ELECTROS'rA'rIC SPRAYING OF PLANA.T{ SURFACES

SIMULATING TURFGRASS

by

R. C. ANANTHESWARAN

B. Tech, Indian Institute of Technology, 1977

A Th2sis Submitted to the Graduate Facultyyf the Un:i.v~rsi ty of Gevrgia in Partial F'ulfill:nent

of theRequirements for the Degree

VASTER OF SCIENCE

ATHENS, G20RGIA

1979

ELECTROSTATIC SPRAYING OF PLANAR SURFACESSIMULATING TURFGRASS

by

R. C • A.~A~THESWARAN

Approved:

~. ~~;(!~Date~~Jlj/9HaJor Professor

~ ~~_ Date jlMY>~Cha~rman,-Read~ng Committee

Approved:

~ d-rf~ /97_q.~, _D- e

ACKN01!JLEDG Ei\1EN'rS

I would like to express my deep sense of gratitudeto Dr. S. Edward Law for his constant encouragement,guidance and understanding throughout this research work.

Thanks to Mr. David Gibson for his invaluableassistance in running the tests in the laboratory.

Thanks are also due to the U.S. Golf Associationand Carolinas Golf Association for part.i3~ support ofthis study.

Appreciation is extended to the faculty ffi1dstaffat the Agricultural Engineering Department of theUniversity of Georgia for making my stay at the Universitya pleasant one.

Appreciation is also expressed to [;Is.Susan Peeblesfor her prompt and efficient typing.

Last, but not the least, I would like to thank mynarents for their numerous sacrifices which enabled meto acquire my formal education.

iii

TABLE OF CONTENTS

. viii

7

111520

21

. . . . 21

1

6

7

24

27

29

212323

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Page

vi

1'urner Fluororr.eter5.1.6

LIST OF FIGURES •

LIST OF TABLES ••••

LIST OF SYMOOLS AND NOI'ATIONS

1.0 INTRODlCI'ION

2.0 OBJErI'IVES

3.0 REVIEW OF LITERATURE

3.1 Electrostatic Spraying .••••

3.2 Spray Drift •••..•3.3 Electrostatic Precipitators

4 • 0 HYPCYI'HESIS ••••••••••

5.0 EXPERIMEN'rAL MlALYSIS •

5.1 Apparatus •••.5.1.1 Ehlbedded-Electrode Spray

Charging Nozzle5.1.2 Field simulator

5.1.3 Turfgrass simulator.5.1.4 High-Voltage Plate Electrostatic

Precipitator ••••.•••••

5.1.5 Dielectric-Barrier Type Electrostatic

Precipitator .••••

iv

Page

5.2 Spray Deposition Analysis . . . . . . . . . 30

5.3 Experime..,talProcErlure 34

5.4 Design of Exper:\m:=>J1t. . . . . 34

5.5 Initial Set-up and Procedure . . . . 35

5.5.1 High-Voltage Plate Electrostatic

Precipitator ••••••••••

5.5.2 Dielectric-Barrier Type Electro-static Precipitator

5.6 Data A"1a1ysis •••

6.0 RESULTS AND DISCUSSIONS7.0 CONCLUSIONS .•••••••••

8.0 SUGGESTIONS FOR FUTURE S'lUDY

9.0 REFERENCES.

10.0 APPENDICES ••

10.1 'Iheoretical Maximum Target Deposi ti01'1

10.2 Average Net Fluorometer Readings .tor High-

Voltage Plate Type Electrostatic Precipi-

35

4445

46

64

65

67

70

71

tator ............... 73

10.3 Average Net Fluorometer Readings ForDifferent Types of Electrostatic Precipi-tators With the Nozzle at 00 Ll1clination. 74

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LIST OFFIGURES

Page

FIGURE5.1 Embedded-Electrode Spray Chargmg Nozzle . 22

FIGURE5.2 'furfgrass Simulator .............. 25

FIGURE5.3 High-Voltage Plate Electrostatic Precipitator 26

FIGURE5.4 Dielectric-Barrier Type ElectrostaticPrecipitator ...••.•••.•.

FIGURE5.5 Effect of Dilution on Fluorometer Readmg

28

32

FIGURE5.6 Ionization Current m the Vicmity of High-Voltage Plate Electrostatic Precipitator .••. 37

FIGURE5. 7 Schematic Diagram of the Experimental Set.-up •. 38

FIGURE5.8 Effect of the High-Voltage Plate ElectrostaticPrecipitator on the Spray Cloud Charge Level .• 40

FIGURE5.9 Set-up with the High-Voltage Plate ElectrostaticPrecipitator ...••...•........ 41

FIGURE5.10 Exhaust System to Catch the Spray Durmg the Set-up for the Tests 43

FIGURE6.1 Effect of Applied Voltage en High-Voltage PlateElectrostatic Precipitator Up:mSpray DeJXIsitionfor 00 Nozzle Inclination Angle .•..•••• 47

FIGURE6.2 Effect of Applied Voltage on High-Voltage PlateElectrostatic Precipitator Upon SprClYDep:>sitionfor 150Nozzle Inclmation Angle ....•.•• 48

FIGURE6.3 Effect of Applied Voltage on High-Voltage PlateElectrostatic Precipitator Ur:onSpray Dep:>sitionfor 300 Nozzle Inclination Angle ••••••• 49

FIGURE6.4 Effect of Applied Voltage on High-Voltage PlateElectrostatic Precipitator Up:mSpray DefOsitionfor 450 Nozzle Inclmation Angle ..•..••. 50

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Page

FIGURE6.5 Comparison of Different Methcx1sof mectro-static F-.cecipitation onto Planar Targetsfor 00 Nozzle Inclination Angle ..••.• 51

vii

LIST OF TABLES

TABLE6.1 Regression MJde1sfor Net F1uorareter Readingswith High-VoltagePlate Electrostatic Precipi-taror .

TABLE6.2 Analysis of Variance for the F.igh-VoltagePlateElectrostatic Precipitator .•.••••.•.•

TABLE6.3 Analysis of Variance for the Dielectric-Barrier'JYpe Electrostatic Precipitator •.••..

TABLE6.4 Analysis of Variance WhenPrecipitators areAbSeI1t ...............•..

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Page

52

54

55

56

LIST OF SYMBOLS AND NOTATIONS

E = Electric field strength (volts/meter)F = Capacitance (farad)F = Electric forceeg = GramsJ = Energy (joule = N.m)L = Liter (10-3m3)

m = Metermin = MinuteN = Force (newton)Pa Pressure (Pascal = rV 2,= ' m I

q = Electric charge (coulomb)

Prefixes:}.l -- r.1icro-(10-6)

c = centi- (10-2)

k = Kilo- (103)

m = lH11i.- (10- 3)

n = Nano- (10-9 )

jx

1.0 INTRODUCTION

The rising costs of pesticides and the desire to con-trol environmental pollutants calls for the development ofa more efficient pesticide deposition process.

At present. conventional methods of turfgrass manage-ment using pesticides result in as much as 80% of the spraybeing drifted to adjacent plots contaminating the environ-ment and the water supplies. Other problems related toinefficient pesticide applications are thei!. ~~noff,leaching and residue hazards which can cause acute set-backs in the environment. Restrictive legislation is beingimposed by the Environmental Protection Agency {EPA) withregards to environmental pollution thl:,oughFederal Insecti-cide. Fungicide and Rodenticide Act (PD!'RA). Fil.rthermore ..

the ever rising costs of pesticides has made it imperativeto improve the pesticide application methods.

Entomologists and plant pathologists have concludedthat smaller droplets, especially less than SOrro aremore effective in the control of pests. Field testsconducted by Buehring, et al. (~973)using mono drop sizesprays revealed that smaller drops and larger carriervolumes of herbicide produce greater weed control. Thesmaller droplet sizes result in better coverage of the

1

2

plant canopy and since control achieved in foliar pesti-cide application is proportional to the total surfacearea of the spray droplets. lesser volume of toxic materialwill be required in the field. This will result in savingsin the cost of pesticides, and reduction in environmentalpollution and resid~al amounts of pesticides in the crops.

Theoretical and experimental studies have indicatedtwo opposing droplet size eff8cts. which becomeincreasingly active as the spray is more finely a~omized.The biological effectiveness of insecticides. fungicidesand herbicides generally increases as the droplet sizeis reduced; however as the size becomes smaller. difficultyis experienced in the deposition of these droplets byinertial and gravitational forces alone, and they are quitesubject to the airborne drift problem.

The drift of these airborne droplets can be reducedby decreasing the time between the point of discharge ofdroplets from the spray nozzle and the deposition ontothe desired target. Since the smaller droplets havevery small gravitational forces associated with them. itis necessary to aid their deposition by additional forces.

At these small droplet sizes electrical forces canbe used to enhance the deposition of pesticide sprays.'l'heelectrostatic forces can be successfully used with themicron sized pal~ticles where the droplet charge-to-massratio is very high.

3

Research aimed at using electrostatics for chargingpesticide dusts originated as early as 1950 at MichiganState University. An electrostatic duster was developedand tested by Dr. Henry Bowen (1952) which worked well underideal atmospheric conditions. In the sixties the chargingof liquid sprays was investigated by Dr. Law (1966) atNorth Carolina State University. His later work at theUniversity of Georgia resulted in the development of aspray charging nozzle which uses the principle of inductionto charge the atomized droplets. This nozzle is beingused as a component in the electrostatic spraying systemswhich is being commercially developed 8nd tested in rowcrops.

It has already been shown for certain row crops thatelectrically charged droplets can increase the d8positionefficiency by almost seven times. The same principle ofelectrostatic charging with different electric field andspray configurati ons can be applied to turfgra.ss sprayingto increase the pesticide deposition.

Most row crops exhibit an extended pl~~t canopyacting as a I F'araday Cage' which attracts all the sprayonto the outer surface avoiding the interior zones whichharbor most of the harmful insects. For this reason thespace charge phenomenon al"1d turbulent transport is usedto deposit the sprays onto the row crops. But in thecase of turfgrass and similar two dirr.ensionaJplanartargets it is also possible to use an external field to

4

force down the charged particles onto the surface as perthe equation:

Fe = qpE (1)~where Fe is the electrical force (newtons) exerted on the

charged particle with a net charge q (coulombs), andp~E is the electric field vector (volts/meter) at the loca-tion of the particle.

The charging of the spray can be accomplished by thepresently developed induction method for row crops. Hence.there arises the need to investigate the several alternatemethods of generating an E field, over and above thatdue to the space-charge distribution of the charged sprays,which may offer practical applications in agriculture too.

Currently, electrostatic precipitators are being usedin the industry to separate particulate matter from smoke.The particulate matter is charged as it enters the smokestack. Along the stack a series of high-voltage electro-static precipitators are maintained at a polarity oppositeto that of the charge on the particulate matter. As aresult this charged particulate matter is attracted ontothese precipitators. Thus. electrical forces are beingutilized to deposit the charged particles onto the precipi-tators.

In a similar manner. charged pesticide sprays can beefficiently deposited onto planar type targets usingexternal electric fields. This will reduce the airbornedrift of the pesticide droplets from the appli.cation area.

5There are several ways of applying an electric field

to accelerate the spray droplets onto the target. Thespace-charge distribution due to the charged spray clouditself generates a strong electric field.

External fields can be imposed by means of a high-voltage plate which passes over the charged spray cloudafter it emerges from the charging nozzle. The high-voltage plate is a possible hazard to the operator, dueto the high electrical energy stored in it, however safeit might be designed and built.

Another way to introduce the driving field is to usea dielectric barrier above the spray cloud. Initialaccumulation of charge by droplet deposition on the barrierpolarizes the dielectric. The resulting free and boundcharges will provide the additional field necessary toaccelerate the charged spray onto the target.

This thesis is devoted to examining the performanceof these electric fields in the deposition of chargedspray droplets onto planar target surfaces like turfgrass.

2.0 OBJECTIVES

The overall objective of this thesis was to increasethe deposition efficiency of airborne spray particlesonto turfgrass by incorporating the electrostatic chargingand precipitation processes.

To evaluate the suitability and adaptability of thepresently available electrostatic-spraying nozzles forrow crops.

To determine the effectiveness of a high-voltageplate type electrostatic precipitator and a dielectric-barrier type electrostatic precipitator for increasingthe deposition of charged spray droplets on planartRrget surfaces.

6

).0 REVIEW OF LITERATURE

Static electricity is the oldest manifestation ofelectricity. The phenomenon of electrostatic attraction,manifested in the extraordinary property acquired by amberon being rubbed by another material. was known to theGreeks way back in 600 B.C. Twenty three centuries later.scientific investigations by Coulomb, Faraday and othersopened the doors for further study of electricity andelectrostatics.

).1 Electrostatic SprayingCharged sprays are being widely used in the industry

to enhance deposition by electrostatic precipi ta-tion.Ransburge first introduced the technique for paint sprayingin 1940. The spray of paint was charged by passing itthrough an ionized field, and then made to deposit bytravelling along the electrical lines of force to thegrounded object being coated.

Around 1950 Michigan State University initiated aproject on electrostatic charging of agricultural dusts.Charging of dusts was done through high-voltage coronadiSCharge. Field tests indicated higher depositionefficiency with charged dusts thw~ uncharged ones. Alsoa better and more even coverage was achieved. 'l'he

7

8

tests also revealed that dusting with charged dust couldbe carried out satisfactorily at 100% hwnidity. However.problems existed in the form of gaseous discharge throughpointed appendages of the plant canopy.

Law and Bowen (1966) investigated the charging ofliquid sprays by electrostatic induction. They found thationized field spray charging by corona from water dischargepoints on the wet induction electrode opposes inductionspray charging when the voltage at the electrode wasincreased beyond -JkV. This was a limiting factor to thedegree of charging to be attained by induction charging.Tests showed that inductive spray charging significantlyincreased the spray coverage of the bottom side of cottonleaves in the field. Induction charging requires thatthe spray be conductive.

Splinter (1968) developed a combined induction andionized field charging nozzle. He used an air streamto keep the induction electrode dry to prevent the onsetof corona discharge. However, this air curtain failedto protect the induction electrode from spray depositionand the resulting wetting above 18kV.

A mathematical model was developed by Bowen et ale(196L~)for a spherical cloud of charged dust or spray.The mathematical model for field intensity and potentialdistribution was experimentally verified with uniformcharged dust cloud with spherical boundary. Some of theconclusions they arri.ved at, based on this study were:

9

a) A uniform cloud of charged particles will experience amaximum potential between two grounded boundaries. b) 'l'hemaximum potential will be centered in case of tYTO parallelboundary pI a.'1es • In case of, a turfgrass-type target witha grounded boundary above the charged cloud, the Tnaximumpotential would occur midway between the target and thegrounded plate. When a charged cloud has only one boundary(in the case of turfgrass. when the grounded plate abovethe spray is absent) the potential and field intensitiesnext to the grounded surface are much greater than whenthe cloud is bounded by two grounded surfaces. Electricfield strength is linearly related to the spacing of thegrour:ded surfaces (e.g .• between two leaves). A reductionin air volume holding a given charge increases fieldintensities. The charged ,dust deposition at a nozzleair velocity of 48.27 km per hour ()O miles per hour) werefound to be twice that at 144.81 km per hour (90 miles perhour) . The deposition was found to be linearly proportionalto the charge density level of the cloud, since the electricfield intensity is linearly related to the charge densitywithin the cloud.

Kelly (1978) theoretically investigated the electro-statie spray injection of benzene into an internal combusti.onengine to compa~:'€'the statist~_cal equili.brium mode ofelectrostatic spraying. Ope of his conclusions was thatelec:trostatic spraying is dorr:inatsd by droplet-surfaceelectric-field strength an:l i~3 independent of' droplet polarity.

10An embedded-electrode electrostatic-induction spray-

charging nozzle has been developed by Law (1978) tosuccessfully charge most pesticide sprays. A commerciallyavailable pneumatic atomizing spray nozzle was modifiedto incorporate a~ annular brass induction electrode.The nozzle charging assembJy utilized a DC-to-DC voltageconverter circuit powered by R low voltage supply (e.g.,

a l2v tractor battery) to induce charge onto the spraydroplets. This nozzle is c~rrently being used as acomponent on a commercial prototype electrostatic row-cropsprayer. The saine type of nozzle was used for producingcharged spray clouds for all experimental work relatedto this thesis.

Lmle (1978) designed laboratory test apparatus tosi:nulate a high clearance electrostatic row-crop sprayer.Variables like spray velocity. target position. spray cloudpath. liquid flow rate and atomizing pressure could becontrolled in this simulator without the extraneousinterventions like crosswind etc. This spray simulatorwas used for the experiments related to this thesis.

Lane (1978) also investigated the corona dischargeoccuring from th8 plill1tsundergoing electrostaticspraying and the effect of drought stress on the chargetransfer capability of the plant. He used a metallicsph8re with a discharge point to document the effect ofpointed appendages on the electrostatic deposition. Theresults revE:aled that with discharge points the deposition

11

was maximum at -4pA of cloud current and was 3.7 timesthat with uncharged droplets. With a cabbage target, themaximum deposit was found to occur at -6/,A of cloudcurrent and was seven times that with uncharged droplets.Beyond -61tA the corona discharge from the leaf edges ofthe cabbage leaf are strong enough to impede furtherincrease in deposition. The test with metallic spherewith discharge points absent resulted in a maximum depositat -8~A and 6.6 times that with uncharged spray. Thetests using metallic cotton leaves revealed a maximumdeposit at -J~A of spray cloud current, corona dischargetaking over much sooner due to increased number of sharpdischarge points. To investigate the effect of droughtstress on the charge transfer capabilit~ a high voltageplate generating an intense electric field was passed overthe plants. The plants showed only a slight reduction intheir abil ity to tr'ansfer charge after many days ofdrought stress (beyond recovery).

3.2 Spray DriftDroplets smaller than lOO~m diameter will drift an

appreciable distance under most meteorological conditions,and droplets larger than l40~m in initial diameter havevery low drift potential during ground applications(Smith, et al. 1975).

Extensive research has been done on drift evaluationat the University of California, Davis (Yates, at ale

121966). Tracer techniques were used to detect the spraywhich has drifted. On the basis of their work withforage crops, Yates, et al. arrived at an empiricalexpression for pesticide residue accumulated on alfalfadue to drift. The expression could be used for quantifyingdrift as far as 0.8 km (0.5 mile) downwind from theborder of the treated area.

Courshee (1959) also investigated the drift ofhydraulically atomized sprays. He devised a spray driftsampling box to measure the part of the spray susceptibleto drift. Both theories of measurement showed thatat ffilY windspeed, drift can best be reduced by keepingthe nozzle, and using flat rather than cone typenozzles, and by avoiding small drops in the sprays. 'l'helatter can be achieved by using low pressure on-groundsprayers and by a combination of low pressure and modifi-cations of the spray liquid when using aerial sprays.

Smith and Threadgill (1973) developed an inexpensivefield sampler to quantify drift. The air sampler hada blower powered by a motor. Air was drawn in througha 10 em diameter opening in the filter at the intake endof the blower. The opening had a thick square wire meshto serve as filter support. The filters used were theones used in common household coffee percolators.

Threadgill and Smith (1975) investigated the potentialdrift of small droplets. Utilizing uniform size dropletsproduced by a spinning disc sprayer, Kromekote paper cards

1.3

were used at different sampling stations to obtain dropletnumber and size data. Also a field sampler (Smith andThreadgill, 1973) consisting of a small air filter unitand a squirrel cage blower was used at each station. Driftsamples were collected 8.13 m (26.67 feet) downwind alongthe top of the plant canopy and to a height of 6.1 m(20 feet) above the canopy. They found an appreciable~~ount of drift of droplets 100ym and less in diameter,at as low windspeed as 2.03 km per hour (1.26 miles perhou~. The specific gravity of 0.82-1.0.3 used in thestudy had no significant effect on the study. On the basisof their study they also developed a model for spraydrift based on meteorological conditions and spraycharacteristics.

Thus, in the absence of additional droplet-depositionforces it has been well established that meteorologicalconditions are the dominant factors controlling drift ofsmall droplets of sizes less than 100J.1m.

Various methods have been suggested to eliminate orreduce drift of these airborne sprays. Smith, et al. (1977)used A.C square wave to create alternate bands of oppositelych;3.rgeddroplets to encourage droplet size uniformity.Their assumption was that the large droplets in a given ba.'1dwould overtake and coalesce with several small droplets ofopposite charge in the band just ahead of them. However,they found no significant net reduction in the small drop-let sizes. This was due to the addition of small droplets,

14caused by disruption of the spray sheet, when the square wavereversed polarity resulting in steep voltage gradient.

Elimination of small droplets from the nozzle wasalso the objective of the research work done by Roth andPorterfield (1966). They positioned ten jet nozzleshorizontally on a 5 em spacing at an angle of 100_150 withrespect to horizontal from the catchment channel (where thespray was received). The jet stream was made to pass througha short length of metallic tubing to which a high voltage of1 kV was applied. Hith the horizontal jet positioned 60 emabove the catchment surface, the uncharged stream deposited74% of the spray in a 5 em wide band while the chargedstream deposited 74% of the spray over a 20 em width.

Kaupke and Yates (1966) worked with sprays whoseviscosity and physical properties were modified by addi-tives. The modified solutions were aerially sprayed,and tracer techniques were used to evaluate the spraydrift. They tested the drift-control efficacy of thethree different types of sprays: namely, particulatespray, invert emulsion and normal emulsion. In particulatespray, the water which forms the substrate is absorbedby polymer particles, and these swollen polymer particlesare separated during the atomization process. An invertemulsion is essentially a water-in-oil type emulsion, aDdthe normal emulsion is oil-in-water type emulsion. Thesedifferent types of emulsions differ in their viscosity,

15which is used as a basis to determine the type of emulsion.Their tests with these three types of sprays showed a two-fold reduction in drift by using invert emulsion ascompared to normal emulsion and a seven-fold reduction indrift with particulate spray as compared to normalemulsion. The pest-control efficacy of the modifiedspray "particles", however, remained questionable.

A vibrating boom sprayer was developed by Reimer(1964) for drift control. The plastic boom had sprayholes spaced 6.25 cm apart and was vibrated by carnmechanism driven by the tractor power take-off. Fieldtests revealed a two-fold reduction in drift with thevibrating boom sprayer.

3.3 Electrostatic PrecipitatorsElectrostatic precipitators are extensively used in

industry to combat air pollution. They are being primarilyused to remove coal fly ash from the eXhaust of powergenerating plants.

The particles of fly ash are charged usually bycorona discharge, as they enter the stack. In one designof these industrial electrostatic precipitators, as theflue gases move upwards in the stack, they are attractedtowards the circular collecting surfaces within thestack. These collectors are maintained at a very highpotential with their polarity being opposite to thatof the charged particles. The solid particles which are

16collected on the surface, are periodically scraped forremoval and disposal.

Abundant literature is available in the area ofindustrial electrostatic precipitators with exactmathematical equations being developed for each type.White (1974) has been engaged in the research, developmentand field application of industrial electrostatic precipi-tation for almost two decades.

Melcher (1977) describes a variety of devices likespace charge precipitator, electrostatic precipitator,self aggloromerator, and charged droplet scrubber asmeans to separate the particulate matter from a gas stream.

Only one literature reference was found on electro-static precipitation of a charged particle cloud. Point(1967) has a patent on a mobile electrostatic sprayingsystem. His system had an ionizing electrode for chargingliquid droplets or powders. Beyond the charging nozzlean auxillary field was established by an high-voltage elec-tric screen. He suggests the use of a flat array of tubularor rod electrodes or wires all interconnected for the elec-tric screen which was maintained at a potential of around200 V. A polyethylene sheet or any plastic material con alsobe used as an electric screen. However, they must possesssufficient mechanical strength and imperviousness toweather the heavy-duty service conditions to which thescreen will be exposed. The chosen material for theelectric screen should also have high electric resistivity_

17The screen may be perforated to reduce its vulnerabilityto wind. The screen should extend rearward from thenozzle a distance equal to that traversed by the tractorin about one second. It is, however, not required tohave the screen longer than the distance covered by thetractor in three seconds. The nozzles in Point'selectrostatic system were oriented 300 forward from thevertical. An actual test was conducted to measure theeffect of this dielectric screen. Four nozzles discharged2g of solid powder and 100 roL of air per second which werecharged by the ionization occuring at -150 kV. Visualobservations showed that the cloud settled more rapidlyover the crops with the dielectric screen rather thanwith the screen absent.

3.4 Electrostatic HazardsStatic electricity can be stored. An excess build

up of the stored charges can result in a dischargethrough ionization. The ensuing spark can act as asource of ignition. A human being who has ~D electricalcapacitance of 300 pF can produce a spark of 15 mJ upondischarge (Moore 1973). Gasoline requires 0.2 mJ toignite.

The three criteria for the existence of an electro-static hazard are: chargin~ the material, leakage ofsuch charges causing local spark, and ignition of materialcausing explosion (Hasse 1977).

18The ch~~ces of hazards from the accumulation of

charges may vary according to given type of environmentand the type of problem involved. Information basedon experience alone can give the precise knowledge ofthe circumstances of these hazards.

Some of the methods being adopted in the industryto avoid charge accumulation are (Hasse 1977): electro-static grounding of all conducting surfaces, increasingsurface conductivity of all rnaterials through raisingrelative humidity or through surface treatment, increasingconductivity of air, proper choice of contact material.

In the area of electrostatic spraying of pesticides,the absence of combustible or explosive materials eliminatesthe need to prevent the buildup of charge from noncon-ducting parts like the dielectric screen as their dischargecaIlnot ignite the spray mixture.

The presence of a high-voltage on the high-voltageplate electrostatic precipitator is a threat to theoperator. Even a small electric current can seriouslydisrupt the cell functions in that portions of the bodythrough which it flows. Electric current can prove tobe fatal either through malfunction in the muscles ofheart and lungs or through burns. At a current of 0.001 Aor more a person can feel a shock. At a current 0.01 A,a person is unable to release the electric wire held inhis hand because of the contraction of his hmld muscles.Currents larger than 0.02 A through the human body can

19paralyze the respiratory muscic~, 1\t curr'ent::3 oJ' about.

0.1 A passing through the regions of the heart wil1 nhod,

the heart muscles causing ventricular fibrillations reld

eventually stopping the heart. F::naJly curn>nts cf 1 A

and greater through the body tis::' e Vi.1.11 C<Xl):,8 se;" 10!:c)

burns. Hence 1 it is necessary trpr'o"tect t.)10 opcJ'atnr'

from coming in contact with the f'.i.{")'j .. '/0] tage on '1:",8 tigh.,

voltage electrode plate.

4.0 HYPOTHESIS

a) The electrostatic sprayin(~ system as developed

for :r:o...'1 crops can be utilized fen.' t\)rfgra.ss~ type ta:;:'getf~

with modification in the or:i.ent8.tior: of th(~ n07,zles and

with the addition of an electrostatj(; pred.pi tator~;o

reduce airborne drift of droplets,

b) The addition of an elec'Ll SLa. tH; precipit; ..l,or'

to provide an external field to acceJerate th(~ chaTU,t;U

spray onto the planar targets can be 2_CCO~:lpl ish 9d E<] the;)'

by means of a conuucti ve high-vol t3.ge electrode pI ~~te

or by a dielectric barrier posi tLonee]. above the eh8_r{~\;d.

spray c} oo.d,

2CJ

5.0 EXPERIMENTAL ANALYSIS

Tests were conducted in the "Particulate EngineeringLaboratory" at the Agricultural Engineering Departmentto analyze the charged spray deposition onto the turf-typetargets when utilizing the various electric fields.

5.1 Apparatus

5.1.1 Embedded-Electrode Spray-Charging NozzleThe electrostatic induction type charging nozzle

developed by Law (1977, 1978) was used in this research.This nozzle uses compressed air to atomize the liquiddropiets and produces a narrow range of droplet sizeswi th a mass median diarneter of approximately 30 I.) m (Fig.

5.1) ~ ~<I

The liquid spray is charged by induction as it goesthrough th0 nozzle. A high voltage is maintained onthe induction electrode which is embedded in a modifiedau' cap so that the spray droplets are charged negatively.The degree of charging is controlled by adjusting theD. C. Po;vcr Supply, supplying powcr to the electroniccircuit within the nozzle which in turn supplies thehigh voltage to the induction electrode.

The liquid flow rate was maintained at 73 mL/rr;in,and the atomizing air pressure was set 206 1!~P8. (30 Psi).

21

EMBEDDEDELECTRODE

CAP

FIG. 5.1

-<t=--HIGH VOLTAGE D.C,

INPUT

~--COMPRESSED AI R

INPUT

~Llo..UID INP.UT

EMBEODED- ELECTRODE SPRAY CHARGING NOZZLE

NN

23The charge levels were measured by an ionization probeand a Keithley 610-c electrometer. The spray cloudcurrent was measured along the centerline of the spraypattern. approximately 3 cm away from the tip of thenozzle. A typical 20ym droplet would attain a charge, .

. ":11~of 2 x 10 coulomb and a charge-to-mass ratio of4.8 x 10~6 coulomb/g.

5.1.2 Field SimulatorThe tests were conducted utilizing the "field

simulator" developed by Lane (1978). It has a nozzlecarriage riding over a frame simulating a high clearaDc/'')spray unit making a pass over the crops. rrtlC carriagehas three embedded electrode induction charging nozzles.Only the central nozzle was used for the tests. Thespray liquid was supplied to this nozzle through a flow-meter connected in series with a reservoir mounted onthe carriage. The ground speed of the nozzle as itsprayed past the target was 4.8 km per hour (3 milesper hour).

5.1.3 Turfgrass SimulatorIt was originally conceived to conduct the whole

series of tests on actual turfgrass in the laboratory.Ho'wever, the ,~()mplexity in the deposition analysis inthe actual turfgrass necessitated the tests being conductedon a flat aluIninum target. The 0.3 m x 0.3 m alu!11inum

target was seated at the center of a 1.20 m by 1.35 m

24

target holder constructed of 1.91 cm (0.75 inch) thickplywood (Fig. 5.2). The target holder had a squarehole cut at its center and had a seat to hold the targetflush with the top surface of the target holder. Thetop of the target holder was covered with aluminumfoil and was electrically grounded to simulate anextended turfgrass target. Ji' ~

The target holder was horizontally positioned 0.) mbelow the face of the electrostatic precipitator withthe center of the aluminum target centered along theline of travel of the nozzle.

5.1.4 High-Voltage Plate Type Electrostatic Precipitator/'The high-voltage electrode plate used by Lane (1978)

to establish a time varying field to study the effectof drought stress on charge transfer capability ofplants. was used here for applying the external drivingfield on the charged spray cloud.

This high-voltage electrode plate was made of1.83 mm thick galvanized sheet metal measuring 1.04 msquare. A 22.2 mm diameter copper pipe was brazed

v1t'\to the edge of this plate to avoid stray corona discharge(Fie. 5.3). The electrode plate was mounted on a plexiglasssheet for safety through 76.2 mm long plexiglaos spacers onthe four corners. The plexiglass sheet was attached to twounistrut channels running along its length with vvhich

the whole assembly was mounted onto the nozzle carriage of

25

FIG. 5.2 Turfgrass simulator

26

FIG. 5.3 High-voltage plate electrostaticprecipitator

27the field simulator.

The high voltage was applied from a Sorenson230-6p-R&D D.C. power supply. A Belden 8866 cathode-raytube lead wire rated for 40 kV was used to supply thepower to the high voltage plate. Since the spray cloud wascharged negatively, the high-voltage plate was maintainedat a negative potential with respect to the ground.

5.1.5 Dielectric-Barrier Type Electrostatic PrecipitatorA 1.04 m square framework was constructed out of

plexiglass to hold the 0.1 mm thick polyethylene sheetto be used as the dielectric-barrier type electrostaticprecipitator. The electrical properties of polyethylene

(}1,)

are given below:6 13D. C. Resistivity = 1. x 10 ohr!t- cm

Dielectric Constant = 2.3Dielectric Strength = 18.11 \rolts/.u.m

Loops were made on either ends of the polyethylenesheet to hold plexiglass rods. The polyethylene sheetwas stretched over the bottom side of the frame, and thetwo rods holding the polyethylene sheet were tied together(Fig. 5.L~).

The dielectric precipitator was mounted 0.3 m abovethe target in the saIne fashion as the high-voltage electrodeplate. Care was taken not to ground any part of thedielectric screen.

28

FIG. 5.4 Dielectric-barrier type electrostaticprecipitator

295.1.6 Turner Fluorometer

A Turner model 111 Fluorometer was used toquantify the spray deposition onto the target. Thefluorometer measured the amount of fluorescent materialin the sample which was introduced inside the fluorometer.

Blaze orange, a DAY -GI,O GT-series fluorescent pigment,was introduced in the spray solution. After spraying andwashing the-target, the w~sh solution was introduced.inside the fluorometer, to quantify the amount offluorescent tracer deposited onto the target, which isdirectly proportional to the amount of spray deposited onthe target.

A 7-60 Turner filter which passes a wavelength bandof 300-l4-00 nm was used as the primary filter in the fluoro-meter. A 2A-15 sharp cutoff filter which passes allwavelength longer than 510 nm was used as the secondaryfilter.

The fluorometer~ initially calibrated with knownconcentrations of tracer solutions. For a range oftracer concentrations the corresponding fluorometerreading minus the background reading due to the distilledwater was recorded. This net fluorometer reading isproportional to the tracer concentration in the solution.Three replications of the data were obtained.

A regression equation for the average observationswas found to "Le:

y = 4.7795042 + 4.2646289 x

where y.- Tracer concentration ( g/liter)x = Average net fluorometer reading

JO(2)

5.2 Spray Deposition AnalysisFluorometry as described by Lane (1978) was utilized

to quantify the spray deposition onto the target. Theobjective of this research work was to increase depositionof the pesticide spray onto turfgrass targets and toreduce drift. An increase in deposition is an equivalentreduction in drift.

Preliminary investigations with the use of fluoro-metry to quantify the spray deposition with flat aluminumtargets presented some unusual problems. r£he aluminum panused to clean the target was found to contribute anexcessive amount of background fluorescence. Replacementof the aluminum pan by one constructed of stainless steelsolved part of the problem. The aluminum target and thestainless steel pan still contributed some fluorescenceeach time a clean target was washed. This fluorescencewas termed as the "background" fluorescence. At thebegInning of each day of the test. the background readingfor the day was obtained by w~shing a clean unsprayedtarget in the same way as a sprayed target.

A standard tracer spray solution (1.5 g of Blaze orange0.1 g of NaCl salt, 1 mL of 'T'riton X-IOO surfactant) witha resisti vit.v of 5 x 103 oht:l-cm all in 1000 mL of distilled

31demineralized water) was used to spray the target. Afterthe target was sprayed the target was carefully retrievedfrom the target holder, and the sprayed side was washedin the stainless steel pan containing 250 mL of washsolution (1 mL of Triton in 1000 mL of distilled water)with a nylon brush for four minutes. Rhythmic motion andconsistent pressure on the brush were maintained to reducethe error between different replications. At the end ofthe four minutes, the target was sloshed inside the panand turned over to be washed for about 15 seconds.

The target was sloshed in the pan again by givingthe pan a rocking motion. The target was then carefullyremoved from the pa'1. The nylon brush was cleaned with50 mL of wash solution makin~ the total amount of washsolutioll used to 300 mL.

Most of the charged sprays would result in enoughtracer to be deposited on the target to get a fluorometerreading above 100. As a result, after washing the target,the wash solution if read directly on the fluorometer,would saturate the maximum reading on the fluorometer.Hence, it was necessary to dilute the wash solution afterclellilingthe target with fresh wash solution.

An investigation on the effect of dilution on theactual fluorometer reading revealed that the straightline plot of results did not pa3s through ths origin(Fig. 5.5). It had a vertical intercept of 11 scale ....un1. ...s

which was roughly the fluorscence reading of the fresh

32

o70

60

50

(!) 40 0/0z0<tWn.::

30n:wI-Wz:0 200::0::J-1LL.

10

0 t _-1~. I I

0 I 2 3 4

DILUTION FACTOR

FIG. 5.5 EFFECT OF DILUTION ONFLUOROMETER READlNG.

J3wash solution with no tracer in it.

All the sprays except the uncharged ones at thenozzle angles of 00 and 150 with respect to the horizontalwere diluted with two parts of fresh wash solution forone part target wash solution (i.e. diluted three times).In the initial stages of the tests, the actual fluoronleterreading was first multiplied by three to take care ofthe dilution, and then the background fluoroIllPterreadine(the actual fluorometer reading of the wash solution aftercleaning a clean unsprayed target) was subtracted from it.When the error in doing this manner was realized afterfInishing the tests with the high-voltage plate electro-static precipitator, the test data was corrected.The background fluorometer reading was first Sl1'h+,Y,'8_cted,

from the actual fluorometer reading. This number wasmultiplied by three to give the net fluorometer reading.

The net fluorometer reading was related to the amountof tracer deposited as follows:

The calibration curve of the fluorometer gavethe concentration of the tracer in the wash solution as:

y ::: o. OOL~779 5042 + o. ooJ}26!+628 x (2)

Tracer concentration in the target washsolution (mg/liter)

x = The net fluorometer readingSince 300 mL of wash solution was used to wash the targ8t,the amount of tracer (mg) in the wash solution would be(O.J)(y). The actual oIrlountof spray (mL) deposited on the

target would be (0 ..3) (Y/1.5)a.3 the concer:tration or tracerspray solution was 1.5 g of tracer per liter of spraysolution.

5.3 Experimental VariablesThe following were the class levels of the different

independent variables; the dependent variable being thefluorometer reading.With the high-voltage plate electrostatic precipitator:(a) spray cloud current - a f-1 A, -2/IA, -4f1A, -6pA and. -~(/A.

(b) voltage applied tothe high-voltage - 0 kV, .-IOkV, -20kV and -)0 kV.electrode plate

(c) angle of the nozzlewith respect to th.e- 0°, 15°, 30°, and 45°.horizonal

With the dielectric-barrier type electrostatic precipitator:( a)

( b)

5.4

spray cloud current - a,u A, -2f1A, -~L(A, -~u..A aYld -~LIA.

angle of the nozzlewith respect to the - 0° (where the inertial effecthorizontal of the spray is minimum)

Design of ExperimentA factorial design ViaS used to doc1..1mel''.tthe effect

of the high-voltage plate electrostatic precipitator ateach nozzle inclination. The angle of inclination ofn07,z.Le was randomly chosen, 'and the factorial design wasu~.;cd to vary the othar two vari abIes: namely, the spray

cloud current 2nd vol t2.f';eaplJlied to the high-voltageeleci;rode plate.

The tests with dielectric-barrier type electrostaticprecipitator were done with the angle of inclination ofthe nozzle at 00 (i.e. horizontal) to minimize inertialeffects in order to emphasize any effect which mightresult from a generated driving field.

Also, a test was run with the nozzle horizontal withno precipitators present to verify the effect of'groundplane above the spray cloud (i.e. with high-voltageplate electrode at 0 kV) on the deposition of the c}largedspr~.1.Yonte the target.

At least three replications for each observationwere obtained. More number of replications were resortedto when the dl.ffprence between the net fluorometerreadingf; in the data was beyond 20 scale readings. 'l'hethree readings that were grouped. together' the closestwere retained in the data on the assumption that the netfluorometer reading for each observation is normallydistributed about its mean.

5.5 Initial Set-up and Procedure

5.5.1 High-Voltage Platt'lElectrostatic PrecipitatorInitial studies were done to determine any effects

which t.hepresence of high voltage might have upon thecharging of the spnW liquid. The two main concerns

35

were: (a) whether the high voltage on the electrodeplate was causing an ionization curren"!;reading on theprobe used to measure spray cloud current. and (b) docs

36

the high voltage on the electrode plate affect thecharging ability of the nozzle's electrode.

Initially, with the high voltage on the electrodeplate, the ionization current in its vicinity was measuredwith no spray. At a distance of 0.2 m from the edge ofthe high voltage plate the ionization current was only15% of the minimum spray cloud current (-2j<A A) to beused in the tests (Fig. 5.6). Hence, it was decided toposition the nozzle orifice, which contains the chargingelectrode, at least 0.2 m away from the plate to ensurethat the ionization current measured by the probe is mostlythe spray cloud current.

The high-voltage plate electrostatic precipitator wasmounted on the carriage of the spray simulator. Thenthe target holder was placed 0.3 m beneath it with thecenter of the target along the line of travel of thenozzle. The electrostatic spraying nozzle was mounted sothat its orifice was coplanar with the high-voltageelectrode plate and trailing 0.2 m behind the precipitatorin its trav91 (Fig. 5.7).

'1'0 study the effect of the hi{::~hvoltage electrodeplate on the charging ability of the nozzle's electrode,the spray 'liasturned on with W3.ter spr:iying out of thenozzle. TIlE: .ionization probe wae, posj tioncd 0.03 In in

front of the nozzle orifice. 'l'hech:..lrgelev2] s on thecloud waa set for different levels of ionization currentwith no voltage on the high-voltage electrode precipita"tor.

37

. A. .rIO~lIGH-VOLTAGE~ 11r- PLATE

TO ELECTROMETER

o 0= 0.'75 m

o 0= O.15m

o

/o 0 0

O~~10 20 30

ELECTRODE PLATE VOLTAGE (- kV)

IONIZATION CUHRENT IN THEVICINITY OF THE HIGH- VOLTAGEPLATE ELECTROSTATIC PRECIPITATOR

I- 0.6zwa:: 0.50:::J0

z 0.40I-<! 0.3Nz0

0.2

0.1

0.00

APPLIED

fiG. 5.6

1.0

0.9

0.8

-«:i. 0.7I.......

CHARGING"./ SPRAY NOZZLE

DIRECTION OF TRAVEL... n"",,_~~

ALUMINUMTARGET./

ELECTROSTATIC'----PRECIPITATOR

O.3m

TARGETSIMULATOR

FIG. 5.7 SCHEMATIC DIAGRAM OF THEEXPERIMENTAL SET UP

wex:>

39

Then the high voltage on the electrode plate was turnedon. As this high voltage was increased, the resultingspray cloud current was monitored. It was found thatthe ionization current so measured increased at a rate of-0.06ftA per kV of applied voltage on the electrodeplate (Fig. 5.8). Since the electric field gradienton the spra~ due to the charging electrode of the nozzle.is very intense compared to that of the high-voltageelectrode plate. it was not clearly knovm as to whythis increase in the ionization current of -0.06 /J- A/kVoccured. It could be that with the spray in the vicinityof the high-voltage plate electrostatic precipitator. theionization current contribution of the high-voltage plateis more than that with no spray beneath the precipitator.

The spray cloud current levels for the tests wereset with no voltage on the high-voltage plate electros.taticprecipi tatol~.

For the tests with 00 nozzle angle, the nozzle wasmounted 0.15 m below the high-voltage plate and 0.2 mav18.Y fy.'omits edge (Fig. 5.9). 'nlis ensured that the spraydid not impinge directly onto the electrostatic precipitator.

Th::!spraying wrts done alonG th~"'.direction of travel,wi.~h the spray--ch::.trgingnozzle traili;lg the precipitator.Thif; way the time of residence of tho charged spraydropl\~ts in the extecnal c1rivi!i{S field was maximized.

The atomizing pressure was set at 206 kPa (30 poundsper ::;qlJ.are li"wh) . The spray cloud CUITent was set at the

_o~o---oI I

10

9

10 20 30

40

APPLIED ELECTRODE PLATE VOLTAGE (- k V)

FIG. 5.8 EFFECT OF THE HIGH - VOLTAGE PLATEELECTROSTATIC PRECIPITATOR ON THESPRAY CLOUD CHARGE LEVEL.

41

FIG. 5.9 Set-up with the high voltage plateelectrostatic precipitator

Lj.2

desired level using tap water as the spray liquidwithout any voltage on the high-voltage electrostaticprecipitator.

To prevent the spray from contaminating the targetwhile the variables were being adjusted, the spray nozzlewas directed into an exhaust system (Fig. 5.10) consistingof an industrial vacuum cleaner with no filters and aset of connecting pipes. Thus, the spray was suckedinto this vacuum cleaner until ready for the test.

Once all the variables were preset, the target wasplaced flush with the target holder after cleaning thetarget seat. Fluorescent tracer solution was pouredinto the o.uxillary reservoir which fed into the testnozzle through a flovmeter. The spray cloud currentwas readjusted, the flow rate was checked for 73 mL/min,and the high-voltage plate was turned on and the voltagewas set at the desired level.

Then the exhaust system seate1 on a revolving stool,was turned around directing 1;hespray toward::.;the targetwhi '.e simul taneously starting the spray nozzle's travelover the tarGet.

At the end of the spraying, the high voltage to theplate mid D.C. input to the char~in[ nozzl~ were turnedoff. After giving sufficient tiDO for the electrode plateto r0(:v~h e:ro'Jndpotential, the target plate was removedfro~ its holder and taken for washinG.

43

FIG. 5JO Exhaust system to catch the spray duringthe set-up for the tests

The nozzle c2.rriage was brought back to the startposition. The nozzle was flushed with tap water andreadied for the next test.

5.5.2 Dielectric-Barrier Type Electrostatic PrecipitatorThe dielectric-barrier type electrostatic precipitator

was mounted on the carriage and the target plate wasplaced 0.) m below it (Fig. 5.7).. The nozzle was setat 00 angle with horizontal and positioned 0.15 mbelow the dielectric surface and 0.2 m away from its edgeto make the test conditioEs the same as that with thehigh-voltage electrostatic precipitator.

The set-up procedure was very similar except forthe absence of the high voltage setting.

Before the stoTt of the test. the charged spraywas allowed to satura~e the dielectric screen with enoughfree and bound cL3.rges by turning the spray on for fiVf!

minutes with the in18t end of the spray catching syst(~lllaway from the spray-path.

The spray cloud current and the liqu.id floiNratewere set using the exhaust system to prevent contcuninationof target area. The target was positioned on the targetholder and was spra:y"8da3 with the high-vol tage electro-static precipitator.

45

5.6 Data AnalysisThe average value of the three replications of each

observation with high-voltage plate electrostatic precipita-tor was utilized in the statistical analysis of high-voltage plate electrostatic precipitator. Using theGeneral Linear Models (GLM) procedure in the StatisticalAnalysis System (SAS) the ANOVA table for the data wasconstructed. Also the regression-model equation wasevaluated.

The three replications associated with eachobservation were entered in the statistical analysisof data from dielectric-barrier type electrostatic precipi-tator. The statistical analysis of data with no precipita-tors present also was conducted with all the threereplications. This was done to increase the error degreesof freedom in the statistical analysis for a strongerstatistic.ANOVA tables and regression- models equationswere evaluated for both the sets of data.

Finally, at a moderate spray charge level of -6~ A,the different treatments were cO:Tlparedfor significantdifferences.