847,438. Valves. SVENSKA FLAKTFABRIKEN A.B. April 29, 1958 [April 30, 1957], No. 13573/58. Class 135. In a ventilating system the flow of air between ducts 2 and 5 is controlled by a channelshaped valve member 6 forming a movable wall portion of the duct 2 and operated by the static pressure of the air in that duct acting against loading springs 12 to vary the area of narrow flow passages 7. To avoid eddies in the flow and thus reduce noise in the normal working position, the length of each of the passages 7 is greater than five times their hydraulic diameter (defined as four times the cross sectional-area divided by the perimeter), and the Reynolds Number does not exceed 2,000 at the maximum speed of flow.