This application is a U.S. national stage application of International Application No. PCT/FI01/00553, filed Jun. 12, 2001, and claims priority on Finnish Application No. 20001404 filed Jun. 13, 2000, the disclosures of both of which applications are incorporated by reference herein.
Not applicable.
The invention concerns a headbox of a paper machine or such.
The making of paper of a good quality and a stable production process make high demands on the headbox of the paper machine. In particular, a headbox meeting qualitative and productive requirements is expected to be able to produce a homogenous and trouble-free lip discharge.
Various applications in operation and further refinement processes make high qualitative demands on paper and board products. In practice, these demands concern the structural, physical and visual characteristics of the products. In order to achieve characteristics suitable for each individual purpose the production processes are optimised at each time for a certain working range, which sets limits usually also limiting the quantity of production. Thus, a product of the desired kind can be made only in a narrow working range of the production process.
Due to the restrictions made by the working range it is very difficult to carry out such changes in the process which aim at increasing the production and at improving the quality of the product. Significant changes usually require long-range research and technological development. Process changes desirable for an increased productivity of the manufacturing process are e.g. new techniques to do with an increased machine speed and a minimised use of water (increased web formation consistency).
In order to make paper of a good quality efforts are made to prevent various disturbances, such as vortexes and consistency streaks, from escaping from the headbox. Such disturbances may occur e.g. in connection with fluidisation (a strong geometrical change) and in the output ends of the pipes of a turbulence generator (disturbances from pipe walls, such as vortexes and consistency and speed profiles). For this reason,
It follows from a low flow rate that the average residence time of the fibre pulp in the headbox after fluidisation is too long as regards avoidance of re-flocculation. Thus, the fibre pulp will not discharge from the headbox in the fluidised state required for a good formation. To improve fluidisation, lamellas have in fact been introduced for use in the headbox. These lamellas are mounted in the lip channel and they bring about more friction surface in the channel. However, the most significant fluidisation-promoting effect of the lamellas relates to their tip turbulences. Although these turbulences are advantageous for the fluidisation, they cause coherent flow structures which will weaken slowly, but which can be seen even in the produced paper. In practice, the added friction surface brought about by lamellas and the increased yield of boundary-layer turbulence are not sufficient to fluidise the flow. However, with the aid of friction surfaces in flow channels and with the aid of boundary-layer turbulence it is possible to maintain the strongly fluidised state brought about in the turbulence generator. An incomplete (cautious) fluidisation carried out in many stages leads to a more disadvantageous floc structure than fluidisation carried out in one go and based on a controlled residence time.
The headbox according to the invention is different from state-of-the-art solutions in that in the headbox according to the invention fluidisation is carried out only once in one stage in each pipeline. Thus, each pipeline includes only one fluidisation element. When the fluidisation has been carried out effectively, the flow is accelerated and the fluidisation level is maintained by using lamellas and suitable flow surfaces. By accelerating the flow the residence time of the pulp in the headbox after the fluidisation point is kept as short as possible, so that the fluidisation level remains good also as the pulp arrives at the formation wire, e.g. into the jaw between the formation wires of the jaw former. Thus, the headbox according to the invention in its turbulence generator 12 includes in each row of pipes only one fluidiser, that is, a fluidisation element, which is used for fluidisation of the pulp. Thereafter the pulp is guided in the flow direction along such flow paths, which do not include any steps or other places that would cause disturbances to the flow.
In the headbox structure according to the invention, it has been found that by increasing pipe-specific flows of the headbox's turbulence generator the paper quality is improved and the web formation consistency can be increased. This is possible by generating more turbulence in the fluidiser and thus bringing about a more complete fluidisation than with traditional headbox solutions. The harmful effects of the raised turbulence level are eliminated by limiting the scale of vortex size of the generated turbulence.
Fluidisation means that the flow characteristics of the fibre suspension are made to correspond with the characteristics of the water flow. That is, multi-phase flow behaves like a single-phase flow. Hereby the wood fibres, fillers and fines in the fibre suspension flow will behave like water. Fibre lumps, that is, fibre flocs, in the fluidisation are broken up.
Thus, in the headbox according to the invention fluidisation is carried out only once and its level is hereby higher than with a conventional headbox. The fluidisation is preferably implemented in a rotationally symmetrical pipe expansion. However, the used total pressure energy is not necessarily higher than before, because other fluidisation elements, such as steps at the ends of turbopipes and at the tips of lamellas, are minimised. The fluidisation level and thus the minimum floc size are controlled by choosing the entity formed by the fluidiser primary pipes, step expansion and vortex chamber to produce the desired loss energy. A higher fluidisation level is achieved with an increased energy supply.
The invention will be described in the following by referring to the figures in the appended drawings and graphic presentations. The description of the inventive theory is based on the graphic presentations, and the illustrations of headbox embodiments of the invention show some advantageous embodiments of the invention, although the intention is not to restrict the invention solely to these.
Since the minimum floc size is reduced logarithmically as the loss power (the flow rate) increases, almost the same fluidisation level is achieved with flow rates exceeding the dimensioning point corresponding with the above-mentioned optimum. However, due to the higher flow rate, a shorter residence time hereby results and thus a better fluidisation level is achieved in the outflow from the headbox. The maximum of the flow rate range is formed by the time needed in the lip channel for disturbance in the lags of turbopipes and lamellas to die out. In the headbox according to the invention, this maximum of the flow rate range is considerably higher than in the traditional headbox, because in connection with the fluidisation a high level of turbulence is brought about, which is kept up with the aid of a high flow rate and a small channel size.
Due to the efficient fluidiser a powerful turbulence is achieved in the headbox according to the invention. Such a step is used as fluidiser, the dimension of which is larger than the average fibre length. In this way a vortex size sufficient for breaking flocs is achieved along with an efficient supply of energy. After the fluidiser the turbulence begins dying out promptly. Although vortexes bigger than the average fibre length are needed for breaking the flocs, they will cause quick re-flocculation after the fluidisation.
In the headbox according to the invention, fibre mobility or the fluidisation level is maintained by using the following procedures:
With the aid of wedge-like lamellas 16a1, 16a2 acceleration of the flow is continued and thus the residence time after the automatic fluidisation unit is shortened in the headbox, and reduction of the channel cross-section (control of the scale) is continued in the lip channel part of the headbox. At the same time the share of the wall surface in the lip channel is optimised. With the aid of wall friction turbulence is brought about, which is used to slow down or even to stop the dying out of the high turbulence level brought about in the fluidiser. In addition, the achieved turbulence takes place in the lip channel divided by lamellas on the desired small scale.
In the headbox according to the invention these trouble situations are controlled with the aid of a high turbulence level, that is, fibre mobility by following the following principles:
and step height h1 is at least equal to the average fibre length, preferably more, preferably in a range of 1 mm–12 mm, and most preferably in a range of 1 mm–6 mm. The average fibre length is typically in a range of 1 mm–3 mm, depending on the pulp used. After the fluidiser, that is, the fluidisation element 14, there is a pipe 15 of the turbulence generator, which pipe includes a rotationally symmetrical mixing pipe part 15a no less than 50 mm long and then an acceleration and reshaping part 15b, which is used to accelerate the pulp flow and the length of which is no more than 200 mm, so that the intensity of turbulence is sufficient to allow the steps in the outlet opening of pipe 15b. The length of lip channel K is chosen so that the flows arriving from pipes 15 have the time to mix in it, but so that re-flocculation is prevented. The length of lip channel K is chosen within a range of 100 mm–800 mm. The cross-section of pipe 15a turns from circular into a square in pipe 15b. The inner diameter Φ1 of pipe part 15a is in the range 20 mm–40 mm. The ratio Φ1/Φ2 between the inner diameters of pipes 15a and 13 is in the range 1.1–4.0. The flow then comes from pipe 15b of the turbulence generator to reach lamellas 16a1, 16a2 in such a way that between the pipe 12a1.1, 12a2.1, . . . and lamella 16a1, 16a2 there is no step or it is no more than 2 mm, that is, equal to the thickness of the pipe wall of the turbulence generator. According to the invention, such lamellas 16a1, 16a2 are used, which narrow in a wedge-like fashion in the flow direction and end in a sharp tip, the height h2 of which tip is in the range 0–2 mm, preferably less than 1 mm. Thus, the headbox according to the invention in the turbulence generator includes only one fluidisation point and after this acceleration arrangements and lamella arrangements to maintain the fluidisation of the flow after the fluidisation point and to minimise the residence time in the headbox before the formation wire H1, H2.
After the fluidisation element 14, the pulp flow speed is accelerated essentially all the time all the way to the lip opening. After the fluidisation element 14 the maximum permissible step expansion in the flow channel in the z direction is less than the average fibre length. The minimum length of pipe 13 of the turbulence generator 12 is 150 mm, the minimum length of the rotationally symmetrical part of pipe 15a is 50 mm and the maximum length of pipe part 15b is 200 mm.
The headbox according to the invention may be used not only in a paper machine but also in board machines, soft tissue machines and pulp drying machines.
Number | Date | Country | Kind |
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2000 1404 | Jun 2000 | FI | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FI01/00553 | 6/12/2001 | WO | 00 | 4/28/2003 |
Publishing Document | Publishing Date | Country | Kind |
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WO01/96657 | 12/20/2001 | WO | A |
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4376014 | Bergstrom | Mar 1983 | A |
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5746889 | Huovila et al. | May 1998 | A |
20030155093 | Lepomaki et al. | Aug 2003 | A1 |
Number | Date | Country |
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69330 | Sep 1985 | FI |
870705 | Oct 1990 | FI |
0121885 | Mar 2001 | WO |
0196657 | Dec 2001 | WO |
Number | Date | Country | |
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20030159792 A1 | Aug 2003 | US |