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Thermal design of two-stage evaporative cooler based on thermal comfort criterion

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Abstract

Performance of two-stage evaporative coolers at various outdoor air conditions was numerically studied, and its geometric and physical characteristics were obtained based on thermal comfort criteria. For this purpose, a mathematical model was developed based on conservation equations of mass, momentum and energy to determine heat and mass transfer characteristics of the system. The results showed that two-stage indirect/direct cooler can provide the thermal comfort condition when outdoor air temperature and relative humidity are located in the range of 34–54 °C and 10–60 %, respectively. Moreover, as relative humidity of the ambient air rises, two-stage evaporative cooler with the smaller direct and larger indirect cooler will be needed. In building with high cooling demand, thermal comfort may be achieved at a greater air change per hour number, and thus an expensive two-stage evaporative cooler with a higher electricity consumption would be required. Finally, a design guideline was proposed to determine the size of required plate heat exchangers at various operating conditions.

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Abbreviations

b:

Width of wall (m)

C p :

Specific heat at constant pressure (J/kg K)

C pv :

Specific heat of water vapour at constant pressure (J/kg K)

d :

Mean pitch of the air channel in a pad (m)

f :

Wettability factor

g :

Gravitational acceleration (m/s2)

H :

Specific enthalpy (kJ/kg)

H fg :

Latent heat of water (kJ/kg)

Le :

Lewis factor

h c :

Convective heat transfer coefficient (W/m2 K)

h m :

Mass transfer coefficient (W/m2 s)

k :

Thermal conductivity (W/m K)

k a :

Volumetric mass transfer coefficient based on unit humidity ratio difference (kg/m3)

L:

Height of channel (m)

Le:

Lewis number

m :

Mass flow rate (kg/s)

m v :

Mass source term (kg/m3 s)

p:

Pressure (Pa)

q v :

Heat source term (W/m3)

T :

Air temperature (K)

U :

Overall heat transfer coefficient (W/m2 K)

u :

Air flow velocity in x direction (m/s)

V :

Volume (m3)

v :

Air flow velocity in y direction (m/s)

w :

Air flow velocity in z direction (m/s)

x, y, z :

Coordinate system (m)

Γeff :

Effective thermal diffusion coefficient of air (kg/m s)

ηeff :

Effective dynamic viscosity of air (kg/m s)

δ:

Thickness of water film (m)

ε:

Cooling effectiveness

μ:

Dynamic viscosity (kg/s m)

ν:

Kinematic viscosity (m2/s)

ρ:

Density (kg/m3)

ω:

Humidity ratio (kg water vapor/kg dry air)

Re:

Reynolds number (ufdhyd/νf)

a :

Ambient air

db :

Dry bulb

f :

Air stream

in :

Inlet

IEC :

Indirect evaporative cooler

out :

Outlet

p :

Primary

pw :

Water vapor

r :

Room

s :

Secondary

w :

Water stream

wb :

Wet bulb

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Acknowledgments

The authors would like to acknowledge College of Engineering, University of Tehran and Iran National science Foundation (INSF) for the esteemed financial support.

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Authors and Affiliations

  1. Fouman Faculty of Engineering, College of Engineering, University of Tehran, P.O. Box 43515-1155, Fouman, 43516-66456, Iran

    Neda Gilani

  2. Department of Mechanical Engineering, University of Guilan, P.O. Box 3756, Rasht, Iran

    Amin Haghighi Poshtiri

Authors
  1. Neda Gilani
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  2. Amin Haghighi Poshtiri
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Correspondence to Neda Gilani.

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Gilani, N., Poshtiri, A.H. Thermal design of two-stage evaporative cooler based on thermal comfort criterion. Heat Mass Transfer 53, 1355–1374 (2017). https://doi.org/10.1007/s00231-016-1889-z

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  • DOI: https://doi.org/10.1007/s00231-016-1889-z

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