Triangular Duct Dimensions Equivalence to Round or Rect Ducts

[HighPotential]

I am trying to size a traingular shaped return air duct and haven't found a table equating it's dimensions to those of a rectangular or round duct for air flow equivalence and inches of water. Does anyone have any guidelines?

Thanks!

P.S. I am doing this to shove the return air into the side of an attic where the rafters meet the attic floor.

 

[briand2]

For an equivalent circular duct having equal VELOCITY and PRESSURE DROP, calculate the hydraulic mean diameter dh thus:

dh = 4 * (cross sectional area / wetted perimeter)

If, on the other hand, you want to find an equivalent circular duct having equal FLOW RATE and PRESSURE DROP, the formula is a little more complicated but it will be something like 10% greater diameter than that given by the above formula.

Regards,

Brian