METHOD AND DEVICE FOR PULP WEB FORMATION

Abstract: The invention relates to a method and an apparatus for pulp web formation. The method comprises feeding pulp, having a dry content of about 1,5 %, from a headbox (2) into a wedge-shaped converging space (5) defined by two wires (3, 4) or the like, said wires or the like being supported on chamber units (6, 7), including at a given time at least one water-receiving chamber (8), having its pressure adjustable in such a way that the pulp can be maintained at an overpressure in the space (5). The method comprises increasing the dry content of the pulp in the converging space (5) to a value which is at least about 10 % to about 12 %. According to the invention, the method comprises subjecting the pulp present between the wires (3, 4) or the like in the downstream section of the converging space, or in its substantially immediate vicinity, to a compression in perpendicular direction (z direction). In the method, the compression performed in the vicinity of the converging space (5) or its downstream section is used for increasing the dry content of the pulp to a value which is at least about 18 %, preferably to a value which is about 20 %.

Description: METHOD AND DEVICE FOR PULP WEB FORMATION The present invention relates to a method as set forth in the preamble of claim 1 for forming pulp web. According to such a method, the pulp drying process, and more specifically the web formation of pulp stock, can be implemented in a preferred fashion. The invention relates also to an apparatus as set forth in the preamble of claim 8. The publication US 3823062 discloses a twin-wire web formation apparatus, comprising a wedge-shaped gap constituted by two wires, the pulp being delivered therein for web formation. It has been stated that the gap constituted by the wires be created by means of drainage boxes, supporting the wires and boundary to the wire with a water permeable surface. According to the publication, the web formation is regulated by measuring and throttling the quantity of percolated water flowing out of the drainage boxes. This presents the method very schematically and it involves a number of shortcomings, particularly if the equivalent method is applied to pulp web formation. It has been discovered that this prior art method provides a very difficult way of controlling the formation of a web and its coherence downstream of a the formation section, especially with a thick web, which is usually the case with pulp. This solution has its limitations also in terms of its dewatering capacity, it is not capable of reaching a dry content substantially higher than about 12%, since the pressure would increase at the downstream end of a wedge-shaped section to such a high level that the pulp would discharge from the gap portion unmanageably, which causes the wire sections to come apart, resulting in the web losing control of its coherence and the failure of web formation. There are prior known pulp drying mechanisms, comprising a Fourdrinier- wire former section, wherein a head box is used for supplying stock on top of the Fourdrinier wire. The stock is dewatered over the wire section, whereby the dry content downstream of the wire section is typically about 20%. One problem with a Fourdrinier wire regarding pulp drying is one-sided dewatering. At a given point, the so-called wet wire shall have developed against itself a layer of web, which impairs the discharge of water from the web. In the prior art, this problem has been addressed by fitting the wire section with so-called underpressurized dewatering devices, such as so- called vacuum foil boxes or vacuum flatboxes, whereby the dewatering is intensified by increasing a pressure difference which brings about or encourages the separation of water. In this type and the like former sections, the vacuum system is an element unavoidable in terms of operation. However, the vacuum system in particular constitutes a significant part of the equipment investment costs as well as operating expenses as regards the forming section of a pulp drying machine. The Finnish publication print 61218 discloses a pulp drying machine, said machine comprising a wet or wedge-shaped section, a register section with a number of twin rolls, a forward press section, as well as a main press section for applying compression to a material being dried. This publication suggests that the wet section be provided with vacuum boxes for absorbing moisture through the wires. Even in this type of solution, the vacuum system brings along the above-mentioned drawbacks. The publication WO 97/42374 describes a sort of wet lap machine, which in turn comprises a variety of sub-assemblies, which thus makes it highly complicated in itself. Another problem in the prior known solutions is generally the efficiency of dewatering. The available solutions are not capable of dewatering pulp at a desired efficiency. In particular, the vacuum boxes designed for a powerful underpressure lead to the compaction of web surface layers, as well as to the clogging of wires, and hence to impaired dewatering. It is an object of the present invention to eliminate the prior art drawbacks and to provide a totally novel type of solution for pulp web formation and drying, wherein the prior art disadvantages are minimized. In order to accomplish the objects of the invention, a method of the invention for forming pulp web, said method comprising feeding pulp from a head box into a wedge-shaped converging space defined by two wires or the like, said wires or the like being supported on chamber units, each including at least one water-receiving chamber, in which method the pulp has its dry content increased in the converging space to the value of at least about 10% to about 12%, is characterized in that the method comprises, in the substantially immediate vicinity of the converging space and/or its downstream end section, subjecting the pulp to an independently adjustable compression force perpendicular (z direction) thereto. An apparatus of the invention for pulp web formation, said apparatus comprising a wedge-shaped converging space defined by two wires or the like, said space being supplied with pulp at a first dry content from a head box, and said wires or the like being supported on chamber units, each including at least one water-receiving chamber, is characterized in that the converging space has its downstream section or its substantially immediate vicinity provided with compaction members for subjecting the pulp between the wires or the like to a compression in the perpendicular direction (z direction). According to one preferred embodiment of the invention, the compaction members are designed as a plane press. The invention will now be described in more detail with reference to the accompanying drawings, wherein: Fig. 1 shows schematically an apparatus of the invention for pulp web formation, Fig. 2 shows schematically one modified way of implementing the invention in the apparatus of fig. 1 , Fig. 3 shows schematically a compaction member for a pulp web forming apparatus of the invention in a side view, Fig. 4 shows the compaction member of fig. 3 in a front end view, and Fig. 5 shows alternative design for a compaction member. Fig. 1 shows schematically an apparatus 1 of the invention for forming a web W. The apparatus comprises a headbox 2, which is used, according to a pulp drying process of the invention, for supplying pulp at a dry content of at least about 1 ,5% into a converging wedge-shaped space 5 defined between two endless wire loops, such as an upper wire 3 and a lower wire 4. The space 5 is formed by adapting the wires 3, 4 to travel around spaced-apart reverse rolls 50 and 51 , respectively, set at the upstream end of the space 5, and thence in a relatively converging fashion for defining a wedge-shaped space. In engagement with the inside surface of the wires 3 and 4 (away from the pulp flow) are mounted chamber units 6 and 7, respectively, which are provided with a number of dewatering chambers 8, the flow rate, and secondarily also the pressure, of water to be discharged from pulp therethrough being regulable by means of valves 22 and which, as the process is going on, are substantially full of water removed from the pulp. Hence, the water discharging from pulp is guided to mix with already removed water in the chambers 8, and further into discharge channels. From the dewatering chambers extend water discharge channels for carrying water removed from the web to a further process. Thus, in the solution of the invention, the flow of water discharging from a pulp web is regulated as desired for this way controlling the dewatering process. Hence, the discharge channels are provided with the valves 22 for regulating the flow rate by variable throttling. It is desirable that the flow rate for each chamber be maintained at a preset value. For the sake of clarity, the figure only depicts the channels from the lower chambers, but it is obvious that the upper chambers are also provided with respective channels, or, alternatively, the respective chambers in an upper and lower tier are connected to a single, common channel. In the illustrated solution, pulp is delivered at such a rate and pressure between the wires 3, 4 while the same are running that said space 5 carries a positive pressure. This overpressure results in the percolation of pulp water through the wires into the dewatering chambers 8. What is essential is that there is a pressure difference between the dewatering chambers and the space 5. According to one preferred embodiment of the invention, a pressure difference between the space 5 and the dewatering chambers 8 is measured continuously and this measuring value is used in process control. Dewatering from the chambers 8 is regulated in such a way that the speed ratio within the area of each chamber is preferably about 0,9-1 ,1. The speed ratio refers to a flow of liquid occurring between the chambers parallel to the web divided by the wire advancing speed. The chambers 8 are otherwise substantially impervious, except that the surfaces in engagement with the wires 3, 4 are such that the water to be removed from pulp is able to migrate into the chambers 8. There are preferably several chambers 8 arranged sequentially in such a way that the surface in engagement with the wires is nevertheless substantially continuous. The separate chamber units 6, 7 commence substantially directly downstream of the reverse rolls 50, 51 for the top wire 3 and bottom wire 4, and extend a certain distance to define the wedge-shaped converging space 5, the upper and lower wire 3, 4 passing therethrough. Preferably, the chamber units 6, 7 are substantially symmetrical and fixedly supported in a suitable position for providing a preferred wedge shape. The converging space 5 defined by the chamber units 6, 7 is provided in its downstream section, or in the immediate vicinity thereof, with a compaction member 9 for adjusting a relative distance between the upper wire 3 and the lower wire 4 or the like as desired. In fig. 1 , the compaction members are depicted to comprise a plane press 9, having its top element 31 , 43 articulated, while its bottom element 30 is solid. During its passage through the space 5, the pulp delivers water in such a way that the pulp has a dry content of at least 12% before the plane press 9, and the plane press is used for removing more water from the pulp in such a way that, after the plane press 9, its dry content is about 18%, preferably about 20%. The dry content can also be increased to about 18-20% as early as within the converging space 5 and the plane press 9 can thus be used merely to substantially maintain the coherence of pulp emerging from the outlet of the space 5. The pulp emerging from the wedge-shaped space comes out of the space 5 at a high pressure and, without the compaction members of the invention, that would occur uncontrollably and the wires 3, 4 would spread apart, resulting in disintegration of the already formed pulp web. If necessary, water is still removed according to the invention downstream of the plane press 9 while the web W remains supported between the two wires 3, 4, for example by compressing the aggregate between two rolls 10, 11. This is followed by guiding the upper wire 3 and the lower wire 4 away from each other and the web W is carried further, in accordance with the invention, on the top surface of the lower wire 4, the web having a dry content of roughly 28%. Thus, according to the invention, the web travels in a fully supported condition. If necessary, the lower wire has its bottom surface fitted with a so-called transfer vacuum box for securing a passage of the web along with the lower wire. The web W is carried further in a supported condition on the top surface of the lower wire 4 through a press nip N, comprising an upper felt 14, i.e. the web W travels with its top side against the felt and its bottom side against the lower wire 4 in the nip N between rolls 12, 13. The nip is preferably constituted by a so-called shoe press. Particularly, a pulp web can be formed and effectively dewatered by means of a comprehensive process, wherein pulp is delivered into the converging wedge-shaped space 5 defined between two endless wire loops, such as the upper wire 3 and the lower wire 4, having a plurality of dewatering chambers 8 in communication with the wedge-shaped space, the flow rate of water to be removed from the pulp therethrough being regulable, wherein the water flow discharging from the pulp web is adjusted as desired for thus controlling the dewatering process, wherein the converging space 5 defined by the chamber units 6, 7 is provided in its downstream section, or in the immediate vicinity thereof, with the compaction member 9 for adjusting a relative distance between the upper wire 3 and the lower wire 4 or the like as desired, wherein the web W is subsequently advanced further, according to the invention, on the top surface of the lower wire 4 in a supported condition through the press nip N, comprising the upper felt 14, i.e. the web travels through the press with its top side against the felt and its bottom side against the lower wire 4 in the nip N between the rolls 12, 13, said nip being especially preferably constituted by a so-called shoe press for providing a sufficient nip pressure and a long action time. In this process, the web W is dried in such a way that its dry content downstream of the nip N is roughly 40%. The above-described method of the invention can be used in a pulp drying machine for the effective and reliable formation of the web W. This solution provides a simplified former section and the web W is comprehensively supported until it has a dry content of roughly 40%, at which point it has a sufficient strength to withstand without breaking a first unsupported passage to the subsequent process phase. In the method of the invention, the pulp web has also a considerable thickness and, thus, according to the invention, the passage of the lower wire 4, to the extent that it supports the web W, is adapted to extend in a substantially straight line. The invention provides major benefits, which include e.g.: - pulp web formation can be performed substantially more effectively than with the prior known apparatus, the web former section of a pulp making machine can be accommodated in a substantially smaller space than the prior known solutions, which may lead to the savings of up to several million marks in investment costs, the passage of a web is substantially more reliable by virtue of a supported run, which reduces downtimes and increases the overall efficiency of a machine. Fig. 2 shows schematically an apparatus 1 which differs from the embodiment of fig. 1 principally in that the plane press 9 is designed to be adjustable regarding both its top element and its bottom element. In addition, fig. 2 depicts an adjustment and control unit 21 , which is connected to appropriate measuring devices transmitting measuring data and which in turn transmits drive commands to various appropriate elements, as shown by way of example with lines 15-20. According to the invention, such data contains some or all of the following: headbox supply consistency, headbox pressure, pressure in space 5, pressure in chambers 8, speed of wires, compression force/pressure of plane press, water flow rate through valves 22. These or some preselected ones of these quantities are forwarded to the adjustment/control unit, wherein drive commands are executed in a pre- desired manner on the basis of these for controlling a drying process. The drive may preferably include the following controls: control of valves 22, loading of plane press, speed of wires, control for headbox supply pump, control of headbox consistency, etc. According to the invention, the plane press 9 may be provided with a loadcell and/or a pressure measuring sensor for measuring and adjusting the compression force. In communication with the chambers 8 is fitted a gauge for measuring of pressure from the chambers and, if desired, also a pressure transmitter for measuring pressure in the space 5 and/or for determining a pressure difference between the chamber 8 and the space 5, whereby it is possible to use e.g. the valve 22 for effecting the adjustment of a pressure difference between the chamber and the space 5. The web W is also measured for its dry content at a suitable point of the drying process, making it possible on the basis of information received therefrom to execute drive commands, if necessary, by means of the adjustment/control unit. Furthermore, according to the invention, the pressure can be regulated as desired in the space 5, as well as by means of the plane press 9 and the press rolls 10, 11 and 12, 13 for adjusting the dry content and dewatering as desired at various points. Since, according to the invention, the discharge of water from the chamber 8 is controlled comprehensively by observing all operating parameters of a drying machine, explicitly on the basis of information originating from the process, and not just the water flow rate out of the chamber, the drying/dewatering process can be driven in such a way that it proceeds as desired and continuously in an optimal fashion. Figs. 3 and 4 depict one embodiment for the plane press 9, including a solid bottom element 30, and a top element 31 whose position relative to the bottom element 30 is adjustable. The top element 31 is articulated at 32 to the downstream portion of a space 5. The pulp is delivered between the wires from the space 5 along a slot 40. The element 31 is secured to upright struts 35 linked to each other at the bottom part thereof by a connecting member 37, which is in turn fastened through the intermediary of cushion elements or bellows 36 to the solid bottom element 30 of the plane press 9. According to the invention, a web to be delivered out of the space 5 can be guided forward in a controlled fashion, the top and bottom elements of the plane press being adjustable regarding the relative position thereof, as well as relative to the position of the chamber units. Furthermore, according to the invention, the loading of the plane press is effected by means of implements which are capable of complying to suitable degree with possible fluctuations in web thickness. Thus, the generation of excessive pressure in the plane press is avoided in possible malfunction incidents. In a normal condition, the plane press is loaded with a desired force by means of regulable adjusting rods 33 or the like, with a certain force against these rods 33, for maintaining a certain stable setting. Consequently, the relative position between the top element 31 and the bottom element 30 can be mechanically defined by means of adjustable implements. The adjusting rods or the like are preferably provided with actuators for setting the same while the machine is running, if, for example, the plane press is in a position which must be reset because of a change in production. The loading can be implemented preferably by means of the cushions 36 using a compressible medium, e.g. air, whereby an occasional increase in web thickness is compensated for by the compressibility of the medium. After the malfunction has passed, the distance returns automatically back to what it was. Thus, by virtue of the elasticity of the cushion elements 36, this solution is capable in a very simple manner of eliminating malfunctions possibly occurring in pulp, causing fluctuations in thickness. It is also possible to provide a different type of distance control, the plane press having its position adjusted by means of pressure and distance measurement. The above-described compaction member is a plane press, but the invention is conceivable also be using a compaction member, which comprises a nip press constituted by two rolls, the nip thereof being located in the immediate vicinity of the outlet of a converging space. This can be implemented, e.g. as shown in fig. 5, by designing a tail-end section 61 of the chamber units to extend to the proximity of the nip constituted by rolls 61 , 62.

Claims: Claims 1. A method for forming pulp web, said method comprising feeding pulp from a headbox (2) into a wedge-shaped converging space (5) defined by two wires (3, 4) or the like, said wires or the like being supported on chamber units (6, 7), each including at least one water-receiving chamber (8), in which method the pulp has its dry content increased in the converging space (5) to the value of at least about 10% to about 12%, characterized in that the method comprises, in the substantially immediate vicinity of the converging space (5) and/or its downstream end section, subjecting the pulp to an independently adjustable compression force perpendicular (z direction) thereto, that said at least one water-receiving chamber is, while the process is going on, substantially full of water removed from the pulp, and that it is provided with a dewatering assembly, as well as with devices (22) for regulating the flow rate of water. 2. A method as set forth in claim 1 , characterized in that the compression force applied in the downstream section of the converging space and/or in the substantially immediate vicinity thereof subjects the pulp to a pressure which is at least equal to the pressure existing in the converging space (5), the web having a sufficient dry content to enable the same to withstand the following prepress treatment. 3. A method as set forth in claim 2, characterized in that the web has its dry content increased by means of said compression force to the value of at least about 18%. 4. A method as set forth in claim 1 , characterized in that, after subjecting the pulp web to a compression force perpendicular (z direction) thereto, the web is carried further between the two wires (3, 4) through a separate prepress zone (10, 11), and that this is followed by guiding the upper wire (3) and the lower wire (4) away from each other, and a web (W) is advanced further on the top surface of the lower wire (4). 5. A method as set forth in claim 1 or 4, characterized in that the web (W) is carried on the top surface of the lower wire (4) in a supported condition through a press nip (N), comprising an upper felt (14), i.e. the web (W) travels with its top side against the felt and its bottom side against the lower wire (4) in the nip (N) between rolls (12, 13), wherein this process comprises drying the web (W) in such a way that its dry content downstream of the nip (N) is at least roughly 40%. 6. A method as set forth in claim 1 , characterized in that said at least one water-receiving chamber (8) is provided with elements for measuring a pressure from the chamber, as well as with adjustment and control elements (21 ) for using this pressure measurement for process control. 7. A method as set forth in claim 1 , characterized in that the devices (22) for regulating the flow rate of water are linked to the adjustment/control unit (21 ), said adjustment and control unit driving the devices for regulating the flow rate of water on the basis of information originating from the process. 8. A method as set forth in claim 7, characterized in that the adjustment and control unit drives the devices for regulating the flow rate of water on the basis of information originating from the process. 9. An apparatus for pulp web formation, said apparatus (1 ) comprising a wedge-shaped converging space (5) defined by two wires (3, 4) or the like, said space being supplied with pulp at a first dry content from a headbox (2), and said wires or the like being supported on chamber units (6, 7), each including at least one water-receiving chamber (8), characterized in that the converging space (5) has its downstream section or its substantially immediate vicinity provided with compaction members (9) for subjecting the pulp present between the wires (3, 4) or the like to a compression in the perpendicular direction (z direction), that said at least one water-receiving chamber is, while the process is going on, substantially full of water removed from the pulp, and that it is provided with a dewatering assembly, as well as with devices (22) for regulating the flow rate of water. 10. An apparatus as set forth in claim 9, characterized in that the compaction members (9) are provided with power units for applying the compression with a pressure which is at least equal to the pressure existing in the converging space (5) for increasing the dry content of pulp to a value which is substantially higher than said first value. 11. An apparatus as set forth in claim 9, characterized in that the compaction members (9) are provided with power units which comprise elements (36) to compensate for momentary machine-directed fluctuations in web thickness. 12. An apparatus as set forth in claim 11 , characterized in that the elements to compensate for momentary machine-directed fluctuations in web thickness include an air cushion (36) to compensate for momentary actions of the compaction member (9). 13. An apparatus as set forth in claim 9, characterized in that the compaction members (9) for the perpendicular compression of pulp present between the wires (3, 4) or the like are provided with a measuring, adjustment and control device (21) for controlling the compression. 14. An apparatus as set forth in claim 13, characterized in that the compaction members (9) for the compression of pulp present between the wires (3, 4) or the like are provided with measuring devices for measuring the compression force and/or pressure, as well as with measuring devices for measuring a pressure from the space (5), as well as with elements for adjusting the pressure existing between the compaction members (9) to be at least equal to the pressure existing in the space (5). 15. An apparatus as set forth in claim 9, characterized in that the compaction members (9) are constituted by a plane press (30, 31 ), wherein the relative position between a top element (31 ) and a bottom element (30) can be mechanically defined by means of adjustable implements (33). 16. An apparatus as set forth in claim 15, characterized in that the implements (33) are provided with actuators for setting the same while the machine is running. 17. An apparatus as set forth in claim 9, characterized in that the compaction members comprise a nip press constituted by two rolls (61 , 62).

Inventors: KOSKINEN JORMA, LUNDEN LEO, MARKKANEN ESA, PAKKALA ILKKA, SAVIA RISTO, KOSKINEN, JORMA, LUNDEN, LEO, MARKKANEN, ESA, PAKKALA, ILKKA, SAVIA, RISTO
Applicants: KOSKINEN JORMA, LUNDEN LEO, MARKKANEN ESA, PAKKALA ILKKA, SAVIA RISTO, VALMET CORP, KOSKINEN, JORMA, LUNDEN, LEO, MARKKANEN, ESA, PAKKALA, ILKKA, SAVIA, RISTO, VALMET CORPORATION
Application year: 1999
Main classification: D21F1
IPC codes: D21F1, D21F9