Step A. Determine the suction pressure at the evaporator outlet with an accurate gauge. If there is no gauge connection, a tee installed in the valve external equalizer line can be used.
Step B. Refer to the temperature-pressure chart (p. 133) for the refrigerant used in the system and determine the saturation temperature at the observed suction pressure.
Step C. Measure the temperature of the suction line at the remote sensing bulb location. This can be accomplished by a “strap on” thermometer or an electric device similar to an “Annie” or “Simpson” meter. Be certain the spot chosen for measurement is clean to ensure accurate readings.
Step D. Subtract the saturation temperature determined in Step B from the suction gas temperature measured in Step C. The difference is the operating superheat. (See fig. 110h.)
For best results, there are two types of TEV on the market today: (1) internally equalized TEV used in regular evaporators where pressure drop inside the evaporator is not significant and (2) externally equalized TEV (see fig. 110h) used on the evaporators in which pressure drop is considerably high.
The pressure inside the sensing bulb is the only opening force in the valve working against the closing forces of the spring and evaporator pressure. When the pressure drop at the evaporator outlet (where the bulb is installed) is substantial, this reduced pressure (low temperature) reduces the opening force applied to the valve diaphragm, and the valve tends to close and starve the evaporator.
To normalize this condition, it only makes sense to reduce the closing force (evaporator pressure) applied to the diaphragm. This is done by connecting the evaporator outlet reduced pressures to the valve by a tube. This allows the low pressures from the evaporator outlet (where the opening force is also affected) to reduce the excessively high pressures that tend to close the valve.
Figures 110d and 110e show the ideal placement (horizontal) of the bulb in relation to suction-line size. Never put the bulb at 6 o’clock because it may sense the temperature of the oil flowing through the pipe rather than the temperature of the refrigerant. And be sure the bulb location is on a free-draining suction line.
A good way to check a suspected punctured sensing bulb is to close one hand around the bulb and its connecting line. The heat transferred to it should cause the refrigerant in it to expand and open the valve. Otherwise, the valve has to be replaced as the refrigerant in the bulb must have leaked out.