Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2002-12-18
2003-12-16
Maust, Timothy L. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S447000, C034S451000, C034S455000
Reexamination Certificate
active
06662468
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
This application claims priority on Finnish Application No.20012514, filed Dec. 19, 2001, the disclosure of which is incorporated by reference herein.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The invention relates to a dryer section of a paper or board machine.
As known from the prior art, the dryer section of paper or board machines has employed cylinder drying in which the dryer section is formed of drying groups. In cylinder drying, a paper or board web is dried against the heated surface of a drying cylinder, against which surface the web is generally pressed by means of a wire or equivalent. The drying cylinders are heated, for example, by steam.
The drying groups in the cylinder dryer sections of a paper or board machine employ twin-wire draw and/or single-wire draw. A drying group is formed by the drying cylinders, reversing rolls or cylinders, alignment and guide rolls associated with the same drying wire/belonging to the loop of the same drying wire in single-wire draw or associated with a pair of drying wires/belonging to the loop of a pair of drying wires in twin-wire draw.
In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against the heated cylinder surfaces. Between the rows of drying cylinders, which are usually horizontal rows, the web has in twin-wire draw free and unsupported draws which are susceptible to fluttering, which may cause web breaks, in particular in those stages of drying in which the web is still relatively moist and, therefore, has a low strength. For this reason, in the last few years, ever increasing use has been made of said single-wire draw in which each group of drying cylinders comprises only one drying wire on whose support the web runs through the entire group so that the drying wire presses the web against the heated cylinder surfaces of the drying cylinders and the web remains at the side of the outside curve of the reversing cylinders or rolls situated between the drying cylinders.
Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the loop. In what is known as normal single-wire draw groups, the drying cylinders are arranged in an upper row and the reversing cylinders or rolls are arranged in a lower row and, correspondingly, in what is known as inverted single-wire draw groups, the drying cylinders are arranged in a lower row and the reversing cylinders or rolls are arranged in an upper row.
The present invention relates to a dryer section of a paper or board machine, which dryer section applies the so-called normal single-wire draw and which, in addition to drying groups that apply single-wire draw, may also include drying groups that apply impingement drying or through-drying. In high-speed paper or board machines that apply single-wire draw, the first cylinder drying group has posed a special problem; it has been necessary to make it short in high-speed paper or board machines having a speed of, for example, over 1200 m/min when manufacturing fine paper, over 1400 m/min when manufacturing SC paper, over 1500 m/min, advantageously over 1600 m/min when manufacturing newsprint, over 1000 m/min when manufacturing fluting and liner, in order to be able to maintain sufficient tension in the web by means of differences in draw so that no runnability problems should arise. The difference in draw refers to a difference in speed both between a press section and a dryer section and between successive drying groups, in which connection, for providing tension, the following drying group uses a speed that is slightly different from the speed used in the preceding press or drying group. In that case, instead of one group, it has been necessary to place two or more cylinder drying groups at the beginning, which has increased manufacturing and operating costs.
At the beginning of the dryer section, the web does not yet contain a lot of solids, which means that its transfer from the press to the dryer section and its runnability in the first cylinder drying group have been problematic because, when the web is passed from a drying cylinder to a reversing cylinder or roll, the web may have separated from the wire and formed bags, the elimination of which has required a high draw value, which has led to the use of short cylinder drying groups, even groups formed by one cylinder, as the first group of the dryer section. The above-mentioned difference in draw from the press has been used specifically for eliminating the bags described above in order to make runnability good enough. The draw difference affects the quality properties of the web and, for the above-mentioned reasons, it has not been possible to optimize the draw difference taking into account quality properties.
With increasing running speeds of paper or board machines, it has been conventionally necessary to also arrange the long drying groups, for example, 8-9 cylinders, used for easy grades in prior art low-speed machines so that they form, for example, two drying groups in order to get runnability under control by thus providing separate drying groups that enable a difference in draw.
In prior art paper and board machines, the length of the first cylinder drying group has become shorter with increasing machine speeds. Although the solids content after the press has increased in recent years, the length of the first cylinder drying group has nevertheless continued to become shorter in prior art dryer sections that apply today's technology.
In prior art cylinder drying groups that apply single-wire draw, the drying wire and the web come from a preceding drying element, for example, from a contact drying cylinder to a reversing suction cylinder or equivalent as a joint straight run, in which connection a closing wedge space is formed between the drying wire and the surface of the last-mentioned suction cylinder, the wedge space being also called a closing nip in the following. The moving drying wire and cylinder surfaces tend to induce overpressure into said wedge space. This in turn produces a pressure difference that affects the web supported by the drying wire and which tends to separate the web from the drying wire, causing runnability problems, wrinkles and even web breaks. On the other hand, in order to improve the efficiency of dryer sections, there is a need to use dryer sections that are more compact than previously and in which the contact drying cylinders and said suction cylinders are as close to one another as possible. All these matters together with increasing web speeds add to the overpressure problems in said closing nip.
A closing wedge space, i.e. a closing nip, i.e. a closing gap is formed when the wire runs towards the next suction cylinder/drying cylinder between the drying wire and the surface of said next cylinder. In a corresponding manner, an opening wedge space, i.e. an opening gap, i.e. an opening nip refers to a wedge-shaped space which is formed between the drying wire and the surface of the suction cylinder/drying cylinder and from which suction cylinder/drying cylinder the wire separates.
In the case of the opening gap, i.e. opening nip, a problem has been that the web starts to follow the drying cylinder and not the drying wire towards a reversing cylinder. The run of the web in this opening nip between the drying cylinder and the wire becomes the more difficult to control, the higher the paper or board machine speeds become, because with increasing speeds the web is more liable to follow the drying cylinder. The solids content of the web is also important for the separation of the web from the cylinder. The wetter the web, the more difficult is it to separate it from the cylinder. If the web does not follow the drying wire, the web will have slackness, which may cause wrinkles and other problems. The separation of the web from the cylinder and
Juppi Kari
Komulainen Antti
Maust Timothy L.
Metso Paper Inc.
O'Malley Kathryn S.
Stiennon & Stiennon
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