When a cold compressor is started, pressure drops rapidly and the temperature of the oil rises. As a result, the solubility of the refrigerant is sharply decreased. There will be a rush of gas out of the oil over a very short period of time. The drop in crankcase pressure causes oil foaming.
When this occurs, an excessive amount of oil may be carried with the refrigerant through and beyond the compressor.
In some cases, sealing off the compressor and evaporator from the rest of the system with a liquid-line solenoid valve keeps refrigerant from dissolving in oil. The evaporator and crankcase then can be pumped down before the compressor stops running. Very little refrigerant remains in the part of the system containing oil. The concentration of refrigerant dissolved in oil when the system is idle would be about the same or lower as when it runs. This is a direct, positive way to prevent refrigerant buildup in the crankcase during the OFF cycles.
Excerpt from the Technical Institute Manual Three from the Refrigeration Service Engineers Society. Visit the online store for more details. RSES.ORG