Study of frost growth on an air heat pump outside coil

The de-rating of both heating capacity and efficiency due to frost formation on the evaporator side of a heat pump operating in heating mode, leads to an increase in energy consumption and to sudden discomfort conditions caused by frequent defrosting cycles. A theoretical finite volume model describing frost formation over a finned coil has been developed for this study. The model is composed by a plate condenser, a finned coil, an electronic expansion valve and a compressor. By this model is possible to consider the effect in the frosting cycle time introduced by different values of refrigerant superheating in the compressor suction line and other physical parameters. These parameters determine some different changes in the general behavior of the heat pump and as consequence, in the running time. These physical phenomena are connected to the changes in frost density, thickness and conductivity, which are time dependent and related to the water vapor diffusivity in the frost layer during the time.