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Electrostatic precipitators are highly efficient Filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. INVENTION OF THE ELECTROSTATIC PRECIPITATOR In 1907 Dr. Frederick G. Cottrell applied for a patent on a device for charging particles and then collecting them through Electrostatic attraction — the first electrostatic precipitator. He was then a professor of chemistry at the University Of California, Berkeley . Cottrell first applied the device to the collection of Sulfuric Acid mist emitted from various acid-making and smelting activities. Cottrell recognized the business potential of his invention and decided to use that to fund scientific research through the creation of a foundation called Research Corporation in 1912 to which he assigned the patents. Research Corporation has provided vital funding to many scientific projects: Goddard's rocketry experiments, Lawrence's cyclotron, production methods for vitamins A and B1 among many others. The organization continues to be active to this day. THE PLATE PRECIPITATOR The most basic precipitator contains a row of thin wires, and followed by a stack of large flat metal plates, with the plates typically spaced about 1 cm apart. The air stream flows through the spaces between the wires, and then passes through the stack of plates. A high-voltage power supply transfers electrons from the plates onto the wires, developing a negative charge of several thousand volts on the wires, relative to the positive charge on the plates. As particulate matter is carried past the strong negative charge on the wires, the particulate matter picks up the negative charge and becomes ionized. The ionized particles then move past the positively charged plates, to which the ionized particles are strongly attracted. Once the particles are in contact with the positive plate, they then give up their electrons and become positively charged like the plate, and thus start acting as part of the collector. Due to this mechanism, electrostatic precipitators can tolerate large amounts of waste buildup on the collection plates and still operate effectively, since the waste itself helps collect more waste from the air stream. Precipitator failure usually only occurs once a very heavy buildup of waste material has formed on the plates. The buildup can become heavy enough to block airflow, or can become thick enough to bridge across insulating gaps and short out the high-voltage power supply. (This typically does not damage the power supply, but effectively stops further electrostatic precipitation.) MODERN INDUSTRIAL ELECTROSTATIC PRECIPITATORS ESPs continue to be excellent devices for control of many industrial particulate emissions, including smoke from electric generating utilities (coal and oil fired), salt cake collection from black liquor boilers in pulp mills, catalyst collection from fluidized bed catalytic cracker units in oil refineries to name a few. These devices treat gas volumes from several hundred thousand ACFM to 2.5 million ACFM in the largest coal-fired boiler applications. The original parallel plate – weighted wire design (described above) has evolved as more efficient (and robust) discharge electrode designs were developed, today focusing on rigid discharge electrodes to which many sharpened spikes are attached, maximizing corona production. Transformer-rectifier systems apply voltages of 50-100 Kilovolts at relatively high current densities. Modern controls minimize sparking and prevent arcing, avoiding damage to the components. Automatic rapping systems and hopper evacuation systems remove the collected particulate matter while on line, allowing ESPs to stay in operation for years at a time. WET ELECTROSTATIC PRECIPITATOR Electrostatic precipitation is typically a dry process, but spraying moisture to the incoming air flow helps collect the exceptionally fine particulates, and helps reduce the electrical resistance of the incoming dry material to make the process more effective. A wet electrostatic precipitator merges the operational methods of a wet Scrubber with an electrostatic precipitator to make a self-washing, self-cleaning yet still high-voltage device. CONSUMER-ORIENTED ELECTROSTATIC AIR CLEANERS Plate precipitators are commonly marketed to the public as Air Purifier devices (such as the much-hyped Ionic Breeze ) or as a permanent replacement for furnace filters, but all have the undesirable attribute of being somewhat messy to clean. A negative side-effect of electrostatic precipitation devices is the production of Ozone . However, electrostatic precipitators offer benefits over other air purifications technologies, such as HEPA filtration, which require expensive filters and can become "production sinks" for many harmful forms of bacteria. With electrostatic precipitators, if the collection plates are allowed to accumulate large amounts of particulate matter, the particles often bond so tightly to the metal plates that vigorous washing and scrubbing may be required to completely clean the collection plates. The close spacing of the plates can make thorough cleaning difficult, and the stack of plates often cannot be easily disassembled for cleaning. Some consumer precipitation filters are sold with special soak-off cleaners, where the entire plate array is removed from the precipitator and soaked in a large container overnight, to help loosen the tightly bonded particulates. SEE ALSO |
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