Here's the scene: A compressor is shut down for a few months and starts up cold. Upon startup it makes a lot of noise and breaks down. The contractor's first thought might be slugging. In this case he is probably correct. However, slugging is only a symptom of a larger problem; in refrigerant-cooled compressors, slugging is the result of aflooded start.

Flooded starts usually are the result of off-cycle refrigerant migration. In other words, while the compressor is off, the refrigerant vapor migrates to the compressor crankcase, where it condenses into a liquid and is absorbed by the oil. The longer the compressor is off, the more time refrigerant vapor has to migrate.

On start-up there is a reduction in crankcase pressure; the refrigerant explodes out of the oil, possibly carrying oil with it out of the crankcase.

During this flooded start, refrigerant exploding from the oil may wash the oil from the lubricated surfaces, such as bearings, journals, and rods. The diluted oil cannot properly lubricate the surfaces. This can be verified during a compressor teardown, where erratic wear or seizure damage will be visible. Telltale signs include:

  • Worn or scored rods or bearings;

  • An erratic wear pattern of crankshaft.


    Off-cycle migration can be prevented. You can start by installing a continuous pumpdown cycle on the unit. This will remove the majority of the refrigerant from the low-pressure side of the unit. If possible, the compressor could be installed in a controlled ambient area.

    A crankcase heater also can be installed. This can help protect a compressor that may not operate for long periods of time. Crankcase heater devices are located at the bottom of the compressor. They are installed so that the oil stays warmer during nonoperational times, thus slowing the attraction of liquid refrigerant.

    Think about refrigerant migration before starting a compressor. For instance, if the compressor has tripped on the oil pressure control, don't write it off as a nuisance trip (reset the control and walk away)! Look at the oil level sight glass.

    If the oil is above the desired level in the glass, this could indicate that refrigerant has migrated to the compressor. You could try to separate the refrigerant from the oil by heating the oil with a crankcase heater a few hours before starting the system, or by "jogging" the compressor. Jogging simply means that you start and quickly stop the compressor.

    Important note: Do not keep the suction service valve closed while you are jogging the compressor. If the valve is closed, the refrigerant and oil could cause a violent flooded start anyway, as there is less space for the initial start-up pressure to be pulled from. And this is precisely what you are trying to prevent.

    A low level in the sight glass could indicate different problems, such as leaks or oil logging. In that case, you need to look for a potential oil-return problem. These will be covered later in this series.

    In summary, for slugging to occur in a refrigerant-cooled compressor, migration has to take place first. In order for migration to take place, the compressor has to be off for some length of time. If you suspect migration has occurred, why was the compressor nonoperational? Fix the cycling problem and your flooded start problems will probably be resolved.

    For more information, click on the Emerson Climate Technologies logo above.