Air-assisted Spraying

Air Blast Sprayers. Air blast sprayers have been used for many years to spray fruit trees. These units consist of a fan that is capable of producing a high volume of air flow into which the spray is dispensed. As the sprayer moves through the orchard, it blows the spray-laden air into the tree canopy, displacing the original air and depositing the spray material on the leaf surfaces Fig. 1.254. Most, but not all, horticultural sprayers use high pressure pumps (1.5–3 MPa) to produce fine droplets that are carried by the air. When low pressure pumps are used, good atomization depends on high-velocity air shear to produce fine droplets. Typically, air blast sprayers direct the air up into the tree from a ground level machine position. Tanks vary in size from 400–4000 dm³. They can apply either high or low volumes of material. Coverage will depend upon the machine and tree size. They may direct output to only one side of a row, both sides or cover double or more rows. Nozzles are typically selected to deliver a higher proportion of the discharge to the top half of the tree. This is discussed in Section 1.5.6, Orchard Spraying.

The latest air-carrier equipment uses diffusers of various shapes (turret, vertical slits, two fans–one in the standard position and a second, less powerful one, placed above) to shift the point of spray emission and the auxiliary air flow closer to the vegetation (Fig. 1.270, Section 1.5.10). On this type of equipment, it is possible to use nozzles with the same atomization characteristics across the entire boom.

Good coverage and penetration along with high spraying capacity are general char­acteristics of the air-blast sprayer. The high annual use rate of these sprayers requires a high level of manufacturing quality. The limitation of these sprayers is their high power requirement, and the potential for increased drift.

Figure 1.254. Air blast sprayer

Tower and Air Curtain Sprayers. There is an increasing trend today to use tower sprayers when spraying orchard trees. These sprayers are constructed to deliver air and spray material along a narrow vertical band that forms an air curtain extending from near the base of the tree to the top. The nozzles discharge closer to the tree foliage than with the conventional sprayers. This improves spray deposition and coverage over that of the conventional air blast sprayers, which attempt to blow the spray up into the treetops from the ground.

The air curtain is produced in some sprayers by use of multiple, long narrow tangen­tial fans mounted vertically (Fig. 1.255). Other sprayers use an air discharge manifold modified to form a long narrow slot. The objective is the same: to have a more even air spray discharge in the tree, and to reduce the distance between the nozzle and target.

The advantage of the tower sprayer is more uniform spray deposition and reduced drift. This is offset by more expensive construction and reduced maneuverability.

Air-assisted Boom Sprayers. Several models of boom sprayers now are equipped with a fan and air manifold with the specific objective to improve spray deposition and reduce drift. Some models are configured with a horizontal air manifold across the boom just ahead of the nozzles. The air curtain discharged from the manifold blow downward, impinges upon the spray and helps to carry it to the target, providing turbulence for good deposition and reducing drift caused by typical atmospheric wind.

Figure 1.255. Tower-air curtain orchard sprayer.

Another approach uses flexible air ducts to conduct air from a central fan to the plants being sprayed. A spray nozzle at the end of the air duct mixes spray with the directed air, which carries it to the target plant, improving coverage and reducing drift.

Aerial Application. Aircraft are used to apply pesticides as sprays or dust, and to seed certain types of crops. A primary advantage of aircraft spraying is the speed with which fields may be covered. Under certain soil or crop conditions, applying chemicals by aircraft may be the only possible or reasonable alternative to ground applications. A disadvantage of aerial application is an increased opportunity for pesticide drift.

This method requires highly trained professional pilots and applicators and reliable aircraft equipment. Aircraft may be either fixed-wing craft, i.e., mono-wings or biplanes, or rotary wing types (helicopters). Aircraft sprayers typically have effective spray swath widths of 12–20 m. They fly at speeds of approximately 150–240 km/h at an altitude of 2–6 m while spraying.

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