The present invention relates to a method of drying the pa¬ per in a paper making machine according to.- the preamble of patent claim 1. The invention also relates to a device de¬ signed to carry out said method, which device is set forth in detail in the first device claim.

When a paper web dries, it will shrink. It has been known for a long time, that the quality of a paper is influenced by this shrinkage. When a paper is produced for e.g. paper bags for cement, it is advantageous to allow the paper to shrink in a completely free way. During such a drying the capacity of the fibers to absorb energy is made the most of, and the paper becomes tough and stretchable. However, when a paper is produced with demands for stiffness, a shrinkage must be stopped as far as possible and the fibers must be kept stretched during the drying process.

The first type of drying usually is carried out in dryers having a so called airborne web, in which the web floats freely above blower boxes and only passes turning rolls, when it passes from one drying area to another. The speed of the turning rolls is adjusted in such a way, that the ten¬ sile stress in the paper web is very small. The second type of drying usually is carried out in so called cylinder dry¬ ers, in which the paper web is heated, when it contacts heated cylinders, via which the web is transported. The speed of these cylinders is adjusted in such a way, that a certain tensile stress in the paper is obtained. In this way a shrinkage in the longitudinal direction of the web is hindered completely or partially.

Unfortunately a fluctuating degree of shrinkage in the trans¬ verse direction is obtained in ordinary cylinder dryers. Thus, the quality will be irregular and the material proper¬ ties cannot be utilized completely. Essentially the central part of the web will become thinner and stiffer than the outer parts. In case specified demands as to e.g. stiffness

are to be met, it means that needlessly a lot of paper will be wasted if the entire production is to meet these require¬ ments.

Thus, SE 8400678-2, which corresponds to EP 85850026.7 and EP 85901109.0, suggests that also the shrinkage in the trans¬ verse direction is to be hindered in order to obtainuniform quality. This document suggests that the edges of the web is to be influenced by forces by means of blowing or bycon¬ tacting the web with bands, which run substantially paral¬ lel to the edges of the web. Said contact with bands can be carried out by e.g. squeezing between two bands or by using an embossed band.

SE 8601291-1, which corresponds to EP 87850093.3, relates to a development of the above-mentioned methods, two bands running parallely with the edges of the paper web and the edges of the paper web being held by said bands by means of adhesive forces.

The methods described in the above-mentioned documents solve the problem with a shrinkage in the transverse direction of the edges of the paper web by means of comparatively compli¬ cated devices. Also , they result in a worse maintenance and repair situation and they constitute obstacles for a normal operation of the paper drying equipment, when it is not im¬ portant that a transversal shrinkage is to be stopped. The method described in the latter application is burdened with the problems that the bands must be cleaned continuously and that the time available for this is very short.

The most important characterizing feature of the present in¬ vention is that the edges of a paper web are sucked into an engagement with the drying cylinders in band-shaped areas on the envelope surface of the cylinders. It is true that it is already known to apply a negative pressure when drying a web-shaped material. This is described in i.a. DE-PS 17 60

226 and SE 7405985-8 . A suction is to be performed, accor¬ ding to these documents, across all the surfaces of the dry¬ ing cylinders, which the material to be dried will contact. The object is to remove moisture from the material to be dried or the drying wire/dryer felt by using a negative pres¬ sure. A condition of making said methods useful is that the material to be dried as well as one or possibly two drying wires/dryer felts will allow air to pass through to a rea¬ sonable extent. Thus, they are only suitable for textile ma¬ terials or porous paper grades, e.g. tissues. These drying methods cannot be used for drying stiff, tight paper grades and consequently they can also not be regarded as related to the method of the present invention. When the methods, which are described in the two last-mentioned documents, are used, always some sliding will take place, even if the wire is fastened to the cylinders, since the paper web will be able to move in relation to the wire.

Consequently the object of the present invention is to sug¬ gest a new method and a new device for drying papers, by means of which the so called transverse shrinkage can be stopped in a simpler way than by means of the already known methods. This object is obtained, in accordance with the present invention, by carrying out a method of the typemen¬ tioned in the introduction mainly according to the charac¬ terizing clause of patent claim 1. Said object is also met by means of a device having the characterizing features set forth in the first device claim.

Additional characterizing features of and advantages of the present invention are set forth in the following descrip¬ tion, reference being made to the accompanying drawings, which show a few preferred but not limiting embodiments. The drawings show in detail:

Fig. 1 a lateral view of an already known paper guide al¬ ternative; Fig. 2 a diametrical sectional view of a first embodiment of

a drying cylinder according to the invention; Fig. 3 a detail of the cylinder according to Fig. 2, on an enlarged scale;

Fig. 4 a lateral view of a preferred arrangement of cylin¬ ders in an embodiment according to the invention; Fig. 5 a lateral view of a cylinder, the difference between a conventional web guide and the web guide according to the invention being illustrated;

Fig. 6 a detail of an embodiment of the drying cylinder shown in Fig. 2, as an alternative to the embodiment shown in Fig. 3;

Fig. 7 a detail of an alternate embodiment of a drying cy¬ linder;

Fig. 8 an additional detail of an embodiment of the drying cylinder according to Fig. 7; and

Fig. 9 a lateral view of the drying cylinder embodiment according to Fig. 7.

In Fig. 1 a conventional paper guide alternative for a cylinder dryer is shown, outlined with dashed lines. A pa¬ per web 3 is guided via and contacting steam heated cylin¬ ders 1, against which it is pressed by means of a drying wire or dryer felt 103. Two drying wires 103 are used, one for each row of cylinders. Conducting rollers 2 for drying wires 3 are used between cylinders 1. The diameter of cy¬ linders 1 is between 1 and 4 m and their length is between 5 and 10 m. The number of drying cylinders can vary between 50 and 100, allotted to 3-5 drive groups. In a cylinder dry¬ er according to Fig. 1 the paper web runs freely a longdis¬ tance between adjacent cylinders 1 and then tends to shrink in the transverse direction without any hinder. Also when in contact with the cylinders the width of the web may be reduced, despite the fact that the dyring wire and the fric¬ tion against the cylinder to some extent counteract this. Unfortunately, the stresses in the paper web will be much larger in the middle of the web than in its edges, and con¬ sequently, the shrinkage will be large in the edges and

very small in the middle, with the above-mentioned quality variations as a consequence.

In accordance with the present invention said shrinkage in a transverse direction in a paper dryer, which like the showed one has two drying wires 103, will be hindered by holding the edges of the paper web onto the cylinders by means of a vacuum and by positioning the cylinders so close¬ ly adjacent each other, that there will not exist any appre¬ ciable free distance between successive cylinders.

In Fig. 2 a drying cylinder 1 with a preferred embodiment of the invention is shown. The cylinder is supported by two bea¬ rings 4,5 and its ends are closed by means of two gables 10, 11. A steam conduit 6 passes in a way known per se through one of the bearings, 5, and into the interior 7 of the cy¬ linder. A conduit 8 for the removal of condensate passes through the other bearing 4 and is provided with a nozzle 9, which is placed immediately above the lowest area within the cylinder.

The envelope surface of cylinder 1 extends beyond closed gables 10,11 and forms extended end sections 12,13, which are shown in more detail in Fig. 3. A zone of these end sections 12,13 is provided with through holes 14 around all the circumference (casing) 15 of the cylinder. A vacuumcham¬ ber 16, within a vacuum box 17, which via a vacuum conduit 18 is connected to a vacuum source, not shown, is designed to interact with said holes on the interior side of the en¬ velope surface along a portion of the circumference of the cylinder. Preferably this interaction takes place merely along that portion of cylinder 1, where the paper web con¬ tacts the cylinder. Consequently, vacuum box 17 is designed as an annular channel, which with its ends 19 contacts the interior surface of end sections 12,13 along a distance of about 3/4 of the circumference of the envelope surface of the cylinder.

Through the negative pressure in vacuum chamber 16 the paper web is slightly sucked downwards into holes 14 and is kept in place in the holes. In this way a shrinkage in the trans¬ verse direction is hindered and a more uniform quality will, as has been stated above, be obtained.

In order to attain a satisfactory sealing action between ro- tatable cylinder 1 and stationary vacuum box 17, ends 19 of the latter can be provided with cavities 20, in which sealing elements 22, actuated by compression springs 21, are disposed.

The zone with drilled holes can have a width of about 50 mm.

In Fig. 4 an arrangement of cylinders 1 according to the in¬ vention is shown, the cylinders being disposed close to each other. The cylinders are provided with vacuum boxes 17 having closed ends 23. The vacuum boxes extend around the same por¬ tion and to the same extent as the paper web. The paper es¬ sentially runs directly from the interaction with the vacuum chamber on one cylinder to the interaction with the vacuum chamber on the next cylinder. In Fig. 5 the difference between the web guide according to Fig. 1 and the web guide sugges¬ ted according to the present invention on a cylinder accor¬ ding to Fig. 4 is shown. Thanks to the comparatively exact connection between successive cylinders and between succes¬ sive vacuum chambers the deviations from the theoretically ideal value will be most minor and the wasted power very minor. The time needed for the web to run from one suction zone to the next one can be as low as 1/100 second, and the paper web then does not have time to alter its position in this very brief period of time. Consequently, the edge hol¬ ding is essentially secured along all the present cylinders, arranged according to Fig. 4.

It is also possible to additionally improve the effects, which counteract the shrinkage by designing the holes or ca¬ vities 14 in such a way, that not merely radially directed

suction effects are obtained but also an axially directed suction effect, the paper web becoming slightly stretched towards the free cylinder ends.

In Fig. 6 the suction zone in the end sections of the cylin¬ ders has been designed differently. In a recess 25 above holes 14 a porous ring 24, made of e.g. a sintered metal, has been countersinked. In this way a more finely divided suction action is obtained.

In Fig. 7 is shown an embodiment with an axial connection to a vacuum chamber through openings 26 in the respective gable and with suction conduits 27 in the cylinder proper, connected to a porous ring in the envelope surface , cor¬ responding to the embodiment shown in Fig. 6. In Fig. 8 the corresponding embodiment, seen in the axial direction, is shown as a detailed magnification. An annular part, inclu¬ ding suction conduits 27, has been mounted by means of screws 28. The corresponding embodiment of all the cylin¬ der gable with a vacuum box is shown in Fig. 9. When the vacuum chambers are axially connected, larger losses will ensue and consequently also a larger power consumption than when the vacuum chambers are radially connected.

The negative pressure in the vacuum chamber suitably is 0.5-0.75 bars, depending on the surface weight of the paper and the quality which is aimed at. Different qualities have different shrinkage forces.

The present invention is not limited to the embodiments de¬ scribed above and shown in the accompanying drawings but can be modified and supplemented arbitrarily within the scope of the inventive concept and the following patent claims. -