A compressor is designed to compress vapour. When liquid enters the cylinder or the scroll pocket there is no volume left to compress into and the pressure rises very high in an instant: the valve plates break, the gasket blows, the connecting rod bends, the scroll wrap cracks. This is called liquid slugging. Its milder form, continuous liquid floodback (wet running), does not break things at once but washes out and dilutes the oil; the bearings are left without oil and within weeks the compressor wears and seizes.
The main ways in which liquid reaches the compressor are these. While running: an expansion valve that is too far open or oversized, a loose valve bulb, an iced-up evaporator, a stopped evaporator fan, overcharge. At the end of defrost: the liquid that has condensed in the evaporator being drawn in all at once. At standstill: the refrigerant migrating to the coldest point of the system, on winter days the crankcase of the compressor outdoors, and at the first start foaming together with the oil and being drawn into the cylinders. Liquid that collects in the oil traps in the suction line can have the same effect.
The key to diagnosis is superheat. If the superheat measured at the compressor inlet is below 5 K there is a risk of liquid floodback; the healthy value is usually 10–20 K. Other signs: a metallic knocking noise at start-up, sweating or frosting of the crankcase and compressor body, oil foaming in the sight glass, low discharge temperature. In a dismantled compressor, broken valves and washed-clean, shiny cylinder surfaces are the signature of liquid slugging. The measurement should be repeated after the system has stabilised and under different load conditions.
Protection is a combination of several measures: a correctly sized and adjusted expansion valve, a suction accumulator in the suction line, a crankcase heater that keeps the crankcase warm at standstill, stopping by pump-down, and a suction line sloping from the evaporator towards the compressor. After a long standstill the crankcase heater is switched on at least 8–12 hours beforehand. Replacing a failed compressor without finding the cause leads the new one to the same end; first it is established where the liquid is coming from. Scroll compressors are more tolerant of a small amount of liquid, but they too have their limits.