Effect Of The Water Inlet Temperature And FlowRate On The Energy And Exergy Performance of AForced Draft Counter Wet Cooling TowerFadhil Abdulrazzaq Kareem  ( fadhiljua91@yahoo.com )

Middle Technical University https://orcid.org/0000-0001-9909-2002Doaa Zaid Khalaf 

Middle Technical UniversityMustafa J. Al-Dulaimi 

Al-Esraa University CollegeYasser Abdul Lateef 

Middle Technical University

Original Article

Keywords: Exergy analysis of cooling tower, exergy e�ciency, thermal e�ciency, exergy of water, exergyof air, exergy of vapor, exergy destruction

Posted Date: June 22nd, 2021

DOI: https://doi.org/10.21203/rs.3.rs-612502/v1

License: This work is licensed under a Creative Commons Attribution 4.0 International License.  Read Full License

Effect of the water inlet Temperature and Flow Rate on the energy and exergy performance of a forced draft counter wet Cooling Tower

Fadhil Abdulrazzaq Kareem*1, Doaa Zaid Khalaf 2, Mustafa J. Al-Dulaimi3, Yasser Abdul Lateef 2

1Institute of Technology Baghdad, Middle Technical University, Baghdad, Iraq.

2Engineering Technical Collage Baghdad, Middle Technical University, Baghdad, Iraq

3Department of air conditioning and Refrigeration Engineering Technologies, Al Esraa University collage, Baghdad, Iraq.

*corresponding author: fadhiljua91@yahoo.com

Abstract

Cooling towers, wherein water and air are contacted directly with each other, are specialized heat

exchangers. These open-topped, tall, cubical or cylindrical shaped are responsible for reducing the temperature of

the water that generated from the industrial or HVAC systems. The performance of the forced draft wet cooling

tower is investigated experimentally. The performance analysis is based on the first and second law of

thermodynamics. . The impact of the inlet water temperature and water inlet flow rate is investigated. The inlet

water temperature is varied from 28 °C to 42 °C for the water flow rates of (0.03, 0.05 and 0.075 kg/sec). The results

reveal that the cooling capacity, cooling range, thermal efficiency and the total exergy destruction increase according

to the increase in the inlet water temperature and the water flow rate. The maximum cooling range is found to be

14.8 °C with the maximum thermal efficiency of 74 %. On other hand, the exergy efficiency decreases with the

increasing of the inlet water temperature and the water flow rate within a range of 11.9 % to 57.8 %.

Keywords: Exergy analysis of cooling tower; exergy efficiency; thermal efficiency; exergy of water; exergy of air;

exergy of vapor; exergy destruction.

1. Introduction

A cooling tower is a direct contact heat exchanger; the process includes both heat and mass transfer between water and

air. The cooling towers are widely used in the large heating, ventilation and air condition system (HVAC) to handle a large

heat load up to more than (100 TR, 325 kW) [1]. The cooling towers are not only specified for (HVAC) only, they also can be

used in industrial power plant and chemical industries. The cooling towers are used to reduce the temperature of the hot water

by evaporating the injected water to the air stream. The hot water stream is injected to fine droplets of water through nozzle to

spray the water on the packing fills. This mechanism helps to increase the rate of water evaporation and leads to reduce the

water temperature [2]. Cooling towers can be classified according to the air flow into a forced and natural draft cooling tower;

moreover, it can be classified according to the direction of air flow and water flow into a cross or counter flow [3]. The first

law of thermodynamics analysis doesn’t indicate the location and magnitude of energy destruction, so in the last few years the

researchers measured the availability of energy which means exergy. Exergy is defined as the maximum work achievable from

gas, fluid, and mass when the final state become equilibrium with the surrounding conditions. The exergy analysis is based on

the second law of thermodynamics, which provides more information to locate the inefficient part of the system. T. Muangnoi

et al. 2007 [3] developed a mathematical model to find the thermodynamic properties of the air and the water through the

cooling tower. The results showed that the water exergy decreased continuously from top to bottom while air exergy decreased

from the top to the bottom. This situation was solved by inserting an efficient filling with a large contact area at the bottom

region where the exergy destruction is high, and inserting a regular one at the top region where the exergy destruction is low.

A. Ataei et al. 2008 [4] developed a mathematical model of counter flow wet cooling tower to predict water and air properties.

The results from the simulation model were vaidated with experimental data. It was noted that the maximum deviation between

the simulation and experimental results was 0.14%. The results showed that the exergy of the water is higher than that of the

air, the water exergy increased from bottom to the top of the cooling tower. As the inlet water temperature increased, the

effectiveness, heat removed and the exergy losses of the cooling tower increased while the second law efficiency decreased.

A. Niksiar and A. Rahimi 2009 [5] provided a model for exergy and energy analysis for a gas spray cooling tower to examine

the impact of some parameter such as cooling tower diameter, length, and liquid drops size. The results showed that the overall

exergy destruction increased as the injected water flow rate and the tower height increased. A.N. Khalifa 2015 [6] formulated

mass, energy and exergy balances according to Merkel theory and evaluated the parameters by using Engineering Equation

Solver software. The results showed that the Lewis number had a constant trend through the cooling tower. The exergy

destruction of the water at the bottom was less than at the top of the tower while the exergy destruction of air at the bottom

was higher than at the top of the cooling tower. As the moisture content increased, the chemical exergy increases too while the

thermal exergy decreased. Q. S. Mahdi and H. M. Jaffal 2016 [7] presented a thermal characteristics analysis of the cooling

tower based on mass and heat transfer methods. The capacity of the cooling tower that used was 9 kW. Dry bulb temperature,

spray water temperature and relative humidity of the air measured at each point of the packing film. The thermal efficiency of

the cooling tower with the cases of the packing film above and under the heat exchanger was approximately (16%) and (28%)

higher than that cooling tower without the packing film. The cooling range, cooling capacity and the first law efficiency

increased as the water flow rate and inlet water temperature increased. Q. S. Mahdi and H. M. Jaffal 2016 [8] conducted an

experimental analysis of the modified Closed Wet Cooling Tower according to the first and second law of thermodynamics.

The results showed that the exergy destruction increased due to the increases of the inlet water temperature, air and water flow

rates and the inlet air wet bulb. M. A. Ghazani et al. 2017 [9] developed a computer simulation to obtain air and water

temperatures at different sections of the cooling tower. The results showed that air exergy increased from the bottom to the top

of the tower, while water exergy decreased as the water descends from the top to the bottom of the tower. Air total exergy had

similar trends as its chemical exergy. The water-to-air mass flow rate ratio was less than one and when the ratio increases. M.

Zunaid et al. 2017 [10] developed a two-dimension numerical model in MATLAB. The numerical model was validated with

experimental data obtained from the literature. The results showed the efficiency of the cooling tower increased with the

increasing of the inlet water temperature. The Maximum efficiency of 55.28% occurred when the water temperature reached

36 ˚C. F.A. Kareem [11] investigated the energy and exergy performance of the cooling tower. The results showed that the

chemical exergy of air, exergy of water and the exergy efficiency increased from the bottom to the top of the cooling tower.

The thermal exergy of air and exergy destruction decreased from bottom to the top.

The objectives of this paper are to study the effects of the inlet water temperature and flow rate on the performance of the

cooling tower according to the first and second laws of thermodynamics. The second law of thermodynamics is used to specify

the exergy distributions of water and air in the cooling tower. The experimental data were obtained from the cooling tower and

the parameters were evaluated by using Engineering Equation Solver software.

2. Experimental Set-up Description

The experimental set up used in the present investigation is shown Fig. (1). The counterflow wet cooling tower is of cross

section 200*200 m and 800 mm in length The packing fill used is of 12 cm . An axial fan is used to blow the air from bottom

of the cooling tower to the top. An adjustable gate is fixed at the suction port of the fan to control the flow rate of the air. A

centrifugal pump is used to circulate the water and to distribute it uniformly on the fills by a spray nozzle. Two heaters (0.5

and 1) kW are used to heat up the entering water. Air mass flow rate is measured by using a manometer that connected on the

sides of an orifice placed at the discharge port of the fan. Water flow rate is measured by using a flow meter placed on the

discharge pipe of the pump. Six thermocouples type-K are used to measure the temperatures of air and water. Four sensors are

3

used to measure the air dry bulb and wet bulb temperatures at the inlet and outlet while the remaining two are used to measure

the temperature of the water at the inlet and outlet. A basin is installed at the bottom of the cooling tower to collect the cold

water and measure its temperature. The Calibration of thermocouples is shown in Fig.2. The relation of the reference

temperature and the thermocouple temperature is a linear with slope of approximately 45 which indicates that nearly there is

a very small error in the measurement

Fig.1. experimental Set-up

Fig.2. Thermocouples Calibration.

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Re

fere

nce

Te

mp

.

Thermocpouple temp.

3. Energy analysis

In the counterflow cooling tower as shown in Fig.3. The air flows upstream while the water flows downstream. For a

steady-state condition of counter flow wet cooling tower, the mass of the evaporated water into air can be calculated by [3]

Water in Air out

Water out Air in

mwi, Twi, hwi

mwo, Two, hwo mai, DBTi,

WBTi, hai,

ωai

mao, DBTo,

WBTo, hao,

ωao

mw, hw

mw - dmw

hw - dhw

ma, ha, ωa

ha + dha

ma

ωa + dωa

dH

Fig.3. control volume of the test rig

v adm m d=

(1)

The heat balance of the cooling tower can be expressed as:

a a w f ,w f ,w am dh m dh h m dω+ = (2)

Where the change in enthalpy can be calculated by:

f ,w w wdh cp dT= (3)

By substitute the above equation into the heat balance, the last becomes:

( )aw a f ,w

w w

mdT dh h dω

m cp= −

(4)

The performance of the cooling tower can be expressed by some parameter such as the cooling rage, which defines as the

water temperature difference of the inlet and outlet streams [7]

wi weR T T= − (5)

Cooling capacity defines as the heat rejected from water to the air stream [7,12]

5

( )w w wi weQ m cp T T −= (6)

Cooling tower efficiency defines as the ratio of actual heat rejected from water to the maximum heat that can be rejected

from cooling tower, and it can be calculated by [7,12]

( )( )

wi we

wi i

T Tη

T WBT

−=

− (7)

4. Exergy Analysis

For a steady-state flow in the counterflow wet cooling tower and by neglecting the effect of potential and kinetic energy,

the exergy analysis is formulated for the counterflow wet cooling tower as follows.

4.1. Exergy of humid air

The total exergy in the humid air, steady-state psychometric process without the effect of kinetic and potential energy,

and by neglecting the change of pressure through the cooling tower can be presented as [10,11]

4.1.1. Exergy of air due to the convective heat transfer

( ) ( )a aa,c a a a o o a a v a o o v

o o

T TEx m cp T T T cp ln ω cp T T T cp ln

T T

= − − + − − (8)

4.1.2. Exergy of air due to evaporative heat transfer

( )( )

a a,oa,o

a,e a a o a v o

a a,o a

ω 1 1.608 ω1 1.608 ωEx m R T ln ω R T ln

1 1.608 ω ω 1 1.608 ω

+ + = + + + (9)

4.2. Exergy of water

By neglecting the mechanical exergy, the total exergy of water can be expressed as [11,13]

( ) ( )w w w,o o w w,o v o oEx h h  T s s R T ln= − − − − (10)

4.3. Exergy of vapor

Considering a reversible approach where the water becomes the water vapor by absorbing the phase change heat in an

isobaric process, where the specific exergy of water vapor at the bulk water temperate (Tw) is given by [13]

( ) ( )wv w o o v v o o

o

TEx  cp T T T cp ln R T ln

Tv

= − − +

(11)

4.4. Exergy destruction

The total exergy destruction of counterflow wet cooling tower represented the differences between the exergy inlet and

the exergy outlet by means of exergy balance equation. [5,11]

dest. in outEx Ex Ex= − (12)

4.5. Exergy efficiency

The second law performance of the cooling tower can be expressed as the ratio of the exergy out to the exergy in of the

system [5,11]

outex

in

ExηEx

= (13)

The constant values of air and water vapor are used [4,11]: cpa = 1.004 kJ/(kg. K), cpv = 1.872 kJ/(kg. K), cpw =4.19 kJ/(kg. K), Ra = 1.004.287 kJ/(kg. K), Rv = 0.461 kJ/(kg. K).

5. Results and Discussions

The performance of a counterflow wet cooling tower has been investigated enterically and exegetically. All the

experiments are conducted for the inlet water temperature range from 28 °C to 42 °C for the water flow rates of (0.03, 0.05

and 0.075 kg/sec), while the surrounding air condition remains constant at (27 °C and 60 % relative humidity). The effect of

the inlet water temperature with the mass flow rate on the cooling tower capacity is shown in Fig.4. It demonstrates that the

cooling capacity increases as the inlet water temperature increases. It is clear that cooling tower capacity increases as the water

flow rate increases due to the increase of the number of water droplets through nozzles per unit time. This leads to more mass

and heat transfer between water and air. The cooling tower capacity increases from (0.4 to 2.1) kW, (0.73 to 3.3) kW and (1.1

to 4.6) kW for the water flow rates of (0.03, 0.05 and 0.075 kg/sec) respectively.

Fig.4. Cooling tower capacity

Fig.5 shows the variation thermal efficiency of the cooling tower the inlet water temperature for different water mass

flow rates. The thermal efficiency of the cooling tower is increases as the inlet water temperature increases. The higher the

thermal efficiency, the better the cooling performance. It also can be seen from the figure that thermal efficiency increases as

water flow rate increases. The thermal efficiency increases from (0.43 to 0.69), (0.51 to 0.71) and (0.6 to 0.74) for the water

flow rates of (0.03, 0.05 and 0.075 kg/sec) respectively. Thus, it can be concluded that the higher water temperature, the less

effect of water flow rate on the thermal efficiency. This behavior is consistent with the experimental results reported by [8, 7,

7

and 4].

Fig.5. thermal efficiency of the cooling tower

Fig.6 illustrates the variation of the cooling range with the inlet water temperature for different water flow rates. It can be

noted that the cooling range increases as the inlet water temperature increases. The cooling range depends on the heat load and

the water circulation rate. If the water pump speed is constant as well as the heat load, the cooling range will not change.

Therefore, the cooling range of the cooling tower varies within 1 °C difference for different water flow rates when the heat

load is constant. The cooling range increase from (2.6 to 13.8) °C, (3.1 to 14.3) °C and (3.6 to 14.8) °C for the water flow rates

of (0.03, 0.05 and 0.075 kg/sec) respectively.

Fig.6. cooling range of the cooling tower

The exergy refers to what degree the energy is convertible to another form of energy. The exergy of moist air that flows

by convection and evaporation, (Exa = Exa,c + Exa,e) is plotted in Fig.7. The exergy difference of air increases as the water

temperature increases due to the convection heat transfer and the mass transfer. At low water temperatures the exergy

difference of air is small, this indicates that neither the air gets heated nor the water gets vaporized. The fact that the small

difference between the temperatures of entering air and water explains that behavior. The exergy difference of air depends on

the amount of water being vaporized but not on the amount of water being circulated. This is why the water flow rate has a

small effect on the exergy difference of air. The later increases from (0.0005 to 0.288) kW (0.0024 to 0.289) kW and (0.0085

to 0.333) kW for the water flow rates of (0.03, 0.05 and 0.075 kg/sec) respectively.

Fig.7. Exergy difference of the air through cooling tower

Fig.8 reveals that the exergy difference of water increases as the inlet water temperature increases. The higher the inlet

water temperature, the more deviated from the dead state, the high level of exergy. The exergy difference of water increases

with the increase of the water flow rate. The increase in the water flow rate leads to increase the heat transfer from water to

air. On the other hand, the exergy out of water decreases when the heat transfer to air increases due to the increase of the water

flow rate. The exergy difference of water increased from (0.37 to 1.96) kW, (0.67 to 3.08) kW and (1.03 to 4.26) kW for water

flow rate (0.03, 0.05 and 0.075 kg/sec) respectively.

Fig. (8) Exergy difference of water through cooling tower

The exergy difference of vapor is shown in Fig.9. It can be observed that the exergy of vapor was increasing as the water

temperature increase, the reason for this because the large amount of water that will be evaporated at temperature increase.

And can be notified that the exergy of vapor remains approximately constant at water flow rate change. The exergy difference

9

of vapor increased from (0.0013 to 0.0262) kW, (0.0009 to 0.025) kW and (0.001 to 0.025) kW for water flow rate (0.03, 0.05

and 0.075 kg/sec) respectively.

Fig.9. Exergy difference of water through cooling tower

The irreversibility of the cooling tower is shown in Fig.10. The total exergy destruction increases with increasing in the

inlet water temperature and with increasing in the water flow rate. The higher the inlet water temperature, the higher potential

of water, the more exergy destroyed. The total exergy destruction increased from (0.37 to 1.69) kW, (0.66 to 2.82) kW and

(1.03 to 3.95) kW for the water flow rates of (0.03, 0.05 and 0.075 kg/sec) respectively.

Fig.10. exergy destruction of the cooling tower

Fig.11 illustrates the exergy efficiency of the cooling tower with the inlet water temperature for different water flow rates.

It can be observed that the exergy efficiency decreased by increasing the inlet water temperature and water flow rate. The

reason is referred to that the more exergy destruction, the less exergy efficiency. The exergy efficiency decreased from (0.578

to 0.43), (0.501 to 0.2827) and (0.409 to 0.119) for the water flow rates of (0.03, 0.05 and 0.075 kg/sec) respectively. The

behavior of the graph trends is corresponding with [4, 5, and 8].

Fig.11. Exergy efficiency of the cooling tower

6. Conclusion

The present works represents an experimental investigation of the performance of a counterflow wet cooling tower in

term of energy and exergy. The influence inlet water temperature is investigated in the range from 28 °C to 42 °C for the

water flow rates of (0.03, 0.05 and 0.075 kg/sec). The main finding of this investigation can be summarized as follows

(1) The cooling capacity increases with the increasing of the inlet water temperature and the water flow rate. The

minimum cooling capacity of 0.4 kW is at the minimum inlet water temperature and flow rate, while the maximum

cooling capacity of 4.6 kW is at the maximum ones.

(2) The thermal efficiency of the cooling tower increases with the increasing of the inlet water temperature and the water

flow rate. The minimum thermal efficiency is 43 % while the maximum is 74 %.

(3) At the minimum inlet water temperature and flow rate, the exergy differences of air and water are 0.0005 kW and

0.37 kW respectively. While At the maximum inlet water temperature and flow rate, the exergy differences of air and

water are 0.333 kW and 4.26 kW respectively.

(4) The minimum exergy destruction is 0.37 kW while the maximum one is 3.95 kW for the minimum and maximum

water temperatures and flow rates respectively.

(5) The exergy efficiency decreases with the increasing in the inlet water temperature and the water flow rate. The cooling

tower’s second law efficiency ranges from 11.9 % at the maximum water temperature and flow rate to 57.8 % at the

minimum water temperature and flow rate.

Ethics approval and consent to participate

The authors declare that they have no competing interests.

Consent for publication

The authors declare that they accepted to publish the manuscript.

11

Availability of data and materials

The datasets used and/or analyses during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests

Funding

Not applicable

Authors' contributions

F K contributed to the overall concept and structuring of the paper, writing the introduction and conclusion and assisting with the overall writing process throughout. Also design the system and applied exergy analysis to the system. D K wrote the Introduction section and literature survey. M A descripted the system and applied calibration to the measurement device A wrote the results and discussion section. All authors read and approved the final manuscript.

Acknowledgements

Not applicable

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