The present invention relates to a dewatering arrangement adapted to use in a twin- wire dewatering section of a stock processing machine, comprising a single-ply head box for supplying stock through a nozzle with an inlet fed by a tube bank with tubes of non-circular cross-section, a closed loop first wire adapted to receive the stock from the head box on its upper surface and with pressure pulse generating blades mounted inside the loop, a closed loop second wire adapted to be fed on top of the first wire, and with pressure pulse generating blades mounted inside the loop, thereby pressing the stock between itself and the first wire.

Background

In a twin-wire roll former, the fibre suspension is dewatered by the pressure generated below the tensioned, curved outer wire. The pressure event can include some fluctuations, but it is in principle mainly constant. This means that much of the fibre flocculation in the headbox jet will remain in the formed paper products.

In a twin-wire blade former, the fibre suspension is dewatered by pressure pulses, generated by local wire deflections over solid blades. These blades thus create a pulsating dewatering pressure, which can improve large-scale formation by floe stretching/breaking effects.

Trials have shown the difference in sensitivity to the headbox jet stability in twin-wire roll and blade forming respectively. With a headbox producing a rather unstable jet the critical sensitivity to jet quality in blade forming compared to that in roll forming shows clearly.

With a more stable jet, well-formed paper products without formation defects were generated in blade forming.

In paper board forming, the separate layers are placed one at a time on a first wire each by a separate headbox and dewatered each by a secondary wire supported and dewatered by blades on the upper side of the secondary wire and by movable blades on the lower side of the first wire. The dewatering is effected by pressure pulses like the dewatering process in the blade former previously mentioned.

Formation problems in industrial twin wire forming In blade forming, the large-scale formation damages can be reduced, and sometimes even avoided, by the choice of a high jet angle relative to the blade-side wire. However, this also has some negative effects on small-scale formation. In pure roll forming, moderate headbox jet angle changes have an insignificant effect on paper formation.

A new design of twin-wire dewatering is previously known which is a combination of initial roll dewatering (low sensitivity to jet instabilities and jet angle), and final blade

dewatering (for large-scale formation improvement). The basic idea is to combine the advantages of the two dewatering principles, while avoiding their drawbacks.

Combined roll/blade-forming is now since many years the standard design for highspeed printing paper machines. At speed in excess of ca 1250 m/min, the industrial experience is that only very limited amounts of fibre suspension may remain between the wires when they leave the forming roll, if formation damages are to be avoided. Even a suspension thickness as small as 1 mm between the wires leaving the forming roll, may cause formation damages at high speeds. This has enforced the application of decreasingly small headbox slice openings, in the order of 5-6 mm, most of which is then dewatered already in the roll part. Such small slice openings in turn means comparatively high forming concentrations, with reduced potential of paper strength and formation. A certain jet angle against the outer wire at the entrance to the roll unit does reduce the formation damages in roll-blade forming, and is therefore applied in industrial roll/blade formers.

The paper machine manufacturers have not provided any relevant explanations of the physical background for the formation damages in high-speed roll/blade forming.

A similar problem with formation damages also occurs in paper board machines. Here the separate layers are placed one at a time on a first wire each by a separate headbox and dewatered each by a secondary wire supported and dewatered by blades on the upper side of the secondary wire and by movable blades on the lower side of the first wire. The pressure pulses from the blade dewatering can cause serious formation damages, like in the roll/blade forming case mentioned above.

In a modern headbox, a tube bank, delivering one individual jet from each tube in the bank, feeds the nozzle. The individual tubes are separated by solid boundaries, downstream of which flow wakes will form. These wakes gradually disappear along the nozzle contraction. When the flow is accelerated along the nozzle, the pressure will drop and the velocity profile across the nozzle will gradually get more even.

The larger the nozzle contraction ratio (upstream headbox nozzle height divided by slice opening), the more even the flow profile at nozzle exit and the more stable the exiting jet. It is then obvious that the larger the inlet height of the nozzle, the higher will the contraction ratio be at a given slice opening and thus the more stable the headbox jet.

Further, the smaller the slice opening with a given inlet nozzle height, the better will the jet stability become.

In today's high speed printing paper machines (newsprint, SC paper, LWC paper and fine paper) of the roll/blade type, the headbox nozzles have a comparatively small inlet height, not above 130 mm. (This has enforced comparatively small slice openings, to avoid formation damages, in the order of 5 mm in newsprint production and 10 mm in copy paper production.)

This small nozzle inlet height seriously limits the possibility to create an acceptable jet stability to avoid formation damages in machines using roll/blade forming.

One aim of the present invention is thus to provide an apparatus capable of lessening the risk of formation damages. This aim is achieved by that the inlet height of the headbox nozzle, defined as the total tube bank height minus the total upstream thickness of eventual nozzle vanes, is at least 150 mm.

A further advantageous embodiment of the present invention discloses an inlet height of the headbox nozzle that is at least 200 mm. A further advantageous embodiment of the present invention discloses an inlet height of the headbox nozzle that is at least 275 mm.

An especially advantageous embodiment of the present invention discloses a roll/blade apparatus where the distance from the wire separation line on the roll to the first blade on the roll side of the wires is less than 100 mm. An especially advantageous embodiment of the present invention together with other embodiments and advantages will be described in further detail in the following description with reference to the enclosed drawings.

Fig. 1 discloses a headbox according to a dewatering arrangement of the present invention. Fig. 2 discloses a fundamental view of an apparatus according to a first embodiment of the present invention involving a roll/blade former.

Fig. 3 discloses a detailed side view of a blade according to a dewatering arrangement of the present invention.

Fig. 4 discloses a side view of a tip of a blade according to a dewatering arrangement of the present invention.

Fig. 5 discloses a fundamental view of an apparatus according to a second embodiment of the present invention involving a paper board former.

Fig. 2 discloses a part of a twin-wire forming apparatus with a forming roll 1 , a first wire 2 partly wrapped directly around the forming roll 1 , a second wire 3 wrapped around the forming roll lying on the first wire 2. Before the forming roll 1 between the two wires 2, 3, a head box 4 is arranged with its opening directed substantially in the feeding direction of the wires 2, 3. The head box feeds stock to the nip between the wires 2, 3 wrapped around the forming roll 1. By changing the angle of the opening of the head box 4 against this nip it is possible to change the direction of the stock fed to the forming apparatus. In fig. 1 the headbox 4 has a tube bank 6, from which is delivered one individual jet of fibre suspension from each tube 7 in the tube bank 6, which feeds a nozzle 8 in the headbox. The individual tubes 7 are separated by solid boundaries 9, to each of which is

arranged a vane 14, downstream of which flow wakes will form. These wakes gradually disappear along the nozzle contraction. When the flow is accelerated along the nozzle 8, the pressure will drop and the velocity profile across the nozzle 8 will gradually get more even.

The larger the nozzle contraction ratio (upstream headbox nozzle inlet 11 height divided by slice opening), the more even the flow profile at nozzle exit 10 and the more stable the exiting jet. It is then obvious that the larger the inlet 1 1 height of the nozzle 8, the higher will the contraction ratio be at a given slice opening and thus the more stable the headbox jet. Further, the smaller the slice opening with a given inlet 11 nozzle height, the better will the jet stability become. So, by increasing the nozzle inlet-height, it is possible to also use higher slice opening, without formation disturbances. This higher slice opening also gives better potential for paper strength and formation. It has to be said here that the inlet-height is defined as the total height of the open tubes 7.

The headbox 4 according to fig. 1 has a height of 320 mm and 9 solid vanes 14 of 5 mm upstream thickness each. The real inlet height is thus 275 mm, with which it was possible to avoid formation damages. However, already with an inlet-height of 150 mm some damages could be avoided and with an inlet-height of 200 mm most of the formation damages could be avoided and at 275 mm no formation damages could be detected. The tubes of the headbox according to the present invention are thus tightly packed and show a non-circular, preferably an essentially rectangular or hexagonal form.

Referring now again to fig. 2, at a distance downstream of the separation line where the wires leave contact with the forming roll surface a first blade 5 is arranged inside of the first wire 2. This blade is designed to support the first wire 2 and to avoid under pressure generation between the first wire 2 and the surface of the forming roll 1 by breaking the fluid capillary between the first wire 2 and the forming roll surface. If this happens early on any substantial force will not form from a local under pressure between roll and wire. This means that any substantial deformation of wire 2 leading to sheet damage will be avoided. This means in turn that the tip of the blade 5 will not be exposed to any significant radial force from the inner wire, thus not causing any significant blade wear by friction. The blade 5 is thus designed so that it supports the inner wire at a distance of at the most approx. 100 mm, and more preferably approx. 20-30 mm downstream of the separation line on the forming roll surface. The blade 5 has for geometrical reasons (the closeness to the roll nip) a very small thickness at its leading edge, of the magnitude of one millimeter, as can be understood from fig. 3. Such a blade gets thereby a low flexural rigidity, and will not be able to carry any loads from the inner wire. Therefore the blade 5 is designed so that the inner side has a curvature adjusted to support against the forming roll while the outer side is essentially plane to support the inner wire.

Referring now to fig. 4, by choosing different tapering of the roll and wire side resp. (see a and b resp.) of the tip of the blade the radial force generated by the flow deflections can be controlled. A higher value of the taper on the roll side gives a resulting force to the wire side. Minimum wear between the roll side of the blade 5 and the roll surface must be ensured. The blade tip must therefore be designed so that a suitable amount lubricating water is let through between the blade and the roll surface. This is effected by the taper (a, α) on the roll side of the tip, see fig 4. As can be seen from the figure, this taper angle is α.

Minimum wear between the wire side of the blade 5 and the first wire 2 is obtained by that a suitable water layer is let through between the wire and the blade 5. This is accomplished by a suitable tapering (b, β) of the wire side of the tip of blade 5, see figure 3. As can be seen from the figure, this taper angle is β.

Downstream of the first blade 5 other blades can be arranged further supporting and deflecting the wires, thus creating pressure pulses for dewatering of the remaining fibre suspension.

Test runs in the experimental paper machine FEX at STFI-Packforsk have shown that with conventional design the inner wire 2 may, already at forming speeds of approx. 1200 m/min, and a suction box vacuum of 10 kPa in the forming roll, separate owing to local under pressures, whereby the free suspension between the wires is redistributed. At a roll surrounding angle of 30° thereby serious formation damages arose. By using the blade according to the invention these damages were completely avoided.

With decreased suspension quantity between the wires, which was obtained from an increase of the wire surrounding angle to 35°, the web structure damages appeared as distinct bands. Also these damages were avoided completely by using a blade according to the invention.

The design of the blade and the headbox according the invention can, of course, be given different forms within the scope of the claims, and is not limited to the example above. The blade may be manufactured from ceramic, polymeric or metallic materials, or combinations thereof. Also other material combinations are possible. In fig. 5, an apparatus for paper board forming is disclosed. The separate layers of stock are placed one at a time on a first wire 2 each by a separate headbox 4 and dewatered each by a separate secondary closed loop wire 3 supported and dewatered by stationary blades 12 on the upper side of the secondary wire 3 and by movable blades 13 on the lower side of the first wire 2. By using headboxes of the type described in reference to fig. 1 serious formation damages caused by pressure pulses are avoided, like in the case with roll/blade- forming in paper machines.