Loss Method for Duct System

[asommer]

If the Loss Method for a duct fitting is set to "Coefficient from table", and I see the Ashrae Table that applies to that fitting, shouldn't the "Pressure Drop" be displayed?

I cannot seem to get a couple of systems to calculate Static Pressure.  Everything is set up perfectly, fittings either have specific loss, or are defined from table.

[julianjameson8403]

In an ideal world - yes you would as the manual method used by Ashrae allows for this. I think that the Loss Co-efficient parameter is a bit misleading as it only becomes active if you wish to place a specific one manually. Perhaps there should have been either been two - an Ashrae Loss Coefficient and a Manual Loss Coefficient or they should allow that parameter to toggle between the greyed out Ashrae version and then have be cleared if a manual input is required instead.

I find that the system volume parameter also struggles to populate half the time in the same way the static pressure is not calculated due to the old Not well connected routine. You find that too?

[asommer]

Yes, there is a "Coefficient From Table" (Ashrae Table) and a "Coefficient" (manual).  Either one does NOT DISPLAY THE PRESSURE LOSS.  Therefore, I don't even know if it is working correctly.  The tables or coefficients should do whatever calculations they calculate, and display the Pressure Drop.  They DO NOT.  SHOULD THEY?

[julianjameson8403]

I understood what you said last time so no need for capitals.

I know there is both options but they are part of the "Loss Method" list parameter and not two separate parameters which is what I was talking about. These separate parameters would display either one co-efficient value or another depending up on selected option and relevant Ashrae Table.

Something in the back of my mind tells me there was a discussion somewhere, either on the web or via an ADSK CSE, where it was determined that no - the co-efficient is not supposed to show and is used in the background for determining the pressure loss in the system and respective sections. The same applies to the pressure drop parameter which is only used for manual input.

To find out the pressure loss for the fittings under 2012 rules and hence whether Revit is doing its job correctly would require a mixture of system inspector, duct schedule and a copy of the Ashrae Duct sizing method - so basically a ball ache.

A link to a short blog on System inspector - in case you hwdn't seen it before.

http://inside-the-system.typepad.com/my_weblog/2011/09/relationship-between-system-inspector-and-sys...

2013 has pressure loss reports and these should solve this issue. Initial impression from brief testing is that they do but I still have to look into this further.

[asommer]

I didn't mean to yell (at you).  I'm so close to getting this stuff to work completely that i'm feeling the static pressure in my head.

I'm really mad at this program for not telling me exactly what the problem is.  It could do this.  It knows what the trouble really is.  It's probably not really a problem, other than it just doesn't understand what I am trying to do.  I am afraid that if I spend too much time on this without accomplishing anything, I will get in trouble.

SO, if I use the Coefficient or Coefficient from Table, It will not display the calculated Pressure Drop.  I hope it is calculating!

[asommer]

By the way, I cannot use the System Inspector because my system is "Not Well Connected".  I really wish I could find a system that someone put together and works completely, so I can see what the connector settings and project settings are.

By the way- one of my systems has more than 50 components.  Will this not allow the System to Calculate the Static?  I remember this from a while back, but I can't remember the details.

[julianjameson8403 · Accepted solution]

No it won't display it - at least it doesn't in my model and the design data is flowwing okay generally. That said my model is small and relates to only a protion of the building.

With regards to the 50 items - I think you may be referring to the 50 unassigned items which remain on default systems. If you had that then you get a warning saying that it stops calculating flows and pressures.

A thought - have you used the disconnection tool under the analyze tab to check for breaks, etc.

I agree with you that it is really frustrating as there is no explanantion as to why and there is no guidance on the rules as to why Revit can't calculate as required. I had a project once where the toilet extract ductwork system would not calculate between basement and first floor until I put a branch in at the ground floor. No ryhme no reason for why Revit behaved like that but it certainly isn't acceptable behaviour from design software.

[asommer]

Yeah, I saw the "View Disconnects" tool.  It is helpful, but everything is connected.

I believe you mentioned something before about "Taps" not being in the center of the duct (in another ALL CAPS post) possibly causing the "Not Well Connected" situation... could you expand on that a bit if you have time?

[julianjameson8403]

To be honest I don't recall that post. Not sure if that would cause the "not well conencted" or if its just something that affects using connection methods and means moving ducts around to allow connection and then move them back to the right place.

I understand why you use CAPS postings - some of these issues are really frustrating and makes life difficult when explaining to the "purse holders" why you have achieved so little for such a large cost.

Have you tried deleting the system and re-building - not that that is guaranteed to solve it?

[asommer]

I might try re-building it, but that's the last resort.  I'm done trying.

Thanks for your posts.

[rmax]

Try this...

http://thebuildingcoder.typepad.com/files/me204-3_connectors_in_revit_mep_content.pdf

[asommer]

I studied that thing for days and used it Religiously for years.  Still no help.  Everything is checked item by item, and they all are correct.

[sachinmenon1990]

Hi,

i have Done Pressure Drop Calculations for Ducting & Piping using Revit, but in all cases the Coefficient for Shoe does not represent any Value although it indicates that the value is being automatically fed from ASHRAE table.

So what we did was ,either go for Specific Coefficient Options or Specific Loss, but you would need to Input Values manually after Finding the Coefficient from ASHRAE table .You will find those tables corresponding to such calculations In Duct Design Pages in ASHRAE handbook.

it might sound like a lot of work, but it is not.we had also created an excel macro so that we just need to input the duct sizes & Flows Corresponding to shoe branch or Fitting ,the excel files gives us the coefficient .We just input that into Specific Coefficient option.We have successfully Delivered our Project with this Solution.

Also in Revit 2017 & 2018,these above mentioned loss options have been removed, so basically if you are working in 2017 or 2018 you have just the default Coefficient form ASHRAE table option .

Do not know why the two manual options are not present, hope its a bug & might be fixed by autodesk in future Updates.

let me know if this solution works for You or you need any assistance in this matter.

[sachinmenon1990]

yes ,they should be, but unfortunately REVIT doesn't display them. No idea why that is happening. That's why i had to assign them either specific loss or Coefficient values from ASHRAE table & they worked.You have to do it manually, that is  the only problem.

but another major problem in 2017 & 2018 version is that the manual options have been removed they are not available anymore.The default coefficient from ASHRAE table is only available but still same issues persist in it.

So i have raised issues with this to Autodesk, hopefully they will look into it.

[sachinmenon1990]

Hi,

even if these parameters were taken care in background, in pressure drop reports these still appear as zeros, whether it is even revit 2016 versions.

only way to avoid these is to manually go for either Specific Coefficient or loss option.In reports it then shows the values instead of zeros.

[sachinmenon1990]

i don't think the number of systems matter , as my project had quite a number of equipment's connected.3 pumps, 2 chillers, valves, some 20 odd FCU's,  4 AHU's, etc. but we got the pressure drop calculation done.the Reports were Ok two.

[sachinmenon1990]

i have mentioned this before that the number of systems does not matter, as long as your connections to individual units ,your system type, the settings for all those units are fine.Because i had so many systems in my project(3 pumps, 2 chillers, 4 AHU, 20+ FCU etc) ,yet they worked fine or we made them to work one way or another. 

Please refer attached image, You can see that we are getting the require Pressure Drop .System inspector is also shown there(Red colour-critical Path) .Its big loop so might not be visible properly.but yeah certain adjustments has to be done to make sure things work out fine .But yes they can be done for sure.

So Could you please upload your file ,so that i could take a look it it & let you know where you went wrong ?

[julianjameson8403]

Thanks for the offer but this was posted 5.5 years ago when we were using 2012/2013. Things have moved on since this post and Revit has improved a lot since then, both in functionality and integration, although it still has a long way to go.