History of Electrical PrecipitationDate16001745182418781885SignificanceWilliam Gilbert, English court physician, publishes De MagneteBenjamin Franklin describes the effects of points “in drawing and throwing off the electric fire.”M. Hohlfeld, German mathematician, describes the precipitation of fog in a jar containing an electrified pointR. Nahrwold notes that the discharge from an electrified sewing needle surrounded by a tin cylinder greatly increases the collection of atmospheric dust. Nahrwold repeats the experiment with a glycerin coating to help particles adhere.Sir Oliver Lodge attempts, unsuccessfully, to remove lead fume from from a smelting works in North WalesHistory of Electrical Precipitation• Frederick Cottrell– Incorporated more reliable rectifier transformer circuits in ESP design - able to sustain higher voltages– Successfully collected sulfuric acid mist in Berkeley, CA laboratory in 1906– First successful commercial precipitator used to collect H2SO4in Pinole, CA 200 cfm capacity– 1912, large scale ESP used to collect cement kiln dust at 1,000,000 cfm in Riverside CAFrederick Cottrell1877 - 1948Source: U.S. Department of AgricultureAdvantages to Electrical PrecipitationElectrostatic Precipitators (ESPs):-collect particles from 0.01 µm to 100 µm with 99% efficiency- operate at high temperatures, up to 1200° F (650° C)- operate at high gas pressures, up to 150 psi (10 atm)- operate at high flow rates, up to 3,000,000 cfm (1500 m3/s)- operate at high particle loadings, 500 grams/m3- have low energy costs, 200 – 1000 Watts/1000cfm- have low pressure dropESPs can be used when:- large volumes of particulate air pollutants are produced- no explosion hazard exists- high efficiency needed- continuous processes (expensive to build but inexpensive to operate)Industries and their pollutants where ESPs are commonly usedProcess Principal Material CollectedElectrical Utility Fly Ash (SiO2, Al2O, Fe2O3)Industrial Boiler Houses Fly AshSteelmaking Furnaces Iron Oxide (Fe2O3)Cement Kilns Calcium Oxide, Silicon OxidePulp and Paper Sodium SulfateMetal Machining Oil MistMulti-stage wire-plate ESPGas inletCollected dust to hopperFlow straightenersCollection platesCorona wire electrodes with rappersElectrical Precipitators in useWire plate type designElectrical Precipitators in useCourtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htmExamples of discharge electrodesCourtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htmPractical considerations:Removing collected dust• Collected particles must be disposed of properly• Dust coated electrodes can- lower electric field strength- increase likelihood of spark - cause back corona• Result: Decreased collection efficiency• Methods used to clean collecting plates- Wire - cylinder design: washing- Wire - plate design: rappingPractical considerations:Removing collected dust: RappingElectrode rappingCollecting plate rappingCourtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htmPractical considerations: Dust resistively• Highly insulated particles are poor conductors– Resistive to charging– Not easily collected• Particle resistivity (Ω-cm) related to:– Elemental composition– Moisture content of air– Gas temperature– Above 1010 Ω-cm, particle collection becomes difficult• Conditioners– Added to gas stream to increase particle conductivity– Examples include: H20, NH3, H2SO4Practical Considerations: Particle re-entrainment• Re-entrainment occurs when collected particles are re-released into the air stream• Sources of re-entrainment:– Highly turbulent flow – velocity concentration– Rapping – observed as ‘puffs’ exiting the precipitator– Back coronaTake 5!Personal ESP sampler developed at UNC+dc powercoronacollectionsubstrateinletionizing wireESP sampler current-voltage characteristics05010015020025030001234567Voltage, kVCurrent, µA10 mil #110 mil #210 mil #3Current ~ ion concentration, NiVoltage ~ electric field strength, EOzone output vs. ESP power01002003004005006000 500 1000 1500 2000Power, mWOzone, ppb20 mil #110 mil #120 mil #210 mil #220 mil #310 mil #3ESP collection efficiency vs. flow0.600.650.700.750.800.850.900.951.000.01 0.1 1 10Particle Diameter, micronsCollection Efficiency2.04.06.08.0Flow, LpmReview: Semi-Volatile Compounds10-8torr < vapor pressure < 10-2torrSemi-volatile aerosols:- exist in both particle and vapor phases- can readily transfer mass between phases- important for exposure – health effect studies- lung deposition behavior- atmospheric transport- emission regulationsevaporationparticlephaseadsorption/absorptionvapor phaseFilters cannot sample semi-volatile mists accurately• Metalworking fluids are semi-volatile• Particles evaporate from filter over time• Underestimation of worker exposure MistVapor lossFilterESP Advantages, DisadvantagesAdvantages• Collection substrate has low surface area– lower vapor adsorption artifact• Collected particles coalesce together– less potential for particle evaporation artifactDisadvantages• Corona discharge generates O3– some potential for chemical artifactComparison of sampling methods for mineral oil mistPVC PTFE GF ESP DataRAM DustTRAK0.00.20.40.60.81.0Mist Concentration, mg/m³filtersoptical particle countersSampling semi-volatile aerosolsSampling semi-volatile aerosolsSampling semi-volatile