WO2010091915A1

WO2010091915A1 - Twin-wire former

Info

Publication number: WO2010091915A1
Application number: PCT/EP2010/050500
Authority: WO
Inventors: Jean Yves Nouaze; Wolfgang Schwarz; Gerhard Weiss; Franz Stelzhammer
Original assignee: Voith Patent Gmbh
Priority date: 2009-02-12
Filing date: 2010-01-18
Publication date: 2010-08-19
Also published as: DE102009000803A1

Abstract

The invention relates to a twin-wire former (16) of a machine for producing a fibrous web, in particular a paper or cardboard web, from at least one fibrous suspension, comprising two continuous revolving wires (18, 20) and a forming roll (22). In the region of the forming roll, the two wires (18, 20) converge to form an inlet gap (24), wherein the forming roll (22) is embraced over a defined wrapping region by the two wires (18, 20) and the drainage of the fibrous web is carried out in said wrapping region through the outer wire (18) in a stream channel (28) and by way of the forming roll (22) into the region of the loop of the inner wire (20). The twin-wire former (16) according to the invention is characterized in that the white water flung into the stream channel (28) is guided by guide means (32) in particular provided in the stream channel and is deflected to the outlet thereof (34) such that it exits as an at least substantially laminar flow at the outlet (34) of the stream channel (28) and/or that at least the majority of the white water flung from the forming roll (22) in the region of the loop of the inner wire (20) is first guided into a closed channel (40), in which guide means (42) are provided in order to create an at least substantially laminar white water flow in the closed channel (40).

Description

twin

The invention relates to a twin-wire former of a machine for producing a fibrous web, in particular paper or board web, from at least one pulp suspension, with two continuous endless screens and a forming roller, in the region of which the two screens converge to form an inlet gap, wherein the forming roller has a certain Envelope of the two wires is looped around and the dewatering of the fibrous web takes place in this wrap on the one hand through the upper wire into a jet channel and on the other hand via the forming roller in the region of the loop of the inner wire.

Twin-wire or gap formers of this type are known, for example, from the publications EP 1 536 062 A1 and DE 198 03 591 A1.

In Gapformern newer design, the forming roller is wrapped over a larger wrap of the two wires. This results in particular in the bullet area, that is in the wrap, high dewatering quantities both in the direction of Formierwalzenmantels and up or out through the top wire in the direction of the jet channel.

1 shows a schematic representation of a typical distribution of the dewatering quantities on a gap former. Thereafter, in the wrap area, drainage through the top wire 10 may be upward or outward, particularly in a range of about 40 to about 60%, and drainage toward the bottom of the forming drum shell 14 are in particular in a range of about 35 to about 55% of the total drainage of the Gapformers.

In particular, for the production of packaging paper, efforts are being made to achieve maximum dewatering capacities in this area, since this has a positive influence on the strength parameters of the paper.

In general, the dewatered quantities are referred to as white water, which can be recycled into the circulation as dilution water, processed in process terms.

Characteristic of the removed from the pulp suspension amounts of water or low-consistency suspensions is in particular the turbulent flow occurring immediately after the exit and water flow under atmospheric conditions, that is, in contact with ambient air. Under these conditions, it is inevitable to strike gas, mainly air, in the amounts of water or low-consistency suspensions. Since more than 90% of the dewatering often takes place in said loop area, this brings about a massive increase in the gas content of the white water.

The said gas content, predominantly free air, has a significant effect on the runnability of the paper machine in question. The disturbances that occur can be, for example, the generation of an inhomogeneous substance jet from the headbox, air bubbles in the fiber mat, etc. Such disturbances usually lead to tearing of the paper web or force the paper manufacturer to reduce the production speed.

For this reason, until today large degassing, namely large occasionally vacuum-charged containers used to post-treat the white water coming from the former and greatly reduce its gas content, so that ultimately the arriving at the headbox suspension amounts have a gas content <1, 5% at speeds above 1,200 m / min.

Technologically, these known devices have proven themselves. A disadvantage, however, is the relatively large space requirement near the paper machine for the pre-venting equipment and the requirement of a correspondingly high side building in the area of the former for accommodating the vacuum-loaded container. In addition, the required drive-side access to the former is made more difficult. The total cost of the paper machine building increases by about 1, 5%. In addition, the piping requires a complex planning, since this requires a single solution depending on the local conditions. In addition, the required units such as in particular vacuum pumps, etc. bring a considerable amount of energy.

From the document EP 1 233 104 A2 it is already known to collect the white water through a collecting basin located inside the loop for discharging the white water arising within the loop of a circulating dewatering screen of a twin-wire former, to remove the collected white water by sucking off the area above the collecting basin still inside the loop Separate sliding loop of the air and remove the white water via at least one subsequent to the reservoir, lying below the water level channel from the reservoir.

In one of the document EP 1 538 258 A1 known longitudinal wire forming white water collected in the Formierbereich is collected in a white water tank and then forwarded in a closed horizontal pipe.

In a device known from the publication DE 10 2006 059 545 A1 for discharging white water arising within a loop of a circulating dewatering sieve, a channel arranged inside the loop is provided for catching the white water, to which an outlet channel for - A -

lateral removal of a portion of the collected white water adjoins. At least one suction opening of a suction channel for a forced discharge of at least part of the collected white water opens into the channel.

The invention has for its object to provide an improved twin-wire former of the type mentioned, in which the aforementioned disadvantages are eliminated. In this case, gas contents of less than 1.5% in the headbox should be achieved without or at least with simplified degassing elements outside the former.

This object is achieved in that the thrown off into the jet channel white water is guided by particular provided in the beam channel guide means and deflected to the output that it emerges at the output of the jet channel as at least substantially laminar flow, and / or that at least the largest Part of the white water thrown by the forming roller in the region of the loop of the inner wire is first led into a closed channel, are provided in the guide means to produce an at least substantially laminar white water flow in the closed channel.

Due to this design, the free gas content of the white water is reduced in a simple and effective manner without additional degassing or with at least simplified additional degassing outside the paper machine to a minimum.

As already mentioned, the impact of air takes place in the entry region, it being possible to differentiate predominantly between a dewatering upwards into the jet channel and a dewatering downwards over the forming roller. In this case, the invention may come into play in one or the other or in both dewatering phases. The white water thrown off into the jet channel is guided with the aid of guide means in such a way that abrupt deflections are avoided. Thus, a beginning separation of white water and gas is already promoted in the jet channel. The flow-calmed amounts of water ideally have laminar characteristics at the exit of the jet channel. The guide means may comprise guide plates or the like, for example.

The minimum radius of curvature occurring in the deflection of the white water in the jet channel is preferably> 50 mm.

To further reduce the free gas content, the white water emerging from the jet channel is expediently guided in a channel that is open in particular generally transversely to the web running direction and open at at least one lateral end. This channel can advantageously be under ambient pressure and / or at least locally be subjected to negative pressure.

According to a preferred practical embodiment of the twin-wire former according to the invention, the channel extending in particular generally transversely to the web running direction has such a cross-section that end-side outflow velocities of the white water are <1 m / s.

Preferably, the ratio between the width generally measured in the web running direction and the height of the channel which extends in particular generally transversely to the web running direction is> 1.5. It can be for example 2.5: 1.

The width of the channel is advantageously in a range of about 500 to about 2,500 mm. The mentioned geometric conditions contribute to the fact that lateral discharge velocities of <1 m / s are established.

In particular, such an embodiment is conceivable in which the channel extending generally transversely to the direction of web travel has a cross-section corresponding to an upwardly widening trapezoid or a cross-section corresponding to an at least substantially ellipsoidal half-shell.

In the embodiment of the channel extending generally transversely to the web running direction in the manner of an upwardly widening trapezium, the width of the channel measured in the web running direction can be in the range of the centroid of the trapezoid, in particular in a range of about 250 to about 2,500 mm.

In the case of a cross section of the channel which extends generally transversely to the direction of travel of the web, corresponding to an at least substantially ellipsoidal half-shell, the ratio between main peak and secondary vertex is advantageously at least 1.5. This ratio can be, for example, 2.5: 1. Preferred lengths of the main vertex of the imaginary ellipse are in a range of about 100 to about 2,500 mm.

The channel extending in particular generally transversely to the direction of web travel can be exposed to the open at the top and / or can be acted upon with vacuum via at least one suction element. The channel can thus be spent on at least one suction element so also under atmospheric pressure, which is the removal of the free air is beneficial. Such a suction also prevents the escape of formed mist or similar amounts of air, which among other things reduces the tendency to fouling in this area.

The negative pressure generated by means of at least one suction element is advantageously in a range from 0 to about 500 mbar. As already mentioned, at least the largest part of the white water which is spun off from the forming roller into the region of the loop of the inner wire, in particular, or down, is preferably first collected in a closed channel. In particular, it is also conceivable that one or the smaller proportion of the centrifuged white water is discharged outside the closed channel. This smaller proportion can be guided in particular on the outer contour of the closed channel up to a channel adjoining the channel.

According to a preferred practical embodiment of the screen former according to the invention, the closed channel initially comprises an at least substantially vertical, in particular downwardly leading portion and an adjoining curved portion preferably with a radius of curvature> 0.5 m for deflecting the white water in a generally horizontal end portion, wherein the vertical portion of the channel is preferably longer than its generally horizontal end portion.

The length of the vertical section of the closed channel is advantageously> 1 m.

To produce an at least substantially laminar white water flow in the closed channel guiding means are preferably provided again in this. For example, these guide means may again comprise guide plates or the like.

It is thus already promoted in the closed channel an incipient separation of white water and gas.

It is particularly advantageous if the closed channel opens with its generally horizontal end portion in an open-topped channel. in the In the area of this open-top channel, the white water comes into contact with ambient conditions.

If, as mentioned above, a smaller proportion of the thrown-off water is discharged outside the closed channel, then this smaller proportion may expediently be led to the outer contour of the closed channel up to the channel.

Guiding means are preferably provided in the area of the end of the open-top channel remote from the closed channel, in order to produce an at least the height of the exit from the closed channel

Stowage height in the gutter the white water in a generally transverse to

Web running direction extending, open at least one side end

Continue discharge channel. Preferably, the channel extending generally transversely to the web running direction is open at at least one lateral end.

The guide means may in particular comprise guide plates or the like.

These guide means provided in the region of the end of the open-topped channel facing away from the closed channel preferably extend so far into the discharge channel that the white water supplied to it only releases below the water level of this discharge channel from the positive guidance of the guide means.

The guide means can thus cause a certain amount of damming in the channel due to pressure or deflection losses, which can correspond at least to the height of the outlet from the closed channel. As long as the white water in the channel is guided, the gas present in the suspension, predominantly air, can escape into the atmosphere. About the guide means, the white water can be flow-calmed deflected to the side discharge in the discharge channel, which advantageously takes place so that the white water is not under the water level of the discharge channel is no longer under the positive guidance of the guide means. This is a renewed hammering of gas, especially air, prevented or minimized ("waterfall effect").

The discharge channel may for example have a generally U-shaped cross-section. Preferably, the ratio between the width measured generally in the web running direction and the height of the discharge channel is> 1.5. The width of this discharge channel is preferably in a range of about 500 to about 2,500 mm. These geometrical conditions contribute to the fact that lateral discharge velocities <1 m / s are established.

In principle, however, in particular, such a design is conceivable in which the discharge channel has a cross section corresponding to an upwardly widening trapezoid. The width is at the height of the line of gravity of the trapezoid advantageously in a range of about 250 to about 2,500 mm.

According to a further advantageous embodiment, the discharge channel may also have a cross section corresponding to an at least substantially ellipsoidal half-shell. The ratio between main peak and secondary peak is preferably> 1.5. So it can for example be 2.5: 1. Preferred lengths of the main vertex of the imaginary ellipse are in a range of about 100 to about 2,500 mm.

The outflow geometry of the discharge channel is expediently designed in such a way that the speeds in the regions decrease towards the open channel. The flow velocities of the white water can therefore be lower in a channel region which is closer to the trough, for example when viewed in the web running direction as in an area that is further away from the gutter in the web running direction. For this purpose, for example, the depth of the discharge channel in the gutter be greater than at a greater distance from the gutter.

This results in maximum discharge velocities <1 m / s.

Viewed in the web running direction after the twin-wire zone further drainage elements may be provided. It is advantageous if in the area or between these dewatering elements stripped quantities of white water are first passed into a preferably upwardly open channel and then to the laterally open discharge channel. In this case, guidance means can again be provided, in particular, in order to steer the quantities of water in a flow-calmed manner below the water level in the discharge channel. The guide elements may in particular comprise guide plates or the like again.

These guide elements or sheets preferably have a minimum radius of curvature> 30 mm.

The discharge channel can be open at the top and / or be acted upon again via at least one suction with vacuum. In the latter case, the generated negative pressure is preferably in a range from 0 to about 500 mbar.

According to the invention, therefore, hydrodynamic and hydrostatic laws such as in particular congestion levels and speed reduction or change in cross section, pressures below the atmospheric pressure, interference-calming forced guidance and large surfaces of the white water quantities are used advantageously to the free gas content of the white water without or at least with simplified degassing outside of the former to reduce a minimum. The invention will be explained in more detail below with reference to exemplary embodiments with reference to the drawing; in this show:

Fig. 1 is a schematic representation of a typical distribution of dewatering amounts at a Gapformer;

FIG. 2 shows a schematic, exemplary representation of a gap former for the purpose of vera n cha ng the be dden E dewatering phases in the contact area of the former;

3 shows a schematic partial representation of an exemplary embodiment of the twin-wire former according to the invention

Illustrating a possible guidance of the white water thrown off into the jet channel or upwards; and FIG. 4 shows a schematic partial representation of an exemplary embodiment of the twin-wire former according to the invention for illustrating a possible guidance of the device via the

Forming roller in the region of the loop of the inner screen or downwardly thrown white water.

Fig. 2 shows a schematic representation of an exemplary embodiment of a twin-wire former 16 of a machine for producing a fibrous web, in particular paper or board web, from at least one pulp suspension. The twin-wire former 16 comprises two continuous endless screens 18, 20 (upper wire 18, lower wire 20) and a forming roller 22, in the region of which the two wires converge to form a wedge-shaped inlet gap 24. In this gap 24 is introduced by means of at least one headbox 26, which may be a single or multi-layer casserole, pulp suspension.

The forming roller 22 is looped over a certain wrap of the two wires 18, 20. In this case, the dewatering of the fibrous web takes place in this wrapping region as indicated in FIG. 2 on the one hand through the outer sieve 18 into a jet channel 28 or upwards (compare the region A) and on the other hand via the forming roller 22 in the region of the loop of the inner wire 20 and down (see the area B).

After the twin-wire zone, further drainage elements 30 can be provided, which in the present case are arranged within the loop of the inner wire 20 which is separate from the outer wire 18 and carries the fibrous web.

The twin-wire former 16 is a so-called gap former. This can be designed to produce a single-layer or to produce a multi-ply fibrous web. As already mentioned, the headbox 26 can therefore basically be a single or multi-layer headbox.

The aforementioned white water is at least partially recycled to the process and used in particular for diluting the pulp suspension. It can be provided as a headbox in particular a headbox with sectioned adjustable dilution water.

3 shows, in a schematic partial representation, an exemplary embodiment of the twin-wire former according to the invention for illustrating a possible guidance of the white water thrown off into the jet channel 28.

In this case, guide means 32 are provided in the jet channel 28, which are designed and / or arranged so that the white water thrown into this jet channel 28 is guided and deflected to the outlet 34 of the channel, that it emerges at the channel outlet 34 as at least substantially laminar flow. The guide means 32 may in particular comprise baffles or the like. By means of the guide means 32, the white water is thus guided in the manner that abrupt deflections are avoided. The minimum radius of curvature occurring in the deflection of the white water in the jet channel 28 is preferably> 50 mm.

About the appropriately arranged or executed guide means a beginning separation of white water and gas is already promoted in the beam channel 28. Ideally, the flow-stabilized quantities of water at the exit 34 of the jet channel 28 have laminar characteristics.

To further reduce the free gas content, the white water emerging from the jet channel 28 can be guided into a channel 36 which extends generally transversely to the web running direction L and is open at at least one lateral end. This channel 36 may, for example, be under ambient pressure and / or at least locally be subjected to vacuum or sucked off.

The generally transversely to the web running direction L extending channel 36 may advantageously have such a cross-section that set the end-side flow rates of white water <1 m / s. Preferably, the ratio between the width B measured in the web running direction L and the height H of the channel 36 which is generally U-shaped in cross-section is preferably> 1.5. For example, a ratio of 2.5: 1 is possible.

The width of the channel 36 is suitably in a range of about 500 to about 2,500 mm.

These geometrical conditions lead to lateral outflow velocities of less than 1 m / s.

In particular, such a design is conceivable in which the channel 36 has a cross-section corresponding to an upwardly widening trapezoid has. In this case, the width at the height of the line of gravity of the trapezoid, for example, in a range of about 250 to about 2,500 mm.

For example, such an embodiment is also conceivable, in which the channel 36 has a cross-section which corresponds to an at least substantially ellipsoidal half-shell. The ratio between

Main and secondary shell in particular be> 1, 5. It is for example one

Ratio between major and minor vertices of 2.5: 1 possible. The length of the

Main vertex of the imaginary ellipse is advantageously in a range of about 100 to about 2,500 mm.

The channel 36 may, as already mentioned, open at the top and / or be acted upon by at least one suction element 38 with vacuum. Via a corresponding suction element 38, the pressure above the channel 36 can be brought below the atmospheric pressure, which further facilitates the removal of the free air. In particular, negative pressures in a range from 0 to about 500 mbar are conceivable. Moreover, such a suction element 38 prevents the escape of formed mist or similar amounts of air, which reduces the tendency to fouling in this area.

4 shows, in a schematic partial representation, an exemplary embodiment of the twin-wire former 16 according to the invention for illustrating a possible guidance of the white water which is thrown over the forming roller 22 into the region of the loop of the inner wire 20 or downwards.

Thereafter, at least the majority of the white water thrown from the forming roller 22 into the region of the loop of the inner wire 20 is first guided into a closed channel 40. In this closed channel 40, guide means 42 are preferably again provided, which are arranged and / or designed so that an at least substantially laminar white water flow results in the closed channel 40. In this case, the closed channel 40 may first have an at least substantially vertical, in particular downwardly leading portion 40 'and an adjoining curved portion 40 "preferably with a Krüm- mungsradius> 0.5 m for deflecting the white water in a generally horizontal end portion 40th '"include. The vertical portion 40 'is preferably longer than the generally horizontal end portion 40'". The length of the vertical portion 40 'of the closed channel 40 is suitably> 1 m.

The majority of the white water thrown down from the forming roller 22 is thus initially collected in the closed channel 40. The guide means 42 provided in the closed channel 40 serve to produce a laminar flow in the channel 40.

In particular, they may again comprise guide plates or the like.

It is thus already promoted in the closed channel 40 an incipient separation of white water and gas.

4, the closed channel 40 with its generally horizontal end section 40 '' can in particular open into an open-topped trough 44. From this point on, the white water is thus exposed to ambient conditions.

In the region of the end facing away from the closed channel 40 of the open-topped channel 44 guide means 46 may be provided to generate a at least the height of the outlet from the closed channel 40 corresponding storage level in the channel 44, the white water in a generally transverse to the web running direction L. continue to carry on at least one lateral end open discharge channel 48 continue. In this case, the guide means 46 preferably extend so far into the discharge channel 48 that this supplied white water is released only below the water level of the discharge channel 48 of the forced operation of the guide means 46.

The guide means 46 may in particular comprise guide plates or the like again.

In particular, it is also conceivable that a smaller proportion of the thrown-off water is discharged outside the closed channel 40. In this case, this smaller proportion can advantageously be performed on the outer contour of the closed channel 40 to the trough 44.

In the region of the upwardly open channel 44, the gas present in the white water or the suspension, predominantly air, can escape into the atmosphere.

About the guide means 46, the white water is flow-smoothed in the laterally outwardly guided discharge channel 48 is deflected. By the white water is released only below the water level of the channel 48 of the forced guidance of the guide means 46, a renewed hammering of gas, mainly air, is prevented or minimized (waterfall effect).

The ratio between the width measured generally in the web running direction L and the height of the discharge channel 48 which is generally U-shaped in cross-section, for example, is expediently greater than 1.5. The width of this channel 48 is advantageously in a range of about 500 to about 2,500 mm. These geometric conditions contribute to lateral discharge velocities <1 m / s.

The discharge channel 48 may in particular also have a cross section corresponding to an upwardly widening trapezoid. In this case, the width can be at the height of the heavy line, in particular in a range of about 250 to about 2,500 mm. However, it is also conceivable, for example, such an embodiment in which the discharge channel 48 has a cross section which corresponds to an at least substantially ellipsoidal half-shell. In this case, the ratio between main peak and secondary peak can be in particular> 1.5. For example, a ratio of major and minor vertices of 2.5: 1 is possible. The length of the main vertex of the imaginary ellipsoid is preferably in a range of about 100 to about 2,500 mm.

The outlet geometry of the discharge channel 48 may in particular also be designed such that speeds decrease towards the open channel 44. The flow velocities of the white water guided in the discharge channel 48 can thus be smaller in an area located closer to the channel 44 in the web running direction L than in a region further away from this channel 44. Accordingly, the cross section of the discharge channel 48, as shown in FIG. 4, in particular also deviate from a U-shape, that is, the bottom of this channel, as shown, also extend obliquely to the horizontal. In this case, the depth of the discharge channel 48 in the present case in the region of the channel 44 is greater than in the areas further away from it.

These measures help to ensure that the maximum discharge rates are <1 m / s.

The stripped quantities in the area or between the dewatering elements 30 after the twin-wire zone can again be opened upwards

Channel 50 may be guided to the laterally open discharge channel 48. It can in the

Region between the channel 50 and the discharge channel 48 advantageously

Guiding means 52 may be provided for directing the flow of water under the water level in the discharge channel 48. The guide means 52 may in particular comprise guide plates or the like again. Preferably, the minimum radius of curvature of these guide means 52 is> 30 mm.

The discharge channel 48 may, as already mentioned, open at the top and / or again be acted upon by at least one suction with vacuum. In this case, the generated negative pressure can be in particular in a range from 0 to about 500 mbar.

Incidentally, the embodiments shown in FIGS. 3 and 4 may, for example, at least substantially again have the same structure as the twin-wire former according to FIG. 2. Corresponding parts are assigned the same reference numerals.

LIST OF REFERENCE NUMBERS

10 upper sieve

12 lower sieve

14 forming roll

16 twin wire formers

18 outer sieve, upper sieve

20 inner sieve, lower sieve

22 forming roll

24 inlet gap

26 headbox

28 beam channel

30 drainage element

32 guiding means

34 output

36 channel

38 suction element

40 Closed channel

40 'vertical section

40 "Curved section 0 '" Generally horizontal end section

42 guiding means

44 gutter

46 guiding means

48 discharge channel

50 gutter

52 guiding means

B width

H height

L web running direction

Claims

DoppelsiebformerPatentansprüche

1. twin-wire former (16) of a machine for producing a fibrous web, in particular paper or board web, from at least one pulp suspension, with two continuous endless screens (18, 20) and a forming roller (22), in whose area the two screens (18, 20) to form an inlet gap (24) converge, wherein the forming roller (22) is looped over a certain wrap of the two wires (18, 20) and the dewatering of the fibrous web in this wrap on the one hand through the outer wire (18) into a Beam channel (28) and on the other hand via the forming roller (22) in the region of the loop of the inner wire (20), characterized in that in the jet channel (28) thrown off white water by in particular provided in the beam channel guide means (32) and so on whose outlet (34) is deflected so that it exits at the outlet (34) of the jet channel (28) as at least substantially laminar flow rode, and / or that at least the majority of the white water thrown from the forming roller (22) in the region of the loop of the inner wire (20) is first led into a closed channel (40), are provided in the guide means (42) to produce an at least substantially laminar white water flow in the closed channel (40).

2. twin wire former according to claim 1, characterized in that the minimum radius of curvature occurring in the deflection of the white water in the jet channel (28) is> 50 mm.

3. twin-wire former according to claim 1 or 2, characterized in that from the jet channel (28) emerging white water in a generally transverse to the web running direction (L) extending, at least one lateral end open channel (36) is guided.

4. twin-wire former according to claim 3, characterized in that the generally transversely to the web running direction (L) extending channel (36) has such a cross-section that adjust the discharge velocities of the white water <1 m / s.

5. twin-wire former according to claim 3 or 4, characterized in that the ratio between the generally measured in the web running direction (L) width (B) and the height (H) of the generally transverse to the web running direction (L) extending channel (36) ≥ 1, 5.

6. twin-wire former according to claim 3 or 4, characterized in that the generally transversely to the web running direction (L) extending channel (36) has a cross section corresponding to a widening upwards

Trapezoid or has a cross section corresponding to an at least substantially ellipsoidal half-shell.

7. twin-wire former according to one of the preceding claims, characterized in that the channel (36) extending generally transversely to the web running direction (L) is open at the top and / or can be acted upon with vacuum by at least one suction element (38), wherein preferably the generated negative pressure is in a range from 0 to approximately 500 mbar.

8. twin-wire former according to one of the preceding claims, characterized in that the closed channel (40), first an at least substantially vertical, in particular downwardly leading portion (40 ') and an adjoining curved portion (40 "), preferably with a radius of curvature ≥ 0.5 m for deflecting the white water into a generally horizontal end portion (40 '"), wherein the vertical portion (40') is preferably longer than the generally horizontal end portion (40 '").

9. twin-wire former according to claim 8, characterized in that the length of the vertical portion (40 ') of the closed channel (40) ≥ 1 m.

10. twin-wire former according to one of the preceding claims, characterized in that the closed channel (40) opens with its generally horizontal end portion (40 '") in an open-topped channel (44).

11. twin-wire former according to claim 10, characterized in that in the region of the closed channel (40) facing away from the end of the open-topped channel (44) guide means (46) are provided to produce at least the height of the exit from the closed

Channel (40) corresponding backwater level in the channel (44) the white water in a discharge channel (48) extending generally transversely to the direction of web travel (L) and extending at at least one end, wherein the guide means (46) preferably extend as far into the discharge channel (48) that the white water supplied thereto only reaches below the water level of the discharge channel (48) from the forced conduct of

Guide means (46) is released.

12. A twin-wire former according to claim 11, characterized in that the ratio between the width measured generally in the web running direction (L) and the height of the discharge channel (48) ≥ 1.5, the width thereof preferably being in a range from about 500 to about 2.500 mm is located.

13. twin-wire former according to claim 11, characterized in that the discharge channel (48) has a cross section corresponding to an upwardly widening trapezoid or a cross section corresponding to an at least substantially ellipsoidal half-shell.

14. A twin-wire former according to claim 13, characterized in that the discharge channel (48) has a cross-section corresponding to a trapezoid widening upwards with a width measured at the height of the heavy line, which lies in a range of about 250 to about 2,500 mm or a cross-section having a ratio between the major and minor vertices corresponding to at least substantially ellipsoidal half-shell, which is ≥ 1.5, wherein the length of the major vertex of the imaginary ellipsoid is preferably in a range of approximately 100 to approximately 2500 mm.

15 twin-wire former according to one of the preceding claims, characterized in that the discharge channel (48) open to the top and / or at least one vacuum can be acted upon with vacuum, wherein preferably the negative pressure generated is in a range of 0 to about 500 mbar.