Numerical and Experimental Study of the Influence of Frost Formation and Defrosting on the Performance of Industrial Evaporator Coils

Abstract

The main objective of an evaporator in any refrigeration system is to extract thermal energy from a conditioned space by recirculating air through a refrigerated coil. However, when an air-cooled evaporator operates at a temperature below the freezing point of water with a coincident entering air dew point temperature that is above the evaporator coil surface temperature, frost will form on the evaporator surface. The presence of frost reduces the performance of an evaporator and the penalty is proportional to the amount of frost that has accumulated. For this reason, the accumulated frost must be periodically removed by the use of a defrost process.

A variety of methods are used to remove frost, however, the most widely-used defrost technique in industry is hot gas defrosting (HGD). The HGD technique depends on temporarily converting the evaporator to a condenser by passing hot gas through the coil; the hot gas is usually obtained directly from the compressor discharge. The HGD technique is a simple and effective method to remove frost rapidly, and the additional hardware required for the HGD process is relatively inexpensive to install. However, during the HGD process, a fraction of the energy supplied to the coil is ultimately transferred to the refrigerated in various forms and becomes a parasitic load (latent and sensible) on the refrigerated space. This additional energy added to the space must be extracted by other evaporators within the freezer space (or, if only one evaporator is in the space then the product temperature must rise). Hence, both the frosting and the defrosting processes penalize the efficiency of the cooling system.

In this research, the performance of a large scale industrial evaporator operating under frosting conditions is experimentally monitored during both cooling mode (which occurs under frosting conditions) and defrost mode. Theoretical models of the evaporator coil during the cooling and the defrosting modes have been developed and validated using the experimental data. The degradation of the performance of the evaporator during the cooling mode and the parasitic heat load associated with the defrost mode are presented. The two models are used to optimize the net cooling by minimizing all the penalties associated with running the refrigeration system. Guidelines relative to the most energy efficient operation of industrial refrigeration systems are presented.