Abstract:
A sheet formation process and associated wet end to produce a pulp web with improved fibre orientation in a paper or board machine. A fluid is added to the pulp suspension from the side through openings ( 9, 9′ ) located after a turbulence block ( 3 ) in a headbox ( 1 ) at the machine wet end.

Description:
BACKGROUND 
       [0001]    The present invention concerns a sheet formation process to produce a pulp web with improved fibre orientation in a paper or board machine, as well as an appropriate wet end for implementing the sheet formation process. 
         [0002]    In order to form a pulp web in a paper machine, a pulp suspension is applied to a wire with the aid of a headbox. The headbox comprises a distributing device to distribute the pulp suspension over the machine width, a turbulence block made up of a large number of turbulence tubes, and a headbox nozzle. In order to achieve good sheet formation it is important that the pulp suspension jet impinges onto the wire as evenly as possible over the entire machine width. The jet from the headbox broadens, however, after exiting from the headbox nozzle, and there is further broadening of the jet after the open jet hits the wire. Thus, a wire edge limitation is needed to guide the open jet and the pulp suspension onto the wire. Without the wire edge limitation the suspension would run off the wire at the sides, which would cause a drop in the basis weight and disrupt the fibre orientation in the areas at the edge of the web. 
         [0003]    The wire edge limitation, however, also has undesirable effects. On the one hand, the fibre orientation is disrupted due to edge friction between the lateral headbox nozzle wall or wire edge limitation and the pulp suspension, and an uneven speed profile develops in the suspension, with a higher speed in the middle of the wire in running direction than at the edges. On the other hand, the wire edge limitation creates a reflected wave towards the center of the wire when the suspension hits the wire edge limitation. This undesirable effect is mitigated by wire edge limitations that widen in the machine running direction. However, the pulp suspension flows into the areas laid open by this widening, which results in formation of a pulp mat with a thinner edge. 
         [0004]    Various solutions to these two problems are known from the state of the art. EP 1 619 298 A2, for example, discloses a headbox in which the turbulence channels at the edge of the turbulence generator are separated from the remaining turbulence generating area. These lateral turbulence channels each have feed pipes for an edge flow that can be used to influence the fibre orientation profile. EP 0 857 816 B1 describes a headbox with an integrated edge feed arrangement in the vicinity of the turbulence block. Infeed of an edge flow in the turbulence block, however has the disadvantage that very large quantities of edge flow must be supplied in order to be able to compensate or reduce subsequent edge friction effects. This considerable overdosing leads to disruption of the flow before the jet exists from the headbox, and thus to disruption of the fibre orientation. 
       SUMMARY 
       [0005]    The object of the invention is to provide an improved sheet formation process and a paper or board machine wet end suitable for this process that circumvents the above mentioned disadvantages, thus reducing disruption of fibre orientation and forming a fibre mat with improved basis weight distribution over the entire machine width. 
         [0006]    This object is achieved by a sheet formation process in which a fluid is added to the pulp suspension from the side through openings located after a turbulence block in a headbox at the machine wet end. This leads to improved suspension guiding and to a reduction or compensation of edge friction. If the wire edge limitation widens in addition, the areas laid open by this widening fill up with the fluid and thus prevent the suspension from running off the wire in these edge areas. 
         [0007]    The fluid used here is preferably white water, which permits a particularly simple process flow because a large quantity of white water is produced at the wet end anyway. 
         [0008]    In another favorable embodiment the fluid used is a pulp suspension that can also have a lower or higher consistency than the pulp suspension fed to the headbox distributor. Due to this additional input of pulp, the differences in basis weight across the machine running direction can be compensated more effectively. With a wire edge limitation that widens in machine running direction, the space formed by widening fills up with pulp suspension because of this additional input of pulp, thus achieving a uniform suspension height on the wire. This favors uniform basis weight distribution over the entire width of the pulp web. 
         [0009]    The fluid used can also be a gas, particularly air. This can also achieve a reduction in edge friction without additional liquid having to be added to the pulp mat forming. 
         [0010]    It is an advantage if the fluid is already added to the suspension inside the headbox nozzle and conventional wire edge limitations can still be used as a result. 
         [0011]    Another possibility, however, is not to add the fluid until the wire edge limitation, as a result of which edge friction directly in the sheet formation area can be reduced particularly well. 
         [0012]    It is practical if the liquid is added largely in the machine running direction as this facilitates formation of a fluid film between the wire edge limitation and the pulp suspension. Here, it is an advantage if the flow speed of the fluid in the machine running direction is greater than the speed of the pulp suspension. 
         [0013]    The fluid can be added in the machine running direction at several points located one after the other, where the fluid can be added at different flow speeds or in different quantities at the individual points. It is preferable if the quantity and/or the flow speed of the fluid increases from one addition point to the next in the machine running direction because the pulp suspension thickens as a result of the dewatering process, thus the effort required to compensate edge friction increases. 
         [0014]    The invention is also directed to a wet end for sheet formation, where feed channels with openings for feeding a fluid to the pulp suspension are provided in the wire edge limitation and/or headbox nozzle after a turbulence block. The fluid exiting from the openings reduces edge friction and improves sheet formation. 
         [0015]    The openings can be located either in a side wall of the headbox nozzle or in a side wall of the wire edge limitation. Addition of fluid in the headbox nozzle has the advantage that existing wire edge limitations do not have to be modified, whereas addition via the wire edge limitation allows improved minimizing of edge friction until final formation of the sheet. 
         [0016]    In a favorable embodiment the feed channels end at the side wall, largely in parallel with the machine running direction. This has the advantage that the fluid flows largely in machine running direction. 
         [0017]    It is favorable here if the openings for adding fluid are provided on both sides of the wet end because this minimizes edge friction and forms a symmetrical basis weight profile. 
         [0018]    It is expedient if the side wall has at least one recess after the opening, which recess has a narrowing cross-section in the machine running direction, where the side wall serves as a guiding surface for the fluid added in the area of the recess. Due to this special design of the side wall, the fluid is added to the pulp suspension roughly in machine running direction and formation of a friction-reducing fluid film is enhanced. 
         [0019]    In a favorable embodiment of the wet end, several exit openings are provided, preferably one after the other viewed in the machine running direction. By adding the fluid at several points at the wet end, the area in which edge friction is reduced and/or pulp is added can be enlarged. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Two examples of embodiments of the invention are described in the following with reference to the drawings in which: 
           [0021]      FIG. 1  shows a schematic diagram of a wet end according to the invention; 
           [0022]      FIG. 2  shows a schematic diagram of a further variant of a wet end according to the invention; 
           [0023]      FIG. 3  shows a schematic side view of  FIG. 1 ; and 
           [0024]      FIG. 4  shows a perspective detailed view of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The configuration shown in  FIG. 1  includes a headbox  1  consisting of a distributor  2 , a turbulence block  3 , which comprises a large number of turbulence tubes  4 , and a subsequent headbox nozzle  5  through which the pulp suspension fed in by the distributor  2  is discharged onto a wire  6 . The machine running direction is indicated by an arrow  15 . At the edge of the wire there is a wire edge limitation  7  that prevents the pulp suspension that impinges onto the wire  6  from running off the wire  6  at the sides. The openings  9  of feed channels  10  are located in the side wall  8  of the wire edge limitation  7 . A fluid, for example water, dilution water or pulp suspension, air, or a mixture of these components is added through these feed channels  10 . As a result of this fluid being added, edge friction between the pulp suspension and the side wall  8  of the wire edge limitation  7  is reduced because the fluid forms a boundary film between the suspension and the side wall  8 . Here, the fluid is largely introduced through nozzles in the machine running direction, where the flow speed of the fluid is greater than the speed of the pulp suspension on the wire. The openings  9  in the side wall  8  of the wire edge limitation  7  are located in recesses  11 . Due to these recesses  11  the fluid is introduced through nozzles as parallel as possible to the machine running direction between the side wall  8  of the wire edge limitation  7  and the pulp suspension coming out of the headbox  1 . 
         [0026]      FIG. 2  illustrates a further embodiment of the invention, where the reference figures correspond to those used in  FIG. 1 . In this embodiment, however, the fluid is added via feed channels  10 ′ through openings  9 ′ in the side wall  8 ′, thus enhancing fluid feed in the machine running direction. 
         [0027]      FIG. 3  shows a side view of  FIG. 1 , where the wire  6  that is deflected by a breast roll  12  under the headbox  1  can be seen particularly clearly. The pulp suspension leaving the headbox nozzle  5  hits the wire  6  at the impingement point  13 . The opening  9  and the recess  1  in the side wall  8  of the wire edge limitation  7  are indicated. 
         [0028]    In  FIG. 4 , a perspective detailed view of  FIG. 1  is provided and clearly shows the shape of the recess  11  in the side wall  8 . The pulp suspension exists here from the nozzle opening  14  of the headbox nozzle  5  and hits the wire  6  at the impingement point  13 . The fluid exits from the opening  9 . The opening  9  is arranged vertically to the machine running direction in the recess  11  of the side wall  8  and in a slot shape. The recess  11  cross-section narrows continuously in the machine running direction. 
         [0029]    During operation a pulp suspension is fed through the distributor  2  to the headbox  1 . Here, part of the suspension can also be removed through a return flow  2 ′. The pulp suspension flows in the headbox  1  through a large number of turbulence pipes  4  into the headbox nuzzle  5 . After leaving the headbox nozzle  5 , the pulp suspension hits the wire  6  at the impingement point  13 . A fluid to alter the speed profile of the pulp suspension is injected through feed channels  10 ,  10 ′ in the wire edge limitation  7  or the headbox nozzle  5 . The flow speed of the fluid in the machine running direction is preferably higher than the speed of the pulp suspension, and also has lower viscosity and consistency than the pulp suspension, if applicable. Due to the higher flow speed of the fluid, the pulp suspension in the edge zones is accelerated in machine running direction. As a result, an even speed profile is formed over the entire width of the pulp suspension. 
         [0030]    The embodiments shown in the drawings merely show some of the embodiment of the invention. The invention also relates to other embodiments, e.g., injecting a fluid through the side wall  8 ′ of the headbox nozzle  5  and through the side wall  8  of the wire edge limitation  7 . It would also be conceivable to have several openings  9 ,  9 ′ arranged in the machine running direction. In this case, the fluid can be added at different speeds and/or in different quantities through each opening. It is an advantage if the speed of the fluid added through the individual openings  9 ,  9 ′ increases in the machine running direction because the pulp suspension thickens on the wire  6  due to the dewatering process, thus requiring greater effort to compensate edge friction.