Operating conditions for retardation and prevention of frost formation in fin-tube heat exchangers

Abstract

A mathematical model to predict the heat and mass transfer rates of fin-tube heat exchangers under wet and frosting conditions is described. This mathematical model was used to analyze the effect of the air-side heat transfer coefficient on the mass transfer rate of water vapors. In addition, the critical mass transfer rate of water vapors was investigated as a function of the air-side heat transfer coefficient, and a mathematical description of the critical air-side heat transfer coefficient was formulated. The critical operating conditions to prevent frosting were established using this model. Heat and mass transfer networks of a fin-tube heat exchanger was constructed, and mathematical models to estimate the interface temperature, and the heat and mass transfer rates were formulated. The interface temperature, i.e., the equivalent temperature of the fin and tube surfaces, was defined as a criterion for determining the surface conditions, because the temperatures of the tube and fin were different. The mathematical model to predict the interface temperature, as well as the heat and mass transfer rates of fin-tube heat exchangers, was constructed as a function of the operating conditions, i.e., the refrigerant temperature, air temperature, absolute humidity of the air, and refrigerant-side and air-side thermal resistances. The mathematical model was verified by comparison with experimental data. The heat and mass transfer rates and the surface conditions of fin-tube heat exchangers were measured under various geometric and operating parameters, including wet and frosting conditions. Five types of heat exchanger were used for the experiments, with various fin pitches and numbers of tube rows. The heat and mass transfer rates from the mathematical model were found to be in good agreement with the experimental values with an error of less than 10%. The mathematical model was used to investigate the mass transfer of water vapors on fin-tube heat exchangers as a function of the air-side heat transfer coefficient. As the air-side heat transfer coefficient increased, the mass transfer coefficient increased and the driving force of the mass transfer decreased. This resulted in a maximum of the mass transfer rate of water vapors as a function of the air-side heat transfer rate, which was defined as the critical air-side heat transfer coefficient. A mathematical description of the critical air-side heat transfer coefficient of a fin-tube heat exchanger was formulated to examine the behavior of the fin-tube heat exchanger as a function of the operating conditions. The critical air-side heat transfer coefficient increased as the refrigerant and air temperature decreased; it also increased with increasing humidity and increasing refrigerant-side heat transfer coefficient. Based on the investigation using the mathematical model, frost growth was found to be inhibited if the temperature of the interface between the air and the cold surface was higher than either the dew point temperature of air or the freezing point of water. Using this criterion, critical operating conditions for the prevention of frosting were established. The frosting and frost-free regions predicted using the mathematical model were in good agreement with the experimental results. These critical operating conditions can therefore be used to develop operating strategies to prevent frosting on air-source heat pumps.