Drying section of a machine for producing a fibrous web and method for operating a drying section

Abstract

The invention relates to a drying section of a machine for producing a fibrous web, wherein: the drying section comprises at least two drying cylinders (1.1, 1.2), at least one suction deflection roller (2), at least one air-permeable drying screen guided over the drying cylinders (1.1, 1.2) and the suction deflection roller (2), and at least one stabilising apparatus (3); the stabilising apparatus comprises three sealing devices (3.3, 3.41, 3.42), a first chamber (3.1) and a second chamber (3.2); a first sealing device seals the stabilising apparatus with respect to the first drying cylinder; a second sealing device and a third sealing device seal the stabilising apparatus with respect to the suction deflection roller, in order to form a first suction region and a second suction region; the first suction region is connected to the first chamber; the second suction region is connected to the second chamber; a portion of an open lateral face of the suction deflection roller, said portion being formed by the openings, is at least 3% of the total lateral face; and a suction angle of the suction deflection roller is at least 60°.

Description

[0001] Drying section of a machine for producing a fibrous web and method for operating a drying section

[0002] The invention relates to the drying section of a machine for producing a fibrous web, in particular a paper, cardboard, or tissue web, wherein the drying section comprises at least two drying cylinders, at least one suction deflector roller, at least one air-permeable drying screen guided over the drying cylinders and the suction deflector roller, and at least one stabilizing device. The operating methods according to the invention relate to the web-width transfer of the fibrous web and a method that is carried out in the event of a web break.

[0003] Drying sections of this type are known from the prior art. For example, DE 10 2009 026 686 A1 discloses such a drying section. The disclosed stabilizing device comprises a first suction area, which serves to lift the fiber web from an upstream drying cylinder, and a further suction area, which acts on the surface of the subsequent suction deflection roller that is free of the drying screen. The disclosed drying section is not intended for transferring the entire web width; rather, the fiber web is transferred as a strip and spread out after transfer.

[0004] Another generic drying process is disclosed in WO 2009 / 141508 A1. This document discloses a method for transferring the fiber web across its entire width and a method that is carried out when the fiber web breaks.

[0005] The object of the invention is to provide an alternative drying section that enables reliable, web-width transfer of the fiber web and a rapid response to a fiber web break. Furthermore, in one embodiment, the drying section according to the invention is robust against contamination of the suction guide roller. Further embodiments of the drying section according to the invention are also characterized by their robustness against mechanical damage that can occur during faulty fiber web transfer.

[0006] The problem is solved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention are found in the dependent claims.

[0007] The invention will be explained below with the aid of figures. The figures show, in detail:

[0008] Fig. 1: Section of a drying section according to the invention. Fig. 2: Section of a drying section according to the invention. Fig. 3: Drying group of a drying section according to the invention.

[0009] Figure 1 shows a schematic representation of a section of a drying section according to the invention in a section perpendicular to the axes of the depicted drying cylinders and the depicted suction deflection roller. The drying section comprises at least two drying cylinders, at least one suction deflection roller, at least one air-permeable drying screen guided over the drying cylinders and the suction deflection roller, and at least one stabilizing device. In Figure 1, only a portion of a first drying cylinder is shown and is labelled 1.1. The curved arrow indicates the direction of rotation of the first drying cylinder 1.1. A suction deflection roller, which is only partially shown in Figure 1 and is labelled 2, is arranged behind the first drying cylinder 1.1 in the machine direction. A plurality of openings are arranged in the casing of the suction deflection roller, connecting the interior of the suction deflection roller with the exterior.In the direction of travel of the machine, behind the suction deflection roller 2, a second drying cylinder is arranged, which is only partially shown in Figure 1 and is labelled 1.2. The air-permeable drying screen, guided over the drying cylinders 1.1 and 1.2 and the suction deflection roller 2, is shown only in the areas where it runs freely and is indicated by the two dashed lines. The fiber web is not shown in Figure 1. Ideally, the fiber web runs parallel to the drying screen and is arranged on the side of the drying screen facing the surfaces of the drying cylinders.

[0010] The length of the section in the direction of travel of the drying screen, in which it runs freely between the first drying cylinder 1.1 and the suction deflection roller 2, is f. The length f is hereinafter referred to as the "free length". The minimum distance between the surface of the first drying cylinder 1.1 and the surface of the suction deflection roller 2 is d. The minimum distance d is hereinafter referred to as the "distance".

[0011] A stabilizing device, designated 3 in Figure 1, is arranged in the space between the first drying cylinder 1.1, the suction deflection roller 2, and the second drying cylinder 1.2. Inside the stabilizing device 3, a first chamber, designated 3.1, is arranged. Inside the stabilizing device 3, a second chamber, designated 3.2, is arranged. The two chambers 3.2 and 3.1 are separated from each other by a partition. The stabilizing device 3 comprises at least three sealing devices. A first sealing device, designated 3.3, seals the stabilizing device 3 against the first drying cylinder 1.1. The other two sealing devices seal the stabilizing device 3 against the suction deflection roller 2. A second sealing device, designated 3.41, is located on the side of the stabilizing device 3 facing the first drying cylinder 1.1.1 arranged, and a third sealing device designated 3.42 and arranged on the side of the stabilizing device 3 towards the second drying cylinder 1.2.

[0012] A first suction zone is defined by the first sealing device 3.3, a portion of the surface of the first drying cylinder 1.1, the free area of ​​the drying screen between the first drying cylinder 1.1 and the suction deflection roller 2, a portion of the surface of the suction deflection roller 2, the second sealing device 3.41, and a portion of the housing of the stabilizing device 3. The first suction zone is connected to the first chamber 3.1. In short, the first suction zone is located between the first sealing device 3.3 and the second sealing device 3.41.

[0013] The second chamber 3.2, the second sealing device 3.41, a portion of the surface of the suction deflection roller 2, and the third sealing device 3.42 form a second suction area. In short, the second suction area is located between the second sealing device 3.41 and the third sealing device 3.42. A portion of the suction deflection roller 2, situated between the second sealing device 3.41 and the third sealing device 3.42, is drawn in via the second suction area. The angle corresponding to the portion of the suction deflection roller 2 drawn in by the second suction area is α. This angle α is referred to below as the "suction angle of the suction deflection roller".

[0014] Optionally, the stabilizing device can include 3 additional sealing devices. For example, a fourth sealing device could be arranged on the side of the stabilizing device 3 facing the second drying cylinder 1.2 to define an additional suction area. Through this additional suction area, the fiber web is drawn to the drying screen after passing around the suction deflector roller, ensuring that the fiber web is guided smoothly on this side of the suction deflector roller as well. A further chamber can be provided to supply the negative pressure for the additional suction area, or the negative pressure can be connected to one of the two chambers 3.1 or 3.2 shown in Figure 1.

[0015] The elements shown in Figure 1 extend, in the direction of view, at least over the entire area of ​​the drying section that can be occupied by the fiber web during operation. Laterally, further elements can be arranged that laterally delimit the elements shown in Figure 1. This applies in particular to the first chamber 3.1 and the second chamber 3.2.

[0016] Figure 2 shows the elements depicted in Figure 1 in a broader context, i.e., together with other elements that play a role in the various functions of the drying section according to the invention. For the sake of simplicity, the drying screen is not shown. The drying section depicted comprises means for providing a first vacuum, which is connected to

[0017] The means 5.1 for providing a first vacuum are connected to the first chamber 3.1, so that the provided first vacuum can act in the first suction area. Furthermore, the illustrated drying section includes means for providing a second vacuum, which are designated 5.2. The means 5.2 for providing a second vacuum are connected to the second chamber 3.2, so that the provided second vacuum can act in the second suction area. The connecting lines between means 5.1 and

[0018] 5.2 with chambers 3.2 and 3.1 are schematically indicated by the dashed lines. These can also be multiple or branched lines. For example, a line or line branch can open into each of the lateral end faces of the chambers. The dry section advantageously includes at least one valve flap, which is arranged and designed such that the negative pressure in the second chamber can be released by opening the valve flap. This valve flap is designated 8.2 in Figure 2. It is also advantageous if the dry section includes a shut-off valve with which the means 5.2 can be separated from the second chamber 3.2. This shut-off valve is designated 9.2 in Figure 2. It is advantageous if a corresponding valve flap 8.1 and a corresponding shut-off valve 9.1 are also provided for the first chamber 3.1 and the means 5.1.The drying section includes a control device, designated 4, which is configured to actuate means 5.1 and 5.2 in order to adjust the applied negative pressures. If one or more valve flaps and / or shut-off valves are present, the control device 4 is configured to actuate these elements as well.

[0019] The drying section further comprises a transfer device, designated 7, which is designed such that, during a transfer operation, it can exert a force on the fiber web conveyed on the first drying cylinder sufficient to detach the fiber web from the drying cylinder across its entire width and convey it towards the drying screen and the suction deflection roller. Such a transfer device 7 can, for example, be designed as a machine-wide blowing device connected to a sufficiently strong air pressure source. The blowing device can include several finger nozzles distributed across the machine width. The transfer device 7 can advantageously be arranged on a scraper device. In this case, it is advantageous if the transfer device 7 is arranged on the scraper body of the scraper device in order to decouple the scraper blade from forces exerted by the transfer device 7.Such a scraper device is schematically indicated in Figure 2 and labelled 6. The supports of the transfer device 7 can be multi-part to reduce the tendency to become soiled. In an advantageous embodiment, the transfer device 7 can be pulled out of the support from one end (e.g., for cleaning purposes). Preferably, the transfer device 7 is arranged behind a cover that prevents paper scraps from accumulating. The transfer device 7 can include a second blower tube that is horizontally oriented and blows the fiber web against the dryer screen at the level of the suction deflection roller. The transfer device 7 is designed to be activated by the control unit.

[0020] The inventors focused primarily on designing the elements described in Figures 1 and 2 to ensure reliable, full-width transfer of the fiber web and a rapid response to any web breakage. They recognized that, in addition to lifting the fiber web from the first drying cylinder, the reliable capture of the fiber web by the subsequent suction deflection roller is crucial during the transfer process. Lifting is achieved through the interaction of the transfer device and the first suction area. The second suction area is responsible for capturing the fiber web. If the fiber web breaks, it must be kept away from the drying cylinders as quickly as possible to prevent them from becoming entangled. For this purpose, the fiber web is fed to the pulpers located below the drying cylinders.The inventors have recognized that the collection of the fiber web and the rapid response to a tear can be significantly improved by ensuring that the proportion of the open surface area of ​​the suction deflection roller is at least 3% of the total surface area – preferably at least 5% – and that the suction angle α of the suction deflection roller is at least 60° (α > 60°) – preferably at least 70° (α > 70°). The open surface area is formed by the openings.

[0021] The actual collection of the fiber web takes place at the openings in the outer surface of the suction deflection roller, which are initially covered only by the drying screen. To ensure a sufficiently high vacuum is created at these openings during the transfer, a sufficiently large airflow must be generated by the means for providing the second vacuum. Air flows from outside the suction deflection roller through the openings in the outer surface of the roller, which are covered only by the drying screen, into the interior of the suction deflection roller, and from there through the openings in the outer surface of the roller located in the second suction area into the second chamber, and from there to the means for providing the second vacuum.The inventors recognized that the aforementioned measures (increasing the open surface area and the suction angle) significantly enlarge the effective cross-section of the relevant flow channel, thereby considerably improving the flow conditions that lead to the formation of a vacuum at the openings in the surface of the suction guide roller. The larger cross-section results in a faster vacuum build-up during the transfer of the web.

[0022] The improved flow conditions also ensure that the vacuum at the openings in the outer surface of the suction deflector roller can be quickly broken in the event of a fiber web tear, allowing the torn fiber web to be quickly discharged via the scraper (6) into a pulper. This is achieved by opening the valve flap 8.2, which opens in the event of a fiber web tear. The valve flap 8.2 is arranged and designed to connect the second chamber 3.2 to the environment. It is advantageous if the cross-section of this connection is as large as possible. Furthermore, it is advantageous if such a valve flap 8.2 is provided for each stabilizing device designed according to the invention. If a shut-off valve 9.2 is present, it closes when the valve flap 8.2 opens to quickly separate the means 5.2 from the second chamber 3.2.

[0023] According to the invention, the flow conditions can be further improved by making the cross-section of the second chamber larger than the cross-section of the first chamber. Preferably, the cross-section of the second chamber is more than twice the cross-section of the first chamber. The cross-sections mentioned refer to the viewing direction used in the figures, i.e., in the direction of the axes of the cylinders or rollers.

[0024] Furthermore, it is advantageous if the increase in the proportion of the open surface area of ​​the suction deflection roller relative to the total surface area is achieved less by increasing the number of openings per surface area, but rather by increasing the cross-section of the openings. According to the invention, the openings in the surface of the suction deflection roller advantageously have a diameter of at least 4 mm. The larger diameters make the openings less prone to clogging and easier to clean.

[0025] Another important factor influencing the capture of the fiber web during transfer is the free length f. It is advantageous if the free length f is no more than 700 mm, as this keeps the deflection of the dryer screen, caused by the negative pressure in the first suction area, within acceptable limits. On the other hand, the free length f should not be too short, otherwise the space between the dryer cylinders and the suction deflection roller decreases, which negatively affects the cross-sectional area of ​​the second chamber and the suction angle α. Therefore, it is advantageous if f is at least 420 mm. For the same reasons, it is advantageous if the distance d is at least 50 mm. A distance d that is too short can lead to damage to the dryer cylinders and the suction deflection roller from paper fragments if the fiber web breaks.

[0026] The following describes the process used to transfer the fiber web across its full width. The drying section is in the state where the fiber web is being fed into the first pulper of the drying section. The steps described below each refer to a stabilizing device. The transfer process begins at the stabilizing device located directly behind the pulper in the machine direction. The described steps are then executed for all other stabilizing devices with a time offset. This time offset depends on the web speed. The process according to the invention comprises the following steps, which are carried out by the control unit:

[0027] - Provision of an initial negative pressure

[0028] - Provision of a second negative pressure

[0029] - Activation of the transfer device

[0030] The first two steps are performed before the third step. The first two steps can be carried out in any order or simultaneously. As long as the fiber web has not been transferred, only a weak vacuum can form in the extraction areas, since the fiber web helps to seal them. Without the fiber web, a significant amount of false air is drawn in. However, since the fiber web is transferred across its entire width, this condition is quickly overcome. It is therefore advantageous to set the power of the means for generating the negative pressure high at the beginning of the transfer process and reduce it after the transfer is complete. Once the transfer is complete, the transfer device is deactivated.By optimizing the flow conditions in the flow channel relevant for the second vacuum according to the invention, the required lead time for the first two steps can be significantly reduced with respect to the third step. It should be noted here that the first vacuum is generally considerably greater than the second vacuum; that is, a stronger vacuum prevails in the first extraction area than in the second. On the other hand, the volume to be extracted in the second extraction area is many times larger than in the first. The extraction capacities required for the two extraction areas by the associated means for providing the vacuums depend primarily on how well the respective extraction areas are sealed. Among other things, the air permeability of the fibrous web plays a role here.

[0031] The following describes the procedure that is carried out when the fiber web breaks. To detect a break, the control unit includes sensors for monitoring the fiber web. These sensors are able to detect whether a fiber web is present at the relevant location or not. The steps described below are only carried out for the stabilizing devices located at points in the drying section where pulpers are arranged. The procedure according to the invention comprises the following steps, which are carried out by the control unit:

[0032] - Detection of a tear in the fiber web

[0033] - Opening the valve flap 8.2

[0034] - Deactivation of the means for providing a second negative pressure (if necessary, closing the shut-off valve 9.2)

[0035] The second and third steps are performed simultaneously. Through the optimization of the flow conditions in the flow channel relevant for the second vacuum, as described in the invention, the vacuum at the suction deflection roller can be quickly broken, so that the torn fiber web can be rapidly fed to the pulpers.

[0036] It is advantageous if, simultaneously with the second and third steps, the other valve flap 8.1 (if present) is also opened and the other means 5.1 are deactivated in order to break the vacuum in the first suction area. The fiber web is then removed from the associated drying cylinder by the corresponding scraper 6 and fed to the pulper.

[0037] Finally, it should be mentioned that a drying section according to the invention can comprise a plurality of the component groups shown in Figure 1, i.e., a plurality of drying cylinders, wherein a suction deflector roller and a stabilizing device are arranged between each pair of successive drying cylinders. This assembly is referred to below as a "drying cylinder pair" (i.e., the suction deflector roller and the stabilizing device are not specifically mentioned). With the exception of the very first and the very last drying cylinder, each intermediate drying cylinder can function as both a "first drying cylinder" and a "second drying cylinder" within the meaning of the present application. In this case, all drying cylinder pairs would be designed according to the invention.However, it can also be advantageous if only some of the drying cylinder pairs are designed according to the invention - for example, only the first two or three drying cylinder pairs of the drying section, since the risk of web breakage is greatest at this point in the drying section.

[0038] To illustrate what was said in the preceding section, a drying unit of a drying section according to the invention can be considered, as shown in Figure 3. The drying unit shown comprises five drying cylinders, five suction deflection rollers, and five stabilizing devices. With the exception of the first suction deflection roller and the first stabilizing device (far left), these elements form four pairs of drying cylinders designed according to the invention. The first suction deflection roller and the associated stabilizing device can be designed in the same way as the other corresponding elements, i.e., according to the invention. This is advantageous, for example, if another drying unit is located upstream of the drying unit shown, in the direction of the machine. If this is the first drying unit of the drying section, then it may be advantageous to design the first suction deflection roller and the associated stabilizing device differently.The control unit can control all pairs of dry cylinders designed according to the invention. However, a separate control unit can also be provided for each pair of dry cylinders designed according to the invention. It is advantageous if separate means for providing the vacuum are provided for each pair of dry cylinders designed according to the invention. For a few closely spaced pairs of dry cylinders designed according to the invention, the means for providing the vacuum can be provided jointly, i.e., supplying more than one pair of dry cylinders with vacuum.

[0039] Reference symbol list

[0040] 1.1 Drying cylinder

[0041] 1.2 Drying cylinders

[0042] 2 Suction rollers

[0043] 3 Stabilizing device

[0044] 3.1 First Chamber

[0045] 3.2 Second Chamber

[0046] 3.3 Sealing device

[0047] 3.41 Sealing device

[0048] 3.42 Sealing device

[0049] 4 Control unit

[0050] 5.1 Means of providing an initial vacuum

[0051] 5.2 Means of providing a second vacuum

[0052] 6 scraper attachment

[0053] 7 Transfer facility

[0054] 8.1 Valve flap

[0055] 8.2 Valve flap

[0056] 9.1 Shut-off valve

[0057] 9.2 Shut-off valve

Claims

Patent claims 1. Drying section of a machine for producing a fibrous web, wherein the drying section comprises at least two drying cylinders (1.1, 1.2), at least one suction deflector roller (2), at least one air-permeable drying screen guided over the drying cylinders (1.1, 1.2) and the suction deflector roller (2), and at least one stabilizing device (3), wherein the suction deflector roller (2) is arranged between a first drying cylinder (1.1) and a second drying cylinder (1.2) and comprises a shell in which a plurality of openings are arranged, and wherein the stabilizing device (3) comprises three sealing devices (3.3, 3.41, 3.42), a first chamber (3.1), and a second chamber (3.2) separated from the first chamber (3.2) by a partition, and wherein a first sealing device (3.3) seals the stabilizing device (3) against the first drying cylinder (1.1), and wherein a second Sealing device (3.41) and a third sealing device (3.42) seal the stabilizing device (3) against the suction deflection roller (2) to form a first suction area which is arranged between the first sealing device (3.3) and the second sealing device (3.41), and to form a second suction area which is arranged between the second sealing device (3.41) and the third sealing device (3.42), wherein the first suction area is connected to the first chamber (3.1), and wherein the second suction area is connected to the second chamber (3.2), characterized in that a portion of an open surface area of ​​the suction deflection roller (2) which is formed by the openings is at least 3% of the total surface area, and wherein a suction angle of the suction deflection roller (2) is at least 60°.

2. Drying section according to claim 1, wherein the proportion of an open surface area of ​​the suction deflecting roller (2) formed by the openings is at least 5% of the total surface area.

3. Drying section according to claim 1 or 2, wherein the suction angle of the suction deflection roller (2) is at least 70°.

4. Drying section according to one of the preceding claims, wherein a cross-section of the second chamber (3.2) is larger than a cross-section of the first chamber (3.1).

5. Drying section according to one of the preceding claims, wherein the drying section comprises at least one valve flap (8.2) which is arranged and designed such that a negative pressure in the second chamber (3.2) can be relieved by opening the valve flap (8.2).

6. Dry section according to one of the preceding claims, wherein the dry section comprises at least one shut-off valve (9.2) which is arranged and designed such that the means for providing the second vacuum (5.2) can be separated from the second chamber (3.2) by means of it.

7. Drying section according to one of the preceding claims, wherein the diameter of the openings is at least 4 mm.

8. Drying section according to one of the preceding claims, wherein a free length in which the drying screen runs freely between the first drying cylinder (1.1 ) and the suction deflection roller (2) is at most 700 mm.

9. Drying section according to one of the preceding claims, wherein a free length in which the drying screen runs freely between the first drying cylinder (1.1 ) and the suction deflection roller (2) is at least 420 mm.

10. Drying section according to one of the preceding claims, wherein the distance between the surface of the first drying cylinder (1.1 ) and the surface of the suction deflection roller (2) is at least 50 mm.

11. Method for transferring a fibrous web across its full width into a drying section according to any one of claims 1 to 10, wherein the drying section comprises means (5.1) for providing a first vacuum in the first chamber (3.1), means (5.2) for providing a second vacuum in the second chamber (3.2), a transfer device (7) and a control device (4) which are configured such that the control device (4) can perform the following steps of the method: - Provision of an initial negative pressure; - Provision of a second negative pressure; - Activation of the transfer device (7); wherein the first two steps are carried out in any order before the third step.

12. A method carried out in the event of a tear of the fiber web in a drying section according to any one of claims 1 to 10, wherein the drying section comprises means (5.2) for providing a second vacuum in the second chamber (3.2), means for monitoring the fiber web, a control device (4) and a valve flap (8.2) which is arranged and configured such that a vacuum in the second chamber (3.2) can be relieved by opening the valve flap (8.2), and wherein the drying section is configured such that the control device (4) can perform the following steps of the method: - Detection of a tear in the fiber web; - Opening the valve flap (8.2); - Deactivation of the means for providing the second negative pressure.

13. The method of claim 12 in conjunction with claim 6, wherein the control device (4) is configured to perform the following additional step of the method: - Closing the shut-off valve (9.2).

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