EP4663847A1

EP4663847A1 - Method for forming a fiber web and a forming section for forming a fiber web

Info

Publication number: EP4663847A1
Application number: EP25174344.9A
Authority: EP
Inventors: Antti Poikolainen; Tuuli SILOMAA; Mika Patjas
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2024-06-10
Filing date: 2025-05-06
Publication date: 2025-12-17
Also published as: FI20245730A1; CN121110410A; FI131993B1

Abstract

The invention relates to a method for forming a fiber web (W) in a forming section comprising a headbox (30) and a forming unit (110), in which method fiber suspension is fed from the headbox (30) to the forming unit (110) comprising at least a twin-wire part comprising a bottom wire (10) and a top wire (20). In the method water is removed from the fiber suspension in the beginning of the forming unit (110) by at least one variable-curvature roll (15; 15A, 15B) comprising a belt loop (53), which variable-curvature roll (15; 15A, 15B) has at least two partial curves such that the radius of curvature of a first partial curve in the first zone (A1) is bigger than the radius of curvature of a second partial curve in the second zone (A2) following the first partial curve in the travel direction of the belt loop (53).). The invention also relates to a forming section for forming a fiber web (W), which forming section comprises a headbox (30) and a forming unit (110) comprising at least a twin-wire part comprising a bottom wire (10) and a top wire (20). In the beginning of the forming unit (110) at least one variable-curvature roll (15; 15A, 15B) comprising a belt loop (53), which variable-curvature roll (15; 15A, 15B) has at least two partial curves such that the radius of curvature of a first partial curve in the first zone (A1) is bigger than the radius of curvature of a second partial curve in the second zone (A2) following the first partial curve in the travel direction of the belt loop (53).

Description

Technical field

The invention relates generally to producing fiber webs. Particularly the invention relates to a method for forming a fiber web according to the preamble of the independent method claim and to a forming section for forming a fiber web according to the preamble of the independent forming section claim.

Background

As known from the prior art in fiber web machines, especially in paper and board machines, the fiber web is produced and treated in an assembly formed by a number of apparatuses arranged consecutively in a process line. A typical production and treatment line comprise a forming section comprising a headbox and a forming unit and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a size press, a calender, a coating section. The production and treatment line also comprise typically at least one winder for forming customer rolls as well as a roll packaging apparatus.
Fiber webs, especially paper and board are available in a wide variety of types and can be divided according to basis weight in two grades: papers and boards with a single ply and a basis weight of 25-300 g/m² and boards manufactured in multiply/-layer technology and having a basis weight of 120-600 m/m². It should be noted that the borderline between paper and board is flexible since board grades with lightest basis weights are lighter than the heaviest paper grades. Generally speaking, paper is used for printing and board for packaging. The present invention is especially suitable for production of board webs.
In the forming section the task of the headbox is to supply fiber suspension, i.e. pulp suspension, also often called also as stock suspension, for the fiber web production into the forming unit. It is known to use single layer headboxes, in which one fiber suspension flow is discharged from the headbox via a flow channel for a stock suspension layer forming a fiber web. It is also known to use multilayer headboxes, in which more than one fiber suspension flows are discharged from the headbox via flow channels for stock suspension layers, each for forming one layer of a multilayer fiber web.
In the forming section the task of a forming unit is to remove water from fiber suspension fed by the headbox. When the web is manufactured of watery fiber stock, water in the stock is removed on the forming section through a forming wire or forming wires for starting the formation of the web. Fibers remain on the forming wire or between the forming wires moving together. Depending on the grade of the web being manufactured, different types of stocks are used. The volume for which water can be removed from different stocks for achieving a web of good quality is a function of many factors, such as e.g. a function of the desired basis weight of the web, the design speed of the machine, and the desired level of fines, fibers and fill materials in the finished product and refining level of used stocks. Many types of devices are known on the forming unit such as foil strips, suction boxes, turning rolls, suction rolls, and rolls provided with an open surface, which have been used in many different arrangements and arrays when trying to optimize the volume, time and location of water being removed when forming the web. The manufacturing a high-quality end-product of desired grade is a function of the volume of dewatering, the dewatering method, the duration of dewatering, and the location of dewatering. When it is desired to improve the water removal capacity and to maintain or improve the quality of the end-product, many times unforeseeable problems are created as the result of which either the water removal volume has to be decreased for maintaining the desired quality or the desired quality has to be sacrificed for achieving the greater water volume.
It is also known from prior art to use as water removal means in the forming unit a variable-curvature roll, often called also a sleeve roll, which variable-curvature roll comprises a stationary support shaft, an belt loop, which is led to circle around the stationary support shaft, which variable-curvature roll further comprises at least one curvilinear dewatering zone consisting of at least two partial curves such that the radius of curvature of a first partial curve is greater than the radius of curvature of a second partial curve following the first partial curve in the travel direction of belt loop. The variable-curvature roll also typically comprises a support element supported on the stationary support shaft and the belt loop led to circle around the stationary support shaft is supported by the support elements. The belt loop is typically impermeable. The variable-curvature roll has proven to be very effective in view of the water removal capacity. The variable-curvature roll is used positions of the forming unit, when the dry solids content of the fiber web has significantly increased from the dry solids content of the fiber suspension fed from the headbox, in order to keep the runnability of the fiber web in the forming section as well as quality of the fiber web as required. Thus, before water removal by the variable-curvature roll, the forming unit has comprised typically several other types of water removal means. Typically, the dry solids content required is at least about 4%.
In patent publication EP2350385B1 is disclosed a forming section comprises a first and a second wire loop which form a twin-wire zone, which comprises and in which are arranged at least one dewatering element by means of which initial dewatering is performed from stock suspension fed by the headbox, and at least one dewatering device following said at least one dewatering element in the travel direction of the web, which dewatering device comprises a stationary support shaft on which support elements are supported at a distance from each other, an impermeable belt loop which is led to circle around the stationary support shaft supported by said support elements, wherein said at least one dewatering device further comprises at least one curvilinear dewatering zone over which the wires are led to travel supported by the belt loop, whereby the degree of curvature of the curve of the at least one curvilinear dewatering zone increases in the travel direction of the belt such that increasing dewatering pressure is applied to the stock suspension travelling between the wires on said at least one curvilinear dewatering zone, which increasing dewatering pressure is dependent on a tension of the wires and a radius of curvature of said at least one curvilinear dewatering zone, wherein the at least one curvilinear dewatering zone is preceded by two dewatering zones prevailing in opposite directions.
In the present days the sustainability in all production has become increasingly important. Thus, there is also a need to find solutions for forming in a way providing possibilities for improved sustainability. One way to improve the sustainability of forming is provide solutions with decreased energy consumption. Furthermore, it is important to find solutions of forming, in which cost-efficient raw materials can be used but not compromising good strength properties of the fiber web.
An object of the invention is to create a method for forming a fiber web and a forming section for a fiber web, in which the disadvantages and problems of prior art are eliminated or at least minimized.
A particular object of the invention is to create a method for forming a fiber web and a forming section for a fiber web, in which the disadvantages and problems of prior art relating especially to energy consumption are eliminated or at least minimized.
A particular object of the invention is to create a method for forming a fiber web and a forming section for a fiber web, in which the disadvantages and problems of prior art relating especially to using cost-efficient raw materials are eliminated or at least minimized, in particular in view of strength properties of the fiber web.

Summary

In order to achieve the above-mentioned objects, the forming section according to the invention is mainly characterized by the features of the characterizing clause of the independent forming section claim and the method according to the invention is mainly characterized by the features of the characterizing clause of the independent method claim. Advantageous embodiments and features are disclosed in the dependent claims.
It has been surprisingly noted that in the forming unit of the forming section a variable-curvature roll can be used already in the beginning stages of forming fiber suspension fed from the headbox to be formed to a fiber web in the forming unit. Inventors found out that when fiber web top surface is still without any dewatering actions (almost in the headbox consistency), it is possible effectively dewater the fiber web by a variable-curvature roll through top surface of the fiber web (lower drainage resistance). The variable-curvature roll can be even the first water removal means. Significantly surprising has been that this provides also that an exceptionally low flow rate of the headbox can be used. Typically, in prior art forming sections known from prior art, the flow rate of the headbox has been about 200 litres per meter, the use of the variable-curvature roll as first water removal means has provided that the flow rate of the headbox can be reduced to 70-100 litres per meter. Astonishingly, also a high headbox consistency can be used, about at level of 1,5-2%. Low frow rate of the headbox means that, significantly less energy is needed for pumping the pulp mass to the headbox, for example if flow rate is decreased to half, also the energy need is decreased to half. Thus, the low flow rate of the headbox combined with the effective water removal of the variable-curvature roll, the forming section according to the invention provides for significant energy savings compared to forming sections known from prior art. The variable-curvature roll provides effective water removal, even about 50% of the headbox flow.
According to an advantageous embodiment of the invention the forming unit of the forming section is a gap former, i.e. in the forming section the fiber suspension flow from the headbox is directly guided to a gap formed between two forming wires, a bottom wire and a top wire, and between two rolls located inside the forming wire loops, respectively, and the two creating the gap of the gap former in between of them. According to this advantageous embodiment of the invention one of the rolls forming the gap of the gap former is a variable-curvature roll. Thus, the advantages described above are achieved. This embodiment is especially suitable for fiber web grades having basis weight of 30-120 g/m². In heavier fiber web grades water removal limitations may occur, which can be avoided according to an advantageous feature of the invention by increasing front-wrap of the variable-curvature roll before the fiber web enters to a tighter curvature area of the variable-curvature roll.
According to another advantageous embodiment of the invention the forming unit of the forming section begins as a short substantially horizontal single-wire part and is followed by a twin wire part and wherein at the beginning of the twin wire part a gap is formed between the two wires, each wire loop comprising at the gap a roll located inside the forming wire loops, respectively and one of the rolls, advantageously the one roll located inside the bottom wire loop, is a variable-curvature roll. The length of the short substantially horizontal single-wire part is 0,5 - 4 m. The direction of the substantially horizontal single-wire part can deviate from the horizontal direction at most +/-30 degrees. This embodiment of the invention provides that consistency of the fiber suspension can be exceptionally low, when the fiber suspension reaches the variable-curvature roll, advantageously the consistency is about 2 %, i.e. remarkably close to the consistency of the fiber suspension, when fed from the headbox, this consistency typically being about 1,2-1,5 %. The variable-curvature roll provides for effective water removal, even about 30-50% of the headbox flow. There is no water removal from the upper surface of the fiber suspension before the variable-curvature roll, which provides that no filtrated fiber layer is formed and the upper surface remains in water-condition and thus, water removal at the variable-curvature roll is effective by the pressure provided and water is well removed through the wire, which provides that the fiber web remains unbroken. According to an advantageous aspect of this embodiment of the invention before the variable-curvature roll there is only non-pulsating water removal provided, for example a non-pulsating forming board, to receive the fiber suspension from the headbox.
According to an advantageous aspect of the invention in the beginning of the forming unit is located at least one variable-curvature roll comprising a belt loop, which variable-curvature roll has at least two partial curves such that the radius of curvature of a first partial curve in a first zone is bigger than the radius of curvature of a second partial curve in a second zone following the first partial curve in the travel direction of the belt loop, and the variable-curvature roll is located not further than 4 meters from the beginning of the forming unit measured from the outermost circumferential point of a first roll to the outermost circumferential point of the variable-curvature. It should be noted that the first roll can be the variable-curvature roll, in which case the location of the variable-curvature roll in the beginning of the forming unit is at the very beginning i.e. at 0 meters.
According to an advantageous aspect of the invention the variable-curvature roll has a front wrap before the fiber web enters to a tighter curvature area of the variable-curvature roll. Advantageously, the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm. Advantageously, the tighter curvature area has radius of curvature of 40 - 120 mm and/or length of the tighter curvature area in running direction of the fiber web is 40-120 mm, advantageously about 70 mm. The wrap angle over the variable-curvature roll is advantageously 55 -150 °.
According to an advantageous aspect of the invention at the variable-curvature roll has at first in direction of the run of the fiber web first a curved surface support element functioning as a sliding surface for the belt loop of the variable-curvature roll, which curved surface has a radius of curvature of 400 - 1000 mm.
According to an advantageous aspect of the invention the variable-curvature roll has at first in running direction of the fiber web a smaller radius of curvature, followed by a larger radius of curvature and there after an area with decreasing radius of curvature on one or more sectors of the variable-curvature roll.
According to an advantageous aspect of the invention no vacuum assisted water removal means are located between the variable-curvature roll and the following roll.
According to an advantageous aspect of the invention the headbox is close to the beginning of the forming unit, the distance between being about 200-300 mm, which provides that the turbulence created by the headbox remains in the fiber suspension and thus, the fiber orientation remains, and good formation is achieved.
According to the invention in the method for forming a fiber web in a forming section comprising a headbox and a forming unit fiber suspension is fed from the headbox to the forming unit comprising at least a twin-wire part comprising a bottom wire and a top wire, whereby in the method water is removed from the fiber suspension in the beginning of the forming unit by at least one variable-curvature roll comprising a belt loop, which variable-curvature roll has at least two partial curves such that the radius of curvature of a first partial curve in a first zone is bigger than the radius of curvature of a second partial curve in a second zone following the first partial curve in the travel direction of the belt loop..
According to an advantageous feature of the invention in the method water is removed from the fiber suspension in a gap forming unit, and the fiber suspension from the headbox is at first fed to a gap formed between two rolls located inside the bottom wire and the top wire, respectively, and water is removed from the fiber suspension at first by the variable-curvature roll being one of the two rolls forming the gap.
According to an advantageous feature of the invention in the method water is removed from the fiber suspension in a hybrid forming unit, and the fiber suspension from the headbox is at first fed to a short single-wire part having length of 0,5 - 4 meters and formed by the bottom wire and the fiber suspension is fed from the single-wire part the twin wire part to a gap formed between two rolls located inside the bottom wire and the top wire, respectively, and water is removed from the fiber suspension by the variable-curvature roll being one of the two rolls forming the gap.
According to an advantageous feature of the invention in the method water is removed from inner surface of the top wire by a support foil arranged on inner surface of the top wire between the variable-curvature roll and a first guide roll or a second variable-curvature roll following the gap inside the top wire loop.
According to an advantageous feature of the invention in the method the fiber web has a front wrap on the first partial curve in the first zone of the variable-curvature roll before the fiber web enters to the second partial curve in the second zone of the variable-curvature roll and the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm.
According to an advantageous feature of the invention the variable-curvature roll comprises a stationary support shaft on which support elements are supported at a distance from each other, the belt loop, which is impermeable and led to circle around the stationary support shaft and supported by the support elements.
According to an advantageous feature of the invention no vacuum assisted water removal means is/are located between the variable-curvature roll and a first guide roll or a second variable-curvature roll.
According to the invention the forming section for forming a fiber web comprises a headbox and a forming unit comprising at least a twin-wire part comprising a bottom wire and a top wire, wherein in the beginning of the forming unit at least one variable-curvature roll comprising a belt loop, which variable-curvature roll has at least two partial curves such that the radius of curvature of a first partial curve in a first zone is bigger than the radius of curvature of a second partial curve in a second zone following the first partial curve in the travel direction of the belt loop.
According to an advantageous feature of the invention the forming unit is a gap forming unit, a gap is formed between two rolls located inside the bottom wire and the top wire, respectively, of the twin-wire part and the variable-curvature roll is one of the two rolls forming the gap.
According to an advantageous feature of the invention the forming unit is a hybrid forming unit comprising a short single-wire part having length of 0,5 - 4 meters and formed by the bottom wire, the single-wire part is followed by the twin-wire part and a gap formed between two rolls located inside the bottom wire and the top wire, respectively, of the twin-wire part, and the variable-curvature roll is one of the two rolls forming the gap.
According to an advantageous feature of the invention a support foil is arranged on inner surface of the top wire between the variable-curvature roll and a first guide roll or a second variable-curvature roll following the gap inside the top wire loop.
According to an advantageous feature of the invention a front wrap is provided on the first partial curve in the first zone of the variable-curvature roll before the second partial curve in the second zone of the variable-curvature roll and the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm.
According to an advantageous feature of the invention the variable-curvature roll comprises a stationary support shaft, on which support elements are supported at a distance from each other, the belt loop, which is impermeable and led to circle around the stationary support shaft and supported by the support elements.
According to an advantageous feature of the invention no vacuum assisted water removal means is/are located between the variable-curvature roll and a first guide roll or a second variable-curvature roll.
By the method of forming and the forming section according to the invention many advantages are achieved: very effective water removal is achieved with low energy consumption, a lower structure of the forming unit is achieved compared to height of forming units known from prior art, the height can be even reduced close to half.

Brief description of the drawings

In the following the invention is explained in detail with reference to the accompanying drawing to which the invention is not to be narrowly limited.

In figures 1A-1B is shown schematically advantageous examples of a forming section of a gap former type according to the invention,
In figures 2A-2B is shown schematically further advantageous examples of a forming section of a hybrid former type according to the invention,
In figures 3A-3B is shown schematically in more detail partial examples of forming unit according to the invention at the location of the variable-curvature roll and
In figures 4A-4B is shown schematically in more detail further partial examples of in more detail partial examples of forming unit according to the invention at the location of the variable-curvature roll.

During the course of the following description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention and its advantageous examples. In the figures some repetitive reference signs have been omitted for clarity reasons.

Detailed description

In figures 1A-1B is shown examples of a forming section for forming a fiber web W, in these examples the forming unit of the forming section is a gap former. This embodiment is especially suitable for fiber web grades having basis weight of 30-120 g/m². The forming section comprises a headbox 30, from which the stock suspension is fed to a forming unit 110 of a gap type, which forming section 110 is followed by a press section 140. The headbox 30 is close to the beginning of the forming unit 110, the distance between being about 200-300 mm. The headbox 30 in the examples can be a single-layer headbox or a multilayer headbox of any as such known type. The flow rate of the headbox is about 70-100 litres per meter and the headbox consistency is about at level 1,5-2%. The forming unit 110, which is a gap type forming unit, comprises a bottom wire 10 and a top wire 20 as well as rolls 12, 16; 21, 22 located inside the forming wire loops 10, 20, respectively, for guiding and driving the bottom and the top wires 10, 20 as endless forming wire loops 10, 20. The forming unit 110 begins with a gap formed between two rolls 15, 25 located inside the bottom wire 10 and the top wire 20, respectively. In the forming section 110 the fiber suspension flow from the headbox 30 is thus directly guided to the gap G formed between the two forming wires 10, 20 and the respective two rolls 15, 25 located inside the forming wire loops 10, 20. One of the rolls 15; 25 forming the gap G is a variable-curvature roll 15A, 15B (figs. 3A and 4A) and is thus, the first water removal means of the forming unit 110. Advantageously, the variable-curvature roll 15A, 15B is located inside the bottom wire loop 10. After the variable-curvature roll 15A, 15B the forming unit 110 continues as the twin-wire forming part and the fiber suspension is formed to the fiber web between the forming wires 10, 20. A support foil 26 is located on the top wire loop 20 between the variable-curvature roll 15A, 15B and a first guide roll 21 following the gap G. The support foil 26 scrapes water away from the inner surface of the forming wire 20 and prevents water transfer back to the fiber web by the first guide roll 21 following the gap G. The wires 10, 20 are guided along the surface of the variable-curvature roll 15A, 15B and the run of the twin wire part is turned to substantially vertical run and there after turned to substantially horizontal run at first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B and located inside the top forming wire loop 20. In the example of figure 1A the first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B is a smooth roll 21. In the example of figure 1B the first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B is a variable-curvature roll 21A, 21B and thus, water is removed also downwards. Thereafter the run of the wires 10, 20 during this water removal on the twin-wire part is substantially horizontal, during which run the stock on the wire 10 is guided past inside the loop of the bottom wire 10 located further water removal means 11, which can comprise suction devices and/or pulsating or non-pulsating water removal means. At the end of the twin-wire part formed between the bottom wire 10 and the top wire 20, the top wire 20 is guided away from the run with the bottom wire 10 and the forming unit 110 continues as a short single-wire run 10C, during which run further water removing can be provides by further water removal means 13, which can comprise suction devices and/or pulsating or non-pulsating water removal means. In the forming unit 110 of the example of figure 1A water removal is downwards by the variable-curvature roll 15A, 15B and by the water removal devices 11 and 13. In the example of figure 1B water is removed also upwards by the variable-curvature roll 21A, 21B. After the forming section the fiber web is guided to a press section 140 to support of a press fabric 40 guided and driven by guide and drive rolls 42. Before the pick-up point to the press section 140 inside the bottom wire loop 10 a suction box 14 can be provided. The press section 140 can be of an as such known type, for example a center roll -based press section 140 or another type of press section such as a linear-type press section. The example of figure 1A is especially good in cases, where the fiber web type to be manufactured is a machine glazed (MG) paper web due to the reason that in the forming section the high dewatering on the variable-curvature roll 15A, 15B through the top wire 20 ensures that fines and solids are locked near the bottom surface of then fiber web. In manufacturing of machine glazed paper webs, in the drying section the bottom surface of the fiber web is directed against a Yankee-cylinder, which further increases smoothness and glossiness of the fiber web. The combination of the variable-curvature roll 15A, 15B in the beginning of the forming section and the Yankee-cylinder in the drying section thus creates an excellent combination for manufacturing of machine glazed (MG) paper webs, which have on one side a really smooth and glossy surface, and additionally low porosity and good densification.
In figures 2A-2B is shown examples of a forming section for forming a fiber web W, in these examples the forming unit of the forming section is a hybrid former. The forming section comprises a headbox 30, from which the stock suspension is fed to a forming unit 110 of a hybrid type, which forming section 110 is followed by a press section 140. The headbox 30 in the examples can be a single-layer headbox or a multilayer headbox of any as such known type. The flow rate of the headbox is about 100-150 litres per meter and the headbox consistency is about at level 1,5%. The headbox 30 is close to the beginning of the forming unit 110, the distance between being about 200-300 mm. The forming unit 110, which is a hybrid type forming unit, comprises a bottom wire 10 and a top wire 20 as well as rolls 12, 16; 21, 22 located inside the forming wire loops 10, 20, respectively, for guiding and driving the bottom and the top wires 10, 20 as endless forming wire loops 10, 20. The forming unit 110 begins with a short single-wire part followed by a twin-wire part arranged as a gap type forming unit corresponding to the examples of figures 1A-1B. Length of the short single-wire part is 0,5 - 4 m meters in the beginning of the forming unit 110 depending on the fiber web W grade to be produced. The single-wire part is formed by the bottom wire 10 arranged as a substantially horizontal run for beginning of forming the fiber web W. Thereafter is arranged the twin-wire part formed between the bottom wire 10 and the top wire 20 followed by the short single-wire run 10C. The forming unit 110 comprises rolls 12, 22 for guiding and driving the bottom and the top wires 10; 20 as endless loops. The pulp suspension M is first fed from the headbox 30 onto the bottom wire 10, advantageously on the area of a non-pulsating forming board 17 with suction. A breast roll 16 is located before the forming board 17 in the machine direction and headbox stock suspension hits the wire after the breast roll 16. The stock suspension is advantageously guided from the headbox 30 directly onto the bottom wire 10 onto the area of the forming board 17, which is the first water removal means of the forming unit 110 in this example, and provides for stabilization, i.e. calming. The fiber orientation may be effected by a wire shaker in the breast roll 16 and calmed on the upper surface of the bottom wire 10 for providing good formation and desired fiber orientation to the board web W. After the short substantially horizontal single-wire run the twin-wire part begins with a gap formed between two rolls 15, 25 located inside the bottom wire 10 and the top wire 20, respectively. In the forming section 110 the fiber suspension flow from the headbox 30 is thus directly guided to the gap G formed between the two forming wires 10, 20 and the respective two rolls 15, 25 located inside the forming wire loops 10, 20. One of the rolls 15; 25 forming the gap G is a variable-curvature roll 15A, 15B (figs. 3B and 4B) and is thus, the first water removal means of the forming unit 110. Advantageously, the variable-curvature roll 15A, 15B is located inside the bottom wire loop 10. After the variable-curvature roll 15A, 15B the forming unit 110 continues as the twin-wire forming part and the fiber suspension is formed to the fiber web between the forming wires 10, 20. A support foil 26 is located on the top wire loop 20 between the variable-curvature roll 15A, 15B and a first guide roll 21 following the gap G. The support foil 26 scrapes water away from the inner surface of the forming wire 20 and prevents water transfer back to the fiber web by the first guide roll 21 following the gap G. The wires 10, 20 are guided along the surface of the variable-curvature roll 15A, 15B and the run of the twin wire part is turned to substantially vertical run and there after turned to substantially horizontal run at first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B and located inside the top forming wire loop 20. In the example of figure 1A the first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B is a smooth roll 21. In the example of figure 1B the first guide roll 21 following the gap G and the variable-curvature roll 15A, 15B is a variable-curvature roll 21A, 21B and thus, water is removed also downwards. Thereafter the run of the wires 10, 20 during this water removal on the twin-wire part is substantially horizontal, during which run the stock on the wire 10 is guided past inside the loop of the bottom wire 10 located further water removal means 11, which can comprise suction devices and/or pulsating or non-pulsating water removal means. At the end of the twin-wire part formed between the bottom wire 10 and the top wire 20, the top wire 20 is guided away from the run with the bottom wire 10 and the forming unit 110 continues as a short single-wire run 10C, during which run further water removing can be provides by further water removal means 13, which can comprise suction devices and/or pulsating or non-pulsating water removal means. In the forming unit 110 of the example of figure 1A water removal is downwards by the variable-curvature roll 15A, 15B and by the water removal devices 11 and 13. Then, the fiber web is guided to a press section 140 to support of a press fabric 40 guided and driven by guide and drive rolls 42. Before the pick-up point to the press section 140 inside the bottom wire loop 10 a suction box 14 can be provided. The press section 140 can be of an as such known type, for example a center roll -based press section 140 or another type of press section such as a linear-type press section.
In figures 3A-3B is shown partial examples of forming unit according to the examples of figures 1A -2B at the location of the variable-curvature roll 15. In these examples one advantageous example of the variable-curvature roll 15A is shown. The variable-curvature roll 15A has at least one zone A1; A2 with variable radius, in this example is a zone with increasing radius A1 is followed by at least one zone A2 with decreasing radius. The variable-curvature roll 15A comprises a stationary support shaft 51 on which support elements 52 are supported at a distance from each other, an impermeable belt loop 53 which is led to circle around the stationary support shaft 51 and supported by the support elements 52. The variable-curvature roll further comprises at least one curvilinear dewatering zone A1; A2 via which the wires 10; 20 are led to travel supported by the belt loop 53. The degree of curvature of the curve of the curvilinear dewatering zone/-s A1; A2 varies in the travel direction of the belt 53 such that variable pressure K1-K4 is applied towards the belt 53. During the zones A1, A2, where the wires 10, 20 and the fiber suspension therebetween runs along the belt surface of the variable-curvature roll water removal pressure is provided to the fiber suspension travelling between the wires 10, 20 on said at least one curvilinear dewatering zone/-s A1, A2. Radius of curvature of the curvilinear dewatering zone/-s A1, A2 consists of two partial curves such that the radius of curvature of a first partial curve in the first zone A1 is bigger than the radius of curvature of a second partial curve in the second zone A2 following the first partial curve in the travel direction of belt loop 53. Radius of curvature of the curvilinear dewatering zone can contain several curves such that the radius of curvatures varies in the running direction of the wires. The variable-curvature roll 15A is located inside the loop of the bottom wire 10. In fig. 3A pulp suspension M is fed from the headbox between the wires 10 and 20 in the beginning of the first dewatering zone A1 area. In fig. 3B web is transferred between the wires 10 and 20 in the beginning of the first dewatering zone A1 area. After the first dewatering zone A1 the fiber web is transferred between the wires 10 and 20 to the second dewatering zone A2. In the first dewatering zone A1 to the fiber web is caused smaller dewatering pressure than in the second dewatering zone A2. In the first dewatering zone A1 web consistency will increase enough that it is able to resist higher dewatering pressure in the second dewatering zone A2. In the example of figure 3A the variable-curvature roll 15A has a front wrap in the first dewatering zone A1 with the bottom wire loop 10 and the top wire loop 20, before the fiber web enters to the tighter curvature zone A2 of the variable-curvature roll. Advantageously, In the example of figure 3A the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm. Advantageously, the tighter curvature area in zone A2 has radius of curvature of 40 - 120 mm and/or length of the tighter curvature area in running direction of the fiber web is 40-120 mm, advantageously about 70 mm. The total wrap angle of the bottom wire loop 10 and top wire loop 20 over the variable-curvature roll is advantageously 55 -150 °. Inventors have found out that when the fiber web upper surface is still in headbox consistency coming on the variable-curvature roll 15A area, it is needed to have long enough front wrap angle such that the fiber web upper surface consistency has decreased to certain level so that it can stand higher dewatering pressure in the tighter curvature area. The front wrap angle of the bottom wire loop 10 and the top wire loop 20 over the variable-curvature roll 15A is advantageously 50 - 150 degrees in the example of figure 3B. Advantageously, dewatering pressure K3 on the variable-curvature roll tighter curvature area is maximum 100-350 kPa. The variable-curvature roll 15A improves strength of the fiber web and provides effective water removal.
In figures 4A-4B is shown further partial examples of in more detail partial examples of forming unit 1A-2B at the location of the variable-curvature roll 15. In these examples another advantageous example of the variable-curvature roll 15B is shown. The variable-curvature roll 15B in examples of figures 4A-4B corresponds to the examples of figures 3A-3B but in addition comprises just before the gap a further zone A10 with variable curvature of a first decreased radius and a second increased radius, forming a zone with bigger curvature. In Fig 4A headbox stock suspension M is directed between the wires 10, 20 on the area of the increased radius area, to so called lower pressure zone K11. By this kind of arrangement headbox stock suspension jet length can be decreased, and it has positive effect to the fiber web formation. In Fig 4B the fiber web is transferred between the wires 10 and 20 on the area of the lower pressure zone K11. In Fig 4A and 4B after the lower pressure zone K11, the fiber web is transferred to the first dewatering zone A1 and to the second dewatering zone A2. In the first dewatering zone A1 to the fiber web is caused smaller dewatering pressure than in the second dewatering zone A2. In the first dewatering zone A1 the fiber web consistency will increase enough such that it is able to resist higher dewatering pressure in the second dewatering zone A2.
In the description in the foregoing, although some functions have been described with reference to certain features and examples, those functions may be performable by other features and examples whether described or not. Although features have been described with reference to the certain examples, those features may also be present in other examples whether described or not.
Above only some advantageous examples of the inventions have been described to which examples the invention is not to be narrowly limited and many modifications and alterations are possible within the invention.

Claims

Method for forming a fiber web (W) in a forming section comprising a headbox (30) and a forming unit (110), in which method fiber suspension is fed from the headbox (30) to the forming unit (110) comprising at least a twin-wire part comprising a bottom wire (10) and a top wire (20), characterized in that in the method water is removed from the fiber suspension in the beginning of the forming unit (110) by at least one variable-curvature roll (15; 15A, 15B) comprising a belt loop (53), which variable-curvature roll (15; 15A, 15B) has at least two partial curves such that the radius of curvature of a first partial curve in a first zone (A1) is bigger than the radius of curvature of a second partial curve in a second zone (A2) following the first partial curve in the travel direction of the belt loop (53).
Method according to claim 1, characterized in that in the method water is removed from the fiber suspension in a gap forming unit, and the fiber suspension from the headbox (30) is at first fed to a gap (G) formed between two rolls (15, 25) located inside the bottom wire (10) and the top wire (20), respectively, and that water is removed from the fiber suspension at first by the variable-curvature roll (15A, 15B) being one of the two rolls (15, 25) forming the gap (G).
Method according to claim 1, characterized in that in the method water is removed from the fiber suspension in a hybrid forming unit, and the fiber suspension from the headbox (30) is at first fed to a short single-wire part having length of 0,5 - 4 meters and formed by the bottom wire (10) and the fiber suspension is fed from the single-wire part the twin wire part to a gap (G) formed between two rolls (15, 25) located inside the bottom wire (10) and the top wire (20), respectively, and that water is removed from the fiber suspension by the variable-curvature roll (15A, 15B) being one of the two rolls (15, 25) forming the gap (G).
Method according to any of claims 1 - 3, characterized in that in the method water is removed from inner surface of the top wire (20) by a support foil (26) arranged on inner surface of the top wire (20) between the variable-curvature roll (15; 15A, 15B) and a first guide roll (21) or a second variable-curvature roll (21; 21A, 21B) following the gap (G) inside the top wire loop (20).
Method according to any of claims 1 - 4, characterized in that in the method the fiber web has a front wrap (A1) on the first partial curve in the first zone (A1) of the variable-curvature roll (15; 15A, 15B) before the fiber web enters to the second partial curve in the second zone (A2) of the variable-curvature roll (15; 15A, 15B) and that the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm.
Method according to any of claims 1 - 5, characterized in that the variable-curvature roll (15A, 15B) comprises a stationary support shaft (51) on which support elements (52) are supported at a distance from each other, the belt loop (53), which is impermeable and led to circle around the stationary support shaft (51) and supported by the support elements (52).
Method according to any of claims 1 - 6, characterized in that no vacuum assisted water removal means is/are located between the variable-curvature roll (15; 15A, 15B) and a first guide roll (21) or a second variable-curvature roll (21; 21A, 21B).
Forming section for forming a fiber web (W), which forming section comprises a headbox (30) and a forming unit (110) comprising at least a twin-wire part comprising a bottom wire (10) and a top wire (20), characterized in that in the beginning of the forming unit (110) is at least one variable-curvature roll (15; 15A, 15B) comprising a belt loop (53), which variable-curvature roll (15; 15A, 15B) has at least two partial curves such that the radius of curvature of a first partial curve in a first zone (A1) is bigger than the radius of curvature of a second partial curve in a second zone (A2) following the first partial curve in the travel direction of the belt loop (53).
Forming section according to claim 8, characterized in that the forming unit (110) is a gap forming unit, that a gap (G) is formed between two rolls (15, 25) located inside the bottom wire (10) and the top wire (20), respectively, of the twin-wire part and that the variable-curvature roll (15A, 15B) is one of the two rolls (15, 25) forming the gap (G).
Forming section according to claim 8, characterized in that the forming unit (110) is a hybrid forming unit comprising a short single-wire part having length of 0,5 - 4 meters and formed by the bottom wire (10), that the single-wire part is followed by the twin-wire part and that a gap (G) formed between two rolls (15, 25) located inside the bottom wire (10) and the top wire (20), respectively, of the twin-wire part, and that the variable-curvature roll (15A, 15B) is one of the two rolls (15, 25) forming the gap (G).
Forming section according to any of claims 8 - 10, characterized in that a support foil (26) is arranged on inner surface of the top wire (20) between the variable-curvature roll (15; 15A, 15B) and a first guide roll (21) or a second variable-curvature roll (21; 21A, 21B) following the gap (G) inside the top wire loop (20).
Forming section according to any of claims 8 - 11, characterized in that a front wrap (A1) is provided on the first partial curve in the first zone (A1) of the variable-curvature roll (15; 15A, 15B) before the second partial curve in the second zone (A2) of the variable-curvature roll (15; 15A, 15B) and that the front wrap is 50 - 150 ° and/or length of the front wrap in running direction of the fiber web is 300 - 1500 mm.
Forming section according to any of claims 8 - 12, characterized in that the variable-curvature roll (15A) comprises a stationary support shaft (51), on which support elements (52) are supported at a distance from each other, the belt loop (53), which is impermeable and led to circle around the stationary support shaft (51) and supported by the support elements (52).
Forming section according to any of claims 8 - 13, characterized in that no vacuum assisted water removal means is/are located between the variable-curvature roll (15; 15A, 15B) and a first guide roll (21) or a second variable curvature roll (21; 21A, 21B).