Duct Diameter ID/OD Convention? 3

[harvey13]

We are having a discussion about duct diameters, and so I have a short question about Ductwork. Is there a generally accepted convention that understands that listing the size of a duct, automatically identifies the ID or OD of the duct? If so, which one? Is the convention the same for HVAC duct and Industrial duct?

I know that SMACNA has standards for duct, and that duct can be fabricated to any diameter and just about any gauge. And I know that drawings and specifications should be explicit, and that in many cases the difference is not critical. But we see many drawings, specs, and so forth that list a "round 16" duct". Didn't I say this was going to be a short question? Ooops.

Thanks
Harvey13

 

[ChrisConley]

Generally accepted HVAC convention is ID, my specifications usually highlight this as well.

The second part of your post isn't actually a question. What exactly are you asking?

 

[harvey13]

Thanks Chris,

The second half was just me running on too long, but also to make it clear that I know that ductwork can be just about any dimension. I would assume that industrial duct would be the same.

 

[cdxx139]

We also call it out as the ID, or free area.

So if the duct is called out to have 1" liner, then the dimension provided is the free area, and the duct must be increased to accomodate the liner and free area requirement

knowledge is power

 

[harvey13]

I've thought about this a little more and I think that industrial pipes (mainly dust collection) are assumed to be O.D. The dimensions of standard rolled angle/flange rings are listed by diameter, and the I.D. of these rings are 1/8" or 3/16" larger than the nominal dimeter of the pipe, i.e. a 16" diameter angle ring has an I.D. of 16 1/8". For this ring to work on any gauge 16" diameter pipe, the nominal pipe diameter implies the O.D. of the pipe.

 

[berkshire]

As someone who used to make this stuff before I retired.
Sheet metal ducting off the shelf (KD) has a nominal diameter, which is usually ID at one end and OD at the other, so that the pieces can be fitted together. The tolerance is pretty generous on this stuff, so parts from one manufacturer may not always fit anothers.
As far as pipes with fitted angle rings, the rings are sized to allow for the thickness of metal of a nominal ID duct. Example 16" ID duct rolled from 16ga galv'd + od of of material = 16.116. Ring = 16.125 gives you just enough clearance to get the ring on. ( Sometimes)
B.E.

 

[harvey13]

Thanks for the input. I think that this discussion highlights that the duct diameter should always be specified as I.D. or O.D.

Arewin

 

[AnotherEllis]

The standard in HVACR is that indicated duct diameter is clear inside dimension. OD is determined by this plus insulation required. We as designers do not show ductwork construction to shop drawing level detail.

I've had some interesting discussions regarding this with mechanical contractors upset at the level of detailing in our drawings.

From a consulting engineering standpoint, we are producing the engineered design. Our drawings are "diagrammatic in nature" and construction details have traditionally been left to the various subs to work out. I tell these guys that a mechanical contractor is a qualified license holder and sheet metal fabrication is a well documented trade, and they had better not be looking to my drawings to figure out how to construct this stuff.

I am interested to see how BIM continues to affect this traditional division of responsibilities. Personally, if I am expected to produce designs as well as coordinate shop drawing level ductwork drawings, I need a bigger fee.

Sorry for the slightly off-topic reply. This is something that gets under my skin from time to time.

http://www.ellisconsultingengineers.com

 

[marcoh]

Typically design drawings show clear internal air size dimensions as this is the engineering requirement, which may or may not be the metal dimensions.

Contractors workshop drawings show metal sizes so the workshop guys fabricating the ductwork don't need to to think.

And as AnotherEllis mentions, BIM and Revit etc are making things interesting, particularly as Revit can only show metal dimensions.