Summer is almost here, and many air conditioning systems will be commissioned for start-up in anticipation of the upcoming heat and humidity loads. As part of this commissioning process, service technicians should inspect the evaporator coil and clean it if necessary.

The evaporator, like the condenser, acts as a heat exchanger. Heat and humidity (sensible and latent heat) from the conditioned space travel through the sidewalls of the evaporator coil and vaporize liquid refrigerant flowing through the evaporator. The pressure drop through the metering device causes vaporization of some of the liquid refrigerant in the evaporator, thus causing a lower saturation pressure/temperature in the evaporator. This temperature difference between the lower pressure/temperature refrigerant in the evaporator and the conditioned air traveling through the evaporator is the driving potential for heat transfer to take place.

The last pass of the evaporator coil acts as a superheater to ensure all liquid refrigerant has been vaporized before entering the suction line. This protects the compressor from any liquid refrigerant from entering its crankcase and mixing with the oil, which may result in valve damage or diluted oil in the crankcase.


Important Yet Ignored

The evaporator coil is one of the most important — yet often ignored — major components in a refrigeration or air conditioning system. In residential and small commercial air conditioning, most evaporator coils are out of sight and are rarely inspected or cleaned.

There should always be an air filter before the evaporator coil to protect it from any dirt, lint, cottonwood fuzz, pollen, grease, or other minute particles and/or debris in the airflow entering the evaporator coil. As debris and dirt particles collect on the evaporator coil, the coil’s ability to absorb both sensible and latent heat is diminished. Debris and dirt on an evaporator coil will also prevent the correct amount of airflow across the coil. Anytime the evaporator coil sees reduced airflow across its face, there will be a reduced heat load on the coil, causing a lowered pressure and temperature.

Low airflow and less heat load across the evaporator coil can cause much of the refrigerant in the coil to remain a liquid and not vaporize as fast. Some liquid refrigerant may travel past the evaporator coil through the suction line and eventually get to the compressor. Compressor damage will soon occur from compressor flooding and/or slugging. Always follow the manufacturer’s recommended procedures for cleaning evaporator coils.


Frosted Coils

Dirty evaporator coils can also cause the evaporator to accumulate frost and ice from the lower pressures and temperatures. Frost insulates the evaporator coil from heat loads trying to be absorbed by the vaporizing refrigerant within the evaporator’s coil. As this more porous type of frost accumulates, the evaporator will see less heat being absorbed from the conditioned space. This will cause the evaporator (suction) pressure and thus the evaporator temperature to drop, and the evaporator coil will become colder.

An iced-up evaporator coil can also cause a reduced airflow across the evaporator, which will in turn cause reduced heat loading on the a/c system. So, for practical purposes, a dirty evaporator coil, a frosted evaporator coil, and a plugged air filter before the evaporator will have the same symptoms.

There are several system problems that can reduce the airflow and heat loading across an evaporator coil and cause frost to accumulate. They include an inoperative defrost heater, dirty filters, evaporator fan burned out, not enough heat load on the evaporator coil, dirty evaporator coil, and high humidity situation. For refrigeration applications, defrost intervals set too far apart and/or defrost component malfunctions are also culprits for coil frostings.

The drop in suction pressure from a frosted or dirty coil will cause higher compression ratios, which will cause lower compressor volumetric efficiencies, reduced mass flow rates of refrigerant, and loss of system capacity. Severe cases of evaporator frosting can cause liquid refrigerant to return to the compressor.


Compressor Damage

The compressor is often the major component in the refrigeration system that will suffer from a severely frosted evaporator coil. Many times a service technician will change out a compressor because of broken internal parts and not find the actual cause of the problem. The compressor having broken parts is not the cause of the problem; instead, any of the above mentioned system problems that reduced the airflow through the evaporator coil are the cause of the problem. The reduced airflow would frost the evaporator coil, causing flooding or slugging of the compressor. This frosting of the evaporator coil, in turn, probably caused the broken internal parts in the compressor.

If the technician did not perform a system checklist and run the system through its modes after changing the compressor, the new compressor is sure to fail from the same reasons. Compressor manufacturers are asking technicians to examine the damaged compressors 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 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. It is suggested that causes and symptoms be listed, and system checklists be made when systematically troubleshooting systems.

Some of the symptoms for a dirty or frosted evaporator coil include:

  • Normal condenser subcooling;
  • Low evaporator (suction) pressures/temperatures;
  • Low superheats;
  • Frosted evaporator coil;
  • Low discharge temperatures;
  • Low head pressures;
  • Low condenser splits;
  • Cold, frosted, or sweaty compressor crankcase;
  • High amp draw if flooding is happening; and
  • Dirty air filter.

It is important to reiterate that if a severely frosted coil leads to a damaged compressor, technicians should always determine the cause of the problem before installing a new compressor. Otherwise, the new compressor is likely to fail for the same reasons.