An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an
induced electrostatic charge. 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.[1]
Invention of the electrostatic precipitator
The first use of corona to remove particles from an aerosol was by Hohlfeld in 1824. However, it was not commercialized until
almost a century later. 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 used proceeds from his invention 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 negative voltage of several thousand volts is applied between wire and plate. If the applied
voltage is high enough an electric discharge ionizes the air around the electrodes. Negative ions flow to the plates and charge
the gas-flow particles.
The ionized particles, following the negative electric field created by the power supply, move to the grounded plates.
Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure
(given from layer resistivity, electric field, and current flowing in the collected layer).
Modern industrial electrostatic precipitators
ESPs continue to be excellent devices for control of many industrial particulate emissions, including smoke from
electricity-generating utilities (coal and oil fired), salt cake collection from black
liquor boilers in pulp mills, and 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, theoretically 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 (WESP)
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 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 toxic ozone and NOx. 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
References
- ^ International Union of Pure and Applied Chemistry. "electrostatic precipitator". Compendium of Chemical Terminology Internet edition.
External links
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