Electrostatic precipitator - Wikipedia, the free encyclopedia. Electrodes inside electrostatic precipitator. Collection electrode of electrostatic precipitator in waste incineration plant. An 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 electrostatic charge minimally impeding the flow of gases through the unit. Cottrell first applied the device to the collection of sulphuric acid mist and lead oxide fumes emitted from various acid- making and smelting activities. Wine- producing vineyards in northern California were being adversely affected by the lead emissions.
The American dream or the history of a company driven by a farsighted philanthropist Companies of the hamon Usa GroUp: The Hamon USA Group now includes Hamon Research-Cottrell, Inc. The operational theory was developed later in Germany, with the work of Walter Deutsch and the formation of the Lurgi company. 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 funded by royalties paid by commercial firms after commercialization occurs. 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. By a decision of the US Supreme Court. 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. Electrophoresis is the term used for migration of gas- suspended charged particles in a direct- current electrostatic field. Traditional CRT television sets tend to accumulate dust on the screen because of this phenomenon (a CRT is a direct- current machine operating at about 3. Plate precipitator. The air stream flows horizontally 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 corona discharge ionizes the air around the electrodes, which then ionizes the particles in the air stream. The ionized particles, due to the electrostatic force, are diverted towards the grounded plates. Particles build up on the collection plates and are removed from the air stream. For shipboard engine rooms where gearboxes generate an oil mist, 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. 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 5. This test is conducted in an air environment containing a specified moisture concentration. An 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 electrostatic charge minimally impeding the flow of gases through.The test is run as a function of ascending or descending temperature, or both. Data is acquired using an average ash layer. 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. Particles can have high, moderate (normal), or low resistivity. Bulk resistivity is defined using a more general version of Ohm. A method for measuring resistivity will be described in this article. The table below, gives value ranges for low, normal, and high resistivity. Resistivity. Range of Measurement. Lowbetween 1. 04 and 1. Normalbetween 1. 07 and 2. At the metal surface of the electrically grounded collection plate, the voltage is zero, whereas at the outer surface of the dust layer, where new particles and ions are arriving, the electrostatic voltage caused by the gas ions can be quite high. The strength of this electric field depends on the resistivity and thickness of the dust layer. In high- resistivity dust layers, the dust is not sufficiently conductive, so electrical charges have difficulty moving through the dust layer. Consequently, electrical charges accumulate on and beneath the dust layer surface, creating a strong electric field. Voltages can be greater than 1. Dust particles with high resistivities are held too strongly to the plate, making them difficult to remove and causing rapping problems. In low resistivity dust layers, the corona current is readily passed to the grounded collection electrode. Therefore, a relatively weak electric field, of several thousand volts, is maintained across the dust layer. Collected dust particles with low resistivity do not adhere strongly enough to the collection plate. They are easily dislodged and become re- entrained in the gas stream. The electrical conductivity of a bulk layer of particles depends on both surface and volume factors. Volume conduction, or the motions of electrical charges through the interiors of particles, depends mainly on the composition and temperature of the particles. In the higher temperature regions, above 5. Volume conduction also involves ancillary factors, such as compression of the particle layer, particle size and shape, and surface properties. Volume conduction is represented in the figures as a straight- line at temperatures above 5. At temperatures below about 4. Surface conduction begins to lower the resistivity values and bend the curve downward at temperatures below 5. Theoretical calculations indicate that moisture films only a few molecules thick are adequate to provide the desired surface conductivity. Surface conduction on particles is closely related to surface- leakage currents occurring on electrical insulators, which have been extensively studied. A sharp rise in current signals the formation of a moisture film on the glass. This method has been used effectively for determining the marked rise in dew point, which occurs when small amounts of sulfuric acid vapor are added to an atmosphere (commercial Dewpoint Meters are available on the market). The following discussion of normal, high, and low resistivity applies to ESPs operated in a dry state; resistivity is not a problem in the operation of wet ESPs because of the moisture concentration in the ESP. The relationship between moisture content and resistivity is explained later in this work. Normal resistivity. Particles with normal resistivity do not rapidly lose their charge on arrival at the collection electrode. These particles slowly leak their charge to grounded plates and are retained on the collection plates by intermolecular adhesive and cohesive forces. This allows a particulate layer to be built up and then dislodged from the plates by rapping. Within the range of normal dust resistivity (between 1. High resistivity. First, the high voltage drop reduces the voltage difference between the discharge electrode and collection electrode, and thereby reduces the electrostatic field strength used to drive the gas ion- charged particles over to the collected dust layer. As the dust layer builds up, and the electrical charges accumulate on the surface of the dust layer, the voltage difference between the discharge and collection electrodes decreases. The migration velocities of small particles are especially affected by the reduced electric field strength. Another problem that occurs with high resistivity dust layers is called back corona. This occurs when the potential drop across the dust layer is so great that corona discharges begin to appear in the gas that is trapped within the dust layer. The dust layer breaks down electrically, producing small holes or craters from which back corona discharges occur. Positive gas ions are generated within the dust layer and are accelerated toward the . The positive ions reduce some of the negative charges on the dust layer and neutralize some of the negative ions on the . Disruptions of the normal corona process greatly reduce the ESP's collection efficiency, which in severe cases, may fall below 5. When back corona is present, the dust particles build up on the electrodes forming a layer of insulation. Often this can not be repaired without bringing the unit offline. The third, and generally most common problem with high resistivity dust is increased electrical sparking. When the sparking rate exceeds the . This causes reduced particle charging and reduced migration velocities toward the collection electrode. High resistivity can generally be reduced by doing the following: Adjusting the temperature; Increasing moisture content; Adding conditioning agents to the gas stream; Increasing the collection surface area; and. Using hot- side precipitators (occasionally and with foreknowledge of sodium depletion). Thin dust layers and high- resistivity dust especially favor the formation of back corona craters. Severe back corona has been observed with dust layers as thin as 0.
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