Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2002-03-13
2004-03-16
Griffin, Steven P. (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S206000, C162S198000, C162S359100, C162S375000, C162S261000, C162S358500, C034S114000, C034S115000, C034S445000, C034S451000, C034S452000, C034S463000, C034S549000
Reexamination Certificate
active
06706150
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to paper and board machine air impingement methods and apparatus for compensating for the curling tendency of a paper or board web.
As known in the prior art, multi-cylinder dryer units of a paper machine employ twin-wire draw and/or single-wire draw. In twin-wire draw, the drying cylinder groups comprise two wires which press the web one from above and the other from below against heated cylinder surfaces. Between the drying cylinder rows, generally horizontal rows, there are free and unsupported draws, in which connection the web is susceptible to fluttering, which may cause web breaks, especially when the web is still moist and therefore weak. For this reason, single wire draw has recently been adopted and applied in the dryer unit in practice without an exception, each drying cylinder group of the single wire draw including only one drying wire, on support of which the web runs through the entire group such that, on the drying cylinders, the drying wire presses the web against heated cylinder surfaces, and on the reversing cylinders or rolls between the drying cylinders, the web runs on the outer surface of the drying wire. Typically, the dryer unit of a paper machine comprises 20-30 drying cylinders and reversing cylinders, in which connection a multi-cylinder dryer has 5-8 wire groups and the groups located at the upstream end of the dryer unit are normally shorter than the groups at the downstream end thereof.
In so-called normal single-wire draw groups of the prior art, the heated drying cylinders are located in an upper row and the reversing cylinders are located in lower rows, which rows are commonly horizontal and parallel to one another. The applicant's FI patent 54627 (corresponding U.S. Pat. No. 4,202,113) proposes placing successively above-mentioned normal single-wire groups and so-called inverted single-wire groups, in which heated drying cylinders are located in a lower row and reversing suction cylinders or rolls are located in an upper row with the main purpose of drying the web symmetrically on both sides thereof. Beloit Corp. have also put forward some proposals for dryer units comprising normal and inverted cylinder groups, in respect of which reference is made to international application publications WO 88/06204 and WO 88/06205 and to U.S. Pat. No. 4,934,067, which proposes inverted groups for a dryer unit for control of curl. According to U.S. Pat. No. 5,269,074 (Beloit Corp.), a long dryer unit applying single-wire draw is followed by a short dryer unit applying twin-wire draw with the purpose of controlling curl of the web.
The use of moist steam for straightening curl has already been known in the art since the 1970's and the 1980's, as appears in U.S. Pat. No. 3,948,721 (Vinheim Karl) or in U.S. Pat. No. 5,557,860 (Voith) and in public FI patent application 821431, which teaches passing the web through a steam treatment station in order to straighten curl. Recently, dryer units provided with single-wire draw have become common in which the upper or lower cylinders are steam-heated drying cylinders, the web coming into direct contact with said cylinders while being pressed by the drying wire, and in which the lower or upper cylinders are cylinders provided with internal suction, for example, Metso Paper, Inc.'s so-called VAC-ROLL™ cylinders in which a vacuum effect is directed through the perforated shell of the cylinders from the interior space of the reversing cylinder to the grooves extending around the shell of the cylinder. Said vacuum effect serves to maintain the web in contact with the drying wire when the web comes to the side of the outside curve on the reversing cylinders. At the same time, the transverse shrinkage of the web is sought to be prevented while drying progresses.
In paper and board machines, the reeling of the web is usually sought to be carried out when the web is as cold as possible, and in order to achieve this aim, it is prior known that a cooling cylinder is used at the end of the dryer unit. In accordance with the commonly known state of the art, the cooling of the web has the following effects:
the relaxation time of the web can be shortened, which leads to smaller differences of stress in the web before the next process stage (e.g. calendering or reeling) as compared with a situation that the web is passed forwards at a higher temperature,
the temperature differences themselves can be reduced by lowering temperature level, which leads to smaller differences in the elastic-plastic behaviour of the web in the next process stage or before it.
The most substantial problem associated with single-wire draw is that drying heating is directed, i.e. by convection from the surface of a heated drying cylinder, more intensely only at one surface of the web from one direction. As a result of this one-direction heating, there arises a strong tendency to curl in the web. This problem is also previously known and in order to deal with it, several different solutions have been proposed in the course of years. However, it is common to all these solutions that there remain in the web more or less internal stresses which will release in an unpredictable manner at a later stage and may cause problems as soon as in connection with finishing, such as coating and reeling, or later at the stage at which the paper product is utilized.
With respect to this complex of problems and the prior art associated with the background of the invention, reference is further made to the publications
FI 902616 describes a steam box disposed in a dryer unit for relaxation of drying stresses and thus for compensating for curl.
FI 931263 describes air impingement against a large cylinder which has a diameter>2 m and which is placed inside a drying wire loop. Said publication proposes the division of air impingement into sections, in which connection each section uses hot air or superheated steam having a temperature, moisture and/or pressure which is different in each section in order to prevent transverse shrinkage of the web, to control drying of the web and to achieve a desired moisture profile for the web.
FI 950434 proposes passing a web, which has a tendency to curl because of the nonsymmetrical forward-drying of the bottom and top surfaces of the web, to finishing in which the tendencies to curl are compensated for by moistening and/or plastically working the web.
FI 951748 describes a dryer unit which applies single-wire draw for control of curl and in which the last group is inverted to allow drying on both sides.
FI 963734 proposes an arrangement for drying a coated paper web in a drying group of an after-dryer unit applying single-wire draw, the web being treated in said arrangement after that by means of a steam box in order to compensate for the tendency to curl.
FI 964830 proposes an arrangement for compensating for the curling tendency of a paper web by means of an air impingement device which is placed above a drying cylinder and by which hot moist air is blown against the web.
FI 971301 discloses an arrangement for controlling the curl of a paper web by means of a dryer unit. According to said arrangement, the necessary operations are carried out in several stages while the temperature of the web is below 85° C. According to the publication, the curl control treatment is accomplished by means of a steam box or a moistening device.
FI 971713 proposes arranging a large-diameter air impingement drying cylinder in connection with a dryer unit which applies single-wire draw and has drying cylinders placed below and reversing cylinders placed above, which air impingement drying cylinder is placed inside a drying wire loop and on top or in the vicinity of which cylinder, at both sides, heated smaller-diameter cylinders are placed, whereby, when the web is supported by the drying wire over the entire length of the dryer unit, uneven transverse shrinkage of the web
Ahonen Pasi
Kiiskinen Harri
Timofeev Oleg
Griffin Steven P.
Halpern Mark
Metso Paper Inc.
Stiennon & Stiennon
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