ASHRAE Duct Fitting Database

[orla.williams]

I know this has been raised before but the thread is now closed

I've set up a duct system and used revit MEP to calculate the pressure drop through the system. I have also done a check calculation manually and the fittings Revit MEP is using is different ASHRAE fittings to those it should be using. For instance for a mitred bend Revit uses CR3-17, which is actually a Z shaped elbow and not CR3-6 which is a mitred bend. At the moment some of my systems are over calculating and some are under calculating the pressure drop by around 15% in revit. This doesn't give me much confidence in the output from revit.

Is Autodesk going to fix this bug? Is there a way to assign the ASHRAE part by type? The way it seems to work is that the fixture is specified as a part type - elbow for instance, and this then pulls up the ASHRAE.xml file which has a program which defines the part based on the size, flow, angle, etc. which would be fine except that it doesn't seem to be working. Is there a way to manually override this in the fitting? I can't find where you would add the part type as a parameter in the types or element properites that you could then change by instance.

i managed to schedule out the pressure loss by system section, but every time i try to schedule out the part types for the system it comes out blank. Also, are the revit MEP calculation algorithims approved by ASHRAE? Have they been checked by ASHRAE to be accurate?

If anyone has any help or if anyone at Autodesk is reading this - help!

[andrew.b]

We've found the same thing, the built in ASHRAE database is pretty much useless. For tap fittings it won't even give an ASHRAE fitting code even though the correct entry is in the ASHRAE folder. And like you mentioned, you can't really schedule anything usefull from the fittings

We've gone ahead and created a whole bunch of shared parameters, which can be scheduled, and added our own calculations within the fitting families to use the AIRAH loss values (you could do the same for ASHRAE or CIBSE too).

Unfortunately with duct fittings at least, you can't link to an external table of values (as far as I've found), so you will need some pretty long nested IF AND statements to work out based on your geometry/flow what the loss value will be. On the connectors, we've changed them to global rather than fitting and assigned the flow and pressure drop shared parameters to the connectors. You can then have calculations within the parameters that will read the flow from the system, work out the loss value, calculate the pressure drop and assign that back to the connector pressure drop.

All this can be scheduled for checking later and will work for the system browser calculations as long as you modify all the ASHRAE .tbl files to have all values set to 0, otherwise you end up with your custom pressure drop added to whatever Revit though was a good number to guess.

A fair bit of effort to set up in the first place, but it is very useful for verifying Revit's calcs

[parleyburnett]

Is this still an issue with recent versions of MEP? Also, you stated that you are mapping the Pressure Drop parameters ... so that the calculations output Pressure Drop. Did you mean you're mapping the K Coefficient parameter so that it can calculate pressure drop given a certain flow etc? Otherwise, your PD calc will be wrong because by giving it the Pressure Drop, it's disregarding the GPM.

Also, I'm getting some very strange PD calculations when using a manufacturers spec data for K Coefficient. Any ideas?