Restricted Airflow: A Common Culprit

September 30, 2000
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What sets a gifted troubleshooter apart from the rest of the hvacr service techs? Many skills are required, but the most important are the ability to gather all the system information needed using the proper instruments — this includes knowing how much information is needed and what types of instruments to use — then applying this system knowledge to the problem at hand.

An ace troubleshooter, therefore, must have certain organizational skills and the ability to critically analyze the situation. These two skills support and enhance each other.

As I mentioned last month, one of the best known ways for service technicians to organize their thoughts is through the use of a service checklist (Table 1). This article will incorporate a service checklist in troubleshooting a frosted evaporator coil or restricted airflow over the evaporator. (See photo, page 26.)

Any time the evaporator coil sees reduced airflow across its face or a frosted coil, there will be a reduced heat load on the coil. No or low airflow, or frost, will cause the refrigerant in the coil to remain mainly a liquid and it will 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.

Why it Happens

Following are ways the airflow over the evaporator can become restricted or the coil frosted:

  • Frosted evaporator coil from malfunctioning defrost heater;
  • Defrost timer motor burned out;
  • Frosted evaporator coil from high humidity;
  • Frosted evaporator coil from evaporator fan not working;
  • Frosted evaporator coil from defrost component malfunctions;

  • Frosted evaporator from no load on the evaporator coil;
  • Dirty evaporator coil;
  • Dirty evaporator air filter;
  • Defrost intervals set too far apart, causing frost build-up;
  • Evaporator coil’s inlet air to the fan is blocked.
  • Many technicians will change out a compressor because of broken internal parts and will not find the actual cause of the problem. The compressor has broken parts, but that is not the cause. The cause could have been a faulty time clock or an open defrost heater not letting the system defrost do its job. This would frost or ice the evaporator coil, causing flooding or slugging of the compressor.

    This, in turn, probably caused the broken internal compressor 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 six to seven times the rate of original equipment.

    Compressor manufacturers are asking technicians to examine broken-down compressors for the cause of failure. Opening a semi-hermetic compressor and examining its internal parts does not void the warranty aslong as all of the parts are returned with 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 heater that is open could be the cause. If the system is not run and put through the defrost mode, or systematically checked with an ohmmeter and voltmeter, the real problem of an open defrost heater will not be found and the replacement compressor will soon fail from flooding or slugging.

    Causes and symptoms should be listed, and system checklists should be made when you are systematically troubleshooting systems.



    Symptoms of Reduced Air Over Evap Coil

    Some symptoms for reduced air over evaporator coil include:

  • Low discharge temperatures;
  • Low head pressures;
  • Low condenser splits;
  • Low evaporator (suction) pressures;
  • Low superheats;
  • Cold compressor crankcase; and
  • High to normal amp draw.
  • Low discharge temperature: 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 cause a cooling of the cylinder walls, taking heat away from the cylinder and leaving 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 can be stripped. The discharge valves can be ruined from hydraulic pressures. These pressures build up from trying to compress liquid refrigerant.

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

    Low condenser splits: Condenser split is the temperature difference between the condensing temperature and the ambient. 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 evaporator (suction) pressures: Because of the reduced heat load on the evaporator coil, the refrigerant vaporization rate and amount will be reduced. This will give lower vapor pressures in the low side of the system.

    However, if the metering device was overfeeding the evaporator, and the airflows were normal with no frost on the coil, evaporator pressures would be higher. This is because of the rapid rate of refrigerant vaporizing in the evaporator from the overfeeding situation. The low suction pressure is a distinguishing symptom that tells the technician the problem is not metering device overfeeding.

    Low superheat: Because the heat load on the evaporator coil is reduced, not much refrigerant will vaporize. 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 and/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. Liquid refrigerant in the compressor’s crankcase will boil off. This will flash the oil and cause compressor damage.

    It is the boiling of refrigerant in the crankcase that makes the crankcase cold to the touch. The crankcase may even sweat or frost if conditions are right.

    High-to-normal amp draw: Since droplets of liquid refrigerant will be entrained with the suction vapors, 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 from the compressor and the amp draw may be a bit high, depending on the severity of the flooding or slugging.

    The use of the system checklist is a sure way to find the real problem. Its use can separate the real service technician from a parts changer.

    Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and author of the book Troublshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by Business News Publishing Co. TO order, call 800-837-1037.

    Publication Date: 10/02/2000

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