GB2140711A - Electrostatic spraying apparatus - Google Patents

Abstract

Spraying apparatus comprises a spray nozzle 20, one or more electrodes 7 disposed adjacent the path of spray emergent in use from the nozzle 20 and a barrier 18 adjacent the said one or more electrodes adapted to reduce contact of the spray with the electrodes, the barrier comprising one or more pointed protuberances 19 arranged so that liquid draining in use from the barrier collects at said protuberances and is discharged The apparatus may be mounted via sprayboom of a tractor. <IMAGE>

Description

SPECIFICATION Electrostatic spraying apparatus This invention relates to electrostatic spraying apparatus, particularly but not exclusively to inductively chargeable spraying apparatus of the kind used for agricultural and horticultural purposes. Electrostatic spraying apparatus also finds industrial applications, for example for paint spraying.

The advantages of electrostatic spraying are clearly illustrated by the applicability of the apparatus to agriculture. Induction of charges of like polarity on the spray droplets inhibits coalescence of the droplets and enhances attraction of the droplets to uncharged foliage at earth. Attraction to under-surfaces of foliage is particularly important since these'surfaces .

may not come into contact with uncharged sprays. Furthermore large droplets formed by coalescence are less biologically active than small droplets.

According to the present invention spraying apparatus comprises a spray nozzle, one or more electrodes disposed adjacent the path of spray emergent in use from the nozzle and a barrier adjacent the said one or more electrodes adapted to reduce contact of the spray with the electrodes, the barrier comprising one or more pointed protuberances arranged so that liquid draining in use from the barrier collects at said protuberances and is discharged from the latter by action of an electrostatic field.

The electrostatic field is preferably generated by said electrodes although separate additional electrodes may be employed.

Electrostatic stress on droplets draining onto the protuberances is enhanced by the pointed shape of the latter. Corona discharge may occur at the points. Many of the remaining surfaces of the barrier may be rounded having large radii to maximize excess liquid drainage to the pointed protuberances.

The protuberances are preferably located at lower extremities of the barrier so that liquid drains onto them under the effect of gravity.

The radii of the pointed protuberances need not be sufficiently small to cause corona discharge since the points are each covered in use by a drop of liquid. The critical radius for breakdown of the electrostatic field is that of the liquid drop as it separates from the point.

A minimum number of pointed protuberances are preferably employed to prevent excessive weakening of the electrostatic field.

Preferred embodiments of the invention comprise not more than eight protuberances, not more than four being especially preferred and two being the most preferred embodiment, wherein a single protuberance is provided adjacent to each electrode. Provision of a small number of protuberances has the advantage that a rugged construction can be used to withstand the hard wear experienced in agricultural use.

The barrier may comprise a shroud or screen surrounding the nozzle. Alternatively, the barrier may comprise two generally parallel plates between which the spray passes.

This latter arrangement is particularly useful with nozzles producing a fan-shaped or fishtailshaped spray. The barrier may also serve to support the electrodes. A conductive material, such as conductive plastics material is preferably used in the construction of the barrier. Earthing of the barrier serves to maintain the electrostatic potential of the points and may avoid the need for earthing of the liquid storage tank and supply line.

Two or more electrodes of like polarity are preferably disposed around the path of the spray emergent from the nozzle. Alternatively, a single annular electode may be used dependent on the shape of the spray.

The electrodes are preferably rounded being generally hemispherical or mushroom-shaped.

The lower portion of each electrode is preferably downwardly pointed to facilitate drainage of liquid. An insulating stem may support the electrode from the barrier.

Spraying apparatus in accordance with this invention may be mounted in use on a spray boom with each electrode arranged vertically.

In this arrangement spray collected on the rounded front surface and generally flat rear surface of the electrode may drain from the latter without causing an electric leakage or short circuit. It has been found that apparatus may be displaced from the vertical by up to 30 on account of flexibility of the boom, uneven ground, wind etc. Tilting of the electrodes impedes or prevents drainage of liquid therefrom. Electrodes known from the prior art have been provided with re-entrant convolutions on the rear surfface and on the other to assist in shedding of liquid. These have been found to fill with liquid when the electrodes are tilted, causing a short circuit.

In order to overcome this problem the present invention may provide a sharp edged plate-like insulator disposed upon the stem.

The plate serves as a physical barrier to movement of liquid along the stem from either the barrier or electrode. Furthermore the plate serves to isolate any charged liquid on the stem adjacent the electrode from uncharged liquid adjacent the barrier. The sharp edge facilitates drainage from the insulator and also increases electrical isolation. An angle of approximately 340 may be provided by the sharp edge, affording a high degree of electrical isolation by disruption of the electrical field and preventing movement of charged droplets towards the rear face of the electrode. Use of a sharp edge is preferable to increasing the path length by use of re-entrant convolutions.

In addition or alternatively the rear face of the electrode may incoporate one or more sharp angled ridges or grooves extending around the attachment to the stem. The sharp angle serves to improve electrical isolation of the stem from the electrode. Furthermore the bottom of the groove also exhibits a Faraday cage effect which resists the ingress of charged droplets. The grooves, which may be concentric, are preferably right angled in cross section. This minimises the effect due to the sharp angle and also allows tilting of the apparatus by up to 45 without impeding drainage of liquid.

The mounting for the apparatus may be provided with means for damping vibrations which occur in use. Transmission of vibrations from the spray boom to the nozzle is preferably also damped. This may be achieved by including a layer of shock absorbent or resilient material within the mounting for the spray head. Vibration of the barrier has been found to hamper discharge of drops from the protuberances causing an uneven pattern of spray.

According to a preferred aspect of the present invention there is provided spraying apparatus comprising a spray nozzle and one or more electrodes arranged adjacent the direction of the path of spray emergent from the nozzle, a high tension source and a conductor extending between said source and said electrodes, the conductor comprising a conductive plastics material.

Use of conductive plastics facilitates formation of electrical connection in the high tension supply. Corrosion of metal conductors is eliminated. Insulation of the conductive material may be simply provided by lamination or encapsulation with non-conductive plastics material.

According to a further preferred aspect of the present invention spraying apparatus comprises a spray nozzle, one or more electrodes disposed adjacent the path of spray emergent from the nozzle and means for adjusting the position of the electrodes to a location adjacent a region of the spray at which the liquid thereof separates into drops.

The position of the electrodes is preferably that at which the sheet of liquid which emerges from the nozzle breaks up into drops.

This position is that at which induction charging of the spray is most effective. Consumption of current is thereby minimised and a wide range of nozzle configurations may be employed.

The means for adjusting the position may, in preferred embodiments, comprise a series of differently sized adaptors or spacers which may be located between the nozzle and support for the electrodes.

The invention is further described by means of example and not in any limitative sense with reference to Figures 1 to 8 of the accompanying drawings,in which: Figure 1 is a plan view of a bus for the apparatus; Figure 2 is a cross sectional view of an electrical connector; Figure 3 is an elevation of the spray apparatus; Figure 4 is a cross-sectional view of an electrode; Figure 5 is a frontal view of the electrode; Figure 6 is an end elevation of the barrier; Figure 7 is a cross-sectional view of the nozzle and lectrical connection thereto; and Figure 8 is a side elevation of an alternative electrode.

Spraying apparatus in accordance with this invention comprises one or more spray heads, for example mounted on a boom, a supply of liquid to be sprayed and an electrostatic charging apparatus. The latter is composed of four parts: a high voltage generator and control unit, a bus or main electrical feed line, electrodes attached to each spray head and the barrier protecting the electrodes.

The high voltage generator may be tractormounted for most agricultural applications although it can be incorporated onto or into the sprayer where appropriate. The high voltage bus can be located along the handle of a knapsack sprayer, or along the boom of a conventional sprayer. For a sprayer having up to a 1 6 metre boom only two voltage buses will be required. A separate outlet on the generator may be connected respectively to the left and right hand sides of the boom. For booms greater than 1 6m in length two further buses may be required, one more for each side of the boom. The current requirement of up to 20 microamps per spray head, gives a more than an adequate safety margin where the nozzle spacing might be is 0.3 metre or below. Each bus should serve an equal number of heads.The bus supplies each head in a parallel circuit with a single pole current and earth. In a preferred embodiment of the design all conductive parts of the equipment other than the voltage generator and control unit are riade of conductive plastic with a total resistance of no more than about 25 Megaohms and preferably not less than 2.5 Megaohms.

The electrosratic charging head comprises the high voltage induction electrodes and the protective electrode barrier. This may have the form of a shroud. The charging head is mounted between the nozzle body and the nozzle cap, tip and seal using a system of adaptors. The charging electrodes are placed on either side of the spray pattern produced by the spray tip but are designed not to physically touch the mass of the spray so produced. The electrodes are opposed physically but have a like charge and impart a charge on the spray droplets by induction.

The electrode barrier and the elongated mushroom-shaped electrode support are designed to prevent both charged and uncharged spray accumulating on the electrodes. The barrier is conductive and earthed via the earth line on the voltage bus. The barrier also comes into physical contact with the spray liquid in the nozzle adaptor and serves to earth the latter.

The electrode support is made of non-conductive material, and therefore serves to protect the electrode from earth potential. The rear surface of the support is convoluted to increase rhe overall surface area and also to provide a Faraday Cage effect, so as to prevent the formation of a liquid path to earth from the electrode. The electrode support is no wider than the shroud and is shaped to a point at its furthest position from the nozzle tip to provide a small surface at which the accumulated spray liquid can be redistributed by the strong electric field. This principle is also used on the earthed electrode barrier whereby the barrier is formed with a series of points so that the accumulated spray liquid can be redistributed into droplets in the atmosphere using the electric field source.This is superior to simply allowing the liquid to fall by gravity off a simple barrier because the drops so produced are smaller in size and therefore likely to be of biological use on the target.

The electrodes are connected to a live output from the generator and the barrier to earth, using a modular system of high voltage cables which run between the charging head and the voltage bus. In this embodiment the conductive parts of the cable are made of conductive plastic. This facilitates the manufacture of fairly simple connections between the cable and the various parts of the head and voltage bus.

The high voltage generator not shown in the drawings, may be either tractor or sprayer mounted in the case of a conventional sprayer, or handle or backpack mounted in a knapsack sprayer, may run either from the tractor nominal 12v DC supply or from a suitable combination of dry cells. It produces a high voltage supply (typically 4000V.) which can be switched over the range 500v to 5000v., if necessary. Where tractor mounted the generator has a voltage regulator fitted to take into account any variations that might occur in tractor battery supply. A stabliizer is preferably fitted since the state of charge of any battery varies throughout its effective life. The control panel may have a warning lamp to show that current is actually being drawn.Alarm circuits can be fitted into either the tractor mounted or backpack versions of the high voltage generator to warn of low charge from the batteries or of excessive current consumption by the spray heads. The high voltage oscillator feeds a separate outlet for each voltage bus to prevent a current drain of any greater than 2 milliamps from any one outlet in order to maximise operator safety.

It is important for the correct operation of the device and for operator safety that all earth lines are earthed correctly. This can be carried out either by having a separate earth for the electrostatic unit for example using a metal braid or wire running along the ground, or using the tractor's earth. The former is used on the knapsack variant. The latter can also be used where conditions are dry and the tractor makes a poor path to earth, it is again better to run a metal wire or other conductor along the ground, possibly connected to the tractor stub axie, to ensure a true earth.

Whilst a good earth connection ensures the proper and safe operation of the electrostatic charging head, it is also important for the operation of the high voltage supply as only one side of the transformed current is actually used. Although the elecrrodes are opposed physically, they are like in polarity.

The high voltage bus or main high voltage feed iine, is illustrated in Figures 1 and 2. In this embodiment it is made of two lengths of conducting plastic, separated from each other but connected by an envelope of flexible insulating material 24. This results, in cross section, in a flattened oval shaped two core wire.

The whole bus is flexible to allow for folding of the boom. One wire is the high voltage supply from the supply unit to the spray head electrodes. The other is the earth connection.

Regular supply 26 and earth 25 socket points are placed at intervals along the bus (typically every 0.125m.). Unused sockets are blocked off with insulated blanking caps. The sockets may be placed in triangular arrangements if two separate cables are used to the electrodes, or in pairs. One socket is used for the earth connection from the charging head and the other one or two sockets are for the supply voltage to the electrodes. The con -struction of the sockets, using all plastics in this example, is shown in cross-section in Figure 2. Figure 1 being a top view of the bus itself. The socket comprises an insulating lip guard 1 surrounding a circular hole in the bus insulation 2.The earth or supply voltage line 3 has a moulded cavity within it directly below the insulation hole 2 which engages and locks with its counterpart on the end of the earth or electrode supply cable 4 when the latter is pushed into it. Supply cable 4 comprises a core of conductive plastics material similar to that in the high voltage bus surrounded by a sheath of insulating material.

The open end of the cable is surrounded by an insulating guard 5 which ensures a positive and electrolyte resistant seal with the lip guard 1. This may be improved where necessary by a bayonet, or similar grip on both the cable and socket lip guards. The high voltage cable 4 is essentially the same at both ends and forms a similar connection with the induction electrodes and the earthed shroud. Alter natively threaded connections incorporating water resistant sealing rings may be em ployed. Flat faced abutting metallic connectors may be employed in such arrangements.

The electrodes are shown in their physical positioning within the shroud in Figure 3, in cross section in Figure 4 and head on in Figure 5. Two electrodes are used, one through each side of the opposing sides of the shroud. They are positioned such that the open, conducting plates that carry the charge directly oppose each other. Each is connected via the high voltage cables to the high voltage line on the supply bus. The electrodes are positioned directly below the spray nozzle tip, the tip being positioned such that any flat fan so produced falls completely between the electrodes. As shown in cross section in Figure 4 each electrode comprises four parts part 6 is made out of similar conductive material to the core of the supply bus and high voltage cable. A conductor 6, composed of plastics material, is located within an insulating stem 8.The stem 8 is provided on the lower surface remote from the nozzle, with a sharp raised edge 1 3 adapted to gather liquid from the stem and to cause it to be atomised by the electrostatic field generated by the electrode 6. Electrode 6 has a connection at one end to positively connect to the high voltage cable, whilst at the other end nearest to the spray fan it terminates in a widened plate 1 4 which offers an enlarged surface area with which to charge the spray. A support head 7 is shaped like an elongated mushroom head, with the pointed end 1 5 furthest from the spray tip (best shown in Figure 5). This helps to collect that charged spray which would otherwise be attacted onto the live electrode and short it to earth.As the liquid runs down the face of the support head it collects at the basal point 1 5 and is redistributed by the strong electric field at that point. This point can also take the form of a plate-like extension to a circular, or similar support. The support head is not wider than the the protective barrier 10. An annular gap 16 separates the stem from the rear face of the support head. This is to prevent a liquid path being formed from the live electrode face 14 to the earthed electrode barrier 10. The stem 8 may be oval in shape to further facilitate removal of liquid from its surface.

The stem 8 passes through the wall of the barrier 10, and terminates in an open screw threaded (or similar fitting) portion, which is held in place by a complementary retaining ring 9. The cable insulating guard 5 fits over this to ensure a tight seal.

The barrier is illustrated in Figure 3 and 6 and the nozzle connection and adaptors in Figure 7. Both inner and outer surfaces of the barrier perform the important task of collecting and re-distributing any charged spray droplets that would otherwise be attracted to, and collect on, the live electrode surfaces. The barrier is preferably made of conducting material which allows easy connection to earth, or may be coated in a conducting material to perform the same function. As the barrier also serves to earth the spray liquid in this embodiment, it obviates the necessity of having to earth the liquid at another point, such as the spray tank or pump.

The barrier takes the form of a rectangular base 1 7 and two generally parallel side portions 1 8. The side portions terminate at their lowermost surfaces in one, two or more collecting points 1 9 or protuberances which serve the same function as similar points on the electrode support head and stem, that is collecting and re-distributing into the charged spray pattern droplets that would otherwise collect on the live electrode surfaces. The collection points 1 9 position the material in a small area in a strong part of the electric field.

As the material leaves the barrier it is disrupted by the electric field and tends to form far more usefully sized droplets than would otherwise be the case.

Ideally, the barrier and hence the whole electrostatic charging head, is connected between the nozzle body 24 and the nozzle cap 23, seal and tip. One such method of doing this is shown in Figure 7. In this design the barrier 10 is common to all makes of sprayer, and two adaptors 11 and 1 2 enable it to be fitted to the nozzle body and cap. An alternative is to have a different barrier for each make of nozzle body. Owing to the frequency with which nozzle tips, and hence caps, are changed, it is essential that the fittings are robust. The barrier can be connected to earth at any point, although in Figure 7 is is shown connected to the earth cable with the attachment 22 at a point close to the nozzle body for convenience.

A layer of cushioning material 30 is provided between the mounting for the spray head and barrier 10 to reduce transmission of vibrations of the boom to the protuberances.

Figure 8 shows an alternative electrode support. The, mushroom-shaped support 31 is provided on the rear surface with a 90 groove 32 surrounding the attachment to the stem 33. The rearwardly projecting, rightangled ridge 34 serves to enhance isolation of the electrode, while the groove 32 exhibits a Faraday cage effect. A sharp edged, plate-like insulator 35, disposed on the stem 33, serves as a physical barrier to movement of liquid between the electrode 31 and shroud 36. The sharp edge 37 facilitates drainage and also enhances electrical isolation by virtue of the angle of approximately 340 at the edge 37.

The collection points on the electrode support head, stem and barrier make prevention of a leakage to earth of the live electrode very much easier and also avoid a very considerable waste of active material. Dependent on nozzle type, and operating spray pressure and volume from 5% to 60% of the total sprayed material can be collected on the barrier and electrode support surfaces. If allowed to simply drip from these surface, the material would tend to fall as large drops and hence be effectively wasted, whereas re-distributed into the spray pattern by using sharp collection points in the electric field the material may be made available in a biologically active form.

The electrostatic charging head shown in the drawings is intended for use in conjunction with flat nozzle tips. The effect of charging the liquid in a hydraulic nozzle is to increase the normal spray angle. Under certain circumstances this effect can offer a particular advantage.

Certain types of sprayer boom have a vertical, as well as horizontal, break-back system.

This is obviously a disadvantage with this particular system as the barrier hangs below the nozzle body. Support runners' may be placed at either end of a boom section for ard of the spray unit i.e. towards the direction of travel. This minimises the unwanted collection of spray material on the support runners where those runners are normally at earth potential.

It is important when fitting the complete head unit to the nozzle body that flat fan nozzle tips align down the boom in the conventional manner i.e. with the fan pattern offset approximately 5 from the line of the boom.

This invention is not restricted to use with sprayers which produce fan-shaped spray patterns, use with swivel cone sprayers also being possible.

Suitable conductive plastics materials may be selected from polyethylenes or other plastics to which conductive materials have been added.

Preferred plastics include VELOSTAT manufactured by the 3M Corporation. Conventional insulating plastics may be employed. It is preferred to use the same plastics materials for the conductor and insulator portions, the former having a conductive additive.

Claims (12)

1. Spray apparatus comprising a spray nozzle, one or more electrodes disposed adjacent the path of spray emergent in use from the nozzle and a barrier adjacent the said one or more electrodes adapted to reduce contact of the spray with the electrodes, the barrier comprising one or more pointed protuberances arranged so that liquid draining in use from the barrier collects at said protuberances and is discharged from the latter by action of an electrostatic field.

2. Apparatus as claimed in claim 1, wherein the radii of the one or more protuberances is not sufficiently small to cause corona discharge in use of the apparatus.

3. Apparatus as claimed in claim 1 or 2, comprising not more than eight protuberances.

4. Apparatus as claimed in claim 3, comprising not more than four protuberances.

5 Apparatus as claimed in claim 4, comprising two protuberances.

6. Apparatus as claimed in any preceding claim, wherein the barrier is composed of a conductive plastics material.

7. Apparatus as claimed in any preceding claim wherein the electrode is supported upon a stem, a sharp edged, plate-like insulator being disposed on the stem between the electrode and support.

8. Apparatus as claimed in any preceding claim, wherein the electrode is supported by a stem and wherein a rear face of the electrode has one or more sharp angled grooves or ridges extending around the attachment to the stem.

9. Apparatus as claimed in claim 8, wherein the said grooves or ridges are right angled in cross section.

10. Apparatus as claimed in any preceding claim comprising a high tension source and a conductive plastics conductor extending between the source and said electrodes.

11. Apparatus as claimed in any preceding claim, wherein said electrodes are disposed adjacent the path of spray emergent from the nozzle, further comprising means for adjusting the position of the electrodes to a location adjacent the region of the spray at which the liquid thereof separates into drops.

12. Apparatus as claimed in any preceding claim comprising two opposed electrodes.

1 3. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.