The principal object of the present invention is to provide a novel gap-roll former in which, on a curve sector of the former roll directly following after the forming gap, the dewatering, in particular its distribution between the inner wire and the outer wire, can be controlled better than in the prior art.

It is a particular object of the invention to provide a former in which, on a twin-wire curve sector of the first former roll, the structure of the paper in its z-direction, in particular the structure of the middle layer in the paper, can be influenced more efficiently than earlier.

It is a particular object of the invention to provide a gap-roll former in which, on the first former roll, it is possible to use a larger curve sector, i. e. wrap angle, of the twin-wire zone, the objective being that the range of basis weight of the paper produced by means of the former can be made wider than earlier and that the measures of said wrap angle can be brought to an optimal level more easily.

As is known from the prior art, on said covering sector of the first former roll, a vacuum zone has been employed. By means of gap pressure measurements, it has,

however, been possible to notice that at the point at which the forming wires are separated from the first forming roll, a considerable vacuum peak is formed. The amplitude of this vacuum peak can even exceed the cavitation limit, so that the fibre mesh in the web that is being formed can be destructed. Thus, one of the objects of the present invention is elimination of this drawback so that, in the area in which the wires are separated from the former roll, it should be possible to regulate the pressure and, thus, to control the deceleration of the stock and thereby to act upon the orientation in the web that is being formed, in particular in the middle layer.

In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that on the curve sector of the twin-wire zone on the first forming roll there are at least two successive suction/pressure zones, by whose means a pressure effect that varies in the direction of progress of the paper web can be applied through the perforated mantle of the first forming roll to the paper web that is being formed, so that the dewatering process on said curve sector can be controlled.

In the present invention, as the first former roll, a former roll is employed which comprises at least two successive pressure zones and by whose means it is possible to act upon the structure of the paper in the z-direction, in particular upon the structure of the middle layer in the paper, more efficiently. By means of a former in accordance with the invention, it is possible to manufacture a paper, in particular a fine paper, which has improved properties, for example Scott Bold.

Owing to the present invention, the wrap angle of the twin-wire zone on the first former roll can, as a rule, be dimensioned optimally, because it is possible to use said wrap angles larger than in the prior art and, in this way, the range of basis weight in the former can be made wider than in the prior art.

In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accom- panying drawing, the invention being not confined to the details of the embodiments.

Figure 1 shows an embodiment of a roll-gap former in accordance with the invention in which the twin-wire zone is vertical and rises from the bottom upwards.

Figure 2 is a schematic side view of a twin-wire zone formed in connection with the first forming roll and of suction/pressure zones fitted in the forming roll.

Figure 3 is a schematic illustration of the effect of the vacuum present in the suction/pressure zone of the first forming roll on the height of the water column present in the perforations in the forming roll.

Figure 4 is a schematic illustration of the initial part of the twin-wire forming zone.

Figure 5 is a graphic illustration of the pressure profiles on the first forming roll with a low wrap angle and with a high wrap angle.

In the following, to begin with, reference being made to Fig. 1, a preferred environ- ment of application of the invention will be described, which environment is a gap- roll former, whose twin-wire zone rises from the bottom upwards vertically, and the first former roll or forming roll 11 is placed inside the loop of the covering wire 10.

In this connection, it should, however, already be emphasized that the invention can also be applied to a number of other gap-roll formers, for example to formers in which the first forming roll 11 is placed inside the loop of the carrying wire 20 and/or the twin-wire zone is horizontal, upwards or downwards inclined.

As is shown in Fig. 1, a stock suspension J is fed through the slice cone 35 of the headbox 30 into the forming gap G defined by the wires 10 and 20. The slice cone 35 of the headbox includes vanes 36, which extend to the vicinity of the slice opening and provide the stock jet J with an adequate level of micro-turbulence as it is discharged into the forming gap G. The forming gap G is defined between the carrying wire 20 coming from the guide roll 21 and the covering wire 10 curved on the first forming roll 11. After the forming gap G, there starts directly a twin-wire curve sector a, in which there are at least two suction/pressure zones lla, llb one

after the other. Of these zones, the latter zone l lob extends preferably at least partly into the opening wedge space or nip No placed between the mantle 12a of the forming roll 11 and the wires 10,20. The covering wire 10 is guided by its guide rolls 18, and the carrying wire 20 is guided by its guide rolls 21 and 28.

The diameter D1 of the first forming roll 11 is, as a rule, in a range D 1 1200... 2000 mm, preferably in a range D = 1400... 1800 mm. The width of the <BR> <BR> <BR> curve sector a is, as a rule, in a range a = 0°... 90°, preferably in a range a z<BR> <BR> <BR> <BR> <BR> 10°... 45°.

As is shown in Fig. 1, the first forming roll 11 is followed by a first forming shoe 22, which has a ribbed deck 22a with a curve radius R1. After this, there follows a MB unit (loading-element unit) 50, and after that a second forming shoe 24 provided with a curved ribbed deck 24a, and after this a second forming roll 29. In Fig. 1, the loading element 13 of the MB unit 50 is placed inside the loop of the covering wire 10, and the support element 23 is placed inside the loop of the carrying wire 20, but corresponding elements can also be placed inside the opposite wire loops 20,10.

In Fig. 1, the first forming roll 11 is placed inside the loop of the covering wire 10, and the second, upper forming roll 29 is placed inside the loop of the carrying wire 20. Both forming rolls 11,29 can also be placed inside the same wire loop. The suction zone 29a of the second forming roll 29 ensures that, after it, the web W follows the carrying wire 20, which is guided by the guide rolls 28 and on which the web W is carried onto a pick-up roll 41. On the suction zone 41a of this roll 41 the web W is transferred onto a pick-up fabric 40, which carries the web W into the press section (not shown). The diameter D21 of the second forming suction roll 29 shown in Fig. 1 is, as a rule, chosen in a range D21 == 800... 1800 mm, preferably in a range D21 = 1000... 1800 mm As is shown in Figs. 1 and 4, on the curve sector a and on a narrow sector in the area of the bottom of the nip space No which is opened at least partly after said

curve sector a, there are two successive zones 11 a and 11 b. In the first zone 11 a, preferably a vacuum is present, so that draining of water takes place to some extent also through the covering wire 10. The suction sector 11 a is followed by a narrow pressure sector 1 lob, in whose area draining of water takes place through the carrying wire 20 only. By means of selection or regulation of pressures in the zones 11 a, 11 b, it is possible to act efficiently upon the dewatering directions and, thus, also upon the properties of the web in its z-direction, for example upon orientation in layers.

By means of the pressure zone l lb, formation of a detrimentally high vacuum peak in the opening nip space No is also prevented. This phenomenon will be described later in more detail in connection with the graphic illustration in Fig. 5.

As is shown in Fig. 2, the first forming roll 11 is provided with three successive zones I la, 1 lb, 1 Ic, whose pressure levels can be regulated by means of a suction/- pressure device 60 by the intermediate of the ducts 60a, 60b, 60c. The zones 1 la, 1 lob, lie are defined by sealing ribs 12c fitted inside the mantle 12a of the forming roll 11, which ribs run against the inner face of the mantle 12a. As is shown in Fig. 2, in the first zone lla directly following after the forming gap G, there is a suitable pressure Pa, in the next zone a pressure Pb, and in the last zone a pressure pu.

Said pressures Pa, Pb, Pc are, as a rule, chosen in a range-40... +20 kPa, preferably in a range-25... + 10 kPa. The last zone 1 lc is placed after the line K on which the wires 10,20 are separated from the roll mantle 12, on a narrow sector, preferably so that, by means of the pressure Pc present in the zone lie, the vacuum that tends to be induced otherwise in the opening wedge-shaped nip space No between the roll mantle and the wires 10,20 is eliminated or reduced (vacuum zone al2 in Fig. 5).

When a pressure Pas Pb or Pc is employed, said pressure is, however, such that by its means, as a rule, the vacuum pulse arising from opening is just attenuated, so that draining of water can also take place upwards, however, not to such an extent as would take place without said pressure.

By means of regulation or setting of the pressures in said zones 1 la, 1 lb, 1 Ic, it is possible to affect the distribution of the draining of water in the area of the curve sector a and directly after that highly efficiently, so that the properties of the web W

that is being produced in the z-direction can be regulated better than earlier in compliance with the requirements of quality imposed on the paper. This regulation is particularly efficient, because in the forming gap G, on the curve sector a, and directly after said sector, the major part, as a rule about 70... 80 %, of the overall quantity of water contained in the discharge jet J is removed.

It comes out from Fig. 3 in what way a vacuum Pt present inside the mantle 12a of the forming roll 11 and in the perforations 12b in said roll mantle 12a affects the height of the water column VP present in the perforations 12b. When Pt < 0, i. e. there is a vacuum in a zone in the interior of the forming roll 11, for example in the zone llb, the water column VPI is almost equal to the thickness do of the roll mantle 12a. When Pt = 0, the height of the water column VP2 is about one half of the thickness do of the roll mantle 12a, and when Pt > 0, the height of the water <BR> <BR> <BR> <BR> column VP3 is little or = 0, in which case the draining of water takes place almost exclusively through the outer wire 20, as is the case, for example, in Fig. 2 in the area of the sector 1 la. The water that has been removed out of the web W through the perforations 12b in the mantle 12a of the forming roll 11 is cast out of the perforations by the effect of centrifugal forces outside the curve sector a.

Fig. 5 is a graphic illustration of the dewatering taking place on the curve sector a of the forming roll 11 in a gap-roll former as shown in Fig. 4; the vertical axis represents the dewatering pressure ve, and the horizontal axis represents the wrap angle of the twin-wire zone on the forming roll 11. The dewatering taking place within the sector a can be divided into three separate stages. In the first stage alo, the water is removed mainly through the outer wire 20 starting from the point of impingement of the slice jet J. Owing to slowing down of the slice jet, a little positive pressure peak is produced, which exceeds the tensioning pressure P = T/R of the outer wire 20, in which equation T is the tension of the outer wire 20, and R is the radius of the forming roll 11. After this, a fibre mesh mat is drained rapidly on the face of the outer wire 20, which mat increases the resistance to draining of water through the outer wire 20. In the second stage a1l, the dewatering pressure P is substantially invariable P = T/R. After the second stage a1 l, there follows a third

stage al2, in which, with a small measure of the curve sector a = a1, there is a considerably high vacuum peak-Pkl. This vacuum peak-Pkl can be so high that it exceeds the cavitation limit and causes destruction of the fibre mesh that is being formed. In the present invention, this can be prevented by means of a pressure zone l lb, l lc as shown in Fig. 4, air being introduced by means of the pressure +Pb, +Pc present in said zone into the opening wedge-shaped nip space No between the wires 10,20 and the roll 11 mantle, and in this way it is possible to reduce the amplitude of the vacuum peak-Pkl. When a larger wrap sector a = a2 is employed, the vacuum peak-Pk2 is lower, in which case, with a vacuum sector 1 lb as shown in Fig. 4 or with a pressure sector l lc as shown in Fig. 2, an equally high pressure is not needed.

As is known from the prior art, in the area of the curve sector a and in the twin-wire zone following directly after said sector, the dewatering can be partly controlled by means of choice of the ratio of the speed of the slice jet J to the speed of the wires 10,20. Since, in particular in the area of the curve sector a, a thin fibre mat is drained quite rapidly on both faces of the wires 10,20 whereas the middle layer remains more freely mobile, by means of choice of said speed ratio, the middle layer can be either decelerated or accelerated in relation to said drained outer layers. In this way it is possible to act upon the properties of the web, in particular upon the formation of layers in the orientation in the z-direction.

In the following, the patent claims will be given, and different details of the inven- tion can show variation within the scope of the inventive idea defined in said claims and differ even to a considerable extent from the exemplifying embodiments described above.