Reduce spray drift with better nozzles choices

Misapplication of pesticides is a potential risk of which you must always be aware. One form of misapplication is pesticide drift. When you apply pesticides, some may escape from the target application area. Drift is a concern because it removes the chemical from the intended target, making it less effective. However, another concern is even more critical: You are liable for off-target drift, which may injure non-target plants and animals.

Although you cannot completely eliminate drift, the use of proper equipment and spraying techniques can keep it within acceptable limits. Nozzle choice has a great deal to do with drift potential. Thus, as a professional in a decision-making position, information about current nozzle technology can help you reduce drift.

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Defining drift Herbicides move off-site in two forms: as particles and as vapor drift. Vapor drift happens when pesticides vaporize and then move as a gas in the atmosphere away from the application site. Nozzles have a limited effect on vapor drift because vaporization takes place after you've applied the spray, not just while droplets are airborne. Particle drift is the off-target movement of tiny airborne spray particles. The amount of particle drift depends mainly on the number of small "driftable" particles that the nozzle produces.

* Spray droplet size. Although it is possible to achieve excellent coverage with extremely small droplets, decreased deposition and increased drift potential limit the minimum size you can use. The relationship between droplet size and coverage is complex, resulting in several common misconceptions regarding droplet size and proper application. For example, many people believe that applying small droplets at high spray pressures provides increased coverage. Research data, as well as a consideration of particle dynamics, do not substantiate this theory. While it is true that atomizing a spray solution into smaller droplets will increase the possible coverage, you must also consider evaporation, drift potential, canopy penetration and deposition characteristics.

As the droplets get smaller, they can travel only a short distance before evaporating. For example, 20-micron droplets travel less than 1 inch from the nozzle before they completely evaporate (in less than 1 second). With typical boom sprayers at 17 to 19 inches above the target, this is not enough distance and time for such small droplets to reach the target. Conversely, droplets greater than 200 microns in diameter resist evaporation much more than smaller droplets due to their smaller surface-to-volume ratio. From these and other research results, we know that the drift potential of droplets rapidly decreases as diameter increases to 200 microns or larger.

* Nozzles and droplet size. Most hydraulic nozzles produce a wide range of droplet sizes. You need to know the actual size-distribution of droplets a nozzle produces before you can make adjustments that affect coverage, deposition and spray-drift potential. Engineers frequently use the percentage of the spray volume that consists of small droplets to estimate the "driftable" fraction of spray a nozzle produces.

Nozzle selection to reduce spray drift Drift potential should be one of the factors you consider when selecting nozzles. Of the many nozzle types available, a few are specifically designed for reducing drift. Several years ago, Delavan introduced its Raindrop nozzle. It effectively reduced the exit pressure at the spray tip resulting in larger spray droplets.

Some nozzles, by design, operate effectively at low pressures. For example, extended-range flat-fan nozzles available from several manufacturers provide uniform spray patterns at pressures down to 15 psi, thereby reducing the amount of small, driftable spray particles in the spray. However, at higher pressures they produce finer spray droplets and more drift. Although manufacturers have developed a large number of nozzle types for practically every kind of spray application, a few types account for most use. Let's review the main types.

* Extended-range flat-fan nozzle. Extended-range flat-fan nozzles are appropriate for soil and foliar applications when good coverage is essential. These nozzles are available in both 80- and 110-degree fan angles. Applicators usually mount the 80-degree fan nozzles on 20-inch centers at a boom height of 17 to 19 inches. The 110-degree nozzles are usually on 30-inch centers at a boom height of 17 to 19 inches, or on 20-inch centers and lowered to 10 to 12 inches.

For soil applications, your pressure range should be between 10 and 30 psi. Pressures from 30 to 60 psi produce smaller drops, increasing the likelihood of drift. You should only use high pressures for foliar pesticides that must penetrate into a dense canopy or that require maximum coverage. Drift potential is a major concern at pressures above 40 psi.

Because the outer edges of spray patterns have reduced spray volumes, you should overlap adjacent patterns along a boom to obtain uniform coverage. For maximum uniformity, this overlap should be about 40 to 50 percent of the nozzle spacing (see figure, above). You can use foam markers to help keep track of swath-width overlap requirements on multiple passes.

* Raindrop nozzle. The RA Raindrop nozzle minimizes spray drift. Within a pressure range of 20 to 50 psi, this nozzle delivers a wide-angle, hollow-cone spray pattern with fewer smaller drops than a flooding nozzle. For a uniform spray pattern, space the nozzles no more than 30 to 40 inches apart and rotate 30 degrees from vertical. Remember that this rotation may lead to increased drift when spraying in windy conditions.

Although this nozzle produces the large droplets that aid in drift control, they may result in less-than-ideal coverage for some foliar pesticides. Therefore, you should take extra care to ensure complete coverage when using the Raindrop nozzles. Heavier application rates (50 to 75 gallons per acre) improve coverage in such cases. A helpful technique with Raindrop nozzles is to adjust them to obtain double coverage (100 percent overlap).

* Wide-angle full-cone nozzle. A wide-angle full-cone nozzle produces large droplets over a range of pressures. The in-line, or straight-through, design of the nozzle uses a counter-rotating internal vane to create controlled turbulence. The design provides you with a 120-degree spray angle over a pressure range of 15 to 40 psi. This nozzle offers a uniform spray distribution and only requires about 25-percent overlap.

More recent low-drift nozzle technology In recent years, one emphasis in design has been the "pre-orifice" concept. A pre-orifice on the entrance side of the nozzle effectively creates a flow restriction without flow-rate reductions, resulting in lower exit spray pressures and larger spray droplets. The term associated with this nozzle design is drift-reduction.

* Drift-reduction flat-fan nozzles. Two styles of drift-reduction flat-fan nozzles are currently available. The RF Raindrop flat-spray nozzle (from Delavan) is available with a 105- to 115-degree fan angle and the Drift Guard flat-spray nozzle (Spraying Systems) is available in both 80- and 110-degree fan angles (see figures, page 29). With a larger droplet size, drift-reduction pre-orifice nozzles can replace conventional flat-fan 80- and 110-degree tips in broadcast applications where spray drift is a problem. The recommended pressure for this type of nozzle is 30 to 60 psi. The pre-orifice nozzle requires the same 50-percent overlap as the extended-range flat-spray tips.

* Drift-reduction turbulation-chamber nozzles. The most recent design improvements incorporate the pre-orifice concept with an internal turbulation chamber. This not only creates larger droplets but also has improved the uniformity of the spray pattern. Turbulation chamber nozzles are available in a Turbo Flood tip and now in the Turbo flat-fan design (both from Spraying Systems).

* Turbo Flood. Turbo Flood nozzles combine the precision and uniformity of extended-range flat-spray tips with the clog-resistance and wide-angle pattern of flooding nozzles. The design of the Turbo Flood increases droplet size and causes an improvement in pattern uniformity. Its recommended range of operating pressure is low--8 to 25 psi--which reduces the number of driftable size droplets in the spray pattern. Thus, Turbo Flood nozzles are a good choice to use in drift-sensitive applications. The Turbo Flood nozzle, because of its improved pattern uniformity, likely needs at least 50-percent overlap to obtain proper application uniformity. Because of improved pattern development, you can orient this nozzle straight down. Thus, it is less likely to produce drift in windy conditions.

* Turbo TeeJet flat-fan. The Turbo TeeJet flat-fan nozzle is a wide-angle pre-orifice nozzle that will create larger spray droplets across a wider pressure range (15 to 90 psi) than comparable low-drift tips, thus reducing the amount of driftable particles with a variety of pressures. The new design also creates a spray pattern similar to the extended-range flat-fan and Turbo Flood nozzles. This is a great improvement in pattern uniformity compared to the extended-range flat-fan and other drift-reduction flat-fan designs. In addition, it significantly reduces driftables over a range of pressures. The nozzle's unique design lets you mount it in a flat-fan nozzle-body configuration. The wide spray angle allows for 30-inch nozzle spacing and 50-percent overlap to achieve uniform application across the boom width. You also can use this nozzle for straight-down spraying.

Rapid improvements in nozzle design and spray equipment are taking place with an emphasis on reducing spray drift. It is important to remember that many other factors influence the amount of drift that actually occurs. Therefore, in addition to selecting the proper type of nozzle, you should use as many approved drift-reduction application techniques as possible to reduce the problems associated with the off-target movement of spray materials.

Dr. Robert E. Wolf is an extension specialist in the Agricultural Engineering Department at the University of Illinois--Urbana.

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