FROSTING IN MEMBRANE ENERGY EXCHANGERS

Abstract

Frost formation in heat/energy exchangers is undesirable because it may reduce air flow through the exchanger, increase the power consumption of fans, decrease the effectiveness of the exchanger, and in extreme cases, cause physical damage to the exchanger. Frosting is more critical in regions with arctic weather conditions, such as Canada and Northern Europe. Membrane-based energy exchangers are believed to be an important step towards frost free exchangers; however, at the beginning of this PhD study, there was no data available in the open literature documenting frosting in membrane energy exchangers. Therefore, the main goal of this PhD work is to determine if membrane energy exchangers are less susceptible to frosting than conventional heat exchangers. In this thesis, an in-depth study between conventional cross-flow air-to-air heat exchangers and membrane energy exchangers is conducted to (1) quantify the frosting limit; the operating conditions at which frost first begins in an exchanger, (2) develop a theoretical model to predict the frosting limit, and (3) quantify the energy impact of frosting and defrosting cycles on energy recovery. To meet these objectives, a test facility to test exchangers under frosting and defrosting cycles is developed and different strategies to detect frosting inside the exchangers are investigated. For the first time in the literature, it is shown that the temperature profile at the exhaust outlet can be used as a reliable and quick method to detect frosting. The frosting limit temperature of the energy exchanger is found to be 5℃ to10℃ lower than the frosting limit of the heat exchanger under the same air flow rate and exhaust air relative humidity. Testing the exchangers under both frosting and defrosting conditions shows that the frost accumulation rate is nearly linear with time, while the frost removal rate decreases exponentially with time. Moreover, the frosting rate in the heat exchanger is found to be three times higher than that of the energy exchanger. A theoretical model to predict the frosting limit using the design parameters of the exchangers and the operating conditions is developed. The model is verified with experiments. Both the experimental and theoretical results show that the indoor air moisture content and the outdoor air temperature have significant effects on the frosting limit. Finally, a method to calculate the energy impact of frosting is introduced. Comparison between different frost control strategies in exchangers shows that frost prevention is preferred over repeated cycle of frosting followed by defrosting.