Expansion Valve: Description and Operation

The thermostatic expansion valve (TXV) "bleeds" regulated amounts of liquid refrigerant into the evaporator, thus, changing the state of the refrigerant from a liquid to a gas. The refrigerant absorbs the heat from the passenger compartment air causing it to evaporate into a low pressure vapor before returning to the compressor.

The TXV has a shaft that moves a ball valve from it's seat to control refrigerant flow. As the ball is unseated, more refrigerant will flow through the evaporator assisting in meeting the demand of the A/C system.

A power dome with two separate sides controls the shaft movement. The sealed thermal side of the power dome has a refrigerant charge. 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 shaft moves down and pushes the ball off of the 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 refrigerant flow is reduced. There is a non-adjustable spring at the bottom of the TXV to help re-seat the ball.

Although the TXV controls the evaporator refrigerant flow, variations in the compressor piston stroke control the A/C system capacity.