efficiencies and maximum rated output.

For example, when mechanical draught towers or dry air coolers are used, it is possible to

operate the system in the most economical way if coolers have several cells. Some of them can

be taken out of operation in order to save water and electric energy without an appreciable loss

of efficiency.

Figure 1.2: Tower plan area as a function of the percentage of time the design wet bulb

temperature is exceeded

(design temperatures 40/24/18oC), [tm083, Adams and Stevens]

With respect to the power industry, Figure 1.2 is not applicable as optimisation of the cold end

is done using validated temperatures:

• for wet or dry cooling towers the wet (or dry) air temperature is taken for all year around,

using one value for every 3 hours;

• for once-through the water temperature is measured and one value per month is

recommended.
Chapter 1

Industrial Cooling Systems 25

Then, optimisation is done taking into account the valorisation of energy for the whole year.

This method, called global actualised balance, is explained in Annex XII.

Generally, in most industries a safety margin is applied to ensure that the cooling system will

meet the cooling demand at any time and especially in the summer months. In circumstances

where the wet bulb temperature at all times remains well below the design wet bulb temperature

or where the heat load appears to be lower, the heat exchanger may has overcapacity. If this is

expected, consideration could be given to operational measures, such as variable fan operation,

that allow the system to run below capacity and thereby reduce the direct energy demand.

In many industries, it is practice to oversize the cooling system at the installation stage to leave

room for capacity growth: the spare capacity is then gradually used up until a new tower needs

to be added. When a licence application for expanding production and cooling capacity is being

considered, an assessment must always be made of the extent to which there is spare capacity in

the existing cooling system, whether inefficiently used (because it is underused) or not well

maintained.

This strategy makes it possible to determine the required cooling capacity (kWth or MWth), the

size of the cooling system (heat exchanger) and perhaps even the choice of coolant (water or

air). In Europe the industries that require large cooling capacities for low process temperatures

(power stations and (petro-) chemical industry) have a preference for sites, where large and

reliable source of water is available and where the use of once-through systems is possible.

Where the water supply is limited large capacity open wet or wet/dry cooling towers are used.

The need to cool (potentially) harmful substances can also affect the size of the cooling system

as well as the possible end temperature (See e.g. VCI-safety concept in Chapter 3 and Annex

VI). In this situation the BAT ”approach” can lead to the conclusion that the effects of leakage

can only be adequately prevented if a secondary (indirect) system is applied. This means that a

second circulation has to be designed and the design temperature will increase, following an

increase of the approach. This will lead to higher end temperatures of the substance to be cooled

and further reduction in the overall efficiency.

1.4.2 Site selection

It is obvious that there is a limit to the extent in which site-specific characteristics can be

optimised by choosing the optimal site. For existing systems the site simply is a given value and

environmental optimisation will have to be considered within the restrictions of the site. For

instance, restricting water use by changing to dry air-cooling might seem to be an obvious

choice. However, climate conditions may not allow this where dry bulb temperature is expected

to exceed the required design temperature for a large part of the year unless a reduced output of

the production process is accepted as well as a simultaneous reduction in the overall efficiency

of the plant.

If site selection is an option, the requirements of the cooling process can be influenced

significantly. Therefore it is important that during the design phase all the following aspects are

considered in the site selection process:

• quantity, quality and costs of cooling medium available (water as well as air),

• available size (area, height, weight of cooling installations),

• effect on the quality of water and on aquatic organisms,

• effect on the quality of air,

• meteorological effects,

• discharges of chemical substances into water,

• noise emissions,

• aesthetic aspects of the building,
Chapter 1

26 Industrial Cooling Systems

• capital expenditure for cooling systems, pumps, piping and water treatment,

• operating costs for pumps, fans and water treatment,

• annual costs for maintenance and repair,

• operating parameters such as minimum service life, annual operating time, average load in

thermal output and water flow rate,

• operating requirements such as required approach and systems availability,

• environmental legislative requirements regarding heat emissions, plume emissions, acoustic

emissions, overall height etc.

• for power plants: plant efficiency losses, capital expenditure to compensate power output

losses, plant lifetime and energy revenue losses due to lower plant efficiency.

Space

Different cooling systems need different amounts of space for the same cooling performance

and vary in area requirements, height and weight. It depends on the heat transfer principle they

follow (See Annex I).

For large systems, space restrictions can be an issue and will be part of the site assessment. The

applies to processes to be cooled entirely by air where large multi-cell constructions are needed

to ensure the required cooling capacity. For smaller capacities, space restrictions should not be a

limiting factor as roof constructions are on the market that are specifically designed for these

situations.

Restrictions on space at existing sites, for instance in densely built urban areas or densely built

industrial sites, are an important factor in the selection of cooling systems. For example, a

cooling tower on top of a building needs no additional ground space, but the roof location may

impose restrictions on its weight.

Space and height requirements are important criteria for air-cooled and hybrid systems. The

ventilation of air can be achieved by natural draught or by ventilation with fans (mechanical

draught). For the same cooling capacity natural draught cooling systems have to be far bigger

and higher than mechanical draught systems.

Site assessment

Regarding site selection in a number of Member States, it is common practice that for a large

site an environmental impact assessment will be required as part of the permitting procedure.

Also, due to the potential high impact of site selection on the cooling performance, there have

been initiatives to pre-select optimum cooling sites in regional planning programmes.

An example of the assessment of a site for cooling systems with a large water requirement such

as those used by power plants, is given in Table 1.5 [tm012, UBA, 1982]. A combination of

local criteria leads towards the classification of a site with its suitability being most,

intermediate or least favourable. It should be remarked that using such an assessment is again

only part of the total assessment and that a grade 3 site (doubtful suitability) could well be

favourable on the overall balance of factors.

An example of the consequences of site selection can be seen when the BAT “approach” is used

for a site qualified as grade 3.
Industrial Cooling Systems Table 1.5: Large cooling demand-related criteria for site selection(Derived from [tm012, UBA, 1982])

Criteria Grade 1 (good suitability) Grade 2 (satisfactory suitability) Grade 3 (doubtful suitability) 

Sufficient cooling watersupply

 WNNQ > -----------------

ζ c ∆T

Plentiful cooling water supply

 WNNQ ≈ -----------------

ζ c ∆T

Sufficient cooling water supply

 WNNQ < -----------------

ζ c ∆T

Insufficient cooling water supplywithout technical measures

NNQW: ζ: c: ∆T:  

Suitable water quality Water quality class

II moderately pollutedII/III critically polluted

Water quality classIII severely contaminated

Irrespective of quality class (Germ

I II II/III III-IV IV 

Complying with permittedevaporation losses

V < A aMinor evaporation losses

V ≈ A a

Bearable evaporation losses

V > A aEvaporation losses not acceptablewithout technical measures

V: e

(

A: pea: 

lt

Impact on drinking watersupply

Cooling water discharge has noimpact on drinking water supply

Cooling water discharge mayunder certain circumstancesimpact drinking water supply,negative effects can be avoided

Cooling water discharge impactsdrinking water supply, negativeeffects cannot be ruled out withoutadditional technical measures

This downobtaior pl

Frequency of long plumeswith low altitude and wasteheat transfer in direct sitevicinity (radius 2 km)

Very low frequency (<2% p.a. onaverage) long (<100 m) plumes withlow altitude (≤300m) and waste heattransfer < 10 000 MW

Long plumes with low altitudemore frequent and waste heattransfer< 10 000 MW

Waste heat transfer > 10 000 MW

Topographic situation in thevicinity of the site

No or only few elevations with analtitude higher than the coolingtower within about 20 km radius ofthe site

Several elevations higher thanabove the cooling tower withinabout 2 -20 km radius of the site

Several elevations higher than thecooling tower within less than 2 kmradius of the site

Possibility of economic useof waste heat

Great potential for economicallyfeasible usage of district heating

Little potential for economicallyfeasible usage of district heating

No possibility of economic wasteheat usage or doubtfull because notthoroughly investigated

The p

wastsite adisaddisch
Chapter 1

28 Industrial Cooling Systems

The assessment should start with a selection of options for reuse of heat, as this could have an effect on the

cooling water demand. This criterion cannot be met as no external heat use is possible and all nonrecoverable heat will have to be discharged. Water supply and permitted evaporation are limited and in

respect of plumes there may be disturbance in the vicinity of the site. If for process reasons a water-cooled

system is necessary, water saving methods will have to be applied and, for example, a recirculating system

(e.g. open wet cooling tower) instead of a once-through system would be recommended. This will imply the

application of some kind of water treatment, depending on water quality and cycles of concentration. The

additional requirement would be plume suppression, which prompts consideration of a hybrid configuration.

If enough space is available and climatic conditions favourable, the use of air-cooling may equally be

considered. The integrated “approach” would follow with a comparison of energy use and costs.

The site selection process requires that for the “final candidate” site, detailed considerations about the

selection of the possible cooling systems be conducted in order to find the overall optimum solution.

1.4.3 Climatic conditions

The climate expressed in terms of wet and dry bulb temperatures is an extremely important site-specific

condition. It influences both the choices of the type of cooling and the possible end temperature of the

process to be cooled. The contradiction of cooling with air and/or water is that when the cooling demand is

high it becomes more difficult to achieve the requirements. Particularly in areas where high air temperatures

and high water temperatures coincide with lower water availability during part of the year a certain

operational flexibility of the cooling system can be very important and may be achieved by combining water

and air cooling. Sometimes, however, a certain loss of efficiency may have to be accepted.

To reach the required process temperature it is an obvious requirement of all cooling systems that the cooling

medium must have a lower temperature than the medium to be cooled, but this depends on the dry and wet

bulb temperatures. For both water and air-cooled systems, seasonal variations in the temperature of the

cooling medium can be limit the choice of cooling system and can demand a certain way of operation.

Wet bulb temperature is always lower than dry bulb temperature (Table 1.6). The wet bulb temperature

depends on the measured temperature of the atmosphere, the humidity and the air pressure. For latent

(evaporative) heat transfer the wet bulb temperature is the relevant temperature. It is theoretically the lowest

temperature to which water can be cooled by evaporation. For sensible heat transfer the dry bulb (dry air)

temperature is relevant, where air is the coolant.

For the selection of the type and design of cooling system, the design temperature is important and usually

relates to the summer levels of the wet bulb and dry bulb temperatures. The bigger the difference between

these temperatures and the higher the dry bulb temperatures, the more difficult it will be to reach low end

temperatures with dry air-cooled systems. As mentioned earlier, this can lead to efficiency losses. Measures

can be taken to overcome the loss, but they require a certain investment. For economic reasons it is useful to

determine the variation of these temperatures throughout the year and what percentage of the year the

maximum temperatures are actually reached.

As an example, Table 1.6 shows how for different climate conditions in Europe the choice for a dry or a wet

cooling system can affect process efficiency losses due to the Carnot cycle. In the example, the approach for

wet cooling is considered to be 4 K and this has to be added to the wet bulb temperature to get the minimum

end temperature of the coolant. The approach for dry cooling is set at 12 K to be added to the dry bulb

temperature. The larger the difference between the wet and the dry end temperatures, the higher the loss of

efficiency (in this example), where losses of 0.35% per K on average occur. At the same time, with for

example 5% efficiency loss, the efficiency of a conventional power plant would be 38.6% instead of 40%

(See Annex XII.6).
Industrial Cooling Systems Table 1.6: Climatic conditions in Europe(derived from [tm139, Eurovent, 1998]

Country and station Dry-bulb temp. (1%)2

(°C)

Wet-bulb temp

(1%)2

(°C)

Difference K

End temp.

Dry system3

(°C)

End temp.

Wet system4

(°C)

Greece Athens 36 22 14 48 26 Spain Madrid 34 22 12 46 26 France Paris 32 21 11 44 25 Italy Rome 34 23 11 46 27 Austria Vienna 31 22 9 43 26 Germany Berlin 29 20 9 41 24 Netherlands Amsterdam 26 18 8 38 22 France Nice 31 23 8 43 27 UK London 28 20 8 40 24 Germany Hamburg 27 20 7 39 24 Norway Oslo 26 19 7 38 23 Belgium Brussels 28 21 7 40 25 Spain Barcelona 31 24 7 43 28 Finland Helsinki 25 19 6 37 23 Denmark Copenhagen 26 20 6 38 24 Portugal Lisbon 32 27 5 44 31 UK Glasgow 23 18 5 35 22 Ireland Dublin 23 18 5 35 22 Notes:1) the given data in Table 1.4 are illustrative of the variation of the climate in Europe. Other references may provide slightly d

a site can be analysed by a meteorological institute.

2) statistically only 1% of the maximum temperatures are above this data3) approach 12 K4) approach for wet system: 4 K5) loss of efficiency 0.35% per ∆T K on average
Chapter 1

30 Industrial Cooling Systems

1.4.4 Mathematical modelling, simulations on models and tests on pilot

loops

For the assessment of the impact of new and existing large cooling systems and for optimising

their performance numerical models can be applied, particularly in case of sensitive ecosystems.

Simulations and tests on pilot loops can be carried out, forecasting thermal changes of the

surface water in the near and far field caused by heat emissions as well as by optimising antifouling treatment.

The purpose of modelling is to study any physical-chemical impacts and adapt the results of this

modelling to the facilities in order to reduce these impacts to the greatest possible extent. It is

particularly important to study:

• water withdrawals and discharges,

• the visual aspects of the site,

• the development of plumes,

• the thermal and chemical impacts on the receiving environment.

The objective of the pilot loop tests is to define the optimum treatment of cooling water both

with regard to scaling and to any biological developments. To do so, pilot facilities representing

real commercial operating conditions are installed on the site for about one year. This makes it

possible to take account of variations in the quality of the waterway in the course of the seasons

and to try out some options on a representative scale (e.g. choice of cooling tower fills, choice

of alloy).

1.5 Selecting a cooling technique in order to meet

environmental requirements

The environmental requirements can affect cooling systems application and are an additional

step in the balanced selection of a new cooling system or in the optimisation of an existing

cooling system. Generally, five major aspects with consequences for cooling systems selection

can be distinguished:

• minimisation of energy use

• minimisation of heat emissions

• minimisation of large plume emissions

• minimisation of emissions to water

• minimisation of noise emissions

• minimisation of immissions to soil and terrestrial habitats

The aspects are cross-linked and each choice potentially has its consequences for one of the

other aspects. The aim is to prevent emissions to the environment from routine operations. In

this assessment step the differences between water-, air- and air/water-cooling should become

clear as well as the operational consequences of the choice of a particular design or a particular

material.

1.5.1 General comparison between air and water cooled systems

Minimisation of the environmental aspects is often translated into a comparison between waterand air-cooling systems. It has been advocated earlier in the document that a judgement of water

versus air cooling should not be made in a general sense as this leaves out the local constraints

that might limit the use of either system. However, it could be opportune to consider or

reconsider water requirements of a cooling system in view of programmes on water
Chapter 1

Industrial Cooling Systems 31

conservation and of the increasing demand on water with good quality for other purposes (civil

and industrial) than cooling.

The economical turning point in the choice between dry air cooling and water cooling systems

is not fixed and according to literature will be somewhere between 50°C and 65°C (as end

temperature) depending on the local climatic conditions.

Some general remarks have been made in a comparison of the features of dry air cooling and

wet cooling systems with the same required cooling capacity: [tm001, Bloemkolk, 1997]

On space requirements:

• Air-cooling demands space because of the low specific heat capacity of air. The space can

be kept to a minimum by installing air-coolers above other process equipment or a pipe

bridge;

• Air-cooling systems have limits to their location as they cannot be placed too close to

buildings because of the resulting air-circulation, blockage of air-supply and the danger of

recirculation;

Maintenance costs

• Generally maintenance costs for air cooling are considered to be lower as they do not

require anti-scaling and mechanical cleaning of the water-contact surface area and do not

require additional surface area to compensate for surface loss caused by pollution on the

water side;

Process control

• Control of the temperature of the process is easier with air-cooling or with a recirculating

flow than with once-through cooling, where the balance of water inlet and outlet restricts

the controllability of the water flow and the temperature increase. With mechanical draught

cooling or evaporative systems there is no limit to the available amount of air, and the

airflow can be adjusted according to the process demand using fan cycling, a multi-fan

installation or modulating capacity control;

• Control of leakage in cooling-water is easier to detect, although detection of leakage in

condensers is reported to be more difficult. Usually this affects the efficiency of the process.

1.5.2 Design factors and choice of materials

Following the BAT “approach”, the design of the cooling system and the choice of materials to

be used are an important preventive step. Both can affect the operation as the required amount

of direct energy consumption, the occurrence of controlled (water treatment) and uncontrolled

(leakage) emissions to the environment, noise emissions, and the direction of heat emissions

(water or air). Also, the selected design and materials will require a certain level of investment.

Again, the balance is sought between the level of prevention of emissions by design and used

materials and the investment costs involved. This again is a site-specific and complex matter in

which the following factors are taken into account:

• type of operation (e.g. once-through or recirculating)

• design of cooler and layout of cooling system (direct/indirect)

• pressure level (condenser)

• composition and corrosiveness of the cooling water

• composition and corrosiveness of the medium to be cooled

• required longevity and costs

A range of materials is available and, in order of increasing resistance, most commonly used are

carbon steel, coated (galvanised) steel, aluminium/brass, copper/nickel, adequate types of

stainless steel and titanium. Within these groups a further sub-classification on the quality is
Chapter 1

32 Industrial Cooling Systems

used. Especially resistance to corrosion, mechanical erosion and biological pollution is greatly

determined by the quality of the water combined with possible conditioning agents.

In Annex IV some considerations on the selection of material are given for once-through and

open recirculating systems. For each industrial cooling system a similar assessment can be

done. In case of water and water/air systems the material that can be selected depends on both

the coolant and the process medium, whereas with primary closed circuit dry cooling, the

process medium is more important.

It is obvious that for different parts of the installation different materials can be used. The

quality of material least sensitive to the corrosiveness of the water or to the conditions of the

process is preferred. If more sensitive materials (alloys) are chosen, the consequence can be that

a complex cooling water treatment and control program is needed, which will lead to emissions

and costs.

Table 1.7 shows an example on the effect caused by differences in design. Three towers are

designed for the same cooling performance and the same required environmental performance.

The choice for a cooling system means different sizes, but in particular a difference in energy

costs as about 7 kW more is needed for the closed circuit cooling tower to be able to give the

same performance with the same sound power level. In this case costs of operation may lead to

choose one of the other options. For other design factors similar comparisons can be made,

which may lead to different effects favouring another option.

Table 1.7: Comparison of different cooling systems with a required maximum sound power level

[tm139, Eurovent, 1998]

Mechanical draught

wet cooling tower

Closed circuit cooling

tower

Hybrid closed circuit

cooling tower

Climate:

dry bulb temperature 26 °C

wet bulb temperature 18 °C

Given Duties:

Capacity 1200 kW

inlet temperature 38 °C

outlet temperature 32 °C

Flow 47.8 l/s

sound power level 90 dB(A) 90 dB(A) 90 dB(A)

Specific Data:

Length 3.7 m 3.7 m 5.2 m

Width 2.8 m 2.4 m 2.0 m

Height 3.2 m 4.2 m 3.0 m

Fan power 5 kW 11 kW 5.0 kW

Spray pump power 1 kW 2.2 kW 1.0 kW

1.5.3 Options for a technological change of existing systems

For a new cooling system, there will be more flexibility to select between complete systems and

to assess the alternative options, whereas for an existing installation a change of technology

often is a drastic solution. Sometimes in specific cases, it is possible to change the technology,

but the number of options to reduce emissions via technological solutions is limited for existing

installations. As the BAT “approach” considers that prevention of emissions prevails, taking

into account also the economical aspects, change of technology is an option that should be

considered before the optimisation of operating a cooling system is to be further assessed. In the
Chapter 1

Industrial Cooling Systems 33

following paragraphs observations and experiences by suppliers are presented to give examples

of possible optimisation steps in the BAT “approach” (See also Annex XI).

1.5.3.1 Retrofit – reasons and considerations

Retrofitting existing installations can be considered for the following reasons:

1. replace existing technology by a different technology with lower operating demands,

2. replace outdated technology equipment by modern equipment with higher efficiency, and

3. modify existing equipment to improve performance or to meet additional demands.

Different from the selection of a new installation, where the site parameters can be more or less

defined, in retrofit scenarios usually the following number of parameters is fixed:

• space - the retrofit installation must fit into the existing space,

• the availability of operating resources – the new installation should not exceed the operating

resources, which were needed for the old one, new infrastructure would result in an increase

in costs, and

• legislative restrictions – environmental impacts, like sound criteria, usually have to be at the

same level or below the ones of the old installation.

Space is often an important reason for retrofitting itself. If a plant or building will be built new

on an existing space-restricted site, it could be a solution to select a new type of cooling system,

which can be placed on the roof of a building or which needs less space than the old one.

The preferred solution would be a new installation with lower operation needs, so that the

retrofit is also associated with lower operating costs. Lower operating cost will be one of the

main reasons for retrofitting. It is preferred, however, to consider a retrofit scenario, which

reduces the emissions as well as the consumption of operating resources. In general this will

require higher investment cost. Considering the operating cost savings and any potential

reduction in emissions, larger investment costs can pay off in short periods of time.

All retrofit scenarios have to consider both the cooling technology and the process to be cooled.

Both have to be seen as one system. Changes in the cooling system may have effects on the

process and vice versa. The first aim of any retrofit must be to maintain, or if possible improve,

the efficiency of the process to be cooled. On the other hand, changes in the process to be

cooled will also result in different demands on the cooling system. This could be another

important reason for retrofitting.

Changes in the process to be cooled can result in a change of demands on the cooling system.

• Due to new technology less waste heat is generated by the process, less cooling capacity is

needed (example: computer terminals, processes with friction).

• The temperature level of the waste heat has changed, both to higher or lower temperatures

(example: incineration processes).

• Larger parts of the generated heat of the process are recuperated, so less waste heat has to

be removed to the environment.

• The temperature sensitivity of the process is increased, a more efficient cooling system is

needed.

Table 1.8 summarises the options for technological upgrading that, according to suppliers

information, can be considered to be technically easy (E), possible (P), difficult (D), not

possible (NP), or does not apply (NA). Generally, each system has a varying number of options

for retrofit. NP-E is an indication that the application of an option is largely dependent on the

specific situation, in which the cooling system operates. (See also Chapter 3 and Annexes).
Chapter 1

34 Industrial Cooling Systems

Table 1.8: Technological upgrading options for existing systems

(pers. comm.)

Industrial cooling systems1

Option

OTCS OWCT OWDCT CCWCT CCDCT CCWDCT

General E E E E E E

Improve capacity E E D D D D

Reduce kWe DEDED D

Reduce water-use NA NP-E D NP-E NA D

Reduce plume NA NP-E NA E NA NA

Reduce noise NA E D E D E

Reduce drift NA E E E NA E

Notes:

1

System code (see also Chapter 2):

OTCS – once-through cooling system

OWCT – open wet cooling tower

OWDCT – open wet/dry cooling tower

CCWCT – closed circuit wet cooling tower

CCDCT – closed circuit dry cooling tower

CCWDCT – closed circuit wet dry cooling tower

There are many possible ways to retrofit a cooling process and some typical scenarios along

with their relevant considerations are listed in the following paragraphs.

1.5.3.2 Change of heat transfer technology

Usually, lower operational costs associated with a new technology or legislative restrictions are

major reasons for the replacement of one heat transfer technology by another technology.

A typical example is the replacement of a once-through system by a recirculating system,

saving on operating costs (water and sewage) and following restrictions on heat emissions to a

surface water. The economic performance of the recirculating system depends on the specific

costs for water, sewage and electrical energy. Assuming average water and sewage costs of 1

[€/m3] and electrical energy costs of 0.1 [€/kWh], the operating costs in this example are

38800 € for the once-through system and 48000 € (2100 € for water and 27000 € for energy) for

the recirculating system. The annual saving is 34000 €, which is higher than the investment

costs of 21000 €. If the balance favours the environment in the first place and investment costs

will be much larger than annual costs, the investment recovery period will become an important

factor.

In this example both the environment with respect to water requirements and the company

benefit from a change in technology at the same time. The environmental costs however are due

to additional energy requirements for extra fan and pump energy. Water use in this example is

by large affected by the evaporation loss which has been calculated by assuming that they

amount to 1.8% of the circulation per 10K of cooling (see Annex V.3).

This example merely shows how to approach changes in technology. With different price levels

the outcome will be quite different and may favour the once-through system. For example, in

Italy, where electricity cost is about 0.05 [€/kWh] and water cost for an open circuit 0.01 [€/m3

]

against 0.1-0.2 [€/m3

] for a closed circuit, the once-through systems would be more favourable

from an economic point of view.
Chapter 1

Industrial Cooling Systems 35

Table 1.9: Example for conversion of a once-through system into a recirculating system

[tm139, Eurovent, 1998]

Example:

air compressor 500 kW Once-through system Recirculating system

inlet temperature 15 °C 27 °C

outlet temperature 35 °C 35 °C

flow rate 6 l/s 15 l/s

annual operating hours 1800 h 1800 h

evaporation loss - 1400 m3/a

blow down - 700 m3/a

annual water use 38800 m3/a 2100 m3/a

extra fan and pump energy - 15kW

investment cost - € 21000

If a change of the cooling configuration is considered, the effects on the overall efficiency must

be taken into account. If possible, the efficiency should be increased. For temperature sensitive

processes, it needs checking whether a cooling technology can provide lower end temperatures

at the same level of safety.

The example of replacing a water-cooled condenser with an open cooling tower by an

evaporative condenser shows an effect on end temperature and system efficiency. Such a

technological replacement can potentially reduce the condensing temperature by 4 – 6 K

depending on actual conditions. The efficiency gain of such retrofit can be estimated in order of

magnitude of 12 – 15 % of the power requirement of the refrigerant compressor [tm139,

Eurovent, 1998]

For temperature sensitive applications in the medium temperature range, the introduction of

hybrid systems could be favourable, where water use and/or water and sewage costs have to be

reduced. Such a change, generally, does not increase electrical demand, but can reduce the

annual water consumption considerably. Depending on actual conditions and required size,

hybrid concepts may require additional space.

1.5.3.3 Replacement of outdated heat transfer technology by modern one

Often a change of cooling technology for different reasons is not suitable. However, also a

modification of the existing technology could lead to better efficiency, better performance, less

emissions and lower operating costs. Development of air moving systems and heat transfer

surfaces, as well as the application of more durable construction materials, are main reasons for

replacement scenarios.

As there is usually no change in process temperatures (same technology) the main focus in this

scenario is to reduce operating resources and environmental impacts as well as to achieve an

extension of equipment’s life. Equipment’s life extension of more than 10 years can be realised

by the use of new durable materials. It is very likely that any equipment installed 15 or 20 years

ago, can now be replaced by modern equipment with higher operating efficiency and better

environmental and economic performance.

A typical example for improvement of once-through cooling systems is the application of the

more efficient plate and frame heat exchangers. For evaporative cooling systems for example,

major developments have taken place to improve the performance of fill packs and of air

moving systems, resulting in a more compact design with higher energy efficiencies. For aircooled systems, new technology to shape fins in various ways has achieved similar results. An

example of what could be the effect on energy use if applying better efficiency is illustrated in
Chapter 1

36 Industrial Cooling Systems

Table 1.10. In this case the investment costs need to be balanced with the yearly operation costs

for energy use and maintenance of fill.

Table 1.10: Example for conversion of an outdated mechanical draught wet cooling tower into

modern design

[tm139, Eurovent, 1998]

Example:

Mechanical draught wet

cooling tower

Outdated design:

induced draught concept

with low-efficiency fill and

fan system

Modern design:

induced draught concept

with high-efficiency fill and

fan system

Capacity 1200 kW

Inlet temperature 38 °C

Outlet temperature 28 °C

Wet bulb temperature 21 °C

Water flow 28.7 l/s

Fan power requirement 7.5 kW 4 kW

Energy consumption for fans 9 MWh/yr 4.8 MWh/yr

Investment cost - € 14000

1.5.3.4 Upgrading existing heat transfer technology

Often it is not necessary to replace the whole cooling system. The performance of existing

cooling systems can also be improved by upgrading. Major components or accessories of the

system are replaced or repaired, while the existing installation remains in situ. Upgrading can

increase system efficiency and reduce the environmental impact. Examples of upgrading are

new and more efficient fill packs of cooling towers and the application of sound-attenuation.

The cases in Table 1.11 and Table 1.12 should be considered as simplified illustrations. For an

integrated assessment of the environmental gain other factors should be considered as well. For

example, with the replacement of cooling tower fill, the environmental costs for the old fill that

has to be disposed of must be included also.

Table 1.11: Example for replacement of outdated fill of a mechanical draught wet cooling tower

with modern high efficiency fill

[tm139, Eurovent, 1998]

Example: mechanical draught wet

cooling tower Outdated fill High efficient fill

Capacity 3600 kW

Inlet temperature 38 °C

Outlet temperature 28 °C

Wet bulb temperature 21 °C

Water flow 86.1 l/s

Existing cell floor space 26 m2

Fan power requirement 22.5 kW 13.5 kW

Energy consumption for fans 81 MWh/yr 48.6 MWh/yr

Investment cost - € 29000

That not all changes have only positive effects can be observed from Table 1.12 where a

considerable reduction of the noise level has been achieved. However, noise abatement usually

leads to fall of pressure, which must be compensated by a higher performance of the fans. This

in its turn raises the direct energy use of the cooling system. It will be a matter of local
Chapter 1

Industrial Cooling Systems 37

preference whether a lower energy use or a lower noise level prevails. Investment and

maintenance costs should be compared with reduced costs for energy consumption.

Upgrading the operational strategy is another example of efficiency improvement. The on and

of cycling of fans can be changed into modulating control with frequency converters. This can

result in significant savings of electrical energy, which, depending on conditions, can be 70%

and more.

Investment costs for upgrading can differ greatly and depend on the type of upgrading and the

age of the existing installation. The investment is accompanied by lower operating costs as a

result of a higher efficiency. Investment costs for upgrading will generally be lower than those

for technology changes or replacements of equipment.

Table 1.12: Example for the improvement of acoustic performance by addition of sound

attenuation

[tm139, Eurovent, 1998]

Example: mechanical draught wet

cooling tower Existing wet cooling tower Upgrading with soundattenuation

Capacity 1200 kW

Inlet temperature 38 °C

Outlet temperature 28 °C

Wet bulb temperature 21 °C

Water flow 28.7 l/s

Fan power requirement 15 kW 18kW

Sound power level 90 dB(A) 81 dB(A)

Investment cost - € 12000

1.6 Economic considerations

Costs are always among the most important factors for the selection of a cooling system and can

only be assessed on an individual project level. Three important types of costs can be

distinguished:

• investment costs,

• maintenance costs,

• operating costs related to energy (and water) requirements,

• environmental costs, such as taxes and costs for waste disposal.

The absolute costs and the relation between the different costs vary and depending on the

cooling system. The cooling system with lowest investment costs is not necessarily also the

system that requires minimal operating resources. Technical solutions to minimise resource

consumption often lead to higher investment cost.

Therefore, it is important that economic considerations not only focus on simple investment cost

comparisons, but also on operating costs of a cooling system. For power plants, operating costs

are linked to the overall energy efficiency. The financial effect of a variation in efficiency

caused by the choice of a different cooling system must be assessed. Generally for power plants,

the comparison of different solutions is done using the earlier mentioned techno-economic

method using an ‘actualised’ or ‘valoridated’ ratio that varies between countries. [tm056,

Caudron, 1991].

Chapter 2

Industrial Cooling Systems 39

2 TECHNOLOGICAL ASPECTS OF APPLIED COOLING

SYSTEMS

2.1 Introduction

This chapter gives brief description of the principles of some of the cooling systems

configurations used in European industry. Within these configurations a variety of applications

can be found all aimed at meeting process, site, environmental and economic requirements. The

size and type of heat exchanger, type of fans and operational practice also vary. The different

types of cooling systems can be classified by different criteria. The standard literature uses the

following criteria:

• dry air-cooled and evaporative wet cooled - according to the dominating

thermodynamic principle – respectively sensible heat transfer and a combination of

latent and sensible heat transfer. In evaporative cooling the two principles are coupled,

but the main part of heat is transferred latent, at dry cooling only sensible heat transfer

takes place.

• open or closed systems – in an open system the process medium or the coolant is in

contact with the environment; in a closed system the process medium or the coolant

circulates inside the tubes, coils or conduits and does not have contact with the

environment.

• direct or indirect systems – in a direct system there is one heat exchanger where coolant

and medium to be cooled exchange heat; in an indirect system there are at least two heat

exchangers and a closed secondary cooling circle, between the process or product to be

cooled and the primary coolant. Due to the additional heat exchanger, indirect systems

have a higher approach (about 5 K). Direct and indirect systems are also known as

primary and secondary systems. In principle every direct cooling system can be

transformed into an indirect system and this option is considered in situations where

leakage of the process medium would endanger the environment.

Direct contact cooling systems (not to be confused with direct/indirect) are not described in this

BREF, because their characteristics depend strongly on the industrial process they are applied to

(e.g. hot steel). Another type of cooling is a once-through system with barometric condensers, in

which a gas flow is cooled directly by dosage water over it. They can be found in the food

industry. These systems are not covered in this document, either are systems that use vacuum

techniques or specific refrigerants, such as HCFC.

In practice a variety of names used for both cooling equipment and cooling configurations found

inside and outside of Europe. Nomenclature is often linked to the purpose of the application and

in power generation plant typology refers to the condensing process (see Annex XII). In general,

the following list of systems commonly applied by European industry can be derived from the

above-given principles.

• once-through cooling systems (with or without cooling tower)

• open recirculating cooling systems (wet cooling towers)

• closed circuit cooling systems

- air-cooled cooling systems

- closed circuit wet cooling systems

• combined wet/dry (hybrid) cooling systems

- open hybrid cooling towers

- closed circuit hybrid towers

For closed recirculating cooling systems, further distinction can be made between small off-thepeg applications and large, tailor-made ones that are constructed or assembled on site.
Chapter 2

40 Industrial Cooling Systems

Generally, once-through systems and open recirculating systems are applied to larger plants in

the power and the (petro-) chemical industries.

The term tower is applied to both shell type constructions (e.g. large natural draught) and cell

type constructions, which can be small and can be found in roof type applications.

For comparison, some technical and thermodynamic characteristics of the most common

industrial cooling systems are summarised in Table 2.1. These data are an example derived from

a given number of assumptions (see legend to the table). It is important to realise that

approaches can vary and depend largely on the design of the heat exchanger and the temperature

of the ambient air. The minimum end temperatures of the process medium will vary

accordingly. For power stations, the approach is calculated in a different way (see Annex I).

Table 2.1: Example of technical and thermodynamic characteristics of the different cooling systems

for industrial (non-power plant) applications

[tm139, Eurovent, 1998]

Cooling system

Cooling

medi

um

Main cooling

principle

Minimum

approaches

(K)4)

Minimum achievable

end temperature of

the process medium5)

(°C)

Capacity of

industrial process

(MWth)

Open once-through

system - direct Water Conduction/Convection 3 – 5 18 – 20 <0.01 - > 2000

Open once-through

system - indirect

Water Conduction/

Convection 6 – 10 21 – 25 <0.01 - > 1000

Open recirculating

cooling system -

direct

Water1)

Air2) Evaporation3) 6 – 10 27 – 31 < 0.1 – >2000

Open recirculating

cooling system -

indirect

Water1)

Air2) Evaporation3) 9 – 15 30 – 36 < 0.1 - > 200

Closed circuit wet

cooling system

Water1)

Air2)

Evaporation

+ convection 7 – 147) 28 – 35 0.2 – 10

Closed circuit dry air

cooling system Air Convection 10 – 15 40 – 45 < 0.1 – 100

Open hybrid cooling Water1)

Air2)

Evaporation

+ convection 7 – 14 28 – 35 0.15 - 2.5 6)

Closed hybrid

cooling

Water1)

Air2)

Evaporation

+ convection 7 – 14 28 – 35 0.15 - 2.5 6)

Notes:

1) Water is the secondary cooling medium and is mostly recirculated. Evaporating water transfers the heat to the air

2) Air is the cooling medium in which the heat is transferred to the environment.

3) Evaporation is the main cooling principle. Heat is also transferred by conduction/convection but in a smaller ratio.

4) Approaches of heat exchanger and cooling tower must be added

5) End temperatures depend on the site’s climate (data are valid for average middle European climate conditions

30°/21°C dry / wet bulb temperature and 15°C max. water temperature

6) Capacity of small units – with a combination of several units or specially built cooling, systems higher capacities

can be achieved.

7) Where an indirect system applies or convection is also involved the approach in this example increases with 3-5K leading to 

an increased process temperature

The example of Table 2.1 shows that configurations have different temperature ranges and that

the desired temperature range for a process might need a certain configuration. For reasons of

space and costs, dry air-cooling systems are generally not used for very large capacities,

whereas water cooling can be applied for the discharge of heat up to 2000 MWth or more.

For condensers the approaches are higher. The approach for once through systems corresponds

to the sum of the “terminal difference” and the temperature rise of the cooling water. The term

“terminal difference” to the temperature difference between the temperature of the steam
Chapter 2

Industrial Cooling Systems 41

entering the condenser (or the condensed steam leaving the condenser) and the temperature of

the cooling medium (water) leaving the condenser. The values differ between 3 and 5 K. The

applicable data are presented in Table 2.2.

Table 2.2: Examples of capacity and thermodynamic characteristics of different cooling systems for

applications in power industry

[Comment EDF/[tm056, Caudron, 1991][tm056, Caudron, 1991][tm056, Caudron, 1991]]

Cooling system Applied approaches (K) Capacity of power generating

process (MWth)

Open once-through systems 13-20 (terminal difference 3-5) < 2700

Open wet cooling tower 7-15 < 2700

Open hybrid cooling tower 15-20 < 2500

Dry air cooled condenser 15-25 < 900

This chapter gives an overview of the most common industrial cooling systems and an

indication of their associated environmental aspects. More detailed information on heat

exchangers and material can be found in Annexes III and V, as well as in the documents in the

reference list. In the following paragraphs the technical terms will be used that have been most

commonly encountered in the literature. As an aid to consulting references, an indication will be

given where other terms are also used.

2.2 Heat exchangers

Heat exchangers are the crucial heat transferring elements, being part of both the process to be

cooled and the cooling system. After the heat exchanger, different systems are used to discharge

the heat into the environment. Two types of heat exchangers are commonly in use: the shell and

tube type (the most common) and the plate and frame type.

2.2.1 Shell and tube heat exchangers

There is a lot of experience with this kind of heat exchanger in the process industry and it has

proven reasonably reliable. There is a range of different designs, where tubes run straight or in a

U-form or where the heat exchanger is particularly designed for high-pressure conditions, high

temperatures, operating with steam or thermal fluids. Usually the tubes contain the cooling

water and the process medium moves around the tubes within the shell. For a more extensive

discussion on shell and tube heat exchangers see Annex II.

2.2.2 Plate and frame heat exchangers

Plate and frame heat exchangers are increasingly used for a range of applications in sugar

refineries, (petro-) chemical industry and power plants. They are particularly suitable for use at

a lower approach as well as in cold applications (< 0oC). However, these exchangers are less

suitable for cooling of steam and high gas volumes, and in situations where there is danger of

sedimentation and/or fouling and for high pressure differences between the process fluid and the

coolant. Some designs have a double construction to guarantee leakage-free operation, but this

is reported to be very difficult to maintain. Plate and frame heat exchangers are economic as

they can be much more compact (e.g. circular) than shell and tubes with the equivalent

exchange surface area.
Chapter 2

42 Industrial Cooling Systems

2.2.3 Environmental issues of heat exchangers

From an environmental point of view the following issues are important for both types of heat

exchangers:

- adequate design for efficient heat exchange;

- proper construction to prevent leakage of the process fluid into the cooling medium;

- choice of material for the efficiency of heat transfer, for resistance to corrosion in water and

to corrosion due to the process medium;

- possibility of using mechanical cleaning devices.

2.3 Once-through cooling systems

2.3.1 Direct once-through cooling systems

Technical description

In direct once-through systems, water is pumped from a source (e.g. a river, lake, sea or estuary)

via large water inlet channels directly to the process. After passing heat exchangers or

condensers the heated water is discharged directly back into the surface water. The heat is

transferred from the process to the coolant through the partition wall in the form of tubes in a

shell & tube or in a plate & frame heat exchanger. Once-through systems are identified by

various names. For example, in the paper industry, many mills refer to their once-through

cooling water as "mill supply". [tm010, Betz, 1991]

Figure 2.1: Schematic representation of a direct once-through cooling system

[tm001, Bloemkolk, 1997]

Cooling capacity

Once-through systems are designed mostly for large cooling capacities (>1000 MWth), but may

also be designed for small systems (< 10 kWth). Typical water flows for large power plants to

cool 1 MWth are in the range 0.02 m3

/s (∆T = 12K) to 0.034 m3/s (∆T = 7K). With once-through

cooling, low end temperatures can be reached with a corresponding approach of 3-5K.

Environmental aspects

For once-through systems the major environmental aspects mentioned are:

• the use of large amounts of water

• heat emission,

• the risk of fish intake,

• sensitivity to bio-fouling, scaling or corrosion

• the use of additives and the resulting emissions to water,

• energy consumption, mainly for pumps,

• the risk of leakage from the process stream, and

• the silting-up of sieves at water intake.
Chapter 2

Industrial Cooling Systems 43

Application

Once-through systems are used by large industrial processes such as the power generating

industry, chemical industry and refineries. The water used for once-through cooling is mostly

surface water. For smaller scale uses, such as pump cooling, tap water or groundwater is also

used. A reliable source of water near the site and at a suitably low temperature is an essential

condition for once-through systems. The quality of the surface water and the discharge limits

can also affect the applicability, but generally water quality and water chemistry are less

restrictive than in the case of recirculating systems. [tm005, Van Donk and Jenner, 1996]

2.3.2 Once-through cooling systems with cooling tower

Because of the power generating process operates under vacuum conditions, leakage in the

condenser of a power plant generally means pollution of the process-water by the cooling water.

On a number of sites, once-through systems can be found combined with a cooling tower to

precool the discharge before it is emitted into the receiving surface water. This configuration is

applied in situations where cooling water may recirculate and raise the temperature of the

cooling water intake of the same plant or other industries. River capacity, tidal movement,

plant-size and temperature of the surface water are also factors. This kind of precooling can be

found at coastal power stations (estuaries) and inland at riverbanks.

Environmental aspects of open wet cooling towers will apply to these cooling systems.

Biological growth and deposits have to be considered when choosing cooling tower fill. In

general, cooling towers with wide spread fill or splash fills are applied.

Figure 2.2: Schematic representation of a direct once-through cooling system with a cooling tower

applied in power industry

[tm132, Eurelectric, 1998]
Chapter 2

44 Industrial Cooling Systems

2.3.3 Indirect once-through cooling systems

Figure 2.3: Schematic representation of an indirect once-through cooling system

[tm001, Bloemkolk, 1997]

Technical description

This cooling configuration is similar to the direct once-through system, but is indirect because

there is no direct transfer from the process fluid/vapor to the coolant that is discharged. Here,

the term secondary cooling system is also used. Heat is transferred from the process medium or

product to a coolant that circulates in a closed circuit (t3 and t4). The coolant in this secondary

cooling circuit transfers its heat via heat exchangers to the coolant (e.g. surface water) that flows

through the heat exchangers only once, the so-called primary cooling water (t1 and t2). This

water is directly discharged into the surface water, whereas the secondary coolant remains in the

closed circuit.

Cooling capacity

With indirect once-through cooling, the same low end temperatures can be reached, but due to

the extra heat exchanger the approach can increase by another 3-5K, depending on the

efficiency of the heat exchanger.

Environmental aspects

See also direct OTS. The design means that the risk of discharge of leaked process fluids to the

surface water is of minimal or zero.

Application

The indirect once-through cooling water system is used where there is a high environmental risk

if process fluids leak into the cooling water. Availability and quality of the surface water are

also important for this cooling system. This system also creates a thermal load in the receiving

surface water. A variant of the indirect once-through system is to recycle a part of the water of

the primary cycle. This part is cooled by air before it is mixed with new incoming cooling

water. This extra cooling capacity can be used in periods of the year when insufficient cooling

water is available.

Generally, as a result of the extra heat exchanger (i.e. higher approach) the process end

temperatures that can be reached are not as low as with direct once-through cooling.
Chapter 2

Industrial Cooling Systems 45

2.4 Open recirculating cooling systems

Figure 2.4: Schematic representation of an open recirculating system

[tm001, Bloemkolk, 1997]

Technical description

Open recirculating cooling systems are also referred to as open evaporative cooling systems. In

these systems, cooling water that is led through the heat exchanger(s) systems is cooled down in

a cooling tower where the majority of the heat is discharged to the environment. In the cooling

tower the heated water is distributed over the cooling tower fill and is cooled by contact with air

and collected in a reservoir, after which it is pumped back to the heat to be reused as a coolant.

The air movement is created naturally or by means of fans that push or pull the air through the

tower. Cooling of the water is a result of evaporation of a small part of the cooling water and of

sensible heat loss by the direct cooling of water by air, also called convection. The wet and dry

bulb temperatures largely influence the level at which these systems can operate.

Most, but not all of the water that is cooled in the tower is recirculated and can be used as

cooling water again. The main causes of water loss are evaporation, blowdown (windage, drift,

purge (intentional blowdown) and leaks. Intentional blowdown is the draining of water from the

circuit necessary to avoid thickening of the cooling water (Annex VI). To compensate for the

blowdown and evaporation, water is added and this is the so-called make-up. Generally, the

make-up water flow used by an open recirculating system is about 1-3% of the flow of a oncethrough system with the same cooling capacity. For the power industry this can be 1-5%. This is

equal to a requirement of approximately “0.25% x cooling range”, which is the make-up water

quantity as a percentage of the circulating water flow. Blowdown generally ranges from 0.15-

0.80 m3

/s per 1000 MWth cooled. (Water-half time varies between one hour and four days.)

This system requires sufficient quantities of water available all year around and generally

cooling water treatment is necessary.

Cooling capacity

Open recirculating systems are mainly used for industrial applications with a heat capacity

ranging from 1-100 MWth, but also for power stations with much larger capacities. These

systems are mostly applied inland where insufficient water is available, or where no further rise

of the water temperature of the receiving water is acceptable, a situation found alongside rivers

with low flows in warm summer months [tm005, Van Donk and Jenner, 1996]. Wet cooling

towers transfer to the atmosphere about 80% of the residual heat in the form of latent heat

(water vapour) and about 20% by sensible heat [tm132, Eurelectric, 1998]. Approaches of 4K

are technically and economically achievable between 15 and 30oC. Approaches and minimum

end temperatures depend on the climatic conditions on site.
Chapter 2

46 Industrial Cooling Systems

Environmental aspects

The environmental aspects of recirculating systems depend particularly on the type of cooling

tower and the way it is operated. They are:

• cooling water additives and their emission through the blowdown to surface water,

• use of energy for pumps and fans,

• emissions into air,

• plume formation, condensation and ice-formation,

• noise,

• waste due to replacement of cooling tower fill, and

• human health aspects.

Application

Recirculating systems are applied in a wide range of processes. One feature is the reduction of

heat load to a receiving waterway by changing the direction of the discharged waste heat from

the surface water to the air. Another feature is the reduction of the amount of water used for

cooling. A common practice therefore is the modification of once-through cooling systems into

open evaporative cooling systems by applying one or more cooling towers.

Open recirculating configurations are:

- open wet cooling towers

- open hybrid or wet/dry cooling towers

2.4.1 Natural draught wet cooling towers

Construction

Large towers nowadays are shell type and made of reinforced concrete. The constructions are

mostly hyperbolic-rotational shells having advantages in thermodynamic/statical aspects.

Investment costs are high, whereas operational costs are comparatively low. Natural draught wet

cooling towers are commonly used for large power stations and large industrial plants.

Water distribution system

The water returning from the heat exchanger is brought into the tower using a water distribution

system. This system creates fine droplets or a water film. Uniform distribution enhances the heat

exchange. Options are offered for partial operation of the water distribution system to lower the

cooling capacity if needed. Also, winter operating modes are offered based on preheating of the

cooling air.

Cooling tower fill

The fill section is the important part of each open wet tower, creating the contact surface for the

exchange of heat from water to air. There can be film fill or splash type fill. Film fill usually

consists of closely packed, corrugated, vertical sheets or sheets of organic materials, which

cause the water to flow down through the tower in a very thin film. This fill is very efficient and

can be used for most applications. Some types may require a certain water quality, because they

are susceptible to fouling.

Splash type fill can be found in different configurations and can be made of a variety of

materials (e.g. wood). Splash fill has a much lower efficiency than film fill, but is used

particularly situations where the water is heavily contaminated or of poor quality, where film fill

would have problems due to a contaminated surface. Where the suspended matter content is

high, fibre cement sheets are also used.

Drift eliminators

To save water, drift eliminators are installed above the water distributors to prevent the water

droplets from being entrained by the airflow. Nowadays, drift eliminators are made of a number
Chapter 2

Industrial Cooling Systems 47

of materials, such as plastic or fibre cement, and designed in such a way that they cause minimal

pressure drop.

Characteristics of natural draught wet cooling towers:

- airflow is a result of air density differences and of shape of tower as in a chimney

construction;

- height is considerable (80-200 m.); [construction height as obstacle for people, aviation,

electronic transmissions and plumes];

- there is no energy requirement for fans, unless fan assisted, which enables lower heights;

- it is designed with counter flow and internal fill, or cross flow and external fill (see Figure

2.5 and Figure 2.6);

- it requires a base load operation, i.e. tower being in operation for more than 60 % of the

year in operation

- it is generally applied for a rejected heat capacity of more than 200 MWth, i.e. large plants

such as power stations or large chemical plants;

- it offers an option for desulphurized flue gas discharge, using the cooling tower as stack

avoiding reheating of the flue gas required for environmental reasons;

Figure 2.5: Natural draught wet cooling tower counter flow

[tm103, BDAG, 1996]
Chapter 2

48 Industrial Cooling Systems

Figure 2.6: Natural draught wet cooling tower cross flow

[tm010, Betz, 1991]

2.4.2 Mechanical draught wet cooling towers

Construction

Mechanical draught cooling towers are applied in different types of constructions. A wide

variety of materials is used for construction of these cooling towers, depending on size and type

and the requirements with respect to the location, service life and capital expenditure. Larger

ones can be built of reinforced concrete, smaller ones can vary considerably, but mostly consist

of synthetic material, steel plate, clad steel constructions and in-situ or pre-cast concrete. For

relatively smaller towers (5 MWth) timber is still in use; it is cheaper, can be built at any time of

the year and can be quicker to build than concrete towers.

It is also possible to use a modular system, i.e. several towers in parallel, in the same concrete

structure. In this way the system can be operated in the most economical way, choosing the

number of elements in operation, depending on ambient conditions and on the amount of heat.

Materials and type of construction and design affect the environmental performance of the

cooling tower. Referring to shape and size or brand, a variety of names is used in the literature

describing the use and application of these towers. Examples are the circular cooling tower and

cell type cooling towers both in forced and induced draught designs.
Chapter 2

Industrial Cooling Systems 49

Design of equipment for water distribution, fill and drift elimination can be different to that

of the natural draught wet cooling tower, but the working principles are the same.

Figure 2.7: Fan assisted natural draught cooling tower

[tm103, BDAG, 1996]

Fans

Mechanical cooling towers use fans to create the airflow and consequently can be much smaller

than the large natural draught type. A large number of different fan types is applied in

mechanical cooling towers (dry, wet or hybrid). Depending on the requirements, fans vary in

fan diameter, blade size and position (radial or axial). Additionally, one-speed or multi-speed

drives enable flexible operation. The choice of the fan type and the drives will affect the energy

demand and the sound emission level of the cooling tower. Depending on the way the airflow is

created distinction is made between forced and induced towers.

The fan-assisted tower is a special design used in a number of cases where the local situation

requires a lower tower.

2.4.2.1 Forced draught wet cooling towers

Characteristics of the forced draught cooling tower:

- fans at the base of the cooling tower push air through the tower;

- thermal performance is adjustable in steps or modulating;

- single-and multi-fan designs are applied;

- the cooling tower size is limited, requiring less space than a natural draught tower;

- tower can be adapted to surrounding terrain (rooftop installation);

- direct energy consumption is assumed to be low;

- it is usually designed with counter flow design;

- it can be designed for a wide variety of applications: for peak load and high heat rejection

and from base load to medium load operating standard;

- it is applied for a rejected heat capacity from less than 100 kWth to a heat capacity of more

than approximately 100 MWth;

- capital investment is low compared to natural draught towers;

- when using mechanical draught cooling towers, regulations with regard to emission of

noise, moisture (plume) and bacteria are to be observed.
Chapter 2

50 Industrial Cooling Systems

Figure 2.8: Schematic representation of a forced draught tower with counter flow design

[tm010, Betz, 1991]

2.4.2.2 Induced draught wet cooling towers

Characteristics of the induced draught cooling tower:

- fans at the top of the cooling tower construction pull air through the tower;

- thermal performance is controllable within limits,

- a relatively simple construction is preferred (prefabricated elements, off-the-peg product),

- the cooling tower size is limited, requiring less space than a natural draught tower;

- cooling capacity can be enlarged by working with several sections

- tower can be adapted to surrounding terrain (rooftop construction);

- cost of direct energy consumption is assumed to be low,

- designed with counter flow or cross flow,

- it is used for a wide variety of applications: for peak load and high heat rejection and from

base load to medium load operating standard; it is applied for a rejected heat capacity from

approximately 100 MWth,

- capital investment is low compared to natural draught towers,

- when using mechanical draught cooling towers regulations with regard to emission of noise,

moisture (plume) and bacteria are to be observed.
Chapter 2

Industrial Cooling Systems 51

Figure 2.9: Schematic presentation of a cell type induced draught cooling tower, cross flow design

[tm010, Betz, 1991]

2.5 Closed circuit cooling systems

2.5.1 Air-cooled cooling systems

In air-cooled cooling systems (or dry cooling systems) the substance (fluid, vapour) is circulated

through coils, tubes or conduits, which are cooled by a passing air stream. Generally dry air

cooling has the following applications:

- cooling of medium of nearly any chemical composition can be applied, requiring

appropriate heat exchanger material only;

- in situations where cooling tower make-up water is not available or for a short period of

time only and

- where formation of plume is not admissible.

Technical description

Depending on the application, closed circuit dry air-cooled systems consist of (finned) tube

elements, coils or conduits of a condenser, fans with drives and a carrying steel construction or a

tower. The process medium itself or a coolant (indirect system) is circulated through the tubes.

An air stream is created, naturally or by fans, that flows past the tubes thus cooling the medium

by conduction and convection. In almost all cases the air flows crosswise through the heat

exchanger. The process medium passes the heat exchanger in a 'one-pass' or a 'multi-pass'

configuration.

The process medium is a fluid, the cooling system is called an air-cooled fluid cooler. If a

vapour (gas or refrigerant) is directly cooled down to condense to liquid, the cooling system is

called an air-cooled condenser. Application can be in a mechanical or natural draught design.

A wide variety of corrosion-resistant materials is used for construction. The options for

construction are numerous. Air cooling systems can be found as large independent units as well

as smaller rooftop units. They can be horizontal, roof type rectangular, vertical or as a Vconstruction to suit the plant layout requirements.
Chapter 2

52 Industrial Cooling Systems

Cooling capacity

In practice, air-cooling is often used to cool process flows at a high temperature level (>80oC)

down to a level at which water-cooling becomes more appropriate. The driving force of the heat

exchange is the temperature difference between the cooling air and the process flow. The

maximum design temperature of the cooling air may in practice only be exceeded a few hours

per year. The design temperature depends on dry-bulb temperature and climatic conditions are

very important.

Because the heat capacity of air is low (1.0 kJ/kg.K) and the coefficient of conduction and

convection is low, a lot of air is needed and a larger heat exchanging surface is required than

with water cooling. For this reason, fins are often placed on the pipe surface to increase the

effective heat exchange surface. Based on economic considerations a minimum approach of 10-

15oC is used in the design of air coolers. This generally results in higher end temperatures

(minimum 40-45oC), although in areas where higher ambient air temperatures occur, the

approaches and the end temperatures exceed the average values mentioned in Table 2.1 and

Table 2.2. For indirect configurations the approach (13-20oC) and the achievable end

temperatures (50-60oC) will increase accordingly.

Environmental aspects

The major environmental aspects are noise and energy use for driving the fans. No water is

being used, unless it is used as secondary coolant in an indirect design. However, being closed,

this water requires little or no maintenance.

Cleaning of the outside of the (finned) tubes is necessary and sometimes problems can arise due

to the accumulation of airborne debris and small insects.

Application

Dry air-cooled heat exchangers are applied in a wide variety of industries in small and large

sizes. They are applied for product cooling in the chemical and petrochemical industries, for

vacuum condensation in power stations and for exhaust cooling.

For the same capacity, dry air-cooling needs a larger surface than a wet cooling system and dry

systems are generally considered to be more expensive. In the power industry, dry air-cooling is

therefore considered in specific situations where power generation is planned at locations with

insufficient water supply for wet cooling.

2.5.1.1 Natural draught dry cooling tower

Characteristics of a natural draught dry cooling tower are:

- base load operation, i.e. more than 60% of the year in operation,

- heat rejection more than 200 MWth, i.e. large plants such as power stations, large chemical

plants, etc.,

- application in situations where absolutely noiseless operation is required,

- application in situations where cooling tower make-up water is not available or available for

a short period of time only.
Chapter 2

Industrial Cooling Systems 53

Figure 2.10: Schematic presentation of principle of a dry natural draught cooling tower

[Eurovent, 2000]

Figure 2.11: Example of natural draught dry cooling tower for a power plant application

[VGB, 2000]
Chapter 2

54 Industrial Cooling Systems

2.5.1.2 Air-cooled liquid cooling systems

Characteristics of an air-cooled liquid cooler tower are:

- thermal performance adjustable by fan control;

- closed circuit necessary;

- forced and induced draught both applied;

- cost of internal power consumption assumed to be higher than for wet cooling towers;

- low heat rejection, i.e. less than 100 MWth;

- change in cooling medium temperature nearly linearly to the (dry bulb) air temperature must

be acceptable for the process to be cooled;

- operating costs nearly entirely consisting of energy costs;

- environmental aspects in particular are noise and energy.

Figure 2.12: Schematic presentation of the principle of a dry air-cooled cooling system

[Courtesy of Eurovent, 2000]

Figure 2.13: Example of a dry air-cooled liquid cooler in a chemical process

[Pers. archive]
Chapter 2

Industrial Cooling Systems 55

2.5.1.3 Air-cooled steam condensers

Air-cooled condensers (see Annex XII) are widely applied in the power industry and chemical

plants for condensation of steam. The air is drawn in by fans under the condenser elements and

pushed through. The passing air cools down the steam entering the condenser tube bundles (see

Figure 2.14). In an indirect system the condenser is cooled by a cooling water stream which in

turn is cooled in a natural draught cooling tower.

Characteristics of air-cooled steam condensers are:

- heat rejection for small to large installations,

- no cooling water needed,

- cost of direct energy consumption is assumed to be higher then for wet condensers or wet

cooling towers,

- requires relatively low overall height,

- short exhaust steam pipes possible,

- considerable space requirement in the immediate vicinity of the steam generator,

- adaptation to load and temperature variations necessary over large ranges, which requires

variable fan speed operation,

- environmental aspects in particular are noise and energy.

Figure 2.14: Schematic presentation of the principle of a direct air-cooled condenser

[Balcke Dürr, 2000]
Chapter 2

56 Industrial Cooling Systems

Figure 2.15: Example of an air-cooled condenser for condensation of turbine exhaust steam

[tm111, BDAG, 1996]

2.5.2 Closed circuit wet cooling systems

In closed circuit cooling systems, the medium to be cooled is circulated in a closed circuit

without contact with the environment. The medium is led through a coil (primary circuit). The

coils are wetted from the outside (secondary or spray circuit). The heat is conducted from the

medium to the spray water (sensible heat transfer). The evaporation of a small part of the water

leads to evaporative cooling and the heat is transferred from the water to the air. There is an

additional sensible heat transfer from the coil to the air. In practice at evaporative cooling

sensible and latent heat transfer are always coupled. The wetting water is treated to avoid

damage to the equipment. Evaporative losses, drift and windage cause concentration, so some

de-sludging (blowdown) is needed and some make-up water has to be added.

Cooling capacity

The heat transfer capability is lower than for open systems due to the lower heat transfer

capacity of the coil. By combinations of units, larger capacities of 150-400 kWth to 2.5 MWth

can be achieved. Approaches of 4 K are typically achievable. The advantage is a contaminantfree closed primary cooling loop (true for all closed cooling), which in some cases eliminates

the need for internal heat exchangers. In terms of resources, the energy requirements for the

spray water loop have to be considered. With closed circuit cooling, end temperatures between

25-30°C are achievable, depending on the climatic conditions of the site [tm139, Eurovent,

1998]

Temperatures in the water film at heat exchanger surfaces are up to 5°C higher than the

temperatures of the bulk water, which typically range from 40 to 50°C, although temperatures

up to 70-80 °C can also be encountered in practice.

Environmental aspects

If closed circuit cooling systems use water as a secondary cooling medium, this is generally

alkalised demineralised water or potable water. Residence times in these systems can be up to 6

months. Make-up water is needed only when leakage and evaporation have occurred at pump

packings or when water has been drained to allow system repairs. Because only little make-up

water is needed, this can usually be of high quality, and as a result, scale deposits are not a

problem. Scaling can be caused by the water used on the outside of the tubes or coils and

treatment (cleaning) may be needed. [tm010, Betz, 1991]. Depending on the technical concept,

the mode of operation and the climatic conditions, plume formation may occur. Water can be
Chapter 2

Industrial Cooling Systems 57

saved, as the tower can be operated as a dry tower when the ambient temperatures are low. Fan

noise may be an issue.

Application

Closed circuit cooling systems are used in many applications. They are well suited to the

cooling of gas engines and compressors and can provide a reliable method of industrial process

temperature control [tm010, Betz, 1991]. They can be applied for both large and small

applications. Applications can be found as liquid coolers (e.g. lubricating oils, cooling water for

compressors), as gas coolers (e.g. diesel engines, process gas) and as air-cooled condensers

(combined cycle plants, steam turbines).

If the process medium in the coil or tubes is a vapour (gas or refrigerant) to be cooled down to

condense into liquid, this cooling system is also called an evaporative condenser.

2.5.2.1 Mechanical draught wet closed circuit cooling systems

Characteristics of the mechanical draught wet closed circuit cooling systems:

- heat rejection for small to large installations,

- low cooling temperatures can be achieved,

- compact design compared to air-cooled equipment,

- low energy requirements,

- need for water supply and spray water circuit,

- plume suppression achievable by air discharge plume abatement coils and/or dry

operation in winter,

- environmental aspects, in particular water treatment and effluent disposal.

Figure 2.16: Schematic presentation of the principle of a closed recirculating wet cooling tower with

induced draught

[Eurovent, 2000]

2.5.2.2 Evaporative steam condensers

Characteristics of evaporative steam condensers:

- heat rejection for medium to large installations,

- lower condensing temperatures than with air-cooled steam condensers,

- low energy requirements,

- generally higher than air-cooled steam condensers but with a smaller footprint,

- environmental aspects in particular are water treatment and effluent disposal.
Chapter 2

58 Industrial Cooling Systems

2.6 Combined wet/dry cooling systems

2.6.1 Open wet/dry (hybrid) cooling towers

Technical description

The open wet/dry cooling tower or hybrid cooling tower is a special design that has been

developed as an important solution to the problem of cooling water use and of plume formation.

It is a combination of a 'wet' and 'dry' cooling tower or, in other words, of an evaporative and a

non-evaporative process. The hybrid cooling tower can be operated either as a pure wet cooling

tower or as a combined wet/dry cooling tower, depending on the ambient temperature. The

heated cooling water first passes through a dry section of the cooling tower, where part of the

heat load is removed by an air current, which is often induced by a fan. After passing the dry

section, water is further cooled in the wet section of the tower, which functions similarly to an

open recirculating tower. The heated air from the dry section is mixed with the vapour from the

wet section in the upper part of the tower, thus lowering the relative humidity before the air

current leaves the cooling tower, which (almost) completely reduces plume formation above the

tower.

Optimising the effect of a hybrid cooling tower means optimising the amount of dry heat

transfer to meet the plume control requirements. At the same time the wet section is being used

for the major part of the cooling.

Figure 2.17: Schematic representation of hybrid cooling tower principle (example applied in power

industry)

[Eurelectric, 1999]

Characteristics of open hybrid cooling towers are:

- base load and partial load operation for all capacities

- cooling medium being water only

- cooling tower make-up water required for most of the operating time

- thermal performance being the same as in the case of wet cooling towers

- reduction in make-up water quantity

- regulations for environmental protection, e.g. reduction in overall height (due to fan

assistance) and plume abatement

- sound attenuation equipment required due to noise regulations.
Chapter 2

Industrial Cooling Systems 59

To operate a hybrid cooling tower efficiently a number of devices are used:

- variable speed fans

- closing devices for air inlet openings (such as louvres or sliding gates)

- valves for water flows to the wet and dry sections

- bypass systems

- booster pumps (for special constructions)

- system for mixing of the wet plume with dry plume.

Hybrid tower construction

Currently only mechanical draught type hybrid cooling towers are available. The hybrid cooling

tower is different from the characteristic open wet tower design in that it has a dry and a wet

section, each with its own air inlet and corresponding fans. Hybrid cooling towers can be found

as package cooling towers, large round cooling towers with forced draught fans or as cell-type

cooling towers with induced draught fans. Fill, water distribution system, drift elimination, and

sound attenuation are features common to both tower designs.

Wet/dry cooling towers of the mechanical draught type are fitted with internal mixing systems

to mix the wet and dry airflows. They can be automatically controlled according to the heat

load, water flow, ambient air and plume conditions.

Cooling capacity

They can be built as package cooling towers, induced draught or forced draught cooling towers

and – on a larger scale - as cooling towers of the cellular type or circular type with the heat

rejection ranging from < 1 MWth up to 2500 MWth.

Environmental aspects

The major difference between a hybrid cooling tower and a conventional cooling tower is its

comparatively lower water use (which is make-up water) amounting to 20% less than that of a

wet cooling tower [tm132, Eurelectric, 1998].

The resulting annual energy consumption of a mechanical draught hybrid cooling tower can be

reduced to a level of 1.1 to 1.5 times that of a comparable mechanical draught wet cooling tower

since in nominal conditions, airflow is almost double (wet and dry sections). Natural draught

cooling towers of the wet/dry design are under consideration.

Application

A decision to install a hybrid cooling tower is made in the light of site-specific requirements

(limitation of height and plume reduction) and several can be found in the power industry,

especially in Germany and in United Kingdom (in cogeneration systems). Its use is restricted to

temperature ranges of 25-55oC, because above 55oC precipitation of calcium carbonate is

observed to occur more easily on the tubes. This does not mean that no precipitation occurs

below 55oC and some care must be taken in using this as a rule of thumb.

2.6.2 Closed circuit hybrid cooling systems

Technical description

For closed circuit cooling hybrid systems, characteristics can be described in a similar way as

for closed recirculating wet cooling systems concerning fans (axial and radial), airflow direction

(cross or counterflow) and noise abatement systems (see § 2.4). Generally, these units have a

small space requirement. Three technical modes can be applied to closed circuit hybrid cooling

towers: sprayed finned coils, adiabatic cooling or combined systems.

Environmental aspects

Closed circuit hybrid cooling towers combine the advantages of closed loop cooling with

significant savings of water when compared to conventional closed circuit wet cooling towers.
Chapter 2

60 Industrial Cooling Systems

Compared to closed circuit dry cooling towers they offer the advantage of lower cooling

temperatures. In terms of size, energy consumption and noise emission they compare with

conventional closed circuit wet cooling towers. Depending on their design (sprayed finned

coils) special attention may need to be paid to the quality of the water treatment. Additional

costs can be more than offset by the significant saving of water, as such products require the use

of water only during a very short period of the year. Closed circuit hybrid coolers also

significantly suppress and, in some designs, even eliminate plume formation.

2.6.2.1 Sprayed (finned) coils

Figure 2.18: Schematic presentation of the principle of a closed circuit hybrid cooling tower

In a closed circuit cooling tower the process medium runs through cooling elements (a

tube/plate bank or the finned coil) in a closed loop, the primary cooling circuit. These cooling

elements are wetted via a secondary water circuit and air is simultaneously moved over the

elements to create evaporative heat. The cooling water that runs off the elements is collected in

a basin and can be recirculated a number of times, sometimes using another cooling tower or

after blowdown (see Figure 2.19). In an indirect configuration, the medium that runs through the

primary cooling circuit is not the process medium but another coolant which in turn cools the

process medium in a second heat exchanger.

2.6.2.2 Adiabatic coolers, wetting and pre-cooling the air that cools the coils

In the adiabatic mode the fluid to be cooled bypasses the prime surface coil. The cooling water

trickles down the wet deck and the air passing the deck is wetted with as much moisture as it

can take up. The wetted air passes the finned coils and will take up more heat than dry air would

do. Compared to conventional evaporative cooling equipment the water consumption is much

reduced. (See Figure 2.19)
Chapter 2

Industrial Cooling Systems 61

Figure 2.19: Combined dry/wet operation of a hybrid cooling system

[tm151, BAC, 1999]

2.6.2.3 Combined technology

In combined technology the finned coils, the sprayed prime surface coil and the wet deck are all

used. In the dry mode it is then possible to close all water sprays and lead the medium to be

cooled through both the finned coils and the prime surface coils, both cooled by dry air only. In

the wet/dry mode the medium after passing the dry coils passes the sprayed prime surface coils

before returning to the process as cooled medium. The warmed water trickling down from the

prime coils will fall over the wet deck surface. Air is drawn in and passes both the prime surface

coil and wet deck surface, where it is saturated and picks up heat. As it passes the finned coil

more heat can be picked up (see also Figure 2.19).

2.6.2.4 Costs of hybrid systems

In the application of hybrid systems reference is always made to the investment and operating

costs involved. In general hybrid systems require higher investment costs. Costs of plume

suppression vary depending on the cooling system. Compared to a cooling tower with the same

cooling performance, Fluor [1995] calculated that for an open wet cooling tower installation of

300 MW the cooling installation costs are about 2.5 times as high as for cooling towers without

plume suppression. For closed circuit wet cooling towers, costs for plume suppression are

reported to be 1.5 to 2 times as high as for towers without plume suppression (Eurovent). The

costs have to be adjusted for cost savings on water intake and operational flexibility. The annual

costs for water, including water treatment and electricity can represent in some cases just about

10 % of the annual costs of a cooling tower. These economic considerations depend of course

on the individual application and the prices of water and energy [tm139, Eurovent, 1998]

Cost indications by the power industry show levels of EUR 40000 to 70000 per MWth for

mechanical draught type hybrid cooling towers. In this sector this means an installation cost

level of 1.3-1.6 times that of towers of similar capacity without plume suppression.
Chapter 2

62 Industrial Cooling Systems

2.7 Recirculating cooling systems

The descriptions given above of major cooling configurations explain the cooling principles and

the different associated technical designs that are applied in industry, depending on

requirements of process, site and environment. Some key definitions have been explained in the

introduction as well as the difference between dry and evaporative cooling, and between open

and closed systems is used in the systems’ descriptions. The application of the criteria direct and

indirect, however, can lead to much confusion if they are not defined in the context of

recirculating cooling systems.

2.7.1 Direct recirculating cooling systems

As stated earlier, in direct cooling systems there is only one heat exchanger level where coolant

and process medium exchange heat and where the coolant (water or air) is in contact with the

environment. Leakage through the wall between process medium and coolant (air or water)

would therefore mean that process medium is discharged into the environment or that, in

vacuum conditions (condensers), the process is affected. Thus, although the cooling of the

coolant as it is done in a cooling tower is also a process of heat exchange, it is still considered a

direct system.

The example of the open cooling tower cooling the water circuit of a water-cooled condenser is

therefore a direct system (although, as mentioned, leakage will affect the process rather than the

coolant).

2.7.2 Indirect recirculating cooling systems

The key element to define an indirect system would be that leakage of the process would not

contaminate the coolant that is in open connection with the environment. This means two levels

of cooling.

In the case of an open recirculating cooling tower, the water leaving the tower would exchange

heat in a heat exchanger with water that is in a closed loop. The water in the closed loop will

leave this heat exchanger to enter another heat exchanger, where it exchanges heat with the

process medium.

In closed recirculating cooling towers the same principle is followed and the coils or tubes are

filled with water, which is cooled by water and /or air. The cooled water enters a heat exchanger

or a condenser within process to exchange heat with the process medium.

Where closed recirculating cooling systems operate in winter and need protection against

freezing, the closed circle usually contains not just water, but also a refrigerant or water mixed

with anti-freeze. In fact, those systems can again be classified as direct systems as the

refrigerant could pollute the cooling medium, which is in open contact with the environment.

2.8 Costs of cooling systems

For each configuration a cost indication has been given, but calculations made of the costs of

cooling systems show a wide variation and it can be concluded that the differences in costs

between the different systems do not necessarily indicate the least expensive variant. Of the

different factors that in the end influence the costs, the users’ requirements and the legal

requirements are very important. For this reason an estimate of the feasibility of a system or the

application of a technique should be made for each individual case. Energy prices always have

to be taken into account. They will be important, for instance, in those cases where heat

recovery is being considered.
Chapter 2

Industrial Cooling Systems 63

An important aspect in calculating the costs of a cooling system and of the possible

improvements is the comparison between the initial investment costs of a system or an applied

measure and the resulting annual costs. In practice, high investment costs can lead to lower

maintenance costs, but also to higher annual fixed costs, which can be an obstacle to investment

itself. For the sake of comparison, costs also have to be expressed in terms of the heat capacity

the system is designed for (kWth or MWth).

For industrial (non-power plant) applications [tm001, Bloemkolk, 1997] listed a number of costdetermining elements for both water-cooled and air-cooled systems, calculated total costs and

compared the different systems. The elements and the approach followed are explained below

and the results have been summarised in Annex X. For power plants a different model applies,

which is explained in Annex XII.

Elements

Generally the following cost-determining elements have to be taken into account:

Table 2.3: Cost elements for water and air cooling systems

[tm001, Bloemkolk, 1997]

Cost type Cost elements Water cooling

systems Air cooling systems

Fixed Heat exchanger(s) (type, size and

model) x x

Heat exchanger (material) x x

Pipelines in process, tube-bridges x x

Pumps/reserve pumps x x

Inlet facilities x

Tube intake/drainage x

Outflow facilities x

Cooling tower(s) (possible) x x

Fans x x

Sound attenuation x x

Indirect system (extra heat exchanger,

pipes, pumps) x x

Variable Water (groundwater, tap water) x

Water discharge fee x

Leakage monitoring x x

Water conditioning x

Energy consumption (pumps and fans) x x

Maintenance x x

Methodology

Different methodologies have been developed for cost comparisons between different cooling

systems. The method used is described briefly in Annex X.

Comparisons

Comparisons should always be made based on the same operational conditions and for the same

capacity and expressed per MWth dissipated.

Calculations have shown that cost sensitivity is to a large extent determined by the investment

level and the consumption of energy. The variation in the costs of heat exchangers (shell &

tube) due to the chosen configuration and to the choice of material is very important. Cheap

materials and models determine the calculated lower limits. Special materials determine the

upper limit. It should not be forgotten here that good materials could considerably decrease the

costs of maintenance and operating and of the use of chemicals.
Chapter 2

64 Industrial Cooling Systems

Calculated as annual costs, investments and operational costs differ significantly. Factors such

as (make-up) water requirement and price, and energy consumption are influential. The choice

of material also has consequences for the annual costs. Where dry air-cooling is applied, the

achievable end temperature is important and the lower the required end temperature, the more

expensive air-cooling will become. With water cooling, the low endtemperature is less

important for cost estimations, unless small approaches are used in the calculation.

Table X.2 in Annex X shows the ranges of costs of various large industrial cooling systems.

From the data used in this method the operational costs of an open wet cooling tower were

shown to be higher than for dry air-cooling. The investment costs for air cooling, on the other

hand, were generally higher than for the other systems. It further suggests that, particularly with

cooling water systems, higher investment can mean lower operational costs (maintenance,

conditioning).

On the basis of the above data, it can be concluded that the cost differences between different

systems do not necessarily indicate the least expensive variant. This clearly depends on the

users' requirements and the requirements on emission levels set by the authorities. For this

reason an estimate of what is feasible should be made for each individual case. The above data

can be used as initial (general) indications and are illustrated in Annex X.
Chapter 3

Industrial Cooling Systems 65

3 ENVIRONMENTAL ASPECTS OF INDUSTRIAL COOLING

SYSTEMS AND APPLIED PREVENTION AND REDUCTION

TECHNIQUES

3.1 Introduction

The environmental aspects of industrial cooling systems are different for each of the

configurations described in Chapter 2. Direct and indirect energy consumption, emissions of

heat and cooling water additives to the surface water, noise and plume formation are

environmental aspects of cooling systems. In every case, the environmental importance of these

issues (such as noise) should always be considered in the light of the total environmental

performance including that of the industrial process to be cooled. Not all issues are equally

important for every system, such as water requirements or plume formation, which play no role

in dry cooling systems. The issues that potentially are relevant and should be taken into account

by the permit writer in considering the industrial cooling system are qualitatively characterized

and summarized in Table 3.1. Obviously, where appropriate measures are taken, the issue will

become less relevant, but this has not been taken into account in this table as it will be part of

the discussion in the following chapters. The character and level of emissions to the

environment are not only a result of the applied configuration, but to a large extent depend on

the way the system is operated and the way in which the resources that are needed to operate a

cooling system are being managed.

This chapter discusses the environmental aspects and microbiological risks (or health risks) that

may have to be taken into account when an application for an environmental permit has to be

considered. At the same time, the principles of the techniques are described that can be

considered in the determination of BAT. In many cases the cooling system will be an existing

installation and it is obvious that the options for improvements are limited compared to

greenfield situations. Generally, the design of the process and the selection of the appropriate

cooling technology and design can reduce consumption and prevent much of the emissions to

the environment. In individual cases it will be a matter of setting the priorities for what should

be done or can be done, where site-specificity plays a major role.

To determine BAT, the BAT-”approach” is described separately for each environmental issue

and each technique, taking into account potential cross-media effects. The evaluation follows

the general “approach” outlined in Chapter 1. It starts with reducing the demand for cooling and

the discharge of heat to the environment. It is followed by the assessment of options for the

minimisation of the resources dedicated to the prevention or reduction of emissions, bearing in

mind that this will also lead to easier cooling process operation:

1. prevention by technological options:

- integrated technical measures

- change of configuration

2. prevention by optimisation of systems operation

3. application of end-of-pipe technology or additional techniques.

The environmental implications of each option are discussed and each technique is evaluated for

its effect on total energy consumption. At first, it is illustrated how changes in cooling

operations can affect energy consumption. Information on particular techniques and their

performances is then given in the annexes.
Chapter 3

66 Table 3.1: Environmental issues of the different industrial cooling systems[tm001, Bloemkolk, 1997]

Energy

Consumption

(direct)

Water

requirement

Fish(2)

entrainment

Emissions to surface

water

Air

emissions

(direct)

Plume

formatio

Cooling system

(§ 3.2) (§ 3.3)(1) (§ 3.3)

Heat

(§ 3.3)

Additives

(§ 3.4) (§ 3.5) (§ 3.5) 

Once-through cooling(direct circuit) Low ++ + ++ + (biocides) -- -- Once-through cooling(indirect circuit) Low ++ + ++ + (biocides) -- -- Open wet coolingtower (direct circuit) + + -- Low +(3) Low (in plume) + Open wet coolingtower (indirect circuit) + + -- Low +(3) Low (in plume) + Open wet/dry coolingtower + Low -- Low Low(3) -- --(5) Closed circuit wetcooling tower + + -- -- Low Low(4) (in plume) -- Closed circuit drycooling ++ -- -- -- -- --/Low -- Closed circuit wet/dry cooling + Low -- -- Low(3) Low -- Notes:-- none/not relevantLow relevance below average+ relevant++ highly relevant

1: paragraph in text2: other species can also be entrained3: biocides, antiscaling, anticorrosion4: potentially in case of leakage5: if properly operated no issue6: waste refers to sludge from water intake and from decarbonization
Chapter 3

Industrial Cooling Systems 67

3.2 Consumption of energy

The energy requirement of industrial cooling systems can be considered as a direct or an

indirect consumption. Direct consumption is the use of energy to operate the cooling system.

The major energy users are pumps and fans. The higher the resistance that has to be

compensated to maintain the required air or water flow, the more energy a cooling system

requires.

If not properly operated, a cooling system may be indirectly responsible for an increased input

of energy or raw material into the production process. To evaluate any change to a cooling

system, the total energy balance of both the cooling system and production processes has to be

taken into account.

3.2.1 Direct consumption of energy

Energy in cooling systems is required to pump the cooling water and/or to create airflow. It is

expressed in specific energy consumption as kWe per MWth dissipated heat. The specific energy

consumption can vary widely and depends on the configuration of the cooling system applied

(design (approach temperatures), pumping pressure) and pattern of operation (year round, only

summer or winter). Local circumstances will cause variation as well, where the same cooling

system in warmer climates typically requires a higher energy input than in cooler climatic

regions. In some cases, energy is required for the on-site preparation of additives. The major

energy users in a cooling system are:

• pumps (used in all systems with cooling water) for water intake as well as for circulating the

cooling water:

- their energy use is determined by the flow rate, the amount of water that has to be

pumped, fall of pressure in the process (number of heat exchangers, design), place

where cooling water is supplied and drained off and the medium to be pumped (gas,

fluid, solid).

- indirect systems have two circuits and will therefore need more pumps.

- in the case of a cooling tower, the lift is higher, which requires more energy in

comparison with a once-through system.

• fans for ventilation are used in all mechanical cooling towers and mechanically cooled

condensers:

- their energy use is determined by the number, size and type of fans, the amount and the

lift of the air,

- dry systems in general need more air for the same cooling capacity than evaporative

(wet) systems, although this does not necessarily lead to a higher energy consumption.

Energy use of associated activities

For an overall assessment of energy requirements of cooling systems auxiliary equipment

essential to operate a cooling system should be included. Not many data have been reported.

On-site production of cooling water chemicals such as ozone is reported as a typical example,

where the production of 1 kg ozone, used for antifouling purposes, varies between 7 and 20

kWh depending on the generator. Given a minimum concentration required at the injection point

between 0.5 and 1 g O3/m3

 and the cooling water volume, it is possible to estimate the energy

requirement.

3.2.2 Indirect consumption of energy

The energy consumption of the production process is referred to as the indirect energy

consumption caused by the cooling process. With inefficient cooling the consumption will

increase. A lower heat transfer (e.g. due to fouling) will raise the temperature on the process
Chapter 3

68 Industrial Cooling Systems

side, requiring more energy, which will have to be generated on or off site. Cooling inefficiency

leads to loss of product and will reduce the efficiency of the process.

In 1.2.1 and 1.4.3 the effects on temperature sensitive applications are discussed. Due to

reduced cooling of the condenser, total energy conversion can be reduced by 0.25%, which is

equal to a reduction of efficiency of about 0.4% per degree Celsius. If an open wet cooling

tower is applied, rather than a once-through system, resulting in for instance a 5°C higher end

temperature, at maximum 2% less power will be produced. If the difference in pumping energy

required for the cooling tower were taken into account (which is 6-8 kWe per MWth cooled), it

would cause an additional one-percent of efficiency loss. For a conventional coal fired power

plant a 1% decrease means that efficiency would drop from 40% to 39.6%.

The evaluation of the environmental impact of cooling should include the evaluation of the

indirect energy consumption. The consequences of a change in indirect energy consumption for

the total energy consumption can be expressed as the effect of a temperature rise on the process

side due to suboptimal cooling. This has been calculated and compared between the major

cooling configurations [tm059, Paping, 1995]. The data in Table 3.2 represent direct and

indirect energy use and CO2-emissions of each cooling configuration. The three variables are

considered to be a linear function of the following design parameters:

- cooling water flow

- pumping pressure

- pumping efficiency (inversely proportional)

In this example the data are calculated with a once-through system as the reference system for

each of the cooling principles. The once-through system has a capacity of 100 m3

/hour per

MWth (or 8.6oC per MWth) and needs a pressure of 3 bar to pump the water to the required

height. This requires about 10 kWe/MWth/year with a pumping efficiency of 75%. For the

indirect once-through system, a pressure drop occurs and 4.5 bar extra has to be generated,

requiring 15 kWe/MWth. A cooling tower system needs extra energy to lift the water over e.g. 8

m and in addition to this about 7 mwg over the nozzles. Compared to a once-through cooling

system, this requires an extra 4.5 bar and 15 kWe/MWth.

Fans are assumed to need 15 kWe/MWth. If they are only operated in summer (4 months) the

average energy required equals 5 kWe/MWth.

In the same table the indirect energy is expressed as a function of an increased cooling water

inlet temperature. This will lead to an increased temperature on the process side. The factor

representing this increase is calculated as 1.4 kWe/MWthoC (see Annex II). It means that per

degree of increase of the temperature on the process side the required energy increases by a

factor 1.4.

Knowing the total energy consumption for each cooling configuration, it is possible to express

this consumption in levels of emitted CO2 per discharged MWth. The energy required in power

generation to produce the energy consumed by a cooling system can be calculated. With an

assumed efficiency of 40% to generate power, each kWe input to operate the cooling system

must be multiplied by 2.5, expressing the energy required to discharge energy (=cooling), or

kWe per kWe (in ‰). For each ‰ an amount of CO2 is being emitted. It is assumed that an

average of 2000 (1500-2500) tonnes CO2 per year per MWe (continuous operation) is emitted or

2 tonnes CO2/‰. (This figure is derived from Dutch emission data and depends on the fuel

mix).

The data in Table 3.2 are within the ranges of relative energy uses generally found when

comparing different cooling systems with similar cooling capacities. The data are not exact and

should not be used as such. Nor do they imply that one system is less preferred than the other.

What the table clearly shows is that the effect due to loss of efficiency of cooling can be

considerable and that the consequences for the total energy balance can be made comparable.

This table shows the importance of considering both the direct and indirect energy consumption

when operating a cooling system.
Industrial Cooling Systems Table 3.2: Example of a comparison of yearly specific direct and indirect energy demand of different cooling systems and the c[tm059, Paping, 1995]

Specific direct energy consumption

(kWe/MWth)

Increased T

pump (oC)

Specific indirect

energy consumption

(kWe/MWth)

Total energy

consumption

(kWe/MWth)

Cooling system

Pumps Fans Total

Applied corr. factor1)

=

1.4 kWe/MWthoC

Total

direct+indirect

Once-through -direct 10 (9-12) - 10 0 0 10 

-indirect 15 (12-18) - 15 5 7 22 

Open wet cooling tower 15 (13-17) 5 20 5 7 27 Hybrid cooling 15 (13-17) 8 23 5 7 30 Closed circuit cooling tower >15 (13-17) 8 >23 8 11 >34 Dry air cooling - 20 20 20 28 48 1) for calculation of correction factor see Annex II
Chapter 3

70 Industrial Cooling Systems

3.2.3 Reduction of required energy for cooling

Reduction of required energy use of cooling systems is a matter of environmental balance.

Again, integrated measures within the industrial process to reuse heat will decrease the need for

discharge of excess heat into the environment. If a smaller cooling capacity is required,

generally and in absolute terms less energy is needed to operate the cooling system. More

efficient equipment and proper operation of the cooling system avoiding temperature increase

on the process side can achieve any further reductions.

The right choice of material and design will reduce the required power consumption of cooling

systems. This is a very complex matter including many factors, where general advice is difficult

to give. The following practices are applied and can be mentioned, as options one should be

aware of:

1. proper lay-out of the cooling system, such as smooth surfaces and as few changes of flow

direction as possible, will avoid turbulence and reduce resistance to the flow of the coolant;

2. in mechanical cooling towers, choice of type and position of fans and possibility of airflow

adjustment are options for reduced energy use;

3. choice of the right fill or packing (in light of the operating conditions) to secure maximum

heat exchange at all times;

4. choice of drift eliminators with minimum resistance to airflow.

Changes in material and design do not seem to be cost-effective options to reduce energy

requirements for existing systems, particularly for large systems. Replacing cooling tower

internals (fans, fill and drift eliminators) are an option in some cases. For smaller systems, such

as the open and closed recirculating wet cooling systems that are on the market as an off-the-peg

product, a change of the cooling system is technically much easier.

A good example of design effect is the application of smooth (finned) supporting pillars at the

inlet of a large (178 m.) natural draught cooling tower for a nuclear power station. The design

enhanced the airflow and reduced the pressure drop enabling 0.3oC colder cooling, where 1oC

cooler for this installation is approximately equal to a saving of EUR 250000/year.

Very little has been reported on the options for reducing the required amount of energy of a

cooling tower through more energy efficient fans or by flexibility of operation system. In

supplier’s information, data can be found on the available fan types and the power needs. Fans

are available that can be operated at variable speeds [tm97, Immell, 1996] or users are advised

to apply a multifan system to have more flexibility in adjusting the required airflow.

With respect to the effect of drift eliminators on the performance of fans due to induced pressure

drop, [tm092, Becker and Burdick, 1994]. Concluded that there will be differences between

different eliminator designs and that the difference in effect on fan performance needs careful

consideration, taking the full systems context into account. This means that a complex

evaluation must be carried out involving tower configuration and flow distribution across the

fan and across the drift eliminator. From this a useful comparison between different fan designs

will be possible.

Examples of changing cooling tower fill reported considerable increases in efficiency of the

heat exchange, lowering the temperature of the cooling water leaving the tower and achieving a

better cooling ([tm034, Hobson et al., 1995], [tm041, Burger, 1994], [tm117, Remberg and

Fehndrich, 1993].

Improving the heat exchange capacity of the fill will improve the cooling of the process in the

heat exchanger. Consequently, fan operation may be reduced whilst achieving the same level of

cooling as before. With an unchanged level of operation, cooling capacity will be larger.

Applying the wrong fill configuration can create unnecessary resistance to the air flowing past

or through, but the tower geometry is also important. Dense film fill packs create larger air
Chapter 3

Industrial Cooling Systems 71

pressure drops and usually require more fan energy. Splash fills have lower air side pressure

drop, but due to its lower efficiency this fill requires larger towers or more cells, and

compensation cannot be made with a higher energy input for fan operation.

Practical experiences report a clear effect of maintenance in reducing the required amount of

energy to operate cooling systems. Generally, for water cooled systems this means proper

systems treatment to reduce the resistance in the system due to scaling, corrosion, fouling etc.

Systems treatment will keep the surface of the exchangers, conduits and packs in cooling towers

smooth. It will prevent resistance to the water flow, reduce required pumping capacity and

enhance the exchange of heat. Appropriate cooling water treatment (see Section 3.4), balancing

the application of cooling water additive against a rise in process temperature, will reduce direct

energy use as well as indirect energy use. No quantification of the reduction of kWe per MWth

discharged due to improved maintenance has been reported.

3.3 Consumption and emission of cooling water

3.3.1 Consumption of water

3.3.1.1 Intake of water and water requirements

Water is an important medium for cooling systems and especially for large once-through

systems, whereas for dry air-cooled systems it is of no importance. Surface water, groundwater

and potable water are used. In principle salt water, brackish water and fresh water can be used

for cooling purposes. Salt water is abundantly available at coastal locations, but the

disadvantage of salt water is its corrosiveness. The use of groundwater for cooling purposes is

expected to reduce in the coming years, because groundwater for low-grade use (such as

cooling) will be increasingly less permitted, unless it is combined with indispensable

groundwater extraction associated with other needs. Examples are the lowering of groundwater

level for unhindered mine exploitation or water from pumping measures for hydropower. Less

groundwater availability could result in an increase in the consumption of surface water for

cooling.

Water use and water consumption are terms both used for the requirements of cooling water

systems. Water use means that the same volume of heated cooling water is directed back to the

source from which it has been taken (once-through). Water consumption mean that only part of

the water used for cooling (blowdown of recirculating systems) is directed back into receiving

water, the remainder having disappeared by evaporation and drift during the process of cooling.

Consumption is particularly important where groundwater is used for cooling purposes in

drought sensitive areas.

The volume of water used is largely connected with the type of industry. Different sources show

that the use of cooling water in Europe is considerable [Correia, 1995]. Generally, the largest

share of (surface) water is required by power stations. The remainder in accounted for by a

small number of larger industries, of which the chemical industry is the largest user.

The volume of water required varies between the various cooling water systems (Table 3.3). For

once-through systems (direct and indirect) the use of water depends on:

• requirement of the process (condenser)

• the temperature of intake water

• the maximum allowed temperature increase of the receiving water

• maximum allowed temperature of cooling water when it is discharged.
Chapter 3

72 Industrial Cooling Systems

Table 3.3: Water requirement of the different cooling systems

[tm001, Bloemkolk, 1997]

Cooling system Average water use

[m3/h/MWth]

Relative water use

[%]1

Once-through system –direct 86 100

Once-through system –indirect 86 100

Open wet cooling tower –direct 2 2.3

Open wet cooling tower –indirect 2 2.3

Open wet/dry (hybrid) cooling tower 0.5 0.6

Closed circuit wet cooling tower variable Variable

Closed circuit dry air cooling tower 0 0

Closed circuit wet/dry cooling tower 1.5 1.7

1) assumption: cooling capacity ∆T 10 K

open wet cooling tower: cycles of concentration between 2 and 4

open wet/dry cooling: 75% dry operation

closed circuit wet/dry tower dry operation ranging from 0 to 25 %

In open recirculating systems, closed circuit wet and closed circuit wet/dry cooling towers, most

of the water is recycled and the heat is dispersed to the atmosphere mainly by evaporation. In

these systems water use varies considerably and no specific data are available as performance

depends on the concentration factor applied (regulated by the intentional blowdown),

evaporation and to a lesser extent to the ambient temperature.

Indirect closed circuit dry cooling towers can use water as secondary coolant but the use is very

low compared to that of water cooling systems. Normally, replenishment or make-up water is

only needed when leakage has occurred, e.g. at pump packages, flanges and valves, or when

water has been drained to allow systems repair. In those circumstances amounts are small and

potable water or even demineralised water can be economically used.

Legislation

In Member States different authorities deal with water as a resource or as a receiving

environment. In any case water use should be part of an integrated environmental permit,

especially where supplies are limited. It is expected that throughout Europe the pressure on

resources of good quality water will increase the pressure on water conservation measures in

cooling systems, limiting volumes allowed to be extracted from a source. With respect to water

use the major legislation on European level is the Water Framework Directive. It focuses on

both the water quality and on the quantitative groundwater status defined in terms of the effect

of the ground water level on associated surface ecosystems and in terms of sustainability of the

water supply. On a national level some Member States have separate legislation for aspects

concerning the intake and use of surface water.

Cross-media issues

The issue of restricting the use of water relates to the following environmental aspects:

- heat emission to the surface water,

- application of cooling water additives,

- energy consumption of both cooling system and production process,

- indirect emissions.

Each of these factors needs to be assessed to evaluate whether reduced water intake for cooling

is the best solution. In the following paragraphs an attempt is made to describe applied reduction

techniques options and their cross-media effects.
Chapter 3

Industrial Cooling Systems 73

3.3.1.2 Applied techniques to reduce water consumption

Reduction of water consumption for cooling is of particular interest where water availability is

low for natural or ecological reasons. This can be drought-stricken areas or areas with seasonal

low rainfall. The danger of depletion of groundwater sources and situations with relatively large

cooling water requirements, where demands approach or could exceed the river flow or where

heat emissions into the surface water are restricted, are other typical examples.

1. Cooling technology

In reducing the amount of water required for cooling, systems choice is important. In a

greenfield situation it is suggested that air cooling be considered e.g. by applying open cooling

towers. For large systems the required cooling capacity could limit the options for dry air

cooling, as it requires a large heat exchange surface. If it is feasible, attention should be paid to

the change in overall efficiency, increased operation costs for fan operation and costs of noise

abatement. Applications of dry cooling systems generally lead to a decrease of the process

efficiency. Consequently, wet systems are to be preferred. Only in the case that no supply with

water (resp. make-up water) is possible dry cooling is unavoidable.

For existing once-through systems the application of recirculating systems (open wet cooling

towers) is an applied option to reduce water requirements. Towers are equipped with drift

eliminators as a standard technique to further reduce water loss through evaporation. Generally,

recirculation means that measures need to be taken to protect the heat-exchanging surface

against scaling or corrosion. On the other hand, application of recirculation of cooling water at

the same time means a reduction of heat emission to the surface water.

2. Systems operation

In recirculating wet cooling systems a commonly applied operation is the increase of the

concentration factor by reducing the frequency of the blowdown. The cleaner the water the

easier this is and proper maintenance of an open wet cooling tower will reduce the

contamination of the cooling water and may enhance a higher number of cycles and

consequently a less frequent blowdown.

Increasing the cycles of concentration generally leads to an increased demand for anti-fouling

chemicals to allow for higher salt concentrations without the risk of deposition. A number of

reports can be found that present water treatment programs designed in particular for operation

with higher cycles of concentration to reduce water requirements and to reduce the volume of

the blowdown [tm094, Alfano and Sherren, 1995]. Under permit conditions, attention should be

given to the potential increase in concentrations of elements in the blowdown.

A critical review of the results of maximising cooling tower cycles and the problems

encountered can be found in [tm095, Cunningham, 1995]. The conclusion is that the ability to

increase the number of cycles depends on many chemical and physical factors (e.g. water

temperature, pH, water velocity) and requires a high level of expertise. Given the variety of

operating conditions and water chemistry, it may not be easy to predict the maximum cycles of

concentration and care must be taken to consider the costs involved before the cooling system

can be operated economically.

3. Additional techniques

For recirculating systems, using relatively limited amounts of water, a number of additional

techniques have been applied. These techniques aim at improving cooling water quality.

Pre-treatment of cooling water (such as flocculation, precipitation, filtration or membrane

technology) can reduce the water requirements, where less blowdown is required to maintain the

same concentration factor. Water treatments however will lead to sludge that will have to be

disposed of (see Annex IV on blowdown).

The evaporative pond is a technique still applied on some older sites and is undergoing further

development. It can be applied to prevent heat emissions to the surface water precooling the

cooling water before discharge, but could serve in a similar way as a cooling tower being part of

the total circulation. In the evaporative pond water is cooled down by spraying it over a large
Chapter 3

74 Industrial Cooling Systems

catchment area, creating large cooling surface, after which it can be reused (Annex XI).

Attention should be paid to the microbiological risks due to the formation of aerosols (see

3.7.3).

Reducing the demand on water resources is also attempted by linking the water flows of

different industrial units on one or more sites. This water conservation method can be quite

successful but needs careful consideration. In an evaluation of alternatives for water

conservation for industrial sites, a number of important considerations are listed [tm065, Meier

and Fulks, 1990] that should be taken into account:

1. survey of available water resources and their chemistry;

2. assessment of quantities of these sources and their fluctuation;

3. assessment of the contaminants and treatments in water sources;

4. the effect of current water resource treatment programmes on the existing cooling water

conditioning methods;

5. effect of potential conductivity increases of recycled water on the process where the water is

used;

6. chemical treatment programme options for the cooling systems;

7. economics of alternative reuse methods.

The factors listed above affect the selection of water sources and the amount of water that can

be reused. Water resources on site are typically blowdowns from cooling towers and

boilers.Tertiary treated municipal waste plant effluent is also used. It is important in all cases to

avoid an increased need for an even more complex water treatment programme to enable water

reuse (Annex XI). Reuse of the blowdown of evaporative ponds is also possible in applications

that are not sensitive to the increased salt content of the water.

The zero discharge system based technology can be applied by treating and reusing the

blowdown. Disposal costs of the resulting sludge need to be assessed against the environmental

costs of conditioning and discharging of blowdown (Annex XI).

3.3.2 Fish entrainment

3.3.2.1 Level of entrainment

Whit a large water intake, such as for once-through cooling water systems, the impingement and

entrainment of fish is an issue. Entrained fish - mainly fish larvae passing through the sieves at

the cooling water intake, the pumps and the condensers - are not generally sampled.

Entrainment is a local matter and the quantity of entrained fish is based on a complex of

technical and hydrobiological factors that lead to a site-specific solution. Water is drawn into

inlet channels in large quantities and at considerable speed. The inlet channels are generally

equipped with debris filters to protect the heat exchangers against clogging and mechanical

damage. Impingement occurs when fish are pressed against the sieves placed the condensers or

heat exchangers. A lot of smaller creatures is taken in with the cooling water and is killed by

mechanical damage, which is called entrainment.

Data on the amounts of fish taken in with the cooling water or caught at the entrance of a

cooling system have not been widely reported. Results of 24-hour-samples have been analysed

on the number of fish impinged by the cooling water of a 600 MWe Netherlands power plant

[KEMA, 1992] on the river Rhine, with a cooling waterflow of 22 – 25 m3

/s. The results show

that the numbers of fish drawn in between years as well as between seasons of the same year

vary widely. Most fish were found in summer.

Studies at a 2000 MW inland power plant on the River Trent in England showed that by far the

majority of entrainment occurred at or soon after dusk, and in the summer. The power plant
Chapter 3

Industrial Cooling Systems 75

does not have a once-through cooling system, and no evidence was found of significant

impingement. This was also the case at another similar sized power plant on the River Thames

[Carter and Reader, in press]. The screens at both plants have approximately 9 mm mesh width.

Research on entrained and impinged fish at nine Dutch power plants shows that more than 95%

of the impinged fish were 0+

-fish, born in the spring of the same year and with a length of less

than 10 cm. This is confirmed by results in the above-mentioned power plant on the River Trent,

although mortality was 100% while there was negligible fish mortality compared with observed

natural death [Carter and Reader, in press].

There is also variation in the number of entrained fish and in entrained fish species between

different power plants. The results of a sampling programme at six Dutch power plants on the

river Rhine, the river Meuse and branches of these rivers show a variation in entrained species

between 12 and 25 species and a variation in impinged fish on the cooling water sieves between

0.02 and 2.45 fish per 1000 m3

 cooling water averaged over the year [Hadderingh et al., 1983].

At power plants on lakes, on estuaries and on the sea coast the observed number of impinged

fish can be much higher than at power plants on rivers, up to 25 fish per 1000 m3

 [KEMA,

1982].

Table 3.4: Fish impingement rates (FIR) at power stations. Annual catches normalised to the

cooling water flow

[tm164, Travade, 1987] and [tm165, Turnpenny et al, 1985]

Water Power Station Power (MWe) FIR

(kg/106 m3)

Sizewell A 480 73

Kingsnorth 2000 4.4

Dunkerque 600 19

North Sea

Gravelines 5400 48

Dungeness A 410 190

Dungeness B 1200 40

Paluel 5200 43 English Channel

Fawley 2000 19

Bristol Channel Hinkley B 1300 24

Estuaries Le blayais 3600 79

Rivers Loire (St Laurent A) 1000 1.8

3.3.2.2 Applied reduction techniques

With varying results a number of techniques has been developed and applied in industry to

prevent the intake of fish due to large cooling water intake. The optimum solutions and results

and the ability to meet BAT requirements are influenced by a wide variety of biological,

environmental and engineering factors that must be evaluated on a site-specific basis. A

comparison of the different techniques is therefore impossible.

1. Cooling technology

Technology changes made to avoid fish entrainment have not been reported. It is obvious that

fish entrainment will not be an issue when changing to open or closed recirculating cooling

systems, which is a costly operation. It may be considered in a greenfield situation. Devices to

prevent intake of fish can be found in e.g. power industry and refineries. Solutions for

prevention are:

- sound devices, positive to divert (a shoal of) scale fish but not for eel;

- light systems with underwater lamps, positive to divert eel;

- position, depth and design of the inlet;
Chapter 3

76 Industrial Cooling Systems

- limits to speed of the water inflow (although the data from studies carried out in England

indicate that the entrained fish allow themselves to be carried by the flow (i.e. deliberately

drifting or dispersing) even when they are physically capable of escaping the flow by

swimming);

- mesh size of the cooling water sieves (against damage to the cooling system). Observations

have shown that, in the same power plant, a mesh size of 5 x 5 mm on average doubles the

number of surviving entrained fish at the cooling water outlet compared with a mesh size of

2 x 2 mm, because impingement mortality of fish larvea is higher than entrainment

mortality [KEMA, 1972] and [Hadderigh, 1978].

Mortality of impinged fish can be decreased by a good system to wash the fish from the cooling

water sieves and to sluice them back to the surface water.

2. Operating practice and end-of-pipe techniques

Lowering the inflow velocity to below 0.1-0.3 m/s clearly showed a positive effect and reduced

the amount of fish drawn in. However, lowering the velocity may mean that larger inlet

channels are required, which may have technical and financial consequences. In general,

changes in operating practice or the application end-of-pipe techniques do not apply to fish

entrainment, but there is also a view –not shared by all- that entrainment could be reduced by

taking account of the diurnal and seasonal patterns of entrainment.

Table 3.5: Available fish protection technologies for cooling water intake devices

Derived from [tm152, Taft, 1999]

Category Protection technique Effects Remarks

Optimizing (increasing) the

mesh-size of the travelling

water screens

Improve survival of

entrained fish larvae and

very young fish stadia

Entrainment mortality of

these fish stadia is lower

than the impingement

mortality of these stadia.

Low pressure water jets to

wash off the fish from the

travelling screens and to

return them to the surface

water

Transport of fish back to

the surface water

Requires a second high

pressure jet system to clean

the travelling screens

Fish buckets on the screens Improve survival of

impinged fish

The fish remain in water

permanently during

transport back to the

surface water

Continuous rotation of the

travelling screens

Improve survival of

impinged fish

Reduction of impingement

time

Fish collection

systems

Fish pumps Transport of fish back to

the surface water

Complicated to keep right

conditions in pipes

Fish diversion

systems

Angled screens or louvres

with a fish by-pass

- Survival harder

species (50-100%) >

fragile species

- Not for fish eggs,

larvae and small

invertebrates

- Requires uniform,

constant low-velocity

flow

- Debris must be

removed

Lights

- strobe lights

- continuous lights

- mercury lights

- other lights

Effects of different light

systems depend on local

situation, fish species and

developmental stadia of the

fish.

In many situations a bypass for diverted fish is

necessary

Behavioural

barriers

Sound Effects depend on local

situation, fish species and

developmental stadia of the

fish.

In many situations a bypass for diverted fish is

necessary
Chapter 3

Industrial Cooling Systems 77

3.3.2.3 Costs of sound devices and light systems

It is obvious that any change made to an existing system will be costly. The power industry

reported additional costs for fish protection technology applied to existing installations of

between EUR 40000 and 200000, including downtime costs. In greenfield situations the

additional investment of alternative intake devices would probably be less substantial.

For a good efficacy the water current through sound devices and light systems must not be

higher than 0.3 – 0.5 m/s. This determines the length of the systems.

The material and construction costs of a light system are EUR 90000 – 140000 for a length of

100 m and of a sound system (BAFF) about EUR 180000 per 100 m.

3.3.3 Heat emission to surface water

3.3.3.1 Levels of heat emission

All heat that is discharged will finally end up in the air. If water is used as the intermediate

cooling medium, all heat will be transferred to the air, either from the water droplets in a

cooling tower or from the surface of the receiving water. Before the heat has left the surface

water it may affect the aquatic ecosystem and this should be avoided.

Heat emission is also an issue closely related to the amount of cooling water used and

discharged. Once-through systems, both direct and indirect, by definition form the largest

source of heat discharged to the surface water, as the heat is entirely discharged via the cooling

water. The cooling water in recirculating systems releases the majority of its heat via a cooling

tower into the air. The amount of heat discharged with the drainage from a cooling tower

amounts to approximately 1.5% of the heat to be discharged, whereas around 98.5% is released

into the air. There is little information on the effects on the aquatic ecosystem of heat emissions,

but there are experiences with high summer temperatures and small receiving waterways.

Temperature rise may lead to increased rates of respiration and of biological production

(eutrophication). The discharge of cooling water into the surface water influences the total

aquatic environment, especially fish. The temperature has a direct effect on all life forms and

their physiology and an indirect effect by affecting the oxygen balance.

Warming reduces the saturation value of oxygen; with high oxygen concentration, that leads to

a reduced oxygen level. Warming also accelerates the microbial degradation of organic

substances, causing increased oxygen consumption. Also, where circulation of the cooling water

occurs or where a number of industries use the same limited source of surface water, heat

emissions need careful consideration to prevent interference with the operation of industrial

processes downstream.

From the specific heat of water amounting to approximately 4.2 kJ/kg/K the temperature rise of

water can be calculated. For example, when cooling water is warmed up by an average of 10K,

1 MWth of heat requires a cooling water flow of about 86 m3/hour. Broadly speaking each kWth

needs 0.1 m3/hour of cooling water. With recirculating cooling water, heat is transferred to the

air through evaporation via cooling water in a cooling tower with the evaporation heat of water

being 2500 kJ/kg (at 20oC).

In the power industry in particular, the factors playing a role in the discharge of large quantities

of heat into the surface water have been researched. A number of physical phenomena have to

be taken into account when heat emissions are being assessed, such as:

• seasonal variation in the temperature of the receiving water;

• seasonal variation in the water level of rivers and the variation in the velocity of the stream;

• the extent of mixing of the discharged cooling water with the receiving water (near field and

far field);

• at coastal sites, tidal movements or strong currents and

• convection in the water and to the air.
Chapter 3

78 Industrial Cooling Systems

The behaviour of the hot water plume in the surface water will not only be valuable in

protecting the receiving environment, but also for choosing the right place for the inlet and

outlet. It will always be important to prevent circulation of the plume affecting the temperature

of the water taken in and consequently the efficiency of the cooling system. As an example, the

extent of a thermal plume, defined as the area within the 1K heating isotherm, without mixing

with strong currents (e.g in a lake), is about 1 ha per MWe for a conventional power plant, or

about 45 km2 for a 5000 MWe power plant. For a more extensive description on heat plume

behaviour see Annex XII.

3.3.3.2 Legislative requirements of heat emissions

Requirements for specific fresh water bodies

European Directive 78/659/EEC (18 July 1978) sets environmental quality standards for certain

substances and for heat discharges in designated freshwater fisheries. The directive

acknowledges local conditions in a provision in Article 11 regarding derogation for Member

States.

Where thermal requirements depend on the fish species, two types of water bodies are

distinguished, according to their fish population:

- salmonid waters

- cyprinid waters.

For each ecological system, three thermal parameters are applied:

- maximum water temperature at the boundary of the mixing zone

- maximum temperature during the breeding period of “cold water species”

- maximum temperature rise.

Table 3.6: Thermal requirements of water temperatures for two ecological systems (European

Directive 78/659/EEC)

Parameter Salmonid waters Cyprinid waters

Tmax at boundary of mixing zone (oC) 21.5 28.0

Tmax during breeding period cold water

species (oC) 10.0 10.0

∆Tmax (oC) at boundary of mixing zone 1.5 3.0

Note: temperature limits may be exceeded for 2% of the time at maximum.

Other receiving water bodies

In the Member States, heat emission to the surface water is regulated in various ways,

depending on the ecological conditions and other factors such as: the sensitivity of the receiving

surface water; the local climatic conditions; the capacity of the receiver to accommodate

thermal loads and the prevailing currents and waves (water hydrodynamics). Regulations often

consider heat emissions in relation to the receiving surface water. Examples are:

• standardisation of the maximum discharge temperature (e.g 30oC in summer in temperate

climates and 35oC in hot countries),

• limiting maximum heating in relation to the water received and to seasonal temperature

differences (e.g. ∆Tmax of 7-10K measured over the entire cooling water distance in the

production process)

• setting maximum acceptable temperature profile of the surface water and the total available

cooling capacity of the surface water.

These requirements are formulated in permits.
Chapter 3

Industrial Cooling Systems 79

Other regulations do not prescribe a generally fixed discharge temperature. Initially the limit

discharge temperature corresponds with the type of cooling system. Additionally, seasonal

variation of the temperature of the surface water plays an important role in setting the discharge

temperature to be permitted. Some regional authorities also further classify the receiving waters

using characteristics of their fauna.

3.3.3.3 Applied reduction techniques

1. Cooling technology

The best way to minimise heat emissions is to reduce the need for discharge by optimising the

primary process or to find consumers for the excess heat. In the case of emission of heat into the

environment, focus is on the problem of heat emissions into surface waters. In considering the

reduction techniques it is important to realise that in the end all heat will disappear to the air and

that the surface water is an intermediate medium. By choosing between different cooling

systems, it can be decided which is preferable. Thus, the environmental impact of heat discharge

can be minimised by discharging more heat into the atmosphere and less heat into surface

waters at the expense of water loss due to evaporation. Minimisation of heat discharges to the

surface water is linked to the minimisation of water use and to the overall energy efficiency.

The more heat is discharged by convection and evaporation, the more E per discharged MWth is

required due to use of fans, unless natural draught is applied. This generally needs large

investments and a lot of space.

In the case of large capacities, a widely practised solution to reduce the heat load to the surface

water of (mainly) rivers and lakes is to choose a suitable heat transfer technology, e.g. instead of

a once-through system a recirculating system with an open wet or wet/dry cooling tower.

2. Operating practice

No particular operational options have been reported to prevent or reduce the discharge of heat

to the surface water.

3. Additional techniques

An old practice still in use in Europe, but on a very small scale and recently attracting fresh

attention, is the use of evaporative (spray) ponds. To enhance cooling of the water, the angle of

the nozzle and the time allowed for the water to remain in the pond before it re-enters the

cooling circuit are important, as is sufficient surface area. To assess this technique it should be

compared with a cooling tower of similar capacity. Attention should be paid to:

• surface area needed,

• loss of water due to evaporation

• energy use

• the need for water treatment, as well as

• to microbiological risks due to formation of aerosols (see also 3.7.3).

Another kind of end-of-pipe technique is the precooling of the discharge of large power plants

by means of a cooling tower. It is a costly technique used where circulation of the discharge in

the surface water can influence the cooling water temperature at the point of inlet. The

additional costs of the extra cooling tower plus the loss of water due to evaporation will have to

be compared with the costs related to a reduced efficiency with a higher temperature of the

intake water.

A measure also suggested to reduce the effect of heat discharge is designing the water outlet of

the cooling system in such a way that by turbulence the water will lose some heat during

discharge. A side effect of this measure is an increase of the oxygen content of the cooling water

compensating for the loss of oxygen due to the higher cooling water temperatures. No data are

available and the extent of the effect is also questioned.
Chapter 3

80 Industrial Cooling Systems

3.4 Emissions from cooling water treatment

Emissions from cooling water treatment into the surface water are regarded as one of the most

important cooling systems issues. Four sources of emissions to the surface water resulting from

wet cooling systems can be distinguished:

• process chemicals (product) and their reactants, due to leakage;

• corrosion products due to corrosion of the cooling system equipment;

• applied cooling water additives and their reactants;

• airborne substances.

To control these emissions different techniques are applied. The risk of leakage can be reduced

as well as the possibility of uncontrolled emissions after leakage and the most adequate material

for the equipment can be selected to reduce corrosion. This section will focus on measures to

reduce the amount and the impact of emissions due to the application of cooling water additives:

• by reducing the need for water treatment;

• by selecting chemical treatments which have lower impact on the environment;

• by applying the chemicals in the most effective way (systems operation).

3.4.1 Application of cooling water treatment

Cooling water is treated to promote an efficient transfer of heat and to protect the cooling

system so as to overcome a number of adverse effects on the performance of the cooling

equipment. In other words, cooling water treatment aims to reduce total energy consumption.

The adverse effects are strongly related to the chemistry of the water taken in for cooling and

the way the cooling system is operated (e.g. cycles of concentration). Salt water will have

different demands from fresh water and industrial emissions of polluted substances upstream

may be a challenge. Also, cooling water can become contaminated by leakage of process fluids

from heat exchangers or, in the case of wet open cooling towers, by the air passing through the

tower introducing dust, micro-organisms and exchange of vapour.

Cooling water additives are used for once-through systems, open wet cooling systems, closed

circuit wet cooling and wet/dry systems. Where water is used as an intermediate coolant in the

coil of dry systems, very low amounts of additives may be used to condition the water in the

closed loop.

Environmentally, additives are important: they leave the cooling system at some stage, being

discharged to surface water or, to a much lesser extent, into air. Generally, the chemistry and the

application of the chemicals applied are known, but the choice of non-oxidizing biocides is

mainly based on “trial and error”. The environmental effects of the chemicals used can be

assessed by means of modelling (risk/hazard) or by measurement. As they are used to improve

an efficient heat exchange, their application is also related to the adverse effects that arise from

a lower exchange efficiency. The industrial process to be cooled can be affected when heat

transfer is inefficient, causing an increase in the use of energy (i.e. similar to an increase in air

emissions) or a higher demand on raw materials to compensate for the loss of production.

Energy consumption of the cooling system can increase due to a higher demand on pumps and

fans to compensate for loss of heat exchange efficiency.

Problems arising from water quality that are commonly encountered are:

• Corrosion of cooling water equipment, which may lead to leakage of heat exchangers and

spills of process fluids into the environment or loss of vacuum in condensers;

• Scaling, predominantly by precipitation of calcium carbonates, sulphates and phosphates,

Zn and Mg;
Chapter 3

Industrial Cooling Systems 81

• (Bio-)Fouling of conduits and heat exchangers (also fill of wet cooling towers) by micro-,

macro-organisms and suspended solids which can lead to blockage of the heat exchanger

tubes by large particulate (shells) or to emissions to air from cooling towers.

Corrosion

D eposition

(Fouling) Bio-film

Scale

Figure 3.1: Graphic representation of interrelation between the different water quality problems

Cooling water problems are often interrelated. Scaling can lead to both corrosion and

biofouling. Spots of corrosion lead to changed waterflow patterns and create turbulence areas,

where biofouling is enhanced. Biofouling may enhance further corrosion of the underlying

surface (Figure 3.1).

The following groups of chemicals are used to condition the water (see also Annex V):

• Corrosion inhibitors:

formerly metals were mainly used, but there is a trend towards azoles, phosphonates,

polyphosphates and polymers. This means that toxicity decreases while the persistency

increases. Recently some better biodegradable polymers have been developed.

• hardness stabilisers or scale inhibitors:

formulas exist mainly of polyphosphates, phosphonates and certain polymers. Recent

developments in this application are also towards better biodegradable compounds

• Dispersion chemicals:

mostly copolymers, often in combination with surfactants. The main environmental effect is

poor biodegradability.

• Oxidising biocides:

chlorine (or a combination of chlorine and bromine) and monochloramine are mainly used.

Chlorine (bromine) is a strong oxidiser (acutely toxic), which means that the half-life is

short, but the side effects of chlorinating are the forming of halogenated by-products. Other

oxidising biocides are ozone, UV, hydrogen peroxide or peracetic acid. The use of ozone

and UV needs pre-treatment of the make-up water and needs special materials. The

environmental effects are expected to be less harmful then halogenated biocides, but the

application needs special care, is expensive and not applicable in all situations.

• Non-oxidising biocides:

isothiazolones, DBNPA, glutaraldehyde and quaternary ammonium compounds etc. These

compounds are in general acutely toxic and often not readily biodegradable, although there

are some which hydrolyse or are degraded by other mechanisms. The environmental effects

are significant.
Chapter 3

82 Industrial Cooling Systems

Table 3.7: Chemical components of cooling water treatments used in open and recirculating wet

cooling systems

Derived from [tm135, Nalco, 1988]

Water quality problems

Corrosion Scaling (Bio-)fouling Examples of

chemical treatment* Oncethrough

systems

Recirculating

systems

Oncethrough

systems

Recirculating

systems

Oncethrough

systems

Recirculating

systems

Zinc X

Molybdates X

Silicates X

Phosphonates X X

Polyphosphates X X

Polyol esters X

Natural organics X

Polymers X X X X X X

Non-oxidizing

biocides

X

Oxidyzing biocides X X

Notes:

chromate is not widely used any more due its high environmental effect

The need for cooling water treatment and type and amount of chemicals applied are more

extensively described in Annex V. The application of cooling water conditioning is a highly

complex and local issue, where selection is based on a combination of the following elements:

- design and material of heat exchanger equipment;

- temperature and chemistry of the cooling water;

- organisms in the surface water that can be entrained;

- sensitivity of the receiving aquatic ecosystem to emitted additive and its associated byproducts.

For proper performance of any of the treatments, control of the cooling water pH and alkalinity

within a specified range is usually required. Good pH and alkalinity control has become more

important where more pH-sensitive treatment programmes are used or where higher cycles of

concentration are applied in open recirculating cooling towers to minimise blowdown and

reduce the water requirement. It is increasingly common practice in industry to have

maintenance programmes developed and carried out by the additive supplier, but the

responsibility for systems operation remains with the owner of the cooling system.

Considering the site and system specificity, it will be difficult to find typical levels of amounts

of additives applied to the different systems. If levels are reported they are expressed in kg or

tonnes per m3

 of cooling water or in kg or tonnes per MWth dissipated. Table 3.8 reports the

result of a recent inventory in The Netherlands of chlorine, which is widely used in Dutch

industry. Data show there is variation between systems as well as between different types of

water. Other sources of water used by recirculating systems are for instance potable water,

groundwater and condensate.
Chapter 3

Industrial Cooling Systems 83

Table 3.8: Consumption of hypochlorite in wet cooling systems in The Netherlands

[tm160, Bijstra, 1999]

Consumption of active chlorine kg/MWth/yr Cooling water source Once-through systems Recirculating systems

Fresh water 85 (10-155)

Salt or brackish water 400 (25-2500) 200 (20-850)

Other water sources 400 (20-1825)

3.4.2 Emissions of chemicals into the surface water

In Europe and U.S. a large amount of work has been carried out on optimisation of cooling

water conditioning, on the application of alternative treatments and on other techniques to

prevent any harmful effects on the aquatic environment due to cooling water additives. By large,

this work focuses on the application of biocides.

Specific emissions resulting from cooling water treatment can be difficult to assess in situations

where analysis methods for the chemicals used for treatment are not available. Besides the

specific chemicals used for treatment of cooling water, by-products originating from the

chemicals used can also contribute significantly to the environmental impact on the surface

water. When chlorine or bromine is used as an additive, 3-5% of the product reacts to haloform

compounds (chloroform or bromoform) [tm072, Berbee, 1997].

Quantification of short-term effects can be achieved by carrying out (acute) toxicity tests on

cooling water streams discharged. These results can be seen as a minimum estimate of the

environmental effects in the surface water, [because long term (chronic) effects,

biodegradability, bioaccumulation (Pow) and carcinogenic effects are not included in these tests].

Recently several studies concerning the use of oxidising biocides (mainly hypochlorite)

([tm001, Bloemkolk, 1997],[tm072, Berbee, 1997] and [tm160, Bijstra, 1999]) and nonoxidizing biocides ([tm001, Bloemkolk, 1997] and [tm149, Baltus et al., 1999]) have been

carried out in the Netherlands.

3.4.2.1 Oxidising biocides

In several countries programmes have been set up to achieve optimum use of hypochlorite in

cooling water. Free oxidant [mg FO/l] is often used as a control parameter in cooling water. In

The Netherlands a concentration of 0.1 – 0.2 [mg FO/l] in the discharge is used as a target

concentration for continuous dosed (once-through) cooling systems. For intermittent or shock

chlorination regimes the FO or FRO concentration is always below 0.2 mg/l as a daily (24h)

average value. But during shock injection the FO or FRO concentrations can be close or equal

to 0.5 mg/l (hourly avarage).

Optimisation by implementation of monitoring and controlled (automatic) dosage of biocides

can significantly reduce the annual consumption of chemicals used in cooling water. This can

result in a reduction of the load of biocides and of their by-products, such as organohalogenated

compounds with bromoform as the main product [tm157, Jenner et al, 1998].

Several companies in the chemical industry and energy production sector have achieved

reductions of the use of hypochlorite in cooling water up to 50 % by introducing the abovementioned optimisation measures [tm160, Bijstra, 1999].
Chapter 3

84 Industrial Cooling Systems

3.4.2.2 Non-oxidising biocides

In 1999 a study was carried out of the environmental effects of the use of oxidising and nonoxidising chemicals in recirculating cooling systems. For the chemicals for which analysis

methods were available, concentrations of the chemicals in cooling water were measured. For

all chemicals applied, toxicity tests were used to evaluate the environmental impact in the

surface water. When cooling water is discharged directly into the surface water the use of non

oxidising chemicals in recirculating cooling systems in many cases resulted in strong

environmental effects in the surface water. For oxidising chemicals (hypochlorite), PEC/PNEC

ratios based on toxicity tests were found in the range of 3 (continuous dosage) and 33 (shock

dosage) and for non-oxidising chemicals, PEC/PNEC ratios of 20 (isothiazolines), 2500 (BNS),

660-13000 (BNS/MBT) and 3700(DBNPA) were found ([tm149, Baltus et al., 1999] see pag

summary pag. 9-10, table 16 p. 64 and chapter 9 p. 75-82)).

Another study carried out indicated that potential risks for the receiving surface water cannot be

excluded when isothiazolines (1,2-benzisothiazolin-3-on, 2-methyl-4-isothiazolin-3-on) are used

in cooling water as additives (see[tm149, Baltus et al., 1999] pag. 13 en 14).

The treatment programmes vary considerably and depend on the factors mentioned earlier and

as such are site-specific. Emissions of additives vary in volume and chemistry (toxicity,

reactivity). Decomposition, interaction and possible purification measures can influence the

actual amount that is finally discharged and, consequently, the resulting impact on the aquatic

environment. Optimisation and controlled conditioning of cooling water by the use of

(automatic) dosage and monitoring can reduce the use of chemicals in cooling water and

consequently the environmental impact in the receiving water significantly.

In the Netherlands the application of hypochlorite and bromine in cooling water is one of the

most important sources of organohalogenated compounds, measured as AOX, in surface water

[tm001, Bloemkolk, 1997] and [tm072, Berbee, 1997].

Sometimes cooling water is treated in a wastewater treatment installation before discharge. An

example is the treatment of the blowdown with other wastewater streams in refineries. This

treatment could possibly reduce the effect of biocides in the surface water. Biological treatment

may be sensitive to low levels of non-oxidising biocides, which could disturb the working of the

treatment plant. Inhibition of active sludge of 60% and more (100%) has been reported.

Physical/chemical treatment for biocides is still on an experimental level. The polarity of nonoxidative biocides will be an obstacle for physical treatment, as they will remain in the water

phase.

The blowdown of open recirculating systems is the more controlled pathway in which biocides

enter the external environment. For closed systems blowdown is not practised. Purges are made,

but they are small and usually discharged into a sewer system. It is obvious that the

concentration of biocides in the cooling water immediately after dosage will be highest and

consequently the concentration in the discharge or purge. By chemical reactions in the systems’

cooling water, such as hydrolysis, the biocide concentration will gradually diminish and this

knowledge can be used to estimate the expected concentration in the discharge. This

information is also used when closing the purge after treatment to prevent discharge of biocides

with a high level of chemical activity. To achieve further optimisation several factors are

important. Besides the concentration in the outlet, process control is also essential here.

3.4.2.3 Factors influencing emissions of biocides

Factors influencing discharge and persistence in the receiving aquatic environment have been

extensively described [tm004, Baltus and Berbee, 1996] for a number of commonly applied

oxidising and non-oxidising biocides. The following factors, in combination with the cooling

process conditions play a role in the selection of a cooling water treatment program:
Chapter 3

Industrial Cooling Systems 85

• Hydraulic half-time (also used is system half-life);

• Hydrolysis;

• Biodegradation;

• Photolyses;

• Volatility.

The volume of the purge determines the hydraulic half time. The larger the purge the smaller the

hydraulic half time and the shorter the retention time of the biocide. The hydraulic half time

does not influence the working of oxidising biocides, because of their fast dissociation and

working, but for non-oxidising biocides it will limit their functioning.

Hydrolysis of a non-oxidising biocide occurs at a certain pH and water-temperature. Generally,

with increasing pH and/or increasing temperature, hydrolysis increases and the biocidal effect

will decrease. Consequently, the lower temperature of the receiving water will further slow

down the hydrolysis and will increase the persistence of non-oxidising biocides in the aquatic

environment.

Biodegradation, photolysis and volatility do not play an important role in degradation of nonoxidising biocides. Photolysis can take place if the aquatic environment is exposed to sunlight.

Evaporation may play a role in the case of oxidising biocides (hypochlorite). Research can be

quoted in which a so called stripping effect of cooling towers has been found to account for a

loss of 10-15% of hypochlorite with each passage of the cooling tower. For hypochlorite the pH

level affects its evaporation.

Biodegradation of biocides depends on the amount of organic and inorganic matter and on the

biodegradability of the biocide itself. A large microbial population, increase of temperature and

higher oxygen content of the cooling water or the receiving water, increases biodegradation.

Surface water contains a lot of suspended organic matter to which biocides can absorb, resulting

in accumulation in the sediment. Also, biocides can be reduced by organic material.

3.4.2.4 Emission levels

It is difficult to report representative levels of concentrations in cooling water emissions to the

surface water. Quantification of emissions of substances in the discharge of cooling water has

been attempted and models have been developed. However, because of site specificity, no

generally applicable model can be reported taking account all aspects. Many assumptions have

to be made and although they give an indication, discharges could easily be overestimated or

underestimated. An example of a model for biocides in an open wet cooling tower is explained

[tm004, Baltus and Berbee, 1996] in Annex IX.

3.4.2.5 Legislation

In many Member States emissions of cooling water chemicals are covered by legislation on the

pollution of surface waters. The laws usually focus on discharge flows with a minimum volume

discharged (in m3

/day). In some legislation (e.g. Italy) receiving waters are classified and each

water has a different level for the relevant emission parameters of the discharged water.

The quality of the discharged water required sets limits on the presence of certain chemicals

(e.g. chromium, zinc or mercury compounds), thereby reducing the use of particular cooling

water additives.

For large and small discharge volumes, requirements are set for temperature and pH-values. The

temperature is generally not allowed to exceed a maximum temperature for most of the year.

Some flexibility is given by varying the discharge temperature limits under unfavourable

seasonal conditions, such as wet bulb temperatures reaching up to 40°C in Mediterranean

climate.
Chapter 3

86 Industrial Cooling Systems

More specific requirements on the chemical composition vary between the Member States, but

generally cover requirements on the concentration of adsorbable organic halogens (AOX),

dissolved oxygen, biological chemical demand (BOD), chemical oxygen demand (COD),

chlorine substances and phosphorus compounds and the residual effect on luminescent bacteria.

Some acts distinguish between different types of cooling systems (once-through or

recirculating) or consider specific operations, such as shock treatment with microbiocidal

substances.

In the Netherlands the abatement efforts are based on the intrinsic properties of substances and

risk assessments. To enable a company and the water authorities to unambiguously identify the

water pollution effect of substances and preparations, a general evaluation methodology has

been developed. This evaluation methodology is based on the European legislation on

classification, packaging and labelling.

Depending on the properties of a substance BTM or BPM must be applied. After applying

BTM/BPM the residual discharge is evaluated against the applicable water quality objectives. If

these objectives are not reached, further measures may be indicated.

European chemical legislation affecting the application of cooling water additives in particular

can be found in:

• the Council Directive on Pollution Caused by Certain Dangerous Substances Discharged

into the Aquatic Environment of the Community (76/464/EEC),

• the Water Framework Directive,

• the Preparations Directive and

• the Biocidal Products Directive 98/8.

3.4.3 Reduction of emissions to the surface water

3.4.3.1 General approach

Techniques for the reduction of emissions to surface water due to the application of cooling

water are:

1. reduction of corrosion of cooling equipment

2. reduction of leakage of process substances into the cooling circuit

3. application of alternative cooling water treatment

4. selection of less hazardous cooling water additives

5. optimised application of cooling water additives

Following IPPC, reduction of emissions due to cooling water treatment should aim at reducing

the need for treatment (prevention) and at selection and optimal application of additives

(pollution control) within the requirement of maximum heat exchange. To reduce the emission

of chemicals in the discharge of cooling water, many options are available. In addition to the

assessment of the appropriate cooling configuration explained in Chapter 1 and in accordance

with the preventive “approach” of applying BAT to industrial cooling systems, the reduction

options can be considered in a certain order. For new large capacity cooling systems, an

“approach” has been developed to reduce emissions to the surface water [tm001, Bloemkolk,

1997].

An “approach” has been developed for the selection of biocides in both new and existing

systems. [tm005, Van Donk and Jenner, 1996] . Both “approaches” involve more or less the

same steps and following these steps will ensure that all important factors involved in the

reduction of additive use are taken into account. The “approaches” are shown in Figure 3.2 and

Figure 3.3.
Chapter 3

Industrial Cooling Systems 87

For the optimisation of biocide use a wide range of possibilities exist, which are often

interrelated. For setting up an optimisation scheme, a structured “approach” offers advantages

Recommendations are now presented by means of two flow charts, one for cooling water

systems in the design phase and another for existing CWS. These charts offer a step-by-step

“approach” for biocide optimisation.

Figure 3.2 is explained as follows [tm005, Van Donk and Jenner, 1996] . In the design phase of

a cooling water system, a decision should be made on the type of cooling that is to be used. If

water-cooling is used, engineering solutions that control the expected biofouling population in

the CWS should be considered. Important issues to consider in the design phase are:

maintenance of sufficiently high flow velocities in all parts of the cooling water system and a

smooth design of conduits and heat exchangers. This will reduce settlement of biofouling

organisms. The application of non-toxic foul-release coatings will help to further reduce the

settlement of organisms. The intake structure should be designed such that entrainment of debris

and organisms is minimised. Filtering devices and trash racks can further reduce the amount of

entrained gross matter. The application of high integrity materials needs to be considered. For

heat exchangers this can be titanium (corrosion resistant, smooth surface). Hydrodynamically

designed inlet and outlet boxes of heat exchangers can be made out of glass reinforced plastics.

This material can also be applied for tubing and joints in the system. Also, relatively simple

provisions can be made in the design phase, such as connections for chemical and biological

monitoring devices, or for dosing (e.g. special dosing racks and points) or more complex ones

for mechanical cleaning, such as mussel sieves or a sponge rubber ball system. In some cases

heat treatment can be used to control macrofouling, and then no biocide is needed at all. For the

application of heat treatment a special loop in the CWS needs to be designed. Further

possibilities for optimisation are similar to those in existing systems.

Figure 3.2: Design scheme for cooling water systems aiming at reduction of biocide application

[tm005, Van Donk and Jenner, 1996]
Chapter 3

88 Industrial Cooling Systems

Figure 3.3: “approach” for reduction of biocide use in industrial cooling water systems

[tm005, Van Donk and Jenner, 1996]
Chapter 3

Industrial Cooling Systems 89

Figure 3.3 shows all the steps to be consider in the selection of biocides [tm005, Van Donk and

Jenner, 1996] . In an existing CWS it is important to characterise the biofouling population and

the size of the biofouling risk. Adequate biological monitoring is a prerequisite for this.

The cause of the biofouling problem should be analysed and addressed. The quality of the

cooling water can be improved by pre-treatment of the water (e.g. micro- and macro-filtration).

This can remove a part of the biofouling risk by reducing the amount of incoming organisms

and nutrients. If process leakages are the main cause of increased biological growth, they should

be eliminated, for instance by application of corrosion resistant materials or by installation of

spare heat exchangers, which allows more frequent mechanical cleaning. In order to improve

system conditions, all options mentioned in scheme 1 should be considered.

In once-through systems, macrofouling is sometimes controlled by the application of heat

treatment, without any biocide use. Sodium hypochlorite is the most important biocide that is

applied. Dosing is shock-wise or continuous. The dosing strategy on macrofouling control

should be preventive, since curative dosing, when a lot of macrofouling has abundantly

developed, requires very high doses over extended periods. It is recommended that

consideration be given to the option of targeted dosage at locations with a high fouling risk,

such as heat exchanger inlet and outlet boxes. Chemical monitoring is essential to establish the

minimum required biocide dose. Since the applied oxidising biocide concentration will decrease

in the CWS, chemical monitors are needed to register the effective residual level of biocide at

the critical points in the CWS. On-line meters should be hand-calibrated with the colorimetric

DPD test on a regular basis. Toxicity-based measurement of biocide concentrations in the

cooling water is also useful for optimisation schemes. Macrofouling monitoring devices give

information on the settlement and growth of macrofouling organisms and on the performance of

the biofouling control program. This information is essential for biocide optimisation programs

in once-through systems that have macrofouling problems.

In open recirculating systems, microfouling is much more important than macrofouling.

Monitors of microfouling, such as the ATP method and the “plate count technique" give

important information on the development and the state of the microfouling population in the

CWS. To prevent entrainment of organisms and nutrients with the cooling water, the intake

water can be pre-treated (e. g. microfiltration, precipitation). Side-stream filtration, the

continuous filtration of a part of the recirculating water volume, further helps to reduce the

amount of organic material in the cooling water. The amount of biocide required for successful

treatment can thus be reduced. Side-stream filters should preferably be closed temporarily

during shock dosing of biocide, which will avoid unnecessary biocide demand by the filter and

avoid killing the microbial population on the filter.

In recirculating cooling water systems, oxidising and non-oxidizing biocides are used. In the

Netherlands approximately 90% of recirculating systems are treated with sodium hypochlorite.

Non-oxidising biocides are only used when oxidising biocides cannot provide sufficient

protection. For oxidising and non-oxidising biocides discontinuous or shot dosing is

recommended to minimise their use, although in some cases continuous halogenation at low

levels may consume less chemcial than shot dosing. This will also reduce the risk of

development of a tolerance of the biology. Accurate chemical analytical field methods for nonoxidising biocides are needed to optimise their use. The possibilities for hypochlorite

measurement have been mentioned before. Biological methods for measurement of biocide

concentrations in the cooling water may also be useful in recirculating systems.

If possible, it is recommended to close or reduce the purge during shock dosing of both

oxidising and non-oxidising biocides, in order to reduce emissions of active biocide. This is

especially effective for fast reacting or disintegrating biocides. It is further recommended to

operate a recirculating CWS treated with hypochlorite at a pH value of 8-9, in order to minimise

evaporation losses of HOCL over the cooling tower. Experience has shown that this does not
Chapter 3

90 Industrial Cooling Systems

necessarily lead to reduced effectiveness of the biocide. It is important though to manage the

risk of scaling.

The combined use of hypochlorite and bromide may be an attractive option in freshwater CWS,

and also in once-through CWS, since some by-products - the brominated amines - have a

stronger biocidal effect than their chlorinated homologues and they degrade more quickly.

In recirculating systems with a high water quality, ozone may be an option. It is important here

to pay attention to the risk of corrosion. A few experiences in Europe have been successful with

the application of ozone. Finally, UV-light may also offer possibilities in recirculating systems

as a supplementary technique. UV-light alone however, cannot attack the biofouling that has

settled on the surfaces of the CWS. In order to be effective, relatively clear cooling water is

needed, since the light must be able to penetrate into the water column.

The “approach” can be summarised as follows:

1. The availability of water, amongst other factors, will decide on the selected cooling

configuration (once-through, open or closed wet cooling tower or hybrid cooling tower).

The configuration chosen may in turn affect the kind of water treatment. These generally

differ between once-through and open wet cooling towers, such as the application of

oxidising or non-oxidising biocides.

2. Once a system choice has been made (see also Chapter 1), a complex assessment scheme

will have to be applied to match the numerous combinations between the following options

which further affect the need for cooling water treatment:

• choice of material and surface treatment of heat exchangers and conduits:

• lay-out of the cooling system to avoid turbulence, sediments or mussel growth, or to

enhance the required water velocity;

• improve the cooling water chemistry by pre-treatment;

• mechanical cleaning of the cooling system;

• alternative treatments, such as thermal, UV and side stream filtration.

Depending on the result of this assessment a wet cooling system might still need a certain

protection against scaling, corrosion or fouling. This depends on the chemistry of the cooling

water, on the way the system is operated, such as the number of cycles of concentration, and the

cooling configuration chosen.

It is clear that for closed circuit dry air-cooling or dry air-cooled condensers no such treatment is

necessary. Chemicals might be used to clean the outside (finned) tubes, but usually not for

operating the system.

Once the need for a cooling water treatment has been established, an accurate selection of the

cooling water treatment program linked to legislative requirements is appropriate. These

requirements can be:

• prohibition of the use of certain substances for cooling water treatment, e.g. chromium,

mercury compounds, organometallic compounds, nitrites, mercaptobenzothiazoles;

• limitation of certain substances or groups of substances (e.g. zinc, phosphorus, chlorine,

AOX) in the cooling water effluent by defining emission limit values;

• requirement of a minimum level of biodegradability for complexing agents;

• limitation of ecotoxicological effects of the cooling water effluent.

The selection of additives for cooling water treatment for both new and existing systems with

the following “approach” will lead to a reduction in emissions of cooling water chemicals:

1. establish the need for cooling water treatment after other physical cleaning methods have

been applied;

2. select the type of additives required;

3. assess the environmental risk of the substances to be applied;

4. apply substances which have lower potential for impact on the environment; where possible.
Chapter 3

Industrial Cooling Systems 91

3.4.3.2 Reduction by selection of material and systems design

For new system’s material and design options can be applied to reduce additive use. Many

different materials are used for cooling systems equipment. Equipment suppliers usually offer

their equipment in a wide range of different metals and alloys so as to enable the operator to

select the material fit for the chemistry of the cooling water and process conditions it is intended

for. Annex IV discusses materials for once-through and open recirculating systems using

brackish or salt water. It is important to realize that some features of a material can have

opposite characters, which may complicate material selection and will affect the cooling water

treatment programme. For example, reduced corrosiveness may go together with a higher

sensitivity to biofouling.

Proper layout and construction of a cooling system can influence the need for cooling water

additives. During assemble, unnecessary ridges should be avoided as well as abrupt changes in

the direction of the water flow. Both lead to turbulence and where possible this should be

avoided as it enhances corrosion or settling of e.g. mussels.

Operating the system with the appropriate minimum water velocity not only maintains the

required cooling capacity, but also reduces settlement of macrofouling and corrosion of

material.

Coatings and paints are applied to reduce the fixation of the organisms, reinforce the velocity

effect and facilitate cleaning. These antifouling paints can contain toxic substances and

therefore non-toxic coatings and paints have been developed. Applicability underwater and

price vary and depend on the size of the cooling system and conditions. E.g. organic coatings

are applied to relatively smaller cooling units by means of thermal curing of the surface. These

are powder coatings, which can be used in wet environments and do not contain toxic

substances, do not use solvents and are corrosion resistant resulting in significant extension of

equipment life.

In larger wet cooling systems coatings are applied as well and experience in the power industry

shows that they have to be renewed every 4 to 5 years. An example is given in Annex XI.

Antifouling paints have been applied that contain toxic substances, such as copper and

tributyltinoxide (TBTO), that are slowly released from the paint. No paints containing TBTO

are still in use in large installations such as power plants. Copper-containing paints may still be

used on a limited scale.

3.4.4 Reduction by application of additional and alternative cooling water

treatment

A number of techniques has been applied to reduce cooling water treatment. The following

techniques have been reported for the reduction of biocide use [tm005, Van Donk and Jenner,

1996] :

- Filtration and pre-treatment techniques

- On-line cleaning

- Off-line cleaning

- Heat treatment

- Coatings and paints

- Ultraviolet (UV)-light

- Sonic technology

- Osmotic shock.

The principle behind these techniques is to improve the biological quality of the cooling water

and to keep surfaces of the cooling systems elements (conduits and heat exchanger) as clean as

possible, creating an environment in the system in which development of fouling will be

reduced. The application of these techniques is summarised in Annex XI and it is obvious that
Chapter 3

92 Industrial Cooling Systems

some may not have general application or are still under investigation. Environmental benefits

should be balanced against the reduced application of chemicals.

For example, UV-light needs relatively clear water, whereas ozone and sonic technology will

need extra energy input. Electric water treatment applied to a very small size cooling system

(< 1 m3

/min) operating at temperatures of 30 – 40 °C gave promising results as a non-chemical

microbial control method, but needs further research.

Financial costs may vary with the size of the system and the extent to which the techniques must

be integrated in the cooling system.

Pre-treatment of water for recirculating wet cooling systems to reduce cooling water additive

use can be considered in the same light as pre-treatment of water to reduce water requirements

(see Section 3.3.1.2). Pre-treatment will affect the chemistry of the cooling water, such as

lowering the salt content, which will affect the required level of scale and corrosion inhibition,

and will affect the way the cooling system is operated.

However, little has been reported on the effect of pre-treatment of cooling water on the

reduction of cooling water additive use, but inverted osmosis for closed cooling circuits and

sidestream filtration for open cooling systems with larger capacities have reportedly given good

results (Annex XI). Lower costs for water intake, for treatment of the blowdown and for dosage

of corrosion inhibitors, scale inhibitors and dispersants were reported. No redesign of the

cooling system was necessary.

3.4.5 Reduction of emissions by assessment and selection of cooling

water additives

After all technological and operational measures have been evaluated, assessment and selection

of additives for cooling water conditioning is the next step to be taken towards the application of

substances having lower potential for causing environmental impact if properly used. In Section

3.4.1 and more extensively in Annex V, the theory behind the treatment of cooling water has

been described and the selection of the right treatment program clearly is a site-specific and

very complex exercise. It takes into account many factors such as the applied material of the

installation, the water quality and the operating practice. As a result of this a large number of

compounds and their combinations have been developed and are currently applied in cooling

water treatment compounds.

Their performance in the cooling circuit is evaluated and balanced with the residual reactivity in

the aquatic environment after discharge. The challenge here is to select an additive effective in

the cooling system, but harmless as soon as it leaves the cooling system and enters the receiving

aquatic system.

The application of different kinds of cooling water chemicals in different kinds of cooling

systems has been reported in the literature. It appears that their environmental impact is

complex and depends on many different factors. Examples show clearly that optimised

operation reduces the required amount of additives and that it can also lead to the application of

different types of additives. (See 3.4.6).

Generally, in the EU, the assessment of chemicals is regarded as necessary and attempts have

been made to develop an integrated methodology to reduce the environmental effects of their

use, but the difficulties encountered on both a national and a European scale are that:

• there is a variety of evaluation methods used for various applications;

• the availability of data on substances and components of preparations is (still) a problem;

• a variety of parties are responsible for the evaluation of substances;

• risk-based assessment in many countries still has to be developed.
Chapter 3

Industrial Cooling Systems 93

To control the application and enhance the use of alternatives, knowledge of characteristics of

treatment chemicals in some member States has been translated into legislative requirements for

cooling water treatment chemicals. German legislation is quoted below as example.

The Annex 31 of the German Federal Water Act on cooling water emissions is an example of

legislation aiming at optimisation of the use of cooling water additives and on the conservation

of the quality of surface waters. It has resulted in restrictions on the input of certain substances,

such as biocides and other substances, and covers all wet-cooling systems (See Annex VI).

This regulations is based on four steps:

1. a list of prohibited substances, which contains:

- chromium compounds

- mercury compounds

- organometallic compounds (e.g. organotin compounds)

- mercaptobenzothiazole

- organic complexing agents which are not readily biodegradable

- no shock treatment with biocidal substances other than chlorine, bromine, ozone and H2O2

2. limitation of certain substances and groups of substances in the effluent concerning:

- chlorine dioxide, chlorine and bromine

- AOX

- COD

- Phosphorous compounds (inorganic and total/ phosphonates)

- Zn

3. requirement of ready biodegradability of all organic substances used, where the applicable

requirement “ready biodegradability” is in accordance with the Chemical Act and Part C4 of

the Annex to Directive 92/69 EC (31 July 1992)

4. limitation of the ecotoxic effects of the total cooling water effluent of biocidal substances

used does not prohibit their use as it would make the application of microbiocidal

substances impossible. However, they can be essential for the operation and proper

functioning of open and semi-open cooling water systems. The regulations require

information on the level and character of toxicity and call for this to be expressed in a

reproducible manner. Use is made of biotests such as dilution factor (TL) to express the

residual toxicity in the discharge compared to the toxicity in the cooling system.

Despite legislative limits on the use of a number of cooling water chemicals, a large number of

additives is available and assessment and selection of alternatives is required taking into account

site-specific factors such as the cooling systems operation and the sensitivity of the receiving

environment. Also, a translation of specific national regulation would fall short of the general

applicability within the framework of a horizontal “approach”.

Thus, substitution of substances by others, which have lower potential for impact on the

environment, is one of the options to reduce the environmental impact of cooling water

discharges. Permitting authorities in the Netherlands have been using the instrument of

substitution for more than 20 years in permitting procedures. The basis of this instrument is a

procedure of approval in permits. By means of a more or less administrative procedure permit

holders have to submit a request for the use and change of cooling water additives. Permitting

authorities use a widely approved procedure for this assessment. It is expected that in the near

future chemical suppliers and industry will move to a system of self-regulation. The revision of

the system is part of the adoption of a general evaluation methodology to identify the water

pollution effects of substances and preparations. This evaluation methodology is based on the

European legislation on classification, packaging and labelling (67/548/EEC, 99/45/EC).
Chapter 3

94 Industrial Cooling Systems

In general terms the assessment of cooling water additives takes place in three steps [Benschop

(1998)]. The first step is an assessment of the intrinsic properties of substances. Substances are

evaluated on the basis of their ecotoxicological characteristics (carcinogenicity, acute aquatic

toxicity, biodegradation, Log Pow and bioconcentration factor). In order to be able to evaluate

the substances and preparations, it is necessary to know the substance or the precise

composition of a preparation. International industrial programmes (Responsible care, ICCA),

European legislation (Biocidal Products Directive) and the development of chemicals

management on the EU-level contribute to those data on properties of substances becoming

more and more available.

The first step results in an indication of the hazard potential of the additive. The evaluation and

selection through the assessment of the hazardousness applies a hazard identification test to

substances and to their preparations. It is a test which has been developed in the Netherlands

([tm070, Benschop, 1997] and [tm071, Niebeek, 1997]) and is particularly based on the

Dangerous Substance Directive (67/548/EEC) and the Directive Hazardous Preparations

(88/379/EEC). The test focuses on the ecotoxicological characteristics of a substance. This

involves mutagenicity, carcinogenicity, acute toxicity and biodegradibility, log Pow and the

bioconcentration factor. However, this needs data that are not always readily available, partly

for reasons of confidentiality and partly because data have not been collected yet.

To evaluate or benchmark additives with alternatives, which have lower impact on the receiving

environment, can assist in the assessment and selection. In Annex VIII.1 an example is given of

how at site level a benchmark method could be applied to get a first indication of the potential

for impact of the proposed alternative additives. This benchmark is designed for an open wet

recirculating cooling system (open cooling tower). It aims to calculate a “standard” PEC for a

preliminary evaluation of PEC in the river. It is a simplified model assuming that there is no

dilution of the additive in the river, thus overestimating the concentration of the substance in the

receiving waterway. Also, PEC is assumed to be independent of the size of the plant and the

operating conditions as the model takes into account the feed rate of the chemical(s). The

availability of environmental quality standards (EQS) of the chemicals as provided by the Water

Framework Directive is important.

The second step involves optimisation steps to reduce the use of the selected additives by all

sorts of operational steps (see next Section 3.4.6). In the third step the residual discharge is then

evaluated in comparison with the applicable water quality objectives or environmental quality

standards (EQS). If these objectives are not reached, further measures may be indicated. When

alternative additives are available the need to take measures can be avoided by substitution of

hazardous additives by less hazardous alternatives.

The added value of this procedure of assessment of additives related to the regulation on

chemicals based on the above-mentioned EU directives is:

1. the ability to identify additives with the lowest environmental impact and

2. the ability to determine whether local water quality objectives are met.

If this assessment method (which is generally applicable for substances and compounds) is

applied to biocides, the first step automatically results in the need to take further measures. In

practice, this means conducting a study for an optimisation programme for the use and dosage

of the biocide. The second step, which more or less runs parallel with the study for optimisation,

involves the evaluation of predicted effects on the local aquatic ecosystem.

With this evaluation three criteria are being checked and, if all three criteria are met, further

reduction measures including effluent treatment and/or substitution of the additive should be

applied. In the Netherlands the predicted concentration of a biocide is checked against the

Maximum Admissible Risk Level (MARL).
Chapter 3

Industrial Cooling Systems 95

Further measures should then be applied if:

1. the concentration of biocide in the effluent is higher than the MARL, and if;

2. the added concentration of biocide in surface water on a certain distance from the discharges

exceeds more than X % of the MARL, and if;

3. the total concentration of biocide in surface water at a certain distance from the discharge

exceeds the MARL.

The assessment procedure described is illustrated in more detail in Appendix VIII.2.

3.4.6 Optimising the use of cooling water additives

Optimising the use of cooling water additives also means selecting the appropriate dosage

regime and monitoring the effects of the water treatment programme both on the emissions to

the surface water and on the performance of the cooling system in terms of heat transfer and

safety. It is obvious that both techniques are linked and that monitoring is a prerequisite for the

appropriate dosage regime.

The selection of a dosage regime should aim to achieve the required concentration at the right

time, without a reduction of the performance of the cooling system. Underdosage can cause

corrosion or scaling and a reduced performance of the cooling system which also leads to

indirect environmental effects, and an overdose of chemicals can result in fouling of heat

exchange surfaces, higher emission levels and higher treatment costs. Graphically this can be

represented as in Figure 3.4. In the improperly designed system no account is taken of the

required minimum concentration of a biocide to keep the system protected against fouling, so

some fouling may occur. At the same time overdosing leads up to such concentration levels that

more than the maximum required concentration is available. At that moment excess additive

will be discharged into the environment.

Time

Concentration

upper spec limit

lower spec limit

target

Figure 3.4: Additive concentration pattern resulting of improperly designed monitoring and dosage

regime

Targeted dosage (Figure 3.5) based on analysis of the cooling water quality aims at maintaining

the minimum required concentration level to give constant protection. Excessive concentration

levels are avoided and thus discharge into the environment is reduced, which will also reduce

costs to purchase the treatment. For this, properly designed dosage regimes will reduce the

amount of additive needed and can be considered as a cost-effective measure.
Chapter 3

96 Industrial Cooling Systems

Time

Concentration

upper spec limit

lower spec limit

target

Figure 3.5: Additive concentration pattern resulting of properly designed monitoring and dosage

regime

3.4.6.1 Dosage of cooling water additives

3.4.6.1.1 Dosage regimes

([tm010, Betz, 1991], [tm005, Van Donk and Jenner, 1996] , [tm157, Jenner et al, 1998])

Dosage of cooling water chemicals is done in the following ways (see also Annexes V and XI):

• continuous

 end of season

 periodic during settlement

 low level during settlement

• intermittent (also called shot, batch)

• semi-continuous or pulse-alternating

• massive or shock dosage, where a large amount of chemical is added to the cooling water at

once.

Continuous dosage is practised in cooling systems where a certain level of additive has to be

maintained constantly. The better systems feed according to the volume being treated and the

chemical demand requirements. It is still applied in once-through systems predominantly against

macrofouling and corrosion. However, experiences show that reduced use through changed

dosage practice can be just as effective.

With intermittent dosage the chemical is fed by an on/off control on a chemical feed pump or by

discharge from a calibrated vessel or measuring chamber. Batch dosage is applied in cooling

systems, bio-oxidation basins, and other places where the system volume to blowdown ratio is

large. In these systems the amount of chemical replenishes lost or consumed material. It is also

used in applications that only require periodic dosage. For instance, antimicrobials for cooling

water systems are usually fed in a discontinuous way. Batch dosage can both be applied in

recirculating systems and once-through systems. It cannot be used in single once-through

systems where a uniform concentration of chemical is needed.

Dosage can also be targeted on specific areas in the cooling system such as at the entrance to

heat exchangers. An important practice is time-targeted dosage matched with seasonal

characteristics of macrobiological growth. Dosage will also depend on the type of cooling

system. In recirculating systems the way the system is operated will affect the timing and

amount of dosage. In once-through system, the point and time of dosage is important to obtain

the highest effect, as contact time between chemical and cooling water is short.
Chapter 3

Industrial Cooling Systems 97

In small systems dosage is done manually, but in larger systems there is usually an automatic

device linked to a monitoring system. As mentioned before, there is a tendency towards

outsourcing the cooling water treatment to specialist companies. For large and complex

installations with several cooling systems, specialised staff from suppliers are permanently onsite to operate these systems. With smaller systems, daily control is often done by site staff and

supported by regular control by the supplier.

With optimisation of dosage reduction of biocide use can be achieved. The way in which

biocides are being dosed depends on the working and persistence of the biocide, the type and

seasonal pattern of fouling (macro/micro), the state of fouling of the cooling system, the system

water temperature and the nutrient status of the cooling water. Biocides are dosed in gaseous,

liquid or solid forms.

Dosage can be continuous or shock. In some of the literature it is advocated that continuous

dosage should take place in once-through systems to extend the contact time of antimicrobials

when dosage low levels. For recirculating systems continuous dosage is also possible, but

intermittent dosage is more common. The purpose of intermittent treatment in these systems is

to generate a high concentration of antimicrobial, which will penetrate and disrupt the biofilm

and eventually dissipate. Compared to continuous treatment intermittent treatment can lead to

lower average annual concentrations in the effluent and can also be more cost effective, where

lower total amounts are needed. However, this has also been argued as observations were made

that continuous dosage could give a reduction of 40% of FO compared to shock dosage. More

research on this will be needed as in general from an operators’ point of view continuous dosage

is easier to operate than shock or intermittent dosage. This will need a monitoring system to

decide on the appropriate moment to apply the treatment. The optimisation of the dosing regime

must take place with achieving a low incidence of failure at the same time.

In recirculating systems use of products composed of synergistic active blends can result in

reduced treatment concentrations in the blowdown water, as well as cost savings. Biocides with

different spectrums can be dosed in combination as to broaden the spectrum of control. With no

increase in the amount of antimicrobial used, the power of a blend can exceed the effect

expected from a single additive effect. This greatly enhanced performance or synergism is

obtained from only certain combinations of additives. The feeding does not necessarily be done

simultaneously, but can be done alternating with similar results. Another effect is that resistance

is less likely to occur in the case of more than one microbial being applied against which it is

unlikely that the microbes will have developed resistance to both (or all) at the same time.

Interactions between different substances will have to be considered to avoid reduction of the

working of any of the biocides dosed and to avoid hazardous reaction products develop in the

cooling water discharge.

The objective of biocidal treatment can be different. Depending on the target organisms and the

extent to which biofouling has progressed, treatments are either preventive or curative. A

biocide that has been researched intensively is sodium hypochlorite. Dosage of hypochlorite

into a once-through system shows that the cooling system will function as a tube reactor, with

many complex reactions taking place between hypochlorite and organic matter. As a result of

such reactions, and for a typical location, using estuarine or coastal cooling water, a

hypochlorite dose at the intake, ranging from 1.5 - 3 mg Cl2/l, will result in a dose of 0.25 – 0.35

mg/1 of TRO at the outlet of the heat exchanger. This refers to approximately 4 - 8 minutes of

reaction time. To reduce the dosage of hypochlorite considerably, pulse-chlorination has been

applied (Annex XI.3.3.2.1 and X.3.3.2.1).

3.4.6.1.2 Dosage systems

([tm010, Betz, 1991])

There are several dosage systems on the market. For the choice of dosage system one should

distinguish between liquid and dry chemicals. For liquid chemicals, pumps are used such as
Chapter 3

98 Industrial Cooling Systems

metering pumps, packed plunger pumps (high pressure) and diaphragm pumps. For dry

chemicals dosage systems are applied such as volumetric feeders (to dispense powdered

material), gravimetric feeders (proportioning chemicals by weight) and dissolving feeders

(dosage into mixing tank). Whether and how the different dosage systems applied actually

reduce the consumption of additives has not been reported. It is beyond doubt that proper

maintenance of the system and calibration will improve the dosage accuracy. The quantity,

location and timing of dosage can only be controlled in an accurate way by properly monitoring

the water cooling system.

3.4.6.2 Monitoring of cooling water

Monitoring of the need for chemicals for treatment of cooling water is essential to reduce

additive use and emissions into the environment, generally the surface water. It can be seen as a

cost-effective method, where treatment of discharge water, if possible at all, is generally more

expensive.

Distinction can be made between the monitoring of the application of biocides and monitoring

of other water treatment chemicals (scale inhibitors, corrosion inhibitors and dispersants),

because in the case of macrofouling an important additional factor is the monitoring of the

behaviour of the biology appearing in the cooling system.

3.4.6.2.1 Monitoring of scale inhibitors, corrosion inhibitors and dispersants

([tm067, Hoots et al, 1993])

The application of inhibitor chemicals and the optimisation of their use is a very complex matter

and specific for each situation. In every case it will be a balance between a number of factors:

• the quality of the cooling water and the options for pre-treatment (softening, filtration),

which in its turn depend on the required flow;

• the need for reducing water requirements by increasing the number of cycles against the

increase of scaling problems due to the increased concentration;

• the cooling water temperature against the solubility of salts;

• the interaction between additives.

Several methods are applied for controlling the dosage of cooling water inhibitor products in

recirculating cooling systems. In an overview [tm067, Hoots et al, 1993]. The following general

techniques were distinguished as being applied in cooling systems:

• manual testing and adjusting

• bleed and feed (blowdown-activated feed)

• water meter controlled cycles

• sidestream chemical analyser (microprocessor based)

• fluorescence.

Each method obviously has its advantages and disadvantages. The principle of the optimised

pattern shown in Figure 3.5 may not be reached. The various monitoring techniques differ in

their potential to dose the proper amount. A variation in the dosage which is not linked to

fluctuations in the cooling system demand should however be avoided as much as possible. It

may lead to an under dose or overdose of chemicals.

Variation in dosage can occur for a number of reasons:

• operator may be insufficiently involved

• equipment has low reliability

• indirect measurement of chemical level

• measurement of wrong variable

• time lag between the analysis and the adjustment is too large
Chapter 3

Industrial Cooling Systems 99

• repeatability of the method of analysis may be low

• variations in cooling load and makeup water quality are not followed accurately.

From experience it is obvious that the most accurate monitoring and dosage systems directly

measure the concentrations of chemicals in the cooling water and have a reduced time between

analysis and the adjustment of the dosage. Monitoring systems should be able to follow the

changes in cooling load and the variation in makeup water quality (Annex XI).

3.4.6.2.2 Monitoring of biofouling

([tm005, Van Donk and Jenner, 1996] and ([tm087, Engstrom and Tully, 1994])

The monitoring of biofouling is based on the monitoring of microbiological activity in the

cooling system as well as the actual microbiocide treatment levels. The key to measuring the

effectiveness of any biocide program is the ability to measure quickly and accurately the

microbiological activity in the cooling system.

To obtain a good dosage regime the following strategy for once-through systems has been

suggested:

• make a problem analysis on the organism(s) to target

• characterise seasonal differences in occurrence (e.g. breeding period of mussels)

• take into account water temperature and water quality (fresh/salt)

• select a dosage programme (e.g. locally per section: continuously or intermittently)

• decide on the dosage units that will reduce consumption especially if linked to a monitoring

system

• decide on the monitoring programme (mussel detection tank (breeding period

determination) or mussel/oyster monitor (concentration detection)

A similar strategy could apply to open recirculating wet systems. However, the dosage

programme for additives used in cooling towers also covers inhibitor chemicals, which further

increases the complexity of a treatment. An additional factor is the effect of operation with an

increaded number of cycles of concentration, which saves water on the one hand, but increases

the possibility of scaling and corrosion and the need for specific additives on the other hand. In

this situation less corrosion-sensitive materials may be the obvious choice in the design phase of

new installations. They could reduce the need for inhibitors (see 3.4.3.2), enabling operation

without adding complex agents, which at the same time saves on costs.

For both new and existing cooling systems it is important to establish the cause of biofouling

(e.g. leakage) and to characterise the organisms first before deciding any further on the required

biocide.

For once-through systems, macrofouling is of major importance. A prerequisite for biocide

treatment is monitoring of macrofouling. This is essential to establish the minimum required

biocide dose and for biocide optimisation, as it will give information on the settlement and

growth of macrofouling organisms and on the performance of the biofouling control program.

An even more targeted dosage regime is the pulse alternating chlorination which takes account

of the variation in residence times in different parts of the process. At different times and at

different points the required levels of chlorine are dosed following the flow patters of the

cooling water stream in the different process stages. At the end of the process and before

discharge of the cooling water, stream dilution occurs by the mixing of the different process

streams. Where only one of the streams is chlorinated and the other is not, the TRO can be

further reduced and emission levels of < 0.1 mg/l are achievable (see Annex XI).

For open recirculating systems microfouling is much more important than macrofouling. As the

amount of water used for makeup generally is much smaller, both pre-treatment of the water and
Chapter 3

100 Industrial Cooling Systems

side-stream filtration of part of the circulating water can prevent the entrainment of microorganisms. In the case of shock dosage of biocides, recirculating systems have the advantage

that the system can be closed temporarily, enabling the biocide to perform and thus lowering the

concentration before blowdown. Also, for recirculating systems, monitoring of the biofouling

control program is a prerequisite for optimizing the use of biocides.
Chapter 3

Industrial Cooling Systems 101

3.5 Cooling air use and air emissions

3.5.1 Air requirements

The use of air as a resource has no direct environmental implications and is not regarded as a

real consumption. Air is used in all but the once-through cooling systems. In mechanical cooling

towers the air requirement is related to the energy required for fan operation.

Table 3.9: Average required airflow for the different cooling systems

[tm134, Eurovent, 1998]

Cooling system Airflow (%)

once-through flow 0

open wet cooling tower 25

open wet/dry (hybrid) cooling 38

closed circuit cooling tower 38

closed circuit wet/dry cooling 60

closed circuit dry air cooling 100

The higher the required amount of air the higher the fan capacity and consequently the level of

energy consumption and noise emission. In Table 3.9 the airflow requirements are compared for

the various cooling systems. The airflow is strongly correlated with the ratio between sensible

and latent heat transfer (Annex I). Dry cooling needs more air than wet cooling.

In some specific areas (e.g. densely industrialised sites) the air quality could be an issue and by

its composition could lead to corrosion of the (finned) tubes or coils or to fouling of the surface,

in both ways adversely affecting efficient heat exchange. Precleaning of cooling air seems

unrealistic and no information on this has been reported. Consequently, cleaning of the surface

of the heat exchanger and/or treatment of cooling water might be necessary depending on the

chemistry of the air.

On the other hand, open wet cooling towers sometimes act as air cleaners, washing several

pollutants out of the air. This can affect the cooling water treatment and potentially the

operation of the cooling system, but no data have been reported.

3.5.2 Direct and indirect emissions

Emissions into air due to the operation of industrial cooling systems can be direct or indirect.

Indirect emissions occur on the level of the production process due to inefficient cooling. They

are due to the fact that inefficient cooling requires a higher input of resources (such as energy)

to compensate for product or performance losses.

The importance of direct air emissions from wet cooling towers is particularly relevant in the

direct vicinity of urban settlements. In comparison with the air emissions of the industrial

process to be cooled, they are considered to be relatively small. Problems that may occur during

operation are:

1. droplets may contain some water treatment chemicals;

2. bacteriae (Legionnaire ́s disease) may develop in case of improper biocide treatment and

cooling tower maintenance (3.7.3).

Open and closed recirculating wet and wet/dry cooling towers may show some emissions due to

drift and volatilisation, which account for some loss of water treatment chemicals and in
Chapter 3

102 Industrial Cooling Systems

particular of biocides. It is known that volatilisation, also called flash-off, of chemicals increases

with temperature, but the mechanism that leads to emissions is complex, with many factors

involved. Therefore, quantification is difficult and no emission data have been reported.

Drift eliminators are considered to be an important reduction measure. All wet cooling towers

nowadays are equipped with drift eliminators, but still a small percentage of the circulated water

stream may be exhausted as water droplets. These droplets containing dissolved particulate and

chemical additives fall out of the exhaust airstream downwind of the cooling tower and can

cause staining or scale deposits on building surfaces [tm046, Vanderheyden and Schuyler,

1994]. Some specific cases are known in which chromium emissions are reported, but most

Member States have prohibited chromium use for environment and health reasons; it is also

reported to give technical problems.

The quality and quantity of direct air emissions from cooling towers will be specific in each

situation depending on the additives used for cooling water treatment, their concentration in the

circulating water and the effectiveness of the drift eliminators. The standard droplet separators

currently used in wet cooling towers make it possible to limit the loss of water by drift to 0.01%

or even less of the total flow rate. An attempt was made to assess cooling tower emissions using

a simplified model [tm046, Vanderheyden and Schuyler, 1994]. From the data obtained it was

concluded that emission concentrations are low (μg/m3

), but not to be neglected, and that design

and positioning of the cooling tower outlet are important to avoid inlets of air conditioning

systems or other cooling installations.

Currently no standardised method exists to calculate drift losses (and environmental

contamination) for given cooling tower configurations. Two test methods exist to verify drift

losses of given configurations (not published):

- the isokinetic method (IK-method)

- sensitised surface drift measurement (SS-method)

Both methods have their advantages and disadvantages. Advantages of the IK-method are:

1. High collection efficiency on all droplet sizes,

2. Analysis of specific elements possible,

3. Provides integrated sample over exit area,

The disadvantages of the IK-method are:

1. Airborne elements may bias results

2. Long sample times required for high efficiency drift eliminators or low mineral

concentrations.

A test code exists describing how to conduct the IK-test (CTI-140).

The SS-method has the following advantages:

1. Provides droplet size characteristics above 30 μm,

2. Not affected by airborne elements,

3. Provides relative indication of drift eliminators effectiveness,

Disadvantages of the SS method are:

1. Poor collection efficiency on small droplets less than 30 μm,

2. Cannot distinguish between condensation and drift,

3. No droplet analysis for specific constituents.

Incidental emissions of asbestos particles have been reported during decommissioning of old

cooling towers in which asbestos-cement has been used, requiring specialised measures to

contain them. A report on the reduction of the emission of asbestos particles during

decommissioning focused on protecting against direct inhalation [tm082, Mittendorf, 1990]. As

the use of asbestos and similar materials has been banned in the EU no asbestos is used in new
Chapter 3

Industrial Cooling Systems 103

or recently built cooling towers. Asbestos can still occur in cooling towers of about 20 years and

older.

Air emission abatement on cooling towers has not been reported and does not seem applicable.

In the light of the origin of the potential contamination and the way it is transported, the

following conclusions have been drawn:

- Reduction of air emissions from cooling towers is positively correlated with the integrated

measures for reducing water intake, in particular application of drift eliminators,

- Reduction of air emissions is positively correlated with the reduction of the need for cooling

water treatment, and

- Reduction of air emissions from cooling towers is positively correlated with the

optimisation of cooling water treatment (optimisation of system operation).

3.5.3 Cooling tower plumes

3.5.3.1 Plume formation

Plume formation can be important in open and closed wet cooling towers when air with a high

moisture content leaves the cooling tower, mixes with the atmosphere and begins to cool down.

During this process some of the excess water vapour that has been absorbed is condensed out

again. Although this is almost 100% water vapour, the horizon marring effect can be

considerable in case of large towers (power industry, chemical industry). The shape and the

extent of the visible plume are influenced by the temperature and the relative humidity of the

atmosphere, and also by the wind. The colder and the more humid the atmosphere, the more

stable and the more persistent the plume will be. It can therefore be considered to be a potential

problem of the more temperate or colder regions in Europe and mainly in wintertime. Extreme

plume formation from large installations (power plants) can also result in fog at ground level in

the case of lower towers (40-50 mtr.). It is also reported that, during extreme weather

conditions, can be formed on roads ice as a result of large plume formation followed by

precipitation.

3.5.3.2 Plume abatement

[tm101, BDAG, 1996], [tm123, Alt and Mäule, 1987]

Plume abatement is a technological integrated measure changing the configuration of the

cooling system. Plume formation can be prevented, drying the wet exhaust air before it is

discharged, by mixing it with some warm dry air. Open hybrid (or wet/dry) cooling towers and

closed wet/dry cooling towers (or recoolers) are designed in particular to prevent plume

formation. (See Section 2.6)

Depending on the climatic conditions and the requirements of the process, the tower can be

operated dry. For northern European climatic conditions it is maintained that it only needs 20%

of the total heat load to be transferred in the dry section for the cooling tower to operate without

a visible plume, under virtually all weather conditions. In certain conditions, such as at very low

ambient temperature and a low thermal load, the tower can also be operated in entirely dry

mode. Regulations sometimes distinguish between day and night operation and allow wet

cooling (with a plume) during the night, whereas in daytime the tower must be operated in

hybrid mode, preventing plume formation. (See also Chapter 2).
Chapter 3

104 Industrial Cooling Systems

3.6 Noise emissions

3.6.1 Sources of noise and noise levels

Noise emissions are important at a local level. Noise emissions from an industrial complex

result from a range of noise-producing sources and in the practice of permit writing cooling

system noise is considered as an integral part of the whole site. Consequently, noise from

cooling systems and the investment on potential attenuation measures should be evaluated

within the total noise emissions of a site. Noise emissions are generally an issue of both

mechanical draught cooling towers and large natural draught wet cooling towers. For

information on characteristics and calculations of noise emissions of sound sources in cooling

towers, reference can be made to the VDI-Directive 3734 and to standards developed in the

German VGB-guidelines for cooling systems of power plants [tm158, VGB, 1998].

Three main sources of noise caused by these cooling systems can be identified:

• fan assemblies (fan, gears, drive) - all mechanical cooling towers;

• pumps - all systems with cooling water

• droplets falling on the cooling water basin/ cascading watermasses - only wet cooling

towers.

Radiation can be directly or indirectly. The sound is directly radiated through:

• points of air intake

• points of air discharge

The sound is indirectly radiated through:

• fan motors

• fan discharge cowls and cooling tower cladding (with concrete constructions there is no

significant contribution)

Noise from dry air-cooled towers is predominantly influenced by the mechanical equipment

applied and the way it is operated. In cases where attenuation has resulted in a very low sound

power level of the equipment, noise from heat exchangers or condenser pipes and conduits may

become predominant.

In wet cooling towers, noise is a result of the falling water droplets only (natural draught) or of

both the falling droplets and the mechanical equipment. Generally the unattenuated noise of fans

is dominant compared to the noise of water droplets. This is reported to be irrespective of the

size of the wet cooling tower. When air-related noise is reduced by attenuation measures, waterrelated noise may become dominant and water noise attenuation may be considered.

For medium to large cooling towers operated in power stations and large industrial plants the

following was reported. For natural draught cooling towers, water flow and tower height are the

most important factors affecting unattenuated noise emission levels. Falling height of the

droplets is important up to 5 m., but with larger falling heights no further influence on the total

noise was reported. The sound power level at the inlet can be calculated according to the

equation:

Lw (dB(A)) = 68 + 10 * (log M/M0) ±2 M0 = 1 tonne/hr

The sound power level at the air outlet of natural draft cooling towers can be approximately

calculated with the following equation:

Lw (dB(A)) = 71 + 10 * (log M/M0) - 0.15 * (H/H0) ±5

M0 = 1 tonne/hr (M = weight rate of water flow)

H0 = 1 m (H = height of the cooling tower)
Chapter 3

Industrial Cooling Systems 105

For fan operated wet cooling towers the spectrum of water noise at the inlet is not much

different. For cooling towers with induced draught (fan in top of tower) the contribution of the

water noise in the air outlet (diffuser) to the total sound power level can be calculated

approximately using the following equation:

Lw (dB(A)) = 72 + 10 * (log M/M0) ±3 M0 = 1 tonne/hr

The most relevant factor in mechanical cooling towers is the mechanical equipment used (fans,

gears, etc). Fan tip speed (25-60 m/s) is a major influence on the total noise level. The type of

fans used (centrifugal or axial) as well as the number and type of blades are also of importance.

It was reported that use of gearboxes can have negative influence on the sound level (at the

same water flow and fan tip speed), if the fan speed is reduced (e.g. nighttime operation), when

they become more dominant.

The sound power level of a fan can be calculated approximately using the equation:

Lw (dB(A)) = 16 + 10 * (log V/V0) + 20 * (∆p/∆p0) ±5

(V0 = 1 m3

 of air/h; ∆p0 = 1 hPa)

This general equation can be used for both forced and induced draught fans. In forced draught

towers the contribution of the fans to the sound power level at the air outlet of medium to large

cooling towers will generally be less than the contribution of a fan of an induced draught tower

with the fans at the top. This difference can be up to 5 dB(A).

The following equation was used showing how the sound power level of axial fans is related to

the tip speed of the fan:

Lw (dB(A)) = C + 30 log Utip + 10 log (Q*P) – 5 log Dfan

(C = fan characteristic shape value, Utip = fan tip speed, Q = fan flow, P = fan pressure

drop, Dfan = fan diameter)

Noise emissions also depend on the construction of the cooling tower. Noise from concrete

towers is entirely emitted through the air inlet and air outlet. For cooling towers made of

different lighter material, emission from the shell will have to be taken into account.

Furthermore, counter-flow or cross-flow design also affects the sound emission of wet cooling

towers, where counter-flow is reported to have more splash noise than cross-flow designs.

Noise emissions can be characterised by the different frequencies they consist of and distinction

can be made between natural wet draught cooling towers and mechanical cooling towers.

Falling water in natural draught towers shows a broad frequency band, whereas the noise of fans

of mechanical cooling towers predominantly consists of low frequencies. This, amongst other

factors, can explain why water noise typically prevails in near-field conditions around the

installation, while the noise of the fans will become progressively predominant with increasing

distance from a fan operated cooling tower.

The sound power levels of different cooling towers show a large variation and each single

source will contribute to the total emission. This is illustrated by examples in Table 3.11 for

power stations [tm158, VGB, 1998] and in Table 3.10 showing values for the different cooling

systems used in a refinery [tm001, Bloemkolk, 1997].

Noise levels of the falling water in wet cooling towers depend on the drop height of the water.

Lower drop height in induced draught wet cooling towers results in about 1 dB(A) lower sound

power level at the air inlet and of a so called cell type cooling tower with induced draught,

3 dB(A) lower.
Chapter 3

106 Industrial Cooling Systems

Table 3.10: Examples of capacity and associated unattenuated sound power levels of cooling

systems equipment of a large refinery

[tm001, Bloemkolk, 1997]

Equipment Capacity1 Lw in dB(A)

Compressors 490/ 2000 kW 108/ 119

Pumps 25/ 100/ 1300 kW 94/ 98/ 108

Steam turbines 1000/ 2000 kW 106/ 108

Air-coolers 7 /20 / 60 kW 89/ 93/ 98

Air-cooler/condenser 170 kW 102

Air-cooler/condenser 2.7 MWth 97

Air-cooler 14.7 MWth/ 18.8 kWe 105

Air-cooler 1.5 MWth/ 7.5 kWe 90

Cooling towers 300 MWth 106

Cooling tower 2000 m3/hour 105

Notes:

1. reference to the capacity of the rotating part, motor, etc., i.e. not cooling capacity.

Table 3.11: Comparison of unattenuated sound power levels at air inlet and air outlet measured at

various types of wet cooling towers of conventional construction

[tm158, VGB, 1998]

Wet cooling tower

construction At air inlet in dB(A) At air outlet (diffusor opening)in dB(A)

Natural draught 84 ± 3 69 ± 3

Open wet cooling tower 86 ± 3 80 ± 3

Open wet cooling tower

(cell type, forced draught) 88 ± 3 85 ± 3

Open wet cooling tower

(cell type, induced draught) 85 ± 3 88 ± 3

For comparison of total sound power levels of various types of cooling systems Table 3.12

documents total noise levels for different types of cooling systems without noise attenuation.

From the above given variation in levels it can be understood that the ranges are wide depending

on the applied design and equipment.

Table 3.12: Noise emissions of different cooling systems without noise attenuation

[tm134, Eurovent, 1998]

Cooling system Noise emission

dB(A)

once-through flow

cooling tower-natural draught 90-100

cooling tower-mechanical draught 80-120

closed circuit cooling tower 80-120

hybrid cooling 80-120

dry air cooling 90-130
Chapter 3

Industrial Cooling Systems 107

3.6.2 Noise abatement

([tm158, VGB, 1998], [tm061, Eurovent/Cecomaf, 1997], [tm086, Van der Spek, 1993],

[tm093, Mirsky, 1995])

Noise abatement should primarily be focused on so called primary measures or “internal”

measures before considering any secondary or “external” measure such as baffles or big

barriers. Different guidelines on noise abatement of cooling towers distinguish between noise

generated by cascading water and noise of mechanical equipment. In general, natural draught

cooling towers are less noisy (unattenuated), but for mechanical draught cooling towers sound

attenuation is more efficient. Obviously, proper maintenance of the noise generating equipment

over time can keep the emission down as well. In most cases only mechanical draught cooling

towers can meet the noise requirements because only the mechanical draught can economically

overcome the additional airside pressure drop. The selection of less noisy radial fans often

implies higher energy consumption and results in higher operating costs than axial fans.

The general “approach” is to apply the primary measures first to optimise noise emission. If

further noise reduction is needed additional attenuation can be considered. Noise attenuation

should be done bearing in mind the effects of abatement measures, such as pressure drop (more

energy required) and other sources becoming more dominant. The contribution of individual

equipment to the sound emission level must be seen as part of the whole noise pattern. This

means that nearby buildings, dispersal and reverberation, amongst many other factors, must be

taken into account as well. To lower the emissions from the different potential noise sources in

cooling systems the above given equations clearly show on what issue the reduction measures

should focus, such as drop height and fan tip speed.

3.6.2.1 Noise control of cascading water (wet cooling towers)

In natural draught cooling towers the sound attenuation focuses on the air inlet, because the air

outlet contributes insignificantly to the total noise level, at least being 5 dB lower. Sound

generated in the basin by falling water is already reduced to some extent by radiation in the

tower, the tower fill and the plume (10-15 dB). Further attenuation can reduce the emission

from the air inlet by a further 5-8 dB. The following measures are suggested and could be

applied to medium to large mechanical cooling towers as well.

3.6.2.1.1 Primary measures

The following techniques have been identified as primary measures:

• Lowering the water surface by faster draining of the basin would use the basin walls as

sound barriers.

• Reducing the drop height of the water is possible by minimising the sectional area of the air

inlet, which is limited.

• Avoiding the impact of droplets in the basin is possible by devices catching the drops and

draining them into the basin (impact deflectors) Effect: 7 dB maximum.

• Water collection troughs underneath the fill also have a deflecting effect: 10 dB maximum.

Disadvantage of the deflection methods is the susceptibility to fouling of the surfaces of the

catching devices, which can potentially contaminate the water.

3.6.2.1.2 Secondary measures

Secondary measures that have been applied are:

• Sound attenuators with baffles at the air intake: 20 dB maximum reduction. Disadvantage

could be the air pressure drop, which can be about up to 10 Pa. Pressure loss can require

20% of the installed fan capacity.

• Earth barriers around the tower base: an attenuation effect of 10 dB.
Chapter 3

108 Industrial Cooling Systems

• Sound walls (or screens) with sound absorbing layers yield a sound attenuation of 20 dB.

With these constructions effectiveness depends on construction and distance to the tower

base.

3.6.2.1.3 Dry-cooling towers

Noise from dry cooling towers is predominantly caused by fans, but for medium to large drycooling towers, the noise of water can become dominating when it flows through the heat

exchangers at higher velocities. In the case of condensers, the sound may become distinct when

low noise equipment is being used, and the flow noise of condenser tubes may reach the same

magnitude as the fan noise. In those cases further sound attenuation may become important and

isolation of connecting pipelines must be considered.

3.6.2.2 Noise control of mechanical equipment (mechanical draught cooling

towers)

For the noise control of cascading water in medium to large mechanical draught cooling towers

the same primary measures as were mentioned for natural draught towers can be referred to.

Additionally applied are:

• grids or woven structures of fine meshes, drifting on the water surface to reduce the splash

noise of falling droplets. No quantification of the reducing effect has been reported.

Noise attenuation of mechanical equipment refers mainly to reduction of fan-related noise in

both wet and dry cooling towers. Applied measures are primary (equipment) or secondary

(absorption). The various sound attenuation measures achieve noise abatement levels of about

20 dB(A) and up to 30 dB(A). For these high noise abatement levels it is necessary to combine

more quiet equipment with additional sound attenuation, such as acoustic baffles or noise

attenuators. Such equipment for passive sound attenuation will increase the investment costs,

but operating costs remain reasonable.

3.6.2.2.1 Primary measures

The following primary measures have been reported:

• Fan measures:

- low power fans (couple of dB(A) reduction);

- larger air cooler fan can make a difference of 2-6 dB (A); number of blades, 6-8 instead

of 4, makes a difference as well (also lowering the energy requirement);

- use of low noise fans with wider blades and lower tip speeds (< 40 m/s) for the same

airflow and air pressure;

• Low noise gear drives (small transmission ratios or multi-pole drive motors), helicoidal

gears instead of right angle gears, belt drives and, if possible, direct drives.

• Belt drive by V belt, flat belt or low noise moulded notch belts. If possible, belts should

encapsulated;

• Low noise fan motors;

• Centrifugal instead of axial fans;

• Largest possible distance between fan blades and support construction;

• Use of flexible support of gears and fan motors;

• Aerodynamic design of the air passages.

Further reduction can be achieved by the way the equipment is operated. The number of

revolutions per minute can achieve further reduction of fan noise. In a period of lower demand

(nighttime) fans could be operated at a lower rotation speed and a 50% reduction of speed can

result in a noise reduction of about 6-10 dB(A).
Chapter 3

Industrial Cooling Systems 109

3.6.2.2.2 Secondary measures

Secondary measures for mechanical draught cooling towers at air intake and discharge are

beneficial. Compared to the advantage of a sound reduction with 10 to 25 dB(A) a higher

pressure drop of 20 to 70 Pa must be accepted, which must be overcome by extra energy input

or larger (noisier) fans.

Examples are:

• The muffling of airflow and housing can contribute with a reduction of 5 dB(A).

• Sound absorbent construction crosses, which are built in the air outlet (diffuser) of smaller

fan cooling towers, reduce the sound emission and also give a better flow line pattern and

by this a better draft.

• Droplet separators in wet cooling towers can be covered

• In addition mounds or walls (buildings or sound barriers) around the air intake opening have

proven to be successful. The sound reduction by shielding can be up to 20 dB(A) in the

vicinity of the cooling tower.

3.6.2.3 Costs of noise reduction

Costs of noise attenuation measures will vary considerably depending on the type of measure

and whether it is part of the design of a new cooling installation or of a measure taken during

retrofitting an installation. For a new installation of a hybrid tower, total costs of the noise

attenuation measures (fan, baffles) accounted for about 20% of the total investment.

An example was reported with respect to costs showing how costs can increase with increasing

reduction of noise. For an axial fan different designs (value C and tip speed) could be applied

without reducing pressure drop and flow/fan efficiency. In case of very low and super low noise

fans, additional measures to the drive are required at extra cost.

Table 3.13: Example of cost increase with different fan design for reduced sound power level

Derived from [tm086, Van der Spek, 1993]

Fan design Sound power level

(dB(A)) Price-index

Classic 100 1

Low noise 95 1.5

Very low noise 90 3

Super low noise 85 4

The data in Table 3.13 just show cost differences for different fan designs, but costs for drive

transmissions, sound attenuation and cooling tower construction should also be taken into

account. The cost of primary measures such as super low noise fans may deliver at the same

time distinctly lower operating costs due to lower power requirements. It can therefore not be

unambiguously concluded that the sound reduction level would not be cost-effective.
Chapter 3

110 Industrial Cooling Systems

3.7 Risk aspects associated with industrial cooling systems

3.7.1 Risk of leakage

[tm001, Bloemkolk, 1997]

3.7.1.1 Occurrence and consequences

Leakage can occur both in water and air cooling systems, but generally leakage is a concern of

water cooled systems. In particular in once-through systems contamination will enter the aquatic

environment immediately via the cooling water. In open and closed circuit wet and wet/dry

systems this will not happen immediately, but leakage will contaminate the coolant and the

chemistry of the coolant will be disturbed with consequences for the heat exchanging process.

This effect of leakage on the working of oxidising and non-oxidising biocides has been clearly

illustrated [tm090, Grab et al, 1994]. Finally, leaked process substances will be discharged with

the blowdown.

In direct air cooling systems leakage may lead to contamination of the cooling air, but generally

leakage is not considered to be an environmental problem of dry air-cooling. This leaves the

responsibility of the operator unchanged to try and prevent leakage from dry cooling systems.

Cooling systems using refrigerants are not within the scope of this document.

Heat exchangers are subject to corrosion, erosion and other forms of wear. Factors such as

choice of material, fluid speeds, wall temperature and pressure levels influence this. As a result

of this, leakage of process fluids and contamination of the coolant or contamination/disturbance

of the process can occur. The type of cooler can also be a potential influence on the risk of

leakage. In practice, an operation that is different from the one intended by design causes

vibrations, and ultimately, leakage. Leakage becomes a relevant problem when the flow to be

cooled contains components that are harmful to the environment. Leakage from condensers at

power stations or condensers of evaporating plants are not considered to be a problem from a

water quality viewpoint, but rather from a process-technical point of view. At power stations,

leakage means a loss of vacuum in the condenser, which will lead to a loss of efficiency of the

power generating process. With cooling in furnaces (steam generation) leakage of water can

lead to explosions.

Product loss by leaching of heat exchangers can be important when, in a corrosive environment,

(such as saltwater) relatively easily corroded material is used (for instance, copper condensers).

Copper is often applied it reduces the chance of fouling, but in practice fouling is also found in

copper condensers. Copper emissions are undesirable and can be prevented with better materials

such as titanium and stainless steel or by adding anti corrosion compounds.

Leakage is also sometimes attributed to the 'sweating' of coolers. This concerns the presence of

small hairline cracks or leaking of packing materials. The most common defects in heat

exchangers, which are reported from actual practice, are caused by:

• corrosion/erosion as a result of chemical pollution (pit corrosion),

• corrosion resulting from biological foulings, chemicals, bacteria,

• mechanical erosion (due to cracks or vibrating mussels),

• vibrations (caused by flow or the resonance of external pumps, etc.),

• leaking, defective packing materials,

• sweating of 'rolled' tube-plate connections,

• dislodged tube-plate attachments,

• stress in materials as a result of incorrect operating pressure and/or temperature and

• temperature gradient of cooler too high; over 50o C can cause problems.

In once-through systems using large volumes, small leaks are hard to detect. In the case of

cooling systems that contain several heat exchangers, there might always be a number of
Chapter 3

Industrial Cooling Systems 111

defective heat exchangers, creating a more or less constant level of pollution in the cooling

water, which is low and barely detectable in the large water flow. Larger levels of leakage can

be detected, but generally this also means a considerable and significant emission. In

recirculating systems with cooling towers, possible volatile compounds are stripped out and

leakage is discharged in the blow down. In this case, because of the small volume of the

discharge flow, detection is easier and the blowdown can be treated if necessary.

The magnitude of leakage usually becomes known as a result of incidents and to a limited

extent by measurement in once-through systems. Larger leaks are detected and are usually

immediately fairly significant. Data from actual cases have shown that levels of 100 to

3000 kg/24 hours with exit flows in mg/l are easily possible with large effluents (10000 m3

/hour

and more). The frequency of failures for different types of heat exchangers shows wide

variation.

3.7.1.2 Reduction of leakage

Heat exchangers should be designed to prevent leakage. Different organisations have given

recommendations regarding the nature of maintenance. This consists of a combination of

preventive and corrective maintenance, because with prevention alone it has been shown that

problems cannot be fully controlled. Preventive maintenance is often part of a production stop,

once every two years. With corrective maintenance, a cooler is turned off and leaks are repaired,

for instance by plugging a leaking tube or replacing a tube bank. For heat exchangers that

cannot be turned off for production-technical reasons it is important that a second reserve cooler

is available. It is becoming increasingly clear that "failure" and leaks are primarily caused by a

fault in design. In the process industry, any extra costs of more expensive construction or better

materials are usually easily outweighed by the costs of failure. Investment costs are low

compared to those entailed by a loss of production. The design of heat exchangers should

therefore take place on a basis of "forecast availability".

The following general measures to reduce the occurrence of leakage can be applied:

• select material for equipment of wet cooling systems according to the applied water quality;

• operate the system according to its design;

• if cooling water treatment is needed, select the right cooling water treatment program;

• monitor leakage in cooling water discharge in recirculating wet cooling systems

(blowdown).

If problems arise in practice, there are a number of options, partially dependent on the cause:

At component level (the heat exchanger):

• check causes of erosion, corrosion

• check operating conditions vs. design conditions

• replace cooler by improved type, checking the construction and material

• drainage of flow polluted by leakage to purification (purification of sub-flow concerned)

• recirculation of cooling the water flow of the cooler concerned over either an air cooler,

and/or indirect water cooling (this option naturally does not remedy production failure

resulting from failure of the cooler).

At a system level (the complete cooling water system or parts thereof):

• maintain as small a pressure difference as possible between the cooling water and the

process water, or create or maintain overpressure in the cooling water

• convert to an indirect system or if technically possible convert to a recirculating system with

cooling tower (taking into account potential volatilisation of components).
Chapter 3

112 Industrial Cooling Systems

The VCI safety concept, applied by the chemical industry, refers to leakage as the temporary

discharge of substances causing long-term detrimental changes to water bodies. To prevent and

control this the chemical process substances are rated (using the R-phrases) and the final rating

is linked to requirements concerning selection of the (indirect) cooling configuration system and

the way leakage is monitored (see Annex VII).

It is obvious that the application of an indirect completely system or a recirculating system with

a cooling tower can control leakage almost 100%. Only if system pressure drops can fouled

water be released, but this flow is small and controllable. Application of both options however,

needs awareness of the requirements of the process to be cooled. An indirect design or

application of a cooling tower will increase the approach and raise the minimum end

temperature of the process substance. If the process to be cooled can tolerate this, the

characteristics of the process substance(s) may justify an indirect design to protect the receiving

surface water against any unwanted emission due to leakage.

Some companies consciously use a cooling system in which parts that are subject to leakage are

provided with indirect cooling, and those that are not subject to leakage are not. As the control

of leakage appears to be difficult, in cooling priority substances or other environmentally

hazardous products, once-through systems should preferably not be used, especially in view of

the alternatives available.

For an existing cooling system an indirect design generally is neither technically nor

economically the most applicable solution. Practical experience with the application of a solid

maintenance and control programme to a large once-through cooling system using seawater has

given good results. Some cooler replacements were necessary, but 90% of the failure of the

different heat exchangers could be reduced by proper antifouling treatment and operating care

(vibration monitoring, pump handling and care with flow pinching). Leakage detection is

applied and, by detecting at the right places, the time between heat exchanger leakage and

detection can be shortened.

Detection of leakages in once-through systems is difficult, but a recommended starting point is

the identification of the heat exchangers that are prone to leakage and that cool harmful

substances. More selective and accurate measurement of leakage will then be possible.

Preventive and corrective maintenance are both important to overcome leakage problems, but

proper design generally tends be most cost-effective.

3.7.1.3 Reduction by preventive maintenance

Visual inspection, hydrostatic testing and additional investigation on pulled tubes are examples

of former inspection methods. The limitations of these methods were that the inspection was

concentrated on the directly visible parts of the tubes. Dirt often masks the early stages of

defects and uniform corrosion is hard to see with the naked eye. Hydrotesting only detects

leaking tubes. The question is how to select a representative tube for further investigation. As a

consequence the former inspection methods described above could not prevent environmental

pollution due to unexpected leakage, breakdown, reduction in capacity and/or occurrence of offspec product. On the other hand a great number of spare tubes had to be in stock for unexpected

retubing.

Experiences with a new kind of investigation of heat exchanger tubes (by means of eddy current

investigation) have shown that the reliability of heat exchanger tubes can be increased

significantly and that emissions due to leakage can be reduced. Because this method is capable

of testing a single tube and giving a prediction of failure of a single tube, the inspection

frequency will be based on facts. As a consequence, inspection methods capable of predicting

the risk of failure of individual tubes of a heat exchanger can lead to a reduction of tube

consumption, better stock management and knowledge about corrosion behaviour in early
Chapter 3

Industrial Cooling Systems 113

stages. This will lead to a reduction of unexpected breakdowns due to leaking tubes with the

environmental benefit of a reduction of emissions via the cooling water.

Application of this method at the site of a chemical plant has resulted in a reduction of more

than 90 % of the percentage of retubing since introducing this method in 1990 [Paping, Dow

Chemical Benelux Terneuzen, 1999]. This has also resulted in a reduction in annual costs. The

average annual savings due to the reduction of number of pipes to be retubed are about 5 times

as high as the inspection costs. The number of unexpected process breakdowns due to leaking

tubes has been reduced by 90 % over the last 10 years.”

3.7.2 Storage and handling of chemicals

The storage and handling of chemicals is potentially an environmental issue of wet cooling

systems. Dosage of cooling systems additives can be continuous or intermittent and the

chemicals can be fed diluted or neat. The amounts of chemical and its characteristics vary

greatly and depend on a range of factors (e.g. water chemistry and heat exchanger material): the

risk due to storage and handling will vary accordingly.

For example, for pH control, concentrated sulphuric acid is used and this is usually stored in

mild steel tanks. Proper ventilation is required to prevent the build-up of explosive hydrogen gas

in the storage tank. Strainers upstream of acid pumps are advisable to remove any residual

corrosion products or other solids that may be present in the storage tank.

Sometimes additives are produced on-site. For example, hypochlorite can be produced on

marine sites by electrolysis of seawater. This process called electrochlorination can be

dangerous for the potential of chlorinegas formation. Also, the installation needs frequent

cleaning using acids. Alternative treatments are applied, where possible to avoid these risks (e.g.

Annex XI.3.4.7).

Cooling additives can be fed by the operator manually or by means of sophisticated computer

controlled systems or can be outsourced to specialised firms, usually the actual supplier of the

additives. Feeding manually has a higher risk of spilling, and for environmental and human

health reasons safe handling procedures should be applied. Automated systems run a risk of

being neglected, but require regular inspection.

In the EU specific regulations on transport, storage or handling of chemicals have to be

observed and environmental permits require site-specific measures. Generally, the aim is to

reduce the risk of spilling and leakage to prevent soil and/or ground water contamination and to

reduce the risk of explosions by defining a restricted area where storage and handling of

chemicals is allowed. Such areas are equipped with impermeable floors or grid floors with a

bund, with segregation to keep reactive chemicals separate and with a minimum required

ventilation rate.

BAT measures for the storage of dangerous substances are described in the BREF concerning

emissions from storage.

3.7.3 Microbiological risk

3.7.3.1 Occurrence of microbes

Microbiological risks from cooling systems relate to the occurrence of different species of

pathogens in cooling water or in parts of the system that are in contact with the cooling water,

such as biofilms in heat exchangers and fill in cooling towers. These risks are not an issue in dry

cooling systems.
Chapter 3

114 Industrial Cooling Systems

The major thermophilic pathogens that are found in wet cooling systems using river waters are

the bacteria Legionella pneumophila (Lp) and the amoeba Naegleria fowleri (Nf). In marine

waters some halophilic vibrios species, pathogenic for fish or man, can develop in once-through

cooling systems. The species mentioned occur in the natural environment in generally low and

harmless concentrations. Due to the raised temperature a favourable climate can occur in

cooling systems enhancing the development of those bacteria, which can create a potential risk

for human health. Development of Legionella is enhanced by fouling, presence of amoeba,

ciliates and algae. It is spread through aerosols. Following some large outbreaks the occurrence

and characteristics of Legionnaire’s disease (LD) and the development of Lp has been widely

researched from a medical/biological point of view. But many points relating to the chemical

and process technology remain unclear.

In the plume of a natural draught wet cooling tower with a considerable height and with a good

functioning drift eliminator, the emission of bacteria is of less importance, but not impossible. A

high concentration of Lp in the plume of a natural draught wet cooling tower has been reported

due to fouling on the inside of the concrete cooling tower wall. The layer had been released

from the wall and fallen onto the drift eliminator [tm145, Werner and Pietsch, 1991].

The appearance of Lp in the plume of industrial mechanical draught cooling towers, which have

a much lower height than natural draught towers, has been reported on a number of occasions

[tm040, Schulze-Robbecke and Richter, 1994], but a clear cause and effect relation between

cooling towers and an LD outbreak could not be established. Where a relation between cooling

systems and an outbreak of LD could be made it always concerned badly maintained systems

[Morton et al., 1986].

Typical conditions in wet cooling towers that enhance the development of Legionella are:

• the water temperature in the cooling tower is between 25 and 50 degrees centigrade;

• the pH between 6 and 8;

• the presence of fouling.

Less information was submitted on the occurrence and treatment of other pathogens such as Nf.

It has been observed that development of Nf is inhibited by brass and enhanced by stainless

steel. The amoebae are also more abundant in open recirculating cooling systems than in oncethrough cooling systems. Research was carried out on the treatment of Nf after increased levels

in plant cooling water (3000 1-1) followed replacement of condensers in a French power station.

Continuous chlorination with a maximum free residual chlorine level in the range of 0.3-0.5

mg/l decreased concentrations of Nf immediately and the levels remained under 4 pathogens/l.

[tm 144, Cabanes et al, 1997].

3.7.3.2 Measuring of bacteriae

Lp-bacteria are measured in colony forming units or CFU per litre and are reported to vary in

concentration in cooling tower water from very low (down to 10 CFU/l) to very high (105

-106

CFU/l). In biofilms Lp has been found in concentrations of up to 106 CFU/cm2.

For air conditioning systems, values of 100-1000 CFU are applied in the UK, but it is not clear

if this can be compared to the levels in well-maintained wet cooling towers and the associated

risk in those situations. A recommendation was made to keep the concentration of Lp below 104

CFU/l. Quantification of representative concentrations of Lp in industrial wet cooling systems

and the level of CFU in wet cooling towers that is still acceptable with respect to human health

may require further research.

3.7.3.3 Techniques to reduce microbiological risks

([038, Millar et al., 1997] and [tm040, Schulze-Robbecke and Richter, 1994], [tm166, Morton et al,

1986] [tm167, Fliermans, 1996], )

The chain of events to create an outbreak of Legionella involves:
Chapter 3

Industrial Cooling Systems 115

- the development of a virulent strain of bacteria in the cooling system

- conditions that enhance the multiplication of bacteria

- contaminated water discharged to the atmosphere as aerosol

- sufficient droplets deeply inhaled by susceptible persons

Prevention of Legionella should therefore be based on prevention of development and

multiplication of bacteria in the cooling system. Particularly in the USA and UK

recommendations have been developed for prevention of LD. Regular analysis of the potential

reservoir (e.g. cooling tower), and additional routine maintenance, proper pH and temperature

levels, adequate levels of residual biocides, and control on quality of the makeup water can

prevent the occurrence of environments that encourage Legionella.

On the prevention of formation of Lp-bacteria (and others) in cooling towers, the following

measures should be applied:

• use clean water and pretreat the cooling water if possible;

• avoid process leakage into the cooling system;

• avoid stagnant zones;

• prevent formation by reduction of light energy within the cooling tower avoiding algae

formation; open water basins are to be avoided;

• easy access for regular cleaning should be provided;

• use of drift eliminators that can be easily cleaned or replaced;

• design cold water temperature as low as possible (small approaches);

• avoid scale and corrosion;

• optimisation of construction to enhance the right water velocity and air speed;

• a minimum distance of the CT from populated areas is impossible to give, but consideration

should be given to avoiding the plume reaching ground level or reaching populated areas, if

space allows;

• minimisation of plume formation could restrict the spread.

With respect to the location of a cooling tower a rating has been suggested of the

microbiological risk associated with a cooling tower based on the host population and the

potential susceptibility of the host. The rating categories are:

- Category 1: highest risk – cooling tower serving or in the vicinity (<200 m) of a hospital,

nursing home or other health care facility caring for persons who may be immunologically

compromised;

- Category 2: cooling tower serving or in the vicinity (>200 m) of a retirement community,

hotel or other buildings accommodating a large number of people are localised;

- Category 3: cooling tower in a residential or industrial neighbourhood;

- Category 4: lowest risk – cooling tower isolated from residential neighbourhood (>600 m

from residential area).

Based on this rating, inspection for the presence of Legionella ranges from monthly (highest

risk), monthly to quarterly (Cat. 2), quarterly to yearly (Cat. 3) to once a year after summer

(Cat. 4).

The following measures are recommended to operators of cooling towers:

• care must be taken in case of process stops and start-ups, especially if the cooling

circulation system is down for more than 4 days;

• operators entering cooling towers should avoid inhaling the air by using mouth and nose

protection (P3-mask has been proven);

• If cleaning a cooling system after Lp has been detected, a combination of mechanical

cleaning and a shock-dosage of a biocide.
Chapter 3

116 Industrial Cooling Systems

Some additional remarks on these recommendations can be made. After a prolonged shutdown

it is imperative to treat cooling systems with a biocide (chlorine). If there is evidence of a dirty

or contaminated system, including accessories such as sound attenuation, it must be cleaned and

receive biocide shock treatment prior to start-up. A competent water treatment company should

conduct such treatment. Disinfection of the system may be needed if the system is heavily

contaminated.

From experience it is clear that chemical treatment mainly treats the bacteria in the water. To

control and clean the cooling system more thoroughly attention must be paid to sediments and

fouling on the cooling system surface, hence the importance of mechanical cleaning.

The level of free chlorine of 50 mg/l mentioned in the literature is clearly a shock dosage level

that has been applied after an outbreak of LD. Because of the large amount of hypochlorite

involved, it is clear that this treatment is not appropriate as maintenance level in a cooling

tower. In any case, after shock dosing, detoxification of the treated cooling water would be

necessary before discharge and treatment with bisulphite has been commonly applied.

A high maintenance level to prevent Lp-development as much as possible is to be favoured. In

general, oxidising biocides are favoured for killing the Legionella in the water. Slower acting

agents are needed to attack bacteria in the biofilms. This would then require treatment with nonoxidising biocides. Of these the QACs have shown better results than the isothiozolines.

In a recent Dutch report [tm155, Berbee, 1999] some results on reducing the CFU-level in

cooling towers were reported which confirmed that a clear minimum concentration level of

biocides has not yet determined. It was concluded that high levels of biocide were needed to

reduce-concentrations, but only showed a temporary effect. The side effect of elevated levels of

toxic by-products must be realised. Lower water temperature appeared to be more effective than

application of biocides (Table 3.14), but this may not be applicable in every case. Investigations

into the effect of treatment of the protozoa revealed that very high concentrations are needed to

kill the protozoa and that cysts are hardly susceptible to the applied non-oxidising biocides.

Table 3.14: Effects of temperature and biocide treatment on CFU-levels in cooling towers

Quoted from Kusnetsov by [tm155, Berbee, 1999]

Cooling tower Effect of lower

Temp

Biocide conc.

(mg/l) Effect of biocide Remarks

A

T 25oC~105 CFU

to

T 15oC~103 CFU

PHMB, 3, shock Temporarily under

detection limit

B

T 25oC~104 CFU

To

T 15oC~103 CFU

BNPD, 5, shock Temporarily under

detection limit

C n.r. PHMB, 2-250,

Shock Not clear Change to tapwater

D n.r. PHMB, 4-11,

Shock

Temporarily,

104 CFU/l to

103

 CFU/l

E n.r. BNPD, 65-190,

shock

Temporarily,

105

 CFU/l to

103 CFU/l

Notes:

PHMB: polyhexamethylenebiguanidechloride (QAC)

BNPD: bromonitropropaandiol
Chapter 3

Industrial Cooling Systems 117

3.8 Waste from cooling system operation

Little has been reported on the wastes or residues from cooling systems operation. For all

cooling systems, decommissioning of part or all of system can be an issue at some stage.

Retrofitting and replacement of equipment as well as the operating methods result in the

following wastes to be disposed of:

• sludge from pre-treatment of intake water (e.g. decarbonisation), treatment of cooling water

or blowdown from the operation of recirculating wet cooling towers (see Annex XI 3.4);

• hazardous waste (e.g. small containers, spillage) associated with the chemical treatment of

cooling water in wet cooling systems;

• waste water of cleaning operations;

• wastes as a result of retrofitting, replacing or decommissioning of the installation.

3.8.1 Formation of sludges

Sludge formation can occur in the collecting basins of wet cooling systems. In quantitive terms,

more sludge results from the decarbonisation process, if this is practised on site. No particular

measures have been reported on reducing the formation of sludge. Appropriate cooling water

conditioning is likely to reduce settlement of sludge. Currently, chemical composition of the

sludge and the local (or national) legislation will determine the disposal method for sludges. In

some Member States sludges can be returned to the surface water of origin, but in others they

have to be treated in more strictly defined ways.

Sludges and mud deposited on the bottom of water basins of cooling towers can contain cysts or

resistance forms of pathogenic bacteria and protozoa (3.7.3). Pathogenic amoebae and

Legionella pneumophila are found at very high concentrations in sludge collected from

condenser tubes during downtime or in clarifying iron chloride sludge [tm145, Werner and

Pietsch, 1991]. Cysts of Lp are also found in the scale on the fill. As a consequence it is

recommended that the microbiological quality of this type of residue be surveyed prior to the

disposal or the reprocessing of PVC fills. A special treatment may be required if the handling

and reprocessing of these wastes cause a significant health risk.

3.8.2 Residues from cooling water treatment and cleaning operations

Treatment of cooling water (especially for larger systems) nowadays is automatic and in many

cases the substances are kept in containers and tanks and are applied stored, transported and

handled by the supplier.

The same applies to waste water resulting from cleaning operations. Here too, more and more

specialised companies are contracted to do this work.

The generation and disposal of this type of waste however is not typical for industrial cooling

systems. The extent to which it represents an environmental problem is closely related to the

way in which the cooling system is operated, to pre-treatment of the intake water and to the

efficiency of the cooling water treatment. No information has been submitted on this

environmental issue.

3.8.3 Residues as a result of retrofitting, replacing and decommissioning

of the installation

Generally, cooling systems are designed and built for a long service life (up to 20 years and

more). Obviously, the better the way they are operated and maintained the longer their

operational life, but they should also be designed and built for the particular circumstances in

which they are to be used. Particular materials should also be considered for their environmental
Chapter 3

118 Industrial Cooling Systems

impact when applying, decommissioning or replacing parts of a cooling system. The following

examples were reported.

3.8.3.1 Use of plastics

Increasingly, different kind of plastics are applied for cooling tower construction, such as

polyvinylchloride, polypropylene, polyethylene and glass reinforced plastics. Their

characteristics make them highly suitable for application in the often corrosive, highly

demanding environment of a cooling tower. Current experiences have been described in a

technical paper of the German Organisation of Power Plant Operators [tm.., VGB, 2000]. The

use of plastics may be an opportunity for waste reduction, if there is a possibility of recycling

after replacement of plastic elements. No experiences have yet been reported that could

illustrate this.

3.8.3.2 Treatment of timber used for wet cooling tower construction

Timber has been and is used for cooling towers, but has to be treated to ensure its longevity.

Timber used in cooling towers for both packing and support structures can be chemically

treated. Treatment has been and still can be based on CCA (copper sulphate, potassium

dichromate and arsenic pentoxide), because of its ability to remain bound to the timber. It is

claimed that over its operating life only 10% by weight is lost.

Quantification of the extent to which emissions from CCA-treated timber occur into the aquatic

environment cannot be quoted. It has been reported that treated timber, although it has time to

drain, still has considerable amounts of chemicals on the wood surface. They may be washed off

in the initial flush of water in the cooling tower and will be sooner or later be discharged into

the receiving water.

Because CCA contains Cr and As, it seems unlikely that will continue to be used for much

longer. CCA treatment of timber is not a best available technique and is expected to be banned.

Alternative treatments for protection of timber have already been developed and applied. It is

therefore expected that emissions into the surface water resulting from CCA will gradually

reduce.

If CCA-treated wood has to be disposed of - some countries allow controlled disposal in an

appropriate landfill site - as low leaching is expected. In other Member States incineration in an

appropriate installation is preferred, where most of the elements will be retained in the dustfilter.

Determining the most favourable technique for dispose of the CCA-treated timber is beyond the

scope of this BREF-document, but also here the final environmental impact of the different

options needs to be evaluated here.

3.8.3.3 Wet cooling tower fill

As soon as cooling tower fill needs to be replaced it will have to be disposed of. Fills are made

of different materials and this will determine the way in which they will have to be treated. No

data on polluting levels of fill have been reported.

A special case is the use of asbestos paper fill. It has not been possible to assess whether this has

been practised in Europe, but asbestos may have been used in many applications in the past,

including cooling tower construction or cooling tower fill. As the hazards of the use of asbestos

are no longer in doubt, it is no longer used in cooling towers. In old cooling towers some

asbestos may still be found and special measures are required to remove it.

One reference reported an example in which crumbling of asbestos fill occurred over a period of

10-17 years of operation, causing reduced heat exchange. Removal and replacement of the fill

were necessary under severe safety conditions [tm082, Mittendorf, 1990].
Chapter 4

Industrial Cooling Systems 119

4 BEST AVAILABLE TECHNIQUES FOR INDUSTRIAL

COOLING SYSTEMS

4.1 Introduction

In understanding this chapter and its contents, the attention of the reader is drawn back to the

preface of this document and in particular the fifth section of the preface: “How to understand

and use this document”. The techniques and methods and associated emission and/or

consumption levels, or ranges of levels, presented in this chapter have been assessed through an

iterative process involving the following steps:

• identification of the key environmental issues for the process; emphasis in the process of

cooling is clearly on energy efficiency increase (increase of the overall energy efficiency of

the process), on the reduction of emissions to surface water by optimization of cooling

water conditioning;

• examination of the techniques most relevant to address those key issues;

• identification of the best environmental performance levels, on the basis of the available

data in the European Union and world-wide; in most cases performance levels are

considered as installation specific.

• examination of the conditions under which these performance levels were achieved; such as

costs, cross-media effects, main driving forces involved in implementation of this

techniques; generally, price indications of techniques in cooling systems have been reported

to a very limited extent;

• selection of the best available techniques (BAT) and the associated emission and/or

consumption levels for this sector in a general sense all according to Article 2(11) and

Annex IV of the Directive.

Expert judgement by the European IPPC Bureau and the relevant Technical Working Group

(TWG) has played a key role in each of these steps and in the way in which the information is

presented here.

On the basis of this assessment, techniques, and as far as possible emission and consumption

levels associated with the use of BAT, are presented in this chapter that are considered to be

appropriate to the relevant cooling systems and in many cases reflect current performance of

some installations applied. Where emission or consumption levels “associated with best

available techniques” are presented, this is to be understood as meaning that those levels

represent the environmental performance that could be anticipated as a result of the application,

under the process and site-specific conditions, of the techniques described, bearing in mind the

balance of costs and advantages inherent within the definition of BAT. However, they are

neither emission nor consumption limit values nor minimum required performance levels and

should not be understood as such. In some cases it may be technically possible to achieve better

emission or consumption levels, but due to the costs involved or cross-media considerations

they are not considered to be appropriate as a BAT for the relevant cooling configuration.

However, such levels or applications may be considered to be justified in more specific cases

where there are special driving forces.

The emission and consumption levels associated with the application of BAT have to be seen

together with any specified reference condition (e.g. climate, site limitations).

The concept of “levels associated with BAT” described above is to be distinguished from the

term “achievable level” used elsewhere in this document. Where a level is described as

“achievable” using a particular technique or combination of techniques, this should be

understood to mean that the level may be expected to be achieved over a substantial period of

time in a well maintained and operated installation or process using those techniques.
Chapter 4

120 Industrial Cooling Systems

Where available, data concerning costs have been given together with the description of the

techniques presented in the previous chapter or the Annexes. These give a rough indication

about the magnitude of costs involved. However, the actual cost of applying a technique will

depend strongly on the specific situation regarding, for example, taxes, fees, and the technical

characteristics of the installation concerned. It is not possible to evaluate such site-specific

factors fully in this document. In the absence of data concerning costs, conclusions on economic

viability of techniques are drawn from observations on existing installations.

It is intended that the general BAT in this chapter are a reference point against which to judge

the current performance of an existing installation or to judge a proposal for a new installation.

In this way they will assist in the determination of appropriate "BAT-based" conditions for the

installation or in the establishment of general binding rules under Article 9(8). It is foreseen that

new installations can be designed to perform at or even better than the general BAT levels

presented here. It is also considered that existing installations could move towards the general

BAT levels or do better, subject to the technical and economic applicability of the techniques in

each case.

While the BREFs do not set legally binding standards, they are meant to give information for

the guidance of industry, Member States and the public on achievable emission and

consumption levels when using specified techniques. The appropriate limit values for any

specific case will need to be determined taking into account the objectives of the IPPC Directive

and the local considerations.

4.2 A horizontal approach to defining BAT for cooling systems

Before summarising the BAT conclusions in this chapter, a short explanation is given on how

the horizontal character of this BREF should be interpreted.

In a horizontal approach it is assumed that the environmental aspects of the applied techniques

and the associated reduction measures can be assessed and that generic BAT can be identified

that are independent of the industrial processes in which techniques are applied.

Industrial cooling systems are an integrated part of the industrial process to be cooled. The

cooling systems within the scope of this document are used in many of the industrial sectors

under the scope of IPPC. Consequently, the variety of applications, techniques and operational

practices is enormous. Additionally, the thermodynamic character of the process leads to further

variations in performance and consequently in the environmental effects.

Due to this large variation, comparisons between techniques leading to general conclusions on

BAT are difficult. The identification of a general preventive approach is considered to be

possible, based on practical experience with reduction of emissions from cooling systems.

In this preventive approach or, primary BAT-approach, attention is firstly given to the process

to be cooled. The design and the construction of the cooling system are an essential second step,

in particular for new installations. Finally, changes of equipment and the way in which the

cooling system should be operated will address new installations, but are particularly important

in existing systems, where technological options are considerably limited and cost-intensive.

Careful evaluations must be performed case by case.
Chapter 4

Industrial Cooling Systems 121

4.2.1 Integrated heat management

4.2.1.1 Industrial cooling = Heat management

Cooling of industrial processes can be considered as heat management and is part of the total

energy management within a plant. The amount and level of heat to be dissipated requires a

certain level of cooling systems performance. This performance level will in turn affect the

system configuration, design and operation and consequently the cooling systems’

environmental performance (direct impact). Reversibly, the cooling performance will also affect

the overall efficiency of the industrial process (indirect impact). Both impacts, direct and

indirect, need to be balanced, taking into account all variables. Every change in the cooling

system has to be considered against the consequences it may have for this balance.

This concept can be used as a starting point to formulate the first principle of BAT for cooling

systems. BAT for all installations is an integrated approach to reduce the environmental impact

of industrial cooling systems maintaining the balance between both the direct and indirect

impacts. In other words, the effect of an emission reduction has to be balanced against the

potential change in the overall energy efficiency. There is currently no minimum ratio in terms

of the environmental benefits and the possible loss in overall energy efficiency that can be used

as a benchmark to arrive at techniques that can be considered BAT. Nevertheless, this concept

can be used to compare alternatives (Chapter 3.2 and Annex II).

4.2.1.2 Reduction of the level of heat discharge by optimization of

internal/external heat reuse

A preventive approach should start with the industrial process requiring heat dissipation and aim

to reduce the need for heat discharge in the first place. In fact, discharge of heat is wasting

energy and as such not BAT. Reuse of heat within the process should always be a first step in

the evaluation of cooling needs. Process-integrated energy measures are outside the scope of

this document, but reference is made to other BAT Reference Documents drafted in the

framework of IPPC describing options for energy measures.

In a greenfield situation, assessment of the required heat capacity can only be BAT if it is the

outcome of maximum use of the internal and external available and applicable options for

reuse of excess heat.

In an existing installation, optimizing internal and external reuse and reducing the amount

and level of heat to be discharged must also precede any change to the potential capacity of the

applied cooling system. Increasing the efficiency of an existing cooling system by improving

systems operation must be evaluated against an increase of efficiency by technological

measures through retrofit or technological change. In general and for large existing cooling

systems, the improvement of the systems operation is considered to be more cost effective than

the application of new or improved technology and can therefore be regarded as BAT.

4.2.1.3 Cooling system and process requirements

Once the level and amount of waste heat generated by the process is established and no further

reduction of waste heat can be achieved, an initial selection of a cooling system can be made in

the light of the process requirements discussed in Chapter 1. Every process has its unique

combination of requirements, where the level of control of the process, process reliability and

safety play an important role. This makes it almost impossible at this stage to make a first

characterisation of BAT, but the following conclusions can be drawn with respect to a number

of process characteristics.
Chapter 4

122 Industrial Cooling Systems

The application of the ambient temperature levels is based on the experiences in Europe in

applying cooling systems under different climatic conditions. Generally, dry bulb temperatures

do not justify cooling away low level waste, heat and water-cooling is preferred. But in areas

with low average dry bulb temperatures dry air-cooling is applied to cool down to lower process

temperatures (after options for reuse have been explored). Water-cooling, if sufficient water is

available, can then dissipate the residual amount of waste heat.

Hazardous process substances, which involve a high environmental risk to the aquatic

environment in case of leakage, should be cooled by means of indirect cooling systems to

prevent an uncontrollable situation.

The selection of a cooling configuration should be based on a comparison between the different

feasible alternatives within all requirements of the process. Process requirements are for

example control of chemical reactions, reliability of process performance and maintenance of

required safety levels. The aim is to minimise the indirect impact of the selected alternative. For

each alternative the environmental performances can be best compared if expressed in direct

and indirect use of energy (kWe) per unit of energy discharged (kWth). Another way to compare

configurations is to express the change in direct energy use (kWe) of the cooling system and the

change in production level of the process in tonnes, both per unit of energy discharged (kWth).

A change in cooling technology to reduce the environmental impact can only be considered

BAT if the efficiency of cooling is maintained at the same level or, even better, at an increased

level.

Table 4.1: Examples of process requirements and BAT

Process

characteristics Criteria Primary BATapproach Remark Reference

Level of dissipated heat

high (> 60oC)

Reduce use of water

and chemicals and

improve overall

energy efficiency

(Pre-) cooling

with dry air

Energy efficiency

and size of cooling

system are limiting

factors

Section

1.1/1.3

Level of dissipated heat

medium (25-60oC)

Improve overall

energy efficiency Not evident Site-specific Section1.1/1.3

Level of dissipated heat

low (<25oC)

Improve overall

energy efficiency Water cooling Site selection Section1.1/1.3

Low and medium heat

level and capacity

Optimum overall

energy efficiency

with water saving

and visible plume

reduction

Wet and hybrid

cooling system

Dry cooling less

suitable due to

required space and

loss of overall

energy efficiency

Section 1.4

Hazardous substances

to be cooled involving

high environmental risk

Reduction of risk of

leakage

Indirect cooling

system

Accept an increase

in approach

Section 1.4

and

Annex VI
Chapter 4

Industrial Cooling Systems 123

4.2.1.4 Cooling system and site requirements

The site-imposed limits apply particularly to new installations, where a cooling system must still

be selected. If the required heat discharge capacity is known it may influence the selection of an

appropriate site. For temperature-sensitive processes it is BAT to select the site with the

required availability of cooling water.

For many reasons new installations are not always erected on a site that is preferred from a

cooling technology point of view, whereas for both new and existing installations the site

characteristics are clear once the site is known. The most important thermodynamic

characteristic of a site is its annual climatic pattern described by the dry and wet bulb

temperatures.

Table 4.2: Examples of site characteristics and BAT

Characteristics

of site Criteria Primary BATapproach Remarks Reference

Climate Required design

temperature

Assess variation in wet

and dry bulb T

With high dry bulb

T dry air cooling

generally has lower

Energy efficiency

Section

1.4.3

Space Restricted

surface on-site

(Pre-assembled) Roof

type constructions

Limits to size and

weight of the

cooling system

Section

1.4.2

Surface water

availability

Restricted

availability Recirculating systems Wet, dry or hybridfeasible

Section 2.3

and 3.3

Sensitivity of

receiving water

body for thermal

loads

Meet capacity to

accommodate

thermal load

- Optimise level of

heat reuse

- Use recirculating

systems

- Site selection (new

cooling system)

Section 1.1

Restricted

availability of

groundwater

Minimisation of

groundwater use

Air cooling if no

adequate alternative

water source is available

Accept energy

penalty Section 3.3

Coastal area Large capacity

> 10 MWth

Once-through systems

Avoid mixing of

local thermal

plume near intake

point, e.g. by deep

water extraction

below mixing zone

using temperature

stratification

Section

1.2.1 /

Section 3.2

/Annex XI.3

Specific site

requirements

In case of

obligation

forplume

reduction and

reduced tower

height

Apply hybrid cooling

system

Accept energy

penalty Ch.2
Chapter 4

124 Industrial Cooling Systems

Other characteristics identified are space, water availability to cool and to discharge and the

surrounding sensitive areas (urban and industrial). With respect to groundwater, it can be BAT

to apply a dry cooling system following the principle to minimise the use of groundwater,

particularly in those areas where depletion of aquifers cannot be ruled out.

In Table 4.2 BAT examples are shown that have been identified for a few site characteristics.

4.2.2 Application of BAT in industrial cooling systems

In Chapter 1 the outline of a preventive approach is presented showing how a step-by-step

evaluation of all constraints can lead to what may be called “Best available cooling technique”.

Within the framework of this approach, Chapter 1 and Chapter 3 and the associated Annexes

discuss the factors and offer techniques involved in the identification of potential BAT for the

major cooling configurations using water and/or air. The optimization of a cooling system to

reduce its environmental impact is a complex exercise and not an exact mathematical

comparison. In other words, combining techniques selected from the BAT-tables does not lead

to a BAT cooling system. The final BAT solution will be a site-specific solution. However, it

is believed that, based on experience in industry, conclusions can be drawn on BAT, in

quantified terms where possible.

In Chapter 3 options for reducing emissions into the environment have been presented based on

the information submitted by the TWG. For each environmental issue and for each relevant

cooling configuration an attempt has been made to identify a general approach and arrive at

BAT. Some techniques are described in more detail in the Annexes. Emphasis is clearly on the

water-related problems with a focus on reduction of the application of biocides and blacklisted

substances.

The proposed techniques are applied techniques. They have proven to be effective, although

quantification is difficult and they may create unrealistic expectations. It can be assumed that all

measures proposed as BAT, and which are not entirely dependant on the local situation, can be

considered for new systems. With respect to existing installations, care must be taken as the

assessment is more difficult where options are limited and depend on a multitude of (process)

factors. There do not seem to be many obstacles to implementation of operational measures in

existing cooling systems, unless the technological design limits the number of options for

modification.

In Tables 4.3 to 4.12 techniques are presented that are considered BAT, following on from the

primary BAT-approach for:

• increasing the overall energy efficiency,

• reduction of use of water and of cooling water additives,

• reduction of emissions to air and water,

• reduction of noise,

• reduction of entrainment of aquatic organisms and

• reduction of biological risks.

No clear BAT has been identified on the reduction of waste or techniques to handle waste

avoiding environmental problems, such as contamination of soil and water, or air in the case of

incineration.

For each environmental issue the consequences for other media of the application of a reduction

technique have been identified. Generally speaking every change made to a cooling system must

be carefully balanced against the associated effects and in this sense the optimisation of

industrial cooling is a cross-media issue.
Chapter 4

Industrial Cooling Systems 125

For some measures BAT-values have been identified. However, addressing the application of

different cooling techniques in a multitude of varying process conditions does not allow for

clear associated levels. In those cases a qualitative description is given.

For new cooling installations it is BAT to start identifying reduction measures in the design

phase, applying equipment with low energy requiring requirement and by choosing the

appropriate material for equipment in contact with the process substance and/or the cooling

water. In this sense the following quotation is exemplary: “in practice... attention to design,

layout and maintenance of the cooling water system has a relatively low priority compared to

the environmental consequences of a poorly designed and/or operated cooling water system.

Since little attention is paid to design factors, treatments often have to make up for bad design,

and therefore need to be chosen in such a way that they minimize risks of fouling. Few changes

of this attitude are to be expected as long as there is a low level of awareness of the long-term

costs of operating and maintaining poorly designed CWS” [tm005, Van Donk and Jenner,

1996].

If dry air cooling systems are the preferred option, measures are primarily related to reduction of

direct energy consumption and noise emissions and the optimization of size with respect to the

required cooling surface.

For existing installations, technological measures can be BAT under certain circumstances.

Generally, a change in technology is cost-intensive where overall efficiency must be

maintained. Cost evaluation should then compare investment costs of the change versus the

change in operational costs and validate the reduction effect versus other environmental

consequences. For example, it would need a comparison between the environmental effect of

recirculating the cooling water - requiring the application of biocidal water treatment - against a

once-through system without biocides, but a large heat emission to the aquatic environment.

In the case of pre-assembled off-the-peg cooling towers, a change in technology seems feasible

both technically and economically. No comparable data have been submitted that can support

this, but supplier experience is that it is relatively easy to change small size cooling towers, for

example, from a closed recirculating wet to a closed recirculating hybrid or wet/dry

configuration. This would not need major process modifications or constructionwork.

For large custom-designed towers that are erected on-site, technological changes are not easy to

make. A different technology generally means a completely new cooling tower.

For existing wet cooling systems, where the focus is largely on environmental measures to

reduce water use and to emissions of chemicals to the surface water, BAT has not so much

technological but rather an operational character. Monitoring, operation and maintenance are the

key issues here.

4.3 Reduction of energy consumption

4.3.1 General

It is BAT in the design phase of a cooling system:

• To reduce resistance to water and airflow

• To apply high efficiency/low energy equipment

• To reduce the amount of energy demanding equipment (Annex XI.8.1)

• To apply optimised cooling water treatment in once-through systems and wet cooling

towers to keep surfaces clean and avoid scaling, fouling and corrosion.
Chapter 4

126 Industrial Cooling Systems

For each individual case a combination of the above-mentioned factors should lead to the lowest

attainable energy consumption to operate a cooling system. Concerning BAT a number of

techniques/approaches have been identified.

4.3.2 Identified reduction techniques within the BAT-approach

In an integrated approach to cooling an industrial process, both the direct and indirect use of

energy are taken into account. In terms of the overall energy efficiency of an installation, the use

of a once-through systems is BAT, in particular for processes requiring large cooling capacities

(e.g. > 10 MWth). In the case of rivers and/or estuaries once-through can be acceptable if also:

• extension of heat plume in the surface water leaves passage for fish migration;

• cooling water intake is designed aiming at reduced fish entrainment;

• heat load does not interfere with other users of receiving surface water.

For power stations, if once-through is not possible, natural draught wet cooling towers are most

energy-efficient than other cooling configurations, but application can be restricted because of

the visual impact of their overall height.

Table 4.3: BAT for increasing overall energy efficiency

Relevance Criterion Primary BAT approach Remarks Reference

Large cooling

capacity

Overall energy

efficiency

Select site for once-through

option

See text above

table

Section

3.2

All systems Overall energy

efficiency

Apply option for variable

operation

Identify required

cooling range

Section

1.4

All systems Variable

operation

Modulation of air/ water

flow

Avoid instability

cavitation in

system (corrosion

and erosion)

All wet systems

Clean circuit/

exchanger

surfaces

Optimised water treatment

and pipe surface treatment

Requires adequate

monitoring

Section

3.4

Once-through

systems

Maintain

cooling

efficiency

Avoid recirculation of warm

water plume in rivers and

minimise it in estuaries and

on marine sites

Annex XII

All cooling

towers

Reduce specific

energy

consumption

Apply pumping heads and

fans with reduced energy

consumption
Chapter 4

Industrial Cooling Systems 127

4.4 Reduction of water requirements

4.4.1 General

For new systems the following statements can be made:

• In the light of the overall energy balance, cooling with water is most efficient;

• For new installations a site should be selected for the availability of sufficient quantities of

(surface) water in the case of large cooling water demand;

• The cooling demand should be reduced by optimising heat reuse;

• For new installations a site should be selected for the availability of an adequate receiving

water, particularly in case of large cooling water discharges;

• Where water availability is limited, a technology should be chosen that enables different

modes of operation requiring less water for achieving the required cooling capacity at all

times;

• In all cases recirculating cooling is an option, but this needs careful balancing with other

factors, such as the required water conditioning and a lower overall Energy efficiency.

For existing water cooling systems, increasing heat reuse and improving operation of the system

can reduce the required amount of cooling water. In the case of rivers with limited availability

of surface water, a change from a once-through system to a recirculating cooling systems is a

technological option and may be considered BAT.

For power stations with large cooling capacities, this is generally considered as a cost-intensive

exercise requiring a new construction. Space requirements must be taken into account.

4.4.2 Identified reduction techniques within the BAT-approach

Table 4.4: BAT for reduction of water requirements

Relevance Criterion Primary BAT

approach Remarks Ref.

Reduction of need for

cooling

Optimisation of heat

reuse

Ch.1

Reduction of use of limited

sources

Use of groundwater is

not BAT

Site-specific in

particular for

existing systems

Ch.2

Reduction of water use Apply recirculating

systems

Different demand on

water conditioning Ch.2/3.3

Reduction of water use,

where obligation for plume

reduction and reduced

tower height

Apply hybrid cooling

system

Accept energy

penalty

Ch.2.6/

3.3.1.2

All wet cooling

systems

Where water (make-up

water) is not available

during (part of) process

period or very limited

(drought-stricken areas)

Apply dry cooling Accept energy

penalty

Section 3.2

and 3.3

Annex

XII.6

All

recirculating

wet and wet/dry

cooling systems

Reduction of water use Optimization of cycles

of concentration

Increased demand

on conditioning of

water, such as use of

softened make-up

water

Section 3.2

and section

XI
Chapter 4

128 Industrial Cooling Systems

Application of dry air-cooling has been suggested on a number of occasions. If the overall

Energy efficiency is taken into account, dry air-cooling is less attractive than wet cooling. With

this the dry technology is not disqualified. For shorter lifetime periods it was calculated that the

differences in costs between dry and wet become less than for longer lifetime periods. When

costs for water and water treatment are taken into account, differences also become smaller. Dry

cooling can be recommended in certain circumstances and for precooling at higher temperature

levels, where excessive water would be needed.

4.5 Reduction of entrainment of organisms

4.5.1 General

The adaptation of water intake devices to lower the entrainment of fish and other organisms is

highly complex and site-specific. Changes to an existing water intake are possible but costly.

From the applied or tested fish protection or repulsive technologies, no particular techniques can

yet be identified as BAT. The local situation will determine which fish protection or repulsive

technique will be BAT. Some general applied strategies in design and position of the intake can

be considered as BAT, but these are particularly valid for new systems.

On the application of sieves it should be noted that costs of disposal of the resulting organic

waste collected from the sieves can be considerable.

4.5.2 Identified reduction techniques within the BAT-approach

Table 4.5: BAT for reduction of entrainment

Relevance Criterion Primary BAT approach Remarks Ref.

Appropriate position

and design of intake

and selection of

protection technique

Analysis of the biotope in

surface water source

Also critical areas,

such as spawning

grounds, migration

areas and fish

nurseries

Section

3.3.3 and

Annex

XII.3.3

All once-through

systems or cooling

systems with

intakes of surface

water Construction of

intake channels

Optimise water velocities

in intake channels to limit

sedimentation; watch for

seasonal occurrence of

macrofouling

Section

3.3.3

4.6 Reduction of emissions to water

4.6.1 General BAT approach to reduce heat emissions

Whether heat emissions into the surface water will have an environmental impact strongly

depends on the local conditions. Such site conditions have been described, but do not lead to a

conclusion on BAT in general terms.

Where, in practice, limits to heat discharge were applicable, the solution was to change from

once-through technology to open recirculating cooling (open wet cooling tower). From the
Chapter 4

Industrial Cooling Systems 129

available information, and considering all possible aspects, care must be taken in concluding

that this can be qualified as BAT. It would need to balance the penalty increase in overall

energy efficiency of applying a wet cooling tower (Chapter 3.2) against the effect of reduced

environmental impact of reduced heat discharge. In a fully integrated assessment at the level of

a river catchment, this could for example include the raised overall efficiency levels of other

processes using the same, but now colder, water source, which becomes available because there

is no longer a large warm water discharge into it.

Where the measures generally aim at reducing the ∆T of the discharged cooling water, a few

conclusions on BAT can be drawn. Pre-cooling (Annex XII) has been applied for large power

plants where the specific situation requires this, e.g. to avoid raised temperature of the intake

water.

Discharges will have to be limited with reference to the constraints of the requirements of

Directive 78/659/EEC for fresh water sources. The criteria are summarised in Table 3.6.

Reference is made to a provision in Article 11 of this directive regarding derogation of the

requirements in certain circumstances.

4.6.2 General BAT approach to reduce chemical emissions to water

Prevention and control of chemical emissions resulting from cooling systems have received the

most attention in Member States’ policies and industry. Next to heat discharge they are still

considered to be the most important issue in cooling.

Referring to the statement that 80% of the environmental impact is decided on the design table,

measures should be taken in the design phase of wet cooling system using the following order of

approach:

• identify process conditions (pressure, T, corrosiveness of substance),

• identify chemical characteristics of cooling water source,

• select the appropriate material for heat exchanger combining both process conditions and

cooling water characteristics,

• select the appropriate material for other parts of the cooling system,

• identify operational requirements of the cooling system,

• select feasible cooling water treatment (chemical composition) using less hazardous

chemicals or chemicals that have lower potential for impact on the environment (Section

3.4.5, Annex VI and VIII)

• apply the biocide selection scheme (Chapter 3, Figure 3.2) and

• optimise dosage regime by monitoring of cooling water and systems conditions.

This approach intends to reduce the need for cooling water treatment in the first place. For

existing systems technological changes or changes to the equipment are difficult and generally

cost-intensive. Focus should be on the operation of the systems using monitoring linked to

optimized dosage. A few examples of techniques with good performances have been identified.

They are generally applicable for certain categories of systems, they are considered cost

effective and do not need large changes to the cooling installation.

After reducing the sensitivity of the cooling system to fouling and corrosion, treatment may still

be needed to maintain an efficient heat exchange. Selecting cooling water additives less harmful

to the aquatic environment and to applying them in the most efficient way is then the next step.

With respect to the selection of chemicals, it has been concluded that a ranking of treatments

and the chemicals of which they are composed is difficult if not impossible to carry out in a

general way and would be unlikely to lead to BAT conclusions. Due to the large variation in

conditions and treatments only a site-by-site assessment will lead to the appropriate solution.
Chapter 4

130 Industrial Cooling Systems

Such an assessment and its constituent parts could represent an approach that can be considered

BAT.

This approach is offered in this BREF and consists of a tool that can assist in a first ranking of

selected chemicals and of an approach to assess biocides, linking the requirements of the

cooling system to requirements of the receiving aquatic ecosystem (Annex VIII). The approach

aims at minimising the impact of cooling water additives and, in particular, biocides. The

Biocidal Products Directive 98/8/EC (BPD) and the Water Framework Directive (WFD) form

the key building blocks for this approach. It is essential to use PEC and PNEC values for the

different substances, where the PEC/PNEC ratio could function as a yardstick for BAT

determination.

On the application of specific substances, much experience has been obtained in once-through

systems with chlorine-derived components (in particular hypochlorite, chloramine) and

chlorine/bromine combinations, as well as with the application of reduced concentration levels.

The same applies to the use of biocides for conditioning of recirculating systems. Treatments for

these systems are often multisubstances. It is clear that some components or substances can be

identified as not BAT or should not be applied at all. A general approach to select the

appropriate biocide will include local aspects, such as the water quality objectives of the

receiving surface water.
Chapter 4

Industrial Cooling Systems 131

4.6.3 Identified reduction techniques within the BAT-approach

4.6.3.1 Prevention by design and maintenance

Table 4.6: BAT for reduction of emissions to water by design and maintenance techniques

Relevance Criterion Primary BAT approach Remarks Reference

Apply less

corrosion-sensitive

material

Analysis of corrosiveness

of process substance as

well as of cooling water to

select the right material

Ch.3.4

All wet cooling

systems

Reduction of fouling

and corrosion

Design cooling system to

avoid stagnant zones

Annex

XI.3.3.2.1

Shell&tube heat

exchanger

Design to facilitate

cleaning

Cooling water flow inside

tube and heavy fouling

medium on tube side

Depending on

design, process T

and pressure

Annex III.1

Reduce corrosionsensitiveness

Application of Ti in

condensers using seawater

or brackish water

Annex XII

Reduce corrosionsensitiveness

Application of low

corrosion alloys (Stainless

Steel with high pitting

index or Copper Nickel)

Change to low

corrosion alloys can

affect formation of

pathogens

Annex

XII.5.1

Condensers of

power plants

Mechanical cleaning

Use of automated cleaning

systems with foam balls or

brushes

In addition

mechanical cleaning

and high water

pressure may be

necessary

Annex

XII.5.1

Reduce deposition

(fouling) in

condensers

Water velocity > 1.8 m/s

for new equipment and 1.5

m/s in case of tube bundle

retrofit

Depending on

corrosion sensitivity

of material, water

quality and surface

treatment

Annex

XII.5.1

Reduce deposition

(fouling) in heat

exchangers

Water velocity > 0.8 m/s

Depending on

corrosion sensitivity

of material, water

quality and surface

treatment

Annex

XII.3.2

Condensers and

heat exchangers

Avoid clogging

Use debris filters to

protect the heat

exchangers where

clogging is a risk

Annex XII
Chapter 4

132 Industrial Cooling Systems

Table 4.6 (continued): BAT for reduction of emissions to water by design and maintenance

techniques

Relevance Criterion Primary BAT approach Remarks Reference

Reduce corrosionsensitiveness

Apply carbon steel in

cooling water systems if

corrosion allowance can

be met

Not for brackish

water

Annex

IV.1

Reduce corrosionsensitiveness

Apply reinforced glass

fibre plastics, coated

reinforced concrete or

coated carbon steel in case

of underground conduits

Annex

IV.2

Once-through

cooling system

Reduce corrosionsensitiveness

Apply Ti for tubes of

shell&tube heat exchanger

in highly corrosive

environment orhigh

quality stainless steel with

similar performance

Ti not in reducing

environment,

optimised biofouling

control may be

necessary

Annex

IV.2

Reduce fouling in

salt water condition

Apply fill that is open low

fouling with high load

support

Annex IV.4

Open wet

cooling towers

Avoid hazardous

substances due to

anti-fouling

treatment

CCA treatment of wooden

parts or TBTO containing

paints is not BAT

Section 3.4

Annex IV.4

Natural draught

wet cooling

towers

Reduce anti-fouling

treatment

Apply fill under

consideration of local

water quality (e.g. high

solid content, scale)

Annex

XII.8.3
Chapter 4

Industrial Cooling Systems 133

4.6.3.2 Control by optimised cooling water treatment

Table 4.7: BAT for reduction of emissions to water by optimised cooling water treatment

Relevance Criterion Primary BAT approach Remarks Reference

Reduce additive

application

Monitoring and control of

cooling water chemistry

Section

3.4 and

Annex

XI.7.3

All wet systems

Use of less

hazardous

chemicals

It is not BAT to use

• chromium compounds

• mercury compounds

• organometallic

compounds (e.g.

organotin compounds)

• mercaptobenzothiazole

• shock treatment with

biocidal substances

other than chlorine,

bromine, ozone and

H2O2

Section

3.4/

Annex VI

Once-through

cooling system and

open wet cooling

towers

Target biocide

dosage

To monitor macrofouling

for optimising biocide

dosage

Annex

XI.3.3.1.1

Limit application

of biocides

With sea water temperature

below 10-12oC no use of

biocides

In some areas

winter treatment

may be needed

(harbours)

Annex V

Reduction of FO

emission

Use of variation of

residence times and water

velocities with an associated

FO or FRO-level of 0.1 mg/l

at the outlet

Not applicable

for condensers

Ch.3.4

Annex

XI.3.3.2

Emissions of free

(residual) oxidant

FO or FRO ≤ 0.2 mg/l at the

outlet for continuous

chlorination of sea water

Daily (24h)

average value

Annex

XI.3.3.2

Emissions of free

(residual) oxidant

FO or FRO ≤ 0.2 mg/l at the

outlet for intermittent and

shock chlorination of sea

water

Daily (24h)

average value

Annex

XI.3.3.2

Emissions of free

(residual) oxidant

FO or FRO ≤ 0.5 mg/l at the

outlet for intermittent and

shock chlorination of sea

water

Hourly average

value within one

day used for

process control

requirements

Annex

XI.3.3.2

Once-through

cooling system

Reduce amount of

OX-forming

compounds in fresh

water

Continuous chlorinating in

fresh water is not BAT

Ch.3.4

Annex XII
Chapter 4

134 Industrial Cooling Systems

Table 4.7 continued: BAT for reduction of emissions to water by optimised cooling water treatment

Relevance Criterion Primary BAT approach Remarks Reference

Reduce amount of

hypochlorite

Operate at 7 ≤ pH ≤ 9 of

the cooling water Annex XI

Reduce amount of

biocide and reduce

blowdown

Application of sidestream biofiltration is

BAT

Annex

XI.3.1.1

Reduce emission of

fast hydrolyzing

biocides

Close blowdown

temporarily after dosage

Section

3.4

Open wet

cooling towers

Application of

ozone

Treatment levels of ≤ 0.1

mg O3/l

Assessment of total

cost against the

application of other

biocides

Annex

XI.3.4.1

4.7 Reduction of emissions to air

4.7.1 General approach

Comparatively, air emissions from cooling towers have not been given much attention, except

for the effects of plume formation. From some reported data it is concluded that levels are

generally low but that these emissions should not be neglected.

Lowering concentration levels in the circulating cooling water will obviously affect the potential

emission of substances in the plume. Some general recommendations can be made which have a

BAT-character.
Chapter 4

Industrial Cooling Systems 135

Avoid

4.7.2 Identified reduction techniques within the BAT-approach

Table 4.8: BAT for reduction of emissions to air

Relevance Criterion Primary BAT approach Remarks Reference

Avoid plume

reaching ground

level

Plume emission at

sufficient height and with a

minimum discharge air

velocity at the tower outlet

Chapter

3.5.3

All wet cooling

towers

Avoid plume

formation

Application of hybrid

technique or other plume

suppressing techniques

such as reheating of air

Need local

assessment

(urban areas,

traffic)

Chapter

3.5.3

Use of less

hazardous material

Use of asbestos, or wood

preserved with CCA (or

similar) or TBTO is not

BAT

Chapter

3.8.3

All wet cooling

towers

Avoid affecting

indoor air quality

Design and positioning of

tower outlet to avoid risk

of air intake by air

conditioning systems

Is expected to be

less important

for large natural

draught CT with

considerable

height

Section 3.5

All wet cooling

towers

Reduction of drift

loss

Apply drift eliminators

with a loss <0.01% of total

recirculating flow

Low resistance

to airflow to be

maintained

Section 3.5

and XI.5.1

4.8 Reduction of noise emissions

4.8.1 General

Noise emissions have local impact. Noise emissions of cooling installations are part of the total

noise emissions from the site. A number of primary and secondary measures have been

identified that can be applied to reduce noise emissions where necessary. The primary measures

change the sound power level of the source, where the secondary measures reduce the emitted

noise level. The secondary measures in particular will lead to pressure loss, which has to be

compensated by extra energy input, which reduces overall energy efficiency of the cooling

system. The ultimate choice for a noise abatement technique will be an individual matter, as will

the resulting associated performance level. The following measures and minimum reduction

levels are considered as BAT.
Chapter 4

136 Industrial Cooling Systems

4.8.2 Identified reduction techniques within the BAT-approach

Table 4.9: BAT for the reduction of noise emissions

Cooling system Criterion Primary BAT approach

Associated

reduction

levels

Ref.

Reduce noise of

cascading water at

air inlet

Different techniques

available ≥ 5 dB(A) Section 3.6

Natural draught

cooling towers Reduce noise

emission around

tower base

E.g application of earth

barrier or noise

attenuating wall

< 10 dB(A) Section 3.6

Section 3.6

Reduction of fan

noise

Apply low noise fan with

characteristics, e.g.:

- larger diameter fans;

- Reduced tip speed

(≤ 40 m/s)

< 5 dB(A)

Section 3.6

Optimised diffuser

design

Sufficient height or

installation of sound

attenuators

Variable Section 3.6

Mechanical draught

cooling towers

Noise reduction

Apply attenuation

measures to inlet and

outlet

≥ 15 dB(A) Section 3.6

4.9 Reduction of risk of leakage

4.9.1 General approach

To reduce the risk of leakage, attention must be paid to the design of the heat exchanger, the

hazardousness of the process substances and the cooling configuration. The following general

measures to reduce the occurrence of leakages can be applied:

• select material for equipment of wet cooling systems according to the applied water quality;

• operate the system according to its design,

• if cooling water treatment is needed, select the right cooling water treatment programme,

• monitor leakage in cooling water discharge in recirculating wet cooling systems by

analysing the blowdown.
Chapter 4

Industrial Cooling Systems 137

4.9.2 Identified reduction techniques within the BAT-approach

Table 4.10: BAT to reduce the risk of leakage

Relevance1) Criterion Primary BAT approach Remarks Reference

All heat

exchangers Avoid small cracks ∆T over heat exchangerof ≤ 50oC

Technical

solution for

higher ∆T on

case-by-case

basis

Annex III

Operate within

design limits Monitor process operation AnnexIII.1 Shell&tube heat

exchanger Strength of tube/tube

plate construction

Apply welding

technology

Welding not

always

applicable

Annex

III.3

Equipment Reduce corrosion T of metal on cooling

water side < 60oC

Temp. affects

inhibition of

corrosion

Annex

IV.1

VCI score of 5-8 Direct system Pcooling water >

Pprocess and monitoring

Immediate

measures in

case of leakage

Annex VII

VCI score of 5-8

Direct system Pcooling water

= Pprocess and automatic

analytical monitoring

Immediate

measures in

case of leakage

Annex VII

VCI score of ≥ 9

Direct system Pcooling water >

Pprocess and automatic

analytical monitoring

Immediate

measures in

case of leakage

Annex VII

VCI score of ≥ 9

Direct system with heat

exchanger of highly

anticorrosive material/

automatic analytical

monitoring

Automatic

measures in

case of leakage

Annex VII

VCI score of ≥ 9

Change technology

- indirect cooling

- recirculating cooling

- air cooling

Annex VII

Cooling of dangerous

substances

Always monitoring of

cooling water Annex VII

Once-through

cooling systems

Apply preventive

maintenance

Inspection by means of

eddy current

Other nondestructive

inspection

techniques are

available

Recirculating

cooling systems

Cooling of dangerous

substances

Constant monitoring of

blowdown

1) Table not applicable for condensers

4.10 Reduction of biological risk

4.10.1 General approach

To reduce the biological risk due to cooling systems operation, it is important to control

temperature, maintain the system on a regular basis and avoid scale and corrosion. All measures

are more or less within the good maintenance practice that would apply to a recirculating wet

cooling system in general. The more critical moments are start-up periods, where systems’

operation is not optimal, and standstill for repair or maintenance. For new towers consideration
Chapter 4

138 Industrial Cooling Systems

must be given to design and position with respect to surrounding sensitive objects, such as

hospitals, schools and accommodation for elderly people.

4.10.2 Identified reduction techniques within the BAT-approach

Table 4.11: BAT to reduce biological growth

Cooling system Criterion Primary BAT approach Remarks Reference

Reduce algae

formation

Reduce light energy

reaching the cooling

water

Section

3.7.3

Reduce biological

growth

Avoid stagnant zones

(design) and apply

optimized chemical

treatment

Cleaning after

outbreak

A combination of

mechanical and chemical

cleaning

Section

3.7.3

All wet

recirculating

cooling systems

Control of pathogens

Periodic monitoring of

pathogens in the cooling

systems

Section

3.7.3

Open wet cooling

towers

Reduce risk of

infection

Operators should wear

nose and mouth

protection (P3-mask)

when entering a wet

cooling tower

If spraying

equipment is

on or when

high-pressure

cleaning

Section

3.7.3
Chapter 5

Industrial Cooling Systems 139

5 CONCLUDING REMARKS

5.1 Timing of the work

The work on this BAT Reference document started in June 1997 with the kick-off meeting on

19-20 June 1997 describing the scope and the key environmental issues. Originally the scope

included vacuum systems as well, but due to their highly process-related characteristics it was

considered to be too complex to cover in a general way and was left out of the work.

Two drafts were issued to the Technical Working group (TWG) for consultation. The first draft

was issued in June 1999 and the second in March 2000. In both consultation periods comments

and new information were submitted.

The final TWG meeting was held on the 29-31 May 2000 and a high level of consensus was

obtained on content as well as on the BAT conclusions was obtained. There was general support

for BAT conclusions on the horizontal subject of industrial cooling systems. The reference to

local aspects and its consequences for the BAT conclusion were strongly debated. The

optimisation of cooling water conditioning as a major aspect of cooling systems operation was

also heavily discussed. Comments and new information submitted during and after the meeting

have been incorporated in the final report.

In the main part of the document, the general approach to arrive at BAT for industrial cooling

systems is explained. The main conclusions on BAT are presented in Chapter 4. A large number

of annexes illustrates the general concepts with practical examples.

5.2 Sources of information

As sources of information a large number of documents, reports and information from cooling

system operators and authorities as well as from suppliers of equipment and cooling water

chemicals has been used to draft the document.

Of these the documents tm001 (NL), tm056 and tm132 (Power Industry) and tm139 (Equipment

suppliers) can be considered as general building blocks. Other information submitted was more

focused on a particular environmental issue, where the emphasis was largely on cooling water

conditioning.

Information was further obtained during site visits and by personal communication on selection

of technology and on experiences with the application of reduction techniques.

5.3 Recommendations for future work

Cooling is an essential element in many industrial processes. The assessment of best available

techniques for cooling systems has revealed that internal heat management, selection and

operation of the cooling system and the resulting emissions to the environment are directly

related. However, the BREF-process has not been able to identify examples that give a

quantified illustration of this principle and a future BREF would benefit from further

investigation.

Within the Technical Working Group there clearly is agreement that BAT for cooling systems is

an approach within which a number of specific techniques can be identified. It is a complex

issue involving thermodynamic principles and interaction with the characteristics of the process.

It is clear that BAT for cooling systems is a balance of the demands of the industrial process to

be cooled, the design and operation of the cooling system and the costs. For this purpose a
Chapter 5

140 Industrial Cooling Systems

BAT-approach is developed with the emphasis on prevention by technological changes and

improving operating practice. This approach makes a distinction between new and existing

cooling systems, but in this document it is emphasised that reduction measures in existing

cooling systems have the same objective. In other words, the same approach applies, but it is

clear that options for reduction are limited in existing cooling systems.

The process of exchange of information has made it possible to identify a number of techniques

that can be considered BAT on a general level, as presented in Chapter 4.

To determine techniques under the primary BAT approach for cooling systems, however, has

been difficult. There seems to be reluctance to identify specific reduction techniques in the

framework of a horizontal issue, where general application may not be so obvious.

On the change in technology with an associated reduction in emissions no detailed information

on a practical example has been made available to illustrate the potential for improvements,

acknowledging that identical changes in similar cooling configurations could still have different

associated reduction levels. Comparisons of the performance of systems would need comparable

units and it is suggested that performance data be expressed per unit of heat dissipated (MWth).

Examples can be found in the document, where this was feasible.

Concerning the environmental issues associated with the operation of the industrial cooling

systems in the scope of this document, emphasis is largely on reduction of emissions to the

aquatic environment. Few data were reported that are considered as representative and an

inventory is recommended to be able to get a better picture, which could serve as a benchmark

for results of (future) reduction techniques.

The TWG considers the selection of cooling water additives to be an important pathway to

reduce potentially harmful emissions to the aquatic environment. A general assessment

procedure which includes local characteristics is necessary to make a selection at a local level.

Two concepts are presented in this BREF to aid the local assessment of cooling water additives.

The TWG considers both concepts as valuable tools, but the benchmark concept (Annex VIII.1)

is still a theoretical model and will need further examination.

Emissions to air from wet cooling towers may contain chemicals or bacteria, but it was a

common view in the TWG that very few data exist. To identify their importance would need

accurate measurement to quantify emissions, given certain water conditioning regimes and

efficiency of the drift eliminators. Further investigation on available data will be needed.

In some Member States a lot of attention is currently paid to the development of Legionella in

wet cooling towers as a result of some recent outbreaks of Legionnaires disease. Hence the

relatively large section paying attention to this aspect. From the information submitted it is clear

that further work is needed to establish representative concentration levels of Legionella and to

improve treatments for systems cleaning after outbreaks as well as for daily maintenance.

A maximum level of acceptable colony forming units (CFU) in a cooling system with an

associated low risk has not been determined. At present it is not clear if such a level can be

identified and future work may identify progress in this field of work.

A number of techniques were identified and considered BAT, but some are still in a stage of

development and may be considered as emerging. Their application and environmental

consequences still have to be assessed. Examples of these techniques are the spray (or

evaporative) ponds and cold and heat storage.

It is recommended that this document be revised in 3 years’ time to assess the above-mentioned

points.
References

Industrial Cooling Systems 141

REFERENCES

[Comment-1, ..]

Information submitted as comment on the first draft of the BREF-document by TWG-member

(Member State or institute)

[Correia, 1995]

Correia F.N. et al, Water availability, uses and institutions in Europe, Summary report and

vertical issues, Congres Center Instituto Superior Técnico (Lissabon (P), 1995).

[Hadderigh, 1978]

Hadderigh, R.H., 1978. Mortality of young fish in the cooling water system of Bergum power

station. Verh. Internat. Verein. Limnol., vol. 20, pp. 1827-1832.

[Hadderingh, 1983]

Hadderingh, R.H. et al., 1983. Fish impingement at power stations situated along the rivers

Rhine and Meuse in the Netherlands. Hydrobiological Bulletin 17 (2), pp. 129-141.

[KEMA, 1972]

KEMA, 1972. Invloed van maaswijdte van de draaizeven op passage en overleving van met het

koelwater ingezogen jonge vis bij de Flevo-centrale. KEMA-memorandum VII 78-87, MO-biol.

[KEMA, 1982]

KEMA, 1982. Visproblemen bij de inlaat van centrales; samenvatting van een voordracht op de

22e

 vergadering van de Kring van Chemici van Centrales. KEMA-memorandum 82-12 MObiol.

[KEMA, 1992]

KEMA, 1992. Bemonstering van ingezogen vis bij eenheid 13 van centrale Gelderland in maart,

juli en november van 1990 en 1991. KEMA-rapport 71176-MOB 92-3652.

[Carter and Reader, in press]

Carter, K.L. and J.P. Reader, (in press), Patterns of drift and power station entrainment of 0+

 fish

in the River Trent, England.

[tm001, Bloemkolk, 1997]

Bloemkolk, J.W. Industrial cooling water discharge. Cooling systems and emissions.

Ministry of Transport, Public Works and Water Management, Directorate-General of Water

Management, RIZA. Report number: 95.050 (Lelystad (NL), 1995, English translation 1997).

ISBN 9036945445

[tm003, Van der Schaaf, 1995]

Schaaf R.J van der., Eindrapport alternatieve koelsystemen industrie. Final report by Fluor

Daniel in order of Ministry of Transport, Public Works and Water Management, DirectorateGeneral of Water Management, RIZA (Lelystad (NL), 1995).

[tm004, Baltus and Berbee, 1996]

Baltus C.A.M., Berbee R.P.M., Het gebruik van biociden in recirculatiekoelsystemen.

Ministry of Transport, Public Works and Water Management, Directorate-General of Water

Management, RIZA. Report number: 95.036 (Lelystad (NL), 1996). ISBN 903694550X.

[tm005, Van Donk and Jenner, 1996]

Donk M van., Jenner H.A., Optimization of biofouling control in industrial cooling water

systems with respect to the environment. Edit. KEMA, by order of RIZA (Arnhem (NL), 1996).
References

142 Industrial Cooling Systems

[tm010, Betz, 1991]

Betz handbook of industrial water conditioning, 9th edition, Trevose, U.S.A (1991).

[tm012, UBA, 1982]

Umweltbundesamt, Abwärmekommission (edit.) Abwärme - Auswirkungen, Verminderung,

Nutzung, Bericht 82-3, Berlin (D) 1982.

[tm014, KEMI, 1991]

Kemikalieinspektionen 6/91 (Solna, (S), 1991). Miljöfarligheten hos mikrobiocider I kylvatten.

(ISSN: 0284-1185).

[tm032, Zimmerman and Hamers, 1996]

Zimmerman, P. and J.P. Hamers, Planung und Bau eines Schwadenfreien geräuscharmen

Hybridkühltürmes mit einer umweltschonenden Wasserbehandlung dusrch Ozon für ein GuDKraftwerk in den Niederlanden, VGB Kraftswerkstechnik 76, 1996, Heft 6 (p.502-505).

[tm034, Hobson et al., 1995]

Hobson, E., P. Lindahl and T. Massey, Leistungssteigerung mit Kühlturmeinbauten aus NPF

(National Power Fill), VGB Kraftswerkstechnik 75, Heft 9 (p. 829-833)

[tm036, Wilsey, 1997]

Wilsey, C.A., Alternative water treatment for cooling towers. ASHRAE Journal April, 1997

(p.43-46).

[038, Millar et al., 1997]

Millar, J.D., G.K. Morris and B.G. Shelton, Legionnaires’ disease: seeking effective prevention,

ASHRAE Journal, 1997 (p.22-29)

[tm039, Strittmatter et.al., 1996]

Strittmatter, R.J., Yang B., Johnson D.A., Ozone application for cooling tower water. ASHRAE

Journal, 1996.

[tm040, Schulze-Robbecke and Richter, 1994]

Schulze-Röbbecke, R. and Richter, M., Entstehung und Vermeidung vonLegionelleninfektionen

durch Kühltürme und Rückkühlwerke, GI Gesundheits –Ingenieur Haustechnik-BauphysikUmwelttechnik 115, 1994 (p.71-77).

[tm041, Burger, 1994]

Burger, R., Select the right cooling tower fill, Hydrocarbon Processing, August 1994 (p. 141-

143).

[tm042, Veil and Moses, 1995]

Veil, J.A., Moses, D.O., Consequences of proposed changes to clean water act thermal

discharge requirements. Paper presented to EPRI, international Clean Water Conference, (Jolla,

CA (USA), 1995).

[tm044, Carhart and Policastro, 1991]

Carhart, R.A. and Policastro, A.J., A second-genration model for cooling tower plume rise and

dispersion – I. Single sources. Atmospheric Environment Vol.25A, no. 8, 1991 (pp. 1559-1576).

[tm046, Vanderheyden and Schuyler, 1994]

Vanderheyden, M.D., Schuyler, G.D., Evaluation and quantification of the impact of cooling

tower emissions on indoor air quality, ASHRAE Transactions of Annual Meeting, Vol. 100,

part 2 (p. 612-620)
References

Industrial Cooling Systems 143

[tm056, Caudron, 1991]

Caudron, L., Les réfrigérants atmosphériques industriels, éditions Eyrolles, Paris (1991).

[tm059, Mortier, 1995]

Mortier, R., Pretreatment and desalting of contaminated and brackish surface water with reverse

osmosis, Proceedings of Watersymposion 1995 (p.144-158), Breda (NL).

[tm059, Paping, 1995]

Paping, L.L.M.J., Energiebesparing door schone koelers, Proceedings of Watersymposion 1995

(p.23-47), Breda (NL).

[tm061, Eurovent/Cecomaf, 1997]

Eurovent/ (WG 9, Cooling towers), Mechanical draught cooling tower acoustics, practical

guide, Paris, Draft December1997.

[tm062, Dallmier, 1997]

Dallier, A.W., J.D. Martens and W.F. McCoy, performance of stabilized halogen biocides in

cooling water, report 398, naperville, Illinois (USA, 1997).

[tm064, Meier, 1990]

Meier, D.A., Zero blowdown - A solution for water conservation, International Water

Conference, October 1990, Reprint nr. 529.

[tm065, Meier and Fulks, 1990]

Meier, D.A., and K.E. Fulks, Water treatment options and considerations for water reuse,

National Association of Corrosion Engineers Corrosion ’90 meeting, Las Vegas (USA), Reprint

nr. 520.

[tm066, Phillips and Strittmatter, 1994]

Phillips, E.C., and R.J. Strittmatter, Reuse of industrial waste stream as cooling tower makeup,

1994 Coolin Tower Institute Annual Meeting, Houston Texas (USA, 1994).

[tm067, Hoots et al, 1993]

Hoots, J.E., B.V. Jenkins and E.C. Ray, Choosing chemical dosage control for cooling water

inhibitors and dispersants, Paper presented at Meeting of International District Heating and

Cooling Association October 27-29, 1993, Tulsa (USA), Reprint 622, Nalco.

[tm068, Ambrogi, 1997]

Ambrogi, R., Environmental impact of biocidal antifouling alternative treatments of sea water

once-through cooling systems (1997). Proceedings of symposium on chlorine dioxide and

disinfection, Rome (I), 1996.

[tm070, Benschop, 1997]

Benschop, P., The evaluation of substances and preparations in the context of the pollution of

surface waters act. Ministry of Transport, Public Works and Water Management, DirectorateGeneral of Water Management, RIZA. Report number: 98.005 (Lelystad (NL), 1997). ISBN

9036951569.

[tm071, Niebeek, 1997]

Niebeek, G. Procedure for the evaluation of substances and preparations in the context of the

pollution of surface waters act. Ministry of Transport, Public Works and Water Management,

Directorate-General of Water Management, RIZA. Report number: 98.006 (Lelystad (NL),

1997). ISBN 9036951577.
References

144 Industrial Cooling Systems

[tm072, Berbee, 1997]

Berbee R.P.M., Hoe omgaan met actief chloor in koelwater? (How to use active chlorine in

cooling water?), Ministry of Transport, Public Works and Water Management, DirectorateGeneral of Water Management, RIZA. Report number: 97.077 (Lelystad (NL), 1997). ISBN

9036951240.

[tm079, CES, 1994]

Consultants in Environmental Sciences Ltd. Biocides in cooling water systems

(Beckenham(UK), 1994) by order of the U.K Department of the Environment.

[tm082, Mittendorf, 1990]

Mittendorf, E.D., Removal of asbestos paper fill from large industrial cooling towers, Paper

presented at 1990 Cooling Tower Institute Annual Meeting, (Houston, Texas (USA), 1990).

[tm083, Adams and Stevens]

Adams, S. and Stevens, J., Strategies for improved cooling tower economy. Paper presented at

1991 Cooling Tower Institute Annual Meeting, (New Orleans, Louisiana (USA), 1991).

[tm084, Rice and Wilkes, 1992]

Rice R.G. and Wilkes J.F.. Biocidal aspects of ozone for cooling water treatment-probable

impacts of bromide ion. Paper presented at 1992 Cooling Tower Institute Annual Meeting,

(Houston, Texas (USA), 1992).

[tm086, Van der Spek, 1993]

Spek, H. van der, Reduction of noise generation by cooling fans, Paper presented at 1993

Cooling Tower Institute Annual Meeting, (New Orleans, Louisiana (USA), 1993).

[tm087, Engstrom and Tully, 1994]

Engstrom, G.G. and J.C. Tully, Monitoring biological control in cooling systems, paper

presented at the 1994 Cooling Tower Institute Annual Meeting, (Houston, Texas (USA), 1994).

[tm090, Grab et al, 1994]

Grab, L.A., J.A. Diemer, M.G. Freid, The effect of process leak contaminants on biocidal

efficacy, Paper presented at 1994 Cooling Tower Institute Annual Meeting, (Houston, Texas

(USA), 1994)

[tm091, Little et.al, 1994]

Little D. A., Mitchell WA., E.S. Lawson, The control of ferous metal corrosion in cooling water

by a novel phophonate corrosion inhibitor. Paper presented at 1994 Cooling Tower Institute

Annual Meeting, (Houston, Texas (USA), 1994).

[tm092, Becker and Burdick, 1994]

Becker, B.R. and L.F. Burdick, Drift eliminators and fan system performance, Paper presented

at 1994 Cooling Tower Institute Annual Meeting, (Houston, Texas (USA), 1994).

[tm093, Mirsky, 1995]

Mirsky, G.R., Cooling tower noise, Paper presented at the 1995 Cooling Tower Institute Annual

Meeting, Fort Worth (Texas (USA), 1995).

[tm094, Alfano and Sherren, 1995]

Alfano, N.J. and D.J. Sherren, Water conservation via new cooling water technology, Paper

presented at the 1995 Cooling Tower Institute Annual Meeting, Fort Worth (Texas (USA),

1995).
References

Industrial Cooling Systems 145

[tm095, Cunningham, 1995]

Cunningham, R.J., Maximizing cooling tower cycles of concentration, Paper presented at the

Cooling Tower Institute Annual Meeting, Fort Worth (Texas (USA), 1995).

[tm096, McCoy et al, 1995]

McCoy, W.F., S.A. Borchardt, and M.R. Hermiller, Traced biocides: a new technology for

industrial water treatment, Paper presented at the 1995 Cooling Tower Institute Annual

Meeting, Fort Worth (Texas (USA), 1995).

[tm97, Immell, 1996]

Immell, W.F., Variable speed fan drives for cooling towers, Paper presented at 1996 Cooling

Tower Institute Annual Meeting, (Houston, Texas (USA), 1996).

[tm101, BDAG, 1996]

Balcke Dürr AG, Hybrid cooling towers, Ratingen (D), 1996.

[tm102, BDAG, 1996]

Balcke Dürr AG, Natural draught wet cooling towers-nature’s good deed, Ratingen (D), 1996.

[tm103, BDAG, 1996]

Balcke Dürr AG, Mechanical draught wet cooling towers-cooling air supplied by fan power,

Ratingen (D), 1996.

[tm104, Tesche, 1997]

Tesche, W., Selection criteria for different types of cooling towers, Balcke-Dürr Ratingen (D),

1997.

[tm109, BDAG, 1996]

Balcke Dürr AG, Air-cooled heat exchangers for the hydrocarbon process industry, Ratingen

(D), 1996.

[tm110, BDAG, 1995]

Balcke Dürr AG, Technical information on cooling towers for salt water applications (pers.

comm.)

[tm111, BDAG, 1996]

Balcke Dürr AG, Air-cooled heat exchangers, Ratingen (D), 1996.

[tm113, Streng, 1996]

Streng, A., Combined wet/dry cooling towers of a cell-type construction, American Power

Conference 58th Annual Meeting, (Chicago (USA), 1996).

[tm117, Remberg and Fehndrich, 1993]

Remberg, H.-W. and B. Fehndrich, Wirkubgsgradverbesserung bei naturzug-Nasskühltürmen

durch Austausch von Asbestzement-Kühleinbauten gegen Kunststoff-Kühleinbauten, Kraftwerk

und Umwelt, 1993 (p. 112-117).

[tm123, Alt and Mäule, 1987]

Alt, W. and R. Mäule, Hybridkühltürme im wirtschaftlichen Vergleich zu Nass- und

Trockenkühltürmen, VGB Kraftwerkstechnik 8, (page 763-768), 1987.

[tm131, Dziobek, 1998]

Dziobek, Th., Ozon stopft Loch in der Kasse (Ozone fills financial gap in the pocket),

Umwelttechnik (Sonderteil Wasser), April 1998.
References

146 Industrial Cooling Systems

[tm132, Eurelectric, 1998]

BAT for Cooling Systems. Eurelectric, The European Grouping of the Electricity Supply

Industry, (Brussels (B), 1997, updated 1998).

[tm135, Nalco, 1988]

The Nalco Water Handbook, 2nd edition, USA (1988).

[tm136, Jäggi/Günthner, 1997]

Jäggi/Günthner, Industrial recoolers, Technical information, edit. 9.97, Bern (1997).

[tm139, Eurovent, 1998]

Proposal for a BREF-document for cooling systems. Drafted by Coopers & Lybrand Umwelt

(1998) by order of Eurovent, the European Committee of Air handling and Refrigeration

Equipment manufacturers. Report number: 61350027.

[tm 144, Cabanes et al, 1997]

Cabanes, P.A., E. Pringuez, F. Siclet, M. Khalanski, and P. Pernin, Continuous chlorination to

control pathogenic free living amoebae in a closed loop power plant, The Environmental

Professional, Volume 19, 1997 (p.192-200)

[tm145, Werner and Pietsch, 1991]

Werner, H.-P. von, M. Pietsch, Bewertung des Infektionsrisikos durch Legionellen in

Kühlkreisläufen von Kraftwerken, VGB Kraftwerkstechnik 71, Heft 8, p. 785-787 (1991)

[tm146, Daamen and Savelkoul, 1999]

Daamen E.J., and J.T.G. Savelkoul, Zijstroom-biofiltratie in open recirculerende

koelwatersystemen, In: proceedings Watersymposium ’99, Breda, NL (1999).

[tm147, Bloemkolk and Van der Schaaf, 1996]

Bloemkolk J.W., R.J. van der Schaaf, Design alternatives for the use of cooling water in the

process industry: minimization of the environmental impact from cooling systems, J. Cleaner

Production Vol.4, No. 1, pp. 21-27, 1996.

[tm148, RIZA, 1996]

RIZA, Visintrek door grootschalige koelwaterinname, Problematiek en aanbevelingen, FWVOnota 96.01 (January, 1996).

[tm149, Baltus et al., 1999]

Baltus, C.A.M., L.C.M., Kerkum and P.G.M. Kienhuis, Acute toxiciteit van koelwaterlozingen

uit recirculatiekoelsystemen, Ministry of Transport, Public Works and Water Management,

Directorate-General of Water Management, RIZA. Report number: 99.025 (Lelystad (NL),

1997). ISBN 9036952492.

[tm150, Paping, 1999]

Paping, L.L., Feasibility study on application of spray ponds, Pers. comm. on study, Dow

Europe (Terneuzen (NL), 1999).

[tm151, BAC, 1999]

Baltimore Aircoil, H2O conserving HXI hybrid fluid cooler, Information Leaflet, Heist-op-denBerg (B), 1999.

[tm152, Taft, 1999]

Taft, E.P., Fish protection technologies: a status report. Alden Research Laboratory Inc., Holden

(USA), 1999.
References

Industrial Cooling Systems 147

[tm153, Paping et al, 1999]

Paping, L.L.M.J., H.A. Jenner, H.J.G. Polman, B.H. te Winkel and M.R. de Potter, Ecological

conditioning and optimisation of a once-through cooling water system, Proceedings of

Watersymposium 1999, p.327 – 355 (Breda (NL), 1999).

[tm154, Besselink et al, 1999]

Besselink, V.V., M.A.M. Beerlage, W. de Jongh and G. Koopmans, Haalbaarheidstudie naar

alternatieve koeling door middel van sproeivijvers, KEMA-report nr 99532763.EP.163 99P01,

Arnhem (NL) 1999.

[tm155, Berbee, 1999]

Berbee, R.P.M., Legionella in oppervlaktewater, in koelwater, in RWZI’s, in.....; waar eigenlijk

niet?, Ministry of Transport, Public Works and Water Management, Directorate-General of

Water Management, RIZA. Report number: 99.057 (Lelystad (NL), 1999), ISBN 9036952867.

[tm156, Schmittecker et al, 1999]

Schmittecker, B.M., K.-P. Henke, and W. Bergmann, Kühlwasserbehandlung mit Ozon.

(Cooling water treatment with ozone), VGB KraftwerksTechniek 4/99.

[tm157, Jenner et al, 1998]

Jenner, H.A., J.W. Whitehouse, C.J.L. Taylor and M. Khalansky, Cooling water managament in

cooling power stations-Biology and control of fouling, Hydroecologie Appliquee, Tome 10,

Volume 1-2, 1998, ISSN: 1147-9213.

[tm158, VGB, 1998]

VGB, VGB-Richtlinie, Lärmminderung in Wärmekraftanlagen, VGB-R 304, April 1998.

[tm159, KEMA/EPON, 1993]

KEMA by order of N.V. EPON, Milieu-effect rapport WKC-Nijmegen (Environmental effect

report Heat and Power Unit Nijmegen), Arnhem (NL), 1993, Ref.nr. KEMA 21446-KEC 92-

363.

[tm160, Bijstra, 1999]

Bijstra, D., Verantwoord omgaan met biociden in koelwater. FWVO-rapport 99.02. (NL, 1999)

[tm161, Borgerding, 1998]

Borgerding, P.H., Koelsystemen. “Koeling is meer dan warmte alleen”. (Cooling systems.

“Cooling is more than just heat”). Ministry of Transport, Public Works and Water Management,

Directorate-General of Water Management, RIZA. Report number: 98.056 (Lelystad (NL),

1998), ISBN 9036952107.

[tm164, Travade, 1987]

Travade F., Aspiration dórganismes aux prises déau des centrales (RGN, 1, pp 59-62, 1987).

[tm165, Turnpenny et al, 1985]

Turnpenny, A.H.W., T.E. Langford and R.J.Aston, Power Stations and Fish. (ECB Research, pp

27-39, 1985).

[tm166, Morton et al, 1986]

Morton, S.J., C.L.R. Bartlett, L.F. Bibby, D.N. Hutchinson, J.V. Dyer, P.J. Dennis, Outbreak of

legionnaires’disease from a cooling water system in a power station. British Journal of

Industrial Medicine 1986; 43: 630-635.

[tm167, Fliermans, 1996]

Fliermans, C.B., Ecology of Legionella: From data to knowledge with a little wisdom.

Microbial Ecology 1996; 32: 203 – 228.
References

148 Industrial Cooling Systems

[tm168, De Potter et al, 1996]

De Potter, M.R., H.J.G. Polman and B.H. te Winkel, Effectiveness and environmental impact of

elevated hypochlorite dosing at Dow Benelux 1995, KEMA-report 64683-KES/WBR 96-3114

[tm169, De Potter et al, 1997]

De Potter, M.R., H.J.G. Polman and B.H. te Winkel, Effectiveness and environmental impact of

elevated hypochlorite dosing at Dow Benelux 1996, KEMA-report 64683-KES/WBR 97-3128

[tm170, De Potter and Polman, 1999]

De Potter, M.R. and H.J.G. Polman, Effectiveness and environmental impact of elevated

hypochlorite dosing at Dow Benelux 1997, KEMA-report 9756058-KPG/CET 99-4403

[tm171, Polman, 2000]

Polman H.J.G., Antifouling optimisation by pulse-alternating chlorination at Dow Benelux

1999, KEMA-report 99550566-KPS/MEC.
Industrial Cooling Systems 149

ANNEXES
150 Industrial Cooling Systems

Table of contents

ANNEX I THERMODYNAMIC PRINCIPLES...........................................................155

I.1 Heat transfer in a shell and tube heat exchanger ..................................................................... 155

I.2 Approach................................................................................................................................. 156

I.3 Capacity of a heat exchanger................................................................................................... 157

I.4 Wet and dry bulb temperatures ............................................................................................... 158

I.5 Relation between heat transfer and heat exchanging surface .................................................. 158

ANNEX II PRINCIPLE OF ENERGY SAVING THROUGH OPTIMISED

COOLING ......................................................................................................161

II.1 Subject..................................................................................................................................... 161

II.2 Summary of conclusions ......................................................................................................... 161

II.3 Introduction............................................................................................................................. 162

II.4 Calculations............................................................................................................................. 164

II.4.1 Principles ..................................................................................................................... 164

II.4.2 Quantity of cooling water ↑......................................................................................... 166

II.4.3 Quantity of cooling air ↑.............................................................................................. 167

II.4.4 Product temperature ↑; gas volume ↑.......................................................................... 168

II.4.5 Product pressure ↑; cooling compressor ↑................................................................... 169

II.5 Total potential energy conservation per °C colder cooling-water boundary layer .................. 170

II.5.1 Efficiency of power generation ↑ ................................................................................ 170

II.5.2 Total water used for cooling in the Dutch industry (excl. power plants) ..................... 171

II.5.3 Total potential energy conservation per °C colder cooling-water boundary layer....... 172

II.6 Examples of calculations for the relative conservation of energy and reduction of the

environmental impact achieved by the use of inhibitors ......................................................... 173

II.6.1 The contribution made by oxidation ............................................................................ 173

II.6.1.1 Once-through cooling system ........................................................................... 173

II.6.1.2 Open recirculating system ................................................................................ 174

II.7 Examples of calculations of the relative savings in energy with colder cooling water ........... 176

II.7.1 Coastal water versus cooling towers............................................................................ 176

II.7.2 River water versus cooling towers ............................................................................... 176

II.7.3 Groundwater versus cooling tower .............................................................................. 177

II.8 Appendix environmental impacts............................................................................................ 177

ANNEX III SHELL AND TUBE HEAT EXCHANGERS FOR INDUSTRIAL ONCETHROUGH COOLING SYSTEMS AND THE OCCURRENCE OF

LEAKAGE......................................................................................................179

III.1 Design of the shell & tube heat exchanger for one through systems....................................... 180

III.2 Leakage in shell & tube heat exchangers ................................................................................ 182

III.3 Alternatives ............................................................................................................................. 183

ANNEX IV EXAMPLE OF SELECTION OF MATERIAL FOR COOLING WATER

SYSTEMS IN INDUSTRIAL (NON-POWER PLANT) APPLICATIONS

..........................................................................................................................185

IV.1 Introduction............................................................................................................................. 185

IV.2 Direct once-through systems (with brackish water)................................................................ 186

IV.3 Indirect once-through system (brackish water/demin-water).................................................. 188

IV.4 Open recirculating cooling systems ........................................................................................ 189

IV.4.1 Fresh water application in open wet cooling tower...................................................... 189

IV.4.2 Salt water application in open wet cooling towers....................................................... 189

ANNEX V OVERVIEW OF CHEMICALS FOR THE CONDITIONING OF

COOLING WATER SYSTEMS...................................................................191

V.1 Corrosion inhibitors ................................................................................................................ 191

V.1.1 Corrosion ..................................................................................................................... 191

V.1.2 Applied corrosion inhibitors ........................................................................................ 191

V.2 Scale inhibitors........................................................................................................................ 192

V.2.1 Scaling ......................................................................................................................... 192

V.2.2 Applied scale inhibition ............................................................................................... 192
Industrial Cooling Systems 151

V.3 Fouling inhibitors (dispersants) ...............................................................................................193

V.3.1 Fouling .........................................................................................................................193

V.3.2 Applied fouling inhibitors ............................................................................................194

V.4 Biocides ...................................................................................................................................194

V.4.1 Biofouling.....................................................................................................................194

V.4.2 Applied biocidal treatment ...........................................................................................197

V.4.3 Oxidising biocides........................................................................................................197

V.4.4 Non-oxidising biocides.................................................................................................199

V.4.5 Factors determining the use of biocides. ......................................................................200

V.4.6 Interactions with other water treatment chemicals .......................................................202

V.5 Cycles of concentration and water balance..............................................................................202

ANNEX VI EXAMPLE OF LEGISLATION IN EUROPEAN MEMBER STATES . 205

ANNEX VII EXAMPLE OF A SAFETY CONCEPT FOR OPEN WET COOLING

SYSTEMS (VCI-CONCEPT)....................................................................... 209

VII.1 Introduction to the concept ......................................................................................................209

VII.2 Requirements of the concept....................................................................................................211

VII.3 Appendix 1 - Automatic analytical monitoring of once-through cooling systems ..................213

VII.4 Appendix 2 – R-phrases used to calculate VCI-score..............................................................214

ANNEX VIII EXAMPLES FOR THE ASSESSMENT OF COOLING WATER

CHEMICALS ................................................................................................ 217

VIII.1 Benchmark assessment concept for cooling water chemicals..................................................217

VIII.1.1 Introduction ...............................................................................................................217

VIII.1.1.1 Background ................................................................................................217

VIII.1.1.2 Relevant legislative background ................................................................218

VIII.1.1.3 The water framework directive (WFD)......................................................218

VIII.1.2 Benchmarking : introduction of the concept..............................................................219

VIII.1.2.1 The PNEC ..................................................................................................220

VIII.1.2.2 The PEC.....................................................................................................221

VIII.1.3 Basic cooling towers material balances .....................................................................222

VIII.1.3.1 Cooling towers basic equations..................................................................222

VIII.1.3.2 Water balance.............................................................................................222

VIII.1.3.3 Material balance.........................................................................................222

VIII.1.3.4 Concentration .............................................................................................223

VIII.1.3.5 Discussion ..................................................................................................223

VIII.1.4 Calculation of PEC and benchmarking......................................................................224

VIII.1.5 Computation methods................................................................................................226

VIII.1.5.1 Single Substances.......................................................................................226

VIII.1.5.2 Complex multi-substances treatments........................................................226

VIII.1.6 Appendix I: extract from technical guidance document ............................................228

VIII.2 Concept of a local assessment method for cooling water treatment chemicals, with a particular

emphasis on biocides ...............................................................................................................231

VIII.2.1 Introduction ...............................................................................................................231

VIII.2.2 Key elements .............................................................................................................232

VIII.2.3 Example of proposed local assessment method.........................................................235

ANNEX IX EXAMPLE OF A MODEL FOR ESTIMATING EMISSIONS OF

BIOCIDES IN THE BLOWDOWN ............................................................ 239

ANNEX X INVESTMENT COSTS AND OPERATIONAL COSTS OF EQUIPMENT

AND ELEMENTS OF COOLING SYSTEMS FOR NON-POWER PLANT

APPLICATIONS........................................................................................... 241

ANNEX XI EXAMPLES OF TECHNIQUES TO BE CONSIDERED WITHIN THE

PRIMARY BAT APPROACH FOR INDUSTRIAL COOLING SYSTEMS

......................................................................................................................... 247

XI.1 Introduction .............................................................................................................................247

XI.2 Cooling water savings through water reuse .............................................................................247

XI.2.1 Reuse of (waste) water for cooling tower make-up......................................................248
152 Industrial Cooling Systems

XI.2.2 Zero discharge system.................................................................................................. 249

XI.2.3 Spray ponds.................................................................................................................. 250

XI.2.4 Cold storage ................................................................................................................. 251

XI.3 Reduction of emissions through optimized cooling water treatment ...................................... 253

XI.3.1 Side-stream-biofiltration in an open recirculating cooling water system..................... 253

XI.3.2 Physical methods ......................................................................................................... 256

XI.3.3 Optimization of biocide use ......................................................................................... 258

XI.3.3.1 Monitoring........................................................................................................ 258

XI.3.3.1.1 Monitoring of macrofouling..................................................................... 258

XI.3.3.1.2 Traced biocides for biocide and microbiological activity ........................ 258

XI.3.3.2 Biocide dosage.................................................................................................. 259

XI.3.3.2.1 Different conditioning regimes to obtain optimum annual total oxidant use

in once-through systems against macro- and microfouling....................................... 259

XI.3.3.2.2 Pulse alternating chlorination in once-through systems ........................... 261

XI.3.4 Alternative cooling water treatments ........................................................................... 264

XI.3.4.1 Ozone................................................................................................................ 264

XI.3.4.2 UV treatment .................................................................................................... 266

XI.3.4.3 Catalytic hydrogen peroxide treatment............................................................. 266

XI.3.4.4 Chlorine dioxide ............................................................................................... 267

XI.3.4.5 Ionic water purification to treat cooling tower water........................................ 270

XI.3.4.6 Stabilizing halogenated biocides in cooling tower water.................................. 270

XI.3.4.7 Filming agent against fouling, corrosion and scaling ....................................... 271

XI.3.4.8 Stable organic corrosion inhibitors in open wet cooling towers ....................... 274

XI.3.5 Treatment of discharged cooling water........................................................................ 275

XI.4 Variable frequency drives for reduction of energy use ........................................................... 276

ANNEX XII SPECIAL APPLICATION: POWER INDUSTRY ....................................277

Synthesis............................................................................................................................................. 277

XII.1 Introduction............................................................................................................................. 277

XII.2 Power plant cooling systems - principles and reminders......................................................... 278

XII.3 Possible environmental impacts of cooling systems ............................................................... 279

XII.3.1 Heat discharges to the atmosphere ............................................................................ 279

XII.3.2 Heating of receiving aquatic-environments............................................................... 281

XII.3.3 Suction of organisms into water intakes.................................................................... 282

XII.3.4 Alteration of the receiving environment by chemical discharges ............................. 283

XII.3.5 Other possible harmful effects resulting from the choice of some cooling systems . 286

XII.4 Prior study of the sites: indispensable tool for the evaluation of their receiving capacity, impact

control and prevention of harmful effects ............................................................................... 287

XII.4.1 Analysis of the situation............................................................................................ 287

XII.4.2 Mathematical modellings, simulations on models and tests on pilot loops, first

indispensable tools....................................................................................................... 287

XII.5 Design of components and choice of materials....................................................................... 288

XII.5.1 Wet cooling............................................................................................................... 288

XII.5.2 Hybrid cooling .......................................................................................................... 289

XII.5.3 Dry cooling ............................................................................................................... 290

XII.5.3.1 Forced draught air-cooled condenser......................................................... 290

XII.5.3.2 Natural draught air-cooled condenser........................................................ 290

XII.5.3.3 Closed recirculating dry cooling towers .................................................... 291

XII.5.4 Cooling towers with discharge of cleaned flue gas ................................................... 291

XII.6 Cost comparison between the various types of cooling towers............................................... 292

XII.7 Choice of the treatment of circulating water alternative methods - monitoring ...................... 295

XII.7.1 Anti-scale treatment .................................................................................................. 295

XII.7.2 Anti-fouling treatments (biocides) ............................................................................ 296

XII.7.3 Monitoring ................................................................................................................ 296

XII.8 Design of the cooling system .................................................................................................. 297

XII.8.1 Design and energy recovery...................................................................................... 297

XII.8.2 Design and noise reduction measures ....................................................................... 297

XII.8.3 Implementation of physical methods ........................................................................ 297

XII.8.4 Modelling and pilot tests........................................................................................... 298

XII.8.5 Choice of the cooling system .................................................................................... 298

XII.9 Conclusions............................................................................................................................. 298

XII.10 Literature................................................................................................................................. 300

XII.11 Illustrations.............................................................................................................................. 306
Industrial Cooling Systems 153

List of tables

Table I.1: Fouling factors for shell and tube heat exchangers, indicative values [Van der Schaaf, 1995]155

Table I.2: Specific heat capacities of air and water ..................................................................................157

Table I.3: Heat transfer coefficients and estimated surface areas A (m2

) per MW and at 20K mean

temperature difference for different industrial applications..................................................158

Table I.4: Effects of the cooling principle on the capacity, approach and cooling surface of a cooling

system ...................................................................................................................................159

Table II.1: Energy consumption in kWe, electricity consumption / MWth, cooling with clean heat exchangers.........163

Table II.2: Mean attainable lowest cooling water inlet temperatures for the various cooling systems during

the months of July and August in the Netherlands................................................................163

Table II.3: Conservation ratios for once-through and recirculating cooling system.................................177

Table II.4: Energy conservation with potential colder cooling water source............................................177

Table IV.1: Cooling water velocity and type of material .........................................................................185

Table IV.2: Materials used for pumps in brackish water..........................................................................186

Table IV.3: Materials used for shell&tube heat exchangers in brackish water.........................................187

Table V.1: Survey of fouling and clogging organisms, and degree of fouling in marine, brackish and fresh

water. In the last column mitigation is presented..................................................................195

Table V.2: Estimated consumption levels of some commonly used oxidising biocides in a few European

Member States ......................................................................................................................198

Table V.3: Estimated consumption levels in some of European Member States of some commonly used

non-oxidising biocides in kg/yr.............................................................................................200

Table VII.1: Score for a number of R-phrases to calculate the total score for process substances...........210

Table VII.2: Requirements of VCI safety concept for cooling technology ..............................................211

Table VII.3: Description of R-phrases used to calculate VCI-score for cooling systems selection..........214

Table VIII.1: Calculation of PEC and Benchmarking ..............................................................................225

Table VIII.2: Assessment factors to derive a PNEC.................................................................................229

Table VIII.3: Predicted concentrations of DBNPA in different surface waters for this example.............237

Table VIII.4: Ecological data of DBNPA.................................................................................................237

Table VIII.5: Consequences of closing the discharge...............................................................................238

Table X.1: Cost elements for water and air cooling systems....................................................................243

Table X.2: Cost indications for water and air cooling systems for industrial applications with the

exception of power plants (1993-1995) ................................................................................244

Table XI.1: Investment and energy costs per MWth for spray pond and cooling tower ...........................251

Table XI.2: Physical techniques to minimize biocide use ........................................................................257

Table XI.3: Effect of applying an optimized dosage regime on the number of leakage caused by mussels

..............................................................................................................................................262

Table XI.4: Typical dosage of chlorine dioxide for once-through and recirculating systems in Europe..268

Table XI.5: Effect of chlorine dioxide applied in once-through system on larvae settlement..................269

Table XII.1: Example of simplified balance of a thermal cycle for conventional new design .................278

Table XII.2: Relationship between the installed capacity and cooling parameters...................................278

Table XII.3: Comparison of different types of recirculating cooling systems with a lifetime of 25 years

and an actualisation ratio of 8% (study on EDF units of 1300 MWe) ..................................293

Table XII.4: Comparison of wet cooling towers and aircooled condenser with a life-time of 20 years and

an actualisation ratio of 8% for a combined cycle unit 290 MWth.......................................293

Table XII.5: Relationship between the concentration factor, the withdrawn water flowrate and the energy

discharged into the receiving waterway (individual example)..............................................295
154 Industrial Cooling Systems

List of figures

Figure II.1: Illustration of areas of potential energy saving by reduction of the temperature gradient

through a fouling layer as well as by using colder cooling water influent ........................... 162

Figure II.2: Graphic representation of pollution factors responsible for extra temperature gradient over the

pipe wall ............................................................................................................................... 164

Figure II.3: Schematic representation of driving force over the length of a heat exchanger.................... 166

Figure II.4: Number of cooling water pumps and change of cooling water low due to fouling............... 167

Figure II.5: Change of temperature gradient of product gas (moles) in a counter flow heat exchanger due

to fouling .............................................................................................................................. 168

Figure II.6: Representation of increase of process pressure to compensate temperature increase due to

fouling .................................................................................................................................. 169

Figure II.7: Efficiency power generation for pumps, fans and compressors............................................ 170

Figure III.1: Major components of shell & tube heat exchanger.............................................................. 179

Figure III.2: Heat exchanger nomenclature (Standards of the tubular Exchanger Manufacturers

Association).......................................................................................................................... 181

Figure V.1: Tower and solids balances for an evaporative cooling system using a cooling tower .......... 203

Figure V.2: Reduction of the make-up flow by concentration in an evaporative cooling system............ 203

Figure VIII.1: Cooling tower material balance ........................................................................................ 222

Figure VIII.2: Combined approach for the assessment of cooling water biocides for existing installations

.............................................................................................................................................. 234

Figure VIII.3: Schematic representation of a recirculating cooling system with the data for the example of

a local selection method of cooling systems chemicals........................................................ 236

Figure XI.1: Optimised hypochlorite dosage (pulse-alternating chlorination) taking account of fouling and

cooling system characteristics .............................................................................................. 263

Figure XII.1: Once-through system.......................................................................................................... 306

Figure XII.2: Wet cooling tower.............................................................................................................. 306

Figure XII.3: Once - through cooling with cooling tower........................................................................ 307

Figure XII.4: Recirculating cooling ......................................................................................................... 307

Figure XII.5: Mechanical draught cooling tower (pressure fans)............................................................. 308

Figure XII.6: Mechanical draught cooling tower (suction fans, cell construction).................................. 308

Figure XII.7: Mechanical draught cooling tower (pressure fans, cell construction) ................................ 309

Figure XII.8: Hybrid cooling tower ......................................................................................................... 309

Figure XII.9: Forced draught air-cooled condenser ................................................................................. 310

Figure XII.10: Natural draught air-cooled condenser .............................................................................. 310

Figure XII.11: Closed recirculating indirect dry cooling tower ............................................................... 311

Figure XII.12: Cooling tower with discharge of cleaned flue gas............................................................ 311

Figure XII.13: Cooling system with fixed concentration factor............................................................... 312

Figure XII.14: Cooling system with sliding concentration factor ............................................................ 312

Figure XII.15: Decision-making logic diagram for the choice of the cooling system ............................. 313
Annex I

Industrial Cooling Systems 155

ANNEX I THERMODYNAMIC PRINCIPLES

Potentially every change in an industrial cooling system can affect the process of heat exchange.

When applying BAT to industrial cooling systems the consequences for the cooling process

have to be assessed also using the thermodynamic principles.

I.1 Heat transfer in a shell and tube heat exchanger

In a counter flow design heat exchanger heat is transferred from a warm source to a cold source

and the heat exchange can be described as follows:

Q = ∆Tm ( ) ln *U * A

Q heat transferred per unit of time (W)

∆Tm(ln) logarithmic mean temperature difference LMTD (K)

U overall heat transfer coefficient (W/m2K)

A heat exchanging surface (m2)

The LMTD for a tube heat exchanger for counter current flow can be determined with the

following equation:

( )( )

{ } ( )( ) 1 2 2 1

1 2 2 1

ln T t T t

T t T t Tm − −− − − ∆ =

T1 temperature inlet warm process side (K)

T2 temperature outlet warm process side (K)

t1 temperature inlet cold side (K)

t2 temperature outlet cold side (K)

Heat transfer is promoted by a large surface area (A). For practical reasons there is a limit to the

size of the surface area and in that case finned tubes are applied. Different sources of resistance

(R) form another limitation to heat transfer. Generally, resistance R is expressed as its reciprocal

value of the heat transfer coefficient 1/U and largely consists of the thickness of the wall

between the two media and its conductivity and the state of fouling of the exchanging surface.,

but also the conductivity of the liquids is important accounting for the effect of different

velocities on heat transfer.

Depending on the nature of the medium that flows through the heat exchanger, the heatexchanging surface gets fouled. During use of the exchanger the resistance to the transfer of

heat increases. For design purposes use is made of a pollution coefficient or fouling factor, that

equals the maximum fouling depending on the nature of the medium or the coolant. In Table I.1

a few examples are given, where a lower fouling factor means a lower state of fouling of the

heat-exchanging surface.

Table I.1: Fouling factors for shell and tube heat exchangers, indicative values [Van der Schaaf,

1995]

Medium Fouling factors (W/m2/K)

River water 3000-12000

Sea water 1000- 3000

Cooling water (cooling tower) 3000- 6000

Light hydrocarbons 5000

Heavy hydrocarbons 2000
Annex I

156 Industrial Cooling Systems

Two thermodynamic principles are responsible for heat transfer:

transfer through conduction (and convection),

transfer through evaporation of a medium.

Heat transfer in a dry cooling system through conduction and convection is called sensible heat

transfer. In a wet cooling system water is cooled down by direct contact with air. Sensible heat

transfer to air in a dry cooling system can be described as follows:

Q = Cρ* ma * ∆T

Q heat transferred

Cρ specific heat capacity of air

∆T temperature range

ma mass of air

In a wet cooling system there is additionally heat transfer through evaporation, latent heat

transfer and can be described with the following formula:

Q = ma * ∆H '

Q heat transferred

' ma mass of air

∆H enthalpy range

With these equations the mass of air necessary for dry (ma) and for wet (ma’) cooling can be

compared:

H

C T

m

m

a

a

∆

∆ = * '

ρ

The ratio depends on the temperature range ∆T and for example with an increase in air

temperature of 10 to 20°C this ratio is about 1:4. In this example it means that dry cooling needs

four times more air than wet cooling. In other words, a larger heat exchange surface is required

for dry cooling.

I.2 Approach

It is always important for a cooling system to provide sufficient driving force in order to achieve

the transfer of heat. A minimal temperature difference is necessary between the incoming and

the outgoing flow, i.e. the difference between the temperature of the process medium leaving

the heat exchanger and the temperature of the coolant (water or air) that enters the heat

exchanger, based on a counter flow design. This difference in temperature is called the

approach. For wet cooling towers the approach is defined as the difference between the wet

bulb temperature of the air and the temperature of the coolant leaving the tower. For dry cooling

towers the approach is defined as the difference between the dry bulb temperature of the air and

the temperature of the coolant leaving the tower.

A cooling system is designed to meet specifications throughout the year. Of course, when water

and air temperatures are low, requirements are easily met. But higher temperatures can cause

problems both in operations and for the environment. Through sustained lowering of process

flow or sustained raising of the amount of cooling medium, the temperature specifications can

eventually be met. There are, however, design limits to this approach. Systems are often
Annex I

Industrial Cooling Systems 157

designed in such a way that specifications may be exceeded to a maximum of 5% or lower on an

annual basis.

For water-cooling, a minimal approach over the heat exchanger of 3-5K is used. Lower values

can be reached, but this requires a larger and therefore more expensive heat-exchanging surface.

With more heat exchangers in a cooling system the different individual approaches have to be

added up and the more exchangers the higher the approach. For a cooling tower an approach

temperature of 7K to 15K is often used.

For condensers of power stations the term “thermal difference”, instead of approach, is used

indicating the difference between the temperature of the condensate (steam) and the temperature

at which the cooling water leaves the condenser. To calculate the temperature at which

condensation can take place the terminal difference and the temperature increase of the cooling

water have to be added. In case a cooling tower is used the approach of this tower will have to

be added as well. For condensers practical experience shows minimal terminal differences in the

range of 3-8K, also depending on the fouling factor [tm056, Caudron, 1991].

The minimum end temperature that can be achieved by a cooling system is determined by the

approach of a cooling system and the design temperature, which depends on the climatic

conditions of the site.

I.3 Capacity of a heat exchanger

The capacity or duty of a heat exchanger is the amount of heat that can be removed. The

required heat transfer area of a cooling system is influenced by the different heat transfer

capacities of the cooling media water and air, by sensible and latent heat transfer and by the

driving force. The design has to consider material requirements, pollution, drop of pressure,

flow speed, spatial restrictions and the volume to be cooled (fluid or vapour).

The total capacity of a cooling system is determined by adding up the duties (or capacities) of

all heat exchangers:

Qtot. = ΣQi[ ] J s orW with Qi = duty of user i

Due to its physical properties, water is an ideal heat carrier because of its high thermal capacity.

It therefore only requires small heat exchange surfaces. The most effective transfer of heat is by

evaporation of water. The latent heat (evaporation of water) compared with the specific heat

capacity per ∆K at 30oC is about 630 times higher (Hv/C). The specific heat capacities of air,

water, and evaporating water are shown in Table I.2.

Table I.2: Specific heat capacities of air and water

Air:

(sensible heat transfer) Cp = 1005.6 + (16.03*10-3*t) J/kg/K

Water:

(sensible heat transfer)

C = 4192 J/kg at 10oC

C = 4182 J/kg at 50oC

Evaporation of water:

(latent heat transfer)

Hv = 2502 kJ/kg at 0oC

Hv = 2431 kJ/kg at 30oC
Annex I

158 Industrial Cooling Systems

I.4 Wet and dry bulb temperatures

The wet bulb temperature is the lowest temperature, to which air can be cooled down by

adiabatic evaporation. The wet bulb temperature always lies below dry bulb temperature and

depends on the measured temperature of the atmosphere, the humidity and the air pressure. The

dry bulb temperature is the temperature of dry air and a very important factor in the design of

air-coolers where sensible heat transfer is the underlying mechanism. Wet and dry bulb

temperatures can be the same when ambient air is fully saturated.

For latent heat transfer the wet bulb temperature is the relevant temperature and it is

theoretically the lowest temperature to which water can be cooled down. In case of wet cooling

towers where the heat is transferred from the cooling water to the air mainly by evaporation, the

wet-bulb temperature (i.e. the degree of saturation) is therefore an important design factor.

I.5 Relation between heat transfer and heat exchanging

surface

A high heat transfer and driving force (approach) will require a relatively small surface,

resulting in a compact and cost effective heat transfer concept. Because of the lower heat

transfer capacity of air, dry cooling systems require much larger heat exchanger surfaces and

driving force for the same cooling capacity. This larger heat exchange surface results in a higher

demand on space and potentially higher investment costs against the absence of costs for water

and cooling water treatment and the associated environmental effects.

The required heat exchanging area also depends on the medium that is to be cooled. As an

example of this Table I.3 shows examples of heat transfer coefficients and the associated

surface areas of various combinations of cooling water and process fluids.

Table I.3: Heat transfer coefficients and estimated surface areas A (m2) per MW and at 20K mean

temperature difference for different industrial applications

[Bloemkolk, 1997].

Hot process side Heat transfer coefficient U

(W/m2 K)

Estimated surface area(2) A

(m2 per MW)

Fluids

-organic solvent

-light oil

-heavy oil

-gases

Condensing vapours(l)

-water vapour

-organic vapour

-vacuum condensers (water)

-organics (partly condensing)

250-750

350-900

60-300

20-300

1000-1500

700- 1000

500-700

200-500

200-600

55-143

166-830

166-2500

33-50

50-71

71-100

100-250

Notes:

1. Starting point is ∆T(ln)=20°C. Cooling with water. Calculation is based on overall heat exchange

coefficient U and is meant as a comparison

2. It should be taken into account that condensing vapours per kg drain off much more heat than cooling

fluids; per MW heat discharged, relatively little vapour is therefore condensed.

3. Q=U.A.∆T(ln)

As an example of what the above could mean two cases of installed cooling systems, one for dry

cooling and one for evaporative cooling, are compared and the results are shown in Table I.4.
Annex I

Industrial Cooling Systems 159

The dry cooling tower with a cooling surface diameter of 20% more, has only 47% of the

capacity of the evaporative cooling system with an approach of 20 K compared to 12.6 K.

Table I.4: Effects of the cooling principle on the capacity, approach and cooling surface of a cooling

system

[132, Eurovent, 1998]

Characteristics Dry natural draught cooling

tower

Wet natural draught cooling

tower

Capacity 895 MWth 1900 MWth

Cooling surface diameter 145 m 120 m

Approach 20 K 12.6 K

Temperature (dry bulb / wet bulb) 14/ 10 °C 11/ 9 °C

Minimum end temperatures 34 °C 21.6°C

Annex II

Industrial Cooling Systems 161

ANNEX II PRINCIPLE OF ENERGY SAVING THROUGH

OPTIMISED COOLING

[tm059, Paping, 1995]

II.1 Subject

This Annex reviews a method of calculation of the potential energy conservation in the event

that cooling is carried out at lower temperatures. The conservation is expressed in terms of

primary energy, with a dimensionless unit of kWth/MWth per K temperature difference in the

cooling process itself. The reduction in energy consumption is achieved by the use of inhibitors

that have been subjected to a standard test method. These inhibitors ensure that the water

coolers remain cleaner during the summer months. The direct and indirect energy use because

of the choice of any of the six main cooling systems in a specific area can also be calculated

with this dimensionless number.

II.2 Summary of conclusions

• In practice the variable temperature gradients across fouling are of the order of magnitude

of 1 to 4K.

• Colder cooling water in the immediate vicinity of the pipe wall results in energy

conservation of 31⁄2 kWth/MWth/K. This is equivalent with EUR 300 per MWth cooling per

annum per K temperature difference.1

• The use of an efficient manner to combat the decrease in the heat transfer coefficient across

fouling of heat exchangers results in a potential energy conservation for the European

industrial sector pro ratio of this energy conservation factor. For every 100 GWth cooling in

Europe the energy conservation potential is 11 PJth per annum2

 per K. This is equivalent

to a reduction of the emissions of carbon dioxide of 700 thousand tons per annum per K for

every 100 GWth cooling in Europe.

• The involved energy consequences of the choice for each of the six main cooling systems

are even more significantly. The difference in minimum attainable cooling water

temperature for each of the six main cooling water systems can also be expressed in

necessary energy expenditure to cool. The same “dimensionless” factor of 31⁄2 kWth/MWth/K

can be applied for comparison of the design alternatives.

• The use of certain inhibitors in cooling water results in significant energy savings. These

savings can exceed the primary energy content of the additives by several factors of ten.

The environmental impact decreases in proportion to these savings.

• The energy conservation achieved by the use of inhibitors exceeds the costs of the relevant

additives by a very wide margin.

An illustration of both effects, as discussed in this Annex, is presented in Figure II.1. A lower

temperature of the coolant as well as the use of antifouling treatment will effect the heat transfer

through the exchanger wall and the waterfilm.

 

1 Based on $14.7 /barrel; with an energy value of 41.87 GJth/ metric tonne oil equivalent and 7.45 barrels/ metric tonne oil

equivalent, representing 5.51 barrels per kWth year or 81 EUR /kWth year (Ministry of Economic Affairs, Netherlands, oil/gas

conversion table NOVEM).

2 PJth = Petajoule thermal = 1015 Joule.
Annex II

162 Industrial Cooling Systems

Tproduct

∆θfouling

∆θwaterfilm

}

Every 0.12 mm = 1 K

Potential energy saving

Extra temperature gradient = Energy loss

pipe

} Potential energy saving

∆θdifference cooling system choice

Tproduct,

minimum energy

through BAT

Tcooling water

Figure II.1: Illustration of areas of potential energy saving by reduction of the temperature

gradient through a fouling layer as well as by using colder cooling water influent

II.3 Introduction

Following many informal contacts a number of multinationals with branches in the Europe

founded the industrial cooling-water group in 1991. They took the initiative to develop a

standard test method for cooling-water inhibitors. The basis of this method was formed by the

research carried out in the nineteen-eighties by DSM and Shell. The “Industrial Cooling-Water

Project” was set up in the form of a joint project in which both the customers for and suppliers

of these inhibitors participated.

The potential conservation in energy - in combination with the reduction in malfunctions of the

production process achieved by the use of cleaner heat exchangers - was the most important

motive for all partners to decide to invest in the project. This Annex discusses the issue in more

detail.

The generation of shaft-power and/or electricity is always accompanied by cooling; this is

necessary since only a maximum amount of useful energy can be extracted from a medium at

any given ambient temperature. A proportion of this energy will pass through a number of

process steps, and will ultimately reach the factory’s cooling-water system. The yearly

averages3

 for the standard direct energy consumption expended in cooling, expressed in terms of

the ratio KWe / MWth, cooling and assuming clean coolers in the summer months, is shown in the

following table.

 

3 ASHREA Handbook, 1983 Equipment volume; American Society of Heating, Refrigeration and Air-conditioning Engineers

Inc., Atlanta, USA, 1983.
Annex II

Industrial Cooling Systems 163

Table II.1: Energy consumption in kWe, electricity consumption / MWth, cooling with clean heat exchangers

Cooling water-system Energy consumption in kWe, electricity consumption / MWth, cooling

with clean heat exchangers.

Σ Cooling-water

pump

Fan

Once-through cooling water ≈ 10; range 5 to 25 5 to 25 n/a

Circulation-cooling water with open

wet cooling tower ≈ 15; range 10 to 25 5 to 20 5 to 10

Closed recirculating system ≈ 30; range 20 to 60 5 to 15 10 to 50

With an efficiency of the generation of electricity of 40%4 the above figures will need to be

multiplied by 2.5 to express the energy consumption in terms of the primary energy carrier. The

relative energy consumption required to cool by means of a water system then becomes

dimensionless.

Moreover this energy consumption will increase in direct proportion to the (higher)

temperatures of the cooling water and/or (increased) fouling of the coolers during the summer

months. To calculate the indirect energy consumption in a more reliable way the previous tables

in this document have to be linked. (The climatic conditions of the European countries

mentioned in Table 1.6, and the technical and thermodynamic characteristics of the different

cooling systems for industrial applications for a specific general site’s climate mentioned in

Table 2.1). The following table shows the lowest attainable water temperatures at inlet for the

various cooling systems during the months of July and August in the Netherlands.

In practice virtually no tap water or groundwater is used for cooling. Moreover it is less

desirable to make use of these kinds of water for this purpose, and the practice is gradually

being discontinued. Consequently it is possible to state that once-through cooling-water systems

located on the coast consume the least amount of primary energy.

Table II.2: Mean attainable lowest cooling water inlet temperatures for the various cooling systems

during the months of July and August in the Netherlands.

Cooling system Mean attainable min.

temp.[°C]

Remarks; Netherlands as

example

Once-through cooling: river 23

Local limitations of heat

discharge by thermal limit

values

Seawater along the coast 19 North sea 12 °C below

mixing zone

tap-water 15 Cost price

groundwater 12 Limited stock

Open wet cooling tower 24 Wet bulb 19°C

Air cooling; (for comparison) 40 (product temp.) Dry bulb 28°C

 

4 Central Bureau for Statistics, CBS, The Hague.
Annex II

164 Industrial Cooling Systems

Moreover the energy consumption will increase when the heat exchangers are fouled. This

sediment can be classified into microfouling and macrofouling. Blockages caused by shellfish

and other solid sediment preventing the flow of water through the pipes may be regarded as

macrofouling5

. Microbial slime, scaling, deposits, that each in turn result in the formation of

corrosion products on or in the warm cooling pipe, are all classified as microfouling6

. A

characteristic common to all the various forms of fouling mechanisms is the concomitant

increase in the internal energy consumption.

This annex discusses the additional energy consumption per K extra temperature gradient in

more detail as well as it presents the calculation basics for the direct and indirect energy

consequences of .the choice of each of the six cooling systems.

II.4 Calculations

II.4.1 Principles

Industrial cooling installation ∆ θ log = 10 K (= driving force)

Φ t = 5 kWth m-2 (= heat flux)

consequently a U total = 0.5 kWth m-2 K-1 (= overall heat

transfer coefficient)

Assume a deposit δ fouling = 0.12 mm (= variable resistance)

with a λ fouling = 0.6 W m-1 K-1(= thermal

conductivity)7

Heat transfer is conceived to be analogous to a series of resistances between the product to be

cooled, the pipe wall, the laminar layer of water, and the variable degree of fouling:

total αproduct αpipe wall αwater layer αfouling

1

+

1

+

1

+

1 = U

1

Tproduct

∆θpo llutio n

∆θwaterfilm

Tcoo ling w ater

}

Every 0.12 m m = 1 K

Potential energy savin g

Pollution = Extra tem perature gradient

pipe

Figure II.2: Graphic representation of pollution factors responsible for extra temperature gradient

over the pipe wall

 

5 Dutch experience with condenser maintenance, E.J. Sneek, H.A. Jenner, KEMA, Arnhem.

6 Practische waterbehandeling (Practical water treatment), Prof. J. Defrancq, de Sikkel Malle.

7 VDI-Wärmeatlas. Berechungsblätter für den Warmeübergang, sechste erweitete Auflage, VDI-Verlag, Düsseldorf.
Annex II

Industrial Cooling Systems 165

= 1K





 





















* 10 K

0 12 mm

0 6 W m K

0 5 kW m K * = U * = U =

Together :

 + + + 1=

Relative temperature gradients : 

 U + U + U + U 1=

Relative heat transfer coefficients :

-1 -1

-2 -1 t

fouling

fouling

total

fouling

total

fouling

product pipe wall water layer fouling

pollution

total

water layer

total

pipe wall

total

product

total

.

.

. log log

log log log log

θ

δ

λ

θ

α

θ

θ

θ

θ

θ

θ

θ

θ

θ

α α α α

∆ ∆ ∆

∆

∆

∆

∆

∆

∆

∆

∆

Consequently a

∆ θ fouling = 1 K

It is assumed that fouled water coolers in industrial installations have virtually no influence on

energy consumption during the eight colder months of the year. This is based on the necessity of

preventing certain process flows from being cooled to too low a temperature. Factories shall

always keep at least one cooling-water pump in operation; a fan that has been stopped is

incapable of a further reduction in its capacity. Savings can be achieved during a longer period

of time in power stations. However, industrial cooling installations are usually deployed to cool

a variety of products, and consequently exhibit a great variety in types of heat-exchange

material. As a result of this diversity, industrial cooling installations have a need for an

appropriate design of the use of inhibitors to accommodate their complex systems, in turn

resulting in the utilization of the maximum benefits of the calculated energy conservation during

only four months in a year. Consequently the calculations in this annex are restricted to this

short period, and are applicable only to the cooling capacity of water systems used in industrial

installations. The reason for these restrictions is that the interpretation of cooling-water

additives would have been undermined if power stations had also been included in these

calculations of the energy conservation.

In practice one or more “solutions” are used to remedy situations in which the design of the

cooling system in combination with the inhibitor is incapable of supplying the required cooling

capacity during the summer months. (These solutions are comprised of increasing the amount of

cooling water and/or cooling air and/or the product temperature and/or the process pressure). All

such solutions share the common characteristic of a concomitant increase in the internal energy

consumption.
Annex II

166 Industrial Cooling Systems

Tin

t

out

Tout, product

tin, cooling water

Length heat exchanger

Pollution compensation through:

* more cooling water

* more cooling air

* higher product temperature

* higher process pressure

Driving force

Figure II.3: Schematic representation of driving force over the length of a heat exchanger

In the following calculations the additional energy consumption resulting from microfouling is

expressed in terms of a 1°C extra temperature gradient across the fouling. The formulae shown

on the previous page indicate that an extra 1 °C temperature gradient already becomes apparent

once a microbial slime layer has attained a thickness of no more than an order of magnitude of

0.12mm. This is also applicable to a layer of boiler-scale with a thickness of the same order of

magnitude. The calculations assume that the heat transfer decreases linearly with the thickness

of the fouling. The reduction in heat-exchanger capacity caused by the first 20% of blocked

cooling pipes due to macrofouling can be compensated by increasing the required temperature

gradient by 1°C. However every additional blocked cooling pipe will result in an exponential

increase in the additional temperature gradient required.

Cooling tower discharging 10 MWth Φ = 1000 m3

 h-1 circulation flow rate

∆ θ = 8.6 K cooling water

inlet/outlet

∆ θ log = 5.0 K cooling tower cooling

limit

II.4.2 Quantity of cooling water ↑

The following is an example of a calculation in which a 1K extra temperature gradient across

microfouling is, in practice, compensated by an increase in the number of cooling-water pumps.

The temperature and flow-rate of the product and the quantity of heat removed from the product

remain constant. The average pressure drop is adjusted to 3.7 bar.

The capacity will be doubled by the parallel operation of two identical centrifugal pumps only

in the event of a purely static pressure. However every successive cooling-water pump brought

into operation in a cooling-water system will result in a change of the dynamic head

characteristics. As a consequence of these changes the nominal capacities of the pumps may not

be totaled. The resultant reduction in the capacity requires increased power consumption8

.

The removal of an equal quantity of heat requires the circulation of an extra 10% of cooling

water (= 20% extra pump energy) to compensate for the extra 1 K temperature gradient:

 

8 Pompen (Pumps), L.W.P. Bianchi, Stam, Culemborg.
Annex II

Industrial Cooling Systems 167

More cooling water >> increased pumping power

1 kWe / MWth per K

Q1 pump

∆Q

H

Q2 pumps< 2 * Q 1 pump

Q2 pumps

Figure II.4: Number of cooling water pumps and change of cooling water low due to fouling

= 30 kW per 10 MW cooling.

s

Nm = 30* 10

s

m* 

s

kg .m

0 7

20* 10 =

0 7

* 37 m water gauge

s

m * 9 8

m

kg * 1000

s/h

m /h

3600

1000 20%* 

=

η

20%* Φ* [ ρ* g * ∆H ] = η∆φ* P W is

e th

3

2

3

3 2

3





 









 

















	
























 





.

.

.

Corrected for 4 summer months per year:

on an annual basis 1 kWe / MWth per K

II.4.3 Quantity of cooling air ↑

The same cooling tower of 10 MWth is used, but the removal of the same quantity of heat is now

achieved by compensating for the 1 K extra temperature gradient across the microfouling by

increasing the quantity of cooling air the fans pass through the cooler. The power consumption

of the fan increases from 54 kWe to 83 kWe per 10 MWth

9

:

 

9 Calculation from POLACEL, Doetinchem (NL), and practical experience with variable pitch in combination with start/stop of

one or more fans.
Annex II

168 Industrial Cooling Systems

Corrected for 4 summer months per year:

on an annual basis 1 kWe / MWth per K

Conclusion: an increase of either the amount of cooling air or the cooling-water flow

rate requires the same annual amount of extra energy.

II.4.4 Product temperature ↑; gas volume ↑

0 °K

1 °K

2 °K

1 °K

Length heat

M ove more gas volume; PV =

polluted heat exchanger >> warmer

Driving

Cooling

Product

2 kWe / MWth per K

Figure II.5: Change of temperature gradient of product gas (moles) in a counter flow heat

exchanger due to fouling

The inlet temperature of the product is assumed to be constant. This means that the decrease in

the outlet temperature of the product discharged from a fouled heat exchanger will be less than

that from a clean heat exchanger. The temperature gradient across the fouling will be virtually

uniformly distributed along the entire length of the heat exchanger. As a result of these two

effects the reduced temperature drop of the product at the outlet of the fouled heat exchanger

will be twice the magnitude of the temperature gradient across microfouling on the waterside of

the heat exchanger. A pure countercurrent heat exchanger in which the energy content per K

temperature change of the cooling-water flow rate (= in fact the specific heat of a quantity) is

greater than that of the product to be cooled will exhibit convergent temperature profiles at the

outlet side of the heat exchanger. The reduced temperature decrease on the product-outlet side

caused by microfouling on the waterside will consequently be less than 2 °C per K temperature

gradient across the fouling. Conversely, energy content per K temperature change of the

cooling-water flow rate smaller than that of the product to be cooled will result in divergent

temperature profiles at the outlet side of the heat exchanger. The reduced temperature decrease

on the product-outlet side caused by the same microfouling on the waterside will then be in

excess of 2 °C per K temperature gradient across the fouling on the water-side.

Under conditions of adiabatic compression, at a constant product mass and compression ratio,

the removal of the same quantity of heat is now achieved by compensating for the extra 1K

temperature gradient across the microfouling by increasing the final temperature of the product

to be cooled by 2 °C.

The formula for the same cooling tower of 10 MWth will than be:
Annex II

Industrial Cooling Systems 169

 - = percent more compressor work per C

 + 

 + 

 = 

 extra compressor power consumtion

T

T

per C product temperature increase 

 - P

P * 

κκ W = V * dP = φ * R * T * 

in

in + C

out

in κ * η

κ - 

v in

 P

P

pol

out

in

 ° 

 





= ° →

=









	




















 







°



1 0.67 2

273 25

273 27

2

compressorwork is a linear function of the inlet- gastemperature.

1

1

2

1

Corrected for 4 summer months per year:

on an annual basis 2 kWe / MWth per K

Conclusion: the displacement of volume is more expensive than mass transport.

II.4.5 Product pressure ↑; cooling compressor ↑

For the same cooling tower of 10 MWth, where, for the same mass flow rate, the removal of the

same quantity of heat is now achieved by means of product condensation (for example, for the

benefit of a refrigerating machine based on adiabatic compression and expansion). This will

raise the process-pressure of the product to be cooled so as to compensate for the 1K extra

temperature gradient across the microfouling:

L L/G G

Molier diagram

P1

P2

H [J/kg]

P

2 kWe / MWth per K

M

Increased dewpoint = increased process

Figure II.6: Representation of increase of process pressure to compensate temperature increase due

to fouling

Assumptions: propene-refrigerating machine

inlet and outlet pressures 1.9 and 13 bar respectively

boiling points - 33.6 and 28.5 °C respectively

pressure-temperature gradient 0.33 bar K-1

compressor and turbine efficiency 0.66
Annex II

170 Industrial Cooling Systems

= 594 percent per K

3 852* 0 66

1 - 1 0151 Extra compressor power cons =

=1 0151

62 14

63 14 =

29 50 -(- 33 64)

273 15 - 33 64

28 50 -(- 33 64)

273 15 - 33 64

=

θ -θ

θ

θ -θ

θ

Cooling factor difference=

=3 852

28 50 -(- 33 64)

273 15 - 33 64 = θ -θ

θ Cooling factor Carnot Cyclus = 

at 29 5 C

at 28 5 C

in out

i.n

in+ C

in out

in

in

in out at 28 5 C

in

 in

0. . .

. .

. .

.

. .

. .

. .

. .

. . .

. .

.

.

1

.





 









 









 









 









 



 



 





°

°

°

°

Corrected for 4 summer months per year:

On an annual basis 2 kWe / MWth per K

Conclusion: “Frigories” are twice as expensive as calories.

II.5 Total potential energy conservation per °C colder

cooling-water boundary layer

II.5.1 Efficiency of power generation ↑

The efficiency of the generation of electricity in the Netherlands is 40% 10.

S [J/kg]

P1

P2

Reuse steam 40 % eff .

Blow off steam 20 % eff .

Power

Latent heat

H [J/kg]

M

Figure II.7: Efficiency power generation for pumps, fans and compressors

The generation and consumption of steam in a petrochemical complex are usually in balance

during the autumn and the spring. In the winter high-pressure and medium-pressure steam will

be diverted to the low-pressure steam network. Conversely, in the summer some of the lowpressure steam will be disposed of by blowing it off to the atmosphere and/or condensing it in

 

10 Central Bureau for Statistics, CBS, The Hague.
Annex II

Industrial Cooling Systems 171

coolers. Moreover a number of steam pumps will be shut down, and some electric motors will

be started up, so as to compensate for the shift in the energy balance.

Steam turbines are also used as auxiliary turbines to power refrigerating machines. In such

situations the extra low-pressure steam produced and exhausted during the summer (the

additional amount generated as a result of fouled product condensers) will be disposed of. The

extra energy consumed by the refrigerating machine will then be accompanied by extra energy

losses, which will be a magnitude of 5 larger. The enthalpy of superheated high, medium and

low-pressure steam is only one-fifth higher than the latent heat of water.

The Dutch Industry uses 20% of the total cooling capacity for compressor gas and refrigerated

cooling 11. Half of this refrigerated cooling is carried out by means of (auxiliary) steam turbines.

“Frigories” (or “negative” calories) are more expensive than calories, and volume displacements

are more expensive than mass transport. In practice this means that during the summer months

preference will be given to starting up cooling-water pumps and fans earlier, and shutting them

down later, rather than increasing the process pressures. Furthermore, in the first instance 80%

of the capacity lost by inefficient cooling due to fouling will be compensated by means of

start/stop commands to motors.

Consequently the average increase in primary energy per K temperature gradient is:

kW / MW per K . . . = 3.5 th th 0 2

10 %* 2

+

0 4

10 %* 2

+

0 4

80 %* 1 



 



 



 



 



 





Note: The above expressed energy factor can be calculated for other European countries as well, if

the relative cooling capacity for compressor gas and refrigerated cooling is known. It is

expected that the outcome will range between 3 and 4 kWth / MWth per K, since the power

generation efficiency in Europe is also around 40%.

II.5.2 Total water used for cooling in the Dutch industry (excl. power

plants)

Numbers regarding cooling water consumption, have to be available to calculate the absolute

direct and indirect energy usage as well as the absolute environmental impact of each of the six

main cooling systems on a European scale. In the Netherlands, these statistical data were

available with a spread over the last decades. The majority of the water consumed by the

national industry (= 3.1 * 109

 m3 per annum) is destined for use in cooling. The total amount of

water used for cooling purposes amounts to 2.7 * 109

 m3 per annum (for power plants 8.3*109

m3 p.a.). Of this water the majority is fresh, salt or brackish surface water. The original

statistical sources differentiate between water used for general purposes and for cooling.

However this data makes no differentiation between the proportions of water used to feed

recirculating and once-through cooling-water systems. Calculations indicate that the amount of

heat removed by water systems in the Dutch industrial sector is of the order of magnitude of 180

PJth per annum 12 (= 5.7 GWth

13) and equally partitioned between cooling towers and once

through systems.

 

11 EST Consult B.V., Woubrugge, December ’90, NESR003. Marktonderzoek naar het electriciteitsverbruik en de mogelijkheden

van electriciteitsbesparing in de Nederlandse industrie (Market research into the electricity consumption of the Netherlands

industry, and opportunities for electricity conservation).

12 CBS, Heerlen: key figure K-261/1991; water provisions for companies 1991, and every five years previously from 1957. CBS,

Voorburg; key figure K-117/1992-1 & -2; Dutch energy consumption, annual figures 1992 and the prior decades from 1972.

13 Also verified with data from:

- Emissie Registratie warmte via water E 260 tot en met de 6e

 ronde (registration of heat emissions via E260 water up to and

including the 6th round (for the most important companies from an environmental perspective) in the 1990’s); RIVM,

Bilthoven.

- Author’s survey of the top ten companies in the Netherlands; data RIZA; Lelystad, Netherlands.
Annex II

172 Industrial Cooling Systems

II.5.3 Total potential energy conservation per °C colder cooling-water

boundary layer

The figure of 180 PJth per annum, removed by cooling-water systems, in combination with an

average extra annual energy consumption of the order of magnitude of 31⁄2 KWth / MWth per °C

colder cooling-water boundary layer, would result in a potential energy conservation for the

Dutch industrial sector amounting to:

0.63 PJth per annum per °C

Or in financial terms:

EUR 1.6 million per annum per °C

Assuming that the Dutch situation represents only 5% of the European industrial production

capacity where cooling is involved, it can be estimated that the cooling capacity in Europe

applied by industry is in the magnitude of 120 GWth, where power plants cool 200 GWth. For

the whole cooling sector in Europe the potential energy conservation would then amount to:

35 PJth per annum per °C

Or in financial terms:

EUR 100 million per annum per °C
Annex II

Industrial Cooling Systems 173

II.6 Examples of calculations for the relative conservation of

energy and reduction of the environmental impact

achieved by the use of inhibitors

II.6.1 The contribution made by oxidation

The following is an example of the use of inhibitors based on oxidants such as sodium

hypochlorite (because this is a relatively well-known degradable additive):

Assumptions:

Basis:

electrolyte (or bulk purchase) 2.2 KWhe / kg chlorine equivalent

- production efficiency 0.7 We / We

- thermal efficiency 0.4 We / Wth

- concentration 15 %

cooling installation

- thickness of slime layer 0.5 mm (no inhibitor in

summer)

- boundary-layer temperature gradient 4 K

- average “conservation ratio” 3.5 kWth / MWth/K

use of inhibitor

- influent ≈ 1.0 mg l-1, stoichiometric oxidation,

- effluent ≤ 0.1 mg l-1, active chlorine

- intermittent chlorination, such as 4 hours in use/out of use

- 1% conversion of dosed chlorine equivalent to halogenated by-products also

expressed as chlorine equivalent which amounts to approximately to 3% of

brominated hydrocarbons [tm160, Bijstra, 1999].

II.6.1.1 Once-through cooling system

usage of hypochlorite 300 kg Cl/MWth [tm160, Bijstra, 1999]14

costs of hypochlorite 114 EUR / metric tonne in tank truck

Consequently both the energy conservation ratio15 and the performance-price ratio of the inhibitor

are of overriding importance. The use of this oxidant achieves environmental energy conservation

several factors of tens in excess of the primary energy content of the inhibitor.

The environmental energy yield of this inhibitor is more than a factor 10 higher than the financial

conservation 16.

The use of hydrogen peroxide or ozone will result in a decrease of the above-mentioned ratios.

 

14 Based on chlorine demand in the north-west European delta.

15 The energy conservation ratio is a dimensionless number that compares the energy conservation achieved by the use of the

inhibitor with the primary energy content of the relevant additive.

16 The financial conservation ratio is a dimensionless number that compares the financial conservation achieved by the use of the

inhibitor with the cost of the relevant additive.
Annex II

174 Industrial Cooling Systems

EURO

EURO = EURO / mt

EURO 1 / kW per year * 

2 mt hypochlorite / year per MW cooling

14 kW / MW

Financial conservation ratio=

J

J =

0 7 W / W * 0,4W / W

2 2 kWh / kg chlorine

3 5 kW / MW .K * 4K *

300 kg " as chlorine" per annum / MW

8760 hr/yr

Energy conservation ratio=

input

th output

th

th th

input

output

e e e th

e

th th

th

5

52



































































 





 















114

8

.

.

.

Note: mt = metric tonne(s)

The dimensionless environmental energy ratio can be supplemented by the calculation of an

additional relative environmental load mass ratio. This also indicates the energy conservation

achieved by the use of inhibitors, but now expressed in terms of the ratio of the reduction in the

emissions of carbon dioxide to the production of unwanted precursors as a result of oxidative

side reactions.

C - X

CO =

0 7 W / W * 0 4W / W

2 2 kWh /kg chlorine* 3 6 MJ / kWh

% conversion to halogenated ones

31 6 MJ / Nm natural gas

1 94 kgCO / Nm natural gas

*

J

J 52

Environmental mass ratio=

2

e e e th

e th th

3

th

3

2

input

output 3000



































 























. .

. .

3

.

.

However the quantity obtained from this formula is not entirely dimensionless. The environmental

mass ratio can be introduced for the estimation of the total environmental effect ratio, for instance,

by introducing the effect ratio of specific chlorinated by-products in seawater, such as bromoform

(84%), dibromo-acetonitril (10%) and trihalomethanes, such as dibromochloromethanes and

bromodichloromethanes (5%) [tm157, Jenner et al, 1998].

Their unwanted formation, which is lineair with the amount of chlorination of the system, can then

be compared with the resulting reduction in the amount of energy used, when both are expressed in

units of CO2 and its corresponding ozone depleting effect.

II.6.1.2 Open recirculating system

water volume; basin + pipes 50 m3 / MWth

metering (for 3 mg/m3) 1.0 l h-1

metering time, discontinuous 1.0 h / day

costs of hypochlorite 160 EUR / metric ton in 1 m3 multi box containers
Annex II

Industrial Cooling Systems 175

For continuous dosage regimes and/or less adequate process controlled dosages approximately 3-

times more chlorine-equivalents per MWth are required.

EURO

EURO = EURO / metric tonne

EURO / kW per year * 

365 l hypochlorite/ year per MW cooling

14 kW / MW

Financial conservation ratio=

J

J =

0 7 W / W * 0 4W / W

15 %* 2 2 kWh /kg chlorine

3 5 kW / MW .K * 4K * 

1l hypochlorite/day

1 MW h* 24 hours/day

Energy conservation ratio=

input

output th

th

th th

input

output

e e e th

e

th th th

20

285





 



 

















































 





 















160

81

. .

.

.

The energy conservation ratio and the performance-price ratio of the same oxidant used in a

circulation system are higher than those for a once-through cooling system. Conversely the

consumption of primary energy required to cool by means of a recirculating system is higher

than that of a once-through cooling system. Consequently most of the largest power stations are

located in the vicinity of the coast.

The relative environmental load mass ratio can also be calculated for this system.

C - X

CO =

0 7W / W * 0 4W / W

2 2 kWh /kg chlorine * 3 6 MJ / kWh

% conversion to halogenated hydrocarbons

31 6 MJ / Nm natural gas

1 94 kgCO / Nm natural gas

* 

J

J 285

Environmental mass ratio=

2

e e e th

e th th

3

th

3

2

input

output 16000



































 























. .

. .

3

.

.

Once again, the mass ratio between the required reduction of emissions of carbon dioxide and the

unrequired emission of halogenated hydrocarbons of the same oxidants – but now applicable to a

recirculating cooling system – is higher than that of a once-through cooling system. Conversely, the

consumption of primary energy by a cooling-tower system is higher than that of a once-through

cooling system.
Annex II

176 Industrial Cooling Systems

II.7 Examples of calculations of the relative savings in

energy with colder cooling water

II.7.1 Coastal water versus cooling towers

Assumptions 17: -influent temperature coastal water 19 °C

cooling tower 24 °C

effluent pressure once-through cooling system 1 mwg

cooling tower system 14 mwg

(height of tower + nozzle)

Raising the cooling water to an additional height and then spraying it through the nozzle results

in an additional power consumption of the pump per MWth heat removed by means of a cooling

tower system:

[ ]

= kW per MW cooling

s

Nm =5* 10

s

kg .m * 

s

m * 

0 7

3 6 * 10 =

0 7

* 13 mwg

s

m * 9 8

m

kg * 1000

 s / h

m / h

3600

100

 =

η

φ * ρ * g * ∆ H 

 = 

η

∆φ * P W = 

e th

3

2

3

3 2

3

5 



 



 



 









 





.

.

.

.

The power consumption of the pump expressed in terms of

primary energy throughout the entire year is 12.5 kWth per MWth cooling

The cooling water is on average 5 °C warmer, so 17.5 kWth per MWth cooling

Together, a difference in the energy consumption of 30.0 kWth per MWth cooling

Consequently in the light of the benefits to energy conservation large cooling systems are, in

practice, preferably built in the form of once-through cooling systems on the coast.

II.7.2 River water versus cooling towers

The temperature difference of a once-through cooling system, supplied with river water, in

comparison with a cooling tower is in the order of magnitude of about 1 K. Together with the

maintenance of the necessary pressure difference across the cooling water the total difference in

energy consumption amounts to 16 kWth per MWth cooling.

 

17 Onderzoek industrieel waterverbruik (Survey of industrial water consumption), final report, F.C.A. Carner, Krachtwerktuigen

Amersfoort, 1992.

Samenwerkende Rijn- en Maas waterleidingbedrijven 1980 – 1992, RIWA, Amsterdam.

Jaarboeken monitoring Rijkswateren (State Water Monitoring Annuals) from 1980.
Annex II

Industrial Cooling Systems 177

II.7.3 Groundwater versus cooling tower

The temperature difference of a once-through cooling system supplied with groundwater in

comparison with a cooling tower is the largest, i.e. 12 K. The total difference in energy

consumption required for cooling is then 42 kWth per MWth cooling. It was assumed that the

power consumed by a pump that pumps up groundwater is of the same order of magnitude as

the power required to maintain a cooling-tower effluent pressure difference. However, the

linited availability of groundwater restricts the use of this way of energy conservation.

II.8 Appendix environmental impacts

Table II.3: Conservation ratios for once-through and recirculating cooling system

Type of cooling system

EnergyConservation ratio

J output / J input

FinancialConservation ratio

EUR output / EUR input

MassRatio environment

CO2 / C-X

Once-through cooling system 52 5 3000

Open recirculating cooling

system

285 20 16000

Table II.4: Energy conservation with potential colder cooling water source

System comparison kW th per MW th Remarks

Coastal waters versus cooling towers 30 geographically specified

Riverwater versus cooling towers 16 local thermal burden

Groundwater versus cooling towers 42 limited stock

The above-presented numbers can be used to show the outcome for specific areas in Europe,

like for instance a high-industrialised area like the Netherlands. The replacement of all industrial

cooling towers by once-through cooling systems supplied with river water would result in a

national energy conservation of 91 PJth

18 * 16 kWth/MWth = 15 PJth per annum (equivalent to a

reduction of the emission of carbon dioxide of 93000 metric tonnes per annum). This would

require the availability of 85 m3

/sec river water throughout the entire year. Conversely, only

those cooling systems located at a relatively small distance from a river would come into

consideration for such a replacement; the distances otherwise involved would negate the energy

advantages offered by the use of this source of water. So it isn’t surprising that most industries

and power stations are located nearby rivers and coastal areas, emphasising the importance of a

proper cooling design and location choice.

 

18 Water symposium 1995; syllabus 43, Nederlands Corrosie Centrum Bilthoven.

Annex III

Industrial Cooling Systems 179

ANNEX III SHELL AND TUBE HEAT EXCHANGERS FOR

INDUSTRIAL ONCE-THROUGH COOLING

SYSTEMS AND THE OCCURRENCE OF LEAKAGE

The design of the heat exchanger is extremely important, as it is the key element of a cooling

system, where the exchange of heat takes place. From an environmental point of view it is there,

where leakage from process substances to the coolant can occur. In once-through cooling

systems the relevance of a well designed, operated and maintained heat exchanger is obvious.

From a preventive approach attention should be paid to the issues presented in this Annex

before considering a move towards an indirect (secondary) system. This Annex gives a

summary of a number of important issues to take into account in the design of the commonly

used shell and tube heat exchanger to avoid environmental problems [tm001, Bloemkolk, 1997]

The shell and tube heat exchanger consists of a shell, a great number of parallel tubes, tube

plates (tube sheets) baffles and one or two heads. Heat exchange between the media takes place

by pumping the one medium through the tubes and the other medium around the tubes. As this

happens the heat is transported through the tube wall. Diagonally on the tubes are baffles. The

baffles ensure a better transfer of heat (through increased turbulence of the flow around the

tubes) and support the tubes. The shell & tube heat exchanger is reproduced in a picture below.

Figure III.1: Major components of shell & tube heat exchanger

[tm001, Bloemkolk, 1997]

There are a great many different types of shell & tube heat exchangers. By making the right

choice from the design parameters below, the design can be adapted to the specific process and

maintenance requirements:

• the type of shell and head

• the type of tube (straight or U-shaped, with or without fin)

• the size of the tube (diameter and length)

• the distance between the tubes (pitch) the configuration (lay-out)

• the number of baffles the type of baffle

• the distance of the baffle (baffle pitch)

• the number of passes through the tubes (tube passes)

• the flow pattern (counter flow, concurrent flow)

• either mechanical or not, cleaning either with (high pressure) water or not
Annex III

180 Industrial Cooling Systems

The Tubular Exchanger Manufacturers Association (TEMA) has drafted a nomenclature for the

various types of shell & tube heat exchangers. TEMA has also drafted mechanical design

guidelines.

The advantages and disadvantages of the shell & tube heat exchanger are listed below.

Advantages:

• available for all applications

• available in almost all materials

• wide range of flows and capacities (duties)

• sturdy, safe construction

• good thermal and mechanical design methods available

Disadvantages

• relatively expensive per m2

 heat exchanging surface area

• not optimal for heat transfer

• cleaning (drawing the tube bank) of the shell side is laborious

Because of the sturdy and safe construction of the shell & tube heat exchanger, refineries prefer

this type of heat exchanger. The choice of this type of shell & tube heat exchanger for oncethrough systems is explained further below.

III.1 Design of the shell & tube heat exchanger for one

through systems

As a rule, the shell & tubes of the TEMA-type AES are used for once-through systems. The

cooling water flows through the tubes and the process medium through the shell. AES refers to

the codes used to describe the different options for shell and tube heat exchangers (Figure III.2)

Allocation media

Because the tube side of the shell & tube heat exchanger can be cleaned easier and better than

the shell side, heavy fouling media are allocated to the tube side. Because of the use of

corrosion-resistant materials for corrosive cooling water, it is also more economic to have the

cooling water flow on the tube side.

A-Type of front end head

Opening the shell & tube heat exchanger for inspection and maintenance is easiest with an Atype 'front end head', because the connecting tubes do not need to be dislodged when opening

this type of head. For this reason, this type of head is almost always used for heat exchangers

with a "polluting" medium on the tube side.

E-Type of shell

The choice for the type of shell depends on the process requirements for the medium on the

shell side. Usually, the E-type is chosen ("one-pass-shell").

S-Type of rear end head

The choice of the type "rear end head" is determined by factors relating to:

• the need to clean (mechanically or with water) the shell side

• the need to clean (mechanically or with water) the tube side

• quality of the cooling water (corrosive, scaling, etc.)

• occurrence of thermal expansion between shell and tube material

• need for counterflow
Annex III

Industrial Cooling Systems 181

Figure III.2: Heat exchanger nomenclature (Standards of the tubular Exchanger Manufacturers

Association)

[tm003, Van der Schaaf, 1995]

Usually the S-type ('floating head' type) is chosen because this type can be cleaned

mechanically (or with water) on both the shell and the tube side. There are also no problems

with this type in the event of thermal expansion differences between the shell and tube material.

However, the S-type is the most expensive type of 'rear end head'.
Annex III

182 Industrial Cooling Systems

III.2 Leakage in shell & tube heat exchangers

Leakage, and with it contamination of cooling water by the process medium, occurs in shell &

tube heat exchangers in most cases as a result of flaws in the tube-tube plate connection, flaws

in the tube itself and flaws in the flanged connection that separates both flows ('floating head').

Leakage can occur primarily as a result of:

1. poor design (about 30% of the cases)

2. poor manufacturing

3. operation that is not within the limits of the design (50-60%)

4. poor inspection and maintenance

1. Poor design

Because a wrong or poor design irrevocably leads to leakage, during the design phase careful

thought must be given to the design parameters below:

• choice of material

• choice of tube-tube plate connection (rolled or welded)

• choice of packing type

• type of detail on packing faces

• design of the flanges (thickness, no rotation)

• design of the tube plate (thickness, no bending)

• design of tube support

In this consideration, attention should be given to the 'operating' conditions. They are:

• occurrence of vibration

• thermal expansion differences

• flow division

• flow speeds

Poor design also includes wrong design of flow speeds and design based on incorrect data.

2. Poor manufacturing

A good design, however, is not sufficient to prevent leakage. Poor manufacturing can also lead

to leakage during operation of the heat exchangers. During manufacturing, the following aspects

play a role when it comes to prevention of leakage:

• tightening procedure of the flange bolts

• smoothness of the treatment of the packing surfaces

• tube hole diameter and tolerance in tube plates and baffles

• rolling or welding procedure for tube-tube plate connection

3. Operation that is not in keeping with the design

Operations that differ from the operating conditions for which the heat exchanger has been

designed can lead to damage and ultimately to leakage in the heat exchanger.

Different operation could be:

• thermal shocks

• 'upset' condition such as over-pressure and/or too high temperature

• increased or reduced through-flow of flows

• vibrations

• mussels in tubes (which have become detached from the cooling water tube) combined with

vibrations

In addition, incorrect treatment can occur during maintenance, for instance during cleaning of

the tubes with steam or warm water (damage as a result of thermal expansion).
Annex III

Industrial Cooling Systems 183

4. Poor inspection and maintenance

During periodic maintenance, the heat exchanger is opened up and the tube bank is pulled,

cleaned and inspected. Inspection aimed at detecting and/or preventing leaks includes control

of:

• corrosion and/or erosion on the inside and outside of tubes and tube-tube plate connection

• corrosion of packing faces of the flanges

• size of tube holes in baffles (has the diameter of the openings increased in size?)

• reduced wall thickness of the tubes (special attention must be given to tube ends, tubes

located at baffles and tube corners)

• bent, twisted or torn off tubes

• tubes pressing against each other, loose tubes

• bent tube plates

• small cracks (or perforations) in tubes and welded tube-tube plate connection

• smoothness and condition of packing faces

Periodic maintenance always ends with a water pressure test, whereby parts of the heat

exchanger are pressurised to verify that the heat exchanger is still suitable to operate under the

desired pressure levels. In this way, too, the tubes, tube-tube plate connections and flange

connections are tested for leaks. To detect leaks, more accurate testing methods are also

available. Air ('air and soapy water' test) or helium is used. If flaws or suspicious areas are

found, their cause will need to be investigated. Once the cause has been found, corrective

measures will need to be taken. If this is not done, and this applies to dealing with the cause as

well as the repair of the parts, there is a large probability of (new) leaks in the future.

Corrective measures include the plugging of tubes and replacing gaskets. If a company has its

own workshop, the repair work on a shell & tube heat exchanger will take one to two days. By

emphasising preventive rather than corrective maintenance, leaks can be prevented. For

instance, tube banks can be replaced sooner. Proper logging of maintenance works done and of

the occurrence of problems enables better planning of maintenance work. It is recommended

that the closing of the heat exchanger and tightening of the bolts is done under supervision to

prevent future leaks. For this, a tool with a regulated momentum can be used.

III.3 Alternatives

The probability of leakage can be reduced by alternative choices of material, TEMA-type, tube

plate connection, type of packing and the level of the process pressure of the cooling system.

Choice of material

Instead of carbon steel, more high-grade materials such as aluminium-brass copper nickel and

titanium can be used for the water side of the heat exchanger. This will make the heat exchanger

considerably more expensive than a heat exchanger whose tubes and tube plates are made of

carbon steel (See also Annex IV).

Different heat exchanger

By choosing another type of heat exchanger, the probability of leakage can be reduced by a

considerable margin.

Alternatives are:

• a U-tube heat exchanger

• a heat exchanger with a double tube plate construction

• both U-tube and double tube plate construction

There is no floating head on the U-tube type heat exchanger and therefore no flange seal on the

rear end head. The U-tube type is 10 to 15% cheaper than the floating head type. If leakage

occurs at the tube-tube plate connection in heat exchangers with a double tube plate
Annex III

184 Industrial Cooling Systems

construction, there will be emission into the atmosphere instead of to the other medium. The

double tube plate connection is fairly expensive.

Tube-tube plate connection

With a welded tube-tube plate connection there is a much smaller chance of leakage than with a

rolled construction. Making a rolled connection into a welded connection can make existing

heat exchangers better leak-proof. In this, there are two welds: a sealing weld (one layer of

weld) or strength weld (usually two layers of weld). A cost indication shows that the price for a

welded construction is about 9 to 11 Euro per tube higher than for a rolled construction.

Type of packing

With the flange seal of a floating head, the packing type can be changed. The usual types of

packing, for instance an 'asbestos-free metal-wound' packing or a cam-profile packing, can be

replaced by a seal with a weld ('Schweissdichtung').
Annex IV

Industrial Cooling Systems 185

ANNEX IV EXAMPLE OF SELECTION OF MATERIAL FOR

COOLING WATER SYSTEMS IN INDUSTRIAL

(NON-POWER PLANT) APPLICATIONS

[tm001, Bloemkolk, 1997]

IV.1 Introduction

The selection of construction materials in cooling systems, and especially the selection of

materials for the coolers (heat exchangers), in many cases is a complex matter. It is the result

being a balance between the requirements due to the chemistry of the water and the operational

requirements (restricted additive use, number of cycles of concentration). To meet these

requirements a large range of materials is offered. This Annex shows a few options for materials

to be used in case of brackish water for open once-through systems.. It gives a qualitative

selection of applications. In the individual case, the final selection will need to involve cost

values to make a fair comparison taking into account also the consequences of a selection for

the operational costs.

Choice of materials for heat exchangers

There are many factors that come together to determine the ultimate choice of materials for heat

exchangers, such as:

• Composition and corrosiveness of the cooling water

• Manner of operation, e.g. through-flow or re-circulating cooling

• Corrosiveness and nature of the medium to be cooled

• Type of cooler

• Life-span

• Costs

These are some of the most important criteria that are taken into consideration in the design of a

new cooler, on the basis of which ultimately a certain choice of materials is made. In many

cases, this ultimate choice is the best possible compromise adhering to the principle that the

cooler must have an economically acceptable 'life-span'. Within this life-span, however, many

coolers will start leaking. An important cause of this is that, in practice, the cooler is not used in

accordance with its design principles, whereas a change of process conditions for a variety of

different reasons is quite common.

Important causes that can lead to leakage are:

• too high or too low speeds in the tube bank and poor circulation in the shell (Table IV.1);

• poor water treatment, i.e. applied method and control;

• too high metal temperatures on the cooling water side.

With regard to temperatures a metal temperature of 60oC is taken as an upper borderline as

above this temperature the majority of corrosion inhibitors is less or not effective. Also in oncethrough systems formation of calcium salts occur.

Table IV.1: Cooling water velocity and type of material

Material Velocity (m/s)

Aluminium brass 1.0 - 2.1

Copper nickel (90-10) 1.0 - 2.5

Copper nickel (70-30) 1.0 - 3.0

Carbon steel 1.0 - 1.8

Austenitic stainless steel (316) 2.0 - 4.5

Titanium 2.0 – 5.0
Annex IV

186 Industrial Cooling Systems

Choice of material for pumps

The choice of material for a pump is less critical because in many cases this equipment is

doubled (has a back up). This means that if a pump breaks clown, the process is often not

disturbed. Another factor is that the available walls are often much thicker than is strictly

necessary (corrosion allowance).

Choice of materials for cooling water tubes

In most cases carbon steel with a sufficiently high corrosion allowance is chosen for the cooling

water tubes. If a corrosion allowance of more than 3 mm is insufficient for an acceptable life

span, alternative materials are chosen, such as plastics, carbon steel with an organic

coating/concrete or, in exceptional cases, alloys of higher quality, such as stainless steel, monel

and other nickel alloys, etc. Tubes have an advantage over machines in that they are much easier

and cheaper to replace, which is why in the choice of materials is less critical.

IV.2 Direct once-through systems (with brackish water)

The composition and corrosiveness of "brackish water" is not fixed and can vary between

"fresh" surface water and seawater. Brackish water is usually found in the transition area

between rivers and/or other "freshwater" outflows and the sea (deltas). The composition and

characteristics can vary widely by location and by season. Local water depth, the level of flow

through (replacement) and the tides can play a role in the corrosiveness of this type of water.

Some kinds of brackish water have a higher level of corrosiveness than seawater. For instance,

in shallow water with an abundance of plant growth sulphur compounds can occur as a result of

decomposition, which can cause serious pit corrosion in copper alloys. In a lot of cases the

amount of suspended silt is considerable, which can heavily pollute the heat exchangers in the

installation. In these cases, the use of stainless steel is dubious because the formation of pits

(pitting) will almost certainly occur quickly. Because all of these factors are not well known in

most cases, it is recommended that the corrosiveness of brackish water be considered equal to

that of seawater with a relatively high level of suspended silt. Another advantage is that there is

considerable knowledge and experience regarding the occurrence of corrosion by seawater.

Pumps

Depending on the situation, the materials in the table below are often used for pumps in

brackish water (other materials are possible, but are usually much more expensive):

Table IV.2: Materials used for pumps in brackish water

Housing Impeller Drive-shaft Comments

Nodular cast iron)*

Aluminium bronze

Aluminium bronze

Tin bronze

Tin bronze

Tin bronze

Stainless steel 316)*

Aluminium bronze

Aluminium bronze

Stainless steel 316

316

Monel

Monel

Monel

Monel

)* Grey cast iron is also possible.

Chance of gratification is

significantly greater. Cast steel is

also sometimes used.

)* Austenitic stainless steel

(Cr-Ni-Mo 18-8-2)

The tendency is to choose a combination for the housing and the fan, whereby the housing is, in

principle, the anodic part of the construction.
Annex IV

Industrial Cooling Systems 187

Tubes

In most cases, carbon steel with a corrosion allowance is used. Another possibility is to provide

the carbon steel with an organic coating, or to concrete it. In both cases the welds are the weak

point of the construction.

Nowadays, glass-reinforced epoxy tubes are used increasingly, especially in underground

systems. The great advantage of this material is that it is almost completely resistant to

groundwater. The installation costs are about the same as those for a carbon steel tubing system

that is coated, internally or externally, with an organic coating. Over time, this solution is often

the cheaper of the two.

Heat exchangers/coolers

As earlier mentioned, the choice of material in a heat exchanger is somewhat more complex

because, where the tube bank is concerned, the corrosiveness of the medium to be cooled must

be taken into account. Assuming that the process medium is not corrosive for the material of the

tube bank and that possible contamination (for instance, by copper ions) of the process is not

significant, the choice of material is mainly determined by the quality of the water.

The possible choice of materials for 'shell & tube' heat exchangers in brackish water is given in

the table below (water through the tubes).

Table IV.3: Materials used for shell&tube heat exchangers in brackish water

Shell/body Water casing Tubes Tube-plate

Carbon steel Carbon steel Carbon steel)*1 Carbon steel

Carbon steel Carbon steel)*2 Aluminium brass Carbon steel

Carbon steel Carbon steel)*2 Aluminium brass Aluminium bronze

Carbon steel Carbon steel)*2 Aluminium brass or and

Cupro-nickel)*4

Carbon steel with a lining

of aluminium brass

Carbon steel Tin bronze stainless steel 316)*3 Carbon steel

Carbon steel Carbon steel)*2 Titanium)*5 Carbon steel

Comments Table IV.3:

There are various possibilities in the list above. The ultimate choice is largely determined by the

level of corrosiveness of the brackish water and by the process conditions.

)

*1 Tubes of carbon steel are only possible when one is certain that the water is not

corrosive (e.g. through practical experience). This is almost never an option.

)

*2 The water casings are usually given an organic coating plus a few sacrificial anodes. If

the tube plate is made of a more precious metal, an organic coating must be applied to this metal

to prevent galvanic effects in the water casing.

)

*3 The use of austenitic material such as 316 is not without risks. In the event of pollution

there is a good probability of pitting. This process can be extremely fast. Another risk is the

possible occurrence of stress corrosion in this type of material. However, practical experience

shows that this does not happen often, which is possibly due to the relatively low temperatures

in this type of system. This risk can be significantly reduced or even prevented if one uses a

higher version of alloy, such as 904L, 254SMO or Incoloy 825. These types of material are also

used when the process side requires this.

)

*4 Cupro-nickel alloys and others are chosen if the design metal temperature is too high for

aluminium brass.
Annex IV

188 Industrial Cooling Systems

)

*5 In many cases, titanium is the best choice. It is often assumed that a heat exchanger of

titanium is too expensive. The price of this material has fallen dramatically in recent decades

and it has been economically applied. Expectations are that decreasing cost differences and

increasing problems with water treatment will lead to an increased use of Ti compared to that of

Cu-Ni alloys now still applied.

In addition to its high resistance to corrosion, even in extremely polluted water, this material has

various advantages:

• Extremely thin walled tubes can be used, so less material per m2 heating surface area.

• The conductivity of heat is very good.

• The scrap value is high and the material well suited for reuse.

• It has a long life expectancy.

A disadvantage is that biological growth is stronger than is the case, for instance, with coppercontaining alloys. It therefore requires extra use of biocides. Another thing is that titanium

cannot be used in a reducing environment, because no protective oxide layer is formed.

IV.3 Indirect once-through system (brackish water/deminwater)

In the indirect (secondary) once-through system, the heat is absorbed in a closed secondary

cooling circuit, after which the absorbed heat is transferred to an open once-through system via

a heat exchanger. Characteristically, in these systems the water quality/corrosiveness is different

for each cooling circuit. The primary part is usually of poorer quality than the secondary part. In

this case, the primary part once again contains brackish water and the secondary part deminwater.

Material selection for primary circulation cooling system

The materials for the primary system, filled with brackish water, are described in Section IV.2.

The heat exchanger between the primary and secondary system is extremely important for

operation. Failure of this heat exchanger has serious consequences, so this should be taken into

account when selecting the materials for it. If the primary cooling agent is brackish water, then

the best choice for the tubes or plates (plate exchanger) is titanium. Other high-quality alloys

such as 254 SMO or better can be considered in certain cases, but in most cases titanium is the

best choice.

Selection of material for secondary circulation system

Essential for a closed secondary system is that making the water oxygen free prevents corrosion.

In this case, demin water has been chosen as the cooling medium. However, in an aerated state,

this demin water is extremely corrosive for carbon steel. This can be suppressed by alkalising

the water (pH=9). Clean tap water with a relatively high chlorine content is, in principle, just as

good as demin water.

If these provisions are present, then the water is 'dead', which means its corrosiveness is

minimal. In principle, all materials, including the tubes in heat exchangers, pumps and tubes

fines, can be made from carbon steel. The conditions in the process must naturally be taken into

account.

It is important that the concentration of oxygen in these systems is checked regularly. In some

cases, nitrates are used as an inhibitor in these systems. By keeping the water alkaline and/or

treating it with nitrates, leaking in of oxygen is less critical.
Annex IV

Industrial Cooling Systems 189

IV.4 Open recirculating cooling systems

IV.4.1 Fresh water application in open wet cooling tower

With respect to the application of material, one of the objectives in the design of an open

recirculating cooling system (open wet cooling tower) is to condition the water in the system in

such a way (inhibitors, pH control, etc.), that use of carbon steel for most parts of the system is

economically acceptable.

This case is based on tap water. Depending on the composition and the cycles of concentration,

the concentration of components in this water will increase (sometimes called 'thickening'),

whereby the number of dissolved salts rises proportionally, which increases corrosiveness. With

inhibitors and the right pH levels, this effect is suppressed. The design of such a system is

usually based on the fact that the water is not corrosive for carbon steel.

Most parts, such as tubes and pumps, are therefore made of carbon steel. The tubes in a heat

exchanger are also often made of carbon steel. In critical systems or to provide more security,

the tubes are also often made of aluminium brass. Problems usually arise when the medium to

be cooled is corrosive. In very many cases, austenitic steel or a better alloy must then be used,

with the same risks as indicated above, for instance pitting or stress corrosion.

It is extremely important that pollution is kept to a minimum. This applies, in principle, to all

cooling water systems. In an open circulation cooling system, 'side-stream' filtration is often

used, or in critical coolers (condensers) a self-cleaning system (for instance with rubber balls) is

installed.

IV.4.2 Salt water application in open wet cooling towers

[tm110, BDAG, 1995]

The use of salt or brackish water in cooling towers requires applications that particularly pay

attention to corrosion of metal materials. A number of observations can be summarised. For salt

water resistant structures good experience has been gained with hardwood species and pressure

treated wood. However, the latter are done with CCA and cannot be regarded as an

environmentally sound method. Sulphate resisting cement for concrete constructions and

reinforcements for external and interior elements are well proven. Silicon, Aluminium Bronze

and/or Stainless Steel can be applied, but coated galvanised only above water distribution level.

Plastic coatings are recommended on Aluminium/Silicon bronze items.

Fill materials should be open low fouling with a high load capacity, where combinations of film

(upper parts) and non-film (lower parts) have shown to be effective. Water velocity should be

low enough to prevent corrosion, but sufficiently high to prevent settlement of heavy solids.

These particular measures amongst others can reduce water treatment requirements in salt-water

applications.

Annex V

Industrial Cooling Systems 191

ANNEX V OVERVIEW OF CHEMICALS FOR THE

CONDITIONING OF COOLING WATER SYSTEMS

In all water-cooled systems additives are applied to treat the cooling water with the aim to

protect the cooling system and to avoid a reduction of heat exchange due to scaling, fouling and

corrosion. A wide range of additives is applied against these cooling water problems. The

Annex gives an overview of the different types of additives that are applied in the different wet

cooling systems. In the final section, treatment of cooling water in a wet open cooling tower is

explained to illustrate the complexity of cooling water conditioning and the elements that are in

play.

V.1 Corrosion inhibitors

V.1.1 Corrosion

Corrosion can be defined as the destruction of a metal by chemical or electrochemical reaction

with its environment. The result is a metal oxide or other salt having little structural ability,

which causes damage to the material. In cooling systems, corrosion causes two basic problems.

The first and most obvious is the failure of equipment with the resultant cost of replacement and

plant downtime. The second is decreased plant efficiency due to loss of heat transfer, which is

the result of heat exchanger fouling caused by the accumulation of corrosion products.

Corrosion is caused or favoured by the presence of oxygen, the salt content, formation of

deposits, or an excessive low pH level.

Corrosion can also result from fouling by the growth of organism, so called micobiologicallyinfluenced corrosion (MIC): acid producing bacteria cause corrosion and vibrating mussels

cause erosion.

V.1.2 Applied corrosion inhibitors

Corrosion inhibitors can be identified by their function. They remove corrosive material,

passivate, precipitate or adsorb it. Passivating (anodic) inhibitors form a protective oxide film

on the metal surface. Precipitating (cathodic) inhibitors are simply chemicals, which form

insoluble precipitates that can coat and protect the surface. Adsorption inhibitors have polar

properties, which cause them to be adsorbed on the surface of the metal.

The use of corrosion inhibitors varies from system to system. In once-through systems

polyphosphates and zinc are applied and there is limited use of silicates and molybdates. In

some countries hardly any corrosion inhibitors are dosed in once-through systems except for

yellow metal inhibitors (e.g. ferrosulphate) dosed in copper alloy heat exchangers or

condensers.

In open recirculating systems normally a more comprehensive corrosion control programme is

required. For many years chromate based programmes were used, but due to its toxicity the use

has been reduced significantly and should not be used anymore as there are good alternatives..

Most current used corrosion programmes are based on phosphates, and zinc is added if water

conditions require this. Often is chosen to operate the system under alkaline conditions (pH of

8-9), but biocide and dispersant treatments might have to be adjusted accordingly under these

circumstances. Water is then inherently less corrosive. Disadvantage of alkaline operation is the

increased potential to scaling. Alkaline conditions in combination with organic phosphonates

are effective against corrosion and scaling.

Theoretically, closed water systems should not require corrosion inhibitors. Any oxygen

introduced with the initial make-up water should soon be depleted by oxidation of system
Annex V

192 Industrial Cooling Systems

metals, after which corrosion should no longer occur. However, closed systems usually lose

enough water and leak enough air to require corrosion protection. Another theory is that the

high residence time of the water, up to several months, is responsible for the heavy treatment

with corrosion inhibitors. For closed systems the three most reliable corrosion inhibitors are

chromate, molybdate, and nitrite materials. Generally, the chromate or molybdate have proven

to be superior treatments. The toxicity of chromate restricts the use, particularly when a system

must be drained. In many cases a non-chromate alternative is available, but in some member

states their use is still permitted. Molybdate treatments provide effective corrosion protection

and are seen as more environmental acceptable as chromate treatments.

Finally, it depends on the systems conditions (materials used and pH) what kind of corrosion

inhibitors is best applicable. For instance the most effective corrosion inhibitors for copper are

the aromatic azoles. Concentrations in evaporative recirculating cooling systems typically range

from 2 to 20 mg/l as active compound. For some anodic inhibitors (such as chromates,

molybdates and nitrites) concentrations used in the past are reported to be 500 to 1000 mg/l. in

closed systems.

V.2 Scale inhibitors

V.2.1 Scaling

If concentration of salt in the water film within the heat exchanger exceeds its solubility,

precipitation occurs, which is referred to as scaling. The main forms of scale are calcium

carbonate and calcium phosphate, but also calcium sulphate, silicates, Zn and Mg deposition can

occur depending on the minerals contained in the water. Scaling reduces the performance of the

heat exchanger, since the thermal conductivity of calcium carbonate is about 25 times lower

than that of steel. Scaling depends on three major factors: mineralisation (alkalinity), higher

temperature and pH of the circulating water and of secondary factors: presence of complexing

organic matters and chemical composition of the heat exchanger surfaces. Also, certain shape of

the heat exchanger body favour scaling. Corrugations, oblique channels and an insufficient ratio

of water flow per film surface area favour scaling. In recirculating systems high cycles of

concentration can lead to scaling as well.

Scaling may cause problems in cooling towers, as the film fill can be very susceptible to various

types of deposition. Because of evaporation (1.8% of the circulation per 10K of cooling) in the

tower, minerals and organic substances in the recirculating water may concentrate to such a

level that scaling can occur.

For power stations in particular, it was reported that scaling occurs due to:

• heating of the water up to 30oC in direct cooling and 45oC in tower assisted circuits,

• evaporation of water to affect cooling in passage through the cooling towers, which gives

rise to concentration of the dissolved salt up to a factor of 1.6 or as high as the concentration

factor determines, and

• the losses of free carbon dioxide during passage of water through the towers causing a rise

in the pH which varies with the flow rate and the type of packing. With older timber statted

packs the pH was 7.5-7.8, but with plastic film flow pack, this increases to 8.2-8.4 in small

towers (250 MWe) as well as in large towers (900 MWe or larger).

V.2.2 Applied scale inhibition

Scale formation can play a role in once-through and open recirculating cooling systems. In

closed recirculating systems it should not be a major issue. It can occur if spills require frequent

additions of make-up water and depending on the factors mentioned above.
Annex V

Industrial Cooling Systems 193

The increase of concentration of salts in cooling water in open recirculating cooling systems

and distribution systems is caused by evaporation in the cooling tower and to be controlled by

the blowndown. The ratio of particulate solute in the recirculating water to that in the make-up

water is called concentration factor. The concentration factor ranges from as low as 2-3 for

large power plants until 8-9 for some recirculating industrial cooling water systems. Typical

concentration factors in industry (not power plants) range between 3 and 5.

In practice, scaling is controlled by adjustment of the pH value by dosing of acid and by the

application of scale inhibitors. Experiences in large systems of power stations equipped with

cooling towers show that treatment with acid (sulfuric acid or hydrochloric acid) does not lead

to a change in pH, which remains alkaline. The acids rather neutralize the alkalinity to avoid the

precipitation of CaCO3.

However, in decarbonated waters pH control can be done by addition of acids. Decarbonation

by precipitation of calcium carbonates depends on three major factors, which are mineralisation

(alkalinity), temperature, and pH of the circulating water. Secondary factors are presence of

complexing organic matters in water and the chemical composition of the heat exchanger

surfaces.

Three alternatives were reported for the chemical treatment of cooling water to avoid scaling in

heat exchangers and wet cooling towers in large wet cooling systems :

• decarbonation of make-up water (resulting in a sludge to be disposed of)

• addition of acid

• addition of organic scale inhibitors

The most important scale control agents are polyphosphates, phosphonates, polyacrylates, copolymers and ter-polymers. Typical concentrations of scale control agents range from 2 to 20

mg/l, as active compound. Hardness stabilisers prevent the formation of crystals and are used in

recirculating systems, but rarely or never in once-through systems.

Closed recirculating systems are not subject to scale formation in the primary system except

when hard make-up water must be used. Many closed systems use zeolite-softened water or

condensates as make-up water to prevent scale problems. Generally, some corrosion occurs due

to loss of water or leakage of air. In the secondary cooling circuit water circulates in an open

evaporative system. Here corrosion can occur on the outside of the coils where wet heat transfer

takes place.

V.3 Fouling inhibitors (dispersants)

V.3.1 Fouling

Fouling occurs when insoluble organic particulates suspended in water of both once-through

and open recirculating cooling systems form deposits on the systems’ surface. Particulate

matter, particle sizes and low water velocities are factors that enhance fouling. Foulants can be

sand, silt, iron oxides and other corrosion products and they can react with some water treatment

chemicals as well. They can either be airborne, can enter the cooling system with the water (silt,

clay) or are introduced by process leaks and can be very finely dispersed with sizes as small as

1-100 nm.

Dispersants are polymers used to prevent fouling by removing particulate (organic) matter (e.g.

microfouling and slime layer) from the heat exchanger surface by increasing the electric charge

resulting from absorption The particles repel each other and as a result remain suspended. To

facilitate penetration of biocides into microfouling and slime layers surfactants often referred to

as biodispersants can be used. Dispersants help to keep the surface of heat exchangers clean,

thereby reducing the risk of corrosion. It is common practice to dose biocides in combination

with dispersants at levels of 1-10 mg/l as active ingredient.
Annex V

194 Industrial Cooling Systems

V.3.2 Applied fouling inhibitors

The most effective and widely used dispersants are low molecular weight anionic polymers. The

most important dispersants are: organic and metal sulphonates, metal phenolate, metal dialkyl

dithiophosphanates, sodium dialkyl sulphosuccinates, polyethylene alkyl and alicyclic amines,

and monoethanolamine phosphate salts, polyacrylates, polymetacrylates and acrylate based

polymers.

V.4 Biocides

V.4.1 Biofouling

Entrainment of organisms through water or air may lead to biofouling. Biofouling is generally

of two main types: macrofouling (e.g. mussels) and microfouling (e.g. bacteria, funghi, algae).

Macrofouling is generally confined to once-through systems and is more severe in marine and

brackish water than in fresh water. Macrofouling may cause gross blockage of pipework and

culverts, and may cause so-called erosion corrosion. Macrofouling is very much location and

water quality specific, both in terms of quantity and species variety.

Microfouling related problems occur both in once-through and open recirculating cooling

systems. Microbial growth on wetted surfaces leads to the formation of biofilms. The result of

uncontrolled microbial growth on surfaces is slime formation. The biological component or

biofilm is produced by the living cells and their metabolic by-products. Microfouling is always

the primary coloniser of surfaces in the development of biofouling.

The predominant effect of biofouling is reduction of heat transfer capacity of the heat

exchangers and energy losses due to increased frictional resistance. Furthermore, where an

exposed metal becomes fouled microbial induced corrosion can occur. In addition microbial

species may threaten human health by spreading via cooling towers.

A number of antifouling techniques and treatments are available. These applications, the type of

cooling water and the associated water problems are summarised in Table V.1.
Annex V

Industrial Cooling Systems 195

Table V.1: Survey of fouling and clogging organisms, and degree of fouling in marine, brackish and

fresh water. In the last column mitigation is presented

(The degree of fouling is indicated as: + some; ++ fair; +++ heavy)

(From: Applied Hydroecology 10, 1-2, 1998)

Type of cooling water, associated

fouling, clogging and scaling Country

Marine Brackish Freshwater

Main antifouling mitigation

techniques

Belgium Hydrozoa +

Slime ++

Slime ++

Zebra mussels +

Asiatic clam +

Bryozoans ++

Gastropods ++

In cooling towers:

scaling ++

Freschwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Discontinuous Chlorination

with hypochlorite

Denmark Mussels +

Slime +

See marine Not used Marine water:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Non-toxic antifouling paints.

France Mussels +++

Barnacles ++

Clogging by:

Seagooseberries

(jellyfish) +++

seaweeds +++

Fouling: no

special

problems due to

wide variations

in salinity in

large estuaries.

Clogging by

driftings

macrophytes +

Zebra mussels ++

Bryozoans ++

Algae ++

Gastropods ++

Asiatic clam +

In cooling towers:

scaling ++

Marine water:

Water filtration, debris filters.

On-line condenser cleaning by

abrasive sponge balls (some

units).

Continuous chlorination at low

dosage (0.5-1.0 mg/L), with

electrochlorination

Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Shock chlorination once or

twice a year

Germany Zebra mussels +

Slime ++

In cooling towers:

scaling ++

Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Discontinuous Chlorination

with hypochlorite.

H2O2, ozone

Ireland Mussels +

Slime +

Clogging by:

Fish +++

See marine Zebra mussels Marine water:

Water filtration, debris filters.

On-line condenser cleaning by

balls.

Continuous chlorination with

hypochlorite
Annex V

196 Industrial Cooling Systems

Table V.1 (cont.): Survey of fouling and clogging organisms, and degree of fouling in marine,

brackish and fresh water. In the last column mitigation is presented

(The degree of fouling is indicated as: + some; ++ fair; +++ heavy)

(From: Applied Hydroecology 10, 1-2, 1998)

Type of cooling water, associated

fouling, clogging and scaling Country

Marine Brackish Freshwater

Main antifouling mitigation

techniques

Italy Mussels +++

Hydroids ++

Tubeworms ++

Barnacles ++

Slime ++

Clogging by:

seaweeds +

Posidonia +

(only one

station)

Clogging by:

seaweeds +

debris +

Zebra mussels

Slime ++

Clogging by:

drifting plants,

leaves +

Marine water:

Water filtration, debris filtrs.

On-line condenser cleaning by

abrasive sponge balls.

Continuous or intermittent

chlorination by hypochlorite or

electrochlorination.

Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls. Intermittent

chlorination (very few cases)

The Netherlands Mussels +++

Slime ++

Clogging by:

jellyfish +++

fish ++

Mussels ++

Slime +

Clogging by:

fish +

Zebra

Mussels +

Slime ++

Clogging by fish

++

Marine & Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Heat treatment.

Chlorination continuous or

discontinous, with hypochlorite

Norway Hydro-power:

only problems

with migrating

fish.

Portugal Mussels ++

Slime +

Asiatic clam + Marine water:

Water filtration, debris filters.

On-line condenser cleaning by

abrasive sponge balls (some

units).

Continuous chlorination at low

dosage (0.5-1.0 mg/L), with

electrochlorination

Freshwater:

Water filtration, debris sfilters.

On-line condenser cleaning by

sponge balls.

Spain Slime +

Tubeworms +

Mussels ++

Oysters +

Slime ++

In cooling towers:

scaling ++

Marine & Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Discontinuous chlorination, low

and shock dosage, with

hypochlorite and

electrochlorination.

United

Kingdom

Barnacles +

Mussels ++

Slime ++

Clogging by:

fish +++

Seaweeds ++

Jellyfish +

See marine Slime ++

In cooling towers:

scaling ++

Marine & Freshwater:

Water filtration, debris filters.

On-line condenser cleaning by

sponge balls.

Continuous chlorination

(intermittent in winter) with

hypochlorite and

electrochlorination
Annex V

Industrial Cooling Systems 197

V.4.2 Applied biocidal treatment

To combat biofouling in industrial open wet cooling systems, biocides are added to the cooling

water. On biocides, their use and their effects, a lot of research has been done and a large

number of publications can be found.

Biocides are substances that slow down the microbiological growth in the cooling water, reduce

the total number of cells in the feedwater and weaken stability of the biofilm matrix, and

thereby minimising organic pollution in the cooling system. Microbiological growth includes

the development of micro-organisms, bacteria, algae and fungi, and also the developments of

macro-organisms, such as oysters, barnacles and mussels.

Generally, biocides are defined as either oxidising biocides or non-oxidising biocides.

Oxidising biocides have a non-specific broad-spectrum biocidal mode of action, which limits

the degree to which fouling organisms can develop resistance to these biocides. Non-oxidising

biocides are more selective and more complex in their working and therefore need a longer

reaction time than oxidising biocides.

The environmental problem of biocides, however, is their inherent toxicity. Some antimicrobials

used in cooling systems are compounds that rapidly break down in water, thereby alleviating

some potential environmental hazards. This chemical breakdown is often accompanied by a

reduction in the toxicity of the compound. The compound can be added to the cooling water

system, accomplish its task of killing the microbes in the system and then break down into less

toxic chemicals.

The consumption of biocides is determined by the type of cooling system, the water resource

(fresh or salt), the season and leakage of organic materials from the process and the systems

half-time. In once-through systems almost solely biocides are used. Usually, these are

oxidative biocides such as hypochlorite or derivatives as hypobromite.

In open recirculating cooling systems use of biocides is made on the basis of an oxidative

biocide alone or in combination with a non-oxidative biocide. The consumption of nonoxidative biocides and other conditioning agents is almost completely determined by

recirculating cooling water systems. Furthermore, it has been observed that in some closed

recirculating systems no biocides are being added at all.

Table V.2 gives an indication of the amounts of biocides used in some European Member

States.

V.4.3 Oxidising biocides

The oxidising biocides commonly used in industrial cooling systems are the halogens, chlorine

and bromine, in liquid and gaseous form, organic halogen donors, chlorine dioxide and ozone,

monochloramine and peroxides. In case of seawater conditions there is a growing interest in the

application of chlorine dioxide, because of its effectiveness and the reduced formation of

bromated hydrocarbons (in particular bromoforme, chlorodibromomethane,

bromodichloromethane and dibromoacetonitrile) and trihalomethanes (THM) compared to

hypochlorite, but on the other hand it produces ClO3

-

 ions.

Chlorine gas (Cl2) is also used in some places being compact and cheap, but has a safety risk

when stored in bulk and gives some difficulties in handling.

Sodium hypochlorite is the most commonly oxidising biocide used in large once-through

systems. It can be produced on marine sites by electrolysis of seawater. This process, called

electrochlorination, avoids the transport and storage of dangerous chlorine gas or solution.

The consumption of sodium hypochlorite as active chlorine demand is generally lower in and

around saltwater systems than on freshwater systems, because of a higher level of dissolved and

particulate organic matter in fresh water. Due to its higher bromide content, the formation of
Annex V

198 Industrial Cooling Systems

halogenated organics in seawater is reported to be lower than in freshwater (rivers), but no

publications could confirm this.

Table V.2: Estimated consumption levels of some commonly used oxidising biocides in a few

European Member States

(kg/yr) (KEMA, 1996)

Group Oxidising biocides United Kingdom

(1993)2

Netherlands

(1995)3

France

(1998)

Chlorine

based Sodium hypochlorite 7310001,5 18000004 8170006

Sodium

dichloroisocyanurate 19300

Chlorine dioxide 13000

Bromine

based Sodium bromide 356000 22500

1-bromo-3-chloro-5,5-

dimethylhydanthoide

(BCDMH)

286000 1000

Other Hydrogen peroxide 910

Peracetic acid 975

Notes:

1

 estimated use as active ingredient, (as formulated product the amounts are much higher)

2

 in all cooling water systems

3

 only in recirculating systems

4

 measured as Cl2 5

 this figure is an underestimation as in the U.K much electrochlorination is employed in most coastal power stations

6

 in Cl2 produced by electrochlorination at fossil fuel power stations

This will also hold for cooling systems using water from heavily polluted harbours. Continuous

‘low” chlorination is often preferred, although discontinuous or semi-continuous chlorination is

more and more practised. However, this requires more intensive monitoring of the cooling

system and the cooling water.

From both the chlorine gas and the sodium hypochlorite solution, the most active chemical

species is the non-dissociated hypochlorous acid. This is a very reactive oxidising agent and

reacts with most organics in the water to form the trihalomethane (THM) chloroform (3-5%)

and other chlorinated organics. Free chlorine can also react with ammonia to produce

chloramines or with diverse dissolved organic compounds forming different types of

organohalogenated compounds (such as THM, chlorophenols). This occurs in the cooling

system itself also and not before this first chlorine demand has been met, the residual chlorine

will be able to do its biocidal work.

The use of the oxidising biocide hypobromous acid (HOBr) could be an alternative to

hypochlorite. The hypobromous acid remains undissociated at higher pH values than is valid for

hypochlorous acid. This implies that at pH 8 and above the free oxidant HOBr is a more

efficient biocide than the dissociated hypochlorous ion OCl-

 . As a consequence, in alkaline

freshwater, the effective dosage of hypobromite can be much lower than the equivalent

hypochlorite. Although, brominated organics are 2-3 times more toxic than the chlorinated

equivalents, they decompose more rapid and given the lower demand could provide a distinct

environmental advantage. However, in marine water, the oxidation of bromide ions by

hypochlorite leads to a rapid formation of hypobromite and the chlorination of sea water is

almost equivalent to the bromination and there may be little environmental benefit in the

hypobromous option compared with the hypochlorous option.
Annex V

Industrial Cooling Systems 199

Bromide and sodium hypochlorite, chloramine and peroxyde are used in combination in

recirculating systems, which is also expected to give less environmentally hazardous substances.

A disadvantage of this treatment could be that at high concentrations of free oxidant (FO)

solutions the formation of carcinogenous bromate could occur. Another possible source can be

the ozonation of natural waters by the oxidation of bromide ions.

The bromate content depends on the bromide concentration in the brine water used to produce

sodium hypochlorite. The theoretical maximum bromate concentration (BrO3

-

) in sodium

hypochlorite solutions produced by electrolysis of seawater is around 100 mg/l or 3 mg per g of

chlorine. A wide range of bromate concentration is found in commercial hypochlorite solutions.

If concentrated brines are used for chlorine production it varies from 0.15 to 4.0 mg BrO3

-

 per g

of chlorine.

V.4.4 Non-oxidising biocides

Non-oxidising biocides are comparatively slow reacting substances that react with specific cell

components or reaction pathways in the cell. The following non-oxidising biocides are reported

to be commonly used: 2,2-dibromo-3-nitrilopropionamide (DBNPA), glutaric aldehyde,

quaternary ammonium compounds (QAC), isothiazolones, halogenated bisphenols and

thiocarbamates, but many others are in the market and within Europe the amount and the

frequency of use of the individual biocide varies considerably. Table V.3 gives an indication of

the consumption of some non-oxidizing biocides.

The application of non-oxidising biocides instead of oxidising biocides is recommended only in

cases where oxidising biocides are not able to give sufficient protection, such as in systems with

high organic loads, or in recirculating wet cooling systems where daily control is not a practice.

In large recirculating wet cooling systems, where mostly sodium hypochlorite is used,

sometimes constant monitoring is applied to ensure that the correct level of free oxidants is

available in the circuit. For many smaller recirculating wet cooling systems however, and for

those operated by water service companies who do not have personnel permanently on-site,

application of non-oxidising biocides that are less influenced by the water quality are preferred

over the oxidising biocides [tm005, Van Donk and Jenner, 1996].

Non-oxidising biocides are mainly applied in open evaporative recirculating cooling systems.

Generally, they are applied to cooling water systems to give water active ingredient

concentrations from about 0.5 ppm up to 50 ppm (exceptionally 100 ppm).

Non-oxidising biocides exert their effects on micro-organisms by reaction with specific cell

components or reaction pathways in the cell. The first reaction involves damage to the cell

membrane and in the other reaction damage is being done to the biochemical machinery in the

energy production or energy utilisation of the cell.

Quaternary ammonium compounds are cationic surface-active molecules. They damage the cell

membranes of bacteria, fungi and algae, thus increasing the permeability of the cell wall

resulting in denaturation of proteines and finally death of the cell. Isothiazolones are nonspecific and they interfere with ATP-syntheses in the cell. Of the others methylene(bis)thiocyanate (MBT) is widely used against bacteria and fungi and this biocide is believed to bind

irreversibly to biomolecules preventing necessary reduction and oxidation reactions.

Glutaraldehyde is used against both aerobic and anaerobic bacteria and its biocidal activity is

based on protein cross-linking.
Annex V

200 Industrial Cooling Systems

Table V.3: Estimated consumption levels in some of European Member States of some commonly

used non-oxidising biocides in kg/yr

(KEMA, 1996)

Group Non-oxidising biocides United Kingdom

(1993)2

Netherlands

(1995)3

France

(1998)

QAC Dimethyl cocobenzyl

Ammonium chloride

 234001

Bezyl-alkonium ammonium

compounds

 21400

Total QAC estimate 71152

Isothiazolines5-chloro-2-methyl-4-

isothiazolin-3-one

 13200

Total isothiazolines 18000 1500

Halogenated bisphenols

(dichlorophen+fentichlor)

 12150

Thiocarbamates 56800

Others Glutaraldehyde 56400 750

Tetraalkyl phosphonium

chloride

 9500

2,2-dibromo-3-nitrilopropionamide 17200 800

Methylene(bis)thio-cyanate

(MBT)

 2270 1450

ß-bromo-ß-nitrostyrene

(BNS)

 231 1950

Fatty amines 200004

Others 4412

Estimated total 234963 6450

Notes:

1

 estimated use as active ingredient (as formulated product the amounts are much higher)

2

 in all cooling water systems

3

 only in recirculating systems

4

 active product used at a marine fossil fuel power station

V.4.5 Factors determining the use of biocides.

[tm005, Van Donk and Jenner, 1996]

The following factors focus on the use of biocides, but could be applied to the use of other

additives as well.

• Effectiveness

Obvious as this may sound, a biocide has to be effective in the specific situation it is used in.

However, it is important to realise that a biocide - or a cooling water treatment program - that is

effective in one system, may not be so in another system, even if these systems are apparently

identical. One of the reasons for this can be the development of a resistant or tolerant population

of micro-organisms. For oxidising biocides this is less of a risk than for non-oxidising biocides.

• System type

The system type determines the hydraulic half-time of the cooling water in the wet cooling

systems, and thus the contact time between the biocide and the cooling water. In once-through

wet cooling systems, where residence times are short, fast reacting - oxidising - biocides are

generally used. At present if biocides are being used in once-through systems in the Netherlands

sodium hypochlorite is applied. Slower reacting non-oxidising biocides are at present only used
Annex V

Industrial Cooling Systems 201

in recirculating wet cooling systems. The majority (> 90%) of the recirculating cooling water

systems is being treated with NaOCl, Cl2, ClO2 or NaOCl./NaBr.

The type of process is an important factor in the choice of a biocide, especially when

considering the reactivity of some biocides with process fluids spilled into the cooling water.

Some processes, like the direct cooling of metal in the metal industry create special conditions

in the cooling water. Process fluids leaking into the cooling water may function as a nutrient for

biological growth.

• Water quality

Chemical and biological water quality affects the choice of a cooling water treatment

programme, and thus the choice of a biocide. Occurrence of macrofouling organisms is very

much water quality related. Generally speaking, an increased quality of the surface water in

biological terms may result in an increased occurrence of macrofouling in wet cooling systems.

For micro-organisms, water type does not play a major role in defining the types of organisms

encountered. In theory a pH value of approximately 7 is optimal for microbial growth. Acid

conditions will favour growth of fungi and pH values higher than 8 will reduce algae growth.

However, in practice micro-organisms prove to be highly adaptable and can colonise a variety

of systems. An illustration of this is the commonly held belief that fungi prefer acidic to neutral

media for growth, and will be supplanted by bacteria in an alkaline system. This is

fundamentally correct, but if such a system is treated with a bactericide with no antifungal

activity, often contamination with fungal spores allows colonisation of the system, even at a pH

value of 9.

In once-through systems, the pH value inside the cooling system is equal to that of the entrained

water, although dosing of sodium hypochlorite may slightly increase the pH value but this is

usually impossible to measure. In open evaporative recirculating systems the pH value is often

controlled, ranging generally from 7 to 9, by addition of acids (often sulphuric acid) or a base

(often sodium hydroxide) or cycling natural alkalinity.

For the application of sodium hypochlorite and sodium hypobromite as a biocide, it is well

known that the pH value strongly influences the equilibrium between hypohalous acid and the

hypohalite ion. Hypohalous acids are approximately hundred times more toxic than their anionic

form. Therefore, in theory, the pH value will affect the toxicity of for example hypochlorite

dose.

In practice, the pH value cannot be influenced in once-through systems. Freshwater oncethrough systems typically use cooling water at pH 7 - 8; seawater cooled systems operate at a

pH value of approximately pH 8. The above-mentioned equilibrium is therefore most relevant

for the effectiveness of the treatment in once-through systems, since residence time of the

cooling water - and thus contact time of the biocide with the organisms - in the cooling system

is relatively short.

Wet recirculating systems generally are operated at a pH value ranging from 7 - 9. Experience

in the chemical industry has shown that a recirculating system operating at pH value of 9 uses

less hypochlorite than a system operating at a lower pH value, without loss of effectiveness of

the treatment. The fate of hypochlorite in recirculating systems has been extensively studied.

The main conclusion from this is that 5-10% of the hypochlorite dosed is lost in the tower when

operating at pH value of 8.5, while at a pH value of lower than 7 this is 30 - 40%.

The explanation for this is that the hypochlorite anion cannot be stripped out of the cooling

tower. This is in contrast with hypohalous acid. It is concluded that hypochlorite dosage at a pH

value of 9 is equally effective, in spite of the fact that only (1 - 5%) is present in the acid form,

because the hypohalous acid consumed will be instantly replenished from the surplus present in

the anionic form. The overall conclusion therefore is that operating recirculating systems at a

high pH value will reduce the amount of hypochlorite needed for effective microfouling control.
Annex V

202 Industrial Cooling Systems

The temperature of the surface influences the growth of the marine biology and can therefore be

used as a factor for the choice of a treatment programme in once-through cooling systems.

Macrofouling in once-through systems in the Netherlands will not grow rapidly during winter

months. Therefore it is not necessary to dose biocides when water temperatures are lower than

12°C. Industries with once-through systems on the Mediterranean coast, where severe

macrofouling growth and spat fall take place all year round, dose oxidising biocides all year

round. In general, water temperature will greatly affect species variability, growth rate and

biocide demand. In once-through systems the water temperature added to the bulk water (∆T) is

8 - 12 °C, restricted by a maximum discharge temperature. Recirculating systems face the same

restriction at the point of discharge, although sometimes higher discharge temperatures are

allowed. Temperatures in the recirculating bulk water can be 20 - 30 °C and higher. Most

macrofouling species in the Netherlands do not tolerate long-term exposure to temperatures of

30 °C, but some species, such as the brackish water mussel, grow very rapidly at these

temperatures.

For recirculating systems with high concentration factors, the hardness of the intake water and

the amount of organic materials are of extreme importance, since this will affect the amount of

scaling and corrosion inhibitors needed. Both for once-through and recirculating systems the

amounts of organic material (dissolved solids, suspended solids) in the cooling water are

important, since they influence biocide demand. The extent into which this influences biocides

varies (e.g. hypochlorite will react with ammonia, chlorine dioxide will not). In general it is

advisable to reduce to a minimum all substances that lead to increased biocide demand.

V.4.6 Interactions with other water treatment chemicals

Other additives such as corrosion and scale inhibitors can also affect the choice of a suitable

biocide. Some biocides limit each other’s effectiveness, but can also be of reciprocal benefit.

For example:

- QACs are known to be partially neutralised by oxidative biocides and anionic

dispersing agents;

- Isothiazolones on the other hand are stabilised by sodium hypochlorite;

- Ozone is such a strong oxidant that it will oxidise almost any other cooling water

additive, which is specifically a problem for corrosion inhibitors that often have to be

applied to some extent adjacent to the ozone application to protect the equipment.

V.5 Cycles of concentration and water balance

The application of additives in open evaporative cooling towers is complex and largely related

to the water balance and the cycles of concentration with which the system is operated. The

blowdown is an important measure to correct the solids balance and plays a role in the

optimisation of cooling systems performance and cooling water treatment. A short explanation

of the principle of blowdown is given below Figure V.1.

A quantity of cooling water (Qc) is circulating through the system in m3 per minute. Passing the

heat exchanger the cooling water is cooled down in the cooling tower by evaporation and

convection. Evaporation (E), drift, windage and some leakage reduce the amount of cooling

water and consequently the concentration of salts in the water increases which could lead to

scaling and corrosion.
Annex V

Industrial Cooling Systems 203

Figure V.1: Tower and solids balances for an evaporative cooling system using a cooling tower

[tm135, Anonymous, 1988]

This is balanced by bleeding the system, which process is called blowdown (B with

concentration Cb) and compensated by adding water called make-up (M with concentration Cm).

As the system has to be balanced use is made of the concentration ratio (CR): CR = M/B =

Cb/Cm (because M x Cm = B x Cb).

Figure V.2: Reduction of the make-up flow by concentration in an evaporative cooling system

[tm135, Nalco, 1988]
Annex V

204 Industrial Cooling Systems

M = E+B, thus CR = (E+B)/B= E/B+1 and from this equation follows that

This is a very useful equation in cooling water treatment. After the cycles of concentration have

been determined based upon the make-up and blowdown concentrations, the actual blowdown

being lost from the system, or the blowdown required to maintain the system at the desired

number of cycles, can be calculated.

B = E/(CR-1)
Annex VI

Industrial Cooling Systems 205

ANNEX VI EXAMPLE OF LEGISLATION IN EUROPEAN

MEMBER STATES

The following text has been integrally included to give an example of legislation that has been

successfully applied in Europe to reduce the emissions of cooling systems.

General Administrative Regulation of 31 January 1994

On the Amendment to the

General Administrative Framework Regulation on

Minimum Requirements for the Discharge of Waste Water into Waters

(currently,august 2000,being revised)

Excerpt of

Appendix 31: Water Treatment, Cooling Systems, Steam Generation

1 Scope

1.1 Waste water in which the contaminant load originates primarily from the treatment of

water from cooling systems of industrial processes.

2 Requirements

The following requirements for the discharge of waste water must be complied with.

The requirements for chemical oxygen demand, for nitrogen as the sum of ammonium,

nitrite and nitrate nitrogen, for inorganic phosphorus compounds and for filterable

substances are based on generally recognised technical rules, the remaining requirements on best available technology.

These requirements shall not apply to waste water discharges of less than 0.5 m3

 per

day.

2.1 General requirements

The waste water must not contain - with the exception of phosphonates and polycarboxylates - any organic complexing agents which are not readily biodegradable in

accordance with the basic level requirements of the Chemicals Act for determining

ready biodegradability by means of the OECD Guidelines 301 A - 301 E of May 1981.

The waste water must not contain chromium compounds or mercury compounds,

nitrite, organometallic compounds (metal-carbon bonds) or mercaptobenzothiazole

originating from the use of operating and ancillary resources.

The requirements in the first and second paragraphs shall be deemed to be complied

with if the substances specified are not used, all operating and ancillary resources used

are listed in an operating journal and manufacturers' information is available showing

that such substances are neither present in the operating and ancillary resources used

nor capable of forming under operating conditions.
Annex VI

206 Industrial Cooling Systems

2.3 Requirements for waste water from cooling systems

2.3.1 Water from single-pass or open-ended fresh water cooling systems

The requirements after shock treatment with microbicidal substances shall be as follows:

Qualified random sample or

2-hour composite sample (mg/l)

Chlorine dioxide, chlorine and bromine

(expressed as chlorine)

0.2

Adsorbable organic halogens (AOX) 0.15

Microbicidal substances other than hydrogen peroxide and ozone must not be present

in the waste water. These requirements shall be deemed to be complied with if such

substances are not used, all operating and ancillary resources used are listed in an operating journal and manufacturers' information is available showing that such substances are not present in the operating and ancillary resources used.

2.3.2 Water from flushing of primary cooling circuits in power stations (flushing water from

recirculation cooling systems)

Random sample (mg/l)

Chemical oxygen demand (COD) 30

Phosphorus compounds as phosphorus,

total2)

1.5

If only inorganic phosphorus compounds

are used, the value for the parameter

phosphorus is increased to 3 mg/l.

The requirements after shock treatment with microbicidal substances shall be as follows:

Random sample (mg/l)

Adsorbable organic halogens (AOX) 0.15

Chlorine dioxide, chlorine and bromine

(expressed as chlorine)

0.3

Toxicity to luminescent bacteria TB 12

The luminescent bacteria toxicity requirement shall also be deemed to be complied

with if the flushing circuit is kept closed until a TB value of 12 or less is reached in accordance with manufacturers' information about input concentrations and biodegradation behavour and this is substantiated by an operating journal.

Zinc compounds from cooling water conditioning agents must not be present in the

waste water. This requirement shall be deemed to be met if all operating and ancillary

resources are listed in an operating journal and manufacturers' information is available

showing that the cooling water conditioning agents used do not contained any zinc

compounds.
Annex VI

Industrial Cooling Systems 207

2.3.3 Waste water from flushing of other cooling circuits

Random sample (mg/l)

Chemical oxygen demand (COD) 40

The value for the parameter COD is increased to 80 mg/l after cleaning with

dispersants.

Phosphorus compounds as phosphorus,

total2)

3

The value for the parameter phosphorus is

increased to 4 mg/l if only zinc-free cooling

water conditioning agents are used. It is increased to 5 mg/l if the zinc-free conditioning agents used contain only inorganic

phosphorus compounds.

Zinc 4

Adsorbable organic halogens (AOX) 0.15

The requirements after shock treatment with microbicidal substances shall be as follows:

Random sample (g/l)

Chlorine dioxide, chlorine and bromine

(expressed as chlorine)

0.3

Adsorbable organic halogens (AOX) 0.5

Toxicity to luminescent bacteria TB 12

The luminescent bacteria toxicity requirement shall also be deemed to be complied

with if the flushing circuit is kept closed until a TB value of 12 or less is reached in accordance with manufacturers' information about input concentrations and biodegradation behaviour and this is substantiated by an operating journal.

2.5 In the case of holding ponds, all values shall be for the random sample. Here the values relate to the quality of the water before emptying.

2.6 Compliance with the requirements in item 2.3 for the parameter COD may also be

checked by determining total organic carbon (TOC). In this case, the COD value is to

be replaced by three times the TOC value as determined in milligrams per litre.

__________________________

2) Determination in the original sample in accordance with DIN 38406 - E22 (March 1988 edition) or an

equivalent measurement and analysis procedure.

Annex VII

Industrial Cooling Systems 209

ANNEX VII EXAMPLE OF A SAFETY CONCEPT FOR OPEN

WET COOLING SYSTEMS (VCI-CONCEPT)

VII.1 Introduction to the concept

This safety concept has been elaborated in order to provide assistance with regard to the

protection of waters from the temporary discharge, via cooling water, of (process) substances

causing long-term detrimental changes to water bodies. The concept specifies measures of

monitoring and changeover in connection with once-through cooling systems and alternatives to

once-through cooling systems as a function of the persistence of water pollution by substances,

which can be discharged into the cooling water.

The capacity of a substance to cause long-term detrimental changes or to pose a hazard to a

water body can be determined on the basis of the R - phrases established under European

legislation on hazardous substances. A certain score is allocated, as shown in the table below, to

each of the R - phrases relating to the protected assets aquatic-environment as well as human

health and soil. The scores of all R - phrases assigned to the substance concerned are added up

to obtain the total score. This total score is then linked with the required safety measure

concerning the contaminated cooling water. Decisions relating to the implementation of such

measures and the technology applied can, of course, only be taken within the individual

companies concerned and with knowledge of the particular circumstances.

It will be recommended to apply this concept immediately to new plants and to refit existing

cooling-systems if they do not meet these requirements within:

- 5 years for substances with total score > 9

- 8 years for substances with total score from 0 - 8

For substances with total score > 5 measures with regard to the monitoring of once-through

cooling systems should be tackled immediately taking into account the requirements of

individual cases.

The requirements of this safety concept refer to all cooling water streams, which are not

connected to industrial purification plants or to an appropriate water-associated purification

plant. The requirements concerning substances with total score < 4 do not refer to indirect

discharges which are fed to a sewage treatment plant.
Annex VII

210 Industrial Cooling Systems

Table VII.1: Score for a number of R-phrases to calculate the total score for process substances

Score 1 2 3 4 5 6 7 8 9

Ecotoxicity

and

Degradation/

bioaccumulation

52/53 51/53 50/53

Ecotoxicity

and/or

Degradation/

bioaccumulation

n.y.d.

* 3) * 2) * 1)

Ecotoxicity 52 50

Ecotoxicity

n.y.d.

*

Degradation/

bioaccumulation

53

Degradation

and/or bioaccumulation

n.y.d.

*

Acute

mammalia

toxicity

(acute orale

toxicity

preferred)

22

20/22

21/22

20/21/22

21

20/21

65

25

23/25

24/25

23/24/25

24

23/24

28

26/28

27/28

26/27/28

27

26/27

Acute

mammalia

toxicity

n.y.d.

*

Carcinogenicity

and/or

mutagenicity

40 45

and/or

46

Irreversible

effect

40/21

40/22

40/20/22

40/21/22

40/20/21/22

39

39/24

39/25

39/23/25

39/24/25

39/23/24/25

39/27

39/28

39/26/28

39/27/28

39/26/27/28

Recurrent

Exposure

33

48

48/21

48/22

48/20/22

48/21/22

48/20/21/22

48/24

48/25

48/23/25

48/24/25

48/23/24/25

Reproduction

toxicity

62

and/or

63

60

and/or

61

Dangerous

reaction with

water

29

15/29
Annex VII

Industrial Cooling Systems 211

Legend to the table on the previous page:

n. y.d. = attributes not yet determined (tested or known)

* = score, if one ore more of the attributes, “ecotoxicity”, “degradation/bioaccumulation" and

“acute toxicity" were not tested or not known

Footnote 1) - Ecotoxicity and degradation and/or bioaccumulation n.y.d. or ecotoxicity n.y.d. and easy

degradation not proved or

- ecotoxicity n.y.d. and potential of bioaccumulation available or classified into R 50 and degradation

and/or bioaccumulation n.y.d.

Footnote 2) - Ecotoxicity > 1 and < 10 mg/l and degradation and /or bioaccumulation n.y.d.

Footnote 3) - Ecotoxicity > 10 and < 100 mg/l and degradation n.y.d.

Footnote 4) - See Appendix 2 for description of R-phrases

VII.2 Requirements of the concept

The requirements put on the cooling technology is determined by the respectively highest total

of points of those process substances that can enter the cooling water. These requirements are

summarized in the following table.

Table VII.2: Requirements of VCI safety concept for cooling technology

score: 0 points score: 1 - 4 points score: 5 - 8 points score: ≥ 9 points

(D1 + Al) (D1 + Al + U1) (D1 + A2 +U1)

(D2 + A1 +U1)

(D3 + A2 + U1) /

(D2 + A2 + U2) /

(Z) (E) / (K) /

(L) / (S)

../.. alternative options

D1, A1 and U1 are always replaceable by the higher measures D2 (or D3), A2 and U2.

The codes in the table are described as follows:

DI Once-through cooling system;

D2 once-through cooling system with a cooling water pressure which is kept clearly and in

a controlled way above the process pressure (cooling water pressure should not fall

below process pressure at any point in the cooling system, also not by hydraulic

processes);

D3 Once-through cooling system with cooler made of high-quality anticorrosive material

and regular maintenance;

Z Intermediate storage with analytical control prior to discharge;

E Cooling via primary/secondary circuits (decoupling);

K Circulation cooling via recooling systems;

L Air cooling system;

S Special cooling system (e.g. heat pumps, absorption cold plants, vapour compression

systems, heat transformers);
Annex VII

212 Industrial Cooling Systems

Al Analytical or other adequate monitoring of cooling water;

A2 Automatic analytical monitoring of cooling water (according to the appendix);

U1 Immediate changeover of the cooling water discharge to holding facilities or a

purification plant provided that such a plant is appropriate for the disposal of the

released substance or immediate changeover to reserve cooling system or switch-off of

the part of the production plant concerned;

U2 Automatic changeover of the cooling water discharge to holding facilities or a

purification plant provided that such a plant is appropriate for the disposal of the

released substance or automatic changeover to reserve cooling system or switch-off of

the part of the production plant concerned.
Annex VII

Industrial Cooling Systems 213

VII.3 Appendix 1 - Automatic analytical monitoring of oncethrough cooling systems

Automatic analytical systems are appropriate for the monitoring of once-through cooling

systems if leakage can be determined with sufficient safety and sufficiently rapid. In this

connection it is sufficient that the analytical system provides trend data. The measurement of

absolute concentrations is not necessary for such systems, but merely the detection of deviations

from normal states.

Measurement can either occur directly by sensors in the cooling water stream or via automatic

sampling semi-continuously outside the cooling water stream.

For the following parameters and analytical methods, equipment is on the market which is

appropriate for automatic monitoring of once-through cooling systems in the above sense. With

regard to the selection of equipment for this function, the reliability of the system is generally

more important than increased demands on its accuracy.

The selection of the appropriate system is determined by the substance(s) released in connection

with a leakage and it is, moreover, strongly dependent on the special circumstances of

individual cases. In this connection it should first be checked whether the automatic analytical

monitoring can be carried out by means of a parameter or of an analytical method of the

following List 1. If this proves not to be possible, the use of systems in accordance with List 2

should be checked.

List 1: - ph-value - photometry

- conductivity - oil warning equipment

- redox potential - foam warning equipment

- turbidity - mercury monitors

- refractometry.

List 2: - TC (total carbon),

- TOC (total organic carbon),

- DOC (dissolved organic carbon),

- purgable substances by means of FID (flame ionization detector),

- TOC/FID combination,

- purgable chloro-organic compounds,

- bacterial toximeters.
Annex VII

214 Industrial Cooling Systems

VII.4 Appendix 2 – R-phrases used to calculate VCI-score

Table VII.3: Description of R-phrases used to calculate VCI-score for cooling systems selection

R 20/21 Harmful by inhalation and in contact with skin.

R 20/21/22 Harmful by inhalation, in contact with skin and if swallowed.

R 20/22 Harmful by inhalation and if swallowed.

R 21 Harmful in contact with skin.

R 21/22 Harmful in contact with skin and if swallowed.

R 22 Harmful if swallowed.

R 23/24 Toxic by inhalation and in contact with skin.

R 23/24/25 Toxic by inhalation, in contact with skin and if swallowed.

R 23/25 Toxic by inhalation and if swallowed.

R 24 Toxic in contact with skin.

R 24/25 Toxic in contact with skin and if swallowed.

R 25 Toxic if swallowed.

R 26/27 Very toxic by inhalation and in contact with skin.

R 26/27/28 Very toxic by inhalation, in contact with skin and if swallowed.

R 26/28 Very toxic by inhalation and if swallowed.

R 27 Very toxic in contact with skin.

R 27/28 Very toxic in contact with skin and if swallowed.

R 28 Very toxic if swallowed.

R 29 Contact with water liberates toxic gas.

R 33 Danger of cumulative effects.

R 39 Danger of very serious irreversible effects.

R 39/24 Danger of very serious irreversible effects in contact with skin

R 39/25 Danger of very serious irreversible effects if swallowed

R 39/23/25 Danger of very serious irreversible effects through inhalation and if swallowed

R 39/24/25 Danger of very serious irreversible effects in contact with skin and if

swallowed.

R 39/23/24/25 Danger of very serious irreversible effects through inhalation, in contact with

skin and if swallowed

R 39/27 Danger of very serious irreversible effects in contact with skin

R 39/28 Danger of very serious irreversible effects if swallowed

R 39/26/28 Danger of very serious irreversible effects through inhalation and if swallowed

R 39/27/28 Danger of very serious irreversible effects in contact with skin and if

swallowed

R 39/26/27/28 Danger of very serious irreversible effects through inhalation, in contact with

skin and if swallowed

R 40 Possible risks of irreversible effects.

R 40/21 Harmful: possible risk of irreversible effects in contact with skin.

R 40/22 Harmful: possible risk of irreversible effects if swallowed.

R 40/20/22 Harmful: possible risk of irreversible effects through inhalation and if

swallowed.
Annex VII

Industrial Cooling Systems 215

Table VII.3 continued

R 40/21/22 Harmful possible risk of irreversible effects in contact with skin and if

swallowed.

R 40/20/21/22 Harmful possible risk of irreversible effects through inhalation, in contact with

skin and if swallowed.

R 44 Risk of explosion if heated under confinement.

R 45 May cause cancer.

R 48 Danger of serious damage to health by prolonged exposure.

R 48/21 Harmful: danger of serious damage to health by prolonged exposure in contact

with skin.

R 48/22 Harmful: danger of serious damage to health by prolonged exposure if

swallowed.

R 48/20/22 Harmful: danger of serious damage to health by prolonged exposure through

inhalation and if swallowed

R 48/21/22 Harmful: danger of serious damage to health by prolonged exposure in contact

with skin and if swallowed.

R 48/20/21/22 Harmful: danger of serious damage to health by prolonged exposure through

inhalation, in contact with skin and if swallowed.

R 48/24 Toxic: danger of serious damage to health by prolonged exposure in contact

with skin.

R 48/25 Toxic: danger of serious damage to health by prolonged exposure if swallowed.

R 48/23/25 Toxic: danger of serious damage to health by prolonged exposure through

inhalation and if swallowed.

R 48/24/25 Toxic: danger of serious damage to health by prolonged exposure in contact

with skin and if swallowed.

R 48/23/24/25 Toxic: danger of serious damage to health by prolonged exposure through

inhalation, in contact with skin and if swallowed.

R 50 Very toxic to aquatic organisms.

R 51 Toxic to aquatic organisms.

R 52 Harmful to aquatic organisms.

R 53 May cause long-term adverse effects in the aquatic-environment.

R 60 May impair fertility.

R 61 May cause harm to the unborn child.

R 62 Possible risk of impaired fertility.

R 63 Possible risk of harm to the unborn child.

R 65 Harmful: may cause lung damage if swallowed.

R 15/29 Contact with water liberates toxic, highly flammable gas.

Annex VIII

Industrial Cooling Systems 217

ANNEX VIII EXAMPLES FOR THE ASSESSMENT OF

COOLING WATER CHEMICALS

VIII.1 Benchmark assessment concept for cooling water

chemicals

VIII.1.1 Introduction

General

It is well established that the cooling BREF is “horizontal” in nature, and that it is not possible

to identify a “BAT cooling system” as such because so much is dependent on the specific

process being cooled, and its location (especially climate, water supply etc.).

Therefore, the approach to be taken in the BREF must be one of providing “tools” to help the

Member State authorities rationalise what options are available, and select an optimal cooling

solution (both in terms of equipment, and “operating conditions”) which will represent BAT for

IPPC permitting purposes.

Firstly, in terms of “plant equipment”, such selections will mainly be made when new systems

are built, but also in the context of upgrades or retrofitting of existing systems.

Secondly, the “operating conditions” which are applied in both existing and new systems have

been singled out for attention in discussions. A key element of these “conditions” relates to the

optimisation of the cooling system in terms of efficiency and plant longevity through the use of

chemicals. BAT-based optimisation decisions will need to be made in terms of what chemicals

are used, and in what quantities, for permitting purposes.

An approach has been developed within the TWG for a simple so-called “benchmarking”

method to help the Member States compare different chemicals one with another on the basis of

potential environmental impact. Without such a tool, the complexity of making such decisions

could be a serious obstacle in determining what is BAT for cooling systems in a rational way at

the local level.

As is described below, most of the main elements for establishing such a risk-based

benchmarking tool can already be found in Community legislation and its official supporting

documentation. The present approach seeks to draw together elements from: the IPPC Directive,

the Water Framework Directive, and Risk Assessment legislation and the supporting “Technical

Guidance Document” in a coherent manner, to provide a tool to help evaluate cooling

chemicals.

VIII.1.1.1 Background

From its earliest meetings, the TWG agreed by consensus that any assessment of cooling water

chemicals should involve both intrinsic properties and local situation characteristics (risk based

approach).

The ensuing Benchmarking Assessment Concept arises as a result of consideration of existing

assessment schemes and methodologies, and seeks to provide a starting point for proper

consideration of both intrinsic properties and the local level situation in the assessment of

different possible treatment regimes.

The assessment concept does not enter into a discussion of the Intrinsic Hazard Approach, but

concentrates on the task of explaining and clarifying the Benchmarking (relative ranking)

Procedure.
Annex VIII

218 Industrial Cooling Systems

It essentially focuses on individual substances, giving brief indications of how the method could

be extended to multi-substance complete chemical treatments.

Also, only the most complex case (and most frequent) of open recirculating systems (cooling

systems with an evaporative cooling tower) is dealt with, with the possibility of later extension

to once-through, closed systems, etc.

VIII.1.1.2 Relevant legislative background

There is no need, here, to evoke in any detail the legislative requirements which have led to the

development of BAT reference documents. It is enough to mention Article 16.2 of the IPPC

Directive on exchange of information, and the initiative of the Commission to develop, through

the institution of the Information Exchange Forum, a tool which should assist and guide

Member States Authorities to set Emission Limit Values (ELV) for IPPC Listed Plants.

It is of importance, though, to underline one of the key aspects of the Directive: the control of

emissions and their impact on the environment through a “combined” method of BAT set

emission limit values to be checked against environmental quality standards.

Also very relevant in this context is the shortly to be adopted Water Framework Directive

(WFD).

Although it might be the case that a correct evaluation of the effects of chemical treatments used

in cooling systems should be subject to a multimedia assessment, it is also correct to state that

the major concerns associated to the use of these chemicals regard the main potential receiver of

polluting substances: the aquatic-environment.

A few words are therefore necessary to briefly review the relevant parts of the WFD.

VIII.1.1.3 The water framework directive (WFD)

While the WFD goes much further than providing elements to prevent and control emissions

from Industrial IPPC Plants, it does in fact supply one key link with the IPPC Directive. It fixes

methods and procedures for the Commission to prioritize dangerous substances and to propose,

for these, emission controls and EQSs (Environmental Quality Standards; or “Quality

Standards”), to be adopted by the Council and the European Parliament.

Furthermore it gives to the Member States the right and the duty to fix Quality Standards for

any other substances that are relevant, in any River Basin, to the achievement of the objectives

set by the Directive itself.

More important than this, it introduces in an annex (Annex V Section 1.2.6) a simple procedure

to be used by the Member States Authorities to calculate Environmental Quality Standards

(EQSs) for chemical substances in water.

In other words, it provides one of the conditions required by the IPPC Directive to implement a

combined approach: methods and procedures to calculate Quality Standards.

According to the Water Framework Directive (WFD) text (Annex V Section 1.2.6), Member

States are to determine EQSs in the following way:
Annex VIII

Industrial Cooling Systems 219

Test Method Safety Factor

At least one acute L(E)C50 from each of three trophic levels of

the base-set 1000

One chronic NOEC (either fish or daphnia or a representative

organism from saline waters) 100

Two chronic NOECs from species representing two trophic

levels (fish and/or daphnia or a representative organism from

saline waters and/or algae)

50

Chronic NOECs from at least three species (normally fish,

daphnia or a representative organism for saline waters & algae)

representing three trophic levels

10

Other cases, including field data or model ecosystems, which

allow more precise safety factors to be calculated and applied. Case by case assessment

While a more detailed analysis of the significance and implications of this table will be made

later, there are a few notes that need to be made at this point:

a) The Quality Standards set on this basis only take into account the protection of the

aquatic system, without considering the indirect-human effects

b) The numbers resulting from the above table are Predicted No Effect Concentrations

(PNEC) (See Technical Guidance Document for Regulation 793/93/EEC)

c) The Commission has developed a Prioritization Procedure, which is based on a system

in which an aquatic effect score is combined with a bio-accumulation score and a

human effect score. The procedure has been used to provide the basis for the

Commission’s proposed “priority list” of substances to be controlled at EU level by

means of emissions controls and EQSs to be adopted under the Water Framework

Directive.

The following benchmarking assessment concept is also based on the above method of

Calculating Quality Standards. This is for the following reasons:

- in the context of the BREF, the method must be clear, simple straight forward,

transparent and easy to use

- it is most probable, even if a lot of work is needed to prove this, that the aquaticenvironment is the weakest link of the chain

- the benchmarking methods will be used in combination with EU chemicals legislation

(Intrinsic Hazards), which, implicitly includes evaluation of potential indirect adverse

effects (both on the aquatic-environment and on humans) through the inclusion of bioaccumulation, the CMT properties (carcinogenic, mutagenic, teratogenic) as well as

chronic effects in the classification of hazardous chemicals.

The WFD also requires that Member States set EQS for waters intended for Drinking

Water Abstraction: this will be another check point to take into account human health in

one of the most important exposure routes.

VIII.1.2 Benchmarking : introduction of the concept

The Benchmarking Assessment Concept is founded on carrying out substance by substance

comparisons using a standardised theoretical measure of the Predicted Environmental

Concentration (here in referred to as PECstandardised). This PECstandardised is compared with the

corresponding Predicted No Effect Concentration (PNEC) or EQS of the substance, determined

in accordance with the method contained in Annex V of the Water Framework Directive. In this
Annex VIII

220 Industrial Cooling Systems

way a ration can be calculated for each substance which permits a preliminary ranking of

substances based on potential impact.

While the terms PNEC and PEC have now entered in the legislative language in the context of

emissions law, and their significance will become common knowledge, it is worth at this point

clarifying the concepts as they apply to the Benchmarking Procedure.

VIII.1.2.1 The PNEC

The Benchmarking Procedure does not attempt to rank the chemicals mentioned in the BREF

itself.

The real-life situation is complicated by the fact that only rarely do chemical treatments for

cooling systems consist of a single substance. The attempt to rank treatments in a BREF would

imply the application of an “additive” procedure of some sort to an enormous list of possible

substances combinations in the treatments. Assuming these combinations can be made

available, this would require a large quantity of work and time, and would almost certainly fail

to be either exhaustive or up to date.

Therefore, this Assessment Concept aims to offer a standard methodology, rather than a

numerical assessment of substances or treatments.

The Member State (MS) Authorities may then use this methodology as they see fit, at MS level,

or better still at local level.

At any rate, aquatic toxicity data must be available and be made available by chemical suppliers

to permit evaluations of PNECs. This is a fundamental aspect of any ranking procedure.

It is worth noting, also, that the Annex V procedure, and the related table, have not been

invented recently by the Commission in the context of the WFD. In fact the approach and the

table have been strictly derived from the Technical Guidance Document on the assessment of

Risk for Existing and New Chemicals.

(An extract of this pertinent portion is attached in VII.1.6 Appendix I).

Only a few words of clarification seem appropriate here.

The fewer data there are available the higher the assessment factor to be applied to convert

toxicity data to PNECs.

The availability of chronic data reduces the factor. Progressing through a set of intermediate

situations, having chronic data on three trophic levels allows the use of a factor of 10, compared

to a factor of a 1000 when only acute toxicity data are available. The costs associated with

performing chronic tests are much higher than the costs for acute testing. Thus it is likely that

more acute than chronic toxicity data will be available.

When the Benchmarking Procedure is applied locally, the available data will have to be used

together with the corresponding assessment factor.

It will be left, in this case, to the supplier of chemicals to decide whether or not to invest

additional resources in obtaining chronic data, as and when this might prove necessary. For

example, for a given plant it could be the case that using only acute data (which implies

obtaining an EQS by dividing the LC50 by a “safety factor” of 1000 to take account of

uncertainty) there may be difficulty in complying with the stringent resultant EQS. In this case

the supplier might opt to obtain the more “certain”, but equally more time-consuming and more

costly, chronic data. Chronic data implies dividing test result concentrations by a safety factor of

only 10, which will lead to a more “certain” EQS, which may also be more achievable.
Annex VIII

Industrial Cooling Systems 221

VIII.1.2.2 The PEC

The “real” PEC, in the context of chemicals used in a cooling system, must be seen and defined

as the final concentration of the chemical in the river water, after discharge and after dilution

with the river water at appropriate distance from the discharge.

When a chemical is used in a cooling system it is subject to a set of physico-chemical conditions

which will determine its fate. As examples can be mentioned:

breakdown in the cooling system due to hydrolysis or photolysis,

adsorption by the system,

partitioning between water and air,

ending up in the sludge,

biodegradation in the cooling system itself, in the waste treatment plants (chemical/biological),

and in the river.

The portion that is not “lost” will end up in the river and will be diluted by the river flow. A

precise appraisal of the final River PEC, is only possible at local level.

Models and algorithms to achieve this task are available, but they must take into account the

very specific conditions of each site. Obviously, also, the final PEC will depend on the amount

of chemical that is fed, and this in turn depends on the size of the system and on the operating

conditions (Number of cycles of concentration, size of the plant and amount of heat to be

removed).

Most of the following analysis will center on describing a simple method to calculate a

“standard” PEC (PEC standardised) which, while not bearing any resemblance with real PEC values,

permits a rapid, preliminary evaluation of chemicals relative to one another. It is stressed that

PEC standardised has only a very restricted, limited value, and can only be used as a general starting

point for the evaluation of the potential effects of chemicals relative to one another.
Annex VIII

222 Industrial Cooling Systems

VIII.1.3 Basic cooling towers material balances

A very simple sketch of a Cooling Tower System is reported in Figure VIII.1 below.

MU (CM)

BD(CB)

E+D

W

RIVER WR (C R)

PHYSICAL/CHEMICAL

BIOLOGICAL

BD(CB)

Figure VIII.1: Cooling tower material balance

VIII.1.3.1 Cooling towers basic equations

MU : Make-Up Rate mc/hr

BD : Blow-Down Rate mc/hr

W : “Windage” (includes drift losses – “D”) mc/hr

E : Evaporation Rate mc/hr

CM : Concentration of Substance in MU mg/l - gr/mc

CB : Concentration of Substance in BD mg/l - gr/mc

NC : Cycles of Concentration = CB/CM

VIII.1.3.2 Water balance

MU = BD + E + W

VIII.1.3.3 Material balance

MU x CM = (BD + W) x CB
Annex VIII

Industrial Cooling Systems 223

VIII.1.3.4 Concentration

Nc = CB/CM = MU/BD+ W

VIII.1.3.5 Discussion

Water enters the cooling system through the make-up line. Water must be “made-up” to

compensate for the losses due to evaporation, “windage” and through the blow-down. The

“windage” represents the quantity of water, which in the form of droplets escapes the cooling

tower. It is assumed that the droplets carry with them the entrained chemicals at the same

concentration as in the blow-down.

As water recirculates through the plant’s equipment, it takes heat with it, which in turn is taken

out in the cooling tower through evaporation. No chemicals are carried-over with the water

vapour (evaporated water).

The Evaporation Rate “ E”, results from design requirements. To compensate for the

evaporation, windage (and blow-down losses - see below) it is necessary to continue to feed an

equivalent amount of make-up water.

Make-up water, be it well water, or surface water, carries with it dissolved and suspended

solids, and the concentration and type of which will vary from case to case. A portion of the

recirculating water must, hence, be “blown-down” (discharged) to avoid the concentration of

input substances to increase above tolerable values due to evaporation.

The value of the blow-down rate must be fixed in order to maintain an optimum concentration

of substances in the recirculating water, which will avoid, together with the appropriate

chemical treatment, fouling (through precipitation and deposition of solids) and corrosion.

The blow-down rate is fixed when designing the chemical treatment and operating conditions.

Frequently in practice, and in older systems, the blow-down cannot be controlled. It results, at

least partially, from unplanned water losses from various parts of the equipment being cooled.

The Number of Cycles of Concentration is the ratio between the concentration of substances in

the blow-down and that in the make up water. For instance if the concentration of the Calcium

ion in the make up is 200 ppm, with a Number of Cycles of 2, the concentration of Calcium in

the recirculating water will be equal to 400 ppm.

The material balance above, shows that the Number of Cycles of Concentration is equal to the

ratio MU/(BD +W), and when ignoring the windage, to the ratio MU/BD.

Chemicals are fed to the cooling system either (rarely) in the make-up line, or into the cooling

tower basin. A certain concentration of chemicals must be “maintained” in the recirculating

water, which is equivalent to saying, maintained in the blow-down water. The higher the

Number of Cycles the lower the blow-down, the more severe the conditions of the system, but

the lower the quantity of chemicals continually lost.

This latest statement is true with the exception that when make-up substances are allowed to

concentrate more and more in the tower, more and different chemicals may be necessary to

maintain appropriate deposition-corrosion balance in the system.

In principle, though, in order to save water and chemicals (consumption, costs, and impact on

the environment) a fine balance must be sought at the highest possible Number of Cycles of

Concentration.

A certain concentration of treatment chemicals must be “maintained” in the recirculating water,

and, hence, in the blow-down in order for the chemicals to perform their function.
Annex VIII

224 Industrial Cooling Systems

Normally, in complex multi-substance treatments, it is prescribed that the level of one easily

measurable substance, is controlled and maintained at the values advised by the chemicals

suppliers. This corresponds to the implicit assumption that the ratio among the different

chemicals stays the same, regardless of different rates of “losses” of the individual chemicals in

the system.

It also corresponds to the important assumption that, if the concentration of chemicals is

measured (one chemical measured, the others calculated) in the blow-down, this value

corresponds to what is available in the cooling system, and that any other losses in the latter

have already been taken into account.

In other words, to evaluate the impact of the chemical(s) on the aquatic-environment, only the

fate, or the variations in the concentration of the chemical downstream of the blow-down line

point, needs to be predicted (i.e. previously mentioned as “system” losses and the reductions in

concentration, due to processes such as hydrolysis, adsorption etc -are already taken into

account).

This assumption will be used in the Benchmarking Assessment Concept.

VIII.1.4 Calculation of PEC and benchmarking

Table VIII.1 below summarises the simple approach suggested for Benchmarking of single

substances.

The suggested approach starts with the concept of calculating the “real” PECriver, and to divide it

by the correspondent EQS, as derived from the WFD.

Table VIII.1 below, shows how the “real” PEC can be calculated, and how, through successive

approximations, it is possible to normalise/standardise the PEC estimation, and so render the

calculation applicable for “benchmarking” purposes. If the BD rate, the River flow, and the

losses of chemicals are known, the Concentration of the substance in the river results from the

very simple equation (1) reported in the table.
Annex VIII

Industrial Cooling Systems 225

Table VIII.1: Calculation of PEC and Benchmarking

PECriver/EQS (EQS from Water Framework)

CB = Concentration in Blow-Down gr/mc

CR = Concentration in River gr/mc = PECriver

BD = Blow-Down Rate in mc/hr

WR = River Flow in mc/hr

t = (1 - % losses in Tower)

w = (1 - % losses in Waste Water Treatment Plant - WTP)

R = (1 - % losses in River)

(1) CR = BD x CB x (t) x (w) x ( r) Local Assessment/Predict

 WR

t = 1 w = 1 r=1 BD = 1

If also WR = 1 we have CB = CR = proportional to PEC river

Equation (1), with all its elements known or calculated, can be used as such only for local level

assessments.

To evaluate the losses in the cooling system, in the waste treatment plant, and in the river itself,

a lot of specific data needs to be available regarding chemical and physicochemical data of each

substance. This ranges from volatility, to biodegradability and deposition rate and concerns the

specific conditions of the system, such as residence time of chemicals in the tower (proportional

to the ratio Volume of System/Blow-Down rate), type and performance of waste treatment

plants (chemical and biological), residence time in the river after initial mixing, and others.

In a “ desk- top “ benchmarking approach these data are not available. Hence the need for

simplification and approximation.

- First of all it is assumed (see Table VIII.1) that the losses in the cooling system are

taken already into account by referring to the concentration of the chemical in the blow

down.

Secondly it is assumed that no losses take place in the waste treatment plant.

- This second assumption is clearly not correct in the “real world” - it puts all chemicals

on the same level, whether or not they may be lost through precipitation in a chemical

treatment plant, or through partial or complete bio-degradation in a biological treatment

plant. Consideration may be given to introducing a correction factor for chemicals with

different degrees of biodegradability; but it would also introduce a differentiation

between different treatment situations, which will vary from case to case and site to site.

- Thirdly, it is assumed that no losses occur in the river, and this is normally done in risk

assessment evaluations.

The key to the proposed Benchmarking approach lies in the next assumptions, that is that the

Blow-Down Rate is equal to 1, and so is the river flow.

This means the PEC value has been normalised (i.e. PECstandardised), to allow the comparison

among chemicals, independent of Blow-Down rate (size of plant and operating conditions), and

of River Flow.

It is obvious that for the same chemical the PEC will be higher in a larger plant with a higher

blow-down rate, and when the plant discharges into a small river.

But this in unimportant when a set of chemicals needs to be compared (i.e. “benchmarked”).

What will count for Benchmarking purposes is the feed rate of the chemical, or in other words,
Annex VIII

226 Industrial Cooling Systems

the concentration which is recommended to be “maintained” in the recirculating system, and

hence in the blow down. Normally chemical suppliers recommend a range of concentrations,

varying from case to case: the average recommended feed rate should be used.

VIII.1.5 Computation methods

VIII.1.5.1 Single Substances

One-substance chemical treatments very rarely apply. In most cases various combinations of

chemicals, inorganic and organic are used in cooling systems.

Examples of single substances are mostly related to the use of single biocides in the system, or

single polymers in Waste Treatment Plants. Still, there is quite likely to remain a desire,

possibly at Member State level rather than at local level, to Benchmark the most typical

individual substances that are on the Market.

At local level it is easier to imagine that the necessity will arise to compare complete treatments,

one against the other, rather than individual substances. A balanced vision of the overall impact

of the various substances on the aquatic-environment can only be acquired at local level, when

different proposed treatments have to be compared.

At any rate, the Benchmarking Procedure proposed here, does imply very simple computations

for individual substances. The average prescribed concentration of the substance in the blow

down is one of the terms that need to be known. It is generally expressed in parts per million

(ppm) or milligrams per litre (mg/l) in the blow-down, and above we refer to this number as

PECstandardised.

The other element of the Equation is the PNEC or EQS. This may either have been already fixed

by the Member State, or it will have to be “agreed” at local level using the WFD Annex V

procedure, based on data furnished by the supplier of chemicals. EQSs are also normally

expressed in ppm, and sometimes in ppb or micrograms per liter.

The PEC/PNEC ratio can hence be easily calculated for all the substances, which need to be

assessed. The resulting value is a purely numerical ratio (if both PNEC and EQS are expressed

in the same units, ppm, or ppb). The lower the ratio, the less the potential impact of the

substance.

It is stressed once more that this Benchmarking Assessment Concept represents a standardised

methodology for assessing potential impacts of cooling water chemicals, which strips out all

local-specific characteristics, and all physico-chemical characteristics of substances except for

toxicity. As such, it can perform a useful function in helping identify areas that require

additional investigation, and in designing chemical treatments into the overall operating-design

procedures of the plant. However, it is not suitable nor intended for use as a decision tool for

local level assessments: the fact that one substance may have a lower PECstandardised : EQS ratio

does not imply that this is necessarily the best choice for a particular situation once other local,

plant and substance-specific factors are taken into account.

VIII.1.5.2 Complex multi-substances treatments

This will in practice most often be the case faced by local authorities and plant operators in

permit applications.

Before carrying out, with the assistance of the supplier, a complete substance by substance real

life PEC evaluation, which may be necessary in certain cases, the simplified benchmarking

approach may be used to help design chemical treatments into the overall operating-design

procedures of the plant.
Annex VIII

Industrial Cooling Systems 227

Consideration may be given to using an additive procedure whereby the PEC/PNEC ratios for

each individual substance is calculated with the method explained above, and they are then

added together to yield a relative “index” number. This approach is similar to that used for the

classification of chemical preparations based on the classification of the individual substances of

which they are composed. The lower the value of the resulting sum the less is the foreseeable

environmental impact of the complex treatment.

Needless to say, a sum of all the values of individual ratios, leading to a figure of less than 1

would be preferable to a result above 1.This would have meaning only when the real local

dilution factor is known and inserted in the evaluation.

However, when values above 1 are found, and it is felt that, from a technical stand point the

specific treatment presents other environmental/economic advantages (less water, less energy) it

will be necessary to go into a more sophisticated risk assessment procedure. This may imply

both a precise calculation of all the losses of the chemical in the system (fate) and a refinement

of the evaluation of the PNEC (Chronic Data instead of Acute).

It would not seem that actually carrying out centrally in the BREF, or even at Member State

level, a general benchmarking exercise including all possible treatments and combinations is a

realistic idea. Multi-substance Benchmarking (i.e. of treatments) is more appropriately to be

considered a local affair and reference is made to Section VII.2 of this Annex.
Annex VIII

228 Industrial Cooling Systems

VIII.1.6 Appendix I: extract from technical guidance document

Chapter 3 (Environmental Risk Assessment), section 3.3.1 of Part II of “Technical

guidance document in support of Commission Directive 93/67/EEC on risk assessment for

new notified substances and Commission Regulation (EC) no 1488/94 on risk assessment

for existing substances”.

3.3 Effects assessment for the aquatic compartment

3.3.1 Calculation of PNEC

The function of risk assessment is the overall protection of the environment. Certain

assumptions are made concerning the aquatic-environment, which allow, however uncertain, an

extrapolation to be made from single-species short-term toxicity data to ecosystem effects. It is

assumed that:

• ecosystem sensitivity depends on the most sensitive species; and

• protecting of ecosystem structure protects community function.

These two assumptions have important consequences. By establishing which species is the most

sensitive to the toxic effects of a chemical in the laboratory, extrapolation can subsequently be

based on the data from that species. Furthermore, the functioning of any ecosystem in which

that species exists is protected provided the structure is not sufficiently distorted as to cause an

imbalance. It is generally accepted that protection of the most sensitive species should protect

structure, and hence function.

For all new substances the pool of data from which to predict ecosystem effects is very limited:

only short-term data are available at the base-set. For most existing substances the situation is

the same: in many cases, only short-term toxicity data are available. In these circumstances, it is

recognised that, while not having a strong scientific validity, empirically derived assessment

factors must be used. Assessment factors have also been proposed by the EPA and OECD

(OECD, 1992d). In applying such factors, the intention is to predict a concentration below

which an unacceptable effect will most likely not occur. It is not intended to be a level below

which the chemical is considered to be safe. However, again, it is likely that an unacceptable

effect will occur.

In establishing the size of these assessment factors, a number of uncertainties must be addressed

to be able to extrapolate from single-species laboratory data to a multi-species ecosystem. These

areas have been adequately discussed in other papers, and may best be summarised under the

following headings:

• Intra- and inter-laboratory variation of toxicity data

• Intra- and inter-species variations (biological variance)

• Short-term to long-term toxicity extrapolation

• Laboratory data to field impact extrapolation

 (Extrapolation is required from mono-species tests to ecosystem. Additive,

 synergistic and antagonistic effects arising from the presence of other substances

 may also play a role).

The size of the assessment factor depends on the confidence with which a PNECwater can be

derived from the available data. This confidence increases, if data are available on the toxicity

to organisms at a number of trophic levels, taxonomic groups and with lifestyles representing

various feeding strategies. Thus, lower assessment factors can be used with larger and more

relevant data sets than the base-set data. The proposed assessment factors are presented in Table

VII.1.
Annex VIII

Industrial Cooling Systems 229

For new substances and assessment factor of 1000 will be applied on the lowest L(E)C50 of the

base-set. Also for existing substances the assessment factor is generally applied to the lowest of

the relevant available toxicity data, irrespective of whether the species tested is a standard

organism (see notes to Table 14). For short-term tests, the L(E)C50 is used, while the NOEC is

used with long-term tests. For some compounds, a large number of validated short-term L(E)C50

values may be available. Therefore, it is proposed to calculate the arithmetic mean if more than

one L(E)C50 value is available for the same species. Prior to calculating the arithmetic mean an

analysis of test conditions has to be done in order to find out why differences in response were

found.

The algal growth inhibition test of the base-set is, in principle, a multi-generation test. However,

for the purposes of applying the appropriate assessment factors, the EC50 is treated as a shortterm toxicity value. The NOEC from this test may be used as an additional NOEC when other

long-term data are available. In general, an algal NOEC should not be used unsupported by

long-term NOECs of species of other trophic levels. However, if a chemical shows a specific

toxicity to algae, the algal NOEC determined from the base-set test should be supported by a

second algae species test.

Microorganisms representing a further trophic level may only be used if non-adapted pure

cultures were tested. The investigations with bacteria (e.g., growth tests) are regarded as shortterm tests. Additionally, blue-green algae should be counted among the primary producers to

their autotrophic nutrition.

Table VIII.2: Assessment factors to derive a PNEC

Description Assessment factor

At least one short-term L(E)C50 from each of three trophic levels of

the base-set (fish, Daphnia and algae) 1000 (a)

One long-term NOEC (either fish or Daphnia) 100 (b)

Two long-term NOECs from species representing two trophic levels

(fish and/or Daphnia and/or algae) 50 (c)

Long-term NOECs from at least three species (normally fish,

Daphnia and algae) representing three trophic levels 10 (d)

Field data or model ecosystems Reviewed on a case by case

basis (e)

NOTES :

(a) The use of a factor of 1000 on short-term toxicity data is a conservative and protective

factor and is designed to ensure that substances with the potential to cause adverse

effects are identified in the effects assessment. It assumes that each of the aboveidentified uncertainties makes a significant contribution to the overall uncertainty.

For any given substance there may be evidence that this is not so, or that one particular

component of the uncertainty is more important than any other. In these circumstances

it may be necessary to vary this factor. This variation may lead to a raised or lowered

assessment factor depending on the evidence available. Except for substances with

intermittent release (see Section 3.3.2) under no circumstances should a factor lower

than 100 be used in deriving a PNECwater from short-term toxicity data. Evidence for

varying the assessment factor could include one or more of the following aspects:

• evidence from structurally similar compounds (Evidence from a closely related

compound may demonstrate that a higher or lower factor may be appropriate).

• knowledge of the mode of action. (Some substances, by virtue of their structure,

may be known to act in a non-specific manner. A lower factor may therefore be

considered. Equally a known specific mode of action may lead to a raised

factor).
Annex VIII

230 Industrial Cooling Systems

• the availability of data from a wide selection of species covering additional

taxonomic groups other than those represented by the base-set species.

• the availability of data from a variety of species covering the taxonomic groups

of base-set species across at least three trophic levels.

In such a case the assessment factors may only be lowered if these multiple data

points are available for the most sensitive taxonomic group.

There are cases where the base-set is not complete: e.g., for substances, which are

produced at < 1 t/a (notifications according to Annex VII B of Directive 92/32/EEC). At

the most the acute toxicity for Daphnia is determined. In these exceptional cases, the

PNEC should be calculated with a factor of 1000. Variation from a factor of 1000

should not be regarded as normal and should be fully supported by accompanying

evidence.

(b) An assessment factor of 100 applies to a single long-term NOEC (fish or Daphnia) if

this NOEC was generated for the trophic level showing the lowest L(E)C50 in the shortterm tests. If the only available long-term NOEC is from a species (standard or nonstandard organism) which does not have the lowest L(E)C50 from the short-term tests, it

cannot be regarded as protective of other more sensitive species using the assessment

factors available. Thus, the effects assessment is based on the short-term data with an

assessment factor of 1000. However, the resulting PNEC based on short-term data may

not be higher than the PNEC based on the long-term NOEC available.

An assessment factor of 100 applies also to the lowest of two long-term NOECs

covering two trophic levels when such NOECs have not been generated from that

showing the lowest L(E)C50 of the short-term tests.

(c) An assessment factor of 50 applies to the lowest of two NOECs covering two trophic

levels when such NOECs have been generated covering that level showing the lowest

L(E)C50 in the short-term tests. It also applies to the lowest of three NOECs covering

three trophic levels when such NOECs have not been generated from that level showing

the lowest L(E)C50 in the short-term tests.

(d) An assessment factor of 10 will normally only be applied when long-term toxicity

NOECs are available from at least three species across three trophic levels (e.g., fish,

Daphnia, and algae or a non-standard organism instead of a standard organism).

When examining the results of long-term toxicity studies, the PNECwater should be

calculated from the lowest available no observed effect concentration (NOEC).

Extrapolation to the ecosystem effects can be made with much greater confidence, and

thus a reduction of the assessment factor to 10 is possible. This is only sufficient,

however, if the species tested can be considered to represent one of the more sensitive

groups. This would normally only be possible to determine if data were available on at

least three species across three trophic levels. It may sometimes be possible to

determine with high probability that the most sensitive species has been examined, i.e.,

that a further long-term NOEC from a different taxonomic group would not be lower

than the data already available. In those circumstances a factor of 10 applied to the

lowest NOEC from only two species would also be appropriate. This is particularly

important if the substance does not have a potential to bioaccumulate. If it is not

possible to make this judgment, then an assessment factor of 50 should be applied to

take into account any interspecies variation in sensitivity. A factor of 10 cannot be

decreased on the basis of laboratory studies.

(e) The assessment factor to be used on mesocosm studies or (semi-)field data will need to

be reviewed on a case by case basis.
Annex VIII

Industrial Cooling Systems 231

For compounds with a high log Kow no short-term toxicity may be found. Also, even in

long-term tests this may be the case or steady state may still not have been reached. For

tests with fish for non-polar narcotics the latter can be substantiated by the use of longterm QSARs (see Section 3.2.1.2 and Chapter 4 on the Use of QSARs). It can be

considered to use a higher assessment factor in such cases where steady state seems not

to have been reached.

For substances for which no toxicity is observed in short-term tests a long-term test has

to be carried out if the log Kow > 3 (or BCF > 100) and if the PEClocal/regional is > 1/100th

of the water solubility (see Section 4.5). The long-term toxicity test should normally be

a Daphnia test to avoid unnecessary vertebrate testing. The NOEC from this test can

then be used with an assessment factor of 100. If in addition to the required long-term

test a NOEC is determined from an algae test of the base-set an assessment factor of 50

is applied.

The effects assessment performed with assessment factors can be supported by a

statistical extrapolation method if the data basis is sufficient for its application (see

Appendix V).

VIII.2 Concept of a local assessment method for cooling water

treatment chemicals, with a particular emphasis on

biocides

VIII.2.1 Introduction

One of the major environmental issues identified in the industrial cooling systems BREF

concerning wet cooling systems is the chemical treatment of cooling water (anti-corrosion; antiscaling, anti-fouling, biofouling control), and the resulting emissions to surface water.

Particular emphasis is placed on biocides, due to their inherent high toxicity, which is necessary

as a result of the particular function they must perform.

The cooling BREF identifies three levels at which techniques may be employed to reduce the

impact of cooling water additives/ biocides on receiving water bodies:

1. Preventive measures (Table 4.7)

2. Optimisation of operation, including monitoring (Table 4.8)

3. Selection & application of additives (Table 4.8)

The three levels of control interact with each other, and discussions in the TWG have

established that the selection of appropriate additives is a complex exercise, which must take

into account a number of local and site-specific factors.

The need to provide an outline of the concepts underlying assessment of cooling water

additives/biocides has been identified as an important BAT measure to help reduce the

environmental impact of additives, and biocides in particular. In this connection, the BREF

contains an Annex which establishes a screening assessment tool based on existing

methodologies and data (“Benchmarking Assessment”); and Chapter 3 also provides some

background information on the assessment regimes used in the Netherlands and Germany.

In a horizontal BREF it is only possible and appropriate to conclude in general terms on the

concepts which will facilitate the application of BAT principles regarding selection of biocides

and other additives. Specific installation characteristics, climatic conditions and the local

environment are key elements in the determination of a BAT compatible approach at the local

level for individual installations.
Annex VIII

232 Industrial Cooling Systems

The justification for the emphasis on biocides in any assessment regime is that their intrinsic

properties result in them being considered to be of higher potential concern in terms of impacts

on receiving water bodies. At the point of exit, discharges from cooling systems using biocides

may exhibit acute toxicity. Local circumstances, the characteristics of the substances employed,

and in particular the actual dilution in the receiving water, determine whether Environmental

Quality Standards (EQSs) can be met. Proper selection and reduction of potential impacts

resulting from biocide use can only be adequately addressed when potential impacts can be

assessed. The yardstick by which a BAT-compatible approach to the application of additives/

biocides can be judged is the environmental status of the receiving water body.

For the above reasons, the horizontal cooling BREF should provide guidance on how to address

site-specific issues in the local level assessment of biocides used in cooling systems. Such a

local assessment can be seen as a subsequent, and more detailed step following an (optional)

preliminary screening exercise such as the Benchmark Method presented in the Annex VI.1.

Therefore, the BREF aims to provide guidance on the concepts relevant to assessment of local

circumstances, without prescribing the methodology itself. Numerous and constantly evolving

methodologies and models are available for assessing local level emissions scenarios (ranging

from simple to highly sophisticated). It should be up to permit applicants and Member State

authorities to select and use methodologies, which are appropriate to the local conditions and

the level of concern about potential environmental effects.

VIII.2.2 Key elements

In the context of how to address minimisation of the impact specifically of biocide use in

cooling systems according to the principles of BAT, there are two key building blocks that it is

important to be aware of:

• The Biocidal Products Directive 98/8/EC (BPD), which since 14/5/2000 has regulated the

placing on the European market of biocidal products. In this context, the EU will examine

exposure scenarios in order to evaluate the risks associated with all the 23 productcategories which are covered. One of the product-types considered for authorisation covers

biocides used in cooling systems (product-type 11). New biocides are to be assessed and

approved immediately according to the Directive. An extension has been made for existing

substances, which will be reviewed in due course.

• The future Water Framework Directive (WFD), which provides a whole host of quality

related objectives. These specifically include a methodology to be used to establish

Environmental Quality Standards (EQSs) for chemical substances, which is laid down in

Annex V of the WFD text. The methodology for setting EQSs is identical to that used to

determine conservative Predicted No Effect Concentrations (PNECs) according to the

testing methods laid down in EU chemicals legislation. The method incorporates a “safety

factor” of up to 1000 in order to take account of the uncertainties involved in extrapolating

from toxicity testing on selected organisms to the protection of the aquatic ecosystem.

Toxicity data for biocides used in cooling systems is either already generally available, or will

be made available together with data on other relevant intrinsic properties (e.g. biodegradability,

bioaccumulation) according to the registration procedures laid down under the BPD. Based on

this data the methodology laid down in Annex V of the WFD can be used to determine the EQS

(i.e. the PNEC value) for a substance in water.

The EQS can then be compared to the Predicted Environmental Concentration (PEC) to help

determine what potential there may be for impact to occur. Since the EQS corresponds to the

PNEC, this is frequently referred to as a “PEC : PNEC comparison”. As noted above, numerous

methods are available to calculate the concentration of substances expected to be found in

receiving waters as a result of a discharge (i.e. PEC).
Annex VIII

Industrial Cooling Systems 233

The PEC/PNEC value can be used as a yardstick for the BAT determination for a BAT

compatible approach for biocides used in cooling systems. It should however be recognised that

a certain distinction in this approach has to be made between new and existing installations. A

PEC : PNEC value of <1 in the receiving water after realistic mixing & dilution could provide

the yardstick (as a limit value) for biocide use in new cooling systems. For existing cooling

systems where many design parameters & other installation characteristics are already

established it will not always be possible to achieve PEC : PNEC <1 at a cost which is

economically viable as described in the definition of BAT. In these cases the PEC : PNEC <1

should remain the target (as a benchmark), but may have to be considered a longer term goal

which fits with equipment replacement cycles etc.

Figure VIII.2 shows a graphical presentation giving an example of how a BAT-compatible

approach might be determined for the use of biocides in existing cooling systems. Properly

optimised operation in a well designed cooling system can be considered BAT when a PEC :

PNEC value <1 is achieved. For installations that cannot achieve PEC : PNEC <1 due to suboptimal design or other local/ site-specific factors, it will be necessary to optimise the operation

of the system as far as is feasible.
Annex VIII

234 Industrial Cooling Systems

SELECTION OF BIOCIDES

- biocides directive 1); 

- technical characteristics of

Installation;

- environmental impact

Advanced measures : 

Pilot research for the application (new)

alternatives in the specific situation

OPTIMIZATION

Good operating practice 2)

(fig. 3.3 of BREF document) ) Advanced measures 3) 4)

LOCAL ASSESSMENT : 

Are local EQS met ?

 BAT

PEC/PNEC <1

START

BAT 5) 

yes no

Horizontal

BAT

Local specific

BAT

Selection

 Aim :

PEC/ PNEC

Information concerning toxicity, persistency 

(biodegradability) and treatability of the selected biocide 

Figure VIII.2: Combined approach for the assessment of cooling water biocides for existing

installations
Annex VIII

Industrial Cooling Systems 235

Legend to Figure VIII.2:

1) implementation of this directive is under preparation;

2) optimisation of the use biocide due to monitoring of parameters relevant for the control

of the cooling system and optimisation of dosage (prefer automatic dosage);

2) measures such as pretreatment, side-stream filtration can be considered. Also end-ofpipe measures can be taken into account. A choice for a measure is often situation

related. A wide variety of end-of-pipe measures can be considered such as biological

treatment, sand filtration, adsorption techniques, oxidation by ozone etc. etc.

4) in this case (normal) BAT criteria towards measures have to be applied; this means an

evaluation of different aspects such as: availability of measures, economic impact of the

necessary measures related to environmental impact of a measure;

5) in this case if we are dealing with an optimised situation in terms of implementation of

measures (process control, optimisation of the use of biocides and implementation of

end-of-pipe measures) all within the normal criteria of BAT for abatement measures

(see 4). The result of the above evaluation represents the solution which comes closest

to the aim PEC/PNEC = 1. Other appropriate additives (with less environmental impact)

are not available. For this reason this can be considered as BAT for existing

installations.

VIII.2.3 Example of proposed local assessment method

[tm004, Baltus and Berbee, 1996] and [tm149, Baltus et al, 1999]

In the following an example has been worked out according to the method which has been

discussed at 29-31st May TWG Meeting in Seville and elaborated since that time into the

proposal for the assessment of biocides in Annex VII of this BREF.

According to the scheme of the proposal three major steps can be distinguished:

1) The SELECTION OF BIOCIDES:

The selection of biocides is a tailor made choice for each and every cooling system, and

normally is the result of expert discussions between plant operators and chemical suppliers. The

benchmark methodology described in appendix VII of this BREF document can be a very use

full support tool in the considerations for the selection of biocides. It should be noted that the

result of this step is only a first prioritisation of possible biocides. The further elaboration in step

2 and 3 might result in a different order of preference of possible biocides.

2) The OPTIMISATION STEP :

The optimisation step includes all kinds of process-, dosage- and monitoring techniques as well

as purification of make up water, side-stream filtration and process control measures such a

temporally closure of the bleed of a recirculation system.

3) The LOCAL ASSESSMENT :

The local assessment is the final step in the assessment of biocides and provides plant operators,

chemical suppliers and regulators a yard stick which enables them to determine to what extent

operations, control techniques and measures have to be applied in order to meet local EQSs.

As example the following situation has been elaborated: a recirculating cooling system has to be

treated with chemicals to prevent microbiological fouling of the cooling system. The

dimensions of the cooling system are presented in Figure VIII.3.
Annex VIII

236 Industrial Cooling Systems

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

V-cooling system = 4500 [m3]

Concentration factor (n) = 3

discharge = 203 [m3/hr]

Intake = 609 [m3/hr]

Surface water

flow [m3/sec]

Optimisation measures

(monitoring and dosage)

Pretreatment measures

Recirculating flow = 18000 [m3/hr]

End of pipe measures

biotreater

PEC :

Predicted concentration of biocide in

surface water

Figure VIII.3: Schematic representation of a recirculating cooling system with the data for the

example of a local selection method of cooling systems chemicals

For this example the assumption is made, that the result of step 1 (Benchmark method) resulted

in the selection of the biocides hypochlorite in combination with dibromonitrilopropionamide

(DBNPA).

The optimisation in terms adequate monitoring and dosage of the hypochlorite shows that the

average concentration in the effluent should not exceed a concentration of 0.2 [mg FO/l].

For the non-oxidising biocide DBPNA the optimisation results in a shock dosage at a

concentration of 4 [mg/l] (frequency: once a day).

DPBNA is an additive which readily hydrolyses in water (τ1/2 = 2 hr). This property of the

additive can be a benefit in reducing the emissions from the cooling system and the realisation

of a more effective use of the biocide. By closing the discharge during and after dosage for a

certain period the concentration of the biocide will be reduced in the system. In this particular

case, where DPBNA is been considered, the temporally closure of the bleed provides an

additional (optimisation) option to reduce the amount of biocides discharged into the

environment. From the operators point of view the question is: to what extent will it be possible

to close the bleed of the recirculating system, in order to reduce the concentration of DBPNA

through hydrolysis to a sufficient level, without hampering a good operational performance of

the cooling system? This sufficient level is a concentration of DPBNA in the effluent leading to

a concentration in the recipient (PEC: predicted environmental concentration) which will not

exceed the EQS.

In the next table the predicted concentration of DBNPA in several types of surface water is

calculated and in the last column the reduction percentage required to meet the EQS for these

surface waters has been determined.
Annex VIII

Industrial Cooling Systems 237

Table VIII.3: Predicted concentrations of DBNPA in different surface waters for this example

Situation:

Recirculating cooling system ; discharge volume (bleed) : 203 [m3/hr]; biocide used : DBPNA ;

Dosage : shock (daily) : concentration : 4 [mg/l] ; EQS : 7 [μg/l].

Dimensions

Receiving water Flow

[m3/sec]

Width

[m]

Depth

[m]

Velocity

[m/sec]

Dilution

after

discharge

PEC

[ug/l]

Necessary

reduction [%]

to meet EQS

Average river 25 50 2,6 0,192 110 36,4 80,5

Large river 262 125 3,8 0,552 770 5,2 0

Small river/brook 1 10 1,5 0,067 10 400 98,5

Large canal 40 200 6 0,033 92 43,5 83,9

Small canal 2 25 2 0,04 14 286 97,6

Ditch 0,15 5 1 0,03 3 1333 99,5

Lake - - 1,5 0,01 3 1333 99,5

The Table VIII.3 shows that a direct discharge leads to an exceeding of the EQS for most of the

selected surface waters. Only a discharge of the effluent in a large river leads to an acceptable

concentration of DBPNA in surface water.

For this example the PEC is calculated using a model which is generally excepted in the

Netherlands and is used by permitting authorities for a local impact assessment after BAT in a

more general sense has been determined (combined approach). The Dutch model is based on the

Fisher equations. The PEC is calculated at a distance of 10 times the width of the receiving

water system with a maximum of 1000 m (for lakes at a distance of 1⁄4 of the diameter).

It is expected that most member states will have their own methodologies or will use dilution

factors for different type of recipients to determine the PEC.

The Environmental Quality Standard for DBPNA is calculated according the methodology that

has been laid down in Annex V of the Water Framework Directive. The data listed in the table

below result in a n EQS for DBNPA of 7 [ug/l]. (one NOEC and 3 acute data result in a safety

factor of 100; lowest concentration /100  7 [ug/l] [1]).

Table VIII.4: Ecological data of DBNPA

Parameter Concentration

LC-50 (fish) 96-hr 2 [mg/l]

MIC (algea) 2 [mg/l]

LC-50 (crustacean) 0.7 [mg/l]

NOEC (fish) 4 [mg/l]

Temporarily closure of the bleed of the system is a good option to optimise the use of biocides

and to decrease the load of biocides discharged when readily degradable additives are used.

In the following table the necessary time needed to reduce the emission of biocides to such a

level that EQS after discharge can be met in the surface water is presented. In the last column

of this table the consequences of this closure in terms of the increase of the salt concentration in

the recirculating water has been calculated.
Annex VIII

238 Industrial Cooling Systems

Table VIII.5: Consequences of closing the discharge

Situation:

biocide DBPNA : k = 0,3 [sec-1] ; τ1/2 = 2 hr ; C0= 4 [mg/l] ; C = C0 * exp -(Qv/V+k)*t [1];

V = volume of the system [m3] ; Qv = discharge [m3/hr] ; t = time [hr].

Receiving water

Necessary

reduction

[%]

Necessary time the

discharge is closed

[hr]

Remarks

Average river 80.5 3.7 increasing concentration of salts : factor 1.2

Large river 0 0

Small river/brook 98.5 10.7 increasing concentration of salts : factor 1,8

Large canal 83.9 4.3 increasing concentration of salts : factor 1,2

small canal 97.6 9.7 increasing concentration of salts : factor 1,7

Ditch 99.5 14.2 increasing concentration of salts : factor 2,5

Lake 99.5 14.2 increasing concentration of salts : factor 2,5

Depending on the specific situation it has to be evaluated whether or not the above-mentioned

consequences in terms of concentration of the inert fraction (salts) are acceptable. On the other

hand it is always possible to anticipate on these consequences by means of extra discharge

before dosage and closing, which will create lower concentrations of inert fraction in the

cooling system.

Further Measures:

If EQS cannot be met it has to be evaluated whether alternative biocides should be considered

and/or other measures can be taken.

Examples of measures are:

• pre-treatment of the cooling water used (side-stream filtration);

• optimisation by improvement of dosage and monitoring;

• end-of-pipe treatment, e.g. treatment of blow down in biological treatment.
Annex IX

Industrial Cooling Systems 239

ANNEX IX EXAMPLE OF A MODEL FOR ESTIMATING

EMISSIONS OF BIOCIDES IN THE BLOWDOWN

RIZA developed a simplified model to estimate the discharge of biocides of an open

recirculating cooling tower [tm004, Baltus en Berbee, 1996]. This model assumes:

• that the major way by which biocides are lost is via the blowdown and via volatilisation,

adsorption etc.

• that this blowdown is very small compared to the circulating amount of water;

• that pH and temperature are constant;

• that with shock dosage, the initial concentration is the same in the entire cooling system

immediately after dosage;

• that hydrolysis is a first order chemical reaction and that the resulting dissociation rate is

known;

• the volume of the purge is much smaller than the volume of the recirculating water flow.

Resulting from these assumptions, which slightly simplify reality, the following equation could

be derived to calculate the fraction of a biocide that will finally be released into the receiving

environment:

Fraction (%) = Φv x 100 %/ (Φv + kV)

Φv = purge (m3

/hr)

k = dissociation factor (hr -1) (k=0 if substances do not dissociate)

V = volume of the system (m3

)

The difference between 100% of the substance and the actual fraction released is assumed to be

hydrolysed. With no other chemical reactions assumed to take place, this model could be seen

as describing the worst case. It is realistic to expect the percentage of biocide emitted in reality

to be lower than the result of this model. It is important to acknowledge that this is just a model

to roughly estimate the discharge and that it does not give any information on the toxicity of the

purge. Especially, in the case of largely hydrolysed biocides the resulting substances can be

even more harmful than the original treatment.

The dissociation factor (k) is an important factor as it is a measure of the speed with which a

biocide disappears from the system by dissociation. If this happens in a very short period of

time, it could be worthwhile to close the blowdown and to wait for the moment that the

concentration of the biocide reaches its lowest level. To prevent the system from salting up the

water in the system should be refreshed just before dosage. The blowdown has to be opened

after a few hours to prevent salt concentrations to raise. It is obvious that this is more successful

with fast hydrolysing than with slow hydrolysing biocides. Fast hydrolysing biocides are e.g. ßbroom-ß-nitrostyreen or DBNPA. A slow hydrolysing biocides is e.g. isothiazolines.

Some results from the use of this model were that with a pH of 8 and temperatures between 25-

40°C biocides can still be quite persistent and can be emitted in the blowdown for more than

80%. The percentage of fast hydrolysing biocides in the blowdown appeared to be much lower

(25%). It shall not be concluded that they are more favourable as their toxicity or that of their

dissociation products can be very high and thus create an even less favourable situation in the

receiving water.

Annex X

Industrial Cooling Systems 241

ANNEX X INVESTMENT COSTS AND OPERATIONAL COSTS

OF EQUIPMENT AND ELEMENTS OF COOLING

SYSTEMS FOR NON-POWER PLANT

APPLICATIONS

[tm001, Bloemkolk, 1997]

In this annex some data are presented on cost data for large industrial cooling systems. Prices in

practice will vary widely as indicated by the ranges given. For smaller systems (series) costs on

investment and operation will be different again, but will also show a wide variation.

The general picture shows that higher investment costs go together with lower operational costs.

It is suggested that this simultaneously could indicate a lower environmental impact.

For each configuration a cost indication has been given, however calculations made on the costs

of cooling systems show a wide variety and it can be concluded that the differences in costs

between the different systems do not necessarily indicate the least expensive variant. Of the

different factors that in the end influence overall costs the users’ requirements and the legal

requirements to be met are very important. For this reason an estimate of the feasibility of a

system or the application of a technique should be made for each individual case. Also, the

energy price has to be taken into account and especially in those cases where heat recovery is

being considered it plays an important role. Costs are based on the year 1995.

An important aspect in calculating the costs of a cooling system and of possible improvements

is the comparison between the initial investment costs of a system (or an applied measure) and

the resulting annual costs. In practice high investment costs can lead to lower maintenance

costs, but also to higher annual fixed costs, which can be an obstacle to investment itself. For

the sake of comparison cost should be expressed in the heat capacity the system is designed for

(kWth or MWth).

Elements and costs

For industrial (non-power plant) applications a number of cost-determining elements Table X.1

were listed for both water-cooled and air-cooled systems to calculate total costs and to compare

the different systems. The costs are beased on the cost levels of the separate parts of a cooling

system. In the same reference costs and cost variations of separate elements of cooling systems

were described.

Fixed costs

Costs of heat exchangers depend on type, material and size.. Plate heat exchangers are cheaper

than shell & tube even in more expensive material such as titanium, but are limited in allowed

pressure level. Condensers are approximately 25% more expensive than shell and tube

exchangers. Materials such as stainless steel or special CuNi are more expensive than steel (up

to 2-5 times). Special tubes may be 10-15% more expensive.

Costs for air-coolers primarily depend on the heat exchanging surface area and the type of fan.

The required end temperature is also reported as a deciding factor. The material of the aircooled heat exchanger generally is less important than in water cooled systems, but this also

depends on the corrosiveness of the substance to be cooled.

Costs for conduits and distribution vary strongly with diameter, material and length.

Inlet and outlet provisions are an an extremely location-dependent issue. Especially the length,

diameter and construction of the supply and drainage pipes can determine the level of the costs.

A cost level of about Euro 13000 per MW was indicated for a 300 MW installation. These costs

will remain relatively high for smaller installations.

Cooling-water systems are fitted with pumps to pump the water around. Indirect systems have

two cooling-water circuits and therefore need extra pumps. Pump investments vary according to
Annex X

242 Industrial Cooling Systems

lift, capacity and material. The cleaner the cooling water the less critical is the choice of

required materials.

Cooling tower prices strongly depend on model and size. Plume suppression may be required in

which case the investments for the cooling tower will be about 1.5-2 times higher. Collection

water trays are also part of the cooling tower installation.

The cooling tower price is partially dependent on the design space desired. A smaller approach

over the cooling tower leads to a larger and more expensive cooling tower, both as regards the

investment for the cooling tower itself, and as a result of energy consumption. The table below

shows this with an example:

Variable costs

The variable costs of cooling systems are highly system-dependent. The most striking cost

factors are (1995):

• energy (0.05-0.06 euro per kWh)

• groundwater including levy, tax and pumping (0.09-22 euro per m3)

• groundwater excluding pumping (0.09-0.11 euro per m3)

• including levy and tax drinking water (0.4-1.4 euro per m3)

• in some cases, semi-manufactured products are also used, for instance flocculated river

water or raised condensation. The costs of these are lower than those of purchased water.

The determining operational aspects of cooling-water systems are the pump energy and, in the

case of a cooling tower, the extra ventilator and supplement water. Furthermore cooling water

treatment will add to the cost, but this varies on the applied treatment which is associated with

the cooling system. Once-through generally only need biofouling control, whereas recirculating

systems need additonal dispersing and anticorrosion agents.

The operational costs of dry air-cooled systems consist primarily of energy costs. The energy

costs of air coolers result from the use of fans. The maintenance costs of air-coolers are onethird to half of those of systems with shell & tubes.

Methodology

Different methodologies have been developed for cost comparisons between different cooling

systems. The following approach is used as an example, but other costs methods are based on

the same principle. The method is not accurate in absolute terms and, in other words, is not

meant to be used for accurate investment estimates. It is however suitable for comparing

investment costs of different cooling systems.

For the various systems universal cost factors have to be included and can be expressed as a

fixed percentage of the equipment installation costs (Direct Field Costs, or DFC). These cost

factors and the associated percentages in this example are:

-indirect costs (5% of investment costs)

-engineering (8% of investment costs)

-unforeseen (15% of equipment installation costs)

The investment costs and the cost factors make up the total investment costs (TIC).

Annual costs are the total of fixed costs (interest + depreciation) and variable (operational)

costs. It should be kept in mind that a higher investment not only leads to higher annual fixed

costs, but can also be an obstacle to investment itself. Also included in the annual costs are the

maintenance costs.

Comparisons

Based on the elements above, the investment for the various cooling systems was calculated and

compared. A calculation was also made of the accompanying operational costs. The total is

summarised in Table X.2. In calculating annual costs the period of depreciation at a certain
Annex X

Industrial Cooling Systems 243

interest has to be applied. The operational costs are also calculated. Annual maintenance costs

are based on the total investment costs (TIC).

Table X.1: Cost elements for water and air cooling systems

[tm001, Bloemkolk, 1997]

Cost type Cost elements Water cooling

systems Air cooling systems

Fixed Heat exchanger(s) (type, size and

model) X x

Heat exchanger (material) X x

Pipelines in process, tube-bridges X x

Pumps/reserve pumps X x

Inlet facilities X

Tube intake/drainage X

Outflow facilities X

Cooling tower(s) (possible) X x

Fans X x

Sound attenuation X x

Indirect system (extra heat exchanger,

pipes, pumps) X x

Variable Water (groundwater, tap water) X

Water discharge fee X

Leakage monitoring X x

Water conditioning X

Energy consumption (pumps and fans) X x

Maintenance X x

Calculations have shown that the investment level and the consumption of energy determine

cost sensitivity to a large extent. The variation in the costs of heat exchangers (shell & tube) due

to the chosen configuration and to the choice of material is very important. Cheap materials and

models determine the calculated lower limits and special materials determine the upper limit. At

the same time it should not be forgotten that good materials could considerably decrease the

costs of maintenance and operating, and of the use of chemicals.

Calculated as annual costs, investments and operational costs differ significantly. Factors such

as (make-up) water requirement and price, and energy consumption are influential. The choice

of material also has consequences for the annual operating costs. In the case dry air-cooling is

applied, the achievable end temperature is important and the lower the required end

temperature, the more expensive air-cooling will become. With water-cooling, low endtemperature have less affect for cost estimations, unless small approaches are used in the

calculation.
Annex X

244 Table X.2: Cost indications for water and air cooling systems for industrial applications with the exception of pow[tm001, Bloemkolk, 1997]

System Installation x 1000 (EUR/MWth) Total investment (TIC) x 1000 (EUR/MWth) Investment determined by Operational costs 

(EUR/MWth

Once-through(range 0.2- 10 MWth)

(range > 10 MWth)

to elements-heat exchangers2

–tubes etc.-pumps-supply/drainageTotalIndirect once-throughTotalRecirculating with openwet cooling tower(range 0.2-1 MWth)

(range>1MWth)

to elements-cool tower-heat exchangers-tubes/pumpsTotalIndirect recirculatingwith open wet coolingtowerTotal

 68 – 182 34 - 91 68 (36 - 136) 9.1 - 14 4.5 - 9.1( 9.1 - 14)' 59 - 17318 - 502,3 (extra)

 59 - 136 45 - 68 18 - 454

 36 – 136 14 - 23 68 - 203 18- 452,3 (extra)

86-255

 77 - 227100 - 269 89 - 266112 - 331

- material, model- length, material- capacity, DP- location(supply/drainage)extra heat exchangersextra heat exchangers- model- material, model- material heatexchangersextra heat exchangersextra pumps

- energy 4.- conditioning 0.- maintenance5 2.

 7.10 - 19- supplement 6.- energy 6.- maintenance 2.- conditioning 1.19-41- energy 9.- maintenance 2.- conditioning 1.- supplement 6.20 - 43
Industrial Cooling Systems Table X.2 continued

System Installation x 1000 (EUR/MWth) Total investment (TIC) x 1000 (EUR/MWth) Investment determined by Operational costs 

(EUR/MWth

Dry air coolingDirectTotalIndirectTotal

 81-220extra 14-452,3

 95-266

105-288123-351

end temperature8

end temperature8

- energy 1.- maintenance 1. 2.- energy 3.- maintenance 1. 5.

1. see text2. costs for extra heat exchanger depending on type

General costs factors materials:• steel 1 special (bv Cu/Ni alloy) 1.5-5.0• coated steel 1.3-1.7 copper 1.5-2• rvs 304/316 1.5-3 titanium 1.7-2.5

3. cost depend on heat exchanger plus extra pumps and distribution; often plate heat exchangers.4. system is factor 2-2.5 times more expensive with steam plume suppression5. maintenance costs 3.5%; for air-cooling 1-1.5%6. assumes depreciation at 5% interest, where the annual fixed costs come to approx. 13% of the investments (annuities).7. number of operating hours per year 80008. upper price limit for deep cooling functions up to 30o C; lower price limit for 60o C9. no cost data known

Annex XI

Industrial Cooling Systems 247

ANNEX XI EXAMPLES OF TECHNIQUES TO BE

CONSIDERED WITHIN THE PRIMARY BAT

APPROACH FOR INDUSTRIAL COOLING

SYSTEMS

XI.1 Introduction

Many options are available for reduction of the environmental effects of industrial cooling

systems. The general approach aims at prevention by proper design and construction, which in

case of new industrial cooling systems is generally easier to achieve than for existing systems.

The application of reduction measures depends on the cooling configuration as well as on-site

related limits, such as space. Other factors such as energy consumption, operational

requirements and economics will play an important role as well. Based on the general approach

presented in chapter 1 and applied to reduce environmental impact as described in chapter 3 this

Annex describes techniques and alternatives in more detail. The techniques can be considered in

the optimisation of cooling systems in line with the BAT-approach.

The list is an overview of the more detailed information on a number of reduction techniques

brought forward by the TWG in the exchange of information on BAT for industrial cooling

systems. For each technique that has been reported by the TWG a short description is given

followed by the reducing effect (quantitatively/qualitatively), cross-media effects, plant size

limits, costs, and example plants. Similar to the assessment of the appropriate cooling technique

the application of any of the techniques presented hereafter will have to be assessed in the light

of the cooling configuration used or planned. As far as selecting between techniques with a

similar environmental objective is possible, their environmental performance and their technical

applicability should be the initial selecting criteria followed by investment and maintenance

costs and the crossover effects on other environmental compartments. Generally, for many of

the techniques described neither cost data, nor cross-media effects were as yet available and

need further research.

With respect to the application and feasibility of the techniques care must be taken. The

environmental results obtained are achieved under certain process conditions and give no

guarantee for similar quantitative results in a different process environment. The results are

useful to illustrate the direction of the improvement. Particularly in the case of industrial cooling

systems, process requirements and cooling system’s size and operation vary widely and will

affect the results of any abatement measure applied.

XI.2 Cooling water savings through water reuse

The use of water for cooling is or might become restricted either in general or, temporarily, by

seasonal changes in availability thus creating periodic shortage. In several European Member

States an increasing pressure is put on industry to limit and optimise their water use. Thus,

emphasis has been placed on industry to change the technology and change once-through

systems into recirculating cooling systems if possible or to operate their recirculating cooling

towers at higher cycles of concentration. Other options in cooling towers that are commonly

applied are drift eliminators.

Also, a number of water treatment options can be to regain the water used and prepare it for reuse in the cooling cycle. Also, some policies aim at increasing the share of dry air cooling,

where this does not require water and thus none of the associated problems,whereas other

considerations might be a limit for this option (climate, investment costs, space).

An overview of treatment methods revealed the following options [tm065, Meier and Fulks,

1990]:
Annex XI

248 Industrial Cooling Systems

• Cold lime-softening

• Hot process softening

• Brine concentrators

• Biological treatment

• Reverse osmosis

• Electro-dialysis reversal

• Evaporation ponds

Of these options reverse osmosis and electro-dialysis reversal are very energy demanding

processes and thus turn out to be relatively expensive. Hot process softening is very efficient,

but has the disadvantage that additional cooling or heat recovery is necessary. Biological

treatment is used to remove organic matter from water and is especially interesting as part of a

treatment programme for wastewater treatment effluent to be used as make-up water.

Evaporation ponds are an easy method of plant effluent reduction. Their size requirements and

limits to the disposal of the remaining sludge may prohibit their use.

XI.2.1 Reuse of (waste) water for cooling tower make-up

[tm066, Phillips and Strittmatter, 1994] and [tm064, Meier, 1990]

Description

Water from within or outside the plant can be used as make-up water for cooling towers. Both

process effluents from within the same plant as well as effluents of municipal wastewater

treatment plants can be applied. The chemistry of the water is important. A water audit can

provide a complete water balance for each portion of the plant. This audit should give

information including the tower water chemistry, on cycles of concentration, holding time

index, velocity, system metallurgy, temperatures and the current treatment chemistry and

operating performance. Some times the water needs to be filtered first and an wide range of

filtration methods can be applied, but in the scope of this paper are not addressed.

The chemistry of the water decides upon the chemical treatment required in the cooling tower to

maintain the number of cycles. Especially, an increasing corrosivity level has been reported to

occur. In some cases the limiting concentration factor can be extended by using scale inhibitors

to increase the cycles of concentration or by using techniques as reverse osmosis to remove

dissolved solids.

Reduction:

The percentage of reduction is largely depending on the demand of the recirculating cooling

system and the availability of reusable water at the required moment. Percentages of up to 15%

are being reported.

Cross-media effects:

Waste as filter-residue due to filtering the water before use might have to be disposed. The

saving of fresh water use will have to be assessed against the environmental and financial costs

of the extra use of additives to condition the waste-water. The chemical treatment of the stream

to be reused can be very complex and may require extra manpower to operate the system.

Application limits:

Water reuse is an option for both new and existing plants and irrespective of plant size, although

for larger demands the supply of alternative water resources might not be sufficient. Organic

content (BOD) can be a limiting factor that must be checked.

Costs:

Cost indications vary widely and are quite specific for the plant. No indicative data are known.
Annex XI

Industrial Cooling Systems 249

Example plants:

For refineries it has been demonstrated that municipal waste water can be used as make-up

water [066, Phillips and Strittmatter, 1994]. An example of applying zero blowdown was

described in [tm064, Meier, 1990].

Considerations:

Typical problems to be encountered using waste streams are:

• the higher microbiological activity due to dissolved nutrients;

• increased risk of scaling due to increased level of dissolved salts

• fouling problems as a result of high levels of iron and/or suspended solids;

• corrosion problems due to high levels of total dissolved solids (TDS).

Possibilities to encounter the aforementioned problems successfully depend on the composition

of the waste stream. Municipal waste waters will differ greatly in water quality (it contains

typically relatively high levels of ammonia and phosphate, in addition to significant levels of

dissolved organic matter. Furthermore, municipal wastewater typically contains relatively high

concentration hardness, which may cause scaling. High levels of iron and/or suspended solids

can lead to fouling problems. Total effluent of a refinery may contain high oil and grease and

suspended solids levels, which may increase the demand for oxidising biocides.

XI.2.2 Zero discharge system

[comment, D]

Description:

A staged cooling system is used to eliminate any liquid discharge from the cooling tower

blowdown.

Blowdown from the primary tower is performed to maintain the balance of salts within the

limits of good operating practice. Water which has a high level of highly insoluble salts

(calcium salts) is converted to water which has a high level of highly soluble salts (Sodium

salts). This process occurs in a softener reactor/clarifier.

After this the blowdown flows to a direct osmosis (DO) membrane concentrator which pulls

water from the blow down through membranes into a sodium chloride brine. The brine is

reconcentrated in a secondary cooling tower, the so called brine cooling tower, using waste heat

from the main condenser as an energy source. The brine cooling tower will have a much lower

waterflow than the primary tower. A typical ratio of the brine flow to the primary flow is 1 to

750.

The concentrate from the DO membrane system is further concentrated in a small crystallizer,

with the solids being removed and disposed of off-site. The liquid discharge of the crystallizer is

recycled to the brine tower.

Reduction:

The reuse of primary blowdown water is reported to be about 75%, where the remaining part

evaporates in the secondary cooling tower (about 16%) or is contained as residual humidity in

the solid disposal.

Using waste heat in the brine-cooling tower reduces the cooling load on the primary tower with

about 3.5 MW at a blowdown of 45m3

/h.

Cross-media effects:

Lower cooling load on the primary tower. Some energy is required to operate the staged cooling

system. Emissions in the blowdown are not discharged in a receiving surface water but

transformed into waste. Waste will need a way for disposal.

Application limits:

The system will be efficient where strict environmental limits exist with regard to waste water

discharge. The system is an option for new power and chemical plants, but can be a retrofit

option for existing installations.
Annex XI

250 Industrial Cooling Systems

Costs:

The capital investment for this system is higher than for stand-alone wet cooling towers. It is

claimed that the capital investment costs of a wet cooling tower with this system are

significantly lower than for an air-cooled system of the same capacity. Operating costs with

respect to power requirements can be lower due to the use of waste heat of the main condenser.

The operating costs of the staged cooling system and the costs of the disposal of solids are to be

assessed against the environmental costs for conditioning and discharging the wastewater.

Example plant:

Within the European Community no installation is reported to exist. Several applications can be

found in the USA.

Considerations:

In conventional technologies for water reuse (for instance brine concentrators) the high

temperature brines that are generated are extremely corrosive, leading to exotic materials and

continuing maintenance issues. In the system described, the low pressure (ca. 1.5 bar) and low

temperatures (less than 32°C) operation of the DO allows the use of HDPE PVC and other noncorrosive materials in areas where corrosion may be of concern. Further experiences are that the

crystallizer is smaller than in conventional systems. Both lead to lower maintenance.

Operation is simple and does not require specialised training. No additional biological treatment

is required.

It needs local consideration if the environmental costs of no discharge to the surface water

outweigh the environmental costs of waste disposal.

XI.2.3 Spray ponds

[tm154, Besselink et al, 1999]

Description:

In the past spray ponds have been used to cool down the cooling water and some may still be

operated in Europe. Currently research is carried out on the applicability of spray ponds for

industrial use to reduce the thermal discharge and to save water. A feasibility study looks into

the use of a spray pond and into the energy saving compared to a cooling tower with a capacity

of 18-21 MWth. A model has been developed that calculates the cooling efficiency of a spray

pond depending on weather conditions, on the dimensions of the spray nozzles and on the

characteristics of the spray pond (surface area, water quality). With the model it should then be

possible for any specific local conditions to design the required spray pond.

All or part of the cooling water flow is led to a pond through spraying nozzles. The spraying

enhances the cooling and theoretically the cooling efficiency of a spray pond is about 36 times

higher than that of cooling pond. Spray ponds cool by evaporation, conduction and convection.

Evaporation is most important at high air temperatures, but conduction and convection are more

important under cold weather conditions. The capacity depends on surface, weather conditions

(wind speed), spray nozzles and spraying characteristics. The heat dissipation of an effective

spray pond can amount to 722 J/m2

K.

Reduction:

Results for the studied cooling system show a potential energy saving compared to the energy

use of a cooling tower that can amount to approximately 6.5 kWe per MWth of cooling. This is

equivalent with a decrease of CO2-emissions of about 38 tons per MWth per year.

Cross-media effect:

By spraying the water aerosols occur. They play an important role in the spreading of biological

contamination. Therefore, and particularly in summer, operating a spray pond needs an adequate

water treatment programme.
Annex XI

Industrial Cooling Systems 251

Application:

Lack of sufficient space on-site will limit the options for a spray pond depending on the required

capacity of the cooling system. For large systems it may not be an option for the entire but for

part of the cooling water need and can reduce the water intake. In the US several conventional

power plants (up to 500 MWe) use spray ponds, nuclear power stations use them for emergency

cooling.

Costs:

Spray pond investments are slightly advantageous to cooling tower investments if it includes the

costs for power supply and also considering the costs for purchase of land. The difference is

larger in case purchase of land is not taken into consideration, but this of course is dependent on

the price of land.

Table XI.1: Investment and energy costs per MWth for spray pond and cooling tower

[tm154, Besselink et al, 1999]

Costs Spray pond Cooling tower

Investment (‘000 EUR/MWth) 39 (25) 48

Spraying and fan energy

(kWe/MWth) 4 11

Designability reliable Reliable

Notes:

Capacity 18-21 MWth

Cooling from 32°C to 24°C

Reference plant:

Dow Europe, Terneuzen (NL).

Considerations:

Although based on an existing technique the current modifications are still in a stage of

research. Application could be particularly interesting in those circumstances where restrictions

of heat discharge may lead to restrictions in production capacity, which occurs during summer

months with power plants. Also, a spray pond may be considered with the expected increase in

restrictions on the use of groundwater.

XI.2.4 Cold storage

[Comment-1, Belgium]

Description:

A special application for smaller industrial use is the underground storage of water to cool it

down. Here groundwater, warmed up after use, is stored at an adjacent location underground for

an extensive period of time, where it cools off. It is also possible to cool the water above

ground, using air coolers in winter, for instance, after which the water is stored underground and

used (in summer). This application is mainly used when there is need for cooling at a level of

about 6 - 9o C.

Reduction:

Reduction of energy and operating costs reported to be 40- to 80% less compared to using small

cooling towers.

Cross-media effect:

unknown
Annex XI

252 Industrial Cooling Systems

Application limits:

Application becomes interesting above a minimum of 150 kW and as addition to cooling with

cooling towers in case of industrial application. Cooling of several MW has already been

realised. As yet, use is limited. Examples are utility construction and greenhouse horticulture.

Costs:

Not indicated

Reference plant:

Not available

Considerations:

Technique is still in stage of development. Full industrial application was not mentioned.
Annex XI

Industrial Cooling Systems 253

XI.3 Reduction of emissions through optimized cooling water

treatment

In the introduction to the paragraph on cooling water savings a number of treatments were listed

that can be applied to prepare effluent to be re-used as make-up for recirculating cooling

systems. The same techniques could be applied to water from a natural source to optimise the

chemistry of the water at the same time minimising the need for an extensive water treatment

programme. As presented before the application is strongly related to the chemistry of the water

and the demand of the cooling system.

XI.3.1 Side-stream-biofiltration in an open recirculating cooling water

system

[tm 146,Savelkoul, 1999]

Description:

For many reasons it is economically interesting to operate an open recirculating cooling system

on a minimum blowdown level. This however results in increased biological activity in the

cooling water, which often is treated by applying biocides.

Amongst other factors, biological activity and growth is primarily dependent on the availability

of nutrients. Irrespective of the cooling system, water circulation or climate, biological activity

will not survive under conditions of nutrient shortage. Every treatment therefore should aim at a

reduction of the biological growth by removal of dissolved nutrients from the cooling water

circuit. For an effective treatment the so-called dead volume (or the circuit volume) of the

cooling water system is important. It is in fact, this dead volume that is treated in a filter and

subsequently chlorinated at low levels and frequency.

This can be done by applying a continuous sand filter on a side-stream breaking down the

dissolved nutrients and at the same time filtering suspended micro-organisms and other

dissolved solids. Consequently less chlorine is needed and higher cycles of concentration are

possible.

This technique can be improved by creating an active biology in the sandfilter with a high

concentration of micro-organisms, which is called side-stream biofiltration. To maintain an

active biology the sand filters are by-passed during periods of high concentration levels of

biocide (chlorine) in the cooling water circuit, because this high level would break down the

biology in the sand filter as well as their effect in the cooling water. As soon as the chlorinelevel is reduced the cooling water is led through the sandfilter again. In effect it means that the

cooling water only needs to pass the filter a limited number of times down to once or twice a

day only.

The application has been applied to an open recirculating system with a capacity of 152 MWth, a

water circulation of 11000 m3

/h and a circuit volume of 3500 m3. The case system applied two

filters of 5 m2

 of filtersurface and about 10 kg VSS/m3 of filter, 4 m filterbed height, filtering

sand (of 1.4-2.0 mm and 0.8 – 1.25 mm). The design model was based on the first order reaction

mechanism of nutrients removal with a reaction rate constant of 6.0 /houre (= sand 0.8 – 1.25

mm, so 3.800 m2

/m3) and 4.5 /houre (= for sand of 1.4 – 2.0 mm, so 2.250 m2/m3) in the sand

filter. This significantly decreased the growth rate of organisms at the filter effluent side

compared with the circulating water.

Reduction:

The resulting reductions depended on the optimized combination of blowdown, biocide use and

application of side-stream biofiltration. For example, the functioning of the sandfilter depends

on the side-streamflow, washwaterflow, sandcirculation, filter resistance and water temperature.

The efficiency of the filter is reduced by higher through flow (higher hydraulic pressure), which

is similar to a lower contact period, and by using larger sandparticles, which means a lower

specific surface.
Annex XI

254 Industrial Cooling Systems

Results show an increased concentration factor (5.0 to 5.5) with a simultaneous reduction of Cldosage frequency of less than once every two days (0.42/day). This means a reduced blowdown

of 12 %, reduced water intake of 2.4% and reduced additive use of 12% or seven times less

chlorine for the same effect.

Due to the lower dosage of chlorine, the level of corrosive elements (expressed as the

summation chlorine and sulphate) remain within the required range for that system (max. 86

ppm Cl-

 and 77 ppm HSO4

-

 respectively). This explains the achieved 12 % reduced blowdown

due to the biofilter, which is based on the same corrosiveness of the water.

Cross-media effect:

If the starting point for application of side-stream biofiltration is to reduce the amount of

chlorination, all other results mentioned are regarded as positive cross-media effects. Data on

extra energy requirements for pumps were not mentioned.

Separate pumping equipment is avoided and the performance of the biofilter is maximised by

sending the hot effluent cooling water directly to the side-stream filter and this filter effluent is

send directly to the cooling tower basin. A slip stream of hot filter effluent is then de facto

bypassing the cooling tower fill and will warm up the cooling tower basin with average 0.15 K,

which is equivalent with an increased indirect energy usage of 0.5 kWth/MWth cooling. A separate

pump to transport the biofilter effluent back to the cooling tower header where the static

pressure is 14 mwg, can avoid this drawback. On average, 11⁄2 m3

/hr per MWth cooling have to

be pumped, which is equivalent with a direct energy consumption of 0.1 kWe/MWth cooling or 0.25

kWth/MWth cooling (with 40% efficiency of power plant). This is also quite limited in comparison

with the standard (in) direct energy use for cooling tower processes.

The saved energy based on the decreased sodium hypochlorite consumption is also very limited

(a 1 litre 15 % solution per day per MWth cooling is equivalent with the oxidant “production” of 1

kWth for one hour per day per MWth cooling or 0.04 kWth/MWth cooling). The saved energy, based on

the reduced make-up due to the 12 % reduced blowdown of 0.04 m3

/hr per MWth, is also

negligible even if the transportation energy of the reduced make-up is included.

In general, the net energy balance of all these energy differences due to the biofilter are

negligible and in the magnitude of 1% of the standard direct energy consumption for cooling

tower processes of 20 kWth/MWth cooling (See Table 3.2). These small numbers are expected since

only 1 to 2 % of the cooling water circulation flow is in general necessary for the biofilter to

avoid microfouling in the heat exchangers.

Application limits:

With a corresponding upgrading of the filter capacity there does not seem to be an application

limit. It can be applied to existing cooling systems.

Costs:

Costs depend on the scale of application and the results obtained as expressed in reduced

operating costs. The operational costs of chlorination were reduced with 85 %. For the given

example the expected investment return period was estimated to be three to four years.

Reference plant:

DSM, Geleen (Netherlands) and Dow Benelux, Terneuzen (Netherlands)

Considerations:

The side-stream filter was designed on a purposely chosen low efficiency removal of nutrients,

by choosing a high linear velocity of 25 m/hour, instead of the standard of 10 - 14 m/hour,

which is normally applied for suspended matter removal only. High removal rates of nutrients

will be achieved with a counter-current sand bed filter, if the influent is at least above 200 RLU

as ATP (= colony counts units expressed as decrease of adenosine tri phosphate) and preferably

600 RLU as a criteria to start the shock dosage of sodium hypochlorite. This occurs together

with the high linear velocities to prevent anaerobic conditions.
Annex XI

Industrial Cooling Systems 255

The chlorination criteria of the cooling tower system, without a side-stream biofilter, was on

halving the micro organisms expressed as 500 to 250 RLU as ATP by means of a first order

reaction (= 0.5 [1/hour]), which was achieved with 1 litre sodiumhypochlorite solution of 15%

per MWth for every shockdosage.

The organic load on the filterbed becomes 10 kg organics per m3

 filter bed, and together with a

desired contact time of 10 minutes it becomes necessary to design a sand height of 4 m, which

influences the capital cost lesser than enlarging the filter diameter. The filter surface was for

every 15 MWth 1 m2

, which results in 1.7 m3/houre filtration flow per MWth, and this is almost

equal to the evaporation rate of 1.3 m3

/houre per MWth together with a slightly decreased

blowdown of 0.3 m3

/hour due to the installed filter. In fact every drop of circulating cooling

water passes the side-stream filter 1.7 times a day. By doing so, the hydraulic half time of the

whole cooling system decreases from 40 to 7 hours. At the same time the blowdownrate

decreased with 12% as well as the conditioning chemicals and the frequency of chlorination

decreased drastically from 3 times a day to only once every 2.4 days. The corrosiveness of the

circulating cooling water, expressed as the summation of chlorine and sulphate, remains the

same.

Based on this model, the outcome for most existing cooling towers systems in Europe will be in

the magnitude of 1 - 2 times per day that the known dead water volume as warm water will pass

over the filter. This will occur together with a limited one hourly shock dosage of an oxidative

biocide only twice a week. Expressed in units: 1 m2

 filter surface only per 15 MWth is expected

to be sufficient in most cases together with a filter height of 4 m to create a residence time of

several minutes. For a common situation, the reduction in water and chemical usage delivers a

pay back time of 3 - 4 years for the capital cost of the side-stream filter. It is expected that other

filtermedia then sand like basalt can result in even smaller filters per MWth.
Annex XI

256 Industrial Cooling Systems

XI.3.2 Physical methods

Cleaning devices for water cooled systems can be on-line (or continuous) cleaning such as the

use of sponge rubber balls or brushes, or off-line cleaning by using for instance high-pressure

water jets and shooting of so-called 'pigs' through the condenser tubes. The better the cleaning

the less need there is for application of cooling water chemicals, not only because fouling of the

tube surface is removed mechanically, but also because the applied additives will be more

effective as they can reach the surface more easily. It has been stated that mechanical cleaning

could be considered as a prerequisite for the use of a macrofouling control programme.

Cleaning of dry air cooling systems is restricted to the fin side of the heat transfer surface. For

maintenance of the heat transfer (also avoiding indirect emissions) and longevity of the coils

cleaning should be implemented.

A number of physical methods to combat macrofouling and the experiences in industry are

listed Table XI.2. [005, Van Donk and Jenner, 1996]

• Reducing entrainment of (bio)fouling in the system. Intake structures should be designed in

such a way that entrainment of fish, debris, organic and inorganic material, including

suspended matter is kept to a minimum. Additionally, side-stream filtration can be an option

in open recirculating systems.

• Maintaining velocities at a level high enough to avoid fixation of organic organisms

(velocity higher than 2 m/s). Too high velocities, however, may introduce a risk of

corrosion. Critical water velocities depend very much on the type of material used.

• Sudden temperature increase by raising the temperature of the cooling water beyond 40 °C

for some dozens of minutes; this techniques eliminates the fixed organisms (mussels), but

nevertheless requires an appropriate design of the cooling system (recirculation of the

cooling water). Also it limits the cooling capacity of the system and can only be done during

an interruption in the process in case this cannot withstand a rise in temperature.

• Non-toxic coating and paints, which reduce the fixation of the organism, reinforce the

velocity effect and facilitate cleaning.

• The use of sonic technology. The principle underlying the application of sound is that the

vibration created by the energy associated with the transmission of sound will remove

deposits on surfaces, by 'shaking' the deposit free.

• Osmotic shock. This physical-chemical based method applies osmotic shock by either freshor seawater systems, by subjecting them to seawater and freshwater, respectively. As a

result, cells of organism may come under the effects of internal pressure, which may result

in death.
257 Table XI.2: Physical techniques to minimize biocide use(derived from [tm005,Van Donk and Jenner, 1996])

Technique Equipment Field experience 

Macrofouling: drumscreens, band sieves, trashracks,mussel sieves

Yes, power station Both 

syste

Microfouling: rotating drum and sand filters Yes, chemical industry Not 

Filtration/waterpretreatment

Microfouling: continuous backwashed microfilters(50-100μm)

Yes, desalination plant For w

Side-stream filtration Rapid sand filters

Continuous backwashed filters

Yes, chemical industryYes, glass industry

Only All bFilte

Sponge rubber balls Yes, power stations Larg

Not 

On-line cleaning

Brush and cage system Limited, chemical industry

and power plant

Once

Off-line cleaning Yes, power stations and

industry

Need

Macrofouling: 38-40oC Yes, seawater and fresh

water systems

Heat treatment

Microfouling: 70-80oC ?

OptidesigRepl

Toxic coatings Varying Base

restr

Coatings and paints

Non-toxic coating Power stations in U.S. For n

Silic

U.V. light Small scale tests Prev

biofosyste

Sonic technology No, only test results High Electric water treatment High frequency transformer No, only test results Test Osmotic shock Yes, once-through system

using sea-water

MateFreswate
Annex XI

258 Industrial Cooling Systems

XI.3.3 Optimization of biocide use

XI.3.3.1 Monitoring

[tm005, Van Donk and Jenner, 1996]

For monitoring of microfouling the plate count technique and the ATP measurement are used.

For monitoring of macrofouling use is made of exposure panels or glass windows. Scaling and

corrosion indirectly influence biocide use and therefore monitoring of the occurrence of these

effects can also be important to measure the occurrence of biofouling. Examples of monitoring

techniques given in the reference are the KEMA Biofouling Monitor and underwater video

equipped robotic devices are used to detect macrofouling, and effects of biocide treatment. For

more accurate measurements particularly for microfouling and biocide treatment techniques are

applied using features as valve movement and as light emission of microorganisms as a result of

their metabolic process. Of both techniques an example is given to illustrate the principle, but

many more are on the market.

XI.3.3.1.1 Monitoring of macrofouling

[tm157, Jenner e.a., 1998]

To be able to target the dosing of biocides to combat macrofouling in once-through systems a

so-called biofouling monitoring system has been developed. The KEMA Biofouling Monitor

consists of a closed cylindrical container, made of PVC, with a vertical water flow from top to

bottom. It can be used to monitor all macrofouling organisms in freshwater, brackish water and

seawater systems. It allows direct observation, weekly and monthly counts of bivalve spat

settlement. Spat are the metamorphosed larvae of the last larval stages of bivalves (so called

"pediveligers"). In order to obtain adequate information on macrofouling development in the

cooling system it is recommended to place a biofouling monitor at the intake, before the dosing

point, and another one at a critical spot in the cooling system, after the dosing point.

When installed parallel to a cooling water conduit as a bypass, the monitor is an effective tool to

detect all possible macrofouling in the cooling system. The water velocity in the monitor is

much lower than in the cooling system. This provides an optimal environment for settlement of

bivalve-spat, and allows easy inspection of the point of time of settlement, growth, and

effectiveness of control measures. Based on the information from the monitor biocide

application can be limited to the periods where it is really necessary. Further research on the

behaviour of the organisms can further target the dosage concentration of the biocide.

Other techniques are applied, such as those using immersed plates near the inlet channels. They

give the operator of the plant an indication of the periods when to avoid chlorination.

XI.3.3.1.2 Traced biocides for biocide and microbiological activity

[tm096, McCoy e.a, 1995]

The tracer diagnostic system consists of an analyser, a data collection system, analytical

software and a luminescent reagent. The analyser measures the light output of the microorganisms. Within minutes, the test can determine biocide concentration and biological activity

in the cooling water. The method is based on a bioluminescence bioassay of biocide active

ingredient. It is aimed at optimising the use of non-oxidising biocides in recirculating cooling

systems by measuring systems’ consumption.
Annex XI

Industrial Cooling Systems 259

XI.3.3.2 Biocide dosage

XI.3.3.2.1 Different conditioning regimes to obtain optimum annual total oxidant use

in once-through systems against macro- and microfouling

To avoid macro- as well as microfouling different conditioning regimes can be applied in oncethrough cooling water systems. This can be low-level chlorination executed as continuous,

semi-continuous as well as discontinuous, also called shock chlorination, of twice an half hour a

day, targeted chlorination in only a part of the heat exchanger or a part of the cooling system

itself,

pulse-chlorination and alternating pulse-chlorination. The different regimes objectives are to

achieve and remain a high energy efficiency by operating with clean heat exchangers through

the year and at the same time minimise adverse environmental effects.

The environmental assessment of chlorination can be split into two main categories: oxidants

and non-oxidants. They differ in their ecotoxicological risk expressed as component life time,

bioaccumulation and toxicity towards aquatic organisms. The non-oxidants like chlorinated

hydrocarbons are persistent and some components will accumulate in the fats of aquatic

organisms as well as show a chronicle mutagenic and carcinogenic toxicity. The oxidants react

quite rapidly with reductants and will only be available for antifouling after overstoichiometric

dosage. Only under those circumstances the result of the dosage is acute toxic at even low

concentrations, but without bioaccumulation of the free oxidants.

Acute toxicity is what is needed in the cooling system including the heat exchangers to prevent

settlement and to keep the heat exchangers clean, but this toxicity is unwanted in the cooling

water discharge.

Since even a continuous low-level chlorination regime has a significant low PEC/PNEC ratio,

the main environmental issue is to reduce the formation of halogenated hydrocarbons, also

called chlorinated by-products, which are due to the inefficiently used oxidant mass. However,

these components are not measured easily on a regular or even continuous basis and also don’t

have any potential acute toxicity. Therefore, performance of a conditioning regime is monitored

in terms of free oxidants, which is also more applicable on a continuous control basis. All

oxidative conditioning regimes have in common that continuous measurement of free oxidants

is favoured regarding the necessary process control. At the same time natural waters have a

minimum detection limit and threshold around 0.1 mg/l (± 0,05 mg/l) depending on the applied

analytical technique and in relation to components present in the natural cooling water that have

no direct contribution to the effect of the conditioning regime itself. Since chlorine just like

other oxidants is non-selective and non-specific and reacts practically with all reducible

components present in (natural) waters and together with the analytical threshold, this can be an

explanation why reported successful conditioning regimes use at least 0.2 mg/l free oxidants

before the condensers.

Production of the halogenated hydrocarbons is an almost linear function of the dosed oxidant

mass irrespective of the conditioning regime.

Comparing discontinuous and continuous low level conditioning regimes it may look as if a

discontinuous conditioning regime (with higher oxidant mass) results in a higher measurable

concentration of halogenated by-products. If the mass balance is corrected for those periods that

the dosage was stopped, then the yearly-emitted mass of a discontinuous regime can be even

lower than that of a continuous low-level regime. In fact, not the applied conditioning regime,

but the water quality does affect the minimum amount of oxidant needed. The higher initial free

oxidant concentration, needed in the case of discontinuous chlorination, is necessary to

compensate for the lower contact time to achieve the same treatment results. This does not mean

that the yearly needed oxidant amount is higher with a higher associated quantity of halogenated

by-products in the outlet. Due to mixing in the receiving water rapid decay occurs of any

oxidant conditioning regime, of acute toxicity as well as of formation of halogenated byproducts.
Annex XI

260 Industrial Cooling Systems

The effectiveness of a conditioning regime is a combination of the level of temporary acute

toxicity and the availability and quantity of nutrients in the water and the deprivation of the

filter rate of the filter feeding organisms, such as oysters and mussels (or bivalvia).

It is necessary that the region from the pumppit until the exchangers as well as the stagnant

zones becomes temporarily acute toxic to prevent settlement in the water distribution piping and

the conducts and to keep the heat exchangers clean. The shorter the periods chosen the higher

the temporary acute toxicity must be for the same effect. Or the longer the contact time is, the

lower the acute toxicity needed to achieve the same result.

All oxidative conditioning regimes have in common that they take advantage of the reduced

food intake of filter feeding organisms like mussels and oysters under stress. All oxidative

conditioning regimes have also in common that spat settlement and growth is mitigated by

prevention of opening their shells for prolonged periods of time. If they are forced to close their

shells – which is the natural fugitive capacity – the organisms switch to anaerobic metabolism

and live on their food reserves. Depending on their condition and local water temperature they

can survive such stress periods for many months. Spat as well as youngster however avoids

these circumstances through a second fugitive behaviour by non-settling or detaches its byssus

wires, which explains the potential failures of intermittent regimes in specific areas.

Favourable conditions for spat settlements and growth are present in eutrophicated waters found

at specific coastal areas and some harbours and it is further accelerated by increased water

temperatures within limits. For that reason all conditioning regimes have in common that

chlorination is not necessary when the nutrients are scarce at low water temperatures (12 °C).

However in specific areas its threshold is 10 °C due to the rich nutrient availability even at

relative low temperatures.

All these circumstances determine the necessary free oxidant concentration measured before or

direct behind the heat exchangers with the related chosen time intervals between the intermittent

dosages.

Continuous and discontinuous conditioning regimes show different levels of chlorination. In

most waters, if the prevention of settlement is done with a continuous low-level chlorination

nearby the pumppit, a FO-level before the heat exchangers of 0.3 mg/l has to be maintained. It

results in a level of 0.2 mg/l at the outlet, which can generally be expected in cooling water

circuits with a time span of 15 minutes. However, in nutrient rich waters the biofouling is so

severe that higher inlet and thus higher outlet concentrations become necessary and sporadically

can reach levels of 0.7 mg/l FO at the outlet to preserve its efficacy.

Discontinuous low-level chlorination in case of an ideal plug flow cooling water system will

need higher FO-levels up to 0.5 mg/l before the exchangers to reach the same result and will

automatically result in higher temporary outlet FO-concentrations. Indissoluble, the production

of halogenated hydrocarbons will be higher also during these temporarily elevated oxidant

dosages. Low frequency shock dosages are rarely done, which is based on their low

conditioning efficiency towards bivalvia. These organisms will fully use the offered long

respiration periods to recover. In general, intermittent regimes applied in nutrient rich waters are

only effective if applied as frequently repeated chlorine dosages to minimise the recuperation

ability of bivalvia. If these non-dosage periods are reduced to a quarter of an hour it is called

pulse-chlorination. The organisms will interpret this as a continuous chlorination regime, since

the frequency gives period too short for mussels and oysters to recover after being exposed to

short successive periods of oxidation. Time intervals between the oxidant dosing periods have a

much larger influence on the behaviour of these organisms than the free oxidant concentration,

as long as this concentration is high enough to produce an initial stress effect on bivalvia.

If the whole cooling system is not an ideal plug flow system, than even higher frequency dosage

regimes called alternating pulse chlorination (XI.3.3.2.2) can be successfully applied. It will

make full advantage of the present reductants availability in a part of the cooling water that will

mix just before the outlet system with previously chlorinated cooling water. Essential is that a
Annex XI

Industrial Cooling Systems 261

part of the oxidised cooling water will have a different residence time in the system and will

reach at different times either as previously or future non-chlorinated cooling water, which still

contains reductants. By shortening the dosage periods to an extend of three quarter of the

residence time of the mechanical water system, an understoichiometric oxidant/reductant

mixture is then created in the outlet zone. At the same time an overstoichiometric

oxidant/reductant condition is created between the dosage point and the area where the different

cooling water streams meet.

In summary, the (alternating) pulse conditioning regime reduces the annual use of additives and

is particularly effective against macro-fouling. However, it may produce peak free oxidant

concentrations at the outlet of the cooling system, which do not meet permissible release levels.

XI.3.3.2.2 Pulse alternating chlorination in once-through systems

[tm153, Paping et al, 1999], [tm168, De Potter et al, 1996], [tm169, De Potter et al, 1997], [tm170, De

Potter and Polman, 1999], [tm171, Polman, 2000]

Description:

For an existing once-through system using seawater with flows up to 11 m3/s, a number of

measures was developed and applied as part of an integrated system (“total care system”). The

system comprises of 200 heat exchangers (mostly copper/nickel 90/10 and coated carbon steel)

connected by 4 km of main conduits. Failure of the system as a result of damaged tubes was

predominantly due to erosion corrosion failure (65%). The measures reduced the number of

leakage incidents and at the same time the amount of biocide applied could be reduced. In the

underlying situation hypochlorite had been dosed as antifouling treatment. As a result of long

term experience it was considered to be the most adequate biocide for this system and for the

available cooling water quality. Thus, no alternative biocide was being considered as a solution.

Optimisation was achieved by applying different levels of biocide treatment. The environmental

impact of different regimes was evaluated by measuring and comparing the amount of

chlorination by-products (predominantly bromoform) and the potential toxicity formed. The

effectiveness was evaluated by looking at:

• the incidence of leakage of heat exchanger tubes caused by mussels,

• the amount of biological growth (macrofouling attachment)

• the behaviour of the valve movement of oysters.

Accordingly, the conditioning regimes were improved. It is important to realize that in this case

use is made of knowledge about the local biotope. This is essential to reach the required

accuracy of the treatment and the associated results.

Reduction:

The results of the optimisation show that an initial increase of hypochlorite (dosage A) did not

reduce the leakage incidents in the first place, but was able to remove almost completely the

macrofouling from the system as was observed from the mussel monitors. Once the cooling

system was clean, reduced levels of hypochlorite (dosage B and C) were applied in the

following years removing the macrofouling completely as well as reducing the number of

leakages down to zero. The methodology applied is able to maintain the required level of FO at

the right place. It is based on knowledge of the life cycle of macrofouling species, of

microfouling areas in the system and of varying residence times and water velocities in different

parts of the cooling system.

By maintaining low oxidant concentrations during longer periods the settlement and growth of

bivalves in the distribution conduits can be prevented. Short term alternating dosage near the

heat exchangers leads to temporarily high concentrations and is able to control microfouling.

Over-stoichiometric dosage is applied in the influent at areas where water velocity rapidly

decreases forming stagnant zones. The results are summarised in the following table.
Annex XI

262 Industrial Cooling Systems

Table XI.3: Effect of applying an optimized dosage regime on the number of leakage caused by

mussels

[tm153, Paping et al, 1999]

Number of leakages caused by

mussels Period Regime

Unit 1 Unit 2

Hypochlorite

(metric

tonne/year)

Year 1 A 28 4 1222

Year 2 A 28 12 2095

Year 3 A+B 32 10 2817

Year 4 B 16 1 2480

Year 5 C 0 2 1994

Year 6 C 0 0 2013

Year 7 C + freq. 1 0 1805

Year 8 C + freq. 0 0 1330

C + freq. = regime C with higher frequency (i.e. 5 minutes dsoage per 20 minutes interval)

An even more targeted dosage regime is the pulse alternating chlorination that takes account of

the variation in residence times in different parts of the process (Figure X.1.). At different times

and at different points the required levels of chlorine are dosed following the flow patterns of

the cooling water stream in the different process parts. At the end of the process and before

discharge of the cooling water stream, dilution occurs by the mixing of the different process

stream. Where only one of the stream is chlorinated and the other is not, the FO is further

reduced and emission levels of < 0.1 mg/l are achievable.

Cross-media effect:

Considerable lower frequency of heat exchanger failure reduced necessary maintenance and

consequently non-production periods. Cleaner heat exchanger increased cooling and reduced

emissions from production process.

Application limits:

The conditioning regime cannot be applied to once-through systems without varying cooling

water residence times.

The optimisation of dosage intervals the system needs careful monitoring of the free oxidant

levels in the cooling system and of required stress periods of the bivalves.

Costs:

Research costs amounted to EUR 1 million during first 5 years. The first dosage installation was

EUR 0.2 million, and further modifications again EUR were 0.2 million. The pay back period of

the dosage installation was in the order of one year and based on the following costs:

- reduced cost level for yearly maintenance and use of sodiumhypochlorite,

- increased costs for yearly preventive and predictive maintenance, and

- costs for analyses.

Costs did not include research cost since these costs were spend to obtain the fundamental

knowledge of pulse alternating chlorination in once-through systems.

Reference plant:

Dow Europe, Terneuzen (NL)
Industrial Cooling Systems 

Plant 2

Plant 1

piping main distribut. piping; FO “stable”

0 minutes 6 to 8 minutes 13 to 

X

Y1

2 mg/l FO

3 mg/l FO

  

O.3 or 1 mg/l FO

1

2

Y2

Pulse alternating chlorination; X, or Y

Mixingzone

Mixing of flow

Figure XI.1: Optimised hypochlorite dosage (pulse-alternating chlorination) taking account of fouling and cooling Derived from [153, Paping et al, 1999]
Annex XI

264 Industrial Cooling Systems

XI.3.4 Alternative cooling water treatments

In this document the alternative cooling water treatment techniques consist of non-chemical

methods as well as alternative chemicals or combinations of chemicals. It has already been

illustrated how proper monitoring can lead to a more effective dosage regime reducing the

amount of additive required and simultaneously maintaining a low incidence of system’s failure.

XI.3.4.1 Ozone

[tm032, Zimmermann and Hamers, 1996], [039, Strittmatter et.al., 1996], [tm084Rice and Wilkes,

1992], [tm096, Mc Coy et al. 1990], [131, Dziobek, 1998] and [tm156, Schmittecker, 1999]

Description:

Extensive experience has been obtained with ozone treatment of drinking water. In Germany

and the United States varying experiences have been obtained with ozone applied to

recirculating cooling system. Ozone is a strong oxidant, more so than chlorine dioxide, which in

turn is a stronger oxidant than sodium hypochlorite. Being very reactive ozone reacts with

practically all organic material present in the cooling water and the residual effect is low if not

absent. Also, ozone has considerable potential to destroy other cooling water additives, e.g.

some corrosion inhibitors.

The reactivity of ozone depends on the pH of the water. When ozone is added to cooling water

with a pH value higher than 8 - which can be often encountered in recirculating cooling system -

it decomposes to form free hydroxyl radicals, which are stronger oxidising agents than

molecular chlorine, but of microsecond half-life. In case that bromide ions are present in natural

surface waters they will react with ozone to produce hypobromous acid, which is what is

actually measured as residual ozone rather than ozone itself. Another important factor is the

hardness of the water and it has been recommended to keep this between 100 and 400 ppm

CaCO3 and the chlorides below 200 ppm Cl-

.

Reduction:

The reduction of microfouling, measured in microbiological activity, is variable and can be

compared with a chlorine/bromine treatment. Reduction of up to 90% of the initial activity was

measured resulting in concentrations of 20-50 colonies per ml. [tm156, Schmittecker, 1999].

Not only the amount, but also the character of the microbiology showed a change as a result of

ozone treatment. A reduced number of colonies forming species could be detected in

comparison with no treatment.

The concentration of ozone present in a system showed no direct influence on corrosion or

scaling rate, although it was also concluded that mild steel and yellow metals corrode easily

when ozone concentrations are too high (1.0 ppm). With the appropriate ozone-concentration

those materials will initially corrode and form a corrosion layer. This layer will protect against

increased corrosion, particularly pit corrosion. In the example, steel (C 1010) corrosion was

reduced with 50% to 0.05 mm/year and brass (CuZn28Sn1) corrosion was reduced to less than

0.004 mm/yr.

Reduction of about 50% of AOX- and COD-levels in the effluent has been reported on different

occasions due to ozone treatment. The resulting levels were less than 0.01 mg/l (AOX) and

10 mg/l (COD) respectively. The COD level was reached in the presence of a dosed hardness

stabilizer.

Cross-media effect:

Although the formation energy of ozone is high, the application of ozone is often referred to as

more environmentally acceptable than hypochlorite, since it leads to less formation of

trihalomethanes (THMs) and extractable organic halogens (EOX). Ozonation may lead to the

formation of by-products e.g. bromate and bromohydrins, but compared to chlorination byproduct formation, relatively little attention has been paid to by-product formation resulting

from ozonation.
Annex XI

Industrial Cooling Systems 265

As far as was reported, ozone emission into the air has not been observed from any part of the

cooling systems.

Applicability:

Ozone is predominantly applied in the chemical and petrochemical industry and in refineries

and to a limited extent in power industry, but from recent experiences wider application can be

expected for smaller industries. The advantages are:

• Efficiency,

• low concentration of by-products,

• low stability of ozone with low or no ozone in the discharge as a result,

• reduction of COD and AOX.

In the U.S, it was concluded that ozone is not a “total” package for cooling water treatment

covering all purposes, but does represent an alternative for a limited number of users.

Furthermore, it is only useful as a stand-alone treatment in cooling water systems, which require

no supplemental corrosion or scale control. Its reactivity limits the application of other biocides

that would be destroyed immediately and if any biofouling occurrs beyond the radius of ozone

activity, it may not be dealt with.

Ozone is preferably to be used in very clean recirculating cooling system, and it is commented

that its high reactivity makes ozone unsuitable for application in once-through system or long

line systems. The speed with which ozone can disappear from a system is illustrated by the

example, in which after the first application of ozone in a contaminated system the ozone

concentration was reported to decrease below the detection limit within 50 meters from the

point of dosage. For conditioning of a cooling tower of power plants a minimum concentration

of 50 μg/l in the cooling basin is currently suggested, but positive results have been reported

also with lower levels. It is considered best practice to apply ozone continuously at low level.

Costs:

Ozone production requires a considerable amount of energy and is relatively expensive due to

the fact that the efficiency of the ozone generators is very low (100 g to 150 g O3/1000 g O2,

10 kWh/ kg O3). Observations on costs may however differ as they depend on the alternative

treatments available. For instance, observations were made that the ozone treatment can be

compared to that of treatment with chlorine gas and that other chlorine or bromine based

treatments could contain additional costs. Care must be taken when interpreting costs, where

investment costs of equipment may or may not have been included and reference is made only

to operating cost.

Reference plants:

Hoechst (Germany), power station Seraing (Belgium), power station EZH-Rotterdam Capelle

(Holland) [see references]

Considerations:

A minimum concentration in the aquatic-environment of the cooling system is required to

obtain the required biocidal effect of ozone. Recent work shows that to overcome the

contamination already present, the initial dose may have to be between 0.1 and 0.3 mg/l and

depending on the systems’ environment it may take months before any residual ozone can be

measured in the residual effluent [tm131, Dziobek, 1998]. It is claimed that in 60% of the

applications ozone treatment can be reduced to 50% within 9 to 12 months with the surfaces

being clean. This type of treatment resulted in a residual ozone concentration in the cooling

tower basin of 0.05 mg/l. It was further reported that cleaner cooling tower fill led to an

increased number of cycles and to a corresponding 70% decrease in water losses.

The point of dosage is very important in order to maintain the required ozone concentration in

the right area of the cooling system. To avoid the breakdown of ozone, sensitive inhibitors and

other cooling water treatment chemicals, side-stream treatment of the make-up was suggested.

Ozone could also be applied in the cooling tower itself [tm131, Dziobek, 1998].
Annex XI

266 Industrial Cooling Systems

Ozone can be generated on-site by subjecting dry air or oxygen to an electric discharge. After

this the product needs to be absorbed in the cooling water. According to the American Congress

of Industrial Hygienists, 0.1 mg/l is the recommended maximum concentration for continuous

exposure [tm059, Swinnen, 1995]. Ozone is very volatile compared with other oxidising

biocides. [tm096, Mc Coy et al. 1990] investigated the volatility of various biocides in cooling

towers. From this study the following order of volatility appeared: ozone> chlorine dioxide >

chloramine > hypochlorous acid > hypobromous acid, with ozone being about 167000 times

more volatile than hypobromous acid at 20°C.

XI.3.4.2 UV treatment

Description:

UV treatment of water to be used in recirculating cooling systems needs clear water for a start to

give good transmission of UV irradiation and prefiltering of water might be necessary.

Reduction:

Particularly in summertime the treatment effected in reducing amoebe formation in the

blowdown before discharge in the river.

Cross-media effect:

Energy costs were not reported.

Application limits:

It is mentioned that in sunny locations algae growth has been observed if no anti-algae was

applied due to lack of residual effectiveness. To overcome this an anti algae will have to be

added or the tower basin will have to be kept clean and free from sludge to prevent microorganisms to grow in the sludge. UV-lamps also need frequent cleaning.

Costs:

Not reported in full scale application.

Reference plant:

Hydro Power Station, Ontario, Canada, EDF Nuclear power Station, Poitiers, France (both

experimental, 1999)

Considerations:

With the tendency to increase the re-use of water it is uncertain whether the required clearness

of the water can still be reached in the future without considerable and expensive treatment..

XI.3.4.3 Catalytic hydrogen peroxide treatment

[comment, D]

Description:

The catalytic hydrogen peroxide treatment is a technique for decontaminating cooling water

from the presence of microorganisms. The technology aims at meeting low limit values for

levels of bacteria in the water. The system acts to prevent the formation of biofilms and algae

and this in turn prevents the spread of bacteria, including legionellae colonies, through the

systems. Hydrogen peroxide is used as an oxidizing agent, which in the instance of the metal

catalyst generates a significant amount of •OH-radicals. These radicals have a very strong

oxydizing effect, which is reported to be larger than that of e.g. ozone or chlorine.

The technology is reported to have a broad spectrum, which means that the radicals are effective

against a large range of microorganisms including legionellae. Genetic resistance has not been

reported and therefore shock dosing is not required. A relatively small concentration of H2O2 in

the water is maintained and keeps the water practically bacteria free.
Annex XI

Industrial Cooling Systems 267

Reduction:

Catalytic hydrogen peroxide treatment will reduce the levels of AOX and COD. It is reported

that it does not produce any residual hazardous chemicals in the discharge flow. It extends the

operating period as it reduces the frequency of maintenance where the whole installation

remains free from biofilms, algae and bacteriae.

Cross-media effect:

The catalyst treatment does not need any additional energy applied. The typical concentration of

hydrogen peroxide (0.5 to 2 ppm) has shown no influence on the corrosion or scaling rate.

Hydrogen peroxide acts as a corrosion inhibitor as well.

Application limits:

For each application the best option for installing the catalyst (in the form of light knitted wire

mesh, normally on a stainless steel or PE mount) has to be selected individually. The catalyst

may be placed on the bottom of the water basin or in the water distribution if an open system is

used. A concentration of 30% of hydrogen peroxide solution was determined as very effective

with respect to storage and application. It has been applied in cell type cooling towers. Limits

with respect to the size of the cooling tower have not been reported. The data point at

applications in small to medium capacity towers, but application for large capacity installations

is under development.

Costs:

This treatment requires the investment for a metal catalyst. It is reported that with usual

depreciation time for the catalyst of 4 to 5 years, the operation costs, including depreciation for

the catalyst and the dosing system, are substantially lower than for either the application of a

biocide (including hypochlorite) or the ozone treatment, when applied to the same cooling tower

capacity.

Reference plant:

Ausimont Deutschland GmbH, Bitterfeld (D).

Considerations:

From experience two alternatives can be chosen as dosage point for the hydrogen peroxide:

H2O2 can be applied in the common pump intake chamber of the cooling tower or direct into the

rising pipes of each cell.

XI.3.4.4 Chlorine dioxide

Description:

Chlorine dioxide (ClO2) has been considered as an alternative to hypochlorite (HOCl) for

seawater conditions and as a freshwater biocide due to its effectiveness as a disinfectant and to

its strong reduction in the formation of organohalogenated by-products in the effluent. It has

been reported as an effective and economical application in cooling water systems for control of

micro-organisms at relatively low dosages. It may be used over a broad pH range and is

effective over the entire spectrum of micro-organisms. It has been reported to be effective

particularly in systems with the following contaminants: ammonia and ammonia salts, alkanes,

alkenes and alkynes, alcohols, primary amines, glycols, ethers, unsaturated aromatics, most

inorganic acids, organic acids, diols, saturated aliphatics.

The conditions under which the application of chlorine dioxide is considered attractive are:

1. process contamination

2. alkaline pH systems

3. effluent chlorine discharge limitations

4. elimination of gaseous chlorine from the site.

The last advantage may be at question, where chlorine dioxide has been reported as difficult to

transport and must be therefore produced on-site [tm059, Swinnen, 1995]
Annex XI

268 Industrial Cooling Systems

Chlorine dioxide does not react with water and is highly soluble in water. It has been found that

blowing air through aqueous solutions of chlorine dioxide can cause chlorine dioxide to be

expelled from the solution. Therefore solutions treated with chlorine dioxide should not be fed

into areas of strong aeration such as splash tanks prior to going over a cooling tower. Aqueous

solutions of chlorine dioxide are subject to photodecomposition upon long exposure to

ultraviolet light. As with chlorination, treatment with chlorine dioxide could be scheduled

during the hours of darkness to achieve greatest effectiveness.

On the treatment it was further observed that after initial high feeds of ClO2, shortly after the

start-up, total plate count should normally decrease. After this initial period, ClO2 begins to

clean up the biomass accumulations of slime and entrapped debris. As the slime masses are

attacked, micro-organisms break loose into the recirculating water. Consequently, the total plate

counts, calcium and turbidity readings will increase for a period of time and then subside to

normal levels. Moreover, foaming may also occur during this period.

Reduction:

Experiments in Italy were confirmed by observations in Spain for a large coastal power station

with a once-through system [068, Ambrogi, 1997]. It appeared to be possible to lower the

dosage of ClO2 after an initial concentration of 0.22 mg/l. (8 kg/hour) during the growth season

to about 0.18 mg/l. (6.5 kg/hour) and to reduce this even further in wintertime. These levels

correspond well with other reported dosage levels. Dosage was continuous and effective in

restricting mussel growth. The resulting concentrations of trihalomethane formations (THM)

were considerably lower than in case HOCl was being used irrespective of reaction temperature

or reaction time. They ranged from 0.31 μg/l with a dose of 0.50 mg/l ClO2 at 10 minutes and

15oC to 460.48 μg/l with a dose of 0.40 mg/l at 60 min. and 60oC.

Considering the maximum initial dosage at 0.22 mg/l, needed to get an effective treatment, the

expected concentration in the seawater at the end of the channel would be considerably lower

than the LC50 (96 h.) of 54.7 μg/l. It has been proven that efficient antifouling of ClO2 occurs at

concentrations in the range 0.05 – 0.25 mg/l.

In open recirculating cooling water systems the typical dosage is 1-5 ppm chlorine based on the

estimated volume plus make-up during feed to the entire system. Typically at start-up, chlorine

dioxide is fed to a clean system at about 1 ppm for one hour, three times per day. Contaminated

or dirty systems may require increased doses (3-5 ppm) and feed times. Mechanical cleanup of

systems may be required to further optimise the programme.

It was recommended for systems where process contamination is suspected , to run a chlorine

dioxide demand on the system. From the determined demand value the initial dosage can be

derived, which from experience shows to be 30-50% of the demand value may be used for the

initial dosage.

Table XI.4: Typical dosage of chlorine dioxide for once-through and recirculating systems in

Europe

[CEFIC Sodium hypochlorite group, comment]

Cooling system Mode of application Time of application Typical dose

(mg/l)

Once-through system continuous 8 hours a day/

during 8 months a year 0.4

discontinuous 6 times for 1 hour/day 0.3

Recirculating system

continuous all year round 0.2 in winter

0.5 in summer
Annex XI

Industrial Cooling Systems 269

Table XI.5: Effect of chlorine dioxide applied in once-through system on larvae settlement

[U.S. data, Van Hoorn, comment]

Dose Frequency Settlement-reduction

0.25 mg/l 4 x 15 min/d 95%

0.25 mg/l 2 x 30 min/d 35%

Cross-media effect:

Although it does not form any THM or chlorophenols it is expected to find reaction products

like aldehydes, ketones and quinones or even epoxydes under certain circumstances. The latter

are known to be carcinogous or mutagenous.

Application:

Treatment with chlorine dioxide requires an installation for in situ production. Due to its

sensitivity for pressure and temperature the gas cannot be compressed and transported in

cylinders. Three ways of in situ generation are mentioned by [tm059, Swinnen, 1995]:

1. from sodiumchlorite/chlorous gas

2. from sodiumchlorite/sodiumhypochlorite/(chlorine acid)

3. from acid activation or sodiumchlorite by means of chlorine acid

Feeding point

To obtain the best results, chlorine dioxide should be fed directly into the recirculating water of

a cooling tower, at a point of good mixing, such as below the water line in the cold well or just

ahead of the equipment that is most critical. A side-stream of chlorine dioxide can be fed to the

basin at the far end (opposite cold well) to create a "sweep" effect across the basin or into the

return riser for additional control within the tower.

Monitoring

When chlorine dioxide is used as a microbiocide in cooling water it is important to monitor the

amount used and its effectiveness. Close control of chlorine dioxide residuals with attention to

plate counts provides the best results at the most economical cost.

At times a free chlorine dioxide residual can be found in the return water of a recirculating loop

or the effluent of a once-through system. The free residual is less than 0.5 ppm in most cases as

tested by the Chlorophenol Red method. In systems where no free chlorine dioxide residual is

found, visual observation of the biomass, through biological organism counts or differential

pressure measurement can judge results.

In the U.S., Redox control is most often used as on-line monitoring technique. Typical ORP

values for good control are reported to be 350 – 500mV.

Costs:

Costs have not reported, but it was concluded that further research was needed on application

strategy to reduce the amount needed in once-through cooling systems and thus lower the

price. At the moment of investigation it was considered too high for full scale operation (1996).

Reference plant:

Brindisi Nord power station, Italy (experiment).

Considerations:

With respect to the application in once-through systems, a comparison with chlorination would

need to normalize dosage regimes to be able to consider the features of both chlorine dioxide

and chlorination as a biocide and in the discharge. Further research seems necessary based on

the above-mentioned promising results.
Annex XI

270 Industrial Cooling Systems

XI.3.4.5 Ionic water purification to treat cooling tower water

[tm036, Wilsey, 1997]

Description:

Based on an already existing concept of supplemental ionic water purification is an alternative

method to treat cooling tower water with copper ions. It is claimed that a chemical-only water

treatment system can be replaced by this technique leading to less harmful substances being

more economical to the environment.

Reduction:

Microbiologists are quoted that have determined that small amounts of copper acting as

supplement to chlorine at 0.4 ppm have the same efficiency as 2.0 ppm free chlorine.

Cross-media effect:

Costs have not been reported, but could be energy input for copper ion generator.

Application limits:

To apply this treatment a copper ion generator is needed together with a device to control total

dissolved solids, a magnetic water conditioning system and a system to analyse the water

composition. With these systems the treatment can be optimised.

A number of factors have to be taken into account. The composition of the make-up water has to

be such that the tower sump contains an alkalinity level between 40 ppm and 130 ppm and the

pH between 7 and 8. The effects of copper are that it functions as a coagulant to reduce scale,

forming larger complexes to be separated and filtered more easily. It also acts as a bacterial

disinfectant forming copper compounds that are lethal to bacteria and algae. Finally it has a

function as algaecide to blue-green algae in particular.

Attention should be given however to the amount of cycled copper that will also dictate the

concentration in the purge or blowdown. Also the residual concentration of the lethal copper

compounds need further examination as the discharge to the receiving water could cause

harmful effects.

Costs:

Not reported

Reference plant:

Not reported.

Considerations:

Results will have to be proven yet in full scale application.

XI.3.4.6 Stabilizing halogenated biocides in cooling tower water

[tm62, Dallmier et.al, 1997]

Description:

Particularly chlorine and bromine based products are often applied. As the biocidal effect of a

halogenated biocide depends on the total halogen residual and it is important to prevent any

reactions that can decrease the amount of residual in the cooling water. Reactions with other

corrosion and scale inhibitors can occur (e.g Br with tolyltriazole). Halogens can be stabilised to

reduce halogen volatility and to increase compatability with inhibitors and to maintain sufficient

effective halogen. Stabilising of bromine was achieved with the application of hydantoines. On

the stabilising process no further information is reported.

Reduction:

Application of stabilised bromine in cooling towers revealed the following effects:
Annex XI

Industrial Cooling Systems 271

1. the loss of Br due to volatization is less when Br is stabilised thus leaving more free Br

present in the cooling water;

2. stabilised Br appeared to be one third faster in killing slime-forming bacteria than

unstabilised Br;

3. stabilised Br appeared to be very effective in removing mixed culture biofilm, the

removal of 45% of the biofilm was measured to create a 47% reduction of the pressure

across the tube;

4. application in an office chiller system showed effective against Legionella

pneumophila;

5. more than 95% of the yellow metal corrosion inhibitor totyltriazole was maintained in

the cooling water when stabilised Br was added.

Cross-media effect:

No information was available on the effects of the application of an additional chemical to

stabilise the halogen.

Application limits:

It has been applied in recirculating systems (wet cooling towers). Comments on the applicability

of this technique concerned the application of hydantoines. It was reported that handling of

hydantoines (in pellet form) is difficult requiring dissolving devices. This limited the application

to small size cooling systems. Currently a liquid stabilised bromine product is on the market,

which can be applied in large size systems as well.

Costs:

Costs of the stabilizing have not been reported.

Reference plant:

On the stabilized liquid product 2 refineries (in Germany and Austria) and 1 chemical plant in

Germany were reported.

Considerations:

Above-mentioned effects were confirmed by the results of field experiments. Two remarks have

to be made. On the addition of stabiliser and its behaviour in the cooling system or the purge no

observations were reported. The hazardousness or environmental acceptability could not be

established.

Its working on Legionella was tested in a chiller system, but a translation to conditions in

cooling tower systems has not been made.

XI.3.4.7 Filming agent against fouling, corrosion and scaling

Description:

Filming agents are applied that cover the surface of conduits on the waterside to prevent or

reduce fouling and corrosion or scaling and do not treat the cooling water flow. A commercially

available compound, called Mexel®432/0, is applied primarily constituting of long chain

aliphatic amines. In aqueous emulsions, this product forms a film on the cell membranes

causing destruction of tissues in a varying proportion depending on the dosage. The efficiency

of this technological alternative is not linked to the modification of the water chemistry

properties nor the water biology of the cooling circuit, but rather to the adsorption or integration

of Mexel® onto all the surfaces present in the circuit. The anti-fouling effect can be explained by

the integration of the Mexel constituents into the biological membranes, and into the biofilm.

This integration disturbs the cohesion of the biological structure and at high concentrations

results in the destruction of the membranes. At sublethal concentrations, the constituents of the

Mexel insert themselves into the membranes and disrupt the ionic or gaseous transfer of the

membranes. In this case, the treatment produces a state of stress for the animal (mussels, etc)

sufficient to avoid their definitive installation in the circuit treated.
Annex XI

272 Industrial Cooling Systems

It has a broad spectrum of action on microfouling and macrofouling in both seawater and

freshwater. The product has also anti corrosion and anti-scaling properties and the treatment

procedure is generally intermittent, intended to renew the film on the surfaces to be protected.

Suggestions were made that periodic treatments can be more effective on macrofouling. The

film lasts for 10-20 days.

Dosage is done automatically and starts with an initial dosage to establish the film.

Concentration in the discharge is measured and dosage levels are reduced as soon as Mexel 432

can be detected in the discharge. The initial period for a large seawater once-through system is

about 10 days.

In once-through systems, the analysis of the product is performed either by a spectrophotometric

laboratory method or by a colourimetric field analysis method. This second analysis method

allows a rapid control of product concentration at various points in the circuit.

Results:

The treatment conditions, defined by the concentration to be injected as well as the duration of

the injections, are dependant upon the results researched (bio-fouling, corrosion, fouling or

scale), the physico-chemical properties of the water medium and the circuit characteristics (type,

temperatures, surface condition, materials, flow rates, etc).

It can be effective against bivalves on a periodic treatment basis and at a residual concentration

of 3.5 mg/l. The efficacy of long-term intermittent treatments on zebra mussels has also been

shown in experiments: a dosage of 3 hours per day at 6 mg/l kills 100% of zebra mussels.

At the example plant, for corrosion protection of aluminium brass a dosage of 5 ppm for 30 min.

per day was sufficient. For macrofouling a dose was applied of 0.5 ppm at 5 hrs/day. By way of

bio-monitoring reactions of mussels on the dosage were researched to identify an optimised

treatment regime.

Cross-media effect:

On the visited site there was clearly an advantage as no electrolysis of seawater was anymore

needed. This also ended the maintenance of the elctrolyser, which is costly, both in terms of

environment (human health) and finance.

Three processes are involved in the disappearance of Mexel in solution: immediate demand,

turbulence of the water, and bacterial degradation in aerobic conditions. Bacteria showed

degradation of up to 98% of the product in 10 days.

Regarding the toxic effects of the product on freshwater organisms, it showed rapid

disappearance in natural waters of the product in its toxic form, and absence of detectable

toxicity during its degradation.

Application limits:

Application is independent of the metal tested, (bronze, copper nickel alloys, iron and stainless

steel 304L & 316L) or the water medium (fresh or seawater), Mexel can be an effective

chemical or biological corrosion inhibitor.

It allows the treatment of open or semi-closed recirculating hydraulic systems which have flow

rates ranging from a few cubic meters per hour (air conditioning systems) to 100.000 m3

hr-1 in

fresh water, brackish or sea water. On a world-wide basis, the product is used to treat the

hydraulic systems (cooling, fire fighting, etc) of electrical power plants, geothermal

installations, shipping, the chemical industry, steel mills, refineries, off-shore platforms, and air

conditioning units using water as the thermal fluid.

Costs:

Cost data have only been given in comparison with the application of chlorination and

particularly in comparison with electrolysis. Confirmation could not be obtained on the cost

balance of Mexel compared to chlorination. Costs depend on the surface to be treated and not on

the cooling water volume.
Annex XI

Industrial Cooling Systems 273

Reference plant:

EDF Power Station, Le Havre (F).

Considerations:

Environmentally its low toxicity and the absence of any detectable toxic degradation products

make it acceptable as an alternative to cooling water treatment.

As the product is easily biodegradable, this advantage may be a disadvantage when it comes to

the amount needed for initial treatment of the surface. Its reactiveness may raise the amount

needed and the costs involved. Where fresh water generally has higher dissolved solid contents

than seawater this could indicate a preferred application in seawater conditions.
Annex XI

274 Industrial Cooling Systems

XI.3.4.8 Stable organic corrosion inhibitors in open wet cooling towers

[tm091, CTI, Little et. al]

Description:

Organically based treatment used in an open wet cooling tower can be counteracted because of

their susceptibility to strong oxidising agents, their sensitivity to high heat flux conditions, their

tendency to precipitate as calcium salts at high hardness levels, and the need for constant water

flows. To overcome these problems Ethanolamine Bisphosphono-methyl, N-oxide (EBO) was

developed EBO is an organic phosphonate that can be used in cooling water treatment as an

anodic corrosion inhibitor. EBO is reported to exhibit good stability against halogens. Its

stability at calcium levels of 500 mg/l calcium as CaCO3, buffered at a pH of 8.3 and a

temperature of 60oC, was compared with HEDP.

Reduction:

It was found that over 100 mg/l of EBO could be added without precipitation, whereas this was

only 7 mg/l of HEDP under the same conditions. No adverse effects on yellow metal corrosion

could be indicated. Corrosion was considerably reduced compared with an organic treatment

without EBO.

Cross-media effect:

Lower water requirements occur due to potential option for higher cycles of concentration, as

EBO is less sensitive to high calcium hardness levels.

Application limits:

Only applied in open recirculating system.

Costs:

Not reported.

Reference plant:

Pilot cooling tower, no full-scale application reported.

Considerations:

Application of EBO and similar cooling water treatment chemicals with improved working need

further research of the levels of toxicity in the discharge respectively blowdown of the systems

they have been applied to.
Annex XI

Industrial Cooling Systems 275

XI.3.5 Treatment of discharged cooling water

Minimisation of emissions from an integrated approach starts with minimisation of resources

used. As such Chapter 1 presents the initial approach that should be taken. Within the

limitations of cooling system and site-specifications a certain amount of chemicals might still

have to be applied with a certain discharge as a consequence. Monitoring and optimised

treatment are able to reduce the discharge further.

In some cases before discharging cooling water streams are treated in wastewater treatment

facilities. For information on wastewater treatment reference is made to the concerning BREF.

Particular examples of cooling water treatment have not been reported. With respect to

treatment a few remarks can be made:

• blowdown treatment containing peak concentrations after dosage can be collected in a

buffer basin to prevent the aquatic-environment or the water treatment facilities to be

affected. In the basin further hydrolysis of the biocides can take place down to less toxic

substances before the water is discharged or reused.

• Because of concentration of process substances, the blowdown of closed recirculating

systems of refineries may have to be treated before it is sent to a wastewater treatment plant

to avoid affecting the equilibrium of the treatment plant. It is maintained that the oil level of

this blowdown generally is much lower than the residual oil level in pre-treated process

water of other installations and therefore can be led to the waste water treatment plant

without pretreatment.
Annex XI

276 Industrial Cooling Systems

XI.4 Variable frequency drives for reduction of energy use

[tm097, Immell, 1996]

In operating a cooling system the required direct energy input can be reduced by reducing the

need for pumping capacity and by optimising the use of fans. In a greenfield situation many can

be done on the design (e.g. cooling tower construction, type of fill, pump configuration), but in

an existing installation options are more limited and comprise of changes of equipment.

Description:

The application of variable fan speed drives is an option to adapt the fan speeds specifically to

the required cooling duty. One technique is the use of variable frequency drives (VFD). A VFD

is a combination of a converter of the voltage and an inverter of the current (DC to AC).

Cooling towers are typically designed to provide a specified cold water leaving the tower for a

specified heat load at a certain wet bulb temperature, that is only exceeded at minimum

percentage of the year (1-2.5%). Most of the time they will perform at a lower wet bulb

temperature than the design, but on a variable level based on seasonal variation of wet bulb

temperatures.

With the VFD system this variation is translated into different fan speeds to obtain the required

water temperature. VFD are commercially available from different suppliers.

Reduction:

Reduction in energy use as well as in reduced noise levels and reduced vibration due to slower

operating speeds is achieved. Also, higher longevity of rotating equipment due to smoother

speed change of the motor (so called soft start) was observed.

Cross-media effect:

See section on reduction.

Application limits:

To apply a VFD a number of specifiable features to be checked were mentioned, such as: an

automatic temperature control, the proper layout of the VFD to the demands of cooling tower

fan motor, and an analysis of resonance of the equipment.

Costs:

A cost indication was not given

Example plant:

Not mentioned with respect to experiences with application.
Annex XII

Industrial Cooling Systems 277

ANNEX XII SPECIAL APPLICATION: POWER INDUSTRY

[tm132, Eurelectric, 1998]

Synthesis

In order to synthesise the specific knowledge and enable other industries to benefit from it, this

Annex has been drawn up within EURELECTRIC. It is the outcome of collaboration mainly

between ELECTRICITÉ DE FRANCE, ELECTRABEL, LABORELEC and VDEW resp. VGB

representing German Power Plant Operators. The results of the permanent various working

groups of UNIPEDE19 and CORECH20 have also been included.

The Annex is meant to give some background information for a better understanding of the

information presented in the main document. It explains in a simplified way the operation of

thermal power plants. It sets out the main functions of the cooling systems of the condenser and

auxiliaries. The possible environmental impacts of cooling systems are then examined in more

detail. This part of the Annex particularly concerns heat discharges, the suction of living

organisms into water intakes, any discharges of treatment reagents and other potential

detrimental effects, such as noise.

Also, an analysis of the various cooling techniques conceivable is made. It is referred mainly to

the design of a new system and to be used as additional information for the determination of

BAT. It deals with not only the technical and economic aspects, but also and especially with the

ecological and energy impact of the various solutions. Its conclusions, although specifically

aimed at the power industry, fall within the general BAT conclusions of Chapter 4 of the main

document.

The main conclusions that have emerged from the analysis are:

• The impact of a cooling system on the receiving environment must be studied before the

power plant is designed; to do so, numerical modelling and on-site tests in pilot loops are

recommended;

• The design of the cooling systems must be studied while taking into account the ecological

and energy impact to a maximum;

• The implementation of physical processes intended to limit fouling must be sought

systematically (continuous mechanical cleaning, temperature increase, filtration, etc.);

• Chemical solutions must be studied on a case-by-case basis so as to limit their utilisation to

the utmost;

• One best solution cannot be selected as too many local factors influence the choice of the

cooling system of a power plant. They include not only the flow-rates available, but also

visual aspects.

XII.1 Introduction

The thermodynamic cycle of conventional thermal power plants obeys CARNOT's principle.

Efficiency levels reach about 40% for conventional new design but can achieve 47% in

advanced design and under very favourable climatic conditions in particular when cooling water

conditions are suitable (once-through cooling system), even with hard coal firing. The result is

that nearly 45% of the amount of energy provided by combustion must be dissipated at the

condenser level.

The condenser is the key point of the facility. Regardless of the mode of cooling adopted, it is in

fact one of the main interfaces between the power plant and the surrounding environment. The

 

19 The International Union of Producers and Distributors of Electrical Energy.

20 Committee on Research.
Annex XII

278 Industrial Cooling Systems

efficiency and availability of a power plant depend to a great extent on the integrity and

cleanness of the condenser. These are reasons why specific solutions have been adopted for a

long time now: continuous mechanical cleaning by foam balls, corrosion-resistant alloys, such

as titanium and stainless steel, etc. Also cooling water treatment systems have been developed

and are in operation, in particular for circulating cooling systems.

Likewise, as cooling flowrates may reach several dozen m3

/s, the modes of treatment adopted

and solutions selected may be difficult to extrapolate to other industries.

XII.2 Power plant cooling systems - principles and reminders

The operation of power plants is governed by CARNOT's principle. The heat source, the boiler,

provides the energy required for water vaporisation. The cold source, the condenser, condenses

the steam coming out of the low-pressure turbine.

One of the main characteristics of a power plant, from the technical and economic standpoints,

is its specific consumption, in other words, the amount of heat needed to produce one kWh of

electrical energy. This specific consumption results from the thermal cycle balance (table 1).

Table XII.1: Example of simplified balance of a thermal cycle for conventional new design

Energy transformation Energy

(kJ) (%)

Efficiency

(%)

Energy from combustion 9000 100 100

Steam generator loss 1050 - 11.7 88.3

Condenser “loss” 4200 - 46.5 41.8

Feedwater heating (2000) (22.2) (Looping)

Turbogenerator losses 65 - 0.75 41.05

Power supply of auxiliaries 65 - 0.75 40.3

Loss in main transformer 25 - 0.2 40.1

Overall efficiency of the facility 40.1

The presence of the cold source is the main consideration. Not always can cooling systems use

water drawn directly from a river, sea or lake. It may be necessary to use a recirculating system

with a cooling tower. A look at the thermal cycle balance shows that 4200 kJ must be yielded

for each kWh generated. In addition, this energy cannot be recovered because its exergy is low.

New generation systems, especially combined cycles (or gas-steam turbines), make it possible

to obtain higher efficiencies of even more than 55%.

The cooling system, which serves to evacuate this energy, is generally called the circulating

system. The condenser tube bundle contains cold water drawn from a river, the sea or a lake.

The heating and flowrate of this water depend on the installed capacity (table 2).

Table XII.2: Relationship between the installed capacity and cooling parameters

(Values as an example, depending on type of the circulating system, the ambient air temperature,

cooling water resource temperature)

Rated capacity of the unit

(MW)

Circulating water flowrate

(m3/s)

Heating of water in the

condenser (K)

125 3 – 5 7 – 12

250 6 – 10 7 – 12

600 14 – 24 7 – 12
Annex XII

Industrial Cooling Systems 279

Each unit also has an auxiliary cooling water system:

• generator seal oil coolers,

• compressor coolers;

• etc.

Closed-loop cooling systems, supplied with demineralised water, are systems for:

• generator stator cooling water coolers;

• generator hydrogen coolers;

• etc.

Depending on the units, the cooling water flowrate of this cooling system of the auxiliaries

represents normally about 4 to 8% of the circulating water flowrate. Heating is limited and

amounts up to 10 K, according to the auxiliaries in operation. However, even with a low thermal

load, it can remain in service several days after the shut down of the unit to evacuate the

residual heat.

XII.3 Possible environmental impacts of cooling systems

The heat releases at the cold source mainly concern two receiving environments: air and water.

But, in fact, even if the discharge occurs into an aquatic-environment, the ultimate heat sink

remains the atmosphere. Indeed, the water gradually transfers the heat received by various

natural processes: evaporation, conduction, radiation. For economic reasons, water is the first

area where one looks.

Before wondering what techniques may be acknowledged as BAT for cooling systems, it seems

desirable to make an analysis of any detrimental effects on the natural environment, estimate

their nature and amplitude and judge them, in other words decide whether they remain tolerable

or not.

XII.3.1 Heat discharges to the atmosphere

Regardless of the type of cooling system, all of the heat conveyed to the cold source is

transferred to the atmosphere. This is carried out on a specific basis in the case of cooling

towers, air-cooled condensers and dry cooling towers. In the case of once-through cooling

systems on a river, lake or the sea, the heat is transferred via the surface of the receiving water

body, over a large area and with a certain time lag, depending on the local situation.

In power plants cooled by a once-through system (Figure XII.1, Section XII.11), pumped

water is generally heated from 7 to 12 K when the units operate at their rated capacity. The

discharged cooling water is progressively cooled by mixing with the receiving water. The heat

is then transferred to the atmosphere by three conventional processes: evaporation (35 to 45% of

the energy released), by radiation of the water surface (25 to 35%) and by conduction with air

(20 to 30%). Depending on the local situation the outlet temperature could be limited by the

local authority.

Energy transfer by evaporation represents a vapour flowrate of 20 kg/s per 100 MWth..

Considering the rapid decrease of water heating process downstream of the discharge, the only

atmospheric phenomena likely to be modified are occurrence frequencies and the persistence of

evaporation fog in the area close to the release, where temperature differences are still

considerable, but the extent of which is limited.

It is worthwhile noting that, all things considered equal, the temperature of the formation or

disappearance of evaporation fog is higher above salt water than above soft water. This

circumstance is therefore favourable to power plants sited in estuaries or along seacoasts.
Annex XII

280 Industrial Cooling Systems

For power plants equipped with wet cooling towers (Figure XII.2, Section XII.11), everything

occurs as if the heat was released directly to the atmosphere. Two kinds of operation methods

are in practice:

• once-through cooling with cooling tower (Figure XII.3, Sector XII.11) and

• recirculating cooling (Figure XII.4, Sector XII.11).

The discharge takes place in a concentrated way over a small area. Wet cooling towers transfer

to the atmosphere about 70% of residual heat in the form of latent heat (wet vapour) and about

30% by sensitive heat. Thus, the vapour flowrate released to the atmosphere is roughly twice

that resulting from once-through cooling without cooling tower. Air saturated with humidity is

released to the atmosphere at a temperature of about 10-20 K above ambient temperature and at

a velocity up to 3-5 m/s in the case of natural draught cooling towers. This velocity is doubled

in the case of mechanical draught cooling towers. This air saturated with humidity, by cooling

through turbulent mixing with ambient air, may give rise to the formation of artificial clouds or

plumes.

The risks of fog formation on the ground resulting from the lowering of the condensation plume

may be relatively frequent especially with mechanical draught cooling towers (Figure XII.5

XII.6 and XII.7, Section XII.11) due to their low height and in conditions of cold humid weather

without wind. The relevant area is about 500 m from the emission source.

The frequency is considerably reduced, when the cooling towers get higher. In plains, one may

estimate that the lowering of plumes reaching the ground is exceptional as of a height of 50 to

75 m depending on the local situation.

The formation of frost may result from the contact with the ground frozen either by the fog due

to the lowering of the plume, or by precipitation linked to priming, or by sprays from the base of

the cooling towers. However, the impact of such sprays remains confined to an area near the

cooling tower and concerns at the very most the several dozen metres close to the base of the

cooling tower.

The main climatic modification due to the operation of wet cooling towers concerns a local

increase of nebulosity by the development of the condensation plume which results in the

reduction of sunshine and light in the vicinity of the power plant.

For power plants equipped with dry cooling towers (Figure XII.11, Section XII.11) or aircooled condensers (Figure XII.9 and 10, Section XII.11), the absolute humidity of the air is not

changed, but its temperature is higher by about 15 to 20 K above the ambient temperature. All

of the heat is released in sensitive form and the non-saturated hot air, which rises in the

atmosphere, seldom leads to cloud formation.

Hybrid cooling towers (wet/dry) (Figure XII.8, Section XII.11) make it possible most of the

time to avoid the formation of plumes. Water consumption (i.e. make up water) is 20% less than

that of a wet cooling tower. However, at the present time, the only hybrid cooling towers

available are of the mechanical draught type. The annual balance of a power plant with

mechanical draught type hybrid cooling tower can be in the same range to that with a

comparable mechanical draught wet cooling tower. This takes into account the operation mode.

Since a few years at fossil fired power stations the discharge of desulphurised flue gases via

cooling tower is state of the art at least in Germany. It is an alternative to traditional discharge

by stack and has ecological and economical advantages.
Annex XII

Industrial Cooling Systems 281

XII.3.2 Heating of receiving aquatic-environments

Although the final heat sink is the atmosphere, in most cases, a fairly large part of the discharge

of a thermal power plant takes place in the aquatic-environment. Various physical phenomena

come into play here:

• turbulent diffusion,

• convection in water,

• flow of fluids of variable density,

• evaporation, radiation, convection in the air.

Depending on the extent of the discharge and according to the receiving environment, such a

phenomenon is preponderant and affects the way the heat is distributed in the receiving

environment.

The near field of the cooling water discharge should be distinguished from the far field.

The near field is defined in a river as the area in which the mixing of the warm water plume

with river water is incomplete.

The water temperature in the near field depends upon the mixing of water released by the power

plant with the water of the receiving environment. Heating can be reduced in this area by

rapidly mixing the effluent with the water of the receiving environment by means of specific

devices.

The far field is the warm water geometry that is fully mixed with depth within the water

column and is thus a background heat field. The excess temperature in the far field is gradually

reduced due to the dilution with ambient waters and heat exchange with the atmosphere.

As concerns discharges in a tidal sea or sea with strong currents, the warm water plume

formed by the discharge of the power plant is mainly governed by the existence of major

velocities in the receiving environment. They bring about a rapid mixture of the water

preventing any stratification caused by the difference in density between the warm water and

cold water. The temperature drop in the warm water plume principally comes from the mixing

and not from heat losses at the surface of the water area. The extent of the warm water plume in

a tidal sea, defined as the area within the 1 K heating isotherm, covers an area from 2 to 10 km

2

for a discharge corresponding to that of a 5 000 MWe nuclear power plant.

The behaviour of the warm water plume in a tideless sea is first of all that of a stratified flow.

The temperature drops very quickly through dilution due to friction and turbulence. In a tideless

sea (or lake) the spreading or transport of cooling waters is strongly influenced by wind-induced

currents and thermocline conditions and is estimated as roughly 1 ha/MWe.

Normally for coastal power plants the cooling water is discharged to the sea surface through an

open discharge channel.

The behaviour of the warm water plume in an estuary is similar to that in a tidal sea with strong

currents. The alternative movement of water plays an essential role. The river flow will tend to

transport the heat towards the sea. The incoming tide will slow down or change the direction of

flow, and will thus affect the spreading of the warm water plume in the estuary.

The assessment of the heating of a river subsequent to a warm water discharge is relatively

complex. Indeed, the cooling mechanism of the river downstream mainly results from the

exchange of energy between the river and the atmosphere. The energy flow between the stretch

of water and the atmosphere fluctuates considerably depending on the meteorological conditions

and the time of day.
Annex XII

282 Industrial Cooling Systems

In a river, diffusers distributed all along the width of a waterway serve to carry out the mixture

over a distance of several dozen to several hundred metres. If the discharge is performed along

the bank, complete mixing by natural flow is carried out over a few kilometres instead.

In all cases, recirculation in the river must be avoided or the recirculation rates for discharges

into the sea and especially in the estuary reduced to a minimum to ensure efficiency and safe

operation of power plants. The position and design of water intake and outlet structures are

determined to eliminate the risk of recirculation.

Preliminary studies make it possible to design water intake and outlet structures and devices

best adapted to avoid recirculation and favour the initial mixing of heated water discharges.

They rely on physical models (hydraulic models) and numerical models. Where possible the

numerical modelling etc. should be based on site-specific hydrographic survey data.

The use of these tools as part of the impact study of projected facilities serves to give assurance

that regulatory thermal limitations will be respected, whether they concern maximum heating in

the mixing area or the temperature level after the mixing.

XII.3.3 Suction of organisms into water intakes

When pumping the water needed for cooling, thermal power plants draw in microscopic

organisms (algae and plankton), as well as organisms which swim in the open water (some

crustacea and fish). The plankton passes through the rotating filters the mesh of which is

generally between 1 mm and 5 mm. This does not hold true for crustacea and fish, which are

flattened against the filtering panels, drawn up and discharged with the washing water of the

filters.

Some studies have shown that most of the organisms drawn into the water intakes are small in

size: Shrimps, larvae and alevin in the sea and estuary or alevin in rivers. The case of young

salmon migrating downstream, which are particularly drawn into the water intakes, is specific

for the behaviour.

To limit the entrainment of these species, three types of measures may be taken:

1. Place the intakes outside critical areas, such as spawning grounds and "fish nurseries" on the

seaside, or migration routes for eel larvae in estuaries;

2. Design intake structures which minimise the drawing in of organisms;

3. Equip intakes with repulsive devices or equipment, which restore organisms to the

environment without damage.

A lot of deterrent systems (repulsive devices) have been developed and installed at water intakes

of hydropower and thermal power stations over the last decades:

• In freshwater bodies, electric fish screens can frighten away fish of specific stages, but do

not affect fingerlings or even attract them into the intakes;

• Air bubble curtains generally had very bad results;

• Light is partly efficient on certain species, but fish can be acclimated and the deterrent

effect is not constant;

• Some results with sound deterrent systems are promising, but there are contradictory results.

The investment costs depends on the size of the intake and the flowrate and can be roughly

estimated to be in the range of Euro 40000 to 200000.

4. Equip intakes with recovery systems, which restore organisms to the aquatic-environment

without damage.

In large water intakes with travelling screens, the organisms can be removed with a fish pump

or washed out of the screen by low-pressure water jets (1 bar). At a power station on the
Annex XII

Industrial Cooling Systems 283

Gironde Estuary (France), such systems showed relatively good efficiencies with survival

results of 80% to 100% for shrimps, plaices and eels. Other attempts have been less effective or

very costly.

The first two actions, of a preventive nature, are preferable to curative actions the efficiency of

which presently remains problematical. A universal, widely applicable, solution is not available.

XII.3.4 Alteration of the receiving environment by chemical discharges

The water withdrawn for cooling purposes may sometimes be the cause of chemical releases

into the receiving environment. The following may be concerned in particular:

• reagents used to avoid the scaling of cooling systems equipped with cooling towers;

• reagents used to fight against biological developments, reaction products of some of them;

• iron sulphate anti-corrosion treatments to protect, in some cases, copper alloy condensers;

• corrosion products of heat exchangers and piping.

As concerns the marine environment, the purpose of the biocide treatment is to maintain the

systems sufficiently clean so as to ensure their proper operation. For the sea intakes, the main

problem is to avoid the development of molluscs (mussels, oysters, etc.) inside the cooling

system. The current practice is the injection of chlorine. It is generally produced on-site by sea

water electrolyses. This process avoids the risk involving the transport of NaOCl by truck. The

chlorination can be made on continuous or discontinuous (seasonal) basis depending on many

factors such as meteoclimatic characteristics of the site, water quality, cooling circuit design and

biofouling typology (settlement periods and growth rates).

Mainly the injection takes place in low doses so that the concentration in free chlorine in the

discharge is generally between 0.1 and 0.5 mg/l normally (sporadically 0.7 mg/l). The value of

this limit concentration is set by local regulations.

However, when it reacts with some organic matter, chlorine may lead to the formation of organo

halogenated substances (mainly bromoform in seawater). Some studies nevertheless show that

bromoform concentrations in the plumes of warm water discharges from coastal-sited power

plants remain extremely low (about 15 μg/l).

It would be advisable here to compare this figure with the natural production of

organohalogenated substances in the oceans. According to Grimvall and deLeer (1995), the

annual production of a number of organohalogens is:

• chloromethane : 5.000.000 t;

• bromomethane : 300.000 t;

• iodomethane : 300.000 t to 1.200.000 t;

• chloroform : 90.000 t to 360.000 t;

• bromoform : 500.000 t to 1.000.000 t;

• iodoform : not detectable in sea water.

The natural concentrations in AOX ranging from 6 to 17 μg Cl/g of sediment in the Gulf of

Bothnia and from 50 to 180 μg Cl/g of sediment in the Gulf of Finland were measured. The

presence of these organohalogenated molecules has been attributed to biohalogenation

reactions.

Chlorination is the anti-fouling chemical treatment method that is the most commonly used to

protect the systems of coastal-sited power plants. Another oxidant, chlorine dioxide, has

nevertheless been tried with success on thermal power plants.

For a great many years, the choice of the alloy for the tubes of heat exchangers in coastal-sited

power plants has gone towards titanium. Under these conditions, the contribution of corrosion

products is insignificant, or even inexistent. However, there are still condensers in copper alloy
Annex XII

284 Industrial Cooling Systems

which are protected by a film of ferric hydroxide produced by adding ferrous sulphate to the

cooling water.

For river-sited power plants the contribution of chemical reagents will depend to a great extent

on the type of cooling system and any biological problems.

Generally, operation with recirculation increases the risks of scaling. This often requires setting

up a specific treatment of make up water or cooling water. The modes of treatment that may be

used are as follows:

• no treatment when the water is not very mineralised,

• lime softening of make up water,

• acid vaccination of circulating water,

• treatment with precipitation retarder,

• combined treatments of the type: acid vaccination and scale inhibitors or lime softening and

acid vaccination.

The choice of the mode of treatment depends on many criteria the following of which are

mentioned for example:

• concentration factor,

• chemical composition of the river water,

• design of the cooling system.

The treatment depends on the concentration factor of the cooling system:

• with a low concentration factor (1.05 to 1.2), it is not generally necessary to treat the water

of the system,

• with an average concentration factor (1.2 to 2), an acid vaccination of the circulating water

is necessary when the hardness of the water is high,

• with a high concentration factor (3 to 7); the lime softening of make up water often becomes

the only choice possible, and may be supplemented by a light acid vaccination.

The acid vaccination of circulating water can be carried out in three different ways:

either by maintaining the pH within a range generally included between 7.5 and 8.5, or by

limiting the total alkalinity to 100 mg CaCO3/l (for make up water with low sulfate-content), or

by respecting regulation instructions which take into account the alkalinity, calcic hardness and

temperature. Sulphuric acid is used in most cases.

The purpose of the lime softening of make up water is to raise the pH of the water up to 10 so

as to precipitate the calcium and part of the magnesium in the form of carbonate and hydroxide.

At the outlet of the decarbonator, the concentration of residual calcium varies between 0.5 and 1

mequivalent. However, it is combined with carbonate, which makes the treated water extremely

scaling. To restore the balance of decarbonated water, a post vaccination with sulphuric acid is

often carried out. The lime softening results in the production of a large amount of sludge. In

addition, by increasing the pH, lime softening may result in the precipitation of some heavy

metals present in the withdrawn water.

The sludge produced by precipitation in the softening process is collected in the bottom of a

clarifier. It is normally pumped to a sludge thickener, where the solids concentration increases

by further sedimentation usually assisted by injection of polyelectrolyte. The clear water returns

to the clarifier while the concentrated sludge is further dehydrated in vacuum drum filters or belt

filters.

The cake produced by dehydration with remaining water content of approximately 50% is

removed for disposal in landfills. No environmental effects have been reported from softening

sludge landfilling sites.
Annex XII

Industrial Cooling Systems 285

The continuous chlorination of the circulating systems so as to eliminate the formation of

biofilm on the condenser tubes was given up a long time ago since mechanical methods have

been applied (Taprogge, Technos systems, etc.). But chlorination as such is still an effective

treatment. In practice, five chlorination treatments can be applied:

• end-of-season; for example continuous chlorination at low level (0.5 mg/l) for 2 to 4 weeks

at the end of the settlement period of the freshwater Zebra mussel Dreissena plymorpha;

• periodic treatment: several periods of continuous addition of biocide during the settlement

season;

• intermittent treatment: frequent dosing (every day or three days for example) for short

periods of time (some minutes to hours);

• continuous treatment at low level during the settlement period; for example in the North Sea

and the English Channel, chlorination at 0.5 to 1.0 mg/l., 7 months a year, to eliminate

marine mussels. Residual oxydant at the outlet 0.1-0.2 mg/l;

• semi-continuous treatment consisting of short term periods of treatment (15-60 minutes)

then stopped for equally brief periods. A semi-continuous chlorination or pulse-chlorination

at low level is used in Canada against the Zebra mussel and in France and the Netherlands

to control the marine mussels in power stations.

The massive chlorination or shock dosing is a specific procedure that has been developed to

eliminate filamentous algae which develop in the basins and the fills of cooling towers. The

concentrations at the injection point vary between 5 and 25 mg Cl2/l. In order to avoid the

release of chlorine into the receiving environment, the blowdowns are closed for a few hours.

They are open when the concentration of free chlorine in the circulating water is lower than the

discharge limit. Depending on the authorisations, this limit varies between 0.1 and 0.5 mg

TRO/l. Some discharge authorisations are expressed in flows. These treatments are not carried

out on all the sites.

The frequency of massive treatments depends to a great extent on the quality of the water, the

concentration factor and the general state of cleanness of the circulating system. It may be

weekly, monthly or quarterly.

The reaction of chlorine with humic and fulvic matter leads to the formation of

organochlorinated compounds. In fact, bromide ion concentrations in river water are generally

insignificant. Under these conditions, only organochlorinated compounds can be formed.

Volatile compounds such as chloroform, dichloromethane, (POX) and adsorbable compounds

(AOX) can be distinguished.

However, as is the case for sea water, the presence of organohalogenated compounds in inland

surface waters is not solely due to the chlorination of the cooling systems. Among the other

possible sources, particular mention should be made of agriculture and natural production. In

unpolluted lakes - for example in Sweden - AOX concentrations ranging from 10 to 190 μg Cl/l.

The highest concentrations have been measured in highly eutrophicated lakes.

Among the parameters which affect the reactions resulting in the formation of

organochlorinated compounds during the disinfection of cooling water, the following should be

mentioned:

• humic or fulvic concentration,

• free chlorine concentration,

• reaction time,

• pH of the environment,

• reaction temperature,

• presence of ammonium ions.
Annex XII

286 Industrial Cooling Systems

These complex reactions can be modelled and validated by measurements carried out on the

sites.

The chlorination of the once-through systems does not result in significant increases of

organochlorinated compounds. Indeed, the contact times are short, about 10 minutes at most,

and the concentrations of free chlorine are low. According to the chlorination procedures used,

the POX and AOX concentrations measured at the peak vary between 0 and 10 μg Cl/l and

between 20 and 150 μg Cl/l, respectively. These values correspond to free chlorine

concentrations at the injection included between 0.5 and 10 mg/l.

The chlorination in a closed loop of circulating systems may lead to higher concentrations of

organochlorinated compounds. The following factors play an infavourable role here:

• the contact time is longer,

• the recirculation increases the concentration of precursors.

It should nevertheless be mentioned that the increase of pH linked to the degassing of CO2 is

favourable to the formation of POX. The latter are easily transferred to the atmosphere via the

cooling tower.

Under these conditions, the concentrations of POX are included between 0 and 10 μg Cl/l and

the concentrations of AOX between 200 and 2 500 μg Cl/l. For concentrations of free chlorine

at the injection included between 5 and 25 mg/l and stay times varying between 2 and 70 hours.

One should note, however, that the presence of low concentrations of ammonium ion in natural

water may considerably reduce POX and AOX concentrations. Actually, the kinetics of the

chlorine-NH4

+

 reaction is more rapid than those of the reactions taking place between the

chlorine and aromatic compounds.

XII.3.5 Other possible harmful effects resulting from the choice of some

cooling systems

The use of natural draught, forced draught and hybrid cooling towers, or also of dry condensers

and cooling towers, makes it possible to considerably reduce the water flow requirements of a

power plant and, consequently, to limit the possible impact on the aquatic-environment.

However, the presence of cooling systems on a site may pose other problems. They concern in

particular problems of aesthetics and noise of wet cooling towers. For dry cooling towers and

condensers, in addition to the two previously mentioned aspects, there is also the possible

dissemination into the air of corrosion products from the heat exchange surface area, in

particular, when the heat exchangers consist of finned tubes made of galvanised steel.

Natural draught wet cooling towers, the sober form of which is generally not unpleasant, are

nevertheless structures that can be seen from afar and that cannot be concealed in a fairly flat

landscape.

On the other hand, mechanical draught wet cooling towers or hybrid cooling towers, the

aesthetics of which is itself much more debatable, present the advantage of generally being

lower than the main part of the power plant.

Nevertheless beyond that it is required to quote comparative factors between different

technologies since a lot may depend on the assumptions made by manufacturers in giving cost

data.

Similar remarks may be made for dry cooling towers and air-cooled condensers. The size

effect is nevertheless much more considerable. This is because the low exchange properties of

air require much larger structures. In addition, in the case of mechanical draught systems, the

power necessary for air supply is about 2% of the unit's net electrical output. For the same
Annex XII

Industrial Cooling Systems 287

thermal power to be dissipated, the size effect is therefore three times higher than that required

for wet cooling towers and combined systems.

A certain detrimental effect that can be caused by a cooling system resides in the emission of

noise at the air inlet and outlet. Even for a natural draught wet cooling tower, the sound level

may reach 60 dBA at 100 m. For a mechanical draught wet cooling tower and hybrid

cooling tower, the noise level comes to about 70 dBA under the same conditions. It is close to

80 dBA for air-cooled condensers.

XII.4 Prior study of the sites: indispensable tool for the

evaluation of their receiving capacity, impact control and

prevention of harmful effects

XII.4.1 Analysis of the situation

The cold source is one of the determining elements in the choice of a site. That's why great care

is taken at a very early stage with regard to the environmental problems posed by the cooling of

a power plant. As mentioned earlier on, these problems may be of several kinds:

• water heating by once-through systems,

• effect on the quality of water and on aquatic organisms, in the case of wet cooling towers,

• effect on the quality of air, in the case of dry cooling towers,

• meteorological effects, discharges of chemical substances and problems of noise regardless

of the mode of cooling adopted.

The designer is not powerless in the face of the problems posed. The knowledge acquired

through great many observations made in the vicinity of existing power plants constitutes a solid

experimental basis serving to effectively orient the studies to be undertaken prior to the

installation of a new power plant.

XII.4.2 Mathematical modellings, simulations on models and tests on pilot

loops, first indispensable tools

The interest of numerical models has been mentioned for forecasting thermal changes in the

near field as in the far field.

In the near field, fairly sophisticated tools serve to describe the dilution conditions of thermal

discharges. They are used at the local discharge level. These models serve to dimension the

outfall structures to the best possible extent so as to ensure the optimum dispersion of the warm

water plume in the receiving environment as quickly as possible and thus limit its impact to a

minimum (meteorological and hydrobiological data).

In the far field, the parameters that have to be taken into account are much more complex. They

concern not only the characteristics specific to the receiving environment, but also discharges

originating from other companies. Much more complex models have been developed to this

effect. They take into account biological parameters of water quality and take into account the

presence of chemical pollutants. They integrate various pollution sources and provide an

assessment of response of waterways or lakes to thermal and chemical disturbances or the

excessive contribution of nutriments (eutrophication phenomenon).

There are also other models used to simulate the accumulative impact of several wet cooling

towers installed on the same site.

The forecast making use of numerical models must rely on field data and experimental

knowledge. These in situ and laboratory studies are required to define and optimise the antifouling treatment or systems cleaning periods. Biological studies make it possible to know the
Annex XII

288 Industrial Cooling Systems

periods of reproduction and fixation of larvae, as well as the rate of growth of the main

biological species. These field and laboratory studies are long. Indeed, in the ecological field,

forecast analytical tools have not yet been wholly validated.

To determine the mode of treatment of the recirculating systems, systematic tests on pilot loops

are carried out. The purpose of these tests is to grasp the scaling risks on the one hand and on

the other hand to define the optimum mode of treatment, as well as the operating instructions.

Among the laboratory studies are to be found model simulations, such as for the visualisation of

water vapour and warm water plumes phenomena.

XII.5 Design of components and choice of materials

XII.5.1 Wet cooling

As mentioned previously the problems encountered in wet cooling systems may be of three

types:

• corrosion,

• scaling,

• biological developments.

For many years now, and almost naturally, the choice of materials used in the cooling systems

of power plants has been oriented towards corrosion-resistant materials. It must be mentioned

that the pressure within the condenser of a power plant is about 35 mbar but can be lower in

units optimised to achieve higher efficiency - in particular when climatic conditions are suitable

or may be even higher when climatic conditions are unfavourable. Under these conditions, the

slightest leak in the tubes leads to the ingress of impurities into the water-steam cycle. The

damage incurred by these intrusions may be substantial and reduce the efficiency of the unit, or

even lead to its shutdown.

In order to avoid the ingress of raw water into the water-steam cycle, the choice of materials

went to highly resistant alloys. Titanium is thus almost always used in seawater and brackish

water. For river water, condensers are equipped with 316L (or even higher Mo-content if

concentration of chloride ions is higher than 100 mg/l)) stainless steel tubes most often or brass

or sometimes made of titanium. In order to limit the formation of deposits (sediments and

biological developments) in the tubes, the minimum average velocity is fixed at 1.8 m/s for

brass. For other material as stainless steel or titanium, the maximum average velocity is much

higher. The choice of the average velocity is in fact the result of the optimisation of the cold end

taking into account the power required for the pumping that is the function of the velocity in the

tubes. Generally, the optimisation for stainless steel or titanium leads to a velocity in the range

of 1.8 - 2.5 m/s.

The tube plates are often made of carbon steel or titanium cladding. Appropriate painting

(epoxy or ebonite coating) protects the side in contact with the recirculating water. In some

particular cases, cathodic protection devices have been installed to solve galvanic corrosion

problems among others.

Even if rich, alloys such as stainless steels may be the subject of particular corrosion, like

corrosion beneath deposits. To avoid these phenomena, the tubes must remain clean under all

circumstances. This objective may be met in two ways:

• either by a continuous injection of rapid-action biocides, generally an oxidising biocide,

such as chlorine;

• or by a continuous mechanical cleaning process. There are various processes in existence.

They consist in the injection of brushes or foam balls, which are recovered and reused on a

continuous basis.
Annex XII

Industrial Cooling Systems 289

The desire to reduce the use of chemical reagents imposed the second solution.

To avoid corrosion of stainless steel, a particular procedure is recommended for the

conservation of the tubes that is carried out during long shut down of the installation. This

conservation consists of draining, washing and drying the inside of the tube.

The main heat exchangers of auxiliary cooling systems generally consist of coolers made of

steel or stainless steel. The distance between the plates is relatively short, which sometimes

leads to silting up. However, these systems operate according to the one of two or two of three

principle. In other words, the operation of one or two systems is sufficient to carry out the

cooling of the auxiliaries, with the additional train playing the security role. This design choice

serves to schedule periodic cleaning operations. These operations consist in dismounting the

unused heat exchanger and in cleaning the plates with pressurised water.

The make up and outfall structures, main circulating water conduits and cooling towers are

made of reinforced concrete. The choice of cement used depends on the mode of treatment to be

adopted for the circulating water. Thus, in the case of sulphuric acid vaccination, it is sometimes

indispensable to use special cements. The addition of fly ash is recommended. In the case of

higher sulfur acid concentration the use of special coatings is required.

The fill of cooling towers is usually made of thermoplastic materials. Specific loads are often

added during fabrication so as to make them fire resistant. The risk of fire in the fills is

particularly high during the maintenance operations. This choice avoids the problems of

asbestos encountered with the packings of the previous generations. In addition, recent

developments have made it possible to substantially increase the thermodynamic properties of

the fill. The choice of lighter synthetic materials and enhanced performances has served, for an

identical thermal load, to appreciably reduce the size of cooling towers. However, some present

profiles show a larger sensitivity to (bio)fouling and scaling.

As can be seen, the choice of a fill depends on several factors. More than the performances

sought, it is rather the quality of water (presence of suspended matter, scaling tendency) which

imposes the choice of the profile. The manufacturers still have a great deal of progress to make

with regard to this particular point. The ideal profile is, of course, the one which guarantees high

performances while being not very sensitive to (bio)fouling and scaling.

The standard drift eliminatorscurrently used make it possible to limit the amount of water

drawn in by priming to 0.01% or even less of the total flowrate. For facilities built near major

trunk roads, these values can be further reduced. A compensation of loss of capacity is

necessary in this case. Separators are also made of plastics.

Part of the pumping energy can be recovered by installing cooling towers equipped with

recuperators located under the fill. However, these cooling towers are extremely sensitive to

frost. Before opting for this solution, a study of local climatic conditions is absolutely necessary.

XII.5.2 Hybrid cooling

Hybrid cooling towers are recommended for special site conditions. The essential characteristic

of hybrid cooling towers is to combine an evaporative process with a non-evaporative process.

This results in a decrease of relative humidity and therefore the almost complete disappearance

of the plume at the outlet of the cooling tower. In connection with mechanical draught it is

possible to reduce the tower height considerably. The investment costs are higher than for a wet

cooling tower.

In general the energy consumption related to the operation of fans and the higher temperature of

the cold source, result in lower cycle efficiencies and higher fuel consumption.
Annex XII

290 Industrial Cooling Systems

XII.5.3 Dry cooling

Dry cooling is mainly used in regions with insufficient water supply.

XII.5.3.1 Forced draught air-cooled condenser

(Figure XII.9, Section XII.11)

In an air-cooled condenser arrangement the exhaust steam from the steam turbine is ducted to

the air-cooled condenser (ACC) where the steam is distributed through a large number of finned

tubes. Cooling air is forced over these tube bundles by fans. The steam rejects heat directly to

the atmosphere via the finned tubes, condenses and flows by gravity into a condensate tank.

From the condensate tank it is returned to the boiler. A typical design for the heat exchanger is

the “A“ frame condenser (other framework designs are also possible to accommodate the tube

elements, fans, and steel structure).

Large dry condensers tend to have long and complex steam tube systems, which may cause

siting, and pressure drop problems. To minimise pressure losses in the steam ductwork the

cooling bundles are generally located immediately adjacent to the turbine hall. Depending onsite conditions, the ACC concept is technically feasible to cover a wide range of power plant

unit sizes.

Compared to wet cooling systems, the ACC ́s efficiency of heat transfer to the atmosphere is

relatively low with the recooled water temperature being determined by the dry bulb air

temperature. The system needs to be designed to exclude formation of dead zones by noncondensable gases and thus eliminate the danger of undercooled condensate or freezing. The

design of tube bundles also needs to be robust to allow for periodic high-pressure water cleaning

of the external surfaces to maintain efficiency and plant output. However, this method of dry

cooling with an ACC avoids the need for large cooling towers, eliminates the vapour plume and

greatly reduces the consumption of cooling water. In particular using low noise fans and drives

can meet stringent noise restrictions.

In comparison to indirect dry cooling systems, then the ACC provides a greater temperature

difference between the condensing steam and the cooling air, and consequently the ACC system

will have a relatively smaller heat transfer surface. The indirect dry cooling system, which has

two heat transfer processes (i.e. steam condenser and Air-cooled heat exchanger) would need to

compensate by either adopting a larger cooling surface and/or by increasing its cooling airflow.

The investment costs for an ACC will be smaller than an indirect dry cooling system as the

latter will have to include the costs of the cooling water recirculation pumps and surface

condenser. On the other hand, the auxiliary power consumption and maintenance requirements

for the mechanically forced draught ACC will be significantly greater than the natural draught

dry tower.

XII.5.3.2 Natural draught air-cooled condenser

(Figure XII.10, Section XII.11)

Although the characteristics of placing a direct air-cooled condenser inside a natural draught

tower make it as feasible as a forced draught air-cooled condenser, the disadvantages are that

the height of the natural draught tower structure will be larger and so will its investment costs.

For example, the cost of the tower itself, the routing of the large steam exhaust ducts to the

cooling tower, and the larger heat transfer surface required as the natural air draught may be

only half that of a forced air draught tower.
Annex XII

Industrial Cooling Systems 291

The advantages of this natural draught ACC system would include:

• reduced/no sound emissions

• reduced/no air recirculation due to high tower structure

• no maintenance for fans, drives or circulation water pumps

• no auxiliary power consumption for steam condensation.

XII.5.3.3 Closed recirculating dry cooling towers

(Figure XII.11, Section XII.11)

In dry cooling towers water flows through the cooling elements in a closed system. Waste heat

is exclusively transmitted by convection. The lack of heat dissipation throough evaporation loss

leads to a significant increase of the temperature of the cooling water and thus a low efficiency

compared to wet-cooling.

In case of dry cooling two flow arrangements are possible:

• closed circuit cooling with dry-type cooling towers as direct cooling in connection with a

surface condenser and

• closed circuit cooling with dry-type cooling tower as direct cooling in connection with

injection condenser.

Advantages of dry cooling are as follows:

• no visible plume formation,

• simple set-in and examination of chemical parameters of the circulating cooling water,

• no need to make up water during operation, only replacement of possible leakage losses.

Compared to wet cooling, dry cooling has the following disadvantages:

• considerably higher investment and operation costs,

• larger dimensions of the building,

• stronger influence of the ambient air temperatures (summer/winter) on cooling performance,

• operation in winter requires special ice prevention measures during standstills,

• the tendancy to fouling of the cooling elements requires an efficient stationary cleaning

device.

XII.5.4 Cooling towers with discharge of cleaned flue gas

(Figure XII.12, Section XII.11)

During the last years the emission of desulphurised flue gas via cooling towers (as an alternative

to the emission via chimneys) in fossil-fired plants has proved to be favourable regarding

environmental and economical aspects. The effect of the transport of flue gas to higher

atmosphere areas is achieved in this case by differences in density between the flue gas/cooling

tower plume mixture inside the cooling tower and the relatively cold ambient air, and not by the

high temperature of the flue gas itself. Using this method an increase of the efficiency of the

power plant is obtained.

Flue gas desulphurisation plants of coal-fired power plants often work according to the principle

of wet desulphurisation. Wet cleaning cools down warm flue gases to between 50 oC and 70 oC.

For environmentally-compatible and troublefree emission of these cleaned flue gases via a

chimney a heating under additional utilisation of energy is necessary. An alternative for

reheating is the clean gas emission via a natural draught cooling tower: up to now this principle

has been exclusively used for wet cooling. The clean gases are led into the cooling tower above

the packing and thus emitted into the atmosphere together with the cooling tower plumes.
Annex XII

292 Industrial Cooling Systems

The inner side of the cooling tower shell including the upper ring beam must be completely

coated against corrosion. During the inlet of clean flue gases into the cooling tower,

condensate can flow down the cooling tower shell, which is, compared to concrete, heavily

aggressive due to its low pH value.

Concrete parts of the internal structure, e.g. the top framing of the fill supporting structure of

channel segments and riser heads respectively, must also be coated similar to the inner side of

the shell.

Steel parts, e.g. slides or handrails that might get into contact with acid condensate from the

plumes must be made out of special stainless steel.

The clean gas channel conducts the clean gases from the FGD21 building to the middle of the

cooling tower. It can be inserted into the cooling tower at the height of the FGD outlet (high

elevation) or right above the internal tower fill (low elevation). The maximum channel diameter

is about 8 m.

The clean gas channel should be made out of glass fibre reinforced vinylester or equivalent. To

this end, especially chemical-resistant moulding materials on the basis of penacryl resins and, as

textile processing, especially acid-resistant fibres out of ECR glass are to be used.

Due to the condensate formation inside the channel, it should have a slight inclination against

the cooling tower. For the outlet of the condensate, an outlet facility at the clean gas channel

inside the cooling tower is to be provided, leading into the cooling tower basin.

XII.6 Cost comparison between the various types of cooling

towers

The cost elements of cooling systems are mainly of three kinds:

• investment costs,

• costs related to energy consumption (i.e. efficiency),

• and maintenance costs.

For power plants, operating costs related to energy have to take into account the financial gain

that is linked to the difference in efficiency between different options. Generally for power

plants, the comparison of different options is executed through a socio-economic method based

on an ‘actualised’ balance with an ‘actualisation’ ratio that varies from country to country (e.g.

8% for France, 5% for Germany and Italy, 10% for Portugal). This method is described in

reference L. Caudron, "Les réfrigérants atmosphériques industriels", Collection de la Direction

des Études et Rechèrches d ́Électricité de France, 1991.

The ‘actualised’ balance is composed of:

• investment added with indirect charges to technical solutions considered I;

• algebraic ‘actualised’ expenses (maintenance of equipment) and receipts in operation

(production during the estimated life « tf »);

• « Pi » is the balance of expenses and receipts of the year « i », supposed in the middle of the

year.

The balance is represented by the following enumeration with « α »as the actualisation ratio:

I + Σi =1

i= tf Pi /((1 + α)

I-1⁄2)

With the expenses counted positively, the criterion of choice between different solutions is the

lowest actualised balance.

 

21 Flue gas desulphurisation.
Annex XII

Industrial Cooling Systems 293

In the case of mechanical draught cooling towers, one may suppose that the maintenance costs

are very similar because they are mainly linked to the maintenance of the fans. By taking into

account the first two criteria and by selecting the least expensive solution as the reference, Table

XII.3 shows that the wet system is much more economical than the dry system, natural draught

more than mechanical draught. From an economic point of view dry systems would be less

recommendable as they are more expensive and have a higher influence on the cost of kWh. So

dry systems may be recommended only in the case of lack of water.

Table XII.3: Comparison of different types of recirculating cooling systems with a lifetime of 25

years and an actualisation ratio of 8% (study on EDF units of 1300 MWe)

[L. Caudron, "Les réfrigérants atmosphériques industriels", editions Eyrolles]

Wet cooling tower

Wet/dry

cooling

tower

Dry cooling tower Type of refrigeration

system Natural

draught

Induced

draught

Induced

draught

Natural

draught

Induced

draught

Approach K (dry air

11°C/wet air

9°C)

12.5 12.5 13.5 16 17

Nominal condensation

pressure (mbar) 63 63 66 82 80

Thermal power (MWth) 2458

Electrical power

Delivered (MWe) 1285 1275 1275 1260 1240

Fan power MW 0 10 12 0 26

Pump power MW 13 13 8 14 13

Cost of refrigerant 1 1.25 2.3 5.7 4.8

Cost of cold end 1 1.1 1.6 3.6 3.1

Difference of cost of

kWh/cost of kWh (%) 0 1.0 2.4 8.4 8.9

Table XII.4: Comparison of wet cooling towers and aircooled condenser with a life-time of 20 years

and an actualisation ratio of 8% for a combined cycle unit 290 MWth

Wet cooling tower Type of refrigeration

system Once-through Natural

draught

Induced

draught

Air-cooled

condenser

Approach K (dry air

11°C/wet air 9°C) / ≈8 ≈8 ≈29

Nominal condensation

pressure (mbar) 34 44 44 74

Thermal power (MWth) 290 290 290 290

Difference of electrical

power delivered (MWe) + 0.6 0 0 - 1.8

Fan and pump power

(MW) 1.9 1.95 3 5.8

Global difference on

electrical power in

(MEuro)

-4.7 -2.9 0 12.6

Difference in cost on

water consumption

(MEuro)

-8.9 -8.9 0 0

Difference in cost of

cooling system in

(MEuro)

-3.0 1.9 0 8.9

Cost of cooling system 0.82 1.11 1 1.54

Global balance of costs

(MEuro) -16.5 -1.0 0 12.6
Annex XII

294 Industrial Cooling Systems

The same comparison can be done for combined cycle plants. Table XII.4 shows that dry

systems are more expensive again than wet systems, but the difference is smaller than in case of

conventional power plants. The difference between mechanical and natural draught is small and

is more or less comparable. Wet systems are preferred to dry systems. Maintenance costs,

eventual taxes for make-up or blowdown flows of water and costs of chemical products

necessary to the treatment of water are not taken into account in this table, which may

underestimate the cost of wet systems or overestimate the cost for dry cooling. Thus, dry

systems may be recommended depending on the price of water and water treatment for wet

systems or taking into account the lifetime of the plant, where a shorter lifetime reduces the

differences between dry and wet systems.

An important factor in cost comparisons is the efficiency or rather the loss of efficiency due to

cooling with less efficient cooling systems. This loss is measured in the dimensionless energytemperature factor kWth/MWth per degree temperature difference of the cooling water (in K). In

the following theoretical example this factor is derived [Paping, pers. comm.].

From the definition that 100 bar steam of 530 °C is equal to 3451 kJ/kg it follows (using Mollier

diagram) that:

50 [mbar] 32.7 [°C] 2110 [kJ/kg]

60 [mbar] 35.6 [°C] 2130 [kJ/kg]

70 [mbar] 38.8 [°C] 2150 [kJ/kg]

Above mentioned vacuum pressures and their related condensation temperatures are also related

to an average cooling water temperature in Europe of 15 °C together with a increase of 10 °C of

the cooling water in the condenser itself.

Including the heat transfer coefficient of the condenser, the condensate will leave it with a total

temperature of 30 °C and an inseparable vacuum pressure of about 43 mbar (see Table XII.3

and Table XII.4). Thus, to calculate the energy-temperature factor for increasing cooling

influent temperatures the calculation for is started with 50 mbar.

The efficiency is calculated following the Carnot cycle resulting in an efficiency, which is in

line with the commonly used 40% for conventional power plants:

at 50 mbar = (3451 - 2110) / (3451 - 4.18 * 32.7) x 100= 40.4609%

at 60 mbar = (3451 - 2130) / (3451 - 4.18 * 35.6) x 100= 40.0037%

at 70 mbar = (3451 - 2150) / (3451 - 4.18 * 38.8) x 100= 39.5583%

The minimum efficiency loss expressed per degree temperature difference under ideal

(thermodynamic) circumstances:

efficiency difference between 50 mbar and 60 mbar = 4.572%o per 2.9 K difference

efficiency difference between 60 mbar and 70 mbar = 4.454%o per 3.2 K difference

efficiency difference between 50 mbar and 70 mbar = 9.026%o per 6.1 K difference

This efficiency loss can be further expressed with respect to the total efficiency and per K:

4.572%o / (2.9 K * 0.4) = 3.9 kWth/MWth per K difference

4.45%o / (3.2 K * 0.4) = 3.5 kWth/MWth per K difference

9.026 %o / (6.1 K * 0.4) = 3.7 kWth/MWth per K difference

From this simplified calculation it appears that for an efficiency of about 40% the loss or gain

per degree of temperature difference of the cooling water can be estimated using the factor of

3.5 kWth/MWth per K.
Annex XII

Industrial Cooling Systems 295

XII.7 Choice of the treatment of circulating water alternative

methods - monitoring

As mentioned previously, problems of corrosion seldom arise in the cooling systems of power

plants. The use of corrosion inhibitor products is therefore unnecessary for cooling systems

cooled by raw water.

XII.7.1 Anti-scale treatment

With a wet cooling system, the only way of reducing heat discharges into the aquaticenvironment consists in the recirculation of the cooling water. This practice results in increasing

the concentration factor (Table XII.5). It is often applied for power plants located on inland

waterways and in estuaries.

This concentration tends to result in the precipitation of calcium salts that are not very soluble:

carbonate, sulphate, phosphate. The scale most commonly encountered is calcium carbonate. It

settles on the tubes of condensers and in the fill of cooling towers, which leads to a reduction in

efficiency. Two prevention techniques are generally used to avoid the precipitation of calcium

carbonate in the cooling systems of power plants. One is the lime softening of make up water

and the other is the vaccination of circulating water with sulphuric or hydrochloric acid.

Table XII.5: Relationship between the concentration factor, the withdrawn water flowrate and the

energy discharged into the receiving waterway (individual example)

Concentration factor

Withdrawn water flowrates

(m3/h)

Energy discharged into the receiving

waterway (%)

1 36000 100

1.2 3600 8.3

1.3 2600 5.5

1.4 2100 4.2

1.5 1800 3.3

2.0 1200 1.7

3.0 900 0.8

4.0 800 0.5

5.0 750 0.4

6.0 720 0.3

Only organic scale inhibitors for which there are ecotoxicological data are used within the limits

laid down by regulations. Their use is extremely limited. Indeed, current ecotoxicological data

are often insufficient. Furthermore, these substances released into the receiving waterway may

disturb the water treatment operations of industries situated downstream of the discharge point.

Coastal-sited power plants are generally cooled by a once-through system. Cooling towers

operating on an after cooling basis can be installed in order to reduce the thermal load. This

choice will mainly depend on local conditions (tides, mixture, etc.).

On the other hand, operation with sea water recirculation is exceptional. Indeed, high

concentration factors may cause the precipitation of a great many salts (calcium carbonate,

calcium sulphate, barium sulphate, etc.). Although the formation of calcium carbonate may be

avoided by adding acid, the same does not hold true for the other salts which can only be

stabilised by organic inhibitors (phosphonates, polyacrylates, copolymers, etc.).
Annex XII

296 Industrial Cooling Systems

XII.7.2 Anti-fouling treatments (biocides)

A recent review of the experience acquired in EUROPE in methods to reduce biological fouling

enables to draw the following conclusions:

Mechanical cleaning of the systems and water filtration are the most commonly used

processes. They involve the continuous cleaning of the tubes of condensers by foam balls or

brushes, manual cleaning, use of trash rakes, filters with meshes of different widths.

Three other physical methods are also regularly used for the anti-fouling treatment of

industrial systems. They concern the following:

• maintaining of velocities high enough to avoid the fixation of organic organisms (v > 2

m/s), this recommendation is applied to a large extent today;

• temperature increase which consists in raising the temperature of the cooling water beyond

40 oC for some dozens of minutes; this technique eliminates the fixed organisms (mussels),

but nevertheless requires an appropriate design of the cooling systems;

• non-toxic coatings and paints, which reduce the fixation of the organisms, reinforce the

velocity effect and facilitate cleaning; these coatings are nevertheless expensive and must be

renewed every 4 to 5 years.

Other techniques are sometimes used, the following in particular:

• dryout;

• installation of specific filters (mussel filters).

The physical methods can be applied both in sea water and soft water.

A non-chemical treatment applied in a few cases is UV.

Chemical treatment can be applied in cases where physical methods are not appropriate or

show insufficient results. There are oxidising products, chlorine, monochloramine, ClO2 and

ozone, which can be used as antifouling treatments. Some degradable organic compounds

applicable intermittently and non toxic in the receiving environment, might be an alternative to

chlorination. Among these, some amine filminducing polymers appear to be promising as anticorrosion chemicals, but sofar intermittent treatment of ferro-sulphate is more efficient.

XII.7.3 Monitoring

Given the flowrates of power plant cooling systems, one cannot conceivably operate them

without an advanced monitoring and control system. This reasoning is applicable both for

problems concerning scale and biological development.

To avoid scaling, the regulation of acid injection into the circulating water is generally subject

to the continuous monitoring of physico-chemical parameters such as: alkalinity, calcic

hardness, conductivity, temperature at the condenser outlet. A computer uses these various

parameters as a basis to calculate a specific scaling index and compares it with the operating instructions. If necessary, the regulator adapts the injection pump flowrate. Finer control methods

are also implemented in high-risk sites. They concern in particular the measurement of the

critical pH 4 and other scale monitors.

As concerns the follow-up of biological developments, many types of sensors exist and are

implemented. Among these should be mentioned biomonitors, and electrochemical sensors.

A control of the quality of drainage water is desirable in order to monitor parameters like

temperature, oxygen concentration, pH, conductivity etc.
Annex XII

Industrial Cooling Systems 297

XII.8 Design of the cooling system

As a non-neglecting requirement it should be recognised that the adoption of cooling water

systems at a particular site can be the collaboration of many different factors. The most obvious

one is the site-specific characteristics.

XII.8.1 Design and energy recovery

In conventional thermal power plants, the thermodynamic cycle imposes the overall efficiency

of the facility. The economiser, superheater and reheater optimise the operation of the boiler.

The low and high-pressure superheaters raise the temperature of feedwater by recovering part of

the energy withdrawn by means of steam extractions. In order to reduce the electrical

consumption of the auxiliaries, turbine-driven feedwater pumps are also used, these being also

supplied by steam extractions. Combustion air is also heated by air heaters prior to entering the

boiler. All of these devices have one objective: reduce the energy losses of the cycle.

Thermodynamic laws govern the energy loss in the condenser.

If energy gains can be obtained in the cooling systems, it is mainly at the level of design and the

resulting choices that it is possible to do so. Some golden rules may be applied:

• limit the number of pumps,

• avoid mechanical draught cooling towers,

• if cooling tower is needed, prefer wet cooling tower to recovery systems (recuperators),

• if deconcentration flowrates are sufficient, install a hydraulic recovery turbine on the

deconcentration blowdown;

• where flowrates must not be constant, use frequency variators on the pumps or fans.

The following conclusions therefore emerge from these observations:

• two sets of pumps are sufficient, one for the supply of the auxiliary cooling system, the

other for the main cooling system;

• if once-through cooling is not possible, natural draught wet cooling towers should be

preferred to other cooling systems;

• two schemes are therefore conceivable (Figure XII.13 and Figure XII.14 in Section XII.11)

and make it possible to eliminate the heat from the auxiliary system via the cooling tower.

XII.8.2 Design and noise reduction measures

The reduction of noise of the cooling systems can be carried out in different ways:

• installation of anti-noise walls around cooling towers,

• modification of the relief of the site (wooded slopes),

• choice of "low noise" fans,

• utilisation of anti-noise panels.

These various solutions generally make it possible to meet stringent noise restrictions.

XII.8.3 Implementation of physical methods

Right from the design stage, it is absolutely necessary to reflect on the possibilities of

implementing physical methods, particularly so as to avoid biological developments. It concerns

the following in particular:

• guarantee an adequate velocity in all the portions of the system;

• install continuous cleaning systems on all heat exchangers whenever this is technically

possible;

• provide for mussel filters on sites at risk;
Annex XII

298 Industrial Cooling Systems

• design the systems so as to be able to carry out manual cleaning operations under normal

operating conditions (alternate heat exchanger operation);

• design the systems so that temperature increase is possible (recirculation with cooling tower

in by-pass);

• in natural draught wet cooling towers prefer fill with a suitable surface and/or structure to

reduce fouling; periodic cleaning should be optional, e.g. in case of high contents of solids

in the used cooling water.

XII.8.4 Modelling and pilot tests

The purpose of modelling is to study any physico-chemical impacts and adapt to the facilities

so as to reduce these impacts to the greatest extent possible. It is particularly important to study:

• water withdrawals and discharges,

• the visual aspect of the site,

• the evolution of plumes,

• the thermal and chemical impacts on the receiving environment.

The objective of the pilot loop tests is to define the optimum treatment of cooling water both

with regard to scaling and to any biological developments. To do so, pilot facilities

representative of real commercial operating conditions are installed on the site for up to one

year. In general those pilot test should last for a period with a minimum length which makes it

possible to integrate the variations of the quality of the cooling water due to seasonal

differences. This also serves to assess the opportunity of some choices on a representative scale

(examples: choice of cooling tower fill, choice of alloys, etc.).

XII.8.5 Choice of the cooling system

As can be seen in the examination of the previous analysis, the choice of the type of cooling

system essentially depends on local site-specific conditions. It is therefore extremely difficult

and may not be appropriate to offer a unique recommendation. The decision-making logic

diagram shown in (Figure XII.15, Section XII.11) gives an idea of all the conceivable cases

involved.

From the energy standpoint, wet cooling (once-through cooling if necessary with wet cooling

tower) is by far the most economical solution combined with the ecological advantage of saving

energy and avoiding flue gas emissions. Whether it is carried out using the once-through

technique or via a circulating system with wet cooling tower, the energy balances are favourable

to this solution.

Of course, such wet cooling can only be envisaged if the receiving waterway is able to

accomodate it. Within the scope of the sustainable management of water resources, it is

absolutely essential that this point has to be examined carefully, particularly by taking into

account future developments. A long-term modelling, integrating statistical data, is a necessary

tool for estimation and assessment of the environmental impacts. It is in the basis of this

essential approach that the choice of the cooling mode, concentration factor and any treatments

must be made.

XII.9 Conclusions

A BAT approach for the cooling systems of new thermal power plants requires a series of

reflection points:
Annex XII

Industrial Cooling Systems 299

1. the need to carry out prior studies concerning site-conditions;

2. the choice of corrosion-resistant materials for the heat exchange surface of condensers and

cooling towers;

3. the implementation of local protection (paints, cathodic protection, etc.):

4. the reduction of energy consumers (fans, pumps);

5. the installation of anti-noise systems (walls, panels, modification of the site-relief, etc.) or

the choice of solutions resulting in lower emissions (low noise fans);

6. the optimisation of the use of treatment reagents and the setting up of (bio)monitors and

chemical monitoring and control devices;

7. the study of systems so as to be able to carry out temperature increase operations;

8. the design of water intakes to limit the drawing in of living organisms;

9. a control of the quality of water discharges by the drain (temperature, oxygen, etc.).

Points 3, 4, 5, 6 and 9 are also relevant for existing power stations as they concern the way a

plant is operated and maintained. The other issues relate to the site assessment, which in for an

existing installation is a given fact. With respect to those points, the result of an evaluation for

an existing installation may lead to a considerable change in the design of an existing cooling

system, which generally is expensive and likely to be not cost effective. In those situations the

depreciation time of the installation (heat exchanger, intake structure), will affect any possible

change resulting from a site assessment.

From the experience no single solution seems to have emerged. Each case is a specific one and

depends, for example, on the cycle of the power plant. In the case of units with recirculating

systems, the choice of the water treatment will depend on the concentration factor selected,

maximum temperatures and the quality of the water withdrawn. The same holds true for the

fight against biological developments. Although macroorganisms can generally be eliminated

by thermal shocks, this solution cannot be applied to eliminate biofilm.
Annex XII

300 Industrial Cooling Systems

XII.10 Literature

-B. Vincent, "Exploitation et maintenance de la source froide", Revue Générale Nucléaire, n° 3,

247-257, mai/juin 1986.

-R. Gras et J. Jacquet "Problèmes d ́environnement liés à la source froide des centrales

thermiques", École d ́Eté de mecanique des fluides, 353-393, 6-10 octobre 1975.

-H.A. Jenner, C.J.L. Taylor, M. Van Donk, M. Khalanski, "Chlorination By-products in

Chlorinated Cooling Water of some European Coastal Power Stations", Marine

Environmental Research, vol. 43, n°4, pp. 279-283, 1997.

-R. Ambrogi, "Environmental Impact of Biocidal anti-fouling Treatment by Chlorine Dioxide".

Proceedings of the First European Symposium on Chlorine Dioxide. Collana ambiante,

vol. 17, pp. 119-132, Rome, 1996.

-L. Duvivier, "Etude des divers procédés de lutte contre l ́entartrage des circuits de réfrigeration

des centrales électriques", Travail de Fin d ́études, Université Catholique de Louvain,

Facultes des Sciences Appliquées, juin 1988.

-G. Gutner, "Aéroréfrigérants secs et mixtes", École d ́Eté de mecanique des fluides, 60-69, 6-

10 October 1975.

-Proceeding: Cooling Tower and Advanced Cooling Systems Conference, EPRI TR-104867,

February 1995.

-L. Duvivier, "Concentration et conditionnement des eaux de surface intérieures dans les

circuits de refroidissement atmopshériques des grandes centrales thermiques", 36 émes

Journees internationales du CEBEDEAU, 205-234, Liege, 25-27 mai 1983.

-Deleval, Duvivier, Hosdain, "Lutte contre l ́entartrage des circuits de réfrigeration des centrales

électriques", 9th Cooling Tower and Spraying Pond Symposium-International

Association for Hydraulic Research, Institut von Karman, 20-23 septembre 1994.

-L. Duvivier, "Considérations générales relatives à l ́entartrage des circuits de refrigeration

industriels", Proceedings Journées Information Eaux, Vol. 1, 311-3111, 18-20

septembre 1996.

-H.A. Jenner, J.P.M. Mommen, "Driehoeksmosselen en aangroeiproblemen", H2O, 18de

jaargang, nr. 1, pp. 2-6, 1985

-Guide de l ́environnement non radioactif - Protocole de chloration massive des réfrigerants,

EDF-SPT, Départment Sécurité - Radioprotection - Environnement, juillet 1984.

-Aprosi, Bidard, Nepveu de Villemarceau, "Expérience d ́expolitation des réfrigérants des

centrales françaises: hydrobiologie - chimie aquatique", Revue Generale Nucléaire, n°

5, 425-429, septembre/octobre 1986.

-Rook, "Formation of Haloforms during Chlorination of Natural Waters", Water Treatment and

Examination, 23 (part 2), 234, 1974.

-Bellar, Lichtenberg, Kroner, "The Occurrence of Organohalides in Clorinated Drinking

Waters", JAWWA, 66, 73, December 1974.

-Amy et al., "Evaluation of THM Precursor Contributions from Agricultural Drains", JAWWA,

57-64, January 1990.
Annex XII

Industrial Cooling Systems 301

-G. Asplund, A. Grimvall, "Organohalogens in Nature - More widespread than previously

assumed", Environmental Science and Technology, 25, pp. 1346-13450, 1991.

-L. Duvivier, "Formation et élimination des organo-halogénés lors de la desinfection des eaux",

Thése de Doctorat en Sciences Appliquées, Université Catholique de Louvain, juin

1993.

-C.T. Jafvert, R.L. Valentine, "Reaction Scheme for the Chlorination of Ammoniacal Water",

Environ. Sci. Technol., Vol. 26, n° 3, 1992.

-Bilello, Singley, "Removing Trihalomethanes by Packed Column and Diffused Aeration",

JAWWA, 62-71, February 1986.

-Munz, Roberts, "Air-Water Phase Equilibria of Volatile Organic Solutes", JAWWA, 62-69,

May 1987.

-Isaac, Morris, "Rates of Transfer of Active Chlorine between Nitrogenous Substances", Water

Chlorination - Environ. Impact and Health Effects, Vol. 3, 1980.

-Weil, Morris, "Kinetics Studies on the Chloramines. The Rates of Formation of

Monochloramine, N-chloromethlamine and N-chlorodimethylamine", Jour. Am. Chem.

Soc., Vol. 71, 1979.

-4 S.A. Hubbs et al., "Use of Chlorine Dioxide and Chloramines as alternate Disinfectants al

Louisville", Proceedings AWWA Seminar, AWWA Conference, pp. 51-60, Atlanta, 15

June 1980.

-Neden et al., "Comparing Chlorination and Chloramination for Controlling Bacterial

Regrowth", JAWWA, 80-88, July 1992.

-"Centrale de CHOOZ B1 B2-Examen des Salissures biologiques- Rapport de synthèse",

Rapport LABORELEC CO3-800-95-005F/LDU/RVM, 22/03/05.

-"CHOOZ B - Essais sur poste pilote. Conditionnement des eaux de réfrigération", Rapport

LABORELEC CO3/06578 - 1, 15/01/1991.

-"Eaux de réfrigération: Centrale de Seraing-Résultats des essais en station pilote", Rapport

Laborelec SER/ER/LD-CO3, 04/11/92.

-"Eaux de réfrigération: Centrale de Drogenbos-Résultats des essais en station pilote", Rapport

Laborelec DROG/ER/LD-CO3, novembre 91.

-L.C. Neale, "Cooling Problems of Thermal Power Plants - Physical Model Studies", École

d ́Eté de mécanique des fluides, 159-180, 6-10 October 1975.

-J.P. Benque, Modéles mathématiques des rejets thermiques", École d ́Eté de mécanique des

fluides, 184-200, 6-10 octobre 1975.

-J. Smitz, "Pégase-Planification et gestion de l ́assainissement des eaux", Ministere de la Région

Wallonne, Rapport de Synthese, 1991.

-Ernst, Winkler, "Flow Patterns of Cooling Tower Plumes in the near atmosphere Results from

a mathematical model", 9th Cooling Tower and Spraying Pond Symposium -

International Association for Hydraulic Research, Institut von Karman, 20-23

September 1994.

-„Cycle Chemistry Guidelines for Fossil Plants: All Volatile Treatment", EPRI, TR-105041,

April 1996.
Annex XII

302 Industrial Cooling Systems

-Mortensen, Conley, "Film Fill Fouling in Counterflow Cooling Towers: Mechanism and

Design", Cooling Tower Institute, Paper TP94-05, Houston, February 13-16, 1994.

-Montjoie, Noble, Mirsky, "Research of Fouling Film", Cooling Tower Institute, New Orleans,

February 17-19, 1993.

-Duvivier, van Damme, Bolsée, "Le choix des corps d ́échange des tours de réfrigération et leur

influence sur les phénomènes d ́entartrage par le carbonate de calcium", Proceedings

Journées Information Eaux, Vol. 1, 371-3711, 18-10 septembre 1996.

-Dedieu, "Etude expérimentale de l ́entartrage des corps d ́échange de réfrigérants industriels",

Thése de doctorat INSA Toulouse, 05/06/97.

-J.P. Fesson, "Le primage et la séparation dans les réfrigérants", École d ́Eté de mecanique des

fluides, 202-215, 6-10 octobre 1975.

-G. Ribier, "Optimisation dans le calcul thermique et la détermination d ́un réfrigérant

atmosphérique à tirage naturel"; École d ́Eté de mecanique des fluides, 217-229, 6-10

octobre 1975.

-Manoha, Lepeintre, Pechon, "Le Modéle TELEMAC-3D pour les écoulements

tridimensionnels: de nouvelles perspectives pour les études d ́environnement",

Hydroécologie Appliquee, Tome 4, Vol. 1, 13-20, 1992.

-Nagel, "New Developments in Air-Cooled Steam Condensers and Dry cooling Towers", 9th

Cooling Tower and Spraying Pond Symposium - International Association for

Hydraulic Research, Institut von Karman, 20-23 September 1994.

-van der Spek, "Advanced Low Noise Cooling Fan", 9th Cooling Tower and Spraying Pond

Symposium - International Association for Hydraulic Research, Institut von Karman,

20-23 September 1994.

-Kosten, Wyndrum, Wet, Dry and Hybrid Systems - A Comparison of the Thermal

Performance", EPRI - Conference on Cooling Towers and Advanced Cooling Systems,

St Petersburg, August 3, 1994.

-Burns, Nicholson, Annett, Alexander, "The Impacts of Retrofitting Cooling Towers at a Large

Power Station", EPRI - Conference on Cooling Towers and Advanced Cooling

Systems, St Petersburg, August 3, 1994.

-Lindahl, Jameson, "Plume Abatement and Water Conservation with the Wet/Dry cooling

Tower", EPRI - Conference on Cooling Towers and Advanced Cooling Systems, St

Petersburg, August 3, 1994.

-Gill et al. "A Mechanistic Approach to the Development of Chemical Solutions for Fouling of

Cooling Tower Film Fills", EPRI - Conference on Cooling Towers and Advanced

Cooling Systems, St Petersburg, August 3, 1994.

-J.F. Commaille, "Perturbations apportées par le rejet de produits tartrifuges dans les rivières sur

les traitements physico-chimiques ultérieurs de ces eaux", Thèse de doctorat, INSA

Toulouse, 25 octobre 1982.

-L. Duvivier, "Elimination des tartrifuges avant rejet. Conséquences industrielles", Tribune du

Cebedeau, n° 487-488, 37, 247-252, 1984.

-"Zebra Mussel Monitoring and Control Guide", EPRI TR-101782, December 1992.
Annex XII

Industrial Cooling Systems 303

-Leyen et al., "Mitigation of Macrofouling: A Comparative Study of Ozone and on Organic

Filminducing Polymer", 7th International Zebra Mussel and Aquatic Nuisance Species

Conference, New Orleans, January 28-31, 1997.

-Duvivier et al., "Do we need Chlorine for Water Treatment?", Proceedings Power-Gen  ́96

Europe, Vol. 1, 757-770, June 96.

-Duvivier et al., "Fighting Zebra Mussel Fouling with Ozone", EPRI-Service Water System

Reliability Improvement Seminar, Daytona Beach, June 25-27, 1996.

-K. D ́Hondt, "On-line Analyse van de "Kritische pH", Applitek, Vergadering Scheikunde,

Laborelec, 28/04/93.

-Lin et al., "Surveillance de la menace d ́entartrage du circuit de refroidissement d ́une centrale

nucléaire par un capteur électrochimique automatique", Proceedings Journées

Information Eaux, Vol. 1, 331-3316, 18-20 septembre 1996.

-G.J. Licina, "Monitoring Corrosion and Biofilm Formation in the Emergency Service Water

System at Susquehanna", EPRI-Service Water System Reliability Improvement

Seminar, Daytona Beach, June 25-27, 1996.

-L. Caudron, "Les réfrigérants atmosphériques industriels", Collection de la Direction des

Etudes et Recherches d ́Electricité de France, 1991.

-Y. Coeffe, P-M. Clique, B. Manoha, "Thermal Impact Studies for French Coastal Nuclear

Sites", 18th ICCE. Cape town, pp 2342-2356, 1982.

-M. Darras, J. Montfort, J.F. Parent, "Ouvrages de prise et de rejet d ́eau, conception et

réalisation. A. The Dilution of Heated Discharges of Power Plants. B. Seashore Power

Stations and Protection of the Marine Environment", Rapport EDF DER HE/ 42/85.18,

1985.

-F. Bordet, "Conception des ouvrages d ́eau des centrales nucléaires littorales", Rapport EDF

DE E-3011, 1983.

-B. Manoha, "Three-dimensional numeric Modelling of Thermal impact for Gravelines Nuclear

Power Plant", 23rd IAHR Congres. Ottawa, 1989.

-A. Gilbert, R. Gras, D. Roult, "Numeric Computation of Natural River Temperatures",

International Conference on Water Quality Modelling in the Inland Natural

Environment. Bornemouth England, paper M1, pp. 457-472, 10-13 June 1986.

-A.W.H. Tumpenny, T.E. Langford, R.J. Aston, "Power Stations and Fish", CEGB Research,

pp. 27-39, April 1985.

-F. Travade, "Aspiration d ́organismes aux prises d ́eau des centrales", Revue générale

nucléaire, n°1, pp. 59-62, 1987.

-EDF International, "Cordemais Coal-Fired Power Station. Units 4 and 5. Environmental Impact

Study", 1987.

-J.W. Whitehouse, M. Khalanski, M. Saroglia, "Marine Macrofouling Control Experience in the

UK with an overview of European Practices", Proceedings of the Symposium on

Condenser Macrofouling Control Technologies, Hyannis, Mass. EPRI report CS 3343,

pp. 17.1-17.16, Dec., 1983.
Annex XII

304 Industrial Cooling Systems

-J.W. Whitehouse, M. Khalanski, M. Saroglia, H.A. Jenner, "The Control of Biofouling in

Marine and Estuarine Power Stations", Joint report of CEGB, EDF, ENEL and KEMA,

48 pp., 1985.

-M. Khalanski, G. Aprosi, F. Travade, "Biofouling Control in Power Station Circuits. Overview

of Electricité de France ́s experience", Condenser Biofouling Control Symposium; the

State of the Art, EPRI Meeting, Lake Buena Vista, Florida, 1985.

-M. Khalanski et Ph. Lutz, "La chloration de l ́eau de réfrigération", Revue Générale Nucléaire,

n°1, pp. 52-58, 1987.

-NCASI, "Naturally occurring Organohalogen Compounds - A Survey", Technical Bulletin,

n°629, 1992.

-H. Palm, R. Lammi, "Fate of Pulp Mill Organochlorines in the Gulf of Bothnia Sediments",

Environmental Science and Technlogy, 29, pp. 1722-1727, 1995.

-H. Kankaanpää, J. Tossari, "Background Levels of EOX and AOX in Sediments of the Gulf of

Finland. Molecular Weight Distribution of EOX in Sediments", Chemosphere, 28, pp.

99-116, 1994.

-3 G. Asplund, "The Origin of Organohalogens found in the Environment", Linköping Studies

in Arts and Science, n° 77, Linköping University, Sweden, 1992.

-Ph. Gosse, M. Khalanski, "La modelisation numerique appliquee a l ́evaluation d ́impact

hydrobiologique", Revue Generale Nucleaire, n° 1, pp. 46-51, 1987.

-M.J. Saleçon, J.M. Thébault, "Modélisation d ́écosystème lacustre" Masson, ISBN 2-225-

85627-3, 1997.

-G. Aprosi, C. Nepveu de Villemarccau, "French operational Experience of fouling Problems

(Algae, Bryozoa) in cooling Towers", 6th IAHR Cooling Tower Workshop, Pisa, Oct.

4-7, 1988.

-J.C. Moretteau, M. Khalanski, "Settling and Growth of D. polymorpha in raw Water Circuits of

the Cattenom Nuclear Power Plant (Moselle, France)", Proceedings: Fourth

International Mussel Conference, Madison, EPRI Report, March 1994.

-M. Khalanski; "Zebra Mussel and other invasive Species in cooling Water Circuits of French

Power Stations on Rivers", Seventh International Zebra Mussel and Aquatic Nuisance

Species Conference, New-Orleans, 28-31 Jan. 1997.

-H.A. Jenner, J. Whitehouse, C. Taylor, M. Khalanski, "Cooling Water Management in

European Power Stations", to be published in Hydroécologie Appliquée.

-B. Comby, "Un écologiste pour le nucléaire", La compagnie du livre, ISBN 2-84155-035-4,

1994.

-VGB-Richtlinie für die Planung von Kühlturmanlagen, R 135 P, 1997.

-VGB-Richtlinie „Empfehlung für den Winterbetrieb von Kraftwerks-Naßkühltürmen“, R 129

P, 1988

-VGB-Richtlinie „Einsatz von Holz im Kühlturmbau“, R 611 U, 1996

-DIN 1947: Wärmetechnische Abnahmemessungen an Naßkühltürmen.
Annex XII

Industrial Cooling Systems 305

-VGB-Richtlinie: "BTR Bautechnik bei Kühltürmen", R 610 U, 1997.

-VGB-"Kühlwasser-Richtlinie", R 455 P, 1990.

-VGB-Merkblatt "Aufbereitung von Betriebswasser und Kühlturm-Zusatzwasser durch

Entcarbonisierung bzw. Flockung und Entcarbonisierung", M 403 G, 1992.

-VGB-Fachbuch: Bautechnik in Wärmekraftwerken, B 009, 1993.

-Tesche, W.; Planerische Berücksichtigung von Rezirkulation bei Zellenkühltürmen in

Reihenanordnung. VGB-TB 110, VGB-Kraftwerkstechnik GmbH, Essen (1996).

-VDI-Wärmeatlas: Berechnung von Rückkühlwerken.

-Held: Kühlwasser-Verfahrenstechnik und chemische Methoden für die Kühlwasserbehandlung

in Industrie und Kraftwerken, Vulkan-Verlag, Essen (1994).

-DIN EN 45 531: Bestellrichtlinien Turbinenhilfseinrichtungen, Trockenkühlturm.

-DIN EN 45 532: Bestellrichtlinien Turbinenhilfseinrichtungen, Naßkühlturm.

-VDI 3734 B12: Emissionswerte technischer Schallquellen; Rückkühlanlagen; Kühltürme,

1990.

-Abwärme-Auswirkungen, Verminderung, Nutzen. Bericht 82-3 der Abwärmekommission

1982.

-Auswirkungen von Kühltürmen. Bericht 82-1 der Abwärmekommission, 1982.

-Ernst, Wurz: Naturzug-Naßkühlturm des Kernkraftwerks Philippsburg (Block 1).

Untersuchungen des Betriebsverhaltens der Emissionen und der Schwadenausbreitung.

Fortschritt-Bericht R. 15, Nr. 25 der VDI-Zeitschriften, 1983.

-Ernst Schnabel: Naturzug-Naßkühlturm des Kernkraftwerkes Philippsburg (Block 1).

Ergebnisse der Schwadenausbreitungsrechnungen. Fortschritt-Berichte R. 15 Nr. 30 der

VDI-Zeitschriften, 1984.

-Baer et al.: Thermodynamische Untersuchungen am Naturzug-Naßkühlturm des Kraftwerks

Neurath und Modelle für das Betriebsverhalten und die Schwadenausbreitung,

Fortschritt-Berichte R. 15 Nr. 7 der VDI-Zeitschriften.
Illustrations

306 Industrial Cooling Systems

XII.11 Illustrations

Figure XII.1: Once-through system

Figure XII.2: Wet cooling tower
Illustrations

Industrial Cooling Systems 307

Figure XII.3: Once - through cooling with cooling tower

Figure XII.4: Recirculating cooling
Illustrations

308 Industrial Cooling Systems

Figure XII.5: Mechanical draught cooling tower (pressure fans)

Figure XII.6: Mechanical draught cooling tower (suction fans, cell construction)
Illustrations

Industrial Cooling Systems 309

Figure XII.7: Mechanical draught cooling tower (pressure fans, cell construction)

Figure XII.8: Hybrid cooling tower
Illustrations

310 Industrial Cooling Systems

Figure XII.9: Forced draught air-cooled condenser

Figure XII.10: Natural draught air-cooled condenser
Illustrations

Industrial Cooling Systems 311

Figure XII.11: Closed recirculating indirect dry cooling tower

Figure XII.12: Cooling tower with discharge of cleaned flue gas
Illustrations

312 Industrial Cooling Systems

Figure XII.13: Cooling system with fixed concentration factor

Figure XII.14: Cooling system with sliding concentration factor
Illustrations

Industrial Cooling Systems 313

Figure XII.15: Decision-making logic diagram for the choice of the cooling system