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Meshal Khaled Al-Saeed423105653Dr. Malik Al-Ahmad
King Saud UniversityCollege of EngineeringChemical Engineering
Dept.Chemical Engineering LaboratoryChE 403
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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.
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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.
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Theory Overall Mass Balance:In Put = Out PutL2 + 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)
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To determine the number of transfer unit (NTU):
To calculate height of transfer unit (HTU):
To calculate the mean driving force:
Simpsons rule:
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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].
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Schematic Apparatus Figure 1: Photo of cooling tower.
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Figure 2: Schematic apparatus for cooling tower.
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ResultsTable1: Temperature at Q = 1.0 kW
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
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Table2: Temperature at Q = 1.5 kW
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
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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.
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ConclusionFrom 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.
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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.
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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.
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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
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References1. Chirstie J. Geankoplis, "Transfer Process and Unit
Operation", 3rd edition.2. Dep. of Chemical Eng Chemical
Engineering Laboratory II Manual.3. Perrys, "Chemical Engineers
Handbook", 5th edition.4. From web side:
http://www.armfield.co.uk/