Electrostatic Precipitator

Abstract

An electrostatic precipitator in which particles entrained in the gas stream are first electrically pre-charged by induction and conduction from a closed and non-emitting electrode configuration then further charged toward their saturation charge in an optimized particle charging section by corona field charging mechanism and finally collected in an optimized precipitation field. To retain the collected particles a controlled and confined electric wind turbulence is provided at the collection plates together with ion showers of the same polarity as of the charged particles. Particle reentrainment even during mechanical dislodgement is eliminated by the effective use of turbulence mixing and diffusion of the confined electric wind near at the collection wall, and is further assured by the recharging and recollection of particles accomplished by the ion showers in an optimized precipitation field.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the electrostatic precipitation of particles entrained in a gas stream and, more particularly, to a method of and apparatus for, more efficient precipitation by reducing the reentrainment of particulate matter.

Electrostatic precipitation of particles entrained in a gas stream is widely known and used in many industrial applications and for air cleaning. There are two types of electrostatic precipitators in use, namely, the Cottrell type and the two-stage type. Structurally, the Cottrell type has a plurality of fixed collectors with emitters equally spaced and centrally located with respect to the collectors; the two-stage type has a short section that contains a number of fixed emitters followed immediately by a relatively long section containing a plurality of fixed collectors and passive electrodes centrally located with respect to the collectors. Functionally, entrained particles are charged and collected through the entire length of collectors in the Cottrell type; whereas particle charging and collection are separately accomplished in the two separate sections in the two-stage type. The first section is called the particle charging stage; the second section is called the particle collection stage. In practice, the Cottrell type is used in general for high particle loading, low gas velocity and continuous applications; whereas the two-stage type is used exclusively for low particle loading, high gas velocity and intermittent applications. The prior art systems thus do not provide a universal type precipitator for high particle loading, high gas velocity and continuous applications of gas separation.

The major shortcomings in the Cottrell type are due to particle reentrainment and low precipitation rate. Particle reentrainment may be caused by aerodynamic or electric wind turbulence or by improper mechanical dislodgement for particle removal. Low precipitation rate is characterized by low particle drift velocity due to an unoptimized precipitation field intrinsically limited by corona sparkover voltages. Another shortcoming in the Cottrell type under heavy particle loading is that particle depositions occur on emitters. In the past, it has been attempted to compensate for these shortcomings by a lengthening of the collection zone to compensate for ineffective particle charging and collection and reentrainment.

The major shortcoming in the two-stage type of precipitator is due to the inability to hold precipitated particles onto the collection plates in the collection stage even though the precipitation field has been optimized. Prior attempts to hold the collected particles for this type of precipitator have included the use of some kind of chemical adhesives coated on the collector plates which must be cleaned and recoated with adhesives after each use. This approach prevents continuous operation and the handling of high dust loading.

Precipitators have also been proposed in which the particle collection surfaces are in the form of endless belts, conductive, semi-conductive of dielectric, charged or non-charged, that continuously or periodically move through the collection zone. Although, particle reentrainment due to mechanical dislodgement is reduced, the complexity of a moving system after all is impractical for many reasons. It is the object of this invention to provide a simple yet efficient precipitator system by combining the advantages of the prior art systems and by eliminating their shortcomings such that the present invention is capable of for all applications of gas separation, i.e., any particle loading, high gas velocity and continuous applications.

SUMMARY OF THE INVENTION

The present invention deviates from the prior art systems by having three separate functional stages, namely, a particle pre-charging section, a particle charging section and a particle collection section. Each section can then be independently optimized for its respective design function. The function of the particle pre-charging section is to assure that all particles passing this section will carry a small amount of charge of the same polarity as obtained later in the particle charging section. Advantageously, pre-charging of a particle may be done by induction and conduction charging so that any particle deposition on the pre-charging electrodes will not offset its pre-charging function. It is however understood that induction and conduction charging is most effective with respect to non-dielectric particles.

Further charging of the particles to saturation is accomplished in the particle charging section which employs corona discharges from emitting electrodes. By the use of the particle pre-charging section no particle deposition will occur on the emitting electrodes in the particle charging section because there are no uncharged or oppositely-charged particles in the zone. Structurally, the particle pre-charging section is accomplished by placing a row or rows of rod electrodes of a closed configuration porous to inlet particles and gas flow at the inlet of the precipitator system of my invention. These electrodes, passive, i.e., non-emitting, are raised to high potentials of the same polarity as that of the emitters in the particle charging section. All particles passing these electrodes are then inductively charged to a small degree but are charged to the same polarity obtained in the following particle charging section. Pre-charging electrodes can be of other closed configurations, such as rings, honeycombs, screens or meshes porous to particle and gas flow. The main point is that they be non-emitting and of the same polarity as the emitters in the particle charging section. Non-emitting electrodes are essential here for the reentrainment of any particles on these electrodes results in these particles being charged to the correct polarity by conduction.

All particles leaving the pre-charging section are then further charged up to saturation by field charging in the particle charging section. The important attribute of the operation of the particle charging section of my invention is that all particles entering the particle charging section carry a small amount of charge of the same polarity as that of the emitters, and therefore particles are kept away from the emitters resulting clean emitters with consistent charging properties. Advantageously, the particle charging section may have any configuration suitable for particle charging by corona discharge and the geometry thereof may be optimized for particle charging to saturation and yet minimized for particle collection.

In further accordance with my invention, electric wind turbulence and ion showers are utilized to enhance particle collection and to prevent particle reentrainment in the particle collection section. In an exemplary embodiment, a duct-type collection section comprises a centrally located passive electrode and a plurality of emitters located at a small distance away from the collector plate surfaces of the duct but a large distance away relative to the centrally located passive electrode. A high voltage of the same polarity of the emitters in the particle charging section is applied to the central electrode. Another high voltage, of suitable magnitude less than that of the central electrode, and of the same polarity as of the central electrode, is applied to the emitters in the particle collection section. The emitters in the collection section are small in diameter and are located closely to the collector plate surfaces but far away from the central electrode. Accordingly, the optimized precipitation field resulting from the maximum voltage appliable to the central electrode without sparkover is practically undisturbed. Furthermore, the emitters in the collection section function to provide ion showers to hold particles onto the collector surfaces until deliberate dislodgement is allowed to take place. It is an aspect of the operation of the emitters when located closely to the collectors that the electric wind turbulence is confined to small eddies in the region of the emitters and the collector surfaces. This region is also the aerodynamic boundary layer region. Accordingly the small eddies along the collector wall help to bring in particles aerodynamically from the center region toward the collection plates in addition to the particles being driven by the optimized precipitation field. Whenever the collected particles become thick enough to fall by gravity, a proper mechanical dislodging force may be applied to the collector for particle removal. Any particles that are broken loose and therefore which would otherwise be susceptible of becoming reentrained during mechanical dislodgement will, however, be charged up immediately by the near-by emitters. Further, because the emitters in the particle collection zone of my invention produce ionization of the gas stream only in the immediate vicinity of the collection plates, the electric wind accompanying this ionization is confined to little eddies adjacent the collection plates. These eddies sweep up the re-charged particles and carry them to the collection plates.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be obtained by referring now to the following detailed description in conjunction with the figures of the accompanying drawing, in which:

FIG. 1 is a top horizontal sectional view of one embodiment of the electrostatic precipitator of the present invention;

FIG. 2 is a vertical end sectional view of the embodiment of FIG. 1;

FIG. 3 is an expanded horizontal sectional view of the pre-charging section of the apparatus of FIG. 1;

FIG. 4 is a horizontal sectional view of an alternate embodiment of the electrostatic precipitator of the present invention; and

FIG. 5 is an expanded detail of the particle collection section of FIG. 4.

DETAILED DESCRIPTION

in FIG. 1, a representative embodiment of the invention includes a gas stream duct 60 divided into three functional sections: the particle pre-charging section 20, the particle charging section 30, and the particle collection section 40. A number of non-emitting rod electrodes 22 are arranged in a closed configuration and positioned within gas stream duct 60 in the pre-charging section 20 by the use of a suitable number of insulating supports (not shown). The interior walls of duct 60 define the collector plate surfaces 50. Electrodes 22 and the adjacent areas of plates 50 define the particle pre-charging section 20.

The particle charging section 30 is located downstream from the pre-charging section 30 and includes two wire emitters 32 located at the center of the duct 60. Wire emitters 32 are mounted within duct 60 via suitable insulating means (not shown) to insulate them electrically from collector plates 50.

The particle collection section 40 comprises a central electrode sheet 42 which is insulated from plates 50 and which partitions the main duct 60 into two subordinate gas ducts 70. A number of wire emitting electrodes 44 also insulated from collector plate walls 50 are positioned at equal distance from each other and closely spaced with respect to collection plates 50. It is understood that the number and configuration of the electrodes in each of sections 20, 30, and 40 shown in the drawing are for illustration purpose only. For instance, electrode 20 need not be limited to only one row, but can include a number of rows; nor need the closed configuration be of rod-shaped elements, but rings, honeycombs, screens, or meshes may be employed so long as electrode 20 is porous to particle and gas flow and yet of closed configuration and non-emitting.

All electrodes 22, 42, and emitters 32, 44 are connected to high voltage power supplies that have the same polarity with respect to the collection plates 50; as illustrated in FIG. 1, they are all at negative potentials with respect to the collection plates 50 which are, as illustrated, at ground potential. The potential of pre-charging electrode 22 is maintained at a level which will not result in corona discharge. The emitters 32 are however placed at the optimum voltage for particle charging by corona discharge. The center electrode 42 is maintained at an optimum voltage just below sparkover threshold for achieving the maximum precipitation field for particle collection. The voltage applied on wire emitters 44 is kept at a minimum magnitude just sufficient to maintain adequate corona discharges and ion showers to provide particle holding force when collected.

Particles and gas flow are indicated by arrows in FIG. 1. At the inlet of gas flow to duct 60 it is to be expected that most of the particles in the gas stream will pass through the pre-charging electrode 22. Nevertheless, some particles will collide with electrode 22 and therefore deposit right on it. As illustrated in FIG. 3, particles that pass the pre-charging electrode 22 without colliding will carry a small amount of negative charge by induction. Particles that are deposited on electrode 22 will pick up negative charges by conduction. If the particles that have been deposited on electrode 22 should somehow become reentrained in the gas stream, they will nevertheless carry a small amount of negative charge with them. Accordingly, all particles leaving the pre-charging section 20 will be negatively charged.

Further charging of particles toward saturation is accomplished by the corona field emanating from emitters 32 in the particle charging section 30. Since the only function of the corona field in the particle charging section is for charging particles (and not for causing particles to be precipitated therein), it then can be optimized for this purpose and the ducts can be designed taking into account the fact that my emitters 32 are clean emitters. Further, the corona field in section 30 emanating from emitters 32 need only extend over a relatively short axial length of the duct in the direction of gas flow since the particle charging time constant is only of the order of fractional milliseconds and particle collection is not the goal in this section. Additionally, the shorter the particle charging section, the less likelihood there will be of any particle collection within this section. Since there will be no particle collection, mechanical alignment of emitters 32 with respect to walls 50 will be easier than in prior art Cottrell type systems in which the emitting field is the principal mechanism for particle collection.

In the particle collection section 40, a uniform electric field optimized for precipitation of already charged particles is provided by the potential applied to central electrode plate 42. A plurality of emitters 44 are arranged in close proximity to the collector plates 50 on each side of central electrode 42. Emitters 44 are equally spaced along both walls 50 to provide an electric wind turbulence that is confined to small eddies in the region of the aerodynamic boundary layer adjacent to each of walls 50 in the collection section 40. In effect, the controlled electric wind created by emitters 44 acts like a plane of electric air pumps altering the gas flow in the immediate vicinity of the collection plates 50. Although this pumping effect may be negligible in an aerodynamic sense, it plays a role in enhancing the particle precipitation rate as compared to that produced by an electrostatic precipitation field alone.

As the particles carrying their saturation charges enter particle collection section 40, they are electrically driven toward the collection plates due to the optimized electric field from center electrode 42. Furthermore, as the particles drift close to the vicinity of the emitters 44, they are immediately subjected to the electric wind turbulence occurring due to the ionization of the air between the emitters 42 and the adjacent one of the collecting walls 50. Furthermore, emitters 44 also provide ion showers to replenish charges to the particles collected on plates 50 such that a strong electrostatic force continuously acts on the collected particles militating against particle reentrainment in the gas stream. In this respect, the present invention is entirely different than prior art systems of the two-stage type where particles after collection lose their charge in a matter of time depending on thier conductivity and to the point that electrostatic force is no longer sufficient to prevent particle reentrainment.

As the collected layer of particles on the collection plates 50 becomes thick enough to fall by its own weight due to gravity forces, a conventional mechanical dislodging force may be applied to the collection plates 50 for particle removal. Such rapping mechanisms are well known. Should any particles become broken loose during the rapping procedure they will however be charged up immediately by the near-by emitters 42 and be brought back for immediate collection by turbulence mixing and diffusion in the small aerodynamic eddies existing in the region. It should be noted that the optimized precipitation field provided by central electrode 42 is not appreciably altered by the ionization field emanating from emitter 44 because these emitters are closely positioned to the collecting plate walls 50. Thus, particle reentrainment due to mechanical dislodgement presents no problem in the present invention.

It is thus seen that in my invention the collection zone 40 has a passive central electrode, i.e., one which does not cause ionization of the main gas stream. Accordingly, there is no electric wind due to the central electrode and the field from the central electrode may be optimized for particle deposition. This differs from the particle collection mechanism in the Cottrell type of precipitator in which an electric wind blows against the full cross section of the gas stream duct causing large eddies and whirlpools that militate for continual particle reentrainment. In the aerodynamic boundary layer vicinity of the collection plate walls 50, however, I provide a number of emitting electrodes that perform a dual function, that of creating a confined electric wind turbulence in the boundary layer region and also that of providing ion showers to the already collected particles. These ions showers maintain charge on the collected particles and prevent them from being reentrained.

Another embodiment of the precipitator of the present invention is illustrated in FIGS. 4 and 5. This embodiment also includes a particle pre-charging section 220, a particle charging section 230 and a particle collection section 240. However, the particle pre-charging section 220 and the particle charging section 230 are now divided into four narrower subducts formed by collection plates 250 and auxiliary ground plates 250'. The outermost of these subducts include negatively poled pre-charging electrodes 222 in pre-charging section 220 and negatively poled emitting electrodes 232 in the particle charging section 230. The innermost of the subducts includes positively poled pre-charging electrodes 222' in pre-charging section 230 and positively poled emitting electrodes 232' in particle charging section 230.

The central electrode 242 in the particle collection section 240 is connected to an externally applied voltage of the polarity opposite to that of the nearest preceding emitters 232' in the particle charging section. Two rows of new emitters 244' are included near the central electrode 242, in addition to the emitters 244 near the collection plates 250. The voltage applied on emitters 244' has the same polarity as but is lower in magnitude than that of the central electrode 242. As illustrated in FIG. 5, emitters 244' energized with voltage of lesser magnitude than that of the central electrode 242 are electrically at a positive potential with respect to the central electrode 242. Emitters 244' therefore provide positive ion showers to the central electrode 242. Small aerodynamic eddies are also generated and confined to the surface boundary region near the central electrode 242. In analogous fashion, negative ion showers and eddies in the surface boundary region near the collection plates 250 are provided by emitters 244.

As particles leave the particle pre-charging section 220 they are charged by induction and conduction to a small degree with the polarity according to the polarity of electrodes 222 and 222', respectively. The particles are then further charged to their saturation by the corresponding emitters 232 and 232' in the particle charging section. In the particle collection section 240 the particles with positive charges will now be collected on the central electrode 242, and particles with negative charges will be collected on the collection plates 250. With this embodiment, the collection surfaces for particles has effectively been doubled over that of the embodiment of FIG. 1 of the same duct volume.

Accordingly, I have shown an electrostatic precipitator having three stages, a first pre-charging stage employing passive, closed configuration electrodes, a second saturation charging stage employing corona discharge electrodes but in a region confined to a short axial length of gas flow to minimize particle deposition on the emitters and, finally, a particle collection stage which employs a passive electrode for providing an electric field optimized for precipitation of the already charged particles and in addition, in the aerodynamic boundary layer region of the collection plate surfaces, a plurality of small emitting electrodes biased to provide a confined electric wind turbulence in the boundary layer region and to shower the immediate adjacent collection plate surfaces with ions to maintain electric charges on the collected particles.

It will be apparent to those skilled in the art that modifications may be made in the above-described illustrative embodiment. Moreover, to assist in the construction of an illustrative embodiment of my invention, I suggest the following dimensions and voltages: In the particle pre-charging section 20 the voltage V.sub.pc applied to the passive electrode 22 may be approximately 10 kilovolts for rod-shaped electrodes 22 that are between 0.125 and 0.250 inches in diameter. In the particle charging section 30, the voltage V.sub.pe applied to emitting electrodes 32 may be approximately 35 kilovolts for wire-shaped emitting electrodes that are from 0.020 to 0.040 inches in diameter. It will be apparent that emitting electrodes 32 may be of other geometries such as that of a saw-tooth, a string of needlepoints, barbs, etc. With the voltage V.sub.pe of 35 kilovolts, the desired electric field intensity E.sub.32 in the particle charging section will be approximately 30 to 32 kilovolts per centimeter. In the particle collection section 40, the voltage V.sub.p applied to the central passive electrode 42 may be approximately 38.1 kilovolts per inch of distance d between the central electrode and the collection plate wire 50 wherein an electric field intensity E of 15 kilovolts per centimeter is desired. The voltage V.sub.dis applied to emitting electrodes 44 may be of approximately 10 kilovolts for emitting electrodes 44 which are of 0.020 to 0.040 inch diameter wire rods. The wire rod electrodes 44 may be positioned approximately a quarter inch away from the adjacent collection plate wall 50.

Claims

I claim:

1. A method of electrostatically precipitating particles entrained in a gas stream comprising

passing the gas stream through a particle pre-charging zone to impart at least a minimal charge to substantially all of the particles,

passing all of the particles so pre-charged through a corona field zone to impart a saturation charge to all of the said pre-charged particles,

passing the particles so saturation-charged through a non-emitting precipitation field to collect the particles on a collection surface, and

providing local ion showers to the particles on the collection surface to maintain charges on the collected particles.

2. The method according to claim 1 wherein a controlled electric wind turbulence limited to the aerodynamic surface boundary region of said collection surface is provided.

3. The method according to claim 1 wherein said collection surface comprises first and second plates biased to opposite polarities and wherein first and second ion showers each having a polarity matching the polarity of the corresponding one of said plates are provided.

4. In a particle precipitator wherein particles entrained in a gas stream are subjected to a corona saturation charging field and thereafter to a passive precipitation field in a particle collection zone, the improvement characterized in that

a particle pre-charging zone is established prior to the corona field to impart at least some minimal charge to substantially all particles entering the corona field, and in that

local ion showers are provided over the aerodynamic surface boundary region of the collection surface of the particle collection zone.

5. Apparatus including a gas stream duct for electrostatically precipitating particles entrained in a gas stream comprising

non-emitting particle pre-charging means position in said gas duct,

corona field particle charge means positioned in said duct downstream of said pre-charging means,

non-emitting collection field producing means positioned in said duct downstream of said corona field particle charging means and defining a particle collection region of said gas duct, and

a plurality of emitting means positioned adjacent the surface boundary region of said particle collection region.

6. Apparatus according to claim 5 wherein said plurality of emitting means is biased to provide a confined electric wind turbulence in said surface boundary region of said particle collection region of said duct walls.

7. Apparatus according to claim 6 wherein said plurality of emitting means provides ion showers to said surface boundary region of said duct.

8. Apparatus according to claim 5 wherein said particle pre-charging means includes a plurality of closed configuration electrodes porous to particle and gas flow.

9. Apparatus according to claim 5 further comprising gas stream channeling means inserted in said gas duct in the area thereof defined by said particle pre-charging and particle charging means for dividing said gas stream duct into a number of subducts each including one of said particle pre-charging means and one of said particle charging means, and

means for individually biasing the particle pre-charging and particle charging means located within a continuous one of said subducts.

10. Apparatus according to claim 9 wherein said particle pre-charging means and said particle charging means contained within the same one of said subducts are each biased to the same electric polarity.

11. Apparatus for electrically precipitating particles entrained in a gas stream comprising

a gas stream duct having an inlet and an outlet and conductive wall surfaces connected to a source of reference potential,

particle pre-charging means disposed at the inlet of said duct and connected to a potential source for establishing a non-emitting electric field in the vicinity of said duct inlet,

emitting electrode means disposed in said duct downstream of said particle pre-charging means for establishing an emitting electric field extending over a short axial length of said duct, said emitting electrode means defining a particle saturation charging region in said duct,

central electrode means having a substantial particle collection surface area positioned in said duct downstream of said emitting electrode means, said central electrode being connected to a source of potential to establish a passive, non-emitting particle collection field extending over a substantial axial length of said duct, said particle collection field being directed from said central electrode to the adjacent conductive wall surfaces of said duct, and

a plurality of emitting electrode means disposed throughout the length of said central electrode for providing ion showers over the aerodynamic boundary surface of said central electrode.

12. Apparatus according to claim 11 wherein said central electrode is connected to a source of potential of polarity opposite to that of the nearest of said emitting electrodes in said particle saturation charging region.

13. Apparatus according to claim 11 wherein the combination further comprises a second plurality of emitting electrode means disposed in said duct adjacent to said conductive wall surfaces thereof over an axial length thereof substantially equal to the axial length in said duct of said central electrode, said second plurality of emitting electrodes being connected to a source of potential for establishing ion showers over the aerodynamic boundary layer portion of the adjacent conductive wall surfaces of said duct.

14. Apparatus according to claim 13 further comprising channeling means inserted in said duct in the areas thereof defined by said particle pre-charging electrode and said particle saturation charging region for dividing said gas stream into a plurality of subducts, each of said subducts including a particle pre-charging electrode and at least one particle saturation charging electrode, wherein at least one of said subducts includes a particle pre-charging electrode connected to a source of potential of opposite polarity to that to which the particle pre-charging electrode of another of said subducts is connected.

15. Apparatus according to claim 14 wherein the particle pre-charging and particle saturation charging electrodes within the same subduct are connected to respective sources of potential having the same polarity.

16. Apparatus according to claim 13 wherein said first plurality of emitting electrodes is biased to provide ion showers of opposite polarity to that provided by said second plurality of ion showers.