The lubricant, or oil, in a refrigeration or air conditioning system is needed for lubrication of the compressor’s moving mechanical parts. Oil also minimizes mechanical wear by reducing friction, and maintains a seal between the high and low side of the compressor. Without proper lubrication, the compressor’s valves, mating scrolls, screws, and vanes would not properly seal. The results would be high-side refrigerant pressures entering the low side of the refrigeration system. Piston rings in a reciprocating compressor and rotating vanes in centrifugal compressors rely on the lubricating oil to prevent blow-by around the pistons and vanes.

Oil also acts as a noise dampener within the compressor and transfers heat away from moving and rotating parts within the compressor. However, refrigeration oil has a very low vapor pressure, and refrigerant will flow to it whether the refrigerant is in the vapor or liquid state. In fact, refrigerant oil has such a low vapor pressure that it will not vaporize even when a 100-micron vacuum is pulled on the refrigeration system. Some refrigeration oils have a vapor pressure as low as 5 to 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 when a vacuum was pulled on the crankcase.

Refrigerant Migration

Because refrigerant migration can occur with refrigerant vapor, the migration can occur uphill or downhill. During a long off cycle, once the refrigerant vapor reaches the crankcase through the migration process, it will condense and settle to the bottom of the crankcase under the oil. This happens because the refrigerant is heavier than the oil. However, this only happens if the compressor remains off long enough.

Oil and refrigerant are very soluble in one another. When the compressor is finally turned on after a long shutdown, the sudden crankcase pressure drop will cause the oil and refrigerant mixture to flash in the crankcase. The oil level in the crankcase will then drop, and mechanical parts can be scored. Oil foaming will appear, and a combination of oil and refrigerant can be forced around piston rings to be pumped by the compressor, and high current draws, motor overheating, and broken valves can occur.

If the compressor is located in a cold ambient, migration will take place even faster. This happens because a cold ambient will cause an even lower vapor pressure in the compressor's crankcase. Migration will even take place from a suction line accumulator to a compressor because of the difference in vapor pressure. Many air conditioning and refrigeration systems have their condensing units located outdoors in order to take advantage of the cooler outdoor ambient temperatures to reject the heat and to reduce noise pollution.

The condensing unit usually consists of a compressor, condenser coil, outdoor condenser fan, contactor, starting relay, capacitors, and a solid state board with circuitry. Receivers are often incorporated in refrigeration system condensing units. Within the condensing unit, the compressor will often have a heater connected in some way to its bottom section or crankcase, which is why it is referred to as a crankcase heater. The crankcase heater is an electric resistance heater that is usually strapped on, clamped to the crankcase bottom or inserted into a well within the compressor's crankcase. The crankcase heater is often seen on compressors that operate in ambient temperatures lower than the system’s operating evaporator temperature.

Refrigeration Systems

Before shutting down a refrigeration system for a long period of time, it is important to manually pump down the system, as this will clear all the refrigerant (liquid and vapor) from the evaporator and suction line and store it in the high side (receiver and condenser) of the system. A low-pressure controller can terminate the on-cycle once the evaporator and suction lines are void of any refrigerant. This will ensure that there is no refrigerant in the evaporator or suction line to migrate towards the compressor during the long off cycle.

A manual pump down is accomplished by front seating the receiver outlet (king) valve while the system is running. It is recommended to let the system run until the pressure in the low side of the system reaches 5 to 10 psig. Any lower pressures can do damage to the compressor on some systems. Once the system shuts off on low-pressure control or is shut off manually by the technician between 5 and 10 psig, the king valve should be left front seated for the entire long shutdown. If the power is going to be off during the long shutdown period, a manual pump-down method is preferred.

When it is time to restart the system, simply turn power back on and take the king valve off of its front seat. This allows refrigerant to flow from the receiver through the liquid line and into the evaporator. The low-pressure control will sense the pressure increase and start the compressor. The compressor should have a clean start without any dissolved refrigerant in the crankcase oil. A crankcase heater can be energized during the long shutdown just in case the king valve may leak. At least the crankcase will be warm and drive away any refrigerant from entering the crankcase to condense under the oil. Air conditioning systems can use this same shutdown methods if they have a king valve and receiver.

Air Conditioning Systems

Residential homeowners often will turn off the electrical disconnect to their air conditioner’s outdoor condensing unit during the heating season and/or cooler months when air conditioning is not needed. This will cause the compressor to be without crankcase heat because of a de-energized crankcase heater, and migration of refrigerant to the crankcase is sure to take place during this long off-cycle.

Once the cooling season begins, if the homeowner doesn’t turn the breaker back on at least 24 to 48 hours before starting the air conditioning unit, serious crankcase foaming and pressurization will occur from the long off-cycle refrigerant migration. This can rob the crankcase of a proper oil level and will score bearing and cause other mechanical failures within the compressor. So, always turn the breaker on while the system is not in the cooling mode for at least 24 to 48 hours to prevent this from happening. This will ensure a warm crankcase without any dissolved refrigerant in the compressor’s oil.