ELECTROSTATIC ATOMISER

Abstract

The present invention relates to (with reference to FIG. 1) an electrostatic fluid atomiser which has a fluid inlet (2) and a fluid channel (3) leading fluid to a wall (6) in which orifices (100) are formed. Fluid passing out of the orifices (100) is atomised into droplets which fall into a first set of larger droplets (12) and a second set of smaller droplets (13). Two charging electrodes (4,6) charge fluid passing through the channel (3). A droplet separator (200) has a separator electrode (16) which applies an electrical force on the droplets and deflects the smaller droplets (13) to he collected by a droplet collector (15) while the larger droplets (12) continue undeflected out of a droplet (201) of the atomiser.

Description

The invention relates to electrostatic atomisers which may have a wide variety of applications, particularly in the fields of drying, coating and mixing, where, despite a need for large flow rates, it is very important that the drops are of a consistent size, i.e. their diameters fall within a selected range of diameters.

The closest prior art known is the applicant's own prior published patent application, PCT/GB2004/000458. This prior published patent application discloses the use of multiple orifice atomisers with a primary charging electrode spanning an array of orifices. With constant fluid flow such electrostatic atomisers produce two distinctly differently sized sets of droplets. Due to the physics of droplet formation each pair of larger sized droplets are separated by a smaller droplet. For example, the smaller sized droplets may have diameters between 20 and 80 microns, whilst the larger sized droplets may have diameters between 150 and 350 microns. Due to such differences in size, the smaller droplets will have much lower mass whilst having much higher specific electrical charge than the larger droplets. Since in many applications it is desirable to obtain fluid droplets of a consistent size, i.e. within a selected range of diameters, there is a need for an electrostatic atomiser which acts to separate larger droplets from smaller droplets.

In a first aspect, the present invention provides an electrostatic fluid-atomiser, comprising: a fluid inlet; one or more orifices out of which fluid emerges in an atomised form which comprises at least a first set of droplets of comparable size to each other and second set of droplets of comparable size to each other and of a smaller size to the droplets of the first set; a fluid channel connecting the fluid inlet to the orifice(s); and at least two charging electrodes for applying a charge to fluid passing through the fluid channel; wherein the electrostatic atomiser comprises a fluid droplet separator downstream of the orifices having a separator electrode which is either earthed or electrically charged and which applies an electrical force on the droplets which deflects the second set of smaller droplets to a droplet collector of the atomiser while allowing the first set of larger droplets to continue out of a droplet outlet of the atomiser.

This configuration is advantageous because it allows the electrostatic fluid atomiser to output only fluid droplets of sizes falling within a selected range.

The droplet collector can comprise an absorbent wall or a porous wall. The wall can be cylindrical and disposed surrounding the droplets as they leave the orifices. The fluid droplet connector can be connected to a fluid return line. The fluid return line can easily avoid crossing the path of the orifices when the droplet collector comprises a cylindrical wall surrounding the droplets emerging from the orifices.

The fluid return line returns the collected droplets to the fluid input. This configuration maximises the efficiency of the electrostatic atomiser; there is no waste or only minimal waste.

The orifices may comprise multiple orifices in close proximity to a substantially flat surface of one of the charging electrodes which spans the multiple orifices. This configuration will provide an ideal combination of evenly spread atomisation across multiple orifices and efficient collection of a second set of droplets.

The orifices can be angled to generate a converging or a diverging stream of atomised droplets. In certain applications, generating directed streams will allow improved collection by the droplet collector, when compared to the use of parallel straight orifices.

The orifices are preferably provided in an orifice wall and the droplet collector can then be provided with a plurality of portions projecting from the orifice wall between the orifices in a downstream direction. This will allow improved collection of small droplets within a central portion of an array of orifices as well as maintaining excellent small droplet collection from the outer portions of the ejected jet of fluid.

The separator electrode of the fluid droplet separator could be electrically connected to one of said charging electrodes.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view, partly in cross-section, of a first embodiment of an electrostatic fluid atomiser according to the present invention with components external to the atomiser shown schematically;

FIGS. 2a and 2b show alternative geometries of orifices of the atomiser of FIG. 1 (or of the atomiser of FIGS. 3 and 4);

FIG. 3 shows in cross-section a second embodiment of electrostatic fluid atomiser according to the present invention, with components external to the atomiser again shown schematically; and

FIG. 4 shows a third embodiment of electrostatic fluid atomiser according to the present invention, with components external to the atomiser once more shown schematically.

FIG. 1 shows an electrostatic atomiser 1 with a fluid inlet 2. A fluid channel 3 connects the fluid inlet 3 to an array of multiple orifices 100 provided in an orifice wall 6. Fluid passing through the channel 3 is electrostatically charged by a pair of charging electrodes comprising an electrode 4 and the orifice wall 5. In the figure it can be seen that a high voltage source 18 is connected to apply a charge of a first polarity to the charging electrode 4 and to apply a charge of the opposite polarity to the wall 6. The electrode 4 has a tip 5 which is substantially flat and spans the array of multiple orifices 100. The fluid, which may be of the any kind of chargeable fluid, is pumped into the fluid inlet by a pump 7. Prior to pumping, the fluid is filtered by a filter 8.

On emerging from the orifices 6 the fluid is atomised, initially forming so-called “ligament” jets 9, 10 and 11, which shortly thereafter break up into substantially two sets of droplets which differ in size, a first set comprising droplets such as droplet 12 (a relatively large droplet) and a second set comprising droplets such as droplet 13 (a relatively small droplet). The small droplets will each inevitably have a lower mass with a much higher specific electrical charge than the large droplets.

The atomiser 1 is provided with a fluid droplet separator 200 for separating the set of small-sized droplets (e.g. 12) from the set of large-sized droplets (e.g. 13), so that only the large-sized droplets leave a fluid outlet 201 of the atomiser. The separator 200 has a cylindrical wall 16 which surrounds the droplets leaving the orifices 100. The wall 16 functions as a separator electrode and is either appropriately charged or earthed, in order to attract the droplets. In the figure the wall 16 is connected to the high voltage source and charged with the same polarity as the wall 6. The smaller droplets are attracted to a greater extent than the larger droplets. Consequently, the smaller droplets are deflected to the wall 16 whilst the larger droplets will continue largely undeflected and exit the fluid outlet 201. The separator 200 has a layer 14 of a porous absorbent material located radially inwardly of the wall 16. The wall 16 has a downstream portion which extends around a downstream end of the porous layer 14, the wall 16 extending radially inwardly to form a fluid channel 15 at the downstream end of layer 14.

Collected fluid is withdrawn from channel 15 by a scavenge pump 17. The scavenge pump 17 draws the fluid from the droplet collector 100 and relays the fluid to an accumulator tank 101. Fluid from the tank 101 is then filtered and pumped back into the fluid inlet 2.

The voltage applied to the charging electrodes 4, 6 and to the wall 16 (which functions as the separator electrode) and the sizes of orifices 100 may be adjusted in accordance with the properties of the fluid used, the flow rate chosen and the desired output droplet sizes.

In the FIG. 1 embodiment the wall 6 is provided with multiple straight parallel orifices 100. An alternative geometry of orifices is shown in FIGS. 2a and 2b. FIG. 2a shows a diverging array of orifices 110 which would provide diverging streams of droplets. FIG. 2b shows a converging array of orifices 111 which would provide converging streams of droplets. The use of the orifices 110, 111 of FIGS. 3a or 3b may be preferred for certain fluids.

FIG. 3 shows a second embodiment of electrostatic atomiser 19. This embodiment shares many components in common with the first embodiment of FIG. 1 and identical components are given identical reference numerals. The atomiser has two charging electrodes 26, 27. They are both connected to the high voltage source 18 and a voltage of a first polarity is applied to electrode 26 and a voltage of a second opposite polarity is applied to the electrode formed by orifice wall 27. The orifice wall 27 has a number of orifices referenced 20, 21, 22, 23. Fluid passing through orifices 20, 21, 22, 23 is atomised and forms droplets in two sets, one set of smaller-sized droplets and one set of larger-sized droplets. The droplets all pass into a droplet separator 24. The droplet separator 24 has an outer wall 25 which is charged or earthed to attract and collect small droplets from outer jets 28, 29. In the figure it can be seen that the separator wall 25 is connected to the high voltage source 18 and is charged with the same polarity as the orifice wall 27. The droplet separator 24 also has a central collecting rod 30, which is charged or earthed to attract and so collect droplets of central streams 31 and 32. In the figure the electrode 30 is electrically connected to charging electrode 27 and is charged with the same polarity. Both the outer wall 25 and the rod 30 are provided with an absorbent layer; there is a layer 34 for wall 25 and a layer 35 for rod 30. A scavenge pump 33 is provided to extract fluid collected by the rod 27 through a pipe 36 to be passed to an accumulator tank 101. The pipe 36 passes through a central passage provided through electrode 26.

The lowermost surface of charging electrode 26 is provided with a roughened surface opposite the orifices 20, 21, 22, 23 to improve the charging of the fluid, e.g. by the provision of faceted elements in a diamond coating or similar (as described in the applicant's own previous patent application PCT/GB2004/000458). Only the part of the surface opposite the orifices 20, 21, 22, 23 need be roughened/coated.

The invention also envisages using several spaced apart rods of the same type as rod 30 in a large multi-orifice array so as to minimise the occurrence of any small droplets exiting the atomiser. The rods would extend from the orifice wall 27 downwardly (i.e. downstream), would be spaced apart from one another and would extend from parts of the wall 27 located between the orifices in the wall 27.

FIG. 4 shows a further embodiment of electrostatic atomiser 39, which has components identical to those of FIG. 1; identical components having identical reference numerals. As with the FIG. 1 embodiment, it has an input fluid line 40, charging electrode 41, an array of multiple orifices 42, 43, 44, 45 in an orifice wall 46 which also functions as a second charging electrode, and a droplet separator 47 with a charged or earthed external cylindrical droplet collector wall 48. As shown in the figure the collector wall 48 is connected to the high voltage source 18 and charged with the same polarity as the orifice wall 46. Additionally, a central rod 50 is located within a spray channel 49 in the separator 47 and the rod 50 is appropriately charged to repel the small charged droplets towards the wall 48 where they are collected. The top of the rod 50 is separated from the orifice wall 46 by an insulator 51. The rod 50 is electrically connected to the charging electrode 41 by connector 52. The planar bottom face of electrode 41 is provided with faceted elements only in the region facing the orifices 42, 43, 44, 45.

Claims

1. An electrostatic atomiser, comprising:

a fluid inlet;

one or more orifices out of which the fluid emerges in an atomised form which comprises at least a first set of droplets of comparable size to each other and a second set of droplets of comparable size to each other and of a smaller size to the droplets of the first set;

a fluid channel connecting the fluid inlet to the orifice(s); and

at least two charging electrodes for applying a charge to fluid passing through the fluid channel; wherein

the electrostatic atomiser comprises a fluid droplet separator extending downstream of the orifices having a first separator electrode which is either earthed or electrically charged and which applies an electrical force on the droplets which deflects the second set of smaller droplets to a droplet collector while allowing the first set of larger droplets to continue out of a droplet outlet of the atomiser; and

the droplet collector is located upstream of the fluid outlet whereby only the larger droplets of the first set are output by the atomiser out of the droplet outlet, the smaller droplets of the first set having been collected within the atomiser by the droplet collector.

2. An electrostatic atomiser according to claim 1 wherein said droplet collector comprises an absorbent layer.

3. An electrostatic atomiser as claimed in claim 1 wherein the first separator electrode applies an attractive electrical force on the fluid droplets attracting the fluid droplets to move towards the first separator electrode.

4. An electrostatic atomiser as claimed in claim 1 wherein the first separator electrode is a cylindrical wall disposed around the orifices and extending downstream.

5. An electrostatic atomiser as claimed in claim 4 comprising a second separator electrode extending spaced apart from and parallel to the cylindrical wall and either earthed or charged to apply an attractive electrical force on the droplets, the atomiser having a second fluid droplet collector which collects the droplets attracted by the second separator electrode.

6. An electrostatic atomiser as claimed in claim 5 wherein the second fluid droplet collector is connected to a fluid return via a conduit which passes through passages formed in the charging electrodes, the fluid return returning fluid collected by the droplet collector to the fluid inlet.

7. An electrostatic atomiser as claimed in claim 1 wherein the orifices are provided in an orifice wall and said fluid droplet separator comprises a plurality of fluid collectors spaced apart from each other and extending from locations on the orifice wall located between the orifices and extending from the orifice wall in a downstream direction.

8. An electrostatic atomiser as claimed in claim 7 wherein each of the fluid collectors is associated with a separator electrode individual thereto which is electrically charged to attract the fluid droplets to the fluid collector.

9. An electrostatic atomiser as claimed in claim 7 wherein the orifice wall functions as a charging electrode.

10. An electrostatic atomiser as claimed in claim 3 comprising a second separator electrode spaced apart from the first separator electrode and electrically charged with a voltage of a different polarity to the first separator electrode, and which applies a repelling electrical force on the fluid droplets repelling the fluid droplets away from the second electrode towards the droplet collector.

11. An electrostatic atomiser as claimed in claim 1 wherein the orifices are provided in an orifice wall which also functions as one of the charging electrodes.

12. An electrostatic atomiser as claimed in claim 1 further comprising a fluid return for returning fluid collected by the droplet collector to the fluid inlet.

13. An electrostatic atomiser as claimed in claim 1 wherein the atomiser incorporates multiple orifices in close proximity to a substantially planar surface of a charging electrode which spans said multiple orifices.

14. An electrostatic atomiser as claimed in claim 1 comprising orifices angled to generate converging streams of droplets.

15. An electrostatic atomiser as claimed in claim 1 comprising orifices angled to generate diverging streams of droplets.

16. An electrostatic atomiser as claimed in claim 1, wherein the first separator electrode is electrically connected to one of said charging electrodes.

17. An electrostatic atomiser as claimed in claim 5 wherein the second separator electrode is electrically connected to one of said charging electrodes.

18. An electrostatic atomiser as claimed in claim 2 wherein the first separator electrode applies an attractive electrical force on the fluid droplets attracting the fluid droplets to move towards the first separator electrode.

19. An electrostatic atomiser as claimed in claim 3 wherein the first separator electrode is a cylindrical wall disposed around the orifices and extending downstream.

20. An electrostatic atomiser as claimed in claim 19 comprising a second separator electrode extending spaced apart from and parallel to the cylindrical wall and either earthed or charged to apply an attractive electrical force on the droplets, the atomiser having a second fluid droplet collector which collects the droplets attracted by the second separator electrode.

21. An electrostatic atomiser as claimed in claim 20 wherein the second fluid droplet collector is connected to a fluid return via a conduit which passes through passages formed in the charging electrodes, the fluid return returning fluid collected by the droplet collector to the fluid inlet.

22. An electrostatic atomiser as claimed in claim 21 wherein the orifices are provided in an orifice wall and said fluid droplet separator comprises a plurality of fluid collectors spaced apart from each other and extending from locations on the orifice wall located between the orifices and extending from the orifice wall in a downstream direction.

23. An electrostatic atomiser as claimed in claim 22 wherein each of the fluid collectors is associated with a separator electrode individual thereto which is electrically charged to attract the fluid droplets to the fluid collector.

24. An electrostatic atomiser as claimed in claim 23 wherein the orifice wall functions as a charging electrode.

25. An electrostatic atomiser as claimed in claim 3, wherein the first separator electrode is electrically connected to one of said charging electrodes.