Note: Descriptions are shown in the official language in which they were submitted.
` ` ~10'~()250
. ~ .
.
The present invention generally relates to electrostatic - -
precipitators, and more specifically, to a method and apparatus
for electrostatically precipitating particles of different sizes
from a gaseous medi~, including those having a diameter less than
5 microns.
The increased emphasis being given to reducing levcls of
air pollution has culmina~ed in a wealth of local, state, and fed~ral
legislation setting rigorous standards ~or particle removal from
industrial and other gaseous emissions. Since the problems of air
10 pollution directly affect a vast majority of the public, particularly
in those urban areas where industry is concentrated, it is assumed
that the standards may become even more rigorous in the future. While
improvements continue in the design and effectiveness of particle
removal apparatus, including electrostatic precipi*ators, the rigorous
15 standards that are now being adopted have shown that many present
types of precipitators are relatively ineffective in removing very
small particles. This is coupled with the recent realization that
the greatest number of particles in industrial gaseous effluents
are in the range of about 0.1 to 10 microns in diameter, and also
20 that the smallest particles remain suspended in the air for the longest
time. ~oreover, the greatest health ha~ard is posed by particles
in the range of about 0.1 to 5 microns in diameter, according to
the National Bureau of Standards Technical News Bulletin, dated December
1972.
All electrostatic precipitators use two charging mechanisms
to build up the charge on a dust particle. These two mechanisms
are diffusion charging and field charging. In field charging, ions
are accelerated by the electric field of the precipitator. These
accelerated ions strike a dust particle and combine with it. As
30 the dust particle accumulates these charges, it takes on the same
. - ' ~ - -
107V2S~
charge as the ions. Whon the ~u~t particla ~ecome~ chargod and hss
the s~me charge A~ tha ion, th~ lon and chargcd partlcle tend to
repel each other, which makaq it more ~lfflcult ~or other ions to
add add$tlonal ¢harg~ to the pnrtlcl~. For a glva~ ~loctric iol~
5 ~trength and a giYen ulze of du~t pnrticlo, th~re will b~ n lLmlt
beyond which the ~u~t partlcl~ will no loncJer Acoept ndditlonal charga~
by ~ld ch~rglng. Fox ~mall partlol~ ln aonventlo~al preolpltators~
thi8 llmit 18 v~ry qulokly ~eaohad. Th~ other chargln~ machanlsm,
dl~fu~ion chnrging, utllize3 thermally aa~lvated ion~ that pO~9~9S `
su~flclent energy to pan~tra~e the r~p~lling ~ield and add additional
charges to the dust particle~ This aharg~ng mechanl~m will charge
~mall pArticl~s~ but ls ~ulte ~low compared to the mechanism o~ ~ield
chargincy.
It is gen~rally Xnown that presently used indu3trial 2reoip~-
lS tators are relati~aly inef~ectiv~ in removlng partlcles havin~ a 9ize
rang~ of nbout 0,1 to 3 micron3. Conventional electro~tatlc precipl-
tator~ fail to colleot thesa ~ine particle~ as rapldly as the larser
particles because the dif~u~io~ mechani~m 1~ th~ mechani~m ~lat is
us~d to depo~it el~ctff cal charge on the ~mall particle~ and it operatq~
too 910wly for such pastlcle~. Ion~ drift onto the partlcl~ by thermal
mot~on ~o that n~ ~ particle begln~ to acquire a charqe, lt repels
th~ ~lower ~ovlng lon~ whlch could brinq further charge~ to the ;
partlcle. Stated ~n other worda, larqe particl~s are predomlnately
charqed by the char~inq mechani~m o~ field charging w~ich is ~ub~ect
to a limlt based upon ~he electro~tatlc repulsion ~ the charged
pastlcle aq~lnst furSh~r charges approaahlng lt. ~ho~e charges are
typically dri~en by an electrio fiel~ applle~ by remote alectrodes. - -
Thus~ in the prlor ~rt ~pparatus and 1~ the pre~ent iAveAtion tha
bal~nce between ~hd drl~ng and repelllng orce~ aetermines the
30 m~x~mum oharqe whioh can be aaqulre~ N8- ;
N - S2 ~e ~D
+ 2
.
-2
.
.~ , . .
10'7t)ZS0
whero N~ 18 the ~Aturatlon number o~ el~c~ronlc magnltuae charg~,
E lo the appl~ed elea~rlc fleld ln ~llovelts per centim~tor~ D 1B
tho partlcle dlameter ~n mlcrometur~ and ~ i~ t2~e partialo dielsctrlo
constant. ~lowev~r~ in conventlonsl ~l~otro~tatic prcclpitato~s,
the mean char~lng and collectlon fiold 1~ limited ~o ~bout ~ kV~cm
becaus~ 1~ i8 lln~d to a hl~her fleld whlch ~upport~ ~ coron~
dls¢harge Ad~acent n smAll~ ld 0nh~ncln~ ol~c~rod~ an~ hlgher
~loldu tend ~o cau~ spArk broakdown in th~ gns. ~hun~ ~or A 0~3
mlcron dl~mo~or partiole~ the n~AxlmUm ~for l~r~o ~) s~ ratlon
chArge produced by the elea~rla ~lald in nn ordlnary electro~t~tlc
precipitator i~ about 20 olactron chargcs.
In conventional ~lectro~tatlc preclpltators, the only
e~ectiv~ oharglng method $or charging small partlcle~ 1~ by dl~fu~lon
ch~rging because o~ the low electrla fleld. ~he number of charges
added ~ 8 given approxlmately by the ~ollowinq aquationt
N - 0.03 T D ln (1 * 7.6 x 10 4 No D t/T1~2)
whsro T is lon ~netlc temperature ln degreee Kelvin, No 18 the ambien~
concentration of ~on~m3 and t i3 the tlme ~n s~conds after the ~ield
charglng has been completed. Since the charye attalned a~er a
long tlme by dlf~uslon 18 proportlonal to D ln D~ lt will exceed
the fleld produced charga for small particlas. In typical Cottrell
pseclpltatore! for ex~mple~ ion den~l~ie~ ~re aeveral tlme~ 107~cm3.
At thls lon density, about 0~3 second is reqnired to deposlt 20
charge on a 0.3 ~lcron dlameter pasticle whlle 24 second~ would
be require~ to doubla th~s charge ana the tran~it t1me of g~ through
~ypl¢al preclpltators i8 only ~bout 8 s~condsO
In other woxd~ con~ent~onal electrosta~lc precipitator~
oper~ta by prod~cing ions of both polariti~o in a GOrOna dl~ah~-ge
pln~ma ne~r one 8~11 elQct~od~ aro-~nd whlch the electrlc ~leld concen-
trat~. Tho ~tr~ngth of the ~i81~ i8 guit~ high noar the e~ectrode~n~ drop~ dramAtlcally away fro~ th0 elactrode ana thereby provlda~
n nonunlfon~ lA, Ions o~ ona pol~rity ¦usuallY negative) ~re
107V2S~
withdra~n ~rom thl~ roglon and ~ th~y drlft townr~ th~ other olectrods,
th~y beco~ att~ched to the aoroDol p~xticlo~ in th~ e~luent~
~o produce th~ flel~ onhancement noc~aary for coron~ di~ch~xga at
one elQctrode wlthout cau~ln~ olectrlcal bre~lcdown bo~woen ~he two
~lectroda~, conv~ntion~l preclpitntor~ o~ton ma~o uaa of co~xlal
geQm~try wlth a amall dla~Q~ar wlre a~ tho c~nter elootrodo and a
largo diamoter outer cylindar. Tho ~rl~t o~ ~ho ion~ i3 c~u~ by
the ln~ractlon o~ th~ chnrge on tha ~on and the nonunl~onm, gonorally
wonk aloctric ~ield. ~a ~h~ ion~ dxlXt, they charge th~ partl~le~
by ~ttnchlng to them, thexeby cAu~inq the parti~'le~ to be drlven
by the olootrlo 101d toward nnd ~ttachod to tha collectln~ electxode.
The officl~ncy of all oleat~os~ntlc precipltators lncluding
tho8a 0~ the prior art ~nd al~o of the pre~en~ ~nvention 1~ limlted
by three ma~or factor~9 ~speclally ~or the a~roaol partlcles whlch
are lo~ than flve mlc~ons in dlameter. The ~ir~t ur~ s~a bQcau~e
the charglng rate of the anroaol par~lcl~ decrenses rapidly R~ the
radius of the particl~s decrea~as~ Thus~ a9 the slze of the pas-
tlclas decsea~es, the paxtlcle charge 1~ le~ ~nd the dri~t velocity,
l.e., the component o~ tha ~vex~ge velocl~y e~ the par~lcle~ direc~ed
toward the ~lectrod~s, decsea~e~, The second actor i~ tbat or
a given charge the drift velocity decresses a~ the electric leld
stxength decrease~ Thus~ thQ dr~ft velocity of a glven 8ize particle
decr~aaes as it move~ in the direction toward th~ collecting electrode
bacause of the decre~slng ~eld in tha coaxial electrode co~flguratlon.
ThQ third ~actor 1~ the att~chment e~iclency of the collecto~ elec~
txode~ i.e., the partlcles which are ~rl~ted to ~ho collec~or
electrGde may rebound o~ be dislodged by the lmpaat o~ o~her paxtlclcB
o~ b~ ~wcpt away by ~he tuxbulent flo~ o th~ gaxeou~ e~ luent n~t~r
they have been ~nltlally collec~a upon lt becau~ the charge on
the part~cl~0 and the electrlc fteld they expe~ienc~ are not 8uf~
o~ently l~rge.
1070250
It h~3 genarnlly been recognlæ~d that lmpro~d opera~ion
o~ an electro~tatic p~clpltator rQsult~ ~rom increa~lng the electrlc
- field str~n~th provld~d, however, that el~ctrlcsl break~own or
arcing do~ not renult fro~ the hi~her eloctria ~l~ld ~tr~ngth~
The prior art al~o disclos~s pr~clpi~atin~ ~ppnr~tu~ ~1hlch ind~p~ndontly
produ~o tho ~onu and the electric fiald xath~r tbnn a configuration
that u~e~ ~ ~mall wiro cantral nloatrode ~n~ outor cylindrical electrodo
to ~imultanoou~ly croa~o the ion~ ~nd ~he ~l~ct~la ~ield. Whllo
rndloactlvo ~Atorial~ and photoionlzation ~ource~, o.~. light tubes
such a~ ultraviolet l~mps, have been di3clo~ed to provide a sourae
o~ lon~ lndependently of the produc~lon o~ th~ electrlc f~eld, these
ion source~ hav~ practical operational and other disa~vantage~ and
it i~ not believed that any commerclal apparatus have been developed.
~ disadvantage of radloactive ~ources ~8 the difficulty in varying
thP energy and quantity of par~icle~ e~ltted by such nources~ Yurther,
the psychologlcal i~pact of u~ing a radioactlve source o~ lon3 in
a precipitating apparatuq, particularly in an urban area, would b~ ~
quite negat~ve. .~oreo~er, there could be a signifi~ant problem of ~ -
radioactiv~ contamlnatlon of the atmo~phere if a rupture or br~akdown
of 60me portion of the apparatus occurred. Preclpi~a~ors that U8
ul raviolet or other lamps to pro~ide pho~ons ~or creatlng *ha neceY-
sary ionlzatlon within the prec~pltator ar2 also ~ub~ect to many
practlcal oparational d~sadvantage~. The lamps are sub~ect to duætlng
o~ uding over by the part~cle~ in th~ gaseou3 medium or effluent
25 ~nd will becoma d~rty qulte r~pidly. Thi~ dustlng o~er ~ay easily ~ ~
-occur ln only a few seconds and greatly decreases th~ ef~clen~y a~
their operatlon. Moreov~r, the photon energy created by ~uch la~p~
cannot be continuously and convenlently controlled~
~owaver, the pre~ent inventlon does not ~u~f~r ~r~m ~ha -~
disndvantages of these rcadioactiYe a~d p~otoionlzation source~ and,
ln fact, exhi~ito many de~lr~ble at~ributes th~t e~ables lt to ~chieve
th~ result~ sough~ by the abo~o ~ource~ ln additlon to other ~lgnllcant
,
1~70%5~
advantages.
More particularly, the present invention utilizes an
electron generating source (often also referred to as an elec-
tron beam generator, E-beam generator or the like) to bombard
the gaseous medium within the pxecipitator with high energy
electrons and produce a plasma region therein. The electron
generating source has the advantages of being able to accurakely
control the penetration and density of the electrons that are
injected into the gaseous medium and th~reby control the extent
of the plasma region. Further, the "window" or surface through
which the electrons are injected into tbe medium, i.e. the
surface through which the electrons pass which is in contact
with the gaseous medium, is self cleaning and will not dust up
or become dirty from the particl~s within the gaseous medium or
effluent. These and other advantages will be described in
detail hereinafter.
Accordingly, it is an object of the present invention
to provide an improved method and apparatus for precipitating
particles from a gaseous medium such as a gaseous effluent,
which method and apparatus are effective to remove extremely
small particles, i.e., those particles between about 0.1 and 5
microns in diameter and particularly those less than 1 micron
in diameter. -
Yet another object of the present invention is to
provide an improved method and apparatus for removing particles
from gaseous effluents with high volume throughput, high
efficiency, and only moderate power re~uirements.
The above objects are met by the present invention
which provides a method of electrostatically precipitating
particles from a gaseous medium carrying the same, comprising:
passing the medium through a channel in a precipitating station
wherein the particles are brought into a first region containing
Y(
25(~
ions of only one sign; s~ecting the medium to a supply of
electrons from an electron beam generator to generate a supply
of ions of both signs in a second region, the ions of one sign
in the first region being supplied from the second region; sub-
jecting the medium to a generally uniform, strong electric fieldto drive the ions of one sign onto the particles, the average
field strength of the electric ield approachiny ~he maximum
field strength therein; the electric field causing attraction of
the charged particles to one or more electrodes hav:Lng a charge
of opposite polarity relative to the charged particles to there-
by precipitate the particles out of the medium.
The above objects are also met by the present inven-
tion which provides apparatus for removing particles from a
gaseous medium passing therethrough comprising: an inlet for
receiving and an outlet for expelling the medium; a central
portion located between and being in communication with the
inlet and outlet, the central portion guiding the medium through
the apparatus; one or more positively charged electrodes being
located in the central portion; one or more negatively charged
electrodes located in the central portion for attracting par-
ticles having a net positive charge from ~he medium; means for
charging the electrodes in the central portion to provide a
uniform, high electric field in the central portion of the
apparatus; an electron beam energy source means for injecting
high energy electrons into the central portion for producing a
supply of positive ions which bombard particles and cause them
to be attracted to the negatively charged electrode.
The inventive aspects of the invention wi11 become ap-
parent from the following description and accompanying drawing,
in which:
FIGURE 1 is a diagrammatic representation of precipi-
tating apparatus embodying the present invention and which is
- ~ -6a-
1~)';'0250
.
useful for practicing tha method of the present invention;
FIGURE 2 is a perspective view of one :Eorm of the
apparatus that may be used to practice the method of the present
invention; and,
-6b-
~0~ 50
FIGUnE 3 1B a ~Chemat10 illu~tratlon o~ anoth~r embodl~.en~ o~
the present inventlon.
~ roadly sta~ed, tho pre~nt inventlon iB dlrectod to Appar~tus
~ well A9 a method ~or preclplt~tlng or romovlng particla~ ~rom n
~tre~m of gA~eOu~ ~Pluent whtch prof~rably u~ n gencrally wlL~orm,
~tron~ electric fl~ld for chargln~ the p~rticle~ wlth lonu, with the
ion~ belng supplled lndepend~ntly o~ the uourc~ of tho olQatrla ~leld
~rom a plasma ~hat i~ ~oxmod by hlyh enorl~y alectron~. ~ pre~ipitatl~g
~tation includeu at l~A~t one po~itlvoly nnd one negatively char~ed
el~atrode for s0tting up ths elaatrla flold, and a source o~ ion~
~hich charge tbe pRrticlo~. ~he particles charged ln the pro~en~e o~
th~ el~ctrla field are therbby precipih~0d or rem~Y~d fro~ the g~seou~
o~fluent and collected at ons o~ ~be electrode~. ~lgh-energy electron~
nro directed ~o a8 to produce a plas~a in thc g~eeou~ medlum or effluent
15 near one o~ the electrode~ and the partlcle~ have no net positlve or
negatiue charge withln tb~ neutral region of pl~sma. ~owever, ~he
charqed el0ctrode~ and pla~ma produce a charged rsglon between the
plasma and the collectlng electrode, 80 that once thc partlcle3 ara
withln tbe chnrged electrlcal region, they will acqulre a net chsrge,
und be AttrA~-ted to ~he opposi~ely charged collectlon electrodeO
Refarring to the dr4wings and partlcularly F~GUR~ 1, an
ideal~zed s~hematlc cro~s-~ectional ~i~gram o~ apparatu~ which ~ay be
used to casry out the method o~ the present inventlon 1~ ~hown. The
appar~tu~, lndicated generally at 10, communlcatea ~ ga~eou~ madlum or
ef~luent from ~he lower lnlet 12 throuqh tD the outlet 14 ln an upward
dlrectlon a~ ~hown. Slde wall~ 16 ~nd 18 dlr~ct the ~low ~hrough the
apparatu~. An eleotron gen~r~tlng ~ourc~ 20 1~ po31tioned wlt~ln an
openlng ln ~he side wall 18 ~nd produces high enargy elactron0 indl-
cated by the arso~s 22 ~hlch penetrat~ ~ ~hln tr~nsml~slon window 24
~s well a~ a positlvely oharged elect~ode or ano~3 26 lnto th8 ga8~eUO
~e~lum. A neqatlvely ~harged electro~e 28 1~ po~tloned ad~acent ~e
slde w~ll 16 ~o tha an alectrlc ~leld ~9 s~t iup b~waen th~ anod~
~70Z~;iO
and the cathode aeroa~ ~ubstRntlally the entire chann~l wlath a~ ~ho~n.
~he anode 26 and cathode 28 are charg~d by a direct current ~ource 30
havlnq it~ poBit~ ve termlnal connected to the anodo 26 through llne 32
and lts negatlva termlnal connect~d to the catho~e 2B thxough llne
~a. ~B i~ deplated by the curv~d ~rrow~ withl~ the oha~nel os nrea
ln~ld~ betw~en the lnlet and outlet o~ tho ~pparAtu~ the a~fluent
pr~ferably has 80mo turbulence ~o thAt lnrg~cal~ mlxin~ o~ tho
particle~ occurs a~ it p~3~e8 ~hrough ~ha app~ratu~. Becau~e a~
the mixing actlon, virtually n~ particle~ will remain for any length
o~ tima ln the re~ion contalnin~ lons of both ~igna clo~c to the
po~ltively aharged electrode 260 The partlale~ wlll b~ s~ept into
the r~gion batwaen alectrode~ 26 and 2B dusing thls pas~age. The
electrodes 26 and 28 are preerably generally fla~, planar membess
havlng arcuate edges ~hat are charged by the external ~ource 30 to
positlve and negatl~e potential~, se~pQctively. ~ha ~nslde surface
of electsode 26 1~ sho~n to ba generally coplanar with ~ide wall
18 slnce ~he flat electrode fits an opening in the right slde wall.
~he generally flat configuration3 and curved edges of ths cath~de
~nd anode are preferred to minimize electx~c ~leld ma~ima, i.e. lt
18 ds~irable that the average field strength approach the maxlmum
field strangth withln the apparatu~. 5tated ~n othcr word~, it 1~ -
deslrabl~ tha~ the electric fleld be uniform 80 that i~ oan be ma~imlzed
~ithout experienclng electrlcal braakdown or arcing. The electsode
26 separates tho stre~m o~ gaseou~ medium on lts left ~ide a~ ~hown
in thu drawing ~rom a quiesce~t g~seous medium on it~ rlght ~lde that
is prefer~bly sealed from the left ~ide to prevent du~t to accumulate - -
between the ~lectrode 26 and the w~ndow 24. ~he thin wall or window
24 separata~ the qulascent meaium from a region o~ vesy lo~ pre~ur~
l.e, as ~uch as 3 x 10 4 tor. The wlndow 24 be fabrlcated from ~ny
material that will tran3~1t electron~ ~herethrough that l~ o capablo
o~ ~eparating the lo~ pre~sure ~ in the electron generator 20 ~ro~
tha ext~rlor pre~ure. Thus, the ~indow 2~ ~an ~e mado ~om tit~nlum,
- . . . . ~ . -
~07(~250
alhmlnum, ~talnle~ st~el, nylon ~nd th~ llke. ~he anode plnt~ 26
may be mad~ o~ ~ tbln she~t o~ conduotiv~ mAt~lnl such ~ t~nl~,
~luminum, or ~tainla~s sta~l wlth th~ combln~d ~hickne~ o~ tho nnodo
and window bolng pre~ara~ly le~ than nhou~ two mllo.~,002 lnoh~o) to
perml~ p~natration o~ the olectronu throu~h th~m. When th~ wlndow 24
L8 madn of conductiv~ mnterl~l, it ~nn b~ dn~igned to nl~o ~erv~ A~
the anode 26. Ilowov~r, 1~ gonexnl, it m~y ba pxofoxred to we A ~parate
Anod~ 26 to ~lmpli~y tho ~ervlclng of tha el~ctron boam ~onerator.
Further, ln c~rtAin embodlment~ it may be pre~esred to orm the electrod~
26 ~rom A ~creen ma~erial o~ ~rom rod~. Means for constrlc~lng or
expandlng the gaa ~low to ad~ust lto velocity and to control the mlxing
action or to control turbulence of the flow are not shown. ~he an~de
26 L~ preforably chargod to produce a8 hl~h a field strength a~ posslble,
generally of about 12 to 18 kLlovolts per contimeter in the gaseous
medlum. i~owaver, any potentlal up to the breakdown po~ential of the
gaseous medium may be us~d. The elactron beam ~enerator 20 i~ posi~loned
between the inlet 12 and outle~ 14 ~o as to irradlate tha gaseou~
medlum or effluent wlth elec~ron~ paseing through window 24 and anode
26 and tho generator prefera~ly ha~ pow~r to provide a~ eleatron bea~
havlng an enesgy den~ity ~u~ficlent to gener~te enough lons to charge
all the particle~ ~n the ga~eous medium to nearly saturat~on.
electron gen~rator i8 pre~erably po~itioned so tha~ it
lrradi~te~ only th~ volum0 immediately ad~acent the anode ~urfaca. This
~ achi~ved by using electron~ that can only penetrate a ~hort di~tanc~
into the ga~eou~ medium. Tha el~ctron generator 20 pr~ferably operate~
at ~ufficient vDlt~ge to produce lon~zatlon and ~u~ficlen~ ourrent to
generate tbR quant~ty o2 lon~ that are ¢apabie o~ c~arging the partlcle~
in ~he gassous streRm. In thi~ regard, the el~c~ron g~nsrator prefer-
- ably operate3 to provide eleotron~ Rntesing th~ gs~eou~ medium wlth an
energy o~ betwaen about 1 ~8V and about 12 KeV p~r contimeter of plato
~ep~r~tion ana at a current lqval~ o~ ~bDut ono mlcroamper~ per ~et~r
o2 eluctrod~ width perpen~$culnr to the ga~ ~lo~. For a con~lg~r~tlon
~070ZS~)
havlng n .5 mil thlck t1tanlum wlndow whero the wl~d~w al~o ~unctlons
a9 the anod~ Rnd wlth a 10 cm ~p~clng between the ano~e an~ ca~hode,
the ~ppar~tu~ per~Qrmed ~atlsfActorlly wlth alectron~ o~ betweon 100
XeV ~nd 115 XeV ~nt~r1n~ the wlnflow.
In Accor~nnc~ with nn lmpoxt~nt ~ap~c~ ~ thn pres~nt lnv~n-
tion, ~ the window ~ of tho electron boam qenor~tor nl30 ~at~ a~ the
olectrode 26, th~ wlndow 24 oxpo~d t~ the partiale lnd~n qaBeOUB modlum
1B ~al~ ¢lQaning. ~ n sepnrAto ~leakrode 26 i~ usud, the ele~tron b~am
from tho qenerAtor ~dt~ to prevent particle bulldup on the anod~ 26.
While there may be some partloles on the expo~ed ~ur~ace o~ elther
confl~urnt~on at nny one tlme, thera 1~ no bulldup o~ particles on it
dua to lt~ s~l~ cleanlng operatlon. ~he expo~ed surfAce doe~ not ex-
perlance any accumulation of small partlcles beQau~e they axe repelled
be~ora they can reach the sur~ac~. A$ ~he qmall partlcles ~re bombards~
-by the electron~ produced by the electron generator, the ~lectrona go
completely through them causing ~econdary emls~ion and tha smalI part~-
cl~ becGmes posltlvely charged and ~ rep~lled by the po~itively charged
Qurface. ~hu~, ~mall particle~ never reach the ~urf~se and cannot
accumulate on ~t.
Wlth reApQct ~o larger partlcle~, however, the electro~
bomhardlng ~he particle wlll not travel through the partlcle and s0cond-
Rry .em~s~ion affect~ wlll not be ~igniflcant compared to the piling
up of electron~ withln the part~cle. Thu~, voltage on the lnslde
d the part~cle build~ up w~thln the partlcle and lt becomes qulte
25 negat~Ye. If the partlcle 1~ in contac~ with the ~rf.ac~ lt wlll
~i~charge to th~ polnt o~ con~aat b~tween the partlcle and ~ho surfac~. -
Thia dl6charge pro~uco~ ~ dlscharg~ path that c~n ~Q analOgl%0d
to the ~h~pe o~ a tree, 1~ h~ d~charge path goes from the branche~
~nd comblnes ~n a large~ trunk portlon wher~ lt contacta ~he ~urface.
~he paths ar~ hol~s ln the part~ala cau~ed b~ vaporl~ing th~ solld
9 the p~rtlcle to ~ g~s, ~h~ ~aporl~atlon produce~ a ~housa~d fold
Yolume lncreaae whlch e~c~pes through ~he dl~charg~ pAth8~ ~h~
:
-10-
~ 0702~0
vaporlzatlon procQ~a producas a grP~t f~rc~ that blows th~ particle
from the surfacQ or d~stroy~ th~ pnrtiqle lt~l4, ~lther re~ult being
e~eative to rld the ~urface of the partlale. ~or~ov~r, the ~orco
o~ ona partlcl~ be~ng removed will e~ctlv~ly xemove ~ovoral othera
as well,
Thi~ cl~aniny not~on can b~ inarea3ad b~ incr~Aslng th~
op~rating volt~g~ o~ the ol~ctron ~on~ra~or. It ~hould therePor~
be understood that tha operating voltage c~n ba varlad, perhap~ p~rlod-
1CA11Y~ tO control the cleanin~ ~ctlon. ~n op~lmum duty cycl~ can
10 be eotablished tl~at woul~ eff~at ad~quate cloaning and minlm~ze tha . .
povQr r~ulrements for th~ ~va~all opera~ion o~ the apparatus.
The upper ~lQctria field ~tr~ngth limit ls deter~ined by
the dlel~ctric strength of ~ha ga~ou~ ~edium at operating pre~ure.
~or a ten centlmetsr separation di~tanco b~tween cathode and anode,
a separation distance used ~n one ambodlment of the apparatu~, tha
unlfonm field breakdown ~rength of alr at normal dan~lty i~ about
25 kV/cm. Since the ab301ute tcmperaturc il~ a typical gaseous e~fluent ~ -
will be ln the rangc of about 400K ~o 500~K, the gas density vill . ;.
bs about a factor o~ two lower than no~mal atmospherlc den~ty, and
the llmltlng field ~tr~ngth would be about 13 kV/cm. ~owever, electron-
~ttaching ga~e~ ~uch a~ sulfur dioxide for example~ will o~ten
be pre~ent in a gaseou~ ef~lueDt, and the pxes~nce of Shese ga~e~
may~enable operatlon at a higher electrlc ~eld value ~han the descri~ed
13 kV/c~.
It ~hould al80 be under~tood that the electron genera~or
~ay generatu ~ singla b~oad steady be~m or one ox morQ narrow beam~
~nd may al~o be adapted to ~can the area withln the apparatu~ in
a pradeterm~ned p~ttern. For examplo, the pattern may have the beam
gollow a moving ga~oou~ medium through a volume for an average dwell
time for partlcle~ w~thin ~hat Yolume, then ts~at other volume8s~cco~siveIy in llke manner an~ then nf~er an average dl~fu~lo~ tlmc
require~ to repopulate the ~irst xffglon wlth partlelen~ seturn to
--11~
~070250
thAt fir~t volume.
Tha rQsldu~l, ambi~nt mlxing nctlon or turbulence o~ the
flow of tho gns~ou~ m~d~um through tho nppnr~tus carrie~ ~lo p~rtlclu-
l~don gaseous modlum to wlthln n dlatanoa d~lnlng the lmnln~r ~low
S bvundary ~ubl~yor of th~ c~rgod eloutrodou. ~llthln n r~tglol- o~
thlcknuo~ compnr~blu wlttl tho rnngo Or tho ol~otronrl ln th~ modlum,
~hn cllnrgo oll du~t p~tlcla(l la nenxly noutrnll-o~ ~onAu~o o~ ~.h~
pro3onad o~ lon~ o~ bot~ nlgn~, In ~ha r~t ~f thu volum~ however,
tholr ch~rqng r~t~ nre no lonqcr nautrAll~ed nnd bui~d xapidly
0 80 that by the t~me the oddy ~otlon carries the ga~eous ~low to
and thsn nw~y from tbe cnthode 28, dust particles which have positlve
cb~xge rsmain becau~e o~ the electrical force that is exerted upon
the charges. ~he partlcle~ may acquire additional charges by i~pinge~ent
of qaaeous lons wh~le tbsy are attached to the csthode. Thii would
l~ lncrea3e the holding force 90 that they uould not be inclined to
be diYlodged. ~o~ever, lf the du~t is of vary high sesistiv~ty,
excesslve local field strength can result from thiY chsrge buildup
~nd cause harmful local breakdown. Thl3 local breakdown can be prevented
by keeplng the ion denRlty in the qaseous ~edium low except in the
rogion o~ lnltla~ partlcle chasging. Partlcles of all slzes rapldly -
collect on ths cathode 28 because the electric ~leld, no longer limited
to about 4 kV~c~ in the bulk of the qas by th~ requirement of corona
generation at one edge, ~an be rai~ed to between 13 a~d about 18
kV/cm. In thi-~ preferred form of operation, the hign fleld covers
25 virtually all the dlatance betveen the electrodes. It ~hould be ~nder- -
stood that while the above de~crlptlon deals wlth p~lllng po~ltive
ions fro~ th8 reglon of neutrol plasma, the present lnvention ls
aPP1$Cab1e tO 10D~ of negative polarlty. ~o~ever, ~he use of posltive
ion~ ha~ the advantage ~n that elsctrons and negative lons are pulled
bac~ toward the anode ~nd the thlckneaa o~ the reglon of neutral
pla~sa 1~ ~inlmlzed, a~ lo de~lred. As prevlously mentioDed~ the
satu~atlon ch~rge by the u~ual mech~nl~m of ~leld cb~rglng 1~ ~ub~ect
-12-
1070Z50
to a limlt cnu~d by the el~ctro~tatlc repul~ion b~twsen the pnrtlcles
that have ~cquir~d ~ ch~rge ~nd additlonal chorge~ which ~pproAch
~t~
~n nccord~nco wLth tho pros~nt lnvontlon, howovor, tho
Aatur~tlon ohnrge on ~11 p~rtlol~n 1~ muoh ~treAtor b~aau~o ~u ~n
oluctrlo ~iold ~tr~n~ oAn bo rnlood by nbout n ~nctor o~ botw~on
~bout 3 nnd 5. ~hu~ a mnxlm~ o~ botwoon nbout G0 ~nd ~0 ahA~o~
uould bo dnl)oD~to~l ~n ~ 0,3 mloron pn~tlcl0 ln nn 18 kV/~m ~lol~,
~)~llo only nbout 20 eo 30 ch~r~ Ar~ ~ypi~lly depo~it~d durlng
tho tr~n~lt o~ such ~ pArtlolo throu~h ~n ordinary eleatro~tatio
preoipi~ator. Wlth re~poct to Pleld oharglng, the ini~ial charglng
rnte 18 glven by
!dt 4~7 x 10 5 ~ No~ D2
E ~ 2
wherfl D i~ the particle diameter ln mlorons, ~ is the electric field
strength in kllovolts per centimeter, F LS the dlelectric constant
of the partlole, and No i3 the amblent ion concentratlon in nu~ber
per cublc centimeter. Values ~or ND- are nbout 3 x 107 per cublc
cent~meter in conventional preclpitators. In the present lnvention,
No iY controlled lndep~ndently o~ tho field stsength E, wherca3 thsse
two values are interllnked ln conventlonal pr~clpitator~. The field
~trength can be controlled independently of ~O to achieve particular
advantages, ~.e., the ~leld strength can be reduced to mLnimize power
consu~ptlon or ~ncreased to maximize the charging rate. ~or example,
ln an 18 ~v~cm fleld, wlth an No o~ 3 x 107/cm3, dN/dt equal~ between
abo~t B00 and 2200 per second for ~ 0.3 mlcron partlcle BO that the
pnrtLcle very rapidly approache~ its saturation chnrgs of about 60 to
80. I~ for other reasons, lt 18 necessary to reduco the field, the
chsrglng rat~ c~n be ~alntalned by increaslng No~
From the aboYe, it ~hould be under~tood that a l~rge decre~e
~n charqing tlme ~8 well ns a lnrge lncre~se ln totnl charge occurs for
0-3 ~lcron dL~m ~ter pnrtlcle belng-charged in the lnrge electrla
flelds thnt c~n exlst in ~ ch~nnel where lon~ are nuppli~d by the agency
-~3-
1~70~S0
of hlgh-energy electron3 ~rom an ~l~ctron generator, for ex~mple, rnther
than ln ~he small~r overall ~lelds typicnl of Q convQntlon~l preclpl-
t~tor, Thus, electron beam supportod charglng may exceed charging cur-
r~ntly u~Qd ln prlor nrt preclpltAtor~ ln both tho rnto an~ mnxlmum
charge attnlnnblo ln roA~onAblo dwoll tlm~l o~ pnrtl~lo~ ln tho I)ro~lp-
lt~tox, ~nd ~l~o mny roqulro loo~ pow~r ~urln~ oporAtlon~ Purthen~ro,
tho olectrlc ~lold nctln~ on onch o~ t~Q~o ch~r~o~ lo l~r~or by n fac~or
o~ nbout 4 nnd wlll provido nn nverAqo preal~it~lon vqloalty that will
bo abo~t 12 ~ln~e~ l~rcJer thnn th~t whlch would~e~perlenced by 0.3
mlcron par~lol~ ln pre~ent conventlon~l pxcclpltator~. ~lowev~r, ~lnce
only ono o~ the two aur~Aces colleat3 thefle partlcle~, the effectlve
oollection rate per unlt AreA wll~ only be lncrea3ffd by a ~actor of
about 81x. An alternatlve deslgn uslng two electron guns on opposlte
side3 of a central colleotlng cathode would lncrease the collection rate
by a,factor o~ about twelve. Alternatively, an electron beam may be
pro~ected down the center of the preclpltator to produce a plasma. ~ach
electrode would then attract it3 respective opposltely charged partlcles
and would be preclpitated out o~ the gaseou~ medium.
- ~eferring to FIGURE 1, it should be understood that the thln
curved electron beam wlndow 24 is preferably covered wth the thin
metal anode 26 to protect the ~tressed window 24 from corroslve
gases and largu particles in the gaseous medium or effluent. The
thin flat protectlve cover anode 26 also produces a smoother electrlc
field distribution and thereby allows a higher average fleld strength.
Turning now to FIGURE 2 which illustrates one form of
app~-atus th~t i~ useful in practicing the method of the prQsent
~nvention, the apparatus 40 ha~ ~n inlet 42 at lts lower end and ~n
outlet 44 at its upper end, with gaseous medium or effluent flowlng
Yert~cally upwardly a~ shown by the arrows. The dust laden gaseou~
effluent preferably flows in the prealpitation channel at 5 to 10
meters per ~econd, An electron generator 4~ 19 posltloncd to lrradlate
the effluent whlle ~t i8 wlthin the channel 46. ~ cathode 18 provldod
1070~2S0
an~ mny be ln th~ for~ o~ a ~lexlble ~t~lnlo~a ~teel b~lt 50 ~8 ~ho~n
which travel~ around upper and lowur roller~ 52 and 54, raspectl~ly,
~lth on~ o~ tha lowor rollers belng drlven by a motor 56. ~he belt .,
ha~ n frone ~ld~ axposed to the qa~eous medlum or e~luent cont~nlng
S hlqh ros13tlvlty duu~ pas~1ng through tho ahannol ~nd n hnok Hldo
eh~t 1~ out~1do o~ the ohnnnol~ on~blln~ th~ pnrt1closl to ~n roll~ov~
from eho bolt bofor~ tha bolt roontoru tho oh~nnol an~l n~Jaln b~com~n
oxpoood to tno ~luont. Ono ~vnn~ngo o~ tho ~pp~rntun nhown ln
F~CUR~ 2 ln ~hn~ lt l~ o~ ~ rolntlvaly llm~ll hel~ht aompAred wltll
10 laas ~f~a~lv~ prlhr n~t pr~lpi~or~ ~or A ~lven througl~put rate.
~ n aooord~nco with Another A0peCt o~ the presant lnventlon
and referrlng to the cro~s-~ectlon~l vlew ~hown ln ~IGUI~ 3, appnratus,
lndlcAted generally At 6~, nnd al~o embodying ~ho present lnvention,
communlcntes a qasaoUs me~ium or effluent ln A dlrection tow~rd the
re~der. A~ l~ deplcted by tho curve~ arrows with~n th~ apparatus,
th~ effluent l~ preferably glven Bome turbulence 80 that large scale
~lx1ng of the partlcle~ occurs as it passes through the apparatus.
Because o~ the mlxlng act~on, the particles wlll be swept around ~nd
brought ln close proxlmlty to negatlvely charged cathode~ 62 as well
a8 the posltlvely charged anode 64 durlng thls pa~age. The turbulent
actlon se~oves p~rtlcles from the reglon o~ neutral charge den~ity
noar tha electron ~ea~ wlndow, brlnglng them through the reglon of
posltlve charge d~nslty to ulthln close range o~ the aathodes. This
~110~B all partlcles to be attracted to tha cathode~ ~o that they may
bo sub~ect to prwlpitation out o~ the gaseous medlum before lt 1Q dis-
charged at ths outlet. It ~hou~a be understood that while the dlagrHm-
mat~c representation sh~n ln FIGURE 3 does not lllustrate elther the
~de or end exterlor ~nllJ o~ the Appnratus, the eleotrode~ 62 and 64
- wlll be posltloned wlthln tha outer slda wall~ whlch guide the ~low
33 o~ a~luant through tha apparntu~. The eloctron generatorn pre erably
comprlsQ a number o~ thln wlres or roughened rod~ 66 enclo~d wlthin
ovacuated tub-~ 68 ln the ~node our~Ace 64. ~hese ~ro3 nra small and
-15-
~Q70250
~harged to a su~lclently lArg~ n~gntlv~ potentl~l that they emlt
electrons by fleld emi~lon, Altornatlvoly, the wlros 66 mAy bo he~te~
and ~mlt electxons thermionlcally. ~hese electron~ are attr~cted to
the thin u~ode wall tubes 68 nnd, becau~e of the hlgh voltage dl~ference,
S hn~o sufrlcient enor~y to penetrate the thln Metal ~node 64. ~node
supports (not ~hown~ con~l~t Or atructurnl ~oln~orclng loop~ o~ motal
t~nt nro ~pnco~ porlo~onlly ~ltllln tho tub~n G3. Tl~o oporntlon lo
~ub8t~nti~11y slmllnr to thnt doocxlbo~ wlth roapoot to tho ~ppnrntuo
ot FlGURE 1. ~ ~v~ntn~ o~ thu con~l~urntlon Or FS~Un~ 3 ln tl~At l~
th~ ~aouum soal no~ ono o~ tho wl~ 66 l~ bxoXon~ v~lta~e c~n b~
romoved ~rom the brokon wlro 66 wlthou~ Aub~t~ntiAlly ndver~ely
arfqctlng the op~ration o~ the npparatu~.
From the to~egolng detAlled d~crlptlon, lt ~hould be under-
~tood that ~ method ~nd flpparatus for electro3tatically preoipltating
partlcles from ~ partlcle carrylng gaseous medlum has been illustrated
and described whlch l~ more efflclent than conventlonal designs and 18
e~ectlve ln removlng extremely ~mall partlcles, even to such ~mall
slzes as 0.1 micron in dlameter. In addition to effectlvely preclpl-
tating ~uch small particle~, the pr~sent lnvention provides rapld
charqlng and rapid preclpl~atlon of ~uch small aR well as larger par-
tlole~, and enables fa~t throughput of the gaseou~ medlum or ef$1uent.
Although particular embodlment~ of the present lnventlon have
been lllustrated and de~crlbed, var~ou~ modiflcation.s, ~ubstltutlona
~nd Alternativ~s wlll be apparent to tho~e ~klllcd ln the art, and
25 accordlngly, the scope o~ the inventlon ~hould be only de~insd by the ~ ~
nppended claims and equ~valents th~reof. - -
Various ~eatures of the inventlon are ~et forth in thQ
follo~ing clAims.
-16-
