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Collection electrode of electrostatic precipitator in waste incineration plantAn electrostatic precipitator (ESP) is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced minimally impeding the flow of gases through the unit.In contrast to which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy (in the form of electricity). The first use of to remove particles from an aerosol was by Hohlfeld in 1824. However, it was not commercialized until almost a century later.In 1907, a professor of chemistry at the, applied for a patent on a device for charging particles and then collecting them through attraction—the first electrostatic precipitator. Cottrell first applied the device to the collection of mist and fumes emitted from various acid-making and activities. Wine-producing in northern California were being adversely affected by the lead emissions.

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At the time of Cottrell's invention, the theoretical basis for operation was not understood. The operational theory was developed later in Germany, with the work of Walter Deutsch and the formation of the Lurgi company.Cottrell used proceeds from his invention to fund scientific research through the creation of a foundation called in 1912, to which he assigned the patents. The intent of the organization was to bring inventions made by educators (such as Cottrell) into the commercial world for the benefit of society at large. The operation of Research Corporation is perpetuated by royalties paid by commercial firms after commercialization occurs. Research Corporation has provided vital funding to many scientific projects: 's rocketry experiments, 's, production methods for and, among many others.By a decision of thethe Corporation had to be split into several entities. The Research Corporation was separated from two commercial firms making the hardware: Research-Cottrell Inc. (operating east of the Mississippi River) and Western Precipitation (operating in the western states).

The Research Corporation continues to be active to this day, and the two companies formed to commercialize the invention for industrial and utility applications are still in business as well.is the term used for migration of gas-suspended charged particles in a direct-current. Traditional television sets tend to accumulate dust on the screen because of this phenomenon (a CRT is a direct-current machine operating at about 35 kilovolts). Conceptual diagram of an electrostatic precipitatorThe most basic precipitator contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented vertically, with the plates typically spaced about 1 cm to 18 cm apart, depending on the application. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the stack of plates.A negative voltage of several thousand is applied between wire and plate. If the applied voltage is high enough, an electric ionizes the gas around the electrodes.

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 (due to layer resistivity, electric field, and current flowing in the collected layer). A two-stage design (separate charging section ahead of collecting section) has the benefit of minimizing ozone productionwhich would adversely affect health of personnel working in enclosed spaces. For shipboard where generate an, two-stage ESP's are used to clean the air, improving the operating environment and preventing buildup of flammable oil fog accumulations. Collected oil is returned to the gear lubricating system. Precipitator performance is very sensitive to two particulate properties: 1) Electrical resistivity; and 2).

These properties can be measured economically and accurately in the laboratory, using standard tests. Resistivity can be determined as a function of temperature in accordance with IEEE Standard 548. This test is conducted in an air environment containing a specified moisture concentration. The test is run as a function of ascending or descending temperature, or both. Data is acquired using an average ash layerfurther explanation needed electric field of 4 kV/cm. Since relatively low applied voltage is used and no sulfuric acid vapor is present in the test environment, the values obtained indicate the maximum ash resistivity.In an ESP, where particle charging and discharging are key functions, resistivity is an important factor that significantly affects collection efficiency. While resistivity is an important phenomenon in the inter-electrode region where most particle charging takes place, it has a particularly important effect on the dust layer at the collection electrode where discharging occurs.

Particles that exhibit high resistivity are difficult to charge. But once charged, they do not readily give up their acquired charge on arrival at the collection electrode.

On the other hand, particles with low resistivity easily become charged and readily release their charge to the grounded collection plate. Both extremes in resistivity impede the efficient functioning of ESPs. ESPs work best under normal resistivity conditions.Resistivity, which is a characteristic of particles in an electric field, is a measure of a particle's resistance to transferring charge (both accepting and giving up charges).

Resistivity is a function of a particle's chemical composition as well as flue gas operating conditions such as temperature and moisture. Resistivity Values of Various Chemicals and Reagents as a Function of TemperatureResults for Fly Ash A (in the figure to the left) were acquired in the ascending temperature mode. These data are typical for a moderate to high combustibles content ash.

Data for Fly Ash B are from the same sample, acquired during the descending temperature mode.The differences between the ascending and descending temperature modes are due to the presence of unburned combustibles in the sample. Between the two test modes, the samples are equilibrated in dry air for 14 hours (overnight) at 850 °F (450 °C). This overnight annealing process typically removes between 60% and 90% of any unburned combustibles present in the samples.

Exactly how carbon works as a charge carrier is not fully understood, but it is known to significantly reduce the resistivity of a dust. Resistivity Measured as a Function of Temperature in Varying Moisture Concentrations (Humidity)Carbon can act, at first, like a high resistivity dust in the precipitator. Higher voltages can be required in order for corona generation to begin.

These higher voltages can be problematic for the TR-Set controls. The problem lies in onset of corona causing large amounts of current to surge through the (low resistivity) dust layer. The controls sense this surge as a spark. As precipitators are operated in spark-limiting mode, power is terminated and the corona generation cycle re-initiates. Thus, lower power (current) readings are noted with relatively high voltage readings.The same thing is believed to occur in laboratory measurements. Parallel plate geometry is used in laboratory measurements without corona generation. Vlc patch. A stainless steel cup holds the sample.

Another stainless steel electrode weight sits on top of the sample (direct contact with the dust layer). As voltage is increased from small amounts (e.g. 20 V), no current is measured.

Then, a threshold voltage level is reached. At this level, current surges through the sample. So much so that the voltage supply unit can trip off. After removal of the unburned combustibles during the above-mentioned annealing procedure, the descending temperature mode curve shows the typical inverted “V” shape one might expect.

Low ResistivityParticles that have low resistivity are difficult to collect because they are easily charged (very conductive) and rapidly lose their charge on arrival at the collection electrode. The particles take on the charge of the collection electrode, bounce off the plates, and become re-entrained in the gas stream. Thus, attractive and repulsive electrical forces that are normally at work at normal and higher resistivities are lacking, and the binding forces to the plate are considerably lessened. Examples of low-resistivity dusts are unburned carbon in fly ash and carbon black.If these conductive particles are coarse, they can be removed upstream of the precipitator by using a device such as a cyclone.The addition of liquid ammonia (NH 3) into the gas stream as a conditioning agent has found wide use in recent years. It is theorized that ammonia reacts with H 2SO 4 contained in the flue gas to form an ammonium sulfate compound that increases the cohesivity of the dust.

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This additional cohesivity makes up for the loss of electrical attraction forces.The table below summarizes the characteristics associated with low, normal and high resistivity dusts.The moisture content of the flue gas stream also affects particle resistivity. Increasing the moisture content of the gas stream by spraying water or injecting steam into the duct work preceding the ESP lowers the resistivity. In both temperature adjustment and moisture conditioning, one must maintain gas conditions above the dew point to prevent corrosion problems in the ESP or downstream equipment. The figure to the right shows the effect of temperature and moisture on the resistivity of a cement dust.

As the percentage of moisture in the gas stream increases from 6 to 20%, the resistivity of the dust dramatically decreases. Also, raising or lowering the temperature can decrease cement dust resistivity for all the moisture percentages represented.The presence of SO 3 in the gas stream has been shown to favor the electrostatic precipitation process when problems with high resistivity occur. Most of the sulfur content in the coal burned for combustion sources converts to SO 2. However, approximately 1% of the sulfur converts to SO 3. The amount of SO 3 in the flue gas normally increases with increasing sulfur content of the coal. The resistivity of the particles decreases as the sulfur content of the coal increases. A smokestack at coal-fired in emits brown smoke when its ESP is shut downESPs 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 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 to 2.5 million ACFM (1,180 m³/s) in the largest coal-fired boiler applications. For a coal-fired boiler the collection is usually performed downstream of the air preheater at about 160 °C (320 °F) which provides optimal resistivity of the coal-ash particles. For some difficult applications with low-sulfur fuel hot-end units have been built operating above 370 °C (698 °F).The original parallel plate–weighted wire design  has evolved as more efficient (and robust) discharge electrode designs were developed, today focusing on rigid (pipe-frame) discharge electrodes to which many sharpened spikes are attached (barbed wire), maximizing production. Transformer-rectifier systems apply voltages of 50–100 at relatively high current densities. Modern controls, such as an, minimize and prevent arcing (sparks are quenched within 1/2 cycle of the ), avoiding damage to the components.

Automatic plate-rapping systems and hopper-evacuation systems remove the collected particulate matter while on line, theoretically allowing ESPs to stay in continuous operation for years at a time. A wet electrostatic precipitator (WESP or wet ESP) operates with saturated air streams (100% relative humidity). WESPs are commonly used to remove liquid droplets such as sulfuric acid mist from industrial process gas streams.

The WESP is also commonly used where the gases are high in moisture content, contain combustible particulate, or have particles that are sticky in nature.The preferred and most modern type of WESP is a downflow tubular design. This design allows the collected moisture and particulate to form a moving that helps to keep the collection surfaces clean. Plate style and upflow design WESPs are very unreliable and should not be used in applications where particulate is sticky in nature. Portable electrostatic air cleaner with cover removed, showing collector platesPlate precipitators are commonly marketed to the public as devices 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 potential production of toxic.

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However, electrostatic precipitators offer benefits over other air purifications technologies, such as 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 can sometimes 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.

One solution, suggested by several manufacturers, is to wash the collector plates in a.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.A study by the testing a variety of furnace filters found that ESP filters provided the best, and most cost-effective means of cleaning air using a forced-air system.The first portable electrostatic air filter systems for homes was marketed in 1954 by Raytheon. (the 'Gold Book') (1997).

Online corrected version: (2006–) '. Ali Farnoud 'Electrostatic Removal of Diesel Particulate Matter', ProQuest, 2008, p.

GEA Bischoff. Retrieved 25 January 2014. Johnson, F. 'Adsorbed Moisture Film on the Surface of Glazed Porcelain'. 24: 797. Davidson, J.

H.; McKinney, P. Chemical vapor deposition in the corona discharge of electrostatic air cleaners. Aerosol Science and Technology 29 (2).

Retrieved 2008-09-01. Popular Science, July 1954, p. 70, bottom of page.