Simplified thermal modeling of indirect evaporative heat exchangers

Abstract

A simplified model using a modified effectiveness–Number of Transfer Units (NTU) method for thermal performance simulation of indirect evaporative heat exchangers is presented and then validated utilizing experimental data from the literature. The objective of this model is to facilitate efficient computer simulation of indirect evaporative cooling or hybrid indirect evaporative cooling/direct expansion vapor compression systems, both of which include an indirect evaporative heat exchanger as a core component. Through some approximations, the governing differential equations that describe indirect evaporative heat exchanger heat/mass transfer behavior are modified to produce a methodology that is analogous to the effectiveness–NTU method for sensible-only heat exchangers. The simplified set of equations can then be solved within short computation times and with numerical stability. An examination of the Lewis factor impact on evaporation processes using this model indicates that the approximation of unity for the Lewis factor may not be valid when the inlet air to the wet side is cold and dry.

Acknowledgments

The authors would like to acknowledge support from the California Energy Commission that funded this work (grant 500-08-042). They also thank Eric Lee (Halperis Energy) for providing test data on the turbulent IEHX for the model verification.

Zhijun Liu, Student Member ASHRAE, is PhD student. William Allen, PhD, is Senior Engineer. Mark Modera, PhD, PE, Fellow ASHRAE, is Professor.