A theoretical model to predict frosting limits in cross-flow air-to-air flat plate heat/energy exchangers

Abstract

In cold climates, the water vapor in the warm and moist exhaust air may condense and form frost on the heat/energy exchangers’ plate. In this study, a simplified theoretical model to predict the inlet conditions under which frost will form in the flat plate heat/energy exchangers is developed. The model uses the exchanger design parameters and operating conditions to determine the frosting limit. Experimental tests are conducted to validate the frosting limits model. The results show that the predicted frosting limits have consistent agreements with experiments, and energy exchangers have a lower risk of frosting than the heat exchangers. Furthermore, a parametric analysis using the theoretical model is performed to estimate the impacts of number of heat and moisture transfer units (NTUh and NTUm), aspect ratio of aluminum spacer, air flow rate, and membrane permeability on frosting limits of heat/energy exchangers. It is found that air flow rate has significant impacts on frosting limits. The combinations of sensible and latent effectiveness ensuring no frost inside energy exchanger are studied theoretically. The model can be applied to improve the design of exchangers to reduce or avoid frosting for cold climates. The frosting limits of cross-flow membrane energy exchanger can also be regarded as criteria to conduct selection and feasibility analysis of energy exchanger and compare with counter-flow exchanger as a reference in future study.