a flow control device has a duct section with a plurality of damper blocking elements , each having a major plane . the damper blocking elements are pivotably connected to the duct section and movable in a range that is limited to ensure that , when the duct section is mounted in a preferred orientation , the damper blocking element major planes always form an angle of at least 45 degrees from the horizontal throughout the range . the range is such that the plurality of damper blocking elements can selectively close and open the duct . the blocking elements can completely close the duct , for example , to block natural convection .