Air conditioning thermal expansion valves TXV
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TXV - Thermal Expansion Valves
Refrigerant flow to the 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.
The TXV controls the refrigerant flow by using a system of opposing pressures which will call:
Temperature sensing capillary tube.
Sealed tube filled with refrigerant. This refrigerant is also filled above the diaphragm. The capillary tube sensing bulb is attached to the evaporator outlet tube surface.
Pressure compensation tube.
This is a hollow tube connected to the evaporator outlet tube and senses the pressure of the R134a refrigerant leaving the evaporator coil. (Other TX valves may not use this tube as pressure is provided internally within the valve).
Pressure spring.
This spring is located under the ball valve.
Operation Open.
When the evaporator outlet tube temperature increases, the refrigerant in the capillary tube , forcing the diaphragm downwards and thus pushing pin also downwards causing the ball valve to move away from the metering orifice, allowing more R134a to enter the evaporator inlet side.
Operation Closed.
As the evaporator outlet tube becomes cooler, the refrigerant in the capillary tube. Forces F2 and F3 cause the diaphragm and pin to move upward allowing the ball valve to move towards the metering orifice, restricting the R134a flow. The outlet tube gets warmer and the process starts over.
TXV - Thermal Expansion Block Valves
The block valve differs from the previously mentioned expansion valve in that it has four passages, although the basic operation is exactly the same.
Operation of the block valve is still via refrigerant expansion/contraction within a diaphragm.