Expansion Valve: Description and Operation

Air Conditioning Thermostatic Expansion Valve

The purpose of the thermostatic expansion valve is to regulate the flow of refrigerant through the evaporator to optimize its cooling performance. This is accomplished by monitoring the refrigerant conditions at the evaporator outlet and adjusting the inlet flow accordingly. This assures that all the liquid is evaporated before it is returned to the compressor.

The refrigerant flow is controlled by the movement of a shaft that pushes a ball from its seat. As the ball is unseated, more flow is allowed through the evaporator.

Shaft movement is controlled by the power dome which has two separate sides. The thermal side of the power dome is sealed and charged with refrigerant. The system side of the power dome is exposed to the pressure of the evaporator outlet or low side. Refrigerant exiting the evaporator flows over part of the thermal side of the power dome. 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 pressure on the thermal side of the power dome is greater than the pressure on the system side (the evaporator outlet or low-side pressure), the shaft moves down and pushes the ball off the seat allowing more refrigerant flow through the evaporator. When the opposite is true, the shaft moves up and re-seats the ball reducing flow. A spring at the bottom of the thermostatic expansion valve is not adjustable.

Although the thermostatic expansion valve controls evaporator refrigerant flow, air conditioning capacity control (to prevent ice build-up) is managed by compressor cycling which is controlled by the powertrain control module (PCM).