FI120979B - The forming part of a paper or board machine - Google Patents

Description

The forming part of a paper or board machine

FIELD OF THE INVENTION The present invention relates to a two-wire web forming section 5 of a paper or board machine, comprising a single-wire initial dewatering section and a two-wire zone having a dewatering box with an upper curved loop side In connection with the dewatering box, there is a first dewatering zone, wherein the dewatering lid is shaped to provide pulse-10 biblical dewatering of the pulp between the wires, and immediately followed by a second dewatering zone, wherein the dewatering fiber mass.

In the pre-dewatering dual wireformer, the pulp is fed from the headbox to the bottom wire, where it is dewatered, and then fed to the double wire zone, where it is dewatered both downstream and upstream. At the beginning of the double-wire zone, the top wire is brought into contact with the fibrous mass layer on the bottom wire. The top wire can be guided over the fibrous mass layers by means of a forming roll with an open surface. The dewatering of the forming roll is essentially non-pulsed, but a strong vacuum pulse is produced on the back side of the forming roll, which in the worst case can break the web. Solutions are also known in which the upper fabric is guided directly from the breastplate to the pulsating dewatering zone so that the upper fabric is impermeable when it encounters the surface of the fibrous mass layer. In this case, the adjustability of the upper drainage is poor.

The pulsating dewatering is accomplished by dewatering strips adapted to support the wires transverse to their direction of travel. When the dewatering rollers are located only on one side of the wire loop, the outer wire is free to flex as the thickness of the fibrous layer between the wires changes and the pressure pulses generated by the dewatering rolls remain substantially constant as the basis weight of the paper or board changes. Alternatively, dewatering strips may be provided on either side of the wire loops, alternating the direction of pressure pulses and dewatering. In order to make the magnitude of the pressure pulses remain constant, the 5 second side of the dewatering foils are usually adjustably loadable, whereby they are called loadable blades.

The use of dewatering strips and loading strips improves dewatering and improves web formation, since pressure pulses create shear forces in the fibrous mass layer that break down already formed flocs. On the other hand, pressure pulses and alternation of the drainage direction reduce the retention as the fines and fillers are leached with water away from the surface layers of the web. Strong pressure pulses increase the orientation of the fibers and thereby increase the strength difference between the machine and transverse directions of the paper. Dewatering also increases wire wear. 15 The use of load strips is limited by the thickness of the pulp layer between the wires and by large thickness variations, which means that load strips may not always be used. The loading lists typically reduce the puncture and pile strength, which is very important for certain board grades. In order to maximize strength, it is sometimes desirable to omit lists or move them to a later stage.

20

Figure 7 of WO 2004018768 shows a prior art solution in which at the beginning of the double wire zone there is a suction box on the side of the upper wire loop, which is connected to at least two successive dewatering zones. The first dewatering zone is formed by a forming shoe having a sloping cover against which the upper wire is supported, with the lower wire unsupported in the forming shoe region. The forming shoe has a substantially open surface and, by the action of a vacuum placed on the shoe, provides substantially non-pulsatile dewatering of the pulp between the wires. The second dewatering zone is formed by transverse dewatering strips on the upper fabric side, supported against the interwoven pulp, together with dewatering strips on the lower wire, which can be adjustably loaded on the wires.

Against the fibrous pulp therein, with solid dewatering at the gaps between the strips to provide a pulsatile dewatering of the pulp in the second dewatering zone.

When the biaxial zone begins with non-pulsed dewatering, retention is maintained because the fiber network formed on the surface of the pulp layer retains very fine and filler in subsequent dewatering steps where the dewatering pressure is pulsating. When making high-density cardboard or using slowly draining pulps, the non-pulsed forming shoe at the beginning of the twin-belt zone 10 may not always be able to remove enough water from the pulp so that the web immediately after the forming shoe can be transferred without dewatering. If the fiber core layer is too thick or its consistency is too low, there is a risk that the web will not withstand strong pulsation of the loading strip region but will break.

15

It has been found that by increasing the vacuum of the non-pulsed forming shoe, the dewatering capacity can be increased only to a certain extent, after which the dewatering capacity begins to decrease despite the increase in vacuum. The deterioration in dewatering is likely due to the fact that as the vacuum increases, the surface facing the fabric of the fiber-20 pulp layer becomes densified, making dewatering through the surface layer more difficult. Therefore, the effect of the vacuum should sometimes be interrupted, thereby partially restoring the thickness of the fibrous mass layer and opening the porous structure of its surface. Even relatively light pressure pulses are sufficient to open the surface structure of the fiber network so that dewatering can continue.

25

The object of the invention is to solve the problems of the prior art described above.

The invention is characterized by what is set forth in the characterization part of claim 1.

4

At the beginning of the twin-wire zone, as is known per se, there is a first dewatering zone which produces non-pulsatile drainage and a second dewatering zone which provides pulsatile drainage. In the new solution, the second dewatering zone is constructed so that the dewatering strips are only on the side of the 5 upper fabric loops, with the lower fabric free of load strips, whereby the position of the lower fabric may automatically change as the thickness of the interlayer fiber mass changes. Further, after the dewatering box, before the transfer suction box, there is a forming shoe with a curved lid on the bottom wire, which increases the drainage through the bottom wire.

10

When only the top wire is supported against the dewatering strips and the bottom wire is free from the load strips, the second dewatering zone provides only a gentle pulsation sufficient to open the fiber network surface structure so that dewatering can continue even if the web surface is at the last dewatering step.

In one embodiment of the invention, the dewatering box comprises three suction chambers, of which the last or third suction chamber is also free from the loading lists on the lower side. At the last suction chamber, the slot spacing in the machine-20 direction is small, so that relatively high vacuum can be used. This results in high dewatering pressure without separation of the wires.

In a preferred embodiment of the invention, the single-wire initial dewatering section begins on a board comprising a first dewatering zone at which the dewatering begins and a second dewatering zone providing a pulsating dewatering.

The initial drainage section can be very short, whereupon the two-wire zone begins immediately after the breastboard. Alternatively, the initial dewatering portion may be longer, comprising one or more dewatering elements disposed on the underside of the lace loop after the breast table before the start of the two wire zones.

5

The web forming part according to the invention is particularly suitable for the production of paperboard and thick paper grades. It is also suitable for the production of multilayer cartons.

5

Because the overhead wire unit has sufficient drainage capacity, the length of the flat wire section can be selected so that the ratio of up and down water discharges is 50/50. In this case, the layer purity of the web is good, since it is not necessary to remove water in one direction from each layer.

10

The invention will now be described with reference to the figures in the accompanying drawings, the embodiments of which are, however, not to be construed as solely limited thereto.

Figure 1 is a side elevational view of a portion of the web forming part of the invention.

Figure 2 shows the evolution of dewatering as a function of the vacuum applied in the non-pulsed forming shoe.

Figure 3 is a side elevational view of a portion of a web forming section having a short initial drainage portion comprising a chest provided with a suction.

The web forming section shown in Fig. 1 comprises a lower wire loop 11 which forms a single-wire initial drainage section and an upper wire loop 21 which, together with the lower wire 11, defines a double-wire zone, followed by a web 11 of a lower wire 11. wherein it is dewatered by means of dewatering elements located below the bottom wire 11 (not shown). The upper wire 21 forms a loop whose passage is guided by the guide rolls 22. Inside the upper wire loop 21 is a dewatering box 23 which guides the upper wire 21 over the fibrous mass layer. At this stage, the upper surface of the fibrous mass layer is still relatively dilute stock, while its lower surface is felted into a fibrous mesh against the lower wire 11.

The dewatering box 23 is provided with a curved lid and comprises three 5 successive suction chambers 24, 25, 26, preferably with different vacuum pressures. By the action of the suction chambers 24, 25, 26, at least two consecutive dewatering zones ZBi and ZBn are formed in connection with the dewatering box 23, the first being non-pulsed and the second being pulsating. The first suction chamber 24 is provided with a cover structure 27 which provides a pulsatile dewatering of the pulp between the wires 11, 21. The lid 27 has a large open area and the first dewatering zone ZBi is connected to the vacuum in the suction chamber 24 via the openings in the lid. The structure of the cover 27 is characterized in that it avoids transverse, uniform and elongated slits of the machine which would create a pulsating dewatering pressure. Cover structures providing non-pulsatile drainage are described, for example, in US 6,372,091 and WO 2004018768.

At the second and third suction chambers 25, 26, the cover of the dewatering box 23 is formed by dewatering strips 28 transverse to the machine direction, which spring 20 overlies 21. There are gaps between the slots 28 through which the second dewatering zone ZBn communicates with the vacuum in the suction chambers 25,26. On the side of the bottom wire 11, there are no moldings limiting or controlling the movement of the bottom wire 11, which allows the bottom wire 11 to move flexibly in the height direction. The distance between the wires 11 and 21 is then directly dependent on the thickness of the fibrous mass layer therebetween. The dewatering strips 28 provide only cautious pressure pulses of substantially constant size, regardless of the thickness of the pulp layer between the wires 11 and 21. The solution is ideally suited for the manufacture of thick board grades. At the third suction chamber 26, the dewatering strips 28 are denser and have a higher vacuum than the second suction chamber 25.

7

Following the dewatering box 23 is further followed by a forming shoe 16 with a curved strip cover on the side of the bottom wire loop 11 and two suction boxes 17 which serve not only to dewater the web but also to ensure that the web two downstream of the wire zone.

5

Figure 2 illustrates the development of dewatering in the area of unpulsated dewatering zone ZBi as a function of vacuum in suction chamber 24. The upper curve represents the desired dewatering and the lower curve represents the actual dewatering when fabricating the web from hard-to-drain masses or high ne-10 masses. The figure shows that as the suction increases, the dewatering does not always increase as expected, but after a certain maximum value, the dewatering begins to decrease (vi-novated area). This is due to the fact that the surface of the web is compacted by the effect of vacuum, which makes it more difficult to drain water through the dense surface layer. The compression of the fiber network can be reduced by applying to it the pressure pulses produced by the dewatering strips. To achieve this, the web forming unit according to the invention has a non-pulsated dewatering dewatering zone, followed by a pulsation dewatering zone, in which the thickness of the fibrous mass layer is not limited by loading strips.

Figure 3 shows an alternative solution in which the double wire zone is preceded by a very short initial drainage section. Initial dewatering takes place on a chest 13 which is provided with a vacuum and comprises two successive dewatering zones ZAi and ZAii. The dewatering begins in the area of the first dewatering zone ZAi and continues in the second dewatering zone ZAn, where the cover 25 of the chest 13 is formed by transverse dewatering strips 15 which provide a pulsating dewatering of the pulp on the lower wire 11.

The dewatering box 23 corresponds to the dewatering box 23 described above in connection with Figure 1, comprising a first non-pulsed dewatering zone ZBi and a second pulsed dewatering zone ZBn, which is further divided into two parts having different vacuum pressures and mold densities.

8 The goal is to provide a good filtering fiber layer on each surface of the web at the beginning of the dewatering process, which allows water to pass through but retains fine and filler material. By the time the fiber mass layer enters the twin-wire zone, its lower surface 5 is already felted as a fibrous mesh, while the upper surface still has a dilute fiber stock. The first dewatering zone ZBi of the dielectric zone efficiently removes water, whereupon the fiber web layer is felt on the surface of the fibrous mass layer opposite the upper wire 21. In the first suction chamber 24, a relatively low vacuum, for example 1-15 kPa, can be used. The vacuum in the second suction chamber 25 is preferably 5 to 15 kPa and at that point the dewatering strips 28 are still relatively sparse. The vacuum in the third suction chamber 26 is of the order of 20-25 kPa and the dewatering strips 28 are spaced more frequently than at the second suction chamber 25.

Preferably, the length of the flat wire section from the point of the headbox lip jet to the beginning of the double wire zone is less than 3 meters. Thus, the consistency of the upper surface of the fibrous mass suspension is low and the properties of the upper surface of the web can still be effectively influenced in the double-wire zone. 1

Many variations of the invention are possible within the scope defined by the following claims.

Claims (5)

9 A paper or board machine two-wire forming section comprising a single-wire starter drainage section and a two-wire zone having a dewatering box (23, 123) with a curved lid on the side of the upper wire (21) to guide the upper wire (21) into contact with the lower wire. a dewatering box (23) having a first dewatering zone (ZBi) in which the dewatering lid (27) is shaped to provide pulsatile dewatering of the pulp between the wires (11, 21), and a second dewatering zone (ZBn), wherein the lid of the dewatering box consists of transverse dewatering strips (28) providing pulsatile dewatering of the pulp between the wires (11, 21), characterized in that the second dewatering zone (ZBn) 28) against the bottom wire (11) being 15 unsupported, whereby that is, the lower wire (11) is free to flex as the thickness of the fibrous mass layer between the wires (11,21) changes, and that after the dewatering box (23), the forming wire (16) has a curved lid on the lower wire loop (11). 1 2 3 4 5 6 7 8 9 10 11 A web forming section according to claim 1, characterized in that the dewatering box (23) comprises three suction chambers (24, 25, 26), of which the first suction chamber (24) and its lid (27) form the first dewatering zone (ZBi). ) and the second and third suction chambers (25, 26) and their dewatering strips (28) on the cover 5 form a second dewatering zone (ZBn). 6 7 A web forming section according to claim 1 or 2, characterized in that at the beginning of the 8 single-wire initial drainage section there is a chest table (15) comprising 9 first dewatering zones (ZAi) within which dewatering is adjustable by vacuum and a second dewatering zone (ZAn). which provides a pulsating dewatering of the pulp of fiber on the bottom wire (11). 10 A forming unit according to claim 3, characterized in that the two wire zones begin immediately after the breast table (15). A web forming part according to claim 3, characterized in that after the rin-5 pin table (15) there is at least one dewatering element before the twin-wire zone. 11