GB1583968A - Papermaking machines - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO PAPERMAKING MACHINES (71) We, J.M. VOITH GMBH, a German Company, of D-7920 Heidenheim, St. Pölt- ener Strasse 43, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a papermaking machine of the kind having an inner screen and an outer screen arranged to pass together over a portion of the periphery of a forming roll, means for collecting water centrifuged from the screens arranged adjacent the roll; and a preliminary dewatering zone upstream of the dewatering roll, this zone being formed by a substantially horizontal section of the inner screen.

In such a papermaking machine, water is extracted from a web inwardly of the inner screen on the horizontal screen section of the inner screen upstream of the dewatering roll and outwardly of the outer screen at that part of the roll periphery carrying the web. In this way papers are obtained that have approximately the same paper properties on both sides. The web formed on the inner screen already has most of the water extracted therefrom on the horizontal screen section of the inner screen, this extraction normally being assisted by suction boxes placed beneath the horizontal screen section. For the subsequent extraction of water in the opposite direction by the centrifugal forces to which the web is subject as it passes round the roll, it has hitherto been considered sufficient merely to change the direction of movement of the web.It was only thought possible to attain a more substantial water extraction through measures of positive or negative pressurisation, for instance through suction boxes arranged beneath the horizontal screen section as disclosed in the German Published Specification No. 2,411,739, or through the provision of scrapers arranged downstream of the roll as disclosed in German Published Specification No. 2,226,029. This view applied to both high-speed and slow-running paper making machines: In the case of high-speed machines a large amount of water will be thrown from the web even though it may be turned only slightly, as a result of the high centrifugal forces which occur, while in the case of slow-running machines less water will be expelled, because of the lower centrifugal forces.Consequently, those skilled in this art were of the opinion that, in both instances, the provision of longer suction boxes or a greater vacuum in the horizontal screen section of the inner screen was the only efficient manner of intensifying this water extraction.

The dewatering roll of the papermaking machine disclosed in German Published Specification No. 2,411,739 has a perforated shell. The two screens leave the roll at a location adjacent the top thereof, i.e. within the upper quadrants of the roll. Subsequent to leaving the roll water is present on both the inner and the outer screen, this water being removed by scrapers provided therefor. A large amount of water will be present on the outer screen as it leaves the dewatering roll, because water is flung from the two screens outwardly as they pass over the roll, as a result of the tension forces acting on the screens and of the centrifugal forces to which they are subjected. Although part of this water will be flung off, the remainder of the water is collected on the outer screen. Moreover, water is flung back onto the inner screen from the perforations in the roller shell.This comparatively large quantity of residual water present on the screens downstream of the dewatering roll results in the undesirable remoistening of the paper web. Furthermore, the scrapers inflict wear on the screens and may damge the screens if they, or the screens, have the slightest irregularity in shape.

The dewatering roll of the paper making machine disclosed in the German Published Specification No. 2,226,029 also has a perforated shell. Asaresult thereof, a large part of the water pressed from the paper web is held in the inner screen and is removed therefrom by scrapers arranged downstream of the roll.

An object of the present invention is to provide a papermaking machine in which water is extracted from a web more satisfactorily without the incorporation of costly devices and without enlarging the bulk of the machine at both high andl ow web speeds. A further object is to provide a dewatering roll which is cheap to manufacture and by means of which the aforementioned problems associated with the use of scrapers are avoided.

According to the present invention there is provided a papermaking device having an inner screen and an outer screen which are led together over a part of the periphery of a forming roll having an imperforate shell, said screens being arranged to run onto the roll in the region of its apex and to leave the roll at locations in the lower half thereof; a collecting device disposed outwardly of the outer screen for collection of water centrifuged from the screens; a preliminary water extraction zone upstream of the forming roll, which zone is formed by a substantially horizontal section of the inner screen alone, and a guide having a convex surface arranged upstream of the roll for leading the inner screen onto the roll, the radius of curvature of said surface being greater than that of the roll shell.

A machine having a forming roll with an imperforate shell is known from the U.S.

patent specification No. 3,876,498. Quite different conditions prevail in this machine, however. The fibre suspension is enclosed in a gap defined by two screens as they run onto the forming roll. In this instance, the inner screen runs onto the forming roll before the fibrous suspension is charged to the screen, and the enveloped part of the forming cylinder constitutes the only means whereby water is extracted from the web being formed. Moreover this type of papermaking machine is only used for comparatively high web speeds.

The use of an imperforate shell in the machine of the U.S. patent is merely to improve the separation of the two screens. The advantages attained with the present invention are neither sought for nor attained with the machine according to the U.S. patent. The point at which respective screens leave the dewatering roll is thought to be of importance. It might be thought that centrifugal forces are effective to throw water outwardly, of the screen. Instead, a vacuum is built up in the wedgeshaped gap occurring downstream of the location at which the inner screen leaves the shell of the roll which vacuums the water in the undesired direction, namely towards the roller shell.In order to overcome this, the locations at which respective screens leave the dewatering roll are in the lower half of the roll, so that the resdiual water (which at said locations is substantially to be found in the zone of the outer screen, as a result of the action of centrifugal force) is flung downwards .

In this Way re-moistening of the web is avoided.

Conveniently the outer screen leaves the dewatering roll prior to the inner screen.

Through this a particularly simple machine construction is attained. If both screens leave the roll together and are separated from one another on a screen separating surface by means of a convex surface (a guide or another roller) disposed downstream of the roll, then in a longer stretch results over which any residual water that may still be present can drip off the outer screen.

The papermaking machine in accordance with the invention has a guide with a convex curved surface disposed upstream of the roll, over which surface the inner screen is guided onto the roll, the radius of curvature of the guide surface being greater than the radius of the shell of the roll. The guide provides a gentler pressure transition for the water extraction pressure alternating from inwards to outwards at this pont, through which the reduction of fine material on the outside of the paper is influenced. The content of fine material on the outside of the paper affects its appearance and adhesive capacity. This latter is, for instance, of significance in the production of carboard, in which several layers have to be couched together.

The simplest form of embodiment consists in the sliding surface of the guide disposed before the roller being a surface impenetrable by water.

In one particular form of the invention an influence upon the water extraction is attained through the outer screen not being led first onto the roller, but initially over the sliding surface of the guide on the inner screen or on the paper web lying thereon. For a given radius of the sliding surface and the roller, the screen tension of the outer screen substantially determines the extent to which the water is extracted.

The screen water removal device beneath the substantially horizontal screen section of inner screen preceding the roller preferably contains suction boxes, and/or water scraper bars (foils).

One particular application of the invention is in machines for the production of multilayer papers or carboard, with which a plurality of papermaking units are disposed above a common belt (screen or felt) receiving the individual webs one after another, one at least of which is formed in the manner in accordance with the invention.

Exemplary embodiments of the invention will now be described with reference to the accompanying schematic drawings, in which: Figure 1 is a side view of one embodiment of a papermaking machine according to the invention; Figures 2 and 3 are schematic side views illustrating modified separation and transfer arrangements; and Figure 4 is a side view of a machine for producing multilayer paper or carboard with two papermaking units according to the invention In Figure 1 there is shown a headbox 1 arranged adjacent an upper guide-roller 2 for feeding stock onto the substantially horizontal section 3 of an inner forming wire 4 to form a web thereon. Arranged beneath the horizontal section 3 are two scrapers 5 and a suction box 6 for removing water from the web.

Downstream of the suction box 6 there is a guide 7 having a curved surface and a roller 8, hereinafter referred to as the "forming roller" having an imperforate roller shell, the inner wire 4 being guided onto the roller 8 via said curved surface. Subsequent to passing round the roller 8, the inner wire 4 then passes over a suction box 9, a roll 10 and a further guide roller 11, back to the upper guide roller 2. The web lying on the inner wire 4 is passed to a web-receiving belt 12 (screen or felt) between the roll 10 and a further roller 12 located opposite thereto. An outer screen 14 is led over guide rollers 15 adjacent the guide surface 7 and passes over the curved surface of said guide onto the paper web carried by the inner screen 4 and leaves the inner screen downstream of the forming roller 8 in the vicinity of the suction box 9 as illustrated in Figure 1.The smaller the diameter of the forming roller 8, the more water that is centrifuged off through the outer screen 14 at the periphery of said roller. Consequently, the radius of curvature of the roller should be comparatively small, and should in all events be smaller than the radius of curvature of the sliding surface of the guide 7. In the zone in which water is thrown off the forming roller 8, there is disposed a water collector 16 having a sump 17 which extends to the vicinity of the screen-separation suction box 9, so that it is able to catch water dripping from the screen.

The suction box 9, by means of which the two screens are separated downstream of the roller 8, may be replaced by a guide with a curved slide surface on which the two screens 4 and 14 are mutually separated. The directions of movement of the two screens 4 and 14 and the belt 13 are indicated by related arrows.

In Figure 2 elements having counterparts in Figure 1 have been identified by the same reference numbers.

In the Figure 2 embodiment, the roll 10 has been replaced by a second guide roller 111. The transfer of the paper web is effected here from the outer screen 14 onto the belt 13 by means of a roller 18, which preferably has an imperforate shell. A sucker or a roller 19 (suction roller or a roller with an imperforate shell) serves to separate the web from the inner screen 4.

In Figure 3, those elements which have counterparts in Figure 1 have been identified by the same reference numbers. The mode of operation of the Figure 3 embodiment is similar to that of the Figure 1 embodiment.

In the Figure 3 embodiment the two screens 4 and 14 are separated from one another at the forming roller 8. As already mentioned, it is possible for a region of sub-pressure to exist at the location where the outer screen 14 leaves the roller shell, through which water under a sub-pressure is liable to be sucked back inwardly out of the outer screen 14 and into the web on the inner screen 4. In order to centrifuge all this water from the web on the inner screen 4, provision is made for the inner screen 4 and the web thereon to be taken further around the roller 8 than the outer screen 14 by a predetermined distance.The respective locations at which the outer screen 14 and the inner screen 4 leave the roller 8 are conveniently so far apart that the two sections of screen running off the roll include an angle oc of at least 5 therebetween. The paper web is transferred from the inner screen 4 to the belt 13 by means of three rollers 101 and 121 having imperforate roller shells.

Figure 4 shows how more than one papermaking machine of the kind shown in Figure 1 can be arranged over a common belt 13 (screen or felt) on which separate webs can be deposited by said machines and bonded together to form multi-layer paper or cardboard.

A web produced in the aforedescribed papermaking machine will have the water efficiently removed therefrom. The reason for this is as follows: because the roll shell is imperforate no water is able to pass out of the fibrous web through the inner screen in the direction of the roller shell within that area of the shell enveloped by the web. Because of the tension in the screen, water will flow in the opposite direction in this zone, that is outwardly, and will be thrown off the outer screen by centrifugal force. The roll is less expensive to produce, since it is not perforated. It is also possible to dispense with the use of scrapers downstream of the forming roll, thereby losing all their disadvantages.In the case of a machine according to the invention it is now possible to attain a balanced ratio between the quantity of water led away in the preliminary water-extraction zone and that removed by the dewatering roll, which is an additional advantage.

WHAT WE CLAIM IS 1. A papermaking device having an inner screen and an outer screen which are led together over a part of the periphery of a forming roll having an imperforate shell, said screens being arranged to run onto the roll in the region of its apex and to leave the roll at locations in the lower half thereof; a collecting device disposed outwardly of the outer screen for collection of water centrifuged from the screens; a preliminary water extraction zone upstream of the forming roll, which zone is formed by a substantially horizontal section of the inner screen alone, and a guide

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. two papermaking units according to the invention In Figure 1 there is shown a headbox 1 arranged adjacent an upper guide-roller 2 for feeding stock onto the substantially horizontal section 3 of an inner forming wire 4 to form a web thereon. Arranged beneath the horizontal section 3 are two scrapers 5 and a suction box 6 for removing water from the web. Downstream of the suction box 6 there is a guide 7 having a curved surface and a roller 8, hereinafter referred to as the "forming roller" having an imperforate roller shell, the inner wire 4 being guided onto the roller 8 via said curved surface. Subsequent to passing round the roller 8, the inner wire 4 then passes over a suction box 9, a roll 10 and a further guide roller 11, back to the upper guide roller 2. The web lying on the inner wire 4 is passed to a web-receiving belt 12 (screen or felt) between the roll 10 and a further roller 12 located opposite thereto. An outer screen 14 is led over guide rollers 15 adjacent the guide surface 7 and passes over the curved surface of said guide onto the paper web carried by the inner screen 4 and leaves the inner screen downstream of the forming roller 8 in the vicinity of the suction box 9 as illustrated in Figure 1.The smaller the diameter of the forming roller 8, the more water that is centrifuged off through the outer screen 14 at the periphery of said roller. Consequently, the radius of curvature of the roller should be comparatively small, and should in all events be smaller than the radius of curvature of the sliding surface of the guide 7. In the zone in which water is thrown off the forming roller 8, there is disposed a water collector 16 having a sump 17 which extends to the vicinity of the screen-separation suction box 9, so that it is able to catch water dripping from the screen. The suction box 9, by means of which the two screens are separated downstream of the roller 8, may be replaced by a guide with a curved slide surface on which the two screens 4 and 14 are mutually separated. The directions of movement of the two screens 4 and 14 and the belt 13 are indicated by related arrows. In Figure 2 elements having counterparts in Figure 1 have been identified by the same reference numbers. In the Figure 2 embodiment, the roll 10 has been replaced by a second guide roller 111. The transfer of the paper web is effected here from the outer screen 14 onto the belt 13 by means of a roller 18, which preferably has an imperforate shell. A sucker or a roller 19 (suction roller or a roller with an imperforate shell) serves to separate the web from the inner screen 4. In Figure 3, those elements which have counterparts in Figure 1 have been identified by the same reference numbers. The mode of operation of the Figure 3 embodiment is similar to that of the Figure 1 embodiment. In the Figure 3 embodiment the two screens 4 and 14 are separated from one another at the forming roller 8. As already mentioned, it is possible for a region of sub-pressure to exist at the location where the outer screen 14 leaves the roller shell, through which water under a sub-pressure is liable to be sucked back inwardly out of the outer screen 14 and into the web on the inner screen 4. In order to centrifuge all this water from the web on the inner screen 4, provision is made for the inner screen 4 and the web thereon to be taken further around the roller 8 than the outer screen 14 by a predetermined distance.The respective locations at which the outer screen 14 and the inner screen 4 leave the roller 8 are conveniently so far apart that the two sections of screen running off the roll include an angle oc of at least 5 therebetween. The paper web is transferred from the inner screen 4 to the belt 13 by means of three rollers 101 and 121 having imperforate roller shells. Figure 4 shows how more than one papermaking machine of the kind shown in Figure 1 can be arranged over a common belt 13 (screen or felt) on which separate webs can be deposited by said machines and bonded together to form multi-layer paper or cardboard. A web produced in the aforedescribed papermaking machine will have the water efficiently removed therefrom. The reason for this is as follows: because the roll shell is imperforate no water is able to pass out of the fibrous web through the inner screen in the direction of the roller shell within that area of the shell enveloped by the web. Because of the tension in the screen, water will flow in the opposite direction in this zone, that is outwardly, and will be thrown off the outer screen by centrifugal force. The roll is less expensive to produce, since it is not perforated. It is also possible to dispense with the use of scrapers downstream of the forming roll, thereby losing all their disadvantages.In the case of a machine according to the invention it is now possible to attain a balanced ratio between the quantity of water led away in the preliminary water-extraction zone and that removed by the dewatering roll, which is an additional advantage. WHAT WE CLAIM IS

1. A papermaking device having an inner screen and an outer screen which are led together over a part of the periphery of a forming roll having an imperforate shell, said screens being arranged to run onto the roll in the region of its apex and to leave the roll at locations in the lower half thereof; a collecting device disposed outwardly of the outer screen for collection of water centrifuged from the screens; a preliminary water extraction zone upstream of the forming roll, which zone is formed by a substantially horizontal section of the inner screen alone, and a guide

having a convex surface arranged upstream of the roll for leading the inner screen onto the roll, the radius of curvature of said surface being greater than that of the roll shell.

2. A machine as claimed in claim 1, wherein the two screens-viewed in the direction of travel thereof-are arranged to leave the roll at locations in the first lower quadrant thereof.

3. A machine as claimed in claim 1 or 2, wherein the inner screen is arranged to be carried further round said roll than said outer screen.

4. A machine as claimed in Claim 3, wherein the locations at which respective screens leave said. roll are spaced apart such that an angle of at least 50 is subtended between said screens at said locations.

5. A machine as claimed in Claim 1 or 2, wherein both screens are arranged to leave the roll at one location and wherein means having a curved surface is arranged to separate said screens downstream of said roll.

6. A machine as claimed in any one of claims 1 to 5, wherein said convex surface is water impermeable.

7. A machine as claimed in any one of claims 1 to 6, wherein the inner screen is arranged to pass over said surface of said guide, and the outer screen is arranged to meet the inner screen over said surface of said guide.

8. A machine'as claimed in any one of Claims 1 to 7, comprising a dewatering device arranged beneath the water extraction zone, wherein said device includes a suction box and/or a water scraper.

9. A machine as claimed in any one of Claims 1 to 8, wherein a belt or felt is provided onto which a paper web is passed subsequent to being separated from said outer screen downstream of said forming roll.

10. Apparatus for producing multi-layer paper or cardboard, comprising a plurality of papermaking machines arranged above a common belt or felt for receiving paper webs sequentially from said machines, wherein at least one of said machines is a machine as claimed in any one of Claims 1 to 9.

11. A papermaking machine substanially as hereinbefore described with reference to figure 1, 2 or 3 or figures 1 and 4 of the accompanying dawings.