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Controlled droplet appliCation

Controlled Droplet Application (CDA) offers economic and environmental benefits for the application of agrichemicals and has won a permanent place in the equipment used to control weeds, pests and diseases. Unlike pressure nozzle sprayers, which produce a wide range of droplet sizes, controlled droplet applicators are designed to produce droplets of a uniform size. The droplets are less likely to run off the target or be moved away by the wind. The spray fluid is therefore used more efficiently and water volumes can be reduced to 5 to 50 litres per hectare. For broadcast applications, CDA sprayers produce droplets in the range 100 to 400 microns. Larger droplets are used to apply herbicides and smaller droplets for fungicides and insecticides.

applications. The droplets formed by the rotary atomiser are blown to the target by a stream of air from a fan. This technique increases penetration of the droplets into the target area.

Advantages of CDA

As a rule of thumb, for a given spray volume CDA sprayers will cover eight to ten times more area than pressure nozzles when applying herbicides and more than 20 times more area when applying insecticides and fungicides. This is arguably the greatest advantage of CDA. Other advantages are: · The production of efficient even-sized droplets. · Less affected by the wind compared with pressure sprayers. · Good spray coverage at low volumes. · Low to very low pressures. · Simple calibration. · Droplet size is easy to change. · Light manoeuvrable equipment requires only light vehicles such as ATVs.

How CDA works

Rotary atomisers are used to generate the droplets. Fluid is metered through an orifice into the inside of a spinning atomiser housing. Centrifugal forces cause the fluid to rise up the grooved walls of the housing where it is broken up into droplets. Carefully designed fingers throw the droplets horizontally outward from the spinning atomiser. They then fall to the target under the force of gravity. Droplet size is determined by the speed of rotation of the atomiser. The spray pattern is circular with a diameter of 1.2 to 1.8 metres. Shrouded atomisers are used for some specialist applications such as band spraying. Various means are used to spin the atomisers. Most common are atomisers driven with small electric motors. These are used on large booms for pasture and crop spraying as well as smaller hand-held units and units used in enclosed situations. Air driven atomisers are common on aircraft booms and orchard sprayers. Some early models used hydraulic motors but these have been largely superseded. Some CDA sprayers incorporate air assistance. These are generally used in enclosed situations such as tunnel houses or in orchard

Disadvantages of CDA

The droplets produced by CDA equipment are more difficult to see and the nozzles generally rely on gravity to get the droplets to the target. This can limit effectiveness when using contact herbicides and fungicides with limited systemic action and those requiring thorough coverage. Some formulations such as wettable powders may be too viscous to flow properly through the rotary atomisers, and some concentrated solutions may corrode the plastic components of some CDA sprayers. The use of foam markers on larger spray equipment and dye markers on smaller rigs overcomes the problem of marking where the spray droplets have fallen. Careful selection of herbicides and fungicides will overcome most efficiency problems. An increasing range of plastic materials has allowed manufacturers to offer equipment which resists a wide range of chemicals used in agriculture and horticulture.

NZ Novachem Agrichemical Manual


Controlled droplet & eleCtroStatiC appliCation

Perhaps a greater concern is the application of the correct amount of chemical. Where specific instructions for application of a chemical using CDA equipment have not been published, label rates should be used and the water rate adjusted to compensate for the lower volume of spray fluid applied.

Maintenance of equipment

Maintenance of CDA spray equipment involves the same principles as conventional sprayers. Cleanliness and calibration are most important to ensure proper operation. Good quality equipment will be fitted with filters which must be inspected and cleaned regularly. Rotary atomisers and their drive mechanism must also be regularly inspected and, where appropriate, lubricant applied. Any damaged components must be replaced. This is especially important where atomiser disks, bearings, or drive belts have been damaged. Correct droplet formation and control of droplet size will be adversely affected by damaged disks or if the disks are not running at optimum speed. Because of the low fluid pressures CDA sprayers do not suffer from nozzle tip wear to the same degree as pressure sprayers. However, it is good practice to carry out regular calibration of the spraying system to ensure that it is performing to specifications. ADVISORY


To calibrate a CDA sprayer, first determine from the manufacturer's data the correct fluid flow rate from the atomisers. The Micromax 84 used on C-DAX Systems pasture sprayers will deliver 1 litre per minute when large droplets are selected. To check this, simply capture the fluid from one atomiser for one minute and measure the contents. Generally it is not necessary to have the atomisers spinning while the fluid is being collected. An exception is shrouded types, such as the Micromax 120. These employ a fluid recycling system for the fluid captured by the shroud, which is dependent on fluid flow. This type must be calibrated with the atomiser running. A typical flow rate would be 380 ml per minute from this type. Coarse adjustments to the flow rate are made with flow plates in the fluid lines. These are usually located in the check valve fitting. Fine adjustments are made by altering the fluid pressure to the atomisers. Typical fluid pressures for the Micromax 84 and 120 fall in the range 1 to 1.3 bar (15-20 psi). A bypass-type control unit is provided for this purpose.

against gravity when approaching the leaf surface. This results in much improved droplet deposition. In some tests under-leaf coverage was shown to be improved more than 70-fold. Electrostatics is a low-volume system. Sprayers use 10 to 25 times less water carrier than standard hydraulic sprayers. However, because the droplets are very small, typically in the 30 to 60 micron range, and they are all charged with the same electrostatic charge, the droplets repel one another. This results in uniform droplet deposition over the plant surface. This decreases the risk of chemical burn seen with some other types of low-volume spraying equipment, and increases exposure of the pests to the chemical treatment.

Electrostatic equipment

Several types of electrostatic spraying systems are commercially available in New Zealand. For indoor applications, handgun units are popular. These are generally supplied with compressed air from an external source and require about 13 to 17 m3 of air per hour (8-10 cfm). The spray fluid is generally displaced from a pressurised tank. Typical fluid flow rates are around 250 ml per minute. Electrostatic boom sprayers are also available. Electrostatic spray nozzles are arranged at 200 to 300 mm intervals. Sometimes nozzles are arranged on droppers to suit the crop being treated. Each nozzle requires about 6 m3 of air per hour (3-4 cfm) which is supplied from an engine or PTO driven compressor. Spray fluid is pumped to the nozzles at a rate of about 70-150 ml per minute per nozzle using a small electric pump. The method of charging the spray droplets is critical. The air-assisted ESS systems discussed here use induction charging of the droplets. This occurs after they have been atomised but before they leave the spray nozzle. This results in a space charge of about 3 millicoulombs per kg of charged material. A 1000 volt charging voltage is supplied from a high voltage power supply mounted in the spray gun or on the boom. The power requirements for induction charging are very low. Quite satisfactory performance can be obtained from dry cells.

eleCtroStatiC appliCation

Electrostatic spraying (ESS) has been used to apply industrial coatings for many years. In agriculture and horticulture electrostatic spraying systems offer an advanced method of low volume agrichemical application. The main advantage is the accuracy with which the chemical is applied to the target. Spray drift and run-off are reduced markedly. Electrostatics was first developed by ICI in the 1970s as the "Electrodyne" system. It required special formulations and a patented container known as a "bozzle". It was not a commercial success. In the 1980s the technique was refined by the University of Georgia to allow its use with agrichemical sprays in agriculture. In the early 1990s, the NZ Forest Research Institute developed the "Lektraspray", an electrostatic nozzle which was initially used in wood treatment and has since been developed for use in agriculture. More recently, agrichemical sprayers that use air-assisted electrostatic technology have been introduced. Air-assisted electrostatic sprayers produce electrically charged spray droplets which are carried into the plant canopy in a high speed air stream. As the charged droplets enter the canopy, opposite charges on the plant surface attract the droplets. Although the charge is small, the force of attraction between the small, uniform sized droplets and the plant surface is strong, up to 40 times the force of gravity. Droplets will therefore move upwards

NZ Novachem Agrichemical Manual


Maintenance of ESS equipment is similar to other low volume sprayers. Cleanliness and calibration are most important to ensure proper operation. Because of the low flow rates, filters and flow plates must be regularly inspected and cleaned.


As with all spraying equipment regular calibration is necessary to ensure the equipment is performing to specifications. Excessive flow rates in an ESS system can result in flooding of the charging chamber. This will result in poor fluid atomisation and inadequate droplet charging. The electrostatic charging system requires no special maintenance other than routine inspection of wiring to ensure that damage does not go undetected, and replacement of batteries where applicable.

Acknowledgement - This section was provided by C-DAX Systems.


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