CA1128607A - High potential discharge control circuit having distributed resistance elements especially suitable for induction-charging electrostatic spraying system - Google Patents
High potential discharge control circuit having distributed resistance elements especially suitable for induction-charging electrostatic spraying systemInfo
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- CA1128607A CA1128607A CA325,242A CA325242A CA1128607A CA 1128607 A CA1128607 A CA 1128607A CA 325242 A CA325242 A CA 325242A CA 1128607 A CA1128607 A CA 1128607A
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Abstract
HIGH POTENTIAL DISCHARGE CONTROL CIRCUIT HAVING
DISTRIBUTED RESISTANCE ELEMENTS ESPECIALLY SUITABLE FOR
INDUCTION CHARGING ELECTROSTATIC SPRAYING SYSTEM
Abstract of the Disclosure A high voltage discharge control circuit is provided for an induction-charging spraying system. The circuit comprises an induction-charging electrode having an electrically conductive wall that provides a surface from which an induction-charging field is established. A first resistance means connected between the electrode surface and the potential applying means retards transport of electric charge to the electrode surface and thus to electrode edges or surface discontinuities which are most sus-ceptible to arcing to an electrical ground point. The circuit may also contain additional current limiting series resistors in the high voltage cable and in the power supply to inhibit current surges to the electrode surface from other circuit elements. Shunt resistors are provided between high potential circuit elements and electrical ground to drain accumulated charge from the circuit.
DISTRIBUTED RESISTANCE ELEMENTS ESPECIALLY SUITABLE FOR
INDUCTION CHARGING ELECTROSTATIC SPRAYING SYSTEM
Abstract of the Disclosure A high voltage discharge control circuit is provided for an induction-charging spraying system. The circuit comprises an induction-charging electrode having an electrically conductive wall that provides a surface from which an induction-charging field is established. A first resistance means connected between the electrode surface and the potential applying means retards transport of electric charge to the electrode surface and thus to electrode edges or surface discontinuities which are most sus-ceptible to arcing to an electrical ground point. The circuit may also contain additional current limiting series resistors in the high voltage cable and in the power supply to inhibit current surges to the electrode surface from other circuit elements. Shunt resistors are provided between high potential circuit elements and electrical ground to drain accumulated charge from the circuit.
Description
~ 136~7 Background of the Invention Field of the Invention:
Electrostatic spraying devices which provide spray streams of ~ -charged liquid particles by an induction-charging mechanism are well known.
Of particular interest herein is circuitry for controlling or reducing the incidence of high electric potential discharges from elements of an in-duction charging spray.ng system.
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~2~7 State of the Art:
A need has long been recogni~ed for an electrostatic spray device that can safely and efficiently deposit charged particles of paint or coating material onto a substrate. Corona-discharge electro-static spray systems, for example, can provide fairly efficlent depo~
sition of charged paint particles onto a substrate. These corona devices, howèver, typically utilize needle-like electrodes that establish corona-producing electric fields by application of potentials of about 100,000 volts to the electrode with resulting corona-d~scharge currents approximating 50-300 microamps. Such high-power electric discharges present potential shock hazards to equipment operators.
Moreover, there is great likelihood in corona systems of high potent-lal electric discharge by arcing from the electrode to a ground point or by sparks from the electrode to air-borne particula~e matter, which elec-tric discharges can ignite flammable paint vapors. This ha~ard of `
fire and explosion from corona-discharge ignited paint vapors has, for example, substantially precluded use by ma~or household appliance manufacturers of electrostRtic spray devices for spraying organic-based paints onto the interior surfaces of appliance cabinets.
In response to a need for an electrostatic spray device tha~
can safely and efficiently spray flammable paints without ha~ards of fire or explosion or electrical shock ~o equipment operators, there has been recently provided an improved electrostatic spray device as disclosed in U.S. Patent No. 4,0099829 to J. E. Sickles. This electro-static spray device comprises an induction-charging e7ectrode positioned exteriorly of
Electrostatic spraying devices which provide spray streams of ~ -charged liquid particles by an induction-charging mechanism are well known.
Of particular interest herein is circuitry for controlling or reducing the incidence of high electric potential discharges from elements of an in-duction charging spray.ng system.
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~2~7 State of the Art:
A need has long been recogni~ed for an electrostatic spray device that can safely and efficiently deposit charged particles of paint or coating material onto a substrate. Corona-discharge electro-static spray systems, for example, can provide fairly efficlent depo~
sition of charged paint particles onto a substrate. These corona devices, howèver, typically utilize needle-like electrodes that establish corona-producing electric fields by application of potentials of about 100,000 volts to the electrode with resulting corona-d~scharge currents approximating 50-300 microamps. Such high-power electric discharges present potential shock hazards to equipment operators.
Moreover, there is great likelihood in corona systems of high potent-lal electric discharge by arcing from the electrode to a ground point or by sparks from the electrode to air-borne particula~e matter, which elec-tric discharges can ignite flammable paint vapors. This ha~ard of `
fire and explosion from corona-discharge ignited paint vapors has, for example, substantially precluded use by ma~or household appliance manufacturers of electrostRtic spray devices for spraying organic-based paints onto the interior surfaces of appliance cabinets.
In response to a need for an electrostatic spray device tha~
can safely and efficiently spray flammable paints without ha~ards of fire or explosion or electrical shock ~o equipment operators, there has been recently provided an improved electrostatic spray device as disclosed in U.S. Patent No. 4,0099829 to J. E. Sickles. This electro-static spray device comprises an induction-charging e7ectrode positioned exteriorly of
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an external-mixing spray-forming nozzle. Since charge imposition on spray particles occurs by the method of induction, there is practically no like-lihood, under ideal conditions, of any substantiaL arc or high energy discharge. The absence of any substantial discharge is assured by an electrode surface configuration that is devoid of sharp edges and points and by the application of high voltage potentials to the electrode of about 25,000 volts or less, with normal current dissipation by the elec-trode being at a level of about 1 to 3 microamps or less. With the induction-charging electrode operating at these substantially lower voltage and current levels as compared to a typical corona-discharge electrode, any incidence of arcing or sparking is substantially reduced.
~loreover, operator injury resulting frGm electric shock is avoided by the practically insignificant current availab]e to be delivered by the electrode.
In addition to the aforementioned improved safety features, the described induction-charging spray device provides improved charged particle atomization. It has been found that a spray device comprising an induction-chargirlg electrode disposed exteriorly of, or outwardly from, an external-mixing nozzle provides an assembly oE particles characterized by a high degree I of fineness and uniform size and having a relatively high average charge-; 20 to-mass ratio. These factors are important in achieving maximum transfer of-coating material from the spray device to the target substrate and for achieving levelling or flow of the material into an evenly deposited, uni-` formly coalesced film.
This unique combination of safety and deposition efficiency features of the described induction-charging spray device is responsible for the significant commercial success of the device in overcoming problems inherent with corona-charging types of electrostatic spray equipment. It
an external-mixing spray-forming nozzle. Since charge imposition on spray particles occurs by the method of induction, there is practically no like-lihood, under ideal conditions, of any substantiaL arc or high energy discharge. The absence of any substantial discharge is assured by an electrode surface configuration that is devoid of sharp edges and points and by the application of high voltage potentials to the electrode of about 25,000 volts or less, with normal current dissipation by the elec-trode being at a level of about 1 to 3 microamps or less. With the induction-charging electrode operating at these substantially lower voltage and current levels as compared to a typical corona-discharge electrode, any incidence of arcing or sparking is substantially reduced.
~loreover, operator injury resulting frGm electric shock is avoided by the practically insignificant current availab]e to be delivered by the electrode.
In addition to the aforementioned improved safety features, the described induction-charging spray device provides improved charged particle atomization. It has been found that a spray device comprising an induction-chargirlg electrode disposed exteriorly of, or outwardly from, an external-mixing nozzle provides an assembly oE particles characterized by a high degree I of fineness and uniform size and having a relatively high average charge-; 20 to-mass ratio. These factors are important in achieving maximum transfer of-coating material from the spray device to the target substrate and for achieving levelling or flow of the material into an evenly deposited, uni-` formly coalesced film.
This unique combination of safety and deposition efficiency features of the described induction-charging spray device is responsible for the significant commercial success of the device in overcoming problems inherent with corona-charging types of electrostatic spray equipment. It
-3 has been found, however, that because of the rather large amounts of energy stored within capacitive elements of the charging circuit, rela-tively intense electrical discharges may occur from the electrode to an object at a lower electrical potential under certain less than ideal operating conditions. For example, when an energi~ed, hand~gun mounted electrode is brought too close to an electrically grounded object, there may be a sudden arc from the electrode to the grounde,l object. Also, during use of induction-charging spraying equipment the metallic surfaces of electrodes may become nicked or scratched, thereby establishing ideal sites of surface discontinuity for corona or sparking discharges. .~fter periods of spraying, dried paint may build up on the electrodes and pro-vide sites for producing corona or sparking, especially where the dried paint contains metallic or electrically conductive pigments.
A high intensity electrical energy discharge occurs as an arc of current between an electrode maintained at a relatively high potential and an object at a relatively lower potential. The object at a lower potential may be another portion of the spray device, or it may be an electrically grounded article such as the target to be coated, or other object in the spraying environment. Electrical arcs or sparking may also occur between the electrode and air-borne dust or spray particles which may be at lower potentials than the electrode. While these sudden dis-charges may be merely irritating to the touch of a spray operator, the arcing or sparking phenomena may have potentially lethal consequences where they occur in environments of flammable vapors, such as contributed by many organic paint systems.
Cne solution to the sparking problem is that of merely decreasing the voltage applied to the electrode to a level so that energy stored in , . , . : .
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the charging circuit lacks suf',icient potential to cause intense current-carrying arcs to leave the electrode surface. Decreasing the electrode potential, however, also causes particle charging and coating deposition efficiencies to decrease.
Another solution to the dilemma of providing high charging voltages while substantially eliminating flalmmable vapor-igniting sparks is described in U. S. Patents No. 3,641,971 and No. 3,795,839 to Walberg.
For the Walberg corGna or contact charging spraying system, there is : provided a complicated, expensive circuit "deenergizing means" for dis-connecting the high potential source when a certain threshold current is reached above which arcing occurs. The Walberg circuitry, while appro-priate for corona spraying systems, which typically require heavy and expensive high voltage and current power supplies, is unsuitable for an induction-charging system which is typically adaptable to small portable ?~ ~5 power supplies. ~loreover, the interruption of the charging circuit de-creases particle charging and coating deposition efficiencies.
~nother hazard common to electrostatic spraying environments is the sudden discharge or sparking that may occur when a break or short circuit occurs in the high voltage circuit. For example, breaks and short circuiting to ground may occur in the high voltage cable that delivers high potential from a power supply to an induction-charging electrode. There may follow a sudden discharge of electrical energy stored in other high capacitive elements of the electrostatic system, such as the power supply, or in the induction-charging electrode or in the cable itself, each of which is capable of storing significant amounts of electrical energy. Sudden electrical discharge may produce sparking conditions at the short circuit point with consequent haza-rds of fire or explosion of paint vapors and may also constitute a significant shock hazard to equipment operators.
,' _5_ Summary of the Invention The incidence of high potential electric discharges from components of an induction-charging electrostatic spraying system may be substantially reduced by including in the charging circuit resistance means of appropriate resistance values to retard the flow of charge stored in one portion of the circuit to another circuit portion where short circuit or likely arc-producing conditions exist. An induction-charging electrostatic spraying system is of a type which comprises liquid particle spray-forming means for discharging a spray stream of liquid particles along the axis of the spray-forming means, induction-charging means disposed in operable association with the spray-forming means to establish a charging æone in which charge is induced on liquid particles as the particles are formed by the spray-forming means, and means for connecting a high voltage DC electric potential to the induction-charging means. A control circuit for controlling high potential electric discharges from elements of the induction-charging electrostatic-spraying system comprises at least one induction-charging electrode disposed exteriorly of, or outward]y from, the spray-forming means. The electrode comprises a substrate having a wall made of electrically conductive material.
~he wall faces the axis of the spray-forming means and provides a surface from which an induction-charging field is established when an electric po-tential is applied to the electrode wall. A Eirst resistance means is connected between the high voltage DC electric potential connecting means and the electrode wall, with the first resistance means being disposed in close physical proximity to the electrode wall. In "close physical proximity"
is intended to indicate that the electrical connection between the electrode wall and the first resistance means is as short as possi~le so that any substantial capacitive or charge storing capability created within the connection is significantly less than the capacitance of the elec-trode circuitry or of the charging circuit. The first resistance means has sufficient ohmic resistance to retard or impede the transport of electric charge to the electrode surface at the magnitude of voltage applied to the electrode required to establish the induction-charging field. A first resistance means of an appropriate resistance value and disposed in close physical proximity to the electrode wall will impede transfer of electric charge to the electrode surface such that substantia'ly no electric discharges will occur which have intensities sufficient to ignite flammable yaint vapors.
The incidence of a spark-ignited fire or explosion of flammable paint vapors is a function of the intensity and duration of the spark or arc, as well as the type and concentration of'the vapor. Arc intensity and duration are, in turn, related to the energy imparted to the arc by the charging circuit. If the quantum of arc energy remains below that energy required to ignite the most flammable vapor-air mixture found in typical spraying operations, then ignition and explosion are not likely to occur. It has been determined that for a saturated mixture of toluene and air at 62F., or of xylene and air at l:L5F., which are the most flammable concentrations of these typically utilized solvents, the threshold energy of ignition is about 15 millijoules. Thus, a relatively safe electrostatic chargingsystem for use in spraying xylene- or toluene-containing paints in confined spaces may be provided by an electrode that provides substantially no electrical discharges or, if discharges do occur, by an electrode that electrically discharges arcs or sparks having energies less than 15 millijoules.
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The first resistance means may comprise a discrete resistor of the conventional carbon-filled or wire-wound type that may be embedded in an electrode substrate of electrically non-conductive or dielectric material. The first resistance means may comprise a siab of resistive material that may make up most of the material of the electrode sub-strate. Combinations of resistive material in series with dlscrete resis~ors may also be used.
An induction-charging electrostatir spraying system typically comprises, in addition to an induction-charging electrode, a power supply for furnishing a suitable high voltage DC electrical potential to the induction-charging electrode. In many industrial appllcations the power supply may be remote from the spraying device. The use of a remote power supply typically requires a cable designed for trans-mitting high voltages safely over a distance of six to twenty feet or more from the power supply to the induction-charging electrode~ In other applications, a portable power supply may be fitted to the spray device itself, eliminating the need for a high voltage conveying cable.
Second and third resistance means may be included in the high electric potential discharge control circuit. The second resistance means comprises a current limiting resistor connected in series with a high voltage output terminal of the power supply and with the first resistance means associated with the induction-charging electrode.
Typically, the series resistor is located in the high voltage transmitting cable at a :
point near the connection of the cable to the high voltage connecting means of the induction-charging electrode. The th:ird resistance means comprises a current limiting resistor in series with the power supply high voltage output terminal and the connecting means for connecting the high voltage cable to the power supply. A purpose of these current limiting resistors is to inhibit transfer of charge stored in one portion of the circult to another circuit portion. With the retardation of flows of substantial charge either from the power supply to the cable or from the induction-charging electrode to the cable, the likelihood of sparking from the cable to ground points is minimized, in the event that the cable is pulled from the power supply terminal, or the cable is severed while power is being applied to the system, when significant amounts of charge remain stored in the charging circuit.
Fourth and fifth resistance means may be included in the high electric potential discharge control circuit. The fourth resistance means comprises a current limiting resistor connected between the power supply high voltage output terminal and an electrical ground connection. The fifth resistance means comprises a current limiting resistor connected at a point near the high voltage connecting means of the induction-charging electrode and an electrical ground connection. A purpose of these resistors is to provide a shunt path to electrical ground to drain or bleed off accu-mulated charge stored within circuit elements.
As another aspect of the invention, there may be provided an induction-charging adapter for mounting on a spray device, which adapter ~5 includes a high electric potential discharge control circuit. The adapter comprises (a) housing means fabricated of a dielectric material, the housing means having an exterior wall and having an interior wall, (b) mo~mting means _g_ on the interior wall of the housing means for detachably mounting the housing means onto a spray device so that the interior wall faces the axis of a spray device when the housing means is mounted on a spray device, (c) induction-charging means including at least one induction-charging electrode attached to the housing means, the electrode com-;; prising a substrate having a wall of electr cally conductive material that faces the axis of a spray device when the housing means is mounted on a spray device, the wall providing a surface from which an induction-charging electrostatic field is established when a high voltage DC
electric potential is applied to the electrode, (d) means for connecting a high voltage DC electric potential to the induction-charging electrode, and (e) first resistance means connected between the electric potential . connecting means and the electrode wall, the first resistance means dis-posed in close physical proximity to the electrode wall and having lS sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to the electrode required to establish the induction-charging field, whereby electrical discharges of energies ~hich are sufficient to ignite a saturated xylene vapor-in-air mixture at about 62F. are substantially suppressed.
The first resistance means may comprise a discrete resistor or a slab of resistive material in series with the conductive electrode wall and the high voltage electric potentlal connecting means for the purposes as set forth above in discussion of these resistor elements.
Also mounted upon the adapter housing may be a current limiting resistor of the type described above comprising the fifth resistance means. This current limiting resistor is connected between the high voltage connecting means for the first resistance means and an electrical ground connection.
~86~7 The resistor provides a shunt path to groun~ for bleeding charge to electrical ground that is stored in the electrode circuit of the induction-charging adapter.
Brief Description of the Drawillgs The accompanying drawings illustrate examples of embodiments of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:
FIG. 1 is a diagrammatic presentation of an electrostatic sprayincg system illustrating preferred physical locations of resistor elements;
FIG. 2 is a schematic diagram of a high voltage discharge control circuit for an induction-charging electrostatic spraying system;
FIG. 3 is a perspective view of an induction-charging adapter fitted to an external-mixing spray gun;
FIG. 4 is an exploded view of the nozzle assembly of the spray gun illustrated in FIG. 3;
FIG. 5 is a rearward elevation of an induction-charging adapter with a downstream vieM into the adapter;
FIG. 6 is a top view of an induction-charging adapter showing an embodiment of the induction-charging electrode in section; and FIG. 7 is a top view of an induction-charging adapter showing another embodiment of the induction-charging electrode in section.
Description of Preferred Embodiments An electrostatic spraying system of the induction-charging type is depicted in FIG. 1. The system comprises a spray device 8, .
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an induction-charging adapter 9 fitted to the spray device, a power supply 10 for providing a suitable high voltage DC electrical potential at output terminal 11 relative to an electrical ground potential at ground connection 12, a cable 13 connecting high voltage cable comlecting S terminal 14 of the power supply with a high voltage input terminal 15 located on the induction-charging adapter, liquid coating material supply 16, a compressed air supply 17 and feed hoses 18 and 19 for de-livering liquid coating material and compressed air, respectively, to spray device 8.
~ high electric potential discharge control circuit for use in combination with the induction-charging electrostatic spraying system is illustrated in the schematic diagram of ~I~. 2. A first resistance means associated with an electrode of the induction-charging adapter is designated Rl, second resistance means in series with hi.gh voltage cable connecting terminal 14 and the high voltage input terminal 15 on adapter 9 is designated R2, within power supply lO third resistance means connected between high potential output terminal 11 and cable connecting terminal 14 is designated R3, fourth resistance means connected between high voltage oueput terminal ll and electrical ground connection point 12 is designated R4, and fifth resistance means connected between high voltage input termi-nal 15 and an electrical ground connection point is designated R5.
~ more detailed description of spray device 8 may be found with reference to FIGS. 1 and 3, wherein there is illustrated a conventional air-atomizing hand-held spray device 8 having a handle 20, a barrel 21 and a nozzle assembly 22. ~ trigger 23 serves to operate a valve assembly (not shown) within barrel 21 to regulate flows of liquid coating material and an atomizing gas, such as air, to nozzle assembly 22. A liquid coating .
material, such as a paint having a condllctivity generally greater than 0.001 ~mho/cm, is fed to the spray device from paint supply 16 through paint feed hose 18, which is connected to spray device 8 by mating threaded members forming connecting means 24 for paint ~eed hose 18.
As indicated in FIGo 1, the paint supply including its container is preferably electrically grounded. From a compressed air supply 17, - feed hose 19 delivers atomizing air under pressure to connecting means 25, which again is an assembly of mating threaded members.
The spray device as illustrated is a commercially available hand-held gun of the air~atomizing siphon type (Model 62, Binks Mfg. Co., Chicago, Ill.). Nozzle assembly 22 is depicted as an external-mixing spray-forming nozzle of the type described in U. S. Patent No. 4,009,829 to J. E. Sickles, incorporated herein by reference. As shown in FIG. 4, nozzle assembly 22 comprises a liquid discharge nozzle 26 having a liquid-conveying passageway and comprises an air cap 27. The assembly of liquid discharge nozzle 26 and air cap 27 defines an annular-shaped atomizing-air discharge port 28 that is concentric with a liquid discharge port 29.
~ir and liquid discharge ports 28 and 29, respectively, lie in the plane of face 30 of nozzle assembly 22 and are disposed generally coaxially with respect to the axis of the liquid-conveying passageway of liquid discharge nozzle 26. Streams of atomizing air and liquid coating material discharged from ports 28 and 29, respectively, coact to form a spray stream of particles that is discharged generally coaxially with respect to the liquid nozzle axis and ln a downstream direction with respect to nozzle face 30.
Liquid discharge nozzle 26 may be fabricated of electrically conducting or non-conducting materials. It is preferred that within the liquid-conveying passageway of liquid nozzle 26 near its discharge port 29 .
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a conductive grounding element (not shown) be provided in electrical contact with the stream of liquid coating material. This grounding element will be grounded to a common electrical ground shared by the paint supply and power supply as indicated in FIG. 1. Preferably, air cap 27 is fabricated of a dielectric material, such as acetal resins, epoxy resins, glass-filled nylon resins and the like. If air cap 27 is made of a metallic or conductive material, then air cap 27 should be electri-cally isolated from ground potential. Also located on nozzle face 30 are additional air discharge ports 31. Projecting downstream from nozzle face 30 and integrally formed with air cap 27 is a pair of air llorns 32. Located on air horns 32 on faces oriented toward the spray stream axis are additional air discharge ports 33. Air discharge ports 31 and 33 cooperate to shape the spray stream into a fan configuration.
Mounted on barrel 21 at its downstream-oriented or forward end is induction-charging adapter 9. The adapter comprises a housing 34 Eabricated of a dielectric material. The dielectric material s~ould be capable of withstanding stresses associated with the high voltages pro-vided by the power supply without electrical breakdown or tracking. Use-ful dielectric materials include those set forth above for fabricating air cap 27. Housing 34 is mounted upon barrel 21 by a friction fit between a pair of shoulders 35 located at the rearward portion of housing 34 and upon interior wall 36. Each of shoulders 35 is positioned diametrically opposed to the other and is shaped to mate with a comple-mentary-shaped surface of barrel 21 so that housing 34 is rigidly secured to spray device 8. When the adapter housing is mounted upon spray device 8, housing interior wall 36 faces the axis of the spray stream discharged from nozzle assembly 22, while housing exterior wall 37 faces generally in a direction outwardly of the spray stream axis.
~3 ~ousing 34 is characterized in having its wall portions ex-tending downstream to form a pair of lobes 38. ~ounted on the inner wall of each lobe 38 is an induction-charging electrode 39. Each of electrodes 39 comprises a substrate 40 h~ving an electrically con-ductive wall 41 with a surface 42 facing the spray stream axis. When an appropriate high voltage DC voltage is applied to electrode 39, an electric field is established between wall surface 42 and a region sur-rounding liquid discharge port 29 through which liquid coating material is discharged. Means for connecting a high voltage DC electrical po-tential to each induction-charging electrode 39 includes a high voltage contact that forms high voltage input terminal 15 and a conductor 43 connecting terminal 15 to a resistive element typically located within electrode substrate 40. As illustrated in FIG. 5, terminal 15 may be a "banana" plug rigidly fixed within a portion of electrode substrate 40 that provides electrical connection for a suitable mating member incorpo-rated into high voltage cable 13.
Adapter 9 is mounted upon spray device 8 such that electrodes 39 are positioned exteriorly of, or radially outwardly from, external-mixing nozæle assembly 22. Preferably, electrodes 39 are positioned with respect to nozzle assembly 22 so that at least a portion of surface 42 of elec-trode wall 41 intersects a plane containing liquid discharge port 29.
Thus, with respect to a plane containing nozzle assembly face 30, elec-trode wall surface 42 intersects the plane~ with at least a portion of wall surface 42 extending downstream from, or forwardly of, nozzle assembly face 30. The radial distance of electrodes 39 outwardly from the axis of liquid discharge noæzle 26 will generally determine the magnitude of the voltage required to be applied to electrodes 39 to ;
provide an induction-charging ~ield. For the adapter illustrated in FIG. 3 having each electrode 39 spaced outwardly about 3/4 inch from the liquid discharge nozzle axis, DC voltages between about 5,000 volts to about 25,000 volts will produce an effective induction-charging field in a region surrounding liquid discharge port 29, which field has an average potential gradient in the range from about 7 kilovolts per inch to about 33 kilovolts per inch. Voltages that are so high as to cause corona discharge from electrodes 39 are to be avoided. In this respect, the induction-charging electrode 39 may be characterized as one which is substantially non-corona producing, that is, electrode 39 has a configuration which is substantially free of sharp angles, points, or surface discontinuities that may tend to produce corona discharges in the aforementioned voltage range.
In an induction-charging device such as that utiliæed in the present invention, liquid coating material atomization and electric charge imposition occur substantially simultaneously so as to create a stream of discrete particles bearing an induced electric charge. For example, the stream of liquid coating material which passes through liquid discharge port 29 of nozzle assembly 22 is thrust into contact with a flow of air or gas from concentrically disposed atomizing-air discharge port 28, which flow of gas or air impinges upon and mixes with the liquid stream and tends to distort the stream into an irregular con-figuration comprising surface discontinuities. Formation of cusp-like, liquid stream discontinuities or l'liquid termini" is aided by the high intensity electric field existing between high voltage electrode 39 and the grounded liquid stream. The electric field flux lines tend to con-centrate at the sharp-pointed liquid termini and to induce electric 8~7 charge redistribution within the liquid stream, with charge of sign opposite that of the high voltage electrode migrating to the extreme sharp portions of the liquid tennini. Since the charges on the liquid termini and on the electrodeare opposite in sign, electrical attractive forces cooperate with the mechanicaldistresses furnished by the flow of gas or air to separate the liquid termini from the liquid stream so as to form discrete coating material particles bearing electric charge.
Connected between high voltage input terminal 15 of the high DC
electric potential connecting means and electrode wall surface 42 is a first resistance means. An electrically conductive material comprises the electrode wall 41 which provides surface 4~. from which an induction-charging electro-static field is established. Electrode wall 41 may be fabricated of practically any electrically conductive material. Suitable materials include aluminum, copper, nickel, magnesium, titanium and tin, and alloys such as stainless steel, bronze and brass. Also suitable are commercially available conductive plastics such as carbon-filled polyester plastics of relatively high con-ductivity (Velostat conductive plastics; 3M Co.). Generally, electrode conductive wall 41 may be in the form of a thin foil having a thickness of about one to ten mils, although it may be considerably thicker. It is general]y desirable that the electrical path through the connecting means between electrode wall 41 and the first resistance means be as short as practicable.
The first resistance means may be provided by any one of several embodiments. In a preferred embodiment illustrated in FIG. 6, the resistance means may comprise one or more discrete resistors 44 embedded within electrode substrate 40 which is fabricated of an electrically non-conductive material oE the aforementioned type described for fabricating adapter housing 34.
Resistor 44 may be a commercially available carbor.-filled composition type or a wire-wound element. A preferred arrangemen-t as illustrated in FIG. 6 utilizes two high-voltage stable miniature resistors 44 co~nercially designated A~ ~
as "Victoreen Minimox" resistors sold by Victoreen Instrument Div., Cleveland, Ohio. Typical values for these discrete resistors may range from about 10 megohms to about 50 gigohms. An electrical connection to discrete resistors 44 is made at a contact point 45 locsted near a rear-ward, or upstream, portion of conductive wall 41 ad~acent substrate 40, about halfway between the upper and lower edges of electrode 39. The other end of resistor 44, or a series of resistors 44, is connected to conductor 43.
The first resistance means may comprise a slab 46 fabricated of resistive material. This resisti~e material may be formed from a synthetic resin-graphite composition having a bulk resistivlty as determined by the ratio of the amount of elec~rically conducting graphite component to the amount of electrically non-conducting resinous component.
Suitable for use as resistive materials are the non-conductive thermo-plastic polyester compositions sold by General Electric under the trade-mark Valox~ which compositions may be doped with graphite to achieve a material of suitable resistivity.
A purpose of the first resistance means is to inhibit or retard transfer of arc-forming amounts of current from other portions of the electrode circui~ry, or cable or power supply circuitry, to po~entially arc-producing sites on the electrode so that arcing ~rom the electrode to an ob~ect at a lower pot~ntial is subs~antially suppressed. This arc suppressing capability is related to, or dependent UpOII, several factors, such as electrode configuration, charging potential applied to the elec-trode, resistance of the first resistance means and the charge-storing capacity of the electrode circuitry and the capacitance of the cable and power supply. The precise resistance value selected for the first re-sistance means may thus depend upon several factors. A criterion or test for selecting a proper resistance may be based upon whether, in actual practice under spraying conditions, arcs or sparks are generated from the electrode surface of sufficient intensity to ignite a concentration of ~ . ~
36~7 ~lammable solvent vapors of a paint that is sprayed. Hence, a choice of resistance for a particular electrode system, which does not produce electrical discharges of sufficient intensity to ignite the most flam-mable vapor-air mixture conceivable, is generally a suitable choice of resistance for practically any spraying operation utilizing flammable paints. ~ suitable highly flammable solvent vapor-air mixture against which arc suppression may be tested is a concentration of saturated toluene-in-air mixture at about 62F. Another rather sensitive test concentration of flamnable vapor-in-air mixture is provided by a concen-tration of a saturated xylene-in-air mixture at about 115F. These test concentrations are intended to be exemplary of flammable mixtures for testing arc suppressing capability of the high potential discharge control circuit of the invention. More sensitive or more highly flam-mable mixtures may also be used to determine an appropriate resistance value for the first resistance means, since any resistance value from about 10 megohms to about 50 gigohms may be used as the first resistance means. In this regard it should be mentioned that the total resistance of the series resistance between the high potential output terminal 11 of the power supply and electrode wall 41 may comprise the first re-sistance means.
Another criterion useful in determining a suitable arc suppressing circuit is the physical proximity of the first resistance means to elec-trode wall 41. As mentioned, it is desirable that any capacitance -Ln the connection or conducting means between the resistance means and electrode wall 41 be relatively less significant than the capacitance of the elec-trode circuitry. Hence~ the connecting means from resistor 44 to wall 41 of the embodiment of FIG. 6 should be a relatively short conductor so that $~3~
resistor 44 is in close physical proximity to electro(le wall 41. ~s de-picted in FIG. 7, electrode wall 41 is directly adjacent to, and thus in close physical pro~imity with, resistive material slab 46.
It has been found that the portions of the electrode most S susceptible to arcing are the portions forming the downstream or forward ends 48. It is preferred, therefore, that the dielectric substrate ma-terial which forms electrode substrate 40 encase the forward edges of electrode wall 41, as indicated in FIGS. 6 and 7. The dielectric ma-B terial may thus form a bead 49~that runs along the circumference of electrode wall 41.
The preclse resistance values of the first resistance means may be selected from a range from about 10 megohms to about 50 gigohms, with the exact choice being determined by the aforementioned parameters and criteria. A set of convenient measuring points Eor a particular resistance element incorporated in the electrode circuitry consists of high potential connecting means terminal 15 and the surface of electrode wall 41. The resistance between these two points may be considered the "working" resis-tance of the first resistance means. Values of this working resistance may be selected from resistances in the aforementioned range. A value of about 0.1 to 5 gigohms is a typical choice for the first resistance means.
The high voltage discharge control circuit may comprise second resistance means in a series circuit comprising the high voltage potential supplied by power supply 10 and the first resistance means. This second resistance means comprises a current limiting resistor 49 connected between high voltage input terminal 15 on adap~er 9 and the high voltage cable con-necting terminal 14 of power supply 10. Typically, resistor 49 may be physically located in high voltage cable 13 as depicted in FIG. 1. Or, .
~20-resistor 49 could be containecl within th~ spray gun in its handle 20 - or barrel 2~; a location of resistor 49 in spray gun barrel 21 is preferred for systems utilizing portable, barrel-mounted power sup-plies as disclosed in aforementioned references.
In systems utilizing a power supply located remotely from the induction-charging adapter, as shown in FIG. 1, a conventionally-available shielded cable 13 rated to carry voltages of about 25,000 volts DC may be utilized to connect power supply 10 to the induction-charging electrodes of adapter 9. Cable 13 is usually wrapped about air feed hose 19 and thus where cable 13 includes resistor 49, the resistor may be physically attached to hose 19 by tape, heat shrink tubing, or other means at a location near air hose connecting`means 25. Typically, resistor 49 may have a value in a range from about 100 megohms to about 50 gigohms, depending on the choice of resistance values for other re-sistor elements of the circuit.
During operation of the electrostatic spray system, consider-able amounts of electric charge are stored in the portion of cable 13 leading to the induction-charging electrodes and in the induction-charging electrode circuit. In the event cable 13 is disconnected from power supply terminal 14 while the spray device is in operation, this stored charge may discharge to an electrical ground point with the likeli-hood of electric sparks being generated. Also, curre~t discharge and sparking may result upon accidental severance of the cable by heavy equip-ment used in many industrial spraying environments. The flow of substantial amounts oE stored charge through cable 13 to some ground point can be re-tarded by the presence of resistor 49 in series with high voltage cable 13.
The high voltage discharge control circuit may comprise third resistance means in a series circuit comprising the power supply and the first resistance means. This third resistance means may comprise a currcnt limiting resistor 50 connected between the high voltage output appearing at circuit point 11 within power supply 10 and high voltage cable connecting terminal 14. Typically, the value of resistance for resistor 50 may be in a range from about 0.1 to about 50 gigohms, the precise value depending upon the choice of resistance values of other circuit resistors. In con-ventional spraying operations, considerable amounts of electric charge may be stored in the power supplies utilized. The accidental disconnection of cable 13 from power supply 10, while the power supply is in its oper-ating mode, may result in sudden discharge or arcing of electrical energy to a ground point. Current limiting resistor 50 serves to retard large surges of current stored in the power supply and thus minimizes arcing or sparking tendencies.
The high voltage discharge control circuit may comprise fourth resistance means between the high voltage output of the power supply and an electrical grolmd point. For example, current limiting resistor 52 may be connected from power supply high voltage output circuit point 11 to electrical gro~md connection 12. One purpose of resistor 52 is to provide a shunt path for discharging stored energy in the power supply to a ground point. Hence, in the event cable 13 becomes disconnected during a spraying operation or when the power supply is routinely turned off, stored energy may be discharged safely without arcing or sparking occurrences.
Another purpose of resistor 52 is disclosed in aforementioned U. S. Patent No. 4,073,002 in that shunt resistor 52 may cooperate with series resistor 50 to provide automatic regulation of voltage applied to the electrodes by a "ballast" effect. For example, during periods of low :
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current drain, as when voltage i5 applied to the electrodes without simultaneous discharge of coating material from the spray noz~le, the voltage at the power supply output can increase to undesirable levels in the absence of a shunt or bleed resistor as a constant load across the output. Since the current and voltage characteristics of the shunt resistor remain substantially constant during operation of the gun, variations in the voltage imposed at the induction-charging electrode effect relatively less change in the absolute load at the power supply output. Where the series resistance is large in comparison to the load resistance, the variations in the spacial impedance between the electrode and grounded nozzle elements which produces changes in voltages at series resistor 50 are rendered relatively small. In practice, it has been found - that the voltage regulation function can be achieved with a series control resistor 50 and a shunt resistor 52 of approximately equal ohmic value, although better regulation is provided where series resistor 50 is on the order of ten times the ohmic value of shunt resistor 52.
Typically, shunt resistor 52 will have a resistance value in a range from about 0.1 to about 10 gigohms.
The high voltage discharge control circuit may contain fifth resistance means comprising a current limiting resistor 53 connected be- -tween high voltage input terminal 15 on adapter 11 and an electrical ground point, as depicted in FIGS. 1 and 6. Resistor 53 is connected in common electrical connection to terminal 15 and connecting point 54 of the first resistance means resistors 44. The ground side of resistor 53 may be con-nected in common to ground connection point 55 which is also connected to ground shields 56. Each of ground shields 56 comprises a conductive foil attached to adapter housing exterior wall 37. The structure and functions 36~7 of ground shields 56 are set forth more fully in U. S. Patellt No.
A high intensity electrical energy discharge occurs as an arc of current between an electrode maintained at a relatively high potential and an object at a relatively lower potential. The object at a lower potential may be another portion of the spray device, or it may be an electrically grounded article such as the target to be coated, or other object in the spraying environment. Electrical arcs or sparking may also occur between the electrode and air-borne dust or spray particles which may be at lower potentials than the electrode. While these sudden dis-charges may be merely irritating to the touch of a spray operator, the arcing or sparking phenomena may have potentially lethal consequences where they occur in environments of flammable vapors, such as contributed by many organic paint systems.
Cne solution to the sparking problem is that of merely decreasing the voltage applied to the electrode to a level so that energy stored in , . , . : .
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the charging circuit lacks suf',icient potential to cause intense current-carrying arcs to leave the electrode surface. Decreasing the electrode potential, however, also causes particle charging and coating deposition efficiencies to decrease.
Another solution to the dilemma of providing high charging voltages while substantially eliminating flalmmable vapor-igniting sparks is described in U. S. Patents No. 3,641,971 and No. 3,795,839 to Walberg.
For the Walberg corGna or contact charging spraying system, there is : provided a complicated, expensive circuit "deenergizing means" for dis-connecting the high potential source when a certain threshold current is reached above which arcing occurs. The Walberg circuitry, while appro-priate for corona spraying systems, which typically require heavy and expensive high voltage and current power supplies, is unsuitable for an induction-charging system which is typically adaptable to small portable ?~ ~5 power supplies. ~loreover, the interruption of the charging circuit de-creases particle charging and coating deposition efficiencies.
~nother hazard common to electrostatic spraying environments is the sudden discharge or sparking that may occur when a break or short circuit occurs in the high voltage circuit. For example, breaks and short circuiting to ground may occur in the high voltage cable that delivers high potential from a power supply to an induction-charging electrode. There may follow a sudden discharge of electrical energy stored in other high capacitive elements of the electrostatic system, such as the power supply, or in the induction-charging electrode or in the cable itself, each of which is capable of storing significant amounts of electrical energy. Sudden electrical discharge may produce sparking conditions at the short circuit point with consequent haza-rds of fire or explosion of paint vapors and may also constitute a significant shock hazard to equipment operators.
,' _5_ Summary of the Invention The incidence of high potential electric discharges from components of an induction-charging electrostatic spraying system may be substantially reduced by including in the charging circuit resistance means of appropriate resistance values to retard the flow of charge stored in one portion of the circuit to another circuit portion where short circuit or likely arc-producing conditions exist. An induction-charging electrostatic spraying system is of a type which comprises liquid particle spray-forming means for discharging a spray stream of liquid particles along the axis of the spray-forming means, induction-charging means disposed in operable association with the spray-forming means to establish a charging æone in which charge is induced on liquid particles as the particles are formed by the spray-forming means, and means for connecting a high voltage DC electric potential to the induction-charging means. A control circuit for controlling high potential electric discharges from elements of the induction-charging electrostatic-spraying system comprises at least one induction-charging electrode disposed exteriorly of, or outward]y from, the spray-forming means. The electrode comprises a substrate having a wall made of electrically conductive material.
~he wall faces the axis of the spray-forming means and provides a surface from which an induction-charging field is established when an electric po-tential is applied to the electrode wall. A Eirst resistance means is connected between the high voltage DC electric potential connecting means and the electrode wall, with the first resistance means being disposed in close physical proximity to the electrode wall. In "close physical proximity"
is intended to indicate that the electrical connection between the electrode wall and the first resistance means is as short as possi~le so that any substantial capacitive or charge storing capability created within the connection is significantly less than the capacitance of the elec-trode circuitry or of the charging circuit. The first resistance means has sufficient ohmic resistance to retard or impede the transport of electric charge to the electrode surface at the magnitude of voltage applied to the electrode required to establish the induction-charging field. A first resistance means of an appropriate resistance value and disposed in close physical proximity to the electrode wall will impede transfer of electric charge to the electrode surface such that substantia'ly no electric discharges will occur which have intensities sufficient to ignite flammable yaint vapors.
The incidence of a spark-ignited fire or explosion of flammable paint vapors is a function of the intensity and duration of the spark or arc, as well as the type and concentration of'the vapor. Arc intensity and duration are, in turn, related to the energy imparted to the arc by the charging circuit. If the quantum of arc energy remains below that energy required to ignite the most flammable vapor-air mixture found in typical spraying operations, then ignition and explosion are not likely to occur. It has been determined that for a saturated mixture of toluene and air at 62F., or of xylene and air at l:L5F., which are the most flammable concentrations of these typically utilized solvents, the threshold energy of ignition is about 15 millijoules. Thus, a relatively safe electrostatic chargingsystem for use in spraying xylene- or toluene-containing paints in confined spaces may be provided by an electrode that provides substantially no electrical discharges or, if discharges do occur, by an electrode that electrically discharges arcs or sparks having energies less than 15 millijoules.
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The first resistance means may comprise a discrete resistor of the conventional carbon-filled or wire-wound type that may be embedded in an electrode substrate of electrically non-conductive or dielectric material. The first resistance means may comprise a siab of resistive material that may make up most of the material of the electrode sub-strate. Combinations of resistive material in series with dlscrete resis~ors may also be used.
An induction-charging electrostatir spraying system typically comprises, in addition to an induction-charging electrode, a power supply for furnishing a suitable high voltage DC electrical potential to the induction-charging electrode. In many industrial appllcations the power supply may be remote from the spraying device. The use of a remote power supply typically requires a cable designed for trans-mitting high voltages safely over a distance of six to twenty feet or more from the power supply to the induction-charging electrode~ In other applications, a portable power supply may be fitted to the spray device itself, eliminating the need for a high voltage conveying cable.
Second and third resistance means may be included in the high electric potential discharge control circuit. The second resistance means comprises a current limiting resistor connected in series with a high voltage output terminal of the power supply and with the first resistance means associated with the induction-charging electrode.
Typically, the series resistor is located in the high voltage transmitting cable at a :
point near the connection of the cable to the high voltage connecting means of the induction-charging electrode. The th:ird resistance means comprises a current limiting resistor in series with the power supply high voltage output terminal and the connecting means for connecting the high voltage cable to the power supply. A purpose of these current limiting resistors is to inhibit transfer of charge stored in one portion of the circult to another circuit portion. With the retardation of flows of substantial charge either from the power supply to the cable or from the induction-charging electrode to the cable, the likelihood of sparking from the cable to ground points is minimized, in the event that the cable is pulled from the power supply terminal, or the cable is severed while power is being applied to the system, when significant amounts of charge remain stored in the charging circuit.
Fourth and fifth resistance means may be included in the high electric potential discharge control circuit. The fourth resistance means comprises a current limiting resistor connected between the power supply high voltage output terminal and an electrical ground connection. The fifth resistance means comprises a current limiting resistor connected at a point near the high voltage connecting means of the induction-charging electrode and an electrical ground connection. A purpose of these resistors is to provide a shunt path to electrical ground to drain or bleed off accu-mulated charge stored within circuit elements.
As another aspect of the invention, there may be provided an induction-charging adapter for mounting on a spray device, which adapter ~5 includes a high electric potential discharge control circuit. The adapter comprises (a) housing means fabricated of a dielectric material, the housing means having an exterior wall and having an interior wall, (b) mo~mting means _g_ on the interior wall of the housing means for detachably mounting the housing means onto a spray device so that the interior wall faces the axis of a spray device when the housing means is mounted on a spray device, (c) induction-charging means including at least one induction-charging electrode attached to the housing means, the electrode com-;; prising a substrate having a wall of electr cally conductive material that faces the axis of a spray device when the housing means is mounted on a spray device, the wall providing a surface from which an induction-charging electrostatic field is established when a high voltage DC
electric potential is applied to the electrode, (d) means for connecting a high voltage DC electric potential to the induction-charging electrode, and (e) first resistance means connected between the electric potential . connecting means and the electrode wall, the first resistance means dis-posed in close physical proximity to the electrode wall and having lS sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to the electrode required to establish the induction-charging field, whereby electrical discharges of energies ~hich are sufficient to ignite a saturated xylene vapor-in-air mixture at about 62F. are substantially suppressed.
The first resistance means may comprise a discrete resistor or a slab of resistive material in series with the conductive electrode wall and the high voltage electric potentlal connecting means for the purposes as set forth above in discussion of these resistor elements.
Also mounted upon the adapter housing may be a current limiting resistor of the type described above comprising the fifth resistance means. This current limiting resistor is connected between the high voltage connecting means for the first resistance means and an electrical ground connection.
~86~7 The resistor provides a shunt path to groun~ for bleeding charge to electrical ground that is stored in the electrode circuit of the induction-charging adapter.
Brief Description of the Drawillgs The accompanying drawings illustrate examples of embodiments of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:
FIG. 1 is a diagrammatic presentation of an electrostatic sprayincg system illustrating preferred physical locations of resistor elements;
FIG. 2 is a schematic diagram of a high voltage discharge control circuit for an induction-charging electrostatic spraying system;
FIG. 3 is a perspective view of an induction-charging adapter fitted to an external-mixing spray gun;
FIG. 4 is an exploded view of the nozzle assembly of the spray gun illustrated in FIG. 3;
FIG. 5 is a rearward elevation of an induction-charging adapter with a downstream vieM into the adapter;
FIG. 6 is a top view of an induction-charging adapter showing an embodiment of the induction-charging electrode in section; and FIG. 7 is a top view of an induction-charging adapter showing another embodiment of the induction-charging electrode in section.
Description of Preferred Embodiments An electrostatic spraying system of the induction-charging type is depicted in FIG. 1. The system comprises a spray device 8, .
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an induction-charging adapter 9 fitted to the spray device, a power supply 10 for providing a suitable high voltage DC electrical potential at output terminal 11 relative to an electrical ground potential at ground connection 12, a cable 13 connecting high voltage cable comlecting S terminal 14 of the power supply with a high voltage input terminal 15 located on the induction-charging adapter, liquid coating material supply 16, a compressed air supply 17 and feed hoses 18 and 19 for de-livering liquid coating material and compressed air, respectively, to spray device 8.
~ high electric potential discharge control circuit for use in combination with the induction-charging electrostatic spraying system is illustrated in the schematic diagram of ~I~. 2. A first resistance means associated with an electrode of the induction-charging adapter is designated Rl, second resistance means in series with hi.gh voltage cable connecting terminal 14 and the high voltage input terminal 15 on adapter 9 is designated R2, within power supply lO third resistance means connected between high potential output terminal 11 and cable connecting terminal 14 is designated R3, fourth resistance means connected between high voltage oueput terminal ll and electrical ground connection point 12 is designated R4, and fifth resistance means connected between high voltage input termi-nal 15 and an electrical ground connection point is designated R5.
~ more detailed description of spray device 8 may be found with reference to FIGS. 1 and 3, wherein there is illustrated a conventional air-atomizing hand-held spray device 8 having a handle 20, a barrel 21 and a nozzle assembly 22. ~ trigger 23 serves to operate a valve assembly (not shown) within barrel 21 to regulate flows of liquid coating material and an atomizing gas, such as air, to nozzle assembly 22. A liquid coating .
material, such as a paint having a condllctivity generally greater than 0.001 ~mho/cm, is fed to the spray device from paint supply 16 through paint feed hose 18, which is connected to spray device 8 by mating threaded members forming connecting means 24 for paint ~eed hose 18.
As indicated in FIGo 1, the paint supply including its container is preferably electrically grounded. From a compressed air supply 17, - feed hose 19 delivers atomizing air under pressure to connecting means 25, which again is an assembly of mating threaded members.
The spray device as illustrated is a commercially available hand-held gun of the air~atomizing siphon type (Model 62, Binks Mfg. Co., Chicago, Ill.). Nozzle assembly 22 is depicted as an external-mixing spray-forming nozzle of the type described in U. S. Patent No. 4,009,829 to J. E. Sickles, incorporated herein by reference. As shown in FIG. 4, nozzle assembly 22 comprises a liquid discharge nozzle 26 having a liquid-conveying passageway and comprises an air cap 27. The assembly of liquid discharge nozzle 26 and air cap 27 defines an annular-shaped atomizing-air discharge port 28 that is concentric with a liquid discharge port 29.
~ir and liquid discharge ports 28 and 29, respectively, lie in the plane of face 30 of nozzle assembly 22 and are disposed generally coaxially with respect to the axis of the liquid-conveying passageway of liquid discharge nozzle 26. Streams of atomizing air and liquid coating material discharged from ports 28 and 29, respectively, coact to form a spray stream of particles that is discharged generally coaxially with respect to the liquid nozzle axis and ln a downstream direction with respect to nozzle face 30.
Liquid discharge nozzle 26 may be fabricated of electrically conducting or non-conducting materials. It is preferred that within the liquid-conveying passageway of liquid nozzle 26 near its discharge port 29 .
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a conductive grounding element (not shown) be provided in electrical contact with the stream of liquid coating material. This grounding element will be grounded to a common electrical ground shared by the paint supply and power supply as indicated in FIG. 1. Preferably, air cap 27 is fabricated of a dielectric material, such as acetal resins, epoxy resins, glass-filled nylon resins and the like. If air cap 27 is made of a metallic or conductive material, then air cap 27 should be electri-cally isolated from ground potential. Also located on nozzle face 30 are additional air discharge ports 31. Projecting downstream from nozzle face 30 and integrally formed with air cap 27 is a pair of air llorns 32. Located on air horns 32 on faces oriented toward the spray stream axis are additional air discharge ports 33. Air discharge ports 31 and 33 cooperate to shape the spray stream into a fan configuration.
Mounted on barrel 21 at its downstream-oriented or forward end is induction-charging adapter 9. The adapter comprises a housing 34 Eabricated of a dielectric material. The dielectric material s~ould be capable of withstanding stresses associated with the high voltages pro-vided by the power supply without electrical breakdown or tracking. Use-ful dielectric materials include those set forth above for fabricating air cap 27. Housing 34 is mounted upon barrel 21 by a friction fit between a pair of shoulders 35 located at the rearward portion of housing 34 and upon interior wall 36. Each of shoulders 35 is positioned diametrically opposed to the other and is shaped to mate with a comple-mentary-shaped surface of barrel 21 so that housing 34 is rigidly secured to spray device 8. When the adapter housing is mounted upon spray device 8, housing interior wall 36 faces the axis of the spray stream discharged from nozzle assembly 22, while housing exterior wall 37 faces generally in a direction outwardly of the spray stream axis.
~3 ~ousing 34 is characterized in having its wall portions ex-tending downstream to form a pair of lobes 38. ~ounted on the inner wall of each lobe 38 is an induction-charging electrode 39. Each of electrodes 39 comprises a substrate 40 h~ving an electrically con-ductive wall 41 with a surface 42 facing the spray stream axis. When an appropriate high voltage DC voltage is applied to electrode 39, an electric field is established between wall surface 42 and a region sur-rounding liquid discharge port 29 through which liquid coating material is discharged. Means for connecting a high voltage DC electrical po-tential to each induction-charging electrode 39 includes a high voltage contact that forms high voltage input terminal 15 and a conductor 43 connecting terminal 15 to a resistive element typically located within electrode substrate 40. As illustrated in FIG. 5, terminal 15 may be a "banana" plug rigidly fixed within a portion of electrode substrate 40 that provides electrical connection for a suitable mating member incorpo-rated into high voltage cable 13.
Adapter 9 is mounted upon spray device 8 such that electrodes 39 are positioned exteriorly of, or radially outwardly from, external-mixing nozæle assembly 22. Preferably, electrodes 39 are positioned with respect to nozzle assembly 22 so that at least a portion of surface 42 of elec-trode wall 41 intersects a plane containing liquid discharge port 29.
Thus, with respect to a plane containing nozzle assembly face 30, elec-trode wall surface 42 intersects the plane~ with at least a portion of wall surface 42 extending downstream from, or forwardly of, nozzle assembly face 30. The radial distance of electrodes 39 outwardly from the axis of liquid discharge noæzle 26 will generally determine the magnitude of the voltage required to be applied to electrodes 39 to ;
provide an induction-charging ~ield. For the adapter illustrated in FIG. 3 having each electrode 39 spaced outwardly about 3/4 inch from the liquid discharge nozzle axis, DC voltages between about 5,000 volts to about 25,000 volts will produce an effective induction-charging field in a region surrounding liquid discharge port 29, which field has an average potential gradient in the range from about 7 kilovolts per inch to about 33 kilovolts per inch. Voltages that are so high as to cause corona discharge from electrodes 39 are to be avoided. In this respect, the induction-charging electrode 39 may be characterized as one which is substantially non-corona producing, that is, electrode 39 has a configuration which is substantially free of sharp angles, points, or surface discontinuities that may tend to produce corona discharges in the aforementioned voltage range.
In an induction-charging device such as that utiliæed in the present invention, liquid coating material atomization and electric charge imposition occur substantially simultaneously so as to create a stream of discrete particles bearing an induced electric charge. For example, the stream of liquid coating material which passes through liquid discharge port 29 of nozzle assembly 22 is thrust into contact with a flow of air or gas from concentrically disposed atomizing-air discharge port 28, which flow of gas or air impinges upon and mixes with the liquid stream and tends to distort the stream into an irregular con-figuration comprising surface discontinuities. Formation of cusp-like, liquid stream discontinuities or l'liquid termini" is aided by the high intensity electric field existing between high voltage electrode 39 and the grounded liquid stream. The electric field flux lines tend to con-centrate at the sharp-pointed liquid termini and to induce electric 8~7 charge redistribution within the liquid stream, with charge of sign opposite that of the high voltage electrode migrating to the extreme sharp portions of the liquid tennini. Since the charges on the liquid termini and on the electrodeare opposite in sign, electrical attractive forces cooperate with the mechanicaldistresses furnished by the flow of gas or air to separate the liquid termini from the liquid stream so as to form discrete coating material particles bearing electric charge.
Connected between high voltage input terminal 15 of the high DC
electric potential connecting means and electrode wall surface 42 is a first resistance means. An electrically conductive material comprises the electrode wall 41 which provides surface 4~. from which an induction-charging electro-static field is established. Electrode wall 41 may be fabricated of practically any electrically conductive material. Suitable materials include aluminum, copper, nickel, magnesium, titanium and tin, and alloys such as stainless steel, bronze and brass. Also suitable are commercially available conductive plastics such as carbon-filled polyester plastics of relatively high con-ductivity (Velostat conductive plastics; 3M Co.). Generally, electrode conductive wall 41 may be in the form of a thin foil having a thickness of about one to ten mils, although it may be considerably thicker. It is general]y desirable that the electrical path through the connecting means between electrode wall 41 and the first resistance means be as short as practicable.
The first resistance means may be provided by any one of several embodiments. In a preferred embodiment illustrated in FIG. 6, the resistance means may comprise one or more discrete resistors 44 embedded within electrode substrate 40 which is fabricated of an electrically non-conductive material oE the aforementioned type described for fabricating adapter housing 34.
Resistor 44 may be a commercially available carbor.-filled composition type or a wire-wound element. A preferred arrangemen-t as illustrated in FIG. 6 utilizes two high-voltage stable miniature resistors 44 co~nercially designated A~ ~
as "Victoreen Minimox" resistors sold by Victoreen Instrument Div., Cleveland, Ohio. Typical values for these discrete resistors may range from about 10 megohms to about 50 gigohms. An electrical connection to discrete resistors 44 is made at a contact point 45 locsted near a rear-ward, or upstream, portion of conductive wall 41 ad~acent substrate 40, about halfway between the upper and lower edges of electrode 39. The other end of resistor 44, or a series of resistors 44, is connected to conductor 43.
The first resistance means may comprise a slab 46 fabricated of resistive material. This resisti~e material may be formed from a synthetic resin-graphite composition having a bulk resistivlty as determined by the ratio of the amount of elec~rically conducting graphite component to the amount of electrically non-conducting resinous component.
Suitable for use as resistive materials are the non-conductive thermo-plastic polyester compositions sold by General Electric under the trade-mark Valox~ which compositions may be doped with graphite to achieve a material of suitable resistivity.
A purpose of the first resistance means is to inhibit or retard transfer of arc-forming amounts of current from other portions of the electrode circui~ry, or cable or power supply circuitry, to po~entially arc-producing sites on the electrode so that arcing ~rom the electrode to an ob~ect at a lower pot~ntial is subs~antially suppressed. This arc suppressing capability is related to, or dependent UpOII, several factors, such as electrode configuration, charging potential applied to the elec-trode, resistance of the first resistance means and the charge-storing capacity of the electrode circuitry and the capacitance of the cable and power supply. The precise resistance value selected for the first re-sistance means may thus depend upon several factors. A criterion or test for selecting a proper resistance may be based upon whether, in actual practice under spraying conditions, arcs or sparks are generated from the electrode surface of sufficient intensity to ignite a concentration of ~ . ~
36~7 ~lammable solvent vapors of a paint that is sprayed. Hence, a choice of resistance for a particular electrode system, which does not produce electrical discharges of sufficient intensity to ignite the most flam-mable vapor-air mixture conceivable, is generally a suitable choice of resistance for practically any spraying operation utilizing flammable paints. ~ suitable highly flammable solvent vapor-air mixture against which arc suppression may be tested is a concentration of saturated toluene-in-air mixture at about 62F. Another rather sensitive test concentration of flamnable vapor-in-air mixture is provided by a concen-tration of a saturated xylene-in-air mixture at about 115F. These test concentrations are intended to be exemplary of flammable mixtures for testing arc suppressing capability of the high potential discharge control circuit of the invention. More sensitive or more highly flam-mable mixtures may also be used to determine an appropriate resistance value for the first resistance means, since any resistance value from about 10 megohms to about 50 gigohms may be used as the first resistance means. In this regard it should be mentioned that the total resistance of the series resistance between the high potential output terminal 11 of the power supply and electrode wall 41 may comprise the first re-sistance means.
Another criterion useful in determining a suitable arc suppressing circuit is the physical proximity of the first resistance means to elec-trode wall 41. As mentioned, it is desirable that any capacitance -Ln the connection or conducting means between the resistance means and electrode wall 41 be relatively less significant than the capacitance of the elec-trode circuitry. Hence~ the connecting means from resistor 44 to wall 41 of the embodiment of FIG. 6 should be a relatively short conductor so that $~3~
resistor 44 is in close physical proximity to electro(le wall 41. ~s de-picted in FIG. 7, electrode wall 41 is directly adjacent to, and thus in close physical pro~imity with, resistive material slab 46.
It has been found that the portions of the electrode most S susceptible to arcing are the portions forming the downstream or forward ends 48. It is preferred, therefore, that the dielectric substrate ma-terial which forms electrode substrate 40 encase the forward edges of electrode wall 41, as indicated in FIGS. 6 and 7. The dielectric ma-B terial may thus form a bead 49~that runs along the circumference of electrode wall 41.
The preclse resistance values of the first resistance means may be selected from a range from about 10 megohms to about 50 gigohms, with the exact choice being determined by the aforementioned parameters and criteria. A set of convenient measuring points Eor a particular resistance element incorporated in the electrode circuitry consists of high potential connecting means terminal 15 and the surface of electrode wall 41. The resistance between these two points may be considered the "working" resis-tance of the first resistance means. Values of this working resistance may be selected from resistances in the aforementioned range. A value of about 0.1 to 5 gigohms is a typical choice for the first resistance means.
The high voltage discharge control circuit may comprise second resistance means in a series circuit comprising the high voltage potential supplied by power supply 10 and the first resistance means. This second resistance means comprises a current limiting resistor 49 connected between high voltage input terminal 15 on adap~er 9 and the high voltage cable con-necting terminal 14 of power supply 10. Typically, resistor 49 may be physically located in high voltage cable 13 as depicted in FIG. 1. Or, .
~20-resistor 49 could be containecl within th~ spray gun in its handle 20 - or barrel 2~; a location of resistor 49 in spray gun barrel 21 is preferred for systems utilizing portable, barrel-mounted power sup-plies as disclosed in aforementioned references.
In systems utilizing a power supply located remotely from the induction-charging adapter, as shown in FIG. 1, a conventionally-available shielded cable 13 rated to carry voltages of about 25,000 volts DC may be utilized to connect power supply 10 to the induction-charging electrodes of adapter 9. Cable 13 is usually wrapped about air feed hose 19 and thus where cable 13 includes resistor 49, the resistor may be physically attached to hose 19 by tape, heat shrink tubing, or other means at a location near air hose connecting`means 25. Typically, resistor 49 may have a value in a range from about 100 megohms to about 50 gigohms, depending on the choice of resistance values for other re-sistor elements of the circuit.
During operation of the electrostatic spray system, consider-able amounts of electric charge are stored in the portion of cable 13 leading to the induction-charging electrodes and in the induction-charging electrode circuit. In the event cable 13 is disconnected from power supply terminal 14 while the spray device is in operation, this stored charge may discharge to an electrical ground point with the likeli-hood of electric sparks being generated. Also, curre~t discharge and sparking may result upon accidental severance of the cable by heavy equip-ment used in many industrial spraying environments. The flow of substantial amounts oE stored charge through cable 13 to some ground point can be re-tarded by the presence of resistor 49 in series with high voltage cable 13.
The high voltage discharge control circuit may comprise third resistance means in a series circuit comprising the power supply and the first resistance means. This third resistance means may comprise a currcnt limiting resistor 50 connected between the high voltage output appearing at circuit point 11 within power supply 10 and high voltage cable connecting terminal 14. Typically, the value of resistance for resistor 50 may be in a range from about 0.1 to about 50 gigohms, the precise value depending upon the choice of resistance values of other circuit resistors. In con-ventional spraying operations, considerable amounts of electric charge may be stored in the power supplies utilized. The accidental disconnection of cable 13 from power supply 10, while the power supply is in its oper-ating mode, may result in sudden discharge or arcing of electrical energy to a ground point. Current limiting resistor 50 serves to retard large surges of current stored in the power supply and thus minimizes arcing or sparking tendencies.
The high voltage discharge control circuit may comprise fourth resistance means between the high voltage output of the power supply and an electrical grolmd point. For example, current limiting resistor 52 may be connected from power supply high voltage output circuit point 11 to electrical gro~md connection 12. One purpose of resistor 52 is to provide a shunt path for discharging stored energy in the power supply to a ground point. Hence, in the event cable 13 becomes disconnected during a spraying operation or when the power supply is routinely turned off, stored energy may be discharged safely without arcing or sparking occurrences.
Another purpose of resistor 52 is disclosed in aforementioned U. S. Patent No. 4,073,002 in that shunt resistor 52 may cooperate with series resistor 50 to provide automatic regulation of voltage applied to the electrodes by a "ballast" effect. For example, during periods of low :
. .
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current drain, as when voltage i5 applied to the electrodes without simultaneous discharge of coating material from the spray noz~le, the voltage at the power supply output can increase to undesirable levels in the absence of a shunt or bleed resistor as a constant load across the output. Since the current and voltage characteristics of the shunt resistor remain substantially constant during operation of the gun, variations in the voltage imposed at the induction-charging electrode effect relatively less change in the absolute load at the power supply output. Where the series resistance is large in comparison to the load resistance, the variations in the spacial impedance between the electrode and grounded nozzle elements which produces changes in voltages at series resistor 50 are rendered relatively small. In practice, it has been found - that the voltage regulation function can be achieved with a series control resistor 50 and a shunt resistor 52 of approximately equal ohmic value, although better regulation is provided where series resistor 50 is on the order of ten times the ohmic value of shunt resistor 52.
Typically, shunt resistor 52 will have a resistance value in a range from about 0.1 to about 10 gigohms.
The high voltage discharge control circuit may contain fifth resistance means comprising a current limiting resistor 53 connected be- -tween high voltage input terminal 15 on adapter 11 and an electrical ground point, as depicted in FIGS. 1 and 6. Resistor 53 is connected in common electrical connection to terminal 15 and connecting point 54 of the first resistance means resistors 44. The ground side of resistor 53 may be con-nected in common to ground connection point 55 which is also connected to ground shields 56. Each of ground shields 56 comprises a conductive foil attached to adapter housing exterior wall 37. The structure and functions 36~7 of ground shields 56 are set forth more fully in U. S. Patellt No.
4,009,829 to J. E. Sickles, the disclosure of which is incorporated herein by reference.
A purpose of resistor 53 is to provide a shunt path to electrical ground for charge stored in the induction-charging circuit and in portions of c~ble 13. Upon interruption of the high voltage applied to the electrodes, either by an operator purposefully turning off the power supply, or by one of the aforementioned accidental dis-connections of the power supply or cable 13, stored electrical energy may be safely discharged to ground without sparking or arcing from these circuit elements. Shunt resistor 53 is preferably located as close as possible to the induction-charging electrodes. For example, the resistor may be mounted on adapter housing 9, as indicated above.
Also, resistor 53 may be mounted on the spray gun barrel 21 or upon paint feed hose 18. Resistor 53 is shown as attached to paint feed hose 18 in FIG. 1 for purposes of illustration only, and is not in-tended to be a particularly preferred position for resistor 53.
Typically, resistor 53 has a resistance value in a range from about 0.5 to about 10 gigohms.
A particularly preferred high electric potential discharge control circuit for an induction-charging spraying system of the type described includes an induction-charging electrode having a wall 41 of conductive material that provides a surface for establishing an induction-charging electrostatic field. A preferred discharge control circuit includes in series with each induction-charging electrode wall 41 two discrete resistors each having a resistance value of about 100 megohms mounted in the electrode substrate 40 which is fabricated of a dielectric material, a resistor of about 1 gigohm included in the high voltage cable, and a resistor of about 1 g~gohm included in the power supply at its high voltage output. The total resistance of a preferred discharge circuit is in a range from about 2 gigohms to about 3 gigohms; a total resistance value of 2.5 gigohms is particularly preferred, but in any case the total series resistance of the circuit between the power supply and the elec-trode may be in the range from about 1 to about 50 gigohms.
Within power supply 10, converter 60 provides a high potential DC output at terminal 11 from a 115 volt AC source. The high potential required to be provided by converter 60 should be adjustably between
A purpose of resistor 53 is to provide a shunt path to electrical ground for charge stored in the induction-charging circuit and in portions of c~ble 13. Upon interruption of the high voltage applied to the electrodes, either by an operator purposefully turning off the power supply, or by one of the aforementioned accidental dis-connections of the power supply or cable 13, stored electrical energy may be safely discharged to ground without sparking or arcing from these circuit elements. Shunt resistor 53 is preferably located as close as possible to the induction-charging electrodes. For example, the resistor may be mounted on adapter housing 9, as indicated above.
Also, resistor 53 may be mounted on the spray gun barrel 21 or upon paint feed hose 18. Resistor 53 is shown as attached to paint feed hose 18 in FIG. 1 for purposes of illustration only, and is not in-tended to be a particularly preferred position for resistor 53.
Typically, resistor 53 has a resistance value in a range from about 0.5 to about 10 gigohms.
A particularly preferred high electric potential discharge control circuit for an induction-charging spraying system of the type described includes an induction-charging electrode having a wall 41 of conductive material that provides a surface for establishing an induction-charging electrostatic field. A preferred discharge control circuit includes in series with each induction-charging electrode wall 41 two discrete resistors each having a resistance value of about 100 megohms mounted in the electrode substrate 40 which is fabricated of a dielectric material, a resistor of about 1 gigohm included in the high voltage cable, and a resistor of about 1 g~gohm included in the power supply at its high voltage output. The total resistance of a preferred discharge circuit is in a range from about 2 gigohms to about 3 gigohms; a total resistance value of 2.5 gigohms is particularly preferred, but in any case the total series resistance of the circuit between the power supply and the elec-trode may be in the range from about 1 to about 50 gigohms.
Within power supply 10, converter 60 provides a high potential DC output at terminal 11 from a 115 volt AC source. The high potential required to be provided by converter 60 should be adjustably between
5,000 and 25,000 volts DC. A description of an AC to DC converter suitable for an induction-charging system of the invention is found in the aforementioned U.S. Patent No. 4,073,002 to J. E. Sickles et al.
The interralationship of capacitive and resis~ive elements of the high potential discharge control circuit which provide arc suppression for an induction-charging spraying system may be found in the following example showing approximate values of resistive-capacitive elements of a ~ypical control circuit for an induction-charging system wlth two electrodes as depicted in Figure l. A typical arc-producing situation may occur when a charging surface of one of ~he two electrodes is ;
brought to a position in close proximity with a ground point, such tha~
a discharge may occur from the conducti~e electrode surface to the ;
ground point. The intensity of the discharge is dependent upon the amoun~ of charge stored in the circuit available for discharge from the conductive electrode surface. Within the illustrated circuit there may be found in the arcing electrode .
circuitry a apacitance, Cl, of about lO pF and in the high voltage cable 13 being approximately twenty Eeet in length (15 pF/ft) a capacitance, C2, of about 300 pF. For this calculation contributions from stray capacitance,such as from the power supply, may be neglected since this capacitance is relatively insignificant in comparison with the total quantified capacitance, CT = 2Cl + C2. For a charging system operating at 25 kilovolts DC, there may be a total stored energy, ET, within the system of ET = 1/2 CTV
1/2 (320 10-12F) (25 x 103v)2 = lO0 mlllljoules An electric discharge in the form of an arc of this energy, ET, may be suppressed by providing sufficient resistance between the electrode conductive surface from which the discharge may occur and the charge-storing elements of the circuit from which the currents must flow to provide the arc energy. The arc energy is thus comprised of the energy, E~, supplied from the arcing electrode capacitance, the energy, EB, supplied from the rest of the circuit capacitance, and the energy, Ec, supplied from the non-arcing electrode capacitance. The illustrated system has a series resistance, Rl, at each of the electrodes, which series resistance is in close proximity to the conductive electrode surface and has a value of about one gigohm, and a series resistance, R2, in the cable with a value of about one gigohm. The maximum energy available at the arcing electrode surface discharge point~ Ear , would be the sum of the component energies supplied by the circuit capacitive elements, EA + EB + EC For the arcing electrode circuit at a potential of 25 kilovolts DC, EA = 1/2 ClV
= 1/2 (10 x 10 F) (25 x 10 ) = 3.1 milli~oules ~ .
, . .
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Assuming an arc discharge occurs from the arcing electrode over a duration of one capacitive time constant, r 1~ (which is a conservatively chosen upper limit), the capacitive discharge time constant may be calculated as ~ 1 ~ RlCl = (10 ohms) (10 x 10 12F) = 0.01 sec.
The capacitive time constant, ~2' for the dissipation of charge stored in the cable through the cable resistance, R2, and the arcing electrode resistance, Rl, would be r2 = (Rl + R2) C2 = (2 x 10 ohms) (300 x 10 12F) = 0.60 sec.
The charge stored in the cable portion of the circuit is Q2 = C2V = (300 x 10 F) (25 x 103v) = 7.5 x 10 6 Coul.
This charge, Q2' drains from the cable capaci~ance during time constant, ~2~ so that an average current, IB, may be expressed as Q2 7.5 ~ 10 6 Coul.
I = - 0.60 sec.
= 12.5 x 10 6 amps Similarly, a calculation may be made for the dissipation of charge stored in the non-arcing electrode to the arcing electrode surface through the combined resistances of the two electrodes, as follows: -= 2RlCl = (2 x 109 ohms) (10 x 10 12F) ` = 0.02 sec.
so that Q3 = ClV = (10 x 10 F) (25 x 103v) = 0.25 x 10 6 Coul.
providing an average dissipation current, IC
Q3 0 02 sec = 12.5 x 10 6 amps The effective current, I ff~ available for discharge at the arcing electrode surface as provided by the sum of currents from the cable, IB, and from the non-arcing electrode, Ic, would be I ff = IB + IC = 25 x 10 amps Assuming an average electrode potential of 20 kilovolts during the discharge period, ~1' the maximum energy for forming an arc, E rc' would be = EA + EB + EC
where E + E = 1/2 QV = 1/2 I ff rlv = 1/2 (25 x 10 6 amps) (0.01 sec.) (20 x 103V) = 2.5 millijoules so that EarC = 5.6 millijoules ~ The ealculated energy o:E the suppressed eleetric discharge is thus sub-stantially less than the 15 milli~oules threshold energy required to ignite a flammable test eoneentration of a saturated mixture of toluene-~; in-air at about 62E. Hence, an arc discharge of the calculated energy would not ordinarily present a hazard to use oE an induction-charging ; system having the exemplified high potential discharge control circuit used in spraying toluene-containing paint compositions.
It should be mentioned that in addition to the arc suppression capability of the exemplified circuit, a suitable high pvtential dis-charge circuit may contain circuit elements selected according to the aforementioned criteria such that no electrical discharges occur oE any measurable energies. Also, discharge control circuits may be designed within the ambit of the invention that provide slight corona dissipation of electrical energy rather than discrete arc discharges. Additional resistance means may also be incorporated into the discharge control circuit to provide flexibility in the spraying operation. For example, additional induction-charging electrodes may be utilized having first resistanee means of the aforementioned type, as depicted in phantom as element Rl'in FIG. 2. Also, additional current-limited high voltage outputs may be provided at cable connection 14' through current-limiting resistor 50', as depieted in phantom in FIG. l.
Those skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.
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The interralationship of capacitive and resis~ive elements of the high potential discharge control circuit which provide arc suppression for an induction-charging spraying system may be found in the following example showing approximate values of resistive-capacitive elements of a ~ypical control circuit for an induction-charging system wlth two electrodes as depicted in Figure l. A typical arc-producing situation may occur when a charging surface of one of ~he two electrodes is ;
brought to a position in close proximity with a ground point, such tha~
a discharge may occur from the conducti~e electrode surface to the ;
ground point. The intensity of the discharge is dependent upon the amoun~ of charge stored in the circuit available for discharge from the conductive electrode surface. Within the illustrated circuit there may be found in the arcing electrode .
circuitry a apacitance, Cl, of about lO pF and in the high voltage cable 13 being approximately twenty Eeet in length (15 pF/ft) a capacitance, C2, of about 300 pF. For this calculation contributions from stray capacitance,such as from the power supply, may be neglected since this capacitance is relatively insignificant in comparison with the total quantified capacitance, CT = 2Cl + C2. For a charging system operating at 25 kilovolts DC, there may be a total stored energy, ET, within the system of ET = 1/2 CTV
1/2 (320 10-12F) (25 x 103v)2 = lO0 mlllljoules An electric discharge in the form of an arc of this energy, ET, may be suppressed by providing sufficient resistance between the electrode conductive surface from which the discharge may occur and the charge-storing elements of the circuit from which the currents must flow to provide the arc energy. The arc energy is thus comprised of the energy, E~, supplied from the arcing electrode capacitance, the energy, EB, supplied from the rest of the circuit capacitance, and the energy, Ec, supplied from the non-arcing electrode capacitance. The illustrated system has a series resistance, Rl, at each of the electrodes, which series resistance is in close proximity to the conductive electrode surface and has a value of about one gigohm, and a series resistance, R2, in the cable with a value of about one gigohm. The maximum energy available at the arcing electrode surface discharge point~ Ear , would be the sum of the component energies supplied by the circuit capacitive elements, EA + EB + EC For the arcing electrode circuit at a potential of 25 kilovolts DC, EA = 1/2 ClV
= 1/2 (10 x 10 F) (25 x 10 ) = 3.1 milli~oules ~ .
, . .
`~\
Assuming an arc discharge occurs from the arcing electrode over a duration of one capacitive time constant, r 1~ (which is a conservatively chosen upper limit), the capacitive discharge time constant may be calculated as ~ 1 ~ RlCl = (10 ohms) (10 x 10 12F) = 0.01 sec.
The capacitive time constant, ~2' for the dissipation of charge stored in the cable through the cable resistance, R2, and the arcing electrode resistance, Rl, would be r2 = (Rl + R2) C2 = (2 x 10 ohms) (300 x 10 12F) = 0.60 sec.
The charge stored in the cable portion of the circuit is Q2 = C2V = (300 x 10 F) (25 x 103v) = 7.5 x 10 6 Coul.
This charge, Q2' drains from the cable capaci~ance during time constant, ~2~ so that an average current, IB, may be expressed as Q2 7.5 ~ 10 6 Coul.
I = - 0.60 sec.
= 12.5 x 10 6 amps Similarly, a calculation may be made for the dissipation of charge stored in the non-arcing electrode to the arcing electrode surface through the combined resistances of the two electrodes, as follows: -= 2RlCl = (2 x 109 ohms) (10 x 10 12F) ` = 0.02 sec.
so that Q3 = ClV = (10 x 10 F) (25 x 103v) = 0.25 x 10 6 Coul.
providing an average dissipation current, IC
Q3 0 02 sec = 12.5 x 10 6 amps The effective current, I ff~ available for discharge at the arcing electrode surface as provided by the sum of currents from the cable, IB, and from the non-arcing electrode, Ic, would be I ff = IB + IC = 25 x 10 amps Assuming an average electrode potential of 20 kilovolts during the discharge period, ~1' the maximum energy for forming an arc, E rc' would be = EA + EB + EC
where E + E = 1/2 QV = 1/2 I ff rlv = 1/2 (25 x 10 6 amps) (0.01 sec.) (20 x 103V) = 2.5 millijoules so that EarC = 5.6 millijoules ~ The ealculated energy o:E the suppressed eleetric discharge is thus sub-stantially less than the 15 milli~oules threshold energy required to ignite a flammable test eoneentration of a saturated mixture of toluene-~; in-air at about 62E. Hence, an arc discharge of the calculated energy would not ordinarily present a hazard to use oE an induction-charging ; system having the exemplified high potential discharge control circuit used in spraying toluene-containing paint compositions.
It should be mentioned that in addition to the arc suppression capability of the exemplified circuit, a suitable high pvtential dis-charge circuit may contain circuit elements selected according to the aforementioned criteria such that no electrical discharges occur oE any measurable energies. Also, discharge control circuits may be designed within the ambit of the invention that provide slight corona dissipation of electrical energy rather than discrete arc discharges. Additional resistance means may also be incorporated into the discharge control circuit to provide flexibility in the spraying operation. For example, additional induction-charging electrodes may be utilized having first resistanee means of the aforementioned type, as depicted in phantom as element Rl'in FIG. 2. Also, additional current-limited high voltage outputs may be provided at cable connection 14' through current-limiting resistor 50', as depieted in phantom in FIG. l.
Those skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.
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Claims (29)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A high electric potential discharge control circuit for an induction-charging electrostatic spraying system of a type having liquid particle spray-forming means for discharging a spray stream of liquid particles along the axis of the spray-forming means, induction-charging means disposed in operable association with the spray-forming means to establish a charging zone in which charge is induced on liquid particles as the particles are formed by the spray-forming means, said induction-charging means including at least one induction-charging electrode disposed outwardly of the spray-forming means, said electrode comprising a substrate having a wall of electrically conductive material facing the spray discharge axis of the spray-forming means, said wall providing a surface from which an induction-charging electrostatic field is established when an electric potential is applied to said electrode, and high voltage connecting means on the induction-charging means for receiving a high voltage potential from a power supply for application to the induction-charging means, said control circuit comprising:
first resistance means connected between said high voltage potential connecting means and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall and having sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is dischargeable at the electrode surface is prevented from dis-charging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.
first resistance means connected between said high voltage potential connecting means and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall and having sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is dischargeable at the electrode surface is prevented from dis-charging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.
2. The control circuit of Claim 1 wherein the flammable gas-and-air mixture is a saturated mixture of toluene-in-air at a temperature of about 62°F.
3. The control circuit of Claim 1 wherein said electrically-conductive electrode wall is fabricated of a metal selected from the group consisting of copper, aluminum, nickel, magnesium, titanium, tin, steel, bronze and brass.
4. The control circuit of Claim l wherein said first resistance means comprises at least one discrete resistor of the wire-wound or compo-sition type connected between said electrode wall and said high voltage connecting means, said resistor having a resistance value in a range from about 10 megohms to about 50 gigohms.
5. The control circuit of Claim 1 wherein said first resistance means comprises electrically resistive material forming at least a portion of said electrode substrate adjacent said electrically-conductive electrode wall, said resistive material providing a resistance path from said high voltage connecting means to said electrode wall, said resistance path having an average value in a range from about 10 megohms to about 50 gigohms.
6. The control circuit of Claim 1, further comprising a power supply having means for connecting high voltage carrying means at the high voltage output of the power supply, and further comprising second resistance means connected between said high voltage connecting means of the induction-charging means and said high voltage carrying connecting means on said power supply.
7. The control circuit of Claim 6 wherein said second resis-tance means has an ohmic value in a range from about 100 megohms to about 50 gigohms.
8. The control circuit of Claim 6, further comprising a gun-like housing for supporting said spray-forming means, said second resistance means mounted within said housing.
9. The control circuit of Claim 6, further comprising high voltage carrying means connectable between said power supply connecting means and said induction-charging means high voltage connecting means, said high voltage carrying means comprising a shielded cable, said cable including said second resistance means within said cable in series with the high potential source and said first resistance means.
10. The control circuit of Claim 6, further comprising a third power resistance means connected between the high potential source of said power supply and said power supply high voltage carrying connecting means.
11. The control circuit of Claim 10 wherein said third resistance means has an ohmic value in a range of from about 0.1 gigohm to about 50 gigohms.
12. The control circuit of Claim 11, further comprising fourth resistance means connected between the high potential source of said power supply and electrical ground potential connecting means.
13. The control circuit of Claim 12 wherein said fourth resis-tance means has an ohmic value in a range from about 0.1 gigohm to about 10 gigohms.
14. The control circuit of Claim 12, further comprising fifth resistance means connected between said high voltage connecting means of the induction-charging means and electrical ground connecting means.
15. The control circuit of Claim 14 wherein said fifth resistance means has an ohmic value in a range from about 0.5 gigohm to about 10 gigohms.
16. An induction-charging adapter for mounting on a spray-forming device capable of discharging a stream of spray particles along the axis of the spray device, said adapter comprising:
(a) housing means comprised of dielectric material, said housing means having an exterior wall and having an interior wall;
(b) mounting means on the interior wall of said housing means for detachably mounting said housing means onto a spray device, so that said interior wall faces the axis of a spray device when the housing means is mounted on a spray device;
(c) induction-charging means including at least one induction-charging electrode attached to an interior wall of said housing means, said electrode comprising a substrate having a wall of electrically conductive material that faces the axis of a spray device when the housing means is mounted on a spray device, said electrode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage DC electric potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode; and (e) first resistance means connected between said high voltage potential connecting means and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall and having sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is available for discharge at the electrode sur-face is prevented from discharging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.
(a) housing means comprised of dielectric material, said housing means having an exterior wall and having an interior wall;
(b) mounting means on the interior wall of said housing means for detachably mounting said housing means onto a spray device, so that said interior wall faces the axis of a spray device when the housing means is mounted on a spray device;
(c) induction-charging means including at least one induction-charging electrode attached to an interior wall of said housing means, said electrode comprising a substrate having a wall of electrically conductive material that faces the axis of a spray device when the housing means is mounted on a spray device, said electrode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage DC electric potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode; and (e) first resistance means connected between said high voltage potential connecting means and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall and having sufficient resistance to retard transport of charge to the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is available for discharge at the electrode sur-face is prevented from discharging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.
17. The induction-charging adapter of Claim 16, wherein the flammable gas-and-air mixture is a saturated mixture of toluene-in-air at a temperature of about 62°F.
18. The induction-charging adapter of Claim 16, wherein said electrically-conductive electrode wall is fabricated of a metal selected from the group consisting of copper, aluminum, nickel, magnesium, titanium, tin, steel, bronze and brass.
19. The induction-charging adapter of Claim 16, wherein said first resistance means comprises at least one discrete resistor of the wire-wound or composition type connected between said electrode wall and said high voltage connecting means, said resistor having a resistance value in a range from about 10 megohms to about 50 gigohms.
20. The induction-charging adapter of Claim 16, wherein said first resistance means comprises electrically resistive material forming at least a portion of said electrode substrate adjacent said electrically-conductive electrode wall, said resistive material providing a resistance path from said high voltage connecting means to said electrode wall, said resistance path having an average value in a range from about l megohm to about 50 gigohms.
21. The induction-charging adapter of Claim 16, further comprising shunt resistance means connected between said high voltage connecting means of the induction-charging means and electrical ground connecting means.
22. The induction-charging adapter of Claim 21, wherein said shunt resistance means has a resistance value in a range from about 0.5 gigohm to about 10 gigohms.
23. A high electric potential discharge control circuit for an induction-charging electrostatic spraying system, the induction-charging electrostatic spraying system comprising:
(a) a spray-forming device comprising a liquid discharge nozzle having a liquid-conveying passageway terminating in a liquid dis-charge port and comprising an atomizing air discharge port concentric with said liquid discharge port, said spray-forming device capable of discharging a spray stream of liquid particles along the axis of said spray-forming device as defined by said liquid-conveying passageway;
(b) a housing fabricated of dielectric material disposed around said spray-forming device, said housing having an exterior wall and an interior wall, said interior wall facing the axis of said spray-forming device;
(c) induction-charging means including at least one induction-charging electrode attached to said interior wall of said housing, said electrode having a wall of electrically-conductive material that faces the axis of the spray-forming device, said elec-trode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode;
(e) power supply means for providing a high voltage potential at an output terminal;
(f) high voltage cable connecting means located on said power supply means;
(g) electrical grounding means located on said power supply means;
(h) a high voltage transmitting cable for connecting to said power supply connecting means and for connecting to said high voltage connecting means of said induction-charging electrode;
said high electric potential discharge control circuit comprising:
(i) first resistance means connected between said high voltage connecting means of the induction-charging electrode and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall, (j) second resistance means connected between said high voltage connecting means of the induction-charging means and said high voltage cable connecting means of the power supply means;
(k) third resistance means connected between the high voltage potential output terminal of said power supply means and said high voltage cable connecting means; and (l) fourth resistance means connected between the high voltage potential output terminal of said power supply means and said electrical ground connecting means on said power supply means;
wherein the total resistance of said first, second and third resistance means has a value from about 1 to about 50 gigohms.
(a) a spray-forming device comprising a liquid discharge nozzle having a liquid-conveying passageway terminating in a liquid dis-charge port and comprising an atomizing air discharge port concentric with said liquid discharge port, said spray-forming device capable of discharging a spray stream of liquid particles along the axis of said spray-forming device as defined by said liquid-conveying passageway;
(b) a housing fabricated of dielectric material disposed around said spray-forming device, said housing having an exterior wall and an interior wall, said interior wall facing the axis of said spray-forming device;
(c) induction-charging means including at least one induction-charging electrode attached to said interior wall of said housing, said electrode having a wall of electrically-conductive material that faces the axis of the spray-forming device, said elec-trode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode;
(e) power supply means for providing a high voltage potential at an output terminal;
(f) high voltage cable connecting means located on said power supply means;
(g) electrical grounding means located on said power supply means;
(h) a high voltage transmitting cable for connecting to said power supply connecting means and for connecting to said high voltage connecting means of said induction-charging electrode;
said high electric potential discharge control circuit comprising:
(i) first resistance means connected between said high voltage connecting means of the induction-charging electrode and said electrode wall, said first resistance means disposed in close physical proximity to said electrode wall, (j) second resistance means connected between said high voltage connecting means of the induction-charging means and said high voltage cable connecting means of the power supply means;
(k) third resistance means connected between the high voltage potential output terminal of said power supply means and said high voltage cable connecting means; and (l) fourth resistance means connected between the high voltage potential output terminal of said power supply means and said electrical ground connecting means on said power supply means;
wherein the total resistance of said first, second and third resistance means has a value from about 1 to about 50 gigohms.
24. The discharge control circuit of Claim 23, wherein said first resistance means comprises one or more discrete resistors in series and having a resistance value in a range from about 10 megohms to about 50 gigohms.
25. The discharge control circuit of Claim 23, wherein said second resistance means has a resistance value in a range from about 100 megohms to about 50 gigohms.
26. The discharge control circuit of Claim 23, wherein said third resistance means has a resistance value in a range from about 0.1 gigohm to about 50 gigohms.
27. The discharge control circuit of Claim 23, wherein said fourth resistance means has a resistance value in a range from about 1 gigohm to about 10 gigohms.
28, The discharge control circuit of Claim 23, further com-prising electrical ground connecting means located on said housing and further comprising fifth resistance means connected between said high voltage connecting means of the induction-charging electrode and said electrical ground connecting means.
29. The discharge control circuit of Claim 29, wherein said fifth resistance means has a resistance value in a range from about 0.5 gigohm to about 10 gigohms.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US91164678A | 1978-06-01 | 1978-06-01 | |
| US911,646 | 1978-06-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1128607A true CA1128607A (en) | 1982-07-27 |
Family
ID=25430629
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA325,242A Expired CA1128607A (en) | 1978-06-01 | 1979-04-10 | High potential discharge control circuit having distributed resistance elements especially suitable for induction-charging electrostatic spraying system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1128607A (en) |
-
1979
- 1979-04-10 CA CA325,242A patent/CA1128607A/en not_active Expired
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