The evaporator is one of the major components of any refrigeration or air conditioning system. The evaporator is a heat exchanger, and heat gains from the product load inside the refrigerated space and from the outside ambient travel through the sidewalls of the evaporator’s tubing. These heat gains will vaporize any liquid refrigerant traveling inside the evaporator coil. The temperature difference between the lower pressure refrigerant inside the evaporator and the product load inside the refrigerated space is the driving potential for heat transfer to take place.

When the evaporator’s coil temperature drops below the dew point temperature, dew will begin to collect on the evaporator’s cool surface. If the temperature of the evaporator coil continues to drop below the freezing point of water, the moisture on the evaporator’s coil will begin to freeze into a thin layer of solid ice. The first thin layer of ice on the evaporator coil is very hard and will actually enhance the evaporator’s efficiency. Not only is this first layer of solid ice a good conductor of heat, but it will also increase the surface area of the evaporator tubes. Because of this increase in tube surface area, the heat transfer into the evaporator’s coil will be increased. Once the evaporator tubes have an increased surface area, the area for airflow around the tubes will be reduced. This will cause a slight increase in the air’s velocity through the coil, which will also increase heat transfer from the refrigerated space to the evaporator.

Depending on humidity, evaporator temperature, and airflow, the subsequent layers of frost that accumulate on the evaporator will be much more porous. These layers of frost will hold more entrained air, insulating the evaporator and severely reducing its heat-absorbing ability. As this type of frost accumulates, the evaporator will see a reduced heat load from the refrigerated space, which will cause the evaporator or suction pressure to drop and the evaporator coil to become colder. Higher compression ratios cause lower compressor volumetric efficiencies, reduced mass flow rates of refrigerant, and loss of system capacity. A result of this insulating effect is that the temperature of the evaporator coil will be “reduced” to maintain the same desired refrigerated space temperature. The temperature difference between the evaporator coil and the refrigerated space will then become greater.

Reduced Airflow

Frost on an evaporator coil will 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. The reduced 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, and compressor damage will soon occur from flooding and/or slugging.

Listed below are ways the airflow over the evaporator can become restricted from frost accumulation:

• Defrost component malfunction;
• Reduced heat load across the evaporator coil;
• High humidity situation in refrigerated space;
• Evaporator fan malfunction;
• Dirty evaporator coil;
• Defrost intervals set too far apart;
• Faulty or open defrost heater; and
• Faulty defrost timer.

Compressor damage occurs from flooding and/or slugging because compressors are designed to pump refrigerant vapors, not refrigerant liquids. Often, service technicians 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. The cause could have been a faulty time clock or an open defrost heater not letting the system defrost. This would frost the evaporator coil, causing flooding or slugging of the compressor. This in turn probably caused the broken internal parts. If the technician did not analyze a system check list and operate the system through its modes after changing the compressor, the new compressor is sure to fail. 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 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, an open defrost heater could be the cause. If the system is not put through the defrost mode or systematically checked with an ohmmeter and voltmeter, the real problem of an open defrost heater will never be found and the replacement compressor will soon fail.

Here are the symptoms for an evaporator coil with major frost accumulation:

• Low compressor discharge temperatures;
• Low head (condensing) pressures;
• Low condenser splits;
• Low to normal suction (evaporator) pressures;
• Low superheats;
• Cold compressor crankcase; and
• High to normal amp draw.

Below is a system check for an R-134a system with an evaporator that has major frost accumulation.

Measured values:

Calculated values:

Analysis:

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 compressor discharge temperature. Discharge temperatures could even be cooler than the condensing temperature, which would be a sure sign of liquid being vaporized by the compression stroke. In other words, wet compression is taking place, and if it 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.

The restricted airflow over the evaporator coil will cause the refrigerant in the evaporator to see a reduced heat load, thus not be completely vaporized. With reduced heat load to be rejected in the condenser, the condensing pressure and temperature do not have to elevate to reject heat to the ambient, resulting in low condensing pressures.

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

Because of the reduced heat load entering 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, thus, the evaporator pressure will be lower.

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 will likely lose control. Compressors can slug and/or flood in these situations.

Since the compressor (total) superheat is low, sometime the compressor will flood or slug during the on-cycle, and there will be liquid refrigerant in the compressor's crankcase boiling off. This will flash the oil and 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.

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 the liquid form, which will require more work from the compressor. The amp draw may be a bit high depending on the severity of the flooding or slugging.

It is for the above reasons that a service technician must complete a system check sheet and systematically analyze and troubleshoot any system that has a frosted evaporator.