Advantages of electrostatic spraying

Several studies demonstrate that the use of small drops provides better results in controlling phytosanitary problems. However, due to their small masses, these drops have little kinetic energy and reach targets with greater difficulty. Thus, research with electrostatic spraying has demonstrated positive results in the use of small droplets.

Since 1984, Embrapa Meio Ambiente has carried out dozens of studies with electrostatic spraying and developed different types of sprayers with this technology.

It was found that the best droplet electrification processes are basically two. Indirect electrification by induction, where the liquid is kept grounded and electrification occurs due to a high voltage electrode, 1.000 to 5.000 Volts, which is kept close to the nozzle droplet formation zone; and direct electrification, where the syrup is electrified with a voltage between 20 thousand and 40 thousand Volts.

In indirect electrification, the drops are strongly attracted to the induction electrode because they acquire charges of opposite polarity, causing them and the body that supports them to become wet, favoring the emergence of a short circuit between the grounded nozzle and the electrified electrode. To solve this problem, it is necessary that the induction electrode assembly and its support are kept dry with the help of a high-speed air jet. However, sometimes the electrostatic attraction to the induction electrode is so strong that the high-speed air cannot eliminate the effect of the attraction and the nozzle becomes completely wet.

Wetting of the induction electrode was a technical impediment to adapting the indirect electrification system to the hydraulic nozzles traditionally used in sprayers. Thus, in the oldest and still commercial system, the hydraulic nozzle and its induction electrode were maintained in a high-speed air flow, to avoid wetting the system.

In direct electrification, the high voltage between 20 and 40 Volts is connected directly to the liquid, as the electrostatic field at the tip of the nozzle will be extremely high, eliminating the need for an external electrode. This technique is ideal for traditional hydraulic nozzles, but the entire hydraulic system will remain at high voltage, requiring isolation measures for the entire hydraulic circuit, such as the pump, hoses and pipes and the liquid storage tank.

Research carried out with a 40kV direct electrification system, with a hydraulic nozzle with a flow rate of 180ml/min, indicated that deposition on artificial targets is seven times greater, with electrified drops when compared to uncharged drops. Based on these results, Embrapa Meio Ambiente adapted a knapsack sprayer to carry out electrostatic spraying studies on some crops. In cotton, staked tomato, eggplant, strawberry and chrysanthemum crops, electrostatic spraying showed greater deposition on the underside of plant leaves, in some cases reaching six times greater than the conventional process. In onion cultivation, there were no statistical differences comparing electrostatic and conventional spraying.

In experiments carried out in chrysanthemum culture, four spray nozzles (TXVK-3, AXI 110015, AXI 12002 TWIN and AXI 11003) were used combined with electrostatic and conventional spraying techniques to study deposition and control of the two-spotted mite. The nozzles that presented drops with smaller volumetric median diameter (TXVK-3 and AXI 110015) showed greater deposits on the lower surface of the leaf when electrostatic spraying was used. Electrostatic spraying promoted greater control of the mite population when compared to conventional spraying. The direct electrification system is, without a doubt, the best technology for electrostatic spraying with hydraulic nozzles. Unfortunately, despite the good agronomic results, there are industrial problems that must be overcome to make the technology available on the market. For the development of knapsack sprayers, the problems are related to the cost of the high voltage source, which requires imported electronic components, replacement of plastic material to make the hydraulic pump, modification of the tank design and development of a new type of valve (trigger). with better electrical insulation characteristics.

Some technological difficulties have discouraged industries from developing electrostatic sprayers. In some cases, there have been international failures in the commercialization of electrostatic machines, which did not present the expected results of increased deposition. These facts occurred due to incorrect designs, with the use of nozzles producing drops with diameters greater than 120 micrometers and, mainly, with insufficient electrostatic charge intensity, that is, with an electric current in the droplet jet of less than two microamperes per ml/s. During electrostatic spraying, the attraction of the drops is directly proportional to the charge they acquire and it can be said that the deposition increment is 10% for each microampere of current for each milliliter of droplet jet per second. Due to the great variability of targets, micrometeorological conditions and other factors, the ideal is for the electrical current verified in the droplet jet to be greater than two microamperes per milliliter of sprayed syrup per second. Another important factor that affects electrostatic deposition is the size of the droplets, as only those with diameters less than 100 micrometers benefit from electrostatic attraction, with the ability to deposit on the lower pages of the leaves.

For the development of tractor-driven sprayers, there are some options: a) system using hydraulic nozzles with indirect electrification, with auxiliary air jet, to avoid wetting the induction electrode; b) system that uses an electrostatic pneumatic nozzle, with indirect electrification. In the first situation, it is necessary to develop and adapt a fan to supply the nozzles with jets of air at an appropriate speed to keep the induction system dry. In the second situation, a pneumatic compressor is needed to supply dozens of nozzles, which individually consume 200 liters of air, at a pressure of approximately 1kg/cm. In the direct electrification system, serious insulation problems arise, as the entire hydraulic circuit is subjected to high voltage, requiring sophisticated technological solutions.

What's New

International research demonstrated that wetting the induction electrode negatively affected the electrification of the drops, but Embrapa Meio Ambiente noticed in some experiments that the electrode continues to work even when completely wet.

The most serious fact that prevented the development of this technology for the application of agrochemicals is that all known devices for induction electrification in hydraulic nozzles used highly electrically insulating electrode supports, but which become wet, causing a short circuit between the high electrode voltage and the spray nozzle, requiring them to be placed in environments with air flow.

Embrapa Meio Ambiente solved all the problems by building the induction electrode support with special material that does not allow the formation of a short circuit between the electrified electrode and the grounded liquid. A conversion kit is low cost, below R$100,00 per unit, does not require auxiliary air and can be adapted to any type of hydraulic sprayer on the market.

Aldemir Chaim, Embrapa Environment

Article published in issue 159 of Cultivar Máquinas.