PROCESS ARRANGEMENT FOR THE SHORT CIRCULATION IN A PAPER OR BOARD MACHINE

Abstract: A process arrangement for the short circulation in a paper or board machine, which arrangement comprises stock chests (20i) for component stocks (Mi), metering pumps (21i) for component stocks, cleaning devices (115, 120, 140, 190, 195), pumps (110, 130, 170, 180), a headbox (150) and a wire section (160) as well as a system of pipes connecting said apparatuses, together with regulation devices. After the metering pumps (21i), the component stock flows (Mi) have been passed into a closed mixing volume, in which the component stocks (Mi) are mixed and diluted with a first dilution water flow (F10). From this closed mixing volume the stock has been passed in a closed space, by means of a first feed pump (110) of the main line of the process, through a screen (115) and a centrifugal cleaner (120) to the suction side of the second feed pump (130) of the main line, where a second dilution water flow (F20) is passed to the stock. The second feed pump (130) feeds the stock through a machine screen (140) to the inlet header in the headbox (150).

Description: Process arrangement for the short circulation in a paper or board machine The invention concerns a process arrangement for the short circulation in a paper or board machine as defined in the preamble of claim 1. Regarding its principal features, the stock feed at a paper machine is, as a rule, as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers the stocks are fed into stock chests, and from them further into a common blend chest, in which the stock components are mixed with each other. From the blend chest the stock is fed into a machine chest, and from the machine chest there is an overflow back into the blend chest. From the machine chest, the stock, which is, as a rule, at a consistency of about 3 %, is fed into a wire pit placed in the short circulation. In the wire pit the high- consistency stock is diluted to a headbox consistency, which is, as a rule, about 1 % . The fibres and fillers which are used as the raw-material are passed onto a wire through the headbox while carried by water. The filtrate that has passed through the wire, which filtrate contains an abundance of fibrous material and fillers, is returned, as a diluting agent for the high-consistency stock coming from the machine tank, through the headbox back onto the wire. The flow loop thus formed is called the short circulation. The short circulation, together with the headbox connected with it, is commonly considered to be the most sensitive part of the papermaking process. Even little changes in the consistency, in the flow, or in other parameters have an immediate effect on the quality of the paper produced or cause web breaks in the paper machine. Along with the high-consistency stock or along other paths, impurities may enter into the short circulation, which impurities must be removed before the headbox. This takes place by means of cleaning devices of the short circulation, which are, for example, centrifugal cleaners, screens and machine screens. Ever stricter requirements of protection of the environment have resulted, in connection with paper and board machines, in more closed systems and also in a more closed short circulation and in as efficient recycling of raw-materials as possible. On the other hand, improved efficiency of production and minimizing of disturbance in the production are also aimed at. This is why, among other things, a higher level of wire retention is used, which requires increased use of retention agents. The short circulations used in the present-day paper and board machines are rather complex, and the main line of the process includes an abundance of equipment, in which case the process space required by the devices must be large. One reason for the complex nature of the short circulation of a paper or board machine is the binding of air in the circulation water in an open wire section. In order to remove the air from the water, it is necessary to construct one or even several deaeration systems. Air is bound in water in the wire section because the process portion after the wire is open and the circulation water is in direct contact with the surrounding air. Air is present in the circulation water both as air bubbles and in dissolved form. When the stock that is used for manufacture of paper is diluted with circulation water that contains air, the content of air in the water produces disturbance of many sorts in the formation of the paper web. Among other things, the content of air lowers the capacity, deteriorates the quality of the paper, and causes contamination of the process, formation of slime, blocking of cleaning devices, and wear. Predicting of vibrations in a system of short circulation is substantially more difficult than predicting of purely mechanical vibrations. This comes, among other things, from the fact that the coefficient of elasticity of flowing liquid also depends, to a great extent, on the air contained in the liquid. Also, the rigidity of the pipe systems and of the tanks affects the rigidity of the system and, thus, the natural frequencies. Further, the velocity of progress of a pressure pulse in the stock slurry is slowed down substantially in compliance with the amount of undissolved air. Resilience of the walls in the pipe systems also has an effect slowing down the velocity of a pressure pulse. The variations arising from these factors have direct effects on the quality of the paper and are noticed as defects in the final product. Changes in the content of air in the stock also cause faults in the flow rate in the headbox. Firstly, air worsens the vibrations of the short circulation in the way mentioned above. Also, the air affects the density of the liquid to be pumped, and thereby it affects the pressure produced by a pump, and further it affects the basis weight. For removal of air from the circulation water, a number of complex solutions are known from the prior art, which solutions involve additional devices and combinations of additional devices which cause costs of investment and operation, such as deaeration equipments, pumps, and tanks. It is partly out of this reason that the process volume of the main line becomes relatively large, as a result of which changes of paper grade in a paper machine require a long grade change time. Further, in the prior-art processes, mixing tanks and stilling tanks have been used in order to keep the process conditions as invariable as possible. In the applicant's FI Laid-Open Publication 88,415, a process arrangement is described for production of headbox stock for a paper machine in the short circulation. In this arrangement, no fresh stock is mixed with the circulation water passing to the deaeration tank. In order to achieve this, in the arrangement, a combination wire pit is employed, which has been divided into two compartments or into two jointly operative tanks. The first tank has been arranged as a feed tank for deaeration, and the second tank as a dilution tank for headbox stock, into which latter tank the fresh stock is fed. By means of this process arrangement, the principal objective has been to eliminate the essential problems produced by variations in consistency and by variations in pressure in the headbox. On the other hand, in the FI Laid-Open Publication 93,132 (Oy Tampella Ab), an integrated headbox and former solution is described, in which the stock is not in contact with the surrounding air as it is transferred from the headbox to the former. Also, the gap former used in the solution is closed, so that the stock and the white water cannot reach contact with the surrounding air. The draining of water in the former takes place by means of water drain boxes. For this integrated headbox-former unit, the designation CFF unit (Control Flow Former) is used. In the FI Laid-Open Publication 81,965 (Oy Tampella Ab), a gap former is described in which the wires are supported on deck elements of closed box-like water drain spaces. The deck elements in the water drain space at the side of one of the wires are loaded resiliently against the wire in the desired way. Thus, herein pressure is used as an aid for the draining, in which way it is possible to enhance the separation of the solid matter from the suspension. The object of the present invention is a simplified short circulation suitable for a paper or board machine, by means of which simplified short circulation it is possible to solve or at least substantially to reduce the problems related to the prior art. It is a further object of the present invention to be able at least to reduce the binding of air in water in the short circulation of a paper or board machine to a substantial extent, as compared with the prior art. The principal characteristics of the process arrangement for the short circulation in a paper or board machine in accordance with the present invention are disclosed in claim 1. In the process solution in accordance with the invention, for precise regulation of the basis weight, the following solutions have been found: • the dilution of the component stocks to the metering consistency takes place before the stock chests of the component stocks, • the regulation of the basis weight takes place from the stock chests of the component stocks by means of regulation of the flows of the component stocks, • the dilution to the headbox consistency takes place in two stages, of which the first one has an invariable flow, and in the second stage the flow is regulated by means of a control signal received from the headbox pressure regulation. With respect to the regulation of the basis weight related to the process arrangement in accordance with the present invention, reference is made to the applicant's FI Patent Application No. 981329. With respect to the metering of a component stock related to the process arrange- ment in accordance with the present invention, reference is made to the applicant's FI Patent Application No. 981328. In the solution in accordance with the present invention, the main process line of the short circulation is closed. In one embodiment of the invention, the headbox and the former are also closed, in which case no air can be mixed with the white water in the wire section. In this embodiment, the circulation water departing from the wire section is kept in a closed space as slightly pressurized, in which case it is possible to prevent binding of air in the white water. The white water needed for dilution of stocks is pumped along closed pipes to the dilution sites, where the dilution takes place in a closed space. For mixing of stock and water, pumps, stock cleaning devices, screens, and centrifugal cleaners normally needed in the process are used. Any excess water is removed from the short circulation as overflow from between the wire section and the circulation water pump or the deaeration tank to atmospheric pressure. From the stock cleaning devices, the rejects are removed for possible further treatment. The process arrangement in accordance with the invention for the short circulation can be applied both in a paper machine and in a board machine. In a board machine, it is possible to use a number of parallel process arrangements in accordance with the invention for the short circulation at the same time. By means of a solution in accordance with the invention, mixing of air with the white water can be minimized, in which case the losses of fibre in the process are also minimized. The equipment is simple, and in it fewer components are needed than in the prior-art systems. This is why, as compared with the prior-art solu- tions, the equipment is less expensive both in respect of the cost of acquisition and in respect of the servicing costs. The process arrangement also requires clearly less space, and it requires less spare parts than the prior-art solutions do. Further, a change of paper grade is very rapid, because the basis weight of paper can be regulated very quickly. The quality of the paper produced is uniform, because disturbance arising from variations in the content of air is avoided. Further, a considerably smaller amount of broke paper is produced in connection with change of paper grade, as compared with the prior-art systems. In the process arrangement in accordance with the present invention, in the main line, blend chests and a machine chests and related pumps etc. auxiliary devices are not needed. Further, in the system, there is no wire pit, for which reason the overall volume of water in the short circulation can be made smaller. This again improves the level of hygiene of the water, because the dwell of the water and the fibre in the process is shorter than in the prior art, whereby microbiological contamination of the water is reduced. This is why it is possible to reduce the use of auxiliary chemicals, such as slime inhibiting agents, which results in economies in the costs of operation. In the following, some preferred embodiments of the invention will be described with reference to the figures in the accompanying drawings, the invention being, however, not supposed to be confined to the details of said illustrations. Figure 1A is a schematic illustration of a conventional prior-art process arrangement of the stock feed in a paper machine. Figure IB is a schematic illustration of a conventional prior-art process arrange- ment of the short circulation in a paper machine. Figure 2 is a schematic illustration of a process arrangement in accordance with the present invention for the short circulation in a paper machine. Figure 3 is a schematic illustration of a modification of the process arrangement shown in Fig. 2 for the short circulation in a paper machine. Figure 4 is a schematic illustration of a second modification of the process arrangement shown in Fig. 2 for the short circulation in a paper machine. Figure 5 shows a modification of the process arrangement shown in Fig. 4 for the short circulation in a paper machine. Figure 6 shows a second modification of the process arrangement shown in Fig. 4 for the short circulation in a paper machine. Fig. 1A is a schematic illustration of a conventional prior-art process arrangement of the stock feed in a paper machine. In the figure, just one component stock is shown. In the figure, the recovery of fibres, the regulation of the flow of the component stock, or the regulation of the surface level in the stock chest of the component stock have not been illustrated. In Fig. 1A, the component stock M^ is fed from a storage tower 10 by means of a first pump 11 into a stock chest 20. To the component stock, a dilution water flow is passed through a regulation valve 18 into connection with the first pump 11. Further, the component stock is diluted in the bottom portion of the storage tower 10 by means of a dilution water flow 9 passed to said bottom portion. From the stock chest 20, the component stock M j is fed by means of a second pump 21 through a regulation valve 22 and through a feed pipe 23 to the main line 60 of the process, which passes into a blend chest 30. From the blend chest 30 the stock is fed by means of a third pump 31 into a machine chest 40. From the machine chest 40 the machine stock M is fed by means of a fourth pump 41, through a second regulation valve 42, into the short circulation. Moreover, from the machine chest 40, there is an overflow 43 passing back to the blend chest 30. The blend chest 30 and the machine chest 40 form a stock equalizing unit, and in them the stock is diluted to the ultimate metering consistency. Further, by their means, uniform metering of the machine stock is secured. The metering of the component stocks M j into the blend chest 30 takes place so that attempts are made constantly to keep an invariable surface level in the blend chest 30. Based on changes in the surface level in the blend chest 30, which changes are measured by a surface level detector LT, the surface level controller computes the total requirement Q tot of stock to be metered, which information is fed to the component stock metering-control block 25. Also, a pre-determined stock proportion value KQJ of the component stock M j and a consistency value CSJ of the component stock M j are fed to the metering-control block 25. Based on the total requirement Q tot of stock M and on the pre-determined proportions K Q J of component stocks, the metering-control block 25 computes the requirement Q j of feed of component stock. Based on the component stock feed requirement Q j and on the data CSJ on the consistency of the component stock M j , the component stock metering-control block 25 computes the flow target F j of the component stock M j . Based on this flow target F j , the regulation valve 22 is controlled so as to produce said flow F j into the mixing tank 30. The flow F j of the component stock M j is also measured constantly by means of a flow detector FT, whose measurement signal is fed through the flow controller FC to the component stock control valve 22. From the blend chest 30, the stock is fed at an invariable flow velocity by means of the third pump 31 into the machine chest 40. At this pumping stage, the consistency of the stock is also regulated to the desired target consistency of the machine chest. This is accomplished by means of dilution water, which is fed through the regulation valve 32 to the outlet of the blend chest 30 to the suction side of the third pump 31. By means of the dilution water, the stock present in the blend chest 30, which is, as a rule, at a consistency of about 3.2 %, is diluted to the ultimate metering consistency of about 3 % . To the dilution water regulation valve 32, the metering signal of a consistency detector AT is fed, which detector AT has been connected to the pressure side of the pump 31. To the basis weight controller, the measurement signal Cs-p of the consistency detector AT is fed, measured either after the third pump 31 or after the fourth pump 41. The regulation of the basis weight takes place so that the basis weight controller 50 controls a regulation valve 42 placed after the fourth pump 41. By means of this regulation valve 42, the flow of the stock to be fed into the short circulation is regulated, which flow again affects the basis weight of the paper web obtained from the paper machine. When the flow is increased, the basis weight becomes higher, and when the flow is reduced, the basis weight becomes lower. Fig. IB illustrates a conventional prior-art short circulation in a paper machine. The stock flow M passing into the wire pit 60 and shown in Fig. IB is fed by means of the fourth pump 41 shown in Fig. 1A. In Fig. IB, the headbox 150 feeds the stock suspension jet through its slice opening into the wire section 160. In the wire section 160, there are water collecting means, which pass the water drained through the wire, as a flow F^Q, into the wire pit 60. To the mixing area 60a in the wire pit 60, a fresh stock flow M j is fed, whose consistency is, as a rule, of an order of 3 % . In the wire pit 60, the fresh stock is diluted to the headbox consistency, which is of an order of 1 % . To the mixing area 60a of the wire pit 60, the suction side of the first mixing and feed pump 70 has been connected. From the pressure side of the first pump 70, the stock flow F^Q, which has been diluted to the headbox consistency, is passed through vortex cleaners 120 into a deaeration tank 200. In the deaeration tank 200, there is an air space subjected to a vacuum above the free surface of the stock. The stock surface level is determined by an overflow 201 of the deaeration tank 200, over which a stock flow F^ Q flows, from which the air has been removed. This flow F^ Q is passed to the mixing area 60b of the wire pit 60. To said mixing area 60b, further, a return flow F^^ from the vortex cleaners and the fresh stock flow M are passed. From the bottom part of the deaeration tank 200, a stock flow F^ Q is passed to the suction side of the second stock pump 130. This second stock pump 130 feeds the intake stock flow F n through a machine screen 140 into the headbox 150 inlet header. The bypass flow F out from the headbox 150 inlet header is returned to the bottom part of the deaeration tank 200. The reject Fγ j from the machine screen 140 is passed to reject treatment. Fig. 2 is a schematic illustration of a process arrangement in accordance with the present invention for the short circulation in a paper machine. In the figure, three components stocks M j , M2, Mg are shown, but, from the point of view of the invention, the number of component stocks can be N, wherein N is a positive integer number ≥ 1. In Fig. 2, each component stock M j is fed from its stock chest 20 j by means of a pump 21 j through a component stock feed pipe 23 j into a feed line 100 between the deaeration tank 200 and the first pump 110 in the main line of the process. The first pump 110 in the main line feeds the stock through a screen 115 and through a centrifugal cleaner 120 to the suction side of the second pump 130 in the main line. The second pump 130 in the main line feeds the stock through the machine screen 140 into the headbox 150. The white water F^ Q recovered from the wire section 160 is fed by means of a circulation water pump 170 into the deaeration tank 200. Any excess white water is passed by means of an overflow F4 Q to atmospheric pressure. In the deaeration tank 200, also in this solution, there is an air space subjected to a vacuum above the free surface of the stock. In the screen 115, for example, shivers and debris are removed from the stock, and in a centrifugal cleaner 120, for example, sand and other particles heavier than fibres are removed from the stock. The component stocks M j are metered from component stock stock chests 20 j precisely to the mixing volume of the stocks in the dilution water feed pipe 100 coming from the deaeration tank 200. The precise invariable pressure of the component stock to be metered is produced so that the surface level and the consistency in the component stock stock chest 20 j are kept invariable and so that an invariable back pressure is arranged at the mixing point of the component stocks M j . A precise invariable pressure of the mixing volume is produced so that a sufficient reduction in pressure occurs between the nozzle of the component stock M^ and the mixing volume, in which case changes of pressure in the mixing volume do not interfere with the metering. Here the mixing volume is composed of the dilution water pipe 100 passing to the first feed pump 110 and of the feed pipes 23 j of the metering pumps 21 j and of connection arrangements between them. The diluting of the stock is carried out in two stages. The dilution of the first stage is carried out at the suction side of the first pump 110 in the main line when the component stocks M j are fed into the feed line 100 between the deaeration tank 200 and the first pump 110 in the main line. In the deaeration tank 200 the surface level is kept invariable by means of a surface level controller of the primary side. The surface level is measured at the point A, and, by means of the surface level controller LIC, the rev. controller SIC is controlled, which controls the speed of rotation of the circulation water pump 170. The flow into the feed line 100 takes place with a ram pressure at an invariable pressure, in which case the feed pressure of the dilution water flow F^Q remains invariable. This secures an invariable back pressure for the component stocks M j when they are fed into the feed line 100. By means of the first pump 110 in the main line, an invariable volume is pumped constantly to stock cleaning 115, 120 and to the dilution of the second stage. In the dilution in the first stage, the stock is diluted to a consistency of about 1.5 % in order that the stock could be fed through the screen 115 and through the centrifugal cleaner 120. The dilution in the second stage is carried out at the suction side of the second feed pump 130 in the main line, to which suction side a second dilution water flow F20 of invariable pressure is passed with a ram pressure from the deaeration tank 200. The regulation of the pressure in the headbox 150 controls the speed of rotation of the second feed pump 130 in the main line. In the dilution in the second stage, the stock is diluted to a headbox consistency of about 1 % . Further, a third dilution water flow F3 Q is fed to the dilution headbox 150 from the deaeration tank 200 by means of a dilution water feed pump 180 through a screen 190. By means of this third dilution water flow F3 Q passed into the dilution headbox 150, the stock consistency is profiled in the cross direction of the machine. Fig. 3 illustrates a modification of the process arrangement shown in Fig. 2, in which modification the deaeration tank 200 is placed below the wire section 160. In such a case, the white water can be passed from the wire section 160 directly by means of ram pressure into the deaeration tank 200, in which there is an air space subjected to a vacuum above the free surface of the stock. From the deaeration tank 200, the dilution water is fed by means of the circulation water pump 170 into the first FJQ and second F20 dilution stage in the main line of the process. Further, into the dilution headbox 150, a third dilution water flow is fed by means of a dilution water feed pump 180 through a screen 190. In the first F^ Q and second F20 dilution water flow, an invariable pressure can be maintained by means of regulation of the speed of rotation of the circulation water pump 170 and/ or by means of throttles in the feed lines 100, 101. Also in this case, there is an overflow F^Q between the wire section 160 and the deaeration tank 200, from which overflow any excess white water is passed to atmospheric pressure. From the deaeration tank 200, the surface level is measured at the point A, and by means of the surface level controller LIC the flow controller FIC is controlled, which controls the valve 201 provided in the line passing from the wire section 160 to the deaeration tank 200. In this way, the surface level in the deaeration tank 200 is kept at an invariable level. Fig. 4 shows a second modification of the process arrangement shown in Fig. 2, in which modification the deaeration tank 200 has been removed completely. In such a case, the headbox 150 and the wire section 160 must be closed so that the stock does not reach contact with the surrounding air. The white water collected from the closed wire section 160 is then fed directly, by means of the circulation water pump 170, into the first F^Q and second F20 dilution stage in the main line of the process. In this embodiment, the process is closed in relation to the surrounding air. Then, it is only the overflow F^Q of the white water, the reject Fg Q from the vortex cleaning 120, and the reject Fg j from the second screen 195 that communicate with the surrounding air. Fig. 5 shows a modification of the process arrangement shown in Fig. 4. In this embodiment, application of fillers and admixtures in layers in a three-layer headbox is used. Here, the main line of the process is divided into three branches after the centrifugal cleaning device 120. In each branch, there is a feed pump 130^ ... I3O3, by whose means the stock fed is fed through the machine screen I4O1... I4O of each branch into each portion 150ι ... I5O3 in the headbox. Here, the middle portion 1502 of the three-layer headbox forms the middle layer in the web, and the first 150^ and the third I5O3 part of the headbox form the surface layers in the web. Into each branch, to the suction side of the feed pumps 130 } ... I3O3, it is possible to feed starch, fillers and retention agents in the desired proportions. Further, retention agents can be fed into each branch in the desired proportion between the machine screens 140- j ... I4O and the headbox 150^ ... I5O . In addition to dilution water, starch and fillers can also be fed into the closed mixing volume preceding the first feed pump 110. The component stocks M^ ...M3 can be, in fine paper, pulp of long fibres, pulp of short fibres and broke, and, in SC paper, mechanical pulp, chemical pulp and broke. Fig. 6 shows a second modification of the process arrangement shown in Fig. 4. In this embodiment, besides application of fillers and admixtures in layers, application of fibres in layers is also used. Here, two separate main lines are used, into which component stocks M1...M can be metered from component stock stock chests in the desired proportion. The main line placed at the bottom in the figure corresponds to the main line in Fig. 4, and by means of this first main line, stock is fed into the middle portion 1502 of the headbox which forms the middle layer in the web. The second main line is divided into two branches after the centrifugal cleaner I2O2, by means of which branches stock is fed into the first 150^ and the third part I5O3 in the headbox, which parts form the surface layers in the web. Here, the first F^Q and the second F20 dilution water flow are passed into both of the main lines. To the suction side of the feed pumps 130 j ... l3θ of the branches of the headbox, starch, fillers and retention agents can be fed in the desired proportions. Further, retention agents can be fed into each branch in the desired proportion between the machine screens 140 ^ ... I4O and the headbox I5O1... I5O3. In addition to dilution water, starch and fillers can also be fed into the closed mixing volume which precedes the first feed pumps llO j , IIO2 in the main lines. In the manufacture of fine paper, in stead of the three component stocks MJ...M3 illustrated in the figure, it is possible to use four component stocks, which are pulp of long fibres, first pulp of short fibres, second pulp of short fibres, and broke. Also, the broke may be divided into broke of long fibres and broke of short fibres, in which case five component stocks are used. Thus, component stocks can be metered in the desired proportions into the middle layer in the web and into the surface layers in the web. The solutions shown in Figs. 5 and 6 are, of course, not restricted to a three-layer headbox, but the principles described in them can also be applied to a two-layer headbox or to a headbox consisting of more than three layers. The solutions illustrated in Figs. 5 and 6 can, of course, also be employed in connection with the embodiments illustrated in Figs. 2 to 4. In the embodiments shown in Figs. 2 to 6, the rejects from the first screen 115 or screens 115 j , 1152, ^ rom ^ e mac hine screen 140 or machine screens I4O-J ... I4O3, and from the dilution water screen 190 of the headbox are passed into the second screen 195, whose accept F^ is fed into the first dilution water line 100. The reject Fg Q from the centrifugal cleaner 120 or centrifugal cleaners 120^, I2O2 and the reject Fg^ from the second screen 195 are removed from the process. In Fig. 2, the feed pipes 23 j of the component stocks M j have been passed directly to the dilution water feed pipe 100. In Figs. 3 to 6, the component stock feed pipes 23 j have been passed first into a common pipe, which common pipe has then been passed to the dilution water feed pipe 100. From the point of view of the present invention, the coupling between the component stock M j feed pipes 23 j and the first dilution water feed pipe 100 can be of any kind whatsoever, provided that the mixing together of the component stocks and the mixing of the component stocks with the dilution water can be made efficient. In Figs. 2 to 6, no bypass flow of stock or dilution water at the inlet header of the headbox 150 has been illustrated. These bypass flows are arranged here by means of short feed-back connections. Figs. 2 to 6 illustrate a situation in which a dilution headbox is employed, but the invention can also be applied in connection with a headbox of a different sort. In such a case, a second circulation water pump 180 and a related screen 190 are not needed at all. In the situation shown in Figs. 2 to 6, white water is used in the main line of the process at the suction sides of both of the main line feed pumps 110, 130 for dilution of the stock, and in the dilution headbox 150 for profiling of the basis weight. In addition to this, white water can be used in earlier stages of the process for dilution of stocks. The main line screen 115 or screens 115 j , 1152 anc ^ ^e centrifugal cleaner 120 or centrifugal cleaners 120^, 1202 shown in Figs. 2 to 6 can comprise one or several stages. The first feed pump 110 or feed pumps 110^, HO2, the screen 115 or screens 115 j , 1152, ^ e mac hine screen 140 or machine screens 140^, 1402, and the centrifugal cleaner 120 or centrifugal cleaners 120^, 1202 shown in the main line in Figs. 2 to 6 can be omitted completely in a situation in which the component stocks M j have already been cleaned to a sufficiently high level of purity before the stock chests 20 j . In such a case, in the main line of the process, just the feed pump 130 or feed pumps 130 j ... l30 is/are needed. In the following, the patent claims will be given, and different details of the invention can show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.

Claims: Patent claims 1. A process arrangement for the short circulation in a paper or board machine, which arrangement comprises stock chests (20 j ) for component stocks (M j ), metering pumps (21 j ) for component stocks, cleaning devices (190, 195), pumps (130, 170, 180), a headbox (150) and a wire section (160) as well as a system of pipes connecting said apparatuses, together with regulation devices, characterized in that, after the metering pumps (21-), the component stock flows (M j ) have been passed into a closed mixing volume, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (F^Q), from which said mixing volume the stock has been passed in a closed space, by means of the feed pump (130) of the main line of the process, to the inlet header in the headbox (150). 2. A process arrangement as claimed in claim 1, characterized in that, after the metering pumps (21 j ), the component stock flows (M j ) have been passed into a closed mixing volume, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (FJQ), after which said mixing volume the main line of the process has been divided into at least two branches, in which the stock has been passed in a closed space, by means of the feed pumps (130^ ... I3O3) of the branches, to the inlet header of a multi-layer headbox (150 j ... I5O3). 3. A process arrangement for the short circulation in a paper or board machine, which arrangement comprises stock chests (20 j ) for component stocks (M j ), metering pumps (21 j ) for component stocks, cleaning devices (115, 120, 140, 190, 195), pumps (110, 130, 170, 180), a headbox (150) and a wire section (160) as well as a system of pipes connecting said apparatuses, together with regulation devices, characterized in that, after the metering pumps (21 j ), the component stock flows (M j ) have been passed into a closed mixing volume, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (FI Q) > from which said mixing volume the stock has been passed in a closed space, by means of the first feed pump (110) of the main line of the process, through a screen device (115) and a centrifugal cleaning device (120), to the suction side of the second feed pump (130) of the main line, at which suction side a second dilution water flow (F20) has been passed to the stock flow, and from which suction side the second feed pump (130) of the main line feeds the stock through a machine screen (140) to the inlet header of the headbox (150). 4. A process arrangement as claimed in claim 3, characterized in that, after the metering pumps (21 j ), the component stock flows (M j ) have been passed into a closed mixing volume, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (F^Q) , from which said mixing volume the stock has been passed in a closed space, by means of the first feed pump (110) of the main line of the process, through a screen device (115) and a centrifugal cleaning device (120), to the suction side of the second feed pump (130) of the main line, at which suction side a second dilution water flow (F20) has been passed to the stock flow, after which the main line of the process has been divided into at least two branches, in which the stock has been passed in a closed space, by means of the feed pumps (130^... I3O3) of the branches, through machine screens (140^,... I4O ) to the inlet header of a multi-layer headbox (150-^... I5O3). 5. A process arrangement for the short circulation in a paper or board machine, which arrangement comprises stock chests (20 j ) for component stocks (M j ), metering pumps (21 j , 21 ' j ) for component stocks, cleaning devices (115^, 1152, 120J, 120 2 , 140J...1403, 190, 195), pumps (110^ 110 2 , 130 1 ...130 3 , 170, 180), a headbox (150-^... I5O3) and a wire section (160) as well as a system of pipes connecting said apparatuses, together with regulation devices, characterized in that, after the metering pumps (21 j , 21' j ), the component stock flows (M j ) have been passed, in the desired proportion, into at least two closed mixing volumes, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (F^Q), from which said mixing volumes the stock has been passed in a closed space, by means of the feed pumps (130^...130 3 ) of the main lines of the process, into each inlet header of the multi-layer headbox (I5OJ ... I5O3). 6. A process arrangement as claimed in claim 5, characterized in that, after the metering pumps (21 j , 21' j ), the component stock flows (M j ) have been passed, in the desired proportion, into two closed mixing volumes, in which the component stocks (M j ) are mixed and diluted with a first dilution water flow (FJQ), from which said mixing volumes the stock has been passed in a closed space, by means of the first feed pumps (HO j , IIO2) of the main lines of the process through screen devices (115^, 1152) an< i through centrifugal cleaning devices (120 j , I2O2) to the suction side of the second feed pumps (130^ ... I3O3) of the main lines, at which suction side a second dilution water flow (F Q) has been passed to the stock flow, and from which suction side the second feed pumps (130^ ... I3O ) of the main lines feed stock through machine screens (140ι ... HO3) into the inlet headers of the multi-layer headbox (I5OJ... I5O3). 7. A process arrangement as claimed in claim 6, characterized in that, after said centrifugal cleaning devices (I2O1... I2O ) one of the main lines of the process is divided into at least two branches, by means of which branches stock is fed into two layers in the multi-layer headbox (150^ ... I5O3). 8. A process arrangement as claimed in claims 1...2 or 5, characterized in that the white water (F^Q) recovered from the wire section (160) has been passed into a deaeration tank (200) placed in the top portion of the process space, in which tank air is removed from the white water by means of a vacuum, and from which tank the white water free from air has been passed by means of a ram pressure, as a first dilution water flow (F^Q), into the mixing volume or volumes at the suction side of the feed pump (130) or feed pumps (130^... I3O3) in the main line. 9. A process arrangement as claimed in claims 3...4 or 6...7, characterized in that the white water (F^ ) recovered from the wire section (160) has been passed into a deaeration tank (200) placed in the top portion of the process space, in which tank air is removed from the white water by means of a vacuum, and from which tank the white water free from air has been passed by means of a ram pressure, as a first dilution water flow (F^Q), into the mixing volume or mixing volumes at the suction side of the first feed pump (110) or feed pumps (110 j , I IO2) in the main line and, as a second dilution water flow (F20), to the suction side of the second feed pump (130) or feed pumps (130 j ... I3O3) in the main line. 10. A process arrangement as claimed in claims 1...2 or 5, characterized in that the white water (Fen) recovered from the wire section (160) has been passed by means of a ram pressure into a deaeration tank (200) placed in the bottom portion of the process space, in which tank air is removed from the white water by means of a vacuum, and from which tank the white water free from air has been passed by means of a circulation water pump (170), as a first dilution water flow (F^Q), into the mixing volume or volumes at the suction side of the feed pump (130) or feed pumps (130^ ... I3O3) in the main line. 11. A process arrangement as claimed in claims 3...4 or 6...7, characterized in that the white water (F^Q) recovered from the wire section (160) has been passed by means of a ram pressure into a deaeration tank (200) placed in the bottom portion of the process space, in which tank air is removed from the white water by means of a vacuum, and from which tank the white water free from air has been passed by means of a circulation water pump (170), as a first dilution water flow (FJQ), into the mixing volume or volumes at the suction side of the first feed pump (110) or feed pumps (llO j , HO2) in the main line and, as a second dilution water flow (F20), to the suction side of the second feed pump (130) or feed pumps (I3OJ... I3O3) in the main line. 12. A process arrangement as claimed in claims 1...2 or 5, characterized in that the white water (F^Q) recovered from the wire section (160) has been passed directly to the suction side of the circulation water pump (170), from which suction side the circulation water pump (170) feeds the white water, as a first dilution water flow (F^Q), into the mixing volume or volumes at the suction side of the feed pump (130) or feed pumps (130^... I3O3) in the main line. 13. A process arrangement as claimed in claims 3...4 or 6...7, characterized in that the white water (F^ Q ) recovered from the wire section (160) has been passed directly to the suction side of the circulation water pump (170), from which suction side the circulation water pump (170) feeds the white water, as a first dilution water flow (F^ Q ), into the mixing volume or volumes at the suction side of the first feed pump (110) or pumps (llO j , IIO2) in the main line and, as a second dilution water flow (F20), t0 t ^ ιe suct i° n side of the second feed pump (130) or pumps (130 ...13θ3) in the main line. 14. A process arrangement as claimed in any of the preceding claims, characterized in that white water recovered from the wire section (160) is also fed by means of a dilution water pump (180) of the headbox through a dilution water screen (190) of the headbox as a flow (F 0) into the dilution headbox (150).

Inventors: HAUTALA JOUKO, KINNUNEN JUHA, PEKKARINEN TIMO, SILANTERAE MARI, SUONPERAE ANTTI, VERKASALO LAURI, HAUTALA, JOUKO, KINNUNEN, JUHA, PEKKARINEN, TIMO, SILANTERAE, MARI, SUONPERAE, ANTTI, VERKASALO, LAURI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1999
Main classification: D21F1
IPC codes: D21F1