Evaporative Cooling and Humidification

7/30/2019 Evaporative Cooling and Humidification

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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 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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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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CONTENTS

Summary 4

Results 5

Discussion of Results 8

Answers to Questions 9

Conclusion 11

References 11

Appendix 12


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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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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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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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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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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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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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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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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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3. Appendix


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Evaporative Cooling and Humidification

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