US2976951A

US2976951A - Electrostatic precipitator

Info

Publication number: US2976951A
Application number: US767967A
Authority: US
Inventors: John S Lagarias
Original assignee: Koppers Co Inc
Current assignee: Beazer East Inc
Priority date: 1958-10-17
Filing date: 1958-10-17
Publication date: 1961-03-28
Anticipated expiration: 1978-03-28

Description

March 28, 1961 J. 5. LAGARIAS 2,976,951

ELECTROSTATIC PRECIPITATOR Filed Oct. 17, 1958 2 Sheets-Sheet 1 INVENTOR. 51 JOH /S. Leseleras BEMQW March 28, 1961 J. 5. LAGARIAS ELECTROSTATIC PRECIPITATOR 2 Sheets-Sheet 2 Filed 001. 17, 1958 CURRENT VOLTAGE. CHARACTER ISTICS OF H VW PZECIFITATOZ VOLTAGE I N K V IN VENTOR. JOHA/ S. Lasezres.

69W, TTOEI/QY United States PatentO 2,976,951 ELECTROSTATIC PRECIPITATOR John S. Lagarias, Pittsburgh, Pa, assignor to Koppers 1 I This invention relates generally to electrostatic precipitators and more particularly to a method and ap paratus for removing material deposited on the electrodes of electrostatic precipitators.

It has become conventional practice to remove the particles which are entrained in a gas stream from the gas stream by the use of electrostatic precipitators. Such precipitators have suspended in the gas stream two sets of electrodes, a corona electrode and a collector electrode. As the gas passes these electrodes, it is ionized and the entrained particles are given an electrical charge thereby so as to be attracted mainly to the collector electrode. The clean gas leaves the precipitator and the collected material is removed to a hopper and thereafter re moved from the precipitator.

The removal of the deposited material from the electrodes are conventionally of two general types: the impulse type, wherein a hammer is actuated periodically by a spring, magnetic or pneumatic actuating means against an anvil to cause an impulse and the vibrator type, wherein a plunger is reciprocated rapidly against an anvil connected to the electrodes by means of spring, magnetic or pneumatic actuating means. I

It is desirable that the dust be dislodged from the electrodes in the form of large agglomerates of such size that they fall against the dust stream into the dust'hopper.

Therefore, the rapping blows should not be so powerful as to disperse the dust and increase the re -entrainment or,;stated differently, since the material from the electrodes'ca'n be re-entrained, the magnitude of the rapping blowsshoud be as light as possible because re-entrainment of the dust will considerably efiect the efl'iciency of the precipitator. Additionally, heavy blows of high intensity may damage the electrodes and their supporting structure.

The mechanism which carries the particles of entrained material from the gas stream to cling to the precipitating electrodes and which causes the particles of material to cling to each other so as to form agglomerates is not .of the current flow through the dust results in a pasting effect which holds the dust more rigidly to the plate than it'would adhere if there isno'current fiowpresent. The

glegree of adherencedepends uponsuch factors as the "ice intensity of current flow, the electrical conductivity of the dust and the degree and type of packing which the dust undergoes. To cause the material to be deposited at all on the electrodes, the potential supplied between the electrodes must generate a corona-effect which is great enough to ionize the gas between the electrodes and cause a current flow through the gas between the electrodes. Under normal conditions in a typical precipitator, for example, the potential difierence which causes these effects may be about 60 kilovolts; and in such a case, if the potential diiference be reduced to 25 kilovolts or less no current will flow between the electrodes and the ions already present in the gas stream and on the pre cipitated material will flow to the grounded electrodes.

According to this invention, the rapping of the electrodes is eifected by vibrations of low intensity when the potential between the electrodes is momentarily reduced to a predetermined level thus effecting the cleaning of the electrodes without damage to the electrodes and without appreciable re-entrainment in the gas stream. Accordingly, this invention provides a novel process and apparatus for reducing the potential diiference between the electrodes to the extent necessary during the period the electrodes are being rapped so that no further current flows between the electrodes and thereafter raising the potential difference to the predetermined optimum so that current flow between the electrodes again takes place,

thereby obtaining the advantage of making the change in potential as slight as possible so that the power loss is minimized.

The above and further novel features of the invention will appear more fully from the detailed description when the same is read in connection with the accompanying :drawings; It is to be expressly understood, however,

that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

another embodiment of the rappers of Fig. 1.

Fig' 5 is a current voltage curve of a typical electrostatic precipitator.

An electrostatic precipitator 11 is shown (Fig. 1) with its shell removed. For the sake of simplicity only two preoipitator lanes are shown, although advantageously a plurality of precipitator sections are provided in parallel. Conventionally, each section is provided with two sets of

electrodes

13 and 15; Set 13 is advantageously in the form of wires which are suspendedfrom' a supporting member '17 that is supported in the precipitator shell from suitable insulating means (not shown), while set 15 is advantageously in the form of plates suspended from a supporting member 21 supported from the precipitator she'll. At the bottom of electrodes 13- are weights 19 which properly distend the electrodes 13 in the precipitator shell whereas no weights are necessary to distend the plates 15. i

Electrodes 13 are given a high potential with respect to electrodes 15 which may be grounded so that a corona current is produced therefrom and thus, as dirty gas passes into the vprecipitatorbetween the electrodes described, the gas is ionized and the dust entrained in the gas thus picks up a charge so that'it is mainly attracted to collecting electrodes 15. .Clean gas then passes out the precipitator. The potential to

electrodes

13 and 15 is supplied from a conventionalalternating power source 28 through a circuit comprised of a conventional voltage regulator ;3l-,-.lead=29, contact 32, armature 33, lead 35,

aura-es 1 primary winding 37 of step-up transformer .39, ,lead 41, junction 43 and lead 45. Stepped-up voltage from secondary winding 47 or transformer 39 is supplied through leads 4 3- and '52,

conventional rectifier tubes

51 and 53, and leads 57 and 63 as a pulsating direct current so as to produce a negative potential to -electrodes 13 and a positive potential to electrodes 15 relatively between the electrodes. Junction 61 of lead 63 may be connected to ground by means of suitable connections (not shown) connected to the precipitator shell which is grounded.

It has been found, as described above, that to cause material to be deposited on the electrodes, the potential supplied to the electrodes must be high enough to generate a corona current. Under normal conditions in con ventional precipitators the potential difference is about 60 kilovolts to generate the required corona. Such a corona is advantageously just below the level at which excessive sparking occurs between

electrodes

13 and 15.

.In such sense, if the voltage is reduced, the current flow from electrodes 13 to electrode 15 will be reduced as illustrated in Fig. 5. Also, under normal conditions, a certain amount of voltage Ec must be impressed on electrodes 13 in order to ionize the surrounding gas and produce a flow of current from electrodes 13 through the ionized gas to electrodes 15. Thus when that voltage is reduced below a certain level there is no flow 'of current switching systems, to be described in-more detail hereinafter, may be applied to the primary circuit during the voltage interruption period to cause the voltage to fall.

If the time'constant of the precipitator is not excessive the normal characteristics of the power supply will reduce the voltage below Be rapidly enoughso that no additional load is necessary.

Fig. 3 shows a circuit with a .timing'controlifor sequentially lowering and raising the voltage at predetermined intervals. To this end, a conventional timer 65 is'provided with a suitable power source 66 for energizing relay coil 71 in a predetermined sequence through

leads

67 and 69. Thus when leads'67 and 69 are energized, relay'coil 71 pulls annatures 33 and 73 downwardly.

When armature 33 is pulled downwardlyit disengages contact 31 and engages contact 75. Thus high voltage current from'source 28 no longer flows from 'lead 29 to lead 35 and low voltage current begins to flowfrom lead 77 through contact 75, armature 33, lead 35, coil 37 and lead 41 back to a 220 volt source of current 28a to efiectively lower the voltage on transformer 39. This relay coil 71 may be part of a conventional contactor, typical of which is the Westinghouse-type 15-825-NT45A. When relay coil 71 is de-energized this pulls armature .33 upwardly to re-engage it with contact 31; Thus, by

varying the primary voltage input, the voltage supplied to transformer 39 is raised and lowered in a sequence so that corona current is generated by the

electrodes

13 and 15, removed and then reinstated.

The duration of'the voltage reduction period depends upon the thickness of the dust, the resistivity of the dust, and the movability of the charged particles entrained in the dirty gas stream. The dust movement depends on the location of the charged particle, particle size and field strength. Most of the current is not carried by the dust but by the gaseous ions which move through the gas in approximately 0.01 ofa second. Through the dust,1 however, the flow of current is iess rapid.

At steady state the current is flowing through the dust at a. uniform. rate. If the voltage is interruptedthe rate 4 of .currentflow would be approximately that of the time constant RC involving the capacitance and resistance of the dust. The time constant is a measure of the rate of voltage fall and may be approximated to give the rate of current decay.

Based on test results of such current decay, it is estimated that the interruptionpjeriod may vary between 0.02 to 5.0 seconds depending upon the material of the dust and the rapping efiect desired. 'It'is consistent that for very conductive dust the interruption of the voltage may be sufiicient whereby no .zrapping .atall will be required to remove deposited material from the electrodes.

Normally, in accordancewith this invention, rappers 25 (Figs. 1 and 3)

vibrate electrodes

13 and 15 to remove material deposited thereon. This causes the deposited material to be removed from the electrodes and the deposited material then falls by gravity into a hopper (not shown) at the bottom of the precipitator. For'this purpose, advantageously pneumatic reciprocating rappers 25, as shown in Fig. 2, which are attached to the electrodes, as shown in Fig. l, are actuated. To this end an air sup ply (notshown) is attached to the rapper housing 78 so as to reciprocate rapidly hammer 79 against anvil 80 connected to the supporting members 17 and 21 .respectively by suitable connecting rods 81. The intensity of the rapper vibrations are easily adjustable. For example, the air volume and pressure supplied thereto from a suitable air source (not shown) may be increased or decreased.

When pneumatic reciprocating rappers (Fig. 3 are used in accordance with this invention, advantageously air is supplied to the rappers intermittently. To'this end, the conventionaladjustable electrical timer 65 is set to energize solenoid 71 at predetermined intervals, as described above. Solenoid 71 when energized closes armatures :33 and 73 with

contacts

75 and 82 respectively. The engagement of armature 73 with contact 82 energizes solenoid 83 which actuates two-Way valve 91 to cause air to be supplied to rappers 25 causing vibrations to be imparted totheelectrodes 13 and 15. When armature 73 is disengaged from contact 82 this actuates the twoway valve 91 to cut oif the air supply to the rappers causing-the vibrations to these electrodes to cease.

in the normal operation of the electrostatic precipitator 11 according to ;this'invention,-the voltage supplied to the discharge electrodes from the 440 volt current source 28 is-typically about 60 kilovolts. At this'voltage there is a current flow of about 6.75 milliamperes through the gaseous ions moving through the gas and through the dust layer deposited on the precipitator electrodes. This current flow is primarily through the gas but the small current fiow through the dust apparently causes a cementing action which tends to hold deposited dust on the electrodes with great'tenacity.

The voltage is then decreased sequentially for appropriate time intervals depending,.for example, on the dust layer on the electrodes, therresistivity of the dust and the mobility of the charged particles below that voltage which causes corona around the precipitator electrodes. To this end at appropriate-intervals timer 65 causes armature 33 to disengage contact 31 and to engage contact 75 so that 220 volt current is supplied to the precipitator electrodes from a suitable source (not shown). The time 'is:no:current :flowthrorighthe dust layer the cementing action is snbstantially'removed and the vibration in the electrodes 1 increases the cleaning action of the electrodes. 'l hus the low intensity rapping of-thisinvention removes reassess inaterin rrsm'the pree' itator'electrodes f-when tlie vatage' on '--'the electrodes is "reduced in accorda'nce with this invention. Thus the cleaning action of the rappingis increased and the overall efifect is to effectively clean the electrodeswhilereducing the r'e-entr'ainr'n'entof material cleaned from the electrodes. g

It is understood by one skilled in the art, from the above, that using one 440 volts power source and a resistance or reactance connected in series between leads 29 and 77 may operate aswell "as'when using 220and 440 volts power sourcesbut it is-inoie'economical to switch from one voltagetoanother asdescribed.

A further embodiment is also contern'plated by this invention using the double contact switch described above. This embodiment incorporatesan impulse rapper-such as a magnetic rapper 25a shown in'Fig. 4'which' is energized :atthe same time that the voltage on the precipitator elec- "trodes is reduced. Tothis end-armature l3 iscorinectedso that when it engages contact 82 current is suppliedfrom ia conventional power source through the rapper coil to actuate the hammer therein. Thus in this embodimentas "well as in the last mentioned above, the opening and closing of the armature 73 takes place during the period of voltage interruption so that the cleaning of the electrodes takes place during the voltage'interruption.

It is understood, however, that a separate timer from timer 65 may be used for'theclosing and opening ofarmature 73 in which case rapper 25 maybe caused to perate in like manner as described above before, during,a'nd after the voltage reduction which 'was caused in a manner as described above. v I The foregoing has described novel apparatus and method for electrode rapping. In accordance with this invention, voltage is reduced and the electrodes vibrated whereby the entrained material which has collected on the electrodes is effectively removed as ag lomerates under blows which are relatively light and 'whichdo not cause appreciable damage to the electrodes or re-entrainment of the material in the gas.

What is claimed is:

1. Apparatus for cleaning electrodes in an electrostatic precipitator having first and second 'electrodes' 'suspended therein on whichparticles are deposited from a dirty gas comprising a source of high current flow for supplying a predetermined first direct electrical voltage of onelpolarity to said first electrodes to produce a potential difierence between said first and second electrodes which is just below the level at which excessive sparking occurs between ;-said first and seconde'le'ctrodes and which is great-enough 1to ionize' said gas 'and'to' transmit electrical current be- :tween said first and second electrodes, said particles being :charged when said current is transmitted so that said par- .ticles migrate to said electrodes and are deposited thereon,

.a source of low current flow for supplying a predetermined second direct electrical voltage of the same polarity to said first electrodes to produce a potential difference between said first and second electrodes which is just below the level which produces current to be transmitted between said first and second electrodes by means of said ionized gas, means for automatically and alternately connecting said high and low source of current flow so as to cause said first and second direct electrical potential differences to be selectively supplied between said first and second electrodes, and vibrating means connected to said first and second electrodes operative when said second direct electrical potential difference is supplied between said first and second electrodes for vibrating said electrodes so as to remove particles deposited thereon.

2. The invention of claim 1 wherein said vibrating means is' a pneumatic actuating rapper which produces continuous low intensity vibrations in said electrodes.

3. The invention of claim 1 wherein said vibrating means is a magnetic impulse rapper which produces inter- ,mittent low intensity vibrations in said electrodes.

4. Apparatus for cleaning electrodes in an electrostatic precipitatorlthaving adischarge-electrode :and 'a collecting electrode comprising a 'first alternating 'electrical energy source of a predetermined firstvoltage; a second alternating electrical energy sour'ce of :a predetermined second :voltage'whi'ch is less than?said firstipredeterinined voltage, common means' 'forstepping-up said first and second predetermined voltages andsupplying them to said discharge electrode, and means for alternately connecting said common means withsaidfirst and second alternatingelec- 'trical energysources so as to alternately :supplvpredetermined third 'and'fourth voltages of the-samep olaritymespectively to said discharge electrode and means operative for vibrating said collecting electrodes only when-said fourthvoltage is supplied thereto.

'5. In an electrostatic precipitatorhaving first and' secon'd electrodes, said first electrodes being adapted tohaye voltage impressed thereon so' as to cause material entrainedjin'a dirty gas to be accumulated primarily on said "second electrodes, apparatus for changing the voltage 'supplied'to said first electrodes comprising a first alternating electrical 'energysource of predetermined relatively high first voltage, a second alternating electrical energy source of predetermined relatively low second voltage, common means for stepping-up and rectifying "said first occurs between s'aid first and second electrodes andzbeing great'enough to causesaidentrained material to accumulatcprimarilyonsaid second electrodes, said 'fourth' voltage being just below the level which causes said'entrained material to-accumulate on said second electrodes, means having a double contact switch for selectively connecting said common means alternately with'said first and second 'saidfourth voltage issupplied to said first electrodes'said material does not-accumulate on said electrodes and may be cleaned easily therefrom and means operative for vibrating saidelectrodes when said fourth voltage is supplied to s'aidfirst' electrode. 7

*6. In combination with an electrostatic precipitator havin'g electrodes which are adapted to accumulate material 'fr'ornagas having entrained material therein, a cleaning systemcornpris'inga first source of current for supplying a predetermined first electrical voltage of one polarity. :to saidelectrodes to p'roduce aforce which is great enough to cause material entrained in said gas to accumulate on said electrodes, a second source of current for supplying a predetermined second electrical voltage of the same polarity to said electrodes to produce a force which is just below the force necessary to cause material entrained in said gas to accumulate on said electrodes, means for selectively causing said first and second sources of current to alternately supply said first and second predetermined electrical forces to said electrodes, and vibrating means connected to said electrodes operative when said second predetermined el'ectrical force is being supplied to said electrodes for vibrating said electrodes so as to remove material accumulated on said electrodes.

7. In an electrostatic precipitator having high and low potential electrodes wherein a high potential difference is impressed between said electrodes so as to cause ionization of the surrounding gas and wherein said electrodes are adapted to be suspended in a gas stream having particles entrained therein so that there is a current flow between said electrodes due to ionization of said gas stream by said high potential and wherein said particles are charged so that they are caused to deposit on said electrodes, a source of high current flow and a source of low current flow apparatus for selectively connecting said high and low current flow sources thereby increasing and decreasing the potential between'said electrodes whereby current flow between said electrodes is reduced and whereby low intensity vibrations in said electrodes will efiectively remove particles deposited thereon comprising means for vibrating said electrodes with low intensity vibrations, first means having a first alternating electrical energy source of a first tension for supplying a first high voltage of one polartiy to said high potential electrode which is just below the voltage which causes excessive spark-over between said electrodes so as to cause high current flow between said electrodes, second means having a second alternating electrical energy source of lower tension than the tension of said first alternating source for supplying a second voltage of the same polarity to said high potential electrode which is just below the voltage which causes current to flow between said electrodes, and means for selectively connecting said first and second means with said high potential electrode at predetermined time intervals whereby said current flow is reduced and particles deposited on said electrodes are removed from said electrodes by said low intensity vibrations on said electrodes.

8. An electrostatic precipitator for cleaning foreign matter from a gas stream having foreign matter entrained therein, comprising first and second electrodes suspended in said gas stream, vibrating means for producing low introdes, means for alternately connecting said first and second means to said first electrodes, and means for actuating said vibrating means to remove matter accumulated on said electrodes when said second'means is connected to said first electrodes whereby matter accumulated on said electrodes can be removed therefrom by low intensity vibrations.

9. An electrostatic precipitator for cleaning foreign matter from a gas stream having foreign matter entrained therein, comprising first and second electrodes suspended in said gas stream, vibrating means for producing low intensity vibrations attached to said electrodes, first means for supplying a first voltage of one polarity to said first electrodes so as to produce a first high current fiow between said first and second electrodes which causes said foreign matter to be accumulated on and to cling strongly to said electrodes, second means for supplying a'second lower voltage of the same polarity to said first electrodes so as to reduce said first high current flow whereby said matter accumulated on said electrodes clings less strongly thereto, means for alternately connecting said first and second means to said first electrodes, and

means for actuating said vibrating means to remove matter accumulated on said electrodes during and after the connection of said second means to said first electrodes whereby matter accumulated on said electrodes can be removed therefrom by low intensity vibrations while still preventing said gas from passing through' said precipitator Without being exposed to a current flow between said first and second electrodes.

10. An electrostatic precipitator for cleaning foreign matter from a gas stream having foreign matter entrained therein, comprising first and second electrodes suspended in said gas stream, vibrating means for producing low intensity vibrations attached to said electrodes, a first source of high current flow for supplying a first voltage of one polarity to said first electrodes so as to produce a first high current flow between said first and second electrodes which causes said foreign matter to be accumulated on and to cling strongly to said electrodes, a second source of low current flow for supplying a second lowervoltage of the same polarity to said first electrodes so as to produce a second lower current flow between said first and second electrodes whereby matter accumulated on said electrodes clings less strongly thereto, and means for alternately connecting said first and second sources of current flow to said first electrodes and having means for actuating said vibrating source of current flow when said second means is connected to said first electrodes whereby matter accumulated on said electrodes can be removed therefrom by low intensity vibrations while preventing said gas from passing said electrodes without being exposed to a current flow between said first and second electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,247,361

UNITED STATES PATENT OFFICE CERTIFICATIN 0F QGRECTION Patent No, 2,976351 March 28, 1961 Johns, Lagarias It is hereby certified that error eppears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 1, line 54, for "shoud" read should -g line 59, for "carries" read causes Signed and sealed this 5th day of September 1961a (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents