Expansion Valve: Description and Operation

The thermostatic expansion valve (TXV) regulates the flow of refrigerant through the evaporator in order to optimize the A/C system cooling performance. This ensures that all liquid is evaporated before the liquid is returned to the compressor.

The movement of a shaft that pushes a ball from a seat controls the refrigerant flow. As the ball is unseated, more refrigerant flow is allowed through the evaporator. A power dome with two separate sides controls the shaft movement. The sealed thermal side of the power dome has a refrigerant charge. The system side of the power dome is exposed to the pressure of the evaporator outlet or low side. The refrigerant exiting the evaporator flows over part of the thermal side. The sealed refrigerant in the thermal side responds to the temperature changes of the refrigerant flowing out of the evaporator. As the refrigerant temperature rises, the pressure of the thermal side increases.

If the thermal side pressure is greater than the system side pressure (the evaporator outlet or low side pressure), the shaft moves down and pushes the ball off of a seat, allowing more refrigerant flow through the evaporator. Likewise, when the system side pressure is greater, the shaft moves back up and re-seats the ball, and the refrigerant flow is reduced. There is a non-adjustable spring at the bottom of the TXV.

Although the TXV controls the evaporator refrigerant flow, variations in the compressor piston stroke actually control the A/C capacity in order to prevent ice build-up.