2272 Valve and Damper Actuators

Like other instruments, actuators come in a variety of styles and quality levels, and each has its pros and cons. Significant differences exist between manufacturers that make generalizations and rules of thumb difficult. One thing is certain about actuators: they are a moving part, with mechanical components, and will require maintenance; therefore consideration should be given to life cycle cost and maintainability. Some of the lower cost actuators are not intended to be serviced. For each of the types listed there are serviceable and throwaway variations, as well as spring-return/non-spring return types. Types of actuators include:

Pneumatic spring return Pneumatic air-to-open / air-to-close Electric motor/gear reduction in oil bath Electric motor/gear reduction-open air Electric hysterisis/stalling motor Hydraulic

Wax motors (thermal expansion)

System powered actuators/using air or water system pressure as the motive force

Self-powered actuators/using a capillary bulb and bellows

22.7.2.1 Actuator "Normal" (spring return) Position

For valves and dampers, the phrases normally open or (N.O.) and normally closed (N.C.) refer to the device position with no power applied, where a spring-returning mechanism exists to drive it to one position or another. Valves required to have a "fail position" are necessary in many applications to provide an increased measure of reliability if control power is lost. In the case of comfort heating and cooling, the choice is made by asking "upon a loss of power, control signal or air pressure, would I rather have full heat, full cool, or don't care?" In other cases, there are other operational issues like overheating, over-humidifying, etc. that should be considered. Without the spring return feature, the actuator will simply remain at its last position prior to the power interruption. Spring-

return actuators are more costly, so should be used prudently, where the added cost is justified.

For large pneumatic cylinder actuators, a measure of fail-safe control can be provided without the expense of a spring system; using an air-to-open/air-to-close actuator (no spring) and a small spring return air solenoid valve, the position of the actuator can be relatively assured on power loss, as long as compressed air remains available.

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