Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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.,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
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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.
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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-
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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
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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.
