Electrostatic Spraying Apparatus

Abstract

A spray gun comprising a spray nozzle and a high-voltage generator consisting of a passage for conveying a gaseous fluid toward said nozzle, an electrode for imparting electric charges to said gaseous fluid, and a convergent-divergent portion in said passage downstream of said electrode for imparting a supersonic speed to the gaseous fluid carrying the electric charge. The gun comprises means for transmitting electric charges from the gaseous fluid to the nozzle.

Parent Case Text

This application is a continuation of Ser. No. 797,996, filed Feb. 10, 1969, and now abandoned.

Description

The present invention relates to electrostatic spraying apparatus of the type having an electric generator incorporated in the body of the sprayer.

It has been proposed to incorporate, in an electrostatic spray gun, the high-voltage generator which supplies the electric charge at high potential to the product to be sprayed. Up to now such proposals have not resulted in any practical use, mainly because the generators used are relatively heavy and cumbersome. However, the incorporation of the generator in the spray gun offers numerous advantages. Thus it eliminates the high-voltage connecting cable which is heavy to handle and which presents a great danger through the capacitive energy which it retains and which it releases in the case of an accidental earthing of the charge electrode, or of the product to be sprayed, at the outlet from the spray nozzle.

According to the invention a spray gun comprising a spray nozzle incorporates a high-voltage generator consisting of a passage for conveying electric charges to a convergent-divergent nozzle for imparting a supersonic speed to a gaseous fluid carrying the said electric charge, and means for transmitting electric charges disposed in the region of the neck of the said convergent-divergent nozzle. Advantageously the fluid of the generator serves another function, either as a driving agent for a rotating spray head for example, or as a gaseous spray fluid for a liquid.

The invention will be further described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of one embodiment of a spray gun according to the invention,

FIG. 2 is a schematic view of a spraying installation,

FIGS. 3 to 5 are sections of other embodiments of the spray gun according to the invention,

FIG. 6 is a view along the line VI--VI OF FIG. 7,

FIG. 7 shows a further embodiment of spray gun,

FIGS. 8 and 9 are sectional views of two further embodiments of a spray gun according to the invention,

FIG. 10 is an enlarged view of the cartridge-generator used in the spray gun of FIG. 9; and

FIGS. 11 to 18 are sections of other embodiments of spray gun.

Referring to FIG. 1, an electrostatic paint spray gun comprises an insulating body 1 extending between a spray nozzle 2 and a rear part forming a handgrip. The insulating body 1 is provided with a passage 4 for the paint, connected by a union 5 to a flexible pipe 6 leading to a paint source under pressure, or to a volumetric pump supplying the paint. As can be seen in the drawing, the paint passage 4 communicates at 4' with a chamber 7 located upstream of a nozzle passage 8 which can be closed by a valve 9 operable by a valve rod 10 controlled by a trigger 11.

The insulating body 1 also contains another passage 12, which communicates at its upstream end with a humidification or damping chamber 13 and also with a passage 14 for the admission of air under pressure. This passage is provided with a valve 15, also controlled by the trigger 11.

In the passage 12, on the upstream side in he direction of the air circulation, is provided a means for the transmission of electric charges, constituted by a needle 16 axially placed at the center of a ring 17 forming a convergent-divergent nozzle. The needle 16 is connected by a conductor stud 18 to a plug 19 in the end o f a cable 20 joined to an excitation source of the order of 2 to 5 kilovolts. The ring 17 forming a counterelectrode is itself connected by a spring 21 to a conductor cover 22 extending towards the rear part including the handgrip of the spray gun 3 and maintained at earth potential by metal braiding 23 of a cable cover covering both the pipe 6 for the admission of the paint and the excitation cable 20.

The outlet at the downstream end of the passage 12 is sealed by a ball valve 24 subjected to the action of a compression spring 25, the while being located in a chamber 26 communicating via passage 27 with an annular distribution chamber 28 disposed between the nozzle body 29 and a nozzle cap 30. This chamber 28 communicates in a normal way, for example by any means imparting a swirling effect to the air, with an annular passage 31 concentric with and surrounding the paint outlet.

One end of a porous wick 32 extends into the damping chamber 13, its other end being located in a tank 33 formed in the handgrip and filled with water.

In operation, pressure on the trigger 11 opens the valve 9 to allow the ejection of the paint; and also opens the valve 15 to allow the circulation of air in the passages 14, 12 and 31 (beyond which it pulverizes the paint). It may also cause the application of the voltage over the excitation cable 20 to the extent where, by any appropriate means, the excitation is controlled by the circulation of the air so that the needle 16 transmits electric charges into an air current circulating at high speed, at least partially supersonic, in the passage 12, the valve 24 being displaced towards the open position under the effect of repelling the air from the passage 12. The electric charges transmitted by the needle 16 are, due to the rapid circulation of air in the passage 12 carried to a high potential and these charges are collected by the ball 24, the spring 25 and from there, are transmitted to the conductive nozzle body 29. This allows a correct charge of the paint flowing through the passage 8 and this paint is moreover pulverized by the air which has served to convey the electric charges. The humidification chamber 13 is particularly useful because it allows the air under pressure which arrives from the passage 14 to acquire a higher hygrometric value which is favorable to the formation, by condensation and/or elimination of liquid and/or solid particles making up the transporter of electric charges.

In the embodiment described, the humidifier consists of a water tank 33 closed by a stopper 34 and is refilled from time to time, but it is obvious that a pipe could be used to supply water to the humidification chamber 13.

Referring to FIG. 2, a spray gun 41 of the type described with reference to FIG. 1 is used, but which does not incorporate the humidification chamber. The humidification is effected by making the air under pressure pass into a saturated atmosphere of a water tank 42 with a porous wall 47, the air in the pipe 43 being thus sufficiently charged with water vapor to be able to transport electric charges. The paint pipe 44 and excitation conductor 45 pass through the handgrip 46.

By way of example, the air admitted upstream of the valve 17 may have a pressure from 2 to 4 bars and a flow of 6 to 12 m..sup.3 /h. into a passage 12 having a diameter of 3 mm. and a length of 5 cm.

Referring now to the embodiment of FIG. 3, a spray gun body 50 of insulating material comprises a passage 51 between a spray nozzle 52 and a rear part forming a handgrip 53. The passage 51 is connected at its upstream end to a charge transmitter of the type previously described, having a needle 55 in the center of a ring 56 forming a counterelectrode and at its downstream end to the spray nozzle 52. The passage 51 is connected by a pipe 57 to a source of air transporting the powder to be sprayed, while the needle 55 is connected by a cable 58 to an excitation source. The counterelectrode 56 is connected electrically to earth, that is to say to the potential of the handgrip 53. In operation, the powder entrained by the air through the pipe 57 passes through the perforations of the needle-centering device 59, passes at high speed in the area of this ionization needle 55, where the particles receive electric charges and whose potential is considerably increased by the circulation at high speed in the passage 51, so that the particles emitted through the nozzle 52 are charged at a sufficiently high potential to ensure their electrostatic attraction on to the object to be sprayed, which is generally at earth potential.

FIG. 4 shows a modification of the embodiment in FIG. 3 wherein the air transporting he powder is conveyed through a pipe 54 discharging between the nozzle 56 and the support 59 for the needle 55, so that it is no longer necessary to perforate the support 59.

Referring to the embodiment of FIG. 5, an atomization nozzle 60 is made up of a plurality of channels 61 located in an atomization chamber 62. This chamber 62 is fed by a liquid inlet passage 63 connected by a union 64 and a pipe 65 to a paint source (not shown). Opposite the outlet of passage 63 is disposed a passage 66 provided in an insulating support 67. This passage 66 serves to transport the electric charges from an upstream zone where a transmitter of electric charges 68 is located to the atomization chamber 62. The passage 66 is connected, upstream of the charge transmitter 68 to a source of air under pressure.

Thus, as has been explained previously, from the air circulating at great speed in passage 66, the charges which this air acquires at the place of the charge transmitter 68 are thus carried to a raised potential at the level of the atomization chamber 62. It is understood that the jet of liquid from the passage 63 and the jet of gas from the passage 66 meet in the chamber 62 which ensures, as is known, pneumatic atomization in the said chamber. But moreover, the electric charges transported by the air into the passage 66 are transmitted to the pulverized liquid particles so that these liquid particles ejected from the channels 61 of the nozzle 60, which can also be produced of insulating material, are charged electrically at a potential sufficient to ensure their attraction to an object at earth potential to be painted.

In the embodiment of FIGS. 6 and 7, a powder spray device comprises a spray nozzle 70 mounted at the end of and transversely of an insulating support 71, having a passage 72 provided with an ionizer 73. The passage 72 communicates with a passage 74 connected through a valve 75 to a gas source under pressure (not shown).

The spray nozzle 70 comprises a cylindrical cavity 76, into which passage 77 discharges axially, this passage being jointed to a pipe 78 serving to feed the air transporting particles to be sprayed. A deflector 79 faces the outlet from the passage 77. As can be seen from FIG. 6, the passage 72 discharges tangentially into the cavity 76 so that the air from the passage 72 moves the particles from the conduit 77 in a whirling manner and imparts an electric charge to them. At the outlet from the nozzle 70 these particles create an electric field which extends to an object to be spray coated. The whirling effect produced by the tangential inlet of the air from the passage 72 produces not only the electric charge of the particles but also in the chamber defined by the bottom of the cavity 76 and the deflector 79, a centrifugal effect on the particles. This effect ensures the spreading of the jet at the outlet nozzle 70 and a permanent sweeping of the nozzle, thus avoiding the risk of clogging by solid particles.

In the embodiment of FIG. 8, a powder spray 80 comprises a body of insulating material 81 in which is located an insulating sleeve 82 carrying a metallic member 84 forming an annular counterelectrode 83, while the needle 85 is connected by a resistance 86, a plug 95 and a contact 96 to a supply cable 87. The member 84 is joined to an earth wire 88. The outlet from the sleeve 82 communicates with an annular chamber 89 of an atomization nozzle 90 entirely made of an insulating material. The powder to be sprayed is fed to the nozzle by a passage 91. Air under pressure is admitted into pipe 92 which discharges through perforations 93 into the member 84 and from there into the passage 82 in the vicinity of the ionizer needle 85. In this embodiment the air admitted through the pipe 92 transports into the passage 82 electric charges which are, as described previously, carried to a high potential at the outlet of the passage 82. This air is used to spray the powder issuing from the nozzle 90. As the spraying is being carried out, the electric charges are transmitted to the powder particles to be sprayed which are thereby attracted by an object to be coated.

It will be noted that in this embodiment, the sleeve 82 transporting the electric charges is easily detachable and the unit can be detached and put back into place by simply releasing the nozzle 90 and a locknut 94.

Referring now to FIGS. 9 and 10, an electrostatic spray device 100 comprises a barrel of an insulating material 101 with an atomization nozzle 102, the whole being made of an insulating material. The atomization nozzle is supplied with paint through a channel 103, while an annular chamber 104 communicates with the outlet of a detachable cartridge 106 constituting the voltage elevator; shown more clearly in FIG. 10. This cartridge 106 is received in the passage 107 of the barrel 101, it abuts against a joint 108 and is retained in position by a cap 110. The cartridge 106 (FIG. 10) is made up of a cartridge body in which is inserted a tubular sleeve 111 with the means of ionization mounted in fixed positions; namely a needle 112 supported by a cruciform block 113, the needle being engaged axially in the interior of an annular metallic ring 114 of convergent-divergent form. The needle 112 is connected within the cartridge to a resistance 115 whose other terminal is connected by a spring 116 to a conductor terminal 117 covered by an insulating cap 118. The terminal 117 and the counterelectrode 114 are in contact with a socket 127 and 119 respectively. As shown in FIG. 9, contact and spring assemblies 120, 123 and 121, 122 mounted in the body of the spray gun electrically connect the needle 112 and the counterelectrode 114 respectively to the excitation cable 124 and to earth via conducting cover 125, which forms the handgrip.

In this embodiment, the air issuing from the sleeve 111 serves both for atomization and for direct transmission of the charge to the particles, given that the nozzle is entirely made of an insulating material. It will be noted that the generator is mounted in a cartridge which is easily detachable and which can thus be replaced by a new cartridge in case of breakdown.

FIG. 11, shows an embodiment of paint spray gun 130 for a fixed installation, comprising a paint supply passage 131 feeding an atomization nozzle 132 consisting of an axially arranged conductor nozzle 133 and a nozzle cap 134 of an insulating material. The axial nozzle 133 can be sealed by an insulating pin 135 controlled in position by a piston 136 movable in a cylindrical body 137. The piston moves against the action of a spring 138 under the action of compressed air admitted through inlet 139 to withdraw the pin from the nozzle 133. This compressed air can escape through a passage 140 discharging into the generator 141 composed, as previously, of an insulating passage 142 and an ionization means 143. The downstream end of the passage 142 discharges into an annular chamber 144, then through passages 145 towards an outlet atomization orifice. A collecting electrode 146 is positioned in the chamber 144, this electrode being connected to the metallic nozzle 133 through the insulating nozzle member.

In this embodiment, the electric charges are thus picked up by the electrode 146 and finally transferred to nozzle 133. It can be seen that a resistance for stabilizing the discharge 147 is fixed in the axial plug 148 of a connector 149, this axial plug 148 being connected to earth and to the counterelectrode 150 of the ionization device by a wire 151.

The embodiment of FIG. 12, has the same general arrangement as that in FIG. 11, with the difference that the nozzle 152 is also made of an insulating material like the cap 134 and a connection 153 is established from the outlet of the passage 142 to penetrate into the internal cavity of the nozzle 152 so as to establish an electric link picking up the charges at the outlet of the passage 142 and conveying them to the interior of the liquid destined to be sprayed, within the nozzle 152, slightly upstream of the discharge from this nozzle. Here, the discharge stabilization resistance 154 is no longer in series with the needle 143, but with the ionization counterelectrode 150 in the insulating body 155 of the spray gun. In this embodiment, the pin 156 for sealing the nozzle is controlled by a piston 157 acted on by the air admitted through connection 158, while the air admitted into the passage generating the high-voltage current is fed through connection 159.

In FIG. 13, a paint spray gun comprises an insulating body with a passage 151 for the generation of electrical charges at a raised potential discharging into an annular chamber 152 and then through passages 153 between a metallic nozzle member 154 and an insulating cap 155. The paint arrives through the axial channel 156. In this embodiment, the air charged with particles at raised potential communciates these charges to the conducting nozzle 154, which then applies a potential to the jet of paint which circulates within this nozzle member 154.

Referring supported the embodiment of FIG. 14, a powder spray gun comprises an insulating body 160 with a channel 161 supplied with fluidized powder and a passage 162 for the generation of particles at high potential. This latter passage 162 discharges into an annular chamber 163 which communicates with the interior of a metallic nozzle 164 through tangential conduits 165. A deflector 166 is positioned in the end of the conduit 161 at some distance from its outlet. The nozzle 164 is supported by a conductor means 167 which serves for collecting the electric charges transmitted by conductivity to the discharge edge 168 of the nozzle 164.

In the embodiment of FIG. 15, an electrostatic paint spray device 170 of the with with hydrostatic pulverization comprises an insulating body 171 with a metallic nozzle 172 of very fine section, generally in the form of a slot, placed on an insulating support 173 located in a cover 174. The support 173 abuts a member 175 forming the seat for an insulating pin 176. A channel 177 for paint under high pressure communicates by appropriate sealed joints with a chamber 178. A tube 179 for generating particles at raised potential is arranged in the cover body 171 of the sprayer and its end leads into an annular chamber 180 with forwardly directed orifices 181. A rigid wire 182 is mounted in the cover 174 and extends from the interior of the chamber 180 to a small distance from the jet of paint sprayed an atomized by hydrostatic action.

In this embodiment, the air coming from the passage 179 generating the particles at raised potential escapes through the openings 181 so as to form a series of air jets protecting the paint against any return towards the rear of the pulverized particles. The air, before leaving the chamber 180, is freed from the electric particles which are picked up by the electrodes 182 whose other end serves to change by discharge the particles from the atomized paint spray.

Referring to the embodiment of FIG. 16, a hydrostatic pressure spray gun 190 comprises a paint supply pipe 191 discharging into a chamber 192 having a pin valve 193 and communicating with a nozzle 194 through a passage 195. The passage 196 for generating particles at high potential discharges into a chamber 197. All the components of this nozzle are made of an insulating material, with the exception of a threaded rod 198 which penetrates into the chamber 197, and which at its other end holds an axial discharge needle 199 in position which protrudes beyond the nozzle in the middle of the atomized spray jet. In this embodiment, it can be seen that the paint is atomized by the jet of air issuing from the passage 196 and from the annular chamber 200, which communicates with the exchange chamber 197 for the electric charges in which the charges are collected by the rod 198.

Referring now to FIG. 17, a portable paint spray gun comprises a metal handgrip 201 connected to earth to which is fixed an insulating barrel 202 at the end of which is positioned a pneumatic spray nozzle 203.

Within the insulating barrel 202 is arranged a passage forming a nozzle 204 supplied through a chamber 25 itself joined through a pipe 206 to a source of compressed air--not shown. The nozzle 204 comprises, in the longitudinal direction, first of all a part 204a of a convergent-divergent shape immediately adjacent to the chamber 205, followed by a divergent part 204b, while the rest of the nozzle 204c is cylindrical.

In this way a nozzle of the Laval type is produced, that is to say a convergent-divergent nozzle creating in the cylindrical part a flow at supersonic speed presenting the character of regularity and parallelism of streams of air peculiar to this type of flow.

As can be seen, this cylindrical part is followed by a main part 204d following the arc of a circle and discharging into the atmosphere at a lateral position from the spray. A spray passage 204e feeds the spray nozzle 203, which can be of the type creating a whirling action of the air. Upstream of the nozzle 204, in the region of the convergent-divergent part 204a, 204b, a means of ionization is provided consisting of a pointed electrode 207 connected by a cable 208 to a DC source of medium voltage, not shown, the said electrode 207 being situated axially in the center of a counterelectrode 209 connected to earth by a conductor 210 of annular form and delimiting the convergent divergent part 204a, 204b, of the nozzle 204. At a position spaced from the ionization means 207, 209 and as near as possible to the nozzle 203 is placed in the nozzle 204 a charge-collecting member, for example, in the form of a tensioned wire 11. This extends across the part 204d of the nozzle and is directly connected to a metal nozzle part 212 of the nozzle 203, while the remainder of the nozzle member is insulating.

When the diameter of the part 204a is small (lower than 3 or 4 mm.) a resistance of 10 to 50 megohms is inserted in the ionization circuit so as to stabilize the outlet charge and to avoid the discharge of the junction cable, in series with the pint 207 of the counterelectrode 204a. The member 204a can also be produced in a semiconducting material.

The arrival of paint at the nozzle 212 is affected by a passage shown schematically in 213, this passage being fed from a tank under pressure, or a pump.

In the usual way, the spray gun is provided with a control device or trigger 214 controlling the opening and closing of the compressed air pipe 206, and also possibly the admission of paint and the feeding of the low voltage to the ionizer 207, 209. These latter can alternatively be controlled by the airflow in the pipe 206.

In this embodiment the nozzle 204 between the discharge means 207, 209 and the collector 211 has a length of 5 to 8 centimeters while the diameter in the cylindrical part 204c is about 6 mm.

Experience has shown that by using a source of compress air of 4 bars, a supersonic flow speed of about450 meters per second was obtained, with a DC excitation voltage on the ionizer 207, 209 of 5 kilovolts and a series resistance with the point 207 of 20 megohms, which provides at the nozzle 212 a DC voltage of 40 kilovolts at a current of 30 microamperes.

As can be easily shown, these results show that the spray gun in question is completely suitable to be used as a portable electrostatic spray gun, since it does not comprise any element of weight additional with respect to the spray guns used at present with a separate high-voltage source.

In the embodiment described and shown, the presence of air under pressure is employed in order to derive by means of the nozzle 204e, the quantity of air sufficient to supply the pneumatic spraying nozzle 203. In certain cases, there is not an overabundance of air and the shunt 204d to the atmosphere can be suppressed.

Referring finally to FIG. 18, a spray gun is arranged so its its rear part (handle 221 and body 222) are similar to those described in FIG. 17, but here the atomization head is represented in the form of a cap 223 mounted to rotate continuously on a pivot 224 constituted by an inlet head 225, comprising the paint feed channel 226 communicating with the radial channels 227 discharging opposite one face of the cap 223. The cap 223 is provided at the rear with a driving device 228 of the blade type opposite which the compressed airpipe 229 discharges, forming the terminal part of the electrostatic generator described in the previous figure.

In this embodiment, when the cap 223 is metal or semiconducting, it is not necessary to provide a charge collector in the pipe 229, the role of the latter being played by the blades 228 of the motor member. However, it is obvious that if an insulating cap is used with a metal or resistive electrode, within or adjacent the edge of this cap, it is then necessary as in the previous embodiment, to arrange a charge collector in the pipe 229 connected electrically to the said electrode.

The invention is not limited to the embodiments shown or to the uses described. It comprises other uses such as, for example, the formation of stable aerosols, the atomization of fuels, the generation of high-voltage electricity with the aid of a noncombustible gas such as (nitrogen, carbon dioxide, argon, etc.) or with the aid of a gas and/or chemically active particles with a sprayed product. The invention also applies to flock spraying as well as to the spraying of agricultural products and to the spraying of powdered metals.

Claims

What is claimed is:

1. Electrostatic spraying apparatus including a spray nozzle and a high-voltage generator, said generator comprising a passage for transporting a gaseous fluid to said spray nozzle, said passage having a convergent-divergent portion for imparting a supersonic speed to at least part of said gaseous fluid, and electrode means adjacent said convergent-divergent portion for transmitting electric charges to said gaseous fluid.

2. Apparatus according to claim 1, including means for injecting solid particles into the passage upstream of the the electrode means.

3. Apparatus according to claim 2, including means for injecting a volatile liquid into said passage upstream of the electrode means.

4. Apparatus according to claim 1, wherein the outlet of the passage for the gaseous fluid transporting the electric charges is adjacent to the outlet of said spray nozzle, in a position such that the gaseous fluid contributes to the atomization of a liquid sprayed out of the nozzle.

5. Apparatus according to claim 1, including means for injecting particles of a product to be sprayed close to the electrode means.

6. Apparatus according to claim 1, including a chamber into which a passage transporting the product to be sprayed discharges, as well as the passage for the gas transporting the electric charges, and a further passage leading from said chamber towards said spray nozzle.

7. Apparatus according to claim 1, wherein the nozzle is made of an insulating material and the passage for the gas transporting the electric charges discharges into the atmosphere adjacent and coaxially to the periphery of a passage transporting the product to be sprayed.

8. Apparatus according to claim 1, including means for picking up electric charges situated in the passage for the gas transporting the electric charges downstream of said electrode means and convergent-divergent portion, and electric conductor means electrically connecting said pickup means to a passage for transporting the product to be sprayed.

9. Apparatus according to claim 8, wherein the conductor means is in contact with the product to be sprayed.

10. Apparatus according to claim 9, wherein the conductor means and the pickup means comprises at least one wall of the spray nozzle.

11. Apparatus according to claim 8, wherein the conductor means terminates in an ionizing electrode protruding beyond the spray nozzle.

12. Apparatus according to claim 1, wherein the high-voltage generator is constructed as a detachable cartridge.

13. Apparatus according to claim 3, wherein the means for injecting the volatile liquid is a wick immersed in a tank and discharging into the passage.

14. Apparatus according to claim 1, in which said nozzle is rotatable and driven in rotation by said gaseous fluid.

15. Electrostatic spraying apparatus comprising a body carrying a spray nozzle, a passage in said body leading to the region of said nozzle, a convergent-divergent portion located upstream of said passage, an electrode arranged adjacent said convergent-divergent portion, means for applying a voltage to said electrode, means for feeding a gaseous fluid to said convergent-divergent portion so as to impart a supersonic speed to at least part of said fluid, as said fluid passes along said passage carrying an electric charge to the region of said spray nozzle.

16. In electrostatic spraying apparatus, a high-voltage generator comprising a passage, a convergent-divergent portion communicating with said passage, electrode means adjacent said convergent-divergent portion, means for applying a voltage to said electrode means, and means for feeding a gaseous fluid to pick up electric charges from said electrode means and so that said charged fluid flows down said passage at least partially at supersonic speed.

17. Electrostatic spraying apparatus including a spray nozzle and a high-voltage generator, said generator comprising a barrel made of insulating material and defining a passage adapted to carry a pressurized gaseous fluid from a source of such fluid to said nozzle, and electrode means in said passage for transmitting electrical charges to said fluid, said passage having a convergent-divergent portion adjacent said electrode which imparts a supersonic speed to at least part of said gaseous fluid.

18. Electrostatic spraying apparatus as claimed in claim 17 comprising a counterelecrode in said passage adjacent said electrode and adapted to be connected to ground.

19. Electrostatic spraying apparatus as claimed in claim 18 in which said counterelectrode defines said convergent-divergent portion.

20. Electrostatic spraying apparatus as claimed in claim 18 in which a barrel defines a second passage for conducting a material to be sprayed from a source of such material to said nozzle, and said nozzle comprises means for causing said gaseous fluid to impinge on said material to be sprayed, and thereby transmit thereto electrical charges transmitted to said fluid by said electrode.

21. Electrostatic spraying apparatus as claimed in claim 18 in which said barrel defines a second passage for conducting material to be sprayed to said nozzle and said nozzle comprises electrically conductive means positioned adjacent the ends of said passages to contact both said gaseous fluid and said material to be sprayed, and thereby transmit to said material to be sprayed electrical charges acquired by said fluid from said electrode.

22. Electrostatic spraying apparatus as claimed in claim 18 comprising means for introducing a material to be sprayed into said passage upstream of said convergent-divergent portion.

23. Electrostatic spraying apparatus as claimed in claim 18 comprising means for introducing a material to be sprayed into said passage upstream of said electrode.

24. Electrostatic spraying apparatus as claimed in claim 18 in which said barrel defines a chamber connecting said passage to said nozzle and comprising an additional passage for material to be sprayed which terminates in said chamber.

25. Electrostatic spraying apparatus as claimed in claim 18 in which said electrode is upstream of said convergent-divergent portion and comprising means for introducing a material to be sprayed into said passageway upstream of said convergent-divergent portion but downstream of said electrode.

26. Electrostatic spraying apparatus as claimed in claim 21 in which said nozzle comprises a head of insulating material defining a separate opening aligned with each passage, and said electrically conductive means is a conductor extending into both openings.

27. Electrostatic spraying apparatus as claimed in claim 21 in which said nozzle comprises an electrically conductive part defining an opening aligned with said second passage, and positioned to be contacted by fluid emerging from the passage having said convergent-divergent portion.

28. Electrostatic spraying apparatus as claimed in claim 21 in which said nozzle is electrically conductive, and the passage containing said electrode reaches said nozzle through a conductive collector ring electrically connected to the walls of said nozzle, said nozzle being provided with a deflector positioned to deflect said material to be sprayed against the wall of said nozzle, whereby electrical charges imparted to said gaseous fluid by said electrode are conducted from said gaseous fluid by said collector to said nozzle and imparted by said nozzle to said material to be sprayed.

29. Electrostatic spraying apparatus as claimed in claim 18 in which said nozzle comprises an electrically conductive part at the end of said second passage, the passage having said convergent-divergent portion being divided into two branches downstream of said convergent-divergent portion, with one of said branches leading to said nozzle so as to conduct to said nozzle particles of gaseous fluid charged by said electrode, and the other bypassing said nozzle and leading to the ambient atmosphere, and said apparatus further comprising an electrical conductor athwart said other branch and leading to the electrically conductive part of said nozzle.

30. Electrostatic spraying apparatus as claimed in claim 18 in which said nozzle comprises a stationary electrically conductive part at the outlet of said second passage, and a rotatable charge collector made of electrically conductive material, said charge collector being positioned in electrical contact with said stationary electrically conductive part and comprising blades athwart the nozzle end of the passage having said convergent-divergent portion, whereby said gaseous fluid acquires an electrical charge from said electrode, is accelerated by said convergent-divergent portion, rotates said charge collector by impinging on said blades, and transmits to said charge collector, and through said charge collector to said stationary nozzle portion the electrical charge acquired from said electrode.

31. Electrostatic apparatus as claimed in claim 18 comprising valve means for controlling the flow of fluid in said passage.

32. Electrostatic apparatus as claimed in claim 21 comprising valve means for controlling the flow through each passage, and single actuating means for simultaneously actuating both valves.

33. In combination, electrostatic spraying apparatus as claimed in claim 18, a source of gaseous fluid under a pressure of several atmospheres connected to said passage, and a source of electric power connected to supply to said electrode an excitation voltage of several kilovolts.

34. Electrostatic spraying apparatus for use with a source of gaseous fluid under a pressure of several atmospheres and a source of direct voltage adapted to supply a voltage of several kilovolts, said spraying apparatus including a spray nozzle and a high-voltage generator, and said generator comprising a barrel made of insulating material, said barrel defining a passage for connecting said gaseous fluid source to said nozzle, an electrode in said passageway adapted to transmit electric charges from said voltage source to the gaseous fluid in said passage and a counterelectrode in said passageway adapted to be connected to ground, said passage having a convergent-divergent portion therein adjacent said electrode which imparts a supersonic speed to at least part of said gaseous fluid, thereby increasing the voltage of the electrical charges transitted thereto, and means for introducing a fluent material to be sprayed into said passage so that at least some of said electrical charges are transferred from said gaseous fluid to said material.

35. Electrostatic spraying apparatus for use with a source of gaseous fluid under a pressure of several atmospheres and a source of direct voltage adapted to supply a voltage of several kilovolts, said spraying apparatus including a spray nozzle and a high-voltage generator, and said generator comprising a barrel made of insulating material, said barrel defining a passage for connecting said gaseous fluid source to said nozzle, an electrode in said passageway adapted to transmit electric charges from said voltage source to the gaseous fluid in said passage and a counterelectrode in said passageway adapted to be connected to ground, said passage having a convergent-divergent portion therein adjacent said electrode which imparts a supersonic speed to at least part of said gaseous fluid, thereby increasing the voltage of the electrical charges transmitted thereto, said barrel defining an additional passage adapted to lead a fluent material to be sprayed to said nozzle, and electrically conductive means positioned to conduct electric charges from said gaseous fluid to said fluent material to be sprayed.