US2386827A - Electrical precipitator - Google Patents

Description

Filed 'Feb. 12, 1944 3 Sheets-Sheet l Oct. 16, 1945. H. J. WHITE I 2,386,827

, ELECTRICAL PRECIPITATOR Filed Feb. 12, 1944 I s Sheets-Sheet 2 EHH HHHH M magi 11 1111111111 Haw/ MAME H. J. WHITE ELECTRICAL PRECIPITATOR Oct. 16, 1945.

Filed Feb. 12 1944 3 Sheds-Sheet 3 \WMWVVVVVV 3mm: Hal-f7 "W I Patented Oct 16, 1945 ELECTRICAL PRECIPITATOR Harry J. White, Cambridge, Mass, assignor, by direct and mesne assignments, of one-half to Research Corporation, New York, N. Y., a corporation of New York and one-half to Western Precipitation Corporation, a corporation of California Los Angcles, Calil'.,

Application February 12, 1944, Serial No. 522,115

6 Claims.

This invention relates to the electrical precipitation of suspended particles from gases and is particularly directed to improved collecting electrode structures for electrical precipitators and to electrical precipitators including the new collecting electrode structures.

Briefly, the operation of electrical precipitation consists in passing gases, containing suspended fine particles of either solid or liquid, through an electric field in which the particles become electrically charged by attachment of electrons or ions and are then attracted to an electrically charged member upon which the charged particles are collected. It has been common practice to effect charging of the suspended particles by passing them between two opposed electrodes between which a high potential difference is maintained, one of the two electrodes being a discharge electrode at which there is silent or corona electrical discharge that ionizes the gas and causes the suspended particles to become charged with the same'electrical sign as the discharge electrode. This is termed charging action.

In the "single stage type of precipitator, the charged particles migrate under the influence of the electric field between the electrodes'toward the other electrode which is a non-discharge electrode of extended surface, and collect or become precipitated upon the surface of that electrode which is consequently termed the collecting electrode. field type of precipitators, the gas containing all or a substantial proportion of the charged particles passes into a second or precipitating field, typically maintained between opposed nondischarge electrodes, and, under the influence of the precipitating field, the charged particles migrate to one of the electrodes between which the field is maintained and are deposited thereon.

The latter electrode is likewise termed a collecting electrode. The novel collecting electrode structures of the present invention may be advantageously utilized as collecting electrodes in either single-stage or two-stage precipitators.

In the following description and in the appended claims the term "discharge electrode will be understood to designate an electrode that facilitates corona discharge therefrom, because it has a configuration that establishes a sufllciently high potential gradient at or near its surface to create corona discharge before there is a disruptive discharge or spark-over. For this purpose the discharge electrode usually takes the form of a member of small surface area, such as a small In the "two-stage or separated or points, whereby there may be created in the immediate vicinity thereof a sufiiclently high electric field intensity to .cause ionization and corona discharge. The term non-discharge electrode" will be understood to designate an electrode that minimizes or prevents corona discharge therefrom because it has a configuration that establishes a sufilciently low field concentration at or near the surface to suppress corona discharge at elevated potentials lower than the voltage required for disruptive discharge or spark-over. For this purpose. a non-discharge electrode usually is one of extended surface area, substantially free from sharp corners or other parts of sharp surface curvature at all portions which are located within the electric field, so as to substantially avoid ionization of corona discharge at that electrode.

Dust particles in the gas stream become electrically charged in the ionizing field and there migrate'to the collecting electrode under the infiuenceof the electrical forces exerted on them, and when the particles reach and are precipitated on the electrode the electric charges are neutralized or lost. The layer of dust particles on the electrode is exposed ordinarily to the gas stream which tends to blow oif loose individual particles or agglomerations or particles, the erosion being a result of the gas velocity. The particles are held on the electrode more or less securely against erosion either by electrical forces or by the physical properties of the particlesv themselves. When the collecting electrode is within the range of the charging field, as in the single field type of precipitator, the field recharges and reprecipitates most of the particles that may be blown off the electrode, but even then some particles are blown along the electrode surface and off the outlet end of the electrode when the dust has such physical properties that the particles offer little resistance to gas erosion. In the two-stage type of precipitator in which there is typically no corona discharge in the second or precipitating field, material removed from the collecting electrode by erosion is not recharged and is carried out of the precipitator by the gases.

- There are some dusts that are particularly hard to collect for this reason, one typical example being fly ash" which is composed of particles that have a generally spherical shape and have very little inherent tendency to adhere to the electrode or to each other. Characteristics such as these facilitate erosion of the precipiwhich the critical velocity is relatively low, since.

the amount of these dusts carried into the outgoing gas stream is considerable and makes it difliciflt to obtain high overall collection efllciencies at economic velocities. Obviously the critical velocity for a givendust limits the capacity of a given treater if a minimum collection efliciency is to be obtained.

This collection difiiculty is inherent in the dust being collected and has the efiect of reducing the capacity of a unit of a given size operating at a given normal efliciency, because of the limitation on dimensions required in order to keep the gas velocity below critical values. Expressed differently, the reduction in collection efficiency caused by erosion losses increases the size of the plant and equipment required to treat a given volume or gas without falling below a minimum efilciency.

In my U. S. Patent No. 2,192,250 there is described a double field screen pocket precipitator including a collecting electrode structure comprising a collecting electrode member shielded by a foraminous non-discharging electrode member positioned between the collecting electrode member and a discharge electrode and electrically energized to provide a non-discharging electric field-i between the collecting electrode member and the foraminous electrode member.

In the precipitator of the patent a certain fraction of the corona discharge penetrates the screen electrode and passes to the shielded collecting electrode. By making the screen electrode more or less open the fraction of the current thus passing to the shielded collecting electrode can be controlled, but experience has'shown that the degree of porosity of the screen electrode required to obtain a suitably high intensity of corona current at the collecting electrode is often so great that an excessive gas circulation back of the screen occurs. In other words, the necessary compromise between the conflicting factors of corona current penetration and gas stream penetration is such that full advantage cannot be taken of the possibilities of the double field arrangement.

Moreover, the double field screen pocket collecting electrode of the patent is of limited use- Iulness in the second or collecting stage of a two-stage precipitator in conjunction with a nondlscharge type precipitating electrode.

These disadvantages are avoided in the collecting electrode structure of the present invention which provides corona discharge inside the shielded spaces of a double field screen pocket collecting electrode, in addition to the corona discharge coming from the primary discharge electrode or in place of such discharge when the collecting electrode is used with non-discharging primary precipitating electrodes.

A principal object of the invention is to provide an improved electric precipitator in which loss or collected material by erosion or redisabove which erosion loss increases persion in the gas stream is substantially reduced or eliminated.

A further object of the invention is the provision of an improved form of collecting electrode construction for electrical precipitators.

These and other objects and advantages which will be apparent. from the following description of the invention are attained by the provision of a collecting electrode structure including extended surface members providing primary material collecting surfaces and defining passages shielded by said members from the stream of gas being treated, secondary collecting surfaces positioned within said passages, and discharge electrode elements carried by the members providing the primary material collecting surfaces and positioned to precipitate material on the secondary collecting surfaces.

Typically the primary collecting surfaces are provided by a plurality of vertically-extending members arranged in spaced parallel relation to define at least one vertically-extending passageway, within which is positioned an extended surface member spaced and electrically, insulated from the vertically-extending members, said vertically-extending members including discharge electrode elements, such as edges and prongs, directed toward the extended surface member within the passageway.

A typical electrical precipitator embodying the invention includes a precipitating electrode structure anda collecting electrode structure spaced therefrom to define a gas passage therebetween and comprising extended surface members defining a vertically-extending passageway and providing primary material collecting surfaces and a plurality of openings establishing communication between the gas passage and the vertically-extending passageway, a further extended surface member spaced and insulated from the first extended surface members and providing secondary material collecting surfaces within the vertically-extending passageway, and discharge electrode elements carried by the first extended surface members within the verticallyextended passageway.

The term "precipitating electrode structure" is used herein to designate electrode means opposed to and spaced from the collecting electrode structure and cooperating therewith to establish the electric field which causes charged particles'to precipitate on the collecting electrode.

The invention will be more particularly described for the purpose of illustration with reference to the accompanying drawings in which:

Fig. 1 is a sectional elevation of an electrical precipitator embodying the principles of the invention;

Figs. la and 1b are fragmentary details of alternative embodiments of the invention;

Fig. 2 is a partial plan view of the precipitator of Fig. 1 with the top cover plate removed;

Fig. 3 is a partial sectional elevation on line 3-3 of Fig. 2, and

Figs. 4 .to 8 are diagrammatic representations of illustrative embodiments of the invention.

In Figs. 1-3, H) is a precipitator casing provided was gas inlet ll, gas outlet l2 and a collecting hopper I3 for precipitated material.

Within the casing III are complementary precipitating and collecting electrode structures spaced apart to define longitudinal gas passages through the casing.

The precipitating electrode structures comprise discharge lectrodes ll, consisting of wires suspended from horizontal tubes l8 and maintained taut and in proper spaced relationship by tubes i8. Tubes 15 are carried on I-members H which are supported on insulators l8 and insulator bushing is contained in insulator housings 20. The precipitating electrode structure is energized through the insulator bushing i9.

The collecting electrode structure comprises a plurality of metal T-members 2i positioned in spaced relation and cooperating with end wall members 22 and partition members 28 to define vertically-extending passageways 26 having a plurality of vertical openings 24a. The web portions of the T-members 2i project into the passageways 2d and provide discharge electrode edges provided on the inner surfaces of the members, as shown in Fig. la, or the web of the members 2| may be notched to form toothed edges Zlb, as shown in Fig. 2b.

The plate collecting electrode members 26 may be replaced by other extended surface. members. such as a curtain of rods of such diameter as to be non-discharging under the conditions oi operation.

Other advantageous arrangements and constructions of the collecting electrode structures are shown in Figs. 4 and 5, in which corresponding elements are given the same numbers as in Figs. 1-3.

therein. The members El, 22 and 23'are supported from the end walls of casing Ill by beam members 25 extending horizontally adjacent the upper and lower ends of members 2!, 22 and 28.

Within each of the vertical passageways 24 is positioned a collecting electrode member comprising a plate 26 provided with rounded lateral edge members 2 by means of which the plates are suspended from tubes 28. The tubes 28 are carried on I-beams 29 which are supported from insulators 30 and insulating bushing 3i through which the collecting electrode plates 23 are energized.

Battles 32 aid in confining the gas flow within the interelectrode spaces.

In operation, the precipitating electrodes it are preferably maintained at a negative polarity and collecting electrode members 26 at a positive polarity, with the primary collecting surfaces including members 2i at ground potential. However, the polarities may be reversed and any of the three electrode elements of the precipitator may be maintained at ground potential.

In general, it is preferable to maintain a potential difference between the primary precipitating electrode and the collecting electrode structure at about 30 to 50 kv. with a spacing of 4 inches, for example, if the primary precipitating electrode is of the discharge type, and average field strengths of the same order of magnitude are preferably maintained between the secondary discharge electrode elements and the secondary collecting members 26. If the primary precipi tating electrode is of the non-discharge type a somewhat higher potential diflerence, say 40 to 60 kv. for a. 4-inch spacing between the precipitating electrode and the collecting electrode structure is maintained.

In operation, suspended material contained in a gas stream passed horizontally through the apparatus is initially collected in part on the extended surfaces of the members 2i facing the precipitating electrodes I4 and in part projected through the slots 24a into passageways 28. All or part of the material collected on the members 2i may further be eroded therefrom and thereafter carried or projected into passageways 23 through the slots. The corona discharge emanating from the discharge elements within the passageways effectively precipitates all of the suspended material entering the passageways upon the secondary collecting surfaces of plates 26 and maintains the deposited material on the plates. I

The collecting electrode structures may be rapped during operation to cause the collected material to drop into a hopper or other suitable receiver in the bottom of the apparatus.

Instead of utilizing the edge of the web of members'2l as discharge elements, prongs Ila may be In the construction of Fig. 4 the members H which define the passageways- 24 and carry discharge elements within the passageways are shaped to provide lips to direct gas flow into the passageways. The gas flow through the passage- ;gays may be regulated by adjustment of dampers In the construction of Fig. 5, the passageways 24 are defined by extended surface members 2|" extending normal to the direction of gas flow, the rear edges of the members 2|", which may be serrated, providing the discharge elements of the structure. either against or with the flow of gas in the main gas passage and may be curved or crimped in the cross-section shown to promote stiffness.

As is shown more particularly in Figs.-8, 7 and 8, the primary collecting surfaces and the secondary collecting surfaces may be provided by a series of parallel vertically-extending members of substantiaily similar configuration except that alternate members are insulated from the next adjacent members to provide two alternating sets, the members of one of which bear discharge elements, the sets being differentially energized to provide a precipitating discharge from said discharge elements directed toward the extended surfaces of adjacent members.

In the construction of Figs. 6, 7 and 8, the passageways are defined by extended surface members SI, SI, 6|", respectively, providing vertically-extending collecting surfaces and bearing discharge elements 62 directed generally toward the surfaces of the alternating members 63, 63', 653". In each construction memberslil, 6|, 6|" are insulated from members 63, 63', 63" and a potential difference is maintained between the two groups of electrodes as hereinbefore described. A suitable construction for electrical precipitators including composite collecting electrode structures of the type shown in Figs. 8, '7 and 8, is described in my application Serial No. 489,833, filed June 5, 1943.

In Figs. 6 and 8 the precipitating electrode structure is shown as comprising discharge electrodes ll, while in Fig. 7 it consists of an extended surface electrode ll. These forms of precipitating electrode structures are interchangeable in the various structures, the form shown in Fig. 7 being more particularly suitable for use in the second stage of a two-stage precipitator.

I claim:

1. An electrical precipitator comprising opposed precipitating and collecting electrode structures spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising extended surface members defining a vertically-extending passageway within said collect ing electrode structure and outside saidgas passage and providing primary material collecting surfaces opposing said precipitating electrode The members 2!" may be inclined extended surface electrode within said passageway spaced and insulated from said foraminous wall members.

3. In an electrical precipitator, a collecting electrode structure comprising a plurality of vertically-extending members positioned in substantially parallel spaced relation to form slotted walls of a vertically-extending passageway and having discharge elements of small radius of curvature projecting therefrom into said passageway, and an extended surface electrode within said passageway spaced and insulated from said verticallyextending members. i

4. An electrical precipitator comprising opposed precipitating and collecting electrode structures spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising grounded foraminous wall members defining a vertically-extending passageway within said collecting electrode structure and outside said gas passage and providing a plurality of discharge electrode elements extending into said passa eway, an extended surface electrode within said passageway spaced and insulated from said foraminous wall members, and means for electrically energizing said precipitating electrode structure at one polarity and said extended sur- .spaced face electrode at the opposite polarity with respect to said grounded foraminous wall members.

5. In an electrical precipitator, a collecting electrode structure comprising a plurality of parallel vertical plate members defining vertical passageways therebetween, further vertical plate members intermediate and parallel to said first plate members and insulated therefrom, and discharge elements of small radius'of curvature pro- Jecting from said first plate members into said passa eways. I

.6. An electrical precipitator including opposed precipitating and collecting electrode structures apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of extended surface members spaced apart to define a plurality of pockets open to the gas passage but shielded by said extended surface members from the flow of gas therein, alternate extended surface members bearing discharge electrode elements projecting into said pockets and being insulated from and maintained at an electrical potential with respect to the remainlng extended surface members to impel charged particles in said pockets toward the surface of said remaining members.

HARRY J. WHITE.

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