Machine having a device for transferring a fibrous web, and operating and retrofitting method

Abstract

The invention relates to a machine for producing a fibrous web, comprising a wire section and a press section. The wire section comprises a first guide roll (1), a second guide roll (2), and a wire (5) running over the guide rolls (1, 2), the inner face of the wire facing the guide rolls, and the press section comprises a removal suction device (3) and a removal felt (6), the removal suction device being positioned such that the removal suction device can suction the inner face of the removal felt and such that the removal felt can remove the fibrous web from the wire at a removal line situated between the first guide roll and the second guide roll. The machine also comprises a retaining suction device (4) which is positioned such that the retaining suction device can suction the inner face of the wire in a suction region, said suction region overlapping with the removal line, and the retaining suction device is designed to be movable such that the retaining suction device can be coupled to the wire and can be decoupled from the wire.

Description

[0001] Machine with a device for transferring a fibrous web and method for operation and retrofitting

[0002] The invention relates to a machine for producing a fibrous web, comprising a device for transferring the fibrous web from the forming section to the pressing section of the machine. The invention further relates to a method for transferring a fibrous web from the forming section to the pressing section in such a machine. The device and method according to the invention enable the reliable transfer of fibrous webs with low basis weights and at high web speeds, particularly in machines with modern, compact pressing sections. The invention also relates to retrofitting an existing machine for producing a fibrous web with a device according to the invention.

[0003] Devices and methods for transferring a fibrous web from the screen section to the press section of a machine for producing a fibrous web are known from the prior art. For example, DE 27 53 201 A1 discloses such a device and such a method. The disclosed screen section comprises, among other things, two guide rollers for guiding the screen. The fibrous web is removed from the screen in its entirety in the area between the two guide rollers. The disclosed press section comprises various rollers for guiding a take-off felt and a web-wide take-off suction box. During the take-off process, the take-off felt is brought into contact with the screen in the area of ​​the take-off suction box. The take-off suction box then draws the fibrous web onto the take-off felt, thereby removing the fibrous web from the screen. The fibrous web is then guided with the take-off felt to a central roller with a smooth surface, which is part of the press section.As an alternative to the suction box, a suction roller is also often used. Therefore, one can generally speak of a suction element.

[0004] In the known arrangement described, the fiber web is not usually transferred from the screen in such a way that a clean, continuous web immediately reaches the central roller. Instead, shreds are initially transferred until the web stabilizes. This process is, however, relatively insensitive to such variations, as the shredded web and the complete web adhere to the smooth surface of the central roller and are conveyed into a pulper by a scraper until the web stabilizes. Further transfer through the press section and the remaining parts of the machine is carried out from the central roller using a previously cut edge strip. The fiber web is then spread out using a coucher. During this process, large quantities of fiber are transferred from the central roller into the pulper.

[0005] However, using a central roller with a smooth surface in a press section has two disadvantages. First, it requires a relatively large amount of space, which means the press section cannot be designed very compactly. Second, a free pull always occurs at a central roller, with the associated risks. Therefore, increasingly, modern press sections are being designed without a central roller.

[0006] In machines with such a press section without a central roller, the transfer of the fiber web from the wire section to the press section can no longer be carried out across the entire web width, as the necessary removal of large quantities of fiber into a pulper within the press section is no longer technically feasible. Therefore, the transfer is carried out directly from the wire section using a knurled belt, with the remaining fiber web (remaining web) staying on the wire and then being conveyed to a pulper. For the removal of the knurled belt, the removal suction box or the removal suction roller comprises at least two sub-zones, so that the removal vacuum can initially be focused on the knurled belt. Such a removal suction roller is disclosed, for example, in DE 10 2011 084 016 A1.

[0007] As usual, after the knurled belt passes through the machine, the fiber web is spread out. During this process, the take-off vacuum in the take-off suction element is extended across the entire web width. Particularly with fiber webs of low basis weight, difficulties can arise due to an effect known as self-take-off of the remaining web. In this case, the remaining web, or parts of it, lifts off the wire prematurely and is carried along by the take-off felt. This leads to a failed transfer, requiring the transfer process to be repeated. Ultimately, self-take-off of the remaining web is usually triggered by the vacuum build-up in the take-off suction element not being synchronized precisely enough with the web spreading. A significant factor is the amount of false air drawn in by the take-off suction element, which decreases progressively during the spreading process.Further factors that negatively impact reproducible vacuum control arise from the many possible variations. These include, for example, the basis weight of the fiber web, the felt type of the take-up felt, and felt wear. A low web speed during the spreading process facilitates the synchronization of the vacuum build-up.

[0008] It was proposed to prevent the self-removal of the remaining web by moving the removal suction element a short distance away from the screen after the knurled belt has passed through the machine, thus preventing contact between the removal felt and the fiber web still adhering to the screen. This necessitates a single, albeit short, free path for the knurled belt and the wide-running web. Experience has shown that, due to its low strength at low dryness levels, the fiber web cannot withstand the tensile forces occurring at this point in the machine.

[0009] A reliable and reproducible transfer using the known devices and methods described in the preceding sections is limited approximately to the following areas:

[0010] - At speeds exceeding 1200 m / min when spreading the web, the basis weight of the fiber web must be at least 120 gsm (grams per square meter).

[0011] Fiber webs with a basis weight of less than 120 g / m² can only be spread at a speed of less than 300 m / min (where the long fiber content should be at least 50%). EP 1 903 142 B1 further discloses a system for spreading fiber webs and an associated method. This document also addresses the problem that, during transfer and spreading, the remaining web is carried along uncontrollably with the edge band. To solve this, this document proposes the use of a stabilizing device by which the remaining web is deflected from and supported by the actual guide rail. The proposed stabilizing device includes, among other things, a suction device. The embodiments disclosed in EP 1 903 142 B1 relate to the transfer of a fiber web from the press section to the drying section of a machine for producing a fiber web.Applying the solution from EP 1 903 142 B1 to a transfer process from the sieving section to the pressing section results in an arrangement and a different method compared to the present invention. The differences will be explained in more detail after the description of the solution according to the invention.

[0012] The object of the invention is to enable the reliable transfer of the fiber web from the forming section to the pressing section, even at low basis weights and higher speeds during spreading. Furthermore, the inventive solution is characterized by the fact that the device according to the invention can be easily retrofitted into an existing machine.

[0013] The problem is solved according to the invention by an embodiment according to the independent claims. Further advantageous embodiments of the present invention are found in the dependent claims.

[0014] The invention will be explained below with the aid of figures. The figures show, in detail:

[0015] Fig. 1: Machine according to the prior art

[0016] Fig. 2: Machine according to the invention. Figure 1 shows part of a machine for producing a fibrous web according to the prior art. The illustrated part of the machine is arranged in the transition area between the screen section and the press section of the machine. The screen section comprises, among other things, a screen and a first and a second guide roll. The first guide roll is designated 1, the second guide roll is designated 2, and the screen is designated 5. The screen 5 runs over the guide rolls 1 and 2, with the inside of the screen facing the guide rolls 1 and 2. The press section comprises, among other things, a take-off suction device and a take-off felt. The take-off suction device is designated 3, and the take-off felt is designated 6. The take-off suction device 3 is arranged such that it draws suction onto the inside of the take-off felt 6 as soon as the take-off suction device 3 is subjected to a vacuum.The vacuum acting in the suction device 3 is also referred to as the "pick-up vacuum".

[0017] Figure 1 shows the machine in a state corresponding to normal operation. A fiber web is transported from the wire section to the press section. In Figure 1, the fiber web is indicated by the dashed line. The fiber web is located on the outside of the wire 5 and, together with the wire 5, is guided over the first guide roller 1 and moves from there to the point labeled A in Figure 1. Point A is the so-called take-off point. There, the fiber web is taken off the wire 5 by the take-off felt 6 and from then on runs along the outside of the take-off felt 6. The take-off point A in Figure 1 corresponds to a take-off line that extends perpendicular to the plane of the illustration. The take-off line A is arranged perpendicular to the machine direction. In Figure 1, the machine direction is indicated by the arrow labeled MD.

[0018] The removal suction device 3 is arranged so that the removal felt 6 can remove the fibrous web located on the sieve 5 between the first guide roller 1 and the second guide roller 2.

[0019] The arrangement shown in Figure 1 represents an ideal state where the take-off occurs at a single point A. In reality, this often happens over a more extended area because (if one imagines the fiber web removed) the take-off felt 6 then contacts the screen 5 not only along the take-off line A, but along a strip of finite width. This causes the screen 5 to be deflected slightly inwards from the straight connecting line between the guide rollers 1 and 2. The contact strip then forms an area that can be referred to as the take-off area. In this document, the convention is that the take-off line or take-off point A is located centrally within any extended take-off area that may exist. The ideal arrangement of Figure 1 is also referred to as the so-called "Kiss position" of the take-off suction device 3.

[0020] The machine further comprises a pulper, which is labelled 7 in Figure 1. During a transfer process, the entire fiber web is initially transported into the pulper. As the transfer process continues, the remaining web is transported into the pulper, becoming progressively narrower as it spreads and finally disappearing completely, resulting in the state shown in Figure 1.

[0021] The inventor was guided by the idea that heavier fiber webs can be transferred at high speeds, while lighter fiber webs cause problems. The inventor recognized that the force of gravity causes the fiber web to be pressed against the screen in the take-off area and is therefore not as easily and unintentionally carried along by the take-off felt as a lighter fiber web. The solution according to the invention therefore consists of a device that artificially assists the force of gravity acting in the take-off area. For this purpose, the inventor proposes the use of a suction device, which is arranged so that it can draw suction onto the inside of the screen in the take-off area.

[0022] Figure 2 shows a machine according to the invention for producing a fibrous web in an analogous representation to Figure 1. The suction device according to the invention is designated 4 and is referred to below as the retaining suction device for the sake of distinction. The retaining suction device 4 is arranged such that it can suction the inside of the screen in the take-off area, i.e., the suction area of ​​the retaining suction device overlaps the take-off line A.

[0023] The retaining suction device 4 extends in the transverse direction of the machine at least over the area occupied by the remaining web at the end of the transfer process, as this is where most problems arise. The width of this area is at least 1 m to 1.5 m. The retaining suction device 4 can equally well extend over the entire maximum width of the remaining web, or over the entire width of the fiber web, or even over the entire width of the machine.

[0024] The vacuum in the retaining suction device 4 is adjustable and is set so that the fiber web is securely held against the screen 5 by the additional holding force caused by the retaining suction device 4 as long as no pick-up vacuum is acting in the take-up suction device 3, but is not sufficient to hold the fiber web against the pick-up vacuum of the take-up suction device 3 against the screen. This is the case, for example, when the additional holding force is approximately equal to the adhesive force of the fiber web on the take-up felt.

[0025] The vacuum in the holding suction device 4 can also be adjusted experimentally. It is particularly advantageous if the holding suction device 4 extends over the knurled belt area, as the adjustment can then be made using the belt itself. To do this, the vacuum in the holding suction device 4 is initially set too high – for example, to the maximum possible value. When the pick-up vacuum is then applied in the removal suction device 3, the knurled belt remains attached to the screen 5, i.e., it is not removed from the screen 5. The vacuum in the holding suction device 4 is then gradually reduced until the knurled belt is removed from the screen 5. At this point, the vacuum in the holding suction device 4 is correctly adjusted. To account for variations in vacuum across the machine width, it may be advantageous to set the vacuum in the holding suction device 4 slightly lower.The retaining suction device 4 can comprise one or more slots. It is advantageous if the retaining suction device 4 comprises no more than three slots. Furthermore, it is advantageous if the retaining suction device 4 comprises several separate chambers in the transverse direction of the machine, which can be pressurized separately with a vacuum. This ensures that an equal vacuum is generated in all chambers of the retaining suction device 4. The separate chambers prevent the vacuum in the retaining suction device 4 from decreasing too much during spreading due to the ingress of false air. The increasing ingress of false air during spreading is caused by the fact that the fiber web, viewed in the transverse direction of the machine, continuously lifts away from the screen 5 during spreading, thereby facilitating the ingress of false air into the retaining suction device 4.Due to the separate chambers, this process is repeated for each chamber during the widening phase. Since the maximum amount of inflowing false air is proportional to the width of the respective chamber, the undesirable decrease in vacuum is reduced by increasing the number of chambers. It is particularly advantageous if the retention suction device 4 has a separate chamber at least in the last 1 to 1.5 m of the remaining web, or if several separate chambers are arranged in this area.

[0026] The retaining suction device 4 is designed such that it can be decoupled from the screen 5. That is, the retaining suction device 4 is movable, allowing it to move away from the screen 5. In this way, the retaining suction device 4 can be decoupled from the screen 5 during normal operation, thus preventing unnecessary wear on the screen caused by the retaining suction device 4. Such mobility of the retaining suction device 4 is also necessary because the position of the discharge suction device 3 is usually also adjustable, and the position of the retaining suction device 4 must be able to follow the position of the discharge suction device 3. The retaining suction device 4 is only subjected to a vacuum during the transfer process. The transfer method according to the invention comprises the following steps:

[0027] - S1: Transferring a marginal band

[0028] - S2: Coupling the retention suction device 4 to the sieve 5 and activating a vacuum in the retention suction device 4

[0029] - S3: Widening of the rim

[0030] - S4: Disconnecting the retention suction device 4 from the sieve 5 and deactivating the vacuum in the retention suction device 4

[0031] The specified steps can be performed in the specified order. Step S2 can also be performed before step S1. Step S2 must in any case be performed before step S3, and step S4 may only be performed after the widening process (S3) has been completed. Steps S1 and S3 are performed according to the prior art; that is, the method according to the invention differs from the known method only by steps S2 and S4, so that the method according to the invention can be easily learned by the operating personnel after retrofitting an existing machine.

[0032] Existing machines for producing fibrous webs can easily be retrofitted with the suction device 4 according to the invention. For this purpose, the suction device 4 according to the invention is installed in the screen section of the machine, which does not pose any major problems, as there is usually sufficient space available. Suitable means for providing the vacuum for the suction device 4 are also installed along with the suction device 4. No further modifications to the existing machine are necessary. The suction device 4 can easily be manually connected to the screen 5. The vacuum can also be switched on manually.

[0033] Finally, it should be mentioned that if the system disclosed in EP 1 903 142 B1 for spreading a fibrous web were applied to the transfer process from the forming section to the pressing section, the person skilled in the art would proceed as follows. The embodiments disclosed in EP 1 903 142 B1 relate to the transfer of a fibrous web from the pressing section to the drying section. When applying the embodiments disclosed in EP 1 903 142 B1, the person skilled in the art would recognize that they would have to replace the elements of the pressing section with suitable elements of the forming section and the elements of the drying section with suitable elements of the pressing section. As can be seen from Figures 1a, 3d, 3c2 and 3d1, the system disclosed in EP 1 903 142 B1 for spreading a fibrous web (reference 3) and any associated stabilizing device (reference 15) is always arranged behind a drying cylinder (reference 7 or 7.1).In this transfer, a person skilled in the art would therefore arrange the system for spreading a fibrous web behind a suitable roller in the press section. Since the solution of the present invention provides that the retention suction device according to the invention is arranged within the screen section, a person skilled in the art, starting from EP 1 903 142 B1, would not arrive at the subject matter of the present invention.

[0034] Reference symbol list

[0035] 1 First guide roller

[0036] 2 Second guide roller

[0037] 3. Suction unit

[0038] 4 Retention suction device

[0039] 5 sieves

[0040] 6 Acceptance felt

[0041] 7 Pulper

[0042] A. Acceptance point or acceptance line

[0043] MD Machine direction

Claims

Patent claims 1. Machine for producing a fibrous web with a screen section and a press section, wherein the screen section comprises a first guide roll (1), a second guide roll (2) and a screen (5) extending over the guide rolls (1, 2), and wherein an inner side of the screen (5) faces the guide rolls (1, 2), and wherein the press section comprises a take-off suction device (3) and a take-off felt (6), and wherein the take-off suction device (3) is arranged such that the take-off suction device (3) can draw suction from an inner surface of the take-off felt (6) and the take-off felt (6) can take the fiber web from the screen (5) at a take-off line (A) which is arranged between the first guide roller (1) and the second guide roller (2), characterized in that the machine comprises a retaining suction device (4) which is arranged such that the retaining suction device (4) can draw suction from the inner surface of the screen (5) in a suction area which overlaps the take-off line (A), and wherein the retaining suction device (4) is designed to be movable so that the retaining suction device (4) can be coupled to and uncoupled from the screen (5).

2. Machine according to claim 1, wherein the screen section is designed such that the fibrous web can be separated into an edge band and a residual web, and that the width of the edge band can be widened in order to extend the edge band over the entire width of the fibrous web, and wherein the retaining suction device (4) is designed such that the suction area of ​​the same extends in a transverse direction of the machine at least over an area in which, when the edge band is widened, the last 1 to 1.5 m of the residual web are located.

3. Machine according to claim 1 or 2, wherein the retaining suction device (4) is designed such that the suction area of ​​the same is located in a The machine's transverse direction extends across the entire width of the fiber web.

4. Machine according to one of the preceding claims, wherein the retaining suction device (4) comprises at least one slot.

5. Machine according to one of the preceding claims, wherein the retaining suction device (4) comprises at most three slots.

6. Machine according to one of the preceding claims, wherein the retaining suction device (4) comprises several chambers separated from one another in the transverse direction of the machine, which can be separately subjected to a vacuum.

7. Method for transferring the fibrous web of a machine according to one of the preceding claims from the screen section to the press section, comprising the following steps: - S1: Transfer of a marginal band; - S3: Widening of the edge band; characterized in that the procedure comprises the following further steps: - S2: Coupling the retention suction device (4) to the sieve (5) and activating a vacuum in the retention suction device (4); - S4: Disconnecting the retention suction device (4) from the sieve (5) and deactivating the vacuum in the retention suction device (4); wherein step S2 is performed before step S3 and step S4 is performed after step S3.

8. Method for retrofitting an existing machine for producing a fibrous web, comprising a screen section and a press section, wherein the screen section comprises a first guide roll (1), a second guide roll (2), and a screen (5) extending over the guide rolls (1, 2), and wherein an inner side of the screen (5) faces the guide rolls (1, 2), and wherein the press section comprises a take-off suction device (3) and a take-off felt (6) comprises, and wherein the take-off suction device (3) is arranged such that the take-off suction device (3) can suction an inside of the take-off felt (6) and the take-off felt (6) can take the fiber web from the screen (5) at a take-off line (A) which is arranged between the first guide roll (1) and the second guide roll (2), and wherein the method for retrofitting comprises the following step: Installation of a retention suction device (4) in the machine, wherein the retention suction device (4) is arranged in such a way that the retention suction device (4) can draw suction from the inside of the screen (5) in a suction area which overlaps the discharge line (A), and wherein the retention suction device (4) is designed to be movable, so that the retention suction device (4) can be coupled to and uncoupled from the screen (5).

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