This insures that all the liquid refrigerant in the holding plates is pumped into the condenser and receiver where it is stored, thereby isolating the refrigerant during periods when the compressor is not in operation. In some cases the compressor may cycle on for short periods of time after the completion of the cooling cycle as small amounts of liquid refrigerant that have remained in the holding plates boil off, raising the low side pressure. Pumping the system down removes the refrigerant from the holding plates and the crankcase of the compressor eliminating several problems. No liquid refrigerant is allowed to collect in the holding plates during off cycle, where it could be pumped into the compressor on start up damaging the compressor. No liquid refrigerant can collect in the compressor crankcase, diluting the lubricating oil.
Diluted oil can cause damage to the bearing surfaces in the compressor.
In addition, because the holding plates and compressor crankcase are not left under pressure during off cycle, the chances of refrigerant leaks are reduced. In short, a Pump Down Cycle insures that liquid refrigerant cannot reach the compressor and damage it. All systems that use holding plates and expansion valves should employ some means of protecting the compressor from liquid refrigerant and a Pump Down Cycle is the most positive way to achieve this.