U.S. patent application number 11/569200 was filed with the patent office on 2007-11-29 for gas treatment appartatus.
This patent application is currently assigned to EMINOX LIMITED (A BRITISH COMPANY). Invention is credited to Duncan Arrowsmith, Phillip David Bush, Carl Robert Taylor.
Application Number | 20070274877 11/569200 |
Document ID | / |
Family ID | 32947643 |
Filed Date | 2007-11-29 |
United States Patent
Application |
20070274877 |
Kind Code |
A1 |
Bush; Phillip David ; et
al. |
November 29, 2007 |
GAS TREATMENT APPARTATUS
Abstract
The invention relates to a gas treatment apparatus (1) for
treating a gas stream. The gas treatment apparatus (1) comprising
addition means (14) for adding a reducing agent (12) to a gas
stream passing through the apparatus (1), a guide chamber (8), a
mixing conduit (18) and a catalytic treatment chamber (16)
containing a catalytic treatment element (20). The apparatus (1) is
arranged such that gas must pass through the mixing conduit (18) to
pass from the guide chamber (8) to the catalytic treatment chamber
(16). The mixing conduit (18) includes an outer wall (22)
surrounding a mixing axis (20) within the guide chamber (8). The
outer wall (22) including at least one aperture (30) through which
a gas can enter the mixing conduit (18) from the guide chamber (8).
The at least one aperture includes guide means (32) associated
therewith for causing at least some of a gas passing through the at
least one aperture (30) to flow circumferentially about the mixing
axis (20) within the mixing conduit (18) before entering the
catalytic treatment chamber (16).
Inventors: |
Bush; Phillip David;
(Lincoln, GB) ; Arrowsmith; Duncan; (Lincoln,
GB) ; Taylor; Carl Robert; (Scunthorpe, GB) |
Correspondence
Address: |
BOURQUE & ASSOCIATES;INTELLECTUAL PROPERTY ATTORNEYS, P.A.
835 HANOVER STREET
SUITE 301
MANCHESTER
NH
03104
US
|
Assignee: |
EMINOX LIMITED (A BRITISH
COMPANY)
NORTH WARREN ROAD
GAINESBOROUGH
GB
|
Family ID: |
32947643 |
Appl. No.: |
11/569200 |
Filed: |
July 26, 2005 |
PCT Filed: |
July 26, 2005 |
PCT NO: |
PCT/GB05/02934 |
371 Date: |
November 16, 2006 |
Current U.S.
Class: |
422/176 |
Current CPC
Class: |
F01N 3/2066 20130101;
B01D 53/9431 20130101; B01F 5/0451 20130101; Y02A 50/20 20180101;
F01N 2610/02 20130101; B01D 2258/012 20130101; F01N 13/009
20140601; Y02A 50/2325 20180101; Y02T 10/12 20130101; B01F
2005/0017 20130101; Y02T 10/24 20130101; F01N 3/2892 20130101; B01D
2251/20 20130101; B01F 5/0618 20130101 |
Class at
Publication: |
422/176 |
International
Class: |
F01N 3/08 20060101
F01N003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 29, 2004 |
GB |
04166917.3 |
Claims
1. A gas treatment apparatus for treating a gas stream, the gas
treatment apparatus comprising addition means for adding a reducing
agent to a gas stream passing through the apparatus, a guide
chamber having an end wall, a mixing conduit within which the
addition means are located and a catalytic treatment chamber
containing a catalytic treatment element, wherein: the mixing
conduit has a smaller cross sectional area for gas flow than the
catalytic treatment chamber and guide chamber such that, in use,
gas flows faster within the mixing conduit than in the catalytic
treatment chamber and the apparatus is arranged such that gas must
pass through the mixing conduit to pass from the guide chamber to
the catalytic treatment chamber; the mixing conduit extends into
the guide chamber and includes an outer wall surrounding a mixing
axis therein, the mixing conduit having an outlet into a downstream
chamber which is substantially coaxial with a central axis of said
downstream chamber; the outer wall includes at least one aperture
through which a gas can enter the mixing conduit from the guide
chamber, the at least one aperture including guide means associated
therewith for causing at least some of a gas passing through the at
least one aperture to flow circumferentially about the mixing axis
within the mixing conduit.
2. A gas treatment apparatus as claimed in claim 1, in which there
are a plurality of apertures through the outer wall of the mixing
conduit and each aperture includes guide means for causing at least
some of a gas passing through the aperture to flow
circumferentially about the mixing axis within the mixing
conduit.
3. A gas treatment apparatus as claimed in claim 1, in which the
mixing conduit has a substantially circular cross section.
4. A gas treatment apparatus as claimed in claim 1, in which the
catalytic treatment chamber has a substantially circular cross
section.
5. A gas treatment apparatus as claimed in claim 1, in which the
guide chamber has a substantially circular cross section.
6. A gas treatment apparatus as claimed in claim 2, in which the
plurality of apertures are substantially equally circumferentially
spaced around the outer wall of the mixing conduit.
7. A gas treatment apparatus as claimed in claim 1, in which the
guide means comprise a deflection wall, the deflection wall being
angled such that at least some of a gas passing substantially
radially through the, or each, aperture is deflected by the
deflection wall to flow substantially circumferentially.
8. A gas treatment apparatus as claimed in claim 1, in which at
least one aperture is substantially rectangular.
9. A gas treatment apparatus as claimed in claim 8, in which at
least one aperture is arranged such that a long axis of the
rectangular aperture is substantially parallel with the mixing
axis.
10. A gas treatment apparatus as claimed in claim 8, in which the
guide means associated with the, or each, aperture comprises a
substantially rectangular deflection wall, the deflection wall
extending from a side of the aperture into the mixing conduit.
11. A gas treatment apparatus as claimed in claim 1, in which the
guide means and aperture in the wall of the mixing region are
formed by forming a cut in the wall of the mixing chamber and then
deforming a region of said wall adjacent to the cut into the mixing
conduit such that an aperture is opened and a guide means
formed.
12. A gas treatment apparatus as claimed in claim 1, in which the
catalytic treatment chamber includes an entry portion located
between the mixing conduit and the catalytic treatment element such
that a gas leaving the mixing conduit can disperse across the
entire cross section of the catalytic treatment chamber before
entering the catalytic treatment element.
13. A vehicle having a diesel engine, the vehicle being equipped
with a gas treatment apparatus as claimed in claim 1, for treating
the exhaust gas from the diesel engine.
Description
[0001] The present invention relates to gas treatment apparatus
and, in particular, to gas treatment apparatus for treating the
exhaust gases from a diesel engine of a vehicle. The invention
extends to a vehicle equipped with such gas treatment
apparatus.
[0002] Diesel engine exhaust gases contain a number of noxious
gases, such as nitrogen oxides, sulphur oxides and carbon oxides,
as well as un-burnt hydrocarbons, carbon and other particles. Some
of these compounds can be treated so as to render them less
obnoxious.
[0003] It is therefore common practice to pass the exhaust gases
through one or more treatment elements such as catalytic converters
and filters. The exhaust gases can be subjected to reduction of
nitrogen oxides to nitrogen by injecting a reducing agent,
typically urea dissolved in water, into the gas stream and then
passing it through a catalytic treatment element to convert
residual ammonia from the urea to nitrogen and water, which are
acceptable exhaust emissions. This process is known as Selective
Catalytic Reduction (SCR). In practice, the efficiency of the
process is partly dependent upon the quality of the mixing of the
reducing agent and gas before the mixture enters the catalyst.
[0004] Such a technology may also be combined with other
technologies such as catalysed Diesel Particulate Filters, CRT.TM.
(Continuously Regenerating Trap) technology or other treatment
methods to further reduce undesirable emissions from diesel
engines.
[0005] To ensure acceptable mixing of the reducing agent and gas
stream a mixing region of the gas treatment apparatus can be large
and therefore have a long residence time during which mixing and
chemical breakdown of the urea-water solution can occur.
[0006] It is an object of the present invention to address some of
the above issues.
[0007] According to the present invention there is provided a gas
treatment apparatus for treating a gas stream, the gas treatment
apparatus comprising addition means for adding a reducing agent to
a gas stream passing through the apparatus, a guide chamber having
an end wall, a mixing conduit within which the addition means are
located and a catalytic treatment chamber containing a catalytic
treatment element, wherein: [0008] the mixing conduit has a smaller
cross sectional area for gas flow than the catalytic treatment
chamber and guide chamber such that, in use, gas flows faster
within the mixing conduit than in the catalytic treatment chamber
and guide chamber, the mixing conduit extends through the end wall
from the guide chamber to the catalytic treatment chamber and the
apparatus is arranged such that gas must pass through the mixing
conduit to pass from the guide chamber to the catalytic treatment
chamber; [0009] the mixing conduit extends into the guide chamber
and includes an outer wall surrounding a mixing axis therein;
[0010] the outer wall includes at least one aperture through which
a gas can enter the mixing conduit from the guide chamber, the at
least one aperture including guide means associated therewith for
causing at least some of a gas passing through the at least one
aperture to flow circumferentially about the mixing axis within the
mixing conduit; [0011] the gas treatment apparatus being arranged
such that, in use, gas flowing through the apparatus passes from
the guide chamber into the mixing conduit and it is caused to
accelerate and to flow circumferentially around the mixing axis is
mixed with reducing agent as the rotating gas flow passes along the
mixing conduit and the rotating mixed gas flow leaving the mixing
conduit expands radially and slows as the cross sectional area for
gas flow increases.
[0012] Positioning at least a portion of the mixing conduit within
the guide chamber and including at least one aperture in the outer
wall of the mixing conduit within the guide chamber causes gas to
enter the mixing conduit substantially radially. In this case, the
term `radially` means that the gas must have a component of its
flow velocity towards the mixing axis. It should be understood that
the flow velocity could also include radial or axial components
with respect to the mixing axis as the gas enters the gas treatment
apparatus. The radial entry of the gas into the mixing conduit
allows simple guide means to cause at least some of the gas to flow
substantially circumferentially. The term `circumferentially` means
that the gas has a component of its flow velocity circumferentially
around the mixing axis. It should be understood that the gas may
still include an axial component to its motion so that the gas
moves through the mixing conduit. Simple guide means are easy to
construct and may have a low resistance to gas flow which enables a
reduction in the pressure drop across the gas treatment apparatus
caused as a gas stream flows through the apparatus. The reduction
in pressure drop may increase the torque output of an engine to
which the gas treatment apparatus is coupled. The circumferential
flow, or swirl, of the gas within the mixing conduit increases
turbulence within the gas which promotes mixing of the gas and
reducing agent as the mixture passes along the mixing conduit.
[0013] By locating the addition means within the mixing conduit and
having the rotating gas stream expand radially after leaving the
mixing conduit the requirements for providing satisfactory mixing
and satisfactory distribution of the mixture in downstream chambers
are effectively de-coupled. This means that the mixing parameters
or the flow distribution parameters can be altered substantially
without affecting the other. This can be achieved because the
rapidly rotating gas flow is turbulent and this promotes rapid
mixing, but the turbulent flow through the mixing conduit has an
overall flow pattern which is rotating about a mixing axis. This
rotating overall flow pattern causes the gas exiting the mixing
conduit to expand rapidly radially and therefore distribute the gas
across the downstream chamber.
[0014] The guide chamber of the apparatus may be a conduit from an
inlet into the apparatus, or from an upstream treatment chamber to
the mixing conduit. In the guide region it is possible that no
treatment is performed on a gas passing therethrough. The guide
chamber may comprise means for removing particulates such as a
filter, performing a catalytic treatment on a gas, or attenuating
noise that may be generated as a gas flows through the apparatus,
or transmitted by the gas flow. Gas may enter the guide chamber a
any angle, for example axially or radially.
[0015] The catalytic treatment chamber contains a catalytic
treatment element that preferably catalyses a reaction between a
gas stream passing through the apparatus and the reducing agent
added such that NO.sub.x in the gas stream is converted to N.sub.2
and water. The preferred reducing agent is a urea in water solution
and this is preferably added through an injector into the gas
stream. The reducing agent is preferably injected so that at least
some of the reducing agent reaches the rapidly swirling gas flow
near the wall of the mixing conduit as this results in more
efficient mixing. Such an arrangement can be achieved in many ways,
for example using a central injector injecting radially, or one or
more injectors arranged adjacent the wall of the mixing conduit
injecting reducing agent substantially axially. It should be noted
that the catalytic treatment element may any other catalytic
treatment element, for example it may act as a hydrolysis
catalyst.
[0016] Although it is preferred that the mixing conduit discharges
directly into the catalytic treatment chamber it should be
understood that the apparatus may include other chambers between
the mixing conduit and the catalytic treatment chamber. Gas must
still flow through the mixing conduit to pass from the guide
chamber to the catalytic treatment chamber, but may additionally
have to pass through other chambers. It should be understood that
no further mixing is required in these chambers as the gas and
reducing agent leaving the mixing conduit are substantially well
mixed.
[0017] It is preferred that the mixing conduit, catalytic treatment
chamber, and guide chamber have substantially circular cross
sections as this facilitates manufacture and reduces the likelihood
of damage to the apparatus at weak spots that may occur at corners
of the apparatus. It should be understood that the cross section of
one or all of the sections mentioned above may not be circular, for
example one or more may have elliptical cross sections, or other
shapes depending upon the desired final use and location of the
apparatus.
[0018] The outer wall of the mixing conduit preferably includes a
plurality of apertures through which gas may flow, and each of
which includes a guide means that causes at least some of the gas
passing through the aperture to flow substantially
circumferentially. All the guide means preferably cause the gas to
flow circumferentially in the same direction about the mixing axis.
It should be understood that there may be additional apertures
through the outer wall of the mixing conduit which do not include
guide means and, in this case, a circumferential flow of gas about
the mixing axis would be created by the guide means provided.
[0019] If a plurality of apertures are provided through the outer
wall of the mixing conduit, it is preferred that the apertures are
substantially equally circumferentially spaced as this helps to
maintain a substantially equal distribution of gas flow within the
guide chamber and mixing conduit and this may help to avoid poor
distribution of the reducing agent within the gas stream.
[0020] The guide means may be any size, shape or configuration but
are preferably formed integrally with the outer wall rather than
formed separately and then attached. A deflection wall which is
angled such that at least some of a gas passing substantially
radially through the aperture is deflected by a surface of the
deflection wall and caused to flow substantially circumferentially
is preferred due to the simplicity in construction and low pressure
drop of such a design.
[0021] The apertures are preferably substantially rectangular and
are preferably arranged such that a long axis of the rectangular
aperture is substantially parallel with the mixing axis.
[0022] The apertures and guide means can be formed in a plurality
of different ways. An embodiment of the aperture and guide means is
a "louver" design which comprises a substantially rectangular
aperture with a substantially rectangular deflection wall attached
to a long edge of the aperture. A mixing conduit having such an
embodiment can be readily fabricated using sheet metal, the sheet
metal being stamped such that three of the sides that will define
the rectangular aperture are cut (in a preferred example, two short
sides and one long side) and the resulting flap of metal is bent so
that the aperture is opened, the folded edge of the flap defines
the aperture together with the three cut sides and the flap is
arranged to form a deflection wall. A plurality of such apertures
and deflection walls are formed in the sheet and the sheet is then
rolled to form the outer wall of the mixing conduit. The seam where
two ends of the sheet meet may then be welded to form the mixing
conduit.
[0023] Other examples of aperture and guide means include a
"twisted ribbon" design and an "out of plane" design. In the
"twisted ribbon" design two slits are made through the wall of the
mixing conduit. The slits are preferably made such that they are
substantially parallel with the mixing axis and have a
substantially equal length. The two slits define two sides of a
`ribbon` of material which is attached to the wall of the mixing
conduit at each end. To create the guide means and aperture the
ribbon is twisted by forcing a first side of the `ribbon` into the
mixing conduit and a second side of the ribbon out of the conduit,
simultaneously opening an aperture through the wall and creating
guide means.
[0024] The "out of plane" design is based on the "louver" design.
Two adjacent sets of cuts through the wall of the mixing conduit
are formed such that the resulting deflection walls are attached to
adjacent long edges of the two apertures. One deflection wall is
bent into the mixing conduit and one out of the mixing conduit.
[0025] The deflection walls can be curved or otherwise shaped to
aid in the deflection of gas in the required direction. It should
be understood that the deflection walls and apertures can be formed
in a variety of other ways, which may include, for example,
separate fabrication of deflection walls and subsequent attachment
to the mixing conduit wall. It should be understood that the
apertures may have any suitable shape.
[0026] The guide chamber and catalytic treatment chambers may be
separate chambers, joined by an elongate mixing conduit, or the two
chambers may have a common outer wall and a gas be prevented from
passing from the guide chamber to the catalytic treatment chamber
by a wall through which the mixing conduit passes. Such a wall is
preferably substantially perpendicular to a chamber axis around
which the common outer wall extends, but may be at any suitable
angle.
[0027] The mixing conduit has a smaller cross sectional area for
gas flow than the catalytic treatment chamber. This allows the
swirling gas flow from within the mixing conduit to expand radially
upon entry into the catalytic treatment chamber. This slows the
rotation of the gas flow and since the cross sectional area for
flow is increased, the axial speed of the gas flow is also reduced
which allows a longer residence time within the apparatus. The
catalytic treatment chamber may include an entry portion located
between the mixing conduit and the catalytic treatment element.
This allows gas leaving the mixing conduit to disperse across the
entire cross section of the catalytic treatment chamber before
entering the catalytic treatment element. This radial dispersal
reduces the rotational velocity of the gas stream which reduces the
pressure loses associated with a rapidly rotating gas flow entering
a catalytic treatment element.
[0028] The mixing conduit has a smaller cross sectional area for
gas flow than the guide chamber. This means that the average gas
speed is increased as a gas stream flowing through the apparatus
passes from the guide chamber to the mixing conduit. The increased
gas speed is both axial and circumferential and this results in
greater turbulence within the mixing region and therefore more
efficient mixing.
[0029] In a particularly preferred embodiment the guide chamber,
mixing conduit and catalytic treatment chamber have substantially
circular cross sections and are therefore substantially cylindrical
in shape, with the mixing conduit have a smaller diameter than the
guide chamber and catalytic treatment chamber. It is preferred that
the portion of the mixing conduit within the guide chamber is
substantially co-axial with guide chamber and that the exit from
the mixing conduit is substantially co-axial with the downstream
chamber into which the rotating gas flow passes. This facilitates
control of the distribution of the mixed gas flow across the cross
section of downstream chamber and catalytic treatment element.
[0030] The invention will now be further described, by way of
example, with reference to the accompanying drawings, in which:
[0031] FIG. 1 shows a schematic view of a gas treatment
apparatus;
[0032] FIG. 2 shows a cross section through a portion of an
embodiment of gas treatment apparatus according to the
invention;
[0033] FIG. 3 shows a detailed view of an aperture of FIG. 2;
[0034] FIG. 4 shows the cross section of FIG. 2 including arrows
indicating gas flows;
[0035] FIG. 5 shows a cross section through a mixing conduit and
illustrates the possible path of reducing agent;
[0036] FIGS. 6, 7, 8 and 9 show possible arrangements of gas
treatment apparatus according to the invention; and
[0037] FIGS. 10, 11 and 12 show cross sections through possible
guide wall and aperture arrangements.
[0038] FIG. 1 shows a schematic representation of a gas treatment
apparatus 1 having an inlet 2, an outlet 3 and a surrounding wall
4. It should be understood that, although the surrounding wall is
shown as a single wall, it may be formed from two or more wall
sections welded or coupled together. Arrows 6 show the direction of
gas flow through the apparatus 1. Gas enters the apparatus 1
through the inlet 2 and enters a guide chamber 8. The guide chamber
8 guides gas towards a mixing region 10 in which a reducing agent
12 is added to the gas stream using addition means 14 and in which
the gas and reducing agent 12 are mixed. Upon leaving the mixing
region 10, the gas enters a catalytic treatment chamber 16
comprising an entry portion 17 and a catalytic treatment element
20. The catalytic treatment element 20 includes a catalyst that
catalyses a reaction between at least one chemical within the gas
stream and the reducing agent 12. The gas leaving the catalytic
treatment element 20 then exits the apparatus 1 through the outlet
3.
[0039] Although the inlet 2 and outlet 3 are shown in FIG. 1 as
having a smaller diameter than the surrounding wall 4 of the
apparatus 1, it should be understood that the inlet 2 and outlet 3
may be any suitable size or shape. It should also be understood
that the gas passing through the inlet 2 may already have undergone
one or more gas treatments, for example filtration, a catalytic
treatment, or other treatment.
[0040] FIG. 2 shows a cross section through the gas treatment
apparatus shown in FIG. 1. This Figure shows the mixing region 10,
an end of the guide chamber 8 and a start of the catalytic
treatment chamber 16.
[0041] The mixing region 10 includes a mixing conduit 18 which
includes an outer wall 22 that surrounds a mixing axis 20 and an
endwall 24. The outer wall 22 extends into the guide chamber 8. The
addition means 14 extends through the surrounding wall 4 and
through the endwall 24 of the mixing conduit 18 so that outlets 26
from the addition means 14 are arranged such that reducing agent 12
is injected into the gas stream substantially radially outwardly
from the mixing axis 20. In this case there are ten outlets 26
substantially equally circumferentially distributed around an
outlet end 28 of the addition means 14. It should be understood
that any suitable number of injector outlets may be used and
injection may occur in directions other than radially.
[0042] The mixing conduit 18 also includes apertures 30 through the
outer wall 22 through which gas from the guide chamber 8 can enter
the mixing conduit 18. In this example the endwall 24 includes no
apertures, and prevents gas from entering the mixing conduit
without passing through the apertures 30 in the outer wall 22. It
should be understood that the endwall 24 may also include
apertures.
[0043] Guide means 32 are associated with each of the apertures 30.
The guide means 32 (in this case guide walls 34) cause at least
some of the gas passing through the apertures 30 to flow
circumferentially around the mixing axis 20. The guide means 32 and
apertures are better shown in FIG. 3.
[0044] The guide chamber 8 and catalytic treatment chamber 16 are
separated by a wall 13 through which the mixing conduit 18 passes.
The wall 13 is, in this case, both the end wall of the guide
chamber 8 and the catalytic treatment chamber 16.
[0045] FIG. 3 shows a detailed view of the apertures 30 of FIG. 2.
The aperture 30 is rectangular having long sides 36 and short sides
38. The long sides 36 are substantially parallel with the mixing
axis 20. The guide means 32 is a deflection wall 34 that extends
from a long side 36 of the aperture 30 into the mixing conduit
18.
[0046] Gas passing from the guide chamber 8 into the mixing conduit
18 through the aperture 30 may contact the deflection wall 34 and
be deflected so that it moves substantially circumferentially
around the mixing axis 20. The deflected gas from the apertures 30
cause the gas flow within the mixing conduit 18 to swirl or rotate
about the mixing axis 20 which increases turbulence and therefore
promotes mixing.
[0047] The deflection wall 34 of FIG. 3 is formed by cutting three
sides of the aperture 30 and then bending the resulting flap into
the mixing conduit to form the deflection wall 34 and the fourth
side of the aperture 30.
[0048] FIG. 4 shows the same view as FIG. 3, but includes arrows 6
showing the direction of gas flow. In the guide chamber 8 the gas
flow is substantially parallel with the surrounding wall 4. As the
gas enters the mixing region 10, the gas is prevented from passing
directly to the catalytic treatment chamber 10 by wall 13. The gas
is therefore `funnelled` through the apertures 30 and into the
mixing conduit 18. As the gas passes through the apertures 30 it
contacts the guide means and the gas flow is therefore caused to
rotate about the mixing axis 20. Into the rapidly swirling gas
stream a reducing agent 12 is added by the addition means 14. The
reducing agent 12 and the gas stream mix in the rapidly swirling
gas and exit the mixing conduit 18 into the catalytic treatment
chamber 16. As the gas leaves the mixing conduit and enters the
catalytic treatment chamber 16 the gas slows and the flow expands
radially as the diameter of the catalytic treatment chamber 16 is
bigger than that of the mixing conduit 18. The gas is still
rotating within the catalytic treatment chamber 16, but slower. The
rotation of the gas facilitates the radial expansion of the gas to
fill the larger diameter flow path of the catalytic treatment
chamber 16.
[0049] The deflection wall 34 helps to re-entrain droplets of
reducing agent 12 that may form within the mixing conduit. FIG. 5
shows a cross section through a mixing conduit 18 and shows
reducing agent 12 being added from addition means 14, in this case
an injector having 4 substantially radially directed outlets 26.
Again, arrows 6 are used to show the direction of gas flow.
[0050] FIG. 5 shows a droplet 40 of reducing agent on a face 42 of
a deflection wall 34 facing an outlet 26. The rapidly swirling gas
flow within the mixing conduit 18 moves the droplet 40 towards an
edge 44 of the deflection wall 38. The droplet 40 is then entrained
in the gas flow as shown by the path 46.
[0051] FIGS. 6 to 9 show different arrangement of gas treatment
apparatus. In each case, the different parts of the apparatus that
have the same function as in the apparatus 1 shown in FIG. 2 will
retain their reference numeral, but these will be incremented by
100, 200, 300 or 400 respectively.
[0052] FIG. 6 shows a gas treatment apparatus 101 having a similar
arrangement to that shown in FIG. 1 in that the apparatus 101 is
substantially enclosed by a substantially cylindrical surrounding
wall 104, with all the chambers 108, 116 and regions 110 arranged
in a substantially linear arrangement. In this case the guide
chamber, mixing conduit and catalytic treatment chamber are all
substantially co-axial with the mixing axis so that the mixing
conduit is substantially central within the guide chamber and the
outlet from the mixing conduit is substantially central with
respect to the catalytic treatment chamber.
[0053] FIG. 7 shows a gas treatment apparatus 201 having a
arrangement to FIG. 6, except that the mixing axis 220 along which
a portion of the mixing region 218 extends is arranged
substantially perpendicular to the surrounding wall 204 and a bend
50 in the mixing conduit 218 redirects the swirling gas flow
towards the catalytic treatment chamber 216.
[0054] FIG. 8 shows a gas treatment apparatus 301 having a twin
tube arrangement. The guide chamber 308 and catalytic treatment
chamber 316 are separate chambers linked by a transfer conduit 52
that extends from the mixing conduit 318 into the catalytic
treatment chamber 316. The mixing axis 320 extends substantially
perpendicular to a central axis of the guide chamber 308. In the
gas treatment apparatus 301 the gas flows through the guide chamber
308 substantially parallel with the central axis of the guide
chamber 308 and then enters the mixing conduit 318 and is caused to
swirl.
[0055] The swirling gas flows through the mixing conduit 318 and
then into the transfer conduit 52. The transfer conduit 52 directs
the gas into the catalytic treatment chamber 320 where the gas
decelerates before entering the catalytic treatment element 320. In
this arrangement, the gas flow through the catalytic treatment
chamber 316 in a direction opposite to that in which the gas flows
through the guide chamber 308. It should be understood that this
need not be the case, the transfer conduit 52 could include bends
to redirect the gas and/or the catalytic treatment chamber 316
could be orientated in a different direction.
[0056] FIG. 9 shows a gas treatment apparatus 401 which has
substantially the same arrangement as the gas treatment apparatus
301 in FIG. 8. In this case, the transfer conduit 152 includes a
bend 54 within the catalytic treatment chamber 416 to redirect the
gas stream towards the catalytic treatment element 420. In this
case, the outlet of the mixing conduit is substantially centrally
located with respect to the catalytic treatment chamber.
[0057] FIG. 10 shows a cross section through a guide wall and
aperture 70 according to a "twisted ribbon" design. The cross
section is taken so that only half the aperture and guide wall 70
is shown so that the structure can be more easily understood. The
guide wall and aperture 70 is formed by two parallel slits 72 which
define a ribbon 74 therebetween. The ribbon 74 is twisted and
deformed such that a leading edge 76 is raised out of the mixing
conduit and a trailing edge 78 is within the mixing conduit.
[0058] FIG. 11 shows a cross section through a guide wall and
aperture 170 according to an "out of plane" design. The cross
section is taken so that only half the aperture and guide wall 170
is shown so that the structure can be more easily understood. The
guide wall and aperture 170 in fact comprise two guide walls 78, 80
which are formed in a similar way to the wall 34 of the louver
design shown in FIG. 3. The wall 78 is bent so that it extends away
from the mixing conduit and the wall 80 is bent so that it extends
into the mixing conduit. In this case, both walls 78, 80 are curved
to direct gas flow. The long edges 136,236 of the walls 78, 80 that
remain attached to the mixing conduit wall 22 are adjacent.
[0059] FIG. 12 shows a cross section through a guide wall and
aperture 270 according to a "cheese grater" design. The cross
section is taken so that only half the aperture and guide wall 270
is shown so that the structure can be more easily understood. The
aperture and guide wall 270 is formed by a single slit 82 through
the mixing conduit wall 22. A region 84 of the wall 22 adjacent the
slit 82 is deformed to form a guide wall 86 that extends into the
mixing conduit.
[0060] It should be understood that the invention has been
described above by way of example only and that modifications in
detail may be made without departing from the scope of the
invention as described in the claims.
* * * * *