Suction device for use in a papermaking machine and a papermaking machine using a suction device

The invention relates to a suction device (15) for use in a papermaking machine and being suitable for acting on a wet fibrous web through a permeable fabric (4, 6), e.g. a through-air-drying (TAD)-fabric. The suction device has a fabric-facing surface (18) formed by a plurality of end surfaces of solid elements (21, 22). Channels (24, 25) are defined between the solid elements. The end surface of the final solid element (22) does not lie in the same plane as the end surfaces of the previous solid elements (21). Thereby, a gap A is formed between the fabric (4, 6) and the final solid element (22). This allows air to flow through the gap and into a final channel (25). The air flow will then catch fines, small fibers and water droplets and draw them into the final channel (25) thereby counteracting a build-up of large lumps of fines and fibers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Application, filed under 35 U.S.C. § 371, of International Application No. PCT/SE2015/051208, filed Nov. 12, 2015; the contents of which as are hereby incorporated by reference in their entirety.

BACKGROUND

Related Field

The present invention relates to a suction device for use in a papermaking machine. The suction device may be, for example, a moulding box intended for a through-air drying machine. The invention also relates to a papermaking machine that uses the inventive suction device.

Description of Related Art

In papermaking machines, various kinds of suction devices are used that act on fibrous webs through a fabric that is permeable to air and water. One use for such suction devices is as moulding boxes in through-air drying machines, i.e. TAD machines. In a TAD machine, a wet fibrous web is dried as it is carried over one or several TAD cylinders on a permeable fabric with an imprinting pattern. To imprint a three-dimensional patterns into the fibrous web that is manufactured, the web and the permeable fabric can made to pass a moulding box before the web is carried by the permeable fabric to the TAD cylinder or cylinders. The moulding box is a suction device that draws the fibrous web towards the permeable fabric as the web and the permeable fabric passes the moulding box. This causes the wet fibrous web to be drawn into the permeable fabric such that the pattern of the permeable fabric is imprinted into the wet fibrous web. The pattern which has been imprinted into the wet fibrous web will remain when the fibrous web has been dried. An example of a moulding box is disclosed in, for example, U.S. Pat. No. 5,718,806. The inventors of the present invention have noted that, in paper making machines using suction devices such as moulding boxes, there may be disturbances in the area downstream of the suction device and that this may sometimes be caused by the suction device itself. Therefore, it is an object of the present invention to provide an improved suction device that can be used, for example, as a moulding box in TAD machines.

BRIEF SUMMARY

The invention relates to a suction device for use in a papermaking machine. The inventive suction device is suitable for acting on a wet fibrous web through a permeable fabric that runs through a part of the papermaking machine. The suction device according to the invention has a length that extends from a first end to a second end which first end is an upstream end when the suction device is used in the papermaking machine and which second end is a downstream end when the suction device is used in the papermaking machine and the direction from the first end to the second end is the machine direction when the suction device is used in the papermaking machine. The suction device further has a width which is perpendicular to the length of the suction device and extends in the cross-machine direction when the suction device is used in the papermaking machine. The inventive suction device further has a surface which, during use of the suction device in the papermaking machine, will be facing the permeable fabric and thus be a fabric-facing surface. The fabric-facing surface has a first part which begins at the first end of the suction device and extends towards the second end but ends before the second end of the suction device. The first part of the fabric-facing surface is formed by a plurality of planar surfaces which planar surfaces constitute end surfaces of a plurality of solid elements. The planar surfaces of the solid elements in the first part of the fabric-facing surface lie in the same plane such that, during operation, they can contact a fabric gliding over the fabric-facing surface along a straight path. The solid elements are separated from each other along the length of the suction device such that channels are defined between the solid elements and the suction device is configured to be connected to at least one source of underpressure in such a way that the at least one source of underpressure is in communication with the channels defined between the solid elements such that, when the suction device is used and the permeable fabric runs over the fabric-facing surface, the suction device can act on the permeable fabric through the channels and draw the permeable fabric by suction effect towards the fabric-facing surface. The first part of the fabric-facing surface is followed by a second part of the fabric-facing surface which second part of the fabric-facing surface is a surface formed as an end surface of a final solid element and which end surface of the final solid element does not lie in the same plane as planar surfaces of the solid elements belonging to the first part of the fabric-facing surface. Instead, it is spaced from the plane in which the planar surfaces of the solid elements belonging to the first part of the fabric-facing surface lie such that, when the permeable fabric passes over the suction device and moves past the final solid element along the plane of the planar surfaces of the solid elements belonging to the first part of the fabric-facing surface, the permeable fabric will not contact the final solid element. The final solid element is separated from a preceding solid element such that a final channel is defined between the final solid element and the preceding solid element. The final channel is capable of communicating with at least one source of underpressure for the final channel when the at least one source of underpressure for the final channel is connected to the suction device such that an underpressure can be generated in the area between the permeable fabric and the surface of the final solid element.

The solid elements are ribs that extend in a direction perpendicular to the machine direction and the cross machine direction, i.e. perpendicular to the length and width dimension of the suction device. The ribs having ends at the fabric-facing surface which ends constitute the surfaces of the solid elements.

In embodiments of the invention, at least the ends of the ribs that constitute surfaces of the solid elements in the first part of the fabric-facing surface are made of a ceramic material.

In embodiments of the invention, at least a part of the final channel formed between the ribs that constitute the final solid element and the solid element immediately preceding the final solid element has a smaller cross-sectional area than the preceding channels. In such embodiments, the final channel formed between the ribs that constitute the final solid element and the solid element immediately preceding the final solid element may be provided with a flow restrictor.

In embodiments of the invention, the suction device is configured such that the channels defined between the solid elements of the first part of the fabric-facing surface can be connected to and communicate with at least one first source of underpressure and that the final channel can be connected to and communicate with at least one second source of underpressure which is separate from the at least one first source or underpressure.

In some embodiments, the suction device may be formed as a single unit.

In other embodiments, the suction device may be formed by a first unit and a second unit which first and second units are physically separate from each other. In such embodiments, the first part of the fabric-facing surface can be formed on the first unit and the second fabric-facing surface formed on the second unit.

In embodiments of the invention, the suction device may be configured such that both the channels defined between the solid elements of the first part of the fabric-facing surface and the final channel all can be connected to and communicate with at least one common source of underpressure.

The invention may also be defined in terms of a papermaking machine that makes use of the inventive suction device. Such a papermaking machine may comprise a forming section and a drying section and the machine being will be arranged to cause a newly formed fibrous web to move along a path of travel that extends in the machine direction from the forming section to and into the drying section. The inventive papermaking machine comprises an endless permeable fabric and a plurality of guide rolls around which the endless permeable fabric runs in a loop. The endless permeable fabric is arranged to carry the fibrous web at least a part of the path of travel of the web and the inventive suction device is arranged inside the loop of the endless permeable fabric with the fabric-facing surface facing the endless permeable fabric.

The papermaking machine may be a machine that has a drying section that comprises a through-air-drying cylinder. The endless permeable fabric may then be a through-air-drying fabric which is arranged to wrap a part of the through-air-drying cylinder and the inventive suction device is then placed within the loop of the permeable fabric such that it can act on the permeable fabric and on a fibrous web that is carried by the permeable fabric and separated from the suction device by the permeable fabric. The inventive suction device is then placed upstream of the through-air-drying cylinder such that it acts on the permeable fabric before the permeable fabric reaches the through-air-drying cylinder and the fabric-facing surface of the suction device then has an orientation that is more vertical than horizontal.

In other embodiments, the endless permeable fabric may be a forming fabric in the forming section and the fabric-facing surface of the suction device may have an orientation that is horizontal or deviates from a horizontal orientation by no more than 15°. In such embodiments, the fabric-facing surface of the suction device is facing downwards such that the permeable fabric passes below the suction device.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

With reference toFIG. 1, a papermaking machine1is shown. The papermaking machine1ofFIG. 1is a machine intended for manufacturing a tissue paper web with high bulk such as, for example, TAD paper. The machine may be intended for manufacturing tissue paper that has a basis weight that may be in the range of, for example, 12 g/m2-40 g/m2and may include such grades as, for example, toilet paper or kitchen towel. The papermaking machine ofFIG. 1comprises a forming section2with a first forming fabric3and a second forming fabric4. The forming fabrics3,4may be, for example, foraminous forming wires that are air and water permeable. Each of the forming fabrics3,4is arranged to run in a loop supported by guide rolls7. During operation, the forming fabrics3,4will run in the direction indicated by the arrows S. A head box5is arranged to inject stock in a gap between the forming fabrics3,4as is known in the art to which the invention pertains. The reference numeral34indicates a forming roll. The forming section is followed by a drying section8that comprises a through-air-drying cylinder9(TAD cylinder9). From the second forming fabric4, the newly formed fibrous web W is transferred to a subsequent permeable fabric6. In the embodiment ofFIG. 1, the permeable fabric6is a through-air drying fabric (TAD fabric) that is arranged to run in a loop supported by guide rolls7in the direction of arrow S. A pick-up suction device14may be arranged within the loop of the TAD fabric6to assist in transferring the fibrous web W from the forming fabric4to the TAD fabric6. The web W is then carried on the TAD fabric6around a part of the circumference of the through-air drying cylinder9(TAD cylinder9). The TAD cylinder9is arranged in a hood10. The hood10and the TAD cylinder may be arranged to operate such that hot air passes from the hood10, through the fibrous web W and the TAD fabric6and into the TAD cylinder9and the air may then be evacuated from the TAD cylinder9in an axial direction. Embodiments are conceivable in which the hot air goes the other way, i.e. from the inside of the TAD cylinder9and out through the TAD fabric6and the fibrous web W and into the hood10. The fibrous web W is dried as it passes the TAD cylinder9and is then passed further, for example to a reel-up. The reel-up is not shown in the figures but, with reference toFIG. 1, it would normally be located to the left of the drying section8. Such a reel-up could take many forms and it may be, for example, such a reel-up as is disclosed in U.S. Pat. No. 5,901,918. The arrangement of the reel-up in relation to the rest of the machine may be such that the reel-up is arranged to receive the fibrous web from the drying section8, for example from the TAD fabric6. The fibrous web may be sent from the drying section8in an open draw to the reel-up or supported by something, for example supported by a fabric. Embodiments are also conceivable in which the TAD cylinder9is followed by a Yankee drying cylinder (not shown) and in which the fibrous web is subsequently passed to a reel-up. Before the fibrous web W reaches the TAD cylinder9, it passes a suction device15that is placed inside the loop of the permeable TAD fabric6. At this stage, the fibrous web W may have a dryness of about 25% or perhaps even less and the web W can easily be shaped. The suction device15may be a moulding box intended to assist in creating a three-dimensional structure into the fibrous web W. As the fibrous web W passes the suction device15, a suction effect from the suction device15pulls the fibrous web W against the TAD fabric6. As the TAD fabric6has a three-dimensional pattern with knuckles, fibers in the fibrous web W will be partially sucked into the TAD fabric such that the TAD fabric creates a three-dimensional pattern in the surface of the fibrous web W, or at least into the side of the fibrous web that faces the TAD fabric. The suction device15plays an important role in this process.

With reference toFIG. 2, a machine with a different layout is shown. The machine ofFIG. 2has a forming section2with a head box5, a first forming fabric3and a second forming fabric4. The fabrics3,4(both of which are water and air permeable fabrics) run in the direction of arrows S and they are supported in their loops by guide rolls7.

The drying section8comprises a drying cylinder11which may be a Yankee drying cylinder11. In operation, a newly formed fibrous web W travels on the lower side of the permeable fabric4(the fabric4may be a felt) to a nip N between a press roll12and the Yankee drying cylinder11. In the nip N, the web W is transferred to the surface of the Yankee drying cylinder11. The surface of the Yankee drying cylinder11is smoother than the surface of the fabric4which would typically be a felt. Since the surface of the Yankee drying cylinder is smoother than the surface of the fabric4, the web W will follow the surface of the Yankee drying cylinder11after the nip N. On the Yankee drying cylinder11, the web W is dried and subsequently creped from the surface of the Yankee drying cylinder by a doctor13.

In the machine shown inFIG. 2, a suction device15is placed inside the loop of the permeable fabric4and the suction device15may serve the purpose of dewatering. It should be understood that other machine configurations are also possible and the suction device15may be placed inside the loop of a wire, for example in order to achieve additional dewatering of the fibrous web.

A suction device acting on a fibrous web through a permeable fabric such as, for example, a TAD fabric, typically has a number of slots or openings through which underpressure can act. The underpressure (vacuum) sucks the fibrous web and the permeable fabric against the suction device. The inventors have noted that in some applications using a suction device inside the loop of a permeable fabric, there may be disturbances in the process downstream of the suction device. The inventors have now identified the operation of the suction device as one source of such disturbances. When a fibrous web carried on a permeable fabric passes a suction device, some fines and fibers will inevitably be sucked through the permeable fabric and the inventors have found that such fines and fibers tend to accumulate at a downstream end of the suction device and clog the suction device at its downstream end. At irregular intervals, pieces of wet fines and fibers may then fall of the suction device and follow into subsequent parts of the process and cause disturbances. The present invention aims at eliminating or at least reducing this problem such that disturbances of the papermaking process also can be reduced.

One embodiment of the present invention will now be explained with reference toFIG. 3-FIG. 7.FIG. 3shows a suction device15for use in a papermaking machine1. The suction device15is suitable for acting on a wet fibrous web W through a permeable fabric6that runs through a part of the papermaking machine1. The permeable fabric6may be a TAD fabric which has a three-dimensional pattern that can be formed by, for example, longitudinal yarns extending in the machine direction and transverse yarns extending in the cross machine direction. The three-dimensional pattern may include knuckles and recessed portions surrounding the knuckles. As best seen inFIG. 5, the suction device15has a length L that extends from a first end16to a second end17. The first end16is an upstream end when the suction device15is used in the papermaking machine1and the second end17is a downstream end when the suction device15is used in the papermaking machine1. InFIG. 3,FIG. 4andFIG. 5, the direction from the first end16to the second end17is the machine direction MD when the suction device15is used in the papermaking machine1. As can be seen inFIG. 5, the suction device15further has a width B which is perpendicular to the length L of the suction device15. The width B extends in the cross-machine direction (the CD direction) when the suction device15is used in the papermaking machine1. With reference toFIG. 5andFIG. 6, the suction device15has a surface18which, during use of the suction device15in the papermaking machine, will be facing the permeable fabric4,6and thus be a fabric-facing surface18. As can be seen inFIG. 4, the fabric-facing surface18has a first part19which begins at the first end16of the suction device15and extends towards the second end17but ends before the second end17of the suction device. The first part19of the fabric-facing surface18is formed by planar surfaces23(seeFIG. 6) on a plurality of solid elements21which planar surfaces23form a part of the fabric-facing surface18. The planar surfaces23of the solid elements21in the first part19of the fabric-facing surface18lie in the same plane such that, during operation, they can contact a fabric4,6that glides over the fabric-facing surface18along a straight path. The solid elements21are separated from each other along the length of the suction device15such that channels24are defined between the solid elements21. The spacing between the solid elements21may vary from case to case but in some embodiments contemplated by the inventors, the distance that separates the solid elements21from each other in the machine direction may be on the order of 15 mm-25 mm, for example 18 mm. The spacing between the solid elements21may be the same for all solid elements21but it may conceivably also vary. The suction device15is configured to be connected to at least one source of underpressure which is symbolically indicated as a fan26inFIG. 3andFIG. 4. In this way, the at least one source of underpressure26may communicate with the channels24defined between the solid elements21such that, when the suction device15is used and the permeable fabric6runs over the fabric-facing surface18, the suction device15can act on the permeable fabric6through the channels24and draw the permeable fabric6by suction effect towards the fabric-facing surface18. According to the present invention, the first part19of the fabric-facing surface18is followed by a second part20of the fabric-facing surface18. The second part20of the fabric-facing surface comprises a surface33(seeFIG. 6) that does not lie in the same plane as the planar surfaces23of the solid elements belonging to the first part19of the fabric-facing surface18but is spaced from the plane in which the planar surfaces23of the solid elements21in the first part of the fabric-facing surface18. It should be understood that the plane in which the surface33of the second part20of the fabric-facing surface18will normally be parallel or substantially parallel to the plane in which the planar surfaces23of the solid elements belonging to the first part of the fabric-facing surface18but the two planes are separated from each other in a direction which is normal to the two planes, i.e. perpendicular to the two planes. The surface33which does not lie in the same plane as the planar surfaces23in the first part of the fabric-facing surface18is formed on a final solid element22and can be seen as that end of the final solid element22which, during operation, will be facing the permeable fabric6. InFIG. 3, it can be seen how the plane in which the planar surface33of the final solid element22lies is separated from the plane in which the planar surfaces23lie by a distance D. Therefore, when the permeable fabric6passes over the suction device15and moves past the final solid element22along the plane of the planar surfaces23of the solid elements21in the first part19of the fabric-facing surface18, the permeable fabric6will not contact the final solid element22. The exact value of the distance D (seeFIG. 3) depends on the circumstances of each individual case but in some embodiments contemplated by the inventors, the distance D may be in the range of 3 mm-10 mm, for example 5 mm or 7 mm but other values for the distance D are also conceivable and the distance D may conceivably be larger than 10 mm. The final solid element22is separated from a preceding solid element21such that a final channel25is defined between the final solid element22and the preceding solid element21. The final channel25is capable of communicating with at least one source of underpressure for the final channel25when the at least one source of underpressure for the final channel25is connected to the suction device15such that an underpressure can be generated in the area between the permeable fabric4,6and the surface of the final solid element22. In the embodiment shown inFIG. 3andFIG. 4, the source of underpressure26is the same for both the final channel25and the preceding channels24but embodiments are conceivable in which this is not the case.

With reference toFIG. 3andFIG. 4, it can be seen that the suction device15may have a housing36that holds the solid elements21,22such that the solid elements21,22are secured/fastened to the housing36or in the housing36.

With reference toFIG. 4andFIG. 5, the solid elements21,22can be understood as ribs that extend in a direction substantially perpendicular to the machine direction, i.e. in the cross machine direction and that also have a certain extension in a direction perpendicular to the plane of the planar surfaces23. The ribs have ends at the fabric-facing surface18which ends constitute the surfaces23,33of the solid elements21,22.

With reference toFIG. 9, the ends of some or all solid elements21,22may optionally (but not necessarily) be formed by a piece28of a ceramic material such that one or several of the planar surfaces23,33may be formed on a ceramic material. In some embodiments, at least the ends of the ribs that constitute surfaces of the solid elements21in the first part of the fabric-facing surface18are made of a ceramic material28. The use of a ceramic material means that friction can be kept low and the resistance to wear is improved.

With reference toFIG. 8andFIG. 9, it may be so in certain embodiments that at least a part29of the final channel25formed between the ribs that constitute the final solid element22and the solid element21immediately preceding the final solid element22has a smaller cross-sectional area than the preceding channels24. This can be achieved in several different ways. For example, the final channel25formed between the ribs that constitute the final solid element22and the solid element21immediately preceding the final solid element22may be provided with a flow restrictor30, for example a flow restrictor which, in cross section, is “pyramid-shaped” as shown inFIG. 9but other shapes are also conceivable, for example rectangular shapes or flow restrictors30which, in cross section, have a curved shape.

With reference toFIG. 10, an embodiment is shown in which in which the suction device15is configured such that the channels24defined between the solid elements associated with the first part19of the fabric-facing surface18can be connected to and communicate with at least one first source of underpressure26and that the final channel25can be connected to and communicate with at least one second source of underpressure27which is separate from the at least one first source or underpressure26. InFIG. 11, the first and second source of underpressure26,27are symbolically shown as fans (and they may conceivably be or comprise fans).

In embodiments of the invention, a common source of underpressure may be used while the air flow through the final channel25is regulated by means of a control valve (not shown in the figures).

The suction device15may be formed as a single unit but with reference toFIG. 11, the suction device15may be formed by a first unit31and a second unit32. The first unit31and the second unit32are physically separate from each other and the first part19of the fabric-facing surface18is formed on the first unit31while the second part20of the fabric-facing surface18is formed on the second unit32.

The function of the inventive suction device will now be explained with reference toFIG. 3,FIG. 7,FIG. 8andFIG. 9. As can be seen inFIG. 3andFIG. 8, there is a gap A between the permeable fabric6and the surface33of the final solid element22. This is because the surface33does not lie in the same plane as the planar surfaces23of the previous solid elements21. This allows air to flow between the fabric and the final solid element22as indicated by the arrow C inFIG. 7andFIG. 9. The air flow through the final channel25will then catch fines, small fibers and water droplets and draw them into the final channel25thereby counteracting a build-up of large lumps of fines and fibers. Testing of the suction device has showed that, during operation, the final channel will quickly be blinded by fibers that lump together and act as a porous vacuum assisted wicket or gate. An effect of this blinding is that the air flow required is reduced and no significant additional vacuum capacity is required. The invention has been tested on a paper machine configuration where a suction device according to the present invention was used as a moulding box in a position upstream of a TAD cylinder (i.e. with a suction device15placed as shown inFIG. 1). The trials showed that the occurrence of breaks and defects was significantly reduced. Without wishing to be bound by theory, the inventors believe that the inventive form of the suction device has the effect that fines and fibers that get blind the final channel are dewatered by the air flow and that, instead of falling off in large lumps, they fall off the suction device in the shape of individual fines or fibers that are substantially dry and that are individually too small to cause any noticeable disturbances.

With reference toFIG. 9, it can be seen that the final solid element22may have an extension35that is shaped as a triangle pointing in the machine direction. This extension35is optional and need not be present. When present, the extension35may serve to extend the length of the gap A. It may also serve to guide away condensated water droplets. The extension35(if present) may also be used simply for the purpose of securing the final element22to the housing36.

In embodiments where the final channel25has a part29with a smaller cross-sectional area than the preceding channels24, for example if it has a flow restrictor30, this entails the advantage that it will be easier to achieve the effect that the final channel25gets blinded such that the required air flow is reduced.

In the configuration ofFIG. 1, wet lumps of fines and fibers may fall downwards in a direction towards the through-air drying cylinder9if the suction device15is a conventional moulding box. However, when a suction device15according to the present invention is used, this problem can be at least significantly reduced.

In the configuration ofFIG. 1, the suction device15is vertically oriented or substantially vertically oriented. However, the inventive suction device need not necessarily take the form of a moulding box in a TAD machine but could also be used in other places in a paper making machine, for example in such configurations in which the endless permeable fabric is a forming fabric4in the forming section. In such cases, the fabric-facing surface18of the suction device15need not be vertically arranged but could have an orientation that is horizontal or deviates from a horizontal orientation by no more than 15°. The fabric-facing surface18of the suction device15is then facing downwards such that the permeable fabric4passes below the suction device15. An example of such a configuration is shown inFIG. 2where a suction device could be employed for additional water removal in the forming section but it should be understood that the configuration ofFIG. 2is only an example since suction devices may be placed in a horizontal or substantially horizontal position inside the loop of practically any permeable fabric in a paper making machine. Of course, it may also be placed in a vertical position for a number of reasons or in such a position that its orientation is somewhere between a vertical orientation and a horizontal orientation.

The invention may thus be understood also in terms of a papermaking machine that makes use of the inventive suction device. The machine according to the invention comprises a forming section2and a drying section8and the machine is arranged to cause a newly formed fibrous web W to move along a path of travel that extends in the machine direction from the forming section2to and into the drying section8. The papermaking machine1comprises an endless permeable fabric4,6and a plurality of guide rolls7around which the endless permeable fabric4,6runs in a loop and the endless permeable fabric4,6is arranged to carry the fibrous web W at least a part of the path of travel of the web W and the papermaking machine1comprises a suction device15according to the invention and the suction device15is arranged inside the loop of the endless permeable fabric4,6with the fabric-facing surface18facing the endless permeable fabric.

In one embodiment of the inventive machine, the drying section comprises a through-air-drying cylinder9and the endless permeable fabric6is a through-air-drying fabric which is arranged to wrap a part of the through-air-drying cylinder9and the suction device15is placed within the loop of the permeable fabric6such that it can act on the permeable fabric6and on a fibrous web W carried by the permeable fabric6and separated from the suction device15by the permeable fabric6. The suction device15is then placed upstream of the through-air-drying cylinder9such that it acts on the permeable fabric6before the permeable fabric6reaches the through-air-drying cylinder9, and the fabric-facing surface18of the suction device15has an orientation that is more vertical than horizontal.

The surface33of the final solid element22may be planar just as the planar surfaces23of the solid elements belonging to the first part19of the fabric-facing surface18but it could also have another shape, for example a round shape. Since the final solid element22will not contact the permeable fabric4,6, the final solid element22and the surface33may very well be formed in a material such as High Density Polyethylene (HDPE) which is less expensive and which can easily be machined.

The inventive suction device is in particular useful as a moulding box in a TAD machine (a Through Air Drying machine) for making through-dried tissue paper such as tissue paper having a basis weight in the range of, for example, 10 g/m2-30 g/m2but it could also be used as, for example, a transfer suction box in a tissue paper making machine or as a dewatering element in, for example, the forming section of a tissue paper making machine such as a TAD machine. While the inventive suction device may be particularly useful in TAD machines, it may also be used in other tissue machines than TAD machines, for example as a transfer suction box or dewatering element.

While the inventive suction device is in particular intended for tissue machines (e.g. TAD machines), it may also be applied in machines for heavier grades than tissue.

It should also be understood that the invention may be defined in terms of a method of operating the inventive papermaking machine. In such a method, the newly formed fibrous web would be caused to travel along a path of travel extending in the machine direction from the forming section and to the drying section and thereby pass the inventive suction device while at least one source of underpressure was connected to the inventive suction device and the source of underpressure is active (operated) to produce underpressure such that a suction effect through the suction device is generated and underpressure acts through the permeable fabric on the fibrous web as the fibrous web passes the inventive suction device.