Charge injection device

In charge injection apparatus comprising essentially a charge injector (1) and a fluid supply arrangement (2) for the charge injector, fluid to be charged is introduced into a mixing chamber (20) via a supply line (25) where it becomes mixed with a volatile fluid, such as a high vapor pressure hydrocarbon or a halogenated component supplied through line (28). The resulting fluid mixture is introduced into the charge injector and, on emerging through the exit orifice (5) of the charge injector into ambient atmosphere, the volatile fluid volatilizes to form a blanket of gas of higher dielectric strength than that of the ambient atmosphere. In this way, resistance to dielectric breakdown is increased which enables the charge injector to be operated at higher potentials then would be the case in the absence of the volatilized fluid, without dielectric breakdown occurring.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus and a method for injecting charge into 
a fluid and finds particular application where it is desired to maximise 
the output charge density levels obtainable from a charge injector. 
DESCRIPTION OF THE PRIOR ART 
Electrostatic free charge injectors are known in the art. An example of 
such an injector is disclosed in U.S. Pat. No. 4,255,777 obtained from 
Ser. No. 853,499, filed Nov. 21, 1977 in the name of Arnold J. Kelly and 
assigned to the present assignees. The injector is designed to 
electrostatically charge a liquid stream and discharge it into ambient 
atmosphere, the stream breaking up into finer droplets or atomising under 
the influence of the injected free charge to form a spray. The charge 
injector comprises essentially a chamber through which liquid can flow, a 
low-voltage electrode at one end of the chamber defining a discharge 
orifice, a pointed high-voltage electrode arranged with its tip close to, 
and in axial alignment with, the discharge orifice and an earthed 
electrode outside the chamber downstream from the discharge orifice so as 
to complete the electrical circuit of the charge injector. Various 
applications of the charge injector are disclosed in U.S. Pat. No. 
4,255,777 such as electrostatic coating or spraying or the atomisation of 
hydrocarbon fuel delivered to the combustion chamber of domestic and 
industrial oil burners. 
In certain circumstances, it may be required to maximise the output charge 
density levels produced by the charge injector. However, at a certain 
operating potential, the charge injector fails to continue to operate 
normally and very significantly reduced charging levels result for the 
fluid exiting the charge injector. 
Reference is directed to U.S. Pat. application Ser. No. 601,253, filed on 
Apr. 17th, 1984 and assigned to the present assignees, which addresses 
itself to situations where it is desired to reduce the ambient pressure 
downstream of the injector. One example is an electrostatic separation 
technique to separate water droplets suspended in oil in which firstly 
free charge is injected into the mixture using a charge injector and then 
the charged mixture passes as a spray or continuous stream through a gas 
or vapor space and into a treatment vessel, avoiding contact with the 
separation vessel walls while passing through the gas or vapor space. In 
the separation vessel, the charged emulsion comes into contact with a bed 
or porous collector beads on which water droplets coalesce, subsequently 
become re-entrained into the oil, and then settle out under gravity. The 
removal of the water from the oil is facilitated by exposure of the 
contaminated oil stream issuing from the charge injector to reduced 
pressure or vacuum conditions. The reduced pressure, however, reduces the 
charging level and charge transport efficiency achieved with the charge 
injector. 
The aforesaid U.S. Pat. application Ser. No. 601,253 overcomes this problem 
by raising the ambient pressure above the reduced pressure value in an 
enclosed region immediately downstream of the discharge orifice of the 
charge injector. The enclosed region can be the internal space within a 
second chamber which is supplied directly with compressed air and has an 
outlet orifice in alignment with the discharge orifice so that the charged 
fluid passes, together with the compressed air, through the chamber and 
out through the outlet orifice into the low pressure downstream region. In 
order to avoid dielectric breakdown downstream of the discharge orifice of 
the charge injector, sulfur hexafluoride or any other blanketing gas which 
can act to reduce or avoid breakdown can be used in place of air for 
pressurising the interior of the second chamber. 
The following patents are also of some interest. U.S. Pat. No. 1,838,930 
(H. F. Fisher et al) relates to an electrical treater in which emulsion to 
be treated is firstly passed through a primary electrical treatment stage, 
and the lighter and heavier parts of the liquid from that stage are 
respectively passed through secondary and tertiary electrical treatment 
stages. The dielectric strength of the emulsion can be increased by 
introducing gas in suitable quantities which forms into bubbles which 
prevent a continuous electrically conductive path from being formed 
between the electrodes of the treater. 
In U.S. Pat. No. 1,405,126 (F. W. Harris), an emulsion to be dehydrated is 
injected into a body of relatively dry emulsion and the dry emulsion is 
circulated over a closed path between charged electrodes in a separation 
vessel. Water precipitates to the bottom of the body of emulsion where it 
is withdrawn and the desired product is withdrawn from the top. Air is 
introduced into the vessel under pressure through a nipple, so as to cause 
rapid circulation of the emulsion through the electrode region. This rapid 
circulation helps to reduce the risk of dielectric breakdown in the inter 
electrode region. 
Reference is also made to the oil dehydrating process disclosed in U.S. 
Pat. No. 1,559,036 (Egloff et al), in which an electrolyte substance, 
which may be a gas, liquid or solid, is added to the oil and water to 
reduce the interfacial film between the oil and water and increase the 
conductivity of the emulsion. 
Finally, reference is made to U.S. Pat. No. 3,073,775 (Waterman) in which a 
complicated electrical treater is used for treating oil-continuous 
dispersions. Air is introduced as various points in the treater, to 
maintain certain pressure levels at those locations for the purpose of 
determining residence times there. 
It is remarked that, in contrast with the present invention, none of 
Fisher, Harris, Egloff and Waterman is concerned in any way with charge 
injectors or the problems of dielectric breakdown in such charge 
injectors. 
SUMMARY OF THE INVENTION 
The present invention is based upon the hitherto unrecognized fact that 
although the charge injection process occurs in a charge injector within 
the inter-electrode region inside the charge injector, the breakdown of 
the gaseous media surrounding the exiting jet, in the immediate vicinity 
of the outlet orifice, can influence the overall charge injection process. 
In particular, it has now been recognized that ionization of the 
background air or gas enveloping the exiting jet causes the low voltage 
electrode to act as if it were a downstream-extending blunt electrode with 
the result that the charge density profiles in the exiting jet can relax 
to a minimum charge density configuration prior to jet break-up into 
droplets. 
In accordance, then, with the present invention, there is provided charge 
injection apparatus which comprises a charge injector having a high 
potential electrode with a pointed tip for injecting charge into the fluid 
to be charged, and an exit orifice, downstream of the point of the 
electrode, through which the charged fluid issues as a jet. The jet may 
take the form of a spray but it is immaterial to the invention whether the 
jet is a continuous stream or a spray (i.e. fine droplets). Additionally, 
the charge injection apparatus comprises means for introducing into the 
first-mentioned fluid, a volatile liquid which volatilizes on emerging 
from the exit orifice. The dielectric strength of the vapor of the 
volatilized fluid is such as to oppose any tendency to dielectric 
breakdown occurring. Normally, the dielectric strength of the vapor of the 
fluid to be charged is greater than that of the ambient atmosphere into 
which the jet of charged fluid issues from the charge injector. In that 
event, it is necessary only that the dielectric strength of the vapor of 
the volatile fluid is greater than that of the ambient atmosphere, in 
order to secure the improved performance which is achievable with this 
invention. However, optimum improvement is obtained when the dielectric 
strength of the volatile fluid vapor exceeds that both of the ambient 
atmosphere and of the fluid to be charged. 
It will be appreciated, then, that the present invention poses a 
particularly simple solution to the problem of dielectric breakdown. 
Specifically, the spray fluid produced by the charge injector is modified 
by the presence of the volatilized liquid which serves as a blanketing gas 
upon volatilization. It will be further appreciated that the introduction 
of a volatile component into the fluid to be charged is in most 
atmospheric applications (such as paint and agricultural spraying, medical 
and grooming aided dispersal) a much simpler and more direct means to 
attain performance from the charge injector than by making provision for 
an external supply of gas directly to the region where dielectric 
breakdown is prone to occur. Furthermore, the invention does not require 
the charge injector to have a special construction but can be put into 
effect merely by a simple modification to the fluid supply line to an 
existing charge injector, or by the addition of appropriate volatile 
adjuvants to the fluid to be charged, i.e. the fluid supplied to the 
charge injector can be a two-component fluid, comprising a blend of a 
component to be charged and a volatile component which serves as a 
blanketing gas when it volatilizes on emerging from the charge injector. 
It should be noted that to incorporate the volatile fluid initially as 
part of the fluid to be supplied to the charge injector represents a 
particularly convenient way of achieving the intended result. 
Any convenient way of introducing the volatile fluid into the fluid to be 
charged can be adopted, but a particularly convenient and simple way is to 
use a mixing chamber for mixing together the two fluids before they enter 
the charge injector. The mixing chamber may comprise respective inlets for 
the two fluids and an outlet connected by a conduit to the inlet of the 
charge injector. 
For optimum performance, the relative proportions of the fluid to be 
charged in the volatile fluid are carefully chosen and maintained. For 
this purpose, the charge injection apparatus may preferably comprise 
respective flow control valves for regulating the flow rates of the two 
fluids supplied to the mixing chamber inlet, respective means for 
monitoring the flow rate determined by the flow control valves, and 
control means for controlling the settings of the flow control valves for 
opposing changes in the monitored flow rates from respective predetermined 
values. 
A preferred embodiment of the invention comprises: 
(a) a chamber having an inlet and an exit orifice; 
(b) means for mixing together a fluid to be charged and a volatile fluid 
whose vapor is of higher dielectric strength than the ambient atmosphere 
into which the exit orifice discharges; 
(c) conduit means connecting the mixing means to the chamber inlet for 
conveying the fluid mixture to said chamber; 
(d) first, high potential, electrode means in said chamber adjacent said 
exit orifice, said electrode means having a pointed tip for injecting 
charge into the fluid mixture before it exits the chamber through said 
orifice; 
(e) second, low potential, electrode means in said chamber between the 
first electrode means and said exit orifice; and 
(f) third, earth potential, electrode means located downstream of said exit 
orifice; 
the arrangement being such that the charged fluid mixture issues through 
the exit orifice as a jet and the volatile fluid component of the fluid 
mixture volatilizes on emerging from the exit orifice so as to oppose any 
tendency to dielectric breakdown occurring. 
In accordance with another aspect of the invention, there is provided a 
method of operating a charge injector having a high potential electrode 
with a pointed tip for injecting charge into a fluid to be charged and an 
exit orifice, downstream of the pointed electrode, through which the 
charged fluid issues as a jet, said method comprising passing through the 
charge injector a two-component fluid comprising a first component which 
it is desired to charge and a second component which is a volatile fluid 
whose vapor is of such a dielectric strength that said volatile fluid 
volatilizes, as the two-component fluid emerges from said exit orifice, to 
oppose any tendency to dielectric breakdown occurring. 
In accordance with one way of putting the invention into effect, the first 
fluid component is a liquid saturated with a gas which is the second fluid 
component. The dissolved gas then vaporizes from the charged jet emerging 
from the charge injector and forms a blanketing protective sheath around 
the jet. The second fluid component could be a dichloro-difluoro methane 
(Freon 12). 
According to another way of performing the inventive method, the premixing 
at source of a volatile additive with the fluid to be sprayed provides a 
very simple and direct means by which the benefits of a high dielectric 
breakdown strength blanketing vapor can be obtained. Therefore, in one way 
of putting the method into effect, the two-component fluid is supplied 
from a source comprising a mixture of the two fluid components. 
Alternatively, the second fluid component is blended with the first fluid 
component flowing from a source of that first fluid component to the 
charge injector, to form said two-component fluid. 
In accordance with a preferred method of performing the invention there is 
provided a method of operating a charge injector having a high potential 
electrode with a pointed tip for injecting charge into a fluid to be 
charged and an exit orifice, downstream of the pointed electrode, through 
which the charged fluid issues as a jet into an ambient atmosphere, said 
method comprising introducing into the first-mentioned fluid, a volatile 
fluid of higher dielectric strength than that of said ambient atmosphere, 
so that said volatile fluid volatilizes on emerging from said exit orifice 
to oppose any tendency to dielectric breakdown occurring. 
As indicated earlier in this specification, the volatile fluid may be a gas 
(e.g. Freon-12, which is dichloro difluoro methane) which is introduced 
into the first-mentioned fluid which itself is a liquid, so that the 
liquid becomes saturated with the introduced gas and that gas vaporizes 
from the jet issuing from the charge injector. 
As indicated above, a preferred method of introducing the volatile fluid 
into the fluid to be charged is by mixing the two fluids in a mixing 
chamber and introducing the resulting mixture into the charge injector. 
Suitably the volatile fluid is a hydrocarbon, e.g. a high vapor pressure 
hydrocarbon having four or more carbon atoms per molecule. Alternatively 
or in addition, the hydrocarbon may have one or more double bonds. An 
example of such a hydrocarbon is 1,3 butadiene. As an alternative, the 
volatile fluid may be a halogenated or oxygenated compound. 
Specific examples of preferred volatile fluids are propane, n-butane, 
iso-butane, ethylene, propylene, butene, acetylene, hexene, and 
cyclohexane.

Referring to FIGS. 1 and 2, there is shown charge injection apparatus for 
charging a fluid which comprises a charge injector 1 and fluid supply 
equipment, denoted generally by reference numeral 2. The charge injector 1 
comprises a cylindrical housing 3 having a fluid inlet 4 in one end and an 
exit orifice 5 in the other end. Mounted centrally within housing 3 is a 
high potential negative electrode 6 connected, through electrical 
connecting lead 15, to a high voltage, negative biasing, source or battery 
7 which is earthed at 8. The electrode 6 tapers at one end to a conical 
tip 9 and is arranged with its axis co-linear with the axis of exit 
orifice 5 and with its conical tip 9 closely adjacent to orifice 5. An 
intermediate potential electrode 10 is disposed within housing 3 between 
the conical tip 9 of electrode 6 and the exit orifice 5 and a central 
aperture 11 in electrode 10 is arranged co-linearly with the common axis 
of electrode 6 and exit orifice 5. Intermediate potential electrode 10 is 
connected by electric lead 16 to earth 8 through biasing resistor 12. A 
third electrode 13, which completes the electrical circuit of the charge 
injector, is arranged outside the cylindrical housing 3 and connected to 
earth 8 so as to be maintained at all times at earth potential. In the 
embodiment illustrated, electrode 13 is located, spaced well away from 
exit orifice 5. Where the equipment is used for electrostatic paint 
spraying for example, electrode 13 would provide the surface to be 
painted. However, other arrangements are possible such as where the charge 
injector is located above a separation vessel of an electrostatic, charge 
injection, separation apparatus, in which event the separation vessel 
itself can serve as earth electrode 13 or, alternatively, electrode 13 can 
be located immersed in the charged fluid in the separation vessel. These 
various alternative arrangements all fall within the scope of the 
invention as defined by the appended claims. 
FIG. 2, in which electrode 13 has been omitted for simplicity, shows that 
the connecting leads 15 and 16 of electrodes 6 and 10 are led out through 
the wall of the cylindrical housing 3 by means of insulating bushings 17, 
18, respectively. 
The fluid supply equipment 2 in this embodiment comprises a mixing chamber 
20 having respective inlets 21, 22 (shown only in FIG. 1) for the fluid to 
be charged and for a volatile fluid to be mixed with the fluid to be 
charged. Mixing chamber 20 is also provided with a fluid outlet 23 which 
is connected by a supply conduit or pipe 24 to the fluid inlet 4 of charge 
injector 1. 
The supply line 25 to inlet 21 includes a solenoid-operated, flow control 
valve 26 and a flow rate detector 27 which produces an electrical output 
signal representative of the flow rate determined by the setting of 
control valve 26. Similarly, supply line 28 to inlet 22 includes a flow 
control valve 29 and flow rate detector 30. The settings of flow control 
valves 26, 29 are regulated by a controller 31 which respond to 
differences between the electrical output signals from the flow rate 
detectors 27, 30 and input electrical signals from a manually set, desired 
flow rate, input circuit 32, so as to oppose any changes in measured flow 
rates for the fluid to be charged and the volatile fluid from desired flow 
rates. 
In operation, fluid to be charged and a suitable selected volatile fluid 
are introduced in desired proportions determined by the manual setting of 
input circuit 32 are introduced into mixing chamber 20 and the fluid 
mixture passes along pipe 24 and into the chamber defined within 
cylindrical housing 3 of the charge injector 1. As the fluid mixture 
passes the tip 9 of the high potential electrode 6, excess charge carriers 
are induced to be emitted into the fluid mixture at or near the electrode 
tip when this electrode is maintained at a sufficiently high negative 
potential with respect to the intermediate potential electrode 10. The 
charge carriers are then swept from the pointed electrode 6 by the cross 
flow of the fluid mixture which then issues as a jet or spray axially 
through the exit orifice of the charge injector. As the jet emerges 
through the exit orifice 5, it experiences a reduction in the ambient 
pressure which is lower outside the cylindrical housing 3 than inside it, 
and this reduction in pressure causes the volatile fluid to volatilize and 
thereby form a blanket of gas enveloping the exiting fluid jet or spray 
which, because it is of higher dielectric strength than the ambient 
atmosphere, serves to resist any tendency to dielectric breakdown on 
emerging from the exit orifice 5. In this way, the charge injector can be 
operated at higher potentials than would otherwise be the case while at 
the same time avoiding dielectric breakdown. 
EXAMPLE 
The benefit of blanketing the exiting spray from a charge injector with a 
blanketing gas by introducing into the charge injector the liquid to be 
charged which is saturated with that gas has been demonstrated by the 
inventor using apparatus, very diagrammatically represented in FIG. 3. The 
charge injector 1 was of similar construction to that of the charge 
injector in the FIGS. 1 and 2 embodiment and therefore is not described in 
detail but its corresponding components are denoted by the same reference 
numerals as in FIGS. 1 and 2. 
In the experimental apparatus, the charge injector 1 was centrally located 
in an upright 15" diameter right circular cylindrical test enclosure 40 
made of Lucite (polymethylmethacryte). The enclosure top was closed with a 
Lucite disc 41 while the bottom rested on a shallow collection pan 42, in 
which a one-inch thick section of 1/8" cell aluminium honeycomb 43 was 
placed to provide a splash-free spray collection surface spaced below the 
exit orifice of the charge injector at a distance in the range 10 to 30 cm 
so that the exiting spray from the charge injector was intimately exposed 
to the gas inside the test enclosure. This honeycomb 43 was connected to 
ground 8 and served to ensure that all incident droplet charge would have 
ample opportunity to drain off and be properly monitored. 
The spray liquid collected in the pan 42 are recirculated, via a pump 44 
with pump reservoir 45, to the charge injector 1, so that at any one time 
between 2 and 3 liters of test fluid was contained within the flow circuit 
(reservoir, pump, charge injector, pan and plumbing). Under normal 
operating conditions, the nominal 1 mL/sec charge injector flow rate 
produced a fluid recycling time of about half an hour. In tests (not 
described in detail herein), to avoid the possibility of spray ignition by 
electrical discharge, the test enclosure 40 was continuously purged by 
nitrogen from a laboratory supply. The nitrogen was supplied along purge 
line 46 to an annular gas distributing ring 47 resting on the honeycomb 
section 43 and delivering a blanket of purge nitrogen completely 
enveloping the exiting spray 48 from the charge injector. For this 
purpose, the distribution ring 47 was made from 3/4" diameter plastics 
tubing formed with 3 mm diameter holes at 3 cm intervals. This arrangement 
provided a reasonably uniform and low turbulence means for filling the 
test enclosure with gas. For tests where Freon-12 was used to fill the 
enclosure 40, the nitrogen purge line 46 was simply connected to a 
Freon-12 source. Because the gas was introduced by an annular distributor, 
the dense Freon gas quite literally behaved like a liquid and filled the 
container 40 from the bottom up during the test. 
Spray fluid that collects in the enclosure pan 42 was returned directly to 
the pump reservoir 45. This process was assisted by use of a laboratory 
vacuum system 49 which maintains a slight subatmospheric reservoir 
pressure. By holding the reservoir at below ambient, a positive flow of 
fluid from the pan is ensured. This was found to be particularly important 
with viscous spray fluids which have a tendency to puddle and not gravity 
feed very effectively. 
In addition to guaranteeing effective siphoning of the collected spray 
fluid, the low reservoir ullage pressure served to maintain a continuous 
flow of the enclosure purge gas into the pump reservoir. The purge gas 
mixed with the returning fluid as it passed in slug and bubbling flow to 
the reservoir and this gas then bubbled through the liquid in the pump 
reservoir and formed a protective blanket 50 over the stored liquid. This 
not only effectively prevented an explosive vapor mixture from forming but 
also quaranteed that the test liquid would be saturated with the purge 
gas. This latter factor was particularly pertinent to the Freon-12 purge 
test results. 
Evidence for charge injector performance being influenced by the evolution 
of a volatile blanketing gas from the spray fluid is seen in FIG. 4. This 
Figure represents a time history of the behavior (in the form of the mean 
spray charge density (in Coulombs per cubic meter)--see plot A) of the 
charge injector operating on a recirculating fluid (Marcol-87 which is a 
white oil manufactured by Exxon Company, U.S.A) during purging with 
Freon-12 (dichloro difluoro methane). Despite significant data scatter due 
to extraneous operating problems unrelated to the experiment the measured 
mean charge density of the charge injector spray displayed an increasing 
trend from time 20 minutes onwards. The enclosure Freon-12 level was 
inferred from the O.sub.2 concentration readings (taken from an O.sub.2 
monitoring tube (not shown) located at the same height and within 5 cm of 
the charge injector exit orifice 11) which are also plotted (plot B). 
As shown, the charge injector performance stayed approximately constant 
throughout the active Freon-12 purge portion of the test (the first 20 
minutes). During this phase, the enclosure was actively purged with 
Freon-12 from a 50 pound capacity cannister. Complete displacement of the 
initial air in the enclosure occurs at about the ten-minute mark when the 
free O.sub.2 concentration has dropped to approximately 2% (plot B). At 
this point, expansion cooling of the Freon started to seriously reduce the 
cannister pressure. Cooling persisted to such an extent that virtually no 
flow to the enclosure could be obtained with the cannister valve being 
shut at about 25 minutes. At this point, air which had diffused into the 
test enclosure had effectively displaced the Freon as indicated by the 
O.sub.2 level on plot B returning to 20%. In view of the proximity of the 
O.sub.2 monitoring tube to the exit orifice 11 of the charge injector, 
this indicated that the charge injector was immersed in ambient air with 
only a minor Freon content. 
Despite the enclosure background gas having returned to ambient conditions, 
the charge injector output charge density shows a monotonic rise starting 
at about the 20 minute mark. This time is consistent with the pump system 
recirculation time scale. That is, a parcel of fluid (Marcol-87) that had 
been sprayed and intimately exposed to the enclosure Freon-12 early in the 
test, would be saturated with Freon, and would be expected to be recycled 
to the charge injector starting at this time. 
There is no other explanation for the observed .about.20% increase in 
performance exhibited by these data that can be used to refute the 
proposed self blanketing of the Freon-12 saturated test fluid.