Most people's impressions of the inside of ducts come from action movies. Heroes gain access or victims find escape through rows of pristine ducts. However, movie ducts have no obstructions or protruding objects to inhibit travel within them. Ducts are connected at each joint with screws or steel rivets that protrude into the ducts.
Steel rods may be placed at various intervals through the ducts to keep them from collapsing. Volume dampers may be placed at any point in a duct to moderate the air flow.