7. Cooling Tower 

7. COOLING TOWER 

Syllabus 

Cooling Tower: Types and performance evaluation, Efficient system operation, Flow 

control strategies and energy saving opportunities, Assessment of cooling towers 

7.1 Introduction

Cooling towers are a very important part of many chemical plants. The primary task of a 

cooling tower is to reject heat into the atmosphere. They represent a relatively 

inexpensive and dependable means of removing low-grade heat from cooling water. The 

make-up water source is used to replenish water lost to evaporation. Hot water from heat 

exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back 

to the exchangers or to other units for further cooling. Typical closed loop cooling tower 

system is shown in Figure 7.1. 

Figure 7.1 Cooling Water System 

Cooling Tower Types 

Cooling towers fall into two main categories: Natural draft and Mechanical draft. 

Natural draft towers use very large concrete chimneys to introduce air through the media. 

Due to the large size of these towers, they are generally used for water flow rates above 

45,000 m3

/hr. These types of towers are used only by utility power stations. 

Mechanical draft towers utilize large fans to force or suck air through circulated water. 

The water falls downward over fill surfaces, which help increase the contact time between 

the water and the air - this helps maximise heat transfer between the two. Cooling rates of

Mechanical draft towers depend upon their fan diameter and speed of operation. Since, 

the mechanical draft cooling towers are much more widely used, the focus is on them in 

this chapter. 

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7. Cooling Tower 

Mechanical draft towers 

 Mechanical draft towers are available in the following airflow arrangements: 

1. Counter flows induced draft. 

2. Counter flow forced draft. 

3. Cross flow induced draft. 

In the counter flow induced draft design, hot water enters at the top, while the air is 

introduced at the bottom and exits at the top. Both forced and induced draft fans are used. 

 In cross flow induced draft towers, the water enters at the top and passes over the fill. 

The air, however, is introduced at the side either on one side (single-flow tower) or 

opposite sides (double-flow tower). An induced draft fan draws the air across the wetted 

fill and expels it through the top of the structure. 

 The Figure 7.2 illustrates various cooling tower types. Mechanical draft towers are 

available in a large range of capacities. Normal capacities range from approximately 10 

tons, 2.5 m3

/hr flow to several thousand tons and m3/hr. Towers can be either factory built 

or field erected – for example concrete towers are only field erected. 

Many towers are constructed so that they can be grouped together to achieve the 

desired capacity. Thus, many cooling towers are assemblies of two or more individual 

cooling towers or “cells.” The number of cells they have, e.g., a eight-cell tower, often 

refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon 

the shape of the individual cells and whether the air inlets are located on the sides or 

bottoms of the cells. 

Components of Cooling Tower 

The basic components of an evaporative tower are: Frame and casing, fill, cold water 

basin, drift eliminators, air inlet, louvers, nozzles and fans. 

Frame and casing: Most towers have structural frames that support the exterior 

enclosures (casings), motors, fans, and other components. With some smaller designs, 

such as some glass fiber units, the casing may essentially be the frame. 

Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by 

maximising water and air contact. Fill can either be splash or film type. 

With splash fill, water falls over successive layers of horizontal splash bars, continuously 

breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill 

promotes better heat transfer than the wood splash fill. 

Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, 

forming a thin film in contact with the air. These surfaces may be flat, corrugated, 

honeycombed, or other patterns. The film type of fill is the more efficient and provides 

same heat transfer in a smaller volume than the splash fill. 

Cold water basin: The cold water basin, located at or near the bottom of the tower, 

receives the cooled water that flows down through the tower and fill. The basin usually 

has a sump or low point for the cold water discharge connection. In many tower designs, 

the cold water basin is beneath the entire fill. 

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7. Cooling Tower 

some forced draft counter flow design, however, the water at the bottom of the fill is 

Figure 7.2 Cooling Tower Types 

In

channeled to a perimeter trough that functions as the cold water basin. Propeller fans are 

mounted beneath the fill to blow the air up through the tower. With this design, the tower 

is mounted on legs, providing easy access to the fans and their motors. 

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7. Cooling Tower 

Drift eliminators: These capture water droplets entrapped in the air stream that otherwise 

would be lost to the atmosphere. 

Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an 

entire side of a tower—cross flow design— or be located low on the side or the bottom of 

counter flow designs. 

Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to 

equalize air flow into the fill and retain the water within the tower. Many counter flow 

tower designs do not require louvers. 

Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the 

top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can 

either be fixed in place and have either round or square spray patterns or can be part of a 

rotating assembly as found in some circular cross-section towers. 

Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, 

propeller fans are used in induced draft towers and both propeller and centrifugal fans are 

found in forced draft towers. Depending upon their size, propeller fans can either be fixed 

or variable pitch. 

A fan having non-automatic adjustable pitch blades permits the same fan to be used over 

a wide range of kW with the fan adjusted to deliver the desired air flow at the lowest 

power consumption. 

Automatic variable pitch blades can vary air flow in response to changing load conditions. 

Tower Materials

In the early days of cooling tower manufacture, towers were constructed primarily of 

wood. Wooden components included the frame, casing, louvers, fill, and often the cold 

water basin. If the basin was not of wood, it likely was of concrete. 

 Today, tower manufacturers fabricate towers and tower components from a variety of 

materials. Often several materials are used to enhance corrosion resistance, reduce 

maintenance, and promote reliability and long service life. Galvanized steel, various 

grades of stainless steel, glass fiber, and concrete are widely used in tower construction as 

well as aluminum and various types of plastics for some components. 

Wood towers are still available, but they have glass fiber rather than wood panels 

(casing) over the wood framework. The inlet air louvers may be glass fiber, the fill may 

be plastic, and the cold water basin may be steel. 

Larger towers sometimes are made of concrete. Many towers—casings and basins—

are constructed of galvanized steel or, where a corrosive atmosphere is a problem, 

stainless steel. Sometimes a galvanized tower has a stainless steel basin. Glass fiber is 

also widely used for cooling tower casings and basins, giving long life and protection 

from the harmful effects of many chemicals. 

 Plastics are widely used for fill, including PVC, polypropylene, and other polymers. 

Treated wood splash fill is still specified for wood towers, but plastic splash fill is also 

widely used when water conditions mandate the use of splash fill. Film fill, because it 

offers greater heat transfer efficiency, is the fill of choice for applications where the 

circulating water is generally free of debris that could plug the fill passageways. 

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7. Cooling Tower 

Plastics also find wide use as nozzle materials. Many nozzles are being made of PVC, 

ABS, polypropylene, and glass-filled nylon. Aluminum, glass fiber, and hot-dipped 

galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated 

from galvanized steel. Propeller fans are fabricated from galvanized, aluminum, or 

molded glass fiber reinforced plastic. 

7.2 Cooling Tower Performance

The important parameters, from the point of determining the performance of cooling 

towers, are: 

Figure 7.3 Range and Approach 

i) “Range” is the difference between the cooling tower water inlet and outlet 

temperature. (See Figure 7.3). 

ii) “Approach” is the difference between the cooling tower outlet cold water 

temperature and ambient wet bulb temperature. Although, both range and 

approach should be monitored, the `Approach’ is a better indicator of cooling 

tower performance. (see Figure 7.3). 

iii) Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal 

range, i.e., difference between cooling water inlet temperature and ambient wet 

bulb temperature, or in other words it is = Range / (Range + Approach). 

iv) Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass 

flow rate of water, specific heat and temperature difference. 

v) Evaporation loss is the water quantity evaporated for cooling duty and, 

theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works 

out to 1.8 m3

. An empirical relation used often is: 

 

Evaporation Loss (m3

/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) 

 T1-T2 = Temp. difference between inlet and outlet water. 

 Source: Perry’s Chemical Engineers Handbook (Page: 12-17) 

 

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7. Cooling Tower 

vi) Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water 

to the dissolved solids in make up water. 

vii) Blow down losses depend upon cycles of concentration and the evaporation losses 

and is given by relation: 

Blow Down = Evaporation Loss / (C.O.C. – 1) 

viii) Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the 

air mass flow rates. Against design values, seasonal variations require adjustment 

and tuning of water and air flow rates to get the best cooling tower effectiveness 

through measures like water box loading changes, blade angle adjustments. 

Thermodynamics also dictate that the heat removed from the water must be equal 

to the heat absorbed by the surrounding air: 

where: 

 L/G = liquid to gas mass flow ratio (kg/kg) 

 T1 = hot water temperature (0

C) 

 T2 = cold water temperature (0

C) 

 h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature 

 (same units as above) 

 h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same 

 units as above) 

Factors Affecting Cooling Tower Performance 

Capacity 

Heat dissipation (in kCal/hour) and circulated flow rate (m3

/hr) are not sufficient to 

understand cooling tower performance. Other factors, which we will see, must be stated 

along with flow rate m3

/hr. For example, a cooling tower sized to cool 4540 m3/hr 

through a 13.9o

C range might be larger than a cooling tower to cool 4540 m3/hr through 

19.5o

C range. 

Range 

Range is determined not by the cooling tower, but by the process it is serving. The range 

at the exchanger is determined entirely by the heat load and the water circulation rate 

through the exchanger and on to the cooling water. 

Range o

C = Heat Load in kcals/hour / Water Circulation Rate in LPH 

Thus, Range is a function of the heat load and the flow circulated through the system. 

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7. Cooling Tower 

Cooling towers are usually specified to cool a certain flow rate from one temperature to 

another temperature at a certain wet bulb temperature. For example, the cooling tower 

might be specified to cool 4540 m3

/hr from 48.9oC to 32.2oC at 26.7oC wet bulb 

temperature. 

Cold Water Temperature 32.2oC – Wet Bulb Temperature (26.7oC) = Approach 

(5.5o

C) 

As a generalization, the closer the approach to the wet bulb, 

the more expensive the cooling tower due to increased size. 

Usually a 2.8o

C approach to the design wet bulb is the 

coldest water temperature that cooling tower manufacturers 

will guarantee. If flow rate, range, approach and wet bulb 

had to be ranked in the order of their importance in sizing a 

tower, approach would be first with flow rate closely 

following the range and wet bulb would be of lesser 

importance. 

 Heat Load 

The heat load imposed on a cooling tower is determined by the process being served. The 

degree of cooling required is controlled by the desired operating temperature level of the 

process. In most cases, a low operating temperature is desirable to increase process 

efficiency or to improve the quality or quantity of the product. In some applications (e.g. 

internal combustion engines), however, high operating temperatures are desirable. The 

size and cost of the cooling tower is proportional to the heat load. If heat load calculations 

are low undersized equipment will be purchased. If the calculated load is high, oversize 

and more costly, equipment will result. 

 Process heat loads may vary considerably depending upon the process involved. 

Determination of accurate process heat loads can become very complex but proper 

consideration can produce satisfactory results. On the other hand, air conditioning and 

refrigeration heat loads can be determined with greater accuracy. 

 Information is available for the heat rejection requirements of various types of power 

equipment. A sample list is as follows: 

* Air Compressor 

 - Single-stage - 129 kCal/kW/hr 

- Single-stage with after cooler - 862 kCal/kW/hr 

 - Two-stage with intercooler - 518 kCal/kW/hr 

- Two-stage with intercooler and after cooler - 862 kCal/kW/hr 

* Refrigeration, Compression - 63 kCal/min/TR 

* Refrigeration, Absorption - 127 kCal/min/TR 

* Steam Turbine Condenser - 555 kCal/kg of 

 steam

* Diesel Engine, Four-Cycle, Supercharged - 880 kCal/kW/hr 

* Natural Gas Engine, Four-cycle - 1523 kCal/kW/hr 

 (18 kg/cm2

 compression) 

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7. Cooling Tower 

Wet Bulb Temperature 

Wet bulb temperature is an important factor in performance of evaporative water cooling 

equipment. It is a controlling factor from the aspect of minimum cold water temperature 

to which water can be cooled by the evaporative method. Thus, the wet bulb temperature 

of the air entering the cooling tower determines operating temperature levels throughout 

the plant, process, or system. Theoretically, a cooling tower will cool water to the entering 

wet bulb temperature, when operating without a heat load. However, a thermal potential is 

required to reject heat, so it is not possible to cool water to the entering air wet bulb 

temperature, when a heat load is applied. The approach obtained is a function of thermal 

conditions and tower capability. 

Initial selection of towers with respect to design wet bulb temperature must be made on 

the basis of conditions existing at the tower site. The temperature selected is generally 

close to the average maximum wet bulb for the summer months. An important aspect of 

wet bulb selection is, whether it is specified as ambient or inlet. The ambient wet bulb is 

the temperature, which exists generally in the cooling tower area, whereas inlet wet bulb 

is the wet bulb temperature of the air entering the tower. The later can be, and often is, 

affected by discharge vapours being recirculated into the tower. Recirculation raises the 

effective wet bulb temperature of the air entering the tower with corresponding increase 

in the cold water temperature. Since there is no initial knowledge or control over the 

recirculation factor, the ambient wet bulb should be specified. The cooling tower supplier 

is required to furnish a tower of sufficient capability to absorb the effects of the increased 

wet bulb temperature peculiar to his own equipment. 

It is very important to have the cold water temperature low enough to exchange heat or to 

condense vapours at the optimum temperature level. By evaluating the cost and size of 

heat exchangers versus the cost and size of the cooling tower, the quantity and 

temperature of the cooling tower water can be selected to get the maximum economy for 

the particular process. 

The Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. 

The towers included were sized to cool 4540 m3

/hr through a 16.67oC range at a 26.7oC 

design wet bulb. The overall width of all towers is 21.65 meters; the overall height, 15.25 

meters, and the pump head, 10.6 m approximately. 

 Table 7.1 APPROACH Vs. COOLING TOWER SIZE

(4540 m3/hr; 16.67oC Range 26.7oC Wet Bulb; 10.7 m Pump Head)

Approach oC 2.77 3.33 3.88 4.44 5.0 5.55

Hot Water oC 46.11 46.66 47.22 47.77 48.3 48.88

Cold Water oC 29.44 30 30.55 31.11 31.66 32.22

No. of Cells 4 4 3 3 3 3 

Length of Cells Mts. 10.98 8.54 10.98 9.76 8.54 8.54 

Overall Length Mts. 43.9 34.15 32.93 29.27 25.61 25.61

No. of Fans 4 4 3 3 3 3 

Fan Diameter Mts. 7.32 7.32 7.32 7.32 7.32 6.71 

Total Fan kW 270 255 240 202.5 183.8 183.8

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7. Cooling Tower 

Approach and Flow

Suppose a cooling tower is installed that is 21.65 m wide × 36.9 m long × 15.24m high, 

has three 7.32 m diameter fans and each powered by 25 kW motors. The cooling tower 

cools from 3632 m3

/hr water from 46.1oC to 29.4oC at 26.7oC WBT dissipating 60.69 

million kCal/hr. The Table 7.2 shows what would happen with additional flow but with 

the range remaining constant at 16.67o

C. The heat dissipated varies from 60.69 million 

kCal/hr to 271.3 million kCal/hr. 

 Table 7.2 Flow vs. Approach for a Given Tower 

 (Tower is 21.65 m × 36.9 M; Three 7.32 M Fans; Three 25 kW Motors; 

 16.7oC Range with 26.7oC Wet Bulb)

Flow m3/hr Approach o

C 

Cold Water o

C 

Hot Water o

C 

Million 

kCal/hr 

3632 2.78 29.40 46.11 60.691

4086 3.33 29.95 46.67 68.318

4563 3.89 30.51 47.22 76.25

5039 4.45 31.07 47.78 84.05

5516 5.00 31.62 48.33 92.17

6060.9 5.56 32.18 48.89 101.28

7150.5 6.67 33.29 50.00 119.48

8736 8.33 35.00 51.67 145.63

11590 11.1 37.80 54.45 191.64

13620 13.9 40.56 57.22 226.91

16276 16.7 43.33 60.00 271.32

For meeting the increased heat load, few modifications would be needed to increase the 

water flow through the tower. However, at higher capacities, the approach would 

increase. 

Range, Flow and Heat Load 

Range is a direct function of the quantity of water circulated and the heat load. Increasing 

the range as a result of added heat load does require an increase in the tower size. If the 

cold water temperature is not changed and the range is increased with higher hot water 

temperature, the driving force between the wet bulb temperature of the air entering the 

tower and the hot water temperature is increased, the higher level heat is economical to 

dissipate. 

If the hot water temperature is left constant and the range is increased by specifying a 

lower cold water temperature, the tower size would have to be increased considerably. 

Not only would the range be increased, but the lower cold water temperature would lower 

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7. Cooling Tower 

the approach. The resulting change in both range and approach would require a much 

larger cooling tower. 

Approach & Wet Bulb Temperature 

The design wet bulb temperature is determined by the geographical location. Usually the 

design wet bulb temperature selected is not exceeded over 5 percent of the time in that 

area. Wet bulb temperature is a factor in cooling tower selection; the higher the wet bulb 

temperature, the smaller the tower required to give a specified approach to the wet bulb at 

a constant range and flow rate. 

 A 4540 m3

/hr cooling tower selected for a 16.67oC range and a 4.45oC approach to 

21.11o

C wet bulb would be larger than a 4540 m3/hr tower selected for a 16.67oC range 

and a 4.45o

C approach to a 26.67oC wet bulb. Air at the higher wet bulb temperature is 

capable of picking up more heat. Assume that the wet bulb temperature of the air is 

increased by approximately 11.1o

C. As air removes heat from the water in the tower, 

each kg of air entering the tower at 21.1o

C wet bulb would contain 18.86 kCals and if it 

were to leave the tower at 32.2o

C wet bulb it would contain 24.17 kCal per kg of air. 

 In the second case, each kg of air entering the tower at 26.67o

C wet bulb would

contain 24.17 kCals and were to leave at 37.8o

C wet bulb it would contain 39.67 kCal 

per kg of air. 

 In going from 21.1oC to 32.2oC, 12.1 kCal per kg of air is picked up, while 15.5 

kCal/kg of air is picked up in going from 26.67o

C to 37.8oC. 

Fill Media Effects 

In a cooling tower, hot water is distributed above fill media which flows down and is 

cooled due to evaporation with the intermixing air. Air draft is achieved with use of fans. 

Thus some power is consumed in pumping the water to a height above the fill and also by 

fans creating the draft. 

 An energy efficient or low power consuming cooling tower is to have efficient designs 

of fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor. 

Power savings in a cooling tower, with use of efficient fill design, is directly reflected as 

savings in fan power consumption and pumping head requirement. 

Function of Fill media in a Cooling Tower

Heat exchange between air and water is influenced by surface area of heat exchange, time 

of heat exchange (interaction) and turbulence in water effecting thoroughness of 

intermixing. Fill media in a cooling tower is responsible to achieve all of above. 

Splash and Film Fill Media: As the name indicates, splash fill media generates the 

required heat exchange area by splashing action of water over fill media and hence 

breaking into smaller water droplets. Thus, surface of heat exchange is the surface area of 

the water droplets, which is in contact with air. 

Film Fill and its Advantages 

In a film fill, water forms a thin film on either side of fill sheets. Thus area of heat 

exchange is the surface area of the fill sheets, which is in contact with air. 

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7. Cooling Tower 

Typical comparison between various fill media is shown in Table 7.3. 

Table 7.3 Typical Comparisons Between Various Fill Media

Splash Fill Film Fill Low Clog Film Fill 

Possible L/G Ratio 1.1 – 1.5 1.5 – 2.0 1.4 – 1.8 

Effective Heat Exchange Area 30 – 45 m2/m3 150 m2/m3 85 – 100 m2/m3

Fill Height Required 5 – 10 m 1.2 – 1.5 m 1.5 – 1.8 m

Pumping Head Requirement 9 – 12 m 5 – 8 m 6 – 9 m

Quantity of Air Required High Much low Low 

Due to fewer requirements of air and pumping head, there is a tremendous saving in 

power with the invention of film fill. 

Recently, low-clog film fills with higher flute sizes have been developed to handle high 

turbid waters. For sea water, low clog film fills are considered as the best choice in terms 

of power saving and performance compared to conventional splash type fills. 

Choosing a Cooling Tower 

The counter-flow and cross flows are two basic designs of cooling towers based on the 

fundamentals of heat exchange. It is well known that counter flow heat exchange is more 

effective as compared to cross flow or parallel flow heat exchange. 

Cross-flow cooling towers are provided with splash fill of concrete, wood or perforated 

PVC. Counter-flow cooling towers are provided with both film fill and splash fill. 

Typical comparison of Cross flow Spash Fill, Counter Flow Tower with Film Fill and 

Splash fill is shown in Table 7.4. The power consumption is least in Counter Flow Film

Fill followed by Counter Flow Splash Fill and Cross-Flow Splash Fill. 

Table 7.4 Typical Comparison of Cross flow splash fill, Counter Flow Tower 

 with Film Fill and Splash Fill 

Number of Towers : 2 

Water Flow : 16000 m3

/hr. 

Hot Water Temperature : 41.5o

C 

Cold Water Temperature : 32.5o

C 

Design Wet Bulb Temperature : 27.6o

C 

Counter Flow 

Film Fill 

Counter Flow 

Splash Fill 

Cross-Flow 

Splash Fill 

Fill Height, Meter 1.5 5.2 11.0

Plant Area per Cell 14.4 × 14.4 14.4 × 14.4 12.64 × 5.49 

Number of Cells per Tower 6 6 5 

Power at Motor Terminal/Tower, kW 253 310 330 

Static Pumping Head, Meter 7.2 10.9 12.05 

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7. Cooling Tower 

7.3 Efficient System Operation 

 

Cooling Water Treatment 

Cooling water treatment is mandatory for any cooling tower whether with splash fill or 

with film type fill for controlling suspended solids, algae growth, etc. 

With increasing costs of water, efforts to increase Cycles of Concentration (COC), by 

Cooling Water Treatment would help to reduce make up water requirements significantly. 

In large industries, power plants, COC improvement is often considered as a key area for 

water conservation. 

 

Drift Loss in the Cooling Towers 

It is very difficult to ignore drift problem in cooling towers. Now-a-days most of the end 

user specification calls for 0.02% drift loss. 

 With technological development and processing of PVC, manufacturers have brought 

large change in the drift eliminator shapes and the possibility of making efficient designs 

of drift eliminators that enable end user to specify the drift loss requirement to as low as 

0.003 – 0.001%. 

Cooling Tower Fans 

The purpose of a cooling tower fan is to move a specified quantity of air through the 

system, overcoming the system resistance which is defined as the pressure loss. The 

product of air flow and the pressure loss is air power developed/work done by the fan; this 

may be also termed as fan output and input kW depends on fan efficiency. 

The fan efficiency in turn is greatly dependent on the profile of the blade. An 

aerodynamic profile with optimum twist, taper and higher coefficient of lift to coefficient 

of drop ratio can provide the fan total efficiency as high as 85-92 %. However, this 

efficiency is drastically affected by the factors such as tip clearance, obstacles to airflow 

and inlet shape, etc. 

As the metallic fans are manufactured by adopting either extrusion or casting process it 

is always difficult to generate the ideal aerodynamic profiles. The FRP blades are 

normally hand moulded which facilitates the generation of optimum aerodynamic profile 

to meet specific duty condition more efficiently. Cases reported where replacement of 

metallic or Glass fibre reinforced plastic fan blades have been replaced by efficient 

hollow FRP blades, with resultant fan energy savings of the order of 20-30% and with 

simple pay back period of 6 to 7 months. 

Also, due to lightweight, FRP fans need low starting torque resulting in use of lower 

HP motors. The lightweight of the fans also increases the life of the gear box, motor and 

bearing is and allows for easy handling and maintenance.

Performance Assessment of Cooling Towers 

In operational performance assessment, the typical measurements and observations 

involved are: 

• Cooling tower design data and curves to be referred to as the basis. 

• Intake air WBT and DBT at each cell at ground level using a whirling 

pyschrometer. 

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7. Cooling Tower 

• Exhaust air WBT and DBT at each cell using a whirling psychrometer. 

• CW inlet temperature at risers or top of tower, using accurate mercury in glass or a 

digital thermometer. 

• CW outlet temperature at full bottom, using accurate mercury in glass or a digital 

thermometer.

• Process data on heat exchangers, loads on line or power plant control room

readings, as relevant. 

• CW flow measurements, either direct or inferred from pump motor kW and pump 

head and flow characteristics. 

• CT fan motor amps, volts, kW and blade angle settings 

• TDS of cooling water. 

• Rated cycles of concentration at the site conditions. 

• Observations on nozzle flows, drift eliminators, condition of fills, splash bars, etc. 

The findings of one typical trial pertaining to the Cooling Towers of a Thermal Power Plant 

3 x 200 MW is given below: 

Observations 

* Unit Load 1 & 3 of the Station = 398 MW 

* Mains Frequency = 49.3

* Inlet Cooling Water Temperature o

C = 44 (Rated 43oC) 

* Outlet Cooling Water Temperature o

C = 37.6 (Rated 33oC) 

* Air Wet Bulb Temperature near Cell o

C = 29.3 (Rated 27.5oC) 

* Air Dry Bulb Temperature near Cell o

C = 40.8oC 

* Number of CT Cells on line with water flow = 45 (Total 48) 

* Total Measured Cooling Water Flow m3

/hr = 70426.76

* Measured CT Fan Flow m3

/hr = 989544

Analysis 

* CT water Flow/Cell, m3/hr = 1565 m3/hr (1565000 kg/hr) 

(Rated 1875 m3

/hr) 

* CT Fan air Flow, m3

/hr (Avg.) = 989544 m3/hr 

(Rated 997200 m3

/hr) 

* CT Fan air Flow kg/hr (Avg.) 

@ Density of 1.08 kg/m3

= 1068708 kg/hr

* L/G Ratio of C.T. kg/kg = 1.46 

(Rated 1.74 kg/kg) 

* CT Range = (44 – 37.6) = 6.4o

C 

* CT Approach = (37.6 – 29.3) = 8.3o

C 

* % CT Effectiveness = 100

( )

x

Range Approach

Range

+

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7. Cooling Tower 

= 100

(6.4 8.3)

6.4

x

+

= 43.53

* Rated % CT Effectiveness = 100 * (43 – 33) / (43 – 

27.5) 

= 64.5%

* Cooling Duty Handled/Cell in kCal = 1565 * 6.4 * 103

(i.e., Flow * Temperature Difference in 

kCal/hr) 

= 10016 * 103 kCal/hr (Rated 

18750 * 103

 kCal/hr) 

* Evaporation Losses in m3

/hr = 0.00085 x 1.8 x circulation 

rate (m3

/hr) x (T1-T2) 

= 0.00085 x 1.8 x 1565 x (44-

37.6) 

= 15.32 m3

/hr per cell 

* Percentage Evaporation Loss = [15.32/1565]*100 

= 0.97%

* Blow down requirement for site COC of 2.7 = Evaporation losses / 

(COC–1) 

= 15.32/(2.7–1) per cell i.e., 

9.01 m3

/hr 

* Make up water requirement/cell in m3

/hr = Evaporation Loss + Blow 

down Loss 

= 15.32 + 9.01 

= 24.33

Comments 

• Cooling water flow per cell is much lower, almost by 16.5%, need to investigate 

CW pump and system performance for improvements. Increasing CW flow 

through cell was identified as a key result area for improving performance of 

cooling towers. 

• Flow stratification in 3 cooling tower cells identified. 

• Algae growth identified in 6 cooling tower cells. 

• Cooling tower fans are of GRP type drawing 36.2 kW average. Replacement by 

efficient hollow FRP fan blades is recommended. 

7.4 Flow Control Strategies 

Control of tower air flow can be done by varying methods: starting and stopping (On-off) 

of fans, use of two- or three-speed fan motors, use of automatically adjustable pitch fans, 

use of variable speed fans. 

On-off fan operation of single speed fans provides the least effective control. Two-speed 

fans provide better control with further improvement shown with three speed fans. 

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7. Cooling Tower 

Automatic adjustable pitch fans and variable-speed fans can provide even closer control 

of tower cold-water temperature. In multi-cell towers, fans in adjacent cells may be 

running at different speeds or some may be on and others off depending upon the tower 

load and required water temperature. Depending upon the method of air volume control 

selected, control strategies can be determined to minimise fan energy while achieving the 

desired control of the Cold water temperature.

7.5 Energy Saving Opportunities in Cooling Towers 

− Follow manufacturer’s recommended clearances around cooling towers and 

relocate or modify structures that interfere with the air intake or exhaust. 

− Optimise cooling tower fan blade angle on a seasonal and/or load basis. 

− Correct excessive and/or uneven fan blade tip clearance and poor fan balance. 

− On old counter-flow cooling towers, replace old spray type nozzles with new 

square spray ABS practically non-clogging nozzles. 

− Replace splash bars with self-extinguishing PVC cellular film fill. 

− Install new nozzles to obtain a more uniform water pattern 

− Periodically clean plugged cooling tower distribution nozzles. 

− Balance flow to cooling tower hot water basins. 

− Cover hot water basins to minimise algae growth that contributes to fouling. 

− Optimise blow down flow rate, as per COC limit. 

− Replace slat type drift eliminators with low pressure drop, self extinguishing, 

PVC cellular units. 

− Restrict flows through large loads to design values. 

− Segregate high heat loads like furnaces, air compressors, DG sets, and isolate 

cooling towers for sensitive applications like A/C plants, condensers of captive 

power plant etc. A 1o

C cooling water temperature increase may increase A/C 

compressor kW by 2.7%. A 1o

C drop in cooling water temperature can give a 

heat rate saving of 5 kCal/kWh in a thermal power plant. 

− Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations. 

It would help to increase water load during summer and times when approach 

is high and increase air flow during monsoon times and when approach is 

narrow. 

− Monitor approach, effectiveness and cooling capacity for continuous 

optimisation efforts, as per seasonal variations as well as load side variations. 

− Consider COC improvement measures for water savings. 

− Consider energy efficient FRP blade adoption for fan energy savings. 

− Consider possible improvements on CW pumps w.r.t. efficiency improvement. 

− Control cooling tower fans based on leaving water temperatures especially in 

case of small units. 

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7. Cooling Tower 

− Optimise process CW flow requirements, to save on pumping energy, cooling 

load, evaporation losses (directly proportional to circulation rate) and blow 

down losses. 

Some typical problems and their trouble shooting for cooling towers are given in Table 

7.5. 

Table 7.5 Typical Problems and Trouble Shooting for Cooling Towers 

Problem / Difficulty Possible Causes Remedies/Rectifying Action

1. Voltage Reduction Check the voltage 

2a. Incorrect angle of axial fan blades Adjust the blade angle 

2b. Loose belts on centrifugal fans 

 (or speed reducers) 

Check belt tightness 

3. Overloading owing to excessive air 

 flow-fill has minimum water 

 loading per m2

 of tower section 

Regulate the water flow by means 

of the valve 

Excessive absorbed 

current / electrical load 

4. Low ambient air temperature The motor is cooled 

proportionately and hence delivers 

more than name plate power 

1. Uneven operation of spray nozzles Adjust the nozzle orientation and 

eliminate any dirt 

2. Blockage of the fill pack Eliminate any dirt in the top of the 

fill 

3. Defective or displaced droplet 

 eliminators 

Replace or realign the eliminators 

Drift/carry-over of 

water outside the unit

4. Excessive circulating water flow 

 (possibly owing to too high 

 pumping head) 

Adjust the water flow-rate by

means of the regulating valves. 

Check for absence of damage to 

the fill 

Loss of water from 1. Float-valve not at correct level Adjust the make-up valve 

basins/pans 2. Lack of equalising connections Equalise the basins of towers 

operating in parallel 

1. Water flow below the design valve Regulated the flow by means of 

the valves 

2. Irregular airflow or lack of air Check the direction of rotation of 

the fans and/or belt tension 

(broken belt possible) 

3a. Recycling of humid discharge air Check the air descent velocity 

3b. Intake of hot air from other sources Install deflectors 

4a. Blocked spray nozzles (or even 

 blocked spray tubes) 

Clean the nozzles and/or the tubes 

4b. Scaling of joints Wash or replace the item

Lack of cooling and 

hence increase in 

temperatures owing to

increased temperature 

range 

5. Scaling of the fill pack Clean or replace the material 

(washing with inhibited aqueous 

sulphuric acid is possible but long, 

complex and expensive) 

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7. Cooling Tower 

QUESTIONS

1. What do you understand by the following terms in respect of cooling 

towers? 

a) Approach, b) Cooling Duty c) Range d) Cooling Tower Effectiveness 

2. Explain with a sketch the different types of cooling towers. 

3. What do you mean by the term of Cycles of Concentration and how it is 

related to cooling tower blow down?

4. Explain the term L/G ratio?

5. CT Observations at an industrial site were 

* CW Flow : 5000 m3

/hr 

* CW in Temperature : 42o

C 

* CW Out Temperature : 36o

C 

* Wet Bulb Temperature : 29o

C 

What is the Effectiveness of the cooling tower? 

6. What is the function of fill media in a cooling tower? 

7. List the factors affecting cooling tower performance. 

8. List the energy conservation opportunities in a cooling tower system. 

9. Explain the difference between evaporation loss and drift loss?

10. What is the Blow-down Loss, if the Cycles of Concentration (COC) is 

3.0?

REFERENCES

1. ASHRAE Handbook 

2. NPC Case Studies 

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