WO2008034947A1

WO2008034947A1 - Paper or board machine and method for manufacturing paper or board

Info

Publication number: WO2008034947A1
Application number: PCT/FI2007/050496
Authority: WO
Inventors: Antti Poikolainen
Original assignee: Metso Paper, Inc.
Priority date: 2006-09-21
Filing date: 2007-09-18
Publication date: 2008-03-27
Also published as: FI20065577A0; DE112007001849T5; FI120747B; FI20065577A

Abstract

Paper or board having a low MD/CD ratio of tensile strength is manufactured on a paper or board machine, the forming section (1) of which comprises a single-wire initial portion which is followed by a twin- wire portion, at the beginning of which there is a forming shoe (Ha) guiding a top wire (9) onto the surface of a fibre suspension layer laid on a bottom wire (5). The surface (36) of the forming shoe (Ha) is provided with dewatering openings, which together with a vacuum prevailing in the forming shoe (1 Ia) produce non-pulsating dewatering of a fibre suspension layer sandwiched between the wires (5, 9). The forming shoe (Ha) is followed by at least one dewatering element (l ib, l ie, l id) producing pulsating dewatering of the fibre suspension layer sandwiched between the wires (5, 9). The press section (2) includes only one press nip (N), which is an extended nip through which the web (W) is passed sandwiched between two press fabrics (15, 17). This kind of combination of a forming section and a press section makes it possible to produce paper at reasonable costs and with good cross-direction stiffness.

Description

Paper or board machine and method for manufacturing paper or board

The invention relates to a paper or board machine for manufacturing a paper or board web having a low MD/CD ratio of tensile strength, comprising a forming section for forming a web from a fibre suspension, a press section for dewatering the formed web, and a drying section for drying the pressed web.

The invention also relates to a method for manufacturing a paper or board web having a low MD/CD ratio of tensile strength.

The press section of the paper machine has traditionally comprised at least two press nips, through which the web has been passed on support of at least one press felt. The development of extended-nip presses has enabled the introduction of single-nip press sections, which allows a substantial cost saving to be achieved and the space needed for the press section to be reduced. The advantages of the arrangement also include its simplicity, closed draws and reduced two-sidedness of paper.

Single-nip press sections are described, for example, in the publications US 6,638,395 and WO 2006/018388, in which both the web is passed to the press section at a dry solids content of at least 18 %, and it is passed from the forming section to the drying section without open draws. In US patent 6,638,395 the forming section comprises a pre-press nip to assure a sufficient dry solids content of the web before the press section. The first cylinder group of the drying section comprises three drying cylinders at the most, which makes it more possible to compensate for the stretching of the web immediately at the initial stage of drying by means of speed differences between dryer groups. In the publication WO 2006/018388 there is a drying unit based on air blowing at the beginning of the drying section. A conventional gap former is used as the forming section. To achieve sufficient dry solids levels, more dewatering capacity is needed at high speeds and at high basis weights than that produced by one press nip. This limits the possibilities of using a single-nip press section. Precise control of the process and uniform quality of the press felts are also required of the press because there are no subsequent press nips in which it would be possible to correct and smooth out the areas of unevenness in the pressed web.

Moreover, it has been found that a sufficiently good cross-direction stiffness of paper cannot always be achieved on known single-nip press sections. A good cross-direction stiffness is an important property, for example, of copy papers. A reason for a low cross-direction stiffness of the web can be the anisotropic stretch and shrinkage of the web in the press section and in the drying section.

An object of the invention is a method and an apparatus enabling a high-quality paper or board having a low MD/CD ratio of tensile strength to be manufactured at high production. An object is also to minimize the known drawbacks associated with the manufacture of paper or board.

With a view to achieving this object as well as those coming out later, the paper or board machine according to the invention is characterized by what is stated in claim 1.

Correspondingly, the method according to the invention for the manufacture of paper or board is characterized by what is stated in claim 12.

The paper or board machine in accordance with the invention comprises a certain type of hybrid former and a single-nip press section as a combination. The twin- wire portion of the hybrid former begins with a forming shoe, which guides the top wire onto the surface of a fibre suspension layer placed on the bottom wire and provides non-pulsating dewatering. This kind of hybrid former produces a web in which fibre orientation and resultant anisotropy are relatively insignificant.

The method for manufacturing paper or board having a low MD/CD ratio of ten- sile strength comprises the steps of feeding a fibre suspension from a headbox to a single-wire initial dewatering portion formed by a bottom wire; guiding a top wire by means of a forming shoe onto the surface of a fibre suspension layer laid on the bottom wire to provide a twin-wire dewatering portion in which a web is dewa- tered first by means of the forming shoe producing non-pulsating dewatering and after that by means of at least one dewatering element producing pulsating dewatering; transferring the web as a closed draw from the forming section to a press section in which it is passed through a single press nip sandwiched between two press fabrics, the press nip being an extended nip; and passing the web as a closed draw from the press section to a drying section where it is dried.

Anisotropy in the structure of paper affects almost all physical properties of paper but it is particularly important from the point of view of strength properties. In- plane anisotropy occurs such that the physical properties are different in the machine and cross-machine directions of paper. Anisotropy is generated, on the one hand, as a result of forming causing orientation in the forming section and, on the other hand, as a result of anisotropic dimensional changes brought about in the web in the press and drying sections. Machine-made paper always has more machine-direction than cross-machine-direction fibres. Machine-direction stretching and cross-direction shrinkage of the web occur in the press and drying sections.

In most cases, a low orientation index is sought to be achieved since paper properties are then almost identical in all in-plane directions. The MD/CD ratio of tensile strengths or another MD/CD ratio of physical properties (i.e. the ratio of properties measured in the machine direction and in the cross direction) can be used as an indirect indicator of the orientation index. The variation of fibre orientation in the thickness direction of the web is also of significance with respect to paper properties. Orientation difference between the surface of the web and its interior parts increases the stiffness of paper, while orientation difference between the opposite surfaces of the web increases the tendency of paper to curl.

For example, copy papers, which are required to have an insignificant tendency to curl and a sufficient cross-direction stiffness, should have an MD/CD ratio of tensile strength that is 2.5 at the most.

At the web formation stage, several hydrodynamic forces affect the distribution of fibre orientation in paper. The most important factor is the speed difference between the slice jet and the wire. Other significant factors include the acceleration and deceleration of the fibre suspension flow in the headbox and in the wire section, and the random variations arising from turbulence. The fibre orientation of paper manufactured on a fourdrinier is clearly stronger on the wire side than on the top side. The z-direction orientation profile of paper manufactured on a gap former is symmetric. A hybrid former makes it possible to manufacture paper whose orientation profile is clearly the lowest. The anisotropy of the physical properties of paper manufactured on a hybrid former is therefore generally slighter than that of paper manufactured on a fourdrinier or on a gap former.

The fibre suspension in a twin-wire former provided with pre-dewatering is supplied from the headbox onto the bottom wire, on which it is dewatered in a downward direction, after which the fibre suspension moves on support of the bottom wire to a twin-wire zone, where it is dewatered both through the bottom wire in a downward direction and through the top wire in an upward direction. At the beginning of the twin- wire zone, the top wire is brought into contact with the fibre suspension layer on the bottom wire. In blade hybrid formers (for example, SymFormer MB, Duoformer D, BelBond), the web is dewatered at the beginning of the twin- wire zone using pulsating dewatering, with the result that the web is easily broken if the consistency of the web is too low. When pulsating dewatering at the beginning of the twin-wire zone is replaced with non-pulsating dewatering, negative pressure pulses are avoided, which contributes to reduced fibre orientation and to a lower degree of in-plane anisotropy of paper properties.

When the twin-wire zone begins with non-pulsating dewatering, retention can be maintained good since the fibre network formed on the surface of the fibre suspension layer is able to retain fines and fillers in the subsequent dewatering stages, in which dewatering is pulsating. By increasing the negative pressure of the non- pulsating forming shoe, the dewatering capacity can be increased only to a certain limit, after which the dewatering capacity starts to decrease in spite of the increase in negative pressure. The deterioration of dewatering is probably due to the fact that, with increasing negative pressure, the surface of the fibre layer lying against the wire is compacted so that dewatering through the top layer becomes more difficult. For this reason, the effect of negative pressure must be interrupted at times, with the result that the thickness of the fibre layer is partially restored and the pore structure of its surface opens. Relatively light pressure pulses are sufficient to open the surface structure of the fibre network so that dewatering can continue. Pulsating dewatering is produced using dewatering blades, which are arranged to support the wires as transverse to the running direction of the wires. The use of dewatering blades and loading blades opposing the dewatering blades enhances dewatering and improves the formation of the web since pressure pulses generate shear forces in the fibre suspension layer, the shear forces breaking up already- formed floes. On the other hand, the pressure pulses and the alternation of the dewatering direction reduce retention, fines and fillers being flushed away with water from the vicinity of the surface layers of the web. Too strong pressure pulses increase the orientation of fibres and, hence, increase the strength difference between the machine and cross-machine directions of paper.

To achieve good formation it is required that the fibre suspension layer shall arrive at the loading blade area of the twin-wire zone at a sufficiently low consis- tency. This requirement easily leads to a high headbox flow rate, the handling of which is problematic with today's technology. A problem may be, for example, the splashing of stock when it impinges on the wire (stock jump), difficult control of the orientation profile, and the crushing of the web at the beginning of the twin- wire portion.

Headbox dilution control makes it possible to considerably improve the orientation profile as compared with conventional control accomplished by means of a headbox profile bar.

The splashing of stock, i.e. "stock jump", occurring at the beginning of the fourdrinier wire section is a phenomenon in which liquid particles separate from the liquid flow as vertically moving drops. Splashing is caused by rebounding or by a change of impulse when the slice jet hits the fabric or the foπning board. With increasing running speeds, the problems caused by splashing tend to increase. Splashing can be prevented, among other things, by a suitable alignment of the breast roll and the forming board or by using a downstream bevelled profile bar.

The crushing of the web at the beginning of the twin-wire portion can be prevented by placing a stationary forming shoe at the beginning of the twin- wire por- tion, which forming shoe guides the top wire onto the surface of the fibre suspension layer on the bottom wire.

The hybrid former described above of itself alone produces a very low MD/CD ratio of tensile strength, which substantially improves the CD stiffness of paper. When the arrangement is further complemented with the shaking of the breast roll, the ratio of tensile strength can be further lowered. The CD vibration of the breast roll decreases the orientation of fibres in the machine direction.

Practice has shown that when conventional forming sections are used, it is diffi- cult to impart controlled and sufficiently high CD stiffness to the end product on a single-nip press. The use of the hybrid former described above as a web forming section improves the possibilities of using a' single-nip press section without the properties of the paper to be manufactured suffering from it. When the orientation profile of the paper coming from the forming section is relatively low, a single-nip press section can be used as the press section without the physical cross-direction properties of the paper deteriorating unreasonably. This enables sufficiently stiff paper to be produced on a paper machine the manufacturing costs of which are lower than those of conventional arrangements. Moreover, the arrangement in accordance with the invention makes it possible to manufacture paper with lower consumption of energy than before.

As the extended-nip press it is possible to use a shoe press or another known press arrangement that provides an extended dwell time in the press nip. The length of the press nip in the extended nip is typically 30 - 450 mm. The web is passed through the nip between two press felts, which receive water pressed out of the web in the nip.

The usability of the single-nip press section can be further improved by installing after it at least one impingement dryer, which improves runnability and moisture profile control before cylinder drying. Impingement drying can be accomplished in the manner disclosed, for example, in the publication WO 2005/068713.

At high speeds, a drying section comprising only drying cylinders becomes long. Some of the drying cylinders can be replaced with impingement dryers, in particular at the beginning of the drying section where full steam pressure cannot be used in the drying cylinders because the web may stick to the cylinder surface. Space can be saved by using a vertical impingement dryer comprising blowers on both sides of the vertical run of the web. Since air is blown directly towards the paper web without a drying fabric placed in between, it is possible to use a relatively high temperature of the blowing air (250 - 700 ⁰C), thereby achieving a very effi- cient heating effect. The impingement dryer comprises arrangements for supporting the paper web as well as a blow chamber provided with openings on the side facing the web to blow hot air or another gas to the surface to be dried. An advantage of the vertical dryer is the saving of space. The paper machine is shortened when a part of the dryer is below or above the basic level of the paper machine. An advantage is also that the force of gravity cannot adversely affect the run of the paper web. Efficient vertical impingement drying requires that the opposite side of the web shall be pre-dried in a horizontal impingement dryer. The horizontal impingement pre- dryer heats the first side of the web before the drying of the second side of the web is started in the vertical impingement dryer. Two vertical impingement dryers cannot be installed one after the other because of their need for a large space. Advantageously, the vertical impingement dryer dries the same side of the web as the drying cylinder group following after that. It is advantageous to place the different drying units close to one another in order that the web shall not have time to cool before transition to the next drying unit.

The running speed of the paper or board machine in accordance with the invention is typically in a range of 600 - 1700 m/min, advantageously 600 - 1300 m/min, and it can manufacture paper or board having a basis weight in a range of 40 - 250 g/m². The arrangement in accordance with the invention is particularly suitable for the manufacture of fine paper.

In the following, the invention will be described with reference to the examples illustrated in the figures of the appended drawings, but it is not desired to limit the invention exclusively to these examples.

Figure 1 is a side view of a paper machine in accordance with the invention.

Figure 2 is a side view of a forming section of a paper machine in accordance with the invention. Figure 3 is an enlarged view of a forming board located at the beginning of the forming section.

Fig. 1 shows a paper machine in accordance with the invention, comprising a forming section 1, a press section 2 and a drying section 3. The forming section 1 is a hybrid former, the press section 2 comprises only one press nip N and the drying section 3 has impingement dryers 23, 24 before a first cylinder group 25.

The forming section 1 of Fig. 1 comprises a headbox 4, a bottom wire 5 forming an endless loop while guided by a breast roll 6, a suction roll 7 and guide rolls 8, and a top wire 9 forming a second endless loop while guided by guide rolls 10. The bottom wire 5 and the top wire 9 define between themselves a twin-wire portion, which is preceded by a fourdrinier portion on the bottom wire 5. A fibre suspension is fed from the headbox 4 onto the bottom wire 5, on the fourdrinier por- tion of which it is dewatered through the bottom wire 5 in a downward direction by means of dewatering elements (not shown). At the beginning of the twin-wire portion, within the top wire loop 9 there is a dewatering box 11 provided with a curved deck and guiding the top wire 9 onto the surface of the fibre suspension layer laid on the bottom wire 5.

The dewatering box 11 comprises, in the example of Fig. 1, three successive suction chambers 11a, l ib, l ie, each of which can be adjusted to have a vacuum of a desired magnitude. The structure of the first suction chamber 11a is such that it produces non-pulsating dewatering, whereas the second and the third suction chamber l ib and l ie produce pulsating dewatering. In addition, at the suction chamber 1 Ib on the side of the bottom wire 5 there are a number of loading blades 12, which generate strong pulsation. The structure of the dewatering box 11 is described in greater detail in connection with Fig. 2.

After the dewatering box 11 on the side of the bottom wire 5 there is a transfer suction box 13, which ensures that a web W follows the bottom wire 5 at the stage when the top wire 9 is separated from the bottom wire 5. The twin- wire portion is again followed by a fourdrinier portion, on which the web W is dewatered through the bottom wire 5 in a downward direction by means of dewatering elements (not shown). At the end of the fourdrinier portion there is the suction roll 7, which turns the web W to run obliquely downward. The suction roll 7 is further followed by a suction box 14, on which the dry solids content of the web W is raised before its transfer to the press section 2.

After the suction box 14, the web W is passed from the downward oblique portion of the bottom wire 5 to a pick-up felt 15 of the press section 2 by means of a pickup roll 16. The only press nip N of the press section 2 is formed between two press rolls 18, 19, of which the upper is a shoe roll 18 and the lower is a counter roll 19. The shoe roll 18 is provided with a static concave surface, around which a flexible shell is rotating. The web W is passed through the nip N sandwiched be- tween the top felt 15 and a bottom felt 17, which both receive water being pressed out of the web. At the press nip N, the counter roll 19 presses the fabrics 15, 17 and the web W against the concave surface of the shoe roll 18, thereby forming a so-called extended nip, the length of said nip being greater than a nip formed by two hard-surfaced rolls. After the nip N on the side of the bottom felt 17 there is a suction roll 20, one function of which is to ensure that the web W follows the bottom felt 17 at the stage when the top felt 15 is separated from the web W. After that, the web W is transferred from the bottom felt 17 by means of a transfer belt 21 to a first drying fabric 22 of the drying section 3.

The drying section 3 first comprises a horizontal impingement dryer 23, which is followed by a vertical impingement dryer 24 and at least one drying cylinder group 25. The horizontal impingement dryer 23 comprises a blow unit 26 and the drying fabric 22 forming an endless loop while guided by support rolls 27. Heated air is blown from the blow unit 26 towards the web W when it travels under the blow unit 26 while supported by the drying fabric 22 and the support rolls 27, so that the web W is heated and some water evaporates from its surface. After that, the web W is transferred from the first drying fabric 22 to a second drying fabric 28, which conveys the web W through both the vertical impingement dryer 24 and the first drying cylinder group 25. The drying fabric 28 first forms, while guided by support rolls 29, a vertical loop, blow units 30 and 31 being disposed on two vertical portions of the vertical loop. The first blow unit 30 blows heated air to the surface of the web W while it runs vertically in a downward direction and the second blow unit 31 blows heated air against the web W while it runs back in an upward direction.

After the vertical impingement dryer 24, the web W is passed, while supported by the drying fabric 28, along a meandering path through the first drying group 25 comprising a number of steam-heated drying cylinders 32 in an upper row and a number of reversing rolls 33 in a lower row. After the first drying cylinder group 25 there can be one or more other drying cylinder groups, which can comprise one or two wires.

The web is passed as a closed draw the whole distance from the forming section 1 through the press section 2 to the beginning of the drying section 3, wherefore the web is not stretched unnecessarily and the number of breaks is minimized.

Fig. 2 shows in greater detail a forming section 1 of a paper machine in accordance with the invention. A fibre suspension is fed from a headbox 4 over a breast roll 6 onto a forming board 34, the structure of which is described in more detail in connection with Fig. 3. After the forming board 34 there is a suction box 35 within a bottom wire loop 5. At the beginning of the twin- wire zone, within a top wire loop 9 there is a dewatering box 11 whose curved deck 36 guides the top wire 9 to the surface of a partly couched web placed on the bottom wire 5. The dewatering box 11 of Fig. 2 comprises four successive suction chambers 11a, l ib, l ie and 1 Id, each of which can be controlled to have a different vacuum. The structure of the deck 36 of the first suction chamber 11a placed against the top wire 9 is such that it produces a non-pulsating dewatering pressure that remains substantially constant. This kind of structure of a forming shoe is described, for example, in the publication WO 2004/018768. The deck of the second, third and fourth suction chambers l ib, l ie and 1 Id is formed of dewatering blades 37, between which there are gaps through which the vacuum in the respective suction chamber l ib, l ie, 1 Id affects the web sandwiched between the top wire 9 and the bottom wire 5. The dewatering blades 37 generate pressure pulses that enhance dewatering and improve the formation of the web. At the third suction chamber l ie, within the bottom wire loop 5 there are a number of loading blades 12, each of which is located at a gap remaining between two dewatering blades 37 located above. This arrangement provides intensely pulsating dewatering, which alternates in direction, at the third suction chamber l ie.

After the dewatering box 11 there are further a curved suction box 38 and a transfer suction box 13 within the bottom wire loop 5, after which the top wire 9 is separated from the web W, which remains on support of the bottom wire 5.

Fig. 3 shows the structure of the forming board 34 and above it there is a graphic representation of dewatering pressure in different areas of the forming board. The forming board 34 comprises two successive dewatering boxes 39, 40, which are provided with different vacuums and deck constructions. The deck of the first suction chamber 39 has holes 41, 42, which are so placed that pressure pulses cannot be created but dewatering pressure is substantially constant. Air 46 brought with the wire 5 and with a slice jet 45 is removed through the first holes 41 and water is removed through the subsequent holes 42. The deck of the second suction chamber 40 is formed of dewatering blades 43 extending in a cross direction with respect to the machine direction, which dewatering blades produce pressure pulses

Pi, P₂ alternating in direction and doctor water from the bottom surface of the wire 5. A front part 44 of the forming board advantageously has a curved surface before the point where a slice jet 45 discharging from a headbox 4 hits the form- ing board 34. This allows a smaller angle of impingement of the headbox slice jet 45 against the forming board 34.

In the area of the first suction chamber 39 of the forming board 34, dewatering is gentle, whereby a thin fibre layer capable of retaining fines and fillers is couched against the wire 5. In the area of the second suction chamber 40 of the forming board 34, the dewatering blades 43 generate pressure pulses P₁, P₂, which enhance dewatering, break up fibre floes and improve formation.

Attempts are made to arrange the supply of the fibre suspension from the headbox onto the forming board so that the splashing of the web (stock jump) does not occur even at high speeds. This is achieved, for example, by placing the breast roll in a position in which its uppermost point is close to the level of the upper surface of the lower lip of the headbox and the vertical plane passing through the centre axis of the breast roll is at such a distance from the headbox on its outlet side that the slice jet discharging from the slice opening of the headbox can be arranged to hit the fourdrinier wire in a direction substantially parallel to it or at a very small angle of impingement either at the breast roll or after the breast roll in the rumiing direction of the web. One arrangement of this kind is described in FI application 990432. Stock jump can also be reduced using a downstream bevelled profile bar in the headbox.

Numerous modifications of the invention are feasible within the scope of protection defined in the following claims.

Claims

1. A paper or board machine for manufacturing a paper or board web having a low MD/CD ratio of tensile strength, comprising a forming section (1) for forming a web (W) from a fibre suspension, a press section (2) for dewatering the formed web (W), and a drying section (3) for drying the pressed web (W), characterized in that it comprises, as a combination, the following features: the forming section (1) comprises a single-wire initial portion which is followed by a twin- wire portion, at the beginning of which there is a forming shoe (1 Ia) which guides a top wire (9) onto the surface of a fibre suspension layer laid on a bottom wire (5), a surface (36) of which forming shoe (1 Ia) is provided with dewatering openings, which together with a vacuum prevailing in the forming shoe (Ha) produce non-pulsating dewatering of the fibre suspension layer sandwiched between the wires (5, 9), and which forming shoe (Ha) is followed by at least one dewatering element (l ib, l ie, l id) producing pulsating dewatering of the fibre suspension layer sandwiched between the wires (5, 9), and

- the press section (2) includes only one press nip (N), which is an extended nip through which the web (W) is passed sandwiched between two press fabrics (15, 17).

2. A paper or board machine as claimed in claim 1, characterized in that the forming section (1) comprises a headbox (4) for feeding a fibre suspension onto the bottom wire (5), which headbox is a dilution headbox.

3. A paper or board machine as claimed in claim 2, characterized in that the forming section (1) comprises a forming board (34) onto which the fibre suspension is fed from the headbox (4), which forming board (34) comprises a first suction chamber (39), which is provided with a deck structure which together with a vacuum prevailing in the first suction chamber (39) produces a non-pulsating dewatering pressure, and a second suction chamber (40), which is provided with a deck structure which together with a vacuum prevailing in the second suction chamber (40) produces a pulsating de watering pressure.

4. A paper or board machine as claimed in claim 3, characterized in that it com- prises a breast roll (6) disposed before the forming board (34), which breast roll

(6) is provided with means for causing the shaking of the breast roll (6).

5. A paper or board machine as claimed in claim 3 or 4, characterized in that it comprises a breast roll (6) before the forming board (34), which breast roll (6) is placed with respect to the headbox 4 in such a position that a slice jet (45) discharging from the headbox (4) hits the bottom wire (5) after the breast roll (6) in a direction substantially parallel to the bottom wire (5).

6. A paper or board machine as claimed in any one of the preceding claims, char- acterized in that the length of the single-wire initial dewatering portion is at least

500 mm.

7. A paper or board machine as claimed in any one of the preceding claims, characterized in that the extended nip (N) is formed between a shoe roll (18) and a- counter roll (19).

8. A paper or board machine as claimed in any one of the preceding claims, characterized in that the web (W) is passed from the press section (2) to the drying section (3) as a closed draw.

9. A paper or board machine as claimed in any one of the preceding claims, characterized in that the drying section (3) comprises at least one drying cylinder group (25), which is preceded by at least one impingement dryer (23, 24).

10. A paper or board machine as claimed in claim 9, characterized in that the drying section (3) comprises a vertical impingement dryer (24).

11. A paper or board machine as^' claimed in claim 10, characterized in that the drying section (3) comprises a horizontal impingement dryer (23) disposed before the vertical impingement dryer (24).

12. A method for manufacturing paper or board having a low MD/CD ratio of tensile strength, characterized by the steps of feeding a fibre suspension from a headbox (4) to a single-wire initial de- watering portion formed by a bottom wire (5); - guiding a top wire (9) by means of a forming shoe (1 Ia) onto the surface of a fibre suspension layer laid on the bottom wire (5) to provide a twin- wire dewatering portion in which a web (W) is dewatered first by means of the forming shoe (Ha) producing non-pulsating dewatering and after that by means of at least one dewatering element (l ib, l ie, l id) produc- ing pulsating dewatering; transferring the web (W) as a closed draw from a forming section (1) to a press section (2) in which it is passed through a single press nip (N) sandwiched between two press fabrics (15, 17), the press nip (N) being an extended nip; and - passing the web (W) as a closed draw from the press section (2) to a drying section (3) where it is dried.

13. A method as claimed in claim 12, characterized by the step of producing paper or board having an MD/CD ratio of tensile strength of 2.5 at the most.

14. A method as claimed in claim 12 or 13, characterized by the step of feeding a fibre suspension onto the bottom wire (5) by means of a dilution headbox (4).

15. A method as claimed in claim 14, characterized by the step of feeding the fibre suspension from the headbox (4) onto a -forming board (34) comprising a first dewatering zone, in which dewatering is non-pulsating, and a second dewa- tering zone, in which dewatering is pulsating.

16. A method as claimed in claim 15, characterized by the step of shaking the bottom wire (5) to reduce fibre orientation.

17. A method as claimed in claim 14 or 15, characterized by the step of guiding a slice jet (45) discharging from the headbox (4) to hit the bottom wire (5) after a breast roll (6) in a direction substantially parallel to the bottom wire (5).

18. A method as claimed in any one of the preceding claims, characterized in that the length of the single-wire initial dewatering portion is at least 500 mm.

19. A method as claimed in any one of the preceding claims, characterized by the step of drying the web (W) by means of at least one impingement dryer (23, 24), after which it is dried by means of at least one drying cylinder group (25).

20. A method as claimed in claim 19, characterized by the step of drying the web (W) by means of a vertical impingement dryer (24).

21. A method as claimed in claim 20, characterized by -the step of drying the web (W) first by means of a horizontal impingement dryer (23) and after that by means of a vertical impingement dryer (24).