The Professor: Restricted Airflow Over an Evaporator

May 11, 2009
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There are a number of ways that the airflow over an evaporator can become restricted. These include:

• Frosted evaporator coil from a 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 coil from low heat load on the evaporator;

• Dirty evaporator coil;

• Defrost intervals set too far apart;

• Defrost heater malfunction causing frost; and

• Defrost time clock malfunction.

Anytime the evaporator coil sees reduced airflow across its face, there will be a reduced heat load on the coil. No airflow will cause a lot of 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 the crankcase, causing oil foaming and diluting the oil. Liquid refrigerant and/or oil slugging can also occur in the compressor’s cylinders from this phenomenon.

A lot of times, 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 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 check list and run the system through its modes after changing the compressor, the new compressor is sure to fail from 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 the technician to examine the broken-down 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.

As mentioned before, an open defrost heater 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 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.

A CHECKLIST

The checklist that follows is for a low-temperature, R-134a refrigeration system that has a receiver and a TXV for its metering device.

RESTRICTED AIRFLOW OVER EVAPORATOR
(Measured values °F):
Compressor Discharge Temp.: 88
Condenser Outlet Temp.: 82
Evaporator Outlet Temp.: -8
Compressor in Temp.: -8
Ambient Temp.: 75
Box Temp.: 25
Compressor Volts: 230
Compressor Amps: Bit High
Lowside (evaporating) pressure (psig): 2.8 (-8°)
Highside (condensing) pressure (psig): 104 (90°)

(Calculated values °F):
Condenser Split: 15
Condensing Subcooling: 8
Evaporator Superheat: 0
Compressor Superheat: 0

SYMPTOMS

Some of the symptoms for reduced air over an evaporator coil are:

• 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.

Here is a look at those symptoms in more detail.

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 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 split is the difference between the condensing temperature and the surrounding ambient around the condenser. 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 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 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 amount of liquid refrigerant flooding or slugging the compressor.

Publication date: 05/11/2009

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