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Old Jun 25, 2009, 05:51 PM
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Willoughby, Ohio
Joined Jan 2002
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I always us 80% of the fan sweep area (FSA). The fan sweep area is the doughnut shaped ring that the fan blades push air through. You need to measure the diameters on the fan you are using. The calculate the exit diameter is easy the simplest way to do is like this:


De= sqrt(.80) * sqrt(D*D - d*d)
De= diameter for 80% FSA
D= rotor diameter
d= diameter of rotor spinner/motor (depends on fan used)
.80 = 80% FSA

I will post a photo and do an example.

Don
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