When the pressure of p1 is higher than the
combined pressure of p2 and p3, the valve
opens. This valve is equalized internally
because the evaporator pressure is sensed
through an intemal port in the valve. Figure
14-22(C) provides another view showing how
a TEV is equalized internally. When a TEV
is used on a large evaporator or an evaporator
with a pressure drop of 6 to 7 pounds across
the evaporator, the valve will prematurely
cause hunting (valve fluctuates toward
opening and closing). In the case of valve
hunting, install a TEV equipped with an
external equalizer line. Figure 14-22(D)
shows the TEV installed with an external
equalizer line. The external equalizer line
compensates for a pressure drop from the
inlet of the evaporator to the end of the tail
coil and eliminates valve hunting. During
installation of an equalizer line, ensure that it
is located downstream from the sensing bulb.
Air-conditioning refrigeration units come
equipped with a metering device that the
manufacturer has engineered for the system.
You should never change the recommended
type of metering device for a system without
consulting the manufacturer.
Figure 14-22(B).Dial thermometer.
predetermined superheat setting and tested at
the factory before they are shipped. If you
need a different superheat setting, follow the
steps in table 14-1(A).
Figure 14-22(C).Thermal expansion