2 DESIGN ASPECTS OF DEFROSTING REFRIGERATORS

This paper shows that the intermittent operation of a defrosting vapour-compression-cycle refrigerator can be optimized with respect to: (1) the frequency of on/off operation, and (2) the way in which the supply of heat exchanger surface is divided between evaporator and condenser. The method used is that of thermodynamic optimization (or entropy generation minimization, or finite-time thermodynamics), in which heat transfer and thermodynamic aspects are accounted for simultaneously to produce a realistic description of the in-time operation of the installation. The optimal on/off frequency and surface allocation ratio are reported in non-dimensional charts, which show the effect of the heat exchanger size, cycle temperature ratio, defrosting time, compressor efficiency, and refrigerant type. The charts are based on the real properties of refrigerants R12 and R134a. The heat exchanger equipment constraint is that of fixed total heat transfer surface. It is shown that, under certain conditions, the optimization conclusions reached in this study are similar to those that would be obtained based on other constraints proposed in the literature.