Flow loss due to friction in galvanized steel duct

[Swanny76]

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.

 

[gepman]

Since you are asking basic fluid mechanics questions and do not know where to find the answers it would probably be best for you to use a ductulator. There is an online version (which I haven't used) at

http://efficientcomfort.net/jsp/ResDuct_Web.jsp

or can get the handheld version of them at:

http://www.professionalequipment.com/trane/brand.html

No endorsement of any product is intended.

 

[trashcanman]

Look in ASHRAE handbook of Fundamentals, Duct Design.

 

[CountOlaf]

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.

 

[EmeraldCoastHVACR]

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

 

[trashcanman]

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).

 

[imok2]

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