Liquid slugging
Also known as: liquid slug, liquid floodback, wet operation
Short definitionThe compressor swallowing liquid refrigerant or oil instead of gas. Liquid does not compress, so valves, connecting rods and scroll spirals break; it is among the most expensive compressor failures.
A compressor compresses vapour only. If liquid fills the cylinder or the scroll pocket, pressure spikes instantly and mechanical parts break. In the milder form liquid returns continuously but in small amounts; nothing breaks, but the oil is washed out and thinned, the bearings run dry and the compressor wears and seizes within a few weeks.
The liquid usually comes from an oversized or wide-open expansion valve, an iced evaporator, the end of a defrost, or refrigerant migrating to a cold crankcase during a winter standstill. The design measures are a suction accumulator, a crankcase heater, pump-down and a suction line sloped towards the compressor. Superheat below 5 K at the compressor inlet means danger; the healthy range is 10–20 K.