A solenoid valve opens when current reaches its coil and the magnetic field lifts the plunger, and closes by spring when the current is cut. In refrigeration it is used most often in the liquid line, before the expansion valve. When the room thermostat reaches the set temperature it closes the solenoid; refrigerant flow to the evaporator is cut off.
This simple shut-off is the basis of the "pump-down" arrangement: when the solenoid closes, the compressor draws the remaining refrigerant out of the evaporator and collects it in the receiver, and the low-pressure switch stops the compressor. So no liquid accumulates in the evaporator during standstill and no liquid reaches the compressor on restart; compressor life is extended. In systems where one unit cools several rooms, every room has its own solenoid; refrigerant goes to whichever room calls for it.
The solenoid valve is fitted according to flow direction (the arrow on the body), fed at the correct voltage to avoid coil burn-out, and a high-temperature type is chosen for hot gas defrost lines. Coil failure, dirt getting into the plunger and wrong orientation are the most frequent problems.