FILTERING DEVICE WITH OPEN-ENDED FILTER TUBES

Abstract: A filter in the form of a bundle of open-ended filter tubes housed within and extending along the length of a support tube (7). Seals (3) are arranged at each end of the support tube such that each filter tube extends through them, the seals preventing axial flow of fluid through the ends of the support tube except through the filter tubes. Fluid to be filtered is delivered to the interior of the support tube between the seal. Fluid to be filtered is distributed to the ends of the filter tube bundle adjacent the seals and/or to a distribution tube extending axially through the bundle and opening in the radial direction into inner portions of the bundle. The fluid distribution structure is such that during backwashing an axial flow is established throughout all or a large proportion of the filter bundle.

Description: FILTERING DEVICE WITH OPEN-ENDED FILTER TUBES The present invention relates to a filter. Filters are known which comprise a bundle of open-ended filter tubes housed within and extending axially along the length of a support tube, seals being arranged at each end of the support tube such that the filter tubes extend through the seals. The seals prevent axial flow of fluid through the ends of the support tube except through the filter tube. A fluid to be filtered is delivered to the space defined within the support tube between the seals and through which the filter tubes extend. Filtrate which has passed through the walls of the filter tubes flows to the ends of the support tube through the seals. Retentate is either retained within the support tube, if the system is being operated in "dead-end" mode, or is at least partially flushed from the support tube by maintaining a steady flow of the fluid to be filtered through the support tube. This can be achieved by delivering the fluid to a support tube inlet and ensuring that the flow is sufficient to maintain a substantial flow out of the support tube through an outlet. In both modes of operation, particles build up on the outer surfaces of the filter tubes and these particles must be periodically washed out of the filter if the filter throughput is to be maintained. The maximum throughput of a filter is dictated by the available surface area of the filter tubes. Manufacturing filters from bundles of filter tubes as described above ensures a relatively high surface area in a relatively low volume structure. When a filter is first used, substantially all of the surface area of the filter tubes is available, but as particles build up on the filter tubes the throughput of the filter decreases. In conventional structures, the fluid to be filtered is fed into the space within the support tube around the filter tube bundle through an opening in the support tube wall. The filter tube bundle is not tightly packed within the support tube as to achieve tight packing would be very difficult in terms of manufacture. Simply feeding fluid to be filtered into the space around the filter tube bundle initially achieves good fluid distribution as the fluid can flow axially through the space between the radially outer filter tubes and the surrounding support tube. As particles build up on the filter tubes however there is a tendency for the particles to block the spaces between adjacent filter tubes in the outer portions of the bundle and as a result the throughput falls relatively rapidly as particles build up. In an attempt to overcome the above-described problem it has been proposed to distribute fluid to be filtered throughout the filter bundle by delivering the fluid to an axially extending central fluid distribution tube from which apertured secondary tubes project in the radial direction. In one known arrangement, the secondary tubes are arranged in sets of three which are offset in the circumferential direction to each other. As a result fluid is distributed throughout the filter tube bundle so that all sections of the filter tube bundle are located relatively close to a point to which fluid to be filtered is delivered. Unfortunately, with such an arrangement, over time particles build up throughout the filter tube bundle and it is difficult to wash these particles out by simply pumping filtrate back through the filter. Thus although the known filters with internal fluid distribution structures do not become clogged so quickly when they are first used, this benefit is lost as particles build up within the body of the bundle. Investigations of particle distribution in the known filters with fluid distribution networks have been conducted and these investigations have revealed that it is difficult to wash out deposited particles because "dead spaces" are formed within the filter tube bundle in which flow rates are zero or very low when the filter is backwashed. A particle located in such a dead space, for example midway between two outlets of the tube network which are at substantially the same pressure, is not exposed to a flow of sufficient rate to dislodge it and carry it to one of the outlets. Thus although particles can be washed out of sections of the filter bundle close to outlets of the fluid distribution network, portions of the filter remote from those outlets rapidly become clogged and cannot be properly cleaned. Accordingly the maximum throughput of filter assemblies falls substantially and filters cannot be properly regenerated without relying upon chemical cleaning. It is an object of the present invention to provide an improved filter which obviates or mitigates the problems outlined above. According to the present invention, there is provided a filter comprising a bundle of open-ended filter tubes housed within and extending axially along the length of a support tube, seals arranged at each end of the support tube such that the filter tubes extend therethrough, the seals preventing axial flow of fluid through the ends of the support tube except through the filter tubes, and at least one conduit communicating with the interior of the support tube between the seals to deliver fluid to or remove fluid from the space defined within the support tube around the filter tubes, wherein the said at least one conduit is connected to at least one passageway extending through the bundle of filter tubes and opening into spaced regions of the filter tube bundle, the spaced regions being distributed such that the filter may be backwashed in a manner which establishes a flow within the said space which is predominantly axial. The term "predominantly axial" is used in the present application in the sense that flow at all but limited and generally peripheral regions of the filter tube bundle is in a direction with an axial component towards the nearest adjacent outlet for backwash fluid. Generally the axial flow will be directed towards only one end of the filter support tube. It has been found that particles distributed in the space defined within the bundle outside the individual filter tubes can be efficiently dislodged by establishing a flow through the bundle which is generally in the axial direction. A substantial proportion of this flow may be through an annular space which is generally defined outside the bundle but within the support tube. The required axial flow can be achieved either by distributing fluid flow passageways in an appropriate manner, or by a combination of fluid flow passageway distribution and control of hydrodynamic pressure occurring during the backwashing process. The backwashing process may be implemented only by pumping filtrate radially outwards through the walls of the filter tubes, but may also include pumping flushing fluid through the bundle between the filter tubes. The passageway may open into radially and circumferentially spaced regions which are located only adjacent one or both of the seals. Typically networks of passageways are provided adjacent each seal, fluid to be filtered being pumped in through one of the networks and out of the other, or if the filter is operating in "dead-end" mode, flow through the other network being prevented by closing a valve in series with it. There is thus no direct distribution of fluid to be filtered to the bundle except at one end. It has been found however that fluid to be filtered can travel relatively easily in the axial direction between adjacent filter tubes and therefore particles build up in a generally uniform manner throughout the bundle. When the filter is backwashed a valve in series with the one network is opened, and filtrate is pumped into the open ends of the filter tubes and radially outwards through the filter tube walls. The filter bundle is flushed by the resultant axial flow towards the other network. Alternatively, additional flushing fluid may be pumped into the one network and out of the other network. As a result, the flow of fluid is predominantly axial, and the velocity of flow through substantially all of the bundle is sufficient to displace particles from the filter tubes. The filter can as a result be regenerated in a relatively efficient manner. The passageways which distribute fluid to be filtered to the filter bundle may be defined by a plurality of radially extending tubes the walls of which define openings communicating with spaces between the filter tubes. Some of the radially extending tubes may be supported at their radially inner ends by a support member that extends axially through the filter tube bundle. Such a structure is convenient to provide support for the filter bundle during the insertion of the filter bundle into the support tube. Further radially extending tubes may be located between each pair of the tubes which are supported by the axial support member. The support member itself may support radially extending members distributed along its length to provide further support for the filter tube bundle. In an alternative arrangement, the said at least one passageway may be defined by at least one distribution tube extending axially through a central portion of the filter tube bundle, the distribution tube having a wall defining a plurality of apertures opening radially outwards into axially and circumferentially spaced portions of the bundle, and the distribution tube having a length and internal diameter such that backwashing establishes a substantial pressure gradient along the length of the distribution tube. With such an arrangement, providing a substantial pressure gradient is established along the length of the distribution tube, when the filter structure is backwashed, a predominantly axial flow of fluid is established in the bundle and as a result deposited particles can be efficiently swept from substantially all sections of the structure. Further distribution tubes may be located adjacent at least one end of the filter tube bundle, the further distribution tubes communicating with the axially- extending distribution tube and opening into radially spaced regions of the filter tube bundle. The axially extending distribution tube may support radially extending members distributed along its length to provide support for the filter tube bundle, but it will be appreciated that these radially extending members will not define fluid distribution passageways communicating with the spaces between the filter tubes. The invention also provides a method for backwashing a filter comprising a bundle of open ended filter tubes housed within and extending axially along the length of a support tube, seals arranged at each end of the support tube such that the filter tubes extend therethrough, the seals preventing axial flow of fluid through the ends of the support tube except through the filter tubes, and at least one conduit communicating with the interior of the support tube between the seals to deliver fluid to or remove fluid from the space defined within the support tube around the filter tubes, wherein the filter tube bundle is backwashed by pumping filtrate radially outwards through the walls of the filter tubes and establishing a flow within the said space which is predominantly axial. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic partially cut-away illustration of a known filter structure; Figure 2 is a section through one end of a first embodiment of the invention; Figure 3 is a section on the line 3-3 of Figure 2, showing the position of the section of Figure 2 by line 2-2; Figure 4 is an axial section through a second embodiment of the present invention; Figure 5 is an enlarged sectional view through an axially extending fluid distribution tube of the embodiment of Figure 4; Figure 6 is a partly cut-away side view of a third embodiment of the invention; and Figure 7 is a perspective view of the partly cut-away end of the filter shown in Figure 6. Referring to Figure 1, the illustrated conventional structure comprises an outer support tube 1, a bundle of filter tubes 2 extending axially along the length of the support tube, and seals 3 adjacent each end of the support tube. Each of the filter tubes 2 extends through both of the seals 3. The seals 3, which may be formed from for example epoxy resin, are effective to prevent flow of fluid through the ends of the support tube 1 except through the filter tubes 2. An inlet port 4 delivers fluid to be filtered to the space defined within the tube 1 around the filter tubes 2. An outlet port 5 enables a continuous flow of the fluid to be filtered to be established through the structure, that flow being effective to flush some deposited particles from the system. In a filter structure design which was superseded by that illustrated in Figure 1, the ports 4 and 5 simply opened into the interior of the tube 1. Such an arrangement did not prove satisfactory however as there was a tendency for particles to build up around the outer periphery of the tube bundle, preventing ready access of fluid to be filtered to filter tubes within the bundle. Furthermore, there was a tendency for the fluid flow to damage the filter tubes extending across the openings with which the ports 4 and 5 communicated. To overcome these problems the structure as shown in Figure 1 was produced in which the ports 4 and 5 communicate with each of three radially extending tubes 6 that in turn communicate with an axially extending tube 7. Secondary tubes 8 communicate with the tube 7, the tubes 8 being apertured such that fluid pumped in through inlet port 4 issues from each of the holes defined by the large number of secondary tubes 8. Fluid flow was thus distributed throughout the body of the filter bundle. In normal operation of the structure shown in Figure 1, filtrate was delivered to each of the open ends of the support tube. When it came to backwashing the structure, filtrate was pumped in through the two open ends of the support tube, travelling down the individual filter tubes and lifting particles off the external surfaces of the filter tubes. Unfortunately, it has been found in practice that particles which are located in regions which are not close to secondary tubes 8 cannot be flushed out of the tube bundle as they are in what is effectively a dead space where the fluid flow velocity during backwashing is minimal. This is the case even if an axial flow of fluid is established in the tube 7 given the distribution and number of the tubes 8. The illustrated structure therefore required chemical cleaning at regular intervals. Referring to Figures 2 and 3, the illustrated first embodiment of the invention does not distribute flow through an axially extending tube but rather distributes flow only to the two ends of the structure. Using where appropriate the same reference numerals as used in Figure 1, in the arrangement of Figures 2 and 3 the inlet port 4 communicates with each of six apertured tubes 9 that penetrate the support tube 1 and are supported at their radially inner ends by a solid or tubular support rod 10. The inlet port 4 also communicates with six shorter apertured tubes 11 which project about halfway towards the rod 10. The outlet port 5 is connected to an array of apertured tubes identical to those associated with the inlet port 4. The filter tube bundle fills the spaces between the apertures tubes 9 and 11 and the support rod 10. The support rod does not act as a fluid distribution passageway but simply acts as a support for the filter tube bundle and the radial apertured tubes. To provide additional support for the bundle the rod 10 may support radial arms similar to the secondary tubes 8 of Figure 1, but those radial arms do not define fluid distribution passageways. Accordingly in the arrangement of Figures 2 and 3, the primary fluid distribution arrangement is not such that fluid to be filtered is distributed throughout the body of the bundle as in the case of Figure 1. In contrast, fluid distribution relies upon passage of the fluid axially between the adjacent tubes of the bundle. This flow in itself is an effective means for dislodging deposited particles. Generally, fluid to be filtered is pumped in through inlet post 4, and fluid which does not pass into the filter tubes exits through the outlet post 5. The filter may operate in dead end mode however, in which case the outlet post 5 would be closed. During backwashing, inlet post 4 is closed, and outlet post 5 is open. Filtrate is pumped into the filter tubes, passes radially outwards through the filter tube walls, and then flows axially towards the outlet post 5. As a result, substantially all of the volume of the filter tube bundle is swept by the axial flow and particles lifted from the filter tubes by filtrate passing radially outwards through the walls of the filter tubes are carried axially towards the downstream end of the support tube. Therefore the structure can be more efficiently cleaned as compared with the structure of Figure 1. It will be appreciated that alternative fluid distribution structures can be arranged adjacent one or both of the seals 3, the only requirement being that there is a reasonable distribution of fluid to be filtered throughout a full cross- section of the bundle adjacent the seal. Optionally, during backwashing, an additional flow of flushing fluid may be pumped through the filter from inlet post 4 to outlet post 5. Referring now to Figures 4 and 5, a second embodiment of the invention will be described. Once again the same reference numerals are used as in Figure 1 where appropriate. As in the case of the structure of Figure 1, the inlet and outlet ports 4 and 5 communicate with an axially extending fluid distribution passageway 7 through tubes 6. In contrast to the arrangement of Figure 1 however, in which the interior of the distribution tube 7 communicates with the interiors of radially extending secondary tubes 8, in the case of the embodiment of Figures 4 and 5 the distribution tube 7 is itself apertured such that the interior of the tube 7 opens directly in the radial direction into the filter bundle. Thus fluid to be filtered is introduced into the centre of the structure and travels radially outwards through the filter tube bundle. Although there will be a tendency for the filter tubes immediately adjacent to the distribution tube 7 to become clogged, the flow of fluid to be filtered will in effect expand the loosely packed filter tube bundle so as to ensure that fluid continues to flow into the filter tube bundle. When the structure of Figures 4 and 5 is backwashed, filtrate is of course distributed uniformly throughout the bundle via the interior passageways defined by the filter tubes. Fluid is also pumped through the tube 7 such that a pressure gradient is established along the length of the tube. The bundle of filter tubes is expanded by the filtrate flow, and tests have shown that given the pressure gradient along the tube 7 particles are displaced predominantly in the axial direction. Arrows 12 in Figure 5 represent the paths followed by particles flushed out of the tube bundle. It will be noted that particles adjacent the inner and outer peripheries of the filter tube bundle move in directions with a substantial radial component but that all particles nevertheless move in a direction with an axial component. Accordingly the movement of the particles is predominantly in the axial direction. It will be noted that there is a substantial flow through the annulus 13 defined within the support tube 1 around the outside of the filter tube bundle. Typically the size of this annulus will be selected such that approximately 50% of the flow of fluid in the axial direction is through the annulus. The rate of flow through the tube 7 is controlled to achieve a desired pressure gradient along the axis of the filter. In one tested structure, the tube 7 had an internal diameter of 35mm, a length of 1.35m and a flow rate of more than 3m per second. This was in a 200mm diameter bundle. It may be the case that there is a portion of the filter tube bundle in which there is no radial component in the direction of movement of individual particles. This is not a problem providing the axial component is not zero. With the exception of peripheral regions of the filter tube bundle, for example radially outer portions of the filter tube bundle adjacent the ends of the structure, an axial component will be established throughout the filter tube bundle. Preferably the overall axial pressure drop (between the ends of the tube 7) is higher than the radial pressure drop (between the interior of tube 7 and the annulus 13. It may be that given the radially inwards flow of fluid flushing out the filter tube bundle adjacent the downstream end of the filter structure there will be a tendency in that part of the structure for particles to become compacted this effect can be overcome by short periods of fluid flow direction reversal. Referring now to Figures 6 and 7, a third embodiment of the invention is illustrated. This third embodiment incorporates features of both the first and second embodiments in that it comprises an axially extending apertured fluid distribution tube 7 that supports and communicates with further radially extending apertured fluid distribution tubes 9. During backwashing, a pressure gradient is established along the tube 7. In the illustrated arrangement, the tube 7 defines circumferentially extending slots 14, and each tube 9 defines a single radially extending slot 15. The tube 7 also supports radially extending solid arms 16 which serve to support a tube bundle (not shown) through which the tube 7 extends. In the arrangement of Figures 6 and 7, a relatively high flow velocity is achieved within the tube bundle near the exit end of the system during backwash. None of the particles which should be removed during the backwash process have to travel far through the tube bundle. Experiments conducted to compare the performance of the prior art arrangement of Figure 1 and the structure illustrated in Figures 6 and 7 have demonstrated that the embodiment of Figures 6 and 7 outperforms the prior art design. These tests have also shown that during operation particles tend to accumulate closer to the end of the structure to which particulates are flushed when the structure is backwashed. Maximum flow velocities are achieved at this end of the structure during backwash and this assists in the removal of particulates.

Claims: CLAIMS 1. A filter comprising a bundle of open-ended filter tubes housed within and extending axially along the length of a support tube, seals arranged at each end of the support tube such that the filter tubes extend therethrough, the seals preventing axial flow of fluid through the ends of the support tube except through the filter tubes, and at least one conduit communicating with the interior of the support tube between the seals to deliver fluid to or remove fluid from the space defined within the support tube around the filter tubes, wherein the said at least one conduit is connected to at least one passageway extending through the bundle of filter tubes and opening into spaced regions of the filter tube bundle, the spaced regions being distributed such that the filter may be backwashed in a manner which establishes a flow within the said space which is predominantly axial. 2. A filter according to claim 1, wherein the spaced regions are distributed such that at most a relatively small proportion of the filter tube bundle is located between and equally spaced from any two of the spaced regions. 3. A filter according to claim 1 or 2, wherein the passageway opens into radially and circumferentially spaced regions which are located only adjacent one or both of the seals. 4. A filter according to claim 3, wherein the passageway is defined by a plurality of radially extending tubes the walls of which define openings communicating with spaces between the filter tubes. 5. A filter according to claim 4, wherein at least some of the radially extending tubes are supported at their radially inner ends by a support member that extends axially through the filter tube bundle. 6. A filter according to claim 5, wherein a further radially extending tube is located between each pair of the tubes which are supported by the support member. 7. A filter according to claim 5 or 6, wherein the support member itself supports radially extending members distributed along its length to provide support for the filter tube bundle. 8. A filter according to claim 1, wherein the said at least one passageway is defined by at least one distribution tube extending axially through a central portion of the filter tube bundle, the distribution tube having a wall defining a plurality of apertures opening radially outwards into axially and circumferentially spaced portions of the bundle, and the distribution tube having a length and internal diameter such that backwashing establishes a substantial pressure gradient along the length of the distribution tube. 9. A filter according to claim 8, wherein further distribution tubes are located adjacent at least one end of the filter tube bundle, the further distribution tubes communicating with the axially-extending distribution tube and opening into radially spaced regions of the filter tube bundle. 10. A filter according to claim 8 or 9, wherein the distribution tube supports radially extending members distributed along its length to provide support for the filter tube bundle. 11. A method for backwashing a filter comprising a bundle of open ended filter tubes housed within and extending axially along the length of a support tube, seals arranged at each end of the support tube such that the filter tubes extend therethrough, the seals preventing axial flow of fluid through the ends of the support tube except through the filter tubes, and at least one conduit communicating with the interior of the support tube between the seals to deliver fluid to or remove fluid from the space defined within the support tube around the filter tubes, wherein the filter tube bundle is backwashed by pumping filtrate radially outwards through the walls of the filter tubes and establishing a flow within the said space which is predominantly axial. 12. A filter substantially as hereinbefore described with reference to Figures 2 and 3, or Figures 4 and 5, or Figures 6 and 7 of the accompanying drawings.

Inventors: CARDEW PETER TOWSEND, GALLAGHER PAUL MARTIN, HANNON JOSEPH PATRICK, POWELL NICHOLAS IAIN, CARDEW, PETER, TOWSEND, GALLAGHER, PAUL, MARTIN, HANNON, JOSEPH, PATRICK, POWELL, NICHOLAS, IAIN
Applicants: CARDEW PETER TOWSEND, GALLAGHER PAUL MARTIN, HANNON JOSEPH PATRICK, POWELL NICHOLAS IAIN, USF LTD, CARDEW, PETER, TOWSEND, GALLAGHER, PAUL, MARTIN, HANNON, JOSEPH, PATRICK, POWELL, NICHOLAS, IAIN, USF LIMITED
Application year: 1997
Main classification: B01D46
IPC codes: B01D29, B01D46, B01D63, B01D65