WO2009118449A1

WO2009118449A1 - Fibre web machine concept, a fibre web machine and method of producing fibre web

Info

Publication number: WO2009118449A1
Application number: PCT/FI2009/050213
Authority: WO
Inventors: Jyrki Savela
Original assignee: Metso Paper, Inc.
Priority date: 2008-03-28
Filing date: 2009-03-20
Publication date: 2009-10-01
Also published as: DE112009000676T5; FI20085258A0; FI127367B; FI20085258A

Abstract

The present invention relates to a concept (10) of a fibre- web machine comprising a web former (215), a press section (220) and a dryer section (225) forming at least three separate drive groups. Transfer arrangements (217, 227; 240, 242; 234, 238) for the web (W) without suction rolls are arranged at least between said drive groups for transferring the web (W) from one drive group to another. The invention also relates to fibre-web machine and a method of producing a fibrous web.

Description

FIBRE WEB MACHINE CONCEPT, A FIBRE WEB MACHINE AND METHOD OF PRODUCING FIBRE WEB

[001] The present invention relates to a concept of a fibre-web machine in accordance with the preamble of claim 1 , a fibre-web machine in accordance with the preamble of claim 4 and a method of producing a fibrous web in accordance with the preamble of claim 17.

[002] In particular, the invention is related to paper and board machines. A fibrous web is typically formed of aqueous pulp, the dry content of which is first increased by mechanical means and, in case of the dry content is higher, also by evaporating water from the web. This takes place at the same time as the fibrous web runs at a relatively high speed, normally at over 1000 m/s. Therefore, it is important that the fibre-web machine used for producing a fibrous web is arranged such that the web can be carried on concurrently with the production process. The operation of the machine needs to be reliable and efficient. It is typical of the fibrous web to be produced that its strength increases, while the dry content gets higher, whereby the production method and the fibre-web machine employing the method need to be such that the supporting of the web at every dry content is sufficient, especially at low dry contents. Thus, the web transfer during the production, especially from one section of the fibre-web machine to another, has to be reliable.

[003] Critical points in the web transfer are especially the points, where the web is separated from the first support surface, such as the first felt, and transferred to the next support surface. The risk here is either a web break or web tear, if the separation is not performed carefully. The web separation point is most often connected to a felt roll, around which the first press felt is looped, while the second felt continues relatively straight on past the roll. While the first felt is looped around the roll, the web is exposed to a centrifugal force, which is obviously dependent on the diameter and rotation speed of the roll, but in any case is relatively high, which force naturally aims at detaching the web from the first felt. It has, however, been realised that the use of a centrifugal force alone cannot be relied on.

i [004] Typically, the critical points in producing a fibrous web are the web transfers taking place on the press section and the web run from the press section to the dryer section. At this point, the dry content of the web is still relatively low and its strength is poor, as well. Generally, the separation of the web from one support fabric and its adhesion to the surface of the other support fabric need to be assisted by a suction arrangement of some kind.

[005] Two basic types of suction arrangements, i.e. suction rolls and transfer suction boxes, are known from prior art. The use of these is explained for instance in the US-patent 5,888,354. Both of these operate basically so that suction is provided through the felt that still supports the web so as to both detach the web from the felt located on the opposite side with respect to the suction device and to keep the web on top of the felt located on the same side as the suction device. Suction rolls are, as such, reliable devices, but the use of suctions rolls causes certain problems. Firstly, the adjustment of the opening of the suction sector is challenging in suction rolls. Moreover, suction rolls are technically demanding to manufacture, consume a great deal of energy in operation and require relatively much maintenance.

[006] The other alternative, i.e. a transfer suction box, is considerably more economical in terms of acquisition and operation. The US-patent 4,113,557, among others, discloses the use of a transfer suction box for separating a paper web from a wire. The publication describes in detail how a paper web is taken, together with the wire and felt, around the roll and close to the roll, after which a transfer suction box is arranged so as to separate the web from the wire. The direction of the felt, and thereby also the direction of the web, is deviated by means of the transfer suction box from the direction of the wire. In other words, the run path of the felt is curved by the concave cover of the suction box and thereby, also the detrimental wear of both the cover and the felt is increased. The wear is further increased by the high speed of the felt when passing over the suction box cover. For this reason, the use of a transfer suction box has gradually decreased, while the speeds of paper machines have increased. The use of a transfer suction box also involves the problem of the length it requires in machine direction. In other words, when using a transfer suction box, the transfer needs to be sufficiently long for a box like this to be able to operate, and still it is necessary to have guide rolls.

[007] On the other hand, in order to improve the overall efficiency, a general target is also to simplify the fibre-web machine and to some extent reduce its size. The publication US 6,638,395 discloses a fibre-web machine and a process for producing a fibrous web. The machine comprises a press section with a press nip for dewatering the fibrous web, a former section located before the press section, relative to the web run direction, for forming a fibrous web, and a dryer section comprising a first dryer group with less than three dryer cylinders located after the press section. Two press rolls form a press nip and at least one press felt is placed on both faces of the fibrous web so that the fibrous web and the at least one press felt on both sides of the fibrous web are led through the press nip. The fibrous web is dewatered in the former section to a dry content of at least 18 % and the fibrous web is led, at least partially by looping it, via less than three dryer cylinders and the fibrous web is constantly supported from the former to the end of the first dryer group. The fibrous web is constantly supported between the former section and the end of the first dryer group and the web transfer from one section to another is realised by utilising suction rolls. Suction rolls provide a suction effect only to the extent the web covers the suction roll and the suction roll cannot in any way have an impact on the support of the web e.g. before or after the covered area.

[008] Even if the fibre-web machine and the process disclosed in the publication US 6,638,395 are as such advantageous, there has recently arisen a need to develop the machine and production process of this kind further so as to achieve a simpler, more economical and reliable solution.

[009] The above-mentioned target is achieved by the concept of a fibre-web machine according to claim 1 , the fibre-web machine according to claim 4 and the production process according to claim 17. [0010] The aims of the invention are achieved according to one embodiment of the invention mainly by a fibre-web machine for producing a fibrous web, which comprises at least a press section with at least one press nip for dewatering the fibrous web, and a dryer section comprising at least a first dryer group and a support fabric included therein, and in which the press section comprises a first press roll above the nip and a second press roll below the nip and a first and second endless fabric loop, of which the first fabric loop is arranged to run around the first press roll and the second fabric loop is arranged to run around the second press roll, and where the fibrous web is arranged to follow the second fabric loop after the press nip. In the fibre-web machine according to the invention at least one guide roll is arranged inside the second fabric loop, to which roll the second support fabric is arranged to pass from the press nip, and on which the run direction of the support fabric changes, and at least one guide roll is arranged inside the support fabric loop of the dryer group in such a way that the support fabric of the dryer group passing via the guide roll of said dryer group passes to said guide roll of the second fabric loop, and that a suction device is arranged within the support fabric loop of the dryer group, at the guide roll, to provide a suction effect on the web through the support fabric of the dryer group at least at a point, where the second support fabric is separated from the support fabric of the dryer group in order to guide the web run onto the lower surface of the support fabric of the dryer group.

[0011] A fibre-web machine of this kind is substantially simpler and accordingly, more economical to manufacture and use than a fibre-web machine according to prior art, and still its operation is very reliable and safe.

[0012] The operation of the fibre-web machine may be further improved by arranging a second guide roll inside the first fabric loop, and by arranging both the first support fabric and the second support fabric to pass from the press nip to the guide roll, whereby the fibrous web remains between the fabrics, and by arranging said guide roll to change the direction of both the first and the second support fabric so that the change of direction of the first support fabric is essentially larger than that of the second support fabric. [0013] In this manner, the web run after the press nip is defined precisely and supported efficiently, and at the same time it is independent of the web adhesion properties of the upper and lower fabrics.

[0014] Preferably, a suction device is arranged inside the second support fabric, at the guide roll within the first fabric loop after the press nip, to provide a suction effect on the web through the second support fabric at least at a point, where the first support fabric is separated from the second support fabric in order to guide the web run onto the upper surface of the second support fabric. This ensures the web run forward on the surface of the second support fabric.

[0015] The suction device is a transfer suction box, which is provided with a cover and one or more suction slots, and which is placed at the roll so that at least one of the slots in the cover is located substantially at the point of the roll where the surface of the roll starts diverging from the direction of the fabric facing the transfer suction box.

[0016] Preferably, the cover of the transfer suction box consists of at least two strips, the first one of which is placed in machine direction so that its rear edge is located before the separation point of the fabric from the roll and the second one of which strips is placed in machine direction so that its front edge is located at a distance from the rear edge of the first strip. The above-mentioned two strips form one suction slot.

[0017] The length of an individual suction slot is 8 - 40 mm, preferably however 15 - 20 mm, in machine direction. This ensures a sufficient suction effect for successful web transfer.

[0018] According to another embodiment of the invention a simpler, more economical and reliable solution than prior art solutions is provided by a concept of a fibre-web machine comprising a web former, a press section and a dryer section forming at least three separate drive groups, where transfer arrangements for the web without suction rolls are arranged at least between said drive groups for transferring the web from one drive group to another. [0019] Preferably, the web transfer arrangement without a suction roll between the drive groups comprises a guide roll for the web support fabric included in the previous drive group and a transfer suction box for the web support fabric included in the following drive group arranged in interaction with one another. The web transfer is performed so that the transfer suction box is arranged inside the support fabric included in the following drive group, at said guide roll included in the previous drive group, to provide a suction effect on the web through the support fabric at least at a point, where the support fabric included in the first drive group is separated from the support fabric included in second drive group in order to guide the web run onto the surface of the support fabric included in the second drive group.

[0020] In the method according to the invention of producing a fibrous web by a fibre-web machine, in which method the formed fibrous web is dewatered on the press section, where the press section comprises a first press roll above the nip and a second press roll below the nip and a first and second endless fabric loop, of which the first fabric loop runs around the first press roll and the second fabric loop runs around the second press roll, the fibrous web is taken through at least one press nip in order to dewater the fibrous web by pressing it by means of the first and second fabric loop, and the fibrous web is taken at least via the first dryer group supported by the support fabric included therein, at which dryer group water is evaporated from the fibrous web. The method is mainly characterised in that at least one guide roll is arranged in the fibre-web machine inside the second fabric loop, to which roll the second support fabric is arranged to pass from the press nip, and by which the run direction of the support fabric is changed, and at least one guide roll is arranged inside the support fabric loop of the dryer group in such a way that the support fabric of the dryer group passing via the guide roll passes to said guide roll of the second fabric loop, and that a suction device is arranged inside the support fabric loop of the dryer group, at the guide roll of said second fabric loop, to exert a suction effect on the web through the support fabric of the dryer group at least at a point, where the second support fabric is separated from the support fabric of the dryer group in order to guide the web run onto the lower surface of the support fabric of the dryer group.

[0021] Advantageously, at least one guide roll is arranged inside the first fabric loop, and both the first and second support fabric passes from the press nip to the guide roll, while the fibrous web remains between them. The guide roll of the first fabric loop changes the direction of both the first and second support fabric so that the change of the direction of the first support fabric is essentially larger than that of the second support fabric.

[0022] Moreover, a suction device is preferably arranged inside the second support fabric, at the guide roll of said first fabric loop, by which suction device a suction effect is exerted on the web through the second support fabric at least at a point, where the first support fabric is separated from the second support fabric in order to guide the web run onto the upper surface of the second support fabric.

[0023] Preferably, the method is further practised so that the suction of the suction device is exerted on the web at the guide roll located on the opposite side of the fibrous web with respect to the suction device and the fibrous web being separated from the fabric facing the guide roll substantially at a point, where the direction of the movement of the fabric facing the guide roll deviates, together with the guide roll, from the direction of the movement of the fabric facing the suction device.

[0024] According to one embodiment of the invention, a tail threading step is included in the invention, in which the vacuum level of the suction device or suction devices is on a first, increased vacuum level, and after the tail threading step the vacuum level of the suction device is on normal level, while the web runs normally. On the increased vacuum level the pressure is preferably 10 - 50 kPa below the ambient pressure and on the normal vacuum level the pressure is 1 - 10 kPa under the ambient pressure. This normal vacuum level is advantageous, as in that case the fabric does not typically come into contact with the cover of the suction device. In a more demanding transfer, or for instance in conjunction with heavier grades, the normal vacuum level can be raised up to 20 kPa. Even if the fabric may be in contact with the cover, the wear of the fabric is not yet substantial.

[0025] Other characteristic features of the fibre-web machine and the method according to the invention are disclosed in the appended claims.

[0026] Several advantages are achieved by the present invention, of which the following can be mentioned as examples.

- the fibre-web machine according to the invention is essentially more economical to manufacture and to use than a fibre-web machine according to prior art, since no complicated suction rolls are required for performing a web transfer in the fibre-web machine and the web transfer technique is as such, i.e. in terms of construction, more advantageous than a suction roll, not to mention the operation of the suction roll and its energy consumption, in the fibre-web machine according to the invention the rate of wear of the fabrics is less than in a fibre-web machine according to prior art and/or in conjunction with present suction box concepts,

- the web run is stable, since during the web transfer, the support fabrics diverge fast from one another at a certain angle at the transfer point,

- the solution is advantageous, as neither malfunctions due to pressure fluctuations originating from suction rolls nor a foil force caused by a small diverging angle of the support fabrics according to prior art appear,

- by the present solution it is even possible to repair already existing web edge damages caused by the pulp, dry content, felt or the like. [0027] In the following, the method and apparatus according to the invention will be explained in more detail with reference to the appended schematic figures, of which

Figure 1 shows one embodiment of the fibre-web machine according to the invention,

Figure 2 shows schematically the web transfer location in the fibre-web machine according to Figure 1 ,

Figure 3 shows in more detail a suction device according to Figure 1 ,

Figure 4 shows a suction slot in the suction device and its position,

Figure 5 shows a transfer suction box according to the invention,

Figure 6 shows still another transfer suction box according to the invention,

Figure 7 shows still another embodiment of the fibre-web machine according to the invention, and

Figure 8 shows yet another embodiment of the fibre-web machine according to the invention.

[0028] Figure 1 shows one embodiment of the fibre-web machine 200 according to the invention. The fibre-web machine comprises a web former 215, of which only one section is shown herein. In machine direction MD the web former is followed by a press section 220 and further, by a dryer section 225. The press section comprises one press nip 222 in the embodiment according to Figure 1. The press nip of the press section is formed by a first press roll 224 and a second press roll 226, of which the first press roll is located above the nip and the second press roll is located below the nip. The press is preferably a so- called shoe press. Thus, one of the rolls is a shoe roll provided with a loading element (a shoe element), the shell of which is flexible, i.e. a so-called belt. The other one of the rolls is typically a counter roll with variable crown. The loading of the shoe elements takes place with built-in hydraulic cylinders. By this arrangement, an adjustable nip load is provided typically in the area of 800 - 1500 kN/m. A press section of this kind with one nip is particularly advantageous due to its short structure and web transfers without suction rolls as well to its low energy consumption. [0029] A first fabric loop 228 and a second fabric loop 230 are connected to the press. The fabric loops are arranged to pass via the guide rolls of the press section 220 as endless loops. The fabrics act as support fabrics for the fibrous web W and they also have an impact and function related e.g. to web drying. On the press section the support fabric is a press felt. The first press roll 224 in the fibre-web machine is arranged inside the first fabric loop 228 and the second press roll 226 is arranged inside the second fabric loop 230. The first fabric loop is arranged to pick up the web W from the fabric 216 of the web former, i.e. from the wire, on its lower surface. This is realised so that a guide roll 217 is arranged on the opposite with respect to the wire surface facing the web W to guide the wire 216 of the web former, over which guide roll, in addition to the wire 216 of the web former, the first support fabric 228 of the press section is arranged to pass by deflecting the support fabric. The first support fabric 228 is arranged to pass via the guide rolls 221 and 223 so that the incoming and outgoing directions of the support fabrics differ from the incoming and outgoing directions of the wire of the web former with respect to the guide roll 217. Then in machine direction, the length of the area, where the web W runs between two support fabrics included in different drive groups (wire section - press section), may be made relatively short and the possible disadvantages due to the draw differences between the groups will be minimised. The roll 223 is preferably, as opposed to smooth guide rolls, a so- called deep grooved roll, the grooves of which are at least 15 - 20 mm deep. In other positions smooth guide rolls are preferably used.

[0030] At guide roll 217 of the support fabric 216 of the web former a suction device 227, i.e. a so-called pick-up suction box, is arranged facing the first support fabric 228 to provide a suction effect on the web W through the support fabric 228. The suction effect is arranged at least at a point, where the support fabric 216 of the web former is separated from the first support fabric 228 for guiding the web run onto the lower surface of the first support fabric. The suction effect provided by the suction device extends to a distance from the separation point of the first support fabric from the guide roll in the direction of the first support fabric. [0031] The web W runs through the press nip 222 between the first and second support fabric, i.e. the press felts. Water is removed from the web in the nip, pressed by the nip load, over to press felts, i.e. to the first and second support fabric, from which the water is discharged after the web W has been detached from the felt surface.

[0032] In the embodiment of Figure 1, the web W is transferred to the dryer section 225, to its first dryer group, after the web has run through the only press nip 222 of the press section. A guide roll 234 is arranged inside the second fabric loop 230, at which guide roll the run path of the web W is led so as to separate from the second support fabric 230 coming from the press nip. Said guide roll 234 is the first guide roll arranged inside the second fabric loop in the run direction of support fabric 230. In this situation, the web W is arranged to run on the upper surface of the second support fabric (press felt) 230, and the support fabric 232 (dryer fabric) included in the dryer section is also arranged to run on the upper surface of the second support fabric 230 so that the fibrous web W remains between these two preferably only on the shell of the guide roll 234. The dryer fabric 232 is arranged to run between the guide roll 236 and the first dryer cylinder 246 so that the incoming and outgoing directions of the dryer fabric 232 differ from the incoming and outgoing directions of the second support fabric 230 at the guide roll 234. Then, the length in machine direction of such an area, where the web W is between two support fabrics included in different drive groups (press section - wire section), may be made relatively short and the possible disadvantages due to the draw differences between the groups will be minimised.

[0033] To enable this, a guide roll 236 is arranged inside the support fabric loop 232 of the dryer group, which guide roll is adapted so that the support fabric 232 of the dryer section passing through it runs also guided by the guide roll 234 of the second fabric loop 230, i.e. over it.

[0034] A suction device 238, a so-called wire suction box 238, is also arranged at the guide roll 234 of the second fabric loop 230, facing the support fabric 232 of the dryer group 225, to provide a suction effect on the web W through the support fabric 232. The suction effect is arranged at least at a point, where the second support fabric 230 is separated from the support fabric 232 of the dryer group for guiding the web run onto the lower surface of the support fabric of the dryer group and for supporting the web W onto the support fabric 232. While the web transfer from the press section to the dryer section is completely supported, the risk of a web break is minimised. In the fibre-web machine according to the invention the suction effect supporting the web during the transfer is specifically exerted on such areas, which have a significant impact on the web run. The structure of the wire suction box of the dryer section is shown in more detail e.g. in Figures 2 and 3.

[0035] The fibrous web is separated from the support fabric facing the guide roll essentially at a point, where the direction of the movement of the support fabric facing the guide roll deviates, together with the guide roll, from the direction of the movement of the support fabric facing the suction device and at this point, an air layer from the side of the edge of the fibrous web , in cross direction from the outside the fibrous web, is drawn essentially at the separation point of the web through the fabric. Preferably, the slot in the cover of the suction device, or a array of separate apertures arranged in cross direction of the fibrous web, is arranged to start on the plain that passes via the centre line of the guide roll shaft and via the line, where the fibrous web is separated from the fabric that starts following the guide roll. Then, the slot or the array of separate apertures may be longer than the width of the fibrous web and shorter than the width of the fabric facing the suction device. In order to enhance the support at the edge zones of the web the arrangement may include separate, so-called trim suction boxes, or trim suction zones, by which the vacuum effect on the web edges is intensified.

[0036] In order to ensure a stable run of the web W on the lower surface of the support fabric 232 of the dryer section a web stabilisation device 244 is arranged inside the support fabric, i.e. on the opposite side with respect to the web W. By the stabilisation device a vacuum effect keeping the web W attached to the support fabric 232 is provided between the web and the support fabric and between the support fabric and the stabilisation device. The stabilisation device extends in cross direction over the width of the web and in machine direction MD from the first suction device 238 over the junction of the support fabric 232 and the first dryer cylinder 246 of the dryer section 225. The stabilisation device 244 is also at such a distance from the first suction device 238 that the air jet 248 at the front edge of the stabilisation device and the air flow induced by it may discharge unobstructed between these away from the surface of the support fabric 232. The stabilisation device and the suction device 238 may also be structurally integrated with one another.

[0037] The first fabric loop 228 is provided with an inner guide roll 240 also after the press nip 222 in the run direction of the web W. The guide roll 240 of the first fabric loop 240 after the press nip is arranged in the fibre-web machine so that also the second fabric loop 230 runs deflected by the guide roll 240. Accordingly, the web W is arranged to pass from the press nip 222 between the first support fabric 228 and the second support fabric 230 up to the guide roll 240, and not until at the guide roll 234, the first support fabric is guided so as to separate from the run path of the web. Thus, it can be guaranteed that the web coming from the press nip follows the exactly correct fabric.

[0038] At the guide roll 240 of the first fabric loop after the press nip, the deflection angle of the first support fabric 228 is essentially larger than that of the second support fabric 230 and the web W. The deflection angle also determines the magnitude of the change of direction and therefore, the change of direction of the first support fabric is essentially larger than that of the second support fabric.

[0039] At the guide roll 240, the web W is arranged to follow the second support fabric 230. This is realised so that a suction device 242, a so-called felt suction box 242, is arranged inside the second support fabric 230, at the guide roll 240 to provide a suction effect on the web W through the second support fabric at least at a point, where the first support fabric 228 is separated from the second support fabric 238 in order to guide the web run onto the upper surface of the second support fabric. The first support fabric 228 is separated from the second support fabric at such a point, where the directions of their respective tangents differ from one another, while they are guided by the surface of the roll 240.

[0040] The felt suction device 242 is a transfer suction box 242, which is thus arranged on the other side of the felt 228 with respect to the guide roll 240, i.e. on the inside of the second press felt 230 at the roll 240. The cover 243 of the transfer suction box 242 is provided with two slots 245, 247 in the embodiment according to the figure. The first slot 247 in the run direction of the web is preferably located in the opening gap formed by the guide roll 240 and the fabric 228, starting at the latest from the same plain that passes via the centre line of the shaft of the felt roll 240 and the separation line of the fibrous web that is separated from the first felt 228 that starts looping around the felt roll. The second slot 247 is arranged after the first slot 245 in the run direction of the web to ensure that the web stays attached to the surface of the second support fabric 230.

[0041] The cover 243 of the suction box is essentially concave in machine direction in the area, where the suction box is adapted against the roll and the cover follows the shape of the roll surface. Elsewhere, the shape of the cover is essentially straight coinciding with the direction of the fabric passing by, in other words the direction of the cover plain of the suction box is essentially the same as the run direction of the fabric 230. The cover 243 of the suction box does not, as such, cause any change of direction of the fabric 230, but the run of the press felt is affected, starting from the guide roll 240, by the position of the next guide roll along the run of the fabric and the impact of the vacuum prevailing in the suction box.

[0042] The fibre-web machine according to Figure 1 may be successfully utilised for paper or board manufacture within the speed area of 800 - 2000 m/min. The advantages of the invention emerge most clearly within the speed range of 1200 - 2000 m/min. This being the case, the web former and the press section are operated so that the dry content of the web after the press section is 40 - 60 %, whereby the strength of the web is on such a level that the web transfer in the fibre-web machine according to the invention from one fabric on another is reliable.

[0043] In the fibre-web machine according to the invention following draw differences are employed between the various sections. The press rolls, the first and second fabric loop run all at the same speed, but at a speed 0.3 - 1 % higher than the web former before the press section. The draw difference between two press nips is preferably 0.3 - 1 %. Further, the dryer section following the press section runs at a speed 0.5 - 4 % higher than the press section preceding it.

[0044] Even if Figure 1 does not show it, the ends of the suction device are supported by the frame structure of the fibre-web machine so that its position is adjustable. Figure 1 shows, however, that the suction device 238 is supported by the frame structure of the fibre-web machine 252 over a portion between the ends of the suction device, preferably in cross direction in the middle. The frame structure 252 and the suction device are connected to one another at least by one adjustable support device 254, by which the position of the suction device with respect to the support fabric 232 and the roll 234 may be adjusted. More precisely, the purpose of the support device is to make the deflection of the suction device in cross direction so as to correspond essentially to the deflection of the roll 234, whereby the distance of the suction device from the roll is essentially the same over the entire length of the slot in the suction device.

[0045] In order to adjust the deflection of the suction device also other kind of arrangements can be used. One thinkable alternative could be a cable wire inside the box extending from one end to the other and a support structure connected thereto, where the points of support are located at the ends, in the vicinity of the cover, and in the central area of the cover at a distance towards the bottom of the box from the cover. Then, the curvature degree of the cover can be affected by the tension of the cable wire. A relevant option is also to arrange the strips adjustably, whereby loading means, e.g. an air hose, are placed under the strips. The strips of the box that are aligned with the guide roll are locked in place by locking means.

[0046] In the embodiment of Figure 1 each web transfer from one support fabric to another is secured by a suction device, whereby the attaching abilities as well as the changes in the attaching abilities during the use of the fabric are less important when choosing the fabric. When the web transfer, especially between the drive groups, is arranged so that there are no suction rolls, the fibre-web machine concept according to the invention is particularly advantageous and technically simple.

[0047] Figure 2 shows the transfer location of the web W when employing the first suction device 238 of Figure 1 , seen from below without a press felt or roll. The figure shows the fibrous web W and its transverse width L_w, the support fabric 232 and its transverse width L_f and the slot restricted in machine direction by the strips of the suction device and its transverse width L_s. The width L_s of the slot in the suction device can be adjusted by moving the sealing body arranged in the slot, as illustrated by arrows A. The width L_s of the slot is set so as to be wider than the web width L_w, but narrower than the width L_f of the support fabric.

[0048] Figure 2 shows clearly that the support fabric 232, the slit and the web W, in this order, have a descending width. In other words: the support fabric is wider than the slit and the slit is wider than the web. This structure makes it possible to draw air from both edge areas of the web W. The inventors have observed that this procedure stabilises the web transfer further. Figure 2 also shows an adjustable vacuum source 250, by means of which a vacuum is provided in the suction device. The illustrated devices operate in the suction box with pressures, which are preferably less than 50 kPa. In normal operation a sufficient vacuum is generally 1 - 10 kPa. The magnitude of the vacuum is dependent e.g. on the dry content, the density and surface of the fabrics and the web and on the adhesive abilities of the support fabric pair. Especially in tail threading procedures, a higher vacuum than in normal operation can be applied, for instance a vacuum of 10 - 50 kPa. By this the fluency of tail threading can be improved considerably. This normal vacuum level is advantageous, as in that case the fabric does not typically come into contact with the cover of the suction device. In a more demanding transfer, or for instance in conjunction with heavier grades, the normal vacuum level can be raised up to 20 kPa. Even if the fabric may be in contact with the cover, the wear of the fabric is not yet substantial.

[0049] It is, however, essential to notice that in the situation of Figure 2 the entire width of the web is such that the web can be taken substantially undisturbed through the fibre-web machine. This indicates that a web formed by the present web formers needs to be trimmed at the web edges. Consequently, it is clear that if the trim strips have been removed just before the transfer suction box, it has to be ensured that the trim strips are not entrained with the web W itself further on at the transfer suction box, but follow the support fabric, from which the actual web is transferred to the next support fabric. In a situation like this, as opposed to the above description, the width L_s of the suction slot has essentially the same width as the trimmed web, i.e. L_s is approximately the same as L_w. By suction or blow devices arranged at the trim strips it can be ensured that the trim strips run e.g. to a pulper.

[0050] In addition, it has turned out that it is advantageous, if the portion of the support fabric 232 exceeding the slot 82 is larger than the portion of the slot 82 exceeding the web W. In other words, the dimension Y must be larger than the dimension X in Figure 2. Preferable dimensions for X are 5 - 150 mm, 40 - 70 mm in particular. Good results were achieved by using values between 100 mm and 200 mm for the dimension Y. It is to be noted that in all embodiments of the transfer locations the slot or slots may also be embodied as one or several rows of separate apertures extending in cross direction (lateral direction) of the web. Then the apertures are simply round holes or even short slots or the like.

[0051] Figure 3 shows schematically a side view of the transfer location applied in the invention. The suction device is a transfer suction box 238. The transfer suction box comprises a cover 260 formed by two strips 262, 264. The first one of these in machine direction MD is positioned so that its rear edge is located before the fabric separation point P from the roll. At this point, the directions of both support fabrics 230, 232 start diverging from one another. A slot 266 in machine direction MD is formed between the strips 262, 264 so that the front edge of the second strip 264 is located at a distance from the rear edge of the first strip 262. The suction slot is adapted into a point, where one support fabric 230 is separated from the other support fabric 232. The length 268 of an individual slot is 8 - 40 mm, preferably however 15 - 20 mm, in machine direction. It has been observed that with larger slot widths the felt curves into the slot, whereby both the strips and the support fabric wear excessively. The dimension of the strips in machine direction, i.e. the strip width, is 5 - 60 mm, preferably however 10 - 40 mm. When a slot with a length of less than 20 mm is used, the suction effect becomes very strong and sharp. The suction area extending farther from the web transfer location in machine direction ensures that the web remains adhered in spite of the transverse air flows occurring in the opening gap. The chamfer radius of the strip angles is preferably 2 - 6 mm so as to reduce the wear of the fabric.

[0052] In addition, Figure 4 shows the bearing system of the strips 262, 264 provided with springs and restricting the suction slot 266 in the cover of the transfer suction box 238. As schematically indicated, there is preferably a compressed-air hose 280 carrying the strip 262, 264. If, e.g. during a disturbance, a wad of fibrous web is caught in the slot between the guide roll 234 and the transfer suction box 238, the strips 262, 264 in Figure 4 are allowed to move upwards and will not be damaged. The spring suspension of the strips may also be provided with elastomers or other, e.g. metallic, spring elements. Figure 4 also shows the distance Z that may be set between the cover (the lower edge of the strips 262, 264 is shown here) of the suction box 238 and the support fabric 232 receiving the web. It has turned out that good results are to be achieved in the web transfer also if there is a gap (distance Z) between the cover of the suction box and the receiving support fabric. Good results have been achieved by using the values within the area of 0.5 - 5 mm for the distance Z and especially within the area of 1 - 3 mm. Thus, there is a gap of 0.5 - 5 mm between the suction device and the support fabric, while the fibre-web machine is stopped, but the fabrics are in their operating tension. In normal operation of the machine, the gap may, however, be reduced due to the vacuum even so much that the gap will be entirely closed. In practise, this does not, nevertheless, cause substantial friction between the fabric and the cover, as the normal force is very small.

[0053] Optionally, the cover may also be hinged, whereby the strips of the cover are moved by a common actuator or a so-called detachable cover part, which is provided with two or more strips are moved by two compressed-air hoses, may be used.

[0054] The roll 234 is movably adapted in the fibre-web machine by means of actuator cylinders 256. Of these, only the cylinder arranged at one end is shown. It is obvious that the movability requires also other arrangements, but those are not shown here for the sake of clarity. The actuator cylinders are provided with pressure accumulators 258 to compensate the run of the web layer between the roll and the suction device caused by an accidental disturbance. Also the roll 240 is provided with a corresponding apparatus (not shown).

[0055] Figure 5 shows the structure of one end of a pick-up suction box 227 at the transfer location of the web W seen from below without a support fabric 216 or a roll 217. The figure shows the web W, from which so-called trim strips W have been cut off from both edges for instance by a water jet. Only one edge is shown here. The pick-up suction box differs from the other transfer suction boxes shown in Figure 1 in that there are blow sections 227.2 arranged at its edges. The blow section 227.2 is formed by arranging a partition wall 227.3 in the header of the transfer suction box. The partition wall divides the suction box into portions with vacuum respective overpressure, of which the portion with vacuum extends over the width of the trimmed web W and the portion with overpressure to the area of the trim strips W. The suction slot 227.4 of the portion with vacuum and the suction slot 227.5 of the portion with overpressure are separated from one another by a sealing body 227.6, which extends essentially to the level of the strips thus minimising direct air flow between the portions with vacuum respective overpressure. The inner edge of the sealing body is in such a position that the suction slot with vacuum extends to the area of the trimmed web. Figure 5 shows an adjustable vacuum source 250, by means of which a vacuum is provided in the suction device within its suction zone. Figure 5 shows an adjustable overpressure source 251 , by means of which an overpressure is provided in the portion with overpressure in the blow section of the suction device. The pressure in the suction zone is preferably under 50 kPa. In normal operation, a sufficient vacuum is generally 1 - 10 kPa. The magnitude of the vacuum is dependent e.g. on the dry content, the density and surface of the fabrics and the web and on the adhesive abilities of the support fabric pair. Especially in tail threading procedures, a higher vacuum than in normal operation can be applied, for instance a vacuum of 10 - 50 kPa. By this the fluency of tail threading can be improved considerably. The overpressure prevailing in the blow section is preferably 1 - 8 kPa. This overpressure is able to guide the trim strips away from the support fabric, onto which the trimmed web is to be transferred.

[0056] Figure 6 shows a transfer suction box 242 according to yet another embodiment of the invention. In the embodiment according to Figure 6 the concave cover of the transfer suction box after the first press nip shown in Figure 1 comprises a suction slot 247 facing the opening gap and, in addition, also a suction slot 247.1 facing the closing gap. Further, there is a slot 247.2 between these suction slots, on which no suction effect is exerted. The slot 247.2 of the transfer suction box, on which no suction effect is exerted, is plugged for instance by a plate 242.1. A thinkable alternative to the solution shown in Figure 6 could be such an assembly, where the suction slots are in connection with separate transfer suction boxes and a zone without suction is left between the slots.

[0057] This solution is particularly advantageous when used in such a position, where the fabric, from which the web is transferred, is thick, poorly air- permeable and/or has such surface properties that it tends to keep the web adhered to its surface. Smoothness is one factor affecting e.g. the surface force and the web has a tendency to stay on the surface of a smooth fabric. According to the solution, the first suction slot 247.1 in the run direction of the web either weakens the contact of the web with the fabric surface or momentarily even detaches it from the outer fabric with respect to the transfer suction box 242. The second, i.e. the former, suction slot 247 in the run direction of the web transfers the web eventually onto the surface of the fabric facing the transfer suction box. The suction slots are located at a distance from one another and since the slot 247.2 is located between these, on which no suction effect is exerted, the contact that wears the fabric remains within acceptable limits, if it is ensured at the same time that the final transfer of the web to the fabric facing the transfer suction box takes place at the desired point.

[0058] Figure 7 shows yet another embodiment of the fibre-web machine 200 according to the invention. The fibre-web machine comprises at least one preceding press nip 220' in the run direction of the web, from which nip is shown here only allusively. In machine direction MD, the web former is followed by a press section 220 and further, by a dryer section 225. The press section comprises one smoothing press nip 222 in the embodiment according to Figure 7. The smoothing press nip of the press section is formed by a first press roll 224' and a second press roll 226', of which the first press roll is here located above the nip and the second press roll below the nip. In this embodiment, the smoothing press comprises only a second endless fabric loop 230 around the second press roll 226' and the first smoothing press roll 224' is without a fabric. The second fabric loop is arranged to run around the second press roll and the fibrous web W is arranged to follow the second fabric loop 230 after the press nip so that a suction device 226.1 provided with at least one suction slot is arranged inside the second fabric loop to provide a suction effect on the web W through the fabric 230 at least at a point, where the fabric 230 is separated from the first press roll 226' in order to guide the web run onto the upper surface of the fabric. Thus, the web W is not between two fabrics here, but the web is transferred from the surface of the smooth roll 226' so as to follow the surface of the fabric. To perform and/or to ensure the transfer to the vicinity of the roll and fabric, there is a suction device 226.1 provided with at least one slot arranged on the side of the opposite fabric surface with respect to the side of the roll. Then, while the machine is running, the fabric is arranged to follow the surface of the smooth roll after the nip over a certain distance, whereby space is provided for the suction device 266.1 inside the fabric loop.

[0059] The solution according to Figure 7 can also be applied, in addition to said smoothing press, for instance in conjunction with the centre roll of a press.

[0060] Figure 8 shows yet another fibre-web machine according to the invention. To some extent it corresponds to the embodiment of Figure 1 and the reference numbers according to Figure 1 are used, where applicable. The fibre- web machine shown in Figure 8 comprises a web former 215, of which only a part is shown here. In machine direction MD, the web former is followed by a press section 220 and further, by a dryer section 225. Also in the embodiment according to Figure 8, the press section comprises one press nip 222. The press nip of the press section is formed by a first press roll 224 and a second press roll 226, of which the first press roll is located above the nip and the second press roll is located below the nip. The press is preferably a so-called shoe press. A press section of this kind with one nip is particularly advantageous due to its compact structure and web transfers without suction rolls as well to the low energy consumption. The transfer of the web W is essentially similar to the one shown Figure 1. The web W runs through the press nip 222 between the first and second support fabric, in other words between the press felts. Water is removed from the web in the nip, pressed by the nip load, over to press felts, i.e. to the first and second support fabric, from which the water is discharged after the web W has been detached from the felt surface.

[0061] In the embodiment of Figure 8, the web W is transferred to the dryer section 225, to its first dryer group, after the web has run through the only press nip 222 of the press section. A guide roll 234 is arranged inside the second fabric loop 230, at which guide roll the run path of the web W is led so as to separate from the second support fabric 230 coming from the press nip. Said guide roll 234 is a guide roll arranged inside the first fabric loop in the run direction of the second support fabric 230. In this situation, the web W is arranged to run on the upper surface of the second support fabric (press felt) 230, and the support fabric 232 (dryer fabric) included in the dryer section is also arranged to run on the upper surface of the second support fabric 230 so that the fibrous web W remains between these two preferably only on the shell of the guide roll 234. The dryer fabric 232 is arranged to run between the guide roll 236 and the first reversing suction roll 246' so that the incoming and outgoing directions of the dryer fabric differ from the incoming and outgoing directions of the support fabric 230 at the guide roll 234. Then, the length in machine direction of such an area, where the web W is between two support fabrics included in different drive groups (press section - wire section), may be made relatively short and the possible disadvantages due to the draw differences between the groups will be minimised.

[0062] To enable this, a guide roll 236 is arranged inside the support fabric loop 232 of the dryer group, which guide roll is adapted so that the support fabric 232 of the dryer section passing through it runs also guided by the guide roll 234 of the second fabric loop 230, i.e. over it.

[0063] A suction device 238, a so-called wire suction box 238, is also arranged at the guide roll 234 of the second fabric loop 230, facing the support fabric 232 of the dryer group 225 to provide a suction effect on the web W through the support fabric 232. The suction effect is arranged at least at a point, where the second support fabric 230 is separated from the support fabric 232 of the dryer group for guiding the web run onto the lower surface of the support fabric of the dryer group and for supporting the web W against the support fabric 232. While the web transfer from the press section to the dryer section is completely supported, the risk of a web break is minimised. In the fibre-web machine according to the invention the suction effect supporting the web during the transfer is specifically exerted on such areas, which have a significant impact on the web run.

[0064] Here, the transfer of the web W to the dryer section takes place so that the web is taken on the lower surface of the support fabric 232 to the first deflecting suction roll 246' of the dryer section, which deflects the web so as to run toward the first dryer cylinder 246. Here, the first member changing the web run in the dryer section is thus the deflecting suction roll 246'. The deflecting suction roll is preferably for instance a so-called VAC-roll, via which a suction effect is arranged to be exerted on the web through the fabric 232.

[0065] A transfer procedure of this kind is particularly advantageous in a low press structure.

[0066] It is to be noted that only a few most advantageous embodiments of the invention are described in the above. Thus, it is clear that the invention is not limited to the above-described embodiments, but may be applied in many ways within the frame of the appended claims. The individual features described in conjunction with the different embodiments may be used in conjunction with other embodiments as well and/or various combinations of the described features may be made within the frame of the basic idea of the invention, if so desired and if technical feasibility for this exists.

Claims

1. A concept (10) of a fibre-web machine comprising a web former (215), a press section (220) and a dryer section (225) forming at least three separate drive groups, characterised in that transfer arrangements (217, 227; 240, 242; 234, 238) for the web (W) without suction rolls are arranged at least between said drive groups for transferring the web (W) from one drive group to another.

2. A concept of a fibre-web machine according to claim 1 , characterised in that the transfer arrangement for the web (W) without a suction roll between the drive groups comprises a guide roll (217; 240; 234) for the support fabric included in the previous drive group and a transfer suction box (227; 242; 238) for the support fabric included in the following drive group arranged in interaction with one another.

3. A concept of a fibre-web machine according to claim 2, characterised in that a transfer suction box (227; 242; 238) is arranged inside said support fabric included in the following drive group, at said guide roll included in the previous drive group, to provide a suction effect on the web (W) through the support fabric at least at a point, where the support fabric included in the first drive group is separated from the support fabric included in the second drive group in order to guide the web run onto the surface of the support fabric included in the second drive group.

4. A fibre-web machine (10) for producing a fibrous web, which machine comprises at least a press section (20) with at least one press nip (222) for dewatering the fibrous web (W), and a dryer section (225) comprising at least a first dryer group and a support fabric (234) included therein, and in which the press section (20) comprises a first press roll (224) above the nip (222) and a second press roll (226) below the nip and a first and second endless fabric loop (228; 230), of which the first fabric loop is arranged to run around the first press roll and the second fabric loop is arranged to run around the second press roll, and in which the fibrous web (W) is arranged to follow the second fabric loop (230) after the press nip, characterised in that at least one guide roll (234) is arranged inside the second fabric loop, to which roll the second support fabric (230) is arranged to pass from the press nip (222), and on which the run direction of the support fabric changes, and that at least one guide roll (236) is arranged inside the support fabric loop (232) of the dryer group in such a way that the support fabric of the dryer group passing via the guide roll runs over to said guide roll (234) of the second fabric loop, and that a suction device (238) is arranged inside the support fabric loop of the dryer group, at the guide roll (234), to provide a suction effect on the web (W) through the support fabric of the dryer roll at least at a point, where the second support fabric (230) is separated from the support fabric (232) of the dryer group in order to guide the web run onto the lower surface of the support fabric (232) of the dryer group.

5. A fibre-web machine according to claim 4, characterised in that at least one guide roll (240) is arranged inside the first fabric loop (228), and that both the first support fabric (228) and the second support fabric (230) are arranged to pass from the press nip (222) to said guide roll (240), whereby the fibrous web remains between the fabrics, and that said guide roll (240) is arranged to change the direction of both the first and the second support fabric so that the change of direction of the first support fabric is essentially larger than that of the second support fabric.

6. A fibre-web machine according to claim 5, characterised in that a suction device (242) is arranged inside the second support fabric (230), at the guide roll (240) inside the first fabric loop after the press nip, to provide a suction effect on the web (W) through the second support fabric at least at a point, where the first support fabric (228) is separated from the second support fabric (230) in order to guide the web run onto the upper surface of the second support fabric.

7. A fibre-web machine according to claim 4 or 6, characterised in that the suction device is a transfer suction box (242), which has a cover (243) provided with one or several suction slots (245; 247), and which is placed at the roll (240) so that at least one of the suction slots (245; 247) in the cover (243) is located substantially at that point of the roll (240) where the surface of the roll starts diverging from the direction of the fabric (230) facing the transfer suction box (242).

8. A fibre-web machine according to claim 4 or 6, characterised in that the suction device is a transfer suction box (238) with a cover (260) and that the cover consists of at least two strips (262; 264), the first (262) one of which in machine direction is placed so that its rear edge is located before the separation point (P) of the fabric from the roll and the second one (264) of which strips in machine direction is placed so that its front edge is located at a distance from the rear edge of the first strip.

9. A fibre-web machine according to claim 8, characterised in that the cover is provided with at least one suction slot (266), and that the suction slot is adapted into a point, where the second support fabric is separated from the support fabric of the dryer group.

10. A fibre-web machine according to claim 7 or 9, characterised in that the length of the suction slot (266) is 15 - 20 mm in machine direction.

11. A fibre-web machine according anyone of the preceding claims 4 - 10, characterised in that there is a gap of 0.5 - 5 mm between the suction device (227; 242; 238) and the support fabric (228; 230; 232), while the fibre-web machine is stopped, but the fabrics are in their operating tension.

12. A fibre-web machine according to claim 7 or 10, characterised in that the transfer suction box (227; 242; 238) is adjustably mounted on the frame of the fibre-web machine.

13. A fibre-web machine according to claim 7 or 10, characterised in that the transfer suction box (238) is supported at its ends and at least at one point between the ends by the frame of the fibre-web machine.

14. A fibre-web machine according to claim 7, characterised in that the cover (243) of the transfer suction box (242) comprises a straight portion in machine direction.

15. A fibre-web machine according to claim 7, characterised in that the cover (243) of the transfer suction box (242) comprises a concave portion in machine direction.

16. A fibre-web machine according anyone of the preceding claims 7 , characterised in that the cover (243) of the transfer suction box (242) is provided with two slots (247; 245), of which the first one (247) in the run direction of the web is used for separating the fibrous web (W) from the outer fabric (228) with respect to the transfer suction box (242) and the second one (245) for ensuring that the web (W) remains supported by the fabric (230) facing the transfer suction box.

17. A method of producing a fibrous web by a fibre-web machine (10), in which method the formed fibrous web (W) is dewatered on the press section (20), the press section comprising a first press roll (224) above the nip (222) and a second press roll (226) below the nip and a first and second endless fabric loop (228; 230), of which the first fabric loop runs around the first press roll and the second fabric loop runs around the second press roll, in which method the fibrous web (W) is taken through at least one press nip (222) in order to dewater the fibrous web by pressing by means of the first and second fabric loop, and in which the fibrous web is taken at least via the first dryer group supported by the support fabric included therein, by which dryer group water is evaporated from the fibrous web, characterised in that at least one guide roll (234) is arranged inside the second fabric loop in the fibre-web machine, to which roll the second support fabric (230) is arranged to pass from the press nip (222), and by which the run direction of the support fabric is changed, and at least one guide roll (236) is arranged inside the support fabric loop (232) of the dryer group in such a way that the support fabric of the dryer group running via the guide roll passes to said guide roll (234) of the second fabric loop, and that a suction device (238) is arranged inside the support fabric loop of the dryer group, at the guide roll (234) of said second fabric loop, by which suction device a suction effect is exerted on the web (W) through the support fabric of the dryer roll at least at a point, where the second support fabric (230) is separated from the support fabric (232) of the dryer group in order to guide the web run onto the lower surface of the support fabric (232) of the dryer group.

18. A method according to claim 17, characterised in that at least one guide roll (240) is arranged inside the first fabric loop (228), and that both the first support fabric (228) and the second support fabric (230) run from the press nip (222) to the guide roll (240), whereby the fibrous web remains between these, and that the guide roll (240) of the first fabric loop changes the direction of both the first and the second support fabric so that the change of direction of the first support fabric is essentially larger than that of the second support fabric.

19. A method according to claim 17, characterised in that a suction device (242) is arranged inside the second support fabric (230), at the guide roll inside said first fabric loop, by which suction device a suction effect is exerted on the web (W) through the second support fabric at least at a point, where the first support fabric is separated from the second support fabric in order to guide the web run onto the upper surface of the second support fabric.

20. A method according to claim 17 or 18, characterised in that the suction of the suction device (242) is exerted on the web at the guide roll (240) located on the opposite side of the fibrous web (W) with respect to the suction device and the fibrous web (W) is separated from the fabric facing the guide roll (240) substantially at a point, where the direction of the movement of the fabric (230) facing the guide roll (240) deviates, together with the guide roll (240), from the direction of the movement of the fabric facing the suction device (242).

21. A method according to claim 20, characterised in that the fibrous web (W) is adhered to the fabric (230) facing the suction device (242) by continuing the suction effect of the suction device (242) by means of a second slot (245) located farther away from the guide roll (240).