US2990912A

US2990912A - Electrical precipitator and charged particle collecting structure therefor

Info

Publication number: US2990912A
Application number: US847847A
Authority: US
Inventors: William H Cole
Original assignee: Research Cottrell Inc
Current assignee: Research Cottrell Inc
Priority date: 1959-10-21
Filing date: 1959-10-21
Publication date: 1961-07-04
Anticipated expiration: 1978-07-04

Description

July 4, 1961 w. H. COLE ELECTRICAL PRECIPITATOR AND CHARGED PARTICLE COLLECTING STRUCTURE THEREFOR 2 Sheets-Sheet 1 Filed Oct. 21. 1959 INVENTOR WILLIAM H. COLE uZ/M BY 5; 1. JM

ATTORNEY/ July 4, 1961 Filed Oct. 21, 1959:

W. H. COLE ELECTRICAL PRECIPITATOR AND CHARGED PARTICLE COLLECTING STRUCTURE THEREFOR (D In p 3 i h l 3;

M g ,Z

(O I I l l I q k I m O 8 8 Q 8 1014301333 NOLL'JBT'IOQ 03% 03mm 2 Sheets-Sheet 2 BEAD DIAMETER- MILLIMETERS INVENTOR WILLIAM H. COLE ,/ZJM BY ghig/z AM ATTORNEU United States Patent Jersey Filed Oct. 21, 1959, Ser. No. 847,847

8 Claims. (Cl. 183-7) This invention relates to an improved two-stage electrical 'precipitator and an improved charged-particle collecting structure therefore.

Many electrical precipitators and particularly air cleaning precipitators are of the two-stage type and generally comprise a relatively short particle-charging section followed by a particle-collecting structure having a relatively large surface area for'the collection of the charged particles. It is a principal object of the present invention to provide an improved two-stage type electrical precipitator and a charged-particle collecting structure therefor hav ing improved collecting efliciency.

It is a particular object of the present invention to provide such a precipitator and charged-particle collecting structure therefor wher ein the collecting structure has a large collecting surface area per unit of installed space.

A further object is to provide such a device that has simplicity of design and manufacture and is relatively inexpensive, lending itself to the possibility of throw-away charged-particle collecting surfaces.

A further object is to provide an improved chargedparticle collecting structure for a two-stage electrical precipitator which may be operated at high average electrical field strength in the range of 30 kv. per inch.

A further object is to provide an electrical precipitator and charged-particle collecting structure therefor having low ozone-discharge properties.

Another object is to provide an improved chargedparticle collecting structure for an improved two-stage electrical precipitator which may be used to simultaneously collect charged particles and remove ozone formed in the charging section. These and other objects and advantages of the present invention are provided in an electrical precipitator for collecting charged gas-suspended particulate material having a charged-particle collecting structure comprising a pair of spaced gas permeable electrical conductive members with the spaces between said members packed with semi-conductive spheroidal bodies having a range of from about 3 to about 4.5 mm. in diameter and by an electrical precipitator for gas-suspended particulate material which generally comprises a housing through which the gas passes, an electrical charging section extending transversely across the housing adjacent the gas inlet thereto and a charged-particle collecting structure extending transversely across the housing downstream from the charging section, the charged-particle collecting struc ture comprising a pair of spaced generally gas-permeable electrical conductive members with the space between said memberspacked with semiconductive, generallyspherical bodieshaving a range of from about 3 to about 4.5 mm. in diameter. V

The invention will be more particularly described with reference to the illustrative embodiments thereof wherein: FIGJ is a horizontal sectional view of an electrical precipitator constructed in accordance with the teachings of the present invention including aszthe charged-particle collecting structure the improvement ofthe present [invention;

FIG. 2 is an enlarged diagrammatic view illustrating the electrical charge pattern within the improved collecting electrode structure illustrated in FIG. 1; and

FIG. 3 is a chart illustrating the collecting eificiency of the charged-particle collecting structure provided with various diameter collecting surfaces.

Many of the conventional air cleaning electrical precipitators are of the two-stage type and include a particlecharging section followed downstream by a collecting electrode structure having a large surface area for the collecting of the particles charged in the charging section. Typically, such air cleaning two-stage precipitators utilize collecting electrodes of closely spaced parallel plates positioned with the broad surfaces of the collecting plates generally parallel to the direction of movement of the gas stream. In such structures, alternate plates are insulated from ground and connected to a source of high potential to provide the electrical collecting field. While such two-stage electrical gas cleaning precipitators have a relatively large collecting surface area per unit of installed space, there are certain inherent disadvantages in such designs. For example, the spacing and thus the number of collecting plates per unit width is limited by practical consideration of plate alignment tolerances and electrical edge effects which become increasingly important as electrode-spacing is reduced. Further, only one-half of the plate surface area in said structures is effective as a collecting surface and such collecting electrode devices are diflicult to clean.

These inherent disadvantages in conventionally designed collecting electrode structures for two-stage type electrical precipitators are greatly reduced by a packed bed-type collecting cell composed of macroscopic, generally-spherical semi-conductive bodies oriented in a gen erally rhombohedral pattern in the space between a high potential and ground electrode wherein the high potential and ground electrodes comprise gas permeable screens or grids oriented perpendicular to the direction of gas flow downstream of the charging section.

Referring generally to FIG. 1 of the drawings an improved air-cleaning electrical precipitator of the twostage type is illustrated and generally designated 10. The electrical precipitator comprises a shell or housing 12 having a gas inlet 14 at one end and a clean gas exit 16 at the other end. Within the shell 12 and suspended from a wall thereof is a particle-charging section generally designated 18. The charging section 18 may be of any conventional type and as is illustrated in FIG. 1 of the drawings, the charging section includes a frame 20 supported within the shell of the electrical precipitator on

insulators

22 and 24. Insulator 22 is provided with an electrical lead-in 26 having external connection to a source of high potential direct current diagrammatically illustrated at 28.

Preferably the polarity of the high potential direct current applied to the charging section is negative and the positive terminal of the source is grounded. The illustrated charging section comprises a plurality of Wire type electrodes 30 supported on top and bottom from cantilever spring wire supports 31 which are connected to the voltage frame 20. The wires 30 are centered between flat plate ground electrodes 33 which extend from top to bottom of the housing.

Downstream of the charging section 18 is a particle collecting section 32 which comprises a pair of spaced gas permeable screens or

grids

34 and 36. The extended surfaces of the

screens

34 and 36 are oriented perpendicular to the direction 'of gas flow downstream of the charging section. The upstream screen 34 of the collecting section 32 is energized by a high voltage source of electricity of the same polarity as the preceding charging section and thus of the same polarity as the incom- 7 ing charged particles to be removed from the gas stream.

As illustrated in FIG. 1, the upstream screen '34 is suspended within the shell 12 by insulator means 38, one of which is connected to the source of high potential direct current and the downstream screen 36 is connected to ground as illustrated.

The space between the pair of generally parallel spaced screen-

type members

34 and 36 is filled with a plurality of generally-spherical semi-conductive members 40. The particles 40 are in a rhombohedral pattern which normally occurs when spheres are poured at random into a container and agitated.

It has been found that a collecting cell as illustrated in FIG. 1 provides very efficient collecting of charged particles when the spheres 40 have diameters of from about 3 to about 4.5 mm. and preferably in a range of from about 3.5 to about 4 mm. and are constructed of a semiconductive material such as glass. While glass beads or spheres of the stated range provide very efficient collecting surfaces, spheres constructed of plastic or other materials of a semi-conductive nature may be used.

While the illustrated form of the present invention shows a bed of generally spherical bodies, it will be appreciated by those skilled in the art that the bed of bodies may be of the fluidized type wherein the spheres would enter the space between the pair of

screen members

34 and 36 at the top and removed at a controlled rate from the lower end so that removal of the collected particles from the spherical bodies would be a continuous process or the removed spheres could be disposed of and new spheres added at the top of the bed. Further, where the fluidized bed type structure is employed, the spheres removed adjacent the lower end of the charged-particle collecting structure could be passed at a controlled rate through a cleaning agent and returned to the top of the cell in a continuous stream.

Further, as hereinbefore pointed out, the improved bed type collecting cell may be employed to collect or to remove ozone in addition to removing charged particles. The removal of ozone may be accomplished by coating the spheres with an appropriate material such as nickel oxide or manganese dioxide. Where both particulate material and ozone is removed from the air stream, the use of higher voltages in the charging section would be permissible in air cleaning precipitators where the voltages are normally limited by ozone generation.

Referring particularly to FIG. 2 of the drawings, there is illustrated a simplification of the field orientation believed to be the general pattern in packed bed chargedparticle collecting structures. In FIG. 2 the spherical bodies 40 are shown as not touching each other for easier visualization of the field orientation; and the spheres are illustrated as dipoles with the electrical charges aligned in accordance with the direction of the electrical field lines. It will be particularly noted that the direction of the field lines generally designated 42 is such as to attract the incoming charged particles to a collecting sphere. Furthermore, the presence of the spheres increases the electrical field in the interstices or voids to a level sufiiciently higher than the average field of the packed bed.

Also from FIG. 2 of the drawings, it will be seen that probably only one-half of the surface area of the spheres is elfective as a charged-particle collecting surface area as it would be difficult to visualize the total surface area of the bed as an effective collector since the collection of the particles on the back-half of a sphere would require a change in direction of the moving particles opposite to the direction of the gas flow through the packed bed. The assumption that only one-half of the surface area is elfective as a collecting surface is generally borne out by the collecting performance of such charged-particle collecting structures; Even assuming that only onehalf of the total surface area of the spheres in a packed 4 bed are elfective for the collection of charged particles, the total collecting area is. substantially larger than the collecting surface area on a conventional plate type collecting electrode.

In a collecting plate type, two-stage precipitator with 0.17 inch plate-spacing, there is provided a collecting surface area of about 64 square feet per cubic foot 'of installed space. The comparable value for a packed bed of 4 mm. spheres is between 160 and 320 square feet, depending on whether one-half or the total surface area is assumed to be elfective for the collection of charged particles.

The spacing between the pair of confining

screen members

34 and 36 determines the collecting area of the electrode for a given diameter sphere. However, it will be apparent to those skilled in the art that as the depth of the packed bed increases, the pressure drop through the bed also increases and, in general, a bed thickness of from about 0.5 to about 2 inches has been found to be particularly elfective for sphere diameters of from about 3 to about 4.5 mm.

Referring to FIG. 3 there is shown a chart of a packed bed charged-particle collecting structure performance on a pro-charged oil-fume aerosol of 0.7 micron particle size at concentrations in the range of 6 to 7 milligrams per cubic foot. The performance data plotted in FIG. 3 summarizes the performance of the packed bed over a range of sphere sizes from 2 to 8 mm. for two selected bed surface areas of 0.65 and 1.30 square feet. It is pointed out that the plotted efliciencies include only the packed bed efiiciencies and do not include the charging section efficiency which was in the range of about 50%.

In FIG. 3, horizontal broken line 50 illustrates the target efliciency of the bed for overall efliciency of the unit. Curve 52 is for a bed surface area of 0.65 sq. feet with the packed bed collector unenergized; curve 54 is for the same packed bed collector energized at about 25 kv. per inch. Curve 56 is for a bed having a surface area of 1.30 sq. ft. with the bed unenergized, and curve 58 is for a bed having a surface area of' 1.30 sq. ft. energized at about 25 kv. per inch.

The performance data plotted in FIG. 3 indicates two unusual results of packed bed charged-particle collecting structures which may be summarized as follows: (1) there is an optimum bead size in the range of from about 3 to about 4.5 mm. for both the energized and de-energized beds; and (2) the efliciency of a de-energized bed is particularly high and cannot be attributed to any significant mechanical collection which was consistently less than 10% at velocities below 10 ft. per second. The collection efficiency of the de-energized bed is considered to be due to a build-up of an electric charge in the bed which creates the electrical field.

From the foregoing description it will be seen that the present invention fully accomplishes the aims and objects hereinabove set forth and that an improved twostage electrical precipitator and particle-collecting structure is provided. It will also be apparent to those skilled in the art that various modifications may be made in the improved electrical precipitator and charged-particle collecting structure without departing from the scope of the present invention. For example, it may be convenient to employ particle-collecting cells having small cell depths for low voltages or more than one cell in series. Where more than one cell is employed as the particle-collecting means, the serially arranged cells may be energized with alternate polarities or serially arranged energized and de-energized cells may be employed in a single installation.

I claim:

1. A charged-particle collecting structure comprising a pair of spaced gas-permeable members, spheroidal semiccnductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said members.

2.- A charged-particle collecting structure comprising a pair of spaced gas-permeable electrically conductive members, spheroidal semi-conductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said gas-permeable electrically conductive members, and means maintaining a high electrical potential between said members.

3. A charged-particle collecting structure comprising a pair of spaced gas-permeable electrically conductive members, spheroidal glass bodies about 3 to about 4.5 mm. in diameter packed in the space between said electrically conductive members, and means maintaining a high potential between said electrically conductive members.

4. In an electrical precipitator for collecting charged gas-suspended particulate material; a particle collecting structure comprising a pair of spaced gas-permeable members, spheroidal semi-conductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said gas-permeable members,

5. In an electrical precipitator for collecting charged gas-suspended particulate material, a particle collecting structure comprising a pair of spaced gas-permeable electrically conductive members, spheriodal semi-conductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said electrically conductive members, and means maintaining a high potential between said electrically conductive members.

6. An electrical precipitator for gas-suspended particulate matter comprising a housing, a gas inlet and a gas outlet for the passage of gas through said housing an electrical charging section extending transversely across the housing adjacent the gas inlet thereto and a particle collecting structure extending transversely across the housing downstream from said particle charging section, said particle collecting structure comprising a pair of spaced gas-permeable members, spheroidal semi-conductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said electrically conductive members.

7. An electrical precipitator for gas-suspended particulate matter comprising a housing, a gas inlet and a gas outlet for the passage of gas through said housing, an electrical charging section extending transversely across the housing adjacent the gas inlet thereto and a particlecollecting structure extending transversely across the housing downstream from said particle charging section, said particle-collecting structure comprising a pair of spaced gas-permeable electrically conductive members, spheroidal semi-conductive bodies about 3 to about 4.5 mm. in diameter packed in the space between said members, and means for electrically energizing said charging section and said particle-collecting structure at the same polarity.

8. The invention defined in claim 7 wherein said spheroidal semi-conductive bodies comprise glass and the spacing between said gas-permeable electrically conductive members is from about .5 to about 2 inches.

References Cited in the file of this patent UNITED STATES PATENTS 1,001,683 Pur tle Aug. 29, 1911 2,297,601 Williams Sept. 29, 1942 2,847,082 Roos Aug. 12, 1958 FOREIGN PATENTS 96,717 Sweden Sept. 5, 1939