cooling tower exp
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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
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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.
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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
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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.
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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:
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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.
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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
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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
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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
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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
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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℃
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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,
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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
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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
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