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Duct Design Transition and Equivalent Lengths

brian_wiley | Posted in Mechanicals on

Hi all, 

I’ m redesigning some of my rectangular (H/W=1) duct work in anticipation of a new air handler, and had a quick question regarding a large radius transition on the return side.

Both radius elbows have similar measurements (see attached drawing), but have different outside radiuses which give them a different footprint and area. Because of the way Manual D calculates equivalent length, they both have the same R/W ratio of .714 though and therefore the same EL. 

Being very new to duct design, is one better than the other, perhaps in terms of turbulence? Or, am I overthinking this and there is little to no actual difference between the two?

Thanks for the feedback!

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  1. Expert Member
    Akos | | #1

    There is not enough difference between the two to worry about, I would guess the 2nd one is slightly less restrictive.

    Just make sure your duct person can build either one, they sometime have standard radius they prefer to use for certain size ducts.

    1. brian_wiley | | #2

      Thanks so much for the feedback. I’ll make sure to give them the option should they have a preference.

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