Two endless belts are formed of steel mesh. The belts are run side-by-side spiralling into the center of the apparatus, reversed and spirally wound out of the apparatus with the outgoing convolutions interleaved between the incoming convolutions. The belts present an inlet at one side of the apparatus and an outlet at the other side of the apparatus. The web is fed into the inlet and conveyed to the outlet. Two or more spiral sections can be run in tandem.