Screening unit of a screen section of a machine for producing a fibrous web

Abstract

A sieve unit (1) of a sieve section of a machine for producing a fibrous web, comprising a sieve (2) which functions as a lower sieve, wherein the sieve unit comprises at least two suction zones in which a vacuum can be built up, and at least two suction boxes (3, 4), wherein the least two suction zones are arranged in the at least two suction boxes, wherein the sieve unit comprises a control device (5), and wherein at least one suction box (3) comprises exactly one suction zone and is designed to be movable, so that it can be moved into a first position and into a second position, and wherein in the first position it can draw suction from the sieve and in the second position it cannot draw suction from the sieve, and wherein the control device is designed such thatthat it can move at least one movable suction box into the first and second positions and activate and deactivate the negative pressure in the suction zone of the same suction box.

Description

[0001] The invention relates to a screening unit of a screening section of a machine for producing a fibrous web, comprising a circulating screen functioning as a bottom screen and a plurality of suction boxes, each with at least one zone. The invention further relates to a method for operating such a screening unit. The invention also relates to a computer program for carrying out this method and a computer program product comprising such a computer program.

[0002] Screening units of this type are known from the prior art. For example, JP 2006-132074 A discloses such a screening unit. The disclosed screening unit comprises a circulating first screen functioning as a lower screen and a total of three suction boxes (designations 11, 12 and 13 in [reference]). Fig. 1 of JP 2006-132074 A), which are arranged one behind the other in the machine direction below the first screen and can draw the fiber web through this screen. In addition, the screen assembly disclosed in JP 2006-132074 A comprises a further screen functioning as an upper screen and four foil boxes, each with several foils (designations 7, 8, 9 and 10 in Fig. 1 of JP 2006-132074 A). The foils are hydrodynamic dewatering elements arranged below the first screen. The four foil boxes are arranged in the machine direction upstream of the suction boxes. JP 2006-132074 A also discloses a method for controlling the dewatering capacity in the screen section. For this purpose, individual foils (designations 35, 36 and 37 in JP 2006-132074 A) are used. Fig. 5 of JP 2006-132074 A) the foil boxes are designed to be movable, so that they can be moved downwards away from the first sieve in order to influence the dewatering capacity of the respective foil box. Furthermore, JP 2006-132074 A discloses that the dewatering capacity of the suction boxes can be adjusted by setting the negative pressure in the suction boxes. The method disclosed in JP 2006-132074 A solves the problem of preventing the fiber web from tearing.

[0003] Document US 5,489,365 A discloses a sieve assembly with two sieve units, i.e., with an upper and a lower sieve, wherein movable suction boxes are arranged in the lower sieve, which serve to adjust the geometry of the forming area.

[0004] Within the context of this document, the term "suction box" refers to a hydrostatic dewatering device, i.e., a device in which dewatering is based on the application of a vacuum beneath the screen and the fiber suspension above it. The phrases "beneath the screen" and "fiber suspension above it" are to be interpreted broadly, meaning that the direction of the gravitational force can also be oblique to the screen. Therefore, such suction boxes can also be located in areas of the screening unit where the screen is inclined to the horizontal. In this case, only a portion of the gravitational force acts in the direction of dewatering. The same applies to the term "lower screen."

[0005] Within the scope of the present invention, such suction boxes are preferably designed as flat suction boxes.

[0006] The object of the invention is to provide a screening unit in which the energy efficiency can be increased compared to a screening unit of the generic type. Therefore, the operating method according to the invention is a method for increasing the energy efficiency of a screening unit designed according to the invention.

[0007] Modern fiber web production machines are designed to produce a wide range of different fiber web types. Therefore, a screen unit in a screen section of such a machine must be capable of providing a dewatering capacity sufficient for all fiber web types being produced. This means that the maximum dewatering capacity of the screen unit is geared towards fiber web types with maximum dewatering requirements, and is therefore oversized for many fiber web types with lower dewatering demands. It is logical to adjust the dewatering capacity of the associated suction boxes by reducing the applied vacuum to the reduced requirements for producing fiber web types with lower dewatering demands.The inventors recognized, however, that this is only practical within certain limits, as the negative pressure in the suction boxes cannot be reduced arbitrarily. This is because the vacuum not only serves to dewater the fibrous web but also prevents the suction slots of the boxes from becoming clogged with fibrous material. The inventors therefore propose making some of the suction boxes movable, so that they can be moved away from the associated screen. This allows the relevant suction boxes to be completely deactivated when necessary, thereby reducing the energy consumption and increasing the energy efficiency of the screening unit without increasing the risk of the suction box slots becoming clogged.

[0008] 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.

[0009] The invention will be explained below with the aid of figures. The figures show, in detail: Fig. 1: Screening section according to the state of the art with three screening units Fig. 2: Sieve unit according to the invention Fig. 3: Movable suction box according to the invention in side view Fig. 4: Movable suction box according to the invention - View MD

[0010] Fig. Figure 1 shows a highly schematic example of a screen assembly according to the state of the art. The screen assembly depicted comprises a total of three screen units, which are labeled 1.1, 1.2, and 1.3. Each screen unit includes a rotating screen that functions as a bottom screen. The horizontal arrows indicate the direction of rotation of these screens. The screen unit labeled 1.1 includes another screen that functions as an upper screen. Screen assemblies with more than one screen unit are used in machines for the production of multi-layer fibrous webs. Fig. Figure 1 shows a headbox in the shape of a triangle for each lower screen. Fiber suspension is applied to the corresponding lower screen via these headboxes, thereby forming a layer of fiber web on that lower screen. The resulting layers of fiber web are then couched together using couching rollers to create a three-layer fiber web.

[0011] Fig. Figure 2 shows a section of a sieving unit according to the invention. The sieving unit is designated 1 and can itself constitute a sieving batch or be part of a sieving batch with more than one sieving unit. In the latter case, the other sieving units can also be constructed according to the invention. The sieving unit 1 comprises a circulating sieve, which functions as a bottom sieve. The sieve is designated 2. The horizontal arrow indicates the direction of rotation of the sieve 2. The sieving unit 1 can also comprise further sieves, which then function as top sieves; that is, only one sieve of a sieving unit 1 can function as a bottom sieve 2. Or, in other words: A sieving unit comprises a bottom sieve, a headbox, and optionally one or more top sieves. A sieving batch comprises at least one sieving unit or a combination of several sieving units.

[0012] The sieve unit 1 comprises a plurality of suction boxes arranged below the sieve 2. At least some of the suction boxes are movable. The movable suction boxes are in Fig. 2 is characterized by the vertical double arrow. One of the movable suction boxes is designated 3. Each movable suction box 3 comprises exactly one suction zone, i.e., an interior space in which a vacuum can be created. According to the invention, the vacuum is at least -20 kPa. Advantageously, the vacuum can vary in a range of -20 to -65 kPa, and particularly advantageously in a range of -20 to -55 kPa.

[0013] A movable suction box 3 can be moved into a first position in which the suction box 3 can draw the fiber web through the sieve 2. Fig. In section 2, the movable suction boxes 3 are in the first position. A movable suction box 3 can be moved to a second position in which it cannot draw in the fiber web. In the second position, the suction box 3 does not touch the screen 2. In the second position, the distance between the screen 2 and the suction box 3 is advantageously at least 10 mm, preferably 10 to 50 mm, and particularly preferably 15 mm. It is advantageous if the movement of the suction box between the two positions is in a vertical direction, i.e., perpendicular to the horizontally extending screen surface. In principle, however, it is also possible for this movement to be oblique to the vertical. An oblique movement can be advantageous, for example, if a vertical movement of the suction box in question is hindered by other components of the screen unit 1.

[0014] The in Fig. The sieve unit shown in Figure 2 comprises a further stationary, i.e., non-movable, suction box, which is designated 4. The stationary suction box 4 comprises two suction zones, i.e., two separate interior spaces in which two independent negative pressures can be established. According to the invention, the negative pressure in the suction zones is at least -20 kPa. Advantageously, the vacuum can vary in a range of -20 to -65 kPa, and particularly advantageously in a range of -20 to -55 kPa. A stationary suction box 4 can just as easily comprise only one suction zone.

[0015] A sieve unit 1 according to the invention comprises a plurality of suction zones arranged in suction boxes one behind the other in the direction of movement of the sieve 2. It is advantageous if the two suction zones furthest forward in the direction of movement of the sieve 2 are arranged in two stationary suction boxes 4 or in one stationary suction box 4. In the first alternative of the preceding sentence, each stationary suction box 4 comprises exactly one suction zone, while in the second alternative, the stationary suction box 4 comprises two suction zones. Fig. Figure 2 shows this latter case.

[0016] It can also be advantageous if the three suction zones furthest forward are arranged in fixed suction boxes 4. These can then be arranged in one suction box 4 with three suction zones, or in two suction boxes 4 (2+1 suction zones), or in three suction boxes 4, each with one suction zone.

[0017] It is also advantageous if at least the suction box located furthest back in the direction of movement includes a humidification spray tube. Fig. 2 is the suction box, which is labelled 3. The humidification spray tube is positioned so that it can moisten the surface of the skirting boards (see the explanations regarding...). Fig. 3).

[0018] It is advantageous if the movable suction boxes 3 are arranged as close together as possible in the direction of movement of the sieve 2 in order to minimize unwanted rewetting. It is advantageous if the distance between two adjacent movable suction boxes 3 in the direction of movement of the sieve 2 is in the range of 10 to 50 mm. This distance is measured along the sieve 2, i.e., at the adjacent points where the suction boxes 3 each contact the sieve in their first position.

[0019] As mentioned above, suction boxes can also be arranged in an area of ​​the sieve unit 1 where the sieve 2 does not run horizontally. This could be in Fig. 2. For example, at the point on the sieve 2 indicated by the designation "2". In this case, the suction box in question is tilted so that its suction side is directed towards the sieve surface. Both a stationary suction box 4 and a movable suction box 3 can be arranged in this way. If a movable suction box 3 is arranged at a point on the sieve 2 where the sieve runs at an angle, then the movement of the suction box 3 can be vertical or, particularly advantageously, oblique to the vertical (e.g., perpendicular to the sieve surface).

[0020] The control device 5 is designed to control the movement of the movable suction boxes 3 between the two positions. The control device 5 can control the movement of several movable suction boxes together or separately. Furthermore, the control device 5 is designed to activate and deactivate the negative pressure in the movable suction boxes 3. Naturally, energy is required to activate (and maintain) the negative pressure. This is not the case when the negative pressure is deactivated.

[0021] The control unit 5 can be part of the machine control system or form a separate unit connected to the machine control system. The control unit 5 is designed to execute the aforementioned control operations depending on predefined input variables. The input variables relevant to the invention are explained in more detail below in connection with the method according to the invention.

[0022] In connection with the Fig. 3 and Fig. 4. Possible embodiments of the movable suction boxes 3 are explained in more detail below. Based on these explanations, a person skilled in the art can easily find further embodiments without having to make an inventive contribution.

[0023] Fig. Figure 3 shows a movable suction box 3 in a side view, i.e., viewed in the machine transverse direction (CD). The viewing direction of this view is from the guide side towards the drive side. The suction box 3 comprises a box body, which is designated 6. In the upper area of ​​the box body 6 are the ribs with which the suction box 3 contacts the screen 2 in the first position. The suction slots are arranged between the ribs. Alternatively, the top of the suction box can comprise a plate with integrated slots. The ribs or the plate are provided on the surface with a wear-resistant coating. This is usually a ceramic coating. The suction box 3 also includes two actuators, only one of which is in Fig. 3 is visible and marked with 7, four or six guide rails, of which only two are in Fig. Three guide rails are visible, one of which is labelled 8, and for each guide rail 8 there is at least one stop, one of which is labelled 9. The stops 9 are adjustable and are set so that the first position and / or the second position of the suction box 3 can be reliably reached by means of the actuators 7.

[0024] Fig. Figure 4 shows the suction box 3 of Fig. 3 in a view along the machine direction (MD). The guide side is on the right and the drive side on the left. The representation of the Fig. Figure 4 is based on the fact that the suction box 3 is shown separated in the area of ​​the box body 6, with the drive side of the suction box 3 being in the first position (i.e., at the screen) and the guide side of the suction box 3 being in the second position (i.e., away from the screen). The resulting vertical offset of the two sides is indicated by the section designated D. As mentioned above, the section D is advantageously at least 10 mm, preferably 10 to 50 mm, and particularly preferably 15 mm long.

[0025] The box body 6 is connected on the right side to a guide-side console, designated 12, and on the left side to a drive-side console, designated 13. Each console comprises an actuator 7 and two or three guide rails 8. A vacuum line, designated 11, is located on the far left and can be used to supply the box body 6 with a vacuum. The connection to the vacuum line 11 is made via a detachable seal, designated 10. The seal is designed to provide its sealing effect when the suction box 3 is in the first position. The seal can advantageously be made of plastic. Instead of the seal 10, a flexible hose section could also be used, extending between the vacuum line 11 and the drive-side console 13.

[0026] The control unit 5 is connected to the actuators 7 in such a way that it can control the actuators 7 to move the box body 6 into the first and second positions.

[0027] It should be noted here that the phrase "when the suction box 3 is in the first position" actually means that the box body 6 of the suction box 3 is in the first position. The same applies to the second position.

[0028] The actuators 7 can optionally operate pneumatically, hydraulically, hydro-pneumatically, or electrically. A pneumatic or hydro-pneumatic design is preferred.

[0029] For the following description of the method according to the invention, it is assumed that the sieving unit according to the invention comprises L suction zones, M movable suction boxes, and N stationary suction boxes. In general, L ≥ 2, M ≥ 1, N ≥ 0, and L ≥ M + N ≥ 2. The L suction zones are distributed among the suction boxes, with exactly one suction zone being arranged in each movable suction box, and the remaining L suction zones being arranged in the stationary suction boxes.

[0030] In a particularly preferred embodiment, L = 6, M = 4 and N = 1 (as in Fig. 2 shown). In a further advantageous embodiment, L = 5, M = 3 and N = 1. In a further advantageous embodiment, L = 4, M = 2 and N = 1. In a further advantageous embodiment, L = 3, M = 1 and N = 1.

[0031] The method according to the invention comprises the following steps: - S1: Detection of predefined input variables by the control unit; - S2: Depending on the input variables detected in step S1, at least one movable suction box is moved by the control unit into the first or second position; - S3: The control unit deactivates the vacuum in at least one suction box if it is moved to the second position in step S2, or activates the vacuum in at least one suction box if it is moved to the first position in step S2.

[0032] Step S1 is executed periodically so that changes in the input variables can be detected promptly by the control unit. Steps S2 and S3 can be executed sequentially or simultaneously in any order. It is clear that steps S2 and S3 can only be executed after step S1 has been executed at least once.

[0033] There are several advantageous options for the input variables that are detected by the control unit and used to activate or deactivate the movable suction boxes. In a first embodiment, important operating parameters of the higher-level machine for producing a fiber web serve as input variables for the method according to the invention. For example, the input variables could include the basis weight of the fiber web currently being produced by the machine and the machine speed at which the machine is currently producing the fiber web. In this case, the control unit can form an integral part of a machine control system (MCS), so that the method according to the invention can be executed automatically when the relevant operating parameters of the machine for producing a fiber web change.

[0034] In another advantageous embodiment, measured quantities generated by certain suitable sensors serve as input quantities for the method according to the invention. Fig. 2 such a sensor is indicated by the rectangle labeled 14, which is connected to the control device.

[0035] Suitable sensors include, for example, those capable of measuring the dewatering rate in the dewatering elements of the fibrous web production machine. The dewatering elements preferably include the suction boxes of the relevant screen unit (i.e., both movable and stationary suction boxes). Other dewatering elements of the relevant screen unit are also suitable, such as a suction roller or a suction box positioned above an upper screen and thus also above the lower screen. Furthermore, all dewatering elements of the fibrous web production machine located downstream of the relevant screen unit in the machine direction are also suitable.If the sensor measurements thus recorded indicate to the control device that one or more drainage elements have an insufficient drainage quantity, then it is advantageous if one or more movable suction boxes are deactivated by the method according to the invention.

[0036] Sensors capable of detecting the moisture content of the fiber web are also suitable. For example, the control unit can deactivate one or more movable suction boxes if the moisture content of the fiber web, in the machine direction, falls below a predefined value at the end or after the relevant screening unit.

[0037] Other suitable sensors include those that measure the moisture content of a take-off felt through which the fiber web is removed from a sieve in the sieving section. Fig.2. Such a suction felt is indicated by the element designated 15. For example, the control unit can deactivate one or more movable suction boxes if the moisture content of such a suction felt falls below a predefined value.

[0038] Virtual sensors are also a possible option. For example, such a virtual sensor could be based on several material preparation process parameters of the machine used to produce a fiber web.

[0039] Sensors that measure steam consumption in the drying section can also be used. In this case, the control unit could deactivate one or more movable suction boxes for a certain period of time. If deactivating a suction box does not increase steam consumption in the drying section, then the suction box in question can remain deactivated.

[0040] It should be noted that deactivating one or more suction boxes according to the invention may lead to an increase in the suction capacity of other drainage elements of the respective screening unit. Nevertheless, deactivating one or more suction boxes according to the invention results in an increase in the energy efficiency of the respective screening unit. Reference symbol list 1.1 Sieve unit 1.2 Sieve unit 1.3 Sieve unit 1 sieve unit 2 sieves (lower sieve) 3 Movable suction box 4 Fixed suction box 5 Control unit 6 box bodies 7 Actuator 8 guide rail 9 attacks 10 Seal 11 Vacuum line 12 Console Guide Side 13 Console drive side 14 Sensor 15 Acceptance felt

Claims

[1] Screening unit (1) of a screen section of a machine for producing a fibrous web with a screen (2) which functions as a bottom screen, and wherein the screening unit (1) comprises at least two suction zones in which a vacuum can be built up and at least two suction boxes (3, 4), and wherein the least two suction zones are arranged in the at least two suction boxes (3, 4), and wherein the screening unit (1) comprises a control device (5), characterized by, that at least one suction box (3) comprises exactly one suction zone and is movably designed so that it can be moved into a first position and into a second position, and wherein the same suction box (3) can draw the fiber web through the sieve (2) in the first position and cannot draw the fiber web through the sieve (2) in the second position, and wherein the control device (5) is designed such that the control device (5) can detect predefined input variables and, depending on the detected input variables, move the at least one movably designed suction box (3) into the first and into the second position and activate and deactivate the negative pressure in the suction zone of the same suction box (3), and wherein the control device (5) is designed such that the control device (5) can deactivate the negative pressure in the suction zone of the same suction box (3) if it has been moved into the second position, and can activate it,if it has been moved to the first position. [2] Sieve unit (1) according to claim 1, wherein a vacuum can be built up in each suction zone which is at least -20 kPa. [3] Sieve unit (1) according to claim 1 or 2, wherein the distance between the sieve (2) and the at least one movable suction box (3) in the second position is at least 10 mm. [4] Sieve unit (1) according to one of the preceding claims, wherein the sieve unit (1) comprises at least two movable suction boxes (3), and wherein the distance between the least two movable suction boxes (3) in a direction of movement of the sieve (2) is in a range of 10 to 50 mm. [5] Sieve unit (1) according to one of the preceding claims, wherein the sieve unit (1) comprises three to six suction zones, one to four movable suction boxes (3) and a stationary suction box (4), wherein two suction zones are arranged in the stationary suction box (4). [6] Sieve unit (1) according to one of the preceding claims, wherein the movable suction box (3) comprises a box body (6), a guide-side console (12), a drive-side console (13) and a vacuum line (11), and wherein each console (12, 13) comprises an actuator (7) and two or three guide rails (8). [7] Method for operating a sieve unit (1) according to any of the preceding claims, comprising the following steps: S1: Detection of predefined input variables by the control unit (5); S2: Depending on the input variables detected in step S1, at least one movable suction box (3) is moved by the control device (5) into the first or second position; S3: The control unit deactivates the vacuum in at least one suction box if it is moved to the second position in step S2, or activates the negative pressure in at least one suction box if it is moved to the first position in step S2. [8] Method according to claim 7, wherein the input variables recorded in step S1 include a basis weight of the fiber web just produced and a machine speed at which the fiber web is just being produced. [9] Method according to claim 7, wherein the machine for producing a fibrous web comprises at least one sensor (14), and wherein the measured quantity generated by the sensor (14) forms an input quantity detected by the control device (5) in step S1. [10] Method according to claim 9, wherein a sensor (14) is designed such that it can detect a drainage quantity from at least one of the suction boxes (3, 4). [11] Method according to one of claims 9 or 10, wherein a sensor (14) is designed such that it can detect the moisture content of the fiber web just produced. [12] Method according to any one of claims 9 to 11, wherein the machine for producing a fibrous web comprises a take-off felt (15), and wherein a sensor (14) is designed to detect moisture in the take-off felt (15). [13] Method according to any one of claims 9 to 12, wherein the machine for producing a fibrous web comprises a drying section, and wherein a sensor (14) is designed to detect steam consumption in the drying section. [14] Computer program which is designed to perform the method according to any one of claims 7 to 13. [15] Computer program product comprising a computer program according to claim 14.

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