Industrial applications throughout Australia generate gas streams amounting to millions of cubic meters per hour. These gases are hot and contain hundreds of tonnes of particulate matter such as dust, fume and mist. In order to comply with EPA emission control laws, these components must be removed from the gas stream before discharging to atmosphere.
Electrostatic precipitators (ESP’s) clean these industrial gases using high voltage. High voltage is applied to the ‘discharge electrode system’. The discharge system is comprised of an arrangement of vertical wires or masts centrally suspended between grounded parallel plates.
The voltage is high enough to cause a corona discharge in the gas; hence the name ‘discharge electrode’. The parallel plates form passages through which the gas flows. The suspended dust burden is ‘precipitated’ on these collecting surfaces as the gas stream moves through the unit. At suitable time intervals, the collecting surfaces are ‘rapped’ with electromagnetic or pneumatic hammers or vibrators to transfer the precipitated dust to the hoppers below for removal by a conveyor system.
Electrostatic precipitators can be designed to collect up to 99% or more of the suspended particulate matter equating to emissions less than 30mg/Nm3. ESP’s are used in many industries and processes operating over a wide range of temperatures up to 400oC. They offer negligible resistance to the flow of the gas stream, so do not require additional fan capacity or power in contrast to other devices or methods of gas cleaning.
Most industrial precipitators have a plate-type collecting surface. The gas flow of such units is usually horizontal. There are also pipe-type precipitators, water flushed precipitators and vertical (gas) flow types.
In any precipitator, it is important that the gas flows through with near uniform velocity, usually about one-tenth of the velocity in the flue delivering the gas to the precipitator. Devices to achieve the necessary uniformity are part of the installation. The collecting surface plates (or curtains), the high voltage electrode system and power supplied are all carefully engineered to assure the best possible combination of accurate alignment, easy installation, efficient precipitation, effective rapping, dependable operation, long life and minimum cost.
Several different types of rappers are available for various types of dust (fly ash, cement etc); the rapping intensity and schedule are adjustable for best results with each individual application.
Fowlerex Electrostatic Precipitators can be designed for sizes ranging from a few thousand cubic meters per hours up to several million cubic meters per hour. For maximum collection efficiency, microprocessor control continually keeps the corona current regulated so a peak vale of the voltage is attained.
Fowlerex precipitators are usually constructed of mild steel, however, other materials used for special applications such as in the pulp and paper industry for sulfuric acid mist and others.
Fowlerex has used many types of collecting surfaces in its long history – rod curtains, pockets, slot boxes, expanded metal and module type plates.
The Adjust-a-flow gas distribution plate show is one type of device that aids in making the velocity near uniform through the electrode system.
The plate type collecting surfaces shown are manufactured independently hung and aligned modules, allowing freedom of movement to cater for changes in process temperature effects on expansion.
The high voltage system shown is of wire-weight design. Shrouded discharge wires eliminate arcing and burning of electrodes where they pass adjacent closer tolerance collecting system components.
High voltage discharge system using mast electrodes is currently used.
The rappers shown are electric impact. European style tumbling hammer rappers are available. The Fowlerex tumbling hammer rapper is top mounted, providing maximum impact shear forces.
Hopper baffles are located to effectively prevent gas sneak-by in the hopper without interfering with the free flow of collected dust.
Not shown is an Insulator Compartment Vent and Heat System to provide clean warm air to keep the vital support insulators clean and dry.
The construction features are typical of all Fowlerex electrostatic precipitators. Some features are modified to suit the style of ESP, eg irrigated weir, acid mist models.
Fowlerex has one of the world’s largest ranges of electrostatic precipitators and some of these are indicated in the following brochures. As opposed to suppliers who offer electrostatic precipitators but have not built them in Australia, Fowlerex has an extensive installation list of Australian installations stretching over some 38 years. All design, engineering and manufacture is carried out in Australia.
Principles of Operation
High Voltage DC is applied to the high voltage discharge system, developing a corona. Particulate laden air enters the ESP and passes between the collecting surfaces. The particulates are ionized by the centrally located HV discharge system receiving a negative charge. The negatively charged particles are attracted to the positively charged, but grounded, collecting surface. Upon reaching the collecting surface, the particle gives up its charge, but is held to the plate by an EMF generated by the ionized current flow through the particulate. A dust layer builds up on the plates.
At periodic intervals, the collecting surfaces are rapped, shearing the accumulated layer particulate from the plates. The particulate is released by the rapping force which is subsequently collected in the hopper. At less frequent intervals, the HV discharge system is also rapped.