IL46046A

IL46046A - Corona charging apparatus

Info

Publication number: IL46046A
Application number: IL46046A
Authority: IL
Original assignee: Coffee R
Priority date: 1973-11-21
Filing date: 1974-11-13
Publication date: 1977-01-31
Also published as: IL46046A0; FR2251934A1; DE2455162A1; IT1025848B; JPS50112440A; AU7551674A; GB1487310A; BR7409675A; FR2251934B1; HU171248B; CA1029433A

Description

Corona charging apparatus RONALD ALAN COFFEE 9 This invention relates to corona charging apparatus for applying an electric charge to particulate material such as liquid droplets or a powdered solid. The use of corona discharge apparatus for applying an electric charge to a powder is already known. Commonly, the powder is blown through a tube formed of electrically conductive material and connected to earth. An electrode of a shape suitable for establishing a corona discharge projects into the duct and is connected to a generator producing an output voltage of several kilovolts in order to establish a corona discharge within the duct. In use, powder is blown along the duct. Part of the current from the generator goes to charge the powder and the remainder is used to maintain the corona discharge and flo*iS to earth via the walls of the duct.

The present invention provides apparatus of this type in which the current drawn from the high voya e generator may be reduced.

According to the invention, apparatus for applying an electric charge to particulate material comprises a duct having at least the inner surface of its walls formed of an electrically conductive material, means for causing a stream of electrically non-conducting fluid containing said particulate material to flow through the duct, an earthed corona discharge electrode within said duct and means for applying a voltage to the inner surface of said duct.

In a preferred embodiment, the corona discharge electrode comprises an electrically conducting wire extending in the axial direction within, and preferably '9 coaxially with, said duct.

The non-conducting fluid will normally be a gas and most conveniently is air.

Since, in accordance with the invention, the corona discharge electrode is earthed, that part of the corona current which goes to charge the material is drawn from earth and only that part of the current which flows to the walls of the duct need pass through the voltage source.

In accordance with one embodiment, the charge induced on the particulate material may be used to provide a high voltage source. In this embodiment, the duct forms part of a conduit defining a path for the stream of fluid, and the apparatus further includes capacitor means which are located outside the walls of said conduit and are electrically insulated so as to avoid charge leakage, and electrical discharge means electrically connected to said capacitor means tmd iocated in but electrically insulated from said conduit downstream of said duct for transferring electrical charge from the particulate material charged in said duct to said capacitor means whereby to build up a charge on an outside surface thereof.

If the conduit is arranged to define a closed loop path for the stream of fluid, very high surface charges can be obtained on the capacitor means.

Conveniently, the conduit defining the closed loop may comprise in addition to said duct two tubes of electrically insulating material, connected one at each end of said duct. The capacitor means may suitably comprise a toroid surrounding one of said tubes downstream of the duct but it may also, for example, be spherical in shape.

The invention will now be described in more detail with reference to one embodiment thereof and with the aid of the accompanying drawing which is a schematic diagram illustrating the application of the invention to the provision of a very high voltage source.

Referring to the drawing, a tube 10, of electrically conductive material, such as aluminium, is connected to form a closed circuit with two tubes 12 and 14 of electrically insulating material and a fan 16. The tubes 10, 12 and 14 contain powder so that, when the fan 16 is in operation, such powder is borne round the closed circuit in a stream of air.

Within the tube 10, a co-axial electrically conductive wire 18 is stretched between end supports 20 and 22 of insulating material. The wire 18 is electrically connected to a second wire 19 which passes through an insulator in the wall of the tube 10 and has its outer end connected to earth. The wall of the tube 10 is connected to the output of a generator 26 which produces an output voltage in the range 5 to 5 kV.

The tube 14 which, it will be recalled, is of electrically insulating material, is surrounded by a toroid 28 of electrically conductive material. A needle 30 of electrically conductive material projects from within the toroid 28, to which it is electrically connected, into the tube 14. The fan 16 is arranged to circulate powder within the tube in a clockwise direction as viewed in the drawing so that the needle 50 is downstream of the tube 10.

In use, with the generator 26 in operation, the fan '» 16 is caused to circulate powder within the tubes 10, 12 and 14. A corona discharge is established from the wire l8, drawing current from earth through the wire I9. Part of this current goes to charge the powder passing through the tube 10 and the remainder flows through the walls of the tube 10 and the generator 26. With a powder flow rate of 10 kg per hour, typically as much as 75# of the current flowing from earth through the wire 22 may go to charge the powder. Thus, if the generator 26 is capable of supplying a current of 25 A, the total current flowing through the wire 19 can be as high as lOO^A.

With prior art apparatus, the total current would have been limited to the current which could have been supplied by the generator 26.

As the charged powder circulates into the tube 14, a corona discharge is established at the tip of the needle 30 so that charge is transferred from the powder to the toroid 28. This charge transfers to the outer surface of the t.oroid 28 through the Faraday cage effect and this surface rises to a very high voltage of the order of 250 kV, or even higher, depending upon the efficacy of the insulation.

In order to prevent the risk of corona discharges being established at any sharp edges which may occur at the ends of the tube 10, the tubes 12 and 14 are formed with spigots 32 and 3 respectively which cover the inner surface of the ends of the tube 10.

In some circumstances, a significant amount of triboelectric charging of the powder may take place.

In this case, the materials of which the tubes 10, 12 and 14 are made are preferably chosen to be such that, in conjunction with the powder used, the polarity of the charge on the powder due to triboelectric charging is the same as that due to corona charging.

The invention may also be used in the form of a straight through system as an apparatus for the electrostatic deposition of particulate material, which may be a solid in powdered form or liquid droplets. In this case, the particulate material with which an object is to be coated is ejected from the duct towards said object after being charged by corona discharge during passage through said duct.

If desired, a capacitor means may bo provided having a surface in the vicinity of the path of the ejected materiel, together with means for charging said surface to a high potential of the same polarity as the charge on the particulate material.

Whereas the illustrated embodiment is described with reference to the use of powder as the particulate material, liquid droplets may be used instead by providing a source of liquid and means for atomising the liquid into a gas stream to provide a "mist" which is maintained at least while the droplets are charged and travel past the electrical discharge means.

Claims

1. Apparatus for applying an electric charge to particulate material, said apparatus comprising a duct having at least the inner surface of its walls formed of an electrically conductive material, means for causing a stream of electrically non-conducting fluid containing said particulate material to flow through the duct, an earthed corona discharge electrode within said duct and means for applying a voltage to the inner surface of said duct.

2. Apparatus as claimed in claim 1 in which the corona discharge electrode comprises an electrically conductive wire extending in the axial direction within said duct.

3. Apparatus as claimed in claim 2 in which the electrically conductive wire is coaxial with said duct.

4. . Apparatus as claimed in claim 1 or claim 2 in which the duct forms part of a conduit defining a path for the stream of fluid, said apparatus further including capacitor means which are located outside the walls of said conduit and are electrically insulated so as to avoid charge leakage, and electrical discharge means electrically connected to said capacitor means and located in but electrically insulated from said conduit downstream of said duct for transferring electrical charge from the particulate material charged in said duct to said capacitor means whereby to build up a charge on an outside surface thereof.

5. Apparatus as claimed in claim 4 in which the conduit defines a closed loop path for the stream of fluid.

6. Apparatus as claimed in claim 5 in which the conduit comprises in addition to said duct two tubes of electrically insulating material, conneoted one at each end of said duct.

7. Apparatus as claimed in claim 6 in which the capacitor means comprise a toroid surrounding one of said tubes downstream of said duct.

8. Apparatus as claimed in any of claims 1 to 3 further including means for ejecting particulate material from the duct after it has been charged, capacitor means located with a surface thereof in the vicinity of the path of charged particulate material ejected from said duct and means for charging said capacitor surface to a high potential of the same polarity as the charge on the particulate material.