GB2436195A

GB2436195A - Atomiser chamber with flow return baffle

Info

Publication number: GB2436195A
Application number: GB0602261A
Authority: GB
Inventors: Graham Staniforth; Chuanjie Zhou
Original assignee: Novel Technical Solutions Ltd
Current assignee: Novel Technical Solutions Ltd
Priority date: 2006-02-03
Filing date: 2006-02-03
Publication date: 2007-09-19
Also published as: GB0602261D0

Abstract

An atomiser features an internal baffle 33. This may prevent undesirable flows 31 of entrained material from re-entering the atomisation zone 35, and thus interfering with the atomisation process, resulting in non uniform or wrongly shaped or sized particles from forming. Also disclosed is an associated apparatus in which it is further specified that this baffle 33 limits the migration of particles from a colder void zone T3, to a warmer atomisation zone T1. Also disclosed is a method in which a baffle is not specified.

Description

1 2436195

ATOMISATION CHAMBER

Field of the invention

This invention relates to an atomisation chamber for atomising liquid media to produce a powder, in particular -but not exclusively -a polymer powder.

Back2round to the Invention Such powders are comprised of many individual particles, and for many applications it is highly desirable if the method used to produce a given powder can be controlled so that the particle morphology, shape, size and size distribution of the resultant powder is appropriate for a given application that the powder has been produced for.

From an application point of view, powder size and powder morphology influence the engineering properties (for example the flowability, packability, and compressibility) of the powder and as such it is important for these characteristics to be carefully controlled so that the powder is actually suitable for the particular application it is intended for.

From a manufacturing point of view, the particle size distribution of the powder is important because it indicates what proportion of a given initial powder product may actually be useful for a particular application. For example, if an initial powder product has a particle size distribution of from 1 micron to 1000 microns, and a given application requires the powder particles to be no smaller than 10 microns and no larger than 30 microns, then it is likely that only 2% of the initial powder product will be of use for that application. As the proportion of uscable material directly affects the commercial viability of any given powder production business, it is desirable for the yield to be as high as possible so that the level of waste is reduced.

From the application point of view, it would be desirable if a powder could be produced that consisted of particles which are of substantially the same morphology, shape and size, and from the manufacturing point of view it would be desirable for the proportion of "useful" particles (i.e. those particles which meet the morphology, size and shape requirements of a given application) to comprise a large proportion of the powder product.

In one conventional method of powder production, extruded polymer pellets are ground (typically under cryogenic conditions) to form an initial "first grind" powder that is then subjected to additional grinding steps until the final resulting powder product is generally of a predetermined size that is suitable for a given application.

Unfortunately, whilst it is possible to control the powder particle size in a grinding process, it tends to be difficult to control the particle shape as the nature of the grinding process tends to produce powder particles with irregular shapes. It is also the case that as the powder particle morphology is reliant to a large degree on the morphology of the initial pellets, it is difficult to ensure that all of the particles (or at least all of the useful particles) have the same or similar morphology unless the homogeneity of the pellets is checked before grinding commences. A further problem is that grinding processes necessarily raise the possibility of contaminating the powder product, either with contaminants from the immediate environment or the grinding equipment used to grind the pellets to produce the powder.

One previously proposed alternative to grinding extruded polymer pellets (for the production of a polymer powder) is the atomisation of a liquid media that comprises the polymer, for example a liquid consisting of the polymer in molten form. In this atomisation process, high velocity (typically supersonic) gas jets are directed at a stream of liquid, and the impingement of the gas on the liquid stream causes the stream to be broken up into a fine mist of particles that are subsequently * I allowed to solidify to thereby provide the desired powder. US Patent No. 5,228, 620 discloses one example of a method and apparatus for atomizing liquid media.

Whilst the arrangement disclosed in this US Patent, for example, has proven to be effective in producing regular homogeneous particles, it has been observed that relatively minor environmental variations in the vicinity of the region where atomisation takes place can have an undesirable effect upon the shape, homogeneity and size of the resulting powder particles. For example, it has been noted that when a liquid polymer is undergoing atomisation in order to produce a polymer powder, temperature variations in the vicinity of the zone in which atomisation is occurring can cause the generally spherical homogeneous particles that might usually be produced to change into longer strands of material, generally referred to as "whiskers", that are unsuitable for many applications.

Atomisation, as is known in the art, is accomplished within an atomisation chamber, and it is usual to provide a decreasing temperature gradient within the chamber (in a direction away from the liquid stream nozzle) so that the particles of the atomised liquid stream can solidify into a powder. Necessarily the ambient temperature in the vicinity of the nozzle needs to be kept relatively high to ensure that the material to be atomised appears at the nozzle in liquid form.

However, the contradictory requirements for a relatively high ambient temperature in the vicinity of the nozzle and a decreasing temperature gradient within the atomisation chamber have caused problems in the manufacture of homogenous, uniformly sized and shaped particles. The present invention has been devised with the aim of mitigating such problems.

Statement of Invention

To this end a presently preferred embodiment of the present invention provides an atomisation chamber comprising a peripheral wall that defines an internal void, and a baffle positioned within the void to define a plurality of regions within the void. In a particularly preferred embodiment, the invention provides an atomisation chamber for use in the atomisation of a stream of liquid material by an atomising jet that is arranged to impinge on the stream in an atomisation zone of the chamber to thereby atomise the stream into droplets that are carried from the zone by the jet, the chamber comprising a peripheral wall that defines an internal void; and a baffle arranged in the void to define a region of the void that includes the atomisation zone and that is warmer in use than another colder region of the void; wherein the baffle is configured to allow the jet to carry said droplets from the warmer region to the colder region and to limit the passage of material from said colder region to said warmer region to thereby limit entrainment of said material in the liquid stream and said jet in said atomisation zone.

This arrangement has helped mitigate against the problem of temperature variation within the atomisation zone. Our postulation, without wishing to be limited to this explanation, is that this arrangement is useful because it limits entrainment of colder material (for example air) in the liquid stream that would otherwise cause the temperature of that stream to vary (and hence the properties of the powder particles to change).

Entrainment of gases in high velocity jet streams is not a new phenomena, but in the field of liquid stream atomisation (in particular, but not exclusively, of liquid polymer atomisation) we believe that it has hitherto never previously been appreciated that the entrainment of relatively colder material (e.g. air) in the gas stream may affect the temperature of the atomisation zone and as a consequence the properties of the powder produced by the atomisation process. Entrainment is particularly, but not exclusively, a problem in the field of liquid polymer atomisation as polymer materials tend to be less good at retaining heat and as such relatively minor local variations in a I temperature can adversely affect the temperature of the stream and the resulting powder particle shape and morphology. By implementing the teachings of the present invention such variations can be avoided, or at least reduced to the point where they are less of a concern.

In one arrangement the baffle may extend from an inner wall of said chamber to divide said chamber into said warmer and colder regions. Alternatively, the baffle may be suspended from a top wall of the said chamber to divide said chamber into said warmer and colder regions.

The baffle may comprise an annular member having an aperture configured to permit the egress of said atomised stream from said warmer region to said colder region. The baffle may also comprise a shroud in the vicinity of said aperture for inhibiting the ingress of material, particularly gaseous material, from said colder region into said warmer region during operation of said chamber.

The shroud may comprise a side wall extending from a circumferential region of said aperture. The side wall may extend to form an inner chamber. An end of said inner chamber distal from said warmer region may narrow to form a neck region and open to form a further inner chamber.

The neck region may be configured to permit the egress of material from said inner chamber into said further inner chamber and to inhibit the ingress of material into said inner chamber from said further inner chamber.

The inner chamber further comprises a further baffle configured to extend from a region of an inner side wall of said inner chamber distal from said warmer region and into said colder region in a direction generally towards said warmer region to form an aperture. The further baffle may be formed of an open-ended truncated cone having its major circumference extending from the inner side wall of said inner chamber into said colder region. b

Preferably the inner chamber comprises a fluid inlet in a region proximal said neck.

In particularly preferred embodiment the chamber comprises series of inner chambers each successive inner chamber coupled to a preceding inner chamber by a neck region configured in accordance with said neck region described herein.

In this arrangement respective fluid inlets proximal a neck region for successive inner chambers may be configured to permit the ingress of successively greater amounts of fluid into respective inner chambers.

The chamber may comprise a nozzle with a plurality of bores opening to said warmer region, at least one of said bores comprising a passageway for liquid material ingress, and at least one other of said bores comprising a passageway for said jet. The at least one jet bore may be configured to accelerate the jet passing therethrough. The at least one jet bore may comprise one or more resonator cavities configured to oscillate said jet, preferably at an ultrasonic frequency.

Another aspect of the present invention provides a method of providing warmer and colder regions in an atomisation chamber, the method comprising inhibiting entrainment of material from the colder region in an atomisation zone of the warmer region during operation of the chamber. The method may include forming one or more distinct zones in said colder region, and optionally cooling said one or more distinct zones to successively lower temperatures. As a further option, the method may include heating said warmer region.

In accordance with another embodiment of the invention, there is provided an atomisation chamber, the chamber comprising a baffle arranged in said chamber to inhibit entrainment of gas in the chamber thereby to provide a first region of a first temperature and a second region of a second temperature different to said first temperature during operation of said chamber and to permit the egress of an atomised stream from said first region to said second region. Yet another embodiment provides a chamber for atomising a liquid stream, comprising a liquid delivery nozzle for ejecting a liquid, a gas stream nozzle for directing a gas stream jet at a liquid stream exiting the liquid delivery nozzle to break the liquid stream up into droplets, said chamber further comprising a baffle configured to permit the egress of droplets from an atomisation region where the droplets are formed to a particle formation zone and to inhibit the ingress of material from said particle formation zone into said atomisation zone.

Yet further embodiments of the present invention, and particularly preferred features of those embodiments are set out in the claims, and elsewhere in the present application.

Brief Description of the Drawin2s

Presently preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figs. I and 2 are schematic representations of a previously proposed atomisation system; Fig. 3 is a schematic representation of an atomisation chamber in accordance with a first embodiment of the present invention; Figs. 4(a) to 4(d) are schematic representations of baffle configurations; Fig 5 is a schematic representation of an atomisation chamber in accordance with a second embodiment of the present invention; Fig. 6 is a schematic representation of an atomisation chamber in accordance with a third embodiment of the present invention; Fig. 7 is a schematic representation of an atomisation chamber in accordance with a tburth embodiment of the present invention; Fig. 8 is a schematic representation of an atomisation chamber in accordance with a fifth embodiment of the present invcntion; and Fig. 9 is a schematic representation of an atomisation chamber in accordance with a sixth embodiment of the present invention.

Detailed Description of Preferred Embodiments

Preferred embodiments of the present invention will now be described with particular reference to the atomisation of polymer materials. However, it will be appreciated that the teachings of the present invention are applicable to the atomisation of many different types of materials, and as such that the following description should not be interpreted as limiting the scope of the present invention only to the atomisation of polymer materials. With the above in mind certain presently preferred embodiments of the invention will now be described by way of illustrative example only.

Fig. 1 is a schematic representation of an atomisation system 1. The system comprises an atomisation chamber 3 with an internal void 5, and a nozzle 7 located in one end of the chamber 3.

In this example, solid polymer material is input to a heated extruder (or other equivalent heated supply device) 11 via a hopper 13. The polymer material is heated to beyond its melting point as it passes through the extruder 11, and the resulting molten polymer forms a liquid stream 15 which enters the chamber void 5 via a first bore 9 of the nozzle 7. In alternative arrangements, the polymer material may be supplied to the system in a liquid form, in which case the hopper and extruder may not need to be provided.

The nozzle 7 includes, in this example, two gas bores 17 through which pressurised gas streams are supplied from respective reservoirs 21 to the internal void of the chamber 3. In the preferred arrangement, the reservoirs are pressurised (for example at a pressure of 2 to 16 bar), and the nozzle bores 17 are shaped (in a known manner) to accelerate the gas streams from the reservoirs so that the streams emerge from the nozzle as high velocity gas jets 19, typically travelling at supersonic speeds.

The jets 19 may be pulsed, for example by forming a series of resonator cavities, such as so-called Hartmann shock tubes, in the gas bores 17 of the nozzle 7, and the resonator cavities may be configured to impose an ultrasonic frequency -for example within the range of 20 to 80 kHz -on each of the jets.

The gas bores are arranged to direct the gas jets 19 to impinge on the liquid stream 15 in an atomisation zone 23 of the void to break-up the liquid stream 15 up into a plurality of droplets that are then carried by the gas jets from the atomisation zone and into the chamber void. There may be, as shown, more than one gas jet directed towards the liquid stream 15, and the nozzle bores from which the jets emerge may be symmetrically arranged around the nozzle 7. In the preferred arrangement, gas heaters 25 are provided in the gas stream supply so that the gas jets are at a roughly comparable temperature to that of the liquid stream.

Following atomisation, the high velocity gas jets carry the resulting polymer droplets from the atomisation zone into the void of the chamber, and as the droplets move from the relatively warm atomisation zone to the relatively colder void, the droplets solidify to from substantially spherical powder particles which may be extracted from the base 27 of the chamber, for example by means of a cyclone extractor 29.

The liquid stream nozzle bore 9 may be a simple tube (as depicted), or it may comprise a plurality of adjacent tubes that are arranged to deliver a plurality of liquid streams for atomisation. As a further alternative, the liquid stream nozzle bore 9 may be configured to deliver a sheet of liquid for atomisation. It is, of course, preferred to atomise as much of the liquid stream as possible (ideally the whole of the liquid stream), and to this end gas jets may be arranged in a regular pattern around the nozzle so that the emerging gas jets impinge on all parts of the emerging liquid stream. The nozzle may also be vibrated.

In theory, the atomisation system functions as follows: the liquid stream and gas jets are heated to a temperature TI that exceeds the melt temperature Tm of the polymer material, and the jets and stream, at or near temperature TI, emerge from the nozzle and the stream is atomised in the atomisation zone whilst it is at or near to temperature T 1. As the droplets formed by the atomisation of the liquid stream by the jets are carried by the jets into the void, the ambient void temperature reduces to a temperature T2 (that is less than Ti) and the droplets begin to solidify so that a powder forms in the base of the chamber in a region that is at a temperature T3 which is colder than the region at temperature T2.

Whilst this is how the system functions in theory, the applicant has noted that what actually tends to happen in practice is that the injection of high velocity gas jets into the chamber causes entrainment to occur, and this entrainment creates a flow path 31, depicted in Fig. 2, that tends to draw material (for example gas and/or powder) from the colder region towards the base of the chamber into the atomisation zone 23 and ultimately into the gas jets and liquid stream.

As this material is at a substantially colder temperature than the atomisation zone, the entrainment of the material in the jets and liquid stream causes Tl (the temperature of the jets and liquid stream in the atomisation zone) to decrease towards the melt temperature Tm of the polymer. As the temperature Ti decreases towards Tm, so the liquid properties of the polymer stream change, and this change affects the size and/or morphology of the powder particles. In particular, it would appear that the polymer liquid stream becomes less liquid as Ti approaches Tm, and this reduction in liquidity tends to cause the formation of undesirable irregular polymer particles, for example polymer whiskers (i.e. relatively long strands of material), instead of desirable substantially spherical particles.

In order to address this problem, it is proposed (as depicted in Fig. 3) to arrange a baffle 33 within the void to provide a physical barrier to the aforementioned flow path and thereby avoid, or at least reduce, the entrainment of material in the jet or liquid stream in the atomisation zone and the associated cooling of the atomisation zone that, as explained above, would appear to at least contribute to the formation of undesirable particles.

As depicted the baffle 33 has an aperture 35 substantially collinear with the nozzle 7 SO that the atomised particles can be carried from the atomisation zone into the chamber as before. The baffle defines a warmer region 37 of the void in the vicinity of the nozzle that includes the atomisation zone 23, and in the particular embodiment depicted in Fig. 3; this region is supplied with air (or other gas) substantially at temperature TI, that is pumped into the region via a gas heater 41 by a pump 39. The baffle and supplied warm air inhibit cooler material (e.g. air) from a colder region 43 on the other side of the baffle from entering the atomisation zone thereby avoiding unwanted cooling of the zone and the associated effect that that cooling has on the particle shape and/or morphology.

The baffle aperture edge may be as depicted in Fig. 3, or in alternative embodiments, it may have any configuration that is suitable for inhibiting the ingress of material into the vicinity of the atomisation zone yet permitting the egress of atomised particles therethrough. Non-limiting examples of different baffle aperture edge configurations are illustrated in Figs. 4(a) to 4(c). For example, figure 4(a) illustrates a baffle in which the baffle aperture edge includes a side wall 45 that extends both above and below the baffle 33, whilst in Figs. 4(b) and 4(c) the side wall extends oniy above and only below the baffle, respectively. The aperture edge walls may be generally perpendicular to the baffle, as depicted, or may instead may be at any angle to the baffle (as shown schematically in outline in Figs. 4(a) to 4(c)).

As will be apparent to persons skilled in the art, the provision of a baffle cannot prevcnt the phenomena of entrainment by high velocity gas jets, but it is anticipated that the provision of a baffle will change the location in the void where such entrainment occurs. In particular, by providing a baffle entrainment may occur outside of the atomisation zone on the side of the baffle in which the colder region is located, or it may be that the warmer air supplied to the warmer region (and flowing out of the warmer region with the jets and atomised particles) will act as a warm air curtain between cooler material circulating in the chamber and the droplets of atomised material. In either case it is anticipated that entry of cooler circulating material into the atomisation zone will be limited.

A second embodiment of the present invention is illustrated in Fig. 5. In this embodiment the side wall 45 has been extended to form an inner chamber 47 which comprises a neck region formed of the side walls 45, the neck region opening up to form a cavity 49 and then narrowing in a second neck region 51 to form an exit aperture.

This embodiment has been conceived with the aim of limiting entrainment of colder material in the jets and liquid stream both in the atomisation zone and in the region of the void where the droplets solidify to form a powder. Such an arrangement may prove useful for those materials where it is desired to carefully control the rate at which the droplets solidify.

A third embodiment of the present invention is depicted in Fig. 6. This embodiment is similar to that of Fig. 5 except that in this case an upper part 53 of inner chamber 47 is provided with a plurality of inlets 55 that permit cooler material (for example cooler gases) circulating in the chamber to enter into the inner chamber 47. This arrangement allows the inner chamber to be maintained at a cooler temperature than the warmer region 37 in the vicinity of the nozzle, and effectively provides for maintaining a plurality (in this instance three) discrete temperature zones -namely the region 37 proximate the nozzle, the inner chamber 47 and the region outside of the inner chamber.

In a fourth embodiment, illustrated in Fig. 7, a further inner chamber 59 extends from inner chamber 47 and has substantially the same shape. Again, in the upper regions of inner chamber 59 inlets 55 are provided to allow material circulating within the chamber to enter into the further inner chamber 59. This further cools chamber 59 (in comparison to chamber 47) thereby providing a sequence of regions of decreasing temperature which assists in the formation and solidifying of particles to form a powder.

A fifth embodiment is illustrated in figure 8. In this embodiment at the lower end of inner chamber 47 a narrowed exit aperture is formed by upwardly and inwardly extending side walls 61. This arrangement has the advantage that any particulate material not directed through the exit aperture is directed away from the exit aperture towards the sides of the inner chamber 47, and hence avoids the possibility that a build up of particulate material could accumulate, and ultimately block, the exit aperture. Although not depicted in Fig. 8, a plurality of inlets (as previously described) may be provided in the upper part 53 of the inner chamber if desired. Fig. 9 is a sixth embodiment similar to that of the fifth embodiment, where the upwardly and inwardly extending side walls 61 additionally function to divide an inner chamber 47 from a further inner chamber 59.

In light of the foregoing, it will be appreciated that the teachings of the present invention provide a means whereby entrainment of cooler material in the liquid stream and/or gas jets in the atomisation zone can be limited, and a means whereby problems associated with local cooling caused by entrainment can be mitigated. It will also be appreciated that the principle of extending the baffle to provide one, two or more inner chambers allows for the creation of better defined temperature zones within the chamber to enhance the controlled solidification of droplets to form a powder.

Whilst several preferred embodiments have been described in detail herein, it should be noted that these arrangements are merely illustrative and that modifications may be made to the particular arrangements described without departing from the scope of the present invention.

For example, whilst the baffle is depicted as being generally U-shaped and hung from a top wall of the chamber, it will be appreciated that the baffle could readily be affixed to either side of the chamber instead of being hung from the top wall. It is also the case that the principle of providing upwardly and inwardly extending walls defining an exit aperture could be applied to one or both of the chambers depicted in the embodiment of Fig. 7, and furthermore that in a modification of the embodiment depicted in Fig. 7 the inlets to inner chamber 47 may be omitted (thereby providing inlets only in the upper part of further chamber 59).

As a further modification, it will be appreciated that instead of supplying heated air to the warmer region above the baffle it may instead be possible to provide heaters in that region and supply air at ambient temperature to the region.

It should also be noted that whilst certain combinations of features have been explicitly enumerated in the accompanying claims, the scope of the invention is not limited to those combinations and instead extends to encompass any combination of features herein described irrespective of whether or not that particular combination is explicitly recited in the accompanying claims.

Claims

CLAIMS: I. An atomisation chamber for use in the atomisation of a

stream of liquid material by an atomising jet that is arranged to impinge on the stream in an atomisation zone of the chamber to thereby atomise the stream into droplets that are carried from the zone by the jet, the chamber comprising a peripheral wall that defines an internal void; and a baffle arranged in the void to define a region of the void that includes the atomisation zone and that is warmer in use than another colder region of the void; wherein the baffle is configured to allow the jet to carry said droplets from the warmer region to the colder region and to limit the passage of material from said colder region to said warmer region to thereby limit entrainment of said material in the liquid stream and said jet in said atomisation zone.

2. A chamber according to Claim 1, wherein said baffle extends from an inner wall of said chamber to divide said chamber into said warmer and colder regions.

3. A chamber according to Claim 2, wherein said baffle is suspended from a top wall of the said chamber to divide said chamber into said warmer and colder regions.

4. A chamber according to claim 1 or 2, wherein said baffle comprises an annular member having an aperture configured to permit the egress of said atomised stream from said wanner region to said colder region.

5. A chamber according to claim 4, wherein said baffle comprises a shroud in the vicinity of said aperture for inhibiting the ingress of material, particularly gaseous material, from said colder region into said warmer region during operation of said chamber.

6. A chamber according to claim 5 wherein said shroud comprises a side wall extending from a circumferential region of said aperture.

7. A chamber according to claim 6, wherein said side wall extends to form an inner chamber.

8. A chamber according to claim 7, wherein an end of said inner chamber distal from said warmer region narrows to form a neck region and opens to form a further inner chamber.

9. A chamber according to claim 8, wherein said neck region is configured to permit the egress of material from said inner chamber into said further inner chamber and to inhibit the ingress of material into said inner chamber from said further inner chamber.

10. A chamber according to any of claims 7 to 9, wherein said inner chamber further comprises a further baffle configured to extend from a region of an inner side wall of said inner chamber distal from said warmer region and into said colder region in a direction generally towards said warmer region to form an aperture.

11. A chamber according to claim 10, wherein said further baffle is formed of an open-ended truncated cone having its major circumference extending from the inner side wall of said inner chamber into said colder region.

12. A chamber according to claim any one of claims 8 to ii, wherein said inner chamber comprises a fluid inlet in a region proximal said neck.

13. A chamber according to any one of claims 7 to 12, comprising a series of inner chambers each successive inner chamber coupled to a preceding inner chamber by a neck region configured in accordance with said neck region of any one of claims 8 to 12.

14. A chamber according to claim 13, wherein respective fluid inlets proximal a neck region for successive inner chambers are configured to permit the ingress of successively greater amounts of fluid into respective inner chambers.

15. A chamber according to any preceding claim, further comprising a heating arrangement for heating said warmer region.

16. A chamber according to claim 15, wherein said heating arrangement comprises one or more heating elements disposed in or adjacent said warmer region.

17. A chamber according to claim 16, wherein said heating arrangement comprises one or more conduits for supplying a heated fluid to said warmer region.

18. A chamber according to claim 17, wherein said heated fluid is a gas.

19. A chamber according to claim 18, wherein said gas is air.

20. A chamber according to any preceding claim, comprising a nozzle with a * plurality of bores opening to said warmer region, at least one of said bores comprising a passageway for liquid material ingress, and at least one other of said bores comprising a passageway for said jet.

21. A chamber according to Claim 20, wherein said at least one jet bore is configured to accelerate the jet passing therethrough.

22. A chamber according to Claim 20 or 21, wherein said at least one jet bore comprises one or more resonator cavities configured to oscillate said jet, preferably at an ultrasonic frequency.

23. A chamber according to any of claims 20 to 22, comprising means for vibrating said nozzle.

24. An atomisation chamber comprising a peripheral wall that defines an internal void, and a baffle positioned within the void to define a plurality of regions within the void.

25. A method of providing warmer and colder regions in an atomisation chamber, the method comprising inhibiting entrainment of material from the colder region in an atomisation zone of the warmer region during operation of the chamber.

26. A method according to claim 25, further comprising forming one or more distinct zones in said colder region.

27. A method according to claim 26, further comprising cooling said one or more distinct zones to successively lower temperatures.

28. A method according to any one of claims 25 to 27, further comprising heating said warmer region.