Back draft damper
A back draft damper includes a rectangular shell having a central opening which air can pass through. A plurality of vanes rotatably mounted in the shell are rotatable between open positions where air can pass through the shell and closed positions where air cannot pass through the shell. The axes the vanes rotate about are slightly offset from vertical.
1 . A damper comprising:
a. a shell having a central opening through which air passes;
b. a plurality of vanes mounted rotatably in said shell;
c. said vanes are rotatable, about axes which are slightly off set from vertical at a predetermined angle, between open positions where air can flow through said central opening and closed positions where air substantially cannot flow through said central opening.
2 . The damper of claim one wherein at least some of said vanes rotates independently of at least some of the other vanes so that when in its open position each said blade is aligned with the localized direction of the air flowing over it.
3 . The damper of claim 1 wherein said shell is rectilinear in cross section.
4 . The damper of claim 1 wherein said shell is elliptical in cross section.
5 . The damper of claim 1 wherein said shell is circular in cross section.
6 . The damper of claim 1 wherein said vanes have leading edges and trailing edges, and said axes are proximate said leading edges.
7 . The damper of claim 1 wherein said vanes have top ends and bottom ends and said axes are closer to said inlet at said top ends than at said bottom ends.
8 . The damper of claim 1 wherein said shell has a vertical center line and said axes are closer to said vertical center line at said top ends than at said bottom ends.
9 . The damper of claim 6 wherein said shell has a depth and said vanes have a width which is not greater than said depth.
10 . The damper of claim 9 wherein said vanes are substantially located within said shell.
11 . The damper of claim 1 wherein said shell has a vertical center line and a portion of said vanes located on one side of said vertical center line rotate clockwise when rotating toward their closed positions and a portion of said vanes located on the other side of said vertical center line rotate counterclockwise when rotating toward their closed positions.
12 . The damper of claim 11 wherein there are an even number of vanes with one half of said vanes on one side of said vertical center line and the other half of said vanes on the other side of said vertical center line.
13 . The damper of claim 1 wherein said vanes are symmetrical in cross section and have a shape which minimizes drag.
14 . The damper of claim 1 wherein said predetermined angle is between 0.5 degrees and 45 degrees.
15 . The damper of claim 1 wherein said predetermined angle is 1.6 degrees.
16 . The damper of claim 1 wherein said predetermined angle is such that said vanes are free to start rotating towards said open positions when there is any airflow through said shell from said inlet to said outlet.
17 . The damper of claim 1 where said predetermined angle is such that said vanes move to their closed positions when there is no airflow through said shell from said inlet toward said outlet.
18 . The damper of claim 1 wherein said vanes have tops and bottoms and there is a bearing between the top of each vane and said shell and the bottom of each vane and said shell.
19 . The damper of claim 18 wherein said bearing is a ball bearing.
20 . The damper of claim 1 wherein said vanes have a thickness of between 0.250 inches and 0.400 inches at a leading edge thereof and of between 0.050 inches and 0.100 inches at a trailing edge of said vane.
21 . The damper of claim 20 wherein said vanes have a thickness of 0.32 at said leading edge and a thickness of 0.086 at said trailing edge.
22 . The damper of claim 1 wherein said vanes weight approximately 2 pounds.
23 . The damper of claim 1 wherein said shell has a horizontal center line and a vertical center line and said axes are offset from vertical by positioning said shell such that said vertical center line is offset from vertical.
24 . The damper of claim 1 wherein said shell has a horizontal center line and a vertical center line and said axes are offset from vertical by positioning said shell such that said horizontal center line is offset from horizontal.