A reasonable desire is for the refrigerant liquid that flows out to the system to be subcooled. Conveying saturated liquid in pipes is always fraught with the drawback of flashing some liquid into vapor, which can restrict the mass flow when the refrigerant reaches an expansion valve or a level-control valve. Liquid can be subcooled in the tubes of the evaporative condenser by operating with liquid backed up into the tubes. This condition is not desirable, however, because heat-transfer area that should be available for condensation is reduced. The result will be a higher-than-necessary condensing temperature. The latter part of this chapter is devoted to proper drain piping, which has the objective of not backing liquid up into the condenser.
Even if subcooled liquid is produced in the condenser, this subcooled liquid flows to the receiver, as in Fig. 7.18, and here the liquid warms to a saturated state. Because both vapor and liquid are present in the receiver, the liquid is saturated. The heat-transfer process that takes place in the receiver is that of condensing some vapor to warm the subcooled liquid. In order to develop subcooled liquid a separate coil must be provided in the condenser that draws saturated liquid from the receiver and subcools it in the coil, as shown in Fig. 7.18.