Twin-wire former

Abstract

The present invention relates to a twin-wire former (10) comprising a first forming wire (12) and a second forming wire (16), wherein the first forming wire (12) is guided over a first closed breast roll (14) and the second forming wire (16) over a second closed breast roll (18) in such a way that they form a gap for receiving a jet of a fibrous suspension, wherein the first forming wire (12) and the second forming wire (16) are guided in the production direction downstream of the inlet gap to form a twin-wire zone over a stationary forming shoe (22) arranged in the loop of the first forming wire (12) for an initial dewatering of the fibrous suspension for the purpose of forming a fibrous web,wherein the first forming fabric (12) and the second forming fabric (16) are guided over a deflection roller (28) downstream of the forming shoe (12) in the production direction, and the twin-wire zone extends in the production direction at least from the forming shoe (22) to the deflection roller (28). The deflection roller (28) is arranged such that the fibrous web is guided in the twin-wire zone between the forming shoe (22) and the deflection roller (28) in a first direction that is inclined downwards relative to the horizontal, and that the fibrous web is guided downstream of the deflection roller (28) on the first forming fabric (12) in a second direction that is inclined upwards relative to the horizontal. Furthermore, the invention relates to a method for producing a fibrous web, in particular a paper, board, or tissue web, using such a twin-wire former (10).

Description

[0001] The present invention relates to a twin-wire former comprising a first forming fabric and a second forming fabric, wherein the first forming fabric is guided over a first closed breast roll and the second forming fabric over a second closed breast roll in such a way that they form a gap for receiving a jet of a fibrous suspension, wherein the first forming fabric and the second forming fabric are guided in the production direction downstream of the inlet gap to form a twin-wire zone over a stationary forming shoe arranged in the loop of the first forming fabric for an initial dewatering of the fibrous suspension for the purpose of forming a fibrous web, wherein the first forming fabric and the second forming fabric are guided downstream of the forming shoe over a deflection roll in the production direction, and the twin-wire zone extends in the production direction at least from the forming shoe to the deflection roll,The second forming fabric is guided away from the first forming fabric behind the deflection roller in the production direction, while the fibrous web remains on the first forming fabric to be subsequently transferred from it to another treatment unit. At least one suction box is arranged in the loop of the second forming fabric between the forming shoe and the deflection roller in the production direction. Furthermore, the present invention relates to a method for producing a fibrous web, in particular a paper, board, or tissue web, using such a twin-wire former.

[0002] The former or forming section of a machine for producing a fibrous web, particularly paper, board, or tissue web, plays a crucial role in shaping the fiber arrangement within the fibrous web. The former ensures controlled dewatering of the fiber suspension to achieve the desired fiber distribution for a specific fibrous web grade. Unlike a conventional fourdrinier former, twin-wire formers with two forming wires offer the option of dewatering the fiber suspension between the two forming wires in both directions. In most twin-wire formers, at least one of the two breast rolls, over which the respective forming wire is guided to form an inlet nip for a jet of the fiber suspension from a headbox, is vacuumed.In these cases, the initial dewatering of the fiber suspension already takes place on the at least one vacuumed breast roll, which is then sometimes called the forming roll. However, it has often proven advantageous, particularly with regard to contamination in the area of ​​the inlet gap, but also with regard to costs and energy consumption, to design the two breast rolls with a closed surface and thus without vacuum. Instead, a stationary forming shoe, which is arranged behind the two breast rolls in the direction of production, is used for the initial dewatering of the fiber suspension. The forming shoe can be made essentially of a ceramic material. For example, it can comprise a plurality of ceramic strips, each of which forms a gap to which a negative pressure can be applied.The blades are preferably arranged in such a way that the two forming fabrics experience a slight directional deflection in the area of ​​the forming shoe. Such a twin-wire former is also called a "shoe blade gap former" in the English-speaking world. Such twin-wire formers are marketed, for example, by Voith under the brand name "DuoFormer CBh."

[0003] In the known twin-wire formers of the type mentioned above, the twin-wire zone between the forming shoe and the deflection roller is either inclined upwards at a more or less pronounced angle to the horizontal, or the twin-wire zone is oriented essentially vertically upwards. In the first case, the performance of the twin-wire former is limited by the fact that the skimmer is always filled with water. The skimmer is a slot, or rather the first slot, in particular a suction slot, which is arranged at the front of the suction box in the loop of the second forming wire and which serves to drain away water that accumulates in front of the suction box after the first forming wire and the second forming wire come together. In the second case, the twin-wire zone between the forming shoe and the deflection roller is severely limited by the available height of the hall in which the vertical twin-wire former is located.In addition, due to the short twin-wire zone, the deflection roll must always be designed as a suction deflection roll to provide sufficient dewatering capacity for the fiber suspension or the resulting fiber web. However, a suction-operated deflection roll is expensive to maintain and expensive. Finally, the accessibility to the breast rolls and the headbox, for example, required for maintenance and cleaning work, is not optimal in this case.

[0004] The object of the present invention is to solve or at least mitigate the aforementioned problems. In particular, the twin-wire former described above is to be provided, which is characterized by high dewatering performance at low cost and good accessibility and / or ease of operation without compromising energy consumption.

[0005] This object is achieved by the features of the independent claims. The dependent claims relate to advantageous developments of the present invention.

[0006] Specifically, according to a first aspect of the present invention, the object is achieved by a generic twin-wire former described above, which is particularly characterized in that the deflection roller is arranged such that the fibrous web is guided in the twin-wire zone between the forming shoe and the deflection roller in a first direction that is inclined downwards relative to the horizontal, and that the fibrous web is guided downstream of the deflection roller on the first forming wire in a second direction that is inclined upwards relative to the horizontal. The deflection roller is preferably arranged in the loop of the second forming wire.

[0007] Thanks to the inventive design of the twin-wire former, the dewatering section in the twin-wire zone is not limited by the hall height, and there is no need to design the deflection roller as a suction deflection roller. Because the deflection roller represents a local low point for the fiber web guide, there is no—or only a significantly lower—risk of rewetting of the fiber web by water thrown off during the deflection of the fiber web clamped between the two forming wires. Access to the inlet gap and headbox is also easily provided. Furthermore, the inventive design allows the suction box to be arranged in the loop of the second forming wire in such a way that the water from the skimmer can easily flow away due to gravity and / or the dewatering flow energy, so that the skimmer is not permanently filled with water.This saves energy costs, as the skimmer does not need to be vacuumed, or only needs to be vacuumed to a lesser extent, and increases the efficiency of the twin-wire former.

[0008] An advantageous development of the present invention provides that the twin-wire zone between the forming shoe and the deflection roll is oriented such that the fibrous web is transported obliquely downwards at an angle between 20° and 80°, preferably between 30° and 70°, to the horizontal. The transport direction in this twin-wire zone can vary slightly over the entire length, for example, if the two forming wires are guided over a dewatering element. However, the transport direction in this twin-wire zone should preferably not deviate from the aforementioned angle range.

[0009] The first forming fabric transporting the fibrous web can also be oriented in the production direction immediately after the deflection by the deflection roller in such a way that the fibrous web is transported there obliquely upwards in an angular range between 10° and 70°, preferably between 20° and 60°, to the horizontal.

[0010] Preferably, the deflection roller is not vacuum-assisted. This saves operating costs. Furthermore, vacuum-assisted deflection rollers are also cheaper to purchase and less expensive to maintain. As previously described, the twin-wire former according to the invention advantageously allows for the elimination of vacuum-assisted deflection rollers.

[0011] In order to achieve high dewatering performance at the deflection roller, particularly when the deflection roller is not vacuumed, it is proposed that the deflection roller comprise a roller shell which is guided over a non-rotating support element with a radius of curvature, wherein the radius of curvature is designed such that the degree of curvature increases in the direction of production such that an increasing dewatering pressure is applied to the fibrous web when it is clamped between the first forming fabric and the second forming fabric and deflected by the deflection roller. The increasing dewatering pressure depends in particular on the tension of the first forming fabric and the second forming fabric, as well as on the radius of curvature of the support element. With such a dewatering roller, the water can be pressed out of the fibrous web even more effectively and then centrifuged off.

[0012] Furthermore, it is advantageous if at least one, preferably adjustable, dewatering blade is provided in the loop of the first forming fabric between the forming shoe and the deflection roller. For example, between three and five, in particular four, such adjustable dewatering blades, also known as "forming blades" in the English-speaking world, can be arranged. These can be arranged directly behind the forming shoe in the direction of production.

[0013] Preferably, the at least one suction box is provided in the loop of the second forming fabric opposite the at least one dewatering strip. This allows for particularly efficient dewatering of the fiber suspension or the resulting fiber web.

[0014] A further suction box can also be provided in the loop of the first forming fabric between the forming shoe and the deflection roller.

[0015] Furthermore, a further deflection roller can be arranged downstream of the deflection roller in the production direction such that, viewed in the production direction, the fibrous web is transported upstream of the further deflection roller at an angle to the horizontal, and downstream of the further deflection roller at an angle to the horizontal, it is transported downstream. The further deflection roller can be vacuumed if necessary, which can be advantageous, particularly at high production speeds, to reliably prevent the fibrous web from lifting off the first forming fabric.

[0016] In addition, it can be provided that a separating suction device is arranged in the loop of the first forming fabric behind the (first) deflection roller, viewed in the direction of production, in order to hold the fibrous web on the first forming fabric while the second forming fabric is guided away from the first forming fabric.

[0017] In a preferred embodiment of the present invention, the suction box can have a skimmer in the loop of the second forming fabric. The skimmer is preferably oriented such that gravity and / or drainage flow energy assists the removal of the water absorbed into the skimmer. This is made possible by the inventive design of the twin-wire former and saves energy, as well as operating and investment costs for the vacuum system.

[0018] A further aspect of the present invention relates to a method for producing a fibrous web, in particular a paper, board or tissue web, using a twin-wire former according to the invention as described above.

[0019] In this case, a jet of fiber suspension can be sprayed from a headbox into the gap between the first forming fabric and the second forming fabric of the twin-wire former at an angle of between 20° and 80°, preferably between 30° and 70°, to the horizontal. Due to the downward inclination of the jet and the closed surfaces of the two breast rolls, less mist formation and thus less contamination occurs in the area of ​​the inlet gap than with conventional "shoe blade gap formers." Furthermore, a lower pressure is required at which the fiber suspension leaves the headbox, since the fiber suspension does not have to be sprayed upwards into the inlet gap against gravity.

[0020] The invention is further explained below with reference to a schematic drawing which is not to scale.

[0021] Fig. Figure 1 schematically shows an embodiment of a twin-wire former 10 according to the invention. In this, a first forming fabric 12 is guided over a first breast roll 14. In addition, a second forming fabric 16 is guided over a second breast roll 18. Both breast rolls 14 and 18 have a closed surface and are not suction-applied. During normal operation, Fig. 1 the first forming fabric 12 counterclockwise and the second forming fabric 16 clockwise. In the area of ​​the first breast roll 14 and the second breast roll 18 there is an inlet gap for the jet of a fiber suspension from a headbox 20. In the direction of production, i.e. in the direction in which the fiber suspension or the resulting fiber web is transported on the first forming fabric 12, a stationary forming shoe 22 is located within the loop of the first forming fabric 12 immediately behind the first breast roll 14. The stationary forming shoe 22 preferably has only a single dewatering zone. Thus the same negative pressure is present everywhere. This is essentially where the dewatering of the fiber suspension begins.On the same assembly that supports the forming shoe 22, a plurality of dewatering strips 24 can also be attached in the loop of the first forming fabric 12. These strips rest against the second forming fabric 12 behind the forming shoe 22 in the production direction. The dewatering strips 24 are preferably arranged to be adjustable, in particular adjustable with regard to their contact pressure. Unlike the forming shoe 22, the dewatering strips 24 are preferably not connected to a vacuum source. Instead, they can be connected to a compressed air source to press them against the first forming fabric 12.

[0022] Furthermore, at least one suction box 26 is provided in the loop of the second forming fabric 16. The suction box 26 is preferably arranged opposite the dewatering strips 24. The suction box 26 can have several, in particular three, dewatering zones. The forming shoe 22 has a surface facing the second forming fabric 16, which is designed such that the direction of movement of the two forming fabrics 12 and 16 undergoes a slight deflection in the region of the forming shoe 22. Thus, the two forming fabrics 12 and 16 form a twin-wire zone at the latest in the region of the forming shoe 22, wherein, during normal operation, the fiber suspension or the resulting fiber web is located in the twin-wire zone between the two forming fabrics 12 and 16. The twin-wire zone extends in the production direction at least as far as a deflection roller 28.According to the invention, the deflection roller 28 is arranged such that the fibrous web is guided in the twin-wire zone between the forming shoe 22 and the deflection roller 28 in a first direction that is inclined downwards relative to the horizontal, and that the fibrous web is guided downstream of the deflection roller 28 on the first forming wire 12 in a second direction that is inclined upwards relative to the horizontal. The deflection roller 28 is arranged in the loop of the second forming wire 16.

[0023] Optionally, a further suction box 30 can be arranged in the twin-wire zone between the forming shoe 22 and the deflection roller 28 in the loop of the first forming fabric 12. In the production direction, behind the deflection roller 28, the twin-wire zone can continue to a separating suction device 32. The separating suction device 32 is arranged in the loop of the first forming fabric 12 and serves to reliably hold the fibrous web on the first forming fabric 12 using negative pressure, while the second forming fabric 16 is guided away from the first forming fabric 12 and the fibrous web located thereon.

[0024] Optionally, it is possible to provide at least one further suction box 44 in the loop of the first forming fabric 12 in the production direction between the deflection roller 28 and the separating suction device 32, as shown in dashed lines in Fig. 1 is shown.

[0025] Optionally, further suction boxes 34, 36 can also be arranged in the loop of the first forming fabric 12 in the production direction behind the deflection roller 28, in particular behind the separating suction device 32. In addition, a further deflection roller 38 can be located in the loop of the first forming fabric 16. By means of the further deflection roller 38, the fibrous web, which was transported diagonally upwards at an angle to the horizontal before the further deflection roller 38, viewed in the production direction, is transported diagonally downwards at an angle to the horizontal after the further deflection roller 38. The further deflection roller 38 is preferably vacuumed in order to reliably hold the fibrous web on the first forming fabric 12 even at high production speeds. However, in order to save energy costs, it can be provided that instead of vacuuming the further deflection roller 38, it is not vacuumed, but preferably grooved.In this case, it may further be provided to provide a suction box behind the unsuctioned but grooved deflection roller 38 in the direction of production.

[0026] Viewed in the direction of production behind the further deflection roller 38, the fibrous web can be removed from the first forming fabric 12 by a removal roller 40 and passed to a further, in Fig. 1, in particular a press section. For this purpose, the take-off roller 40 can be vacuumed and / or partially wrapped by a covering not shown here, in particular a press felt.

[0027] The suction box 26 in the loop of the second forming fabric 16 can comprise a skimmer 42, i.e. a slot that is arranged at the front of the suction box 26 and that serves to drain water that accumulates in front of the suction box 26 after the first forming fabric 12 and the second forming fabric 16 come together. The skimmer 42 is preferably oriented so that gravity supports the drainage of the water absorbed in the skimmer 42. Specifically, an inlet end of the skimmer 42 facing the second forming fabric 16 (right in Fig. 1) be at the same height or preferably higher than an outlet end opposite the inlet end (left in Fig. 1) of the skimmer 42. In this way, no negative pressure 42 needs to be applied to the skimmer to remove the water from the skimmer 42, or a relatively small negative pressure is sufficient. This saves energy and increases the efficiency of the twin-wire former 10 according to the invention.

[0028] But even if the inlet end of the skimmer 42 facing the second forming screen 16 (right in Fig. 1) are slightly lower than the outlet end opposite the inlet end (left in Fig. 1) of the skimmer 42, the flow velocity and energy (drainage flow energy) allow the water to be drained from the skimmer even without a vacuum. However, if necessary, the skimmer can be connected to a vacuum source as a precautionary measure. List of reference symbols: 10 twin-wire formers 12 first forming screen 14 first breast roll 16 second forming fabric 18 second breast roll 20 Headbox 22 Forming shoe 24 drainage strips 26 Suction box 28 Deflection roller 30 additional suction boxes 32 vacuum cleaners 34 additional suction box 36 additional suction box 38 additional deflection rollers 40 take-off roller 42 skimmers 44 additional suction box

Claims

[1] Double screen former (10) comprising a first forming screen (12) and a second forming screen (16), wherein the first forming screen (12) is guided over a first closed breast roll (14) and the second forming screen (16) is guided over a second closed breast roll (18) such that they form a gap for receiving a jet of fiber suspension, wherein the first forming screen (12) and the second forming screen (16) are guided in the production direction after the inlet gap, forming a double screen zone, over a stationary forming shoe (22) arranged in the loop of the first forming screen (12), for initial dewatering of the fiber suspension to form a fiber web,wherein the first forming screen (12) and the second forming screen (16) are guided in the production direction after the forming shoe (12) over a deflecting roller (28) and the double screen zone extends in the production direction at least from the forming shoe (22) to the deflecting roller (28), wherein in the production direction behind the deflecting roller (28) the second forming screen (16) is guided away from the first forming screen (12), while the fiber web remains on the first forming screen (12) to be subsequently transferred from it to a further processing unit, wherein at least one suction box (26) is arranged in the loop of the second forming screen (16) between the forming shoe (22) and the deflecting roller (28) in the production direction, , characterized by, that the deflecting roller (28) is arranged such that the fiber web in the double screen zone between the forming shoe (22) and the deflecting roller (28) is guided in a first direction which is inclined obliquely downwards relative to the horizontal, and that after the deflecting roller (28) the fiber web is guided on the first forming screen (12) in a second direction which is inclined obliquely upwards relative to the horizontal. [2] Double sieve former (10) according to claim 1, characterized by , that the double sieve zone between the forming shoe (22) and the deflecting roller (28) is oriented such that the fibrous web is transported obliquely downwards at an angle between 20° and 80°, preferably between 30° and 70°, to the horizontal. [3] Double sieve former (10) according to claim 1 or 2, characterized by, that the first forming screen (12) transporting the fiber web is oriented in the production direction immediately after the deflection by the deflection roller (28) such that the fiber web is transported there at an angle between 10° and 70°, preferably between 20° and 60°, to the horizontal at an angle upwards. [4] Double sieve former (10) according to any one of the preceding claims, characterized by , that the deflection roller (28) is unvacuumed. [5] Double sieve former (10) according to any one of the preceding claims, characterized by, that the deflecting roller (28) comprises a roller shell which is guided over a non-rotating support element with a radius of curvature, wherein the radius of curvature is designed such that the degree of curvature increases in the production direction in such a way that an increasing dewatering pressure is applied to the fiber web when it is deflected by the deflecting roller (28) while clamped between the first forming screen (12) and the second forming screen (16). [6] Double sieve former (10) according to any one of the preceding claims, characterized by , that at least one, preferably adjustable, dewatering bar (24) is provided in the loop of the first forming sieve (12) between the forming shoe (22) and the deflecting roller (28). [7] Double sieve former (10) according to claim 6, characterized by , that in the suction box (26) the loop of the second forming sieve (16) is provided opposite the at least one drainage bar (24). [8] Double sieve former (10) according to any one of the preceding claims, characterized by , that in the loop of the first forming sieve (12) between the forming shoe (22) and the deflecting roller (28) a further suction box (30) is provided. [9] Double sieve former (10) according to any one of the preceding claims, characterized by , that in the production direction behind the deflecting roller (28) a further deflecting roller (38) is arranged such that the fiber web, viewed in the production direction, is transported obliquely upwards at an angle to the horizontal in front of the further deflecting roller (38), and is transported obliquely downwards at an angle to the horizontal after the further deflecting roller (38). [10] Double sieve former (10) according to any one of the preceding claims, characterized by, that in the loop of the first forming screen (12) viewed in the production direction, a separating suction cup (32) is arranged behind the deflecting roller (28) to hold the fiber web on the first forming screen (12), while the second forming screen (16) is guided away from the first forming screen (12). [11] Double sieve former (10) according to any one of the preceding claims, characterized by , that the suction box (26) has a skimmer (42) in the loop of the second forming sieve (16). [12] Double sieve former (10) according to claim 11, characterized by , that the skimmer (42) is oriented such that gravity and / or the drainage flow energy assists in the removal of the water taken up into the skimmer (42). [13] Method for producing a fibrous web, in particular a paper, cardboard or tissue web, using a double screen former (10) according to one of the preceding claims. [14] Method according to claim 13, characterized by , that a jet of fibrous suspension is injected from a headbox (20) into the gap between the first forming screen (12) and the second forming screen (16) of the double screen former (10) at an angle between 20° and 80°, preferably between 30° and 70°, to the horizontal.

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