A refrigeration engineer notes excessive cycling of the temperature control on a reciprocating compressor unit. The crankcase temperature is high, the discharge temperature is low, and the suction line temperature is high. Which fault is most likely causing these conditions?

• A. Faulty compressor valves
• B. Low oil level
• C. Overcharged refrigerant
• D. Faulty thermostat switch

Answer: (B)

The key idea is that oil level in a reciprocating compressor isn’t just about lubrication—it also governs heat removal from the machine. When oil is low, the moving parts don’t get enough lubrication and the oil film that carries heat away from the compression process is diminished. That leads to overheating of the compressor’s crankcase because more friction and less heat transfer are happening inside.

With excessive temperature cycling, the control system is trying to protect the unit from this overheating, so it short-cycles as the crankcase overheats and then cools. The crankcase getting hot is the most direct clue that lubrication and heat management are failing.

The suction line temperature rising can be explained by heat being conducted from the overheated crankcase toward the suction side, or by the compressor operating under poor lubrication and inefficiency, which allows more heat to be transferred into the refrigerant as it returns to the evaporator. The discharge gas temperature being lower than expected fits with reduced lubrication and poorer heat transfer in the compression chamber, so the gas doesn’t heat up as much on discharge as it would with proper lubrication and cooling.

Other faults would typically show different patterns—for example, valve problems or an overcharged refrigerant usually push discharge temperature higher rather than lower, and a faulty thermostat switch would alter cycling behavior without the same crankcase overheating pattern.