In a thermostatic expansion valve, three forces act on the diaphragm. Which sequence describes how these forces affect the valve?

In a thermostatic expansion valve, three forces act on the diaphragm. Which sequence describes how these forces affect the valve?

• A. Evaporator pressure closes; Spring pressure closes; Bulb pressure opens
• B. Evaporator pressure closes; Spring pressure opens; Bulb pressure opens
• C. Evaporator pressure opens; Spring pressure opens; Bulb pressure closes
• D. Evaporator pressure opens; Spring pressure closes; Bulb pressure closes

Answer: (A)

The valve’s diaphragm is held by three opposing forces: the evaporator pressure on one side, the pressure from the sensing bulb on the other, and a spring pushing toward closing. The spring provides a constant closing bias, while the evaporator pressure tends to push the diaphragm toward the valve seat (closing). The bulb pressure, which rises with higher sensed temperature in the evaporator, pushes in the opposite direction and tends to open the diaphragm, allowing more flow. Thus, in this arrangement the evaporator pressure acts to close, the spring adds a closing force, and the bulb pressure acts to open. The valve settles at a position where these forces balance, opening more when the bulb pressure rises (to reduce superheat) and closing more when the evaporator pressure or spring force dominates.

The valve’s diaphragm is held by three opposing forces: the evaporator pressure on one side, the pressure from the sensing bulb on the other, and a spring pushing toward closing. The spring provides a constant closing bias, while the evaporator pressure tends to push the diaphragm toward the valve seat (closing). The bulb pressure, which rises with higher sensed temperature in the evaporator, pushes in the opposite direction and tends to open the diaphragm, allowing more flow.

Thus, in this arrangement the evaporator pressure acts to close, the spring adds a closing force, and the bulb pressure acts to open. The valve settles at a position where these forces balance, opening more when the bulb pressure rises (to reduce superheat) and closing more when the evaporator pressure or spring force dominates.