WO1987006633A1

WO1987006633A1 - Method for regulating the cross-direction profile of a paper web and equipment for the application of the method

Info

Publication number: WO1987006633A1
Application number: PCT/FI1987/000051
Authority: WO
Inventors: Viljo Klemetti
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-04-30
Filing date: 1987-04-23
Publication date: 1987-11-05
Anticipated expiration: 1988-10-30
Also published as: FI861811A7; FI861811A0

Abstract

Method for regulating the cross-direction pulp profile of a paper web produced in a paper machine, which paper web is formed from hydrous pulp suspension (3) or similar material fed from the headbox (1) on the wire (2), water being removed from the paper web in the wire section after the headbox. The invention is furthermore concerned with equipment for the application of the method. In present paper machines, the cross-direction pulp profile of the paper web cannot be regulated after the headbox. With the method presented in the invention, the problem is solved by that the cross-direction profile of the paper web is regulated in the wire section after the headbox (1) by regulating the removal of water from the paper web in the cross-direction of the paper web in such a way that at the same time the retention, i.e. the amount of solid material remaining in the web, is regulated.

Description

METHOD FOR REGULATING THE CROSS-DIRECTION PROFILE OF A PAPER WEB AND EQUIPMENT FOR THE APPLICATION OF THE METHOD

This invention is concerned with a method described in the introductory part of Claim 1 and with equipment for the application of that method.

Already known are methods for regulating the water removal in the direction of motion of the suspension. In the production of paper, this is usually done by changing the quality, quantity, placing and setting of the water removal equipment of the wire section. With these methods the qualities of the paper web can be considerably regulated in its direction of motion.

Modern, fast paper machines use for the water removal in the wire sections so-called foils, which in their present form are ceramic or plastic laths that reach across the machine and against the leading edges of which the wire leans. The lath forms a small angle (0-4°) with the wire. When the wire goes over the leading edge of the lath, a vacuum directly proportional to the square of the speed of the Mire is formed on its trailing side. The highest value of the vacuum is about half of that of a comparable table roll.

There is a tendency to make the water removal elements - forming board, table rolls, single-foil boxes, multifoil boxes, vacufoil boxes and wet suction boxes - as straight and rigid as possible so that they remove water evenly throughout the width of the machine. The water removal of the foils depends on the direction of their top surfaces and on the length of the surfaces in the direction of motion of the wire. Even very small deviations from the intended direction cause differences in the amount of water removal. Building long foil boxes and installing the foils for them does not always succeed with sufficient accuracy, as for instance the heavy torsional loading caused by water jets can bend the boxes and distort them in such a way that the deformation is greater in the middle than at the ends. Consequently, the water removal is uneven, and so is the retention.

The theory of web formation on which this invention is based is explained in the following.

In the web formation unit of the paper machine, dilute pulp is drained on the wire. With 'drainage' is usually meant the separation of a suspended solid phase from a flowing liquid or gas phase by mechanical means. When employing the drainage process, generally a compromise between the efficiency of drainage (retention) and the capacity of the filter is necessary. When draining pulp in a paper machine, the requirements regarding the structure of the fiber layer created by means of the drainage are in this respect a restricting factor that limits for instance the means of process engineering that can be used for water separation. The aim with the drainage in the wire section of a paper machine is as good a retention as possible and a fast drainage for the pulp in such a way that the evenness of the structure of the sheet produced meets the requirements of the paper grade in question. In regard to its properties concerning drainage, pulp has turned out to be a complicated material. When the basic mechanisms of the drainage of pulp are known, it is possible to explain qualitatively the formation of the structure of a sheet and to evaluate in individual cases the effects of various factors on the rate of drainage and on the structure of the sheet.

Regarding the smallest fiber material and the filler particles of the pulp, the efficiency of drainage, i.e. retention, and the related fine material distribution in the thickness direction of the fiber web are important questions both for the process engineering point of view and for the structure of the sheet. Both the retention and the fine material distribution depend on how much the smallest particle material can move inside the fiber mat that is being formed during the drainage.

The movement of the small particles is essentially influenced by the actual process of drainage, which determines the density of the fiber mat formed from a certain fiber suspension and the variations of that density. Especially in the early stages of the drainage, the properties of the wire are also of great importance in this respect.

The distribution of fiber material in the direction of the plane of the sheet essentially depends on how homogeneous the pulp is when it comes to the wire, but the actual process of drainage too influences the evenness of the distribution of material. The smallest headbox dilution possible to use with each furnish in the machine in question is nearly always determined by the requirement of evenness in the distribution of material. The inhomogenousness of the pulp coming from the headbox sharply rises with the increasing of consistency. Too great a dilution in the headbox may also have an adverse effect on the evenness of the distribution of material. Likewise, the water removal has to happen in such a way that the distribution of material in the sheet will be as even as possible. The distribution of material resulting in the drainage stage is a very critical factor for the process, determining what kind of water removal conditions are possible in the web formation. About the effects of the drainage mechanism on the evenness of the sheet very little is known so far, which is explained by the fact that there are very many variables, as the properties of pulp vary a lot from paper grade to paper grade. To optimize the process, it would be necessary to be able to carry out test runs separately for each pulp in every paper machine. With present constructions, carrying out tests in the wire section, where the conditions of drainage are determined, is difficult because changing the water removal elements, which influence the actual process of water removal, into other kind of gear for carrying out the tests is so difficult and time-consuming that it would not be practicable.

The drainage base and the drainage resistance of the continuously growing drained material layer determine the rate of drainage.

The flow of liquid through a (not contractible) porous material layer depends on four factors:

- pressure difference which causes the flow

- nature of the flowing liquid

- chemical nature of the surface of the pores

- structure of the material layers

The actual process of drainage is influenced by the pressure difference caused on the web by means of various water removal elements of the web formation section. For the flow of liquid, an important property of the porous material is also its homogeneity. Internal variations in the porosity of the material are difficult to measure, yet they influence the flow essentially. Inhomogenousness of the fiber mat, caused for instance by flocculation of fibers, weakens the drainage resistance of the pulp.

Due to frictional forces, the porous material layer causes in the liquid flowing through it a pressure loss which is in direct proportion to the amount of flow if the flow is laminar. The friction that retards the liquid naturally affects the porous material with an equal force which tries to contract it in the direction of the flow. The pulp mat drained from the fiber suspension is contractible, so pressure causes it to contract, and its porosity decreases with the growing of the contracting pressure. With the porosity decreasing, the drainage resistance increases, and because the contracting force is different at different distances from the drained mat, the drainage resistance is also dependent on the distance from the pulp mat.

Under a certain contracting pressure, the concentration of a fiber layer is the greater, the more contractions and recoveries there have earlier occurred in it, and likewise the smaller, the shorter time the pressure lasts.

Consequently, with liquid flowing through a fiber layer, the following phenomena can be stated in reference to what was said above:

- average specific drainage resistance is a function of the pressure difference,

- average specific drainage resistance is a function of the flow,

- average specific drainage resistance under a certain pressure difference depends on the earlier contractions and recoveries the layer has experienced.

- it can be approximately assumed that if the flow situation is stable, a certain pressure difference is unambiguously equalled by a certain fiber concentration, which in turn together with the specific weight and specific volume of the fiber material determines the drainage resistance.

During the drainage of a particle suspension, particles in the suspension at the beginning of the drainage settle on the drainage base, and later, as the drainage continues, they settle on the material layer so far formed. As the drained material layer becomes thicker, its drainage resistance increases and the rate of drainage decreases.

If the particles have a broad distribution of size, smaller particles penetrate with the flowing liquid into an already drained material layer and adhere to it partly mechanically andgenerally partly also through forces of chemical nature. The likelihood that a particle should penetrate into an already drained particle layer depends besides the size and form of the particle also on the course of the actual process of drainage, first of all on the flow rate, and if the particle layer is contractible, also on the amount of the pressure difference and on how the pressure difference relates to time. When observing a certain spot inside a particle layer, it is seen that the amount of particles coming to it per unit of time decreases as a function of time, the reason being that while getting thicker, the layer that lies above the spot under observation retains an ever-increasing proportion of the finer particle material coming to it. But on the other hand, as the drainage goes on, the spot under observation retains an ever-increasing proportion of the small particles coming to it because its density increases as new particles adhere to it. If the material layer under formation is contractible, the specific drainage resistance also depends on the contracting pressure prevailing in different parts of the material layer. Pulp often contains a lot of fine-grained material, fine material and filler particles, which penetrate into a layer of fibers and also through it.

The drainage resistance especially of pulp depends essentially on the duration of pressure. With a fast drainage, the drainage resistance is smaller than with a slower one.

In the web formation in a paper machine, a mere precipitation is out of question because it would require so great a turbulence that a fiber mat could not be formed on the drainage base, as the turbulence would continuously break the fiber mat. This would lead to a very poor wire retention. The consistency of pulp under drainage is generally so great in the production of paper that fibers can form a solid net. Three different layers can be discerned in pulp. The lowest is a fiber mat of drained fibers, where the fibers are already fixed in relation to one another. Above the fiber mat is a transitory layer, where the fibers are in closer contact to one another than in free pulp. The highest is a free fiber layer approximately in the same consistency as when it came to the wire.

Fine material and filler particles move with drained water relatively freely in the fiber mat already formed on the wire. Throughout the water removal stage, fine material particles keep coming from above with water into each layer of the fiber mat. Part of these particles is retained in the layer observed, the rest go on with water into lower layers of the fiber mat and either remain in them or ultimately leave the wire with water. For a particle to remain in one of the layers, it has to collide with a fiber and in the collision cling to it. The likelihood of collision increases, the larger the particles are and the greater the density and thickness of the fiber layer is. Not all the particles, however, that collide with fibers cling to them. The clinging is the more likely, the lower the speed of the water flowing through the layer is. The force of the flow going through a fiber layer causes particles already adhered to it to loosen from it. The likelihood of particles loosening is the greater, the faster the water flows through the layer.

The above-mentioned factors influencing the adhering and loosening again of particles determine in all web formation methods both the z-distribution of the fine material content and the amount of wire retention. The reciprocal importance of adhering and loosening depends on the conditions of drainage. In discontinuous drainage, such as in a Fourdrinier machine, in which variations of the speed of water and of the density of the fiber mat during the drainage contribute to the loosening of particles from the fiber mat, the wire retention is weaker in comparison with the retention in continuous drainage. So the fine material content, the retention, and the form of the fine material content's z-distribution in the paper are primarily determined by how discontinuous or continuous the water removal is in the drainage stage.

With foils it has been possible to regulate the drainage and thus also the web formation in the direction of motion of the paper, but with very insufficient means of influence, as the use of foils has generally been based on very limited tests about their effect on each web formation at hand. Only a perfect controllability of foils makes it possible to utilize their possibilities effectively.

When using foils, the drained layer on the wire side can remain whole thereby preventing fibers and additives from freely escaping from the web with water. The retention is thereby improved. The amount of retention can be influenced by shifting water removal to a later stage, in which case the drained layer remains and protects against loss of fibers and additives, i.e. the retention increases, or by intensifying the water removal, in which case the retention decreases in this place because the denser layer protecting the surface of the web breaks as a result of the forceful water removal. The first-mentioned operation is according to this invention carried out by reducing the negative pressure caused by the foil in some place, as a consequence of which the water removal gets weaker and the suspension has more time to get drained to that part of the suspension layer that lies on the water removal side. In this stage of the water removal, more fibers, fine materials and additives have remained in said place of the paper web than in the adjacent places. The water content too is greater, but in whichever stage of the process the water removal is later done, the ratio of solid material amounts can no more be changed with it, as the water removal then occurs through a drained layer, which to a much greater degree prevents the leaving of fibers and additives from the web than is the case in connection with an earlier water removal.

The retention can also be reduced by causing on the underside of the wire a pressure impact that breaks the fiber mat, as a consequence of which more fine materials and additives can leave the web through it. One way to create the pressure impulse is by turning the foil slightly higher, for instance 1°, than the plane of movement of the web. The purpose of the headbox is to reach a good distribution of material for the paper. Yet in the pulp suspension flow coming to the headbox from the short circulation system there are flaws that cannot be completely corrected with headbox adjustments.

There is variation in where the slice jet hits the forming board, there are differences in the installation of foils, there is bend on foil boxes, the wire does not get dirty evenly, the wire stretches differently in different places, and other such things cause that the water removal and the retention are different in different places.

The above-mentioned faults lead to many faults in the paper web, but there are no means actually to correct them. In other words, the solid material amount that has remained in the web in the wire section cannot be changed in any subsequent stage.

One essential aspect of the present formers with curved gaps in regard to process control is that they can only to a very limited degree, if at all, influence how the water removal pressure develops in the area of drainage; consequently, there are no means of control available when trying to optimize the web formation. So it is especially important to utilize this invention in connection with these formers, for it would otherwise be necessary to set to the headbox and the former ever greater constructional requirements, with which the web formation and the profile regulation still could not be influenced. By using an adjustable forming board in connection with gap formers, it is possible to influence the web formation essentially. Likewise, by mounting adjustable foils on a curved shoe the web formation can be guided in desired direction.

The height and duration of the vacuum impulse, and thus the water removing capacity of the foil at a certain speed of the machine, depend on the size of the angle between the foil and the wire, on the width of the foil, and on the form of the surface of the foil. The water removing capacity increases, the greater the size of the foil angle and the greater the width of the foil are.

Indeed, this invention covers a method for removing water in the wire section of a paper machine in the cross-direction, the water removal being carried out in different places at different times and different intensities, so that the retention and thus also the basis weight will be different.

The method based on the invention is characterized by what is presented in the novelty part of Claim 1. For the technical realization of the method, equipment has been developed with which the size of the clearance angle of the foil, or the width of the foil, or the suction between foils is changed while the paper machine is running, either manually or automatically on the basis of results measured from the paper web.

The possibility to regulate the cross-direction profile of the basis weight in the wire section of a paper machine brings many considerable advantages to the production of paper.

Below, the invention is explained in detail by means of advantageous applicational examples, with reference to the following drawings where

Fig. 1 shows a side-view of certain equipment for forming a paper web.

Fig. 2 and 3 show from different directions an application of the invention, partly in section.

Fig. 4 and 5 show from different directions another application of the invention, partly in section.

Fig. 6 and 7 show from different directions a third application of the invention, partly in section.

Fig. 8 and 9 show from different directions yet another application of the invention, partly in section.

Fig. 10 shows yet another application of the invention, partly in section.

Figure 1 shows a paper machine's paper web formation section where hydrous pulp suspension 3 coming from the headbox 1 is fed on the moving wire 2. Number 4 refers to forming board, number 5 to single-foil boxes, and number 6 to multifoil boxes. In the paper machine section shown in Figure 1, water is, in a manner explained further on, removed from the pulp suspension 3 that forms the paper web.

Figures 2 and 3 show a way of adjusting the foil 7. The foil is mounted on a base lath 8, which in turn is mounted on a single-foil box 5. The trailing side of the foil 7 forms an angle with the wire 2. The amount of water leaving the pulp suspension and thus the final quality of the paper depend on the size of the angle. The larger the angle between the wire and the foil is, the greater is the negative pressure that is created in the space between them and the more water is removed. In the cross-direction of the wire 2, several power devices 9 are attached to the base lath 8 of the foil 7 at regular distances. The distance is most advantageously of the order of a few hundred millimeters. The base lath 8 is dimensioned suitably flexible, so that the power device 9 can bend the base lath and the foil, by which means the angle between the foil and the wire 2 can be changed with the power device 9 in comparison with the clearance angles in areas governed by adjacent power devices. Foils are nowadays made of plastic, ceramics, or various other materials provided with hard, durable coatings. Ceramic laths are made from short (a few dozen millimeters in length), straight-sided pieces that are attached to a relatively rigid frame that can be bent to some degree. A foil of plastic can withstand even very large bendings, and so can one of steel.

As power devices can be used for example single-acting or double-acting hydraulic, pneumatic, electrical or mechanical devices. In the applicational example shown in Figures 2 and 3, the power devices are hydraulic. In them, a piston 10 is moved in desired direction by means of a hydraulic medium. To the piston is connected a rod 11, which in turn is attached to the base lath 8. The power device 9 is fixed in place with a supporting structure 12.

In Figures 4 and 5 is shown an application in which the foil is adjusted by means of pressure air bellows. The foil 7 is mounted with a T-shaped slot joint 13, 14 on a base lath 8 which is dimensioned to have a suitable torsional rigidity, so that it can be bent at the trailing edge by means of supporting bars and pressure air bellows 15, yet keeping the front end essentially straight. At its front edge 16, the base lath 8 is fixed on the body 5, and at its rear edge, it is pushed upwards by means of a lower support 17 dimensioned as a spring, by tightening a nut 18 against the lower support 17. The foil is adjusted to its highest position by means of an adjustor nut 20. The clearance angle can be increased thereby that the lower support 17 and the support bar 19 attached to it are pushed downwards by increasing the air pressure of the pressure air bellows 15. With this construction, the foil is well supported in every position, as it is supported on one hand by the bending stress of the lower support 17 and on the other hand by the bellows 15.

Figures 6 and 7 show a construction similar to that in Figures 4 and 5, the only difference being that the foil 7 is strained to the greatest angle with respect to the wire by means of the lower support 17, and that the angle is decreased by adding pressurized air to the bellows 15.

In Figures 8 and 9 is shown an application in which the negative pressure between the foil 7 and the wire 2 is regulated and thus also the amount of water removal. The regulation is carried out by reducing the negative pressure by blowing air into the space in question, or by increasing the negative pressure by sucking even more air away from said space. To realize this, through the foil have been formed air channels 21 through which the changes in the negative pressure can be effected.

In Figure 10 is shown a very effective way of regulating the water removal on the forming board 4 by adjusting a broad front foil 22 and/or other foils 23. When the hydrous pulp suspension jet 3 from the headbox 1 hits the forming board 4, there is much water removal and with it also much loss of solid material. If the trailing edge of the broad foil is turned downwards, the wire gets separated from the foil earlier, as a consequence of which more water and more pulp can leave than in the straight area which is supported by the foil. The turning of the foil also causes a negative pressure between the foil and the wire, which too increases the water removal. The same phenomenon is also repeated in the area of the other foils 23. In Figure 10, the adjustment of the foil is carried out by means of a spiral spring 24 and a pressure air bellows 25 in a manner obvious from the figure.

It is clear to a professional that the invention is not confined to the applicational examples presented above, but can vary within the framework of the patent claims presented below. The adjustment of the foil can be controlled either manually, or automatically on the basis of impulses received from the process of the paper machine, and the adjustment can be a single operation or happen continuously on the basis of change impulses. Likewise, a particular water removing capacity of the foil that has been found the most suitable can be kept by measuring the negative pressure between the foil and the wire and adjusting the angle between the foil and the wire so wide or narrow that the desired negative pressure is maintained or by adjusting the amount of the pressurized air so great or small that the right negative pressure is reached.

Claims

1. Method for regulating the cross-direction pulp profile of a paper web produced in a paper machine, which paper web is formed from hydrous pulp suspension (3) or similar material fed from the headbox (1) on the wire (2), water being removed from the paper web in the wire section after the headbox, c h a r a c t e r i z e d by that the cross-direction profile of the paper web is regulated in the wire section after the headbox

(1) by regulating the removal of water from the paper web in the cross-direction of the paper web.

2. Equipment for the application of the method presented in Claim 1 comprising a headbox (1), a wire (2), and water removal devices arranged after the headbox under the wire, such as forming board (4), single-foil box (5), multifoil box (6) or a comparable device, which for removing water from the pulp suspension (3) has at least one foil (7, 22, 23) crosswise in relation to the wire

(2) and in contact with it, the water removing effect of said foil depending at least on the size of the angle between the foil (7) and the wire (2) and on the length of the foil in the direction of movement of the wire, since a vacuum is created between the wire and the top surface of the foil, c h a r a c t e r i z e d by that in the cross-direction of the wire, directly or indirectly connected with the foil (7) or the space between the foil and the wire (2), there are one or more separately-driven power devices (9, 15, 25) which either regulate the angle (clearance angle) between the foil (7) and the wire (2) or otherwise influence the pressure conditions between the foil and the wire in comparison with areas governed by adjacent power devices.

3. Equipment conforming to Claim 2, c h a r a c t e r i z e d by that the power devices (9, 15, 25) are arranged in the cross-direction of the wire at regular distances (2), the distance being most advantageously about 200 - 1000 mm.

4. Equipment conforming to Claims 2 and 3, c h a r a c t e r i z e d by that the power device (9, 15, 25) is a single-acting or double-acting hydraulic, pneumatic, electrical or mechanical device.

5. Equipment conforming to any of the Claims 2-4, c h a r a c t e r i z e d by that the power devices (9, 15, 25) affect the foil's (7) trailing edge, i.e. in the direction of the wire (2) the rear edge, which can be bent most suitably

+1°... -4°, while the leading edge remains unmoved.

6. Equipment conforming to any of the Claims 2-4, c h a r a c t e r i z e d by that the power units (9, 15, 25) affect the leading edge of the foil (7), while the trailing edge remains essentially unmoved.

7. Equipment conforming to any of the Claims 2-4, c h a r a c t e r i z e d by that in the foil (7), there are holes (21) through which the pressure between the foil (7) and the wire (2) can be influenced by conducting there pressurized air or negative pressure.