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ESP Terminology Back to







Air Pollution Control Association


American Society of Mechanical Engineers


Automatic Voltage Control


Clean Air Act


Code of Federal Regulations


United States Environmental Protection Agency


Electric Power Research Institute


Electrostatic Precipitator


Institute of Clean Air Companies


Institute of Electrical and Electronic Engineers


Industrial Gas Cleaning Institute


National Electrical Manufacturers Association


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New Source Performance Standards


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Specific Collecting Area


Silicon Controlled Rectifier


Southern Research Institute



  ACCESS DOORS: A hinged or detached cover provided with a hand operated fastening device where accessibility is required. Typically it is interlocked using a key interlock system to prevent access to the ESP when the ESP is energized.

  ACFM: Actual Cubic Feet per Minute of gas volume at the actual condition temperature, pressure and composition.  See gas flow rate.

  ACID DEW POINT: The temperature at which combustion gases are saturated with sulfuric acid.

  AIR LOAD: This term refers to energizing the ESP during an outage period at ambient temperatures. The object is to verify the electrical integrity of the bus section, to obtain electrical readings of voltage and current and to determine whether sparkover occurs before reaching a specific limit of the TR Set rating. The advantage of this type of measurement is to relate the internal integrity of the ESP BUS SECTIONS to one another as to electrode damage or extent of ash buildup.

These readings can be obtained with or without fan operation, or at different temperatures but, all conditions should be recorded. Measurements can be recorded as maximum readings or as V-I CURVES. Generally, SPARKOVER should not be observed during these readings.

ALIGNMENT: This term refers to the maintenance of optimum clearances between the high voltage system and the grounded surfaces of the precipitator. Operation at the highest possible ESP voltages depends to a large measure on good mechanical integrity and elimination of reduced distances where sparkover could occur. This means the high voltage frame of a BUS SECTION must be held plum relative to the grounded collecting surfaces. Ideally, discharge electrodes should be centered in the gas passages between the collecting plate surfaces, and centered between the vertical ribs / baffles of the collecting plates. On a practical basis for the 9" (228mm) wide GAS PASSAGES commonly found in WEIGHTED WIRE ESP designs, proper alignment generally means no corona producing wire closer than 4" (102mm) to the flat surface of the collector plate or 5 1/2" (140mm) to any vertical rib of the collector plate.   The emitters, or corona producing elements of RIGID DISCHARGE ELECTRODES in GAS PASSAGES spaced at 12" (305mm) should generally be within 1/2" of center (51/2", 140mm) of the flat surface of the collecting plates, and be a minimum of 6" - 71/2" (152 - 190mm) to any vertical rib of the collector plate.  Allowable tolerances will vary somewhat by manufacturer, electrode design, and from a practical standpoint, by age and condition of the ESP.

ANODE: Positive electrical terminal of high voltage power supply; this is the collecting plate surface (ash surface) which is maintained at ground potential.  Precipitator (ESP) sparking starts at the anode.

ANTI-SWAY INSULATOR: These insulators are used to prevent the bottom high voltage frames, which position and retain the DISCHARGE ELECTRODES, from swinging or drifting out of alignment. The insulators are either a ceramic bar or a shaft type and are usually secured to the hopper wall. Some designs connect directly between the collecting plates and the lower high voltage frames.

ANTI-SNEAKAGE BAFFLES: Internal baffle elements within the precipitator to prevent the gas from bypassing the active field or causing hopper reentrainment.

ARC:  A severe electrical breakdown between the negative high voltage component and ground, usually caused by, or aided by, some internal mechanical defect. A poor AUTOMATIC VOLTAGE CONTROL circuit, as well as operation of the control in the MANUAL MODE, can aggravate this problem. Meter swings are usually 2 to 3 times greater those than observed during normal SPARKOVER. Length of breakdown could last several CYCLES.

ASPECT RATIO: The ratio obtained by dividing effective length of the precipitator by the effective height.

AUTOMATIC MODE: Modern controls will often feature the ability to transfer from an automatic mode of AUTOMATIC VOLTAGE CONTROL, which is normal operation, to a MANUAL MODE for maintenance or trouble periods. The automatic mode allows the TR Set voltage control to adjust for the level of sparkover.

AUTOMATIC VOLTAGE CONTROL: The normal method of controlling the amount of secondary current to the ESP is by controlling the magnitude of voltage on the primary winding of the TR Set. This is accomplished by detecting the transient disruption in the electrical circuitry caused by sparkover, or an arc, in the ESP. A feedback circuit then adjusts the gate signal of an SCR (thyristor) so as to provide a level of voltage necessary to maintain the desired sparkover rate.

BACK CORONA : A term that signifies that the ash layer on the collector surface has reached a level of RESISTIVITY that produces a flow of positive IONS back towards the negative high voltage discharge electrode. Most commonly observed back corona conditions result in SPARKOVER and a reduction of POWER INPUT. SEVERE BACK CORONA (generically referred to as back corona and also referred to as reverse ionization), this condition is extremely detrimental to precipitator performance and will result in dramatic reentrainment of material for the collecting plates at high current levels.

BACT: (Best Available Control Technology) An emission limitation based on the maximum degree of emission reduction achievable.  Under Title I of the CAAA, EPA will establish BACT standards for serious, severe and extreme non-attainment areas.

BALANCED DRAFT: The condition where the absolute pressure in a boiler furnace is exactly equal to the absolute atmospheric pressure outside the furnace or it is slightly negative.

BUS: A conductor enclosed within a grounded duct.

BUS SECTION: Is the smallest portion of high voltage structure, containing a fixed group of DISCHARGE ELECTRODES, that can be independently energized by a single Transformer (TR). More than one bus section can be controlled through a TR either in parallel or series arrangement.

CASING: The precipitator shell or casing is designed to confine the flue gas within a specific collection zone, and it must provide structural support for the discharge and collecting electrode systems, rapping systems, gas distribution system, and other precipitator components.  The precipitator casing is usually constructed of fabricated steel panels fitted with external columns, beams, and stiffeners and is designed so that the final assembly provides a gas tight unit able to withstand both internal and external loading.   The precipitator casing includes ACCESS DOORS located in the side walls and on the roof that permit access to the precipitator interior.  A key interlock system prevents opening the doors while the precipitator is energized.  Access walkways, platforms, stairs, and ladders are attached to the casing at various internal and external locations.

CATHODE: This is the negative polarity, high voltage DISCHARGE ELECTRODE of a precipitator.  It is the cathode that suffers metal erosion due to ESP repetitive sparkover – a common cause of localized wire thinning, draw-out due to weight tension, and ultimate breakage with sharp points.

CELLS: A cell is an arrangement of bus sections across the width of the ESP.  Typically the number of cells times the number of fields equals the number of bus sections.

CFM: Cubic Feet (of any gaseous matter) per Minute.  See gas flow rate.

CHAMBER:  Refers to a gas tight longitudinal subdivision of the precipitator (a precipitator without any internal dividing walls is a single chamber precipitator; a precipitator with a single internal dividing wall is a two chamber precipitator, etc.).  Very wide precipitators may have non gas tight load bearing walls that are used for structural purposes.  Technically these non gas tight walls would not be considered as chamber dividing walls.

CHEVRON DESIGN: Refers to the inlet transition design that places 2 parallel ESPs at a small angle to each other.  This layout typically is used to minimize space requirements.

CLR - CURRENT LIMITING REACTOR: This is primarily a ballast of inductance placed in the low voltage circuit to provide current limiting ability under sparkover in the ESP. Another major advantage of a properly sized reactor is to better shape the waveform of the input voltage to the TR Set thereby gaining a greater conduction angle of secondary current flow. This feature has benefits for ash or dust layers that exhibit high resistivity characteristics.

COLLECTION EFFICIENCY: The weight of dust collected per unit time divided by the weight of dust entering the precipitator during the same unit time expressed in percentage.  The computation is as follows:

                                    (Dust in) – (Dust out)

            Efficiency =              (Dust in)           X 100

COLLECTING SURFACE: Is the term for the sheet metal collector plate that serves as the point of deposition for the particulate that is negatively charged within the gas passage of the ESP.  Collecting surface plate design differs between manufacturers but, all are secured to the shell of the ESP at ground potential and serve as the positive anode of the gas passage.

COLD SIDE ESP: An ESP which is installed downstream of the air heaters 

COLLECTION SURFACE AREA: The total flat projected area of collecting surface exposed to the active electrostatic field (length x height x 2 x number of gas passages).

CONDUCTION ANGLE: The CORONA CURRENT flows in pulses rather than as pure direct current. The percent conduction during each half cycle, out of an available 8.33 milliseconds, (60 Hz), represents the length of time this current flows relative to the off time of the CYCLE. Operating at the current rating of the TR Set usually means a conduction angle of 86% exists. As the operating level of the TR Set is reduced, this angle decreases in a somewhat linear fashion.

CONTROL EQUIPMENT: The necessary electrical components required to regulate the potential of the high voltage system by regulating the voltage or current applied to the primary of the high voltage transformer (also metering and protection equipment, and controls for auxiliary electrical items such as rappers, heaters, etc.).

CONTROL CABINET: This cabinet contains the control and monitor apparatus of the power supply. Features mainly involve low voltage breaker, overload controls, metering, and the automatic voltage control components.

CORONA : A gaseous discharge found near an ESP discharge electrode resulting in a faint glow caused by ionization of gas molecules due to the electric field.

CORONA CURRENT: This term signifies all the measured current flow passing from the CORONA DISCHARGE of the DISCHARGE ELECTRODE through the gas space of the ESP to ground, for a collecting plate area controlled by a single TR Set. It is also commonly known as the secondary DC current and is read on the SECONDARY AMMETER either as milliamperes or fraction of an amp.

CORONA DISCHARGE: This term represents an electrical breakdown of the flue gas at localized small zones on the surface of the DISCHARGE ELECTRODE. Approximately 20 - 25 thousand volts is required to start this process on a smooth 0.1" (2.54mm) diameter wire. Corona onset will vary with electrode design and gas passage spacing. Electrons coming out of this localized tuft of activity primarily produce the negative gas IONS required to properly charge the ash particles negatively so they can migrate toward the positive ground COLLECTING SURFACE.

CORONA POWER (KW): The product of secondary current and secondary voltage. Power density is generally expressed in terms of: (1) watts per square foot of collecting surface, or (2) watts per 1000 ACFM of gas flow.

CURRENT DENSITY: The amount of secondary current per unit of ESP collecting surface.  Common units are ma/ft2 and nA/cm2.

CURRENT WAVE SHAPE: Usually refers to the pattern of the pulsating secondary CORONA CURRENT as observed on an oscilloscope. As conduction angles decrease, the waveshape tends toward peakiness. As conduction angles increase, the wave shape approaches the look of a sine-wave.  

CYCLE: Generally refers to an alternating current of 60 cycles per second which is the standard energizing mode of the TR Sets. This means that 3600 alternating cycles per minute are rectified into 7200 half cycles per minute and are fed into the area of the precipitator controlled by one TR Set.  

DEW POINT: The temperature at which the equilibrium vapor pressure of a liquid is equal to the existing partial pressure of the respective vapor.  (For air containing water vapor, it is the temperature at which liquid water begins to condense for a given state of humidity and pressure as the temperature is reduced.  For flue gaining water vapor and SO it is the set of conditions at which liquid sulfuric acid begins to condense as the temperature is reduced.)

DIELECTRIC FLUID: A substance used to keep the transformer operating at moderate temperature levels, and as a dielectric where space is concerned.

DISCHARGE ELECTRODE: Refers to the high voltage component which ionizes the process gases and creates the electric field. It is shaped to provide a corona discharge when the impressed voltage breaks the gas down at the electrode surface. This breakdown of the gas creates corona tufts on the discharge surface. Typically, voltage applied to the discharge electrode is of negative polarity. In many WEIGHTED WIRE designs, the discharge electrode is a smooth round wire slightly larger than 0.1" (2.54mm) in diameter. Barbed wire is also frequently used in part, or all of the ESP, to enhance corona characteristics. Discharge Electrodes of the RIGID and RIGID FRAME or Mast variety are also widely available. They can vary widely in style and shape

Deutsch-Anderson Equation: The Deutsch-Anderson Equation is a mathematical formula that can be used to determine the collection efficiency of an ESP.  It states that efficiency is related to the ratio of collecting area (A) divided by the gas volume (V) multiplied by the particle migration velocity (w).  This formula is the basis for all ESP design and performance models. 

Collection Efficiency             N = 1 - e – (A / V) w 

DUST OR MIST CONCENTRATION: The weight of dust or mist contained in a unit of gas, e.g. pounds per thousand pounds of gas, grains per actual cubic foot of gas, or grains per standard dry cubic foot (the temperature and pressure of the gas must be specified if given as volume).  

EFFLUENT: A discharge or emission of a fluid (liquid or gaseous).

ELECTRONIC TUBE RECTIFIER: A rectifier consisting of high vacuum rectifier tubes.  Can be either Air-cooled or Liquid-immersed

ELETROSTATIC PRECIPITATOR: (ESP) A single precipitator is defined by all parts that are contained by a independent casing.

EMISSION: Release of pollutants into the air from a source.

exceedance: is defined as a condition that is detected by the CAM plan monitoring that provides data in terms of an emissions limitation or standard and that indicates that emissions (or opacity) are greater than the applicable emissions standard, consistent with any specified averaging period.

EXCESS AIR: Air in excess of the amount necessary to combust all the available fuel.

excursion:  is defined as a departure from an indicator or indicator range established for monitoring under the CAM plan, consistent with any specified averaging period. 

FIELD:  Refers to an arrangement of one or more bus sections, oriented perpendicular to the direction of flue gas flow, which is energized by one TR SET. The number of TR sets / power supplies positioned in series (parallel to the gas flow), each one controlling the collection of particles in a specific area, will typically identify the number of fields of a precipitator.

FLY ASH: Particulate matter entrained in the flue gas stream leaving a fossil fuel fired boiler.  It consists of both ash and combustible matter.

FOSSIL FUELS: Coal, oil and natural gas; so-called because they are the remains of ancient plant and animal life.

FUGITIVE EMISSIONS: Emissions not caught by a capture system.

FULL WAVE: This electrical term means that the 7200 rectified half cycles per minute are fed into the full precipitator area energized or controlled by one TR Set.

FUME: Solid particulates generated by condensation from the gaseous state, generally after volatilization from molten metal, and often accompanied by a chemical reaction, such as oxidation.  Fumes flocculate and sometime coalesce.

GAS DISTRIBUTION DEVICES: Internal elements in the transition or ductwork to produce the desired velocity contour at the inlet and outlet face of the precipitator example: turning vanes or perforated plates. 

GAS FLOW RATE, CUBIC FEET PER MINUTE (CFM): The volume of process gas at any point of the plant exhaust system measured in terms of minutes.  There are several units of measurement:

            ACFM-The actual gas flow measured (Actual Cubic Feet per Minute:

SCFM-The gas flow volume reduced to 70OF (standard temperature) by calculation (Standard Cubic Feet per Minute)

DSCFM-The gas flow reduced to 70O (standard temperature) and without volume of steam or water vapor contained in the exhaust gas (Dry Standard Cubic Feet per Minute

GAS PASSAGE: Is the passage formed by two adjacent collector plates, normally on 9" - 10" (228 - 254mm) centers with WEIGHTED WIRE systems; 11" - 16" (279-406mm) with Rigid Discharge and Rigid Frame Electrode systems. The passage can be considered to consist of two capacitors with the negative DISCHARGE ELECTRODE at centerline and the positive ground collecting plates forming the other electrode. This passage is where the action takes place within the precipitator.

GAS VELOCITY: A figure obtained by dividing the volume rate of gas flow through the precipitator by the effective cross-sectional area of the precipitator.  Gas velocity is generally expressed in terms of ft./sec. And is computed as follows:

 Velocity =         Gas Volume (ft3/sec.)
                          Effective cross-section area (ft2)

Effective cross-section is construed to be the effective field height X width of gas passage X number of passages.

GRAIN: A dust weight unit commonly used in air pollution control.  Equal to one seven thousandth of a pound.  One grain = 1/7000 lb.

GRAIN LOADING: The rate at which particles are emitted form a pollution source.  Measurement is made by the number of grains per cubic foot of gas emitted.

HALF WAVE: This term means the TR Set is energizing more than one BUS SECTIONS and that these separate areas of the precipitator are receiving alternate pulses or 3600 rectified half cycles per minute. The TR Set will always have two outlet bushings with this mode of hook-up. Even though each bus section operates electrically independent, the overall operating level of the TR Set is controlled by the weakest point of the areas controlled. On balance, with the larger TR Sets in use today, the half-wave mode is not generally recommended.

HIGH VOLTAGE BUS SYSTEM: The high voltage (HV) bus system is used to transfer power from the power supplies (Transformer Rectifiers) to the HV discharge electrode frames.  The bus is the conductor and is usually made of pipe/bar, cable, or a combination of the two.  Bus runs between the interlocked insulator compartments or penthouses are enclosed in watertight bus duct.  The bus is supported with insulators, usually of the standoff / post insulator type. Thru-bushing insulators may or may not be used at the insulator compartment / penthouse and switch housing penetrations.   Ground and/or disconnect switches may be part of the HV bus arrangement.

HIGH VOLTAGE CONDUCTORS: Conductor to transmit the high voltage from the transformer-rectifier to the precipitator high voltage system.

HIGH VOLTAGE SELECTOR SWITCH: Is the means to selectively energize a separate bus section when more than one bus is controlled by a TR Set. There are several methods of high voltage isolation, but all must be accomplished with the TR Set shut down and properly locked out.  One type of switch is internal to the transformer and immersed in the same oil as the transformer winding. Another type of switch, external to the TR tank, isolates the high voltage circuit with a blade mechanism by withdrawing a blade from a clip or pan disc. A third mode of isolation on the high voltage side involves actual disconnection of a flexible lead from one TR output bushing and physically placing a jumper between the two bushings (if one is not already in place).

HOPPERS: Hoppers located at the bottom of the precipitator casing and are used to collect the material that has been collected and that falls off of the internal components that are cleaned.  The typical shape is pyramidal with the sides of each hopper being steep sloped and the outlet opening is sized so that fly ash may be easily removed by an ash removal system.   Baffles are usually placed in the hoppers; they extend below the dust level to minimize undesirable gas sneakage below the collection plates.  Typically hopper are equipped with level detectors to alarm high levels and hopper heaters which are used to reduce corrosion and to keep the material fluidized.  Hoppers are also equipped with access doors, strike plates for manually rapping the hopper walls, and poke holes to unclog the hopper outlets.

HOPPER CAPACITY: Total volumetric capacity of hoppers measured from a plane 10” below high voltage system or plates, whichever is lower.

HOT SIDE ESP: An ESP which is installed upstream of the air heaters.

HOT WIRE ANEMOMETER: A device used for the measurement of flow velocities and turbulence in an ESP and its associated ductwork.  It has the advantage of high sensitivity at very low velocities and produces an electrical readout.

HUMIDITY, ABSOLUTE: The weight of water vapor per unit volume, pounds per cubic foot or grams per cubic centimeter.

HUMIDITY, RELATIVE: The ratio of the actual partial pressure of water vapor in a space to the saturated pressure of pure water vapor in a space to the saturated pressure of pure water at the same temperature.

INLET DUST LOADING: A measure of the particulate matter entering an ESP expressed in grains of particulate matter per actual cubic foot of flue gas.

IONS:  Generally refers to the flue gas molecules within the gas passage that become primarily charged negatively by the action of free electrons initiated from the CORONA DISCHARGE. It is this ionic flow that basically charges and pushes the ash particles toward the positive ground COLLECTING SURFACE under the influence of the VOLTAGE FIELD.  In the complex action of the gas space, some positive gas ions are also formed which tend to promote particle deposition on the negative discharge electrode. Negative ions by far are the most numerous in the gas space and help constitute a space charge in the precipitation process.

IN SITU RESISTIVITY: Particle resistivity as determined by a probe inserted into the flue gas stream.  See Resistivity

INSULATOR COMPARTMENT: Enclosure for the insulator(s) supporting the high voltage system (may contain one or more insulators, but not enclosing the roof as a whole).

LAER (Lowest Achievable Emission Rate): The rate of emissions which reflects either the most stringent emission limit contained in the implementation plan of any state (unless it is proved that such limitations are not achievable) or the most stringent emission limit achieved in practice, whichever is most stringent.

LINEAR REACTOR (CLR): This is primarily a ballast of inductance placed in the low voltage circuit to provide current limiting ability under sparkover in the ESP. Another major advantage of a properly sized reactor is to better shape the waveform of the input voltage to the TR Set thereby gaining a greater conduction angle of secondary current flow. This feature has benefits for ash or dust layers that exhibit high resistivity characteristics.

key interlock system: A system of locks that prevents opening the ESP access doors while the precipitator is energized.

MACT: (Maximum Achievable Control Technology) – the standard with which source of HAPs will have to comply; the CAAA defines MACT as “the maximum degree of reduction in emissions… achievable for new or existing sources… taking into account the cost of achieving such reductions.”  MACT standards for existing sources must be at least as stringent as the average level of control achieved at the best controlled 12 percent of facilities, and MACT for new sources will have to be even stricter.

MANUAL MODE: This refers to the ability to remove the automatic voltage control features from the electrical circuit by a switch in the control cabinet. In this mode, the amount of power input to the ESP is fixed at the manual setting chosen. If excessive sparkover occurs at this manual setting the control will not recognize nor correct this condition. This could cause deterioration in performance as well as possible internal damage to the DISCHARGE ELECTRODES.

MECHANICAL COLLECTOR: Devices that are functionally dependent on the laws of mechanics governing the motion of bodies in space.  They can be operated dry or wet.  When operated wet, devices are generally called scrubbers.  Examples of mechanical collectors are cyclones, settling chambers and various types of impingement collectors.

MECHANICAL RECTIFIER: A device consisting of a disc or arms with appropriately placed contacts rotating at a synchronous speed to produce an unidirectional voltage at its output.

MIGRATION VELOCITY: A parameter in the Deutsch-Anderson equation used to determine the required size of an electrostatic precipitator to meet specified design conditions.  Other terminology used are “W” value and precipitation rate.  Values are generally stated in terms of ft/min or cm/sec

NOX (NITROGEN OXIDES): Chemical compounds containing nitrogen and oxygen; react with volatile organic compounds, in the presence of heat and sunlight, to form ozone.  They are also a major precursor to acid rain.  Nationwide, approximately 45% of NOX emissions come from mobile sources, 35% from electric utilities and 15% from industrial fuel combustion.

OHMS LAW: The formula used to determine the relationship between Voltage (V), Current (I) and Resistance (R).   V = IR

OPACITY: Refers to the amount of light that can pass through expressed in percent reduction of light intensity.  At the stack it normally refers to the degree of visibility of an exhaust plume. Normally measured by opacity monitors mounted in the ductwork or stack. EPA method 9 is used to measure it visually at the stack.

OZONE: A compound consisting of three oxygen atoms, that is the primary constituent of smog.  It is formed through chemical reactions in the atmosphere involving volatile organic compounds, nitrogen oxides and sunlight.  Ozone can initiate damage to the lungs as well as damage to trees, crops and materials.  There is a natural layer of ozone in the upper atmosphere, which shields the earth from harmful ultraviolet radiation.  

PARTICLE SIZE: The diameter in mms (micrometers) of a particular piece of particulate matter.

PARTICLE MATTER: Solid or liquid particles entrained a gas stream.  

PENTHOUSE:  A weatherproof gas-tight enclosure over the precipitator to contain the high voltage insulators.

PIGGY-BACK LAYOUT: Refers to the orientation of two or more parallel ESPs in a piggy back arrangement (one on top of the other). This layout typically is used to minimize space requirements.

PITOT TUBE: A common instrument used for velocity determination in ducts leading to and from air pollution control devices.

PM 10: A EPA standard for measuring the amount of solid or liquid matter suspended in the atmosphere (“particulate matter”).  Refers to the amount of particulate matter under 10 micrometers in diameter.  The smaller PM10 particles penetrate to the deeper portions of the lung, affecting sensitive population groups such as children and people with respiratory diseases. PM 2.5: New EPA standard that limits the amount of particulate matter emissions under 2.5 micrometers in diameter.

PPM (PARTS PER MILLION): The number of parts of a given pollutant in a million parts of air.  Units are expressed by weight or volume.

PRECIPITATOR CURRENT: The rectifier or unidirectional average current to the precipitator measured by a milliampmeter in the ground return leg of the rectifier.


1.      Effective Length: Total length of collecting surface measured in the direction of gas flow.  Length between fields is to be excluded.

2.      Effective Height: Total height of collecting surface measured from top to bottom.

3.      Effective Width: Total number of gas passages multiplied by spacing dimension of the collecting surface.

4.      Effective Cross-Sectional Area:  Effective width times effective height.

PRECIPITATOR VOLTAGE: The average DC voltage between the high voltage system and grounded side of the precipitator.

Preventive maintenance (PM): This includes the actions that detect, preclude, or mitigate degradation of functional equipment to sustain or extend its useful life by controlling degradation and failures to an acceptable level. There are three types of preventive maintenance: periodic, predictive, and planned.

Periodic maintenance is a form of preventive maintenance consisting of servicing, parts replacement, surveillance, or testing at predetermined intervals of calendar time, operating time, or number of cycles.

Predictive maintenance is a form of preventive maintenance performed continuously or at intervals governed by observed condition to monitor, diagnose, or trend the equipment’s functional or condition indicators. Results indicate current and future functional ability or the nature and schedule for planned maintenance.

Planned maintenance is a form of preventive maintenance consisting of refurbishment or replacement that is scheduled and performed prior to failure of the equipment.

Corrective maintenance includes actions that restore, by repair, overhaul, or replacement, the capability of any failed equipment so it can function within acceptance criteria.

PRIMARY AMMETER: This meter measures the current flow through the low voltage primary winding of the TR Set in alternating current amperes. The meter normally receives its signal from a current transformer in the primary circuit. Dividing this indicated current by the turns-ratio of the TR Set will provide the level of AC current in the secondary winding.

PRIMARY CURRENT: Current in the transformer primary as measured by an AC ammeter.

PRIMARY VOLTAGE: The voltage as indicated by an AC voltmeter across the primary of the transformer.

PRIMARY VOLTMETER: This meter measures the voltage drop across the primary winding of the high voltage transformer in the TR Set. The voltage can be measured in various manners, but the object is not to include any other equipment or apparatus within the measurement point located at the main power cables going directly to the TR Set. With recent SCR controls, the true value of this voltage varies with the waveform at different levels of load current.

POWER INPUT: Generally refers to the corona power of the ESP which is the average DC voltage multiplied by the corona DC current. Without actual secondary circuit meters, an approximate DC power input in watts would be between 65 to 70% of the primary AC circuit volt-amperes. A key point here is not to rely on power as the criterion of good precipitation. Good precipitation requires adequate VOLTAGE FIELDS; the current observed will be a reflection of many factors. Thus, we have to evaluate the values of proper power input more in terms of how the voltage relates to the current than by the product of the two terms.

RACT (Reasonably Available Control Technology): An emission limitation on existing sources in nonattainment areas, defined by EPA in a Control Techniques Guideline (CTG) and adopted and implemented by states. Under Title I of the CAAA, EPA will establish RACT standards for marginal, moderate and serious nonattainment areas.

RAPPERS - COLLECTING SURFACES: These are devices, generally located at the top of the ESP or bottom of the collecting plates, which periodically impart a shock to the collecting surfaces to help dislodge the collected material into the hopper system. The final collection efficiency of the precipitator is often determined by how well this process is conducted. The object is to dislodge the material from the collector surface in small clumps or patches without building excessive dust layer thicknesses. This is a complex part of precipitation, but it is more important to know that reliability of rapper operation holds priority over timing, impact force and other aspects of this system.

RAPPERS - HIGH VOLTAGE: These rapper devices impart a vibration or shock to the high voltage frame supporting the discharge electrodes. The object is to keep the buildups on these electrodes from affecting the corona discharge pattern. The discharge electrodes will generally exhibit irregular coatings of various size and shape. Whether the buildups observed during outage inspections are detrimental can usually be determined by an analysis of electrical readings during periods of operation. It is usually better to operate with some buildup than employ excessive rapping forces that can result in failure of DISCHARGE ELECTRODES.

RAPPER INSULATOR: A device to electrically isolate discharge electrode rappers yet transmit mechanically, forces necessary to create vibration or shock in the high voltage system.

RAPPING INTENSITY: The “g” force measured at various points on collecting or discharge electrodes.  Measured forces should be specified as longitudinal or transverse.

REENTRAINMENT: Reentrainment, usually is associated with rapper reentrainment, which refers to the reintroduction of particulate back into to the gas stream from the discharge electrodes and collecting surfaces during rapping. Reentrainment can also result from gas sweepage when gases bypass the treatment zone of the ESP and disturb collection zones such as hoppers. High velocity zones and external influences, such as ambient air infiltration of the casing or hopper, can also promote particulate reentrainment. Reentrainment can substantially lower the ESP collection efficiency.

RESISTIVITY:  This term is most critical for the fly ash precipitator because it directly controls the levels of voltage and current observed at most installations. Resistivity refers to the electrical resistance of the ash layer after it forms on the positive ground COLLECTING SURFACE. If the resistance level is high, the corona current passing through the ash layer must be generally reduced or BACK CORONA effects will reduce performance of the ESP. The range of resistivity is primarily affected by the chemistry of the ash, moisture in the flue gas, levels of sulfur trioxide, and flue gas temperature. Resistivity effects are generally observed by the occurrence of SPARKOVER on most ESP fields at some reduced level of voltage and current. Operation in a good zone of resistivity allows the ash layer on the collector plate to bond sufficiently for optimum ESP performance and helps to reduce REENTRAINMENT.  When resistivity drops to low levels, the ash layer on the collecting surface allows current to flow through it without restriction and it is easily reentrained back into the gas stream.  This condition is generally characterized by high corona current levels without the occurrence of sparkover.

RIGID DISCHARGE ELECTRODE (RDE) DESIGN: This term refers to precipitators utilizing rigid discharge electrodes, such as the pipe and spike variety, for it's discharge electrode rather than a weighted wire type of high voltage system.

RIGID FRAME DESIGN: This term refers to precipitators utilizing rigid frames with tensioned discharge electrodes between supporting members. Frame shapes can vary from rectangular tubular pipes with horizontal cross members to Mast frames with a vertical primary support and horizontal cross members in a T configuration. Electrode styles and shapes can also vary widely. Rigid frame designs are almost exclusive to European design precipitators and are typically rapped by tumbling hammer rappers located within the gas stream.

SAFETY GROUNDING DEVICE: A device for physically grounding the high voltage system prior to personnel entering the precipitator.  (The most common type consists of a conductor, one end of which is grounded to the casing, the other and attached to the high voltage system using an insulated operating lever.

SATURABLE CORE REACTOR: This is a method of voltage control for the precipitator that has been superseded in recent years by the SCR. The saturable core reactor passed the primary current through a substantial winding on an iron core. The voltage output of this device varied by the level of DC current through a control winding and it's subsequent affect on the saturation flux of the reactor core. This control was used extensively for years and still exists at some locations. A disadvantage was a slower response to sparkover conditions.

SCA – SPECIFIC COLLECTING AREA: The quotient of the total collecting area (A) divide by the total gas volume (V) handled by the ESP multiplied by 1000. SCA is commonly expressed as ft2/1000 acfm (m2 / (m3/sec). 

SELENIUM RECTIFIER: A rectifier consisting of selenium cells (fluid immersed).

SPECIFIC COLLECTING AREA (SCA): A figure obtained by dividing total effective collecting surface of the precipitator by gas volume, expressed in thousands of actual cubic feet per minute.

SPECIFIC CORONA POWER: The quotient of the total corona power of all precipitator bus sections divided by the total gas volume handled by the precipitator, multiplied by 1000 Units are expressed as watts/1000 acfm.

SCFM (STANDARD CUBIC FEET PER MINUTE): The volume that a gas would occupy at standard temperature and pressure conditions (70OF and 14.7 PSIA). See gas flow rate.

SCR:  Selective Catalytic Reduction is the most advanced and efficient process for NOx reduction.  SCR technology is based upon the conversion (reduction) of NOx (NO2 and NO) with ammonia (NH3) into water (H2O) and nitrogen (N2).  A catalyst is used to speed up the conversion rate. The term SCR is used to describe both for the technology and the apparatus that that is used. 

SCR CONTROLS (Silicon-Controlled Rectifier): Silicon controlled rectifiers are the most extensively used method of voltage control in recent years, and consists of two silicon rectifiers mounted in an inverse parallel fashion in the primary AC circuit of the TR Set. Thyristors are also used instead of silicon diodes, but the principle is basically identical. These devices are normally open in both directions until a small gate signal is applied which allows the SCR to conduct in one direction. The output is controlled by the strength of the current flow in the gate circuit which receives it's signal from either the AUTOMATIC or MANUAL mode operation of the voltage controller.

SCRUBBER: A device that uses a liquid spray to remove aerosol and gaseous pollutants from an air stream.  The gases are removed either by absorption or chemical reaction.  Solid and liquid particulates are removed through contact with the spray.  Scrubbers are used for both the measurement and control of pollution.

SECONDARY AMMETER: This meter measures the average DC secondary current, which is actually the precipitator corona current passing through the ground path on it's return to the rectifier connection of the TR Set so as to complete the electrical circuit. This meter has a low resistance movement and the scale reads in milliamperes or amps depending on the size of the TR Set. The secondary current waveform can usually be observed by connecting an oscilloscope across the meter. There is usually a shorting device or surge arrestor across the meter for protection. Under no circumstance should the leads be removed from this type of meter with the TR Set energized. Another method generally used is a meter measuring a voltage across a resistor and calibrated as a current meter.

SECONDARY VOLTMETER: This measurement is made between the rectifier output and the outlet bushing of the TR Set by use of a voltage divider installed inside the tank. With older TR Sets, it is possible to obtain the average precipitator voltage, usually read as average DC kilovolts, by installation of a retrofit voltage divider at the outlet bushing of the TR Set. The indicated voltage represents the voltage from the discharge electrode to ground, comprising both the voltage drop across the gas space as well as the ash layer on the collecting surface. It actually is a measurement of the dielectric resistance and represents all the characteristics of the precipitator load.

SHROUD:  Refers to a steel tube or rod installed at the top and bottom of a wire discharge electrode. The purpose of this shroud, approximately 3/8" (9.5mm) outside diameter, is to eliminate the corona discharge opposite the top and bottom termination of the collector plate. The enlarged radius of the shroud would require a higher voltage than required by the wire to initiate a corona tuft. The shrouds usually extend about 4 - 6 inches (102 - 152mm) into the gas passage. Prevention of localized sparkover at these critical points eliminates premature wire failure due to electric erosion.

Silicon Diode Bridge : The diode assembly converts the high voltage AC to DC voltage.  These bridges are normally made up individual diodes connected in series. The number of diodes used will vary by manufacturer from 20 to several hundred.

SNCR: Selective Non-Catalytic Reduction, As the name implies this form of NOx reduction technology does not use a catalyst as with SCR NOx reduction technology.  With the SNCR technology, ammonia or ammonia based compounds, such as urea, are injected into the furnace at specific temperatures. These temperatures are much higher than those for an SCR, the optimum being around 1400 to 2000 F.   The NOx reduction reactions are very sensitive to temperature and ammonia slip is a common problem.

SO2: Sulfur dioxide is an invisible, nonflammable acidic gas, formed during combustion of fuel containing sulfur.

SO3: Sulfur trioxide oxidized from SO2; combines with atmospheric moisture to form sulfuric acid mist (H2SO4).

SPARKOVER:  Is a localized electrical breakdown in the gas space between the high voltage system and ground. This generally occurs between the DISCHARGE ELECTRODE and COLLECTING SURFACE. This breakdown, or flashover, can occur when the physical clearance has been reduced so that the operating voltage is greater than the space will allow. More often, sparkover will occur when the resistivity of the ash layer on the collecting surface reaches critical levels. Premature sparkover at extreme low levels of voltage can often be observed with a combination of higher resistivity and internal difficulties such as reduced electrical clearances.

During a significant sparkover, the basic collapse of the VOLTAGE FIELD occurs which should always cause a downward flick of the VOLTMETER needles. A small number of sparks per minute is generally desirable in fly ash precipitators as long as it occurs at reasonable levels of voltage and current.

SPARKMETER:  This meter attempts to represent the number of sparks per minute by integrating these transient surges by some type of capacitance circuit. In most locations where spark-meters still exist, replacement of the automatic voltage controls will discontinue its use. In all cases, representation of the ESP sparkover by a meter can be misleading. For all practical purposes, it is recommended that a visual count or evaluation of the flicks of the voltmeter needle be made on a per minute basis to better gauge the spark rate. Spark rates as high as 60 to 70 per minute can be easily observed. This is generally at the higher range that should exist on most modern installations. At this higher sparkover level, the meter needle must still come to rest many times during any given minute.

Structural Steel: The precipitator casing is support by a structure steel system that is typically only tied to the ESP in one fixed location to allow the ESP casing to grow and expand independently during normal operation.  The casing columns rest on a slide plate or slide bearing in all other areas.

SUPPORT INSULATOR: This term refers to the ceramic component that supports and isolates the high voltage frame from ground potential. Recent designs involve an alumina cylinder that also acts as a gas seal at the top frame locations. The surface of the insulator is sensitive to electrical leakage to ground if condensation or contamination is allowed to occur. Purge air and heater applications are two methods used to minimize insulator failures.

TR RATING: The transformer - rectifier (TR) should be sized to supply sufficient current for the area of the precipitator to which it is connected. The nameplate shows a KVA size, primary and secondary voltage ratings, and primary and secondary current ratings. These values are of most interest. The nameplate should show whether the primary winding is tapped for more than one voltage connection.  A key point here is that the actual electrical performance of the ESP may in no way resemble any of the values shown on the nameplate.

TR SET: Is the term for the high voltage transformer and rectifier that provides the electrical energy for a given precipitator area. These components involve a specially wound transformer that supplies a RMS secondary voltage sized on the basis of GAS PASSAGE spacing and discharge electrode design. An RMS secondary voltage of about 53,500 volts AC (45 KV DC average) is utilized for the 9" (228mm) wide GAS PASSAGE of most weighted wire precipitators; 77,300 volts AC (65 KV DC average) for most of the 12" (305mm) wide GAS PASSAGE of rigid electrode precipitators.

This AC voltage is usually rectified through a silicon diode bridge circuit in most existing TR Sets. Rated DC voltages are usually specified at the 45,000 - 50,000 volt level for 9"-10" (228 - 254mm) plate spacing; the 55,000 - 65,000 volt level for 11"-12" (279 - 305mm) plate spacing; the 70,000 - 90,000 volt level for 15"-16" plate spacing. Other pertinent data can be observed on the metal nameplate of each tank. While the voltages are generally similar between TR Sets, the current ratings vary greatly based on the anticipated load requirements of the particular ESP FIELD. While the KVA rating is used, it is also common practice to specify the size of the TR Set by it's corona current rating in milliamperes.

TR TURNS RATIO: Expresses the number of turns in the secondary winding of the transformer for every turn in the primary winding. For example, a TR Set with a 400 volt primary rating relative to a 53,500 volt secondary would have an approximate ratio of 133 turns in the secondary winding.

TRANSITION: An aerodynamically designed inlet or outlet duct connection to the precipitator.  Transitions are normally included as part of the precipitator.

TREATMENT TIME: A figure, in seconds, obtained by dividing the effective length, in feet, of a precipitator by the precipitator gas velocity figure calculated above.

TURNING VANES: Vanes in ductwork or transition to guide the gas and dust flow through the ductwork in order to minimize pressure drop and to control the velocity and dust concentration contours.

V-I CURVE: Usually refers to a plot of secondary voltage versus secondary current for a single TR set in which the shape of the plotted curve might indicate a number of internal operating characteristics of the precipitator. An important part of these measurements is the indicated voltage at the threshold of corona current. While normally obtained during air load, these curves sometimes can be developed during operating periods

VOLTAGE DIVIDER: A means for supplying a low voltage feedback signal that is proportional to the KV output of the TR.

VOLTAGE FIELD: Refers to the high voltage field generated between the negative discharge electrode and positive collecting surface at ground potential. This field supplies the charging mechanism and driving force for the removal of the particles from the flue gas stream. Desirable values of this field strength would lie between 4 to 5 kilovolts per inch of space.

VOLTAGE WAVE SHAPE: The pulsating DC voltage in the precipitator will show peak and minimum values that vary in magnitude about the average as observed on a secondary KV meter. The peak voltage is based somewhat on the peak AC magnitude while the minimum voltage is based on the capacitance resistance effect on the decay characteristics of the voltage field in the gas passage. The conduction time of the secondary current will be a factor. With low capacitance conditions, the minimum point might coincide with the threshold corona producing voltage on the discharge electrode.


Upper – A non gas-tight enclosure on the roof of the precipitator to shelter equipment (TR sets, rappers, purge air fans, etc.) and maintenance personnel.

Lower – A non gas-tight enclosure at base of precipitators to protect hoppers from wind and/or detrimental weather condition.

WEIGHTS:  This term refers to the cast iron weight attached to the bottom of the wire discharge electrode to keep it taut, much like the effect of a plumb-bob. These weights are about 25 lbs. (11.3kg) for most installations. The weights are positioned and retained in a bottom guide frame for maintenance of wire alignment at the center line of the gas passage.

WEIGHTED WIRE DESIGN: This term refers to precipitators utilizing the wire and weight for it's discharge electrode rather than a rigid type of high voltage system.


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Last updated: May 10, 2009.
Copyright © 1997 TRK Engineering Services, Inc. All rights reserved.
For more information contact: TRK Engineering Services - 95 Clarks Farm Road - Carlisle, MA 01741 - Telephone: 978-287-0550 - Fax: 978-287-0569 - email: trkeng@apcnetwork.com