The automatic pumpdown cycle is the only sure remedy for refrigerant migration to a compressor during the off cycle. The cycle will also prevent slugging of liquid refrigerant or oil caused from a severe migration problem and at cycle start-ups.
The automatic pumpdown system consists of a normally closed liquid-line solenoid valve installed in the liquid line of a refrigeration system. The solenoid is a normally closed electric shut-off valve that is controlled by a thermostat. The thermostat is located somewhere in the refrigerated space.