WO2001051704A1 - Method for pressing a paper web and a press device for a paper web - Google Patents

Abstract

The press device for a paper web contains at least one calender nip (N) for applying pressure to the paper web, said calender nip (N) being formed between a first roll (1) equipped with a flexible shell (1a) and a second roll (2). A shoe element (3) is placed inside the first roll (1) at the location of the nip (N) in such a manner that it guides and supports the shell (1a) of the first roll against the second roll (2). The centre of curvature of the surface of the shoe element (3) that supports the shell (1a) and guides the same through the nip (N) is on the side of the first roll (1) from the nip (N). The press device can be a calender for finishing the surface of paper or a press for removing water from the wet web.

Description

Method for pressing a paper web and a press device for 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 two rolls, of which the first roll is equipped with a flexible shell which is supported in the area of the nip against the second roll from inside the roll with a shoe element. The invention also relates to a press device of a paper web.

The paper web formed in the wire section is pressed at two points in the papermaking line: 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 a different purpose 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 given 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 roll with a soft surface and a hard roll, which nip extends 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 roll with a soft surface is made of an elastic flexible belt, and the roll with a hard surface is a metal roll which is a heated roll functioning as a thermoroll that brings heat to the web. The shell of the soft roll is loaded from inside the roll against the hard roll by a loading shoe with a concave surface, 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 within 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 weave that reinforces the same. Thanks to 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 to on-line calendering of printable paper or paperboard grades. Said calender type is called a shoe calender, and it is known by the trademark "OptiDweH". One further embodiment of the shoe calender is described in the international publication WO 99/28551.

The surface of the shoe guiding the shell of the soft roll through the nip is concave to form a long nip, and it corresponds approximately to the curvature of the surface of a hard roll. In a shoe calender of this type fibre roughness has been detected on the side which has been pressed against the soft belt. Supposedly, the problem is due to the fact that the surface of the belt which is against the web to be calendered is shortened considerably in the area of the nip as a result of the shoe with the concave surface, and thereby the web to be calendered is also urged to shorten.

In addition, US patent 5400707 discloses a calender in which the second nip is formed between a belt loop brought over a shoe with a convex surface, and a heated calender roll with a smooth surface. This belt, called finishing belt, requires many guiding rolls and occupies space in the machine.

Finnish patent 98843 and the corresponding US patent 5908536, as well as US patents 5084137, 5262011 , 5639351 and the international publication WO 99/19562, in turn, disclose long nip presses comprising a shoe element with a concave surface in the press section. In these publications, inside a so-called hose roll there is a shoe element to be 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. In some paper grades, the dry matter content of the paper is already rather high when the web enters the last nip in the press section. Thus, the area has clearly limitations to pressing, and a high-pressure press impulse is required to further improve the dry matter content.

In a conventional "sharp" roll nip the length of the nip is approximately 40 to 50 mm, because the diameters of the press rolls to be used are large, for example in a roll pair 1000 mm/1600 mm. Thus, the pressure in the nip remains rather low, because the maximum linear pressure available, resulting from the structure of the rolls is 130 to 150 kN/m.

In the present long nip technique to which reference was made hereinabove, with a shoe length of 150 to 200 mm, it is possible to attain a high pressure in the nip, but in that case a large linear pressure must be used.

It is an aim of the present invention to eliminate the aforementioned drawbacks and to present a pressing method and a press device by means of which, on one hand, it is possible to produce a nip which extends in the machine direction as well as to produce an advantageous pressure effect either in dewatering by pressing or in finishing by a calender, and, on the other hand, to avoid the problems in the surface quality of the paper caused by the nip. To attain this purpose, the method according to the invention is primarily characterized in that the centre of curvature of the surface of the shoe element that supports the shell and guides the same through the nip is on the side of the first roll from the nip. In practice this means that said surface is convex or straight, wherein in the latter case it can be assumed that the centre of curvature is located in the infinity on the side of the soft roll. By the selection of the radius of curvature of the shoe (infinite - definite numerical value) together with the properties of the elastic compressible shell it is possible to affect the extension of the nip in the machine direction. Solutions 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 considerably increased anymore, the dry matter content of the ingoing paper is typically at least 85 %.

By means of the solution, it is possible to reach a typical good calendering result in a shoe calender also for the part of the surface structure, and for example surface sizing, which has heretofore been utilized for elimination of problems related to aforementioned fibre roughness, is not necessary. By means of the solution it is possible to form a nip in the press section, in which the member applying the loading is a shoe of a long nip press, which has a straight or convex surface and for example a radius of curvature which is substantially smaller than a radius of the normal press roll, and the other member of the nip is a normal or advantageously lighter counter roll. If the shoe element is composed of a convex surface with a radius of e.g. 100 to 200 mm, the nip becomes substantially shorter than the present roll nip length of 40 to 50 mm. Thus, the pressure in the nip is easily increased to a high level even at a low linear pressure.

The flexible shell is capable of adjusting to the shape of the guiding surface of the shoe element when it travels over the surface. The concept of an elastic shell, in turn, refers to such a shell which is capable of deforming under the effect of the loading pressure effective in the nip, and so producing a nip which extends in the travel direction of the web.

The shoe element may contain loading devices, which load the shoe against the roll located on the other side of the nip. The loading devices may be adjustable, wherein the loading and thereby also the nip pressure as well as the shape of the nip can be adjusted. In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a solution according to the state of the art in a shoe calender,

Fig. 2 shows a solution according to the first embodiment of the invention in a shoe calender,

Fig. 3 shows a second embodiment of the invention in a shoe calender in a view of Fig. 2,

Fig. 4 shows a third embodiment of the invention in a shoe calender,

Fig. 5 shows a fourth embodiment of the invention in a shoe calender,

Fig. 6 shows a more detailed view of the structure of the calender nip,

Fig. 7 shows an embodiment of the invention in a press, and

Fig. 8 shows another embodiment of the invention in a press.

Fig. 1 shows a solution according to the state of the art, in which a paper web W has been guided through a calender nip N formed between two rolls. The first roll 1 is a calender roll equipped with an elastic flexible shell 1 a. The second roll 2 is a roll with a hard surface, against which the shell of the first roll 1 is supported by means of a shoe element 3 with a concave surface. The shoe element is loaded against the inner surface of the shell 1a by means of loading devices 4. For lubrication, a static oil pocket is formed between the shoe element 4 and the inner surface of the shell 1 a. Fig. 2 shows a first embodiment of the calender according to the invention in a side-view similar to Fig. 1. In the calender the web W to be calendered is guided through a nip N formed between two superimposed rotating calender rolls. The lower first roll 1 comprises a shell 1a made of an elastic material, which rotates around the rotation 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 smooth, for example a polished metal surface or a surface equipped with a smooth coating. The shell 1a that is resilient in its thickness direction (in the direction of the radius of the calender roll 1) is pressed into the shape defined by the shoe element 3 guiding the shell from inside and by the surface of the second roll 2, thereby forming a long nip N, in which the web W travels between the surface of the second roll 2 and the surface of the compressed 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 roll, the radius of curvature of the cylinder and the corresponding centre of curvature being 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, i.e. the centre of curvature and the rotation axis of the cylinder incide.

The shoe element 3 is loaded against the inner surface of the shell 1a by means of loading devices 4, which effect an adjustable loading force. There are several loading devices 4 in the transversal direction of the shoe (in the cross-machine direction) and they can be for example pressure medium operated cylinders. The shoe element 3 forms a continuous element extending on the entire width of the web W in the cross-machine direction, and the surface guiding the shell 1a extends continuously across the machine.

Fig. 3 shows a second embodiment, in which the calender contains a first calender roll 1 and a second calender roll according to Fig. 2. Here, the radius of curvature R of the surface of the shoe element 3 located against the shell inside the first calender roll 1 is smaller than the inner radius of the calender roll 1. Thus, the shoe 3 guides the inner surface of the shell along a convex path, whose convexity is steeper than the overall curvature of the inner surface of the shell.

Fig. 4 shows a third embodiment in which the radius of curvature R of the surface of the shoe element 3 is larger than the inner radius of the shell of the calender roll 2. Thus, the shoe element 3 guides the shell at the location of the nip N along a convex path, whose convexity is less steep than the curvature of the inner surface of the shell.

Fig. 5 shows a fourth embodiment in which the radius of curvature R of the guiding surface of the shoe element 3 is infinite, i.e. the surface located against the inner surface of the shell is straight. The front and rear edges of the shoe element are formed so that their shape corresponds to the curvature of the shell, so that the shell is capable of moving on the straight surface and off the surface without problems while rotating.

One advantage of the aforementioned shapes of the surface guiding the shell is that the inner surface of the shell does not curve at the location of the nip N in the direction of the first roll 1 , but it remains at least straight or travels along a path that is curved towards the hard- faced second roll 2.

The flexible hose-like shell can be fixed at its ends to the rotating end structures of the roll by means of prior art techniques, for example according to a solution described in US-patent 5098523, incorporated herein by reference. The shoe element 3 is supported by means of loading devices 4 to an axially extending static element arranged inside the roll shell, said static element being brought through the ends of the roll and supported to carriers outside the roll. The roll shell is clearly wider than the shoe element, so that it can have a circular shape at its fixing point to the ends and can conform along the width of the nip line to a shape defined by the shoe element, which shape possibly deviates from the circular shape. The thickness of the shell 1a is generally between 1 to 20 mm, advantageously between 3 to 8 mm.

Fig. 6 shows a more detailed side-view of the nip N. The paper web W to be calendered first enters in contact with the hot surface of the second roll 2 and is guided along this surface on the outer surface of the shell of the first roll 1 , travels between said surfaces through the nip N and exits the nip N along the surface of the second roll 2, whereafter it is detached from the second roll. The length of the nip N in the machine direction is affected by the compressibility of the shell of the first roll 1 , the shape of the guiding outer surface of the shoe element 3, as well as by the loading effected by the shoe element against the second roll 2. This loading can be adjusted by means of loading devices 4. There is no static oil pocket on the outer surface of the shoe element 3, but the lubricating oil is supplied from the front edge of the shoe element 3, between the outer surface of the shoe element and the shell as is illustrated by the arrow L.

Thanks to the straight or convex shape of the guiding outer surface of the shoe element 3, the structure becomes less complex, and it is easier to manufacture the shoe element 3. In addition, in the calendering process itself, the nip N becomes shorter, and the constant pressure in the nip is attained with a smaller outer loading. This, in turn, reduces the diameter of the thermoroll used as a second roll 2 and the size of other components, respectively.

According to yet another advantageous embodiment, the material of the shoe element 3 is advantageously more elastic than the material of the second roll 2. When the material of the second roll 2 is metal, the material of the shoe element 3 can be a more elastic polymer or elastomer (rubber). Consequently, the shoe can be deformed by compression. By using a greater nip pressure, it is for example possible to widen the shoe in the cross direction. Similarly, it is possible to attain concavity in the shape of the guiding surface of a shoe with a straight surface in the rest position when loading the same, wherein the surface bends very slightly to a concave shape under a load. Another advantage of the elastic shoe is its behaviour in stroke situations. Thus, in "wad situations", the elastic shoe element 3 yields, letting the wad through without damaging the shell of the first roll 1. The shoe element can also be manufactured of such a material in part, for example in such a manner that the part of the element contacting the shell 1a, i.e. the part bordered by the guiding surface, is made of such an elastic material. The loading devices can then be connected to the rigid part of the shoe element 3. This possibility is illustrated in Fig. 6 by a broke line that denotes the border between the rigid part and elastic part and by a hatching that denotes the elastic part of the shoe.

The shell 1a of the first roll 1 can be a belt, which is compressible in its thickness direction, and the belt can thus be made of e.g. reinforced resilient polymer, e.g. fibre reinforced polyurethane. Such a belt can have a lower dynamic elastic modulus, in the order below 0.5 GPa in the temperature range of 25 to 125 °C. It is also possible to use a belt disclosed in the international publication WO 99/28551 , the material of which belt has a compressible volume, for example as a result of polymer with a cellular structure.

In the above-presented embodiments, the second roll 2 is a hard roll, but alternatively it can also be a calender roll 2 with a soft surface, wherein its surface is also deformed as a result of the loading effective in the nip. Such a calender roll pair can be used for matte calendering.

In addition to a calender, the invention can also be applied in the press section, taking into account its special requirements. Fig. 7 shows the use of the invention in the press section, wherein a press nip N is formed by means of a flexible hose-like shell 1 a arranged rotatable and a shoe element 3 guiding the same, in a manner known from so-called long or extended nip structures. The flexible shell 1 a is fixed to the roll structure in a manner similar to the one described above in connection with the calender rolls. Differently from the long nips of presses, whose patents have been discussed hereinabove, the shell 1a is guided along a path curved towards the counter roll 2 located on the other side of the nip N. This is attained by means of a suitably dimensioned radius of curvature R of the guiding surface of the shoe element 3, which is approximately 100 to 200 mm. The shell can be substantially incompressible, wherein a nip is attained which is as short as possible, or it may be resilient in its thickness direction (in the direction of the radius of press roll 1), wherein it can be more efficiently pressed to the shape determined by the shoe element 3 guiding the shell from inside and by the external elements, thereby forming a longer nip N. Two press elements 5, 6 are also brought via the nip, said press elements being in the form of an endless felt or belt passed as a loop around the corresponding press roll 1 , 2. On the side of the press roll 1 provided with the shoe element, the element 5 is a water receptive press felt, and on the side of the press roll 2 functioning as the counter roll there may also be a water receptive press felt as the element 6, but it can also be a belt with a closed surface, which is substantially non- receptive to water and, unlike the porous press felt, capable of intaking at the most an amount of water corresponding to its surface roughness. This element 6 can function as a transfer belt for transferring the paper web that has travelled between the elements through the press nip N between the elements 5, 6 to the drying section, wherein the press nip shown in the figure is the last nip in the press section. A load can be applied to the shoe element 3 by means of loading devices, as in calender rolls.

Fig. 8 shows a press nip which resembles the one shown in Fig. 7 as far as the placement of the shoe element 3 and the structures of the rolls are concerned, but in this nip the paper web W travels exposed over the smooth-faced counter roll 2 through the nip N, on the other side of which there is a water receptive press felt as an element 5, which travels on the shell 1a of the press roll 1 through the nip and receives the water pressed from the web W.

In the press nip embodiment of Figs. 7 and 8, the radius of curvature R of the guiding surface of the shoe element 3, 100 to 200 mm, is smaller than the radius of the corresponding press roll. It is also possible that the radius of curvature of the guiding surface of the shoe element 3 is larger, or that the shoe element is straight. In this case, the radius of curvature R of the surface of the shoe element 3 can in a proportion according to Fig. 2, 4 or 5 to the radius of the roll, i.e. the radius of curvature R is equal to or larger than the radius of the roll 1 , or it is infinite. Also in these cases, the shell 1a can be substantially incompressible, or resilient in its thickness direction, thus enabling the deforming of the nip under the effect of elements on both sides of the shell.

In both above-described press nip alternatives it is also possible that the roll shell 1a is open-faced, i.e. it is capable of receiving water, and/or the counter roll 2 is an adjustable-crown roll.

For the reasons stated in the introduction, the press nip according to Figs. 7 and 8, in which the radius of curvature of the shoe element is smaller than the radius of curvature of the corresponding roll 1, e.g. 100 to 200 mm, is especially well suited as a last press nip of the press section.

The method according to the invention can be used for the processing of all continuous paper webs, in which the webs are pressed with the aim of producing a finished paper which, after possible aftertreatment steps, fulfils certain quality requirements and can be wound to rolls. Such paper in the form of rolls can later be printed or it can be used for other purposes irrespective of the paper grade. In this context the term paper refers to all paper and paperboard grades made of fibrous pulp in the form of a continuous web, whose production involves a stage where it is possible to remove water from the web by pressing the web by means of the method according to the invention, or it is possible to finish the surface of the web by calendering it with the method according to the invention.

Claims

Claims:

1. A method for pressing a paper web, in which the paper web (W) is guided through a nip (N), which is formed between two rolls (1 ,2), the first one of the rolls (1) being equipped with a flexible shell (1a), which is supported in the area of the nip (N) against the second roll (2) by means of a shoe element (3) from inside the roll, said shoe element at least partly determining the shape of the nip, characterized in that the centre of curvature of the surface of the shoe element (3) supporting the shell (1a) and guiding the same through the nip (N) is on the side of the first roll (1) from the nip (N).

2. The method according to claim 1 , characterized in that the radius of curvature (R) corresponds to the inner radius of the first roll (1).

3. The method according to claim 1 , characterized in that the radius of curvature (R) is smaller than the inner radius of the first roll (1).

4. The method according to claim 1 , characterized in that the radius of curvature (R) is larger than the inner radius of the first roll (1).

5. The method according to claim 1 , characterized in that the radius of curvature (R) is infinite.

6. The method according to any of the preceding claims, characterized in that the shoe element (3) is loaded with an adjustable force against the second roll (2).

7. The method according to claim 6, characterized in that the shoe element (3) and the second roll (2) are loaded against each other from inside the first roll (1).

8. The method according to any of the preceding claims, characterized in that it is used in calendering when finishing the surface of the web (W).

9. The method according to any of the preceding claims, characterized in that it is used when water is removed from the web (W) by pressing.

10. A press device for a paper web containing at least a nip (N) for applying pressure to the web (W), the nip being formed between a first roll (1) equipped with a flexible shell (1a) and a second roll (2), wherein a shoe element (3) is placed inside the fist roll (1) at the location of the nip (N) in such a manner that it guides and supports the shell (1a) of the first roll against the second roll (2), characterized in that the centre of curvature of the surface of the shoe element (3) supporting the shell (1a) and guiding the same through the nip (N) is on the side of the first roll (1) from the nip (N).

11. The press device according to claim 10, characterized in that the radius of curvature (R) corresponds to the inner radius of the first roll (1).

12. The press device according to claim 10, characterized in that the radius of curvature (R) is smaller than the inner radius of the first roll (1).

13. The press device according to claim 10, characterized in that the radius of curvature (R) is larger than the inner radius of the first roll

(1 ).

14. The press device according to claim 10, characterized in that the radius of curvature (R) is infinite.

15. The press device according to any of the preceding claims 10 to 14, characterized in that inside the first roll (1) there are loading devices (4) which are coupled to the shoe element (3) to load the same towards the second roll (2).

16. The press device according to any of the preceding claims 10 to 15, characterized in that the shoe element (3) is at least partly made of a material which is more elastic than the surface material of the second roll (2), for example of polymer or elastomer.

17. The press device according to any of the preceding claims 10 or 16, characterized in that it forms the calender nip of a calender.

18. The press device according to any of the preceding claims 10 to 16, characterized in that it forms the press nip of a press section.

19. The press device according to claim 18, characterized in that the radius of curvature (R) is 100 to 200 mm.