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 from an inlet pulp concentration of 3-8 percentages by weight to a 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 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 section between the dewatering tables where dewatering occur.

Alongside the longitudinal direction of the wires, in the wedge-shaped dewatering space, there are perforated dewatering elements that are arranged against the wires

outside the dewatering space, through which dewatered filtrate is lead away to upper and lower outlet boxes, respectively, arranged at the dewatering tables for receiving filtrate that flows from the dewatering space through upper and lower wires, respectively. The dewatering elements which against the upper and lower wire, respectively, rests against in the wedge zone constitutes together an upper and lower dewatering surface, respectively, where each dewatering surface respectively may be composed of one or more dewatering elements. Upper and lower outlet box, respectively, may be divided into several chambers whereby a filtrate through upper and lower dewatering surface, respectively, may be divided into partial filtrate in two or more chambers in respective outlet box.

A traditional dewatering space in a twin-wire press has a wedge-shape with a fixed design that is not changeable when the twin-wire press is in operation. The outlet boxes are not sealed and thus works against atmospheric pressure. The geometry in the table and the pulp suspension flow creates the operating pressure difference over the wire that controls the dewatering. The wedge shape decide the pressure built-up in the twin-wire press and the dewatering process is to a large extent dependant on the shape of wedge, which is difficult to change.

Changes of the wedge-shape requires new, extensive settings of the dewatering tables, change of side sealings to the dewatering tables, etc.. Owing to that the wedge- shaped dewatering space has its larges cross-sectional area adjacent the inlet box, and thus narrows linearly in the direction of movement of the web, a well adapted pressure difference, also called delta P, is not obtained over the wire in the area at the dewatering space inlet

end, which result in that the built-up of the fibre web becomes defect. Small fibres accumulate closest to wire surface and creates a layer difficult to penetrate for the filtrate. The problem is to create a geometry for the dewatering table that gives an optimal pressure difference for the most effective dewatering and formation of the fibre web. The dewatering begins first some way inside the dewatering space in the position where a first dewatering element is located. The inlet box comprises extended sealing blades whose free ends are arranged against the insides of the wires in the dewatering space. The. ends of the sealing blades terminates in a position usually situated about 100 mm in advance of the location of the first dewatering element, which results in that a relatively extensive backward flowing leakage of thick, fibre-rich flow occurs on the both sides of the wires, where the ends of the sealing blades terminate.

The present invention aims for providing an easier and improved adjustment of the pressure levels, and thereby more favourable dewatering, over the whole length of the dewatering table in a twin-wire press without changing the geometry of the dewatering table. Yet an object is to minimise leakage of fibres, particularly at the inlet end of the dewatering space.

This object is achieved with the method for dewatering of a fibre-suspension in a twin-wire press according to-the present invention, in which method the fibre suspension is fed to an oblong dewatering space, that is defined by an endless lower wire and an endless upper wire of the twin- wire press, against which wires lower and upper perforated dewatering elements are arranged outside the dewatering space, and the fibre suspension is dewatered in the

dewatering space during displacement of the wires, such that a flow of filtrate from the dewatering space through the wires and the perforated dewatering elements is formed, filtrate that flows from the dewatering space through the wires is collected in outlet boxes, and that the pressure difference between the dewatering space and at least an outlet box is adjusted by controlling at least one counter pressure that is applied on at least a part of the flow of filtrate.

By means of control of the pressure in the dewatering space in accordance with the present invention, the dewatering can be corrected without needing to change the geometry of the wedge-shaped dewatering space, desired basis weight of the fibre web can be maintained and new settings of the dewatering tables need not to be done. The invention facilitates that different pressures can be obtained at different areas along the whole dewatering space, whereby a perfect pressure for the dewatering required can be achieved. The built-up of the fibre web is improved and the fibre web. can be prepared in a desired way before the roll nips. The invention makes it possible to decrease the pressure difference in a first section of the dewatering space such that the smaller fibres do not accumulate closest to the surface of the wire.

The fibre suspension enters into the dewatering space via an inlet end. Preferably the pressure difference is adjusted between the dewatering space and outlet boxes by control of at least a counter pressure that is applied on at least a portion of the flow of filtrate. Suitably at least a counter pressure is applied on the portion of the flow of filtrate that is formed on both sides of the lower and upper wires directly adjacent to the inlet end of the

dewatering space, particularly in the area around the sealing blades. By inlet end of the dewatering space is meant the position where the dewatering space begins. The possibility to create a first dewatering area with separate pressure control for separation of a first filtrate is particularly advantageous, because this first filtrate consists of a relatively thick, fibre-rich flow because the built-up of the fibre web not begins until some way into the dewatering space. Accordingly, fibre leakage at the dewatering inlet end can be minimised.

According to a preferred embodiment of the present invention it is possible to apply a plurality of separate counter pressures on respective portions of the flow of filtrate, which are separated, whereby the pressure level over the whole length of the dewatering space can be adjusted optimally. Thus, the fibre web can be better prepared and controlled before the nips. The separate counter pressures can suitably be controlled independently of each other for an optimal adjustment.

The present invention also relates to a twin-wire press for dewatering of a fibre suspension, comprising an endless lower wire and an endless upper wire, which defines an oblong dewatering space in which the fibre suspension will be dewatered during displacement of the wires, an inlet box for supplying the fibre suspension to the dewatering space, lower and upper perforated dewatering elements that are arranged against wires outside the dewatering space, a lower outlet box for receiving filtrate that flows from the dewatering space through the lower wire and lower perforated dewatering element, an upper outlet. box for receiving filtrate that flows from the dewatering space through the upper wire and

upper perforated dewatering element, a pressure control device arranged to control one or more counter pressures in the lower and upper outlet box, respectively, for adjustment of a pressure difference between the dewatering space and the outlet boxes.

The outlet boxes can be separated from each other whereby a pressure control device is arranged for each of the outlet boxes in order to be able to receive different counter pressures in the outlet boxes. The pressure control device may for example be provided with a pump and a valve or the similar, that facilitates that a control of the pressure difference can be achieved between the dewatering space and the outlet boxes. According to a preferred embodiment the pressure control device comprises a filtrate outlet pipe connected to the outlet boxes, and a spillway overflow in the filtrate outlet pipe, whereby the spillway overflow is located at a level above the dewatering space. The level of the spillway overflow is suitably adjustable, thus its height over the dewatering space can be changed which means that the pressure difference can be adjusted optimally between the dewatering space and the outlet boxes.

As been described above, according to a preferred embodiment of the present invention, at least a counter pressure can be applied on both sides of the upper and lower wires directly adjacent to the inlet end of the dewatering space, in particular in the area around the sealing blades. In that respect the twin-wire press comprises initial perforated dewatering elements arranged directly adjacent to sealing blades, extended from the inlet box, whose free ends are arranged against the insides of the wire in the dewatering space, whereby a

first partial filtrate is separated to a first chamber of respective outlet box.

One or more upper and lower perforated dewatering elements, respectively, forms an upper and lower dewatering surface, respectively. According to an embodiment, the outlet boxes of the twin-wire press comprises two or more chambers for receiving a respective partial filtrate, which facilitates that the pressure level over the whole length of the dewatering space can be controlled optimally. Preferably, a plurality of separate pressure control devices can be arranged to control the counter pressures in the chambers independently of each. other.

The chamber or those chambers that are arranged to use a counter pressure are sealed in order to operate above atmospheric pressure. In the case the outlet boxes are divided in two or more chambers, the first chamber is the one that is arranged to receive filtrate from the first section of the dewatering space.

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 shows schematically in a partial view a longitudinal cross-section through a twin-wire press according to the present invention, and fig. 2 shows schematically in an overview a longitudinal cross-section through the twin-wire press according to fig. 1.

Fig. 1 shows schematically a partial view through the twin-wire press 2 according to the present invention, more precisely a section at an inlet end 14 of a dewatering space 4 in the area at and around an inlet box 16. The fibre suspension that will be dewatered is supplied from the inlet box 16 to the dewatering space 4 defined by an endless lower wire 6 and an endless upper wire 8 during displacement of the wires 6, 8. Initial perforated dewatering elements 10, 12 are arranged outside the dewatering space. Such perforated dewatering elements are used over the whole length of the dewatering space for removal of filtrate from the dewatering space. The perforated dewatering elements 10,12 shown in fig. 1 that are arranged at the inlet end 14 are formed of a bent plate with holes in and replaces those rolls that traditionally are used at an inlet end of a twin-wire press. Filtrate flows through the wires 6, 8 from dewatering space and is collected in outlet boxes 18,20.

The lower outlet box 18 receives filtrate that flows from the dewatering space through the lower wire 6 and lower dewatering element while the upper outlet box 20 receives filtrate that flows from the dewatering space through the upper wire 8 and upper dewatering element. A pressure control device 22 controls a counter pressure in the lower and the upper outlet box 18,20, respectively, and is arranged to adjust the pressure difference between the dewatering. space and the outlet boxes 18,20.

Fig. 1 shows a pressure control device 22 that comprises a vertical filtrate outlet pipe 24 connected to the outlet boxes, and a spillway overflow 26 in the filtrate outlet pipe, whereby the spillway overflow is located at a level 28 above the dewatering space 4. The level of the spillway overflow is thus the difference in height between the

spillway overflow and the dewatering space. The filtrate outlet pipe 24 is arranged vertically displaceable whereby the level 28 of the spillway overflow 26 relatively the dewatering space, and the outlet boxes therein, are adjustable. Thus, the pressure difference between the dewatering space 4 and the outlet boxes 18,20 may easily be controlled by displacement of the filtrate outlet pipe 24 whereby the position of the spillway overflow 26 can be moved. In that respect, the filtrate outlet pipe 24 can for example be formed of a plurality of pipes telescopically arranged to each other.

As is evident from fig. 1, the invention according to a preferred embodiment can comprise initial perforated dewatering elements 10,12 arranged directly adjacent to sealing blades 34,36 extended from the inlet box 16, whose free ends are arranged against an inside 38,40, respectively, of the wires 6,8 in the dewatering space, whereby a first filtrate is removed to a first chamber 30, 32 of the outlet boxes 18, 20, respectively.

Thus, each outlet box 18,20 may comprise of more than one chamber for receiving of a partial filtrate, respectively, from the dewatering space 4 and the pressure control device 22 can be arranged to control the counter pressures in the chambers independently of each other. Thus, also the pressure control device 22 may for example be arranged to control the counter pressure in the chambers 30,32 independently of each other.

During operation of the twin-wire press 2 according to fig. 1, displacement of the wires 6, 8 occurs during rotation of the rolls (not shown). The fibre suspension that will be dewatered is fed to an oblong dewatering

space 4 via an inlet end 14 thereof. The fibre suspension is dewatered in the dewatering space during displacement of the wires, such that a flow of filtrate from the dewatering space 4 through the wires 6,8 and the perforated dewatering elements 10,12 is formed.

Filtrate that flows from the dewatering space through the wires is collected in the separate chambers 30,32 of the outlet boxes 18,20. The pressure difference between the dewatering space 4 and the chambers 30,32 is adjusted by applying at least a counter pressure on the portion of the flow of filtrate that is formed on both sides of the upper and lower wires 6, 8 directly adjacent to the inlet end 14 of the dewatering space. The magnitude of the counter pressure is adjusted by the location of the level 28 of the spillway overflow 26. In that respect, a relatively thick, fibre-rich first filtrate can be removed at the inlet end 14 and the built-up of the fibre web, during favourable conditions, can begin almost immediately after feeding into the dewatering space 4.

Fig. 2 shows in a view the whole twin-wire press 2 in fig.

1 according to the present invention. The twin-wire press 2 comprises three lower rolls, more specifically a drive roll 50, a guide roll 52 and a stretch roll 54. The above mentioned (fig. 1) endless lower wire 6 runs in a path around the lower rolls 50,52, 54. In corresponding way runs the above mentioned (fig. 1) upper endless upper wire 8 in a path around three upper rolls, specifically a drive roll 56, a guide roll 58 and a stretch roll 60. An upper dewatering table 62 that supports the upper wire 8, and a lower dewatering table 64 that supports the lower wire 6, forms the dewatering space 4 between the wires 6,8. Fig.

2 shows also the pressure control device 22, that has been

described above with reference to fig. 1."Press section" in fig. 2 indicates an ordinary roll arrangement according to the state of the art.