American Petroleum Institute

Characterizing 

Cooling Water Systems 

in Petroleum Refineries

Briefing Report
Page i

TABLE OF CONTENTS

Page

Executive Summary............................................................................................v

I. Introduction..........................................................................................................1

II. Cooling Water Use Description............................................................................3

II.1 What drives the need for cooling in petroleum refineries?.................................3

II.2 What types of equipment require cooling? ........................................................5

II.3 What are the types of cooling water systems?..................................................7

II.4 Are there specific requirements/limitations associated with cooling water 

usage?..............................................................................................................9

II.5 How are these cooling needs and requirements/limitations met?....................11

II.6 What dictates which cooling approach is used?..............................................12

II.7 What volumes of cooling water are required for various approaches? ...........14

III. Recirculated Cooling Water Technology............................................................17

III.1 What are the components of a Recirculated cooling tower system? ...............17

III.2 What are the limitations of using recirculated systems?..................................18

III.3 What must be considered when increasing cycles of concentration in 

cooling towers? ?............................................................................................19

III.4 What quality/quantity of make-up water is required and how is it 

determined?....................................................................................................22

III.5 Effects of brackish vs. fresh water? ................................................................24

III.6 Blowdown flows, handling, treatment?............................................................25

III.7 Factors affecting drift and evaporation losses?...............................................26

III.8 Factors affecting chemical usage and concentrations?...................................27

IV. Converiting from Once-through to Cooling Towers ............................................28
Page ii

IV.1 What would be involved in changing from once-through cooling to cooling 

towers?...........................................................................................................28

IV.2 Effects on wastewater discharge volumes and quality? ..................................30

IV.3 Effects on metallurgy in piping and exchangers? ............................................31

IV.4 Effects on refinery piping configurations? .......................................................32

IV.5 Effects on heat exchanger capacity? ..............................................................33

IV.6 Additional space requirements?......................................................................35

IV.7 Changes in water treatment requirements? ....................................................36

IV.8 Changes in water pressure requirements? .....................................................37

V. Changing to Dry Cooling....................................................................................38

V.1 What would be involved in changing to dry cooling?.......................................38

VI. Cooling Water Recycle and Reuse ....................................................................39

VI.1 What are the options for recycle/reuse of cooling water? Practicality?...........39

VII. Cooling Water Intake Structures ........................................................................40

VII.1 What types of intake structures are used at refineries.....................................40

VII.2 What is involved in retrofitting refinery intake structures (e.g., moving 

screens)?........................................................................................................41

VII.3 Other information on cost-effectiveness of intake structure controls? .............42

VIII. Cost Implications ...............................................................................................43

VIII.1 Cursory review of the EPA’s “Draft Initial Cost Estimates”-- May 17, 2001?..43

VIII.2 Estimate the cost of installing a 17,400 gpm cooling tower .............................46

IX. References ........................................................................................................51

Appendix A EPA Draft Initial Cost Estimate

Appendix B Quote for Cooling Tower

Appendix C Quote for Pumps
Page iii

Appendix D Quote for Clarifier

Appendix E Quote for Traveling Water Screen
Page iv

LIST OF FIGURES

Page

Figure 1 Typical Distillation System .............................................................................4

Figure 2 Once Through Cooling System ......................................................................7

Figure 3 Closed Loop Cooling System.........................................................................7

Figure 4 Evaporative Cooling System ..........................................................................8

Figure 5 Makeup & Blowdown vs. Cycles of Concentration........................................23

Figure 6 Cooling Tower System for Estimate .............................................................46
Page v

Executive Summary

Section 316(b) of the Clean Water Act requires EPA to ensure that the location, design, 

construction and capacity of cooling water intake structures reflect the best technology 

available for minimizing adverse environmental impact. EPA is developing national 

standards in three phases: Phase I for new facilities, Phase II for existing electric 

generating plants that use large amounts of cooling water, and Phase III for manufacturing 

plants and for electric generating plants using smaller amounts of cooling water. By June 

2003, EPA must propose the Phase III regulations addressing existing manufacturing 

facilities (which would apply to existing petroleum refineries) and EPA must issue final 

Phase III regulations by December 2004. The proposed Phase II regulations include a 

requirement to limit cooling water intake flow to that commensurate with closed loop 

recirculation of cooling water. This requirement is an option EPA may consider for the 

Phase III regulations, as well as an option to require air cooling in some cases as a means 

to limit cooling water intake flow.

This report details the significant technical issues and costs associated with implementing 

either closed-loop flow reduction or air cooling at petroleum refineries. The environmental 

benefit realized by either of these options would not justify the high cost to the industry. 

EPA should take the information presented in this report into account when drafting its 

Phase III rule, and should not require the petroleum industry to install air cooling or reduce 

intake flows to that commensurate with closed loop recirculation. EPA should instead 

provide the petroleum industry the option of minimizing adverse environmental impact 

through the installation, operation, and monitoring of control technologies effective at 

reducing impingement and entrainment at the cooling water intake. Depending on sitespecific conditions and the particular technology selected, control technologies have been 

shown to be up to 90 -100 percent effective at reducing impingement and entrainment. 

Technical and Economic Feasibility of Closed-Loop Recirculation and Air Cooling

EPA may believe that the choice of cooling medium, and the switch from once-through 

cooling to complete recirculation of cooling water with the installation of cooling towers, is 

readily achieved at manufacturing facilities, with little effect on process operations. In 

practice, however, the choice of once-through cooling vs. recirculation is a complex 

decision based on numerous site-specific considerations. In the refining industry, a 

requirement mandating a particular cooling medium (water vs. air) or configuration (oncethrough vs. recirculation) will have significant technical and cost implications to process 

operations. EPA needs to understand the complex nature of cooling operations at 

refineries and the significant potential impacts to the refining industry associated with 

these regulations. EPA should take the following implications to the refining industry into 

account when considering Phase III regulations that mandate cooling water recirculation, 

cooling tower installation, and/or air cooling:
Page vi

 A recirculation system cannot provide cooling water at temperatures as low as a 

once-through system, which provides cooling water at the temperature of the water 

source. This has significant process implications at refineries. For example, a key 

factor to refinery profitability is to cut as deeply into the vacuum tower residue as 

possible. This is achieved by pulling as much of a vacuum as possible in the 

vacuum column, but this vacuum requires low condensing temperatures, which in 

turn requires cooler water. 

 The recirculation temperature affects the design, sizing, and cost of all water-cooled 

heat exchange equipment within the refinery; the higher cooling temperature will 

require larger and more costly heat exchange equipment to achieve the same 

cooling duty as a once-through system. Depending on specific space and pressure 

drop limitations, it may not be possible to add incremental heat exchange; wholesale 

replacement may be required. 

 Installation of a cooling tower and related equipment (sumps, pumps, piping, etc.) is 

dictated by the unit operations being conducted at the particular refinery and by the 

space available. While it might be optimal to install one large cooling tower to 

replace an existing once-through system, space and logistics will likely require 

building multiple cooling towers. This invariably leads to increased costs and 

operational complexity.

 Recirculation systems depend on evaporation to remove heat and thereby increase 

the concentration of hardness and dissolved solids in the cooling water system. If 

the dissolved solids content of the water is high it will limit the recirculation rate. 

Unless the quality of the intake water is very high, intake water pretreatment will be 

needed to avoid scaling or corrosion in the recirculation system. Such pretreatment 

may entail precipitation, water softening, pH adjustment, and suspended solids 

removal. 

 It is necessary to treat recirculating cooling water with a variety of chemicals to 

control scaling, deposition, biological growth and corrosion. Sulfuric acid is most 

often used to control pH. Corrosion inhibiters must be added to prevent corrosion. 

Slimicides are used to control biological growth.

 Facilities will typically treat the blowdown from cooling tower systems in the existing 

wastewater treatment plant and the extra loadings may drive the need for increased 

capacity. Cooling tower blowdown contains high dissolved solids as well as those 

chemicals added to prevent corrosion, scale, and biological growth. The need to 
Page vii

treat blowdown can significantly reduce residence time and therefore overall 

efficiency in the treatment system.

 Direct air cooling used in refineries is dictated by the dry bulb temperature at that site 

and conditions of the process streams such as inlet temperature and required outlet 

temperature. Air cooling can be used in refineries only for specific applications. 

Many process requirements will still require the installation of cooling water 

exchangers downstream of the air coolers. Air-cooled exchangers tend to be large, 

require significant plot space, and can be a source of noise. 

 Replacement of once through cooling with air-cooled heat exchangers would 

demand an economically infeasible number of changes to the existing process 

equipment. Furthermore, whereas cooling water shell-and-tube exchangers can be 

stacked vertically in process units, air coolers cannot be stacked (because lower 

units would send heated air to higher units) and so it may be physically difficult or 

impossible due to space constraints to replace water-cooled exchangers with aircooled exchangers.

 Were conversion of a once-through cooling water system a refinery even feasible, 

API’s estimate of the cost of such a conversion is much higher than that estimated by 

EPA. For a 17,400 gallon/minute system at a refinery, API estimates an installed 

cost of $9.2 million, with an annual operation and maintenance cost of $334,600. 

EPA estimated an installed cost of $1.34 million and an operation and maintenance 

cost of $179,000 for such a facility. This cost is an extremely burdensome cost to 

achieve a limited environmental benefit.
Page viii

 
Page 1 of 50

I. Introduction

Section 316(b) of the Clean Water Act requires EPA to ensure that the location, design, 

construction and capacity of cooling water intake structures reflect the best technology 

available for minimizing adverse environmental impact. For many years, this provision has 

been implemented without federal standards in place, on a resource intensive, site-by-site 

basis. EPA is developing national standards in three phases: Phase I for new facilities, 

Phase II for existing electric generating plants that use large amounts of cooling water, 

and Phase III for manufacturing plants and for electric generating plants using smaller 

amounts of cooling water. EPA issued the Phase I regulations for new utilities and 

manufacturing facilities on December 18, 2001. In March 2002, EPA plans to issue the 

Phase II proposed rules for existing utilities, with the goal for making them final by August 

2002. By June 2003, EPA must propose the Phase III regulations addressing existing 

manufacturing facilities (which would apply to existing petroleum refineries) and EPA must 

issue final Phase III regulations by December 2004. API expects the provision for the 

existing utilities (Phase II) will be the basis for developing the Phase III rules.

Consequently, the Phase II regulations can have significant cost impacts on petroleum 

refineries. Some of the potential costs range from costs for studies and restoration 

projects for offsetting perceived adverse environmental impacts, to adding or retrofitting 

impingement reduction equipment to the intake structure, relocating the entire intake 

structure, reducing volumes by changing cooling tower operations (e.g., increasing 

cycles), reducing volumes from one-through cooling systems by converting to recycle 

cooling towers, reducing volumes by reuse or recycle of cooling water for process uses, 

reducing volumes by installing dry cooling technology, and/or any combination of these 

options.

EPA needs some additional information to better understand the nature of cooling 

operations at refineries and to understand the significant potential impacts to the refining 

industry associated with these regulations. EPA has invited API to meet and discuss 

these topics. This report will be the basis for the discussions at that meeting.

API Project Objectives

API senses EPA may have incorrect assumptions and/or incomplete understanding about 

petroleum refinery cooling operations, including how and why cooling water moves 

through a refinery, practical vs. theoretical options for recycle/reuse, implications of 

increasing cycles in existing cooling towers, the process consequences (e.g., 

thermodynamic effects, effects in stream composition, material and equipment 
Page 2 of 50

requirements, etc.) in changing from once through cooling to cooling towers, and feasibility 

vs. practicality of dry cooling.

API commissioned the preparation of a briefing paper to address various issues 

associated with changing from a once through cooling system to a recirculating cooling 

tower system. This briefing paper provides answers to basic questions about refinery 

cooling operations and clarifies the significant technical issues and extreme costs 

associated with implementing either closed-loop flow reduction or air cooling at petroleum 

refineries. The environmental benefit realized by either of these options would not justify 

the extremely high cost to the industry. 
Page 3 of 50

II. Cooling Water Use Description

II.1 What drives the need for cooling in petroleum refineries?

In petroleum refineries crude and refined oils are separated and subdivided into various 

fractions based on boiling point. This is accomplished by fractional distillation. The 

distillation is carried out in distillation column where the oil is heated up and vaporized in 

a fuel- (fuel oil, natural gas or refinery fuel gas) fired heater. Various fractions are 

separated by condensing and cooling products that are withdrawn from the tower. From 

an overall heat balance point of view, the heat that is put into the system by burning fuel 

and/or the introduction of steam has to be removed or “rejected”. This is accomplished 

in various ways such as:

 Heat exchange with boiler feed water to generate steam

 Heat exchange with other process streams

 Rejection of heat using air coolers

 Rejection of heat to cooling water

Figure 1 depicts a typical distillation system in a refinery. In this system three types of 

heat rejection systems are shown. The crude oil is preheated by exchanging with 

another process stream and fed to a fired heater. The partially vaporized products are 

sent to the distillation tower where different side streams are withdrawn based on the 

boiling point range of the product. The side streams are sent to strippers (a type of 

distillation column) where the boiling point range of the product is adjusted further by the 

addition of steam. The bottoms product from these strippers is cooled and sent to 

storage tanks or on to further processing. The vapors from these side strippers are sent 

back to the main tower. The overhead vapors from the main tower are condensed using 

an air-cooled exchanger and then further cooled using a cooling water heat exchanger. 

Three types of heat exchangers are shown in this system:

 Type 1 heat exchangers such as steam generators and process stream heat 

exchangers

 Type 2 heat exchangers which use cooling water

 Air coolers
Page 4 of 50

Feed

P-42--

Water

Steam

Gas To Recovery

P-36--

Light Cut

Steam

Steam

Towe Bottoms

Medium Cut

Heavy Cut

Distillation Tower

Side Stripper

Side Stripper

Type 2

Fired Heater

Air Cooler

Type 1

Type 1

Type 1

Type 1

Type 1 - Heat Exchanger where the stream is

heated/cooled against another Process

stream

Type 2 - Heat Exchanger where heat is

rejected to cooling water

Figure 1

Typical Distillation System
Page 5 of 50

II.2 What types of equipment require cooling?

Various types of heat exchangers in the refinery constitute the largest requirement for 

cooling. These are listed below:

Distillate Condensers: These are used to cool and condense the overhead vapors from 

distillation towers. Since this service involves condensation of vapors, the heat duty of 

these exchanges tends to be relatively large; and due to the fact that there are 

numerous distillation towers in a refinery, distillate condensers represent the largest use 

of cooling water in a refinery. It is not uncommon to find scores of distillate condensers 

in a refinery.

Steam Condensers: In these exchanges steam is condensed against cooling water, 

and the heat duty of these exchangers also tends to be relatively large since the service 

involves the condensation. Refinery distillation towers that run under vacuum conditions 

typically use steam ejectors to generate the required vacuum. The steam needs to be 

condensed, and this is accomplished using cooling water exchangers.

Product Coolers: Various refinery products need to be cooled before they are sent to 

storage tanks. This cooling is accomplished using cooling water exchangers, because of 

the low product outlet temperature requirements. It is not uncommon to find scores of 

product coolers in a refinery.

Additionally, various process streams (liquid and vapor) are transported by rotating 

equipment:

Compressors: Compressors fall into three general categories: centrifugal, rotary and 

reciprocating. Compressors are usually equipped with oil lubrication systems to 

dissipate some to the heat of compression. The heat that is picked up by the lubricating 

oil is rejected from the system using air coolers or cooling water depending on the 

amount of heat rejection required and the temperature requirements of the circulating oil. 

In addition, when multistage compressors are used, the process stream is cooled 

between stages using intercoolers and after coolers.

Vacuum Pumps: Vacuum that is required in distillation processes is generated by 

using vacuum pumps, steam ejectors or using a combination of the two. When vacuum 

pumps are used, the cooling requirements are similar to compressors in that the heat 

picked up by the lubricating system needs to be rejected using air coolers or cooling 

water exchangers. When ejectors are used to create the vacuum, all the steam that is 

added needs to be condensed and cooled. This is typically done with cooling water 

exchangers.
Page 6 of 50

Pumps: Pumps that are used in liquid service fall into two general categories: 

reciprocating and centrifugal. Pumps are also equipped with lubricating system and can 

require cooling, depending on the operating efficiency of the pump.
Page 7 of 50

II.3 What are the types of cooling water systems?

There are three types of cooling water systems:

a. Once Through Cooling Water System: In this type of system where the 

cooling capacity of the water is used only once without contacting the fluid or vapor 

being cooled. These systems use of water withdrawn from a surface water source such 

as a lake, river or estuary and typically are returned to the same source. Figure 2 shows 

a typical once-through cooling water system.

Figure 2

Once Through Cooling Water System

b. Closed Loop Cooling Water System: In this system water is circulated in a 

closed loop piping system and is subject to cooling and heating without evaporation or 

air contact. Heat that is absorbed by the water in a closed loop system is normally 

rejected using a heat exchanger to a once through cooling system. Figure 3 shows and 

example of a closed loop system.

Figure 3

Closed Loop Cooling Water System

Warm Water Discharge

Cold Water Intake

Process Heat Exchanger Process Heat Exchanger

Process Heat Exchanger Process Heat Exchnager

Cooler

Surge Tank

Cold Water Intake

Warm Water Return
Page 8 of 50

Closed loop cooling water systems in U.S. refineries are rare. Since closed loop 

systems are not typical, they are not discussed further in this report.

Evaporative Recirculating Cooling Water System: In this type of system, the heat 

that is picked up by the recirculating cooling water is rejected in a cooling tower by 

evaporation. In the cooling tower the hot water is sprayed against a rising stream of 

atmospheric air. The heat in the cooling water is removed by heating the air as well as 

by evaporation. An example of an evaporative recirculating cooling tower system is 

shown in Figure 4.

Figure 4

Evaporative Recirculation Cooling Water System

Blowdown

Process Heat Exchanger Process Heat Exchanger

Makeup Water Intake

Cooling Tower

Evaporation
Page 9 of 50

II.4 Are there specific requirements/limitations associated with cooling water 

usage?

Cooling water must be of a quality that allows for control over scaling, deposition of 

suspended solids and corrosion in the equipment through which the cooling water 

passes. Scale formation limits heat transfer and is destructive to heat exchangers and 

other plant equipment; therefore, scale prevention chemistry is extremely important in 

recirculated cooling water system design. Scale can be controlled by optimization of 

chemical concentrations, pH and water temperature.

In a recirculated cooling water systems as shown in Figure 4, water is evaporated which 

concentrates the dissolved solids contained in the feed water. Make-up water is added 

to the cooling water system to replace the water that is evaporated. The maximum 

cycles of concentration allowable and still minimize scale formation is a function of water 

chemistry. Impurities that form scale will determine the maximum cycles of 

concentration. Silica and Calcium concentrations are typically used to determine the 

cycles of concentration.

Typical allowable concentrations of impurities in open recirculated cooling water are as 

follows:

ppm

Total Dissolved Solids 3,000

Total Suspended Solids 200

Oil and Grease 10

pH Range 6.8 to 8.0

Aluminum (as Al) 1

Iron (as Fe) 1

Ammonia (as NH3) 10

Calcium (as CaCO3) 1000

Copper (as Cu) 0.2

Chlorides (as Cl) 1000

Silica (as SiO2) 150

The quality of the intake water is an important factor in the design of cooling water 

systems. In open recirculated systems, the intake water can be pretreated to lower the 
Page 10 of 50

concentration of specific ions in order to achieve an optimized number of cycles of 

concentration. 

There can be limitations on the maximum return water temperature in both closed loop 

and open recirculated systems. In once-through systems there is usually a limitation on 

the return temperature depending on environmental considerations of the source of the 

water. In open recirculated water systems, the return temperature is often dictated by 

the need to limit corrosion in the cooling water piping in the refinery. Cooling water 

return temperatures of 100oF to 110oF are typical.

Corrosion in the cooling water piping and heat exchanges is caused by the dissolved 

oxygen contained in the recirculating water, dissolved salt concentration and the 

temperature of the cooing water return. This is controlled by limiting the temperature of 

the return cooling water, and by the addition of corrosion inhibitors.
Page 11 of 50

II.5 How are these cooling needs and requirements/limitations met?

In once-through cooling water systems, the intake water can be clarified to remove 

suspended solids and any limitation on the return water temperature can be met by 

increasing the flow of water.

Makeup water to recirculated cooling water systems may be clarified to remove 

suspended solids and/or softened prior to being used as cooling water make-up. 

Frequently a surface water or well water source is found that does meet minimum quality 

specifications without prior treatment. The circulating cooling water is treated with a

variety of chemicals to control scaling, deposition, biological growth and corrosion. 

Sulfuric acid is most often used to control pH.

Softening and other methods of removing dissolved solids prior to using the water for 

cooling tower makeup are usually not cost effective because of the relatively large 

volume and low concentration of solids. Dissolved solids removal from a cooling tower 

with sidestream softening is usually more cost effective because the impurities are more 

concentrated.
Page 12 of 50

II.6 What dictates which cooling approach is used?

When cooling systems are designed for use in refineries a series of evaluations and 

decisions are made. The following flowchart illustrates such a decision tree.

No

Yes

Yes

No

No

Yes

Yes

No

Determine

temperature to

which process

stream needs to

be cooled

Is stream hot

enough for steam

generation

Is stream hot

enough for heat

recovery

Design steam

generator and

determine process

outlet temperature

Is air cooling

feasible

Use cooling water

exchanger

Design heat

exchange with

other process

streams

Design air cooler

and determine

outlet temperature

Is further cooling

required

Done
Page 13 of 50

In general, lower process outlet temperatures can be achieved using water coolers than 

using air coolers. This is because the dry bulb temperature of the ambient air 

determines the lowest achievable temperature in air coolers and the wet bulb 

temperature of ambient air is the controlling factor for cooling water exchangers.

After going through the analysis as shown in the chart above, the heat load that needs to 

be rejected to cooling water is determined. Then an evaluation of once through cooling 

versus recirculating cooling water is carried out. The factors that go into this evaluation 

include:

 Location of the refinery with respect to the water source. In some cases local 

regulations might preclude the use of once through cooling water.

 The required cooling water supply temperature based on the cooling needs in the 

refinery.

 Plot space considerations within the refinery.

 The quality of the fresh water with respected to contaminants such as total dissolved 

solids, chlorides etc. needs to be evaluated. If the dissolved solids content of the 

water is high it will lower the number of cycles of concentration in the cooling tower. 

Consequently, the makeup water and blowdown treatment requirements will be 

higher.
Page 14 of 50

II.7 What volumes of cooling water are required for various approaches?

Once-through cooling system: In a once through system the volumetric flow of the 

water required is dictated by the amount of heat that needs to be rejected (Btu/hr) and 

the maximum allowable discharge temperature. 

For example, if a refinery needs to reject 130,500,000 Btu/hr with an intake temperature 

of 85oF and a maximum discharge temperature of 100oF, the flowrate of water is 

calculated as follows:

(100 85) 500

130,500,000 ( )

 

Flowrate gpm 

= 17,400 gpm

Even though this circulation rate is small compared to a mid sized refinery, it is used for 

illustration purposes because EPA used this size in the EPA study. The next higher 

EPA case was 104,000 gpm, which was considered too high to use as a representative 

case.

Open Evaporative cooling system: The cooling water required in this type of system is 

the water required to makeup the losses due to:

 Evaporation

 Drift (entrainment)

 Blowdown (% of the makeup rate to prevent buildup of ions in the system)

Blowdown (B)

Evaporation (E) Drift (D)

Cooling Tower

Process Cooler

Makeup (M)
Page 15 of 50

Makeup Requirement

The make up water rate (M) is the sum of the evaporative losses (E), drift losses (D) and 

the blowdown (B).

M  E  D  B (1)

The evaporative loss is a function of the heat rejected by the cooling tower and the drift 

loss is a function the circulation rate. 

The amount of dissolved solids (TDS) coming into the system with the makeup water 

has to go out of the system with the blowdown stream to prevent TDS buildup in the 

circulating cooling water. Cycles of concentration (n) is calculated by:

n = TDS concentration in circulating water (y)

TDS concentration in makeup water (x)

y  (n)(x) (2)

and since the TDS concentration of the blowdown is the same as the circulating water

M (x)  (B  D)( y) (3)

Combining equations (2) and (3)

M (x)  (B  D)(nx) (4)

Therefore M  n(B  D) (5)

Combining equations (1) and (3)

1

( 1)



 



n

E n D

B (6)

and 

( 1)

( )





n

n E

M (7)

Where:

M = amount of makeup water

E = amount of evaporated water

D = amount of water loss by drift (specified by tower manufacturer)

B = amount of blowdown

n = number of cycles of concentration

x = mass fraction of TDS in the makeup water
Page 16 of 50

The same equations apply when considering blowdown constituents other than TDS 

(e.g, silica, hardness, etc.)The following example illustrates the calculation of these 

parameters

Assumed Cooling Tower Circulation rate 17,400 gpm

Assumed Temperature Difference 15oF

Calculated duty (circ rate x temp. diff) 131 MM Btu/hr 

Heat of Vaporization (HV) 1037 Btu/lb 

Evaporation ( 

HV  500

Duty ) 252 gpm

Drift (assumed to 0.1% of circulation) 17 gpm

Assumed cycles of concentration 5

Blowdown (by equation 4) 46 gpm

Makeup (by equation 3) 315 gpm
Page 17 of 50

III. Recirculated Cooling Water Technology

III.1 What are the components of a recirculated cooling tower system?

Recirculated cooling tower systems can be divided into three parts:

 Intake structure including the intake screens, pumps and piping 

 Cooling Tower and cooling water circulation pumps, including any pretreatment 

systems

 Recirculation piping and heat exchangers

a) Intake Structure: Consists of the piping from the intake source including screens to 

remove suspended solids and marine organisms, intake pumps (number of pumps 

depends on the size and includes operating and spare pumps), and the piping from 

the intake pumps to the refinery.

b) Cooling Tower: The cooling tower consists of basin capable of providing about 

10 minutes of holdup based on the circulation rate of the system, a tower equipped 

with fill (wood or fiberglass), cooling tower sump, distribution piping and nozzles to 

maximize air/water contact, fans to force atmospheric air through the tower and 

circulating pumps. 

c) Pretreatment: Depending on the quality of the intake water, a clarifier is used to 

remove suspended solids. If the dissolved solids content of the intake water is too 

high, other pretreatment methods such as lime softening, ion exchange and reverse 

osmosis are included to increase the cycles of concentration allowable in the cooling 

tower. Pumps are required to transfer the water from the clarifier to the cooling tower 

and the recirculating cooling water.

d) Cooling Water Treatment: Chemical treatment of the recirculating water requires 

that chemical feed systems be installed. This includes tanks and pumps for each of 

the chemicals required.

e) Recirculation piping: This consists of the cooling water supply and return piping to 

the various process units in the refinery. Also includes all the sub-headers in the 

refinery to carry the cooling water to heat exchangers.

f) Blowdown Treatment: The blowdown from the cooling tower system is typically 

sent to the wastewater treatment plant. This stream would contain high dissolved 

solids and would contain hydrocarbons if heat exchanger leaks have occurred.
Page 18 of 50

III.2 What are the limitations of using recirculated systems?

The makeup water to a cooling tower may come from wells, reservoirs, lakes, rivers, 

and even treated wastewater. The impurities in these makeup water sources may 

cause a number of water related problems in the cooling system. Corrosion, scale, 

biological fouling, sludge formation, and chemical attack on the cooling tower 

components may be experienced when the chemistry of the cooling water is not 

properly controlled.

The quality of the makeup water determines the amount and type of internal 

chemical treatment required. Chemicals are added to protect the metal surfaces 

from the following problems:

 Corrosion.

 Scale deposits.

 Biological fouling.

In a cooling tower, the recirculating water is saturated with atmospheric air as it 

passes through the tower. This condition, combined with warm water temperatures 

and increasing dissolved salt concentration, results in water that is very corrosive to 

carbon steel. Corrosion inhibitors are required for all recirculated cooling tower 

systems. Selection of the optimum inhibitor and dosage is based on the analysis of 

the water, materials of construction, environmental regulations, and other factors 

including cost of chemicals. Typical inhibitors are phosphates, molybdates, zinc and 

polyphosphates.

Several corrosion inhibitor chemical treatment programs have been developed in the 

past fifteen (15) years. The most successful of these programs have been the 

stabilized phosphate and zinc phosphate treatments, which incorporate the use of 

copolymer dispersants.

Scale is an adherent insoluble deposit laid down on surfaces of the cooling water 

system during operation, causing impaired heat transfer. Calcium carbonate is the 

principal scale formed when calcium bicarbonate breaks down and becomes even 

more insoluble as the temperature rises (reverse solubility). Scale control can be 

accomplished through limiting the cycles of concentration.

Refinery cooling towers also require continuous chlorination for control of 

microbiological activity and usually acid for pH control, which prevents calcium 

carbonate scaling .
Page 19 of 50

III.3 What must be considered when increasing cycles of concentration in 

cooling towers? 

Cycles of concentration are limited by makeup water quality, the design of the cooling 

tower itself, the design of the cooling system piping, heat exchangers and by the type of 

pretreatment available for the makeup water.

Variability of the makeup water quality must be considered: changes in total suspended 

solids, iron, calcium to magnesium ratio, alkalinity, sodium, silica, sulfate and chloride 

can lead to control problems, corrosion and deposition. Blowdown rates are difficult to 

control with variable quality makeup water, since the blowdown rate is set or adjusted 

based on laboratory tests. Variations in makeup water quality make setting this rate 

difficult without either exceeding chemistry limitations or wasting water. The only 

solution to this problem is on-line analysis of some components of the circulating water 

quality, such as conductivity, pH and phosphate. These continuous analyses are used 

to drive automatic blowdown valves, treatment chemical pumps and sulfuric acid pumps 

via a PLC controller. The cost for such automation is about $100,000 installed for a 

17,400 gpm system. The level of reliability is questionable as the various probes become 

fouled with deposits, requiring a preventative maintenance program to operate 

dependably. Similarly, chemical dosages are set based on makeup water characteristics 

and the cycled up components in the circulating water. Variable quality makeup water 

makes it difficult to control dosages without either overshooting or undershooting the 

proper dosage. As stated above, the use of automated systems is the only means of 

addressing the situation, adding both equipment and labor costs to the operation of the 

cooling system.

Commonly used values (based on industry practice) exist for each of the potential 

contaminants that may be found in the makeup water stream. These values are set for 

cycled up (concentrated) cooling tower water and therefore directly limit the maximum 

cycles of concentration that can be achieved before equipment fouling, equipment 

scaling or equipment corrosion problems are encountered. The various limits are set 

based on many years of industry experience as well as extensive testing by various 

suppliers of treatment products and by the end users themselves. Industrial water 

treatment has been practiced extensively at least since the 1950’s so there is a large 

body of knowledge on the subject.

Calcium carbonate deposition is controlled by limiting the calcium level, generally no 

higher than 1,000 parts per million, by the addition of sulfuric acid to control pH within a 

range of 6.8 to 8.0 and by the addition of dispersants specific to calcium carbonate. The 

general limit of 1,000 ppm calcium as CaCO3 assumes the use of such dispersants.
Page 20 of 50

Magnesium silicate limits are set to control scaling of surfaces contacted by the 

recirculated cooling water. The magnesium silicate limit is an empirical value (product of 

the magnesium concentration x silica concentration) set at 100,000 ppm, and can only 

be controlled by limiting the levels of magnesium and silica in the recirculating water via 

blowdown.

Because carbon dioxide is stripped from the water as it passes through a cooling tower, 

the rise in pH (alkalinity) is controlled via the use of sulfuric acid and/or blowdown.

Sodium and sodium chloride are limiting factors because of potential damage to the 

wooden structures of cooling towers and due to the corrosive potential for steel and 

stainless steel equipment components in the cooling water system. The general limit for 

chlorides is 1,000 ppm.

Silica itself must be limited to no more than 200 ppm as SiO2 to avoid scaling.

Sulfates must be limited in cooling systems having concrete components, such as the 

cooling tower basin, to avoid leaching of calcium from the concrete matrix. The general 

limit for water containing sulfates in contact with concrete systems is 3,000 ppm as SO4.

Copper and iron in the makeup water can cause severe deposition and corrosion 

problems at quite low levels. Copper levels above 0.2 ppm and iron levels above 1 ppm 

are unacceptable in the recirculated cooling tower water.

Suspended solids in excess of 200 ppm in the cooling tower water are generally not 

tolerated due to fouling and plugging problems.

Typical design flow velocities for refinery heat exchangers range from 3 to 5 feet per 

second. Older systems may have flow velocities as low as 1 to 2 feet per second. A 

calcium limit of 1,000 ppm as CaCO3 would be acceptable for the higher velocities; 

however, a limit of 500 ppm as CaCO3 would be required for the lower flow velocities.

Examples of makeup water quality affecting cycles of concentration would be as follows. 

Given adequate flow velocities, one could assume an upper calcium limit of 1,000 ppm 

as CaCO3. A makeup water containing 100 ppm of calcium could theoretically be 

cycled 10 times, 200 ppm of calcium in the makeup would limit cycles to 5 times, 300 

ppm would limit cycles to 3.3 times, and so forth.

Clarification is often used as a pretreatment of makeup water. Clarification of the 

makeup water can control the levels of suspended solids in the makeup water; however, 

it will have no effect on the concentration of dissolved solids contained in the cooling 

water.
Page 21 of 50

Cold lime softening of cooling tower makeup can be used to lower the calcium and silica 

levels of the makeup water. Effective cold lime softening can only reduce calcium levels 

to the 30 to 50 ppm range while silica can generally be reduced to the 2 to 5 ppm range. 

Cost for cold lime softening is in the range of $50 per million gallons of water treated. In 

addition, the use of cold lime softening produces 200 to 300 pounds of sludge for each 

million gallons of water treated. Sludge disposal adds additional costs to the process.
Page 22 of 50

III.4 What quality/quantity of make-up water is required and how is it 

determined?

As illustrated in section II.7 makeup water is a function of the evaporation rate and the 

number of cycles of concentration. Equation (6) and (7) in section II.7 are used to 

calculate the blowdown and makeup rates

1

( 1)



 



n

E n D

B

( 1)

( )





n

n E

M

where

B is the blowdown rate

D is the drift rate

M is the makeup water rate

E is the evaporation rate

n is the number of cycles of concentration

In the example illustrated in section II.7 the makeup and blowdown rates are as follow:

Makeup water – 315 gpm

Blowdown rate – 46 gpm

Using the equations shown above the following chart illustrates the makeup and 

blowdown as a function of number of cycles of concentration.
Page 23 of 50

Figure 5

Makeup & Blowdown vs. Cycles of Concentration

As can be seen from the chart, the rate of change of both makeup and blowdown rates 

decreases rapidly until the cycles of concentration reaches about 7 after which the rate 

of decline is small. This is the reason most cooling towers are designed with a cycle of 

concentration of between 4 and 7. Another consideration is the TDS content of the 

makeup water and the allowable TDS content of the circulating water. 

-

200

400

600

800

0.0 5.0 10.0 15.0

Number of Cycles

Makeup/Blowdown Water Rate, GPM

Makeup Water Rate

Blowdown Rate
Page 24 of 50

III.5 Effects of brackish vs. fresh water?

Brackish water contains higher dissolved solids such as chlorides than fresh water. The 

chloride content of the brackish water could be as high as 10,000 ppm. This will impact 

on the following:

 Cycles of concentration – The cycles of concentration could be reduced to as low as 

1.5 to 2 depending on the chloride content of the makeup water. This will require a 

larger volume of makeup water and also result in a larger blowdown flow.

 Cooling tower – Due to the higher chloride content of brackish water, the cooling 

tower will be subject to greater corrosion and could require an upgrade in materials 

of construction, resulting in higher capital costs.

 Treatment Chemicals – Depending on the TDS define content of the circulating 

water, more treatment chemicals could be required to control corrosion and 

biological growth. This will result in higher operating costs.

 Circulation Pumps and Piping – Due to the higher chloride concentration, the 

material of construction of the pumps and the distribution piping will require an 

upgrade to corrosion-resistant alloy material, which will result in significant capital 

expenditure.

 Heat Exchangers – The material of construction of the heat exchangers will require 

an upgrade to alloy materials and result in significant capital costs. These costs 

could be minimized somewhat by passing the cooling water on the tubeside of the 

exchanger.
Page 25 of 50

III.6 Blowdown flows, handling, treatment?

In a cooling tower system, part of the circulating water is removed as blowdown to 

prevent the buildup of TDS in the system. The example shown in section II.7 illustrates 

in detail the calculation method. In that example, the blowdown is calculated to be 

46 gpm. This is for system in which the cooling tower makeup water has a calcium 

concentration of ppm as CaCO3 and is operated with five cycles of concentration.

Cooling tower blowdown is typically sent to wastewater treatment in refineries via the 

sewer. This is because in many cases the pressure on the process side of heat 

exchangers is higher than the cooling water pressure and any leaks in a heat exchanger 

would result in the contamination of the cooling water with hydrocarbons. This will 

impose a hydraulic load on the existing wastewater treatment. The full impact on 

wastewater treatment needs to be evaluated on a case-by-case basis.
Page 26 of 50

III.7 Factors affecting drift and evaporation losses?

In order to obtain efficient heat transfer in a cooling tower, the return cooling water (hot) 

is sprayed on to the packing to increase heat and mass transfer efficiency. This results 

in small droplets of water distributed throughout the tower. The high flow of atmospheric 

air passing through the tower results in the entrainment of some of these droplets, which 

are released to the atmosphere and is referred to as drift. Cooling tower manufacturers 

have developed various methods such as baffling, packing etc. to minimize drift. The 

manufacturers often consider these design methods and know-how proprietary. The 

manufacturers usually provide a guarantee on the maximum expected drift from cooling 

towers. Typical quantity of drift is <0.1% of the circulation rate of the cooling tower.

The cooling tower operates on the principle of evaporative cooling by a combination of 

heat and mass transfer. Circulating water is cooled when some of it evaporates, 

removing the heat of vaporization. The evaporation loss in a cooling tower is a function 

of the total heat duty that needs to be rejected. The evaporation loss calculated by 

dividing the heat duty by the latent heat of vaporization of water. In the example 

illustrated in section II.7, the evaporation is 252 gpm for a heat duty of 131 MM Btu/hr.
Page 27 of 50

III.8 Factors affecting chemical usage and concentrations?

The factors affecting treatment chemical usage and concentration are complex and interrelated. Most cooling tower vendors will utilize a modeling program such as WaterCycle

to determine the exact requirements and limitations for a given makeup water. In 

general, the higher the concentration of a scaling or fouling component, the greater the 

treatment requirement. 

In almost every instance, the use of sulfuric acid will be required in a recirculating 

cooling system in order to control the pH of the water. The dosage is dependent upon 

makeup water alkalinity where the acid requirement is based on alkalinity in the cooling 

tower; so as number of cycles increases, and consequently the alkalinity increases, the 

sulfuric acid requirement also rises.

Dispersants to control calcium carbonate, magnesium silicate and silica are dosed 

based on the levels of magnesium and/or silica contaminant in the cooling water. For 

example: 200 ppm of calcium would generally require 20 ppm of dispersant while 1,000 

ppm of calcium would require 100 ppm of dispersant.

Use of chlorine or bleach to control microbiological growth and fouling is higher in 

cooling towers than in once through systems. Continuous chlorination is generally 

required for adequate control in refinery cooling towers with the dosage in the range of 

2 to 5 pounds per 1,000 gallons of recirculation capacity. Some operators of cooling 

towers find that intermittent additions of chlorine are more effective.

Corrosion inhibitors are not generally required in once through systems while 

recirculating systems will require inhibitors to control corrosion of copper alloys and 

carbon steel piping and heat exchangers. Again, the higher the cycles of concentration, 

the higher the requirement for corrosion inhibitors.
Page 28 of 50

IV.Converting from Once-through to Cooling Towers

IV.1 What would be involved in changing from once-through cooling to cooling 

towers?

The typical petroleum refinery is complex system of process components that evolved 

over decades. Designers provided heat removal systems based upon the needs of the 

individual components and the available cooling utilities. Plot siting issues are major 

design problems at refineries. Changing once-through cooling systems to recirculating 

systems will involve site-specific piping and equipment location issues that may not be 

apparent to those more familiar with power plants.

Each refinery component may have several different heat transfer needs, ranging from 

pump cooling to steam eduction-generated vacuum. If the cooling tower can provide 

similar cooling capacity, then the process heat exchangers will not need replacement. 

The problem is that the temperature difference provided by once-through cooling is not 

readily replicated with cooling tower-based systems. Consequently, the complex nature 

of a petroleum refinery will demand site-specific cooling system designs having multiple 

cooling towers (possibly placed far away from the process unit), the replacement of 

pumps, and/or the replacement of process heat exchangers (and possible associated 

piping, depending upon system hydraulics).

There are practical aspects that need to be taken in account if changing from a oncethrough system to a recirculating system is being considered, such as:

 A key factor to refinery profitability is to cut as deeply into the vacuum tower residue 

as possible. This in turn means pulling as low a vacuum as possible in the vacuum 

column. Vacuum is generated using steam jet ejectors, and lower column vacuum 

requires more steam. Lower vacuum results in lower condensing temperatures, 

which in turn requires cooler water. 

 Installation of a cooling tower in an existing refinery is dictated to a large extent by 

the space available and not by optimum location to minimize pipe runs, access to 

utilities etc. While it might be optimum to install one cooling tower to replace the 

existing once through system, space and logistics might require building multiple 

cooling towers. This invariably leads to increased costs.

A cooling tower requires a sump or basin for collection and storage of the water 

recirculated over the tower and through the system. Hydraulic and pumping 

requirements set a minimum volume for the sump. Generally, the sump is sized to 

provide for at least 10 minutes of holdup based on the recirculating rate. 
Page 29 of 50

New recirculation pumps with enough head to overcome the hydraulic losses of the 

supply piping, heat exchangers and return piping plus the static head of the cooling 

tower will also be required.

New cooling water supply and return piping to and from the cooling tower would need to 

be installed.

Since the flow of makeup water in the case of recirculation system is significantly smaller 

than a once through system, 315 gpm versus 17,400 gpm, (for the case illustrated in 

section II.7) new intake structure, supply pumps and lines to the refinery may be 

required.

The thermal rating of heat exchangers in the refinery will need to be reviewed to account 

for any differences in the cooling water supply and return temperatures. This could be a 

very significant cost issue. The example shown in section IV.5 illustrates a case where 

more than 20% more heat exchanger area would be required.
Page 30 of 50

IV.2 Effects on wastewater discharge volumes and quality?

In a once through cooling water system there are no discharges of wastewater from the 

system. In an evaporative cooling water system, the blowdown, which is a function of 

evaporative and drift losses and the number of cycles of concentration (shown in the 

example in section II.7 to be 46 gpm) is usually sent to wastewater treatment. The 

contaminant in the blowdown is high dissolved solids, and possibly hydrocarbons, if 

leaks from heat exchangers occur. Cooling tower blowdown is considered a process 

wastewater and is subject to refinery effluent guidelines.
Page 31 of 50

IV.3 Effects on metallurgy in piping and exchangers?

In refineries that use fresh water as the source of once through cooling water, the piping 

and heat exchangers (cooling water side) are usually designed with of carbon steel but 

in some cases corrosion resistant material are used. If the system is changed to a 

recirculated cooling water system the TDS content of the circulating water can be higher 

that the once through water. If the TDS is kept within reasonable limits (< 3000 ppm), 

carbon steel would be an acceptable material of construction.

If brackish water is used for the cooling tower makeup, higher alloys such as admiralty, 

copper nickel alloys etc. would need to be considered and would result in higher capital 

costs. As mentioned before, if the pumps, distribution piping and exchangers in the 

refinery need to be upgraded, it would require significant capital expenditures as well as 

result in significant costs in order to reduce impacts on refinery production capability.

In refineries that use salt water as the source of once through cooling water, the piping 

and heat exchangers would most likely be made of alloy material. In these cases, 

changing to a recirculating cooling water system would not require changes provided the 

dissolved solid concentration in the recirculating water are kept within reasonable limits 

(described previously in this report).
Page 32 of 50

IV.4 Effects on refinery piping configurations?

If the temperature difference between the cooling water supply and return is the same 

for both once through and evaporative cooling water systems, resulting in the same 

circulation flow, the impact on piping configuration in the refinery should be minimal. 

If the temperature difference is substantially different resulting in different circulation 

flows, the piping inside the refinery will need to be evaluated to ensure that the flows are 

within the allowable pressure drop and velocity limits. Typical limits are as follows:

Pressure drop psi/100ft Maximum velocity (ft/s)

1.0 – 2.0 10

The pressure drop limits are usually imposed in order to keep the hydraulic losses in the 

system within reasonable limits. Lower pressure drops will result in lower velocities and 

could result in excessive scaling and corrosion. Higher pressure drops will require the 

circulating pump differential head to be higher resulting in increased capital and 

operating costs.
Page 33 of 50

IV.5 Effects on heat exchanger capacity?

In once through cooling water systems the supply temperature is the bulk temperature 

of the water source. Any seasonal variations in the cooling water supply temperature 

are also taken into account in the design of heat exchangers in the refinery.

In evaporative cooling towers the local wet bulb temperature (1) determines the cooling 

water supply temperature. Typically, a 10oF approach (2) between the wet bulb 

temperature and the cooling water supply temperature is taken in the design of cooling 

towers – i.e., the cooling water supply temperature is 10oF above the design wet bulb 

temperature. Consequently, the cooling water supply temperature from the cooling 

tower in a recirculating system is typically warmer than the supply temperature for once 

through system. This will lower the log mean temperature difference (3) in heat 

exchangers in the refinery and in certain cases require significantly more heat exchanger 

area. For example:

Units

Once 

Through

Cooling 

Tower

Heat Exchanger duty MM Btu/hr 1.0 1.0

Process Inlet Temperature oF 180 180

Required Process Outlet Temperature oF 120 120

Cooling Water Inlet Temperature oF 80 90

Cooling Water Outlet Temperature oF 95 105

Log Mean Temperature Difference (LMTD) oF 60 49

Heat Transfer Coefficient (U) Btu/hr-ft2125 125

Required Heat Exchanger Area (

LMTD U

Duty



) ft2

133 163

As illustrated by this example, 22% extra heat exchanger area would be required if the 

system is changed from a once through to a cooling tower system. For the case 

illustrated in section II.7 this would mean that an additional 3,900 ft2of heat exchanger 

would be needed. But for practical reasons, in some cases, new heat exchangers would 

need to be installed instead of installing only the additional area required. This would 

lead to significant capital costs. A detailed analysis of all the exchangers in the refinery 

should be carried out in order to understand the impact on the whole refinery. 

(1)Wet Bulb Temperature: The wet bulb temperature is a measure of the amount of moisture, in the 

form of invisible water vapor contained in the air. As the name implies it is measured by a standard 

thermometer whose bulb is covered by a muslin sleeve and that has been moistened by pure water.
Page 34 of 50

The principle of the wet bulb thermometer is as follows; Water evaporates from the muslin cover 

passing into the air in the form of invisible water vapor. This is accomplished by absorbing heat from the 

thermometer bulb and the mercury it contains. The thermometer therefore indicates a lower 

temperature than that of the dry bulb thermometer.(4) The difference between the readings of the dry 

and the wet thermometers is called the depression of the wet bulb.

If the air contains nearly all the moisture it can possibly hold, evaporation from the muslin will be slight 

and the depression of the wet bulb will be small. However, if the air is very dry, containing little moisture, 

evaporation will be quite rapid and the depression of the wet bulb will be quite large. 

In hot dry desert climates wet bulb depressions of over 45°F have been observed, but at sea the 

depression is seldom more than 10°F. If the air contains all the moisture it can possibly hold, there is no 

evaporation from the muslin, and the dry and wet bulb thermometers will read the same. When this 

condition exists the air is said to be saturated.

(2)Approach Temperature: The lowest possible temperature to which water can be cooled in a cooling 

tower corresponds to a wet bulb temperature of the air. This is not practical because at the wet bulb 

temperature, the partial pressure of the water in the air is equal to the vapor pressure of water resulting 

in a zero driving force. The difference between the water outlet temperature and the wet bulb 

temperature is called the approach temperature in a cooling tower. This is typically about 10oF.

(3)LMTD: Logarithmic Mean Temperature Difference (LMTD) is a measure of the difference in 

temperature between the hot side and cold side of a heat exchanger. It is defined as





















  



2

1

1 2

T

T

Ln

T T

LMTD

Where, T1 and T2 are the difference in temperature between hot and cold side fluid at two ends of 

the exchanger. LMTD is an indication of the driving force available for heat exchange.

(4)Dry Bulb Temperature: The dry bulb temperature is the atmospheric temperature of air at a specific 

location and at a specific time. It is measured by exposing a thermometer to the atmosphere. The 

temperature reported by the meteorological stations is the dry bulb temperature.
Page 35 of 50

IV.6 Additional space requirements?

Installing a new cooling tower will require additional plot space in the refinery. For the 

cooling tower example used in this study (17,400 gpm circulation rate), the plot space 

required for the cooling tower is 100 feet x 60 feet. This includes the space required for 

the ancillary equipment such as the circulation pumps, chemical feed systems etc.

In an existing refinery, the location of the cooling tower is often dictated by the space 

available and not by optimum location to minimize pipe runs, access to utilities etc. 

While it might be optimal to install one cooling tower to replace an existing once-through 

system, space constraints and logistics might dictate the use of multiple cooling towers 

resulting in increased capital expenditures. Other considerations include impact of 

cooling tower drift on roads, houses and process units.

The routing of the interconnecting piping between the cooling tower system and the 

existing cooling water distribution system needs to be considered.

The cooling water intake structure for recirculation systems is significantly smaller that a 

once through system (315 gpm vs. 17,400 gpm in the example illustrated in section II.7) 

and, therefore, the space required for the new intake structure would be significantly 

smaller.
Page 36 of 50

IV.7 Changes in water treatment requirements?

Once through cooling systems are in general not chemically treated. There may be a 

need to chlorinate the once through cooling system intermittently to control 

microbiological activity and fouling. Some once through systems use low levels of 

dispersant, 1 to 5 ppm, to control calcium carbonate scaling and suspended solids 

fouling. 

The change to cooling towers would require the feed of several different types of 

chemicals for adequate control and protection of the cooling system. 

a) Sulfuric acid would be required to control alkalinity.

b) Chlorination would be required to control microbiological fouling. 

c) The continuous feed of polymeric dispersants would be required to control calcium 

carbonate scaling, magnesium silicate scaling and suspended solids fouling. 

d) The continuous feed of corrosion inhibitors would be required to control the corrosion 

of copper alloys and carbon steel components of the system. Corrosion is an 

electrochemical process; the higher the TDS of the cooling water, the higher the rate 

of electrochemical reaction and the greater the concern with corrosion issues for 

cooling towers over once through systems.

e) There may be a requirement for clarification, to remove suspended solids to produce 

makeup water suitable for cooling tower use.

f) There may be a requirement for cold lime softening, to produce makeup water with a 

calcium concentration suitable for cooling tower use.

The following table lists the quantities of chemicals required for treatment for a 17,400 

gpm cooling tower. As calculated in section II.7 the blowdown for this cooling tower 

would be 46 gpm and makeup water would be 315 gpm.

Chemical

Concentration(ppm) in 

Circulating Water Usage (lbs/yr)*

Dispersant 100 27,600 

Corrosion Inhibitor 20 5,520 

Sulfuric Acid 150 41,400

Chlorine 150 41,400

*assumes operation 24 hours per day, 365 days per year
Page 37 of 50

IV.8 Changes in water pressure requirements?

The typical arrangement in a once through cooling system is to provide enough pressure 

in the supply pumps to overcome the hydraulic losses in the cooling water supply piping, 

heat exchangers and cooling water return piping in the refinery. As discussed, the 

volumetric capacity of once through cooling water pumps is significantly lower for cooling 

tower systems. Therefore, new pumps and supply piping will need to be installed to 

provide makeup water for the evaporative cooling system. New cooling water circulating 

pumps need to be installed inside the refinery to carry the cooling water to all the 

exchangers and return it to the cooling tower. The table below gives a comparison of 

the typical pressure requirement of the circulating pumps in a once through system and 

a recirculating cooling water system.

Component

Differential Pressure Allowance (psi)

Recirculating Cooling 

Water System

Once Through Cooling 

Water System

Cooling Water Supply Main Header 15 15

Cooling Waster Supply Branches 5 5

Cooling Tower Static head (~45 ft.) 20 0

Cooling Water Return Branches 5 5

Cooling Water Return Main Header 15 15

Differential Head Required 60 40

Differential Head Required 140 ft 93 ft
Page 38 of 50

V. Changing to Dry Cooling

V.1 What would be involved in changing to dry cooling?

There are two types of dry cooling systems: direct dry cooling and indirect dry cooling. 

Direct dry cooling systems utilize air to directly condense or cool the process stream 

while indirect dry cooling systems utilize a closed cycle water cooling system to cool the

process stream and the heated water is then air cooled. Indirect air cooling is used in 

power plants while direct cooling is used for certain applications in refineries depending 

on process outlet temperature and plot space considerations. 

Indirect air cooling is not used in refineries because the lowest achievable cooling water 

supply temperature would be about 10oF higher than the dry bulb temperature for that 

location. Frequently, process streams in refineries need to be cooled to a temperature 

lower than the dry bulb temperature and, therefore, indirect dry cooling is not technically 

feasible. 

Direct air cooling used in refineries is dictated by the dry bulb temperature at that site 

and conditions of the process streams such as inlet temperature and required outlet 

temperature. As previously discussed, air cooling can be used in refineries only for 

specific applications and even in these applications cooling water exchangers are 

installed downstream of the air coolers in order to meet the process requirements. 

Additionally, since the heat transfer coefficient for air coolers is relatively low, air cooled 

exchangers tend to be large, require significant plot space, and can be a source of 

noise. 

Replacement of once through cooling with air-cooled heat exchangers would demand an 

economically infeasible number of changes to the existing process equipment. 

Furthermore, whereas cooling water shell-and-tube exchangers can be stacked vertically 

in process units, air coolers cannot be stacked (because lower units would send heated 

air to higher units) and so it may be physically difficult or impossible due to space 

constraints to replace water-cooled exchangers with air-cooled exchangers.
Page 39 of 50

VI.Cooling Water Recycle and Reuse

VI.1 What are the options for recycle/reuse of cooling water? Practicality?

Petroleum refineries in the U.S. practice water recycling where possible. For example, 

steam condensate is recycled back to the to system since it has a low dissolved solids 

content. Condensate that has been in contact with process fluids is also recycled, such 

as distillation overhead condensate and stripped sour water recycled to the desalter.

In general, water needs to have a low dissolved solids content in order to be considered 

as a candidate for recycle in a petroleum refinery. Cooling tower blowdown usually has 

a high dissolved solids content and is sent to wastewater treatment. 
Page 40 of 50

VII. Cooling Water Intake Structures

VII.1 What types of intake structures are used at refineries

Refinery water intake structures were built to accommodate design flow rates and to 

screen out trash, aquatic life, and debris that could damage pumps and heat exchange 

equipment. An intake structure typically consists of a pipe submerged in the waterway or 

a concrete channel at the shore, each with bar screens or trash racks. More 

sophisticated designs can include traveling screens, fixed screens, perforated pipe, and 

velocity caps; however, it is believed that most facilities use simple intake structure 

designs.
Page 41 of 50

VII.2 What is involved in retrofitting refinery intake structures (e.g., moving 

screens)?

Upgrading refinery intake structures involves a variety of considerations, and will depend 

on the type of upgrade being implemented.

 Permitting. Any construction at the shore of a waterway will require permits from 

the Army Corps of Engineers under Section 404 of the Clean Water Act. Such 

permits can involve significant studies to evaluate the impact of new construction on 

the waterway. In some cases, certain technologies (e.g., velocity caps) may be 

difficult to permit because of potential interference with navigation.

 Redesign of intake structure to reduce intake velocity. Meeting potential 

maximum intake velocity requirements could mean extensive redesign of the 

structure to make the intake area larger or to replace pipes with intake channels. 

 Redesign to accommodate new technology. Retrofit of new technology would 

likely require substantial disruption of the existing operation. For example, 

installation of traveling screens will likely require increased intake channel width and 

depth. If the existing intake is a pipe, installation of traveling screens could require 

shutdown of the cooling water flow for days to weeks. 

 Fish return system. For traveling screens, a fish return system would be needed to 

return fish and other aquatic life to the waterway. Such a system would require 

adequate distance from the intake to insure that the organisms are not re-entrained.

 Utility Access. Traveling screens and some other technologies would require 

electrical service to operate pumps, motors, lighting, etc. In many cases, the intake 

can be remotely located, and installation of electrical service can be prohibitive. For 

passive screens, an air supply would be required to provide periodic pulse-cleaning 

capabilities.

 Tie-in/refinery down time. Installation of new intake structures would likely require 

that the cooling water flow be curtailed or stopped completely while the tie-in is 

made. Depending on the nature of the tie-in, the activity could take days or even 

weeks to complete. Since the refinery would not be able to operate at capacity 

during this time, the cost of lost production can be substantial.

 Land availability. In some cases, access to land at the water body may be limited, 

and installation of larger equipment and associated utilities may require the purchase 

of additional property.
Page 42 of 50

VII.3 Other information on cost-effectiveness of intake structure controls?

The cost-effectiveness of intake controls can be highly variable for the population of 

refinery intakes. In some cases, intake controls can be very cost-effective; that is, the 

technology provides a significant reduction in adverse environmental impact (e.g., 

ambient fish population increases through reduced impingement and entrainment) of the 

cooling water intake. On the other hand, in some cases, even the best controls will have 

little impact on the populations of aquatic species in the water body. Therefore, 

evaluation of cost-effectiveness of controls should be done on a site-specific basis. 

Some of the factors that will impact cost-effectiveness include ambient species that 

would be potentially affected, water body size/flow/tidal exchange, prevailing currents, 

ambient water quality for supporting aquatic life, and seasonal variations in all these 

factors.

In the Technical Development Document (EPA-821-R-01-036, November 2001) for the 

316(b) regulations for new facilities, EPA has provided documentation of entrainment 

and impingement performance of various control technologies at power plants. The 

table below summarizes the performance information for some of the technologies that 

may be applicable to refinery intakes. However, the TDD notes that, although these 

technologies are promising, the performance is subject to site-specific considerations, 

and that there are relatively few successful examples of full-scale installations. 

Therefore, information on qualitative costs and technical feasibility considerations has 

been included in this table to provide further insights on technology selection.

Technology Effectiveness Relative cost Technical 

Impingement Entrainment Feasibility

Diversion/behavior

al devices

Highly variable, 

depending on species

No effect Low Generally feasible

Fabric Screen 100% >90% Low Subject to damage 

under severe 

conditions

Velocity cap Species dependent, up 

to 90% reported

No effect Moderate For offshore 

intakes

Wedge-wire screen Reported to be very 

effective ("no 

impingement")

Variable, 

depending on slot 

size

Moderate Prone to plugging, 

biofouling; 

adequate ambient 

water current 

needed.

Traveling finemesh screenUp to 90% Variable, 

depending on mesh 

size, over 90% 

reduction reported 

for certain species

Moderate Prone to plugging, 

biofouling. Little 

operating 

experience.

Traveling screen 

with modified fish 

return system

70-80% reduction over 

conventional traveling 

screens

Generally not 

effective, depends 

on mesh size

Moderate Generally feasible
Page 43 of 50

VIII. Cost Implications

VIII.1 Cursory review of the EPA’s “Draft Initial Cost Estimates”-- May 17, 2001?

Section 316(b) of the Clean Water Act requires EPA to ensure that the location, design, 

construction and capacity of cooling water intake structures reflect the best technology available 

for minimizing adverse environmental impact. EPA prepared a draft report dated May 17, 2001, 

that contains the preliminary analyses to estimate the cost of cooling water intake structures and 

cooling system technologies that are in place at existing facilities.

The draft document was reviewed and we offer the following comments:

Chemical Addition Systems: Chemical addition system costs are not addressed in the draft 

report. Typically, once through cooling systems in refineries do not have chemical addition 

systems other than intermittent chlorination. The chemical use is cooling tower systems 

contribute significantly to operating costs

Salvage Credit: In the EPA report, credit is taken for the salvage value of equipment when 

changing over from once through to recirculating cooling water system. Credit for old 

equipment in the case of refineries is location specific and depends to a large extent on the 

condition of the old equipment. Our experience shows that salvage credit for equipment in good 

condition is no more than 10 cents on the dollar. The analysis of costs should be based on no 

credit for salvage on old equipment and piping.

Installed Costs: An installation factor of 30% on equipment costs has been used to determine 

installed costs. Our experience with refineries, supported by the literature, indicate that this 

factor should be 300 to 600% of equipment costsdepending on the location of the refinery and 

the general standards (for piping and equipment) that are typically applied to equipment and 

piping in refineries.

Dry Cooling: The costs associated with the Dry Cooling option are incomplete. They do not 

include the costs of replacement of turbines, which would represent significant capital costs. 

 



Installed cost should reflect the total fixed capital investment, including not only equipment installation, 

but costs of piping, utilities, instrumentation, yard improvements, service facilities, buildings, contractor’s 

fees, and contingency. Fixed capital investment typically ranges from 371% to 501% of the purchased 

equipment cost; or up 566% under inflationary economic conditions. From Plant Design and Economics 

for Chemical Engineers, Second Edition, Peters, Max S. and Klaus D. Timmerhaus, McGraw-Hill, New 

York, 1968, p.105.
Page 44 of 50

Switching entirely to dry cooling for refineries is not a viable option since, as discussed earlier in 

this report, the cooling needs frequently require the process streams to be subcooled and it is 

not possible to achieve this using air coolers. 

Economic & Engineering Analysis of Proposed 316(b) New Facility Rule, August 2000

The EPA draft report stated that with certain exceptions, EPA developed all of the draft initial 

cost estimates using the cost data presented in the Economic and Engineering Analysis of 

Proposed 316(b) New Facility Rule, August 2000 (EEA).

After review of the EEA, taking into account the referenced exceptions, it appears that the cost 

estimates presented to convert once through cooling to a recirculating system are based on a 

number of assumptions that are not representative of conditions at petroleum refineries and do 

not recognize the inherent differences between refinery and power plant cooling systems. If 

these assumptions were changed and the differences were recognized, the cost for conversion 

of once through to recirculating systems (cooling towers) at refineries would be significantly 

higher. Examples of these assumptions and factors are briefly described below along with the 

reference in the EEA.

a) Differences between Refinery and Power Plant Cooling Systems: The cost for conversion of 

once through cooling to cooling towers at refineries would be significantly higher than for 

power plants. This is due to the inherent differences between petroleum refineries and 

power plants. Based on the greater number of refinery sources that require cooling water 

and the typical geographic layouts of these various refinery sources, petroleum refineries 

would tend to end up with more cooling towers, of smaller size, than power plants. For equal 

intake cooling tower flows, the cost for conversion at petroleum refineries would therefore be 

significantly greater than for power plants. The reference that discusses the cost of 

switching to a recirculation system is found on page 6-4 of the EEA.

b) Differences in Refinery Cooling Tower Intake Flows: The average cooling tower intake flow 

is presented for the category of "petroleum and coal products", Appendix A, Table A-2. The 

number presented is 320 gpm, for 3,509 facilities. There are only approximately 

150 refineries operating in the United States, so this sample base of 3,509 facilities is not 

representative of refineries. 

A limited survey of five refineries using once through water shows the average intake flow to 

be twenty times higher than the EPA estimate.

Consequently, the actual average intake flow rate for refineries is significantly greater than 

used in the EEA. This suggests that the cost curves that were developed and included in 

Appendix A, which correlates capital cost and flow may be inaccurate.
Page 45 of 50

c) Effect of Additional Refinery Heat Exchanger Capacity. As described in the response to 

question II-13, use of cooling tower water instead of once through cooling water will result in 

a 22% increase in heat exchanger area. Considering that refineries are more likely to have 

significantly more sources that require cooling than power plants, the costs for additional 

heat exchanger capacity, for the same cooling water intake flow, will be greater at refineries 

than for power plants. This factor is not included in the discussion on page 6-4 of the EEA 

on switching to a recirculating system.
Page 46 of 50

VIII.2 Estimate the cost of installing a 17,400 gpm cooling tower

Capital Costs:

For comparison purposes, the cost estimate for a 17,400 gpm (circulation flow) cooling tower 

was developed. Even though this circulation rate is small compared to a mid sized refinery, it is 

used for illustration purposes because EPA used this size in the EPA study. The next higher 

EPA case was 104,000 gpm, which was considered too high to use as a representative case. 

The cost estimate includes the cost of the raw water intake system, a clarifier for suspended 

solids removal, induced draft cooling tower, circulating pumps and interconnecting piping. The 

costs do not include any changes that might be required to the heat exchangers in the refinery 

due to the change from once through to a recirculating system. The costs also do not include 

demolition costs and does not take any credit for salvage of old equipment and piping. Figure 6 

shows a schematic of the system that illustrates the systems included in the cost estimate.

Typically, a travelling screen would not be installed for a small intake flow (315 gpm) as is the 

case for this tower system. However, since the purpose of the cost estimate is to compare it to 

the estimate in the EPA draft document, a travelling screen has been included in the estimate.

The costs are based on vendor written and oral quotes for major equipment and ENSR’s 

database for other items.

Basis:

Cooling Water Circulation (gpm) 17,400

Design Temperature Difference (oF) 15

Design Dry Bulb Temperature (oF) 91

Design Wet Bulb temperature (oF) 80

Cooling Water Supply Temperature (oF) 90
Page 47 of 50

Figure 6

Cooling Tower System for Estimate

CW to

Process

Intake Pumps

Capacity: 400 GPM

Head: 120 Ft

Clarifier

Cooling

Tower

Feed Pumps

Capacity: 400 GPM

Head: 30 Ft

Circulating Pumps

Capacity: 20,000 GPM

Makeup Head: 160 Ft

Water

CWR from

Process

200'

3000'

500'

River

Cooling Tower

Battery Limit

500'
Page 48 of 50

a) The following table provides a summary of the cost estimate.

1 Process Equipment Items (Note 1) Quantity Unit Cost Total Cost

2 Intake System

3 Travelling Water Screens 2 70,000 $140,000

4 Intake Pump (315 gpm x 1.25 (SF) = 400 gpm, 120 Ft) 2 10,700 $21,400

5 Clarifier Mechanism (40' Diameter and 16') 1 180,000 $180,000

6 Clarifier Tank (40' D; 16' L Field Erected) 1 235,000 $235,000

7 Feed Pump (315 gpm x 1.25 (SF) = 400 gpm, 30 Ft) 2 8,200 $16,400

8 Cooling Tower

9 Circulation Pump (20,000 gpm, 160 ft) 2 279,000 $558,000

10 Basin (36' x 72') 3 100,000 $300,000

11 Treatment System

12 Dispersant Tank - 2000 gallon 1 3,000 $3,000

13 Feed Pump and Controls 2 2,000 $4,000

14 Corrosion Inhibitor Tank - 500 gallon 1 1,000 $1,000

15 Feed Pumps and Controls 2 2,000 $4,000

16 Sulfuric Acid Tank - 2000 gallon 1 7,000 $7,000

17 Feed Pump and Controls 2 19,000 $38,000

18 Bleach Tank - 5000 gallon 1 8,000 $8,000

19 Feed Pumps and Controls 2 19,000 $38,000

20 Total Process Equipment (T.P.E.) $1,553,800

21 Piping & Mechanical [20]T.P.E. x 0.80 $1,243,000

22 Civil – Site Work, Structures, Foundations & Earthwork [20] T.P.E. x 0.90 $1,398,000

23 Electrical [20] T.P.E. x 0.30 $466,000

24 Instrumentation [20] T.P.E. x 0.15 $233,000

25 Fire Protection [20] T.P.E. x 0.01 $16,000

26 Insulation [20] T.P.E. x 0.02 $31,000

27 Painting [20] T.P.E. x 0.1 $155,000

28 Sub Total (Equipment and Installation T.E & I.C) $5,096,000

29 Project Management & Engineering

30 Home Office Engineering [28] x 0.15 $764,000

31 Construction Management [28] x 0.05 $255,000

32 Line Item (Note 1)

33 Cooling Tower (9000 gpm 2 operating, one spare) 3 $1,034,000 $3,102,000

34 (Field erected, marked up for engineering and civil works)

35 Total Installed Cost (T.I.C.) $9,217,000

36 Rounded up Cost $9,200,000
Page 49 of 50

b) Cooling tower cost obtained from vendor has been marked up:

 20% for engineering

 90% for civil, sitework and foundation

c) Process Equipment Cost: Costs of all process equipment are based on vendor 

quotes and ENSR database. It does not include the cost of any spare parts, taxes, 

freight and future escalation. It also does not include buyer's indirect costs.

d) Cost estimate is based on using ANSI pumps. If API pumps are required the cost of 

the pumps will increase by a factor of three.

e) Piping & Mechanical: The cost of installing pipes interconnecting the process 

equipment. It includes all the valves, fittings and installation.

f) Civil, Site Work, Structures, Foundation and Earthwork: Includes the concrete pad for 

installation of process equipment. It does not include construction of any buildings. 

Does not include demolition and removal of any equipment. It also does not include 

installation of any sewer system. 

g) Electrical: Includes the cost of connecting the new equipment to existing motor 

control centers. Does not include the cost of upgrade of the MCC. Does not include 

the cost of bringing additional power or upgrade of transformers.

h) Instrumentation: Connecting all the necessary process equipment into a new control 

loop and connecting it to an existing control room.

i) Fire Protection: Installation of fire protection.

j) Insulation: The cost of installing insulation on pipes and equipment. It does not 

include any heat tracing of pipes and equipment.

k) Painting: The cost of painting process equipment.

l) Home Office and Engineering: Includes process engineering and detailed 

engineering for the cooling tower systems.

The following is the list of items that are not included in the cost estimate. This list is not 

intended to be a comprehensive list of exclusions but highlights important items:

1. Does not include the cost of any spare parts, taxes, freight and future escalation. It 

also does not include buyer's indirect costs.

2. Cost estimate is based on using ANSI pumps. If API pumps are required the cost of 

the pumps will increase by a factor of three. 

3. Does not include construction of any buildings. Does not include demolition and 

removal of any equipment. It also does not include installation of any sewer system. 

4. Does not include the cost of upgrade of the MCC if required. 

5. Does not include the cost of bringing additional power to the site or upgrade of 

transformers.
Page 50 of 50

6. Does not include the cost of engineering analysis to upgrade heat exchangers in the 

refinery.

7. Does not include the cost of replacing the heat exchangers in the refinery.

8. Does not include the cost of handling and treatment of sludge from the makeup 

water system.

9. Does not include the cost of handling and treatment of the blowdown stream.

10. Does not include the cost land acquisition (if necessary).

11. Operating Costs

The following table defines the operating and maintenance costs for the cooling tower 

system. The operating costs are based on 8,000 hrs/yr of operation.

Category

Cooling 

Tower ($/yr)

Once Through 

($/yr)

Net Costs 

($/yr)

Pumps and Blowers (@ 8,000 hrs/yr, 0.07 $/kwh)

Cooling Tower Fan 156,700 156,700

Intake Pump 6,800 6,800

Feed Pump 2,300 2,300

Recirculating Pump 452,600 452,600 _

Chemical Addition Pumps 2,800 2,800

Chemicals (@ 8,000 hrs/yr)

Dispersant 42,000 42,000

Corrosion Inhibitor 9,600 9,600

Sulfuric Acid 5,000 5,000

Chlorine 27,600 27,600

Spare Parts (2% of Total Purchased equipment -$2,963,800)

59,300 59,300

Labor (Operating and Maintenance (1 @ $30 ea, 8760 hrs) 262,800 262,800 _

Total Operating Cost per Year (rounded) 1,050,000 715,400 334,600

Cost Comparison

The following table compares the costs included in the EPA May 17 2001, draft 

document to the costs developed by ENSR for refineries:

17,400 gpm CT w/travelling screen ENSR estimate EPA estimate

Capital Costs (installed) $ 9,200,000 $ 1,340,000

Incremental Operating Costs $ 334,600 $ 179,000
Page 51 of 50

IX.References

1. Gary, James H., & Handwerk, Glenn E., Petroleum Refining Technology and 

Economics, Marcel Decker Inc, New York, 1975

2. Kern, Donald Q., Process Heat Transfer, McGraw-Hill Book Company, 1950

3. Rase, Howard F., & Barrow, M.H., Project Engineering for Process Plants, John 

Wiley & Sons Inc., 1957

4. Evans, Frank L., Equipment Design Handbook for Refineries and Chemical Plants, 

Gulf Publishing, 1974

5. Nelson, W.L., Petroleum Refinery Engineering, McGraw Hill Company, 1958

6. Perry, John H., Chemical Engineers Handbook, Fourth Edition, McGraw Hill Book 

Company 1963

7. Peters, Max S. and Klaus D. Timmerhaus, Plant Design and Economics for Chemical 

Engineers, Second Edition, McGraw-Hill, New York, 1968.