The center line will appear when your model graphics style of your view is set to wireframe. Wireframe lets you see through the duct. If you change the model graphics style to hidden line, the center lines will go away. This will also generate the mechanical hidden lines when two ducts or pipes are over top of each other. Another way to turn off the center lines (even in wire frame) is to uncheck the centerline box under the duct category in the Visibility graphics overrides. I hope this helps.
Glen Walson, PE
Ideate Inc.