US2244279A - Electrode for electric precipitators - Google Patents

Description

June 3, 194i. H. J. WHITE 2,244,279 ELECTRODE FOR ELECTRIC PREIPITATORS Filed March 1, 1940 N VENT OR.

ATToNEY;

'the precipitator. now coming into extensive use for the purpose of cleaning, air that is used for ventilation inA atented June 3i i EIEUIBJODE 170B ELECTRIC PRECIPITATUBS Harry J. White, Los Angeles, Calif., assigner to Research Corporation, New York, N. Y., a corporation of New York Application Maren ri, 1940, sensi' No, 321,185

(ci. iss-v) 8Clalms.

'/l'he'present invention is generally concerned with the art of electrically precipitating susgpended particles from a .stream of gas; and it -is more particularly concerned with improvements in electrodes for precipitators of the type in which the particle-laden gas. is iirst passed l 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 iield which is substantially free from corona discharge, in order to precipitate the charged particles. The gas is subjected successively to two functionally distinct elds, and for thisreason a precipitator of this type has been termed a precipitator.

"two-stage or separated field` 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 coniiguration that establishes a sufliciently high potential gradient at or near its surface to create corona discharge before there is a disruptive discharge or sparkover. pose, 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 af sufii'ciently high electric field intensity'to cause ionization and corona discharge. non-'discharging electrode" will be understood to designate`an electrode that minimizes Vor prevents corona discharge therefrom because it has a conguration that establishes a suiliciently low eld 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, a riondischarging electrode is usually one of extended surface area, substantially free from sharp corners or other partsV of sharp surface curvature at all portions which are located within the elec'- tric field, so as to substantially avoid ionization or corona discharge atvtliat electrode.

`Electric precipitators of lvarious types have in the pastbeen used chiefly for industrial purposes, and in thisfleld it has beenfimmaterlal that small amounts of ozone or oxides o f nitrogen are formed by the corona discharge and introduced into the stream of gas iiowing through But electric precipitators are homes, ofces and the like; and in this type of useit is of prime importance that the precipitator not produce substances that are objectionable because of .their odor or possible dangerI to For this pur-v oxides of nitrogen, formerly of little signicance,

has now become a factor of very considerable importance in designing an electrical precipitator for cleaning Ventilating air, because bothl these gases are distinctly irritating to many people, and, according to some medical jauthorities,

ozone may be harmful in excessive concentrations. The permissible upper limit of ozone concentration has not been deiinitely established although various3 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 vozone is the chief product of interest,`it will be particularly referred'to, but it Iwill be understood that reference to ozone includes also the oxides of nitrogen since conditions/which bring about a decrease in the production of vozone also reduce the' production of oxides of nitrogen, K

Ozone is generated in the eld of corona discharge at the discharge electrode. It has been shown experimentally that the rate of production is roughly proportional to the current consumed and to thelength of the discharge elec.- trode, and it decreasesrapidly with decreasing Aelectrode diameter for electrodes smaller than about 60 mils. Consequently all of these -factors should be kept at a minimum in order to reduce the rate of ozone production; but it "is to be kept inl mind that various other factors also inuence the net rate of production, such as. applied voltage, vozone concentration, configur-ation of electrodes, etc,

iin

In separated eld precipitators of the tubular" type, one form of precipitator has comprised 4a tube through which the gas stream ows and which acts as a non-discharging electrode that opposes a central electrode placed co-axially of the tube, the central electrode having both discharge and non-discharging sections. The upper section of the central electrode is generally cylindrical of relatively large diameter to prevent discharge therefrom, while the lower section is a wire or rod of relatively small diameter that acts as a discharge electrode and around which the corona discharge takes place. A precipitator of this construction is disclosed in my co-pendhealth. Consequently, production of ozone and .60 tion of the electrode are to some extent superimposedv upon each other. This physical overlapping of the electric fields of the non-discharging sectionk'andlzggdischarge section oi the electrode creteszonditions which tend to increase the amount of ozone generated by this conven tional type of.\electrode structure.

'I'he very close proximity of the two adjoining electrodes causes the electric field inithe neighdischarging electrode. i l

According to the present invention, this in-` .termediate supporting member is made of elec-` 5 `rijrically non-conducting material, as for example. iglass or porcelain, and has a metallic conductor;

(discharge electrode with the non between the discharge electrode and the non- `running lengthwise to electrically connect th l borhood 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 currentfrom it. To increase the corona current to the desired relectrode. Although 4this conductor may be a lli/very fine wire which is a continuation of the value for charging suspended particles, the ap plied potential must be increased. But a more important result ofthe electrical shielding effect is that the shielding increases the rate oi ozcne production per unit corona current. The net\result Ais that in the conventional electrode structure, the rate of ozone production is comparable to the production rate of an unshielded discharge electrode of larger diameter, and the benefits of-smail diameter electrodes are impossible oflfull 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. theA non-discharging electrode of relatively large d- `\ameter in such va. way that the discharge elec-- trode is substantially uninuenced by'the electric ileld produced by the non-discharging electrode. r

A further object of my invention is to Vprovide an velectrode structurecom rising both discharge and non-discharging sec in which the discharge electrode is a verf fine wire.

'Ihese objects havebeen accomplished in my invention by spacing the" discharge electrode from the non-discharging electrode in the diintermediate member.

rection of gas flow by meansof an intermediate member which supports the discharge electrode on the non-discharging electrode.. This construction provides a unitary electrode structureA around whichthe gas flow is, in general, parallel to the electrode surfaces. Ina preferredem bodiment ofwmy invention adaptedto upward gas ilow. an intermediate member is attached at its`upper'end to a cylindric non-discharging electrode of relatively large., ameter and at its lower end carries the discharge electrode which is a've fine wire.` The minimum length of the intermediate electrode is-approximately equal to the diameter of the non-discharging electrode, measured -in a plane transverse to the direction ofgas flow, as a memberof this length removes the discharge electrode from the .influence` of the non-discharging field sumciently that there is nc material shielding effector reduction in coronapdischarge. Althou h inthe subsequent description of a preferr embodiment of my invention I show and-describe an electrode structure having a cylindrical non-discharging scction-it will be understood that my invention is not necessarily limited thereto; 'and 'it is within the scope of my invention to use non-discharg'.

Y of unifqrm 'discharge-electrode, the conductor is preferably substantially larger in order to impart some mechanical strength to the intermediate member. In either case the thickness of electrically non-conducting material should be suillclent to prevent production of corona discharge from the In the present application,.this type of in vtermediate member is described and claimed: but

Fig." 1 is a vertical section through a simplified form of tubular, separated iield precipitato; with an electrode constructed according to my invention;

Fig. 2 is a fragmentary elevation, partially in vertical section, of the lower endof a. preferred form ofl central electrode structure;

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

Fig. 4 is a side elevation of another modied formof discharge electrode and support therefor; and

Fig. 5 is 'a 'plan view of another modied forml of discharge electrode and support therefor.

` There is shown in'FigA 1 a tubular type of precipitator comprising a single tube I Il of substantially circular cross-section, and preferably diameter; or nearly so, throughout its entire length. At the lower end of 'tube Il 1S gas inlet means in the form of housing II which is connected to a conduit I2 that communicates with some source, not shown, of gas containing` dust or other suspended particles to be removed.

. At the upper end of tube Il isgas outlet means ing electrodes' that are not circular in crosssection, as-for example, oval, and in this case the minimum diameter in'a plane transverse to the direction of gas ilow determines thespacing ixrthe form of housing I4 that is connected to conduitll which either may open to the atmos phere or may communicate with some spaccato wmcnrit is desired te conduit the cleaned sus.

Although a uniform diameter for 'tube il is not necessary. yet it is much preferred, because it is much easier and less expensive to makea straight its entire tube of uniform diameter thro length thanto construct a tube th varying diameter or two or more different diameters.

The rigid central electrode, generally indicated at Il, is mounted at` its upper end; on insulator bushing I8 which passes im opening in the top' wall o'f 'housing' Il, the opening being slightly larger in diameter thanthe bushing. The bushing and electrode assembly are sup- .POrtedon the top wall of housing Il by anged phere 25.

' the central electrode can be moved laterally to the extent of the clearance between bushing i8 and the top wall of housing i4, and the electrode jaaa/'rave also can be moved angularly as the result of the compressibility of gasket 2i, by tightening selected ones of bolts 22 which hold the central electrode and ring Zll'in place. v

vAlthough ectrode H. may-be constructed in other ways than shown and described here, the present form illustrates the general principles of construction as well ascertain preferredv details. Electrode i7 comprises ahemispherical body 25. into the upper side of which vertically extending rod 25 is screwed and held in position by locknut 27, as may be seen in Fig. 2. The hemisphere 25 closes the lower end of cylindrical sleeve 28, the two members being of the same diameter to provide a smooth joint. The upper end of hemisphere 25 is preferably provided'with Aa shoulder From a, standpoint of function, wire es n discharge electrode, ando sleeve 28 together with himisphere 25 is a nondischarging electrode, the

two electrodes being separated by an intermediate member 3| 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'he diameter of sleeve 28 is considerably larger than that of intermediate member 3|, and is usually ofthe orderof 2" to 4", though the size depends on the diameter of tube i0 and a larger or smaller diameter for the non-discharging electrode can be used as may be found suitable under circumstances encountered. The diameter of sleeve 28 serves as a measure of the distance from the electrode that the non-discharging eld is ,particularly active and beyondA which it is desired to place discharge electrode 35. The lines of force in the electric eld maintained between electrodes 28 and It do not extend radially outward from the bottom -of hemisphere for along e distance, but soon bend toward the opposing elec- 25a. shown in 2, in order tohold cylinder 28 and hemisphere 25 in proper position with re spect to each other. The upper end of sleeve 28 rests against the lower -face of insulator bushing i8, which also may have a vshoulder to position the cylinder. Rod 26 passes upwardly through the insulator, and locknuts 3d are screwed .on to the upper end of rod 26 and 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.

At the lower end of hemisphere 25, there is attached the intermediate member indicated generally at 3i which extends vertically and is substantially co-axial with sleeve 28 and electrode be' less than the diametervof sleeve 2B, and thatv 'tube' i0. Intermediate member 3i comprises a.

ternal metallic electrical conductor 33 and a metallic base member 3d which is threaded to provide a convenient means for attaching the intermediate member to the lower end of hemis- For structural reasons, it is preferred that conductor 33 be about .O3-.10 inch in dialn-v eter, a typical size usedbeng .05 inch diameter, as a wire of this size reinforces the non=conduct ing body 32 and provides a strong and conveninet meansrfor attaching non-c nducting rod 32 to metallic base 3d. `Base 3d provided at its lower end with an axially extending hole'in which the upper endof conductor 33 is inserted, then the hole is lled with metallic solder to securely join the two members together with an electrically conducting joint. To the lower end oi' conductor 33, there is attached a very fine wire which serves as a discharge electrode 35. Then there is applied fused dielectric material 32 over the entire length of conductor 33 and a short portion of the ieri-ed since, without being 'unnecessarily large, y it imparts desirable mechanical strength to the standpoint, a wire oi the order of aboutY .intermediate member 3i and is suiciently rigidv lthat no elaborate means are necessary to secure a tight load-carrying connection between dielectric 32 and metallic base 3d.

-diameter of approximately 1/8"1%;".

trade i0. For this reason, the eld strength decreases rapidly in the directionof the axis of sleeve 28, and beyond about one diameter from member 25 the iield is 'so reduced in strength that it exerts relatively little adverse eiect'on a discharge electrode. In order to be deiinitelyjn a position at which the discharge electrode is substantially -unaffected from a practical standpoint by the non-discharging iield, i preferredthat the length of the intermediate section 3i between hemisphere 25 and the upper end of discharge -`electrode 35, be approximately one and one-half times the diameter of sleeve 28. It has been found by experiment that this length'should not little advantage is gained by increasing it to more thantwice the diameter of sleeve 28. It is not to be inferred that there is no part of the nondischarging field in this vicinity, at or near the axis of the tube it, where the discharge electrode is placed; but rather that the existing iield is so weakened because of the path followed by the lines of force that the iield exerts substantially no diminishing induence on the corona discharge.

Typically, dielectric rod 32 has an external A rod of this diameter provides a sufiicient thickness of dielectric to prevent any corona discharge :from` conductor 33, and there is no particular advantage to be gained by increasing the thickness beyond an amount sumcient for this purpose. In general, the diameter of dielectric rod 32will be as small as possible consistent with adequate insulation and structural strength. Provision o the dielectric body around conductor -33 "has the.

particular advantage that corona discharge from this conductor is prevented andthat this conductor has no appreciable shielding effect on the .corona dicharge from electrode 35;

' Discharge electrode 35 may be described as a very fine' wire, which term is used to includea wire of such small diameter that a lengthuequal to about one and one-half or two times the ameter of sleevev 2S is not sufciently rigid to pro-v ject from the non-discharging electrode without support at its lower end, as in the conventional construction described. From a quantitative mils or largerpossesses this amount of `rigi 'l a wire of a diameter substantially less than' y50 mils is so iiexible 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 III, with resultant unevenidistribution of the discharge and frequently a sparkover.

Of course (the flexibility cfa piece of wire del limited to any specic size of wire. In practice,

l'. use a very small diameter to minimize ozone, and preferably use a discharge electrode approxe imately 5. to `15 mils diameter and of a lengthto project beyond the lower end of electrode support 3l for a distance of about one half inch. It has beenjfound by experiment that an electrode of this Vlength gives an adequate amount of corona discharge vwhen lsubstantially free from the influence of the non-discharging electrode section. In general, it isdesirable 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 has also been kept to a minimum in order to keep the electrode suillciently rigid that it need be supported at only one end.

It is preferred that wire electrode 35 be made of tungsten, tantalum'orother material having similar characteristics. 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 Ui because, when such a sparkover occurs, the temperature generated is sufficient to melt the more common metals.

In operation, a high potential is supplied to the central electrode I1 by conductor 36 which is connected to the upper end of rod 26 and is ordinarily connected to the negative terminal of a suitablesource of unidirectional Vcurrent at high potential, such as a transformer and mechanical rectier or like equipment familiar to persons skilled in the art, adapted toereate a high potential betweenthe central electrode and tube lil which is suitably grounded, as indicated diagrammatically at 31. Instead, tube l! may be connected -to the positive Vterminal of the.

power source. Fine wire 35 and the surrounding section ofthe opposing electrode i form one pair of electrodes; andthe high potential applied to thedischarge electrode is sufficient to create corona discharge at and in the vicinity of discharge electrode Il, but is not suiiicientf-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 2l, and the surrounding' portion of opposing electrodel form a second electrode pair: and the applied high potential maintains between these 4two electrodes a non-discharging electric eld.

The gas stream enters houing lvl through con. duit I2 and passes upwardly within tube Il. The gas stream rst passes through the ionizing or are not shown as they are familiar to those skilled in the art.

According to my invention, discharge section ,35 and non-discharging section. of the central electrode I1 -are separated by the length of inter- Vmediate section 3|, thus. removing the discharge electrode and the ionizing field around it from active or substantial iniluenceby the non-discharging eldmaintained around electrode Il.

I Because of this spacing betweenelectrodes, the

discharge electrode functions with maximum eiliciency, i. e-it maintains a given ,corona discharge or corona'currnt flowing to the opposing electrode il at a lower voltage than is possible when the discharge electrode immediately adioins the non-discharging section. My improved construction permits the use of a shorter discharge electrode and operation with lower voltage' and in the preferred form described, the len h of discharg\electrod e is reduced to ,ja minimum consistent with suitable charging of the particles in the gas stream. All of these factors control the production of ozone, and when reduced to a minimum, as th are with my improved type of "discharge electrode,vthe production of ozone is substantially reduced compared with 'production rates `of conventionalv types ofelectrodes; and

with an electrode\constructed according to my invention, the ozone concentrati'ois about one part per hundred million part's of air, or even less. E

When it is desired: have a longer discharge electrode to `provide a i er. chargingfield, it is necessary that a wire asgrmall asthe wire used here vbe supported at both-ends, and such an embodiment is shown \in Fig. 3.V Inthis form, the intermediate electrode section Ila that supports discharge electrode 35a is the same as cept that it is longer and previously described, is provided with four short radial arms III on Y whichthe wire electrode a is mounted. Arms charging ileld discharge electrode II and in this field the suspended particles in the gas stream become electrically charged with the same polarity Aas the-discharge electrode,A The gas next passes through the non-discharging field around the non-discharging electrode 2 8;

and'this eld is of sumcient length in the direction of gas iow to effect precipitation of substantially all the charged particles entering the ileld. These precipitated particles collect on the` interior walls of'tube i0. l The precipitated particles may beV removed Afrom tube l0 by any suit- I0 are both angularly and axially spaced to-hold the discharge electrode'in the 'tormof a spiral of one complete turn. Arms ll are made'of dielectric and are attached to the Vbody of electrically non-conducting material 12a, although they may be reinforced if desired by providini! inte al metallic members which are attached to. central conductor ua. Electrode lla is connected tointernal conductor 33a through a short length of electrode wire which preferably through' the lowermost arm 4I. The up passes per end of thedischarge electrode is spaced froml the hemispherical end of the non-dischargin electrode by an interval of about one and one\ half times-the diameter of sleeve Il, a distance that is determined by the considerations above described in connegtion with electrode' Il of Fig. 2. c

The spiral form of discharge electrode 35a provides a longer and more intensive charging field, .thus not only bringing about more nearly complete charging of. all particles in the gas stream but .allowing .a higherV gas velocity through the charging iield. The higher gas velocity to some extent oilsets a'higher'V ratey oi ozone production, as the vottone concentration is inversely proportional to (the rate of ilow; but

evenv without. a higher gas velocity, they rate of ozone production is still sovlowwas to be weil under generally accepted tolerance limits. be-

' cause the full advantage of 'a very mail diam- V'Iii eter electrode is secured.` One factor contributing to the high chargingeiliciency of the spiral electrode is that the supporting member 31a isinside the spiral and does not appreciably shield the electrode with respect to opposing electrode Hl, either. by setting up a ileld that interferes with the action of the discharge electrode or by 'beinginterposed between the discharge electrode and electrode lll.

Fig. 4 illustrates another embodiment designed to provide a longer charging eld, and having much the same characteristics as the form of Fig. 3. Here, intermediate section 3Ib supports a plurality of lengths of discharge electrode 35h; and though four wires 35h are here shown, a larger or smaller number may be used. Each electrode Wire 35h is held in the vertical position by a radial arm ll at either end of the electrode wires. Intermediate member Sib is composed as previously described of a body 32h of electrically non-conducting material covering a conducting core 3th attached to a ibase' 3S. The intermediate member is preferably oi suiilcient length that the closest part of each discharge electrode 35h is spaced fromthe non-discharging electrode above by at least one and one-half times the diameter of the latter. It will be ap-v preciated that in the forms of Figs. 3 and 4 the It will be understood from the foregoing description that various changes inthe form and arrangement of parts may be made without departing from the spirit and scope of my invention andthat the specic embodiments disclosed are but typical and illustrative forms of the in- Vention. 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:

l. An electrode for an velectric precipitator comprising a non-discharging electrode section: an intermediate section attached to one end of the non-discharging section and comprising an electrically non-conducting body and an internal electric conductor covered by said non-conducting body to prevent corona discharge from the conductor; and a discharge electrode section mounted on the intermediate section at a distance from the non-discharging section at least equal to the minimum diameter of the non-discharging section in a plane transverse to the direction of gas flow,` said discharge electrode section comprising a very ine wire.

2. An electrode for an electric precipitator discharge electrode section mounted on the intermediate section at a distance from the nondischarging section of at least the diameter oi the non-discharging section, said discharge section comprising a short straight length of very ne Wire of about five to fteen mils diameter.

3. An electrode for an electric precipitator comprising a non-discharging electrode section of relatively large diameter; an intermediate section attached to one end of the non-discharging section and comprising an electrically non-coni ducting body and an internal electric conductor covered by said non-conducting body to prevent corona discharge from the conductor; and a discharge electrode sectionmounted on the intermediate section at a distance from the non-discharging section of at least the diameter of the non-discharging section, said discharge section comprising a very fine wire in the shape of a spiral.

4. An electrode for an electric precipitator comprising a non-discharging electrode section of relatively large diameter; an intermediate section attached to one end of the non-discharging section and comprising an electrically non-conf ducting body and an internal electrical'conductor covered by said non-conducting body to prevent corona discharge from the conductor; and a discharge electrode section mounted on the intermediate section at a distance from the non-discharging section of at least the diameter of .the non-discharging section, said discharge section comprising a very fine Wire in the shape of a circle.

5. In an electric precipitator, the combination of a cylindrical non-discharging electrode; avery fine wire discharge electrode; and an intermedi-l ate member supporting the discharge electrode from the non-discharging electrode at a position sumciently removedv from the non-discharging electrode to be substantially unaffected by the non-discharging eld around it, said intermediate member comprising a conductor electrieally'connecting the two electrodes and a dielectric body completely covering said conductor to prevent corona discharge therefrom.

6. In an electric precipitator, the combination as set forth in claim 5 in which the length of the intermediate member is approximately one and one half times the diameter of the non-discharging electrode.

initial 7. An electrode fora separated eld type of electric precipitator comprising a hollow cylin1 drical tube of relatively large diameter and a hemispherical member oi the same radius cf external curvature as the tube closing the lower end of the tube, the tubel and hemispherical member forming a non-discharging electrode section; a rod-like intermediate electrode section attached to the hemispherical member and comprising an electrically non-conducting body and an internal electric conductor of substantially smaller diameter than the non-discharging section covered by said non-conducting body to prevent corona vdischarge therefrom; and' a very iine wire discharge electrode section mountv ed on the intermediate section at a distance from the non-discharging section of at least the diameter of the non-dischargingsection. 7^

- 8. An electrode asin claim 7 in which the discharge electrode section is a short straight lengthV of wire about five to iifteen mils diameter, and all three electrode sections are substantially co-

Images