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 1B 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 1A.

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 underpressure 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 6A.

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 1A 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 N. 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 pick- up 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 N, 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 N, 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 1 A, 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 1A. 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 N,, in which the web passes guided by the felt on one side only.

Occasionally, the arrangement of Figure lA in the area between the vacuum transfer roll 31 and the second press nip N, may cause faults in the paper web.

Therefore, an alternative arrangement is presented in Figure 1B when transferring the web W from the lower felt 20 of the first press to the second press nip N,. 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 N. 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 N,. In the run common to the web W and the fabric 30 between the transfer suction roll 31 and the nip N,, a first web support device Cl, a support roll 36 and a second web support device C 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 1C presents an alternative arrangement, in which the support roll 36 of Figure 1B is replaced by a web support unit C4, so that three web support units C3, C4, Cs 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 1B is more advantageous.

Figures 2 and 3 present two state-of-art blow boxes 57 placed in position B in Figure 1A, 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 underpressure prevailing in the suction sector 3 la 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 overpressure 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 underpressure 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 underpressure of almost constant magnitude in the area of the entire blow box 57.

Owing to the influence of the underpressure, 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 underpressure 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,, p, p3 and p4 can be supplied into each blow nozzle slot 61a, 61b, 62a, 62b. The first blow nozzle slot 61 a 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 underpressure-p, 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 underpressures-ps and- ? 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 underpressure 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, underpressure 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 underpressure 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 underpressure field to be generated in the space 66 with the aid of the web support device 60 of the invention. In the exemplary case, underpressures-Pa, of equal size prevail in the area of the front and back edge of the web support device, underpressures-Pb prevail on the side edges and an essentially uniform underpressure-Pc prevails in the large middle area, being clearly lower than the underpressures-Pa and-pb prevailing on the edges. The underpressures-pa affecting on the front and back edges of the device are not necessarily of equal magnitude, instead, the underpressure can be stepped according to the locations where more support is needed. For the same reason, also the underpressures-Pb affecting in the edge areas of the fabric and the web can be different from the underpressures-pa affecting the areas of front and back edges of the device 60.

Alternatively, the underpressure of the front and/or back edge can be profiled by varying the size of the nozzle slot and focussing greater underpressures-Pa'and- p,"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 underpressure 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 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, which aids in maintaining the steaming distance constant and therefore, makes it easier to achieve a uniform 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, guided by the fabric 30, directly into a press nip N formed between an extended nip roll 32 and a backing roll 42, without being supported by the transfer belt 40 (cf. also Figures 1B and 1C). 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 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 underpressure in the area of the front edge and the back edge 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 N,, supported solely by the felt 30 and without any support of the transfer belt 40. The stabilization of the transfer of the web W between the transfer suction roll 31 and the press nip N2 is arranged with the aid of three consecutive web support units 60C3,60C460C ;, which together provide an underpressure 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 underpressure, 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 60C ;. 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 60Cs, there are conventional, simple blow nozzles to generate relatively small underpressure in the middle area of the support distance.

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

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 underpressure-Pa 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 underpressure-pc is distinctly lower than the underpressure- Pa 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 underpressure 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 underpressure 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 underpressure 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 underpressure-p3 is prevailing and in the suction channel ending in the suction slot 64, the underpressure-p4 is prevailing, said underpressures being selected to be of desired magnitude in order to achieve a desired underpressure profile. With the arrangement as proposed above, an underpressure 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 underpressure-P6 is prevailing in the adjacency of the edges of the web W than the underpressure-Pc prevailing in the middle area of the web.

As a comparison, an underpressure field is presented in Figure 10, which can be generated with a prior art blow box 57 shown in Figure 11. 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 underpressure-p is prevailing in the entire area of the space 66.

A uniform cross-machine underpressure 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 underpressure on the edges.

Figures 12 and 13 present modifications of the web support device of Figure 6A, 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 underpressurized, that is, the location where underpressure 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 underpressure 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 underpressure 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 overpressure (+p3) or underpressure (-p3) for maintaining the underpressure 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 underpressures 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 underpressures-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 overpressure (+p3) or underpressure (-p3) for adjusting the underpressure level in the middle of the space 66 to a level which is lower than the underpressure 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 underpressure 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 60C, of Figure 6C, so that members are arranged, according to the invention, on the front edge thereof for generating a great local underpressure 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 60C, or 60C3 of Figure 6C, that is, either a conventional blow box for generating uniform underpressure or a web support device is conceivable, in the back edge area of which, members are provided for generating a great local underpressure 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 underpressure 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.