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The butterfly or wafer damper can be a stack damper where leakage is not all-important.  This type has also been tried as an isolating damper for flue-gas scrubbers.  The butterfly damper is lightweight, simple, quickly actuated, and often comparatively low in cost.  Its frame shape is a handicap in power plant work, however, where much ducting is rectangular.  In process work, oval shapes adapt to that shape of duct, but conversion of a round butterfly to a rectangular one results in a louver damper.  Occasionally, a double-disc butterfly appears, resembling a circular two-bladed louver damper. 

Disadvantages of the butterfly damper include high leakage, blade flutter, tendency of the blade to warp, and large clearance needs for the open blade.  The frame of most butterfly dampers is light, so twisting and distortion are dangers. 

Most butterfly blades or discs are flat and single-thickness, so reinforcement is necessary for pressurized duct service.  Either angles or simple bars on edge will stiffen the plate.  Welding of the reinforcing can warp the plate badly, requiring straightening.  Another disadvantage of reinforcing is the flow resistance it causes when the damper is open.  Seals for the butterfly damper are usually simple, either flat metal-on-metal, or a soft seat resembling a tadpole gasket.

The drive for a butterfly damper is also simple, needing only 90° of motion.  The torque requirement can vary widely over damper life, if corrosion and thermal effects are severe.  Electric motors, air cylinders, and oil cylinders are able to actuate butterfly dampers with little difficulty

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