Fig.20. Air-assist systems provide better coverage with less drift.
One of the most effective ways to prevent drift of small droplets is using high-velocity air flow to aid the process of transporting droplets from the nozzle to the target. Several studies have shown that air assistance reduces drift deposits from boom sprayers. One common air-assist system uses a stream of air to direct the spray to the target by entraining droplets in a downward-moving air curtain. With these types of air-assisted sprayers the air assistance system is an add-on to the existing conventional boom sprayer. Droplets are generated by conventional nozzles using hydraulic pressure. Nozzles direct the spray droplets into the air jets. These air jets offer some protection against drift, and improve the penetration of spray into target canopies. As shown in Figure 20, there is less drift from a sprayer when there is air-assistance compared to the same sprayer operating with no air assistance. However the angle and air velocity of the jet must be adjusted to provide optimum results. Factors that must be considered when operating air-assisted sprayers include the wind velocity and direction, travel speed and direction with respect to the wind direction, and ground cover (plant canopy) being sprayed. For example, some studies indicate that more drift can be created by using this type of air-assisted system if high air flow and air velocity is used when spraying ground with no vegetative cover.
Some other air-assist systems use specially designed air atomizers to break up the liquid solution into droplets and to carry these droplets to the target. Air atomizers may use just air pressure, or both air and liquid pressure to atomize and carry the spray solution to the target. The sprayer operator can vary droplet size independently of nozzle flow rate. In windy conditions, larger droplets can be used to reduce drift. Where a thorough coverage is required (both upper-side and under-side of the leaves covered), smaller droplets can be generated.
Currently two true air atomizers are available in the United States. The air atomizer "AirJet®" (Figure 21), manufactured by Spraying Systems Co. (Wheaton, Illinois), works by mixing air with the spray solution inside the nozzle to atomize the spray. A metering insert determines the liquid capacity. An on-board compressor provides about 140 cubic feet per second air supply to the atomizer. The air enters the nozzle body through a very narrow port from one side of the atomizer body. By adjusting both air and liquid pressures, the operator can control droplet size over a wide spectrum without changing nozzles.
The other air atomizer, "Shear GuardTM PLUS" (Figure 22) manufactured by Spray-Air USA Inc. (Grangeville, Idaho), is inserted at 8 inch intervals in a boom assembly that may span up to 90 ft.
The boom, a hollow fiberglass tube 1 inch in diameter, delivers air from a fan, powered by the tractor PTO, to the atomizer at low pressure. The air is used to atomize liquid into droplets, as well as to develop a high momentum jet to agitate the crop canopy thereby increasing droplet penetration and coverage. Air travels through the hole at the center of the atomizer as well as two narrow ports on both sides of the center port. The air exiting the side ports is directed toward the center of the main vertical air port where liquid is discharged. The main function of the air coming from the side ports is to give the discharging liquid a spray pattern that is similar to the pattern obtained from a standard flat fan nozzle. Liquid is pumped to each atomizer on the boom via a separate manifold attached to the boom. It is introduced to the air stream in the center of the main air jet port of the atomizer, between the two cross-flow pattern-shaping jets. When the air and liquid meet, the liquid is atomized. An orifice disc is used to control the amount of liquid flow to each atomizer. With Shear GuardTM PLUS, it is possible to choose the droplet size that will give the best performance and still minimize drift. The operator can adjust the velocity of air and thus change the droplet size from the cab regardless of the ground speed. The range of droplet size produced by this atomizer is from 100 to 500 microns.