A spiral or round duct is typically made from galvanised steel and occasionally aluminium. If not from sheet metal, spiral ducts can be fiberglass board or a flexible plastic and wire composite.
Spiral ducts are being used more frequently and can be any length, but the most common cut is 6 mL in order to fit in a standard truck. The ducts can be connected using slip joints or flange-to-flange connection methods, depending on additional performance requirements.