Managing Spray Drift: Boom Height and Spray Nozzles

Ground Boom Sprayers

Boom Height. The higher the nozzle, the more opportunity the wind or air flow has to move droplets from the intended target. Determine the optimum boom height for the particular nozzle from the nozzle literature. Often 20 inches above the target is the optimum boom height for broadcast boom applicators with 20-inch nozzle spacing. To prevent the boom ends from striking the ground, boom heights are frequently much higher on large application equipment traveling at higher speeds.

Nozzle selection. No one nozzle will be the best for all spraying conditions, although some work well over a range of uses. To determine which nozzle may be best suited for the intended use, first consult the pesticide label. If there are specific carrier rates, droplet sizes, or drift precautions, they should be listed on the label. An example label statement would be: "Apply with 15 or more gallons per acre using a nozzle producing a medium droplet."

Also, determine equipment limitations when selecting nozzles. The recommended pressure or flow rate for a particular nozzle may exceed the capacity of the equipment. Consult the nozzle manufacturer to determine specific requirements and expected spray quality for each nozzle being considered. Below is a listing of manufacturers of many of the spraying nozzles and drift reducing equipment sold in the USA.

Conventional Nozzles (flat fan). The extended-range flat-fan nozzle has been the standard for boom pesticide application. Variations of this design are available from several manufacturers. The design is available in a wide range of flow rates and fan angles to fit many application needs. A nozzle with an 80° discharge angle produces a larger spray droplet than a 110° angle nozzle at the same flow rate and pressure. Spray quality is directly dependent upon the nozzle orifice size and boom pressure. Larger orifices and lower pressures produce coarser droplets that reduce drift potential while higher pressures, above 30 to 40 psi, produce finer droplets that may be more susceptible to drift. These nozzles produce a uniform spray pattern when patterns are overlapped 30 to 50% and when operated at 15 to 60 psi.

Drift-reduction nozzles

Pre-Orifice Nozzles. A pre-orifice nozzle reduces the internal operating pressure in the nozzle, producing a larger spray droplet than a standard flat-fan nozzle at the same operating pressure. The pre-orifice design contains a metering orifice (pre-orifice) that restricts the amount of liquid entering the nozzle. This is what drops the pressure within the nozzle.

Pre-orifice nozzles are available in 80° and 110° discharge angles with a general pressure range from 30 to 60 psi. The nozzle has a narrower pressure-operating range as compared with a conventional extended-range flat-fan tip. Pressure should not drop below 30 psi due to the internal pressure drop in the nozzle or exceed 60 psi, as considerable driftable fines may be produced. The optimum pressure for this nozzle is 40 psi. Pre-orifice nozzles can reduce drift by 50% over extended-range flat-fan nozzles, but they are more difficult to clean due to the metering orifice. Several manufacturers make nozzles using this technology.

Turbulence Chamber, Flood type. A few manufacturers offer a flood-type nozzle with a pressure-reducing turbulence chamber. These nozzles produce wide, uniform spray patterns of approximately 120° and can be mounted 15 to 18 inches above the spray surface when using a 20-inch nozzle spacing. This will provide about a 50% overlap over the adjoining nozzle spray pattern. At low to moderate operating pressures, these nozzles produce few driftable droplets. As pressure increases, the number of fine droplets also increases. Flood-type nozzles are capable of operating over a wide pressure range (15 to 90 psi), which makes them an excellent choice for use with automatic rate controllers that use pressure to adjust flow rate in response to a change in travel speed.

Air induction. All "air induction" nozzles have the same basic design feature-two orifices, one to meter liquid flow and the other larger orifice to form the pattern. Between these two orifices is a venturi or jet, used to draw air into the nozzle body. In the body, air mixes with the liquid and forms an air-entrained spray pattern at a lower pressure. The coarse spray contains large, air-filled droplets and very few drift-susceptible droplets. Venturi nozzles differ from conventional low-pressure spray nozzles by producing more coarse droplets and fewer fine droplets.

Dramatic drift reductions have been observed with these tips while good spray coverage has generally been maintained. The manufacturers suggest that the droplets are filled with air bubbles that cause the droplets to shatter on impact with the leaf, providing better coverage. Getting the maximum benefit from these nozzles, however, requires careful selection of the right nozzle for your needs and proper operation. Many air induction nozzles have an optimum operating pressure between 60 and 80 psi, while others will operate well at lower pressures. You need to be aware of differences in pressure operating ranges, ease of cleaning, and the ability to fit into existing nozzle caps.

Publications to aid nozzle selection

http://www.oznet.ksu.edu/library/ageng2/mf2541.pdf
http://agbiopubs.sdstate.edu/articles/FS919.pdf

Authored by Jim Wilson, PhD  
South Dakota State University