SHOE PRESS DEVICE FOR PRESSING A WET FIBRE WEB, AND METHOD FOR PRESSING A WET FIBRE WEB AT HIGH MACHINE SPEEDS

Abstract: A shoe press device (200) for pressing a wet fibre web, comprising a first press surface (201), a second press surface in the form of a flexible belt (202), and at least one press shoe (203). The first press surface and the press shoe form a press nip (204) having an effective pressure zone (205) during the pressing. The press shoe comprises at least one supply area (207) for a pressurized lubricant. A heat transport connection (208, 209) is integrated in the press shoe for controlling the temperature of the lubricant and the flexible belt within and after the effective pressure zone during the pressing, and comprises at least one outlet (208) for removing lubricant, heated in the effective pressure zone (205) during the pressing, in close proximity to or immediately downstream the end of the effective pressure zone. Furthermore, a method for pressing by means of the shoe press device, which is particularly advantageously implemented for pressing paper webs intended for liner, paperboard, printing paper, newsprint, tissue paper, or the like, at machine speeds exceeding 1500 m/min.

Description: Title Shoe press device for pressing a wet fibre web, and method for pressing a wet fibre web at high machine speeds. Technical field The present invention relates to shoe press device for pressing a wet fibre web. The invention further relates to a method for pressing a wet fibre web at a high machine speeds by means of such a shoe press device. The shoe press device and the method according to the invention are particularly advantageously utilised for pressing wet fibre webs at machine speeds exceeding 1500 m/min. However, the shoe press device can be utilised for all pressing of wet fibre webs, and particularly advantageously when pressing paper webs for the manufacture of liner, paperboard, printing paper, tissue paper, or the like. Background of the invention A paper manufacturing process comprises a number of different steps such as stock preparation, sheet formation, drainage, pressing and drying. Thereby, the function of the pressing is to dewater the paper sheet, which has been formed and drained on one or several wires, even more before the subsequent drying. The pressing also strongly influences the physical properties of the finished paper, such as density and strength properties. The pressing normally takes place in one or several press nips between pairs of press rolls, wherein the paper web normally is carried through the press nips by one or several water-absorbing press felts which pass through the press nip or nips together with the paper web. The pressure in, and the length and geometrical shape of, the press nip plays an important role for the pressing result, when the dryness of the pressed fibre web and the density and strength properties of the finished paper are concerned. It is previously known to provide an extended press nip through a so-called shoe press device (extended nip press). A shoe press device extends the effective press nip and, consequently, enables a higher pressing impulse in spite of the maximum pressure being at a relatively low level. The use of a shoe press unit provides a pressing which is more gentle to the fibre web but with a retained dryness increase. A conventional shoe press unit comprises a concave, stationary surface, a so-called shoe. The concave surface is facing the counter roll in the pair of press rolls, wherein a flexible endless belt runs in a loop around the shoe, so that the belt will pass between the shoe and the counter roll. Such a shoe press unit also comprises a loading device for pressing the shoe against the counter roll, and a support beam supporting the shoe. Furthermore, a system for supplying a suitable lubricant between the stationary shoe and the movable belt is present. In some previously known shoe press units, a solid counter roll is utilised, but it occurs that also the counter roll is provided with one or several internal loading means, such as a system with press shoes or hydraulic pressure zones, which are able to influence the geometrical shape of the press nip and the pressure profile across the machine direction. Accordingly, US 5, 302, 252 discloses a shoe press unit having a reversed, declining pressure curve through the press nip. The object is stated to be to provide a mechanical drainage apparatus which provides a better drying result and which at the same time prevents delamination of the fibre web. This is reported to be achieved by means of an apparatus in which the fibre web is passed between at least two press surfaces, which form a press nip and which exercise a drainage pressure on the fibre web by means of one press surface being designed as an impermeable belt and which can be pressed towards the other press surface by means of a hydraulic loading device. A supporting pocket, located in the loading member substantially in the area between the inlet end for the fibre web and the centre of the loading means, generates a hydrodynamic pressure field at the loading member by means of setting a lubricant into circulation. The hydrodynamic pressure field provides the desired pressure characteristics when the paper web passes through the pressure zone. According to US 5, 302, 252, the lubricant/hydraulic fluid which follows the belt can be removed from this by means of a perforated guiding plate downstream the loading member and a subsequent elongate scraper, in order to be collected in a supply volume of hydraulic fluid and to be returned to the supporting pocket. Furthermore, US 5,676,799 discloses a device for treating a fibre web, which device comprises two press surfaces forming an elongated press web for exerting a treatment pressure onto the fibre web, wherein at least one of the press surfaces is formed by an impermeable band movable in a web moving direction, and the impermeable band is pressable against an opposing press surface by means of a hydraulic pressure apparatus. According to US 5,676,799, the impermeable band is lubricated both hydrodynamically and hydrostatically via the pressure device. For this purpose, a support element facing the band in a direction lateral to the web moving direction can be provided, wherein the support element can have at least one bearing pocket, which is/are operatively connected with pressure chambers of hydraulic piston-cylinder units. The bearing pocket or pockets can also be replaced with one or several bores. In US 5,676,799, it is claimed to be advantageous to remove the pressure medium via a scraper or remover at the web exit end of the pressure contact apparatus, since the pressure medium is heated during its journey along the support element. Furthermore, it is discloseed that the impermeable band can also be cooled during the continued process via spraying on of a temperature-controlled pressure or lubricating media onto the impermeable band. US 5,110,417 discloses a shoe press having an extended press zone with an increasing or level pressure curve through the press nip. The disclosed shoe press comprises a rotating press roll, a press shoe, and a slide band sliding along the surface of the press shoe, a web being passed between felts through the press where it is pressed between the roll and the slide band. The press shoe, which is designed to operate hydrodynamically, has a surface provided with a pocket area formed by one or more pockets of small average thickness. If required, pressurized lubricant can be introduced to the pocket area to achieve the desired press effect and speed. According to US 5 , 110,417, the pocket or pockets has/have an area for lubricant supply communicating with at least one lubricant conduit, wherein the area for lubricant supply is formed by at least one groove for lubricant supply which has a larger depth than the pocket or pockets and which extends in the axial direction of the press roll. EP 0 818 572 A2 discloses a press roll and method for its operation. The disclosed press roll is reported to have a stationary support and a flexible roll shell encircling the support. The roll shell is supported via a press shoe, which can be pressed against the inner surface of the roll shell, exhibiting a supporting surface, wherein a fluid lubricant is supplied between the supporting surface and the inner surface in order to reduce the friction between the press shoe and the roll shell. With the stated purpose of obtaining a reliable operation, a long lifetime of the roll shell and an extended lifetime of the lubricant, the total quantity of lubricant supplied to the press roll is chosen so that the temperature of the roll shell, depending on the heat quantity emitted to the lubricant and transported away by the lubricant, does not exceed a maximum allowable limit. According to the above-mentioned EP 0 818 572 A2, the lubricant supply can be accomplished by means of spraying lubricant onto the inside of the roll shell, preferably close to the exit of the roll shell from the area of the supporting surface. Additionally or alternatively, lubricant can be provided to the region of the supporting surface by means of being pressed in via the press shoe. It is stated that part of the lubricant, and particularly the part which does not enter between the supporting surface and the inner surface of the roll shell, can be transported away to cleaning and conditioning (e.g. cooling), in order to thereafter be returned by means of spraying or pressing in via the press shoe. The removal is reported to be accomplished by means of a suctioning device for excess lubricant arranged at the roll shell in connection to its entry onto the press shoe. Accordingly, as has become evident in the foregoing, the press shoe of a shoe press device has to be lubricated continuously with a suitable lubricant in order to reduce the friction. The lubrication also provides a certain cooling of the flexible belt. Impulse pressing is another previously known technique for dewatering paper webs. In principle, this technique implies that mechanical dewatering by means of pressing in a press nip is combined with heating of, for example, one of the press rolls. Since the heating takes place simultaneously as the paper web is subjected to a very high pressure in the press nip, the heating temperature has to be considerably higher than 100 °C, and is normally 150 - 400 °C. In order to allow the intended heat transfer to the fibre web to take place, impulse pressing requires a relatively long retention time in the press nip. Therefore, it has been suggested previously that press devices having an extended press nip, e.g. shoe presses, also should be utilised for impulse pressing. When pressing wet fibre webs in shoe press units having an extended nip, the previously known shoe press devices and methods for pressing have proved to be associated with potential problems at high machine speeds which result in raised temperatures of the lubricant, the shoe and/or the flexible belt. A raised temperature of the lubricant results in a viscosity reduction of the lubricant, which impairs the lubrication and causes an additional increase of the temperature, in some case to levels high enough for damaging or deforming the belt, which normally consists of a polymer material having a limited resistance against high temperatures. The high temperatures can also result in the lubricant being degraded so that its function is impaired. These problems become particularly pronounced at high machine speeds, particularly from 1500 m/min and upwards. When shoe press devices utilised for impulse pressing are concerned, the above-mentioned problem is made even worse because of the heat which is supplied to the counter roll from the environment. Summary of the invention Accordingly, the first object of the present invention is to provide a shoe press device for pressing wet fibre webs, which device reduces the risk of the above-mentioned problem with raised temperatures occurring and which therefore enables very high machine speeds, primarily when pressing with an extended press nip, but also when utilising impulse technique. In accordance with claim 1, this first object is achieved by means of the shoe press device having a first press surface, a second press surface in the form of a flexible belt, and at least one press shoe. Thereby, the first press surface and the press shoe are arranged for forming a press nip with a pressure zone effective during the pressing through which the wet fibre web runs together with the flexible belt, wherein the surface of the press shoe facing the flexible belt comprises at least one supply area for a pressurised lubricant. Thereby, at least one heat transport connection is integrated in the press shoe for controlling the temperature of the lubricant and the flexible belt within and after the effective pressure zone during the pressing. According to the invention, the heat transport connection/connections comprises at least one outlet for removal of lubricant which has been heated in the effective pressure zone during the pressing, wherein the outlet/outlets is/are arranged in close proximity to or immediately downstream the end of the effective pressure zone. A second object of the present invention is to provide a method for pressing a wet fibre web at high machine speeds. In accordance with claim 12, this object is achieved by means of the method comprising that the pressing takes place in a shoe press device according to the invention, and that the temperature of the lubricant and the flexible belt within and after the effective pressure zone during the pressing are controlled for remaining below a predetermined value. Brief description of the drawings In the following, the invention will be described in greater detail with reference to the attached drawings, in which Fig. 1 schematically shows a sectional view through a portion of a shoe press device according to a first preferred embodiment of the invention, and Fig. 2 schematically shows a sectional view through a portion of a shoe press device according to a second, particularly preferred embodiment of the invention. Detailed description of preferred embodiments The attached Figs. 1 and 2 schematically show sectional views through portions of shoe press devices according to different embodiments of the invention, which in real life would have the appearance of pairs of press rolls. In the drawings, only details in connection with the press nip are shown, since the remaining components of presses with extended press nips should be well known to the skilled person. The shoe press device 100; 200 according to the invention has a first press surface 101 ; 201, a second press surface in the form of a flexible belt 102; 202, and at least one press shoe 103; 203. Thereby, the first press surface preferably is constituted of a shell surface of a metal roll while the second press surface, i.e. the flexible belt, advantageously by means of stiff gables is arranged in order to obtain a roll-like shape. However, it is also conceivable with embodiments of the invention where the flexible belt runs in a path over a guide roll assembly which guides the belt in a path with another shape than a roll shell. Different designs of the first and second press surfaces should be well known within the field of extended press nips, and will therefore not be described in any greater detail herein. Furthermore, in the shoe press device according to the invention, the first press surface and the press shoe are arranged, in a way previously known er se, for forming a press nip 104; 204 having an effective pressure zone 105; 205 during the pressing through which the wet fibre web 106; 206 runs together with the flexible belt. Thereby, the surface of the press shoe facing the flexible belt comprises at least one supply area 107; 207 for a pressurized lubricant. In the embodiments shown in Figs. 1 and 2, the supply area comprises a hydrostatic pocket, of a type previously known per se, which in addition to providing lubricant for lubricating the press shoe and the flexible belt also provides possibilities to influence the pressure build-up in the effective pressure zone through the press nip and thereby also the pressing course. Furthermore, at least one heat transport connection is integrated in the press shoe for controlling the temperature of the lubricant and the flexible belt within and after the effective pressure zone during the pressing. According to the invention, the above-mentioned heat transport connection(s) comprise(s) at least one outlet 108; 208 for removing lubricant which has been heated in the effective pressure zone 105 ; 205 during the pressing, wherein the outlet or outlets is/are arranged for removing the heated lubricant during the pressing in close proximity to or immediately down stream the end of the effective pressure zone. As a result of this feature of the present invention, the lubricant which has been heated during the pressing is removed where it has the highest temperature, and is prevented from following the flexible belt to the beginning of the effective pressure zone and from generating an additional temperature increase there, etc . This first preferred embodiment makes it possible to maintain the desired temperatures of the lubricant and the flexible belt in the effective pressure zone also at very high machine speeds and/or during a longer time of operation. Accordingly, the specific temperature control provided by means of the invention reduces the risk of the lubricant and/or the flexible belt being heated to an exceedingly high temperature in the effective pressure zone during the pressing, and therefore enables very high machine speeds also when utilising impulse technique. The outlets 108; 208 advantageously are constituted of or comprise apertures, rows of apertures, bores, grooves, or one or several pockets in the surface of the press shoe. The actual design of the outlets has to be adapted to the application in question. During the pressing, the outlets advantageously can be connected to a conduit at a reduced pressure, created by a pump or the like, in order to obtain a very effective evacuation of the lubricant. However, it is also conceivable with embodiments where the lubricant is pressed out into the outlets by an positive pressure prevailing in the pressure zone and thereafter is transported further by means of self-transport. According to an alternative embodiment of the shoe press device according to the invention, the heat transport connection or connections further comprise(s) at least one inlet 209 for supplying a lubricant cooled down to a temperature lower than the temperature prevailing in the lubricant which is present within the effective pressure zone 205 during the pressing. The inlet or inlets can be designed as one or several small pockets in the surface of the press shoe, which pockets via one or several channels in the press shoe are connected to a system for supplying cooled lubricant. However, as will become evident from the following, it is also conceivable with another suitable design of said inlets. According to another embodiment of the shoe press device according to the invention, one or several of the above-mentioned inlets 209 is/are arranged for supplying the cooled lubricant upstream the supply area 207 for the pressurized lubricant during the pressing. By means of supplying cooled lubricant in this way immediately before or in the beginning of the effective pressure zone, the effect of lubricant, which has been heated during the pressing and has followed the flexible belt from the end of the effective pressure zone, entering the effective pressure zone and causing an additional temperature increase is prevented and/or reduced. According to still another embodiment (not shown in the drawings) of the shoe press device according to the invention, one or several of the inlets is/are arranged for supplying the cooled lubricant downstream the supply area for the pressurized lubricant during the pressing. By means of supplying cooled lubricant in this way at the end of or immediately after the effective pressure zone, in connection with or after the above-mentioned outlets, in this embodiment the advantage is obtained that the temperature can be reduced specifically in the position where the highest temperatures of the lubricant and the flexible belt normally occur during the pressing. The above-mentioned inlet(s) for lubricant 209 advantageously is/are constituted of or comprise(s) apertures, rows of apertures, bores, or grooves in the press shoe 203. Thereby, the inlets are arranged in a suitable pattern, amongst other things depending on the characteristics of the temperature control which is required for the application in question. At its/their orifice, the inlet or inlets 209 can advantageously be designed as a pocket in the surface of the press shoe 203 which is hydrostatic during the pressing, however with considerably smaller dimensions than which is the case with the hydrostatic pockets illustrated within the supply areas 107; 207 in Figs. 1 and 2. According to a particularly preferred embodiment, illustrated in Fig. 2, the press shoe 203 comprises at least one inlet 209 arranged upstream the supply area 207 for pressurized lubricant and least one outlet 208 downstream said supply area 207. By means of this particularly preferred embodiment, lubricant which has been heated during the pressing can be removed at the exit from the effective pressure zone where the temperature of the lubricant has its maximum, and instead be replaced with cooled lubricant at the entrance to the effective pressure zone. Thereby, a particularly effecient and stable control of the temperature level of the lubricant and the flexible belt can be obtained in the effective pressure zone. In all embodiments, the heat transport connection or connections 108; 208, 209 advantageously are connected to an oil-cooler which is adapted for cooling lubricant which has been heated in the effective pressure zone 105; 205 during the pressing before the cooled lubricant is returned to the effective pressure zone. Thereby, the cooled lubricant advantageously can be returned to the effective pressure zone 105; 205 via a lubricant reservoir (not shown), a suitable pump (not shown) and a system of channels (partially shown) through one or several of the above-mentioned supply areas 107; 207 and/or inlets 209. In a particularly advantageous embodiment, the shoe press device 100; 200 comprises a heater (not shown), which is arranged for heating the first press surface 101 ; 201 during the pressing. In this embodiment, the shoe press device is intended for impulse pressing, wherein the above-discussed temperature control according to the invention will be particularly useful. In the following, a method according to the invention for pressing a we fibre web at high machine speeds will be described, when applicable with reference to the attached Figs. 1 and 2. The method according to the invention comprises that the pressing takes place in a shoe press device 100; 200 according to the invention, and that the temperature of the lubricant and the flexible belt 102; 202 within and after the effective pressure zone 105; 205 during the pressing is controlled in order to remain below a predetermined value. Thereby, the predetermined value primarily is determined by the properties of the lubricant ,and the heat resisting properties of the flexible belt, which are utilised in the application in question. According to a preferred embodiment of the invention, the pressing takes place at a machine speed higher than about 1500 m/min. At such high machine speeds and resulting high temperatures, the advantages provided by the present invention will be particularly useful. However, it is also conceivable with less advantageous embodiments within the scope of the invention where the above-discussed shoe press device according to the invention is utilised for pressing wet fibre webs at lower machine speeds than 1500 m/min. According to one advantageous embodiment, the temperature is controlled by means of removing lubricant, heated to a temperature higher than the predetermined value during the pressing, from or immediately after the effective pressure zone 105; 205. According to another, particularly advantageous embodiment of the method according to the invention, the temperature is controlled by means of removing lubricant, heated to a temperature higher than the predetermined value during the pressing, from or immediately after the effective pressure zone 205, and by means of additionally supplying lubricant cooled down to a temperature lower than the predetermined value to the effective pressure zone 205. The present invention should by no means be regarded as being limited to what has been described above in connection with the different embodiments, or to what is shown in the attached drawings, but the scope of the invention is defined by the attached claims. Accordingly, it should be noted that the expression "heat transport connection", as used herein, refers to a connection integrated in the press shoe the main task of which is to enable supply or removal of a medium for heat transport, which medium at least has to be compatible with and preferably is constituted of the same type of lubricant which normally is supplied between the press shoe and the flexible belt with the intention of creating a hydrostatic and/or a hydrodynamic pressure. It should also be noted that water-receiving fabrics 110; 210 are shown in Figs. 1 and 2, with dashed lines. It is previously known that such a water-receiving fabric, for example, can be constituted of a suitable felt or wire material, and should be led in a suitable path for the configuration in question. In all embodiments, when implementing the invention, it is preferred that the fibre web 106; 206 is supported by at least one water receiving fabric 110; 210 through the press nip, and it is also conceivable with embodiments where the fibre web with both its faces contacts water-receiving fabrics through the press nip.

Claims: Claims 1. A shoe press device for pressing a wet fibre web, said shoe press device (100; 200) having a first press surface ( 101 ; 201 ), a second press surface in the form of a flexible belt (102; 202), and at least one press shoe (103; 203), wherein the first press surface and the press shoe are arranged for forming a press nip (104; 204) having an effective pressure zone (105; 205) during the pressing through which the wet fibre web (106; 206) runs together with the flexible belt, wherein the surface of the press shoe facing the flexible belt comprises at least one supply area (107; 207) for a pressurized lubricant, and at least one heat transport connection is integrated in the press shoe for controlling the temperature of the lubricant and the flexible belt within and after the effective pressure zone during the pressing, characterised in that said heat transport connection(s) comprise(s) at least one outlet (108; 208) for removing lubricant which has been heated in the effective pressure zone (105; 205) during the pressing, and that said outlet(s) is/are arranged in close proximity to or immediately downstream the end of the effective pressure zone. 2. A shoe press device according to claim 1, characterised in that said outlet(s) (108; 208) is/are constituted of or comprise(s) apertures, rows of apertures, bores, grooves, or one or several pockets in the surface of the press shoe (103; 203). 3. A shoe press device according to claim 1 or 2, characterised in that said outlet(s) (108; 208) is/are arranged for being connected to a conduit at a reduced pressure during the pressing. 4. A shoe press device according to any one of the preceding claims, characterised in that said heat transport connection(s) further comprise(s) at least one inlet (209), for supplying a lubricant which has been cooled down to a temperature lower that the temperature prevailing in the lubricant present within the effective pressure zone (205) during the pressing. 5. A shoe press device according to claim 4, characterised in that one or several of said inlets (209) is/are arranged for supplying said cooled lubricant upstream the supply area (207) for the pressurized lubricant during the pressing. 6. A shoe press device according to claim 4 or 5, characterised in that one or several of said inlets is/are arranged for supplying said cooled lubricant downstream the supply area for the pressurized lubricant during the pressing. 7. A shoe press device according to any one of claims 4-6, characterised in that said inlet(s) (209) is/are constituted of or comprise(s) apertures, rows of apertures, bores, or grooves in the press shoe (203). 8. A shoe press device according to any one of claims 4-7, characterised in that said inlet(s) (209) at its/their orifice is/are designed as a, during the pressing, hydrostatic pocket in the surface of the press shoe. 9. A shoe press device according to any one of claims 4-8, characterised in that the press shoe (203) comprises at least one inlet (209) arranged upstream the supply area (207) and at least one outlet (208) arranged downstream said supply area (207). 10. A shoe press device according to any one of the preceding claims, characterised in that said heat transport connection(s) (108; 208, 209) is/are connected to an oil-cooler, which is arranged for cooling lubricant heated in the effective pressure zone (105; 205) during the pressing before the cooled lubricant is returned to said effective pressure zone. 11. A shoe press device according to any one of the preceding claims, characterised in that the shoe press device (100; 200) comprises a heater, which is arranged for heating the first press surface (101 ; 201) during the pressing. 12. A method for pressing a wet fibre web at high machine speeds, characterised in that the pressing takes place in a shoe press device (100; 200) according to any one of claims 1-11, and that the temperature of the lubricant and the flexible belt (102; 202) within and after the effective pressure zone (105; 205) during the pressing is controlled for remaining below a predetermined value. 13. A method according to claim 12, characterisedin that the pressing takes place at a higher machine speed than about 1500 m/min. 14. A method according to claim 12 or 13. characterised in that the temperature is controlled by means of removing lubricant, heated to a temperature higher than the predetermined value during the pressing, from or immediately after the effective pressure zone (105; 205). 15. A method according to any one of claims 12 - 14. characterised in that the temperature is controlled by means of removing heated lubricant at a higher temperature than the predetermined value from or immediately after the effective pressure zone (205), and by means of additionally supplying cooled lubricant at a lower temperature than the predetermined value to the effective pressure zone (205).

Inventors: BROX ERIK, ERIKSSON JONAS, GUSTAVSSON LARS, ILMARINEN ANTTI, KILIAN MALIN, LAURINOLLI TERO, SNELLMAN JORMA, BROX, ERIK, ERIKSSON, JONAS, GUSTAVSSON, LARS, ILMARINEN, ANTTI, KILIAN, MALIN, LAURINOLLI, TERO, SNELLMAN, JORMA
Applicants: BROX ERIK, ERIKSSON JONAS, GUSTAVSSON LARS, ILMARINEN ANTTI, KILIAN MALIN, LAURINOLLI TERO, SNELLMAN JORMA, VALMET KARLSTAD AB, BROX, ERIK, ERIKSSON, JONAS, GUSTAVSSON, LARS, ILMARINEN, ANTTI, KILIAN, MALIN, LAURINOLLI, TERO, SNELLMAN, JORMA, VALMET - KARLSTAD AB
Application year: 2000
Main classification: D21F3
IPC codes: D21F3