The thermostatic expansion valve (TXV) is located at the evaporator inlet.

The TXV divides the high and the low pressure sides of the A/C system. Changing the refrigerant pressure, by creating a pressure differential, from high to low allows the refrigerant to absorb heat. The TXV also regulates the amount of liquid refrigerant that will flow into the evaporator.

The TXV is controlled by two separate forces:

The low side system pressure operates directly on the TXV.

A refrigerant charged sensing bulb or capillary tube responds to the temperature of the refrigerant flowing out of the evaporator. As the refrigerant temperature rises, the pressure inside of the capillary tube increases and forces the TXV to open.

Internally, the TXV has a shaft that moves a ball valve, against spring pressure, from the ball valve seat to control refrigerant flow. As the ball valve is unseated, more refrigerant will flow through the evaporator to meet the demand of the A/C system.

When the capillary tube pressure is greater than the low side system pressure, the shaft in the TXV moves down and pushes the ball valve off of the seat, allowing more refrigerant flow through the evaporator. When the capillary tube pressure becomes less than the low side system pressure, the shaft moves back up and re-seats the ball valve and refrigerant flow is reduced.

Although the TXV controls the refrigerant flow to the evaporator, variations in the compressor displacement control the capacity of the A/C system.