WO2001040573A1

WO2001040573A1 - Method for supporting web and web support device

Info

Publication number: WO2001040573A1
Application number: PCT/FI2000/001046
Authority: WO
Inventors: Jorma Laapotti
Original assignee: Metso Paper, Inc
Priority date: 1999-12-02
Filing date: 2000-11-30
Publication date: 2001-06-07
Also published as: AU2013301A; DE10085256T1; JP2003515685A; FI113973B; FI19992588A; DE10085256B4

Abstract

A method and device for stabilizing the movement of a web in the press section of a paper or board machine in rectilinear transfer of the web, while supported by a fabric (20), from a roll (23) to a second roll (31). With the aid of the web support device (60) according to the invention, the vacuum effect is intensified in critical areas such as in the opening and closing gaps, and also in the edge areas of the web (W). To said areas, greater blowing speeds than earlier are directed from ejection nozzles (61a, 61b, 62a, 62b) and/or to said areas, suction slots (63a, 63b) are connected to intensify the suction. In the wide transverse central area of the web in machine and cross-machine direction, the vacuum effect is limited by arranging a controlled leakage flow (64) or vacuum reduction flows implemented with limited counterblow. In this manner, a vacuum effect greater than before is concentrated to the adjacency of the support rolls (23, 31) of the fabric and to its edge areas.

Description

Method for supporting web and web support device

The present invention relates to a method for stabilizing the movement of a web in its rectilinear transfer in the press section of a paper or board machine, while supported by a fabric, from a first roll or equivalent to a second roll or equivalent.

The invention also relates to a web support device which is particularly appropriate for stabilizing the movement of the web in the press section of a paper or board machine in rectilinear web transfers, while supported by a fabric, from a first roll or equivalent to a second roll or equivalent.

In addition, the invention relates to an arrangement in the press section of a paper or board machine for stabilizing the movement of the web in its rectilinear transfer, while supported merely on one side by the fabric, from a vacuum transfer roll or equivalent to a press nip.

In OptiPress™ type press sections, which are characterized by a nearly rectilinear movement of the web through the press section, the transfer of a paper or board web from a fabric to another fabric is accomplished with the aid of so-called felt vacuum rolls and vacuum transfer rolls. Between said rolls or thereafter the web is passed along a linear path sandwiched between two fabrics or supported by one fabric only, utilizing blow or suction boxes as an aid, which generate an underpressure field in the space between the fabric and the box with the aid of ejection blows or direct suction.

A problem related to blow boxes known in the art is in the relatively weak vacuum effect they produce between the fabric and the web because the press felts in particular are, because of their flock layers and high water content, rather impermeable in the vacuum levels currently used. A second problem lies in the fairly long support distances of the fabrics used in rectilinear transfers and the limited tightnesses of the fabrics, because of which the blow boxes cause a considerable curving of the fabrics both in machine and cross-machine directions. Owing to said curving in two directions, also the web has to stretch differently in the center compared to the edges. The web may, probably at least partly, slide on the surface of the support fabric and, consequently, the web may disengage from the fabrics and become bagged. Moreover, when in the transfer preceding the press nip a steam box is used in order to raise the pressing temperature of the web and to intensify the water removal, the curving of the fabric and the web is harmful in the steaming area, since the steaming distance will not be constant.

In a paper or board machine running at high speed, the air flows that tend to release the web from the fabrics are powerful. In the opening and closing gaps, the web has to be adhered tightly to the fabrics, neither are the web edges allowed to be released from the fabric in any phase in the press section because otherwise there will be problems in the runnability of both the press section and the drying section. A need for underpressure differs therefore in different phases of support. The highest levels of underpressure would be needed in the opening and closing gaps and in the edge areas of the web, whereas in the central part of the support area, even a lesser underpressure would be enough to keep the web adhered to the fabric. With the blow and suction box techniques currently used, an almost uniform vacuum has to be maintained in the entire area to which the impact of a blow or suction box extends.

In the US. patent specification No. 4, 757,619, a suction transfer device is disclosed which is intended for use in transferring the web from the press section to the drying section and which is arranged to apply a higher vacuum effect to the edge areas of the web than to the central area of the web. In the travelling direction of the web, the underpressure profile is almost similar throughout the entire transfer length.

The objective of the present invention is to develop a novel design for supporting the web and stabilizing its transfer, with the aid of which the problems concerning the technology known in the art can be minimized. The aim is to achieve a support arrangement with which the web can reliably be maintained adhered to the fabric also at high speeds of running and, nevertheless, to be able to reduce the harmful curving of the fabric in the support space.

For achieving the above aims and those to be disclosed below, the method of the invention is characterized in what is presented in the characterizing part of claim 1. The web support device of the invention is in turn characterized in what is presented in the characterizing part of claim 3. Various applications of the method and the device of the invention are moreover characterized in what is presented in independent claims 13 and 14.

In the method of the invention, a vacuum effect is applied to the fabric guiding the movement of the web from a first roll to a second roll, the magnitude of which varies in the support area between the rolls so that at the beginning of the support distance, in the area of a gap opening between the first roll and the fabric, and respectively, at the end of the support distance, in the area preceding the second roll, the underpressure is substantially greater than the underpressure exerting an effect on the middle area left therebetween. In order to achieve this, the web support device of the invention comprises means for applying a high local underpressure in said gap between the web support device and the fabric in the front edge area of the web support device and means for applying a high local underpressure in said gap in the back edge area of the web support device, and means for applying an underpressure which is substantially lower than those earlier in said gap in the central area of the web support device. Preferably, members are also arranged on the ends of the web support device for arranging high underpressure in the edge areas of the web.

With the aid of the web support device of the invention, the vacuum effect is intensified in critical areas like in the opening and closing gaps, and advantageously also in the edge areas of the web. Higher blowing speeds than earlier are directed to said areas from ejection nozzles and/or suction slots to intensify the underpressure are connected to said areas. In the wide central areas of the web in machine direction and cross-machine direction, the vacuum effect is limited by arranging controlled leakage flow or vacuum reduction flows implemented by means of limited counterblow. In this manner, a higher vacuum effect than earlier is directed to the adjacency of the support rolls of the fabric and the edge areas. On the other hand, a lower underpressure than earlier is generated in the middle area, which will reduce the harmful curving of the fabric, but which is however enough to keep the web adhered to the fabric. Advantageously, the magnitude of the underpressure in the edge areas of the web support device is about 500 to 4,000 Pa and in the middle area, about 50 to 400 Pa.

According to an embodiment of the invention, the desired underpressure profile in the support area between the rolls is achieved by arranging in the support distance a number of consecutive web support units, the first of which includes members for providing a high local underpressure at the beginning of the support distance and the last of which includes members for providing a high local underpressure at the end of the support distance and in addition, all units include members for providing a lower underpressure than the foregoing in the middle area of the support distance.

The web support method and the device of the invention are appropriate for use not only in the transfers of the press section but also elsewhere in the paper manufacturing line, like in supporting the web to the drying wire right at the beginning of the drying section. Other potential uses are e.g. supporting the web to a permeable drying wire in a planar impingement dryer unit and supporting the web to a conveyor fabric in a surface treatment unit.

The invention is described below more in detail, reference being made to the figures of the accompanying drawing, to the details of which the invention is not, however, intended to be strictly confined.

Figure 1A presents the press section of a paper machine and the potential positions of blow boxes/web support device in the press section.

Figures IB and 1C present alternative press section constructions, in which the transfer of the web to a second press nip is implemented in a manner deviating from that presented in Figure 1 A.

Figures 2 and 3 present state-of-art blow boxes.

Figure 4 presents schematically a web support device of the invention.

Figure 5 demonstrates an underpressure field to be obtained with the web support device of the invention in a perpendicular view in the direction of the fabric.

Figure 6A presents stabilization of web movement with the aid of the web support device of the invention while transferring it from the fabrics of the first press of the press section to the fabrics of the second press.

Figures 6B and 6C present two alternative ways of arranging the stabilization of web movement while transferring it from the fabrics of the first press of the press section to the fabrics of the second press.

Figure 7 demonstrates an undeφressure field to be obtained with the web support device of Figure 6A viewed laterally to the paper machine.

Figure 8 demonstrates the shape of the underpressure field to be obtained with the web support device of Figure 6 A. Figure 9 presents a cross-sectional view of the web support device in cross- machine direction.

Figure 10 demonstrates an underpressure field to be obtained with the web support device as in Figure 11.

Figure 11 presents the structure of the web support device producing a uniform underpressure field in transverse direction when viewed in cross-machine direction.

Figures 12 and 13 present modifications of the web support devices of Figure 6A.

Figure 14 presents a web support device of a second embodiment.

Figure 15 presents a web support device of a third embodiment.

Figure 1 A presents a press section marketed by the applicant under product name OptiPress™, including two press nips, the first of which is a roll nip N, and the second of which is an extended nip N₂. The press section includes moreover three water-receiving press felt loops 10, 20 and 30 and a water non-receiving transfer belt loop 40, which guide the web W along a relatively rectilinear path through the press nips N, and N₂ and further to the dryer section.

At the end of the former section, the web W is transferred from a wet wire 5 with the aid of a pick-up roll 11 onto the lower side of a first top felt 10. From the pickup roll 11, the web W is conducted, supported by the top felt 10, to a felt roll 21 guiding the course of a lower felt 20 and further, sandwiched between the felts 10 and 20 to the first press nip N,, formed between two press rolls 12 and 22 that are loaded against each other. After the nip N,, the course of the top felt loop 10 is guided by a felt roll 13 and the course of the lower felt loop 20 is guided by a felt vacuum roll 23 and a felt roll 24, under the combined influence of which the web W is after the felt vacuum roll 23 separated from the top felt 10 and made to follow the lower felt 20. From the rectilinear run of the lower felt 20, the web W is transferred with the aid of a vacuum transfer roll 31 onto the lower side of the second top felt 30 and, supported thereby, it is transferred to a leading roll 41, where the web W gets into contact with the top side of the transfer belt 40. Sandwiched between the top felt 30 and the transfer belt 40 the web W is guided, touching on the felt roll 33, to the second press nip N₂ formed by an extended nip roll 32 and its back roll 42 that are loaded against each other. In order to avoid rewetting of the web W right after the nip N₂ by the water absorbed into the top felt 30, the top felt 30 is guided immediately after the extended nip N₂ off from the web with the aid of a felt roll 34. The web W follows the transfer belt 40 gripped on its surface by adhesion until it is before the leading roll 43 transferred with the aid of a vacuum transfer roll 51 onto a drying wire 50, which conducts the web into the dryer section including, as is known in the art, drying cylinders 52 and reversing suction rolls 53.

Figure 1A presents also a steam box 35 arranged in the rectilinear transfer of the web W preceding the second press nip N₂ and, as an option, the positioning of an equivalent steam box 14 in a transfer preceding the first press nip N,, the function of said steam boxes 14,35 being to raise the temperature of the web W and to increase dewatering in the nip. In the dryer section, a blow box 54 known in itself in the art is illustrated, being positioned in a pocket formed between two drying cylinders against the reversing suction roll 53.

With the press section shown in Figure 1A, four rectilinear transfers are associated, in which a wet web W is supported merely on one side by a press fabric or a drying fabric. The transfers are as follows: from the pick-up roll 11 supported by the top felt 10 to the felt roll 21 ; from the felt vacuum roll 23 supported by the lower felt 20 to the vacuum transfer roll 24; from the vacuum transfer roll 31 supported by the top felt 30 to the leading roll 41; and from the vacuum transfer roll 51 supported by the drying wire 50 to a first drying cylinder 52 of the dryer section. In the three last-mentioned positions, blow boxes have usually been positioned, whereto letters B, C, D, and D₂ generally refer in the figure and with the aid of which a differential pressure is generated between the two sides of the fabric to adhere the web W onto the felt 20,30 or the wire 50. In the transfer after the pick-up roll 11 , a blow box has not yet been used, but a potential location for positioning the web support device A is depicted with dotted lines in the figure.

The invention relates to a web support device of novel type, which can be advantageously positioned in any of the positions A-D of Figure 1 A. If, deviating from what is presented in the figure, a porous press felt is provided in the second press nip N₂ as the lower fabric, instead of the smooth and impermeable transfer belt 40, the web support device can be used also in the rectilinear felt transfer after the second press nip N₂, in which the web passes guided by the felt on one side only.

Occasionally, the arrangement of Figure 1A in the area between the vacuum transfer roll 31 and the second press nip N₂ may cause faults in the paper web. Therefore, an alternative arrangement is presented in Figure IB when transferring the web W from the lower felt 20 of the first press to the second press nip N₂. In this design, the web W is passed after the vacuum transfer roll 31, supported by the top felt 30, directly to the second press nip N₂. The leading roll 41 guiding the course of the transfer belt 40 is placed in separation from the path of the web so that the web W and the transfer belt 40 do not meet until in the nip N₂. In the run common to the web W and the fabric 30 between the transfer suction roll 31 and the nip N₂, a first web support device C_b a support roll 36 and a second web support device C₂ are arranged, positioned inside the press felt loop 30, all of which together secure a good support of the web W to the felt 30 in the entire support distance.

Figure IC presents an alternative arrangement, in which the support roll 36 of Figure IB is replaced by a web support unit C₄, so that three web support units C₃,C₄,C₅ are provided, in succession in the support distance, to stabilize the movement of the web, all of which together provide the support impact sought by means of the invention. A problem related to this design can be the long support distance, whereby the steaming distance of the steam box 35 will be indefinite, because of which the press section geometry of Figure IB is more advantageous.

Figures 2 and 3 present two state-of-art blow boxes 57 placed in position B in Figure 1 A, in which the web W is passed after the suction sector 23a of the felt vacuum roll 23, while supported by the lower felt 20, into contact with a second top felt 30, onto the under side whereof it is adhered by the effect of the undeφressure prevailing in the suction sector 31a of the vacuum transfer roll 31. In order to keep the web W in contact with the felt 20 in the rectilinear transfer between the felt vacuum roll 23 and the vacuum transfer roll 31, a blow box 57 is arranged inside the lower felt loop 20, wherein oveφressure p, is prevailing. On the front and back edges of the blow box 57, blow nozzle slots 58a and 58b are provided, directed to blow air at a great speed away from a space between the blow box 57 and the fabric 20. In Figure 2, the blow nozzle slot 58a of the front edge of the box is directed at a gap opening between the roll 23 and the fabric 20, whereas an equivalent gap 58a in Figure 3 is directed against the surface of the roll 23. A powerful air blowing causes an ejection flow in the space between the blow box 57 and the fabric 20, and in the front edge area of the box, also in a gap opening between the roll 23 and the fabric 20, absorbing air from said spaces and producing undeφressure needed for supporting the web W. The nozzle slots 58a,58b blowing air under the same blow pressure p, usually encircle the entire box 57, also extending to the area of the side edges, so that they provide an undeφressure of almost constant magnitude in the area of the entire blow box 57. Owing to the influence of the undeφressure, the fabric 20 is curved and stretched in the middle part of the blow box 57 both in longitudinal and transverse directions. A curve E of the fabric 20, meaning the distance between the rectangular distance between the support points (broken line J) and the actual path of the fabric 20, is at most about 30 to 60 mm, depending on the length of the support distance, the tightness of the fabric 20 and the undeφressure level caused by the blow box 57.

Figure 4 presents a web support device 60 of the invention placed in the support distance between the felt vacuum roll 23 and the vacuum transfer roll 31 parallel with the fabric 20 so that a gap-like space 66 is formed between the web support device 60 and the fabric 20. Across its entire length, the web support device 60 is provided with four blow nozzle slots 61a,61b,62a,62b extending transversely to the travelling direction of the fabric 20, each whereof being connected via its individual air channel to a source generating blow air (not shown). Thanks to the separate air channels, blow air under various pressures p,, p₂, p₃ and p₄ can be supplied into each blow nozzle slot 61a,61b,62a,62b. The first blow nozzle slot 61a is positioned on the front edge of the device 60 and the second blow nozzle slot 61b is positioned on the back edge of the device 60, and they are arranged to blow air at high flow speed away from the web support device 60. The third blow nozzle slot 62a is positioned at a little distance from the first blow nozzle slot 61a and the fourth blow nozzle slot 62b is positioned at a little distance from the second blow nozzle slot 61b, and they are arranged to blow air into the space 66 between the web support device 60 and the fabric 20 at a flow speed which can be lower than the speed of the air flows blown from the first and the second blow nozzle slots 61a,61b. In the middle area of the web support device 60, a first suction slot 64 or equivalent is provided, wherethrough the air blown into the space 66 from the nozzle slots 62a,62b is removed into a suction channel, where undeφressure -p₇ is prevailing. In addition, the web support device 60 comprises a second suction slot 63a, being positioned on the front edge of the device, in a space between the first blow nozzle slot 61a and the third blow nozzle slot 62a, and a third suction slot 63b positioned on the back edge of the device, in a space between the second blow nozzle slot 61b and the fourth blow nozzle slot 62b. Said suction slots 63a and 63b are arranged to intensify the vacuum effect in the area of the front edge and the back edge of the device 60 by removing air into suction channels where undeφressures -p₅ and -p₆ are prevailing.

The blow air channels ending in the blow nozzle slots 61a,61b,62a,62b are convergent at the end part in the blow direction, and the outflow of the air takes place through about 0.5 to 4 mm nozzle slots at 30 to 300 m/s speed. The dimension of the nozzle slot of the front and/or back edge of the blow box can be different in size in cross-machine direction, whereby e.g. a greater nozzle size or a greater suction slot in the edge areas of the machine generates a greater volumetric flow of air to the edge areas. A greater volumetric flow intensify now the vacuum effect maintaining the web adhered to the fabric in the front and/or back edge area. Said area of greater undeφressure may extend from the machine edges to the middle area by about 50 to 400 mm.

The air flow blown at a great speed from the nozzle slots follows the curved surface bordering the nozzle slot, which is usually a pipe of 40 to 80 mm diameter. The air flow generates an ejection effect drawing the ambient air with the air jet. The air flows blown from the outermost nozzle slots 61a,61b produce an ejection flow away from the space 66 and from the opening or the closing gap. The air flows blown from the innermost nozzle slots 62a,62b cause an ejection flow into the space 66 between the web support device 60 and the fabric 20, wherefrom the air exits through the suction slot 64. In this manner, undeφressure is generated between the web support device 60 and the fabric 20, which causes a differential pressure and a force keeping the web W tight to the fabric 20. The suction slots 63a and 63b on the front and back edges of the web support device 60 intensify further the vacuum effect in said areas.

To ensure that the undeφressure will not escape from the gap opening between the roll 23 and the fabric 20 via the sides, on the sides of the front part of the web support device 60, sealing plates 65 are arranged very close to the surfaces of the fabric 20 and the roll 23. Similar sealing plates can, whenever needed, be arranged also on the sides of the back part of web support device 60 to seal the gap being about to close, as shown in Figure 6A.

Advantageously, also on each side edge of the web support device 60 is provided with the blow nozzle slot and suction slot arrangements of the type described above, as will be described below in connection with Figure 9.

Figure 5 demonstrates the shape of the undeφressure field to be generated in the space 66 with the aid of the web support device 60 of the invention. In the exemplary case, undeφressures -p_a, of equal size prevail in the area of the front and back edge of the web support device, undeφressures -p_b prevail on the side edges and an essentially uniform undeφressure -p_c prevails in the large middle area, being clearly lower than the undeφressures -p_a and -p_b prevailing on the edges. The undeφressures -p_a affecting on the front and back edges of the device are not necessarily of equal magnitude, instead, the undeφressure can be stepped according to the locations where more support is needed. For the same reason, also the undeφressures -p_b affecting in the edge areas of the fabric and the web can be different from the undeφressures -p_a affecting the areas of front and back edges of the device 60.

Alternatively, the undeφressure of the front and/or back edge can be profiled by varying the size of the nozzle slot and focussing greater undeφressures -p_a' and- p_a" e.g. onto the side areas of the front and/or back edge. The need of support of the web onto the fabric is usually greater in the edge areas of the opening and the closing gap than in the middle area.

In Figure 6A, a transfer of web W is presented from the fabrics 10,20 of the first press to the fabrics 30,40 of the second press while supported by the web support device 60B,60C. The first device 60B is positioned, as shown in Figure 4, in the transfer between the felt vacuum roll 23 and the vacuum transfer roll 31. The second device 60C is positioned between the vacuum transfer roll 31 and the leading roll 41 guiding the travelling of the transfer belt 40 so that the impact area of the undeφressure produced with the web support device 60C extends from a gap opening between the vacuum transfer roll 31 and the fabric 30 into a closing gap between the felt roll 33 and the fabric 30. Opposite to the latter device 60C, a steam box 35 is positioned under the web W, being arranged to blow steam 5 towards the underside of the web W in order to raise the temperature of the web prior to the subsequent press phase. The differential pressure caused by the device 60C between the different sides of the fabric intensifies penetration of steam 35 into the web W. In addition, the web support device 60C reduces the curving of the fabric 30 and the web W in the support area between the rolls 31 and 41, 0 which aids in maintaining the steaming distance constant and therefore, makes it easier to achieve a unifomt end result.

Figures 6B and 6C introduce two alternative ways of how to implement the supporting of the web W while it is travelling from the vacuum transfer roll 31, 5 guided by the fabric 30, directly into a press nip N₂ formed between an extended nip roll 32 and a backing roll 42, without being supported by the transfer belt 40 (cf also Figures IB and IC). In Figure 6B, in the middle of the support distance, a support roll 36 is positioned to prevent the press felt 30 from vibrating and curving in the relatively long support distance. The support roll 36 can be a 0 smooth or grooved roll. Alternatively, it can be a perforated roll suctioning all around or a roll provided with a suction box, forming a narrow suction sector against the fabric. The web support devices 60C, and 60C₂ according to the invention are positioned on both sides of the support roll 36 and they are arranged to generate a greater undeφressure in the area of the front edge and the back edge 5 of the device than in the area of the middle part of the device.

Also in Figure 6C the web W is passed from the vacuum transfer roll 31 along a linear path into the press nip N₂, supported solely by the felt 30 and without any support of the transfer belt 40. The stabilization of the transfer of the web W o between the transfer suction roll 31 and the press nip N₂ is arranged with the aid of three consecutive web support units 60C₃,60C₄60C₅, which together provide an undeφressure field of desired size in the support distance for supporting the web W to the fabric 30. At the beginning of the support distance, the front edge of the first web support unit 60C₃ is provided with suction and blow nozzles generating intensified undeφressure, and equivalent suction and blow nozzles are to be found also at the end of the support distance, on the back edge of the last web support unit 60C₅. On the back edge of the first unit 60C₃, on the front and back edges of the centremost unit 60C₄ and on the front edge of the last unit 60C₅, there are conventional, simple blow nozzles to generate relatively small undeφressure in the middle area of the support distance.

Between the vacuum transfer roll 31 and the press nip N₂, two boxes, or even only one long box can be used alternatively, whereby the zones of great undeφressure are formed in the areas of the opening gap of the transfer suction roll 31 and the closing gap of the press nip N₂.

Figure 7 demonstrates the vacuum effect stabilizing the course of the web W, provided with the web support device as those in Figure 6A. It can be seen in the figure that in both support distances, the undeφressure -p_a is greatest in the area of the gap opening at the beginning of the support distance and in the area of the gap closing at the end of the support distance, whereas in the middle area of the support distance, the undeφressure -p_c is distinctly lower than the undeφressure - p_a of the edge areas.

Figure 9 presents the structure of the web support device of the invention in transverse view and Figure 8 presents the shape of the undeφressure field produced therewith. On both side edges of the web support device 60, two blow- nozzle slots 67,68 are provided in the direction of the end, the outer blow nozzle slots 67 of which are arranged to blow air at high speed away from the web support device 60; respectively, the inner blow nozzle slots 68 are arranged to blow air into the space 66 between the device 60 and the fabric 20. Into the outer blow nozzle slots 67, blow air is conducted at pressure p,, which is most advantageously greater than the pressure p₂ of the blow air blown into the inner nozzle slots 68. In the middle area of the web support device 60, the suction slot 64 is connected to an undeφressure source, wherethrough air is removed through the space 66 between the device 60 and the fabric 20. Suction slots 69 are arranged between the air channels ending in the outer and inner blow nozzle slots 67 and 68, with the aid of which the undeφressure effect can be intensified in the area of the side edges of the web support device 60. In the suction channels ending in the suction slots 69, the undeφressure -p₃ is prevailing and in the suction channel ending in the suction slot 64, the undeφressure -p₄ is prevailing, said undeφressures being selected to be of desired magnitude in order to achieve a desired undeφressure profile. With the arrangement as proposed above, an undeφressure field like in Figure 8 can be produced in the space 66 between the web support device 60 and the fabric 20, in which field a substantially greater undeφressure -p₆ is prevailing in the adjacency of the edges of the web W than the undeφressure -p_c prevailing in the middle area of the web.

As a comparison, an undeφressure field is presented in Figure 10, which can be generated with a prior art blow box 57 shown in Figure 1 1. On both side edges of the blow box 57, a blow nozzle slot 58 is provided, being arranged to blow air at great speed away from the box 57, so that a substantially uniform undeφressure -p is prevailing in the entire area of the space 66.

A uniform cross-machine undeφressure distribution as in Figure 10 may be conceivable as an alternative also in the web support device of the invention when no suction is used for intensifying the undeφressure on the edges.

Figures 12 and 13 present modifications of the web support device of Figure 6 A, in which modifications an outer nozzle slot 61a of the front edge of the web support device 60B',60C is placed on the front wall of the device 60B',60C to blow air towards the surface of the roll 23 or 31 defining an opening gap. In Figure 13, also the blow nozzle slot 61b of the back edge of the web support device 60C is likewise placed on the back wall of the web support device 60C, wherefrom it blows air flow against the surface of the roll 33 defining a closing gap. When the blow nozzles 61a are positioned to blow against the surface of a roll, a gap opening after the rolls 23 and 31 can in general be better undeφressurized, that is, the location where undeφressure is most urgently needed. Hereby, the sealing side plates 65 are, however, indispensable. An arrangement such as the one described above is advantageous, particularly in association with rolls of large dimensions, whereby the opening gap is low and the nozzle cannot be entered very deep into the gap.

Figure 14 presents another embodiment of the web support device, in which on the front and back edges of the device 80, blow nozzle slots 81a,81b are provided, wherein blow air is supplied at elevated pressure p,, p₂ through the blow nozzle channels. Through the blow nozzle slots 81a,82b, air is blown at high speed away from the web support device 80 for achieving undeφressure based on an ejection effect. In the adjacency of the blow nozzle slots 81a,81b, inside the gap 66, anti- flow means 82 are arranged on the surface of the web support device 80 to be in parallel direction with the slots 81a,81b, to limit the ejection effect of the air blows in the middle area of the web support device 80 and as a result, generation of undeφressure in the space 66. For anti-flow device 82, for instance laths, rods or pipes can be used. Equivalent blow nozzles and anti-flow means are arranged advantageously also on the ends of the web support device 80. In addition, in the middle area of the web support device 80 an air channel 83 is provided, which can optionally be connected to a source generating oveφressure (+p₃) or undeφressure (-p₃) for maintaining the undeφressure of desired magnitude in the middle area of the web support device 80 left between the anti-flow device 82. To the spaces between the blow nozzles 81a,81b and the anti-flow means 82, also a direct suction effect can be directed for intensifying the vacuum effect on the web. Suction flow undeφressures to be directed into said intermediate spaces are demonstrated with the aid of arrows -p₄ and-p₅ in Figure 14. Figure 15 presents a third embodiment of the web support means. Therein, the force drawing the web W towards the fabric 20 is provided by a direct suction effect through suction slots 91a,91b positioned on the front and back edges of the web support means 90, into which slots undeφressures -p, and -p₂ are directed via suction channels. Preferably, similar suction slots are also arranged on the sides of the web support means 90 to support the edges of the web W. In the middle area of the web support means 90 an air channel 92 is arranged, being optionally connectable to a source generating oveφressure (+p₃) or undeφressure (-p₃) for adjusting the undeφressure level in the middle of the space 66 to a level which is lower than the undeφressure in the edge areas of the web support means 90.

The range of use of the web support device is not restricted only to the press section, but it is also appropriate for use in other positions of the paper machine in which the web is conveyed supported by a fabric and utilizing undeφressure in the support. One conceivable use is presented in Figure 1A, in which the travelling of the web in its rectilinear transfer, while supported by the drying wire 50, from the vacuum transfer roll 51 to the drying cylinder 52 is stabilized with blow boxes placed in positions D, and D₂. The first unit D, is advantageously equivalent to the unit 60C₂ of Figure 6C, so that members are arranged, according to the invention, on the front edge thereof for generating a great local undeφressure in a space opening between the vacuum transfer roll 51 and the drying wire 50. A second unit D₂ can be equivalent in structure to the unit 60C₂ or 60C₃ of Figure 6C, that is, either a conventional blow box for generating uniform undeφressure or a web support device is conceivable, in the back edge area of which, members are provided for generating a great local undeφressure at the end of the support area. Preferably, also on the sides of the first web support device D,, nozzle arrangements according to the invention are provided, wherewith undeφressure can be provided to support the web edges onto the fabric.

Other potential uses for the web support device of the invention could be, for instance, supporting the web onto a permeable drying wire in a planar impingement drying unit and supporting the web onto a conveyor fabric in connection with a surface treatment unit.

Claims

1. A method for stabilizing the movement of a web in its rectilinear transfer in the press section of a paper or board machine, while supported by a fabric (20,30) from a first roll (23,21) or equivalent to a second roll (31,41) or equivalent, characterized in that a vacuum effect is directed to the fabric (20,30), the magnitude of which varies in the area between the rolls so that at the beginning of the support distance, in the area of a gap opening between the first roll (23,31) and the fabric (20,30) and at the end of the support distance in the area preceding the second roll (31,41), the undeφressure (-p_a) is substantially greater than the undeφressure (-p_c) having an effect on the central area left therebetween.

2. A method according to claim 1, characterized in that the vacuum effect is directed to the fabric (20,30), the magnitude of which varies in cross-direction to the fabric (20,30) and the web (W), so that the undeφressure (-p_b) affecting in the area of the edges of the web (W) is substantially greater than the undeφressure (- p_c) having an effect on the transverse central area of the fabric (20,30).

3. A web support device, which is particularly appropriate for stabilizing the movement of a web in the press section of a paper or board machine in rectilinear transfers of the web (W), while supported by the fabric (20,30) from a first roll (23,31) or equivalent to a second roll (31,41) or equivalent, which web support device (60,80,90) is intended to be positioned between the rolls on the side to the fabric, to extend substantially across the entire width of the web (W) in parallel direction with the run of the fabric (20,30) so that between the fabric (20,30) and the web support device (60,80,90) a space (66) is formed and which web support device (60,80,90) includes members for generating undeφressure into the space (66) between the fabric (20,30) and the web support device (60,80,90), characterized in that the web support device includes means (61a,63a,81a,91a) for generating great local undeφressure (-p_a) into said space (66) in the front edge area of the web support device (60,80,90) and means (61b,63b,81b,91b) for generating great local undeφressure (-p_a) in said space (66) in the back edge area of the web support device (60,80,90), and means (62a,62b,64,82,83,92) for generating a substantially lower undeφressure (-p_c) than those before into said space (66) in the central area of the web support device (60,80,90).

4. A web support device (60) according to claim 3, characterized in that it comprises furthermore means (67,68) for generating high local undeφressure into the space (66) between the web support device (60) and the fabric (20) in the area of its ends.

5. A web support device according to claim 3, characterized in that in the area of its front edge, a first blow nozzle slot (61a) is provided, and in the area of the back edge, a second blow nozzle slot (61b) is provided, said blow nozzle slots (61a,61b) being arranged to blow air at great speed away from the space (66) between the web support device (60) and the fabric (20), and on the surface against the fabric of the web support device (60) after the first blow nozzle slot (61a), a third blow nozzle slot (62a) is provided and respectively, on the surface against the fabric of the web support device (60) before the second blow nozzle slot (61b), a fourth blow nozzle slot (62b) is provided, said blow nozzle slots (62a,62b) being arranged to blow air into the space (66) between the web support device (60) and the fabric (20) and that in the central area of the web support device (60), a suction slot (64) connected to an undeφressure source or equivalent is provided, which is arranged to remove air from the space (66) between the web support device (60) and the fabric (20).

6. A web support device according to claim 5, characterized in that in the area of its front edge between the first and the second blow nozzle slot (61a,62a), a second suction slot (63a) is provided; respectively, in the area of the back edge between the second and the fourth blow nozzle slot (61b,62b), a third suction slot (63b) is provided, said suction slots (63a,63b) being arranged to remove air from the space (66) in the proximity of the front edge and the back edge of the web support device (60).

7. A web support device (60B',60C) according to claim 5 or 6, characterized in that the first blow nozzle slot (61a) is positioned on the front wall of the web support device (60B',60C) to direct an air blow against the surface of the first roll (23,31) defining the opening gap.

8. A web support device (60C) according to any one of claims 5 to 7, characterized in that the second blow nozzle slot (61b) is positioned on the back wall of the web support device (60C) to direct an air blow against the surface of the second roll (33) defining the closing gap.

9. A web support device according to claim 4, characterized in that in the area of its both ends, two parallel blow nozzle slots (67,68) are arranged, the first blow nozzle slots (67) whereof being arranged to blow air at a great speed away from the space (66) between the web support device (60) and the fabric (20), and the second blow nozzle slots (68) being arranged to blow air into the space (66) between the web support device (60) and the fabric (20), said speed being lower than the speed of the air flow blown from the first blow nozzle slots (67).

10. A web support device according (60) to claim 9, characterized in that on both ends of it, a suction slot (69) is arranged between the first and the second blow nozzle slots (67,68), being provided to direct air away from the space (66) in the adjacency of the ends of the web support device (69).

11. A web support device according to claim 3, characterized in that in the front edge area thereof, a first blow nozzle slot (81a) is provided and in the back edge area, a second blow nozzle slot (81b) is provided, said blow nozzle slots (81a,81b) being arranged to blow air at great speed away from the space (66) between the web support device (80) and the fabric (20), and that onto the surface of the web support device (80) towards the fabric (20), sealing members (82) are arranged that are transverse to the travelling direction of the web (W), the puφose of which is to reduce the ejection effect of the air blows of the front and back edges in the central area of the web support device (80), and that in the central area of the web support device (80), an air slot (83) or equivalent is provided, which can optionally be connected to an undeφressure source or an oveφressure source for adjusting the undeφressure to be as desired in the central area of the web support device (80).

12. A web support device according to claim 3, characterized in that in the front edge area thereof a first suction slot (91a) is provided and in the back edge area, a second suction slot (91b) is provided, through which suction slots (91a,91b) air is conducted away from the space (66) between the web support device and the fabric, and that in the middle area of the web support device (90), an air slot (92) or equivalent is provided, which can be optionally connected to an undeφressure source or an oveφressure source for adjusting the undeφressure in the central area of the web support device (90) to be of desired magnitude.

13. An arrangement in the press section of a paper or board machine for stabilizing the movement of the web in its rectilinear transfer, while supported on one side only by a fabric (30), from a vacuum transfer roll (31) or equivalent to a press nip (N₂), said arrangement comprising, when positioned in the support area left between the vacuum transfer roll (31) and the press nip (N₂), at least one web support device (60C,, 60C₂), including members for generating undeφressure into a space (66) between the fabric (30) and the web support device (60C,,60C₂), characterized in that in the central area of the support distance, a support roll (36) to guide the common run of the web (W) and the fabric (30) is arranged, including means for generating a great local undeφressure in the space (66) between the web support device (60C],60C₂) in the front edge area thereof, and means for generating a great local undeφressure in said space (66) in the back edge area of the web support device (60C_!,60C₂) and means for generating a substantially lower undeφressure than the previous in said gap (66) in the central area of the web support device (60Cj,60C₂).

14. An arrangement in the press section of a paper or board machine for stabilizing the movement of the web in its rectilinear transfer, while supported by a fabric (30) on one side only, from a vacuum transfer roll (31) or equivalent to a press nip (N₂), said arrangement comprising, positioned in the support area left between the vacuum transfer roll (31) and the press nip (N₂), at least one web support unit (60C₃, 60C₄,60C₅), including members for generating undeφressure in the space (66) between the fabric (30) and the web support unit (60C₃,60C₄,60C₅), characterized in that in the support distance, at least two and most advantageously three web support units (60C₃,60C₄,60C₅) are positioned in succession, on the front edge of the first unit (60C₃) of which means are arranged for generating great undeφressure in the front edge area of said unit (60C₃) and on the back edge of the last unit (60C₅) means are arranged to generate great undeφressure in the back edge area of said unit (60C₅), and both web support units (60C₃,60C₅) and possibly the units (60C₄) therebetween include also means for generating an undeφressure substantially lower than those described above in the portion of the support area left between the front edge of the first unit (60C₃) and the back edge of the last unit (60C₃).