Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~201~3~;
1 B 32802
ectrostatic Spraying
hi eQtion relates to electrostatic ping. ye form
of electrostatic spraying apparatus, for example for agricultural or
horticulture use, comprises a portable spray gun including a spray
nozzle means for applying a high potential to said nozzle, and mean
for supplying to said nozzle the liquid -to be sprayed from a container
of the liquid mounter on the spray guy. Examples of such electron
station spraying apparatus are described in, inter alias ~S-~-4356528.
It has been proposed Lo U~P-A-3212211 to produce the Nazis
Mary high voltage for a portable electrostatic spraying device from
a low voltage power supply, e.g. batteries, by means of a high volt-
age generator producing rectified high voltage pulses which charge
a capacitor connected across the generator -output The charge on
the capacitor is used to maintain the requisite potential at the
prune nozzle.
Clearly, to obtain electrostatic atomization the potential
at the nozzle has to be maintained at above a certain minimum voltage,
but should not be so high that corona discharge takes place. Gffner-
all, to effect electrostatic atomization, the potent 1 at the nozzle
will need to be in an excess of 5 TV, and often above 10 I although
the precise minimum value required will depend, inter alias ox the
nozzle design. Ike maximum voltage required is generally jot more
than 25 TV
In low cost generators it it generally necessary to
employ a switching System Lo the generator which produces rapid
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changes of current in the primal of a step-up transformer, The
magnitude and rapidity of the current changes in the primary
determine the magnitude and shape of the high voltage pulses: the
magnitude is restricted by the need to avoid excessive voltages
at the nozzle which would give rise to corona discharge. The
rapid change of current in the transformer primary is conveniently
achieved by periodically effecting the rapid discharge of a
capacitor Lo the primary circuit through the transformer primary.
Such rapid discharge may be effected by means of a triggering unit
lo connecters in series with the transformer primary, across the
primary circuit capacitor The triggering unit is cringed to
discharge the primary circuit capacitor, via the transformer
primary, typically through a thruster or a gas gap discharge
tube, when the voltage across the primary circuit capacitor, and
hence across the triggering unit, reaches a predetermined value.
the frequency of operation of the triggering unit and
hence the frequency with which the high voltage pulses are goner-
axed, thus depends on the rate of charging of the primary circuit
capacitor.
this rate of charging will of course depend on the
capacitance of the primary circuit capacitor and the current
supplied thereto. In order to obtain high voltage pulses of
adequate magnitude to achieve the desired nozzle potential under
load, the primary circuit capacitor will generally need to have
a fairly large capacitance. Consequently to keep the current
drake on the low voltage power source small, the charging rate
of the primary circuit capacitor and hence the rate of actuation
of the triggering device, and thus the frequency of the high
voltage pulses must be relatively low.
As mentioned herein before, the high voltage pulses
are rectified and used to charge a capacitor in the high voltage
circuit to maintain the required potential at the spray nozzle
If the capacitance of this capacitor in the high voltage circuit
is sufficient, there will be little variation of the potential at
the nozzle between pulses since -the load represented by the
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transfer of charge at the nozzle to the liquid to effect electron
static atomization, together Ruth leakage currents, trill represent
dissipation of only a small proportion of the charge on the
capacitor.
However, if the capacitor has a high capacitance, the
high voltage circuit will have a high stored energy A high
stored energy is undesirable as it may present satiety hazards,
for example electric shocks to the operator from accidental con-
tact with the nuzzle Desirably the stored energy is below 10 mJ0
The stored energy is given by C2V where V is the voltage and C
is the capacitance. Hence to achieve a stored energy below 10 my
the capacitance must be below 2 v21 pi where V is the voltage
expressed in kilovolts, ire. below 50 pi Then the voltage is 20 TV
The load current, represented by the transfer of charge
to the liquid at the nozzle, required to effect atomization is
relatively small and, provided that the leakage currents are
small, it would be possible to use a high voltage circuit having
a stored energy below 10 my.
However, not only are capacitors capable of operation
at high voltages expensive, but, even those capacitors of the
relatively WOW capacitance required, exhibit considerable leakage
currents at such high voltages.
t these relatively low values of capacitance the charge
dissipated as a result of the leakage currents represents a signify-
cant proportion of the charge on the capacitor with the result that between the pulses applied to the capacitor the voltage at the
no ale is liable to drop to below that required for spurring
While this could be counteracted by increasing the
frequency of the high voltage pulses applied to the capacitor in
the high voltage circuit, as explained herein before, increasing
-the frequency results in an increase in the current drawn on the
power supply. Consequently to maintain the current drain at an
acceptable level, e.g. to give an adequate life where dry batteries
are employed as the low voltage power source, the frequency with
which the pulses can be applied to the high voltage capacitor is
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limited, generally to below about 50 Ho.
We have now devised an arrangement, having a low stored
energy high voltage circuit, that can be operated at a frequency
that gives an acceptable current drain on the power source.
According to the present invention we provide a portable
electrostatic spraying apparatus including
(a) a spray nozzle,
- (b) means to supply liquid to be sprayed to said
spray nozzle,
10 (c) a low voltage power source,
(d) a high voltage generator powered by said low voltage
power source, whereby rectified high voltage pulses
may be produced across its output,
(e) a capacitor connected to said nozzle and to one side
of said generator output whereby said capacitor may
be charged by said rectified high voltage pulses so
that said nozzle may be maintained at a sufficiently
high potential, with respect to the other side of
said generator output, to cause electrostatic atoms-
anion of said liquid at said nozzle,
characterized in that capacitor has a value below pi,
where V is the average voltage, expressed in kilovolts, that said
generator is capable of maintaining at said nozzle, and in that
said capacitor is formed by the capacitance between a lead coy-
netting said one side of the generator output to said nozzle and
a lead connected to said other side of the generator output,
said generator being capable of producing said high
voltage pulses of such magnitude and frequency that the potential
at said nozzle may be maintained at a sufficient value to cause
electrostatic atomization of the liquid but without corona disk
charge.
the use of the lead from one side of the generator
output to tune nozzle, in conjunction with a second lead connected
to the other side of the generator output as the capacitor surf-
fishnet capacitance can be obtained with negligible leakage
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current. the two leads should be in sufficiently close proximity to give the requisite capacitance which is generally within the
range
Z X2lo to F Pi (where V is in TV).
5 the capacitance is preferably within the range 10 to 50pFo
For example two separate insulated wires each having a
length of about 0.5 m may be twisted together as necessary to
give the requisite capacitance the leads may of course be longer
but spaced sufficiently far apart over some or all of their length
that the capacitance is at -the requisite level. Alternatively a
suitable length of a twin core or Cole cable Jay be employed.
Since a capacitor formed by two such leads will give
negligible leakage current, the leakage current between pulses
will be markedly reduced, enabling sufficient potential to be
maintained at the nozzle
As mentioned herein before the average potential at the
nozzle will depend on the frequency and magnitude of the high
voltage pulses applied to the capacitor: the magnitude is no-
striated by the need to avoid voltages that would give rise to
corona discharge The frequency of the pulses is typically in
the range 10 - 40 Ho, and preferably is in the range it - 30 Ho
the requisite frequency will depend ox the load applied by the
liquid being sprayed which in turn will depend on the properties,
ego resistivity, of the liquid and on the volumetric flow rate.
the latter is preferably below 0025, particularly below 0.1 ml/s.
rate ox 0.05 ml/s typically represents a load of less than
100 nay
If desired the generator may be provided with means
for varying the frequency Audrey magnitude, Leo peak voltage,
of the high voltage pulses as the volumetric flow rate is
varied.
though, as a result of using the leads from the
high voltage generator to form the capacitor, the leakage cur-
rent through the capacitor is virtually eliminated, leakage of
charge from the capacitor will occur between pulses, inter alias
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as a result of the reverse leakage current of the rectifier
the rectifier reverse current may be significant in relation
to the load presented by transfer of charge to the liquid
being sprayed and will affect the minimum frequency required
of the generator. We prefer to employ as the rectifier a high
voltage diode rated at a leakage current of less than l at
37 TV at 20C. Such a diode will have a reverse leakage current
of less than about lo no at 20 TV at 20C~
the spraying apparatus preferably comprises an elongated
member intended to be held in the hand with the low voltage power
supply, ego batteries, cud high voltage generator in one end
thereof with the spray nozzle at the other end. the leads forming
the high voltage circuit capacitor thus can extend along the
elongated member to connect the nozzle to the generator
In a preferred arrangement one lead is connected to the
nozzle while the other is connected to, orprcvides, an electrically
conductive member adjacent to but spaced from the nozzle. In
assessing the lead capacitance, the capacitance between the nozzle
and such an electrically conductive member should be taken into
accost The electrically conductive member is preferably main-
twined substantially at earth potential, for example by proving
a connection to earth from that lead via the operator. Such an
earthed electrically conductive member can then act as a field
adjusting electrode as described in aforementioned SUE
In one form of the apparatus on elongated holder having
the high voltage generator and a receptacle for receipt of the low
voltage power source, erg. batteries, at ore end is provided, at
the other end, with a receptacle for receipt of a canister of the
liquid to be sprayed. the nozzle may form part of the huller or
may be attached to the canister In the latter case meats are
provided in the holder for making electrical connection between
the lead from the one side of the high voltage generator and the
nozzle
the apparatus is of particular utility for the spray-
in of liquids, such as pesticides, polishes, and the like at low
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volumetric flow rates The liquid preferably has a resisti~Jity
of 107 to 1011 ohm. cm.
The liquid may be supplied to the spray nozzle by
simple gravity feed. However this is disadvantageous in many
cases since it restricts the spatial orientations of the nozzle
that can be used Tins problem can be overcome by supplying the
liquid to the nozzle from a pressurized container; in particular
the liquid can be supplied from a container containing the liquid
and a compressed pressurizing agent.
It is preferred that the container is arranged so that
the pressuring agent is not dispensed through the nozzle with
the liquid to be sprayed. In this way the atomization of the
liquid by the electrostatic forces is not affected by the
emergence of the pressurizing agent In one preferred arrangement
the count inter comprises a barrier pack with the liquid to be
sprayed contained within a collapsible inner container located
within the outer container with the pressurizing agent fluid in
the space between the inner and outer containers.
The rate of delivery of the liquid to the spray nozzle
will depend on the pressure exerted by the pressurizing agent
(which is often a gas at ambient temperatures and atmospheric
pressure, but is liquid at the pressure prevailing within the
container). We have found that the pressure exerted by the pros-
surmising agent is liable to considerable fluctuation as the
ambient temperature varies, with the result that the liquid
supply rate to the nozzle is also liable to considerable
fluctuation: indeed o'er the range of ambient temperatures
liable to be encountered in use ox the spray gun particularly
where such use is outdoor, the pressure exerted by the pros-
surmising agent, and consequently the flow rate, may vary, insole cases by a factor of four or more.
Variations in flow rate will affect the size, and
size distribution, of the liquid droplets formed by elect
static atomization Such variation in droplet size is undesir-
able since for an joy liquid there is an optimum droplet size
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or size range, for the intended use of the liquid.
or example, when spraying plants with a pesticide formulation, if the droplets are too large, the amount of
"wrap-round", giving coating on the underside of plant leaves,
is reduced; whereas if the droplets are too small, they are
liable to be unduly affected by factors such as wind strength
and so may drift onto plants other than those intended and/or
on to the operator
As a further feature of the invention we have devised
a way of overcoming these difficulties by varying the nozzle
potential to control the droplet size
Accordingly the present invention further provides,
in electrostatic spraying apparatus of the type herein before
described for spraying a liquid as droplets from a nozzle sup-
plied with said liquid from a pressurized container by applying high voltage to said nozzle, the improvement comprising means
to monitor the ambient temperature and to vary the average volt-
age applied to said nozzle in response to said monitored temper
azure to maintain the average droplet size within a predetermined
range.
he average voltage at the spraying nozzle can be
varied by variation of the amplitude, frequency and/or shape
of the high voltage pulses. Such variations can be brought
about by appropriate variation in the low voltage circuit, ego
of the magnitude and/or frequency of the current changes in the
transformer primary winding and/or the rate of change thereof.
y incorporating a temperature sensitive electrical
component, e.g. a thermistor, into the spray apparatus and using
the variation in the electrical properties of this component
with temperature to modify the transformer primary current
changes, the average high voltage applied to the nozzle can be
varied.
he average nozzle voltages required to give a specified
droplet size or size distribution at various flow rates of a given
liquid can readily be diatom by e~perimenta Typically for a
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given liquid at a given flow rate, an average voltage of 15 TV
may be required at the nozzle. If the flow rate is increased
by a factor of two, the average voltage required to obtain the
same, or a similar, droplet size is typically increased to 20 kVo
Likewise variation in pressurizing agent pressure, and
hence liquid flow rate, with temperature can also be readily
determined.
From this data, and from the temperature characteristics
of the temperature sensitive component, the appropriate circuitry
can be devised to provide the necessary variation in nozzle volt-
age to maintain the droplet size within the desired range.
The invention it illustrated by reference to the
accompanying drawings wherein:
Figure 1 is an elevation of one form of the
apparatus,
Figure 2 is a longitudinal section of the
spurred part of the apparatus,
Figure 3 is a longitudinal section of -the
handle part of the apparatus
Figure 4 is a circuit diagram,
Figure 5 shows a modification of part of
the circuit depicted in figure I
Referring first to Figure 1, the apparatus comprises
an elongated member 1 having a handle portion 2 incorporating a
trigger 3 and a spurred assembly 4 comprising a sleeve 5 in
which a cartridge containing the liquid to be sprayed is inserted.
the cartridge has a mechanically actuated valve and a nozzle to
which a high voltage can be applied When the cartridge valve is
open Ed a high voltage is applied to the nozzle, the liquid is
electrostatically atomized a a spray through an orifice at the
lever end of the spurred assembly I Jo enhance the spray
there is disposed around the sleeve 5, but insulated from the
nozzle, an Allah æ conductor 6 constituting a field intensifying
electrode ego as described in aforementioned ASP 4356528.
the shaft of the elongated member 1 comprises a casing
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formed by two shell moldings of an electrically insulating
material.
Referring now -to Figure 2 one of the shell moldings
is indicated by reference numeral 7. The sleeve 5 is mounded
prom an electrically insulating material and is of generally
cylindrical configuration. Sleeve 5 is located on the shell
moldings by means of an integrally mounded, open-sided, box
structure 8 which engages with a hollow projection 9 on mound-
in 7 and a corresponding projection on the other shell mound
in. Sleeve 5 is provided with integrally mounded projections
10 in tush one end 11 of a v~Jve-actuating member 12 is pivot-
ally mounted.
Sleeve 5 is also provided with an opening 13 through
its wall, through which the other end 14 of the valve-actuating
15 member 12 passes, and integral flanges 15, 16 which act as a
guide for the end 14 of the valve-actuating member 12.
Screw mounted on the end of the sleeve 5 is a nose
cone 17 having an opening 18 through which the end of the cart-
ridge nozzle can project
the cartridge 19, which is shown partly in section
in Figure 2, is a metal can 20 provided with a closure 21 in-
corporating a valve assembly, typically of the type commonly
employed in aerosol canisters Inside can 20 a flexible bag
22 is mounted on the inlet 23 to the valve assembly. the liquid
to be sprayed is contained within bag 22 while the space between
bag 22 and the walls of the can 20 is charged with a volatile
liquid pressurizing agent, ego a fluorocarbon such as dichloro-
difluoromethane. the cartridge 19 also has a nozzle 24 having a
fine bore (not shown) extending longitudinally there through.
the nozzle 24 is formed integrally with a flange 25 forming part
of the valve assembly. Movement of flange 25 axially towards
the base 26 of cartridge 19 effects opening of the valve to
permit liquid to flow from the reservoir out of the cartridge
via the fine bore extend through nozzle 24. the bore is
typically of 1 my diameter while the tip of the nozzle 24 is
,
Lo 33~
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typically of hemispherical configuration of 3 - 5 mm diameter.
Cartridge lo is held in place by a rib 27 on a cap 28
engaging with the base 26 of the cartridge and holding the
flange 25 against the valve actuating member 12. the cap 28 is
5 mounded from an electrically insulating plastics material and
is pivotal mounted in a boss 29 in shell mound 7 and a cores-
pounding boss in the other shell mound. Cap 28 has an integral
latch 30 engaging it a projection 31 mounded integrally with
sleeve 5.
Extending through an opening 32 in sleeve 5 is a
spring metal contact strip 33 which is held in place between
-the shell moldings and the wall of sleeve I Electrically con-
netted, ego soldered, to strip 33 is a high voltage lead 34 from
a generator located in the handle portion of the apparatus, On
application of a high voltage to lead 34, the high voltage is
applied, via contact strip 32, to the metal can cartridge 19
and hence, via conduction through the cartridge and its contents,
to the nozzle 24.
the valve-actuating member 12 is a mounding of an
electrically insulating plastics material of such cross section
that the portion in the vicinity of nozzle 24, flange 25? and
mounting 10 is relatively rigid but the free end 14 is relatively
flexible, the valve-actuating member 12 is provided with an
opening 35 through which Nazi 24 projects, and projections 36
which engage with flange 25 on either side of nozzle 24. It is
then seen that longitudinal movement of the free end 14 of the
valve-actuating member 12 away prom mounting lo causes flange 25
to be depressed thus opening the valve the tree end 14 of the
valve-actuating member 12 is provided with a slot 37 which engages
with a hook 38 of a metal wire 39 which extends along the shaft
of the elongated member to the trigger JO
As mentioned herein before, extending round sleeve 5 is
a metal wire 6 acting as a field adjusting electrode. A flexible
extension 40 of wire 6 passes through a groove (shown dotted in
Figure 2) in shell mounding 7 and is electrically connected, ago.
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12 3 32802
soldered to wire 39.
Wires 39 and 40 thus provide an electrical connection
from the trigger 3 to the field adjusting electrode 6 and wire
39 also provides a mechanical connection from trigger 3 to the
5 valve actuating member 12,
he handle portion 2 of the apparatus is shown in
figure 30
Provided within the handle portion 2 of the casing
is a compartment 41 for receipt of a series train of two dry
cell batteries 42, a high voltage generator assembly 43; and
a trigger assembly 44~
The generator assembly comprises a printed circuit
board 45 on which are mounted the various components shown in
figure 4 as enclosed within the dotter box For simplicity
15 these components are not shown in Figure 3. o'er 45 is mounter
in a mounding 46 of electrically insulating plastics material.
Also mounted in mounding 46 is an output step transformer 47
which is connected to board 45 by leads 48, 49. the high volt
age output from transformer 47 is fed, via a high voltage diode
20 50, (not shown in Figure 3), to the high voltage lead 34 via a
contact within sleeve 51 attached to transformer 47~ me
generator assembly 43 is located by projections 52, 53, 54 an 55
integral with shell mounding 7 and by corresponding projections
(not shown) in the other shell molding
Board 45 is provide with two electrical contacts 56,
57. Contact 56 is a spring metal s-trip which extends round
mouldi~g 46 to the trigger assembly 44 while contact 57 projects
into the battery compartment 41 wherein it contacts the positive
terminal of the triune of batteries 42. Extending the length of
30 compartment 41 is a wire 58~ At the rear end ox compartment 41
wire 58 is wormed as a coil spring contact 59 which urges the
trains of batteries 42 into engagement with context 57. Wire 58
also serves to connect the negative contact of the battery trait
to the trigger assembly 440
Ike trigger assemblage 44 comprises a trigger lever 3
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made of an electrically conductive plastics material pivot-
ably mounted on bosses 60 in the shell moldings. The free
end of wire 58 from the battery compartment extends through
a hole in lever 3 to form a contact pin 61. Also mounted in
lever 3 is a pin 62 formed from an electrically insulating
material. Pin 62 engages with the spring contact strip 56
from board 45 to hold the strip 56 out of engagement with
pin 61 Lyon the trigger lever 3 is in the "off" position.
Strip contact 56 is laterally spaced from lever 39 and hence
insulated therefrom when the trigger is in the "off" position.
Rotation of lever 3 from the '70ff" position causes the contact
pin 61 to engage with strip contact 36 thus completing the
circuit to supply power from the batteries 42 to the generator.
rooked round an integral extension 63 to trigger
15 lever 3 is the connecting wire 39~ A return spring (not shown)
is provided to bias lever 3 to the "off" position
In use the operator's finger contacting trigger lever
3 provides a connection -through the operator, to earth thus
earthing the field intensifying electrode 6 and the negative
20 side of the batter try
Referring now to Figure 4, the low voltage part of the
high voltage generator circuit consists of a conventional tray-
sistorised saturation oscillator formed by the primary 64 of a
first step-up transformer 65, resistor 66 and a transistor 67.
25 Typically this oscillator has a frequency of the order of 10
to 100 kHz. The secondary of transformer 65 is connected, via
; a diode 68, to a capacitor 69. Connected in parallel with
capacitor 69 is a gas-gap discharge tube 70 connected in series
with the primary of the output step-up transformer 47~ Shown
dotted in the high voltage output circuit of figure 4 is a
capacitor 71. this capacitor is not a discrete component but
represents the capacitance between the high voltage lead 34,
the cartridge 19, and the nozzle 24 and the adjacent "earthed"
components, ego wires 39 and 40, and -the field intensifying
35 electrode I
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Jo ensure that the capacitor 71 has the desired
value, typically 20 - 40 pi, guides (not shown) may be provided
in the shell moldings to hold ire 39 in the desired spatial
relationship to the high voltage lead OWE
In operation the saturation oscillator gives rise
to current pulses in the secondary of transformer 65 which
charge capacitor 69 via diode 68. When the voltage across
capacitor 69 reaches the striking voltage of gas-gap discharge
tube 707 the latter conducts discharging capacitor 69 through
the primary of output transformer 47, until the voltage across
the gas-gap discharge tube falls to the extinguishing voltage.
Typically the striking voltage is 150 - 250 V and the extinguish-
in voltage is less than 10 V.
the discharge of capacitor 69 through the primary of
transformer 47 produces high voltage pulses in the secondary
thereof: these high voltage pulses charge capacitor 71 via
diode 50 and thus maintain a sufficiently high potential be-
tweet nozzle 24 and the field intensifying electrode 6 for
electrostatic atomization of the liquid from nozzle 24,
the frequency with which the high voltage pulses are
produced is determined by the value of capacitor 69, the impede
ante of the secondary of transformer 65 and the magnitude and
frequency of the pulses produced by the saturation oscillator.
In an example a pesticide composition of receptivity
8 x 107 ohm. cm was sprayed from apparatus of the type shown in
figures 1 to 4. The voltage at noble 24 was about 18 XV, the
liquid flow rate 1 ml/min, the frequency of the high voltage
pulses about 25 I The capacitance of capacitor 71 was about
20 pi and primarily formed by the capacitance between wires 34
and 39 which were each about OWE m long and spaced apart by an
average of about 2 cm. The series train of batteries 42 gave a
voltage of 301 V and the current drain thereon was about 150 Mao
In the modified circuit of figure 5 the arrangement of
the generator is modified by the replacement of the gas-gap disk
charge tube 70 by a thruster 72 and by the incorporation of a
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temperature dependent triggering circuit 73, the output of~rhich is applied to the gate of thruster 72.
his temperature dependent triggering circuit incorp-
orates a temperature sensitive component, e.g. a thermistor,
and is arranged such that as the temperature increases, thruster
72 is triggered to conduct, thus discharging capacitor 69 through
the primary of output transformer 47, at increasing voltages
across capacitor 69, Although this results in a reduction of
the frequency of discharge of capacitor 69, the rate of trays-
for of energy to the high voltage circuit is increased thus giving an increased voltage at the nozzle 24.
As the temperature increases the pressure exerted by
the volatile liquid in can 20 increases, thus increasing the
liquid flow rate through nozzle 24. the characteristic of the
temperature dependent triggering circuit 73 is arranged so that
the voltage at the nozzle 24 is increased, as the flow rate
through nozzle 24 increases, so as to give the desired droplet
size spectrum.
PANG
7 June 19~4
