Thermostatic expansion valve (TXV) (Air conditioning)

Thermostatic expansion valve (TXV)

The thermostatic expansion valve meters the flow of refrigerant into the evaporator to match the refrigerant flow with the heat load of the air passing through the evaporator matrix.

The thermostatic expansion valve is attached to the evaporator, in the cooling unit behind the passenger’s side of the fascia. The valve is the parallel charge type, consisting of an aluminium housing containing inlet and outlet passages. A ball and spring metering valve is installed in the inlet passage and a temperature sensor is installed in the outlet passage. The temperature sensor consists of a push rod connected to a diaphragm. The bottom end of the push rod acts on the ball of the metering valve. Pressure on top of the diaphragm is controlled by evaporator outlet temperature conducted through the push rod. The bottom of the diaphragm senses evaporator outlet pressure via internal passages.

Liquid refrigerant flows through the metering valve into the evaporator. The restriction across the metering valve reduces the pressure and temperature of the refrigerant. The restriction also changes the solid stream of refrigerant into a fine spray, to improve the evaporation process. As the refrigerant passes through the evaporator, it absorbs heat from the air flowing through the evaporator matrix. The increase in temperature causes the refrigerant to vapourize and increase in pressure.

The temperature and pressure of the refrigerant leaving the evaporator are sensed by the temperature sensor, which regulates the metering valve opening to control the volume of refrigerant flowing through the evaporator. The warmer the air flowing through the evaporator matrix, the more heat available to evaporate refrigerant and thus the greater the volume of refrigerant allowed through the metering valve.

Evaporator

The evaporator absorbs heat from the exterior or recirculated air being supplied to the heater assembly.

The fin and plate, brazed aluminium evaporator is installed in the cooling unit behind the passenger’s side of the fascia. Inlet and outlet connections on the evaporator are attached to the thermostatic expansion valve.

Low pressure, low temperature refrigerant changes from liquid to vapour in the evaporator, absorbing large quantities of heat as it changes state.

Refrigerant lines

To maintain similar flow velocities around the system, the diameter of the refrigerant lines varies to suit the two pressure/temperature regimes. The larger diameters are installed in the low pressure/temperature regime and the smaller diameters are installed in the high pressure/temperature regime. Low and high pressure charging connections are incorporated into the refrigerant lines for system servicing.