US3400513A - Electrostatic precipitator - Google Patents

Description

Sept. 10, 1968 R. H. BOLL 3,400,513

- ELECTROSTAT IC PRECIPITATOR 2 Sheets-Sheet 1 Filed Sept. 8. 1966 FIG. 1

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[ ENTE LIN E .R T\ K WXEL RADIAL FIELD AT WALL INVENTOR. Richard H. Boll 6 AT TORNEY R. H. BOLL ELECTROSTATIC PRECIPITATOR Sept. 10, 19 68 2 Sheets-Sheet 2 Filed Sept.

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H n n United States Patent 3,400,513 ELECTROSTATIC PRECIPITATOR Richard H. Boll, Alliance, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Sept. 8, 1966, Ser. No. 578,435 6 Claims. (Cl. 55-103) ABSTRACT OF THE DISCLOSURE An elestrostatic precipitator having an open ended housing of dielectric material with hoods positioned on said opposite ends of the housing to direct a flow of dust laden gas into and clean gas from the precipitator. The dust in the gas is electrostatically charged upon entering the housing and the dust collected after leaving the housing while the electrostatic charge on the dust is increased by electrogasdynamics in passing through the housing for enhanced dust collection.

The present invention relates to an improved electrostatic precipitator and more particularly to a precipitator the performance of which is improved by utilization of the electrical potential generated by a moving gas stream in apparatus adapted for applying electrogasdynamic principles so that dust collection in the collector section of the precipitator is enhanced and the electrical power requirements are minimized.

The use of electrostatic principles for separating dust from gases is well known. In this instance the dirty gas contains dust particles which are cleaned by the action of the electrostatic force which acts on charged particles within an electrostatic field. The particles are usually charged by passing them through or near the region of corona discharge, and precipitation occurs when the gas stream passes through a region containing an electrostatic field. The latter region may be either identical with or separated from the region of particle charging, but in any event the electrostatic field is supplied by an external power supply. This precipitating power supply is usually of higher voltage than actually required for particle charging and is the source of several practical problems. For example, because of its high voltage, the precipitating power supply is expensive; arcing between precipitator electrodes presents the possibility of current surges whose effects must be guarded against by elaborate electronic control. Moreover, if a single electrode pair shorts out completely due to dust build up and arcing, a large section of the precipitator is rendered inoperable because all of its elements are fed from a common power supply which is shorted. While these difficulties can be overcome to a degree in conventional practice by various means, they nevertheless result in increased cost, lower reliability, and loss of precipitating efiiciency.

In the present invention, I utilize a corona discharge for charging the particles in much the same fashion as accomplished in conventional two-stage precipitators. This charging process, which occurs in an ionizer section, requires a power supply of modest voltage (e.g., 5,000 to 10,000 volts) and, therefore, modest cost compared to that required by a single-stage precipitator. However, I achieve precipitating fields of magnitude comparable to those obtained in a single-stage precipitator without employing a corresponding external power supply. The voltage required for the precipitating field is generated internally by a plurality of electrostatic generators operating on electrogasdynamic principles and using the gas containing charged particles as the working fluid. Thus, not only is the cost of high voltage power supply avoided, but operating difficulties due to current surges 3,400,513 Patented Sept. 10, 1968 and short circuits are eliminated. For example, interelectrode arcing causes no damage to the apparatus and only very brief and slight impairment of collection efficiency; even complete short-circuiting of a few electrode pairs causes only slight decrease in overall collection efiiciency because of the large plurality of electrogasdynamic generators and collectors that are placed in flow parallel. Additionally, such high voltages are attainable by electrogasdynamic principles that relatively large interelectrode spacings can be utilized, which results in an additional cost advantage. Moreover, since maximum particle charging is not essential, overall pressure drop can be held to not greater than about one inch of water.

Electrogasdynamic generation of the high precipitating voltage is achieved using the dust particles themselves as charge carriers. The dust-laden gas is first passed through an ionizing section wherein the gas velocity is increased somewhat and particle charging occurs by corona discharge between a number of corona needles and oppositely disposed attractor electrodes'Thereafter, the gas flows into a dielectric conduit wherein the electrostatic potential increases parabolically in the flow direction due to the space charge which resides on the charged dust particles. At a suitable distance down this channel, the high electrostatic potential is communicated to a series of needle electrodes which are electrically connected to repulser electrodes located still further downstream. Between the region of the needle electrodes, which I call repulser corona needles because a corona will sometimes form between them and the highly charged dustladen gas, there is a continuation of the dielectric flow conduit and the electrostatic potential falls again towards zero. Actual particle collection takes place within a collector section at the end of the dielectric section, wherein are placed a multiplicity of grounded collector electrodes. The electrostatic field necessary for particle collection is generated by the potential difference between repulser electrodes, which are placed within or adjacent to the collector electrodes, the repulser electrodes being electrically connected to the repulser corona needles. Thus, by virtue of the space charge on the dust particles, there exists between the ionizer and collector sections a parabolic electrostatic potential, which may achieve a quite high peak voltage; the repulser corona needles are situated approximately at the plane of maximum electrostatic potential, and they achieve potentials only slightly lower than the maximum electrostatic potential by means of corona discharge from the particles to the needles. This charge transfer from particles to the repulser electrodes occurs mainly during startup; once the repulser electrodes have attained full potential, there is no current drain from them, except for that due to an occasional spark. Such electrical power as is dissipated in occasional spark discharging of the repulser electrodes is generated from the gases by their forcing the charged particles against the electrostatic field in the upstream part of the generator section; if no discharging occurs, such fluid power as is required to push the charged particles against the electrostatic field in the upstream portion of the generator is returned to the fluid from the charged particles by their pushing the gas stream in the downstream portion of the generator; but in any event, the pressure drop in the generator section is less than a few tenths of an inch of water because of the large volumetric flow of gas and the comparatively small amount of electric power required.

Preferably, the collector section is provided with a water washing arrangement for removing the collected dust from the surfaces of the collector electrodes. Alternately, mechanical means may be used to remove collected dust; however, the type of dust removal means will be largely dependent upon the temperature of the gases a cleaned by the electrostatic precipitator of the present invention.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a schematic showing of the electrostatic precipitator of the present invention;

FIG. 2 is a diagramatic showing of the electrical characteristics of the various sections of the precipitator; in relation to the gas being treated;

FIG. 3 is a schematic illustration of a precipitator constructed according to this invention;

FIG. 4 is a section taken along the line 44 of FIG. 3; and

FIG. 5 is an enlarged section of a single element of the unit shown in FIG. 3.

As illustrated in FIGS. 1 and 2 a single element of an electrostatic precipitator constructed according to the present invention includes a dielectric body or housing enclosing the precipitator element where the housing may be constructed of plastic, fiberglass, or other ceramic materials having the desired dielectric characteristic. Isopotential lines Z are shown qualitatively to help envision the electrostatic fields involved. As shown, the incoming gases enter the housing as at Y, and are accelerated in passing through a restricted throat 11. The throat is formed with a metallic insert 12 defining at least part of the walls of the throat, and is provided with a centrally located corona needle 13 which projects substantially through the full extent of the throat 11. As shown, power from a DC source 14 is applied to the inserts 12 through a connection 15 and through the connection 16 to the corona needle 13. The supply to the two components being of opposite polarities.

The dirty gases in passing through the ionizing section 17 are partially ionized and the dust therein contained is electrostatically charged with a polarity corresponding to that of the ionizer needles. Thereafter the gases proceed through a passageway 18 of increasing cross-sectional flow area which terminates in a collector section 20. The walls of section 20 are defined by collector electrodes 21 connected through line 22 to ground potential. Intermediate the ionizing section 17 and the collector section 20 and within the passageway 18 is positioned a repulser corona needle 23, which is exposed to the ionized gases passing thereover and is connected with a repulser electrode 24. Electrode 24 is concentrically arranged or centrally located with respect to and co-extensive with the collector electrode 21 and connected with the repulser corona needle 23 by an electrically conducting wire 25.

The electrostatic potentials and fields of the apparatus of FIGURE 1 will be qualitatively as indicated in FIG- URE 2 during operation. It will be noted that the center line electrical potential resulting from the electrogasdynamic efiect due to the gases flowing through the passageway 18 will provide a substantial voltage on the repulser electrode 24 resulting in a very substantial increase in the radial electrostatic field between the repulser electrode 24 and collector electrode 21, thus providing a high dust-collecting effectiveness.

As shown in FIGS. 3-5 a multiple arrangement of precipitator units may be used to process gas quantities exceeding the capacity of one unit. In such a commercial installation, the housing surrounding the precipitator is formed of dielectric material and is provided with a hood 31 of steel adjacent the upper end of the housing and with a steel outlet hood 32 adjacent the outlet end of the housing. The connections from external power source 26 include a connection 27 to each of the ionizer elements or bars 33 situated in the gas restrictor or throats 34 positioned in the upper end of the housing 30. The opposite side of the power source is connected to ground, and to the inlet and outlet hoods by line 28.

As shown particularly in FIGS. 3 and 5 the restrictors or throats 34 adjacent the inlet end of the housing 30 are formed in the general shape of Venturi tubes, with the metal ionizer bars 33 imbedded in the dielectric material which is shaped to define the throats 34. Corona needles 36 are suspended from above by Wires connected to support grid 37 and are arranged to extend into the throat of the associated restrictors so as to be concentrically positioned with respect to the bars 33. Corona needles 36 are electrically grounded by connection through the wires 35, support grid 37, and inlet duct 31.

- The depending ends of some or all of the corona needles are provided with bars or strands 38 of a dielectric material, such as aluminum oxide, to provide support for subjacent repulser corona needles 40 positioned intermediate the ionizing section A and the collector section B of the unit. Each of the repulser corona needles 40 is connected by an electrically conducting wire 41 to a repulser electrode 42 suspended in the collector section B. The repulser electrodes 42 are concentrically positioned relative to the collector electrodes 43 in collector section B whether the cross-sectional shape of the electrodes is circular, square, or of other cross-sectional configuration. Although not shown in the drawings, the corona needle 40 and repulser electrodes 42 may if necessary be supported by a grid or grids comparable with the support grid 37 supporting the corona ionizer needles 36 since it is desirable to maintain a generally fixed concentric relationship between each repulser electrode 42 and its corresponding collector electrode 43 so as to avoid electrical shorting of the assembly.

An enlarged view of an individual precipitator unit of FIG. 3 including the needle and repulser electrode assembly is shown in FIG. 5. It will be noted that both FIGS. 3 and 5 illustrate an arrangement whereby water introduced through pipe 39 as in FIG. 3 is utilized to wash the collected dust from the collector electrodes 43. This is accomplished by utilizing riser passageways 44 so that the water will flow, generally in film form, over the upper end of the collector electrodes 43 and then downwardly over the surface of each electrode. As shown particularly in FIG. 3 the water, with the collected solids, discharges downwardly from the collector section B into a sump 45 formed in the outlet hood 32. The mixture of water and collected solids is removed from the sump through a pipe 46 as required for discharge to Waste, or if the solids are of a type having industrial values, the sump discharge may be transmitted to settling tanks and the like for separation and reclamation of the solids from the liquid.

In operation the dust laden or dirty gases entering the upper hood 31 of the electrostatic precipitator as indicated at X in FIG. 3 pass successively downward over the support grid 37 and through the throats 34. In passing through the throats of ionizer section A the gases are temporarily ionized and the dust particles electrostatically charged. In the course of their continuing flow through passageway C, the gases continue to build up electric potential in accordance with known electrogasdynamics principles prior to entering collector section B. In the collector section B the collector electrodes are spaced from each other laterally to provide passages for flow of water therethrough, the discharge being generally as a film flowing over the surfaces to dislodge the accumulated dust particles from the collector surfaces. Thereafter the water and separated dust is discharged to the bottom of the precipitator for subsequent disposal. As previously pointed out, the shell or housing 30 of the generator section is formed of dielectric material which may be a ceramic, formed of fiberglass or various forms of plastic, for example, so that the static electricity built up within the generator portion will have a mainly axial electrical field. Ordinarily no appreciable surface discharge from the dielectric housing 30 will occur and thus the axial field will develop on account of the surface charge represented by the charged dust particles and such ions as may be present and in transit through'the precipitator element. The potential distribution will in general follow the form illustrated in FIG. 2. The arrangement is such that there will be a minimum probability of an electrical breakdown or leakage between the repulser electrode 42 and the ground. This is due to the spacing of the electrodes 42 and the repulser needles 40 with respect to wall leakage of the housing 30 and to the manner of support of the electrodes 42, i.e. by suspension from corona needles 36 and support grid 37. It is realized that leakage between the repulser needles 40 and/ or the repulser electrodes 42 and the housing 30 may occur at infrequent intervals, with consequent discharge of high potential electricity by means of short circuit through the gas within the system. Nevertheless the potential charge will immediately build up to an effective separating charge so that such short circuiting will not appreciably effect the efliciency of dust collection.

It will be noted in the FIG. 3 showing of the apparatus that there will be more than one corona needle 36 in the ionizer section for each repulser electrode in the collector section. This is due to the desirability of maintaining adequate spacing in the collector section between the repulser electrode 42 and the collector electrodes 43. Thus in installation there may be from 24 or even more, corona needle electrodes 36 in the ionizing section for every repulser electrode 40 in the collector section.

It should be realized that the power charge initially applied to the ionizer section A will be of the order of 10,000 volts direct current with relatively low amperage. By using the electrogasdynamic generator principle within the generating section C the static electricity will ordi narily build up to a range of from 50,000 to 200,000 volts in the collector section B with consequent enhanced collection efliciency in the apparatus. Thus the present electrostatic precipitator utilizes considerably less external power for comparable collection efficiencies than has been usual in the ordinary electrostatic precipitator. Likewise,

' arcing across the electrodes will not seriously affect or interrupt the power supply to the precipitator. For any arcing that may occur, the duration is short and infrequent, and does not adversely affect the power to the ionizer section A. Furthermore, efficient electrostatic separation can be attained without excessive gas flow velocity and high pressure drop in the apparatus. In apparatus such as disclosed the pressure drop through the system will be of the order of but one inch of water, which is comparable with that of conventional electrostatic precipitators having comparable capacities. On the other hand, the amount of material utilized in the electrostatic precipitator in the present invention is considerably less than that previously considered necessary in electrostatic precipitators of comparable capacity so that the present invention is more economical both to install and to operate than the precipitators which have been utilized heretofore.

While the precipitator described is limited in the temperature of the gases it can handle, due to the use of water washing in the collector section B it will be understood that higher gas temperatures can be accommodated using other means for removing the collected dust from the collector electrodes. For example, such dust removal could be accomplished by wrapping or scraping of the electrode surfaces 43. However, for most normal installations the described apparatus is satisfactory and will efficiently remove the dust from the gas with very modest expenditure for external power supply, minimum cost of electrical power consumed, negligible difficulty due to interelectrode arcing, and continuously maintained high efficiency in spite of several interelectrode short-circuits.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. An electrostatic precipitator comprising an open ended elongated housing formed of dielectric material, an inlet hood formed of conductive material and attached to one end of said housing for passing dust-laden gas into said housing, an outlet hood formed of electrically conductive material attached to the opposite end of said housing for passing dust-free gas out of said housing, charging means positioned in the inlet end portion of said housing for electrostatically charging the particles of dust in the dust-laden gas, a plurality of transversely spaced repulser and collector surfaces positioned in said outlet hood and adjacent said housing for electrostatically collecting the dust from said gas, and a plurality of transversely spaced repulser corona needles within said housing and spaced apart a substantial distance from said charging means, said repulser corona needles being intermediate the length of said housing and between said charging means and said outlet hood for increasing the electric potential between said repulser and collector surfaces by electrogasdynamics to enhance the collecting effect of said repulser and collector surfaces, a plurality of electrically conductive wires, each of said wires connecting a respective one of said corona needles with a respective one of said repulser surfaces, means connecting said inlet hood, said outlet hood and said collector surfaces to an electrical ground, means connecting said charging means to a source of high voltage direct current electrical potential, and support means within said housing and operatively associated with each of said repulser corona needles, said wires and said repulser surfaces for supporting said repulser corona needles, said wires, and said repulser surfaces out of contact with said collector surfaces.

2. An electrostatic precipitator according to claim 1, wherein said electrostatic charging means includes a gas flow restrictor for accelerating the gases entering said housing, and a corona needle coaxially extending through said restrictor.

3. An electrostatic precipitator according to claim 2, wherein said housing is vertically oriented and gas flow is downwardly therethrough.

4. An electrostatic precipitator according to claim 3, wherein said repulser needle is suspended from an upwardly adjacent corona needle by said support means which comprises a dielectric strand.

5. An electrostatic precipitator according to claim 3, wherein washing means are provided to Wash the dust from said collector surfaces.

6. An electrostatic precipitator according to claim 1, wherein means are provided for removing the collected dust from said collector surfaces.

References Cited UNITED STATES PATENTS 1,337,489 4/ 1920 Strong. 2,142,128 1/1939 Hoss et a1. 2,615,530 10/1952 Hodson et a1. 2,662,608 12/1953 Fields 55-137 2,715,944 8/1955 Dohrer 55--113 X 2,789,657 4/1957 Fields 55-437 2,798,572 7/1957 Fields 55137 2,813,595 11/1957 Fields 55-137 2,814,360 11/1957 Beaver.

(Other references on following page) UNITED STATES PATENTS FOREIGN PATENTS McDonald et a1. 55137 792,068 3/1958 Great Britain.

Roos er 55 137 820,415 9/1959 Great Britain. Vicard.

De Seversky- 5 HARRY B. THORNTON, Primary Examiner. 32:32: at D. TALBERT, Assistant Examiner.

Weindel et a1.

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