The ice formed on the evaporator surface is disadvantageous for the energy efficiency and performance of the refrigeration system. In order to achieve lower temperatures than ambient air, whether for food cold storage purposes or thermal comfort, the evaporator coils generally operate at lower temperatures than the freezing point of water.
Alongside the effect of relative humidity, the paper also considers the effects of other performance parameters on the processes of frosting and defrosting such as cooling time and length of previous defrost cycle, and discusses ways in which some of these parameters may be used to implement defrost on demand. Using relative humidity as a control parameter the defrost frequency can be reduced considerably without affecting cabinet performance and product integrity. The results show that for medium-temperature refrigeration applications where the environment temperature is kept reasonably constant, the store humidity is the primary parameter influencing the rate of frost formation. This paper reports on field and experimental investigations on the processes of frosting and defrosting of medium-temperature display cabinet evaporator coils. Implementing defrost only when it is needed or on ‘demand’ should reduce the number of defrost cycles and lead to savings in energy and improved product quality. It is widely acknowledged, however, that time-based defrost may cause a number of unnecessary defrost cycles and this reduces the energy efficiency of the refrigeration systems as well as the accuracy of temperature control of the cabinets. Although different defrost control strategies can be employed, for simplicity and cost considerations, defrosting in supermarket refrigeration systems is usually controlled by a pre-set time cycle. In order to maintain satisfactory performance, evaporator coils are defrosted periodically. With continuous operation of the coil, frost will accumulate on the coil surface leading to a decrease both in the air flowrate and in the overall heat transfer coefficient. The air which is circulated over the evaporator coil is cooled below its dew point and water vapour present in the air condenses and eventually freezes on the coil surface. In order to achieve the required air and product temperatures in refrigerated food display cabinets, the evaporator coils, which are normally located in the base of the cabinets, operate at temperatures below the freezing point of water.
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