The Professor: Foiling Frosted Evaporator Coils
Reduced airflow over the coil can cause a multitude of woes
Frost on an evaporator coil will prevent the correct amount of airflow across the coil. Any time an 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.
Here are some of the ways airflow over the evaporator can become restricted:
• Frosted evaporator coil from bad defrost heater;
• Frosted evaporator coil from high humidity;
• Frosted evaporator coil from evaporator fan out;
• Frosted evaporator coil from defrost component malfunctions;
• Frosted evaporator from no load on the evaporator coil;
• Dirty evaporator coil; and
• Defrost intervals set too far apart.
Figure No. 1 shows a frosted evaporator coil in a walk-in cooler blocking a lot of airflow from entering the coil.
A COMMON CULPRIT: FROST
Frost is the most common cause of inefficiency in evaporator coils. When the evaporator’s coil temperature drops below the dew point temperature, dew will begin to collect on the evaporator’s cool surface. (The dew point temperature is the temperature where dew starts to form.) 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 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 around the evaporator coil a good conductor of heat, it also will increase the surface area of the evaporator tubes. Thus, heat transferred into the evaporator’s coil will be increased. Now that 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 to the evaporator.
Depending on humidity, evaporator temperature, and airflow, the subsequent layers of frost that accumulate on the evaporator may be more crystalline or snow-like. These layers of frost will hold more entrained air and will be very porous. This type of frost — often referred to as radiant frost or hoar frost — will insulate the evaporator and severely reduce its heat-absorbing ability.
As the hoar frost accumulates, the evaporator will see less heat from the refrigerated space. This will cause the evaporator or suction pressure to drop, and the evaporator coil will become colder. Higher compression ratios cause lower compressor volumetric efficiencies, less dense refrigerant return gases entering the compressor, reduced mass flow rates of refrigerant, and loss of system capacity. As a result of this insulating effect, the temperature of the evaporator coil must be reduced to maintain the same desired refrigerated space temperature. Now, the temperature difference (TD) between the evaporator coil and the refrigerated space will be greater.
In many cases, technicians will change out a compressor because of broken internal parts and not find the actual cause of the problem. Broken compressor parts are 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 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 from the same causes.
Compressor manufacturers are asking technicians to examine broken 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.
Technicians should then make a list of the causes that could be blamed for this failure and, once the system is up and running, eliminate them, one by one. As mentioned before, an open defrost heater could be the cause. If the system is not test-run and 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. It is suggested that causes and symptoms be listed and system checklists be made when systematically troubleshooting systems. Shown above is a system check for an R-134a system with an evaporator that has major frost accumulation.
SYMPTOMS OF REDUCED AIRFLOW
• Low discharge temperatures;
• Low head pressures;
• Low condenser splits;
• Low-to-normal 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 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 strip and discharge valves may ruin due to 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, it will 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 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 lower 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 percent saturated vapor point in the evaporator will crawl down past the end of the evaporator, and the thermostatic expansion valve (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. 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 — 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. 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.
Publication date: 8/3/2015