Shoe press in a wire part of a board or paper machine and a wire part The present invention relates to a shoe press in a wire part of a board or paper machine for manufacturing a continuous web, which wire part comprises a headbox and at least one forming wire running in a loop, which shoe press comprises a shoe press roll arranged outside the loop of the forming wire, a counter roll arranged in the loop of the forming wire and, together with the shoe press roll, forming an extended press nip, and an outer clothing running in a loop outside the forming wire through said press nip to carry and transfer the web after the press nip to a subsequent press section, which shoe press roll comprises a rotatably journalled, flexible shoe belt and a press shoe arranged on the inside of the shoe belt and having a curved, convex inlet surface and a concave press surface for cooperation with the counter roll, the transition between the inlet surface and the press surface being in a point Ps intersected by a tangent Ts to the inlet surface, whereby the forming wire encounters the counter roll in a point Pw intersected by a tangent Tw to the curved envelope surface of the counter roll, which tangent Tw coincides with the draw of the forming wire upstream of said point Pw, and wherein the shoe press comprises at least one water-receiving member arranged in the loop of the forming wire in connection to the press nip, and has water-receiving, uniformly distributed voids that are open at least in the direction of the side of the forming wire not carrying the web.

The invention also relates to a wire part of a board or paper machine for manufacturing a continuous web, which wire part comprises a headbox, at least one forming wire running in a loop, and one or more presses, at least one of which is a shoe press, which shoe press comprises a

shoe press roll arranged outside the loop of the forming wire, a counter roll arranged in the loop of the forming wire and, together with the shoe press roll, forming an extended press nip, and an outer clothing running in a loop outside the forming wire through said press nip to carry and transfer the web after the press nip to a subsequent press section, which shoe press roll comprises a rotatably journalled, flexible shoe belt and a press shoe arranged on the inside of the shoe belt and having a curved, convex inlet surface and a concave press surface for cooperation with the counter roll, the transition between the inlet surface and the press surface being in a point Ps intersected by a tangent Ts to the inlet surface, whereby the forming wire encounters the counter roll in a point Pw intersected by a tangent Tw to the curved envelope surface of the counter roll, which tangent Tw coincides with the draw of the forming wire upstream of said point Pw, and wherein the shoe press comprises at least one water-receiving member arranged in the loop of the forming wire close to the press nip, and has water-receiving, uniformly distributed voids that are open at least in the direction of the side of the forming wire not carrying the web.

The web produced on the forming wire has a low dry-solids content, typically between 10 and 20 per cent, and is therefor sensitive to external strain. Such external strain occurs when the forming wire runs towards and is bent over a larger or smaller portion of the curved inlet surface of the shoe, resulting in damage to the wet web.

It has also been found that the shoe belt immediately prior to the press nip does not follow the circle arc it has in rest position, but instead bulges out just before and/or within the inlet radius of the curved shoe due to its rigidity. This deflection affects the adjacent process belt so that it is pressed down into the wet paper web. The paper web is subjected to shearing damage

due to the low dry-solids content of the paper web, in combination with the difference in speed that arises between the forming wire and the process belt when the shoe belt is deflected in the manner described.

When the web is pressed in a press nip at the end of a wire part, large quantities of water are released, as the dry-solids content of the web is very low, as mentioned typically between 10 and 20 per cent. Part of the water volume can be accumulated in the open volume of the forming wire, formed by its voids. When the forming wire reaches the press nip, however, these voids are already filled with water to a greater or lesser extent from the draining of the web along the preceding forming zone.

Considerable difficulties are therefore encountered in controlling the volume available in the forming wire to receive the water pressed out of the web in the press nip.

When using one press felt or two press felts in a single-felted or double-felted press nip, respectively, the press felt will absorb part of the released quantity of water. However, the absorbing capacity of the press felt diminishes after a relatively short period of use as it is clogged by small fibres that accompany the water pressed out in the press nip and deposited on the surface of the press felt as well as penetrating it. Even if the press felt has some elasticity in its thickness direction, this elasticity is exhausted because of the continuous compression to which the press felt is subjected in the press nip so that the thickness of the press felt decreases, which results in reduced absorption capacity. Continuous conditioning of the press felt is therefore also more difficult. If the press nip is single-felted and the clothing in contact with the web is an impermeable belt, the strain on the press felt is increased. The use of suction equipment to create a

suction zone in the press nip to remove the water pressed out also entails disadvantages as explained below. The difficulties to remove water increase as running speeds increase. At the same time demands are constantly made by industry for higher running speeds. It would be possible to produce a wire for direct web contact with a greater open volume than presently existing forming wires. Such a solution is not available, however, when taking into account that the wire is designed to serve not only as a press clothing in the pre-press but also as a forming wire, it being important that the fibres are retained on the forming wire and do not accompany the drainage water and that the forming wire does not carry with it excessive quantities of water after the press nip, which water is physically retained in said imagined greater open volume and burdens the forming wire in an undesirable way.

WO 97/13030 describes a wire part with a pre-press, having a press nip, through which a forming wire passes together with an impermeable transfer belt. The press nip is formed by a press shoe roll and a press roll that has an envelope surface provided with openings (Figure 2, page 11, second paragraph). The transfer belt carries the web on its underside from the press nip to the clothing of a subsequent shoe press. This publication does not discuss dealing with the water pressed out of the web in a shoe press nip or the capacity of the perforated press roll. Neither does it mention anything about the shearing damage that occurs in the inlet part of the shoe press nip or just prior to this. The publication therefore proposes no solution to this problem.

The object of the invention is to provide a shoe press that makes it possible to prevent damages to the wet and therefore sensitive web, in an efficient and controllable manner.

The shoe press and the wire part in accordance with the invention are characterized in that the press shoe is so arranged in relation to said point Pw that said tangent Ts and said tangent Tw intersect each other at an acute angle a that is greater than 2°, preferably greater than 6°.

The invention will be further described with reference to the drawings, in which Figure 1 is a side view of parts of a paper machine with a shoe press in accordance with a first embodiment.

Figure 2 is a cut-out section of a water-receiving belt in the shoe press in accordance with Figure 1.

Figure 3 is a side view of parts of a paper machine with a shoe press in accordance with a second embodiment.

Figure 4 is a side view of parts of a paper machine with a shoe press in accordance with a third embodiment.

Figure 5 shows parts of the shoe press substantially in accordance with Figure 1, with the shoe mounted in a specific position in relation to the point where the forming wire meets the counter roll.

Figure 6 shows the shoe press in accordance with Figure 5 with the shoe mounted in a different position.

Figure 1 shows, schematically, parts of a paper machine comprising a wet section 1 and a press section 2. The wet section 1 comprises a wire part which in the embodiment shown consists of a fourdrinier former, comprising a headbox 3 and a forming wire 4, running in an endless loop around a plurality of guide rolls, which comprise a

breast roll 5, a wire turning roll 6, a wire roll 7, an alignment roll 8 and a tension roll 9. Stock is ejected from the headbox 3 onto the forming wire 4 and dewatered to form a continuous web 10. At the end of the fourdrinier former, a plurality of dry suction boxes 11 are arranged in the loop of the forming wire 4. However, any type of wire part may be used, e. g. a twin wire part with a forming roll.

The fourdrinier former further comprises a shoe press 12, which constitutes a pre-press, and which is arranged downstream of the dry suction boxes 11. The shoe press 12 comprises a shoe press roll and a counter roll 13. The counter roll 13 is arranged in the loop of the forming wire 4, while the shoe press roll 14 is arranged outside the loop of the forming wire 4. The counter roll 13 and shoe press roll 14 co-operate with each other to form an extended press nip between them. The forming wire 4 runs through the press nip, whilst encompassing a pre- -determined sector part of the counter roll 13. Further, the shoe press 12 comprises an outer impermeable belt 15, running in an endless loop around a plurality of guide rolls 16 and through the press nip whilst encompassing a pre-determined sector angle of the shoe press roll 14.

The impermeable belt 15 co-operates with a lower press felt 17 of a subsequent shoe press 18 in the press section 2, which shoe press 18 also has an upper press felt 19. The impermeable belt 15 has a smooth surface, which faces the forming wire 4 and to which the formed web 10 adheres after the web 10 has passed the press nip.

The shoe press 12 further comprises a first water- -receiving member 20 in the form of a non-compressible inner belt, running in a loop inside the loop of the forming wire 4 around a plurality of guide rolls 21 and around the counter roll 13 and three of the guide rolls of the forming wire 4. In the embodiment shown in

Figure 1, the belt 20 is guided directly onto the counter roll to meet the forming wire in or shortly before the extended press nip, where the forming wire 4 reaches the counter roll 13. The belt 20 is driven around in its loop by its friction engagement with the forming wire and the counter roll in the press nip.

The belt 20 has water-receiving voids 22, open towards the outside of the belt 20, which outside, in the extended press nip, is in contact with the side of the forming wire 4 not carrying the web. The water-receiving voids 22 are visible to the naked eye and are evenly distributed in the longitudinal and transverse directions of the belt 20 in such a way that the water-receiving capacity is uniform or virtually uniform per unit of area along and across the belt 20. In one embodiment of the belt 20, the voids 22 are also open towards the inside of the belt, which inside, in the extended press nip, is in contact with the envelope surface of the counter roll. In other words, the voids 22 form through-openings in the thickness direction of the belt 20. A preferred embodiment of such a belt is a wire, in which the through-openings are defined by the intersecting threads 23,24 of the wire, see Figure 2.

In another embodiment (not shown), the belt 20 has straight, uniform through-holes which give the belt a perforated appearance.

In a third embodiment (not shown), the belt has recesses on its outer side facing the forming wire, which recesses are preferably as deep as possible and have any shape, for instance round depressions and/or long grooves that are longitudinal, transverse or diagonal or which cross each other.

In a fourth embodiment (not shown), the belt 20 has voids 22 that consist of a combination of said second and third embodiments.

The belt 20 is thus either permeable or impermeable to water.

The thickness of the belt 20 and the design and number of the voids 22 per unit of area are chosen in such a way that the belt obtains an open volume equal to or substantially equal to or greater than the volume of water pressed out of the web in the press nip, for instance 1-100 per cent greater. To be able to ensure a constant open volume during an extended period of production it is consequently necessary that the belt is not compressed in the press nip so that its thickness is diminished. For a wire with suitable thickness and thread dimensions, the water-receiving capacity can be in the range 300-2,000 g/m2 and may normally be 800-1,300 g/m2 (corresponding to a void volume of 300-2,000 cm3/m2 and 800-1,300 cm3/m2, respectively).

To ensure continuous operation with sustained removal of a constant quantity of water from the press nip and to be able to control this removal, it is important that the press also comprises conditioning members arranged at suitable locations downstream of the press nip for continuous removal of water from the voids 22 of the belt 20, so that the belt is dry and free of fibres when it runs into the press nip. In the embodiment shown, such conditioning members comprise a high-pressure squirter 25 for air and two suction boxes 26. One or both of the suction boxes can be replaced by at least one blow box.

Figure 3 shows parts of a shoe press in accordance with a second embodiment of the invention. The shoe press in Figure 3 is similar to the one in Figure 1 but comprises,

additionally, a second water-receiving member in the form of an open counter roll 13, the shell of which has water-receiving, evenly distributed voids 27. The embodiment shown in Figure 3 uses a perforated counter roll 13, said voids 27 of which constitute radially extending openings. The water that runs through the openings 27 is collected in an inner trough 28, from which the water is drained through one of the end walls of the counter roll. When using an open counter roll 13, the water-receiving belt 20 is always permeable.

Alternatively, the open counter roll 13 may be a blind-drilled or grooved counter roll, which thus has water-receiving voids that are outwardly open but inwardly closed. When using an open counter roll 13, its voids 27 and the voids 22 of the belt 20 together have an aggregate open volume equal to or substantially equal to or greater than the volume of water pressed out of the web in the press nip. In this context, it is suitable for the voids 22 of the belt 20 to constitute at least 50 per cent, preferably at least 60 per cent, of said aggregate open volume. It is also suitable to arrange some form of conditioning member by the open counter roll 13, when this helps in handling the press water.

Figure 4 shows parts of a shoe press 12 in accordance with a third embodiment. The shoe press in Figure 4 resembles that in Figure 3 but has only one water-receiving member, namely in the form of an open counter roll 13, the shell of which has water-receiving, evenly distributed voids 27. A perforated counter roll 13 is used in the embodiment shown in Figure 4, the voids 27 of the counter roll forming radially extending through-openings. The water that runs through the openings 27 is collected in an inner trough 28, from which the water is drained through one of the end walls of the counter roll. Alternatively, the open counter roll 13 may be a blind-drilled or grooved counter roll, which

thus has water-receiving voids that are outwardly open but inwardly closed. When using an open counter roll 13, its voids 27 have an aggregate open volume equal to or substantially equal to or greater than the volume of water pressed out of the web in the press nip. It is suitable to arrange some form of conditioning member by the open counter roll 13.

To achieve the best results as regards the handling of the press water, it is necessary that the open volume available in a wire or, alternatively, in a wire and an open counter roll, or only an open counter, is essentially greater than, e. g. 50-100% greater, than the volume of water pressed out of the web. This is presumably due to the time being so limited that accessibility to the open volume must be good. This also means that the open volume must be evenly distributed over the surface.

Figures 5 and 6 show parts of the shoe press 12 substantially in accordance with Figure 1 within the area for the extended nip. The shoe press roll 14 has two rotatably journalled end walls 30, a flexible shoe belt 31 fitted to the end walls and forming the shell of the shoe press roll, and a press shoe 32 which is arranged on the inside of the shoe belt 31 and which, together with the counter roll 13 defines the extended press nip. The press shoe 32 is provided with a plurality of hydrostatic pressure pockets 33 arranged in a row side by side. The press shoe 32 has a concave press surface 34 and a convex, curved inlet surface 35 which transits into the concave press surface 34 in a point Ps intersected by a tangent Ts to the curved inlet surface 35.

The forming wire 4 meets the counter roll 13 in a point Pw intersected by a tangent Tw to the curved envelope

surface of the counter roll, which tangent Tw coincides with the forming wire 4 upstream of said point Pw.

The two tangents Ts and Tw intersect each other at an acute angle a that is greater than 2°. At this lower limit value the point Pw is situated downstream of the point Ps and considerably closer to this point Ps than the front end of the pressure pocket, as is clear from Figure 5. At increasing values of the angle a the point Pw approaches the point Ps of the shoe, and passes this point when the angle a is increased further, so that the point Pw will be situated upstream of the point Ps of the shoe, as illustrated in Figure 6. The angle a is set by moving the journalling axle of the shoe press roll in a path that is concentric with the envelope surface of the counter roll, after which the shoe press roll is fixed, when the angle a has the desired value. At the desired value, which may vary depending on different operating conditions, the shoe is so concentrically displaced in relation to the point Pw where the forming wire 4 meets the counter roll at a tangent, that the forming wire 4 runs into the press nip without being bent around the curved inlet surface 35 of the shoe and, furthermore, without the web carried by the forming wire 4 coming into contact with the deflections the shoe belt is subjected to immediately prior to the press nip due to its change of direction and rigidity. It will be understood that the tangent Ts is always situated above the forming wire 4 (the tangent Tw), i. e. seen generally, is always on the same side of the forming wire 4 as the process belt 15.

In accordance with a preferred embodiment said angle a is greater than 6°.