WO2012022844A1 - Impacteur électrique - Google Patents

Impacteur électrique Download PDF

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Publication number
WO2012022844A1
WO2012022844A1 PCT/FI2011/050731 FI2011050731W WO2012022844A1 WO 2012022844 A1 WO2012022844 A1 WO 2012022844A1 FI 2011050731 W FI2011050731 W FI 2011050731W WO 2012022844 A1 WO2012022844 A1 WO 2012022844A1
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WIPO (PCT)
Prior art keywords
impactor
electrical
particles
measurement
particle
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PCT/FI2011/050731
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English (en)
Inventor
Kauko Janka
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Kauko Janka
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Application filed by Kauko Janka filed Critical Kauko Janka
Publication of WO2012022844A1 publication Critical patent/WO2012022844A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
    • G01N27/68—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas
    • G01N27/70—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas and measuring current or voltage
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
    • G01N15/06—Investigating concentration of particle suspensions
    • G01N15/0656—Investigating concentration of particle suspensions using electric, e.g. electrostatic methods or magnetic methods

Definitions

  • the present invention relates to an electrical apparatus for measuring particles according to the preamble of claim 1 and specifically to an electrical impactor comprising a component which switches or modulates at least a parameter affecting the measurement mode of the electrical impactor.
  • the present invention further relates to a process for measuring particles according to the preamble of claim 12.
  • Particle measurements are frequently carried out by cascade impactors, where particulate matter is withdrawn (preferably isokinetically) from a source and segregated by size
  • Cascade impactors use the principle of inertial separation to size segregate particle samples from a particle laden gas stream.
  • Conventional cascade impactors cannot be used in real-time particle size distribution (PSD) measurement and especially in measuring changes in PSD in real time.
  • PSD particle size distribution
  • ELPITM Electrical Low Pressure Impactor
  • the particles are first charged into a known charge level in the corona charger. After charging, the particles enter a cascade low pressure impactor with electrically insulated collection stages.
  • the particles are collected in the different impactor stages according to their aerodynamic diameter, and the electric charge carried by particles into each impactor stage is measured in real time by sensitive multi-channel electrometers.
  • This measured current signal is directly proportional to particle number concentration and size.
  • the particle collection into each impactor stage is dependent on the aerodynamic size of the particles.
  • Measured current signals are converted to (aerodynamic) size distribution using particle size dependent relations describing the properties of the charger and the impactor stages. The result is particle number concentration and size distribution in real-time.
  • ELPITM without operating corona charger, it can be used for particle charge distribution measurements.
  • the electrical impactor collects the particles, it is possible also to carry out post-sampling measurements, i.e. weight the samples collected on each impactor stage and/or analyze the composition of the particles collected on each stage.
  • determination of the final charging state is the fact that the original charging state of particles affects the results. In many cases, e.g. when measuring particles from combustion process or particles after electrostatic precipitators, the resulting measuring errors may be significant.
  • the prior art technique uses a separate aerosol neutralizer prior to the charger unit to neutralize the particles before they are charged. However, the neutralizes are not usually used, because it is an extra component producing cost and man work, and it increases the particle losses yielding inaccuracy to the measurement result. There are no reliable and easily applied criteria to decide when use of a neutralizer is necessary.
  • electrical impactors face the problem of deposition of charged fine particles on the collection stages for larger particles. Due to the collection mechanism, i.e.
  • cascade impactors including electrical impactors need to be constructed so that the 1 st (upper) collection stage (in the direction of the sample flow) collects the largest particle fraction, the 2 nd stage collects the 2 nd largest particle fraction, etc.
  • This is a major difference to e.g. collection devices based on diffusion, where the smallest particle fraction is collected first (in the direction of the sample flow).
  • the Coulombic force accumulates smaller particles on the first or on the few upper collection stages, these fine particles cause an error both on the real-time measurement and on the post- sampling analysis.
  • the measurement error generated by the problem mentioned above may be significant. Fine particles have a high electrical mobility and thus are highly affected by Coulombic force. In a typical particle sampling the amount of large particles collected by the few upper collection stages is low and thus the electrical current erroneously generated by the fine particles accumulated on the upper stages may even be as large as or larger than the current generated by the large particles.
  • Finnish patent publication FI 104127 B Dekati Oy, 28.7.1999, describe a method for minimizing Coulombic losses in impactors and an impactor, in which the Coulombic losses have been minimized.
  • the method according to the invention is based on restricting the force effect of the charges accumulated in impactor' s insulators on the charged particles. This restriction of the force effect can be realized, for example, by placing an electricity conductive surface between the insulator and the flow, or by forming an insulator (24) so that the force effect is minimized.
  • the solution described in FI 104127 B leads to difficult mechanical constructions and requires high-purity electrical insulators, as even small impurities on the insulator surfaces cause leaking currents which adversely affect the measurement.
  • An electrostatic instrument for measuring particle concentrations and possibly sizes in aerosols suffers from errors which limit the useful response bandwidth of the device.
  • United States Patent application Publication US 2004/0080321 Al, Kingsley St. John Reavell, et al., 29.4.2004 describes modifications made to the design of electrostatic particle measurement instruments to compensate or eliminate the transient currents produced by the rate of change of charge near the sensing electrodes, and hence reduce the transient errors in measured particle concentrations.
  • the invention minimizes or eliminates these transient errors which are caused by changing particle concentrations in the aerosol.
  • a system may be added to an otherwise conventional instrument to compensate for the transient effects based on a model of the charge production mechanism.
  • a screening electrode placed over the sense electrodes in the instrument, and held at controlled electrical potential difference is added to the instrument to eliminate the effect.
  • a third embodiment adds compensating electrodes which provide a direct measurement of the transient effect which can be subtracted from the signal.
  • the publication also provides a hint for volumetric flow measurement where the travel time between two electrodes, the penultimate electrode and the final electrode could potentially be used. However, this measurement method suffers from the problem discussed earlier, i.e. the need for at least two sensing elements. Also, the publication specifically states that the screening electrode is held at controlled potential difference and does not teach potential difference modulation.
  • the patent publication also describes an apparatus for characterization of particles in an aerosol, said apparatus comprising: an essentially closed chamber comprising a gas inlet, a gas outlet, and means for establishing an aerosol flow through said chamber; means mounted in the interior of said essentially closed chamber in the region of said inlet for electrically neutralizing said aerosol; means for heating said aerosol, said heating means comprising measuring and control means for establishing a predetermined aerosol temperature; radiation means arranged within the interior of said essentially closed chamber and defining an irradiation zone for activating the heated aerosol flowing there through; said irradiation zone comprising a wall surface held at a defined electric potential and having a conductivity which is high enough to conduct the charge of ions produced by irradiation and movable to said wall surface to said defined electric potential; and at least one collector electrode mounted in the interior of said essentially closed chamber downstream of said irradiation zone for collecting electrically charged particles and electrically connected to a current or charge meter; said at least one collector electrode comprising a size
  • the patent publication US 4,837,440 describes a measurement method which is based on the use of filters connected in series.
  • the probability that a particle is deposited in the filter depends on its diffusion coefficient which, in turn, depends on the size of the particle. Smaller particles have a higher diffusion velocity resulting in a higher probability that they are deposited in the filter. As the probability that a particle is deposited in the filter is also increased, if the filaments of the cluster or web of the filter are arranged tightly to each other, it is obvious for a person skilled in the art that in order to provide size selectivity to a measurement unit described in US 4,837, 440, the smaller particles are collected first (in the direction of the sample flow) and then, subsequently, larger and larger particles.
  • Hameri describes a transfer function which is used to determine the PSD. This is not the same as a dynamic transfer function which can be used to describe the time response of the collection stage.
  • One way for describe the time-wise behavior of a collection stage is to model it by a delay 3 ⁇ 4 and a first-order low-pass filter (which, as obvious for a person skilled in the art is equivalent to the time-wise behavior of a completely mixed reactor).
  • ⁇ (s) i + T s' wnere > ° re f er t0 me input and output of the collection stage, s is frequency and ⁇ is the time constant .
  • the transfer function for the PSD determination as well as the dynamic transfer function changes due to different operational conditions and thus theoretical calculation of the transfer functions or even experimental determination of the transfer functions with fixed operational conditions will not provide accurate results for real-life measurements in continuously changing operational conditions.
  • information on the electrical charge of the particles under measurement is often required.
  • Real-time particle charge measurement as a function of the particle size with the prior art technology requires two measurement units, such as two ELPF M s, one for measuring the particle concentration (as a function of the particle size) and another for measuring the particle charge, in which case the corona charger used to charge the particles entering the measurement unit is switched off.
  • Such a measurement method is expensive and the use of two parallel instruments increases the measurement errors.
  • the measurement error is further increased by the "naturally" charged particles (i.e. particles which carry charge before entering the measurement unit), as also the charging may differ between the sample flows of the two measurement units.
  • the inventor has surprisingly found a method which will solve the prior art electrical impactor problems described above.
  • the problems are solved by implementing a process where at least either (1) the particle charging or (2) free ion and small particle collection is switched, preferably on and off, or modulated, over time.
  • the invented electrical cascade impactor in the following text also referred to as “electrical impactor” or “impactor” ensures reliable, size-selective particle concentration measurement, makes sure particle charge measurement, provides a method for eliminating a measurement error caused by naturally charged particles, confirms reliable particle measurement results even with low particle concentrations and with long measurement periods and provides tools for precise correction algorithms due to the determination of time responses of different collection stages. All this can be achieved simultaneously with decreased soiling or particle deposition into the measurement unit and thus the maintenance period is prolonged.
  • the invented method also provides a method to measure or control the sample flow flowing through the electrical impactor.
  • the impactor comprises an electrical discharging unit, typically a corona charger, which charges the particles flowing into the impactor, and at least one collection stage which is based on inertial separation of particles so that the largest particles are collected on the first (upper) collection stage.
  • the collection stage or typically collection stages lays usually an ion trap which removes free ions or extremely small particles (typically having a diameter of few nanometers or less than 10 nanometers) from the sample flow.
  • the ion trap can remove particles up to about 100 nm particle diameter, i.e. also particles larger than the nucleation mode particles. Obviously such measurement mode is not used when measuring particles smaller than the particles which are trapped.
  • the impactor may also comprise a neutralizer which neutralizes the electrical charge of the particles before they enter to the collection stage(s).
  • the neutralizer may be a separate unit or the neutralization may be carried out by the electrical discharging unit, e.g. by such a way that the corona charger comprises two charging units with opposite electrical potential or by using a single corona charger in AC (alternating current) mode which produces ions with opposite charges.
  • the electrical discharging unit of the present invention has a unique feature that it can be switched or modulated between different charging modes.
  • the electrical discharging unit is a corona charger which is switched periodically between the ON-mode and OFF-mode, i.e. the corona voltage is periodically switched ON and OFF.
  • Essentially continuous comparison of the measurement results between the ON and OFF modes provides the base for calculating the natural charge of the particles as well as for calculating the particle concentration.
  • the term "periodical switching" means that the switching may have a fixed frequency or the frequency may vary and also the length of the ON and OFF modes may vary. The length of the ON mode may be less than 100 seconds, less than 10 seconds or even less than a second.
  • the duty cycle may vary between 1 and 99%, preferably between 5 and 50% and more preferably between 5 and 20%.
  • the response time of the impactor should be as short as possible.
  • the response time is decreased by using, in the calculation of the particle concentration or PSD, the time constants of each collection stage which can be obtained from the response which the switching, or more generally modulating, of the corona charger voltage creates to the current measured from the collection stage(s).
  • Another alternative is to modulate or switch the trap voltage.
  • the time constants can even be determined continuously and thus the changing operational conditions, such as temperature or pressure, will not adversely affect the measurement accuracy.
  • the particle concentration is determined from the total charge collected from a collection stage.
  • the concentration is determined from the difference between the collection stage current of the ON-mode and the OFF-mode. Using the switched mode measurement makes it possible to continuously determine the drifting of the measured current and eliminate it.
  • the electrical discharging unit is switched between two opposite voltages, ON + and ON " .
  • the particle concentration or PSD is determined from the difference of the response between these two modes, the natural charge of the measured particles affects the measurement significantly less than if the result is determined from the difference between the ON and OFF modes.
  • the electrical discharging unit is switched between three different modes: ON-, OFF-, and NE-modes, where the NE mode describes a mode where either a separate neutralizer is used or the electrical discharging unit is used to neutralize the particles entering the impactor.
  • ON-, OFF-, and NE-modes where the NE mode describes a mode where either a separate neutralizer is used or the electrical discharging unit is used to neutralize the particles entering the impactor.
  • the natural charge of the particles can be determined by using a single measurement apparatus. Obviously there is a practical advantage of avoiding the use and handling of two measurement units. Also, because the aerosol sampling is taken from a single point, the differences between two sampling points do not affect the measurement accuracy and neither do the calibration differences and instabilities of two different measurement equipment.
  • the electrical discharge switching decreases the deposition of the charged particles on the measurement unit interior and especially beneficially decreases the deposition of fine particles on the upper collection stage(s) due to the Coulombic force.
  • the best results are achieved when the duty cycle is small, preferably around 10% or less. It has been surprisingly found that in typical realizations of the detection electronics of electrical impactors the low-frequency (1/f) noise dominates the noise level of the measurement result. The consequence of this feature is that even low duty cycles and synchronous detecting technique may yield better noise and sensitivity performance to the instrument than the prior-art measurement mode without switching/modulating. So, this improved noise performance can be achieved in addition to the elimination of current drifting. Additionally, as the soiling of the measurement unit is considerably decreased, the
  • the electrical discharge unit can be switched between ON, OFF and NE-modes.
  • the length of the NE-mode is longer than the length of the ON- and OFF-modes and thus the Coulombic deposition can be minimized.
  • the measurement error caused by the fine particles accumulated on the upper collection stage(s) is determined by switching the operation mode of the ion trap.
  • the collection stage(s) lays usually an ion trap which removes free ions or extremely small particles (typically having a diameter of few nanometers or less than 10 nanometers) from the sample flow.
  • Increasing the ion trap voltage increases the trapping of fine particles and with high trap voltage the fine particles which would accumulate on the upper collection stage(s) are removed.
  • the ion trap may be switched between high and low voltages, V hjgh and V low and measuring the response, i.e.
  • the measurement error on the upper collection stage(s) caused by the fine particles in the sample may be determined and simply decreased from the measurement current of the upper stage(s). This measurement result may also be used to estimate the error of the lower collection stage(s), which receive too little amount of fine particles, as compared to the actual sample flow.
  • the duty cycle or the length of the ON + , ON " , OFF and NE-modes can be varied during the measurement and thus optimize the operation of the measurement apparatus.
  • the duty cycle control may be based on an internal signal of the measurement apparatus e.g. the particle concentration or on the time wise derivative of the particle concentration.
  • the duty cycle control may also be based on an external signal, e.g. when measuring particle exhaust from a combustion engine; the external signal may include a change in the momentum of the engine or in the revolution speed of the engine.
  • the response from the switched or modulated mode of the electrical discharge unit is determined by synchronic detection.
  • Synchronic detection can be realized by using either analog electronics or digitally. The digital realization can obviously be carried out in a separate computing unit or it may be integrated to a common controller or computing unit, where other control functions of the electrical impactor are carried out as well.
  • the switching or modulation of the electrical discharge unit is used in determining the dynamic transfer function of each collection stage, keeping in mind that the typical main parameters, delay time, t s , and characteristic time, ⁇ , may vary for each collection stage.
  • the main parameters are determined by modulating the electrical discharge unit or trap voltage, i.e. the switching of the electrical discharge unit between at least two of the modes ON + , ON " , OFF or NE.
  • the main parameters can be determined even continuously and thus the time response of each collection stage may be determined even continuously and the measurement result of each collection stage may be corrected even continuously and thus the changes in the measurement environment do not adversely affect the measurement result. If the changes in the measurement environment are not remarkable on a short time interval, and when the maximum time response of the measurement is required, the determination of the main parameters may be carried out with longer intervals, and thus use the determined parameters over longer intervals.
  • the correction of the measurement result of each collection stage is based on a model where the time wise behavior of each stage is modeled by delay and filter units which are connected together.
  • the optimal equivalent circuit used in the modeling depends on the flow characteristics of the collection stage, the serial collection of delay and transfer units is usually a reasonably accurate model.
  • the filter units are typically first-order low-pass filters, the behavior of which is determined by the characteristic time constant r.
  • the time correction of the collection stages may be realized by compensating the combined delay times t s by analog or digital means and by analog or computational (digital) filter units carrying out inverse time constants ⁇ .
  • the time correction may be carried out continuously, it may be carried out with longer intervals or it may be even carried out during the calibration of the electrical impactor and store the time correction parameters on the control unit of the measurement equipment.
  • the interval time for determining the time correction parameters depends mainly on the use of the instrument, i.e. if it is used in steady or varying environmental conditions.
  • a surprising benefit of using a modulated signal instead of one sensor signal is that, in spite of simpler and cheaper practical solution, it can yield better performance.
  • the reason of this feature is that one of the signals to be compared/correlated has practically no noise or other disturbances, whereas in prior-art solutions two noisy signals are compared/correlated.
  • FIG. 1 shows a summary of the invention with its benefits to the electrical impactor
  • Fig. 2 shows a schematic drawing of the invented apparatus
  • Fig. 3 describes the time behavior of the collection stages
  • Fig 4 describes the determination of the parameters used in the time correction of the collection stages
  • Fig 5 describes the time wise correction of the measurement signal
  • Fig 6 describes an embodiment where the determination of the parameters used in time correction and the time wise correction of the measurement signal are integrated together
  • Fig 7 describes another embodiment where the determination of the parameters used in time correction and the time wise correction of the measurement signal are integrated together.
  • the present invention is an electrical impactor 1 (numbers refer to Figure 2)where the particles entering the impactor 1 are classified by inertial separation on at least one stage 4, characterized in that impactor 1 comprises a functional component 22 which switches or modulates the operational mode of the electrical discharge unit 20 or/and the ion trap.
  • the electrical discharge unit 20 is switched or modulated at least between two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE).
  • the electrical discharge unit 20 or the ion trap is preferably switched or modulated on frequency higher than 0,01 Hz, more preferably higher than 0, 1 Hz and most preferably higher than 1 Hz.
  • the duty cycle of the switched/modulated signal is preferably between 1-99%, more preferably between 5-50% and most preferably between 5-20%.
  • the frequency and the duty cycle can vary freely during the measurement and also the form of the modulated pulse may vary. Especially with synchronic detection such variations do not adversely affect the measurement.
  • the electrical impactor 1 further comprises functional component 24 connected to functional component 22 or to the electrical discharge unit 20 and to electrometer 6 for providing the natural charge of the particles on stage 4, as well as functional component 25 connected to functional component 22 or to the electrical discharge unit 20 and to
  • electrometer 6 for providing particle concentration of the particles on stage 4.
  • Electrical impactor 1 further comprises functional component 26 connected to functional component 22 or to the electrical discharge unit 20 and to electrometer 6 for eliminating measurement current drifting in electrometer 6.
  • Electrical impactor 1 comprises functional component 27 for determining the essential parameters of transfer function Fi of stage i 4.
  • Functional component 23 connected to functional component 27 for monitoring the volumetric flow flowing through impactor 1.
  • the present invention also includes a process for particle measurement where the particles are classified by inertial separation, characterized in that unwanted particle deposition on the upper stages used for inertial separation is minimized by using essentially continuous switching or modulating either the electrical source used for particle charging or the ion trap used to trap free ions and ultrafine particles.
  • the electrical source used for particle charging is switched or modulated at least between two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE).
  • the electrical source used for particle charging is switched or modulated on frequency higher than 0,01 Hz, preferably higher than 0,1 Hz and most preferably higher than 1 Hz.
  • the electrical source used for particle charging is switched or modulated with duty cycle between 1-99%, preferably between 5-50% and most preferably between 5-20%.
  • the difference in the measurement results between at least two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE) is used to determine particle concentration, or particle size distribution, or natural charge of particles and to eliminate the measurement current drifting as well as to determine the essential parameters of the transfer function Fi of stage i.
  • the transfer function Fi of stage i comprises a first-order low-pass filter transfer function and the essential parameters are delay time t i of stage i and time constant rof the first-order low-pass filter.
  • the process further includes correcting the measurement signal Soi by the inverse of the transfer function Fi.
  • the switching or modulation frequency as well as the duty cycle can be adjusted during the measurement.
  • the duty cycle does not have to stay constant during particle monitoring, but a beneficial advantage of the present invention is that the duty cycle can be dynamic, i.e. its value may be optimized during the measurement. Not only the duty cycle may be varied, but also the length of the t 0 N may be varied as well. The variation and optimization may be based e.g. on the sensor measurement result (particle concentration, time-wise derivative of particle concentration) or on external factors (like combustion engine revolution speed or torque).
  • Figure 2 shows an embodiment of the present invention, which comprises an electrical impactor 1 where the particles entering the impactor 1 are classified by inertial separation on at least one stage 4.
  • Impactor 1 comprises a functional component 22 which switches or modulates the operational mode of the electrical discharge unit 20 or/and the ion trap.
  • Impactor 1 also comprises means for switching or modulating the electrical discharge unit 20 at least between two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE).
  • the switching or modulating frequency is preferably higher than 0,01 Hz, more preferably higher than 0,1 Hz and most preferably higher than 1 Hz and the duty cycle is between 1-99%, preferably between 5-50% and most preferably between 5-20%.
  • Electrical impactor 1 also comprises functional component 24 connected to functional component 22 or to the electrical discharge unit 20 and to electrometer 6 for providing the natural charge of the particles on stage 4 and functional component 25 connected to functional component 22 or to the electrical discharge unit 20 and to
  • Electrometer 6 for providing particle concentration of the particles on stage 4.
  • Impactor 1 also comprises functional component 26 connected to functional component 22 or to the electrical discharge unit 20 and to electrometer 6 for eliminating measurement current drifting in electrometer 6.
  • electrical impactor 1 may comprise functional component 27 for determining the essential parameters of transfer function Fi of stage 4i. It is possible to monitor the volumetric flow passing through the impactor lby determining the essential parameters of transfer function Fi and thus impactor 1 may comprise functional component 23 connected to functional component 27 for monitoring the volumetric flow flowing through impactor 1.
  • the present invention also includes a process for particle measurement where the particles are classified by inertial separation, so that unwanted particle deposition on the upper stages used for inertial separation is minimized by using switching or modulating either the electrical source used for particle charging or the ion trap used to trap free ions and ultrafme particles.
  • the electrical source used for particle charging is switched or modulated at least between two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE).
  • the electrical source used for particle charging is switched or modulated on frequency higher than 0,01 Hz, preferably higher than 0,1 Hz and most preferably higher than 1 Hz and the electrical source used for particle charging is switched or modulated with duty cycle between 1-99%, preferably between 5-50% and most preferably between 5-20%.
  • the difference in the measurement results between at least two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE) is used to determine particle concentration, or particle size distribution, or natural charge of particles.
  • the difference in the measurement results between at least two of the modes: high positive voltage (ON+), high negative voltage (ON-), essentially zero voltage (OFF) and neutralization (NE) may also be used to eliminate the measurement current drifting and to determine the essential parameters of the transfer function F, of stage i.
  • the transfer function F, of stage i comprises a first-order low-pass filter transfer function and the essential parameters are delay time of stage i and time constant rof the first-order low-pass filter.
  • the invented process further includes correcting the measurement signal Soi by the inverse of the transfer function E,.
  • Figure 1 shows a summary of the invention, describing the beneficial effects that the invention has on the operation of the electrical impactor.
  • the main inventive step of the present invention is that the modulation (time wise change) of the electrical discharge element input/output (e.g. modulation of the voltage of a corona discharge unit resulting in the modulation of the ion current emitted from the corona needle) and/or modulation of the ion trap voltage decreases the particle deposition on the interior of the electrical impactor and especially minimizes fine particle deposition on the upper collection stages caused by the electrical discharge element input/output (e.g. modulation of the voltage of a corona discharge unit resulting in the modulation of the ion current emitted from the corona needle) and/or modulation of the ion trap voltage decreases the particle deposition on the interior of the electrical impactor and especially minimizes fine particle deposition on the upper collection stages caused by
  • the modulation (time wise change) of the electrical discharge element input/output e.g. modulation of
  • Modulation may be a step-wise switching, e.g. switching between ON and OFF modes, or it may be an essentially smooth change between at least two separate modes.
  • the ion trap voltage is modulated
  • the correction can be made both to the upper stages (additional deposition of fine particles) or lower stages (reduced deposition of fine particles).
  • the correction can be made both to the electrical signal measured from the stages and/or to the weighted mass on the stages.
  • the modulation provides additional benefits to the electrical impactor in addition to the decrease of the particle deposition.
  • These additional beneficial effects include definition of the natural charge of the particles in the sample flow, elimination of measurement current drifting, reduction of the low-frequency noise, and elimination of the measurement error caused by the deposition/accumulation of the fine particles on the upper collection stages (which not only affects the measurement signal of the upper stages but also of the lower stages collecting fine particles) and improving the representativeness of the sample.
  • the present invention also allows the determination of the time response which can be typically presented by two parameters: the delay time tj and the time constant Tof each collection stage, it may be used to speed up the time response of the electrical impactor, improve the PSD determination and monitor the sample flow through the measurement apparatus.
  • FIG. 2 shows the schematic drawing of the invented apparatus.
  • Electrical impactor 1 comprises an electrical discharge unit 20 for charging particles which enter impactor 1 through the inlet nozzle 12.
  • the electrical discharge unit 20 is preferably a corona charger or a dielectric barrier discharge unit and it may also comprise a neutralizer or it may be used in a neutralizing mode, e.g. by using an alternating current (AC) power supply with corona charger.
  • Impactor 1 comprises at least on chamber 2, having side surfaces and forming collection stage 4, which is electrically connected to an electrometer 6. Collection stage 4 is electrically isolated from the impactor body 10 and potential other stages 4 by electrical insulator 8. Aerosol sample flow 14 is passed through the inlet nozzle 12 into chamber 2.
  • the bottom of chamber 2 comprises a nozzle 16 which is essentially perpendicular to the sample flow 14.
  • Nozzle 16 is in galvanic contact to stage 4.
  • Nozzle 16 comprises holes through which sample flow 14 flows. After nozzle 16 the sample flow impacts on a collection plate 18 and particles larger than a designed particle size attach on the collection plate 18.
  • the collection plate 18 may be in galvanic contact to stage 4, but impactor 1 may also be constructed so that only the collector plate 18 is connected to the electrometer 6, or impactor 1 may also be constructed so that only the surface of collector plate 18, e.g. a metal foil, is connected to the electrometer 6.
  • the particle collection in electrical impactor 1 is based on inertial separation, where particle collection is based on sudden change in the direction of the aerosol flow.
  • Impactor 1 further comprises a exit nozzle 30 to guide flow 14 to the next chamber 2 or to exit the flow from impactor 1.
  • Apparatus 1 of the embodiment shown in Figure 2 further comprises component 22 to modulate the operation mode of the electrical discharge unit 20.
  • component 22 controls the corona voltage of a corona charger 20 between ON and OFF modes with 0,01 Hz - 1 Hz frequency.
  • the duty cycle is preferably about 10%, so that 90% of time the corona charger is in OFF-mode and thus the particles entering apparatus 1 are not charged by the corona charger 20.
  • the embodiment of Figure 2 further comprises component 24, which component 24 should be considered as a functional component, i.e. component 24 may be a separate component carrying out the required function or it may be an integral part of some other component such as the main control unit of the electrical impactor 1.
  • Component 24 may carry out the required function by analog or by digital means.
  • Component 24 is connected to the electrical discharge unit 20 or to the controlling component 22 or to both and it is also connected to the electrometer 6.
  • the function of component 24 is to provide information on the natural charge of the particles entering the impactor 1, based on the current measured by the electrometer 6.
  • Component 24 compares the current values provided by the electrometer 6 at the different operation modes of the electrical discharge unit 20, which is controlled by the control unit 20.
  • the electrical discharge unit 20 is a corona charger and its voltage is switched or modulated between OFF and NE modes. Switching between OFF and NE modes provides an additional benefit as it allows the elimination of the current drift of the electrometer 6. Elimination of the current drift is carried out by another functional component 26, which may be a separate component carrying out the required function or it may be an integral part of some other component such as the main control unit of the electrical impactor 1. Component 26 may carry out the required function by analog or by digital means.
  • the embodiment of Figure 2 further comprises component 25, which component 25 should be considered as a functional component, i.e. component 25 may be a separate component carrying out the required function or it may be an integral part of some other component such as the main control unit of the electrical impactor 1.
  • Component 25 may carry out the required function by analog or by digital means.
  • Component 25 is connected to the electrical discharge unit 20 or to the controlling component 22 or to both and it is also connected to the electrometer 6.
  • the function of component 25 is to provide information on the concentration of the particles entering the impactor 1, based on the current measured by the electrometer 6.
  • Component 25 compares the current values provided by the electrometer 6 at the different operation modes of the electrical discharge unit 20, which is controlled by the control unit 20.
  • the electrical discharge unit 20 is a corona charger and its voltage is switched or modulated between ON and OFF modes or between ON and NE modes or between ON + and ON " modes. Switching between different modes provides an additional benefit as it allows the elimination of the current drift of the electrometer 6. Elimination of the current drift and low-frequency noise suppression is carried out by another functional component 26.
  • the embodiment of Figure 2 further comprises component 27, which component 27 should be considered as a functional component, i.e. component 27 may be a separate component carrying out the required function or it may be an integral part of some other component such as the main control unit of the electrical impactor 1. Component 27 may carry out the required function by analog or by digital means.
  • Component 27 is connected to the electrical discharge unit 20 or to the controlling component 22 or to both and it is also connected to the electrometer 6.
  • the function of component 27 is to determine the main parameters, especially delay time ⁇ and time constant rof the transfer function of stage 4. These parameters can be used to optimize the time-wise behavior of stage 4.
  • the embodiment of Figure 2 further comprises component 23, which component 23 should be considered as a functional component, i.e. component 23 may be a separate component carrying out the required function or it may be an integral part of some other component such as the main control unit of the electrical impactor 1.
  • Component 23 may carry out the required function by analog or by digital means.
  • Component 23 is connected to the component 27.
  • the function of component 23 is to use the delay and time constants obtained by component 27 to monitor the flow passing through the electrical impactor 1.
  • the function of component 27 is realized by describing the time- wise behavior of stage 4 so that it comprises a delay component t s and a time constant which describes the behavior of a first-order low-pass filter.
  • the time-wise behavior of the firs-order low-pass filter is similar to the one of a fuU-mixed reactor.
  • the first-order low-pass filter can be described in Laplace notation as
  • the derivative of function F(s ) in time domain is dSo(t) _ Si(t)-So(t) (r) .
  • component 27 receives signals at a certain frequency and with a certain duty cycle, the frequency being preferably higher than 0,01 Hz, more preferably higher than 0,1 Hz and even more preferably higher than 1 Hz and the duty cycle being preferably between 5-20%.
  • the differential equation (2) can be approximated by difference equation (3):
  • the stages 4 are connected to a cascade and thus the time response of the whole impactor can be described as a serial connection of first-order low-pass filters and delay times, as shown in Figure 3.
  • Delay time ts and time constant 3 ⁇ 4 of each collection stage A can be determined by setting them to such values that the model describing the time-wise behavior of each stage has a maximum correlation to the actual measured signal.
  • the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and OFF modes. In another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and NE modes. In yet another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON + and ON " modes. In yet another embodiment of the present invention, the required impulse for the response is generated by modulating the trap voltage of the ion trap.
  • FIG. 4 shows a block diagram of an embodiment where the time- wise parameters are determined.
  • Component 27 providing the time-wise model receives an excitation Cy, from component 22 controlling the corona charger 20.
  • Component 27 comprises a function Con,- which is either an analog or digital unit connected to correlator X.
  • Correlator X compares the measurement signal So, from stage 4j to the calculated signal E, and the time parameters t ⁇ u and Ti are set to the values where the correlator X provides a maximum signal C Von i.e. the correlation between the model and the measured signal is set to maximum.
  • the time response of the measured signal can be compensated or correlated by modifying the signal with the inverse function.
  • the transfer function following the Laplace notation this means multiplying the signal with the inverse of the transfer function F(s).
  • Compensating the time delay is merely applying a time shift.
  • FIG. 5 An embodiment with the compensation algorithm based on time delay and first-order low-pass filter is shown as a block diagram in Figure 5.
  • the corrected output signals Set from each stage i are calculated by modifying the stage output signals So, with the inverse transfer functions 7/F,-. It is obvious for a person skilled in the art that modeling the time-wise behavior with a first-order low-pass filter is only given here as an example and any suitable model which describes the behavior of stage 4 and which can be presented in analog or digital form can be used for modeling. The best model depends on the construction of the electrical impactor.
  • the current from each stage is not measured only from the collection plate 18, but from essentially all surfaces of collection stage 4, i.e. stage 4 forms a Faraday cage into which sample 14 enters from nozzle 12 and exits from nozzle 30, as shown in Figure 2.
  • the current from the charged particles which stay in collection stage 4 is measured with electrometer 6.
  • the charged particles leaving stage 4 obviously carry current with them to the next stages.
  • Figures 6 and 7 show two embodiments for determining the correction parameters and realizing the signal correction. Only two first stages are shown in the figures, but it is obvious for a person skilled in the art that similar protocol may be followed also for the subsequent staes 3,4, and so on.
  • the embodiment of Figure 6 maximizes the correlation between the signal Soi measured from stage i and the modeled transfer function into which signal Ch provides the impulse.
  • the embodiment of Figure 7 maximizes the time-wise correlation between the corrected signal Sc, and the signal Ch representing the control or output signal of the electrical discharge unit 20. Both embodiments lead in principle to the same result.
  • the volumetric flow passing the electrical impactor 1 is inversely proportional to the sum of the time delay ⁇ and time constant ⁇ .
  • the determination of the time constants tj and T provides also a tool to monitor or measure the volumetric flow.

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Abstract

L'invention porte sur un impacteur électrique (1) dans lequel les particules entrant dans l'impacteur (1) sont classées par séparation inertielle sur au moins un étage (4). L'impacteur (1) comprend un composant fonctionnel (22) qui commute ou module le mode de fonctionnement de l'unité de décharge électrique (20) et/ou du piège à ions. L'invention porte également sur un procédé permettant la mesure des particules dans lequel les particules sont classées par séparation inertielle. Un dépôt indésirable de particules sur les étages supérieurs utilisés pour la séparation inertielle est minimisé grâce à l'utilisation d'une commutation ou d'une modulation soit de la source électrique utilisée pour la charge des particules, soit du piège à ions utilisé pour piéger des ions libres et des particules ultrafines.
PCT/FI2011/050731 2010-08-20 2011-08-19 Impacteur électrique WO2012022844A1 (fr)

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FI20100360U FIU20100360U0 (fi) 2010-08-20 2010-08-20 Sähköinen hiukkasmittauslaite
FIU20100360 2010-08-20
FI20110067A FI20110067A0 (fi) 2010-08-20 2011-02-28 Hiukkasanturi
FI20110066A FI20110066A0 (fi) 2010-08-20 2011-02-28 Hiukkasanturi
FI20110066 2011-02-28
FI20110067 2011-02-28
FI20110065 2011-02-28
FI20110065A FI20110065A0 (fi) 2010-08-20 2011-02-28 Sähköinen impaktori

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PCT/FI2011/050729 WO2012022842A1 (fr) 2010-08-20 2011-08-19 Procédé et appareil pour la mesure de particules

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CN104487817B (zh) * 2012-03-06 2017-11-03 皮卡索尔公司 用于颗粒质量浓度测量的设备和过程以及对用于颗粒质量浓度测量的设备的使用
JP5960619B2 (ja) * 2013-01-28 2016-08-02 日本特殊陶業株式会社 外部ガス流を利用した微粒子センサ
DE202014007548U1 (de) 2014-09-16 2014-12-02 Pegasor Oy Vorrichtung zur Spülung einer Partikelmessvorrichtung
US9791361B2 (en) 2015-10-26 2017-10-17 Dekati Oy Method and apparatus for measuring aerosol particles of exhaust gas
US9791360B2 (en) 2015-10-26 2017-10-17 Dekati Oy Method and apparatus for measuring aerosol particles suspended in gas
FI20155760A (fi) 2015-10-26 2017-04-27 Dekati Oy Varaajayksikkö hiukkasmonitorointilaitteistoa varten sekä hiukkasmonitorointilaitteisto
EP3371582A4 (fr) 2015-11-02 2020-02-12 Pegasor Oy Appareil et procédé de mesure de caractéristiques de flux de particules
GB201609868D0 (en) 2016-06-06 2016-07-20 Cambridge Entpr Ltd Particle measurement apparatus
AT523371B1 (de) * 2019-12-18 2021-11-15 Avl List Gmbh Vorrichtung und Verfahren zur Messung von Aerosolen

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FI20110066A0 (fi) 2011-02-28
FI20110067A0 (fi) 2011-02-28
WO2012022842A1 (fr) 2012-02-23
FI20110065A0 (fi) 2011-02-28
FIU20100360U0 (fi) 2010-08-20
EP2606344A1 (fr) 2013-06-26

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