Liquid floodback is liquid refrigerant returning to the compressor’s crankcase during the running cycle. Refrigerant migration is refrigerant liquid or vapor returning to the compressor during the off cycle. Migration usually happens because of a pressure difference. The compressor can be located in a cold ambient causing a lower pressure, or the oil can be attracting the refrigerant because of its lower vapor pressure. In either case, the refrigerant will migrate to the compressor because of its lower pressure.
Liquid refrigerant floodback is a compressor’s worse nightmare. Floodback will dilute the compressor’s oil with liquid refrigerant and cause foaming in the compressor’s crankcase, which causes bearing wash. Even if the compressor superheat is kept over 20 degrees F, there are times when evaporator heat loads are low, defrost periods have just ended, mechanical valves fail or are misadjusted, or a lack of system maintenance like a dirty evaporator coil can cause compressor floodback. If there is no way to prevent these unexpected periods of floodback to the compressor, a suction line accumulator is needed on the system.