1.0 Introduction

Power plants, large air-conditioning systems, and some industries generate large

quantities of waste heat that is often rejected to cooling water from nearby lakes or rivers. In

some cases, however, the cooling water supply is limited or thermal pollution is a serious

concern. In such cases, the waste heat must be rejected to the atmosphere, with cooling water

recirculating and serving as a transport medium for heat transfer between the source and the

sink (the atmosphere). One way of achieving this is through the use of wet cooling towers.

A wet cooling tower is essentially a semienclosed evaporative cooler. An induced-

draft counterflow wet cooling tower is shown schematically in Figure 1-1. Air is drawn into

the tower from the bottom and leaves through the top. Warm water from the condenser is

pumped to the top of the tower and is sprayed into this airstream. The purpose of spraying is

to expose a large surface area of water to the air. As the water droplets fall under the

influence of gravity, a small fraction of water (usually a few percent) evaporates and cools

the remaining water. The temperature and the moisture content of the air increase during this

process. The cooled water collects at the bottom of the tower and is pumped back to the

condenser to absorb additional waste heat. Makeup water must be added to the cycle to

replace the water lost by evaporation and air draft. To minimize water carried away by the

air, drift eliminators are installed in the wet cooling towers above the spray section.

Figure 1-1: Schematic diagram for an induced-draft counterflow wet cooling tower

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The air circulation in the cooling tower described is provided by a fan, and therefore it

is classified as a forced-draft cooling tower. Another popular type of cooling is the natural-

draft cooling tower, which looks like a large chimney and works like an ordinary chimney.

The air in the tower has a high water-vapour content, and thus it is lighter than the outside air.

Consequently, the light air in the tower rises, and the heavier outside air fills the vacant

space, creating an airflow from the bottom of the tower to the top. The flow rate of air is

controlled by the conditions of the atmospheric air. Natural-draft cooling towers do not

require any external power to induce the air, but they cost a lot more to build than forced-

draft cooling towers. The natural-draft cooling towers are hyperbolic in profile, as shown in

Figure 1-2 and Figure 1-3, and some are over 100 m high. The hyperbolic profile is for

greater structural strength, not for any thermodynamic reason.

Figure 1-2: Schematic diagram for a natural-draft cooling tower

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Figure 1-3: Natural-draft cooling tower

The idea of a cooling tower started with the spray pond, where the warm water is

sprayed into the air and is cooled by the air as it falls into the pond, as shown in Figure 1-4.

Some spray ponds are still in use today. However, they require 25 to 50 times the area of a

cooling tower, water loss due to air drift is high, and they are unprotected against dust and

dirt.

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Figure 1-4: Spray pond

We could also dump the waste heat into a still cooling pond. As shown in Figure 1-5,

a cooling pond is basically a large artificial lake open to the atmosphere. Heat transfer from

the pond surface to the atmosphere is very slow, however, and we would need about 20 times

the area of a spray pond in this case to achieve the same cooling.

Figure 1-5: Cooling pond

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2.0 Experimental Procedure

Figure 2-1: Cooling tower

Figure 2-2: Schematic diagram for the cooling tower

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Firstly, the power button of cooling tower was switched on. The pump was started and

the water flow rate was adjusted to 50 L/h as indicated on the flow meter. Next, the fan was

started and the fan speed was measured by using air velocity meter. After that, the heater was

turned on. The current was immediately adjusted to 3 A. The following temperatures were

measured and recorded after a steady value had achieved: the water temperature at the tower

outlet, T 1, the wet-bulb temperature of air at the column top, T 2, the water temperature at the

heater outlet, T 3, the dry-bulb temperature of air at the column top, T 4, the water temperature

at the tower inlet, T 5, the wet-bulb temperature of air at the column bottom, T 6, the water

temperature at the tank, T 7, and the dry-bulb temperature of air at the column bottom, T 8. The

aforementioned steps were repeated by increasing the current to 4 A, 5 A, and 6 A. This

experiment was repeated by adjusting water flow rate to 100 L/h and 150 L/h.

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3.0 Experimental Result

Table 3-1: Experimental data for water-flow-rate setting of 50 L/h

Temperatures (℃)

H1: 240 V , 3 A H2: 240 V , 4 A H3: 240 V , 5 A H4: 240 V , 6 A

T 1 21.0 21.1 22.0 22.4T 2 20.6 20.7 21.6 22.0T 3 27.7 29.4 32.5 36.1T 4 21.3 21.6 22.6 23.1T 5 26.8 28.6 31.8 35.3T 6 20.1 19.9 20.3 20.1T 7 20.6 20.4 20.5 20.6T 8 21.7 21.6 21.5 21.3

Table 3-2: Experimental data for water-flow-rate setting of 100 L/h

Temperatures (℃)

H1: 245 V , 3 A H2: 245 V , 4 A H3: 245 V , 5 A H4: 245 V , 6 A

T 1 21.8 21.6 22.1 23.0T 2 22.3 22.0 22.2 23.0T 3 26.0 26.8 28.6 31.5T 4 22.9 22.7 23.1 24.1T 5 25.2 26.2 28.0 30.7T 6 20.1 20.1 20.2 20.0T 7 21.0 20.9 20.9 21.2T 8 21.3 21.3 21.3 21.3

Table 3-3: Experimental data for water-flow-rate setting of 150 L/h

Temperatures (℃)

H1: 240 V , 3 A H2: 240 V , 4 A H3: 240 V , 5 A H4: 240 V , 6 A

T 1 22.1 21.9 22.3 23.1T 2 22.9 22.6 22.7 23.5T 3 25.2 25.8 26.9 29.4T 4 23.4 23.2 23.6 24.5T 5 24.6 25.3 26.4 28.9T 6 19.9 19.7 19.7 19.6T 7 21.4 21.1 21.2 21.6T 8 21.3 21.2 21.3 21.3

Table 3-4: Recorded fan speed for each water-flow-rate setting

Water-flow-rate setting (L/h) Fan speed (m /s)50 4.50100 4.75

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150 4.69

4.0 Discussion

Consider this cooling tower experiment is an adiabatic saturation process and a

steady-flow process.

Taking subscript ‘1’ as the inlet condition and subscript ‘2’ as the outlet condition, the

mass balance equation for dry air can be written as

ma1=ma2

=ma (4.1)

The mass balance equation for water vapour can be written as

mw1+mf =mw2

ma ω1+mf =ma ω2

mf =ma (ω2−ω1 ) (4.2)

where mf is the rate of droplet evaporation. Therefore, if inlet specific humidity, ω1 and outlet

specific humidity, ω2 are known, the rate of droplet evaporation can be determined.

The energy balance equation for the process can be written as

ma h1+mf h f 2=ma h2

ma h1+ma ( ω2−ω1 ) hf 2=ma h2

h1+( ω2−ω1 ) hf 2=h2

(cp T 1+ω1 hg1 )+( ω2−ω1 ) hf 2=(cp T 2+ω2 hg2 )

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ω1=cp (T 2−T 1 )+ω2 hf g2

hg1−hf 2

(4.3)

ω2=0.622Pg2

P2−Pg2

(4.4)

Table 4-1: Unit conversion for water-flow-rate setting

Water-flow-rate setting (L/h) Water-flow-rate setting (m3/s)50 1.389 x 10-5

100 2.778 x 10-5

150 4.167 x 10-5

Table 4-2: Data analysis for water-flow-rate setting of 50 L/h

P2 (kPa) Pg2 (kPa) ω2 h f g2

(kJ /kg) hg1 (kJ /kg) h f 2

(kJ /kg) ω1

2.622 2.356 5.513 2453.26 2539.77 84.33 5.5082.605 2.327 5.197 2453.74 2540.31 83.50 5.1902.588 2.389 7.454 2452.79 2542.13 85.17 7.4412.555 2.356 7.351 2453.26 2543.04 84.33 7.333

Table 4-3: Data analysis for water-flow-rate setting of 100 L/h

P2 (kPa) Pg2 (kPa) ω2 h f g2

(kJ /kg) hg1 (kJ /kg) h f 2

(kJ /kg) ω1

2.555 2.356 7.351 2453.26 2542.68 84.33 7.3342.555 2.356 7.351 2453.26 2542.31 84.33 7.3362.555 2.372 8.075 2453.03 2543.04 84.75 8.0572.555 2.339 6.737 2453.50 2544.86 83.92 6.715

Table 4-4: Data analysis for water-flow-rate setting of 150 L/h

P2 (kPa) Pg2 (kPa) ω2 h f g2

(kJ /kg) hg1 (kJ /kg) h f 2

(kJ /kg) ω1

2.555 1.682 1.198 2453.74 2543.59 83.50 1.1932.539 1.683 1.223 2454.21 2543.22 82.66 1.2192.555 1.683 1.200 2454.21 2543.95 82.66 1.1952.555 1.683 1.201 2454.45 2545.59 82.24 1.195

Table 4-5: Rate of droplet evaporation for each water-flow-rate setting

Current, I (A)Rate of droplet evaporation, mf (kg /s)

50 L/h 100 L/h 150 L/h3 7.445 x 10-8 4.538 x 10-7 1.885 x 10-7

4 1.001 x 10-7 4.213 x 10-7 1.911 x 10-7

5 1.888 x 10-7 5.131 x 10-7 2.101 x 10-7

6 2.431 x 10-7 6.128 x 10-7 2.640 x 10-7

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As can be seen from Table 4-5, two general trends can be observed. The first one is,

as the current increases, the rate of droplet evaporation increases. However, the rate of

droplet evaporation for water-flow-rate setting of 100 L/h and heater setting of 245 V, 5 A is

slightly deviated from the trend. A possible reason is, the raw data were recorded before it

had settled down to a steady value. The second trend that can be observed from Table 4-5 is,

as the flow rate of water increases, the rate of droplet evaporation increases. However, the

rates of droplet evaporation for water-flow-rate setting of 100 L/h are heavily deviated from

the trend. This is because the voltage setting for the heater was 245 V instead of 240 V.

Therefore, the rates of droplet evaporation for water-flow-rate setting of 100 L/h are higher

than that of 50 L/h and 150 L/h. It should be noted that wet-bulb temperatures are used in

calculations instead of the actual adiabatic saturation temperature. Therefore, there are slight

discrepancies from the actual value. Nevertheless, the results are accurate enough for any

practical use.

The specific heat transfer from heater can be written as

qheater=c p (T 3−T 7 ) (4.5)

where c p=1.005 kJ /kg K , T 3 is the water temperature at the heater outlet, and T 7 is the water

temperature at the tank.

The specific heat transfer between the droplets and the air can be written as

qdroplet=c p (T 5−T 1) (4.6)

where c p=1.005 kJ /kg K , T 5 is the water temperature at the tower inlet, and T 1 is the water

temperature at the tower outlet.

In this case, the rate of heat transfer can be written as

Q=mq

Q=ρV c p ΔT (4.7)

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Since it is known that the density of water is approximately 1000 kg/m3, the rate of heat

transfer can be computed for both cases.

Table 4-6: Heat transfers for each water-flow-rate setting

V (L/h)

I (A) qheater (kJ /kg) qdroplet (kJ /kg) Qheater (kJ / s) Qdroplet (kJ / s)

50

3 7.136 5.829 0.0991 0.08104 9.045 7.538 0.1256 0.10475 12.06 9.849 0.1675 0.13686 15.58 12.96 0.2164 0.1801

100

3 5.025 3.417 0.0698 0.04754 5.930 4.623 0.0824 0.06425 7.740 5.930 0.1075 0.08246 10.35 7.740 0.1438 0.1075

150

3 3.819 2.513 0.0530 0.03494 4.724 3.417 0.0656 0.04755 5.730 4.121 0.0796 0.05726 7.840 5.830 0.1089 0.0810

As can be seen from Table 4-6, two general trends can be noticed. The first one is, as

the current increases, the heat transfer increases. This is expected because a heater with high

current setting will produce more heat compared to a heater with low current setting. Since

the heat transfer between the droplets and the air is directly proportional to the heat transfer

from heater, when the heat transfer from heater increases, the heat transfer between the

droplets and the air increases. Another interesting trend observed is, as the volumetric flow

rate increases, the heat transfer decreases. This is because, as the volumetric flow rate

increases, the contact time between the water and the heater decreases, therefore the heat

transfer from the heater decreases. As a consequence, the heat transfer between the droplets

and the air decreases.

A graph of heat transfer between the droplets and the air versus heat transfer from the

heater is plotted.

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5.0 Conclusion

In this experiment, the two major factors affecting the cooling effect (heat transfer

between the droplets and the air) of the cooling tower are volumetric flow rate and current

setting of the heater. As the volumetric flow rate increases, the cooling effect of the cooling

tower decreases because the heat transfer between the droplets and the air decreases. As the

current of the heater increases, the cooling effect of the cooling tower increases because the

heat transfer between the droplets and the air increases. It should be noted that the wet-bulb

temperature was used in the calculations instead of the adiabatic saturation temperature.

There are discrepancies in the computed results. However, since the wet-bulb temperature is

approximately equal to the adiabatic saturation temperature, the computed results are accurate

enough for any practical use.

6.0 References

Cengel, Y. A., & Boles, M. A. (2011). Thermodynamics: An engineering approach (7th ed.).

Boston: McGraw-Hill.

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