The expansion valve separates the two pressure regions of the cycle: as liquid refrigerant from the high-pressure side passes through the valve’s small opening, its pressure and temperature fall, it partly evaporates and it enters the evaporator as a cold liquid–vapour mixture. Without the valve the evaporator would either flood with liquid (liquid goes to the compressor) or be starved of refrigerant (capacity falls).
The thermostatic expansion valve (TXV) measures the temperature with a sensing bulb clamped to the pipe at the evaporator outlet and opens and closes just enough to hold the superheat constant (usually 5–8 K). As load rises it feeds more refrigerant, as it falls it throttles back; it is mechanical, needs no electricity and is reliable. There are internally and externally equalised models and separate orifices for each refrigerant.
A wrongly selected or wrongly adjusted valve is the most frequent source of faults: too large a valve hunts (opens and closes continuously), too small a valve cuts capacity; if the bulb is clamped in the wrong place the superheat is read wrongly. The service technician adjusts the valve by measuring superheat. The electronic expansion valve (EEV) does the same job with a stepper motor and a controller, offering a more precise and efficient alternative.