Duct Tee Pressure Drop

[Anonymous]

Getting into exploring some more of Revit's calculation and sizing functionality on ducts. For the most part things are working pretty good but when i use system inspector I'm noticing the pressure drop associated with tees are much much higher than expected.

I manually calculated some of the tees using the same ashrae chart referenceed in the ASHRAE table settings and come up with a more realistic pressure drop. 

For example a tee that I calculate having a pressure drop of .15 is calculating over 2" w.c. with Revit. 

Anyone run into this or have any advice?

[Martin__Schmid]

@Anonymous  - can you provide substantiating data using the Duct Pressure Loss Report data and a simplified model?

[Anonymous]

See attached for leg 3 sketch. It is a 22" tee with 8" branch. 5335 CFM into the main and 300 out the branch.

See first 3 lines of report below. You see Section 3 has a Loss Coefficient of 96.8. There is a diverging tee (SD5-9) and a 5-gore 90 in this section. The 5-gore fitting has a realistic pressure drop listed when looking in properties. Based on a the ASHRAE tables the loss coefficient  should be 1.2. 

The only way i see numbers getting this high in the SD5-9 chart would be if the Ab/Ac approaches 1.0 (aka branch size similar to main size which is not the case.

Total Pressure Loss Calculations by Sections

Section	Element Flow Size Velocity Velocity Pressure Length Loss Coefficient Friction Total Pressure Loss	Section Pressure Loss
1	Fittings 150 - 0 FPM 0.08 in-wg - 0.030556 - 0.00 in-wg Equipment 150 - - - - - - 0.00 in-wg	0.00 in-wg
2	Duct 150 6"ø 760 FPM - 4' - 0 9/32" - 0.18 in-wg/100ft 0.01 in-wg Fittings 150 - 760 FPM 0.04 in-wg - 1.4625 - 0.05 in-wg	0.06 in-wg
3	Duct 300 8"ø 860 FPM - 15' - 0" - 0.15 in-wg/100ft 0.02 in-wg Fittings 300 - 860 FPM 0.05 in-wg - 96.851734 - 4.46 in-wg	4.49 in-wg

[Martin__Schmid]

@Anonymous  - These are difficult things to validate.. check my math, but based on SD5-9 for the 22x8 fitting, with those airflows, I am getting a coefficient of 96.61841, which is nearly equivalent to Revit's computed value of 96.618401.  

Here's a simplification of the duct network:

The flow and area ratios are as follows:

Therefore, the loss coefficient should be somewhere in the boxed range (so, 1.2 would be the minimum value.. it is certainly going to be more.)

Interpolating for the missing values:

Which matches nearly identically to the Pressure Loss Report

Spreadsheet is attached so you can confirm.

[Anonymous]

Martin,

Thanks for the reply. Everything you posted makes sense and the math checked out.

What I am wondering is where the 0.01 column's in your chart comes from as I have never seen an ASHRAE table that goes below 0.1 for both Ab/Ac or Qb/Qc. Are these 0.01 columns extrapolated from the ASHRAE data? Interpolation is how I've always used these charts and I'm not sure extrapolation beyond the limits of the chart is valid.

In the example we've been discussing if we increase the branch CFM too 533.5 (or 0.1 Q fraction) we increased the flow but the C-factor is now a more realistic 2.13. Logically it doesn't make sense that the same tee with 300 CFM branch flow would have a 4.5 "w.c. pressure drop and a tee 533.5 CFM would have a 0.31" w.c.

[Martin__Schmid]

Hi @Anonymous  - we license the data from ASHRAE directly.  

If you take a look at ASHRAE's iOS App, you'll get the same results.

[Anonymous]

Martin,

Is there a way for me to create or edit my own fittings with C-factor tables?

I've been impressed with the calculations performed by Revit and it appears these tees with branches that have low A and Q fractions are the one stumbling block I'm having for the the pressure calculations. 

These situations are common in VAV systems near the AHU. The fan in this system will ultimately see much much less static than the 4+"w.c. Revit is calculating. In practice we have never seen these types of pressure drops showing up on balancing reports of installed systems. 

[Martin__Schmid]

Hi @Anonymous  - in theory, you could modify the files in C:\Program Files (x86)\Common Files\Autodesk Shared\Revit Shared\Ashrae, but it is not a supported workflow.  What we think is needed is a better data management, audit, and override mechanism, with the ability to also support the detailed fabricatable components.  This idea comes the closest to articulating some of the potential improvements.  Feel free to add any comments/votes as you see fit.

[Anonymous]

Thanks Martin.

That other post, while i don't disagree with the premise, is really getting into the weeds. 

I don't intend to rely on Revit to fully size duct systems but can see it as a first run and as a visual calculation verification tool. I think if these C-factor situations that are at the edge of the lookup table (similar to the situation occurring in this thread) are addressed the pressure drop calculations will be instantly more usable and provide more meaningful data. 

As it sits now these 0.01 columns, seem like extrapolations ASHRAE performed for the computer world and are not based on actual test information. Every published chart I've seen whether it's from ASHRAE, SMACNA or a duct manufacturer does not drop below 0.1 for the A and Q ratios.