Web formation station at a multi-layer papermaking/cardboard prodn machine has an initial water extraction zone at a fourdrinier with a structured gap between the stock inlet and the suction box

摘要

The web formation zone (1) where pulp is converted into a multi-layer web by extraction of the water content, at a papermaking or cardboard prodn. machine, has a double-fourdrinier structure (D) where the lower fourdrinier (16) has a longitudinal initial water extraction zone (VEW1) as a long fourdrinier. The length (L1) of the gap between the jet of the stock inlet (22) and the water extraction suction box (32) is 0.75-2.5 m and pref. 1.25-2.0 m. The initial water extraction zone (VEW1) has entry and exit bars (17a,17b), across the direction (R) of travel by the first layer of the paper/cardboard web. At least one parallel support bar (17c) and at least one vacuum box (17d) is between the other bars (17a,17b). The vacuum box (17d) is operated with an overpressure against the atmospheric pressure, with holes to the side of the initial water extraction zone (VEW1), or slits with a slit width of = 20 mm and pref. = 15 mm. The water extraction rate in the double-fourdrinier zone is -50%. The time span between the outflow of the pulp suspension from the jet of the stock inlet (22) and the entry into the double-fourdrinier zone (D) is = 0.1 secs. An assembly of bars (L), to form the web, develops the first web layer in front of the web shaping roller (24) of the double-fourdrinier zone (D), in the direction (R) of web travel, fitted with pressure bars (30). The double-fourdrinier zone (D) has a secondary stock inlet, aligned into the gap between the secondary fourdriniers (16,18) as they come together. The primary fourdrinier (10) of a web shaping unit, with a former, has an initial water extraction zone with a gap between the stock inlet (14) and the suction box (32) in a length of 0.75-2.5 m and pref. 1.25-2.0 m. A second web forming unit is at the first, which forms a web layer to be laid on the first layer from the double-fourdrinier zone (D), in an identical structure, and followed by additional and identical units to overlay additional web layers for the final multi-layer web.