Fluid flow control valve

Claim 1

A valve for controlling air flow through ducting, comprising an axially extending valve casing having opposite first and second end portions for connection respectively to ducting through which the air flow is to be controlled, said first end portion being upstream of said second end portion as viewed in the direction of air flow; said casing further having an axial air inlet and an axial air outlet, said second end portion constituting said air outlet; and a flared inner face widening continuously from said air inlet to said air outlet, whereby the cross-sectional area of said valve casing continuously increases from said air inlet to said air outlet; the maximum of said cross-sectional area of said valve casing being identical to the cross-sectional area of the ducting to which said valve casing is connected; a disc-shaped valve member mounted coaxially within said valve casing for axial movement relatively to said valve casing to vary the area of a valve opening defined between the outer periphery of said valve member and said inner face; said valve casing providing a substantially unobstructed axial air flow passage from said air inlet to said valve opening and from said valve opening to said air outlet; the axial length of said valve member being small in relation to the axial length of said inner face; said valve member comprising a generally flat front face directed towards said air inlet, a more sharply curved peripheral shoulder extending outwardly and rearwardly from said front face to define a radiused entry to said valve opening, and a rear edge bounding the outer periphery of the valve member to define a stepped exit from said valve opening; and operating means for setting said valve member in any selected position substantially along the entire path between said air inlet and said air outlet; the shape of said inner face of the valve casing being such that the variation in the area of said valve opening with axial movement of said valve member compensates for the corresponding variation in the drag coefficients due to said more sharply curved shoulder, said rear edge of the valve member and the portion of the inner face of the valve casing forming the valve opening, whereby for any given pressure drop across a combination of duct and valve, the variation in the volume of air flow in relation to the position of said valve member is linear.