The invention concerns a twin-wire roll-gap former in a paper machine, which former comprises a covering wire guided by guide rolls and a carrying wire guided by guide rolls, from which carrying wire the web that has been formed is removed onto a pick-up fabric or equivalent, and which former comprises a forming gap, into which the stock suspension jet can be fed out of the headbox and which forming gap is defined while guided by the rolls placed inside the loops of said wires, and which wires define a twin-wire zone, in which there are at least two successive water draining and web forming units, of which units one unit is a rib loading unit which comprises a set of guide and support ribs as well as a set of loading ribs operating opposite to said set of guide and support ribs, by means of which said sets or ribs it is possible to produce a controlled loading pressure which presses said wires against each other, and which water draining and web forming units comprise a vacuum box or boxes.

With increasing running speeds of paper machines, several problems in web forma- tion have come out with higher emphasis. In the former section of a paper machine, phenomena that act upon the fibre mesh and upon the water that is still relatively free in connection with said fibre mesh, such as centrifugal forces, are intensified, as a rule, in direct proportion to the second power of the web speed. The highest web speeds of many modern paper machines are of an order of 1600 metres per minute (m/min). Machines are, however, being planned in which a web speed of up to about 2000 metres per minute or even higher is aimed at.

With respect to the prior art most closely related to the present invention, reference is made to the applicant's following patents and patent applications FI Patent 91,788, FI Patent Applications 934667 and 972640 and to the published EP Patent Applica-

tion 0454989 of Sulzer Escher Wyss GmbH and to the published DE Patent Applica- tion 19632509 Al of Voith Sulzer Papiermaschinen GmbH.

One significant problem which occurs in the prior-art gap formers in whose twin- wire zone a prior-art web forming and water draining unit or units is/are used is unequalsidedness of the paper produced, in particular in respect of the distribution of fines and fillers. This drawback comes from the fact that, in the twin-wire zone, readily mobile fillers tend to gather at the side of the wire through which a vacuum is effective. In prior-art units of said sort, the vacuums in successive suction boxes act, as a rule, from the side of the same wire.

Said unequalsidedness also has unfavourable effects on other properties of the web, such as smoothness of the paper faces and oil absorbancy of the paper. The draw- backs referred to above partly arise from the fact that, in prior-art gap formers, said units have usually been placed in an area in which the consistency of the stock layer that is being formed has been decisively higher than the headbox consistency, i. e. clearly higher than about 1.5 %, so that, in the area of the rib loading unit, it has no longer been possible to apply a major effect on the distribution of fines and fillers.

Thus, the principal object of the present invention is development of the prior art coming out from the papers cited above and development of corresponding prior art so that the drawbacks discussed above can be substantially eliminated.

It is an object of the present invention to provide a former with which it is possible to produce a paper which is highly symmetric in the z-direction and which has good formation. Symmetry of the structure of paper and equal good quality of both faces of paper are required in particular from printing and writing papers, and the former in accordance with the present invention is supposed to be particularly well suited for manufacture of such papers.

It is a further object of the invention to provide a former in which the twin-wire zone can be controlled in a very precise and stable way in order to secure uniformity of the profiles of the web in the cross direction.

It is an object of the present invention to provide a roll-gap former that is suitable for ever higher speed ranges, which are of an order of 1500... 2000 metres per minute and even higher.

In relation to this, it is an object of the invention to provide a former which has sufficiently efficient water drain properties so that, while the speed of the former is increased, for example, from 1500 m/min to a speed of about 2000 m/min, however, a former of reasonably short length is obtained so that, in spite of said increase in speed, the length of the former is not increased, at least not to a significant extent, or that the length becomes even shorter in preferred embodiments of the invention.

Thus, it is an object of the invention to provide a former in which the length of the twin-wire zone can be made considerably shorter or at least be kept substantially unchanged ion spite of a considerable increase in the web speed.

In relation to the above, it is an object of the present invention to provide a former that can be accomplished as a construction of lower height, as compared with the prior art, so that, for example, in connection with modernizations, a new former can be fitted in the place of an earlier fourdrinier wire part without necessity to increase the height of the paper machine hall. On the other hand, in the invention, it is an aim of the compact construction of low height that, since high and massive frame parts of the former can be avoided, the frame can be made more stable at reasonable cost, so that problems of vibration are also reduced.

It is also an object of the invention to provide a former in which the large quantities of draining of water arising from the high speed of the former can be removed out of the former more favourably than in the prior art without unreasonable formation of splashes and/or water mist.

It is a further object of the invention to provide a former in which the construction of the area of the carrying wire placed after the twin-wire zone, in which there are the web pick-up means, can be modernized so that, in respect of this area, a former is obtained which is of reasonably short length in the machine direction and in which the web has a more linear draw than in the prior art, which for its part permits high speeds and a higher dry solids content of the web or at least an unchanged dry solids content of the web in spite of a considerable increase in the web speed.

It is also a further object of the invention to provide a former in which it is possible to take advantage of the relatively high kinetic energy of the large quantities of water drained from the web without having to raise the large quantity of water removed from the web to a substantial extent by means of relatively expensive suction energy in a direction opposite to the direction of the force of gravity.

Even though it is an important object of the present invention to provide a former in which the run of the twin-wire forming zone is substantially vertical running from the bottom towards the top, the ideas of the invention can also be applied in formers in which the twin-wire zone is substantially horizontal, for example, when an existing fourdrinier wire part is modernized and its constructions are used in the constructions of the lower wire and the carrying wire in the new twin-wire former.

In view of achieving the objectives stated above and those that will come out later and in view of eliminating or substantially avoiding the drawbacks discussed above, the former in accordance with the invention is mainly characterized in that, after the forming gap confined to the area of the first forming roll, the twin-wire zone comprises the following web forming and water draining elements: (a) a curve zone a fitted on the first forming roll, preferably substantially on its suction zone, the magnitude a of the sector of said curve zone a being chosen in a range a = 0°... 60°, preferably in a range a z 10C. 45°;

(b) a straight run that starts from said curve zone a, within which straight run there are (bl) a pre-loading element or elements, which is/are placed at a short distance from said curve sector a inside the loop of the carrying wire and which element/elements is/are preferably a rib element/elements whose loading force can be regulated, (b2) a rib loading unit, which guides the twin-wire zone substantially along a straight run and in which there are (b21) a set of ribs placed inside the loop of the covering wire, and (b22) a suction box placed inside the loop of the carrying wire and loading elements placed inside said suction box, by means of which loading elements said linear run can be loaded against said set of ribs; (c) a second forming-suction roll or an equivalent static suction box, by means of whose suction zone it is ensured that the web is separated from the covering wire and follows the carrying wire.

In a particularly advantageous embodiment of the invention, the web formation and water drain elements (a), (b), (bl), (b2) and (c) defined above and placed after the forming gap in the twin-wire zone are placed in the sequence given above.

By means of the present invention, a possibility has been created to provide a very compact former, whose height is lower than the height of a corresponding prior-art former and whose length is not increased or becomes even shorter as compared with prior-art former concepts even if the web speed were increased substantially.

In the present invention, in the relatively short twin-wire zone, the draining of water is highly intensive and symmetric so that the web obtains a structure symmetric in the z-direction and an adequate content of fines and fillers in both of the opposite

faces, which is important in particular in the case of writing and printing papers, for whose manufacture the former in accordance with the present invention is supposed to be particularly well suited.

In the invention, an important parameter in the web formation is the vacuums in the different forming elements, which vacuums act in the twin-wire zone preferably alternatingly from opposite sides of the web, so that symmetric draining of water and the other effects aimed at can be achieved.

In the present invention, the rib loading unit, which was earlier also called MB unit, has been placed in the twin-wire zone in an area in which the dry solids content of the web that is being formed is still sufficiently low so that, by means of varying pressure impulses produced by means of said rib loading unit, it is possible to affect the draining of water and the formation of the web so that the objectives stated above are carried into effect in this respect.

The invention is also related to a novel geometry of the carrying wire and the pick- up area after the twin-wire zone, which geometry, for its part, reduces the total length of the former and of the press section and by means of which geometry both a compact construction and a draw of the web as linear as possible from the former to the press section without a risk of web breaks even at high web speeds are achieved, which factors constitute objects of the present invention.

In the following, the invention will be described in detail with reference to some preferred exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments.

Figure 1 is a schematic side view of a preferred embodiment of a former in accord- ance with the invention.

Figure 2 is an illustration more detailed and on a larger scale than Fig. 1 of a preferred embodiment of a twin-wire former.

Figure 3 illustrates an embodiment of the first forming roll and of a water drain box provided with a static ribbed shoe and placed above said forming roll.

Figure 4 is a schematic side view of an application of the invention in a former whose former part is substantially horizontal.

Figs. 1 and 2 illustrate a roll-gap former in accordance with the invention, in which the twin-wire zone is substantially vertical running upwards. Being guided by the rolls 11,21, the covering wire 10 and the carrying wire 20 form a forming gap G between them. Into this forming gap G, a stock suspension jet J is fed out of the slice channel of the headbox 15. After the bottom of the forming gap G, the twin- wire zone is curved on the sector lla of the first forming roll 11, the magnitude a of said sector being, as a rule, in a range a = 0... 60°, preferably a = 10... 45°.

The length of the sector lla has been chosen, as a rule, in a range 100... 700 mm, preferably in a range 200... 400 mm After said sector lla, the runs of the wires 10,20 are separated from the forming roll 21 and continue their run as a short straight run while guided by an adjustable pre-loading element 28 placed inside the covering wire 20. The length LS of said straight twin-wire run is preferably in a range LS = 200... 500 mm. The pre-loading element 28 acts in the area of the middle portion of said straight run. The first pre- loading element 28 is followed, in the twin-wire zone, inside the loop of the covering wire, by a water drain box 17, in which there is a ribbed shoe 18L1... 18LN. The number N of the ribs 18L is, as a rule, in a range N ~ 4... 10, preferably N = 5... 7.

The ribs 18L1 N are static, and they guide the twin-wirs zone as a straight run.

Opposite to the static set of ribs 18L1 N, there is a suction box 27, in whose interior there are loading elements 29, which have been illustrated schematically by

means of arrows only. The loading elements 28 and 29 are provided with a rib part, whose wear-resistant front face rubs against the inner face of the carrying wire 20.

The friction between the front face of said rib and the inner face of the wire 20 and the wear of the parts placed against each other are reduced partly by the water drained from the web. These loading elements 29 act, in a way in itself known, preferably in the middle areas of the gaps between the ribs 18L with an adjustable force F. Thus, in a configuration as shown in Fig. 2, the number of the loading elements 29 is N-1, i. e., as a rule, in a range 3... 9, preferably 4... 6. The loading elements 29 are most appropriately loaded with a pneumatic pressure medium, if necessary, so that the force of each of the different loading elements 29 can be adjusted independently. The construction of the loading elements 29 is preferably such that their loading force remains substantially unchanged irrespective of the position of the element. With respect to the details of the construction of such a loading element, reference is made to the applicant's FI Patent 95,935. <BR> <BR> <BR> <BR> <BR> <BR> <P>The first static rib 18L1 of the box 17 is placed at the distance of a short gap ~ LS/2 from the adjustable pre-loading element 28, and it is placed at the front edge of the curved wall 17a of the suction box 17. The water drain boxes 17,27 with their sets of ribs 18L and 29 form a rib loading unit 40, which has also been called MB unit in the applicant's earlier patent applications. This rib loading unit 40 guides the twin-wire zone precisely and in a stable way so that, in respect of this unit 40, the cross-direction profiles of the web W can be made sufficiently uniform.

The linear set of ribs 18L, 29 is followed by a forming suction box 19 fitted inside the loop of the covering wire 10, in which box 19 there is a static set of guide ribs 19L. This set of ribs 19L guides the twin-wire zone 10,20 to be curved with a curve radius Ri towards the loop of the covering wire 10. The curve radius Ri is, as a rule, chosen in a range R1 = 2000... 10,000 mm, preferably in a range R1 ~ 5000... 8000 mm. After the last rib 19L, at a short distance, there follows a curve sector of the twin-wire forming zone 10,20 placed on the suction zone 23a and 23b of the second forming suction roll 23. By means of a vacuum present in the latter suction zone 23b of the roll 23, it is secured that the web W is separated from the

covering wire 10 and follows the carrying wire 20. The magnitude of the suction sector lazzis about al z 90°, and the magnitude of the suction sector a2 is about a2 z 30°, in which case, on the second forming suction roll 23, the twin-wire zone is curved about 90° and turns on the suction sector 23b into a downwards inclined run at an angle b z 20... 40°.

On the run 20A of the carrying wire 20, which is downwards inclined at said angle b, near the pick-up point P, there is a suction flatbox 23A, by whose means, in respect of the former, the ultimate dry solids content of the web W is produced, <BR> <BR> <BR> <BR> which is of an order k6 = 15... 30 %. The pick-up point P is placed at a very short distance before the wire 20 guide roll 22a. At the pick-up point P the web W is transferred as a relatively linear run without major changes in direction onto the pick-up felt 30 of the press section on the suction zone 31a of the transfer suction roll 31. The angle c of the change in the direction of the web taking place at the pick-up point P is relatively little and substantially smaller than in the prior-art pick- up arrangements. Said angle c of change in direction is, as a rule, in a range c = 5... 30°, preferably c = 10... 20°. This for its part secures a reliable transfer free of disturbance of the web W from the forming wire 20 onto the pick-up felt 30 even at high web speeds and even at speeds of an order of 2000 m/min or higher. The fact that the pick-up point P has been fitted on said downwards inclined run 20A of the carrying wire 20, for its part, makes the total length of the former and of the following press section shorter and makes the construction more compact. After the guide roll 22a, the lower wire 20 has a drive roll 22b, after which the carrying wire 20 continues its run, being guided by the guide roll 22, onto the breast roll 21. The covering wire 10 is guided outside the twin-wire zone by the first forming roll 11, by the drive roll 13, and by the guide rolls 12. The suction zones on the second forming roll 23 are covered by a water drain channel 13 a, which communicates with a water drain trough 13b.

In the following, with reference to Figs. 1,2 and 3, the construction and operation of the water drain equipments placed in the twin-wire zone will be described in more detail.

As is shown in Figs. 1 and 2, the curved wall 17a of the water drain box 17 placed inside the loop of the covering wire 10 defines an intermediate space RR between said wall and the perforated mantle 11'of the forming roll 11. The radial measure of said intermediate space is of an order of AR == 50... 200 mm. Through this intermediate space RR, a substantial amount of water is removed through the wire 10 out of the perforations 11'on the roll 11, which draining of water is illustrated by the arrow DW2. Before that, a large proportion of water has been drained in the area of the suction zone 11 la of the roll 11 on the curve sector a through the carrying wire 20, which is illustrated by the arrow DW1. In the vicinity of the rib 18L1, into the front wall of the wall 17a of the box 17, a gap 17R has been made, which extends across the entire width of the machine. This gap 17R operates as an ejector and removes water from the interior of the box 17. This draining of water is illustrated by the arrow DW4, which is caught along with the water flow DW2 departing at a high velocity by means of an ejector effect, and the water flow DW2 + DW4 formed in this way is removed through the water drain duct 14a into a water drain channel 16a passing to the other side of the machine. In the draining of water DW2 + DW4, the kinetic energy of the water is utilized by passing the water further into the duct 14a and into the water drain channel 16a. In such a case, the large quantities of water DW2 + DW4 that are removed, which are of an order of ~ 100 litres per second per metre (1/s/m), need not be raised against the force of gravity by means of suction energy, and the draining of water takes place without splashes and without unreasonable formation of water mist.

Similarly, the draining of water taking place through the carrying wire 20 in the areas of the forming gap G and of the curve zone a is illustrated by the arrow DW1.

The waters DW1 are passed, in the way shown in Fig. 1, through a downwards inclined duct 24a into a cross-direction channel 26a without undue splashing or water mist. The suction box 27 of the rib loading unit 40 has been connected by means of a duct 27a to a suction source 45, which is illustrated schematically. The draining of water taking place from the suction box 27 is illustrated by the arrow DW3. The waters DW3 pass, in the way shown in Fig. 1, through a downwards inclined duct 24b into a channel 26b.

On the curve sector a on the roll 11, water is removed by the effect of the tensioning pressure P = T/Ro (T = tightening tension of the carrying wire 20, and Rp = radius of the roll 11) of the carrying wire 20 through both wires 10 and 20, which draining of water through the wire 10 is promoted by a vacuum po in the suction sector lla of the roll 11, and the draining of water DW1 taking place through the wire 20 is promoted by the centrifugal forces on the curve sector a. The vacuum po in the suction zone 11 of the roll 11 is, as a rule, chosen in a range po 0... 25 kPa.

As has come out from the above, the zones subjected to a vacuum Po, pi, P2, P3, P4 are placed alternatingly at both sides of the web, which for its part produces a symmet- ric draining of water and a sufficiently high content of fines and fillers at both of the opposite faces of the web W that is being formed.

If the different water drain proportions DW1: DW2: DW3: DW4 are compared with each other, their relative ratios are of an order of 4: 4: 1: 1. If necessary, the water drain box 17 can also be connected to a suction source, such as to the last suction box 19, which has been connected through the duct 19a above the wall 17b between the boxes 17 and 19. Said draining of water is illustrated by the arrow DW5 in Fig.

2, and these waters DW5 are passed through the downwards inclined duct 14b into the channel 16b, which is illustrated in Fig. 1.

In the twin-wire zone of a former of the sort shown in Fig. 1, the development of the dry solids content k of the web is typically as follows. The consistency ko of the stock discharge jet J is typically ko = 0.7... 1.5 %. The consistency of the stock layer after the curve zone a on the roll 11 is kl = 1.5... 2.5 %. At the first rib 18L1 in the set of ribs 18L, the dry solids content k2 of the web is k2 =--'1.1. 6... 2,8 %, <BR> <BR> <BR> <BR> after the rib loading unit, k3 = 4.0... 6.0 %, after the forming suction box, k4 z 6... 9 %, after the latter suction zone 23b on the roll 23, k5 = 11... 14 %, and in respect of the former, the ultimate dry solids content directly before the pick-up <BR> <BR> <BR> <BR> point is k6 = 15... 22 %. As comes out from the example described above, the dry solids content of the web W at the time of its arrival in the rib loading unit 40, k2

1.6... 2.8 %, is so low that in the area of the set of ribs 18L/29, a substantial amount of water is still drained out of the web W by the effect of the vacuum P1 in the suction box, and a sufficient amount is also drained through the carrying wire 20, so that the web W can obtain a symmetric structure and a sufficiently high content of fines and fillers on both of its faces. By means of a rib loading unit 40 fitted in accordance with the invention, the web W can be given good formation and a symmetric structure partly because the set of ribs 18L/29 causes pressure pulsation in the web, and the effect of said pulsation is increased further at high speeds.

Further, by means of the loading element 28 and the set of ribs 18L/29, a stable run is obtained for the twin-wire zone, even though said run is substantially linear in the area of said elements.

As is seen best from Fig. 2, the construction of the twin-wire zone is particularly compact, and the efficiency of dewatering in said zone is high, which factors contribute to a sufficiently short twin-wire zone even at high web speeds and to a reasonable height H of the former.

As can be seen directly from Fig. 1, the maximal height H of the former shown in it is relatively little. As a rule, said height H is H ~ 7... 8 m. The length of the former in the machine direction up to the pick-up point P is L = 15... 18 m. The length LW of the twin-wire zone from the forming gap to the point at which the web W is separated from the covering wire 10 is, as a rule, Lw « 9... 13 m.

In Fig. 2, the pressurized forming zones are shown with a darker shading, and, thus, from Fig. 2, the significant feature of a preferred embodiment of the invention comes out directly that the vacuum zones 10a, 29,19,23a+23b are placed alternatingly inside the opposite wire loops 10,20. This partly promotes the symme- try of dewatering, and by means of regulation of the vacuums Po, PI, P2, P3 and p4 in said zones it is possible to act efficiently upon the dewatering and the web formation in the twin-wire zone.

Fig. 2 illustrates the, based on present-day knowledge, preferred former geometry in accordance with the invention when the invention is applied to a vertical former.

However, within the scope of the inventive idea, many different modifications of the concept as shown in Fig. 2 are possible. It is one possible modification that the forming suction box 29 is arranged as filled with water, and the draining of water from said box is controlled by means of a throttle valve which is fitted in connection with the water drain duct departing from the box 29. It is a second modification that the water drain boxes in the rib loading unit 40 are also connected to a suction source 45, in which way draining of water through the covering wire 10 is pro- moted.

In the following, reference being made to Fig. 4, a preferred exemplifying embodi- ment of a horizontal version of a roll-gap former in accordance with the invention will be described. The lower-wire loop 20 is guided by the breast roll 21, guide rolls 22, by a wire suction roll 22a provided with a suction zone 22aa, and by a drive roll 25. The loop of the upper wire 10 is guided by guide rolls 12 and by the first forming suction roll 11. The initial parts of the water drain elements in the twin-wire zone are substantially similar to that described above and comprise a first adjustable loading element 28, a rib loading unit 40A, in which there are loading elements 29 which load the twin-wire zone against a stationary set of ribs 18L. After this, there follows a set of ribs 19L, which curves the twin-wire zone with a large curve radius R1. At the end of the twin-wire zone, inside the lower wire 10, there is a suction box 23B provided with a curved R2 ribbed deck, which suction box secures that the web W is separated from the upper wire 10 and follows the lower wire 20 on its horizontal plane run 20B, on which there can be further water drain elements (not shown) placed inside the wire 20. After this, the web W is curved over the suction zone 22aa of the wire suction roll 22a to the pick-up point P.

As is shown in Fig. 4, inside the loop of the upper wire 10, three water drain boxes 17A, 19A, 19B have been fitted. Out of these, the last box 19B or all the boxes 17A, 19A, 19B have been connected to a suction source 45. Through the box 17A, those waters are drained that flow, by the effect of dynamic forces, through the intermedi-

ate space RR further through the opening 17B into the box 17A. Through the next box 19A, the waters are drained that come in the ribbed zone 18L/29 through the upper wire 10, and through the last box 19B, the waters are drained that are removed through the intermediate spaces in the curved R1 set of ribs 19L and through the upper wire 10. In the other respects, the construction and the operation of the twin-wire zone in the former are substantially similar to those described above in relation to Figs. 1 to 3.

In the following, the patent claims will be given, and the different details of the invention can show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.