Farm Machinery and Equipment-I 3(2+1)

Lesson 29. Spraying and dusting equipment: atomising devices & pumps for sprayers, constructional details

TYPES OF EQUIPMENT:

a) Boom-type field sprayers

b) High-pressure orchard and general purpose sprayers

c) Air-blast sprayers which utilize an air stream as a carrier for sprays

d) Air-craft sprayers

e) Granular applicators

f) Ground- rig dusters

g) Air – craft dusters

h) Aerosol generators, which atomize liquids by thermal or mechanical means and are widely used for control of mosquitoes and other diseases – transmitting vectors and have limited application for agricultural pest control.

MANUALLY OPERATED:

1) Hand atomizer: Used in house.

2) Bucket type: Used for small fields.

3) Knapsack

Sprayers are used for various purposes:

1. Application of insecticides to control insects on plants

2. Application of fungicides to control plant diseases

3. Application of herbicides to kill weeds, either indiscriminately or selectively

4. Application of pre-harvest sprayers to defoliate or condition crops for mechanical harvesting

5. Application of hormone (growth regulating) sprays to increase fruit set or prevents early dropping of fruit 

6. Application of sprays to thin fruit blossoms

7. Application of plant nutrients (sprays) directly to the plant foliage

8. Application of biological materials such as viruses and bacteria, in spray to control pests

- Because dusts have  much greater drift hazard and lower deposition efficiency than sprays, most pesticides application other than granular soil treatments are now in the form of sprays, usually water emulsions, solutions or suspensions of wet table powders.

 The main function of sprayers are:

1) To break the liquid into droplets of effective size

2) To distribute them uniformly over the plants

3) To regular the amount of liquid to avoid excessive application

Desirable quality of sprayer:

1) The sprayer should produce a steady stream of spray materials in the desired fineness of particle so that plants to be treated may be covered uniformly.

2) They should deliver the liquid at sufficient pressure so that it reaches all the foliage and spreads entirely over the sprayed surface.

3) It should be light yet sufficiently strong, easily workable and repairable.

BASIC COMPONENTS OF A SPRAYER:

1.	Nozzle body	6.	Pressure regulator	11.	Nozzle disc
2.	Swirl plate	7.	Cut-off value	12.	Nozzle cap
3.	Filter	8.	Spray boom	13.	Nozzle tip
4.	Over-flow pipe	9.	Drop legs	14.	Spray lance
5.	Relief- value	10.	Nozzle boss	15.	Spray gun

 

Nozzle Body: Main component on which other components of nozzle fit.

Swirl Plate: It is a part of  cone nozzle which imparts rotation to liquid passing through it.

Nozzle Disc: Component containing the final orifice of nozzle usually in cone nozzle.

Nozzle Cap: Component which retains the assembled parts in or on a nozzle body. The nozzle disc or tip may be integral with the cap.

Spray Gun: It is a lance from which the spray is readily adjustable during the operation.

Spray Boom: It is spray lance with spray nozzles fitted to a head mounted at right angle to the lance.

Filter: Component to remove suspended matter larger than a pre-determined size from fluid.

Over-flow Pipe: It is a conduit through which excess fluid from a pump is by-passed by the action of relief value or pressure regulator.

Relief-value: It is an automatic device which opens when the pressure of fluid or gas reaches a pre-determined value.

Pressure regulator:  It is an automatic device to control the pressure of fluid or gas within a range of settings.

Cut-off value: It is mechanism between pump and nozzle to control the flow of liquid from sprayer.  It is operated by hand.

Nozzle boss: It is a lug on spray boom or spray lance to which a nozzle body or cap is screwed.

Nozzle tip: It is the component containing final orifice of nozzle usually a fan nozzle.

Spray lance: A hand-held pipe through which liquid reaches the nozzle mounted at free end.

TYPE OF SPRAY: Sprays can be:

1) High volume spray:    (More than 400 litres spray/ha).

The dilute liquids are applied by hydraulic machines. It consumes more time and labour. Hand operated - pressure developed 1-7 Kg/cm, mechanically operated or power sprayer - pressure 3-8 Kg/cm².

2) Low Volume Spray:   ( 5 to 400 li/ha)

It uses air stream from fan as  pesticide carried with small quantities of liquid. There is a saving of material and labour.

3) Ultra-low volume  (ULV):      (Less than 5 li spray/ha)

ULV spraying can be defined as plant protection operation in which total volume of liquid applied amounts to a few ml per acre.  It is mainly used in air craft spraying.  Undiluted, technical-grade liquid pesticides (i.e. no water added). Grasshopper and co-deal leaf petal.  Selection of technique depends on type of vegetation, kind of pests and approach to field.

Foam Spraying: In this system a foaming agent (chemical additive) is added to spraying solution. The spray is passed through a special nozzle. This system is economical.

Ultra-low volume sprayer:

            The sprayer has a motor powered by 6 to 12 volt battery. Spinning disc is attached to a motor, having grooves or teeth and rotates at a very high revolution per min. (4000-9000). The spinning disc receives the concentrated chemical from a plastic container having a capacity of 1 li. (approx).  Average droplet size varies between 35-100 micron.  It is used for application of weedicides and for spraying small trees and crops.

Power sprayer:

-         Operated usually with internal combustion engines.  Prime mover capacity varies from 1 to 5 HP. The pressure pump is operated by a small power unit ensuring a constant steady pressure. They are operated at a pressure of 20 to 55 kg/cm² and are usually portable type. Sometimes it is operated by PTO shaft of tractor. The main parts are:

i)        Prime mover:    needed to supply power to the power sprayer. It is usually internal combustion engine.  Power generally varies from 1 to 5 hp.

ii)       Tank:    Steel tank is used to prevent corrosion.  Plastic tanks are also widely used due to prevention from corrosion and ease of molding into smooth shapes. A covered opening fitted with a removable strainer is provided for easy filling, inspection and cleaning.  A drain plug is there at bottom of tank for draining the liquid.

iii)     Agitator: Agitator is needed to agitate the liquid of tank.  Propeller or paddle type mechanical agitators are provided for agitating liquid. Horizontal shaft may be used with flat blades rotating at about 100 to 120 rev./min. Paddle tip speeds in excess of 2.5 m/s may cause foaming.

iv)     Air Chamber:    Air chamber is provided on the discharge line of the pump to level out the pulsation of pump thereby providing a constant nozzle pressure.

v)      Pressure Gauge: Pressure gauge is provided on discharge line to guide the operator regarding spray pressure.

vi)     Pressure regulator: It is meant for adjusting the pressure of the sprayer according to the requirement of crops in field.

vii)   Strainer: A strainer is included in the suction line between the tank and pump to remove dust, dirt and other foreign materials.

viii)  Boom:  Field sprayer to be driven by a tractor has a long boom in a horizontal plate

on which nozzles are fixed at specified spacing.  Boom can be adjusted vertically to suit the height of plants in different fields.

     ix) Nozzle:  Used to break the liquid into the desired spray and deliver it to plants. A nozzle consists of (a) body, (b) screw cap, (c) disc, (d) washer  (e) vortex plate and(f) strainer.

SPECIFYING PARTICLE SIZES AND SIZE DISTRIBUTION:

-         Atomizing type of devices produce a wide band or spectrum of droplet sizes under any given set of conditions.

-         The range and distribution of droplet sizes and measure of average size are important in pesticide applications.

-         graphical representation of droplet size distribution comparing different atomizing devices and different conditions are:

-         Aero droplet size is expressed by one or more of several forms of median or mean diameters.

-         Median diameter divides spray into two equal portions on the basis of number, cumulative length (dia.), surface area or volume.

-         Volume median diameter (VMD) divides droplet spectrum into two portions such that total volume of all droplets smaller than VMD is equal to total volume of all droplets larger than VMD.

-         Mass median diameter (MMD) is also sometimes used in place of VMD.

-         VMD or MMD and number median dia. (NMD) are more commonly used parameters for agricultural sprays.

-         VMD is larger than NMD as it places relatively more emphasis on larger droplets.

-         Various mean diameters are based on arithmetic averages of dia., surface areas, volumes of individual droplets or upon ratios of totals of any 2 of these 3 measures.

-         Dusters have no direct influence on particle size, except they affect agglomeration of particles during application, but average size and range of sizes controlled by processor.

-         Size distribution is determined by sieving and average is expressed by No. median dia.

Particle size in relation to effectiveness of drift:

-         Size of particle is the significant parameter in relation to penetration and carrying ability of hydraulic sprayers,  efficiency of ‘catch’ of sprays or dusts by plant surfaces, uniformity and completeness of coverage on plant surfaces, effectiveness of individual particles after deposition, drift of material  is out of the treated area.

-         Coarse atomization is good for drift control but more coverage of plant surface with smaller droplets give more effective control with fungicides, herbicides and insecticides.

-         Large droplets give satisfactory results with sprays of translocation type.

-         For a given Q number of droplets L    1/d³

-         Particle size also important to particles impinge upon plant surface when carried by air stream.

-         Efficiency of dynamic catch is defined as % age of total frontage of approaching air stream that is cleaned of droplets of a particular size.

-         Catch e.g. 100% means air sweeping through the foliage would be stripped of that size in a cross sectional area equal to that presented by foliage.

-         Increasing size of particle increases %age of catch because of greater momentum of particles.

-         Catch varies inversely with size of obstruction.

FACTORS AFFECTING DRIFT:

-         Rate of fall of particles

-         Initial height and other effects of application equipment

-         Wind velocity and direction

-         Atmospheric stability

-         Other meteorological factors

Size is the most important particle property affecting the rate of fall and associated drift distances

-         Small particle settle more slowly than large particles because aerodynamic drag forces are greater in relation to particle mass

-         Evaporation of water or other volatile materials from droplets reduces the droplet size and thus, adversely affects both deposition efficiency and drift

-         Small droplets evaporate more rapidly than large droplets

-         At 30% RH and 25.6⁰C temperature theoretical time for water droplet to be reduced to 10% of its initial volume is 0.8 s for 40 µm droplets and 4.2 s for 100 µm droplet.

-         Drift is minimized by employing devices that produce sprays having large VMD.

-         Increasing VMD increases the sizes throughout the distribution spectrum and reduces the number of small droplets.

-         Increased size of largest droplets reduces the efficiency in regard to uniform coverage.

-         Small droplet size is best for coverage and effectiveness and large droplet size reduces drift thus, ideal situation would to produce a spray having uniform sized droplets or a narrow size spectrum.

-         Drift is also influenced by discharge height and direction and air-turbulence and air currents induced by equipment.

-         Aircraft and air-blast sprayers create considerable air movement.

-         Hydraulic ground-rig sprayers have min. discharge heights and minimum air turbulence which reduce drift.

-         Drifts are more with dusts because of smaller particle sizes.  Most commercial dusters have NMD 1 to 10 μm.

10 μm dust particles with sp. gravity 2.5 require 100 s to settle 0.g m an1 gm require 3hr.

-         Test indicates that 70% of dust particle applied by air plane may drift away from treated area.

ELECTROSTATIC CHARGING OF DUSTS AND SPRAYS:

-         For electrostatic precipitation of pesticide dusts commercial electrostatic dusters are developed.

-         Primary objective of charging spray as dust particles are to increase %age deposition on plant surfaces.

-         Electrostatic spray has no effect on large particles also does not affect the basic trajectory from the application equipment to the target.

-         If a charged particle reaches the plant or target area and has insufficient inertia to cause impingement charge increases the probability of depositions.

-         Charging dust improved control of insects and diseases on number of different crops. Charging dust or spray has increased deposits on cotton plants by ratio of 2 or 3 to one.  Increased deposition efficiency for small particles reduces drift.

-         Electrostatic sprayers and dusters are more complicated and more expensive than conventional one.  And there are number of practical problems regarding design and effectiveness.

ATOMIZING DEVICES

Generally atomizing devices utilize following principles to atomize liquid:

1. Pressure or hydraulic atomization depends upon liquid pressure to supply the atomizing energy.  The liquid stream from an orifice is broken up by its inherent instability and its impact upon atmosphere etc. 
2. Gas atomization, in which liquid is broken up by a high-velocity gas stream. The break-up may occur either outside the nozzle or within a chamber ahead of exit orifice.
3. Centrifugal atomization, in which liquid is fed under low pressure to a centre of high-speed rotating devices, like disk, cup, cylindrical screen and is broken up by centrifugal force as it leaves the periphery.
4. Low-velocity jet break-up, in which non-viscous, low-velocity stream after emerging from a small orifice or tube breaks up into droplets as a result of external/internal disturbances and effect of surface tension.

Pneumatic atomizing (two-fluid) nozzles, in which compressed air is employed for atomization and is used to some special low volume sprayers because fine atomization can be obtained at low liquid pressure.  Drift hazards from extremely fine particles limit the use of this equipment.  High Speed rotating cages covered with fine-mesh screen gauge (40-80 mesh) are used on air-craft and  air blast sprays having rotary screen atomizers are developed.  Rotary atomizers operating in still air at relatively low liquid flow rate produce uniform droplets of controlled sizes for lab studies but when they are employed on aircraft or air blast sprayers rapidly moving air stream affects the atomization process and produces droplets with considerable range of sizes. VMD is less with hydraulic nozzles used in agricultural spraying.

Hydraulic nozzle:

a)      Side-entry hollow cone

b)      Disk-type solid  cone

c)      Core-insert hollow cone

d)      Fan spray

e)      Flooding

f)        Disk-type hollow cone

g)      Jet or solid stream nozzle

Hollow-con nozzle: liquid is fed into whirl chamber through a tangential side entry passage or through spiral passages in whirl plate or core insert to give it a rotary velocity component.  Orifice is located on axis of whirl chamber, liquid emerges in form of hollow conical sheet and then breaks up into droplets. 

Core-insert is mainly in small size nozzles and used limited in agricultural spraying. 

Solid-cone nozzle: there is addition of internal axial orifice which strikes the rotating liquid within the orifice of discharge.  The breaking of droplet is due to impact.

Fan-spray nozzle forms narrow elliptical spray pattern.  The liquid is forced to come out as a flat fan shaped sheet which is then broken into droplets. It is mostly used for low-pressure spraying.

Flooding nozzle liquid emerging through a circular orifice impinges upon a curved deflector which produces fan-shaped sheet having relatively wide spray angle.

Flow rate of a particular nozzle is proportional to square root of pressure.

Discharge rate is proportional to orifice area.

 

Nozzles on field sprayers have spray angle 60⁰ to 90⁰ (Fan, Hollow cone).

Flooding nozzles have spray angle 100⁰ to 150⁰ and operating pressure below 1.5Kg/cm² is undesirable as nozzle does not work satisfactorily.

Low-velocity jet break-up are used to obtain uniform, predictable droplet sizes and reducing drift. Pressure is low that produce non-turbulent flow because of liquid to emerge from a circular orifice or capillary tube as a cylindrical column or filament.

Factors affecting droplet size:

            Degree of atomization depends upon:

-         Characteristics and operating conditions of the atomizing devices.

-         Characteristics of liquid being atomized. 

Principal fluid properties affecting droplet sizes are surface tension and viscosity. Increased surface tension and viscosity increases droplet sizes.  Emulsifiers or water in oil emulsions are used for increasing viscosity to increase droplet size.  For a given flow rate, pressure and spray angle hollow cone nozzles have smaller droplet sizes than fan spray nozzles.  Flooding type produces coarser spray.

Droplet sizes and number can be determined by collecting samples of spray on glass slides coated with silicone, magnesium oxide or other similar material or a glossy-surfaced printing material. Correction factor is used to determine original sphere diameter from observed stains.

Droplet sizes can be measured by immersion method.  The spray droplets are caught in a shallow dish containing liquid or material in which droplet can sink.  There, they remain spherical.  Cellulose – thickened water solution containing detergent or soap can be used for oil droplet.

Sizing and counting of collected droplets or stains can be done directly with a microscope, or sample photograph scanned with electronic analyzer.  Direct automatic scanning of droplets in flight can also be done.

Field measurement of uniformity of distribution made by collecting sprayed material on mylar sheet or metal plates.  Known concentration of trace material is added.  The material from each plate is washed into specified volume of water.  And concentration of tracer is measured.

Nozzle distribution pattern is determined in lab.  By spraying onto a surface that consists of series of adjacent, sloping V-troughs and measuring the liquid collected from each trough. Uniformity of coverage on plant surfaces can be checked by adding fluorescent dyes or insoluble fluorescent materials to spray and viewing surfaces with fluorescent light of the dark.

Pumps for sprayer:

1)      Piston or plunger pump: The displacement pump used on sprayers with piston plunger, rotary and diaphragm.

-         Self priming.

-         Automatic (spring loaded) by-pass valve to control pressure and protect equipment from mechanical damage if flow is shut off.

-         Piston plunger well suited for high-pressure orchard sprayers multi-purpose sprayers.

-         Designed for both low and high pressure sprayers.

-         More expensive, occupy more space, more heavy, durable and handle abrasive material without excessive wear.

-         Volumetric efficiency high (90% or more), discharge rate is function of crank speed and volumetric displacement. Crank speed of small spray pump - 400 to 600 r/min., high – pressure of 125-300 r/min.

-         Mechanical efficiency 50 to 90% depends on size and condition of pump.

2)      Rotary – Used for low pressure sprayers.

-         Common types gear pump, roller pump.

-         Nylon used for rollers, rubber, steel and carbon also

-         Rollers held against with centrifugal force.

-         Roller rotary pumps are compact, inexpensive and operated at speed suitable for direct connection of PTD.

-         Pumping action depends on maintaining close clearance between housing and gear or impellers.

-         Classed or positive displacement pump.

-         Pressure above 100 psi is recommended.

-         Gear pumps cannot be used for wettable powder and abrasive material as rapid wear and short life.

-         Roller pumps are better than gear as roller can be replaced.

3)      Centrifugal :   depends on centrifugal force.

-         High speed, high volume devices and do not have positive displacement pump.

-         Pressure developed is function of discharge rate.

-         Discharge rate varies directly with speed.

-         Head varies with square of speed.

-         Power varies with cube of speed.

-         Multistage increases pressure without increase pressure.           

-         Simple and handles abrasive material easily.

-         Well suited to air blast sprayers, air craft sprayers in high flow rate, low pressure needed.

-         Speed ranges 1000 to 4000 r/min. depending on pressure required and diameter of 1 impeller.

-         They are not displaced, not self brimming and do not require pressure relief valve.

4)      Miscellaneous: Diaphragm pumps used for flow rates 19 to 23 l/min.

-         Valves and diaphragm only moving part.

-         Handle abrasive material.