Theoretical Analysis On Heat And Mass Transfer In A Direct Evaporative Cooler

Applied Thermal Engineering 29 (2009) 980–984

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Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng

Theoretical analysis on heat and mass transfer in a direct evaporative cooler
J.M. Wu a,b,*, X. Huang b, H. Zhang a
a
b

College of Power Engineering, University of Shanghai for Science and Technology, Shanghai200093, China
School of Environment and Chemical Engineering, Xi’an Polytechnic University, Xi’an 710048, China

article

info

Article history:
Received 19 October 2005
Accepted 13 May 2008
Available online 27 May 2008
Keywords:
Direct evaporative cooler
Heat and mass transfer
Theoretical analysis

abstract
The heat and mass transfer between air and water film in the directevaporative cooler is theoretically
analyzed in present paper. A simplified cooling efficiency correlation is proposed based on the energy balance analysis of air. The correlation may be applied to the water-drip cross-flow direct evaporative cooler,
in which the wet special durable papers with different wave angles form the air channel. The Influences
of the air frontal velocity and the thickness of padmodule on the cooling efficiency of a direct evaporative
cooler are discussed. An optimum frontal velocity of 2.5 m/s is recommended to decide the frontal area of
pad module in the given air flow. The simplified correlation of cooling efficiency is validated by the test
results of a direct evaporative cooler.
Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction
Evaporative cooling may be analternative to mechanical refrigeration cooling for air conditioning depending on climatic conditions and building load characteristics. This concept is enhanced
again at the present status when energy-saving and environment-preserving are two subjects in all engineering fields due to
its characteristics of zero pollution, energy efficiency, simplicity
and good indoor air quality. The directevaporative coolers have
been widely used in many arid areas of the world such as southwestern United Stated, Australia, western Asia and northwestern
China.
The early studies mostly focused on the testing of the overall
performance of the evaporative cooling systems, and the results
in special region provided guidance for local engineering applications. The engineering design also depended onthe experiences
of designer who did his best to improve the new design by avoiding the existing problem of the old system. Recently theoretical
and numerical studies on the heat and mass transfer process of
evaporative cooling systems were reported by Zalewski and Gryglaszewski [1], Yan [2], San José Alonso and Rey Martínez [3], Halasz [4,5], Stabat et al. [6], Dai and Sumathy [7], Fisenko et al.[8],
Hawlader and Liu [9], and Tan et al. [10]. Hales [4,5] provided a
general dimensionless mathematical model for the description of
all types of evaporative cooling devices in use today (water cooling
towers, evaporative condensers, evaporative fluid cooler, air
washer, etc.). The differential equations describing non-adiabatic
evaporative processes are transformed to purely dimensionless* Corresponding author. Tel./fax: +86 29 82663401.
E-mail address: wjmxjtuhd@yahoo.com.cn (J.M. Wu).
1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.applthermaleng.2008.05.016

forms by the introduction of dimensionless coordinates and
parameters and by the substitution of the real air saturation curve
by a linear air saturation curve line. In sucha way, the overall performance of any such device can be expressed in terms of only few
parameters and presented with one or only a few diagrams. For
every type of device, a unique rating procedure is established.
However, more experimental data will be needed to determine
the accuracy of the application of the general model to different
evaporative cooling devices. Stabat et al. [6]...