A particle counter with high counting efficiency for both nanoparticles and multi-micron sized particles and high energy efficiency

Abstract

A particle counter comprises; a horizontal inlet 1 for carrier gas / aerosol flow; a heated mixer chamber 2 where gas saturated with a working fluid vapour is added to the aerosol; a horizontal cooled c

Description

This invention relates to counters for particles suspended in a gas stream where the size of the particles may range from a few nanometres to a few microns. Optical particle counters are a long-established method to measure the concentration of particles suspended in a gas, otherwise known as an aerosol. The aerosol flows through a fine nozzle into an optical chamber where the particles are illuminated by a laser causing light to scatter and fall upon a light sensor (for example, a photodiode). Each pulse of light caused by a scattering particle causes an electrical signal to be produced by the light sensor, and the frequency of electrical pulses produces equals the frequency of particles passing through the nozzle, provided that the incidents of two or more particles passing at the same side can be kept to a minimum with a low enough concentration. The frequency of the particles is combined with the flow rate which is either measured and / or controlled, and this gives the particle concentration entering the instrument. Light will only scatter strongly from particles similar in size or greater than the wavelength of that light, therefore optical particle counters are only suitable in practice for particles larger than a few hundred nanometres. To count particles smaller than this a condensation particle counter (CPC, also known as a condensation nuclei counter, CNC) is used. In a CPC, a vapour is condensed on the particles before entering an optical particle counter. The particles grow a coating of the condensate until they are large enough to scatter the light in the OPC and hence be counted. Often butanol is used as the working fluid to generate the vapour. Butanol vapour is generated by passing a flow of gas through a heated saturator - in some cases the aerosol flow itself passes through the saturator [e.g. Agarwal and Sem (1979), DOI 10.1016 / 0021-8502(80)90042-7]; in others a flow of filtered gas is passed through the saturator and then mixed with the aerosol flow [US6567151], Once the sample is saturated, it enters a cooled region known as the condenser in which the vapour condenses on the particles to grow them, before they finally enter the optical particle counter. Methods of measurement of aerosol properties are highly influenced by the transport properties of the suspended particles, in particular the various mechanisms by which particles may be lost from the gas stream before measurement. These mechanisms include, but are not limited to: diffusion where the random motion of the smallest particles can cause them to be lost to the walls of a tube, impaction where larger particles end up being lost as they cannot follow the streamlines of the gas where bends or obstructions are present in the sample path, gravitational sedimentation where the largest particles settle on pipe walls by the force of gravity, electrophoresis where static charge causes migration of particles out of the stream, and thermophoresis where particles are lost due to a temperature gradient causing migration to the walls. As these different mechanisms are highly size dependent, particularly diffusion, impaction and sedimentation, the designs of CPCs (for the smallest particles, where diffusion loss dominates) and simple OPCs (for the largest particles, where impaction and sedimentation loss dominate) have been quite different. OPCs usually have a straight (“line of sight”) flow path without 17 01 25 bends and obstacles, to prevent impaction loss, and the flow path is often vertically orientated such that gravity does not act to lose particles by sedimentation to the walls. In a CPC it is vital that the condensed working fluid does not come into contact with the optical surfaces (such as lenses and windows) in the optical particle counter itself, as any contamination will affect the light scattering and may render the instrument unusable. The likelihood of such an occurrence can be reduced by heating the optical components sufficiently to avoid direct condensation. However, in order to avoid condensed fluid running from the condenser through the nozzle into the OPC, CPCs have always been designed with the entry to the optics being vertically upwards, such that the OPC is at the uppermost point in the saturator - condenser - OPC path. Any excess condensate then simply falls back to the saturator, or in more advanced designs, to avoid contamination of the saturator fluid, the excess condensate is removed from the entry end of the condenser by a pump. Due to the requirement of the OPC being uppermost, if a vertical sample flow were to be used, the entry could only be into the bottom of the instrument, which would usually be very inconvenient in a bench laboratory setting. CPCs usually have a horizontally arranged sample inlet for laboratory convenience; arranging the components of an experiment along a bench is clearly simpler than arranging them in a vertical tower or stack and sedimentation is not significant for particles in the nanometre size range. As the sample entry and flow path through the optics are orthogonal, a bend is required at some point in the sample flow path, which can cause larger particle losses by impaction. Currently available CPCs have claimed upper size limits below 5 pm. Other reasons may include loss of particles in designs where the particles pass through the saturator, or abrupt changes in pipe diameter causing further impaction loss. Therefore, the state of the art in aerosol science currently employs CPCs for nanoparticles, and OPCs for larger particles. CPCs are often used as part of an aerosol size spectrometer, where they are placed downstream of a particle size classifier which is scanned across a range of sizes, with the CPC measuring the concentration at each size, such that a plot of concentration versus size can be built up. When the size classifier is an electrical mobility classifier (known as a Differential Electrical Mobility Classifier, DEMC; historically referred to as a Differential Mobility Analyser, DMA), the combined spectral sizer is known as a Differential Mobility Analysing System (DMAS). A particular embodiment of a DMAS, where the DEMC is scanned continuously rather than in steps, is commonly known as a Scanning Mobility Particle Sizer (SMPS, Wang and Flagan (1989), DOI 10.1016 / 0021-8502(89)90868-9). DEMCs classify nanoparticles by their electrical mobility diameter, which involves electric charges being imparted to the particles, followed by their migration in a sheath flow under the effect of an electric field. DEMCs are fundamentally limited to the nanoparticle size range, because their accuracy relies on each particle gaining only one fundamental charge. As particles become larger, the stochastic charging process becomes more likely to impart two or more charges per particle, which can lead to gross inaccuracies in size and concentration reported by the DMAS. Whilst software algorithms can correct to an extent for multiple charging, the charging process is material dependent and such algorithms struggle with the largest particles. This practically limits the use of DMASs to the sub-micron size range, and thus the limited upper range of CPCs does not matter in this case. 17 01 25 However, in recent years two techniques have emerged which make extending the upper size range of CPCs compelling. Firstly, a new type of particle classifier was developed which can classify from the nanometre to the multi-micron size range without using electrical charging. This is the Aerodynamic Aerosol Classifier, or AAC [US8966958], which uses the deflection of particles flowing in a sheath flow under the effect of a centrifugal field (imparted by rotating the classifier), to classify by relaxation time (which is directly related to aerodynamic diameter). The currently commercially available embodiment of this device can classify particles from 25 nm to 6.8 pm. It can be used with a CPC downstream to form a size spectral instrument, in this case known as a Scanning Aerodynamic Size Spectrometer (SASS, [Johnson, Symonds, Olfert &Boies (2021), DOI 10.1080 / 02786826.2020.1830941]). However, the SASS has been restricted up until now to an upper size limit of around 3 pm due to losses in CPCs above this size [Johnson et al. (2018), DOI 10.1080 / 02786826.2018.1440063], Secondly, a technique called the Mass and Mobility Aerosol Spectrometer (M2AS, Walker (2020), via https: / / cambridgeparticlemeeting.org / proceedings / 2020) has recently been developed which can produce a mass spectral density plot, a size spectral density plot and also actually measure the charge state of the particles, allowing empirical charge correction to be applied and overcoming the upper practical limit of a DMAS system. The M2AS is also however limited by the upper size range of state-of-the-art CPCs to around 3 pm. In both these cases, in theory a CPC and OPC could be used in parallel, but this would be cumbersome, involving taking a larger sample flow and also requiring some sort of algorithm to stitch the data together in the region where the effectiveness of one takes over from the effectiveness of the other. The present invention extends the working size range of a condensation particle counter to around 10 pm, allowing it to be used as part of size spectrometers where the classification system extends beyond that of traditional DMASs, and also encompassing the regulatory air quality size range known as “PM10” (Particulate Matter less than 10 pm). In order to reduce loss by impaction, the present invention’s sample flow path is in a direct straight line all the way from sample inlet to the optical particle counter. For laboratory convenience a horizonal sample entry is used - classifiers such as the AAC (and many other aerosol science devices) have a horizonal sample outlet, and so to couple to a CPC with a vertical entry would require bends in pipework, with associated impaction loss of larger particles. Gravitational sedimentation is not significant in the sub-10 pm size range at the flow rates and tube sizes used. Therefore the present invention has a horizontal sample path all the way from entry to optical counter, which means that gravity cannot be relied upon to prevent excess condensate contaminating the optics. Potential losses in the saturator means that the present invention is designed in a mixing configuration rather than a sample-through-saturator configuration, and that mixing is designed to be laminar [Keady, Denier and Sem, DOI 10.1007 / 3-540-50108-8_1045] rather than turbulent [US6567151] to prevent large particle loss. Larger particles will not be grown to any great extent by the working fluid, however they will be easily directly detected by the OPC part of the CPC. 17 01 25 Figure 1 shows a preferred embodiment of the invention. Sample enters though an inlet (1) which is preferably heated such that the sample is at a known controlled temperature. Sample meets in a heated mixer (2) with a flow of gas which is pre-saturated with vapour from a working fluid. The saturated flow enters in an anulus around the sample flow such that they gently mix by diffusion under laminar flow. The mixing sample and saturated flows continue down the flow path until they enter a cooled condenser (3). The profile of the condenser is “keyhole” or “figure of 8” shaped - the detail of which is shown in Figure 2. The upper, substantially circular in cross section, part (4) is where the mixture passes through and condensation of the working fluid onto the particles occurs. The particle flow continues in the upper section of the condenser until it enters a nozzle (5) which accelerates the flow through the optical particle counter (6). The optical particle counter (6) consists of a laser (7) which is collimated into a thin sheet by a lens (8) and illuminates the particles causing scatter which is detected by light sensor (9, for example, a photodiode). The optical particle counter (6) is heated to stop direct condensation of the working fluid on the optical elements. In this embodiment, scattered light is detected at close to 180° from the laser source, so a beam dump (10) is used to block the full laser beam entering the light sensor (9). The flow exits the optical particle counter and is filtered (11) and then split into two (12). One resultant flow passes through a flow control means (13) and a pump (14) and exits the invention via an exhaust (15). The rate of this flow is the same as the rate of flow entering via the inlet (1) and is thus used in the calculation of particle concentration. The second flow after the split (12) enters a flow controlling means (16) and a pump (17) and passes into a heated saturator block (18). In this preferred embodiment, the saturator is a block of metal with internal baffles; and a pump (19) recirculates working fluid around the block encouraging saturation of the gas. The saturated gas exits the saturator and enters the mixer (2). The saturator gas path is effectively in a loop. Excess condensation in the condenser (3) runs into the lower section of the condenser (20) from which it is collected by a slow running pump (21, for example a peristaltic pump) and removed to a waste container (22). The saturator (18) is filled with working fluid from a reservoir (23) either by gravity, or in this preferred embodiment, by a peristaltic pump (24). In this embodiment, the pump (24) can also be reversed upon initiation of instrument shutdown to remove as yet uncontaminated working fluid from the saturator (18) to avoid working fluid flowing through the sample path onto the optics if the instrument is tipped during subsequent transport. An electronic circuit or computer (25) combines the electrical pulse train from the light sensor (9) and the flow rate from the flow controlling means (13) to give the particle concentration. In a conventional CPC all particles are usually grown to a similar size, and thus the height of pulses (dependent up size according to Mie Theory) will be similar. In the present invention, for larger particles, which are not substantially grown, there will be variation in the pulse height for these according to their size. The electronic circuit or computer (25) needs to take this into account when its pulse height threshold is set. In this preferred embodiment, butanol is used as the working fluid. In this case the saturator (18), inlet (1) and mixer (2) are all heated to a similar temperature of >=45 °C, the condenser (3) is cooled to a temperature <15 °C and the optical particle counter (6) is heated to around 40 °C. The condensation chamber (3) is cooled by two thermoelectric coolers (26) and (27) each of which has a hole in the middle (commonly commercially available) which here allows the aerosol sample to pass. Cooler (26) removes heat from the condensation chamber (3) and pumps it into the mixer (2) thereby heating it. Cooler (27) removes heat from the condensation chamber (3) and pumps it into the body of the optical particle counter (6) thereby heating it. Any additional heating the mixer (2) requires to reach the required temperature is provided by a resistive heating element (28); any additional heating the optical particle counter body (6) requires to reach the required temperature is provided by a resistive heating element (29). The use of the heat removed from the condenser (3) which is otherwise waster in other designs to partially heat the mixer (2) and the optical particle counter (6) reduces the overall power consumption of the device and reduces the time taken for the instrument to reach overall stability. CXI The likelihood of gravitational sedimentation is increased by slow passage through the tubing, therefore the inside diameters of the inlet (1), mixer (2) condenser (3) should be kept as small as possible to allow a faster transit time for a given flow. In this preferred embodiment, the flow controlling means (13) and (16) consist of measuring the pressure drop across an orifice plate and controlling a proportional solenoid valve to maintain said pressure drop corresponding to the desired flow. 17 01 25

Claims

1. A particle counter comprising:A substantially horizontal particle inlet for a suspension of particles in a carrier gas flow or aerosol;A heated mixing chamber, in which a gas saturated with a working fluid vapour is added to said aerosol;A substantially horizontal cooled condensation chamber of figure of eight or keyhole cross-section in which aerosol flow occurs predominantly in the upper section where cooled vapour originating from a working fluid condenses upon the particles in the sample flow and condensate from the working fluid which has condensed on the walls of the condensation chamber collects in the lower section and is removed;A heated optical particle detector consisting of a nozzle conveying the aerosol flow from the condensation chamber and forming it into a jet, a laser generating light which is focused into a sheet through which the jet flows and a photodetector which detects the light scattered by the particles in the aerosol producing an electrical pulse corresponding to each particle which is counted by an electronic circuit;A first thermoelectric cooler which removes heat from the condensation chamber and pumps it into body of the mixing chamber;A second thermoelectric cooler which removes heat from the condensation chamber and pumps it into the body of the optical particle detector;A particle flow path from particle inlet, mixing chamber, condensation chamber and optical particle detector which is in a straight line and is substantially horizontal;A saturator consisting of a heated chamber which is partially filled with working fluid and a gas flow path entering the saturator above the fluid level and leaving via a connection to the mixing chamber;A flow control system which pumps and controls the flow leaving the optical particle detector and splits off a controlled portion to enter the saturator;An electronic circuit or software which combines the sample flow rate and pulse count rate to give particle concentration.

2. A particle counter as in claim 1 in which a recirculation pump raises the working fluid from the bottom to the top of the saturator to allow it to fall back to the bottom of the saturator via the saturator’s internal walls to promote saturation of the gas flow.

3. A particle counter as in claim 2 in which the working fluid is raised by a peristaltic pump.LO CXI4. A particle counter as in any of the above claims in which the saturator is filled with working fluid by a peristaltic pump which can also be reversed in direction to remove working fluid from the saturator.

5. A particle counter as in any of the above claims where the condensate is removed from the lower section of the condenser using a peristaltic pump.

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