Slugging Vs. Overheating

HVACR technicians have been keenly aware of compressor failures caused by slugging for many years. It used to be a much greater problem, and more emphasis was put on it during training.

Today many compressor failures are still chalked up to slugging. However, there can be other causes for all the noise and damage. Sometimes the difference can only be detected during a teardown.

Essentially, slugging is the result of trying to compress liquid refrigerant and/or oil, in the cylinders. Slugging indicates extreme floodback in air-cooled compressors and severe flooded start on refrigerant-cooled compressors. When slugging occurs in a semi-hermetic compressor, liquid is between the top of the piston and bottom of the valve plate.

During a teardown a contractor could see one or more of the following:

  • Broken reeds, rods, or crankshaft;

  • Loose or broken backer bolts;

  • Blown head gaskets.

    Correct And Prevent

    Conditions that lead to slugging can be prevented, mainly through the use of proper system sizing and refrigerant charge calculations.

  • Maintain proper evaporator and compressor superheat. If the system was incorrectly charged with refrigerant - in other words, if the technician didn't measure superheat and subcooling to determine the proper charge - it may result in liquid floodback from the evaporator.

  • Correct abnormally low load conditions. (Ensure proper evaporator airflow. If airflow is lost the refrigerant will not evaporate. This airflow problem could be caused by a fan motor failure, dirty, iced up or blocked coil.)

  • Install accumulators to help control liquid return. This modification may be used if liquid returning to the compressor cannot be controlled by other means.

  • Locate the compressor in a warm ambient area.

  • Install automatic pumpdown. News contributor and Ferris State professor John Tomczyk describes this cycle:

    "The automatic pumpdown cycle can help prevent refrigerant migration to a compressor during the off cycle. The system consists of a normally closed liquid-line solenoid valve (an electric shut-off valve controlled by a thermostat) installed in the liquid line of a refrigeration system. The thermostat is located somewhere in the refrigerated space.

    "When the desired temperature is reached in the space, the thermostat opens and de-energizes the liquid-line solenoid valve, which then closes.

    "The compressor continues to run, evacuating refrigerant from the solenoid valve's outlet. This includes part of the liquid line, evaporator, suction line, and crankcase. This refrigerant is stored in the condenser and receiver on the high side of the system. Most of the refrigerant is stored in the receiver. The compressor will be shut off by the action of a low-pressure switch. This will ensure that no refrigerant will migrate to the compressor during the off cycle.

    "On a call for cooling, the thermostat will close and energize the liquid-line solenoid valve. Liquid refrigerant is sent to the expansion valve and into the empty evaporator, thus increasing the evaporator pressure. Once the cut-in pressure of the low-pressure control is reached, the compressor will start and resume a normal refrigeration cycle.

    "The cut-in pressure of the low-pressure control should be set at a high enough pressure to ensure that the system will not short-cycle if residual pressure remains in the low side of the system once it is pumped down."

    Be aware that pumpdown will help put not prevent migration in all cases. For instance, if you have a power outage, pumpdown will not occur and liquid refrigerant can still migrate from the low side to the compressor.

    Changing Perspective

    While slugging can and still does occur, it is not as much of a problem as it used to be. Today's compressors have more safeguards for their own protection.

    According venerable compressor manufacturer Copeland Corp., "Twenty years ago liquid refrigerant slugging and liquid floodback were very serious problems. Compressor valves and bearings were more vulnerable, and system designs were not always the best.

    "For 20 years, every educational and training program has hammered at the dangers of liquid refrigerant and the need for proper compressor superheat, and few words have been said about the dangers of high return gas temperatures. In effect, a whole generation of service personnel has worried only about liquid floodback.

    "Possibly because of the old fears of liquid refrigerant, we as compressor manufacturers and in general the industry have been slow in recognizing and identifying high-temperature failures. At cylinder temperatures of 315 degrees to 325 degrees F, the lubricating film is literally evaporated off the cylinder walls like water on a hot griddle, but modern refrigeration oils are so resistant to breakdown, carbon is not formed on the valve plate. As a result many high temperature failures are misdiagnosed as liquid failures."

    So today, a failure that sounds like slugging according to the customer's description, may actually be caused by the effects of high temperatures over time. How to tell the difference? Open up that compressor.

    Next month: Overheating causes.

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

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