Duct Question

[Dymalica]

What do you think has more loss? A rectangular duct that is soft bend to it or a elbow with turning vanes. I know that the turning vanes take up some of the cross section of the duct, but do you think it is significant compared to a rounded rectangular duct elbows? Also, what is the cost associated with a rounded rectangular duct elbows compared to a elbow w/ turning vanes? Lets assume that the duct is about 24x24.

 

[GreaterSDAC]

The best place to get duct loss coefficients is look in the SMACNA HVAC duct system design manual....it has all that info.
Costs?? call your local duct supply company...cost vary

 

[marcoh]

Depends on the quality/dimensions of the turning vanes and the radius of the round bends.

I asked a duct manufacturer a similar question and they preferred to make round bends with a 50% inner radius as it took less time (ie less cost) to fabricate round bends compared to square bends with turning vanes.

Note that contractors will always try to reduce the bend radius (ie use less sheetmetal) which increased pressure losses and a a square bend with good quality turning vanes will have a lower pressure loss than a round bend with a 50% inner radius.

 

[VinceNY]

1 1/4 radius will have a lower pressure loss than the 1/2 radius. Problem is the elbow gets larger and clumsier to handle and install. Square throat w\ vanes are labor intensive.

 

[KiwiMace]

Generally agreeing with the comments above... a radius bend will normally exhibit a larger pressure drop than an angle with [tightly spaced] vanes.

* ASHRAE lists the Cp factor of 0.15 to 1.18 for increasingly tighter bends.

* The mitred elbow with tightly spaced vanes has a Cp of 0.11.

However, going to wider spaced vanes will quickly increase the pressure drop to something that can easily be achieved with a cheaper bend.

So, use a long bend where possible, but don't sweat the tight installs where an elbow is required.