CA1039125A - Electrostatic deposition of powder or liquid droplets in air by triboelectric or corona discharge means - Google Patents
Electrostatic deposition of powder or liquid droplets in air by triboelectric or corona discharge meansInfo
- Publication number
- CA1039125A CA1039125A CA213,555A CA213555A CA1039125A CA 1039125 A CA1039125 A CA 1039125A CA 213555 A CA213555 A CA 213555A CA 1039125 A CA1039125 A CA 1039125A
- Authority
- CA
- Canada
- Prior art keywords
- duct
- capacitor
- particulate material
- charge
- electrically
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
- B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
- B05B5/025—Discharge apparatus, e.g. electrostatic spray guns
- B05B5/047—Discharge apparatus, e.g. electrostatic spray guns using tribo-charging
Landscapes
- Electrostatic Spraying Apparatus (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
- Pretreatment Of Seeds And Plants (AREA)
- Electrostatic Separation (AREA)
- Elimination Of Static Electricity (AREA)
Abstract
ABSTRACT
A method and apparatus are provided for the electrostatic deposition of solid particles or liquid droplets on to an earthed surface.
The method involves in the vicinity of the surface to be treated directing a flow of gas carrying electrically charged particulate material adjacent a capacitor surface which has been charged to a high potential by the transference of electric charges developed on another particulate material which is being transported round a closed circuit path including a charging region remote from where the charge is transferred to the capacitor. Charging of both particulate materials is preferably by triboelectricity although corona discharge may also be used.
Apparatus comprises tubing forming a closed loop, a compressor for circulating airborne powder round the loop, a corona discharge needle projecting through an insulated portion of the tubing wall and electrically connected to a toroidal capacitor surrounding said insulated portion and a barrel associated with a second compressor for projecting a separate supply of airborne powder adjacent the capacitor surface.
The invention is particularly suitable for crop spraying or dusting or the electrostatic coating of products e.g. with plastics, and represents a more efficient and thus improved method over known processes.
A method and apparatus are provided for the electrostatic deposition of solid particles or liquid droplets on to an earthed surface.
The method involves in the vicinity of the surface to be treated directing a flow of gas carrying electrically charged particulate material adjacent a capacitor surface which has been charged to a high potential by the transference of electric charges developed on another particulate material which is being transported round a closed circuit path including a charging region remote from where the charge is transferred to the capacitor. Charging of both particulate materials is preferably by triboelectricity although corona discharge may also be used.
Apparatus comprises tubing forming a closed loop, a compressor for circulating airborne powder round the loop, a corona discharge needle projecting through an insulated portion of the tubing wall and electrically connected to a toroidal capacitor surrounding said insulated portion and a barrel associated with a second compressor for projecting a separate supply of airborne powder adjacent the capacitor surface.
The invention is particularly suitable for crop spraying or dusting or the electrostatic coating of products e.g. with plastics, and represents a more efficient and thus improved method over known processes.
Description
~39~'~5 This invention relates to electrostatic deposition of a particulate material which may be liquid droplets or a solid in particulate, e.g. powder, form. The invention has particular but by no means exclusive application to the : 5 treatment of vegetable growth, e.g. as by the dusting or spraying of crops,but it is also applicable, for example, to the electrostatic coating of products with powder or liquids.
In accordance with the present invention, there is provided a method of electrostatically depositing particulaté materia1 on to an earthed surface, said m~thod comprising in the vicinity of the earthed surface directing a flow of gas carry"ng charged particulate material adjacent ~o a capacitor surface which is charged to a high voltage of the same polar- ~y as the charge on the particulate D~terial by trarsf~rring t~ said capacitor electrical charges developed on a separate particulate material which is being transported by a non-conducting fluid medium round a closed circuit path which includes a region where the particles are charged remote from where the charge is transferred to the capacitor.
The separate particulate material may be charged triboelectrically and/or by other means such as corona dis-charge.
Apparatus in accordance with one aspect of the 25 invention comprises in combination a duct defining a closed loop path for a flowing medium;
means for causing a flow through the duct of a medium comprising particulate material suspended in a non-conducting fluid vehicle, the arrangement ~bi
In accordance with the present invention, there is provided a method of electrostatically depositing particulaté materia1 on to an earthed surface, said m~thod comprising in the vicinity of the earthed surface directing a flow of gas carry"ng charged particulate material adjacent ~o a capacitor surface which is charged to a high voltage of the same polar- ~y as the charge on the particulate D~terial by trarsf~rring t~ said capacitor electrical charges developed on a separate particulate material which is being transported by a non-conducting fluid medium round a closed circuit path which includes a region where the particles are charged remote from where the charge is transferred to the capacitor.
The separate particulate material may be charged triboelectrically and/or by other means such as corona dis-charge.
Apparatus in accordance with one aspect of the 25 invention comprises in combination a duct defining a closed loop path for a flowing medium;
means for causing a flow through the duct of a medium comprising particulate material suspended in a non-conducting fluid vehicle, the arrangement ~bi
2 - ~ -.. . . . . . . . .. . ... . . . .. .. . . ....
1~3~5 being such that electrical charges are induced triboelectrically on the particulate material carried by the vehicle along a length of the duct;
capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically cornected to said capacitor means and located in the duct but electrically insulated from ar,y conducting portion of the duct for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for sus-pending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.
Triboelectric charging is the charging of a body by friction contact with a different body.
The triboelectric charging of the particulate material will generally occur primarily as a result of frictional contact between the particles and the wall of the duct "` ~ c;) i~ ~
as the particulate material travels along the duct length. However, some incidental triboelectric charging may occur in the container or containers in which the material is stored and from which it is transferred into the duct, and also during the transference of the material to the duct. Triboelectric charging may also occur as a result of the passage of the particulate material through the flow causing means, e.g. fluid compressor and also as a result of the passage of the particulate material through other portions of the duci~
In one embodiment of the invention, a part of the duct wall is formed of electrically conductive material or other material capable of dissipatir.g electrical charge built up ~ said part of th~ duct wa~l, e.g. resin impregnated fabric the inner sGrface of which may be lined with rubber to provide a soft inner surface to increase the triboelectric charge produced, and another part of the duct wall is of electrically non-conductive material, with the electrical discharge means extending through the non-conductive part. Generally, the electrically non-conductive part of the duct wall will comprise a length of the duct wall and the capacitor means is located adjacent that length.
Alternatively, the inner face of the duct for substantially the entire length thereof may be formed of an . .
I~3~5 electrically non-conductive material such as polyethylene or poly-methylmethacrylate since the softer nature of such materials considerably increases the area of contact with the particulate material which is being transported through the duct by the flowing vehicle, thus tending to increase the triboelectric charge produced. However, it is then necessary to prevent the build-up of charge on the surfaces of the duct and this can be done, for exarnple, by providing a series of earthed needles protruding through the duct wall and spaced at appropriate intervals along the duct or by providing an earth wire extending within the duct and axially of the duct.
The electrical discharge means which is electrically connected to the capacitor means may suitably comprise a corona f discharge electrode, e.g. pOi:lt or knife edge, exten~ing into the duct. Alternatively, it may comprise a length of duct the inner face of which is of electrically conducting material which - is insulated from the remainder of t~e duct ~nd from earth.
The capacitor to which the eleotrical discharge means is connecte-l may be, for example, spherical or toroidal and may be arranged to surround a part of the length of the duct. It has been found that in some circumstances a higher potential can be induced on the capacitor by providing one or more earthed needles extending into the duct through a non-conducting part of the wall immediately upstream and/or downstream of the electrical discharge means.
Advantageously,means such as baffles may be located in the path of the flowing medium to increase the turbulence of the flow and hence the level of induced triboelectric charge.
The particulate material which is conveyed in the duct is preferably a powder and the fluid medium will usually be air.
However, it is envisaged that the particulate material could .
.,5 alternativel~ be provided in the rorm of liquid droplets, for example by providing a liquid supply and means for ato~ising the liquid into a flowing gas stream to provide a mist which is maintained at least while the liquid droplets are charged and carried past the electrical discharge means.
The separate supply of particulate material which is to be deposited electrostatically may also be a powder or liquid droplets and usually, although not necessarily, the gas in which this separate particulate material is suspended will be air.
Conveniently, the material is electrically charged to the same polarity as the charge induced on the capacitor surface triboelectricaIly by projecting it in an air stream along a duct whose inner surface is of a suitable material. hlternatively or additionally, hcwever, the - charging may be effec'ed other than by triboelectricity. For exampls, a part of the charge on the capacitor surface may be used for this purpose or c~ar~ing may be effected by corona discharge from a separate energy source.
` When the gas-suspended flow of thus charged particulate material is projected adjacent the charged surface of the capacitor, it will be repelled by it and deposited on the surface of any earthed object in the vicinity~ -An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which is a partially broken away schematic elevational view.
Referring to the drawing, an alu~inium tube 10 is connected between a tube 12 made of an insulating material such as polymethylmethacrylate and one inlet 14 of a blower unit 16. The blower unit 16 comprises two centrifugal fans driven by the same motor but each having an air path which is completely separate from that of the other. The outlet 18 of the blower unit 16 which is associated with the inlet 14, is connected by a second aluminium tube 20 to the other end of the tube 12 so as to form a completely closed duct.
The tube 12 is surrounded by a 'coroid 22 formed of electrically ~3~
conductive material. A needle 24, of electrically conductive material, electrically connects the toroid 22 with the inside of tube 12.
The inlet 26 of the other part of the blower unit 16 is connected to a hopper 28 from which a mixture of air and a powder to be deposited can be fed into the blower 16. The corresponding outlet 30 is connected by a tube 32 to a nozzle 34 which directs powder into the region adjacent to the toroid 22.
A powder such as silicon carbide is placed in the closed circuit formed by the tubes 10, 12 and 20. In use, the blower unit 16 causes this powder to be circulated within the closed circuit. Tribo electric or contact /separation charging takes place due to contact and friction between the pow-der and the walls of the tubes 10 and 20. Charge build-up on the tubes 10 and 20 doeb~lngt take place since these members are made of aluminium and are earthed. A corona discharge forms at the tip of the needle 24 and since the needle is electrically connected to the toroid 22 the resulting charge is transferred to the outer surface of the toroid acts as a capacitor due to the Faraday cage effect. Thus the toroid 22 is charged to a high voltage.
Meanwhile, a separate supply of powder from the hopper 28 is blown by the blower unit 16 along the tube 32 where tribo electric charging takes place. The material of which the tube 32 is made is chosen to be such that the polarity of the charge on the powder to be deposited is the same as that on the toroid 22. Thus, although the voltage on the particles of the powder emerging from the nozzle 34 may be relatively low, they are strongly repelled by the high voltage on the toroid 22 and deposited on any convenient earthed object in the vicinity, for example the object 36 which is the article to be coated. Thus, the closed circuit formed by the tubes 10, 12 and 20 performs the function of charging the toroid 22 to a high voltage to establish an electric field between the toroid 22 and an object 36 to be coated. At the same time, the powder to be deposited is charged in the tube 32.
11~3~5 Since the particulate material flowing in the duct in which charg-ing takes place is separate from the particles being deposited, the material in the duct can be chosen to give optimum charging in conjunction with the material of which the duct is made. In addition, since no material or air need be removed from the closed circuit, it can be completely sealed thereby reducing the risk of charge leakage and weak tribo charging, due to humidity.
Various modifications of the closed circuit part of the apparatus can be made.
For example, an earthed needle may project into the tube 12, up-stream and/or downstream of the needle 24 or a plurality of such needles may be provided which may be located upstream or downstream or both upstream and downstream of the needle 24. It has been found in some circumstances that the provision of such needles may lead to a higher potential on the capacitor.
Baffles may be provided inside the tubes 10 and 20 to increase the turbulence of the flow therein. These are preferably inwardly inclined in the direction of the flow when the fan is running and are of the same mat-erial as the tubes 10 and 20, which are also preferably of the aame material.
Instead of being made of aluminium, the tubes 10 and 20 may be formed of another metal or of a material having a soft inner surface and sufficient electrical conductivity to reduce the build up of stored charge on the tube, and thereby inhibit the formation of corona discharge on the parts of the tube where charging takes place. One suitable material is a resin-impregnated fabric having a thin rubber lining on its inner surface.
Alternatively, the whole of the tubing fo i ng the closed circuit may be made of an eaectrically non-conductive material. In this case, how-ever, it is necessary to provide means for removing surface charges from the walls of the tubes remote from the electrical discharge means. Conveniently, this may be achieved by a coaxial earth wire extending the full length of the tubes 10 and 20 to remove surface charge from the walls thereof. Alterna-~3~.,5 tively, earthed needles may be provided to project through the walls of tubes 10 and 20 at suitably spaced intervals along the length of these tubes.
It is to be understood that the illustration of the embodiment in the drawing is schematic and that, for example, in practice a variety of different configurations for the tubes forming the closed circuit are pos-sible. Thus, in one embodiment which is preferred for its ease of manipu-lation, the two tubes 10 and 20 may extend parallel and closely adjacent each other for at least some of their length and the tube 12 may comprise a U-bend which connects the adjacent ends of the tubes 10 and 20. The toroid 22 may then be so constructed and arranged as to fit over the two arms of the U-bend.
It may also be desirable to twist the tubes 10 and 20 around each other alorg their long axes so that the flow through each of them follows a helical path.
As an alternative, or in addition to triboelectric charging, the - particles in the closed circuit may be charged by other means, suitably corona discharge. In accordance with this embodiment, the apparatus com-prises in combination a duct defining a closed loop path for a flowing medium;
means for causing a flow through the duct of a medium comprising particulate material suspended s in a non-conducting fluid vehicle;
means for inducing an electrical charge on the particulate material carried by the vehicle through the duct;
- 5 capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically connected to said capacitor means and located in but electrically insulated ~ from the duct remote from said charging means for ; ~ 10 transferring electric charge from the charged particulate material to said capacitor means, whereby to build up - a ch~lge on an outside surface thereof; and ; means separate from said duct for suspending a separaGe supply of pa~ticulate material in a gas flow, electrical y charging said material with a charge of tne same polarity - as that induced on the outside surface of the capacitor means ~nd projecting the flow of charged particulate material adjacent said capacitor surface.
With this embodiment, for example, one or more corona discharge electrodes, e.g. in the form of needles or knife edges, may extend into the charging region of the duct. Such electrodes are electrically insulated from the adjacent waIls of the duct and a potential difference of sufficiently high voltage to establish a discharge is applied between the corona discharge electrode and the adjacent walls of the duct. With conventional sharp-edge electrodes a voltage of about 1 to 20 kV would be sufficient.
In one preferred form, the corona discharge electrode, which may suitably comprise an electrically conducting wire extending within and axially along a length of the duct, is earthed, and a _ 10 -1~3~ S
suitably high voltage, e.g. of 5-25 kV from an appropriate voltage source, is applied to the inner face of the duct which for that length of the duct is conductive. By this means, that part of the corona current which goes to charge the particulate material is drawn from earth and only that part of the current which Mows to the duct wall need pass through said voltage source.
For electrical and mechanical reasons, it is preferred that the wire is coaxial with the duct.
The same arrangement may be used to charge the particulate material in the tube 32, or to supplement the triboelectric charge on said material.
The invention may be used, for example, for the spraying or dusting of crops, seed dressing, weed killing, mineral dressing, artificial pollina-tion, the coating of metals with plastics, the coating of rubber with talc, and the application of corrosion inhibitors such as borax to exposed metal surfaces.
By way of example, utilising the apparatus illustrated in the accompanying drawing in which the tubes 10, 12, 20 have an internal diameter of 8 mm and the length of the closed loop is 2 metres, and using an air flow velocity of about 35-S0 metres per second and a loading of 5-25 g of powder in the airstream, potentials of up to 200kV can be obtained on the capacitor surface.
1~3~5 being such that electrical charges are induced triboelectrically on the particulate material carried by the vehicle along a length of the duct;
capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically cornected to said capacitor means and located in the duct but electrically insulated from ar,y conducting portion of the duct for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for sus-pending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.
Triboelectric charging is the charging of a body by friction contact with a different body.
The triboelectric charging of the particulate material will generally occur primarily as a result of frictional contact between the particles and the wall of the duct "` ~ c;) i~ ~
as the particulate material travels along the duct length. However, some incidental triboelectric charging may occur in the container or containers in which the material is stored and from which it is transferred into the duct, and also during the transference of the material to the duct. Triboelectric charging may also occur as a result of the passage of the particulate material through the flow causing means, e.g. fluid compressor and also as a result of the passage of the particulate material through other portions of the duci~
In one embodiment of the invention, a part of the duct wall is formed of electrically conductive material or other material capable of dissipatir.g electrical charge built up ~ said part of th~ duct wa~l, e.g. resin impregnated fabric the inner sGrface of which may be lined with rubber to provide a soft inner surface to increase the triboelectric charge produced, and another part of the duct wall is of electrically non-conductive material, with the electrical discharge means extending through the non-conductive part. Generally, the electrically non-conductive part of the duct wall will comprise a length of the duct wall and the capacitor means is located adjacent that length.
Alternatively, the inner face of the duct for substantially the entire length thereof may be formed of an . .
I~3~5 electrically non-conductive material such as polyethylene or poly-methylmethacrylate since the softer nature of such materials considerably increases the area of contact with the particulate material which is being transported through the duct by the flowing vehicle, thus tending to increase the triboelectric charge produced. However, it is then necessary to prevent the build-up of charge on the surfaces of the duct and this can be done, for exarnple, by providing a series of earthed needles protruding through the duct wall and spaced at appropriate intervals along the duct or by providing an earth wire extending within the duct and axially of the duct.
The electrical discharge means which is electrically connected to the capacitor means may suitably comprise a corona f discharge electrode, e.g. pOi:lt or knife edge, exten~ing into the duct. Alternatively, it may comprise a length of duct the inner face of which is of electrically conducting material which - is insulated from the remainder of t~e duct ~nd from earth.
The capacitor to which the eleotrical discharge means is connecte-l may be, for example, spherical or toroidal and may be arranged to surround a part of the length of the duct. It has been found that in some circumstances a higher potential can be induced on the capacitor by providing one or more earthed needles extending into the duct through a non-conducting part of the wall immediately upstream and/or downstream of the electrical discharge means.
Advantageously,means such as baffles may be located in the path of the flowing medium to increase the turbulence of the flow and hence the level of induced triboelectric charge.
The particulate material which is conveyed in the duct is preferably a powder and the fluid medium will usually be air.
However, it is envisaged that the particulate material could .
.,5 alternativel~ be provided in the rorm of liquid droplets, for example by providing a liquid supply and means for ato~ising the liquid into a flowing gas stream to provide a mist which is maintained at least while the liquid droplets are charged and carried past the electrical discharge means.
The separate supply of particulate material which is to be deposited electrostatically may also be a powder or liquid droplets and usually, although not necessarily, the gas in which this separate particulate material is suspended will be air.
Conveniently, the material is electrically charged to the same polarity as the charge induced on the capacitor surface triboelectricaIly by projecting it in an air stream along a duct whose inner surface is of a suitable material. hlternatively or additionally, hcwever, the - charging may be effec'ed other than by triboelectricity. For exampls, a part of the charge on the capacitor surface may be used for this purpose or c~ar~ing may be effected by corona discharge from a separate energy source.
` When the gas-suspended flow of thus charged particulate material is projected adjacent the charged surface of the capacitor, it will be repelled by it and deposited on the surface of any earthed object in the vicinity~ -An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which is a partially broken away schematic elevational view.
Referring to the drawing, an alu~inium tube 10 is connected between a tube 12 made of an insulating material such as polymethylmethacrylate and one inlet 14 of a blower unit 16. The blower unit 16 comprises two centrifugal fans driven by the same motor but each having an air path which is completely separate from that of the other. The outlet 18 of the blower unit 16 which is associated with the inlet 14, is connected by a second aluminium tube 20 to the other end of the tube 12 so as to form a completely closed duct.
The tube 12 is surrounded by a 'coroid 22 formed of electrically ~3~
conductive material. A needle 24, of electrically conductive material, electrically connects the toroid 22 with the inside of tube 12.
The inlet 26 of the other part of the blower unit 16 is connected to a hopper 28 from which a mixture of air and a powder to be deposited can be fed into the blower 16. The corresponding outlet 30 is connected by a tube 32 to a nozzle 34 which directs powder into the region adjacent to the toroid 22.
A powder such as silicon carbide is placed in the closed circuit formed by the tubes 10, 12 and 20. In use, the blower unit 16 causes this powder to be circulated within the closed circuit. Tribo electric or contact /separation charging takes place due to contact and friction between the pow-der and the walls of the tubes 10 and 20. Charge build-up on the tubes 10 and 20 doeb~lngt take place since these members are made of aluminium and are earthed. A corona discharge forms at the tip of the needle 24 and since the needle is electrically connected to the toroid 22 the resulting charge is transferred to the outer surface of the toroid acts as a capacitor due to the Faraday cage effect. Thus the toroid 22 is charged to a high voltage.
Meanwhile, a separate supply of powder from the hopper 28 is blown by the blower unit 16 along the tube 32 where tribo electric charging takes place. The material of which the tube 32 is made is chosen to be such that the polarity of the charge on the powder to be deposited is the same as that on the toroid 22. Thus, although the voltage on the particles of the powder emerging from the nozzle 34 may be relatively low, they are strongly repelled by the high voltage on the toroid 22 and deposited on any convenient earthed object in the vicinity, for example the object 36 which is the article to be coated. Thus, the closed circuit formed by the tubes 10, 12 and 20 performs the function of charging the toroid 22 to a high voltage to establish an electric field between the toroid 22 and an object 36 to be coated. At the same time, the powder to be deposited is charged in the tube 32.
11~3~5 Since the particulate material flowing in the duct in which charg-ing takes place is separate from the particles being deposited, the material in the duct can be chosen to give optimum charging in conjunction with the material of which the duct is made. In addition, since no material or air need be removed from the closed circuit, it can be completely sealed thereby reducing the risk of charge leakage and weak tribo charging, due to humidity.
Various modifications of the closed circuit part of the apparatus can be made.
For example, an earthed needle may project into the tube 12, up-stream and/or downstream of the needle 24 or a plurality of such needles may be provided which may be located upstream or downstream or both upstream and downstream of the needle 24. It has been found in some circumstances that the provision of such needles may lead to a higher potential on the capacitor.
Baffles may be provided inside the tubes 10 and 20 to increase the turbulence of the flow therein. These are preferably inwardly inclined in the direction of the flow when the fan is running and are of the same mat-erial as the tubes 10 and 20, which are also preferably of the aame material.
Instead of being made of aluminium, the tubes 10 and 20 may be formed of another metal or of a material having a soft inner surface and sufficient electrical conductivity to reduce the build up of stored charge on the tube, and thereby inhibit the formation of corona discharge on the parts of the tube where charging takes place. One suitable material is a resin-impregnated fabric having a thin rubber lining on its inner surface.
Alternatively, the whole of the tubing fo i ng the closed circuit may be made of an eaectrically non-conductive material. In this case, how-ever, it is necessary to provide means for removing surface charges from the walls of the tubes remote from the electrical discharge means. Conveniently, this may be achieved by a coaxial earth wire extending the full length of the tubes 10 and 20 to remove surface charge from the walls thereof. Alterna-~3~.,5 tively, earthed needles may be provided to project through the walls of tubes 10 and 20 at suitably spaced intervals along the length of these tubes.
It is to be understood that the illustration of the embodiment in the drawing is schematic and that, for example, in practice a variety of different configurations for the tubes forming the closed circuit are pos-sible. Thus, in one embodiment which is preferred for its ease of manipu-lation, the two tubes 10 and 20 may extend parallel and closely adjacent each other for at least some of their length and the tube 12 may comprise a U-bend which connects the adjacent ends of the tubes 10 and 20. The toroid 22 may then be so constructed and arranged as to fit over the two arms of the U-bend.
It may also be desirable to twist the tubes 10 and 20 around each other alorg their long axes so that the flow through each of them follows a helical path.
As an alternative, or in addition to triboelectric charging, the - particles in the closed circuit may be charged by other means, suitably corona discharge. In accordance with this embodiment, the apparatus com-prises in combination a duct defining a closed loop path for a flowing medium;
means for causing a flow through the duct of a medium comprising particulate material suspended s in a non-conducting fluid vehicle;
means for inducing an electrical charge on the particulate material carried by the vehicle through the duct;
- 5 capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically connected to said capacitor means and located in but electrically insulated ~ from the duct remote from said charging means for ; ~ 10 transferring electric charge from the charged particulate material to said capacitor means, whereby to build up - a ch~lge on an outside surface thereof; and ; means separate from said duct for suspending a separaGe supply of pa~ticulate material in a gas flow, electrical y charging said material with a charge of tne same polarity - as that induced on the outside surface of the capacitor means ~nd projecting the flow of charged particulate material adjacent said capacitor surface.
With this embodiment, for example, one or more corona discharge electrodes, e.g. in the form of needles or knife edges, may extend into the charging region of the duct. Such electrodes are electrically insulated from the adjacent waIls of the duct and a potential difference of sufficiently high voltage to establish a discharge is applied between the corona discharge electrode and the adjacent walls of the duct. With conventional sharp-edge electrodes a voltage of about 1 to 20 kV would be sufficient.
In one preferred form, the corona discharge electrode, which may suitably comprise an electrically conducting wire extending within and axially along a length of the duct, is earthed, and a _ 10 -1~3~ S
suitably high voltage, e.g. of 5-25 kV from an appropriate voltage source, is applied to the inner face of the duct which for that length of the duct is conductive. By this means, that part of the corona current which goes to charge the particulate material is drawn from earth and only that part of the current which Mows to the duct wall need pass through said voltage source.
For electrical and mechanical reasons, it is preferred that the wire is coaxial with the duct.
The same arrangement may be used to charge the particulate material in the tube 32, or to supplement the triboelectric charge on said material.
The invention may be used, for example, for the spraying or dusting of crops, seed dressing, weed killing, mineral dressing, artificial pollina-tion, the coating of metals with plastics, the coating of rubber with talc, and the application of corrosion inhibitors such as borax to exposed metal surfaces.
By way of example, utilising the apparatus illustrated in the accompanying drawing in which the tubes 10, 12, 20 have an internal diameter of 8 mm and the length of the closed loop is 2 metres, and using an air flow velocity of about 35-S0 metres per second and a loading of 5-25 g of powder in the airstream, potentials of up to 200kV can be obtained on the capacitor surface.
Claims (26)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for electrostatically depositing particulate material and comprising in combination a duct defining a closed loop path for a flowing medium; means for causing a flow through the duct of a medium com-prising particulate material suspended in a non-conducting fluid vehicle, the arrangement being such that electrical charges are induced triboelect-rically on the particulate material carried by the vehicle along a length of the duct; capacitor means located outside the duct and electrically insul-ated so as to avoid charge leakage; electrical discharge means electrically connected to said capacitor means and located in the duct but electrically insulated from any conducting portion of the duct for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for suspending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.
2. Apparatus for electrostatically depositing particulate material and comprising in combination a duct defining a closed loop path for a flowing medium; means for causing a flow through the duct of a medium comprising particulate material suspended in a non-conducting fluid vehicle;
means for inducing by corona discharge an electrical charge on the particulate material carried by the vehicle through the duct; capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically connected to said capacitor means and located in the duct but electrically insulated from any conducting por-tion of the duct remote from said charging means for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for suspending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.
means for inducing by corona discharge an electrical charge on the particulate material carried by the vehicle through the duct; capacitor means located outside the duct and electrically insulated so as to avoid charge leakage;
electrical discharge means electrically connected to said capacitor means and located in the duct but electrically insulated from any conducting por-tion of the duct remote from said charging means for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for suspending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.
3. Apparatus as claimed in claim 2 in which the electrical charge inducing means includes at least one electrode extending into the duct and electrically insulated from the adjacent duct wall, and means for establish-ing a corona discharge between said electrode and the adjacent duct wall.
4. Apparatus as claimed in claim 1 in which one part of the duct wall is of electrically conductive material or other material capable of dissipating electrical charge built up on said part of the duct wall and another part of the duct wall is of electrically non-conductive material, and the electrical discharge means extends through said non-conductive part.
5. Apparatus as claimed in claim 4 in which a part of the length of the duct is formed of electrically non-conductive material and the capacitor means is located adjacent that length.
6. Apparatus as claimed in claim 5 in which the duct includes two tubes of electrically conducting material connected together by a tube of electrically non-conductive material.
7. Apparatus as claimed in claim 6 in which the two tubes of elect-rically conducting material extend parallel to each other and in close proximity to each other and the tube of electrically non-conductive material comprises a U-bend connecting the adjacent ends of said tubes of electrically conducting material together, said capacitor means being in the form of a toroid fitting over the two arms of the U-bend.
8. Apparatus as claimed in claim 6 or claim 7 in which the two tubes of electrically conductive material are twisted about one another in helical fashion.
9. An apparatus as claimed in claim 4 including one or more earthed needles projecting into the tube through said non-conductive part of the duct wall each needle being upstream or downstream of said electrical dis-charge means.
10. Apparatus as claimed in claim 4 in which at least the inner face of said one part of the duct wall has a soft surface.
11. Apparatus as claimed in claim 10 in which said one part of the duct wall comprises a duct length formed of resin impregnated fabric the inner surface of which is lined with rubber.
12. Apparatus as claimed in claim 1 or claim 2 in which the inner face of the duct for substantially the entire length thereof is of soft electrically non-conductive material, and means are provided remote from the electrical discharge means for removing surface charges from the wall of the duct.
13. Apparatus as claimed in claim 12 in which said charge removing means comprise an earthing wire extending axially of and within the duct and/or earthed corona discharge electrodes protruding into the duct at spaced intervals.
14. Apparatus as claimed in claim 1 or claim 2 wherein the electrical discharge means comprise corona discharge means extending into the duct.
15. Apparatus as claimed in claim 1 or claim 2 wherein the electrical discharge means comprise a length of duct the inner face of which is formed of electrically conducting material which is electrically insulated from the remainder of the duct and from earth.
16. An apparatus as claimed in claim 1 or claim 2 in which the capacitor is toroidal and surrounds a part of the length of the duct.
17. An apparatus as claimed in claim 1 or claim 2 including means for increasing the turbulence of the flow of the medium in the duct.
18. Apparatus as claimed in claim 17 in which said turbulence in-creasing means comprise baffles located in the flow path.
19. Apparatus as claimed in claim 1 or claim 2 in which the means for electrically charging the particulate material to be projected adjacent said capacitor surface includes means for transferring a part of the charge from the capacitor surface to said material.
20. Apparatus as claimed in claim 1 or claim 2 in which the means for electrically charging the particulate material to be projected adjacent said capacitor surface includes corona discharge means.
21. An apparatus as claimed in claim 1 or claim 2 including a re-servoir for-the separate supply of particulate material.
22. A method of electrostatically depositing particulate material on to an earthed surface, said method comprising in the vicinity of the earthed surface directing a flow of gas carrying charged particulate material adjacent to a capacitor surface which is charged to a high voltage of the same polarity as the charge on the particulate material by transferring to said capacitor electrical charges developed on a separate particulate material which is being transported by a non-conducting fluid medium round a closed circuit path which includes a region where the particles are charged remote from where the charge is transferred to the capacitor.
23. A method as claimed in claim 22 in which the separate particulate dielectric material is charged triboelectrically and/or by corona discharge.
24. A method as claimed in claim 22 in which the non-conducting fluid medium is air and the separate particulate dielectric material comprises a powdered solid.
25. A method as claimed in claim 22 for electrostatic coating of surfaces with particulate material.
26. A method as claimed in claim 22 for applying particulate material to vegetable matter.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB53939/73A GB1487325A (en) | 1973-11-21 | 1973-11-21 | Electrostatic deposition of particles |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1039125A true CA1039125A (en) | 1978-09-26 |
Family
ID=10469465
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA213,555A Expired CA1039125A (en) | 1973-11-21 | 1974-11-13 | Electrostatic deposition of powder or liquid droplets in air by triboelectric or corona discharge means |
Country Status (10)
| Country | Link |
|---|---|
| JP (1) | JPS5930468B2 (en) |
| BR (1) | BR7409676A (en) |
| CA (1) | CA1039125A (en) |
| DE (1) | DE2455161C2 (en) |
| FR (1) | FR2251378B1 (en) |
| GB (1) | GB1487325A (en) |
| HU (1) | HU173050B (en) |
| IL (1) | IL46037A (en) |
| IT (1) | IT1025847B (en) |
| ZA (1) | ZA747232B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10137463B2 (en) | 2012-01-27 | 2018-11-27 | Durr Systems Gmbh | Coating agent line with grounding element |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4168327A (en) * | 1976-03-05 | 1979-09-18 | Research Corporation | Space-charge controlled electrostatic spraying |
| DE3529703C1 (en) * | 1985-08-20 | 1986-08-28 | Ransburg-Gema AG, St. Gallen | Spraying device for electrostatic powder coating |
| GB8611035D0 (en) * | 1986-05-06 | 1986-11-26 | British Aerospace | Protecting articles from particle bombardment |
| DE59301228D1 (en) * | 1992-03-06 | 1996-02-08 | Frei Siegfried | Method and device for monitoring the spray flow in a powder coating system |
| DE19636234C5 (en) * | 1996-09-06 | 2005-02-17 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method and device for the electrostatic coating of substrates |
| DE19730231A1 (en) * | 1997-07-15 | 1999-01-21 | Abb Research Ltd | Process for electrostatic coating |
| KR101713791B1 (en) | 2012-03-05 | 2017-03-08 | 엠파이어 테크놀로지 디벨롭먼트 엘엘씨 | Flexible circuits |
| DE102017002623A1 (en) | 2017-03-20 | 2018-09-20 | Reinhold Gregarek | Improved tribostatic I-I-P process, tribostatic powder nozzle and use for the production of electro-technical multilayer composites |
| CN110203701B (en) * | 2019-06-05 | 2024-05-17 | 中国石油大学(北京) | Bent pipe electrostatic protection device, pneumatic conveying experiment system and experiment method |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1577757B2 (en) * | 1951-01-28 | 1973-09-06 | METHOD AND DEVICE FOR ELECTROSTATIC COATING OF AN OBJECT WITH PULVERIZED COATING MATERIAL |
-
1973
- 1973-11-21 GB GB53939/73A patent/GB1487325A/en not_active Expired
-
1974
- 1974-11-12 ZA ZA00747232A patent/ZA747232B/en unknown
- 1974-11-12 IL IL46037A patent/IL46037A/en unknown
- 1974-11-13 CA CA213,555A patent/CA1039125A/en not_active Expired
- 1974-11-19 BR BR9676/74A patent/BR7409676A/en unknown
- 1974-11-20 HU HU74CO309A patent/HU173050B/en not_active IP Right Cessation
- 1974-11-20 IT IT29625/74A patent/IT1025847B/en active
- 1974-11-21 FR FR7438268A patent/FR2251378B1/fr not_active Expired
- 1974-11-21 JP JP49134206A patent/JPS5930468B2/en not_active Expired
- 1974-11-21 DE DE2455161A patent/DE2455161C2/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10137463B2 (en) | 2012-01-27 | 2018-11-27 | Durr Systems Gmbh | Coating agent line with grounding element |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2251378A1 (en) | 1975-06-13 |
| IT1025847B (en) | 1978-08-30 |
| IL46037A (en) | 1976-09-30 |
| ZA747232B (en) | 1975-12-31 |
| JPS5930468B2 (en) | 1984-07-27 |
| AU7551574A (en) | 1976-05-20 |
| HU173050B (en) | 1979-02-28 |
| DE2455161A1 (en) | 1975-05-22 |
| BR7409676A (en) | 1976-05-25 |
| JPS50112439A (en) | 1975-09-03 |
| IL46037A0 (en) | 1975-02-10 |
| FR2251378B1 (en) | 1979-08-10 |
| GB1487325A (en) | 1977-09-28 |
| DE2455161C2 (en) | 1986-12-04 |
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