When framing a basement, there is a good chance of having to deal with air ducts.
If you have a wall that involves air ducts, weather it is a rectangular plenum or a round heating duct just frame it as if you were framing a basement window except leave the window part out. Always remember to leave some breathing room between the framing and the duct itself. If you have to run electrical wires inside the framing. I recommend to insulate the wires with either pink insulation or better yet with foam tubing (the kind used around water lines).
Below is a simple way of framing around air ducts:
