An objective to be pursued in the next several sections is to effectively drain the condenser of liquid so that the maximum amount of surface area will be available for condensation. Before addressing that goal, however, a technique sometimes used in commercial refrigeration (e.g., supermarkets) that operate year-round will be shown. The condenser pressure must be prevented from dropping so low that the thermal expansion valves (typically used in that type of system) are unable to pass a sufficient flow rate of refrigerant. The condensing pressure must be artificially prevented from dropping during cold weather operation, and a procedure, such as the one shown in Fig. 7.26, backs liquid into the condenser to reduce its heat-transfer capacity and maintain the desired level of condensing pressure.
The operating efficiencies offered by lower condensing pressures are thus not available, although the subcooling coil does reduce the enthalpy of liquid passing to the expansion valve and thereby increases the refrigerating effect.