Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1:100~88
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to ionizers, and more par-
ticularly, to an electrode structure capable of producing a
high-intensity, radially and circumferentially uniform elec-
trostatic field through which a gas flows for ionizing the
gas for charging particles entrained in the gas.
Description of the Prior Art
Many industrial processes discharge considerable
amounts of atmospheric contaminants as particulates in the
sub-micron range. This type of particulate is most diffi-
cult to control. Eine particulate emission is becoming a
major source of air pollution as the larger particulate
problems have been easier to bring under control.
Currently, there are three basic approaches to the
problem of handling sub-micron sized particulates in contam-
inated gases. The first approach is the traditional elec-
trostatic precipitator system. The application of electro-
static precipitators to fine particulate control has severalinherent problems.
The second basic type of cleaning system is the
wet scrubbing approach. The wet scrubbing approach, as
applied to the control of fine particulates, generally is of
the high-energy venturi type. In order to capture the sub-
micron particulates in water droplets, large quantities of
water must be injected and high relative velocities em-
ployed. Both of these factors increase the pressure drop of
the system, and operating cost is directly related to this
pressure drop.
The third basic type is generally referred to as
the dry filter system. A problem with equipment of this
type, however, is the temperature limitation of the filter
elements, the related problem of the high cost of reducing
this temperature, and the difficulty in handling certain
types of particulates, such as "sticky" dusts.
Efforts have been made to improve the efficiency
of the various techniques by electrostatically precharging
the contaminants upstream of the primary collecting system.
These efforts have generally been unsuccessful due primarily
to the lack of an effective mechanism to produce a continu-
ous, sufficiently intense field to adequately charge and
affect the sub-micron sized particles.
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Ionizers for charging particles or ionizing gases
have heretofore been of the wire/cylinder, wire/plate or
needlepoint type and have been limited to field intensities
of about 10 kv/cm3 in the interelectrode region. As a re-
sult, the usefulness and effectiveness of such ionizers havebeen limited.
S UMMARY OF TH E I NVENTI ON
It is an object of this invention to provide a
process and apparatus for efficiently removing sub-micron
sized contaminants along with the larger particles from
contaminated gases such that the gases can be discharged
into the atmosphere without accompanying air pollution.
A further objective of this invention is to accom-
plish the removal of the contaminants with equipment of com-
petitive initial sales price.
A still further object of this invention is to
accomplish the removal of the contaminants with equipment of
low installation cost.
A still further objective of this invention is to
provide a process and apparatus which will substantially re-
duce operating costs, both from power consumption and main-
tenance, and still accomplish the desired removal of sub-
micron contaminants.
These and other objects of the invention are ac-
complished by a high-intensity ionizer including one or more
planar discharge electrodes concentrically positioned in a
tubular outer electrode and spaced apart from each other, or
any other corona-producing structure, by a predetermined
distance. A high voltage is connected between the discharge
and outer electrodes to effect a corona current producing
electrostatic field within the electrode gap between the
discharge and outer electrodes. The electrostatic field
expands radially and circumferentially away from the dis-
charge electrode so that the field is radially and circum-
ferentially uniform, thereby allowing the average intensity
of the field to be extremely high. The ionizer may be used
to charge particulate matter entrained in gases or to ionize
gases flowing axially within the outer electrode through the
electrostatic field in each electrode gap. The average
intensity of the electrostatic field in the high-intensity
ionizer and in conventional wire/plate and wire/cylinder
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ionizers may be increased as the velocity of the gas flowing
through the field is increased.
BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING
-- 5 Fig. 1 is a longitudinal section of one embodiment
of an apparatus embodying the principles of the invention.
Figs. lA and lB are schematic illustrations of
contaminated particle paths in a conventional wet scrubber
and in a system highly charged according to the principles
of this invention, respectively.
Fig. 2 is a fragmentary, enlarged section of the
portion of the apparatus shown in Fig. 1.
Fig. 3 is a transverse section taken along the
line 3-3 of Fig. 2.
Fig. 4 is a transverse section taken along the
line 4-4 of Fig. 2.
Fig. 5 is a transverse section taken along the
line 5-5 of Fig. 2.
Fig. 6 is a fragmentary, diametrical section of
the throat of a modified venturi wall.
Fig. 7 is a diagram of the electrostatic field
between the electrodes of the invention.
Fig. 8 is an axial section of a form of ionizer
illustrating the principles of a second invention.
Fig. 9 is a transverse section of the embodiment
of Fig. 8.
Figs. 10A-lOD are various edge radius shapes.
Fig. 11 is another embodiment of an ionizer.
Fig. 12 is an isometriv view of another embodiment
of the invention utilizing a plurality of axially spaced
discharge electrodes enclosed in a tubular outer electrode.
Fig. 13 is a cross-sectional view illustrating a
single precipitator unit.
Fig. 14 is a cross-sectional view of an alterna-
tive embodiment of a precipitator unit utilizing dischargedelectrodes of varying transverse dimensions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, the gas containing the con-
taminants is directed through an inlet duct 1 by a blower lato the entrance of a gas contaminant-charging venturi sec-
tion 2. The gases and contaminants are accelerated to an
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elevated velocity that will be a maximum in the venturi
throat. The principles of the invention, however, are
applicable also to a constant velocity gas stream in which a
venturi is not employed. A highly intense corona discharge
is maintained in the venturi throat by a high-voltage DC
power supply 3. The discharge D propagates from a highly
stressed planar discharge electrode, such as a disc 4, cen-
tered in the venturi throat, to the outer wall 5 of the
venturi in a radial direction. The corona discharge is
extremely thin (less than one diamter of the outer electrode
5) in the direction of the gas flow and, hence, the resident
time of the contaminant particles in the electrostatic field
is short. A high level of electrostatic charge is imposed
on the particles, however, for several unique reasons.
In accordance with conventional practice, the term
"electric field," as used herein, shall designate a non-
corona field, while the term "electrostatic field," as used
herein, shall designate a corona field.
Although an electrode having the shape of a circu-
lar disc is shown and will be described in detail, a toroid,ellipsoid, polygon, such as a square or other configuration,
may also be used as long as the shape of the outer electrode
approximately conforms to the shape of the discharge elec-
trode. Similarly, the outer edge of the electrode 4 need
not be smoothly curved as viewed in Fig. 2. Other designs
that can be used include, for example, blunt edges, sharp
edges, or edges having a plurality of closely spaced pro-
jections. It is also possible to use electrodes with ser-
rated edges. The term discharge electrode or inner elec-
trode as used herein is intended to cover all such config-
urations.
While optimum performance is obtained by centering
the discharge electrode 4 concentrically within the venturi
throat wall 5 (outer electrode), it will be understood by
one skilled in the art that the apparatus will function
effectively with off-center positioning as well.
The outer electrode 5 has a radius Ro (the radius
of the outer electrode as shown in an axial cross-section
through the outer electrode) which forms the transition
between the inlet cone and the throat of the venturi. How-
ever, the outer electrode need not be in the form of a ven-
turi since other structures, such as cylindrical or square
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88
structures having straight sidewalls, may also be used. The
venturi recovers the energy imparted to the gas stream by
accelerating the stream in the venturi throat, and it pro-
vides a smooth flow of the gas past the discharge electrode
4 so that a film of cleaning fluid flowing along the wall of
the outer electrode 5, as explained hereinafter, is not dis-
rupted. ~lthough the structure of the outer electrode may
vary considerably, best results are obtained with a venturi
having a radius Ro which has a ratio of above 50:1 relative
to the inner electrode edge radius r.
The axial location of the discharge electrode 4
within the venturi throat can be varied within limits.
Shifting the locating upstream increases the gap R3 to reduce
the field intensity and requires higher voltage requirements
but reduces the velocity of the contaminated gas stream.
Reducing velocity both aids and detracts from ionizing effi-
ciency within limits which will be described.
All of the above variations of the preferred
illustrated configuration will degrade the performance to
some degree; however, many operations or uses of the inven-
tion will not be necessary to obtain maximum operating con-
ditions, and more economical construction techniques may
suggest the use of one or more variations with acceptably
lower ionizing efficiency.
Thus far the invention has been described as an
ionizer for use upstream of a contaminant cleaning appara-
tus, such as a scrubber or precipitator, to substantially
increase the efficiency of the cleaning apparatus. The ion-
izer, however, has other applications as well. For example,
it may be used merely to charge particles for electrical
power generation; i.e., EGD (electro-gas-dynamic) genera-
tion, or ionize gas streams for gas phase reactions, for
example, generating atomic oxygen for oxidizing reactions,
such as ozone generation for odor removal or sulphur dioxide
to sulphur trioxide reactions. In these applications, a gas
stream at the velocities described herein is directed past
the ionizer in the same manner as the contaminated gas
stream, but it may be desirable to limit the passage of the
gas through the field to a specific radial location. How-
ever, surface cleaning of the outer electrode is not neces-
sary if particle deposition does not occur.
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The electrostatic field Eo sustained between the
discharge electrode 4 and outer venturi throat wall 5 is
comprised of two elements, an electric field Ee and a space
charge influence, as shown in the chart of Fig. 7. The
electric field is related to the applied voltage and the
electrode geometry. The space charge influence, comprised
of ions, electrons, and charged particles in the interelec-
trode region, is created after corona discharge has been
initiated. As shown in Fig. 7, the space charge influence
tends to amplify the field in the region closer to the outer
venturi throat wall and suppresses the highly intense field
closer to the electrode. This effect causes the field to be
radially uniform, thereby stabilizing the corona discharge
while allowing a high electrostatic field to bridge the
entire interelectrode region R3. This is accomplished with-
out spark breakdown by electrode design, maintaining a high
velocity in the region and a clean surface on the outer
electrode 5.
To best understand the uniqueness of the inventive
electrostatic field, reference is made to electrostatic
fields produced by two types of well-known prior art elec-
trode configurations. In a wire/cylinder electrode configu-
ration, a wire extends along the axis of a cylinder. The
electric field between the cylinder and the wire is entirely
radial, with no components (neglecting edge effects at the
ends of the cylinder) extending along the axis of the cylin-
der. When a voltage is initially placed between the wire
and cylinder, a space charge is not present. The intensity
of the electric field at any point between the wire and cyl-
inder is inversely proportional to distance from the wire sothat the intensity of the field continuously decreases radi-
ally outward from the wire toward the cylinder. As the
voltage between the wire and cylinder increases to a corona-
starting voltage, the electrons in the air adjacent the wire
(where the field is the greatest) are accelerated toward the
cylinder (assuming a negative voltage on the wire), imping-
ing on gas molecules driving off additional electrodes.
Since the molecules have now lost an electron, they become
positive ions, which, by virtue of the wire's negative
potential, accumulate adjacent the wire. The space charge
continues to build up by a phenomena commonly known as the
"avalanche process." In the avalanche process, the high-
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energy electrons accelerated radially outward by the elec-
trostatic field strike additional molecules. The extremely
high energy of the electrons allows them to separate elec-
trons from the nucleus of the molecule, thereby creating
additional free electrons e- and additional positive ions.
It is to be emphasized that the avalanche process occurs
only near the wire since it is in this region that the field
is the greatest. As the electrons migrate radially toward
the cylinder, the deceleration caused by striking molecules
exceeds the acceleration caused by the field since the
intensity of the field is reduced away from the wire. At
points where the field is approximately 30 kv/cm and below,
the free electrons, instead of freeing additional electrons
by the avalanche process, attach to electronegative gas
molecules to form negative ions, such as from 2 to 2~ when
air is the gas between the electrons. Oxygen is the only
major electronegative component of air. Thus, for air the
only negative ion is the 2 ion. However, other negative
ions may be formed for other electronegative gases. Other
commonly produced electronegative stack gases include SO2,
water vapor, and CO2. The 2 ions are accelerated toward
the positive potential cylinder where, en route, they form a
negative ion space charge in the interelectrode region. In
summary, in the area adjacent the wire where the field is
greater than 30 kv/cm, electrons which are accelerated by
the field have sufficient kinetic energy that when they
strike molecules, a free electron and a positive ion are
formed by the avalanche process. Away from the wire, the
electrons of reduced energy attach to oxygen molecules,
forming 2 ions. As positive ions accumulate adjacent the
negative potential wire and 2 ions accumulate between the
positive space charge and the cylinder, the electric field
is modified so that the intensity of the electrostatic field
adjacent the wire is reduced, while the intensity of the
electrostatic field toward the cylinder is increased. The
reduced electrostatic field adjacent the wire reduces the
quantity of free electrons and positive ions produced by the
avalanche process, and the increased electrostatic field
adjacent the cylinder increases the migration of 2 ions
toward the cylinder. The result is a stabilizing, or nega-
tive feedback, effect which maintains the space charge dens-
ity relatively constant with time. Even though the 2 ions
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increase the intensity of the field adjacent the cylinder,
the increase is insufficient to prevent the intensity of the
field from continuously decreasing toward the cylinder.
Electrostatic fields produced by the wire/cylinder
electrode configuration are relatively inefficient since the
relatively large area of the cylinder walls results in a
large interelectrode current flow to maintain a given aver-
age field intensity.
The second conventional electric field to be exam-
ined is that of the wire/plate, in which a potential isplaced between a wire positioned in parallel between a pair
of parallel plates. With this type of electrode configura-
tion, the gas stream passes through the field along a line
perpendicular to the wire and parallel to the plates. The
electrostatic field generated by a wire/plate electrode con-
figuration produces a space charge in the same manner as
the wire/cylinder electrode configuration. However, since
the electrostatic field adjacent the plates is more intense
directly across from the wire, the space charge formed by
the 2 or negative ions from other electronegative gas is
more concentrated in this area. This is in contrast to the
negative ion distribution of the wire/cylinder electrode
configuration, where the negative ion concentration is uni-
formly distributed around the periphery of the cylinder.
The space charge amplification of the wire/plate electro-
static field is most intense opposite the wire so that the
intensity of the electrostatic field between the wire and
plate is greatly increased toward the plate. The high elec-
trostatic field at the plates and high local current deposi-
tion cause sparkover unless the average field intensity ismaintained at a relatively low value. Therefore, with a
wire/plate electrode configuration, it is not possible to
achieve relatively large, average electrostatic fields.
With the wire/plate electrode configuration, as with the
wire/cylinder, the electrostatic field (neglecting edge
effects at the ends of the wire) does not vary along the
axis of the wire. In other words, the components of the
electrostatic field extend in only two directions of a car-
tesian coordinate system and result in a non-uniform elec-
trostatic field.
In the inventive electrode configuration, theplanar discharge electrode 4, concentrically placed within
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the cylindrical outer electrode 5, creates an electrostatic
field having components which extend along the three dimen-
sions of a cartesian coordinate system. The X and Y compon-
ents of the Eield (or the R components in a cylindrical
coordinate system) are substantially identical to the elec-
tric field of the wire/cylinder when viewed along the axis
of the cylinder. Thus the concentration of negative ions in
the interelectrode gap in a plane transverse to the outer
electrode 5 is uniform about the periphery of the discharge
electrode 4, resulting in uniform space charge amplification
of the electrostatic field. However, the electric field
between the discharge electrode 4 and the outer e~ectrode 5,
when viewed in a plane axial of the outer electrode 5, is
substantially identical to the electric field of the wire/
plate electrode configuration. The concentration of nega-
tive ions in a plane passing through the axis of the outer
electrode 5 is greater in the plane of the discharge elec-
trode 4 than at points axially spaced therefrom.
The intensity of the electric field as a function
of the distance from the axis of the discharge electrode 4,
as illustrated by the solid line in Fig. 7, continues to
decrease from the electrode 4 toward the outer electrode 5.
The electrostatic field, including the space charge amplifi-
cation, as illustrated by the broken line in Fig. 7, is sub-
stantially constant throughout a substantial distance fromthe outer electrode 5 toward the discharge electrode 4 so
that the electrostatic field is substantially uniform within
a generally wedge-shaped volume diverging outwardly in a
direction perpendicular to the gas flow, as illustrated at D
in Fig. 1. Thus the field has a radial dimension approxi-
mately equal to its axial dimension at the outer electrode
5. The uniform space charge distribution occurring in a
plane transverse to the discharge electrode 4 has some of
the characteristics of a wire/cylinder-type electrostatic
field in which the field continues to decrease as the outer
electrode 5 is approached. The non-uniform ion concentra-
tion occurring in the axial plane of the discharge electrode
4 has some of the characteristics of a wire/plate-type elec-
trostatic field in which the intensity of the field is
increased toward the outer electrode 5. The inventive elec-
trostatic field, by simulating a wire/cylinder electrostatic
field in one plane and a wire/plate electrostatic field in
l~V(~388
an orthogonal plane, combines the continuously decreasing
electrostatic field of the wire/cylinder with the continu-
ously increasing electrostatic field of the wire/plate to
produce an electrostatic field having an intensity which is
substantially uniform in a radial direction throughout a
substantial distance from the outer electrode 5 toward the
discharge electrode 4 within a generally wedge-shaped volume
diverging outwardly in a direction perpendicular to the gas
flow. Thus the field between the electrodes 4,5 has a
radial dimension equal to the spacing between the electrodes
4,5, which is approximately equal to the axial field adja-
cent the outer electrode 5, i.e., the dimension of the field
adjacent the outer electrode 5 in a direction perpendicular
to a plane passing through the electrodes 4,5. The uniform-
ity of the electrostatic field allows a highly intense aver-
age field without producing sparkover since there is no
point at which the field becomes excessively intense, such
as at the plate of a wire/plate system, which limits the
average intensity of the field which can be applied without
sparkover. The electrostatic field configuration is also
less prone to sparkover because it delivers far less current
per unit area at the outer electrode at a given field inten-
sity than the wire/plate electrode configuration. The radi-
al symmetry of the electrode structure produces a field
which is also constant along a circumferential path of con-
stant radius.
Both the wire/plate and wire/cylinder electrode
structures generate electrostatic fields which are elongated
systems and contact relatively large areas, thus causing a
relatively large current flow. In contrast, the inventive
electrode configuration produces current flow in a relative-
ly small area so that a highly intense electrostatic field
is maintained utilizing a minimum of current (and hence pow-
er) without causing sparkover between the electrodes.
Cleaning of the outer electrode surface is neces-
sary only to maintain the surface relatively clean in order
to minimize spark breakdown. Where maximum field intensity
is not necessary and lower voltages can be applied, or the
ionizing occurs in clean gas streams, or during other condi-
tions not producing serious buildup on the surface, cleaning
or flushing is, of course, not required. Also, intermit-
tent cleaning may be used.
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The inner electrode design introduces large
amounts of current (ions) by corona discharge due to the
intense field close to the electrode surface. The electrode
design also provides a concentrated electric field region
all the way to the venturi throat wall 5. This concentrated
residual field directs the space charge on this path in its
migration to the wall and is responsible for the proper
field amplification. The smoothly curved, generally radial
periphery of the inner electrode causes the space charge to
expand circumferentially in the throat, reducing the current
deposition per unit area at the outer electrode to reduce
potential spark breakdown.
Since the electrostatic field is relatively thin
in the direction of the gas stream, higher velocities of gas
flow through the field tend to diffuse the ion concentration
axially from the plane of the inner electrode. This adds
further stability by expanding the space charge region in
the direction of flow to decrease the electrostatic field
between the space charge region and outer electrode 5. This
"velocity enhancement" effect is maximized at gas velocities
of 50 fps and above. In addition, turbulence at these high
velocities may also provide stability by mechanically dis-
rupting the mechanism which causes spark breakdown.
To maintain the corona and, hence, the performance
of the charging unit from contamination and degradation, the
discharge electrode 4 is isolated from other leakage paths
besides the corona discharge. As best shown in Fig. 2, a
probe 10 supports the electrode 4 in its proper location in
the outer electrode 5 and provides high resistance to elec-
trical leakage, both internally and on its surface. Althoughnot shown, the probe can be moved axially or laterally if
desired. The resistance is provided between the electrode
and the support structure 12 of the probe in the upstream
duct 1. Surface resistance is improved by providing one or
more clean air bleeds 14 which are continuous slots (.030")
around the circumference of the probe just upstream of the
electrode 4. Clean air, provided by an outside supply 15,
is fed through the probe body and passes out these slots at
high velocity. This action maintains a positive high-
resistance path that the surface leakage would have to"bridge" to short the electrode 4 to ground.
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The probe body includes a high-voltage cable 16
supported by dielectric hubs 18 which secure the probe to
the duct 1. The upstream end of the probe body is contained
in a closed shroud 20 and a hollow, corrugated cover 22.
Openings 23 allow passage of the air axially to a plurality
of spaced rings 26, each with corresponding slots 24 (Fig.
3). The spacing forms the series of continuous slots 14 for
bleeding the air, as mentioned above.
Electrode 4 also has slots 24 which allow airflow
downstream of the electrode. The rings and electrode disc
are secured to the cable 16 by a bolt 28 fitted in a nose
30. The nose and clean air from the downstream side of the
electrode prevent stagnation of charged contaminants down-
stream of the discharge electrode 4 and prevent deposition
of particles on the surface of the electrode 4.
In order to allow the field to expand axially
between the discharge electrode for an outer electrode 5, it
is important to isolate the discharge electrode 4 from any
other structure that generates a corona current producing
electrostatic field by at least 1.25 electrode gaps. In the
embodiment illustrated in Figs. 1-11, this is accomplished
by mounting a single discharge electrode within the outer
electrode S. However, in the embodiment illustrated in
Figs. 12-14, multiple discharge electrodes are used, as ex-
plained hereinafter, so that this axial spacing requirementbecomes more critical than in the embodiment illustrated in
Figs. 1-11.
The outer electrode 5, because of contaminant
buildup, is kept smooth and reasonably clean for a short
distance of several times the corona gap R3. This assures
that disturbances in the corona from the outer electrode
surface, such as contaminant buildup, will be eliminated.
This cleaning can be accomplished in several ways; one
technique is shown in Figs. 1 and 2. Water or a similar
fluid is injected by an external pump 32 in a smooth layer
on the surface of the converging cone section of the venturi
wall 5. Where the outer electrode 5 is a venturi, the angle
of convergence phi of the venturi is held at about 12.5
half-angle to minimize turbulent flow effects. The venturi
in use is pointed in a downward direction and the water film
is accelerated as it approaches the throat, both from grav-
ity and friction with the moving gases. The point of water
1101; ~8~38
injection is about 1.5 electrode gap R3 lengths line-of-
sight upstream from the electrode 4 or approximately 1.12
electrode gap R3 lengths axially upstream from the discharge
electrode 4 intersecting the outer electrode. The expansion
of the downstream divergent cone of the venturi is less than
3.5, again to minimize effects from flow separation. The
radius Ro that forms the transition between these angles
should be no smaller than about two inches. Water injection
is accomplished by a weir arrangement including a thin
(.010-.025"), continuous slot 40 formed by a surface 40 on
the circumference of the converging cone with a nozzle
direction beta of about 12.5 half-angle to the sidewall of
the venturi. The action of the water on the wall of the
venturi maintains a smooth, clean surface without degrading
corona performance for velocities of gas flow up to about 75
fps. Water consumption varies with venturi size and ranges
from .2 to 2 gpm/1000 acfm for 5 inch to 50 inch venturi
diameters.
Water is prevented from migrating upstream along
the outer electrode 5 by providing an inwardly directed band
or deflector 42 insulated from the cooler water. The water
from pump 32 is directed under pressure tangentially into a
housing 44 and leaves the housing through slot 40 in an
axial direction to minimize spiralling of the water as it
passes through the electrostatic field.
The stable, high-intensity electrostatic field of
the present invention can be produced with a wide variety of
electrode designs as long as the discharge electrode 4 is
isolated from other corona curent producing electrostatic
fields by at least 1.25 electrode gaps and the shape or
periphery of the discharge electrode conforms to the shape
of the outer electrode 5. The variations in electrode
designs may include, for example, square or hexagonal dis-
charge electrodes 4 in square or hexagonal outer electrodes
5, respectively, and discharge electrodes 4 having blunt,
sharp, serrated, or barbed edges. However, for best per-
formance, based on present experimental data, the discharge
electrode 4 preferably has an edge radius r designed such
that the ratio of electrode gap R3 to the discharge electrode
4 radius r is about 100:1. If the ratio is set below 50:1,
sparking will occur at low applied voltage, yielding a low
operating current and field. If the ratio exceeds 400:1,
the electric field contribution in the gap is reduced, which
llOU8~38
results in higher operating current to maintain the high
fields. Where the ou~er electrode 5 is in the form of a
venturi, radius Ro (the venturi throat radius in axial cross-
section should be set no less than a ratio 50:1 with the
discharge electrode radius r. Smaller radii will induce
sparking at lower applied voltages. The diameter of the
probe 10 and, hence, the overall diameter of the discharge
electrode 4, should be set such that the probe occupies
around 10% of the cross-sectional area of the outer elec-
trode 5. A practical minimum is 5% and a practical maximumis 40%. A probe occupying a smaller percentage of the outer
electrode 5 causes an increase in discharge electrode sur-
face power density. More importantly, smaller values also
increase the electrode gap for constant flow capacity of the
unit, thereby increasing power supply voltage requirements
significantly. Values greater than 40% increase size of the
outer electrode 5 and probe cost and increase probe isola-
tion air bleed requirements and, hence, operational cost.
In summary, the ratio of the electrode gap R3 to
the edge radius r should be between 50:1 and 400:1, and pre-
ferably about 100:1; the fill area (the percentage of the
outer electrode occupied by the discharge electrode 4)
should be between 5% and 40~, and preferably about 10%; the
discharge electrode 4 must have a shape corresponding to the
shape of the outer electrode 5; and the discharge electrode
4 must be spaced apart from all structures which generate
corona current producing electrostatic fields by at least
1.25 electrode gaps. With these electrode geometries, typi-
cal high-voltage requirements are such that an average field
of about 18-20 kv/cm can be maintained across the electrode
gap R3 at standard atmospheric conditions (a pressure of
29.92 inches of mercury and a temperature of 70 degrees) and
zero velocity. With gas velocities greater than about 50
fps, the field can be increased to about 24-28 kv/cm without
sparking.
Several important functions occur in the highly
intense corona region of the charging unit. The suspended
contaminants are field-charged by the strong applied fields
and ion impaction in the high ion-dense region R3. It is
presumed that the diffusion charging mechanism has minor
contribution here on the fine particles due to the short
residence time of the particles in the corona. There will
! 15
',~
11~()888
be a slight displacement of the particles outward radially
as they become charged and miyrate in the strong fields of
the corona. The amount of this displacement will vary with
the size of the particles, so some mixing, impaction, and
- 5 possible agglomeration can occur. This is seen as a minor
effect in view of the thermal agitation and flow turbulence
present. In the case of liquid aerosols, however, the
effects of strong applied fields (greater than 10 kv/cm),
high temperatures, and turbulent mixing cause significant
agglomeration to occur, and this effect can be witnessed
downstream of the corona. This can be of great benefit in
the collection of fine aerosols as particles agglomerate and
"grow" to larger, more easily collected sizes.
Velocity of the gases through the highly charged
corona area affects the charging efficiency of the system.
Above about 50 fps, the space charge region of the field
becomes axially spread by the gases to reduce the possibili-
ty of a spark breakdown; that is, greater stability of the
corona is achieved. With the increases in velocity, how-
ever, the advantage of increased stability begins to becomeoffset by the disadvantage of the shorter resident time of
the contaminants in the field, and thus a reduction in
charge on the particles, and increased disruption of the
water film on the outer electrode wall if water cleaning is
used. Up to about 125 fps, there is a gain in stability of
the corona, but with a decrease in charging efficiency. For
one system tested, the maximum charge on the particulate
appears to occur at about 100 fps. To a great extent, how-
ever, gas velocity must be a trade-off between the capacity
needed for efficient operation of the industrial gases being
cleaned, electrode voltage requirements, and venturi wall
cleaning capability.
A second method of venturi wall cleaning is illus-
trated in Fig. 6. In this embodiment, a perforated or por-
ous air bleed section 70 is provided on the outer electrode5 adjacent the discharge electrode 4 to provide an air film
over the downstream wall of the outer electrode 5 rather
than a water film. Downstream of the air bleed section 70
for a distance of several electrode gap R3 lengths, the out-
er electrode 5 wall surface is coated with a material ofhigh electrical resistivity for providing electrical isola-
tion of the particles deposited in this area.
ll~U888
Still another method is the use of gas stream
erosion to limit the thickness of the deposition to permis-
sible levels.
Still another method is to vibrate or shock the
wall to intermittently or continuously dislodge the contami-
nants before buildup.
The suspended particulate contaminants having
passed through the electrostatic field are highly charged,
are of like polarity, and are migrating to the outer venturi
wall downstream of the corona. Deposition on the wall which
occurs is minor and represents only those particles travel-
ing near the wall on their original trajectories. Since the
applied field in this region is primarily of the space
charge element and, therefore, the migration velocities are
low in comparison to stream velocities, the bulk of the par-
ticles remain in the stream for considerable distances. At
least two forms of collection of these highly charged, sus-
pended particulates can be employed.
One technique for collecting the charged particles
is a conventional electrostatic precipitator. Another tech-
nique is a wet scrubber 50 to be described. The gas contam-
inant charging section of the outer electrode 5 is directly
attached to the throat 52 of the venturi scrubber 50. In
general, the design velocity of the outer electrode is con-
sistent with the desired velocity in the scrubber venturisuch that the charging section divergent cone angle is set
at about 0. The charged particle-laden gases pass through
the scrubber venturi with the particles collected onto water
drops by impaction and interception enhanced by the electro-
static forces. Water enters the venturi scrubber in a con-
ventional manner as through a continuous slot 54 and is
atomized by the gas stream. The water droplets are oppo-
sitely charged to the particles by induction because the
atomization process occurs in a residual field region.
Preferably, at low venturi velocities (below about 75 fps),
the injection point should be at least two gaps R3 down-
stream of the discharge electrode 4 to prevent prematurespark breakdown. At higher velocities, greater separation
distances are required due to ions drifting downstream of
the corona which tend to foul the induction process by un-
desirably charging the water droplets with the same polarity
as the charged particles. By extending bolt 28, the induc-
U888
tion charging field is increased axially, even though theseparation distance between the electrode 4 and the injec-
tion point is increased. This also provides for a cylin-
drical field emitting from the bolt which drives the ions
toward the outer wall 5 downstream of the electrode 4.
The collection efficiency of a conventional
venturi scrubber depends upon the inertial impaction of par-
ticles on water droplets. The impaction is accomplished by
high relative velocity of the contaminated airstream and
water droplets injected at low velocity. The sub-micron
sized particles escape impaction by following the slip
stream around the water drops instead of impacting. (An
example is illustrated schematically in Figure lA.) This is
due to their high aerodynamic drag-to-inertia ratio. Par-
ticle bounce and rebound also become important considera-
tions in cases of marginal impaction and interception ener-
gies. Particles with low impaction energies fail to pene-
trate the water droplets due to surface tension effects.
Particles containing a high (10 kv/cm saturation
charge) electrostatic charge and with induced charge on the
water droplets, as in this invention, have an attractive
force between the charged particles and water droplets suf-
ficient to significantly affect their impaction trajector-
ies, as shown schematically in Fig. lB. This effect results
in a substantial improvement in collection efficiency over
the basic scrubber efficiency. The impaction improvement
effect varies with the particle size and the relative velo-
city between the particles and water droplets.
The sensitivity to particle size is minor with a
variation in effect of only + or - 20% when considering 0.1
micron through 10 micron size particles. Since the longer
the electrostatic forces have time to act, the more effec-
tive they become, lower relative velocities between charged
particles and water drops yield a larger improvement effect.
Since lower velocities also yield less effective atomization
of the scrubber fluid and larger equipment sizes, an optimum
velocity range becomes apparent.
Below about 50 fps relative velocity, atomization
in the venturi scrubber degrades rapidly; therefore, liquid
requirements increase substantially to maintain efficiency.
About 200 fps relative velocity, the pressure drop across
the system due to water droplet acceleration losses becomes
18
11C~(~888
excessive. Therefore, the optimum improvement in collection
efficiency of the gas contaminant-charging unit on a venturi
scrubber collector occurs with venturi scrubber designs
around 125-150 fps in the throat.
One tested embodiment of the invention employed a
gap radius R3 of 1-1/2 inches, a discharge electrode edge
radius r of 1/64 of an inch, a peripheral radius Rl of .875
inch, an outer electrode radius R2 of 2-3/8 inches, a con-
verging cone angle phi of 12.5, and an outer electrode
(axial cross-section) radius Ro of 3-4 inches. The embodi-
ment had a 750 cfm capacity, with gas flow of about 120 fps
in the scrubber venturi. Typical prior art "scrubber only"
collection efficiency of this design is approximately 81% at
a .5 micron particle size. Collection efficiency is in-
creased to approximately 95% at .5 micron particle size when
the gas contaminant-charging unit of this invention is acti-
vated. The system at this condition consumes approximately
7.5 gpm/100 acfm of water, 150 watts/1000 acfm charging unit
power, and has 4 inches of water system pressure drop.
A second tested embodiment employs a gap radius
R3 of 2.15 inches, a discharge electrode edge radius r of
1/64 of an inch, a peripheral radius Rl of .875 inch, an
outer electrode radius R2 of 3.03 inches, a converging cone
half-angle of 15, and a venturi radius Ro of 2 inches.
The embodiment had a 1000 cfm capacity, with gas flow of
about 150 fps in the scrubber venturi. The typical prior
art "scrubber only" collection efficiency of this design is
approximately 94.6% at a 1.25 micron particle size. Collec-
tion efficiency is increased to approximately 97.5% at 1.25
micron size when the gas contaminant charging unit of this
invention is activated. The system at this condition con-
sumes about 6 gpm/1000 acfm of water, 150 watts/1000 acfm
charging unit power, and has 6 inches of water pressure
drop.
Typical corona-ionizing apparatus in the prior art
have generally been limited to operating field intensities
of 3-6 kv/cm. With the ionizer of this invention using the
optimum electrode design and fluid velocity past the elec-
trodes, operating field intensities up to 15 kv/cm are
obtainable without spark breakdown.
1~0~888
One incidental advantage of the invention occurs
from the discovery that the velocity effect which axially
diffuses the space charge to assist in reducing potential
breakdown can be used advantageously alone with more conven-
tional precipitation designs to greatly increase their oper-
ating field strength. For example, Figs. 8 and 9 illustrate
a known ionizer using a single wire electrode 80 placed
transversely across a venturi throat 81 of a rectangular
duct 82. Insulators 83 isolate the wire from the duct in a
known manner. The wire is connected to power supply 3 as in
the preferred embodiment.
Normally, a single wire/plate ionizer must be
operated at low applied voltages such that the average field
between the electrodes does not exceed about 10 kv/cm before
spark breakdown. Velocities are kept low, at about 10 fps.
A typical example of this operation is a home electrostatic
air cleaner. Using the higher velocities of about 50 fps of
this invention, average field intensities of above 10 kv/cm
can be obtained without spark breakdown since the velocity
sweeps the excess space c'narge downstream out of the most
intense field.
By the same mechanism, multiple transverse wire
precipitators having transverse wires spaced axially along a
duct are also limited to low voltages, even with higher
fluid velocities since the displacement of ions from one
wire region will then be exposed to the next downstream
field region.
Multiple transverse, axially spaced wires can be
used, of course, if spaced axially sufficient distances
apart to allow ions from each next upstream wire to migrate
to the outer electrode (duct) prior to entering the ionizing
field of the downstream wire.
Fig. 11 illustrates another embodiment having
electrode ends 80a of a radial configuration and central
electrodes 80b of linear configurations. Preferably, the
duct 82 is again rectangular, but could be curved to match
the electrode. Air ports 24 are provided as shown in Figs.
3-5. All of the shapes of Figs. lOA-lOD can be used, of
course, for the edge radius r. This electrode configuration
will perform most like the wire/plate electrode of Figs. 8
and 9, but also will obtain some of the advantages of the
more radial-type electrodes.
1110~888
Another embodiment of the precipitator device
illustrated in Fig. 12 utilizes several parallel precipi-
tator units 110. A particle-entrained gas enters an inlet
manifold 112 through an annular duct 114. The manifold 112
is partially filled with water 116, and a plurality of plan-
ar, spaced baffles 118,120 are positioned in the manifold
112 such that the gas passing through the manifold 112 must
flow through the water 116 around the baffles 118,120. The
water 116 is provided to cool the gases entering the inlet
duct 114 to promote particle formation by condensation.
Additional cooling is provided by a pre-quench spray includ-
ing a plurality of nozzles 112 communicating with a water
inlet 124 through pipes 126. The cooling water is recycled
by a pump (not shown) with moderate pressure.
After the particulate-laden gases have been suf-
ficiently cooled and saturated, they are directed to the
portion of the manifold 112 beneath the precipitator units
110. As best illustrated in Fig. 13, a sloped collection
surface 128 is placed beneath the precipitator units 110,
and an annular aperture 130 having a concentric, cylindrical
shield 132 is placed beneath each precipitator unit 110 with
the shield 132 extending into the lower end of a tubular
outer electrode 134. Each of the precipitator units 110 in-
cludes a cylindrical outer electrode 134 enclosing a planar
discharge electrode 136 mounted at the lower end of an elon-
gated support electrode 138. As explained in detail herein-
after, the particles in the gas passing through the aperture
130 are charged within the relatively thin, radially and
circumferentially uniform electrostatic field extending
between the discharge electrode 136 and the outer electrode
134. The term "electrostatic field," as used herein, desig-
nates a field producing a corona discharge, while the term
"electric field" designates a non-corona-producing field.
The term "relatively thin" designates a field hav-
ing a radial dimension which is substantially greater than
its axial dimension. The charged particles are then accel-
erated toward the outer electrode 134 by the relatively low-
er intensity electric field extending between the support
electrode 138 and the outer electrode 134. The particles
deposited on the inside walls of the outer electrode 134 are
collected by a film of water covering the inner walls of the
outer electrode 134 and flow downwardly, where they are
:.
888
deposited on a collection surface 128. The shield 132 pre-
vents the liquid film from being entrained in the upward gas
stream when the liquid falls off the end of the outer elec-
trode 134 onto the collection surface 128. Re-entrained
liquid from the liquid film covering the inside walls of the
outer electrode 134 can cause electrostatical field insta-
bility when gas velocities within the outer electrode 134
exceed 10 fps.
The upper end of the outer electrode 134 projects
into an exhaust plenum 140 where it is surrounded by a cy-
lindrical weir 142. Water is continuously supplied through
line 143 by a pump (not shown) so that a thin film of water
144 flows over the weir 142 and down the inside walls of the
outer electrode 134. A cover plate 146 extending over the
weir water has a cylindrical flange 147 projecting into the
outer electrode 134 to allow a smooth airflow from the outer
electrode 134 into the exhaust plenum 140 so as not to dis-
turb the weir head water.
- The upper ends of the support electrodes 138
extend into a high-voltage housing 148 where they are con-
nected to a high-voltage bus bar 150 supported on the floor
of the housing 148 by insulators 152 and a feed-through
shroud 154. The bus bar 150 is connected to an external
high-voltage power supply or transformer/rectifier set (not
shown) through a high-voltage conductor 156 (Fig. 1) which
passes through the high-voltage housing 148 through a feed-
through insulator 160. Access to the electrodes 134,136,138
is provided by a plurality of annular access covers 162 on
the top surface of the housing 148.
The electrostatic field extending between each
discharge electrode 136 and the outer electrode 134 is sub-
stantially identical to the electrostatic field produced in
the embodiment of Figs. 1-11. Consequently, the manner in
which the electrodes 136,134 form a corona current-producing
electrostatic field, as well as the characteristics of the
electrostatic field, are not repeated herein. The major
diameter of the discharge electrode 136 should be between
0.2 and 0.5, preferably about 0.35, of the inside diameter
of the outer electrode 134. The diameter of the support
electrode 138 is between 0.15 and 0.25, preferably about
0.2, of the inside diameter of the outer electrode 134, and
between 0.4 and 0.8, preferably O.S to 0.6 of the major dia-
11~)t~888
meter of the discharge electrode 136. ~ larger diametersupport electrode 138 can overly suppress the axial expan-
sion of the electrostatic field between the discharge elec-
trode 136 and outer electrode 134, thereby reducing the
stability of the field. A smaller support electrode 138
tends to produce excess corona and sparkover between the
support electrode 138 and outer electrode 134.
By securing the discharge electrode 136 directly
to the support electrode 138, a single electrode mounting
system may be used for both the charging stage of the pre-
cipitator as well as the collecting stage of the precipi-
tator. Further, by varying the diameters of the discharge
electrode 136 and the support electrode 138 with respect to
each other and the outer electrode 134, the intensities of
the fields extending between the discharge electrode 136 and
the outer electrode 134 and between the support electrode
138 and the outer electrode 134 can be independently select-
ed even though both electrodes 136,138 are powered by a com-
mon transformer/rectifier unit. Consequently, the cost and
complexity of the precipitator compared to those of conven-
tional precipitators are greatly reduced.
In order to increase the efficiency of the preci-
pitator, particularly where various sized particles are to
be removed, multiple electrode staging may be employed, as
illustrated in Fig. 14. A plurality of discharge electrodes
180,182,184 are secured to a support electrode 186 at axial-
ly spaced points greater than 1.25 discharge electrode gaps
apart from each other. A spacing between electrodes 180,
182,184 of greater than 2 discharge electrode gaps is pref-
erable to prevent the fields extending from adjacent dis-
charge electrodes from interfering with each other and to
provide an electric field extending from the support elec-
trode 186 having a length sufficient to collect charged par-
ticles. Interelectrode spacings of less than 1.25 discharge
electrode gaps cause interference between adjacent fields
which prevents axial expansion of the fields, thereby reduc-
ing the stability of the corona discharges. As the gas
passes through the outer electrode 134, the particles en-
trained in the gas are thus subjected to a plurality of
charging fields, each of which is followed by a collecting
field. If desired, the diameters of the discharge elec-
trodes 180,182,184 may be varied so that the average inten-
llOU888
sities of the fields extending between the discharge elec-
trodes 180,182,184 and the outer electrode 134 are different
for each electrode 180,182,184. Large particles are gener-
ally easier to precipitate and are thus removed with greater
ease at less intense applied fields. Consequently, where
the particles to be collected vary in size, it may be desir-
able to progressively increase the diameter of discharge
electrodes along the gas flow so that particles are subject-
ed to increasingly more intense fields as they flow through
the outer electrode 134. The largest particles are thus
collected on the inner walls of the outer electrode 134 just
beyond the smaller diameter discharge electrode 180, smaller
particles are collected just beyond the intermediate dia-
meter discharge electrode 182, and the smallest particles
are collected beyond the larger diameter discharge electrode
184.
The larger charged particles produce localized
field anomalies which tend to produce sparkover in these
localized regions. By removing the larger particles near
the smaller diameter discharge electrode 182, the intensity
of the field between the larger diameter discharge electrode
184 and the outer electrode 134 can be increased, thereby
removing a greater percentage of smaller particles and
increasing the overall collection efficiency of the system.
If desired, a low field corona discharge may be
produced between the support electrode 138 and the outer
electrode 134 by helically wrapping a conductor, such as a
wire, around the support electrode 138. The low field coro-
na discharge is particularly useful where the outer elec-
trode is intermittently cleaned since the corona current
holds the particles on the inner wall of the outer electrode
until they are removed by any suitable technique, such as
rapping. Where the particles are continuously removed by a
water film, for example, there is no necessity to retain the
particles on the outer electrode in this manner.
24
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