A frosting limit model of air-to-air quasi-counter-flow membrane energy exchanger for use in cold climates

Abstract

Membrane energy exchangers (MEEs) can reduce or avoid frosting in cold climates since the moisture transfer lowers the dew point of the exhaust air and as a result, the exchanger can be frost-free at lower outdoor air temperatures or higher indoor humidities. A frosting limit is necessary to predict at which conditions the onset of frost occurs for a given membrane energy exchanger. The frosting limit model provides criteria for energy exchanger selection and frost control methods to avoid frosting.A theoretical frosting limit model is developed in this study for a quasi-counter-flow MEE. The frosting limit model uses analytical relationship between the onset of frosting at the coldest location of the exchange and the inlet air conditions. The model is validated with experimental data and consistent agreements are obtained between the theoretical and experimental data. Parametric studies are conducted using the validated model. The influence of airflow rates, exhaust air temperature and channel spacing on the frosting limit is rather limited compared to the diffusive resistance of moisture transfer. A membrane with improved moisture transfer properties is crucial to implement frost-free operation at normal indoor relative humidities.