Thermal Expansion Valve - General Description

The thermal expansion valve is installed on the evaporator inlet and outlet pipes. The expansion valve converts the high-pressure liquid refrigerant from the receiver drier to a low-pressure liquid refrigerant by forcing it through a small port before entering the evaporator.

When the heat load increases or decreases, the expansion valve will supply the correct quantity of refrigerant to the evaporator for maximum heat transfer.

The expansion valve consists of the block and body.

The body includes a temperature sensor for sensing the temperature of vapor. A diaphragm transmits changes in pressure inside the temperature sensor bulb stem which is connected to the diaphragm.

The stem is tapered and moves in response to changes in the pressure inside the temperature sensor bulb. This controls the flow of refrigerant by changing the size of the valve opening.

The body has an equalizing pressure port through which the vapor pressure of the evaporator acts on the bottom of the diaphragm.

When the evaporator heat load increases, the stem is pushed down and the flow of refrigerant to the evaporator increases. As a result, the vapor pressure and temperature of the refrigerant on the evaporator outlet side changes. When the evaporator heat load decreases, the situation is reversed.