The function of the thermostatic expansion valve (TXV or TEV) is to hold a constant evaporator superheat. When set and operating properly, the TXV will keep the evaporator active throughout its entire length.

The conventional TXV is controlled by springs, bellows, and push rods.  The spring force is a closing force on the TXV. The evaporator pressure, which acts under the thermostatic element’s diaphragm, is also a closing force. An opening force is the remote bulb force, which acts on top of the thermostatic element’s diaphragm.

There is also a liquid force from the liquid line, which acts on the face of the needle valve and has a tendency to open the valve. However, this force is cancelled out when using a balanced port TXV. Working together, these forces maintain a constant evaporator superheat in a refrigeration system. There are no electronic devices associated with a conventional TXV.