Construction of the Thermostatic Expansion Valve
Construction details of the internally equalized thermostatic expansion valve are shown in the figure to the right. It is comprised of a metallic body which encloses the following parts:
1) Metallic Diaphragm: The metallic diaphragm is a flexible metallic plate that can expand due to pressure of refrigerant inside the refrigeration or air conditioning plant.
2) Power Element: This is the upper portion of the thermostatic expansion valve which is filled with gas. In the case of the air conditioning system, the gas filled in element is the same as the gas filled in the air conditioning system. For instance if the air conditioning system is has R22 refrigerant, the gas filled in the power element is also R22. The power element is connected to the feeler bulb, via thin tubing. The same gas is also filled in the tubing and the feeler bulb. Thus the feeler bulb, the connecting tubing and the power element all form a single flexible chamber.
The feeler bulb is connected to the evaporator and senses the temperature inside the evaporator of the refrigeration system. The volume of the gas inside the power element changes as per the temperature sensed by the feeler bulb inside the evaporator. If the temperature inside the evaporator is high the gas in the feeler bulb will expand; the gas in power element will also expand and its pressure will increase. The gas pressure inside the power element tends to open the thermostatic expansion valve.
3) Valve Seat and Needle: The valve seat and the needle are located in the lower side of the thermostatic expansion valve. The valve seat is the metallic plate that provides passage for the flow of the refrigerant. The needle is connected to the lower part of the diaphragm and it moves inside the opening of the valve seat. When the diaphragm moves down due to high pressure inside the power element the needle also moves down thus opening the thermostatic expansion valve and when the needle moves up the valve closes. The valve seat and the needle form the orifice that allows the flow of the refrigerant through it.
4) Spring: The spring is located at the bottom of the thermostatic expansion valve. It is under compression and tends to move the needle of the valve in an upward direction and close the valve. The pressure of the spring is adjusted by the manufacturer and it depends on the degree of superheat in the evaporator. You should purchase the TEV of the required spring pressure. Though there is a screw for changing the spring pressure, it is preferrable not to change its setting unless you are sure that the plant is working satisfactorily.
5) Liquid Inlet Port and Outlet to Evaporator: The liquid inlet port is connected to the tubing coming from the condenser. The refrigerant enters the thermostatic expansion valve via this port. After passing through the orifice of the valve seat and the needle the refrigerant leaves to the evaporator in flashed conditioned via outlet to the evaporator.
7) All refrigeration systems require equalization lines. For more information, see this source: https://www.swtc.edu/ag_power/air_conditioning/lecture/expansion_valve.htm
Information and Images from Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited. See the trial version of the book here.