Double-walled structure with perforated inner tube

摘要

The double walled structure includes an inner tube (2) with a perforated wall. An outer casing is spaced from and encloses the tube. A filler material is inserted between the tube and the casing, and a fluid permeable cover is tightly applied on the outside of the tube covering the perforated wall. The tube is a helically wound lock-seam tube formed from a perforated strip. The cover is formed from a second strip and is helically wound together with the first strip. The second strip has its longitudinal edges secured in the helical lock-seam of the tube. The two strips are the same width.