Method and equipment for forming of a paper or board web

Abstract

 Method and equipment for the forming of a paper or board web. The headbox (17) is sealed in relation to the wires (11, 12) so that the pulp suspension is transferred from the headbox (17) to the forming zone as a closed jet without free boundary surfaces. The forming zone comprises the first dewatering element (23), which is located inside the first wire loop (11), and the second dewatering element (24), which is located inside the second wire loop (12), so that between them, they provide the borders for the initial dewatering zone which narrows in the flow direction. In the initial dewatering zone, water is removed from the pulp suspension through the openings (26, 27) in the cover of the dewatering elements (23, 24). After the initial dewatering zone, the wires (11, 12) are led over the curved cover of the forming shoe (29) while at the same time pressure pulses are exerted on the pulp suspension located between the wires (11, 12) by means of dewatering blades (28) located on the cover of the forming shoe (29). The length (L₀) of the initial dewatering zone bordered by the dewatering elements (23, 24) is at the most 50 % of the length (L₁) of the forming shoe (29) following it.

Description

[0001] The present invention concerns a method for the forming of a paper or board web as defined in the introduction of claim 1. The invention also concerns equipment for the forming of a paper or board web as defined in the introduction of claim 6.

[0002] At the moment, probably the most practicable gap former type is the roll and blade former. In it, the pulp suspension is fed from the slice opening of a headbox as an open jet into a curved forming gap bordered by two wires. After the forming gap, there are a number of dewatering blades which are arranged on the side of either wire loop. The curved forming gap is achieved by means of a forming roll over which the wires are arranged to run. The inner wire is supported against the surface of the forming roll while the outer wire is unsupported so that the outer wire receives some of the pressure of the slice jet. The tension of the outer wire contributes to dewatering from the pulp suspension fed between the wires. Dewatering can be intensified by using a roll with an open surface as the forming roll, and vacuum can also be arranged in the roll. Initial dewatering on the forming roll essentially takes place at constant pressure. The multifoil shoe following the forming roll creates pressure pulses which intensify dewatering and improve the formation of the web.

[0003] The dewatering pressure on the forming roll follows the below formula:

where: p is the dewatering pressure,

T is the tension of the outer wire, and

R is the radius of curvature of the wire on the forming roll.

[0004] In the curved forming gap, the pulp suspension between the wires is affected by centrifugal force which tends to push the outer wire off from the surface of the forming roll. In order to avoid this, the wire tension must follow the below formula:

where: ρ is the density of the pulp suspension,

h is the thickness of the pulp layer between the wires, and

v is the velocity of the pulp suspension.

[0005] As the running speed of the paper machine increases, the tension of the wires must usually be raised. This imposes new requirements on both the wires and other paper machine structures, which need to be designed to withstand increasingly higher stresses. Higher wire tension increases, among other things, the price of rolls and the space required by the rolls.

[0006] When a forming roll revolves, a vacuum pulse is created on its outgoing side. This vacuum pulse may damage the structure of the partly dewatered web located between the wires. In this case, the damaged web can no longer endure the strong pulsation in the pulsating dewatering zone following the forming roll. Another problem of roll and blade formers is insufficient dewatering capacity at great speeds. The forming roll is also expensive. Having a large size, the forming roll requires much space, and a backup roll is always required by it.

[0007] In addition to roll and blade formers equipped with an adjustable gap, there are also gap formers equipped with a fixed gap, into which the pulp suspension is fed as a closed jet without free bordering surfaces and where the web is formed between two wires which advance under mechanical control. FI patent 85885 presents such web-forming equipment, where a forming gap which narrows in the machine direction is restricted by a dewatering box on both sides. The pressure created in the gap presses water from the pulp suspension located between the wires through the holes in the cover of the dewatering boxes. The goal is to carry out the dewatering as slowly as possible in a long and narrow gap. After the mechanically-controlled forming gap, the web is led to the forming roll via a wet suction box. The formation of flocks must be prevented as early as at the area influenced by the dewatering boxes, because this is no longer possible in subsequent dewatering stages as the fibers already form a uniform fiber network.

[0008] The problem with the above solution is that the range of influence of the dewatering boxes is relatively long. After the mechanically-controlled initial dewatering zone, the dry matter content of the web on both sides is so high that formation cannot be improved after it significantly by means of pulsating dewatering.

[0009] The objective of the present invention is to solve the problems related to prior art technology.

[0010] The characteristics of the method according to the invention are presented in the characterizing part of claim 1.

[0011] The characteristics of the equipment according to the invention are presented in the characterizing part of claim 6.

[0012] In the present invention, the headbox and the forming zone are integrated to each other in a known manner so that the pulp suspension is transferred from the headbox to the forming zone as a closed jet without free boundary surfaces. The transfer of the pulp suspension in an enclosed space improves the evenness of the web, since the wind effect of surrounding air exerted on a free jet, with this wind effect causing small waves on the surface of the free jet, is eliminated. Initial dewatering takes place by means of dewatering elements which support the wires and which guide the wires mechanically closer to each other. After the short initial dewatering zone, dewatering continues on the curved surface of the forming shoe, where the tension of the outer wire and the pressure pulsation created by the dewatering blades promote dewatering and improve web formation. The maximum length of the initial dewatering zone bordered by the dewatering elements is 50 %, preferably at the most 30 %, of the length of the forming shoe following it.

[0013] Initial dewatering is preferably carried out using non-pulsating dewatering elements, in which case water is removed from the pulp suspension gently, only by the effect of pressure difference. After initial dewatering, the dry matter content of the pulp suspension is still relatively low, preferably less than 3 %. This gives an opportunity to influence web formation in the subsequent dewatering stages. After the initial dewatering zone, dewatering is intensified and web formation is improved by means of the blades and curvature of the forming shoe.

[0014] The present invention helps to avoid problems caused by high wire tensions. Wire rolls are not required to be as rigid as in roll and blade formers. An expensive forming roll is not needed. Initial dewatering can be adjusted easily by adjusting the internal pressure of the dewatering box. This adjustment also changes the propagation velocity of the pulp suspension in relation to the wires and hence influences fiber orientation in the paper web formed.

[0015] In what follows, one solution according to the invention is described by making reference to the figure in the enclosed drawing, with the invention not being limited to the details of the figure.

[0016] The forming section shown in the figure comprises the first wire loop 11 and the second wire loop 12, which border the two-wire forming zone between them. Inside the first wire loop 11 are the first breast roll 13, the first guide strip 14 and the first dewatering element 23, which guide the travel of the first wire 11 in the entry area of the forming zone. Inside the second wire loop 12 are the second breast roll 15, the second guide strip 16 and the second dewatering element 24, which guide the travel of the second wire 12 in the entry area of the forming zone. The headbox 17 is located between the breast rolls 13, 15 so that its slice channel 18, which narrows in the flow direction, extends deep between the wires 11, 12. Plate-like support elements 21, 22 have been attached after the walls 19, 20 which border the slice channel 18. The support elements 21, 22 extend into contact with the wires 11, 12. The flexible support elements 21, 22 are installed so that their outer surfaces are in close contact against the wires 11, 12 which move along the guide strips 14, 16, so that the pulp suspension is transferred from the headbox 17 to the forming zone without free boundary surfaces. The pulp suspension is accelerated in the headbox 17 essentially to the same velocity with the wires 11, 12.

[0017] The pulp suspension is discharged from the slice channel 18 to between the wires 11, 12 at a point where both wires 11, 12 are supported by the dewatering element 23 and 24. Between them, the dewatering elements 23 and 24 provide the borders for a gap which narrows in the flow direction, with the gap being a direct continuation to the slice channel 18 which is bordered by the walls 19, 20 and support elements 21, 22. The pressure created in the gap presses water from the pulp suspension located between the wires 11, 12 through the openings 26, 27 in the water removal elements 23, 24. The openings 26, 27 are preferably arranged so that they accomplish non-pulsating dewatering. There are no pressure pulses for example when the openings 26, 27 are holes, or slots which run across the cover diagonally.

[0018] One of the dewatering elements 23, 24 comprises a dewatering box 25, within the region of which dewatering can be adjusted by changing the internal pressure (overpressure or vacuum) in the dewatering box by means of known methods used on paper machines. Alternatively, both dewatering elements 23, 24 can be equipped with an adjustable dewatering box.

[0019] The stationary dewatering elements 23, 24 are followed by a forming shoe 29 equipped with a curved blade cover, with the forming shoe 29 placed inside the second wire loop 12. The forming shoe 29 can be composed of one or more consecutive suction boxes depending on the application, and its total length L₁ is preferably 500-1500 mm. The forming shoe 29 comprises a number of dewatering blades 28 which are in cross direction to the travel direction of the web. The dewatering blades 28 create pulsating dewatering, which intensifies dewatering from the pulp suspension located between the wires 11, 12 and improves the formation of the web. If necessary, loading elements 30 can be used for improving the formation. The loading elements 30 are located on the side of the opposite wire loop 11 so that they meet the openings remaining between the dewatering blades 28.

[0020] Between them, the dewatering elements 23, 24 provide the borders for the initial dewatering zone, which is relatively short. Its length L₀ is at the most 50 %, preferably at the most 30 %, of the length L₁ of the forming shoe 29 following the initial dewatering zone. In practice, the length L₀ of the initial dewatering zone is usually 50-500 mm, preferably 100-300 mm. The length of the initial dewatering zone L₀ depends on the properties of the pulp suspension. When making paper or board from an easily dewatered pulp, the initial dewatering zone can be shorter than when making paper or board from a dense pulp which is more difficult to dewater.

[0021] The many variations of the invention are possible within the scope of protection defined by the below claims. The dewatering elements 23, 24 can have a straight or curved surface. By using known mechanisms, they can be adjusted to a suitable position in relation to the frame of the machine in order to adjust the width of the gap.

Claims

1. Method for the forming of a paper or board web, wherein a pulp suspension is transferred from the headbox (17) as a closed flow to the initial dewatering zone bordered by two dewatering elements (23, 24), over which the forming wires (11, 12) are arranged to run while at the same time water is removed from the pulp suspension located between the wires (11, 12) through the openings (26, 27) in the cover of the dewatering elements (23, 24), characterized in that after the initial dewatering zone the wires (11, 12) are led over the curved cover of the forming shoe (29) while at the same time pressure pulses are exerted on the pulp suspension located between the wires (11, 12) by means of dewatering blades (28) located on the cover of the forming shoe (29) and in that the length (L₀) of the initial dewatering zone bordered by the dewatering elements (23, 24) is at the most 50 % of the length (L₁) of the forming shoe (29) following it.

2. Method according to claim 1, characterized in that the length (L₀) of the initial dewatering zone is at the most 30 % of the length (L₁) of the forming shoe (29).

3. Method according to claim 1 or 2, characterized in that the openings (26, 27) of the dewatering elements (23, 24) are arranged so that they accomplish non-pulsating dewatering.

4. Method according to any of the above claims, characterized in that at least one of the dewatering elements (23, 24) is equipped with a dewatering box (25), the dewatering of which is adjustable.

5. Method according to any of the above claims, characterized in that pressure pulses are exerted on the pulp suspension in the area of the forming shoe (29) by means of blades (30) which can be loaded, with the blades (30) being arranged inside the wire loop (11) which is opposite to the forming shoe (29).

6. Equipment for the forming of a paper or board web, wherein the equipment comprises a headbox (17) and two wire loops (11, 12) which border the forming zone between them, which headbox (17) is sealed in relation to the wires (11, 12) so that the pulp suspension is transferred from the headbox (17) to the forming zone as a closed jet without free boundary surfaces, which forming zone comprises the first dewatering element (23), which is located inside the first wire loop (11), and the second dewatering element (24), which is located inside the second wire loop (12), so that the first and second dewatering element (23, 24) border the initial dewatering zone, which narrows in the flow direction, between them, which dewatering elements (23, 24) are provided with openings (26, 27) for removing water from the pulp suspension fed between the wires (11, 12), characterized in that the initial dewatering zone bordered by the dewatering elements (23, 24) is followed by a forming shoe (29) equipped with a curved cover, which forming shoe (29) is arranged inside either wire loop (11, 12) and equipped with dewatering blades (28), and in that the length (L₀) of the initial dewatering zone bordered by the dewatering elements (23, 24) is at the most 50 % of the length (L₁) of the forming shoe (29) following it.

7. Equipment according to claim 6, characterized in that the length (L₀) of the initial dewatering zone is at the most 30 % of the length (L₁) of the forming shoe (29).

8. Equipment according to claim 6 or 7, characterized in that at least one of the dewatering elements (23, 24) is equipped with a dewatering box (25), the dewatering of which is adjustable.

9. Equipment according to any of the above claims 6-8, characterized in that at least one of the dewatering elements (23, 24) is curved.

10. Equipment according to any of the above claims 6-8, characterized in that both dewatering elements (23, 24) have a straight surface and comprise a dewatering box, the dewatering of which is adjustable.

11. Equipment according to any of the above claims 6-10, characterized in that the openings (26, 27) of the dewatering elements (23, 24) are arranged so that they accomplish non-pulsating dewatering.

12. Equipment according to any of the above claims 6-11, characterized in that at least one blade (30), which can be loaded, is arranged inside the wire loop (11) which is opposite to the forming shoe (29), which blade (30) is fitted so that it touches the said wire (11) at the slot which remains between two dewatering blades (28).

Drawing