US1959374A - Method and apparatus for electrical precipitation - Google Patents

Description

Patented May 22, 1934 UNITED STATES PATENT OFFICE;

METHOD AND APPARATUS FOR ELEC- TRICAL PRECIPITATION Marcel A. Lissman, Temple City, Calif., assignor to International Precipitation Company, Los Angeles, Calif., a corporation of California Application October 1, 1932, Serial No. 635,829

11 Claims.

The principal object of the invention is toprovide a highly efficient method and apparatus for electrical precipitation, by means of which a relatively large volume of gas may be effectively cleaned in an apparatus of minimum size and with a minimum expenditure of electrical energy.

In certain cases, and particularly in the electrical precipitation of poorly conducting solids from gases, it has been found that when the voltage applied to the electrodes is reasonably constant, such as is obtained by the usual method of energization by rectified alternating current, either single phase or polyphase, of ordinary commercial frequency, it is difficult or impossible to maintain a sufficiently high voltage between the electrodes'to produce uniform copious corona discharge at the discharge electrodes, without leading to the production of arcing or disruptive discharge therebetween. If the voltage is in such cases kept sufiiciently low to substantially prevent arcing, the corona discharge and ionization are insuflicient to bring about the desired electrical charging of the suspended particles' which is required for eflicient electrical precipitation, while, on the other hand, if the voltage is increased in an effort to increase the ionization, serious difliculties are encountered due to excessive arcing or disruptive discharge which produces a momentary short circuit be- 40 tween the electrodes, and thus not only reduces the force tending to produce precipitation but also greatly increases 'the current flow and theconsequent power consumption. The use of discharge electrodes of extremely small cross-section and sharp surface curvature, such as wires or rods of small diameter, tends to overcome these difficulties to a certain extent, in that it promotes corona discharge and permits a relatively high degree of ionization to be maintained without the use of excessively high voltages. However, it is sometimes desirable for other reasons to employ discharge electrodes of somewhat larger cross-section, and in such cases the difllc'ulty of maintaining adequate corona discharge uniformly over the surfaces of the discharge electrodes is particularly apparent. The use of such relatively large diameter discharge electrodes with ordinary relatively constant unidirectional electrical potentials tends to cause formation of isolated brush discharges at only 0 certain points on the discharge electrodes, which brushes grow rapidly and are across to the collecting electrodes before any appreciable corona or ionization occurs at the remaining portions of the discharge electrode surfaces.

An important object of the present invention is to provide a method and apparatus whereby a relatively uniform corona discharge or ionization may be maintained over substantially the entire surface of the discharge electrode members, so as to provide a sufllcient number of ions to cause rapid charging of the suspended particles, while at the same time the formation of arcs or disruptive discharges between the discharge and collecting electrodes is substantially prevented, and particularly, to permit these desirable operating characteristics to be maintained even when discharge electrode members of relatively large cross-section are employed and even though the precipitated material is of a poorly conducting nature.

It has already been proposed to apply to the. electrodes, instead of rectified alternating current impulses whose form corresponds substantially to the wave form of ordinary commercial B5 alternating current after rectification, sharp unidirectional potential impulses of relatively short duration and steep wave form, such impulses having preferably a duration of about 10- second or less, and to deliver these impulses at such intervals that the period between successive impulses is relatively long as compared to the duration of each impulse, for example, on the order of one-hundredth of ajsecond or greater. It has been proposed to produce such impulses by charging an electric condenser or capacity means with unidirectional high potential energy, for example by means of rectified alternating current, and connecting said condenser or capacity means to the electrical precipitator through a spark gap adapted to break down when the condenser or capacity means becomes charged to a certain potential, so that the electrical energy stored in the condenser is applied to the electrodes of the precipitator only by means of the short high po- 105 tential impulses of steep wave front which occur at the time of each such breakdown of the spark gap. One disadvantage of such methods, however, is that condensers and spark gaps of relaatively large size are required, since the entire 111) electrical energy for precipitating purposes must be delivered therethrough. Furthermore, after eachsuch impulse the voltage between the electrodes is quickly reduced to the point at which ionization substantially ceases, so that during the periods between successive impulses there is not only substantially no ionization taking place, but furthermore, the potential difference between the electrodes is relatively low and consequently exerts only a relatively low force on the charged suspended particles to effect precipitation thereof.

According to the present invention, these objections are overcome and a highly efficient operation obtained, by continually maintaining between the electrodes a unidirectional potential of relatively constant value, for example by means of rectified alternating current, such continually maintained .potential being preferably approximately equal to or somewhat above the minimum potential required for ionization but being at a safe value below arcing potential, and also superimposing upon said continually maintained potential, sharp unidirectional high potential impulses of extremely short duration and steep wave front, and of the same direction. as said continually maintained potential. In this manner, there is at all times maintained between the electrodes a sufficient potential to cause effective electrical precipitation of charged suspended particles and to also preferably maintain some ionization, while the intermittently applied sharp impulses momentarily increase the potential above this value, to a point where an increased corona is produced, substantially uniformly over all portions of the discharge electrode surfaces. After each of such impulses, the voltage again quickly falls to approximately the value of the continually maintained potential, and the formation of large brushes or arcs is thus prevented. According to this mode of operation, the energy delivered to the precipitator during each of the superimposed high potential impulses is insufficient to effect a breakdown or disruptive discharge between the electrodes, but is sufiicient to bring about the desired degree of ionization due to the uniform corona at the discharge electrode surfaces. The relation between the peak voltage produced by each of the superimposed impulses and the relatively steady voltage which is continually maintained may be varied in different cases, but I have found that, in actual operations, advantageous results have in some cases been obtained by the use of peak voltages approximately twice as great, or at least 50% greater than the continually maintained voltage.

Various means may be'employed for producing the high potential impulses and causing the same to be superimposed upon the voltage continually supplied from the main source of unidirectional potential, but I have found that good results may be obtained by the use of a system of condensers and spark gap means acting as a voltage doubling circuit or voltage multiplying circuit, such a circuit consisting of a modified .form of Marx circuit impulse generator. This voltage multiplying circuit may be so connected as to be energized by the same source of unidirectional potential which is utilized for maintaining the substantially constant unidirectional potential between the electrodes of the precipitator. Such a circuit comprises essentially a plurality of electric condensers connected for charging in parallel from the unidirectional power supply means and connected in series, through intermediately disprevent discharging of said condensers otherwise than in the proper impulse circuit, which includes the respective condensers, spark gap means and electrical precipitator in series, and for preventing any high frequency surges from reaching the main unidirectional power supply means.

The invention is not necessarily restricted to the use of the same source of electrical power supply for maintaining the steady unidirectional potential at the precipitator and for energizing the impulse generating means. The desired results may also be obtained, although in a somewhat less economical manner, by the use of impulse generating means provided with a'separate' power supply source for energization thereof.

The accompanying drawing illustrates diagrammatically certain forms of electric circuit means which may be employed in accordance with this'invention, and referring thereto:

Fig. 1 represents diagrammatically one form of circuit for this purpose.

Figs. 2 and 3 are diagrammatic representations of other circuits which may be used.

Fig. 4 is a graph representing approximately the voltage-time relationship in an electrical precipitator operated in accordance with this invention.

Referring to Fig. 1, an electrical precipitator is sh wn at 1, comprising collecting electrode means 2 having ground connection 3 and high tension discharge electrode means 4, suitably spaced and insulated from said collecting electrode means. The collecting means may consist of a pipe, plate, screen or other member of extended surface, while the discharge electrode means may consist of a small diameter rod or wire, or other member of relatively small discharging surface and sharp surface curvature. The power supply means is shown as comprising a transformer 5 whose secondary winding 6 is connected to a suitable rectifying device such as mechanical rectifier 7. Such rectifier may comprise a disc 8 of insulating material, adapted to be rotated in synchronism with the alternating current supply to transformer 5 and provided with four equally spaced contact points 9 connected, as shown, by diametrically opposed conductors 10, and four stationary contact shoes 11 equally spaced around said rotating disc. Two of said fixed contact shoes 11, diametrically opposite one another, are connected to the terminals of secondary winding 6. One of the other fixed contacts 11 is grounded as at 12, while the remaining fixed contact is connected by conductor 13 to the discharge electrode means 4 of the precipitator.

The circuit thus far described is adapted to deliver unidirectional current to the precipitator, as in the ordinary method of operation of such apparatus, so as to maintain a steady unidirectional potential difference between the discharge electrode means 4 and the collecting electrode means 2. According to this invention, however, the output voltage of transformer 5 is so selected that the potential difference thus maintained in the precipitator is only about equal to or slightly above the minimum voltage required to 1,959,374 produce ionization or corona discharge, and is considerably below the voltage at which any tendency to arcing or disruptive discharge between the electrodes occurs.

In this form of the invention, the impulse generating means is shown as connected to the same source of unidirectional potential above described. Such impulse generating means comprises two electrical condensers 15 and 16, which are so connected as to be charged in parallel from said power supply means, and to be discharged in series to the electrical precipitator. For this purpose, condenser 15 is connected by conductor 1'? to the high tension lead 13, and by conductor 18 to a ground connection 19, while condenser 16 is connected by conductor 21 to the high tension lead 13 and by conductor 22 to a ground connection 23. Impedance means of low capacity, such as resistances 24 and 26, are provided in conductors 17 and 22. These impedances not only regulate the rate of charging of condensers 15 and 16, but also act as chokes against the sharp impulses or surges produced at the time of discharge of such condensers. A spark gap 28 is connected between the condensers 15 and 16, one side of said gap being connected by conductor 29 to the conductor 17 between condenser 15 and'the impedance means 24, and the other side of said gap being connected by conductor 31 to conductor 22 between condenser 16 and impedance means 26. In order to prevent high frequency oscillations created by the impulse generating means from reaching the main power supply means some suitable impedance means of low capacity, such-as inductances Y33 and 34 are also preferably inserted inthe high tension conductor 13. Said conductor may also include resistance, as. indicated at 35, for the purpose ofstabilizing the load at the power supply source.

In the operation of the circuit above described, the unidirectional current delivered by rectifier '7 through conductor 13 serves to charge the condensers 15and 16 in parallel, this current consisting of rectified alternating current impulses of ordinary frequency so that the resistance offered to 1 this charging current by impedance means 33, 34, 35, 24 and 26 is relatively low. It

will be noted that spark gap 28 isgtsonnected to the high potential side of condenser 15 and to the grounded side of condenser 16, so that as said condensers are charged a potential difference is created across said gap. When the condensers become charged at suflicient voltage to cause the spark gap 28 to break down, the energy stored in said condensers is suddenly discharged through the precipitator, through an impulse circuit including condenser 15, conductors 17 and 29,

spark gap 28, conductors 31 and 22, condenser 16, conductor 13, electricalprecipitator 1, ground connections 3 and 19, and conductor 18. It will be seen that the condensers 15 and 1.6 are connected in series in this circuit, so that the voltage of the impulse thus appliedbetween the dis-. charge electrode means 4 and collecting electrode means 2 is approximately equal to the sum' of the voltages at which the condensers were charged. This impulse voltage may, therefore,

be madeconsider-ably greater than the charging voltage maintained between the high tension lead 13 and ground, the exact value thereof depending upon the characteristics of the spark gap.. As a maximum, this impulse voltage may be made approximately twice as great as the charging voltage, and'the spark gap is preferably soadjusted -able arcing produced due to the fact that the as to make the impulse voltage at least greater than the steady unidirectional potential applied directly to the precipitator from the power supply means. I

At the time of discharge of the condensers, as above described, the potential impulse is of such steep wave front that it is substantially blocked by the low capacity impedance means 33, 34, 35, 24 and 26, and is thus prevented from reach- -ing the power supply source including transformer 5 and rectifier 7, and is caused to be dissipated substantially entirely in the above described circuit including the precipitator.

Between successive operations of the impulse generating means, the discharge electrode means is at all times maintained at a relatively-high potential with respect to the grounded collecting electrode means, by energy supplied from the rectifier '7 through conductor 13 to the precipitator, and the potential difference thus constantly maintained is preferably at or somewhat above the minimum potential required to produce ionization, but is considerably below the voltage at which arcing or disruptive discharge occurs. by the impulse generating circuit above described, there is superimposed upon this steady potential difierence, a sharp impulse potential of greater magnitude, sufficient to produce unusually copious ionization or corona discharge at the discharge electrode, resulting in the production of an increased number of ions in the gas adjacent said electrode, but this potential impulse is of such short duration as to prevent formation of At the time of each impulse delivered complete arcing paths between the electrodes.

Before any appreciable arcing or disruptive discharge can occur, the energy of the impulse generating means is completely dissipated and the pared to the duration of each impulse, and the potential difference during this period has some such maximum value as indicated at V1, materially lower than V2; As stated above, the value 1' of V1 is preferably about equal to or somewhat greater than the minimum voltage at which ionization cr corona discharge occurs, but is materially less than the voltage at which appreciable arcing or disruptive discharge occur. For exam- 1 ple, the minimum ionization voltage may be as indicated by the dotted horizontal line B-B, and the voltage at which arcing or disruptive discharge would occur if such voltage were main tained for an extended period may be as indicated by the dotted horizontal line 0-0. It will be noted that the peak voltage V2 of each-impulse may lie somewhat above the arcing potential C-C, but neverthelessthere is no apprecitime during which the voltage remains above this value is extremely small, for example, on the order of 10- seconds or less. The interval T1, between successive impulses, is many times as great as the duration of the impulses, and is preferably on the order of one hundredth second or greater.

It will be seen that the curve of the relatively steady potential continually maintained during the intervals T1, between impulses, is somewhat undulating, due to the fact that it is produced by successive impulses of rectified alternating current. The duration of one cycle of the alternating current supplied to rectifier 7 is indicated at T, and it will be noted that with a full wave rectifier, such as that above described, there are two crests per cycle,'produced by the two rectified half waves of the alternating current. The variations in this continually maintained potential, however, are relatively small as compared to the increased potential produced at the time of each impulse, and the points of minimum potential are preferably not permitted to fall materially below the ionization potential. It should also be pointed out that the frequency of occurrence of the sharp high potential impulses, such as at T2, is entirely independent of the frequency of the rectified alternating current.

Due to the relatively short duration of the high potential impulses as compared to the interval therebetween, the amount of energy required to be delivered by the impulse generating means is relatively small, which permits condensers and other equipment of relatively small size and cost to be used in the impulse circuit, a considerable portion of the total energy consumed being delivered directly from rectifier 7 to the precipitator during the relatively long intervals between impulses.

If a, gas containing suspended particles of solid or liquid materials is passed between the discharge and collecting electrodes, the suspended particles are rapidly caused to become electrically charged by the ions produced as above described, and the charged particles are then precipitated by the electrical field, upon the electrodes and principally upon the surface of the collecting electrode. It will be seen that a sufficiently high electrical field for effecting rapid migration of the charged particlesis continually maintained by the steady unidirectional potential between said electrodes.

The invention, therefore, permits highly efficient electrical precipitation to be obtained, and thus permits a maximum volume of gas to be handled in a precipitator of a given size, or permits a maximum degree of removal of suspended particles to be obtained, for a given rate of gas flow. Furthermore, the cost of electrical equipment is relatively small for the reasons above set forth, and the power consumption is reduced to a minimum due to the substantial elimination of arcing between the electrodes.

In the apparatus shown in Fig. 2 the steady unidirectional potential is maintained in the same manner as in the usual form of electrical precipitation apparatus now employed. The power supply means for this purpose is shown as comprising transformer 3'7 and rectifier 38,

' one terminal of said rectifier being connected to the high tension line 39 and the other terminal thereof being grounded as indicated at 41. As before, the collecting electrode means 2 is grounded at 3, while the discharge electrode means 4 is connected to the high tension line 39.

The impulse generating means in this case is shown as comprising a source of unidirectional current, such as transformer 42 and rectifier 43. One of the output terminals of said rectifier is grounded as indicated at 44, while the other terminal thereof is connected by conductor 45 to one side of a spark gap 46, the other side of which is connected by conductor 47 to the high tension line 39. An electrical condenser or capacity 48 is connected between the conductor 45 and a ground connection 49.

Suitable impedance means, such as resistance 51 and inductance 52 may be provided in the conductor 45, betweenthe rectifier 43 and the point of connectionof condenser 48 to said conductor. Impedance means, such as inductance 53 is also preferably inserted between the rectifier 38 and the high tension line 39.

In the operation of this form of apparatus, the main unidirectional power supply source including transformer 37 and rectifier 38 serves to maintain a steady unidirectional potential between the electrodes of the precipitator, while the impulse generating circuit serves to produce sharp potential impulses, of somewhat greater potential than said steady potential, and these potential impulses are also delivered through the high tension line 39 to the precipitator. The secondary voltage of transformer 42 is necessarily somewhat greater than the secondary voltage of transformer 37, in order to maintain the abovementioned relationship.

The mode of operation of the impulse generating circuit may be readily understood by reference to the drawing. The unidirectional current delivered by rectifier 43 serves to charge the condenser 48 through impedance means 51 and 52, until the voltage across the spark gap 46 beto cause such gap to break down, whereupon the energy stored in said condenser is delivered through said spark gap to the high tension line 39 and thence to the precipitator.

The apparatus shown in Fig. 3 is substantially the same as that shown in Fig. 1, with the exception that three condensers 55, 56 and 5'7 are provided in the impulse circuit, for charging in parallel and discharging in series, and two spark gaps 58 and 59 are connected between said condensers for controlling the time of discharge thereof. The condensers and 5'7 are connected between the high tension line 13 and ground in the same manner as condensers 15 and 16 in Fig. 1, impedance means such as resistances 61 and 62 being provided in the same manner as resistances 24 and 26. denser 56 is connected through resistance or impedance means 63 to the high tension line 13 and through resistance or impedance means 64 to ground. Spark gap '58 is connected between the high tension side of the condenser 55 and the grounded side of the condenser 56, while spark gap 59 is connected between the high tension side of condenser 56 and the grounded side of condenser 57. It will be understood that this form of circuit will operate in substantially the same manner as that shown in Fig. 1, with the exception that the-impulse potential is approximately three times the potential at which the individual condensers are charged, and a somewhat greater ratio of impulse potential to line potential may therefore be obtained.

I claim:

1. The method of electrical precipitation which comprises passing gas containing suspended material between opposing discharge electrode and collecting electrode means, continually maintaining a unidirectional electrical potential between said electrodes, and intermittently applying to said electrodes sharp unidirectional high potential impulses of short duration and in the The intermediate consame direction as said continually maintained potential, the potential between successive impulses being materially less than arcing potential and the potential at the instant of each such impulse being considerably greater than said continually maintained potential.

2. The method as set forth in claim 1, said continually maintained potential being at all times sui'liciently high to produce corona discharge from said discharge electrode means.

3. The method as set forth in claim 1, in which the maximum potential of each impulse is above that at which arcing would occur between the electrodes if such potential were maintained for an extended period, but in which the duration of said impulse is sufliciently small to prevent such arcing.

4. The method as set forth in claim' 1, the duration of each high potential impulse being on the order of 10- seconds or less.

5. The, method as set forth in claim 1, the duration of each high potential impulse being on the order of 10- seconds or less, and the interval between successive impulses being on the order of one-hundredth second or greater.

6. Themethod as set forth in claim 1, in which the maximum potential at the instant of each impulse is at least 50% greater than the continually maintained potential.

7. In an electrical precipitation apparatus, a collecting electrode means, discharge electrode means spaced from said collecting electrode means, a source of unidirectional current at high potential connected to said electrode means and adapted to continually maintain between said electrodes a unidirectional potential materially less than arcing potential, and impulse generating means also connected to said electrode means and adapted to intermittently deliver sharp unidirectional high potential impulses of short duration in the same direction as said continually maintained potential and of considerably greater magnitude than said continually maintained potential.

8. An apparatus as set forth in claim '7, said impulse generating means being connected to said first-named source of electric current in such manner as to receive unidirectional electrical energy therefrom for charging said impulse gen erating means.

9. An apparatus as set forth in claim 7, said impulse generating means being connected to said first-named source of electric current in such manner as to receive electrical energy therefrom for charging said impulse generating means, and the connections of said impulse generating meansto said current source including impedance means so positioned as to prevent the discharge of said impulse generating means from reaching said current source.

10. In an electrical precipitation apparatus, collecting electrode means, discharge electrode means spaced from said collecting electrode means, a source of unidirectional current athigh potential connected to said electrode means and adapted to continually maintain between said electrodes a unidirectional potential materially less than arcing potential, and impulse generating means adapted to intermittently produce sharp unidirectional high potential impulses of short duration and of considerably greater magnitude than said continually maintained potential, and electric circuit means connecting said impulse generating means to said electrode means in parallel with said source of unidirecto said source of unidirectional current in par allel with said electrode means so as to provide for charging of said impulse generating means by said source, and the connection of said impulse generating means to said source including impedance means so positioned as to substantially prevent said high potential impulses from reaching said source.

MARCEL A. LISSMAN.

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