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.
If refrigerant migration occurs in a system that has a crankcase 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. Slugging is liquid refrigerant or liquid oil trying to be compressed in the cylinders of the compressor. As we know, liquids cannot be compressed, and tremendous reversal forces are generated, often resulting in broken parts. Slugging is especially likely if the compressor is located outdoors in a cold ambient. The cold ambient will amplify the lower vapor pressure area and help condense the refrigerant vapor to liquid. The crankcase heater helps keep the oil in the crankcase free of refrigerant arriving via refrigerant migration.
Because refrigeration migration can occur with refrigerant vapor, the migration can travel uphill or downhill. Once the refrigerant vapor reaches the crankcase, it will be absorbed and condense in the oil. Refrigerant and oil have a strong attraction for one another and mix very well. Since liquid refrigerant is heavier than oil, the liquid refrigerant will be on the bottom of the oil in the crankcase. On short off cycles, the migrated refrigerant does not have a chance to settle under the oil, but 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 causes violent foaming in the crankcase. The oil level in the crankcase will then drop, and mechanical parts will be scored from inadequate lubrication. The crankcase pressure will rise and the mixture of refrigerant and oil foam will now 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, it can also cause a mild form of slugging in the compressor’s cylinders. High compressor current draw, which will lead to motor overheating, usually follows. Also, broken or warped valves can occur as a result of overheating and/or slugging.
ONE SURE SOLUTION
The only sure solution to avoiding migration is to get rid of all the refrigerant in the evaporator, suction line, and crankcase before the off cycle. This can be accomplished by an automatic pumpdown system. In such a system, a thermostat controlling box temperature is wired in series with a liquid line solenoid. When the box temperature is satisfied, the thermostat contacts open. This de-energizes the liquid line solenoid and initiates a pumpdown cycle. Soon, all the liquid and vapor refrigerant from the solenoid forward through the compressor will be pumped into the high side (condenser and receiver) of the system. Once the low-side pressure reaches about 10 psig, a low-pressure controller will interrupt the compressor circuit, initiating an off cycle. The system is now pumped down, and migration cannot occur due to a lack of refrigerant vapor and liquid in the evaporator, suction line, and crankcase.
When the box thermostat calls for cooling, the liquid line solenoid is energized, and refrigerant pressure will now travel through the metering device to the low side of the system. This pressure will cause the cut-in pressure of the low-pressure control to close its contacts and bring the compressor to another on cycle. The cut-in pressure for the low-pressure control is system- and refrigerant-dependent. It has to be high enough to prevent any short-cycling of the compressor during an on cycle, but low enough to allow the low-side pressure to reach it when an on cycle is initiated by the box thermostat. Trial and error will allow a service technician to determine the low-pressure control’s settings.
It’s important not to let the low-side pressure get too low before shutting off the compressor. If the low-side pressure was allowed to drop to 0 psig before the low-pressure control terminated the cycle every off cycle, damage could occur to the compressor from lack of refrigerant mass flow rate and high compression ratios. This severely unloads the compressor and may cause overheating from loss of the cooling effect on the compressor’s windings. A cut-out pressure of 10 psig is low enough to ensure most of the liquid and vapor refrigerant has been cleared from the evaporator, suction line, and crankcase to prevent refrigerant migration during the off cycle.
Publication date: 6/29/2015