1.0 ABSTRACT

This experiment is conducted for students to evaluate and study the performances of the

cooling tower at difference operating conditions. This experiment is divided into two parts.

Initially for both experiments, the load tank is filled with distilled water to the wet bulb sensor

reservoir o the fullest while the makeup tank is also filled with distilled water up to zero mark on

the scale. The temperature set point of temperature controller is set up to 45⁰C then the 1 kW

of water heater is switched on and heated until 40 ⁰C. The damper is set to fully open, and the

fan is switched on. The valve V4 and V5 are opened and adjusted to check the pressure across

the orifices while the valve V3 and V6 is to check the different pressure across the column. Then,

for the first part of the experiment, the power of the heater is set at 0.5 kW, 1.0 kW and 1.5 kW

then, for each different power, the differential pressure from the orifice and column, and all the

related temperatures are recorded. For second part, firstly the general start-up procedure are

repeated for three times and this times the manipulating variables is the per cent % of damper

opened; 0%, 50% and fully opened and the differential pressure from the orifice and column

and the related temperatures are also recorded at different % opened of damper. As in results,

it shows that, the increment in cooling load power provides a better efficiency as it increases

when cooling load increases. However, the cooling tower shows the best performance when the

damper is half-opened as its efficiency is the highest when it is 50% opened. The cooling range

and approach to wet bulb temperature also varies with different amount of heat load and

condition of damper. The cooling range increases greatly as well as heat load when the cooling

load increases. As for the second experiment, the cooling range increases when the damper is

half-opened while the approach to wet bulb temperature decreases as the damper is fully

opened. Heat load and efficiency is the highest when the damper is half-opened. The

experiment was a success.

2.0 INTRODUCTION

Regarding to Ananthanarayanan (2005), cooling tower is used in conjunction with the water-

cooled condenser. Water passed through the condenser water tubes only to get warm up but does not

get any contamination. Therefore, the water can be used again after cooling process. The cooling water

also cools the warmed water for recirculating it into the condenser making it as water-conservation

1

equipment. Heat that is removed by refrigeration system to the space is thrown to the atmosphere

through cooling tower in a water-cooled condenser system.

The cooling process is brought about the sensible heat transfer with air as well as through

evaporative cooling by the evaporation of a very small portion of water. In order to get the maximum

heat exchange between water and air, the surface area of water exposed to the air stream is increased

by breaking the water stream into droplets. There are two ways of air circulation within the tower,

which is either by mechanical means or natural air movement. Therefore, there are two types of cooling

tower existed which are atmospheric (natural draft) and mechanical draft towers. The selection of

cooling tower is based on the heat rejection load on the cooling tower.

In United States, the natural draft tower is used for large electric utility condenser cooling with

the flows that could reach as high as 500,000 gpm. Basically, the natural draft tower is designed to take

advantage on the temperature differences between ambient air and the hotter air inside the tower. This

design could create a chimney effect as it causes the cold air at the bottom of the water to push the

warmer out of the top (GC3 Special Chemicals Inc., 2013).

Figure 2a : The component of natural draft cooling tower that is being implemented in industry.

Source: GC3 Special Chemicals Inc., Houston.

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3.0 AIMS

At the end of this experiments, students will be able to: -

To determine the performance characteristics of cooling tower.

To compare the effects of variation in temperature, flow speed and packing filling packages.

To understand the vaporization towers operating principles.

4.0 THEORY

When a gas flows over a surface of liquid, the heat and mass transfer that occur is referring to the

process of evaporative cooling. According to Incropera (2005), evaporation occur from the surface of

liquid, which the phase change utilizes the energy in the form of latent heat of vaporization of the liquid.

3

Figure 4a: Latent and sensible heat exchange at a gas-liquid interface.

(Incropera, 2005)

Based on Figure 4a, evaporation process initiated when liquid molecules embedded on the surface

experience higher collision activity thus increase their energy level. The ever increasing energy later

overcome the energy that are needed to overcome the surface binding energy. This deduces that the

energy of evaporation must come from internal energy of liquid, which later would reduce their

temperature (cooling effect).

To maintain steady state condition of evaporation, the latent heat of evaporation losses by the liquid

must be replaced with energy transfer from surrounding liquid molecule. This will result in further

evaporation process across the liquid surface. The transfer is made possible by convection of sensible

energy where the liquid undergo heat addition from heat source (eg; electric heater), where radiation

effect is considered negligible.

The conservation of energy in a fixed surface area is

q”conv + q”add = q”evap (Eq. 1.1)

where

q”evap = nA¿hfg (Eq. 1.2)

4

where

nA¿= evaporative mass flux

hfg = latent heat of vaporization

In order to understand the principle of evaporative cooling, cooling tower is used as the model. Cooling

tower is an essential equipment to be operated in a plant, which act as the cold utility unit where all the

excessive heat from streams (hot stream) is dissipated. Then, the energy is to be used to prime the feed

by heating them in feed preheater. By practising this efficient energy management in long run, the plant

is more cost-efficient and effective in reducing cost related to its energy expenditure.

Figure 4b: Cross-section of simple cooling tower.

(http://www.thermopedia.com/content/663/)

Based on Figure 4b, water stream is introduced through overhead inlet of cooling tower. The entering

water falls over packing material that function to increase the surface area for heat transfer by many

fold. At the same time, air is let to flow through the bottom inlet of tower and flowing upward inside

there. Once water and air in contact through series of packing, the water evaporates into air stream at

5

the water-air interface. Latent heat of evaporation is carried away by evaporated water vapour, thus

lowering the water temperature.

In order to detect the difference in temperature of air and water at their respective inlet and outlet, the

cooling tower is equipped with various thermocouple which measure the dry and wet bulb temperature

of air. From this, the change in enthalpies of water and air and their energy balance through evaporative

cooling process can be determined. The water flow is controlled by the gate valve, monitored by flow

meter and pumped from a load tank to distribution cap for even and efficient droplet distribution all

over packing area.

Since water eventually will flow down to the bottom, one thermometer is fitted there for water outlet

temperature reading. The remaining water then reheated and re-circulated through the column. For air,

the air is pulled from surrounding by a damper. The damper cover is adjustable where the level of

openness can be set as variable. The wet and dry bulb temperature of the air are taken at various points

along the length of the column. The temperature of air is taken again after it pass by a droplet arrestor

and exit to surrounding via orifice. The pressure drop through the orifice can be used to estimate the air

flow rate.

The principle theory applied in cooling tower is the First Law of Thermodynamics. According to Newton,

this conservation of energy theory stated that energy neither created nor destroyed, they only change

form. This is prove as entering water lose heat through evaporative cooling, its temperature reduce. The

heat eventually gain by counter-current flowing air thus increase its heat energy, resulting in higher

temperature at the outlet. From the theory, enthalpy is defined as

H=U+PV (Eq. 1.3)

Where H is the enthalpy,U is internal energy, P is pressure and V is volume. The enthalpy is in equal

amount as heat as

q=∆H (Eq. 1.4)

To do a heat balance, enthalpy of water of any temperature can be obtained in the steam table as the

example of the table is shown in Appendix 1.1. The water temperature of inlet and outlet of cooling

tower is used as reference to measure the enthalpy of air at dry and welt bulb temperature of both inlet

and outlet tower.

6

The equation for the energy balance is as below:

in = out (Eq. 1.5)

where ∆H = Hin - Hout. For air, the enthalpy is

∆H = Cp∆T (Eq. 1.6)

Where ∆H is the change in enthalpy, Cp is the specific heat with respect to constant pressure and ∆T is

the temperature change. Note that due to low pressure of air, the air can be treated as an ideal gas and

Eq. 1.6 can be used. After obtaining the dry bulb and wet bulb temperature of the inlet and outlet air,

psychometric chart is used to cross both temperatures to obtain enthalpy of air.

Figure 4c: The layout of the cooling tower.

5.0 Experimental Apparatus

7

Figure 5 : The Water Cooling Tower ( model : HE 152-5 )

1

6.0 Experimental Procedure.

6.1 General start-up procedure:

8

1. The valves V1 to V6 are closed and valve V7 is partially opened. The valve V8 is also closed

before the experiment is started.

2. Then, the load tank is filled with distilled water or deionized water to the wet bulb sensor

reservoir to the fullest.

3. The makeup tank is filled with distilled water or deionized water up to zero mark on the scale.

4. In addition, the distilled water is added to the wet bulb sensor reservoir to the fullest.

5. The cooling tower packing for the experiment is installed appropriately.

6. All tubing is connected to the different pressure sensor.

7. After that, the temperature set point of temperature controller is set up to 45⁰C. The 1.0 kW

water heater is switched on the water is heated until 40⁰C.

8. The pump is switched on and valve V1 is opened and the water flow rate is set to 1.6 of 1.2 LPM.

A steady operation is obtained where the water is distributed and flow uniformly through the

packing.

9. Next, the fan damper is fully opened, and the fan is switched on. The differential pressure

sensor is checked either it give reading or not.

a) The valve V4 and V5 are opened while the valve V3 and V6 are closed and different pressure

across the orifices is measured.

b) The valve V3 and V6 are opened and valve V4 and V5 are closed. The different pressure

across the column is measured.

10. The unit is run for 10 minutes to 15 minutes for the float valve to correctly adjusted the level in

the load tank. The makeup tank is refilled as required.

11. Then, the unit is ready for use.

6.2 Experiment 1 : The power of heater is varied.

1. The general start-up procedure are repeated,

2. The power of the heater is set as 0.5 kW before the differential pressure from the orifice and

column are recorded.

3. The step 1 and 2 are repeated by increasing the power of heater to 1.0 kW and 1.5 kW.

9

6.3 Experiment 2 : The percentage of damper opener is varied.

1. The general start-up procedure are repeated for three times.

2. The step 9 is changed for each steps the repeated by only 50% opened the damper and fully

closed damper.

6.4 General shut-down procedure

1. The heater is switched off and the water is circulated through the cooling tower system for 3 to

5 minutes until the water is cooled down.

2. The fan is switched off and the fan damper is fully closed.

3. The pump and power supply are switched off.

4. The water in the reservior is retained for the following experiment.

5. Lastly, the water is completely drained from the unit if it is not in used.

7.0 RESULTS

7.1 Experiment 1

10

Constant variable: Flow rate = 2.0 LPM

Damper = Fully-open

Manipulated Variable: Heater

Parameter Unit 0.5 kW 1.0 kW 1.5 kW

Air inlet dry bulb, T1oC 31 31 31.12

Air inlet wet bulb, T2oC 26.8 26.7 26.9

Air outlet dry bulb, T3oC 28.3 28 29.1

Air outlet wet bulb, T4oC 28.7 28 28.7

Water inlet, T5oC 32.5 32.9 36.4

Water outlet, T6oC 26.5 26.8 27.3

∆P Orifice Pa 93 93 93

∆P Column Pa 11 10 11

Heater Power W 422 811 1231

Table 7a: Values of Cooling range, Approach wet bulb, Heat Load (Q), Overall heat Transfer coefficient

(U), and Efficiency (η¿ calculated at different Cooling Load (kW)

Cooling

Load,kW

T5 T6 T2 Cooling

range

(T5-T6), ℃Approach

To wet bulb

(T6-T2), ℃Heat

Load, Q

(kW)

Overall

Heat Transfer

Coefficient, U

Efficiency,

η ,( %)

0.5 32.3 26.5 26.8 5.8 -0.3 0.801 0.00135 160.2

1.0 32.9 26.8 26.7 6.1 0.1 0.843 0.00135 84.3

1.5 36.4 27.3 26.9 9.1 0.4 1.257 0.00135 83.8

Then 2 graphs are plotted;

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-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.2

0.4

0.6

0.8

1

1.2

1.4

Approach wet bulb, ℃

Heat

Lo

ad, Q

Figure 7a: Graph shows the Heat Load, Q versus Approach wet bulb,

0.4 0.6 0.8 1 1.2 1.4 1.60

20

40

60

80

100

120

140

160

180

CoolingLoad,kW

Effici

ency

, 𝜂,( %

)

Figure7b : Graph shows the Efficiency versus cooling load

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7.2 Experiment 2

Constant variable: Flow rate = 2.0 LPM

Heater = 1.5 kW

Manipulated Variable: Damper

Parameter Unit 0% opened 50% opened 100% opened

Air inlet dry bulb, T1oC 33.3 31.2 31.4

Air inlet wet bulb, T2oC 28.1 27 26.9

Air outlet dry bulb, T3oC 38.2 29.7 29.1

Air outlet wet bulb, T4oC 35.0 29.4 28.7

Water inlet, T5oC 45.9 37.7 36.4

Water outlet, T6oC 39.4 27.9 27.3

∆P Orifice Pa 0 83 93

∆P Column Pa 0 8 11

Heater Power W 1214 1224 1231

Table7b : Values of Cooling range, Approach wet bulb, Heat Load (Q), Overall heat Transfer coefficient

(U), and Efficiency (η¿ calculated at different Blower change (%)

Blower

changes

(%)

T5 T6 T2 Cooling

range

(T5-T6)

Approach

To wet bulb

(T6-T2)

Heat

Load, Q

Overall

Heat Transfer

Coefficient, U

Efficiency,

η

13

0 45.9 39.4 28.1 6.5 11.3 0.898 0.00135 59.6%

50 37.7 27.9 27 9.8 0.9 1.354 0.00135 90.2%

100 36.4 27.3 26.9 9.1 0.4 1.257 0.00135 83.8%

Then , 2 graphs are also plotted

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Approach wet bulb, ℃

Heat

Lo

ad, Q

Figure 7c: Graph shows the Heat Load, Q versus Approach wet bulb

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0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100

Blower changes (%)

Effici

ency

,𝜂,%

Figure 7d: Graph of Efficiency versus Blower changes %

8.0 CALCULATIONS

Volume=height ×width×depth

Volume=1.25m×0.91m×0.45m

Volume=0.512m3

Area=volume × packing density

Area=0.512m3×200 m2

m3

Area=102.4m2

15

Sample calculation:

Experiment 1:-

Independent Variable:

Cooling load (kW)

Rate of evaporation=Heat Load=Q

Q=mc p ΔT

Constant Variables:

Flow rate (2.0 LPM)

Blower (fully open)

Conversion: 2 L 1 min 1 m3 1000 kg m= 0.033kg

min 60 s 1000 L m3 S

To find cooling range

CoolingRange=(T hot water−T coldwater )

¿ (T 5−T 6 )

¿ (32.3−26.6 )

¿5.8

16

To find Approach wet bulb temp

Approach¿wet bulb temp=(T coldwater−T wet bulb )

¿ (T 6−T 2 )

¿ (26.5−26.8 )

¿−0.3

To find Heat Load, Q

Heat Load ,Q=mc p ΔT

Q=(0.033 kgs )×(4.186 kJkg .℃ )× (5.8℃ )

¿0.801 kJs

¿0.801kW

To fond Overall Heat Transfer Coefficient, U

Overall heat transfer coefficient ,U= QA . ΔT

¿ 0.801kW

102.4m2×5.8℃

¿0.00135 kW

m2℃

17

To find Efficiency, η

Efficiency ,η= Heat LoadHeat Supply

×100%

¿ 0.8010.5

×100%

¿160.2%

Then, same step are repeated to find the cooling range, Approach wet bulb, Heat Load (Q), Overall heat

Transfer coefficient (U), and Efficiency for experiment 1 at cooling load 1.0 kW and 1.5 kW. All the values

calculated are tabulated as in table below:

Table : Values of Cooling range, Approach wet bulb, Heat Load (Q), Overall heat Transfer coefficient (U),

and Efficiency (η¿ calculated at different Cooling Load (kW)

Cooling

Load,kW

T5 T6 T2 Cooling

range

(T5-T6), ℃Approach

To wet bulb

(T6-T2), ℃Heat

Load, Q

(kW)

Overall

Heat Transfer

Coefficient, U

Efficiency,

η ,( %)

0.5 32.3 26.5 26.8 5.8 -0.3 0.801 0.00135 160.2

18

1.0 32.9 26.8 26.7 6.1 0.1 0.843 0.00135 84.3

1.5 36.4 27.3 26.9 9.1 0.4 1.257 0.00135 83.8

Then 2 graphs are plotted;

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50

0.2

0.4

0.6

0.8

1

1.2

1.4

Approach wet bulb, ℃

Hea

t Lo

ad, Q

Figure : Graph shows the Heat Load, Q versus Approach wet bulb,

19

0.4 0.6 0.8 1 1.2 1.4 1.60

20

40

60

80

100

120

140

160

180

CoolingLoad,kW

Effici

ency

, 𝜂,( %

)

Figure : Graph shows the Efficiency versus cooling load

For experiment 2 the same step calculation also repeated in order to find the cooling range, Approach

wet bulb, Heat Load (Q), Overall heat Transfer coefficient (U), and Efficiency(η¿ but in this experiment,

the value of cooling load (kW) are constant, while the manipulated variable are the Blower change (%) at

0% opened, 50% opened and 100% opened.

All the values calculated are tabulated as in table below

20

Table : Values of Cooling range, Approach wet bulb, Heat Load (Q), Overall heat Transfer coefficient (U),

and Efficiency (η¿ calculated at different Blower change (%)

Blower

changes

(%)

T5 T6 T2 Cooling

range

(T5-T6)

Approach

To wet bulb

(T6-T2)

Heat

Load, Q

Overall

Heat Transfer

Coefficient, U

Efficiency,

η

0 45.9 39.4 28.1 6.5 11.3 0.898 0.00135 59.6%

50 37.7 27.9 27 9.8 0.9 1.354 0.00135 90.2%

100 36.4 27.3 26.9 9.1 0.4 1.257 0.00135 83.8%

Then , 2 graphs are also plotted

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Approach wet bulb, ℃

Hea

t Lo

ad, Q

Figure : Graph shows the Heat Load, Q versus Approach wet bulb

21

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100

Blower changes (%)

Effici

ency

,𝜂,%

Figure : Graph shows the Efficiency , % versus Blower changes, %

9.0 DISCUSSION

This cooling tower experiment was carried out to determine the performance characteristics of a cooling

tower and to compare the effects of variation in some parameters such as cooling load and condition of

damper. Besides that, this experiment was also carried out to understand the operating principle of

vaporization towers.

Generally, cooling tower is a heat rejection device that extracts waste/undesired heat to the

atmosphere by cooling of a stream of water to a lower temperature. This heat rejection is also known as

‘evaporative’ in which it allows a little portion of the water being cooled to evaporate into a moving air

stream which provide significant cooling to the remaining in the water stream. The heat that was

released by the water stream transferred to the air stream, thus raising the air’s temperature and its

relative humidity to 100% which later discharged into the atmosphere (Cooling Tower Institute, 2012). In

designing a cooling tower for a chemical plant, several parameters should be considered, in which each

will affect the size, capacity and efficiency of a cooling tower. For a better understanding, the effect is

simplified in terms of parameters such as heat load, range, approach and wet-bulb temperature. If one

parameter is changed while the rest are held constant, this will indeed affect the tower size and

efficiency (Marley, 2013). However in this experiment, two parameters are being manipulated that

causes the cooling range (T5-T6) of cooling tower varies which are the variation of heater power (cooling

load) and damper opener.

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In the first experiment, the cooling load parameter was manipulated. The experiment was repeated

three times with different electric power of 0.5 kW, 1.0 kW and 1.5 kW respectively. The graph of heat

load versus approach wet bulb in result section showed that the approach wet bulb temperature

increases as heat load increases. The approach wet bulb temperature refers to the difference between

the temperature of water leaving the tower and the wet bulb temperature (Ananthanarayanan, 2005).

The wet bulb temperature of air is the determining factor for the amount of water vapour that the air

can take. This is based on the wet bulb depression of air, which is the difference between the dry and

wet bulb temperature. The greater value of wet bulb depression can take more water vapour, thus the

amount of water that can evaporate will naturally depend on the capability of the circulating air to take

in water. The cooling range also increases greatly as cooling load increases. However, the efficiency

calculated decreases as cooling load increases. This is because heat load increases as cooling load

increases. A bigger heat load provides better efficiency. Since the rate of evaporation is equal to heat

load, therefore heat load also refers to the load on the condenser. However, the pressure drop is

constant despite the increment in cooling load.

In the second experiment, the manipulated variable is the condition of the damper. Damper functions as

capacity controlling accessory item of cooling tower. In this case, blower is equipped to the cooling

tower, in which it provides airflow stream to absorb heat that was rejected by water stream in the

cooling tower (Yunus A.Cengel, 2008). In this experiment, observations and results were recorded in

three conditions, which are when the damper was 0% opened, 50% opened and 100% opened. The

observations recorded that the cooling range increases when the damper is half-opened while the

approach to wet bulb temperature decreases as the damper is fully opened. Heat load and efficiency is

the highest when the damper is half-opened. The observations are illustrated as in both graph of heat

load versus approach to wet bulb and the graph of efficiency versus blower changes. In the fully opened

condition, they are like a thin piece of sheet metal in a moving airstream oriented parallel to airflow. As

the damper is closed, the sheet metal become less parallel to airflow, hence turbulence disrupts the air

stream. This damper is usually adjusted when exiting water temperature becomes too low, which is to

adjust the airflow (Nicholas P. Cheremisinoff, 2000). Damper also creates a difference in pressure inside

the cooling tower. The change in reading of pressure drop in orifice and column when the damper is in

open and close position proves that damper creates a change in pressure.

10.0 CONCLUSION

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In conclusion, the performance of cooling tower can be characterized by heater power(cooling load) and

the condition of damper. Increment in cooling load provides a better efficiency as it increases when

cooling load increases. However, the cooling tower shows the best performance when the damper is

half-opened as its efficiency is the highest when it is 50% opened. The cooling range and approach to

wet bulb temperature also varies with different amount of heat load and condition of damper. The

cooling range increases greatly as cooling load increases. Heat load increases as cooling load increase

hence the approach wet bulb temperature also increases as heat load increases. As for the second

experiment, the cooling range increases when the damper is half-opened while the approach to wet

bulb temperature decreases as the damper is fully opened. Heat load and efficiency is the highest when

the damper is half-opened. The experiment was a success.

11.0 Recommendations

Based on the experiments, it is highly recommended for students to:-

11.1 To repeat three times the general procedure before starting the experiment 2 as to ensure the

efficiency of the performances are not being affected with temperature of the power provided for the

first experiment.

11.2 To make sure in experiment2, pressure difference and the temperature reading is stable before the

readings are recorded to ensure the accuracy of the results.

11.3

12.0 References

1. Cooling Technology Institute (2012) retrieved from

http://www.cti.org/whatis/coolingtowerdetail.shtml

2. 2. P.N Ananthanarayanan (2005), Basic Refrigeration and Air-Conditioning, published by Tata

Mc-Graw Hill Education.

3. Yunus A. Cengel (2008), Fundamentals of Heat and Mass Transfer Sixth Edition, published by

Wiley.

24

4. Nicholas P. Cheremisinoff (2000), Handbook of Chemical Processing Equipment, published by

Butterworth-Heinemann.

5. Fundamentals of Heat and Mass transfer, 6th edition, Incropera/DeWitt/Bergmann/Lavine,

ISBN-13:978-0-471-794714. Evaporative Cooling, 381-384

6. (http://www.thermopedia.com/content/663/) ACHE, Singham J.R.

DOI: 10.1615/AtoZ.c.cooling_towers

7. Thermodynamics: An Engineering Approach, 7th Edition in SI units, Cengel/Boles,

ISBN 978-007-131111-3. Enthalpy, 124, properties of gas mixtures:ideal and real gas, 697-698.

Property tables and charts, 904-908.

8. P. N. Ananthanarayanan (2005), Basic Refrigeration and Air Conditioning Third Edition; Tata

McGraw Hill

9. GC3 Special Chemicals Inc.; Houston, retrieved from website

http://www.gc3.com/Default.aspx?tabid=90 on 17th November 2013.

13.0 APPENDICES

25

Figure 13.1 : The temperature reading for water cooling tower.

Figure 13.2 : The water cooling tower

26