US2283964A - Electrical dust precipitator - Google Patents

Description

y 1942- P. H. WYCKOFF ,2 3,9

ELECTRICAL DUST-PRECIPITATOR Filed Feb. 29, 1940 2 Sheets-Sheet 2 WITNESSES: INVENTOR gg Pew/H Wycf/(off I BY ATTORNEY y 26, 1942- P. H. wYcKoFF 2,283,964

ELECTRICAL DUST-PRECIPITATOR Filed Feb. 29, 1940 2 Sheets-Sheet l WlTNES SE 1 I INVENTOR W, Peter H. Wyc/mfi g K f 246W ATTORN EY Patented May 26, 1942 ELECTRICAL DUSTPRE('JIIPITATOR,

Peter H. Wyckofl', Avalon, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 29, 1940, Serial No. 321,464

4 Claims.

My invention relates generally to electrical precipitators for removing dust from flowing gases, the dust being suspended or contained in the gases, and more particularly relates to the type of precipitator which is provided with an ionizing zone for electrically charging the gas-borne dust, and a separate precipitating zone for subsequently precipitating the charged dust.

It is a broad object of my invention to provide an electrode for an electrical precipitator, which is substantially self-cleaning.

It is another object of my invention to provide an electrode for a precipitator employed for cleaning gases having high dust concentrations, such as are encountered in some industries. An

' example of such an application may be found in boiler rooms, especially the more recently-designed stations employing powdered coal as a fuel. With modern methods and devices for rap id burning, the flue gases may contain as much as 1.4 grains of dust per cubic foot, and even more, and in larger installations the flue gases may carry as much as 2 tons of dust, or more,'per hour, the dust being a fine powdery ash comprising particles ranging in size from a fraction of 2. micron to about 200 to 300 microns, and varying in electricity-conducting properties from highlyconductive carbon particles to practically insulating solid oxides or other forms of ash.

. The usual precipitator of the multi-zone or multiestage type comprises at least two zones through which the dust-laden gases flow, in the first of which the dust is charged or ionized, and in the-second of which the charged dust is precipitated. However, in the general form of precipitators of this type there may be some deposit of dust particles on the elements of the first ionizing zone which either collect on these elements, or, if they be conducting, lose their charge after contacting the elements, and then flow uncharged through the precipitating zone and, consequently, are not deposited or collected but flow out with the treated gases. In either case, the result is a lowering. oi the overall efiiciency of the unit. Where a large volume of dust must be continuously removed, these disadvantages are considerably aggravated and require the deenergization of the elements so that they may be cleaned by jarring or by some other means.

Moreover, during such cleaning it may be necessary to divert the gas flow so that smaller particles or agglomerated particles which burst from the falling dirt, will not be blown out of the precipitator.

My invention is particularly directed to providing a form of electrode for the ionizing zone of a multi-stage precipitator which will be substantially self-cleaning so that it will be continuously 'efilcient and relatively clean during continued use.

It is a further object of my invention to provide a multi-stage electrical precipitator for cleaning gases having high dust concentrations in which the dust contains a good proportion of carbon particles or other particles having fairly good electricity-conducting properties, that is, particles of relatively-low resistivity or relativelyhigh conductivity.

It is a further object of my invention to provide an electrical precipitator for cleaning gases in which the amount or dust to be removed may rise to considerable proportions; a more particular object being to provide an electrode in the ionizing zone of such a precipitator, which electrode will remain eiiicient with continued use, and will automatically discharge any dust deposited thereon into a convenient receptacle.

The ionizing zone, for the type of precipitator to which my invention especially relates, comprises electrodes defining an ionized electrostatic field, with one side or end of the field being relatively highly concentrated and the other,

relatively distributed. The electrode, or electrodes, at which the field is concentrated are the means for creating the ions which flow toward the other electrode, or electrodes, which bound the less concentrated field. In practice, the former may assume the shape of a wire or other equivalent elements having a relativelysmall cross-sectional area, as compared to the latter, which are spaced from the wire, and which have a relatively-large surface area. These characteristics and functions may conveniently serve to identify the various electrodes. For example, the smaller electrodes may be designated as the high-gradient electrodes, while the others may be called the low-gradient electrodes. Or, if the basis for identification be the functions of the electrodes, then the smaller electrodes may be called the ionizing-wires or ionizing-electrodes while'the larger electrodes can be called the collector-electrodes, or receivingelectrodes. Other common terms for the respective electrodes, which are frequently employed in the prior art, are ionizing-wire for the smaller electrode, and ground electrode" for the larger. In the specific embodiments to be described, I prefer to employ the terms ionizing-electrode for the smaller electrode, and "refled, perhaps, by adjectives descriptive of the O mD S 8 plurality of no -d elephysical attributes of the diverse electrodes. tively-insulated precipitator- plates 20 and 24, al-

Other' objects and features of my invention temately spaced, and having a unidirectional will be gathered from the following description voltage between them yielding a unidirectional thereof, which is to be taken in conjunction with gradien of approximately 13,000 volts per i the accompanying drawings, in whch: of spacing, with the series of plates 20 at one Figure 1 is a horizontal sectional view, sympolarity a d t eries of plates 4 at the other bolically' drawn, of a precipitator built in acp01ari y, Si ycordance with my invention, i In order to determine the relationship of my Fig. 2 is a perspective view of one of the receivreceiving-electrodes in a precipitator of this kind ing electrodes of Fig. 1, compared to continuous, planar receiving-elec- Fig. 3 is a fragmentary perspective view showtrodes of the ,same dimensions, I carried out a ing a modified form of an ionizing electrode. series of tests with both types of electrodes, the

Fig. 4 is a fragmentary sectional view of 9. results being shown in Fig. 5. In this figure the further modification of an electrode adapted to efli y 0f the units were determined in accordbe used in an ionizing zone comprising more than ance with the so-oalled blackness test" evolved one ionizer unit; and, by the United States Bureau of Standards, and

Fig. 5 is a graph ofcleaning efliciency against the solid line, represents the efilciency of a unit gas velocity for a precipitator employing, planar constructed with the planar electrodes, that is. electrodes in the ionizing zone, and for a precipielectrodes in which the front side was made a tator employing electrodes in accordance with solid metal rather than spaced wires as shown my invention. 7 in Fig. 2. The short-dash line indicates the drop A precipitator built along the lines of my inin efiiciency of such a unit as the solid electrodes vention comprises an ionizing zone 2 and a precoat over with a layerof dirt and, as may be excipitating zone 4 successively in the direction of Dected, the efllciency rops at commerciallythe gas-flow which is indicated by the arrows in usable gas velocities. However, the l n sh l e Fig. 1. shows the efllcie'ncy of the unit with open-mesh The ionizing zone comprises one or more ionizer G yp i -elec odes bu in accordunits, each of which includes anionizing-eleeance. with my invention, with the same total trode 6, which in this instance is shown in the amount of 8 d dust P g throu h t form of a round wire, and two receiving-elecunit as was the case in determini t s orttrodes B disposed on each side of the ionizingdash-line curve. The dustusedinobtainin! these electrode 6, and relatively insulated therefrom so Curves a y 8511 Such 88 passes with fl 88885 that the terminals of a source of energy may be, resulting from the burning of powdered coal. It in effect, connected, on the one hand, to the sat once appa n that w the use of y elecionizing-electrode, and on the other hand to the tlflde t eflieiency of the unit is considerably electrodes 3, thereby creating an electrostatic incr as d. sp ia ly at th hi h r rat s of gas field between the ionizing-electrode and the reoc y. us. for an efilciency of about 85%. calving-electrodes. Preferably, for precipitators it unit w receiving-electrodes having a confor industrial application a unidirectional source tinuous metallic surface can be ope a e with 8 of power is employed with the ionizing-electrode gas-flow of about 400 feet p r min whereas 6 maintained negative and the receiving-electhe S unit with y electrodes can a e.

trodes positive, with the same efilciency, a gas-flow of above 500 A receiving-electrode in accordance with my in- 15 et p nutevention may take any one of a number of forms nvestigating e causes of the difference in but the specific form, hereindescribed comprises efilcieney between a precipi r employing the a hollow-fiat, channel or box-like structure open yp electrodes and One employing P at the bottom and at the side which faces the t d s n the i nizin z n I have disc vered ionizing-electrode. As shown in Fig. 2, the rethat one of the primary reasons for the difference ceiving-electrode 8 comprises a rear side In relais due to the fact that the dust contained a good tively elongated in the direction of gas-flow, as percentage of conducting particles, in this case compared to sides I! transverse tothe direction carbon. From observation of the action of a preof gas-flow. The front side comprises a plurality cipitator with continuous-surface receiving-elecof spaced wires ll secured tofianges or edges l6 trodes, and from chemical analyses determining along the top and bottom of the front side of 5 the carboncontent of the gases before entering the electrode. In one specific embodiment I emthe precipitator, and the dust deposited on the ploy No. 10 wire having a length which will be, various electrodes, I explain the operation of of course, dependent upon the capacity of the such precipitatorsin'the following manner.

unit. The wires are spaced apart between. The carbon particles are quickly charged in centers, the end wires being spaced from side to the ionizing zone and are attracted to the large edges is of the front side, which parallel the electrodes of this zone before they can pass out wires. The ionizing-electrode 6 maybe of any of it. They will adhere to these electrodes it suitable size, and a wire of approximately 30 some adhering factor is present, or, if not, will mils has been found satisfactory. ultimately pass into the precipitating zone, per- I have found that with a distance of'approxihaps with their charges neutralized. If neumately 2" from the plane of the wires to the tralized, the particles rapidly fiow through the center of the wire 6, with a unidirectional voltage precipitating zone with the gases, and if not neuof approximately 26 kilovolts between the wire 6 tralized, they merely rebound or bounce over the and electrodes 8, and an ionizingcurrent of surface, of the precipitator-plates, and finally about 350 microamperes per foot of ionizing wire, also flow along with the gases out of the prea depth of /2" to 1" between the front and rear cipitating zone.

sides of the electrode I yielded satisfactory re- If dust particles adhere to the electrodes,'the sults. ionizing emciency is soon decreased. If they fiow The precipitating non-ionizing zone of the unit out of the unit with the gases, the gases are not thoroughly cleaned. In either case, a decrease in cleaning efficiency results.

However, a unit with trough-shaped receivingelectrodes of metal covered with a grid on the side facing the ionizing-electrode, operates in a considerably different manner. In this case, when the conducting particles are attracted to the grid electrode, they pass through the spaces between the wires and are discharged against the back of the trough which acts as a confining pocket for the particles, the particles falling down into a small hopper or receptacle which may be arranged below the open bottom of the electrode. I explain this action by the difference in the intensity of the field in front of the grid .as comtated again, inside the electrode, by contact with the ion stream.

I have found that the grid wires of my electrode did coat with a thin layer of dust but that pared to the intensity of the field between the grid and the rear side of the electrode, the former being relatively intense so that the dust particles will be charged, and the latter being relatively weak since it has metal at the same polarity along all its sides. The spaces between the wires permit some of the lines of force from the ionizing-electrode to extend to the rear side of the trough and this factor appears to be important in the design of a receiving-electrode in ac cordance with my invention, since the spacing should be such that the field within the electrode will be insufiicient to recharge the carbon particles by induction, or otherwise, after they contact the rear side, so that they will fall down of their own weight into a receiving receptacle below. i

A large percentage of the ionizing current is taken by the grid wires, and only a small percentage is permitted to leak through the grid wires to the back wall of the electrode. Thus when a carbon particle is charged in the ionizing zone, it is attracted towards the grid-type receiving-electrode in the same manner as in the caseof a solid receiving-electrode. When the particle reaches the grid-type electrode, it passes through the grid wires, is substantially neutralized, and finds itself in a region'of very weak field, and falls by gravity down the inside chute into a convenient receptacle. As it is falling down the chute, any tendency to wander out through the grid will be repulsed by the electrostatic field and by bombardment by other particles entering the grid structure. Accordingly, any particles which tend to precipitate prematurely on the receiving-electrode will be removed as quickly as they are precipitated. It is highly probable that some very fine particles actually agglomerate on the back side until they attain a size suflicient to permit them to fall.

Should a conducting particle strike a grid wire, it would bounce oif after losing its charge but would be struck by the stream of ions flowing from the ionizing-electrode, or other charged dust particles, and consequently enter the next vacant space and thus be trapped. High-resistance particles which strike a grid wire will tend to build up a film over the wire, which builds up to a definite thickness. When this thickness reaches equilibrium, any further high-resistance dust which is deposited merely rolls over the surthis layer would automatically peel off in small sections, or chunks, which were sufliciently large to drop by their own weight against the forces of the electrostatic fleld and gas-flow. Any fine spray accompanying a falling chunk of dust would agglomerate, or be recharged sufliciently to enter one of the succeeding spaces of the grid and pass into the hollow of the electrode where it would be discharged and fall as described before.

I am fairly convinced that a precipitator in accordance with my invention collects, in the ionizing zone, practically all of the carbon content of the fly ash. With fly ash containing 17% carbon, a precipitator without 'my electrode showed only 9% carbon in the dust collected, whereas with a grid type electrode the deposit gathered from the hopper beneath the receivingelectrodes showed a carbon content of 40%, and further indicated that practically all of the carbon in the gas had been precipitated.

I further found that the thickness of the builtup dirt on the grid-type receiving-electrode was only about A," before it began to break or chunk oif, whereas with a continuous, planar, receivingelectrode of metal the dirt would build-up to a thickness of /2", or more, before breaking off in chunks. Moreover, the latter dirt stuck quite tenaciously, probably due to the'constant ionic bombardment from the ionizing-electrode.

The use of a grid-type receiving-electrode has, consequently, certain pronounced advantages such as those described. Additionally, other practical advantages are present. For example, with only a thickness of dirt'building-up on each of the grids as compared to or more on a flat plate, a number of deleterious effects are avoided depending on the conductivity of the particles. Thus, if the dust deposited is'highly conducting, it would decrease the spacing or clearance between the ionizing and receiving electrodes, and consequently excessive arcing may occur which would momentarily drop the voltage due to the heavy drain on the power supply, or would jar collected dirt from the precipitating elements into the gas stream. If the dirt is of a medium resistivity, the effect of a thicker layer of dirt would be the same as a series resistor due to the high voltage drop across the dirt layer, and it is common to observe a 50% drop in ionizing current as a flat field electrode coats over. With a layer of dirt of high resistivity, a thinner film coating, such as' obtained with my invention, decreases the effect of back ionization, so that the tendency to introduce ions of opposite sign from those desired for dustcharging, and consequently neutralization of charged particles is considerably decreased. Accordingly, by substituting an electrode in accordance with my invention in a multi-zone precipitator, I improve its performance in a number of respects.

In Fig. 3 I show a modified form of receivingelectrode in which the spaced wires are replaced by expanded metal which is secured, as by'welding, to the edges of the front side of the electrode. I have found that, with conditions as previously described, expanded metal .094" thick with a diamond shaped mesh of ,5 x 2" would operate satisfactorily with air velocities up to 500 feet per minute and even greater. Smaller meshes tend to clog up and render the unit less efiective.

In Fig. 4 I show a further form of modified unit for use in an ionizing zone comprising more than one aligned ionizer unit, each unit comprising two ionizing-electrodes and three cooperating receiving-electrodes. The central electrode of this unit is made relatively deep, and has a central partition 30 dividing it into the equivalent of two separate receiving-electrodes, one facing toward an ionizing-electrode 32 disposed at one side of the combined unit, and the other facing another ionizing-electrode 34 on the other side.

While I have described my invention in the form in which I now believe to be the best mode of application thereof, it is obvious that many equivalent forms can be conceived.

I claim as my invention:

1. An electrical precipitator for cleaning a flowing particle-laden gas, comprising a first zone; a second zone closely spaced from the first zone in the direction of gas-flow; ionizing-electrode-means substantially centrally disposed in said first zone, and extending transversely with respect to the gas-flow, for charging substantially all of the particles carried by the gas; precipitating-electrode-means disposedin said first zone in laterally spaced relation to said ionizing-electrode-means, considered with reference to the direction of gas-flow, said precipitating-electrodemeans comprising means whereby charged particles of certain types including substantially all of the particles of relatively higher conductivity are selectively precipitated and continually removed from the boundary of the gas-stream in said first zone; the particle-charging efl'ect of the ionizing-electrode means being sufllciently strong, the extent of the precipitating-electrode-means in the direction of gas-flow being sufliciently limited, and the velocity of the gas-flow being sufficiently high, so that other charged particles remain in charged condition in the gas as it leaves the first zone and enters the second zone; and a plurality of relatively closely spaced precipitating-electrode-means of alternately opposite potentials substantially fllling said second zone for removing said remaining particles with a much smaller mean transverse-flow path than in the case of the particles precipitated in said first zone,

2. The method of removing particles from gases laden with particles of various degrees of electrical conductivity, including particles of relatively higher conductivity, which method\comprises passing the gases through an ionizing electrostatic zone in the gas-flow path for charging substantially all of the particles and causing a predominating part of the charged particles of relatively higher conductivity to be precipitated in a pocketed area in said ionizing electrostatic zone, whereby these particles of relatively higher conductivity are removed from the gas-flow path, and then passing the gases through a non-ionizing electrostatic zone in the gas-flow path for causing charged particles remaining in the gases, leaving the ionizing electrostatic zone, to be precipitated in said non-ionizing electrostatic zone. 3. The method of cleaning flowing flue gases containing gas-borne particles comprising a relatively small percentage of conductive carbon particles, and a larger percentage of. ash particles, which comprises simultaneous electrical charging of substantially all of the particles and electrically precipitating substantially all of the charged carbon particles in a pocketed area in an ionizing zone in the path of the flowing gases, while permitting charged ash particles to pass beyond the ionizing zone, and then electrically precipitating substantially all of the charged ash particles remaining in the gases in a non-ionizing electrostatic zone which is next to and follows the said ionizing zone in the path of the flowing gases.

4. A gas-cleaning electrical precipitator device for cleaning a flowing gas, such as flue gas, containing particles of varying degrees of resistivity, such as carbon and ash, said precipitator device having an ionizing zone and a precipitating zone successively in the direction or gasflow, said ionizing zone having ionizing-electrode means and spaced receiving-electrode means cooperating with said ionizing-electrode means for establishing an ionized electrostatic field for charging the dust particles in the gas flowing through said field, said receiving-electrode means being of limited extent in the direction of gas flow and including chamber means providing a collecting pocket having a grid-front presented to said ionizing-electrode means for causing charged dust particles of relatively higher conprecipitating means comprising a plurality ofalternately relatively insulated and non-insulated precipitator-plate-electrodes of limited extent in the direction of gas-flow, upon which the last said dust particles precipitate.

PETER, H. WYCKOFF.

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