ELECTROSTATIC PRECIPITATORS

Description

An electrostatic precipitator is used for the efficient removal of particles from gas streams. The gas is passed through high voltage ionisers, and the particles become charged. The particles are then attracted to oppositely charged collection plates where they agglomerate. In dry electrostatic precipitators the agglomerated particles are then removed in a dry form by mechanical rapping or vibration to give a powder for disposal. In wet electrostatic precipitators the particles are sprayed and wash off as a slurry, which may require treatment before discharge.

The high voltage electrodes are usually wires or plates but can be spiked rods and either positive or negative corona discharges are employed. The collector electrodes are usually plates or tubes. In single-stage electrostatic precipitators the charging and removal of particles takes place in the same chamber. Two stage systems, where collection is separate, are more commonly employed for small gas volumes with a high proportion of submicron particles.

Application

Electrostatic precipitators are able to operate at temperatures up to 400 C and under a wide range of other conditions. Units can be arranged in parallel to increase capacity or in series to achieve higher collection efficiencies.

Electrostatic precipitators are characterised by high collection efficiencies (>995%) and low operating costs. Such benefits are offset by relatively high capital costs.

Dry electrostatic precipitators are used for the removal of smoke, fumes and condensible organic vapours from the printing, textile, vinyl coating and food industries. Wet electrostatic precipitators can be used for the removal of odour in the food industry, and the removal of organic smoke in the wallpaper industry. They can achieve very high collection efficiencies as dust re-entrainment is eliminated. However, they can create a steam plume and disposal of the effluent is generally more expensive.

Sizing

Electrostatic precipitators are not usually suitable for gas streams less than 10 m3/second, although small units may be incorporated into ventilation systems. A typical dry electrostatic precipitator dealing with a gas flow of about 3000 m3/hour would be rated at 100 W, and would also require a 15 kW fan. A wet electrostatic precipitator used for odour removal in a gaseous stream of 25,000 m3/hour would require a 15 kW fan, a 15 kW recirculation pump, and a 1 kW electrostatic power pack.