Defrost Circuit Problems

January 9, 2003
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Can a defrost circuit problem cause a compressor to quit running?

You bet it can! And these types of problems are quite often overlooked. Anytime there is a malfunction in the defrost circuit, the compressor will soon feel the results.

Problems such as a burned-out or opened defrost clock motor, defrost contacts that are stuck open, a defrost limit switch that is stuck open, a defrost termination switch that is stuck closed, or an opened defrost heater will cause the evaporator to build up frost, restricting airflow across its face.

Frost Buildup

Anytime the evaporator coil sees reduced airflow across its face, there will be a reduced heat load on the coil. No airflow will cause the refrigerant in the coil to remain a liquid and not vaporize. This liquid refrigerant will travel on past the evaporator coil and eventually get to the compressor. Compressor damage will soon occur from flooding and/or slugging.

Sometimes technicians will change out a compressor because it has broken internal parts without finding the actual cause of the problem. The compressor having broken parts is not the cause of the problem. In this case, the cause would be a faulty defrost circuit not letting the system defrost. This would frost or ice the evaporator coil, causing flooding or slugging of the compressor. This, in turn, probably caused the broken internal parts. If the technician did not run a system checklist and run the system through its modes after changing the compressor, the new compressor is sure to fail for the same reasons. In fact, compressors installed by service technicians are failing at a rate 6 to 7 times greater than the rate for original equipment.

Compressor manufacturers are asking technicians to examine the broken-down compressor for the cause of failure. Opening a semi-hermetic compressor and examining its internal parts does not void the warranty, as long as all of the parts are returned to the old compressor.

The technician should then make a list of the causes that could be blamed for this failure and eliminate them one by one once the system is up and running. As mentioned before, a defrost circuit problem was the cause here. When a service technician approaches a broken-down compressor, usually many hours — if not a day — has passed, and the evaporator has naturally defrosted. Thus, the technician never sees the frosted or frozen evaporator coil.

After the compressor is fixed or replaced, if the system is not run and put through a defrost mode (or systematically checked with an ohmmeter and voltmeter), the real problem — the defrost circuit — will not be found and the replacement compressor will soon fail.

It is suggested that causes and symptoms be listed, and a system checklists be made when systematically troubleshooting problems like these.

Operating System

If the system is still running with a frosted or frozen evaporator coil, certain symptoms will show in the system check. Figure 1 is a system check for a forced-air commercial refrigeration system using R-134a as the refrigerant. This system incorporates a TXV metering device with a receiver.

The Symptoms

The symptoms for a frosted evaporator coil include:

  • Low discharge temperatures;

  • Low condensing (head) pressures;

  • Low condenser splits;

  • Low to normal evaporator (suction) pressures;

  • Low superheats;

  • Cold compressor crankcase; and

  • High to normal amp draw.

    Low discharge temperatures: Since the superheats are low and the evaporator and compressor could be flooding, the compression stroke could contain liquid entrained with vapor (wet compression). The heat of compression will hopefully vaporize any liquid. This vaporization process needs heat and will get it from the heat of compression.

    This will take heat away from the cylinder and leave a colder discharge temperature. Discharge temperatures could even be cooler than the condensing temperature. This would be a sure sign of liquid being vaporized by the compression stroke. In other words, wet compression is taking place.

    If wet compression is severe enough, head bolts have been known to be stripped and discharge valves have been known to be ruined from hydraulic pressures. These pressures build up from trying to compress liquid refrigerant.

    Low condensing (head) pressures: The restricted airflow over the evaporator coil will cause the refrigerant in the evaporator not to see a heat load, thus not be completely vaporized. With no heat load to be rejected in the condenser, the condensing pressure and temperature does not have to elevate to reject heat to the ambient. Low condensing pressures are the result.

    Low condenser splits: With condensing pressures and temperatures low, the condensing split will be low. The condenser does not have to elevate its temperature to reject the small heat load.

    Low to normal evaporator (suction) pressures: Because of the reduced heat load seeing the evaporator coil, the refrigerant vaporization rate and amount will be reduced. This will give low vapor pressures in the low side of the system.

    Low superheats: Because the heat load on the evaporator coil is reduced, not much refrigerant will be vaporizing. The 100% saturated vapor point in the evaporator will crawl down past the end of the evaporator and the TXV usually loses control. Compressors can slug or flood in these situations.

    Cold compressor crankcase: Since the compressor (total) superheat is low, sometime during the on cycle the compressor will flood or slug. There will be liquid refrigerant in the compressor’s crankcase boiling off. This will flash the oil and will cause compressor damage. It is the boiling of refrigerant in the crankcase that will make the crankcase cold to the touch. The crankcase may even sweat or frost if conditions are right.

    High to normal amp draw: Droplets of liquid refrigerant will be entrained with the suction vapors and the density of the refrigerant coming from the suction line will be high. Some refrigerant may even be in liquid form. This will require more work in the compressor and the amp draw may be a bit high depending on the severity of the flooding or slugging.

    Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at tomczykj@tucker-usa.com.

    Publication date: 01/13/2003

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