I have an 18"diameter duct that is connected to a dust collector that has chemical charge for deflagration control. The vendor states that the duct must be rated for 3.8 psi but no one can tell me what gauge the duct needs to be to meet that. The engineer tells me that the safe way is to use schedule 10 pipe but that seams excessive. No trick question here.
Check the SCACNA HVAC Duct Construction Standards. I don't have a copy around but it should tell you there. No less than 24 gauge min. possibly thicker.
You might talk to Fab-Tech Inc. Their primary products are SS ducts coated with PTFE for chemical resistance, but they also sell it uncoated. As I recall it is rated into this pressure range. (I'm not clear if you are talking about positive or negative pressure rating? On a dust collection system it could be either)
A critical part of your system is going to be the joints. It may have to be a welded system.
Review the SMACNA Industrial Standards. I just checked the 8/2004 Rectangular Industrial Construction Standards - the tables are good to 150" wg (5.4 psi) @650 F on carbon steel. I cannot find the new round standards, the old (9/1999) industrial round are only good to 50" wg. You may want to ask a local design firm if you can review a copy before you purchase.
