The present invention relates to a twin-wire web former in a paper machine, comprising a single-wire initial web forming portion, preferably a substantially horizontal initial portion, or an equivalent web forming zone such as a twin-wire zone placed in connection with a forming roll. After the initial web forming portion, a twin-wire forming zone starts in the area of or in proximity to a ribbed deck of the first forming shoe placed inside one of the wire loops. The present invention also relates to a method for dewatering a web in a twin-wire forming zone.
In the assignee's Finnish Patent No. 75,375(corresponding to U.S. Pat. Nos. 4,614,566 and 4,744,866 assigned to the assignee herein, the disclosures of which are hereby incorporated by reference herein), a web former of a paper machine is described which constitutes one of the starting points of the present invention. The web former described in FI '375 is utilized in particular for modernizations of Fourdrinier wire parts. This web former comprises a lower-wire loop placed in connection with the headbox of the paper machine and forming a single-wire, usually horizontal initial portion of the dewatering zone, in which zone water is drained through the lower wire by means of dewatering members out of the web that is being formed. The web former also comprises an upper-wire unit, in which an upper wire is guided by guide and web-forming rolls. The upper wire together with the run of the lower wire form a twin-wire dewatering zone which starts at the first forming roll. The first forming roll is placed inside the loop of the upper wire and is provided with an open hollow face. After the first forming roll, a forming slope is arranged in the twin-wire dewatering zone inside the loop of the lower wire. The forming shoe has a curved deck that guides the loop of the lower wire and whose curve centers or center are/is placed at the side of the lower wire loop. After the forming shoe, an additional forming roll is arranged inside the loop of the lower wire. The forming roll guides the twin-wire forming zone and has a guide sector on which the run of the twin-wire forming zone is curved downward.
Substantially the entire twin-wire forming zone in accordance with FI '375 is situated above the level determined by the single-wire initial portion. The twin-wire forming zone is curved upward from the plane of the initial portion on a sector of the first forming roll. After the initial dewatering has taken place to a suitable extent, e.g., through the lower wire in the single-wire initial portion, additional dewatering takes place in the twin-wire dewatering zone initially on the sector of the first open forming roll in two directions through both of the wires. In the area of the next forming shoe, the dewatering takes place primarily upwards through the upper wire. Thereafter, the dewatering pressure is increased further in the area of the second forming roll while the dewatering still takes place primarily through the upper wire.
The web formers described in FI '375 and other, corresponding formers have been marketed by the assignee during the last 10 years (since the early 1980's ) under the trade marks "Sym-Former" and "Sym-Former R". From the use of these formers in several different applications, a considerable amount of operating experience has been obtained and has provided the background and the starting point for the present invention.
With increased running speeds of paper machines, several problems in the web formation have been manifested with increased emphasis. In the former section of a paper machine, the phenomena that act upon the fiber mesh and upon the water, which is still relatively free in connection with the fibre mesh, such as centrifugal forces, are generally increased in proportion to the second power of the web speed. The highest web speeds of some existing paper machines are already of an order of about 1500 meters per minute. However, machines are being planned in which a web speed of about 2000 m/min or even higher web speeds will be possible.
With respect to the prior art related to the present invention, reference is also made to U.S. Pat. No. 4,769,111 granted to Messrs. A. Ahlstrom Corporation, to the assignee's Finnish Patent Application No. 885609, as well as to the Finnish Patent Application No. 885606 and 885607 of Messrs. Valmetit Ahlstrom Inc., in which formers marketed under the assignee's trade mark "MB-former" are described.
Further, reference is made to the assignee's Finnish Patent Application No. 904489(corresponding to U.S. pat. No. 5,215,568, the specification of which is hereby incorporated by reference herein), 911281(corresponding to U.S. patent application Ser. No. 07/850,505, the specification of which is hereby incorporated by reference herein), 913112(corresponding to U.S. Patent application No. 07/903,603, the specification of which is hereby incorporated by reference herein), and 920228(corresponding to U.S. pat. application Ser.No. 08/006,372, the specification of which is hereby incorporated by reference herein), in which various combination concepts of an MB formers and hybrid and gap formers are described.