Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/32—Washing wire-cloths or felts
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/48—Suction apparatus
  • D21F1/50—Suction boxes with rolls
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F7/00—Other details of machines for making continuous webs of paper
  • D21F7/08—Felts
  • D21F7/12—Drying

Definitions

• the invention relates to an apparatus for removing a liquid, dusty or solid substance from a moving fabric or web.
• the invention relates to an apparatus for applying overpressure on a moving web path.
• the conventional function of the wet end of a paper machine is based on, for example, the so-called Fourdrinier principle, by which an endless rotating wire is covered with pulp which consists of a liquid and the fibres required for paper manufacture and which is herein also called a web. Water is removed from the web at several different steps by means of devices placed under the wire and including, for example various suction boxes.
• suction box is presented in EP 0 639 667 A1.
• Said suction box com- prises a roll rotating with the wire and having a jacket that is, for example, meshed.
• the roll is placed in the box under a negative pressure. Air flows through the wire and the jacket of the roll.
• the document also discloses the use of overpressure to make the removal of liquid more efficient.
• a device for producing suction in the suction box is presented in WO 99/64667 A1. Appa- ratuses equipped with a sealing are disclosed in publications WO-00709914,
• WO 95/07387 A1 discloses an apparatus used for controlling a felt and a web supported on it, applying a rotating roll whose perforated jacket is permeable to air. Other rolls are also known which are provided with a perforated jacket and equipped with suction by means of a separate apparatus.
• One apparatus is disclosed in US 5,347,728.
• a suction device according to Fig. 1 is also known, by which it is possible to support a moving wire so that the friction is reduced when compared with, for example, foils.
• a perforated rotating roll In an opening in the upper surface of the device, against the wire, a perforated rotating roll is provided, through which a negative pressure is effective on the wire.
• the device is used to provide a sufficient suction effect and high-capacity dewatering, wherein it can also be applied on wires with a high speed.
• the device can be used to replace suction boxes of prior art.
• the device is suitable for the removal of liquid as well as solids, for example dust, from a wire, a felt or various other fabrics which are used, for example, in a paper machine.
• the device of Fig. 1 is also applicable for the removal of dust developed during the manufacturing process of various fabrics or the like, or for the removal of liquid, dust and solids accumulated during their cleaning. Consequently, the device can be used as a cleaning device or a dewatering device for removing liquid or solids from moving pulp, a web or a fabric, for example from a wire and a felt, which are sufficiently permeable to air or liquid.
• a suction device and an apparatus according to Fig. 1 an apparatus according to publication WO-2006/090012-A1 is used.
• the aim of the invention is to eliminate the above-presented drawbacks of prior art.
• Another aim of the invention is to make the operation of the suction device more efficient, particularly by affecting the removal of liquid.
• the arrangement according to the invention for making the operation of the suction device more efficient comprises an apparatus, by means of which overpressure is effective on the opposite side of the wire when compared with the effect of negative pressure.
• the overpressure makes, for example, the removal of liquid from the web path and the operation of the suction device more efficient.
• the overpressure moves liquid from the web path to an area subjected to negative pressure, and the negative pressure removes the liquid further from the web path.
• the solution can be applied so that the web path contains two wires and the web therebetween. The solution can be applied especially in situations where the web path contains one wire and the web on top of the same.
• the overpressure apparatus does not comprise sealings positioned against the wire, but in one embodiment of the invention, the sealing is an air gap, wherein the overpressure apparatus can be positioned above the web.
• overpressure is effective on an area that is also subjected to the negative pressure of the suction device.
• the sealing between the overpressure apparatus and the suction device by means of air gaps is provided by defining the area which is subjected to the overpressure. When a suction roll is used, the overpressure is restricted within the effective area of negative pressure.
• the overpressure is not effective in the point where the web path is to be detached from the top of the roll, wherein the loosening takes place more easily and friction is not unnecessarily increased.
• the overpressure is not effective in the point where the web path is to move on top of the roll, wherein the loosening for example from the even guide surface takes place more easily and friction is not unnecessarily increased.
• the pressures and amounts of air in the overpressure apparatus and the suction device are set so as to minimize the spurting and splashing of liquid between the overpressure apparatus and the web path.
• the settings are selected so as to direct the air flow in said lo- cations towards the inside of the configuration formed by the suction device and the overpressure apparatus, or to minimize said air flow.
• Correct settings may also be determined by means of tests, wherein by means of increasing the overpressure the changes in the amount of removed water are observed and the behaviour of the web path is examined at the location of the air gap. The overpressure prevails within a suitable area when the amount of removed water is not substantially increased and for example spurting does not occur. It is also possible to determine the correct settings by measuring the pressures of the overpressure apparatus.
• the overpressure apparatus comprises a chamber where overpressure prevails and affects a predetermined area in the web path.
• Significant changes occurring in the overpressure have been equalized or eliminated by means of air gaps both between the apparatus and the web path, as well as by conveying air out of the chamber, if neces- sary.
• the pressure peaks occurring in the chamber may be eliminated, wherein the effect of negative pressure and overpressure on the web path remains steady, which increases the uniform quality of the web path.
• the spurting of the liquid as well as the resulting web breaks are eliminated, because the pressure is capable of discharging from the chamber through a route other than the air gap between the apparatus and the web path, wherein the route is a special channel or gap.
• the pressure or air flow remains constant in the air gap between the web path and the chamber.
• the overpressure apparatus contains at least one overflow chamber alongside with said overpressure chamber, to which the air from the overpressure chamber can be discharged, if necessary. Furthermore, air can be sucked from the overflow chamber through the web path to the suction device and the ambient pressure typically prevails in the overflow chamber.
• the overflow chamber extends at least on top of the point where the web path is to be detached for example from the top of the roll, and where the underpressure area ends.
• at least one overflow chamber extends at least on top of the point where the web path is intended to move on top of the roll, and where the underpressure area begins. There is a small air gap as a sealing between the overflow chamber and the web path.
• the overflow chamber covers either entirely or partly the stationary supporting surface along which the web path is positioned before it moves on top of the roll or off the same.
• the overpressure chamber extends transversely across the web path, and so does the roll of the suction device, and the overflow chamber also runs next to the overpressure chamber.
• the overpressure chamber forms a tapering gap or throat, in which the narrower lower part of the throat faces the web path and the wider upper part is directed away from the web path. In the narrow throat the movement of the air is accelerated.
• the upper part of the throat is sealed with a cover and air is conveyed thereto, but in the upper part of the throat, for example between the cover and the throat there is a channel or a gap, through which excess pressure and air are capable of being dis- charged, preferably to the overflow chamber.
• the channel or gap is larger than the corresponding gap between the web path and the overpressure chamber, and therefore the pressure and air are more easily discharged from the chamber through the aforementioned channel or gap, and not through the gap functioning as a sealing between the web path and the overpressure chamber. This results in that the pressures and air settle down by the web path between the web path and the overpressure chamber.
• the overflow chamber has preferably the form of a widening chamber comprising a narrower upper part at the lo- cation of said channel and a wider upper part opening towards the web path.
• the overflow chamber restricts an area from the web path, using the air gap for sealing.
• the arrangement according to one embodiment of the invention comprises, for the suction device, a sealing device that is capable of following the transverse movement of a fabric, for example a wire.
• the function can be described as being active, because the sealing device follows the movement of the fabric in different situations.
• the sealing device is capable of keeping the ends of the opening of the suction device closed and sealed even if the fabric moved in the transverse direction.
• the sealing device is placed onto the ends of the opening and glides above the roll. Using the sealing device, the openings can be made so long that they cover the full width of the fabric, irrespective of even large movements of the fabric.
• the suction device is coupled to the sealing device, with which the overpressure apparatus moves.
• the sealing device utilizes a movable carriage which can also be utilized for moving the overpressure apparatus as such, without properties relating to the sealing.
• the overpressure apparatus is equipped with height adjustment, wherein it is possible to adjust the size of said air gaps.
• FIG. 1 illustrates the principle of a suction device according to prior art, the suction device comprising a rotating roll against a moving fabric
• FIGs. 2, 3, 4, and 5 illustrate suction devices equipped with overpressure apparatuses according to the invention
• Fig. 6 illustrates the pressure box according to Fig. 2, in a cross section seen from its end.
• the dewatering suction device comprises a box-like and sealed chamber 1 used as a basin, in which a rotating roll 2 has been placed.
• the upper surface 1a of the box, on and against which a fabric 3 is placed, is substantially flat and typically horizontal.
• the upper surface 1a may also form a very gently sloping angle, the roll 2 being placed in its angular point.
• the chamber 1 is elongated and extends preferably underneath the fabric 3, substantially across its entire length.
• the chamber 1 is placed crosswise, primarily perpendicularly to the travel direction of the fabric 3, in the same way as the opening 1 b in the upper surface 1a of the chamber 1.
• the opening 1 b is uniform and continuous and extends over the full width of the fabric 3.
• the chamber 1 is closed with end parts 1c.
• a roll 2 is placed inside the chamber 1 and is allowed to rotate with the fabric 3.
• the opening 1 b exposes at least part of the outer jacket 2a of the roll 2 in such a way that the moving fabric 3 or web that is tangential to the chamber 1 can be supported to the outer jacket 2a.
• the roll 2 extends slightly higher than the upper surface 1a and causes a gently sloping ridge or a shape having a curved cross-section in the fabric 3, when the wire is placed or aspirated against the outer jacket 2a.
• the roll 2 and the upper sur- face 1a form a surface, against which the fabric 3 is pressed by the effect of the suction.
• This part of the outer jacket 2a covers a sector-like part limited for instance by beams 4 and 5 at the front and back edges of the opening 1 b.
• the exposed sector-like part encompasses about a quarter or less of the outer jacket 2a shown in Fig. 1.
• said beams 4 and 5 at the same time, constitute the upper surface 1 a and are placed as close to the roll 2 as possible.
• the beams 4 and 5 are connected to the upper part of the chamber 1 or to supporting structures above the chamber 1.
• the fabric 3 travels along the upper surface 1a, and the negative pressure effective on the fabric 3 removes, for example, liquid from the fabric 3 further into the chamber 1.
• the roll 2 is perforated, and the suction prevailing inside it pulls the fabric 3 against the outer jacket 2a of the roll and the liquid to the inside of the roll 2 and into the chamber 1.
• the roll 2 is arranged to rotate around a rotation axis X, and the rotation axis X is parallel to the longitudinal axis of the roll 2 and also to the opening 1 b.
• the width of the opening 1 b is a fraction of the length of the roll 2 and the opening 1 b.
• the longitudinal axis is parallel to the chamber 1 and transverse to the travel direction of the fabric 3.
• the roll 2 rotates or is rotated at a speed that corresponds to the speed of the fabric 3.
• the roll 2 comprises the structure of a shaft 6, for example a solid shaft, appropriately mounted with bearings at its ends.
• the ends of the shaft 6 protrude from both ends of the chamber 1 , wherein the bearings can be easily arranged outside the chamber 1.
• the gap between the end of the chamber 1 and the shaft 6 is sealed with appropriate sealings.
• At least one end of the shaft 6 may be connected to a separate rotating device, typically an electric motor, to rotate the roll 2 via the shaft 6.
• the roll 2 comprises an outer jacket 2a with a circular cross-section.
• the outer jacket 2a can be made of a sheet or, for example, as a wire-cloth that is permeable to liquid and air.
• the outer jacket 2a may be perforated or equipped with holes or openings, through which the suction is effective on the moving fabric 3.
• the liquid or air flow can enter the inside of the roll 2.
• the air flow effected by the suction and passing through the fabric 3 and the web carries liquid, dust and possibly other solids that can penetrate the fabric 3 and the outer jacket 2a.
• the ends of the upper surface 1a of the chamber 1 are provided with inclined sealing surfaces 8 which are positioned partly underneath the fabric 3.
• the fabric 3 rises from the upper surface 1a onto the sealing surface 8.
• the sealing surfaces 8 are placed onto the roll 2, and simultaneously they cover the ends of the opening 1b.
• the sealing surface 8 is connected to the chamber 1 or to the beams 4 and 5.
• the sealing surface 8 is formed in a part, whose lower side conforms to the shape of the roll 2 at the opening 1b. In Fig. 1 , said part is connected next to a cover 7 fixed on the end part 1c to close the opening 1 b.
• the sealing surface 8 and the cover 7 may also constitute a single integrated element.
• the chamber 1 is coupled, for example by means of a connection 9, to a separate suction system to provide a suction effect.
• the pressure difference causes a liquid or air flow through the outer jacket 2a and simultaneously through the fabric 3 or web into the chamber 1.
• the beams 4 and 5 as well as the sealing surfaces 8 define a suction area, i.e. the underpressure area, or the area subjected to negative pressure, having a length Y1 and a width X1.
• the suction area is placed at the opening
• the suction area may also be shorter than the distance between the two sealing surfaces 8 or the carriage 11 , because the length Y1 of the suction area is also confined to that area of the outer jacket 2a of the roll 2 which is provided with holes, openings or gaps. More specifically, the underpressure area X1 can be defined as an area within which the fabric 3 is in contact with the outer jacket 2a.
• the overpressure apparatus is installed as a pair to the suction device of Fig. 1 , on the opposite side of the web path.
• the overpressure apparatus comprises a box or a hood that is also called a pressure box 19 in this description, wherein the web path is placed between the pressure box and the roll 2.
• the height of the pressure box 19 in relation to the web path can be adjusted by adjusting means, such as screw members attached to the chamber 1.
• the pressure box 19 is placed at the location of the underpressure area defined by the suction device, and the size of the pressure box is selected so that the pressure is effective at least on the underpressure area (X1 , Y1).
• the pressure box 2 comprises both an overpressure chamber and two overflow chambers, wherein the pressure box extends within the entire shared width of the upper surface 1a and the outer jacket 2a, and beyond.
• the pressure box is coupled, for example by means of one or several connections 20, to a separate pressure system by means of which a pressure effect and an air flow to the pressure box is attained.
• the pressure box can be constructed in a variety of ways and of various materials.
• the pressure box may be made for example of metal or plastic.
• the web path consists of a fabric 16, a web 17 being placed on top of the same.
• the pressure box is sealed against the fabric 16 by means of the air gap 14 located by the outer jacket 2a, and by means of an air gap 15 located at the outer edge of the pressure box.
• the outer jacket 2a of the roll is sealed directly against the fabric 16.
• the pressure box is installed at a distance from the suction device, and it is supported by structures whose construction may vary.
• the pressure box is also supported, for example, to structures on top of the web path, or struc- tures placed on both sides of the web path.
• the suction device of Fig. 1 has an opening 1b, through which the suction is effective on the wire and the web which is narrower than the wire. If the wire and the web move vigorously in the cross direction, the ends of the opening 1 b are covered to a varying extent, which will affect the negative pressure maintained inside the apparatus and makes it more difficult to maintain a uniform suction. If the opening 1 b is not sealed under the wire, the suction becomes weaker.
• the web path for example the fabric 3, moves the sealing device which comprises parts placed at both edges of the fabric 3 and coupled to each other for a synchronized movement.
• Figure 3 shows in more detail a suction device according to one embodiment of the invention, equipped with a moving sealing device and an overpressure apparatus.
• the principle of operation of the suction device of Fig. 3 corresponds primarily to the operation of the suction device of Fig. 1.
• Figure 3 shows one of two carriages 11 detached from a coupling 13 and separated from a bearing part 10, for better illustration and to expose the bearing part 10, and the pressure box 19 is also detached.
• the overpressure apparatus is installed as a pair to the suction device, comprising a pressure box 19 placed at the underpressure area defined by the suction device.
• the pressure box 19 extends over the carriages 11 or the end of the pressure box 19 is positioned at the carriages 11.
• the bearing parts 10 close the ends of the openings 1 b, being placed at the edges of the beams 4 and 5, and they are connected e.g. on top of the end parts 1c.
• the bearing parts 10 are also placed as close as possible to the ends of the roll 2.
• the shape of the upper surface of the bearing parts 10 corresponds to the joint shape of the upper surface 1a and the outer jacket 2a of the roll 2.
• the upper surface of the bearing part 10 is provided with a cut grooving 12, to which a liquid is supplied, for example pressurized water, to carry the carriage 11 and to act as a bearing between the bearing part 10 and the moving carriage 11 , and also between the carriage 11 and the beams 4, 5.
• the water also seals the gap between the bearing part 10 and the carriage 11 , so that no suction into the roll 2 takes place via said gap, and variations of negative pressure are avoided.
• the liquid is supplied, for example, via an inlet at the end of the bearing part 10. From the inlet, a channel extends inside the bearing part to the grooving 12.
• Figure 4 shows an example in which the suction device of Fig. 3 is used, but the end of the pressure box 19 is located by the carriages 11. At the end of the pressure box 19 there is a groove in which the carriage is positioned and in which it is capable of moving back and forth in relation to the pressure box.
• the invention can also be implemented in such a manner that the pressure box is supported on top of the carriages 11 , wherein they move together.
• two carriages are provided at the same end of the roll 2, the first carriage moving with the fabric 3 and taking care of the sealing, and the second carriage supporting the pressure box.
• the second carriage is stationary and is moved, if necessary, for example by actuators.
• the single carriage 11 can be utilized for sealing independently of the pressure box, as shown in Fig.
• the pressure box 19 can also be arranged to be movable in another way, for example, for monitoring the transverse movement of the carriage 11 or the web path.
• the shape of the lower surface of the moving carriage 11 corresponds to the joint shape of the upper surface 1a and the outer surface 2a of the roll 2 so that the carriage 11 can be placed tightly on top of them. Because the shapes are the same, the carriage 11 can glide on the bearing part 10 and the beams 4, 5. In the example shown in Fig. 4, the carriage 11 has a substantially vertical surface, against which the moving edge of the fabric 3 is sealed.
• the other carriage 11 on the other side of the fabric 3 is coupled to the carriage 11 that the fabric 3 forces to move.
• a coupling 13 the carriages 11 follow each other's movements, wherein no gap is formed between said second carriage 11 and the fabric 3 either.
• the coupling 13 is, for example, mechanical, comprising two bars coupled to both sides of the carriage 11 and to both carriages 11.
• the coupling 13 is placed so that it will not be hit by the fabric 3.
• the coupling 13 may have a control function, by means of which the distance between the carriages 11 can be changed as desired.
• the coupling 13 can also be placed underneath the beams 4 and 5 or inside channels formed therein.
• the pressure box coupled on top of the two carriages 11 can function as a corresponding coupling.
• the beams 4 and 5, the bearing parts 10 and the carriages 11 are made of, for example, a plastic material having good sliding properties to reduce friction.
• an inclined sealing surface 8 according to Fig. 1 can be placed next to the carriage 11 , underneath the fabric 3, the fabric 3 rising above the sealing surface 8, which is connected to the carriage 11. The fabric 3 pushes and moves the carriage 11 , being pressed against the sealing surface 8. The carriages 11 slide on the suction device, crosswise to the fabric 1 , and the web path is placed therebetween.
• the above-presented principles can also be applied when the upper surface 1a is substantially flat throughout and it does not comprise a roll. Even in such a case, the shape of the upper surface of the bearing part conforms to the upper surface 1a, and the lower surface of the carriage 11 conforms to the shape of the upper surface of the bearing part and the upper surface 1a.
• the carriage 11 covers those openings, gaps or holes in the upper surface 1a, which are not placed underneath the fabric 3.
• Figure 4 shows an example in which the suction device of Fig. 3 is used, but the end of the pressure box 19 is located by the carriages 11.
• the underpressure area restricted by the carriages 11 moves along with the web path and remains inside the pressure box, and due to the arrangement the overpressure is always effective by the underpressure area.
• Figure 5 shows an example in which the suction device of Fig. 2 is used, but the end of the pressure box is supported on top of the covers 7. Alternatively, the pressure box is supported on top of the bearing part 10 or the end 1c, in a movable or stationary manner.
• Fig. 6 shows in more detail an embodiment of the invention in a cross- section, seen from the side of the web path.
• the figure also shows the web path to illustrate the mutual placement of the web path and the overpressure apparatus.
• the web path is elevated when it is positioned on top of the outer jacket 2a of the roll and the elevated section substantially corresponds to the width X1 of the underpressure area, which is also positioned between the beams 4 and 5.
• Fig. 6 also shows the pivot of the roll 21 , and in this example the overpressure apparatus and the suction device are positioned symmet- rically in relation to the plane Z that extends via the pivot 21 and perpendicularly to the web path.
• the pressure box 19 comprises at least one overpressure chamber 22 which is positioned at the underpressure area and extends transversely across the web path.
• the cross section of the overpressure chamber has the form of a narrowing slit or throat 23.
• the overpressure chamber 22 is open towards the web path and it narrows down towards the web path.
• the opening angle of the walls 26 restricting the overpressure chamber, i.e. the angular difference in their mutual position is substantially the same or preferably larger than that of the sector 24 which has the pivot 21 as its tip and whose sides travel via the narrowest point of the throat 23. At the same time the sector 24 defines the overpressure area.
• the sector 24, in turn is narrower or smaller than the sector 25 whose tip is formed by the pivot 21 and whose sides extend via those points of the throat 23 in which the web path moves on top of the outer jacket 2a and leaves from the same, thus determining the width X1 of the underpressure area more precisely.
• the pressure of the overpressure chamber 22 affects the web path within the area of the sector 24, but not outside the same. Between the sectors 24 and 25, in the edge of the underpressure area X1 there is a narrow area in which only the suction affects the web path.
• the narrowest point of the throat 23 is in its lower part, and in the narrowest point the distance of the wall 26 from the web path is in its smallest, thus forming an air gap 28 of 3 to 8 mm, preferably 5 mm.
• the throat 23 is positioned between the walls 26.
• the gap must be larger than the air gap 28, preferably twice as large.
• the height of the wall 26 is 50 to 70 mm.
• the width X1 of the underpressure area is approximately 130 to 150 mm, when the size of the roll of the suction device is approximately 170 to 180 mm.
• the largest width of the overflow chamber 30 is thus approximately 130 to 150 mm.
• the channel 29 is located for example between the upper edge of the wall 26 and the cover 27.
• the channel 29 may be implemented in other ways as well in the upper part of the throat 23, for example by means of an opening or perforation of corresponding size, via which the air and pressure of the pres- sure of the overpressure chamber 22 are capable of being discharged more easily than through the gap 28.
• the discharging preferably takes place by the side of the overpressure chamber 22, where, according to an embodiment, there is an overflow chamber 30 evening out the air flow and the pressure.
• the air gap 28 also leads to the overflow chamber 30.
• the throat 23 is constructed by means of two walls 26 extending crosswisely across the web path and inclining with respect to the web path.
• the walls 26 are for example plate structures fastening for example to the ends 32 and/or the cover 27 of the pressure box 19, which cover is made for example of metal.
• the walls 26 forming the side walls of the throat 23, are directed almost radially in relation to the outer jacket 2a or to the curved web path.
• the walls 26 are substantially perpendicular to the cover 27, which comprises inclinations 14 above the overflow chambers 30.
• the cover 27 is substantially parallel to the web path and horizontal above the overpressure chamber 22. Thanks to the inclinations 14 the pressure box 19 becomes lower in height towards the front and back edge of the pressure box, wherein it can be more easily positioned closer to the other structures, such as rolls.
• the end walls 32 of the pressure box 19 are substantially vertical.
• the overflow chamber 30 is positioned on top of the web path in an area located outside the sector 24, extending in the example of Fig. 6 over the beam 4 and beam 5, in parallel to the web path.
• the overflow chamber 30 is restricted by an air gap 15 of 3 to 8 mm, preferably 5 mm.
• the overflow chamber 30 preferably has a widening shape, starting from the channel 29 or at its vicinity, and it is at its largest between the air gaps 15 and 28 by the web path. Thanks to the widening shape, the movement of the air coming through the channel settles down rapidly and does not affect the web path.
• the air can also be sucked to the suction device through the narrow edge area (the distance between the edges of sectors 24 and 25) of the underpressure area X1 (distance between the outer edges of sectors 24 and 25), which is affected by the pressure of the overflow chamber 30, which is preferably substantially 0 bar.
• the pressure of the overpressure chamber 22, in turn, is approximately 0.3 bar, 0.7 bar at the highest.
• the pressures can be monitored by means of meters 31.
• the width of the edge area corresponds to the width of the air gap 28, or it is slightly larger, 5 to 10 mm.
• the aforementioned edge area belongs to the area X2 restricted by the overflow chamber 30 and subjected to the pressure of the overflow chamber 30. In this example the area X2 and the overpressure area are also located in parallel to each other and there is an air gap 28 in the transition point.
• the pressure box 19 is symmetrical in relation to the underpressure area X1 (sector 25), but the placement of the walls 26 may also be asymmetrical and the size of the air gap 28 may vary, as well as the distance of the wall 26 from the edge of the underpressure area.
• the reason may be, for example, that the air flows behave in different ways in the opposite edges of the suction area, resulting from the fact that on one edge the moving web path arrives in the suction area (inlet side) and on the other it leaves the suction area (outlet side).
• the apparatus comprises overflow chambers 30 on both sides of the overpressure chamber, as well as on the inlet side and on the outlet side of the web path.
• the shapes of the cover 27 and the inclinations 14 may vary, wherein the cross-sections of the overflow chambers 30 may vary slightly from each other.
• the shape of the walls 26 may also differ from the planar shape, but the throat 23 substantially keeps its tapering or narrowing shape when moving from the upper part to the lower part towards the web path.
• the air gaps 28 in the lower end of the throat 23 restrict the area subjected to the over- pressure.
• the ends of the pressure box 19 may comprise a similar air gap, but typically the overpressure area is restricted for example by a sealing carriage 11 in accordance with Fig. 3, or a sealing surface 8 in accordance with Fig. 1.

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• 2008
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