Filter module for a rotary disc filter and rotary disc filter

Abstract

The invention relates to a filter bag (1) for a rotary disc filter used for filtering wastewater. The filter bag (1) has a filter material (2) with a mounting opening (3) over which it slides on a support frame (4) of the rotary disc filter during assembly to form a filter module (5). The filter material (2) has an outlet opening (6), preferably opposite the mounting opening (3), which, in the intended use of the filter bag (1), allows the filtrate to flow from inside the filter module (5) into a central tube (7) of the rotary disc filter once the filter module (5) has been connected to said tube. According to the invention, a reinforcement (26), connected in particular to the filter material (2), is provided in the region of the rim (8) that defines the mounting opening (3), which serves to secure the filter bag (1) to the support frame (4).Also described are a filter module (5) and a rotary disk filter.

Description

[0001] The present disclosure relates to an unclaimed filter bag for a rotary disc filter used for filtering wastewater, wherein the filter bag has a filter material with a mounting opening through which the filter bag can be slipped over a support frame of the rotary disc filter during an assembly step to form a filter module, wherein the filter material has an outlet opening, preferably opposite the mounting opening, which in the intended use of the filter bag serves to allow filtrate to escape from the interior of the filter module into a central tube of the rotary disc filter after the filter module has been connected to the central tube.

[0002] Furthermore, a filter module for a rotary disc filter is claimed, wherein the filter module has a support frame and a filter bag that surrounds the support frame at least partially.

[0003] Finally, the invention relates to a rotary disc filter with a central tube and several filter discs arranged adjacent to each other in the longitudinal direction of the central tube and connected to the central tube, wherein each filter disc comprises several filter modules arranged adjacent to each other in the circumferential direction of the central tube.

[0004] A rotary disc filter of this type is known, for example, from DE 39 26 962 A1. Such rotary disc filters, and this also applies to the rotary disc filter according to the present invention, are used, for example, to remove solids from wastewater.

[0005] GB 836 086 A discloses a device for clamping cover segments of a filter medium for rotating disc filters. A base part supports two parallel structures with a sliding crossbeam that has hooks for clamping the cover segment. Furthermore, a cover segment with eyelets is provided, which is held in a clamped position by a clamping system with hand-tightened screws and by sewing or tying the eyelets.

[0006] Publication GB 861 751 A discloses a filter system, particularly for vacuum filters, in disc form. A cover is proposed that carries filter cloths on both sides over openings in a flexible frame and can be slipped over the rigid frame. This is intended to facilitate assembly and operation.

[0007] US Patent 4,865,732 A1 discloses a mesh filter bag that fits over a metal support. The bag has opposite edges that extend lengthwise along one side from the bottom to the top. A rope runs from the bottom along one edge, over the top, and along the other edge back to the bottom.

[0008] In principle, a rotary disc filter comprises a container in which the individual filter discs are arranged adjacent to one another on the central tube. The central tube, along with the individual filter discs, can be driven around its longitudinal axis by a drive mechanism. When the wastewater to be treated is introduced into the container, it flows from the outside through the filter surface of the individual filter modules (from which the individual filter discs are composed), into the interior of the filter modules, and from there into the central tube. Solids present in the wastewater are retained by the filter surface of the filter modules, with the degree of solids removal depending on the properties of the filter surface. The filtrate that enters the central tube is then discharged from the rotary disc filter.It is also already known to realize the filter surface by means of a filter cloth, which is bag-shaped and is placed over a support frame of the respective filter module during its manufacture and then fixed to the support frame by clamps.

[0009] A disadvantage of this solution is that the filter cloth can slip relative to the support frame during operation of the rotary disc filter, so that in the worst case wastewater can bypass the filter surface and enter the interior of the filter modules, ultimately contaminating the filtrate already obtained.

[0010] The object of the present invention is to further develop the described prior art.

[0011] The problem is solved by a filter module and a rotating disk filter with the features of the independent patent claims.

[0012] The disclosure relates to an unclaimed filter bag for a rotary disc filter used for filtering wastewater, wherein the filter bag has a filter material with a mounting opening through which the filter bag can be slipped over a support frame of the rotary disc filter during an assembly step. The mounting opening is a slot or other interruption in the filter material, the dimensions of which are such that the support frame can be inserted into the interior of the filter bag through the mounting opening (alternatively, the filter bag can also be pulled over the support frame). After the filter bag has been pulled onto the support frame, which is preferably made of plastic, or after the support frame has been inserted into the filter bag, the filter bag has approximately the same shape as the support frame, which in turn preferably has a trapezoidal base shape.The individual sides of the trapezoidal shape do not necessarily have to be straight. For example, it is also conceivable that the side in the intended use of the filter modules (support frame with filter bag) that rests against a central tube of a rotary disc filter and / or the side that is outside the central tube has a circular arc shape.

[0013] The filter material further comprises one or more outlet openings, preferably opposite the mounting opening, which, in the intended use of the filter bag, serve to allow filtrate to exit from the interior of the filter module into a central tube of the rotary disc filter after the filter module has been connected to the central tube. During operation of the rotary disc filter, wastewater thus flows from the outside through the filter bag into the interior of the respective filter module, with solids up to a certain size being retained by the filter bag.

[0014] If the filter bag were simply pulled over the support frame and the resulting filter module connected to a central tube of a rotary disc filter, there would be a risk of the filter bag slipping relative to the support frame. While it has been suggested to fix the filter bag to the support frame at selected points using clamps, the filter bag is typically made of a fabric-like material with little or no inherent stability. If the rotation of the individual filter discs of the rotary disc filter (each comprising several filter modules arranged around a central tube) exerts forces on the filter bag, there is a risk that the filter bag will slip relative to the support frame during operation of the rotary disc filter.To prevent this, it is now proposed that a reinforcement be arranged in the area of ​​an edge limiting the mounting opening, which serves to fix the filter bag to the support frame.

[0015] The reinforcement is a section of the filter bag containing the reinforcement that differs from the rest of the filter bag in terms of its material, material thickness (especially its thickness), material composition, or other properties. For example, the filter bag in the area of ​​the aforementioned edge could have a greater material thickness than the majority of the rest of the filter bag. This greater material thickness allows the filter bag to be more reliably fixed in this area. This fixation can be achieved, for example, by clamping the edge between a first section of the support frame and another section of the support frame opposite the edge. This second section can also be formed by a separate clamping strip (as described below).

[0016] For example, it would be conceivable that the filter bag's edge could be folded over several times during manufacturing and then sewn, creating a reinforced edge. It is also conceivable, as will be described in more detail later, that one or more elements in the edge area could be connected to, fixed by, or encased in the filter bag, with the element(s) acting as reinforcement and ensuring reliable fixation of the filter bag to the support frame.

[0017] The filter bag consists of a sack- or tube-like filter material into which a support frame can be inserted through a mounting opening. In the area of ​​the mounting opening, the filter bag has a property (e.g., material type, thickness, or composition) that differs from the property in the area of ​​the actual filter surface. This difference is designed such that the area surrounding the mounting opening is thicker than the area of ​​the filter surface through which the wastewater will later flow.

[0018] Furthermore, it is advantageous if the filter material is designed as a pile material. A pile material is a fabric in which pile threads are firmly bound to a preferably textile base fabric, creating a three-dimensional structure. The solids carried in the wastewater to be filtered are retained by this pile material, while the purified water enters the interior of the respective filter module and can be discharged via the central pipe connected to the filter module.

[0019] It is also advantageous if the mounting opening is essentially slot-shaped. Slot-shaped in this case means that the mounting opening has a shape whose length is at least twice, preferably three times, the width of the mounting opening perpendicular to its length. In particular, the mounting opening should have a shape and dimensions that allow the support frame to be completely inserted into the interior of the filter bag during assembly of the filter module containing the filter bag. Preferably, the reinforcement completely surrounds the mounting opening, or at least 70% of it, and more preferably at least 80%.

[0020] It is also advantageous if the reinforcement is formed by at least one elongated, preferably cable-shaped, profile, for example, a wire, wherein the reinforcement is made of metal and / or plastic. The filter bag preferably includes a tubular section in the area of ​​the reinforcement, into which the reinforcement is inserted. For example, the tubular section can be produced by folding over the filter material in the area of ​​the edge defining the mounting opening during the manufacture of the filter bag and sewing it accordingly. It is also conceivable that the reinforcement is woven directly into the filter material or bonded to it.

[0021] It is also advantageous if the reinforcement is attached to the filter material by means of one or more seams. The reinforcement is preferably surrounded by the filter material, with the seam(s) either penetrating or merely surrounding the reinforcement.

[0022] It also offers advantages if the reinforcement is at least partially encased by the filter material, for example, by a tube-like section of the filter bag, or in particular, by a tube-like section of the filter material itself. This tube-like section of the filter material can be created by folding it over. Alternatively, the filter material can be connected to an additional section that forms the tube-like section and is part of the filter bag.

[0023] It is also advantageous if the filter bag has a seal at the outlet opening, ensuring a tight seal against the central tube of the rotary disc filter during normal use. As described above, the filter bag is placed over a support frame during assembly to form a filter module. This filter module is then connected to the central tube of the rotary disc filter, creating a connection between the filter module and the central tube. To prevent unfiltered wastewater from entering the central tube in this area, the aforementioned seal is beneficial. This seal could, for example, be a rubber profile surrounding the outlet opening on one side of the filter bag, bonded or sewn to the filter material.

[0024] The invention relates to a filter module for a rotary disc filter, wherein the filter module comprises a support frame and a filter bag that at least partially surrounds the support frame. According to the invention, the filter bag has a filter material with a mounting opening through which the filter bag can be slipped over the support frame of the rotary disc filter during an assembly step to form the filter module. Furthermore, the filter material has at least one outlet opening opposite the mounting opening, which, in the intended use of the filter bag, serves to allow filtrate to escape from the interior of the filter module into a central tube of the rotary disc filter after the filter module has been connected to the central tube. A reinforcement is arranged in the area of ​​an edge bordering the mounting opening, which serves to fix the filter bag to the support frame.The support frame preferably has a substantially trapezoidal shape, wherein one or more sides of the trapezoidal shape may also be curved, preferably arc-shaped. The filter bag surrounds the support frame almost completely or entirely. Furthermore, the support frame includes one or more cavities into which filtrate can flow from the outside to the inside during operation of the rotary disc filter. The support frame also includes at least one opening through which the flowing filtrate can enter a central tube of the rotary disc filter, to which the filter module is connected during intended use.

[0025] The filter module includes a clamping strip (preferably made of plastic) with which the filter bag is pressed against the support frame in the area of ​​its mounting opening and fixed to the support frame by the clamping effect caused by this.

[0026] It is particularly advantageous if the clamping strip is designed to completely cover and thus seal the mounting opening. This ensures that no solids carried by the wastewater can enter the interior of the filter module through the mounting opening, i.e., unfiltered. The filter material is preferably folded over an outer edge of the support frame in the area of ​​the mounting opening and covers it. Furthermore, the reinforcement of the filter material's edge is located between the support frame and the clamping strip, so that the reinforcement is clamped and thus securely fixed by the interaction of the clamping strip and the support frame. The clamping strip is preferably connected to the support frame with one or more screws. Alternatively, it is also conceivable that the reinforcement is located adjacent to the clamping strip and prevents the filter material from being pulled out of the clamping area.

[0027] It is also advantageous if the supporting frame is composed of two, preferably mirror-symmetrical, half-shells. Both half-shells preferably have the trapezoidal shape described above (preferably with two arcuate bases and two straight legs).

[0028] It is particularly advantageous if the clamping strip has extensions, especially in the form of bolts, that engage in corresponding recesses of both half-shells and preferably penetrate the filter material. The bolts engage the recesses of both half-shells like clamps, thus creating a connection and spatial alignment between them. The two half-shells can, for example, be additionally screwed, glued, or positively connected by snap-fit ​​mechanisms. It is also advantageous if the two half-shells have spacer bolts that maintain a distance between them after they are joined, creating an internal cavity into which the filtrate can flow during normal use of the filter module. The interaction of the bolts and recesses ultimately ensures that the two half-shells remain correctly positioned relative to each other.

[0029] It offers particular advantages if the support frame and / or the clamping strip has an uneven surface, preferably a ribbed one, which is in contact with the filter bag in order to increase the clamping effect of the clamping strip compared to a flat surface. When the filter module is fully assembled, the uneven surface is in contact with the reinforcement or the material surrounding the reinforcement of the filter bag.

[0030] It is also conceivable that the uneven surface is located between the reinforcement and the section of the filter bag that forms the actual filter surface of the filter module. The reinforcement would therefore have to pass over the uneven surface before it disengages from the clamping strip. This virtually eliminates the possibility of the filter bag accidentally coming loose.

[0031] It is advantageous if the support frame has a large number of honeycomb-shaped openings through which the filtrate passing through the filter material can flow into the interior of the filter module. Of course, individual holes or slots that allow the desired filtrate flow are also conceivable.

[0032] Furthermore, it is advantageous if the support frame has retaining projections, particularly in the form of hooks, that engage in or pass through holes or flaps in the filter bag. The holes or flaps are preferably located in the area of ​​the rim surrounding the mounting opening, or at most 1 cm to 5 cm away from it. When the support frame is inserted into the filter bag, the retaining projections engage through the corresponding holes or flaps. This already secures and aligns the filter bag relative to the support frame, thus simplifying subsequent securing with the clamping strip. Additionally or alternatively, it is advantageous if one or more holes or flaps are also present in the area of ​​the filter bag's outlet opening(s).In this case, the support frame should have corresponding retaining projections in the area where the aforementioned holes or tabs are located after the support frame has been inserted into the filter bag. The interaction of these retaining projections with the holes or tabs ensures that the filter bag does not slip in the area of ​​the outlet opening before the filter module is connected to the central tube. Preferably, after connection to the central tube, the filter bag is held in place by clamping between the central tube and the support frame.

[0033] The rotary disc filter according to the invention comprises a central tube driven by a drive mechanism and several filter discs arranged adjacent to one another in the longitudinal direction of the central tube and connected to the central tube, each filter disc comprising several filter modules arranged adjacent to one another in the circumferential direction of the central tube. According to the invention, the filter modules are designed as described above and / or below. Preferably, the individual filter modules comprise a rod penetrating the respective filter module in the radial direction of the filter discs, which is connected to the central tube. On the opposite side of the central tube, the rod also penetrates the filter module. Finally, the filter module is pulled against the central tube by means of a nut and thus fixed to it.

[0034] It is also advantageous if the central tube has openings through which the filtrate can flow from the filter modules into the interior of the central tube, with each filter module having retaining projections that extend into the opening assigned to that filter module and prevent the filter module from slipping relative to the central tube. The filter modules are thus positively fixed within the area of ​​the central tube.

[0035] Further advantages of the invention are described in the following exemplary embodiments. These show, schematically: Figure 1 Filter modules mounted on a central tube, Figure 2 a support frame and a clamping strip of a filter module according to the invention, Figure 3 a filter bag Figure 4a a filter module according to the invention before mounting the terminal strip, Figure 4b a filter module according to the invention after mounting the terminal strip, Figure 5a sectional view of a filter module according to the invention in the area of ​​the terminal strip, and Figure 6 A filter module according to the invention viewed from the side which, after assembly, is in contact with the central tube.

[0036] In In the following figures, similar elements or components are shown in the same way, and therefore sometimes only one of several similar elements or components is marked with a reference symbol in order to ensure the best possible overview.

[0037] Figure 1Figure 1 shows a central tube 7 of a rotary disc filter with several filter modules 5 attached to the central tube 7. Several of the filter modules 5 each form a filter disc 17. The components shown are, in their intended use, located in a closed container (not shown) which serves to hold wastewater to be filtered. The wastewater flows through the filter discs 17 from the outside to the inside, whereby solids up to a certain particle size, which depends on the type of filter bag 1 described below, are retained. The filtrate passing through the filter bags 1 finally enters the interior of the central tube 7 via openings 18 and can be discharged from the rotary disc filter via a filtrate outlet (not shown).

[0038] The structure of the individual filter modules 5 is described in more detail below.

[0039] Each of the filter modules 5 has an internal support frame 4, which preferably comprises two interconnected half-shells 10. The half-shells 10 in turn have individual, for example honeycomb-shaped, openings 14 (see Figure 2 ), via which the filtrate can flow from the outside into the interior of the support frame 4 and finally, via an outlet opening of the same, into the interior of the central tube 7.

[0040] Furthermore, the filter module 5 includes a clamping strip 9 for fixing a filter bag 1, which is shown in the view of Figure 2 not yet connected to the support frame 4. How Figure 2 As can be seen, the support frame 4 includes a groove 19 which essentially corresponds to the negative shape of the clamping strip 9.

[0041] Figure 3Figure 1 shows a filter bag 1. This bag essentially consists of a filter material 2 with a defined mesh size, such that solids carried by the wastewater to be filtered are retained by the filter material 2 above a certain particle size. The filter bag 1 essentially has a shape that corresponds to the shape of the support frame 4.

[0042] Furthermore, a mounting opening 3 is provided into which the support frame 4 (referring to Figure 3 The filter material 2 can be inserted into the filter bag 1 (from above) (of course, the filter bag 1 can also be pulled over the support frame 4). The mounting opening 3 therefore has a cross-section that is large enough to allow the support frame 4 to be positioned inside the filter bag 1. Preferably, the filter material 2 is designed as a pile material.

[0043] In Figure 4aA support frame 4 is shown from above (i.e., from the side facing away from the central tube 7) and without the clamping strip 9, after the filter bag 1 has been pulled over the support frame 4 or the support frame 4 has been inserted into the filter bag 1. The filter material 2 is either flexible enough that the support frame 4 can be moved through the mounting opening 3, or the filter bag 1 has individual slits in the area of ​​the mounting opening 3 (not shown), thus enabling the aforementioned insertion.

[0044] After the filter bag 1 and the support frame 4 have been arranged accordingly Figure 4a Once aligned, the clamping strip 9 is connected to the support frame 4, for example by means of screws 22 which pass through holes 20 (see Figure 2 ) inserted and e.g. into corresponding threads 30 of the support frame 4 (see Figure 4a ) be screwed in.

[0045] Furthermore, the Figures 4a and 4bthat the support frame 4 can have a central bore 21. The same applies to the clamping strip 9, wherein this is in Figure 4b The central screw 23 is not visible. The central screw 23 preferably extends through the entire filter module 5 and is finally connected to the central tube 7, so that the filter module 5 is fixed to the central tube 7 via the central screw 23.

[0046] Figure 4b Furthermore, it shows that it can be advantageous if the clamping strip 9 has a protrusion 25 and a corresponding recess 24. If several such filter modules 5 are now mounted circumferentially around a central tube 7 to form a filter disc 17, a protrusion 25 of each filter module 5 engages in the corresponding recess 24 of the adjacent filter module 5. The filter disc 17 thus formed exhibits high mechanical stability.

[0047] In Figure 5Figure 1 shows a cross-section of a filter module 5 according to the invention in the area facing away from the central tube 7. The cross-sectional surface runs with respect to Figure 1 in a plane through which the axis of rotation of the central tube 7 passes and extends radially to it.

[0048] Two alternative methods are shown for providing the rim 8 of the filter bag 1 surrounding the mounting opening 3 with a reinforcement 26, which ensures reliable and durable fixation of the filter bag 1 to the support frame 4. As shown from Figure 5 As can be seen, the filter bag 1 has, in the area of ​​its edge 8 surrounding the mounting opening 3, on both sides (referring to Figure 5) a reinforcement 26. While the reinforcement 26 on the right side is formed by a thickened area of ​​the filter material 2 itself, a separate reinforcement 26 in the form of a wire 28 is present on the left side. Preferably, the reinforcement 26 extends over the entire rim 8 surrounding the mounting opening 3. Alternatively, it is also possible that the reinforcement 26 is only present in the area of ​​the rim 8 that extends in the longitudinal direction of the clamping strip 9.

[0049] To further increase the strength of the fixation of the filter bag 1, individual ridges 13 can be present in the area of ​​the support frame 4. This increases the static friction between the filter bag 1 and the support frame 4.

[0050] Furthermore, it is conceivable and also in Figure 5The clamping strip 9 is shown to have several bolts 11 that extend into corresponding receptacles 12 of the support frame 4. In particular, when the support frame 4 is composed of two half-shells 10, these bolts 11 ensure that the half-shells 10 are always correctly aligned with each other.

[0051] The clamping strip 9 is fixed to the support frame 4 or pulled against the support frame 4 by means of several screws 22 and corresponding nuts 27, so that the filter bag 1 is reliably pressed against the support frame 4 and fixed by the resulting clamping effect.

[0052] In conclusion, in Figure 6 The area shown is where the filter module 5 rests against the central tube 7. To ensure a reliable seal between the filter module 5 and the central tube 7, the filter bag 1 preferably has one or more seals 29. The seals 29 surround in Figure 6Each has an outlet opening 6 through which the filtrate can flow from the interior of the filter module 5 into the central tube 7. There is also a space between the two outlet openings 6. Figure 6 The central bore 21 can be seen, through which the center screw 23 (not shown) can be inserted and connected to the center tube 7.

[0053] Finally, filter bag 1 shows in the case of Figure 6 Several tabs 16 are attached, which are slid over corresponding retaining projections 15 of the support frame 4 during the assembly of the filter module 5. This fixes the filter bag 1 in a fixed position relative to the support frame 4, preventing it from shifting in the area of ​​the outlet openings 6 relative to the support frame 4. The retaining projections 15 extend upwards through the tabs 16 relative to the plane of the sheet, thus preventing the tabs 16 from shifting within the plane of the sheet.

[0054] The present invention is not limited to the embodiments shown and described. Modifications within the scope of the claims are possible, as is any combination of the described features, even if they are shown and described in different parts of the description or the claims, or in different embodiments, provided that no contradiction arises with the teaching of the independent claims. Reference symbol list

[0055] 1 Filter bag 2 Filter material 3 Mounting opening 4 Support frame 5 Filter module 6 Outlet opening 7 Center tube 8 Edge 9 Clamping strip 10 Half shell 11 Bolt 12 Receptacle 13 Ribbed 14 Honeycomb opening 15 Retaining extension 16 Tab 17 Filter disc 18 Opening 19 Groove 20 Bore 21 Center bore 22 Screw 23 Center screw 24 Recess 25 Bulge 26 Reinforcement 27 Nut 28 Wire 29 Seal 30 Thread

Claims

1. Filter module (5) for a rotary disc filter, wherein the filter module (5) has a support frame (4) and a filter bag (1) which surrounds the support frame (4) at least in sections, wherein the filter bag (1) has a filter material (2) with a mounting opening (3), via which the filter bag (1) can be placed over the support frame (4) of the rotary disc filter during a mounting step in order to form the filter module (5), wherein the filter material (2) has at least one outlet opening (6) which lies opposite the mounting opening (3) and which, when the filter bag (1) is used as intended, serves for the outlet of filtrate from the interior of the filter module (5) into a central tube (7) of the rotary disc filter after the filter module (5) has been connected to the central tube (7), wherein a reinforcement (26) is arranged in the region of an edge (8) which delimits the mounting opening (3) and which serves to fix the filter bag (1) to the support frame (4), characterized in that, the filter module (5) comprises a clamping bar (9), with the aid of which the filter bag (1) is pressed against the support frame (4) in the region of its mounting opening (3) and is fixed to the support frame (4) by the clamping effect caused thereby.

2. Filter module (5) according to the preceding claim, characterized in that the support frame (4) is composed of two, preferably mirror symmetrical, half shells (10).

3. Filter module (5) according to one of the preceding claims, characterized in that the clamping bar (9) has extensions, in particular in the form of bolts (11), which engage in corresponding receptions (12) of the two half shells (10) and preferably penetrate the filter material (2).

4. Filter module (5) according to one of the preceding claims, characterized in that the support frame (4) and / or the clamping bar (9) has an uneven surface, preferably a ribbing (13), which is in contact with the filter bag (1) in order to reinforce the clamping effect of the clamping bar (9) in comparison with a flat surface.

5. Filter module (5) according to one of the preceding claims, characterized in that the support frame (4) has a plurality of honeycomb-shaped openings (14), through which the filtrate passing through the filter material (2) can flow into the interior of the filter module (5).

6. Filter module (5) according to one of the preceding claims, characterized in that the support frame (4) has retaining extensions (15), in particular in the form of hooks, which engage in holes or tabs (16) of the filter bag (1) or pass through these.

7. Rotary disc filter having a central tube (7) and multiple filter discs (17) which are arranged adjacent to one another in the longitudinal direction of the central tube (7) and are connected to the central tube (7), wherein each filter disc (17) comprises multiple filter modules (5) which are arranged adjacent to one another in the circumferential direction of the central tube (7), characterized in that the filter modules (5) are designed according to one of the preceding claims.

8. Rotary disc filter according to the preceding claim, characterized in that the central tube (7) has openings (18), via which the filtrate can flow from the filter modules (5) into the interior of the central tube (7), wherein each filter module (5) has retaining extensions (15) which each project into the opening (18) assigned to the filter module (5) and prevent the filter module (5) from slipping with respect to the central tube (7).

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