An analytical model for predicting frosting limit in membranes

Abstract

Membrane energy exchangers have been used in the HVAC industry recently in order to reduce energy consumption and reduce the risk of frost formation. In this paper, frosting in membranes is studied with an analytical model based on thermal and mass resistances to determine the temperature and relative humidity at the surface of a membrane at steady state. The model is verified with experimental and numerical data. The main goal of this paper is to investigate the effects of design and operating parameters on the onset of saturation in order to prevent or delay frosting in a membrane. The maximum cold temperature which leads to frost formation on a surface at a specific air relative humidity is defined as the frosting limit. The frosting limit for a membrane is presented for different temperature and humidity conditions. A new parameter, allowable dew point depression of the surface (ΔDP), is defined as the difference between the dew point temperature of the air stream and the temperature at the top surface of the membrane. A sensitivity study of ΔDP demonstrates that the water vapor permeability of the membrane, heat transfer coefficient, and thickness of the membrane impact the frosting limit. The main conclusion is that increasing the moisture transfer rate through the membrane decreases the frosting limit.