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Determining that a compressor has failed on a refrigeration system is normally a relatively simple procedure. But, discovering why the original compressor failed may not be as simple. Any time a compressor is replaced, the root cause of the problem should be identified. Not identifying why it failed can lead to multiple compressor change-outs on the same system. This is not a good business practice for any contractor as it can lead to lost revenue and a very unhappy customer.
There are several common system problems that can lead to a failed compressor, one of which is refrigerant floodback. This occurs when liquid refrigerant flows through the suction line into the compressor during the running cycle. This can cause compressor damage.
The liquid refrigerant will mix with the compressor’s lubricating oil diluting its lubricating properties and cause the moving parts of the compressor to wear, seize, or break. On compressors where the returning refrigerant directly enters the compression chamber, if a sufficient amount of liquid returns, not all of the liquid will vaporize within the compression chamber and the compressor will attempt to compress the remaining liquid, which will lead to severe compressor damage.
Refrigerant floodback commonly occurs when the system loses control of the refrigerant entering the evaporator. This can be the result of low evaporator loading or sudden changes to the evaporator load. Low evaporating loading can be the result of:
• Excessive buildup of ice, frost, or dirt on an evaporator coil;
• Restricted airflow from poor product placement within a case;
• Low airflow caused by a defective evaporator fan motor(s); or
• The use of oversized equipment for the required load.
On equipment using a capillary tube metering device, floodback can occur when an excessive amount of refrigerant is added to the system. However, this is not likely the cause on equipment using an expansion valve. If the valve is working properly it should still control the amount of refrigerant entering the evaporator. But, on these systems, if the expansion valve is defective or misadjusted it could definitely result in refrigerant floodback.
Part of inspecting a system after replacing a failed compressor should always include looking at the refrigerant’s superheat value leaving the evaporator. If this is overlooked, liquid refrigerant returning to the compressor may not be identified and the true problem not discovered. What is the correct superheat value of the refrigerant leaving an evaporator? It is based on the system’s application and the manufacturer’s design. As a rule of thumb, medium-temperature systems will generally have a superheat of 8-10°F and low-temperature applications, 4-6°. But again, always check with the system’s manufacturer for the recommended values.
Sometimes a floodback issue is not an event on startup. The evaporator load is normally high and the refrigerant is sufficiently superheated leaving the evaporator. However, after some runtime and the load of the evaporator is lessened, floodback may occur. So it may be wise to also check the system when the case has reached its design temperature or when the load is low on the system.
Remember, not identifying why a compressor has failed will not only take away from the contractor’s profit margin, but it will also tarnish the company’s professional image. Finding the root cause and repairing it not only keeps the customer happy, it also keeps a contractor profitable.
Publication date: 5/6/2013