GB2472681A

GB2472681A - A gas treatment cell

Info

Publication number: GB2472681A
Application number: GB1012569A
Authority: GB
Inventors: Ronald Robert Codling
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-08-14
Filing date: 2010-07-27
Publication date: 2011-02-16
Anticipated expiration: 2030-07-27
Also published as: GB2472681B; GB201012569D0; WO2011018603A2; WO2011018603A3; GB0914242D0

Abstract

A gas treatment cell 1 for use in a gas treatment apparatus, for example for treating air to generate ozone comprises a tube 2 defining a multiplicity of apertures 3 along its length such that a gas pathway 8 is defined through the tube and through the apertures. A coil of wire 4 is located around or within the tube in close proximity to said apertures. The wire forms one of a pair of electrodes between which a corona discharge is generated when a potential difference is applied to them such that, in use, gas passing through the apertures as it moves into or out of the tube passes around the wire and thereby through said corona discharge. A gas treatment apparatus (25, fig. 6) comprising such a cell (1; 15) or a plurality of them is disclosed.

Description

GAS TREATMENT CELL AND

APPARTUS INCORPORATING SAME

The present invention relates to a gas treatment cell, in particular but not exclusively for treating air to generate ozone, and a gas treatment apparatus incorporating same.

Gas treatment apparatus has a variety of uses. It can be employed as an ozone generator by treating air to produce ozone-rich air that is then used for various purposes such as odour control in kitchen ventilation systems and the like. It can a'so be used to treat waste gases, in particu'ar air containing contaminants, to remove pollutants such as volatile organic compounds (VOCs) therefrom.

Conventionally, ozone is generated by passing air through or past a corona discharge which ionizes the air and results in the formation of ozone from the oxygen molecules contained in it. Such generators typically incorporate gas treatment cells in the form of one of three types, as follows.

1. A plurality of concentric tubular elements is provided that define parallel annular passages through which air is passed. Each element comprises a gas treatment cell in the form of a dielectric sleeve with an outer metal sleeve and a metal core that comprise electrodes. A potential difference is applied between the metal sleeve and the metal core to produce a corona discharge that generates ozone in the air passing down the annular passages of the apparatus. Such an arrangement is described in GB580141.

2. Spaced, parallel flat-plate pairs of electrodes separated by dielectric gaskets or similar are arranged with air passageways extending between them. Air to be treated is pumped between the plates. This is a flat plate corona discharge method such as described in U5454596o and U55512254.

3. A thin dielectric sheet is located between a sheet of perforated foil or wire mesh and a solid metal plate. A corona discharge forms around the edges of the holes in the perforated sheet or mesh.

Ozone is then generated in air passing over the sheet or mesh.

This is a surface ozone generator such as is described in US2003/1o846o and in JPioo534o3.

All of these arrangements have disadvantages. In the first two the dielectric materials used tend to be made of glass or of ceramic material and this makes the equipment fragile. In the third arrangement in particular, the corona discharge does not occur uniformly between the electrodes and generally occurs haphazardly between them. This means that not all of the air passing over the electrodes passes through or past a corona discharge.

This considerably lowers the efficiency of these arrangements as ozone generators.

It is an object of the present invention to provide a gas treatment cell for incorporation in a gas treatment apparatus that overcomes or substantially mitigates the aforementioned disadvantages.

According to a first aspect of the present invention there is provided a gas treatment cell for use in a gas treatment apparatus comprising a first tube defining a multiplicity of apertures along its length and adapted such that a gas pathway is defined through the tube and through the apertures; and a pair of electrodes between which a corona discharge is generated when a potential difference is applied to them, at least one of the electrodes comprising a coil of wire located around or within the tube in close proximity to said apertures such that, in use, gas passing through the apertures as it moves into or out of the tube passes around the wire of said one electrode and thereby through said corona discharge.

In a first embodiment, the first tube preferably comprises the other electrode and the corona discharge is generated between the first tube and the coil of wire.

In a second embodiment, the other electrode preferably comprises a second coil of wire that wound around the first tube such that each loop of wire of the first coil lies adjacent at least one loop of wire of the second coil and vice versa.

In both embodiments said one electrode preferably comprises an insu'ated wire wound helically around the exterior of the first tube such that gaps are left between each turn of the helix.

According to a second aspect of the present invention there is provided a gas treatment apparatus comprising a gas treatment cell according to the first aspect of the present invention, a means for creating movement of gas to be treated along the gas pathway of the cell, and a control means for controlling the application of a potential difference to the electrodes of the cell.

Preferably, the apparatus comprises an outer containment tube in which the gas treatment cell is located, the gas pathway of the gas treatment cell being in communication with the interior of the containment tube and a sealing means being provided whereby gas passing down the containment tube and into the gas treatment cell is separated from gas exiting from the gas treatment cell.

Preferably also, the control means comprises a potentiometer which allows the potential difference applied to the electrodes of the cell to be varied as required.

Preferably also, the control means is adapted to apply an AC voltage of up to 15,000 V to the electrodes.

Other preferred but non-essential features of the various aspects of the present invention are described in the dependent claims appended hereto.

Embodiments of the various aspects of the present invention Will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a diagram showing schematically a first embodiment of a gas treatment cell for incorporation in an apparatus in accordance with the first aspect of the present invention; Fig. 2 is a longitudinal cross-section through the cell shown in Fig. 1 showing diagrammatically how a corona discharge is generated in the cell; Fig. 3 is a view similar to Fig. 2 but of a modified arrangement; Fig. 4 is a view similar to Fig. 2 showing a second embodiment of gas treatment cell; Fig. 5 is a view similar to Fig. 4 but of a modified arrangement; Fig. 6 is another view similar to Fig. 4 but of a second modified arrangement; Fig. 7 is a perspective view of a third modified arrangement with an area where wire coils are broken away to reveal the underlying structure; Fig. 8 is a diagram showing an embodiment of a gas treatment apparatus in accordance with the second aspect of the present invention; and Fig. 9 is a diagram similar to Fig. 8 but showing a modified gas treatment apparatus.

Throughout this description, similar components or parts in the various embodiments and modifications are given the same reference numeral.

A first embodiment of gas treatment cell 1 for incorporation in a gas treatment apparatus is shown in Figs. 1 and 2. This comprises a metal tube 2 that defines a multiplicity of apertures 3 along its length and that has an insulated wire 4 helically wound around its exterior. Preferably, the tube 2 comprises a perforated stainless steel cylinder and the insulated wire 4 comprises a wire 4 with a silicone or other suitable insulating sheath 5. A narrow gap 6 is left between each turn of the helix that is approximately of the order of 0.5 mm. One end of the tube 2 is blanked off by a plate 750 that a gas pathway 8, as indicated the arrows, is formed through the open end of the tube 2 and out through the apertures 3 around the wire 4.

In use, if the cell 1 is to be used to generate ozone by treating air, a potential difference, typically via a high AC voltage, is applied between the insulated wire 4 and the tube 2, which form a pair of electrodes. The AC voltage can be at any suitable level to cause ionisation or corona discharge and is typically between 1,000 V and 15,000 V. The tube 2 is preferably retained at earth potential. The high voltage stress around the wire 4 causes ionisation of the gas that surrounds it and a corona discharge 9 forms around the insulating sheath 5, particularly on the surface closest to the tube 2. The lines of flux forming the corona discharge 9 are labelled 10 in Fig 2.

As the gas to be treated passes along the gas pathway 8 it must pass through the corona discharge 9 as passes around the wire 4. If the gas contains oxygen, a proportion of this will be converted into ozone. Similarly, contaminants in the gas will be treated by their passage through the corona discharge.

It wifl be appreciated that various modifications can be made to the cell 1. The metal tube 2 need not be made from a perforated metal sheet but could, instead, be made from a wire mesh, an expanded metal sheet or any other electrically conducting material that has apertures allowing gas to pass through it. Although sheet made from stainless steel is preferable for most purposes, the tube 2 could be made from any conducting material and can be cylindrical or define any curved, polygonal or flat surfaces. The tube 2 may also be bare metal, coated with insulation or other protective coating or with a catalyst to improve the performance of the cell 1. If the tube 2 is sheathed in an electrically insulating material, then the wire 4 need not be provided with the insu'ating sheath 5.

The blanking plate 7 need not completely seal off the end of the tube 2 and it is possible in a variation to use a closure means that allows some gas to pass around or through it whereby the quantity of gas treated by the cell 1 can be varied as desired.

The wire 4 need not be helically wound around the exterior of the tube 2 but can be laid in any manner provided it touches or is in close proximity to the surface of the tube 2. The coil of wire 4 could also be located inside the tube 2 rather than being wound around its exterior. Similarly, the coil can comprise single or multiple windings. Either one end or both ends of the wire 4 can be connected to the high voltage electricity supply. In a variation, a non-conducting mesh or fabric can be laid over the tube 2 between the tube 2 and the insulated wire 4, thereby introducing an air space between the tube 2 and the wire 4 in which the corona discharge is generated.

A modified arrangement of the cell 1 is shown in Fig. 3. Here, an outer electrically conducting tube 11 is located around the exterior of the coil of wire 4 and in close proximity to it. In some embodiments the tube 11 may contact the wire 4. The tube 11 is also an electrode and is maintained at the same potential as the first tube 2. Hence, a corona discharge 12 forms between the wire 4 and the tube 2 and also forms between the wire 4 and the tube 11 in a gap 14 between the tubes 2 and ii. The tube ii also defines a multiplicity of apertures 13 along its length. The gas pathway 8 through the cell 1 is therefore through the tube 2, past the wire 4 through the gap 14 between the tubes 2 and 11 out through the apertures 13 of the tube 11. Gas travelling through the gap 14 must, therefore, pass through a corona discharge.

As with tube 2, the tube 11 can be made from a perforated sheet, a mesh, an expanded sheet or any other electrically conducting material. It may also be bare metal, coated with insulation or other protective coating or with a cata'yst to improve the performance of the cell 1.

A second embodiment of cell 15 is shown in Fig.4. Here a tube 16, which is provided with apertures 17, has two separate coils of wire 18 and 19 wound around it in place of a single coil of wire. The tube 16 is non-conducting and at least one of the coils of wire i8, 19 is provided with an insulating sheath 20. The coils of wire 18, 19 are wound around the tube 16 such that each loop of wire from one coil 18, 19 lies adjacent at least one loop of wire of the other coil 19, 18. A small, uniform air gap 21 is left between the wires 18, 19. In use, a high AC voltage is applied to the wires i8 and 19, which in this embodiment comprise the electrodes of the cell 15. A corona discharge develops in the air gaps 21 between each wire i8, 19 and its neighbouring wires 19, i8, as shown in Fig. 4. Hence, gas to be treated that travels along the gas pathway 8 passes out of the tube 16 and between the wires 18, 19, thereby passing through the corona discharge which effectively sheaths the tube 16.

In a modification of this embodiment as shown in Fig. , the first coil of wire i8 is wound helically in a single layer around the tube 16 with a gap 22 between each turn. An insulating mesh or perforated sheet 23, for example a layer of woven glass-fibre mesh, is then wrapped around the tube i6 over the coil or wire i8. The second coil of wire 19 is then also wound helically in a single layer over the sheet 23 such that each turn of the wire 19 is located in the gap 22 between the turns of the first coil of wire 18. The sheet 23 acts as a spacer between the coils 18 and 19 creates an air gap between them. As before, a high AC voltage is applied to the wires 18, 19 and a corona discharge 24 develops in the air gaps between them. As before, gas to be treated passes out of the tube 16 and between the wires 18, 19 thereby passing through the corona discharge 24 formed between them.

The winding of the wire coils 18, 19 around the tube i6 during manufacture such that a uniform spacing is maintained between the wires of the coils 18, 19 can be difficult to achieve. In order to overcome this problem, in a further modification as shown in Fig. 6, one or preferably both of the wires 18 and 19 may comprise a close-fitting exterior braid 25 made of a suitable insulating material such as glass-fibre. The braided wires 18 and 19 can then be wound helically around the tube 16 in a touching relationship, the wall thickness of the braid 25 determining the air gap between adjacent wires i8, 19. When a high voltage is applied to the wire coils i8, 19, a corona discharge is created between them that envelopes the braid 25. Air passes through the braid 25 and therefore through the corona discharge.

In this modification, the tube i6 may also be electrically conducting, for example by being made from a perforated metal sheet, a wire mesh, or an expanded metal sheet, provided that the air gap defined between the tube i6 and the wire coils 18, 19 is greater than the air gaps defined between adjacent wires of the coils 18, 19. This minimizes the likelihood of a corona discharge occurring between the wires of the coils 18, 19 and the tube 16.

Preferably, therefore, if the tube 16 is electrically conducting an electrically insulating mesh or electrically insulating perforated sheet 26, for example a layer of woven glass-fibre mesh, is wound around the tube 16 between the tube 16 and the wire coils 18, 19 to create the necessary air gap.

It will also be appreciated that such a braid 25 could also be used to sheath the wire 4 or the insulated wire 4, of the embodiments shown in Figs. 1 to 3 as an aid to uniform spacing of the wire 4 when it is wound around the tube 2.

Maintaining a uniform spacing between the adjacent wires of the coils 18, 19 can also be accomplished in other ways, for example as shown in Fig. 7, by the use of a cage 27. The cage 27 is located around the tube i6 and the first and second coils of wire i8, 19 are wound over it. One end of each of the wire of the coils i8, 19 is adapted for connection to a high AC power supply and the other end of each of the wires is sealed off. In the illustrated embodiment, the cage 27 is made of an electrically insulating material such as PTFE (poytetrafluoroethyene) or other plastics materia' and comprises a series of spaced longitudinal struts 28 that are provided with notches or grooves 29 along their length to provide a seating for the wires. The struts 28 are themselves supported by being slotted into a series of rings 30 that are located at the ends and spaced in between the ends along the length of the tube i6. Preferably, the notches or grooves 29 in adjacent struts 28 are off-set slightly from one another so that the wires can be wound helically around the tube i6 such that each loop of wire from one coil i8, 19 lies adjacent at least one loop of wire of the other coil 19, i8. One of the rings 30 at the end of the cage 27 may comprise or support the blanking plate at the end of the tube 16. In the latter case the plate is made of the same material as the rest of the cage 27. If the tube i6 is to be made of an electrically conducting material, for example from stainless steel, the cage 27 can also be used to provide an provide an air gap between the tube 16 and the wire coils 18, 19 that is greater than the gap between adjacent wires of the coils i8, 19.

Such a cage 27 may also be used with the tube 2 of the embodiments shown in Figs. 1 to 3 as an aid to uniform spacing of the wire 4 or the insulated wire 4, 5 when it is wound around the tube 2.

The gas treatment cells 1, 15 and their modifications can be incorporated into gas treatment apparatus 31 (see Figs. 8 and 9) that comprises, in addition, a means 32 for creating movement of gas to be -10 -treated along the gas pathway of the cell, and a control means 33 for controlling the application of a potential difference to the electrodes of the cell. As indicated above, the AC voltage applied to the electrodes is preferably between 1,000 V and 15,000 V. The means 32 for creating gas movement may comprise a blower or fan upstream of the cell in order to blows the gas to be treated into the cell, as shown schematically in Figs. 8 and 9, or a fan downstream of the cell that sucks the gas through it. The control means 33 is preferably housed in a compartment that is totally segregated from the gas passing through the cell and that preferably comprises a potentiometer (not shown) which can be used to vary the potential difference applied to the electrodes of the cell. In a ozone generator this enables the rate of ozone production to be set at a desired level between zero and the maximum possible for the cell. In some embodiments, the control means may be adapted to respond to signals from a remote location using digital or analogue signals connected to the control circuit.

It will be appreciated that the gas treatment apparatus may comprise a plurality of gas treatment cells as described above that are arranged either in series or in parallel within the apparatus. In the embodiment of the gas treatment apparatus 31 shown in Fig. 8, a gas treatment cell 1, 15 such as described above is located in an outer containment tube 34, which is preferably insulated or non-conducting. The cell 1, 15 is supported in the containment tube 34 and an annular sealing ring 35 is located at the open end of the first tube 2, 16 of the cell 1, 15 in order that gas forced down the containment tube 34 from one end 36 is directed along the gas pathway 8 of the cell 1, 15 for treatment. The sealing ring 35 also separates gas exiting from the gas treatment cell 1, 15 from the untreated gas and may also serve to support the cell 1, 15 within the containment tube 34. The treated gas is then collected for use at the other end 37 of the containment tube 34.

In a modification, as shown in Fig. 9, two annular sealing rings 35, 38 are used. The first ring 35 is located in the same location as in the arrangement shown in Fig. 8 but the second ring 38 is located approximately -11 -two thirds of the distance along the first tube 2, 16 of the cell. In addition, the tube 2, 16 is provided with a blanking plate 39 which is located approximately a third of the distance along its length in addition to the plate 7 at its end. Hence, in this apparatus the gas pathway through the cell 1, 15 is modified so that the gas passes three times through the tube 2, 16 and therefore through the corona discharge. This significantly increases the gas treatment that takes place.

The treatment apparatus shown in Fig. 8 is a single-pass arrangement where the gas to be treated only passes once through the gas treatment cell 1, whereas the apparatus shown in Fig. 9 is a triple-pass arrangement. It will be appreciated that the apparatus could be further modified in a similar way to provide any number of passes through the treatment cell as appropriate for the use to which the apparatus is to be put.

In this regard, if the treatment apparatus is to be used to generate ozone from air in order to treat contaminants contained within the air, the ozone-rich discharge gas can be contained in an outer receptacle of suitable ozone-resistant material to permit reaction of the ozone with the contaminants before discharge to atmosphere. Alternatively, the ozone-rich discharge gas can be ducted in a duct of suitable ozone-resistant material to a point of application as part of a treatment process. In other treatment apparatus, the feed gas need not be drawn from the ambient air space; it can be ducted from a remote location and connected to an inlet port of the treatment apparatus. Typically, this would be for treatment of noxious fumes or purification of air. In some applications it may not be permissible to discharge air containing significant quantities of ozone gas, in these cases it may be necessary to install a filter bed to remove the excess ozone prior to discharge. The media in the filter bed can be a catalyst or carbon.

Alternatively or in addition downstream of the cell, the air can be passed through a suitable catalytic medium or combination of media to further reduce the VOC concentration of the output air and/or to eliminate by-product gases produced by the breakdown of some VOCs.

-12 -The treatment apparatus of the invention although principally intended for operation at, or slightly above, ambient atmospheric pressure, by using a suitable fan or blower system, it will operate over a range of pressures.

The gas treatment cell of the present invention has several advantages over conventional treatment cells. It is robust, compact and self-cooling as the gas flow through it cools the cell. In conventional cells where the gas to be treated passes over the surface of a mesh or similar where corona discharges occur, on'y a portion of this gas actually passes though the ionization zones. In contrast, within the cell of the present invention, virtuaHy all the gas to be treated passes through the corona discharge and ionization zones around the wire coil or coils where these are subjected to the high electrical stresses. Conventional apparatus also rely on turbulence to mix the treated gas, for example the generated ozone, with the bulk of the gas, in the present invention, virtually of the gas is treated owing to the gas pathway through the apertures in the cell.

Experimentation using a cell in accordance with the present invention to treat air to produce ozone with an air flow through it of around 1,000 1/rn (3 cfm) produces up to i8g/hr of ozone. The power consumption at this level is up to 550W including the fan used to blow air through the cell.

Claims

-13 -CLAIMS1. A gas treatment cell for use in a gas treatment apparatus comprising a first tube defining a multiplicity of apertures along its length and adapted such that a gas pathway is defined through the tube and through the apertures; and a pair of electrodes between which a corona discharge is generated when a potential difference is applied to them, at least one of the electrodes comprising a coil of wire located around or within the tube in close proximity to said apertures such that, in use, gas passing through the apertures as it moves into or out of the tube passes around the wire of said one electrode and thereby through said corona discharge.
2. A cell as claimed in Claim 1, wherein the wire and/or the first tube is provided with an electrically insulating sheath.
3. A cell as claimed in Claim 1 or Claim 2, wherein said one electrode comprises an electrically insulated wire wound helically around the exterior of the first tube such that gaps are left between each turn of the helix.
4. A cell as claimed in any of Claims 1 to 4, wherein one end of the first tube is open to permit gas to be treated to enter or to exit the cell and the other end of the tube is closed by a closure means that forces at least some of the gas passing through the cell to exit or to enter the cell through the apertures in the tube.
5. A cell as claimed in any of Claims 1 to 4, wherein the first tube comprises the other electrode and the corona discharge is generated between the first tube and the coil of wire.

-14 -
6. A cell as claimed in Claim 5, wherein an electrically conducting second tube is located around the exterior of the coil of wire and also comprises an electrode that is adapted to be maintained at the same potential as the first tube.
7. A cell as claimed in Claim 5 or Claim 6, wherein at least the first tube of the first and second tubes is made from a perforated metal sheet, a wire mesh, or an expanded metal sheet.
8. A cell as claimed in any of Claims 5 to 7, wherein the first tube is maintained at earth potentiaL
9. A cell as claimed in any of Claims 1 to 4, wherein the other electrode comprises a second coil of wire that is wound around the first tube such that each loop of wire of the first coil lies adjacent at least one loop of wire of the second coil and vice versa.
10. A cell as claimed in Claim 9, wherein the second coil of wire comprises an electrically insulated wire.
ii. A cell as claimed in Claim 9 or Claim 10, wherein an electrically insulating mesh or electrically insulating perforated sheet is located between the first and second coils of wire.
12. A cell as claimed in any of Claims 1 to 11, wherein the coil or at least one of the coils of wire comprises an exterior braid comprised of an insulating material.
13. A cell as claimed in Claim 12, wherein the braid is comprised of glass-fibre.
14. A cell as claimed in Claim 12 or Claim 13 when dependent on Claim 9 or Claim 10, wherein the first tube is electrically conducting and an air -15 -gap is defined between the first tube and the first and second coils of wire that is greater than air gaps defined between adjacent wires of the first and second coils of wires.
15. A cell as claimed in Claim 14, wherein an electrically insulating mesh or perforated sheet is located between the first tube and the first and second coils of wire.
i6. A cell as claimed in any of Claims 1 to 13, wherein the coil or coils of wire are wound around a cage located around the exterior of the first tube.
17. A cell as claimed in Claim i6, wherein the cage comprises a series of spaced longitudinal struts that are provided with notches or grooves along their length to provide a seating for the wire of the coil or coils.
18. A cell as claimed in Claim 17, wherein the notches or grooves in adjacent struts are off-set from one another to facilitate winding of the wire of the coil or coils helically.
19. A gas treatment apparatus comprising a gas treatment cell as claimed in any of Claims 1 to i8, a means for creating movement of gas to be treated along the gas pathway of the cell, and a control means for controlling the application of a potential difference to the electrodes of the cell.
20. An apparatus as claimed in Claim 19, comprising an outer containment tube in which the gas treatment cell is located, the gas pathway of the gas treatment cell being in communication with the interior of the containment tube and a sealing means being provided whereby gas passing down the containment tube and into the gas treatment cell is separated from gas exiting from the gas treatment cell.-16 -
21. An apparatus as claimed in Claim 19 or Claim 20, wherein the treatment cell is provided with at least one blanking plate intermediate its length that forces gas to pass through the apertures at least twice during is passage through the cell.
22. An apparatus as claimed in any of Claims 19 to 21, wherein the control means comprises a potentiometer which allows the potential difference applied to the electrodes of the cell to be varied as required.
23. An apparatus as claimed in any of Claims 19 to 22, wherein the control means is adapted to apply an AC voltage of up to 15,000 V to the electrodes.
24. An apparatus as claimed in any of Claims 19 to 23, which is adapted to generate ozone by the treatment of ambient air.
25. A gas treatment cell substantially as described herein with reference to Figs. 1 and 2 or to any one of Figs. 3 to 7 of the accompanying drawings.
26. A gas treatment apparatus substantially as described herein with reference to Fig. 8 or Fig. 9 of the accompanying drawings.