FI101819B - Compressed air wiring table - Google Patents

Description

101819

Pressurized wire table - Med tryckluft verkande virabord

The present invention relates to the initial production of paper 5 in paper machines in which an aqueous slurry called pulp is deposited on a continuously moving wire cloth.

In such a machine, a pulp containing from about 0.1% to about 1.5% by weight of both fibers and other substances is fed to a wire cloth moving from the lips of 10 pressure boxes. Water is continuously removed from the pulp through the wire cloth as it passes over various dewatering elements, with the pulp containing about 2-4% by weight of solid material at the end of the forming section. At this point, the distribution and orientation of the fibers and 15 other solid ingredients in the still rather wet pulp become approximately determined and do not change much with the subsequent papermaking steps. Thus, the formation of the paper is approximately completed at the end of the forming section.

20

The forming section of a conventional paper machine includes a wire cloth supported at the lip opening of the pressure box by a chest roller, followed by a wire table and then sequential dewatering devices placed at the end of the forming section. The nature and placement of these dewatering devices vary considerably depending, at least in part, on whether a conventional open wire fabric forming compartment or a double wire or slit former is used.

: 30

It is well known that the mass flow fed to the wire cloth from the mouth of the pressure box is often quite non-uniform. In a uniform pulp, all the various component parts would be evenly and uniformly distributed throughout. Similarly, the mass flow exiting the pressure box orifice would also have a constant depth, a linear linear velocity, and no ripple or other interference over the entire width of the pressure box orifice. Such ideal conditions are rarely, if ever, achieved in practice. Within the pulp itself, there is also a significant local change in the distribution of different materials in the pulp, both vertically and horizontally. It is desirable that these harmful factors affecting the pulp could either be eliminated or at least controlled so that they do not become worse before they are transferred to the paper web as a result of dewatering of the pulp in the forming section. Rapid and uncontrolled dewatering at the beginning of the forming section tends to transfer these detrimental factors to paper formation. This fact effectively prevents 15 all remedies.

It has now been found that if this very rapid dewatering in the early stages of the forming process could be reduced, the entire papermaking process would be improved. The other dewatering elements of the forming section 20 must, of course, be able to handle the increased dewatering load caused by this arrangement.

The principles of this invention can be applied to both a twin wire machine and a conventional open surface forming section to improve mass formation in the area of the forming section immediately adjacent the lip opening of the pressure box.

30 Slowing down the dewatering speed in a conventional wire table seems simple: all you have to do is reduce the dewatering area in use by expanding the support surfaces and / or moving them closer together. However, experiments have shown that this is not the case and that the water-35 removal rate is not significantly reduced. In addition, considerable streaks are formed in the paper web as the fiber weight is unevenly distributed over the width of the paper. In other words, such changes can significantly degrade paper quality.

5 It has now been found that these difficulties can be overcome and a much lower dewatering rate achieved by converting the wire table so that a low air pressure (above the ambient air pressure) can be applied to the machine side of the wire fabric. In addition, means can be used to advantageously mix the pa-10 base mass in connection with this new wire table.

The first embodiment of the present invention thus relates to an apparatus for improving pulp formation in a papermaker's machine, the forming section includes at least one continuous wire cloth passing through a pressure box lip gap through which an aqueous paper pulp is deposited on the wire cloth over an adjacent breast roll, said apparatus comprising: (a) a perforated wire table surface supporting the wire cloth; (b) a dewatering box supporting the wire table surface; (c) an air supply device including a positive air pressure source and air pressure monitoring means, wherein a positive air pressure is maintained in the dewatering box sufficient to prevent dewatering of the pulp through the perforated wire table surface but not sufficient to affect paper formation on the wire cloth 30; and (d) dewatering means for the dewatering box, both the means and the box containing a positive air pressure and allowing an unrestricted flow of removed circulating water, wherein: 101819 (i) the wire table surface forms a pressure-tight seal between the wire fabric and the dewatering box; (ii) the perforated wire table surface forms a channel-5 age through which the wire cloth passes, thereby using a controlled level of periodic harmonic mixtures at uniform intervals; and (iii) the dewatering box includes at least one compartment 10 extending across the width of the wire cloth, provided with separate air pressure monitoring means and dewatering means.

15 The positive pressure preferably decreases from a high value next to the lip of the pressure box to a value approaching the ambient atmosphere at the end of the wire table.

The air supply device preferably produces a positive pressure of about 0.1 kPa to 2.5 kPa (1 to 25 cm in height of the water column).

The wire table surface is preferably provided with a plurality of elongate blades extending across the entire width 25 of the wire cloth and positioned to assist in the controlled mixing of the pulp on the wire cloth. It is even more desirable that the shape of the top surface of at least some of these blades provide for controlled mixing of the pulp on the wire cloth.

Another embodiment of the present invention provides a method of improving pulp formation in a paper machine having a forming section comprising at least one continuously moving wire cloth passing over a breast roll 35 adjacent to a pressure box lip through which aqueous pulp is deposited on the wire cloth, the method further comprising : (i) feeding an aqueous pulp to a moving wire cloth; (Ii) transferring the wire cloth over a dewatering device comprising a pressurized dewatering box containing at least one chamber subjected to a controlled pressure level sufficient to prevent dewatering but insufficient to affect paper formation on the wire cloth, the chamber being provided with a torn coating support surface and forming a pressure seal between the wire cloth and the dewatering box; and (ii) forming the desired level of uniformly spaced harmonic mixing in the pulp on the wire cloth of the wire table.

The air pressure is preferably controlled to a value of about 0.1 kPa (1 cm water column height) to about 2.5 kPa (25 cm water column height) above the ambient air pressure.

The desired level of mixing is preferably achieved and monitored within the pulp by monitoring the distance between the support surfaces forming the wire table. Even more preferably, the desired level of mixing is achieved within the pulp by means of a support surface, the shape of the top surface of which provides the desired mixing within the pulp on the wire table.

30

In all these embodiments, the dewatering box preferably comprises either a single box divided into separate compartments by means of pressure-tight transverse walls or a plurality of continuous separate boxes to each of which a different pressure level can be fed.

The invention will now be described in more detail with the aid of a conventional open-surface forming section. It is clear that the invention is not limited to this application and that it can also be used in other paper machines of the double-wire, double-wire or slot-forming type. In the accompanying drawings:

Figure 1 schematically shows the beginning of a paper machine;

Figure 2 schematically shows a wire table 10 according to the present invention;

Fig. 3 shows several possible support surfaces of the wire table according to Fig. 2; Figure 4 schematically shows an alternative wire table according to the present invention.

In these figures, the same parts are denoted by the same reference numerals.

20

Figure 1 shows the beginning of a paper machine provided with a wire cloth 1 moving in the direction of arrows IA and IB. This vii-fabric passes over the chest roll 2 and several tensioning and intermediate pins 3. The paper pulp is deposited on the wire cloth 1 in the form of a pressure box marked with the number 4, through a lip opening 5 which passes across the wire cloth 1.

Adjacent to the breast roll 2 and the lip 5 of the pressure box is a wire table followed by the first of a plurality of dewatering units forming the forming compartment, the first unit being shown as a film unit and designated 7, originally described in Wrist U.S. Patent 2,928,458.

The wire table unit 6 is provided with a plurality of support surfaces, 7 101819, denoted by the numbers 12, 13 and 14, of which the one closest to the lip opening of the pressure box, 12, is generally wider than the others. Each of these surfaces is mounted on a T-beam as indicated by reference numeral 10. As can be seen in the figure, the dewatering box 6 is left open only at its bottom to remove water by gravity.

Figure 2 shows an embodiment of a wire table according to the present invention. Two differences with respect to the conventional wire table 10 of Fig. 1 are immediately apparent: the device unit in this case is a closed box 20 provided with a compressed air supply pipe 28, the outlet pipe 20 containing a fixed pressure branch for the applied pressure. The support surfaces of the wire table again comprise a T-beam 10 with an elongate blade 17 extending across the width of the wire table. Closure pieces (not shown) are placed on each side of the wire table between the blade units to seal the wire cloth tightly to the top surface of the drain box. The shape of the upper surface and also the mutual distance 20 of the closure pieces will be described in more detail below.

The first blade 18 must receive the impulse caused by the jet of pulp as it leaves the lip opening 5 and enters the wire cloth 1. Conventionally, as shown in Figure 2, this first blade is usually much wider, being twice the width of the other blades. It has now been found that this need not be the case and that it is possible to use a first blade which is essentially as wide as the other blades. This first blade also includes 30 generally flat top surfaces. Like all other blades, the first blade is mounted on a conventional T-beam structure.

The placement and shape of the other blades 19 are important. It is now known that almost every surface used to support the wire cloth 101819 8 influences the behavior of the pulp on the wire cloth. The properties of this surface can be selected for pulp mixing, which may involve lifting the pulp sufficiently efficiently from the fabric, visible macroscopic mixing, or microscopic mixing that can only be detected by careful photography and probe illumination (i.e., using a simple flat blade such as a first blade 18). ). In addition, it is now also known that the placement of these blades on a wire-10 fabric is an important issue, especially for static dewatering devices. Although the hydraulic phenomena causing mixing are not fully known, they are known to have harmonic properties. This information can be utilized in connection with the wire table of the present invention. 15 In the first step, the blades 19 can be positioned so that the microscopic mixing they cause is harmoniously distributed over the length of the wire table. It then follows that the amount of mixing caused by any blade can be controlled by controlling the overall level of mixing of the pulp.

20

The amount of mixing provided by the blades is mainly determined by the shape of the support surface. A simple flat blade causes mixing in the microarray, which may be sufficient for some purposes. In general, this blade used for micro-blending the pulp causes little or no dewatering. Suitable blade types are described by Johnson in U.S. Patent 3,874,998 and Saad in U.S. Patent 4,420,370. These blades can be fabricated using currently standard T-beam supports. This kek-30 invention does not preclude the use of equipment that; cause dewatering and provide the desired amount of mixing.

Figures 3A and 3B show blades that mix paper pulp without removing water therefrom, as described, for example, in U.S. Patent No. 3,874,998 to Johnson. In either case, the blade 37 or 38 is mounted on the T-beam 10. Referring first to Fig. 3A, the blade 37 is provided with a central recess 39 in its upper surface, whereby a slot in the shape of an isosceles triangle with a depth x is formed below the plane of the wire cloth. Alternatively, as shown in Fig. 3B, this recess may be shallow and concave instead of triangular in shape. In either case, as shown in Figure 3A, water enters this recess from the pulp on the wire cloth and returns to the pulp according to the arrows shown in Figure 3A. By carefully selecting the distances 10, yx, y2, and z, the mixing effect on the pulp can be controlled. The amount of mixing required in this initial stage of papermaking is small, on the order of magnitude of the above-mentioned micromixing, and thus it is possible that not all blades are provided with a recess, because, as mentioned above, a flat blade surface also causes some mixing.

Typical dimensions of the blade shown in Figure 3A or 3B when used in micromixing are as follows: 20 (i) overall width (i.e. yx + y2 + z): 25 to 75 mm (ii) flat surface width: generally yx and y2 are equal but not necessarily; the minimum value of each is about 5 mm, with a minimum value of 10 mm being desirable (iii) the width z of the recess: 15 to 65 mm; (iv) recess depth x: 0.25 to 2.5 mm for both triangular and curved recesses.

30 The recommended value for the recess width z is approximately half the total blade width. In this case, sufficiently flat front and rear parts (yx and y2) remain to obtain a water trap with the cloth in the blade in these areas. Since this depth also affects the amount of mixing, a wider blade does not necessarily require a deeper recess. In several cases, it has been found that when the blade is expanded, the depth x should not be changed, even if the width z increases, but should be considered to be about half the total width of the blade. Care must be taken when selecting a blade for a particular situation. A narrower blade should be provided which provides adequate support, which means that the wire table according to the present invention usually uses more support blades than the conventional table. In addition, all the blades used can be the same width. Similarly, the lowest cavity that provides the desired amount of mixing should be used. If the blade is made too wide and the recess too deep, then the amount of mixing may exceed the micromixing required in this area of the wire cloth and extend to the level where the wire cloth with the pulps therein rises from the blades. This does not help to make good paper. Although it is simpler to use a blade with a single recess 15, it will be appreciated that in certain situations it may be desirable to use a blade with more than one recess. If such a blade is used, the flat central portion between the recesses should be approximately as wide as the flat front and rear surfaces.

20

Figure 4 shows a preferred embodiment. Generally, the drainage box labeled 6 is divided into a plurality of smaller pressure-tight compartments 20, 21, 22, and 23 by transverse walls 24, 25, and 26. Four sections have been shown 25, but their number is not critical. Similarly, a series of separate boxes could be used, with their side walls being continuous. Each compartment 20 to 23 extends over the entire width of the wire cloth. In each case, a blade, such as a blade 27, is placed on each transverse wall 30. Each compartment is provided with a compressed air supply pipe 28, 29, 30 and 31 to which a pressure monitoring device (not shown) is connected and also a drainage branch 32, 33, 34 and 35 which includes suitable means for maintaining pressure in the compartment while also allowing free drainage. This arrangement allows a different limit pressure in excess of 35 ambient air pressure to be applied to each box 20, 21, 22 and 23.

11 101819

In most cases, the applied pressure decreases stepwise from the box 20 adjacent the orifice of the pressure box to the box 23, which is subjected to only low pressure or even no pressure at all. It is also possible that situations arise where such a staggered arrangement is not desirable and in which, for example, the highest pressure could advantageously occur in the box 21.

Claims (11)

101819 A paper machine, the forming compartment of which comprises at least one continuous wire cloth (1) passing over a breast roll (2) adjacent to the lip gap (5) of the pressure box through which the aqueous paper pulp is deposited on the wire cloth, and comprising a perforated wire table surface (17). , 18, 19), supported by a dewatering box (6), this surface supporting both the wire cloth and providing a pressure-tight seal between the wire cloth 10 and the dewatering box, characterized in that: (a) an air supply device (28) is attached to the dewatering box; 29), which includes a positive air pressure source and air pressure monitoring means so that a positive atmospheric pressure above the ambient atmospheric head is maintained in the dewatering box sufficient to prevent water from leaking from the pulp through the perforated wire table surface but insufficient to interfere with pulp web formation. wire cloth kaalla; (b) the dewatering box also includes a drying means 20 (32, 33), both of which contain a positive air pressure in the dewatering box that exceeds the ambient atmospheric pressure and allows the drained circulating water to flow out of the dewatering box; (c) the perforated wire table surface forms a passageway through which the wire cloth passes, thereby providing a controlled level of intermittent harmonic mixing at uniform intervals; and (d) the dewatering box (6) includes at least one compartment (20, 21) extending across the width of the wire cloth, 30 provided with separate air pressure monitoring means and dewatering means (32, 33). Device according to claim 1, characterized in that the wire table surface comprises a plurality of thin elongate blades (17, 18, 19) spaced at equal distances 35, which extend over the entire width of the wire cloth (1) together with closing strips placed between their ends. 101819 Device according to Claim 2, characterized in that the first blade (18) adjacent to the lip opening (5) of the pressure box is wider than the other blades. Device according to claim 2, characterized in that the shape of the upper surface (39, 62) of the at least one elongate blade facilitates the mixing of the pulp on the wire cloth. Device according to claim 4, characterized in that the upper surface of the blade is flat and comprises a front and rear part of the wire cloth moving in the direction of movement, separated by at least one shallow recess (39, 62) extending along the blade the width of the wire cloth, and - that the rear part 15 and any flat parts between the shallow recesses are of sufficient width to provide a hydraulic closure on the machine side of the wire cloth, and that the level of mixing is controlled by monitoring the depth or shape of the shallow recess or both depth and shape. 20 Device according to claim 1, characterized in that the dewatering box (6) comprises either a single box extending from the lip opening (5) of the pressure box to the end of the wire table and provided with a plurality of pressure transverse walls (24, 25, 26), a separate controlled positive pressure may be applied to each compartment between them, each compartment being provided with a separate drainage means (32, 33, 34, 35), or a plurality of continuous adjacent boxes (20, 21, 22, 23) extending 30 adjacent the orifice of the pressure box, and each of which can be subjected to a separate controlled positive pressure and each of which is provided with a separate dewatering means (32, 33, 34, 35). Device according to claim 2, characterized in that the dewatering box (6) comprises either a single box extending from the lip opening (5) of the pressure box to the end of the wire table and provided with a plurality of transverse walls (24, 25, 26) of pressure pressure. each compartment between which can be subjected to a separate controlled positive pressure, each compartment being provided with a separate drainage means (32, 33, 34, 35), or a plurality of 5 contiguous adjacent boxes (20, 21, 22, 23) extending adjacent the orifice of the pressure box, and each of which can be subjected to a separate controlled positive pressure and each of which is provided with a separate dewatering means (32, 33, 34, 35), and that the elongate blade (18, 17, 10 27) is positioned in the first, last and all intervening transverse (24, 25, 26) on the pressure-tight walls in the dewatering box (6). A method for improving the formation of pulp in a papermaking machine, the forming section comprising at least one continuous wire cloth (1) passing through a breast roll (2) adjacent to the lip gap (5) of the pressure box through which the aqueous pulp is deposited on the wire cloth; a wire table surface arranged in connection with the lip gap of the pressure box and supported by the dewatering box (6), this surface both supporting the wire cloth and providing a pressure-tight seal between the wire cloth and the dewatering box, the method comprising feeding an aqueous paper pulp to the wire moving through the lip gap the following steps: (i) the wire cloth passes over a perforated wire table surface (17, 18, 19) having a pressurized dewatering box (6) containing at least one chamber (20) subjected to a controlled pressure level sufficient to prevent water run-off 30 and (ii) the desired level of intermittent harmonic mixing at uniform intervals is caused by the pulp placed on the woven fabric on the torn vi-35 table. a. A method according to claim 8, characterized in that the perforated surface forms a passage through which the web moves, causing a controlled level of mixing in the pulp. Method according to Claim 9, characterized in that the perforated surface is provided with a plurality of thin elongate blades (17, 18, 19) arranged at regular intervals and running along the entire width of the wire cloth together with the closing strips placed between their ends. Method according to claim 10, characterized in that the dewatering box (6) comprises either a single box extending from the lip opening of the pressure box to the end of the round table and provided with a plurality of pressure-tight transverse walls (24, 25, 26) between each A separate controlled positive pressure may be applied to the compartment 15, each compartment being provided with a separate drainage means (32, 33, 34, 35), or a plurality of continuous adjacent boxes (20, 21, 22, 23) extending adjacent the orifice of the pressure box, each of which may be A separate controlled positive pressure is applied, each of which is provided with a separate dewatering means (32, 33, 34, 35), and that an elongate blade (17, 18, 19) is placed on the first, last and all intermediate transverse pressure-tight walls in the dewatering box -25 sa. λ p | ^ η * r ·, IU 10