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(D). - Externally equalized TEV.

Figure 14-22(B). - Dial thermometer.

TEV Adjustment Most TEVs are adjusted at a 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 valve.