FI93872C - Method and apparatus in the web forming part of a paper machine - Google Patents

Description

93872

FIELD OF THE INVENTION force field zone.

10

The invention further relates to an apparatus for improving the formation of a web produced in a two-wire forming zone of a paper machine forming section without impairing its retention, which apparatus comprises forced guide members opposed inside both wire loops by which the two-wire web forming zone is forced in a direction of varying direction.

As is already known, in the two-wire zone of paper machine twin-wire formers or hybrid formers, pressure pulses and shear forces are applied to the fiber suspension layer between the wires by various means, e.g. forming strips, with the aim of simultaneously improving the web formation and dewatering the web. However, the disadvantage of said devices is that, although they provide good formation, the retention deteriorates because the web additives, fillers and fines escape from the web with the water leaving it.

Another disadvantage of the known formers is that they have to use a relatively low feed consistency of the pulp suspension (~ 0.7%), which results in rather large headbox flows. This in turn causes relatively high energy costs as well as problems in keeping the paper machine clean.

It is already known in both flat wire formers and twin wire formers to limit dewatering at the beginning of the forming zone by using various limited water permeable strip members acting against the inner surface of the wire or wires and running parallel to the wire (s). With these strip members, dewatering has not been completely prevented 2 93872, and no substantial formation improvement can be achieved in their area. With regard to the above-mentioned prior art, reference is made, by way of example, to the applicant's public FI patent application 850321, J.M. Voith GmbH's FI patent application 1220/73 (cf. DE application).

2,219,302) and U.S. Patent 3,578,559.

5

The object of the present invention is to provide a new web-forming method and a device with which the disadvantages discussed above can be avoided and good formation and in particular a smooth structure in the Z-direction of the web can be realized while maintaining a good retention level.

10

It is a further object of the invention to provide a web forming method and apparatus in which dewatering and improving the formation are separated into their own separate steps so that the final dewatering can be performed using even less pulsation and turbulence, which is intended to improve web retention.

15

It is a non-essential additional object of the invention to provide a web-forming method and apparatus in which even lower headbox flows, i.e. higher headbox densities, can be used and thus considerably saves energy costs, especially pulp pumping energy costs.

20

In order to achieve the above and later objects, the method of the invention is mainly characterized in that in order to improve the formation of the web to be formed without impairing its retention in the above zone, dewatering from the web is substantially prevented through both wires .

The device according to the invention, in turn, is mainly characterized in that said forced guide members are opposed to each other in a substantially identical shape in the direction of travel of the web and only separated from each other by the wires and the paper web.

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The invention has resolved the contradiction between prior web forming methods and apparatus which has prevailed between good formation by strong pressure pulses applied to the web and poor retention due in part to them. In the invention, the formation improvement step is separated from the actual dewatering step or 5 steps, with which both good retention and good formation can be realized. After the formation improvement step according to the invention, the dewatering of the web is carried out with a low pressure pulsation or even a uniform dewatering pressure so that the already achieved level of formation is not deteriorated.

In the method and apparatus according to the invention, a forming step is used in the two-wire zone, during which dewatering through both wires is essentially prevented. At this stage, dynamic shear forces are applied to the resulting paper web, which is obtained by guiding the forming wires and the path between them a winding path, the changes in direction of which cause a dynamic oscillating acceleration field and thus a formation-improving small-scale turbulence in the path between the wires.

It should be emphasized in this connection that the invention does not in fact seek to increase or control dewatering of the web, but to smooth the Z-direction structure of the formed paper web and to break up any fibrous flocs in the web into softness while substantially preventing dewatering.

The invention provides a remarkably large, mainly Z-direction small-scale turbulence in the fiber suspension layer by a force field oscillating in the two-wire zone, which smooths the substantially Z-direction structure of the web and improves the formation.

The formation improvement area according to the invention is adapted to the area of the two-wire zone in which the dry matter content ka of the pulp web is in the range ka <2 ... 10%. Optimal conditions for the invention are achieved in the dry matter range of the track by about 3 ... 6%.

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The length L of the dewatering barrier and the formation improvement area used in the invention in the direction of travel of the track is generally in the range L «50 ... 1500 mm and optimal results are achieved with a length L» 70 ... 300 mm.

One way to consider the effect of the formation region of the invention is to determine the frequency at which accelerating forces of varying directions are applied to the mass trajectory between the wires and at which the force-enhancing force field vibrates. This frequency f = 1 / T, where T is the residence time of the wires and the web at one wavelength λ. Said frequency f in the invention is generally selected from the range f = 50 ... 4000 Hz. The above-mentioned frequency range is in the middle range of audio frequencies. These vibrational frequencies of the force field are so high that they produce a sufficiently small-scale, mainly Z-direction turbulence, which is particularly effective in improving the formation and achieving the flatness of the Z-direction structure of the web. Em. the frequency range f = 50 ... 4000 Hz corresponds to the wavelength range λ = 2.5 mm ... 500 mm of the oscillating force-15 in the web speed range v = 10-25 m / s with small amplitude values.

In the following, the invention will be described in detail with reference to some application examples of the invention shown in the figures of the accompanying drawing, to the details of which the invention is in no way narrowly limited.

20

Figure 1 shows a two-wire zone vertical guitar former with a • forming member according to the invention after the forming roll and then a second forming roll.

Fig. 2 shows a two-wire former corresponding to Fig. 1, in which the forming member according to the invention is arranged in the region of the forming crystal.

Figure 3 shows a hybrid former in which the forming member according to the invention is applied at the beginning of the two-wire zone.

30 5 93872

Figure 4 schematically shows a machine direction vertical cross-section of a forming element according to the invention and the essential parameters of its dimensioning.

Fig. 5 schematically shows a machine direction vertical section of a forming member according to the invention 5, in which two dewatering barrier belts and alternating static strips guiding them are used.

Fig. 6 shows a device corresponding to Fig. 5, in which the static strips of Fig. 5 have been replaced by rotating guide rollers.

10

Figure 7 shows a device with stationary wire guide members with two opposite closed guide surfaces.

Figure 8 shows a static forming member according to the invention arranged in the region of a forming crystal.

Figure 1 shows a schematic side view of a web forming section according to the invention comprising a first wire loop 10 and a second wire loop 20 and a first forming roll 21 and a second forming roll 25 and a breast roll 11. The forming rollers 21 and 20 25 are each inside the wire loop 20 and the breast roll 11 is inside the wire loop 10. The wire 10 is a so-called covering wire and wire 20 ns. a load-bearing wire followed by a web W • after a two-wire forming zone. From the lip portion 18 of the headbox 17, a pulp jet J is directed by the wires 10 and 20 to delimit a forming roll K formed by the position of the rollers 11 and 21. K jaw restrict the other side of the wire 25 to 20, which is guided by the guide roller 26 tangential to the first forming roll 21 and on the other side of the wire 10 which is passed over the breast roll 11 face 21 of the forming wire <20, however, the fiber suspension layers separated. In Fig. 1, the forming roll 21 is a roll provided with a hollow surface 2Γ. The common coverage sector a of the wires 20 and 10 in the region of the roll 21 is a <40 ° -60 °. The dewatering in the region of the forming roll 21 takes place in two directions, partly through the wire 10 away from the forming roll 21 and partly through the wire 20 either into the cavities of the surface 21 of the roll 21 and / or into the suction chamber 21a inside the roll 21 shell.

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A forming member 40 according to the present invention is placed in the two-wire zone between the first and second forming rolls 21 and 25, the structure and operation of which will be described in more detail later. Inside the wire loop 20, a suction box 24b or the like is placed downstream of the forming member 40 to supplement dewatering. The final dewatering with the two-wire section 5 is achieved by the forming roll 25 mainly in its suction sector 25a and 25b, after which the wires 20 and 10 differ from each other. As the formed web W follows the carrier wire 20, the web W advances at the support P of the wire 20 to a point P where it is detached from the wire 20 by the suction zone 30a of the pick-up roll 30 and transferred to the pick-up fabric 31 which takes the web W to the press section of the paper machine 10 (not shown).

The forming member 40 shown in Figs. 1 and 4 comprises a guide portion 41 inside the loop of the covering wire 10 having a corrugated guide surface 43 and a second guide portion 42 opposite the guide portion 41 inside the loop of the supporting wire 20. This guide part 42 comprises a corrugated guide surface 44. The guide surfaces 43 and 44 are of the same shape and opposite to each other, separated by the wires 10,20 and the track W. The guide portions 41 and 42 are pressed against the wires 10,20 so that the guide surfaces 43, 44 of the forming member 40 forcibly guide the two-wire zone and the web W passing therebetween with a small amplitude A wavy path. At the same time, the closed guide surfaces 43 and 44 20 prevent dewatering in the area of the forming member 40. In this connection, it should be emphasized that when we refer to the formation member 40; 50 in the following, it means: a member which, by means of an oscillating force field, provides the web W with both turbulence to improve its formation and dewatering by the same member.

Fig. 2 shows a double-wire former otherwise as shown in Fig. 1, except that the chest roll 11 and the forming roll 21 have been replaced by a forming member 50 according to the present invention, which further acts as a guide device for guiding the wires 10,20 to form K and a wavy path in its region. in the same manner as in Fig. 1 in the region of the forming member 40. In addition, in Fig. 2, after the forming member 50, inside the wire loop 20 30, a forming shoe 24a curving a two-wire zone with a large radius of curvature R has a strip cover 24c and then a planar box 24b. In Figure 2, the forming member 50 comprises two guide portions 51 and 52, the former 51 being within a loop of the covering wire 10 and the second 52 being within a loop of the supporting wire 20. The guide portions 51 and 52 have corrugated closed guide surfaces 53 and 54 which forcibly guide the wires 10,20 in the jaw K and thereafter. In addition, the guide portions 51 and 52 have curved guide surfaces 53a and 5 54a which correspond to the chest or forming rollers leading the wires 10,20 to the forming roll K.

Figure 3 shows an embodiment of the invention as a hybrid former comprising a lower wire 20 having a single wire initial portion 20a to which a pulp suspension jet J at the breast roll 26a is fed from the lip slot 18 of the headbox 17. The horizontal flat wire section 10 20a has suction boxes 27a and 28a and foils 27b. The single-wire flat wire section 20a is followed by a two-wire zone bounded from above by the upper wire 10 of the upper wire unit 16, which is guided by the guide rollers 15. The two-wire zone begins at the open forming roll 11a, followed by a forming member 40 according to the invention, similar to that described above in connection with Figure 1, comprising two opposite guide portions 41 and 42. This is followed by a pivot roller 29 and flat boxes 28b inside the lower wire 20 loop. At the guide roll 14 of the upper wire 10, the upper wire 10 differs from the web W following the lower wire 20. The web W is transferred at the pick-up point P in the suction zone 30a of the pick-up roll 30 to the pick-up fabric 31 which takes it to the press section (not shown).

20

The formers shown in Figures 1, 2 and 3 above, with the exception of the formation members 40; 50, are essentially known in advance and are described here only as some application environments of the invention.

Figure 4 illustrates a more detailed embodiment of the forming member 40 in machine direction vertical section. The guide portions 41 and 42 of the forming member 40 have e.g.

«Substantially sinusoidally curved, uniformly closed, low-friction guide surfaces 43, 44 which forcibly guide the masses 10 and 20 and the mass path between them into a substantially sinusoidal path, causing dynamic acceleration forces of varying direction and oscillation in the mass 30 between the wires 10,20, preferably substantially sinusoidally, improving the formation and breaking the fluff of the fibrous web W fiber 93872 8. As described above, a force field oscillating mainly in the Z direction of the web W is obtained, the oscillation frequency of which is quite high, usually in the middle range of the audio frequencies. In this way, a sufficiently strong small-scale turbulence is obtained in the web W, which is mainly in the Z-direction of the web, i.e. in the direction of the vibrations of S. It is also essential that, in the region of the oscillating formation zone F, dewatering through both wires 10,20 is substantially prevented.

According to Fig. 4, the upper guide part 41 is fixedly connected to the body parts of the paper machine (not shown) and the lower guide part 42 is pressed against the opposite guide part 41 10 by actuators 48a and 48b with, for example, hydraulic cylinders or pneumatic beams. When adjustable pressures are applied to the actuators 48a and 48b between the fixed body portion 49 and the lower guide portion 42, the compression pressure between the wires 10,20 of the vibrating formation zone F, in which dewatering is prevented, can be adjusted. Also, the pressure distribution in the direction of travel of the wires 10,20 can be adjusted by applying different pressures to the actuators 48a and 48b.

In Fig. 4, the wavelength of the forced-controlled oscillations in the formation zone F is denoted by X and the wave amplitude by A (wave height from bottom to peak = 2A).

In the present invention, the length of the formation zone F shown in Fig. 4 is wires. 10.20 in the direction of travel L is in the range L «50 ... 1500 mm, preferably • L« 70 ... 300 mm. The wavelength λ is generally in the range λ «2.5 mm ... 500 mm, preferably λ« 10 mm ... 200 mm and said amplitude A »0.01 mm ... 10 mm, preferably A« 0.1 mm. .. 2 mm. Depending on the paper quality and speed, the oscillation frequency f = 1 / T <50 ... 4000 Hz, preferably f «1000 ... 2500 Hz, for the formation area F, where T = travel time of the wires 10,20 and the web W over one wavelength λ, i.e. * eg from peak to peak of a wave. In Fig. 4 L «2 x λ and in the invention in general L« N x λ, where N is the wavelength at the length L of the formation region F; usually N ~ 1 ... 100, preferably N «3-8.

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Formation member 40 according to the invention; 50 is generally placed in an area in the two-zone zone where the dry matter content ka of the web W is in the range ka »2 ... 10%. Optimal applications of the invention are generally obtained in the dry matter range of about 3 to 6%. In the embodiment of Fig. 2 ka »2 ... 4% and in the embodiment of Fig. 1 ka» 2 ... 5%, and in the embodiment of Fig. 4 ka ka 2-5%. The solution according to Figure 2 is also advantageous in that, when applied, the headbox can be driven with a fairly high consistency, e.g. ka »2% instead of the previous ka» 0.7, so a considerable amount of pumping energy is saved.

Figures 5-8 show a schematic machine direction vertical section of some preferred embodiments of the invention. According to Figure 5, a belt loop 61 is traced inside the loop of the upper wire 10, which is a substantially, or preferably completely, water-impermeable low-friction belt. The belt 61 is guided by guide rollers 63a and 63b over the upper strips 45a. The strips 45a are supported on a fixed body 47a. Inside the loop 15 of the lower wire 20 is arranged a low or impermeable low-friction belt 62 corresponding to the strip 61, which is guided by the guide rollers 64a and 64b against the inner surface of the wire 20 and over the lower strips 46a. The lower strips 46a are supported by springs 48 or similar pressure-loaded actuators in the fixed body part 49. In Fig. 5, a formation zone and a dewatering blocking area F are formed between the belts 61 and 62, which is undulating by a small amplitude due to forced control of alternating strips 45a and 46a. The strips 45a and 46a extend over the entire transverse width of the web W and the wires 10,20.

• ·

In some cases, the strips 61 and 62 may be slightly water permeable, but most preferably they are completely impermeable low friction belts that can be provided with a drive if necessary.

25

The forming member 40b shown in Fig. 6 differs from that shown in Fig. 5 in that the dewatering belt loops 61 and 62 are guided by a wavy path by guide rollers 45b and 46b, of which upper rollers 45b are mounted on a fixed body 47b and lower rollers 46b are alternately spaced with upper rollers 45b. springs 48 or 30 with corresponding power devices to be loaded against the inner surface of the lower wire 20 to guide the two-wire zone and the web W therebetween to form a gently undulating track 10 93872 to form a formation zone and a dewatering barrier area F. When using the forming member according to the invention, the tension T of the dewatering barrier strips 61 and 62 can be used as one of the control parameters. The lower the tension T of the strips 61,62, the smaller the transverse force, e.g. the force of the springs 48, gives a certain wave amplitude A of the formation region F.

Figure 6 shows, before the forming member 40, a forming roll 21b arranged inside the lower wire loop 20 and then a forming shoe 24a, the open strip cover 24c of which guides the two-wire zone with a straight radius of curvature R.

10

The dry matter content kaQ of the web before the formation zone F is preferably kaQ ~ 2 ... 5%.

Fig. 7 shows a stationary forming member 40c substantially similar to that shown in Fig. 4, in which the length of the formation zone is L »3 x λ. The lower guide portion 42 is loaded with springs 48 or the like with a suitable force against the inner surface of the wire loop 20 and thereby against the upper guide portion 41. The strips 61 and 62 used in Figures 5 and 6, which are dewatered in the formation zone F, are preferred in that they prevent the wires 10,20 from wearing in the formation zone F. The strips 61 and 62 can furthermore be made relatively short in loop and quick to change.

• ·

Figure 8 illustrates a stationary forming member 50 according to the invention arranged in the region K of the forming crystal. The forming member 50 comprises a guide portion 51 arranged inside the loop of the upper wire 10, which has a closed guide surface 53 and a steplessly connected guide surface 53a which acts as a breast roll. In Fig. 8, said surface 53a is shown in broken lines, if necessary, to be replaced by a rotating chest roll 57. The forming member 50 includes a loadable guide portion 52 inside the lower wire loop 20 having a corrugated guide surface 54 and an associated guide surface 54a guiding the lower wire 20 to form the forming roll. The guide member 52 is loaded with springs 55 or the like against the fixed upper guide member 51 using the fixed body portion 56 as a support member.

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The forming grating arrangement shown in Fig. 8 is also advantageous in that the guide surfaces 53a and 54a can be made to have a relatively small radius of curvature, so that the lip opening 18 of the headbox lip cone 17 can be brought very deep into the forming gravel K. This has the advantage of free travel M , whose lip shower without guidance and support flies in the air before coming into contact with the wires 10,20, is made even shorter. When the forming member 50 according to the invention, which at the same time replaces the breast rolls, is used, said free flight distance is M <100 mm, while in normal formators with forming and / or breast rolls it is of the order M <250-300 mm, which is a long distance for the stability of the lip jet J 10.

It is also essential for the operation of the forming member 40; 50 according to the invention that the formation improvement caused by the forming member is maintained in the subsequent dewatering steps, which requires a certain dry matter content from track W after the forming zone F and / or dehydration caution in later dewatering steps. mainly maintained. The above means that after the formation zone F according to the invention, in the area of which dewatering is prevented, the dewatering is carried out with a sufficiently lower pulsation or even a constant pressure, since a sufficient level of formation has already been reached in the previous formation steps.

Finally, it should be emphasized that although paper and paper web have been discussed above, the invention is also suitable for use in the manufacture of paperboard.

The following claims, within the scope of the inventive idea defined by it, the various details of the invention may vary and differ only from those exemplified above.

Claims (20)

A method in a paper machine, with its former removing water from the fiber layer to be formed into a paper web (W) pressed between two opposite wires 5 (10,20), in which method the web forming zone (10; W; 20) with double white forceps on a changing direction path, which directional changes provide a vibrating force field zone (F) in the fiber layer formed, characterized in that to improve the formation on the web (W) to be formed without retarding the dewatering, the dewatering by both wires ¢ 10.20) in zone 10 (F) of the web (W), and that forced control is carried out in the direction of flow of the paper web by uniformly opposed, generally closed guide surfaces. Method according to Claim 1, characterized in that the double-wire zone is forced along a slightly undulating, most preferably substantially sinusoidal path, at least over a path length (λ). Method according to claim 1 or 2, characterized in that the solids content ka of the web (W) formed in force field zone (F) is in the range ka «2% ... 10%, most preferably in the range ka« 2% ... 6%. 20 Method according to any of claims 1-3, characterized in that the length L of the force field zone (F) in the direction of flow of the wires (10,20) and the web (W) is in the range L ~ 50 ... 1500 mm, most preferably L »70 ... 300 mm. Method according to any of claims 1-4, characterized in that the vibrational power f of the dynamic forces of the force field zone (F) is chosen depending on the paper quality and the speed (v) of the paper machine in the range f = 50 ... 4000 Hz, most preferably in the range f = 1000 ... 2500 Hz, where f = 1 / T and T = the delay time of the wires (10.20) over a distance of a path length (λ). 17 93872 A 0 0.1 ... 2 mm and that the path length λ and / or that the ratio of the amplitude λ / A is selected in the range λ / Α = 20 ... 2000, most preferably in the range λ / Α = 50 ... 1000 . Method according to any of claims 1-5, characterized in that the path length λ of the vibrating force field zone in the method is selected in the range of λ »2.5 ... 500 mm, most preferably in the range of λ« 10 ... 200 mm and the amplitude. A on the road is selected within the range A «0.01 ... 10 mm, most preferably within the range Process according to any of claims 1-6, characterized in that in the process a substantial amount of water is removed from the paper web (W) to the single wire zone 10 (20a), with the forming roller (21), with the curved forming shoe (24a). ) and / or with other corresponding dewatering elements, after which the double-wire zone is guided through a vibrating force field zone (F) after which water is removed from the web (W) so that a substantially improved formation level on zone (F) is maintained. 15 Method according to any one of claims 1-6, characterized in that the method is applied to the gap area (K) of a double twist gap former in such a way that a pulp suspension jet (18) is fed from the lip opening (18) of the inlet carriage (17). J) to the forming gap (K), on whose area the zone (10; W; 20) is double-guided along a road-shaped vibrating path (Figures 2 and 8). 20 9. A method according to claim 8, characterized in that the thickness ka of the pulp suspension measured in the process for the forming gap (K) is larger than average and selected in the range ka «1 ... 4%, most preferably in the range ka« 2 ... 3%. 25. 10. Device on a double-wire forming zone in the forming portion of a paper machine to improve formation »! on a web produced without impairing its retention, which apparatus comprises forced guide members (41,42,43,44; 45a, 46a; 45b, 46b, 61.62; 51,52,53,54) within the two the wire loops 30 (10, 20) with which the double guide web forming zone (10; W; 20) is double guided along a path of changing direction, characterized in that the double guide means (41, 42, 43, 44; 45a) 46a; 45b, 46b, 61.62; 51.52.53.54) are substantially uniform to each other in the running direction of the web and are separated only by the wires (10 and 20) and the paper web (W). 11. Apparatus according to claim 10, characterized in that, by means of control means, at least those which are inside one of the wire coils are provided with spring or loading devices (48a, 48b, 48) which are substantially perpendicular to the main running direction of the zone with double wire presses on the control means (43; 53; 45a, 45b, 61) inside the wire loop (10) opposite the control means or control means (44; 54; 46a, 46b, 62) with a definite or controllable load force or forces. 12. Device according to claim 10 or 11, characterized in that the device is arranged in a double-wire zone after a single or double-wire dewatering zone. 15 Device according to claim 10 or 11, characterized in that the device is arranged in the area of a forming gap (K) of a double-wire gap former (Figures 2 and 8). Device according to any one of claims 10-13, characterized in that the device comprises two opposite control means (41,42,51,52) for the wires having a compact, most suitable road-shaped forced guide surface (43,44; 53,54) for the wire, which is of uniform shape in both guides (41, 42), in the area of which the surface dewatering through both wires is substantially prevented (Figures 1,2,7 and 8). 25 Device according to any one of claims 10-13, characterized in that the device comprises load strips (45a) arranged within one of the winding loop (10), most preferably evenly in the forward direction of the dual wire zone, the device further comprising load strips (46a) on the inside of the opposite wire loop (20), which are arranged between load strips (45a), most preferably at the center, that substantially impermeable strips (61, within) are provided for the two wire loops (10, 20). 62) which is guided by articulated rollers (63a, 63b and 64a, 64b), which load strips (45a, 46a) control and press against the inner surfaces of the wires (10,20) by guiding the wires (10,20) along a weak roadway. Device according to claim 15, characterized in that said load strips (45a, 46a) are replaced by correspondingly arranged rotatably stored articulated rolls (45b, 46b) (Figure 6). Device according to any one of claims 10-16, characterized in that 10 of guide means (41, 42), load strips (46a) and / or rollers (46b) are at least those which are arranged on the inside of one wire loop (20). to be loaded in a resilient manner (48) or by force devices (48a, 48b) against the opposite wire (10,20) or the belt (61.62). Apparatus according to any of claims 10-17, characterized in that formation means (40; 50) are arranged in the region of the double-wire zone, where the dry matter content ka of the web (W) is in the range ka ~ 2 ... 20% , most preferably 13% ... 6% 19. Device according to any one of claims 10-18, characterized in that the length L of formation means (40; 50) in the main direction of the advancement of the double * zone * is L 50, 1500 mm, most preferably L 70. .300 mm. Device according to any of claims 10-19, characterized in that in the device the formation zone and the drainage zone for the dewatering are guided along a slightly road-shaped, generally sinusoidal road-shaped path, the path length λ of which path is within the range λ «2, 5 ... 500 mm, most preferably within the range λ »10 ... 200 mm and the amplitude A of the roadway is within the range A« 0.01 ... 10 mm, most preferably within the range A 1 0.1 ... 2 mm and / or that depending on paper quality and the speed of the paper machine 30, the length of time λ is thus dimensioned that the delay time T of the double-wire zone at a distance of a path length λ corresponds to the frequency f »1 / T« 50 ... 4000 Hz, 100 .. .2500 Hz. The double wire at said distance of said path length λ corresponds to the frequency f «1 / T« 50 ... 4000 Hz, most preferably f «100 ... 2500 Hz. 5 1