In nature, most fluids travel from a place of higher pressure to a place of lower pressure. Refrigerant is no different. During a refrigeration system’s off cycle, or especially during a long shutdown, refrigerant will want to travel, or migrate, to the place in the system where pressure is the lowest. Therefore, refrigerant migration is defined as refrigerant traveling to the compressor’s suction line or crankcase during the 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 regardless of if the refrigerant is in the vapor or liquid form. In fact, refrigerant oil has such a low vapor pressure it will not vaporize, even when a 100-micron vacuum is pulled on the system. Some refrigeration oils have a vapor pressure as low as 10 microns. If the oil did not have a very low vapor pressure, it would vaporize every time a low pressure existed in the crankcase, or a vacuum was pulled on it.