Refrigerant Migration, a Low-Pressure Trip
During the off cycle, or especially during a long shutdown, refrigerant will want to travel, or migrate, to a place where the pressure is the lowest. In nature, most fluids travel from a place of higher pressure to a place of lower pressure. The crankcase usually has a lower pressure than the evaporator because of the oil it contains.
Oil has a very low vapor pressure. Refrigerant will flow to it whether the refrigerant is in vapor or liquid form. In fact, refrigerant oil has such a very low vapor pressure, it will not vaporize even when a 100-micron vacuum is pulled on the refrigeration system. Some refrigeration oils have a vapor pressure of 5 microns.
If the oil did not have a very low vapor pressure, it would vaporize every time low pressure exists in the crankcase, or a vacuum was pulled on it. If the crankcase has a heater, the vapor will be forced away from the crankcase and end up in the suction line.
This refrigerant may condense in the suction line and cause slugging in the compressor’s cylinders on startup. There is a greater chance of this happening if the compressor is located outdoors in a cold ambient temperature. The cold ambience will amplify the lower vapor pressure area, and also help condense the vapor to liquid.
The crankcase heater helps keep the oil in the crankcase free of refrigerant from migration. Even if the compressor does not slug a mixture of refrigerant-oil foam, the amp draw will be very high from the high-density saturated vapors being pulled into the cylinders from the crankcase.
Because refrigerant migration can occur with refrigerant vapor, it can occur uphill or downhill. Once the vapor reaches the crankcase, it will be absorbed and condense in the oil. Refrigerant and oil have a strong attraction for one another; they mix very well. Because liquid refrigerant is heavier than oil, the liquid refrigerant will be on the bottom of the oil in the crankcase.
The amount of refrigerant absorbed in the oil depends on the temperature of the oil and the pressure in the crankcase. The lower the temperature and the higher the pressure, the more refrigerant is absorbed. During short off cycles, the migrated refrigerant does not have a chance to settle under the oil, but it does still mix with the oil in the crankcase.
When the compressor turns on, the sudden pressure drop on the crankcase (containing liquid refrigerant and oil) will cause the refrigerant in the oil to flash to a vapor. This is called a flooded start. It causes violent foaming in the crankcase. The oil level in the crankcase will now drop and mechanical parts, especially bearings, will be scored due to inadequate lubrication.
The foaming action of the oil and refrigerant can also create restrictions in the oil passages and cause pressure to build. The crankcase pressure will now rise, and the mixture of refrigerant and oil foam can be forced through compressor passages, around piston rings, and be pumped by the compressor.
Not only does this situation cause loss of oil from the crankcase to the system, but it can also cause a mild form of slugging in the compressor’s cylinders. High compressor current draw, which can lead to motor overheating, follows. Also, broken or warped valves can occur as a result of overheating and/or slugging.
PUMPDOWN REMEDYThe only sure remedy to refrigerant migration is through the use of an automatic pumpdown cycle. This allows all refrigerant in the evaporator, suction line, and crankcase to be stored in the high side of the system (condenser and receiver).
A thermostat that controls box temperature is wired in series with a liquid line solenoid valve. When the box temperature is satisfied, the thermostat contacts open. This will de-energize the liquid-line solenoid; a pumpdown cycle will be initiated.
Once the low-side pressure reaches about 10 psig, a low-pressure control will de-energize the compressor circuit, initiating an off cycle. The system is now pumped down and refrigerant migration cannot occur because of lack of refrigerant vapor and liquid in the evaporator, suction line, and crankcase.
Publication date: 03/12/2007