This type of duct is made of non-woven fabric. The inner part is filled with highly tensile spiral steel wires to support the duct. Fiberglass with 25mm thickness 16kg-24kg/m3 is used for thermal isulation. The outer layer is covered by reinforced aluminum foil and glass fiber coating. The duct is generally applied on low to medium air velocity with temperature variantion system. This product has passed GB8624B1 fire resistant test.