Crankcase heaters

Flooding, Migration, and Slugging of Compressors. Causes and cures for these troublesome conditions.

(Crankcase heaters will keep the compressor’s crankcase warm and prevent migration to the compressor’s oil)

When it comes to servicing compressors, technicians often encounter three issues: flooding, migration, and slugging. This article will look at how they differ, as well as what usually causes these conditions to occur. (Copy from NEWS, hope can bring some useful information for you.)


Flooding occurs when liquid refrigerant enters the compressor’s crankcase while the compressor is operating or running. It only occurs to a compressor during the on or run cycle. Some causes of compressor flooding include:

  • Expansion bulb loses on evaporator outlet;
  • Oversized expansion valve;
  • After hot gas termination;
  • Heat pump changeover;
  • Defrost termination;
  • Wrong thermostat expansion valve (TXV) setting (no compressor superheat);
  • Overcharge;
  • Evaporator fan out;
  • Low load on evaporator;
  • End of the cycle (lowest load);
  • Capillary tube overfeeding;
  • Capillary tube system overcharged;
  • Defrost clock or heater out (iced coil); and/or
  • Dirty or blocked evaporator coil.

Since liquid refrigerants are heavier than refrigeration oils, liquid refrigerants returning to the compressor will settle underneath the oil in the bottom of the compressor’s crankcase. This liquid refrigerant will gradually be boiled off from the low pressures in the crankcase. However, since the liquid refrigerant being boiled off is under the oil in the crankcase, very small oil particles will be entrained in this vaporization process. The oil level in the crankcase will then drop and rob mechanical parts of vital lubrication.

Refrigerant-cooled semi-hermetic compressors often have check valves located on a partition between the crankcase and motor barrel to prevent oil and liquid refrigerant from mixing. Air-cooled semi-hermetic compressors and hermetic compressors are often more prone to flooding. Suction accumulators can help a flooding condition, but if the situation is severe, accumulators can also flood.

A telltale sign that a compressor’s crankcase is being flooded with refrigerant will be a cold, frosted, or sweaty crankcase. … Higher-than-normal current draws will also be present when flooding occurs.

Crankcase pressures can become excessively high from liquid refrigerant boiling in the crankcase, which can cause refrigerant and entrained oil particles to escape around the rings of the pistons during its down-stroke. Once in the compressor’s cylinders, they will be pumped by the compressor into the discharge line. The compressor is then pumping oil and refrigerant, robbing the crankcase of lubrication. Oil in the system and not in the crankcase will coat the inner walls of the tubing and valves and cause unwanted inefficiencies.

Higher-than-normal crankcase pressures, caused by the higher density refrigerant and oil mixture being pumped through the cylinders, will cause a high compressor current draw. This may overheat and even trip the compressor. Broken valves can also occur from this phenomenon. A telltale sign that a compressor’s crankcase is being flooded with refrigerant will be a cold, frosted, or sweaty crankcase. A foaming oil sight glass with a low oil level is also a sign of flooding. Higher-than-normal current draws will also be present when flooding occurs.


Migration occurs when refrigerant liquid or vapor returns to the compressor’s crankcase or suction line during the off-cycle. Migration only occurs during the off-cycle of the compressor and can be caused by:

  • No automatic pump-down system or
  • Leaky liquid line (pump down) solenoid.

During the compressor’s off-cycle, both liquid and vapor refrigerants will have a tendency to migrate to the compressor’s crankcase and settle under the oil. Refrigeration oil has a much lower vapor pressure than the liquid or vapor refrigerant; thus, migration or flow of refrigerant takes place to the compressor, because this is where the lowest pressure exists. If the compressor is located at a cold ambient temperature, 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. 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 it instead. 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 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 and be 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. This involves clearing all the 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 toward the compressor.

Crankcase heaters will keep the compressor’s crankcase warm and prevent migration to the compressor’s oil. However, the 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 start-ups. 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 start-ups from liquid flood back unless used in conjunction with a properly sized suction line accumulator.


Slugging occurs when liquid refrigerant — or liquid refrigerant and oil — enters the compressor’s cylinder during an on-cycle or run-cycle. Causes of this condition can include:

  • End of the cycle (lowest load);
  • Evaporator fan out;
  • Iced evaporator coil;
  • Defrost timer or heater out;
  • Dirty evaporator;
  • Capillary tube overfeeding;
  • Overcharge;
  • No compressor superheat;
  • Migration (off-cycle);
  • Bad TXV;
  • TXV hunting; and/or
  • Low load.

Air-cooled semi-hermetic compressors are more prone to slugging liquid 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.

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 draws will be noticed here from dense refrigerant vapors entering the compressor’s cylinder.

Most hermetic compressor suction lines end at the shell of the compressor. If the liquid refrigerant is entering the compressor, the liquid will fall directly into the crankcase oil and eventually be flashed. As mentioned earlier, 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 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 pressures 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 red valves, piston rods, bearings, and many more mechanical parts.

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