Which sequence correctly describes the operation of a thermostatic expansion valve with respect to the bulb, spring, and evaporator?

Which sequence correctly describes the operation of a thermostatic expansion valve with respect to the bulb, spring, and evaporator?

• A. bulb closes, spring closes, evaporator opens the valve
• B. bulb closes, spring opens, evaporator opens the valve
• C. bulb opens, spring closes, evaporator opens the valve
• D. bulb opens, spring closes, evaporator closes the valve

Answer: (D)

A thermostatic expansion valve uses a bulb on the evaporator to sense temperature (and thus superheat). The temperature rise in the bulb raises its pressure, which pushes on the diaphragm against the opposing spring. This combined action tends to close the valve to throttle refrigerant flow when the evaporator gets hotter. Conversely, when the evaporator cools and the bulb pressure drops, the spring’s closing force is reduced and the valve opens more to allow more refrigerant in. So when the evaporator temperature rises above the setpoint, the ultimate effect is the valve closing. That matches the sequence in which the bulb’s effect leads toward closing against the spring, and the evaporator condition ends the cycle by closing the valve.

A thermostatic expansion valve uses a bulb on the evaporator to sense temperature (and thus superheat). The temperature rise in the bulb raises its pressure, which pushes on the diaphragm against the opposing spring. This combined action tends to close the valve to throttle refrigerant flow when the evaporator gets hotter. Conversely, when the evaporator cools and the bulb pressure drops, the spring’s closing force is reduced and the valve opens more to allow more refrigerant in.

So when the evaporator temperature rises above the setpoint, the ultimate effect is the valve closing. That matches the sequence in which the bulb’s effect leads toward closing against the spring, and the evaporator condition ends the cycle by closing the valve.