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7/30/2019 Evaporative Cooling and Humidification
1/16
University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 1
EVAPORATIVE COOLING AND HUMIDIFICATION
Flow diagram of a generic heat exchanger
cooling tower system.
Summary:
The basic principle of the cooling tower operation is that of evaporative condensation and
exchange of sensible heat.
The air and water mixture releases latent heat of vaporization which has a cooling effect on water
by turning a certain amount of liquid into its gaseous state thereby releasing the latent heat of
vaporization. In this experiment, certain parameters were measured to determine the height and
number of transfer units.
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 2
POST LAB REPORT
Chemical Engineering Laboratory 2
EVAPORATIVE COOLING
AND HUMIDIFICATIONAugust 23, 2013
JOHN KEVIN G. SAN JOSE
Kazandra M. Aquino
JERICKO L. SAMSON
Charlette Ritz O. Magtrayo
5ChEA
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 3
CONTENTS
Summary 4
Results 5
Discussion of Results 8
Answers to Questions 9
Conclusion 11
References 11
Appendix 12
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 4
1. Summary
The objectives of this experiment are to know the principle behind humidification and
evaporative cooling tower, know its parts, and to estimate the height and number of transfer
units of the tower.
To achieve this purpose, 6 trials were conducted before attaining steady state condition.
Wherein at least 3 data points remain constant, or have at least 0.5 difference between any
three consecutive data.
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 5
Certain parameters were recorded to attain these goals such as air conditions, water
conditions and the flow rates of water and air.
In determining the entering and exit air conditions, a digital thermometer was used to
determine the dry bulb temperature of air and a thermometer with wet cotton on its bulb was
used to get the wet bulb temperature. For its relative humidity, ____ was used.
As for the water conditions, the entering temperature of water was provided by the
thermometer on the exit water outlet of the shell and tube heat exchanger. Whereas a digital
thermometer was used to determine the exit temperature of water to the tank reservoir.
Water and air flow rates was also recorded. The entering volumetric flow rate can be read
through the tube in the entering water outlet at the shell and tube heat exchanger. And to
measure the waters exit mass low rate, a manual operation was done by letting a 1L beaker
be filled by the water from the cooling tower and recording the time it reached the 1L mark.
As for the entering and exit air velocity, readings on two different points were recorded so as
to account for the difference in reading across the cooling tower.
2. Results
In this lab, the cooling tower performance was measured by computing for certain parameters
such as its range, approach and its cooling factor. Also, the height and number of transfer
units was also obtained using the gathered data.
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 6
Steam from the boiler heated the water in the heat exchanger that served as the feed to the
cooling tower. The hot water entered on the top of the tower and was cooled by the air
counter flow to the direction of the water. The cooled water left at the bottom of the tower
and was delivered in a reservoir that supplies the water to the heat exchanger.
Certain air conditions were measured like the wet and dry bulb temperature, the relative
humidity, and the velocity of the entering and leaving air stream. The entering flow rate of
water was also obtained from the reading on the flow meter on the heat exchanger while the
exit flow rate of water was measured on the inlet pipe to the reservoir where the water from
the tower enters the tank. Inlet and outlet temperature of water was also measured for
computations.
The procedure was repeated until steady state was obtained or when three consecutive data
points for all conditions has a difference of 0.5. Based on the data gathered, not all the
conditions of air as well as of the water reached steady state due to instrumental and personal
errors. Steady state condition was observed on Relative Humidity, 64.1%, and the exit wet
bulb temperature, 28oC of air stream, the entering volumetric flow rate, 60.0 L/min, and exit
mass flow rate, 1.7 s/L, of water.
Other observed parameters were the wet and dry bulb temperature of entering air which was
26.8oC and 31.2
oC respectively. The wet and dry bulb temperature and the relative humidity
of exiting air which were 28.0oC, 32.7
oC and 68.0% respectively. The entering and exiting
velocity of airstream were 1.6m/s and 6.1m/s respectively. Entering and exiting water
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 7
temperature were recorded as 42.5oC and 36.9
oC respectively. Table 1.0 shows the tabulated
data of this experiment.
Table 1.0
Data from the Evaporative Cooling and Humidification experiment
AIR CONDITIONS
Entering
Wet Bulb Temperature 26.8oC
Dry Bulb Temperature 31.2oC
Relative Humidity 64.1
Velocity 1.6 m/s
Exit
Wet Bulb Temperature 28 oC
Dry Bulb Temperature 32.7 oC
Relative Humidity 68.0
Velocity 6.1 m/s
WATER CONDITIONS
Entering Temperature 42.5 oC
Exit Temperature 36.9 oC
FLOW RATE
Entering Volume Flow Rate60 L/min
Exit Mass Flow Rate 1.7 s/L
Using the gathered data from Table 1.0, the performance of the cooling tower was evaluated
by calculating the range, approach and effectiveness. The summary of performance
evaluation is shown on Table 2.0
Table 2.0
Cooling Tower Performance Evaluation
Cooling Tower Performance Parameter Equation Result
Range T2-T1 5.6
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 8
Approach T1-tw1 10.1
Effectiveness Range/(Range+Approach) 0.356687898
Cooling Factor L/G 0.368808534
And for the result for the transfer units which includes the height and number of transfer
units, the overall mass transfer coefficient and heat load were summarized on Table 3.0
Table 3.0
Tabulated results for transfer units
Transfer Units Result
Height of Transfer Units 0.367749251
Number of Transfer Units 0.848404175
Overall Mass Transfer Coefficient 8.08 kg/(m3s)Heat Load -23.4303
3. Discussion of ResultsThe basic principle of the cooling tower operation is that of evaporative condensation and
exchange of sensible heat. The cooling of the process fluid, water, from 42.5oC to 36.9oC which
resulted to a range of 5.6 illustrates the capacity of the tower to lower the temperature of water.
The low yield of range may be a result of some errors in reading of certain parameters and also
some instrumental errors. On the other hand, the temperature of the air entering and exiting the
cooling tower increases due to the heat transfer process involving the latent heat (80%) and the
sensible heat (20%).
On the other hand, the approach was computed by getting the difference between the leaving
temperature of water and the wet bulb temperature of air. The approach indicates the possible
heat loss by the water to the air. From the calculation (refer to appendix), a 10.1oC approach was
obtained. This value of approach is dependent on the tower design.
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 9
As seen from Table 2.0, the effectiveness of the cooling tower was computed based from the
ration of the approach and the sum of the approach and the range, giving a value of 0.3567.
For the cooling factor, L/G ratio, it is the minimum required coefficient which can also be
computed by another equation (refer to appendix) which give a value of 0.3688. A high value of
cooling factor indicates a more water to less air ratio, a low evaporation loss, air is more
saturated, a greater residence time of water, etc.
Meanwhile, the number of transfer units indicates the measure of difficulty of the separation
between the water vapour and the air mixture. It was computed to give a value of 0.8484. While
the value of the height of transfer units illustrates the measure of the separation effectiveness of a
particular packing which has a value of 0.3677.
And finally, the value of the overall mass transfer coefficient was obtained to have a value of
8.08 kg/m3*s. While the heat load gives a value of -23.4303kW. The negative sign signifies a
heat loss from the tower due to the heat transfer process.
4. Answers to Questions1. How does the number of transfer units affect the cooling factor?
Number of transfer units (NTU). Also called the tower coefficient, the NTU is a numerical
value that results from theoretical calculations based on a set of performance characteristics.
The value of NTU is also representative of the degree of difficulty for the cooling process.
The NTU corresponding to a set of hypothetical conditions is called the required coefficient
and is an evaluation of the problem. The same calculations applied to a set of test conditions
is called the available coefficient of the tower involved. The available coefficient is not a
constant but varies with operating conditions. The operating characteristic of a cooling tower
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 10
is developed from an empirical correlation that shows how the available coefficient varies
with operating conditions.
Liquid-to-gas ratio (L/G). The L/G ratio of a cooling tower is the ratio of the liquid (water)
mass flowrate (L) to gas (air) mass flowrate (G). Cooling towers have certain design values,
but seasonal variations require adjustment and tuning of water and air flowrates to get the
best cooling tower effectiveness.
A high L/G ratio means:
More water to less air
Air is more saturated driving force is reduced
More residence time of water needed
Less cooling in given time
Increase in required fan power
Decrease in height of tower
Low evaporation loss (under same water flowrate)
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 11
2. Given that the boiler will be supplying steam to several equipment such as heat exchanger
and dryers present in the laboratory
a. How would the operation of cooling tower be affected?
There will be a lesser supply of steam which will remove process heat and cool the working
fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers,
rely solely on air to cool the working fluid to near the dry-bulb air temperature.
b. What parameters would be affected?
The parameters that will be affected will be mass flow rates, temperature and enthalpy.
5. ConclusionThe group learned how to use the heat exchanger and the cooling tower. For the entering
air, the relative humidity reached its highest value on Trial 4 and remained constant until
Trial 6. For the exiting air, the range of the relative humidity is close to each other but did
not become constant averaging to 68%. Its highest relative humidity happened in Trial 4
with 70.2%.
6. References1. Perry, Robert H. And Don Green (editor), Perrys Chemical Engineering Handbook 6th
Ed. New York: McGraw Hill, 1984.
2. Goyal, Jonny (2012). Effective Thermal Design of Cooling Towers. Retrieved fromwww.che.com
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 12
3. Appendix
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 13
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 14
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 15
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University of Santo Tomas Chemical Engineering Laboratories 2Faculty of Engineering Evaporative Cooling and HumidificationChemical Engineering Department
Aquino, Magtrayo, Samson, San Jose August 24, 2013Unit Operation Lab II | 5ChEA 16
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