Replacing a compressor is rarely a simple or inexpensive task. As a technician, you might be able to prevent some premature compressor failures by spending some additional time on a repair or maintenance inspection. Performing a more detailed inspection of a compressor might reveal a system issue that could lead to a failed compressor. This will, of course, require more than just reading its suction and discharge pressures.
If a compressor has an oil level sight glass, make sure the oil level is correct. An oil level of approximately half of the sight glass is generally acceptable, but always consult with the system or compressor manufacturer for their required levels. Also on startup, view the oil sight glass and look for abnormal foaming of the oil. This generally indicates liquid refrigerant mixed with oil, which could indicate a refrigerant migration issue during the off cycle. Unfortunately, not all compressors have an oil level sight glass, but those that do should be observed.
A common cause of premature compressor failure is overheating. A compressor that is running too hot will surely fail before its time. You should always look for this potential problem while servicing any refrigeration system. Measuring the temperature of the discharge line approximately 6 inches from the outlet of the compressor is a good place to check and see if a compressor is running too hot. For example, a discharge line temperature below 225°F is preferable for reciprocating compressors. A discharge line temperature between 225°F and 275°F is acceptable with some caution, but it should not go above 275°F.
Measuring the returning refrigerant’s superheat value is another valuable inspection point. It is very important to ensure that liquid refrigerant is not returning to the compressor, because that will surely cause it to fail. A superheat level that is too low can also be a concern. An extremely low superheat value can change to a no superheat value if the load on the system changes.
Measuring the amperage draw of a compressor is another good inspection point. A compressor that is not consuming the correct amperage is generally not pumping properly. Unfortunately, many times this means the damage is already done — but not always. Single-phase compressors with a weakened or failed run capacitor will not consume the correct amperage. The amperage draw of a compressor is not fixed — it varies based on the compressor’s operating suction and discharge pressure. You will need to determine the correct amperage for a compressor by referencing its performance chart. When measuring the amperage draw of a compressor, make sure to read only the amperage drawn by the compressor. Choose a location on the wiring diagram that will isolate the compressor from any other loads such as the condensing fan motor. It is important to measure amperage draw of the compressor only when trying to determine if it is operating properly.
Taking the extra time to measure some of these additional parameters can help prevent a compressor failure and save your customer from an expensive repair.