Another specification that often varies from the compressor catalog of one manufacturer to another is the number of degrees of superheat of the vapor entering the compressor. If a catalog specifies some superheat of the vapor, it is almost always understood that the superheat can be counted as useful refrigeration. Whether this assumption is true depends on the configuration of the evaporator. If the superheating occurs within the evaporator, as shown in Figure 4.17a, the refrigerating effect is useful. On the other hand, if the vapor that enters the compressor is superheated because of heat absorbed in the suction line, as in Figure 4.17b, the amount of this superheating is not productive, and the refrigerating capacity shown in the catalog must be reduced by the amount of this superheating. Whether the superheating occurs within the evaporator is usually a function of the type of expansion valve, and applies only when a superheat controlled expansion valve is used.
A perspective on the question of how liquid subcooling and vapor superheating are specified is that equipment selected without reading the fine print may be short of capacity by about 5% when actually installed and set into service. Because load calculations can hardly ever be precise, and because plants are usually installed with more capacity than absolutely necessary, the difference in behavior of the plant may not be noticed. On the other hand, in the bidding stage, a price difference of 5% may shift the choice from one supplier to another. The subcooling and superheating issue may, therefore, be more important from a marketing than from an engineering standpoint.