US2249801A

US2249801A - Method and apparatus for electrical precipitation

Info

Publication number: US2249801A
Application number: US315111A
Authority: US
Inventors: Harry J White
Original assignee: Research Corp
Current assignee: Research Corp
Priority date: 1940-01-22
Filing date: 1940-01-22
Publication date: 1941-07-22
Anticipated expiration: 1958-07-22

Description

H. J. WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECiPITATION July 22, 1941.

Filed Jan. 22, 1940 4 Sheets-Sheet l INVENTOR ARRY rI WHITE,

BY v I ATTORNEY y 1941- H. J. WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4 Sheets-Sheet 2 & w

INVENTOR HARRY a: WH/TE,

ATTORNEY July 22, 1941. H,- WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4 Sheets-Sheet s :5 31 19 Z0 gag, I 15 I L I I al 1 A I l, l\ I 30 28a 31 -INVENTOR H4 REY r2 WH/ TE,

' ATTORNEY July 22, 1941. H. J WHITE 2,249,801

METHOD AND APPARATUS FOR ELECTRICAL PRECIPITATION Filed Jan. 22, 1940 4 Sheets-Sheet 4 INVENTOE ff/LQRY 11 710-1175,

ATTORNEY Patented July 22, 1941 UNITED STATES PATENT OFFICE ME'rnonaNn APPARATUS ron ELECTRICAL rnuorrrrarron Harry J. White, Los Angeles, cum, assignor to Research Corporation, New York, N. Y., a corporation of New York Application January 2:, 1940, Serial No. 315,111

Claims.

The present invention is generally concerned with the art of electrically precipitating suspended particles from a stream of gas; and it is In its basic form, a precipitator of this type has a first field maintained between two electrodes of which one is a discharge electrode and the other is a non-discharging electrode, and a second field maintained between two' substantially non-discharging electrodes.

In the following description and appended claims, the term discharge electrode will be understood to designate an electrode that facilitates corona discharge therefrom, because it has a configuration that establishes a sufllciently high potential gradient at or near its surface to create corona discharge before there is a disruptive discharge or spark-over. For this purpose, the discharge electrode usually takes the form of a member of small surface area, such as a-small diameter wire or a rod provided with sharp edges or points, whereby there may be created in the immediate vicinity thereof a sufllciently high electric field intensity to cause ionization and corona discharge. The term non-discharging electrode will be understood to designate an electrode that minimizes or prevents corona discharge therefrom because it has a configuration that establishes a sumciently low field concentration at or near the surface to suppress corona discharge at elevated potentials lower than the voltage required for disruptive discharge or spark-over. For this purpose, a non-discharging electrode is usually one of extended surface area, substantially free from sharp comers 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.

The construction and advantages of an electrical precipitator of the two-stage or separated field type are discussed in Patent No. 1,343,285 granted June 15, 1920, to W. A. Schmidt and also in my co-pending application Serial No. 248,348 filed December 29, 1338, for Method and apparatus for electrical precipitation." Briefly, these advantages are economy of installation and operation; advantageous inherent operating characteristics with certain types of dust; a much reduced tendency to the detrimental phenomenon of back-corona" in the collecting section; and possibility of maintaining a. relatively higher potential gradient between the precipitating electrodes in the collecting section, thus bringing about very effective collection.

The phenomenon of back-corona arises when dust of relatively non-conducting characteristics is being collected in a precipitatora For the purposes or this disclosure, back-corona may be considered to be corona discharge at or in the immediate vicinity of a non-discharging electrode that is opposing a discharge electrode. The

20 back-corona discharge is consequently opposite in electrical sign tn the main discharge and therefore produces ions of an opposite sign to the main supply of ions. Since a discharge electrode usually produces negative ions, the back 85' discharge ordinarily is thought of as producing positive ions. The mechanics of the occurrence of back-corona are not entirely understood, but apparently are as follows.

Particles of non-conducting dust collect upon the surface of a non-discharging electrode opposing a discharge electrode and eventually the collection is suillcient that, even though the layer is very thin, it is substantially continuous and has the effect of insulating the electrode from the space around it. As a result of the presence of this insulating layer, there is an alteration in the potential and field distribution around the electrode in such a way that a relatively larger proportion of the total potential drop between the electrodes occurs across the dust layer. The immediate result" is a decrease in voltage across the gas and a decrease in corona current, with a consequent reduction in efflciency of the pre- 5 cipitator. Eventually the voltage drop across the dust layer becomes sufiiclently great that the layerbreaks down locally, a small hole being formed in' the layer into which the gas flows. In the vicinity of this hole, the gas becomes a good conductor and there is a concentration of the field and thedischarge at this hole. The result is either a spark-over between the electrodes or back-corona discharge from the normally nondischarging electrode. Whether spark-over or back-corona appears depends upon various factors, and sometimes the back-corona gives way to a spark-over.

Several undesirable conditions that reduce the precipitator eiilciency result from back-corona. The spark-over voltage is greatly lowered and it is necessary that the applied voltage be reduced in order to maintain corona discharge at the discharge electrode. Also, there is a'concentration of discharge at the points of local rupture in the dust layer with the result that there may be large sections of the space through which the gas stream flows that have relatively few ions.

This means that there is a lower average charge on the dust particles, or in other words that the charging ability of the preclpitator is decreased. Probably the most important result of back-corona is that positive ions produced at the points of back-corona enter the gas stream. Not only do the positive ions reduce the charging and collection eificiency of the precipitator by neutralizing the negative ions producing negative charges on the dust particles, but the positive ions produce positively charged dust particles which tend to collect upon the discharge electrode and thus interfere with maintenance of the proper ionizing field.

In a two-stage precipitator, back-corona is not ordinarily present in the precipitating section, even though there is a heavy dust layer present, because of the almost complete absence of current between the electrodes, as the only current is that carried by the dust particles and usually is of the order of less than 1% of the current in the ionizing field. However, there is often a very strong tendency toward back-corona in the is obtained. However, there are some circumstances under which humidification is definitely not practical, especially since the degree of humidification often runs from 50% to 95%. It may be undesirable under some circumstances to collect the material in damp form, and again the gas stream may be rendered exceedingly corrosive by the presence of water vapor with the result "that replacement or maintenance "costs on the precipitator are excessive unless corrosion resistant materials are used, and the cost of these materials may be prohibitive. Also, humidifying apparatus is expensive and requires constant attention to insure its proper functioning.

It is thus a general object of my invention to eliminate back-corona from the charging section of a follow-up or two-stage precipitator and at the same time to collect the suspended particles charging section of a two-stage precipitator. It

has been found that in charging sections having non-dischargingelectrodes upon which dust particles accumulate, very considerable trouble with back-corona arises when the dust is of a particularly non-conducting nature. It will be appreciated that the amount of dust collected upon a non-discharging electrode in the charging section is relatively small compared with the quantity of dustcollected in the precipitating section, but that nevertheless .an amount which cannot be neglected always accumulates.

Since a given voltage will be required tobreak down a given layer of dust, and since that voltage is numerically equal to the product of the current density through the layer times the specific resistance of the layer times the thickness of the layer, the tendency to produce back-corona may be considered as the product of these three factors. v

Reduction or elimination of back-corona in a charging field has heretofore generally been accomplished by reducing the specific resistance of the material, and this has been done by humidification of the entire gas stream at a point well in advance of the precipitator. It has been found by experience that where the specific resistance of the material is less than approximately 10 ohms/cm. little or no trouble with back-corona is encountered. But for dusts having a specific resistance in excess of this approximate value, and which are referred to herein as those having a relatively high specific resistance or having relatively non-conducting characteristics, the tendency towards back-corona becomes increasingly great with the increase in resistance. By adequate humidification of the gas stream, the material absorbs moisture andthe average resistance of the layer can be kept below this value, with the result that suitable collection emciency in a substantially dry condition.

It is an object of my invention to provide electrical precipitation apparatus of the .twwstage type which maintains a relatively high collection efficiency at all times because it is operated and constructed so as to prevent the occurrence of back-corona discharge that would reduce the normal collection efiiciency.

It is also an object of my invention to eliminate the necessity of humidification of the gas stream, since by eliminating humidification there are also eliminated various other problems and costs such as the difficulties of handling a wet precipitate, thecost of the humidification system, and the high costs of installation or maintenance as a result of corrosion produced by the wet gases.

These objects of my invention are attained according to my improved and novel method of operating electrical precipitation apparatus, by

' charging electrode in the first field with liquid to prevent accumulationpf particles on-the surface of the electrode and collecting the particles in the second field in a substantially dry condition on dry collecting surfaces of the non-discharging electrodes. In a two-stage precipitator constructed according to my invention and operating according to the novel method above set forth, I provide, in addition to the conventional elements mentioned above, means for maintaining a substantially continuous liquid film'only on the non-discharging electrode or electrodes opposing the discharge electrode in the first or charging field, the film being exposed to the gas stream and flushing the electrode surface clean of any precipitated particles. In the collectingsection, I provide collecting electrodes with dry surfaces upon which the suspended particles are precipitated in order that the particles be collected and maintained in a substantially dry condition.

These novel improvements may be added to two-stage precipltators of otherwise conventional construction, though it may be necessary in some cases to modify the electrodes to provide a precipitator constructedand operated according to my invention. For purposes of fully disclosing my invention and showing a typical embodiment thereof, I hereinafter describe my invention as applied to a. preoipitator of the type described in my co-pending application mentioned above, in

which all electrodes extend transversely of the stream now, However, I am not to be limited thereto, since my invention may also be applied to other types, as for example pipe or tubular types in which the electrodes are parallel to the gas flow.

In a preferred form of my invention I provide a gas directing member ahead of each non-discharging electrode of the charging field to prevent direct impingement of the entering gas stream on the liquid film maintained over the surface of the non-discharging electrode. This gas-directing member also serves to direct the gas stream into the space occupied by the discharge electrode, thus making for more efilcient charging of the particles in suspension in the stream.

How the above objects and advantages of my invention, as well as others not specifically mentioned, are achieved will be more readily understood by reference to the following description and the annexed drawings, in which:

Fig. 1 is a vertical median section through a each electrode l8, pipe I! carrying angularly spaced studs which center the pipe within the electrode and maintain the electrode in a vertical position after inlet pipe I9 is properly positioned. Each pipe 19 has a valve 2| which regulates the flow of wash liquid into the electrode interior, and pipes l9 are connected to a header 22 which is connected to a main supply line 23, as shown in Fig. 3. In order to facilitate assembly and inspection of the parts, housing II is provided with a cover plate 24 over the top of compartment 1811, as the plate may be removed to permit header 22 and the attached inlet pipes l9 to be lifted vertically upward out of the electrodes. In operation, liquid from the interior of each electrode l6 overflows the open top end and maintains on the exterior surface of the electrode a substantially continuous film of liquid. Wall means l8 shields the upper ends of the tubes from the gas stream so that the water films are formed in a space isolated from the gas stream, an arrangement that favors formation two-stage precipitator constructed in accordance with my invention;

Fig. 2 is a horizontal section taken on line 2-2 of Fig. 1;

Fig. 3 is an elevation looking from the left of Fig. 1, showing the system of electrodes in the charging field in elevation; and

Fig. .4 is a vertical transverse section taken on line 4-4 of Fig. 2.

From Fig. 1 it will be seen that the electric precipitator com-prises generally three main parts, the ionizing or charging section-indicated generally at ill at the entrance side of housing II, the collecting or precipitating section indicated generally at l2 located to the right of the charging section and removed from the charging section in the direction of gas flow (indicated, by the arrows), and the dust collecting hopper" indicated generally at I 4 located below the collecting section of the precipitator to receive the dust which falls from the electrodes.

Charging section ll) of the preci'pitatorcontains the first system of electrodes that comprises in general one or more pairs of electrodes between which an ionizing field is established in which are charged the particles suspended in the gas stream, each pair of electrodes comprising a discharge electrode and anon-discharging electrode. As will be apparent from the subsequent description, ;the. system of electrodes shown in Figs. 1, 2 and 3 disclosing a preferred embodiment of my invention comprises a plurality of spaced, parallel, non-discharging electrodes IS with a discharge electrode l5 spaced midwaybetween each two non-discharging electrodes. Non-discharging electrodes 16 are yertically extending cylindrical members placed in a common plane extending transversely of housing II and the gas stream. These electrodes are located adjacent the gas inlet and rest in sump ll in the bottom of the housing. The upper end of each electrode 16 passes with clearance through an opening Na in plate l8 which has a U-shaped portion that forms a rectangular compartment lab at the top of the housing.

Electrodes l6 are preferably hollow tubes in order to provide a convenient means for maintaining over their exterior surface, whichv is exposed to the gas stream, a substantially continuous film of water or other suitable wash liq uid. For this purpose a liquid inlet pipe I! exof an uninterrupted film. This liquid runs down the outside surface of the electrode, openings [8a being large enough for this purpose but not large enough to admit the gas stream to compartment I8b. Liquid collects in sump l1, and is discharged from the precipitator through. Pipe connection 25 to which a suitable drain-pipe, not shown, may be attached.'

Other means of forming water films are known in the art and may be used instead of the overflow arrangement shown. For example, a film may be formed by jets or sprays directed onto the upper end of the electrode, as may be seen from U. S. Patent 1,250,088 issued toBurns on Dec. 11, 1917; or the electrode may be provided with suitable openings in the walls through which liquid flows to reach the electrode surface in a manner shown by Schmidt Patent 1,309,221, issued July 8, 1919.

Non-discharging electrodes l6 are grounded through housing I1 and oppose the discharge electrodes I5 which are maintained at a relatively high potential with respect to the grounded electrodes. Discharge electrodes I5 are mounted on an open rectangular frame 28, each discharge electrode being mounted between a pair of forwardly projecting arms 28a. and 28b which extend outwardly from the upper and lower sides of the frame respectively. All electrodesv I5 are preferably in a common'transverse vertical plane. It is notnecessary that the discharge electrodes be exactly'in the vertical plane established by the axes of electrodes l6, but each discharge electrode is spaced equidistant from. the two electrodes [6 on either side. i

Electrode frame 28 is suspended from transverse cross-bar 30 and is attached thereto by a pair of hooks 3| which slide downwardly into suitable openings in the crossbar. can be detached by lifting it vertically. Frame 28 is held in a vertical position and kept from pivoting around cross-bar 30 by engagement of an intermediate horizontal frame member 280 with a rigid depending stop member 30a. attached to crossbar 30. Supporting bar 30 is electrically insulated from housing, I] and is supported at either end on an electrical insulator 33 which is enclosed withina separate insulator compartment 34 outside the wall of housing II in order to keep the insulator out of the main gas stream and as free as possible from any accumulation of dust Particles. v i

,A high potential diiference is maintained be The frame,

tween electrodes i5 and electrodes l6 by appli-.

cation of a high tension current to cross-bar 30 and the attached electrodes I5 through electrical cable 35 which is attached to a suitable source of high tension current, not shown. It is common practice to connect lead 30 to the negative side of a source of unidirectional current; and since suitable power sources are well-known in the art, it is not necessary to describe one here. Lead-in 35 enters insulator compartment 34 through an insulator 36 and is connected to bar 36 by strap 31.

A single pair of electrodes l 5 and i5 can maintain an ionizing field only within a limited space, and consequently it may be necessary to provide a plurality of pairs of these electrodes. The number of pairs provided is determined by the size of the gas conduit formed by housing II, and is suflicient to maintain a substantially continuous charging field entirely across housing ll so that the entire stream of gas passing through the housing is subjected to the action of this field in order to charge the particles suspended in the gas stream.

Ahead of each non-discharging electrode i6 is placed a vertically extending gas directing member 40, as may be seen most clearly from Fig. 2. Bailles' 40 extend the full height of the gas passage and are attached at their upper ends to plate IS. The exact shape of these members is not critical but it i preferred that the maximum horizontal width of each baille be greater than the diameter of the electrode I6 with which it is aligned in the direction of gas flow. The gas directing members prevent the gas stream from directly impinging on the liquid film, which has two main advantages. Continuity of the film is more easily maintained since the force of the gas stream against the film is lessened, and there is less tendency to precipitate dust particles on the film since the volume of gas coming in direct contact with the electrode is decreased. There is a further advantage in that the shape of members 40 is such as to direct the gas stream toward or around discharge electrodes l5 and thus concentrate the gas stream where charging of the suspended particles is best effected.

Normally the parts described may be made of ordinary grades of steel or iron, as so little moisture is picked up by the gas stream that no corrosion occurs; but should the liquid absorb sufficient gas to become corrosive, only the wetted parts need be made of corrosion resistant metals. This is a great saving over prior practice in which the entire precipitator and the ducts leading to and from it were subjected to any corrosive action.

At a location in housing I I close to but spaced from the charging section in the direction of gas flow, is the second electrode system contained in the precipitating or collecting section of the precipitator. This second system of electrodes comprises, in general, one or more pairs of nondischarging electrodes between which an electrostatic field substantially free from corona discharge is established. In -the preferred embodiment of my invention herein described, the second system comprises a plurality of pairs of nondischarging electrodes. Each pair of electrodes comprises a grounded electrode 43 and a high tension electrode 44 which is electrically insulated from the housing, these two electrodes having dry surfaces and being placed in parallel planes extending transversely across the housing and the stream of gas flowing through the housing.

Grounded electrodes 43 are larger than electrodes 44 and, as may be seen from Fig. 4, substantially fill the entire cross-section of housing ll, except at the four corners. The construction of the grounded electrodes may be seen in the left-hand half of Fig. 4 wherein one such electrode is shown in elevation. Although it is to be understood that other types'of construction are suitable for the electrodes. the type described is typical. The electrode comprises .a frame member 45 which is preferably a hollow tube of suitable diameter that forms the rectangular central portion of the electrode. Across this rectangular frame is placed a foraminous metal sheet 46 which is provided with a sufficiently large number of gas openings to render the electrode easily fluid-pervious. Although the metal sheet 46 may be formed in various ways, such as by woven-wire screening, it is preferred that the electrode have a fiat surface such as provided by rolling expanded metal fiat or punching necessary openings in a flat metal sheet. Metal sheet 46 extends over the entire portion of the electrode formed by rectangular frame member 45, and, as is seen from Fig. 4, this portion of the electrode extends out to the vertical side walls of housing il.

At the top and bottom of each electrode (except the end electrodes as described later) is an impervious plate portion 41 and 48 respectively,

mounted on frame 45. Plate 41 extends upwardly from frame 45 into contact with the top wall open bottom side of the housing. These impervious portions of the electrode do not extend the full horizontal width of the housing, in order to provide electrical clearance for longitudinally extending members located at the four corners of the housing, as will be more fully described. At each vertical edge of the electrode there is provided a marginal plate 50 which is imperforate and reduces the gas pervious portion of the electrode 43 to a size that corresponds approximately to the fluid pervious portion of the high tension electrodes 44. It is desired that the fluid pervious portions of all transverse collecting electrodes be approximately the same in size and shape in order to reduce the tendency for the gas stream to by-pass any of the electrodes.

Other electrode constructions and also other means of baiiling that may be used to further reduce by-passing of the gas, are described in my co-pending application Serial No. 248,348, referred to above.

Each electrode 43 is held in a vertical plane across the gas stream by two pairs of pins 6| located in each housing side wall and engaging the vertical side of the electrode. Horizontal pins 52 in the electrode frame 45 engage the upper set of pins 5| and support the weight of the elec- The materials used in the high tension elec- .trodes 44 are the same as those used in the grounded electrodes 43 Just described. The details of construction of an electrode 44. are illustrated in the right-hand half of Fig. 4 wherein one half of such an electrode is shown in elevation. Each electrode .44 has a rectangular frame of tubing 45a of approximately the same size and shape as the corresponding rectangular frame of electrode 43, except that the high tension electrodes are not as wide because they must be spaced from the side-walls of the housing in ings in longitudinally extending bus-bars 55 located at the four corners of the housing. A transverse pin through the top of each extension of the side frame members supports the weight of the electrode from the upper two bus-bars 55, while the lower two bus-bars 55 engage the bottom of the electrode frames to keep the electrode in vertical alignment and parallel to grounded electrodes 43. High tension electrodes 44 have no imperforate

marginal portions

41, 48 or 50 as described in connection with electrodes 43, the entire electrode being fiuid-pervious and spaced around-its periphery by the required electrical clearance from the grounded housing.

As may best be seen from Fig. 2, the two longitudinal bus-bars 55 at the top of the housing are supported at their ends on two transverse bars 58 which extend through the side-walls of the electrical insulators 51 enclosed within separate insulator compartments 58 outside the housing walls. In this way insulators 58 are removed from the main gas stream and protected from adeposit of dust. The lower two bus-bars 55 are suspended from similar transverse members 88 which are supported at their ends by depending insulators 8| (see Fig. 4) placed in separately enclosed insulator compartments 82.

A high potential from some suitable source, not shown, is applied to the high tension electrodes 44 by electrical lead-in cable85 which enters one jacent the top and-bottom margins respectively .of the first electrode 48 in order to prevent the .85 housing and are supported at their ends'uprui" of the insulator compartments 58 through insulator 88. Electrical connection to supporting

busbars

58 and 55 is completed through strap 81 attached to one of the transverse members 58. Sources of high potential are common knowledge in the art and need not be described here, but it is common practice to apply to the high ten.-

sion electrodes a negative potential from a source 3 of unidirectional rectified current.

Grounded electrodes l6 and 43 are connected electrically to housing II to which a suitable ground connection is attached, as indicated diagrammatically at 88 in Fig. 4. 4

In will be observed from Fig. lthat the firfi and last two of the grounded collecting electrodes 48 are modified slightly from the others in that these electrodes have no top or bottom bafiles'" and 48. Also, the rectangular frames45 of the second and next to the last electrodes are slightly smaller vertically than the othersin order to afford the proper electrical clearance between the electrodes and the transverse members 58 and .68.

- the collecting electrodes.

Top and bottom baflies 41 and 48 are replaced at gas stream from lay-passing the electrode. A similar construction is used at the outlet end of the collecting section where baffles 12 and 18 are placed at the top and bottom of the last electrode 48. These baflies are likewise attached to the housing walls and extend into engagement with the last electrode.

Underneath collecting section II is located hopper l4 which has downwardly converging sidewalls that terminate at their lower ends at an opening into the upper side of the housing of screwconveyor 18. Conveyor I5 is driven by chain 18 from motor 11 and is designed for automaticdischarge of the dust from the conveyor, since plate 18 closing the discharge end of the conveyor housing isnormally held closed by spring I8 butcan be moved to the right against the spring by the pressure of material moved by the conveyor. Discharged dust falls down out of the right-hand end of the conveyor housing. Housing H is open on its under side throughout that portion of its length which is underneath the collecting electrodes, and dust falling from these electrodes passes through this opening in the bottom of the housing and enters hopper H. In order to reduce or eliminate any tendency of the gas stream to flow downwardly into thejropper and thus by-pass the collectingrelectrodes, vertically extending baflles amm suitable number are provided at intervals throughout the length of 'hopper l4. The'location, spacing and number of these is such as to accomplish the desired end of preventing gas flow through the hopper, but at the same time not to interfere with the proper collection of dust.

Although the collecting electrodes are, as stated above, self-cleaning to a large extent, it may be desirable under some circumstances to effect a more thorough cleaning of them, and for this purpose mechanical cleaning means is provided. Such a mechanical cleaning .means may take other forms than the one shown, but the manually actuated cleaning means illustrated in Fig. 1 is simple to operate and construct. This cleaning means comprises a rotatable shaft 8| which extends horizontally the full length of hopper i4 and is journaled in bearings mounted on the end walls of the hopper. The shaft is capable of limited axial movement in these hearings. On one end of shaft 8| extending outside the hop.- per is placed handle 82. A plurality of hammers 83 are attached to shaft 8| and are all aligned in a single axial plane, each hammer comprising a weight on the. end of a rod of such length that, when the shaft 8| is rotated from the position shown in Fig. 1, the weights engage

lower frame members

45 and 45a of

electrodes

43 and 44. The electrode cleaning mechanism normally occupies the position shownin Fig. 1 when itis not in operation. To clean the electrodes, the operator grasps handle 82 and rotates shaft 8| 9. half-tum, which places the weight ends of hammers 83 in the spaces between v The operator thenreciprocates shaft 8| longitudinally, thus striking the lower frame members of the electrodes with hammers 83 to jar loose accumulated deposits of precipitated dust. A few blows of this nature are 'suflicient to loosen the dry dust which falls through the bottom of housing 'into hopper l4, and after the operation is completed, the hammers of their own weight return to the inoperative position shown in Fig. 1.

When it is desired to place the precipitator in operation. but before the gas stream is passed through the housing, valves 2| are open to admit wash liquid to the interior of electrodes 16 and the electrodes are adjusted in position until the liquid overflows evenly around the entire upper rim of the electrode to form a thin film of liquid flowing down the entire outside surface of the electrodes. It is desirable that this film be unbroken over the entire surface of each tube l6, since the gas stream swirls around the electrodes and comes in contact with the entire outside surface. gas is passed through housing II, and it first comes into engagement with gas directing members 40 which direct the gas stream away from the non-discharging electrodes I6 and toward the discharge electrodes i5. This concentration of the gas around the discharge electrodes facilitates charging of all suspended particles. Bailles 40 prevent the gas stream from directly impinging upon the liquid films maintained over the exterior surface of each electrode i6, thus keeping the film continuous at all times and preventing breaks in the film caused by the gas stream striking the film. Also, there is less tendency for particles to precipitate upon the electrodes l6, since a smaller portion of the gas stream comes into contact with the electrodes than when members 40 are omitted.

It is inevitable that there will be some precipitation of dust particles upon the non-discharging electrodes of the first electrode system, since the gas stream cannot be kept out of any contact with the electrodes and no matter how short the time in which the gas stream is exposed to the charg- After the liquid films are established,

ing field, that time is still of finite duration and sufficient to cause a small amount of precipitation. However, the precipitation is now upon the film rather than upon the dry metal surface of the electrode itself, and all the precipitated particles are immediately carried away by the flushing action of the liquid film with the result that there is no insulating layer of poorly conducting material built up on the exterior of the non-discharging electrode. In this manner, the condi tions essential to the formation of back-corona are prevented from occurring and back-corona from this source is completely eliminated in the charging field.

Experience has shown that there is no material increase in humidity of the gas stream, be-

cause the'time of exposure to the liquid is so short that very little evaporation occurs.

As the gas stream passes the first system of electrodes, the suspended particles pass through the harging field maintained across the entire width of the housing between the pairs of electrodes l5 and I6. Assuming standard practice is followed in supplying unidirectional current to the high tension electrodes, the discharge electrodes ii are of negative polarity and conse quently the particles suspended in the gas stream become charged negatively. These dry charged particles are then carried by the gas stream toward the first transverse collecting electrode which may be a high tension electrode 44 but which is here shown as a rounded electrode 43 which is consequently oppositely charged or of positive polarity with respect to the suspended particles. Some of the negatively charged particles are attracted to the positively charged electrode 43 and are precipitated on the front or upstream side thereof in a dry condition, these precipitated particles becoming neutralized or even positively charged.

The remaining negatively charged particles pass through the gas openings in the first electrode and enter the precipitating field maintained between the first two collecting electrodes. This field is in a direction to force the charged particles toward the grounded electrode against the movement of the gas stream, and consequentl part of the remaining particles are precipitated in a dry condition on the rear or downstream face of the first electrode 43. This deposition'is also facilitated by eddying of the gas stream as it passes through the electrode, since the eddies .tend to carry dust particles toward the back face. Negatively charged particles as yet unprecipitated pass through the first high tension electrode 44 and enter the second precipitating field, oppositely directed tothe first, that tends to precipitate particles on the front face of the second grounded electrode 43. This action is'repeated at each electrode 43, and the negative particles are subjected to a series of successively oppositely directed fields that eiTect substantially complete initial precipitation in a dry condition on the first few grounded electrodes.

If for any reason there are positively charged particles in the gas stream, these particles tend to go through the first electrode and precipitate upon the first high tension electrode 44, since the high tension electrode is negatively charged and is opposite in sign to the positively charged particles.

The collecting section is preferably composed of transverse fluid pervious electrodes as described, the advantages of this construction being more fully discussed in my co-pending application referred to above. However, my invention is not limited thereto, as any suitable collecting electrodes with dry surfaces may be used, as, for example, plates parallel to the gas flow as illustrated by Schmidt Patent 1,343,285. Likewise, other changes in arrangement and construction of parts may be made without departing from the spirit and scope of my invention. Consequently, the foregoing description is to be considered as illustrative of a typical embodiment of my invention rather than limitative thereon,

and the appended claims are to be construed accordingly. I claim:

1. The method of removing suspended particles from a gas stream which comprises subjecting the gas stream to the action of corona discharge in a high tension electric field maintained between a pair of electrodes of which one electrode is substantially non-discharging, and in which field the suspended particles are charged and some of them are removed from the gas stream; and subsequently passing the gas and the remaining suspended particles through a second high tension electric field substantially free from corona discharge; maintaining in said first field a subs'tantially continuous film of liquid over the nondischarging electrode; and collecting charged particles in said second field in a substantially dry condition.

2. The improvement in the method of removin'g substantially non-conducting suspended particles from a stream of gas by passingthe gas stream through successive electric fields in the first of which the particles are charged by corona discharge and some are precipitated and in the second of which particles charged in the first field are precipitated in an electric field substantially free from corona discharge, which comprises continually fiushing the surface of a nondischarging electrode in the first field to prevent accumulation of particles on the electrode, and collecting particles in the second field in a substantially dry condition.

3. In an electrical precipitation apparatus through which moves a stream of gas containing suspended particles to be removed, the combination of a first complementary electrode system comprising a non-discharging electrode and a discharge electrode adapted to create an ionizing field; a second complementary electrode system comprising spaced non-discharging electrodes with dry surfaces; means for directing a gas stream to be treated successively between the complementary members of said electrode systems; means for impressing a high potential across the complementary electrodes of each of said electrode systems; and means for maintaining a liquid film only on the non-discharging electrode of the first system.

4. Electrical precipitation apparatus as in claim 3 in which a gas directing member is placed ahead of the non-discharging electrode of the first system to prevent direct impingement of the entering gas stream against the liquid film.

5. In an electrical precipitation apparatus through which moves a stream of gas containing suspended particles to be removed, the combination of a first pair of electrodes of which one is a non-discharging electrode and the other is a discharge electrode; means for maintaining a substantially continuous liquid film on said nondischarging electrode of said first pair; means for maintaining a high potential difference between said electrodes to establish a charging field; a plurality of non-discharging collecting electrodes having dry surfaces disposed beyond said first pair of electrodes in the direction of stream flow, each of said non-discharging collecting electrodes being fluid-pervious and extending transversely to the direction of stream flow; and means i or applying a potential difference between successive ones of the non-discharging collecting electrodes to establish a plurality of successive precipitating fields in which suspended particles are collected in a substantially dry condition.

6. In an electrical precipitation apparatus through which flows a stream of gas containing suspended particles to be removed, the combination of a first electrode system disposed transversely to the gas stream and comprising a finewire discharge electrode spaced between a pair of opposing non-discharging electrodes; means for maintaining a substantially continuous liquid film on the outside surfaces of said non-discharging electrodes of the first system; means for applying a high potential difference between the electrodes of said first system to establish a charging field; a second electrode system comprising a plurality of non-discharging collecting electrodes having dry surfaces disposed beyond said first system of electrodes in the direction of stream fiow, each of said non-discharging collecting electrodes being fiuid pervious and extending transversely in the direction of stream fiow; and means for applying a potential difference between successive ones of the non-discharging collecting electrodes to establish a plurality of successive precipitating fields in which suspended particles are collected in a substantially dry condition.

7. Electrical precipitation apparatus as in claim 6 in which a gas directing member is placed ahead of each non-discharging electrode of the first system to prevent direct impingement of the entering gas stream against the liquid film on that non-discharging electrode.

8. In electrical precipitation apparatus for cleaning a gas stream of the separated field type having a first pair of electrodes between which an ionizing field is maintained and a second pair of electrodes between which a non-discharging precipitating field is maintained, the combination comprising: a non-discharging electrode in the ionizing field; means for maintaining a substantially continuous liquid film upon the surface of said non-discharging electrode exposed to the gas stream as the gas stream moves through the ionizing field; and an electrode in the precipitating field of the same polarity as said non-discharging electrode and having a dry surface upon which the particles are collected in a substantially dry condition.

9. In an electrical precipitation apparatus through which fiows horizontally a stream of gas containing suspended particles to be removed, the combination of wall means forming a compartment isolated from the gas stream; a plurality of vertically extending non-discharging electrodes spaced transversely of the gas stream, the upper ends of the electrodes extending with clearance through the wall means into the compartment; means at the upper ends of the electrodes for forming and-maintaining a liquid film over the exterior surface of each electrode, each film being formed inside said compartment away from the gas stream; andmeans at the lower ends of the electrodes for collecting and discharging the liquid from the precipitator.

10. Electrical precipitation apparatus as in claim 3 in which the non-discharging electrode of the first system is a substantially cylindrical member extending vertically and transversely of the gas stream, and in which a gas directing member extending substantially the full height of said non-discharging electrode of the first system is aligned therewith and shields the electrode and liquid film thereon against direct impingement of the entering gas stream.

' HARRY J. WHITE.