Classifications

• • 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
  • G01N15/02—Investigating particle size or size distribution
  • G01N15/0266—Investigating particle size or size distribution with electrical classification
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/017—Combinations of electrostatic separation with other processes, not otherwise provided for
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/02—Plant or installations having external electricity supply
  • B03C3/04—Plant or installations having external electricity supply dry type
  • B03C3/06—Plant or installations having external electricity supply dry type characterised by presence of stationary tube electrodes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/02—Plant or installations having external electricity supply
  • B03C3/04—Plant or installations having external electricity supply dry type
  • B03C3/12—Plant or installations having external electricity supply dry type characterised by separation of ionising and collecting stations
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/34—Constructional details or accessories or operation thereof
  • B03C3/36—Controlling flow of gases or vapour
  • B03C3/368—Controlling flow of gases or vapour by other than static mechanical means, e.g. internal ventilator or recycler
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/34—Constructional details or accessories or operation thereof
  • B03C3/40—Electrode constructions
  • B03C3/41—Ionising-electrodes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/34—Constructional details or accessories or operation thereof
  • B03C3/40—Electrode constructions
  • B03C3/45—Collecting-electrodes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/34—Constructional details or accessories or operation thereof
  • B03C3/40—Electrode constructions
  • B03C3/45—Collecting-electrodes
  • B03C3/49—Collecting-electrodes tubular
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F5/00—Measuring a proportion of the volume flow
  • G01F5/005—Measuring a proportion of the volume flow by measuring pressure or differential pressure, created by the use of flow constriction
• • 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
• • 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
  • G01N2015/0038—Investigating nanoparticles

Definitions

• This invention relates to particle sensors, in particular ultrafme particle sensors that operate based on the principle of measuring an electrical current that results from the precipitation of ultrafme particles that have been electrically charged by diffusion charging.
• Diffusion charging refers to a particle charging process wherein airborne particles are electrically charged in a particle charging section by collisions with gaseous ions (typically positive ions) that have been generated by an ionization electrode such as a needle- tip electrode or corona wire.
• gaseous ions typically positive ions
• an ionization electrode such as a needle- tip electrode or corona wire.
• a known particle sensor of this type typically comprises a means for establishing an airflow through the sensor (for example a ventilator, a fan or a pump).
• the airflow passes through the particle charging section, and then through a particle precipitation section for removing substantially all airborne particles from the flow.
• the sensor further comprises a particle measurement section having a current meter for measuring the electrical current that results from the deposition of particle-bound charge per unit time in the particle precipitation section.
• the apparent ultrafme particle number concentration is equal to the ratio of the particle length concentration (i.e. the total length of the string of all airborne UFPs in a unit air volume when they would be lined up therein as a string) and a predetermined average particle diameter.
• the particle precipitation section may comprise a mechanical particle filter disposed within a Faraday cage, or a parallel-plate electrostatic particle precipitator.
• a parallel-plate electrostatic particle precipitator charged positive particles are precipitated at the negative plate of the capacitor and the resulting current measured is proportional to the particle concentration times particle diameter.
• the particle charging section of the ultrafme particle sensor is for example designed with a high voltage ionization electrode in the form of a needle-tip electrode that is surrounded by an electrically conductive enclosure.
• the enclosure is at least partly provided with openings.
• gaseous ions can be generated that are drawn from the needle-tip electrode towards the enclosure, to escape through the openings establishing a region containing gaseous ions next to the enclosure.
• the enclosure forms a corona ionization chamber and the gaseous ions emanate through a metal grid into the adjacent gas flow channel.
• the performance of the needle-tip electrode reduces over time. It has been found out that this is due to a contamination of the electrode's outer surface by compounds that are present in the surroundings of the electrode.
• silane compounds may form a silicon dioxide layer on the outer surface of the ionization electrode which acts as an electrically insulating layer. Such contaminants are typically present in the airflow carrying the ultrafme particles that are to be monitored.
• a particle sensor comprising:
• an electrostatic particle charging section comprising an ionization electrode within an ionization chamber, wherein the gas flow passes past the ionization chamber and partially enters the ionization chamber;
• a sensor unit for detecting the precipitated particles to produce a sensor signal
• the particle sensor comprises a sensor arrangement for producing a signal which is representative of the amount of gas flow between the outside of the ionization chamber and the inside of the ionization chamber.
• the invention is based on the recognition that although the ionization electrode is separated from the main airflow by the outer enclosure of the ionization chamber, at least a part of the air flow can still enter the enclosure via openings in this enclosure, as a result of an electrohydrodynamic phenomenon called the "ionic wind".
• the repulsion of gaseous ions from the ionization electrode creates an expanding ion cloud, which in turn creates this ionic wind emanating from the electrode.
• a pressure difference resulting from the presence of this wind can cause at least part of the airflow to enter through the openings into the enclosure thereby exposing the ionization electrode to harmful contaminants.
• This ionic wind within the ionization chamber in particular leads to a gas flow redistribution so that a large fraction (or all) of the gas flow is travelling through the ionization chamber.
• the performance of the ionization electrode will gradually decrease over time as a result of being exposed to contamination from the airflow. This performance decrease can be manifested as flow instabilities, ionization electrode (i.e. corona needle) lifetime problems and loss of charged particles.
• the voltage applied to the ionization electrode may be adjusted to compensate for this decrease in performance.
• the invention provides a sensor system for producing a signal which is representative of the amount of gas flow between the outside of the ionization chamber and the inside of the ionization chamber.
• the signal exceeds a predetermined threshold level, the magnitude of the ionic wind and/or the flow rate of the airflow may be adjusted. In this way, the operational lifetime of the device is increased.
• the sensor arrangement comprises an arrangement of flow rate meters. These flow meters measure the flow rate of the airflow. By measuring flow rate conditions at different locations, the flow conditions can be determined.
• the arrangement of flow meters may comprise a first flow meter outside the ionization chamber in the vicinity of an inlet end and/or an outlet end of the particle charging section, and a second flow meter outside the ionization chamber in the vicinity of a tip of the ionization electrode.
• the flow rate outside the ionization chamber but nearest the ionization electrode will have the greatest change in reading when there is a change in the amount of flow between the inside and the outside of the ionization chamber.
• the second flow meter is for example between the inlet end of the particle charging section and the tip of the ionization electrode.
• the greatest change in flow rate is at the inlet side (i.e. just before) the ionization electrode.
• the sensor arrangement comprises an arrangement of pressure sensors. By measuring pressure conditions at different locations, the flow conditions can be determined.
• the arrangement of pressure sensors may comprise a first pressure sensor inside the ionization chamber and a second pressure sensor outside the ionization chamber.
• the first pressure sensor may be at the inlet end of the ionization chamber and the second pressure sensor may be at the outlet end of the particle charging section.
• the pressure difference as measured by the first and second pressure sensors may then be determined. When the pressure difference exceeds a predetermined threshold level, the airflow is again considered to be pulled in to the enclosure too strongly.
• the particle sensor may further comprise a controller for controlling the drive level applied to the ionization electrode.
• the controller is adapted to select the drive level in dependence on the signal. In particular, when the flow conditions indicate that there is too large a flow into the ionization chamber, the drive level may be reduced.
• the particle sensor may instead or additionally comprise a controller for controlling a flow rate through the particle sensor, and the flow rate is selected in dependence on the signal. In particular, when the flow conditions indicate that there is too large a flow into the ionization chamber, the flow rate may be reduced.
• the precipitation section may comprise a parallel-plate particle precipitation section.
• a non-metallic shield may be provided in the ionization chamber. This non- metallic shield may be provided to direct the ion flow produced near the needle-tip electrode towards the grid separating the ionization chamber from the flow channel.
• the particle sensor may comprise a pre-filter to remove harmful contaminants (such as larger particles) from the airflow.
• the output of the sensor system may then also be used to determine when the pre-filter has deteriorated to such an extent that it should be replaced, by measuring the decrease in gas flow through the main channel.
• a predetermined factor for example by a factor of two
• an indication can be given to the consumer that the filter needs to be replaced.
• an electrostatic particle charging section comprising an ionization electrode within an ionization chamber, wherein the gas flow is provided past the ionization chamber but partially enters the ionization chamber;
• This method generates a signal which can be used to assess when flow conditions in and around the ionization chamber have become unsuitable, so that corrective action may be taken.
• Generating a signal may comprise measuring a first flow rate outside the ionization chamber in the vicinity of an inlet end and/or an outlet end of the particle charging section, and measuring a second flow rate outside the ionization chamber in the vicinity of a tip of the ionization electrode, wherein the signal is based on the relative magnitudes of the first and second flow rates.
• Generating a signal may instead comprise measuring a first pressure inside the ionization chamber and a second pressure sensor outside the ionization chamber, wherein the signal is based on the difference between the first and second pressures.
• a drive signal to the ionization electrode may then be adjusted in response to the signal and/or the flow rate through the particle sensor may be controlled.
• Figure 1 shows a first known example of a particle sensor
• Figure 2 shows a second known example of a particle sensor
• Figure 3 shows first flow conditions in and around the particle charging section
• Figure 4 shows second flow conditions in and around the particle charging section
• Figure 5 shows third flow conditions in and around the particle charging section
• Figure 6 shows a first example of flow sensor arrangement which may be used to modify the sensor of Figure 1 or 2;
• Figure 7 shows a second example of flow sensor arrangement which may be used to modify the sensor of Figure 1 or 2.
• the invention provides a particle sensor which uses an electrostatic particle charging section in the form of an ionization chamber.
• a flow sensor arrangement is used to produce a signal which is representative of the amount of gas flow between the outside of the ionization chamber and the inside of the ionization chamber. This information is indicative of the flow conditions, and can be used to determine when flow conditions are present which may adversely affect the performance or lifetime of the particle sensor.
• UFP electrical ultra- fine particle
• the sensor comprises an inlet 10 for receiving air at a flow rate ⁇ .
• a UFP charging section 12 comprises an air- ionizing high voltage ionization electrode 14 surrounded by a porous screen electrode 16.
• a UFP precipitation section 18 comprising a Faraday cage 20 containing a particle filter that is capable of substantially filtering all airborne particles from the sampled airflow that passes through the UFP sensor.
• a current meter 22 is connected to the Faraday cage. It measures the amount of particle-bound charge that deposits per unit time inside the Faraday cage as an electrical current I sen sor. Isensor constitutes the sensor signal.
• a means 24 for moving a sampled airflow comprising the airborne UFPs through the sensor This can be a ventilator, fan, pump, or an ionic wind device.
• a means 24 for moving a sampled airflow comprising the airborne UFPs through the sensor.
• This can be a ventilator, fan, pump, or an ionic wind device.
• the ionization is created by the high voltage V cor applied to the high voltage ionization electrode 14.
• the voltage is variable, under the control of a controller 27.
• the inferred apparent UFP number concentration N app (as defined above) in the sampled airflow relates to the measured signal I sen sor according to:
• N is the total UFP number concentration
• d p,av is the count mean particle diameter
• N app is proportional to the product of N and d p , av .
• the product Nd p , av denotes the particle length concentration (m/m 3 ).
• Knowledge of only N app is sufficient to assess the relative severity of the UFP-associated air pollution level. Separate knowledge of both N and d p , av is not required for that purpose.
• Equation 2 is valid for an average UFP particle size range 25 nm ⁇ d p , av ⁇ 120 nm. This range encompasses the typically encountered UFP size distributions throughout the UFP diameter range 10 nm ⁇ d p ⁇ 500 nm.
• a parallel-plate electrostatic particle precipitator 30 is provided and the current meter 22 is attached to the plate electrode whereupon the charged particles are precipitated.
• the parallel-plate precipitator can, for example, be embodied as two parallel flat electrode plates or as a concentric electrode set comprising an inner electrode that is surrounded by an outer electrode. Alternative embodiments will be apparent by the person skilled in the art. Use of the embodiment in Figure 2 instead of the embodiment in Figure 1 is preferred for ease-of-assembly reasons when UFP sensors are to be mass-produced at low cost and small size.
• the invention relates to the detection and/or prevention of undesired flow conditions in the ionization chamber.
• Figure 3 shows an example of a desired gas flow situation within the particle charging section 12.
• the axes are in units of mm.
• the main gas flow channel is at the bottom of the figure with flow entering from the right and exiting to the left.
• the ionization chamber is located above the metal grid 16 (which is vertically at -2.75 mm, with the tip of the high voltage ionization electrode 14 used as the origin). Only a small fraction of the gas flow is travelling through the ionization chamber.
• Figure 4 shows an example of an undesired gas flow situation within the particle charging section 12.
• This flow pattern has been created by adding an oval metal shield 40 above the high voltage electrode 14, at a vertical location of +1.5 mm, and at the same potential as the high voltage ionization electrode 14.
• the shield blocks the upward pointing part of the ion current and is therefore creating a strongly asymmetric "ionic wind" force field which is oriented downwards only. This is a known approach.
• Ion currents and gas velocities are identical in both examples.
• Figure 4 shows that a metallic shield is strongly disadvantageous when the flow conditions are considered, since it raises the operating voltage of the corona discharge device by several kilo volts. Furthermore it leads to a strongly directional flow of "ionic wind” and as a consequence to a dramatic increase of gas flow from the flow channel through the ionization chamber.
• Figure 5 shows that if a non-conducting shield 42 is used above the high voltage electrode 14, the change to the flow pattern is much less significant.
• a non-metallic shield for the same purpose as the metallic shield, is greatly preferred.
• the invention relates to measures to detect and prevent disadvantageous flow conditions.
• a first aspect relates to the detection of undesired flow conditions.
• a first approach is to measure the flow velocity in the center of the main gas flow channel (for example the region 1 mm ⁇ x ⁇ 4 mm in Figures 3 to 5).
• This velocity should not be less than a set fraction of the velocity at the entrance or exit of the main flow channel.
• This set fraction may for example be in the range 0.4 to 0.7, for example 0.4, 0.5 or 0.6.
• Figure 6 shows the particle charging section 12 with a first flow sensor SI at the inlet side, a second flow sensor S2 between the inlet and outlet sides and a third flow sensor S3 at the outlet side.
• the sensors in this example are all outside the ionization chamber, i.e. in the main air flow channel.
• sensors may be at only one angular position around the annulus, or at multiple positions around the annulus.
• the design does not need to be rotationally symmetric.
• openings may be provided in the top or bottom of the ionization chamber, not only in the side walls.
• the second sensor S2 is generally aligned with the high voltage electrode tip, preferably slightly towards the inlet side so that it is positioned where the greatest flow reduction is experienced as shown in Figure 4. For example it may be positioned at least 75 % of the way along the section 12 between the inlet and the high voltage electrode tip.
• the distance dl is the distance of the flow sensor from the inlet end of the ionization chamber (in particular the component parallel to the general flow direction) and the distance d2 is the distance of the tip of the ionization electrode from the inlet end of the ionization chamber.
• Figure 7 shows the particle charging section 12 with a first pressure sensor PI at the inlet end of the ionization chamber, at a corner (for measuring p cor ) , and a second pressure sensor P2 at the main channel exit (for measuring ⁇ ⁇ 3 ⁇ 4).
• a second aspect relates to how to change the operation of the sensor when adverse flow conditions have been detected.
• the current delivered to the high voltage electrode may be reduced depending on the measured signals, i.e. the relative flow velocities or pressure differences.
• the high voltage electrode drive voltage V cor is variable by a controller 27 as shown in Figures 1 and 2, and it may be adjusted making use of the information from the flow sensor arrangement.
• the flow rate thought the particle sensor may additionally or alternatively be controlled to influence the flow conditions.
• the measures above relate to the detection and prevention of undesired flow conditions. It is also possible to design the system to prevent and reduce the occurrence of the undesired flow conditions.
• Design rules for example include:
• a horizontal corona wire may be used which spans the corona chamber in flow direction (inlet to outlet direction). This yields a broad ion cloud and a broad "ionic wind" distribution.
• the sensor arrangement may be considered generally to be a "flow sensor arrangement" in that the sensed signals are influenced by flow conditions, or it may be considered to be a "flow and/or pressure sensor arrangement". It may comprise flow rate sensors or pressure sensors or combinations of these, each of which provide measurements which relate generally to flow conditions. There may be sensor elements inside and/or outside the ionization chamber.
• One example has been given of a pressure sensor inside the chamber, but other examples may provide one or more flow rate sensors inside the ionization chamber. The aim is essentially to be able to distinguish between flows of the general type shown in Figure 3 and those of the general type shown in Figure 4. Various combinations of pressure sensors and flow rate sensors may be used to achieve this.

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• 2016
  • 2016-06-29 EP EP16733497.8A patent/EP3317640B1/en active Active
  • 2016-06-29 US US15/736,832 patent/US10508982B2/en active Active
  • 2016-06-29 WO PCT/EP2016/065072 patent/WO2017005560A1/en not_active Ceased
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