Many plants serve evaporators at several different temperature levels. Examples are particularly frequent in the food refrigeration industry where a facility maintains some space at, for example, -23°C (-10°F) for storing frozen food and another space at 2°C (36°F) for unfrozen fruits and vegetables. The flow diagram of a refrigeration system to meet such varied temperature needs is shown in Figure 3.19. This system uses a subcooler for the liquid passing to the lowtemperature evaporator. The intermediate- temperature evaporator is fed with liquid from the condenser or in some cases liquid mechanically pumped from the subcooler/intercooler. While it would be possible to direct the refrigerant leaving the medium-temperature evaporator directly to the suction line of the high-stage compressor, this stream is normally passed to the vessel. This arrangement is mandatory when the medium-temperature evaporator receives pumped liquid and operates with liquid overfeed. Return to the vessel is also a good precaution as well when expansion valves control the liquid flow to the evaporator and may fail, which would allow liquid to flood out of the evaporator.
When the plant operates with two temperature levels of the evaporation, the intermediate pressure is regulated to meet the temperature needs of the medium-temperature evaporator. The intermediate pressure thus may not match the optimum for minimum total power, but as Figure 3.18 shows, a moderate variation in this pressure results in only a small penalty in increased power. In designing a system with two levels of evaporating temperature, the flow rate of refrigerant through the low-stage compressor is unaffected by the existence of the medium-temperature evaporator. The high-stage compressor, on the other hand, must accommodate the flow attributable to the mediumtemperature evaporator plus that normally associated with the low-stage compressor, flash-gas removal, and desuperheating.