Abstract: A hot air duct is formed from thin walled metal tubing. The duct is provided with a plurality of linearly spaced reinforcing rings. Each reinforcing ring is formed from sheet metal and is channel-shaped in cross-section. The rings are rigidly secured to the outer surface of the duct and positioned so they project radially outwardly from the duct with the channel portion inwardly open and facing the outer surface of the duct. With this arrangement the channel portions are closed off, defining ring shaped chambers around the duct. Each ring shaped chamber surrounding the duct is provided with holes to provide communication between the ring-shaped chamber and the ambient air around the external surface of the duct.