Calculating the pressure drop in junction by referencing the loss coefficient to the downstream duct section (i.e., outlet section)

[zakarya.kabbara]

I am wondering why the simulation framework in Revit estimates the pressure drops in junctions by referencing the dynamic pressure drop to the outlet section of the junction. Although, as referred by ASHRAE fundamentals handbook 2001, the pressure drops from junctions should be referenced to the dynamic pressure loss to the main section (common straight section upstream), please refer to the screenshot below. Could someone enlighten me, please?

[dyp4f]

It is not only that. It’s a bigger problem.

I mean, the problem is not just that Revit calculates a possibly wrong Pressure Drop value, referencing it to a possibly wrong section of the junction.

The truth is worse: Revit is clearly malfunctioning on that specific matter, failing to make any junctions Pressure Drop Calculations at all. It returns “0”, it cannot calculate anything at all, as a Pressure Drop for a junction.

Look for example in my post here:

https://forums.autodesk.com/t5/revit-mep-forum/ashrae-table-implementation-in-revit/m-p/11346965

Further, if you make a search in this forum, you will find many posts on that matter.

[HVAC-Novice]

Some fitting information are based on the branch. Look at SMACNA design manual. I don't have the ASHRAE tables, but assume this is the case there as well. In addition coefficients are different depending on which way air flow. Like the same fitting in supply or return should use different values. 

I ended up having pressuredrop manually calculated in the fitting. then you can assign the pressuredrop to whatever connector you want. i used SMACNA as a source. 

In cases where they have many different C-values (like for different aspect rations, and in and out flow), I used a reasonably worst case to be conservatives. I didn't want to use different fittings that look the same, but are different for supply, return and different aspect rations. i guess You could make types to do that. But it would lead to errors in design since I likely forget to correct that all the time. 

there are multiple threads about this ASHRAE issue. My conclusion is, the way the ASHRAE tables are implemented is bad, and the ASHRAE tables themselves seem to be lacking. Revit works much better for pipe fittings. Looks like the developer made pipe fittings work reasonably well, but hasn't gotten around to fix the duct fittings yet :-0) Admittedly, pipe fittings are much simpler and don't have the variations duct fittings have. 

[zakarya.kabbara]

Thanks for your response.
Well, that's unfortunate! Do you think anything can be done about it?

[zakarya.kabbara]

Thanks! indeed! Maybe an additional reason why pipework work reasonably and ductwork doesn't, is that in ductwork the pressure losses generated by fittings are often dominant in the system, and that's not the case for pipework. Though, in both systems, there are discrepancies in estimating the local pressure drop coefficients for fittings (C)

[dyp4f · Accepted solution]

@zakarya.kabbara 

You asked: “Do you think anything can be done about it?”

Answer:

Yes, there are some workarounds for this problem, but none of them solve it properly.

Personally, I would prefer the following manual method, if this were not so hard and time-consuming.

Since I cannot have a junction fitting’s Pressure Drop value automatically calculated for the main way and the branch, I manually insert 2 volume dampers on those 2 exits, simulating the pressure drop that the fluid flowing into the junction will have passing through those 2 ways. The volume damper OOTB Revit families allow for the manually setting of their specific Pressure Drop.

In this point, ASHRAE Tables come into play. I calculate independently in the ASHRAE Duct Fitting Database Software (ASHRAE DFDB) the proper values for those exits (main & branch) by inserting the corresponding geometry and flow data to take the ASHRAE calculated values. I then insert those calculated values manually to Revit, into the corresponding preset values of the Pressure Drop of each one of the 2 dampers (look at the Properties window).

All those are Instant Parameters, and you can easily imagine how hard is doing something like this manually if you work on a large duct network, containing a lot of fittings. Plus, you are not going to do that only once (!), because every time you set  some values in, those change all the pressure values across the system, and you will have to adjust it again with new settings, and repeat it again and again, until you will finally have a properly balanced duct system !

That is why those calculations should normally be executed automatically and not manually.

I have 2 screenshots below, showing my method in a very small and simple test duct system.

[zakarya.kabbara]

Thanks for your clear explanation! I highly appreciate it.

[lim.wendy]

Hi Zakarya,

Thank you for posting. It looks like @dyp4f  provided you with detailed explanations, so for now, I am going to mark this thread as "Answered." This won't prevent other folks from commenting on this question though, in the event other ideas come up!

Remember to mark your posts as having an "Accepted Solution" if a solution is found as it helps other forum users with the same issue – so don't be shy about marking it as a solution! Accepted solutions get viewed more and help more users find the answer to their problems.