Patent Description:
Filter candles for hot gas applications according to the prior art are often based on porous filter elements in the shape of hollow cylinders with one closed end. The gaseous fluid to be purified usually passes through the wall of the filter element from the outside in a substantially radial direction, and exits the filter element through the one axial opening. A plurality of such filter candles can be arranged parallel to each other to form a filtering assembly. Filter candles of this type can also be operated as blowback filters, wherein the filter elements are cleaned by a high-pressure gas stream flowing in the opposite direction of the normal filtering process.

The filter candles described above, which are typically made of a ceramic material, are rigid and self-supported. A major drawback, however, is their limited mechanical strength, in particular their limited tensile and bending strength. Consequently, there is a risk of failure of a filter candle if it is subjected to a mechanical stress, which might be the case, for example, when dust bridging between filter candles occurs. Document <CIT> discloses a kit of parts for a ceramic filter tube, the kit comprising two or more shorter porous ceramic tubes, characterised in that the shorter tubes have ends designed to mate with the end of an adjacent shorter tube when assembled to form the desired filter tube and a tie bar to pass longitudinally through the assembled tube , the tie bar being connectable at one end to a support plate into which one open end of the filter tube can be attached and at its other end to a closure plate to seal the other end of the filter tube whereby the shorter tubes are held together in compression.

It is an object of the present invention to provide a filter candle with an improved mechanical strength.

This problem is solved by a filter candle according to claim <NUM>.

In the inventive filter candle of the present invention, a support element is provided within the one or more cylindrical filter elements. This support element comprises a metal tube which has by itself a considerably higher tensile and bending strength than the porous material of the filter element. The gaseous fluid, after passing through the filter element, enters the metal tube through the plurality of perforations, and exits the filter candle through the opening at one end of the tube.

In order to provide for an unhindered flow through the complete inner surface area of the one or more filter elements, there is an annular gap between said inner surface and the outer surface of the metal tube. If a force is exerted onto the filter element in a radial direction, the filter element will move and/or bend until it abuts onto the metal tube. In this way, the metal tube supports and stabilizes the filter element and improves the mechanical strength of the filter candle as a whole, in particular its bending strength.

According to the invention, the one or more filter elements are compressed between the first and second terminal sealing disks, taking advantage of the relatively high compression strength of the porous material of the filter elements. However, the compression of the filter element(s) is such that it allows for a radial movement between a filter element and an adjacent terminal sealing disk, until the filter element comes into contact with the metal tube as described above. During this movement, the axial end faces of the filter element(s) have to be covered tightly by the sealing discs.

According to a first preferred embodiment of the invention, the filter candle comprises only one filter element. In this case, only two sealing disks are provided at the two axial end faces of this filter element. The filter element preferably has an axial length of about <NUM> to about <NUM>, more preferably from about <NUM> to about <NUM>.

According to a second preferred embodiment of the invention, the filter candle comprises a plurality of filter elements disposed in line. In this case, in addition to the two terminal sealing disks, further internal sealing disks are provided between adjacent filter elements. Such a segmentation of the filter candle provides for further flexibility and resilience against mechanical stress, in particular against a radial force exerted onto the filter candle.

The total axial length of the plurality of filter elements of the second embodiment can be within the same preferred range of the length of the single filter element of the first embodiment. However, a segmentation is particularly preferred for larger lengths, such as from about <NUM> to about <NUM>. In the second embodiment, the filter candle preferably comprises two to eight filter elements, more preferably three to five filter elements. The filter elements can have the same or different lengths.

The one or more filter elements preferably have an outer diameter of about <NUM> to about <NUM>, more preferably from about <NUM> to about <NUM>, and an inner diameter of about <NUM> to about <NUM>, more preferably from about <NUM> to about <NUM>. The wall thickness of the hollow cylinder is typically in a range from about <NUM> to about <NUM>.

The radial distance between the filter element(s) and the metal tube is from about <NUM> to about <NUM>, more preferably from about <NUM> to about <NUM>. The outer diameter of the annular sealing disks is equal to or larger than the outer diameter of the filter element(s), and the inner diameter of the sealing disks is smaller than the inner diameter of the filter element(s), so as to provide for a full contact of the axial end face of the filter element with the sealing disks, even if the filter element abuts on the metal tube.

In present invention each sealing disk comprises an annular metal disk, preferably a steel disk, and a gasket provided on one or both sides of the metal disk in contact with a filter element; i.e., the terminal sealing disks will be provided with gaskets on one side and the internal sealing disks will be provided with gaskets on both sides. In this way, a gastight seal between the filter element(s) and the sealing disks is provided, while at the same time allowing a radial movement of the filter element(s) relative to the sealing disks.

Preferably, the gasket used for the sealing disks of the inventive filter candle comprises graphite, metal fibers, a metal mesh, a polymeric material or a combination thereof. These and further suitable gasket materials are already known from the prior art. The selection of a specific gasket material will also be dependent on the intended use of the inventive filter candle and the respective operating conditions. For example, most polymeric gasket materials are limited with regard to their operation temperature.

The gaskets used for the sealing disks preferably have a thickness of about <NUM> to about <NUM>, preferably from about <NUM> to about <NUM>.

One or more of the sealing disks may comprise a resilient compensating element. In particular, it is preferred if one of the terminal sealing disks comprises a resilient compensating element. In the embodiment of the invention with more than one filter element, it is also preferred if the internal sealing disks comprise a resilient compensating element. Thereby, an axial bending of two adjacent filter elements relative to each other is enabled. The internal sealing disks can comprise such a compensating element in addition to or in place of an annular metal disk.

The one or more filter elements of the inventive filter candle are typically made of a ceramic material, preferably of a material comprising sintered silicon carbide. These ceramic materials, which exhibit a high porosity, are known from the prior art. Furthermore, the filter element(s) can comprise one or more catalytic materials, in particular for removal of nitrogen oxides.

The metal tube acting as the supporting element of the inventive filter candle is preferably a steel tube, more preferably having a wall thickness of about <NUM> to about <NUM>, in particular of about <NUM> to about <NUM>. However, the use of other metals or alloys might also be preferable in certain cases, for example if the gaseous fluid to be filtered requires a higher corrosion resistance or chemical resistance of the support element. This applies accordingly to the annular metal disks of the sealing disks.

Typically, the metal tube will have an axial length larger than the total axial length of the filter element(s), wherein a middle section of the metal tube extends within the filter element(s) and two end sections of the metal tube extend outside of the filter element(s). In this case, the perforations in the wall of the metal tube are distributed uniformly over the middle section, wherein the wall of the two end sections is not perforated.

The number and size of the perforations is preferably selected in such a way that the total area of the perforations is large enough to promote the radial flow of the gaseous fluid into the metal tube, but small enough to maintain a sufficient mechanical stability of the metal tube. With regard to the latter requirement, also the wall thickness of the metal tube has to be taken into account.

According to a preferred embodiment of the invention, a first end section of the metal tube extends outside of the filter element(s) and through the first terminal sealing disk, wherein the first terminal sealing disk is fixed to said first end section, preferably by welding or screwing. It is further preferred if the first end section comprises an axial discharge opening for the gaseous fluid.

In the embodiment described above, a second end section of the metal tube can also extend outside of the filter element(s) and through the second terminal sealing disk, wherein the second terminal sealing disk is axially movable relative to the metal tube. It is further preferred if the second end section is closed at its axial end, thereby allowing a discharge of the gaseous fluid only through the first end section.

By providing a first terminal sealing disk being fixed to the metal tube and a second terminal sealing disk being movable relative to the metal tube, the differences in the thermal extension of the metal tube and the porous material of the filter element(s) are taken into account.

Preferably, the one or more filter elements are compressed between the first and second terminal sealing disks by an axial force exerted onto the second terminal sealing disk through a spring element, preferably a high temperature spring, which abuts against the second terminal sealing disk and a flange element fixed to the second end section of the metal tube.

Since the metal tube is preferably closed at the second end section, as explained above, and the gaseous fluid is discharged only through the first end section, a spring cover can preferably be provided to enclose the second end section including the spring element, thereby protecting the spring element from dust etc. from the outside. The spring cover preferably abuts against the second terminal sealing disk.

A plurality of the filter candles of the present invention can preferably be arranged parallel to each other to form a filtering assembly, in particular for hot gas filtration.

The present invention also relates to the use of the inventive filter candle, or of a filtering assembly comprising a plurality of the inventive filter candles, for hot gas filtration, in particular for purification of industrial flue gases or exhaust gases.

The exemplary embodiments described hereinafter serve to illustrate further details of the invention with reference to the drawings, wherein.

In <FIG>, a first exemplary embodiment of the inventive filter candle <NUM> is shown in a longitudinal cross section. The representation of the filter candle <NUM> is schematic and not necessarily true to scale.

In this first embodiment, the filter candle <NUM> comprises one filter element <NUM> in the shape of a hollow cylinder. The filter element <NUM> can have, as an example, a length of <NUM>, an outer diameter of <NUM> and an inner diameter of <NUM>. The filter element <NUM> is made of a porous material, typically a porous ceramic material. For example, porous filter elements of a sintered silicon carbide material, also known as filter candles, are sold by the applicant under the trademark "Dia-Schumalith".

Inside of the filter element <NUM>, and coaxially extending with the same along a rotational axis <NUM>, is disposed a metal tube <NUM> (typically a steel tube) as a support element. This metal tube <NUM> comprises a middle section <NUM> extending within the filter element <NUM>, as well as first and second end sections <NUM> and <NUM> extending outside of the filter element <NUM>. The wall <NUM> of the metal tube <NUM> has a plurality of perforations <NUM> which are distributed uniformly over the middle section <NUM> of the metal tube <NUM>, whereas the wall <NUM> in the first and second end sections <NUM> and <NUM> is not perforated.

The metal tube <NUM> can have, for example, an outer diameter of about <NUM> and a thickness of <NUM> of its wall <NUM>. In any case, the outer diameter of the metal tube <NUM> is smaller than the inner diameter of the filter element <NUM>, resulting in an annular gap <NUM> between the filter element <NUM> and the metal tube <NUM>. This gap <NUM> can have a width of, for example, about <NUM>.

In a typical application of the filter candle <NUM>, for example, for use in hot gas filtration, a gaseous fluid to be filtered passes through the filter element <NUM> in a substantially radial direction from the outside into the annular gap <NUM>, and through the perforations <NUM> into the metal tube <NUM>. The gaseous fluid then leaves the metal tube <NUM> through an axial discharge opening <NUM> at the first end section <NUM>, whereas the second end section <NUM> is closed at its axial end <NUM>.

The filter candle <NUM> further comprises two annular sealing disks, namely a first terminal sealing disk <NUM> and a second terminal sealing disk <NUM>. The first terminal sealing disk <NUM> surrounds the first end section <NUM> of the metal tube <NUM> and is fixed to the same by welding, as indicated by reference numeral <NUM>, thus providing for a gastight closure of the annular gap <NUM> at the first end section <NUM>. In contrast, the second terminal sealing ring <NUM>, which surrounds the second end section <NUM> of the metal tube <NUM>, is axially movable relative to the metal tube <NUM>, to allow different thermal extensions of the metal tube <NUM> and the filter element <NUM>.

The filter element <NUM> is compressed between the first and second terminal sealing rings <NUM> and <NUM>, wherein the compressive force is exerted by a spring element <NUM>, preferable a high temperature spring. This spring element <NUM> surrounds the second end section <NUM> of the metal tube <NUM> and abuts against the second terminal sealing disk <NUM> and a flange element <NUM> fixed to the second end section <NUM>, for example by means of screw nuts <NUM>.

The second end section <NUM> of the metal tube <NUM> including the spring element <NUM> is covered by a spring cover <NUM> which abuts against the second terminal sealing disk <NUM>. The spring cover <NUM> protects the spring element <NUM> against dust etc. from the outside, and it also withholds the gaseous fluid which may pass through the small annular gap between the metal tube <NUM> and the second terminal sealing disk <NUM>.

Each of the terminal sealing disks <NUM> and <NUM> comprises a metal disk <NUM> (typically a steel disk) and a gasket <NUM>. This gasket is in direct contact with the respective axial end face of the filter element <NUM>, providing for a gastight seal, but at the same time allowing a radial movement of the filter element <NUM>. The first terminal sealing disk <NUM> may additionally comprise a resilient compensating element.

As a result of an external force acting on the filter element <NUM>, the filter element <NUM> will move radially along the gaskets, until it comes into contact with the metal tube <NUM>. At this point, the metal tube <NUM> supports and stabilizes the filter element <NUM>, ideally preventing the filter element <NUM> from bending further and breaking. The metal tube <NUM> thus acts as a support element which increases the overall tensile and bending strength of the filter candle <NUM>.

In order to provide for a gastight seal at the end faces of the filter element <NUM> in every position, the outer diameter of the annular sealing disks <NUM> and <NUM> is larger than or of the same diameter as the outer diameter of the filter element <NUM>, and the inner diameter of the sealing disks <NUM> and <NUM> is smaller than the inner diameter of the filter element <NUM>. In particular, the difference between the outer and inner diameters of the sealing disks <NUM> and <NUM> should be at least twice as large as the radial distance between the filter element <NUM> and the metal tube <NUM>.

In <FIG>, a second exemplary embodiment of the inventive filter candle <NUM> is shown in a longitudinal cross section. The filter candle <NUM> of the second embodiment corresponds to the filter candle <NUM> of the first embodiment except for the differences described below. Identical or corresponding elements in the first and second embodiments are provided with the same reference numerals.

The filter candle <NUM> of the second embodiment comprises, instead of a single filter element, a plurality of shorter filter elements <NUM> of a porous material. In this example, four filter elements <NUM> are shown, but a smaller or larger number of filter elements is also possible. The individual filter elements <NUM> are shaped as hollow cylinders with identical inner and outer diameters, being disposed coaxially in line along the rotational axis <NUM> of the filter candle <NUM>.

The length of the individual filter elements <NUM> can be the same or different, and their total length can be the same as that of the single filter element of the first embodiment (for example, <NUM>). However, the use of a plurality of filter elements <NUM> is also particularly advantageous for larger total lengths, such as total lengths of up to <NUM>.

Between each two adjacent filter elements <NUM>, an internal annular sealing disk <NUM> is disposed. The filter elements <NUM>, together with the internal sealing disks <NUM>, are compressed between the first and second terminal sealing disks <NUM> and <NUM>, as in the filter candle <NUM> of the first embodiment.

The segmentation into a plurality of filter elements <NUM> provides for a further flexibility and resilience of the filter candle <NUM> against mechanical stress, in particular against a radial force exerted onto the filter candle <NUM>. To that effect, the internal sealing disks <NUM> typically comprise a resilient compensating element <NUM> which allows an axial bending of the two adjacent filter elements <NUM> relative to each other. The resilient compensating element <NUM> is provided with a gasket <NUM> on both sides.

The bending strength of an inventive filter candle in accordance with the first exemplary embodiment was determined in a <NUM>-point bending test. The filter element of the tested filter candle was a hollow cylinder of a ceramic material based on sintered silicon carbide (Dia-Schumalith) with a length of <NUM>, an outer diameter of <NUM> and an inner diameter of <NUM>.

In the <NUM>-point bending test, the filter element of the inventive filter candle cracked at a force of about <NUM>,<NUM> N.

The corresponding filter element alone, without a supporting metal tube, which is conventionally used as a filtered candle, typically cracks at a bending force in the range from <NUM>,<NUM> to <NUM>,<NUM> N.

Claim 1:
A filter candle (<NUM>; <NUM>) for gaseous fluids, in particular for hot gas filtration, comprising
- one or more filter elements (<NUM>; <NUM>) in the shape of hollow cylinders made of a porous material, wherein the filter elements (<NUM>; <NUM>) have identical inner and outer diameters and are disposed coaxially in line with each other;
- a support element (<NUM>) comprising a metal tube which is disposed within the one or more filter elements (<NUM>; <NUM>), wherein said metal tube (<NUM>) has an outer diameter which is smaller than the inner diameter of the filter element(s) (<NUM>; <NUM>), wherein said metal tube (<NUM>) has a wall (<NUM>) with a plurality of perforations (<NUM>), wherein the metal tube (<NUM>) has an axial length larger than the total axial length of the filter element(s) (<NUM>; <NUM>), and wherein the perforations (<NUM>) are distributed uniformly over a middle section (<NUM>) of the metal tube (<NUM>) which extends within the filter element(s) (<NUM>; <NUM>); and
- at least two annular sealing disks (<NUM>, <NUM>; <NUM>) having an outer diameter equal to or larger than that of the filter element(s) (<NUM>; <NUM>) and an inner diameter smaller than that of the filter element(s) (<NUM>; <NUM>), wherein first and second terminal sealing disks (<NUM>, <NUM>) are disposed at opposite axial end faces of a single filter element (<NUM>) or of a plurality of filter elements (<NUM>) disposed in line, and wherein optionally further internal sealing disks (<NUM>) are disposed between two adjacent filter elements (<NUM>) of a plurality of filter elements (<NUM>),
wherein the one or more filter elements (<NUM>; <NUM>), and optionally the internal sealing disks (<NUM>), are compressed between the first and second terminal sealing disks (<NUM>, <NUM>),
characterized in that the radial distance between the filter element(s) (<NUM>; <NUM>) and the metal tube is from <NUM> to <NUM>, and in that each sealing disk (<NUM>, <NUM>; <NUM>) comprises an annular metal disk (<NUM>), preferably a steel disk, and a gasket (<NUM>) provided on one or both sides of the metal disk (<NUM>) in contact with a filter element (<NUM>; <NUM>), so as to provide a gastight seal between the filter element(s) (<NUM>; <NUM>) and the sealing disks (<NUM>, <NUM>; <NUM>), and to allow a radial movement of the filter element(s) (<NUM>; <NUM>) relative to the sealing disk (<NUM>, <NUM>; <NUM>).