910 Freeport Parkway, Suite 100 Coppell, TX 75019
+ (877) 342-5526

Model Specific Air Conditioning
1930's to 1980's Cars & Trucks
Since 1977

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Thermal Expansion Valves

In the following text we will explain Thermal Expansion Valves and their function. Refrigerant flow to the Classic Auto Air evaporator must be controlled to obtain maximum cooling. While ensuring that complete evaporation of the liquid refrigerant takes place. This is accomplished by the thermal expansion valve (TXV).

Pressures in control

As shown in the illustration, TXV controls the refrigerant flow by using a system of opposing pressures which will call:

F1- Temperature sensing capillary tube

Sealed tube fills with refrigerant. This refrigerant is also filled above the diaphragm (7). The capillary tube sensing bulb (3) is attached to the evaporator outlet tube surface.

F2- Pressure compensation tube

This is a hollow tube connected to the evaporator outlet tube and senses the pressure on the R134a refrigerant leaving the evaporator coil. (Other TX valves may not use this tube as pressure is provided internally within the valve).

F3- Pressure spring

This spring (6) is located under the ball valve (5).



When the evaporator outlet tube temperature increases, the refrigerant (3) in the capillary tube expands, forcing the diaphragm (7) downloads and this pushing pin (A) also downwards causing the ball valve (5) to move away from the metering orifice (4), allowing more R134a to enter the evaporator inlet side.


As the evaporator outlet tube becomes cooler. The refrigerant in the capillary tube (3) contracts. Forces F2 and F3 cause the diaphragm
(7) and pin (A) to move upward allowing the ball valve to move towards the metering orifice (4), restricting the R134a flow. The outlet tube gets warmer and the process starts over.

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