During the compressor's off cycle, both liquid and vapor refrigerant will have a tendency to "migrate" to the compressor's crankcase and lay under the oil. Refrigeration oil has a much lower vapor pressure than the liquid or vapor refrigerant. So migration or flow of refrigerant takes place to the compressor because this is where the lowest pressure exists. If the compressor is located in a cold ambient, migration will take place much faster. 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.
Migration can take place with liquid or vapor refrigerant. Because migration can take place as a vapor, the refrigerant flow can occur uphill or downhill. Once the refrigerant vapor reaches the crankcase, it will condense and settle to the bottom of the crankcase under the oil only if the compressor remains off long enough. Oil and refrigerant are very soluble in one another. On short off cycles, the refrigerant will not have a chance to settle under the oil, but will mix with the oil. When the compressor turns on, the sudden crankcase pressure drop will cause the oil/refrigerant mixture in the crankcase to flash.
The oil level in the crankcase will now drop and mechanical parts can be scored. Oil foaming now appears and a combination of oil and refrigerant can be forced around piston rings and pumped by the compressor. High current draws, motor overheating, and broken valves can occur.
The only sure remedy for compressor migration is an automatic pump-down system. One must clear all refrigerant (liquid and vapor) from the evaporator and suction line before every off cycle. Automatic pump-down is accomplished with a thermostat controlling a liquid line solenoid in combination with a low-pressure controller terminating 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.
It is often thought that a crankcase heater will prevent migration. Crankcase heaters will keep the compressor's crankcase warm and prevent migration to the compressor's oil. However, condensed migrated refrigerant will be driven from the compressor and will sit in the suction line near the compressor waiting for the next on cycle. If excessive liquid refrigerant has been driven to the suction line, severe liquid slugging may occur during startups.
Frequently, compressor damage such as broken valves and damaged pistons will occur. Crankcase heaters can be effective in combating migration, but they will not remedy slugging at startup from liquid floodback unless used in conjunction with a "properly sized" suction line accumulator.
Air-cooled semi-hermetic compressors are more prone to slugging than refrigerant-cooled semi-hermetic compressors. This is because refrigerant is often drawn directly into an air-cooled semi-hermetic compressor's cylinder without passing through the motor barrel. (See Figure 1.) Slugging can result in broken valves, broken head gaskets, broken connecting rods, and other major compressor damage.
Refrigerant-cooled semi-hermetic compressors will often draw liquid from the suction line through hot motor windings in the motor barrel, which will assist in vaporizing any liquid. Even if liquid refrigerant gets past the motor windings, the check valve in the partition between the crankcase and the motor barrel will prevent any liquid refrigerant from entering the crankcase. High current draw will be noticed here from dense refrigerant vapors entering the compressor's cylinder.
Most hermetic compressor's suction lines end at the shell of the compressor. If liquid refrigerant is entering the compressor, liquid will fall directly into the crankcase oil and eventually be flashed. As mentioned in the Sept. 5 column, this is referred to as flooding. This causes oil foaming and excessively high crankcase pressures. Refrigerant and oil droplets will soon reach the compressor's cylinder and slugging will soon occur.
Slugging in hermetic compressors can also occur from a migration problem. As mentioned before, foaming oil and refrigerant in the crankcase due to migration will generate excessive crankcase pressure when the on-cycle occurs. These oil and refrigerant droplets can now get past piston rings and other small openings and enter the compressor's cylinder. The end result is slugging of refrigerant and oil. Slugging can damage reed valves, piston rods, bearings, and many more mechanical parts.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at firstname.lastname@example.org.
Publication date: 10/03/2005