7. C ool i ng Tower



## 7. COOLING TOWER

Syllabus Cooling Tow er: Types and perform a nce evaluation, Efficient system operation, Flow control strategies and energy saving opportunities, Assessm ent of cooling towers



### 7.1 Introduction

Cooling towers are a very im portant part of m a ny chem ical plants. The prim ary task of a cooling tower is to reject heat into th e atm o sphere. They represent a relatively inexpensive and dependable m eans of rem oving low-grade heat from cooling water. The

m a ke-up water source is used to replenish wate r 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 furt her cooling. Typical closed loop cooling tower system is shown in Figure 7.1.

Fi gure 7.1 Cool i n g Water System

Cooling Tow er Types

Cooling towers f a ll into two m a in categories: Natural draf t and Mechanical draf t.
Natural draft towers use very large concrete chim neys to introduce air through the m e dia. Due to the large size of these towers, they ar e generally used for water flow rates above 45,000 m 3 /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, wh ich help increase the contact tim e between the water and the air - this helps m a xim i se heat transf er between the two. Cooling rates of Mechanica l draft towers depend upon their fan diam eter and speed of operation. Since, the m echanical draft cooling towers are m u ch m o re widely used, the focus is on them in this chapter.

Bureau of Energy Efficiency 1357. C ool i ng Tower

Mechanical draft tow ers
Mechanical draft towers are available in the following airflow arrangem e nts:

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 f l ow induced draf t towers, the water enters at the top and passes over the f ill. 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. Norm al capacities range f r om approxim a tely 10 tons, 2.5 m 3 /hr flow to several thousand tons and m 3 /hr. Towers can be either f actory built or field erected – for exam ple concrete towers are only field erected. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, m a ny cooling towers are assem b lies of two or m o re individual cooling towers or “cells.” The num ber 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 bottom s of the cells.

Components of Cooling Tow er

The basic com ponents of an evaporative tower are: Fram e and casing, fill, cold water basin, drift elim inators, air inlet, louvers, nozzles and fans.

Frame and casing: Most towers have structural fram es that support the exterior enclosures (casings), m o tors, fans, and other com ponents. W ith som e sm aller designs, such as som e glass fiber units, the casing m a y essentially be the fram e.

Fill: Most towers em ploy fills (m ade of plastic or wood) to facilitate heat transfer by

m a xim i sing water and air contact. Fill can either be splash or f ilm type.

W ith splash fill, water falls over successive la yers of horizontal splash bars, continuously breaking into sm aller droplets, while also wetting the fill surface. Plastic splash fill prom otes better heat transfer than the wood splash fill.

Film fill consists of thin, closely spaced pl astic surfaces over which the water spreads, form ing a thin film in contact with the air. These surfaces m a y be flat, corrugated, honeycom bed, or other patterns. The film type of fill is the m o re efficient and provides

sam e heat transf er in a sm aller volum e than the splash f ill.

Cold w a ter basin: The cold water basin, located at or near the bottom of the tower, receives the cooled water that flows down th rough the tower and fill. The basin usually has a sum p or low point for the cold wate r discharge connection. In m a ny tower designs, the cold water basin is beneath the entire f ill.

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s o m e forced draft counter flow design, how ever, the water at the bottom of the fill is Fi gure 7.2 Cool i n g T o w er T y pes

I n channeled to a perim e ter trough that functions as the cold water basin. Propeller fans are

m ounted beneath the fill to bl ow the air up through the tower. W ith this design, the tower is m ounted on legs, providing easy access to the fans and their m o tors.

Bureau of Energy Efficiency 1377. C ool i ng Tower

Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atm o sphere.

Air inlet: This is the point of entry f o r the air entering a tower. The inlet m a y 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 inle t louvers. The purpose of louvers is to equalize air flow into the fill and retain th e water within the tower. Many counter flow tower designs do not require louvers.

N o zzl e s : These provide the water sprays to wet the f ill. Unif orm 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 assem b ly as found in som e circular cross-section towers.

Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towe rs and both propeller and centrifugal fans are found in forced draft towers. Depending upon thei r size, propeller fans can either be fixed or variable pitch.

A fan having non-autom a tic adjustable pitch blad es perm its the sam e fan to be used over a wide range of kW with the fan adjusted to deliver the desired air flow at the lowest power consum ption.

Autom a tic variable pitch blades can vary air flow in response to changing load conditions.

Tow er Materials

In the early days of cooling tower m a nufactur e, towers were constructed prim arily of wood. W ooden com ponents included the fram e, casing, louvers, fill, and often the cold water basin. If the basin was not of wood, it likely was of concrete. Today, tower m a nufacturers fabricate towe rs and tower com ponents from a variety of

m a terials. Often several m a terials are used to enhance corrosion resistance, reduce

m a intenance, and prom ote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fiber, and conc rete are widely used in tower construction as well as alum inum and various types of plastics for som e com ponents. W ood towers are still available, but they have glass fiber rather than wood panels (casing) over the wood fram ework. The inlet ai r louvers m a y be glass fiber, the fill m a y be plastic, and the cold water basin m a y be steel. Larger towers som e tim es are m a de of concrete. Many towers—casings and basins— are constructed of galvanized steel or, wh ere a corrosive atm o sphere is a problem
, stainless steel. Som e tim es 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 harm ful effects of m a ny chem icals. Plastics are widely used for fill, including PVC, polypropylene, and other polym ers. Treated wood splash fill is still specified for wood towers, but plastic splash fill is also widely used when water conditions m a ndate th e use of splash fill. Film fill, because it of

f

e rs greater heat transf er ef f i ciency, is the f ill of choice f o r applications where the circulating water is generally f r ee of debris that could plug the f ill passageways.

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Plastics also find wide use as nozzle m a teri als. Many nozzles are being m a de of PVC, ABS, polypropylene, and glass-filled nylon. Alum inum , glass fiber, and hot-dipped galvanized steel are com m only used fan m a teri als. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are fa bricated from galvanized, alum inum , or
m o lded glass fiber reinforced plastic.

7.2 

### Cooling Tow er Performance

The im portant param e ters, from the point of determ ining the perform a nce of cooling towers, are:

Figure 7.3 Range and Approach
i) “Range” is the difference between th e cooling tower water inlet and outlet tem p erature. (See Figure 7.3). ii) “Approach” is the difference between the cooling tower outlet cold water tem p erature and am bient wet bulb te m p erature. Although, both range and approach should be m onitored, the `Approach’ is a better indicator of cooling tower perform a nce. (see Figure 7.3). iii) Cooling tower ef f

ectiveness (in percenta ge) is the ratio of range, to the ideal range, i.e., difference between cooling wa ter inlet tem p erature and am bient wet bulb tem p erature, 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 m a ss flow rate of water, specific heat and tem p erature difference. v) Evaporation loss is the water quan tity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works out to 1.8 m 3 . An em pirical relation used often is:

Evaporation Loss (m 3 /hr) = 0.00085 x 1.8 x circulation rate (m 3 /hr) x (T 1 -T 2 )

T 1 -T 2 = Tem p. difference between inlet and outlet water. Source: Perry’s Chemical Engineers Handbook (Page: 12-17)

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vi) Cycles of concentration (C.O.C) is the ra tio of dissolved solids in circulating water to the dissolved solids in m a ke 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 m a ss flow rates. Against design values , seasonal variations require adjustm e nt and tuning of water and air flow rates to get the best cooling tower ef f

ectiveness through m easures like water box loading changes, blade angle adjustm e nts.

Therm odynam i cs also dictate that the heat rem oved from the water m u st be equal to the heat absorbed by the surrounding air:

where: L/G = liquid to gas m a ss flow ratio (kg/kg) T 1 = hot water tem p erature ( 0 C) T 2 = cold water tem p erature ( 0 C) h 2 = enthalpy of air-water vapor m i xture at exhaust wet-bulb tem p erature (sam e units as above) h 1 = enthalpy of air-water vapor m i xture at inlet wet-bulb tem p erature (sam e units as above)

Factors Affecting Cooling Tow er Performance

Capacity

Heat dissipation (in kCal/hour) and circulated flow rate (m 3 /hr) are not sufficient to understand cooling tower perform a nce. Other factors, which we will see, m u st be stated along with flow rate m 3 /hr. For exam ple, a cooling tower sized to cool 4540 m 3 /hr through a 13.9 o C range m i ght be larger than a cooling tower to cool 4540 m 3 /hr through 19.5 o C range.

Range
Range is determ ined not by the cooling tower, but by the process it is serving. The range at the exchanger is determ ined entirely by th e 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

.

Bureau of Energy Efficiency 1407. C ool i ng Tower

Cooling towers are usually specified to cool a certain flow rate from one tem p erature to another tem p erature at a certain wet bulb te m p erature. For exam ple, the cooling tower

m i ght be specified to cool 4540 m 3 /hr from 48.9 o C to 32.2 o C at 26.7 o C wet bulb tem p erature. Cold Water Temperature 32.2 o C – Wet Bulb Temperature (26.7 o C) = Approach (5.5 o C) As a generalization, the closer the approach to the wet bulb, the m o re expensive the cooling tower due to increased size. Usually a 2.8 o C approach to the design wet bulb is the coldest water tem p erature that cooling tower m a nufacturers will guarantee. If f

l ow rate, range, approach and wet bulb had to be ranked in the order of their im portance in sizing a tower, approach would be fi rst with f l ow rate closely following the range and wet bulb would be of lesser im portance.

Heat Load

The heat load im posed on a cooling tower is determ ined by the process being served. The degree of cooling required is controlled by th e desired operating tem p erature level of the process. In m o st cases, a low operating tem p erature is desirable to increase process efficiency or to im prove the quality or quantity of the product. In som e applications (e.g. internal com bustion engines), however, high operating tem p eratures are desirable. The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low undersized equipm ent will be purchased. If the calculated load is high, oversize and m o re costly, equipm ent will result. Process heat loads m a y vary cons iderably depending upon the process involved. Determ ination of accurate process heat loads can becom e very com p lex but proper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determ ined with greater accuracy. Inform ation is available for the heat re jection requirem e nts of various types of power equipm ent. A sam p le list is as f o llows:

* Air Com p ressor

- 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, Com p ression - 63 kCal/m in/TR * Refrigeration, Absorption - 127 kCal/m in/TR * Steam Turbine Condenser - 555 kCal/kg of

s t e a m * Diesel Engine, Four-Cycle, Supercharged - 880 kCal/kW / hr * Natural Gas Engine, Four-cycle - 1523 kCal/kW / hr (18 kg/cm 2 com p ression)

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Wet Bulb Temperature

W e t bulb tem p erature is an im portant factor in perform a nce of evaporative water cooling equipm ent. It is a controlling factor from th e aspect of m i nim u m cold water tem p erature to which water can be cooled by the evapor ative m e thod. Thus, the wet bulb tem p erature of the air entering the cooling tower determ ines operating tem p erature levels throughout the plant, process, or system . Theoretically, a cooling tower will cool water to the entering wet bulb tem p erature, when operating without a heat load. However, a therm a l potential is required to reject heat, so it is not possibl e to cool water to the entering air wet bulb tem p erature, when a heat load is applied. Th e approach obtained is a function of therm a l conditions and tower capability.

Initial selection of towers with respect to design wet bulb tem p erature m u st be m a de on the basis of conditions existing at the tower site. The tem p erature selected is generally close to the average m a xim u m wet bulb for th e sum m e r m onths. An im portant aspect of wet bulb selection is, whether it is specif i ed as am bient or inlet. The am bient wet bulb is the tem p erature, which exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb tem p erature 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 tem p erature of the air entering the tower with corresponding increase in the cold water tem p erature. Since th ere is no initial knowledge or control over the recirculation factor, the am bient wet bulb s hould be specified. The cooling tower supplier is required to f u rnish a tower of suf f i cient cap ability to absorb the ef f

ects of the increased wet bulb tem p erature peculiar to his own equipm ent.
It is very im portant to have the cold water tem p erature low enough to exchange heat or to condense vapours at the optim um tem p erature le vel. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and tem p erature of the cooling tower water can be selected to get the m a xim u m econom y 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 m 3 /hr through a 16.67 o C range at a 26.7 o C design wet bulb. The overall width of all to wers is 21.65 m e ters; the overall height, 15.25

m e ters, and the pum p head, 10.6 m approxim a tely.

Table 7.1 APPROACH Vs. COOLING TOWER SIZ E (4540 m 3 / h r; 16.67 o C Range 26.7 o C Wet B u l b ; 10.7 m Pump Head) Approach o C 2

. 7 7 3

. 3 3 3

. 8 8 4

. 4 4 5

. 0 5

. 5 5 Hot W a ter o C 4 6

. 1 1 4 6

. 6 6 4 7

. 2 2 4 7

. 7 7 4 8

. 3 48.88 Cold W a ter o C 2 9

. 4 4 3 0 3 0

. 5 5 3 1

. 1 1 3 1

. 6 6 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 Diam eter 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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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 diam eter fans and each pow ered by 25 kW m o tors. The cooling tower cools from 3632 m 3 /hr water from 46.1 o C to 29.4 o C at 26.7 o C W B T dissipating 60.69

m illion kCal/hr. The Table 7.2 shows what w ould happen with additional flow but with the range rem a ining constant at 16.67 o C. The heat dissipated varies from 60.69 m illion kCal/hr to 271.3 m illion kCal/hr.

Table 7.2 Flow vs. Approach for a Given Tow er (Tow er is 21.65 m × 36.9 M; Three 7.32 M Fans; Three 25 kW Motors; 16.7 o C Range w i th 26.7 o C Wet Bulb)

Flow m 3 /hr Approach

o C Cold Water

o C Hot Water

o C Million kCal/hr
3 6 3 2 2

. 7 8 2 9

. 4 0 4 6

. 1 1 6 0

. 6 9 1
4 0 8 6 3

. 3 3 2 9

. 9 5 4 6

. 6 7 6 8

. 3 1 8
4 5 6 3 3

. 8 9 3 0

. 5 1 4 7

. 2 2 7 6

. 2 5
5 0 3 9 4

. 4 5 3 1

. 0 7 4 7

. 7 8 8 4

. 0 5
5 5 1 6 5

. 0 0 3 1

. 6 2 4 8

. 3 3 9 2

. 1 7

6 0 6 0

. 9 5

. 5 6 3 2

. 1 8 4 8

. 8 9 1 0 1

. 2 8

7 1 5 0

. 5 6

. 6 7 3 3

. 2 9 5 0

. 0 0 1 1 9

. 4 8

8 7 3 6 8

. 3 3 3 5

. 0 0 5 1

. 6 7 1 4 5

. 6 3
1 1 5 9 0 1 1

. 1 3 7

. 8 0 5 4

. 4 5 1 9 1

. 6 4
1 3 6 2 0 1 3

. 9 4 0

. 5 6 5 7

. 2 2 2 2 6

. 9 1
1 6 2 7 6 1 6

. 7 4 3

. 3 3 6 0

. 0 0 2 7 1

. 3 2

For m eeting the increased heat load, few m odifications 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 f unction of the quantity of wa ter 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 tem p erature is not changed and the range is increased with higher hot water tem p erature, the driving force between the wet bulb tem p erature of the air entering the tower and the hot water tem p erature is increas ed, the higher level heat is econom ical to dissipate.

If the hot water tem p erature is left constant and the range is increased by specifying a lower cold water tem p erature, the tower size would have to be increased considerably. Not only would the range be increased, but the lower cold water tem p erature would lower

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the approach. The resulting change in both range and approach would require a m u ch larger cooling tower. Approach & Wet Bulb Temperature The design wet bulb tem p erature is determ in ed by the geographical location. Usually the design wet bulb tem p erature selected is not exceeded over 5 percent of the tim e in that area. W e t bulb tem p erature is a factor in c ooling tower selection; the higher the wet bulb tem p erature, the sm aller the tower required to give a specified approach to the wet bulb at a constant range and flow rate. A 4540 m 3 /hr cooling tower selected for a 16.67 o C range and a 4.45 o C approach to 21.11 o C wet bulb would be larger than a 4540 m 3 /hr tower selected for a 16.67 o C range and a 4.45 o C approach to a 26.67 o C wet bulb. Air at the highe r wet bulb tem p erature is capable of picking up m o re heat. Assum e th at the wet bulb tem p erature of the air is increased by approxim a tely 11.1 o C. As air rem oves heat f r om the water in the tower, each kg of air entering the tower at 21.1 o C wet bulb would contain 18.86 kCals and if it were to leave the tower at 32.2 o 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 26.67 o C we t b u l b wo u l d contain 24.17 kCals and were to leave at 37.8 o 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 26.67 o C to 37.8 o C.

Fill Media Effects

In a cooling tower, hot water is distribut ed above fill m e dia which flows down and is cooled due to evaporation with the interm ixing air. Air draft is achie ved with use of fans. Thus som e power is consum ed in pum ping the water to a height above the fill and also by fans creating the draft. An energy efficient or low power consum i ng cooling tower is to have efficient designs of fill m e dia with appropriate water distribu tion, drift elim inator, fan, gearbox and m o tor. Power savings in a cooling tower, with use of efficient fill design, is directly reflected as savings in fan power consum ption and pum ping head requirem e nt.
Function of Fill media in a Cooling Tower

Heat exchange between air and water is infl uenced by surface area of heat exchange, tim e of heat exchange (interaction) and tu rbulence in water effecting thoroughness of interm ixing. Fill m e dia in a cooling tower is responsible to achieve all of above.
Splash and Film Fill Media : As the nam e indicates, splash f ill m e dia generates the required heat exchange area by splashing action of water over f ill m e dia and hence breaking into sm aller water droplets. Thus, su rface of heat exchange is the surface area of the water droplets, which is in contact with air. Film Fill and its Advantages

In a f ilm f

ill, water f o rm s a thin f ilm on eith er side of f

ill sheets. Thus area of heat exchange is the surface area of the fill sh eets, which is in contact with air.

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Typical com p arison between various fill m e dia is shown in Table 7.3.

Ta ble 7

. 3 Ty pica l Co mpa r iso n s Between Va rio u s Fill Media

Spla s h Fill Film Fill Lo w Clo g Film Fill

Possi bl e L/ G R a t i o 1.1 – 1.5 1.5 – 2.0 1.4 – 1.8

Effective Heat Exchange Area 30 – 45 m 2 /m 3 150 m 2 / m 3 85 – 100 m 2 /m 3

Fill Heig h t Req u i red 5 – 1 0 m 1

. 2 – 1

. 5 m 1

. 5 – 1

. 8 m

Pum p i ng Head R e qui rem e nt 9 – 12 m 5 – 8 m 6 – 9 m

Qu an tity o f Air Req u i red Hig h Mu ch lo w Lo w Due to fewer requirem e nts of air and pum pi ng head, there is a trem endous saving in power with the invention of f

ilm f

ill. Recently, low-clog film fills with higher flute sizes have been developed to handle high turbid waters. For sea water, low clog f ilm f

ills are considered as the best choice in term s of power saving and perform a nce com p ared to conventional splash type fills. Choosing a Cooling Tow er
The counter-flow and cross flows are two basi c designs of cooling towers based on the fundam e ntals of heat exchange. It is well known that counter flow heat exchange is m o re effective as com p ared to cross flow or parallel flow heat exchange. Cross-flow cooling towers are provided w ith splash fill of concre te, wood or perforated PVC. Counter-flow cooling towers are provi ded with both film fill and splash fill. Typical com p arison of Cross flow Spash Fill, Counter Flow Towe r with Film Fill and Splash fill is shown in Tabl e 7.4. The power consum ption 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

Num b er of Towers : 2

Water Flow : 16000 m 3 /hr.

Hot Water Tem p erature : 41.5 o C
Cold Water Tem p erature : 32.5 o C Design Wet Bulb Tem p erature : 27.6 o C

Counter Flow Film Fill Counter Flow Spla s h Fill Cross-Flow Spla s h Fill Fill Heig h t , Meter 1

. 5 5

. 2 1 1

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

Num b er of C e l l s per Tower 6 6 5

Power at M o t o r Term i n al / T ower, kW 253 310 330

St at i c Pum p i ng Head, M e t e r 7.2 10.9 12.05

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### 7.3 Efficient System Operation

Cooling Water Treatment

Cooling water treatm e nt is m a ndatory f o r any cooling tower whether with splash f ill or with f ilm type f ill f o r controlling suspended solids, algae growth, etc. W ith increasing costs of water, efforts to increase Cycles of Concentration (COC), by Cooling W a ter Treatm e nt would help to reduce m a ke up water requirem e nts significantly. In large industries, power plants, COC im prove m e nt is often considered as a key area for water conservation.

Drift Loss in the Cooling Tow ers
It is very difficult to ignore drift problem in cooling towers. Now-a-days m o st of the end user specification calls for 0.02% drift loss. W ith technological developm ent and pr ocessing of PVC, m a nufacturers have brought large change in the drif t elim inator shapes and the possibility of m a king ef f i cient designs of

drif t elim inators that enable end user to specif y the drif t loss requirem e nt to as low as 0.003 – 0.001%. Cooling Tow er Fans The purpose of a cooling tower fan is to m ove a specified quantity of air through the system , overcom ing the system resistance whic h is defined as the pressure loss. The product of air flow and the pressure loss is ai r power developed/work done by the fan; this

m a y be also term ed as fan output a nd input kW depends on fan efficiency. The fan efficiency in turn is greatly dependent on the profile of the blade. An aerodynam i c profile with optim um twist, taper a nd higher coefficient of lift to coefficient of drop ratio can provide the fan total effi ciency as high as 85-92 %. However, this efficiency is drastically affected by the factor s such as tip clearance, obstacles to airflow and inlet shape, etc. As the m e tallic fans are m a nufactured by adopting either extrusion or casting process it is always difficult to generate the ideal aerodynam i c profiles. The FRP blades are

norm a lly hand m oulded which facilitates the generation of optim um aerodynam i c profile to m eet specific duty condition m o re effici ently. Cases reported where replacem ent of

m e tallic 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 sim p le pay back period of 6 to 7 m onths. Also, due to lightweight, FRP fans need low starting torque resulting in use of lower HP m o tors. The lightweight of the fans also increases the life of the gear box, m o tor and bearing is and allows for easy handling and m a intenance. Performance Assessment of Cooling Tow ers In operational perform a nce assessm ent, the typical m easurem ents and observations involved are:

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

• Intake air W B T and DBT at each ce ll at ground level using a whirling pyschrom e ter.

Bureau of Energy Efficiency 1467. C ool i ng Tower

• Exhaust air W B T and DBT at each cell using a whirling psychrom eter.

• CW inlet tem p erature at risers or top of tower, using accurate m e rcury in glass or a digital therm o m e ter.

• CW outlet tem p erature at full bottom
, usi ng accurate m e rcury in glass or a digital t h e r mo me t e r

.

• Process data on heat exchangers, loads on line or power plant control room readings, as relevant.

• CW flow m easurem ents, either direct or inferred from pum p m o tor kW and pum p head and flow characteristics.

• CT fan m o tor am ps, volts, kW and blade angle settings

• TDS of cooling water.

• Rated cycles of concentration at the site conditions.

• Observations on nozzle f l ows, drif t elim inators, condition of f

ills, splash bars, etc. The findings of one typical trial pertaining to the Cooling Towers of a Therm a l Power Plant 3 x 200 MW is given below:

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

* M a i n s Frequency = 4 9

. 3
* Inlet Cooling Water Tem p erature o C = 44 (Rated 43 o C)
* Outlet Cooling Water Tem p erature o C = 37.6 (Rated 33 o C)
* Air Wet Bulb Tem p erature near Cell o C = 29.3 (Rated 27.5 o C)
* Air Dry Bulb Tem p erature near Cell o C = 40.8 o C
* Num b er of CT Cells on line with water flow = 45 (Total 48)
* Total Measured Cooling Water Flow m 3 / h r = 7 0 4 2 6

. 7 6
* Measured CT Fan Flow m 3 / h r = 9 8 9 5 4 4 Analysis
* CT water Flow/Cell, m 3 / h r = 1 5 6 5 m 3 /hr (1565000 kg/hr) (Rated 1875 m 3 /hr)
* CT Fan air Flow, m 3 /hr (Avg.) = 989544 m 3 /hr (Rated 997200 m 3 /hr)
* CT Fan air Flow kg/hr (Avg.) @ Density of 1.08 kg/m 3 = 1 0 6 8 7 0 8 k g / h r

* L/G Ratio of C.T. kg/kg = 1.46 (Rated 1.74 kg/kg)
* CT Range = (44 – 37.6) = 6.4 o C
* CT Approach = (37.6 – 29.3) = 8.3 o C
* % CT Effectiveness = 100 ) ( x Approach Range Range

+

Bureau of Energy Efficiency 1477. C ool i ng Tower

= 100 ) 3

. 8 4

. 6 ( 4

. 6 x

+

= 4 3

. 5 3
* Rated % CT Effectiveness = 100 * (43 – 33) / (43 – 27.5)

= 6 4

. 5 %
* Cooling Duty Handled/Cell in kCal = 1565 * 6.4 * 10 3
(i.e., Flow * Tem p erature Difference in kCal/hr) = 10016 * 10 3 kCal/hr (Rated 18750 * 10 3 kCal/hr)
* Evaporation Losses in m 3 /hr = 0 .00085 x 1.8 x circulation rate (m 3 /hr) x (T 1 -T 2 )

= 0 .00085 x 1.8 x 1565 x (44- 37.6)

= 15.32 m 3 /hr per cell
* Percentage Evaporation Loss = [ 15.32/1565]*100

= 0

. 9 7 %
* Blow down requirem e nt for site COC of 2.7 = E vaporation losses / (COC–1)

= 15.32/(2.7–1) per cell i.e., 9.01 m 3 /hr
* Make up water requirem e nt/cell in m 3 /hr = E vaporation Loss + Blow down Loss

= 15.32 + 9.01

= 2 4

. 3 3 Comments

• Cooling water f l ow per cell is m u ch lowe r, alm o st by 16.5%, need to investigate CW pum p and system perform a nce for im provem e nts. Increasing CW flow through cell was identified as a key re sult area for im proving perform a nce of cooling towers.

• Flow stratif ication in 3 cooling tower cells identif ied.

• Algae growth identif ied in 6 cooling tower cells.

• Cooling tower fans are of GRP type drawing 36.2 kW average. Replacem ent by efficient hollow FRP fan blades is recom m e nded.



### 7.4 Flow Control Strategies

Control of tower air flow can be done by va rying m e thods: starting and stopping (On-off) of fans, use of two- or three- speed f a n m o tors, use of autom a tically adjustable pitch f a ns,
use of variable speed fans. On-off fan operation of single sp eed fans provides the least effective control. Two-speed fans provide better control with further im provem e nt shown with three speed fans.

Bureau of Energy Efficiency 1487. C ool i ng Tower

Autom a tic adjustable pitch f a ns and variable-s peed fans can provide even closer control of tower cold-water tem p erature. In m u lti-cell towers, fans in adjacent cells m a y be running at different speeds or som e m a y be on and others off depending upon the tower load and required water tem p erature. Depending upon the m e thod of air volum e control selected, control strategies can be determ in ed to m i nim i se fan energy while achieving the desired control of the Cold water tem p erature. 

### 7.5 Energy Saving Opportunities in Cooling Tow ers

− Follow m a nufacturer’s recom m e nded cl earances around cooling towers and relocate or m odify structures that interfere with the air intake or exhaust.

− Optim ise 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, re place old spray type nozzles with new square spray ABS practically non-clogging nozzles.

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

− Install new nozzles to obtain a m o re uniform water pattern

− Periodically clean plugged cooling tower distribution nozzles.

− Balance flow to cooling tower hot water basins.

− Cover hot water basins to m i nim i se algae growth that contributes to fouling.

− Optim ise blow down flow rate, as per COC lim it.

− Replace slat type drift elim inators with low pressure drop, self extinguishing, PVC cellular units.

− Restrict flows through large loads to design values.

− Segregate high heat loads like furnaces, air com p ressors, DG sets, and isolate cooling towers for sensitive applications like A/C plants, condensers of captive power plant etc. A 1 o C cooling water tem p erature increase m a y increase A/C

com p ressor kW by 2.7%. A 1 o C drop in cooling water tem p erature can give a heat rate saving of 5 kCal/kW h in a therm a l power plant.

− Monitor L/G ratio, CW f l ow rates w.r.t. design as well as seasonal variations. It would help to increase water load during sum m e r and tim es when approach is high and increase air flow during m onsoon tim es and when approach is

narrow.

− Monitor approach, effectiveness and cooling capacity for continuous optim isation efforts, as per seasonal variations as well as load side variations.

− Consider COC im provem e nt m easures for water savings.

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

− Consider possible im provem e nts on CW pum ps w.r.t. efficiency im provem e nt.

− Control cooling tower fans based on leaving water tem p eratures especially in case of sm all units.

Bureau of Energy Efficiency 1497. C ool i ng Tower

− Optim ise process CW flow requirem e nts, to save on pum ping energy, cooling load, evaporation losses (directly proportional to circulation rate) and blow down losses. Som e typical problem s and their trouble shooti ng for cooling towers are given in Table 7.5.

T a bl e 7.5 T y pi cal Probl ems and T r oubl e Shooti n g for Cool i n g T o w ers

Problem / Difficulty Possible Caus e s R e m e d i e s / R e c t i f y i n g A c t i o n 1

. V o l t a ge R e duct i on C h eck t h e vol t a ge 2a. Incorrect angle of axial fa n blades Adjust the blade angle 2b. Loose bel t s on cent r i f ugal fans (or speed reducers) C h eck bel t t i ght ness

3. Overl o adi ng owi ng t o excessi ve ai r flo w -fill h a s m i n i m u m water loading per m 2 of t o wer sect i on Regulate the water flow by m eans of t h e val v e Excessive absorbed current / electrical load

4. Low am bient air tem p erature The m o tor is cooled proport i onat e l y and hence del i v ers
m o re than nam e plate power 1

. U n e v e n o p e r a t i on of spray nozzl es Adjust t h e nozzl e ori e nt at i on and elim inate any dirt 2

. Blo c k a g e o f th e fill p ack Elim in ate an y d i rt in th e to p o f th e fill 3. Defective or displaced droplet elim inators Replace or realign the elim inators Drift/carry -over of wat e r out si de t h e uni t

4. Excessi ve ci rcul at i ng wat e r fl ow (possi bl y owi ng t o t oo hi gh pum pi ng head) Adjust the water flow-rate by

m eans of t h e regul at i ng val v es. Check for absence of dam a ge to th e fill 1. Fl oat

- val v e not at

correct l e vel Adjust t h e m a ke-up val v e Loss of water from basi ns/ p ans 2. Lack of equal i s i ng connect i ons Equal i s e t h e basi ns of t o wers
o p e ratin g in p a rallel 1. W a t e r fl ow bel o w t h e desi gn val v e R e gul at ed t h e fl ow by m eans of th e v a lv es 2. Irregular airflow or lack of ai r Check the direction of rotation of t h e fans and/ or bel t t e nsi on (broken bel t possi bl e) 3a. R ecy cl i ng of hum i d di scharge ai r Check the air descent velocity 3b. Int a ke of hot ai r from ot her sources Inst al l defl ect ors 4a. B l ocked spray nozzl es (or even bl ocked spray t ubes) Clean the nozzles and/or the tubes

4b. Scaling of joints W a sh or replace the item Lack of cool i ng and hence increase in t e m p erat ures owi ng t o increased tem p erature range

5. Scaling of the fill pack Clean or replace the m a terial (washi ng wi t h i nhi bi t e d aqueous sul phuri c aci d i s possi bl e but l ong,
com p l e x and expensi v e)

Bureau of Energy Efficiency 1507. C ool i ng Tower



### QUESTIONS 1. W h at do you understand by the following term s 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. W h at do you m ean 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 m 3 /hr * CW in Tem p erature : 42 o C * CW Out Tem p erature : 36 o C * W e t Bulb Tem p erature : 29 o C

W h at is the Ef f

ectiveness of the cooling tower?

6. W h at is the f unction of f

ill m e dia in a cooling tower? 7. List the factors affecting cooling tower perform a nce.

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

.

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

10. W h at is the Blow-down Loss, if the Cycles of Concentration (COC) is 3.0?



### REFERENCES

1. ASHRAE Handbook 2. NPC Case Studies

Bureau of Energy Efficiency 151