Triangular Duct Dims for Flow Equivalence to Round or Rect Ducts 2

[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.

 

[Tremolo]

HighPotential,

I do not know of any tables that will provide equivalent dimensions for the triangular duct. However, you can perform some simple calculations to determine the equivalent duct dimensions yourself.

Assuming you want the flow and pressure drop to be identical for the circular and triangular duct, and assuming the friction factors, f, are roughly equivalent, then the triangular duct will be equivalent to the circular duct when,

(D*A^2) triangular = (D*A^2) circular

where,

D = hydraulic diameter
A^2 = cross-sectional area squared

For the circular duct, D is simply the duct diameter. For the triangular duct, D = 4*A/P, where A is the triangular duct cross-sectional area (1/2*base*height) and P is the triangular duct perimeter.

The above expression is obtained by equating the gas flow equations for a round and circular duct. So, when the expression is satisfied, you should get identical flow rate and pressure drop for the different duct shapes.

Tremolo

 

[HighPotential]

Thanks! Those formulas are very helpful, just FYI, I calculated that a 31"x21" (right angle) triangular duct is about equivalent to a 19" round duct and a 25"x17" is significantly less than a 16" round duct.

 

[owlscreech]

From Fried and Idelchik
"Flow Resistance: A Design Guide for Engineers"
Taylor & Francis 1989
Paragraph 2-1, p. 5

"It should be noted that the concept of hydraulic diameter will provide values of pressure losses of acceptable engineering accuracy for most shapes, where there are no secondary flows of significance. Triangular ducts, and shapes where laminar flows can persist locally, are the exception. Empirically derived values are best for such
cases, where available."

There are diagrams and tables that provides corrections. Send an email address to riverpilot@earthlink.net and I will send a scanned copy (two pages).

 

[Tremolo]

HighPotential,

I double checked your calculations and arrived at the same conclusion.

Owlscreech,

That is intersting information. It makes sense that a different answer could be obtained for shapes where laminar flow is present since the friction factor would be much higher for laminar flow as opposed to turbulent flow.

I would be interested in your 2 pages of material and will send my e-mail address to you.

Tremolo

 

[BobPE]

owlscreech:

Idelchic is the man!!!! I love reading his work, espically with the Soviet variables in his equations!!!

BobPE

 

[Tremolo]

BobPE,

Idelchic must have had one long, cold winter to sit down and catalog all those obscure correlations for loss coefficients.

His is the ultimate source for loss coefficients.

Tremolo.