GB2346821A

GB2346821A - Apparatus for concentrating gasborne particles in a portion of a gas stream

Info

Publication number: GB2346821A
Application number: GB9903503A
Authority: GB
Inventors: Andrew Ian Briggs
Original assignee: Notetry Ltd
Current assignee: Dyson Technology Ltd
Priority date: 1999-02-17
Filing date: 1999-02-17
Publication date: 2000-08-23
Anticipated expiration: 2019-02-17
Also published as: GB2346821B; GB9903503D0

Description

Apparatus for Concentrating Gasborne Particles in a Portion of a Gas Stream The invention relates to apparatus for concentrating gasborne particles in a portion of a gas stream in which the particles are entrained. The apparatus is particularly, but not exclusively, suitable for use in the separation of particulates from the exhaust of an internal combustion engine.

By the term"concentration", we mean to include concentration of a specific type of particle within the gas stream (such as particles of a particular mass, size or mobility) so as to effect classification of the particles within the gas stream.

There is an ever increasing demand for gases which are emitted to the atmosphere to be vigorously cleaned before being so emitted. This is particularly true of vehicle exhausts and strenuous efforts are continuously being made to reduce the particulate concentration in vehicle emissions. New and improved methods of reducing the particulate concentration are constantly being sought.

Electrostatic agglomeration is also well known as a method of increasing the average particle size in a gas stream. Particles carrying a specific charge are mixed with or brought into close contact with particles carrying an opposite charge. US 4765803 discloses apparatus which divides a gas stream into two paths, applies opposite electrostatic charges to the particles in each path and then recombines the gas stream paths so that the particles agglomerate. The disadvantages of this and similar arrangements are that the agglomeration rate is not very good, even after the process has been repeated, that the agglomerated particles remain finely dispersed within the gas stream, and that it very difficult to attach an electrostatic charge to very small particles such as those normally present in a diesel exhaust.

Other electrostatic precipitators attract charged particles onto one or more plates. This causes build-up of particle layers on the plates which reduces efficiency of the precipitator over time. With solid particulates, electrostatic precipitators of this type require regular maintenance or active plate cleaning devices to clear away the deposited particles and maintain adequate operation. Some precipitators require their plate areas to be extremely large for a given volumetric flow rate in order to achieve practical results. Large sized precipitators are impractical for applications such as diesel exhausts and vacuum cleaners.

The use of electrostatic forces to move particles between different zones in a gas body or gas stream have been considered. Electrostatic movement of charged particles within an electrostatic field is a well known principle. However, the desired effect of moving particles from one area to another is achieved only if the gas in which the particles are entrained has a significant amount of residence time between the elements. Inevitably, the charged particles will follow electric field lines and often impinge upon a surface within the device. The risk of these particles becoming lodged within the specific apparatus instead of continuing to flow with the gas stream is therefore significant and can lead to air channels becoming blocked, increased pressure drops within the device and electrical breakdown of the gas.

A method and apparatus for concentrating aerosol particles within a portion of a gas stream flowing within an electrostatic field is proposed in US 4 734 105. In the disclosure, the gas stream passes between a plurality of pairs of plate-like electrodes whose interelectrode spacings decrease in a stepwise manner in the direction of flow.

An electrostatic field is established and maintained between the electrodes and across the gas stream and each electrode is a source of ions which are injected into the passing gas stream on the side thereof on which the respective electrode is arranged. The aerosol particles entrained within the gas stream acquire a charge by collision with ions in the area near to one of the electrodes and are then attracted towards the opposite electrode. As the particle crosses the central area, it loses its original charge and acquires an opposite charge so that it then becomes attracted to the electrode away from which it was originally repelled. The particles thus move in a reciprocating manner across the path of the gas stream. However, as the particles move across the central area, they are uncharged and therefore uninfluenced by the electrostatic field so the apparatus and method of the patent is unable to exert a concentrating force on the particles in the central area. This means that the apparatus disclosed in the patent is not capable of concentrating the aerosol particles of the gas stream in less than about a third or a quarter of the volume of the gas stream.

Moving gasborne particles between different zones or areas of a gas stream can also be achieved in other ways. One way of achieving the same effect is to use thermophoretic effects. The thermophoretic principle has been known for many years. Particles entrained within a gas stream tend to move away from hotter zones and towards colder zones within the body of the gas, or away from the hot gas towards a cold surface nearby. This effect is due to the particles being buffeted by the hotter gas molecules which have a higher kinetic energy than the cooler gas molecules and this moves the particles away from the hotter zone of higher gas molecular activity and towards the cooler zone of lower gas molecular activity.

Thermophoretic devices have previously been used for a number of different purposes.

One use which originated early on is for sampling aerosols for toxicology research in coal mines. Thermophoretic forces are used to deposit aerosol particles onto a cold plate such as a microscope slide ready for immediate analysis. The small thermal forces involved do not subject the particles to any great physical stress and sensitive structures are therefore preserved allowing accurate analysis to be carried out. The most common type of sampler used for this type of purpose is the"wire and plate"type in which a heated wire is placed near to a cold plate or between two cold plates.

Particles entrained within a gas body move away from the heated wire as they pass it and towards the cold plate or plates on which the particles are then deposited. An alternative arrangement uses opposing hot and cold plates to achieve a similar effect.

The particles move away from the hot plate towards the cold plate and are deposited thereon. The"wire and plate"type of sampler produces a high localized force but uses only a relatively small amount of power. The use of hot and cold plates requires more power consumption in order to maintain a thermal gradient along the length of the apparatus because the hot plates warm the cold plates which reduces the thermal gradient. In each case, the required effect of depositing particles on a cold plate is achieved only if the gas from which the particles are to be removed has a significant amount of residence time between the appropriate elements. Also, in order to achieve the desired deposition onto a cold plate, the particles must overcome the resistance offered by the boundary layer present adjacent the surface of the plate.

Thermophoresis has been used in other types of device in which it has been appropriate to repel particles entrained within a gas stream or gas body away from a specific area.

A thermophoretic separation device is shown and illustrated in US 4 572 007. In this device, a heated membrane is used to repel aerosol particles from an area within a contaminated atmosphere in order to allow gaseous samples to be taken without hindrance from the aerosol particles. The device makes use of simple thermophoretic repulsion in order to maintain a particle-free area in the contained atmosphere for sampling purposes. Simple thermophoretic effects are also made use of in US 4 519 061 which describes a method of reducing contamination of an optical disk by repelling debris generated during the recording process. The repulsion of the debris away from the recording surface reduces the contamination of the surface and improves the quality of the disk.

Thermophoresis has also been employed in order to contain an aerosol stream. US 4 650 693 describes apparatus for producing an aerosol stream which is surrounded by an annular sleeve of an essentially aerosol-free vapour and/or gas stream to prevent undesirable precipitation of the particles contained in the aerosol stream beyond the desired boundary thereof. By heating the gas forming the annular sleeve to a high temperature, thermophoresis effects prevent the aerosol particles from passing through the gas stream.

Another way of achieving the same effect is to use ultrasonics to agitate the gas molecules near to a particle-repelling element so that the gasborne particles are buffeted away therefrom. This can be achieved by vibrating the particle-repelling elements at an ultrasonic frequency thus causing the gas molecules closest to the elements to vibrate with more energy than the molecules further away from the elements. The particles entrained within the gas will therefore be repelled away from the elements in a similar fashion as that achieved by thermophoretic effects.

It is an object of the present invention to provide apparatus suitable for cleaning or enhancing the cleaning of a relatively high velocity, high volume gas stream in which particles are entrained whilst minimising any build-up of the particles within the said apparatus. It is a further object of the invention to provide apparatus which is suitable for cleaning or facilitating the cleaning of the exhaust gas of an internal combustion engine, again minimising any build-up of particles within the apparatus. It is a further object of the invention to provide apparatus which is suitable for cleaning or facilitating the cleaning of an airflow making use of electrostatic effects and which minimises deposition of particles within the apparatus. It is still a further object of the present invention to provide apparatus which reduces the emission of particles into the atmosphere.

The invention provides apparatus for concentrating unipolar-charged, gasborne particles in a portion of a gas stream in which the particles are entrained, comprising a conduit for conducting the gas stream between an inlet and an outlet, first and second electrodes arranged within the conduit, and means for creating an electrostatic field between the first and second electrodes so that, in use, the particles are urged towards either the first or the second electrode by electrostatic effects, characterised in that the apparatus further comprises repulsion means for causing the particles to be repelled by nonelectrostatic effects away from the electrode towards which they are urged by the electrostatic effects.

The apparatus according to the invention makes use of electrostatic forces to concentrate particles or particulates within an area of the gas stream close to one of the electrodes. However, the repulsion means oppose the action of the electrostatic forges so as to repel the particles away from the surface of the electrode towards which they are urged so that the particles are prevented from impinging upon the electrode.

Preferably the repulsion forces generated by the repulsion means exceed the electrostatic forces only in an area very close to the electrode in question so that the zone of concentrated particles is close to the electrode but spaced slightly away from its surface. Since the repulsion forces are generated by non-electrostatic means, they do not interfere with the effect of the electrostatic field in other areas. Therefore, the particles are concentrated without causing deposition of the particles within the apparatus, which would block flow channels and reduce the performance of the apparatus.

Thermophoretic or ultrasonic forces can be used to repel the particles away from the electrode. If thermophoretic effects are used, the electrode is heated by any suitable heat source. Appropriate heat sources could be an electrical heater or the exhaust manifold of an internal combustion engine. Hot exhaust gases themselves can also be usefully employed. An internal combustion engine is a readily available heat source when the apparatus is used in conjunction with an intemal combustion engine.

The ability to progressively concentrate the particles into a specific zone of the gas stream may be advantageous because natural agglomeration can take place simply and effectively. When sufficient agglomeration is achieved, separation by way of, for example, a centrifugal separator can then result in a higher efficiency of separation.

Progressive concentration can be achieved using a series of electrode plates or rings, each plate or ring being required merely to repel approaching particles sufficiently far away therefrom in order to maintain the particle within a particle-containing zone. This means that the residence time of the gas stream within the apparatus as a whole is not excessive.

Further and advantageous and preferred features of the invention are set out in the subsidiary claims.

Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure la is a schematic cross-sectional view through a first embodiment of apparatus according to the invention; Figure lb is a sectional view taken on line I-I of Figure la; Figure lc is an enlarged schematic view of part of the apparatus of Figure la illustrating the operation thereof; Figure 2a is a schematic cross-sectional view of a second embodiment of apparatus according to the invention; Figure 2b is a sectional view taken on line II-II of Figure 2a; Figure 2c is an alternative sectional view taken on line II-II of Figure 2a illustrating a third embodiment of apparatus according to the invention; and Figure 3 is a cross-sectional view of a third embodiment of apparatus according to the invention.

Apparatus according to a first embodiment of the invention is shown in Figures la, lb and lc. The apparatus 10 for concentrating particles within a portion of a gas stream comprises a conduit 12 having a generally rectangular cross-section as shown in Figure lb. The conduit 12 has a tapering upstream end 14 connected to an inlet pipe 16. The inlet pipe 16 leads directly or indirectly from a source of the gas in which the particles are entrained. The end 18 of the conduit 12 remote from the inlet end 14 is tapering in shape and is connected to an outlet pipe 20. The dimensions of the outlet pipe 20 are similar to that of the inlet pipe 16.

A first electrode 22 and a second electrode, 24 are arranged within the conduit 12. The electrodes 22,24 are plate electrodes having dimensions to one another and are arranged facing one another on either side of the longitudinal axis 25 of the conduit 12.

The electrodes 22,24 extend along the length of the conduit 12 and are supported by and meet the side walls of the conduit 12. Baffle plates 23 extend between the electrodes 22,24 and the upper and lower walls respectively of the conduit 12 so as to prevent gas from entering the space between each electrode 22,24 and the conduit 12.

The first electrode 22 is maintained at a high potential (positive or negative) by commonly known means whilst the second electrode 24 is earthed. An electrostatic field is thereby generated between the electrodes 22,24. The electrodes 22,24 are essentially aligned with one another so as to provide a uniform field perpendicular to the direction of flow of the gas stream which lies parallel to the axis 25 of the conduit 12. In the preferred embodiment, the first electrode 22 is maintained at a high negative potential. Of course, it will be understood that the first electrode 22 may equally carry a positive charge with respect to the second electrode 24 or that the second electrode 24 need not be earthed but may be biased with an electrical potential having a polarity opposite to that applied to the first electrode 22. Further, the first and second electrodes 22,24 need not be planar plate electrodes but may be slightly curved to follow the contour of the interior wall of the conduit 12. The outer surface of the first and second electrodes 22,24 are electrically insulated.

A second outlet, formed by an outlet mouth 26a connected to an outlet conduit 26b is provided inside the conduit 12 and is located so that the outlet mouth 26a is positioned immediately downstream of the second electrode 24 on the side thereof facing the first electrode 22. The outlet conduit 26b leads to a separator device 28, which may be a cyclonic separator.

The second electrode 24 is connected to a heat source 30. The heat source 30 can be any appropriate heat source, for example, the exhaust manifold of an internal combustion engine. At least one thermal conductor 32, preferably in the form of a rod, connects the heat source 30 to the second electrode 24. The outer surface of the second electrode 24 and the external surface of the or each conductor 32 are coated with a thermal insulating material. Therefore, only the inner surface of the second electrode 24 (ie, that facing towards the first electrode 22) transfers heat to the gas passing through the conduit 12. The second electrode 24 is heated to a temperature substantially greater than the temperature of the gas stream by heat transfer between the heat source 30 and the electrode 24.

The operation of the apparatus 10 will now be described. A gas stream in which particles are entrained and from which the particles are to be removed, for example the exhaust of an internal combustion engine, is introduced to the particle concentrating apparatus 10 via the inlet pipe 16. Prior to its introduction to the apparatus 10, a charge is imparted to the particles. This can achieved by introducing ions from an ion source (not shown) into the inlet pipe 16 and mixing the ions with the gas. Alternatively, the charge may be imparted by means of a Corona discharge electrode located in or upstream of the inlet pipe 16. The charge imparted to the particles has the same polarity as the charge applied to the first electrode 22 relative to the second electrode 24 so that, as the gas stream passes along the length of the conduit 12, the particles are urged away from the first electrode 22 and towards the second electrode 24 by means of the electrostatic field created between the electrodes 22,24.

Since the second electrode 24 is heated to a temperature significantly higher than that of the gas stream, thermophoretic effects simultaneously repel any particles entrained within the gas stream and located close to the second electrode 24 away therefrom so as to prevent any particles which are approaching the second electrode from impinging thereupon. A particle-free zone 40 is therefore maintained immediately adjacent the second electrode 24 (see Figure lc). Due to the combined effects of the electrostatic field and the thermophoretic repulsion, the particles entrained within the gas stream therefore become concentrated into a zone close to the second electrode 24 but spaced away from the inner surface thereof by the width of the particle-free zone 40.

As the gas stream passes along the conduit 12, the particles are concentrated within a zone of decreasing volume and therefore the concentration of particles within that zone increases. As the concentration of particles increases, random collisions between particles may cause agglomeration and there will then be a natural increase in the average size of the particles contained within the zone.

At the downstream end of the conduit 12, the zone of concentrated particles can be "skimmed"off from the area immediately adjacent the second electrode 24 by way of the outlet mouth 26a and fed via the outlet conduit 26b to the separator 28. The separator 28 is of known design and separates the agglomerated particles from the gas stream. If the particles have been agglomerated before separation, the efficiency of the separator 28 may be higher than might otherwise be expected and a higher proportion of the agglomerated particles are reliably separated from the gas stream. Furthermore, if the separator 28 is required to extract only particles of a certain minimum diameter from the gas stream and/or to extract particles from a relatively low-volume airflow, then the size of the separator 28 can be reduced below the size which might otherwise have been required. After separation within the separator 28, the cleaned gas stream is then returned to the outlet pipe 20 of the apparatus 10 for exhaustion to the atmosphere.

The separated particles remain contained within the separator 28 and can be disposed of in an appropriate manner at a convenient time. The cleaned gas stream could alternatively be returned to the inlet 16 of the apparatus 10 so that the cleaned gas stream is sent through the conduit 12 again in case any particles remain entrained.

It has been mentioned that a preferred application of apparatus of this type is for use in separating particles from the exhaust of an internal combustion engine, particularly a diesel engine. The exhaust manifold of the diesel engine provides a good source of heat for heating the second electrode 24. If desired, the exhaust gases from the internal combustion engine can be fed directly through the second electrode 24 in order to supply the necessary heat source before being channelled away from the apparatus, allowed to cool and then returned to the inlet pipe 16 so that the particles entrained therein can be concentrated, agglomerated and then separated. Such an arrangement would be highly advantageous in that the particulate concentration within the final emissions from the engine would be greatly reduced. The apparatus 10 could also operate as a silencer which would advantageously obviate the need for a traditional silencer.

A second embodiment of the apparatus according to the invention is shown in Figures 2a and 2b. The conduit 112 of the apparatus 110 and its inlet and outlet pipes 116,120 are the same as those shown in Figure la. However, the first electrode 22 of the first embodiment has been replaced by a pair of plate electrodes 122a, 122b which are arranged on either side of the second electrode 124. The second electrode 124 is a plate electrode arranged along the longitudinal axis 125 of the conduit 112. The pair of first electrodes 122a, 122b are negatively charged with respect to the second electrode 124 which is itself earthed. The second electrode 124 is again connected via a conductor 132 to a heat source 130 as in the first embodiment. However none of the external surfaces of the second electrode 124 are thermally insulated so that all surfaces of the second electrode 124 transfer heat to the gas. In this second embodiment, the outlet mouth 126a, which leads to the separator 128 via the outlet conduit 126b, is arranged to collect gas and concentrated particles from either side of the second electrode 124.

The operation of the apparatus 110 of the second embodiment are similar to that of the first embodiment. Charged particles are introduced into the conduit 112 via the inlet 116 and caused to pass between the electrodes. The electrostatic field created between the electrodes causes the particles to be urged away from the first electrodes 122a, 122b towards the central, second electrode 124. The particles are therefore concentrated within a relatively small portion of the gas stream adjacent the second electrode 124.

However, due to heating of the second electrode 124, the particles are prevented from impinging upon the surfaces of the second electrode 124 by thermophoretic effects.

The zone in which the particles are concentrated, and then agglomerated as before, therefore remains spaced by a slight distance from the second electrode 124. The portion of the gas stream in which the particles are concentrated is then passed to the separator 128 whereupon the agglomerated particles are separated from the gas stream.

Clean gas is exhausted to the atmosphere through the first outlet pipe 120.

A third embodiment of the invention is illustrated in Figure 2c, which shows an alternative cross-section through the apparatus shown in Figure 2a. In the third embodiment, the conduit 212 is circular in cross-section and the first electrode 222 is cyclindrical in shape whilst the second electrode 224 comprises a rod lying along the axis 225 of the conduit 212. As before, the first electrode 222 is negatively charged whilst the second electrode 224 is earthed. The only other difference between the third embodiment and the second embodiment is the shape of the outlet mouth 226a which, instead of having a rectangular shape extending across the width of the conduit 112 as shown in Figure 2b, has a circular shape in Figure 2c. The diameter of the mouth 226a is a little larger than the diameter of the second electrode 224 in this third embodiment.

The operation of the apparatus according to the third embodiment is very similar to that of the second embodiment described above. However, the charged particles are urged by the electrostatic effects towards the centre of the conduit 212 where they are prevented from impinging upon the second electrode 224 by thermophoretic effects.

This embodiment is advantageous over the embodiment described above with reference to Figures 2a and 2b because the zone of the gas stream in which the particles are concentrated is annular instead of being elongate and this increases the ease with which the outlet mouth 226a can be constructed. A fourth embodiment of apparatus according to the invention is shown in Figure 3. The apparatus 310 is similar to that shown in Figure 2c except that the first electrode 222 of Figure 2c has been replaced with a plurality of annular rings 322 arranged coaxially around a central rod electrode 324. The central electrode 324 corresponds to the second electrode of Figure 2c and is made from a thermally conductive material and lies along the longitudinal axis 325 of the conduit 312. The annular rings 322 are fixedly mounted on the central electrode 324 by struts 334. The mounting struts 334 are made from a thermally and electrically insulating material. Each successive ring 322, seen in the direction of flow, has a slightly smaller diameter than the preceding ring. The smallest ring has a diameter which is only slightly larger than the diameter of the central electrode 324. A small annular gap 336 is formed between each pair of adjacent rings 322. Downstream of the smallest ring, there is provided a cylindrical wall 338 which leads to the outlet conduit 326 and to the separator 328. The length of each ring 322, seen along the longitudinal axis 325 of the conduit 312, is the same as all of the other rings 322. In this way, a"ladder"of annular rings is formed in the general shape of a cone with small annular gaps spaced therealong.

As in the second embodiment, the central electrode 334 is earthed and also connected to a heat source 330 by way of thermal conductors 332. A relatively high electrical potential is applied to the annular rings 322 so that the rings are negatively or positively charged with respect to the central electrode 324. An electrostatic field is thereby created between the annular rings 322 and the central electrode 324. A constant electrostatic field can be created by applying different potential differences to each annular ring 322 with respect to the central electrode 324. As in the third embodiment, charged particles entrained within the gas stream are introduced into the conduit 312 via the inlet 316. The charged particles carry charge of the same polarity as that of the annular rings 322 with respect to the central electrode so that the particles are urged by electrostatic forces away from each annular ring 322 and towards the central electrode 324. The particles are moved away from each annular ring 322 to a sufficient extent that, by the time the gas stream reaches the next annular ring, the particles do not pass through the annular gap 336 to the area of the conduit 312 outside the rings. Therefore, the particles are progressively concentrated within the annular rings 322 in an area having a decreasing diameter around the central electrode 324. Since the central electrode 324 is heated to a temperature significantly greater than that of the gas stream, the particles are thermophoretically repelled away from the surface of the central electrode 324 and therefore do not impinge or collect on the surface of the central electrode 324.

The clean gas stream outside the annular rings is exhausted to the atmosphere via the outlet pipe 320 whereas the particle-containing gas stream passes to the separator 328 via the cylindrical wall 338 and the outlet conduit 326. The particles are then separated from the gas stream by the separator 328.

It will be appreciated that the arrangement of the first electrode as a series of annular rings 322 can be varied without altering the effect achieved. One alternative arrangement would be to provide pairs of opposing plates arranged symmetrically about the longitudinal axis of the conduit 312 with small gaps therebet electrodes is not regarded as essential to the invention; all that is required is that an electrostatic field is created through which the gas stream in which the particles are entrained is forced to pass. The particles must be charged but the means by which they are give a charge is not material to the invention. What is required is a means of applying a repulsive force to the particles in a direction which prevents them from impinging upon the electrode towards which they are urged, when they approach that electrode. Thermophoretic and ultrasonic means of achieving this effect have been disclosed. The aim of the invention is to achieve concentration of originally dispersed particles within a small area of the gas stream in which the particles are entrained whilst minimising the risk of the particles being retained within the apparatus during the concentration process. The concentration process allows separation apparatus to operate more efficiently or else to be reduced in size. It will be understood that the term"concentration"is intended to include concentration of a specific type of particle within the gas stream (such as particles of a particular mass, size or mobility) so as to effect classification of the particles within the gas stream.

Claims

CLAIMS: 1. Apparatus for concentrating unipolar-charged, gasborne particles in a portion of a gas stream in which the particles are entrained, comprising a conduit for conducting the gas stream between an inlet and an outlet, first and second electrodes arranged within the conduit, and means for creating an electrostatic field between the first and second electrodes so that, in use, the particles are urged towards either the first or the second electrode by electrostatic effects, characterised in that the apparatus further comprises repulsion means for causing the particles to be repelled by non-electrostatic effects away from the electrode towards which they are urged by the electrostatic effects.
2. Apparatus according to claim 1, wherein the repulsion means comprise a heating device connected to the electrode towards which the particles are urged by the electrostatic effects so that, in use, the particles are repelled away from the said electrode by thermophoretic effects.
3. Apparatus according to claim 2, wherein the heating device is an electric heater.
4. Apparatus according to claim 2, wherein the heating device is an exhaust manifold of an internal combustion engine.
5. Apparatus according to claim 2, wherein the heating device is a conduit arranged to carry the exhaust gas of an internal combustion engine.
6. Apparatus according to any one of claims 2 to 5, wherein the electrode towards which the particles are urged by the electrostatic effects is thermally insulated on the side thereof facing away from the other electrode.
7. Apparatus according to claim 1, wherein the repulsion means comprise means for causing the electrode towards which the particles are urged by the electrostatic effects to vibrate at an ultrasonic frequency, so that, in use, the particles are repelled away from the said electrode by ultrasonic effects.
8. Apparatus according to any one of the preceding claims, wherein the first electrode comprises at least one annular ring and the second electrode lies coaxially with the first electrode.
9. Apparatus according to claim 8, wherein the first electrode comprises a series of annular rings decreasing in diameter with distance from the inlet.
10. Apparatus according to any one of claims 1 to 8, wherein the first electrode comprises a series of annular rings increasing in diameter with distance from the inlet.
11. Apparatus according to any one of claims 8 to 10, wherein the second electrode comprises a rod.
12. Apparatus according to any one of claims 1 to 7, wherein the first and second electrodes comprise a pair of plate electrodes.
13. Apparatus according to claim 12, wherein the first electrode comprises a pair of plate electrodes arranged on either side of the second electrode.
14. Apparatus according to any one of claims 1 to 7, wherein the first electrode comprises a series of plates arranged so as to be offset from one another in the direction of flow of the gas through the conduit, and the second electrode comprises a single plate which extends alongside the first electrode.
15. Apparatus according to claim 14, wherein the first electrode comprises a second set of plates, the second set of plates being arranged to converge towards the said series of plates, the second electrode being arranged between the first and second sets of plates.
16. Apparatus according to claim 14, wherein the first electrode comprises a second set of plates, the second set of plates being arranged to diverge away from the said series of plates, the second electrode being arranged between the first and second sets of plates.
17. Apparatus according to claim 15 or 16, wherein the first and second sets of plates are arranged symmetrically.
18. Apparatus according to any one of claims 8 to 17, wherein each annular ring or plate has the same axial length as an adjacent annular ring or plate.
19. Apparatus according to any one of the preceding claims, wherein, in use, the electrostatic field has a substantially constant strength along the length of the conduit between the first and second electrodes.
20. Apparatus according to any one of the preceding claims, wherein the apparatus further comprises means for imparting an electric charge to the gasborne particles at a point upstream of the first and second electrodes.
21. Apparatus according to claim 20, wherein the means for imparting the electric charge comprises means for generating ions and means for introducing the ions into the conduit.
22. Apparatus according to any one of the preceding claims, wherein a separator device is located downstream of the first and second electrodes.
23. Apparatus according to claim 22, wherein the separator device is a cyclonic separator.
24. Apparatus for concentrating unipolar-charged, gasbome particles in a portion of a gas stream substantially as hereinbefore described with reference to any one of the accompanying drawings.
25. A method of concentrating unipolar-charged, gasborne particles in a portion of a gas stream in which the particles are entrained, comprising the steps of creating an electrostatic field between two electrodes and passing the charged particles through the electrostatic field so as to urge the charged particles towards one of the electrodes, characterised in that the method further comprises the step of causing the particles to be repelled by non-electrostatic effects away from the electrode towards which the particles are urged by electrostatic means.
26. A method according to claim 25, wherein the electrode towards which the particles are urged by electrostatic means is heated so that the particles are repelled away therefrom by thermophoretic effects.
27. A method according to claim 25, wherein the electrode towards which the particles are urged by electrostatic means is caused to vibrate at an ultrasonic frequency so that the particles are repelled away therefrom by ultrasonic effects.
28. A method of concentrating unipolar-charged, gasbome particles in a portion of a gas stream in which the particles are entrained substantially as hereinbefore described with reference to any one of the accompanying drawings.