In most refrigeration and air conditioning systems, refrigerant will want to travel, or migrate, to a place where the pressure is the lowest. During the off-cycle or especially during a long shutdown, the compressor’s crankcase is usually the location that has the lowest pressure. Refrigerant migration happens because fluids travel from a place of higher pressure to a place of lower pressure. In fact, refrigerant migration is defined as refrigerant, either liquid or vapor, traveling to the compressor’s suction line or crankcase during the compressor’s off-cycle.
The crankcase usually has a lower pressure than the evaporator because of the oil it contains. Oil has a very low vapor pressure, and refrigerant will flow to it whether the refrigerant is in vapor or liquid form. If the oil did not have a very low vapor pressure, it would vaporize every time a low pressure exists in the crankcase, or a vacuum was pulled on it.