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Experimental and numerical analysis of across-flow closed wet cooling tower

Article in Applied Thermal Engineering November 2013

Impact Factor: 2.74 DOI: 10.1016/j.applthermaleng.2013.08.043

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Xiaohua Liu

Tsinghua University

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Yi Jiang

Tsinghua University

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Retrieved on: 21 June 2016
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Experimental and numerical analysis of a cross-ow closedwet cooling tower

Jing-Jing Jiang, Xiao-Hua Liu*, Yi Jiang

Department of Building Science, Tsinghua University, Beijing 100084, PR China

h i g h l i g h t s

A cross-ow closed wet cooling tower (CWCT) is experimentally analyzed.

Empirical correlations of the heat and mass transfer coefcients are obtained.

Numerical model of the CWCT is established and validated by experimental data.

Heat and mass transfer driving forces inside a cross-ow CWCT are more uniform.

Performance of a cross-ow CWCT is better than parallel/counter-ow patterns.

a r t i c l e i n f o

Article history:

Received 3 January 2013

Accepted 31 August 2013

Available online 10 September 2013

Keywords:

Closed wet cooling tower

ExperimentNumerical model

Flow pattern

Cross ow

a b s t r a c t

Closed wet cooling tower (CWCT) is an indirect-contact evaporative cooler, in which ambient air, spray

water and process water function together. In this study, a cross-ow CWCT unit based on the plateen

heat exchanger was designed and tested under various conditions in an environmental chamber. The test

results suggest that the heat and mass transfer coefcients and the cooling efciency are remarkably

affected by the temperature of the process water and the ow rates of the air, the spray water and the

process water. Heat and mass transfer coefcients were correlated based on the sensitive parameters.

Two-dimensional steady-state numerical model of the cross-ow CWCT was established and validated

by the experimental data. The numerical analyses revealed that the cross-ow CWCT could breakthrough

the structure limitation of the commonly parallel/counter-ow conguration and obtain more uniform

driving forces, which is benecial for the cooling performance. The ow pattern optimization of the

CWCT shows that air and process water in the opposite direction, spray water and the other uids in the

cross direction is the best ow pattern, which is distinct from the general knowledge of the researches.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Closed wet cooling tower (CWCT) has been adopted in a wide

range of application elds [1], such as refrigeration, air-

conditioning, manufacturing, power generation, etc. CWCT is an

indirect-contact evaporative cooler mostly based on tubular heatexchanger structure. Three uids function together in the CWCT,

which are ambient air and spray water owing outside the tubes

and process water running inside the serpentine tubes. The prin-

ciple of CWCT can be split into evaporative heat and mass transfer

process between the ambient air and the spray water, and heat

transfer process between the spray water and the process water. As

the uid inside the tubes never contact the ambient air, the CWCT

can be used to cool uids other than water and prevent contami-

nation of the airborne dirt and impurities. Furthermore, CWCT

could operate as an air cooling tower by stopping spray water in

severe cold days which makes it possible to run continuously year-

round in hospitals, schools, data centers, etc. However, the cost of

CWCT is often higher since tubular heat exchanger needs quantityof metallic materials[2].

Series of experiments have been conducted for the fundamental

researches of the heat and mass transfer processes in CWCTs. Niitsu

et al. [3] tested the performance of the plain and nned tubes,

including the lm heat transfer coefcient and airewater mass

transfer coefcient. Experimental tests by Heyns and Krger [4]

showed the water-lm heat transfer coefcient was a function of

spray water temperature, spray water and air ow rates, while the

airewater mass transfer coefcient was a function of air and spray

water ow rates. Sarker et al. [5] assessed CWCTs with staggered

arranged bare-type or nned tubes, from the perspectives of* Corresponding author. Tel.: 86 10 6277 3772; fax: 86 10 6277 0544.

E-mail address:[email protected](X.-H. Liu).

Contents lists available at ScienceDirect

Applied Thermal Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / a p t h e r m e n g

1359-4311/$e see front matter 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043

Applied Thermal Engineering 61 (2013) 678e689
http://-/?-https://www.researchgate.net/publication/223872513_Experimental_analysis_of_heat_and_mass_transfer_phenomena_in_a_direct_contact_evaporative_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/256718226_Numerical_simulation_of_a_closed_wet_cooling_tower_with_novel_design?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-https://www.researchgate.net/publication/245213693_Experimental_investigation_into_the_thermal-flow_performance_characteristics_of_an_evaporative_cooler?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==mailto:[email protected]://www.sciencedirect.com/science/journal/13594311http://www.elsevier.com/locate/apthermenghttp://dx.doi.org/10.1016/j.applthermaleng.2013.08.043http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043https://www.researchgate.net/publication/223872513_Experimental_analysis_of_heat_and_mass_transfer_phenomena_in_a_direct_contact_evaporative_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213693_Experimental_investigation_into_the_thermal-flow_performance_characteristics_of_an_evaporative_cooler?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/256718226_Numerical_simulation_of_a_closed_wet_cooling_tower_with_novel_design?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043http://dx.doi.org/10.1016/j.applthermaleng.2013.08.043http://www.elsevier.com/locate/apthermenghttp://www.sciencedirect.com/science/journal/13594311http://crossmark.crossref.org/dialog/?doi=10.1016/j.applthermaleng.2013.08.043&domain=pdfmailto:[email protected]://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


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cooling capacity, wet-bulb efciency and pressure drop. Experi-

mental tests showed that the n-tube CWCT had better thermal

performance although the pressure drop was higher than that of

the bare-tube one. Zheng et al. [6] investigated the thermal

behavior of an oval tube CWCT under different operating condi-

tions. The results showed that the oval tube had a better combinedthermal-hydraulic performance. Some novel CWCTs consisting of

indirect evaporative cooling stage and direct evaporative cooling

stage (or heat transfer stage) were proposed, constructed and

tested by Xia et al.[2]and Heidarinejad et al.[7].

Besides the experimental researches, a number of theoretical

and computational analyses have been conducted aiming to a morerealistic description of the transport phenomena taking place in-

side a CWCT. Hasan and Sirn[8]presented a computational model

to simulate the performance of the CWCT. The variation of the spray

water temperature was taken into consideration and the saturation

enthalpy was calculated from psychometric relations for moist air.

The coefcients of mass transfer were derived from experimental

data and then implemented in the computational model. Koschenz

[9]presented an analytical model for a CWCT for use with chilled

ceilings, assuming that the spray water temperature kept constant

along the wayand the constant temperature was equal to the outlet

process water temperature. However, the accuracy levels of these

assumptions were not quantied with respect to other approaches

or relevant experimental works. Hasan and Gan[10]compared the

cooling performances calculated by the computational modelestablished by Hasan and the analytical models utilizing the as-

sumptions raised by Koschenz. Gan and Riffat[11]conducted a CFD

Fig. 1. The cross-ow CWCT unit: (a) the schematic diagram of the three uids; and (b) the photo from the front view.

Fig. 2. The louver structure of the n.

Fig. 3. The schematic diagram of the testing con

guration.

J.-J. Jiang et al. / Applied Thermal Engineering 61 (2013) 678e689 679
https://www.researchgate.net/publication/251667832_Experimental_and_computational_analysis_of_thermal_performance_of_the_oval_tube_closed_wet_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/256718226_Numerical_simulation_of_a_closed_wet_cooling_tower_with_novel_design?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/223479744_Experimental_investigation_of_two-stage_indirectdirect_evaporative_cooling_system_in_various_climatic_conditions?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222577271_Theoretical_and_computational_analysis_of_closed_wet_cooling_tower_and_its_applications_in_cooling_of_buildings?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-https://www.researchgate.net/publication/229644662_Simplification_of_analytical_models_and_incorporation_with_CFD_for_the_performance_predication_of_closed-wet_cooling_towers?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222026576_Numerical_simulation_of_closed_wet_cooling_tower_for_chilled_ceiling_systems?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/229644662_Simplification_of_analytical_models_and_incorporation_with_CFD_for_the_performance_predication_of_closed-wet_cooling_towers?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222026576_Numerical_simulation_of_closed_wet_cooling_tower_for_chilled_ceiling_systems?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/251667832_Experimental_and_computational_analysis_of_thermal_performance_of_the_oval_tube_closed_wet_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/223479744_Experimental_investigation_of_two-stage_indirectdirect_evaporative_cooling_system_in_various_climatic_conditions?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/256718226_Numerical_simulation_of_a_closed_wet_cooling_tower_with_novel_design?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222577271_Theoretical_and_computational_analysis_of_closed_wet_cooling_tower_and_its_applications_in_cooling_of_buildings?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


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method to predict the performance of CWCT according to the

cooling capacity and the pressure drop.

Plenty of works have been done by the researchers concerning

experimental tests and theoretical analyses, which give a good

description of the performance of the CWCT. Moreover, many novel

designs of structures or components have been put forward and

have already reformed the CWCT to a great extent. However, the

ow pattern analysis of the CWCT has barely been involved in

available literature. The majority of the ow pattern of the CWCT

[3e6,8e16] is air owing from the bottom to the top, while the

process water inside the serpentine tubes and the spray water

outside the tubes owing in the opposite direction, which has been

proved the best one-dimensional ow pattern of CWCT by Ren and

Yang[17]. Little work[18,19]has been carried out on the cross-ow

CWCT. The performance of a cross-ow CWCT, with a counter ow

between the airand the process waterand two crossows between

the air and the other two uids, will be analyzed in present study.

Experimental tests will be carried out to investigate the behavior

and inuencing factors of the CWCT. Numerical models of parallel/

counter-ow and cross-ow CWCTs will be established and vali-

dated to optimize the ow pattern of the CWCT.

2. Experimental test of the CWCT unit

2.1. Description of the cross-ow CWCT unit

Fig. 1(a) indicates the ow directions of the three uids in the

cross-ow CWCT unit, which are spray water owing from top to

bottom, ambient airowing from front to back through thens and

process water owing in the serpentine tubes from back to the

front. In other words, the ambient air and process water are in

counter ow and spray water is in cross ow with the other two

uids. As shown in Fig. 1(b), the CWCT unit employs n-tube

structure to expand the heat and mass transfer area. The tubes

and ns of the unit are made of stainless steel to ensureperfect heat

transfer between the spray water and the process water. Further-

more, there are discontinuous louvers on the ns, presented in

Fig. 2, to strengthen the heat and mass transfer performance and

the wettability of the ns by disturbing the boundary layer of the

spray water.

The size of the unit is 570 mm (H, the spray water ow

direction) 310 mm (W, width) 180 mm (L, the air ow direc-

tion). Its n thickness is 0.127 mm, and the distance between the

ns is 2.2 mm. The inner and external diameters of the tubes are

9.42 mm and 10.02 mm, respectively. The tube bundle consists of 8

rows of steel tubes. The tubes are 0.31 m long and are arranged in a

triangular pattern at a transversal pitch of 25.4 mm. There are 20

tubes per tube row. The external surface of the whole unit is

24.336 m2 (Fm), 0.784 m2 of which is the external surface of the

tubes and 23.552 m2 of which is the surface of the ns. The specic

surface area of the CWCT unit is 790 m2/m3.

2.2. The CWCT testing conguration

The tests for assessing the performance of the CWCT unit were

performed in an environmental chamber. The system conguration

can realize wide range of air, spray water and process water states,

as displayed inFig. 3. The cooling coil, heater A and humidier can

regulate the air inlet temperature and humidity independently. The

variable frequency fan can control the volume ow rate of the inlet

air. Also, the temperature and ow rate of the process water can be

controlled by Heater B and the water valves.

The environmental chamber provided the measuring in-

struments for the ow rates and inlet/outlet temperatures of the

air, the spray water and the process water. As listed in Table 1, theow rate of the air a was measured by standard nozzles

(GB14294) with the accuracy of 1%. Theow rate of the spray water

swas measured by a rotameter with the range from 60 to 600 L/h

and the accuracy of 1.5%. The ow rate of the process water wwas

measured by water meter with the accuracy of 3 L/h. The temper-

atures of the three uids were measured by T-type thermocouples

with the accuracy of 0.2 C.

2.3. Verication of the experimental data

In order to study the CWCT unit, a series of experiments were

conducted which intended for nding out the effects of the inlet

parameters on cooling performance. The main parameters are the

owrate of the three uids and the inlet temperature of the process

water etc. Variable condition analyses were conducted with 11

operating conditions and 46 sets of data. Each set of data was

recorded under approximately steady states, which required all the

temperature points uctuated within 0.2 C for longer than

20 min. The typical experiment data is listedin Table 2. To verify the

reliability of the experiment data, energy balance of the air, spray

Table 1

Accuracies of the measuring instruments.

Parameter Sensor Accuracy

Air dry/wet bulb temperatures T-type thermocouple 0.2 [C]

Air ow rate Standard nozzle (GB14294) 1 [%]

Spray water/process water

temperatures

T-type thermocouple 0.2 [C]

Spray water ow rate Rotameter 1.5 [%]

Process water ow rate Water meter 3 [L/h]

Table 2

The experimental data of the CWCT test.

No. Inlet parameters Outlet parameters QckW

ta,in C twb,in

C ts,in C tw,in

C akg/s skg/s wkg/s ta,out C twb,out

C ts,out C tw,out

C

1 24.8 20.1 24.7 30.3 0.35 0.13 0.32 24.2 23.9 24.4 26.7 0.36 4.94

2 25.3 20.9 25.8 32.9 0.35 0.12 0.32 25.3 25.0 25.8 28.8 0.35 5.65

3 27.0 22.0 27.9 36.6 0.35 0.12 0.32 27.3 27.0 27.8 31.2 0.37 7.26

4 27.2 21.1 26.2 30.2 0.19 0.12 0.32 26.0 25.7 26.0 27.7 0.28 3.31

5 26.7 20.6 25.6 30.8 0.27 0.12 0.32 25.2 24.8 25.3 27.6 0.32 4.31

6 27.3 22.9 26.1 30.2 0.35 0.12 0.31 25.6 25.2 25.8 27.5 0.36 3.49

7 25.1 20.8 24.8 30.4 0.35 0.12 0.26 24.4 24.0 24.5 26.6 0.40 4.10

8 25.1 21.1 24.6 30.2 0.35 0.12 0.20 24.2 23.8 24.3 26.1 0.45 3.39

9 26.8 22.6 26.2 30.2 0.35 0.06 0.29 25.1 24.6 25.6 27.6 0.34 3.22

10 26.9 22.8 25.9 30.2 0.35 0.09 0.30 25.3 25.0 25.5 27.4 0.38 3.58

11 27.0 22.6 25.8 29.9 0.35 0.13 0.30 25.3 25.0 25.4 27.0 0.40 3.77

J.-J. Jiang et al. / Applied Thermal Engineering 61 (2013) 678e689680
http://-/?-http://-/?-http://-/?-https://www.researchgate.net/publication/229377791_An_analytical_model_for_the_heat_and_mass_transfer_processes_in_indirect_evaporative_cooling_with_parallelcounter_flow_configurations?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/292730103_Experimental_research_on_cross-closed_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/232808578_Numerical_study_on_indirect_evaporative_cooling_performance_comparison_between_counterflow_and_crossflow_heat_exchangers?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-https://www.researchgate.net/publication/292730103_Experimental_research_on_cross-closed_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/229377791_An_analytical_model_for_the_heat_and_mass_transfer_processes_in_indirect_evaporative_cooling_with_parallelcounter_flow_configurations?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/232808578_Numerical_study_on_indirect_evaporative_cooling_performance_comparison_between_counterflow_and_crossflow_heat_exchangers?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


5/13

water and process water was adopted. As shown inFig. 4, the un-

balance ratios of the heat gained by the ambient air and the heat

lost by the process water and the spray water are within20%. The

average absolute unbalance ratio is 7.4%, which means the data are

reliable.

To better describe the cooling processes, some indexes are

introduced, seen in Eqs.(1) and (2). The wet-bulb cooling efciency

[5,13,16]illustrates the distance between the outlet process water

temperature (tw,out) and the ambient wet bulb temperature (twb,in),

which is the limit of the outlet process water temperature. The

cooling capacity Qc [5,12,15]presents the cooling capacity of the

CWCT unit.

tw;intw;out

tw;inta;wb(1)

Qc cp;w _

mw

tw;intw;out

(2)

3. Experimental results and inuencing factors

3.1. Effect of the airow rate

In the environmental chamber shown inFig. 3, the ow rate of

the ambient air is easily conditioned and controlled by regulating

the rotate speed of the fan. The owrateof the air was at the lowest

rate of 0.19 kg/s and up to the maximum of 0.35 kg/sFig. 5displays

that mass transfer coefcient between the spray water and the air

(Km) increased greatly by increasing the air ow rate. While the

heat transfer coefcient between the spray water and the process

water (Kh) was generally constant since the increase of the air ow

rate had little relationship with the heat transfer between the spray

water and the process water. As a result of the strengthening of the

heat and mass transfer performance between the spray water and

the air, andQcincreased with the increase ofa. Thus, increasing

air ow rate is a good way to improve the performance of the

CWCT. However, the air ow rate is not the bigger the better if

taking the fan power consumption into consideration. There is an

optimal value of air ow rate depending on the balance of the

CWCT performance and the fan power consumption.

Uncertainty analyses for the experimental results, based on the

accuracies of the measuring instruments introduced in Section2.2,were conducted in this study using the method proposed by Kline

and McClintock [20] according to the following expression (Eq. (3)):

Dy

vf

vx1

2Dx1

2

vf

vx2

2Dx2

2/

vf

vxn

2Dxn

21=2

(3)

The uncertainties of , Qc, Km, and Kh were calculated and

expressed inFig. 5in the way of error bars. Results show that the T-

type thermocouples are the main sources of errors. TakingKmas an

example, the uncertainties of the air wet-bulb temperature and

spray water temperature account for 81.3% and 16.6% of the total

uncertainty, respectively, while that of the air mass ow rate only

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8

Gainedh

eatoftheair(kW)

Heat loss of process/spray water (kW)

+ 20

- 2 0 %

Fig. 4. Energy balance of the CWCT test.

Fig. 5. Effects of the air

ow rate (

a) on the CWCT: (a) wet-bulb ef

ciency; (b) cooling capacity; (c) Km; and (d) Kh.

http://-/?-http://-/?-https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/227412720_Simplified_model_for_indirect-contact_evaporative_cooling-tower_behaviour_Appl_Energy?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222680559_Thermal_behavior_of_closed_wet_cooling_towers_for_use_with_chilled_ceilings?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/239340592_Thermal_performance_analysis_of_heat_exchanger_for_closed_wet_cooling_tower_using_heat_and_mass_transfer_analogy?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/233057718_Performance_Characteristics_of_a_Closed-Circuit_Cooling_Tower_With_Multiple_Paths?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-https://www.researchgate.net/publication/239340592_Thermal_performance_analysis_of_heat_exchanger_for_closed_wet_cooling_tower_using_heat_and_mass_transfer_analogy?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/222680559_Thermal_behavior_of_closed_wet_cooling_towers_for_use_with_chilled_ceilings?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/227412720_Simplified_model_for_indirect-contact_evaporative_cooling-tower_behaviour_Appl_Energy?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/233057718_Performance_Characteristics_of_a_Closed-Circuit_Cooling_Tower_With_Multiple_Paths?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==https://www.researchgate.net/publication/245213656_Enhancement_of_cooling_capacity_in_a_hybrid_closed_circuit_cooling_tower?el=1_x_8&enrichId=rgreq-a1cb0eea477efd5b1a1fdd0b881cbbe0-XXX&enrichSource=Y292ZXJQYWdlOzI3MDM1NjEyNztBUzoyNjgzNjA2ODAyMTA0MzJAMTQ0MDk5MzU4MDYxNg==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


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accounts for 0.8%. Thus improving the temperature measurement

accuracy is the key point of improving the accuracy of the test.

3.2. Effect of the spray waterow rate

The ow rate of the spray water was regulated by changing the

valves in the pipelines. The ow rate of the spray water was from

0.06 kg/s to 0.13 kg/s. When the ow rate of the spray water

increased, wetting degree of the CWCT was improved and heat and

mass transfer area was expanded to a certain extent. Also, the in-

crease of the spray water ow rate would strengthen the heat

transfer process between the spray water and the process water, so

as to take away more heat from the process water. As a result, ,Qc,

and Kh increased, as shown in Fig. 6. On the other hand, the

Fig. 6. Effects of the spray water ow rate (s) on the CWCT: (a) wet-bulb efciency; (b) cooling capacity; (c) Km; and (d) Kh.

Fig. 7. Effects of the process water

ow rate (

w) on the CWCT: (a) wet-bulb ef

ciency; (b) cooling capacity; (c) Km; and (d) Kh.

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7/13

Fig. 8. Effects of the process water temperature (tw,in) on the CWCT: (a) cooling capacity; (b) spray water inlet temperature; (c) Km; and (d) Kh.

(a) (b)

Fig. 9. Calculated results of Eqs.(4) and (5): (a) Kh; and (b) Km.

Table 3

Comparison of the experimental parameters in literature [4,6,15,16].

Source Flow pattern Ga(kg/m2s) Gs(kg/m

2s) Gw(kg/m2s) Kma(kg/m

3s) Kha(kW/m3 K) a(m2/m3) Correlations

Heyns[4] Parallel/counter 0.7e3.6 1.7e4.5 - 0.5e3.2 42.0e67.2 24 e Kh 470Ga0.1Gs

0.35ts0.3

Km 0.038Ga0.73Gs

0.2

Zheng[6] Parallel/counter 2.5e5.0 1.2e3.2 2.8e5.3 2.7e5.0 23.9e60.4 31 0.11e0.19 Kh 350.3(1 0.0169ts)Ga0.59Gs

1/3

Km 0.034Ga0.977

Shim[15] Parallel/counter 1.2e4.2 1.1e3.3 0.9e4.8 6.6e21.5 18.2 31.4 33 e e

Faco[16] Parallel/counter 0.7e2.4 0.3e1.9 0.6e1.1 1.6e4.3 5.5e17.5 25 0.2e0.65 Kh 700.3(s/1.39)0.6584

Km 0.1703(a/1.7)0.8099

Present study Cross 1.3e2.4 1.1e2.3 1.1e1.8 10.3e19.0 30.8e45.0 790 0.28e0.46 Kh 31.79Gs0.238Gw

0.547

Km 0.00154tw0.471Ga

0.694Gs0.512

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8/13

improvement of the spray water ow rate enhanced heat and mass

transfer between the ambient air and the spray water, which

resulted in the growth ofKm.

3.3. Effect of the process waterow rate

The ow rate of the process water is also a key parameter

inuencing the CWCT performance. It was from 0.20 kg/s to

0.32 kg/s in this set of experiments. Apparently, the rise of the

process water ow rate promoted the heat and mass transfer be-

tween the spray water and the process water, therefore Khincreased. Thus process water could release more heat to the spray

water, which led to the increase ofQc, as shown inFig. 7(b). Since

the inuence ofwto the outlet temperature of the process water

was more remarkable than that of Kh, the cooling effect of the

process water per unit mass was denitely worsened, though the

total cooling capacity was improved. In this way, the outlet tem-

perature of the process water increased and dropped. Since the

process water barely touched the ambient air, Kmbetween the air

and the spray water scarcely changed.

3.4. Effect of the process water temperature

This set of experiment was meant to study the performance of

the CWCT at different process water temperatures. The inlet tem-

perature of the process water was from 30.1 C to 36.6 C. The re-

sults showed that Qc and Km increased with the growth of the

process water temperature, seen in Fig. 8(a) and (c), due to the

increase of the heat and mass transfer driving forces between the

threeuids. Since the temperature of the spray water was decided

by the air and process water states, it rose with the growth of the

process water temperature, as shown inFig. 8(b).

3.5. Comparison with experimental results from previous studies

From the sensitivity analyses we could see that Kh is mainlyinuenced by the ow rates of the spray water and the process

water, whileKmis dominated by the ow rates of the air and spray

water and the inlet temperature of the process water. Therefore, for

the specic geometry of the tower, the correlation equations forKhandKmcould be presented as follows:

Kh 31:79G0:238s G

0:547w (4)

Km 0:00154t0:471w;in G

0:694a G

0:512s (5)

whereGa is the air velocity in the minimum ow area,Gs G/do,Gw w/(H$L) is the process water ow rate per ow area

(1.3< Ga< 2.4 kg/m2 s; 1.1< Gs< 2.3 kg/m

2 s; 1.1< Gw< 1.8 kg/

m2 s; 30.1< Tw,in


9/13

transfer coefcient Kma, due to that the n-tube structure

tremendously extends the contact area, which is exactly the re-

striction point of the heat and mass transfer between the air and

the spray water. In contract, the performance improvement of the

n-tube structure for waterewater heat transfer process is insig-

nicant. Thats why the volume heat transfer coefcientKha is no

bigger than the others.

4. Numerical model of the cross-ow CWCT

4.1. Theoretical model

Two-dimensional steady-state model of the cross-ow CWCT, in

which airand process waterowing in the opposite direction, spray

water owing in the cross direction with the other twouids, seen

inFig. 11, will be illustrated in this section. In the CWCT, process

water releases heat to the metal nned tubes while the tubes are

cooled by the spray water. At the same time, heat and mass transfer

takes place between the spray water and the air. The main as-

sumptions for the numerical model are presented as follows[8,13,17]: 1) Heat and mass transfer processes are at steady state; 2)

The heat and mass exchange between the CWCT unit and the sur-

roundings is negligible; 3) The specic heat of the uids are

assumed to be constant; 4) The spray water lm uniformly covers

all the wall of the tubes and ns, so that Fh equals to Fm and the heat

exchange between the air and process water is negligible; and 5)

The ow of the process water and the air is approximately counter

ow.

Dene the air ow direction aszaxis and the spray water ow

direction asxaxis. The energy balance equation for the three uids

is:

_maH

vhavz

1

L

v _mshs

vx cp;w

_mwH

vtwvz

0 (6)

Mass conversion equation of the spray water and the air is given

by:

_maH

vdavz

1

L

v _msvx

0 (7)

Heat transfer between the spray water and the process water

driven by the temperature difference between them is shown as:

vtwvz

KhFhcp;w _mwL

twts (8)

As well known, there is a thin lm of saturated air at the

interface between the spray water and the air. The temperature of

the saturated air is close to that of the spray water. The humidity

ratio of the saturated air is also called the equivalent humidity ratioof the spray water, which is de. Heat transfer driven by the tem-

perature difference of the saturated air lm and the air ow and

mass transfer driven by the water vapor partial pressure difference

between the two streams take place simultaneously. Thus the mass

transfer equation and the energy balance equation for the air ow

can be expressed by the following equations separately:

vdavz

KmFm

_maL deda (9)

vhavz

K0hFm

_maL tsta r

vdavz

(10)

The Lewis factor or Lewis relation Lefcould be de

ned to indi-cate the relation between the heat and mass transfer in an evapo-

rative process[21e23]. The denition ofLefis as follows:

Lef K0hKmcp;m

(11)

Substitute Eq.(11)into Eq.(10):

vhavz

KmFm

_maL

hLef$cp;atsta rdeda

i (12)

As the enthalpy of the air can be expressed asha cp;mtar$da; Eq.(12)can be transformed into:

vhavz

KmFm

_maL $

Lef heha r

1

Lef 1

deda

(13)

Thus we get all the governing equations of the cross-ow CWCT.

The boundary conditions are shown as follows:

ta ta;in; da da;in; ha ha;in; z 0 (14)

tsjx0 tsjxH (15)

tw tw;in; z L (16)

By discretizing the governing equations, the heat and mass

transfer process could be numerically solved. When solving the

model,Lefcould be equal to 1[6,8,14]. The model of the cross-ow

CWCT was validated by the experimental results described in

Fig. 12. Comparison of the calculated values and the experimental results of the cross-ow CWCT: (a) the variance of the air humidity ratio; and (b) the variance of the temperature

of the process water.

Table 4

Simulated condition of the cross-ow CWCT.

ta,in C da,inkg/kg tw,in

C akg/s skg/s wkg/s KmFmkg/s KhFhkW/K

27.2 0.0136 30.3 0.19 0.12 0.32 0.365 1.17

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10/13

Section 3. Asshownin Fig.12, the maximal differences between the

calculated results and the experimental values are within 8.0%, and

the average absolute differences are 3.0% and 4.0% for the variance

of the air humidity ratio and the process water temperature

respectively. On the whole, the calculated parameters by the nu-

merical model agree well with the experimental results, the model

could be used to analyze the heat and mass transfer performance of

the CWCT unit in the following content.

4.2. Typical simulation result of cross-ow CWCT

Since the distribution parameters of the cross-ow CWCT are

two-dimensional, it is difcult to describe it through experimental

results of limited measurement points. Therefore, numerical

modeling results were introduced to investigate the performance of

the CWCT. The boundary conditions from the experimental results

are displayed in Table4. Asseenin Fig.13, the wet-bulb temperature

of theair increasesin theairow direction, while the temperature of

the process water decreases in the opposite direction. Heat is

transferred fromthe processwater to the air. Without the circulation

of the spray water, the temperature distributions of the air and the

process water should be one-dimensional and the temperature

gradients of the two uids along zshould be consistent. Once the

spray water is brought in, the consistency will be disturbed.

To better explain the heat and mass transfer process of cross-

ow CWCT, we could divide it into enough control volumes along

Fig. 13. Simulated eld distribution of the cross-ow CWCT: (a) the air wet-bulb temperature; (b) the spray water temperature; (c) the process water temperature; (d) thetemperature difference between the spray water and the process water; and (e) the temperature difference between the spray water and the air (wet-bulb).

Fig. 14. Simulated temperatures of the x sections: (a) x

0.05H; (b)x

0.5H; and (c) x

0.95H.

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zaxis and assume that the heat and mass transfer between each

two control volumes could be ignored, which means the perfor-

mance of the spray water is only affected by the air and the process

water inside the volume. If the inlet temperature of the spray water

is lower than those of the process water and the air (wet-bulb),

spray water will absorb heat from both the uids when falling

along thexaxis. As a result, the spray water temperature will go up

until it gains the same heat from the process water as the heat

released to the air. Vice versa, when the inlet temperature of the

spray water is higher than those of the other uids, spray waterwill

discharge heat to the air and the process water until it transfers the

same quantity of heat from the process water to the air. In this way,

there is an equilibrium temperature of spray water in each control

volume, which is somewhere between the temperatures of the air

and the process water, determined by the heat and mass transfer

ability of the CWCT. As shown in Fig. 13(b), on the bottom of the

CWCT, the spray water temperature barely changes along the way,which means it already reaches the equilibrium temperature.

Fig.14shows the temperatures of the three uids at the sections

ofx 0.05H,x 0.5Handx 0.95H, which represent the states of

the spray water from the inlet to the outlet. It can be observed from

Fig. 14(c) that the equilibriumtemperaturerises from the airinlet to

the process water inlet. Since there is only one sink at the outlet,

spray water of different control volumes with different

temperatures must be mixed to the medium temperature before

going to the inlet. Because of the mixture, the heat and mass

transfer driving forces at the inlet are not uniform, shown in

Fig. 14(a). Fortunately, the spray water of the cross-ow CWCT has

the self-adjust ability to achieve proper equilibrium temperature.

As the simulated mass ow rate of spray water is relatively small in

this article, according to Table 4, its state is easy to be inuenced by

the other two uids and it reaches the equilibrium temperature

very quickly (at aboutx 0.2H). Whenx 0.5H, shown in Fig.14(b),

the three uids have already reached the equilibrium states and

had rather uniform heat and mass transfer driving forces. On the

whole, the heat and mass transfer driving forces are relatively

uniform in the cross-ow CWCT, especially in the lower part.

5. Effect ofow pattern on the performance of the CWCT

For two-ow heat and mass transfer system, scholars haveagreed that counter ow achieves the best performance, followed

by the cross ow and the parallel ow. By analogy, the recom-

mended ow pattern in the CWCT is two counter ows and one

parallel ow between the three uids, achieving as many counter

ows as possible and abandoning the mediocre crossow. Ren and

Yang [17] studied all the parallel/counter-ow patterns of the

CWCT, nding that theow pattern shown in Fig. 15(a) achieves the

best cooling performance. Following this conclusion, we simulated

the performance of the model to test the analogy. The boundary

conditions are listed inTable 4.

For the parallel/counter-ow CWCT, the distribution parameters

are one-dimensional. As shown in Fig. 15(b), the process water is

cooled along the way while the air is continuously heated. Since the

inlet and outlet temperatures of the spray water should be thesame, the one-dimensional spray water temperature could not

keep pace with the temperature gradient along x. As a result, the

heat transfer driving force between the process water and the spray

water is not uniform. Neither is the heat and mass transfer driving

force between the air and the spray water. This phenomenon is also

stated by many other researchers[4,10]. Unfortunately, the uneven

driving forces could not be avoided by improving the heat and mass

transfer area or regulating the ow ratios of the three uids. In

other words, the one-dimensional parallel/counter-ow CWCT has

the structural limitation. As mentioned in Section 4.2, the cross-ow CWCT could achieve uniform heat and mass transfer driving

forces in the most part of the module. In this way, the cross-ow

CWCT turns up the ideal ow pattern, although the two-ow

cross-

ow heat and mass transfer performance is not the best.

Fig. 15. (a) Parallel/counter-ow CWCT conguration; and (b) simulated temperatures.

Fig. 16. Flow pattern optimization of the CWCT.

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The cooling capacity comparison of the parallel/counter-ow

and cross-ow CWCTs under different KmFm are shown inFig. 16.

The ow rates of the air, the spray water and the process water are

all 0.5 kg/s, while the other boundary conditions are presented in

Fig.16. Asshownin Fig.16, the cross-ow CWCT wouldalways have

a larger cooling capacity Qc, and the superiority will be amplied

when the heat and mass transfer coefcient is increased, which

means the cross-ow CWCT has better cooling performance than

the commonly used parallel/counter-ow CWCT.

6. Conclusion

A cross-ow CWCT based on the n-tube structure was

designed and tested in present study. The ow arrangement of the

airand the process water was counterow, while that of the air and

the spray water was cross ow. Experimental tests were conducted

to investigate the cooling performance and the inuencing factors

on the basis of a good energy balance discrepancy. The main con-

clusions are:

1) Effect of the process water temperature and ow rates of the

air, spray water and process water on the cooling capacity, wet-

bulb efciency, heat and mass transfer coefcients were stud-ied. Empirical correlations of the heat and mass transfer co-

efcients based on the inuencing factors were obtained.

2) Compared to the bare-tube structure in literature, the n-tube

structure tremendously extends the contacting area between

the air and the spray water, thus improves the heat and mass

transfer coefcient. While for the heat transfer coefcient be-

tween the spray water and the process water, the n-tube

structure has little impact.

3) Two-dimensional steady-state numerical model of the cross-

ow CWCT was built and validated by the experimental data.

The deviation between the model and the experimental data

was less than 8%, which ensures the accuracy of the model.

4) The numerical results show that the spray water temperature

of the cross-ow CWCT would automatically form a gradient inthe air/process water ow direction to match the temperature

variances of the air and the process water. As a result, the heat

and mass transfer driving forces of the cross-ow CWCT are

fairly uniform, which is benecial for the behaviorof the CWCT.

5) The ow pattern optimization of the CWCT shows that the

cooling performance of the cross-ow CWCT is better than that

of the commonly studied parallel/counter-ow CWCT due to

more uniform driving forces. The superiority will be amplied

when heat and mass transfer coefcients are increased.

Acknowledgements

The research described in this paper was supported by National

Natural Science Foundation of China (No. 51138005) and thefoundation for the author of National Excellent Doctoral Disserta-

tion of China (No. 201049).

Nomenclature

a specic surface area (m2/m3)cp specic heat capacity (kJ/kg C)

d humidity ratio (g/kg)

do external diameter of the tube (m)

Fh heat transfer area (m2)

Fm mass transfer area (m2)

G mass ow rate per ow area (kg/m2s)

H height of the CWCT unit (m)

h enthalpy (kJ/kg)

Kh heat transfer coefcient between spray water and process

water (kW/m2 K)

K0h heat transfer coefcient between spray water and air

(kW/m2 K)

Km mass transfer coefcient between airand spray water (kg/

m2 s)

L thickness of the CWCT unit (m)

Lef Lewis factor (dimensionless)_m mass ow rate (kg/s)

Qc cooling capacity (kW)

r vaporization latent heat (kJ/kg)

t temperature (C)

W width of the CWCT unit (m)

Greek symbols

G spray waterow per unit breadth (kg/m s)

D change of or difference between parameters

wet-bulb cooling efciency

Subscripts

a air

e air in equilibrium with spray waterin inlet

m moist air

out outlet

s spray water

w process water

wb wet-bulb

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Experimental and Numerical Analysis of a Cross-flow Closed Cooling Tower --

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Transcript of Experimental and Numerical Analysis of a Cross-flow Closed Cooling Tower --