EP0438681B1 - De-watering apparatus for a twin-wire former - Google Patents

Abstract

The dewatering apparatus for a twin-wire paper machine is constructed in such a way that, after the two wires (1, 2) are first brought together at a forming cylinder (7), a uniform pressure is exerted on the suspension layer between the wires. The two wires then run past stationary dewatering strips (7 or 10) which may be both in the upper and in the lower wire. In this process, reduced pressure is applied at least at the strips (7) in the upper wire (1). There then follows at least one further dewatering element (11) before the two wires are separated again. The dewatering apparatus can be used both downstream of a preliminary dewatering section, for example a long wire, or in the immediate vicinity of a headbox. <IMAGE>

Description

The invention relates to a twin wire former according to the preamble of claim 1.

Twin-wire formers of this type can be provided with a pre-dewatering section, in which care is taken to ensure that the suspension coming from the headbox is guided over a certain distance on a wire, with some of the water contained in the suspension already entering the twin-wire former. in particular downwards - can be led out. The pre-dewatering section is then followed by the twin-wire area, the pre-dewatered suspension generally remaining on the wire and the twin-wire former being formed with the addition of a further wire.

The prior art on which the invention is based can be found in EP 0 296 135 A2. It describes a twin wire former with a one-way pre-dewatering section in front of a forming cylinder, which is followed by stationary forming bars on both screens. In order to achieve a sufficient effect, the molding strips must be pressed hard, which can damage the formation and increase wear.

Another twin-wire former with an open, rotatable forming cylinder initially located in the top wire, then a non-rotating drainage section and a further open, rotatable forming cylinder following this is known, for example, from European Patent EP 0 101 709. In this device, all measures are taken in such a way that the drainage in the area of the twin wire is predominantly upward. In other embodiments, which are described, for example, by the patent US Pat. No. 4,414,061, only a part of the water is discharged through the drainage section located in the top sieve, while in front of and behind it Drainage element arranged in the lower sieve, generating negative pressure drainage elements, which can be either circumferential or stationary, some of the water is drawn off downwards. Yet another embodiment is known from the applicant's German patent number 31 38 133. There it is proposed to arrange deflection elements in the top and bottom screen area for the entire dewatering taking place in the twin-screen area, so that the screens passed between them can form in an expedient form.

EP-A-0 296 135 proposes deflectors on both sides, which are adjustable towards the sieve depending on the requirements for drainage.

Despite the large number of known devices of the type of the subject matter of the invention, it has not hitherto been possible to solve all the problems which arise from the fact that various requirements must be met in the area of this device, which can at least partially contradict one another: firstly, a structure that is as homogeneous as possible the solidifying fibrous web should be formed (the formation should be optimal), secondly, the fine material present in addition to the fibrous material in the fibrous web should remain in the fibrous web to the greatest possible extent (highest possible retention), although due to its size and nature with the Thirdly, the course of the concentration of the fine substance, viewed over the thickness of the web, should have a desired shape, and fourthly, a considerable proportion of the water contained in the suspension should be removed.

The object of the invention is to achieve the above-mentioned objectives in a manner which is as favorable as possible to one another and to keep the structural and operational outlay for the device as low as possible.

These goals are fully achieved by the measures mentioned in the characterizing part of claim 1. Further features of the twin wire former are specified in dependent claims 2 to 22.

The device according to the invention is particularly suitable for a desired and partly intensive pressure treatment to improve formation and generally to ensure a higher drainage performance. This is made possible, among other things, by first pulsation-free, uniform drainage over a short distance, eg 200 mm, and then by subsequent drainage with frequent reversal of direction. In the area of the drainage section which is not running in the top wire, the distance between the successive strips is advantageously kept relatively small, for example between 10 and 60 mm, in order to frequently reverse the direction of the drainage flow.

As a division for successive strips in the drainage sections, amounts between 30 and 130 mm are generally advantageous.

Because the dewatering section located in the upper sieve can be subjected to negative pressure, the mechanical pressure of the strips on the sieves can remain relatively low with the same dewatering performance, as a result of which the formation can be advantageously influenced. The pressure of the strips on the sieves leads to a fiber orientation in the machine direction. This increases the tear length ratio lengthways / crossways, which can be undesirable depending on the type of paper.

If pulsations of the same intensity are generated on both sides and they start at about the same time, the filler is removed relatively strongly from the peripheral layers of the suspension. In this way, a desired course of the filler content seen over the sheet thickness can advantageously be set, e.g. such that the filler content is greater in the middle of the sheet than on the top and bottom. A sheet so produced is e.g. well suited for printing in offset printing. Paper or cardboard of this type have a particularly high surface strength. With a corresponding adjustment of the pulsations, the production of a symmetrical sheet is also possible.

Further advantages result if the drainage element located in the top sieve is divided into different zones with different pressures.

At the beginning, an "auto-slice" (registered trademark) can be advantageous, which sucks off the water in and above the top wire and, especially when it is attached close to the forming cylinder, achieves effective dewatering even at a relatively low wire speed.

In the case of strips located in the lower sieve, for example resiliently fastened in the pressing direction or adjustable with particular advantage, the achievement of an optimal formation is effectively supported. The strips can be adjustable individually or in groups relative to the bottom wire. If necessary, it can be provided to apply underpressure or overpressure to the spaces between the strips. In cooperation with the strips of the drainage section present in the top wire, there is a frequent reversal of the drainage direction in the fiber suspension.

If, on the other hand, the lower dewatering section is formed without stationary parts touching the lower sieve, then when the two merged sieves pass the arch-shaped forming shoe located in the upper sieve, a force-controlled controllable contact takes place, with which changes in basis weight changes in the included suspension layer are flexible can be reacted to. The adjustment of counter strips is not necessary.

Another advantage of the subject matter of the invention is the relatively short length of the drainage sections when viewed in the direction of the wire. Because of the very intensively dewatering area that adjoins the first forming cylinder, the wrap around the first forming cylinder can be kept very low, which in turn serves good formation.

Also, on the strips of the drainage section located in the top wire, due to the shortness of this area, there is a steady and constant width across the web White water flow is present, so that contamination from deposits from the white water is almost impossible. A desired streak-free paper can thereby be produced. The sheet formation is completed by at least one subsequent further dewatering element.

Basically, the subject of the invention enables a variety of control options, e.g. due to good accessibility and - if available - good adjustability of the lower strips, which enables flexible adaptation of the drainage device to many conceivable operating states.

Fig. 1

schematische Darstellung einer nicht beanspruchten Entwässerungsvorrichtung, eingesetzt nach einer Langsieb-Vorentwässerung;

Fig.2-4

schematische Darstellungen des Zusammenwirkens der Leisten des unteren Entwässerungsabschnittes mit den Leisten des oberen Entwässerungsabschnittes;

Fig. 5

schematische Darstellung eines erfindungsgemäßen GAP-Formers;

Fig. 6

eine andere Ausführungsform eines erfindungsgemäßen GAP-Formers;

Fig. 7

eine nicht beanspruchte Entwässerungsvorrichtung ohne stationäre Leisten im Untersieb;

Fig. 8

eine andere Ausführungsform eines erfindungsgemäßen Gap-Formers ohne stationäre Leisten im Untersieb.

In Fig. 1 sind ein Obersieb 1 mit Umlenkwalze 3 und ein Untersieb 2 sichtbar, welche im Wirkungsbereich eines - insbesondere - offenen Formierzylinders 4 zusammengeführt werden und dadurch eine Doppelsiebzone bilden. Der Formierzylinder 4 kann dabei von beiden Sieben mit einem Winkel zwischen 15° und 45° umschlungen sein. Im Umschlingungsbereich kann auch Unterdruck anliegen. In Laufrichtung gesehen hinter der Stelle der Siebzusammenführung befinden sich ein Entwässerungsabschnitt 5 mit Leisten 7 im Obersieb und ein Entwässerungsabschnitt 6 mit Leisten 10 im Untersieb. Der Entwässerungsabschnitt 5 weist anfänglich einen "Auto-Slice" 13 (registriertes Warenzeichen) auf, dem hier zwei weitere mit Unterdruck beaufschlagbare Saugzonen folgen, wobei das abgesaugte Wasser im Wasserkasten 8 gesammelt und über die Leitung 9 abgeführt wird. Die Lage des Entwässerungsabschnittes 5, relativ zu den Sieben, ist durch Verstellvorrichtungen 14 variierbar. Der im Untersieb 2 befindliche Entwässerungsabschnitt 6 ist hier mit Entwässerungsleisten 10 ausgerüstet, welche im Wirkungsbereich des Entwässerungsabschnittes 5 in das Doppelsieb 1, 2 eintauchen können. Zusätzlich ist eine weitere Leiste dargestellt, welche bereits vor dem Wirkungsbereich des ersten Entwässerungsabschnittes 5 gegen das Sieb 2 drückt. Die vom unteren Entwässerungsabschnitt kommende Wassermenge wird über einen Wasserkasten 12 gesammelt und abgeleitet.The device according to the invention and particularly advantageous embodiments are explained by drawings as follows. Are represented by

Fig. 1

schematic representation of a drainage device, not claimed, used after a wire pre-dewatering;

Fig. 2-4

schematic representations of the interaction of the strips of the lower drainage section with the strips of the upper drainage section;

Fig. 5

schematic representation of a GAP former according to the invention;

Fig. 6

another embodiment of a GAP former according to the invention;

Fig. 7

an unused drainage device without stationary strips in the bottom wire;

Fig. 8

another embodiment of a gap former according to the invention without stationary strips in the bottom wire.

1 shows a top wire 1 with a deflection roller 3 and a bottom wire 2, which are brought together in the effective area of an - in particular - open forming cylinder 4 and thereby form a twin wire zone. The forming cylinder 4 can be wrapped by both sieves at an angle between 15 ° and 45 °. There may also be negative pressure in the loop area. Seen in the direction of travel behind the point of the screen merging there is a drainage section 5 with strips 7 in the top screen and a drainage section 6 with strips 10 in Bottom sieve. The drainage section 5 initially has an “auto-slice” 13 (registered trademark), which is followed here by two further suction zones which can be subjected to negative pressure, the extracted water being collected in the water tank 8 and being discharged via the line 9. The position of the drainage section 5, relative to the screens, can be varied by adjusting devices 14. The dewatering section 6 located in the lower sieve 2 is equipped here with dewatering strips 10 which can be immersed in the double sieve 1, 2 in the effective area of the dewatering section 5. In addition, a further bar is shown, which presses against the screen 2 before the area of action of the first drainage section 5. The amount of water coming from the lower drainage section is collected and discharged via a water box 12.

Behind the two dewatering sections 5 and 6 - seen in the direction of the wire run - is followed by a further drainage element 11 - here as a circumferential roller in the lower wire 2, which is provided with grooves which are open in the circumferential direction and from which the water coming from the suspension flows into the water tank 15 is thrown out and can be removed. It is also conceivable to use a suction roll or a fixed suction box here. Behind a further deflection roller 16 and a subsequent suction box 17, the sieves are separated, the upper sieve 1 is returned to the roller 3 and the lower sieve 2 with the material material thereon is fed to the further treatment of the material.

Fig. 2 shows the embodiment in which the strips 10 of the drainage section 6 located in the lower sieve 2 press against the strips 7 of the drainage section 5 located in the upper sieve, while in Fig. 3 the variant is shown in which the lower strips 10 in the Press gaps between the upper strips 7 and a wavy shape of the twin wire 1, 2 can set. The strips 7 are individually adjustable here against the lower sieve, for example by means of hydraulic pressure elements 18. They can also be fastened in a pressing manner in the pressing direction.

Fig. 4 shows a modified form of the strip arrangement of the drainage section 5 located in the top wire 1 such that the twin wire is guided in an arc around this section. This allows a concave surface to be spanned there. The possibly existing strips of the drainage section located in the lower sieve are not shown here.

5 shows the subject matter of the invention used on a paper machine without a pre-dewatering section. A headbox 20 forms a flat jet 21 which is entered directly into the space between the two converging screens. A twin wire former designed in this way is also called a GAP former in specialist circles. The embodiment shown has in the lower wire 2 the forming cylinder 4, which the screens advantageously contact at a wrap angle alpha of less than 25 °, which corresponds to a length of less than 200 mm. With this very compact arrangement, which is favorable for certain types of paper, the advantages of the subject matter of the invention can also be fully exploited. As shown, the two screens are fed in the running direction behind the dewatering sections 5 and 6 via a curved shoe 19 to a suction roll 11 'located in the bottom wire and then to a suction roll 11 "located in the top wire. The screening is then carried out.

While the embodiment with straight surfaces on the dewatering sections is shown by FIG. 5, FIG. 6 shows a device with curved dewatering sections in which the dewatering effect can be further increased by screen tension and centrifugal action.

7 shows a top wire 1 with a deflection roller 3 and a bottom wire 2, which are brought together in the effective range of an - in particular - open forming roller 4 and thereby form a twin wire zone. Seen in the running direction behind the point of the screen merging there are a drainage section 5 with strips 7 in the top screen in such a way that the twin screen is arched around this section are guided around, thus a device with curved drainage sections, in which the drainage effect can be further increased by screen tension and centrifugal action. The drainage section 5 initially has an auto-slice 13, which is followed here by two further suction zones which can be subjected to negative pressure, the extracted water being collected in the water tank 8 and being discharged via the line 9. The position of the drainage section 5, relative to the sieves, can be varied by an adjusting device, not shown. The amount of water coming from the lower drainage section 6 is collected and discharged via a water tank 12.

8 shows a form of the subject matter of the invention, used on a paper machine without a pre-dewatering section. A headbox 20 forms a flat jet 21 which is entered directly into the space between the two converging screens. A twin wire former designed in this way is also called a GAP former in specialist circles. The embodiment shown has in the lower wire 2 the forming cylinder 4, which the screens advantageously contact at a wrap angle alpha of approximately 15-45 °, which corresponds to a length of less than 400 mm. With this very compact arrangement, which is favorable for certain types of paper, the advantages of the subject matter of the invention can also be fully exploited. As shown, the two screens are fed in the running direction behind the dewatering sections 5 and 6 via a curved shoe 19 to a suction roller 11 'located in the bottom wire and then to a suction roller 11 "located in the top wire. The screening is then carried out.

Claims (22)

1. Double-screen former comprising

   1.1) an upper screen (1) guided over guide rolls (3) and a lower screen (2) guided over guide rolls, with a receiving space for the suspension to be dewatered forming between the screens,

   1.2) a forming cylinder (4), in particular an open forming cylinder, located at the start within one screen (1, 2) when considered in the running direction and surrounded in part by both screens (1, 2),

   1.3) further stationary dewatering sections (5, 6) which follow the forming cylinder (4) of which

   1.3.1) one is located in the upper screen and contains laths (7) arranged transverse to the running direction and contacting the screen from which the water is led off from the upper screen (1) out of the screen area, and

   1.3.2) another dewatering section (6) in the lower screen (2) lies predominantly in the region of the dewatering section (5) located in the upper screen, and

   1.4) a further dewatering element (11) lying thereafter in the screen running direction,

   characterised in that

   1.5) the suspension to be dewatered is introduced through a headbox (20) as a flat jet (21) into the material receiving space arising by the leading together of the screens (1, 2)

   1.6) in that the dewatering section (5) located in the upper screen (1) is provided with a suction box through which the dewatering section (5) can be subjected to a partial vacuum, whereby the water is directed away from the upper screen (1) into a water receiving box (8) from which a drainage of the water collected therein is possible.
2. Double-screen former in accordance with claim 1, characterised in that the dewatering section (6) in the lower screen (2) is provided with transversely arranged laths (10) which contact the lower screen from below and are suitable to exert pressure impacts onto the material located between the two screens as the screen passes by.
3. Double-screen former in accordance with claim 1, characterised in that the contact surfaces facing the screen of the laths (7) of the stationary dewatering section (5) located in the upper screen span a convex surface.
4. Double-screen former in accordance with claim 1 or claim 2, characterised in that the laths (7) of the stationary dewatering section (5) located in the upper screen (1) lie with their contact surfaces facing the screen in one plane.
5. Double-screen former in accordance with claim 1, characterised in that the laths (7) of the stationary dewatering section (5) located in the upper screen are made with their position relative to the screen adjustable.
6. Double-screen former in accordance with claim 1, characterised in that the laths (7) of the stationary dewatering section (5) located in the upper screen are resiliently mounted in the contact pressure direction.
7. Double-screen former in accordance with claim 1, characterised in that the contact surfaces facing the screen of the laths (7) of the stationary dewatering section (5) located in the upper screen span a concave surface.
8. Double-screen former in accordance with claim 1, characterised in that the contact surfaces facing the screen of the laths (7) of the stationary dewatering section (5) located in the upper screen form a wave-shaped or zigzag contour in the running direction of the screen.
9. Double-screen former in accordance with claim 1 or claim 2, characterised in that the pitch of the laths of one or both dewatering sections (5, 6) has a magnitude between 30 and 130 mm.
10. Double-screen former in accordance with one of the claims 2 to 9, characterised in that the laths (10) of the dewatering section (6) in the lower screen (2) are so arranged that they respectively act in the area of the intermediate space between the laths (7) located in the upper screen.
11. Double-screen former in accordance with one of the claims 2 to 9, characterised in that the laths (10) of the dewatering section (6) are so arranged in the lower screen (2) that they are respectively active in the region of a lath (7) located in the upper screen (1).
12. Double-screen former in accordance with one of the claims 2 to 11, characterised in that the laths (10) contacting the lower screen (2) are adjustable individually or in groups relative to the lower screen.
13. Double-screen former in accordance with one of the claims 2 to 12, characterised in that a partial vacuum can be applied to the lath intermediate spaces of the dewatering section (6) located in the lower screen.
14. Double-screen former in accordance with one of the claims 2 to 12, characterised in that excess pressure can be applied to the lath intermediate spaces of the dewatering section (6) located in the lower screen.
15. Double-screen former in accordance with claim 3, characterised in that the lower dewatering section (6) is formed without stationary parts contacting the lower screen (2).
16. Double-screen former in accordance with one of the preceding claims, characterised in that the further dewatering element (11) lying in the running direction of the screen after the stationary dewatering sections (5, 6) brings about a dewatering predominantly through the lower screen (2).
17. Double-screen former in accordance with one of the preceding claims, characterised in that the further dewatering element (11) lying after the stationary dewatering sections (5, 6) in the running direction of the screen bring about a dewatering predominantly through the upper screen (1).
18. Double-screen former in accordance with claim 16 or 17, characterised in that the dewatering element (11) is a rotatable closed roll which is arranged within the screen, which lies opposite to the one through which dewatering takes place here.
19. Double-screen former in accordance with claim 16 or claim 17, characterised in that the dewatering element (11) is a rotatable open forming roll which is arranged within the screen through which dewatering takes place here.
20. Double-screen former in accordance with claim 16 or claim 17, characterised in that the dewatering element (11) is a stationary suction box or a rotatable suction roll which is arranged within the screen through which dewatering takes place here.
21. Double-screen former in accordance with one of the preceding claims, characterised in that the screens (1, 2) surround the forming cylinder (4) with a angle of 15° to 45°.
22. Double-screen former in accordance with one of the preceding claims, characterised in that the forming cylinder (4) is arranged within the lower screen (2).

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