WO2012150958A1

WO2012150958A1 - Aerosol collection apparatus and methods for ultrasonic care thereof

Info

Publication number: WO2012150958A1
Application number: PCT/US2011/059487
Authority: WO
Inventors: Peter Ariessohn; Igor Novosselov; Evan DENGLER
Original assignee: Enertechnix, Inc
Priority date: 2011-05-02
Filing date: 2011-11-06
Publication date: 2012-11-08

Abstract

The lifetime of aerosol monitoring, concentration and collection equipment is extended by acoustic cleaning of accreted particle deposits from internal surfaces where fouling occurs by application of acoustic energy to the particle accretion surface, optionally in combination with a liquid wash or sampling volume. In one application, acoustic cleaning or sampling of particle deposits for analysis is triggered by a signal indicating changes in gas flow associated with particle loading. In another application, electro-acoustic transducers may be used to prevent particle buildup without interruption of particle monitoring.

Description

TITLE:

AEROSOL COLLECTION APPARATUS AND METHODS FOR ULTRASONIC CARE THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to US Patent Application 13/284937, filed 30-Oct-2011, titled "Aerosol Collection Apparatus and Methods for Ultrasonic Care Thereof, which is a continuation of US Patent Application 13/099295, filed 02-May-2011, titled "Aerosol Collection Apparatus and Methods", from which priority are claimed and which are herein incorporated in full by reference.

FIELD OF THE INVENTION

This invention is related to acoustic care and cleaning of aerosol concentrators and collectors. BACKGROUND

"Air-to-air aerosol concentrators" such as aerodynamic lenses and virtual impactors, are frequently used to fractionate and concentrate particles in a gas flow prior to collection or detection and offers a significant advantage when coupled to an analytical method. Using a variety of devices known in the art as "virtual impactors", for example, aerosol particles to be sampled from a larger volume of air are concentrated into a particle-enriched gas stream of smaller volume (the "minor flow" or "particle-enriched flow") while the bulk of the sampled air, depleted of particles, (also termed the "major flow", "bulk flow", or "particle-depleted flow") is discarded. Such aerosol concentrating devices are described for example in US Pat. Nos. 7,704,294 and 7,873,095 co-assigned to the Applicant. Other air-to-air concentrators include virtual impactors such as described in US Pat. Nos. 3901798, 4670135, 4767524, 5425802, 5533406 and 6698592, and others.

Particle collectors are designed to reversibly trap of particles for further study and include inertial impactors, centrifugal impactors, vortex separators, and electrostatic collectors. It is desirable to be able to periodically sample the particles accumulating in a particle collector. Samples subjected to analysis reveal a great deal about the environment from which they are collected, and can be used to detect hazardous conditions, for example. However, operation of air-to-air concentrators and particle collectors are hampered by fouling considerations. Upstream filters may be used to prevent entry of oversized materials such as dust, fibers, or aerosolized salt crystals which would block gas flow, but the downsteam channels and features also become fouled with accretions of smaller particles when overloaded or in extended use. Accumulation of micron- or submicron-sized particles will result in blockage over time. As a result, these devices must be continuously monitored for performance, for example by monitoring backpressure and/or continuity of flow and must be serviced when performance deteriorates due to partial or complete blockage. This problem has adversely impacted the wider use of particle traps for a variety of industrial and security applications in favor of particle collection devices that rely on wetted wall or liquid impingement technology, both of which are comparatively less sensitive and less portable.

For example, particle deposits can accumulate around the mouth of a virtual impactor, often termed a "skimmer", where the gas flow is split into a "minor flow" enriched in particles and a "major flow" (sometimes termed "bulk flow") depleted of particles. Particles may also accumulate in the collector channel where the minor flow first encounters slower air at the mouth and downstream from the virtual impactor. Particles may also accumulate on the forward faces of aerodynamic lenses, or more generally at points in the air ducting where stagnation, turbulence, or bluff body impaction is unavoidable, such as also are noted in inlet particle separators, vortex particle separators, and electrostatic precipitators, for example. Particle accumulation on the surfaces of these devices unacceptably alters device performance, and even unintended particle collection surfaces nevertheless become progressively fouled. Unfortunately, deterioration of performance accelerates over time: i.e., as deposits become larger the fouling rate increases in a vicious cycle.

Removal of particle deposits can be technically difficult. Particle impaction can result in bonding of the particles to the surfaces of the particle trap or aerosol concentrator. For instance, because of the small dimensions and radii of channels often used in inertial impactors, a mechanical arm such as a pipe cleaner inserted into the channel to clear the channel must be thin and flexible, and excess force in cleaning can result in formation of a packed mass that cannot be physically removed. Disassembly for cleaning, such as by removal of cathodic and anodic plates of an electrostatic impactor, can be inconvenient or not possible. Aggressive chemical cleaning solutions can damage the inside surfaces of the channels, which may be made of metal or plastic. Also, any of these cleaning methods will likely result in destruction of the structure and/or composition of the captive particles, defeating a basic purpose of particle collection and sampling for analysis, and hence are not satisfactory.

Call, in US 6938777, describes a method for removing concentrated spots of collected particles from an impactor surface, which involves first removing the surface bearing the spot from the collection device, and then subjecting the spot to a blasting jet of fluid or using a mechanical scraper to dislodge the spot. The mechanical scraper can be rinsed with liquid or vibrated to remove the particles. In practice, the impactor surface is a moveable solid support, for example in the form of a roll of tape or a rotating disk, so that the impactor surface can be translocated from the collection apparatus, and the method is thus not generally applicable. As to cleaning of the internal surfaces of particle traps or virtual impactors or as to sampling of particles entrapped within a particle trap where the internal surface cannot be removed from the aerosol collector, no solution is provided.

In US 7578973, Call goes on to point out that particulate "wall loss," i.e. unintended deposition of particles on various surfaces of virtual impactor structures, especially the curved or bent portions, remains an unsolved problem.

Thus, there is a need in the art for a method and apparatus to clean particle traps and aerosol concentrators such as virtual impactors, either in response to a change in gas flow associated with accumulation of particles therein, or periodically as prophylaxis against accumulation of particles. Preferably the method also facilitates particle sampling and reduces equipment down time.

SUMMARY

An unsolved problem in the field of aerosol analysis and aerosol hazard detection relates to aerosol monitoring equipment that has become blocked or fouled and must be replaced or rebuilt, which can result in down time of critical monitoring systems. We have found that acoustic cleaning "on the fly" removes buildup of particle deposits without the need to interrupt ongoing monitoring to service the equipment. Devices that benefit from acoustic cleaning in response to particle accumulation and fouling include aerosol concentrators, aerodynamic lenses, virtual impactors, inertial impactors, centrifugal impactors, vortex particle separators, bluff body impactors, inlet particle separators, and electrostatic impactors, for example. In contrast, wetted wall devices are relatively resistant to fouling but suffer from losses in sensitivity.

Performance of aerosol monitoring equipment is measured by cut size, by efficiency of particle concentration (or capture), by flow split, and by detection sensitivity. Particles in the gas stream are typically concentrated in a part of the gas stream and separated from the gas flow by impaction or precipitation onto a solid surface. Prior to separation, the gas stream may be split at a virtual impactor so that particles are concentrated in a smaller gas volume. During these processes, some particles are inadvertently deposited on other internal surfaces of the equipment. The net effect of excessive accumulation of particles on internal surfaces is a deterioration of performance efficiency in capturing particles from the gas stream. Of interest here is the use of acoustic energy to excite the internal solid surface or surfaces of the aerosol monitoring equipment and thereby dislodge particle deposits for cleaning or sampling.

Surprisingly, application of ultrasound can improve particle collection efficiency, does not require interruption of operation, and is not associated with increased losses. Periodic application of ultrasound is found to result in extended operation of particle collection equipment that would otherwise be subject to frequent blockage. Ultrasound is applied to the internal solid surfaces while a gas flow is directed across the surfaces of interest. In this "dry" cleaning method, particles that are dislodged are entrained in the flowing gas and removed from the equipment (or transferred to a downstream trap). Deterioration in performance associated with accumulation of particles can be corrected in most instances.

Surprisingly, periodic application of ultrasound may also be used preventatively so that performance is not affected during extended use without interruption of monitoring. The benefits of "on the fly" prophylactic acoustic treatments can be achieved with very low power consumption and without down time. Intermittent pulsatile application of acoustic energy can prevent fouling over an extended lifetime of use, for months or even years, and in fact improves particle collection efficiency by routine intermittent application of dry acoustic treatments.

Acoustic energy may be applied to a solid substrate directly, using contacting transducers, or indirectly, using non-contacting air-coupled transducers, and the particles dislodged into the liquid may be sampled by collecting the liquid. Contacting and/or gas-coupled acoustic transducers may be used, either one or the other, or together.

For cleaning, the surface to be treated may be an internal surface of a particle trap or any internal surface of a gas-to-gas particle concentrator subject to fouling or blockage. With respect to the operation and cleaning of aerosol concentrators, in a preferred method, the invention further includes a step for sensing a flow rate, a flow velocity, or a backpressure with a sensor and outputting a digital or analog signal from the sensor that is used to initiate a cleaning cycle if the signal is consistent with an undesirable level of accumulation of particles.

Where cleaning or sampling is triggered by a change in performance or particle loading, a buildup of particle mass on an internal surface of a particle collector or concentrator may be detected by a change in gas flow in the apparatus, such as a change in velocity or backpressure, or by monitoring a change in light transmittance, absorbance, or reflectance of an internal surface; and a positive signal output from a sensor or detector triggers the acoustic cleaning or sampling treatment. If the particle mass is not of interest, it may be cleaned away using dry ultrasound so that the unwanted particle mass is discarded while continuing to monitor gas flow.

Any suitable signal can be used to trigger a cleaning cycle. In one embodiment, an ultrasonic flow velocity sensor is used, where increases in flow velocity are associated with stenotic buildup of particle deposits in a particle trap or around a collection duct. In yet another embodiment, a backpressure sensor or densitometer is monitored on the feed side of the inlet port, and increases in backpressure or density on the feed side are associated with a downstream constriction or blockage. In another embodiment, transmitted, attenuated or reflected light, or fluorescent emissions, are used to monitor particle buildup on an internal surface.

Surprisingly, a completely unanticipated performance improvement results. Instead of having to replace or service particle monitoring equipment that has become blocked or fouled, captive particles that are trapped on and fouling aerosol collector or concentrator surfaces may be periodically mobilized and removed by application of a brief pulse of acoustic energy, essentially eliminating the progressive deterioration of performance resulting from fouling as is commonly seen with particle monitoring equipment of this type— without the need to interrupt gas flow and with only a very low increase in power consumption.

Thus in a first aspect, the invention is a particle monitoring apparatus comprising an acoustic transducer for on-the-fly dry cleaning of particle monitoring equipment having narrow internal channels with air: so lid inside surfaces subject to particle impaction and fouling.

More generally the invention is an integrated aerosol concentration apparatus incorporating acoustic transducers for improved particle monitoring and sampling. In particle monitoring devices having at least one internal channel with intake end and outlet end for conveying aerosol particles in a gas stream flowing under suction therethrough, and which is subject to fouling by accretion of particle deposits at an air: solid interface by a process of impaction on an inside surface thereof, the invention may be described as an apparatus for dry acoustic cleaning of the inside surface as needed, which comprises at least one acoustic transducer operatively coupled to the inside surface, the acoustic transducer with power supply for applying pulses of modulated acoustic energy to the inside surface prophylactically at regular intervals during continuous uninterrupted operation, wherein the modulated acoustic energy is configured with on/off pulse duration, frequency, bandwidth, resonance, harmonics, soft start or amplitude characteristics selected for extending the service life of said particle monitoring device.

Realized is an aerosol particle sampling method, which comprises a) flowing a gas stream containing an aerosol through internal channels of a particle concentrator, the particle concentrator having a solid body with one or more internal particle accretion surfaces subject to particle accretion; b) applying acoustic energy to at least one the internal surface, wherein the acoustic energy is applied from an acoustic transducer acoustically coupled thereto; c) acoustically dislodging any accreted particles and entraining the dislodged accreted particles in the flowing gas stream; and d) sampling the accreted particle or particles from the flowing gas stream in a downstream particle collecting, sampling or analyzing apparatus. The method may be performed on the fly without interrupting the flow of the gas stream.

The particle concentrator may be an aerodynamic lens, an aerodynamic lens array, a skimmer, or a virtual impactor, for example, having one or more particle accretion surfaces therein. Particle collectors may also be treated in this way.

Acoustic energy may be applied in bursts, pulses or trains of pulses of acoustic energy according to a fractional duty cycle. In other instances acoustic energy may be applied continuously or semi-continuously.

The acoustic transducer is generally a piezoelectric, magnetostrictive, or electrostatic transducer acoustically coupled to the solid body of the particle monitoring device, or a member thereof. In a preferred case, the acoustic transducer is an ultrasonic transducer and the particle accretion surface is an enclosed surface of a gas-to-gas aerosol particle concentrator.

In yet another aspect, the invention incorporates a sensor or sensors to trigger dry cleaning in a feedback loop where operating parameters are continuously sensed and cleaning is performed to restore the operating parameters to optimal efficiency by dry acoustic cleaning without interruption of gas flow. The method may employ a feedback loop for prophylactic cleaning of particle monitoring equipment, which comprises using a sensor or sensors to sense a flow rate, a flow velocity, or a backpressure of the gas stream flowing in the particle concentrator and outputting a digital or analog signal indicative of the flow rate, flow velocity, or backpressure to a control circuit. The method may comprise modulating the sensor signal by applying acoustic energy, under control of a controller with circuits connected to the sensor or sensors and the acoustic transducer or transducers, to the at least one internal surface subject to particle accretion, thereby forming a feedback control loop in which acoustic energy is emitted to modulate the sensor output. Thus, the method also includes a step for acoustically interrogating the flowing gas stream to sense a flow rate, a flow velocity, or a backpressure, and generating a signal indicative of accretion of particles on the particle accretion surface - acoustic energy may then be used to maintain the signal within defined operating limits.

Also realized is a particle sampling method having steps for: a) flowing a gas stream containing an aerosol through a particle collector, the particle collector having a solid body with one or more particle collection surfaces subject to particle accretion; b) applying acoustic energy to at least one the internal surface, wherein the acoustic energy is applied from an acoustic transducer acoustically coupled thereto; c) acoustically dislodging any collected particles and entraining the dislodged accreted particles in the flowing gas stream; and d) sampling the dislodged particle or particle from the flowing gas stream in a downstream particle collecting, sampling or analyzing apparatus. In this case the particle collection surface is often an inertial impactor surface, a centrifugal collector surface, a bluff body surface, or an electrostatic precipitator surface used to trap particles of interest for subsequent analysis, either in situ or remotely by techniques known in the art.

In another embodiment, the invention is a self-cleaning apparatus for sampling aerosolized particles, which comprises: a) a particle concentrator with solid body having an internal channel or channels fluidly connected to a suction pressure source for flowing a gas stream, the particle concentrator having one or more internal particle impaction surfaces subject to particle accretion; b) a sonic transducer or transducers in acoustic contact with the solid body, the sonic transducers each for applying energy to an internal particle accretion surface and entraining the dislodged accreted particles in the flowing gas stream; and

wherein the particle concentrator is an aerodynamic lens, an aerodynamic lens array, a skimmer, or a virtual impactor. In a preferred embodiment, the apparatus includes a particle concentrator, and the particle impaction surface is proximate to a virtual impactor mouth of a skimmer and comprises the lips of a lateral flow channel and the inlet of a collector channel. The apparatus may also include aerodynamic lenses with particle accretion surfaces in need of periodic cleaning. The apparatus may also include a controller for actuating the sonic transducer according to a fractional duty cycle, wherein the acoustic energy is intermittently applied on the fly without interrupting the flow of the gas stream. In alternate embodiments, the

acoustic energy is applied continuously or in bursts or pulses of acoustic energy according to a fractional duty cycle.

These and other aspects of the invention are described in the accompanying drawings and detailed description contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be better understood by considering the description in conjunction with the accompanying drawings, in which:

FIG. 1 is a view of a first air-to-air particle apparatus adapted for on-the-fly ultrasonic cleaning.

FIG. 2 is a view of a second air-to-air particle apparatus adapted for on-the-fly ultrasonic cleaning.

FIG. 3 is a view of as third air-to-air particle apparatus with feedback system adapted for ultrasonic cleaning.

FIG. 4A depicts schematically a skimmer assembly with acoustic window aligned transversely through the skimmer, perpendicular to the exposed face of the intersecting channels. The structure is rendered with more detail FIG. 4B.

FIG. 5 shows a cross-section through a skimmer with upstream aerodynamic lenses and depicts an interface with an array (dotted lines) of piezoelectric, magnetostrictive, or electrostatic transducer elements.

FIG. 6 is a plot monitoring backpressure in an aerosol concentrator with and without periodic application of ultrasonic acoustic energy to the concentrator body.

FIGS. 7A and 7B depict chimney pressure drop and collector pressure drop respectively, comparing performance parameters of a skimmer and a particle trap assembly under particle loading with and without pulsed application of ultrasonic acoustic energy. FIGS. 8A and 8B are views of a skimmer mouth (FIG. 8A) that has been fouled by progressive accretion of particle deposits over time and a corresponding skimmer mouth (FIG. 8B) treated preventatively with periodic application of pulsed ultrasound.

FIG. 9 is a conceptual view showing the use of acoustic emission in care and maintenance of particle concentration and collection equipment.

FIG. 10 is a schematic view of care of a particle concentrator with centrifugal particle trap and analytical module using acoustic cleaning and maintenance.

FIG. 11 is a schematic view of care of a particle concentrator with electrostatic particle trap and analytical module using acoustic cleaning and maintenance.

NOTATION AND NOMENCLATURE

Throughout the following description and claims, certain terms are used to refer to particular elements, features, steps or components and are defined here as intended by the inventors, i.e. they are intrinsic meanings. As one skilled in the art will appreciate, different persons may refer to the same element, feature, step or component by different names. This document does not intend to distinguish between elements, features, steps or components that differ in name but not in function, action or result. Other words and phrases used here take their meaning as consistent with usage as would be apparent to one skilled in the relevant arts.

The drawing figures are not necessarily to scale. Certain elements, features, steps or components herein are shown in somewhat schematic form and some details familiar to those skilled in the art might not be shown in the interest of clarity and conciseness.

A "particle" is a generally diminutive or lightweight body of solid, liquid or gel-like matter suspended or dispersed in a gas volume or resident on a surface. This can include, without limitation thereto, dust motes, exfoliated skin cells, fibers, spores, vegetative cells, mists, condensates, virus particles and aggregates, bacteria, yeasts, mucous droplets, microdroplets of saliva and bronchial secretions, pollen grains, fly ash, smog condensate, smoke, fumes, dirt, fogs (as in industrial or agricultural spray application), salt, silicates, metallic particulate toxins, tar, combustion-derived particles, particulate toxins, and the like. The aerosol particle may be a composite, containing both solid and liquid matter. Such particulate bodies can remain suspended in a column of air for long periods of time, can be carried on currents in the air, or can settle onto surfaces and may be hazardous if inhaled. Aerosol particles ranging from 0.01 to 25 microns are particularly hazardous and are often of particular interest. Particles less than 10 microns in apparent aerodynamic diameter pose a significant respiratory threat, and those less than 2.5 microns are of particular concern, as these can be inhaled deep into the lung. Aerosols may include bioaerosols and biotoxins. Bioaerosols refer generally to aerosolized living cells and infectious lifeforms. Aerosols may also include evidence of contraband or explosive substances, and thus are a matter of significant interest for screening cargo and passengers.

A particle "agglomerate" is a cluster of particles. A "particle deposit" is at least one particle, generally more than one particles, deposited or "accreted" on an inside surface of a particle monitoring apparatus.

"Aerosol concentrator module" - includes aerodynamic lens concentrators, aerodynamic lens array concentrators, and micro -aerodynamic lens array concentrators, when used in conjunction with a "virtual impactor", "skimmer", or other means for separating a gas flow into a particle- enriched core flow (also termed "minor flow") and a "major flow" or "bulk flow" that is particle depleted, and which may be discarded or in some instances is sampled for any vapors. Also included are cyclone separators, ultrasound concentrators, and air-to-air concentrators generally for generating a flow split, where the "flow split" refers to the ratio of the minor flow to the bulk flow or total flow. The particle-enriched gas stream is delivered to an outlet of the aerosol concentrator module and may be conveyed to an aerosol collector module with particle trap. Also included are inertial particle separators and electrostatic impactors which capture charged particles by electrostatic attraction.

"Aerodynamic lens" (ADL) - is a device having a passage for a gas stream characterized by constrictions (lenses) that have the effect of focusing the particle content of the gas into a core flow region or "particle beam" surrounded by a sheath of particle-depleted air. ADLs may be micro-ADLs as described in US Pat No 7704294, which is co-assigned.

"Virtual impactor" is an air-to-air aerosol concentrator which separates a particle fraction of an aerosol having a higher inertia from a surrounding air mass having a lower inertia, thereby concentrating the particle fraction in a smaller fraction of the gas. In operation, particles in the gas stream are first focused into a particle-rich core flow surrounded by a particle-poor sheath flow. The gas stream is then directed at an obstructing surface, where the obstructing surface includes a smaller orifice at the precise point where the particle rich core flow is targeted to impact the surface, and thus the core flow is admitted through the orifice without impact while the sheath flow is diverted by the obstruction on a new vector away from the core flow. The bulk of the gas stream (typically termed the "major flow") is diverted by the obstruction while a smaller fraction of the gas stream (typically termed the "minor flow") containing the particle is admitted through the orifice. Heavier or denser particles exceeding a "cutpoint" fail to change direction and pass through the orifice. Finer particles remain entrained in the deflected major flow. Particulate "wall loss," i.e., unintended deposition of particles on various surfaces of virtual impactor structures, especially at curved or bent portions, remains a hithertofore unsolved problem because this unwanted particle deposition leads to fouling. Examples of virtual impactors and virtual impactor arrays include US Pat. Nos. 3901798; 4670135; 4767524; 5425802; 5498271; 5533406; 6062392 and in US Pat. No. 7875095, which is co-assigned.

"Skimmer" - a virtual impactor device for separating a bulk flow from a particle-enriched core flow to form an aerosol concentrator. Skimmers having slit-type and annular-type geometry are realized.

"Particle trap" - as used here, refers to a collector channel for a gas flow having the property of reversibly capturing aerosol particles by virtue of their inertia or their electrostatic charge. Particles are captured by inertial or electrostatic impaction on an internal surface or surfaces of the particle trap. Examples of particle traps and particle trap arrays include US Pat Appl Ser No 12/364872 and US Prov Pat Appl No 61/224861, which are co-assigned.

"Inertial Impactor" - a particle collector with a body or member having an impactor surface which is disposed in a gas flow such that streamlines of the gas flow are deflected around the impactor surface but particles with inertia exceeding the cutpoint of the device collide with the impactor and are captured on it. Inertial impactors of interest here include two classes, "centrifugal impactors" and "bluff body impactors". "Plate impactors" are considered here as a sub-class of bluff body impactors.

"Centrifugal Impactor" - describes a family particle collectors having impactors for capture of aerosol particles (i.e. aerosol or aerosols) from the core of a streaming laminar flow of a gas, in which a channel for conducting the gas flow is bent or curves. Where the concavedly curving inner wall intersects or impinges on the long axis of gas flow, inertial force will cause more dense aerosol particles to impact what is termed here an "inertial impactor surface" or "centrifugal impactor surface", the area of the inside wall surface crossing or impinging on the long axis of flow. In a collector channel, an impactor surface is formed wherever an internal wall of the concavoconvex passage intersects or impinges the long axis of gas flow, deflecting the gas streamlines. The channel geometry for an inertial impactor is generally tubular, with circular, ovoid, or rectangular cross-section, and may be tapered as described in US Pat No 7704294 and US US Provisional Patent Application 61/224861, which are co-assigned.

"Bluff Body Impactor" - Inertial impactors are also formed by flowing a gas stream around an obstacle in the path of the stream so as to cause a sharp change in the direction of the gas stream. In some cases, the gas flow is "split" around the obstacle. The obstacle, which is termed a bluff body, is not streamlined. The bluff body may be for example a pillar or a plate positioned to intersect the gas stream. Higher momentum particles do not deviate with the gas stream around the bluff body and instead collide with the windward surface of the obstacle, termed here an "inertial impactor surface".

"Electrostatic Impactor" - refers to a particle collector having a pair of capacitively electrified plates for capturing charged aerosol particles by impaction on the plates. The plates are thus "electrostatic impactor surfaces" or particle precipitators. Positive and negatively charged plate surfaces attract and bind oppositely charged particles. Particles may be natively charged or may acquire charge by contact with a source of ions, such sources including but not limited to a "corona wire," a source of ionizing radiation, or a radio-frequency discharge.

"Particle accumulation surface" - an inside surface of a particle concentrator or collector sensitive to fouling. The inside surface is generally not accessible for cleaning without disassembly of the particle monitoring device.

"Cut size" - The cut size parameter of an inertial impactor, D_p ¹⁵⁰, is defined as the aerodynamic particle diameter at which 50% of the particles entering the impactor are captured on an internal impactor surface. The cut size of a virtual impactor, Dp^V50, is defined as the aerodynamic particle diameter at which 50% of the particles entering the virtual impactor follow the minor flow stream.

The behavior of the impactor requires consideration of the Stokes number (Stk), which is the ratio of the particle stopping distance at a mean throat velocity to the throat width), and the Reynolds number (Re) since they govern particle and gas phase flow behavior, respectively, in the impactor or virtual impactor.

The Stk primarily governs the collection efficiency in impactor theory. It is the ratio of the particle stopping distance at a mean throat velocity to the throat width. p_p ^> p; · < /.. - u_B

Stk,.

where:

IX; = particle diameter

P_P = particle density

C_e - slip correction factor

Ϊ = mean velocity at acceleration nozzle exit

Ik - fliii d kiiiema ti c v sc osity

I,_r = critical dimension

The stopping distance is defined as the maximum distance a particle can travel with an initial velocity in still air without any external forces. For a Stk » 1 , particles should follow a straight line as the gas turns and for a Stk « 1, particles should follow the gas streamlines

"Critical dimension" (or "L_c") - for circular jets L_c is the nozzle radius (D/2) for Stk and nozzle diameter (D) for Re; for slot jets L_c is the nozzle half-width (W/2) for Stk and the full-width (W) for Re. When the Stk is approximately 1, the highest amount of wall losses occur since this is near the cutpoint. The critical dimension is generally made as small as practically possible to minimize the required suction blower power. Examples of critical dimensionalities associated with proximate fouling conditions include the virtual impactor throat, where the width or diameter is a critical dimension, and a centrifugal impactor, where the concavoconvexedly bending tube has a critical width or diameter proximate to the impactor surface. Also critical are inside clearances between aerodynamic lens elements, chimney passages, and collector tube manifolds.

"Acoustics" is an interdisciplinary science that deals with the study of sound, ultrasound and infrasound. Acoustic waves propagate in solids, fluids or gases as waves or disturbances in the ambient pressure level. The periodicity of the waves may lead to resonance. "Acoustic energy" is associated with the amplitude and the frequency of the waves. "Acoustic cleaning" as used here refers to the use of acoustic energy to clean an inside surface of a particle monitoring device.

"Ultrasound" - generally is defined as an acoustic wave having a frequency of greater than about 15-20 KHz, but may usefully extend into the megahertz range, up to perhaps 20 MHz. Ultrasound is a form of energy consisting of alternating waves of compression and rarefaction.

"Acoustic transducer" - a device for conversion of electrical to acoustic energy or vice versa. These transducers may be used to generate a waveform for carrying acoustic energy, generally by consumption of electricity. A first class of acoustic transducers are formed of a piezoelectric material such as lead zirconate titanate (PZT), barium titanate or a polymeric materials such as polyvinylidene fluoride (PVDF). Another class of acoustic transducers may be formed of magnetostrictive materials such as domain-microstructured materials containing terbium, dysprosium, or gallium in a nickel or ferrite crystalline base. Capacitive acoustic transducers (ultrasonic transducers of this variety are termed "CUTs") are also known and are used for non- contact inspection. These devices are composed of a thin pliant membrane film and a rigid conducting backplate to form an electrostatic capacitor. Cyclically applied voltages cause the membrane to vibrate, and hence can generate ultrasound, whereas a change in voltage across the membrane can be used for detection. Metallic or ceramic backplates can be used; the backplate may be machined to improve the acoustic properties of the transducer. The ultrasonic emission may be collimated or focused to a point as desired, for example as described in US Pat. Appl. No. 2009/0158851. Micro fabricated, layered structures having a resonant cavity or cell formed between a thin metallized membrane and a backplate electrode may be formed, the cell having a charged state and a relaxed state and the membrane capable of rapid flexion in the manner of a diaphragm (US Pat. No. 5287331).

PZT transducers also may be coupled in air by applying a quarter-wavelength thick impedance matching layer to the front surface, but with loss of bandwidth and with generally poorer efficiency.

Hybrid cells in which the conductive membrane is driven simultaneously in both piezoelectric and electrostatic modes are known, as are resonant cavities having varied depth to achieve broader operating bandwidths (US Pat. No. 6775388). Acoustic transducers may be formed as rectilinear slabs, as arrays of elements, as annular arrays, as cylinders, as coin-like disks, as membranes, and so forth. In one class, concavedly spherical transducers are formed having the property of emitting focused acoustic waves, so that energy density of the propagated wave increases to a peak at a point away from the surface of the transducer.

Acoustic waves can be amplitude modulated or frequency modulated. A frequency sweep within the transducer's harmonic bandwidth is particularly useful in mobilizing particles from surfaces. Arrays of transducers having different bandwidths may be used to increase the frequency range.

Couplant - a medium for transmitting a sound wave across an interface, such as from a transducer to a solid surface, often a gel.

Means for Analyzing - as used herein, refers to an apparatus for detecting and/or characterizing a particle or particle constituent, or a sample of a mass of particles. Detectors for analysis and identification of particles or vapors are known in the art and may be selected for physical, chemical or biological analysis. Detection methods include visual detection, machine detection, manual detection or automated detection. Means for detecting include laser particle scattering, liquid chromatography (LC), high pressure liquid chromatography (HPLC), high pressure liquid chromatography with mass spectroscopy (HPLC/MS), gas chromatographic mass spectroscopy (GC/MS), gas chromatography coupled to electro capture detection (GC-ECD), atmospheric pressure ionization time-of- flight mass spectrometry (TOFMS), ICP-mass spectrometry, ion mobility spectroscopy (IMS), differential ion mobility spectroscopy, secondary electrospray ionization - ion mobility spectrometry, electrochemistry, polarography, electrochemical impedance spectroscopy (EIS), surface plasmon resonance (SPR), fast atom bombardment spectroscopy (FABS), matrix-assisted laser desorption ionization mass spectrometry (MALDI/MS), inductively coupled plasma mass spectroscopy (ICP/MS), Raman spectroscopy (RS), FTIR, SAW spectroscopy, surface-enhanced Raman spectroscopy (SERS), laser induced breakdown spectroscopy (LIBS), spark-induced breakdown spectroscopy (SIBS), lateral flow chromatography, NMR, QR (quadrupole resonance), fluorescence, luminescence, and so forth. Detection systems are optionally qualitative, quantitative or semi-quantitative. Also included are analytical devices such as spectrophotometers, fluorometers, laser particle counters and laser scattering devices, luminometers, photomultiplier tubes, photodiodes, nephelometers, photon counters, voltmeters, ammeters, pH meters, capacitive sensors, and so forth. Magnifying lenses, optical windows, lens flats, waveguides, and liquid waveguides, may be used to improve detection. Detection methods may also rely on molecular biological techniques such as hybridization, amplification, immunoassay, PCR, rtPCR, electroimpedance spectroscopy, ELISA, and the like. Means for detecting include "labels" or "tags" such as, but not limited to, dyes such as chromophores and fluorophores; radio frequency tags, plasmon resonance, radio labels, Raman scattering, chemiluminescence, or inductive moment as are known in the prior art. Fluorescence quenching detection (FRET) is also anticipated. A variety of substrate and product chromophores associated with enzyme-linked immunoassays are also well known in the art and provide a means for amplifying a detection signal so as to improve the sensitivity of the assay, for example "up-converting" fluorophores. Explosives detection was recently reviewed by Moore (Moore, DS. 2007. Recent advances in trace explosives detection instrumentation. Sens Imaging 8:9-38).

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. "Conventional" - refers to a term or method designating that which is known and commonly understood in the technology to which this invention relates. Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including, but not limited to". The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase "means for."

DETAILED DESCRIPTION

Although the following detailed description contains many specific details for the purposes of illustration, one of skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.

Referring to the figures, FIGS. 1 and 2 depict use of a contacting acoustic transducer in combination with a flowing gas stream to clean particles from a particle monitoring device with air-to-air concentrator. As shown in FIG. 1, a representative particle concentrator 100 is depicted; the particle concentrator having a solid body 101 with intake 102 for receiving a gas stream 103, an aerodynamic lens array (104) combining a series of focusing lenses (104a, 104b, 104c, 104d) for separating the gas stream into a particle rich core flow (termed "minor flow" M) and a particle depleted sheath flow (termed "bulk flow" B) under laminar conditions, and a skimmer 105 for separating the minor and bulk flows so as to achieve particle concentration according to the flow split (the ratio of the bulk flow to the minor flow) for a given particle size as is known in the art (see for example US 7875095 to Ariessohn). In a preferred embodiment of the invention, the concentrator includes a slit-type virtual impactor, however annular virtual impactors may also be used.

At the base of the device is a collection channel 106, immediately below the virtual impactor 107, which is vulnerable to particle accumulation. The walls of the collection channel are prone to accumulate fine particle agglomerates due to wall impaction losses over extended use. Devices of this type may be driven using positive pressure at the intake, but are more commonly driven by applying suction pressure at the collection channel 106 and lateral flow arms and chimneys (108a, 108b).

In acoustic contact with the solid body 101 bilaterally on either side of the collection channel are piezoelectric transducers (1 10a, 110b). Shown schematically is an ultrasound generating circuit 111 designed for driving both piezoelectric transducers in parallel. Wiring is simplified for clarity. Sonic waves, such as at a frequency of 20 KHz or 40KHz, are used to dislodge the accumulated particles. These particles can be collected in a downstream particle trap without loss.

A couplant may be used to increase efficiency of propagation of sound across the solid:solid interface between the transducer and the solid body of the particle monitoring device. The acoustic waves may be applied continuously or intermittently. A duty cycle of less than 0.5% has been found to be effective, and is more preferably about 0.1%. Pulse interval can range from minutes to hours depending on the particle load.

The acoustic transducer may be a piezoelectric, magnetostrictive, or capacitive electrostatic transducer, or a hybrid thereof. Shown schematically is a programmable, low energy, transducer LC driving circuit. Transducers are for example Steiner & Martins Inc SMUTK2500RS112 (2.5 MHz, 26VDC, 800 niA), Nanhai Techsin Electronic Co DK-24 (40 KHz, 24V AC, 950 niA), or custom piezoelectric atomizers available from Microflow Engineering of Sweden which run on AA batteries and are the size of a postage stamp. Battery operated circuits using piezoelectric driver circuits at 3600 Hz with peak currents of 20 mA at 36 mV are described in "Piezoelectric Transducer Driver Circuit with Adjustable Output Level" by Gary Pace (Motorola Inc Technical Developments, Vol 17 Dec 1992). Ultrasonic driving circuits are described for example in US Pats No 4113809, 4632311, 4641053, 4689515, 5803362, 6361024, RE39671, and US Pat Appls 2007/0235555 and 2009/0095821. Such driving circuits are well known in the art and are not described in detail here, but include frequency control, resonance tuning, impedance sensing, soft start, and solid state features. The acoustic transducer and associated circuitry produces vibrations to dislodge particle deposits from internal surfaces of the particle monitoring apparatus. If desired, acoustic excitation may be conveyed to selected surfaces of the particle monitoring device using acoustic waveguides of the type described in US Pat Appl 20080237366 to Ehlert.

In FIG. 2, a second particle monitoring device 200 is shown. The geometry of the skimmer is modified so that the chimney passages 208a, 208b for drawing the bulk flow are directed downward and parallel to the collection channel 207. In this configuration, sonic transducers applied to the lateral surfaces of body 201 are effective in insonating aerodynamic lense array 204 and the upper surfaces of the virtual impact void 205.

An alternate skimmer assembly 300 is shown schematically in FIGS. 3A and 3B. Since acoustic energy propagates with higher efficiency through solids than through air ducts, the acoustic transducers 301, 302 are shown on the face surfaces rather than on the lateral surfaces, and may be disposed on both front and back of the device over areas in the ductwork where particle deposition is likely. FIG. 3A illustrates the basic geometry in cross-section. Top center is the intake channel 303, where the dark arrow indicates a flowing gas stream. Aerodynamic lenses 304a and 304b are characterized by sharp front facing steps which can become coated with particles over time. Chimney arms 305a, 305b on the right and left bound the central virtual impactor aperture 306 which forms the mouth of the collection channel 307. The mouth of the collection channel includes lips 308 that define the lateral entrances to the chimney passages 305a and 305b, but most particle deposition occurs immediately beneath the virtual impactor 306 within the collection channel. The dotted circle indicates the area of the detailed view of FIG. 3B.

The skimmer assembly is depicted in more detail in FIG. 3B. Shown is a cross-section through a skimmer with upstream aerodynamic lenses and depicts locations of acoustic windows 301, 302 where transducers may be aimed. A first sonic transducer (301, dotted line) is shown having an acoustic footprint above one of the aerodynamic lenses 304b. The second transducer (302, dotted line) is shown having an acoustic footprint over the virtual impactor 306 and mouth of the collection channel 307. In this way, adherent particles in the collection channel may be dislodged and restored to the minor flow (M, 310), minimizing impaction losses in the skimmer. For slit-type particle concentrators, these acoustic windows extend the length of the skimmer, which lies perpendicular to the plain of the paper and is shown here shown in vertical sagittal section, dividing the body of the device into right and left halves on a mirror plane of symmetry.

FIG. 4 shows a cross section through a device 400 with body 401 and intake 402. The virtual impactor void 403 is provided with lateral chimneys 404a, 404b folded back against the inlet channel. A series of aerodynamic lenses are again shown for concentrating the particle ribbon. The direction of inlet flow of the gas feedstream is downward through the skimmer; a minor flow 405 exits at the bottom of the device through a collection channel 406 and can be routed to a particle collector, sampler, or analytical device. Minor flow is exhausted from the particle monitoring apparatus at 410 Bulk flow of the particle-depleted gas (B) is directed back and up along the two chimneys, which extend out of the plane of the image.

As shown, a pair of piezoelectric transducer elements 41 la, 41 lb are attached on opposite sides of the body. Optionally a couplant may be used to improve transmission efficiency. Experience has shown that acoustic energy is propagated through the solid body and excites the internal surfaces of the skimmer, dislodging any buildup of particles around the virtual impactor aperture mouth of the skimmer, in the collection duct 406, and on the internal surfaces of any upstream aerodynamic lens elements before performance-altering fouling can occur. The dislodged particles are resuspended in the gas stream so as to be cleaned from the apparatus. Particle deposits that have resulted in a decrement in performance may also be cleaned away in this manner. In the case of particles that cannot be cleared because of blockage, the gas flow direction may be reversed and dry application of ultrasound applied to dislodge the plug.

A feedback control loop is shown demonstrating the integration of the acoustic transducers with a sensor 450 for measuring performance of a particle trap to control activation and to modulate the acoustic energy emitted by the transducers. As particles accumulate around the skimmer or in the particle trap, the sensor detects the particle deposits directly or indirectly by measuring gas velocity or backpressure, and emits a signal 451 to a controller 452 with microprocessor, dynamic and non-volatile memory and encoded program instructions, which controls the transducer driving circuit 453, shown here schematically. In this way, the sensor signal results in application of acoustic energy to the concentrator module body (or to the particle trap body if independently mounted) which breaks up accreted particle deposits and resuspends them in the gas stream, cleaning the inside surfaces and restoring performance only as needed. The signal- actuator- sensor circle thus functions as closed feedback loop to enhance and prolong particle monitoring function without the need for service or disassembly.

In addition to sensors for detecting an increase in backpressure or flow velocity associated with accretion of particle deposits, the downstream apparatus 450 may include a particle trap such as a centrifugal impactor. In operation, a gas stream with entrained aerosol particles enters the concentrator at the top of the device body. Aerosol particles not deflected with the gas streamlines in the bending portion of the channel are captured by inelastic impaction on the inertial impactor surface in a particle trap. Particles having an inertia greater than a critical limit (the cutpoint of the impactor) cannot round the bend and are captured by impaction. The aerosol-depleted gas stream exits the collector body at the outlet arm. Under influence of acoustic energy applied with a transducer contacting the body of the device or applied through an acoustic window in the body, particles in the trap are re-aerosolized in the gas stream to effect acoustic cleaning of the trap.

The acoustic concentrator of the figure may be used in conjunction with a sensor to detect particle accumulation on inside surfaces of the internal channel or proximate to the skimmer and chimneys. The particle trap may be provided with means for detecting particles or particular particle types based on spectrophotometry, fluorometry, conductivity, resistivity, electro impedance spectroscopy, acoustic impedance, and the like, without limitation thereto. Means may also be provided for extracting the particles or particle constituents from the trap for further analysis or archiving.

In a related application, the particle trap is incorporated in the solid body with the aerodynamic lenses and skimmer in a single integrated functional unit and the body of the unit is supplied with ultrasonic transducers, for example, and control circuitry linked to sensors mounted in the gas flow path or mounted with waveguides in the solid body to optically monitor particle buildup in the particle trap or around the skimmer mouth. In other applications, combinations of acoustic transducers, including transducers for treating skimmer orifices and transducers for treating particle trap inside surfaces are used in combination with sensors to monitor gas flow velocity, backpressure, and optical sensors to directly monitor accretion of particle deposits. FIG. 5 again shows a cross section through a particle monitoring device 500 with skimmer of the slot type depicted in FIG. 4, the section representing a view along the virtual impactor 501 and through the long axis of mirror symmetry of the concentrator body. Also shown are an intake 502, upstream aerodynamic lenses, and collector channel 503. Superimposed on a superior surface of the device body is an acoustic interface with an array of piezoelectric, magnetostrictive, or electrostatic transducer elements (dotted lines, 505). Acoustic energy is delivered to the surfaces of the aerodynamic lenses and skimmer through the solid body, or through the air spaces (acoustic windows) occupied by the flowing gas stream. Particles that have accumulated on the internal surfaces of the device are readily dislodged and entrained in the flowing gas stream by the application of acoustic energy using the array as shown. While not shown to scale, a variety of array configurations are effective. Multiple parallel aerosol concentrators in a single body may be treated in this way.

Blockage or partial occlusion of internal surfaces may occur almost any point in particle monitoring equipment, but is more likely to occur around the mouth of the skimmer and in the particle trap. The acoustic transducers or array are configured so that these surfaces are selectively excited by the acoustic energy. Application of acoustic energy can be intermittent or triggered by a signal from an upstream or downstream sensor positioned to monitor performance of the equipment.

Also shown schematically is a downstream particle trap 510. The particle trap is used for capturing particles on a solid surface, such as an inertial impactor or electrostatic impactor, and is optionally configured with in situ detection capability. The particle trap may also be acoustically cleaned as required, either on a regular schedule, for example as part of a sampling regime, or in response to a signal from a sensor that monitors particle accumulation in the particle trap or indirectly monitors gas flow resistance through the particle trap.

FIG. 6 is an experimental plot monitoring backpressure in an aerosol concentrator with and without periodic application of ultrasound to the concentrator body. Particle dust in a gas stream is generated using a dry nebulizer and passed through the particle monitoring equipment. As evidenced by the increase in backpressure, particle fouling results in progressive deterioration of performance and buildup of solid deposits in the equipment (see FIG. 8A). FIG. 6 is described in more detail in Example 3. With periodic cleaning of the skimmer and particle trap, the problem is alleviated. Curves 601 and 602 represent backpressure in the chimney arms and collection channel, respectively, of the skimmer, absent ultrasonic treatment. Backpressure in the concentrator becomes progressively worse during a 30 min exposure to ASHRAE dust. Contrastingly, curves 611 and 612 represent backpressure in the chimney arms and collection channel, respectively, with intermittent ultrasonic treatment.

FIGS. 7A and 7B are plots extrapolating experimental results for fouling associated with particle monitoring over a timecourse of 200 days. The extrapolation is done by overloading the particle monitoring equipment experimentally and then projecting the effect of a more typical loading rate as described in more detail in Example 4. A significant gain in equipment life is achieved by use of intermittent ultrasonic cleaning. Curve 621 shows the progressive effect of particle loading on chimney pressure; curve 622 the effect of periodic sonic treatment. Curve 623 shows the progressive effect of particle loading on collector pressure (i.e., in the collection channel); curve 624 the effect of periodic sonic treatment. A beneficial effect is achieved which is an advance in the art.

The acoustic transducer was coupled to a face of the device body and acoustic energy propagated through the solid. Any of a variety of acoustic transducers may be used, including ultrasonic and sonic frequencies. Optionally, miniature transducers may be embedded in the solid body during fabrication. A driver circuit board was also provided with the body and includes a heatsink for drawing heat away from the power transistor, or is provided in a separate module and is electrically connected to the transducer.

Any particle concentrate captured in the device may be analyzed for a physical, chemical or biological property. A variety of analytical means are known in the art, and include without limitation methods of:

1. inducing fluorescence or detecting auto fluorescence of specific constituents of the liquid sample and detecting emitted fluorescent radiation;

2. measuring optical absorption of the liquid sample at various wavelengths;

3. measuring light scattered from the liquid sample in various directions, as by laser scattering;

4. subjecting the liquid sample to at least one spectroscopic measurement technique such as Raman spectroscopy (RS), laser induced breakdown spectroscopy (LIBS), spark-induced breakdown spectroscopy (SIBS), and mass spectroscopy (MS); 5. subjecting the liquid sample to nucleic acid amplification and real-time PCR;

6. subjecting the liquid sample to an immunological assay; or

7. measuring radiation emitted from the liquid sample;

the analysis or analyses having the purpose of identifying those constituents of interest. In a first instance, using optical means, analysis may be performed by detecting light transmittance, light reflectance, fluorescence, or luminescence of the liquid. In another instance, using electrical means, the conductance, impedance, or capacitance may be monitored. In yet other instances, analysis may be performed by detecting an alpha-particle emission, beta-particle emission, or gamma emission, or by detecting a molecular species or fragment thereof in the liquid by an affinity binding technique or an enzymatic reaction.

FIG. 8A is a representation of a particle concentrator 800 opened for inspection so that the channels of the skimmer are exposed. In this view of the skimmer, the horizontal slit 801 is the entrance to a lateral chimney 802, and one half face of the structure is shown. Large numbers of particles have accumulated on the upper faces of collection duct 803 and around the mouth of the skimmer in slit 801, dramatically increasing resistance to flow through the skimmer and changing the flow split. FIG. 8B shows the effect of periodic acoustic cleaning for prophylaxis under equivalent operating conditions. Essentially no particle accumulation is noted and operational performance is within specification.

Accordingly, the invention is a method for cleaning particle deposits from an internal surface of a particle monitoring apparatus or for preventing accumulation of particle deposits, which comprises steps for:

a) applying acoustic energy to the surface, wherein the acoustic energy is applied from an acoustic transducer operative ly coupled thereto;

b) acoustically mobilizing any particle deposits from the surface; and

c) removing the particle deposits in a flowing gas stream.

Steps a) - c) may be repeated if needed to satisfactorily clean the particle accumulation surface. In a preferred method, the cleaning cycle is performed "on the fly", where gas flow and particle monitoring are not interrupted and the method is applied as needed to prevent of particle buildup, either on a regular schedule or when conditions indicate it is needed. Optionally, the method may include a step for sensing a performance parameter of the particle monitoring apparatus and triggering the cleaning cycle when the operational parameter deviates from an acceptable value or range. The sensor may be an acoustic transducer, where the transducer sonically interrogates the flowing gas stream to sense a flow rate, a flow velocity, or a backpressure, and generates a signal. Other sensors are also useful. Flow velocity for example may be detected by Doppler shift methods. Backpressure may be monitored using suitable pressure sensors, for example air-coupled acoustic transducers where energy output is highly sensitive to gas density, or piezoelectric, micro cantilever or diaphragm-type pressure sensors. Alternatively, optical or electrical characterization of the condition of the internal surfaces is used to determine a need for cleaning. Optical sensors include laser scattering detectors, spectrophotometers, and fluorometers. Electrical sensors include circuits for measuring capacitance, resistance or conductance of the inside surface associated with fouling. Sensors may be used in combination to detect an operating condition of the particle monitoring apparatus and a composition of the accreted particle deposit, if desired.

Advantageously, cleaning can be combined with periodic sampling particle residues by means known in the art. Continuous in situ monitoring may also be employed.

FIG. 9 is a conceptual view showing the use of acoustic emission in care and maintenance of particle concentration and collection equipment. By way of illustration, a particle monitoring apparatus 900 having three functional modules or subassemblies is illustrated, an aerosol concentrator 901, a particle collector 902, and a sample analysis unit 903. These may be independent units with fluidic connections or may be fully integrated in both structure and function. Suction pressure used to operate the apparatus is indicated by direction of gas flows (arrows). The body of each module is indicated by a dotted line. Acoustic transducers are placed so as to be acoustically coupled to the body or to a body member of the module. The aerosol concentrator may be an air-to-air concentrator, for example.

An aerosol entering at the intake 904 is concentrated by separating the particulate solids from the excess air, generally using an air-to-air concentrator 901, and the particle depleted air 905 is exhausted while the particulate solids are captured in a particle collector 902 or trap. As shown here, the aerosol is first split in an air-to-air concentrator into a bulk flow 905 and a minor flow 906 and the minor flow enriched in particle mass is conveyed to the particle collector. Generally the sample mass is captured on a solid substrate and the particle depleted minor flow 907 is exhausted from the unit. The particulate mass 908 is then conveyed to a sample analysis unit for detailed compositional analysis or other characterization. The particulate mass may also be evaluated in situ in the particle collector, and advantageously, the results of a preliminary evaluation may be used to determine whether a sample is transferred to the analysis unit for further study or archiving.

The aerosol concentrator and particle collector may be combined in a single functional unit. Similarly, the particle collector and sample analysis unit may be combined as a single assembly. Full functional integration of all three modules is also contemplated.

Conversely, it may be advantageous to supply each of the three modules separately, so that, for example, the aerosol collector module 450a and/or the liquid sample analysis module 460a are disposable. Alternatively for example, the aerosol collector module 902 is fluidly plugged into an apparatus containing the aerosol concentrator module 901 for an analytical run, and following the run, the aerosol collector module 902 is then removed and forwarded to a separate workstation 903 for sample preparation and analysis.

According to another embodiment of the invention, the aerosol concentrator module 901 and sample analysis module 903 are framed in an apparatus with supporting pumps, fans, vacuum pumps, waste sinks, reagent reservoirs, electrical supplies, temperature controls, spectrophotometers, analytical instrumentation, and so forth, and the aerosol collector module 902 is a disposable part that is fluidly plugged in for each analytical run.

Acoustic energy, conducted through the solid body, is used to maintain the aerosol concentrator in good working order. The supporting power supply and control circuitry for insonation is either built into the modules or is attached as part of a supporting apparatus that interfaces with the modules and is operated in concert with other systems of the apparatus. Insonation may be used to prevent fouling and to clean fouled surfaces without use of a liquid as a coupling agent.

Acoustic transducers (909,910) in acoustic contact with the body or a body member of the aerosol concentrator or collector are actuated periodically on a schedule or in response to a decrement in performance so as to displace accreted particulate material that is fouling internal surfaces of the apparatus and reducing performance efficiency. Acoustic transducer 909 is operatively coupled to internal surfaces of the aerosol concentrator subjected to fouling and acoustic transducer 910 is operatively coupled to internal surfaces of the particle collector where particles accumulate. A sound guide of denser material may be used to drive radiated sound to particular internal features of the apparatus if desired. In some instances, acoustic energy is used to assist in sampling solid materials that have accumulated in the particle collector. Dry sampling and sampling in the presence of a liquid wash volume are contemplated. Acoustic energy may also be used to assist in comminuting the sample material or in lysing cells such as bacterial spores and the like. Generally an ultrasonic transducer is preferred for this purpose.

Sample material 908 that is conveyed to the sample analysis unit may be handled and analyzed by techniques known in the art. Archival samples are also commonly desired and may be acquired by conventional material handling technology.

FIG. 10 is a schematic view of care of a particle concentrator with centrifugal particle trap and analytical module using acoustic cleaning and maintenance. The use of a liquid in transfer of a sample of particles from a particle trap is illustrated here.

In this instance, modular elements of a concentration-collection-analysis apparatus are interfaced as a fully functional aerosol concentrator/liquid sample collector/and analytical apparatus 920. Modules 921, 922, and 923 are fluidly connected and are configured to function as an aerosol- to-liquid converter. Module 921, an aerosol concentrator module, is capable of processing 20, 30, 1000 or more liters per minute of a gas at intake 924 and diverting a major fraction of that gas, depleted of particles, to bulk flow exhaust 925. The "particle-enriched gas stream" 926 is then routed into module 922, an aerosol collector module with particle trap 922a and micro- elution capability via micro fluidic duct 922b which is adapted for interfacing with analysis module 923. While not limiting, micro fluidic duct 922b is part of a fluid handling system for eluting particles collected on impaction surfaces in the particle trap 922a and conveying solubilized or suspended liquid sample 928 for downstream analysis in module 923, which is configured as a "liquid sample analysis module". Liquid sample analysis module 923 may be an integrated micro fluidic analytical workstation for performing one or several analytical subroutines, such as liquid chromatography, lateral flow chromatography, ELISA, nucleic acid amplification and detection, PCR, fluorescence spectroscopy, and other means for detecting as are desired and are known in the art.

Thus, modules 921, 922, and 923 are fluidly connected and are configured to function as an aero so 1-to -liquid converter. The aerosol collector module includes collector channel 922c, centrifugal impactor 922a, outlet arm 922c, and elution duct 922b which is adapted for delivering a liquid sample 928 to liquid sample analysis module 923. In a preferred instance, a liquid solvent is injected into the particle trap during insonation and then withdrawn, carrying with it any suspended and dissolved material that have been dislodged by the ultrasonic action and the action of the solvent. Elution systems having features of this type are described in US Pat. Appl. Ser. No. 12/364672, which is incorporated herein in full by reference.

Acoustic transducer 929 is used to prevent or clean fouling of internal air-to-air concentrator channels of module 921. The control circuitry and power supply may be built into the module or may be supplied from an apparatus of which the concentrator module is a sub-assembly. The transducer is operatively coupled to the internal surfaces in need of cleaning by the solid body of the concentrator, shown here by dotted lines, or by a member thereof. A sound guide of denser material may be used to drive radiated sound to particular internal features of the apparatus if desired. Acoustic energy is applied periodically according to a preventive maintenance schedule or in response to a signal indicative of concentrator performance, as in a feedback control loop. Acoustic energy may also be applied continuously at levels that do not disrupt the laminar flow regime within the concentrator in the case of an aerodynamic lens, for example. Acoustic transducer 930 is used to apply acoustic energy to the particle collector body, and may be used to maintain performance of the collector by preventing excessive buildups, or to assist in particle sampling as described above. Alternatively, the highly concentrated sample liquid can instead advantageously be analyzed in situ in the particle trap with a suitable analytical apparatus, such as spectrometric or electro metric analysis via an optical window in the collector body prior to downstream analysis or archiving of the sample. Use of ultrasound, for example, to disrupt particles such as cells so as to release any intracellular contents aids in this analysis.

FIG. 11 is a schematic view of care of a particle concentrator with electrostatic particle trap and analytical module using acoustic cleaning and maintenance. Modular elements of a concentration-collection-analysis apparatus are interfaced as a fully functional aerosol concentrator/electro static sample collector/and analytical apparatus 940. Modules 941, 942, and 943 are fluidly connected and are configured to function as an aerosol-to-particle sample converter. Module 921, an aerosol concentrator module, is capable of processing particle-laden gas at intake 944 and diverting a major fraction of that gas, depleted of particles, to bulk flow exhaust 945. The "particle-enriched gas stream" 946 is then routed into module 942, an aerosol collector module. Particles may be natively charged or may acquire charge by contact with a source of ions, such sources including but not limited to a "corona wire" (942a), a source of ionizing radiation, or a radio -frequency discharge. A pair of capacitively electrified plates 942b for capturing charged aerosol particles by impaction on the plates is then used to collect the charged particles. Shown here is a duct 942c for carrying a particle concentrate 948 from the space between two such plates into the sample analysis unit 943.

Insonation applied via acoustic transducer 949 may be used to prevent or clean fouling of the aerosol concentrator. Other transducers may also be used to assist in particle transfer from the impactor surfaces to the sample analysis module. After neutralizing charge on the plates 942b, an acoustic vibration is conveyed to the plates by a suitable acoustic guide and the dislodged material is conveyed by suction aspiration or by liquid instillation and elution into the analytical module.

EXAMPLES

Example 1. Acoustic cleaning of an Aerosol Concentrator

An aerosol concentrator of the type shown in co-assigned US Pat. No. 7875095 was set up with flowing air and instrumented to monitor backpressure. ASHRAE dust was then introduced into the feed and backpressure was monitored. After a suitable interval, backpressure in the major and minor flow channels had substantially increased. The skimmer assembly was then subjected to acoustic energy using a piezoelectric horn contacted to the body of the skimmer. Backpressure immediately returned to pre-fouling levels. As shown in Table I below, backpressure in the major flow channels was seen to rise from 5.5 to 12 in H₂0 with increased stenosis due to accumulation of ASHRAE dust in the channels. Upon application of ultrasound to the body of the assembly, backpressure immediately returned to baseline. Similarly, in the minor flow channel, backpressure rose from 0.3 to 1.8 in H₂0, but returned to 0.3 in H₂0 upon application of ultrasound to the device. Inspection showed that the internal workings of the concentrator were essentially free of particle deposits following this treatment.

Table I. Acoustic Cleaning

Example 2. Acoustic Prophylaxis

In a second example, prophylactic treatment was demonstrated. Using the setup of example 1 , ASHPvAE dust was again introduced into an aerosol concentrator. A flow split of 40: 1 was used; with 10 Lpm flow rate in the chimneys and 0.25 Lpm in the collection channel. Rather than permit fouling to occur, ultrasound (33 KHz, 50W) was applied for 1 second at 2 minute intervals. Backpressure was again monitored.

After 30 minutes, no increase in backpressure was noted in any of the channels of the device. Contrastingly, backpressure had noticeably increased under control conditions without ultrasonic prophylaxis of fouling. Visual inspection confirmed that particle deposits were prevented by periodic ultrasonic treatments and the results are shown in Table II.

Table II. Acoustic Propylaxis

The reduced duty cycle (1 sec ON per 2 min intervals) reduced energy consumed in the ultrasonic treatment to less than a Watt. Low power consumption is desirable for portable applications, for example, such as where power is battery supplied or supplied by a solar cell.

Rechargeable AA-sized batteries based on lithium ion chemistry are of use. These batteries are rated at 3.6 volts and are incompatible with most AA-based devices. AA lithium batteries have a relatively low internal resistance that effectively provides very high current if shorted. RCR-V3 batteries having a nominal voltage of 3.7 V are capable of performing 3.6 Watt -hours of work (computed as 1200 mAh * 3V). AA batteries yielding 2.4 Watt-hours are also suitable. For more extended application at higher loads, cell phone batteries or combinations of 12VDC batteries may be configured in a portable battery case and will operate pulsed acoustic transducers for days or even months without recharging or replacement. This permits the bulk of the electrical capacity to be directed to the suction blowers. Example 3. Timecourse for Fouling Under Heavy Loading

Using the setup described in the examples above, the graphical data of FIG. 6 was obtained by monitoring backpressure over a thirty minute interval. Backpressure is reported as percent over baseline. Backpressure in the chimney of the untreated channel continued after ten minutes but increases are not shown because the gauge had reached its maximum reading.

Example 4. Longterm Equipment Operation

A surprising and unexpected finding from extrapolations of these results (FIGS. 7 A and 7B) is that, by the inventive application of ultrasound at periodic intervals, an aerosol concentrator of this construction can be operated for months without maintenance. Periodic pulses of ultrasound applied to the concentrator body were shown not to interfere with particle collection and analysis.

INCORPORATION BY REFERENCE

All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or cited in accompanying submissions, are incorporated herein by reference, in their entirety. When cited works are incorporated by reference, any meaning or definition of a word in the reference that conflicts with or narrows the meaning as used here shall be considered idiosyncratic to said reference and shall not supersede the meaning of the word as used in the disclosure herein.

While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents.

Claims

CLAIMS We claim:

1. An aerosol particle sampling method, which comprises:

a) flowing a gas stream containing an aerosol through a particle concentrator, said particle concentrator having a solid body with one or more internal particle accretion surfaces subject to particle accretion;

b) applying acoustic energy to at least one said internal surface, wherein said acoustic energy is applied from an acoustic transducer operatively coupled thereto;

c) acoustically dislodging any accreted particles and entraining said dislodged accreted particles in said flowing gas stream; and

d) sampling said accreted particle or particles from said flowing gas stream in a downstream particle collecting, sampling or analyzing apparatus.

2. The method of claim 1, wherein said method is performed on the fly without interrupting the flow of the gas stream.

3. The method of claim 1 and wherein said acoustic energy is applied in bursts or pulses of acoustic energy according to a fractional duty cycle.

4. The method of claim 1, wherein said acoustic transducer is a piezoelectric, magneto strictive, or electrostatic transducer operatively coupled to said solid body.

5. The method of claim 1, wherein said acoustic transducer is an ultrasonic transducer.

6. The method of claim 1 , wherein said particle accretion surface is an enclosed surface of an air-to-air aerosol particle concentrator.

7. The method of claim 1, wherein said acoustic energy is applied as a pulse or train of pulses having a fractional duty cycle.

8. The method of claim 1, wherein said acoustic transducer is battery powered.

9. The method of claim 1 , further comprising using a sensor to sense a flow rate, a flow velocity, or a backpressure of said gas stream flowing in said particle concentrator and outputting a digital or analog signal indicative of said flow rate, flow velocity, or backpressure to a control circuit.

10. The method of claim 9, comprising modulating said signal by applying acoustic energy to said at least one internal surface subject to particle accretion, thereby forming a feedback control loop.

11. The method of claim 1, further comprising acoustically interrogating said flowing gas stream to sense a flow rate, a flow velocity, or a backpressure, and generating a signal indicative of accretion of particles on said particle accretion surface.

12. The method of claim 1, wherein said particle concentrator is an aerodynamic lens, an aerodynamic lens array, a skimmer, or a virtual impactor having one or more particle accretion surfaces therein.

13. The method of claim 1, wherein said sampling step comprises an analysis of said accreted particle by

a) analyzing optical absorption of a sample at various wavelengths; b) analyzing optical scattering;

c) analyzing by mass chromatography;

d) analyzing by ion mobility spectroscopy;

e) analyzing fluorescence or autofluorescence of specific

components of said sample,

f) analyzing a Raman, SERS, LIBS, SIBS, or SPR spectrum;

g) analyzing by hybridization or nucleic acid amplification; or h) analyzing by immunoassay.

A particle sampling method, which comprises a) flowing a gas stream containing an aerosol through a particle collector, said particle collector having a solid body with one or more particle collection surfaces subject to particle accretion;

b) applying acoustic energy to at least one said internal surface, wherein said acoustic energy is applied from an acoustic transducer acoustically coupled thereto;

c) acoustically dislodging any collected particles and entraining said dislodged accreted particles in said flowing gas stream; and

d) sampling said dislodged particle or particle from said flowing gas stream in a downstream particle collecting, sampling or analyzing apparatus.

15. The method of claim 14, wherein said particle collection surface is an inertial impactor surface, a centrifugal collector surface, a bluff body surface, or an electrostatic precipitator surface.

16. The method of claim 14, wherein said step for sampling comprises an an analysis of said accreted particle by

c) analyzing by mass chromatography;

d) analyzing by ion mobility spectroscopy;

e) analyzing fluorescence or autofluorescence of specific

components of said sample;

f) analyzing a Raman, SERS, LIBS, SIBS, or SPR spectrum;

17. A self-cleaning apparatus for sampling aerosolized particles, which comprises:

a) a particle concentrator with solid body having an internal channel or channels fluidly connected to a suction pressure source for flowing a gas stream, said particle concentrator having one or more internal particle impaction surfaces subject to particle accretion; b) an acoustic transducer or transducers in acoustic contact with said solid body, said acoustic transducers each for applying energy to an internal particle accretion surface and entraining said dislodged accreted particles in said flowing gas stream; and

wherein said particle concentrator is an aerodynamic lens, an aerodynamic lens array, a skimmer, an inertial particle separator, or a virtual impactor.

18. The self cleaning apparatus of claim 17, wherein said particle impaction surface is proximate to a virtual impactor mouth of a skimmer and comprises the lips of a lateral flow channel and the inlet of a collector channel.

19. The apparatus of claim 17, further comprising a controller for actuating said acoustic transducer according to a fractional duty cycle, wherein said acoustic energy is intermittently applied on the fly without interrupting the flow of the gas stream.

20. The apparatus of claim 17, wherein said acoustic energy is applied continuously or in bursts or pulses of acoustic energy according to a fractional duty cycle.