Modify display properties to display centerlines for round duct.
This video demonstrates the following:
For MEP elements such as round duct, you can turn on the display of centerlines in hidden line plan and elevation views. Centerlines are not displayed for oval or rectangular duct and are not available for 3D views.
To display centerlines for duct, modify the display properties for Duct and Duct Fittings in the Visibility/Graphic Overrides dialog. For complex fittings, you can override the default centerline by editing the family, adding a model line that represents the centerline of the fitting geometry, and setting its subcategory to centerline.
Centerlines can also be enabled for pipe and conduit.