Which sequence describes the thermostatic expansion valve response as the sensing bulb reacts?

Which sequence describes the thermostatic expansion valve response as the sensing bulb reacts?

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

Answer: (B)

The key idea is that the thermostatic expansion valve is controlled by the pressure on a diaphragm created by the sensing bulb. When the evaporator experiences more load, its temperature rises and the refrigerant in the sensing bulb boils more, raising the pressure inside the bulb. That higher bulb pressure pushes on the diaphragm and tends to move it to open the valve, working against the spring. As a result, the valve opens and more refrigerant is sent to the evaporator to meet the increased demand. If the evaporator cools and the bulb pressure falls, the spring then pushes the diaphragm to close the valve, reducing flow. So the bulb’s reaction to higher temperature leads to valve opening (overcoming the spring) and increased flow to the evaporator.

The key idea is that the thermostatic expansion valve is controlled by the pressure on a diaphragm created by the sensing bulb. When the evaporator experiences more load, its temperature rises and the refrigerant in the sensing bulb boils more, raising the pressure inside the bulb. That higher bulb pressure pushes on the diaphragm and tends to move it to open the valve, working against the spring. As a result, the valve opens and more refrigerant is sent to the evaporator to meet the increased demand. If the evaporator cools and the bulb pressure falls, the spring then pushes the diaphragm to close the valve, reducing flow. So the bulb’s reaction to higher temperature leads to valve opening (overcoming the spring) and increased flow to the evaporator.