Design and Experiments on Droplet Charging Device for High-Range Electrostatic Sprayer

It is wide foreground that air-assisted electrostatic spraying technology is applied for the forest pest prevention and control of tall tree. The droplet charging device was designed including a high-voltage generator and a double acicular corona charging device for a high-range electrostatic sprayer. The device can make droplets charging more effectively and fully. The droplet charging theory by the double acicular corona charging device was introduced and an experimental system was set up. Some experiments and analysis about spraying characteristics were finished such as coverage rate, droplet diameter, charge-to-mass ratio and spray breadth. The electrostatic spraying can make the front target coverage rate increase 21droplets / cm2. Droplet can be found on front and back of target as far as 45m. When charging voltage is 20kV, electrostatic spray can averagely increase the spray breadth for 0.84m, improve the droplet distributing uniformity with the average volume medium diameter (VMD) of 80.8µm and obtain the maximum charge-to-mass ratio of 2.35mC/kg. The results showed that electrostatic spray could produce uniform and fine droplets with better droplet adhesion and distribution, higher depositing efficiency, lower environmental contamination, lower pesticide application rate. Air-assisted high-range electrostatic sprayer will take an important role for the forest pest prevention and control of tall tree.


Air-assisted electrostatic sprayer
Based on the theoretical analysis and practical experiments and the requirements for the forest pest control of tall trees, an axial-flow, air-assisted, ultra-low-volume, electrostatic sprayer was developed (China Patent No ZL03259151.9) as show in figure 1. Because adopting separating design with the sprayer from vehicle, the sprayer can be trailed by fourwheel or other type wheel tractor. A diesel engine mounted on the vehicle produced the power supply to drive axial-flow blower, pump, swing mechanism and generator ( Figure 2). The featured parameters of the sprayer (not including vehicle) are listed as follows: configuration dimension of 1900x1400x1100 mm, net weight of 400 kg, and pesticide liquid tank of 400 L. an axial-flow blower (Model of Z50) with the outlet of 500 mm in diameter, 2920 rpm rotation speed and 25m/s maximum air flow rate, spraying height of 20 to 25 m. spraying breadth of 38 to 50m, flow rate of 40 to 460L/h. Droplet size could be regulated by adjusting liquid pressure, from 50 to 150μm. Bellows can swing at range of -15°to 85°in vertical plane. An electrostatic generator was used to deliver voltage of 24-36V into a high-voltage generator with output voltage of 20 kV in common use and 30 kV in maximum. Both automatic control system and remote control system were used on the sprayer, so only operator was required.

Electrostatic charging device
The charging device was designed using corona charging methods, supplying 20kV high voltage by double acicular electrostatic device (figure 3). Double acicular stainless steel electrodes were mounted on both sides around atomizing area for each nozzle. It charges droplets in the atomizing area, without intervening the droplet transporting and subatomization through the air-assisted system. High-voltage wires pass through the fixture and connect with stainless steel electrodes. The fixture is connected with support by screw thread in order to adjust the location between the electrode and the atomizing area. Nozzle passes through support and connects with spraying conduit. For desired charging effect, both fixture and support are made of nylon. The material for nozzles is copper as for better connection and not easy leak. There were six sets of electrostatic charging devices which were mounted around the 6HW-50 air-assisted electrostatic sprayer (Figure 4).

High-voltage generator
High-voltage electrode mounted on the nozzle produces high voltage corona which makes droplets charged. In order to ensure the formation of corona field, the high-voltage generator should be supplied with limited voltage and steady current. The highest voltage can be set by a regulating knob. Under steady power supply, stabilized current will decide current density of corona field. Voltage amplitude can be adjusted according to conditions of corona field. In this manner, output current keeps steady and charging equipment can work normally under wider variation range of corona field.
The nonlinear DC high-voltage power supply technique was introduced in the high-voltage power supply equipment, i.e. the typical DC high-voltage switch power supply technique in DC-DC mode (figure 5). The MOSFET, IGBT, quasi-resonant frequency conversion technique (the frequency of power supply can be 2500Hz) and the silicon voltage multiplier technique were introduced. Compared with the linear power supply, it has outstanding characteristics of high efficiency, small volume, low weight, fast reaction, low power storage and short period of designing and manufacturing. The ±10～±30kV/5mA DC high-voltage power was generated by the high voltage generator, the regulation and display systems were combined fixed in the same control box. The regulation switch、shunt supply insurance、trimmer pot、fault display LED、two ±HV digital display and the power supply of the digital display were included in the control box. The control box has the significant advantage of small volume(only 15cm×10cm×10cm), low weight(only 250g), convenient moved and installed for monitoring and manual adjusting by driver.  Between August 10-12, 2009, a long trade tree of a fast growing poplar in Jiangsu Province were sprayed by high range sprayer with electrostatic spraying system to control micromelalopha troglodyte, a length of 10Km had been sprayed (figure 6). 10 kg raw pesticide was mixed with 200kg water for spraying. The vehicle speed was 8 km / h, each side lasted color appeared. After the forestry targets receive the color droplets, the light-green color was useful for observing and stating droplet deposition distribution and deposition density. After three days, the pest mortality or the viability of the pre-arranged standard plants (gauze was setup for observation) was investigated till the seventh day.

Experiments
The experiment was performed in a void workshop with the area of 150m×80m. As measured, the ambient wind speed was 0.8m/s, the atmosphere temperature was 28°C, relative humidity was 60% and the wind speed at the outlet of the sprayer was 25m/s. An electrostatic spray experimental system was built for atomization tests, charge-mass ratio measurements and droplets deposit investigations. A charge-mass ratio measurement device was designed and applied to measure the charge attenuation of charged droplets along the spraying swath. A laser granularity-measuring system was used to evaluate distribution of droplets granularity on sectional planes along the spray jet (figure 7). Three stainless steel balls, mounted on metal pole for simulating tree, were well-distributed on metal pole from spray boundary to spray center from anywhere within the distance of 25m apart from sprayer, (figure 8). Away from 25m, stainless steel balls were distributed at the space out 0.8m on metal pole. To collect droplets, a number of white papers were attached to the balls. Red dye was filled in reagent for test so that red droplets can be seen distinctly on the paper. In practical operation, the spray vehicle moved at speed of 10km/h and collecting set was fixed on a wheelbarrow pushed through spray area at equal speed.

Droplet size measurement
Droplet size can be measured in different positions and heights with different spraying condition and different charging voltages. The chart in figure 8 shows droplet size is measured in different charging voltages (15kV and 20kV) at the same positions (15m apart from sprayer) and in the same spraying condition. VMD is 108.4μm for non-electrostatic spraying, while it is 96.7μm at 15kV charging voltage and 80.6μm at 20kV charging voltage for electrostatic spraying. Comparing a with b in figure 9, curve of b was extend to small size direction, which showed more small size droplets were distinctly generated. So electrostatic spraying can make the droplets atomization better. Comparing b with c, curve of c was offset to small size direction, droplet were atomized deeply. Curve of c was steeper than curve of b which shows that droplet size was more uniformity when charging voltages increased. Fig. 9. Droplets granularity chart

Charge-to-mass ratio test
One of the most important indexes to evaluate the effectiveness of electrostatic spraying is the charge-to-mass ratio. Generally speaking, the higher the charge-to-mass ratio, the more effective the electrostatic spraying should be. The test facility involves a bucket for collecting droplets, a micro-amperemeter for measuring the electric current and an insulated frame. The charge could be determined by measuring the current with the micro-amperemeter and the mass rate was determined by collecting spray liquid for a specified time. The charge-tomass ratio q cm was calculated by dividing the current by the mass rate. Charge-to-mass ratio was measured in three different position (10m, 20m, 30m) and different voltages (15kV to 25kV) with a certain spraying parameters. The figure.10 showed that the q cm rises as the charging voltage increases and the droplet diameter decreases, and the q cm goes down in a lower descent rate as the distance increases. This means that the charged droplet still carry charge at distant spots.

Droplet coverage rate
After collecting droplets, droplets volumes were estimated by a square window with area of 1×1cm 2 . The test results were listed in table 1. As just as figure 7, number 1, number 2, number 3 display position of collecting ball. Table 1 shows test result for two mode of spraying such as non electrostatic spraying and electrostatic spraying. Because paper on front of collecting ball had been fully dyed into red within 15m, coverage rate had not been counted. Droplets volumes on the bottom collecting ball were more than others at the same location. Compared with non-electrostatic spraying, more droplets from electrostatic spraying deposited on the targets as far as 45m and fewer droplets drifted away. The electrostatic spraying can make coverage rate of target front increase 21 droplets per cm 2 . Therefore, it can obviously decrease the probability of pesticide poisoning in other surface and the wastage of pesticide.

Mode of spraying Ball number
Coverage ratio(droplets/ cm 2 ) 10m 15m 20m 25m 30m 35m 40m 45m *front is the side face to sprayer on collecting ball, back is the side far away sprayer on collecting ball. Blank shows paper on front of collecting ball had been fully dyed into red within 15m, coverage rate had not been counted.

Spraying breadth
The spraying breadth experimental results showed that electrostatic spraying could increase spraying breadth and averagely increase spraying breadth to 0.84m ( Table 2). The reason is that charged droplets will repel each other exerted by the like charges.

Field test result
After field test, some result were received. with the electrostatic spraying, the average cumulative mortality of pest was 95.4% and with the non-electrostatic spraying, the average cumulative mortality of pest was 74.8%. The highest mortality of pest appeared on the fourth and the fifth day after spraying and the total number of dead pest reached up to 75% of total deaths. There was a sharp decline in deaths after the seventh day. It was shown in the tests that with the electrostatic spraying, the mortality of pest was significantly higher than that of the non-electrostatic spraying. It was positively correlated with the fact that the droplet deposition effect with electrostatic spraying was obviously better than that of nonelectrostatic spraying. The reason was that with the electrostatic spraying, the droplet deposition density was larger, the distribution was more uniform and the larval had more chance to contact pesticide, so higher mortality; on the contrary, the droplet deposition density was smaller, less even distribution and the larvae had less chance to contact pesticide, so lower mortality. with the high-range electrostatic spraying for controlling Micromelalopha troglodyte, to achieve the desired effect, the dosage of raw pesticide was only 300g/mu. If controlling Micromelalopha troglodyte for 20,000ha, fund can be saved $90,000，wood loss can be reduced 30,000m 3 ,economical loss can be retrieved $6,600,000，utilization ratio of patricide can be improved 60%.
It was shown that the effect of high-range electrostatic spraying was superior to conventional aerial spraying. It was beneficial for reducing spraying drift losses, improving the density of droplet deposition. Therefore, it had the outstanding advantages of high spraying efficiency and low spraying cost.

Conclusions
Nanjing forestry University has started the research work on basic theory, testing, measurement and practical applications in electrostatic spray since 1990s. The test research result shows superiority for electrostatic spraying. The electrostatic spraying can make the front target coverage rate increase 21droplets / cm 2 . Droplet can be found on front and back of target at far as 45m. When charging voltage is 20kV, electrostatic spray can averagely increase the spray breadth for 0.84m, improve the droplet distributing uniformity with the average volume medium diameter (VMD) of 80.8μm and obtain the maximum charge-tomass ratio of 2.35mC/kg. The electrostatic spray could produce uniform and fine droplets with better droplet adhesion and spread, higher deposit efficiency, lower environmental contamination, lower application rate, less application expenses and longer residual action than conventional sprays. On the bases of the experimental results and practical production examinations in the laboratory and field, it showed that combining high-range spray technique with electrostatic spray technique, the invented air-assisted high-range electrostatic sprayer was provided with scientific design, rational structure, convenient operation, high productivity and high efficiency. There was not droplet backward phenomenon during the electrostatic spray, and thus the possibility, of pesticide contamination by the sprayer was greatly reduced.