hook shaped tip portion 28 of which is shown sealed with respect to the hook shaped portion 30 of its adjacent blade as seen in fig1 the lower terminal portion 32 of blade 26 has a truncated hook shaped portion which is adapted to create a seal with respect to upstanding flange 22 the blade 26 also has defined thereon a hook shaped hinge portion 34 which is complementally conformed with respect to the hook shaped hinge element 24 of the inwardly depending flange 14 such that the blade 26 will freely pivot therearound blade 26 is articulated with respect to the remaining blades in the damper by means of blade bracket 38 which is pivotally attached to operator rod 40 by pivot 42 similar brackets are attached to adjacent blades such that upon axial movement of rod 40 the remaining blades in the damper will assume the same position as blade 26 a first preferred embodiment locking clip designated generally 100 in fig1 comprises a serpentine bimetallic element 102 a base plate 104 which is riveted by rivet 106 to frame bottom 18 and a restraining member 108 which is pivotally attached to base 104 in a standard hinging arrangement around hinge pin 110 restraining member 108 is defined into pivoting portion 112 which is complementally conformed with respect to the exterior surfaces.