US2244278A - Electrode for electric precipitators - Google Patents

Description

J'une3, 1941. H. J. WHITE ELECTRODE FOR ELECTRIC PRECIPITATORS Filed March 1', 1940 m-m gy 316% I I I mom.

. the precipitator.

' NITED STATES PATENT OFFICE,-

memoirs non Emornm f rancrrmroas Harry J. White, Los iingeles, Calilfi, assignor to Research Corporation, New York, N.

poration of New York Application March 1, 1940,- Serial No. 321,784

(on. i1837) g omces and the like; and in this type of use it is Claims.

The present invention is generally concerned with the art ofelectrically precipitating suspended particles from a stream of gas; and it is more particularly concerned with improvements in electrodes for precipitators of the type inwhich the particle-laden gas is first passed through a charging field in which corona dis-- charge is produced, in order to electrically charge the suspended particles, and is then passed through a spatially separate electrostatic precipitating field which is substantially free from corona discharge, in order to precipitate the' The gas is subjected succes-.

charged particles. sively to two functionally distinct fields, and for this reason a precipitator of this type has been termed a two-stage or separated field" precipitator.

In the following description and appended Y 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 sufiiciently 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 sufficiently 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 sparkover. For this purpose, anon-discharging now coming into extensive use for the purpose of cleaning air that is used for ventilation in homes,

Y., a cornow become a factor of very considerable importance in designing an electrical precipitator for cleaning ventilating air, because both these gases are distinctly irritating to many people, and, accordingto some medical authorities, ozone may be harmful in excessive concentrations The permissible upper limit of ozone concentration has not been definitely established although various arbitrary limits have been established at approximately one part per 10 million parts of air, a concentration that has been greatly exceeded by precipitators of conventional type used for industrial purposes. Since ozone is the chief product of interest, it will be particularly referred to but it will be understood that reference toozone includes also the oxides of nitrogen since conditions which bring about a decrease in the production of ozone also reduce the production of oxides of nitrogen.

Ozone is generated in the field of corona discharge at the discharge electrode. It has been shown experimentally that the rate of productrode, and it decreases rapidly with decreasing electrode diameter for electrodes smaller than about 60 mils; Consequently all of these factors 4o electrode is usually one of extended surfacearea,

should be kept 'at a minimum in order to reduce the rate of ozone production; but it is to be kept in mind that various other factors also influence the net rate of production, such as applied voltage, ozone concentration, configuration of electrodes, etc.

In separated field precipitators of the tubular typ one form of precipitator has comprised a tube through which the gas streamflows and which acts as a non-discharging electrode that opposes a central electrode placed co-axially of the tube, the central electrode having both dis-- charge and non-discharging sections. The upper section of the central electrode is generally But electric precipitators are cylindrical of relatively large diameter to prevent discharge therefrom, while the lower section-is a wire or'rod of relatively small diameter that acts .ssra discharge electrode and around which the corona discharge takes place. A precipitator of application filed July 6, 1938, Serial No. 217.748

entitled "Apparatus and method for cleaning gases," now Pat. No. 2,192,249 issued Mar. 5, 1940.

A central electrode structure of the conventional type described usually has an upper nondischarging section of a diameter of two inches or more, while the lower discharge section is usually of the order of 50 mils or less in diameter. The two sections are Joined directly together and consequently the fields set up by each portion of the electrode are to some extent super-imposed upon. each other. This physical overlapping of the electric fields of the nonedischarging section and the discharge section of the electrode creates conditions which tend to increase the amount of ozone generated by this conventional type of electrode structure.

The very close proximity of the two adjoining electrodes causes the electric field in the neighborhood of the discharge electrode to be reduced and consequently the corona discharge current at a given applied potential is reduced, that is, the non-discharging electrode partially shields, in an electrical sense, the discharge electrode and thereby reduces the corona current from it. To increase the corona current to the desired value for charging suspended particles, the applied potential must be increased. But a more important result of the electrical shielding 'efiect is that the shielding increases the rate of ozone production per unit corona current. The net result is that in the conventional electrode struc-- ture, the rate of ozone production is comparable to the production rate of an unshielded discharge electrode of larger diameter, and the benefits of small diameter electrodes are impossible of full realization.

It thus becomes a general object of my invention to provide a discharge electrode with a minimum rate of ozone production.

It is also an object of my invention to provide an electrode structure comprising both discharge and non-discharging sections in which the discharge electrode is supported from the non-dismeasured in a plane transverse to the direction of gas flow, as a member of this length removes the discharge electrode from the influence of the non-discharging field sufiiciently that there is no material shielding effect or reduction in corona discharge. description of a preferred embodiment of my invention I show and describe an electrode structure having a cylindrical non-discharging section, it will be understood that my invention is not necessarily limited thereto; and it is within the scope of my invention to use non-discharging electrodes that are not circular in crosssection, as forexample, oval, and in'this case the minimum diameter in a plane transverse to the direction of gas flow determines the spacing between the discharge electrode and the nondischarging electrode.

According to the present invention, this intermediate supporting member is made of metal or other electrically conducting material, which construction ofiers important advantages from the standpoint of economy and cost of manufacture as well as durability of the structure. Reference should also be made to my co-pending application entitled Electrode for electric precipitator" filed concurrently herewith in which is described and claimed an intermediate member embodying non-conducting-or dielectric material; and the present application contains generic claims as well as specific claims to a metallic member.

How the above objects and advantages of my invention, as well as otherobjects and advantages not'specifically mentioned, are attained, will become more readily apparent by reference to the following description and the annexed drawing, in which:

Fig. 1 is a vertical median section through a simplified form of tubular, separated field precipitator with an electrode constructed accordvertical section, of the lower end of a preferred charging electrode of relatively large diameter in such a way that the discharge electrode is substantially uninfiuenced by the electric field produced by the non-discharging electrode.

A further object of my invention is to provide an electrode'structure comprising both discharge and non-discharging sections in which the discharge electrode is a very fine wire.

These objects have been accomplished in my invention by spacing the discharge electrode from the'non-discharging electrode in the direction of gas fiow by means of an intermediate member which supports the discharge electrode on the non-dischargingelectrode. Thisconstructionprovides a unitaryelectrode structure around which the gas fiow is, in general, parallel to the electrode surfaces. In a preferred embodiment of my invention adapted to--upward gas fiowfa cylindrical intermediatemember is attached at its upper end to a cylindrical non-discharging electrode of relatively large diameter and at its lower end carries the discharge electrode which is a very fine wire. The intermediate member may be made of metal and is of such character that no corona discharge is produced around it, but it ismaterially smaller in diameter than the nondischarging electrode so that the field around it does not appreciably influence the action'of the discharge electrode by reducing corona discharge,

means in the form of he form of central electrode structure:

Fig. 3 is a side elevation of a modified form of intermediate member with a spiral discharge electrode;

I Fig. 4 is a side elevation of'another modified form of discharge electrode and support therefor; and

Fig. 5 is a plan view of another modified form of discharge electrode and support therefor.

There is shown in Fig. 1 a tubular type or precipitator comprising a single tube III of substantially circular cross-section and preferably of uniform diameter, or nearly so, throughout its entire length. At the lower end of tube II is gas inlet II which is connected to a conduit 12 at communicates with some source, 'not shown, of gas containing dust or other suspended particles to be removed. Gas flows upwardly within tube I l, and at the upper end of tube It is gas outlet means in the form of housing I 'thatis connectedto conduit Ii which either may open to the atmosphere or may communicate with some space to which it is desired to conduct the cleaned gas. Although a diameter of the non-discharging electrode, 7

uniform diameter of tube In is not necessary. yet.- it is much preferredbecause it is much easier and less expensive to make a straight tube of uniform diameter throughout its entire length than to construct, a tube with varying diameter or with' two or more diiferent diameters.

The rigid central electrode, generally indicated at H, is mounted at its upper end on insulator Although in the subsequent bushing l8 which passes'through an opening in the top wall of housing ll, the opening being slightly larger in diameter than the bushing.

v 3 meter of rod 33 should not greatly exceed the minimum diameter, as above described; and for this reason the maximumdiameter of rod 33 will ordinarily not exceed about one-half inch.

The diameter of sleeve 28 is considerably larger than that of rod 33, and usually is of the .order of 2" to 4", though the size depends on the diameter of tube It and a larger-or smaller diameter for the electrode can be used asmay be the extent of the clearance between bushing I8 and the top wall of housing II, and the electrode also can be moved angularly, as a result of the compressibility of gasket 2|, by tightening selected ones of bolts 22 which hold the central electrode and ring in place.

Although electrode il may be constructed in other ways than shown and described here, the present form illustrates the general principles of construction as well as certain preferred details. Electrode l'l comprises a hemispherical body 25 into the upper side of which vertically extending rod 26 is screwed and held in position by lockout 21, as may be seen in Fig. 2. The hemisphere 25 closes the lower end of cylindrical sleeve 28, the

found suitable under circumstances encountered.

'The diameter of sleeve 28 serves as a measure of the distance from the electrode that the nondischarging field is particularly .active and beyond which it is desired to place discharge electrode-'35. The lines of force in the electric field maintained between electrodes 28 and Ill, do not extend radially outward from the bottom of hemisphere 25 for a long distance, but soon bend toward the opposing electrode l0. For this reason, the field strength decreases rapidly in the direction of the axis of sleeve 28, and beyond about one diameter from member 25 in the direction of gas fiow the field is so reduced in strength that it exerts relatively little adverse two members being of the same diameter to provide a' smooth joint. The upper end of hemisphere 25 is preferably provided with a shoulder 2511, as shown in Fig. 2, in order to hold cylinder 28 and hemisphere 25 in proper position with respect to each other. The upper end of sleeve 23 rests against the lower face of insulator bushing I8, which also may have a shoulder to position the cylinder. Central rod 23 passes upwardly through the insulator, and locknuts 30 are screwed on to the upper end of rod .28 to hold the parts assembled in the position-shown in Fig. 1. Hemisphere 25 serves to eliminate corners or edges that would produce corona discharge from sleeve 28.

Into the lower end of hemisphere 25, there is.

screwed rod-like intermediate member 83 which is co-axial with sleeve 28 and carries at its lower end a very fine wire which serves as a discharge electrode 35. The wire is preferably secured to rod 33 by drilling an axially extending hole in the lower end of rod 33, inserting wire 35 in the drilled hole, and then filling the hole with metallic solder.

From the standpoint of function, wire 35 is a dischargeelectrode, and sleeve 28 together with hemisphere 25 is a non-discharging electrode, the two electrodes being separated by an intermediate member 33 which has no function as an electrode. From the standpoint of construction. these same parts are but sections of a single, continuous electrode structure. I

Electrode support 33 is a metallic rod-and is preferably cylindrical in order to eliminate any portions of sharp surface curvature which might produce corona discharge. The largest diameter of the rod which will produce corona discharge depends on various factors, butfor any given set of conditions it can be determined and it is preferable that the minimum diameter actually used be somewhat in excess of this diameter, as by say 25%, in order to provide some factor of actual use, rod 33 will have a diameter of not less than about one-fourth inch. Since it is desirable that the fleld exerted by intermediate member 33 be kept at a minimum, the maximum dieffect on a discharge electrode. In order to be definitely in a position at which the discharge electrode is substantially unaffected from a practical standpoint by the non-discharging field, it is preferred that the length of the intermediate section 33 between hemisphere 25 and the upper end of electrode 35, be approximately oneand one-half times the diameter of sleeve 28. It has been found by experiment that this length should not be less than the diameter of sleeve 28, andthat little advantage is gained by increasing it to more than twice the diameter of sleeve 28. It

. is not to be inferred that there is necessarily no safety. As typical of a diameter satisfactory in part of the field from the non-discharging electrode in this vicinity, at or near the axis of tube to, where the discharge electrode is placed; but ratherthat the existing field is so weakened because of the path followed by the lines of force that the field exerts substantially no diminishing influence on the corona discharge.

With the foregoing description of a specific non-discharging section in mind, it will be understood that the non-discharging electrode section may be non-circular in cross-section, e. g. oval or elongated. In this case, the minimum diameter, as measured in a plane transverse to the gas flow, is used to determine the length of intermediate member 33in the same manner as already described.

Discharge electrode 35 may be described as a very fine wire, which term is used to include a wire of such small diameter that a length equal to about one and one-half or two times the diameter of sleeve 28 is not 'sumciently rigid, to project from the non-discharging electrode without support at its lower end, as in the conventional construction described. From a quantitative standpoint, a wire of the order of about 50 mils or larger possesses this amount of rigidity; buta wire of a diameter substantially less than 50mils is so flexible that, when say three or four inches long and supported only at the upper end; the gas stream blows it away from the axis of tube ID, with a resultant uneven distribution of the discharge and frequently a sparkover. Of course the flexibility of a piece of wire depends on the physical characteristics of the particular wire, as well as the ratio of length to diameter, so that no hard and fast limits can be set to the diameter, and my invention is not limited to any is of sufiicient length in the direction of small diameter to minimize ozone, and preferably use a-discharge electrode of approximately 5 to mils-diameter and of a length to project beyond the lower end of electrode support 33 for a distance oi'about one-half inch.

length gives an adequate amount of corona discharge when substantially free from the influence of the non-discharging electrode section. In general, it is desirable to keep both the diameter and the length of electrode 35 as small as possible in order to minimize the production of ozone; and the length is also then kept to a mini- It has been found by experiment that an electrode of this mum in order to keep the electrode sufliciently rigid that it need be supported at only one end.

It is preferred that wire electrode 35 be made of tungsten or other refractory material having similar characteristics. Another suitable metal is tantalum. Materials of this character are used in order that the electrode may not be completely destroyed in the event of a sparkover from the electrode to opposing electrode l0 because, when such a sparkover occurs, the heat generated is sufficient to melt the more common metals In operation, a high potential is applied to the central electrode by conductor 36 which is connected to the upper end of rod 26 and is ordinarily connected to the negative terminal of a suitable source of uni-directional current at high potential, such as a transformer and mechanical rectifier or like equipment familiar to persons skilled in the art, adapted to create a high potential between the central electrode and tube l0 which is suitably grounded, as indicated diagrammatically at 31. Instead, tube III maybe connected to the positive terminal of the power source. Fine wire 35 and-the surrounding section of the opposing electrode l0 formone pair of electrodes; and the high potential applied to the discharge electrode is suflicient to create coron'a discharge at and in the vicinity of discharge electrode 35, but is not sufiicient to cause arcing or disruptive discharge between the two electrodes of this pair. The upper non-discharging electrode section formed by sleeve 28 and hemisphere 25, and the surrounding portion of opposing electrode ll form a second electrode pair; and the applied high potential maintains between these two electrodes a non-discharging electric field.

The gas stream enters housing ll through conduit l2 and flows upwardly within tube Ill. The gas stream first passes through the ionizing or charging field surrounding discharge electrode 35 when the suspended particles in the gas stream become electrically charged with the same polarity as the discharge electrode. The gas next passes through the non-dischar8 ns field around the non-discharging electrode 28; and thisfield as flow to effect precipitation of substantially all the charged particles entering the field.v These precipitated. particles collect on the interior wall of tube It. The precipitated particles maybe removed ii'rom tube ID by any suitable means, as, for example, by mechanically rapping the tube with suitable hammers. which are not shown as they are familiar to those skilled in. the art.

According to my invention, discharge section II andnon-discharging section 28 of the central electrode H are separated by the length of the intermediate section 33, thus removing the discharge electrode and the ionizing field around'it from active or substantial influence by the nondischarging field maintained around electrode 28.

Because of this spacing between electrodes, the discharge electrode functions with maximum efflciency, i, e., it maintains a given corona discharge or corona current flowing to the opposing electrode ll with a minimum current consumption and at a lower voltage than is possible when the discharge electrode immediately adjoins the non-discharging section. My improved construction permits the use of a shorter discharge electrode and operation with lower voltage and less current; and in the preferred form described the length of discharge electrode is reduced to a minimum consistent with minimum charging of particles in the gas stream. All of these factors control the production of ozone, and when reduced to a minimum, as they are with my improved type of discharge electrode, the production of ozone is substantially reduced compared with production rates of conventional types of electrodes. An electrode constructed according to my invention can keep ozone concentration at about one part per hundred million parts of air, even less. A further advantage is the economy of operation resulting from decreased power consumption.

When it is desired to have a longer dischargeelectrode to provide a longer charging field, it is necessary that a wire as small as the wire used here be supported at short intervals, and such an embodiment is shown in Fig. 3. In this form, the intermediate electrode section 33a that supports discharge electrode 35a, is the same as described except that it is longer and is provided with four short radial arms 40 on which the wire electrode 35a is mounted. Arms 4| are both angularly and axially spaced to hold the discharge but allowing a higher gas velocity through the charging field. The higher gas velocity to some extent oifsets a higher rate of ozone production, as the ozone concentration is inversely proportional to the rate of flow; but even without a higher gas velocity, the rate of ozone production is still so low as to be well under generally accepted tolerance limits, because the full advantage of a very small diameter electrode is secured. One

factor contributing to the high charging efliciency' of the spiral electrode is that the S pporting member as is inside the spiral and does not appreciably shield the electrode with respect to the opposing electrode III, either by'the rod 33a setting up a field or by being interposed between the wire and; electrode ll.

Fig. 4 illustrates another embodiment designed to provide a longer charging field, and having much the same characteristics as the form of Fig. 3. Here, intermediate section 33b supports a plurality. of lengths of discharge electrode 35b; and though i'our wires 35b are here shown, a larger or smaller member may be used. Each electrode wire 35b is held in a vertical position by a radial arm ll on rod 3312 at either end of the wires. preferably sufliciently long that the closest part The intermediate member 331) is.

of *e'ach. discharge electrode 35b is spaced from the non-discharging electrode by at least one and one-half times the diameter of the latter. It will be appreciated that in the forms of Figs. 3 and 4 the minimum spacing from the nondischarging electrode is not so critical since with. the longer electrode the lower end is well beyond the minimum spacing and therefore more certain to be substantially unaffected by the nondischarging field. t

Fig. 5 shows an inverted plan view of a modified form in which the discharge electrode 350' is in the shape of a circle mountedon three spaced radial arms 42 at the lower end of inter mediate member 330.;

it will be understood from the foregoing description that various changes in the form and arrangement of parts may be made without departing from the spirit and scope of my invention and that the specific embodiments disclosed are but typical and illustrative forms of the invention. Consequently, it is to be understood that the foregoing description is to be construed as illustrative of, rather than limitative upon the claims appended hereto.

-I claim:

1. An electrode for a separated field type of electrical precipitator comprising a cylindrical non-discharging electrode section terminating in a rounded portion, the entire surface of said section-having a radius of curvaturenot sub.- stantially less than about one inch; an intermediate section carried by said non-discharging section comprising a rod-like member extending from said non-discharging, section to a distance at leastequal to the smallest diameter of said non-discharging section, the entire surface of said intermediate section having a radius of curvature not substantially less than about one- I eighth inch;- and a very fine wire discharge said electrode sections being axial. e

3. An electrode for a separated field type of electrical precipitatorcomprising a cylindrical non-discharging electrode section terminating in a rounded portion, the entire surface of said section having a radius of curvature not subsubstantially costantially less than about one inch; an intermediate section carried by said'non-discharging section comprising a rod-like member extending from saidnon-discharging section to, a distance diameter of the non-discharging section, said discharge electrode section comprising a wire of about five to fifteen mils diameter.

4. An electrode for a. separated field type of electrical precipitator comprising a' cylindrical non-discharging electrode section terminating-in a rounded portion, the entire surface of said section having a radius of curvature not substantially less than about one inch; an intermediate section. carried-by said non-discharging section comprising a rod-like member extending from said non-dischargingsection to a distance at least equal to the smallest diameter of said. nondischarging section, the entire surface of said intermediate section having a radius of curvature not substantially less than about one-eighth inch; and a very fine wire discharge electrode section in the shape of a spiral mounted on the intermediate section and spaced from the nondischarging section ata distanceat least equal to the smallest diameter of the non-discharging section.

' 5. An electrode for a separated field type of section at a distance at least equal to the smallest diameter of the non-discharging section.

2. An electrode for a separated field -type of a rounded portion, the entire surface of said section having a radius of curvature not substantially lessthan about one inch; an intermediate section carried by said non-discharging section comprising a rod-like member-extending from said non-dischargingsection to a distance at least equal to. the smallest diameter of said non-discharging section, the entire surface of said intermediate section having a radius of electrical precipitator comprising a cylindrical non-discharging electrode. section terminating in curvature not substantially less than-about oneeighth inch; and a very fine wire discharge electrode section mounted on the intermediate diameter of the non-discharging section, all of electrical precipitator comprising a cylindrical non-discharging electrode section terminating in a rounded portion, the entire surface of said .section having a radius of curvature not subelectrode section in'the shape of a circle mounted on the intermediate section and spaced from the non-discharging section at a distance at least equal to the smallest diameter of the non-discharging section.

v HARRY J. WHITE.

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