Pump-down
Also known as: pump down, pump-down cycle
Short definitionWhen the room reaches temperature, the solenoid valve in the liquid line closes, the compressor collects the refrigerant from the evaporator in the receiver and stops on the low-pressure switch. It protects the compressor from drawing in liquid at start-up.
If the compressor is stopped directly by the thermostat, liquid stays in the evaporator. During the off period this liquid drains into the crankcase, mixes with the oil and wears the valves, pistons and bearings at the next start. In a pump-down arrangement the thermostat closes the solenoid valve, not the compressor. The compressor keeps running, empties the evaporator and stores the refrigerant as liquid in the receiver. When suction pressure falls, the pressure switch stops the compressor. When cooling is needed, the solenoid opens, pressure rises and the compressor starts on dry suction.
Three things are checked when choosing a unit: a solenoid valve in the liquid line, a receiver large enough to hold the whole charge and a low-pressure switch set for pump-down. A one-time pump-down control is used so the compressor does not restart again and again during long off periods. An electronic expansion valve closes at standstill, so no separate solenoid is needed. The same arrangement helps in service; while an evaporator is replaced, the refrigerant is held in the receiver and no gas is released to the atmosphere.