Fundamentals of Electrostatic Spraying: Basic Concepts and Engineering Practices

Fundamentals of Electrostatic Spraying: Basic Concepts and Engineering Practices

Manoj Kumar Patel (Academy of Scientific and Innovative Research, India & CSIR-Central Scientific Instruments Organisation, India) and Chirravoori Ghanshyam (CSIR-Central Scientific Instruments Organisation, India)
DOI: 10.4018/978-1-4666-8682-3.ch018
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The desired attributes of electrostatic spraying are uniform deposition onto both directly exposed or obscured crop surfaces which minimize the off-target losses of active ingredients to soil, water, atmosphere and provide more effective and economical pest control. This chapter presents an overview of electrostatic spraying technologies in the field of agriculture emphasizing the key role of advanced electrostatic instrumentation and chronicles the scientific innovations in the parlance of providing cost effective and reliable commercial systems along with an insight on the needs of future research perspectives and directives. It is aimed primarily at a familiarization with spraying concepts and engineering practices. This text is to bridge the knowledge and experience gap among researchers and technology developers and the people involved in electrostatic processes applied to agriculture and food processing. It will also introduce the engineering aspects of design and development of an electrostatic spraying nozzle for agricultural applications.
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Air-assisted electrostatic sprayers are advanced agri-instruments for efficient use of pesticides to agricultural crops, orchards, plants, trees etc. The electrostatic spraying technique is all about reducing the use of pesticide by increasing the efficiency and bio-efficacy. Bio-efficacy is a measure of the biological efficacy of an active ingredients of agro chemical such as insecticide etc. The methods used to perform the function of bioremediation are known as bioremediators. Electrostatic spraying is to be the one among the available spraying techniques in precision agriculture and food processing. For example, the electrostatic spraying technique can be used for protective biomaterial coatings to fruits and vegetables for resistance towards microbial attacks, to enhance the transportation life, to control spoilage microorganisms, antimicrobial sprays for enhanced food safety etc. It is a method which reduces the environmental pollution by reducing contamination to soil as well as air. In totality, it reduces the chemical consumption which is used indiscriminately through conventional methods such as pedestal-mounted sprayers, the high pressure spray guns, the hand pressure swirl nozzles and the consecutive high volume spraying systems etc. The trans-disciplinary aspects of the embryonic field of electrostatic spraying have provided a major motivation to agricultural and food processing researchers for the development of novel techniques for spraying liquid pesticides to crops and orchards, protective coatings to food and food packaging, in addition to other applications of sprays to industrial, manufacturing and transportation, medical facilities and devices etc. This chapter is to be one among the motives behind the renewed curiosity in the usage of the electrostatics in liquid based spraying.

Although, organic measures for crop protection are being preferred, chemical intervention is still the fastest and most economical way for crop protection. However, due to lack of awareness and ignorance, pesticides are being used indiscriminately leading to side effects on human health and ecosystem. Electrostatic method of pesticide application reduces off-target drift, environmental pollution and human health risks and increases the bio-efficacy and mass transfer efficiency onto the biological surfaces of crops and trees with uniform back deposition. Law, (1978); Jia, Xue, Qui, & Wang, (2013) explained the design and development of induction based electrostatic sprayer for agricultural usage and evaluated the performance. So far, the equipment available in the market are uncontrolled in terms of spraying variability. Pesticide application control, targeted pesticide delivery and variable pesticide spraying are the key to improve operation quality, reduce chemical waste, environmental pollution and operational costs. This entices to develop a sensing mechanism which would discriminate between the presence and absence of pesticide application surfaces. He, Yan, & Chu, (2003) developed the automatic target detecting air-assisted electrostatic orchard sprayer. In this spraying system, the sensing mechanism is based on infrared proximity sensors which determine the presence and absence of target to be sprayed. Other than infrared proximity sensors, ultrasonic sensor mechanism is another substitute for target detection and canopy mapping. Sensory attributes stipulate a good approximation of target and canopy mapping for targeted delivery of pesticides to actual target. Automation and mechanization with respect to agricultural pesticide spraying is one of the naive research topics in the present scenario.

Key Terms in this Chapter

Faraday Cage: Faraday cage, a specially designed wire mess like structure which is called a cage, commonly used to collect the charged droplets coming out from the spraying system. The contact of the charge droplets onto the wire meshes of Faraday cage and transfer of the charge to the earth caused an electrical current which is detected by a microampere meter, the charged liquid spray is then collected at a specific time and weight. Then the spray current is divided by the mass flow rate to determine the charge to mass ratio.

Electrode Material: The amount of charge present in the fine droplet depends on many parameters, the one is electrode material. Appropriate and suitable electrode material and its dimensional specifications may enhance the chargeability of finely divided particulate matter. Selection of electrode material for spray charging is as much important as other parameters such as electrical and mechanical properties of liquid to be sprayed. So far, most frequently used materials for electrode in electrostatic nozzle systems are nickel, copper, stainless steel and brass.

Wraparound Effect: In electrostatic spraying, the pesticide deposits uniformly onto both directly exposed or obscured crop surfaces. The deposition of pesticide onto the backside of the crop surface is called wraparound effect. The wraparound effect occurs due to electrostatic phenomena takes place during the spraying.

In-Flight Trajectory: The path followed by the charged particulate matter in the presence of many forces including electrostatic force is called in-flight trajectory. Once the droplets have been charged inductively, the charged spray-cloud has to travel in a harsh and transient environment, and thus, a charged spray-cloud will encounter some degree of neutralization. The charged droplets are governed by many forces such as gravitational force, force due to surface tension, electrostatic forces, drag force etc.

ESPART Analyzer: Electronic Single Particle Relaxation Time (ESPART) analyzer is an instrument which is used in many applications to determine both size and charge of micrometer sized particles in electrostatic spraying processes. ESPART analyzer consists of two major units: Laser Doppler Velocimeter (LDV) and particle relaxation apparatus which measures the velocity of a particle and aerodynamic diameter passing through the intersection of two coherent light sources, such as lasers and the signal processing unit, a simulation model for the ESPART analyzer to measure the time lag. The ESPART analyzer can measure the statistical distribution of particle size and charge in electrostatic spraying processes.

Bio-Efficacy: In agrochemical, bio-efficacy is a measure of the biological efficacy of an active ingredients of agrochemicals such as insecticide etc. Bio-efficacy of an insecticide is determined by the minimum dose required for maximum control of the disease. For an agrochemical, the bio-efficacy of an insecticide is determined by the minimum dose required for complete kill of the insects or diseases.

Volume Median Diameter: It also refers to average volumetric size or mean size. The Volume Median Diameter (VMD) refers to the midpoint droplet size, where half of the volume of spray is in droplets smaller, and half of the volume is in droplets larger than the mean. For example, A VMD ( DV 0.5 ) of 50µm, indicates that half of the volume is in droplet sizes smaller than 50µm, and half the volume is in droplet sizes larger than 50µm.

Rayleigh Limit of Charge: A drop can hold a maximum charge, defined by the Rayleigh limit, or a limit defined by electrical breakdown strength. The maximum limits of charge that can charge mother droplet carry without rupture into the daughter droplets.

Charge to Mass Ratio: Experimentally the performance of the air-assisted electrostatic nozzle can be evaluated in terms of charge to mass ratio, which signifies the chargeability of the spray droplets by the charging electrode. It also signifies the efficiency and performance of the spraying system, higher the charge to mass ratio, better the performance. Charge to mass ratio depends on electrical and mechanical properties of the liquid as well as material of the charging electrode.

Induction Charging: Induction charging is the most commonly used method to charge the finely divided particulate matter in electrostatic spraying processes. In electrostatic induction charging, direct charge-transfer to droplet formation zone of a liquid jet results from electrostatic induction of electrons on to the continuous jet and in order to maintain it at ground potential the presence of a closely positioned electrode of positive polarity is required. Induction electrification process reduces the chances of shock and hazardous to operators of the nozzle system.

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