The first consideration is the evaporating temperature. This will be set by the required load condition and the appropriate temperature differential (AT) across the evaporator. In the context of evaporator selection, the AT used is the difference between the evaporating refrigerant and the temperature of the fluid entering the cooler, not the log mean temperature difference.
In systems where the evaporator cools air, the air itself becomes the heat transfer medium and its temperature and humidity must be considered in relation to the end product. Where the product cannot suffer dehydration, the AT may be high, so as to reduce the size and cost of the coil, but the lower the evaporating temperature falls, the lower will be the capacity of the compressor and its COP. In these circumstances, a first estimate might be taken with a AT of 10-12 K and cross-checked with alternatives either side of this range. In each case, the ‘owning’ cost, i. e. taking into account the running costs, should be considered by the user. Table 10.1 illustrates this for a cold store comparison, running 8760 hours per year. The data should be used for comparative purpose only.
Table 10.1 Comparison of running costs for various cooler sizes
Annual electricity costs
Cost (Ј) AT Fans Compressor Total
Unsealed products will be affected by low humidity of the air in the cooled space and may suffer dehydration. Conversely, some food products such as fresh meat will deteriorate in high humidity. Since the dew point of the air approaches the fin surface temperature of the evaporator, the inside humidity is a function of the coil AT. That is to say, the colder the fin surface the more moisture it will condense out of the air, and the lower will be the humidity within the space. Optimum conditions for all products likely to be stored in cooled atmospheres will be found in the standard reference books, or may be known from trade practice. The following may be taken as a guide:
Products that dehydrate quickly, such as most fruits and vegetables Products requiring about 85% saturated air Products requiring 80% saturation or drier Materials not sensitive to dehydration
AT = 4K AT = 6K AT = 8K
AT = 10 K upwards
A further consideration may be the possibility of reducing ice buildup on the evaporator, whether this is in the form of frost on fins or ice on the coils of a liquid chilling coil. Where temperatures close to freezing point are required, it may be an advantage to design with an evaporator temperature high enough to avoid frost or ice — either for safety or to simplify the defrost method.
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