In HVAC control systems, a bellows is used in which controllers?

In HVAC control systems, a bellows is used in which controllers?

• A. Pressure controllers
• B. Flow controllers
• C. Both pressure and temperature controllers
• D. Temperature controllers

Answer: (C)

Bellows act as a flexible sensing element that converts a physical change into motion that can drive a control valve. In a pressure controller, the process pressure pushes on the bellows, and the resulting displacement is used against a spring or feedback mechanism to set and hold the desired pressure by adjusting the valve. In a temperature controller, the sensing system (often a bulb connected by capillary to the bellows) transfers heat to the bellows; the gas inside expands or contracts, causing the bellows to move and drive the actuator to maintain the set temperature. Because a bellows responds to both pressure and temperature changes, it serves as the sensing element in both types of controllers. This dual use is why bellows are associated with both pressure and temperature controllers.

Bellows act as a flexible sensing element that converts a physical change into motion that can drive a control valve. In a pressure controller, the process pressure pushes on the bellows, and the resulting displacement is used against a spring or feedback mechanism to set and hold the desired pressure by adjusting the valve. In a temperature controller, the sensing system (often a bulb connected by capillary to the bellows) transfers heat to the bellows; the gas inside expands or contracts, causing the bellows to move and drive the actuator to maintain the set temperature. Because a bellows responds to both pressure and temperature changes, it serves as the sensing element in both types of controllers. This dual use is why bellows are associated with both pressure and temperature controllers.