GB2378510A

GB2378510A - Method to improve electrostatic particle measurement

Info

Publication number: GB2378510A
Application number: GB0119091A
Authority: GB
Inventors: Nicholas Collings; Kingsley Stjohn Reavell
Original assignee: Cambustion Ltd
Current assignee: Cambustion Ltd
Priority date: 2001-08-04
Filing date: 2001-08-04
Publication date: 2003-02-12
Anticipated expiration: 2021-08-04
Also published as: GB0119091D0; GB2378510B

Abstract

At least one of the electrodes (5) which create the electric field in an instrument which classifies particles by their electrical mobility is modified such that the electrode is no longer at an uniform electrical potential so that the variation of collection location along the classifier column (6) with electrical mobility more closely approaches that desired for resolution of a wide range of particle sizes. The electrode may be connected in different places to different electrical potentials, with the current flowing through it creating the potential distribution; or it may be divided into separate sections controlled at different potentials.

Description

IMPROVEMENT OF ELECTRICAL PARTICLE SIZE CLASSIFICATION.

This invention relates to improvement of electrical particle size classification.

The size distribution of particles in an aerosol (a suspension of particles, which may be solid or liquid, in a gas) is often inferred from measurement of the electrical mobility of these particles when charged. The electrical mobility is dependent on the equivalent diameter of the particle: thus, from a measurement of the electrical mobility, the equivalent particle diameter can be inferred.

Instruments which operate on this principle include the Scanning Mobility Particle Sizer (SMPS) and the Electrical Aerosol Spectrometer (EAS). In these instruments, a sample of the aerosol is passed through a charging device which applies an electrical charge to the particles; the charged aerosol then passes through a classifier which separates the particles according to their electrical mobility; and particles with given ranges of electrical mobility are counted by discrete detectors. In the classifier section (itself sometimes referred to as a Differential Mobility Analyser, or DMA), the aerosol is introduced adjacent to a sheath gas flow containing no particles, between conductive electrodes held at different electric potentials (typically one at ground and the other at a large positive or negative voltage), which create an electric field. The electric field causes a drift of the particles relative to the gas with a velocity equal to the product of their electrical mobility and the electric field strength, in the direction of the applied field, which is approximately perpendicular to the direction of the gas flow. The time taken for the particles to drift a given distance across stream is therefore inversely proportional to their electrical mobility.

In an instrument such as the EAS which measures particles in a number of size ranges simultaneously, an array of detection devices, often electrodes (which also serve as one of the field generating electrodes), connected to earth via electrometers, is mounted along a flow channel which forms the classifier. Often, particles of a very wide range of diameters are of interest: for example, the exhaust from an internal combustion engine may contain particles from a few nanometres to tens of microns. The exact relationship between diameter and electrical mobility depends on the operating conditions of the classifier and the method used to charge the aerosol, but for practical charging techniques, this wide diameter range results in a similarly wide range of electrical mobilities. For example, one common method of charging, diffusion charging, results in electrical mobility approximately inversely proportional to particle diameter over a significant part of the size range of interest. This gives a transfer function from particle size to position of impact with the electrode along the array which is linear. This has the result that conditions which allow detection of the whole range of particle diameters cause the

majority of smaller particles to land so close together on the detector array that they cannot practically be resolved.

Logarithmic spacing of size classes over a range of several orders of magnitude would give useful resolution at any given particle size throughout the whole range. To more closely approach this, the detectors for small particles are sometimes mounted more closely together than those detecting the large particles, but this cannot practically give the desired spacing of classes over the entire size range. Alternatively, analysers have been suggested with flow channels that change in cross section along their length, but this cannot achieve the desired transfer function over the full range. Alternatively, analysers with parallel multiple classifiers, with different operating conditions, have been constructed, but this results in a large and complicated instrument.

According to the present invention, modifications are made to at least one of the electrodes creating the electric field within an electrical mobility classifier so that its surface is at a non-uniform electric potential.

This invention is applicable to many geometries of electrical mobility classifier regardless of what charging method and detection method is used. This includes annular classifiers where a long annular flow chamber is formed between coaxial electrodes, which may be of constant or varying cross section along their length, in which case the ends of the electrodes are considered the ends in the direction of the axis of the assembly. The invention is also applicable to a radial geometry where the flow passes between two plates either radially inward to an exit in one or both plates, or radially outward from an entry in one or both plates; in this geometry the axis of the classifier is the radial direction and one end of the electrodes is considered the point of entry or exit and the other the circumference of the plates: these plates need not necessarily be flat. The invention is also applicable to a rectangular section channel with one electrode forming one face of the cross section and the other electrode the opposite face: in this case the axis of the classifier is the flow direction. The invention equally applies to other geometries.

In order to achieve the desired non-uniform electric potential, one or both of the electrodes may be divided into a number of sections held at more than one potential; or a current, in addition to any current due to charged particles colliding with the electrode, may be passed through the electrode; or a combination of these two methods.

In the case that the instrument uses electrometers connected to electrode rings which serve as one of the electrodes, it is preferable that the other electrode is modified in the method of this invention.

In the case of the electrode divided into sections, the boundaries between sections are preferably approximately perpendicular to the flow direction in the classifier. The sections may be constructed of a material with good electrical conductivity and separately connected to electrical circuitry to hold them at separate voltages. These sections should preferably either be electrically isolated from one another, for example with insulators, or electrically connected to one another with components of high resistance such that the current draw from the electrical circuitry is not excessive. If said sections are insulated from one another, it is preferable that any solid insulation material does not extend to the surface of the electrode forming the side of the flow channel, but that there should be a narrow gap: insulating material very near the aerosol flow may accumulate charged particles by diffusion which will then create an unwanted electric field.

In the case that the non-uniform potential is achieved by passing an additional current through the electrode, two or more electrical connections must be made to the electrode.

Typically, the electrodes are operated at very high electrical potentials, often of the order of kilovolts, and the potential differences between the connections to the electrode may be of a similar order. Therefore, in order to prevent very large power requirements from the electrical circuitry and excessive heat dissipation in the electrode, the current flow should preferably be small and the electrical resistance of the electrode from one connection to another should preferably be very high, typically of the order of 109 ohms. In order to achieve such a resistance, the electrode should preferably be manufactured of a material with a very high resistivity: preferably in the range 106 to 1012 ohm. metre. The exact requirement depends on the geometry of the electrode and the placement of the connections. The maximum practical resistance is determined by the need to conduct away to the circuitry any currents due to charged particles which collide with the electrode without generating significant additional potential differences. This current is typically very small in such instruments, generally of the order of picoamps or less, but will probably preclude the use of materials conventionally recognised as good insulators such as most pure polymers or many ceramics. For instruments with predominantly unipolar (where unipolar means that all particles receive a charge of the same sign) charging of the ions, this current will be less for an electrode which is held at a potential which repels the charged particles, and therefore it is preferable that in such an instrument, if only one of the electrodes is modified according to this invention, that it is the electrode held at a potential which repels the majority of the charged particles.

The electrical connections to the electrode should preferably provide for contact over an area rather than at a point, because the high resistivity of the electrode material may otherwise lead to significant undesired field concentrations. Suitable methods for making the electrical connections to the electrode include conductive bands or strips of conductive material secured to the surface of the electrode, conductive paint or ink

printed or otherwise placed on the surface of the electrode or conductive components recessed into the electrode.

The electrode may have a uniform cross-section with as few as two electrical connections, which may be placed at the ends or at intermediate locations. To further optimise the electrical potential distribution, additional electrical connections may be made to the electrode which may be connected to different electrical potentials to the other connections, or some of the connections may be held at the same electrical potentials. The different electrical potentials may be derived from a resistor ladder from a single high voltage supply or other circuitry may be used.

The electrical potential distribution may be modified by arranging for a non-uniform cross sectional area of the electrode between the electrical connections. This may be achieved by varying the outside dimensions of the electrode or by making it hollow. With a non-uniform cross sectional area, the local potential gradient in the region between two electrical connections is approximately inversely proportional to the cross sectional area for electrical conduction, cross sectional area being defined as perpendicular to the direction of current flow. The electrode may take the form of a thin conductive covering over an insulating component which may provide physical support: this may have the added benefit of making the high required resistance easier to achieve. In this case, the electrical connections may be formed from more conductive regions between the insulator and the conductive covering, or by any of the methods described above. The thickness of such a thin conductive coating may be varied to modify the potential distribution.

The electrical potential distribution may also or alternatively be modified by making the electrode from a material with varying electrical resistivity over its length.

In the case of the electrode being divided into a number of sections, any or all of the sections may be modified by any of the above methods to further modify the electric potential distribution.

Materials which would be suitable for the high resistance components or electrode above include polymers containing additives to provide some electrical conductivity, which may be those sometimes described as dissipative or conductive; or semiconductor materials including silicon, or some ceramic materials.

The modified electrode or any electrode not modified with this invention may be constructed in such away that charged particles may travel through it to a detection device mounted on the other side. For example, the electrode may be perforated or constructed of a gauze or mesh.

In order to reduce unwanted ionic current generation by the corona effect, it is preferable that all electrode constructions are free of sharp comers and rough surfaces which would lead to local electric field concentrations.

One embodiment of this invention will now be described with the aid of the following figures.

Figure 1 is a cutaway diagram of an annular Electrical Aerosol Spectrometer (EAS) with a centre electrode constructed to allow a current to flow along it between different potentials at the two ends.

Figure 2 is a graph showing the predicted collection locations of a range of particle sizes for this classifier compared with a classifier with a constant voltage along its length.

The Differential Mobility Analyser (DMA) shown in Figure I consists of a charger 4, where particulate matter becomes diffusion charged by impact with ions formed in a corona discharge such that all charge particles have a positive potential. The aerosol sample containing the charged particulate matter then flows into a cylindrical classifier 6, where it is introduced parallel to sheath air containing no particulate matter (2 and 3 on Figure 1) such that there is minimal mixing between the particulate sample and the sheath flow.

The cylindrical classifier 6 is 700mm long with an internal diameter of 50mm, and contains several conducting rings 7 along its length, which are connected to electrometer amplifiers with input running near to ground potential in order to measure any charge flow incident upon them (note that only one amplifier is shown in Figure I for clarity).

Along the axis of the classifier 6 is a conductive electrode 5 made from modified acetyl plastic with an outside diameter of 25mm. The upstream end of this electrode is connected to ground potential and the downstream end is connected to an HT supply of lOkV. Connections are made by screwing metal terminals into each end of the electrode.

The conductivity of the material forming the central conducting electrode is such that a current flow of the order of IOnA flows with this applied potential and the voltage at the surface of this electrode varies approximately linearly along its length between ground potential and lOkV.

Charged particulate matter within the sample moves in the electric field from the central electrode 5 towards the conducting rings 7 near ground potential. The trajectory of the charged particles depends on their electrical mobility and the flow velocity within the classifier which is maintained at 1.7 m/s. Charged particles collected on the conducting rings 7 cause a current flow at each ring.

Figure 2 is a graph showing the predicted collection location, in mm along the column from the entry, as a function of particle size.

The graph shows the collection location along the classifier for particles with equivalent diameters from I to 1000 nm.

Line 1 shows an ideal response for the instrument where the classification is logarithmic over the particle size range.

Line 2 shows the modelled response for a DMA with a constant central electrode voltage of 5kV. It is clear that whilst the classification of particles at the large end of the size range is clear, small particles (from I to 20nm) are all collected at the start of the column and cannot be easily differentiated.

Line 3 shows the modelled response for a DMA with a linearly varying voltage along the central electrode as described in this embodiment. Differentiation between the smallest particle sizes is now possible and the response more closely approaches the logarithmic ideal.

Adjusting the axial voltage distribution to be non-linear (for example by adjusting the conducting cross section as described earlier) would enable a closer approximation to the desired response.

Claims

CLAIMS 1. An electrical mobility particle classifier where modifications are made to one or more of the electrodes creating the electric field within the classifier such that at least one of said modified electrodes is divided into a number of sections which are controlled to more than one electric potential.
2. The instrument of claim I where at least two of the electric potentials on said modified electrodes differ by at least 100 volts.
3. The instrument of claim I where said sections of said modified electrodes are connected together by conductive but highly resistive components.
4. The instrument of claim 3 where said conductive but highly resistive components have resistance greater than 1 megaohm and less than 100 teraohm.
5. An electrical mobility particle classifier where modifications are made to one or more of the electrodes creating the electric field within the classifier such that current is caused to flow between at least two locations on each of said modified electrodes, resulting in a non-uniform electric potential along said electrodes.
6. The instrument of claim 5 where the electric potential over said modified electrodes varies by at least 100 volts.
7. The instrument of claim 5 where said modified electrodes are constructed of material with a resistivity between 106 ohm. metre and 1010 ohm. metre.
8. The instrument of claim 7 where said modified electrodes are constructed of a plastic which contains additives which increase their conductivity
9. The instrument of claim 5 where said modified electrodes are connected to electrical ground at one end and a high voltage at the other end.
10. The instrument of claim 9 where said high voltage is greater than I kilovolt.
11. The instruments of claims 1 and 5 where said classifier is in the form of an annular channel with said modified electrode forming the inner wall of the channel and at least one other outer electrode forming the outer wall.
12. The instrument of claim 11 where said outer electrode is perforated and one or more detection devices are mounted outside it.

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13. The instrument of claim 12 where said detection devices are electrometers connected to electrodes in the form of rings around said perforated outer electrode.
14. The instrument of claim 11 where said detection devices are mounted externally and draw a small flow through holes in said outer electrode.