Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSTATIC SPRAYING APPARATUS
FIELD OF THE INVENTIO~
This invention relates to electrostatic spraying
apparatus.
BACKGROUND OF THE INVENTION
One of the advantages of electrostatic spraying is
that the charged spray tends to wrap around the target.
This can be of particular use in, say, agricultural spraying
because the spray will cover both sides of the leaves of a
plant, not merely the outer or upper surfaces as would be
achieved with conventional spraying. Another feature i9
that the attraction of the spray to the target may reduce
the amount lost by drift. More or most of the spray reaches
its intended target. This reduces the total amount of spray
which has to be used which reduces the cost of effective
treatment and is thought to be generally better for the
environment.
Electrostatic spraying apparatus is known in which a
spray head has a spraying edge, an electrically conducting
or semiconducting surface and means for delivering liquid to
be sprayed to the edge via the surface; an electrode spaced
from the edge; and high voltage supply means for generating
a high voltage between the surface and the electrode so
that, in use, when covered by the liquid to be sprayed, the
electric field strength at the edge is intensified
sufficiently, that the liquid at the edge is drawn out
preponderantly by electrostatic forces into ligaments which
break up into electrically charged droplets.
An apparatus falling within this broad type is
disclosed in British patent specification No. 1569707.
An advantage of this apparatus is that the ligaments
break up into droplets having a very narrow spectrum of
diameters. This is preferred because if a droplet of a
particular size is required to carry a lethal dose of an
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insecticide, say, smaller droplet~ are wasted as ineffective
while larger droplets require a larger amount of insecticide
to provide the same number of sites.
In order to treat iarge areas spraying can be
effected from aircraft. Although aerial electrostatic
spraying has been proposed, e.g. European patent application
No. EP-A1-186353, a problem which has not been addressed is
that caused by the airstream past the aircraft. In fixed
wing aircraft used for spraying, there is an airstream past
the vehicle due to its movement and possibly accentuated by
the slipstream from a propeller, of the order of 70 mph.
The problem caused by the airstream, is that turbulence
around the electrostatic spray head interferes with the
formation of the ligaments and thus spoils the spectrum of
droplet diameters or even prevents spraying.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided
apparatus for spraying liquid electrostatically into an
airstream comprising: a spray head having a spraying edge,
an electrically conducting or semiconducting surface and
means for delivering liquid to be sprayed to the edge via
the surface; an electrode spaced from the edge; and high
voltage supply means for generating a high voltage between
the surface and the ~lectrode, the sprayhead and the
electrode being mounted for part of the airstream to pass
between them, the shape and position of the sprayhead and
the electrode producing a sufficiently low turbulence wake
in the region of the spraying edge, and the electric field
at the edge being intensified sufficiently when covered by
liquid to be sprayed, that: the liquid at the edge is drawn
out preponderantly by electrostatic forces into ligaments
which break up into electrically charged droplets.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be
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described, by way of example, with reference to the
accompanying drawings, in which:
Figures 1 and 2 show the general disposition on a
light aircraft of apparatus embodying the invention;
Figure 3 shows a cross section through spraying
apparatus embodying the invention;
Figures 3a and 3b show details of Figure 3;
Figure 4 shows an end view of an alternative
embodiment;
Figure 5 shows the fluid delivery arrangement of the
embodiment of Figure 3; and
Figure 6 shows the electrical circuit of the
embodiment of Figure 3.
DETAILED DESCRIPTION
Referring to Figures 1 and 2, a linear sprayhead 2a,
2b is mounted beneath the trailing edges of each wing of a
light aircraft 3. The position of the sprayhead is chosen
so that the sprayhead is in turbulence free air and such
that the spray is directed substantially parallel to the air
flow and does not end up in any substantial quantity on the
tail plane of the aircraft. The sprayhead is supported by
arms 6 (Figure 1) attached to brackets 8 (Figure 3) at
intervals of about one half metre.
The spray head 2, shown in Figure 3, is in the form
of an aerofoil body at the trailing edge of which is a
linear nozzle. The body comprises a nose assembly 12 formed
generally of insulating materials and a nozzle assembly 14
formed in this case of a semi insulating material, e.g.
composite sold under the trade mark ~ite Brand by Tufnol
Limited of Birmingham England. The nozzle assembly 14
provides the trailing edge 16 of the aerofoil. The trailing
edge 16 also acts as a spraying edge. The nozzle assembly
14 comprises two parts 14a and 14b secured together with a
thin spacer therebetween leaving a slot 18, defined by the
thickness of the spacer, just forward of the
trailing/spraying edge 16.
In use a liquid agrochemical is delivered through
the slot 18, via a conducting or semi conducting surface 20,
across an exterior surface 21, to the spraying edge 16 from
which spraying takes place. The spraying edge is directed
between the two opposed electrodes 4.
The electrodes 4 comprise a core 22 of conducting
material sheathed by a body 24 partly of semi-insulating
material 26 and partly of insulating material 28. The
insulating part 28 of the body 24 is formed from glass
reinforced plastics by pultrusion. The semi insulating part
of the body 24 is a round tube 25 of a material having a
resistivity in the range 101 to 1014, more preferably
5xlOll to 5X1013 ohm cms. Examples of suitable materials
are certain grades of soda glass and phenol-
formaldehyde/paper composites. The composite sold under the
trade mark Rite Brand by Tufnol Limited of Birmingham
England has been found particularly suitable. The core 22
is packed iron filings or carbon granules. The tube 25 is
bonded and faired to the insulating part 28, by an epoxy
filler or adhesive 23.
The conducting or semiconducting surface 20 is
connected via one of a pair of supply leads (not
illustrated) to one of the output terminals of a high
voltage generator 50 or 52 (Figure 6). The electrode cores
22 are connected by the other of the pair of high voltage
supply leads to another voltage output terminal of the high
voltage generator, 90 that in use a high potential
difference, e.g. 10 to 35 Rv, is maintained between the
surface 20 and the electrode cores 22. Various voltage
configurations can be used. Assuming the target is
essentially at earth potential, either the electrodes 22 or
(as will be explained later) the surface 20 may be at earth
potential. Alternatively, the electrodes 22 may be
maintained at a potential intermediate that of the surface
20 and that of the target. In our preferred arrangement,
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the surface 20 is maintained at + 35 Kv and the electrodes
are maintained at an intermediate potential of + 17.5 Kv.
The electrodes thus have a potential of similar polarity to
that of the droplets in the spray. Once past the electrodes
the droplets thus tend to be repelled by the electrodes. If
the electrodes are at ground potential, there is a tendency,
especially at high flow rates, for the droplets to be
attracted back to the electrodes.
Any suitable circuit arrangement may be used to
provide the voltages required at the surface 20 and the
electrode cores. In Figure 6 each generator is illustrated
with two high voltage outputs. In another alternative the
electrode core voltage is derived by a potential divider
from a single output generator.
The edge 16 is sharp to a degree, that combined with
the closeness with which the electrode cores 22 are spaced
therefrom, enables spraying to take place at a relatively
low high-voltage. In use the electric field is defined
between the semi insulating part 26 of the electrode sheaths
and the liquid arriving at the edge 16. Assuming the
surface 20 has a positive potential relative to the
electrode cores 22, negative charge is conducted away from
the liquid at its contact with the conducting or
semiconducting surface, leaving a net positive charge on the
liquid. The presence of the electrodes 4 intensifies the
electric field at the liquid/air boundary at the edge 16,
sufficiently that the liquid is drawn out into ligaments
spaced along the edge 16.
The liquid becomes positively charged, negative
charge being conducted away by the conducting or
semiconducting surface 20, leaving a net positive charge on
the liquid. The charge on the liquid produces internal
repulsive electrostatic forces which overcome the surface
tension of the liquid forming cones of liquid at spaced
intervals along the edge 16. From the tip of each cone a
ligament issues. At a distance from the edge 16, mechanical
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forces on the ligament produced by travelling through the
air cause it to break up into charged droplets of closely
similar size. Mutual repulsion between the droplets causes
the spray to expand in a direction transverse to the
ligaments. The number of ligaments, which are formed
depends on the flow rate of the liquid and on the electric
field intensity amongst other factors such as the
resistivity and the viscosity of the liquid. All other
things being constant, controlling the voltage and the flow
rate controls the number of ligaments, which enables the
droplet size to be controlled and very closely similar.
If the conducting surface is separate from the
spraying edge 16, we find it necessary to dimension the
spacing therebetween suitably, in relation to the
resistivity of the liquid being sprayed. We find that
spraying will not take place if, given a spacing, the
resistivity of the liquid is too high or, conversely, given
a particular resistivity, the spacing is too great. A
possible explanation for this observation is that in
addition to the liquid becoming charged as it passes over
the conducting or semiconducting surface, there is also
conduction of charge away from the liquid at edge 16 through
the liquid. The resistance of this path must not be so high
that the voltage drop across it results in the voltage at
the edge 16 being too low to produce an atomising field
strength. The distance between the edge 16 and the
conducting or semiconducting surface must therefore be
sufficiently small to allow for the resistivity of the
liquid being used. We have found that a suitable position
can be found for the surface even when spraying, say, a
liquid having a resistivity in the range 106 to 101 ohm cm.
Since the electrical connections are made to the core
22, the surface of the body 24 is not at a uniform
potential. The surface potential will be lowest on the semi
insulating material 26 near the core 22 and it is here that
the flux in the electric field between the edge 16 and the
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electrodes will concentrate. In order to permit maximum
electrical stress to be applied between the edge 16 and the
electrode body in the region of the core 22, without surface
tracking leading to corona discharge between more closèly
spaced points, the core and the sheath are so shaped and
positioned as to be closest to the spray head at the
spraying edge. In the example illustrated, the core is
packed iron filings or carbon granules.
It is important that the area near the spraying edge
where the ligaments are formed is substantially free from
airflow transverse to the ligaments. This would spoil or
even prevent formation of the ligaments. To this end, the
three aerofoils are so shaped as to each leave a low
turbulence wake, when the spray head is substantially
aligned with the general direction of the airstream. In the
area downstream of the spraying edge 16, it is desired to
give the droplets as little opportunity as practical to
deposit on the electrodes. To this end, the electrode
bodies curve away from each other towards their trailing
edges creating an expanding passage. The airstream passing
through this passage is thus decelerating, creating an
environment in which it is difficult to remove turbulence
completely. However, sufficiently low turbulence can be
achieved to permit the formation of stable ligaments by
electrostatic forces and in practice the arrangement can
have an angle of incidence of 10 or 15 degrees or so to the
general direction of the air stream, before it stalls
creating a turbulent wake. This enables spraying to take
place through the normal range of attitudes of the aircraft.
At high spraying rates and/or hi8h potential
differences between the surface 20 and the electrode cores
22, there is a tendency for droplets of the spray to
contaminate the electrodes. This tendency will be reduced
by the air flow over the edge 16 which will a~sist the
droplets away from the electrodes faster than they can
migrate transverse to the airflow.
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In the arrangement illustrated in Figure 3, it is
found that the forward movement of the aircraft produces
sufficient airflow to remove the droplets before they
contaminate the electrodes. Too great an airflow would
produce air shear on the liquid on the surface 21, tending
to strip it off the surface before it reaches the edge 16.
It is, however, possible to enhance the effect by the
particular shape and position of the electrode aerofoils. A
circumstance in which it might be desired to enhance the
airflow is if the droplets are found otherwise to be
depositing on the electrodes. Such a condition might arise
if it were necessary to make the electrodes large in order
to achieve stiffness. Suitable aerofoils to enhance the
flow are illustrated in Figure 4. These are substantially
flat in section on the side remote from the edge 16, so
encouraging the beneficial air flow through the space
between them and the spray head, at the expense of the
airflow around the outside. In this arrangement, the
position of the electrode bodies encourages the airflow to
be in the direction of the ligaments, with no substantial
transverse component, assisting the spray head to resist
stalling.
Without the provision of the air flow between the
electrodes and the spraying edge, if the conducting or
semiconducting surface 20 were arranged at earth potential
and the electrodes 4 were at a high (positive or negative)
voltage, most of the droplets would deposit on the
electrodes. With the present provision of an air flow , it
is possible to spray using such an arrangement. A
sufficient flow of non turbulent air can protect the
electrodes even in this extreme case.
The nose assembly 12 of the sprayhead comprises two
parts: a skin 12a and a generally I-section beam 12b. Both
are manufsctured from glass reinforced plastics by
pultrusion. The skin 12a and beam 12b are assembled
together by screws leaving a hollow cavity 38 through which
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pipework and high tension electrical leads (not shown in
Figure 3) supply liquid to be sprayed and high voltage
respectively, to the nozzle. The nozzle assembly 14
conforms to the exterior shape of the nose assembly 12 so as
to form part of the aerofoil section. The rozzle assembly
14 has a pro~ection 40 along its length, which is a push fit
between the flanges of the beam 12b. Push together
electrical and fluid connectors (not shown) are provided
between the beam 12b and the projection 40, so that the
nozzle assembly can be plugged into the nose assembly and
easily removed for service or replacement. The fluid
connector communicates with a distribution channel 44 in the
interior surface of the nozzle part 14b. The distribution
channel 44 conveys liquid to be sprayed from the passage 40
to the slot 18.
As can be seen in Figure 2, the sprayheads are not
horizontal but conform to the dihedral of the aircraft's
wings. During spraying the liquid is supplied under
positive pressure from a metering pump (not shown) and the
dihedral causes no problem. However, when the aircraft
reaches the end of its run over one strip, the spray is
turned off while it turns to spray an adjacent strip. If
there were one continuous slot 18 throughout the length of
the sprayhead, there would be a tendency for the liquid to
run towards the lower end of the slot leaving the upper end
empty when the liquid. This would leave a short lag between
the time the metering pumps were switched on and the time
spraying commenced, which would leave an indeterminate and
unacceptable area unsprayed. This problem is overcome by
dividing the slot 18 into short independent sections, each
supplied with liquid separately and each short enough that
capillary action is sufficient to keep the sections full
from end to end at normal attitudes and in manoeurers normal
during spraying.
Referring to Figure 5, the sprayhead is manufactured
in standard length sections. Eight sections 14.1 to 14.8 of
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nozzle 14 are illustrated schematically. In each section
there are three separate sections of slot 1-8, the sections
being isolated by separators provided in the spacer
defining the slot 18.
Each section of slot 18 is fed by a respective
separate distribution passage 44 from a respective separate
fluid connector. Between the fluid connector and the
respective distribution gallery 44 is a non return valve 46
which prevents one section of distribution gallery 44
draining into another. Each section of nozzle 14.1 to 14.8
has three isolated sections of slot 18 and distribution
gallery 44, these being fed from a common duct 41 via a non
return valve 48 and a flow regulator 42.
A problem is caused by the need to use non return
valves to isolate the separate sections of the distribution
gallery 44 and slot 18, in that the sorts of solvents used
for pesticides for electrostatic spraying, are highly
damaging to most elastomeric materials. Non return valves
not using elastomers as a seal tend to rely on high spring
pressures to keep them shut. This in turn leads to the
valve not opening at low forward pressure and variations
between valves of the flow rate at a particular pressure.
This does not matter so much for the valves 48 as each is
associated with a flow regulator 42. However no such
regulator is sssociated with the valves 44. This
problem can be overcome by the use in the non return valves
44, of a PTFE 0-ring as a seal. Suitable 0-rings are
available under the trade name "CHEMRAZ!' from Green Tweed
and Co Inc, Detweiler Road, ~ulpsville, USA.
As there is no direct connection possible,
maintaining a voltage reference relative to ground poses
something of a problem. A solution is described in EP-A2-
0186353. Applied to the present apparatus, the circuit
arrangement is shown in Figure 3.
As shown in Figure 6, the aircraft carries two spray
head/electrode assemblies 2a, 4a and 2b, 4b. These are
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mounted one on each side of the aircraft, a~ shown in Figure
2. There are two high voltage generators 50, 52, each
powered by a battery 54. Each generator has two high
voltage outputs reference to a respective ground terminal
58, 60. both of which are connected to the body or airframe
61 of the aircraft. A -35 ~v output 62 of the generator 50
is connected to the surface 20a of the spray head 2a. A -
17.5 Rv output 64 of the generator 50 is connected to the
associated electrodes 4a. Similarly, a +35 Kv output 66 is
connected to the surface 20b of the spray head 2b and a
+17.5 Kv output is connected to the associated electrodes
4b. The generators 50 and 52 are preferably mounted in the
nose assemblies of their respective sprayheads. This
removes the need to make high voltage electrical connections
to the sprayheads, only low voltage external connections
being necessary.
It will be appreciated that atomised liquid emerging
from the spray head 2b is charged positively. Liquid
emerging from the spray head 2a is charged negatively.
During spraying, positive current from the generator 52
flows to ground via the terminal 66, the conducting or
semiconducting surface 20b in the spray head 2b and the
liquid emerging from the spray head. In the absence of the
connection between terminals 58 and 60, there would be no
return lead for current to flow back to the generator 52
from the ground (i.e. the target). Accordingly a negative
charge would build up on the generator 54.
This build up of charge on the generator 52 reduces
the potential with respect to the electrode 4b, which is
applied to the conducting or semiconducting surfaces 20b,
thus reducing the atomising field and the charge applied to
the spraying liquid. There is therefore an increase in the
size of the droplets of liquid and a deterioration of
spraying quality. The generator 50 would be affected
similarly.
In practice if one of the generators 50 or 52
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supplie~ more current than the other, a charge builds up on
the generators. The polarity of the charge is such as to
reduce the atomising field on the spray head supplied by the
generator supplying the greater current. This reduces the
quality of the ~pray from the associated spray head and the
spray current from the generator is also reduced.
Conversely, the atomising field on the spray head supplied
by the other generator supplying the smaller current, is
increased. The quality of the spray from this spray head is
therefore improved and the spray current increases until it
matches that from the first generator.
In an alternative arrangement, the slot 18 can be
at, rather than ahead of, the trailing edge 16. Although
such an arrangement may appear to create two spraying edges
because the slot, naturally, has two sides, the
electrostatic effect is that of one edge. That is to say
only one set of ligaments is formed centrally. If the
electrostatic effect were that of two edges, ligaments would
be produced o~f the "edges" at both sides of the slot. This
concept of one edge fed by a central slot may, perhaps be
better understood by considering that the fluid to be
sprayed has significant conductivity and will, in use,
bridge the slot.
In a yet further arrangement~ more than one slot 18
can be arran3ed to feed liquid to a single spraying edge.
