System and method for controlling the screen tension of a drying group

Abstract

The invention relates to a drying group and to a method for controlling the screen tension of a drying group, wherein, in a first operation, a screen tensioning device applies a first screen tension to the screen. The invention is characterised in that after the operation has been interrupted, the drying group is operated in a second operation, wherein the screen tensioning device applies a second, greater screen tension to the screen. The invention permits a reliable re-feeding of the fibrous web, in particular after operation has been interrupted.

Description

[0001] DEVICE AND METHOD FOR CONTROLLING THE SCREEN TENSION OF A DRY GROUP

[0002] The invention relates to a method for controlling the screen tension of a drying unit in a machine for producing a fibrous web, in particular a paper web, wherein the drying unit is designed as a single-layer drying unit and comprises a screen, a screen tensioning device, at least one heatable drying cylinder, at least one reversing roll, in particular one capable of being suctioned, and at least one web stabilizer; the screen is guided from the drying cylinder to the reversing roll as a screen train, the screen is capable of being suctioned in a low-pressure zone via the web stabilizer at least in a region of the screen train, and the low-pressure zone can be sealed via at least one seal of the web stabilizer acting in the direction of the screen; wherein the moist fibrous web to be dried can be guided through the drying unit, and the fibrous web can be guided on the screen through the low-pressure zone;wherein in a first operation with fibrous web, the screen tensioning device imparts a first screen tension to the screen. The invention further relates to a drying unit for a machine for producing a fibrous web, in particular designed as a paper web, especially for carrying out the process according to the invention.

[0003] Machines for producing a fibrous web, particularly a paper web, are known from the prior art, and the fibrous web is sometimes also thermally dried. The thermal drying of the fibrous web typically takes place in a drying section, e.g., a pre-drying section or a post-drying section, wherein a drying section comprises at least one drying group.

[0004] EP0383744A2, for example, discloses a multi-cylinder dryer for a paper machine with several single-wire pull sections arranged in succession, wherein two transfer suction rollers are used, the first transfer suction roller being arranged within the loop of a first drying wire and the second transfer suction roller being arranged within the loop of a second drying wire, and wherein a roller guiding the first drying wire is adjustable such that the overlap of the second transfer suction roller can be adjusted. DE102008042282A1 discloses a method for transferring a

[0005] Fiber web in a machine for producing a fiber web comprising a wet section and a drying section downstream of the wet section with at least one drying device and a transfer arrangement for transferring the fiber web to the drying device, forming a transfer area, and is characterized in that, in the event of a fiber web break or a start-up of the machine, an edge strip is transferred up to the full fiber web width free from support in the transfer area by the coverings when the transfer area is opened.

[0006] This involves regulating the tension of the screen in the drying group, with the aim of ensuring a uniform screen tension during operation, e.g. measured as force per unit length [kN / m], where the unit length is related to the working width of the screen - corresponding to the width of the fiber web.

[0007] In particular, the regulation of the screen tension is intended to ensure that any elongation of the screen resulting from operation is compensated for by the screen tensioning device and that the screen tension is kept at the desired level.

[0008] It is also known that after an interruption, e.g. a standstill of the drying unit or in particular a break in the fiber web, guiding the fiber web through the drying unit or re-feeding the fiber web can be difficult and time-consuming.

[0009] The aim of the invention is to improve the re-feeding of the fiber web by the drying group after an interruption of operation, whereby the time until successful re-feeding is reduced or the number of possible breaks until successful re-feeding is reduced.

[0010] According to the invention, this is achieved by the fact that the first operation is terminated by an interruption, e.g. a standstill of the drying group or in particular a break in the fiber web, and no fiber web is guided through the drying group in the meantime; wherein, after the interruption, a second operation of the drying group is started to refeed the fiber web, the screen tensioning device imposes a second screen tension on the screen, and the second screen tension is greater than the first screen tension.

[0011] According to the invention, a drying unit comprises a screen tensioning device, at least one heated drying cylinder, at least one reversing roller (which is preferably suction-fed), and at least one web stabilizer, wherein the aforementioned machine elements interact with a screen (preferably a single screen). A screen tension can be applied to the screen via the screen tensioning device, with the drying cylinder and the reversing roller being at least partially enclosed by the screen. A moist fiber web to be dried is guided on the screen by the drying unit, supported by the screen. The fiber web, arranged between the screen and the drying cylinder, is pressed against the drying cylinder by the screen according to the screen tension. The fiber web is guided on the screen from the drying cylinder to the reversing roller as a screen pull, with the screen being guided around the reversing roller between the fiber web and the reversing roller.A corresponding arrangement is known to those skilled in the art as a single-felted drying unit or by the technical term "single felted". According to the invention, the screen is capable of being drawn into a vacuum zone via the web stabilizer, at least in one area of ​​the screen section, wherein the vacuum zone can be sealed by at least one seal of the web stabilizer acting towards the screen. The vacuum zone, in particular, ensures a reliable separation of the fiber web from the drying cylinder, so that the fiber web is guided on the screen from the drying cylinder via the screen section to the reversing roll. The seal of the web stabilizer acting towards the screen is necessary to create a vacuum in the vacuum zone.According to the initial screen tension imposed on the screen by the screen tensioning device during the first operation with fibrous material, it is ensured that a harmful deflection of the screen - especially in the direction of the seal of the web stabilizer - is avoided, which could lead to the formation of folds or warping of the screen or the fibrous web guided on the screen.

[0012] The inventor recognized that unexpected temperature behavior of the screen, particularly the drying screen for a drying unit of a fibrous web production machine, has serious consequences for the refeeding of the fibrous web after an interruption of operation, especially a standstill of the drying unit or a break in the fibrous web. In these cases, the screens or drying screens expand or lengthen locally, causing a local drop in screen temperature, which leads to an unexpected deflection of the screen in the area of ​​the screen pull and especially in the low-pressure zone. Furthermore, it was recognized that these local expansions or elongations do not necessarily correspond to an overall elongation of the screen and are therefore not detectable by the screen tensioning device and—if the screen tension is kept constant—are not compensated for by the screen tensioning device.

[0013] Immediately after the interruption, the screen is passed over at least one heated drying cylinder before the fiber web is re-feeded. Without heat being drawn from the drying cylinder, temperatures develop on its surface that exceed those during regular operation, i.e., the initial run. Consequently, the screen will exhibit higher temperatures than during regular operation, i.e., the initial run. However, as soon as the wet and comparatively cooler fiber web is transferred to the screen—at the start of the re-feed—a greater local cooling of the screen temperature occurs due to the fiber web compared to steady-state operation, particularly between the initial transfer of the fiber web to the screen and the subsequent drying cylinder.The significant cooling of the screen, due to its observed temperature behavior, leads to unexpectedly pronounced deflections, particularly in the area of ​​the screen pull and especially in the low-pressure zone. This promotes the formation of wrinkles or distortions in the screen or the fiber web guided on the screen. The screen tension control according to the invention counteracts this behavior. By operating the drying unit after the interruption and at least during the refeeding of the fiber web, whereby the screen tensioning device imposes a second, higher screen tension on the screen, the local deflections of the screen are reduced, and effective refeeding of the fiber web by the drying unit is enabled. The time required for successful refeeding, as well as the number of potential breaks, is reduced.As a further effect, increased contact between the at least one seal acting towards the screen and the screen is avoided during re-feeding, thus preventing increased wear of both the seal and the screen, resulting in lower operating costs, particularly for consumables such as seals and screens. In an advantageous embodiment of the method, in a third operation of the drying unit following the second operation, after a complete re-feed of the fibrous web, the screen tensioning device applies a third screen tension to the screen, wherein the third screen tension is lower than the second screen tension.According to the invention, applying a second screen tension that is higher than the first allows for more effective refeeding of the fiber web. After successful refeeding and the achievement of quasi-steady-state thermal conditions, the observed unexpected temperature behavior of the screen recedes. Following complete refeeding, the screen tension can then be reduced again, i.e., from the second screen tension for refeeding the fiber web to the third screen tension for operation after refeeding. For example, the third screen tension can essentially correspond to the first screen tension. Similarly, for example, the screen tension can be reduced from the second screen tension to the third screen tension no later than 30 minutes, preferably no later than 15 minutes, and particularly preferably no later than 5 minutes, after complete refeeding of the fiber web.Reducing the screen tension can, in particular, reduce the stress on the screen resulting from the higher screen tension, which is advantageous for the service life of the screen.

[0014] In an equally advantageous embodiment of the method, in the first operation with a fibrous web, a first power is applied to create a first vacuum in the vacuum zone; in the second operation, at least intermittently, a second power is applied to create a second vacuum in the vacuum zone; wherein the second power for creating the second vacuum is greater than the first power for creating the first vacuum. Typically, in the regular first operation, the first power for creating the vacuum is applied, for example, as drive power for a device for creating a vacuum.Especially during the re-feeding of the fiber web, applying a higher power output to create the second vacuum in the vacuum zone is advantageous, for example, to ensure effective guidance of the fiber web on the screen, and in particular to guarantee the reliable transfer of the fiber web from the drying cylinder to the screen. Furthermore, the higher power output for creating the vacuum does not necessarily result in a higher measurable vacuum in the vacuum zone or greater screen deflection, as the permeability of the screen can vary considerably with and without the fiber web.

[0015] In a further advantageous embodiment of the method, the screen tension is increased to the second screen tension after an interruption of operation, compared to the first screen tension. This increase is more than 0.1 kN / m, preferably more than 0.5 kN / m, and particularly preferably more than 1 kN / m. The first screen tension is advantageously in the range of 2.5 kN / m to 5.5 kN / m, and preferably in the range of 3.0 kN / m to 5.0 kN / m. The significant increase in screen tension during the refeeding of the fiber web minimizes the detrimental deflection of the screen—particularly in the direction of the web stabilizer seal—and thus prevents the formation of creases or distortions in the screen or the fiber web guided on the screen.

[0016] A particularly advantageous method is one in which the vacuum zone is formed between a first seal – acting in particular against the drying cylinder or the screen train – and a second seal – acting in particular against the screen train or the reversing roller. During operation, especially during the first or third operation, the vacuum zone is operated to achieve a vacuum greater than 500 Pa, preferably greater than 1500 Pa, and most preferably greater than 2500 Pa, which is easily accomplished by appropriately selecting the drive power of the associated device for generating the vacuum.

[0017] In an equally preferred embodiment of the method, the temperature of the screen in the drying unit is measured using a temperature sensor, and the second screen tension is controlled as a function of the measured temperature. The temperature sensor is connected to a control device, which determines a setpoint for the second screen tension based on the screen temperature measured by the temperature sensor. This setpoint is then transmitted to the screen tensioning device to achieve an actual value of the second screen tension corresponding to the setpoint. By measuring the screen temperature, the screen tension can be increased, i.e.,the second screen tension can be limited to a required level, thus avoiding a general over-increasing of the screen tension, which in particular reduces the maximum screen tension acting on the screen and the corresponding load time, which is advantageous for the service life of the screen.

[0018] In a particularly advantageous embodiment of the method, when operating with a fibrous web, especially in the first operation and / or in the second operation, at least one of the drying cylinders is operated as a high-temperature drying cylinder. The steam-heated high-temperature drying cylinder is operated at a steam pressure of more than 1 bar g, particularly more than 3 bar g, and preferably more than 6 bar g. Alternatively, a heated high-temperature drying cylinder is operated such that a surface temperature of more than 100°C, particularly more than 130°C, and preferably more than 150°C, is achieved. The effect according to the invention is particularly pronounced in drying units comprising a high-temperature cylinder, since the temperature behavior of the screen, as identified by the inventor, is particularly effective in these cases.

[0019] In a further advantageous embodiment of the method, the screen tension is increased to the second screen tension after the interruption of operation. In a first alternative, the increased second screen tension is applied to the screen at least during the widening of the fiber web, and preferably also during the feeding of a strip of the fiber web through the drying unit. The method according to the invention is thus particularly suitable for refeeding the fiber web, wherein initially only a portion of the fiber web is fed through the drying unit as a strip. After successful feeding of the strip through the drying unit, the strip is spread to the width of the fiber web, whereby the entire width of the fiber web is finally fed through the drying unit.In a second alternative of the process, it is equally advantageous to imprint the increased second screen tension on the screen at least before the fiber web is re-feeded. This applies to a process in which the fiber web is re-feeded immediately at its full width. The invention also relates to a drying unit for a machine for producing a fiber web, particularly one designed as a paper web, and especially for carrying out a process according to the invention, wherein the drying unit is designed as a single-layered drying unit, comprising:

[0020] - a sieve

[0021] - a screen tensioning device

[0022] - at least one heated drying cylinder

[0023] - at least one reversing roller that is particularly suitable for suction, e.g. guide roller, heated roller, suction roller, etc.

[0024] - at least one track stabilizer

[0025] - a control device and

[0026] - a sensor for detecting an interruption of operation, in particular for detecting a break in the fiber web, wherein the sensor is connected to the control device, wherein the screen can be guided from the at least one drying cylinder to the at least one reversing roll via a screen train, the screen can be drawn into a vacuum zone via the web stabilizer at least in one area of ​​the screen train, and the vacuum zone can be sealed via at least one seal of the web stabilizer acting in the direction of the screen; wherein the moist fiber web to be dried can be guided through the drying unit and the fiber web on the screen can be guided through the vacuum zone; and wherein an interruption of operation, e.g.a standstill of the drying group or, in particular, a break in the fiber web, which can be detected by the sensor, wherein the control device is designed to control the screen tensioning device on the basis of the detected interruption of operation in such a way that, in order to refeed the fiber web, the screen tensioning device imposes a second screen tension on the screen, which second screen tension is greater than a first screen tension before the detection of the interruption of operation.

[0027] The drying unit according to the invention particularly includes a sensor connected to the control device for detecting an interruption of operation, whereby, after an interruption of operation, the control device can automatically activate the screen tensioning device such that the relatively higher second screen tension is applied to the screen to refeed the fibrous web. Thus, the advantageous effect of the method according to the invention can be achieved automatically in the system. Preferably, the drying unit includes a temperature sensor for detecting the temperature of the screen in the drying unit, wherein the temperature sensor is connected to the control device, wherein the control device is configured to control the screen tensioning device based on the detected temperature of the screen, and wherein the second screen tension is adjustable as a function of the measured temperature.As already described in relation to the inventive method, this allows the control of the screen tension so that the second screen tension is increased only to the necessary extent, whereby the increase in screen tension depends on the measured temperature of the screen. A person skilled in the art can easily determine the required dependency through routine tests. The temperature sensor is particularly preferably arranged between a first transfer of the fiber web onto the screen of the drying unit and the subsequent drying cylinder. This arrangement of the...

[0028] The temperature sensor allows for advantageous measurement of the sieve's temperature, since the sieve and the fibrous web exhibit a significantly large temperature difference.

[0029] In an advantageous embodiment of the drying unit, the vacuum zone is formed between a first seal – acting particularly against the drying cylinder or the screen train – and a second seal – acting particularly against the screen train or the reversing roller – and is capable of being drawn in via the web stabilizer. For example, the vacuum zone can be formed between a first seal acting against the drying cylinder and a second seal acting against the screen train. Similarly, the vacuum zone can be formed between a first seal acting against the screen train and a second seal acting against the screen train. Finally, the vacuum zone can be formed between a first seal acting against the screen train and a second seal acting against the reversing roller, or the vacuum zone can be formed between a first seal acting against the drying cylinder and a second seal acting against the reversing roller.

[0030] In an equally advantageous embodiment of the drying unit, at least one drying cylinder designed as a high-temperature drying cylinder is included, wherein the steam-heated high-temperature drying cylinder is designed for operation in the drying unit with a steam pressure of more than 1 bar g, in particular more than 3 bar g, and preferably more than 6 bar g. Alternatively, the high-temperature drying cylinder is designed for operation in the drying unit with a surface temperature of more than 100°C, in particular more than 130°C, and preferably more than 150°C. Such high-temperature drying cylinders can, for example, be heated by thermal oil, gas, or electricity.

[0031] The effect according to the invention increases with the temperature achievable on the high-temperature cylinder.

[0032] The invention will now be described using the drawings as an example.

[0033] Fig. 1 shows a drying group according to the invention.

[0034] Fig. 2a shows a track stabilizer.

[0035] Fig. 2b shows another track stabilizer.

[0036] Fig. 3 illustrates a first step of the method according to the invention.

[0037] Fig. 4 illustrates a second step of the method according to the invention.

[0038] Fig. 1 shows a drying unit 1 according to the invention for a machine for producing a fibrous web 2 – in particular designed as a paper web – which is particularly suitable for carrying out the process according to the invention. The drying unit 1 comprises: a screen 3; a screen tensioning device 4 (e.g., a screen tensioner with a movable tensioning roller); at least one heated drying cylinder 5; at least one, in particular, suction-fed reversing roller 6; and at least one web stabilizer 7. The drying unit 1 is designed as a single-layer drying unit 1, i.e., the drying unit 1 comprises exactly one screen 3, wherein the fibrous web 2 is guided on the screen 3 from the drying cylinder 5 to the reversing roller 6 as a screen pull 8, and the screen 3 is guided around the reversing roller 6 between the fibrous web 2 and the reversing roller 6.

[0039] The drying unit further comprises a control device 12 (not shown) and a sensor 13 (not shown) for detecting an interruption of operation, in particular for detecting a break in the fiber web, wherein the sensor 13 is connected to the control device 12.

[0040] The moist fiber web 2 to be dried is guided through the drying unit 1, whereby the fiber web 2 can be guided on the screen 3 through a low-pressure zone 9 (not shown). If an interruption of operation, e.g., a standstill of the drying unit 1 or, in particular, a break in the fiber web 2, is detected, for example, via the sensor 13, a second screen tension is applied to the screen 3 via the screen tensioning device 4 to refeed the fiber web 2. This second screen tension is greater than the first screen tension before the interruption of operation was detected.

[0041] Fig. 2a shows a web stabilizer 7, wherein the vacuum zone 9 is formed between a first seal 10 acting against the drying cylinder 5 and a second seal 10 acting against the reversing roller 6, and is capable of being drawn in via the web stabilizer 7. The screen 3 is guided from the drying cylinder 5 to the at least one reversing roller 6 via the screen train 8, wherein the screen 3 is drawn in at least in a vacuum zone 9 via the web stabilizer 7 in at least one region of the screen train 8, and wherein the vacuum zone 9 is sealed by at least one seal 10 of the web stabilizer 7 acting in the direction of the screen 3.

[0042] Fig. 2b shows a further web stabilizer 7, wherein the vacuum zone 9 is formed between a first seal 10 acting against the drying cylinder 5 and a second seal 10 acting against the screen train 8, and is drawn in via the web stabilizer 7. The screen 3 is guided from the drying cylinder 5 to the at least one reversing roller 6 via the screen train 8, wherein the screen 3 is drawn in at least in a region of the screen train 8 within the vacuum zone 9 via the web stabilizer 7, and the vacuum zone is sealed by the first and the second seal 10 of the web stabilizer 7 acting towards the screen 3.

[0043] Fig. 3 illustrates a first step of the inventive process, wherein the increased second screen tension is imposed on the screen 3 at least during a widening of the fiber web 2, and preferably also during a transfer of a strip of the fiber web 2 through the drying group 1 .

[0044] In Fig. 3, the upper section indicates the fiber web 2, which, during operation, spans the full width between the operator side and the drive side. Initially, in a first operation, the drying unit 1 operates with the fiber web 2 at its full width, during which time the operation is interrupted. After the interruption, operation continues without the fiber web, followed by a second operation to re-engage the fiber web 2. This second operation can include the transfer of a strip of the fiber web 2 through the drying unit 1. Once the strip of fiber web 2 has been successfully transferred through the drying unit 1, the fiber web 2 can be widened, resulting in operation with the fiber web 2 at its full width.According to the invention, in the first operation with the fiber web 2, a first screen tension is applied to the screen 3, wherein the first operation is terminated by an interruption, e.g., a standstill of the drying unit 1 or, in particular, a break in the fiber web 2. After the interruption, the second operation of the drying unit 1 is resumed to re-feed the fiber web 2, wherein the screen tensioning device 4 applies the second screen tension to the screen 3, and the second screen tension is greater than the first screen tension. Thus, Fig. 3 shows by way of example that the first screen tension is applied to the screen 3 during the first operation before the interruption and also partially during operation without the fiber web 2. In a first alternative, the second, higher screen tension is applied to the screen 3 before or during the transfer of the strip. In a second alternative, the second, higher screen tension is applied before the fiber web 2 is spread out.The temperature profile of the screen 3 is also shown as an example, whereby the screen temperature rises significantly when operation is interrupted and only drops to a temperature corresponding to the initial operation once the fiber web 2 has been spread out. Corresponding to the increased temperature of the drying screen 3 during the transfer or spreading of the strip, the temperature of the screen 3 drops sharply locally when the screen 3 comes into contact with the strip of the fiber web 2, or, during spreading, with the fiber web 2 itself. This causes the screen 3 to expand or lengthen locally, leading to the unexpected deflection of the screen 3 in the area of ​​the screen tension 8 and, in particular, the low-pressure zone, as observed by the inventors. The advantages of the invention are achieved by applying the second increased screen tension.Finally, a typical power curve for generating the negative pressure at the negative pressure zone 9 is also shown, wherein in the first operation with fiber web 2, a first power is applied to generate the first negative pressure at the negative pressure zone 9; in the second operation, the second power is applied at least intermittently to generate the second negative pressure at the negative pressure zone 9; and wherein the second power for generating the second negative pressure is greater than the first power for generating the first negative pressure. Fig. 4 illustrates a second operation of the method according to the invention, wherein the increased second screen tension is applied to the screen 3 at least before the fiber web 2, which is to be fed in its full width, is re-engaged. Again, the fiber web 2 is indicated in the upper area, which, during operation, has its full width between the operator side and the drive side.Initially, in the first operation, the drying unit 1 operates with the fiber web 2 at its full width, with an interruption at one point. After the interruption, operation continues without the fiber web 2, followed by a second operation to re-feed the fiber web 2 at its full width. After successful re-feeding, operation with the fiber web 2 at its full width is achieved. According to the invention, in the first operation with the fiber web 2, the first screen tension is applied to the screen 3, with the first operation being terminated by the interruption, e.g., a standstill of the drying unit 1 or, in particular, a break in the fiber web 2. After the interruption, the second operation of the drying unit 1 to re-feed the fiber web 2 is initiated, with the second screen tension being greater than the first. In Fig.Figure 4 illustrates that the first screen tension is applied to the screen 3 during the initial operation before the interruption and also partially during operation without the fiber web 2. Before restarting, the second, higher screen tension is applied to the screen. The temperature profile of the screen is also shown as an example, where the drying unit 1 is operated with high-temperature drying cylinders, and the high-temperature drying cylinders have a surface temperature significantly higher than the screen temperature. With the interruption of operation, the cooling effect of the fiber web 2 ceases, thus exposing the screen 3 directly to the higher temperature of the high-temperature drying cylinders; consequently, the screen temperature rises after the interruption. Typically, during operation without the fiber web, the temperature of the high-temperature drying cylinders decreases, which is evident from the subsequent decrease in the screen temperature.Typically, the temperature of the high-temperature drying cylinders is raised again before the fiber web 2 is re-engaged, which also leads to a renewed increase in the temperature of the screen 3. Once the fiber web 2 is re-engaged and operation with the fiber web 2 at its full width is established, the screen temperature drops back to a normal operating level. Again, during the re-engagement of the fiber web 2, the elevated temperature of the drying screen 3 drops sharply locally due to the cooling effect of the fiber web 2, causing the screen 3 to expand or lengthen locally. This leads to the unexpected deflection of the screen 3 in the area of ​​the screen pull 8 and, in particular, the low-pressure zone 9, as observed by the inventors.Furthermore, a typical power curve for creating the negative pressure at the negative pressure zone 9 is also shown, wherein in the first operation with fiber web 2 a first power is applied to create the first negative pressure at the negative pressure zone 9; in the second operation, at least intermittently, the second power is applied to create the second negative pressure at the negative pressure zone 9; wherein the second power for creating the second negative pressure is greater than the first power for creating the first negative pressure.

[0045] The present invention offers numerous advantages. For example, after an interruption of operation, the inventive method allows for improved refeeding of the fiber web by the drying unit, reducing the time required for successful refeeding and the number of potential breaks. In particular, the formation of creases or distortions of the screen or the fiber web guided on the screen during refeeding is reduced. By minimizing local screen deflections, effective refeeding of the fiber web is enabled. Likewise, increased contact between the at least one seal acting towards the screen and the screen is avoided during refeeding, thus preventing increased wear of both the seal and the screen, resulting in lower operating costs, especially for consumables such as seals and screens.

[0046] Reference sign

[0047] 1 Dry group

[0048] 2 Fiber web

[0049] 3 Sieve 4 Sieve clamping device

[0050] 5 drying cylinders

[0051] 6 Reversing roller

[0052] 7 Track stabilizer 8 Sieve train

[0053] 9 Low-pressure zone

[0054] 10 Seal

[0055] 11 Temperature sensor

[0056] 12 Control device 13 Sensor for detecting an interruption of operation

Claims

Patent claims 1. Method for controlling the screen tension of a drying unit (1) in a machine for producing a fibrous web (2), in particular a paper web, wherein the drying unit (1) is designed as a single-threaded drying unit and comprises a screen (3), a screen tensioning device (4), at least one heatable drying cylinder (5), at least one, in particular, suction-filled reversing roll (6) and at least one web stabilizer (7); the screen (3) is guided from the drying cylinder (5) to the reversing roll (6) as a screen train (8), the screen (3) is suction-filled at least in a region of the screen train (8) in a low-pressure zone (9) via the web stabilizer (7), and the low-pressure zone (9) is sealable via at least one seal (10) of the web stabilizer (7) acting in the direction of the screen (3); wherein the moist fiber web (2) to be dried can be guided through the drying group (1 ) and the fiber web (2) on the sieve (3) can be guided through the low-pressure zone (9);wherein in a first operation with fiber web (2) the screen tensioning device (4) imposes a first screen tension on the screen (3), characterized in that the first operation is terminated by an interruption, e.g. a standstill of the drying unit (1) or in particular a break in the fiber web (2) and in the meantime no fiber web (2) is guided through the drying unit (1); wherein after the interruption a second operation of the drying unit (1) is started to refeed the fiber web (2), the screen tensioning device (4) imposes a second screen tension on the screen (3), and the second screen tension is greater than the first screen tension.

2. Method according to claim 1, wherein in a third operation of the drying group (1) following the second operation, after the complete re-feeding of the fibrous web (2), the screen tensioning device (4) imposes a third screen tension on the screen (3), the third screen tension being smaller than the second The sieve tension is, and particularly preferably the third sieve tension corresponds essentially to the first sieve tension.

3. Method according to claim 2, wherein the screen tension is increased to the second screen tension after the interruption of operation compared to the first screen tension; and wherein, at the latest 30 min, preferably at the latest 15 min and particularly preferably at the latest 5 min, after complete re-feeding of the fibrous web (2), the screen tension is reduced from the second screen tension to the third screen tension.

4. Method according to claim 1 or 3, wherein in the first operation with fibrous web (2) a first power is applied to create a first negative pressure at the negative pressure zone; in the second operation a second power is applied at least temporarily to create a second negative pressure at the negative pressure zone; wherein the second power to create the second negative pressure is greater than the first power to create the first negative pressure.

5. A method according to any one of claims 1 to 4, wherein the screen tension is increased to the second screen tension after the interruption of operation compared to the first screen tension, and the increase in screen tension is more than 0.1 kN / m, preferably more than 0.5 kN / m and particularly preferably more than 1 kN / m.

6. Method according to one of claims 1 to 5, wherein the vacuum zone (9) is formed between a first seal (10) - in particular acting against the drying cylinder or the screen train - and a second seal (10) - in particular acting against the screen train or the reversing roller - and the vacuum zone (9) is operated in particular to achieve a vacuum greater than 500 Pa, preferably greater than 1500 Pa and particularly preferably greater than 2500 Pa.

7. Method according to any one of claims 1 to 6, wherein the first screen tension is in a range of 2.5 kN / m to 5.5 kN / m and preferably in a range of 3.0 kN / m and 5.0 kN / m.

8. Method according to any one of claims 1 to 7, wherein the temperature of the sieve (3) in the drying group (1) is measured with a temperature sensor (11), and the second sieve tension is controlled as a function of the measured temperature.

9. Method according to any one of claims 1 to 8, wherein in operation with fibrous web(2), in particular in the first operation and / or in the second operation, at least one of the drying cylinders (5) is operated as a high-temperature drying cylinder; wherein the high-temperature drying cylinder is operated with a vapor pressure of more than 1 bar g, in particular more than 3 bar g and preferably more than 6 bar g, or alternatively wherein the high-temperature drying cylinder is operated such that a surface temperature of more than 100°C, in particular more than 130°C and preferably more than 150°C is achieved.

10. Method according to any one of claims 1 to 9, wherein the screen tension is increased to the second screen tension after the interruption of operation, wherein in a first alternative the increased second screen tension is imposed on the screen (3) at least during a widening of the fiber web (2), and preferably also during a transfer of a strip of the fiber web (2) through the drying group (1); and wherein in a second alternative the increased second screen tension is imposed on the screen (3) at least before a refeed of the fiber web (2) to be fed in full width.

11. Drying unit (1) for a machine for producing a fibrous web (2) in particular designed as a paper web and for carrying out a method according to one of claims 1 to 10, wherein the drying unit (1) is designed as a single-layered drying unit (1), comprising: - a sieve (3) - a screen tensioning device (4) - at least one heated drying cylinder (5) - at least one particularly absorbent reversing roller (6) - at least one track stabilizer (7) - a control device (12) and - a sensor (13) for detecting an interruption of operation, in particular for detecting a break in the fiber web, wherein the sensor (13) is connected to the control device (12), wherein the screen (3) can be guided from the at least one drying cylinder (5) to the at least one reversing roller (6) via a screen train (8), the screen (3) being in a position at least in one area of ​​the screen train (8). The vacuum zone (9) can be drawn in via the web stabilizer (7), and the vacuum zone (9) can be sealed via at least one seal (10) of the web stabilizer (7) acting in the direction of the screen (3); wherein the moist fiber web (2) to be dried can be guided through the drying unit (1) and the fiber web (2) can be guided on the screen (3) through the vacuum zone (9); characterized in that an interruption of operation, e.g. a standstill of the drying unit (1) or in particular a tear of the fiber web (2), can be detected via the sensor (13), wherein the control device (12) is designed to control the screen tensioning device (4) on the basis of the detected interruption of operation in such a way that, in order to refeed the fiber web (2), the screen tensioning device (4) imposes a second screen tension on the screen (3), which second screen tension is greater than a first screen tension before the interruption of operation was detected.

12. Drying group (1) according to claim 11, wherein the vacuum zone (9) is formed between a first seal (10) - in particular acting against the drying cylinder or the screen train - and a second seal (10) - in particular acting against the screen train or the reversing roller - and is capable of being suctioned via the web stabilizer (7).

13. Drying unit (1) according to one of claims 11 to 12, comprising a temperature sensor (11 ) for detecting the temperature of the sieve (3) in the drying unit (1 ), wherein the temperature sensor (11 ) is connected to the control device (12), wherein the control device (12) is configured to control the sieve tensioning device (4) on the basis of the detected temperature of the sieve (3), and the second sieve tension is adjustable depending on the measured temperature.

14. Drying group (1) according to claim 13, wherein the temperature sensor (11) is arranged between a first transfer of the fibrous web onto the screen (3) of the drying group (1) and the subsequent drying cylinder (5).

15. Drying unit (1) according to one of claims 11 to 14, comprising at least one drying cylinder (5) designed as a high-temperature drying cylinder, wherein the high-temperature drying cylinder is designed for operation in the drying unit (1) with a vapor pressure of more than 1 bar g, in particular more than 3 bar g and preferably more than 6 bar g, or wherein the The high-temperature drying cylinder is designed for operation in the drying group (1) with a surface temperature greater than 100°C, in particular greater than 130°C and preferably greater than 150°C.

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