The present invention relates to a method and a twin-wire press, for dewatering of a fibre suspension.

Twin-wire presses for dewatering of a fibre suspension and forming of a continuous web thereof are previously known. Dewatering of the pulp is usually done from an inlet pulp concentration of 3-8 percentages by weight to an outlet pulp concentration of 30-50 percentages by weight.

According to the state of the art, such twin-wire presses comprises lower rolls, an endless lower wire running in a path around the lower rolls, upper rolls, and an endless

^■ upper wire running in a path around the upper rolls. The two wires co-operate with each other along sections of said paths that run substantially in parallel with each other for dewatering of the fibre suspension between the wires during displacement thereof. An inlet box provides for supply of the fibre suspension to a wedge-shaped dewatering space between the wires. The twin-wire press further comprises two dewatering tables supporting the respective wire in said sections of the path and forming the wedge-shaped dewatering space between the wires for initially pressing and dewatering the fibre suspension, whereby a web is formed between the wires, and a roll arrangement situated after the dewatering tables in said sections of the paths, as seen in the direction of movement of the wires, for finally pressing and dewatering the web between the wires, so that the web will get a desired dryness. By dewatering space is meant the part of the dewatering tables where dewatering occurs.

A conventional dewatering space in a twin-wire press has a wedge-shape with a fixed shape that is not changeable when

the twin-wire press is in operation. The geometry in the table and the flow of the pulp suspension creates the operating pressure difference over the wire that controls the dewatering. The wedge-shape decides the built up pressure in the twin-wire press and the dewatering procedure is in a large extent dependent on the shape of the wedge, which is difficult to change. Alterations of the wedge-shape requires new, extensive settings of the dewatering tables, exchange of side sealings to the dewatering tables, etc..

The dewatering tables in a twin-wire press is the first step in the dewatering of a fibre suspension and is adapted for relatively slow dewatering and also for preparation of the formed fibre web, formed through dewatering of the fibre suspension, for the much more faster dewatering that occurs in the subsequent roll arrangement through pressing in a roll nip. Too large loading of the dewatering tables results directly in high frictional forces and also very high energy consumption for the operation. The step between the relatively slow dewatering in the dewatering tables and the very fast dewatering in the roll arrangement is considerable and may sometimes give rise to problems in the first roll nip in the roll arrangement. In case the fibre web has far too low pulp concentration when it leaves the dewatering tables, is received and deformed with far too high speed in the first roll nip in the roll arrangement, the fibre web may be destroyed.

The object of the present invention is to at least partially eliminate those drawbacks that are associated with previously known state of the art twin-wire presses that have been described above. A further object is to

achieve an easier and more improved dewatering by the dewatering tables in a twin-wire press without changing the geometry of the dewatering tables. An additional object is to overcome the considerable difference of conventional twin-wire presses between the relatively slow speed at which the fibre suspension/ web is deformed and the relatively high speed the fibre web is deformed by the roll arrangement.

These objects are achieved with the method for dewatering of a fibre suspension in a twin-wire press according to the present invention. The twin-wire press has an endless lower wire and an endless upper wire, and a first and a second dewatering table, which supports the wires such that an oblong wedge-shaped dewatering space is defined by the wires. According to the method, the wires transports the fibre suspension under the compression of the wires by the dewatering tables, such that the fibre suspension is dewatered and forms a fibre web between the wires. The method is characterised in that a separate pressing force is applied against one of the wires, such that this wire presses the formed fibre web via the second wire against one of the dewatering tables and further dewaters the fibre web. The separate pressing force is adjusted such that a desired dryness of the fibre web leaving the dewatering tables is achieved.

The frictional loads can be decreased to the half at the additional dewatering of the fibre web through the use of the separate pressing force, which does not cause any frictional force, against one of the wires in accordance with the method of the present invention, compared to a conventional twin-wire press where upper and lower dewatering tables give rise to frictional . forces.

Application of a separate pressing force at the additional dewatering results in a particularly effective pressing for dewatering that is adjusted independent of the load on the fibre web achieved by the dewatering tables in the wedge-shaped dewatering space. The separate pressing force, respectively the compression of the wires for dewatering of the fibre suspension that is accomplished along the whole wedge-shaped dewatering space by the dewatering tables, acts individually, separately, and thus does not influence each other.

According to a preferred embodiment, the first dewatering table is an upper dewatering table and the second dewatering table is a lower dewatering table. Moving of at least one of the dewatering tables, suitably the upper dewatering table, in a vertical direction, controls the compression of the wires by the dewatering tables. The separate pressing force is preferably applied at an end of the dewatering tables that is situated in connection to a subsequent roll arrangement of the twin-wire press.

The present invention also relates to a twin-wire press for dewatering of a fibre suspension, having an endless lower wire and an endless upper wire. A first and a second dewatering table supports the respective wires, such that a oblong dewatering space is defined by the wires. Further the twin-wire press comprises an inlet box for supply of fibre suspension to the dewatering space, in which the fibre suspension is dewatered under transportation of the wires and compression between the dewatering tables, whereby the dewatered suspension forms a fibre web between the wires. A pressing apparatus is arranged to apply a separate pressing force against one of the wires, such that this wire presses the formed fibre web via the second

wire against one of the dewatering tables and further dewaters the fibre web. The pressing apparatus is arranged to adjust the separate pressing force such that a desired dryness of the fibre web leaving the dewatering tables is achieved.

In a preferred embodiment of the present invention, the pressing apparatus is a press roll, preferably a hole roll or a grooved roll. The press roll can be a solid roll or a roll with an envelope surface and an inner cavity. By the term hole roll is meant a roll which envelope surface is perforated. The holes in the envelope surface can be substantially circular but they may also have another shape. The shape of the grooves or slots is chosen such that a favourable dewatering is achieved. Preferably the twin-wire press comprises at least a perforated dewatering element that is arranged against the second wire outside the dewatering space. Owing to that the surface also comprises grooves or slots, increased dewatering efficiency of the fibre web is achieved by application of the separate pressing force.

Additional preferred embodiments according to the present invention are evident from the following by the detailed description with reference to the accompanying drawings.

The present invention will now be described in more detail by embodiments, with reference to accompanying drawings, without restricted interpretation of the invention thereof, where

fig. 1 schematically shows in an overview a longitudinal cross-section through a twin-wire press, and

fig. 2 schematically shows in a partial view a longitudinal cross-section through a part of the dewatering table of the twin-wire press in fig. 1 according to the present invention.

Fig. 1 shows a twin-wire press 2 according to the present invention and which is suitable for carrying out the method according to the present invention. The twin-wire press 2 comprises three lower rolls, viz. a drive roll 4, a control roll 6 and a tensioning roll 8. An endless lower wire 10 runs in a path around the lower rolls 4, 6 and 8. In a corresponding manner an upper endless wire 12 runs in a path around three upper rolls, viz. a drive roll 14, a control roll 16 and a tensioning roll 18. An upper dewatering table 20, that supports the upper wire 12, and a lower dewatering table 22, that supports the lower wire 10, forms the dewatering space 24 between the wires 10, 12 in which the fibre suspension/web is dewatered under movement of the wires and compression between the dewatering tables (in a direction R, from the right to the left in the drawings) , whereby the dewatered suspension forms a fibre web between the wires. "Press section" refers to an ordinary roll arrangement according to the state of the art that can involve a plurality of roll pairs 25, such as schematically shown in fig. 1. An inlet box 26 is arranged at one end of the press.

As evident from fig. 1 and 2, a press roll 28 is arranged in connection to the dewatering space 24 at the upper dewatering table 20. The press roll 28 is arranged to apply a separate pressing force F on the upper wire 12 (not shown in fig. 2) , such that this wire presses the formed fibre web via the lower wire 10 (not shown in fig. 2) against the lower dewatering table 22 and further

dewaters the fibre web. The press roll 28 is arranged to adjust the separate pressing force F such that a desired dryness of the fibre web that leaves the dewatering tables 20, 22 is accomplished. The separate pressing force F is adjusted independently of the load on the fibre web achieved by the dewatering tables 20, 22.

As evident in fig. 2, the twin-wire press 2 can further comprise perforated upper 30 and lower 32 dewatering elements, respectively, that are arranged against the wires 10, 12 (see fig. 1; not shown in fig. 2) outside the dewatering space 24. The twin-wire press comprises suitably a lower outlet box (not shown) for catching the filtrate that flows from the dewatering space 24 through the lower wire 10 and the lower perforated dewatering element 32, and an upper outlet box (not shown) for catching filtrate that flows from the dewatering space through the upper wire 12 and the upper perforated dewatering element 30.

As evident from the embodiment according to fig. 2, the press roll 28 can be arranged to one of the dewatering tables via an arm 36, attached to a first outer end to the centre 38 of the press roll. The arm 36 is pivotally journalled in bearings about an axis of rotation 40 arranged to the dewatering table, here the upper dewatering table 20. A control device 42, such as a screw- like member, is arranged with adjustable distance D in attached connection to a second outer end of the arm 40 at the dewatering table 20. The control device 42 is arranged to adjust how far down it is possible to press the press roll 28 against the dewatering space 24 and the lower dewatering table 22. Thus, the control device 42 limits the movement of the press roll in direction against the

dewatering space 24, such that it always will be a certain distance between the wires 10, 12. The object of the control device 42 is to prevent the press roll 28 to load the lower dewatering table when there does not exist any fibre suspension/web between the wires, which otherwise would result in damages on the wires, and also to not completely close the space between the wires. For example at a too low inlet concentration of the fibre web, it otherwise can be broken off completely.

The twin-wire press 2, in accordance with the present invention, can be shaped such that the upper dewatering table 20 is arranged for movement in vertical direction V (see fig. 1) relatively the lower dewatering table 22, and the lower dewatering table can be fixed.

According to an embodiment of the present invention, the press roll 28 can be a hole roll or a grooved roll. By means of sharpening the surface 34 of the roll with holes, grooves or slots, increased dewatering efficiency of the fibre web can be achieved by application of the separate pressing force F, since the recesses facilitates the carrying away of the filtrate.

During operation of the twin-wire press 2, by the method according to the present invention, the following occurs: According to the method the wires 10, 12 moves the fibre suspension under the compression of the wires by the dewatering tables 20, 22, such that the fibre suspension is dewatered and forms a fibre web between the wires. A separate pressing force F is applied by the press roll 28 against the upper wire 12, such that this wire 12 presses the formed fibre web via the lower wire 10 against the lower dewatering table 22 and further dewaters the fibre

web. The separate pressing force F is adjusted such that a desired dryness of the fibre web that leaves the dewatering tables 20, 22 is achieved. According to a preferred embodiment, the flow of filtrate from the dewa'tering is conveyed, by means of the applied separate pressing force F, to flow via perforated dewatering elements 30, 32 that are arranged outside the dewatering space 24 of at least the lower dewatering table 22. The filtrate that flows from the dewatering space 24 through the wires is collected in outlet boxes (not shown) .

The compression of the dewatering tables 20, 22 by the wires 10, 12 can be controlled by movement in vertical direction V of at least one of the dewatering tables 20, 22. Said separate pressing force F is suitably applied at an end 36 of the dewatering tables 20, 22 that is situated in connection to a subsequent roll arrangement "press section" of the twin-wire press 2. It is possible, within the scope of the inventive concept according to the present invention, that the twin-wire press comprises more than one press apparatus/roll 28, such as mentioned above, for application of the separate pressing force F. In such a case are those press apparatuses/rolls arranged in, the vicinity of the above mentioned end 36 of the dewatering tables 20, 22.