EP0069925B1 - Wet end of a paper-making machine - Google Patents

Abstract

The disclosure concerns the wire end section of a paper making machine. The wire belt is an endless loop and the pulp suspension is supplied by one or more head boxes. The head box has an upstream and a downstream flow guide wall defining a pulp outlet opening between them. The downstream guide wall has a convexly curved slide shoe which cooperates with the passing wire belt to define a web-forming zone. At the other side of the wire belt, upstream of the outlet opening, another convexly curved wire support surface is defined for leading the wire belt into the web-forming zone. Both of the convexly curved surfaces are displaceable transversely to the direction of pulp flow from the head box. The radius of curvature of the slide shoe is greater at the outlet opening and smaller away from the outlet opening. The radius of curvature of the cooperating supporting surface on the other side of the belt is smaller than the mean radius of curvature of the slide shoe. Where a plurality of head boxes are provided, drainage water from one head box is collected and is pumped to supply water for the other head box, and vice-versa.

Description

The invention is based on the wire section specified in the preamble of claim 1. Such an arrangement is known from DE-B-2 908 791 (= US-A-4 308 097). Before the sieve belt runs there together with the fibrous web that forms over the curved continuation of the one flow guide wall of the headbox, it is guided over the outside of the other flow guide wall of the headbox. A disadvantage of this arrangement is therefore that the channel delimited by the two flow guide walls must have a relatively sharp deflection in the region of the outlet opening. Or in other words:

The central flow thread of the material flow has a smaller radius of curvature in the area of the outlet opening than in the web formation zone. As a result, there is a risk that at high working speeds in the web formation zone - seen in longitudinal section - secondary currents running transverse to the main flow direction will arise. These can lead to inhomogeneous fiber distribution. In extreme cases, cavitation phenomena must also be expected.

Another disadvantage: If several headboxes connected in series are assigned to the screen belt to produce a multi-layer product, it is necessary to transfer it to a felt after the first fiber web layer has been formed. Then the sieve must be fed to the second headbox, after which the fiber web layer formed here must be transferred to the felt. If necessary, this is repeated several times. The headboxes between the screen belt on the one hand and the felt belt on the other hand have to be accommodated in very narrow spaces. You are usually forced to deviate from the proven nozzle-like headbox design and one. «To provide a folded flow channel.

The invention has for its object to further develop the known arrangement in such a way that a fibrous web of high quality, i.e. at extremely high wire speeds. H. can be produced with a homogeneous fiber distribution.

This object is achieved by the combination of features specified in claim 1. The screen belt is then no longer supported by the outside of one of the two flow guide walls of the headbox in order to introduce it into the web formation zone.

• a) Man gewinnt im Bereich der Auslaßöffnung des Stoffauflaufes genügend Platz für eine stabile Bauweise der Stromführungswände, ohne daß der von diesen begrenzte Kanal nennenswert gekrümmt werden muß. Somit kann der Stoffstrom im wesentlichen gradlinig zur Bahnbildungszone hin geführt werden. Dadurch gelingtr es, die Gefahr des Entstehens von Querströmungen, welche die Homogenität der sich bildenden Faserbahn stören würden, zu beseitigen.
• b) Es ist nunmehr möglich, das Siebband zusammen mit einer darauf schon gebildeten Faserstoffbahn in die Bahnbildungszone eines zusätzlichen Stoffauflaufes einzuführen. Dadurch kann - wie an sich bei anderen Mehrlagen-Papiermaschinen bekannt (DE-A-2 552 485, Fig. 4) - eine zweite Faserbahnlage unmittelbar auf der schon vorhandenen gebildet werden. Bekanntlich dient hierbei die schon vorhandene Faserbahnlage als Filterhilfsschicht bei der Entwässerung der zusätzlichen Lage, so daß hierbei weiniger Fein-und Füllstoffe mit dem Siebwasser abgeführt werden. Hinzu kommt, daß man bei der Anordnung der Stoffaufläufe nicht mehr durch einen Filz beengt ist. Man kann also z. B. die bewährten Dusen-Stoffaufläufe verwenden.

Rather, for this purpose, according to feature e) of claim 1, a support device arranged within the sieve belt loop is provided with a convexly curved screen support surface. In this way, a double effect is achieved in cooperation with feature d) of claim 1:

• a) You gain enough space in the area of the outlet opening of the headbox for a stable construction of the flow guide walls without the channel delimited by them having to be significantly curved. The material flow can thus be guided essentially in a straight line to the web formation zone. In this way, it is possible to eliminate the risk of cross currents which would disrupt the homogeneity of the fiber web being formed.
• b) It is now possible to insert the screen belt together with a fibrous web already formed thereon into the web formation zone of an additional headbox. As a result, as is known per se in other multi-layer paper machines (DE-A-2 552 485, FIG. 4), a second fiber web layer can be formed directly on the already existing one. As is known, the already existing fiber web layer serves as a filter auxiliary layer in the drainage of the additional layer, so that less fine and fillers are removed with the white water. In addition, one is no longer cramped by a felt when arranging the headboxes. So you can z. B. use the proven nozzle headboxes.

From Fig. 2 of DE-B 1 931 686 (= US-A-3 582 467) it is known to provide in the area of the outlet opening of a headbox a drainage box located inside a wire loop with a convexly curved wire guide surface. The drainage box can be pivoted in the direction of the outlet opening of the headbox or back.

It has several webs extending transversely to the direction of the wire, between which there are drainage slots. Such a drainage box, however, has the disadvantage, particularly in the case of the arrangement directly in the beginning of the web formation zone, that it causes an uneven fiber distribution in the paper web and a high loss of fine and fillers. Otherwise, it is a two-wire paper machine. Paper machines of this type require a high level of construction for the two screens. A much higher energy requirement for the drive and a higher effort for cleaning the two screens must also be expected. In many two-wire paper machines the sheet formation is also disturbed due to a long free stream of material between the outlet opening of the headbox and the web formation zone. If one wants to avoid this disadvantage, then according to DE-B-1 931 686, flexible flow guide walls must be provided which rest against the sieve belts. In the construction according to the invention, this disadvantage is eliminated from the outset by the fact that the material jet is guided on one side without interruption through the extended current-carrying wall (sliding block).

On the other side of the material jet, the free piece can be kept particularly short by carrying out the screen support device arranged in the screen belt loop according to claim 2. Then a simple solid sieve support strip is provided, which has no drainage slots and whose radius of curvature can thus be chosen to be particularly small. Compared to a rotating roller, this design has the advantage that critical speeds are not taken into account. So you can design them regardless of the machine speed.

In order to compensate for any inaccuracies in the manufacture of the bar or the adjacent flow guide wall of the headbox, the following is provided according to claim 3: The sieve support bar is not only, as is known per se, can be pushed as a whole across the material flow (towards the outlet opening of the headbox or back) Rather, it will preferably be ensured that the displacement of the bar can be set to different sizes across the machine width. For this purpose, e.g. B. several individually adjustable threaded spindles can be provided over the length of the bar.

The claim 4 is based on the knowledge that the sieve belt, when entering the web formation zone, possibly transports air into the material flow which interferes with the web formation process. This danger is countered according to claim 4 in that the air in the sieve meshes can escape behind the drain line of the sieve belt from the sieve support strip when the material jet hits the meshes into the interior of the sieve belt loop.

A screening section is known from FR-A-2 457 340 (= W080 / 02575), in which a screening belt is guided over a suction box in the web formation zone and has a screening support bar only at the inlet and outlet ends. As a result, the vacuum in the suction box causes the sieve belt to bend strongly. The material flow is covered in the web formation zone by a flexible lip which is attached to the upper flow guide wall of the headbox. This lip bends according to the deflection of the sieve belt. A disadvantage of this known design is - in addition to high energy requirements for generating the negative pressure - that the flexible lip can vibrate. This generally results in large fluctuations in the basis weight of the paper produced; there is also a risk of breakage. Furthermore, the course of the curvature of the web formation zone can be influenced at most by changing the screen tension or the negative pressure. The invention, on the other hand, enables a certain course of the curvature to be specified precisely on the fixed sliding shoe, not only in the running direction of the sieve belt, but also transversely thereto, e.g. B. at the edges. The curved slide shoe can be designed as a rigid extension of the associated current guiding wall. Preferably, however, the slide shoe will be designed to be adjustable transversely to the material flow (claim 5). As a result, if necessary, the sliding shoe can be allowed to protrude a little into the fabric channel and thereby adjust the size of the outlet opening in order to change the emerging mass flow. It can also be provided in the case of the sliding shoe that the displacement path can be set to different sizes over the machine width (claim 6). In this way, the material flow can be evened out across the machine width. The measure specified in claim 7 can contribute to the stabilization of the material flow in the outlet opening.

By decreasing the curvature of the sliding shoe in the flow direction (claim 8), the fact can be taken into account that the drainage takes place faster at the beginning of the web formation zone than at the end. The sieve belt in the dewatering zone has an approximately constant curvature. However, it can also be expedient, particularly when the demands on the homogeneity of the web to be formed are high, to have the curvature of the sliding shoe decrease so strongly in the flow direction that a pressure decrease compensating for friction on the sliding shoe is achieved in the flow direction. The average radius of curvature of the slide shoe will generally be between 100 and 800 mm, preferably between 150 and 300 mm.

In order to precisely define the end of the dewatering zone, the sieve belt should expediently be deflected slightly, at most by 5 °, by a drain edge provided at the end of the slide shoe (claim 9).

An overpressure chamber designed according to claim 10 can be provided for further dewatering of the fibrous web formed. The air pressure in this chamber should generally be between 500 and 10,000 pascals, preferably between 2,000 and 7,000 pascals.

If a multilayer fibrous web is to be produced, the features of claims 11 and / or 12 can advantageously be applied. It is assumed that several, for. B. two, headboxes are arranged so that at least a second fiber web layer is formed. As already mentioned above, the first fiber web layer serves as a filter auxiliary layer in the drainage of the second fiber web layer. In this process, a higher retention of fine and fillers is achieved.

• Figur 1 einen überwiegend schematischen Längschnitt durch eine Siebpartie mit einem Stoffauflauf ;
• Figur 2 einen Ausschnitt aus Fig. 1 in vergrößertem Maßstab ;
• Figur 3 einen schematischen Längsschnitt durch eine Siebpartie mie zwei Stoffaufläufen und mit den dazugehörenden Siebwasserkreisläufen :
• Figur 4 einen schematischen Längsschnitt durch eine Siebpartie mit drei Stoffaufläufen.

Embodiments of the invention are described below with reference to the drawing. It shows:

• Figure 1 is a largely schematic longitudinal section through a wire section with a headbox;
• Figure 2 shows a detail of Figure 1 on an enlarged scale;
• FIG. 3 shows a schematic longitudinal section through a wire section with two headboxes and with the associated white water circuits:
• Figure 4 is a schematic longitudinal section through a wire section with three headboxes.

The wire section shown in FIG. 1 has an endless wire belt 10 which runs over guide rollers 11 to 14. One of the guide rolls is a regulating roll 13 and another is a tensioning roll 14. A headbox is generally designated 15. In a known manner, it includes a transverse distribution pipe 16, a tube bundle 17 and an outlet duct 20 delimited by two flow guide walls 18 and 19 with an outlet opening 21.

The flow guide wall designated 19 has a continuation, which extends in the direction of flow beyond the outlet opening 21, in the form of a convexly curved slide shoe 22. The slide shoe 22, like the other parts of the headbox, extends over the entire width of the sieve belt 10. This runs over the slide shoe 22. In the area of the outlet opening 21, the sieve belt 10 is guided over a fixed strip 23 which is arranged in the interior of the sieve belt loop and has a convexly curved sieve support surface 24 (see FIG. 2).

The stream of material emerging from the outlet opening 21 reaches a web formation zone 25 which is curved and tapered in accordance with the slide shoe 22. Intensive dewatering takes place in the interior of the sieve belt loop.

The bar 23 is attached to a cross member 26. which rests at its two ends on a support bracket 27 and there transversely to the flow direction in the web formation zone 25, d. H. is displaceable in the direction of the outlet opening 21 or back. A tub for collecting the white water is designated 28. A suction box 29 can be provided for further dewatering of the paper web formed.

A stiffening wall 30 of the headbox 15 carrying the slide shoe 22 forms a cavity 31 together with the flow guide wall 19. This is supplied with compressed air via a connection 32, which passes through a plurality of openings 33 into an overpressure chamber 34 located behind the slide shoe 22. This is limited on the outside by a labyrinth formed from three strips 35. These strips 35 are located on a strip support 36 which is fastened to the stiffening wall 30.

A felt belt 37 or wire belt runs over a take-off suction roll 38. takes the paper web formed off the wire belt 10 and feeds it to further dewatering and drying devices which are not shown in the drawing.

In Fig. 2 you can see some more details: The fixed sieve support bar 23, the z. B. can be made of ceramic, is attached to a strip holder 23b by means of a terminal block 23a. This is connected to the cross member 26 with the aid of a plurality of screws 41 (only one of which is visible) distributed over the length of the strip. In addition, a plurality of threaded spindles 42, each with two nuts 43, are provided on the cross member and distributed over its length. Each of the threaded spindles 42 extends through an eye 44 which is integrally formed on the strip holder 23b. After loosening one of the screws 41, by turning the nuts 43, the strip holder 23b with the strip 23 can be placed more or less locally on the current-carrying wall 18 than in the area of the other spindles 42. This can compensate for any manufacturing inaccuracies.

After leaving the outlet opening 21, the material jet is a short distance free on the side facing the sieve belt 10 (dashed line 7 in FIG. 2). The jet of material strikes the sieve belt 10 at 8, which previously runs off the bar 23 at 9.

In general, the distance between the bar 23 and the end of the flow guide wall 18 will be set as small as possible so that the length of the free jet of material, that is the distance from the outlet opening 21 until the jet of material hits the screen belt 10, is as small as possible. The radius of curvature k of the bar 23 is generally less than 100 mm, which also contributes to the shortening of the free material jet.

The slide shoe 22 is fastened to a plurality of adjustment rods 45 arranged distributed over its length. Only one of these rods 45 is visible in the drawing. They rest in bores 46 of the stiffening wall 30 and are axially displaceable in them, i. H. towards the outlet opening 21 or back. An adjusting nut 47 is rotatably mounted on each adjusting rod 45 and engages with an external thread in a threaded bore of the sealing strip carrier 36. The adjusting nuts 47 can be rotated individually. As a result, the displacement path of the slide shoe 22 can be set to be different in size over the machine width.

The movable slide shoe 22 touches the end of the flow guide wall 19 on a sealing surface 48. This, viewed in the direction of flow, lies in front of the end of the flow guide wall 18, that is to say in front of the outlet opening 21. Thus, by adjusting the slide shoe 22, the clear width of the outlet opening 21 and so that the emerging volume flow can be changed.

The curved surface of the sliding shoe 22 touched by the material flow can advantageously be designed as follows: in the area of the outlet opening 21 it is flat or only slightly curved with a very large radius of curvature K. This is followed by an area with a relatively large curvature (radius of curvature r ) and finally an area with again weaker curvature (radius of curvature R).

In FIG. 2, a sharp trailing edge is also indicated at 50 on the slide shoe 22, and also a Tagente placed there on the curved surface with t. The Screen belt 10 is preferably guided such that it includes an angle a of approximately 0.5 to 5 ° with the tangent t; measured with the machine stationary with the sieve belt 10 tensioned.

In Fig. 1, the headbox 15 is arranged vertically with the outlet opening 21 below. Instead, however, any other positions are possible. 3 shows a wire section with two headboxes. The first headbox 15a is arranged horizontally and the second headbox 15b as in FIG. 1 vertically. The two headboxes 15a and 15b differ by different sliding shoes 22a and 22b. In the first headbox 15a, the curved surface of the slide shoe 22a is relatively short and only slightly curved. Accordingly, the material flow in the web formation zone is deflected by only about 45 °. In contrast, the slide shoe 22b of the second headbox 15b has a longer curved surface with a greater curvature. This measure compensates for the higher drainage resistance caused by the fiber web layer formed in the first headbox 15a. In the first headbox 15a, the relatively large, average radius of curvature in the web formation of the first fiber layer is intended to achieve the lowest possible dewatering speed. As a result, the fibers are less strongly washed into the mesh, so that at the end of the wire section the fiber web can be more easily removed from the wire. In addition, fewer fine and fillers are swept through the sieve belt.

In Fig. 3, each headbox 15a, 15b is assigned a separate white water pan 28a or 28b. From each of these tubs, a white water line 51a or 51b leads into a white water tank 52a or 52b. As has already been explained above, the white water of the two headboxes is interchanged in the further course. I.e. The circulation pump 53a conveying to the first headbox 15a (via the line 54a) is connected on the suction side to the white water tank 52b and accordingly the other circulation pump 53b (with pressure line 54b) to the white water tank 52a. At 55 there is a connection between the two white water tanks. As a result, part of the white water accumulating in the container 52a, which is not required by the pump 53b, can overflow into the container 52b.

4 shows a wire section for producing a three-layer fibrous web. The three headboxes 15 ', 15 "and 15"' are in turn the same except for the differently shaped sliding shoes 22 ', 22 ", 22"'. Not shown in FIG. 4 (as also in FIG. 3) are the seal supports 36 on the headboxes with the sealing strips 35 (see FIGS. 1 and 2). It goes without saying that the seal carriers must be adapted to the different inclinations of the screen belt. The fixed sieve guide strips 23, which are assigned to the headboxes 15 "and 15" ', are not attached to a separate cross member, but at the end of a suction box 29' or 29 ". This reduces the construction effort.

3 and 4 can be provided to make the individual headboxes pivotable so that they can be easily lifted off the screen belt and that, if necessary, for. B. can drive less with a headbox without the shoe touching the screen belt.

In all of the exemplary embodiments, the paper web can be removed from the screen belt - instead of using the suction roller 38 (FIG. 1) - by means of a strip or smooth roller (“lick-up”) or a suction box, or finally by means of a blow box. The belt is driven by one or more of the wire guide rollers 11 to 14 (Fig. 1). The wire tension should be in the usual range between 3 and 10 kN / m.

A wire section according to one of claims 1 to 13 can also be combined with one of the known web forming units, for example with a conventional wire forming section or a suction breast roller or a twin wire former. 1 (or more), preferably together with the components 23 and 26 to 29, can be arranged in the wire running direction in front of the known web forming unit. In the case of a twin wire former, the arrangement could be such that a headbox 15 according to FIG. 1, but essentially with the flow direction from bottom to top, is placed in front of the twin wire zone on the wire belt which is approaching in the direction from bottom to top of the twin wire zone .

Claims (14)

1. Wire section of a paper machine with the following features :

a) a headbox (15) shows, viewed in a longitudinal section, a duct (20) limited by two stationary flow guiding walls (18, 19), said duct having an outlet opening (21) through which a flow of stock is led to the outside of an endless moving wire belt (10) ;

b) one of the two flow guiding walls (19) shows a convex, stationary continuation (sliding shoe 22) extending beyond the outlet opening (21) ;

c) the stock flow is limited in the area of a sheet forming zone (25) on the one hand by the sliding shoe (22) and on the other by the wire belt (10) ;

characterized in that

d) the flow guide wall (19) showing the continuation (22), including the continuation (22), is less convex from the area of the outlet opening (21) to the beginning (8) of the sheet forming zone (25) than in the area of the sheet forming zone (25) ;

e) in the area of the end of the other flow guiding wall (18) a wire supporting unit (23) is arranged inside the wire belt loop (10) with a convex wire supporting surface (24) is provided for the introduction of the wire belt into the sheet forming zone (25).

2. Wire section according to claim 1, characterized in that the wire supporting unit (23), over which the wire belt (10) is led to the drainage zone (25), is a stationary solid bar, in which the curvature radius (k) of the wire supporting surface (24) is smaller than 200 mm, preferably smaller than 100 mm.

3. Wire section according to claim 2, in which the stationary wire supporting bar (23) is slideable in direction to the sliding shoe (22) or back, characterized in that the displacement of the bar (23) is variably adjustable across the machine width.

4. Wire section according to claim 2 or 3, characterized in such an arrangement of the wire supporting bar (23) that the descending line (9) of the wire belt (10) from the bar (23) lies in direction of the wire travel ahead of the line (8) of impingement of the stock jet on the wire belt (10).

5. Wire section according to one of claims 1 to 4, characterized in that the sliding shoe (22) is adjustable in direction of the wire supporting unit (23) or back, preferably along a sealing surface (48), which is located between the unmoving flow guiding wall (19) and the adjustable sliding shoe (22).

6. Wire section according to claim 5, characterized in that the displacement of the sliding shoe (22) is variably adjustable accross the machine width.

7. Wire section according to claim 5 or 6, characterized in that the sealing surface (48) is arranged in the direction of flow ahead of the outlet opening (21).

8. Wire section according to one of claims 1 to 7, characterized in that in the area of the sheet forming zone (25), the curvature radius (r, R) of the sliding shoe (22) becomes greater in the direction of flow.

9. Wire section according to one of claims 1 to 8, in which the sliding shoe shows a discharge edge characterized in that the wire belt (10) is turned slightly by the discharge edge (50) (angle a).

10. Wire section according to one of claims 1 to 9, characterized in that an overpressure chamber (34), fed with air and extending across the machine width, is arranged in the direction of flow behind the sliding shoe (22), with the chamber (34) being sealed without contact against the fibrous stock web formed on the wire belt by one or more throttle gaps (bars 35).

11. Wire section according to one of claims 1 to 10, in which to the wire belt is allocated at least one additional headbox according to one of claims 1 to 10, characterized in that the bow length of the sliding shoes (22a, 22b) measured in the direction of flow becomes greater in the direction of travel from headbox (15a) to headbox (15b).

12. Wire section according to one of claims 1 to 12, in which to the wire belt is allocated at least one additional headbox according to one of claims 1 to 10, characterized in that the mean curvature radius of the sliding shoes (22a, 22b) becomes smaller from headbox (15a) to headbox (15b).

13. Wire section according to claim 11 or 12, characterized in that a stationary bar (23", 23"') located inside the wire belt loop and allocated to an additional headbox (15", 15"') is arranged at the discharge end of a suction box (29', 29").

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