WO2009118450A1

WO2009118450A1 - Method and apparatus for transferring fiber web from a support fabric to another

Info

Publication number: WO2009118450A1
Application number: PCT/FI2009/050214
Authority: WO
Inventors: Kalle Hirvelä; Petter Honkalampi
Original assignee: Metso Paper, Inc.
Priority date: 2008-03-28
Filing date: 2009-03-20
Publication date: 2009-10-01
Also published as: CN101981252B; AT508470A2; CN101981252A; FI20085259A; FI20085259A0; DE112009000701T5; FI122309B; AT508470A3; AT508470B1

Abstract

Invention relates to a method of transferring a fiber web from a first support fabric (44; 46; 52; 66) to a second support fabric (46; 52; 66), in which method the fiber web (W) is brought close to the location of a suction device (48; 56; 68) on the surface of the first support fabric, the fiber web is detached by the suction device from the first fabric from the opposite side with respect to the suction device and transferred further on surface of the second support fabric on the suction device side. The area of a closing gap (G) of a guide roll (50; 54; 70) on the opposite side with respect to the fiber web (W) is subjected to the suction of the suction device (48; 56; 68) and the fiber web W is detached from the support fabric (44; 46; 52) on the guide roll (50; 54; 70) side before the guide roll (50; 54; 70). Invention relates also to an apparatus for transferring fiber web.

Description

Method of and apparatus for transferring fiber web from a support fabric to another

[001] The invention relates to a method of transferring a fiber web from a first support fabric to a second support fabric in accordance with the preamble of claim 1. The invention also relates to an apparatus in accordance with the preamble of claim 7.

[002] The invention is thus related to transferring a fiber web from a support fabric to another. Thus, the method and the apparatus in accordance with the invention are applicable, for example, for the transfer of a web from a wire section to a press felt and from the press to a dryer section. In particular, the method and the apparatus are applicable for the transfer of a web from a first support fabric to a second support fabric in a paper or board machine.

[003] The method of and apparatus for transferring paper web in accordance with the prior art are discussed more in detail below with reference to an exemplary press section of a paper machine. Currently, the press section following the wire section most frequently comprises two subsequent presses, in which a paper web is mostly transferred with support of at least one felt, more broadly speaking fabric, from a press nip to another, whereby the paper web momentarily runs also with support of/between two fabrics. In this connection, fabric may also refer to a transfer belt. When passing the paper web from a first press, in other words from a first press nip, the paper web has to be detached from the first, for example, upper felt of the first press nip first and maintained attached with the second, for example, lower felt. Thereafter, the web is transferred with support of said second felt to a second press or dryer section, in which the web is transferred to the felts of the second press or drying wire. This takes place in principle in such a way that first the web is brought into contact with the first felt of the second press, detached from the second felt of the first press, maintained attached with the first felt of the second press and further guided to the press nip, or to the dryer section with support of a wire.

[004] Critical points in the web transfer are especially the points, where the web is to be separated from the felt and transferred to the next fabric. The risk is either web break or tear, if the detachment is not performed carefully. The web separation point is most often connected to a felt guide 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, but the web separation has to be facilitated usually by means of a suction arrangement prior to said roll.

[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, and the use of suctions rolls does not generally cause problems in the actual separation of the web from the first felt or in keeping the web on surface of the second felt. However, the use of suction rolls causes other kind of problems. Firstly, the adjustment of the opening of the suction sector in suction rolls is challenging. Moreover, suction rolls are expensive to acquire, use and maintain. [006] The other alternative, i.e. a transfer suction box, is significantly more economical in terms of acquisition and operation. The US patent publication 4,113,557, among others, discloses the use of a transfer suc- tion 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. A transfer suction box is arranged close to the roll, but after it anyway, so as to separate the web from the wire. The direction of the felt, and thereby also the direction of the web, are deviated by means of the transfer suction box from the direction of the wire by less than 45 degrees to bring about an intense suction to the fiber web. In other words, the travel path of the felt is deflected by the convex cover of the suction box and thereby, also the wear of both the cover and the felt is increased.

[007] It is not possible to use a transfer suction box in connection with very fast machines, because, as already mentioned, the cover of the suction box wears and also wears the felt running on surface thereof. The reason for this is that the path of the felt is conventionally arranged in such a way that the felt changes direction on top of the suction box and the tension of the felt presses the felt against the cover of the suction box. The cover of the suction box is elongated in machine direction, which increases the length of the wearing contact. In other words, both the previously described change of the running direction of the felt at the suction box and the vacuum of the suction box draws the felt against the cover of the suction box, whereby both the felt and the cover of the suction box wear fast. Such wearing is yet intensified by high speed of the felt across the cover of the suction box. Thus, for said reason, the use of the transfer suction box is continuously diminishing with the increasing speed of the fiber web machines. [008] The present invention aims at overcoming the disadvantages of the prior art apparatuses by introducing a new kind of solution for transferring a fiber web from a support fabric to another.

[009] The objects of the invention are mainly achieved by means of a method of transferring a fiber web from a first support fabric to a second support fabric, in which method the fiber web is brought to the location of a suction device on the surface of the first support fabric, the fiber web is detached by the suction device from the first fabric on the opposite side with respect to the suction device and is transferred further on surface of the second support fabric on the side of the suction device. A main characteristic feature of the method is that the area of a closing gap of a guide roll on the opposite side with respect to the fiber web is subjected to the suction of the suction device and the fiber web is detached before the guide roll from the support fabric on the same side as the guide roll.

[0010] Thus, the transfer of the web from a support fabric to another is reliable and the speed of the machine facilitates a successful transfer, as the invention utilizes the aerodynamic effects of the motions of the support fabric and the roll. Due to the effect of the motion of the support fabric and the roll an over-pressurized area caused by the airflows is formed to the closing gap between the roll and the support fabric, which intensifies the detachment of the web from the first support fabric.

[0011] When the edge trims cut from the web are already removed, the web as well as the area outside the edges of the web, but inside the edges of the support fabrics is subjected to suction. This promotes a successful transfer of the web. Thereby, especially the edge areas of the web act smoothly during the transfer. [0012] In the method, the second support fabric on the side of the suction device is brought into connection with the first fabric supporting the fiber web by deviating the path of the second support fabric by means of the suction device. The path of the second support fabric forms an angle of the order of preferably 170 - 179°, preferably 173 - 177°, when passing over the suction device.

[0013] The tension of the web can be maintained appropriate in a controlled way in such a manner that the running speed of the second sup- port fabric is higher than the running speed of the first support fabric. The use of speed differences in the web transfer method in accordance with the invention is advantageous, because, for example, the contact length between the first and second support fabric is short enough in machine direction. A short contact length also results in that the contact surface between the support fabric and the suction device can be lubricated efficiently and simply by means of a lubricating water injection device located in running direction of the web in front of a suction device. When the contact surface is short in machine direction, the amount of lubricating water is relatively small, which reduces the risk of the web to rewet.

[0014] The method in accordance with the invention may be used for transferring a fiber web from a wire section of a fiber web machine to a press section, for transferring the fiber web from the press section of the fiber web machine to a dryer section and/or for transferring the fiber web from a preceding press of the press section of the fiber web machine to the next press. Also other applications in the fiber web manufacture and finishing process and/or apparatuses of are possible.

[0015] An apparatus for transferring fiber web from a first support fabric to a second support fabric comprises a suction device, arranged in con- nection with the second support fabric, and by means of which web is transferred from a first support fabric to the surface of a second support fabric; and a guide roll arranged in the other side of the first fabric. The main characterizing feature of the apparatus is that said suction device is arranged to bring about a suction effect to the area of a closing gap on the opposite side of the fiber web with respect to the suction device prior to the guide roll so as to separate the fiber web from the surface of the fabric on the side of the guide roll.

[0016] Such an apparatus is reliable and it utilizes the speed of the machine in the transfer, because the invention utilizes the aerodynamic effects of the motions of the support fabric and the roll. Due to the motion of the support fabric and the roll an over-pressurized area caused by the airflows is formed to the closing gap between the roll and the support fa- brie, which intensifies the detachment of the web from the first support fabric.

[0017] The suction device is a transfer suction box having a cover and at least one suction slot in the cover. Said at least one suction slot is lo- cated in running direction of the support fabric in front of the guide roll in such a way that the at least one suction slot in the cover is located substantially to the point of the first support fabric which is located in the closing gap formed between the support fabric and the mantle of the roll.

[0018] According to an embodiment of the invention, the cover of the transfer suction box is convex. The transfer suction box is preferably arranged in such a way that the cover comprises a first strip and a second strip, and that the suction slot is arranged between the strips. Thereby, it is advantageous that the cover area of the second support fabric on the cover of the suction box extends in machine direction from the first strip to the second strip.

[0019] The transfer suction box is preferably such that the strips are at- tached to a special fitting and the fittings are attached to a counter profile of the suction box by means of profile locking. This enables a simple attachment and quick change of strips.

[0020] According to a preferred embodiment of the invention, a suction slot extends through the width of the web as well as to the area outside the edges of the web, but inside the edges of the fabrics. A slot in the cover of the suction device or a array of separate openings running across the fiber web is/are preferably located to start at the plane which runs through the center line of the axis of the guide roll and the line at which the fiber web is detached from the first fabric. Thereby, the slot or the array of separate openings can be longer than the width of the fiber web and shorter than the width of the fabric on the side of the suction device.

[0021] According to an embodiment, a tail threading step is included in the invention, in which the vacuum level of the suction device or suction devices is at 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 pref- erably 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 example, 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.

[0022] Other features characteristic of the method of and apparatus for transferring the fiber web from a support fabric to another become apparent in the accompanying patent claims.

[0023] Other advantages of the invention worth mentioning, among others, are: - a reliable web transfer from a fabric to another;

- essentially more economical to acquire and use than the present suction roll, because the suction box is already as such, i.e. in terms of construction more advantageous than a suction roll, not to mention the use of the suction roll and the energy consumption thereof;

- the amount of lubrication water required is very low.

[0024] The method and apparatus in accordance with the present invention is discussed more in detail below with reference to the accompany- ing schematic drawings, in which

Fig. 1 illustrates an apparatus in accordance with the prior art for transferring a fiber web from a support fabric to another; Fig. 2 illustrates another apparatus in accordance with the prior art for transferring a support fabric to another; Fig. 3 illustrates a general view of preferred embodiments of a press section of a fiber web machine comprising preferred embodiments of the invention;

Fig. 4 illustrates a side view of a suction box in accordance with preferred embodiment; Fig. 5 illustrates a transfer point of the web in Fig. 3; Fig. 6 illustrates a preferred embodiment for attaching strips to the transfer suction box: and

Fig. 7 illustrates a side view of the suction box in accordance with a second embodiment.

[0025] Fig. 1 illustrates a partial view of a press section of a paper machine in accordance with the prior art and as discussed in US Patent 5,888,354 with an apparatus for transferring a paper web from one part of the press section to another. The press section illustrated in the figure comprises a first press 10 and a second press 20. The first press in the drawing is represented by a shoe roll 12 and a counter roll 14 thereof, through which a paper web W runs in the drawing between two fabrics 32 and 34 from left to right. The first felt 32 is looped, enclosing the shoe roll 12, around a fabric guide roll 16, which in this embodiment is mova- ble in a manner shown in the drawings and runs further upwards. The second felt 34 in turn runs over the counter roll 14. The paper web W is detached from the first felt 32 by means of a transfer suction box 18 within the run between the counter roll 14 and the fabric guide roll 16. Thereby, the suction of the suction box 18 is brought to the fiber web W through the second felt 34, so as to separate the web from the first felt 32. When at the same time running direction of the first felt 32 no longer goes along the cover of the suction box 18, but a bit higher, the paper web W is easily detached from the first fabric. At the same time, the suction of the suction box keeps the paper web W attached with the second felt 34. In this embodiment, the second felt 34 has been elevated by means of the suction box 18 in such a way that it has been possible to ensure that the second fabric remains continuously in contact with the cover of the suction box 18. This ensures in turn that the paper web W is continuously exposed to a uniform suction, by means of which it remains reliably attached to the second felt 34. The cover of the suction box is, however, in this method subjected to a relatively high wearing, resulting in fast wearing of both the cover and the press felt. This applies especially with higher speeds, for example, > 1100 m/min.

[0026] The fiber web W is taken on top of the second felt 34 to a second section 20 of the press section, to a so called second press, which is represented by a third felt 22 and a transfer suction roll 24. The purpose of the transfer suction roll 24 is to catch the fiber web from the second felt 34 of the first press before the second felt 34 is guided down around a roll 26. In principle, the suction roll detaches the fiber web from the second felt 34 as previously described in connection with the transfer suction box 18. By means of suction roll 24, the third felt 22 representing the second press 20 runs the paper web W further, until a fourth felt (not shown) on the second press comes from below, whereby the paper web is again taken between two felts to the nip of the second press or possibly to yet following nips, respectively.

[0027] Fig. 2 illustrates a corresponding press section of a paper machine, also disclosed in US Patent 5,888,354, in which corresponding structures are referred to with the same reference numbers. In fact, the only difference to the embodiment of Fig. 1 is that the transfer suction box 18 of Fig. 1 has been replaced by a suction roll 18', which has the same function as the suction box 18. Thus, this embodiment has no wearing problem with the fabric nor with the suction box owing to the suction roll 18'. However, said problem has been solved by an arrangement, which is expensive to acquire, use and maintain. In other words, compared to the suction box, as a rotating device, the suction roll requires both bearings, rotating pressure sealings and a drive in addition to a vacuum pump and pipings as shared equipment. [0028] Fig. 3 illustrates a general view of an embodiment in accordance with the invention, in other words a press section of a fiber web machine comprising apparatuses in accordance with the invention. The press section in accordance with Fig. 3 consists of a press 40 forming around a first press nip 42. Web W arrives to the press 40, for example, from the wire section of a fiber web machine with support of a wire 44. From the wire 44 the web is transferred to be supported by a first press felt 46 by means of a suction device 48 and a guide roll at a location close to a guide roll 50. In this embodiment, wet wire and press felts act as support fabrics for the web W.

[0029] To be more precise, the suction box is a transfer suction box 48, which is arranged to bring about a suction effect to the area of a closing gap G of the guide roll 50 on the opposite side of the fiber web with re- spect to the transfer suction box 48 prior to the guide roll 50 in order to detach the fabric 44 from the surface. The closing gap G in this connection refers to an area between the mantle of the roll and the fabric running in contact across the roll, said area having an elevated pressure level caused by the airflows created by the motion of the roll surface and the fabric. The structure and the operation of the transfer suction box in the concept of Fig. 3 are discussed more in detail later.

[0030] In the apparatus in accordance with Fig. 3 there is provided to an arrangement for transferring the web from the wire section to the press section, a guide roll 47, 49 for the support fabric 46, to which the web W is transferred, on both sides of the suction device in running direction of the support fabric and within a relatively short distance from each other so as to ensure a smooth run of the fabric. Furthermore, due to the effect of the short run between the rolls, the run of the support fabric across the transfer suction box and thus the transfer of the web is more reliable. Further, to ensure the support of the web, it is possible to use a so called deep grooved roll 49, which is illustrated in patent application Fl 20076039.

[0031] Supported by the first press felt 46, the web W is taken to the press nip 42 between the first press felt 46 and the second press felt 52. From the nip 42, the web runs between the felt 46 and the fabric 52, the so called support fabrics, and further after the nip all the way to the guide roll 54. A transfer suction box 56 has been arranged in running direction of the web in front of the guide roll, opposite to the web relative to the guide roll 54, by means of which transfer suction box the web is detached from the press felt 46 and transferred to the surface of the support fabric 52. The press felt 46 travels on upwards in the drawing. The web W is taken with the support of the press felt 52 to a next section 60 of a fiber web machine in the path of the web, which section may be a drying section or in some cases the next press section. The transfer to the next machine section is performed in such a way that the web W is brought into contact first with a support fabric 66, as with the dryer wire or press felt, whereafter it is detached from the second press felt 52 to be supported by a support fabric 66 by means of transfer suction box 68 and a guide roll 70 arranged inside the second press felt 52 at a location in close proximity of the guide roll 70. In the apparatus in accordance with Fig. 3 there are provided, to an arrangement transferring the web from the wire section to the press section, rolls 72, 74 for the support fab- ric 66, to which the web W is transferred, on both sides of the suction device in the running direction of the support fabric, and within a relatively short distance from each other so as to ensure a smooth run of the fabric. Further, to ensure the support of the web, it is possible to use a so called deep grooved roll 74, which is illustrated in patent application Fl 20076039. The deep grooved roll 74 can also be below the fabric and the web, for example, in the transfer to the drying section.

[0032] Fig. 4 schematically illustrates a transfer suction box 56 to be used as a suction device of Fig. 3, but this applies also the transfer suction box 68 where applicable, in which, deviating from transfer box 56 and 48, there is only one suction slot. The position illustrated in the drawing is not relevant as such, but the mutual position of the elements illustrated in the drawing is more relevant. The transfer of the web from the surface of the first support fabric 46 to the surface of the second support fabric is performed as follows. The fiber web W is taken on surface of the first support fabric 46 (support fabric, from which the web is transferred away) towards a roll 54 in the inner loop of the first support fabric, i.e. inside the so called support fabric loop, from the left to the right in Fig. 4. The transfer suction box 56 is arranged to the inner loop of the second support fabric, in other words inside the so called support fabric loop to the area of a closing gap G of the guide roll 54 on the opposite side of the fiber web in respect of the transfer suction box 56, in the region of the closing gap G prior to the guide roll 54, especially in the way that the suction effect of the device is exerted on the fiber web in this area. Due to the air flows caused by the motion of the support fabric 46 and rotation of the roll, an elevate pressure level compared to the ambient pressure is created in the region of the closing gap. As the cover 80 of the transfer suction box is within a distance Z from the mantle surface of the roll 54, the overpressure of the closing gap facilitates the transfer of the web W from the surface of the support fabric 46. The smallest distance Z is typically 5-50 mm, preferably 7-30 mm. This also has the advantage of the speed of the machine effecting the overpressure prevailing the gap G. When the suction effect is arranged at the same time to the web W of the support fabric, here a second support fabrics 52, on the opposite side in respect of the web, the transfer of the web takes place in principle the more reliably, the higher the speed of the machine is.

[0033] The suction box 56 is arranged in such a way that it deflects the travel path of the second fabric and the fabric arriving and the fabric leaving form an angle α. Angle α is 170 - 179°, preferably 173 - 177°. This way the support fabric, to which the web is transferred, is taken by means of a suction into connection with the support fabric and at the same time into connection with the surface of the web, from which the web W is transferred away by deflecting the travel path of the second fabric 52. At each transfer of the web from one machine section to another, in other words from one drive group to another, it is possible to remove the non-desired stretch in such a way that the running speed of the second support fabric is greater than the travel speed of the first support fabric.

[0034] Fig. 4 also schematically shows a cover 80 of the transfer suction box 56. The cover 80 has been provided with at least one, first suction slot 82, through which the under-pressure arranged to the box, is trans- ferred to the fiber web W to be transferred. Said suction slot 82 performs the transfer of the web W from the surface of the first fabric to the surface of the second fabric, in other words the actual transfer of the web is performed in a suction slot 82 between two strips 86, 88. The transfer suction box has been arranged in such a way that at least one suction slot 82 of the cover 80, a so called web transfer suction slot 82 is located substantially to the point of the first support fabric 46 which is located in the closing gap G formed by the support fabric and the mantle of the roll 54. [0035] The cover 80 comprises a first strip 86 and a second strip 88, between which said web transfer suction slot 82 is formed. The transfer suction box illustrated in Fig. 4 further comprises a third strip 90, arranged in run direction of the web within a distance from the second strip 88. A second suction slot 83 has been arranged between the third strip 90 and the second strip, said slot being substantially located to the area of the gap opening between the support fabric 52 running along the cover 80 of the transfer suction box and the support fabric 45 diverging from said support fabric and running across the roll. The second suction slot is a so called web support suction slot. The transfer suction box is preferably designed in such a way that a vacuum level prevails in the web support suction slot, in other words in the second suction slot 83 than in the web transfer suction slot, in other words in the first suction slot 82. This is performed, for example, in such a way that the second suction slot 83 is connected to the header of the transfer suction box by means of a channel, which has a smaller flow surface area than the first suction slot 82.

[0036] The cover 80 is convex in such a way that the leading edge of the cover on the first strip and the trailing edge of the second strip form an angle, which is at least α. The convexity may be formed by a surface looped around a certain convexity centre point, but the convexity may also be a variably convex surface. The surface of the latter, second strip 88 may join at least close to the trailing edge with the direction of sepa- rating second support fabric. The dimension of the strips in machine direction, in other words the width of the strip, is 5 - 60 mm, preferably 10 - 40 mm.

[0037] The length of a single suction slot in machine direction, which can be considered as the width of the slot in direction of the longitudinal axis of the suction box as well, is typically 8 - 40 mm, preferably 15 - 20 mm. When the slot widths increase, the fabric tends to bend to the slot, whereby the strips and the support fabric wear excessively. Furthermore, the suction requires considerably more energy. The material of the strips 88, 86 of the cover is preferably ceramic and they have been designed in such a way that the corners of the strips are rounded to minimize the wearing of both the strip and the support fabric. The radius of curvature of the rounding is preferably 2 - 6 mm. The ceramic surface is typically made by gluing ceramic pieces subsequently on the surface of the strip body. Sometimes the use of plastic strips may also come into question, especially with transfer suction box to be arranged against the dryer wire.

[0038] Fig. 5 illustrates a similar transfer point as the transfer between the press felt 46 and press felt 52 in Fig. 3. Fig. 5 illustrates a bottom view from Fig. 3, whereby the press felt 46 and the guide roll 50 have been omitted from the drawing. The drawing shows the fiber web W and the traverse width thereof Lw, the support fabric 46 and the traverse width thereof Lf as well as the slot defined in machine direction by the strips of the suction device and the traverse width thereof Ls. The slot width Ls of the suction device can be adjusted by transferring the sealing body arranged in the slot, as shown with arrows A. Fig. 5 clearly shows that the width of the press felt 46, slot 82 and width W has a descending width in said order. In other words, the support fabric 46 is wider than the slot 82 and the slot is wider than the fiber web W. What is gained with said structure is that a layer of air is drawn on both sides of the web W close to the edges. The inventors have observed that by this method, the transfer of the web is more stabilized. Fig. 5 also illustrates a controllable under-pressure source 87, by means of which an under-pressure is provided in the suction device. [0039] It has become apparent that it is advantageous to have the part of the edge of the press felt 46 which passes across the slot 82 greater than the part of the slot 82 passing across the web W, in other words, dimension Y must be bigger than X in Fig. 5. Preferably, the dimensions for X are 5 - 150 mm and most preferably 40 - 70 mm. Good results are gained with Y values 100 - 200 mm. It has to be noticed that in all embodiments of the transfer points, the slot can be formed as one opening or a number of openings extending as a array across the web (in width direction). Thereby, the openings are round holes or short slots or like. The illustrated apparatuses operate in suction boxes under pressures which are preferably below 50 kPa. In normal operation, a sufficient vacuum level (under-pressure) is usually 1 -10 kPa. The vacuum level depends, for example, on the dry content, density of the fabrics and the web as well as the surface and the adhesion characteristics of the pair of support fabrics. Especially in the tail threading, a vacuum level higher than that in the normal operation can be applied, such as a 10-5OkPa under-pressure. Thereby the tail threading can be improved significantly. This normal vacuum level is advantageous, because there the fabric does not typically get into contact with the cover of the suction device. In a more demanding transfer or especially with heavier grades, the vacuum level can be raised to 20 kPa. Though the fabric may be in contact with the cover the wearing of the fabric is not yet significant.

[0040] Thereby, the tail threading can be improved significantly. It is, however, essential to notice that in the situation of Fig. 5, the web is already of the whole width such that it can be taken through the fiber web machine substantially without problems. This means that the trim strips need to be removed from the edges of the web from the web formed by the present formers. Thus, it is clear that if the trim strips are cut just be- fore the transfer suction box, it has to be ensured that the cut trim strips do not entrain with the action web W forward in the transfer suction box, but they follow the support fabric, from which the actual web is transferred to the next support fabric. In such a situation deviating from the description above, the width Ls of the suction slot is substantially of the width of the cut web, in other words Ls is approximately Lw. The run of the trim strips is ensured, for example, by suction or blow devices 51 located close to the edge strips.

[0041] Fig. 6 illustrates a detailed view of the attachment of strips 88, 86 to the transfer suction box 48. The strips 88, 86 are attached to a fitting 120, which is attached by means of profile locking to a counter profile 122 of the suction box 48 so as to facilitate the change of strips. The strips are preferably flexibly mounted with bearings, which can be performed, for example, by means of a compressed-gas hose 150, which is arranged in Fig. 6 between the fitting 120 and the counter profile 122. This arrangement enables the suspension of the strips, if, due to a disturbance, a thicker fiber web layer suddenly arrives between the suction box and the guide roll. Thus, the strips allowed to move downwards in the figure and are not damaged. The suspension of the strips can be per- formed also by elastomers or like, for example, by metal spring elements.

[0042] So as to optimize the location of the fabric guide roll and the transfer suction box assembled beside the roll in all illustrated transfer points, either the guide roll or the box, or possibly both have to be arranged to be movable.

[0043] Fig. 7 discloses another embodiment of a transfer suction box 56 of Fig. 3 to be used as suction device. This corresponds otherwise to the transfer suction box shown in Fig. 4, except the cover 80 of the transfer suction box of Fig. 7 is of different structure and is located in a different way relative to the roll 54. Here, the cover comprises a suction slot 82 on the side of the closing gap G and furthermore, also a second suction slot located in machine direction at a location before the fabrics 46, 52 separate from each other. Furthermore, between the suctions slots there is a gap 83.1 , which is not subjected to suction effect. The slot 83.1 , which is not subjected to suction effect, is plugged, for example, by plate 83.2. A possible alternative for the embodiment of Fig. 7 is an entity, in which the suction slots are in connection with separate transfer suction boxes and between which slots there remains an area without suction. The first slot in running direction of the web in this kind of arrangement does not actually transfer the web to a second (lower) fabric, but the web continues over the plugged slot with support of both fabrics. This, however, facilitates the transfer of the web on surface of the second fabric taking place at the latter slot.

[0044] This is especially advantageous to be used in positions in which the fabric, from which the web is detached, is thick, poorly air-permeable and/or has such surface characteristics that it tends to attach the web to its surface. Smoothness is a factor affecting, for example, the adhesion and the web tends to remain on the surface of a smooth fabric. The arrangement functions in such a way that in the running direction of the web, the first suction slot 82 weakens the contact of the web on the fabric surface. In running direction of the web, the second, latter suction slot 83 finally transfers the web to the surface of the fabric on the side of the transfer suction box. The suction slots are within a distance from each other and as there is a slot 83.2 between them, which is not subjected to suction effect, the contact wearing the fabric remains within acceptable limits as it is ensured that the final transfer of the web to the fabric on the side of the transfer suction box takes place at a desired point, in other words by the latter suction slot 83. [0045] With the previous description, it must be born in mind that an apparatus and a method in accordance with the invention can applied in the manner of the given examples of internal transfers of a press section to other transfer points of the web in the fiber web machine, too. Similarly, it is clear that though merely fiber web machine is discussed above, the invention can as well be applied in connection with paper and board machines. Thus, the term fiber web is understood to refer also to board as a subtype. Generally speaking, the invention is applicable anywhere, where web is transferred from a support fabric to another. It is to be noted that individual features described in connection with different embodiments may be used within the scope of the basic concept of the invention also with other embodiments and/or be combined to form differ- ent entities, if so desired and they are technically feasible.

Claims

1. Method of transferring a fiber web from a first support fabric (44; 46; 52; 66) to a second support fabric (46; 52; 66), in which method the fiber web (W) is brought to the location of a suction device (48; 56; 68) on the surface of the first support fabric, the fiber web is detached by the suction device from the first fabric on the opposite side with respect to the suction device and transferred further on surface of the second support fabric on the suction device side, characterized in that the area of a closing gap (G) of a guide roll (50; 54; 70) on the opposite side with respect to the fiber web (W) is subjected to the suction of the suction device (48; 56; 68) and the fiber web W is detached before the guide roll (50; 54; 70)from the support fabric (44; 46; 52) on the guide roll (50; 54; 70) side.

2. Method according to claim 1 , characterized in that the area outside the edges of the web (W), but inside the edges of the support fabrics (44; 46; 52) as well as the web are subjected to suction traverse to the running direction of the web.

3. Method according to claim 1 , characterized in that in the method the second support fabric (46; 52; 66) on the side of the suction device (48; 56; 68) is brought into connection with the first fabric supporting the fiber web by deflecting the web run of the second support fabric by means of the suction device (48; 56; 68).

4. Method according to claim 3, characterized in that the run of the second support fabric forms an angle (α) of 170 - 179° when passing over the suction device.

5. Method according to any of the preceding claims, characterized in that the running speed of the second support fabric is higher that the running speed of the first support fabric.

6. Method according to anyone of the preceding claims 1 - 4, characterized in that the suction is performed in two stages, of which the first transfers the web (W) from the first fabric to the surface of the second fabric and the second stage operates as support suction maintaining the web on the surface of the second fabric.

7. Apparatus for transferring fiber web from a first support fabric to a second support fabric, which apparatus comprises a suction device (48; 56; 68), arranged in connection with the second support fabric, and by means of which web W is transferred from the first support fabric (44, 46, 52, 66) to the surface of the second support fabric (46, 52, 66); and a guide roll (50; 54; 70) arranged in the other side of the first fabric (44; 46; 52), characterized in that said suction device is arranged to bring about a suction to the area of a closing gap of the guide roll (50; 54; 70) on the opposite side of the fiber web relative to the suction device (48; 56; 68).

8. Apparatus according to claims 7, characterized in that the suction device is a transfer suction box (48; 56; 68), which has a cover (80) and at least one suction slot (82) in the cover (80).

9. Apparatus according to claim 8, characterized in that there are at least two suction slots (82) in the cover.

10. Apparatus according to claim 9, characterized in that the cov- er (80) is provided with a transfer (82) and a support (83) suction slot.

11. Apparatus according to claim 8, characterized in that the suction slot (82) is located in front of the guide roll (50; 54; 70) in the running direction of the support fabric in such a way that the suction slot (82) of the cover (80) is substantially located to the area of the first support fa- brie (44; 46; 52; 66) located in the closing gap (G) which is formed by the support fabric and the mantle of the roll (50; 54; 70) and the support fabric.

12. Apparatus according to claim 8, characterized in that the cov- er (80) of the transfer suction box (48; 56; 68) is convex.

13. Apparatus according to claim 8, characterized in that the cover comprises a first strip (86) and a second strip (88), and that the suction slot (82) is arranged between the strips.

14. Apparatus according to claim 13, characterized in that the cover area of the second support fabric on the cover (80) of the suction box extends in the machine direction from the first strip (86) to the second strip (88).

15. Transfer suction box according to claim 13, characterized in that the strips (86; 88) are attached to a special fitting (120).

16. Transfer suction box according to claim 13, characterized in that the fittings (120) are attached to a counter profile (122) of the suction box (48; 56; 68) by means of profile locking.

17. Transfer suction box according to claim 8, characterized in that the suction slot extends through the width of the web (W) as well as to the area outside the edges of the web, but inside the edges of the fabrics.

18. Use of a transfer suction box according to anyone of the pre- ceding claims 8-17 for transferring fiber web from a wire section of the fiber web machine to a press section.

19. Use of a transfer suction box according to anyone of the preceding claims 8-17 for transferring fiber web from a press section of the fiber web machine to a dryer section.

20. Use of a transfer suction box according to anyone of the preceding claims 8-17 for transferring fiber web from a preceding press to a next press of the press section of the fiber web machine.