WO2004079090A1

WO2004079090A1 - Method and device for pressing a paper web

Info

Publication number: WO2004079090A1
Application number: PCT/FI2004/050022
Authority: WO
Inventors: Markus Laitila
Original assignee: Metso Paper, Inc.
Priority date: 2003-03-04
Filing date: 2004-03-02
Publication date: 2004-09-16
Also published as: FI20030337A0; FI114031B; DE112004000357T5

Abstract

The press device for a paper web comprises a flexible moving element (1a) forming an endless structure, and a counter roll (2). Inside the moving element (1a) there is a shoe element (3) that is arranged to support the moving element (1a) against the counter roll (2) to form a nip (N). In the machine direction, the shoe element is supported at a first point to a fixed pivot (P) and at a second point to a loading means (4) which is arranged to cause the loading of the shoe element towards the nip and its movement in relation to said pivot.

Description

METHOD AND DEVICE FOR PRESSING A PAPER WEB

The invention relates to a method for pressing a paper web, in which the paper web is guided through a nip formed between a moving ele- ment forming a flexible, endless structure and a counter roll, of which the moving element is a flexible shell or a belt loop, wherein the moving element is supported in the area of the nip against another roll from inside the roll with a shoe element. The invention also relates to a press device for a paper web. In this context, the term paper web also refers to such webs made of pulp, the basis weight of which is within a range where products are usually called by the term paperboard.

The paper web formed in the wire section is pressed at two points in the line of making paper or paperboard: in the press section, in which water is removed from the wet paper web by pressing, and in the calender, in which the surface is finished by applying pressure on the already relatively dry paper web. Even though the pressing has different purposes in the press section and in the calender, the web has quite a different dry matter content when it enters these sections, and the technical development of said sections is guided by the phenomena affecting the paper therein, a common feature for both sections is that they both have a nip in which a certain pressure affects the web, the pressure being dependent on the force with which the two moving surfaces forming the nip, normally the shell surfaces of two rotating rolls, are pressed against each other from both sides of the web.

For example in a shoe calender, a nip is formed by combining a soft- faced roll and a hard roll, which nip has some extension in the longitudinal direction of the machine, and thus the paper web to be calendered has a long retention time in the calender nip. The shell of the soft-faced roll is made of a flexible belt, and the hard-faced roll is a metal roll, which is a heated roll functioning as a thermoroll that brings heat to the web in the calendering. The shell of the soft roll is loaded from inside the roll against the hard roll by a loading shoe, and as a result of this, the paper web travelling along the surface of the hard roll is pressed at a given pressure between the surfaces of the soft shell and the hard roll over a long distance in the nip. At the same time, the elastic shell can be compressed in its thickness direction in the area of the nip. The belt forming the shell of the soft roll can be made of a suitable elastic polymer, such as polyurethane, and inside the belt there is a fabric that reinforces the same. As a result of the nip structure, it is possible to attain a good bulk and stiffness for the paper or paperboard, as well as a uniform smoothness of the surface. To sum up, the calender is especially well suited for on-line calendering of printable paper or paper- board grades. Said calender type is called a shoe calender, and it is known by the trademark "OptiDwell". One further embodiment of the same is described in the international publication WO 99/28551.

Extended nip presses comprising a shoe element with a concave surface in the press section, in turn, are disclosed in the Finnish patent 98843 and e.g. in the corresponding US patent 5,908,536, in the US patents 5,084,137, 5,262,011 , 5,639,351 , and in the international publication WO 99/19562. In these publications, inside a so-called hose roll there is a shoe element which is loaded against the inner surface of a flexible roll shell with a given force, and the web is guided together with one or two press felts through a nip formed between the hose roll and a counter roll.

It is known to adjust the pressure exerted on the paper web in the nip by hydraulic cylinders acting on the shoe element and supported on a stationary shaft inside the shell. The cylinders are normally arranged in two rows to achieve a linear loading movement. Such loading systems for adjusting the pressure of the calender nip are disclosed, for example, in US patents 5,582,689 and 6,213,009, and placed in the roll of the press, in US patent 5,639,351.

The Finnish patent 65103 and the corresponding US patent 4,713,147 disclose a method for changing the location of the centre of gravity of the support force of the shoe element in the machine direction, wherein the distribution of the dewatering pressure in the longitudinal direction of the nip can be changed. In practice, this idea is implemented by providing parallel pressurized fluid hoses, which load the shoe, with unequal pressures which can be adjusted independently of each other.

The same aim is presented in US patent 6,306,261 which discloses a concave shoe divided into segments which are hinged to each other and which can each be separately loaded.

In fact, the loading implemented by means of two parallel (successive in the machine direction) rows of loading cylinders, or loading hoses, is difficult to control (unless asymmetry is desired, as in US patent 4,713,147). Examples of this are undesired closings of the nip in an inclined position or at the edges. With the known loading system, a problem has also been the precise positioning required by the shoe.

It is an aim of the invention to eliminate said disadvantages and to present a method, by which the loading in the nip, through the shoe element, can be implemented with a greater reliability, and the positioning of the shoe element is also more accurate. To achieve this aim, the method is primarily characterized in that the shoe element is supported, in the machine direction, at a first point to a fixed pivot and at a second point to a loading means, whose force effects the loading of the shoe element and, correspondingly, the pressure in the nip and the movement of the shoe element, with the pivot as a fulcrum.

The pivot is located in a fixed position with respect to the moving element passing through the nip and over the surface of the shoe element. The pivot is supported to a stationary frame structure, such as the stationary shaft of the shoe roll. To the same frame structure is also supported the loading means, which can be a uniform loading hose, or the like, extending in the cross-machine direction, or a row of loading cylinders extending in the cross-machine direction. There may also be several loading means in the machine direction, at different distances from the pivot. With a combination of the pivot in a fixed position and one or more loading means, it is possible to implement an adjustable tilt, by which the pressure curve of the nip (the pressure distribution in the longitudinal direction of the nip) can be changed during the operation. The shoe element can be designed to have such a surface facing the nip that it takes into account the turning movement taking place in relation to said pivot in the horizontal plane (in the plane perpendicular to the axes of the rolls).

By means of the invention, it is possible to implement a moving shoe element having, however, an improved motional accuracy, thanks to the fixed pivot. The only variable having an effect on the loading force and simultaneously the nip pressure is the force produced by the loading means, which is exerted, in the machine direction, on only one point in the shoe element, by means of either an elongated loading hose or a row of loading cylinders. Similarly, the position (tilt) only needs to be adjusted by means of a single loading means, whereas the arrange- ments of prior art require two successive loading means in the machine direction.

The press device according to the invention, in turn, is characterized in that the shoe element is, in the machine direction, supported at a first point to a fixed pivot and at a second point to a loading means which is arranged to cause the loading of the shoe element in the direction towards the nip and the movement in relation to said pivot.

The loading means can be a hose placed in the cross-machine direc- tion, having an effect on the shoe element at substantially one point in the machine direction, and containing a pressurized medium whose pressure determines the loading force, or it can be a row of loading cylinders extending in a similar manner in the cross-machine direction, the loading being similarly caused by the pressure of the pressurized medium inside the cylinders. The loading means may be supported to a stationary frame, in relation to which the pivot is also in a fixed position.

According to one structural alternative, the shoe element may be fixed to a support plate, said loading means being provided at its opposite side and one edge of the support plate being arranged pivotable around said pivot. The arrangements can be applied both in the press nips of the press section and in the calender nips of the calender, even though there are differences in the aforementioned sections of the papermaking process. A considerable increase in the dry matter content takes place in the press section, in the press nips after the wire section typically from the level of 16 to 25 % to the level of 42 to 55 %. In a calender in which the dry matter content of the paper is not consirerably increased any more, the dry matter content of the ingoing paper is typically at least 85 %.

The flexible element, which can be the shell of a so-called "hose roll" or a belt forming an endless loop, is capable of conforming to the shape of the guiding supporting surface of the shoe element when it travels over the surface. The concept of an elast c moving element, in turn, refers to such a shell or a looped belt that s capable of deforming under the effect of the loading pressure effecti ve in the nip, and produces a nip which extends in the travel direction of the web.

The loading means loads the shoe against a counter roll on the oppo- site side of the nip and thus affects the nip pressure. The loading means is adjustable in a known manner, wherein it is possible to adjust the magnitude of the loading and thereby the nip pressure level.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a side-view of a calender in which the invention can be used,

Fig. 2 shows a side-view of an advantageous embodiment for arranging the loading in the calender of Fig. 1 ,

Fig. 3 shows a side-view of a second embodiment,

Fig. 4 shows a side-view of a third embodiment, and Fig. 5 shows a side-view of a fourth embodiment.

Fig. 1 shows a calender in which the invention can be used. The paper web W is guided through a calender nip N formed between two rolls. The lower first roll 1 comprises a shell 1 a made of an elastic flexible material, which rotates around the rotating axis of the calender roll. The upper second roll 2 is a heated calender roll with a hard surface, for example a roll equipped with a metal shell, the surface of which is harder than the elastic flexible material of the shell. The shell 1a that is elastic in its thickness direction (in the direction of the radius of the calender roll 1) is compressed in the shape defined by the shoe element 3 guiding the shell from inside and the surface of the second roll 1, thereby forming an extended nip N, in which the web W travels between the surface of the second roll 2 and the surface of the com- pressed shell 1a of the first roll 1. The surface of the shoe element 3 that guides the shell is convex, and it forms a part of a cylinder extending in the direction of the rotation axis of the calender, whose radius of curvature and the corresponding centre of curvature is on the side of the first roll 1 from the nip N. In Fig. 2, the radius of curvature R equals the inner radius of the first roll 1 , i.e. the centre of curvature and the rotation axis of the cylinder coincide. The guiding surface of the shoe element 3, over which the shell 1a slides, may also be straight or concave, wherein in the latter case, the centre of curvature of the surface is on the side of the second roll 2.

The shoe element 3 is loaded against the inner surface of the shell 1a by a loading means which exerts an adjustable loading force F on the shoe element 3. The loading means and the fixing of the shoe element 3 will be discussed below, that is, the cylinders can be hydraulic cylin- ders. The loading means is supported to an axial supporting element E inside the roll 1.

Because of the effect of the rolls 1 and 2, the nip contact is not linear but it has a certain dimension in the travel direction of the web. It is a so-called extended nip N. Figure 2 shows the mounting of the shoe element 3 in more detail. The shoe element 3 is arranged pivotable with respect to the pivot P in the horizontal plane. The pivot P is supported to a support element E. Mechanically, the shoe element 3 moves in such a way in relation to the pivot P that the points of its supporting surface facing the nip move along circular paths with the pivot P as their centre point. The shoe element 3 is mounted in such a way in relation to the pivot that it forms a kind of a single-arm lever whose fulcrum is said pivot P, the supporting surface of the shoe element 3 being on the same side of the pivot P. The forces of the supporting surface at different points in its longitudinal direction (machine direction) are thus determined according to the torque calculated in relation to the pivot P. The movement and simultaneously the loading force are produced by a loading means 4 effective between the shoe element 3 and the support element E. The loading means 4 is located at a distance from the pivot P in the machine direction (direction of movement of the shell 1a). In Fig. 2, the loading means 4 is an expanding loading hose placed in the cross-machine direction and containing a pressurized medium, preferably a fluid. By adjusting the pressure p of the pressurized medium, it is possible to adjust the force F exerted on the shoe element and, correspondingly, the nip pressure.

Structurally, the mounting and support of the shoe element 3 are implemented by fixing it to a support plate 5, and the loading hose used as the loading means 4 on the opposite side of the plate is placed between the support plate 5 and the support element E. The edge of the support plate is connected to the support element E by means of a joint which forms the above mentioned pivot P.

In Fig. 2, the shape of the shoe is concave, and it can be designed by taking into account the non-linear or "rotational" character of the loading movement.

The load return movement to the rest position can be effected, in the hose loading of Fig. 2, by means of gravity or, if necessary, by means of return springs (not shown). The loading means 4 can also be used to adjust the tilting of the shoe element 3 in relation to a straight line parallel to the central axis of the roll, to influence the pressure curve in the nip N.

Figure 3 shows the mounting of the shoe element 3 according to another embodiment, with the same principle and possibilities of movement. In this embodiment, however, the loading hose is replaced with a row of cylinders driven by pressurized medium, extending in the cross- machine direction, and forming the loading means 4. The cylinders are supplied with the pressurized medium, preferably hydraulic fluid, having a given pressure p and exerting, in a corresponding manner, the force F on the shoe element 3 and thereby producing the nip pressure in the nip.

The position of the press device, shown in Figs. 2 and 3, is such that the web W enters the nip from the side of the pivot P. However, it is possible that the loading mechanism formed by the shoe element 3 and the support plate 5 is located mirror-symmetrically, that is, the pivot P is on the exit side of the web.

Figure 4 shows, in a side view, the mounting of the shoe element according to a third embodiment of the invention. In the machine direction (movement direction of the shell 1a), at distances from the pivot P, there are more than one loading means 4 effective between the shoe element 3 and the support element E, each at a different distance from the pivot P, on the same side of the pivot. In Fig. 4, there are two such means 4 effecting a force which can be changed, in contact with the support plate 5. The pressures effective in the different loading means can be separately controlled. Thus, the loading means 4 can also be used to implement different functions in such a way that one is used to implement the adjustment of the position (tilt) of the shoe element, or the fine adjustment of the loading. In Fig. 4, the loading means 4 are expanding loading hoses placed in the cross-machine direction and containing a pressurized medium, preferably a fluid. The force is adjusted according to the above-described principle. One of the loading means 4 can also be a series of hydraulic cylinders, according to the principle of Fig. 3. This alternative is illustrated in Fig. 5. Yet another possible alternative is that both loading means 4 consist of a series of hydraulic cylinders. There may also be more than two loading means in different locations in the machine direction (longitudinal direction of the nip N).

Advantageously, the shoe element 3 has a continuous supporting surface, and it has no static oil pocket or pressure pocket on its outer sur- face. Lubricating oil can be supplied between the supporting surface of the shoe element 3 and the shell 1 a from the front edge of the shoe element , as illustrated by arrow L in Figs. 2 and 3. However, the invention also covers the alternative that the lubricating oil is introduced via bores made through the shoe element, which open in a pocket formed on the supporting surface guiding the shell 1 a of the shoe element 3.

In all the above-mentioned shaping alternatives, the front and rear edges of the shoe element are chamferred or rounded in shape, wherein the chamferred or rounded sections are not intended for sup- porting surfaces of the nip contact, but to ensure a problem-free sliding of the shell 1a on top of the supporting surface of the shoe element, and off the supporting surface.

The supporting surface of the shoe element 3 is a part of a continuous structure, meaning that it is not composed of separate blocks in the machine direction, which could be transferred closer to and further away from each other. The supporting surface is turned as a single unit around said pivot P. The rigidity of the support plate 5 bearing the shoe element 3 can be used to affect the even setting of the shoe element towards the nip. The plate 5 may be solid in the cross-machine direction (direction of the shaft of the roll), or it may be composed of different parts in the cross-machine direction.

The invention is not limited to the order of the rolls shown in the figures. It is, for example, possible that the roll 1 or the like equipped with a shoe element is up and the thermoroll 2 is down. Hereinabove, the flexible elastic element 1a is a roll shell, that is tubular or hose-like and fixed at its opposite ends to the ends of the roll which are journalled rotatable. The roll shell is clearly wider than the shoe element supported to a static element inside the shell by means of loading devices, so that it can have a circular shape in its fixing point to the ends and can travel along a path determined by the supporting surface of the shoe element over the width of the nip line. It is, however, possible that the flexible, elastic element is a belt instead of a roll shell, said belt forming an endless belt loop, wherein the belt is brought over the shoe element 3. The support element E, to which the shoe element 3 is fixed and supported, is thus inside the belt loop.

In addition to a calender, the invention can also be applied in the press section, taking into account its special requirements. In the press sec- tion, the flexible hose-like shell 1a arranged rotatable, and the shoe element 3 guiding the same, are used to construct the press nip in a way known from so-called extended nip structures. As the compression pressure can be adjusted in a known way by the loading means 4, the arrangement can be used to affect dewatering in the press nip. It is also possible to bring two press elements through the nip, which press elements are in the form of an endless felt or belt passed as a loop around the corresponding press roll.

Claims

Claims:

1. A method for pressing a paper web (W), in which the paper web (W) is guided through a nip (N) formed between a flexible moving element (1a) forming an endless structure, and a counter roll (2), of which the moving element is a flexible shell or a belt loop, wherein the moving element (1a) is supported in the area of the nip (N) against the counter roll (2) by means of a shoe element (3) which determines the shape of the nip, characterized in that the shoe element (3) is supported, in the machine direction, at a first point to a fixed pivot (P) and at a second point to a loading means (4), whose force is used to cause the loading of the shoe element (3) and a corresponding pressure (p) in the nip and the movement of the shoe element (3) with the pivot (P) as a fulcrum.

2. Method according to claim 1 , characterized in that the loading force is caused by a hose-like loading means (4), by the pressure of a pressurized medium therein.

3. The method according to claim 1 , characterized in that the loading force is caused by power-generating actuators which are placed in a row and which jointly constitute the loading means (4).

4. The method according to any of the preceding claims, characterized in that the loading is effected by two or more loading means (4) at dif- ferent distances from the pivot (P).

5. The method according to any of the preceding claims, characterized in that one or more loading means (4) are used to adjust the position or "tilt" of the shoe element (3).

6. The method according to any of the preceding claims, characterized in that the loading or the position adjustment is caused by a single-arm lever, in which the shoe element (3) is located and whose fulcrum the pivot (4) is.

7. A press device for a paper web, which contains a flexible moving element (1a) forming an endless structure, and a counter roll (2), wherein inside the moving element (1a) there is a shoe element (3), which is arranged to support the moving element (1a) against the counter roll (2) to form a nip (N), characterized in that the shoe element is, in the machine direction, supported at a first point to a fixed pivot (P) and at a second point to a loading means (4) which is arranged to cause the loading of the shoe element towards the nip and the movement in relation to said pivot.

8. The press device according to claim 7, characterized in that the loading means (4) is a hose-like loading means extending in the cross- machine direction.

9. The press device according to claim 7, characterized in that the loading means (4) is a row of actuators extending in the cross-machine direction.

10. The press device according to claim 8, characterized in that the actuators are hydraulic cylinders.

11. The press device according to any of the preceding claims 7 to 10, characterized in that there are two or more loading means (4) at different distances from the pivot (P).

12. The press device accord ng to any of the preceding claims 7 to 11 , characterized in that the p vot (P) is placed on one side of the shoe element (3) in the machine d rection.

13. The press device according to claim 12, characterized in that the shoe element (3) is fixed to a support plate (5), the loading means (4) is arranged on the opposite side of the support plate (5), and the edge of the support plate (5) is arranged pivotable by means of said pivot (P).

14. The device according to any of the preceding claims 7 to 13, characterized in the counter roll (2) has a harder surface than the moving element (1a).

15. The device according to any of the preceding claims 7 or 14, characterized in that it is a calender.

16. The device according to claim 15, characterized in that the counter roll (2) is a heated thermoroll.

17. The device according to claim 15 or 16, characterized in that the moving element (1a) forming an endless structure is the roll shell of a calender roll.

18. The device according to any of the preceding claims 7 to 14, characterized in that it is a press for removing water from a paper web by pressing.