An electrostatic precipitator (ESP) is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The particles are given an electrical charge by forcing them to pass through a corona, a region in which gaseous ions flow. The electrical field that forces the charged particles to the walls comes from electrodes maintained at high voltage in the center of the flow lane. Once the particles are collected on the plates, they must be removed from the plates without reentraining them into the gas stream. This is usually accomplished by knocking them loose from the plates, allowing the collected layer of particles to slide down into a hopper from which they are evacuated. Some precipitators remove the particles by intermittent or continuous washing with water. Types of ESPs ESPs are configured in several ways. Some of these configurations have been developed for special control action, and others have evolved for economic reasons. The types that will be described here are the plate-wire precipitator, the most common variety; the flat plate precipitator, the tubular precipitator; the wet precipitator, which may have any of the previous mechanical configurations; and the two-stage precipitator. Plate-Wire Precipitators Plate-wire ESPs are used in a wide variety of industrial applications, including coal-fired boilers, cement kilns, solid waste incinerators, paper mill recovery boilers, petroleum refining catalytic cracking units, sinter plants, basic oxygen furnaces, open hearth furnaces, electric arc furnaces, coke oven batteries, and glass furnaces. In a plate-wire ESP, gas flows between parallel plates of sheet metal and high-voltage electrodes. These electrodes are long wires weighted and hanging between the plates or are supported there by mast-like structures (rigid frames). Within each flow path, gas flow must pass each wire in sequence as flows through the unit. The plate-wire ESP allows many flow lanes to operate in parallel, and each lane can be quite tall. As a result, this type of precipitator is well suited for handling large volumes of gas. The need for rapping the plates to dislodge the collected material has caused the plat to be divided into sections, often three or four in series with one another, which can be rapped independent. The power supplies are often sectionalized in the same way to obtain higher operating voltages, and further electrical sectionalization may be used for increased reliability. Dust also deposits on the discharge electrode wires and must be periodically removed similarly to the collector plate. The power supplies for the ESP convert the industrial ac voltage (220 to 480 V) to pulsating dc voltage in the range of 20,000 to 100,000 V as needed. The supply consists of a step-up transformer, high-voltage rectifiers, and sometimes filter capacitors. The unit may supply either half-wave or full-wave rectified dc voltage. There are auxiliary components and controls to allow the voltage to be adjusted to the highest level possible without excessive sparking and to protect the supply and electrodes in the event a heavy arc or short-circuit occurs. The voltage applied to the electrodes causes the air between the electrodes to break down electrically, an action known as a “corona”. The electrodes usually are given a negative polarity because a negative corona supports a higher voltage than a positive corona before sparking occurs. The ions generated in logo polo shirts the corona follow electric field lines from the wires to the collecting plates. Therefore, each wire establishes a charging zone through which the particles must pass. Particles passing through the charging zone intercept some of the ions, which become attached. Small aerosol particles fat burning furnace (<1 ?m diameter) can absorb tens of ions before their total charge becomes large enough to repel further ions, and large particles (>10 ?m diameter) can absorb tens of thousands. The electrical forces are therefore much stronger on the large particles. As the particles pass each successive wire, they are the diet solution driven closer and closer to the collecting walls. The turbulence in the gas, however, tends to keep them uniformly mixed with the gas. The collection process is therefore a competition between the electrical and dispersive enlarged prostate forces. Eventually, the particles approach close enough to the walls so that the turbulence drops to low levels and the particles are collected. If the collected particles could be dislodged into the hopper without losses, the ESP would be extremely efficient. The rapping that dislodges the accumulated layer also projects some of the particles (typically 12 percent for coal fly ash) back into the gas stream. These reentrained particles are then processed again by later sections, but the particles reentrained in the last section of the ESP have no golf swing chance to be recaptured and so escape the unit. Practical considerations of passing the high voltage into the space between the lanes and allowing for some clearance above the hoppers to support and align electrodes leave room for part of the gas to flow around the charging zones. This is called “sneakage” and Medifast Coupons amounts to 5 to 10 percent of the total flow. Antisneakage baffles usually are placed to force the sneakage flow to mix with the main gas stream for collection in later sections. But, again, the sneakage flow around the last section has no opportunity to be collected. These losses play a significant role in the overall performance of an ESP. Another major factor is the resistivity of the collected material. Because the particles form a continuous layer on the ESP plates, all the ion current must pass through the layer tourbillon watches to reach the ground-plates. This current creates an electric field in the layer, and it can become large enough to cause local electrical breakdown. When this occurs, new ions of the wrong polarity are injected into the wire-plate gap where they reduce the charge on the particles and ricostruzione unghie may cause sparking. This breakdown condition is called “back corona” Back corona is prevalent when the resistivity of the layer is high, usually above 2 x 1011 ohmcm. For lower resistivities, the operation of the ESP is not impaired by back coronas, but bedroom furniture resistivities much higher than 2 x 1011 ohm-cm considerably reduce the collection ability of the unit because the severe back corona causes difficulties in charging the particles. At resistivities below 108 ohm-cm, the particles are wedding favors held on the plates so loosely that rapping and nonrapping reentrainment become much more severe. Care must be taken in measuring or estimating resistivity because it is strongly affected by variables such as temperature, moisture, gas composition, particle composition, and surface characteristics. Flat Plate Precipitators A significant number of smaller Christian books precipitators (100,000 to 200,000 acfm) use flat plates instead of wires for the high-voltage electrodes. The flat plates (United McGill Corporation patents) increase the average electric field that can be used to collect the particles, and they provide an increased surface area for the collection of particles. Corona cannot be generated on flat Christian Book store plates by themselves, so corona-generating electrodes are placed ahead of and sometimes behind the flat plate collecting zones. These electrodes may be sharp-pointed needles attached to the edges of the plates or independent corona wires. Unlike place-wire or tubular ESPs, buy Twitter followers this design operates equally well with either negative or positive polarity.

The manufacturer has chosen to use positive

polarity to reduce ozone generation. A flat plate ESP operates with little or no corona current flowing through the collected dust, except directly under the corona needles or wires. This has two consequences. The first is that the unit is somewhat less susceptible tax attorney to back corona than conventional units are because no back corona is generated in the collected dust, and particles charged with both polarities of ions have large collection surfaces available. The second consequence is that the lack of current in Hen Party the collected layer causes an electrical force that tends to remove the layer from the collecting surface; this can lead to high rapping losses. Flat plate ESPs seem to have wide application for high-resistivity particles with small (1 to 2 ?m) mass median diameters (MMDs). These applications especially emphasize the strengths of the design because the electrical dislodging forces are weaker for small particles than for large ones. Fly ash has been successfully collected with this type of ESP, but low-flow velocity the diet solution program review appears to be critical for avoiding high rapping losses. Tubular Precipitators The original ESPs were tubular like the smokestacks they were placed on, with the high-voltage electrode running along the axis of the tube. Tubular precipitators have typical applications in sulfuric add plants, coke oven by-product gas cleaning (tar removal), and, recently, iron and steel sinter plants. Such tubular units are still used for some applications, with many tubes operating in parallel to handle increased gas flows. The tubes may be formed as a circular, square, or hexagonal honeycomb with custom band merchandise gas flowing upwards or downwards. The length of the tubes can be selected to fit conditions. A tubular ESP can be tightly sealed to prevent leaks of material, especially valuable or hazardous material. A tubular ESP is essentially a one-stage Free iPhone 4 unit and is unique in having all the gas pass through the electrode region. The high-voltage electrode operates at one voltage for the entire length of the tube, and the current varies along the length as the particles are iPhone deals removed from the system. No sneakage paths are around the collecting region, but corona nonuniformities may allow some particles to avoid charging for a considerable fraction of the tube length. Tubular ESPs comprise only a wedding photographer Berkshire small portion of the ESP population and are most commonly applied where the particulate is either wet or sticky. These ESPs, usually cleaned with water, have reentrainment losses of a lower magnitude than do the dry particulate precipitators. Wet Precipitators Any of the precipitator configurations discussed wedding photographer Hampshire above may be operated with wet walls instead of dry. The water flow may be applied intermittently or continuously to wash the collected particles into a sump for disposal. The advantage of the wet wall precipitator is that it has no problems with rapping reentrainment or with back coronas. The disadvantage is the increased complexity of the wash and the fact that the collected slurry must be handled more carefully than a dry product, adding to the expense of disposal. Two-Stage louis vuitton handbags Precipitators The previously described precipitators are all parallel in nature, i.e., the discharge and collecting electrodes are side by side. The two-stage precipitator invented by Penney is a series device with the discharge electrode, or ionizer, preceding the collector electrodes. For indoor applications, chanel handbags the unit is operated with positive polarity to limit ozone generation. Advantages of this configuration include more time for particle charging, less propensity for back corona, and economical construction for small sizes. This type of precipitator is hair loss treatment generally used for gas flow volumes of 50,000 acfm and less and is applied to submicrometer sources Cleaning may be by water wash of modules removed from the system up to automatic, in-place detergent spraying of the collector followed Fitted Wardrobes by air-blow drying. Two-stage precipitators are considered to be separate and distinct types of devices compared to large, high-gas-volume, single-stage ESPs. The smaller devices are usually sold as preengineered, package systems. Auxiliary Equipment Typical auxiliary equipment associated with an ESP system is shown schematically in Hair Transplant Figure Along with the ESP itself, a control system usually includes the following auxiliary equipment: a capture device (i.e., hood or direct exhaust connection); ductwork; dust removal equipment (screw conveyor, etc.); fans, motors, and starters; and a stack. In addition, spray coolers and Labradoodle mechanical collectors may, be needed to precondition the gas before it reaches the ESP. Capture devices are usually hoods that exhaust pollutants into the ductwork or are direct exhaust couplings attached to a combustor or process equipment. These devices are usually refractory lined, water Meditation for beginners cooled, or simply fabricated from carbon steel, depending on the gasstream temperatures. Refractory or water-cooled capture devices are used where the wall temperatures exceed 800EF; carbon steel is used for lower temperatures. The ducting, like the capture device, should be water cooled, refractory, or stainless steel for hot processes and carbon steel for gas affordable seo services temperatures below approximately 1,150EF (duct wall temperatures <800EF). The ducts should be sized for a gas velocity of approximately 4,000 ft/min for the average case to prevent particle deposition in the ducts. Large or dense particles might require higher link building service velocities, but rarely would lower velocities be used. Spray chambers may be required for processes where the addition of moisture, or decreased temperature or gas volume, will improve precipitation or protect the ESP from warpage. marketing consultant For combustion processes with exhaust gas temperatures below approximately 700EF, cooling would not be required, and the exhaust gases can be delivered directly to the precipitator. When much of the pollutant loading consists of relatively large particles, mechanical collectors, such as cyclones, may be used to sleep aids reduce the load on the ESP, especially at high inlet concentrations. The fans provide the motive power for air movement and can be mounted before or after the ESP. A stack, normally used, vents the cleaned stream to the atmosphere. loan Screw conveyors or pneumatic systems are often used to remove captured dust from the bottom of the hoppers. Wet ESPs require a source of wash water to be injected or sprayed near the top of the collector plates either continuously or at timed intervals. The PLR Articles water flows with the collected particles into a sump from which the fluid is pumped.

A portion of the fluid may be recycled to reduce

the total amount of water required. The remainder is pumped directly to a settling pond or passed through a dewatering stage, with subsequent disposal of the sludge. Gas conditioning equipment to improve ESP performance by changing dust resistivity is occasionally reverse phone lookup used as part of the original design, but more frequently it is used to upgrade existing ESPs. The equipment injects an agent into the gas stream ahead of the ESP. Usually, the agent mixes with the particles and alters their resistivity to promote higher migration velocity, and thus higher collection efficiency. However, stickers electrical properties of the gas may change, rather than dust resistivity. For instance, cooling the gas will allow more voltage to be applied before sparking occurs. Significant conditioning agents that are used include SO, H SO, sodium 3 2 4 compounds, ammonia, and water, but the major conditioning agent 18th birthday ideas by usage is SO. A typical 3 dose rate for any of the gaseous agents is 10 to 30 ppm by volume. The equipment required for conditioning depends on the agent being used. A typical S03 conditioner requires a supply of molten sulfur. It is stored in a heated vessel and supplied debt consolidation loans to a burner, where it is oxidized to SO . The SO gas is passed over a catalyst for further oxidation 2 2 to S0. The S0 gas is then injected into the flue gas stream through a multi-outlet set of probes 3 3 that breach a duct. In place of a sulfur Donington Park burner to provide S0, liquid S0 may be vaporized from 2 2 a storage tank. Although their total annual costs are higher, the liquid SO systems have lower 2 capital costs and are easier to operate than the molten sulfur based systems.

Water or ammonia injection requires a set of spray nozzles in the duct, along with pumping and control equipment. Sodium conditioning is often fat burning furnace done by coating the coal on a conveyor with a powder compound or a water solution of the desired compound. A hopper or storage tank is often positioned over the conveyor for this purpose. Electrostatic Precipitation Theory The theory of ESP operation requires many scientific unlock blackberry 9800 disciplines to describe it thoroughly. The ESP is basically an electrical machine. The principal actions are the charging of particles and forcing them to the collector plates. The amount of charged particulate matter affects the electrical operating point of the ESP. The transport of marketing company the particles is affected by the level of turbulence in the gas. The losses mentioned earlier, sneakage and rapping reentrainment, are major influences on the total performance of the system. The particle properties also colon cleanse leave a major effect on the operation of the unit. Electrical Operating Point The electrical operating point of an ESP section is the value of voltage and current at which the section operates. As will become apparent, the best collection occurs when Succession Planning the highest electric field is present, which roughly corresponds to the highest voltage on the electrodes. In this work, the term “section” represents one set of plates and electrodes in the direction of flow. This unit is commonly called pest control Brisbane a “field”, and a “section” or “bus section” represents a subdivision of a “field” perpendicular to the direction of flow. In an ESP model and in sizing applications, the two terms “section” and “field” are used equivalently Car Payment Calculator because the subdivision into bus sections should have no effect on the model. This terminology has probably arisen because of the frequent use of the word “field” to refer to the electric field. The lowest acceptable voltage is the voltage required GPA Calculator for the formation of a corona, the electrical discharge that produces ions for charging particles. The (negative) corona is produced when an occasional free electron near the high-voltage electrode, produced by a cosmic ray, gains enough energy from the electric field to ionize the gas and produce more Whey Protein free electrons. The electric field for which this process is self-sustained has been determined experimentally. This is the field required to produce “glow” corona, the form usually seen in the laboratory on smooth, clean wires. The glow appears as a uniform, rapidly moving diffuse light around the electrode. After a period of operation, the movement concentrates into small spots on the wire surface, and the corona assumes a tuft-like appearance. The field required to produce “tuft” corona, Truth About Abs the form found in full-scale ESPs, is 0.6 times the value of E. c The voltage that must be applied to the wire to obtain this value of field, is found by c integrating the electric field from the wire to the loans bad credit plate. The field follows a simple “1/r” dependence in cylindrical geometry. This leads to a logarithmic dependence of voltage on electrode dimensions. In the plate-wire geometry, the field dependence is somewhat more complex, but the voltage still shows the logarithmic dependence. No paycheck calculator current will flow until the voltage reaches this value, but the amount of current will increase steeply for voltages above this value. For tuft corona, the current density is zero until the corona onset voltage is reached, when it jumps almost to this value of j within a few hundred volts, directly under a tuft. The region near the wire is strongly influenced by the presence of ions there, and the corona onset voltage magnitude shows strong spatial variations. Outside the corona region, it is quite uniform. Emax The electric field is strongest along the line from wire to medical billing and coding plate and is approximated very well, except near the wire. When the electric field throughout the gap between the wire and the plate becomes strong enough, a spark will occur, and the voltage cannot be increased without ultrasound technician severe sparking occurring. Particle Charging Charging of particles takes place when ions bombard the surface of a particle. Once an ion is close to the particle, it is tightly bound because of the image charge within make money online the particle. The “image charge” is a representation of the charge distortion that occurs when a real charge approaches a conducting surface. The distortion is equivalent to a charge of opposite magnitude to the real charge, located as far below the surface as the real charge is above it. The notion of the fictitious charge is similar to the notion of an image in a mirror, hence the name. As more ions accumulate on a particle, the total charge tends to prevent further ionic bombardment. There are two principal charging mechanisms: diffusion charging and field charging. Diffusion charging results from the thermal kinetic energy of the ions overcoming the repulsion of the ions already on the particle. Field charging occurs when ions follow electric field lines until they cna training terminate on a particle. In general, both mechanisms are operative for all sizes of particles. Field charging, however, adds a larger percentage of charge on particles greater than about 2?m in diameter, and diffusion charging adds a greater percentage on particles smaller tax lien certificates than about 0.5?m. Diffusion charging, as derived by White, produces a logarithmically increasing level of charge on particles. Diffusion charging never reaches a limit, but it becomes very slow after about three dimensionless time units. For fixed exposure times, the charge on a particle is proportional to its radius. The saturation charge is proportional to the square of the radius, which explains why field charging is the dominant mechanism for larger particles. Strictly speaking, both diffusion backlinks and field charging mechanisms operate at the same time on all particles, and neither mechanism is sufficient to explain the charges measured on the particles. Particle Collection The electric field in the collecting zone produces a force on a particle proportional to mis sold ppi the magnitude of the field and to the charge. Because the field charging mechanism gives an ultimate charge proportional to the electric field, the force on large particles is proportional to the square of the field, which shows the advantage for maintaining as high a field as possible. The motion of the particles under the influence of the electric field is opposed by the viscous drag of the gas. By equating the electric force and the drag force component due to the electric field (according to Stokes’ law), we can obtain the particle velocity. The particle velocity, is the rate at which the particle moves along the electric field lines, i.e., toward the walls. For a given electric field, this velocity is usually at a minimum for particles of about 0.5 ?m diameter. Smaller particles move faster because the charge does not decrease very much. Cunningham factor increases rapidly as hermes bag radius decreases. Larger particles have a charge increasing as r2 and a viscous drag only increasing as r1; the velocity then increases as r. Equation gives the particle velocity with respect to still air. In the cheap auto insurance quotes ESP, the flow is usually very turbulent, with instantaneous gas velocities of the same magnitude as the particles velocities, but in random directions. The motion of particles toward the collecting plates is therefore a statistical process, with an average phlebotomy training component imparted by the electric field and a fluctuating component from the gas turbulence. This statistical motion leads to an exponential collection equation, given by. When this collection equation is averaged over all the particle sizes and hermes replica weighted according to the concentration of each size, the Deutsch equation results, with the penetration (fraction of articles escaping). Sneakage and Rapping Reentrainment Sneakage and rapping reentrainment are best considered on the basis of the sections within an ESP. Sneakage occurs when a part of study abroad programs the gas flow bypasses the collection zone of a section. Generally, the portion of gas that bypasses the zone is thoroughly mixed with the gas that passes through the zone before all the gas enters the next section. labels This mixing cannot always be assumed, and when sneakage paths exist around several sections, the performance of the whole ESP is seriously elected. To describe the effects of sneakage and rapping reentrainment mathematically we first consider sneakage by itself and then consider the website design effects of rapping as an average over many rapping cycles. On the assumption that the gas is well mixed between sections, the penetration for each section can be expressed as. The penetration of the entire ESP is the product of the section penetrations. debt relief

The sneakage sets a lower limit on the penetration of particles through the section. To calculate the effects of rapping, we first calculate the amount of material captured on the plates of the section. The fraction of material that was caught is given by. This material accumulates until the plates are rapped, whereupon most of the courier services material falls into the hopper for disposal, but a fraction of it is reentrained and leaves the section. Experimental measurements have been conducted on fly ash ESPs to evaluate the fraction reentrained, which averages about 12 percent.An electrostatic precipitator whole house water filtration system (ESP) is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The particles are given an electrical charge by forcing them to pass through a corona, a region in which gaseous ions flow. The electrical field that forces the charged caribbean holidays particles to the walls comes from electrodes maintained at high voltage in the center of the flow lane. Once the particles are collected on the plates, they must be removed from the plates without planos de saude reentraining them into the gas stream. This is usually accomplished by knocking them loose from the plates, allowing the collected layer of particles to slide down into a hopper from which they are evacuated.

Some precipitators remove the particles by intermittent

or continuous washing with water. Types of ESPs ESPs are configured in several ways. Some of these configurations have been developed for special control action, and others have evolved for economic reasons. The types that will be described here are the plate-wire precipitator, the most common variety; the flat plate precipitator, the tubular precipitator; the wet precipitator, which handbags may have any of the previous mechanical configurations; and the two-stage precipitator. Plate-Wire Precipitators Plate-wire ESPs are used in a wide variety of industrial applications, including coal-fired boilers, cement kilns, solid waste incinerators, paper mill recovery boilers, petroleum refining debt management plan catalytic cracking units, sinter plants, basic oxygen furnaces, open hearth furnaces, electric arc furnaces, coke oven batteries, and glass furnaces. In a plate-wire ESP, gas flows between parallel plates of sheet metal and high-voltage electrodes. These electrodes are long wires weighted and hanging between the plates chiropractor springfield MO or are supported there by mast-like structures (rigid frames). Within each flow path, gas flow must pass each wire in sequence as flows through the unit. The plate-wire ESP allows many flow lanes to operate in parallel, and each lane can be quite tall. As a result, this type of electronic cigarette review precipitator is well suited for handling large volumes of gas. The need for rapping the plates to dislodge the collected material has caused the plat to be divided into sections, often three or four in smokeless cigarettes series with one another, which can be rapped independent. The power supplies are often sectionalized in the same way to obtain higher operating voltages, and further electrical sectionalization may be used for increased reliability. Dust also deposits on the discharge electrode wires and must building inspection be periodically removed similarly to the collector plate. The power supplies for the ESP convert the industrial ac voltage (220 to 480 V) to pulsating dc voltage in the range of 20,000 to 100,000 V as needed. The supply consists of a step-up transformer, high-voltage rectifiers, chiropractic marketing and sometimes filter capacitors. The unit may supply either half-wave or full-wave rectified dc voltage. There are auxiliary components and controls to allow the voltage to be adjusted to the highest level possible without excessive sparking and to protect the supply and electrodes in the event a heavy promotional pens arc or short-circuit occurs. The voltage applied to the electrodes causes the air between the electrodes to break down electrically, an action known as a “corona”. The electrodes usually are given a negative polarity because a negative corona supports a higher voltage than a positive corona before sparking occurs. The ions generated in the corona follow electric field lines from the wires to the collecting plates. Therefore, each wire establishes a charging zone through which the particles electronic cigarette must pass. Particles passing through the charging zone intercept some of the ions, which become attached. Small aerosol particles (<1 ?m diameter) can absorb tens of ions before their total charge becomes large enough to repel further ions, and large particles (>10 ?m diameter) network marketing can absorb tens of thousands. The electrical forces are therefore much stronger on the large particles. As the particles pass each successive wire, they are driven closer and closer to the collecting walls. The turbulence in the gas, however, tends to keep them uniformly mixed with the gas. The collection process is therefore a competition between the electrical and dispersive forces. Eventually, the particles approach close enough to the walls so that the turbulence drops to low levels and the particles are collected. If the collected particles could be dislodged into the hopper without losses, the ESP would be chanel handbags extremely efficient. The rapping that dislodges the accumulated layer also projects some of the particles (typically 12 percent for coal fly ash) back into the gas stream. These reentrained particles are then processed again by later sections, but the particles reentrained in the louis vuitton bags last section of the ESP have no chance to be recaptured and so escape the unit. Practical considerations of passing the high voltage into the space between the lanes and allowing for some clearance above the hoppers to support and align electrodes leave room for part of the gas to flow around the charging zones. This is called “sneakage” and amounts to 5 to 10 percent of the total flow. Antisneakage baffles usually are placed to force the sneakage flow to mix with the main gas stream for collection in later sections. But, again, the sneakage lawn care business flow around the last section has no opportunity to be collected. These losses play a significant role in the overall performance of an ESP. Another major factor is the resistivity of the collected material. Because the particles form a continuous layer on the ESP plates, all the ion current must pass through the wordpress plugins layer to reach the ground-plates. This current creates an electric field in the layer, and it can become large enough to cause local electrical breakdown. When this occurs, new ions of the wrong polarity are injected into the wire-plate gap where they reduce the charge on iphone repairs the particles and may cause sparking. This breakdown condition is called “back corona” Back corona is prevalent when the resistivity of the layer is high, usually above 2 x 1011 ohmcm. For lower resistivities, the operation of the ESP is not impaired by back coronas, phen375 but resistivities much higher than 2 x 1011 ohm-cm considerably reduce the collection ability of the unit because the severe back corona causes difficulties in charging the particles. At resistivities below 108 ohm-cm, the particles gym equipment are held on the plates so loosely that rapping and nonrapping reentrainment become much more severe. Care must be taken in measuring or estimating resistivity because it is strongly affected by variables such as temperature, moisture, gas composition, particle composition, and surface characteristics. Flat Plate Precipitators A significant number of smaller precipitators (100,000 hgh supplements to 200,000 acfm) use flat plates instead of wires for the high-voltage electrodes. The flat plates (United McGill Corporation patents) increase the average electric field that can be used to collect the particles, and dedicated hosting they provide an increased surface area for the collection of particles. Corona cannot be generated on flat plates by themselves, so corona-generating electrodes are placed ahead of and sometimes behind the flat plate collecting zones. These electrodes may be sharp-pointed needles attached to the kindle fire review edges of the plates or independent corona wires.

Unlike place-wire or tubular ESPs.

this design operates equally well with either negative or positive polarity. The manufacturer has chosen to use positive polarity to reduce ozone generation. A flat plate ESP operates with little or no corona current flowing through the collected dust, except directly under the corona needles or wires. This has two consequences. how to get rid of acne The first is that the unit is somewhat less susceptible to back corona than conventional units are because no back corona is generated in the collected dust, and particles charged with both polarities of ions have unlock iphone 4s large collection surfaces available. The second consequence is that the lack of current in the collected layer causes an electrical force that tends to remove the layer from the collecting surface; this can lead to high rapping losses. Flat plate ESPs seem to have wide application for seo wellington high-resistivity particles with small (1 to 2 ?m) mass median diameters (MMDs). These applications especially emphasize the strengths of the design because the electrical dislodging forces are weaker for small particles than for large ones. Fly ash has been successfully collected with this type of ESP, but low-flow velocity appears to be critical cleaning services for avoiding high rapping losses. Tubular Precipitators The original ESPs were tubular like the smokestacks they were placed on, with the high-voltage electrode running along the axis of the tube. Tubular precipitators have typical applications in sulfuric add plants, coke oven by-product gas cleaning (tar removal), and, recently, iron and steel sinter project management methodologies plants. Such tubular units are still used for some applications, with many tubes operating in parallel to handle increased gas flows. The tubes may be formed as a circular, square, or hexagonal honeycomb with dbc network gas flowing upwards or downwards. The length of the tubes can be selected to fit conditions. A tubular ESP can be tightly sealed to prevent leaks of material, especially valuable or hazardous material. A discount k cups tubular ESP is essentially a one-stage unit and is unique in having all the gas pass through the electrode region. The high-voltage electrode operates at one voltage for the entire length of the tube, and the current varies along the length as the particles wartrol are removed from the system. No sneakage paths are around the collecting region, but corona nonuniformities may allow some particles to avoid charging for a considerable fraction of the tube length. Tubular ESPs comprise only a small portion of the ESP population and are most commonly applied where the particulate is either wet or sticky. These ESPs, usually cleaned with water, have reentrainment losses of a lower magnitude than do the dry particulate precipitators. Wet Precipitators Any of the precipitator configurations discussed above may be operated with wet walls instead of dry. The water flow may be applied intermittently or continuously to wash the collected particles into a sump for disposal. The advantage of the wet wall precipitator is that it has no problems with rapping reentrainment or with back coronas. The disadvantage is the increased complexity of the wash and the fact that the collected slurry must be handled more carefully than a dry product, adding to the expense of disposal. Two-Stage Precipitators The previously described precipitators are all parallel in nature, i.e., the discharge and collecting electrodes are side by side. The two-stage precipitator invented by Penney is a series device with the discharge electrode, or ionizer, preceding the collector electrodes. For indoor applications, the unit is operated with positive polarity to limit ozone generation. Advantages of this configuration include more time for particle charging, less propensity for back corona, and economical construction for small sizes. This type of precipitator is generally used for gas flow volumes of 50,000 acfm and less and is applied to submicrometer sources Cleaning may be by water wash of modules removed from the system up to automatic, in-place detergent spraying of the collector followed by air-blow drying. Two-stage precipitators are considered to be separate and distinct types of devices compared to large, high-gas-volume, single-stage ESPs. The smaller devices are usually sold as preengineered, package systems. Auxiliary Equipment Typical auxiliary equipment associated with an ESP system is shown schematically in Figure Along with the ESP itself, a control system usually includes the following auxiliary equipment: a capture device (i.e., hood or direct exhaust connection); ductwork; dust removal equipment (screw conveyor, etc.); fans, motors, and starters; and a stack. In addition, spray coolers and mechanical collectors may, be needed to precondition the gas before it reaches the ESP. Capture devices are usually hoods that exhaust pollutants into the ductwork or are direct exhaust couplings attached to a combustor or process equipment. These devices are usually refractory lined, water cooled, or simply fabricated from carbon steel, depending on the gasstream temperatures. Refractory or water-cooled capture devices are used where the wall temperatures exceed 800EF; carbon steel is used for lower temperatures. The ducting, like the capture device, should be water cooled, refractory, or stainless steel for hot processes and carbon steel for gas temperatures below approximately 1,150EF (duct wall temperatures <800EF). The ducts should be sized for a gas velocity of approximately 4,000 ft/min for the average case to prevent particle deposition in the ducts. Large or dense particles might require higher velocities, but rarely would lower velocities be used. Spray chambers may be required for processes where the addition of moisture, or decreased temperature or gas volume, will improve precipitation or protect the ESP from warpage. For combustion processes with exhaust gas temperatures below approximately 700EF, cooling would not be required, and the exhaust gases can be delivered directly to the precipitator. When much of the pollutant loading consists of relatively large particles, mechanical collectors, such as cyclones, may be used to reduce the load on the ESP, especially at high inlet concentrations. The fans provide the motive power for air movement and can be mounted before or after the ESP. A stack, normally used, vents the cleaned stream to the atmosphere. Screw conveyors or pneumatic systems are often used to remove captured dust from the bottom of the hoppers. Wet ESPs require a source of wash water to be injected or sprayed near the top of the collector plates either continuously or at timed intervals. The water flows with the collected particles into a sump from which the fluid is pumped.

A portion of the fluid may be recycled to reduce the total amount

of water required. The remainder is pumped directly to a settling pond or passed through a dewatering stage, with subsequent disposal of the sludge. Gas conditioning equipment to improve ESP performance by changing dust resistivity is occasionally used as part of the original design, but more frequently it is used to upgrade existing ESPs. The equipment injects an agent into the gas stream ahead of the ESP. Usually, the agent mixes with the particles and alters their resistivity to promote higher migration velocity, and thus higher collection efficiency. However, electrical properties of the gas may change, rather than dust resistivity. For instance, cooling the gas will allow more voltage to be applied before sparking occurs. Significant conditioning agents that are used include SO, H SO, sodium 3 2 4 compounds, ammonia, and water, but the major conditioning agent by usage is SO. A typical 3 dose rate for any of the gaseous agents is 10 to 30 ppm by volume. The equipment required for conditioning depends on the agent being used. A typical S03 conditioner requires a supply of molten sulfur. It is stored in a heated vessel and supplied to a burner, where it is oxidized to SO . The SO gas is passed over a catalyst for further oxidation 2 2 to S0. The S0 gas is then injected into the flue gas stream through a multi-outlet set of probes 3 3 that breach a duct. In place of a sulfur burner to provide S0, liquid S0 may be vaporized from 2 2 a storage tank. Although their total annual costs are higher, the liquid SO systems have lower 2 capital costs and are easier to operate than the molten sulfur based systems. Water or ammonia injection requires a set of spray nozzles in the duct, along with pumping and control equipment. Sodium conditioning is often done by coating the coal on a conveyor with a powder compound or a water solution of the desired compound. A hopper or storage tank is often positioned over the conveyor for this purpose. Electrostatic Precipitation Theory The theory of ESP operation requires many scientific disciplines to describe it thoroughly. The ESP is basically an electrical machine. The principal actions are the charging of particles and forcing them to the collector plates. The amount of charged particulate matter affects the electrical operating point of the ESP. The transport of the particles is affected by the level of turbulence in the gas. The losses mentioned earlier, sneakage and rapping reentrainment, are major influences on the total performance of the system.

The particle properties also leave a major effect on the operation of the unit. Electrical Operating Point The electrical operating point of an ESP section is the value of voltage and current at which the section operates. As will become apparent, the best collection occurs when the highest electric field is present, which roughly corresponds to the highest voltage on the electrodes. In this work, the term “section” represents one set of plates and electrodes in the direction of flow. This unit is commonly called a “field”, and a “section” or “bus section” represents a subdivision of a “field” perpendicular to the direction of flow. In an ESP model and in sizing applications, the two terms “section” and “field” are used equivalently because the subdivision into bus sections should have no effect on the model. This terminology has probably arisen because of the frequent use of the word “field” to refer to the electric field. The lowest acceptable voltage is the voltage required for the formation of a corona, the electrical discharge that produces ions for charging particles. The (negative) corona is produced when an occasional free electron near the high-voltage electrode, produced by a cosmic ray, gains enough energy from the electric field to ionize the gas and produce more free electrons. The electric field for which this process is self-sustained has been determined experimentally. This is the field required to produce “glow” corona, the form usually seen in the laboratory on smooth, clean wires. The glow appears as a uniform, rapidly moving diffuse light around the electrode. After a period of operation, the movement concentrates into small spots on the wire surface, and the corona assumes a tuft-like appearance. The field required to produce “tuft” corona, the form found in full-scale ESPs, is 0.6 times the value of E. c The voltage that must be applied to the wire to obtain this value of field, is found by c integrating the electric field from the wire to the plate. The field follows a simple “1/r” dependence in cylindrical geometry. This leads to a logarithmic dependence of voltage on electrode dimensions. In the plate-wire geometry, the field dependence is somewhat more complex, but the voltage still shows the logarithmic dependence. No current will flow until the voltage reaches this value, but the amount of current will increase steeply for voltages above this value. For tuft corona, the current density is zero until the corona onset voltage is reached, when it jumps almost to this value of j within a few hundred volts, directly under a tuft. The region near the wire is strongly influenced by the presence of ions there, and the corona onset voltage magnitude shows strong spatial variations. Outside the corona region, it is quite uniform. Emax The electric field is strongest along the line from wire to plate and is approximated very well, except near the wire. When the electric field throughout the gap between the wire and the plate becomes strong enough, a spark will occur, and the voltage cannot be increased without severe sparking occurring. Particle Charging Charging of particles takes place when ions bombard the surface of a particle. Once an ion is close to the particle, it is tightly bound because of the image charge within the particle. The “image charge” is a representation of the charge distortion that occurs when a real charge approaches a conducting surface.

The distortion is equivalent to a charge of opposite magnitude

to the real charge, located as far below the surface as the real charge is above it. The notion of the fictitious charge is similar to the notion of an image in a mirror, hence the name. As more ions accumulate on a particle, the total charge tends to prevent further ionic bombardment. There are two principal charging mechanisms: diffusion charging and field charging. Diffusion charging results from the thermal kinetic energy of the ions overcoming the repulsion of the ions already on the particle. Field charging occurs when ions follow electric field lines until they terminate on a particle. In general, both mechanisms are operative for all sizes of particles. Field charging, however, adds a larger percentage of charge on particles greater than about 2?m in diameter, and diffusion charging adds a greater percentage on particles smaller than about 0.5?m. Diffusion charging, as derived by White, produces a logarithmically increasing level of charge on particles. Diffusion charging never reaches a limit, but it becomes very slow after about three dimensionless time units. For fixed exposure times, the charge on a particle is proportional to its radius. The saturation charge is proportional to the square of the radius, which explains why field charging is the dominant mechanism for larger particles. Strictly speaking, both diffusion and field charging mechanisms operate at the same time on all particles, and neither mechanism is sufficient to explain the charges measured on the particles. Particle Collection The electric field in the collecting zone produces a force on a particle proportional to the magnitude of the field and to the charge. Because the field charging mechanism gives an ultimate charge proportional to the electric field, the force on large particles is proportional to the square of the field, which shows the advantage for maintaining as high a field as possible. The motion of the particles under the influence of the electric field is opposed by the viscous drag of the gas. By equating the electric force and the drag force component due to the electric field (according to Stokes’ law), we can obtain the particle velocity. The particle velocity, is the rate at which the particle moves along the electric field lines, i.e., toward the walls. For a given electric field, this velocity is usually at a minimum for particles of about 0.5 ?m diameter. Smaller particles move faster because the charge does not decrease very much. Cunningham factor increases rapidly as radius decreases. Larger particles have a charge increasing as r2 and a viscous drag only increasing as r1; the velocity then increases as r. Equation gives the particle velocity with respect to still air. In the ESP, the flow is usually very turbulent, with instantaneous gas velocities of the same magnitude as the particles velocities, but in random directions. The motion of particles toward the collecting plates is therefore a statistical process, with an average component imparted by the electric field and a fluctuating component from the gas turbulence. This statistical motion leads to an exponential collection equation, given by. When this collection equation is averaged over all the particle sizes and weighted according to the concentration of each size, the Deutsch equation results, with the penetration (fraction of articles escaping). Sneakage and Rapping Reentrainment Sneakage and rapping reentrainment are best considered on the basis of the sections within an ESP. Sneakage occurs when a part of the gas flow bypasses the collection zone of a section. Generally, the portion of gas that bypasses the zone is thoroughly mixed with the gas that passes through the zone before all the gas enters the next section. This mixing cannot always be assumed, and when sneakage paths exist around several sections, the performance of the whole ESP is seriously elected. To describe the effects of sneakage and rapping reentrainment mathematically we first consider sneakage by itself and then consider the effects of rapping as an average over many rapping cycles. On the assumption that the gas is well mixed between sections, the penetration for each section can be expressed as. The penetration of the entire ESP is the product of the section penetrations. The sneakage sets a lower limit on the penetration of particles through the section. To calculate the effects of rapping, we first calculate the amount of material captured on the plates of the section. The fraction of material that was caught is given by. This material accumulates until the plates are rapped, whereupon most of the material falls into the hopper for disposal, but a fraction of it is reentrained and leaves the section. Experimental measurements have been conducted on fly ash ESPs to evaluate the fraction reentrained, which averages about 12 percent.