Compressor manufacturers express refrigerating capacity in their catalogs based on a certain number of degrees of subcooling of the liquid entering the expansion valve and a certain number of degrees of superheat of vapor leaving the evaporator. These two conditions affect the refrigerating effect, Δhev, in Eq. 4.7, and thus affect both the catalog rating as well as the actual refrigeration capacity developed by the compressor. Furthermore, the purchaser of a compressor deciding between offerings of two competing suppliers should know how to adjust the ratings to put them on the same basis.
The temperature of the liquid arriving at the expansion valve must be equal or lower than the condensing temperature in order for the expansion valve to function properly. Every additional degree of subcooling of this liquid lowers the entering enthalpy and adds to the refrigerating effect and the refrigeration capacity. Some manufacturers rate their compressors on the basis of saturated liquid entering the expansion valve, and others on the basis of a specified number of degrees of subcooling. To establish a common basis for comparison, the capacities listed in different catalogs can be multiplied by a correction factor, illustrated in Fig. 4.16, which is the ratio of the refrigerating effects:
An important question is, “What is a reasonable amount of subcooling to expect?” The answer is, “Not much,” because most condensers in industrial refrigeration plants operate with very little subcooling. But there are situations where the liquid subcooling is appreciable, and these are in two-stage systems, as studied in Chapter 3, where subcooled liquid supplies the low-stage evaporators. Realizing that the low-stage subsystem is fed with liquid that may be appreciably subcooled emphasizes that the selection of the low-stage compressor should not be made directly from catalog data. Instead, the required evaporator capacity should be multiplied by the ratio of refrigerating effects Δhev,c/Δhev,a, where Δhev,a is the refrigerating effect with the actual entering enthalpy. The resulting capacity is the value to be sought of a compressor listed in the catalog.