There are lots of authorized ways of making a welded joint for grease exhaust ductwork. The alternatives will depend on the code on your jurisdiction but should incorporate flanged, telescoping, Bell type or overlapping joints. Promat has produced self-supporting units without a steel liner, by way of example PROMATECT®-Advertisement, PROMATECT®-L500 https://grease-tight-extract-duct45567.webdesign96.com/37441687/about-grease-tight-extract-ductwork