CA1100888A - Single and multiple disc ionizer and particulate removal apparatus and method - Google Patents

Abstract

TITLE

HIGH-INTENSITY IONIZER

ABSTRACT OF THE DISCLOSURE
A high-intensity ionizer including one or more planar discharge electrodes concentrically mounted in a tubular outer electrode. In one embodiment, the tubular electrode forms a venturi, and the planar discharge electrode in the form of a disc is mounted on an insulated probe in the throat of the ven-turi. The walls of the venturi are kept free of particulate matter by either a thin layer of water or an airstream along the wall of the venturi. Particulate matter is prevented from building up on the surface of the probe by an air vent extending around the probe upstream of the discharge electrode. A high voltage placed between the discharge and outer electrode gener-ates a high-intensity, corona current-producing, electrostatic field within the electrode gap between the discharge electrode and venturi. The electrostatic field expands radially and cir-cumferentially so that it is radially and circumferentially uniform. In another embodiment, a plurality of planar elec-trodes are mounted on a support electrode. The discharge elec-trodes are spaced apart from each other by at least 1.2 times the distance between the discharge and outer electrodes so that the electrostatic fields extending from adjacent discharge elec-trodes do not interfere with each other. The planar discharge electrodes may have varying diameters so that the intensity of the electrostatic fields through which a gas flowing through the tubular electrode sequentially passes is varied. Contaminants in gases flowing through the outer electrode thus sequentially flow through a plurality of alternating electrostatic charging fields and electric collecting fields.

Description

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.

2 i~

llV~888 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 llQ~888 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 110(~888 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 ~r 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.

1~,0~
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-llOV888 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 ' l~V~388 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 8~8 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.

1~0~888 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.

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-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 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 :.

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.

.~

Claims (16)

CLAIMS:

1. An apparatus for ionizing gases and for removing contaminants from gases, comprising:
a tubular outer electrode adapted to conduct said gases therethrough;
a generally planar discharge electrode having a peri-meter generally corresponding to the shape of said outer elec-trode, said discharge electrode being positioned within said outer electrode and defining an electrode gap therebetween, said discharge electrode being isolated from all corona current-emit-ting structures by at least 1.25 electrode gaps to allow said field to expand axially in a generally wedge-shaped configura-tion between said discharge electrode and said outer electrode;
means for applying a high voltage between said elec-trodes for creating a corona current-producing, high-intensity electrostatic field within said electrode gap; and means for moving said gases in a stream axially through said electrode gap.

2. The apparatus of claim 1 wherein said discharge electrode occupies between 5% and 40% of the transverse area within said outer electrode.

3. The apparatus of claim 1, further including means positioned downstream of said discharge electrode for collecting contaminants entrained in said gases and charged by said elec-trostatic field.

4. The apparatus of claim 3, further including means for cleaning the surface of said outer electrode, including means for injecting a substantially continuous layer of air along said outer electrode to prevent deposition of contaminants thereon.

5. The apparatus of claim 1 wherein said means for applying a high voltage between said electrodes places a voltage between said inner and outer electrodes greater than 12 kv for each centimeter of said electrode gap when air at approximately standard temperature and pressure is within said electrode gap such that the average applied field between said electrodes is greater than 12 kv/cm.

6. The apparatus of claim 1 wherein said planar dis-charge electrode is mounted on the upstream end of an elongated support electrode positioned concentric with said outer elec-trode, said support electrode having a diameter which is between 0.15 and 0.25 the inside diameter of said outer electrode, and between 0.4 and 0.8 the diameter of said discharge electrode such that when a predetermined potential is applied between said outer electrode and said discharge and support electrodes, a radially and axially expanding, radially and circumferentially uniform corona current-producing electrostatic field is generat-ed between said discharge and outer electrodes, and a radial, non-corona current-producing electric field is generated between said support and outer electrodes.

7. The apparatus of claim 1, further including a plurality of generally planar discharge electrodes mounted on a common elongated support electrode positioned concentric with said outer electrode, said discharge electrodes being axially spaced from each other by at east 1.25 discharge electrode gaps such that contaminants entrained in said gases passing through said outer electrode are repeatedly charged within a plurality of radially and axially expanding, radially and circumferential-ly uniform electrostatic fields and then migrate toward said outer electrode under the influence of a radial electric field downstream of each electrostatic field such that said contami-nants are deposited on the inside wall of said outer electrode.

8. The apparatus of claim 7 wherein said discharge electrodes are of different diameters such that different applied average fields are maintained between each of said dis-charge electrodes and said outer electrode.

9. A method for ionizing gases and for removing con-taminants from gases, comprising:
directing said gases through a tubular outer electrode;
placing a generally planar discharge electrode concen-trically within said outer electrode;
generating an electrostatic field between said elec-trodes, the intensity of said field being radially constant throughout a distance from said outer electrode at least to about 50% of the electrode gap toward said discharge electrode such that said field is substantially uniform throughout a distance from said outer electrode at least to about 50% of the electrode gap toward said inner electrode, and is generally wedge-shaped, diverging outwardly in a direction perpendicular to the flow of gases through said outer electrode.

10. The method of claim 9 wherein the average inten-sity of said electrostatic field is equivalent to an average intensity in air greater than 12 kv/cm at standard atmospheric pressure and temperature without the occurrence of sparkover.

11. The method of claim 9, further including the step of adjusting the ratio between the cross-sectional area of said discharge electrode and the cross-sectional area of said outer electrode such that said ratio is between 0.05 and 0.40.

12. The method of claim 9, further including the step of increasing the voltage between said discharge and support electrodes beyond the sparkover voltage in accordance with the velocity of said gas stream such that ions are carried from said field by said gas stream, thereby allowing the average applied intensity of said electrostatic field to be increased without sparkover as said velocity is increased.

13. The method of claim 9, further including the steps of supporting said discharge electrode on the upstream end of an elongated support electrode and generating a non-corona current-producing electric field between said support electrode and said outer electrode such that particles entrained in said gases are charged by the electrostatic field extending between said dis-charge and outer electrodes and subsequently migrate toward said outer electrode under the influence of said electric field extending between said support and outer electrodes.

14. The method of claim 13, further including the step of mounting a plurality of said planar discharge electrodes on said support electrodes at least 1.25 electrode gaps apart from each other such that a plurality of said radially and axially expanding, radially and circumferentially uniform electrostatic fields are formed within said outer electrode axially spaced from each other, and a radially expanding electric field is generated downstream of each electrostatic field such that par-ticles entrained in said gases are repetitively charged by said electrostatic fields and subsequently migrate toward said outer electrode under the influence of said electric fields.

15. The method of claim 14 wherein the transverse dimensions of said discharge electrodes are progressively increased moving in the direction of gas flow such that said electrostatic fields are progressively increased moving in the direction of gas flow, whereby larger particles are removed by upstream fields in advance of downstream fields where finer par-ticles are removed, thereby increasing the stability of down-stream fields such that the collection efficiency of said system is increased.

16. The method of claim 13 wherein said electrostatic fields are spaced apart from each other by at least 1.25 times the radial dimensions of said fields, thereby preventing said fields from interfering with each other.