A reciprocating compressor is making a knocking sound during operation. If worn parts are not the cause, what is probably the reason?

• A. The slugging of the liquid refrigerant
• B. Worn crankshaft bearings
• C. A misadjusted valve
• D. Low oil level

Answer: (A)

Slugging of the liquid refrigerant is the most likely cause. Reciprocating compressors are designed to compress gas, not liquid. When liquid refrigerant makes it into the cylinder, the liquid slug can’t be compressed and instead slams against the piston and valves, producing a knocking or hammering sound. This often happens when there’s too much liquid in the suction side or the evaporator isn’t vaporizing all the refrigerant—common issues include an overcharged system, too low evaporator superheat, or a faulty metering device that allows liquid to reach the compressor. If worn parts aren’t the cause, correcting the refrigerant charge, ensuring proper superheat, and verifying the metering device and oil return will help prevent the knocking by keeping vapor in the compressor.