EP0261146A1

EP0261146A1 - Filter device

Info

Publication number: EP0261146A1
Application number: EP87901151A
Authority: EP
Inventors: Rune Helmer Frykhult; Harry Wilhelm Nilsson
Original assignee: Celleco AB
Current assignee: Celleco AB
Priority date: 1986-01-29
Filing date: 1987-01-29
Publication date: 1988-03-30
Also published as: SE8600379D0; WO1987004640A1

Abstract

Un dispositif de filtrage, destiné de préférence à déshydrater des suspensions de cellulose, comprend des disques de filtrage annulaires (18), disposés en alignement parallèles autour d'un axe. Chaque disque de filtrage comprend une paroi de soutien (1), des parties saillantes (2) situées sur chaque paroi de soutien et au moins une toile de filtrage (7) couvrant les deux côtés de la paroi de soutien et soutenue par les parties saillantes, ladite paroi de soutien, ladite toile de filtrage et lesdites parties saillantes délimitant des espaces (8) pour le filtrat. Des sorties (9), dans lesquelles débouchent les espaces (8) pour le filtrat, sont ménagées dans le disque de filtrage pour permettre l'évacuation du filtrat passant à travers la toile de filtrage (7) et s'écoulant dans les canaux durant la rotation du disque de filtre. Selon la présente invention, un organe de renforcement (5) est placé entre les parties de la paroi de soutien (1) situées à l'extérieur et à l'intérieur par rapport au rayon et entre les parties de la toile de filtrage (7) qui couvre les deux côtés de la paroi de soutien. L'organe de renforcement (5) s'étend essentiellement dans le sens périphérique du disque de filtrage et est relié de façon rigide à la paroi de soutien.A filter device, preferably for dehydrating cellulose suspensions, comprises annular filter discs (18), arranged in parallel alignment about an axis. Each filter disc includes a support wall (1), projecting parts (2) located on each support wall and at least one filter cloth (7) covering both sides of the support wall and supported by the projecting parts. , said support wall, said filter cloth and said projecting parts delimiting spaces (8) for the filtrate. Outlets (9), into which the spaces (8) open for the filtrate, are provided in the filter disc to allow the evacuation of the filtrate passing through the filter cloth (7) and flowing in the channels during rotation of the filter disc. According to the present invention, a reinforcing member (5) is placed between the parts of the support wall (1) located outside and inside with respect to the radius and between the parts of the filter cloth (7 ) which covers both sides of the support wall. The reinforcing member (5) extends essentially in the peripheral direction of the filtering disc and is rigidly connected to the support wall.

Description

Filter device

The present invention relates to a filter device, preferably intended for dewatering of cellulose suspensions, including annular filter discs, which are arranged parallelly and axially aligned and each of which comprising a support wall, protrusions formed on both sides of the support wall, at least one straining cloth, which covers both sides of the support wall and which is supported by the protrusions, said support wall, straining cloth and protrusions delimiting spaces for filtrate, outlets for the discharge of filtrate from the spaces, which filtrate has streamed throuch the straining cloth into the spaces during rotation of the filter disc, and means for stiffening of the support wall in its circumferential direction.

In such a filter device usually each filter disc is composed of a plurality of separate, sector shaped filter elements, each of which being covered by a bag shaped straining cloth. The filt¬ rate spaces in the filter elements are via the outlets of the spaces connected to axial channels in a shaft, which carries the filter elements, so that the filter elements, which in the filter discs are located axially aligned along the carrier shaft, are connected to the same axial channel. The axial channels are usually connected to a barometric leg or a vacuum pump. At rotation of the filter discs in a cellulose suspension, into which they are immersed more or less by halves, the suspen¬ sion is dewatered because of a pressure difference between the latter and the interior of the filter discs caused by the baro¬ metric leg or vacuum pump. Hereby cakes of cellulose fibres are formed on the outside of the filter elements, which are immersed into the suspension. As the filter elements come up out of the suspension, the fibre cakes are removed from them before they return down into the suspension. In a known filter device of the present kind the protrusions of each support wall are formed as radially extending ridges delimiting spaces for filtrate between adjacent ridges. In addition to the fact that the ridges form a support for the straining cloths they also effect a radial stiffening of the support wall. In order to obtain a sufficient stiffness also in the circumferential direction of the support wall this is formed relatively thick. For among other things weight saving reasons the support wall is composed of two wall parts, so that an empty space is formed within this. Alternatively, the support wall of course could be moulded solidly in for instance plastic material. However in that case the entire filter device would be considerably heavier, which would mean an undesirable increase of the load on its carrier shaft. The support walls of the known filter device are relatively expensive to manufacture, wherefore a cheaper design of these is desirable.

Another possible design of the support walls for a filter device of the present kind is disclosed in the Swedish patent 79 654 from 1931. This describes filter discs composed of a plurality of filter elements. Each filter element comprises a support wall of a corrugated sheet-metal, the ridges of the corrugations on each side of the support wall forming support for straining cloths. Such corrugated sheet-metals of course are cheap to manufacture but their stiffness in the direction transverse to the corrugations are weak. Thin such plates therefore are unsuitable for use in modern filter devices, the filter discs of which often having a diameter exceeding four metres, thus having a relatively large area. Already a small difference between the pressures, which during operation are acting against the sides of each filter element, would therefore mean a deflection of the support wall of each filter element. Such a pressure difference often appears during operation because of an uneven or an incomplete coating of fibres on the sides of the respective filter elements. It is very important that the filter elements do not bend during operation for the reason that the removal of the formed fibre cakes will proceed without interruption at use of stationary flush nozzles for a fluid under high pressure, for instance water. Hereby each flush nozzle is arranged to give a very thin jet so that this will hit a narrow area along the border between the straining cloth and the fibre cake. Even if the filter elements would bend just slightly the jet of a flush nozzle would hit outside said border, so that the removal of the filter cake from the straining cloth would be disturbed.

The object of the present invention lε to provide a filter device of the kind initially mentioned, in which the filter discs can be made very thin and of a low weight without risking that they will bend during operation.

This object is obtained by means of a stiffening means situated between the radially outermost and innermost parts of the support wall and between the portions of the straining cloth covering both sides of the support wall, and further, the stiffening means extends substantially in the circumferential direction of the filter disc and is rigidly connected to the support wall.

Hereby the advantage is obtained that the support wall can be made of for instance a light thin sheet-metal at low price, in which protrusions in a simple way can be formed on each side for instance by pressing or bending. The stiffening means secures the stiffness of the support wall in the circumferential direction of the filter disc.

Preferably each filter disc is composed of a plurality of separate sector shaped filter elements, each of which having a support wall section and at least one straining cloth. Hereby the stiffening means comprises at least one stiffening member arranged on each side of the respective filter element between its radially directed delimiting edges.

According to one embodiment of the filter device according to the invention the stiffening means comprises a separate elongated member, which extends substantially in the circum¬ ferential direction of the filter disc and is rigidly connected to the support wall at at least two connection points.

According to another embodiment of the filter device according to the invention the stiffening means comprises at least one stiffening member, which is rigidly connected to the support wall along substantially the entire peripheral extension of the member and is formed such that along said extension it has a surface forming an angle with the support wall.

The invention will be described in the following with reference to the accompanying drawing, in which

Figure 1 shows a front view of a sector shaped filter element, without straining cloth, for a filter device according to the invention, which filter element comprises protrusions in the form of radially extending ridges, and is provided with a stiffening means in the form of separate elongated members, Figure 2 shows a side view of the filter element of Figure 1, Figure 3 shows a section along the line III-III of Figure 1, Figure 4 shows a section along the line IV-IV of Figure 1, Figure 5 shows a side view of a part of a support wall in another sector shaped filter element for the filter device according to the invention, which filter element is provided with a stiffening means in the form of protrusions as well as separate elongated members,

Figure 6 shows a section along the line VI-VI of Figure 5, Figure 7 shows a section along the line VII-VII of Figure 5, Figure 8 shows the filter element according to Figure 1, which is provided with a stiffening means in the form of a plurality of stiffening members integrated in the support wall of the filter element,

Figure 9 shows a section along the line IX-IX of Figure 8, Figure 10 shows the filter element according to Figure 5, which is provided with a stiffening means only in the form of protrusions,

Figure 11 shows a section along the line XI-XI of Figure 10, Figure 12 shows a section along the line XII-XII of Figure 11, Figure 13 shows a front view of a sector shaped support wall for a filter device according to the invention which support wall is composed of three pressed sheet-metals,

Figure 14 shows a front view of a sector shaped filter element comprising the support wall according to Figure 13, Figures 15, 16 and 17 show sections along the lines XV-XV, XVI- XVI and XVII-XVII, respectively, of Figure 14 according to a first embodiment of the filter element according to Figure 14, Figures 15a, 16a and 17a show sections along the lines XV-XV, XVI-XVI and XVII-XVII, respectively, of Figure 14 according to a second embodiment of the filter element according to Figure 14, Figures 15b, 16b and 17b show sections along the lines XV-XV, XVI-XVI and XVII-XVII, respectively, of Figure 14 according to a third embodiment of the filter element according to Figure 14, Figure 16c shows a section along the line XVI-XVI of Figure 14 according to a fourth embodiment of the filter element according to Figure 14, and

Figure 18 shows an axial section through a preferred embodiment of a filter device according to the invention.

The details in Figure 8, corresponding to the details in Figure 1 are provided with the same reference numerals as in Figure 1 supplemented by the notation "a". The details in Figures 10-12 corresponding to the details in Figures 5-7 are provided with ⁼ the same reference numerals as in Figures 5-7 supplemented by the notation "b". The sector shaped filter element shown in Figure 1 comprises a pleated support wall 1 of for instance stainless or acid proof steel. On each support wall side the pleats form protrusions in the form of radially extending ridges 2 and valleys 3 between adjacent ridges. Thus, the ridges 2 on one side of the support wall are forming valleys on the opposite side (Figure 3). A U-shaped frame 4 is arranged along the edges of the filter element. The frame may constitute a separate part or be formed by the support wall 1, for instance by bending the edge parts of the latter. A plurality of separate elongated stiffening members in the form of threads 5 (or rods) are attached to the radially directed parts of the frame 4. The threads 5 cross the ridges 2 and are rigidly connected to at least some of these. A bag shaped straining cloth 7 (indicated by dashed-dotted lines in Figures 3 and 4) is. intended to surround the support wall 1 and to be supported by the ridges 2, spaces 8 for filtrate being formed on both sides of the support wall between the latter and the straining cloth 7. The threads 5 are arranged spaced from and on both sides of a plane located half-way between the portions of the straining cloth 7, which cover the two sides of the support wall 1 (Figure 3). Each thread 5 on one of the sides of the support wall is located just opposite a thread on the opposite side of the support wall (Figure 4). Filtrate which is collected in the filtrate spaces 8 during operation flows out from these through an outlet 9 at the radially innermost part of the filter element. (Alternatively, outlets for filtrate may be arranged at the radially outermost part of the filter element).

Since the breadth of the filtrate spaces 8 reduces in direction towards the outlet 9 the height of the filtrate spaces must increase towards said direction (Figure 2). Otherwise the flow of filtrate will be throttled because of reducing cross-section area of the filtrate spaces. If the flow of filtrate is throttled also the efficiency of the filter device is decreased. Suitably the height of the filtrate spaces 8 increases in direc- tlon towards the outlet 9 to such a degree that also the cross- section area of the filtrate spaces increases towards said direction, since the flow of filtrate in the filtrate spaces increases towards the outlet 9.

To avoid wear damages of the straining cloth 7 the threads 5 at the crossings with the ridges 2 are countersunk into depressions 10 at the ridges (Figures 3 and 4). The threads 5 suitably have circular cross-section so that the contact surface between these and the straining cloth 7 will be small. Alternatively, the threads 5 may be countersunk into the ridges 2, such that the straining cloth will not be in contact with the threads.

The support wall 11 at the filter element shown in Figures 5, 6 and 7 consists of a plane disc of for instance stainless sheet- metal, which is provided with depressions 12 on one side of the support wall, and with depressions 13 on the other side of the support wall. The depressions form protrusions, which are inten¬ ded to support the straining cloth 14 (indicated by dashed- dotted lines in Figure 6 and 7). Alternatively, the protrusions may be formed by separate members, which are attached on a support wall in the form of a plane disc. A plurality of depres¬ sions 12, 13 are on each side of the support wall distributed in the radial direction as well as the circumferential direction. Each depressing 12, 13 comprises two wall portions 15, 16 exten¬ ding in the circumferential direction of the filter disc, which wall portions are rigidly connected to the support wall 1 and the surfaces of which are forming an angle with the latter. Hereby the support wall Is stiffened in the circumferential direction of the filter disc by the depressions. The depressions are oriented in substantially radially directed rows, each row only containing depressions 12 and 13, respectively, on the same side of the support wall. Each row on one side of the support wall 11 is situated next to a row on the opposite side of the support wall. The depressions 12, 13 are formed with rounded upper portions (Figure 7), in order to reduce the contact surface between the depressions and the straining cloth 14.

The closer adjacent depressions 12, 13 are to each other in the circumferential direction of the filter disc, the stiffer the filter element is in said direction. However, the cross-section area of the filtrate spaces in radial direction must be dimen¬ sioned, such that during operation filtrate has time to flow out from the filtrate spaces of the filter element before the formed pulp cake is removed, which may be determining on the smallest possible distance between neighbouring depressions in the circumferential direction of the filter disc.

Filtrate can flow between neighbouring rows of depressions 12 and 13, respectively, on the same side of the support wall in direction towards the filtrate outlet (not shown) which is situated at the radially innermost part of the filter element. (Alternatively, the filtrate outlet may be arranged at the radially outermost part of the filter element). In addition to this filtrate can flow in a direction transverse the rows between the depressions, which will enhance the discharge of filtrate from the filter element when the latter ascends from the suspension. In order that the cross-section area for the formed filtrate space 17 between said rows should not be reduced but should rather increase somewhat in direction towards the narrowing, radially innermost part of the filter element, the depressions 12, 13 are formed with increasing height in said direction. (The same effect may also be obtained if the number of depressions per length unit in the circumferential direction of the filter element reduces in direction towards the radially innermost part of the filter element. As a further a possibility for the gain of said effect the depressions may also, or alter¬ natively, be designed such that the closer they are to the radially innermost part of the filter element, the smaller their extension are in the circumferential direction of the filter element).

For the increase of the stiffness of the support wall 11 in radial direction the depressions 12, 13 are elongated in said direction and placed such that the space between two neigh¬ bouring depressions 12 in one row on one side of the support wall is bridged by a depression 13 in a neighbouring row of the opposite side of the support wall (Figure 5).

For further increase of the stiffness of the support wall in the circumferential direction a plurality of separate elongated stiffening members in the form of threads or rods 18 are attached to the support wall and are crossing and supporting against some of the depressions 12, 13 on both sides of the support wall 11. The threads 18 are also countersunk in the crossed depressions to avoid wear against the straining cloth 14 (Figure 6). The threads 18 may also be countersunk in the crossed depressions such that contact between the threads and the straining cloth is avoided.

The above described embodiment of the filter element according to Figures 5-7 with protrusions distributed In the radial direction as well as in the circumferential direction, has the advantage that the formed fibre -ake on each side of the filter elements is more coherent than the filter elements described earlier according to Figures 1-4, which are provided with pro¬ trusions in the form of radially extending ridges. Along the latter weakened portions arise at the formed fibre cake, which portions thus extend across the entire fibre cake. Therefore the fibre cake at the removal might break into smaller parts, so that a part of the fibre cake will be left on the filter element. The filter element according to Figures 1-4 is shown in Figures 8 and 9 but without separate stiffening threads. The support wall la of the filter element is provided with depressions 19 in the ridges 2a on one side of the support wall and with depressions 20 in the ridges on the opposite side of the support wall. Each depression 19, 20 forms two triangular end portions 21, 22 at two adjacent ridges 2a situated along the same radius of the filter element. Each triangular end portion 21, 22 extends substantially in the circumferential direction of the filter disc and has a surface forming an angle with the support wall la. Hereby the support wall is stiffened in the circum¬ ferential direction of the filter element by each end portion 21, 22.

The depressions 19, 20 are distributed in the ridges 2a along the entire breadth of the filter element seen in the circum¬ ferential direction of the filter element. Each depression 19, 20 has a depth corresponding to half the distance between the portions of the straining cloth, which cover both sides of the support wall la. In spite of the fact that the height of the filtrate space is thus halved in the region of a depression 19, 20 the flow area for filtrate in said region still is not reduced by more than 25 %, because of the triangular cross- section shape of the filtrate space in said region. In addition to this, filtrate can flow in direction transverse the ridges 2a through the depressions 19, 20, which to a certain degree com¬ pensates the throttling effect of the depressions on the filtrate flow along the ridges.

In the region of each depression 19, 20 of course the stiffness of the support wall is reduced in radial direction. For counter¬ acting said stiffness reduction depressions 19, 20 in each ridge on one side of the support wall are so arranged that they in the radial direction of the support wall do not bridge over adjacent ridges on the opposite side of the support wall. Thus each depression 19, 20 is bordered in direction transverse to the ridges by unbroken, radially stiffening ridges. The depressions 19, 20, which are situated in adjacent ridges on each side of the support wall are arranged substantially displaced relative to each other in radial direction. Hereby the stiffness of the support wall in radial direction is substantially retained in spite of the existence of the depressions 19, 20.

Alternatively, the filter element shown in Figures 8 and 9 may additionally be provided with the above described separate stiffening members in the form of threads or rods.

In Figures 10-12 there is shown the support wall according to Figures 5-6 but without separate stiffening threads. For further increase of the stiffness of the support wall lib in the circum¬ ferential direction, the support wall is provided with further depressions 23, 24, forming protrusions of lower height than the protrusions supporting the straining cloth 14b. Each further depression 23, 24 extends in the circumferential direction of the filter disc between two adjacent depressions 12b and 13b, respectively, on each side of the support wall and has a height, which is substantially smaller than the height of each depres¬ sion 12b, 13b. Hereby filtrate in the filtrate spaces 17b can flow in the radial direction of the filter element without being substantially obstructed by the further depressions 23, 24.

The further depressions 23, 24 need not necessarily be arranged between two adjacent depressions 12b and 13b, respectively, but may be arranged on optional portions of the support wall between the depressions 12b, 13b.

Alternatively, the support wall of a filter element shown in Figures 10, 11 and 12 may also be provided with the above described separate stiffening members in the form of threads or rods. The support wall of each filter element may be composed of two or more pre-fabricated wall parts. In Figure 13 there is shown such a composed support wall 25 consisting of three identical wall parts 26-28 of sheet-metal formed by pressing. The parts 29, 30 of the composed support wall 25 projecting from the desired geometry may easily be removed, for instance by sawing. The composed support wall 25 is surrounded by a stiffening frame 31 (Figure 14).

According to a first embodiment of the composed support wall (Figures 15-17) the radially directed edge portions 32 of the support wall are abutting perpεudicularly to the frame 31. Adjacent edge portions 32 of neighbouring wall parts overlap each other and are attached to each other by welding.

According to a second embodiment of the composed support wall (Figures 15a, 16a and 17a) the radially directed edge portions 33 of the support wall are bent transverse to the support wall and are attached to the frame 31 by welding, so that the sides of the edge portions 33 abut against the frame. Adjacent edge portions 33 of neighbouring wall parts are bent transverse to the support wall in the same direction from the latter and are attached to each other by welding, so that the sides of the edge portions abut against each other. The last mentioned edge portions 33 are spaced from the straining cloth, so that the edge portions could not damage the latter.

According to a third embodiment of the composed support wall (Figures 15b, 16b and 17b), the latter at least is lacking radially extending frame parts. Each wall part has one of its radially directed edge portion 34 bent, such that it extends across the entire depth of the support wall (measured between the portions of the straining cloth, which cover both sides of the support wall) and is terminated by a small extension 35 directed inwards towards the support wall (Figure 15b). The other radially directed edge portion 36 of the wall part is bent in a corresponding manner as the first described one, but has a smaller extension transverse to the support wall, so that it is situated spaced from the straining cloth (Figure 17a). Said bent edge portions 34, 36 mean that each wall part is stiffened in radial direction. Adjacent edge portions 34, 36 of neighbouring wall parts are attached to each other by welding, so that the sides of the edge portions abut against each other. Thus, with the edge portion 36 of one of the wall parts in an edge portion joint situated spaced from the straining cloth, pockets with impaired drainage capability are avoided. Such pockets could weaken the formed pulp coating during operation of the filter device, so that the pulp coating will separate at the removal thereof from the straining cloth, which could cause functional troubles.

Adjacent edge portions 34, 36 of neighbouring wall parts in the above described third embodiment of the composed support wall may alternatively be attached to each other through a stiffe- ning, separate wall member 37, which^*is placed between the edge portions 34, 36 of the wall parts (Figure 16c).

The preferred embodiment of a filter device according to the invention shown in Figure 18 comprises a container 38 for cellu- lose suspensions, which carries a horizontal rotor 39 partly sunk into the container. The rotor 39 consists of a number of annular filter discs 40, which are mounted parallelly on a horizontal rotatable drum 41. The filter discs 40 are composed of a number of separate filter elements, for instance of the kind shown in Figure 1. The filtrate spaces of the filter elements communicate with axial channels 42, the filtrate spaces in the filter elements, which are situated axially aligned along the drum 41, communicating with the same axial channel. The axial channels 42 are connected to a barometric leg 43. A collection container 44 for fibre cakes extends centrally through the drum 41 along all the filter discs. A conveyor screw 45 is arranged in the collection container 44 for transporting away the fibre cakes, which have fallen down into the latter.

Claims

1. A filter device, preferably intended for dewatering of cellulose suspensions, including annular filter discs (40), which are arranged parallelly and axially aligned and each of which comprises a support wall (1, la, 11, lib, 25), protrusions (2, 2a, 12, 13, 12b, 13b) formed on each side of the support wall, at least one straining cloth (7, 14, 14b), which covers both sides of the support wall and which is supported by the protrusions, said support wall, straining cloth and protrusions delimiting spaces (8, 17, 17b) for filtrate, outlets (9, 9b), for discharge of filtrate from the spaces, which filtrate has streamed through the straining cloth into the spaces during rotation of the filter disc, and means for stiffening of the support wall in its circumferential direction, c h a r a c ¬ t e r i z e d b y a stiffening means (5, 12, 13, 18-20, 23, 24) situated between the radially outermost and innermost parts of the support wall and between the portions of the straining cloth covering both sides of the support wall, and, further, the stiffening means extends substantially in the circumferential direction of the filter disc and is rigidly connected to the support wall.

2. A filter device according to claim 1, c h a r a c t e r - I z e d i n that the stiffening means comprises stiffening members (5, 12, 13, 18-20, 23, 24) arranged spaced from and on both sides of a plane situated half between said portions of the straining cloth (7, 14, 14b).

3. A filter device according to claims 1 or 2, in which each filter disc (40) is composed of a plurality of separate, sector shaped filter elements, each of which having a support wall section and at least one straining cloth, c h a r a c t e r ¬ i z e d i n that the stiffening means comprises at least one stiffening member (5, 12, 13, 18-20, 23, 24) arranged on each side of the respective filter element between the radially directed delimiting edges of the filter element.

4. A filter device according to claim 3, c h a r a c t e r - I z e d i n that each support wall section comprises a sheet- metal, in which said protrusions (2, 2a, 12, 13, 12b, 13b) are formed, for instance by pressing or bending on both sides.

5._' A filter device according to any of claims 1-4, c h a - r a c t e r i z e d i n that said stiffening means comprises a separate elongated member (5, 18) extending substantially in the circumferential direction of the filter disc (40) and rigidly connected with the support wall^* at at least two connec¬ tion points.

6. A filter device according to claim 5 comprising radially extending ridges (2) at the support wall constituting said protrusions and delimiting valleys (3) between them, the ridges on one side of each support wall forming valleys on the opposite side of the latter, c h a r a c t e r i z e d i n that the separate member (5, 18) passes at least one of said ridges.

7. A filter device according to claim 5, c h a r a c t e r ¬ i z e d i n that said stiffening means also comprises a plurality of protrusions (12, 13), which on each support wall side are distributed in radial direction as well as in circum¬ ferential direction, each protrusion having a wall portion (15, 16) extending in the circumferential direction of the filter disc, which wall portion is rigidly connected to the support wall (11) and the surface of which is forming an angle with the latter, and, further, said separate member (18) passes at least one of the protrusions.

8. A filter device according to any of claims 1-4, c h a - r a c t e r i z e d i n that said means comprises at least one stiffening member (12b, 13b, 21-24), which is rigidly connected to the support wall (la, lib, 25) along substantially the entire peripheral extension of the member and is formed such that along said extension it has a surface forming an angle with the support wall.

9. A filter device according to claim 8, c h a r a c t e r ¬ i z e d b a plurality of elongated protrusions (12b, 13b) extending in radial direction and distributed on each support wall side (lib, 25) in the radial direction as well as in the circumferential direction, said stiffening means comprising at least some of the protrusions.

10. A filter device according to claim 9, c h a r a c t e r - I z e d i n that further protrusions (23, 24) of lower height than said plurality of protrusions (12b, 13b) are formed between the latter, said stiffening means also comprising said further protrusions.

11. A filter device according to claim 8 comprising radially extending ridges (2a) at the support wall (la) constituting said protrusions and delimiting valleys (3a) between them, the ridges on one side of each support wall forming valleys on the opposite side of the support wall, c h a r a c t e r i z e d i n that the stiffening means comprises the end portions (21, 22) of said ridges.