Ground Cover Supplement : GC Supplement - Spray application technology
DRIFT-REDUCTION TECHNOLOGY Bigger droplets produce less drift New research by the Centre for Pesticide Application and Safety shows that nozzles producing coarse droplets reduce the risk of spray drift A SPRAY NOZZLE CLASSIFIED to produce coarse droplets will still produce some larger or smaller droplets that are not coarse -- this is the droplet spectra. The amount of spray that drifts is influenced by pressure, release height and the physical characteristics of the liquid being used. Wind direction is also important; driftable fines always go in the direction the wind is blowing. Research carried out in the advanced pesticide wind tunnel laboratory at the University of Queensland, using a laser diffraction analyser, has determined the droplet size and amount of drift that is generated by an assortment of '02' (a flow rate of 0.2 gallons per minute at 40 psi) hydraulic nozzles (Figure 1). Nozzles tested included the newer technology low-drift/air-induction nozzles as well as standard flat-fan nozzles. Data shown in Figure 1 were obtained using a spray solution of water and 0.1 per cent surfactant (Agral). Research by the Centre for Pesticide Application and Safety (CPAS) has also tested the droplet spectra for nozzles with active product, including 2,4-D ester and glyphosate. In the CPAS trials, large differences in droplet size and downwind drift profiles were recorded depending on nozzle design. Generally the newer technology low-drift/ air-induction-type nozzles generated larger drop sizes as indicated by their volume median diameters (VMDs). However, Dr Chris O'Donnell, who conducted the research at CPAS, considers that VMDs only tell part of the story and that the proportion of droplets that are released as driftable fines is more important when assessing the risk of drift. Spray droplets that are less than 200 microns (or micrometres) -- that is, fines -- will readily drift. A standard 110˚ flat-fan nozzle produces about 45 per cent of its spray droplets as driftable fines. An air-induction nozzle can produce less than 10 per cent of its droplets as driftable fines. In the CPAS trials the airborne drift profile for each nozzle was determined by quantifying the amount of spray deposited on collector strings located two metres downwind from the nozzle in the wind tunnel (Figure 2). This stage of the research identified that nozzles producing coarser droplets produced less driftable fines under the same set-up. An inverse relationship between droplet size Nozzle designs that reduce drift By Paul Miller The proportion of droplets released as driftable fines influences the risk of drift GROUND COVER SPRAY APPLICATION TECHNOLOGY 8 Much of the nozzle development undertaken throughout the world over the past two decades for the application of agricultural pesticides has been directed at spray drift control. Four types of nozzle have emerged as relevant to drift control: The pre-orifice hydraulic pressure flat-fan nozzle In this design, flow through the pre-orifice leads to a substantial pressure drop within the nozzle, so the spray contains large, slower-moving droplets. This results in drift reductions in the region of 50 per cent of that from the reference 110˚ '03' conventional flat-fan nozzle operating at a pressure of 3.0 bar. The air induction nozzle This design has an air inlet between a pre-orifice and a final output orifice, with air being drawn into the nozzle by a Venturi action. Droplets in the spray have air inclusions and are very large and slow moving. Drift reductions in excess of 75 per cent of the reference 110˚ '03' flat-fan nozzle can be achieved. However, different designs have a very wide range of performance, based on the different droplet sizes produced. Twin-fluid nozzles These use both air and liquid under pressure, fed into the nozzle body, to give a wide range of spray characteristics and flow rates depending on the input pressures and flows. Set to generate large droplets that have air included, this nozzle design can give drift reductions of more than 75 per cent of the European reference condition. The design requires a source of compressed air, but has the major advantage of output flexibility in terms of flow and droplet size distribution. Spinning disc units These can deliver drift reductions, particularly at low application volumes, by creating large droplets. Variations in disc speed and flow rate give this system flexibility, but mechanical complexity and the need for care to ensure uniform volume distributions have limited the adoption of such systems.
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