US2192250A - Electrical precipitation apparatus - Google Patents

Description

March 5, 1940.

AFiled Aug. 19. 1958 3 Sheets-Sheet 1 March 5, 1940. H. J. WHITE lELECTRICAL PRECIPITATION APPARATUS I Filed Aug. 19, 1938 3 Sheets-Sheet 2 AfA/em' JI WH/TE, Ma/XVM @M fw/@V 5 Sheets-.Sheet 3 m /fy/ m www H. J. WHITE Filed Aug. 19, 1938 ELECTRICAL PRECIPITATION APPARATUS m d Z W /0 .y j e w a i e ow. .1.. .,f W w v m I III I'IIOI 7l .IIS M 5 J W 6 f f L8 fb. u u J u lj 4. I 0 ..9 u. .IH .ml/ l-|| Il.. u 7N u u W 7 V1 3 m J m E March 5, 1940.

Patented Mar. 5, 1940 2,192,250 l ELEcTarcAL PRECIPITATION APPARATUS Harry J. `White, Los'Angeles, Calif., assignor to Research Corporation, New York, N. Y., a corporation oi' New York Application August 19, 1938,' Serial No. 225,809

UNITED STATES PATENT OFFICE Claims.

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 I, particles by passing them between two opposed become charged with the same electrical sign as l .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 the discharge electrode. 'Ihls is termed charg- 5 ing action. The charged particles migrate, under the influence of the electric ileld between electrodes, toward the other electrode which is a non-discharging electrode of extended surface. and collect or become precipitated upon the surface of the second electrode, which is consequently termed the collecting electrode.

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 conguration that establishes a suillcient- 1y high potential gradient at or near its surface to create corona discharge before there is a disruptive discharge or spark-over. For thispurpose the discharge electrode usually takes the form of a member of small surface area, such as a small diameter wire or a rod provided with sharp edges or points, whereby they may be created in the immediate vicinity .thereof a sufficiently high electric field intensity to cause ionization and corona.` discharge. v'Ihe term non-discharging electrode will be understood to designate an electrode I that minimizes or prevents corona discharge therefrom because it has a configuration that establishes a sufliciently low field concen tration at or near the surface to suppress corona discharge at elevated potentials lower than the voltage required' for disruptive discharge or sparkover. For this purpose. a.V non-discharging electrode usually is one ofextended'surtace area,

substantially free fromsharp corners or other parts of sharp surface curvature at all portions which are located within the electric eld, so as to substantially avoid ionization or 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 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 lgas 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 electricalfforces or by the physical properties of the particles themselves. When the ,collecting electrode is within the range of the charging, eld, as in the single field type of precipitator, the eld recharges and reprecipitates most of the particles that may be blown 01T the electrode, but even lthensome particles areblown along the electrode surface and ofi the outlet end of the electrode when the dust has such physical properties thatthe particles offer little resistance to gas erosion. There are some dusts that are particularly hard to collect for this reason, one typical example being fly ash which is composed of up to some velocity termed the critical velocity above which erosion loss increases rapidly with an increase in velocity and becomes quite appreciable. Itis 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 lfor which the critical velocity is relatively low, since the amount of these dusts carried into the outgoing gas stream isconsiderable and makes it diflicult to obtain high overall collection efliciencies at economic velocities. Obviously, the critical velocity for a given dust limits the capacity of a given treater if a minimum collection efilciency is to be obtained.

This censeuon amenity is inherent in the dust being collected and has the ee'ct of reducing the capacity of a u nit of a given size operating at a caused by erosion losses increases the size of the plant and equipment required to treat a given volume of gas without falling below a minimum efiiciency.

As might be expected, it has been frequently observed that the erosion losses are increased when the collecting' electrode surfaces become coated with 'deposited particles. Consequently, the percentage of loss is increased by relatively heavy dust loading of .the gas stream and -by the usual custom of shaking or rapping the collecting electrode at relatively long intervals. as these circumstances keep down the total amount of time when the electrode is clean and therefore the erosion loss is' the least. At present it is common practice to discontinue the flow through the preciptator section being cleaned forthe durati n of the rapping period, to permit the. deposi` removed from the collecting electrode to settle down into the hopper in a quiet zone free of the gas-stream. Obviously, rapping at short intervals with discontinuous flow is not resorted to becahse it reduces the operation time of that section and, proportionately, the eiective capacity of the treater. A

It is thus a general object of my invention to' provide an improved electrical precipitator in which losses from erosion by the gas stream are minimized or eliminated and the collection efilciency is improved, particularly in the case of dusts having certain physical characteristics making collection vdiiicult.

It is als'o an object of the invention to provide an electrical precipitator adapted to continuous operation without need for discontinuing gas ow during the periods of cleaning, thus improving efficiency and capacity and eliminating the necessity of providing gas by-pass flues and dampers or an additional precipitator. s

A further object isv to increase the capacity of,

s a precipitator of given size by decreasing erosion lossesk at any given gas velocity and so allowing. higher gas velocities, especially in the case of dusts having low critical velocities in treaters of conventional types.

Another object is to provide an electrical precipitator that is adapted to handle a gas stream with a relatively high dust loading without loss of collection enlciency from the accumulation of deposits on the collecting electrode.

'I'hese and other objects are lattained in a prev non-discharging electrode intermediate the discharge and collecting electrodes. diate electrode forms a part of a system of collecting electrodes, as some dust particles collect the space between the discharge and intermediate electrodes, in which space an ionizing field is maintained by suitable means. Another elec-v nleai nele of -a substantially nen-discharging Expressed dif- Ierently, the reduction in collection eciencyv 'of Fig. 5; and

character is preferably maintained between the intermediate and collecting electrodes to aid precipitation and to prevent redlspersion of collected material into the gas stream. especially during rapping Periods.

How the above objects and advantages of my invention, as well as others not specifically mentioned, are attained will be more evident from the following description ings, in which:

Fig. 5 is a vertical transverse section through a variational form of electrical precipitator constructed according to myinvention, using plate type electrodes;v l

- Fig. 6 is a vertical longitudinal section through the variational type of precipitator on line 6-6 Fig. '1.is a horizontal section on line 'I-'I of Fig. 6.

Referring now particularly toFlgs. l and 2, I

shall rst describe my invention Ias applied to a precipitator having four similar sets of electrodes which, with the exception of the discharge electrodes, are ofthe tubular type. 'I'he electrodes are enclosed within-cylindrical housing I0 which,

for convenience in manufacture and assembly, is

l preferably made in three. sections, the bottom,

intermediate and top sections being indicated at Illa, Illb, and I0c respectively. Bottom section Illa contains mechanism for collecting and removing continuously the accumulated dust taken from the gas stream, and this portion of the precipitator will be described in greater detail later. Inside this bottom section is an outer hopper formed by inclined plate members II and the housing sidewalls, the hopper being V- shaped in cross section, as shown in Fig. 2. Within and-slightly above the `outer hopper, is

' an inner hopper I2 generally oi' diamond-shape sloping top walls I6 completely cover over the topof the inner hopper. Walls Il, I5 and I6 ex- -tend horizontally until they intersect the vertical wens of housing-section Ina, as shownln Fig.' 1, sothat the housing walls form the end members of the hopper. As may be seen' best from Fig. 1, gas inlet opening II inthe' housing wall opens Adirectly into the' interior of hopper I2.

5 Within the upper portion of the housing is, header plate 2li-which mayconveniently be held between the upper and intermediate sections Ib and Inc ofthe housing and which forms in the upper section ofthe housing a gas chamber into f which the' gas from the :electrodes is discharged before leaving the treater through gas outlet 2l, which may be connected to any suitable gas" con- 1 duit. openings z2 1n the side and top 'fsf the housing are' normally covered by plates and provide means for obtaining Vaccess to the treater interior for assembly and adjustmentl of parts.

Supported on header plate 20 and located-,centrally of housing I0, is vertically extending inandthe annexed draw-. y

. 10 Fig. 1 is a vertical median section through an sulator 24 which carries at its upper end four equally spaced, radially extending arms 25. A similar insulator arrangement is placed at the lower end'of the housing and comprises insulator 21 which is supported centrally `of the housing on bracket 21a attached to the inside faces of hopper walls I4'. At the upper end of insulator 2l, there are four radially extending arms 28 which are vertically aligned with arms 25. Between ea'ch pair of vertically spaced arms 25 and 28, there is stretched a wire 29 of relatively small diameter which forms a discharge electrode. As may be seen in Fig. 3, 4there are thus four such discharge electrodes spaced equally about the center of housing Il).`

Surrounding each discharge electrode 29, is an intermediate electrode 30 in the shape vof a cylinder or tube open at both ends. Intermediate electrodes 30 .are made of screen, perforated metal, or other similar foraminous material that permits the passage of Asuspended particles from the gasstream through the electrodes to the collecting electrode, as will be more fully explained. Each electrode 30 ts snugly around an imperfor.

late metal nipple 3| at the upperA end of the electrode and against ring 32 on the nipple. Each l nipple 3| is flanged at its upper end, the flange 3|a resting upon the upper surface of header plate 2l) while the nipple itself extends through a suitable opening in the header plate. Nipples 3| are preferably spot-welded to header plate 20 tohold them securely in place. The intermediate electrodes are held at their lower ends by similar construction, each tubular screen itting snugly over an imperforate metal nipple-34 and resting upon a collar 35 attached tothe nipple. Nipples preferably have flanges at their lower ends by which they are welded or otherwise attached to top walls i6 of inner hopper i2, the nipples opening into the interior of the hopper. .The four collecting electrodes 38 are cylin- -drical `members of sheet metal, of somewhat greater diameter than the intermediate electrodes 30, supported at their upper ends by horizontal plate 39 which is suspended by means of insulators 40 from header plate 20. Collecting electrodes 38 are conveniently supported from plate 39 by forming flanges 38a. on the upper ends of the electrodes which are easily spot-welded to the plate.. This holds the electrodes against undesired movement. Collecting electrodes 38 are held against lateral movement at their lower ends by transverse brace 42 which extends across the interior of the housing, and to which all the collecting electrodes are fastened as by welding- The upturned ends of brace 42 are provided with adjusting screw and clamping nut assemblies 43 which extend three horizontally elongated slots therein and bear against insulators 44 on the housing wall, thus electrically insulating .the electrodes from the housing and at the same time permitting close lateral adjustment of the electrodes. I

'Ihe electrodes thus far described are' arranged in four independent groups operating in par-4 allel, each group comprising a tubular collecting electrode 38 and a tubular intermediate electrode 38 arranged concentrically of each other and coaxially of a discharge electrode 28. Pour groups are placed in a single housing to obtain larger capacity than can be had with a single group which is the basis u'nitof the treatcr; and any number of electrode groups may be uled according to the total las capacity dlired. As will become apparent, each group of electrodes may be considered as comprising a discharge electrode and a collecting electrode system that includes the outer impervious tube upon which the bulk of the dust is collected and the inner pervious tube upon which a portion of the dust is collected, but which is referred to as an intermediate electrode because of its position between the discharge electrode and the outer collecting electrode.

The rapping mechanism for cleaning the electrodes of. accumulated deposits of precipitated particles yincludes two separate hammer systems that may be independently operated, there being one for the collecting electrodes and one for the intermediate electrodes. The first of these hammer systems adapted to rap the collecting electrodes consists of a transverse'shaft 48 journaled at opposite ends in suitable bearings 58 mounted g on the housing walls. At the mid-point of the shaft, is fastened hammer 5| which extends radialb from the shaft a suillcient distance to strike mers; and by turning handle 55 in a directionto rotate shaft 48 counter-clockwise through a semi-circular arc, as viewed in Fig. 2, hammer 5| is brought into contact with the other anvil 52 and the second pair of electrodes may be rapped. In order to effectively insulate the collecting electrodes from the housing, shaft 481s made of electrically-non-conducting material such as wood that insulates metal hammer 5| from metal bearings `5|) and housing I0, althoughit'will be understood that it is not necessary that shaft 48 be one continuous insulating member, for it may also be a composite member having the necessary insulating sections. The same results may be secured by making the arms of hammer 5| of insulating material, or placing insulating sections elsewhere in the system.

A similar rapping mechanism is provided for jarring the intermediate electrodes to remove any accumulations on them. 'I'his second rapping mechanism comprises shaft 58 journaled in suitable bearings on the walls of housing section |8a and having a. handle 59 on one end externally of the housing for oscillation of the shaft. Hammer 58 is fastened to the shaft midway of its length at the proper' position to strike against the two U-shaped anvils Gl'which are supported one from each top plate I8 of inner hopper l2 adjacent a pair of nipples 34. I shaft 58 througha semi-circular arc by means of handle 55' causes hammer 60.150 strike against anvils 8| alternately. The impact against an anvil vibrates the top wall i8 and nipples 34 and transmits the vibration to electrodes 38 from which accumulated dust is shaken by the jarring action. It lwill be understood that. although both rapping mechanisms are here shown as being manually operated for. the sake of simplicity in the description and drawings, it is fully within the contemplation of the invention to mechanically operate both rapping mechanisms by any suitable type of mechanical drive. Particular- 60 Oscillation of ly is this so. because mechanical operation of the i ation may'be desired.

rapping mechanisms oers the advantage of automatic actuation of the hammers at short intervals, such as 30 or v60 seconds when the type of y dust being precipitated out of the gas stream is such that better precipitation is obtained by cleaning the electrodes at frequent intervals.. Obviously, the rapping mechanisms may be actuated at longer intervals when the character of the dust collected is such as to make this operation satisfactory. and inV this case manual oper- Dust shaken from the inside surfaces of electrodes 80 `and fallingdown inside the electrodes, passes through nipples 34 into hopper I2 and collects in a narrow trough 6l at the bottom of the hopper. 'I'he dust from the inside surfaces of collecting electrodes 38 and that dust from electrodes 30 which falls von the outside of them, drops out of the electrodes on to the inclined hopper walls I 6 and collects at the bottom of hopper II in a narrow trough 65. The dust in lthese two troughs is removed by a single continuous chain 68 which passes over idler sprocket 69 and driven sprocket 10, the latter being driven Aby motor 1I or any other suitable source of power. As viewed in Fig. 1, the upper run of chain 68 moves towards the right and carries with it the accumulated dust in trough 64, the dust being carried intothe housing around idler sprocket 69V until it builds up suiliciently that it is carried out of the housing by the lower run of chain 68. This dust, together with the dust accumulated in trough 65 is then carried tothe left,'as viewed in Fig. l, by the bottom run of chain 68 and discharged from theV precipitator housing through discharge opening 12, from which the dust fallsinto any suitable collecting means, such as a bin or other conveyor. There is preferably suillcient slack in chain 68 that it scrapes or drags along the bottom surface of both troughs 64 and 65, the troughs being but little wider than the chain itself, -so that the links of the chain keep the troughs scraped clean. The

movement of chain 68 is preferably continuous, and its speed should be determined by the rate at which dust accumulates in hoppers I I and I2.

The electrical conductor 15 connecting the discharge electrodes 29 to a source of high tension continuous or unidirectional current, indicated generally at A, is brought into the precipitator housing through insulator 16. A second conductor 11 is brought' into the precipitator through insulator 18 and is used to connect the collecting electrodes 38 with a suitable sourceof high tension unidirectional or continuous current, indicated 'generally at B.- Ihe precise nature of theelec- Y trical means used to supply these high tension currents is not essential to the present invention, and it is not shown because such means are well known in the art and may take different forms depending upon various engineering consideraof corona discharge before a disruptiveor spark 'alegando discharge takes place between the electrodes '28 and Il. -The charging or ionizing field main? tained between electrodes 2l and 80 is of a character to charge electrically any particles carried through the field bythe ga's stream, the'charge taken on by the particles being of the same sign or polarity as the discharge electrode.

The electrical apparatus also maintains an added non-discharging field between'the interemediate and collecting electrodes, by maintaining a potential difference between electrodes 86 and 28. Since the intermediate electrodes are grounded, the collecting electrodes 38 are maintained at a polarity opposite to that of the disthe discharge and intermediate electrodes. As

typical Vofvoltages which may be applied to the o electrodes, but without any limitation thereto, v

because the actual' voltages used in any given instance will be dictated by sound engineering design, it has been found in practice advantageous with a collecting electrode diameter of 8" and an intermediate electrode diameter .of 6, to apply a potential of from 35,000 to 50,000 volts to the discharge electrode and apotential of opposite polarity of from 12,000 to 18,000 volts to the collecting electrode. In accord with standard practice, itis preferred that the discharge electrodebe negative and that the collecting electrode be' positive.- Thus, the conductor 15 is shown connected conductor 11 connected to the' positive terminal of source B, both'of said sources having their other terminals grounded as indicated at 8 0.

During operation of the precipitator, a stream of gas carrying suspended particles to be separated or precipitated from the stream enters thel treaterthrough inlet I1 from a suitable gas conduit, not shown, which Acanbe attached to the housing by means of the flange around the opening I1. Gas entering inlet I1 is introduced directly into inner hopper I2 which acts as an inlet header to distribute the gas to the four nipples 34 through which the gas ilows into the ionizingl ields. As the gas flows upwardly 'in a direction generally parallel to the electrode surfaces, the suspended particles are -removed by' the electrical precipitating action previously described, the clean gas then leaving the electrodes through nipples 3l and entering the space at the top of the 'treatein e The clean gas then leaves the treater through outlet opening 2| and may be wasted to the atmosphere, or the gas may be conducted away for further use by means of a suitable gas conduit attached to the flange around opening 2 I It will be noticed that, as the gas leaves hopper I2, each nipple 34 directs an individual gas stream upwardly through one set of vertically extending electrodes. thestream being more orvv less confined throughout its entire vertical travel by the electrode130. Becauseintermediate electrode 38 ts snugly around nipple 34 at the bottom end of the electrode, v-all of the gas is ini tially introduced into the ionizing -ileld from y 25. previously chargedin the ionizing eld b etween to the negative terminal of source A, and thev 2,199,250v the nipple 34. Likewise, none of the gas can 1 30 to conilne the gas stream to the space inside the electrode, causes substantially all the gas to flow in one main stream directly through the ionizing iield, though a minor portion of it passes outwardly 'through the foraminous electrode 30 into .the annular space between the intermediate l and 'collecting electrodes, circulating in and out through the intermediate electrode as a result of electric wind eiects. This advantageous con- 'struction' is permitted because the intermediate foraminous electrode is grounded. As pointed out above, the housing is grounded, and grounding the intermediate electrode eliminates 4the necessity of electrically-insulating these two members. Consequently, it is possible to connect some gas impervious directing member, such as Q hopper i2 and nipples 3l, or header plate 20,

to the housing walls, and at the same time have that member engage directlythe intermediate electrodes at their ends where gas enters or leaves theionizing field. The advantage is that all the incoming gas is introduced directly into the ionizing fields, andfnone can ilrst enter di rectly the space behind the tubular electrodes where the gas stream would not be exposed to the ionizing iield.

Although the annular space between the inter,- Imediate and collecting electrodes is open at both the top and bottom of the collecting electrode 38, there is little or no tendency to create an eddy current of gas leaving the collecting electrodes and ilowing into the housing. Consequently, the annular space surrounding each intermediate electrode constitutes a relatively qui'- escent zone in which there is little or no gas movement.

The, ionizing iield radiating outwardly from a discharge electrode terminates in part' where it encounters the metal wires or portions of the opposed intermediate electrode 30; but where the ionizing field encounters an opening in the intermediate electrode, the iield projects or continues on through the opening and terminates on collecting electrode 33. The separate eld inde- Dendently maintained .between the intermediate and collecting electrodes is effective in the annular space between these two electrodes, and is superimposed on the parts of the ionizing ileld extending beyond the intermediate electrode. As a result of the fact that the total area .of the Openings in electrode 4il!) is normally considerably-in excess of the area of 'the metal portions of` the electrode, and as a result of the field distribution mentioned, the bulk of the charged particles are carried on throughy the openings in the intermediate electrode under the action of the electric fields and are deposited upon the inside surface of collecting electrode 38. A portion of the particles strike the intermediate electrode and become neutralized, thus kbecoming precipitated upon the intermediate electrode. A part of the particles so precipitated continue to adhere to this electrode, while a part of them eventually are carried on through the openings in the electrode and are precipitated upon the collecting electrode. It is because of the Iact that both the cylindrical electrodes oppose the -rate of accumulation of dust deposits.

tated upon their surfaces that the two .of them may be considered as constituting a collecting electrode system which i's made up of two separate electrodes that are electrically distinct and. insulated from one another, but both of which .operate to facilitate precipitation of the charged particles.`

The two rapping mechanisms for cleaning the intermediate and collecting electrodes may be operated either mechanically or manually at such intervals as may be necessary according to the It is a particular advantage of this form of treater that 'the rapping may be carried out atvery short intervals, of the order of 30 to 60 secondsif desired, to give the eiect of a continuous shaking or cleaning action onthe electrodes, because the gas ilow through any one of the electrodes. need not be discontinued during the cleaning operation. As mentioned, above, the largest part of the 'precipitated particles collect eventually upon the inside surface i' a collecting electrode 38, and when they are jarred oi the surface they fall downwardly within the annular space be' tween the collecting and intermediate electrodes which is a quiet zoneA relatively free from gas ow. Consequently, the dust deposits from the collecting electrodes 38 and 'from the outer surfaces of the intermediate electrodes 30 fall down-A wardly out of the electrode system without being picked up by the upwardly moving gas stream, and are collected in the outer hopper I l which is entirely separated from the incoming gas. The dust deposits shaken loose from the inner sur- Afaces of the intermediate electrodes tend to fall downwardly through nipples 34. However, this dust is brought into contact with the upwardly rising gas stream. The larger agglomerates of precipitated particles settle down within hopper l2, but a few of the smaller agglomerates or individual particles are again picked u'p and carried suspended in the gas stream.. These resuspended particles are again subjected to the charging and precipitating action of the electric elds Within the electrodes and vwill eventually either be precipitated on the collecting electrode surfaces or will accumulate ln large agglomerates which will fall down in hopper l2 to the trough 64 at the bottom of the hopper. Because of this improved operation of the precipitator constructed as described, the electrodes may be rapped at such-frequent intervals as to be kept in a substantially clean condition at all times efficiency oi' the precipitator with respect to fly ash, which is a dust that is extremely diilcult to collect because of the spherical shape and non-adherent character of its particles, it was found that the loss of dust from the outlet end of a single tubular collecting electrode without any intermediate electrode 30, amountedto 10% of the dust Vintroduced at the inlet end oiE the electrode at a volume o f 80 cubic ieet per minute of gas. Insertion of a foraminous intermediate electrode between the collecting and discharge electrodes decreased the dust loss from the precipitator to 1% at the same gas volume. That is.

fss

I at the same capacity the quantity of dust carried in the outlet gas stream was reduced to one-tenth its former value. It was also found that the capacity of the same precipitating unit could be increased to 190 cubic feet per minute without exceeding the former loss of 10%, that is, with .a collection eiliciency of 90%. A'Ihis last experb ment was carried out with both the 'collecting and intermediate electrodes grounded so that no precipitating'iield was maintained independently between'them. y

The effectivenesse'of the non-discharging eld is emphatically demonstrated by a later set of tests using a slightly higher potential at the discharge electrode, in which the capacity at 90% eiiiciency was determined tol be 215 cubic feet per minute with both -the intermediate and colcollecting electrodes grounded.` When a high potential was applied to the collecting electrode. it was found that the capacity of the treater could be increased to 465l cubic feet per minutewithoutV dropping the collection eflicien'cy below 90%, and that the eiliciency was increased to 98% at the initial ow of 215 cubic feet per minute. All these tests were -run on fly ash becausethat material is'hard to collect. Other tests show that the increase in capacity or efllciency depends o`n the kind of dust treated, beinggreatest for those materials that are hard to collect in a conventional precipitator of the single nem type.'

Experiments on various other types of dust uniformly show a marked increase in capacity or improvedgcollection eiliciency lor both as a result of having a foraminousI intermediate elec- 'trode between the discharge and collecting electrodes to form radjacent the collecting electrode a quiescent zone in which the dust is collected or Aprecipitated onl the collecting electrode and which is suiliciently free from disturbance by the gas stream that there is little or no tendency for .o the dust to be redistributed into the gas stream even when the electrodes are .being shaken for cleaning. 'Ihe tests also showuniformly a still greater increase i'n lcapacity at v the same collecting eiliciency, on a .greater increase inoelciency at the same capacity, as a result ol.' maintaining an independent substantially .non-discharging 'eld betweenthe intermediate and collecting electrodes which facilitates pre-l cipitatton of the particles on the collecting electrodes-and amplies the effectiveness of the collecting electrode system comprising two'separate electrodes.

Flgs. 5, 6 and 7 illustrate a variational form of precipitator which is fundamentally the same as the .precipitator valready described, except that not shown in the.' drawings as. devices are' the intermediate and collecting. electrodes instead of being' tubular in shape are now platelike in shape'and other minor changes in thehoppers in which the dust falls after being shaken from the electrodes.; The dust may be removed v from hopper bottombf housing 85. by vany suitable type of ,conveyor or -valve mechanism.; whichl ls,

welllmow in' the art.

'.'rne typical prooipitacor' illustrated fmthe drawings is composed oi' two'groupsof electrodes,

' fine the entering'gas stream to the .space be- `vertical portions of margins 95 of electrodes 94 guasto each group comprising discharge'. intermediate,

' and collecting electrodes and each group being a substantial duplicate of the'other group. The discharge electrodes 98 are a-series "offwlres of relatively small diameter arranged in a'overtical 6 and to maintain the discharge electrodes of the two groups in parallel spaced vertical planes. This supporting framework is mounted upon insulators 93 carried `on brackets attached tothe walls of housing 85 in order to electrically insulate the discharge electrodes and their support ing structure from the housing walls', which are grounded in accordance with commonv practice. The discharge electrodes may, of course, be held in place by any other' suitable structure electrically insulated from Vthe housing and the other electrodes. I i

On each side of ra seriesof discharge electrodes and parallel to the plane thereof, is a foraminou`s intermediate electrode. which is in the form of a sheet or plate. Intermediate electrodes 94 mayconveniently be suspended from the top wall of housingBS and are connected directly thereto since it is -preferred'th'at these electrodes be grounded .through their eieotriai connection to the housing. -The intermediate electrodes may be of any foraminousconstruction, such as is afforded by the use offwire screen or perforated metal sheets; and arev preferably bounded on their vertical and bottom sides by an imperforate border 95 which strengthens the electrodes against flexing and helps direct and contween the 4discharge-andinternjlediate electrodes.A For'thevlatter reason, borders 95 are gas impervious and at thevertical edges of the intermediate electrodes 94 are shaped toprevent anyl direct ilow of the incoming' gas stream into thev space behindvthe electrodes, thus the only gas behind the electrodes lisv that which vpasses through the perforate portions. o1 electrodes. after having once entered the ionized eld. The

adjacent the side walls ofthe housing arebent` through an angle'of 90 and joined to the verti- 'cal side walls in a suitable gas tight manner.

In the central portion of-the precipitatorl the vertical portions of margins 95 of two electrodes 9i are formed in a U-shape, thus connecting together two intermediate electrodes into the forni of a hollow envelope. `This construction is shown best in Fig. 7. Borders 95 extend upwardly and,-

are connected to the roof of the housing in order the electrode, but maybe left open at the bottom in'A order to facilitate removal of dust from the electrodes. since gas, entry at this point is negligible 1in any event. 'As a consequence of this construction,- the'main Astream Voi' gas flows parallel to' theelectrode surfaces and through the space between the discharge ,and 'intermediatev i electrodes where the ionizing eld is maintained.

bomomeaouorfurtneroonnnmnnem'doww n "4' to prevent gas from leaking'around the edge of alternately against the lower edges of the two the horisontal passages between electrodes 94. A

sidiary baffles 98 of limited vertical extent mayY be placed advantageously between the two intermediate electrodes on opposite sides of a series of discharge electrodes, as shown inFig. 5. The top edges of these subsidiary baffles 88 are placed at the same level as the top edgesof baflles 91,

thatis, lnot within arcing distance of electrode framemembers 89, but the subsidiary bailles extend downwardly only to the bottom of marginal portions 95 of the intermedite electrodes. Main baffles 91 greatly impede any gas flow directly into the lower portion of the housing from the regions at the ends of the electrodes, especially from inlet 86, while the subsidiary baffles act as deflectors that substantially eliminate anyv tendency for the gas to flow downwardly into the hopper bottom of the housing from the spaces between the electrodes. In this manner a quiescent zone is assured not only at the sides of the intermediate electrodes removed 'from the discharge electrodes, but also in the lower portion ofthe housing into which the dust falls when it is jarred from the electrodes.

The collecting electrodes are suspended from thetop wall of housing 85 by insulators |0| and 'are yheld against lateral movement at their lower ends by insulators |02 which are attached to'bafiles .91. In this way these electrodes |00 are electrically insulated from the frame and from the discharge electrodes in order that a separate potential may be applied to the collecting electrodes. The collecting electrodes are in the' form of imperforate plates extending parallel to the intermediate electrodes and the planes of the discharge electrodes, there being a collecting electrode at each side of the precipitator lbetween intermediate electrode 94 and the side wall of the housing, while `at the center a single collecting electrode |00 may be used for both intermediate electrodes, since both faces of the collecting electrode are available for collecting surfaces. A

Thus, as in the form o'i the invention previously described, a foraminous non-dischargingelectrode is provided intermediate each discharge electrode and an opposing collection electrode' trodes which it raps, and the hammers are cf'suf" flcient length that oscillation of shaft |05 through a semi-circular arc causes the hammers to strike intermedite electrodes, as shown in Fig. 5. Dust jarred loose from these electrodes falls down into the hopper bottom of housing 85.

Similar rapping mechanism is provided for the collecting electrodes and comprises a shaft ||0 llocated midway between each pair of collecting electrodes |00 and journaled in bearings mounted on thehouslng walls and bailles 91. A suitable number'of. hammers ||2 are secured to shaft ||0 and the hammers upon oscillation of shaft ||0 through a semi-circular arc, strike against the lower edges of the collecting electrodes. One shaft ||0 and its attached hammers,

is used to clean two collecting electrodes, as the hammers strike alternately against the electrodes; and where an odd number of collecting electrodes is used as shown in Fig. 5, the hammers are staggered alongv the length of the two shafts ||0 in order that the hammers will not interfere with each other if they swing simultaneously towards the same electrode. Shaft l0 is' either made of non-conductingmaterial. or

rapping mechanism during the cleaning opera-A tion. Although no operating mechanism is shown, it will be understood that the several rapping mechanisms may be actuated by either mechanical or manual means attached to shafts |05 and ||0 and adapted to oscillate these shafts at suitable intervals of time.

Suitable electrical apparatus and connections, not shown, are employed to apply a high potential current to the electrodes and maintain between the discharge and intermediate electrodes an ionizing field as a result of the corona discharge around electrodes 88. A separate and independent electric fleld is preferably maintained between the intermediate and collecting electrodes, but this field is of a substantially nondischarging character. It is preferred that the intermediate electrode be grounded and that the discharge and collecting electrodes be maintained at opposite potentials or polarities in order to maintain both the fields in the direction that forces charged particles toward the intermediate and collecting electrodes.

In general, the operation of the last described form of my invention is the same as described in connection with Figs. l to 4; and the collecting electrode .systems comprising a foraminous intermediate electrode and a plate collecting electiode oier the same advantages of increased capacity and collecting efficiency as compared with a single plate collecting electrode, that `are found in the use of electrodes of tubular shape. Since in this form of precipitator the gas stream is directed horizontally through the treater, the lower portion of the housing is free from disturbance caused by the incoming vgas stream flowing through it as through hopper |2, and the collection of dust falling off the electrodes is somewhat simplified, only a single hopper being necessary in this case.

In the first form of the invention described f that the collecting electrodes be imperforate; but in the second form described, it may be desired -above utilizing tubular electrodes, it is preferred v under some circumstances to replace the plate type collecting electrodes with perforate or gas pervious electrodes, as for example, with the rod curtain" type of collecting electrode which is 45 intermediate electrodes ina ymore easily by rapping than is the yplate type A A cles to be removed, the combination of a discharge electrode; a substantially non-discharging collecting electrode; va foraminous substantially non-discharging electrode intermediate the discharge and collecting. electrodes; means for 2. maintaining a nigh potential ionizing meld between the discharge and intermediate electrodes; means for independently maintaining a non-dis-l charging precipitating ileld between the intermediate and collecting electrodes; and means for w' directing the main stream of gas into,'throu'gh,

and out of the space between the discharge and intermediate electrodes in a direction sub stantially parallel to the electrode surfaces.

ilows a stream oi' gas carrying suspendedV particles to be-removed, the combinationof a discharge electrode: a substantially non-discharg"- ing collecting electrode; a foraminous substantially non-discharging grounded electrode intermediate the discharge and collecting electrodes: means for maintaining a, high potential ionizing field between the discharge and intermediate electrodes; means for independently maintaining a non-discharging precipitating eld between v the intermediate and collecting electrodes with the collecting electrode of va polarity opposite to that of the discharge electrode; and means for directing the main stream of gas into.- through. and out of the space between the discharge and direction substantially parallel .to the electrode surfaces. l

3. In an electrical precipitator lthrough which iiows a stream of gas carrying suspended parti-` -cles to be removed, the combination oi' a discharge electrode: a. substantially non-,dischargfing collecting electrode; a foraminous substantially non-discharging electrode intermediate the discharge andl collecting electrodes; means for 5'5 independently maintaining ahigh potential ionizing iield between the discharge vand intermedi-- ate* electrodes: means for maintaining a separate non-discharging precipitating field the intermediate and collecting electrodes, said precipitating 'iield being maintained in a ldirectlonto .assist movement of charged particles toward the. collecting electrode to collect thereon:

and means for directing the main stream of gas f inta-through, and out of the space between the 65 discharge and intermediate electrodes in a.direc tionv substantially parallel to the electrode sur-j i faces.-

4. man electric nreeipiiair through which nows a stream of gascarrying suspended particles to be removed, the combination of a dischargejelectrode; a-'collecting electrode system comprising apair of substantially non-discharging electrodes of which one is a foraminous electrode positioned-between the discharge electrode 2. In an electrical precipitator through which and the other non-discharging mi for independently maintaining a high potential I `ionizing field between the discharge electrode and the collecting electrode system; means for maintaining a separate non-discharging precipitating ileld between the pair of non-discharging 5' -quiescent zone between the electrodes of the collecting electrode system. f

5. In an electric precipitator through which flows a stream of gas carrying suspended particles to be removed,` the combination of a dis- 15 charge electrode; a collecting electrodesystem comprising a pair of substantially non-discharg- I I ing electrodes of which one is a foraminous grounded electrode `positioned between the discharge electrode and the other non-discharging electrode; means for maintaining a high potential ionizing field between the discharge electrode j and the collecting electrode system; means for maintaining a separate non-discharging precipitating held between the pair of non-discharging electrodes with the polarity of said other nondischarging electrode opposite to that of the discharge electrode; and means directing substantially all the gas iiowv into, through', and out of the space between the discharge' andforaminous velectrodes in a direction substantially parallel to the electrode surfaces, and taining a relatively quiescent zone between the electrodes ofA the collecting electrode system: 35

6. In an electric precipitator through which flows a stream of gas carrying suspended particles to be removed, the combination of a discharge electrode; a collecting electrode system comprising a pair of substantially non-discharging ,electrodes of which one is a foraminous 49 grounded electrode positionedl between the discharge electrode and the other non-discharging electrode; means for maintaining a high potential ionizing neld between the discharge electrode" and the collecting electrode system; means for 45 maintaining a separate non-discharging precipitating neld between the pair of non-discharging electrodes: and means directing substantially all the gas ilow into, through, and out of the space -between the discharge and foraminous electrodes 50 in a ldirection substantially parallel to the elec` trode surfaces and maintaining a relatively quiescent zone between electrodes of the collecting '1. In an electric precipitator through which 55 flows asti-'cam of gas carryingl suspended parti. cles to beremoved, the combination of a discharge electrode: a collecting electrode system surrounding the discharge electrode and comprising a pair of substantially non-discharging ,v

cylindrical. c'o-aidal electrodes 'of which one-is an imperforate electrode, and the other is a foraminous grounded electrode between the discharge electrode and the imperforate electrode; -meens fer mniniaininga high potential vionizing 6 neld between the discharge electrode and the collecting electrode system; means for maintaining aseparate non-discharging precipitating eld, bev tween the foraminousv and imperforate electrodes in a direction to assist movement of charged par- 'I ticles toward the imperforate electrode;` and means directing substantially all the gasy flow into, through. and `out of the space between the dischargefand'foraminous electrodes in a direc- .electrode surf' 75r f aces and maintaining a relatively quiescent zone l between the electrodes of the collecting electrode system.

8. In an electric precipitator through which ows a stream of gas carrying suspended particles to be removed, the combination of a discharge electrode; a collecting electrode system comprising a pair of substantially non-discharging electrodes of which one is a. ioraminous electrode positioned between the discharge electrode and the second non-discharging electrode; means for maintaining a high potential ionizing iield between the discharge electrode and the collecting electrode system; means for maintaining a separate non-discharging precipitating iield between the pair of vnon-discharging electrodes; means directing substantially all the gas`ow throughthe space between the discharge and foraminous electrodes and maintaining a relatively quiescent zone between the electrodes oi the collecting ,electrode system; rapping mechanism for jarring deposited dust particles from said second non-discharging electrode; a hopper to catch dust. jarred from said second non-discharging electrode; a second rapping mechanism for jarring deposited 'dust particles from the foraminous electrode; a second hopper to catch dust jarred from the foraminous electrode; and conveying means for removing dust from both hoppers.

9. In an electric precipitator through which flows a stream of ga's carrying suspended particles to be removed, the combination of a discharge electrode; a collecting electrode system comprising a pair oi' substantially non-discharging cylindrical, co-axial electrodes of which onev is an imperforate electrode, and the other is a foraminous grounded electrode positioned bef tween the discharge electrode and the imperiorate electrode; means for maintaining a high potential ionizing field between the discharge electrode and the collecting electrode system; means-for maintaining a separate'non-discharging precipitating iield between the foraminous and imperforate electrodes in a direction to assist movement of charged particles toward the imperforate electrode; means directing substantially all the gas flow through the space between the discharge and ioraminous electrodes and maintaining a relatively quiescent zone between the electrodes of the collecting electrode system: rapping mechanism for jarring deposited dust particles from the imperforate electrode; a hopper to catch dust jarred from the imperforate electrode; a second rapping mechanism for jar ring deposited dust particles from the foraminous electrode; a second hopper to catch dust jarred from the foraminous electrode; and conveying means for removing dust fromrboth hoppers.

10. In an electric precipitator through which I ows a 4stream of gas carrying suspended particles to be removed, the'combinationof a discharge electrode; a collecting electrode system comprising a pair of substantially non-discharging electrodes of which one is an imperforate electrode, and the other is a foraminous electrode between the discharge electrode and the' imperforate electrode; means formaintaining a high potential ionizing iield between the dischargel electrode and the collecting electrode system; means for maintaining a separate non-dis,-

charging precipitating eld 'between the foraminous and imperforate electrodes; means directing substantially. all the gas flow through the space between the discharge and foraminous electrodesand maintaining a relatively quiescent zone between the electrodes of the collecting elec'- trode system; rapping mechanism for :larring deposited dust particles from the imperforate elecl trode; 'an outer housing enclosing all said electrodes and having a hopper bottom adapted to catch dust jarred from the imperforate electrode: a second rapping mechanism for jarring deposited dust particles from the foraminous electrode; an inner housing forming a combined gas inlet header and hopper adapted to catch dust jarred from the foraminous electrode; .and conveying means for removing dust from both hoppers.

1l. In an electric precipitator through which flows a stream of gas carrying suspended particles to be removed, the combination of a discharge electrode; a collecting electrode system comprising a pair of substantially non-discharging electrodes of which one is a. foraminous electrode positioned between the discharge electrode and the other non-discharging electrode; rapping mechanism for jarring deposited dust` particles. from the collecting electrode system; an outer housing enclosing al1 said electrodes and having a hopper bottom adapted to catch'dust jarred from said other non-discharging electrode; an inner housing forming a combined gas inlet 'header and hopper adapted to catch dust jarred from the foraminous electrode; and conveying means for removing dust i'rom both hoppers.

12. In an electric precipitator through which ows a stream of gas carrying suspended particles to be removed, the combination of a diseA charge electrode; a collecting electrode system comprising a pair of substantially non-discharging electrodes of which one is a foraminous electrode positioned between the discharge electrode and the other non-discharging electrode; rapping mechanism for jarring deposited dust particles from the collecting electrode system; an outer housingenclosing all said electrodes and having a hopper bottom'adapted YKto catch dust jarred from said other non-discharging electrode; an inner housing forming a combined gaslnlet header and hopper adapted to catch dust jarred from the foraminous electrode; and a continuous `chain conveyor having" an upper run moving dust from the inner hopper into the outer hopper, and a lower run removing dust from the outer hopper.

13. In an electrical precipitator through which flows a stream of gas carrying suspended particles to be removed, the combination of a grounded housing; a discharge electrode; a substantially non-discharging collecting electrode; a foraminous substantially non-discharging electrode intermediate the discharge and collecting electrodes; means for maintaining a high potential ionizing field between the discharge and intermediate electrodes; means for independently maintaining a separate non-discharging precipitating field between the intermediate and co1- lecting electrodes; and gas impervious members connected to the housing and engaging the ends of the intermediate electrode where the gas enters and leaves the ionizing'eld to direct the main stream of gas into, through, and out of 'the' space between the discharge and intermediate electrodes in a direction substantially par-7 allel to the electrode surfaces.

14. In an electric precipitator through which" ilows a stream'of gas carrying suspended particles to be removed, the combination of a dis.

drical, substantially non-discharging electrodes, lof which one is an imperforate electrode and the other is a foraminous electrode positioned between th discharge electrode-and the imperforate electrode; an outer housing enclosing all said electrodes and having a hopper bottom adapted to catch dust falling from said imperforate electrode; an inner housing connected to the outer housing and forming a. combined gas inlet header and hopper adapted to catch dust falling inside said foraminous electrode; and a nipple connected to the hopper and engaging charge electrode; a collecting electrode system comprising a pair of substantially non-discharging electrodes-of which one is a foraminous electrode positioned between the discharge electrode and the other nonfdischarging electrode; an out# er housing enclosing all said electrodes .and having a hopper bottom adapted to catch dust falll ing from said other non-discharging electrode; an inner housing connected to Vthe outer hous-U ing andforming a combined gas inlet header and hopper adapted to catch dust falling inside said foraminous electrode; and a nipple connected to the'hopper and engaging the lower end of the foraminous electrode to direct the stream of gas upwardly into the, space inside the foraminous electrode.,

. I HARRY J. WHITE.

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