Patent Application
20180133744
This invention relates to a condensation apparatus for use with particle counters. More particularly, the invention relates to a condensation apparatus that can increase the effective size of gas-entrained nano-particles so that they can be detected by an optical particle counter.
There is currently a great deal of concern about the health effects of nano-particles emitted unintentionally into the air. For example, the 500% increase in respiratory illness and allergies in the UK in recent years has been associated in part with particles emitted by diesel engines and other combustion processes. Whilst the main focus has been on diesel emissions, attention is turning to other potential sources such as power generation using fossil fuels, incineration, nuclear power generation and aircraft emissions. All heavy industries involving processes emitting fumes have potential problems with the emission of nano-particles. Such processes include smelting, firing, glass manufacture, welding, soldering, nuclear power generation and incineration. There is also concern amongst consumer companies that enzymes in washing powders, powder coatings and fibres used in disposable nappies and other products could cause problems. In addition, the US EPA is becoming increasingly concerned about gasoline engine emissions.
Nano-particles are known to produce toxic effects. For example, they can cross the blood-brain barrier in humans and gold nano-particles can move across the placenta from mother to foetus. Early studies with PTFE (polytetrafluoroethylene) particles around 20 nm in diameter showed that airborne concentrations of a supposedly inert insoluble material lower than 50 μg/m3 could be fatal to rats. Moreover, nano-tubes produce a more toxic response in rats than quartz dust.
In addition to concerns from a health perspective, the elimination or control of airborne nano-particles is important in maintaining standards in the many thousands of clean rooms in the micro-electronics, pharmaceutical, medical, laser, and fibre optics industries.
Small particles can be classified as shown in Table 1 below.
1 nm < D < 0.1 μm
The term “nano-particles” is used to refer to particles having an aerodynamic particle size in the range from 1 nm to 0.1 μm (100 nm).
For spherical particles, the aerodynamic particle size is the diameter of the particle. Real particles in the air often have complicated shapes. For non-spherical particles, the term “diameter” is not strictly applicable. For example, a flake or a fibre has different dimensions in different directions. Particles of identical shape can be composed of different chemical substances and have different densities. The differences in shape and density cause considerable confusion in defining particle size.
The terms “aerodynamic particle size” or “aerodynamic diameter” are therefore used in order to provide a single parameter for describing real non-spherical particles having arbitrary shapes and densities. As used herein, the term “aerodynamic diameter” is the diameter of a spherical particle having a density of 1 g/cm3 that has the same inertial property (terminal settling velocity) in the air (at standard temperature and pressure) as the particle of interest. Inertial sampling instruments such as cascade impactors enable the aerodynamic diameter to be determined. The term “aerodynamic diameter” is convenient for all particles including clusters and aggregates of any forms and density. However, it is not a true geometric size because non-spherical particles usually have a lower terminal settling velocity than spherical particles. Another convenient equivalent diameter is the diffusion diameter or thermodynamic diameter which is defined as a sphere of 1 g/cm3 density that has the same diffusivity as a particle of interest.
The investigation and monitoring of nano-particles in the atmosphere has been hampered by a shortage of instruments which can measure in the nano-particle range but which are sufficiently inexpensive, robust and convenient to be used on a widespread basis.
Some instruments for measuring nano-particles are known which make use of laser optics to detect and measure particles. However, because optical measurements cannot readily be used to detect particles in the nano-particle size range, techniques have been developed for “growing” particles to make them larger and therefore detectable and this technique forms the basis for Condensation Particle Counters. Condensation Particle Counters (CPCs) work by passing a sample of airborne particles through a chamber containing a vapourised liquid and then through a condenser where the vapourised liquid is condensed onto the airborne particles to form droplets of a size that can be measured. One example of such an instrument is disclosed in WO 02/029382 (Ahn et al). The CPC disclosed in WO 02/029382 comprises a cylindrical evaporation chamber which is lined with a porous absorbent support formed from a material such as nonwoven fabric. At one end of the chamber, the porous absorbent support is in contact with a reservoir of a volatile liquid such as isobutanol so that the liquid can travel along and soak the support by capillary action. The exterior surface of the evaporation chamber is surrounded by a heating element that heats the chamber causing isobutanol to evaporate from the support thereby to create a vapour-filled chamber. Air samples suspected of containing airborne particles are introduced into the chamber at the reservoir end and drawn through the chamber into a condenser where the condensation of the isobutanol vapour onto the airborne particles takes place to form droplets that can be measured using an optical particle counter.
An example of a commercially available CPC making use of the principles discloser) in WO 02/029382 is the Model 3025A Ultrafinc Condensation Particle Counter available from TSI Incorporated, Shoreview, Minn., U.S.A.
Another known apparatus is the handheld CPC 3007 from TSI (www.tsi.com), and the operation of this is described in more detail below in relation to
Existing Condensation Particle Counters suffer from a number of disadvantages. For example, they tend to require a high power consumption in order to heat the working fluid and have a long (10 to 20 minutes) warming up time before they can be used. These disadvantages arise at least in part because the evaporation chamber is heated by means of an external heating element and therefore the entire casing surrounding the chamber must heated before the instrument reaches the operating temperature. Furthermore, with known CPCs, there is a relatively high consumption of the working fluid (e.g. isobutanol) with the result that the working fluid must be topped up on a frequent basis, often before each use. Even in the case of the TSI US 3007 handheld condensation particle counter, the working fluid cartridge with the working fluid must be replaced on a regular basis. A further disadvantage of known CPCs is the unpleasant smell of the working fluids used (e.g. iso-butanol) and the relatively high costs.
At present, therefore, there remains a need for a Condensation Particle Counter that can be used for long periods without topping up the working fluid, which has a greatly reduced warm-up time and which lends itself to miniaturisation.
The present invention sets out to provide a condensation apparatus which, when used in a Condensation Particle Counter, can overcome or at least alleviate some or all of the problems described above in relation to known CPCs.
In a first aspect, the invention provides apparatus for increasing the size of gas-entrained particles in order to render the gas-entrained particles detectable by a particle detector, the apparatus comprising an evaporation chamber and a condenser;
In one embodiment, the evaporation chamber has at least two inlets, one of which serves to admit a sample gas containing gas-entrained particles into the evaporation chamber and another of which is connectable to a source of substantially particle-free carrier gas.
In another embodiment, the condenser has at least two inlets, one inlet being in fluid communication with the evaporation chamber and another inlet serving to admit a sample gas containing gas-entrained particles into the condenser.
In a second aspect of the invention, there is provided apparatus for increasing the size of gas-entrained particles in order to render the gas-entrained particles detectable by a particle detector, the apparatus comprising:
the apparatus being configured such that condensation of the vapour onto gas-entrained particles takes place in the condenser to increase the size of the particles so that they are capable of being detected by the particle detector;
In a further aspect, the invention provides a condensation apparatus for increasing the size of gas-entrained particles in order to render the gas-entrained particles detectable by a particle detector, the apparatus comprising:
As described above in the introductory section of this application, many particle counters, particularly those based on optical methods of particle detection, are unable efficiently to detect and count particles having a particle diameter of less than about 300 nm. The condensation apparatus of the invention enables particles of much smaller size (e.g. an aerodynamic particle diameter down to less than 3 nm) to be detected and achieves this by growing the particles by condensing onto them a vaporisable condensable substance.
The vaporisable substance can be a liquid or a vaporisable solid. Where the vaporisable substance is a solid at room temperature, it is preferably one that melts first to form a liquid and then forms a vapour from the liquid state rather than a substance that sublimes from the solid state.
Examples of solid materials that can be used as the vaporisable substance include solid hydrocarbons and long chain carboxylic acids, e.g. fatty acids such as stearic acid.
It is currently preferred, however, that the vaporisable substance is a liquid.
Liquids that may be used include water and alcohols such as propanol, isopropanol and isobutanol, or higher boiling organic liquids. As discussed above in the introduction, one of the disadvantages of known condensation particle counters is that liquid used as the vaporisable substance is consumed within a relatively short period of time and therefore fresh liquid must be added at frequent intervals. With some known condensation particle counters, it is necessary to add more liquid each time the apparatus is used.
In order to overcome the disadvantages associated with known condensation particle counters, it is preferred to use as the vaporisable substance a liquid having a boiling point at atmospheric pressure of at least 110° C.
One group of preferred vaporisable liquids consists of dimethyl phthalate, dioctyl phthalate and dimethylsulphoxide. One particularly preferred liquid is dimethyl phthalate. By using higher boiling liquids such as dimethyl phthalate, the rate of consumption of the liquid is greatly reduced and hence the liquid does not need to be topped up so frequently.
Where the vaporisable substance is a liquid and the evaporation chamber contains or is linked to a reservoir of liquid, there is a possibility that tipping the apparatus (e.g. while in transit) could cause liquid to leak into any inlets or outlets of the evaporation chamber. In order to prevent or minimise the likelihood of this occurring, the inlet(s) and outlet(s) of the evaporator chamber can be provided with a lip or rim which acts as a barrier to liquid. It will be appreciated that the height of the rim or lip will depend upon the volume of liquid carried in the reservoir. The lip or rim may be defined or provided by the end of an inlet or outlet tube extending into the evaporation chamber. By way of example, the rim or lip may be from 1 to 8 mm high, more preferably 2 to 5 mm high.
The carrier gas may be air or a pure gas or mixture of gases. For example, instead of air, the carrier gas could be nitrogen gas. The carrier gas is preferably filtered so that particles and other impurities are not carried through the evaporation chamber into the condenser. The carrier gas can be provided from a particle-free source, for example a cylinder of gas. Alternatively or additionally, a filter can be located externally of the evaporation chamber. For example, a filter can be located across the first inlet itself, or a filter can be located upstream of the first inlet, so that, in either case, carrier gas entering the evaporation chamber is free from impurities and especially particulate impurities. Examples of filters include HEPA filters and such filters are well known and do not need to be described in detail here.
In an alternative arrangement, the carrier gas can be filtered after it has entered the evaporation chamber. For example, in one embodiment, a filter can consist of or comprise the porous support for the vaporisable substance. In this embodiment, the porous support acts as a filter membrane that extends across the interior of the evaporation chamber dividing it into an upstream compartment and a downstream compartment. Carrier gas entering the upstream compartment through the first inlet may contain particulate impurities which are removed as the carrier gas passes through the porous support, whilst at the same time the vaporisable substance on the porous support is evaporated and the vapour is carried off by the filtered carrier gas. Thus, on the downstream side of the porous support, there are no particulate impurities present. It will be appreciated that by “particulate impurities” is meant particles other than those that are intended to be detected and counted.
The porous support can take various forms and be made from any of a number of different materials. For example, the porous support can be formed from a porous ceramic material or a porous fabric such as glass cloth, quartz fibre filter rockwool or a cotton fabric. The porous material should be stable at the temperatures used to vaporise the vaporisable substance and, when the vaporisable substance is a liquid, should preferably be wettable by the substance.
A temperature sensor e.g. a thermocouple) is typically disposed within the evaporation chamber for sensing the temperature inside the chamber interior. The temperature sensor is preferably in thermal contact with or in close proximity to the heating element. The temperature sensor may be arranged so that it is encircled by the heating element and/or the porous support. The temperature sensor is typically connected to a temperature control device.
The heating element can take various forms but, in each case, the heating element is disposed inside the evaporation chamber and is in close proximity to the porous support rather than surrounding the exterior of the chamber (as is the case in known commercially available condensation particle counters). A significant advantage in placing the heating element inside the evaporation chamber is that it greatly reduces the warm-up time of the instrument and the power consumption of the instrument. Thus, CPCs containing the condensation chambers of the invention can be warmed up to operating temperature in under a minute in contrast to the 10-20 minutes required for known CPCs to reach operating temperatures.
Most preferably the heating element is in direct contact with the porous support.
For example, the porous support can surround the heating element.
In one embodiment, the heating element comprises a rod (e.g. cylindrical rod) portion and the porous support surrounds the said rod (e.g. cylindrical rod) portion. For example, the porous support can comprise a sleeve that fits over the rod (e.g. cylindrical rod) portion of the heating element. Such a form of construction is particularly suitable for use when the porous support is formed from a porous fabric as hereinbefore defined.
The porous support (e.g. sleeve) may have a downwardly depending portion which, in use, extends into a reservoir of the vaporisable substance (when a liquid).
The rod portion of the heating element can have a hollow interior within which is disposed a heater wire or heater probe and optionally a thermocouple. In order to ensure good thermal contact between the heater wire or heater probe and the inner surface of the hollow rod, a thermally conductive filler may be used to hold the heater wire or heater probe and the thermocouple (when present) in place. Examples of thermally conductive fillers include solders and other low melting alloys, and thermally conducting resins such as metal particle-filled resins (e.g. epoxy resins).
Vaporising-devices incorporating heating elements of the aforesaid type are believed to be new and represent a further aspect of the invention. Accordingly, in another aspect, there is provided a vaporiser device for use in a condensation particle counter, the vaporiser device comprising:
The rod portion of the heating element and the porous support may be as defined above.
The retaining means can comprise or consist of a clip or perforated sleeve that fits over the porous support to hold it in place.
The evaporation chamber may vary in cross sectional shape and can be, for example, of circular or rectangular cross section.
The apparatus of the invention may be provided with a second inlet through which is introduced a stream of sample gas containing the gas-entrained particles to be counted. The second inlet can be disposed so that it opens into the evaporation chamber, or into an intermediate chamber between the evaporation chamber and the condenser, or into the condenser.
In one embodiment, the second inlet is arranged so that it opens into the evaporation chamber. The second inlet may have a nozzle that extends into the evaporation chamber. When the second inlet is located in the evaporation chamber, it is preferably in-line with an exit opening communicating with the condenser, e.g. so that a longitudinal axis of the inlet is aligned with a longitudinal axis of the condenser. The second inlet preferably has a cross sectional area less than the cross sectional area of the exit opening; e.g. the second inlet when circular has a diameter less than the diameter (when circular) of the exit opening. In this embodiment, without wishing to be bound by any theory, it is believed that a stream or jet of the sample gas is surrounded by a concentric layer of carrier gas and vapour as it leaves the evaporation chamber, mixing of the two concentric layers taking place as they move along the condenser.
In another embodiment, the second inlet is arranged so that it opens into the condenser. Preferably, an exit opening of the evaporation chamber is provided with a nozzle that extends into the condenser to a position level with or downstream of the second inlet. With this arrangement, without wishing to be bound by any theory, it is believed that a stream of carrier gas and vapour from the evaporation chamber is surrounded by a concentric layer of sample gas as it enters the condenser, mixing of the two concentric layers taking place as they move along the condenser.
Where the second inlet opens into the condenser, the sample gas may be partially or wholly saturated with vapour before it enters the condenser. In this embodiment, the second inlet may be connected to an ancillary evaporation chamber.
In a further embodiment, the apparatus is configured such that:
With the foregoing arrangement, without wishing to be bound by any theory, it is believed that a stream of carrier gas and vapour from the evaporation chamber is surrounded by a concentric layer of sample gas as it exits the nozzle into the upstream intermediate sub-chamber. As the two concentric layers of carrier gas/vapour and sample gas pass through the central hole in the dividing wall into the downstream intermediate chamber, they are surrounded by a further concentric layer of filtered carrier gas entering through the third inlet. Thus there is formed, temporarily, a tri-laminar stream of gas consisting of a central core of carrier gas and vapour, an intermediate layer of sample gas containing gas-entrained particles, and an outer layer of filtered carrier gas. The tri-laminar stream of gas then exits the intermediate chamber through the interior of the cylindrical baffle and into the condenser where mixing of the three layers occurs.
In each of the foregoing embodiments, as the mixture of heated carrier gas, sample gas, gas-entrained particles and vapour passes along the condenser, cooling leads to the gases within the condenser becoming supersaturated with the vapour of the vaporisable substance with the result that it condenses onto the surface of the particles. When the vaporisable substance is a liquid, droplets are formed on or around the particles, whereas when the vaporisable substance is a normally a solid at room temperature, cooling leads to the formation of beads containing or bearing the particles. In this way, the size of the particles is effectively increased from sizes as low as 3 nm to sizes up to and in excess of 1 μm. By increasing the size of the particles, they are rendered detectable by optical particle detectors such as optical particle counters.
The condenser is typically formed from a material of high thermal conductivity and is made sufficiently long to ensure that the mixture of carrier gas, sample gas, vapour and particles to be detected cools sufficiently to allow the vaporisable substance to condense onto the particles to grow the particles to a detectable size (e.g. 1 μm or greater). The condenser can therefore take the form of a tube formed from a metal material such as aluminium or stainless steel. The thickness of the walls of the condenser and other parts of the apparatus can be as thin as practical, but, when there is an intention to use the apparatus under elevated or reduced pressure (e.g. at 10 bar), the wall should be sufficiently thick to withstand such pressures.
In order to assist cooling of the mixture of gases, vapour and particles in the condenser, cooling means may be provided.
In one embodiment, the cooling means comprises one or more fans each directing a flow of air onto the external surface of the condenser. In one embodiment there is one fan. In another embodiment there are two fans.
The fan(s) may or may not be part of an air temperature controlling system. The temperature controlling system enables the air cooling the condenser surface to be maintained at a pre-set temperature which is not influenced by the temperature of the air surrounding the condensation chamber.
Alternatively, a cooling element may be located in contact with the external surface of the condenser. The cooling element can be, for example, a thermoelectric cooling device (e.g. a Peltier cooling device).
In order to facilitate improved cooling, the cross-section of the condenser may be formed in such a way as to enhance the ratio of the circumference to cross-section area.
The condenser preferably has a surface area to volume ratio which is greater than the surface area to volume ratio of a cylinder. By increasing the surface area to volume ratio, the condenser can be made more efficient resulting in more rapid cooling and thereby enabling the size of the condenser to be reduced.
A condenser can be defined as having a length (a dimension corresponding to the distance between the inlet and outlet of the condenser), a width (a dimension orthogonal to the length) and a height (a dimension orthogonal to the length and height). In the case of a tubular condenser of circular cross section, the width and the height are the same and both correspond to the diameter of the tube. In the case of a rectangular condenser of square cross section, the width and height are also the same. However, in this application, where the width and height of a condenser are not the same, the reference to “height” means the lesser of the two dimensions.
The surface area to volume ratio of a condenser can be increased in a number of ways. For example, at least part of the condenser may have a portion of flattened cross section or may be have an elongate oval or rectangular shape in cross section, i.e. a cross section in which the height is substantially less than the width of the condenser. In one preferred embodiment, the condenser is substantially rectangular in cross section wherein the height is less than half the width.
In another embodiment, the condenser may comprise an annular or part annular condenser body. An annular condenser body may be formed from two concentric cylinders with the hot vapour laden gas being directed through the annular space between the inner and outer cylinders and cooling air being directed through the interior of the inner cylinder as well as around or against the outer surface of the outer cylinder.
In order to enable particle sizes to be measured accurately, it is important to ensure that the residence time of each particle in the condenser is substantially the same. This means that the flow velocities and flow paths of the particles through the condenser should ideally be as uniform as possible.
Where a non-cylindrical condenser (e.g. a rectangular condenser) is connected to in-line cylindrical inlets and outlets, there exists the possibility of non-uniform flow between the inlet and outlet, particularly in cases where the width (as defined herein) of the condenser is greater than the diameters of the inlet and outlet. In order to overcome this potential problem, a condenser (e.g. a substantially rectangular condenser) may be provided with a pair of flow distributor tubes which are aligned substantially at right angles with respect to the length (direction of flow) of the condenser. The flow distributor tubes are connected to the inlet and outlet of the condenser and each extend across the width of the condenser and are provided with elongate slots or arrays of holes which open into the interior of the condenser.
Accordingly, in another aspect of the invention, there is provided a condenser for use with an apparatus of the invention as defined herein, the condenser comprising:
The inlet flow distributor tube in use is attached or otherwise in fluid communication with the outlet of the evaporation chamber whereas the outlet flow distributor tube is attached or otherwise in fluid communication with the particle detector.
The configuration of the flow distributor tubes, and the positioning of the slots or holes, is such as to provide a substantially uniform flow of gas through the condenser body to the particle detector.
The flow distributor tubes may be, for example, of circular cross section, oval cross section or polygonal (regular or irregular) cross section. In one embodiment, the flow distributor tubes are of circular cross section.
The internal cross sectional area of each flow distributor tube is preferably greater than the internal cross sectional area (internal width×internal height) of the condenser body. As an example, if the cross-section of the flow distributors is a circle of internal diameter Dt and the condenser internal height is Hc and the internal width We then πDt2>Hc*Wc. The ratio of πDt2/(Hc*Wc) should typically be more than 1.1 or preferably more than 2 or even more preferably the ratio should be more than 3.
As indicated above, fluid communication between the interiors of the flow distributors 102 and 104 and the interior of the rectangular condenser may be achieved by providing the walls of the flow distributor tubes with elongate narrow slots or an array (preferably linear) of holes that open into the hollow interior of the condenser body. Preferably fluid communication between the flow distributor tubes and the interior of the condenser body is provided by means of a narrow slot in the wall of each flow distributor tube. By making the slots narrow, the internal diameters of the flow distributor tubes can be reduced because the uniformity of the flow in the condenser is governed by the ratio of the internal height (Hc) of the condenser to the width (Ws) of the slot. In the present context, the width of the slot means the dimension which is in the same direction as the internal height of the condenser (in contradistinction to the “length” of the slot where the reference to “length” the dimension which is in the same direction as the internal width of the condenser). Typically, the ratio of Hc/Wc should be more than 1.1 or preferably more than 2 or even more preferably the ratio should be more than 3.
In another embodiment, the walls of the flow distributor tubes contain holes evenly distributed along the inlet and outlet of the rectangular condenser 103 instead of slots. The number of holes Nh should be more than 1 or preferably more than 4 or even more preferably more than 10. The diameter of the holes Dh should be sufficiently small and can be evaluated from the expression: πDt2>Nh*πDh2. The ratio of πDt2/(Nh*πDh2) should be more than 1.1 or preferably more than 2 or even more preferably the ratio should be more than 3.
The flow distributor tubes can be made from any of a variety of materials. For example, they can be made from stainless steel tube, PTFE, aluminium, or any suitable metal, glass, ceramic or plastics material. The condenser body is preferably made from a heat conducting material such as a metal, e.g. a steel such as a stainless steel.
One potential problem with the condensers, especially when they have a small internal cross sectional area or width (e.g. less than 2 mm), is that condensation on the walls of the condenser can lead to blockage. In order to overcome this problem, means may be provided for removing condensed substance from the interior walls of the condenser. For example, the condenser may have one or more drainage ducts extending along all or part of its length, the drainage ducts being separated from the interior of the condenser by a semi-permeable wall or membrane through which the liquid condensate can pass, the drainage ducts having one or more outlets connectable to a pump to extract liquid condensate from the ducts. The semi-permeable membranes are constantly filled with the working fluid and therefore the gas flow cannot penetrate through them. By means of such an arrangement, when the vaporisable substance condenses on the inner wall of the condenser, rather than accumulating in and blocking the condenser, it is extracted through the semi-permeable wall into the drainage ducts and away from the condenser interior. Once extracted, the condensate can either be sent to a waste storage compartment for later disposal or recycled back to the evaporation chamber.
The ducts can be formed by partitioning the interior of the condenser over at least part of its length by means of one or more longitudinally extending semi-permeable walls or membranes. The semi-permeable walls or membranes may be provided with capillaries that draw condensate from the interior of the condenser into the drainage ducts. For example, the walls or membranes can be formed from a ceramic or stainless steel filter material having a capillary size of <0.5 mm, e.g. 1-10 μm.
The condensation apparatus of the invention is designed to be connected to a particle detector, typically a particle detector capable of single particle detection and/or single particle counting.
More typically, the condensation apparatus of the invention is designed to form part of a Condensation Particle Counter and, for this purpose, can be connected to a particle counter which can be, for example, a Naneum ‘SAC 1’ particle counter available from Naneum Limited of Canterbury, United Kingdom.
Accordingly, in another aspect, the invention provides a condensation particle counter comprising a condensation apparatus of the invention as defined herein.
In another aspect, the invention provides a method of detecting and counting nano-particles using a condensation particle counter comprising a condensation apparatus of the invention as defined herein.
Further aspects and features of the invention will be apparent from the specific embodiments described below and illustrated in
The invention will now be illustrated in greater detail by reference to the specific embodiments described in the following non-limiting examples.
A stream of air containing small gas-entrained particles (e.g. airborne particles) is drawn into the chamber 1a via inlet 4a by means of a pump (not shown). As it passes through the evaporation chamber, the stream of air is heated and saturated with vapour formed by evaporation of the working fluid. The vapour-saturated stream of air then passes into the condenser where cooling of the air and condensation of the working fluid onto airborne particles takes place. As a result, the particles grow by condensation up to a readily detectable size of about 1 μm.
The enlarged particles pass out through the outlet 5a, and are directed to an optical particle counter where they are counted.
The condensation apparatus illustrated in
One major disadvantage is that the working fluid must be replaced on a regular and frequent basis.
A further disadvantage is that the apparatus is very slow to warm up to an operating state. In the case of the TSI CPC 3007 described above, the apparatus has a 600 second warming up time before it can be used.
Another disadvantage is that the layout of the apparatus does not readily lend itself to miniaturisation. Reducing the size of the apparatus would necessitate using a smaller working fluid cartridge which would therefore need to be refilled more frequently. Thus, miniaturisation would lead to a reduction in the period of time over which the apparatus could be used without refilling.
A further disadvantage is that the abovementioned TSI CPC 3007 instrument cannot be used in an environment of elevated pressure and, according to its product specification, the instrument will only operate when held horizontally.
The relatively rapid depletion of working fluid in the cartridge in the CPC shown in
The condensation apparatus of the invention overcomes or at least alleviates the problems identified above with known CPCs.
A condensation apparatus according to a first embodiment of the invention is shown in
In use, a stream of clean carrier gas (e.g. air) which has been filtered through a filter (not shown) and which contains no (or negligible quantities of) detectable aerosol particles enters through inlet 1 into the evaporation chamber 2. In the evaporation chamber, heating element 3 is positioned to be in a good thermal contact with temperature sensor 4 and a predetermined temperature that is sufficient to evaporate working fluid and to generate conditions necessary to sustain condensation of the working fluid on particles of interest is controlled by temperature control device 5 which is linked to the temperature sensor. The working fluid is contained in a porous support that takes the form of a cover 6 that is placed on the heating element 3. In the case of a cylindrical heating element, the cover will be wrapped around its surface and soaked with the working fluid. As a result, the stream of clean air introduced via the inlet 1 is saturated with the vapour of the working fluid and moved towards the condenser/outlet 7. A gas (e.g. air) sample containing nano-particles of interest is introduced into the chamber through the second inlet 8. In the zone between the inlet 8 and the opening into the condenser 7, supersaturation of the working fluid vapours arises due to mixing of the hot saturated vapour and the unheated stream of sample gas containing the nano-particles of interest. Therefore, in this zone, heterogeneous nucleation of the working fluid on the particles of interest begins to occur. When the mixture of vapour and sample gas containing the nano-particles enters the condenser 7, additional supersaturation occurs due to cooling of the gases and vapour by the walls of the condenser/outlet. The excess of heat is removed from the surface of the condenser/outlet 7 by means of a cooling system 10. In
As an example of the apparatus of shown in
The temperature was controlled by a Digitron Temperature controller 5. It was found that the condensation apparatus thus constructed was able to enlarge nano-particles up to 1 to 2 μm diameter. The condensation apparatus was used over a period of at least 2 months without refilling.
Another embodiment of the present invention is shown in
The mode of action of this embodiment is the same as for the embodiment of
A further embodiment of the invention is illustrated in
The shape of the condenser/outlet 7 can affect its performance. In the embodiment shown in
The second inlet 8 can also have a rectangular or elongated oval cross sectional shape. In practice, the ratio of the height to the width of the rectangle can be from 1 to 100.
In each of the embodiments shown in
The cross sectional shape of the main body of the condensation chamber 2 can also be rectangular as can the cross sectional shape of the heating element 3. The heating element 3 is typically positioned and orientated so as to optimise the efficiency with which the stream of carrier gas entering the first inlet 1 is saturated with the working fluid vapour.
If the width of the rectangular cross section condenser shown in
It is advantageous to control the temperature of the liquid in the working fluid containers 16 to stabilise the temperature of the condenser. This can be done using an external cooling element (e.g. thermoelectric cooling element) as shown in
The chambers 16 each have an additional outlet 18 and the two outlets are connected by a length of tubing. The inlets/outlets 17 are connected via lengths of tubing to a pump 19 and a temperature controller 20. Together, the inlets/outlets 17 and 18, the connecting tubing, the pump 19 and the temperature controller 20 form a circuit around which the working fluid can be pumped. The working fluid flowing around the circuit and through the fluid collection chambers 16 can be maintained at a constant pre-determined temperature by the temperature controller 20 and, in this way, the temperature of the internal surface of the condenser can be controlled.
The circuit is provided with a valve (not shown) that enables a portion of the working fluid to be directed along tube 21 to a reservoir 12 of working fluid in the main body of the evaporation chamber 2 by means of an additional pump (not shown) or other liquid transporting means.
The tubing connections shown in
In addition, it should be appreciated that the left and the right fluid collection chambers 16 can be maintained at different temperatures. This generates extra supersaturation of the vapours of the working fluid in the condenser and enables the rate of growth of the droplets and the droplet size to be increased or decreased as required. In this case each chamber has its own temperature controlling cycle. The temperatures of liquid in the containers can be found experimentally by means of trial and error or calculated according to nucleation theory.
Keeping two fluid collection chambers 16 at different temperatures has another important advantage. When supersaturation in the condenser is sufficiently high, nano-particles of different sizes can form droplets in different locations along their travel through the condenser and, therefore, droplets formed onto nano-particles of different sizes will grow to various sizes. For instance, 50 nm particles will produce 0.5 μM droplets but 100 nm particles will generate 1 μm droplets. This enables the size of the nano-particles to be obtained from the size of the droplets, a facility which can form the basis for methods of characterising aerosol size distributions.
It should be also appreciated that the temperature of the internal surface of the condenser can be non-uniform, for instance, it can linearly decrease with the length of the condenser. This gradually increases supersaturation of the working fluid vapour along the length of the condenser and, therefore, increases the ability of the apparatus to grow nano-particles of different sizes up to droplets of different sizes. Larger nano-particles tend to form droplets earlier (at the beginning of the condenser) whereas smaller particles that require greater supersaturation tend to form droplets only later at the end of the condenser and consequently the smaller particles have less time to grow and therefore grow to smaller droplet sizes in comparison with larger nano-particles. This makes it possible to establish a one-to-one relationship between the size of droplets formed in the condensation chamber and the size of nano-particles. This relationship can be utilised to evaluate the size of nano-particles by analysing the size of the droplets.
Supersaturation in the apparatus of the invention is controlled by the temperature of the walls, the dimensions of the component parts of the apparatus and the flow rates of the carrier gas and sample gas streams through the apparatus. Variation of these parameters enables a skilled person to select the supersaturation conditions. There is a well-known link between the supersaturation and the minimal size of nano-particles that can form droplets. Therefore, it is possible to change the lower detection limit of a condensation apparatus by changing one or several of these parameters, e.g. the temperature of a heating element 3. This is a powerful tool iii determining size distributions of nano-particles and the proportion of nano-particles in various size ranges. It also enables the development of a condensation apparatus with a predetermined lower detection limit, e.g. 100 nm, 30 nm, 10 nm or 3 nm or with a variable lower detection limit. This provides a platform for an aerosol particle sizing in order to obtain nano-particle size distributions.
It also should be appreciated that a plurality of condensation apparatuses of the invention set up to give different supersaturation conditions can be connected to each other sequentially or in parallel. The sequential arrangement enables nano-particles of different sizes to grow up to different size droplets. If the first condensation chamber is set at lower supersaturation than the second then larger particles form droplets in the first chamber but smaller particles form droplets only in the second chamber whereas previously formed droplets are grown further and become distinctly larger in size. The same is true for the second and the third chambers. Thus, a plurality of chambers enables a plurality of droplet sizes to be formed. This allows the size distribution of nano-particles to be retrieved by analysing the size distribution of droplets, e.g. by using an optical particle counter.
In the case of a parallel arrangement of condensation chambers, the stream of nano-particles of interest is divided into several parallel flows and the said flows are directed to different chambers. The chambers should be set to different values of supersaturation so as to have different lower size detection limits. This makes it possible to retrieve nano-particle number size distributions by analysing the numbers of droplets grown in these chambers.
It will also be appreciated that it is possible to vary temperatures and other parameters of the condensation apparatus and therefore vary the supersaturation as well as the lower detection limit during a given measuring cycle. This enables a cumulative particle size distribution to be obtained.
An apparatus according to a further embodiment of the invention is illustrated schematically in
The embodiment of
In order to reduce particle losses, both the nozzle 22 and the condenser 7 have cylindrical symmetry and the nozzle 22 is positioned along the axis of the condenser 7 in such a way that the end of the nozzle extends downstream beyond the second inlet 8. This enables the cooler sample gas stream to be formed around the vapour-containing carrier gas stream.
An advantage of the condensation apparatus of the invention is that it provides reliable data and can be miniaturised to dimensions much smaller than those of known condensation counters
A preferred working fluid in each of the embodiments of the invention is the semi-volatile dimethyl phthalate. A major advantage of using a semi-volatile compound is that it leads to a much lower consumption of working fluid. An apparatus of the invention has been found to work without requiring refilling for more than 10 months.
The choice of flow rates, the temperature of the saturating chamber and the manner in which airborne particles of interest are introduced into the chamber will usually be made according to the nature of the particles and their concentration. The total flow out of the condenser outlet 11 is often in the range from 0.1 to 4 l/min. The clean carrier gas flow accounts for 10 to 90% of the total flow. For dimethyl phthalate, the temperature of the saturating chamber usually is in the range from 80 to 150° C.
In order to reduce the power consumption of the heating element, a thin film heater can be used which has attached to it a porous medium which is wettable by the working fluid. It is advantageous for a part of the porous medium to be long enough to be in contact with working fluid at the bottom of the chamber 2.
The evaporation chamber 2 and condenser 7 may be manufactured from a variety of materials including any metal, glass or ceramic or (in the case of the evaporation chamber) plastics such as PTFE, but it is preferred to use materials or surface treatments that are inert or resistant to oxidation in air or other carrier gases and which do not react chemically with the working fluid. Pyrex glass, quartz, ceramic and stainless steel were used for various modifications of the chambers and their elements.
It will also be appreciated that the sample gas stream containing particles of interest can be introduced through inlet 1 and the clean air via inlet 8. This is preferable for temperature stable particles such as metal particles. However, aerosol particles formed from organic compounds can be affected the high temperatures generated by the heating element and should therefore be introduced via inlet 8.
An apparatus according to another embodiment of the invention is illustrated in
In this embodiment, a sample of aerosol of interest is directed into inlet 8 and a stream of clean air is introduced through inlet 23. The mixing chamber enables the stream of the aerosol sample of interest to be sandwiched between a central core stream of carrier gas containing working fluid vapour and a outer layer formed by the clean air from inlet 23. Using an apparatus of this type, it is found that the best results are obtained when the gas layers in the sandwich are cylindrically symmetrical.
It is advantageous to prolong the working life of the apparatus without the need for frequent refilling with working fluid. A significantly longer operating life between refills can be achieved by means of a combination of two condensation units with means for collecting and recycling working fluid from droplets that have passed though the particle detector and which contain airborne particles. Such an assembly can comprise two condensing units and a system of aerosol flow manipulation with three way valves to redirect the flows. The condensation apparatus used with such an assembly is slightly different from the other specific embodiments described above and an example of a suitable condensation apparatus is shown in
It should be understood from the above that the porous medium 26 should be positioned in such a way to form an airtight seal with the walls of the evaporation chamber so that all of the gas received through inlet 1 is filtered and all of the airborne particles are trapped. Droplets of working fluid collected on the porous support can be re-evaporated and released as vapour into the evaporation chamber on the downstream side of the porous support
An assembly comprising two condensation apparatuses of the aforementioned type consumes negligible or no working fluid and therefore does not need to be refilled. Such an assembly is illustrated in
The assembly shown in
The assembly shown in
Airflow containing nano-particles of interest is drawn via common inlet 35 into one condensation apparatus (e.g. apparatus 28) by appropriate adjustment of the three-way valve 30. Micro-droplets formed on the nano-particles in the apparatus are directed towards optical particle counter 29 by means of three-way valve 31 while valve 34 is closed. After being counted, micro-droplets are directed to the other apparatus 27 by three-way valve 32. In apparatus 27, micro-droplets are collected onto the porous medium and filtered clean air is released into the atmosphere through outlet 35 by opening valve 33. The evaporation chamber in apparatus 27 remains cold because there no voltage is applied to the heating element, and therefore the working fluid collected on the porous medium is stored.
After an appropriate period of time, the apparatus 27 is heated, the valve positions are adjusted and apparatus 28 is allowed to cool so that it is able to collect droplets of working fluid. The previously idle apparatus 27 is then in working mode and the working fluid previously captured by the porous medium is heated and evaporated to form a vapour which is then mixed with a gas sample stream as described above.
After passing through the condenser and particle counter, the stream of particle- and vapour-laden air is directed to apparatus 28 where the air is filtered and the working fluid collected as described above for apparatus 27. The cycle is then repeated.
The time necessary to switch between the chambers can be determined empirically through trial and error. Normally it is only necessary to switch the valves after hundreds of hours of operations. Therefore, the system requires relatively little energy to operate and energy and can be easily implemented.
If necessary, additional specialised gas filters can be attached to outlets 35 and 36 to trap working fluid vapour remaining in the gas stream after filtration by the porous support. However, by using a semi-volatile working fluid such as dimethyl phthalate, for the majority of applications there is no need to use additional filters because the vapour pressure of the semi-volatile compound is very low.
In each of the foregoing embodiments, the condensation chamber can be equipped with a working fluid sensor, e.g. filled glass capillary or dew point type sensors (not shown in the Figures). A sensor placed e.g. inside the chamber enables the depletion of the working fluid to be monitored.
By using a condenser of rectangular cross-section, the size of the condensation chamber can be reduced considerably. However, a potential problem with some rectangular condenser layouts, particularly where the inlet and outlet of the condenser are tubes of circular cross section, is that there may be non-uniformity of the flow velocity in the condenser. This can lead to some particles spending more time in the condenser than others meaning that there is non-uniform growth of the particles in the condenser. This in turn can give rise to inaccuracies in measurement of the numbers and sizes of the particles.
Thus, as shown in
The uniformity of the residence time is achieved by designing the flow distributors such that the internal area of the cross-sections of the flow distributors are sufficiently greater than the internal area of the cross-section of the condenser. As an example, if the cross-section of the flow distributors is a circle of the internal diameter Dt and the condenser internal height is Hc and the internal width Wc then πDt2>Hc*Wc. The ratio of πDt2/(Hc*Wc) should be more than 1.1 or preferably more than 2 or even more preferably the ratio should be more than 3.
Fluid communication between the interiors of the flow distributors 102 and 104 and the interior of the rectangular condenser can be achieved by providing the walls of the flow distributors with elongate narrow slots or a linear array of holes that open into the condenser. It is advantageous to provide a narrow slot between the flow distributors 102 and 104 and the condenser 103. In
In another embodiment, the flow distributor contains holes evenly distributed along the inlet and outlet of the rectangular condenser 103 instead of two slots. The number of holes Nh should be more than 1 or preferably more than 4 or even more preferably more than 10. The diameter of the holes Dh should be sufficiently small and can be evaluated from the expression: πDt2>Nh*πDh2. The ratio of πDt2/(Nh*πDh2) should be more than 1.1 or preferably more than 2 or even more preferably the ratio should be more than 3.
In one embodiment of the condenser arrangement shown in
It will also be appreciated that the shape of the cross-section of the distributor may be rectangular, triangular, ellipsoidal, polygonal or any combination of simple geometric shapes.
The evaporation chamber shown in
A heating element is mounted in one side of the PTFE body 200. The heating element has a mounting portion 206 removably secured in the wall of the PTFE body, and a rod portion 210 which extends into the chamber interior 216. A holder 204 holds the mounting portion 206 in place and an O-ring provides a seal between the mounting portion 206 and the wall of the chamber body. Another O-ring provides a seal between the mounting portion 206 and the rod 210.
The rod portion 212 has a hollow interior within which are disposed a metal heating wire 222 and a thermocouple 224 (see
The heating wire and thermocouple are connected to a controller (not shown).
Beneath the heating element, the body of the evaporation chamber has a well 214/218 for holding a working liquid such as dimethyl lphthalate. The well 214/218 is connected via tube 218 (see
As shown in
In this embodiment, the heating element is in direct contact with the porous support thereby reducing the heat input required and time taken to evaporate the working fluid to form a saturated vapour within the chamber.
Several examples of apparatus according to this invention have been built and tested and these are described below.
In one example constructed as shown in
A comparison was made between aerosol particle number concentrations (N) measured using an apparatus as illustrated in
The results are shown in
It was found that the apparatus of the invention enables nano-particles to be enlarged up to 1.2 μm in diameter. The lower detection limit for the apparatus of the invention was estimated to be 3 nm. It is clear from
It will readily be apparent that numerous modifications and alterations may be made to the specific embodiments of the invention described above without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0808385.9 | May 2008 | GB | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14485221 | Sep 2014 | US |
| Child | 15867672 | US | |
| Parent | 12991535 | Nov 2010 | US |
| Child | 14485221 | US |
