Distance of VCD from fittings

[nabeel3]

In the project I am working now, HVAC specification mentions as follows. “Volume dampers shall be located a minimum of two duct sizes away from fittings”. I am little confused about the requirement two duct size away. Is it the two duct size indicate the length of two standard duct? Or is it the width or height? Can anyone help ?

 

[GT-EGR]

Technical you convert the rectangular dimensions to their equivalent diameter.

de = 1.30 (a*b)^0.625 / (a + b)^0.25

de = equivalent diameter
a = length of major or minor side
b = length of minor or major side

Diagram and explanation of this is here:

https://www.engineeringtoolbox.com/amp/equivalent-diameter-d_205.html

But the duct diameter distance reference is not a perfect science so if you do a normal calculation to get a circle that is the same area as your rectangle that will work most of the time unless you have an extreme aspect ratio

 

[ChasBean1]

Poorly worded on two counts: 1) Stuff that requires straight runs of duct prior to bends refer to duct diameters, not duct sizes. 2) Volume dampers shouldn't need minimum straight duct runs. Volume FLOW STATIONS controlling VAV boxes, however, may.

 

[lilliput1]

In addition to ChasBean1 comment note that volume damper should be located at each branch of the riser (supply or return) duct to facilitate air flow balancing. In the supply duct they are needed to kill off excess pressure so the downstream duct construction do not have to be as strong as the upstream. Note that VAV boxes are only rated up to maximum 3" wg upstream pressure. In exhaust or return duct, they are needed so the upstream duct construction do not need to be as strong as the downstream portion. Duct construction pressure class should be noted or scheduled accordingly. Avoid locating volume dampers too close to outlets or inlet so as not to create noise problem.

 

[Emrelug84]

Generally speaking, two duct sizes away from fittings mean that you should put volume damper two branch duct width away from the wye point in branch line. I think the reason to do so is to provide laminar flow through volume damper to prevent excess pressure drop and noise propogation.