Round /Rectangular Duct Air Flow Q


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Old 01-01-10, 06:24 PM
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Round /Rectangular Duct Air Flow Q

I cant find on line an air flow chart for stack duct, I need to know if 6'' round 90 elbow provides more flow than the existing 3''-1/4x10''-4''x10'' stack head 90 elbow, and also be able to compare rectangular with round.
I understand round duct is more efficient than rectangular for the same cross area but dont know how much percentage wise,any help much appreciated.
Thank you
 
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Old 01-02-10, 04:03 PM
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It's probably best not to get into any specific percentage advantage of round duct vs. rectangular duct because the differences are small and not always directly transferable to real life situations.

That stated, the capacity of a duct is directly related to its cross-sectional area and developed length. The developed length is the length of all straight duct plus the equivalent length of any transitional fittings including elbows and tees. Normally a slide rule device called a Ductulator is used for figuring the developed length of transitions but you may find some table of equivalent lengths on some website.

In your particular situation as I remember it you really don't have too many choices unless you want to get into some big-time modifications. I would simply go by the cross-sectional area and use the duct that was larger. To figure area of a round duct multiply the diameter by itself and then multiply that product by 0.7854 (the formula is A=1/4 pi x d squared.) For the rectangular duct the area is just the the two dimensions multiplied together. From that you can easily see that the 6 inch round duct is only 87% of the size of the 3-1/4 x 10 duct or that the 3-1/4 x 10 duct is just under 115% larger than the 6 inch round duct. (114.96% to be more exact.)
 
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Old 01-02-10, 09:26 PM
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Thank you furd. I see that you remember my previous post. Im traying to get a little more heat upstairs but the limiting factor is the wall and top plate space for the duct
[ 4''x 12''].Besides changing the existing 3''-1/4''x10'' stack for a 3''-1/4'' x 12'' dont know what else I can do
Regards
 
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Old 01-02-10, 11:51 PM
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If you can get a 3-1/4 X 12 in the space then do it. The 3-1/4 X 12 is 20% larger than the 3-1/4 X 10 and that is a significant amount.
 
 

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