It is installed on the inlet of the evaporator with a sensing bulb connected to the valve body by a capillary tube and mounted on the evaporator outlet. (See figs. 110a through 110e.)
The thermal bulb is filled with refrigerant. As the evaporator temperature rises, the refrigerant in the bulb attached to the evaporator coil expands, exerting pressure on the diaphragm within the valve, allowing the refrigerant to flow. As the temperature of the coil drops, the refrigerant in the bulb contracts and reduces pressure on the diaphragm, which closes the valve. The valve is spring loaded, so the spring tension and the
pressure in the evaporator also work to shut off the valve. These three forces control the performance of the thermostatic expansion valves. (See figs. 110a and 110b).
Force F1 is applied to the valve diaphragm by the expansion of the refrigerant in the sensing bulb which tends to open the valve. F2 is the force applied by the evaporator pressure which tends to close the valve. F3 is the spring tension which acts to close the valve.
If force F1 is greater than forces F2 and F3, the valve will open. If forces F2 and F3 are greater than F1, the valve will close.
Both the temperature perceived by the sensing bulb, as well as the pressure inside the evaporator, work together to control the flow of refrigerant and, consequently, the temperature of the unit. On the bottom of most TEV valves, there is an adjusting stem. To gain access to the stem, first remove the seal cap covering the stem. Use a service wrench to turn the stem clockwise to decrease the flow of refrigerant into the evaporator, and counterclockwise to increase the flow. Turn this valve stem only one-fourth turn at a time, then wait for about five minutes for a partial frost pattern to appear on the evaporator coil. A “hissing sound from the evaporator while the unit is turned on indicates a starved evaporator. In this case, turn the stem counterclockwise as described above.
On replacement of any valve, it is most important to obtain an exact duplicate, (one with the same specifications). Just as containers for different types of refrigerant are color-coded, the majority of TEVs are color-coded indicating the type of refrigerant they can handle. If they are not color-coded, they have a label or tag giving the specifications of the unit for which they are suited. The label shows the type of refrigerant and the tonnage of the unit.
Valves can be obtained with flared or brazed connections. For those with flared connections, be sure they are fastened tight enough to prevent Freon leakage, but not too tight to damage the flare. Use Teflon tape on these connections.
The bulb is attached to the evaporator outlet by a couple of screws. It is important not to change the location of this bulb when a new one is installed. (See figs. 110c through 110e.)
Be sure not to bend the bulb line sharply during installation; otherwise, it may inhibit the free flow of refrigerant.
The bulb should be seated on a straight section of the last portion of the evaporator line, not on a curve.
The temperature on a curve is slightly different because the pressure produced at that particular point is different and produces a false temperature.
With the unit running, liquid refrigerant passes through the TEV and gets into the evaporator where it immediately absorbs heat and boils off (becomes superheated vapor). Superheated refrigerant, due to the absorption of heat, loses a great deal of its cooling capacity. A properly adjusted valve admits the correct amount of liquid refrigerant into the evaporator to produce the desired evaporator temperature (by absorbing heat and becoming superheated), and the least amount of refrigerant leave the evaporator in liquid state.
The sensing bulb is installed at the outlet of the evaporator. If all of the refrigerant becomes superheated before leaving the evaporator, the bulb senses the higher temperatures and transmits signals to the valve for more refrigerant. The valve opens wider and admits more refrigerant to a point where all of the refrigerant within the evaporator coil will not get a chance to boil off and become superheated, and a portion of the refrigerant remains in the liquid state while leaving the evaporator at which time the bulb senses the lowered temperatures due to the passage of this liquid refrigerant and transmits a signal to the valve to limit the flow of refrigerant into the evaporator. This cycle continues as long as the unit runs.
Since only superheated refrigerant (with very little heat-absorbing capacity) should reach the TEV sensing bulb, the temperature of the bulb is always above the actual evaporator temperature. This temperature difference is called superheat. The best way to adjust this type of valve is to do it by superheat adjustment.