Patent Description:
Various particle collectors using electrostatic charges to collect particles are known. These devices are known as electrostatic precipitators (ESP) which may either have a general inlet gas flow that is substantially parallel to the electrostatic collection surface (linear ESP), or generally orthogonal to the collection surface (radial ESP) whereby the gas flows radially outwards as it impinges against the collection surface.

One of the drawbacks of linear ESP systems is the generally lower collection efficiency and higher particle size dependency in deposited position compared to radial ESP systems. All conventional ESP's however suffer from one or more drawbacks including: low spatial uniformity in deposition pattern; high size dependency in deposition pattern such that particles in different size are not uniformly distributed; poor collection efficiency in that the yield of particles collected is low compared to the particles in the gas stream; low collection mass flux leading to slow particle accumulation; and high chemical interference whereby reactive molecules such as ozone, NOx and others are produced from the high electric field strength of the ESP electrodes due to corona discharge.

In particle sampling applications, it is important not to generate reactive gases that could modify the properties of collected particles (described herein as chemical interference). For the sampling of various particle containing gases, for instance with spectroscopic measurement devices, it is advantageous to have a uniform spatial distribution with low size dependence such that the observation of the collection area is representative of the particles contained in the sampled gas. In order to perform sampling rapidly with high accuracy, it is also advantageous to have a high particle collection efficiency over a short duration.

Sampling applications may include sample collections for spectroscopy and spectrometry or other types of chemical analyses for studies in air quality, atmospheric science, or industries that involve generation of particles such as in manufacturing industries, construction and e-cigarettes where customer safety is a consideration. The aforementioned advantageous properties of ESP's would also be useful in seeding applications for subsequent epitaxial film growth of crystals that can prove useful in membrane technology and nanocrystal technology. Further applications that use particle collection with ESP systems may include biological samples needed for optical analysis or other in vitro studies. ESP particle collection may also be used in certain coating applications.

An orthogonal electrostatic particle collection device comprising sheath flow is known from <CIT>, however the particles precipitated on the electrode in the disc precipitator portion are not observed, rather it is the particles that pass through the precipitator that are counted. The particle size distribution may be obtained by stepping the precipitation voltage through the entire voltage range and measuring the electrical charges associated with penetrating particles. The purpose of the precipitator is thus to act as a cut-off "filter" that retains particles above a certain size and allow particles below said threshold to pass through, such cut-off threshold being dependent inter alia on the voltage applied across the electrodes which can be varied in order to perform a full analysis of the particles in the gas flow. In such a classification system, the distribution of particles on the electrode in the disc precipitator is unimportant and the problem of having a uniform distribution which is not particle size dependent is not considered.

Various other electrostatic particle collection devices are known from <CIT>, <CIT>, and <CIT>:.

In view of the foregoing, an object of the invention is to provide an electrostatic particle collector apparatus for automated optical analysis of the collected particles that has a high spatial uniformity in the deposition pattern with low size dependence of the particles and low chemical interference.

It is advantageous to provide a particle collector that has a high collection efficiency.

It is advantageous to provide a particle collector that has a high collection mass flux enabling rapid particle accumulation for a given period of time.

It is advantageous to provide a particle collector that is economical to manufacture and operate.

It is advantageous to provide a particle collector that is compact.

It is advantageous to provide a particle collector that is easy to operate and maintain.

Objects of this invention have been achieved by providing a particle collector according to claim <NUM>.

Disclosed herein is an ESP particle collector for collecting particles in a particle containing gas stream, comprising an inlet section, a collector section, and an electrode arrangement, the inlet section comprising a flow tube defining a gas flow channel therein bounded by a guide wall extending between an entry end and a collector end that serves as an inlet to the collector section, the entry end comprising an inlet for the particle gas stream and a sheath flow inlet portion for generating a sheath flow around the particle gas stream, the collector section comprising a housing coupled to the flow tube, and a collector plate mounted therein having a particle collection surface. The ESP particle collector comprises an optical measuring instrument configured to transmit light through the collector plate along a centre axis A orthogonal or substantially orthogonal to the particle collection surface for optical analysis of the collector plate particle collection surface to measure particles collected thereon, and wherein the flow tube has a bent portion such that the entry end is positioned out of the centre axis A to allow the light to be transmitted through the collector plate in the direction of the centre axis and to be picked up without interfering with the gas flow or the gas inlet.

The flow tube comprises a first portion arranged along an initial inclined axis (Gi) connected via the bent portion to a second portion arranged along the centre axis (A) connected to the collector section <NUM>, an angle of inclination (β) of the initial axis (Gi) relative to the centre axis (A) being less than <NUM>°, preferably less than <NUM>°, more preferably less than <NUM>°, for instance in range of <NUM>° to <NUM>°.

In an advantageous embodiment, a length (d2) of the second portion of flow tube is in a range of <NUM>. 3D to <NUM>. 7D, D being an overall length of the flow tube.

In an advantageous embodiment, the optical measuring instrument comprises a spectroscopic optical instrument comprising a light source arranged to project light through the collector plate and a light detector arranged to capture the light transmitted through the collector plate from the light source.

In an advantageous embodiment, the ESP particle collector further comprises a cleaning system comprising one or more nozzles arranged to direct one or more jets of a cleaning gas on the collector plate particle collection surface.

In an advantageous embodiment, the collector plate is mounted on a motorized movable platform to move the collector plate away from the measurement position for the cleaning operation.

In an advantageous embodiment, the ESP particle collector further comprises a purge gas source connected fluidically via a valve to the gas inlet, for instance the sheath gas flow inlet, configured to purge the inlet section and collector section gas flow channels prior to the measurement cycle.

In an advantageous embodiment, the ESP particle collector further comprises a controller connected to various devices of the ESP particle collector allowing the automated measurement of collected particles, said devices including some or all of: the particle charger; gas pumps as for the particle gas flow, for the sheath gas flow, for the outlet; a purge gas valve, a motorized platform for moving the collector plate; a cleaning system; and the optical measuring instrument.

In an advantageous embodiment, the ESP particle collector, in a variant, comprises a pair of devices each having said inlet and collector sections and associated flow tubes, coupled optically to a common said measuring instrument.

In an advantageous embodiment, the electrode arrangement comprises at least a base electrode positioned below the collection surface and a counter-base electrode positioned at a separation distance L2 above the collection surface such that an electrical field is generated between the electrodes configured to precipitate said particles on the collection surface, wherein the electric field is in a range of <NUM> kV per mm to <NUM> kV per mm, with an absolute voltage on any said electrode that is less than <NUM> kV, and wherein a ratio ratio_1 of a radius L1 of said inlet at the collector end divided by said separation distance L2 is in a range of <NUM> to <NUM>.

In an advantageous embodiment, the base electrode preferably has an annular shape that permits the optical beam to pass through the centre.

In an advantageous embodiment, the collector plate is mounted on a collector plate holder, removably mounted in the housing to allow the collector plate to be optically analysed by an external instrument for measurement of particles collected thereon.

In an advantageous embodiment, the ESP particle collector further comprises a particle measurement instrument arranged in the housing above or below the particle collection surface to measure the particles collected on the particle collection surface.

In an advantageous embodiment, a ratio_2 (L1/L4) of the radius L1 of said inlet divided by a radius L4 of the base electrode is less than <NUM>.

In an advantageous embodiment, said ratio_2 (L1/L4) is less than <NUM>, for instance <NUM> or lower.

In an advantageous embodiment, a ratio lims (Ls/L1) of an inner radius Ls of the said sheath flow relative to the inlet radius L1 is less than <NUM>.

In an advantageous embodiment, said ratio lims (Ls/LL) is in a range of <NUM> to <NUM>.

In an advantageous embodiment, a ratio ratio_3 of the radius L1 of said inlet divided by a radius L3 of the collector plate (L1/L3) is in a range of <NUM> to <NUM>.

In an advantageous embodiment, said ratio ratio_3 (L1/L3) is in a range of <NUM> to <NUM>.

In an advantageous embodiment, the electrode arrangement further comprises a tube electrode around the collector end forming the inlet to the collector section.

In an advantageous embodiment, the sheath flow inlet portion comprises a sheath flow gas inlet, a gas chamber and an annular sheath flow gas outlet surrounding the centre of the flow channel and configured to generate an annular sheath flow along the guide wall of the flow channel surrounding the particle gas stream.

In an advantageous embodiment, the ESP particle collector further comprises a particle charger arranged upstream of the inlet section configured to electrically charge the particles of the gas stream entering the inlet section.

In an advantageous embodiment, the particle charger is configured to impart a charge on the particles contained in the gas stream in a range of about <NUM> elementary charge per <NUM> (<NUM>=<NUM>-<NUM>m) to about <NUM> elementary charge per <NUM> diameter of a particle.

In an advantageous embodiment, the particle charger is configured to impart a charge on the particles contained in the gas stream in a range of about <NUM> elementary charge per <NUM> diameter to about <NUM> elementary charge per <NUM> diameter of a particle.

In an advantageous embodiment, the collector plate is made of a transparent conductive or semiconductor material.

The invention will now be described with reference to the accompanying drawings, which by way of example illustrate embodiments of the present invention and in which:.

Referring to the figures, an ESP particle collector <NUM> according to embodiments of the invention comprises an inlet section <NUM>, a collector section <NUM> including a collector plate <NUM>, an electrode arrangement <NUM>, and an optical measuring instrument <NUM> for automated measurement of the particles collected on the collector plate. The particle collector may further comprise a particle charger <NUM> arranged upstream of the inlet section <NUM> configured to electrically charge the particles of the gas stream entering the inlet section <NUM>.

The particle charger is configured to impart a small charge on the particles contained in the gas stream to be sampled preferably in a range of about <NUM> elementary charge per <NUM> (<NUM>=<NUM>-<NUM>m) to about <NUM> elementary charge per <NUM> diameter of a particle. Preferably the charge is in a range of <NUM> elementary charge per <NUM> diameter to about one elementary charge per <NUM> diameter for instance around <NUM> elementary charge per <NUM> diameter. The relatively small charge allows the particles to be charged with a low generation of reactive species such as ions and radicals such as ozone, in order to ensure low chemical interference on the particles contained in the gas stream. Various per se known particle chargers may be used, such known chargers using field charging, diffusion charging, or ultraviolet charging, provided that they have a low reactive species generation on the particles in the gas stream.

An example of a charger that may be used for the invention is for instance described in Han [<NUM>] which describes a wire-wire charger with a low ozone production.

The charging of the particle stream, although optional in embodiments of the invention, advantageously assists in improving uniforms spatial distribution of particles on the collector plate.

According to an aspect of the invention, the optical measuring instrument <NUM> is configured to transmit light through the collector plate along an axis A orthogonal or substantially orthogonal to a particle collection surface <NUM> on the collector plate. The optical measuring instrument <NUM>, in a preferred embodiment, comprises a spectroscopic optical instrument comprising a light source <NUM> arranged to project light through the collector plate <NUM> and a light detector <NUM> arranged to capture the light transmitted through the collector plate <NUM> from the light source.

The inlet section <NUM> comprises a flow tube <NUM> defining a gas flow channel <NUM> therein bounded by a guide wall <NUM> that is preferably of a generally axisymmetric shape at the collector end section. The flow tube at the collector end section may be generally cylindrical as illustrated in embodiment of <FIG> or may have other axisymmetric shapes for instance as illustrated in <FIG> and <FIG>. The flow tube may however also have non-axisymmetric cross-sectional profiles such as polygonal (square, pentagon, hexagon or other polygons).

The flow tube <NUM> extends between an entry end <NUM> and a collector end <NUM> that serves as a gas inlet to the collector section <NUM>. According to an aspect of the invention, the flow tube has bent portion <NUM> such that the entry end <NUM> is positioned out of the centre axis A to allow the light to be transmitted through the collector plate in the direction of the centre axis and to be picked up without interfering with the gas flow or the gas inlet. The gas thus flows in a first portion along an initial inclined axis Gi before joining a second portion along the centre axis A and then entering the collector section <NUM>. The angle of inclination β of the initial axis Gi relative to the centre axis A is less than <NUM>°, preferably less than <NUM>°, more preferably less than <NUM>°, for instance between <NUM>°and <NUM>°, the length d2 of the second portion of flow tube is preferably in a range of <NUM>. 3D to <NUM>. 7D, D being the overall length of the flow tube, in order to ensure a substantially axisymmetrical developed flow of the gas particles and sheath at the collector section <NUM>.

The bent portion <NUM> is preferably curved as illustrated in <FIG> to reduce turbulence and ensure that the gas flow is substantially laminar while maintaining a similar distribution of the streamlines about the centre line.

The entry end <NUM> comprises an inlet <NUM> for the particle gas stream and a sheath flow inlet portion <NUM> for generating a sheath flow around the particle gas stream. By the term "particle gas stream" it is meant the gas stream containing the particles to be collected in the collector section <NUM>.

The sheath flow inlet portion <NUM> comprises a sheath flow gas inlet <NUM>, a gas chamber <NUM> and a sheath flow gas outlet <NUM> surrounding the centre of the flow channel <NUM> and configured to generate and annular sheath flow along the wall <NUM> of the flow channel <NUM> surrounding the particle gas flow. The chamber <NUM> serves to contain a volume of gas with a low or essentially no pressure gradient within the chamber with respect to the sheath gas inlet, such that the radial nozzle defining the sheath flow outlet <NUM> generates an even circumferential sheath flow.

The flow rates of the sheath flow and particle gas flow may be calibrated such that the two gas streams have laminar flow properties and the boundary layer between the sheath flow stream and particle gas stream remains laminar substantially without mixing. The gas flow streams are configured such that the Reynolds number is below <NUM>, preferably below <NUM>, for instance around <NUM>.

The flow tube <NUM> has an overall length D that is configured to ensure that the velocity of the sheath gas stream and particle gas stream at the interface therebetween accelerates such that the velocity profile of the gas stream within the flow channel collector end is a substantially continuous single rounded profile with a substantially flatter profile compared to the gas stream as the entry end. In effect, at the sheath flow outlet, the laminar flow profile is substantially parabolic and joins the particle gas stream at the boundary interface with a velocity close to zero that accelerates as the gas stream flows away from the sheath flow outlet.

The sheath flow separating the particle gas flow from the guide wall <NUM> reduces or avoids deposition of particles on the guide wall <NUM> and has further advantages in improving spatial uniformity of the particle deposition in the collector section <NUM>, reducing also chemical interference, reducing size dependence in the collection and improving collection efficiency. This is not only because it reduces the gradient in axial velocity of the particle gas stream that flows on to the collector plate, but also due to the separation of the gas stream from the flow channel walls, it reduces interference of the charge particles with the flow channels walls.

The collector section <NUM> comprises a housing <NUM> coupled to the flow tube <NUM>, and the collector plate <NUM> mounted therein on a collector plate holder <NUM>.

The collector plate comprises a transparent disc, for instance made of a crystal such as a Silicon, Zinc Selenide, or Germanium crystal, for optical analysis using for instance infrared spectroscopy. The collector disc may be removably mounted within the housing for cleaning, replacement, or for placement in an external measurement instrument for analysing the particles deposited on the collector plate <NUM> as a complementary measurement to the internal measurement. The collector plate <NUM> may comprise a filler material <NUM> arranged around the collector plate <NUM>. The gas stream flow over the collector plate is thus defined not only by the collector end <NUM> of the flow tube <NUM> but also the radius of the collector plate <NUM> and the filler material <NUM> therearound.

The ESP particle collector <NUM> may advantageously comprise a cleaning system <NUM> comprising one or more nozzles <NUM> arranged to direct one or more jets of a cleaning fluid on the collector plate particle collection surface <NUM> of the collector plate to blow away particles on the collection surface. In an embodiment, the collector plate may be mounted on a motorized movable holder or platform <NUM> to lower the housing <NUM> and the collector plate <NUM> away from the counter-base electrode 8b for the cleaning operation. The cleaning system may be operated at the end of a measurement cycle, or at defined intervals. The lowering of the housing <NUM> using motorized movable holder or platform <NUM> can further allow for insertion or removal of collector plate <NUM> or collector plate holder <NUM>.

The cleaning fluid may for instance be liquid/ gas mixture of CO<NUM> with suspended solids, or other preferably inert gases such as argon or nitrogen. The cleaning fluid may be supplied from a dedicated cleaning fluid source, or may be supplied from a filtered purge gas that would improve the portability of the system.

The ESP particle collector <NUM> may further comprise a purge gas source <NUM> connected fluidically via a valve <NUM> to the gas inlet, for instance the sheath gas flow inlet, to purge the inlet section and collector section gas flow channels prior to a new particle collection cycle. The purge gas source may be operated at the beginning of a measurement cycle, or at defined intervals, and may be operated prior to or simultaneously with the cleaning system. The purge gas may for instance be argon or nitrogen, or other inert gases that do not interfere with mid-IR wavelengths. The purge gas may also be dry air supplied from a purge-gas generator, which comprises self-regenerating columns of molecular sieves that only require pressurized air and electricity. This allows to remove the need for supplying and replacing gas cylinders.

The controller <NUM> is connected to the various automated devices of the ESP particle collector allowing the automated measurement of collected particles. The controller may be for instance be connected to all or some of the following components of the ESP particle collector: the particle charger <NUM>, various gas pumps 51a, 51b, 51c such as for the particle gas flow, for the sheath gas flow, for the outlet, the purge gas valve <NUM>, the motorized platform <NUM>, the cleaning system <NUM>, and the measuring instrument <NUM>. The ESP particle collector may further include a graphical user interface including a display <NUM> for inputting data or commands and for displaying measurement results and control information.

The electrode arrangement <NUM> comprises at least a base electrode 8a positioned adjacent or on an underside <NUM> of the collector plate <NUM>, below the collection surface <NUM> where particles are deposited. The base electrode may have a central orifice to allow the light of the measurement instrument to pass through the collector plate. The electrode arrangement <NUM> further comprises a counter-base electrode 8b positioned at a certain separation distance L2 above the collector plate <NUM> and which may be arranged substantially parallel to the base electrode 8a such that an electrical field is generated between the electrodes 8a, 8b.

In embodiments, the electrode arrangement may optionally further comprise a tube electrode 8c around the collector end <NUM> forming the inlet to the collector section <NUM>. The tube electrode 8c may be at the same voltage as the counter-base electrode 8b or at a different voltage therefrom separated by an insulating element from the counter-base electrode 8b.

It may be noted that the various electrodes may be at a certain voltage with respect to ground or one of the electrodes may be connected to ground and the other at a potential different from ground.

The inlet channel at the collector end has a radius defined as L1. The collector plate has a radius defined as L3. The base electrode has a radius defined as L4. The distance between the counter-base electrode 8a and the collector plate <NUM> has a separation distance defined as L2.

are within certain ranges that according to an aspect of the invention allow to provide a high spatial uniformity and low size dependence, as well as a high collection efficiency of particles to be sampled on the collector plate <NUM>.

An optimal ratio_1 (L1 / L2) affects the variation in the electric field under the inlet tube which may be optimized to improve spatial uniformity and collection efficiency.

A lower bound value for an optimal ratio_3 may be constrained by any value where impaction affects the final deposition pattern, however collection mass flux is generally higher if this ratio is more than <NUM>. An upper bound value may be constrained by a fixed limit on operating voltage (and maximum electric field strength) and on ratio<NUM> above, for example by, <MAT>.

The upper bound value may also be constrained by a desired efficiency, for example by, ratio<NUM> ≤ <MAT>.

Advantageously, another ratio L1/L4 of interest for high spatial uniformity and low chemical interference is a ratio between the radius L1 of the inlet channel collector end and the base electrode radius L4, named hereinafter by convention as ratio_2. The ratio_2 controls the electric field concentration effects on the collector plate's edges. An optimal ratio_2 may thus serve to improve spatial uniformity and lowers the electric field strengths in some regions, in particular to lower the variation in electric field strength under the inlet tube.

According to an aspect of the invention, the ratio_1 (L1 divided by L2) is in a range of <NUM> to <NUM>, preferably in a range of <NUM> to <NUM>.

According to an aspect of the invention, the ratio_2 (L1/L4) is less than <NUM>, preferably less than <NUM>, for instance <NUM>,<NUM> or lower.

According to an aspect of the invention, the ratio_3 (L1 divided by L3) is preferably in a range of <NUM> to <NUM>, preferably in a range of <NUM> to <NUM>.

The electric field generated between the base electrode 8a and counter-base electrode 8b is preferably in a range of <NUM> kV per mm to <NUM> kV per mm, preferably from <NUM> kV per mm to <NUM>,<NUM> kV per mm for instance around 1kV per mm, with an absolute voltage on any electrode that is less than <NUM> kV, to reduce chemical interference while ensuring high collection efficiency.

The inner radius Ls of the sheath flow relative to outer radius L1 of the sheath flow at the collector end <NUM> forming the inlet to the collector section <NUM>, is defined herein as ratio lims (Ls/L1). Ratio lims is in a range of <NUM> to <NUM>, preferably in a range of <NUM> to <NUM>, for instance around <NUM>, to ensure a sheath flow layer sufficient to provide a good separation between the gas particle stream and the flow channel wall <NUM> as well as ensuring that the particle gas stream impinging upon the collector plate <NUM> allows optimal uniform spatial distribution of the particles on the collector plate.

The above mentioned ratios are important in achieving the following advantages of embodiments of the invention:.

Embodiments of the invention may advantageously be used in various applications, including:.

An unexpected finding by the inventors of the present invention is that the particle velocity distribution at the plane of deposition (just before collection on the collector plate <NUM>) is not a direct representation of the final distribution of deposited particles. This finding contradicts conventional thinking such as found in the work of Dixkens and Fissans [<NUM>] and Preger et al. [<NUM>], but is an important consideration predicting and therefore optimizing particle deposition patterns.

As illustrated in <FIG> and <FIG>, the ESP particle collector <NUM> according to variants may have a pair of inlet and collector sections <NUM>, <NUM> and associated flow tubes, coupled optically to a common measuring instrument <NUM>.

In the embodiment of <FIG>, the pair of inlet and collector sections are arranged in mirror image symmetry.

In the embodiment of <FIG> the pair of inlet and collector sections are arranged in parallel, with beam splitters and mirrors <NUM> positioned in the optical path between the light source <NUM> and light detector <NUM> so as to split the beam at the source <NUM> into two beams, one passing through each collector plate <NUM>, and then collect the two beams at the detector <NUM>.

The pair of devices 1a, 1b, coupled to a common measuring instrument <NUM> allows to double the number of particles measured and thus increase the sensitivity and/or reduce the measurement cycle time of the measurement. Alternatively, the pair of devices allows to collect gases for measurement from two different positions, for instance in order to provide an average measurement value of a certain volume of measurement or to reduce the impact on measurement results due to a localized pollution at the inlet of one of the devices.

It is important to characterize the composition of aerosol particles in air, which causes adverse health effects and millions of deaths each year. Aerosol, or particulate matter (PM), is difficult to characterize because of its wide range of particle sizes (few nanometers to several micrometers); constituents (various organic and inorganic compounds); concentration (one to hundreds of µg/m<NUM>, for PM < <NUM> µm); morphology; state (liquid or solid); and time-dependent modification.

An ideal collector would enable collecting an aerosol sample that is an identical copy of the aerosol in air at an instant of time. Such a collector, when used with an ideal characterization method, will allow an ideal quantitative measurement of the composition of the aerosol. However, most conventional particle collectors modify or preferentially sample certain size ranges, chemical composition, morphology or state. Furthermore, collected sample is characterized for the constituents and/or their composition using numerous spectrometric techniques, which can induce further modifications. For example, most spectroscopic techniques require collecting aerosol on a surface for a prolonged period to make a confident claim about its constituents, composition.

Infrared (IR) spectroscopy is a non-destructive method, which provides useful chemical information about the constituents. Current methods for collecting samples use filters that are made of material which interferes with the IR spectra and thus lowers detection capabilities. Hence, collection on an IR-transparent substrate (for example, chalcogenide crystals) is desirable. A particle collector according to embodiments of the invention that achieves the advantages mentioned above allows to make a good quantitative measurement using IR-spectroscopy. Specifically, "Low size-dependence", "Low chemical interference" and "High collection efficiency" is required to collect an aerosol sample that is identical to the aerosol in air, "High spatial uniformity in deposition pattern" is required to reduce optical artefacts or spectrometer dependence, and "High collection mass flux" is required to reduce the collection time needed for making a confident claim.

Electrostatic precipitation (ESP) is a versatile method of collection and does not suffer from high pressure drop (which can modify the aerosol chemical composition, for example in filtration), or from bounce-off effects (which preferentially samples the size range and liquids, for example in impaction). ESP is a common device for dust removal but is also used for particle deposition.

Example <NUM>: Referring to the exemplary embodiment illustrated schematically in <FIG>, the plot in 5a(ii), which is aligned along the vertical axis with the particle collector schematically shown in <FIG>, shows results from particle deposition simulations done using a simulation program (COMSOL Multiphysics). The difference between the outer radius of deposition of various sizes is low and the spatial deposition is close to the ideal profile. In this example the collector plate radius is L<NUM> = <NUM> mm.

Example <NUM>: This example shown in <FIG>, has the same collection plate radius and differs from Example <NUM> above mainly in the ratio<NUM> value L1/L3.

Referring to <FIG>, the basic equation for the radial ESP system is : <MAT>.

The equation is different for the two representative theoretical inlet flow profiles illustrated in <FIG>: a) plug-flow and b) parabolic flow, based on the different expression of fv (r<NUM>) and Q for both the cases. For the limiting case of <MAT> the final position for the outermost particle for both the equations become:.

where, Qa = Aerosol flow rate particle containing air stream.

Furthermore, this equation is in terms of the aerosol flow rate, which is the flow rate of interest as it contains the particle and if possible maximizing this flow rate while keeping the collection efficiency high would be ideal. Some key implications of the analytical model:.

Factors important for "High spatial uniformity in deposition pattern".

Factors important for "Low size-dependence ".

Factors important for "Low chemical interference ".

Factors important for "High collection efficiency" and "High collection mass flux".

The analytical model is valid for the case where flow is laminar. Hence, for any given combination of L3 value and ratio<NUM> value, the operating flow rates can be adjusted such that the Reynold's number (Re) is within laminar limit. However, if we operate at the collection volume flux limit (which is related to velocity), and with Re < <NUM> (such that the flow is laminar), we have an upper limit of collector plate radius (L3) for various sheath flow positions (lims) and ratio<NUM>. Some examples of the limit is shown in <FIG> if the system is made to operate at the volume flux limit (ϕmax) where the upper limit is because of Reynolds number and the lower limit is to avoid particle impaction. As the volume flux limit is higher for higher electric field strength, three different operating electric field strengths are used to find the limits on the collector plate radius. <MAT> where, <MAT>.

The analytical model is valid for the case where particles are not impacting onto the surface. Hence, for any given combination of L3 value and ratio<NUM> value, the operating flow rates can be adjusted such that the Stokes number (St) is low (lower than <NUM> as then the impaction efficiency is lower than <NUM>%). However, if we operate at the collection volume flux limit (which is related to velocity), and with St < <NUM> (such that impaction is negligible), we have a lower limit of collector plate radius (L3) for various sheath flow positions (lims) and ratio<NUM>. The examples in <FIG> have the lower limit to have negligible impaction (i.e. impaction efficiency around <NUM>%) for particles with density of <NUM>/cc and diameter <NUM>.

Various considerations in choosing exemplary materials for various parts of embodiments of the invention are presented below:.

Claim 1:
ESP particle collector (<NUM>) for collecting particles in a particle containing gas stream, comprising an inlet section (<NUM>), a collector section (<NUM>), and an electrode arrangement (<NUM>), the inlet section comprising a flow tube (<NUM>) defining a gas flow channel (<NUM>) therein bounded by a guide wall (<NUM>) extending between an entry end (<NUM>) and a collector end (<NUM>) that serves as an inlet to the collector section (<NUM>), the entry end comprising an inlet (<NUM>) for the particle gas stream, the collector section comprising a housing (<NUM>) coupled to the flow tube, and a collector plate (<NUM>) mounted therein having a particle collection surface (<NUM>), wherein the ESP particle collector comprises an optical measuring instrument (<NUM>) configured to transmit light through the collector plate along a centre axis (A) orthogonal or substantially orthogonal to the particle collection surface for optical analysis of the collector plate particle collection surface to measure particles collected thereon, the ESP particle collector being characterised in that the entry end comprises a sheath flow inlet portion (<NUM>) for generating a sheath flow around the particle gas stream, and in that the flow tube has a bent portion (<NUM>) such that the entry end (<NUM>) is positioned out of the centre axis A to allow the light to be transmitted through the collector plate in the direction of the centre axis and to be picked up without interfering with the gas flow or the gas inlet wherein the flow tube comprises a first portion arranged along an initial inclined axis (Gi) connected via the bent portion to a second portion arranged along the centre axis (A) connected to the collector section <NUM>, an angle of inclination (β) of the initial axis (Gi) relative to the centre axis (A) being less than <NUM>°.