Wherever the ductwork leaves the compartment from which it serves (revealed in blue) it will have to preserve integrity and insulation fire ratings for the same timeframe as being the compartment by which it passes. Promat has made self-supporting units and not using a steel liner, by way of example https://marcopfuhw.angelinsblog.com/35950827/a-secret-weapon-for-grease-tight-extract-ductwork