Parallel Blade

Unlike the opposed blade these units will bias the air to one side of the duct if not operated in the fully open position. Very effective for isolation.

For Isolation dampers which are either open or full closed, a good flow characteristic is desirable. Badger Dampers use a blade which is formed longitudinally for rigidity but presents a very small face area to the flow.

The free area is often better than 95%. Other blade designs may be a diamond section blade which will usually have a chord to thickness ratio of 6:1, meaning the free area is likely to be about 85%. Specifications often refer to % leakage. The leakage through a closed damper will be a function of the pressure across the blades, which will be the same irrespective of the volume flow when the damper is open.

When the damper is fully closed it will be subjected to the full pressure that the fan can produce at zero flow. Thus the percentage for a damper in a high volume/low pressure system will will always be much lower than that for a low volume/high pressure system. Similarly, it is not possible to produce a generic curve for flow against blade angle since the flow will depend upon the total system resistance compared to the resistance across the damper. This concept is known as the authority of the damper.