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-   -   total effective length question (http://www.diychatroom.com/f17/total-effective-length-question-36992/)

drewhart 01-27-2009 07:26 PM

total effective length question
 
when calculating the total effective length for a branch, does the size of the fitting change the effective length? for instance, a fitting that transitions from a round duct, up 90 degress to a rectangle register has an effective length of 30. does that number change if the round duct is 6", 7", or 8"?

i assume not, because the whole point in doing this calculation is to determine the duct size. it just seems that if the duct were wider, the fitting would cause less resistance.

beenthere 01-27-2009 08:51 PM

The transitions are calculated to be pretty much standardized.
They aren't actually, but close enough.
The PD changes if the radius changes.
And it will also change if you run above average FPM.

JohnH1 01-27-2009 09:46 PM

A larger size can reduce the resistance but then you will lower the velocity wich is why you are doing proper duct design to begin with. You want the proper cfm as well as the proper velocity.


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