Modify duct velocities in Revit

[msbeityYBPXZ]

Hi all, the velocity shown on the duct properties in revit is based on a certain calculation (i believe flow/cross section area). I tried to search for a global parameter or whatsoever to allow changes but i think the calculation is built-in. How can i modify the result to my needs as the external tools (approved by the consultant) give higher velocities than what revit provided? So any suggestion would be appreciated.

[iainsavage]

I don't understand what you mean. The velocity in the duct IS the flow rate divided by the cross-sectional area. What else could it possibly be? That's not a Revit method it has been around for ever.

Could your external tool be wrong?

[msbeityYBPXZ]

Actually, this is not the result in the tools used worldwide... 

First check the below thread in other forums speaking about the same subject:

https://forums.augi.com/showthread.php?98830-Velocity-in-Ductwork

Also I will attach the 3 results for a flow of 4050cfm with 40" x 12" duct using revit, ductulator, and Mcquay duct sizer.

[iainsavage]

You have to be careful if using ductulators etc - are they calculating the velocity based on the actual duct dimensions or are they using the circular equivalent diameter of a rectangular duct based on equivalent pressure loss per unit length?

4050 CFM in a 40x12" duct has a velocity of 1215 FPM. The ductulator is giving you a higher velocity because it is assuming a cross sectional area based on the circular equivalent duct.

Note that circular equivalents are not geometrically equivalent, they are the equivalent diameter which would give the same pressure loss - its not the same thing.

The one method that you don't mention is using a calculator. 4050/(40/12 * 12/12) = 1215. You can't argue with that.

So its not Revit which is wrong.

[iainsavage · Accepted solution]

As you can see below its the loss per 100 foot which remains constant when taking the 23" dia duct equivalent of the 40x12" rectangular duct.

The cross sectional area differs so the velocity differs.

Your external calculators are calculating the velocity incorrectly.

[msbeityYBPXZ]

Thanks for your reply, but as I was checking Ashrae, they refer to equivalent circular diameter since the results are not consistent, check the below paragraph extracted from Ashrae Fundemantals :

and that is the reasons consultants don't accept the velocities resulting from revit, in all cases my main question is if there was a way to let revit calculate velocity in a customized way different from the built-in functionality.

[iainsavage]

I think you are misunderstanding the principle of circular equivalents for rectangular ducts. That excerpt that you have posted is quite clear that equivalency is based on:

NOT equal velocity.

The velocity in the duct should be based on the cross section of the ACTUAL duct, NOT the cross section of the (equal resistance) equivalent duct.

Forget Revit - how would you do it with a pen and paper or with a calculator?

I'll tell you - you would have to corrupt your calculation by using the cross sectional area of the (equal resistance) equivalent circular duct in your calculation rather than the actual cross section of the actual duct, and you would be WRONG to do so.

I've been doing this for over forty years, long before Revit existed, and I'm sorry to tell you but your consultants are simply wrong in what they are saying.

[iainsavage]

If you want to you could create your own shared parameter and use Dynamo to calculate the value of that parameter using the incorrect formula using the equivalent diameter instead of the actual duct dimensions.

It would not change the correctly calculated built-in Velocity parameter value but would give you a separate secondary value to meet your needs.

[KoosU]

Edit: I need to agree with the other posters that the average velocity can only be calculated on the cross sectional area.

You can see there is a "hydraulic diameter" calculated  but I have never used that to calculate duct velocity.

I do understand your concern though because air does not flow in the corners of a duct  and you do get the highest velocity in the middle of the duct.

For me, I get a lot of value in the Revit calculations. It is very usefull if one needs to draw/design on the fly.

The same goes for pressure drop calculations in large pipe systems. While it helps me a lot, I have yet to see final calculations on large systems that is apart from the flow, correct.

[KoosU]

If you go to MEP settings, there is more than one way that you select the calculations to be done.

[KoosU]

If you wan the velocity based on the hydraulic diameter, it can be done directly from a duct schedule and a simple formula:

I used Velocity = Flow / (Hydraulic Diameter * Hydraulic Diameter *.78)

BTW: It is good that you ask these questions and not blindly accept the results of a software package. This is out job as engineers. In the end, you are responsible for the output. The judge will not accept that you relied on software done by someone else.

[HVAC-Novice]

I think you are mixing up velocity, friction and pressuredrop.

Velocity is just what it is and doesn't 'care about round or rectangular. It is flowrate divided by crossectional area and you get the AVERAGE velocity. note, there is a velocity profile in flow (center flow is faster, flow at the wall is slower). Revit, Excel, or your hand calculation will come up with the exact same result for round and rectangular. 

Those equivalent dimensions are to make up for different pressure drop in a round vs. rectangular duct at a given velocity. Rectangular duct has corners that increase pressure drop etc. There are multiple methods to calculate pressure drop in round and rectangular duct and Revit lets you select them. But for velocity, there is only one method.