US2395927A - Electrical precipitator - Google Patents

Description

March 5, 194%.

H. J. WHITE ELECTRICAL PRECIPITATOR Filed Dec. 10, 1945 I 4 Sheets-Sheet l Patented Mar. 5, 1946' UNITED STATE PATE NT orrice I ELECTRICAL PRECIPITATOR Harry J. White, Cambridge, Mass, assignor, by

direct and mesneassignments, of one-half to Research Corporation, New York, N. Y.-, a corporation of New York, and one-half to Western Precipitation Corporation, Los Angeles, Calif a corporation of California Application December 10, 1943, Serial No. 513,765

3 Claims. (0]. 183-7) -mon practice to effect charging of the suspended particles by passing them between two opposed electrodes between which a high potential differe'nce is maintained, one of the two electrodes being a discharge electrode at whichthere 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 Zvhich is consequently termed the collecting elecrode.

passes into a second or precipitating field, typically .maintained between opposed non discharged electrodes, and, under the influence of themecipitating 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 singlestage or two-stage precipitators.

In the following description and in the ap pended 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 sufflciently 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 or a a member of small surfacearea, such as a small or points, whereby they may be created in the immediate vicinity thereof a sufllciently high electric field intensity to cause ionizationand corona discharge. .The term "non-discharge electrode" will be understood to designatean electrode that minimizes or prevents corona discharge therefrom because it has a configuration that establishes a sufllciently low field concentration at or near the surface tosuppress 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 harp 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 at that electrode.

Dust'particles in the gas stream become elec- I trically charged in the ionizing field and there In the two-stage or "separated field v type of precipitators, the gas containing-all or a substantial proportion of the charged particles migrate to the collecting electrode under the in fiuence or 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 exp sed 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 par ticles are held on the electrode more 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 parelectrode or toeach other.

ticles are blown along the electrode surface and of! 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 typically no corona discharge in the second or precipitating field, most of the material removed from the collectingelectrode by erosion is not recharged and is carried out of the precipitator by the gases. There are some dusts being fiy ash" which. is composed of particles that have a generally spherical shape and have very little inherent tendency to adhere to the Characteristics such as these facilitate erosion of the precipitated dust that are particularly hard to collect for this reason, one typical example cleaned gases. Very little erosion takes place at low gas velocities and may be neglected, though 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, if not all, that there is a critical velocity above which losses are appreciable. The present interest centers particularly around dusts for which the crittained.

This collection difflculty is inherent in the nature of the dust being collected and has the eifect 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 ini be provided at least in part by the extended surcreases the size of the plant and equipment required to treat a given volume of gas without falling below a minimum eiflciency.

A principal object of the invention is to provide an improved electric precipitator in which loss of collected material by erosion or redispersion in thegas 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.

, Another object of the invention is the provision of an improved method for the removal of susaegded particles from gases by electrical precipi- These and other objects and advantages which will be clearly apparent from the following description of the invention are attained by the provision of a collecting electrode structure inface memberswhich provide the primary material collecting surfaces, utilizing, for example, opposite faces of said members to provide the two collecting surfaces. When the secondary material collecting surfaces are provided by different members from those providing the primary material collecting surfaces, the latter members may also provide or support the precipitating electrode members which cooperate with the secondary colture will typically be of the non-discharge type.

In general, the average field strength between the precipitating electrode members within the vertical passageways and the secondary collecting surfaces therein may be of the same order as theaverage field strength between the collecting electrode structure and the precipitating electrode members complementary thereto.

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

F18. 1 is a sectional elevation of an electrical precipitator embodying the principles of the invention;

Fi 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 H of Fig. 2, and

,l igs. 4 to 8 are diagrammatic representations of illustrative embodiments of the invention.

eluding primary material collecting surfaces at least partially exposed to the stream of gas being treated, secondary collecting surfaces fully Y shielded from the stream of gas, precipitating electrode means positioned to precipitate material on said secondary collecting surfaces, and mean for directing a flow of gas including material eroded from the primary collecting surfaces into the space between the discharge electrode means 6 and the secondary collecting surfaces.

A typical electrical precipitator embodyin the invention includes a precipitating electrode structure and a collecting electrode structure spaced therefrom to define a gas passage therebetween and comprising extended surface members defining one or. more vertically-extending passageways f and providing primary material collecting sur- .faces and a plurality of openin s, preferably verments or suitably positioned fans, for directing as, flow from said gas passage into said vertipassageways. means providing secondary Wu! collecting surfaces within the passageways, and'precipitating electrode members; within the verticallyextending passageways spaced and insulated from the secondary material collecting surfaces.

In Figs. 1-3, I! is; a precipitator casing provided with gas inlet ii, gas outlet i2 and a collecting hopper it 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 structure comprise discharge electrodes i4, consisting of wires suspended from horizontal tubes I! and maintained taut and in proper spaced relationship by rods ii. Tubes II are carried on I-members I! which are supported on insulators l8 and insulator bushing it contained in insulator housings It. The precipitating electrode structure is 5 energized through the insulator bushing IS.

The collecting electrode structure comprises a plurality of sheet metal members-2|, shaped and rranged to provide a plurality of vertically-extending passageways 22 having vertical inlet slots 22c and outlet slots 22b. Adjacent the inlet slots 22a the members II are provided with lip member Ila, extending outwardly into the gas pas-' Within'each of the vertical passageways 22 provided by members 2!, is positioned a'discharse electrode 14 suspended from horizontal tubes 25 and maintained tent and in proper position by rods fl. Tubes 28 are carried on I-members 21 The secondary material collecting Q which 7 are supported from insulators 2| and in.

ascaesr sulating bushing 29 through which the discharge erosion. Thus the whole or a major part of the deposited material is carried along the primary collecting surface in the direction of gas flow in'the main gas passages. The lips 2 la projecting from the collecting surfaces adjacent the inlet slots 22a cause a small portion of the gas stream adjacent the collecting surfaces to flow into each of the passageways 22, a corresponding amount of gas flowing out of the passageways through outlets 22b. The amount of flow through the passageprimary collecting electrode surfaces at ground potential with the primary precipitating electrodes being of negative polarity with respect to ground potential, and the secondary precipitating electrodes also being of negative polarity with respect to ground potential in the forms of the ing the angle of the lips Ma or by any combination of these means, and is preferably adjusted to a velocity which will permit a high efliciency of precipitation within the passageways;

Thus the material deposited in the primary precipitating field is transferred by erosion and gas flow into a secondary precipitating field where it is collected under conditions of relatively low gas velocity upon the shielded innersurfaces of members 2|. Very high gas velocities of. 25 to 50 1 feet per second or even higher may be used in g the main gas passages.

Th 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.

Alternative arrangements and constructions of the inlet and outlet'openings and the lip members are shown diagrammatically in Figs. 4, 5 and 6 in which corresponding elements are given the. I same number as in Figs. 1-3.

In the construction of Fig. 4, the primary precipitating electrodes H" are shown as nondischarge electrode members, while'in the construction of Fig. 5 the secondary precipitating electrodes 24' are shown as non-discharge electrode members. charge type primary and secondary precipitating electrodes shown in the constructions of Figs. 1 to 7 may also be made. The use of non-discharge type primary precipitating electrodes, as shown in Fig. 4, together with either'dischargeor nondischarge type secondary precipitating electrode members, is particularly advantageous when using the construction as the second or precipitating stage of two-stage electrical precipitators.

Similar replacement of the dis- In general, it is preferable to maintain a potential difference between the primary precipitating electrodes and the collecting electrode structure at about 30 to kv. with a spacing of 4 inches,

for example, if th primary precipitating elec trode. is. of the discharge type, and somewhat higher, say 40m kv. for the same spacing, if

' the precipitating electrode is of the non-discharge type. Average field strengths of the same order of magnitude are preferably maintained between the secondary precipitating electrodes and the invention shown in Figs. 1, 2, 3, 4, 6 and 7, and being of positive polarity with respect to ground in the form of the invention shown in Fig. 5. In

as the general potential difference relations sug-' gested above are maintained.

In the arrangement shown in Fig. 7, the gas passageways 22 have a plurality of inlets 22a and a single outlet 22b which may be led directly into the gas stream or may be connected to a suitable system of ducts leading to an exhaust fan, as

indicated in the drawings, to provide a more positive control of the gas velocity through the secondary collecting spaces 22. Instead of'the duct system, adjustable dampers, such as are shown in- Fig. 8, may be provided-at the outlets 22b.

In the form of the invention shown in Figs. 8 and 9, the discharge electrode elements llla providing the secondary precipitating corona discharge are carried by extended surface members 40, which provide the primary material collecting surfaces, and-the secondary material collecting surfaces are provided by extendedsurface members 4 l which are insulated from the members I. The members 40 define ,gas passageways 42 witninlet openings 42a and outlets 42b. The outlets 42b may be provided with adjustable dampers 43, as shown, or may be connected to a suitable flue and fan system as indicated in Fig. I.

The discharge precipitating electrodes H of Fig. 8 may. be replaced by non-discharge type precipitatingelectrodes as shown in Fig. 4 and the discharge elements 40a may be eliminated from extended surface members 40, so that the latter act as non-discharge precipitating electrodes with respect to secondary collecting electrode members ll.

I claim:

1. An electrical precipitator comprising: a primary precipitating electrode structure;a collecting electrode structure spaced therefrom to provide a gas passage therebetween and comprising wall means defining a vertically extending space within said collecting electrode structure and shielded from the flow of gas in said passage, said wall means providing primary material collecting surface portions of extended area facing toward said gas passage, secondary material collecting surface portions of extended area facing toward said shielded space, and a plurality of slots establishing communication between said gas passage and said shielded space, said slots being spaced apart in the direction of gas flow in said passage and extending. transversely with respect to said gas flow, and said collecting electrode structure being provided with gas deflecting members disposed adjacent said slots in position to direct gas from said gas passage into said shielded space and withat least one opening for outflow of gas from said shielded space at a position removed from said slots; and secondary precipitating electrode members within said shielded-space and spaced from said secondary material collecting surface portions. v

2. Anelectrical precipitator comprising: a priprecipitating electrode structure: a collect-v electrode'structure spaced therefrom to provide a gas e therebetween and comprising wall means defining a vertically extending space withinfisaid collecting electrode structure and shielded from the flow of gas in said passage, said wall means providing-primary material collecting portions of extended area facing toward said a: e, V secondary material collecting portionso'tjexte'nded area lacing toward said shieldedspace, a plurality or slots establishing'comniunication between saidgas e and *said shieldedspaca'saidmlots being spaced apart lnthe direction or gas flow in said passage and eitending transversely with respect tosaid gas assess-z 3. An electrical preeipitator comprising: a primary precipitating electrode structure; a collecting electrode structure spaced therefrom to provide a horizontally extending gas passage therebetween and comprising wall means defining a plurality of vertically extending passageways within said collecting electrode structure and shielded from the flow of gas in said passage. said wall means providing primary material collectiiow,' and said collecting electrode structure being I provided 'withgas deflecting members disposed ing surface portions of extended area facing toward said gas passage, secondary material collecting surface portions of extended area facing toward the. respective shielded passageways, and a plurality of'horizontally spaced vertical slots establishing communication between said gas passage and the respective shielded passageways,

and said collecting electrode structure being provided with gas deflecting members disposed adjacent said slots in position to direct gas from said gas passage into said shielded passageways and with openings for outflow of gas from the respective shielded passageways at positions removed from said slots; and secondary precipitat- .ing electrode members within. the respective shielded passageways and spaced irom said secondary material collecting surface portions.

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