WO2007137646A1

WO2007137646A1 - Filter element and filter assembly comprising same

Info

Publication number: WO2007137646A1
Application number: PCT/EP2007/002723
Authority: WO
Inventors: Steffen Heidenreich; Walter Haag; Gerhard Messerschmidt; Oliver Baatz
Original assignee: Pall Corporation
Priority date: 2006-05-30
Filing date: 2007-03-28
Publication date: 2007-12-06

Abstract

For providing a filter element having an improved handling and operating characteristic, a filter element is suggested which comprises a filter body with a porous, fluid permeable, rigid filter wall having an inner and an outer peripheral surface, said inner peripheral surface defining an inner cavity receiving filtered fluid, said filter wall comprising on its outer peripheral surface two or more elements reducing fluid flow of non-filtered fluid to selected areas of the outer peripheral surface, said outer peripheral surface of the filter wall having an essentially flat or convex configuration between each of two proximate fluid flow reducing elements.

Description

FILTER ELEMENT AND FILTER ASSEMBLY COMPRISING SAME

Field of the Invention

The invention concerns a filter element, especially of the candle-type, comprising a filter body with a porous, fluid permeable, rigid filter wall having an inner and an outer peripheral surface, said inner peripheral surface defining an inner cavity receiving filtered fluid. Such filter elements are useful in filtration of liquid and gaseous fluids.

The invention furthermore relates to a filter assembly comprising one or more of such filter elements.

Background of the Invention

The afore-mentioned filter elements are used in filtration systems where the filter elements are used in multiple filtration cycles. During each cycle, a filter cake of solids contents of the fluid to be filtered is deposited on the outer peripheral surface of the filter wall. After completion of a filtration cycle, the filter cakes on the filter elements are dislodged during a so called backwashing step. Dislodging of the filter cake may be supported by a gas backpulse which, however, often results in non-complete removal of the filter cake. Filter cakes which contain a high amount of liquid are more easily detached by gas back- pulsing. Disposal of the wet filter cake is more costly since the fluid remaining in there substantially increases its weight.

If the filter cake is the product desired from the filtration step, the fluid remaining in the filter cake may still contain unwanted components and there- fore the purity of the filter cake material is affected and further refining and drying steps are needed.

Because of this high solid contents in the filter cake are greatly desired.

On the other hand, the continued operation of the filter systems using such filter elements requires essentially complete removal of the filter cake so as to provide maximum of filtration surface for the subsequent filtration cycle after the backpulsing step.

This is not only a precondition for consistent filtering conditions but also for the build up for a homogenous filter cake. In addition, such properties are highly wanted in order to extend the operation periods in between servicing of the system which would include cleaning and inspection of the outer surface of the filter elements.

Summary of the Invention

The object of the present invention therefore resides in providing a filter element having an improved handling and operating characteristic.

The object of the present invention is met by the filter element as described above, wherein said filter wall comprises on its outer peripheral surface two or more elements reducing fluid flow of non-filtered fluid to selected areas of the outer peripheral surface, said outer peripheral surface of the filter wall having an essentially flat or a convex configuration between each of two proximate fluid flow reducing elements. It is important that the portions of the outer peripheral surface do not have a pronounced concave configuration which would affect removal of the filter cake and therefore be detrimental for the handling and operating characteristics.

The filter element according to the present invention is especially useful in the fields of brine filtration, e.g., brine polishing filtration in the chlor-alkali process, waste water filtration, catalyst recovery, e.g., from the caprolactam production process, filtration of cleansing agents and filtration of cooling lubricants.

Depending on the filtration application, filter elements of different basic designs are used. For one type of application, the filter element may have a filter wall of an essentially homogenous porous structure. In some cases, it may be useful to precoat the filter wall on its outer peripheral surface to adapt the characteristics of the filter element to a specific filtration task.

For another type of application, the basic design of the filter element comprises an asymmetric porous structure, e.g., a porous filter wall supporting on its outer peripheral surface a membrane layer.

Still further variations in the basic design of the filter element are possible within the scope of the present invention.

Generally, the filter element according to the present invention will have a filter body in the form of a hollow rod and the filter wall will be self-supporting.

Often, the rigid filter wall may be manufactured from sintered particulate matter, selected from the group including, but not limited to, metal particles, ceramic particles and plastic particles or mixtures of these materials.

Preferably the filter wall will have a seamless structure, preferably made of sintered particulate material. A seamless configuration of the filter wall provides for a uniform filtration characteristic in all accessable portions of the filter body.

The rigid filter wall preferably does have a non-pleated structure.

The cross-section of the filter body, with respect to its outer and and/or inner peripheral surface, may be circular or non-circular, e.g., polygonal, elliptical and the like.

Although the geometry of the outer and inner peripheral surface may be of a different type of cross-section, it is preferred that the inner and outer peripheral surface cross-sections have the same geometry resulting in an essentially uniform thickness of the filter wall.

Preferably, the filter body is of the candle filter type structure.

The fluid flow reducing elements on the outer peripheral surface of the filter wall prevent or at least greatly reduce deposition of the solids on the top of the selected areas of the outer peripheral surface of the filter elements which results in a reduced thickness of the filter cake in such areas. Those portions of the filter cake with reduced thickness then act as predetermined breaking points of the filter cake which facilitates breakage of the filter cake during gas backpulsing such that filter cakes with solid contents of about 30 wt. % and more, and even filter cakes with solid contents as high as about 90 wt. % or more may be safely detached during gas backpulsing.

Because of this, a build-up of a homogeneous filter cake is ensured in any of the filtration cycles and a complete draining of the liquid from the filter cake is possible at the end of each filtration cycle.

Removal of filter cakes with a high solids content is a quite challenging task since the flow resistance of the filter cake presented to the gas backpulse decreases with increasing solid content. In order to apply a sufficient force on the filter cake to detach the same from the outer peripheral surface of the filter wall, higher pressure pulses are needed which increases the costs of the equipment as well as the operating costs.

The fluid flow reducing elements on the outer peripheral surface of the filter wall may be provided in various configurations.

According to one embodiment of the present invention, the fluid flow reducing elements have a porosity which is smaller than the porosity of other portions of the filter wall, or may be even essentially non-porous, or have an essentially non-porous surface.

Because of the smaller porosity or the non-porous surface, the areas covered by the fluid flow reducing elements receive less inflowing fluid to be filtered

which leads to a reduced deposit of solid material contained in a non-filtered fluid in these areas.

The areas covered by the fluid flow reducing elements preferably continuously extend along parallel lines extending over at least about 50 %, preferably at least about 80 % of the length of the filter element measured in the longitudinal direction thereof. Alternatively, regularly arranged sections along such lines may be covered by the fluid flow reducing elements which then may be of, e.g., a punctiform or oblong configuration.

The lines may be straight lines extending parallel or somewhat obliquely to the longitudinal direction of the filter element. Alternatively, the lines may be wound is spiral form around the outer peripheral surface of the filter wall.

The outer surface of the fluid flow reducing elements may be essentially flush with the outer peripheral surface of the filter wall.

In another embodiment of the present invention, the fluid flow reducing elements may comprise elements in the form of projections showing an increased flow resistance to the fluid to be filtered which serves to decrease the thickness of the deposit of the filter cake in portions of the surface area of the outer peripheral surface of the filter wall, thus providing predetermined breaking points for the filter cake. The projections may have a smaller porosity than the other portions of the filter wall, or may have an essentially non-porous surface. The projections may have a continuous configuration extending essentially along the length of the filter body and preferably parallel or slightly oblique to the longitudinal direction thereof or spirally wound around the filter body.

Alternatively, the projections may have a punctiform or oblong configuration or the like, groups of such projections being arranged preferably along lines parallel or slightly oblique to the longitudinal direction of the filter body or spirally wound around the filter body.

The projections may have various configurations as noted above but preferably are in the form of elongated ribs which define predetermined breaking lines for the filter cake.

Furthermore preferred is an arrangement of the plurality of fluid flow reducing elements along essentially equidistant lines on the outer peripheral surface of the filter wall.

Independent of how the fluid flow reducing elements are configured, the fluid flow reducing elements preferably form an integral part of the filter wall.

The present invention may be applied to a broad variety of filter elements, for example, those made of ceramic material or those made of sintered grained plastic material.

The present invention also may be used where the filter elements are pre- coated prior to the filtration cycle with a precoat material.

Examples for precoat material include, but are not limited to, cellulosic materials like α-cellulose, diatomaceous earth, activated carbon and graphite.

Precoating of the filter wall of the filter element may be used to adapt the filter characteristics of a filter element to the specific need of a filtration task. Precoating furthermore may be used to facilitate dislodging of the filter cakes from the outer peripheral surface of the filter wall. This is of special importance once the solids contents of the non-filtered fluid contains substantial portions of very fine particles.

Preferably, the filter element of the present invention comprises a filter body which has a self supporting filter wall and therefore does not need a supporting structure.

The present invention further relates to a filtering assembly for filtering particulates from a fluid, especially a liquid fluid, such filter assembly comprising one or more filter elements as described above.

The filter assembly according to the present invention preferably comprises a housing accommodating said one or more filter elements. The housing is provided with a fluid inlet for fluid to be filtered and a fluid outlet communicating with the inner cavity of each filter element for draining the filtrate.

Furthermore, the filter assembly according to the present invention preferably comprises a support element receiving and positioning said one or more filter elements within said housing. Preferably, the filter elements are mounted at one end thereof on such support element, more preferably such that the filter elements are pending from said support element.

In order to facilitate detachment of the filter cake from the filter elements, the filter assembly preferably is connectable to a source of pressurized gas from which the backpulses may be delivered to the inner cavity of the filter elements.

The filter assembly according to the present invention preferably comprises a plurality of filter elements, and in a specific configuration of the inventive filter assembly, such a plurality of filter elements is arranged in two or more groups, each group being selectively connectable to the pressurized gas source.

This allows detaching the filter cakes from the filter elements with a limited volume and pressure of the backflushing gas pulses such that the costs for the pressurized gas source may be kept to a minimum.

Preferably, the inventive filter assembly comprises a draining system including one or more conduits providing a fluid communication of the inner cavity of each filter element to a draining outlet of the housing.

Such a draining system enables removal of filtrate remaining in the inner cavity of the filter elements prior to backpulsing, and furthermore may be used to further drain liquid from the filter cake prior to backpulsing. It is evident that such a draining system is advantageous also in systems equipped with conventional filter elements without the fluid flow reducing elements on the outer peripheral surface of the filter wall. In order to facilitate the draining, the draining system preferably includes pumping means.

In order to achieve very high solids contents in the filter cake, the housing may comprise a gas inlet and a gas outlet for flowing a drying gas through the filter cakes. Alternatively, the fluid inlet and/or fluid outlet may be configured to provide such additional function(s).

Furthermore, preferably the housing of the filter assembly comprises a bottom wall including a solid discharge outlet which allows discharging of the solids or the particulate filtered from the non-filtered fluid after the filtration cycle.

The afore-mentioned advantages and further features of the present invention will be explained in connection with the Figures in more detail.

Brief Description of the Drawings

In the Figures:

Figure 1: An embodiment of a filter element according to the present invention;

Figure 2: a cross-sectional view of the filter element of Figure 1 along line II - II;

Figure 3: a group of inventive filter elements according to an embodiment of the invention mounted on a support element;

Figure 4: a filter assembly according to an embodiment of the present invention;

Figure 5: another embodiment of a filter element according to the present invention;

Figure 6: a cross-sectional view of the filter element of Figure 5 along line VI-VI;

Figure 7a to c: cross-sectional representations of various embodiments of filter elements according to the present invention similar to the filter element of Figure 1 with different types of fluid flow reducing elements; and

Figure 8: a cross-sectional view of an embodiment of a filter element of the present invention with a hexagonal filter wall structure.

Figure 1 shows an embodiment of a filter element 10 in the form of a candle- type filter element with a filter body 12 comprising a porous, fluid permeable rigid filter wall 14 having an inner and an outer peripheral surface 16, 18 (cf. Figure 2), the inner surface 16 defining an inner cavity 20. The illustrated filter element 10 is closed at one end 22 thereof and comprises at its opposite end 24, which is open and in fluid communication with the inner cavity 20, a portion 26 having a reduced diameter (in the following called neck portion) for mounting the filter element 10 in a filtration environment as will be explained below.

The filter wall 14 comprises on its outer peripheral surface 18 a plurality of elements reducing fluid flow of non-filtered fluid to selected areas of the outer peripheral surface 18. In accordance with the present invention, the filter wall can include any suitable number of fluid flow reducing elements.

In the embodiment of the present invention shown in Figures 1 and 2, there are four of such fluid flow reducing elements provided in the form of elongated ribs 28 which are arranged along lines essentially parallel to the longitudinal direction of filter element 10. The fluid flow reducing elements may be likewise arranged along lines slightly oblique to the longitudinal direction of the filter element.

Furthermore, in this illustrated embodiment, the four elongated ribs 28 are arranged equidistantly over the surface 18 of the filter wall 14 of the filter element 10.

During operation of the filter element 10, non-filtered fluid is flowing to the outer peripheral surface 18 of the filter element 10 and solids contents of the non-filtered fluid are deposited on the outer peripheral surface 18. During the filtration process, a so called filter cake is built up which is essentially regularly deposited on the outer peripheral surface 18 of the filter wall 14. Due to the existence of the four elongated ribs 28 which serve as flow reducing elements deposition of solids material contained in the non-filtered fluid is greatly diminished or blocked off in areas covered by these elongated ribs 28, thereby

providing predetermined breaking points in the filter cake. It is not necessary that the filtration is stopped once the thickness of the filter cake reaches the thickness of the elongated ribs 28, but the filtration may continue for quite some more time which may lead to filter cake portions covering the elongated ribs 28 on their outer surface. Nevertheless, the function of the elongated ribs 28 is not hindered and still they provide predetermined breaking points of the filter cake which facilitates removal of the filter cake from the outer surface 18 of the filter wall 14 of the filter element 10.

A more detailed explanation about the functioning and the filter cycles during operation of such filter elements 10 will be given in connection with the explanations relating to Figures 4 and 5 of this description.

Figure 3 shows a group of three illustrative filter elements 10 mounted together on a common support element 30 which receives the filter elements 10 with their neck portions 26 at their open ends 24 in openings 32 provided therein. The filter elements 10 are secured to the support element 30 by nuts 34 which are screwed on a threaded portion of the neck portions 26 of filter elements 10.

Once the filter elements 10 are assembled as shown in Figure 3, they may be inserted into a housing of a filter system which is not shown in Figure 3.

Figure 3 also shows the arrangement of the fluid flow reducing elements in the form of elongated ribs 28 on the outer peripheral surface 18 of the filter wall 14 of the filter elements 10 which, in a preferred embodiment, extend from the closed ends 22 of the filter elements 10 up to the neck portions 26 of the same.

Figure 4 shows in a schematic representation a filter assembly 40 according to the present invention in a first embodiment. The filter assembly 40 shown in Figure 4 comprises a housing 42 with a cylindrical wall portion 44 which may be closed by a domed cover 46 on its upper end and which terminates at its lower end in a cake discharge opening 48. The cake discharge opening 48 may be opened and closed as desired during the continued operation of the filter assembly 40 which will be explained in more detail below.

At the upper end of the cylindrical wall 44 of the housing 42, a support eler ment 50 which may be similarly constructed as the one shown in Figure 3 is sealingly mounted between the cylindrical wall 44 and the domed cover 46.

A volume 51 within the housing delimited by the domed cover 46 and the upper surface of support element 50 receives filtrate from the open ends of the filter elements. A volume 53 defined by the cylindrical wall 44 and the lower surface of support element 50 receives and holds non-filtered fluid.

The support element 50 is at the same time designed as a sealing element which separates the interior space of the housing 42 defined by the cylindrical wall 44 (non-filtrate volume 53) from the inner space of housing 42 defined by the domed cover 46 and delimited by the upper surface of the support element 50 (filtrate volume 51). The support element 50 holds a plurality of filter elements 52 which are of a candle-type shape. While the lower end portion of these filter elements 52 is closed, the opposite ends of filter elements 52 are open and are received in and secured to the support element 50.

The filter elements 52 are of a tubular shape and their inner cavities 54, which extend from the open upper end of the filter elements 52 down to their closed bottom ends, are in fluid communication with the filtrate volume 51 of the housing defined by the upper surface of support element 50 and the domed cover 46.

The housing 42 comprises in its cylindrical wall portion 44 an inlet 56 receiving non-filtered fluid during a filtration cycle.

The domed cover 46 of housing 42 comprises an outlet 58 in fluid communication with the filtrate volume 51 via which filtrate from the inner cavities 54 of the filter elements 52 may be discharged.

Furthermore, the filter assembly 40 as shown in Figure 4 advantageously comprises a drainage system 60 comprising draining conduits 62 which extend from a position near the closed end of each of the filter elements 52 in their inner cavities 54 up to their open ends into the space defined between the upper surface of the support element 50 and the domed cover 46 where they may be merged into a common tube 64 which leads to a draining outlet 66 in the domed cover 46 of the housing 42 of the filter assembly 40. The draining outlet 66 may be connected to a suction system in order to promote draining via the draining system 60 at the end of a filtration cycle.

On top of the domed cover 46 of the housing 42 of the filter assembly 40, a gas inlet 68 is provided which is connected to a gas reservoir 70 which comprises pressurized gas, for example, pressurized air. The gas reservoir 70 is connected to the gas inlet 68 via a valve 72 which allows an introduction of gas pulses into the volume 51 defined by the domed cover 46 and the upper surface of the support element 50 as well as the inner cavities 54 of the filter elements 52.

This allows momentarily building-up a gas pressure in the volume 51 defined by the domed cover 46 and support element 50 as well as the inner cavities 54 of the filter elements 52.

In the following the operation of the filter assembly 40 is described in some detail.

Once the filter assembly 40 has been assembled as is apparent from Figure 4, filtration of a fluid, especially a liquid fluid containing particulate matter as a solids material, may be started by feeding such fluid via inlet 56 into the volume 53 of housing 42, i.e., essentially the space defined by the cylindrical wall 44 delimited by the discharge opening 48 which is in this stage of the operation closed and the lower surface of the support element 50 as well as the outer surfaces of the filter elements 52. The air included in this volume is first of all displaced through the porous filter walls of the filter elements 52 and exits the filter assembly through outlet 58 in the domed cover 46 of the filter assembly 40.

Subsequently, filtrate from the non-filtered fluid enters into the inner cavities 54 of the filter elements 52 and fills these inner cavities 54 until it reaches the upper open ends of the filter elements 52 and enters into the volume defined by the domed cover 46 and the upper surface of support element 50. From there the filtrate is discharged through outlet 58 and collected. During the continued filtering operation, the solids material contained in the non-filtered fluid deposits on the outer surface of the filter elements 52 and builds up filter cakes around each one of the filter elements 52 until a certain pressure drop is reached over the filter elements. The feed of the non-filtered fluid to the housing 42 is stopped.

Because of the fluid flow reducing elements on the outer surface of the filter elements 52 which are not shown in detail in Figure 4, filter cakes built up with predetermined breaking points.

Pressurized gas may be introduced into inlet 56 lowering the level of non-filtered fluid in volume 53 down to the closed ends of the filter elements 52. The remainder of non-filtered fluid is drained from the housing through discharge outlet 48.

In order to complete the filtration cycle, the discharge system 60 is now operated draining the inner cavities 54.

If the solids content of the filter cake deposited on the outer surface of the filter elements 52 is already sufficiently high after the operation of the drainage system 60, a gas pulse may be fed into the interior of the domed cover 46 from gas reservoir 70 by the operation of the valve 72.

The gas pulse extends from volume 51 into the inner cavities 54 of the filter elements 52 and helps to detach and break the filter cakes deposited on their outer surface. The portions of the filter cakes fall down by gravity and are discharged through the discharge opening 48.

In the simplest case, then another filtration cycle may start with feeding non- filtered fluid through inlet 56 into the housing 42.

If the solids content of the filter cakes deposited on the outer surface of the filter elements 52 is not sufficient for the specific needs of the filtering application, a gas stream may be provided, for example, through inlet 56 into the housing defined by cylindrical wall 44 blowing through the filter cakes on the outer surface of the filter elements into their inner cavities 54 and being drained through outlet 58 in the domed cover 46. This drying gas may be heated in addition to promote further drying of the filter cakes deposited on the outside of the filter elements 52.

After the desired solids content of the filter cakes has been reached, the filter cakes are detached from the filter elements by a gas pulse as described before and discharged from the housing 42 via discharge opening 48.

Optionally, the filter cake when still sticking to the filter elements may be washed in a washing step.

Optionally, prior to each filtration cycle, the filter elements may be precoated in order to adapt their filtration characteristics to the specific filtration task performed and/or to further facilitate discharge of the filter cake from the outer surface of the filter elements.

Some of the preferred features of the present invention may be summarized as follows:

Filtrate remaining within the inner cavity of the filter elements after draining of the filter housing is drained off by using a conduit system as shown and explained in Figure 4. The conduits used in the draining system are small as compared to the inner diameter of the inner cavity of the filter elements. The conduits reach down to the closed end of each filter element. Preferably, all of the conduits are connected to a pump in order to suck off the remaining filtrate from the interior of the filter element after draining of the filter housing from non-filtered fluid.

By use of the active suction system within the draining system of the inventive filtering assembly, the filtrate from the inner cavities of the filter elements is quickly and completely removed. Because of the small diameter of the conduits of the draining system present in the inner cavity of each filter element, the cleaning intensity of the gas backpulse does not suffer and may be still efficiently used for the detachment of the filter cakes. Also this allows for a ho-

mogeneous build-up of a precoat layer along the outer peripheral surface of the filter wall of the filter element if needed.

The use of filter elements according to the present invention having on the outer peripheral surface of the filter wall a plurality of elements for reducing fluid flow of non-filtered fluid greatly improves the detachment of the filter cakes from the surface of the filter elements.

Preferably, the fluid flow reduction elements are provided along lines typically extending at least about 50 %, more preferably at least about 80 %, of the distance bet-ween the closed end and the open end.

As an example, fluid flow reducing elements in form of elongated ribs which run from the closed bottom of the filter elements essentially up to their open ends, greatly promote detachment of the filter cake. Filter candles having four to five of such elongated ribs on the outer peripheral surface of the filter wall already worked best. In case of elongated ribs which are arranged in spiral form on the outer peripheral surface, one or two elements for reducing fluid flow may already give very good results.

Therefore, the provision of the elements for reducing fluid flow of non-filtered fluid to selected areas on the outer peripheral surface of the filter wall, does not significantly affect the active filtration area of the filter elements.

For dislodging of the filter cakes by a gas backpulse, a gas reservoir is used which is under higher pressure than the rest of the filtration assembly. The gas

reservoir is connected via a valve to the filtrate side of the filter elements (cf. the explanation given in connection with Figure 4).

In case a large number of filter elements is to be used inside the housing of the filter assembly, the filter elements may be arranged in groups, each group of filter elements preferably having separate draining facilities, and each group being also separately connectable to the gas reservoir for detachment of the filter cakes. This allows the use of smaller valves and smaller gas reservoirs than backpulsing all of the numerous filter elements at the same time. Illustratively, one group of filter elements is backpulsed after the other with a gas pulse.

While the filtrate side of such filter assembly preferably is divided into various compartments, each compartment accommodating the filtrate outlets of the filter elements of one group, such compartments being sealed off against one another, the non-filtrate side of the housing of the filter assembly does not need to be divided and may be used as a common volume for all groups for filter elements.

Figures 5 and 6 show another embodiment of a filter element 80 of the present invention in the form of a candle-type filter element with a filter body 82 comprising a porous, fluid permeable rigid filter wall 84, having an inner and an outer peripheral surface 86, 88 (cf. Figure 6). The inner surface 86 is defining an inner cavity which extends from a closed end 92 of the filter element 80 to the opposite end 94 which is open and in fluid communication with the inner cavity 90. The filter element 80 has adjacent to its open end 94 a portion 96 of reduced diameter (in the following neck portion) for mounting the filter element 80 in a filtration environment as is illustratively shown in Figure 3.

The filter wall 84 is self-supporting such that no additional reinforcement or supporting structure is needed for the operation of the filter element 80, be it during filtration or be it during backpulsing.

The filter wall 84 comprises on its outer peripheral surface 88 two elements reducing fluid flow in the form of spiral continuous ribs 98 which run in parallel approximately from the closed end 92 of the filter element 80 up to the neck portion 96.

Figures 7a to 7c show further embodiments of a filter element according to the present invention in a cross-sectional representation.

The filter elements 100a, 100b and 100c may all have the same overall structure as shown for the filter elements in Figures 1 and 5.

The basic structures of all these three filter elements comprise a filter body 102a, 102b and 102c, respectively, comprising a porous, fluid permeable, rigid filter wall 104a, 104b and 104c, respectively, having inner and outer peripheral surfaces. The inner surfaces 106a, 106b and 106c, respectively, define an inner cavity 108a, 108b and 108c, respectively.

What is different in all these three embodiments of Figures 7a through 7c is the configuration of the fluid flow directing elements 112, 113 and 114, re- spectively, on the outer peripheral surface 110a, 110b and 110c, respectively, of the elements 100a, 100b and 100c, respectively.

In Figure 7a, the fluid flow directing elements 112 are in the form of puncti- form projections arranged along lines parallel running from the closed bottom of the filter element 100a up to the opposite which is an open end and not shown in Figure 7a. The lines are essentially parallel with the longitudinal direction of the filter element. Likewise, the lines could be arranged in a slightly oblique direction with respect to the longitudinal axis of the filter element.

The punctiform projections of the fluid flow directing elements 112 may also be arranged along spirally wound lines, similarly to what is illustrated in Figure 6 for the filter element 80.

In the embodiment shown in Figure 7b, the filter element 100b comprises fluid flow directing elements 113 on the outer peripheral surface 110b along four parallel lines extending in parallel (or slightly oblique) to the longitudinal direction of the filter element 100b.

The fluid flow directing elements 113 comprise a plurality of elongated projections.

As has been explained with respect to the embodiments of Figure 7a already, the fluid flow directing elements 112 may also be arranged along spirally wound lines as is illustrated for the ribs 98 in Figure 6 of filter element 80.

Preferably, the multiplicity of projections forming a fluid flow directing element

112 and 113 are spaced from one another with a distance equal or less than the extension of the projections in circumferential direction of the filter element.

If the distance between two neighboring projections of the fluid flow direction elements 110 and 112 becomes too large, their function as a breaking point for the filter cake deposited during filtration on the outer peripheral surface will be affected.

As is the case with the fluid flow directing elements 98 of the embodiment shown in Figure 5, the extension of the fluid flow directing elements 112 and

113 in radial direction of the filter wall 102a and 102b, respectively, is kept as small as possible in order not to block too much of the available outer peripheral surface 110a, 110b for deposition of a filter cake.

The filter element 100c shown in Figure 7c has fluid flow directing elements in the form of strips 114 as shown in Figure 7 which may be straight and arranged in parallel (or slightly oblique) to the longitudinal direction of the filter element 100c as shown in Figure 7c or may be arranged along spiral lines similar to what is shown in Figures 5 and 6 for the fluid flow direction elements 98.

In Figure 7c, the fluid flow directing elements 114 are made an integral part of the filter wall 104c of filter element 100c and their outer surface is flush with the outer peripheral surface 110c of the filter wall 104c. In order to achieve the fluid flow directional effect, the fluid flow directing elements 114 are made of a more dense material than the filter wall 104c and may be essentially non-porous.

During the filtering operation, a partial deposit of the filter cake may cover the fluid flow directing elements 114. However, this does not impair their function to form breaking points for the filter cake, as has been explained for the projecting fluid flow directing elements in connection with Figures 1 and 2 above.

Figure 8 shows still another embodiment of a filter element 120 of the present invention which comprises a filter body 122 comprising a self-supporting filter wall 124 which is made of a rigid porous material. The filter element 120 may be of a candle type as shown for the filter element 10 in Figure 1 and have a closed end (not shown).

The filter wall 124 comprises an inner and an outer peripheral surface 126, 128, the inner peripheral surface 126 defining an inner cavity 130.

The outer surface 128 comprises as an integral part of the filter wall 124 fluid flow directing elements 132 at each one of the edges of the hexagonal structure. The outer surface of the fluid flow directing elements 132 are essentially flush with the outer peripheral surface of the filter wall 124. The fluid flow directing elements 132 may be in the form of elongated strips as shown in Figure 7c. Alternatively, the fluid flow directing elements 132 may be of, e.g., punctiform configuration similarly to what is represented for projecting fluid flow directing elements in Figures 7a and 7b.

Figure 8 demonstrates that the cross-sectional shape of the filter element according to the present invention need not necessarily be of a cylindrical shape as shown in Figures 1 through 7 but may have a polygonal form as shown in Figure 8. It is noted that the polygonal structure is of course not limited to the hexagonal structure illustratively shown in Figure 8, but may be of any type of polygonal form.

Preferably, the inner cavity 130 has the corresponding polygonal shape as the outer shape of the filter element 120 defined by the outer peripheral surface 128 such that the thickness of the filter wall 124 is uniform.

Nevertheless, filter elements, where the outer peripheral surface 128 is of a polygonal shape, the inner peripheral surface 126 may define a cylindrical inner cavity.

Because of the different thickness of the wall 124, a slightly different distribution of deposition of the filter cake and/or the filtering effect of the filter wall 124 may result.

Claims

1. A filter element comprising a filter body with a porous, fluid permeable, rigid filter wall having an inner and an outer peripheral surface, said inner peripheral surface defining an inner cavity receiving filtered fluid, said filter wall comprising on its outer peripheral surface two or more elements reducing fluid flow of non-filtered fluid to selected areas of the outer peripheral surface, said outer peripheral surface of the filter wall having an essentially flat or convex configuration between each of two proximate fluid flow reducing elements.

2. The filter element of claim 1, wherein the filter wall is self-supporting.

3. The filter element of claim 1 or 2, wherein the filter element is a candle- type filter element.

4. The filter element according to any one of claims 1 to 3, wherein the fluid flow reducing elements have a porosity which is smaller than the porosity of other portions of the filter wall.

5. The filter element according to any one of claims 1 to 4, wherein the fluid flow reducing elements have an essentially non-porous surface.

6. The filter element according to any one of claims 1 to 5, wherein the fluid flow reducing elements comprise elements in the form of projections.

7. The filter element according to claim 6, wherein the projections comprise a plurality of elongated ribs.

8. The filter element according to any one of claims 1 to 7, wherein the fluid flow reducing elements are arranged along two or more parallel lines extending over at least about 50 %, preferably at least about 80 %, of the length of the filter body measured in the longitudinal direction thereof.

9. The filter element according to claim 8, wherein the lines are arranged essentially equidistantly on the outer peripheral surface.

10. The filter element according to any one of claims 1 to 9, wherein said fluid flow reducing elements form an integral part of the filter wall.

11. The filter element according to any one of claims 1 to 10, wherein the filter wall has a seamless configuration.

12. The filter element according to any one of claims 1 to 11, wherein the outer peripheral surface of the filter body is pre-coated with a pre-coat material.

13. The filter element according to claim 12, wherein the pre-coat material is selected from cellulosic material, diatomaceous earth, activated carbon and graphite.

14. The filter element according to any one of claims 1 to 13, wherein the filter body comprises a porous ceramic or plastic material.

15. A filter assembly for filtering particulates from a fluid, said filter assembly comprising one or more of the filter elements according to any one of claims 1 to 14.

16. The filter assembly according to claim 15, wherein said assembly comprises a housing accommodating said one or more filter elements, said housing having a fluid inlet for fluid to be filtered and a fluid outlet communicating with the inner cavity of each filter element.

17. The filter assembly according to claim 16, wherein said assembly comprises a support element receiving and positioning said one or more filter elements within said housing.

18. The filter assembly according to any one of claims 15 to 17, wherein the assembly is connectable to a source of pressurized gas.

19. The filter assembly according to claim 18, wherein said assembly comprises a plurality of filter elements arranged in two or more groups, each group being selectively connectable to the pressurized gas source.

20. The filter assembly according to any one of claims 15 to 19, wherein said assembly comprises a draining system including one or more conduits providing a fluid communication of the inner cavity of each filter element to a draining outlet of the housing.

21. The filter assembly according to claim 200, wherein said draining system includes pumping means.