Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Patent
1995-10-31
1999-02-02
Bennett, Henry A.
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
34453, 34456, 34117, 34170, 34125, D21F 500
Patent
active
058649654
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method in drying a paper web in the dryer section of a paper machine comprising a plurality of drying cylinders for producing a web having substantially maintained width throughout the dryer section, said web, together with at least one drying wire or parts of a drying wire, runs in one or more free draws each extending between two rolls, at least one of which is a drying cylinder, a partial vacuum being maintained by suction means within the longitudinal edge portions of the web.
The expression "free draw" shall in the present context be understood as the distance that the web travels upon passage from a first roll to the next following roll, regardless of whether the web is supported by a drying wire or the like or travels unsupported in a so-called "open draw".
The invention also relates to a device in the dryer section of a paper machine for producing a paper web having substantially maintained width, said dryer section comprising a plurality of drying cylinders and at least one drying wire supporting the web, said drying wire or parts thereof and the web being arranged to run in one or more free draws, each extending between two rolls, at least one of which is a drying cylinder, said device comprising suction means arranged to maintain a partial vacuum within the longitudinal edge portions of the web. The most important application of the invention is in drying paper for use when the bending stiffness, tensile stiffness, tensile strength and compression strength are essential quality properties.
It is a well known phenomenon that a paper web has a tendency to shrink during drying. The physical reason for this has proved to be more that the fibres becomes thinner than that they become shorter. However, this means that at the point of intersection between crossed fibres, the fibres become wrinkled and the strength properties are thus greatly altered. Briefly it may be said that free shrinkage gives a tough material able to absorb a great deal of energy before rupturing, while the paper becomes relatively stiff if drying occurs under restricted shrinkage or even with a certain amount of stretch. Free drying is used, for instance, in the manufacture of sack paper. In this context the ability to withstand strain is important whereas the stiffness is of no great significance. Drying with restricted shrinkage is used, for instance, in the manufacture of paper for envelopes and corrugated fibreboard.
U.S. Pat. No. 4,807,371 describes an apparatus for retaining the lateral edges of a paper web close to a felt. The web and felt pass around a special vacuum roll situated between two drying cylinders so that a space is formed for a special box to be fitted at each end portion of the vacuum roll, the box having lateral walls facing towards and spaced from the felt so that pockets are formed that are laterally sealed by means of curtains of air or mechanical strips. A partial vacuum occurs in the pockets when the felt passes the boxes at high speed. This shall prevent the lateral edges of the web from becoming detached from the felt, which otherwise can lead to web flutter and the risk of wrinkling at the edges of the web. The partial vacuum in the pockets can be increased through the influence of said curtains of air or by placing the pockets in communication with the vacuum roll. The problem of shrinkage in the cross direction of the web is not touched upon in the patent specification and neither can the apparatus be used for such a purpose.
WO 87/05063 (PCT/US86/00364) also relates to a device to prevent web flutter in a pocket formed between a drying cylinder and the free draw of a felt travelling towards and around the drying cylinder and carrying a paper web. A vacuum manifold arrangement extends through the pocket across the felt and is in communication with an inner vacuum chamber formed by two convergently related walls provided with openings. The air driven into the pocket during operation is removed by being sucked in through said openings to the vacuum chamber and on t
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Hansson Torbjorn
Olofsson Greger
Bennett Henry A.
Gravini Steve
Valmet-Karlstad AB
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