GB2339398A

GB2339398A - Concentration of gas-borne particles utilising thermophoretic effects

Info

Publication number: GB2339398A
Application number: GB9903501A
Authority: GB
Inventors: Andrew Ian Briggs
Original assignee: Notetry Ltd
Current assignee: Dyson Technology Ltd
Priority date: 1998-07-10
Filing date: 1999-02-17
Publication date: 2000-01-26
Anticipated expiration: 2019-02-17
Also published as: GB2339398B; GB9903501D0

Description

2339398 Apparatus and Method 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 invention also relates to a method for concentrating gasborne particles in a portion of the gas stream in which the particles are entrained. The apparatus and method are particularly, but not exclusively, suitable for use in the separation of particulates from the exhaust of an internal combustion engine.

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.

The thermophoretic principle has been known for many years. Particles in a gas stream tend to move from hotter zones to colder zones within the body of the gas or from the hot gas to 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 sensit ive 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 "wir e and plate" type in which a heated wire is placed near to a cold plate or between two cold plates.

2 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 and this 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 then-nophoretic 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 3 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.

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.

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. 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. It is a further object of the invention to provide apparatus which is suitable for cleaning or facilitating the cleaning of an airflow through a vacuum cleaner making use of thermophoretic, effects. It is a still a further object of the present 4 invention to provide apparatus which reduces the emission of particles into the atmosphere and a method of reducing such emdssions.

The invention provides apparatus for concentrating gasbome 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, and a plurality of particle-repelling areas, the particle-repelling areas being arranged sequentially within the conduit to define a boundary between a particle-containing zone and a particle-free zone, the boundary incorporating gaps between the particle-repelling areas to allow particlefree gas to pass from the particle-containing zone to the particle-free zone, characterised in that the particle-repelling areas are connected to a heating device so that, in use, the particles are repelled away from the particle-repelling areas and away from the gaps therebetween by thermophoretic effects.

The apparatus according to the invention makes use of thermophoretic forces to repel particles or particulates entrained within the gas stream away from the boundary and thus maintains the particles in the particlecontaining zone whilst particle-free gas is allowed to pass between the particle-repelling areas into the particle-free zone. The effect is a relocation of the particles within the cross-section of the gas stream and a gradually increasing concentration of the particles in a predefined zone within the gas stream. An increased concentration of the particles can result in agglomeration of the particles due to random collision. With sufficient increase in concentration, the particles leaving the apparatus may be considerably larger in size than they were when they entered. Separation of such enlarged particles by any suitable means, such as centrifugal separation, is therefore easier and more reliable. The fact that deposition onto a cold plate is not required means that no cold boundary layer has to be traversed which reduces the energy required to concentrate the particles.

Thermophoretic forces are used in order to repel the particles away from the particlerepelling elements which are 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 internal combustion engine.

An advantageous feature of the apparatus is the inclusion of means for imparting a swirling motion to the gas stream upstream of the boundary. This allows centrifugal forces to enhance the concentration of the particles in the particle-containing zone when the said zone is located radially outwardly of the particle-free zone. The swirling motion can be imparted to the gas stream by way of stationary or rotating vanes, or by way of a tangential inlet.

The invention also provides a method of concentrating gasbome particles in a portion of a gas stream in which the particles are entrained utilising apparatus as described above, comprising the steps of introducing a gas stream with the particles entrained therein through the inlet of the conduit to the particle-containing zone, passing the gas stream across the particle-repelling areas so that the particles are repelled away therefrom, allowing particle-free gas to pass through the gaps between the particle-repelling areas and into the particle-free zone and maintaining the particles in the particle-containing zone, wherein the particle-repelling areas are heated so as to repel the particles away therefrom and away from the gaps therebetween by thermophoretic effects.

The ability to progressively concentrate particles within a particlecontaining zone of the gas stream is highly advantageous. If the concentration is high enough, natural agglomeration can take place simply and effectively. Separation by way of, for example, a centrifugal separator then results in a higher efficiency of the separator. The use of a series of particle-repelling areas, each area being required merely to repel adjacent particles sufficiently far away therefrom in order to maintain the particle within the particle-containing zone, means that the residence time of the gas stream within the apparatus as a whole is not excessive.

6 Further, 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 I is schematic sectional view through a first embodiment of apparatus according to the invention; Figure 2a is a perspective view of three of the particle-repelling elements forming part of the apparatus of Figure 1; Figure 2b is a view similar to Figure 2a showing an alternative configuration of the three particle-repelling elements of Figure 2a; Figure 3 is a schematic illustration of the operation of the apparatus of Figure 1; Figure 4 illustrates an altemative inlet portion to that shown in Figure 1; Figure 5 is a schematic illustration showing a second embodiment of apparatus according to the invention attached to the exhaust manifold of an internal combustion engine; Figure 6a is a schematic view of a first alternative boundary forming part of apparatus according to the invention; Figure 6b is a schematic view of a second alternative boundary fomiing part of apparatus according to the invention; Figure 6c is a schematic view of a third alternative boundary forming part of apparatus according to the invention; 7 Figure 7a is a schematic sectional view, similar to Figure 1, of a third embodiment of apparatus according to the invention; and Figure 7b is a cross-section taken along line VII-VII of Figure 7a.

Apparatus according to a first embodiment of the invention is shown and illustrated in Figure 1. The apparatus 10 for concentrating particles within a portion of a gas stream essentially comprises a cylindrical conduit 12 having a frusto-conical upstream end 12a connected directly to an inlet 14 which is in the form of a pipe or conduit. The pipe or conduit leads directly or indirectly from a source of the gas from which particles are to be removed. The apparatus 10 has an outlet 16 at the end of the conduit 12 remote from the inlet 14. The diameter of the outlet 16 is-similar to that of the inlet 14. The interior of the conduit 12 is divided into a particle-containing zone 18a and a particle-free zone l8b by means of a boundary, generally designated 20. The boundary 20 has an upstream portion 20a and a downstream portion 20b. The upstream portion 20a comprises a series of annular rings 22 arranged in a generally conical manner as will be described in more detail below. The downstream portion 20b comprises a solid cylindrical wall 24 which is arranged downstream of the annular rings 22. The boundary 20 is arranged so as to be coaxial with the cylindrical conduit 12 so that the particle- containing zone 18a and the particle-free zone 18b are also coaxial with the conduit 12. A first outlet path 16a leads from the particle- containing zone 18a downstream of the cylindrical wall 24 to the outlet 16 via a separator device 23. A second outlet path l6b leads directly from the particle-free zone 18b to the outlet 16.

The annular rings 22 and the cylindrical wall 24 are each fixedly mounted on a central rod 28 which is made from a thermally conducting material and lies along the longitudinal axis 26 (see Figure 2) of the conduit 12. The central rod 28 has a finely tapered or sharpened upstream end 28a but is otherwise circular in cross-section. The central rod 28 extends along the entire length of the conduit 12 and supports the annular rings 22 and the cylindrical wall 24 by means of mounting struts 30. For the sake of 8 clarity, only three mounting struts are illustrated in Figure 1. However, it will be understood that each annular ring 22 and the cylindrical wall 24 will be supported by a plurality of mounting struts arranged preferably symmetrically about the central rod 28 and rigidly holding the annular ring 22 and the cylindrical wall 24 in position. The mounting struts 30 are made from a thermally conducting material.

The specific arrangement of the annular rings 22 will now be described in more detail. As can be seen from Figure 1, each successive ring, seen in the direction of flow, has a slightly larger outer diameter than the preceding ring. The smallest ring has an outer diameter which is slightly larger than the diameter of the central rod 28. A small annular gap or channel is formed between each pair of adjacent rings and the leading and trailing edges of each ring are chamfered as illustrated in Figure 3. The difference between the outer diameters of adjacent rings is between 0.4 and 3.6mm. so that adjacent rings are offset from one another by between 0.2 and 1.8mm. The leading edge of each ring is either flush with or arranged to overlap very slightly with the trailing edge of the preceding ring and the length of each ring, seen along the longitudinal axis 26, is the same. In this way, a "ladder" of annular rings is formed in the general shape of a cone with small annular gaps spaced therealong. It is preferred that the gaps are dimensioned so that the proximity of one ring to another will not disrupt the boundary layer which will inevitably develop across the surface of the preceding ring to any significant extent. Furthermore, the length of each ring 22 is selected so that the boundary layer formed at the surface is maintained along the length of each ring and does not break away before reaching the next gap. It is preferred that the size of the gaps between adjacent rings decreases with distance from the inlet 14. However, the gaps can also be equal in their dimensions, with the offset between adjacent pairs of rings being approximately 0.5mm, say between 0.4 and 0.7mm.

As can be seen from Figure 1, the shape of both the particle-containing zone 18a and the particle-free zone l8b is annular at any appropriate point along the length of the conduit 12. In the case of the particlecontaining zone 18a, the outer diameter of the zone remains constant along the cylindrical portion of the conduit 12 whilst the inner 9 diameter increases with the diameter of the boundary 20. The cross- sectional area of the particle-containing zone 18a therefore decreases along the length of the conduit 12. In contrast, the particle-free zone l8b has a constant inner diameter but an increasing outer diameter. The cross-sectional area of the particle-free zone 18b therefore increases along the length of the conduit 12 as the diameter of the boundary 20 increases. The area of each of the particle-containing zone 18a and the particle-free zone l8b remains constant when the boundary 20 is formed by the cylindrical wall 24 instead of the annular rings 22.

The central rod 28 is connected to a heat source 32. The heat source 32 can be any appropriate heat source, but the exhaust manifold of an internal combustion engine, preferably a diesel engine, is regarded as an appropriate heat source. Thermal conductors 34 in the form of rods connect the heat source 32 to the central rod 28. The mounting struts 30 are in thermal connection with the central rod 28 and with the annular rings 22 or the cylindrical wall 24 as appropriate. Thus, heat from the heat source 32 can pass down the conductors 34 and along the rod 28 to the mounting struts 30 and from there to the annular rings 22. The external surfaces of the conductors 34 and the mounting struts 30 are coated with a thermal insulating material, as are the inward-facing surfaces of the annular rings 22 and the cylindrical wall 24 and the surface of the central rod 28 inside the boundary 20. The outward-facing surfaces of the annular rings 22, the cylindrical wall 24 and the upstream end 28a of the central rod 28 are not thermally insulated so as to allow only these specific areas to transfer significant amounts of heat energy to the gas and to enable them to reach a higher temperature than would be achievable if all the surfaces of the conductors 34, the central rod 28, the mounting struts 30 and the annular rings 22 were able to conduct heat to the gas.

Located in the frusto-conical upstream portion 12a of the conduit 12 are a plurality of vanes 36. These vanes are arranged and designed to impart a swirling motion to a gas stream entering the conduit 12 via the inlet 14. The vanes 36 can be fixed or rotary as desired and various different arrangements and configurations will be apparent to a skilled reader. An alternative way of imparting a swirling motion to a gas stream entering the conduit 12 is to provide a tangential inlet to the conduit as illustrated in Figure 4. In this configuration, the inlet 14' is connected directly to an inlet pipe or conduit 15' connected directly or indirectly to the gas source as before. However, in this case, the pipe or conduit 15' is arranged to deliver the gas stream to the inlet 14' in a tangential manner, thus setting up a swirling motion in the frusto-conical portion l2a' of the apparatus.

The operation of the apparatus 10 will now be described with reference to Figures 2 and 3. 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 14. A swirling motion is imparted to the gas stream, either by way of a tangential inlet or by the vanes 36. The gas stream is also slowed considerably as it passes along the frusto-conical portion 12a and the crosssectional area of the conduit 12 increases. This allows a cyclonic swirling motion to be established without there being too much skin friction generated between the gas and the outer wall. As the gas stream passes the upstream end 28a of the central rod 28, which is heated by way of the heat source 32 to a temperature of more than 200'C above the temperature of the gas stream, thermophoresis effects repel any particles entrained within the gas stream and located close to the central rod 28 away therefrom. The slowing of the gas stream is beneficial here because the residence time of the gas stream in the vicinity of the heated upstream end 28a of the central rod 28a is increased, thus allowing time for the entrained particles to be repelled to a greater distance than might otherwise have been the case. An essentially particle-free area is created very close to the upstream end 28a of the central rod 28. By the time the gas stream approaches the first annular ring 22, the portion of the gas stream adjacent the central rod 28 is free of particles because the particles have all been repelled sufficiently far away to remain outside the first annular ring 22 of the boundary 20. Particle-free gas passes between the central rod 28 and the first annular ring 22. The sharpened or chamfered upstream edge of the annular ring 22 helps to maintain a laminar flow past the annular ring 22, reduces the resistance presented by the ring to the gas flow and also reduces the 11 pressure drop caused by the presence of the ring. Thermophoretically propelled particles are able to move more predictably across a laminar flow than across a turbulent flow and therefore a laminar flow is preferred.

It is preferred if the upstream end 28a of the central rod 28 is heated to a temperature which is higher than that of the annular rings 22 in order to ensure that the thermophoretic process is brought into effect.

As the gas stream passes further along the conduit 12, a similar effect occurs at each annular ring 22 because the temperature of the annular ring is significantly higher than the temperature of the gas stream; of the order of 200C higher. Thermophoretic effects repel the particles away from the annular rings across the laminar flow so that, by the time each annular ring is reached, all of the particles entrained within the gas stream are sufficiently far away from the previous annular ring to ensure that only particle-free gas passes through the annular gap between the annular rings. Also, because the particle-free gas entering the particlefree zone has passed between two heated annular rings, it will be hotter than the gas stream outside the boundary and this will enhance the thermophoretic effect. As the gas stream passes along the conduit 12, the particulates are therefore maintained in an area which decreases in volume. The concentration of particles within the particle-containing zone 18a therefore increases as the particles are "walked" up the "ladder". As the concentration of particles increases in the particle- containing zone 18a, random collisions may cause agglomeration of the particles and there can be a natural increase in the average size of the particles contained within the zone. If more larger particles appear in the particle-containing zone 18a, the rate of agglomeration will increase and the effect is that of a cascade. Furthermore, because of the swirling motion applied to the gas stream at the upstream end of the conduit 12, centrifugal effects can also encourage the particles to approach the wall of the conduit 12. This can enhance the agglomeration process. As agglomeration continues, the centrifugal effects will be more effective due to the increase in the average particle size. It does not matter if the gas stream inside the boundary is 12 relatively turbulent because the gas stream inside the boundary has little or no effect on the swirling action outside the boundary.

An additional benefit occurs when the gas stream passing through the conduit 12 is the exhaust of an internal combustion engine which may include soot particles with hydrocarbons condensed thereon. The hydrocarbons on the side of the particles facing the annular rings may evaporate when they become exposed to the hot surfaces of the annular rings which causes the soot particles to move away from the hot surfaces. Furthermore, the particles themselves will absorb heat from the hot rings and then in turn radiate heat to the gas molecules closest to the hot side of each particle. This will increase the kinetic energy of the closest gas molecules which will then cause the particle to be buffeted in the direction of the colder side of the particle.

The particles present in the downstream portion of the particlecontaining zone 18a are passed, together with the relevant portion of the gas stream, to a separator device 23 of any suitable known design which, acting in the normal manner, separates the particles from the gas stream. If the particles have been agglomerated before separation, the efficiency of the separator device 23 may be higher than might otherwise have been expected, particularly if the separator device 23 is a cyclonic separator, and a relatively high proportion of the particles are extracted from the gas stream. Furthermore, if the separator device 23 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 device 23 can be reduced below the size which might otherwise have been required. After separation within the separator device 23, the cleaned gas stream is then returned to the outlet 16 of the apparatus 10 for exhaustion to the atmosphere. The separated particles remain contained within the separator device 23 and can be disposed of in an appropriate manner at a convenient time. The cleaned gas stream could also be returned to the inlet of the apparatus so that the cleaned gas stream is sent though the conduit again in case any particles remain entrained.

C' 13 It has been mentioned that a preferred application of apparatus of this type is 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 central rod 28 and the annular rings 22. If desired, the exhaust gases from the internal combustion engine can be fed directly through the central rod 28 in order to supply the necessary heat source before being channelled away from the apparatus, allowed to cool and then returned to the inlet 14 so that the particles entrained therein can be concentrated 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 could also operate as a silencer which would advantageously obviate the need for an additional silencer.

One way of putting such an arrangement into effect is illustrated in Figure 5. The gas stream from the exhaust manifold 50 leaves the manifold at a high temperature, perhaps of the order of 600'C. The exhaust gas is then led via an inlet conduit 54 to a hollow conducting conduit 52 (which corresponds to the central rod 28 in the apparatus shown in Figure 1) made of a thermally conducting material, eg metal, and which has an enlarged diameter compared to the inlet conduit 54. The exhaust gas is therefore slowed so that heat transfer can take place between the hot gas stream and the conduit 52. The temperature of the conduit 52 is maintained at around 400C. The gas stream is then led out of the conduit 52 and passed through a set of cooling fins 56 in order to drop the temperature of the gasstream to around 200C. It is then introduced to the inlet of the thermal trap 60 (equivalent to the apparatus 10 shown in Figure 1 above) which surrounds the conduit 52. Annular rings 62 are mounted on the conduit 52 by thermally conducting struts 63 so that the rings 62 are at a temperature significantly higher than that of the gas stream as it enters the trap 60. Concentration of the particles in the particle-containing zone 64a takes place as described above and the gas stream containing the particles is passed to a separator device 66 for removal of the particles from the gas stream. Particle-free gas is exhausted from the particle-free zone 64b via the outlet 68. Cleaned gas from the separator device 66 is also exhausted via the outlet 14 68. The exhaust gas enters the atmosphere at a temperature similar to that of existing exhaust systems.

This arrangement is advantageous over the arrangement illustrated in Figure 1 because the heat transfer from the heat source (manifold 50) is more direct which will result in fewer losses. The heat transfer can of course be radiative as well as conductive. The increased diameter of the conduit 52 within which the annular rings 62 are located means that a larger surface area is presented to a slower moving gas stream and the thermophoretic effects are enhanced. Alternatively, by providing an expanded conduit 52, a larger volume of gas can be passed through the trap 60 without increasing the velocity in comparison to that of a trap having a smaller conduit 52. This increases the capacity of the trap 60.

It will be appreciated that, when the above-described apparatus is used in conjunction with an internal combustion engine, there will be no ready heat source when the engine is started from cold. In order to bring the thermal trap into immediate effect, the central rod 28 or conduit 52 could be heated initially by a vehicle battery or other electrical source until the engine is sufficiently warm to provide the required thermal energy.

The device just described is also capable of functioning as a catalytic converter. In order to achieve this, the annular rings 22, 62 can be coated on their interior surfaces with a catalyst material suitable for converting noxious gaseous elements of the exhaust gas into relatively harmless substances in a known manner. The presence of the catalyst material will convert noxious gaseous elements prior to emission into the atmosphere. It is also envisaged that the heat energy expended during the conversion will assist in maintaining the annular rings at the desired temperature. Furthermore, any ozone created during the operation of the electrostatic aspects of the device may assist in the gas reactions occurring during catalytic conversion.

It will be appreciated that the exact construction described above can be varied without departing from the scope of the invention. Specifically, it is envisaged that the shape and arrangement of the particle-repelling elements can be varied to suit different arrangements and needs. An alternative arrangement to the annular rings 22 is illustrated in Figure 2b. In this arrangement, square or rectangular collars 22' instead of rings are arTanged symmetrically about the longitudinal axis 26 with small gaps arranged therebetween. The conduit can then have a square or rectangular crosssection. It is alternatively possible to arrange for pairs of opposing plates to be supported on the sides of a square or rectangular conduit. The operation of the apparatus would then be the same as described above, although the provision of vanes or a tangential inlet to impart a swirling motion to the gas stream is not appropriate and will be dispensed with.

A further alternative embodiment is illustrated in Figure 6a. In this arrangement, the boundary 120 is formed by a series of fins 124 which are arranged in an elongate manner about the longitudinal axis 126 and increase in width as the distance from the inlet 114 increases to form a conical shape. Also, linear gaps are formed between the fins 124 and these also increase in width with distance from the inlet 114. Within the gaps, a wire mesh 125 is arranged, the wire mesh 125 and fins 124 being thermally connected. This construction can be achieved by producing a cone of wire mesh and affixing to it, in a thermally conducting manner, the fins 124. The fins 124 converge at an upstream end 128a and the upstream end 128a and the fins 124 are directly connected to an electrical heater 132 which provides heat to the fins 124 and wire mesh 125. Vanes 136 are arranged upstream of the upstream end 128a in order to impart a swirling motion to the gas stream as described above. The downstream outlet paths and separator device are as illustrated in Figure 1.

The embodiment illustrated in Figure 6a operates in a manner similar to that illustrated in Figure 1. The gas stream entering the inlet 114 is caused to rotate about the longitudinal axis 126 and, as it swirls in a helical manner about the fins 124, particles entrained within the gas stream are repelled away from the fins 124. Particle-free gas is allowed to pass between the fins 124 into the particle-free zone 11 8b inside the boundary 120 whilst the particles remain in the particle-containing zone 11 8a. When 16 the gap between the fins 124 is sufficiently large to allow particles to travel between the fins 124, the mesh 125, which must be present at this stage, prevents the particles from entering the particle-free zone 11 8b by means of further thermophoretic effects.

Another alternative embodiment is similar to Figure 6a but omits the fins 124. The boundary is then formed purely from a conical mesh 140 which is heated to repel particles but will allow particle-free gas to pass between the strands or wires of the mesh and enter the particle-free zone within the boundary. The boundary 140 is illustrated in Figure 6b. The interior of the boundary 140 could be thermally insulated.

A further alternative embodiment is illustrated in Figure 6c in which the boundary comprises a solid cone 150 of thermally conducting material, eg metal, in which gaps or slits 152 are formed over substantially the whole surface thereof. The gaps 152 effectively separate the exterior surface of the cone 150 into a plurality of particlerepelling areas which repel particles as the gas stream passes across the surface. Particle-free gas is allowed to pass through the gaps into the interior. The interior surface of the cone 150 can be thermally insulated and the vanes 154 are prefer-red but not essential.

A further alternative embodiment of the invention is illustrated schematically in Figures 7a and 7b. The apparatus 300 comprises a generally cylindrical conduit 312 having an inlet 314 and a downstream end 315 which tapers inwardly in a frusto-conical manner to merge with an outlet 316. The conduit 312 is thermally connected to a heat source such as an electric heater 332. A series of annular, thermally-conductive rings 322 is located inside the conduit 312, each ring 322 being supported on the conduit 312 by one or more supporting struts 330. The conduit 312 and the supporting struts 330 are coated with a thermally insulating material. The series of annular rings 322 defines a boundary 320 between a particle-containing zone 318a and a particle-free zone 318b. The annular rings 322 are arranged in a generally conical manner such that each successive ring 322, seen in the direction of flow, has a slightly smaller diameter than the preceding ring 322. Between each pair of successive rings 322, there is an annular gap which, as 17 before, can remain constant or else decrease in size with distance from the inlet 314. The rings 322 are arranged coaxially with the conduit 312. The innermost ring, which is furthest downstream of the inlet of the conduit 312, leads into a feed pipe 324 leading to a separator device 323. The outermost ring, which is furthest upstream and closest to the inlet, is joined to the wall of the conduit 312 by means of a flange 322a so that gas entering the conduit 312 via the inlet 314 may not pass between the wall of the conduit 312 and the outermost ring 322.

The annular rings 322 are maintained at a high temperature compared to the temperature of the gas stream from which the particles are to be separated. The entrained particles pass along the conduit 312 through the centre of the annular rings 322. Any particles which approach one of the annular rings 322 are repelled away from the respective ling 322 as the gas stream passes. A particle-free portion of the gas stream is thereby created in the immediate vicinity of each annular ring 322. The particlefree area adjacent each annular ring 322 is sufficiently large, by the time the gas stream reaches the next annular ring 322, for only particlefree gas to pass through the annular gap between the adjacent rings 322 into the particle-free zone 318b. The portion of the gas stream in which particles remain entrained therefore remains within the particlecontaining zone 318a inside the annular rings 322.

As the gas stream passes along the conduit 312 and through the successive annular rings 322, the area of the particle-containing zone 318a reduces and, as a result, the particles become more concentrated and, if the concentration is high enough, agglomeration may occur. Eventually, after the final ring 322 has been passed by the gas stream, the gas from the particle-containing zone 318a passes along the feed pipe 324 to the separator 323. Clean air is exhausted from the separator 323 whilst the separated particles are collected within the separator 323. At the same time, particle-free gas passes out into the atmosphere from the particlefree zone 318b via the outlet 316.

It may be advantageous to use liquid injection and condensation methods in combination with the apparatus and methods described above to further increase the size 18 of particles required to be separated from a gas stream. Since thermophoretic effects are used, liquid injection and condensation are best used downstream of the concentration apparatus to further grow the agglomerated particles so that the efficiency of the downstream separator is enhanced. If the liquid were to be injected upstream of the concentration apparatus, the heat of the apparatus may re-evaporate the condensed liquid surrounding the particulates and negate the effect. Injection directly into the concentration apparatus itself may also affect the cyclone action if a swirling motion has been imparted to the gas stream, or else the particles may be pushed back towards the annular rings or plates.

19

Claims

Claims:

1. Apparatus for concentrating 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, and a plurality of particle-repelling areas, the particlerepelling areas being arTanged sequentially within the conduit to define a boundary between a particlecontaining zone and a particle-free zone, the boundary incorporating gaps between the particle-repelling areas to allow particle-free gas to pass from the particle-containing zone to the particle-free zone, characterised in that the particlerepelling areas are connected to a heating device so that, in use, the particles are repelled away from the particle-repelling areas and away from the gaps therebetween by thermophoretic effects.

2. Apparatus as claimed in claim 1, wherein the boundary is generally conical or frusto-conical in shape.

I Apparatus as claimed in claim I or 2, wherein the conduit has a longitudinal axis and the boundary diverges away from the longitudinal axis as the distance from the inlet increases.

4. Apparatus as claimed in any one of claims 1 to 3, wherein the conduit has an interior wall and the boundary approaches the interior wall as the distance from the inlet increases.

5. Apparatus as claimed in any one of the preceding claims, wherein the particlerepelling areas each form part of a single particle-repelling element.

6. Apparatus as claimed in any one of claims 1 to 4, wherein the particlerepelling areas are arranged on a plurality of particle-repelling elements.

7. Apparatus as claimed in claim 6, wherein the particle-repelling elements comprise a series of annular rings increasing in diameter with distance from the inlet.

8. Apparatus as claimed in claim 7, wherein the annular rings are arranged coaxially with one another.

9. Apparatus as claimed in claim 6, wherein the particle-repelling elements comprise a series of plates arranged so as to be offset from one another in the direction of flow of the gas through the conduit.

10. Apparatus as claimed in claim 9, wherein a second set of plates is provided, the second set of plates being arranged to diverge away from the said series of plates.

11. Apparatus as claimed in claim 10, wherein the first and second sets of plates are arTanged symmetrically.

12. Apparatus as claimed in any one of claims 7 to 11, wherein each annular ring or plate has the same length, seen in the axial direction.

13. Apparatus as claimed in any one of claims 6 to 12, wherein the upstream edge of each particle-repelling element is chamfered or sharpened to reduce turbulence of the gas stream.

14. Apparatus as claimed in any one of claims 6 to 13, wherein the particle-repellingelements are arranged to overlap in the axial direction.

15. Apparatus as claimed in any one of the preceding claims, wherein each pair of adjacent particle-repelling areas is offset by a distance of between 0.2mm and 1.8mm.

16. Apparatus as claimed in claim 15, wherein each pair of adjacent particlerepelling areas is offset by a distance of substantially 0.5mm.

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17. Apparatus as claimed in claim 15, wherein each pair of adjacent particlerepelling areas is offset by a distance which decreases with distance from the inlet.

18. Apparatus as claimed in claim 6, wherein the particle-repelling elements comprise elongate fins spaced around a longitudinal axis of the conduit and diverging away from the said axis as the distance from the inlet increases.

19. Apparatus as claimed in claim 18, wherein the gaps between the fins increase in size as the distance from the inlet increases.

20. Apparatus as claimed in claim 18 or 19, wherein a wire mesh is arranged in the gaps between the fins.

21. Apparatus as claimed in claim 6, wherein the particle-repelling elements comprise the elements of a wire mesh.

22. Apparatus as claimed in any one of the preceding claims, wherein the conduit has an interior wall which is generally cylindrical.

23. Apparatus as claimed in any one of the preceding claims, wherein the conduit has an interior wall which is generally conical or frusto-conical.

24. Apparatus as claimed in any one of the preceding claims, wherein the heating device is an electric heater.

25. Apparatus as claimed in any one of claims I to 23, wherein the heating device is the exhaust manifold of an internal combustion engine.

26. Apparatus as claimed in any one of claims 1 to 23, wherein the heating device is a conduit arranged to carry the exhaust gas of an internal combustion engine.

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27. Apparatus as claimed in any one of the preceding claims, wherein a separator device is located downstream of the boundary between the particle-containing zone and the particle-free zone.

28. Apparatus as claimed in claim 27, wherein the separator device is a cyclonic separator.

29. Apparatus as claimed in any one of the preceding claims, wherein means are provided, upstream of the boundary, for imparting a swirling motion to the gas stream.

30. Apparatus as claimed in claim 29, wherein the means for imparting a swirling motion to the gas stream comprise at least one vane.

31. Apparatus as claimed in claim 30, wherein the or each vane is stationary.

32. Apparatus as claimed in claim 29, wherein the means for imparting a swirling motion to the gas stream comprise a tangential inlet.

33. Apparatus as claimed in any one of the preceding claims, wherein one side of each of the particle-repelling areas is thermally insulated.

34. Apparatus as claimed in claim 33, wherein the innermost side of each of the particle-repelling elements is thermally insulated.

35. Apparatus as claimed in any one of the preceding claims, wherein the crosssectional area of the particle-containing zone decreases with distance from the inlet.

36. Apparatus for concentrating gasbome particles in a portion of a gas stream substantially as hereinbefore described with reference to any one of the embodiments shown in Figures I to 6 of the accompanying drawings.

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37. A method of concentrating gasborne particles in a portion of a gas stream in which the particles are entrained utilising apparatus according to any one of the preceding claims, comprising the steps of introducing a gas stream with the particles entrained therein through the inlet of the conduit to the particle-containing zone, passing the gas stream across the particle-repelling areas so that the particles are repelled sequentially away therefrom, allowing particle-free gas to pass through the gaps between the particle-repelling areas and into the particle-free zone and maintaining the particles in the particle-containing zone, wherein the particle-repelling areas are heated so as to repel the particles away therefrom and away from the gaps therebetween by thermophoretic effects.

38. A method as claimed in claim 37, wherein a swirling motion is imparted to the gas stream upstream of the boundary.

39. A method as claimed in claim 37 or 38, wherein the particle-repelling areas are heated to a temperature of more that IOOT above the temperature of the gas stream.

40. A method as claimed in claim 39, wherein the particle-repelling elements are heated to a temperature of more that 200C above the temperature of the gas stream.

41. A method as claimed in any one of claims 37 to 40, wherein the particlerepelling areas are heated by transfer of heat from the exhaust manifold of an internal combustion engine.

42. A method as claimed in any one of claims 37 to 41, wherein the gas and particles within the particle-containing zone are delivered to a separator.

43. A method as claimed in claim 42, wherein the particles delivered to the separator are separated from the gas stream by centrifugal separation.

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44. A method as claimed in any one of claims 37 to 43, characterised in that a laminar boundary layer formed at the surface of each particlerepelling area is not disrupted as it passes through the gap into the particle-free zone.

45. A method substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings.