King Saud University
College of Engineering
Chemical Engineering Dept.
Chemical Engineering Laboratory
ChE 403
Meshal Khaled Al-Saeed
423105653
Dr. Malik Al-Ahmad
Introduction
The cooling tower is one of the most important device in
chemical industries for example when the hot water come
from heat exchanger we use the cooling tower to cool it.
The purpose of cooling tower is to cool relatively warm
water by contacting with unsaturated air. The evaporation
of water mainly provides cooling.
In a typical water cooling water tower, warm water flows
countercurrent to an air stream. Typically, the warm water
enters the top of packed tower and cascades down through
the packing, leaving at the bottom.
Air enters at the bottom of the tower and flows upward
through the descending water. The tower packing often
consists of slats of plastic or of packed bed. The water is
distributed by troughs and overflows to cascade over slat
gratings or packing that provides large interfacial areas of
contact between the water and air in the form of droplets
and films of water. The flow of air upward through the
tower can be induced by the buoyancy of the warm air in
the tower (natural draft) or by the action of a fan.
The water cannot be cooled below the wet bulb
temperature. The driving force for the evaporation of the
water is approximately the vapor pressure of the water less
the vapor pressure it would have at the wet bulb
temperature.
Theory
Overall Mass Balance:
In Put = Out Put
L2 + G1 = L1 + G2 → (1)
Water Mass Balance:
L2 - L1 = G2 * H2 - G1 * H1 → (2)
L2 - L1 = G * (H2 - H1) → (3)
Energy Balance:
Q = G * (HG2 - HG1) → (4)
G1 =  * air → (5)
G2 = G1 * H2 → (6)
HG = Cs * (T - To) + o * H → (7)
Cs = Cp Liquid + Cp Vapor * H → (8)
 To determine the number of transfer unit (NTU):
H G2
d HG
NTU  
H Gi  H G
H G1
 To calculate height of transfer unit (HTU):
G
HTU 
K G a.M B .P
Z
 d Z  Z  (HTU) (NTU)
0
 To calculate the mean driving force:
ΔH LM
 Simpson’s rule:
x2
H G2  H G1

H G2
ln
H G1
h
x f(x)dx  3 [f(x o )  4f(x 1 )  f(x 2 )]
o
Where:
L2: Flow rate of water in [kg/s.m2].
L1: Flow rate of water out [kg/s.m2].
G1: Air in [kg/s.m2].
G2: Air out [kg/s.m2].
: Volumetric flow rate of air.
H: Humidity of air [kg water/kg air].
HG: Enthalpy of the air [J/kg air].
KGa: Mass transfer coefficient of air [kg mol/s.m3.Pa].
NTU: Number of transfer unit [dimensionless].
HTU: Height of transfer unit [m].
Schematic Apparatus
Figure 1: Photo of cooling tower.
Figure 2: Schematic apparatus for cooling tower.
Results
Table1: Temperature at Q = 1.0 kW
Time
(min)
Air in
Air out
Water
T1
(ºC)
T2
(ºC)
T3
(ºC)
T4
(ºC)
T5
(ºC)
T6
(ºC)
5
21
19
22
22
31
23
10
21
19
20
21
29
22
15
21
19
20
21
29
21
20
21
19
20
21
29
21
Flow rate of water = 40 g/sec
Initial pressure = 31 mm H2O
Final pressure = 38 mm H2O
Pressure drop = 7 mm H2O
Volume of evaporation water at 20 min = 1027 ml
Table2: Temperature at Q = 1.5 kW
Time
(min)
Air in
Air out
Water
T1
(ºC)
T2
(ºC)
T3
(ºC)
T4
(ºC)
T5
(ºC)
T6
(ºC)
0
22
19
22
22
31
21
5
21
19
23
24
32
22
10
21
19
24
24
32
21
15
21
19
24
25
32
22
20
21
19
24
25
33
22
Flow rate of water = 40 g/sec
Initial pressure = 31 mm H2O
Final pressure = 38 mm H2O
Pressure drop = 7 mm H2O
Volume of evaporation water at 20 min = 1025 ml
Where:
T1: Dry bulb temperature in.
T2: Wet bulb temperature in.
T3: Dry bulb temperature out.
T4: Wet bulb temperature out.
T5: Water temperature input.
T6: Water temperature output.
Conclusion
From conclusion point view:
 Cooling tower is used to cool relatively hot water.
 As the humidity of the inlet air decreased, the performance
of the cooling tower will be better. This leads to the better
mass transfer between water and gas phase.
 As the temperature of the inlet air decreased, the
performance of the cooling tower will be better.
 As the temperature increased overall mass transfer
coefficient KGa increased.
 If the air flow rate is increased, the height of the cooling
tower decrease.
Recommendations
It is better to open the windows or doors in the
lap to refresh the air and to make a good
deference in the driving force.
Summary
The main objective of this experiment is to perform mass and
energy balances over a cooling tower and to determine the mean
driving force, the number of transfer units and the overall mass
transfer coefficient.
In this experiment can be calculated:
 The outlets water for a typical cooling tower and compare
it with the measured value.
 The rate of heat transfer.
 The mean driving force.
Also can be get HGi from the Temperature Enthalpy diagram and
the operating line then get the number of transfer unit (NTU) by
determine the area under the curve then find the overall mass
transfer coefficient.
Results:

At Q = 1.0 kW
L1 = 0.04 kg/s
Q = 0.524 kW
NTU = 0.285
HTU = 1.68 m
∆HG Lm = 49.23 kJ/kg dry air
KGa = 2.9 e-6 kg mol/s.m2
 At Q = 1.5 kW
L1 = 0.0401 kg/s
Q = 1.25 kW
NTU = 0.0879
HTU = 5.523 m
∆HG Lm = 60.39 kJ/kg
KGa = 9.73 e-7 kg mol/s.m2
References
1. Chirstie J. Geankoplis, "Transfer Process and Unit
Operation", 3rd edition.
2. Dep. of Chemical Eng “Chemical Engineering
Laboratory II Manual”.
3. Perry’s, "Chemical Engineers Handbook", 5th edition.
4. From web side: “ http://www.armfield.co.uk/ ”