[2010] Static Pressure Calc. Improvements (OPEN)

[john_hazucha]

Currently, the only way to do duct static pressure calcs in Revit is to...er, it's not possible. All the information is there (if all the fittings ad devices are in the system), but there is no way to get it out in a format that is really useful. It was frustrating to discover this. Revit MEP 2010 is at least the fourth release, and this is pretty basic to engineers, and all the information required is already in the model. I can't even get the system inspector to work between the AHU and the VAV boxes, only downstream of the VAV's.

[mjdanowski]

Please elaborate on the changes you would wish to see made to static pressure calculations and their presentation. This will give people something to go off of when discussing this issue.

Thanks!

[tys90]

It took some time to properly learn how systems and connectors work but I haven't had any troubles since I understood how they work. However, just because of the nature of the way taps work in Revit and the current ASHRAE tables you are forced to use tees and crosses instead of taps to get a proper pressure drop. It would be ok if tees and crosses inserted correctly most of the time, but alas, they do not. Not to mention there is no rectangular to round cross, so I have to put in a rectangular cross and transition to rounds if that's what I need, which isn't correct.

[john_hazucha]

Typically, engineers are concerned about the static pressure drop through a piping or duct system at maximum flow, so they can select a pump or fan that will operate at those conditions, and select a motor horsepower that is sufficient to power the pump or fan. In a complete system, Revit "knows" most of the information required to calculate this pressure drop. -flow rate through the duct or pipe, pipe or duct size, and pressure losses - though the actual pressure drop will vary with the fluid. If Revit assumes air or water, I'm sure we could figure out the rest fairly quickly, but it would be preferable if we could input the viscosity of a glycol/water solution and have Revit do the adjustment internally.

For a duct static pressure calculation, there are multiple possible paths. Ideally, Revit should calcualte the pressure drop for every path, and then pick the path with the highest pressure drop, and somehow identify it. Alternatively, we could specify the beginning (an Air Hndling Unit?) and end (diffuser or register) of the path, and have Revit add up all the pressure drops between them. Right now, the closest I can get to this is to use the System Inspector, which will give me the pressure drop from a VAV box to the end of a duct system that is connected to a diffuser. I can't get it to even look at the system between the air handling unit and the VAV. That may be a problem with the way the AHU equipment family was created. I tried a little sample system using out-of-the-box components, but that wouldn't work either. I could use the System Inspector to look at the system both upstream and downstream of the VAV box, but could not get proper flow information in some parts of the duct system, and got a message that there was no pressure loss information in a standard Revit fitting, which was created by Revit when I connected a VAV box into a duct main.

[john_hazucha]

"Just because of the nature of the way taps work in Revit and the current ASHRAE tables you are forced to use tees and crosses instead of taps to get a proper pressure drop."

Thanks for the tip. Yes, it was a tap that was a problem.

[tys90]

"Just because of the nature of the way taps work in Revit and the current ASHRAE tables you are forced to use tees and crosses instead of taps to get a proper pressure drop."

Thanks for the tip. Yes, it was a tap that was a problem.

You can still use taps but you either have to manually specify a pressure drop or specify a coefficient. Depending on the complexity of a system I've just been specifying a conservative coefficient for all taps in the system. If it's a complex system I may input specific drops along any duct runs I suspect might be the critical run and leave the rest alone. It's cumbersome and easy to miss a tap, which probably isn't the end of the world but still annoying.

[john_hazucha]

After experimenting more with a simple test system with all out-of-the-box Revit components, I see that what I asked for is what the System Inspector is supposed to do already. In some cases, it seems to work, but the time spent "fixing" things in your system and manually inputing data makes the effort not worthwhile - it would have been faster to do it by hand or in a spreadsheet.

One of the bigger problems is that System Inspector is really flaky (in RME 2010) - it does not reliably appear on the toolbar when it should, and there is no keyboard shortcut to make it appear. It seems particularly reluctant to appear if you want to examine a system between an air handler and a VAV box.

Add this to the propensity of Revit to create a disconnect or a "not well connected" system during sizing and the necessity to manually input pressure drop data for some commonly-used fittings, and the time required to get Revit to give you the information considerably exceeds the time required to do it manually.

This is not what I want software to do for me.

So, I guess I want the System Inspector to work, even if my system is "not well connected", and if necessary, give me more information about why things are "not well connected". I also want the large majority of the duct fittings on a project not to require manual pressure drop input.

[rrubert]

I second this. I have also noticed that there is no benefit to doing the static pressure calculations in Revit, except if you are following strict BIM standards that require it (and if you are, I'm so, so, sorry for you). The calcs actually work to a certain degree, but the effort required to get them to work is too time consuming.

What I would like is the ability to make a duct fitting family and assign it a typical ASHRAE fitting number. For example, I would like all vaned, mitered elbows to default to ASHRAE elbow type CR3-16. I could make different families for the different types of ASHRAE fittings.

It would be nice if there was some graphic that showed the path with the highest pressure drop, so we could see if changing a fitting affected the critical path without actually going and redoing the calc.

Also, if there could be some kind of schedule or report that looks presentable that shows the pressure drop calculation. I think there has been some sort of discussion on here about how to make a schedule with the required info, but I think it involved a lot of work arounds and was still not what people wanted.

I ditto all of this for piping.

[john_hazucha]

I think that I have narrowed down the problem with the Sytem Inspector. What I had assumed was "flaky' behavior seems to have a pattern. The System Inspector will not recognize systems where there is a connection that is not "Well Connected" according to Revit's "Check Duct Systems" function.

[sasaelshennawy369237]

goooooooooooooooooooooooooood