| 摘要 |
A backdraft damper comprises a frame defining an air control opening and a plurality of vanes mounted on the frame for closing the opening. The frame is assembled from frame sections of lightweight material all having identical cross sections. The frame sections are joined by a corner bracket which is complementally received and engages a large surface area of the sections. The respective sections are pulled together and locked in place as the assembled frame is placed in a jig to stake the corner bracket to the frame sections. The vanes are biased against opening by a spring which is disposed to provide a constant tension on the vanes regardless of the extent to which the blades are already open. The spring is disposed on a shaft and exerts a force tangentially of the shaft. The constant tention assures that any given change in air pressure will result in the same quantity of compensating air flow through the damper regardless of whether the pressure change occurs at near the fully closed or near the fully open position of the vanes. The spring may be wound more or less tightly on its mounting shaft to vary the tension against the vanes as required for different static pressures in different installations.
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