The thermal expansion valve (TEV or TXV), as shown in picture below, it’s used for refrigerant flow control and operates at varying pressures resulting from varying temperatures. This valve maintains constant superheat in the ac evaporator.
How does thermal expansion valve works?
The thermostatic expansion valve needs capillary tube and thermal element (bulb) to work. The capillary tube connects the element to the top of TXV diaphragm.
Here is what it looks like:
The element (sensing bulb or thermal bulb) is partly filled with a liquid refrigerant (usually the same refrigerant used in the system) and maintains some liquid under all conditions of temperature and load.
A cross section of a thermostatic expansion valve components and the operation principle are showing below.
The three forces which control the operation of the thermal expansion valve are:
P1 – The vapor pressure of the thermostatic element (a reaction to the bulb temperature) which acts to open the valve.
P2 – The evaporator pressure which acts in a closing direction below the diaphragm.
P3 – The pressure equivalent of the superheat spring force which is also applied underneath the diaphragm in a closing force.
At any constant operating condition, these forces are balance. Therefore, P1=P2+P3
When a change in temperature in the suction line occurs, the pressure in the thermal bulb also changes similarly. With an increased heat load, the refrigerant in the evaporator coil boils fast.
This results in a rise in temperature at the thermal bulb because of superheating. The higher temperature produces an increase in pressure within the thermal bulb due to superheating.
The higher temperature produces an increase in pressure within the thermal bulb which increases the pressure at P1. The pressure in the evaporator at P2, and the spring pressure in the TEV at P3 remains constant.
Therefore, with the increased pressure at P1, the bellows (diaphragm) expands to force a wider valve opening. As a result, more refrigerant is allowed to enter the evaporator to compensate for the increased heat load.
The increase in flow rate increases to evaporator pressure P2, which establishes a balance control once again. With a decreased load the reverse cycle takes place.
Thermostatic Expansion Valve Capacity
The capacity of a TXV differs due to orifice size, pressure difference between high and low side, and the temperature and condition of the refrigerant in the liquid line.
The amount of liquid that will flash to a vapor will increase with a rise in liquid line temperature. The capacity of most valves, however, maybe determined by the orifice size and needle assembly. The body size remains the same for many capacities.
Thermostatic expansion valves are rated in ton of refrigeration. However, three different tonnage capacities are usually provided for the same orifice. The range of capacity depends on the difference in pressure between the high and the low side of the system.
Using the appropriate capacity valve is very important. If the valve orifice is too small, the evaporator will be starved regardless of the superheat settling. The full capacity of the evaporator will never be obtained.
If the valve orifice is oversized, too much refrigerant will pass into the evaporator and the suction line will sweat or frost before the thermal element can close the valve.
Thermal expansion valve training video
Make sure to press the Play button in the player controls to watch it. Enjoy!
