Working with ASHRAE Tables

[josep.monzo]

I am going to make an explanation about ASHRAE tables in REVIT.

Just for ventilation systems, Revit links any fitting with an ASHRAE table to get the Co value which is the Dynamic pressure coefficient. Then with the velocity on this line and the Co value you can get the pressure drop fitting.

DP(fitting) = 0.6018 x Co x v2

It means Revit needs two values to get the DP, the velocity and the Dynamic coefficient.

First of all ventilation system needs to be linked with flow rates to determinate the velocity on the line, after you should choose the kind of fitting to get the Co value which depends on the shape and fabrication typology.

Just at this moment if you have designed a correct ventilation system you should select the kind of fitting according your requirements.

As an example, if you are choosing a rectangular elbow, this could be made by different shape according radius (small, medium and long) first picture below, however you can also choose the fabrication typology (with blades and kind of them)

One big trouble with REVIT is just at this point. Teorically you should choose any elbow following ASHRAE Rules based on next conditionals into ASHRAE.xml file.

According these two elbows CR3-1 and CR3-10 both must keep some rules:

CR3-1

Theta (angle) must be between 0 and 180 degrees (this is really good parameter than doesn't match with round elbow)

H (Duct height) / W (Duct wide) relation

r (radius) / W (Duct wide) relation

CR3-10

Theta (angle) must be 90 degrees

Ho (entrance duct height) equals H1 (exit duct height)

Each fitting needs to have this information to satisfy the xml file in order to have the possibility to choose the kind of elbow and in consequently the Co value.

However the real situation in REVIT is showing only the next fitfing.

It means only satisfy the next parameters:

As far as I can understand, Revit must be taken the H value as per L value wrongly. If this was the case we can modify the xml file in order to update the H vale by L value to choose the correct fitting at the moment.

There are many mistakes in elbows, junctions and transitions.

I would be really appreciate if we can join to the team in order to help in this mistakes and if there is a possibility to see what value is linked in each fitting to review if match with the xml file.

[josep.monzo]

Going further, I modified the ASHRAE tables within the 2 folders (C:\Autodesk/WII/....ASHRAE/  and C:\Programe Files (86)/...ASHRAE/) and nothing changes internally, so if you change values into these tables nothing happens in the programme, so these tables are only for information or maybe you need to purge these values, because if you delete these 2 folders, Revit doesn't show any possibility to choose any table. Looks strange.

[pawel.romaniuk]

Hi @josep.monzo

Generally editing ASHRAE Table is not recommended. Let me take a closer look on this issue and consult with team.

I will keep you up to date with any new findings.

What is confusing for me that for similar situation I get following ASHRAE Table Settings: what allow me to choose fitting.

[Anonymous]

Hi @pawel.romaniuk,

I'm also interested in this issue. I get the same problem while choosing my ASHRAE Table.

Thank you,

Regards,

GM

[robert.baillet]

I wish I was savvy enough to understand what's going on "under the hood" with the XML file that josep posted, but the reality is that engineers who are using this software shouldn't have to dig into the code like this to get it to work.

I've started using Duct Systems to calculate static pressure drop, but like many I don't fully trust it.  A big reason for that doubt is that I lack direct control over the what ASHRAE table each fitting is using.  We're supposed to trust that it's using the correct data, and that doesn't sit well with a lot of users.  It's like using a locked spreadsheet that uses formulas that you can't view or edit.  If you can't clearly see and control every step of the calculation, that mistrust will remain. IMO, this is the sort of thing that's stalling the full adoption of Revit in the MEP industry.

[josep.monzo]

Hi Pawel,

I know that in some cases you can choose different ASHRAE tables, but is restricted to the formulas.

If you see the XML file, you can see the restrictions.

As an example if you draw an elbow 89.9 degrees by mistake but quite common, then some restriction are

Theta == 90 so in this case doesn't give the posibility to choose them. I

t should be really easy to modify this restriction like 45 < Theta < 135.

I would like to help AUTOCAD staff in these issues as I am experienced in Mechanical projects as I am working as a Senior Mechanical Design Engineer even many years of experience in REVIT MEP.

I only need to understand how is linked internally within ASHRAE tables and REVIT in order to collaborate and improve the program.

[jorge.fernandez.alonso]

Thanks you very much for the info @josep.monzo.

I'm now in the same problem and, even using correct dimension, I haven't been able to make it work properly.  I guess revit takes the values of the parameters for the ASHRAE  tables from the connectors (height and width) and their geometrical relationship (for the angle between them). Anyone who can corroborate it?

[LjubanL]

Hi,

Very interesting behavior, I had the similar problem with Revit few years ago (while I was trying to solve this problem- now I just put loss coeff and finish with this or I use excel) 

I noticed the following:

if I chose same fittings from different data base for families (example US, EURO, and UK) and put in project 

only one work with ashrae tables and the others dont , also interesting is that geometrically the same fittings do not have same data within the family.

Also, when you find the one that works, it becomes a template for everyone else.

I also tried to change the ashrae table, but whatever I do in the table, nothing happens :).

Please find Rectangular elbow from my data base....

BR 

[josep.monzo]

I guess if the solution is working with a pluggin...

I saw some pluggin adding information about the noise reduction within the ducts too.

I want to start making my own pluggin, moreover I need to learn as I want to make different one for booster cold water calculations using loading units with restriction.

Even Rainwater system it doesnt' make sense to use loading units... I always works with flow rate in l/s from roof.

[nhoorzad]

Hello @pawel.romaniuk,

Has there been any progress on this issue? Anything to report?

Thank you,

NH

[Thommy_]

I think the ASHREA tables are useful only in the US and in Europe you need to buy an external tool which uses the api ( https://thebuildingcoder.typepad.com/blog/2013/07/user-mep-calculation-sample.html ) to extract the values from the content to calculate pressure drop according to local standards etc. The Content need to have specific parameters to be useful for the external tool. In most cases the external tool comes with a special content package suitable for specific country. 

The workflows with the ASHREA tables is not comprehensible for me. Documentation non existent how to adjust it to other standards than US with out programming a own tool. If you know a good documentation please give me a link, ty!

Best, 

Thomas

[DARTEC]

how ca i override the ashrae value in rectangular Tee duct fitting . because in report the value is complettly wrong 

thanks

[jan.smolik]

The strange-looking values are caused by setting decimal comma instead of decimal point in your OS. You have to change it in settings of your OS.

[Iev60047]

It looks like @pawel.romaniuk hasn’t posted on these forums since 2019, so it may be better to get another autodesk member engaged on this issue. Does anyone know how we could escalate this issue?