Patent ID: 8122911

Claim:
In combination: a continuous conduit having an outer surface and an inner surface and designed to convey a flow of airborne particulate material therethrough; a slot formed in said conduit at a location where the inner surface is uninterrupted by flanges, seams or the like; and a variable orifice mounted to said conduit for varying the rate of flow between a low, non-zero and a high rate, said orifice comprising a pair of contiguous valve plates, a gear box and a pair of concentric shafts extending from the gear box to said plates through said slot, said shafts being attached to the gear box for simultaneous rotation in opposite directions to move the plate between a first position in which the plates extend across the interior of the conduit and a second position in which the plates are in a substantially folded and mutually parallel adjacent condition, the dimensions of the plates being such as to provide clearance around said plates between the outer edges of said plates and the uninterrupted inner surface of said conduit when the plates are in the first position thereby to establish said low, non-zero flow rate.