Goes together with the inner flange and inner locking band (with centering extension). The outer locking band is on the skin For additional guidance, rendering it a more robust process. Contaminants of combustion aren't trapped in the flue like it is often present in the corners of an oblong design https://fire-resisting-ductwork-i45566.luwebs.com/37645141/top-guidelines-of-grease-tight-extract-ductwork