AIR-ASSIST
AND ELECTROSTATIC SPRAYERS |
Vern Hofman, Extension Ag. Engineer
North Dakota State University
Most agricultural pesticides are applied by pumping chemical through spray nozzles and the nozzle atomizes the liquid into a range of droplet sizes. Current spray applications rely mainly on a combination of inertial and gravitational forces to control and direct the spray drops toward the target. These forces are often unable to overcome the problems of poor deposition, poor penetration into the plant canopy and high drift susceptibility. The upper parts of the canopy and top leaf surfaces often receive much higher deposits than the lower parts of the canopy and bottom leaf surfaces.
Equipment improvements in the application of pesticides are needed to increase the efficiency of spray penetration into plant canopies, spray deposition on the underside of leaves and overall pest control. In 1992, the USDA increased its research effort to improve application technology for pest control. Included in the effort was the evaluation of air-assisted sprayers. Air-assisted sprayers use a high velocity air flow to aid in the atomization of the spray solution to help in the penetration of the spray into the canopy and in the spray deposition on the under-side of leaves.
Research has shown that air-assisted sprayers can increase deposition on plant surfaces, especially covering the under-side of leaves. Air-assisted sprayers are divided into two basic types of sprayers. One is termed an air curtain type sprayer which uses a standard spray nozzle to atomize the spray solution which is then entrained into a curtain of air which forces the spray into the canopy. The other type of air-assisted sprayer is termed an air atomizer sprayer and uses a high velocity air flow to atomize the spray solution and to force the spray drops into the plant canopy.
A study in Israel found that an air-curtain sprayer (Degonia, FMC Corp.) used in a dense cotton canopy found improved overall droplet coverage and better defoliation than a conventional sprayer equipped with hydraulic nozzles. But, when cotton insect pest control was studied , no significant difference was found between spray treatments using the air-assist sprayer and conventional nozzles. Another trial using an air curtain sprayer (Degonia, FMC Corp.) and a conventional hydraulic nozzle sprayer in several different crop canopies found that uniformity of drop distribution from the air curtain system was usually better than the hydraulic nozzle; however, the quantity of spray deposited on different crop canopies was generally similar for both sprayers.
A University of Tennessee study compared an air curtain sprayer (Twin Hardi Inc.), an over-the-top hydraulic nozzle sprayer with two nozzles per row and an over-the-top hydraulic drop nozzle sprayer with three nozzles per row in a mature cotton canopy. They reported that the air curtain sprayer increased spray deposition and chemical efficiency in the canopy middle to 92% of that detected in the canopy top. However, the drop nozzle sprayer tended to control cotton insects on the leaf under-side more effectively than the other applicators.
A USDA study in Mississippi tested air-assist sprayers for penetration and deposition efficiency in a cotton canopy. The air-assist sprayers included both air curtain (Degonia, FMC, Inc.) (Hardi Twin) and an air atomizer (Airtec). Drop size, leaf coverage and deposition on the upper and under-sides of leaves at the top and middle canopy locations were measured and compared to a conventional hydraulic over-the-top ground sprayer. The top of the canopy results using leaf-wash analysis showed that the conventional hydraulic over-the-top sprayer deposited more insecticide on the upper side of leaves and only the Airtec sprayer deposited more chemical on the underside of leaves (Figure 1). Middle of the canopy results showed that all air-assisted sprayers, when compared to the conventional over-the-top hydraulic sprayer, deposited more insecticide on both the upper and under-side of leaves.
Another part of the study measured leaf coverage with water-sensitive paper. The results did not correlate with residue results (Figure 2). All three air-assisted sprayers on the average had a higher percent of card coverage than the conventional over-the-top hydraulic sprayer. The largest difference was on the upper side of the leaf in the top zone of the canopy where the Degonia sprayer had significantly higher deposits as compared to the other three sprayers. There was no significant difference on the under-side of the leaf in the top zone of the canopy or on both the upper and under-side of the leaf in the middle zone of the canopy.
A study done by Dupont Chemical Co. and NDSU compared an air-assist sprayer, high-pressure broadcast and directed applications and low pressure broadcast and directed applications on dry beans. The conclusion arrived at in this study found that the Spray-Air air-assist sprayer and high pressure applications provided the best control of white mold. Directed high pressure applications provided approximately 20% better control than directed low pressure applications. Broadcast high pressure applications also provided approximately 20% better control than broadcast low pressure applications and directed applications provided approximately 25% better control of white mold than broadcast applications. This was due to the spray directed at both sides of the plant. The following is a summary of the control of white mold on navy beans:
| Benlate fungicide | |
| % white mold control | |
| Air Assist | 93.7 |
| Directed high pressure followed by a broadcast high pressure (2 applications) | 100.00 |
| Directed high pressure | 97.7 |
| Directed low pressure | 77.6 |
| Broadcast high pressure followed by a broadcast high pressure (2 applications) | 62.7 |
| Broadcast high pressure | 73.4 |
| Broadcast low pressure | 51.g |
| Untreated check | 0.0 |
Figure l. Insecticide deposition and penetration comparisons for selected canopy location/leaf side and an average of the total deposition using leaf wash analysis.
Top/Top - Top of Plant/Top of Leaf
Top/Bottom - Top of Plant/Bottom of Leaf
Middle/Top - Middle of Plant/top of Leaf
Middle/Bottom - Middle of Plant/Bottom of LeafFigure 2. Percent coverage for selected canopy location/leaf side and an average of the total coverage using water sensitive paper.
Another study at NDSU looked at the relative difference in plant canopy penetration of spray in spring wheat with the Rardi Twin sprayer. The differences measured with a string collector compared the amount of dye collected at 3 levels in the plant canopy with a conventional spray boom applying 20 OPA and with the air curtain sprayer. The main differences in canopy penetration show up at the lower operating pressures. The air-assist is probably breaking up the larger spray drops, directing them to the target and providing better coverage. This does not show up as much at the higher operating pressures. The test results are shown in the following table:Relative Deposition of Spray into Spring Wheat (Hardi Twin Sprayer)
Spray Pressure and String Placement 20 GPA 20 OPA
Air Off Air On
| Relative Deposition of Spray into Spring Wheat (Hardi Twin Sprayer) | ||
| Spray Pressure and String Placement | 20 GPA Air Off | 20 GPA Air On |
| 40 PSI | ||
| Top Plant | 174.0 | 476.9 |
| Middle Plant | 47.8 | 103.2 |
| Bottom Plant | 25.7 | 109.2 |
| 80 PSI | ||
| Top Plant | 494.7 | 491.7 |
| Middle Plant | 194.5 | 279.9 |
| Bottom Plant | 91.3 | 122.7 |
| 160 PSI | ||
| Top Plant | 412.6 | 494.7 |
| Middle Plant | 466.5 | 374.3 |
| Bottom Plant | 202.5 | 205.3 |
Some of the preceding trials look promising for the successful use of air-assist sprayers. But other studies show no improvement in yield over the conventional applications as climatic conditions, weeds and other pests may interfere with crop growth and final yield. Also, two trials in Mississippi and Saskatchewan have shown increased drift with air-assist sprayers. This is due to small spray drops being carried in the high speed air stream and when the air stream approaches the ground, it is dissipated in all directions. As the air stream rebounds, some of the fine drops are carried up as they are too small to break loose from the air stream and deposit on the target. Be sure to use caution when using drift susceptible pesticides in high speed air streams.
Electrostatic forces have been investigated for use in pesticide application for several decades. While many electrostatic sprayers have been tested extensively and provided favorable results in laboratory environments, field results have been inconsistent. The deposition of charged droplets under field conditions varies significantly from laboratory applications. Transport of droplets to the plants is greatly influenced by the size and velocity of droplets, dynamics of the spray vehicle, weather conditions, and physical properties of plants. The electrical phenomena governing the deposition of charged droplets (the electro-deposition process) are the electrical field gradient between the atomizer and the plant, space charge, and image charge effects.
An electrostatic atomizer was developed in the United Kingdom that relied solely on electrostatic forces for atomization of non-conductive spray liquids and subsequent emission of the droplets in air. The United Kingdom invention has been implemented in a hand held sprayer, the Elektrodyne and is being used mainly in developing countries. Several researchers have indicated the need for an air-assist system to successfully develop a field sprayer equipped with this type of electrostatic atomizer.
A similar atomizer was developed in the United States. It is called TotalStatÆ, Terronics Development Corp., and can atomize liquids with viscosities ranging from waterlike liquids to honey. A study using the TotalStat electrostatic atomizer was completed at Ohio State University. The following conclusions found are for sprays produced by the TotalStat which relies primarily on electrostatic and gravitational forces for spray drop transport and subsequent deposition.
_ The charge on small drops was more effective than on large drops.
_ Penetration of spray is poor into a multi-level plant canopy.
_ Electrostatic charging improves the deposit efficiency of the spray immediately behind the atomizer, especially for small drops.
_ Charging of drops also makes a portion of the spray more susceptible to drift due to inter-droplet repulsive electrostatic forces.
This study found that an additional force is required to improve the penetration of the spray into plant canopies and to control the drift susceptibility of the charged drops under varying conditions. Comparisons of the TotalStat electrostatic atomizer with hydraulic sprayers in the laboratory have demonstrated that the electrostatic atomizer has potential to improve chemical efficiency significantly. However, control of pests in field experiments has been inconsistent.
Another study at Ohio State University looked at penetration and deposition of charged spray through air jets into a plant canopy. This study was done in a wind tunnel at 3 wind speeds with 2 inch wide air jets directing charged spray drops at velocities up to 35 mph.
The conclusions determined in this study were:
Electrostatic and air-assist sprayers look to have some potential in pesticide application on the prairies. The information presented is very limited. Much of it was done in the laboratory and much of the field studies were done on cotton. The cotton plant may or may not be representative of crops grown in this area and the answer to usefulness of air-assisted sprayers on the prairies cannot be answered at this time. In the next one to two years, considerable more information will be available.
