JPH01310710A - Partition wall of backwashable precoat filter element - Google Patents

Abstract

PURPOSE: To increase a contamination capacity and to execute filtering and backwashing efficiently for a long period by coaxially arranging a septum in a longitudinally pleated form on the circumference of a perforated metallic core and supporting a prescribed precoat at the septum. CONSTITUTION: The precoat filter element consists of the metallic core 20 consisting of a perforated cylindrical wall 24 regulating a space for discharge and the septum 10 coaxially arranged therewith. The septum 10 has the longitudinal pleats 12 having valley parts 14 in contact with the perforated cylindrical wall 24 and front ends 16 distant from the wall 24 and is coated with the precoat. The precoat consists of, for example, a slurry of ion exchange resin particles suspended in deionized water. The precoat slurry is sent to the pleated septum 10 in a first direction and the precoated is accumulated on the pleats 12. The liquid is sent to the precoated pleats 12 in the same direction and is thus treated. Another liquid is sent in an opposite direction to execute backwashing.

Description

[Detailed description of the invention] The present invention relates to a precoated filter element.

Precord filtration equipment can be used for chemical processing and industrial wastewater treatment. For example, diatom-coated filters may be used to filter festivals, oils, milk, beer, fruit juices, and water. In particular, coated filters are used when the liquid product after processing must be of very high purity and closely conform to defined standards of deionization or chemical composition. Pre-coated filters are used, for example, with an ion exchange resin coating, in the boiler feed water treatment of steam generation systems in nuclear reactors and in the condensation process of nuclear power plants.

Precoated filter elements generally consist of a porous support structure, what is called a septum coated with filter media, and what is called a precoat that can perform the filtration operation and chemically interact with the filtrate. Commonly used in many filtration applications? The filter material is diatomaceous earth. Ion exchange resins can be used in wastewater treatment applications that require high purity effluent. In each case, the filter material is accumulated on the filter septum by forming a slurry of the same type of liquid as that to be filtered and sending this slurry to the porous septum and collecting the suspended coating material of the slurry on the septum. be able to. When diatomaceous earth is used, the particles of the coating material are preferably significantly smaller than the pores of the septum material. The particles of the coating material bridge the pores of the septum, forming a thick but permeable layer that acts to filter contaminant particles from the filtrate once a sufficient depth of coating is deposited and the filtration operation begins. do. Examples of liquids that can be filtered through such coated filters are oils; foods such as milk, syrup, beer, and juice;
water; liquid wax; chemicals; and various chemical solutions. The ability of a coated filter to filter particulates depends on the fineness of the diatomaceous earth or other coating that covers the rough septum.

? As the filtration operation progresses, the pressure drop across the sheathed filter gradually increases until continuous filtration becomes difficult and the sheathed filter must be cleaned, removing the sheathing material, such as diatomaceous earth, and replacing this with new sheathing material. replace. A preferred method of cleaning is generally to reverse the flow of liquid to the filter septum by reversing the pressure gradient across the filter element. The reverse fluid flow is generally driven at a faster rate than during the filtration process to remove the filter layer. The previously filtered liquid may be used for that purpose and then placed in a reservoir to deposit the coating material. - Once the carrier coating material has been deposited, the fluid can be recycled as product by repeating the filtration process. A sudden and violent backflow of liquid is often used to dislodge the filter membrane and carry it away.

Although the pore size of the membrane is generally smaller than the septum, nevertheless, after many successive filtration and backwashings, the septum becomes sufficiently occluded to prevent effective cleaning and the membrane must be replaced. .

Precoated filter elements are often utilized to treat liquids containing both suspended particles and dissolved chemical and ionic contaminants, and the treated effluent is of very high purity and meets defined standards of deionization and chemical contaminants. The composition must be followed exactly. Industrial processes that utilize steam from steam generating systems may generate corrosion products, cooling water internal leaks, and condensate contaminated with various materials used in the process. Contaminants in the condensate or boiler feedwater of nuclear steam generation systems can cause corrosion of heat transfer equipment within the system, damage heat exchange surfaces, and
and may reduce heat transfer efficiency. This ultimately results in overheating of the tube, leading to tube breakage, further equipment damage, and possibly radioactive contamination of the environment. To prevent corrosion in nuclear steam generation systems, water treatment must include conditioning of the raw feed water, condensate returns from the process steam or turbine, and boiler water.

A particular example of water treatment that requires strict quality control and where the use of precoat filters is common is the condenser process in nuclear power plants. The two main types of nuclear power plants are boiling water reactors and pressurized water reactors. Although they use different steam generation methods to drive turbines that produce electricity and require different water treatment, both use similar water purification systems commonly referred to as condensate polyurethanes. In boiling water reactors, if particles enter the reactor through the raw water, they may decompose and become radioactive. The generation of radioactive particles may pose costly disposal problems and pose a significant risk of exposing people to radioactive material.

The presence of particulate contaminants in pressurized water reactors may cause stress cracking of heat exchanger tubes.

Feed water may initially contain many different types of dissolved and suspended substances. Most commonly, these materials include silica, iron, copper, calcium, magnesium, and sodium compounds. Metal components generally occur as bicarbonates, carbonates, sulfates and chlorides. Many of these substances are ions in solution, which can be advantageously used in water treatment to obtain high purity effluent water.

Because raw water may contain a wide variety of harmful contaminants, it is common to use one or more techniques to remove contaminants during this treatment. Typically, the raw water is first purified to mechanically remove suspended large particulate contaminants and then deionized by ion exchange. The deionization process, commonly known as condensate polishing, produces water from ionic contaminants that is reasonably close to its theoretical maximum chemical purity.

Condensate polishing involves reversible exchange of ions between a liquid phase and a solid phase caused by charge carried by the ions. The solid phase typically consists of an ion exchange resin saturated with ionic groups that exchange ions contained in the liquid for new ions when the solid comes into contact with the liquid. In the condensate polishing process, ion exchange can be used to treat contaminated water in two ways; ionic contaminants in the raw water can be replaced with relatively harmless deionized products, and major components within the contaminant's molecular structure can be replaced. Ionic contaminants can be converted to harmless or less harmful products by replacing the ions with other ions that eventually react with the ions present in the liquid to produce relatively harmless products.

In the second case, for example, the replacement ions may react with the remaining ionic contaminants to form further water.

In a method for deionizing boiler feedwater or steam condensate, e.g.
It is common for one tank to replace the metal cations in the water and a series of other tanks to replace the anions in the water. Each resin continues to displace contaminants until its deionization capacity is exhausted.

When deionization capacity is exhausted, ion exchange capacity is restored by replacing the resin in the case of cartridge tubular condensate polishing systems or by regenerating it in deep desalination systems.

In a condensate polishing system used in a condenser process of a nuclear power plant, it is common to perform the filtration process and deionization process of a water conditioning method at the same time, for example, using a filtration device equipped with a precoat filter. The filter elements of this type of device are typically all pre-coated filter elements as described above and include a porous support structure, referred to as a media-coated septum;
It consists of something called a precoat that performs both filtration and ion exchange steps. In pre-cartridge condensate polishing systems where the precoat is disposable, if the precoat becomes clogged with particulate contaminants, analysis of the effluent water will show that the precoat becomes clogged with particulate contaminants, as indicated by an increase in pressure drop, i.e. no ion exchange capacity. As such, the precoat is peeled off from the partition wall and discarded by a backwash operation in which the precoat is washed out of the -11= system.

A new resin precoat is then applied to the septum.

Typical backwashable Precor+ used for deionization applications
->A filtration device 100 is schematically shown in FIG.

The filtration device 100 has a housing consisting of a pressure vessel 101 with a port 102 and an outlet 103. A tubesheet 104 is secured around the inner wall of the container 101, forming a lower low pressure chamber or space 105.
It is partially divided into a high pressure chamber 106.

Outlet 103 communicates space 105 with the exterior of the container. The two-part chamber 106 communicates through an opening in the tubesheet 104 to the exterior of the container and to the inlet 102.

A number of filter elements 109 are located within the two-part chamber 106. Each filter elm l-
109 includes an isolation tube 111 passing through a hole in the tubesheet. Each filter element 109 is connected to the tube seat 1
-104 and its outlet 11 at the base of the isolation tube 111.
It is supported by 0. Each hole is surrounded by a gasket (not shown) to provide a seal between tubesheet 104 and isolation tube 111. Each filter element 109 consists of a medium core (not shown) and a porous partition wall 112 capable of supporting pre-coated resin. When the pressure in the upper chamber becomes greater than the space 105, the porous partition allows water to flow into the core. An isolation tube 111 extending vertically downward from the core connects the element 109 to the space 1.
Serves as a discharge passage for furnace fluid to 05.

The filtration device shown in FIG.
8, above the tubesheet 104 and inlet opening 1
07 includes a baffle plate 113. A baffle plate 113 is supported in spaced relationship with the tubesheet to provide an annular space around the baffle plate through which water may pass.

Thus, water passes through the inlet 102 into the upper chamber 10.
6, the flow hits baffle plate 113 and flows through the annular space throughout upper chamber 106. The purpose of the baffle plate is to reduce turbulence, particularly at the lower center of the chamber near the inlet, but generally throughout the chamber.

Three basic types of commonly used precoat filter partitions are wound porous cores, coarse meshes, and porous metal cartridges. Generally these are cylindrical, but some have other shapes. The wound porous core septum typically consists of a porous cylindrical stainless steel hollow core wrapped with a protective cord or thread. Polymeric threads such as nylon or polypropylene are commonly used. These threads are typically 1.6 mm (1.6 mm)
/16 inch) in diameter and wrapped around a hollow core to provide a septum depth of about 1/2 inch. As the contaminated water flows to the precoat, it attempts to pass through the microporous openings of the yarn filaments without passing through any spaces between adjacent strands of the wound yarn.

Rough mesh septa include septa made from polypropylene mesh and wire mesh. Porous metal cartridges generally consist of a filtration media of sintered or bonded metal particles. The rough mesh septum and porous metal cartridge may be wrapped with thread.

The precoat of these filters typically consists of a slurry of ion exchange resin particles suspended in deionized water. The suspension is formulated with a predetermined ratio of cationic and anionic particles depending on the intended use. The operation of the cleaning device 100 of FIG. 1 can be divided into two stages: (1
) precoat stage and (2) clearing stage. Generally, each filter septum 112 is precoated by introducing a flow of precoat resin slurry into the population 102 across the baffle plate 113 and into the filter element 109 to accumulate a precoat layer on the upstream surface of the septum 112. Optimize the resin particle size distribution, flow rate, and flocculant proportion to achieve a proper precoat. The precoat layer is then applied to each Nilemene L-109 septum 1 by continuously circulating deionized water at the process flow rate for a short period of time to the coated septum.
Compress on 12 surfaces.

A good precoat will have a thickness along the entire length of each septum and no racking during the precoating or processing cycle.
and of uniform thickness on all septa within the cleaning device housing.

During water treatment, contaminated water passes through the inlet 112 to the baffle plate 1
13 into the container 101 and into the upper chamber 106. This water contacts the resin precoat on the surface of each septum 112 and flows radially inward first through the precoat and then into the septum 112. As water flows through the resin in the precoat, the specifically formulated ion exchange resin particles remove or transform minerals and other ionic contaminants in the water by the methods described above. The ion exchange precoat also acts as a filtration medium and typically
It has a finer pore structure than 2. Ideally, particulate contaminants would be captured in the pre-coat and prevented from entering the septum 112, and the treated liquid would then be routed through the septum 112 to the core of the filter element 109 and into the isolation tube 1.
11 into space 105 and exits container 101 through outlet 103.

Because the precoat continues to trap particulate contaminants during the deionization operation, is it necessary to maintain a given flow rate? What is the differential pressure of the filtration device 100? The ion exchange capacity of the resin is exhausted as indicated by the change in effluent water species as the ion exchange continues and increases until a level is reached where the filtration operation becomes inefficient.

At this point, the treatment process is stopped and the water is flowed in the opposite direction to the equipment for a backwash operation? Apply a filtration device. The used precoat is peeled off from each partition wall 112 and washed out of the system by backflow. Then start the next cycle with a new precoat operation.

Known deionized pre-coated filtration systems of this type have several drawbacks. Many conventional precoat filtration systems are inefficient because the backwash frequency is dictated by the maximum allowable differential pressure before precoat depletion. The amount of contaminants that can be retained within the precoat while maintaining the high flow rate required by the critical flow area provided by the cylindrical partition within differential pressure limits is relatively small.

Although the resin precoat is clogged with particles, its deionization properties are not exhausted, and the precoat filter filters less liquid than can be deionized. Thus, tipping is often detected by differential pressure, wasting useful precoat, and increasing the amount of radioactive waste that must be disposed of.

The present invention provides a filter element for a precoated filtration device comprising a central cylindrical core having an upper end, a lower end, a central drainage space and a perforated wall that allows filtrate to flow into the drainage space and into one end of the core. and; a bulkhead surrounding the core and approximately coaxial with the core;
It supports the precoat and includes longitudinal pleats having valleys near the core wall and tips distal from the core wall.

The present invention also provides a filter element for a precoat type filtration device capable of conveying precoated slurry along a flow path to a filtration device, the filter element comprising a partition wall disposed across the flow path and an upstream surface of the pleats. includes a longitudinal pleat with an upstream surface that accumulates precoat.

The invention further provides for directing the precoat slurry in a first direction to the pleated septum to accumulate the precoat on the pleats of the septum, and directing the liquid in a first direction to the precoated pleats of the septum to deposit the precoat material on the pleated septum. process liquids,
A method of disposing of a liquid is also provided which comprises sending another liquid in the opposite direction to the precoated pleats of the septum to strip the precoat from the pleats of the septum.

Because the pleated septum has a significantly larger surface area than current cylindrical septa, the filter element and method provided by the present invention has a longer life and greater efficiency than before.

The pressure difference is lower at a given flow rate during a filtration operation, which gives a larger precoat contamination capacity. Thus, the filter element and method of the present invention efficiently utilizes the septum while infrequently backwashing and maintaining septum pressure differentials during filtration, backwashing, and precoating operations within acceptable differential pressure ranges.

The large surface area of the pleated bulkhead gives significantly higher performance. Depending on the element diameter envelope, the pleated septum can be twice the area of the cylindrical septum, thus halving the flow density. Since contamination capacity is highly dependent on flow density, halving the flow density approximately doubles the contamination capacity per unit area of septum and precoat. Thus, the combined effect of doubling the area of the septum increases the contamination retention capacity of the precoat by a factor of four.

The high contamination capacity of this element allows the use of finer pore size partitions without exceeding differential pressure limits. This brings important advantages. With rough septum elements, low flow rates must be maintained for extended periods after the precoat cycle to prevent excess resin from leaking through the septum from the precoat. Fine septa can increase the initial throughput and bring the vessel to steady state more quickly. Resin leakage during continuous processing cycles is also reduced. Resin leakage results in resin build-up downstream of the deionizer, which may lead to erosion and reduce the efficiency of the heat exchange equipment.

FIG. 2 shows an example of a precoated filter element embodying the present invention. The precoated filter element of Example-20= generally consists of a partition 10 in the form of longitudinal pleats disposed coaxially with a porous metal core 20. The core consists of a perforated cylindrical wall 24 defining a drainage space. Core 2o is placed on pace cap 30. An isolation tube 40 extends vertically downward from the pace cap and is in fluid communication with the drainage space. The porous cylindrical metal core 2o can be made of any suitable material, such as stainless steel, for example. The core 20 has two caps, a top cap 35 and a pace cap 30. The upper end cap is located at the upper end of the core and the pace cap 30 is located at the lower end of the core.

The caps are secured to the cylindrical core wall 24 by welding or any other suitable bonding method, such as resin bonding.

3 is a schematic cross-sectional view of the septum of FIG. 2 taken through a section perpendicular to the axis of the septum, and like reference numbers refer to like parts in these figures; FIG. Generally, the partition wall 1° is a cylindrical core wall 24
It includes a longitudinal pleat 12 having a trough 14 adjacent to the core wall and a tip 16 remote from the core wall. The septum, shown partially coated with Brecord 500, was coated with PMM, a porous metal membrane material commercially available from Pall Corporation, immediately prior to the filtration operation.
, etc., may be formed of any suitable material. The barrier material can be chosen depending on the particular application depending on the nature of the precoat resin mixture used and the chemistry of the water to be treated.

As seen in FIG. 2, peripheral bands 8o may be wrapped around pleats 12 at regular intervals along the vertical length of the septum to support the pleats against bending stresses. These bands can be made from any suitable material, such as stainless steel. These bands may be attached to tip 16 by any suitable means, such as by pressure attachment.

The pleat design must satisfy the requirements of maximizing filtration area while providing adequate space between the pleats for precoat deposition. Furthermore, the differential pressure of the partition wall is 2.1
It must also meet the requirements of providing adequate mechanical strength to withstand the stresses of a backwash cycle, which may be from 30 to 100 psi. Using an optimization method based on analysis,
Determine the pleat number and pleat height of the preferred embodiment based on achieving maximum pleat area for filtration and meeting critical values for minimum precoat thickness and maximum allowable bending stress. Given the center-to-center placement of the elements within the upper chamber of the filter housing and the desired precoat thickness, the outer diameter envelope within which the pleated elements must fit can be determined. Given for a specific use? The composition of the filtration media establishes the minimum thickness of the resin precoat.

As illustrated in Figure 3: t=4Js Thickness of filtration medium, support and drainage composite;
meters (inches) OD = Outer diameter envelope line within which the pleated element must fit, meters (inches) CD-4 Core diameter of the element compatible with critical values of liquid flow rate and pressure drop, meters (inches) (inches) T R = iJsBending radius at the tip of the pleat, determined by the physical properties of the permedium composite and manufacturing critical values, in meters (inches) PT - Minimum precot (thickness, meters (inches) ) PD = maximum pressure drop experienced by the element, bar (psi) SS - allowable bending stress at any pleat, bar (psi)
i) For optimization of pleat form: N = number of pleats in the element CP - circular pitch of pleats - 2π/N radians b -
Spacing of the circumferential bands along the septum axis, meters (inches) h - Height of the pleats, meters (inches) PR - Radius of the pleats, meters (inches) S - Height of the pleats in h inches relative to the distance between the bands The bending stress along the radial direction caused by the number of pleats N and = 2 in a mutually influencing way, bar, (psi)
4 - The core diameter CD can be determined, which has a pleat radius P greater than or equal to the minimum precoat thickness PT.
R is achieved and the resulting stress S along the outer radial direction of the pleat is less than or equal to SS. Model the pleats simply as supported beams with the beam span equal to the pleat height h, the beam depth equal to the composite thickness t, and the beam width equal to the spacing between the surrounding bands. , and applying beam bending theory, the maximum bending stress S can be shown for known variables as give. Although this work reveals pleat troughs of approximately circular cross-section, from the point of view of ease of manufacture, the pleat troughs may have other cross-sections, for example elliptical or trapezoidal.

As an example of the improvements possible with pleated bulkheads, consider a cylindrical element with an outer diameter of 5 centimeters (2 inches) and a core diameter of 2.82 centimeters (1,109 inches), which is the maximum bending stress allowed. 1034 bar (
required to operate with a minimum precoat thickness of 5 millimeters (0.2 inches) and a backwash pressure of 6.9 bar (100 psi) without exceeding 15,000 psi). Such an element may be replaced by the present invention with a pleated element having 11 pleats with a pleat height of 1,13 centimeters (0,446 inches), provided that the minimum precoat thickness is 5.21 millimeters (0,446 inches). 205 inches) with maximum induced stress of only 82
It receives 8 bar (12,000 psi). ``This element provides an 89% increase in surface area over cylindrical elements.

The increased surface area reduces the pressure drop for a given flow rate. This allows the use of finer pore grade septa. Examples of barrier materials are fine mesh porous metal membranes such as PMM from Pall, sintered woven wire mesh, Spramesh, Rigimesh, and fibrous materials.

PMM, Supramerash and Rigimesh are registered trademarks of Pall Corporation.

The increased surface area of the pleated septum allows it to support a greater volume of precoat than a cylindrical septum. This increases precoat volume and contamination capacity. As above, the flow density is halved, doubling the contamination capacity.

The combined effect is an overall 4-fold increase in contamination capacity in the coated element. Due to the increased surface area, optimization of septum and process parameters is required due to depletion of resin deionization capacity rather than due to increased pressure drop due to backwash frequency or precoat blockage. Reducing backwash frequency reduces bulkhead wear and increases filter element operating life. The increased surface area and decreased flow density also increases the residence time of the contaminant within the precoat, which provides a longer contact time between the ionic contaminant and the ion exchange resin. This results in improved precoat utilization.

Longer element lifespans that reduce element replacement rates can reduce overall element replacement costs. More importantly, the cost of rad waste disposal of the used elements is reduced.

As a result of the full utilization of resin deionization capacity, the reduction in precoat volume that must be disposed of also results in considerable savings in rad waste disposal costs.

For example, an analysis of one power plant found that the cycle life of the precoat could be extended from 15 days to 60 days, which cut resin rad waste costs and resin replacement costs in half.

Furthermore, doubling the surface area of the septum reduces the possibility of septum contamination, ensuring long cycle life and eliminating resin leakage. The cost of replacing bulkhead pipes as well as the cost of disposing of old pipes is reduced and the rad waste effort is minimized. In this particular study, it was estimated that the minimum savings over two years of operation would be more than half the cost of constructing a bulkhead according to the present invention.

Although an example of a Precot filter element embodying the present invention has been shown, it should be understood that the present invention is not limited thereto.

Modifications may occur to those skilled in the art, especially in light of the above teachings. It is therefore intended, by the appended claims, to cover any modifications that include the features of the invention or which encompass the true spirit and scope of the invention.

[Brief explanation of the drawing]

FIG. 1 is a partial cross-sectional view of a conventional precoat extinguisher. FIG. 2 is a cutaway perspective view showing a support body, a drainage core, and a surrounding partition according to an example of the present invention. FIG. 3 is a cross-sectional view of the septum and drainage core of one embodiment of the present invention taken perpendicular to the axis of the septum, also showing a portion of the precoat applied to the septum. Engraving of drawings (no change in content) ■ FIG + FIG 3 Procedural amendment (method) % formula % ], Indication of case 1999 Patent Application No. 17425 2, Name of invention Partition wall of pre-coated filter element that can be backwashed 3,
Relationship with the case of the person making the amendment Patent applicant address Name: Paul Corporation 4, Agent address: Shintemachi Building 206-5, 2-2-1 Otemachi, Chiyoda-ku, Tokyo Date of amendment order: 1999 April 25th (shipping stamp 6, subject to correction)

Claims (1)

Claims: 1. A central cylindrical core (20 ); and the core (2
a bulkhead (10) surrounding and substantially coaxial with the precoat (500) and supporting the core wall (24);
); and a septum (10) comprising a longitudinal pleat (12) having a valley (14) close to the core wall (14) and a tip (16) remote from the core wall (24). 2. The filter element according to claim 1, wherein the trough (14) has a larger radius of curvature than the tip (16). 3. The filter element according to claim 1 or 2, wherein the partition wall (10) is made of a porous metal material. 4. The filter element according to claim 3, wherein the metal material comprises a porous metal membrane. 5. Peripheral support bands (80) arranged at regular intervals along the longitudinal direction of the filter element, the core (2
5. A filter element according to claim 1, 2, 3 or 4, further comprising a band (80) coaxial with 0) and holding down the septum tip (16). 6. The surface area of the partition wall (10) and the space between these pleats are such that the shape and number of longitudinal pleats (12) around the partition wall are the maximum precoat life determined by the deionization capacity of the precoat. 3. Using a deionizing precoat selected to provide an optimal relationship with the maximum precoat thickness allowed by
, 4 or 5. 7. Sending the precoat slurry in a first direction to the pleated bulkhead (10) to accumulate the precoat (500) on the pleats (12) of the bulkhead (10); A liquid is directed in a first direction to treat the liquid with the precoat material; and another liquid is directed in the opposite direction to the precoated pleats (12) to remove the precoat (500) from the pleats (12) of the septum (10). Peeling; A liquid processing method consisting of each of the above steps. 8. Pre-coat (on the pleats (12) of the partition wall (10)
8. The method of claim 7, further comprising the step of compressing 500). 9. A method according to claim 7 or 8, wherein the step of directing the liquid in a first direction to the pre-coated pleats (12) of the septum (10) comprises trapping particulate contaminants. 10. Pre-coated pleats (12) of bulkhead (10)
10. A method according to claim 7, 8 or 9, wherein the step of directing the liquid in a first direction to comprises treating the liquid with an ion exchange material. 11. In a precoat type filtration device capable of sending a precoat slurry through a filtration device along a flow path,
A filtration device, wherein the filter element consists of a septum (10) disposed across the flow path and comprises longitudinal pleats (12) having upstream surfaces for accumulating precoat (500) on the upstream surfaces of the pleats (12). 12. The filter element of claim 11, wherein each pleat (12) comprises a trough (14) having a radius and a tip (16) having a radius smaller than the radius of the trough (14). 13. Porous core (20) arranged coaxially within the partition wall (10)
) The filter element according to claim 11 or 12, further comprising: ). 14. The valley (14) of each pleat (12) is the core (20
) and the tip (16) is remote from the core (20).