Exhaust Ducting Thickness

[RHolmes8]

My question is regarding the thickness of Material needed for exhaust ducting from a generator to a heat exchanger. A microturbine generator with (5) engines will be supplying its exhaust to a heat exchanger.

The design criteria is:

Exhaust Temperature: 535-600 F
Exhaust Flow: 11,720 SCFM or 22,0003 ACFM
Exhaust Specific Heat: 0.265 Btu/lb-degF

The ducting will be 30" diameter to keep the back pressure below the maximum threshold of 8 in W.C. but I am unsure on the thickness of material needed for this. I have seen on a previous job 304 stainless steel 16 Ga was used but my question is how did they come to that conclusion. Would 18 GA be sufficient as long as it is supported properly?

 

[willard3]

You will need to refer to SMACNA "High Pressure Duct Construction Standards" manuals.

 

[chicopee]

I searched willard3's response and here is a reference: Smacna Hvac Duct Construction Standards Pdf at

http://www.mabookan.com/sm/smacna...duct-construction-standards

 

[willard3]

I have the publication on my desk. Perhaps it's not in publication currently.

 

[RHolmes8]

Thank you for the suggestion about the SMACNA but I could not find the information in there either.

All I have found regarding thickness is something like this:

Fume Exhaust Ductwork and Plenums:
41. Stainless Steel Construction:
a. Round ducts and fittings: Longitudinal seam 316L 16 gauge stainless steel for all sizes with welded seams and fitting joints.

Which was from a building specifications for a project. Nothing yet that says why they would make decision to choose the stainless to be a thicker gauge then the galvanized steel.

 

[chicopee]

16 gauge is thicker than 18 gauge, so I would go with the thicker wall; however, I would go with the thickest wall commercially available to offset the elements of erosion and corrosion.

 

[RHolmes8]

I understand the mentality of over designing a system to reduce the chance of having issues in the future. It is often cheaper to spend a little more money up front then to have to deal with a problem down the line. My question is regarding the engineering reason behind going with the thicker material. I do not know why 16 Ga would need to be selected over 18 Ga material. Going with the thickest commercially available would not be logical because of the price of the material.

10 GA 304 Stainless Weight per Sqft: 5.67 Lbs
16 GA 304 Stainless Weight per Sqft: 2.50 Lbs
18 GA 304 Stainless Weight per Sqft: 2.01 Lbs

Going from 18 to 16 GA would increase your cost of material by 25% while going to 10 Ga would increase it by 282%.

 

[RossABQ]

Thicker material = longer support spacing
Use of stiffener rings can also extend the support spacing for a given gauge.

There isn't a single solution. Say for instance the ductwork is running along a wall where inexpensive supports can be used at shorter spacing. In that case it may make sense to use a lighter material and more supports (at least from total cost perspective). If the duct is running high in the air between structures, fewer supports are cheaper than the added material cost of thicker ductwork.

There are also forces on the ductwork that need to be resisted; thermal, wind, snow (maybe), insulation, etc. SMACNA's design guide is definitely a big help, but involving the fabricator is the best way to get material and installation costs.

 

[SAK9]

I am not sure if SMACNA or any of the ducting standard deals with duct work handling gases at 600F.Thickness will be decided by thermal stress and corrosion allowances .Have you seen how bright the exhaust pipe shines during operation?

 

[chicopee]

Well, do a hoop stress calculation, followed by bending stress from its own weight and other elements if they exist between supports from the expected highest static pressure and I think that the wall thickness will be paper thin.

 

[berkshire]

I think you need to research the uniform mechanical code for PRODUCT-CONVEYING DUCT

– Ducting used for conveying solid particulates, such as refuse, dust, fumes, and smoke; liquid particulate matter, such as spray residue, mists, and fogs; vapors, such as vapors from flammable or corrosive liquids; noxious and toxic gases; and air at temperatures exceeding250ºF (121ºC).

You are looking at industrial ducting. The SMACNA code does give a minimum wall thickness of 22Ga for spiral and 20 GA for long seam for galvanized unreinforced pressure duct at 10" w.g., 27" -36" diameter. Then in the next paragraph in the commentary section It says " Designers should consult SMACNA's Round Industrial Duct Construction Manual" for : Systems other than clean air. Extended hangar spacing, Engineering design of bolted flanged joints. Etc..
B.E.

You are judged not by what you know, but by what you can do.

 

[IRstuff]

perhaps this will help:

https://law.resource.org/pub/us/cfr/ibr/005/smacna.duct.1995.html

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[berkshire]

Ok,
The section I was quoting from was page 3.3 and the other section I quoted was 3.2 Commentary .
B.E.

You are judged not by what you know, but by what you can do.

 

[willard3]

Not very careful looking, Chicopee, but, then, people see what they want to see

https://law.resource.org/pub/us/cfr/ibr/005/smacna.duct.1995.html

 

[chicopee]

You are right willard3, however, my suggestion is for the inquirer to get a feel on the thickness of the metal under internal static pressure and by no means to suggest that the calculated thickness is the way to go.

 

[willard3]

I would also suggest that the OP hire an Engineer who does this and not try to bootstrap a solution.

Forum policies begin as follows:


>>>>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be a substitute for appropriate professional assistance within your field or geographical region.

 

[hoodsman]

Use SMACNA round industrial duct construction manual and then talk to a fabricator about which gauge he would use. A heavier gauge may be easier to fabricate and might require less reinforcement.