US2440455A - Charging suspended particles - Google Patents

Description

"April 2' 7 194 8; JIWHITE 5,

GHARGING SUSPENDED PARTICLES Filed Julie: 1945,, 3 s eets-sheet 1 April 2 1943- H. .1. WHIT 2,440,455

CHARGING SUSPENDED PARTICLES Filed June 11, 1945 I s Shee1s-Shee t 2 April 27, 1948.

H. J. WHITE CHARGING SUSPENDED PARTICLES Filed June 11, 1945 3 Sheets-Sheet 5 Patented Apr. 27, 1948 PAT ENT G'FF ICE 7 2,440,455 7 eiinnemc SUSPENDED PARTICLES .I-Iarry fJ. 'White,'.Oambridge, Mass, assignor, by direct and mesne-assignments, of one-half to v\ResearcheCorporation, New York, N. Y., a corporation ofiNew York, and one-half to Western 'Precipitation Corporation, *Los Angeles, .acorporation of California cane,

=Application June 11, 1945, Serial'No. 598,732-

*12"Claims. 1 {Elle .invention relates tie-methods! and appara- 'tus'foritheicharging of solid and liquid .particles suspended .in :a .gas. The invention is useful in :thescharging of suspended particles for thepuraposepfzremoving themtfrom the gasby means of :an electric ifielcliandltheiinvention awillfbe particularly; describe'dw-ith re'ferenceto such use, but ltheinventioniiagenerally:useful:for the 'charging 'of l suspended particles.

Typicallmisuspended :particles :are charged, for

example, in :the operation of electrical i precipitation,i-bypassingfthegascontaining?the-suspended particles "through :a high potential electric field maintained :between :an attenuated or discharge :electro'deend an 'extended'suriace or non-dis- :charge electrode. ionization of the gas by the action of corona or silent discharge i rom the at- ;tenuated electrode brings about the :charging of "the particles. Thencharged particles are transported' by the action of the electric field in which the charging is e'fected,=or in a subsequent electric field-through which the gas is passed, upon an :extended surface electrode or collecting electrode. Because the conditions for -most eiTectiVe collection of the charged particles are not always identical with the conditions for the most eifective charging, thewcarry'ing out of the operations of charging and collecting in separate successive stages is advantageous in -many applications of the process of electrical precipitation.

"It has beenfiound that the vcharging of suspended particles by-corona discharge in a gas depends for its eifect'iveness 1on three principal factors: 1 the presence of an adequate density of unipolar ions, typically :of "the order of 10 "to 10 ions/cmF; (2) a strong electric field, typically of the-order of'5 to 1 kv. /cm.; and (3') an adequate charging time, typically of the order of 10- sec- --onds. However, it has been found in the past that attempts to iner-ease the efficiency of the charging operation by in'creasing ion density or field strength or both have encountered serious Particularly disastrous i-n its ultimate effect on the efficiency of charging is the phenomenon of bade-corona, which is a reverse discharge usuallyar ising fromparticles-deposited on the surface of the extended surface electrode. "The conditions under which back corona arises vary greatly with the character of the deposited material and the chemical composition and physical state of the suspending gas, but whenever back corona appears a great reduction in the charging eificiency results, due:to;the formation of ions of opposite polarity to those formed in the discharge from the attenuated velectrode, .toflocalization cof the corona discharge,gand :to otherlfactors;

' Various means FfOI' iovercoming back Ecorona have beenusedor suggested. "Theserincludenonditioning of the; gas, conditioning of the extended surface electrode as by" waterrflushin'g, provision of mechanical n' eans zfor cleaning tor preventing the deposit of material :on' therextended surface electrodes, "and reducing the :c'oron'afcurrent; density as by the use offimpulservoltages. II have 'now found thatlthe Jefiiciency inf. the

to oscillate :in 'the-ffield withoutsstriking rtherextended :sur'ia'ce electrode, Ythus ibuilding up and maintaining =9, high ion density :relativly small net currenti'flomi-second, 'inisuchlan ialterhating field even :fewer aofitheemuchz less mobile charged particles he transported :to the :iex-

tended surface electrodeitosform fcrstcon- :ditions leading to hack :roron'a; third, the alternating' field tends to suppress Tithe formation eof back corona even underrconditinnswhich would otherwise lead to :substantlaleand highly :deleterious back discharge; because tthe alternation of the field has the "effect "of "quenching"'?the points ofhack corona fdischarge "during every other half cycle, and'LtOurth;zthezmaximumrelectrio field which can hermaintained'fhetween the non-discharging alternating potential electrodes is about double that which canabe maintained with the customany electrode 'arran gementtin which one electrode "is 'dfattenuated D! discharging'form, so that th'elmaximumpossible particle charge is approximately doubledssince the particle charge is :proportionalftothe maximum field strength. J

The principles of the invention may be applied in practice in a number of Waysand various modifications and variations of them'ethod and apparatus of the invention may be utilized; general, the method of the invention comprises passing a gas containing suspended particles through an electric field alternating ih'etween spaced non-dischargeelectrodes at a period less "than' half thetransport time (if gaseoussionsrbetween said electrodes and subjecting the :gas

while passing through the alternating field to ionizing discharge impulses between a discharge electrode and at least one of the non-discharge electrodes, the ionizing impulses preferably having an impulse duration substantially less than half the period of the alternating field.

The maximum period or minimum frequency which will just prevent ions in a uniform alternating field from reaching the boundary electrodes can be calculated from the following equation:

ZkV fa dg A where f is the critical frequency, d is the disvolt/cm. for air. Then with an interelectrode distance of 10 cm. and a peak voltage of '70 kv., fc would be 890 cycles per sec. However, the "critical frequency is not at all sharp because of the effect of such factors as space charge and diffusion, and the value of the frequency as determined by Equation 1 is to be taken as a guide to the proper order of magnitude. The advantages of the alternating current field by no means cease at the-critical frequency as even at lower frequencies the formation of back corona is substantially decreased, but, in general, it is desirable to operate at frequencies substantially above the critical range. In practice, the range of useful frequencies will usually be found within the sonic or audio range, that is, within the range of frequencies ofaudible sound although supersonic frequencies will sometimes be preferred. Alternating electric potentials within this rangearexreadily produced by a number of different known devices such as mechanical alternators, vacuumtube oscillators and the like. 1 The electrodes forming the boundaries of the alternating field may consist of plates or other extended surface members free from surface ele- 'ments of small radius of curvature such as rods.

-' metrically positioned between the non-discharge electrodes forming the boundaries of the alter nating field, may be of any form providing surface elements of: small radius of curvature which induce the emission of corona discharge under the conditions of operation.

'As was stated above, one of the principal appli cations of the invention is in the charging of suspended particles as the first stage of a twostage precipitator. Since one of the advantages of the invention is that it permits the charging of the suspended particles at a high rate of 11 W it is desirable to provide a second or collecting stage which can efiiciently operate at a high gas velocity. This may be effectively accomplished by collecting the charged particles by means of a unipolar corona discharge instead of the unipolarv non-discharge electric field typically used in the second stage of two-stage precipitators.

The alternating electric field of the invention is not necessarily symmetrical. It may advantageously be"biased" at one polarity so that the repelling force for the ions produced by the I corona discharge is higher at one of the boundary electrodes. In this form of the invention,

it is particularly advantageous to confine the gas fiow substantially to the region having the higher potential in order to take advantage of 5 the more effective charging rate in that region, or, if the gas is passed transversely through the electrodes, to pass the gas last through this region where the charging conditions are best.

The alternating electric field of the invention need not be bilaterally oriented with respect to the dischargeelectrode structure, but may comprise a series of alternating fields of successively reversed polarity along or around the discharge electrode structure.

Theionizing discharge impulses to which the gas containing suspended particles is subjected are produced by impressing between the discharge electrode and at least one of the extended surface electrodes a succession of unipolar high potential impulses preferably of short duration relative to the period of the alternating field and preferably of a form having a relatively steep wave front. Circuits for the production of such impulses of any desired frequency and duration are well known in the electrical art. Suitable combinations of a condenser and a spark gap provide a simple means of producing high voltage impulses, which may be modified in various known ways to produce impulses of the desired wave form. The impulses may be synchronized with the alternating field by suitable connection of the impulse generating circuit with the alternating field circuit, for example, by operating a mechanical spark gap by means of the alternating current of the alternating field circuit at the frequency or a submultiple of the frequency of the alternating field.

In general, the peak potential of the corona discharge impulses may be from about 10 to 100 kv. The peak potential may be higher the shorter the duration of the impulse.

The duration of the corona discharge pulses is preferably considerably shorter than the half period of the alternating field. For example, when using an alternating field having a frequency of say 1,000 cycles per sec., the pulse duration may advantageously be of .the order of 10 to 100 microseconds and the pulse frequency may be the same as the alternating field frequency or a sub-multiple of it.

The invention will be more particularly described for the purpose of illustration with reference to the accompanying drawings, in which:

Fig. 1 is a plan view of a two-stage electrical precipitator embodying the principles of the in -vention;

Figs. 2 and 3 are sectional elevations on lines 2-2 and 33 of Fig. 1;

Fig. 4 is a diagrammatic representation of electric circuits suitable for energizing the precipitator of Fig. 1 in accordance with the principles of the invention;

Fig. 5 is a diagrammatic plan view of a charg- 2'8 and rectifier '21. current either continuous or disccrrtinuous may be supplied to discharge electrodes 14 'by asepa- "comprises two stages, a 'charglng stage A 'and -a collecting stage B. The side-walls HI 11f ahe precipitator shell :pr'ovide extended surface electrodes complementary to the extended surface plate electrode 'l 1 in "stage A and provide, "with extended surface plateelectrode 11:2 the collecting electrodes of stage l 3 ane the dischargetelectrodes .of stage A and M are the precipitating electrodes, in this embodiment of the invention shown as discharge electrodes or -=stage "B.

' Plate electrode H and discharge electrodes l3 and are suspended in the precipitator'chamber from insulators E5, I B and [1, respectively, through Whichtheleads f-rom the energizing "circuits may be' brought. "The 'precipitator shell and a plate electrode 12 are grounded.

As shown more particularly "in Fig. 4, a high potential field of alternating polarity is provided between plate I and the sidewalls-1 B by -means of high potentialalternating current "from-transformer [8, the midpoint of the potential Winding of the transformer being connected to the discharge electrodes 1 3; 4 4 through resistance B. 'High potential mnipolarliimpulses a're supplied '-to discharge electrodes F3 and l-4 by means of condensers C1 and C2, resistanceR and rotary interrupter 1 9 whichare supplied high potential unidirectional current from transformer If desired, unidirectional rate source.

Because of the al-ternating electric {field in stage A, a strong electric fleld and a high'ion density are maintained-in this stage with subbe eroded from the colle'ctin'g lectrode surfaces due to the high'ga-s velocity; *Other means of reducing the loss of efiiciency due tmerosiomsuch as pocket-type collecting electrodesmay be providcd in stage B in addition to or i-nstead of the use of corona-forming preci-pitait -g 'electrodes.

In the chargingdeviceshown in F L 5, eXtended surface electrodes are provided by the 'side walls 3%! of the grounded shell-of *the device, and by plate 3 which is ihs-u-lated li'rorn th'esh'ell. High potential unipolar impulses are supplied to --discharge electrodes 3-2 hy means oi1 'trans former 3 3 and rectifying switch 34. A -highpotential alterhating current field is supplied between p'lat'e electrode 3| and the shll 3 fl hy means of transformer '35. here isbiased by incr'ea sing the negative potential deplete- 3 {(whehfithe-impdlms supplied to the discharge electrodes are-negative, as is preferred), as diagrammatically indicated by battery '35. This negative bi'as tends to rep'el negative ions which would otherwise strike plate 3| because of space charge and difius ion e'ffeots. In this form of the "invention, it "is desiralcle to substantially confine the gas -flow t'o th'ejregion adjacent plate "3 l, for example, by suitably arranging'the gas inlet and outletcorfiiu'its as indicated diagrammatically inFi-g; 5. The phase relation of the current supply to "transformers Hand 35 is preferably regulated so that the Voltage across one transiormer is at its "maximum when the voltage acros-s 'the ot'her' is (zero,

Ini'thefch'arging. device diagrammatically shown in Rigid, the'alt'emating field is not bil'aterally orientedzwith resp ect to the-discharge Pele'ctrodesc, but the field p'eri'odically reverses in orientation along the discharge electrode structure. .In this form of the invention extendedsurface electrodes fi, which :may be :smoothesurfaced rods, iare electrically connected in alternatelrsetsyra.1and 1); respectively, along the length of the discharge electrode structure to the opposite terminals ro'f the high potential r-side'o'f transformer 4|. High potential unipclarcimpulses are supplied to hischarge electrodes 1: :by means :of transformer 42 rectifier 43. The relative phases of transformers "4i and 42 may be advantageously adjusted as described in connectionwith Fig. :5.

The gas containing suspendedparticles to be chargedma'y be pas'sedithrough the interelectrcde space parallel to the-electrode systems '(ei-therin or normal to the plane of the drawing), orthe gas may be passed x-transversely .throughthe electrode systems.

Arrangements ofthe type shown Fig. 6 are effective in largely eliminating back corona :even when treating suspended materials which; "because of their poor "conductivity,ftend to-rcause the formation of back corona very readily. During the periods when one of the electrode'sets, for example, the :a electrodes areipositwe, practica'lly all of the c'or'on'a rcurren't ifrom.-the discharge electrode willccome toit :(assuming :the usual negativepolarity of the discharge. electrode energization). es the material which deposits "on the electrodes is poorly iconclucting,filocal sparks may pass through Ithedeposited-material harming :craters "which, in the usualarrangement, would constitute points from hich 1 back corona would emanate. But, with theerrarigement 0f "the invention in which the electric 'fie'ld at the 'electrode sur faces is rapidly alternating, before pos'i'tive Lions can travel an appreciable i distance into the gas stream -they are forced b'a'ck to the electrode from which they-oi ihate byiih'e reversal of the new. The shutting o-ff of the corona current together with the elimination ofipositive ions during the reverse ii'el'd :periods efiectively "quenches the back corona points andprevcnts positive ions from-passing into the :main gas "stream. Hence, the gas 'stre'amremains tree of positive ions and the charging action of the unipolar corona discharge is maintained at high effectiveness despite the poorly conductingcharacter of the-suspensoid. Theadvantageous efiect of-the rapidly reversing field' is iur ther enhanced by the fact that it greatly reduces the amount or suspended material which is: iieposi ted on ithe ex-tended surface electrodes.

Figs. "I "to 9 are diagrammatic representations of several modified electrode arrangementsin which the significance of the letter-designations "is same Fig. '6. 1n anyof these arran'gethe same significance as in Fig. 6), particularly adapted for gas fiow parallel to the electrodes, as indicated by the arrows. In this arrangement, the a and brod electrodes, or equivalent shapes, are positioned in alternation in a circle about the discharge electrode wire 0. The gas fiow is largely confined to the cylindrical path defined by the rod electrodes. 7

In each of the forms of apparatus described above, the charging stage comprises two spaced insulated extended surface electrode systems and a discharge electrode system interspaced with and. insulated from said extended surface electrode systems. The two extended surface electrode systems comprise electrodes Iil and electrode I! in Figs. 1, 2 and 4, electrodes 30 and electrode 3| in Fig. 5, and the oppositely energized rod electrodes A and B respectively in the remaining figu'res, while the discharge electrode system comprises the electrodes l3 in Figs. 1, 2 and 4, electrodes 32 in Fig. 5, and electrode or electrodes C in the remaining figures.

It will be seen that the principles of the invention are susceptible of application to a wide variety of conditions and arrangements within the scope of the invention as defined by the claims hereof.

I claim:

1. A method of charging particles which comprises passing a gas containing suspended particles through an electric field alternating between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes and subjecting the gas while passing through the alternating field to ionizing discharge impulses having an impulse duration substantially less than half the period of the alternating field.

2. A method of charging particles which comprises passing a gas containing suspended particles through an electric field alternating between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes and subjecting the gas while passing through the alternating field to ionizing discharge impulses of negative polarity having an impulse duration substantially less than half the period of the alternating field.

3. A method of charging particles which comprises establishing an alternating electric field between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes, biasing the voltage of said field to produce a higher peak potential at one of said electrodes, passing a gas containing, suspended particles through said field, and subjecting the gas while in said field to ionizing discharge impulses having an impulse duration substantially less than half the period of the alternating field.

4". A method of charging particles which comprises establishing an alternating electric field between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes, biasing the voltage of said field to produce a higher peak potential at one of said electrodes, passing a gas containing suspended particles through said field adjacent the electrode of higher peak potential, and subjecting the gas while in said field to ionizing discharge impulses having an impulse duration substantially less than half the period of the alternating field. v

5. A method of charging particles which comnprises establishing an alternating electric field between spacednon-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes, biasing the voltage of said field to produce a higher negative peak potential at one of said electrodes, passing a gas containing suspended particles through said field, and subjecting the gas while in said field to ionizing discharge impulses of negative polarity having animpulse duration substantially less than half the period of thealternating field.

' 6. A method of electrical precipitation which comprises passing a gas'containing suspended particles through an electric field alternating between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes, subjecting the gas while passing through the alternating field to ionizing discharge impulses having an impulse duration substantially less than half the period of the alternating field, and thereafter passing the gas through a high potential unidirectional electric field.

7. A method of electrical precipitation which comprises passing a gas containing suspended particles through an electric field alternating between spaced non-discharge electrodes at a pe riod less than half the transport time of gaseous ions between said electrodes, subjecting the gas while passing through the alternating field to ionizing discharge impulses having an'impulse duration substantially less than half the period of the alternating field, and thereafter subjecting the gas to ionizing discharge impulses in a high potential unidirectional electric field.

8. A method of electrical precipitation which comprises passing a gas containing suspended particles through an electric field alternating between spaced non-discharge electrodes at a period less than half the transport time of gaseous ions between said electrodes, subjecting the gas while passing through the alternating field to ionizing discharge impulses of negative potential having an impulse duration substantially less than half the period of the alternating field, and thereafter subjecting the gas to ionizing discharge impulses of negative potential in a high potential unidirectional electric field.

9. An electrical precipitator comprising a charging stage including two spaced insulated extended surface electrode systems, a discharge electrode system interspaced with and insulated from said extended surface electrode systems, means for establishing an alternating electric field of sonic frequency between said extended surface electrode systems, and means for establishing unipolar high potential impulses between the discharge electrode system and at least one of the extended surface electrode systems; a collecting stage including an extended surface electrode, a precipitating electrode spaced and insulated therefrom, and means for establishing a high potential unidirectional field between said precipitating electrode and said extended surface electrode; and means for directing gas fiow successively through said charging and said collecting stages.

10. An electrical precipitator comprising a charging stage including two spaced insulated extended surface electrode systems, a discharge electrode system interspaced with and insulated from said extended surface electrode systems, means for establishing an alternating electric field of sonic frequency between said extended surface electrode systems, and means for establishing unipolar high potential impulses between the discharge electrode system and at least one of the extended surface electrode systems; a collecting stage including an extended surface electrode, a discharge electrode spaced and insulated therefrom, and means for establishing a high potential unidirectional field between said discharge electrode and said extended surface electrode; and means for directing 'gas flow successively through said charging and said collecting stages.

11. An electrical precipitator comprising a charging stage including two spaced insulated extended surface electrode systems, a discharge electrode system interspaced with and insulated from said extended surface electrode systems, means for establishing an alternating electric field of sonic frequency between said extended surface electrode systems, and means for establishing unipolar high potential impulses between the discharge electrode system and at least one of the extended surface electrode systems; a collecting stage including an extended surface elec- 10 trode, a discharge electrode spaced and insulated therefrom, and'means for establishing high potential unipolar electric impulses between said discharge electrode and said extended surface electrode; and means for directing gas flow successively through said charging and said collecting stages.

12. Apparatus as defined in claim 9 in which the extended surface electrode systems include 10 substantially parallel plates.

HARRY J. WHITE.

REFERENCES CITED The following references are of record in the 15 file of this patent:

UNITED STATES PATENTS

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