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Flow loss due to friction in galvanized steel duct

Flow loss due to friction in galvanized steel duct

Flow loss due to friction in galvanized steel duct

(OP)
I am trying to figure out what the loss of flow due to friction in an 8" square duct versus a 9" round duct would be. This is an exhaust duct 80' in height. How do I relate pressure drop in in wg to cfm and velocity? Or, how do I relate cfm and velocity to pressure? Any help would be greatly appreciated.

RE: Flow loss due to friction in galvanized steel duct

Look in ASHRAE handbook of Fundamentals, Duct Design.

RE: Flow loss due to friction in galvanized steel duct

Yeah, you definitely need a Ductulator.  But to help you with your particular problem this is what the magical ductulator says:

Assuming 300 CFM, an 8" x 8" duct gives you 0.1" W.C. loss per 100 ft of duct, and the 9" round gives you 0.09" W.C. loss per 100 ft.  They are hence insignificantly close but the 9" round is slightly better.  BTW, the velocity is approx 700 fpm at this assumed CFM.  If you have another airflow rate or velocity in mind, let us know and we/I can re-chug the numbers accordingly.

RE: Flow loss due to friction in galvanized steel duct

ASHRAE Fundamentals is definitely your source. There's an equation to relate round duct to equivalent rectangular or square duct.

RE: Flow loss due to friction in galvanized steel duct

Note: The Trane Ductalator is based on the same equations as given in the Carrier Duct Design manual.  The equation uses the Equivalent round diameter of a given duct.  So, if you have square, rect, triangular or what ever, you must convert it to a round dia. for the equations to be correct (if you are hand calculating it).

RE: Flow loss due to friction in galvanized steel duct

I get for an 8 x 8 duct at 300 cfm a velocity of 719 fpm with a pressure drop of 0.127 and for a 9" round duct at 300 cfm a velocity of 670 fpm with a pressure drop of 0.109 for 80 feet, so 9" round is a better selection

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