U.S. patent application number 14/485221 was filed with the patent office on 2015-01-01 for condensation apparatus.
This patent application is currently assigned to PARTICLE MEASURING SYSTEMS INC.. The applicant listed for this patent is PARTICLE MEASURING SYSTEMS INC.. Invention is credited to Harald Wilhelm Julius GNEWUCH, Boris Zachar GORBUNOV.
Application Number | 20150000595 14/485221 |
Document ID | / |
Family ID | 39571015 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150000595 |
Kind Code |
A1 |
GORBUNOV; Boris Zachar ; et
al. |
January 1, 2015 |
CONDENSATION APPARATUS
Abstract
The invention provides an 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 (2) and a condenser (7); the
apparatus is configured so that vapour-laden gas from the
evaporation chamber can flow into the condenser and condensation of
the vaporisable substance onto gas-entrained particles in the
condenser takes place to increase the size of the particles so that
they are capable of being detected by a particle detector.
Inventors: |
GORBUNOV; Boris Zachar;
(Canterbury, GB) ; GNEWUCH; Harald Wilhelm Julius;
(Canterbury, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PARTICLE MEASURING SYSTEMS INC. |
Boulder |
CO |
US |
|
|
Assignee: |
PARTICLE MEASURING SYSTEMS
INC.
Boulder
CO
|
Family ID: |
39571015 |
Appl. No.: |
14/485221 |
Filed: |
September 12, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12991535 |
Nov 8, 2010 |
8869593 |
|
|
PCT/GB2009/001147 |
May 8, 2009 |
|
|
|
14485221 |
|
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Current U.S.
Class: |
118/716 ;
137/334 |
Current CPC
Class: |
B01D 5/0027 20130101;
G01N 15/02 20130101; G01N 33/0011 20130101; B05C 3/005 20130101;
Y10T 137/6416 20150401; B05C 3/02 20130101; Y10T 137/8593 20150401;
G01N 2015/0046 20130101; G01N 15/065 20130101 |
Class at
Publication: |
118/716 ;
137/334 |
International
Class: |
B05C 3/00 20060101
B05C003/00; B05C 3/02 20060101 B05C003/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 2008 |
GB |
0808385.9 |
Claims
1. An 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; a condenser in fluid communication with the evaporation
chamber and having an outlet for connection to a particle detector;
a heating element and a porous support each of which is disposed
within the evaporation chamber, the porous support carrying thereon
a vaporisable substance and the heating element being heatable to
vaporise the vaporisable substance to form vapour within the
evaporation chamber; wherein the heating element is in direct
contact with the porous support; a first inlet for admitting a
stream of carrier gas into the evaporation chamber to carry vapour
through to the condenser; a second inlet which is downstream of the
porous support and through which a stream of sample gas containing
gas-entrained particles can be introduced; the apparatus being
configured so that condensation of the vaporisable substance onto
the gas-entrained particles in the sample gas takes place in the
condenser to increase the size of the particles so that they are
capable of being detected by a particle detector.
2. An apparatus according to claim 1 wherein the porous support
surrounds the heating element.
3. An apparatus according to claim 1 wherein a temperature sensor
is disposed within the evaporation chamber.
4. An apparatus according to claim 1 wherein the heating element
comprises a rod portion and the porous support surrounds the said
rod portion.
5. An apparatus according to claim 4 wherein the porous support is
formed from a porous fabric and comprises a sleeve that fits over
the rod portion of the heating element.
6. An apparatus according to claim 5 wherein the rod portion of the
heating element has a hollow interior within which is disposed a
heater wire or heater probe and optionally a thermocouple.
7. An apparatus according to claim 6 wherein a thermally conductive
filler is used to hold the heater wire or heater probe and the
thermocouple (when present) in place.
8. An apparatus according to claim 1 wherein the vaporisable
substance is selected from dimethyl phthalate, dioctyl phthalate
and dimethylsulphoxide.
9. A condenser configured to be used with an apparatus for
increasing the size of gas-entrained particles in order to render
the gas-entrained particles detectable by a particle detector; the
condenser being connectable to said apparatus such that it is in
fluid communication with an outlet of an evaporation chamber of the
apparatus; and the condenser having an outlet for connection to the
particle detector; wherein the condenser has a surface area to
volume ratio which is greater than the surface area to volume ratio
of a cylinder; and the condenser comprises: a condenser body having
an inlet, an outlet and a hollow interior which has an internal
length, an internal width and an internal height; an inlet flow
distributor tube connected to the inlet of the condenser body and
extending across the internal width of the condenser body; and an
outlet flow distributor tube connected to the outlet of the
condenser body and extending across the internal width of the
condenser body; the internal height of the condenser body being
less than a corresponding internal height of each of the inlet and
outlet flow distributor tubes; the inlet and outlet flow
distributor tubes each being provided in the walls thereof with one
or more slots or holes communicating with the hollow interior of
the condenser body so as to provide a flow path from the inlet flow
distributor tube through the hollow interior of the condenser and
into the outlet flow distributor tube.
10. A condenser according to claim 9 wherein the internal cross
sectional area of each flow distributor tube is greater than the
internal cross sectional area (internal width.times.internal
height) of the condenser body.
11. A condenser according to claim 10 wherein the ratio of the
internal cross sectional area of each flow distributor tube to the
internal cross sectional area (internal width.times.internal
height) of the condenser body is greater than 1.1
12. A condenser according to claim 12 wherein the said ratio is
greater than 2.
13. A condenser according to claim 9 wherein the walls of the flow
distributor tubes with elongate narrow slots that open into the
hollow interior of the condenser body and the ratio of the internal
height of the container body to the widths of the slots is greater
than 1.1.
14. A condenser according to claim 9 wherein and the ratio of the
internal height of the container body to the widths of the slots is
more than 2.
15. A condenser according to claim 14 wherein and the ratio of the
internal height of the container body to the widths of the slots is
greater than 3.
16. A condenser configured to be used with an apparatus for
increasing the size of gas-entrained particles in order to render
the gas-entrained particles detectable by a particle detector; the
condenser in use being in fluid communication with an outlet of an
evaporation chamber of the apparatus; the condenser having an
outlet for connection to the particle detector; wherein the
condenser is provided with means for removing condensed substance
from the interior walls of the condenser; the said means for
removing condensed substance comprising one or more drainage ducts
extending along all of part of its length, the drainage ducts being
separated from the interior of the condenser by a permeable wall or
membrane through which liquid condensate can pass, the drainage
ducts having one or more outlets connectable to a pump to extract
liquid condensate from the ducts.
17. A condenser according to claim 16 wherein the ducts are formed
by partitioning the interior of the condenser over at least part of
its length by means of one or more longitudinally extending
permeable walls or membranes.
18. A condenser according to claim 17 wherein the permeable walls
or membranes are provided with capillaries that draw condensate
from the interior of the condenser into the drainage ducts.
19. A condenser according to claim 18 wherein the walls or
membranes are formed from (a) a ceramic or stainless steel filter
material having a capillary size of <0.1 mm; or (b) a porous
material.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/991,535 (published as U.S. 2011-0056273 A1 on Mar. 10,
2011), which is a U.S. National Phase filing under .sctn.371 of
International Application No. PCT/GB2009/001147, filed May 8, 2009,
which claims priority to GB Application No. 0808385.9, filed May 8,
2008. The entire contents of each of the prior applications are
hereby incorporated herein by reference.
[0002] 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.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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 .mu.g/m.sup.3
could be fatal to rats. Moreover, nano-tubes produce a more toxic
response in rats than quartz dust.
[0005] 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.
[0006] Small particles can be classified as shown in Table 1
below.
TABLE-US-00001 TABLE 1 Term Aerodynamic Particle Size Range Dust D
> 10 .mu.m Coarse particles 2.5 .mu.m < D < 10 .mu.m Fine
particles 0.1 .mu.m < D < 2.5 .mu.m Nano-particles or
ultrafine particles .sup. 1 nm < D < 0.1 .mu.m
[0007] The term "nano-particles" is used to refer to particles
having an aerodynamic particle size in the range from 1 nm to 0.1
.mu.m (100 nm).
[0008] 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.
[0009] 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/cm.sup.3 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/cm.sup.3 density that has the same diffusivity as a
particle of interest.
[0010] 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.
[0011] 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.
[0012] An example of a commercially available CPC making use of the
principles disclosed in WO 02/029382 is the Model 3025A Ultrafine
Condensation Particle Counter available from TSI Incorporated,
Shoreview, Minn., U.S.A.
[0013] 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 FIG. 1.
[0014] 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.
[0015] 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.
SUMMARY OF THE INVENTION
[0016] 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.
[0017] 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; [0018]
the evaporation chamber having an inlet for admitting gas into the
evaporation chamber and an outlet through which vapour-laden gas
may leave the evaporation chamber; [0019] the evaporation chamber
having disposed therein a heating element and a porous support, the
heating element being in direct contact with the porous support,
wherein the porous support carries thereon a vaporisable substance
and the heating element is heatable to vaporise the vaporisable
substance to form vapour within the evaporation chamber; [0020] the
condenser being in fluid communication with the outlet of the
evaporation chamber, and the condenser having an outlet for
connection to the particle detector. [0021] the apparatus being
configured so that vapour-laden gas from the evaporation chamber
can flow into the condenser and condensation of the vaporisable
substance onto gas-entrained particles in the condenser takes place
to increase the size of the particles so that they are capable of
being detected by a single particle detector.
[0022] 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.
[0023] 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.
[0024] 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: [0025] a source of a
vaporisable substance; [0026] heating means to bring about
evaporation of the vaporisable substance to form vapour; [0027] an
inlet for admitting a sample gas containing gas-entrained
particles; [0028] a condenser, the condenser being provided with an
outlet for connection to the particle detector; 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; [0029] characterised in that the vaporisable
substance is selected from dimethyl phthalate, dioctyl phthalate
and dimethylsulphoxide.
[0030] 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: [0031] an evaporation
chamber; [0032] a condenser in fluid communication with the
evaporation chamber and having an outlet for connection to a
particle detector; [0033] a heating element and a porous support
each disposed within the evaporation chamber, the porous support
carrying thereon a vaporisable substance and the heating element
being heatable to vaporise the vaporisable substance to form vapour
within the evaporation chamber; [0034] a first inlet for admitting
a stream of carrier gas into the evaporation chamber to carry
vapour through to the condenser; [0035] a second inlet which is
downstream of the porous support and through which a stream of
sample gas containing gas-entrained particle can be introduced;
[0036] the apparatus being configured so that condensation of the
vaporisable substance onto the gas-entrained particles in the
sample gas takes place in the condenser to increase the size of the
particles so that they are capable of being detected by a particle
detector.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] It is currently preferred, however, that the vaporisable
substance is a liquid.
[0041] 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.
[0042] 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.degree. C.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] Most preferably the heating element is in direct contact
with the porous support.
[0051] For example, the porous support can surround the heating
element.
[0052] 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.
[0053] 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).
[0054] 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).
[0055] 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: [0056] a heating element
comprising a mounting portion for installing in a wall of an
evaporation chamber in the condensation particle counter, and a rod
portion; the rod portion being arranged to extend inwardly into the
evaporation chamber in use; and [0057] a porous support which
surrounds and is in contact with the rod portion, the porous
support carrying or being capable of carrying a vaporisable
substance; [0058] and optionally retaining means for holding the
porous support in place on the rod portion.
[0059] The rod portion of the heating element and the porous
support may be as defined above.
[0060] The retaining means can comprise or consist of a clip or
perforated sleeve that fits over the porous support to hold it in
place.
[0061] The evaporation chamber may vary in cross sectional shape
and can be, for example, of circular or rectangular cross
section.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] In a further embodiment, the apparatus is configured such
that: [0067] the second inlet is arranged so that it opens into an
intermediate chamber between the evaporation chamber and the
condenser; [0068] the intermediate chamber is divided by a dividing
wall into upstream and downstream sub-chambers, a central hole in
the wall providing communication between the sub-chambers, whereby
the second inlet opens into the upstream sub-chamber; [0069] a
third inlet opens into the downstream sub-chamber, the third inlet
being connectable to a supply of filtered gas; [0070] a nozzle is
provided that extends from an exit opening of the evaporation
chamber into the condenser to a position in the upstream
sub-chamber that is level with or downstream of the second inlet;
[0071] the downstream sub-chamber contains a cylindrical baffle
that is aligned with the said nozzle and the central hole in the
dividing wall, and the third nozzle opens into a space surrounding
the cylindrical baffle.
[0072] 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.
[0073] 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 .mu.m. By increasing the size of the particles, they
are rendered detectable by optical particle detectors such as
optical particle counters.
[0074] 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 .mu.m 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.
[0075] In order to assist cooling of the mixture of gases, vapour
and particles in the condenser, cooling means may be provided.
[0076] 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.
[0077] 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.
[0078] 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).
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] 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: [0087] a condenser body
having an inlet, an outlet and a hollow interior which has an
internal length, an internal width and an internal height; [0088]
an inlet flow distributor tube connected to the inlet of the
condenser body and extending across the internal width of the
condenser body; and [0089] an outlet flow distributor tube
connected to the outlet of the condenser body and extending across
the internal width of the condenser body; [0090] wherein the
internal height of the condenser body is less than a corresponding
internal height of each of the inlet and outlet flow distributor
tubes; [0091] inlet and outlet flow distributor tubes each being
provided in the walls thereof with one or more slots or holes
communicating with the hollow interior of the condenser body so as
to provide a flow path from the inlet flow distributor tube through
the hollow interior of the condenser and into the outlet flow
distributor tube.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] The internal cross sectional area of each flow distributor
tube is preferably greater than the internal cross sectional area
(internal width.times.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 Wc then .pi.Dt.sup.2>Hc*Wc. The ratio of
.pi.Dt.sup.2/(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.
[0096] 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.
[0097] 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:
.pi.Dt.sup.2>Nh*.pi.Dh.sup.2. The ratio of
.pi.Dt.sup.2/(Nh*.pi.Dh.sup.2) should be more than 1.1 or
preferably more than 2 or even more preferably the ratio should be
more than 3.
[0098] 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.
[0099] 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.
[0100] 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 .mu.m.
[0101] 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.
[0102] 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.
[0103] Accordingly, in another aspect, the invention provides a
condensation particle counter comprising a condensation apparatus
of the invention as defined herein.
[0104] 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.
[0105] Further aspects and features of the invention will be
apparent from the specific embodiments described below and
illustrated in FIGS. 2 to 12.
BRIEF DESCRIPTION OF THE DRAWINGS
[0106] FIG. 1 is a schematic side sectional view of a known type of
condensation apparatus.
[0107] FIG. 2 is a schematic view of a condensation apparatus
according to a first embodiment of the invention.
[0108] FIG. 3 is a schematic view of a condensation apparatus
according to a second embodiment of the invention.
[0109] FIG. 4 is a schematic view of a condensation apparatus
according to a third embodiment of the invention.
[0110] FIG. 5 is a schematic view of a condensation apparatus
according to a fourth embodiment of the invention.
[0111] FIG. 6 is a schematic side sectional elevation of a
rectangular cross section condenser/outlet provided with working
fluid removal means that can be substituted for the
condenser/outlets of any one of the embodiments of FIGS. 2 to
5.
[0112] FIG. 7 is a schematic side sectional elevation of an
alternative rectangular cross section condenser/outlet provided
with working fluid removal means that can be substituted for the
condenser/outlets of any one of the embodiments of FIGS. 2 to
5.
[0113] FIG. 8 is a schematic view of a condensation apparatus
according to a fifth embodiment of the invention.
[0114] FIG. 9 shows a graph comparing of aerosol particle number
concentrations (N) measured with the condensation chamber of FIG. 2
coupled with a MetOne.TM. laser optical particle counter (black
squares) and handheld 3007 CPC from TSI (white diamonds).
[0115] FIG. 10 is a schematic view of a condensation apparatus
according to a sixth embodiment of the invention.
[0116] FIG. 11 is a schematic view of a condensation apparatus
according to a seventh embodiment of the invention.
[0117] FIG. 12 is a schematic representation of a condensation
apparatus and particle counter assembly according to an eighth
embodiment of the invention, wherein the assembly has the ability
to recycle working fluids.
[0118] FIG. 13 is a schematic view of the rectangular condenser
with a flow distributor in the entrance to the condenser and in the
outlet of the condenser.
[0119] FIG. 14 is a cross-sectional view (vertical plane) of the
rectangular condenser with a flow distributor in the entrance to
the condenser and in the outlet of the condenser.
[0120] FIG. 15 is a cross-sectional view (horizontal plane) of the
rectangular condenser with a flow distributor in the entrance to
the condenser and in the outlet of the condenser.
[0121] FIG. 16 is a cross-sectional view (vertical plane) of the
rectangular condenser with a flow distributor containing a slot in
the entrance to the condenser and another flow distributor with a
slot in the outlet of the condenser.
[0122] FIG. 17 is a sectional view through an evaporation chamber
according to another embodiment of the invention.
[0123] FIG. 18 is a schematic sectional elevation showing the
evaporation chamber of FIG. 17 from another angle but with the
heating element omitted and a working fluid reservoir in place.
[0124] FIG. 19 is a schematic sectional elevation showing a plan
view of the apparatus depicted in FIG. 18.
[0125] FIG. 20 is a more detailed sectional elevation of the
heating element and porous support used in the embodiment of FIGS.
17 to 19.
DETAILED DESCRIPTION OF THE INVENTION
[0126] The invention will now be illustrated in greater detail by
reference to the specific embodiments described in the following
non-limiting examples.
[0127] FIG. 1 is a schematic side sectional view of a known type of
condensation apparatus that can be used in combination with a
particle counter. One such known condensation apparatus is the
apparatus found in the handheld CPC 30007 from TSI (www.tsi.com).
The apparatus comprises an evaporation chamber 1a with a porous
support in the form of a cartridge 2a soaked in a volatile working
fluid. The chamber 1a has a cylindrical shape and an internal
diameter that is greater than the external diameter of the
cartridge 2a. The evaporation chamber 1a is provided with an inlet
4a and outlet 5a, and a heating element 3a, which encircles the
outer wall of the chamber. The temperature of the chamber is
measured by a sensor 6a and controlled by a control unit 7a using
interface 8a connected to the heating element on the outer wall of
the chamber. A condenser 9a is placed between the chamber 1a and
the outlet 5a.
[0128] 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 .mu.m. The
enlarged particles pass out through the outlet 5a, and are directed
to an optical particle counter where they are counted.
[0129] The condensation apparatus illustrated in FIG. 1 can be used
to detect and count particles in the size range from 10 nm to 600
nm. However, the apparatus suffers from a number of
disadvantages.
[0130] One major disadvantage is that the working fluid must be
replaced on a regular and frequent basis.
[0131] 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.
[0132] 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.
[0133] 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.
[0134] The relatively rapid depletion of working fluid in the
cartridge in the CPC shown in FIG. 1 affects the performance of the
condensation unit because it leads to a lower concentration of
vapour of the working fluid in the air inside the chamber. When the
vapour concentration is lower, vapour tends to condense
preferentially on relatively large particles and therefore the
smaller particles are not detected and counted. For instance, the
lower detection limit can increase by as much as 10 nm to 15 or 20
nm due to working fluid depletion. Since it is very difficult to
monitor the extent or rate of depletion in practice, there is a
chance that many small particles will not be detected and
counted.
[0135] The condensation apparatus of the invention overcomes or at
least alleviates the problems identified above with known CPCs.
[0136] A condensation apparatus according to a first embodiment of
the invention is shown in FIG. 2. The apparatus comprises: [0137]
An evaporation chamber 2 with one inlet 1 (the "first inlet") for a
clean gaseous medium, e.g. a clean air where all relevant aerosol
particles have been removed. [0138] Another inlet 8 (the "second
inlet") through which a stream of a sample gas (e.g. air)
containing nano-particles of interest can be drawn into the
evaporation chamber. The inlet 8 is positioned at a distance from
the first inlet, e.g. at the top of the chamber while the first
inlet 1 is positioned at the bottom of the chamber. [0139] A
heating element 3 which is in sufficiently close thermal contact
with a temperature sensor 4, the element being covered by a porous
support 6 wettable by the working fluid. The porous support 6 is
soaked with a working fluid of low equilibrium vapour pressure, for
instance a semi-volatile organic compound such as dimethyl
phthalate. [0140] A temperature controlling device 5 that keeps the
temperature of the surface of the porous support 6 at a level
sustaining formation of a sufficient high vapour density of the
working fluid; [0141] A condenser/outlet 7 made of a material of
high thermal conductivity and sufficiently long to let droplets of
the working fluid be formed on nano-particles of interest and to
grow to a size that enables them to be detected and counted by
means of single particle counting. The condenser/outlet 7 is
positioned near or opposite the second inlet 8 to provide effective
mixing of the carrier gas and sample gas streams entering the
chamber through the first and the second inlets. The
condenser/outlet 7 is positioned in line with the second inlet.
[0142] A temperature cooling device 10 that keeps the temperature
of the surface of the condenser/outlet 7 at a level sustaining
formation and growth of the working fluid droplets. The cooling
device 10 is placed in a position that enables effective cooling of
the condenser/outlet 7 to be sustained.
[0143] 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 FIG. 2 a flow of cold air 9 generated by a blower is
shown. In the condenser/outlet droplets of the working fluid grow
on the nano-particles up to detectable sizes for instance 1 .mu.m.
These droplets can be counted for instance using an optical
particle counter (not shown) that is connected to the exit of the
chamber 11. Therefore every nano-particle of interest is detected
individually.
[0144] As an example of the apparatus of shown in FIG. 2, a
miniature cylindrical condensation chamber of 10 mm ID was built
from PTFE with inlets 1 and 8 made from stainless steel tube of 4
mm OD and a condenser/outlet 7 made from 8 mm OD stainless steel
tube. A K-type thermocouple was utilised as a temperature sensor 4.
The heater 3 used was a NiCr heater covered by a layer of porous
SiO.sub.2 soaked with dimethyl phthalate. Particles were counted
using a MetOne laser particle counter (Hach ULTRA Analytics).
[0145] 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 .mu.m
diameter. The condensation apparatus was used over a period of at
least 2 months without refilling.
[0146] Another embodiment of the present invention is shown in FIG.
3. The apparatus of FIG. 3 is similar to the apparatus of FIG. 2
except that a reservoir 12 of working fluid is provided to extend
the period of time over which the apparatus can be operated without
refilling. In order to prevent the fluid from the reservoir 12 from
escaping and interfering with the condensation process when the
apparatus is not in a horizontal position, the inlets 1 and 8 and
the condenser/outlet 7 are arranged so that they extend inwardly
into the condensation chamber 2. The extensions prevent the escape
of working fluid through the inlets 1 and 8 and outlet 7. A
condensation apparatus of the type depicted in FIG. 3 has been
shown to work for at least 12 months without refilling.
[0147] The mode of action of this embodiment is the same as for the
embodiment of FIG. 2 above.
[0148] A further embodiment of the invention is illustrated in FIG.
4. In this embodiment, which is similar in construction to the
apparatus of FIG. 2, a cooling element 13 is provided which is in
thermal contact with the condenser/outlet. The cooling element 13,
which can be, for example, a thermoelectric cooling element, is
attached to the surface of the condenser/outlet. The cooling
element 13 enables heat to be removed from the condenser/outlet in
a more efficient manner than is possible with a fan. The enhanced
cooling effect of the cooling element 13 increases supersaturation
of the vapour within the condenser and enables droplets of the
working fluid to grow more rapidly. The operation of the cooling
element 13 can be controlled by so that the temperature of the
surface of condenser/outlet 7 is lower than the ambient
temperature, and therefore the apparatus can be used effectively
over a range of ambient temperatures including hot environments.
The performance of this embodiment is not influenced by ambient
temperature variations.
[0149] The shape of the condenser/outlet 7 can affect its
performance. In the embodiment shown in FIG. 5, the
condenser/outlet 7 has a rectangular cross-section, illustrated by
the element labelled as 14. In this embodiment, the cross-section
14 refers to the middle region of the condenser/outlet 7. To either
side of the middle region, the condenser can be of any cross
sectional shape, e.g. circular. The rectangular cross sectional
shape presents a greater surface area onto which the stream of
cooling air 9 from the fan 10 can be directed thereby enhancing the
efficiency of the cooling of the condenser/outlet 7 which in turn
assists the supersaturation of the vapour in the condenser/outlet 7
and improves the efficiency of droplet growth. Although the
condenser/outlet 7 is shown as having a middle region of a flat
sided rectangular cross section, the shorter sides of the rectangle
can be rounded instead of flat so that the cross section is in the
shape of an elongated oval rather than a regular rectangle. An
elongated oval cross section can be created by the simple expedient
of flattening a portion of the tube from which the condenser/outlet
7 is made.
[0150] 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 FIGS. 2 to 5, the second inlet 8 is positioned
in line with the condenser/outlet and the cross sectional area of
the interior of the inlet 8 is less than the cross sectional area
of the interior of the condenser/outlet 7. Without wishing to be
bound by any theory, it is believed that this arrangement results
in the stream of sample gas (e.g. air) containing the aerosol
particles being injected into the centre of a stream of vapour and
carrier gas so that the mixture entering the condenser/outlet 7
outlet consists of a core stream of sample gas containing the
nano-particles surrounded by a sheath of carrier gas and vapour.
Mixing between the concentric layers takes place as gases and
vapours move along the condenser/outlet 7.
[0151] 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.
[0152] If the width of the rectangular cross section condenser
shown in FIG. 5 is small (e.g. less than 2 mm), then there is a
risk that condensed working fluid will build up on the interior
surface of the condenser thereby clogging the condenser. In order
to prevent this from happening, means can be provided for removing
condensed working fluid form the condenser. One way of doing this
is to remove the condensed liquid from the internal surfaces of the
condenser by means of a combination of a capillary action and a
pressure differential. An arrangement for accomplishing this is
shown in FIG. 6.
[0153] FIG. 6 is a cross sectional view of the condenser. The
interior of the condenser is provided with two solid porous
membranes 15 which are wettable by the working fluid and serve to
partition the interior of the condenser into a central passage and
a pair of elongate fluid collection chambers 16. In use, as the
stream of vapour and gases containing the nano-particles and
growing droplets passes along the central passage between the
membranes 15, condensation of working fluid vapour onto the surface
of the membranes takes place. The excess liquid thus formed is
immediately sucked through the membranes 15 and into the collection
chambers 16 by the combination of capillary action and a negative
pressure maintained by a pump (not shown). From the chambers 16,
the liquid is removed through the outlets 17 and can then be
directed to a waste collection chamber (not shown) or, in the case
of an apparatus having a working fluid reservoir 12 as shown in
FIG. 3, recycled to the reservoir.
[0154] 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 FIG. 4 or can be
achieved by circulating the liquid through a heat exchanger as
shown in FIG. 7.
[0155] FIG. 7 is a schematic illustration of a cross-section of
part of a condenser (the rest of the condenser is not shown)
provided with fluid collection chambers 16 which are connected to a
working fluid recycling and temperature control circuit.
[0156] 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
predetermined temperature by the temperature controller 20 and, in
this way, the temperature of the internal surface of the condenser
can be controlled.
[0157] 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.
[0158] The tubing connections shown in FIG. 7 are merely
illustrative and it will be appreciated that the connections can be
arranged differently to enable the liquid to cool the condenser
more uniformly. For example, in an alternative arrangement, the
working fluid can be directed from the controller 20 to the inlets
17 and removed from outlets 18 to the pump 19.
[0159] 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.
[0160] 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 .mu.m droplets but 100
nm particles will generate 1 .mu.m 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.
[0161] 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.
[0162] 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 in 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.
[0163] 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.
[0164] 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.
[0165] 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.
[0166] An apparatus according to a further embodiment of the
invention is illustrated schematically in FIG. 8. In this
embodiment, the apparatus comprises an evaporation chamber
(saturating chamber) 2 containing a heating element 3 powered by a
controlled power supply 5. The heating element 3 is in close
contact with a porous material 6 soaked with a working fluid, e.g.
semi-volatile compound, attached to a temperature sensor 4
connected to the power supply 5. The evaporation chamber has an
inlet 1 and an outlet which extends into a nozzle 22 opening into
the condenser 7. The condenser is provided with an inlet 8 through
which a gas sample containing nano-particles of interest can be
introduced into the condenser. The nozzle 22 extends into the
condenser so that it opens out into the condenser downstream of the
inlet 8. The condenser 7 is equipped with an outlet 11 and a
cooling element 13.
[0167] The embodiment of FIG. 8 works as follows. A stream of the
clean air (carrier gas) is directed into to the saturating chamber
2 via inlet 1 by means of a flow-generating device, e.g. a pump
(not shown). In the chamber, the heating element 3 heats the porous
material 6 soaked in the working fluid to produce vapour. The air
stream containing the vapour is directed to the condenser through
nozzle 22 where the air stream is cooled upon mixing with the
unheated stream of sample gas containing nano-particles entering
through inlet 8. The nozzle 22 is designed to deliver the hot
vapour-saturated air stream into the centre of the sample gas
stream containing nano-particles so that the hot vapour saturated
stream is surrounded by a sheath of cooler sample gas. The combined
gas streams are cooled by the cooling element 13 that controls the
temperature of the walls of the condensing chamber 7. In the
condenser, supersaturation of the working fluid occurs as a result
of the mixing of the hot vapour-saturated air with the cooler
sample gas and cooling by the walls of the condenser 7. Thus leads
to condensation of the working fluid vapour onto the airborne
nano-particles and the formation of droplets of about 1 .mu.m.
These droplets are directed to an optical particle counter via
outlet 11 and counted individually.
[0168] 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.
[0169] 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
[0170] 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.
[0171] 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.degree.
C.
[0172] 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.
[0173] 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.
[0174] 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.
[0175] An apparatus according to another embodiment of the
invention is illustrated in FIG. 10. In this embodiment, in which
the evaporation chamber has a similar layout as the embodiment of
FIG. 8, a mixing chamber (intermediate chamber) 25 is positioned
between the saturating chamber 2 and the condenser 7. The mixing
chamber is divided by a partition 24 having a central orifice into
a downstream sub-chamber and an upstream sub-chamber. The nozzle 22
extends into the upstream sub-chamber and a cylindrical baffle,
which is aligned with the nozzle 22 extends in an upstream
direction from the end of the downstream sub-chamber. These
elements can be made from the same materials as the rest of the
apparatus. The mixing chamber 25 and partition 24 can have
cylindrical symmetry or they can be rectangular in cross section.
This embodiment enables to achieve the lowest low detection size of
nano-particles of 1 nm. A third inlet 23 is provided and this opens
out into the annular space surrounding the cylindrical baffle.
[0176] 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.
[0177] 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 FIG. 11. In the embodiment of FIG. 11, the inlet 1
receives gas (e.g. air) in which is suspended micro-droplets
containing airborne particles recycled from the particle counter.
In order prevent the recycled airborne particles from continuing
through to the condenser and hence contaminating the sample gas
containing the particles of interest, the porous medium 26 is
positioned across the evaporation chamber airflow in the
evaporation chamber. The porous medium is selected so as to be
capable of performing two functions. Firstly, it must be wettable
by the working fluid so that, when heated, it can serve as source
of vapour and, secondly, it must be capable of functioning as a
filter to collect micro-droplets containing microparticles thereby
avoiding contamination of the vapour in the region downstream of
the porous medium. As in the other embodiments described above, a
heating element 3 is positioned near the surface of the porous
medium to evaporate deposited working fluid. However, thermal
contact between the heating element 3 and the porous medium 26 is
not important in this embodiment because of the heating effect
provided by the gas entering the chamber through inlet 1.
[0178] 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
[0179] 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 FIG. 12.
[0180] The assembly shown in FIG. 12 comprises two apparatus 27 and
28 each corresponding to the apparatus illustrated in FIG. 11 and
an optical particle counter 29. Three three-way valves 30, 31 and
32 and two on/off valves 33 and 34 direct the flows of fluid to
permit recycling of the working fluid.
[0181] The assembly shown in FIG. 12 functions as follows:
[0182] 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.
[0183] 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.
[0184] 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.
[0185] 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.
[0186] 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.
[0187] FIGS. 13 to 16 illustrate a further type of condenser for
use in the apparatus of the invention. In this embodiment, the
condenser is of rectangular cross section.
[0188] 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. FIGS. 13 to 16 illustrate a rectangular
condenser which provides a substantially uniform flow velocity of
the vapour laden gases through the condenser. In this embodiment, a
pair of flow distributors is provided, one attached either side of
the rectangular condenser. One of the flow distributors is in fluid
communication with (e.g. connected to) the evaporation chamber, and
the other flow distributor is in fluid communication with (e.g. is
connected to) a particle detector.
[0189] Thus, as shown in FIG. 13, a rectangular condenser 103 is
equipped with the first flow distributor 102 and a second flow
distributor 104, both of which are in fluid communication with the
condenser. The hot gas (e.g. air) saturated with working fluid
enters the first flow distributor 102 via an inlet 101. The flow
distributor 101 is designed to supply a uniform flow to the
condenser 102. Therefore, the opposite to the inlet 101 end of the
flow distributor 102 is blocked, see FIG. 15. At the outlet of the
condenser 103, the second flow distributor is attached to reduce
non-uniformity of the flow at the outlet of the condenser 103. The
gas flow leaves the second flow distributor through an outlet
105.
[0190] FIG. 14 shows the relative positions of the first flow
distributor 101, the rectangular condenser 103 and the second flow
distributor 104.
[0191] FIG. 15 shows schematically the flow stream lines inside the
condenser 103, the flow distributor 102 and the flow distributor
104. In this embodiment, the air parcels moving along trajectories
106, 107 and 108 have substantially the same velocity and therefore
substantially the same residence time in the rectangular condenser
103 and as a result substantially the same sizes of droplets
formed.
[0192] 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 .pi.Dt.sup.2>Hc*Wc. The ratio of
.pi.Dt.sup.2/(Hc*Wc) should be more than 1.1 or preferably more
than 2 or even more preferably the ratio should be more than 3.
[0193] 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 FIG. 16, the vertical cross-sectional view shows
schematically the slots 109 and 110. In this case the internal
diameter of flow distributors can be reduced because of the
uniformity of the flow in the condenser is governed by the
expression containing the width of the slot Ws but not the height
of the condenser Hc: Hc>Ws. 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.
[0194] 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:
.pi.Dt.sup.2>Nh*.pi.Dh.sup.2. The ratio of
.pi.Dt.sup.2/(Nh*.pi.Dh.sup.2) should be more than 1.1 or
preferably more than 2 or even more preferably the ratio should be
more than 3.
[0195] In one embodiment of the condenser arrangement shown in
FIGS. 13 to 16, the flow distributors 102 and 104 were manufactured
from a stainless steel tube of 7 mm ID and the length of 40 mm. The
condenser 103 was manufactured from stainless steel sheet with
dimensions: Wc=20 mm, Hc=1.4 mm and Lc=30 mm. However, it should be
appreciated that other materials can be used to manufacture the
flow distributors, e.g. PTFE, aluminium, or any suitable metal,
glass, ceramic or plastics material.
[0196] 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.
[0197] FIGS. 17 to 19 illustrate an evaporation chamber and
associated working fluid reservoir.
[0198] The evaporation chamber shown in FIG. 17 comprises a body
200 formed from PTFE enclosing a chamber interior 216. The chamber
has a pair of inlets 220 (not shown in FIG. 17 but see FIG. 18) and
has means 202 for connection to a condenser (not shown).
[0199] 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.
[0200] The rod portion 212 has a hollow interior within which are
disposed a metal heating wire 222 and a thermocouple 224 (see FIG.
20). The heating wire 222 and thermocouple 224 are secured in place
by means of a thermal filler (222) which can be, for example, a
solder or a metal filled epoxy resin.
[0201] The heating wire and thermocouple are connected to a
controller (not shown).
[0202] Beneath the heating element, the body of the evaporation
chamber has a well 214/218 for holding a working liquid such as
dimethyl phthalate. The well 214/218 is connected via tube 218 (see
FIG. 18) to a reservoir 216 of the working fluid.
[0203] As shown in FIG. 20, the rod portion is surrounded by a
sleeve 226 of a porous material which, in this embodiment, is a
porous fabric such as quartz fibre filter or glass fibre filter or
a polymer or metal filter. The sleeve of porous fabric has a tail
portion 228 which extends into the well and acts as a wick to draw
working fluid up from the well. The sleeve is held in place on the
rod portion by means of a clip 230 which is provided with holes
through which the working liquid can evaporate and an opening on
its underside to accommodate the tail portion of the sleeve. The
wick is surrounded by a wick holder 231 made from an inert material
e.g. stainless steel.
[0204] 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.
EXAMPLES
[0205] Several examples of apparatus according to this invention
have been built and tested and these are described below.
Example 1
[0206] In one example constructed as shown in FIG. 8, the
evaporation chamber (saturating chamber) 2 was made of stainless
steel tube (12 mm ID) and 30 mm length. Stainless steel tube of 3
mm ID was used for the inlets, nozzle and the outlet. The cooling
element was constructed from a 5V DC micro fan positioned at 15 mm
from the surface of the condensing chamber. The condenser chamber
was made of stainless steel tube 6 mm ID length 60 mm and the
heating element was made a NiCr heating element covered with a
quartz fibre material that was sufficiently long to be positioned
near to the bottom of the stainless steel cylinder of the
saturating chamber. About 0.5 ml of dimethyl phthalate was poured
into the chamber 2 as the working fluid. Micro droplets formed onto
nano-particles were counted with a MetOne laser optical particle
counter. The apparatus was tested against SMPS (TSI), portable SAC
size spectrometer (Naneum) and a handheld instrument 3007 CPC (TSI,
model 3007). Nanoparticles of chromium oxides and atmospheric
aerosols were used for the tests. In the tests, it was found that
the apparatus of the invention enabled nano-particles to be
enlarged up to 1.2 .mu.m in diameter and the lower limit of the
detection range was estimated to be 4 nm.
Example 2
[0207] A comparison was made between aerosol particle number
concentrations (N) measured using an apparatus as illustrated in
FIG. 2 coupled to a MetOne laser optical particle counter and
aerosol particle number concentrations (N) measured using a
handheld 3007 CPC from TSI. The clean air flow through inlet 1 into
the saturating chamber was set at 0.3 l/min and the sample gas flow
through inlet 8 was set at 0.5 l/min. Nano-particles of chromium
oxide and atmospheric aerosols were used for the comparative
tests.
[0208] The results are shown in FIG. 9 where the data points for
the apparatus of the invention are shown as black squares and the
data points for the TSI 3007 CPC instrument are shown as white
diamonds. In FIG. 9, D is the mean diameter (nm) which was obtained
by calibration using a reference method.
[0209] It was found that the apparatus of the invention enables
nano-particles to be enlarged up to 1.2 .mu.m in diameter. The
lower detection limit for the apparatus of the invention was
estimated to be 3 nm. It is clear from FIG. 9 that the lower
detection limit of the apparatus of the invention is lower than the
low detection limit of CPC 3007 (TSI).
EQUIVALENTS
[0210] 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.
* * * * *