US2383030A - Electrical precipitation - Google Patents

Description

Aug.2l, 1945. J, WHITE I 2,383,030

ELECTRICAL PRECIPITATION Filed June 5, 1943 2 Sheets-Sheet 1 Aug. 21, 1945. H. J. WHITE ELECTRICAL PRECIPITATION 2 Sheets-Sheet 2 Filed June 5, 1945 vvvvvvvvvvv @VMVVVVVV Patented Aug. 21 1945 Research Corporation, New York, N. Y., a corporationof New York, and'one-half to Western Precipitation Corporation, a corporation of California Los Angeles, Calif.,

Application June 5, 1943, Serial No. 489,833

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. V

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 withthe 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. 'In the two-stage or "separated 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 non-discharge 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 elec-- trode. The novel collecting electrode structures of the present invention may be advantageously utilized as collecting electrodes in either singlestage 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 sufficiently high potential gradient at or near its surface to create corona discharge before there is a disruptive dis.- charge or spark-over. For this purposethe discharge electrode usually takes the form of a member of small surface area, such as a small diampoints, whereby there may be created in the immediate vicinity thereof a sufficiently 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 sufiiciently 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 or corona discharge atthat electrode.

Dust particles in the gas stream become electrically charged in the ionizing field and there migrate to the collecting electrode under the infiuence of 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 off loose individual particles or agglomerations of particles, the erosion being a result of the gas velocity. The particles are held on the electrodemore or less securely against erosion either by electrical forces or by the physical properties of the particles 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 oil! 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 twostage type of precipitator in which there is typieter wire or a rod provided with sharp edges or cally 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 "fiy' ash which is composed of particles thathave 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 precipitated dust layer by the gas stream, the particles that are picked up being carried away by the outgoing cleaned gases. Very little erosion takes place at the rate increases slowly with increasing velocity up to some velocity termed the "critical velocity" above which erosion loss increases rapidly with an increase in velocity and becomes quite-appreciable. It is probably true of most dusts, it not all, that there is a critical velocity above which losses are appreciable. The present interest centers par icularly around dusts for which the critical velocity is relativelylow, since the amount or these dusts carried into the outgoing gas stream is considerable and makes it diflicult to obtain high overall collection emciencies at economic velocities. Obviously, the critical velocity for a given dust limits the capacity oi a given treater it a minimum collection eflleiency is to be obtained.

- This collection diiiiculty is inherent in the dust being collected and has the effect of reducing the capacity oi a unit of a given size operating at a given normal eiliciency, because oi the limitation on dimensions required in order to keep the gas velocity below critical'values. Expressed diil'erently, the reduction in collection efliciency caused by erosion losses increases the size of the plant and equipment required to trat a given volume of gas without falling below a minimum efllciency.

vide an improved electric precipitator in which loss of collected material by erosion or redispersion in the gas stream is substantially reduced or eliminated.

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

These, and other objects and advantages of the invention which will be clearly apparent from the following description, are attained by the provision in an electrical precipitator of a composite collecting electrode structure spaced from a comshield members. The electrode members in the pockets may be either discharge or non-discharge electrodes. Particularly when the electrode members in the pockets are of the non-discharge type, they may be positioned and shaped to cooperate in shielding the pocket spaces from the adjacent gas stream. The potential difference maintained between the electrode members in the pockets and the shield electrodes is of the same polarity but substantially less than the potential diiIerence maintained across the gas stream between the precipitating electrode structure and the collecting electrode structure.

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.

If the collecting electrode structure of the invention is embodied in a single stage precipitator, the complementary precipitating electrode structure will include discharge elements, while if the collecting electrode structure is embodied in the asaaoso g A principal object or the invention is to prothe complementary precipitating electrode structure will typically be oi the non-discharge type.

In general, the average fleld strength between the electrode members in the pockets and the shield members may be of the same order as the average field strength between the precipitating electrodes and the collecting electrode structure.

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

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

Fig. 2 is a plan view of the precipitator of Fig. l

spaced apart to define longitudinal gas passages through the casing.

The precipitating electrode structures comprise discharge electrodes l4, consisting of wires suspended from horizontal tubes l8 and maintained taut and in proper spaced relationship by tubes It. Tubes I! are carried on I-members il which are supported on insulators i8 and insulator bushing l9 contained in insulator housings 20. The

precipitating electrode structure is energize proper spaced relation by members 23 at their lower ends, and similar plate members 24 parallel to and alternating with plate members 2| but electrically insulated therefrom.

Plate members 24 are supported at their upper ends by means of rods 25 attached to the upper ends of the plate members and extending into engagement with tubes 26 which are carried on I-members 21 supported from insulators 28 and insulator bushing 29 through which plate members 2 are energized. Plate members 24 are maintained in proper spaced position by means of tubes 3! which engage rods 30 attached to the lower ends of the plate members 24.

Plate members 21 and 24 cooperate todefine material collecting pockets shielded from the gas stream in the gas passages between the precipitating and collecting electrode structures. The lateral edges 01' the plate members are preferably rounded as shown to minimize corona discharge from the edges.

Bailles 32 aid in confining the gas flow within the interelectrode spaces. I

The potential diflerence between the precipitating electrode structure and the grounded collecting electrode plates is preferably maintained at from 30 to 50 kv, with a spacing between the structures of 4 inches, for example, and a potentialdiflerence oi the same polarity and of from 5 to 15 kv. is maintained between plates 24 and plates with a spacing therebetween of 1 inch, for example. v

If the precipitating electrode structure includes precipitating stage of a two-stage precipitator, non-discharge electrode members instead of the aasa'oso discharge electrode members I 4 of Figs. 1-3, the potential difference across the gas stream is preferably somewhat higher, for example, about 40 to 60 lrv. for a 4-inch spacing of the electrode structures. If the non-discharge plate members 5 24 of the collecting electrode structure are replaced by discharge members spaced by one inch from the grounded shield members, a potential difference of about 8 to 10 kv. is preferably main- 10 tained therebetween.

A number of the many variations in the character and arrangement of the principal elements of the invention which are possible within the principles of the invention are diagrammatically illustrated in Figs. 4 to 9.

In Fig. 4 the plate members 24 of Figs. 1-3 are I replaced by discharge electrode members 24' positioned in the material collecting pockets formed by plate members 2|.

In Fig. 5 the discharge electrode members ll of Figs. 1-3 are replaced by non-discharge precipitating electrode member l4.

In Fig. 6 the" plate members Na and 24a corresponding to members 2| and 24, respectively, of

Figs. 1-3, are convex in the upstream direction of 2 the gas stream, V

In Fig.7 the plate members Ila, similar to the corresponding members of Fig. 6, are associated with discharge electrode members 24.

In Fig. 8 the plate members! lb and 24b corre- Fig. 5.. Such constructions are particularly adapted for use as the second or precipitating stage of two-stage electrical 'precipitators.

It will be apparent from the foregoing description that the elements of the invention are subject to wide variation in form and arrangement without departing from the scope of the invention as defined in the appended claims.

I claim:-

1. An electrical precipitator including opposed precipitating and collecting electrode structures spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of extended surface members spaced apart longitudinally of the gas passage to define a plurality of pockets open to the gas passage but shielded by said extended surface members from the flow of gas therein, and electrode members electrically insulated from said extended surface members, positioned in said pockets and maintained at an electric potential with respect to said extended surface members to impel charged particles in said pockets toward the surface of said members.

2. An electrical precipitator including opposed precipitating and collecting electrode structures spaced apart to provide a gas passagetherebetween, the collecting electrode structure comprising a. plurality of extended surface members spaced apart longitudinally to define a plurality of pockets open to the gas passage but shielded by said extended surface members from the flow of as therein, alternate extended surface members being insulated and maintained at an electric potential with respect to the remaining extended surface members to impel charged particles in said pockets toward the ing members.

3. An electrical precipitator including Opposed precipitating and collecting electrode structures spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of extended surface members spaced apart longitudinally of the gas passage to define a plurality of pockets open to the gas passage but shielded by said extended surface members from the flow of gas therein, and discharge electrode members electrically insulated from said extended surface members, positioned in said pockets and maintained at an electric potential with respect to said extended surface members to impel charged particles in said pockets toward the surface of said members.

4. An electrical precipitator including opposed I and spaced apart longitudinally thereof and elec- 5 trode members positioned between adjacent plate precipitating and collecting electrode structures.

spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of parallel vertical plate members extending transversely to the gas passage and spaced apart longitudinally thereof and discharge electrode members positioned between adjacent plate members and insulated therefrom.

7. In an electrical'precipitator opposed electrode structures spaced apart to provide a gas passage therebetween, electric circuit means for -establishing an electric field between said opposed electrode structures, one of said electrode structures comprising extended surface elements defining a plurality of pockets open to he gas passage but shielded from the flow of gas therein and intermediate elements positioned in said pockets, and electric circuit means for establishing an electric field between said intermediate elements and said extendedsurface elements of the same polarity and approximately the same average strength as said first electric field.

8. In an electrical precipitator opposed electrode structures spaced apart to provide a gas passage therebetween, electric circuit means for establishing an electric field between said opposed electrode structures, one of said electrode structures comprising extended surface elements defining a plurality of pockets open to the gas passage but shielded from the flow of gas therein and intermediate elements positioned in said pockets, and electric circuit means for establishing an electric field between said intermediate elements and said extended surface elements effective to maintain an electric discharge therebetween.

9. An electrical preclpitator including opposed precipitating and collecting electrode structures surface of said remainspaced apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of extended surface members spaced apart longitudinally of the gas passage to define a plurality of pockets open to the gas passage but shielded by said extended surface members from the flow of gas therein, and electrode members electrically insulated from said extended surface members, positioned in said pockets at a distance from said extended surface members substantially less than. the distance between aid collecting electrode structure and said precipitating electrode structure andmaintained at an electric potential with respect .to said extended surface members to impel assaoso charged particles in said pockets toward, the surface of said members.

10. An electrical precipitator including opposed precipitating and collecting electrode structures spaced apart to provide a gas passage therebetween, the collecting electrode structure comprising a plurality of parallel vertical plate members extending transversely to the gas passage and spaced apart longitudinally thereof by a distance substantially less than the distance between said collecting electrode structure and said precipitating electrode structure and electrode members positionednbetween adjacent plate members and insulated therefrom.

. A HARRY J. WHITE.

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