The electric expansion valve uses a thermistor to detect the presence of liquid refrigerant at the evaporator outlet.
When there is no liquid, the temperature of the thermistor rises, which reduces its electrical resistance, allowing a greater current through the heater installed in the valve. The valve is thus opened, allowing a greater flow of refrigerant.
One of the applications of the electric expansion valve is in heat pumps, where the refrigerant flow is reversed when changing from cooling to heating. Since the control is independent of the refrigerant pressures, the valve can operate in either direction.
The refrigerant saturation fixation point is controlled by the location of the thermistor and can be moved from one point to another by using more than one thermistor, which can be turned on or off as required. The thermistor can be used to control the liquid level in a suction accumulator or current transmitter, to ensure control of the evaporator by flooding or semi-flooding it with dry suction gas that returns to the compressor.
