Duct POrts

[mizzoueng]

Ok, I have a problem. One of my contractors wants to move the centerline of a duct port from the center on the main trunk to 4" back. On all the ports in the main. Now normally I wouldn't worry about this but the ports are 22" wide (square) and the main is 36" wide.

He tells my bosses that this will IN NO WAY effect the air flow through the main or the ports. I know this is wrong as the ports will now be in the turbulent zone of the duct.

Also, the branches cause tubulence around them, and being off-center I believe that will only magnify this effect.

Has anyone delt with anything like this? I know what I know, but I need something to back myself up with. Equations, evidence, something.

I am going to search Knovel for something.

 

[mizzoueng]

Bump. Didn't find anything on Knovel.

 

[quark]

Do you mean by 'duct port' a branch off? Offsetting the centerline will not make much difference though it is advised to keep the center line. The main constraint will be enough space on all sides to make the connection. 1" is necessary for duct construction and then you have insulation thickness.

There will be a laminar sublayer near the duct walls and the velocity distribution will be as per Prandtl's 1/7th power law. The idea of going for a center line is to get the advantage of equidistribution of velocity (along the cross section of a duct)

 

[lilliput1]

It should be alright if you have put in a safety factor in your pressure drop calculation (10% or minimum +0.25" wg) to take care of actual installtion conditions that you can not forsee such as this. Anyway the pressure drop will be a % of the velocity pressure at the section. A good guess would be about 2% of the velocity pressure.