Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/48—Suction apparatus
  • D21F1/483—Drainage foils and bars
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/009—Fibre-rearranging devices
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F9/00—Complete machines for making continuous webs of paper
  • D21F9/02—Complete machines for making continuous webs of paper of the Fourdrinier type

Definitions

• 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.
• 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.
• 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.
• 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.
• 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:
• 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;
• the forming board surface provides a pressure tight seal between th. forming fabric and the drainage box;
• 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;
• 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.
• 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.
• 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.
• 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.
• the air pressure is controlled to be from about 1 cm water gauge (above ambient atmospheric) to about 25 cms water gauge.
• 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.
• 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.
• 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.
• 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.
• FIG 2 is shown one embodiment of the forming board of this invention.
• 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.
• sealing pieces 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.
• 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.
• 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.
• 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.
• 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.
• this depression rather than being triangular, can be a shallow concave shape.
• 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:
• 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 ybal) 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.
• all of the blades used can be of the same width.
• 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.
• FIG 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.
• 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.

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• Paper (AREA)
• Press Drives And Press Lines (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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• 1990
  • 1990-06-07 JP JP2507884A patent/JPH07113194B2/ja not_active Expired - Fee Related
  • 1990-06-07 CA CA 2057880 patent/CA2057880C/en not_active Expired - Fee Related
  • 1990-06-07 EP EP19900908446 patent/EP0475976B1/de not_active Expired - Lifetime
  • 1990-06-07 DE DE69007479T patent/DE69007479T2/de not_active Expired - Fee Related
  • 1990-06-07 WO PCT/CA1990/000184 patent/WO1990015186A1/en active IP Right Grant
  • 1990-06-07 AU AU56748/90A patent/AU633100B2/en not_active Ceased
• 1991
  • 1991-12-05 FI FI915762A patent/FI101819B1/fi active

Non-Patent Citations (1)

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