Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
  • B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
  • B05B5/16—Arrangements for supplying liquids or other fluent material
  • B05B5/1608—Arrangements for supplying liquids or other fluent material the liquid or other fluent material being electrically conductive
  • B05B5/1616—Arrangements for supplying liquids or other fluent material the liquid or other fluent material being electrically conductive and the arrangement comprising means for insulating a grounded material source from high voltage applied to the material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/02—Plant or installations having external electricity supply
  • B03C3/16—Plant or installations having external electricity supply wet type

Definitions

• This invention relates to novel forms of dust mitigatio systems and in particular to a method relying upon the distribution of an electrically charged fluid spray for use therein.
• a disadvantage of such spray systems is that other majo sources of dust, such as roads and remote corners of the terminal site, may escape treatment altogether and remain a major source of dust over the entire area.
• the difficulty of suppressing airborne dust is, in simple terms, related to the relative numbers of airborne dust particles and the number and size of water droplets necessary to effectively capture them.
• the added mass thus causes rapid movement ofthe water droplet to the ground.
• any effective water spray must produce droplets of a sub ⁇ stantial size compared to the dust particles to be effective.
• To remove even a major proportion of airborne dust from a given air volume requires the spraying of a mass of water many times greater than the mass of dust particles. This is clearly impractical except in special circumstances.
• droplet means any self-cohesive volume of water restrained by its surface tension only.
• R I x R l wherein R. is the internal impedance, R-. is the leakage resistance, and V is the voltage generated within the gener-
• any significant leakage currents in such a system will cause electrolytic corrosion of any conducting materials and possibly of adjacent metallic equipment.
• Measures involving supply of spray water to electrostatically charged nozzles from an insulated tank also present problems .
• the electrical capicity of such a large conductive mass creates the danger of electric shock, potentially lethal at the high voltages involved.
• A cross-section area (internal bore)
• Vg generated voltage
• Vs required nett voltage at spray nozzle
• R. internal resistance of generator (ohms)
• a substantially non- conductive second fluid, immiscible with the working liquid is injected into the working liquid supply line at a point at or near the point of earth potential, in such manner that the turbulent mixture so formed is substantially non-conducting.
• the non-conducting fluid may be air or oil.
• a method of electrically charging liquid droplets in a dust mitigation system whereby the applied voltage is amplitude modulated at a rate substantially lower than the rate of generation of liquid droplets, the applied voltage being varied in peak potential, or in polarity to earth, or any combination thereof.
• Fig. 1 schematically represents the apparatus in its most elementary form.
• Fig. 2 illustrates the addition of a surfactant wetting agent to the working fluid of the apparatus of Fig. 1.
• Fig. 3 shows modifications required if the working fluid is of high conductivity.
• Fig. 4 represents the apparatus when applied to boundary dust mitigation installations.
• Fig. 5 is a plan view of a stockpile site layout.
• Fig. 6 is a side elevation of a stockpile controlled by one embodiment of the invention.
• Fig. 7 illustrates a variation of Fig. 1 where the spray nozzles are at or below the level ofthewater feed point.
• Fig. 8 is a cross sectional elevation of the first fluid second fluid junction of the arrangement of Fig. 7, and Fig. 9 illustates a further variation of the present invention wherein the first fluid is subject to nil, electrical potential until departure from the delivery pipe.
• the simplest form is a method for increasing the electrical resistance of the leakage path from the spray nozzle to earth to a magnitude sufficient for connection to a practical high-voltage generator with high internal resistance, when the conductivity of the water being used is low or moderate. See Figures 1 and 2.
• tubing With modern high-strength plastic materials, tubing can be constructed with very high electrical insulation values, yet capable of withstanding high fluid pressures. By accepting some loss of pressure from water flow (i.e. friction loss) in a relatively narrow-bore duct, an insulated spray nozzle can be supplied via a long length of such tubing without significantly reducing the electrical reistance to earth.
• the duct may be linear, or for convenience coiled as a spiral, a helix or as a zig-zag within an insulated casing.
• the ratio of length to cross-sectional area will be determine by several factors:-
• the available water has a conductivity of 150 microsiemens
• the desired voltage is 20 kilovolts at th spray nozzle
• generator capacity is 24 kv
• 2 ma DC
• the internal impedance ⁇ is- - 12 x 10 ohms.
• Minimum leakage path resistance is 60 x 10 ohms.
• the invention in its simplest form as shown in Figure 1, includes an apparatus having a length of electrically non ⁇ conducting ducting (1) , said duct having a ratio of length to internal cross-section of bore satisfying the equation:-
• Vg generated voltage
• Vs required nett voltage at spray nozzle
• R. internal resistance of generator (ohms)
• the additional load on the generator, of charging curren to the water droplets, is very small compared to the leakage current via the water-filled tubing, and negligible for the purpose of determining _ above.
• the duct is supplied with water (4) from a source at earth potential electrically, and feeds spray nozzles (2) insulated from earth.
• a transformer/rectifier unit or electrostatic generator (3) is connected between the nozzles (2) and earth, and supplied with electrical energy from a source (5) , usually mains voltage 230 volts 50 Kz.
• Water droplets (6) are emitted from the spray nozzles (2) bearing an electric charge of voltage determined by the equation:-
• the charged droplets (6) will capture dust particles at a rate determined by the voltage Es, the viscosity of the air, the rate of fall of each droplet, the number and mass of dust particles present per unit volume of air, and by the hydrophobic or hydrophilic property of the dust particle surface, relative to the water being sprayed.
• the voltage on the droplets is the determining factor in the rate of contact between each falling water droplet and dust particles.
• the dust particle is hydrophobic, such particles may simply acquire a similar charge by contact with the droplet, and not be captured by wetting action. The now-charged dust particles will then be repelled by the like-charge effect, and capture efficiency will actually be reduced.
• a suitable surfactant wetting agent being a polar compound with lipophilic and hydrophilic properties, may be added to the water supply at (4).
• a second line of tubing (7) may conduct the surfactant additive (8) to the water at spray nozzles (2) . Since the required proportion of such an additive to the water is only from 0.01% to 1.0% in most cases, relatively small-bore tubing is adequate for the surfactant supply line (8) , and hence of high resistance. The consequent reduction of earth-leakage resistance by the shunting effect of (7) is negligible.
• a second embodiment of the invention is used in situations where the available water supply for dust mitigation is relatively impure, i.e. of high conductivity. See Figure 3.
• the water fee-dduct (1) is again of non-conducting material such as nylon, polyvinyl chloride, etc. but may be of any ratio of length to cross-section.
• the air outlet from the cyclone is conveyed via non-conducting tubing (11) through a suction-relief valve (13) to a small compressor unit (14) and injected again at the base of (1), Surfactant, if required, is injected at the spray nozzle (2) via non-conductive tubing (7) from a supply source (8) into the water outlet from (9) from which a has been removed by the action of cyclone separator (12) .
• the injection of air into the flow of water in (1) redu the electrical conductivity of thewater column to an insignificant figure, such that the voltage output from the generator (3) may be connected directly to (2) without excessive loss by leakage current through (1) .
• the internal resistance of the generator may be made very high, so that accidental contact by personnel does not create an excessive hazard, the relative conductivity of the human body being low enough to effectively short-circuit the generator voltage wit only minute current flow through the person.
• the capacity of the compressor (14) is small, since the nett pressure difference between its input and output is little more than the hydraulic head differential between (4)- and (2).
• the air volume required is only a fraction of the water flow rate. In one example an air volume of 10 percent of the water flow in a tube of 18mm internal diameter, raise the electrical resistance of a 10-metre length of the tubing plus water and air turbulent mixture to 180 x 10**-- ohms.
• a third embodiment of the present invention involves t application of low-frequency modulation of the applied charge. This important feature lies in the design of the electrostatic generator (3) shown schematically in Figures 1, 2 and 3.
• this can be achieved by applying a. voltage at mains frequency (50 Hz) and the appropriate volta (usually 10 - 50 Kv) , to a spray nozzle generating droplets at rates greater than 100 per second, preferably several times this rate.
• the generator is so designed that the voltage applied to spray nozzles (2) may be modulated at low frequencies to change the magnitude and/or polarity of the charge imparted to water droplets (6) .
• the dust-capture effectiveness and precipitation efficiency are materially improved over a charged spray system of constant polarity.
• Each charged droplet collects dust particles by the well-known effect of attraction of opposite charges.
• the charge on a water droplet induces opposite charges on the nearer surface of adjacent dust particles, and like charges on the far side.
• the net effect is a force of attraction between droplet and particle.
• Droplets emitted during one half-cycle of the charging cycle are followed through the air by droplets of opposite charge emitted during the succeeding half-cycle. Due to variations of droplet size and velocity of emission, dronlets of opposite polarity intermingle in flight a short interval after emission, with an average spacing determined largely by the spray nozzle design and applied water pressure.
• each droplet collects dust by the aforementioned action of electrostatic attraction.
• the rate of movement of dust particles towards water droplets is an inverse function of the mass of each dust particle, which is generally very much less than the mass of the droplet.
• each droplet collects a number of dust particles before joining with another droplet of opposite charge, at which point the charge on each is neutralised.
• This increase in total agglomerated mass of water-plus-dust causes a sharp increase in the rate of fall through the air, which for small droplets is a function of size (mass) .
• the assembly shown schematically in Figure 1,2 or 3 may, instead of generating charged droplets (6) from spray nozzles (2) , have the fluid output from the top of (1) carried via an additional length of non-conducting tubing (21) to a length of tubing (15) , via a tee or Y junction piece (18) .
• the tubing (15) is suspended above ground by supporting masts (23) at a minimum height sufficient to clear all stacking cranes, conveyors, and handling equipment. • Support from the masts (23) is by tension insulators (20) and chain or wire stirrups (22) to eye-bolt anchors (19) in the end of each catenary section of tubing (15) , or other alternative supports providing electrical isolation of the suspended tubing from earth.
• Tubing completely surrounds the site in three or more sections, and has its upper surface perforated with fine orifices (17) such that thin jets of fluid are directed upward therefrom. These jets quickly disperse by the action of wind and fluid turbulence into separate droplets (16) .
• tubing (15) Being effectively insulated at each end, tubing (15) is at substantially uniform potential along its length, said potential being that at the output terminal of the high voltage generator (3) as corrected for leakage resistance by equation 1 above.
• the ejected droplets thus carry char ⁇ es at a ⁇ otential approximately that of (2) in the arrangements of Figures 1, 2 or 3 as relevant.
• a further embodiment of this invention is the non ⁇ conducting feedermethods described above, but in which the charge-modulation effect is produced by a pair of adjacent but separate spray nozzles or catenary-tube curtain sprays.
• Each such droplet emitter therein is connected to separate D.C. high voltage generators of opposite polarity, the emitte droplets of opposite charge then mingling in flight as previously described, to produce similar dust-capture effects.
• FIG. 7 Yet a further embodiment of the invention is illustrate by Figures 7 and 8. Where it is possible to locate spray nozzles below or near the same altitude as the water-feed point, (if necessary at some horizontal distance therefrom) the required reduction of conductivity may be achieved by t arrangement shown.
• Water is supplied at moderate to high pressure at the feed point (4) , and passes through an orifice plate (25) or venturi section, with air inlet ports (26) .
• the velocity o the water, and the relative areas of the tubing (1) and venturi throat or orifice (25) are so arranged that a suitably large proportion of air aspirated into the liquid within tubing (1) , thus reducing electrical conductivity of the fluid column.
• High voltage is applied at the spray head, in the sam manner as Figure 1.
• the leakage path to earth via the colu of air and liquid in (1) is of sufficiently high resistance to enable a high potential to be maintained at spray head (2) , from a source (3) of high internal resistance and low current output.
• a jet nozzle (27) containing a venturi section or orifice plate (25) having air-aspiration ports (26) is connected to a water source (4) of high or moderate pressure
• a deflector plate (28) of non-conducting material is mounted on a tubulor support (29) also of non-conducting material.
• a current limiting resistance o 80 to 300 -meg ohms (30) in series with the connection from a high voltage generator (3) via wiring (31) within the tubing to a small electrode (32) on the jet-contact face of the deflector plate (28).
• Water emitted from the nozzle contains air bubbles sucked in through the ports (26), and the jet is therefore of high resistance.
• the jet impinges on the deflector plate (28) , and is broken up into spray droplets on impact.
• the shape of the emitted spray pattern is- determined by the profile and angular setting of the deflector plate (28) and may be designed thus to virtually any desired spray pattern, with a charge on the droplets determined by equation (1) , imparted by the electrode (32) .

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• Electrostatic Spraying Apparatus (AREA)

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• 1981
  • 1981-10-23 WO PCT/AU1981/000153 patent/WO1982001481A1/en not_active Application Discontinuation
  • 1981-10-23 JP JP50330281A patent/JPS57501565A/ja active Pending
  • 1981-10-23 EP EP19810902834 patent/EP0063575A4/de not_active Withdrawn

Non-Patent Citations (1)

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