Patent Publication Number: US-2005126135-A1

Title: Gas flow arrangement apparatus and to apparatus for removing pollutants from gas streams

Description:
FIELD OF THE INVENTION  
      The present invention relates to gas flow arrangement apparatus and to pollutant removal devices, which may incorporate such gas flow arrangements.  
     BACKGROUND TO THE INVENTION  
      Pressure is continuing to grow on vehicle manufacturers to reduce the amount of pollutants, especially particulates in gas streams emitted from vehicle exhausts. Attempts have been made to collect particulates from gas streams using electro-static precipitation, but generally these fail because the performance of the apparatus degrades substantially over time so it cannot be used in a practical environment.  
      The present invention finds particular, but not exclusive, application in the field of the removal of pollutants from vehicle exhaust gas streams. In this technological application, often a filter is used to remove pollutants, especially particulate pollutants. However, as particulate material is built up in the filter, the porosity of the filter decreases thus increasing back pressure on the exhaust system which can reduce engine efficiency. Since environmental concerns are the primary reason for removing pollutants, such a decrease in efficiency, with a resultant increase in pollutants, defeats the object of many such proposed filtration devices.  
      One particular problem area is in relation to the particulate material that is agglomerated. For instance, in a prior art electro-static precipitation apparatus of this type, a central electrode is mounted within a circular cylindrical solid-walled tube, whereby particulates are charged by the electrode and attracted to the solid-walled container. However, once particulates arrive at the tube wall over time they agglomerate and can eventually be swept out through the vehicle exhaust by the continued flow of exhaust gas flow stream over the agglomerated particulate.  
      In other prior art devices filters have been proposed to remove particulates from gas streams. However, in this case over time particulate build up in the filters reduces their efficiency and causes back-pressure reducing engine efficiency also.  
      It is an aim of preferred embodiments of the present invention to obviate or overcome at least one disadvantage of the prior art, whether referred to herein or otherwise.  
     SUMMARY OF THE INVENTION  
      According to the present invention in a first aspect, there is provided a gas flow arrangement apparatus comprising a gas entrance and a gas exit, a first flow path from the gas entrance to the gas exit through a means for at least partly removing at least one pollutant from a gas flow stream and second flow path from the gas entrance to the gas exit other than through the removing means.  
      Suitably, gas passing through the pollutant removing means intersects the first gas flow.  
      Thus pressure differences can be minimised and undue back pressure is avoided. To the extent that gas is blocked from a first it can follow the second flow path avoiding the filter.  
      Suitably, the first flow path diverges from the second flow path upstream of the pollutant removing means.  
      Suitably, the first flow path and the second flow path intersect with each other downstream of the pollutant removing means. Thus the gas in one flow path is reintroduced into the gas of the other flow path.  
      Suitably, the first gas flow splits from the second gas flow path at a separator for diverting pollutant to the pollutant removing means. Suitably, the separator is generally conically shaped with an opening for one of the gas flow paths therethrough.  
      Suitably, the first flow path diverges from the second flow path at a tube through which gas can pass. Suitably, the tube is a perforated tube.  
      The first and second flow paths may be in common for some of their respective passages through the arrangement, but they form discrete flow paths before intersecting downstream of the filter.  
      Suitably, the arrangement comprises a gas flow tube for the second flow path, which gas flow tube comprises a slot for the first gas flow path to join the second gas flow path.  
      Suitably, the arrangement comprises a first chamber, a second chamber and a third chamber, whereby gas enters into a first chamber, passes into a second chamber at which the first flow path diverges from the second flow path, and whereby gas can flow into the third chamber through two openings one of which comprises the pollutant removing means, and in which there is an exit for gas from the third chamber.  
      Suitably, the pollutant removing means comprises a filter.  
      Suitably, the filter comprises a regenerative filter. Suitably, the filter is electrically regenerative.  
      Thus, the arrangement provides a gas flow apparatus.  
      According to the present invention in a second aspect, there is provided a pollutant removal device for at least partly removing a pollutant from a gas flow, the device comprising a gas flow arrangement apparatus according to the first aspect of the invention.  
      Suitably, the device comprises means for at least partially ionising gas flow. Suitably, the ionising means comprises an electrode for electrostatic precipitation. Suitably, the electrode is mounted in the second chamber. Suitably, the electrode is mounted in the first chamber.  
      Suitably, the apparatus comprises a tube through which the gas stream at least partly flows, whereby the tube is at least partly porous to the gas stream.  
      Suitably, the tube is at least partly about the ionising means.  
      Suitably, the tube is perforated. Suitably, the tube comprises a plurality of holes therethrough. Suitably, the holes are evenly spaced. Suitably, the holes are evenly sized. Suitably, the perforated region of the tube is substantially annular. Suitably, the perforated region of the tube extends for a substantial length thereof.  
      Suitably, the tube comprises at least one slot therethrough. Suitably, a plurality of slots is provided. Suitably, the slots are substantially evenly distributed about the tube. Suitably, the at least one slot runs longitudinally along the tube.  
      Suitably, a major portion of the tube is porous. Alternatively a minor portion of the tube is porous.  
      Suitably, the tube is circular in cross-section.  
      Suitably, the tube comprises an inlet and an outlet.  
      Suitably, the cross-sectional area of the tube decreases along its length from the input to the output thereof.  
      Suitably, the tube is at least partly coated with a barrier coating for slowing the discharge time of charged agglomerates.  
      Suitably, the electrode is mounted at one end thereof only.  
      Suitably, the tube is located in the first and second gas flow paths. The tube acts to split the gas flows and concentrate at least one pollutant in one flow path for subsequent removal.  
      Suitably, the apparatus comprises a first expansion tube in fluid communication with an apparatus gas inlet. Suitably, the diverting tube extends from the first expansion tube to a second expansion tube defined by the tube. Suitably, there is a third expansion tube about the diverting tube into which gas can flow through the diverting tube. Suitably, a filter is located between (in respect of gas flow) the second and third expansion tubes.  
      Suitably, the device is arranged whereby at least one pollutant is biased towards the first flow path (ie a substantial majority of an input pollutant flows through the first flow path, subject to being trapped by the filter).  
      Suitably, a catalytic converter is provided in the second flow path.  
      Suitably, the electrode projects from the first chamber in to the second chamber.  
      Suitably, the second flow path includes a catalytic converter.  
      Suitably, the device is for fitting to a vehicle exhaust. Suitably, the device is for fitting in place of the silencer of a vehicle exhaust.  
      According to the present invention in the third aspect, there is provided an apparatus for removing pollutants from a gas stream, the apparatus comprising means for charging particulates in the gas stream and a tube through which the gas stream at least partly flows, whereby the tube is at least partly porous to the gas stream and the apparatus additionally comprises means for collecting at least one pollutant.  
      Suitably, the tube is at least partly about the charging means. Suitably, the charging means comprises an electrode.  
      Suitably, the tube is perforated. Suitably, the tube comprises a plurality of holes therethrough. Suitably, the holes are evenly spaced. Suitably, the holes are evenly sized. Suitably, the perforated region of the tube is substantially annular. Suitably, the perforated region of the tube extends for a substantial length thereof.  
      Suitably, the tube comprises at least one slot therethrough. Suitably, a plurality of slots is provided. Suitably, the slots are substantially evenly distributed about the tube. Suitably, the at least one slot runs longitudinally along the tube.  
      Suitably, a major portion of the tube is porous. Alternatively a minor portion of the tube is porous.  
      Suitably, the tube is circular in cross-section. Suitably, the tube comprises an inlet and an outlet.  
      Suitably, the cross-sectional area of the tube decreases along its length from the input to the output thereof.  
      Suitably, the electrode is mounted at one end thereof only.  
      Suitably, there is a first gas flow path from an apparatus gas inlet to an apparatus gas outlet and a second gas flow path from the apparatus gas inlet to the apparatus gas outlet. The first and second gas flow paths may be in common for a part thereof. Suitably, a filter is located in the second gas flow path. Suitably, the tube is located in the first and second gas flow paths. The tube acts to split the gas flows and concentrate at least one pollutant in one flow path for subsequent removal.  
      Suitably, the apparatus comprises a first expansion tube in fluid communication with an apparatus gas inlet. Suitably, the diverting tube extends from the first expansion tube to a second expansion tube defined by the tube. Suitably, there is a third expansion tube about the diverting tube into which gas can flow through the diverting tube. Suitably, a filter is located between (in respect of gas flow) the second and third expansion tubes.  
      Suitably, the filter comprises an electrically regenerative filter.  
      Suitably, the apparatus is for removing pollutants from an exhaust gas stream, preferably a vehicle exhaust gas stream.  
      According to the present invention in a fourth aspect, there is provided a combustion generator and an apparatus according to the second or third aspects of the invention in which exhaust gas from the generator flows to an apparatus inlet.  
      Suitably, the generator is an internal combustion engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will now be described, by way of example only, with reference to the drawings that follow; in which:  
       FIG. 1  is a schematic perspective (partly cut away) illustration of a gas flow arrangement apparatus according to an embodiment of the present invention.  
       FIG. 2  is a schematic perspective (partly cut away) illustration of the gas flow arrangement shown in  FIG. 1  from a reverse angle.  
       FIG. 3  is a longitudinal cross-sectional view of the arrangement shown in  FIGS. 1 and 2 .  
       FIG. 4  is an enlarged partly cut away and sectional drawing of the filter shown in  FIGS. 1 and 2 .  
       FIG. 5  is a schematic partly cut away illustration of an embodiment of a particulate filtration device according to the present invention.  
       FIGS. 6 and 7  are schematic partly cut away illustrations of two further embodiments of a device according to the present invention.  
       FIG. 8  is a schematic longitudinal cross-sectional view of an electrode mount.  
       FIG. 9  is a schematic partly-sectional elevation of a gas flow arrangement apparatus according to a yet further embodiment of the present invention.  
       FIG. 10  is a perspective view of a second gas flow path tube and filter of  FIG. 9 .  
       FIG. 11  is a sectional view of a further electrode mounting arrangement.  
       FIG. 12  is a plan elevation (external walls cut away) of an apparatus according to a further embodiment of the present invention.  
       FIG. 13  is a side elevation of  FIG. 12 .  
       FIG. 14  is a perspective illustration of  FIGS. 12 and 13 .  
       FIG. 15  is a plan elevation (external walls cut away) of an apparatus according to a yet further embodiment of the present invention.  
       FIG. 16  is a perspective illustration of  FIG. 15 .  
       FIG. 17  is a plan view of a yet further embodiment of the present invention.  
       FIG. 18  is a side elevation of  FIG. 17 .  
       FIG. 19  is a sectional, inverted plan view corresponding to  FIG. 17 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIGS. 1-3  of the drawings that follow, there is shown a gas flow arrangement apparatus within a circular cylindrical tubular body indicated by dashed line  2 . The body  2  is defined internally by wall plates  4 ,  6 ,  8  and  10  respectively into a first chamber  12 , a second chamber  14  and a third chamber  16 . The body  2  is provided with a gas entry tube  18  and gas exit tube  20 . Gas entry tube  18  extends from the exterior wall plate  4  to first chamber  12 . That is, gas enters at the entrance of  18  and exits into first chamber  12 . Gas exit tube  20  extends from the exterior of wall plate  10  to third chamber  16 . Additionally, there is provided a perforated tube  22  extending between first chamber  12  and third chamber  16 , the perforations opening into second chamber  14 . The tube  22  is highly perforated whereby in a given annulus there is more area taken up by holes than by solid. The preferred structure is substantially constant radially and longitudinally.  
      A filter  24  for removing pollutants from the gas stream is mounted in third chamber  16  about an opening  26  between third chamber  16  and second chamber  14 .  
      The filter  24  is an electrically regenerative filter such as the filter identified as 3M part number SK-1739.  
      The filter  24  is shown in more detail in  FIG. 4  of the drawings that follow. The filter  24  comprises a tubular outer body  28  of a NEXTEL 312 filtration mounted on a porous metallic frame  30  which is connected to earth (which may be a floating earth) at one end  32 . The other end  34  provides an electrical connection  36  (see also  FIGS. 1 and 2 ) to a power supply  37  ( FIG. 5 ) to achieve heating and regeneration of the filter  24  as is known in the art.  
      An electrode  38  is mounted on wall plate  10  by a ceramic electrode mount  39  to project into the hollow interior of perforated tube  22  as shown in cross-section in relation to  FIG. 4  of the drawings that follow in which corresponding reference numerals are used.  
      In use, pollutant eg particulate carrying gas enters the arrangement at  18  and passes into first chamber  12  from which its only route is into perforated tube  22 . In operation the electrode is highly charged to between 18 kV-40 kV negative polarity d.c. to ionise or charge particulates in the gas stream forcing them through the perforated holes of the tube  22  in to second chamber  14  (under full load the potential may be about 10 kV). Additionally, it is believed that the gas becomes at least partly ionised.  
      The perforated tube  22  opens into third chamber  16  allowing gas to pass through exit tube  20  to exhaust. Further, gas can flow from second chamber  14  to third chamber  14  through hole  26  through filter  24 . Thus filter  24  can collect particulate material. The filter  24  is regenerative so that at intervals it is electrically regenerated. This need not be on a regular basis. However, if for any reason the filter  24  does not regenerate fully or a heavy loading occurs causing back pressure between filter  24  and second chamber  14 , this is compensated for because gas can still flow to exit tube  20  through perforated tube  22  and third chamber  16 . Thus build up of particulates (or other pollutants) in filter  24  will not cause undue back pressure on the engine providing an exhaust stream to the gas flow arrangement. As a result, the problem of back pressure encountered in relation to prior art filtration arrangements is overcome by embodiments of the present invention and there is provided a geometrically efficient and compact gas flow arrangement.  
      Thus embodiments of the present invention provide a first gas flow path  40  ( FIG. 5 ) from gas entrance  18  to gas exit  20  via first chamber  12 , tube  22 , third chamber  16  through filter  24  and second chamber  14  and a second gas flow path  42  ( FIG. 4 ) from gas entrance  18  to gas exit  20  via first chamber  12 , tube  22  and second chamber  14  which is other than through the filter  24 .  
      Referring to  FIG. 6  of the drawings that follow, there is shown another embodiment of a gas flow arrangement and pollutant removal device according to the present invention. The arrangement and device is similar to that described in relation to  FIG. 5  (and similar reference numerals are used for corresponding integers), except that the first gas flow path  40  through filter  24  is generally straight on, ie the flow path does not diverge substantially from the path of the tube  22  to the filter  24  and the second gas flow path  42  follows the more tortuous route as shown.  
      To bias the particulate pollutants to follow first gas flow path  40  at  FIG. 6 , instead of a highly perforated tube  22  (considered over the length at tube  22 ) a small area  50  of perforated tube  52  with a lower hole density is provided. The less perforated tube  52  is not annular, it just occupies a slot in the tube. As the effect of the corona discharge electrode  38  with the floating earth of the tube  52  is to draw particulates to the side (tube  52 ) walls where they tend to agglomerate, by providing less open area for the agglomerated particulate to pass through, it is less likely that particulates will follow the second flow path  42 .  
      Another difference in the  FIG. 6  embodiment is the provision of a catalytic converter  54  in the second flow path  42  for the removal of hydrocarbons from the gas stream.  
       FIG. 7  is a yet further embodiment of the present invention substantially similar to the embodiment of  FIG. 6 , except that four equally spaced longitudinal slits  60  are provided over a substantial minority of the surface area of tube  62 .  
      Referring to  FIG. 8  of the drawings that follow, the electrode mount  39  is shown in more detail. The electrode mount  39  is a one piece ceramic construction having a longitudinal hole  64  therethrough for the electrode  38  (not shown in  FIG. 8 ). The electrode projects from distal end  66  and is connected to a power source at end  68 . The electrode mount  39  is held by a bracket (not shown) about shoulder  70 . Protrusions  72   a ,  72   b  and  72   c  project from the exterior of electrode mount  39 . The protrusions  72  are partly hollow, rebated conical shapes that provide a tortuous route from the electrode  38  projecting from distal end  66  to earth to reduce leakage.  
      Referring to  FIGS. 9 and 10  of the drawings that follow, there is shown a gas flow arrangement apparatus  80  for use in a pollutant removal device in which outer walls are not shown for clarity. The apparatus  80  comprises an ionising electrode  82  in an electrode mount  83 , partly surrounded by an electrode hood  84 . Electrode  82  extends into an electrode tube  86  which terminates in an outwardly diverging end  88 . Spaced from electrode tube  86  is a second gas flow path tube  90  having a generally conically shaped entrance  92  with a central opening  94 . The opening  94  is substantially inside the diameter of the walls of electrode tube  84 . Tube  90  terminates in an exit  98 . About tube  90  is a catalytic filter  100  for at least partly removing pollutants from a gas stream passing therethrough.  
      Operation of the embodiment of  FIGS. 9 and 10  is similar to that of the embodiments described above. Exhaust gases, carrying pollutants, enter the apparatus  90  upstream of electrode  82 , and pass over hood  84  which serves to help prevent pollutant build up on electrode  82 . The electrode  82  is charged to ionise pollutants in the gas flow, which pollutants are therefore attracted to the walls of electrode tube  86  as they flow downstream, leaving relatively cleaner gas towards the centre of the flowstream. The conical opening of second gas flow path tube  90  serves to help deflect pollutant into a first gas flow path (indicated schematically by arrows labelled  102 , while the second gas flow path is indicated by arrows labelled  104 ). The first gas flow path  102  passes through filter  100 , which removes some pollutants, and rejoins second gas flow path  104  through a slot  96  in tube  172  downstream to the filter  100 . The slot  96  is relatively small compared to the surface area of tube  90 . The pressure difference either side of slot  96  is believed to encourage now relatively cleaner gas from the first gas flow path downstream of filter  100  to rejoin the second gas flow path. Second gas flow path  104  passes through second gas flow path tube  90  carrying relatively cleaner gas. The rejoined gas streams, pass out of the apparatus at exit  98 .  
      In any of the embodiments a resistive organic barrier coating may be provided over the inner surface of the tube ( 22  in  FIG. 1 ) downstream of the beginning of the electrode. The barrier coating is preferably over substantially all of the inner surface of the tube. The coating is TLHB/02 available from Camcoat Performance Coatings on 127 Hoyle Street, Bewsey Industrial Estate, Warrington, WA5 5LR, United Kingdom. It is believed that by reducing the discharge rate of the agglomerated particulates along the tube by providing the coating, the particulates are more likely to stay in the vicinity of the tube.  
      Referring to  FIG. 11  of the drawings that follow, an alternative electrode mounting arrangement is shown. Both the electrode mount  83  and electrode hood  84  are formed from a ceramic high purity alumina material, sold under the trade mark SINTOX FF which is believed to have a dielectric strength of between 30 and 40 kV/mm.  
      The electrode mount  83  comprises a first ceramic mounting portion  88  and a second ceramic mounting portion  90  mounted in bore  86 . The second ceramic mounting portion  90  is of a reduced external diameter compared with the first ceramic mounting portion. The electrode mount  83  can be formed from a single ceramic. Thus the electrode mount  83  has a portion of a first diameter and a portion of a lesser diameter towards the distal end (from which the electrode projects) thereof. The second portion  90  of second diameter extends a substantial distance beyond hood  84  typically at least 30mm.  
      The hood  84  protects a substantial part of the electrode (mounted in central bore  86 ) from the inflow of pollutants containing gas thus minimising the risk of shorting. However, it is believed that at least a 30mm length of the electrode needs to project beyond the hood. It is noted that the gas inlet is not around the electrode but rather alongside it and can be protected from it by the hood  84 .  
      The electrode mount and hood can be glazed to reduce pitting of the surface and hence the build up of articulates thereon. The glaze acts as a means for smoothing the surface of the electrode mount.  
      It is noted that although the maximum exterior diameter of each generally conically shaped protrusion  83  decreases in a downstream direction, the minimum internal diameter are substantially the same ±10%. This is believed to provide additional burn-off points if required.  
      The alumina content of hood and mount is typically at least 80%, normally at least 90%, preferably more than 95%, more preferably more than 97% and most preferably more than 99%.  
      Referring to  FIGS. 12-14  of the drawings that follow, there is shown a further embodiment of a gas flow arrangement and apparatus for removing pollutants according to the present invention. In the  FIGS. 12-14  embodiment, exhaust gas enters through an inlet  100  into a perforated baffle tube  102  from which all of the entering exhaust gases flow into first chamber  104 . In chamber  104 , electrode mount  106  over a substantial part of which lies hood  108  mounts an electrode  110  which projects into a second chamber  112  defined by field tube  114 . Field tube  114  includes an opening in its end to an intermediate chamber  116 , the only exit from which is into filter  118 . An alternative flow path is provided via an opening  120  in the wall of field tube  114 . The opening  120  is provided with an upstanding lip  122  projecting inwardly into the field tube  114  at at least the upstream portion thereof, but in this embodiment along the full length thereof. Further, the opening  120  comprises a generally V-shaped upstanding leading edge  124  at an upstream end thereof.  
      Fluid flow path leads from field tube  114  via opening  120  leads to a perforated exit tube  126 . Perforations  128  in exit tube  126  permit gas passing through filter  118  to re-enter the diverted gas flow leading to exit  130 .  
      It is noted that the leading edge  132  of field tube  114  comprises a returned edge that is curved back on itself whereby the exterior edge of the leading edge  132  of field tube  114  is configured relative to the electrode whereby something else lies between it and electrode and/or electrode mount. In this case, another part of the field tube lies between the external edge and both of the electrode mount  106  and electrode  110 .  
      Upstanding lip  122  and leading edge  124  help to divert particulates away from opening  120  from which it is intended that cleaner gas flows. Together, upstanding lip  122  and leading edge  124  act as means for diverting particulates away from the opening  120 .  
      The electrode, electrode mount and hood are not shown in  FIG. 15 .  
      Referring to  FIGS. 15 and 16  of the drawings that follow, there is shown a further gas flow arrangement apparatus and apparatus for removing pollutants according to the present invention.  
      In  FIGS. 15 and 16 , the apparatus comprises an inlet  150  into which exhaust gas flows into a baffle chamber  152  having first exit ports  154  and second exit ports  156 . First exit ports  154  exit to first clamber  158 . Second exit ports  156  exit into an intermediate chamber  160  having holes  162  permitting the flow of gas back into first chamber  158 . An electrode mount  164  ( FIG. 15  only), covered for a substantial part thereof by hood  166  ( FIG. 15  only), is provided in first chamber  158  for mounting of an electrode  168  ( FIG. 15  only) within a field tube  170 . At its downstream end, field tube  170  terminates in an outwardly diverging portion  172  adjacent a generally conical portion  174  within which is a tube  176  extending to an exit tube  178 .  
      In exit tube  178  is provided an opening  180  prior to the exit  182  of tube  176 .  
      In use, exhaust gas flows in via inlet  150  into field tube  170  via first chamber  158 . Particulates in the field tube are charged by electrode  168  and tend towards the walls of field tube  170 . Thus the particulates are diverted from the central flow of gas through field tube  170 . The central flow of gas enters tube  176  into exit tube  178 . Other gas bearing a higher loading of particulates exits towards the periphery of field tube  170  and therefore tends not to enter tube  176 . The generally conical portion  174  acts as a deflector for the particulates encouraging them not to enter tube  176 . The particulate laden gas exiting field tube  170  other than through tube  176  enters a second intermediate chamber  184  leading to filter  186 . Gas exiting filter  186  can only exit the apparatus via opening  180  and into exit tube  178 . However the gas exiting filter  186  tends to be at a low velocity compared to the high velocity gas exiting tube  176 . The pressure differential causes the gas in third chamber  188  about filter  186  to be drawn through opening  180  into exit tube  178  and hence to outlet  190 .  
      Field tube  170  may include a curved leading edge  192  as described above in relation to  FIGS. 12-14 .  
       FIGS. 17 and 18  show a further embodiment of the present invention. In  FIGS. 17 and 18 , for clarity the electrode mount and electrode are not shown.  
      Referring to  FIGS. 17 and 18 , there is shown a gas inlet into a perforated expansion chamber  202 , from which all the input gas flows into a first chamber  204  and from there into field tube  206  which leads to filter  208 . Alternatively, through opening  210  in field tube  206  gas can flow to exit tube  212  in which there is a concentrically mounted flow tube  214  and in an exterior wall of which an opening  216  mounted behind (relative to the gas flow) the exit  218  of tube  214 . In exit tube  212  a catalytic body  220 , acting as a catalytic converter, optionally can be mounted. In use, gas enters through inlet  200 , passes through expansion tube  202  into first chamber  204  and then into field tube  206  in which particulates in the gas flow are charged. Charged particulates tend towards the side wall of field tube  206  and an upstanding lip may be provided around  210  to divert particulates therefrom. Particulates proceeding from field tube  206  to filter  208  are filtered and the gas flow can continue towards exit  222  via holes  216  into exit  212 .  
      Although the first and second gas flow streams are shown separately in the same tube or area of the apparatus, this is for explanatory purposes only and it will be appreciated that in these regions the gas flows are intermingled.  
      It is noted that there may be a plurality of devices, a plurality of filters and/or a plurality of catalytic converters.  
      Instead of using standard direct current as described above, high frequency superimposed a.c can be used.  
      The reduced gas flow through the filter when compared with a corresponding device in which all of the input gas stream flows through the filter makes the electrical regeneration of the filter more efficient because the thermal effect of the gas flow is correspondingly reduced.  
      Preferred embodiments of the present invention find particular benefit in the application of pollutant, especially particulate removal from exhaust gas streams, especially of internal combustion engines. For such engines the arrangement can be mounted in place of the vehicle silencer to avoid taking up unnecessary space. The device may be upstream or downstream of a catalytic converter.  
      The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.  
      All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.  
      Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.  
      The invention is not restricted to the details of the foregoing embodiment(s). The invention extend to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.