Double louver damper

A double louver damper for a duct system to enable safe entry to be made therein, said damper including a rectangular frame dimensioned to be incorporated in the duct system, two series of the same number of blades of the same size and shape, means rotatably connecting each blade of each series to said frame with one series upstream relative to the other and with the axes of corresponding blades of the two series in the same plane, each blade in the form of an elongated diamond with its pivot axis at the center of the junction of first and second acutely angled portions including coplanar lengthwise and end sealing portions of the same radial extent and resiliently flexible sealing means, one for each series and attached to the frame to surround the flow path, and power operated means to pivot simultaneously all blades of both series between open and closed positions, in said closed position, the sealing portions of the blades of each series substantially in the same plane with the longitudinal and end sealing portions of adjacent blades overlapping and the corresponding portions of the remote blades and the end sealing portions of all blades in sealing engagement with the appropriate portions of the sealing means for that series, and the two series so spaced that in said open position the acutely angled first portions of one blade of each corresponding pair thereof so overlap the second portion of the other blade of that pair as to establish a closed streamlined figure the height of which is the maximum thickness of either blade.