WO2009135992A1

WO2009135992A1 - Forming section of paper or board machine, method for the forming of web, and suction box

Info

Publication number: WO2009135992A1
Application number: PCT/FI2009/050349
Authority: WO
Inventors: Samppa Ahmaniemi; Antti Poikolainen; Tomi Tissari; Jukka Muhonen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2008-05-05
Filing date: 2009-05-04
Publication date: 2009-11-12
Anticipated expiration: 2010-11-05
Also published as: FI20085408A7; FI20085408L; FI20085408A0

Abstract

The forming section of a paper or board machine comprises a double-wire part, where water is removed from the web between two wires, and after the double- wire part there is at least one high-vacuum suction box (20) before a point where the web is transferred to the press section. The vacuum in the suction box (20) is 20-80 kPa, the cover (21) of the suction box is equipped with holes (22), the centre axis (B) of which holes (22) forms a sharp angle (α) with the direction of travel (A) of the wire (10) running over the cover (21), and the open area of the cover (21) is 40-90%. The suction box (20) is placed at the end of the forming section at a location where the dry solids content of the web (W) is at least 6%, preferably at least 10%.

Description

Forming section of paper or board machine, method for the forming of web, and suction box

The present invention concerns the forming section of a paper or board machine, where the forming section comprises a double-wire part, where water is removed from the web between two wires, and after the double-wire part there is at least one high-vacuum suction box before a point where the web is transferred to the press section. The invention also concerns a method for the forming of a paper or board web on such a forming section, and a high-vacuum suction box.

Prior art solutions include the placing of at least one flat suction box on a short single-wire part following a double-wire web forming and dewatering zone, the purpose of the flat suction box being to increase the dry solids content of the web before the web is transferred to the press section. After the double-wire part, the dry solids content of the web is already so high that it is difficult to extract more water out of the web without a considerable vacuum. The use of a high vacuum involves many problems, one of which is the increase in the porosity of the web as a function of vacuum.

Present flat suction boxes generally employ ceramic foil covers, which have de- watering foils in the cross direction to the direction of travel of the web. Such a foil arrangement wears the wire, because the wire bends as a result of vacuum every time the wire runs over a gap between two foils. A high vacuum raises the power required to run the wire and increases the wear of the wire as a result of friction forces. It is often necessary to make compromises in terms of the dry solids content, because the wire wears more at higher vacuum levels and the power required to run the wire increases. The web running over the foil cover of the suction box is subject to a series of very short vacuums created by separate pulses. The porosity of the web increases as a function of vacuum, which is why the prob- lem involved in the use of high vacuums is that there are more pinholes in the web.

One problem in prior art forming sections is the re-wetting of the web after the suction box. FI patent publication 110702 presents a method for preventing the re- wetting of the web. In the method, the web is led, supported by a wire, past at least two vacuum chambers so that in the direction of travel of the web, the vacuum in the first vacuum chamber pulls water from the web and wire, and the vacuum in the second vacuum chamber prevents the transfer of moisture from the wire back to the web, so that the vacuum in the first vacuum chamber is higher than the vacuum in the second vacuum chamber. The vacuum chambers used in the method are ordinary high-vacuum suction boxes with a foil cover.

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US patent 2 487 202 presents a suction box cover where the suction holes are ar- ranged to run through the cover at an inclined angle with respect to the direction of travel of the wire. Due to the inclination of the holes, water removed from the wire side meets the inclined back wall of each suction hole, and the back wall tends to guide the water out of the wire instead of letting the water splash back to the web through the wire. The suction box is used beneath the Fourdrinier forming section, where the web is still relatively wet and it is easy to remove water from it. The suction box as such is not suited for use at the end of the forming section, where a high vacuum is needed. A high vacuum together with suction holes arranged in lines in the machine direction and with a small open area of the cover easily creates streaks in the web produced.

The objective of the present invention is to overcome or at least reduce the disadvantages related to prior art solutions. The specific objective is to intensify dewa- tering after the double-wire part before the web is transferred to the press section. Another objective is to reduce the power needed to run the wire, and to save the wire from wear. The characteristics of the forming section according to the invention are presented in the characterizing part of claim 1.

The characteristics of the method according to the invention are presented in the characterizing part of claim 6.

The characteristics of the suction box according to the invention are presented in the characterizing part of claim 8.

A high- vacuum suction box with a vacuum of 20-80 kPa is placed at the end of the forming section according to the present invention. The cover of the suction box is provided with holes, with the center axis of the holes forming a sharp angle with the direction of travel of the wire running over the cover. The open area of the cover is 40-90%. It has been discovered that by using such a cover with in- clined holes, the power needed to run the wire can be reduced, the dry solids content of the web after the forming section can be improved, and the life time of the wire can be extended as compared to prior art solutions, where the suction box in a corresponding position has a foil cover.

The power needed to run the wire decreases as the friction between the suction box cover and the wire sliding over it is reduced. This is so mostly because the wire makes a considerably smaller bend at each individual suction hole than when the wire runs over a uniform slot in the cross direction of the machine. When the power needed to run the wire is decreased, the wire usually also wears less and its life time is extended.

The dry solids content of the web increases mostly because the holes drilled through the cover at an inclined angle remove water from the wire and web more effectively than slots or holes going through the cover at an angle of 90°. The vac- uum of the suction box pulls water from the web to the lower surface of the wire, and the water is guided downwards in the suction hole along the inclined back wall of the hole. By drilling the holes at an inclined angle through the cover, it is possible to reduce the splashing of water from the back wall of the hole back to the web. The open area of the hole cover can be increased easily from that in a slot cover without compromising the strength of the cover. Friction is also re- duced as the regions between the holes are narrow, so they are lubricated well by water. The creation of streaks in the web is prevented by not placing the suction holes in rows in the machine direction.

The suction box cover is preferably manufactured from steel and coated with a ceramic or other coating which enhances durability. The cover can also be completely of a ceramic material or some other suitable material.

The solution according to the invention accomplishes significant benefits over a conventional forming section, where there is a suction box with a foil cover at the end of the forming section. One essential difference is that on the forming section according to the present invention, the amount of pinholes in the web does not increase even though the vacuum in the suction box needs to be raised. When using conventional suction boxes with a foil cover, it is not always possible to use a sufficiently high vacuum, because in this case there would be too many pinholes in the web, and this is not acceptable in all paper grades.

Another essential difference to the use of a conventional suction box with a slot cover is that a high-vacuum suction box with inclined holes gives a high dry solids content after the press section at a relatively low vacuum level (30-40 kPa). So as to achieve a high dry solids content after the press section, it has been necessary to use a very high vacuum level at the end of the forming section in conventional solutions. This has resulted in high energy consumption.

The suction box with inclined holes used on a forming section according to the invention does not remove as much fines from the lower surface of the web as a suction box with a slot cover, because dewatering is not pulsating. In certain paper grades, a low fines content on the lower surface of the web has been a problem when using a high-vacuum suction box with a foil cover.

The forming section according to the invention allows the use of lower wire ten- sions, because there is no fear of the wire bending at the slot between foils in a high- vacuum suction box. This enables the use of rolls with a lighter structure on the forming section.

The benefits of a suction box with a hole cover with inclined holes include smaller friction and easier dewatering, which enables the use of a smaller vacuum level. There is less re- wetting of the web after the suction box when the center axis of the holes is in a sharp angle with respect to the direction of travel of the wire. It has been estimated that by using the new cover design for the suction box, the power needed to run the wire can be reduced by more than 10%, and the life time of the wire can be extended by more than 10%. The new cover design enables an improvement of more than 1% in the dry solids content of the web after the forming section as compared to a conventional foil cover.

In what follows, the invention is described by making reference to the enclosed figures, with the invention not being restricted narrowly by the figures.

Figure 1 is a side view of the forming section according to the invention.

Figure 2 is a side view of the high-vacuum suction box cover.

Figure 3 is a top view of the suction box cover.

Figure 1 shows a double-wire forming section, which comprises bottom wire 10 and top wire 11, between which there is a double-wire web forming and dewater- ing zone. The pulp suspension is fed from the headbox 12 into the forming gap between the wires 10 and 11. Dewatering from the pulp suspension begins as the wires 10, 11 turn over the suction sector of the forming roll 13. The forming roll 13 is followed by a two-chamber dewatering box 14, at the first dewatering chamber of which there are loaded dewatering foils 15 on the side of the bottom wire loop 10, and the dewatering foils 15 can be loaded against the cover of the dewa- tering box 14. The dewatering box 14 is followed by the second forming roll 16 placed on the side of the bottom wire loop 10, where the forming roll 16 turns the inclined upward run of the wires 10, 11 to an inclined downward run. At the end of the double- wire part on the side of the bottom wire loop 10, there is a transfer suction box 17, which ensures that the web follows the bottom wire 10 when the top wire 11 is separated from the bottom wire 10.

The double-wire part is followed by a short single-wire part, where the web W runs suspended by the bottom wire 10 to the pick-up point P, where the web W is detached from the bottom wire 10 by means of the pick-up roll 18 and transferred to be suspended by the fabric 19 on the press section. On this single- wire part, there is a high- vacuum suction box 20, the purpose of which is to remove as much water from the web W before the web W is transferred from the forming section to the press section. The dry solids content of the web before the suction box 20 is more than 6%, preferably more than 10%.

Figure 2 shows a magnification of the high- vacuum suction box 20, and Figure 3 shows a part of the suction box cover 21 as a top view. Through the cover 21, there a high number of holes 22, the center axis B of which forms a sharp angle a with the direction of travel A of the wire 10 running over the cover 21. The angle a in this case is approximately 45°. Generally, the angle a should be in a range of 30-85°, so that the wire 10 running over the suction box cover 21 meets the back edge of the hole 22 at a sharp angle. In this case, the back edge of the hole 22 scrapes water from the lower surface of the wire 10, and the water is removed downwards along the inclined back wall of the hole 22. The holes 22 are placed in a staggered pattern with respect to each other so that in the direction of travel A of the web there is no uniform line formed by consecutive holes 22, which might create streaks in the web. The wire 10 running over the cover 21 cannot bend significantly at the holes 22, because the wire 10 always has lateral support at each hole 22. The open area of the cover 21 can be relatively high, higher than in conventional slot covers. Figure 3 does not disclose the fact that the cross section of the cylindrical holes 22 in the direction of the plane of the cover 21 is actually oval, which contributes to an increase in the open area of the cover 21.

Typically, the length of the suction box cover 21 in the direction of the paper machine is 100-600 mm, and the width of the cover 21 corresponds to the width of the paper machine. The cover 21 can be uniform or it can be assembled from several cover elements installed side by side in the width direction of the paper ma- chine. The cover 21 can be straight or curved. The open area of the cover 21 can be in a range of 40-90%. The diameter of the holes 22 is usually 5-20 mm, and their center axis B forms an angle of 30-85° with respect to the direction of travel of the wire 10. The holes 22 can be cylindrical, or their cross-sectional area may be oval or polygonal. A vacuum of 20-80 kPa is preferably used in the suction box 20.

The many variations of the invention are possible within the scope of protection defined by the below patent claims. A high-vacuum suction box according to the invention can be placed not only on the single-wire part following the double-wire part, but also at the end of the double-wire part or, on a Fourdrinier forming machine, at the end of the web forming section on an area where the dry solids content of the web is more than 6%, preferably more than 10%. Several suction boxes according to the invention can be placed after one another, or the suction box can be placed after a normal flat suction box.

Claims

Claims:

1. A forming section of a paper or board machine, where the forming section comprises a double-wire part, where water is removed from the web between two wires (10, 11), and after the double- wire part there is at least one high- vacuum suction box (20) before a point (P) where the web (W) is transferred to the press section, characterized in that the vacuum in the suction box (20) is 20-80 kPa, the cover (21) of the suction box (20) is equipped with holes (22), the centre axis (B) of which holes (22) forms a sharp angle (a) with the direction of travel (A) of the wire (10) running over the cover (21), and the open area of the cover (21) of the suction box (20) is 40-90%.

2. A forming section as claimed in claim 1, characterized in that the cover (21) of the suction box (20) is a coated steel cover.

3. A forming section as claimed in claim 1 or 2, characterized in that there is a vacuum of over 30 kPa, preferably over 40 kPa, in the suction box (20).

4. A forming section according to any of the above claims, characterized in that the centre axes (B) of the suction box cover holes (22) form an angle (a) of 35-

85°, preferably 40-60°, with the direction of travel (A) of the wire (10) running over the suction box (20).

5. A forming section according to any of the above claims, characterized in that the suction box (20) is placed at a location where the dry solids content of the web

(W) is at least 6%, preferably at least 10%.

6. A method for the forming of a paper or board web on a forming section, which comprises a double-wire part, after which double-wire part, before the web (W) is transferred to the press section, water is removed from the web by means of a high-vacuum suction box (20), characterized in that a vacuum of 20-80 kPa is used in the said suction box (20) and that the suction box (20) is equipped with a cover (21), the open area of which is 40-90% and which comprises holes (22), the centre axis (B) of which holes (22) forms a sharp angle (α) with the direction of travel (A) of the wire (10) running over the cover (21).

7. A method as claimed in claim 6, characterized in that the dry solids content of the web (W) when the web (W) comes to the suction box (20) is over 6%, preferably over 10%.

8. A high-vacuum suction box (20) intended for the forming section of a paper or board machine, characterized in that there is a vacuum of 20-80 kPa in the suction box (20), the cover (21) of the suction box is equipped with holes (22), the centre axis (B) of which holes (22) forms a sharp angle (α) with the direction of travel (A) of the wire (10) running over the cover (21), and the open area of the cover (21) of the suction box is 40-90%.