Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PCT/DE96/00817
UKA 101/00/WO
Cyclone With Spray Electrode
The invention concerns a cyclone for the continuous
separation of fine particles from a stream of gas, in
particular soot from the exhaust gases of internal
combustion engines in accordance with the pre-charac-
terizing parts of claims 1 and 2. A cyclone is a con-
ventional device in which the particles are enriched in a
particle collection region via flow effects within a
rotating stream formed in the cyclone. The cyclone includes
a casing having an entrance region into which an inlet
feeds, a separation region disposed in the flow path of the
gas downstream of the entrance region, the particle
collection region disposed in the flow path of the par-
ticles downstream of the separation region, and a purified
gas outlet connecting into the inside of the casing.
The entrance region is that region inside the cyclone
immediately adjacent to the inlet. A stream of soiled gas
having particles which are to be separated-out is trans-
formed in the inlet region, and in particular in the
separation region of the cyclone, into a rotating stream
within which the particles are incident on the wall of the
cyclone under the influence of centrifugal force. The
particles slow down at this location and settle into the
particle collection region or are discharged out of the
particle collection region by a boundary layer flow. The
purified gas leaves the cyclone through the purified gas
outlet. The regions in the casing of a cyclone are not
always clearly separable into an entrance region, sepa-
ration region, and particle collection region, since they
often join continuously into another. The purified gas
outlet designates that area section of the inner wall
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inside the casing which surrounds the cross sectional area
through which the purified gas leaves the inside of the
casing. In a cyclone not having a vortex finder, the
purified gas outlet is the border of the hole in the casing
through which the purified gas exits. In cyclones having a
vortex finder, the purified gas outlet is the end of the
vortex finder located inside the casing.
A cyclone has practical technical advantages, since it is
inexpensive to produce, is robust and operates reliably
when properly designed. Moreover, it can be used at high
temperatures. The separation capability of a simple cyclone
is, however, normally insufficient for particle sizes below
a particular separation limit. The limiting grain size is
between 1 ~m and 10 ~m, depending on the size of the
cyclone. A cyclone having a separation limit near 1 ~m is
designated a high efficiency cyclone. The centrifugal
forces caused by the rotating stream are only sufficient to
separate larger particles at the inner wall of the cyclone.
For very fine particles, these forces are, however, insuf-
ficient for effecting a significant degree of separation.
For many applications, a larger fraction of the particles
to be separated is located precisely in the region of
smaller particle sizes. In e.g. diesel motor exhaust gas,
approximately 80 ~ of the particle mass is soot particles
of sizes below 1 ~m. The average particle size is about 0.1
~m. Further steps must therefore be taken in order to use a
cyclone in this size region.
Additional electrical forces are conventionally used to
improve particle separation. A conventional realization of
this idea combines a cyclone with an upstream electric
agglomerator. The upstream agglomerator conditions the
particles which are to be separated-out by forming agglo-
merates in the agglomerator to increase the average size of
the particles into a region in which the downstream cyclone
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exhibits a better degree of separation. Electrical forces
are used for agglomerate formation. The agglomerators, also
designated as electrostatic precipitators, consist
essentially of a flow-through pipe in which an electrode,
connected to a high voltage source, is disposed. The par-
ticles agglomerate in a conventional manner under the
influence of the electrical field so that the agglomerates
having large grain sizes can be separated in the downstream
cyclone.
In order to use this conventional two-step process having
an agglomerator and downstream cyclone, the particles must
have a small primary size and must tend to agglomerate,
i.e. be electrically conducting. Non-conducting particles
form a solid layer in the agglomerator which cannot be
discharged with the gas stream, rather must be removed by
means of a mechanical removal of solids.
A combination of agglomerator and downstream cyclone of
this kind is known in the art from the publication DE 37 32
552 A1. This document describes the manner in which an
exhaust gas stream having particles which are to be
separated-out, in particular diesel internal combustion
engine exhaust gas having soot particles, is initially fed
through an agglomerator designated as an electrofilter. The
particles coagulate or agglomerate into larger agglomerates
in the agglomerator. The gas leaving the agglomerator,
having particle agglomerates, is fed into a downstream
cyclone which is designed in a particularly compact, ring-
shaped manner.
The patent publication DE 32 38 793 C2 also concerns a
combination of an agglomerator and downstream cyclone of
this kind. This publication describes a special feature of
the upstream agglomerator. Namely, instead of using the
otherwise conventional spray wire to produce ions moving
towards a grounded chamber wall which ions, along their
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path, accumulate on and charge the particles, an electric
field strength less than the ionization field strength is
used for agglomerate formation.
The patent publication DE 35 00 373 C2 describes a device
for the removal of particles, in particular soot particles
from the exhaust gas of internal combustion engines, having
an electroseparator comprising spray electrodes with spray
elements and a downstream centrifugal separator. This
device is characterized by the fact that the centrifugal
separator is fashioned from a plurality of mutually coaxial
cyclones adjacent to the electroseparator and disposed in a
ring-shaped manner at the exit side of the electro-
separator.
The conventional combined connection of agglomerator and
downstream cyclone has, in particular for fine particles, a
separation capability which is still not acceptable. The
constructional volume is relatively large and the pressure
loss in the gas stream is substantial. In addition, greater
constructive effort is necessary for manufacture.
Additional electrical forces, also within the cyclone, have
also been proposed for improvement of the degree of sepa-
ration of a cyclone.
For example, publication DE 37 23 153 A1 proposes sub-
jecting soot particles exiting from an electrostatic
precipitator, in which they are charged with the assistance
of spray electrodes, to an electric field in the cyclone
having substantially radial field lines. In this case, the
cyclone funnel should be made from a non-conducting
material and comprise a metallic layer on the inside or
outside circuited as a coagulation or separation electrode.
The applied electric field is intended to improve the
agglomeration or the separation of the particles, so that
the separation of the particles in the cyclone is not only
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based on centrifugal force separation, rather is also
supported by the electric field.
The device described in this document requires compre-
hensive protection measures to guarantee safe operation, in
particular when the voltage-carrying layer is arranged on
the outside of the cyclone. The manufacture of such a
device is also very difficult, since it comprises differing
materials. In addition, the stability with regard to
temperature is questionable in view of the combina~ion of
differing materials among one another as is the stability
of the electrode layer. Finally, the assembly is not
compact, since an upstream agglomerator is still necessary.
Other publications propose not only using a flat shape for
the electrodes inside the cyclone, but also allowing them
to project into the inside.
An arrangement of this kind has been proposed in US patent
publication 4,010,011 for an axial flow cyclone. In an
axial flow cyclone, the gas inlet and gas outlet lie across
from each other so that flow reversal does not occur. In
the device shown therein, the rotating stream is produced
by an axial inlet with the assistance of guide blades. The
spray electrode is a wire strung between the rotation-
producing inlet and the vortex finder. The cyclone is
provided for cleaning the intake air of internal combustion
engines, i.e. it serves primarily for the separation of
mineral dusts. These tend, due to their electrical proper-
ties, to form bonding layers on the precipitation electrode
whose removal is difficult. In order to improve removal,
the patent publication proposes coating the precipitation
electrode with an insulating layer of dielectric material.
Both the inlet as well as the vortex finder must be made
from non-conducting material to guarantee the electrical
insulation of the spray electrode. The arrangement is
therefore difficult to construct.
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Enrichment assistance using electric forces emanating from
a spray electrode disposed inside the cyclone has been
studied for return flow cyclones in a theoretical work by
P. Dietz, Powder Technology, 31 (1982) 221. A conventional
return flow cyclone comprises a cyclone funnel and a vortex
finder passing through a cover and forming the purified gas
outlet at its inside end. This publication assumes that it
is necessary to charge the particles via external spray
devices prior to entrance into the cyclone. The
configuration is therefore two-stepped, since an upstream
ionization stage is required. The described cyclone is
furthermore difficult to construct, since the spray
electrode in the cyclone is disposed on the vortex finder.
This necessitates a high voltage isolation or attachment of
the vortex finder.
Experimental investigations of this type of cyclone having
an internal spray electrode have been published in a
plurality of publications.
The publication by J. Petroll et al. in Freiberger For-
schungshefte A 220 (1962) 175, 189 describes an electro-
cyclone comprising a vortex finder made from insulating
material on which a spray basket is disposed. Construction
of the vortex finder as an insulator is technically
difficult and the publication assumes that the slight
improvement in the separation capability achievable with
the spray electrode does not justify the technical
difficulties.
The publication of B. Rabel et al. in Luft- und Kalte-
technik (1981) 107 likewise shows an electrocyclone
separator having the spray electrodes disposed on the
vortex finder. This publication states that the inner
components of the spray electrode lead to a decrease in the
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separation capability and that electrocyclone separators
represent a non-advantageous solution.
Another publication by J. Petroll et al., Luft- und Kalte-
technik (1987) 198 once more presents the opinion of
experts in this field that the corona discharge fails to
improve separation behavior and that the electrodes
disadvantageously influence flow inside the cyclone.
In summary, prior art considers the incorporation of spray
electrodes in cyclones, in particular in reverse cyclones,
to require technically difficult insulators to insulate the
vortex finder and is generally considered to negatively
influence the flow behavior in the cyclone. The charging
zone formed by the spray electrode in the region of the
purified gas outlet has also been considered to be disad-
vantageous. For these reasons, cyclones having spray elec-
trodes projecting into the inside of the casing have not
found practical application to date.
Departing from this prior art, it is the underlying purpose
of the invention to further improve a cyclone of the above
mentioned kind for the separation of fine particles from a
stream of gas, in particular soot from the exhaust gas of
internal combustion engines, having a spray electrode pro-
jecting into the inside of the casing to which an electri-
cal high voltage can be applied, in such a manner that it
has a higher degree of separation and is of compac~ con-
struction without increased technical difficulty and ex-
pense. This purpose is achieved in accordance with the
features given in claims 1 and 2. Advantageous embodiments
of the invention are given in the dependent claims.
The solution in accordance with the invention, based on a
cyclone of the above mentioned type, consists, in a first
principal aspect, in the fact that the spray electrode is
electrically insulated from the purified gas outlet and the
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purified gas outlet is connected to the casing in an elec-
trically conducting manner. This cyclone configuration has
the advantage that technically difficult measures for elec-
trical insulation of the purified gas outlet are no longer
necessary. The spray electrode is electrically insulated
from the casing via an insulator, but the purified gas
outlet is not. The purified gas outlet or the vortex finder
can be made from metal, in particular from the same metal
as the rest of the casing, and formed on the cyclone using
simple production techniques such as welding.
An additional advantage is that the spray electrode can be
disposed at a separation from the purified gas outlet or
the vortex finder. Another principal aspect of the inven-
tion, which can be realized separately or particularly
advantageously in combination with the first independent
claim, therefore proposes that an insulator bearing the
spray electrode be mounted inside of the casing at a
separation from the purified gas outlet, i.e. without
direct mechanical contact with same. If the insulator
bearing the spray electrode is not mounted to the purified
gas outlet or to the vortex finder, rather to another
location in the casing, unconventional principles of
arrangement and configurations are facilitated having
surprising and advantageous properties.
In a preferred embodiment, the purified gas outlet is
formed on a vortex finder projecting into the inside of the
casing. Also herein, the spray electrode can advantageously
be electrically insulated from the vortex finder and the
vortex finder can be connected to the casing in an
electrically conducting manner. In this case, it is also
advantageous when an insulator bearing the spray electrode
is mounted inside the casing at a separation from the
vortex finder.
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The invention is based on the realization that a partial
destruction of the weakly bound agglomerates occurs in the
turbulent rotating stream of the cyclone. The fine material
which thereby results can only be separated in the cyclone
to a limited extent. The use of electric field forces to
assist particle separation leads to the avoidance of this
break-up effect or to its compensation through reagglome-
ration or at least to its reduction. It has been discovered
within the framework of the invention that arrangement of a
spray electrode in the inside of the cyclone can efficient-
ly affect agglomerate formation or reagglomeration to such
an extent that, through the improved direct use of electric
forces to assist particle separation, it is even possible
to do without the upstream agglomerator and thereby to
achieve a very compact design. It has been discovered that,
with a cyclone in accordance with the invention, even those
particles can be separated which, due to their physical
properties, do not tend to agglomerate under the pertaining
conditions.
It has turned out that the spray electrode can be disposed
and configured in such a manner that its negative influence
on the flow field and the obstruction of the rotating
stream are sufficiently small and sufficiently capable of
compensation by the electric field that the overall degree
of separation of the cyclone is increased. Up to now, one
had attempted to keep the inside of a cyclone as free as
possible from additional elements which could obstruct the
free formation of the rotating stream. It is therefore
surprising that a spray electrode projecting into the
inside of the casing increases rather than decreases the
separation capability.
The experimental work carried out to date led to a less
positive evaluation of spray electrodes disposed inside the
cyclone since, as has been discovered in accordance with
the invention, it is not reasonable to use electrocyclones
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for the classical region of cyclone application, namely for
separation of relatively coarse particles. This is true
since, in this case, the positive effect of the electric
forces is not compensated by the negative influence of the
spray electrodes on the flow field. It has, however, been
discovered within the framework of the invention that
cyclones having spray electrodes can be advantageously used
for the separation of fine dust, i.e. of particles smaller
than 1 to 5 ~m. One has discovered that, in this size
region, separation via the electric forces dominates, so
that the disadvantages to the flow field are insignificant.
In conventional cyclones, the disadvantages to the guiding
of the flow are, in contrast, not acceptable if high
degrees of separation are required.
One reason for the skepticism of experts in the field with
regard to the separation of sub-micron particles with
electrocyclones using electrical field forces is that the
literature imparted the point view that, in this size
range, charging occurs via the relatively slow mechanism of
diffusion charging. To achieve a high degree of separation,
this mechanism requires long residence times not normally
present in a cyclone. Although one can speed up the
kinetics of charging within certain limits via high current
densities, i.e. through the use of spray electrodes having
a plurality of tips, there are however associated problems
with deposition on the precipitation electrode in depen-
dence on the particle material. Within the framework of the
invention, one has discovered that particles and dusts of
sufficiently high electrical conductivity are non-critical.
In particular, one has discovered that diesel soot fulfills
this important criterion so that no deposition problems
normally occur, even with very high electrical current den-
sities desirable for achieving a high degree of separation.
One has also discovered within the framework of the
invention that the charging of sub-micron particles occurs
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more rapidly than expected based on conventional models.
For this reason, a charging and separation of particles, in
particular soot particles, is also possible despite the
short residence times in the cyclone.
A cyclone in accordance with the invention can be confi-
gured as a return flow cyclone with which the entrance
region and the purified gas outlet are disposed at the
entrance side, i.e. in the region of the inlet. A return
flow cyclone effects a flow reversal, since the incident
gas stream must be directed back into the entrance region
after establishment of the rotating stream. Conventional
high efficiency cyclones are manufactured in this fashion,
wherein the purified gas outlet is usually formed on a
vortex finder passing through the cover of the cyclone.
Another advantageous embodiment of a cyclone in accordance
with the invention is as an axial flow cyclone, wherein the
entrance region and the purified gas outlet are disposed in
the casing at end regions axially across from another.
There is no flow reversal in axial flow cyclones. Most con-
ventional axial flow cyclones exhibit a flatter separation
curve than return flow cyclones, i.e. generally have worse
separation for comparable size. This is probably due to the
fact that the rotating stream fundamental to particle sepa-
ration is less advantageously established in such cyclones.
Configuration of prior art high efficiency cyclones as
return flow cyclones having a vortex finder disposed at the
cover side was therefore preferred. It has been discovered
within the framework of the invention that precisely axial
flow cyclones are also suitable for separation of fine
particles if they are configured in accordance with the
invention. An axial flow cyclone has the advantage that the
pressure loss is less.
The measures in accordance with ~he invention, of elec-
trically separating the spray electrode from the purified
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gas outlet and electrically connecting the purified gas
outlet to the casing or of mounting the insulator at a
separation from the purified gas outlet, can be advanta-
geously applied to both return flow as well as to axial
flow cyclones if at least one insulator bearing the spray
electrode is mounted in that region of the casing opposite
the purified gas outlet in the axial direction of the
casing. If the return or axial flow cyclone comprises a
vortex finder, the spray electrode and the vortex finder
can advantageously project into the inside of the casing
from axially opposite ends of the casing. In this manner,
the purified gas outlet or the vortex finder can be
arranged at a separation from the spray electrode. This is
technically advantageous with regard to flow and can be
easily manufactured. A cyclone of this kind has an improved
separation performance compared to conventional cyclones.
This arrangement has proved especially advantageous within
the framework of the invention, in particular, with axial
flow cyclones. This is completely surprising in view of
prior art in which only return flow high efficiency
cyclones were used to achieve optimum separation
performance. The reason for the surprising advantages of
this preferred arrangement is that the separation at small
flow velocities is dominated by electric forces, since the
residence time is sufficiently long. In contrast, the
influence of the centrifugal force dominates the separation
process at larger flow velocities. In general, the
separation performance is therefore better, the smaller the
flow velocity in the cyclone. This behavior is atypical for
conventional cyclones, since the separation therein is
improved with increasing flow velocity.
The simultaneous influence of electrical forces and
centrifugal forces with relative strengths which change
with flow velocity and particle size therefore leads to a
stabilization of the operational performance of a cyclone
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in accordance with the invention. The cyclone becomes less
sensitive to fluctuations in the distribution of particle
sizes or in the flow velocity. The corresponding optimi-
zation and adjustment measures for achieving an optimal
operational performance, i.e. the design, choice, and
dimensioning of spray electrodes and high voltage as well
as the geometric size of the cyclone, can be adjusted by
one of average skill in the art to the requirements in each
case using standard calculations or experimental investi-
gations.
It is generally true that the separation performance is
better the longer the path along which the spray electrode
projects into the inside of the casing. This is also a
surprising and atypical characteristic of the cyclone in
accordance with the invention in view of the constructional
guideline believed to date, that the inside of the cyclone
be kept as free as possible from internal components.
The separation region and/or the particle collection region
can advantageously be cylindrical in shape as is usually
the case for axial flow cyclones A hollow truncated cone
shaped configuration, as found in the cyclone funnels of
conventional high efficiency cyclones, is however
preferred.
A first advantageous embodiment of the arrangement of the
spray electrode has at least one section of the spray
electrode disposed in the entrance region. In this manner,
one guarantees that the particles already enter into the
region of influence of the electric field of the spray
electrodes when they enter into the cyclone. This permits
an efficient, early start of agglomerate formation.
A second advantageous embodiment provides that at least one
section of the spray electrode be disposed in the separa-
tion region. The spray electrode can thereby extend along
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14
the entire separation region or only along a portion there-
of. Current understanding of the inventlon considers this
to be an advantageous embodiment of the invention, since
the very strong and turbulent flow conditions reigning in
the separation region are largely responsible for agglo-
merate destruction. The arrangement of the spray electrode
in this region allows for most efficient reagglomeration
before the fine particles escape through the cross section
of the purified gas outlet.
A third advantageous embodiment can provide that at least
one section of the spray electrode be disposed in the
particle collection region. A particularly advantageous
embodiment further provides that the spray electrode be
disposed in the entrance region and in the separation
region. In this manner, the spray electrode is surrounded
by the gas stream in a manner which particularly assists
agglomerate formation.
A preferred feature proposes that the spray electrode be
axially disposed inside the casing. An axial configuration
minimizes possible negative effects on the flow field, in
particular due to the rotational symmetry of the arran-
gement.
The design of the spray electrode can be effected in any
conventional manner, e.g. as is done in the art for the
agglomerators. Spray electrodes are characterized by ~heir
spray elements. Spray elements are parts of the electrode
with surface portions having very small radii of curvature.
Conventional arrangements comprise spray wires or, if a
larger active region is required, spray baskets or a dis-
tribution of spray tips. A high charge ion concentration
can be achieved using a plurality of spray tips. The spray
tips are advantageously disposed in such a manner that
their separatlons from the wall of the casing are largely
equal to effect an even flashover voltage. The spray tips
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are often disposed about the periphery of spray disks,
which are preferred within the framework of the invention.
An advantageous improvement provides for a plurality of
spray electrodes. The plurality of spray electrode can have
the same or different shapes and advantageously have dif-
fering applied high voltages. In this manner, spray elec-
trodes can be used which are locally adapted to differing
flow conditions in the cyclone to further optimize the
separation behavior.
The advantages of the cyclone of this invention compared to
prior art are that the degree of separation is improved
and, in particular for inventive applications without com-
bined upstream agglomerator, the size is reduced and the
pressure losses decreased. In addition, the amount of
effort required for manufacturing and safe operation of the
cyclone is reduced. The invention expands the range of
application of cyclones to particles well below 0.1 ~m and
to particles which cannot be agglomerated or which agglo-
merate poorly.
The cyclone in accordance with the invention is generally
suitable for the separation of the finest of liquid and/or
solid particles from gas streams. In combination with an
upstream condensation stage, separation of condensable
gaseous substances is also possible. For the separation of
solid particles, it can e.g. be used for diesel soot sepa-
ration for exhaust gas purification of stationary and non-
stationary diesel engines in, for example, power plants,
ships, locomotives, and motor vehicles, or for production
of cleaner inert gases from combustion processes. The
separation of solid particles with the assistance of the
cyclone in accordance with the invention is also possible
in connection with soot separation for product recycling in
soot production or in soot injection in small boiler
installations as well as in salt separation in the ferti-
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16
lizer industry and for separation of sublimates in melting
or casting processes. The separation of fluid particles is
relevant e.g. for separation of so-called "blue haze" from
shredders or in particle board production as well as for
exhaust gas purification in painting and coating instal-
lations. Gaseous substances which can be separated in
combination with an upstream condensation stage include,
for example, solvents in the production of liquid paints,
dyes, and inks or in the form of odorous materials from
smoke-house exhaust. Simultaneous separation of liquid and
solid particles is possible. Examples are the separation of
dust and bituminous materials in asphalt plants and in
roofing paper production or for exhaust gas purification
following smaller anode furnaces in the aluminum industry.
Additional advantageous features and distinguishing charac-
teristics can be recognized through the representation of
the drawing in the following more closely described and
explained embodiment of the invention.
Figure 1 shows a schematic representation of the principle
of operation of a cyclone of prior art,
Figure 2 shows a cut through a cyclone of prior art,
Figure 3 shows a schematic representation of a cyclone
having an upstream agglomerator according to prior
art,
Figure 4 shows a schematic representation of a cyclone of
prior art comprising a spray electrode disposed
therein,
Figure 5 shows a cut through a cyclone in accordance with
the invention having a spray electrode disposed
therein, and
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Figure 6 shows a spray disk having spray tips.
Figure 1 is a schematic representation of the manner of
operation of a high power return flow cyclone 1 of prior
art. The casing 2 comprises an entrance region 4 having an
inlet 3 feeding therein, a separation region 5, and a
particle collection region 6 designated as cyclone funnel
27 and having the shape of a hollow truncated cone. The gas
18 which is to be purified with the particles or agglome-
rates contained therein enters through the inlet 3 tangen-
tially into the inside of the casing 7 into the entrance
region 4. The particles describe a helix which extends into
the separation region 5 and the cyclone funnel 27. Due to
the centrifugal force resulting thereby, the particles are
incident on the outer edge of the cyclone 1, are enriched
in the particle collection region 25, and sink down through
the dust exhaust opening 17 or are rinsed-out with a soiled
gas stream 20. The purified gas 19 leaving the cyclone 1
travels through the cross section of the purified gas out-
let 9 into a central vortex finder 8 which feeds through
cover 28 into the inside of the casing 7. The purified gas
outlet 9 is disposed at the entrance side so that the flow
path of the gas 18 in the casing 7 must be reversed in
order to exit through the purified gas outlet 9.
Figure 2 shows a schema~ic cut through a high power re~urn
flow cyclone 1 of prior art comprising an apex cone 16 for
stabilizing the flow spout. In order to form a stable flow
spout and well-centered rotating stream to achieve a high
degree of separation, prior art has proposed additional
measures for influencing the inward flow of the gas through
the inlet 3 into the entrance region 4 to improve formation
of the rotating stream and thereby the degree of separa-
tion. Towards this end, the installation of e.g. baffles or
a spiral inlet have been proposed. The casing inside 7 of
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18
prior art is, to the extent possible, kept free of elements
obstructing the stream.
Figure 3 shows a schematic representation of a cyclone 1
having an upstream agglomerator 22 according to prior art.
The gas 18 which is to be purified and the particles con-
tained therein are initially introduced to a pipe-shaped
agglomerator 22 comprising a spray electrode 10. The spray
electrode 10 is connected to the agglomerator casing 21 via
an insulator 11 and can be connected to a high voltage
source (not shown) via a high voltage feed-through 23. The
spray electrode 10 has an axially directed electrode finger
15 along which spray disks 12 are disposed. Agglomerates
formed in the agglomerator 22 from the particles introduced
with the gas 18 enter through the inlet 3 into the cyclone
1 and are separated therein in the manner previously des-
cribed. This figure clearly shows the large amount of space
required by the combination assembly of prior art. One also
sees that this design causes relatively large pressure
losses.
Figure 4 schematically shows a high power return flow
cyclone 1 of prior art comprising spray electrodes 10
projecting into the casing inside 7. Its structure corre-
sponds to that of the cyclones shown in figures 1 and 3. It
is distinguished by a so-called spray basket mounted to the
central vortex finder 8 feeding through the cover 28. A
high voltage can be applied to this spray electrode 10
which is introduced via the vortex finder 8. For this
reason, it is necessary to fashion the cover 28 as an
insulator to electrically insulate the spray electrode 10
and the vortex finder 8 from the grounded casing 2. Fur-
thermore, an additional insulator (not shown) is necessary
to insulate the outlet-sided end of the vortex finder 8
from the connected downstream apparatus. Figure 4 clearly
illustrates the complicated nature of the conventional
configuration. The space charge zone emanating from the
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19
spray electrode 10 and surrounding the purified gas outlet
9 is also considered disadvantageous.
An embodiment modified relative to figure 4 has been
proposed in prior art, wherein the vortex finder 8 is not
made from metal rather from an insulating material. In this
arrangement, the non-conducting vortex finder 8 itself
constitutes the insulator bearing the spray electrode 10
and the cover 28 can be made from metal. This has, however,
the disadvantage that the vortex finder 8 itself as well as
its feed through the cover 28 are technically difficult and
an additional high voltage feed-through is necessary for
passing the electrical current to the spray basket.
Figure 5 shows a cut through a cyclone 1 in accordance with
the invention. The casing 2 has an inlet 3 leading into an
entrance region 4. The inlet 3 can be an axial inlet or a
tangential inlet slot. Baffles or similar flow-guiding
elements can be provided for in a conventional manner to
optimize the inward flow. The shown preferred configuration
of the inlet 3 as a helical or spiral inlet effects an
improved centering of the rotating stream compared to the
simple slot inlet. Adjacent to the hollow cylindrically
shaped entrance region 4 along the flow path of the gas 18
which is to be purified is a hollow cylindrically shaped
separation region 5 which passes into a cylindrical and
hollow truncated cone shaped partlcle collection section 6.
A spray electrode 10, disposed in the casing inside 7, pro-
jects into the inner volume and is electrically insulated
from the casing 2 by means of an insulator 11. The spray
electrode 10 can be connected to a high voltage source (not
shown) via a high voltage feed-through 23. The insulator 11
is disposed in a casing extension 24 in which a zone having
low flow is formed. In this manner, soiling and the asso-
ciated leakage current caused by deposited particles is
counteracted. A collimator 26 is additlonally provided to
CA 02220233 l997-ll-0
also protect the insulator 11 from particles. One can also
provide that the insulator 11 be rinsed with inlet air to
keep away particlesO
The spray electrode 10 is axially disposed in the casing
inside 7 and has an axially oriented rod-shaped finger 15
bearing a plurality of spray disks 12. The spray disks 12
are mounted in a parallel, separated manner on the elec-
trode finger 15 and are perpendicularly penetrated by same.
The axial and rotationally symmetric configuration ob-
structs formation of the rotating stream to as small a
degree as possible. The diameter of the spray disks 12 or
of the spray electrode 10 is advantageously 40 to 60 ~ of
the inner diameter D1 of the separation region 5. In the
example shown, the inner diameter D1 is 120 mm, the length
D3 of the entrance region 4, separation region 5 and par-
ticle collection region 6 iS 390 mm, and the inlet 3 has a
width D4 of 25 mm and a height D5 of 65 mm.
The spray electrode 10 is disposed in the entrance region 4
and in a portion of the separation region 5 adjacent there-
to. In this manner, an efficient agglomerate formation or
reagglomeration of broken agglomerates is achieved and a
greater degree of separation effected. In order for the
spray electrode 10 in the entrance region 4 to be surroun-
ded by the entering gas 18, the insulator 11 is accommo-
dated in an upwardly disposed casing extension 24 with
which the casing inside 7 iS extended in the upward direc-
tion. The vortex finder 8 having the purified gas outlet 9
for the purified gas 19 projects into the casing inside 7
at the lower end of the cyclone 1 axially opposite the in-
let 3 from the smaller diameter end of the hollow truncated
cone shaped end of the cyclone funnel 27. It is electri-
cally conducting and connected to the casing 2 in an elec-
trically conducting manner. If necessary, the spray elec-
trode 10 can be borne, at its end facing the vortex finder
CA 02220233 1997-11-0~
8, by an additional insulator which e.g. is mounted to the
vortex finder 8.
The entire assembly can, as is conventional for high
efficiency cyclones, also have a vortex finder 8 projecting
into the casing inside 7 from above. The insulator 11
bearing the spray electrode 10 would then be mounted at the
end of the casing at the cyclone funnel side. With such a
configuration, it is howe~er usually more difficult to have
the spray electrode 10 project up to the entrance region 4.
In both cases, an apex cone 16 can also be provided for
which, in the first case, can e.g. be configured as part of
the vortex finder 8 and, in the second case, as part of the
insulator 11.
A dust exhaust opening 17 is provided in the vicinity of
the smaller diameter end of the cyclone funnel 27 to carry
out the separated particles via a partial volume flow of
the gas stream 18. The opening is configured for tangential
suctioning-off of the particles. The partial volume stream
of soiled gas 20 is about 1 ~ to 5 ~.
The gas 18, having fine particles, passes through the inlet
3 into the entrance region 4 and is set into rotation about
the longitudinal axis of the cyclone 1. This is positively
influenced when the inlet 3 is configured as a spiral in-
let. In the entrance region 4, the gas flows around part of
the spray electrode 10 to which a high voltage is applied.
Agglomerates of particles can be formed under the influence
of the electrical field forces. In any event, the electri-
cal field forces assist in separation of the particles. The
rotating stream extends into the separation region 5 adja-
cent to the entrance region 4. Due to the centrifugal force
which is increased relative to that of finer particle due
to the increased mass of the agglomerates, the agglomerates
are driven primarily radially in an outward direction. In
~his manner, they are separated out of the gas stream via
CA 02220233 1997-11-0~
the downwardly directed border-layer flow in the vicinity
of the wall. The purified gas 19 leaves the casing inside 7
through the cross section of the purified gas outlet 9 of
the central vortex finder 8. The separated particles or
agglomerates are rinsed-out, as soiled gas 20, through the
dust exhaust opening 17 with a partial volume stream.
The strong, turbulent rotating stream destroys agglomerates
inside the casing 7. This negatively influences the degree
of separation, particularly for fine particles, in conven-
tional cyclones without spray electrode 10 due to the re-
duced centrifugal forces. By arranging the spray electrode
10 inside the casing 7, in particular in the entrance
region 4 and in the separation region 5, these disadvan-
tages are compensated for so that the electric forces can
act at an efficient location. In addition, the electric
forces counteract size reduction of the agglomerates by
effecting reagglomeration of the pieces. Furthermore, an
electric field is formed between the spray electrode 10 and
the casing 2 so that the charged particles are subjected to
the outwardly directed Coulomb forces acting in the direc-
tion of the centrifugal forces and assisting particle sepa-
ration. The overall degree of separation is thereby impro-
ved and the range of application extended to non-agglome-
rating fine dust. The design is simultaneously technically
slmple O
The degree of separation ban be optimized through appro-
priate design of the geometry and flow behavior of the
cyclone 1 as well as of the shape, number, arrangement and
individual voltages of the one or plurality of electrodes
10. The installation or operational position of the cyclone
is, of course, arbitrary since gravitational forces can be
neglected.
Figure 5 shows an embodiment of a spray disk 12 having
spray tips 13. It has a diameter of about 66 mm and a
CA 02220233 1997-11-0~
thickness of 0.05 mm. Spray tips 13 are disposed at its
periphery which are curved in the direction of the peri-
phery 14. The voltage on the electrodes lies in the range
between 10 kV and 20 kV. Charge carriers are set free at
the spray tips 13 through application of a voltage above
the so-called corona threshold voltage. This leads to
charging of the particles and thereby to the formation of
agglomerates.
CA 02220233 1997-11-0~
List of Reference Symbols
1 cyclone
2 casing
3 inlet
4 entrance region
5 separation region
6 particle collection region
7 casing inside
8 vortex finder
9 purified gas outlet
10 spray electrode
11 insulator
12 spray disk
13 spray tip
14 periphery
15 electrode finger
16 apex cone
17 dust exhaust opening
18 gas
19 purified gas
20 soiled gas
21 agglomerator casing
22 agglomerator
23 high voltage feed-through
24 casing extension
26 collimator
27 cyclone funnel
28 cover
D1 inside diameter of 5
D2 inside diameter of 8
D3 length
D4 width of 3
D5 height of 3
