Effects of evaporator frosting on the performance of an air-to-air heat pump

This paper shows the development and use of a transient model for evaluatin g frost formation on a parallel-plate hear pump evaporator. A frost formati on model is derived by applying the equations of conservation of mass, mome ntum, and energy, as well as empirical correlations, to calculate the growt h and densification of the frost layer. The frost formation model is valida ted by comparison with experimental results. The frost formation model is t hen incorporated into the evaporator subroutine of an existing heat primp m odel to calculate performance losses due to frosting as a function of weath er conditions and time of operation since the last evaporator defrost. Perf ormance loss calculation includes the effect of air pressure drop through t he evaporator and the reduction in evaporator temperature caused by the gro wth of the frost layer. The results show frost formation parameters and hea t pump COP as a function of time and ambient conditions. It is determined t hat there is a range of ambient temperatures and humidities in which frosti ng effects are most severe, and this range is explored to calculate heat pu mp operating conditions. The heat pump analysis results are expected to be useful in predicting optimum defrosting conditions, and to evaluate alterna tive methods for defrosting.