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Hitachi Infrastructure Systems (Asia) Pte. Ltd.

Principles of Electrostatic precipitator

Electrostatic precipitation is a method of dust collection that uses electrostatic forces, and consists of discharge wires and collecting plates. A high voltage is applied to the discharge wires to form an electrical field between the wires and the collecting plates, and also ionizes the gas around the discharge wires to supply ions. When gas that contains an aerosol (dust, mist) flows between the collecting plates and the discharge wires, the aerosol particles in the gas are charged by the ions. The Coulomb force caused by the electric field causes the charged particles to be collected on the collecting plates, and the gas is purified. This is the principle of electrostatic precipitation, and Electrostatic precipitator apply this principle on an industrial scale. The particles collected on the collecting plates are removed by methods such as (1) dislodging by rapping the collecting plates, (2) scraping off with a brush, or (3) washing off with water, and removing from a hopper.

Image: Diagram of the principle of electrostatic dust collection
Diagram of the principle of electrostatic precipitation

Dust collection efficiency and electrical resistivity of dust

The dust collection efficiency of Electrostatic precipitator is affected by the electrical resistivity of the dust collected.

In the normal resistivity area, the dust collection efficiency is high, so dust collection is stable. Most of the aerosols handled by Electrostatic precipitator are in this electrical resistivity area.

Particles in the low-resistivity area lose their charge as soon as they arrive at the collecting plate, so they are re-entrained in the dust collection area, and the dust collection efficiency is greatly reduced. (Re-entrainment)

The dust collection efficiency in the high-resistivity area reduces as the electrical resistivity of the dust increases. Also, partial discharge occurs within the dust layer collected on the collecting plates, and as a result flashovers frequently occur, the applied voltage is reduced, and the discharge current is reduced. If the electrical resistivity of the dust is further increased, the discharge current increases abnormally, and the applied voltage is reduced (back corona phenomenon). The moving-electrode type electrostatic precipitator we developed is suitable for high-performance collection of high-resistivity dust.

Therefore, an important factor when planning Electrostatic precipitator is the electrical resistivity of the dust to be collected.

Image: Schematic diagram of the dust resistivity and dust collection efficiency

Dust collection efficiency and particle size distribution

The dust collection efficiency of Electrostatic precipitator is affected by the particle size of the aerosol (dust, mist) to be collected. The theoretical migration velocity at which a particle diameter of several µm moves towards the collecting plate is almost directly proportional to the particle diameter. When dust collection is performed on an aerosol with different size particles using an electrostatic precipitator, the collection efficiency is high for the large particles, and low for the small particles.

To obtain the same dust collection efficiency for an aerosol with small particle size, the electrostatic precipitator must be larger (to increase the treating time it takes for the process gas to pass through) than that of large-size particles. Therefore, the particle size distribution of the aerosol to be collected is an important factor when planning an electrostatic precipitator.

Relationship between electrostatic precipitator collection efficiency and the particle size

Image: Relationship between electrostatic precipitator collection efficiency and the particle size

Dust collection efficiency and aerosol concentration

The dust collection efficiency of an electrostatic precipitator is greatly affected by the concentration in the gas of the aerosol (dust, mist) to be collected. For the same required value of aerosol concentration at the outlet of the electrostatic precipitator, the higher the aerosol concentration at the inlet, the longer the treatment time of the gas in the electrostatic precipitator, and the larger the device.

Also, if the inlet aerosol concentration is high and the content of fine particles smaller than several µm is high, in order to create a charge cloud of charged fine particles between the discharge wires and the collecting plates, the corona discharge from the discharge wires is reduced, also reducing the dust collection efficiency (space charging effect).

The aerosol concentration at the electrostatic precipitator inlet and the required outlet concentration are important factors when planning an electrostatic precipitator.