Duct fitting loss coefficient - metric vs. IP and is the ASHRAE duct design guide any good? 1

[EnergyProfessional]

I have the SMACNA "HVAC Systems Duct Design Guide" That includes a good explanation and also duct fitting loss coefficient numbers. There are also free guides, like from McGill

But I wanted to compare what SMACNA says to what ASHRAE recommends. I use Revit for design and that allegedly uses the ASHRAE tables. Since Revit doesn't do that well, I wanted to compare to a manual calculation with the ASHRAE fittings. Problem is, I only find metric data and wonder if there is a conversion from SI duct fitting loss coefficients to IP?

I also wonder if buying the ASHRAE duct Design Guide is worthwhile or if it just duplicates what I already have in SMACNA. They don't have a published table of content or a "preview" (like Amazon has). It also isn't clear if that includes the Fitting database. they sell the duct fitting database separately and it seems to be a subscription. I really just want to see some of the ASHRAE fittings REvit uses and verify it calculates pressured rop correctly. So I don't want to buy it all if I don't have to.

And if there are differences between the ASHRAE and SMACNA duct fitting values, which is better or more correct?

 

[nuuvox000]

I've only used the SMACNA guide unfortunately but I have to think that both guides would be similar if they're using scientific methods so I doubt you can go wrong with either of them.

I got the ashrae duct fitting database subscription for a year to try it out but I was shocked at some of the standard fittings they did not have there. After doing some testing, I realized that my rules of thumb for sizing duct gave me the same results as meticulously using the duct fitting database pressure drops; at least for low pressure duct systems (pretty much all I do).

Personally, I don't want to use Revit's calcs or automatic sizing because I don't trust Autodesk. They're in a lucky position of basically being a monopoly for their niche right now because some of the things they do in their software is bizarre and puzzling. I really don't trust them all that much but I have to use their software until there is a solid alternative.

 

[EnergyProfessional]

Thanks. I actually found there are the ASHRAE tbl-files (can open with text editor) under C:\Autodesk\Revit_2022_G1_Win_64bit_dlm\x64\RVT\PF32\CF32\AS32\Revit Shared\Ashrae
From what I saw for some fittings they do a good job and list the C-values based on diameter. But not for all. And for many fittings they only list the 1.5D.

I've been using Revit for ducts for a long time and think the fittings are the only flaw (and I'm working on correcting this). For diffusers, or devices you can implement formulas to automatically calculate the pressuredrop based on actual flow. For the ducts it also works pretty well. and obviously it does a good job using the correct duct length and size etc. It also does a good job determining total pressure drop and critical path. Where it fails is the use of the ASHRAE fittings. the first problem is, as you pointed out, ASHRAE doesn't always have a fitting and in that case Revit by default just uses 0. Obviously this isn't good. and for some fittings it has multiple ASHRAE options, but the default isn't necessarily correct, and again, ASHRAE may not have the correct one. So for an 1D elbow it will use the 1.5D ASHRAE fitting. By default it also may use a smooth elbow, even if the family is gored. and this is the part I'm working to resolve. overall i think Revit is better than the alternatives as long as you know the limitations.

So I won't buy the ASHRAE database. But based on the text files it looks like the C-value is the same for metric and IP. One fitting listed the C-values and showed the diameter as 75mm (3in).

The SMACNA C-factor seems to equal an ASHRAE factor for around 8-10" duct. That's fine since I don't use smaller ducts anyway. And for lager duct diameters, ASHRAE reduces C, while SMACNA doesn't correct for that. So for larger ducts, SMACNA will be quite conservative.