The purpose of this article is to cover the higher percentage problems and repair procedures of TEVs in supermarket applications.
A brief review of basic TEV operation is in order. The diaphragm is the actuating member of the TEV. There are three fundamental pressures acting on it: Sensing bulb pressure P1, equalizer pressure P2, and the equivalent spring pressure P3 (see illustration at right). The sensing bulb pressure is a function of temperature and acts on the top of the diaphragm causing the TEV to move in an opening direction. The equalizer and spring pressure act together underneath the diaphragm causing the valve to move in a closed position. Under normal operation (disregarding the pressure differential required across the diaphragm to move it), sensing bulb pressure equals equalizer pressure plus spring pressure, i.e.: P1 = P2 + P3. It is important to note that spring pressure is essentially constant once the valve is set. As a result, the TEV is actually controlling the difference between the bulb and the equalizer pressures, which is the amount of the spring pressure. The spring pressure represents the superheat the valve is controlling at the bulb location.