GB2100147A - Electrostatic spraying - Google Patents

Abstract

In a spraying apparatus to achieve control of the velocity with which spray drops land on a target, a spray liquid is subjected to mean pressure to determine the mean velocity of a jet of liquid from a nozzle 16 on a spray liquid reservoir 10 and to a superimposed periodic pressure perturbation, e.g. by means of magnetostrictive transducer 17, to cause the jet to break into drops. An electrical charge is applied to the drops by means of electrodes 22. By control of the mean and periodic pressure for a particular nozzle size the jet velocity and drop size can be arranged to produce a particular drop landing velocity at a known range. In this way the application, and retention, of spray on plants can be controlled and selective application to one type in a mixture of plant types achieved. <IMAGE>

Description

SPECIFICATION Electrostatic spraying The invention relates to a method and apparatus for electrostatic spraying, particularly for the spraying of herbicidal liquid formulations.

It has been proposed to spray insecticides and fungicides on to field crops by means which cause the spray droplets to be charged electrostatically. In such cases it is normally important that the presence of charge enables droplets to become attached to the undersides of. leaves where pests may be found. The overall distribution of the spray is, of course, also important for economy and for avoiding contamination. The general requirements for the spraying of herbicide are similar, but there are additional constraints and it cannot be assumed that a sprayer which is satisfactory for insecticides and fungicides will be safe or effective for herbicide.In particular, the risk from spray drift to neighbouring crops is much greater, and where even a selective weedkiller is being used among a crop, the risk exists that at some stage of growth the crop will be susceptible to the weediciller.

It thus becomes important to determine the most favourable conditions for effective deposition and to control the spraying operation so that such conditions are provided to a significant extent. The achievement of such control, although conceived in the context of herbicide spraying is, of course, likely also to be advantageous for other materials.

In accordance with one aspect of the invention there is provided a method of spraying liquids comprising the steps of providing a volume of liquid in communication with a nozzle aperture, causing liquid to emerge from the aperture as a jet, controlling the mean pressure of liquid at the aperture to determine the mean velocity of flow in an emergent jet, superimposing a selected periodic perturbation on the mean pressure of liquid to cause the jet to break into drops of predetermined and substantially uniform diameter, and exposing the jet to an electrical influence such that the drops are electrically charged, the jet velocity and the diameter of the drops being so related that the landing velocity of drops at a target has a desired value.

The importance of the latter relationship in spraying a herbicide is that the effective deposition rate of liquids on plants of different leaf structure has been found by the inventors to be selectively influenced by the landing velocity. The effective deposition rate is a net rate which takes account of the varying degree of probability that an impacting drop of liquid will bounce off or run off the surface of different kinds of leaf. Thus a desired value of landing velocity in that context is one which provides the best differentiation between a crop plant and a weed. In order to determine the initial jet conditions for a desired landing velocity a substantially uniform spraying distance may be assumed, or a servo signal which is distance-dependant may be derived.

The periodic perturbation may be produced by the excitation of a piezo-electric, magnetostrictive, or electrnmagnetic transducer and may be variable in frequency in order to control drop size in a range related to the size of the aperture.

The transducer may carry a vibratable member for direct contact with the liquid or may transmit motion to the liquid through a flexi- ble isolating wall.

Alternatively, the vibration may be applied direct to the nozzle aperature.

The nozzle may have a plurality of apertures and may be in the form of a spinnerette such as is used for producing fibres of synthetic materials.

The nozzle may have apertures which are isolated from each other and supplied by independent reservoirs. Such an arrangement allows the simultaneous spraying of incompatible materials which are prevented by the electrical charge from mixing in flight.

Preferably the electrical influence comprises an electric field which causes the jet to become charged by induction. The field may conveniently be produced by a potential difference in the range 500v to 1,500v between a charging electrode and the nozzle.

In addition to the effect on deposition at a target, an electric charge on the drops prevents the merging of drops in flight. Mutual repulsion thus acts to maintain a uniform size and uniform distribution of the drops. Such a condition is otherwise impossible to achieve in a stream of drops which undergo a marked deceleration in flight.

In accordance with a second aspect of the invention there is provided an apparatus for spraying liquids, comprising a reservoir for liquid in communication with a nozzle aperture, means to maintain a pressure in liquid in the reservoir, pressure control means for controlling the mean pressure of liquid at the aperture to determine the mean velocity of flow in an emergent jet, drop control means for superimposing a periodic perturbation on the mean pressure of liquid to cause the jet to break into drops of predetermined and substantially uniform diameter and means causing the jet to be electrically charged, the pressure control means and the drop control means being related in operation so that the landing velocity of the drops at a target has a desired value.

The invention will be further explained and the construction and operation of an embodiment of the invention will be described with reference to the accompanying drawings in which Figure 1 is a schematic diagram of spraying apparatus in accordance with the invention; Figure 2 illustrates graphically the variation of spray velocity with distance from the nozzle for the apparatius of Fig. 1; and Figure 3 illustrates graphically the species selectivity of a spraying operation with the apparatus of Fig. 1.

Referring to Fig. 1 a cylindrical reservoir 10 having end walls 11, 1 2 is supplied with liquid from a supply pipe 1 3 in which the liquid is maintained under pressure by a pump 14. Reservoir 10 is closed except for one or more jet apertures in a metallic plate 1 5 forming part of wall 11. Only a central aperture 1 6 is shown. Particularly for purposes of experiment, where highly precise and uniformly sized apertures are required, it is convenient to use as the plate 1 5 a spinnerette such as is manufactured for the spinning of synthetic fibres. A spinnerette has one or more apertures of few tens or hundreds of micrometres in size.The present embodiments use one or more apertures in the range 100 to 200 ym. Liquid under pressure will issue from aperture 1 6 as a jet which breaks up to form a stream of droplets of irregular size and distribution. The initial velocity of the jet can be varied approximately as the square root of pressure by varying the speed of pump 14.

Control in the formation of droplets is provided by superimposing a repetitive pressure pulse on the static pressure which is maintained on the liquid. In the illustrated embodiment the pulse is produced by a magnetostrictive transducer 1 7 comprising an excitation coil 1 8 mounted externally of wall 1 2 of reservoir 10 and a magnetostrictive element 1 9 which lies axially within coil 1 8 and extends through wall 1 2 into the liquid. Transducer 17 is driven by a pulse generator 21 which produces square pulses at a rate which can be varied over a substantial range in the region of 1 OkHz. In one embodiment a range including the band of 8kHz to 1 2kHz was provided.An electrostatic charge is induced on the jet of liquid and thus on the resultant droplets by means of a pair of parallel plate electrodes 22 which are maintained at an elevated potential with respect to the nozzle and the liquid which will normally be held at earth potential. Plates 22 need to be spaced apart from the jet by a distance of only a few mm and the liquid is exposed to a field sufficient for effective charging at a potential as low as 500V. Plates 22 are energised by a power supply 23 output of which is variable over a range from 500V to 1 ,500V so that various required leves of charge can be accommodated. The current requirement is only a fraction of 1yA at low flow rates and is never likely to exceed a few pA so that unit 23 can be compactly and safely constructed.

It is of course necessary that the spray liquid should be sufficiently conductive for charging to occur but the degree of conductivity required is not high and is satisfied by most practical formulations.

For purposes of experiment the apparatus of Fig. 1 has been operated on a conveyor which can be driven at a selected height above a test area. The speed of each traverse is uniform but can be selected to reproduce any spraying rate (volume/unit area/unit time) which might be typical of field use. In the test area, the total deposition is measured in suitable dishes of known area and the effective deposition is measured on growing plants. The dishes and the plants are so positioned that all are equally exposed to the falling spray and collection surfaces are maintained at earth potential through suitable moisture paths. A cultivar of spring barley has been taken as a typical cereal crop for comparison with the Spanish radish which is a broad-leaved plant typical of many weeds.

It is to be expected that in a field population of cereal plants and weeds the broadleaved weed which tends to spread horizon taliy will be more exposed to herbicide deposition than the narrow leaved plant which tends to grow vertically. In addition to this difference it is proposed by the inventors that because the leaf surfaces are different in their nature a further important degree of selectively can be achieved by control of the conditions of spraying. Results obtained in the experimental arrangement described in the preceding paragraph demonstrate the significance of the landing velocity of droplets at the leaf surface, that velocity being a function of the initial velocity and of the droplet diameter for a given height of the nozzle above the leaf surface.The means by which the relevent parameters are controlled will be discussed and the nature of the results will be indicated.

The spraying apparatus of the invention can be aimed in any direction but for consideration of landing speed it will be assumed that deposition occurs vertically and in still air.

Referring to Fig. 2, a droplet size of 280ym is assumed for which a terminal velocity of 1.1 7m/s can be calculated. On a graph of velocity (vertical axis) against distance fallen by the droplet below the nozzle (horizontal axis), a first curve 28 relates to an initial velocity determined by a static pressure of 10 psi (70 kPa) and a second curve 29 relates to an initial velocity determined by a static pressure of 4 psi (28kPa). When an initial velocity has been assigned, by the static pressure value, the subsequent change in velocity is entirely dependent on the drop size. Assuming a drop size of 280 ym in each case the terminal velocity of 1.17 m/s is indicated by a line 30 which is approached exponentially from each initial condition. For the higher initial velocity of curve 28 the terminal value is reached on falling through 0.95 m and consequently a desired value of landing speed within the range 1.17 m/s to 9.2 m/s can be selected by operating at a corresponding spraying distance below 0.95 m. It will be observed similarly that for the lower initial velocity of curve 28 the terminal value is reached in falling through 0.7 m and that a range of landing speed from 1.17 m/s to 5.5 m/s is available for spraying distances below 0.7 m. Curves can be constructed for other values of static pressure, the initial velocity being dependent on (pressure)+ for a given size of aperture and the terminal velocity being recalculated if the drop size is changed.

At least over a limited range the drop size is variable, independently of the aperture size and of the static pressure, in response to a pulsed modulation of the pressure. The purpose of the pulsation is to set up a condition of uniformly repeated instability in the ligament of liquid which initially forms the jet so that the ligament breaks up into droplets which are uniformly sized and uniformly spaced. Each element of the ligament which forms a droplet is detached when the length is equal to one wavelength A at the pulsation frequency fand drop size is thus directly controlled in terms of frequency. The constant of proportionality depends on the initial jet velocity V in accordance with the relationship V = fA.An appreciation of the dimensional relationship can be obtained from criteria established by Rayleigh that for maximum instability A = 4.5d where d is the diameter of the jet. (It cannot be assumed that d is equal to the exit diameter of the aperture and generally it will be rather larger). Simple calculation then shows that the spherical drop formed from a linear thread of length 4.5d has a diameter of about 1.8d. In practice controlled droplet formation is found to be possible for a range of frequency extending at least to 30% above and below the Rayleigh frequency, the drop size varying inversely with frequency.

The preceding discussion has shown that the landing velocity of a droplet on a plant at a known distance below the spray nozzle can be preselected by an appropriate adjustment of pump 14 (Fig. 1) to determine the static pressure. It will also be understood that automatic compensation for substantial variation in the spraying distance can be provided by controlling pump 14 continuously in response to the output from a ground-distance sensor.

In Fig. 1 such a sensor 32, responsive to capacitance for example, is indicated as an optical feature of the apparatus which provides a signal to a pump speed controller 33.

A pressure sensor 34 in supply pipe 1 3 supplies a feedback signal from the output of pump 14 to controller 33. In order to maintain a constant drop size in the presence of variations in static pressure and consequent variations in initial jet velocity, it is also necessary to supply a frequency correction signal from pressure sensor 34 to pulse generator 21. For pressure P, the ratio P;/f must be held constant since V ap+ and v/f = A is then also constant. Any desired variation in drop size by a variation in frequency is of course still available independently of the automatic control system.

The results of experiments will now be presented briefly to show that a clear advantage in the distribution of spray between a crop and a weed is obtained by the use of predetermined landing speeds. The test area was laid out as described above with dishes for monitoring the overall rate of deposition and with whole plants of barley and radish. At the stage of growth when the tests were made, approximating as closely as possible to that when field spraying would in fact be carried out, the barley was about 1 5cm high and the radish about half this height. The nozzle heights to be referred to were measured on the barley to the highest level at which both stem and leaves were present. The spraying conditions were as represented by curve 29 of Fig. 2 i.e. p = 4 psi (28kPa), f = 9.9 kHz, drop size = 280 ,um, charging voltage = 1,500 V.Numerous test runs were made at each of two nozzle heights, 0.5m and 0.1 m. Reference to curve 29 of Fig. 2 shows that these heights correspond respectively to a velocity close to the terminal velocity and a velocity not greatly below the initial velocity.

The spray material was a fluorescin solution but for half of the tests a surfactant was added because in practical herbicidal spraying such an additive is often necessary to enable the biological action to proceed. After each spray run the test surfaces were washed and the amount of the deposit estimated by spectrofluorimetry. In order to take all surfaces of the plant into account the deposit was calculated as yL/gm. dry weight of the plant and further taking into account the overall deposition rate, data for the retained deposit is finally expressed in (yL/gm. dry wt.)/(L/ha).

The economics of field use can thus be assessed.

In a statistical analysis of the measurements a probable error of up to + 10% of the mean value of the retained deposit is found but the basic results shown in Fig. 3 are considered to be significant. The vertical scale represents the mean value of retained deposit in ('iL/gm.

dry wt.)/(L/ha) and the horizontal scale represents nozzle height in metres. Single point values are plotted at 0.1m and 0.5m and the two values for each sample are joined by a broken line which is not intended to suggest linearity over the intervening range. Line 35, marked BS, relates to barley with surfactant spray; Line 36 marked RS, relates to radish with surfactant spray; and respectiveiy corresponding lines 37, B$ and 38, R$ relate to barley and radish sprayed without surfactant.

Rembering that landing speed is higher at the shorter distance, it is clear by observation of Fig. 3 that the retained deposit increases to some extent at lower landing speed particularly for barley, in the presence of surfactant.

More significantly, if the species selectively is considered, (i.e. the ratio of the deposit on radish to the deposit on barley) the values derived from the end points of lines 36, 35 are 11.0 at 0.1m and 2.0 at 0.5m, with surfactant. In the absence of surfactant the ratio at each position is increased but the relative advantage of the higher landing speed is less marked.

The preceding results show species selectivity which is made up of contributions from the wetting characteristics of different leaf surfaces and from the different attitude of the growing leaves. Separate experiments have also been carried out on isoiated leaves laid horizontally and mounted at an inclination to the horizontal. The retained deposit calculated in IlL/cm2 of surface area confirms the trend demonstrated in Fig. 3.

On the basis of these results the spraying of herbicide on barley with broad-leaved weeds can be carried out more productively (or less destructively) by designating a high value of landing speed and operating the appratus to maintain that value. It is expected that the control process can be refined, for example to take account of the variation of electric field with distance, and that in the course of further experiment it will be possible to determine drop sizes which lead to preferentiai retention by the weed, and to characterise different weed types.

It is also visualised that in addition to the usual advantages of an electrified spray, the ability to produce droplets of controlled size and landing speed by the method of the invention will be useful in other applications, for example where a controlled depth of penetration of foliage is required. In spraying insecticides it is obviously advantageous if preferential deposition on the affected plant species or the plant species associated with the rest, say as a host plant, can be achieved. It is also of general interest in controlling drift that droplets of uniform size and distribution are produced at a considerably larger diameter than is commonly found with either rotary atomisers or high pressure jets.

A possible application of the spinnerette type of aperture plate is that holes of different sizes can be provided such that from a single source a range of drop sizes, each having a different but predetermined velocity, can be produced. A compartmented reservoir having a common pulse source or a number of sources is also visualised to enable the synchronous application of two or more chemically distinct (and possibly incompatible) pesticides such that the drops remain separated on the target.

If required the liquid can be circulated back from reservoir 10, e.g. to the inlet of pump 1 4. In this way the liquid can be kept agitated for example to prevent separation into components by a mixture or suspension while a higher liquid speed reduces the risk of blocking of the liquid flow path. The use of a return path, which could include a controllable restriction, offers the ability to control flow and pressure independently which would reduce the dependence of jet flow velocity and nozzle size. As a further aid to reliable operation the transducer 1 7 can be arranged to permit occasional operation at a higher power level to provide a sonic cleaning action in the liquid in the reservoir which action would be helpful in dispersing sediment and blockages in the reservoir and nozzle.

The use of a spinnerette, that is a nozzle with one or more fine apertures, such as round holes, intended for the extrusion of a fibre-forming material, is mentioned. However other forms of nozzle are also suitable and there may be several apertures, these nozzle apertures may be along a line, so that the view in Fig. 1 may be regarded as a crosssection of a manifold supporting the line of nozzle apertures. Spinnerettes have apertures of the order of ten or hundreds of micrometres and this order of size is generally suitable. However particular liquids, e.g. more viscous ones, may require other sizes with appropriate modifications, as will be apparent from the examples given of other dimensions pressures etc.

The generator 21 has been described with reference to Fig. 1 as producing square waves but wave shape is not considered to be critical except that a sinusoidal pulsation is to be avoided as allowing the production of satellite drops when the jet ligament becomes unsta ble.

Other variations in the apparatus will be apparent, for example that transducer 1 7 may be made alternatively for piezo-elecric or electromagnetic conversion of energy. The man ner of charging the spray may for example comprise a corona discharge but inductive charging is likely to be preferred because the current flow is applied directly to produce charge and is not dissipated to other earthed bodies as commonly occurs with corona. The induction electrode may be a single electrode of any convenient form e.g. cylinderical in stead of parallel plates.

Claims (16)

1. A method of spraying liquids compris ing the steps of providing a volume of liquid in communication with a nozzle aperture, causing liquid to emerge from the aperture as a jet, controlling the mean pressure of liquid at the aperture to determine the mean velocity of flow in an emergent jet, superimposing a selected periodic perturbation on the mean pressure of liquid to cause the jet to break into drops of predetermined and substantially uniform diameter, and exposing the jet to an electrical influence such that the drops are electrically charged, the jet velocity and the diameter of the drops being so related that the landing velocity of drops at a target has a desired value.

2. A method according to Claim 1 of spraying liquids on to plants including assessing the desired value spray droplet landing velocity for retention of the sprayed drops on the plants, selecting an appropriate jet velocity and drop diameter to produce the desired landing velocity.

3. A method according to Claim 2 of spraying a liquid on to different types of plants, the plants growing mixed together including assessing the retention of the sprayed drops on each type of plant and selecting an appropriate jet velocity for better retention on a plant type to which retention is required.

4. A method according to Claim 3 in which the types of plants include crop plants, and weed plants and the liquid is a herbicide for weed plants the jet velocity being selected for better retention on the weed plants.

5. A method according to Claim 3 in which the liquid is an insecticide for insect pests, the jet velocity being selected for better retention on the plants associated with the pests.

6. A method according to Claim 2 including spraying the liquid from a uniform distance to maintain the desired landing velocity.

7. A method according to Claim 2 including assessing the distance over which the liquid is to be sprayed and adjusting the spray conditions to maintain the desired landing velocity with variation in said distance.

8. A method according to Claim 1 including selecting a periodic perturbation frequency from a range of such frequencies to produce one of a range of drop sizes from a particular aperture size.

9. A method according to Claim 1 including providing the electrical influence as an electric field and charging the jet drops by induction.

10. A method according to Claim 1 of spraying two liquids including providing independent liquid reservoirs, each having a respective distinct aperture to produce distinct drops and exposing the distinct drops to an electrical influence such that the drops are electrically charged and repel each other to retain their distinctiveness.

11. An apparatus for spraying liquids comprising a reservoir for liquid in communication with a nozzle aperture, means for maintaining a pressure in liquid in the reservoir, pressure control means for controlling the mean pressure of liquid at the aperture to determine the velocity of flow in an emergent jet, drop control means for superimposing a periodic perturbation on the mean pressure of liquid to cause the jet to break into drops of predetermined and substantially uniform diameter and means causing the jet to be electrically charged, the pressure control means and the drop control means being related in operation so that the landing velocity of the drops at a target has a desired value.

1 2. Apparatus according to Claim 11 in which the drop control means includes a transducer responsive to energisation to produce a pressure.variation in a liquid.

1 3. Apparatus according to Claim 1 2 in which the transducer is one of a piezoelectric a magneto-strictive and an electromagnetic transducer.

1 4 Apparatus according to Claim 1 2 in which the transducer carries a vibratable member.

1 5. Apparatus according to Claim 11 in which the drop control means includes a source of variable frequency, variable over a range of at least 8kHz to 1 2kHz and means to couple the variable frequency to the liquid to perturb it.

16. Apparatus according to Claim 11 in which the nozzle has at least one aperture with a size in the range of a few tens to a few hundreds of micrometres and typically between 100 and 200 micrometres.

1 7. A method of spraying substantially as herein described with reference to the accompanying drawings.

1 8. A spraying apparatus substantially as herein described with reference to the accompanying drawings.