Short title: METHOD AND DEVICE FOR FILTERING A FLUID

The invention relates to a method and a device for filtering a fluid.

Many different filters for filtering a fluid, such as a gas or liquid, are known. Generally, such a filter comprises a filter element which may or may not be replaceable. The filter element can in this case contribute to the filtering of the fluid by means of one or more suitable filtration processes. The term filtration is understood to mean any suitable filtration process, including, but without being limited to, for example, deep bed filtration, cake filtration, membrane filtration, surface filtration, adsorption, absorption, microfiltration, ultrafiltration, nanofiltration, reverse osmosis, chromatography, dialysis, demisters, drying of gases and liquids, ion exchange, osmosis in the presence of relatively large molecules, precoat filtration, filtration on the basis of Brownian diffusion, gravity effects, hydrodynamic effects, electrostatic effects, inert collision effects, and interception.

It is known to use a replaceable filter element, so that as little waste as possible is produced upon replacement, since, for example, elements such as a housing and the like may be reused, and in this case as little waste as possible is produced.

It is an object of the invention to provide an improved method and device for filtering a fluid.

The inventors have come to the realization that the moment when the filter element is replaced is not always determined by a time period up to a moment when the filter element is completely saturated, filled with the substance to be filtered and the like. The inventors have realized that, during use of the filter element, the substance to be filtered occurs as material collected in the filter especially in a part of the filter element viewed in a throughflow direction thereof. Depending on the filter technology which is used, a part of the filtered substance to be filtered may have collected on an inflow side of the filter element. It is also possible that a concentration of the material to be filtered out occurs in particular at a certain depth, for example a depth viewed in the throughflow direction which is situated closer to an inflow side of the filter element than to an outflow side of the filter element. The inventors have also realized that, at the point in time when the filter element is replaced, in fact only certain regions thereof (viewed in a throughflow direction of the filter element) shows signs of saturation, pressure drop and the like, while other parts of the filter element viewed in a

throughflow direction thereof, are not consumed, saturated or filled to such an extent.

Based on this understanding, the inventors have realized that it is possible to achieve advantageous filtration by means of a method for filtering a fluid comprising: passing the fluid through a first filter element and a second filter element, in which the first filter element is situated upstream with respect to the second filter element during a first time period and the second filter element is situated upstream with respect to the first filter element during a second time period.

By means of the method, it is for example possible to achieve a more uniform distribution across the filter elements of the material which is to be filtered out by the filter elements, viewed in a throughflow direction thereof, as will be explained in more detail with reference to the figures. It is also possible to improve the use the absorption characteristics of the filter elements. Other advantages may be the possibility of optimizing the design of the filter and/or optimizing operating aspects, such as pressure loss, filter capacity, filter efficiency, filter service life, residual filtration and/or environmental impact.

In this document, the term fluid is understood to comprise a gas, a gas mixture, a liquid, a vapour, a liquid mixture or any combination thereof. The fluid may, for example, comprise an oil, a fuel, a paint, a polymer or a consumable. Filtration can be carried out in order to clean the fluid to a certain degree of a substance which is to be filtered out by the filter element, such as for example wear particles, ingression particles, gel-like substances, ageing residues or pollution, including for example metal, sand, corrosion, plastic, textiles, glass, wood or fibres. It is also possible to carry out filtration in order to recover a desirable substance from a fluid, possible examples thereof being: recovering silver, ore, coal, starch, yeast.

The term filter element is understood to mean any kind of filter element, including for example homogeneous filter media such as woven filter media, heterogeneous filter media such as natural and synthetic fibres, or combinations of homogeneous and heterogeneous filter media. The filter elements may have any desired shape, such as cylindrical, star- shaped, plate-shaped, segment-shaped or rolled.

The terms upstream and downstream are respectively understood to mean a location in a fluid stream, with a fluid stream running from the location situated upstream to the location situated downstream. It should be clear that the upstream or downstream location of the first/second filter element during the various time periods can be achieved in many different

ways, for example by moving the filter elements (for example replacing them), passing the fluid through the filter elements in another way or any combination thereof.

Generally, the flow through the filter elements during the time periods will be in the same direction, in other words, an inflow side thereof will be the same during the time periods. This makes it possible to prevent material which has attached, for example on an inflow side of the filter element, from being released by the filter element in question during a subsequent time period when the flow is in an opposite direction and end up in a fluid stream after all. As will be explained in more detail below, it is, however, also possible for one or more of the filter elements to be flowed through in the reverse direction during a subsequent time period, in order thereby to clean the filter element in question. The duration of the first and/or second time period may be predetermined and may, for example, be a fraction of an expected service life of the filter, but it is also possible for measurements to be taken to monitor the state of the filter element and to determine the duration of the time period from the measurement data obtained. Thus, it is for example possible to measure a pressure difference across the first and/or the second filter element, when the pressure drop across the filter element is increasing, a certain degree of pollution or saturation of the relevant filter element will occur. In a simple embodiment, a sensor is used for measuring a pressure difference across the assembly of the filter elements, as such a pressure difference is already being measured in the prior art for many applications, so that the moment for terminating a time period can be derived from an existing measurement signal.

Instead of or in addition thereto, it is also possible to measure a content of a substance to be filtered out by the filter element in the fluid in order to determine therefrom a final point in time, remaining time or the like for one of the time periods. The content may be measured in fluid filtered by the filter elements, so that a residual content after filtration is in fact measured. It is also possible to measure such a content in fluid flowing towards the filter elements, since it is, after all, possible to derive a measure for the total pollution presented to the filter element or other substance to be filtered out from such a measurement in combination with a flow rate or another indication of an amount of fluid filtered through the filter. With a known performance of the filter element or the filter elements in question, it is possible to determine a desired length of one or more of the time periods. The content could also be measured in fluid which is present between the first and second filter element, in order to thereby be able to determine a measure of filtration by the first filter element. Of course, the content could also be measured in different locations, for example both upstream and downstream of one or more of the filter elements, by means of which a difference can be determined and thus an effectiveness of the relevant filter element or the filter elements

in question can be determined in order to be able to determine therefrom the end of or a remaining time for a time period.

During a service life of the filter elements, these can be used in each case once for one first time period and once for one second time period, respectively, so that for example when two filter elements are used, each of these is used once for one time period during which the filter element is placed upstream with respect to the other filter element and once for one time period during which the respective filter element is placed downstream with respect to the other filter element. By changing the filter elements in question more often during their service life, it is possible to achieve a more uniform load thereon.

Although a description has been given above with reference to two filter elements, it is also possible to use more filter elements, for example three, four or more filter elements, which gives more options with regard to changing a stream or placing the filter elements during successive time periods.

The filter elements may be identical to one another. However, it is also possible to use filter elements which differ from one another. The filter elements may differ from one another in, for example, material, fineness, structure, thickness or other geometric properties, and/or properties which affect the permeability. By using filter elements which differ from one another, it is possible, for example, to achieve a higher capacity, to reduce a pressure drop and/or to improve the recovery of raw materials.

As mentioned above, one of the filter elements can be used during a subsequent time period in an opposite throughflow direction (in other words: with an inflow side which is opposite) in order thereby to achieve, for example, cleaning thereof. In that case, another one of the filter elements may be situated downstream with respect to the filter element which is flowed through in the opposite direction during a later one of these time periods in order to thus collect a substance which may emerge from the respective filter element (for example pollution in the other filter element). Thus, it is for example possible to collect pollution in the other filter element in order to regenerate the filter element which is flowed through in the opposite direction. During maintenance, for example, only one of the filter elements has to be replaced.

In a preferred embodiment, a valve is provided for passing the fluid to at least one of the filter elements, in which a position of the valve is changed between an end of the first time period and a start of the second time period. In this way, it is possible to change the flow of

the fluid in a simple and reliable way. Of course, many embodiments in which use is made of one or more valves are conceivable. The valve may comprise any form of valve, slide or the like.

As an alternative or in addition thereto, one of the filter elements can be provided so as to be displaceable, with the displaceable filter element being displaced between an end of the first time period and a start of the second time period. This makes it possible to achieve a displacement of a filter element in a simple and reliable manner, and thus bring about a change in the order that the filter elements are flowed through. Many embodiments are conceivable. Thus, it is possible, for example, for one or more of the filter elements to be displaceable and/or for the displaceable filter element to be rotatable about an axis.

The invention furthermore comprises a device for filtering a fluid comprising a first filter element for filtering the fluid, and a second filter element for filtering the fluid, the device being adapted to: guide the fluid through the first filter element and the second filter element, with the first filter element being situated upstream with respect to the second filter element during a first time period and the second filter element being situated upstream with respect to the first filter element during a second time period.

By means of the device according to the invention, the same objects, advantages and effects can be achieved as with the method, and identical or similar embodiments are possible.

Further advantages and features of the invention will be described with reference to the attached drawing, in which a non-limiting exemplary embodiment is illustrated, in which:

Fig. 1 shows a graphic representation of particles dispersed across a thickness of a filter medium;

Figs. 2A and 2B show a diagrammatic exemplary embodiment of a filter and a method according to the invention;

Figs. 3A-3D show graphic representations of the particles dispersed in the filter elements shown in Fig. 2;

Fig. 4 shows an exemplary embodiment of a filter and a method according to the invention in which three filter elements are used; Figs. 5A and 5B show a highly diagrammatic representation of a possible embodiment of the filter or the method, respectively, as described with reference to Fig. 4;

Figs. 6A and 6B show a highly diagrammatic representation of a further embodiment of a

filter according to the invention;

Figs. 7A and 7B show a highly diagrammatic representation of another embodiment of a filter according to the invention;

Figs. 8A and 8B show a highly diagrammatic representation of yet other embodiments of a filter according to the invention;

Figs. 9A - 9D show a highly diagrammatic representation of yet further embodiments of a filter according to the invention; and

Fig. 1OA and 1OB each show a highly diagrammatic representation of yet another embodiment of a filter according to the invention; and Figs. 11A and 11 B each show a highly diagrammatic representation of yet a further embodiment of a filter according to the invention.

In the figures, identical or similar parts are denoted by the same reference numerals.

Fig. 1 shows a graphic representation of a distribution of particles collected by a filter element (for example molecules, crystals) across a thickness of the filter medium. On the horizontal axis, a penetration depth of the filter element is plotted. In this case, the penetration depth 0 is to be understood as an inflow surface of the filter element. As is shown in Fig. 1 , generally a distribution of particles will occur resulting in a non-uniform distribution profile. A concentration is plotted on the vertical axis in the graphic representation of Fig. 1. As can be seen in Fig. 1, the highest concentration is reached around a depth dm, with the concentration sinking both in the direction of the inflow surface and in the direction of the outflow surface. It can also be seen in Fig. 1 that a concentration of particles occurs at the inflow surface.

As can be seen in Fig. 1 , a saturation or other phenomenon which will lead to a change in the performance of the filter element will generally occur first around the thickness dm, as the highest concentration of dispersed particles occurs in such a region. If at any time the decision is taken to replace the filter element, then this effectively means that the filter element is only actually consumed around the depth dm, while this still does not have to be the case in other regions of the filter element. The distribution illustrated in Fig. 1 should only be seen as an illustrative example. However, it is fair to say that the general experience is that more dispersed particles are collected on an inflow side of the filter element than on an outflow side thereof.

Fig. 2A and Fig. 2B highly diagrammatically show an exemplary embodiment of a filter according to an aspect of the invention and by means of Fig. 2A and Fig. 2B an embodiment

of the method according to an aspect of the invention will also be illustrated. Fig. 2A highly diagrammatically shows a filter F which is provided with a first filter element FE1 and a second filter element FE2. An inflow of fluid into the filter takes place at I ₁ while an outflow of fluid takes place at O. In Fig. 2A ₁ a flow through the filter element is such that the inflowing fluid flows first from A to B through the first filter element FE1 and subsequently from C to D through the second filter element FE2. The situation shown in Fig. 2A can be maintained during a first time period. In a second time period, which does not overlap the first time period, the filter element can then be flowed through in the manner shown in Fig. 2B. Fig. 2B again shows the filter F with the filter element FE1 and the second filter element FE2. In contrast to the flow in Fig. 2A, the fluid flowing in at I first flows from C to D through the second filter element FE2 and then from A to B through the first filter element FE1 , after which it flows out at O. All this means that in the configuration shown in Fig. 2A, the first filter element forms a filter element which is situated upstream with respect to the second filter element, and that in the situation shown in Fig. 2B, the second filter element FE2 forms a filter element which is situated upstream with respect to the first filter element FE1. In the situation illustrated in Fig. 2A, a larger proportion of the particles to be collected by the filter elements will therefore be collected by the first filter element FE1 , while in the situation illustrated in Fig. 2B, a larger part is collected by the second filter element FE2 than by the first filter element FE1 , as a result of which the load of the filter elements FE1 and FE2 is more uniformly distributed, which may, for example, result in a longer interval before a point in time where one or more of the filter elements FE1 , FE2 have to be replaced.

It should be noted that the configuration shown in Figs. 2A and 2B is not intended to be limiting in the sense that a location where the fluid flows in and/or the location where the fluid flows out of the filter differs between the situations shown in Fig. 2B and Fig. 2A. The configurations illustrated here only show a course of the flow in a highly diagrammatic way. Thus, it is for example possible for the change from the situation shown in Fig. 2A to the situation shown in Fig. 2B to be effected by interchanging the filter elements FE1 , FE2. It is, for example, also possible to achieve this change by changing a flow of the fluid, for example by means of valve means suitable for this purpose, or by means of a different type of guide. Of course, many alternatives are conceivable to this end. Compared to a conventional filter which only uses a single filter element, it is possible to limit the thickness of both filter elements FE1 , FE2 to, for example, half the thickness of a prior-art filter element. If ₁ with the prior-art single filter element, a high concentration of dispersed particles occurs in particular on a side which faces the inflow side, a significant extension of the service life of the filter element could be achieved, for example by a factor which is in the order of magnitude of 2.

Figs. 3A-3D show an example of a dispersion of particles such as could for example occur with the configuration shown in Figs. 2A and 2B. In this case, Fig. 3A and Fig. 3B, respectively, show a distribution in the first filter element FE1 and the second filter element FE2 in the configuration shown in Fig. 2A, and Fig. 3C and Fig. 3D show a similar representation for the situation shown in Fig. 2B for the first filter element FE1 and the second filter element FE2, respectively. In the representation shown here, it is assumed that the filter is first operated in the mode shown in Fig. 2A and then in the mode shown in Fig. 2B. Analogously to Fig. 1 , each of Figs. 3A-3D shows a thickness direction of the filter element plotted along a horizontal axis, in which 0 is to be understood as an inflow side of the respective filter element, and a concentration of dispersed particles is plotted in the vertical direction. A concentration to the left of the inflow side indicated by 0 is to be understood as a dispersion of particles against or on an inflow side of the respective filter element. As is shown in Figs. 3A and 3B, a dispersion of particles will in particular occur in the configuration shown in Fig. 2A in and against the first filter element FE1. The load on the second filter element FE2 is in this case, as is shown in Fig. 3B, very small. In the situation shown in Fig. 2B, the first filter element is only loaded to a small degree in the situation shown in Fig. 2B, while the second filter element in this situation forms the filter element which is situated upstream and is in this case (analogously to the first filter element FE1 in the situation shown in Fig. 2A) loaded in a similar manner. AH this results, in the exemplary embodiment illustrated here, in the situation shown in Fig. 2B to an increase in the dispersion of particles in particular in the second filter element FE2 which forms the upstream filter element in the situation shown in Fig. 2B. It can also be seen that in this exemplary embodiment (as on the left-hand side of Fig. 3D which shows the second filter element FE2 in the situation shown in Fig. 2B) that there is also an increase in the concentration of particles on the inflow surface. When comparing the situation in Fig. 3C and Fig. 3D to that in Fig. 1, it appears that alternately using the first and the second filter element as upstream and downstream filter element, respectively, has resulted in a more homogeneous distribution of particles across a total thickness of the total filter element (i.e. a sum of the thickness of the first filter element FE1 and the second filter element FE2), as a result of which an overall improvement in the filtration performance can be achieved, which can be evidenced by for example a longer service interval, a lower pressure loss, a reduction in waste to be processed, or other parameters in the filtration process.

Fig. 4 shows an exemplary embodiment of the method and device according to the invention which uses three filter elements, denoted here by 1 , 2 and 3. At the top of Fig. 4, a first situation is shown, in which, for example during a first time period, the fluid flows into the filter F as denoted by I, flows through, successively the first, second and third filter element

(denoted here by 1 , 2, and 3, respectively) and leaves the filter at O. For illustrative purposes, it is assumed that particles disperse in the fluid on/in the first, second and third filter element at a ratio of 4 : 2 : 1. Diagrammatically, it has been shown at the top that the load of the three filter elements will go from O, O ₁ 0 to 4, 2, 1 during the first time period. In the filters shown in the centre of Fig. 4, two exemplary embodiments are shown of a configuration which can be used in a subsequent (second) time period. In the left-hand configuration, the fluid first flows through the second filter element and then through the third and finally through the first filter element. In the configuration shown on the right-hand side, the fluid first flows through the second filter element, then through the first and finally through the third filter element. During the first time period, an accumulation of particles has occurred in the first, second and third filter elements which is indicated by 4, 2, 1. On the basis thereof, concentrations which are to be built up are shown symbolically in the two central embodiments. During the first time period, a distribution of 4, 2, 1 has occurred. This leads to an initial situation of 2, 1 , 4 in the left-hand example when the order in upstream/downstream is changed, with a further increase occurring to 6, 3, 5, during the second time period in which the filter is operated in the illustrated configuration. The filter element which is furthest upstream, in this case the second filter element, had a load of 2 during the first time period (this was, after all, the second filter element). On the basis of 2, 1 , 4 and adding a distribution thereto of 4, 2, 1 thus results in 2 + 4, 1 + 2, and 4 + 1 , in other words 6, 3, 5. It should be noted that this is obviously a highly symbolic approximation which may be different from a practical situation and serves only to illustrate a possible effect of the device and method.

Analogously to the left-hand exemplary embodiment during the second time period, in the right-hand exemplary embodiment, starting from a situation with a distribution 2, 4, 1 at the start of the second time period, and adding an amount of particles in a ratio of 4, 2, 1 during the second time period, will eventually result in a load of 6, 6, 2 at the end of the second time period.

The two bottom exemplary embodiments in Fig. 4 show examples of a third time period, in which the left-hand embodiment shows the fluid flowing through the filter from the third filter element to the first filter element and then to the second filter element, and the right-hand embodiment shows a flow from the third filter element to the second filter element to the first filter element. Starting from the situation 6, 3, 5, as is shown in the left-hand embodiment at the second time period, the left-hand embodiment will, in a manner analogous to that described above, result in a final load of 7, 7, 7 at the end of the third time period. In the right-hand example, starting from the abovementioned 6, 6, 2, after changing the

throughflow will result in 2, 6, 6 at the start of the third time period, which at a load during the third time period of 4, 2, 1 results in a distribution of 6, 8, 7 at the end of the third time period. Of course, it will be clear that many other configurations are possible by means of which identical or similar results can be achieved.

In general, it can be stated that using a larger number of filter elements with the filter and the method according to the invention could result in a more uniform distribution across the filter elements of the particles to be collected by the filter.

Figs. 5A and 5B show a further embodiment of the invention. Fig. 5A shows a filter with three filter elements FE1 , FE2 and FE3, in which, for example during a first time period, the fluid flows into the filter at I and successively flows through the first, second and the third filter element, after which the fluid leaves the filter at O. Fig. 5B shows a configuration during a second time period, in which the inflowing fluid from I first flows through the second filter element FE2, then through the first filter element FE1 and finally through the third filter element FE3, after which the fluid leaves the filter F at O. In the situation shown in Fig. 5B (for example the second time period), the filter element FE1 is flowed through in the opposite direction compared to the situation shown in Fig. 5A (for example the first time period). Whenever an opposite throughflow direction is mentioned in this document, this is to be understood as a direction of flow through the filter element such that during the second time period the inflow side is the opposite face with respect to the inflow side during the first time period. Using the configuration shown in Fig. 5B, it is for example possible to release particles which have collected in the first filter element FE1 during the first time period shown in Fig. 5A, from the first filter element FE1 during the second time period and for particles to be collected by the third filter element FE3. This makes it possible, for example, to regenerate the first filter element FE1. Thus, for example, the third filter element FE3 could be replaced by another filter element at the end of the second time period.

Figs. 6A and 6B show in a highly diagrammatic manner an exemplary embodiment of a filter according to the invention. The figures show the filter elements FE1 and FE2 and three valves V1 , V2 and V3. In a first state, shown in Fig. 6A, the inflowing fluid, denoted by I, is passed to the first filter element FE1 via valve V1 , then passed to the second filter element FE2 via valve V2 and then passed to the outside via valve V3. The fluid therefore flows first through the filter element FE1 and then through the filter element FE2. In Fig. 6B, the valves V1 , V2 and V3 are each placed in a different position. By means of the pipelines, ducts or the like which are shown highly diagrammatically in Figs. 6A and 6B, the inflowing liquid (denoted by I) is passed first to the second filter element FE2 via valves V1 and V2, and then

to the first filter element FE1 via valves V3 and V1 , and finally to the outlet O via valves V2 and V3. As is illustrated diagrammatically in Figs. 6A and 6B, each of the valves may comprise a rotatable part which is denoted by VR in Figs. 6A and 6B, it being possible for the respective valve to pass from the position shown in Fig. 6A to the position shown in Fig. 6B by rotation of the rotatable part through for example 90°. Flow directions of the fluid are indicated by arrows.

Figs. 7A and 7B show another exemplary embodiment which also uses valves. However, in contrast to the valves shown in Figs. 6A and 6B, here valves are used, as is diagrammatically indicated in Fig. 7A and 7B by A - G ₁ which can each be in a closed or open position. In the position illustrated in Fig. 7A, the valves A, B, E and F are closed, while the valves C, D and G are open. The fluid will therefore flow from I via valve C to and through the first filter element FE1 and subsequently via valve D to and through the second filter element FE2. Thereafter, the fluid flows via valve G to the outlet of the filter denoted by O. In Fig. 7B, valves B, C, D and G are closed, while valves A, E and F are open. The fluid will therefore flow to the second filter element FE2 via valve A and then from the second filter element FE2 via valve E to the first filter element FE1 , while the fluid, after having flowed through the first filter element FE1 , is passed to outlet O via valve F. It should be noted that in Figs. 7A and 7B the filter elements FE1 , FE2 are illustrated highly diagrammatically by means of a dashed line.

Figs. 8A and 8B each show a filter according to yet another embodiment of the invention. Fig. 8A shows a tank T for holding a fluid, in this example a liquid such as oil, and in this exemplary embodiment a filter comprising three filter elements FE1 , FE2 and FE3. Inflowing fluid successively flows through filter elements FE1 , FE2 and FE3 and is subsequently collected in tank T. A bypass valve BV is provided in order to serve as a bypass in case the pressure difference across the filter is excessive, so that a fluid stream (for example a hydraulic oil or lubrication) can be ensured, even in case the filter becomes blocked, for example. Strainer STR forms an auxiliary filter for filtering out any coarse particles. Over the course of time, the filter elements can be replaced, for example manually, by moving the respective filter element to the disassembly space SP and thereby remove it from the filter. Subsequently, it is possible to replace one or more of the filter elements FE1, FE2, FE3, or they can be switched around. Removing and replacing or swapping the filter elements can be carried out manually or using any suitable drive, such as a motorized drive. A maximum level of the fluid in the tank T is denoted by LMAX.

Fig. 8B shows a variant of the embodiment shown in Fig. 8A, in which the filter elements

FE1 , FE2 and FE3 are installed in the vertical direction and in which, in this exemplary embodiment, a disassembly space SP is provided at the top. Of course, both embodiments shown in Fig. 8A and Fig. 8B can be provided with or without an integrated tank T for holding fluid. Incidentally, indicators IND are shown in Fig. 8A and 8B in order, for example, to indicate an excessive pressure drop or initial pressure drop.

Figs. 9A - C show an operation of a filter according to yet another embodiment of the invention. Fig. 9A shows a first position during the first time period in which the fluid flows from I first through the first filter element FE1 and then through the second filter element FE2. The filter elements FE1 and FE2 are both arranged so as to be movable, in this example rotatable, with respect to an axis of rotation AX. Fig. 9B then shows how the filter elements rotate with respect to the axis of rotation AX. In the position shown in Fig. 9B, the second filter element FE2 is rotated through approximately 180° in, for example, a counterclockwise direction, which is followed by a further rotation of the second filter element FE2 to the position shown in Fig. 9C. In this case, the first filter element FE1 is subjected to a relatively small rotation, as the first filter element FE1 is in fact rotated from the position shown in Fig. 9A to the position of the second filter element FE2 as shown in Fig. 9A. The position shown in Fig. 9C is therefore the result of the filter elements FE1 and FE2 having been swapped in the throughflow direction, that is to say from I to O, in other words the fluid first flows through the second filter element FE2 and then through the first filter element FE1. Obviously, it will be clear that the mode of operation illustrated by means of Figs. 9A - 9C can be implemented in many ways, and only a highly diagrammatic principle representation is shown here. The principle shown by means of Figs. 9A - 9C can of course also be used for filters which are provided with more filter elements, which is explained by means of Fig. 9D. Fig. 9D successively shows three states in which a filter with three filter elements FE1 , FE2 and FE3, each of which is rotatable about axis AX, is in each case taken to a next position by rotating the bottommost of the filter elements about the axis AX in a counter-clockwise direction, with the other two filter elements in each case moving of one position in a counter-clockwise direction. In the top figure, the order of the filter elements is therefore FE1 , FE2 and FE3. When the third filter element FE2 is then rotated to the position of original filter element FE1 , this results in the representation shown in the central figure, in which the first and second filter element FE1 and FE2 have been rotated one position further along in a counter-clockwise direction. Now, the fluid also flows successively through the filter elements FE3, FE1 and FE2. If this same procedure is repeated once more, the position shown in the bottom figure occurs, in which the fluid successively flows through the filter elements FE2, FE3 and FE1.

Figs. 1OA and 10B each show yet another embodiment of a filter according to the invention, in which a rotatable assembly of filter elements is used. It should be noted that Figs. 1OA and 1OB show a highly diagrammatical cross section. As can be seen in Fig. 10A, the filter is fitted in a housing HS which is substantially cylindrical around an axis AX. The filter is provided with a hollow, cylindrical filter element FE1 which is, in this exemplary embodiment, fitted rotatably with respect to the axis AX ₁ as will be described in more detail below. In this exemplary embodiment, the hollow cylindrical filter element is composed of two parts, namely filter element FE1 and FE2, each of which extends across, for example, substantially 180° of a periphery. The filter is furthermore provided with a core, which is arranged substantially concentrically to the filter element. In this exemplary embodiment, the core comprises the partition wall SEP and the perforated or otherwise permeable elements PER which may also be cylindrical and concentric with the filter element. The elements PER may, for example, serve as mechanical support for the filter elements FE1 , FE2. An inflow opening is provided on an outer edge of the housing HS, then flows from the outside of the first filter element FE1 through the first filter element and via the perforated element PER to an inside thereof. The partition wall SEP then ensures that the fluid cannot flow directly from this space to the outlet which is provided centrally with respect to the axis AX at the top of the housing HS, but instead flows to the outside of the second filter element FE2 in order to then flow through the second filter element FE2 and the respective part of the perforated element PER (for example a perforated cylindrical plate), after which the fluid returns to an inner part of the filter element, but now on the other side of the partition panel SEP. The fluid will then leave the filter via the outflow opening at O. At the top of the filter element FE1 , FE2, an annular seal SE1 is provided in order to prevent the flow of fluid past the filter element. Also, a seal SE2 is provided on another side of the filter element, which, at the position shown in Fig. 1OA, is the first filter element FE1 , which seal SE2 prevents the fluid from flowing past the filter element, at an underside thereof. The second seal SE2 will therefore also be semicircular in shape in this exemplary embodiment, in other words extend over substantially 180°. In this exemplary embodiment, the seal SE1 is fitted coaxially with respect to the axis AX. The second seal SE2 forms a semicircular segment which is concentric with respect to the axis AX. An order in which the filter elements are flowed through can be changed in the exemplary embodiment shown in Fig. 10A by rotation with respect to the axis AX. Thus, it is for example possible to rotate the entire filter element, in other words the assembly of the filter elements FE1 and FE2 which together form a hollow, cylindrical filter element. When a rotation about 180° takes place, this means that the filter elements FE1 and FE2 change places. In other words, the fluid will then first flow through the second filter element FE2, which has, through rotation, taken the place of the first filter element FE1 , and subsequently through the filter element FE1, which has taken the place of

the second filter element FE2 through rotation. Of course, many variants are possible: thus, it is possible, for example, instead of carrying out a rotation of the entire filter element, to perform a rotation of a core of the filter, in other words a rotation of the partition wall SEP, optionally together with the perforated elements PER, about the axis AX. A rotation thereof about, for example, 180° can achieve the same effect as a rotation of the assembly of filter elements FE1 , FE2 about 180°. Of course, instead of a rotation about 180°, it is also possible to rotate the assembly of filter elements FE1 , FE2 several times in small steps which makes it possible to achieve a gradual, stepwise swap.

Fig. 10B shows a variant of the embodiment shown in Fig. 10A, in which the partition wall SEP, instead of being arranged diagonally, has a substantially vertical section which is intersected by the axis AX and runs substantially parallel thereto, and two respective end portions which run from the part which in this representation runs vertically to an end of the respective filter elements FE1 , FE2. Depending on the selected fluid, it is possible to achieve, for example, a slightly lower flow resistance for the fluid by means of the embodiment illustrated in Fig. 10B.

As described above, the duration of the time periods may be predetermined. It is also possible to determine the duration or the end of the respective time periods through carrying out a measurement, for example a pressure measurement or a measurement of a content of a substance to be filtered out by the filter element. For example, in the configuration shown in Fig. 2A, a pressure difference can be measured between A and D, or a pressure difference can be measured across the first filter element FE1 between A and B, or a pressure difference can be measured across the second filter element FE2 between C and D.

Fig. 11A shows a highly diagrammatically cut-away top view of a filter according to an embodiment of the invention. Filter elements FE1 and FE2 together form a hollow, cylindrical filter element which is rotatable about the axis AX which is concentric therewith. A partition wall, in Fig. 11 A denoted by SE1 A and SE1 B, extends from the axis AX to an outer periphery of the assembly of filter elements FE1 , FE2. By means of the partition wall, which extends in a direction perpendicular to the plane shown in Fig. 11A, not only is the space inside the circle segment-shaped filter elements FE1 , FE2 is separated into two parts, but the partition wall also forms a partition between the circle segment-shaped filter elements FE1 , FE2 themselves. A cylindrical housing HS provides cylinder segment-shaped spaces on an outer side of the segment-shaped filter elements FE1 , FE2, and these spaces are separated from one another by the partition walls SE2A and SE2B. As a matter of fact, two parts which are

separated from one another are thus created in the housing HS by a partition formed by the partition walls SE1 A, SE2A, SE1 B, and SE2B. The partition walls SE2A and SE2B are in this case connected to the housing HS. The partition walls SE1A and SE1 B are rotatable about the axis AX. Perhaps unnecessarily, it should be noted that the axis AX extends in a direction perpendicular to the plane of the drawing. 11 and 01 respectively denote an inflow opening and an outflow opening on a right-hand side, while I2 and 02 denote an inflow and outflow opening, respectively, on a left-hand side. A fluid to be filtered can thus also be fed into the filter at 11 in order to then be filtered by the filter element FE1 and to be discharged on the right-hand side via 01. The fluid is then passed to inflow opening I2 via a suitable pipeline in order to flow through the second filter element FE2 and to be discharged via 02 on the left-hand side of the filter. By means of the partition wall, two filter elements are in fact created. At the end of the first time period, an exchange of the filter elements FE1 , FE2 with one another can be achieved by rotating the filter elements FE1 , FE2 about the axis AX. In the exemplary embodiment shown here, the partition wall SE1A and the partition wall SE1B will also co-rotate. In the exemplary embodiment shown here, the partition walls SE2A and SE2B are connected to the housing HS, so that these will not undergo the respective rotation. At a rotation through substantially 180° about the axis AX, the first filter element FE1 and the second filter element FE2 have in fact changed places. Inflowing fluid will then first flow through the second filter element FE2 (which has taken the place of the first filter element FE1) via 11 and leave the right-hand side via 01, then flow into the left-hand side via I2 and through the first filter element FE1 (which has taken the place of the second filter element FE2) and leave the left-hand side via 02. In the exemplary embodiment illustrated here, the partition walls SE1A and SE1B are rotatable while the partition walls SE2A and SE2B are stationary. Of course, it is also possible for the partition walls SE2A and SE2B to be rotatable, as well, for example by connecting them to the walls SE1A and SE1B, resulting in them following the movement of the filter elements when these are rotated about the axis AX. Although in the embodiment illustrated here the filter elements are rotated with respect to the housing, many variants are of course conceivable: Thus, it is for example possible for the housing to be rotatable with respect to the filter elements. As has already been indicated briefly above, it is also possible for the partition walls SE2A and SE2B to be connected to the filter elements FE1 and FE2, and follow the filter elements FE1 and FE2 when these are rotated about the axis AX. An advantage thereof may be that this provides a relatively simple sealing, as a sealing between the partition walls SE2A and SE2B, respectively, and a wall of the housing HS can be achieved relatively easily, particularly if the wall of the housing HS forms a relatively smooth surface. The partition walls SE2A and SE2B may, for example, be rubbers, inflatable membranes, etc.

Fig. 11B shows a variant of the embodiment shown in Fig. 11A, which uses three filter elements FE1 , FE2 and FE3. Similarly to Fig. 11 A, the filter elements are segment-shaped and together form a cylindrical unit which is rotatable is about the concentric axis AX. Analogously to Fig. 11 A, partition walls SE1A, SE1B and SE1C are provided between filter elements. These extend up to the axis AX in order thereby to also separate an inner space in the hollow, cylindrical assembly of the filter elements into three compartments. Analogously to Fig. 11A ₁ separating elements are provided in order to also separate the space around the assembly of filter elements into compartments, namely partition walls SE2A, SE2B and SE2C. These can be connected to the assembly of filter elements and/or to the housing HS. Analogously to Fig. 11A ₁ a respective inflow opening 11, 12 and 13 and a respective outflow opening 01, O2 and 03 are provided for each filter element in a corresponding segment of the housing HS. By means of suitable pipelines or other types of ducts, fluid can be fed in, for example at 11, and leave the illustrated assembly at 01 , then be fed in at I2 and leave the assembly at 02 and finally be fed in at I3 and leave the assembly at 03. In the illustrated position of the assembly of filter elements which is rotatable about axis AX, the fluid will therefore successively flow through the filter elements FE1 , FE2 and FE3. If the assembly of filter elements is now rotated through substantially 120° about the axis AX (for example in a clockwise direction), then the fluid will successively flow through the filter elements FE3, FE1 and FE2. After the filter has been used for a certain time period in this position, the assembly of filter elements can again be rotated through 120° in the same direction, so that the fluid then successively flows through the filter elements FE2, FE3 and FE1. The embodiment shown in Figs. 11 A and 11 B makes it possible to produce a filter according to the invention in a very simple manner, only requiring a small number of moving parts and a small number of seals. In particular if the partition walls SE2A, SE2B (SE2C) are connected to the filter element and thus co-rotate therewith, only a minimum degree of sealing is required between the segments and these sealings can be implemented in a relatively simple manner. Of course, many variants are possible: thus, it is for example possible to use four or more segments instead of the two segments shown in Fig. 11A and the three segments shown in Fig. 11B. It is also possible to change the order of throughflow of the inlets H , I2, I3 in Fig. 11 B (and the associated outlets O1 , O2 , 03) by means of for example suitable valves. By changing, for example, the order H 01 , 12 O2, 13 03 described above by means of suitable valves into 11 01, 13 O3 and 12 O2, it is possible to achieve different successive throughflows of the filter elements: in the position shown in Fig. 11B, the first filter element FE1 is thus flowed through first, then the third filter element FE3 and finally the second filter element FE2. In variants which use more than three segments, many combinations and sequences of throughflow of the segment-shaped filter elements are of course possible.

Obviously, the exemplary embodiments shown in Figs. 6 - 11 are also illustrative for the method according to the invention.

Where this document mentions particles or other types of substances which are to be filtered out by the filter, this should be interpreted broadly. Thus, this can for example be understood to mean filtering an undesirable substance out of the fluid (for example a pollutant), but it is for example also possible that this is understood to mean filtering a desirable substance out of a fluid (for example a recovered filter product or residue).

The time periods mentioned in this document succeed eachother continuous, but it is also possible that they are separated by an interim time period.