| 主权项 |
1. A duct comprising a number of wall sections at least partly produced from a refractory fibre material and a binder agent that holds together individual fibers of the fibre material, the fibre material and the binder agent being compressed so that the individual fibres are connected, which together define a closed, elongate flow space, mutually meeting wall sections being united by the intermediary of at least one longitudinal fold or joint line permitting the duct to be foldable to a first condition in which the flow space has a first cross-sectional area and unfoldable to a second, expanded condition in which the flow space has a second cross-sectional area that is greater than the first cross-sectional area, the duct comprising six wall sections arranged in a form of one of a regular hexagon cross-section and a rectangle in cross-section, the rectangle having two long sides each formed by at least two wall sections and two short sides formed by at least one wall section, wherein there are disposed, in the fibre material and binder agent of the duct, longitudinal rigidifying elements and that the duct has, on its outside, longitudinal bead-like projections which comprise the fibre material and the binder agent. |
