Depends on the HP/CFM of your system. I believe they recommend 26 ga for
systems 2-3 HP and Spiral pipe for 5 HP. 30 ga is for HVAC which run at
a positive pressure and Dust collectors run at a negative pressure or
vacuum and require heavier metal to prevent the pipe walls from sucking in.
This website has a lot of good info > http://www.airhand.com /
If you are destined to use thin gauge pipe, I've seen a DIY spring
loaded vacuum release flapper (made from plywood) which works to
prevent pipe collapse. Can't give you a link, but use your
imagination. As a side effect, it gives the pipes a shot of air to
clear out the shavings when the blast gate on the machine is closed.
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