Patent Application
20060118257
This application is a national stage application of International App. No. PCT/FI2004/000271, filed May 5, 2004, the disclosure of which is incorporated by reference herein, and claims priority on Finnish App. No. 20030691, Filed May 8, 2003.
Not applicable.
The invention relates to a method in the treatment of surface-sized paper, in particular fine paper in a finishing section of a paper machine. In addition, the invention relates to a finishing section of a paper machine for treating surface-sized paper, in particular fine paper.
As known in the prior art, multi-cylinder dryers of a paper machine employ twin-wire draw and/or single-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 heated cylinder surfaces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, has a low strength. Therefore, ever increasing use has recently 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 the prior-art so-called normal single-wire draw groups, the heated drying cylinders are placed in the upper row and the reversing cylinders are placed in the lower row, which rows are generally horizontal and parallel to each other. When the terms “normal (drying) group” and “inverted (drying) group” are used in the following, they specifically refer to the above kind of single-wire draw groups of multi-cylinder dryers.
From the prior art it is also known to dry the paper web by means of contact-free drying. One contact-free drying method is airborne web-drying and, with respect to the prior art relating thereto, reference is made to FI patents 98944 and 107623. Impingement drying is also contact-free drying and, with respect to the prior art relating thereto, reference may be made, for example, to FI patent application 20002429 and to FI patent 106269.
When paper is dried by means of normal single-wire draw groups from its bottom side and if this kind of asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place such that the bottom surface side of the paper web dries first and with the progress of drying the drying effect also spreads to the top surface side of the paper web. The thus dried paper generally has a tendency of curling because of asymmetric drying.
As known in the art, the tendency of curling of paper is already affected in connection with the forming of the web, in particular in the sheet forming stage by the selection of a speed difference between the slice jet and the wire and by other running parameters. However, in paper machines, in particular in fine paper machines in which there is a single-wire draw in the major part of the length of the dryer section, other arrangements must also be applied in practice with a view to controlling the tendency of curling of paper in order that drying might be made sufficiently symmetric in the z-direction.
From the prior art it is thus known that paper curls if it is moistened from one side and dried from one or two sides or if paper of equal moisture is dried from one side. It is also known that the curl of paper can be controlled by means of an air dryer placed on one side, for example, by means of an impingement hood or cylinder drying from one side. When paper is dried from one side, the paper dries and curls to the dried side and the curl can also be controlled by moistening the other side of the paper by means of a moistening device and by drying it, whereby the paper is straightened.
For the purpose described above, different finishing groups for paper, in particular for fine paper have been disclosed, among other things, in Metso Paper, Inc.'s FI patent 98387.
With respect to the prior art, reference is also made to FI patent 101987, which discloses a method for drying surface-treated paper, in particular fine paper, in which method a paper web is first dried in a forward dryer section of a paper machine by means of several successive downward open single-wire draw groups on support of a drying wire, after which the paper web is finished in a finishing section, in which finishing section the web is surface-sized or coated. In the method, in the finishing section, hot moist air is supplied into connection with the drying groups to suppress evaporation or hot dry air to promote evaporation at desired sites on the desired side of the web with respect to control of curl, so that the tendency of curling of the paper web created in the paper web in the forward dryer section can be substantially eliminated and/or compensated.
With respect to the prior art, reference may also be made to FI patent 105935, which discloses a method for drying paper, which method comprises the following stages: a) the paper web that is being dried is passed from a press section to a forward dryer section, in which the paper web is dried from the side of its bottom surface in dryer groups that apply normal single-wire draw, which forward dryer section comprises only single-wire draw groups that apply normal single-wire draw, b) the paper web is passed from the forward dryer section to a finishing section, in which the paper web is coated/surface-sized by a coating/surface-sizing apparatus, dried in an after-dryer section, in which the paper web is dried in at least one drying group that applies normal single wire draw, after which the paper web is calendered in a calender and passed to a reeling station, in which the paper web is wound into a machine reel. In the method, the curl of the paper web is controlled by means of at least one steam box in at least the area of the finishing section, the effect of said steam box being enhanced by cooling the web before said steam box, and in the method the curl of the paper web is controlled by elements and/or assemblies and combinations formed of said elements in at least the area of the finishing section.
Further, with respect to the prior art, reference may be made to FI patent 101488, which discloses a method for drying a surface-treated paper web or equivalent in an after-dryer section of a paper machine, in which method the paper web is first finished in a finishing section, in which finishing section the paper web is surface-sized or coated on both sides by means of a finishing device, after which the paper web is dried. In the after-dryer section the paper web is dried in a drying group/drying groups that apply only normal single-wire draw and in connection with or after the drying the paper web is treated by a device/devices to compensate for the tendency of curling of the paper web.
With respect to the prior art, reference may also be made to FI patent 106270, which discloses a method for drying a surface-treated paper web or equivalent in an after-dryer section of a paper machine, in which method the paper web is first finished in a finishing section, in which finishing section the paper web is surface-sized or coated by a finishing device, after which the paper web is dried. In the finishing section, the paper web is dried in at least one drying group that applies single-wire draw and, at the same time, the paper web is dried by an impingement apparatus disposed in connection with at least one cylinder or roll of said drying group.
One object of the invention is to develop further the arrangements described above such that the tendency of curling of paper can be controlled in an after-dryer section more effectively than before.
One problem with the arrangements known from the prior art is that the strong tendency of curling of paper dried in the forward dryer section in the downward open drying groups that apply single-wire draw and the stresses produced by it are not fully relaxed with the moistening caused by surface-sizing, in particular when changing to the use of surface-sizes with increasingly higher dry solids. One object of the invention is to provide an arrangement which makes it possible even in such a situation to eliminate or at least minimize the tendency of curling of the paper web.
One object of the invention is to propose a new arrangement for a method of finishing paper, in particular surface-sized fine paper, in which the problems associated with the curling of paper are eliminated or at least minimized.
The invention relates to a paper machine in which downward open drying groups that apply single-wire draw are used in a forward dryer section, so that a tendency of curling is created in the paper web, or in which paper machine at the end of a forward dryer section there are also drying groups that apply twin-wire draw, in which case the need for curl control is actually smaller. After the forward dryer section, the paper web is surface-sized, after which the paper web is dried in an after-dryer section of a finishing section mainly by means of contact-free drying; airborne web-drying and/or air drying and/or impingement drying and/or infrared drying. Hereafter, the term “after-drying” is also used to describe the drying of the after-dryer section, that is advantageously at least 60 percent, most appropriately at least 75 percent, even as much as 90-100 percent of the power needed for evaporation of the water amount between the moisture after surface-sizing and the final moisture contained in the web is produced by contact-free drying.
The invention is also suitable for use in connection with a finishing section of a paper machine which uses cylinder drying in addition to contact-free drying, however, at least part of the drying takes place by means of contact-free drying.
In one advantageous embodiment of the invention, at least part of the contact-free drying is accomplished by airborne web-drying in which the power of airborne web-dryers placed on the different sides of the web is adjustable in order that the stresses possibly not relaxed during surface-sizing and the curling tendency left by them may be further compensated for. The proportion of airborne web-drying in the after-drying is at least 60 percent, most appropriately at least 70 percent, even as much as 90-100 percent. In connection with the invention, as contact-free drying it is also possible to use contact-free drying produced by an infrared dryer without control of curl and to dry, for example, 10 percent of the after-drying by an infrared dryer. In connection with the invention it is also possible to use heating cylinders as cylinders of a holding group, in which connection they can be used for controlling curl on the bottom side of the web or only for raising drying power. In that case, about 10-20 percent of the water dried in after-drying can be evaporated by means of the holding group cylinders.
In another advantageous embodiment of the invention, at least part of the after-drying is also accomplished on the basis of impingement drying, in which connection impingement units provided with curl control are advantageously used. In that case, the proportion of the drying achieved by impingement is at least 50 percent of the after-drying, most appropriately 70 percent or even as much as 90 percent. Curl can then be taken into account by dimensioning the portions of the impingement drying applied to the different sides of the web so that they are, for example, of unequal length, i.e. more top-side drying in the after-dryer section for paper pre-dried from the bottom side, so that the speeds and temperatures of the blows used in the impingement units still remain as a curl control allowance.
In accordance with a further advantageous embodiment of the invention, it is possible to use different combinations of airborne web-drying and impingement drying to build a suitable contact-free after-dryer section that controls curl.
In accordance with one advantageous embodiment of the invention, in addition to airborne web-drying, normal drying cylinder groups applying single-wire draw are used in which drying takes place as contact drying. In this kind of embodiment, the web is advantageously dried first by airborne web-drying, then by cylinder drying and after that again by airborne web-drying. By this means, the tendency to curling created in the forward dryer section can also be controlled at a high speed and when the final drying takes place as airborne web-drying from both sides of the web, so high a moisture gradient cannot be produced in the paper web as in totally one-sided drying. The drying of the web from both sides also offers more possibilities of regulation for controlling the tendency of curling.
In accordance with one advantageous embodiment of the invention, in the finishing section of a paper machine, airborne web-drying and impingement drying are used in the after-drying as contact-free drying and, in addition to this, normal cylinder drying that applies single-wire draw. In this embodiment, impingement drying is advantageously accomplished as vertical drying, so that the after-dryer section is easily provided with enough impingement drying length. In this advantageous embodiment of the invention, airborne web-drying makes it possible to dry the web from both sides, thus allowing its susceptibility to curling to be controlled by adjusting the parameters of the airborne web-dryers and, when cylinder drying and impingement take place on different surfaces of the web, the web can be dried from both sides and, by regulating the power of impingement, it is possible to control the tendency of curling.
The method and the paper machine in accordance with the invention make it possible to control the tendency to curling created in the forward dryer section with single-wire draw.
In accordance with the invention, the after-dryer section thus uses a surface-sizing technique and contact-free drying, of which at least part is airborne web-drying or impingement drying, for control of curl. By this means, the tendency of curling of the web can be eliminated. In the invention, airborne or impingement dryers are used which are placed on both sides of the web and in which the dryers of both sides, i.e. the drying ratio between the different sides, can be adjusted separately, so that the web can be dried in a manner as required by the elimination of the tendency of curling. Advantageously, it is possible to use dryers that also make it possible to profile the drying power in the cross direction of the web, thereby also enabling the curl profile to be affected, for example, by adjusting blowing speeds and/or blowing temperature in the cross direction of the web.
The nozzle arrangement of the airborne web-dryers used in connection with an advantageous embodiment of the invention is advantageously asymmetric with respect to the top and bottom sides of the web, so that heat transfer can be made asymmetric, thus not losing drying power because of control of curl, but the nozzle arrangement that takes into account an assumed basic curl still leaves full curl control power for use. Alternatively, in connection with the invention it is possible to operate also with a smaller curl control power, for example, with one adjustable airborne web-drying unit.
Cylinder drying is not necessarily used in the after-dryer section in accordance with the invention, but possible cylinder/roll groups must be placed in the after-dryer section to serve as web holding groups also for providing and improving the runnability of the web. However, the cylinders of said holding group can be used for increasing drying power, even though they are situated on the wrong side of the web from the viewpoint of curl. The cylinders that dry the bottom surface of the web can, however, be also used in the control of curl as a control allowance for the opposite side.
In those embodiments of the invention in which cylinder drying is used in the after-dryer section of the finishing section, by regulating the power of drying cylinders it is possible to regulate the intensity of the drying that takes place by means of contact drying as compared to contact-free drying in after-drying, so that curl can also be controlled by means of cylinder drying parameters.
In one advantageous embodiment of the invention, the drying after surface-sizing in the after-dryer section has been arranged mainly by means of airborne web-dryers—the drying accomplished by the airborne web-dryers being at least 60 percent, most appropriately 70 percent—which airborne web-dryers are placed on both sides of the web, so that the asymmetric drying produced in the forward dryer section can be corrected, when the airborne web-dryers placed on both sides of the web have separate control of drying power, advantageously control of blowing speed and/or temperature.
In accordance with another advantageous exemplifying embodiment of the invention, surface-sized paper is dried by air dryers which are placed on both sides of the web and in which the temperatures and blowing speeds of the different sides are advantageously regulated separately. In this advantageous embodiment of the invention, curl is controlled by a two-sided dryer whose different sides can be adjusted independently of each other, thereby avoiding the use of moistening water and the considerable drying costs caused by it.
In accordance with one advantageous exemplifying embodiment of the invention, the control of curl in the after-dryer section is assisted, when needed, with a steam box, which steam box can be used, for example, such that if the paper is naturally straight, the steam box is not used because there is no need for control of curl. If the paper web curls with its edges upwards, as is the most common tendency after a forward dryer section with single-wire draw, the steam box is used, and if the paper web tends to curl with its edges downwards, for example, the cylinders of a holding group of the after-dryer section are used. The steam box is advantageously placed in a position immediately before a reel-up, where the web is colder than in the dryer section, and it is not needed much because drying has mainly taken place by means of contact-free drying.
In accordance with one advantageous embodiment of the invention, impingement drying is used for contact-free drying and with respect to the prior art relating thereto reference may be made to Metso Paper, Inc.'s FI patent application 20002429 disclosing arrangements for paper with no surface size and to the Metso Paper, Inc.'s FI patent 106269 disclosing impingement arrangements which are applied to the forward/after-dryer section and whose applications can be made use of in connection with the invention.
In accordance with one additional feature of the invention, as a possibility of control is used impingement drying in which an impingement drying unit is arranged, for example, in connection with a reversing cylinder or a grooved guide roll or a suction roll or a suction cylinder, advantageously a suction cylinder of the type marketed by Metso Paper, Inc. under the trademark VAC ROLL. The impingement drying units used in connection with the invention may also be so-called vertical impingement units, the details of which are described in greater detail, for example, in Metso Paper, Inc.'s above-mentioned FI patent application 20002429.
When needed, possible curl in a paper that has been lightly coated with a size-press or by other means can also be corrected in the manner described above. However, the control of curl in coating is usually not necessary, but when changing to thin coatings with higher dry solids, it may become necessary to do so with possibly further slight calendering (which also has a curl controlling effect). In coated paper it is then possible to control the movements of water in the paper with a view to also reducing the smoothness two-sidedness caused by fibre roughening in the paper and the dusting caused by binder migration.
In the following, the invention will be described in greater detail with reference to the figures of the appended drawing, but the invention is by no means meant to be narrowly limited to the details of them.
In
In the exemplifying embodiment shown in
In the embodiments shown in
The impingement hood 34 can also be accomplished, when needed, such that its power can be profiled in the cross direction. The roll or cylinder 33, 53 arranged in connection with the impingement hood can be in structure of the drying cylinder type, a paper guide roll or a suction roll, for example, a suction roll, without an internal suction box, of the type marketed by Metso Paper, Inc. under the trademark VAC ROLL.
In the schematic embodiment shown in
In the exemplifying embodiment of the invention shown in
In the exemplifying embodiment of the invention shown in
The exemplifying embodiment of the invention shown in
In the Figures described above, the blowing speeds and powers of the airborne web-dryers and impingement drying units can be regulated, when needed, to accomplish the control of curl and, in addition, they may also enable the control of curl to be profiled in the cross direction of the web W.
In connection with the figure, it shall be particularly taken into account that the dimensions of the impingement hoods and the heights of the units are meant as a general guide and their dimensioning is accomplished as needed.
The invention has been described above only with reference to some of its advantageous exemplifying embodiments, but the invention is not by any means meant to be narrowly limited to the details of them.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20030691 | May 2003 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI04/00271 | 5/5/2004 | WO | 2/2/2006 |
