WO2007042623A1

WO2007042623A1 - Press belt

Info

Publication number: WO2007042623A1
Application number: PCT/FI2006/050440
Authority: WO
Inventors: Mikko Blom; Bo RÖKMAN
Original assignee: Tamfelt Oyj Abp
Priority date: 2005-10-14
Filing date: 2006-10-13
Publication date: 2007-04-19
Also published as: FI118227B; FI20055556A0; FI20055556A

Abstract

The invention relates to a press belt for a shoe press. The press belt (5) is an endless loop made of an elastomer material, whose outer surface (11) is provided with a plurality of grooves (13) for dewatering. The dewatering grooves (13) are shallow, their maximum depth being only 0.8 mm. Furthermore, the grooves (13) are arranged sparsely on the outer surface (11) so that the distance between the center lines (20) of the grooves (13) is at least 4.5 mm. In that case, the press belt (5) has a limited water volume on its outer surface (11).

Description

PRESS BELT

BACKGROUND OF THE INVENTION

[0001] The invention relates to a press belt for a shoe press, the press belt the press belt being an object with the shape of an endless loop and comprising: an elastomer base; a plurality of reinforcement yarns arranged inside the base; an inner surface and an outer surface; a plurality of parallel and endless dewatering grooves on the outer surface of the press belt, each groove having side surfaces, a bottom and a centre line; and a land between adjacent grooves. [0002] Shoe presses are used in pulp, board and paper machines to dewater a fibre web. The surface of the press belt facing the fibre web may be smooth, in which case the press belt has no water space at all. In that case, the water the paper machine textile, such as a press felt, used in a shoe press is unable to receive may pose a problem as the water may overflow in an un- controlled manner in the press zone and cause runnability problems. On the other hand, if the press belt is provided with patterning, such as grooves or blind drillings, for dewatering, removal of water from the water space on the surface of the press belt may pose a problem. This problem is emphasized in slow-speed shoe presses which do not generate a sufficient centrifugal force for emptying the water space. This problem also appears in shoe presses which are not provided with dewatering apparatuses for collecting water from the press belt. In other words, the problem is that there exists no well- functioning press belt for low-speed shoe presses.

BRIEF DESCRIPTION OF THE INVENTION [0003] The object of the invention is to provide a new and improved press belt.

[0004] The press belt according to the invention is characterized in that the maximum depth of dewatering grooves is 0.8 mm or smaller; and that the distance between the centre lines of two parallel and adjacent dewatering grooves is at least 4,5 mm.

[0005] The basic idea of the invention is that the outer surface of a press belt, i.e. the surface facing the fibre web to be dried, is provided with a plurality of parallel and endless dewatering grooves. The grooves are relatively shallow as their maximum depth is only 0.8 mm or smaller. Furthermore, the grooves provided on the outer surface of the press belt are at a relatively large distance from one another as the distance between the centre lines of two parallel and adjacent grooves is 4.5 mm or greater.

[0006] An advantage of the invention is that the shallow dewatering grooves on the outer surface of the press belt form, during the pressing phase of the shoe press, necessary additional volume for the water to be removed from the fibre web. As the grooves on the outer surface of the press belt are long and endless, water may spread in the grooves to a relatively large area in the press zone, which facilitates removal of water from the water space. Furthermore, the hydraulic pressure in the endless grooves is small, in which case the mechanical pressure acting on the lands may be greater, which enables a larger dry solids content. In addition, as the grooves are shallow, it is easy to remove the water accumulated in them after the pressing phase. A further advantage is that the grooves are simple to manufacture since they are located at a large distance from one another and shallow. In addition, structure of this kind is durable.

[0007] An embodiment of the invention is based on the idea that shallow grooves that are at a relatively large distance from one another form only a relatively small additional water space on the outer surface of the press belt. The volume of the water space is between 50 and 220 g/m². As the water volume is relatively small and consists of several shallow grooves, dewatering the press belt is unproblematic.

[0008] An embodiment according to the invention is based on the idea that the proportion of the total open area (OA) of the dewatering grooves to the outer surface area of the whole press belt is between 15 and 28%. [0009] An embodiment according to the invention is based on the idea that the outer surface of the press belt is provided only with several longitudinal, i.e. machine direction, endless grooves and with longitudinal lands between the grooves.

[0010] An embodiment according to the invention is based on the idea that the cross section of the grooves is arranged to increase towards the outer surface. The grooves may thus have a substantially V-shaped gross section or, alternatively, a substantially U-shaped cross section that widens upwards.

[0011] An embodiment of the invention is based on the idea that in a shoe press, there is a press felt arranged or intended to be arranged against the outer surface of the press belt. The press felt is arranged to compress in the press zone and return substantially to its original thickness after pressing. Normalization of the press felt volume generates a suction effect for absorbing the water from the shallow grooves of the press belt into the press felt. The press felt is arranged to transport the water away from the press zone. [0012] An embodiment of the invention is based on the idea that the water volume of the press belt is dimensioned to correspond to the change in the volume of the press felt used in the shoe press between the pressing phase and the normalization phase.

[0013] An embodiment of the invention is based on the idea that the edge portion between the side wall of a groove and the land is bevelled or curved. In that case, the groove widens rapidly towards the outer surface of the press belt. An advantage of this embodiment is that water may move into the groove and out of it more easily. Furthermore, even a thick press felt may better penetrate into the upper section of the groove to some extent and thus fa- cilitate absorption of water from the groove into the press felt after the press zone.

[0014] An embodiment according to the invention is based on the idea that the maximum width of the groove is larger than its maximum depth. It is substantially easier to dewater relatively wide and shallow grooves than deep and narrow grooves. Usually wide grooves do not, however, cause markings on the fibre web since a thick press felt having a relatively large water space is typically used in slow shoe presses. The water space of the press felt may be between 1700 and 2200 g/m². Such a press felt efficiently prevents marking. [0015] An embodiment according to the invention is based on the idea that the outer surface of the press belt is provided with a corrugated cross-sectional profile, where the valleys of corrugations form water discharge grooves and the tops of corrugations lands. In that case, the groove bottoms, groove sides, lands and the edge portions of these parts may be provided with a curved shape. When the outer surface of the press belt has a curved waveform, it is easier for the surface of the press felt to penetrate into the upper part of a dewatering groove, which may enhance water absorption from the grooves.

[0016] An embodiment according to the invention is based on the idea that the press belt is arranged or intended to be arranged in a shoe press, which has a slow operating speed (less than 500 m/min) and which is not pro- vided with water collection troughs or doctor blades for dewatering the press belt.

[0017] An embodiment according to the invention is based on the idea that the thickness of the press belt is between 4.5 and 5.5 mm. Shallow grooves enable a thin structure.

BRIEF DESCRIPTION OF THE FIGURES

[0018] Some embodiments of the invention will be described in greater detail in the accompanying drawings, in which

Figure 1 schematically illustrates the principle of a shoe press, Figure 2 schematically illustrates a perspective view of a press belt having the shape of a closed loop,

Figure 3 schematically illustrates a cross section of a press belt according to the invention from the longitudinal direction, i.e. machine direction (MD), of the press belt, Figure 4 schematically illustrates a cross section of another press belt according to the invention from the longitudinal direction, i.e. machine direction (MD), of the press belt,

Figures 5a and 5b schematically illustrate dewatering in a shoe press from the machine direction (MD), Figures 6a to 6d schematically illustrate sections of the outer surface of some press belts and feasible alignment of dewatering grooves, and

Figure 7 schematically illustrates schematically illustrates a cross section of another press belt according to the invention from the longitudinal direction, i.e. machine direction (MD), of the press belt. [0019] For the sake of clarity, the figures illustrate some embodiments of the invention in a simplified manner. In the figures, similar parts are denoted by the same reference numbers.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

[0020] Figure 1 illustrates a considerably simplified shoe press 1 , which is a drying apparatus typically employed in drying a fibre web. The shoe press 1 comprises a press roll 2 and a press shoe 3, between which there is a press zone 4, through which a press belt 5, at least one paper machine textile 6,7 and the fibre web 8 to be dried are arranged to be run in the machine direction MD. The press shoe 3 and the press roll 2 are pressed against each other at force F, in which case the press belt 5, paper machine textile 6,7 and fibre web 8 running between them are compressed. The press shoe 3 has a curved sliding surface facing the press roll 2, in which case there is an elongated contact area between the press shoe 3 and the press roll 2, known as a "long nip". The press belt 5 may be arranged to be supported by a suitable support mem- ber 9 when it travels around the press shoe 3. The inner surface 10 of the press belt 5 slides against the sliding surface of the press shoe 3 and the outer surface 11 of the belt is towards the fibre web 8. When introduced into the press zone 4, the fibre web 8 is typically supported by one or more paper machine textiles, such as a press felt 7 or a wire 6. When run, the paper machine textile 6, 7 is controlled by control rolls 12 or the like. In the solution according to Figure 1 , the upper paper machine textile may be a press felt 7 and the lower web a wire 6.

[0021] The outer surface 11 of the press belt 5 may be provided with a plurality of grooves 13 for dewatering. In some cases, the inner surface 10 of the press belt 5 may also be patterned, but this patterning has nothing to do with the dewatering grooves 13 on the outer surface 11. When the fibre web 8 travels through the relatively long press zone 4 formed by the press roll 2 and the press shoe 3, water is pressed from it. At least some of this water may flow through the press felt 7 into the dewatering grooves 13 on the outer sur- face 11 of the press belt 5, which provide an additional volume for the exiting water. In that case, the amount of water that the compressed press felt 7 cannot receive may move into the water space offered by the press belt 5. If there were no additional water space in the press belt 5, the water could flow from the press zone 4 towards the forward end and cause runnability problems. The dewatering grooves 13 provided on the outer surface 11 of the press belt 5 prevent overflow, and dewatering occurs in a controlled manner. When the press felt 7 is released from the pressing after the press zone 4, it returns substantially to its original thickness. The change in the volume of the press felt 7 generates a suction effect, which absorbs the water from the shallow dewater- ing grooves 13 of the press belt 5 back into the press felt 7 and carries it away from the press zone 4. This suction effect is described further in connection with Figures 5a and 5b. The cycle of the press felt 7 may be provided with one or more water collection devices 14 or the like for dewatering the press felt 7 before it enters the press zone 4 again. In that case, there is no need for water collection troughs, doctor blades or other similar devices for removing water from the grooves of the press belt 5. It is easier to arrange dewatering appara- tuses in the felt cycle of the press felt 7 than in connection with the press belt 5.

[0022] It should be mentioned that the press belt according to the invention is also applicable to other kind of shoe presses. In an embodiment, the shoe press comprises a rotating press roll, against which a press shoe provided with a curved surface is pressed, the press shoe forming an elongated nip. A separate press belt controlled by control rolls or the like may also be run into the space between the press roll and the press shoe. Furthermore, a fibre web supported by the paper machine textile is run into the press zone. In addition, a separate press belt may be arranged on top of the press roll and the press shoe pressed against the press belt forming the outer surface of the press roll. A feature common to these different embodiments is that the press belt is a component which is always manufactured separately, has the shape of an endless loop and is arranged or arrangeable so that it travels through a long nip between at least one roll and the press shoe.

[0023] Figure 2 illustrates a press belt 5 having the shape of an endless loop, an inner surface 10 and an outer surface 11. The press belt 5 is arranged in the shoe press 1 so that its outer surface 11 faces the fibre web 8. The outer surface 11 is provided with several parallel dewatering grooves 13. The dewatering grooves 13 are endless and may be arranged substantially in the machine direction MD. Between adjacent dewatering grooves 13, there is a longitudinal land 15 in the machine direction. The dewatering grooves 13 may be grooves that go around the press belt or continuous spiral grooves.

[0024] Figure 3 illustrates a cross section of a press belt according to the invention from the machine direction MD. The press belt 5 comprises an elastic base 16, which has a sufficient flexibility and ability to normalize after pressing. The base 16 may be mainly made of an elastomer material, such as a suitable rubber material, or polymer, such as polyurethane. Furthermore, the press belt 5 may be provided with a support structure, which may comprise several reinforcement yarns 17, which may be arranged in one, two or more layers inside the base 16. The yarn layers on top of each other may be separate from or bound to each other. The inner surface 10 of the press belt 5 may be substantially smooth. Instead, the outer surface 11 of the press felt 5 comprises several parallel dewatering grooves 13, between which there are lands 15. The dewatering grooves 13 are typically arranged in the machine direction MD but Figures 6a to 6d below illustrate other options. [0025] The cross section of the dewatering grooves 13 may have the U-shape illustrated in the figure or it may resemble letter V. The groove 13 comprises side surfaces 18 and a bottom 19. The section between the bottom 19 and the sides 18 may be rounded or bevelled to improve durability. Fur- thermore, the groove 13 has a centre line 20 and a maximum depth GD determined from the outer surface 11 to the deepest point of the bottom 19. The dewatering grooves 13 may be relatively shallow. The maximum depth GD of the grooves is only 0.8 mm or smaller. The water volume on the outer surface 11 of the press belt 5 is influenced by the cross sectional area of the dewater- ing grooves 13 and by the fact how densely the dewatering grooves 13 are arranged. The dewatering grooves 13 are arranged at a relatively large distance from one another. The distance LC between the centre lines 20 of the grooves 13 is 4.5 mm or larger. Since the grooves 13 are at a large distance from one another and shallow, the groove 13 width GW may be dimensioned to be relatively large without the water space offered by the press belt 5 becoming unnecessarily large. Furthermore, when the distance between the grooves 13 is large, the width L of the lands 15 may also be relatively large: typically L is 3.5 mm or larger. The large surface area of the lands 15 and small hydraulic pressure in the endless dewatering grooves 13 ensure inten- sive mechanical pressing in the press zone 4. Furthermore, thanks to the shallow grooves 13, the thickness T of the press belt 5 may be between 3 and 6 mm, preferably between 4.5 and 5.5 mm, i.e. the press belt may be clearly thinner than conventional press belts.

[0026] Figure 4 illustrates the cross section of another press belt 5 from the machine direction MD. The dewatering grooves 13 may have a curved bottom 19. Also in this case, the maximum depth of the dewatering grooves 13 is at most 0.8 mm. The edge portion 21 between the groove 13 side 18 and the land 15 may be bevelled as shown in the figure or curved. The edge portion 21 that rapidly opens towards the outer surface 11 of the press belt may facilitate moving of water from the press felt into the dewatering grooves and back into the felt. Also in this embodiment, the distance LC between the centre lines 20 of the dewatering grooves 13 is 4.5 mm or larger.

[0027] Figures 5a and 5b illustrate how the water pressed from the fibre web 8 moves through the press felt 7 into a dewatering groove 13 of the press felt as shown by arrow A. The grooves 13 receive the amount of water that the compressed belt 5 is no longer able to receive. Water can spread in the shallow and endless grooves, in which case the water accumulated in the grooves 13 has a large surface area towards the press felt 8. When the press felt 7 released from the pressing after the press zone 4 returns to its original thickness B, a suction effect is generated, which absorbs the water from the shallow dewatering grooves 13 into the felt 7 as shown by arrows C. The press felt 7 carries the water away. It can further be seen from Figure 5a that, due to the influence of pressing, the surface 7a of the press felt 7 tends to penetrate into the upper section of the dewatering groove 13, which may reduce the surface tension of water and thus enhance absorption of water from the grooves 13,

[0028] Figures 6a to 6d illustrate feasible ways of aligning the dewatering grooves 13. It is common to all these embodiments that the distance between the centre lines LC of two adjacent and parallel grooves 13 is at least 4.5 mm, i.e. the grooves are arranged on the outer surface of the press belt 5 rather sparsely. In addition, the grooves 13 are shallow.

[0029] In Figure 6a, the endless dewatering grooves 13 are substantially in the cross machine direction CMD. In Figure 6b, the dewatering grooves are arranged diagonally. The angle D between the centre line 20 of the groove 13 and the machine direction MC may be selected according to the need. In the embodiment shown in Figure 6c, there are both machine direction grooves 13a and cross-directional grooves 13b on the outer surface 11. In that case, the lands 15 are quadrangular. The distance LC between the centre lines of the machine direction grooves 13a may be equal to or different from the distance LC between the centre lines of the cross-directional grooves 13b. Figure 6d further illustrates an embodiment where diagonal dewatering grooves 13 are arranged crosswise.

[0030] Figure 7 further illustrates the cross section of a press belt 5. The outer surface 11 of the press belt 5 may have a curved and wavy cross section, where wave troughs form dewatering grooves 13 and wave crests lands 15. In that case, the bottoms 19 of the grooves, sides 18 of the grooves, edge portions 21 and lands 15 may be curved surfaces. When the outer surface 11 of the press belt 5 is corrugated, the surface 7a of the press felt 7 may penetrate, due to the influence of pressing, into the upper section of the dewatering groove 13, which may enhance absorption of water from the grooves 13. Also in this embodiment, the distance LC between the centre lines 20 of the dewatering grooves 13 is 4.5 mm or larger and the grooves 13 are shallow. [0031] It is also feasible that the grooves are curved and not linear in the machine direction. Also in this case, the distance between the centre lines of adjacent grooves is 4.5 mm or larger and the grooves are shallow.

[0032] If the cross section of a dewatering groove 13 is asymmetric, the centre line 20 of the groove is determined as the line that divides the cross section of the groove into two same-sized surface areas in the groove's cross direction.

[0033] in some cases, the features described in this application may be used as such, regardless of the other features. On the other hand, features described in this application may, if necessary, be combined to form different combinations.

[0034] The drawings and the related description are only intended to illustrate the inventive concept. The details of the invention may vary within the scope of the claims.

Claims

1. A press belt for a shoe press, the press belt (5) being an object with the shape of an endless loop and comprising: an elastomer base (16); a plurality of reinforcement yarns (17) arranged inside the base (16); an inner surface (10) and an outer surface (11 ); a plurality of parallel endless dewatering grooves (13) on the outer surface (11 ) of the press belt, each groove (13) having side surfaces (18), a bottom (19) and a centre line (20); and a land (15) between adjacent grooves (13), characterized in that the maximum depth (GD) of the dewatering grooves (13) is 0.8 mm or smaller; and the distance (LC) between the centre lines (20) of two parallel and adjacent dewatering grooves (13) is at least 4.5 mm.

2. A press belt according to claim 1 , characterized in that the dewatering grooves (13) are arranged to form a water volume on the outer surface (11 ), the magnitude of the volume being between 50 and 220 g/m².

3. A press belt according to claim 1 or 2, characterized in that the outer surface (11 ) of the press belt (5) is provided only with a plurality of machine direction (MD) endless dewatering grooves (13) and machine direction (MD) lands (15) between them.

4. A press belt according to any one of the preceding claims, characterized in that the proportion of the total open area (OA) of the dewatering grooves (13) to the surface area of the outer surface (11) of the whole press belt is between 15 and 28%.

5. A press belt according to any one of the preceding claims, characterized in that the cross section of the dewatering grooves (13) is arranged to increase towards the outer surface (11).

6. A press belt according to any one of the preceding claims, characterized in that the thickness (T) of the press belt (5) is between 4.5 and 5.5 mm.

7. A press belt according to any one of the preceding claims, characterized in that the width of the land (15) between two adjacent dewatering grooves (13) is at least 3.5 mm.

8. A press belt according to any one of the preceding claims, characterized in that the outer surface (11 ) of the press belt (5) has a wavy cross sectional profile, where the troughs of curved waves form dewatering grooves and the wave crests form lands (15).