These butter-fly valves are offered in both manual and automatic, and light and heavy duty options. The basic operation of all butterfly valves are the same. A circular disc or flap pivots on an internal shaft to open or close the duct space. The automatic valves are pneumatically operated and electronically controlled by a pneumatic cylinder. The cylinder requires 90 psi to operate but uses very little volume. The cylinder is activated by solenoids connected to virtually any electric signal - 12/24 V, AC/DC or 110/220 voltage. These solenoids operate off of milliamps and therefore require minimal wiring. The valve is powered in both directions to open and close the disc/flap; usually constructed from 3/16" steel.
Optional half-moon spacers with gaskets are available for a tighter seal.
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The pivot of an internal disc restricts or opens air-flow in the duct passageway. For applications that require periodic adjustments in air-flow, butterfly valves may be a good option. Consider using ductwork gates, as well.