TECHNICAL FIELD

The invention relates to a forming section according to the preamble of claim 1. BACKGROUND ART FI patent application 20086007 discloses a former comprising a first wire loop passing over a first breast roll, a second wire loop passing over a second breast roll, said first and second wires forming a converging gap on the section after the first and the second breast rolls. A first non-pulsating dewatering section is formed of a first stationary dewatering shoe, which has been arranged inside the first wire loop immediately after the first breast roll. A head box supplies a mass suspension on the first wire on the first dewatering shoe. There is further a second non-pulsating dewatering zone beginning immediately after the first dewatering zone. The second dewatering zone is formed of a second stationary dewatering shoe, which has been arranged inside the second wire loop immediately after the second breast roll. The first wire joins the second wire on the cover of the second dewatering shoe.

US patent 4,154,645 discloses a method and a machine for manufacturing a multilayer paper board. A first wire loop passes over a first breast roll and over a first dewatering means and a subsequent second dewatering means. The first dewatering means constitutes a forming table, the top wall structure of which may be solid, perforated, or slotted provided with dewatering lists. Vacuum could be used in order to intensify the dewatering into the forming table. The surface of the forming table is preferable plane. A forming table provided with transverse dewatering lists will cause pulsating dewatering on the pulp suspension travelling on the first wire on the forming table. The second dewatering means is a wire guiding shoe made up of a wet suction box and a wire guiding shoe proper. The upper part of the wet suction box consists of foil-like ribs. A second wire loop passes over a second breast roll and joins the first wire on the second dewatering means. The second dewatering means is curved throughout its upper surface so as to have a convexly curved upper surface which guides the first wire. The cover of the second dewatering means is divided into two consecutive portions i.e. a first portion provided with foil-like ribs and a second portion provided with a solid cover. The second dewatering means will on the portion provided with foil-like lists cause pulsating dewatering on the pulp suspension travelling on the second dewatering means.

The formers disclosed in these two prior art publications are so called hybrid formers. There are also blade gap formers where the centers of the breast rolls are essentially on an equal distance from the apex of the gap formed by the two forming wires after the breast rolls. The forming wires are unsupported between the breast rolls and the apex of the gap. The twin wire section is supported by consecutive dewatering suction boxes with list covers situated on alternating sides of the twin wire section. The head box slice jet contacts the forming wires either at the unsupported area or at the dewatering list area. The risk of stock jump exists when the pulp suspension impinges the surface of an unsupported wire. The flow rate and the speed of the pulp suspension jet have to be limited when the pulp suspension jet is ejected on an unsupported wire. Also the turbulence of the head box has to be limited to a minimum when the pulp suspension jet is ejected on an unsupported wire. The length of the free flight of the pulp suspension jet discharging from the slice opening of the head box becomes rather long in prior art blade gap formers. This is due to the parallel position of the breast rolls. The outer surfaces of the breast rolls must be near each other in order to form a narrow gap. There is thus no space between the breast rolls for the head box. The distance between the lip opening of the head box and the impingement point of the pulp suspension jet on the forming wire becomes thus inevitably rather long.

SUMMARY OF THE INVENTION

The object of the invention is to achieve a forming section with improved characteristics compared especially to prior art blade gap formers.

The forming section according to the invention is characterized by the features in the characterizing portion of claim 1.

The paper or board machine forming section according to the invention comprises:

- a first wire forming a first wire loop passing over a first breast roll,

- a second wire forming a second wire loop passing over a second breast roll, whereas a converging gap is formed between the first wire and the second wire after the first and the second breast roll,

- a first dewatering zone comprising a first stationary forming shoe situated within the first wire loop immediately after the first breast roll and setting against an inner surface of the first wire, said first stationary forming shoe comprising:

- a first straight portion at the beginning of the first forming shoe, said first straight portion comprising a straight cover provided with through openings being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire on the first straight portion of the first stationary forming shoe,

- a second curved portion immediately following the first straight portion, said second curved portion comprising a curved cover,

- a first head box supplying a pulp suspension jet on the first wire at the beginning of the first portion of the first stationary forming shoe, and

- the second wire loop is brought in connection with the first wire loop at the beginning of the second portion of the first stationary forming shoe. The forming section according to the invention is characterized in that:

- said second curved portion is provided with through openings being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire on the second curved portion of the first stationary forming shoe, and

-that a length of the first straight portion of the first stationary forming shoe in the travel direction of the first wire is in the range of 200 to 1500 mm, preferably 200 to 1000 mm.

The first straight portion of the first stationary forming shoe provided with through openings forms a big open area that "freezes" the slice jet of the head box effectively immediately at the impingement point. Stock jump is thus eliminated. The web is dewatered only in one direction on the first straight portion of the first forming shoe i.e. through the first forming wire into the first forming shoe. The fine particles will thus travel towards the surface of the web, which is situated against the first wire on the first straight portion of the first forming shoe.

It is easy to adapt the length of the first straight portion of the first forming shoe in order to achieve a desired amount of dewatering on the first straight portion of the first forming shoe. In high capacity solutions the first straight portion can be divided from one to three or even to five vacuum zones such that the vacuum goes up step by step to compensate increased filtration resistance. The second curved portion of the first forming shoe dewaters the pulp suspension mainly in an opposite direction through the second wire. This will again cause the fine particles to travel towards the opposite surface of the web i.e. the surface situated against the second wire. It is also easy to adapt the length and the curvature of the second curved portion of the first forming shoe in order to achieve a desired amount of dewatering on the second curved portion of the first forming shoe. The curvature of the curved cover of the second curved portion of the first forming shoe is in the range of 1500 to 15 000 mm. The curvature of the curved cover can be progressive such that the radius goes smaller in a machine direction. With progressive curvature outside dewatering can be kept constant or nearly constant by increasing dewatering pressure to compensate increased filtration resistance in a machine direction.

The forming section according to the invention makes it possible to use a lower draw ratio i.e. the ratio between the speed of the pulp suspension jet and the speed of the wires compared to prior art blade gap formers. This is due to a low pressure in the gap. The possibility to use a wide draw ratio makes it possible to adjust the useful formation window of the former in a wide range. This means that a forming section according to the invention can be used in producing fine paper, liner and fluting. A forming section according to the invention makes it possible to use a wider window in order to optimize e.g. the runnability and the bursting strength. A forming section according to the invention makes it possible to achieve a short free slice jet, makes the forming of the web faster compared to prior art blade gap formers, makes it possible to use a higher turbulence in the slice jet compared to prior art blade gap formers. Existing blade gap formers can advantageously be rebuilt according to the invention. The first suction box with the list cover situated at the beginning of the twin wire section in a prior art blade gap former is substituted by the first forming shoe. The position and/or the diameter of the second breast roll is further changed making it possible to change the position of the head box so that a short free slice jet is achieved. The forming section can be used with two or three layer head box, too. Non-pulsating initial dewatering makes it possible to prevent or minimize mixing of different stock layers in the beginning of the forming section. By adjusting the first straight portion, one layer can be formed with good layer purity.

BRIEF DESCRIPTION OF THE DRAWINGS

Some specific embodiments of the invention are described in the following in detail with reference to the accompanying figures, in which:

Figure 1 shows a first embodiment of a forming section according to the invention.

Figure 2 shows an enlargement of the beginning of the forming section shown in Fig. 1. Figure 3 shows a second embodiment of a forming section according to the invention.

Figure 4 shows an enlargement of the beginning of the forming section shown in Fig. 3.

Figures 5a, 5b and 5c show three examples of how the distance between the centre of the first breast roll and the centre of the second breast roll in the travel direction of the first wire on the first portion of the first forming shoe can vary according to the invention.

DETAILED DESCRIPTION OF SOME SPECIFIC EMBODIMENTS

Figure 1 shows a first embodiment of a forming section according to the invention. The forming section comprises a first wire 1 forming a first wire loop 11 passing over a first breast roll 12 and a second wire 2 forming a second wire loop 21 passing over a second breast roll 22. A twin-wire section Kl is formed after the breast rolls 12, 22. A head box 40 supplies a pulp suspension jet on the first wire 1 after the first breast roll 12. The web W is dewatered in both directions in the upwards directed twin- wire section Kl . The travel direction of the wires 1, 2 is turned with a suction roll 23 at the end of the twin- wire section Kl . The first wire 1 is separated from the second wire 2 after the suction roll 23 and the web W travels on the second wire 2 in a downwards directed slope to a pick-up point P. The web W is transferred in the pick-up point P from the second wire 2 to a fabric 61 with a suction roll 62 being situated within the fabric loop 61. The web W is then carried with the fabric 61 to the press section.

Figure 2 shows an enlargement of the beginning of the forming section shown in Fig. 1. A first dewatering zone Zl is formed of a first stationary forming shoe 30 situated within the first wire loop 11 immediately after the first breast roll 12. The first wire 1 passes over the cover of the first forming shoe 30. The first forming shoe 30 comprises a first straight portion 30A at the beginning of the first forming shoe 30. The first straight portion 30A comprises a straight cover 31A provided with through openings 32A being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire 1 on the first straight portion 30A of the first stationary forming shoe 30. The first forming shoe 30 comprises further a second curved portion 30B, which follows immediately after the first straight portion 3 OA. The second curved portion 30B comprises a curved cover 3 IB provided with through openings 32B being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire 1 on the second curved portion 30B of the first stationary forming shoe 30. A first head box 40 supplies a slice jet on the first wire 1 at the beginning of the first portion 30A of the first forming shoe 30. The second wire loop 21 and the first wire loop 11 are brought together at the beginning of the second portion 30B of the first forming shoe 30. The dewatering in the first portion 30A of the first forming shoe 30 could be intensified by providing suction PA to the first portion 30A of the first forming shoe 30. The dewatering in the second portion 3 OB of the first forming shoe 30 could be intensified by providing suction PA to the second portion 30B of the first forming shoe 30. The first portion 30A of the first forming shoe 30 can be divided from 2 to 4 vacuum zones by a partition wall for heavier grades. Then vacuum level could be increased in running direction when web filtering resistance has increased. The level of the suction PA, PB provided in the first 30A and/or the second 30B portion of the first forming shoe 30 could be regulated in order to regulate the dewatering in said portions 3 OA, 30B of the first forming shoe 30. There will be dewatering in both directions in the second curved portion 30B of the first forming shoe 30. The tension of the wires 1, 2 will cause dewatering of the web through the second wire 2 in the curved portion 30B of the first forming shoe 30. Also the curvature Rl of the second portion 30B of the first forming shoe 30 will cause dewatering of the web through the second wire 2.

The open surface area defined by the openings 32A in the first portion 30A of the first forming shoe 30 is in the range of 30 to 90%, advantageously 40 to 70%> of the total surface area of the first portion 30A of the first forming shoe 30. The open surface area defined by the openings 32B in the second portion 30B of the first forming shoe 30 is in the range of 30 to 90%>, advantageously 40 to 70%> of the total surface area of the second portion 30B of the first forming shoe 30. Openings 32A, 32B of the first forming shoe 30 being formed as holes or narrow slots and slots can be formed cross machine wide with narrow part of cover between the slots. The wire is then supported such that it will not bend in the narrow slots and this will cause essentially non-pulsating dewatering. In a preferred embodiment of the invention the openings 32A in the first straight portion 30A and/or the openings 32B in the second curved portion 30B are narrow slots formed in a cross machine direction, such that a narrow part of the cover 31 A, 3 IB is arranged between the slots. The narrow slots can have a maximum width of 15 mm in a machine direction and the narrow part of the cover 31 A, 3 IB can have a maximum width of 15 mm in a machine direction.

The length LA of the first straight portion 30A of the first forming shoe 30 in the travel direction of the first wire 11 is in the range of 200 to 1500 mm, preferably 200 to 1000 mm. The length LB of the second curved portion 30B of the first forming shoe 30 in the travel direction of the first wire 11 is in the range of 200 to 800 mm. Short length LA of the first straight portion 30A of the first forming shoe 30 ensures that the fine particles of the web will travel towards the top surface of the web in following curved portion 30B.

The curvature Rl of the curved cover of the second curved portion 30B of the first forming shoe 30 is in the range of 1500 to 15000 mm. Curvature can also be progressively changeable such that for example in the beginning the curvature is 10000 mm and in the end it is 1500 mm. The angle a between the first 1 and the second 2 wire in the gap G is in the range of 2 to 10 degrees. The length LI of the free flight of the slice jet of the head box 40 is in the range of 150 to 350 mm. The second breast roll 22 is situated ahead of the first breast roll 12 in the machine direction so that the distance L2 between the centre C ₁₂ of the first breast roll 12 and the center C ₂₂ of the second breast roll 22 in the travel direction of the first wire 1 on the first portion 30A of the first forming shoe 30 is 200 to 2000 mm.

Figure 3 shows a second embodiment of a forming section according to the invention. The forming section shown in figure 3 corresponds essentially to the forming section shown in figure 1, but the twin- wire section Kl is in this embodiment essentially horizontally directed. The forming section comprises a first wire loop 11 passing over a first breast roll 12 and a second wire loop 21 passing over a second breast roll 22. A twin- wire section Kl is formed after the breast rolls 12, 22. A head box 40 supplies a pulp suspension jet on the first wire 11 after the first breast roll 12. The web W is dewatered in both directions in the essentially horizontally directed twin- wire section Kl . The second wire 21 is separated from the first wire 11 on a turning roll 24 at the end of the twin-wire section Kl and the web W travels on the first wire 11 in a downwards directed slope to a pick-up point P. The web W is transferred in the pick-up point P from the first wire 11 to a fabric 61 with a suction roll 62 being situated within the fabric loop 61. The web W is then carried with the fabric 61 to the press section.

Figure 4 shows an enlargement of the beginning of the forming section shown in Fig. 3. The beginning of the forming section corresponds essentially to that shown in Fig. 2, but the direction is horizontal. The dimensions i.e. the length of the first portion 30A and the second portion 30B of the first forming shoe 30, the curvature Rl of the curved cover of the second curved portion 30B of the first forming shoe 30, the angle a between the first 1 1 and the second 21 wire in the gap G, the length LI of the free flight in the air of the pulp suspension jet discharging from the slice opening of the head box 40, and the distance between the breast rolls 12, 22 corresponds to the dimensions of the embodiment shown in figure 2. The length LA of the first straight portion 30A of the first forming shoe 30 in the travel direction of the first wire 11 is in the range of 200 to 1500 mm, preferably 200 to 1000 mm. The length LB of the second curved portion 30B of the first forming shoe 30 in the travel direction of the first wire 11 is in the range of 200 to 800 mm. The length LI of the free flight of the slice jet of the head box 40 is in the range of 150 to 350 mm.

The second breast roll 22 is situated ahead of the first breast roll 12 in the machine direction so that a distance L2 between a centre C ₁₂ of the first breast roll 12 and a centre C ₂₂ of the second breast roll 22 in a travel direction of the first wire 1 on the first portion 30A of the first forming shoe 30 is 200 to 2000 mm.

The open surface area defined by the openings 32A in the first portion 30A of the first forming shoe 30 is in the range of 30 to 90%, advantageously 40 to 70%> of the total surface area of the first portion 30A of the first forming shoe 30. The open surface area defined by the openings 32B in the second portion 30B of the first forming shoe 30 is in the range of 30 to 90%, advantageously 40 to 70%> of the total surface area of the second portion 30B of the first forming shoe 30.

In a preferred embodiment of the invention the openings 32A in the first straight portion 30A and/or the openings 32B in the second curved portion 30B are narrow slots formed in a cross machine direction, such that a narrow part of the cover 31 A, 3 IB is arranged between the slots. The narrow slots can have a maximum width of 15 mm in a machine direction and the narrow part of the cover 31 A, 3 IB can have a maximum width of 15 mm in a machine direction.

Figures 5 a to 5 c show three examples of how the distance L2 between the centre C of the first breast roll 12 and the centre C ₂₂ of the second breast roll 22 in the travel direction of the first wire 1 on the first portion 30A of the first forming shoe 30 can vary. The distance L2 can vary for example from 100% up to 400% of a radius R12 of the first breast roll 12. Figure 5a shows two straight vacuum zones in the first straight portion 30A and a curved second portion 30 B. Figure 5b shows three straight vacuum zones in the first straight portion 30A and a curved second portion 30B. Figure 5 c shows four straight vacuum zones in the first straight portion 30A and a curved second portion 30B.

The examples of the embodiments of the present invention presented above are not intended to limit the scope of the invention only to these embodiments. Several modifications can be made to the invention within the scope of the claims.

CLAIMS

1. A paper or board machine forming section comprising:

- a first wire (1) forming a first wire loop (1 1) passing over a first breast roll (12), - a second wire (2) forming a second wire loop (21) passing over a second breast roll (22), whereas a converging gap (G) is formed between the first wire (1) and the second wire (2) after the first (12) and the second (22) breast roll,

- a first dewatering zone (Zl) comprising a first stationary forming shoe (30) situated within the first wire loop (1 1) immediately after the first breast roll (12) and setting against an inner surface of the first wire (1), said first stationary forming shoe (30) comprising:

- a first straight portion (30A) at the beginning of the first forming shoe (30), said first straight portion (3 OA) comprising a straight cover (31 A) provided with through openings (32A) being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire (1) on the first straight portion (3 OA) of the first stationary forming shoe (30),

- a second curved portion (30B) immediately following the first straight portion (30A), said second curved portion (30B) comprising a curved cover (3 IB), - a first head box (40) supplying a pulp suspension jet on the first wire (1) at the beginning of the first portion (3 OA) of the first stationary forming shoe (30), and

- the second wire loop (21) is brought in connection with the first wire loop (1 1) at the beginning of the second portion (30B) of the first stationary forming shoe (30),

characterized in that:

- said second curved portion (30B) is provided with through openings (32B) being formed as holes or slots in order to provide essentially non-pulsating dewatering of the pulp suspension travelling on the first wire (1) on the second curved portion (30B) of the first stationary forming shoe (30), and