EP0824159A2 - Process and apparatus for making a fibrous web - Google Patents

Abstract

In a process for making a web of fibrous material (especially paper), the suspension of fibrous material (stock) is passed through two dewatering units of a twin wire former, and the dewatering performance of the first dewatering unit is controlled in such a way that the water quantity removed per unit time in the second dewatering unit is held at an at least approximately adjustable rate. Also claimed is a twin wire former having a first dewatering section with an adjustable dewatering performance, and a second which is fitted with a device which measures the amount of water being removed by it. Preferably the quantity of water removed per unit time in the second dewatering unit is measured, and the measurement is fed to a control device (66) in which it is compared with a set point value. In the event of a deviation, the control device adjusts the dewatering performance of the first unit so that of the second unit is restored to approximately the set point value. The first dewatering unit is subjected to a suction which is varied to obtain the required dewatering performance. Both sieve belts (11,12) in the first unit may be passed over at least one curved surface to which the suction device is connected. In the second unit, one sieve belt is passed over fixed rails, whilst the other is subjected to pressure from flexible rails. The controller has input channels for the measured value and the set point, and an output channel for a control signal which effects a change in the performance of the first dewatering unit.

Description

The invention relates to a method and an apparatus for Manufacture of a fibrous web, in particular paper web, from an aqueous fibrous suspension. The invention proceeds from the subject of EP 0489 094 (PA 04668). From this publication are those in the preamble of claim 1 specified characteristics known. These say with others Words: Forming the fibrous web from that of the headbox fed stock suspension takes place in a twin wire former only between two sieve belts. So one is missing so-called single-screen pre-dewatering section. In a first The two sieve belts form a dewatering unit together a wedge-shaped inlet gap; this opens into the Headbox coming jet suspension. This hits on the two sieve belts at a point where the sieve belts preferably run over a curved drainage element; this can be a stationary, curved forming shoe or a rotatable forming roller. In this first drainage unit begins the path formation, with a first part of the Water escapes through the mesh of the sieve belts. Aimed for is possible, fibrous webs (especially paper webs) high quality and with relatively high working speeds to manufacture. Due to the formation of a path between two screens is achieved in particular that the finished fibrous web has largely the same properties on both sides (slight "two-sidedness"). At the same time you try one as even as possible fiber distribution in the finished Fibrous web, so a good "formation" or "review" to achieve.

During the formation of the path there is a constant risk that Bale fibers into flakes. One tries therefore in Headbox a fluff-free material suspension jet to form (e.g. with the help of a turbulence generator).

In addition, efforts are made to drain during the formation of the web to influence the pulp suspension in such a way that a "back flocculation" is avoided as far as possible or that after a possible formation of flakes again a "deflocculation" (i.e. disintegration of the flakes) takes place. This generally succeeds quite well with that from EP'094 known method, according to which in a second drainage unit several times in the remaining part of the fiber suspension Initiates pressure impulses. This is done while the further Drainage and web formation takes place.

The known method and the device used for this have generally proven themselves in practice. Difficulties arise, however, when the composition of the material changes occasionally. This can be caused by an unintentional disturbance in the stock preparation or by an intended change in the paper type or certain paper properties. It is then observed that the entire web formation takes place either too quickly or too slowly, and that consequently the quality, in particular the "formation" of the finished web leaves something to be desired. This problem can also occur in a former with a single-screen pre-dewatering section.

The object of the invention is therefore to ensure that a good "formation" or "review" of the finished web is guaranteed even if a change the composition of the substance.

This object is achieved by the method specified in claim 1 or by the device specified in claim 6 solved. The main idea of the invention is that the following must be observed to solve the task: At the entry point of the two sieve belts into the second dewatering unit the water content of the still existing Suspension as constant as possible, as optimal as possible Have value. It has been found that this is done can achieve that from the second drainage unit escaping amount of water at least approximately constant holds, by influencing the drainage performance the first dewatering unit, e.g. by influencing the vacuum applied to the first drainage unit. With In other words, it is ensured that the dry content, the itself on the border between the first and second drainage units sets a constant as possible (not too high, not too low) value.

Further details and embodiments of the invention will be in the following using exemplary embodiments which are described in the Drawing are illustrated, explained. Each of Figures 1 to 5 shows - in a simplified schematic representation - one of the various embodiments.

The twin wire former shown in Figure 1 has a essentially horizontal twin-wire zone; this comprises three drainage units arranged one behind the other or sections I, II and III. The only partially shown endless sieve belts (lower sieve 11 and upper sieve 12) run in in the immediate vicinity of a headbox 10 via a breast roller 13 or 14, so that the two sieve belts together Beginning of the twin-wire zone a wedge-shaped inlet gap 15 form. The jet of material emitted by the headbox 10 comes only in contact with the two sieve belts 11 and 12 where the lower wire 11 in the first section I of the twin wire zone a stationary, curved forming shoe 16 runs. Whose curved tread is formed with a few strips 16b drainage slots in between. For water drainage serve a line 16a and a container 59. The distance can be varied between the two breast rollers 13 and 14. Of the Forming shoe 16 is preferably operated with negative pressure (Suction line 60 with control valve 61 and suction fan 62).

In the second section II of the twin-wire zone (second dewatering unit) run the two sieve belts 11 and 12 (with the intervening, partly still liquid fiber suspension) between a lower drainage box 17 and an upper drain box 18 therethrough. In the lower drainage box 17 is a series of at least two strips 27 (preferably with an approximately rectangular cross section), from below to the bottom sieve 11 be pressed. They are for this purpose, for example via springs 24 (or via pneumatic pressure pads) on one water-permeable plate 26 supported. It goes without saying that the force of the springs (or that in the pressure pads Pressure) is individually adjustable.

The upper drain box 18 is both at the front also at the rear end, as shown schematically with double arrows, suspended on vertically movable support elements. At the bottom there is a row of at least three strips 28 with a preferably parallelogram cross section, which rest on the top of the upper sieve 12 and are firmly connected to the box 18. Above the strips 28 are a front vacuum chamber in the drainage box 18 21 and a rear vacuum chamber 22 are provided. Suction lines 63 connect the chambers 21, 22 to one Vacuum source 64.

In the area of the forming shoe 16, part of the water is the Fibrous suspension discharged downwards (line 16a); a other part penetrates - due to the tension of the top wire 12 - through the top sieve upwards and through the foremost of the strips 28 into the front vacuum chamber 21 redirected. This water portion reaches the line 21a Container 59.

The water penetrating upwards between the upper strips 28 passes into the rear vacuum chamber 22 and from there Line 22a into a container (e.g. 59 or 59 ').

That between the lower strips 27 through the lower sieve 11 Urgent water is drained down via line 17a.

The amount of water flowing out through line 22a (per unit of time) is kept constant as follows: A control device 66 is assigned to a line 22a Measuring device, e.g. Flowmeter 65, (via measuring line 67) connected, also a setpoint generator 68 (via setpoint line 69). The control device compares the measured value with the setpoint. According to the amount of difference between The control device 66 sends a measured and desired value Manipulated variable (via line 70) to the control valve 61. the amount of water flowing out via line 22a is too small (which a deteriorated "review" of the finished paper Can result), the control device 66 ensures Reduce the vacuum in the forming shoe 16 and thus for reducing the drainage performance of the first drainage unit I. As a result, more water gets back into the second drainage unit II, so that this again for a can provide better formation. Works similarly the control device 66 if the flowing out via the line 22a Amount of water is too large.

In the third section III of the twin-wire zone, both run Sieve belts 12 and 13 over another curved forming shoe 23, which (as shown) preferably in the lower Sieve loop 11 is arranged. Behind this can in the Loop the top wire 12 with an additional bar 29 Vacuum chamber 30 may be provided. In addition, in the Loop of the lower sieve flat suction 31 are provided. There you can (as shown with dash-dotted lines) Upper screen 12 by means of a guide roller 19 from lower screen 11 and be separated from the fibrous web formed. Bottom sieve and the fibrous web then run over a screen suction roll 20. The guide roller 19 can also be further back, so that the top wire 12 only on the wire suction roll 20 from the bottom wire 11 is separated.

It is important that the two drainage boxes 17 and 18 with the alternately compliant and firmly supported last 27 and 28 not in the front or in the back, but in the middle Section II of the twin-wire zone. Because only here they can take full effect, namely intensive drainage the fiber suspension supplied while maintaining the fine, flake-free fiber distribution.

The device according to FIG. 1 is used in many cases modify that (similar to Figure 2) in Section I instead of the stationary formation shoe 16 and the one in front of it Breast roll 13 a known forming roll 40 is provided becomes. You will then make use of this possibility if from the paper making machine in the first place highest productivity is required. It can also be beneficial be, instead of the predominantly horizontal direction of the wire (in the twin-wire zone) a diagonally increasing direction of wire travel to choose, see Figure 4.

In Figures 1 to 3, the distance is in the twin wire zone drawn exaggeratedly large between the two sieves 11 and 12. This is to illustrate that the two Sieves 11 and 12 over a relatively long distance within the Twin wire zone converge to each other and that the process of Web formation on the first forming shoe 16 or on the forming roller 40 (in section I) starts relatively slowly and only ended at the end of Section II or only in Section III becomes. The end of the main drainage zone, in the the two sieves converge towards each other (and thus the end of the web formation process), for example approximately in the Middle of the wrapping zone of the second forming shoe 23 lie, as shown only by way of example in FIGS. 1 to 3 is. The end of the sieve convergence is symbolically there point E shown; there the dryness of the Paper web reached approximately 8%. This point can but also e.g. lie on one of the flat suction cups 31.

The embodiments shown in Figures 2 and 3 differ from the rest primarily in that that the twin wire zone in the wire running direction essentially rises vertically from bottom to top. This simplifies the removal of the water extracted from the fiber suspension; because the water can be largely even on both sides be dissipated. Especially in the middle section II No vacuum chambers are required in the twin-wire zone. However, the forming roller 40 of Figure 2 is a suction roller educated. The formation shoe 23, which in the third section II is arranged, if necessary, also with a suction device be provided.

Further elements of the twin wire former shown in FIG. 2 are water reservoirs 41, 42 and 43, the fixed Last 28 associated baffles 44 and a water drain bar 45. The other elements are the same Reference numerals like the corresponding elements of the Figure 1. The same applies to Figure 3. In particular that Controlling the negative pressure in the forming roller 40 or in Forming shoe 16 takes place in FIG same way as described above with reference to FIG. 1. This also applies to the other exemplary embodiments according to Figures 4 and 5.

Particular features of the construction shown in Figure 4: The forming belts 11, 12 run obliquely from the forming roller 40 up. The upper drain box 18 is weak after bulged at the bottom (with a large radius of curvature R). The control device 66 does not control one arranged in the suction line 60 Valve, but the speed of the motor 61A, which the Suction blower 62 drives (so also in Figure 2 and 5).

Special features of the construction shown in FIG. 5: The forming roller 40 'is not in the lower wire 11, but in Upper screen 12. The forming roller 40 'can (as shown) follow a forming shoe 16 'located in the lower wire; this can also be omitted. The upper drain box 18 is bulged down again. It follows (as in FIG. 4) a separating suction device 23 'located in the lower sieve 11. The Control device 66 controls the vacuum both in the Forming roller 40 'and in the forming shoe 16', if this is available.

Addenda

A. The invention is also applicable in a twin wire former with essentially horizontal wire running direction in the web formation area Similar to Figure 1, but with the section I (the first dewatering unit) a single-screen pre-dewatering section forms. In this case this happens Upper sieve 12 only at the border between sections I and II on the lower sieve 11, more precisely: on the in section I partially dewatered fiber suspension.

B. If it occurs in the control process described above, that when adjusting the drainage performance of the section I (e.g. when adjusting the vacuum) one Operating limit is reached, it is ensured that a signal is given. Because of this signal then for a change in the material supply (e.g. by an intervention on the headbox).

Claims (7)

Method for producing a fibrous web, in particular a paper web, from an aqueous fibrous suspension with the following features: a) the fibrous suspension is fed to a first dewatering unit I (for example section I of a twin-wire zone or single-wire pre-dewatering section) of a twin wire former, with a first part of the water being removed by the wire belts or by one of the wire belts, so that the web becomes closed education begins: b) then the sieve belts (11, 12) with the suspension remaining therebetween and with the web being formed are passed through a second dewatering unit (II) in order to remove a second part of the water, pressure pulses being introduced several times into the suspension in order to to dissolve any flakes of fiber that may have arisen; c) characterized in that the drainage capacity of the first drainage unit (I) is controlled in such a way that the amount of water removed per unit of time (per unit time) is kept at least approximately at an adjustable value. A method according to claim 1, characterized in that the removed by means of the second dewatering unit (II) measures the amount of water over time and the measured value of a Control device (66) supplies, wherein it with a setpoint is compared, the control device at a Deviation of the measured value from the nominal value the drainage performance the first dewatering unit (I) is adjusted in such a way that the measured value approaches the target value. A method according to claim 1 or 2, characterized in that that the first drainage unit (I) with negative pressure acted upon and that one by their drainage performance Varying the negative pressure controls. A method according to claim 1 to 3, characterized in that the two sieve belts (11, 12) in the first Drainage unit (I) via at least one curved one Drainage element (16; 40; 40 ', 16') leads to one Vacuum source (62) is connected. Method according to one of claims 1 to 4, characterized characterized in that one of the two sieve belts (11, 12) in the second drainage unit (II) over some firmly supported ledges (28) leads and at the same time resiliently supported strips (27) on the other sieve belt presses. Device for carrying out the method defined in claim 1, with a twin-wire former which has a first dewatering unit (I),
and comprises a subsequent second dewatering unit (II) which has at least one water discharge line (22a),
characterized by the following features: a) the first dewatering unit (I) has a control device (61, 61A) for varying its dewatering performance; b) the second dewatering unit (II) has on its water discharge line (22a) a measuring device (65) for measuring the amount of water leaving per unit of time. Apparatus according to claim 6, characterized in that a control device (66) is provided with a Actual value input (67) to which said measuring device (65) is connected, as well as with a setpoint input (69), furthermore with an output (70) for a manipulated variable, an adjustment of the drainage capacity of the first Drainage unit (I) triggers such that the Actual value approximates the setpoint.

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