[2011] Duct Calculation Improvements (OPEN)

[jrap]

Since Revit is supposed to be a modeling tool, duct velocities should be calculated the same way designers calculate them, using the Huebscher formula:

De = (1.3*(a*b)^0.625)/((a+b)^0.25)

(From Chapter 35 of the ASHRAE Fundamentals Handbook)


Due to the nature of fluid dynamics, the air velocity in a duct is not simply Flow/Area as Revit calculates it. Since air flow in a duct follows the laws of physics, and not the laws of geometry, Revit should correct this.

We use this tool combined with a color coded duct legend for three main reasons:

1. Accoustical consultants have duct velocity requirements and this function allows us to easily show compliance and areas of concern.

2. When squeezed for space, we can easily show the problem areas to architects in a graphical form (Bright red is too small duct!).

3. Although we know how to size ducts, it is easy as any other function we perform, it is nice to keep accurate model data within the model. The old way was to separately type in AutoCAD on hidden layer or on piece of paper to keep track of duct velocities. As the design changes it is a lot of time to update velocities.


(Thanks to Buzz for his comments on a thread in the general forum)

[tys90]

Just a comment about how Fundamentals uses velocity. Refer to 2009 Fundamentals, page 21.22, Example 6. A quick calculation of any of the velocities listed in Table 9 will reveal that the number is simply flow divided by area, regardless of duct shape. This velocity is used to find the velocity pressure which in turn is used to find the pressure drop in fittings.

Fundamentals finds sectional pressure loss by reading the table on page 21.8, a friction chart for round duct. The chart is based on round duct, which is where the example uses the Huebscher correlation to find the equivalent round duct. Friction is loosely based on geometry of the cross section (all else being the same). For equal friction and flow to happen (which is what the Huebscher equation assumes) with less area (the equivalent round duct) you will have a higher velocity. Actually the study found that for equal friction and flow in a round duct there is less cross sectional area, mostly because a round duct is, well, round and a rectangular duct has corners. The correlation just allows us to estimate what those differences are in numbers. This does not magically make the velocity higher in a rectangular duct as a ductulator might make one think. It shows you what the mean velocity would be in an equivalent round duct at equal friction and flow.

I believe Revit uses the Darcy equation for calculating sectional ductwork friction loss, which uses the ducts mean velocity but accounts for hydraulic diameter. On page 21.7 "From experiments using round, square, and rectangular ducts having essentially the same hydraulic diameter, Huebscher found that each, for most purposes had the same flow resistance at equal mean velocities."

So, are you saying that the mean velocity in a rectangular duct is not flow divided by area? Now that breaks the laws of physics.

Maybe it's more accurate when calculating sound data to use the Huebscher empirical relationship to find the mean velocity in a resulting circular duct?

[jrap]

I believe Revit uses the Darcy equation for calculating sectional ductwork friction loss, which uses the ducts mean velocity but accounts for hydraulic diameter.
....
So, are you saying that the mean velocity in a rectangular duct is not flow divided by area? Now that breaks the laws of physics.
...


tys90,

thanks for the prod in the right direction. If this is the case, I retract my request from Revit.

I admit I glossed over the "mean" in that referenced ASHRAE section, that was my biggest mistake, I assumed center of duct velocity was the variable of concern. We all know what happens when we assume something.

Of course, mean velocity is Q/A, I just made the bad mistake of assuming center of duct velocity, which cannot be simply Q/A with turbulent flow.
....

Now I need to follow up on my other assumption, did the Acoustical Consultant care about center of duct or mean velocity?

edit... and for the record, Lindberg's "Mechanical Engineering Reference Manual for the PE Exam, 12th edition" explicitly states that the ASHRAE charts (and the Trace Ductulator, by extension) are innacurate for rectangular duct velocity.