Question on velocity obtained from Duct-sizing wheels

[jazzgirl]

Does anyone know why the velocity calculated from a "ductolator" is different than what is calculated mathematically?
For a given duct size and cfm, the air velocity indicated on my ductolator is much different than what the velocity would be if it were calculated out using the formula: velocity = CFM/Face Area. When I contacted my local Loren Cook rep and ask the question, he indicated that ductolators add a safety factor to the velocity calculation. When I asked him what that safety factor is and what is it supposed to represent, he could not find an answer for me.
Any thoughts on this?

 

[MintJulep]

How different, and in what direction? Could you give a few example calcs?

 

[AbbyNormal]

Are you looking at the velocity through a round duct then getting an equivalent rectangular duct size from the ductilator?

You are then calculating the velocity through the rectangular area?

 

[trashcanman]

The Trane Ductalator gives very accurate results. It is based on using the velocity in a round duct. Remember that in a square or rectangular duct, there is not much air velocity in the corners, so that effective area is lost. Convert the square/rect duct to an equivalent round duct and get the velocity that way.

 

[KiwiMace]

For the reasons that trashcanman has highlighted; I was told on my first job at work to always use the ductulator and not avarage velocity via flow and area.

 

[AbbyNormal]

Most likely the 'wheels' give rectangular equivalents based on a similar pressure drop for the same volumetric flow, not because of cross sectional area. Look at the areas increasing as the ducts 'flatten out'

An equivalent 'square duct' will result in having very close to the same area as a round duct. Something rectangular will end up with a larger area yet be deemed equivalent for pressure loss. Bigger perimeter, more area for friction I would suppose.

20 inch round similar to 18x18, 35x10 or 50x8