Three forces are at work within a TEV (Figure 1). In the valve-opening direction, bulb pressure acts on the diaphragm from above. In the valve-closing direction, both spring pressure and evaporator pressure oppose it, acting on the diaphragm from the other side.
When the load on an evaporator changes, the change in sensing bulb pressure (PB) is transmitted through the capillary tube to the space above the TEV's diaphragm. The diaphragm moves in res-ponse; its motion is transmitted by one or more pushpins to the orifice, which opens or closes to cause a corresponding change in refrigerant flow through the valve.