GB2346700A - Particulate size detector - Google Patents

Abstract

A number concentration-based sub-micrometre particle size distribution measuring instrument comprises a sampling system, an electrostatic precipitator for removing charged particles from the incoming aerosol sample, a photoelectric charging system for charging particles in the sample and a classification system for separating particles into ranges of differing electrical mobility, in which a series of electrodes and corresponding electrometer amplifiers are provided to detect particles in each range. The sampling system may comprise an intermediate chamber (figure 2) between the sample source and the precipitator, the chamber being maintained at a constant pressure to provide a constant sample flow rate through the instrument. The electrostatic precipitator may be rapidly activated and deactivated or parts of the sample path duplicated to allow discrimination between solid and volatile particles present in the sample.

Description

Ultra-Fast, Sub-Micrometer Particulate Size Detector This invention relates to an instrument for the measurement of the size distribution of submicrometer particles suspended in a gas (aerosol). Measurement of the particle size distribution of an aerosol is important in a wide range of applications, and several instruments are currently available to fulfil this task. However, the time response of currently available instruments for the accurate, high-resolution measurement of particle size distribution is typically on the order of tens of seconds.

In some applications, the ability to accurately measure particle size distributions on much shorter time scales is desirable or necessary. One such application is the measurement of the size distribution of particles emitted from internal combustion engines particularly when operated under transient conditions.

The present invention describes the configuration and overall design of an instrument capable of measuring sub-micrometer particle size distributions with an overall time response of less than 10 milliseconds. The present invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 shows a complete schematic diagram of the layout of the entire instrument, including the sample extraction and conditioning system.

Figure 2 shows a diagram of the sampling system.

Figure 3 shows a diagram of the particle size classification and detection section of the instrument.

As Figure 1 shows, the instrument is comprised of four individual sub-components which are described here briefly and in detail below. The first sub-component is the sample extraction system, which is responsible for extraction of the sample, inertial removal of large particles from the sample, and provision of a stable, controlled flow rate of sample to the rest of the instrument. The extracted sample is then passed through an electrostatic precipitator, which can be turned on to remove any charged particles from the incoming sample. The aerosol sample is then passed into the charging section, where the particles are photo-electrically charged via exposure to ultraviolet radiation. The charged particles are finally passed into the particle detector where they are electrically classified and counted according to their size.

Figure 2 shows a diagram of the sample system. The sample extraction system consists of a sample inlet, two sample transport tubes, and an intermediate chamber. The intermediate chamber is controlled to a constant absolute pressure, which is always below the sample source pressure. This arrangement results in a jet of sample gas being drawn into the intermediate chamber through tube 1. The particle precipitator/charger/detector section is controlled to a constant absoute pressure, which is always below the intermediate chamber pressure. Hence a small fraction of the sample flow in tube 1 is drawn through tube 2 into the particulate precipitator/charger/detector. Tube 1 and tube 2 are arranged to be orthogonal and tube 2 forms a static tapping on tube 1. The pressure difference between the intermediate chamber and the particle precipitator/charger/detector is very close to the pressure drop across tube 2. The mass-flow of gas through tube 2 depends upon this pressure drop.

By careful regulation of the pressures in the intermediate chamber and the particle precipitator/charger/detector, the sample flow through the particle precipitator/charger/detector may be made substantially independent of pressure fluctuations at the sample source. This is of great advantage when sampling from a source which has a rapidly fluctuating pressure, such as the exhaust of an internal combustion engine. In addition, the orthogonal arrangement of tubes 1 and 2 form a"virtual impactor"which ensures that all particles larger than a certain maximum size (much larger than I micro-meter) are removed from the sample entering the precipitator/charger/detector. In this way such large particles are excluded from the sample analysis system; this is an advantage primarily in terms of keeping the instrument clean.

The controlled sample flow passes from the sample extraction system first into an electrostatic precipitator, then into the particle charging system. The electrostatic precipitator can be arranged in a simple parallel plate configuration, and can be turned on or off. When the precipitator is active, it acts to remove any particles from the sample which carry a net charge. This is a function which has two possible uses: > Under instrument operating conditions where the final charge imparted to the particles by the instrument's charging system is limited by particle exposure time within the charger, the precipitator results in a more uniform charge distribution with respect to particle size.

This occurs because all of the particles begin the charging process at the same initial (zero net charge) state. The presence of these pre-charged particles is not normally a consideration under conditions where saturation charge is reached, but it can cause non uniformity of the charge distribution under exposure-time limited situations.

> When the aerosol consists of two classes of particles, one which is charged, and another which is uncharged, the precipitator can be used to remove the charged particles from the sample, thus allowing for discrimination between the two classes of particles. One such application would be in the measurement of a diluted aerosol emitted from an internal combustion engine. Such aerosols generally contain both solid (carbonaceous and metallic) and volatile (sulphates, hydrocarbons, water, etc.) particles. While the solid particles carry some net charge (due to ionising radiation present in the combustion chamber), the volatile particles (which are formed through gas-to-particle conversions during dilution) typically carry no net charge. Consequently, the selective removal of charged particles from a diluted engine exhaust aerosol sample would allow for discrimination between solid and volatile particles.

Exiting the electrostatic precipitator, the aerosol sample is passed into the particle charging section of the instrument. Inside the charging section, the aerosol is exposed to intense ultraviolet radiation from a high efficiency excimer lamp (with photon energies in the range from 5.6-6.4 eV). Bombardment of the aerosol with high-energy UV photons causes ejection of electrons from the particles, resulting in a net positive charge on the particles.

This process is referred to as photoelectric charging. Use of photoelectric charging is key to the operation of this invention, as it provides a fast and efficient charging process which yields very high particle charge distributions.

The net charge given to a particle by this process is dependent on several factors including the particle surface area, particle material surface properties, photon energy, photon exposure intensity, and free electron reattachment probability. For a given instrument configuration, all of these factors except for particle surface area and surface properties, are fixed. While the charge distribution on an aerosol can be reasonably estimated, the efficiency of the instrument charging system will need to be determined through careful calibration in order to yield maximum measurement accuracy.

Once charged, the aerosol is passed into the particle detector section for analysis. The detector employs radial axi-symmetric geometry, and consists primarily of a central rod and an outer concentric cylinder. The aerosol is passed into the detector near the central rod, while particle-free"sheath"air is passed into the remainder of the detector, establishing a stratified flow. Sample aerosol and sheath flow rates are set such that their respective velocities are closely matched and so that the overall flow in the detector remains laminar.

The particles are then attracted radially outwards from their starting point near the central rod, by an applied electric field within the detector (the central rod is held at a positive voltage, with respect to the outer cylinder). The mobility of the particles in the presence of the applied electrical field, Zp, is a function of the particle Stokes diameter dp, and is given by: Zp = #### 37cLdp 3, where n is the net number of charges on the particle, e is the charge of an electron, Q is the Cunningham slip correction factor, u is the gas viscosity, and dp is the particle Stokes diameter. Classification of particles in this manner is referred to as electrical mobility classification.

The electrical mobility of a particle determines how far it travels along the axial length of the detector before it reaches the outer wall of the detector. By arranging a number of sensitive electrometers along the length of the wall of the outer cylinder of the detector, it is possible to measure the current associated with the flow of charged particles landing on each electrometer. The midpoint electrical mobility of the particles landing on any electrometer in the detector is given by: . L-r,-8/2)'ln () +/2)lnh/ (.,+8/2)]" where V is the voltage difference between the central rod and outer cylinder, U, is the axial aerosol sample velocity through the detector, zizis the axial distance from the aerosol inlet of the detector to the leading edge of the electrometer, xe is the width of the electrometer in the axial flow direction, r, and r2 are the radii of the central rod and outer cylinder, respectively, and 8 is the radial width of the aerosol sample inlet (similar expressions can be derived for the minimum and maximum particle mobilities measured by an electrometer as well). Equations 1 and 2 can be combined to directly determine the particle aerodynamic size range which will be detected by a given electrometer. This size range can be varied by changing any of the variables mentioned as well as pressure and temperature that the instrument operates at.

By dividing the total measured current at a given electrometer by the average net charge per particle, one can determine the total number of particles which have landed on the given electrometer. By repeating this process for a number of electrometers, an entire numberweighted size distribution can be measured in a straightforward fashion.

In addition to classification and measurement of particles, the detector section also serves to remove any free electrons (which may have been ejected from particles during photoelectric charging) from the aerosol sample, via electromagnetic attraction to the central rod which is held at positive potential. The removal of free electrons from the aerosol sample is necessary to avoid reduction of particle charge by re-attachment of the free electrons to the positively charged aerosol particles.

Claims (6)

1. Claims 1. A electrical mobility based particulate number concentration analyser having improved response time to changes in particulate number concentration in a source of sample gas, said analyser comprising: A sample tube coupling the sample source to the electrostatic precipitator; An electrostatic precipitator for the removal of pre-charged particles from the incoming aerosol sample; * A photoelectric charging system for charging aerosol particles in the sample at a predictable rate; * A classification system capable of separating aerosol particles into a continuous range of electrical mobilities; * A series of electrodes along the outer periphery of the classifier section for the purpose of collecting particles in a series of discrete size ranges; A series of electrometer amplifiers capable of independently measuring the incoming current associated with charged particles landing on each electrode.
2. 2. An electrical mobility based particulate number concentration analyser according to claim 1, wherein between said sample tube and said electrostatic precipitator, an intermediate chamber is introduced, disposed immediately adjacent to said sample tube and connected to said electrostatic precipitator by a tube arrangement which minimises the length of the path the sample gas must travel in said electrical mobility based particle number concentration analyser thereby to improve its response time, said intermediate chamber being adapted to maintain a substantially constant static pressure, whereby the flow rate of said sample gas through said tube arrangement into said electrostatic precipitator remains substantially constant.
3. 3. An electrical mobility based particle number concentration analyser according to claim 1, wherein an intermediate chamber according to claim 2, is couplable to the sample gas source by means which result in a flow of sample gas into said chamber which is orthogonal to said tube arrangement connecting said intermediate chamber and said electrostatic precipitator whereby a portion of the sample gas enters said tube arrangement substantially at the intermediate chamber static pressure and whereby all particles larger than a given size are inertially removed from the sample gas.
4. 4. An electrical mobility based particle number concentration analyser according to claim 1, wherein said electrical precipitator is arranged to be activated and de-activated rapidly thereby allowing discrimination between solid and volatile particles.
5. 5. An electrical mobility based particle number concentration analyser according to claim 1, wherein the sampling system is duplicated to allow part of the sample to be passed through said electrical precipitator whilst it is active and, simultaneously, part of the sample to be passed through an identical electrical precipitator whilst it is inactive, thereby allowing discrimination between solid and volatile particles.
6. 6. A method according to claim 3, for measuring particle number concentration in the diluted or undiluted exhaust gases of an internal combustion engine.