Rectangular Duct Design per SMACNA

[strainstress]

Hi All,

I have question about rectangular sheet metal duct welding.

When joining the sheet metal to form a rectangular duct, is it required to use a continuous longitudinal seam weld at the welded corners (See Figure 16, SMACNA 1980), or is intermittent seam weld acceptable. Use of continuous seam weld seems like on overkill. What is the common standard practice in duct world ?

Thanks
StrainStress

 

[chicopee]

If I remember correctly, ducts for exhaust air laden with oil vapor generated from commercial deep fat fryers, you'll need liquid tight seams (continuous seam weld) per NFPA. I suppose it depends on the purpose of the ducts, so you'll need to tell us more about the ducting system.

 

[Drazen]

as chicopee said, you do it only when you are mandated to do it, welded duct is something far from common practice.

 

[berkshire]

Grease ducts are typically the only ducts with heavy wall construction and welded seams.
Most other ducts use folded seams, i.e. Lockform, Snaplock.
There is a system that uses a pocket and flange that is tack welded about every 12 to 18 inches with a 1" stitch.
As Drazen and Chicopee say don't do it unless you have to.
B.E.

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

 

[DrRTU]

longitudinal seam would be odd. I could see the application of a transverse seam. Lets say a you turned a flange then used a weld to replace mechanical fasteners, applied sealer it may work. Define pressure class, +/-, use and size (w x d). Use the tables to find an acceptable seams and sealing requirements. Be careful, other codes or AHJ requirements may force a more conventional construction technique. We have built plenum walls for fan housings like this but without the class and use, you cannot build a proven TS. Make sure you apply the correct standard AND year.

 

[berkshire]

DrRTU (Mechanical)
Yes the longitudinal seam machine was an odd duck, I have only ever seen one used in a shop, It looked a bit like an end-cap machine, it turned a pocket on one side and a flange on the other. In use a sealant gun laid a bead in the pocket, then the duct was folded, assembled and tacked with a Mig welder.
I think the snaplock superceded this system.
B.E.

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