A DEVICE IN THE DRYING SECTION OF A PAPER MACHINE

Abstract: The drying section comprises two rows of drying cylinders or rolls, one above the other. The web travels between these cylinders and rolls, meandering from one row to the other, all the time supported by a drying fabric. Herein, the web lies directly against the cylinder or roll surface in one row of cylinders or rolls and similarly the drying fabric is adjacent to the roll surface, while the web is on the outside, in the other row of cylinders or rolls. The means comprises a suction box (13) or a plurality of suction boxes, disposed to operate against the drying fabric (12). Said suction box (13) or suction boxes (13', 13'') extend substantially over the entire length of the joint run of the web (W) and the drying fabric from one cylinder or roll to the other. In addition, the suction of said suction box (13; 13'; 13'') also extends into the wedge space between the drying fabric (12) and the drying cylinder (11) or equivalent roll shell in at least the suction box (13) on the entry side (17) of the drying fabric (12) and web (W) or the respective suction boxes, there are suction apertures through which the subatmospheric pressure prevailing therein acts in said wedge space (16, 17, 18, 19).

Description: A device in the drying section of a paper machine The present invention relates to a device in the drying section of a paper machine for running the web as a closed draw, which device operates in connection with a drying fabric arranged in such a way that one cylinder/several cylinders or roll/rolls of the drying section is/are outside the drying fabric loop and the other cylinder/ cylinders or roll/rolls is/are inside the drying fabric loop so that the web passes from one cylinder or roll to the other cylinder or roll over the whole distance supported by said fabric. Regarding the state of art, reference is made to the Applicant's Finnish patent applications nos. 761953 and 771056, to the Appli¬ cant's Finnish patent No. 54954 and to the Finnish patent appli¬ cation No. 780833 of J.M. Voith GmbH (related DE-GM 7 708 630). Said Finnish patent application No. 761953 shows a procedure for pro¬ viding a* closed paper web draw in the beginning of the multi- cylinder -dryer that comprises two rows of drying cylinders preferably one on top of the other. The first part of one of the rows, preferably the top row, is enclosed within the first wire or felt that supports the wire as it passes from one cylinder row to the other so that the web is, when being in association with one cylinder row, on top of said felt or wire, and when being in association with the cylinders of the other cylinder row, between the felt or wire and the surface of the cylinders of the row in question. In the procedure depicted above, the essentially new feature is that, for holding the web on the surface of the cylinders of that cylinder row whose cylinders are enclosed within the first wire or felt, at this point, another wire or other wires, preferably of a coarse-meshed type and essentially of the width of the web, is/are passed on the web, in order to press the web on the sector on which the web touches said cylinders, at the most, and that, in order to prevent friction between the wire or felt and the wire and the consequent dusting of the wire, the angular speed of the second wire or felt in relation with center points of the drying cylinders in question, is arranged to be automatically adjusted to be the same as the angular speed of the web that runs on these cylinders. The Finnish patent application No. 771056 mentioned above discloses a procedure in the drying section of a paper machine for guiding the web " as a closed draw, in which procedure, in the beginning of the drying section, particularly in its first group of drying cylinders, it is used a drying wire or fabric arranged in such a way that the cylinders of one row are outside the fabric loop and the cylinders of the other row are within the fabric loop so that the web runs zig-zag from one cylinder row to the other at all times supported by the same fabric, from the beginning to the end of the drying cylinder group that is equipped with said fabric. An essentially new feature in this known method is that, at least at some of the cylinders at which the web is outside the fabric, a pressure- difference is, via the grooved surfaces of these cylinders, applied to the web so as to make the pressure outside the web higher as compared with the pressure in the grooves of the cylinder surface, particularly for preventing the web from separating from the fabric and for ensuring the continuous operation of the paper machine. In said Finnish patent No. 54954 we have depicted a procedure for ensuring the draw of the web from the press section to the drying section, in which procedure the main new feature is that between the drying cylinder group and the press section there is a separate lead drying cylinder that does actually not belong to this group; that, for holding the web in contact with said bottom drying belt as this runs around the top cylinders of said special cylinder group, an overhe ' ad drying belt, essentially of the width of the web, is passed onto the web in order to press the web against the bottom drying belt over a sector that is essentially narrower than the sector at which the bottom drying belt supporting the web covers the top cylinders; and that said overhead drying belt can be made touch the lead drying cylinder by means of a lead roll of adjustable position at least for the phase during which the web, when the paper machine is started, is transferred from the press section to the drying section. In the above-mentioned Finnish patent application No. 780833 of J.M. Voith GmbH it is depicted a paper machine drying section, wherein in the wedge-shaped space between the drying belt and drying cylinder at the entry side and/or at the exit side there is an air lock essentially reaching over the whole length of the drying cylinder in such a way that the penetration of air into said wedge-shaped spaces is prevented. With the known device described above it is not possible to completely prevent the web from separating the drying belt and consequent web ruptures. In the above-mentioned procedures and devices, in which the web follows the drying fabric in such a way that, at the bottom cylinders or similar of the drying cylinder group, the web will be the outer¬ most element, the web tends, due to air currents and centrifugal forces, to separate from the drying fabric. The risk of such sepa¬ ration mainly increases with the square of the web speed. Another important reason why the web tends to separate from the drying fabric is that the drying cylinder and the drying fabric induce an air flow that tends to separate the web from the drying fabric. Another draw¬ back related with these facts is that air enters between the drying fabric and the drying cylinders which reduces thermal conduction. At current web speeds it has been possible to eliminate these problems by using suitable felt. As the machine speeds have increased, it has not entirely been possible to avoid said phenomena. For preventing the drawbacks mentioned above and for attaining the objectives of the invention, the principal characteristic feature of the invention is that the device comprises a vacuum box or boxes, which are arranged to function against the drying fabric essen¬ tially on the whole length of the common draw of the web and the drying fabric from one roll to the other, and that the suction of said vacuum box is arranged to reach the wedge-shaped space between the drying fabric and the drying cylinder or roll shell at least at the exit side of the machine. OMPI The operation of a device in accordance with the invention is based on the control of air currents and a suitable use of vacuum. In a device in accordance with the invention the vacuum box is made reach over the whole free space to the felt side. The device corα- prises such baffles so as to minimize harmful air currents by de¬ viating them from their direction. The device of the invention also produces a pressure over the web and the felt so to make it press the web against the felt. In the invention, a .pressure difference is sucked preferably in to the closing pit between the felt and the bottom cylinder with the consequence that the felt is made under- pressurized, and the pressure difference also has an influence on the arc of the bottom cylinder, as with currently used dry matter content the web produced with high-speed machines is almost impervious at pressure differences in question. It is therefore sufficient that an apparatus in accordance with invention be installed at the end of said closing pit only. The efficiency of the invention may be improved by using as the cylinder/roll a groove-surfaced or pitted organ, in which case also the pit opening between the bottom cylinder and the felt is preferably covered or under suction. A device in accordance with the invention should preferably cover the whole width of the web, but the device may be transversally divided to separate compartments for ensuring the web transfer. It may also be necessary, at the exit side, to apply the suction on the web end transfer strip only. In a device in accordance with the invention the distribution of pressure should preferably be arranged in such a way that the pressure difference is at its maximum at the critical points, viz. on the cylinders, and at its minimum on a free run. This is for minimizing the pressure-energy requirements and reducing the arching of the felt in the draw, which might cause friction, or necessitate special structures in order to prevent said arching. An apparatus in accordance with the invention has at least the following advantages: it features an uncomplicated construction and provides for good support at the free runs of the web. Another advantage is an easier transfer of the web. An apparatus in accordance with the invention is also easy to remove from the machine. The vacuum on the circumference of the cylinder contributes to the evaporation of water from the web and improves the thermal conduction between the web and the cylinder. Embodiments of the invention will .now be described in detail, with reference to the accompanying drawings, with no intention to restrict the invention to these details. Figure 1 is a schematic side view of an apparatus in accordance with the invention comprising a vacuum box installed between the drying cylinder. Figure 2 illustrates such an application of the invention in which there are two separate boxes between the drying cylinders. Figure 3 illustrates such an application of the invention in which the suction box is located in the wedge-shaped closing space at the enter side of the drying fabric and the web. Figure 4 illustrates detail A, in figure 3. Figure 5 illustrates detail A« in figure 3. Figure 6 illustrates such a version of a device in accordance with figures 3,4 and 5, in which there is a particular air-removal channel and specially shaped sealing strips of doctor-blade type acting against the drying fabric. Figure 7 is an alternative embodiment of detail A„ in figure 6, The figures illustrate a part of the multicylinder dryer of a paper machine, said multicylinder dryer comprising drying cylinders 10,11 heated by means of steam, electricity or with some other way. The drying cylinders are located in two rows, of which the top row comprise drying cylinders 10 and bottom row drying cylinders 11. Over the drying cylinders passes drying fabric 12, that from now on will be called the felt, though it must be emphasized that, instead of a felt, the invention may make use of a similar fabric such as drying wire or some other similar belt. Web W runs, supported by felt 12, zig-zag from one row of cylinders to the other in such a way that, on top cylinders 10, web W is between felt 12 and the heated surface of cylinders 10, and on bottom cylinders 11, felt 12 is against the heated surface of the cylinder and outside the web W. Thus cylinders 10 of the top row are located outside the loop of felt 12, and cylinders 11 of the bottom row are enclosed within the loop. As shown in figures 1 and 2, vacuum boxes 13,13',13" are arranged between the cylinders so as to operate against the surface of felt 12 said vacuum boxes thus being located within the loop of felt 1-2. In vacuum box 13 of figure 1, against the outer surface of cylinder 11 there is a closed wall 20, and between cylinders 10 there is such a curved wall 21, that, for its part, directs air in the direction of arrow A. In the vacuum box there are such suction slots that the suction is mainly applied to wedge-shaped spaces 16,17,18 and 19 between cylinder 10 and felt 12. As shown in figure 1, at cylinder 10 web W is between felt 12 and cylinder 10, and at cylinder 11 web W is in the outermost position. Vacuum box 13 is installed so that pressure p 1 i p_ ( ___. 1 aty) . Vacuum box 13 is so constructed as to provide maximum vacuums at 16,17 and 18. Top part 21 of vacuum box 13 directs air current A so as not to make it separate web W from felt 12 at 10. A high vacuum also draws web W into contact with felt 12 in pit 16, and a lower vacuum holds the web at the center section of the draw W_. In pit 17, a ' igh vacuum sucks web W solidly against web 12 and brings about a vacuum into the felt, which vacuum, due to the imperviousness of web W, is preserved over the whole arc t , and in pit 18 a high vacuum sucks web W against felt 12, and in area 1 a lower vacuum eontri- butes to the transference of web W into the pit. Cylinder 11 may either be groove-surfaced or smooth, in which latter case felt 12 must have a certain pore volume. The grooved surface of cylinder 11 is illustrated with phantom line 11'. Vacuum p 1 of vacuum box 13 is provided with schematically shown pump 15 and fixture 14, known as such. The apparatus illustrated in figure 2 is principally similar with the apparatus illustrated in figure 1 except for that vacuum box 13 is constructed of two separate parts 13' and 13". Vacuum box 13' can also be used alone, without box 13". In this case, should cylinder 11 be grooved, sector fl> should then preferably be perforated. Vacuum boxes 13,13' and 13" can either be as wide as the whole web W or they can be only as wide as the main draw strip. Boxes 13,13',13", illustrated in figures 1 and 2, may, in transversal direction, be divided in two or more compartments, which division is favourable in the situation of main web transfer. In certain cases it is also favourable to apply a higher vacuum to the edges of web W through making use of said division in compartments. Figures 3,4 and 5 show one possible embodiment of the construction of the vacuum box. It has openings 22 and 23 for sucking the vacuum both to cylinder 10 into the separation angle 16 and to cylinder 11, into the loading angle 17. Said openings 22,23 can be adjustable for instance by means of slide mechanisms, and different in size at different points transversally. Sealing strips 24 and 25 prevent the air current from being induced along with cylinders 10 and 11. At least seal 24 should be flexible. Curved gable sealings 26 prevent air flow in the direction of the cylinder axle; seals 27, located at each end of vacuum box 13, are intended for the same purpose. Seals 28 and 29 prevent the vacuum from spreading into the center area of the draw of felt 12 between cylinders 10 and 11. Vacuum box 13' borders upon a plane-shaped wall 30 that is at distance -^ 1 from the inner surface or felt 12, upon a curved wall 31 that is at distance Δ from the surface of cylinder 11, upon outer wall 32 connecting walls 30 and 31, and upon two gables 33. Figure 6 illustrates one alternative embodiment of the construction of vacuum box 13"'. Sealings 34 and 35 are now bevelled to resemble a doctor blade, and suction opening 36 is located just in front of seal 34 to ensure maximum flow . Felt 12 always induces a small air current a, that follows the felt into the slot area between wall 30 and felt 12. The influences of air current a_ are, as shown in figure 6, eliminated in such a way that air current a_ is allowed to flow through tube 6. Air flow a_ can be so directed as to make air flow a,, exiting from closing space 17 through opening 38. eject air flow a». In the alternative illustrated in figure 7, a hole 39 is substituted for tube 37. Sealing 35 guides flow a-,', that can, through hole 39, discharge into the under-pressurized (p..) interior of box 13'". Should one wish to have slight vacuum only at the- center area of the felt draw, gable seals 40, located at the center region further away from the felt 12, of a shape shown in figure 7, can be used. The shape of edge 41 of seal 40 can for instance be selected so to follow the curved shape of the edge area of felt 12, said curved shape resulting from the pressure difference. As shown in figure 1, cylinder 11 of the bottom row is equipped with groove-surfaced shell 11'. As air is sucked to the grooves of this shell 11' through wedge-shaped spaces 17 and/or 18 as described above, the relating vacuum has an influence on the whole sector "X , as, at used moisture contents, paper web W is virtually impervious. This contributes to the prevention of web W from separating on sector °^ due to centrifugal forces and other factors. The grooves of shell 11' may be wound around the shell of roll 11; this kind of grooving can easily be manufactured by winding profiled tape around the shell. Should one wish to use grooved surface 11 in such appli¬ cations wherein the shell of the roll is not covered or equipped with vacuum box on sector extending the borders of sector c- 5 axial grooves or blind perforation can substituate for said wound grooving. The various details of the invention may vary within the framework of the inventional idea as defined in the following claims.

Claims: Claims 1. An apparatus in the drying section of a paper machine for. leading the web as a closed draw, which apparatus operates in connection with a drying fabric (12) arranged in such a way that one/some of the cylinders or rolls of the drying section are outside the drying fabric (12) loop and the other cylinders or rolls are within the drying fabric (12) loop so that the web (W) runs from one cylinder (10,11) to the other all the way supported by said fabric (12) ; wherein the apparatus comprises a vacuum box (13) or vacuum boxes (13',13"), which are arranged to operate against the drying fabric (12) essentially over the whole length of the common run of the web and the drying fabric from one cylinder or roll to the other, and that the suction of said vacuum box (13;13';13") is arranged to reach up to the wedge-shaped space between the drying fabric (12) and the shell of the drying cylinder (11) or roll at least at the entry side. 2. An apparatus in accordance with claim 1, wherein in said vacuum box (13) or vacuum boxes (13;13") there are such suction openings (22,23,36,38,39) as to mainly guide the suction effect of the under-. pressure of the vacuum box to influence on one or more ' wedge-shaped spaces (16,17,18,19) between the drying cylinders (10,11) or similar rolls and the drying fabric (12) . 3. An apparatus in accordance with claim 1 or 2, wherein in said vacuum box (13) or boxes (13;13") there is a plane-formed, mainly closed wall (30), in association with whose edges, near the edges of the drying fabric (12), there are sealing strips (27,40) and sealings (28, 29;34,35) that are located transversally in relation to the running direction of the drying fabric (12), said sealings (28,29; 34,35) guiding the drying fabric (12) to run at a certain distance ( A ) from said plane-formed wall (30) of the vacuum box (13;13'; 13"'). 4. An apparatus in accordance with claim 3, wherein in association with said transversal sealing strips (28,29;34,35) there are suction slots (22,23;36,38) or a similar set of suction holes, through which the vacuum effect is mainly directed to said wedge-shaped spaces (16,17). 5. An apparatus in accordance with claim 1,2,3 or 4, wherein the shell of those cylinders (11) or similar rolls, in association with which the vacuum box (13) or vacuum boxes (13';13") are installed, is equipped with grooved or pitted surface, into whose grooves a vacuum is sucked via said wedge-shaped shapes (17 and/or 18) , which for its part, holds the web (W) that has the outward position in contact with the drying fabric (12), as the web (W) is bent over sector ( CK) of said roll (11). 6. An apparatus in accordance with claim 5, wherein said grooves (ll 1 ) run circumferentially around the shell of the cylinder (11) , preferably manufactured of a profiled band by winding this around the shell, and that the sector outside said sector ( <x» ) is under- pressurized or covered with a plate. 7. An apparatus in accordance with claim 5, wherein said surface of the cylinder (11) or a similar roll is equipped with axial grooves or blind borings so as to make it possible for the sector outside the sector ( ζ ) on which the drying fabric (12) touches the cylinder (11) to be open to the ambient pressure. 8. An apparatus in accordance with one of claims 1 to 7, wherein the suction box (13) or boxes (13';13") partially border with a curved wall (31), which has, in its both edges parallel with the web run, curved sealing strips (26) or similar arranged in such a way that sai curved wall (31) of the suction box will be located at an appropriate distance ( A_ - ) from the shell of the drying cylinder (11) that is touched by the drying fabric (12). 9. An apparatus in accordance with one of claims 1 to 8, wherein on the edge of the vacuum box (13) or boxes (13';13") there is a flexible sealing strip (24), which operates transversally against the outer surface of the drying fabric (12) and prevents the air, induced OMPI by the drying fabric, from entering the vacuum box. 10. An apparatus in accordance with one of claims 1 to 9, wherein said sealing strips (34,35), transversally operating against the drying f bric (12) , are bevelled to resemble a doctor blade (figures 6 and 7). 11. An apparatus in accordance with one of claims 1 to 10, wherein the apparatus comprises an air-removal channel (37) that is associated with the slot area ( Δ ) between said plane-formed wall of the suction box and the drying fabric (12) . 12. An apparatus in accordance with one of claims 1 to 11, wherein it comprises a vacuum box (13) or vacuum boxes (13';13") that essentially reach over the whole width of the drying fabric (12) . 13. An apparatus in accordance with one of claims 1 to 12, wherein the vacuum box (13) or boxes (13';13") is/are divided transversally in relation to the drying fabric (12) to several compartments located side by side. 14. An apparatus in accordance with claim 13, wherein the vacuum directed to said compartments nearest to edges or to the compartment that is nearest to the edge at the side of the end- transfer band is higher than the vacuum directed to other possible compartments.

Inventors: VEDENPAEAE T
Applicants: VALMET OY, VEDENPAEAE T, VALMET O
Application year: 1980
Main classification: D21F5
IPC codes: D21F1, D21F5, F26B13