Liquid is nearly incompressible, so a slug can break valves, bend rods or crack scrolls, and the compressor knocks loudly when it happens. It usually follows refrigerant migration into the crankcase during off cycles, heavy floodback, or oil logging in the evaporator. Crankcase heaters, accumulators, correct charge and proper airflow are the defenses.
Hear that knock on startup? It's slugging, the crankcase heater is open and the oil is full of refrigerant.
Also heard as
- liquid slugging
- slug
Do not mix it up with
- Floodback: Floodback is a steady return of liquid while running; slugging is a sudden mass of liquid hitting the compressor, often at startup.
Related terms
Floodback
Liquid refrigerant returning continuously to the compressor through the suction line while the system runs.
Accumulator
A tank in the suction line that catches liquid refrigerant before it can reach and damage the compressor.
Compressor
The pump that raises refrigerant vapor from evaporator pressure to condenser pressure and keeps it circulating.
Superheat
How many degrees refrigerant vapor is above its boiling point at the pressure it is at.