Duct Area is Plain Wrong, or am I missing something?

[lee.imbimbo86EM4]

I'm trying to figure out what Revit is reporting in the Properties menu for a duct, but for the life of me it just seems plain wrong.

When I draw a duct, measuring 10" x 4" x 24", under the Mechanical subgroup it reports Area = 4.67sf.  Which is very confusing to me.   The cross-sectional area of this piece of duct should be 40 sq.in or 0.28sf.  Even if you assume that they are somehow accounting for the 24" in length, which would be volume, not area, then the number is 2.22cubic feet.  How are they getting from 0.28sf to 4.67sf in the Area.

It also says that the equivalent diameter is 6 95/128", which again seems odd to me, because that is 35.7sq.in. or 0.25sf.  Which seems to me that the equivalent diameter should be reporting back more like 7.136496 inches.

Am I missing something, or are these functions just plain unreliable and wrong?

[RobDraw]

Isn't that the surface area of the entire duct which is used for fabrication?

[HVAC-Novice]

This may be the surface area of the sheet metal required. Circumference of your duct is 2.33 ft times 2ft long = 4.67 ft².

That value is mainly useful to estimate the material needed. 

[lee.imbimbo86EM4]

It would appear you are correct. Hadn't considered that. So then I guess there isn't a default parameter for Cross-Sectional Area. Any idea what the equivalent diameter is referring to?

[HVAC-Novice]

I assume that would be for a rectangular duct to know what round duct will have the same cross sectional area. Note that this isn't what you need to have the same friction rate. That is what the hydraulic diameter is for. 

I actually never used or paid attention to equivalent diameter, so this is just a guess based on an example I tried out. 

Edit: What is it that you want to accomplish? The cross-sectional area is taken into account when Revit calculates velocity etc.  If you  want to tag or schedule the cross-sectional area, you could have it calculated from the equivalent diameter. 

[lee.imbimbo86EM4]

My Mechanical Engineer on this project is not trained in Revit, and I'm trying to coordinate the ductwork layout with them.  For now, I've just been using equivalent diameter, to convert his schematic circular diameter ductwork to a ductboard layout.

He'll then give me a final design based on the three dimensional model of the ductwork.  I also liked the idea of getting to know the MEP tools in Revit better, just part of getting further into the software.

[iainsavage]

Equivalent diameter is the diameter of a circular duct which would have the same frictional loss as the rectangular duct. It is not the same as the geometric equivalent diameter.

[HVAC-Novice]

You can do all the pipe and duct calcs in Revit. No need to do it separately. Revit is not just a drawing program. It probably takes less time to learn the basics in Revit than to duplicate work.

[lee.imbimbo86EM4]

I would agree.  But in this case, I'm the architect, contractor, and developer.  My project is still in Platting, but I wanted to learn the MEP tools, kind of enjoy learning the ins and outs of a software.  My MEP Engineer on this project is not and is not willing to buy and learn Revit.  So I figured I could take a stab at it and it would help with quantity take-offs, class-detection etc.

I there a video tutorial I could reference that would allow me to just learn some of the stuff you are referring to.  Along the lines of learn something new everyday in this profession, I've always just assumed that equivalent diameter and cross-sectional area was analogous to geometric cross-section.  So chalk that one off as learn something new everyday in this profession.

But while I'm not looking to become an MEP engineer, I don't mind know how the software works and how to get the most out of it kind of thing.

[HVAC-Novice]

I started with Revit (and still consider myself a novice) by studying some good books, googling, this forum and just starting with a project. 

This is a general Revit book that walks you through a whole project

For MEP Autodesk has a MEP fundamentals book and there are others. I'd start with those in addition to what you get for free online. There were no local courses when I started, but i don't think they are really necessary. But they can be a good start, though. 

The hydraulic diameter is different since it accounts for the loss in efficiency in the corners of rectangular duct. A 10"x10" duct will have less velocity than a round 10"duct. But the pressuredrop is about the same since the round duct has les turbulences due to no corners etc. So if you only look at cross section, you would need a 11"round duct, but a 10"round duct would have the same pressure drop. Numbers here are just examples to demonstrate.... 

Unfortunately some laggards hold us back. I know one consultant we sometimes hire who still  doesn't use Revit, and he is short before retirement. Even that isn't an excuse, IMHO. It's kind of a pain to work with him since when I have updates and give him my new Revit model, he'll ask what the changes are since he has to manually extract the dwg from my Revit file (yes, he pays to have Revit for that reason, still doesn't use it for design). Laggards not only make their life worse (less efficient, less quality), they also make our life harder. In my office some architects refuse to use Revit. So when I have to do MEP design for them, I take their PDF and recreate the architecture for my model. All wasted time and their plans are totally insufficient due to lack of phasing, 3D etc. I spend more time on dealing with architecture than the actual MEP design.