Abstract
The influence of frost formation on the heat exchange surfaces in a model heat exchanger is studied. The investigation is based on experiments with a humidified forced air stream between parallel cooling plates. A model based on simplified expressions is used to extract values of heat transfer coefficients, frost layer thickness, deposited frost mass and frost surface temperature as a function of both time position. The model is correlated to experimental data of in- and outlet temperatures, absolute humidities as well as plate temperatures. It is found that the heat transfer coefficient is locally improved during frosting. However, the overall rate of heat transfer is reduced due to the increasing thermal resistance introduced by the frost layer. Frost formation on the heat exchanging surfaces also increased the air flow resistance as a consequence of increasing drop in the static pressure along the path between the plates. Thus, the presence of a frost layer substantially reduces the amount of heat determined from the heat rate equation due to decreasing air flow and also reduces the overall heat transfer coefficient. The results obtained may serve as a basis for understanding the decreasing effectiveness due to frosting and to demonstrate the importance of considering frost formation in the optimization design work of commercial plate heat exchangers.
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