You don't say what you are meshing it for. Is it a Structural model of the duct or are you going to mesh the air in the duct for a CFD model?
If its a structural model of the duct, then open the Geometry in SpaceClaim and extract the midsurface and mesh that with quad shell elements. Don't mesh a thin walled structure like that with hex elements.
If its for a CFD model, you don't have a solid body of the air. You can get the fluid volume in SpaceClaim.