PRESSURIZED FORMING BOARD

Abstract: A paper making machine forming board is described wherein the rate of drainage of the stock through the forming fabric is curtailed and controlled by applying a low positive air pressure above ambient atmospheric to the machine side of the forming fabric. Additionally, a controlled level of agitation is induced in the stock on the forming board, providing better initial paper formation.

Description: PRESSURIZED FORMING BOARD 1 This invention is concerned with the initial formation of paper on machines of the type wherein an aqueous slurry known as the stock is deposited onto a continuous moving forming fabric. In this type of machine, the stock which contains both fibers and other substances in an amount of from about 0.1% to about 1.5% by weight, is fed from a head box slice onto a moving forming fabric. Water is progressively drained from the stock through the forming fabric as it passes over various drainage elements until at the end of the forming section the stock contains some 2% to 4% by weight of solid material. At that point, the distribution and orientation of the fibers and other solids in the still very wet stock is largely determined, and will not change very much in the later paper making steps. Thus at the end of the forming section the formation of the paper is largely completed. In outline, a conventional paper making machine forming section includes a forming fabric which is supported at the head box slice end by a breast roll, which is followed in sequence by a forming board, and thereafter by a sequence of drainage devices which complete the forming section. The nature and placement of these drainage devices varies considerably, and is determined at least in part by whether either a conventional open fabric forming •section or a twin wire or gap former is being used. It is well known that the jet of stock delivered onto the forming fabric from the head box slice is often far from uniform. In a uniform stock, all of the various components therein would be evenly and uniformly distributed throughout. Similarly, the jet of stock that exits the head box slice would also be of constant depth, constant linear velocity, and free from ripples and other disturbances across the full width of the head box slice. These ideals are rarely, if ever, achieved in practice. Within the stock itself, there is also significant point-to-point variation in the distribution of the various materials in the stock, in both a vertical and a horizontal direction. It is desirable that these imperfections in the stock should either be removed or at least controlled so that they get no worse, before they are set into the paper web as a consequence of water drainage from the stock in the forming section. A high and uncontrolled rate of drainage in the initial part of a forming section tends to set these stock imperfections into the formation of the paper. This effectively precludes any remedial action. It has now been realized that if this very high rate of drainage in the initial part of the forming process could be curtailed, then an improvement in the overall paper making process will result. The remaining drainage elements in the forming section must of course be able to handle the increased drainage load that such a step will cause. The concepts of this invention are equally applicable to both a twin wire machine, and to a conventional open-surface forming section, that is to improve stock formation in the region of the forming section immediately adjacent the head box slice. Curtailing the drainage rate through a conventional design forming board would appear to be simple: all that is needed is to decrease the available drainage area, by widening the support surfaces, and/or moving them closer together. Experiment shows that this is not so and the drainage rate is not significantly reduced. Further, the paper web is marked severely with streaks, of uneven fibre weight across the width of the paper. In other words, such changes can cause significant deterioration in paper quality. It has now been discovered that these difficulties can be overcome, and a much reduced rate of drainage obtained, by modifying the forming board in such a way that a low value of air pressure (above ambient atmospheric) can be applied to the machine side of the forming fabric. Additionally, means can be provided to induce some beneficial agitation within the stock on the new design of forming board. Thus in a first embodiment this invention provides in a paper making machine having a forming section including at least a continuous travelling forming fabric which passes over a breast roll adjacent a head box slice through which aqueous stock is deposited onto the forming fabric, an apparatus for improving stock formation comprising: (a) a foraminous forming board surface supporting the forming - fabric; (b) a drainage box supporting the forming board surface; (c) air supply means including a source of positive air pressure; and air pressure control means, such that a positive air pressure sufficient to hinder water drainage from the stock through the foraminous forming board surface but insufficient to interfere with paper formation on the forming fabric on the forming board surface, is maintained in the drainage box; and (d) drainage means for the drainage box which both contains the applied positive air pressure and permits an unrestricted flow of drained white water, wherein: (i) the forming board surface provides a pressure tight seal between th. forming fabric and the drainage box; (ii) the foraminous forming board surface provides a path through which the forming fabric moves which induces a controlled level of uniformly spaced periodic harmonic agitation in the stock; and (iii) the drainage box includes at least one compartment extending across the width of the forming fabric, which has separate air pressure control means and drainage means. Preferably, the positive pressure will generally decrease from a high value adjacent the head box slice to a value approaching ambient atmospheric at the end of the forming board. Preferably, the air supply means provides a positive pressure above ambient atmospheric of from about 1 cm water gauge to about 25 cms water gauge. The forming board surface preferably comprises a plurality of thin elongate blades extending across the full width of the forming fabric, so placed as to contribute toward controlled agitation of the stock on the forming fabric. More preferably, at least some of the thin elongate blades are provided with a top surface contour which will induce controlled agitation in the stock on the forming fabric. In a second embodiment, this invention provides a method for improving stock formation in a paper making machine having a forming section including at least a continuous travelling forming fabric which passes over a breast roll adjacent a head box slice through which aqueous stock is deposited onto the forming fabric, comprising: (i) discharging onto the moving forming fabric an aqueous stock; ( i) causing the forming fabric to move over a dewatering device comprising a pressurized drainage box including at least one chamber to which a controlled level of pressure is applied which is sufficient to hinder drainage but insufficient to interfere with paper formation on the forming fabric, which includes a foraminous top support surface for the forming fabric, and which provides a pressure tight seal between the forming fabric and the drainage box; and (iii) inducing a desired level of uniformly spaced periodic harmonic agitation in the stock on the forming fabric on the forming board. Preferably, the air pressure is controlled to be from about 1 cm water gauge (above ambient atmospheric) to about 25 cms water gauge. Preferably the desired level of agitation is caused and controlled within the stock by controlling the spacing of the support-surfaces comprising the forming board. More preferably, the desired level of agitation is caused within the stock by a support surface with a top surface contour which induces controlled agitation within the stock on the forming fabric. Preferably, in each of these embodiments, the drainage box is either a single box divided into separate compartments by pressure tight transverse walls, or a plurality of contiguous separate boxes, to each of which a different level of pressure can be applied. The invention will now be described in more detail by way of reference to a conventional open-surface forming section. It is to be understood that the invention is not so limited, and may be used with other paper making machines of the 2 wire, twin-wire, or gap former type. In the attached drawings: Figure 1 shows diagrammatically the initial part of a paper making machine; Figure 2 shows schematically one forming board of this invention; Figure 3 shows several possible support surfaces of the forming board of Figure 2. Figure 4 shows schematically an alternative forming board of this invention. In these Figures, relevant like parts have been given the same numbers. In Figure 1, the initial part of a paper making machine is shown, incorporating a forming fabric 1, which moves in the direction of the arrows shown at 1A and IB. The forming fabric moves over a breast roll 2, and various tensioning and idling rollers 3. The stock is deposited onto the forming fabric 1 from the head box shown diagrammatically at 4, through a slice opening 5, which extends transversely across the forming fabric 1. Adjacent the breast roll 2 and the head box slice 5 is a forming board followed by the first of the many drainage units which comprise the forming section, which in this case is shown as a foil unit 7, based on that originally described by Wrist in US 2,928,465. The forming board unit 6 is provided with a plurality of support surfaces shown generally at 12, 13 and 14, of which that nearest the headbox slice, 12, is generally wider, than the others. Each of these surfaces is mounted on a T-bar, as at 10. As shown, the drainage box 6 is merely left open at its bottom to provide gravity drainage. In Figure 2 is shown one embodiment of the forming board of this invention. Two differences to the detail of the conventional forming board of Figure 1 are immediately apparent: the unit is now a closed box 20 with a pressurized air supply pipe 28, and the drain pipe 32 has an integral pressure leg, to contain the applied pressure. The forming board support surfaces again comprise a T-bar 10 onto which is mounted the elongate blade 17, which extends across the width of the forming fabric. At each side of the forming fabric sealing pieces (not shown) are inserted between the blade units to seal the forming fabric effectively to the top surface of the drainage box. The shape of the top surface, and also their lateral spacing, will be discussed in more detail below. The first blade 18 has to counter the impact of the jet of stock as it exits the slice 5 onto the forming fabric 1. Conventionally as shown in Figure 2, this first blade is usually much wider than the others, often being twice the width of the later ones. It has now been found that this need not be so, and a blade substantially the same width as the others can often be used. This first blade will also generally have a flat top surface. As is the case with all of the blades, it is mounted onto a conventional T-bar arrangement. The placement and shape of the remaining blades 19 is important. It is now known that almost any surface used to support a forming fabric has an effect on the behaviour of the stock on the fabric. The characteristics of the surface can be chosen to produce agitation in the stock which can range from sufficient effectively to lift the stock bodily off the fabric ranging downwardly through visible macroscopic agitation to microscopic agitation which can only be observed by using careful photography and strobe light illumination (i.e., with a simple flat surfaced blade such as the first blade 18). Further, it is also now known that the placement of these blades along the forming fabric is important, particularly for static drainage devices. Although the hydraulic phenomena causing the agitation are not fully understood, it is nevertheless known that they exhibit harmonic characteristics. This concept can be usefully utilized in the forming board of this invention. As a first step, the blades 19 can be placed so that the microscopic agitation which they induce is harmonically spaced apart along the length of the forming board. It then follows that by controlling the amount of agitation imparted by any one blade, the overall level of stock agitation can be controlled. The amount of agitation imparted by the blades is largely determined by the shape of the support surface. A simple flat blade generates some agitation in the microagitation range, which can be enough for some purposes. Generally speaking the blade used to provide microagitation in the stock will cause low or zero drainage. Suitable blade types are described by Johnson in US 3,874,998 and by Saad in US 4,420,370. These blades can be constructed using the now standard T-bar support concepts. This invention does not preclude the use of devices which cause some drainage as well as providing the desired amount of agitation. Figures 3A and 3B show blades which agitate without draining, as described by Johnson, in U.S. 3,874,998. In each case, the blade 37 or 38 mounts on a T-bar 10. Referring first to Figure 3A, the blade 37 has a central depression 39, in its top surface, so that a cross-machine gap of a flat triangular shape, having a depth x, is created below the plane of the forming fabric. Alternatively, as shown in Figure 3B, this depression rather than being triangular, can be a shallow concave shape. In both cases, as is shown in Figure 3A, water enters this depression from the stock on the forming fabric 1, and re-enters the stock, as indicated by the arrows 40 in Figure 3A. Careful choice of the distances x, y, , y ? and z then controls the amount of agitation imparted to the stock. The amount of agitation that is needed at this early stage of paper formation is small, in the microagitation range mentioned earlier, and therefore it is possible that not all of the blades will have a depression at all, since, as is noted above, a flat-blade surface also causes some agitation. Typical dimensions for a blade such as those shown in Figure 3A or Figure 3B when used to cause microagitation are: (i) total width (i.e. y, + y 2 + z): 25 mm to 75 mm (ii) flat surface width: generally y, and y„ are equal, but are not necessarily so; the minimum for each is about 5 mm, with a minimum of 10 mm being preferred (iii) the width, z, of the depression: 15 mm to 65 mm (iv) the depth, x, of the depression: 0.25 mm to 2.5 mm, for both a triangular and a curved depression. The preferred value for the depression width z is that it is about half the total width of the blade. This then leaves adequate leading and trailing flat portions (y, and y„) to get a water seal with the fabric onto the blade in these areas. As the depth also affects the amount of agitation, a wider blade will not necessarily require a deeper depression. In many cases it is found that if the blade is widened then the depth, x, should not be changed, although the width z will increase, to maintain it at about half the total blade width. In selecting a blade for a given circumstance, some care is needed. The narrowest blade that gives adequate support should be used, which will mean that a forming board according to this invention usually will use more support blades than a conventional board. Further, all of the blades used can be of the same width. Similarly, the shallowest depression should be used that is needed to cause the desired amount of agitation. If the blade is- made too wide, and the depression is made too deep, then the level of agitation can go far beyond the microagitation needed in this area of the forming fabric to a level where the forming fabric with the stock on it lifts clear of the blades. This is not conducive to the making of good paper. Although it is simpler to use a blade with a single depression, it is realized that in certain circumstances a wider blade with more than one depression might be desirable. If such a blade is used, then the central flat portion between the depressions should be about as wide as the leading and trailing flat surfaces. In Figure 4 a preferred embodiment is shown. The drainage box shown generally at 6 is divided into a series of pressure tight smaller compartments 20, 21, 22, and 23, by transverse walls 24, 25 and 26. Four are shown, but this number is not crucial. Similarly, a series of separate boxes could also be used, with their side walls contiguous. Each of the compartments 20 through 23 extends across the full width of the forming fabric. In each case, a blade, as at 27, is placed over each of the transverse walls. Each of the compartments is provided with a pressurized air supply pipe 28, 29, 30 and 31, which incorporates a pressure control means (not shown), and also with a drainage leg 32, 33, 34 and 35, which includes suitable means to contain pressure in the compartment while also allowing free drainage. This arrangement permits a different marginal pressure above ambient to be applied to each of boxes 20, 21, 22 and 23. In most cases, the applied pressure will decrease in a stepwise fashion from box 20, adjacent the headbox slice, to box 23 to which a low pressure, or even none at all, is applied. It is also contemplated that situations might arise wherein such a progression is not desirable, and for example the highest pressure might be found desirable in box 21.

Claims: WHAT IS CLAIMED IS: 1. In a paper making machine having a forming section including at least a continuous travelling forming fabric which passes over a breast roll adjacent a head box slice through which aqueous stock is deposited onto the forming fabric, an apparatus for improving stock formation comprising: (a) a foraminous forming board surface supporting the forming fabric; (b) a drainage box supporting the forming board surface; (c) air supply means including a source of positive air pressure; and air pressure control means, such that a positive air pressure sufficient to hinder water drainage from the stock through the foraminous forming board surface but insufficient to interfere with stock formation on the forming fabric on the forming board surface, is maintained in the drainage box; and (d) drainage means for the drainage box which both contains the applied positive air pressure and permits an unrestricted flow of drained white water, wherein: (i) the forming board surface provides a pressure tight seal between the forming fabric and the drainage box; (ii) the foraminous forming board surface provides a path through which the forming fabric moves which induces a controlled level of uniformly spaced periodic harmonic agitation in the stock; and (iii) the drainage box includes at least one compartment extending across the width of the forming fabric, which has separate air pressure control means and drainage means. 2. An apparatus according to claim 1 wherein the forming board surface comprises a plurality of uniformly spaced thin elongate blades extending across the full width of the forming fabric, together with sealing strips interposed between the ends thereof. 3. An apparatus according to claim 2 wherein all of the elongate blades are of the same width. 4. An apparatus according to claim 2 wherein the first elongate blade adjacent the headbox slice is wider than the remainder. 5. An apparatus according to claim 2 wherein at least one of the elongate blades has a top surface contour adapted to contribute toward agitation of the stock on the forming fabric. 6. An apparatus according to claim 5 wherein the top surface contour comprises a flat surface having a leading and a trailing portion in the direction of forming fabric movement separated by at least one shallow depression extending along the blade for the width of the forming fabric, and wherein each of the leading and trailing portions, and any flat portions intermediate shallow depressions, are of sufficient width to provide a hydraulic seal to the machine side of the forming fabric. 7. An apparatus according to claim 6 wherein the top surface contour comprises a flat surface having a leading and a trailing portion separated by a shallow depression extending along the blade for the width of the forming fabric. 8. An apparatus according to claim 7 wherein the shallow depression extends for about one half of the width of the blade. 9. An apparatus according to claim 7 wherein each of the leading and trailing portions are about one quarter of the width of the blade. 10. An apparatus according to claim 6 wherein the top surface contour comprises a flat surface having a leading, a central, and a trailing portion separated by two shallow depressions, wherein each depression is about the same width; the leading, central, and trailing portions are about the same width and each are about one half of the width of the depressions. 11. An apparatus according to claim 6 wherein the level of agitation is controlled by controlling the depth, or the shape, or both the depth and shape of the shallow depression. 12. An apparatus according to claim 1 wherein the drainage box comprises either a single box extending from adjacent the headbox slice to the end of the forming board which is provided with a plurality of pressure tight transverse divisions between each of which a separate controlled positive pressure can be applied and each of which has a separate drainage means, or a plurality of contiguously adjacent boxes extending from adjacent the head box slice, to each of which a separate controlled positive pressure can be applied, and each of which has a separate drainage means. 13. An apparatus according to claim 12 wherein the air pressure in the compartments of the drainage box decreases from a high value adjacent the headbox slice to a value approaching ambient atmospheric at the end of the forming board. 14. An apparatus according to claim 13 wherein the air supply means provides a pressure above ambient atmospheric of from about 1 cm water gauge to about 25 cms water gauge. 15. An apparatus according to claim 2 wherein the drainage box comprises either a single box extending from adjacent the head box slice to the end of the forming board which is provided with a plurality of pressure tight transverse divisions between each of which a separate controlled positive pressure can be applied, and each of which has a separate drainage means, or a plurality of contiguously adjacent boxes extending from adjacent the head box slice to each of which a separate controlled positive pressure can be applied and each of which has a separate drainage means, and wherein an elongate blade is placed above the first, the last, and any intermediate transverse pressure tight divisions in the drainage box. 16. An apparatus according to claim 15 wherein the air pressure in the compartments of the drainage box decreases from a high value adjacent the headbox slice to a value approaching ambient atmospheric at the end of the forming board. 17. An apparatus according to claim 16 wherein the air supply means provides a pressure above ambient atmospheric of from about 1 cm water gauge to about 25 cms water gauge. 18. A method for improving stock formation in a paper making machine having a forming section including at least a continuous travelling forming fabric which passes over a breast roll adjacent a head box slice through which aqueous stock is deposited onto the forming fabric, comprising: (i) discharging onto the moving forming fabric an aqueous stock; (ii) causing the forming fabric to move over a dewatering device comprising a pressurized drainage box including at least one chamber to which a controlled level of pressure is applied which is sufficient to hinder drainage but insufficient to interfere with stock formation on the forming fabric, which includes a foraminous top support surface for the forming fabric, and which provides a pressure tight seal between the forming fabric and the drainage box; (iii) inducing a desired level of uniformly spaced periodic harmonic agitation in the stock on the forming fabric on the forming board. 19. A method according to claim 18 wherein the foraminous surface provides a path through which the forming fabric moves which causes a controlled level of agitation in the stock. 20. A method according to claim 19 wherein the foraminous surface comprises a plurality of uniformly spaced thin elongate blades extending across the full width of the forming fabric, together with sealing strips interposed between the ends thereof. 21. A method according to claim 20 wherein at least one of the blades is provided with a top contour which contributes toward- agitation in the stock. 22. A method according to claim 21 wherein the amount of agitation imparted to the stock is controlled by controlling the shape of the blade top contour. 23. A method according to claim 23 wherein the blade top contour includes at least two flat portions separated by a depression, and wherein the amount of agitation is controlled by controlling the depth or the shape, or both the depth and shape of the depression. 24. A method according to claim 18 wherein the drainage box comprises either a single box extending from adjacent the head box slice to the end of the forming board which is provided with a plurality of pressure tight transverse divisions between each of which a separate controlled positive air pressure can be applied and each of which has a separate drainage means or a plurality of contiguously adjacent boxes to each of which a separate controlled positive pressure can be applied and each of which has a separate drainage means. 25. A method according to claim 24 wherein the positive pressure is controlled from a high value adjacent the head box slice to a low value approaching ambient atmospheric pressure at the end of the forming board. 26. A method according to claim 25 wherein the air pressure is controlled to be from about 1 cm water gauge above ambient atmospheric to about 25 cms water gauge. 27. A method according to claim 20 wherein the drainage box comprises either a single box extending from adjacent the head box slice to the end of the forming board which is provided with a plurality of pressure tight transverse divisions between each of which a separate controlled positive air pressure can be applied and each of which has a separate drainage means or a plurality of contiguously adjacent boxes to each of which a separate controlled positive pressure can be applied and each of which has a separate drainage means, and wherein an elongate blade is placed over the first, the last and any intermediate pressure tight transverse divisions in the drainage box. 28. A method according to claim 27 wherein the positive pressure is controlled from a high value adjacent the head box slice to a low value approaching ambient atmospheric pressure at the end of the forming board. 29. A method according to claim 28 wherein the air pressure is controlled to be from about 1 cm water gauge above ambient atmospheric to about 25 cms water gauge.

Inventors: MCPHERSON DOUGLAS ROBERT, PITT RICHARD EDWARD, SMITH DAVID THEODORE, MCPHERSON, DOUGLAS, ROBERT, PITT, RICHARD, EDWARD, SMITH, DAVID, THEODORE
Applicants: JWI LTD, JWI LTD.
Application year: 1990
Main classification: D21F9
IPC codes: D21F1, D21F9