If the compressor is stopped directly by the thermostat, liquid refrigerant remains in the evaporator and suction line; during standstill this liquid flows into the compressor crankcase, mixes with the oil and on restart causes "liquid slugging" that wears the valves, pistons and bearings. Pump-down prevents this: the thermostat closes the solenoid valve rather than stopping the compressor; the compressor keeps running, draws the refrigerant out of the evaporator, discharges it to the condenser and collects it as liquid in the receiver; when the suction pressure falls, the low-pressure switch stops the compressor.
When cooling is needed again, the thermostat opens the solenoid, liquid fills the evaporator, suction pressure rises and the pressure switch starts the compressor. So the compressor draws dry on every start. Because pump-down also collects the refrigerant in the receiver, it allows service work (evaporator replacement, work on the line) without releasing gas to the atmosphere.
Its requirements: a solenoid valve in the liquid line, a receiver large enough to hold the entire system charge, a low-pressure switch set for pump-down (for example cut-out at 0.5 bar g and cut-in at 2 bar g with R404A) and a "single pump-down" control that stops the compressor restarting repeatedly during long standstills. In systems with an electronic expansion valve no solenoid is needed because the valve closes at standstill. Our condensing units are equipped with the receiver and pressure switch needed for pump-down.