Electrostatic precipitator
United States Patent 3915672
An electrostatic precipitator particularly for removing dusts of high electrical resistivity from a gas stream. The precipitator includes parallel, grounded plate electrodes with high voltage corona wires suspended between the plate electrodes. Corona from the wires electrically charges the dust particles, and the electric field adjacent the grounded plates drives the charged particles onto the plates. In order to prevent the occurrence of back-corona due to high resistivity of the dust, the high voltage corona discharges are produced in short pulses with longer intervals between the pulses, and auxiliary electrodes are provided to maintain the necessary electric field during the intervals between pulses. Electrical circuits and control means are also provided for applying a high voltage to the corona wires in short pulses, and for applying a high voltage to the non-discharging auxiliary electrodes during the intervals between pulses with a low voltage on the auxiliary electrodes during the pulses.
US Patent References:
Alternating current ion-filter for electrical precipitators
White - June 1954 - 2682313
Triggered pulse generator
Fischer et al. - June 1959 - 2889457
ELECTROSTATIC PRECIPITATOR
Chiang - July 1973 - 3747299
Alternating current ion-filter for electrical precipitators
White - June 1954 - 2682313
Triggered pulse generator
Fischer et al. - June 1959 - 2889457
ELECTROSTATIC PRECIPITATOR
Chiang - July 1973 - 3747299
Application Number:
05/407663
Publication Date:
10/28/1975
Filing Date:
10/18/1973
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
361/226, 96/25, 96/82, 96/76, 327/183
International Classes:
B03C3/38; B03C3/34; B03C3/41
Field of Search:
55/2,105,123,139,136,137,145-148,150-152 317/262AE 328/65,67
Primary Examiner:
Nozick, Bernard
Attorney, Agent or Firm:
Brown, Murray, Flick & Peckham
Claims:
I claim as my invention
1. A method for electrically charging gas-borne dust particles and effecting deposition thereof on a grounded surface, said method comprising producing short pulses of corona discharges in the gas to cause a corona current to flow in the gas and the deposited dust to said grounded surface, the intervals between pulses being not less than three times the duration of a pulse and not greater than the electrical time constant of said deposited dust, and independently producing a unidirectional electric field with no discharges in the space adjacent the grounded surface during said intervals between pulses, said electric field being of the same polarity as said discharges and of high enough voltage to drive the charged dust particles to the grounded surface.
2. An electrostatic precipitator comprising a plurality of parallel, grounded plate electrodes, a plurality of corona-emitting electrodes and a plurality of non-discharging auxiliary electrodes, said emitting and auxiliary electrodes being disposed in a plane between said plate electrodes and substantially parallel thereto, means for applying high voltage pulses of one polarity to said corona-emitting electrodes at predetermined intervals of time to produce corona discharges and for maintaining a lower voltage on the emitting electrodes during the intervals between pulses, said intervals being of much longer duration than the pulses, and means for applying a high unidirectional voltage of said polarity to said auxiliary electrodes during said intervals between corona-producing pulses and for maintaining low voltage on the auxiliary electrodes during said pulses.
3. An electrostatic precipitator as defined in claim 2 having a plurality of parallel, grounded plate electrodes, said corona-emitting electrodes being disposed between parallel plate electrodes in a plane substantially parallel thereto, and said auxiliary electrodes being disposed in said plane between the corona-emitting electrodes.
4. An electrostatic precipitator as defined in claim 3 in which the duration of the intervals between high voltage pulses is not less than three times the duration of a pulse.
5. An electrostatic precipitator as defined in claim 2 in which at least portions of said emitting electrodes are adapted to produce corona at the voltage applied in said high voltage pulses, and the auxiliary electrodes are of such configuration that no discharges occur at the high voltage applied to the auxiliary electrodes.
6. An electrostatic precipitator as defined in claim 5 in which the corona-emitting electrodes comprise lengths of wire, and the auxiliary electrodes are rod members.
7. An electrostatic precipitator including a plurality of parallel, grounded plate electrodes, at least one corona-emitting electrode and at least one auxiliary electrode, said emitting and auxiliary electrodes being disposed between the plate electrodes in a plane substantially parallel thereto, a first circuit for applying a unidirectional voltage to said corona-emitting electrode, a second circuit for applying a unidirectional voltage of the same polarity to said auxiliary electrode, and means for controlling said circuits to cause the first circuit to apply short pulses of a sufficiently high voltage to cause emission of corona and to maintain a lower voltage for predetermined intervals of longer duration between pulses, said controlling means causing said second circuit to apply a high voltage to the auxiliary electrode during the intervals between said pulses and to maintain a low voltage during the pulses.
8. An electrostatic precipitator as defined in claim 7 in which the configuration of said auxiliary electrodes is such that no corona discharges occur on them and the high voltage applied to the auxiliary electrodes is of sufficient magnitude to maintain a substantial electric field adjacent the plate electrodes.
9. An electrostatic precipitator as defined in claim 7 in which each of said circuits includes a first capacitor connected to the respective electrode to maintain a substantially constant bias voltage thereon, a second capacitor in series relation with the first capacitor, means for controlling the polarity of said second capacitor to place it in additive relation with the first capacitor to supply the high voltage and to reverse the polarity of the second capacitor for applying a low voltage to the electrode.
10. An electrostatic precipitator as defined in claim 9 in which said second capacitor is connected in an oscillatory circuit, said oscillatory circuit including electric valve means of opposite polarities, and in which said controlling means includes means for firing said electric valve means at predetermined times to control the reversals of polarity of the second capacitor.
11. An electrostatic precipitator as defined in claim 10 and including means for supplying energy to said oscillatory circuit during the periods of low voltage.
12. An electrostatic precipitator as defined in claim 11 in which said means for supplying energy comprises a third capacitor in series relation with one of said electric valve means, said third capacitor being of large capacitance as compared to the second capacitor, and means for charging the third capacitor.
1. A method for electrically charging gas-borne dust particles and effecting deposition thereof on a grounded surface, said method comprising producing short pulses of corona discharges in the gas to cause a corona current to flow in the gas and the deposited dust to said grounded surface, the intervals between pulses being not less than three times the duration of a pulse and not greater than the electrical time constant of said deposited dust, and independently producing a unidirectional electric field with no discharges in the space adjacent the grounded surface during said intervals between pulses, said electric field being of the same polarity as said discharges and of high enough voltage to drive the charged dust particles to the grounded surface.
2. An electrostatic precipitator comprising a plurality of parallel, grounded plate electrodes, a plurality of corona-emitting electrodes and a plurality of non-discharging auxiliary electrodes, said emitting and auxiliary electrodes being disposed in a plane between said plate electrodes and substantially parallel thereto, means for applying high voltage pulses of one polarity to said corona-emitting electrodes at predetermined intervals of time to produce corona discharges and for maintaining a lower voltage on the emitting electrodes during the intervals between pulses, said intervals being of much longer duration than the pulses, and means for applying a high unidirectional voltage of said polarity to said auxiliary electrodes during said intervals between corona-producing pulses and for maintaining low voltage on the auxiliary electrodes during said pulses.
3. An electrostatic precipitator as defined in claim 2 having a plurality of parallel, grounded plate electrodes, said corona-emitting electrodes being disposed between parallel plate electrodes in a plane substantially parallel thereto, and said auxiliary electrodes being disposed in said plane between the corona-emitting electrodes.
4. An electrostatic precipitator as defined in claim 3 in which the duration of the intervals between high voltage pulses is not less than three times the duration of a pulse.
5. An electrostatic precipitator as defined in claim 2 in which at least portions of said emitting electrodes are adapted to produce corona at the voltage applied in said high voltage pulses, and the auxiliary electrodes are of such configuration that no discharges occur at the high voltage applied to the auxiliary electrodes.
6. An electrostatic precipitator as defined in claim 5 in which the corona-emitting electrodes comprise lengths of wire, and the auxiliary electrodes are rod members.
7. An electrostatic precipitator including a plurality of parallel, grounded plate electrodes, at least one corona-emitting electrode and at least one auxiliary electrode, said emitting and auxiliary electrodes being disposed between the plate electrodes in a plane substantially parallel thereto, a first circuit for applying a unidirectional voltage to said corona-emitting electrode, a second circuit for applying a unidirectional voltage of the same polarity to said auxiliary electrode, and means for controlling said circuits to cause the first circuit to apply short pulses of a sufficiently high voltage to cause emission of corona and to maintain a lower voltage for predetermined intervals of longer duration between pulses, said controlling means causing said second circuit to apply a high voltage to the auxiliary electrode during the intervals between said pulses and to maintain a low voltage during the pulses.
8. An electrostatic precipitator as defined in claim 7 in which the configuration of said auxiliary electrodes is such that no corona discharges occur on them and the high voltage applied to the auxiliary electrodes is of sufficient magnitude to maintain a substantial electric field adjacent the plate electrodes.
9. An electrostatic precipitator as defined in claim 7 in which each of said circuits includes a first capacitor connected to the respective electrode to maintain a substantially constant bias voltage thereon, a second capacitor in series relation with the first capacitor, means for controlling the polarity of said second capacitor to place it in additive relation with the first capacitor to supply the high voltage and to reverse the polarity of the second capacitor for applying a low voltage to the electrode.
10. An electrostatic precipitator as defined in claim 9 in which said second capacitor is connected in an oscillatory circuit, said oscillatory circuit including electric valve means of opposite polarities, and in which said controlling means includes means for firing said electric valve means at predetermined times to control the reversals of polarity of the second capacitor.
11. An electrostatic precipitator as defined in claim 10 and including means for supplying energy to said oscillatory circuit during the periods of low voltage.
12. An electrostatic precipitator as defined in claim 11 in which said means for supplying energy comprises a third capacitor in series relation with one of said electric valve means, said third capacitor being of large capacitance as compared to the second capacitor, and means for charging the third capacitor.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to electrostatic precipitators, and more particularly to a precipitator adapted for removing dusts or particulate matter of high electrical resistivity from a stream of air or other gas.
In conventional electrostatic precipitators for industrial use, a plurality of grounded plate electrodes is disposed in parallel, spaced relation forming passages between them for the dust-laden gas. Corona wires are suspended in the passages between the plates and a high voltage is applied to the wires to cause corona discharges. The dust particles are electrically charged by the corona, and the electric field between the wires and the grounded plates drives the charged dust particles onto the plates where they collect in a layer and are removed.
In a typical precipitator of this type, for example, the grounded plates may be about 8 inches apart, and the corona wires are suspended in a plane halfway between and parallel to the grounded plates. The wires are typically spaced about 6 inches apart in this plane and are maintained at a negative potential of from 35 to 60 kilovolts. This high potential results in corona discharges surrounding the wires, and a flow of negative ions occurs which constitutes a corona current from the wires to the grounded plate electrodes. This flow of negative ions results in negative charges on the dust particles suspended in the gas in the space between the plates, and the charged particles are driven to the grounded electrodes by the electric field adjacent the plates.
In many cases, this type of precipitator construction operates with entire satisfaction. The corona current, however, is of the order of 10 -7 amperes per square centimeter and must be conducted through the layer of dust on the grounded plates. If the resistivity of the dust is high, the voltage drop across the layer of dust, which is equal to the corona current multiplied by the resistance of the dust layer, may exceed the local breakdown voltage so that local breakdowns or punctures occur. This results in local high voltage gradients at the dust surface, causing glow discharges or so-called back-corona. A flow of positive ions occurs from these glow discharges into the space between the peaks and tends to neutralize the negative charge on the dust particles. The phenomenon of back-corona is well known and is further discussed in a paper entitled "Electrostatic Precipitation of High Resistivity Dust" by G. W. Penney, AIEE Transactions, Vol. 70, Part II, Page 1192 (1951).
Furthermore, these glow discharges at the dust surface frequently trigger premature spark-over between the electrodes. In a precipitator of this type, spark-overs occur from time to time and an automatic control is normally used which reduces the applied voltage if the rate of spark-overs becomes too high. With high resistivity dust, the tendency to spark-over may be such that the operating voltage is reduced to a very low value which may be only slightly above the corona starting voltage. Because of dust deposits and other irregularities on the corona wires, the initiation of corona is quite non-uniform. At the very low operating voltages which may be made necessary by the high dust resistivity, therefore, corona may occur only at scattered locations along the wires. Particle charging may thus occur only at these scattered locations and even there the electric field is quite weak and the particle charges are small. The efficiency of dust collection becomes very poor under these conditions.
SUMMARY OF THE INVENTION
In accordance with the present invention, an electrostatic precipitator is provided which is capable of removing dusts or particulate matter of high electrical resistivity from a gas stream without causing back-corona and with high efficiency.
The invention depends on the fact that the layer of dust deposited on the grounded electrode has electrical capacitance since it can store energy by retaining a charge on its surface. The dust layer, therefore, behaves like a capacitor in parallel with a resistance and has an electrical time constant due to the exponential rate of decay of the stored charge. Thus, for example, if the dust has a resistivity of 10 12 ohm-centimeters and a relative dielectric constant of 3, the dust layer will have a time constant of 240 milliseconds. If short corona current pulses are applied to such a dust layer, the current through the dust layer can be kept relatively constant. For example, if short current pulses are applied to the dust layer with an interval of 80 milliseconds between pulses, then the voltage across the dust layer will drop only to 72 percent of its intitial value in the 80-millisecond interval. The corona current through the dust layer, therefore, remains relatively constant even though the voltage is applied in short pulses, and can be kept below the value at which back-corona would start. Under these conditions, the electric field between the corona wire and the grounded plate electrode is very non-uniform and is quite low at the grounded electrode. In such a relatively long interval as 80 milliseconds with no corona current in the gas stream itself, the field at the grounded electrode would be so low that little precipitation of dust would occur. In order to maintain a strong dust precipitating field in the intervals between current pulses, auxiliary electrodes are provided and a high voltage is applied to the auxiliary electrodes during the intervals. The auxiliary electrodes are relatively large electrodes placed between the corona wires and of such configuration that no discharges occur on the auxiliary electrodes when a high voltage is applied to them.
The precipitator of the present invention, therefore, comprises a structure generally similar to that of conventional precipitators, including parallel grounded plates with corona wires disposed between them. Means are provided for applying a high corona-producing voltage to the corona wires in short pulses, with intervals of much longer duration between the pulses during which a low voltage is applied to the corona wires. Auxiliary electrodes of such configuration that no discharges occur on them are disposed adjacent to and preferably between the corona wires. A low voltage is applied to the auxiliary electrodes during the high voltage corona pulses, and during the intervals between pulses a sufficiently high voltage is applied to the auxiliary electrodes to maintain the dust precipitating field during these intervals. In this way, a relatively low corona current can be maintained through the dust layer to prevent back-corona, but with high voltage pulses for effectively charging the dust particles. By separately controlling the precipitating field, the necessary field can be maintained during the periods between such pulses so that the efficiency of precipitation is kept high.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawings, in which:
FIG. 1 is a side elevation of an electrostatic precipitator embodying the invention, partly broken away on the line I--I of FIG. 2;
FIG. 2 is a transverse sectional view substantially on the line II--II of FIG. 1;
FIG. 3 is a horizontal sectional view substantially on the line III--III of FIG. 1;
FIG. 4 is a schematic diagram showing a preferred electrical circuit for the precipitator of FIG. 1;
FIG. 5 is a circuit diagram illustrating a firing circuit suitable for use in the circuit of FIG. 4; and
FIG. 6 is a diagrammatic view of a control switch adapted for controlling the circuit of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
There is shown in the drawings for the purpose of illustration, an electrostatic precipitator for removing dust from a stream of gas. The word "dust" is used herein to include any particulate or finely-divided material which can be suspended or carried in a gas stream. As previously indicated, this precipitator is particularly adapted for removing dusts of high electrical resistivity, although its usefulness is obviously not limited to this particular purpose.
Referring first to FIGS. 1, 2 and 3, the precipitator is shown enclosed in a generally rectangular housing 10 preferably of sheet metal. A grounded metal plate electrode 12 is disposed centrally in the housing 10 and is grounded by its connection to the sheet metal housing at the ends of the plate. The plate 12 preferably has rounded or rolled edges as indicated at 13 to prevent any possibility of corona discharges from sharp edges of the grounded plate. As shown, the plate 12 is disposed centrally of the housing 10 and parallel to the side walls 14 of the housing which thus constitute additional grounded plate electrodes. It will be seen that this arrangement provides a structure consisting of three parallel, grounded plate electrodes forming two passageways between them, although it will be understood that any desired number of parallel, grounded plates might be utilized. The dust-laden air or other gas enters through an inlet duct 16 at one end of the housing 10 and is discharged through an outlet duct 18 at the other end, as indicated by the arrows, the gas passing through the passageways between the parallel grounded electrodes. The bottom of the housing 10 may be formed as a hopper 20 with a discharge passage 22 for removal of dust collected on the plates 12 and 14.
A plurality of corona-emitting electrodes or wires 24 is disposed between the grounded plates 12 and 14. The corona wires 24 are arranged in planes parallel to the plates and midway between each pair of plates, so that the wires 24 extend parallel to the plates, and the wires are spaced apart in the planes in which they are disposed. The wires 24 are suspended from conducting bars 26 and are preferably provided with weights 28 at their lower ends to hold the wires in position. The bars 26 may be mounted on insulators 30 from the top of the housing 10 and are electrically connected to a suitable voltage source as described hereinafter. The wires 24 are adapted to emit corona discharges when a sufficiently high voltage is applied to them and for this purpose they must be suitably spaced apart. In a typical construction, the plates 12 and 14 would be spaced apart approximately 8 inches with the row of corona wires 24 halfway between them. With this arrangement, the corona wires should be spaced apart approximately 6 inches. The wires themselves should be of small diameter so as to have a very small radius of curvature to insure local breakdown of the gas to produce the corona discharge. The corona-emitting electrodes 24 may, of course, be of types other than wires provided they have a sufficient number of sharp edges or points where corona may occur.
As previously indicated, the corona voltage is applied to the electrodes 24 in short pulses and in order to provide a dust precipitating electric field in the intervals between pulses, a plurality of auxiliary electrodes 32 is provided. The electrodes 32 are disposed between the corona wires 24 in the same planes, as shown, and are made of such configuration that no corona discharges occur on these electrodes. The electrodes 32, therefore, are preferably rod members as shown in the drawings and are suspended from a conductive bar 34 by individual conducting supports 36. The bar 34 may be supported from the top of the housing 10 on insulators 38 and is electrically connected to the electrical system described below.
In the operation of this precipitator, the dust-laden gas enters through the inlet duct 16, passes through the passages between the pairs of grounded electrodes 12 and 14 and is discharged through the outlet duct 18. Corona is produced on the wires or electrodes 24 by applying short pulses of high voltage to the wires to produce the corona discharges. Between pulses, a relatively low voltage is maintained on the wires 24 and during these intervals, which are much longer than the pulses, a high voltage is applied to the auxiliary electrodes 32 to maintain an electric field in the space between the electrodes 24 and 32 and the adjacent grounded electrodes. During the high voltage corona-producing pulses, a low voltage is applied to the auxiliary electrodes 32.
As previously explained, the layer of dust which collects on the grounded plate electrodes is capable of storing a charge on its surface and thus has electrical capacitance. The dust layer, therefore, acts like a capacitor in parallel with a resistor and has a time constant determined by the electrical resistance and capacitance. When the high corona-producing voltage is applied to the wires 24, a discharge occurs surrounding the wires and a flow of negative ions occurs through the gas to the grounded plate. This flow of ions constitutes a corona current which must flow through the dust layer on the plate. Because of the electrical characteristics of this layer referred to above, charge is stored on its surface and only a relatively small current flows through the high resistance of the dust layer itself. When the high voltage pulse terminates and the flow of negative ions ceases, the charge on the dust layer decays and a small current continues to flow through the dust layer. The corona current through the dust layer itself therefore can be kept below the value which would cause the occurrence of backcorona, as discussed above, yet the corona-producing voltage can be made high enough to effectively charge the dust particles in the gas.
The charged dust particles are driven to the grounded plate electrodes by the electric field in the gas and collect there in a layer. The negative charge acquired by the dust particles is approximately proportional to this field. The force driving the particles toward the grounded electrode is proportional to the particle charge multiplied by the field, so that the velocity of the particles is proportional to the square of the field strength. For this reason, the strength of the field is very important to successful operation of the precipitator, and particularly the field strength in the region near the grounded electrodes. Since the field strength is determined by the corona electrode potential, the critical importance of a high voltage on the corona electrodes is apparent. It is also necessary to provide for maintaining a high precipitating field during the intervals between the pulses of corona current, and the auxiliary electrodes 32 are provided for this purpose but are designed so that the necessary high voltage can be applied to them without causing discharges and with as little disturbance to the distribution of corona current as possible.
To further illustrate the operation of the precipitator disclosed above, the following illustrative example may be given for a precipitator having the configuration and spacings described above. The corona electrodes or wires 24 have a high negative voltage, high enough to produce corona discharges, applied to them in short pulses of duration of the order of 1 to 10 milliseconds. The intervals between pulses should be relatively large compared to the duration of the pulses but must be less than the time constant of the dust layer. This time interval, for example, may be of the order of 80 milliseconds and should not be less than approximately three times the duration of a pulse although it is preferably much longer, within the limit indicated above. With this pulsing of the corona-emitting electrodes, the effect discussed above is obtained of relatively low corona current to prevent the occurrence of back-corona even though the corona voltage itself is high enough to effectively charge the dust particles. With usual spacing between the wires 24, the corona current density in the dust layer cannot be uniform, and it has been found that the current density in the dust layer halfway between the wires is about one half of the current density directly opposite the wires. It is desirable for this current distribution to be maintained and the auxiliary electrodes 32 should be designed to cause as little disturbance in the corona current distribution as possible. For the arrangement described above it has been found that the electrodes 32 may be rods of elliptical cross-section having a major axis of about 1-1/2 inches extending parallel to the grounded plates 12 and 14, and a minor axis of approximately 7/8 inch. Other configurations are, of course, also suitable and rod electrodes of circular cross-section having a diameter of from 1 to 1-1/2 inches may also be utilized. With this arrangement, if the auxiliary electrodes 32 are maintained at a relatively low voltage during the corona pulse, the distribution of current density is not substantially affected. The presence of the auxiliary electrodes causes little disturbance to the corona current; therefore, the voltage of electrodes 32 can be high enough between pulses to maintain a sufficiently high electric field at the grounded electrodes 12 and 14 for efficient dust collection.
The electrodes 24 and 32 of the precipitator may be energized by any desired electrical supply circuit which will operate in the manner described. That is, it must apply a high voltage to the corona-emitting electrodes 24 in a series of short pulses separated by longer intervals at lower voltage, and apply a high voltage to the electrodes 32 during the intervals between pulses with a lower voltage during the pulses. A suitable circuit for this purpose is shown by way of example in FIG. 4. As shown in this figure, two pulse-forming circuits 40 and 42 are provided and connected to the electrode bars 26 and 34, respectively. Referring, first, to the circuit 40 which applies the high voltage pulses to the corona-emitting electrodes 24, this circuit includes two capacitors 44 and 46 arranged in series relation between the bar 26 and ground 48. The capacitor 44 is connected directly to the bar 26 with negative polarity to provide a negative voltage to the wires 24. Capacitor 44 is made large enough to maintain relatively constant voltage; that is, the drop in voltage during a corona pulse should not exceed some permissible value such as 15 percent. The voltage and polarity of the capacitor 44 are maintained by a voltage source 50, indicated as a battery, which charges the capacitor between corona pulses through the diode 52.
The capacitor 46 is controlled to reverse its polarity so as to supply a high voltage when the two capacitors 44 and 46 are in additive relation, and to reduce the voltage on the wires 24 to a low value when the polarities of the capacitors are opposite, the voltage of capacitor 44 always being higher so as to maintain the negative polarity of the wires 24. The capacitor 46 is connected in a circuit consisting of the capacitor itself and an inductance 53 of the proper value to produce an oscillatory circuit in combination with capacitor 46. The circuit is not allowed to oscillate freely, however, as it is controlled by a pair of electric valves, shown as thyratrons 54 and 56, which are oppositely connected in parallel to control the direction of current flow in the oscillatory circuit. A capacitor 58 is connected in the anode circuit of the thyratron 56, and a voltage source 60 is connected across the capacitor 58 through an adjustable resistance 62 to keep the capacitor 58 charged. The thyratron 56 is controlled by a firing circuit 64 actuated by a switch 66, and the thyratron 54 is controlled by a firing circuit 68 actuated by a switch 70. The firing circuits 64 and 68 may be any suitable type of circuit, and each firing circuit is connected between the cathode and grid of the corresponding thyratron to make it conductive when the firing circuit is actuated by closing its switch 66 or 70.
In the operation of the circuit 40, a high voltage pulse on the corona-emitting wires 24 occurs when the capacitors 44 and 46 have the same polarity so that their voltages add. This occurs when thyratron 54 is made conductive, and since it is a unidirectionally conductive or valve device, the oscillatory circuit of the capacitor 46 and inductance 53 cannot reverse and the high voltage pulse is maintained although the thyratron 54 ceases to be conductive. To terminate the voltage pulse, the thyratron 56 is made conductive. This permits the oscillatory circuit to reverse and the polarity of capacitor 46 is thus reversed so that its voltage subtracts from that of capacitor 44. A relatively low voltage is thus applied to the wires 24 which is too low to produce corona discharges. During this period when current flows through thyratron 56, energy is supplied to the oscillatory circuit by the capacitor 58 and stored in the oscillatory circuit. The capacitance of capacitor 58 is made relatively large compared to that of capacitor 46 so that the change in potential of capacitor 58 is small. The voltage source 60 in turn supplies energy to capacitor 58 as determined by its potential and the resistance 62. The power supplied in this way by source 60 through capacitor 58 controls the magnitude of the reversing voltage of capacitor 46 and may be regulated by adjusting the resistance 62 or the potential of source 60. The low voltage interval continues until the thyratron 54 is fired which again reverses the polarity of the capacitor 46 and applies another high voltage pulse to the corona wires. The operation continues in this way under the control of the firing circuits 64 and 68 as actuated by their respective switches 66 and 70.
The pulse-forming circuit 42 which supplies the voltage to the auxiliary electrodes 32 may be the same as the circuit 40 and corresponding elements of circuit 42 are designated by primed reference numerals in FIG. 4. In the circuit 42, the thyratron 54' is controlled by a firing circuit 72 actuated by a switch 74, and the thyratron 56' is controlled by a firing circuit 76 actuated by a switch 78. It will be seen that the operation of the circuit 42 is similar to that of the circuit 40; that is, a high voltage is applied to the electrodes 32 for a period of time determined by actuation of the firing circuits, and a low voltage is maintained on the electrodes in the intervening times between periods of high voltage. The auxiliary electrodes 32 may be operated in either of two somewhat different modes. In one case, the voltage applied to the auxiliary electrodes during the corona pulse on the wires 24 is so low that the electrodes 32 may draw a corona current which is greater than any leakage current through the insulators. In this case, an adjustable resistor 80 is connected from the electrodes 32 to ground, as shown, to allow excess corona current to flow to ground, the resistance 80 being adjusted to maintain the desired average potential. The electrodes 32 might also be operated at a somewhat higher voltage so that any corona current to these electrodes is held to a value less than any leakage current that can occur. In this case, the resistor 80 could be replaced by a voltage source and diode similar to the voltage source 50 and diode 52 in the circuit 40.
It will be understood that while the pulse-forming circuits of FIG. 4 are highly satisfactory and give excellent performance, other types of pulse-forming circuits, such as those using solid-state circuitry, for example, could be utilized.
The firing circuits 64, 68, 72 and 76 may be identical and any suitable or known type of firing circuit may be used. A suitable circuit is shown by way of example in FIG. 5. The firing circuit 82 there shown is connected to the thyratron to be controlled by means of leads 84 and 86 connected to the grid and cathode, respectively, of the thyratron. The grid is normally biased negative with respect to the cathode by means of a capacitor 88 connected as shown across a battery 90 through a suitable resistor 92, the time constant of the circuit being made long compared to the time required to fire the thyratron. The firing circuit is controlled by a switch 94 which connects a capacitor 96 to a primary winding 98 on the transformer core 100. The capacitor 96 is charged by a battery 102 or other suitable voltage source through a resistor 104. It will be seen that when the switch 94 is closed, a current pulse flows through the winding 98 which induces a pulse in a secondary winding 106 on the core 100, and thus applies a pulse to the leads 84 and 86 to make the grid of the thyratron positive and thus render the thyratron conductive. To prevent saturation of the transformer core 100 by the unidirectional currents, an opposing magnetomotive force may be provided in a winding 108 by means of a battery 110 through a suitable inductance 112 and resistor 113. Since the cathode and grid of thyratrons 56 and 56' are at high potential, the transformer 100 is essential in the firing circuits for these two thyratrons to isolate the high voltage circuits so that the control switch can be at ground potential as shown. The transformer is also desirable for the other two thyratrons to isolate the control circuit from possible voltage surges.
The pulse-forming circuits 40 and 42 of FIG. 4 apply the desired voltages to the respective sets of electrodes 24 and 32 in a sequence determined by actuation of the control switches of the several firing circuits. These switches may be actuated and controlled in any desired manner to fire the thyratrons in the necessary sequence. An illustrative control device or switch which may be used for this purpose is shown in FIG. 6. The device 116 there shown is a rotating switch adapted to actuate the respective firing circuits in sequence. It will be noted from FIGS. 4 and 5 that each of the firing circuit switches connects a transformer winding to ground. The rotating switch 116, therefore, may be simply a conductive rotating element 118 mounted on a shaft 120 and connected to ground by a spring contact 122 resting on the shaft 120. The rotating element 118 has a contact member 124 extending radially therefrom in position to engage successive switch contacts. Since it is only necessary to connect the successive firing circuit leads to ground, each switch may consist only of a spring contact member disposed in the path of the rotating contact 124. With the element 118 rotating in the clockwise direction as indicated in FIG. 6 the contact 124 will first engage the switch contact 78 to actuate the firing circuit 76. This fires the thyratron 56' to apply a low voltage to the auxiliary electrodes 32. Immediately thereafter, the contact 124 engages the switch contact 70 to actuate firing circuit 68 to fire thyratron 54 and apply the high corona-producing voltage to the wires 24. After a short time interval corresponding to the desired duration of the high voltage pulse, the contact 124 engages switch contact 66 to actuate firing circuit 64. This fires thyratron 56 to terminate the high voltage pulse, as previously explained, and apply a low voltage to the corona wires 24. Immediately thereafter, contact 124 engages contact 74 to actuate firing circuit 72 and fires thyratron 54'. This applies a high voltage to the auxiliary electrodes 32 which is maintained until the contact 124 completes its revolution and again engages contact 78 and the cycle repeats.
It will be seen that the rotational speed of the element 118 determines the frequency of the corona voltage pulses and the circumferential positions or spacing of the stationary contacts determines the duration of the pulses. In general, the pulse-forming circuits should be controlled so that there is only a moderate drop in voltage through the dust layer in the interval between pulses. For a resistivity of 10 12 ohm-centimeters, for example, a pulse frequency of ten pulses per second is reasonable and the voltage and duration of the corona pulse are made as high as possible without causing back-corona, typical values being given above. It is desirable to increase the frequency of the pulses as the resistivity of the dust decreases and this is easily done by controlling the speed of the member 118. The positions of the stationary contacts are preferably made adjustable so that they can be adjusted when the speed of rotation is changed to maintain the duration of the corona pulses approximately constant. It will be understood, of course, that while the device 116 is a very simple and satisfactory means of controlling the operation of the circuit of FIG. 4, any desired device or control means could be utilized.
It will be noted that the operation of the circuit described above for applying voltage pulses to the corona wires is quite different from that of known pulse circuits which are non-reversible. If a voltage pulse is applied to the corona wires by such a circuit, the stored charge on the wires at the end of the pulse can be dissipated only by the corona discharge. The voltage of the wires thus decays slowly, resulting in a period of weak field strength. High voltage could not be applied to the auxiliary electrodes during this period while corona is still being emitted because the resulting distortion of the electric field would tend to concentrate the corona current in a narrow region immediately opposite the wire and thus tend to produce back corona in this region. In the new circuit disclosed herein, the voltage of capacitor 46 is reversed which has the effect of reducing the potential of the corona wires. That is, at the beginning of a voltage pulse, charge flows to the wires and at the termination of the pulse, charge remaining on the wires flows back to the capacitor because its voltage has reversed. The potential of the wires is thus rapidly reduced, permitting immediate application of high voltage to the auxiliary electrodes.
It should now be apparent that an electrostatic precipitator has been provided which will effectively remove dusts of high electrical resistivity from a gas stream without causing back-corona and with high efficiency. A particular embodiment has been shown and described for the purpose of illustration but it will be understood that the invention is not limited to the specific arrangement shown.
The present invention relates to electrostatic precipitators, and more particularly to a precipitator adapted for removing dusts or particulate matter of high electrical resistivity from a stream of air or other gas.
In conventional electrostatic precipitators for industrial use, a plurality of grounded plate electrodes is disposed in parallel, spaced relation forming passages between them for the dust-laden gas. Corona wires are suspended in the passages between the plates and a high voltage is applied to the wires to cause corona discharges. The dust particles are electrically charged by the corona, and the electric field between the wires and the grounded plates drives the charged dust particles onto the plates where they collect in a layer and are removed.
In a typical precipitator of this type, for example, the grounded plates may be about 8 inches apart, and the corona wires are suspended in a plane halfway between and parallel to the grounded plates. The wires are typically spaced about 6 inches apart in this plane and are maintained at a negative potential of from 35 to 60 kilovolts. This high potential results in corona discharges surrounding the wires, and a flow of negative ions occurs which constitutes a corona current from the wires to the grounded plate electrodes. This flow of negative ions results in negative charges on the dust particles suspended in the gas in the space between the plates, and the charged particles are driven to the grounded electrodes by the electric field adjacent the plates.
In many cases, this type of precipitator construction operates with entire satisfaction. The corona current, however, is of the order of 10 -7 amperes per square centimeter and must be conducted through the layer of dust on the grounded plates. If the resistivity of the dust is high, the voltage drop across the layer of dust, which is equal to the corona current multiplied by the resistance of the dust layer, may exceed the local breakdown voltage so that local breakdowns or punctures occur. This results in local high voltage gradients at the dust surface, causing glow discharges or so-called back-corona. A flow of positive ions occurs from these glow discharges into the space between the peaks and tends to neutralize the negative charge on the dust particles. The phenomenon of back-corona is well known and is further discussed in a paper entitled "Electrostatic Precipitation of High Resistivity Dust" by G. W. Penney, AIEE Transactions, Vol. 70, Part II, Page 1192 (1951).
Furthermore, these glow discharges at the dust surface frequently trigger premature spark-over between the electrodes. In a precipitator of this type, spark-overs occur from time to time and an automatic control is normally used which reduces the applied voltage if the rate of spark-overs becomes too high. With high resistivity dust, the tendency to spark-over may be such that the operating voltage is reduced to a very low value which may be only slightly above the corona starting voltage. Because of dust deposits and other irregularities on the corona wires, the initiation of corona is quite non-uniform. At the very low operating voltages which may be made necessary by the high dust resistivity, therefore, corona may occur only at scattered locations along the wires. Particle charging may thus occur only at these scattered locations and even there the electric field is quite weak and the particle charges are small. The efficiency of dust collection becomes very poor under these conditions.
SUMMARY OF THE INVENTION
In accordance with the present invention, an electrostatic precipitator is provided which is capable of removing dusts or particulate matter of high electrical resistivity from a gas stream without causing back-corona and with high efficiency.
The invention depends on the fact that the layer of dust deposited on the grounded electrode has electrical capacitance since it can store energy by retaining a charge on its surface. The dust layer, therefore, behaves like a capacitor in parallel with a resistance and has an electrical time constant due to the exponential rate of decay of the stored charge. Thus, for example, if the dust has a resistivity of 10 12 ohm-centimeters and a relative dielectric constant of 3, the dust layer will have a time constant of 240 milliseconds. If short corona current pulses are applied to such a dust layer, the current through the dust layer can be kept relatively constant. For example, if short current pulses are applied to the dust layer with an interval of 80 milliseconds between pulses, then the voltage across the dust layer will drop only to 72 percent of its intitial value in the 80-millisecond interval. The corona current through the dust layer, therefore, remains relatively constant even though the voltage is applied in short pulses, and can be kept below the value at which back-corona would start. Under these conditions, the electric field between the corona wire and the grounded plate electrode is very non-uniform and is quite low at the grounded electrode. In such a relatively long interval as 80 milliseconds with no corona current in the gas stream itself, the field at the grounded electrode would be so low that little precipitation of dust would occur. In order to maintain a strong dust precipitating field in the intervals between current pulses, auxiliary electrodes are provided and a high voltage is applied to the auxiliary electrodes during the intervals. The auxiliary electrodes are relatively large electrodes placed between the corona wires and of such configuration that no discharges occur on the auxiliary electrodes when a high voltage is applied to them.
The precipitator of the present invention, therefore, comprises a structure generally similar to that of conventional precipitators, including parallel grounded plates with corona wires disposed between them. Means are provided for applying a high corona-producing voltage to the corona wires in short pulses, with intervals of much longer duration between the pulses during which a low voltage is applied to the corona wires. Auxiliary electrodes of such configuration that no discharges occur on them are disposed adjacent to and preferably between the corona wires. A low voltage is applied to the auxiliary electrodes during the high voltage corona pulses, and during the intervals between pulses a sufficiently high voltage is applied to the auxiliary electrodes to maintain the dust precipitating field during these intervals. In this way, a relatively low corona current can be maintained through the dust layer to prevent back-corona, but with high voltage pulses for effectively charging the dust particles. By separately controlling the precipitating field, the necessary field can be maintained during the periods between such pulses so that the efficiency of precipitation is kept high.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawings, in which:
FIG. 1 is a side elevation of an electrostatic precipitator embodying the invention, partly broken away on the line I--I of FIG. 2;
FIG. 2 is a transverse sectional view substantially on the line II--II of FIG. 1;
FIG. 3 is a horizontal sectional view substantially on the line III--III of FIG. 1;
FIG. 4 is a schematic diagram showing a preferred electrical circuit for the precipitator of FIG. 1;
FIG. 5 is a circuit diagram illustrating a firing circuit suitable for use in the circuit of FIG. 4; and
FIG. 6 is a diagrammatic view of a control switch adapted for controlling the circuit of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
There is shown in the drawings for the purpose of illustration, an electrostatic precipitator for removing dust from a stream of gas. The word "dust" is used herein to include any particulate or finely-divided material which can be suspended or carried in a gas stream. As previously indicated, this precipitator is particularly adapted for removing dusts of high electrical resistivity, although its usefulness is obviously not limited to this particular purpose.
Referring first to FIGS. 1, 2 and 3, the precipitator is shown enclosed in a generally rectangular housing 10 preferably of sheet metal. A grounded metal plate electrode 12 is disposed centrally in the housing 10 and is grounded by its connection to the sheet metal housing at the ends of the plate. The plate 12 preferably has rounded or rolled edges as indicated at 13 to prevent any possibility of corona discharges from sharp edges of the grounded plate. As shown, the plate 12 is disposed centrally of the housing 10 and parallel to the side walls 14 of the housing which thus constitute additional grounded plate electrodes. It will be seen that this arrangement provides a structure consisting of three parallel, grounded plate electrodes forming two passageways between them, although it will be understood that any desired number of parallel, grounded plates might be utilized. The dust-laden air or other gas enters through an inlet duct 16 at one end of the housing 10 and is discharged through an outlet duct 18 at the other end, as indicated by the arrows, the gas passing through the passageways between the parallel grounded electrodes. The bottom of the housing 10 may be formed as a hopper 20 with a discharge passage 22 for removal of dust collected on the plates 12 and 14.
A plurality of corona-emitting electrodes or wires 24 is disposed between the grounded plates 12 and 14. The corona wires 24 are arranged in planes parallel to the plates and midway between each pair of plates, so that the wires 24 extend parallel to the plates, and the wires are spaced apart in the planes in which they are disposed. The wires 24 are suspended from conducting bars 26 and are preferably provided with weights 28 at their lower ends to hold the wires in position. The bars 26 may be mounted on insulators 30 from the top of the housing 10 and are electrically connected to a suitable voltage source as described hereinafter. The wires 24 are adapted to emit corona discharges when a sufficiently high voltage is applied to them and for this purpose they must be suitably spaced apart. In a typical construction, the plates 12 and 14 would be spaced apart approximately 8 inches with the row of corona wires 24 halfway between them. With this arrangement, the corona wires should be spaced apart approximately 6 inches. The wires themselves should be of small diameter so as to have a very small radius of curvature to insure local breakdown of the gas to produce the corona discharge. The corona-emitting electrodes 24 may, of course, be of types other than wires provided they have a sufficient number of sharp edges or points where corona may occur.
As previously indicated, the corona voltage is applied to the electrodes 24 in short pulses and in order to provide a dust precipitating electric field in the intervals between pulses, a plurality of auxiliary electrodes 32 is provided. The electrodes 32 are disposed between the corona wires 24 in the same planes, as shown, and are made of such configuration that no corona discharges occur on these electrodes. The electrodes 32, therefore, are preferably rod members as shown in the drawings and are suspended from a conductive bar 34 by individual conducting supports 36. The bar 34 may be supported from the top of the housing 10 on insulators 38 and is electrically connected to the electrical system described below.
In the operation of this precipitator, the dust-laden gas enters through the inlet duct 16, passes through the passages between the pairs of grounded electrodes 12 and 14 and is discharged through the outlet duct 18. Corona is produced on the wires or electrodes 24 by applying short pulses of high voltage to the wires to produce the corona discharges. Between pulses, a relatively low voltage is maintained on the wires 24 and during these intervals, which are much longer than the pulses, a high voltage is applied to the auxiliary electrodes 32 to maintain an electric field in the space between the electrodes 24 and 32 and the adjacent grounded electrodes. During the high voltage corona-producing pulses, a low voltage is applied to the auxiliary electrodes 32.
As previously explained, the layer of dust which collects on the grounded plate electrodes is capable of storing a charge on its surface and thus has electrical capacitance. The dust layer, therefore, acts like a capacitor in parallel with a resistor and has a time constant determined by the electrical resistance and capacitance. When the high corona-producing voltage is applied to the wires 24, a discharge occurs surrounding the wires and a flow of negative ions occurs through the gas to the grounded plate. This flow of ions constitutes a corona current which must flow through the dust layer on the plate. Because of the electrical characteristics of this layer referred to above, charge is stored on its surface and only a relatively small current flows through the high resistance of the dust layer itself. When the high voltage pulse terminates and the flow of negative ions ceases, the charge on the dust layer decays and a small current continues to flow through the dust layer. The corona current through the dust layer itself therefore can be kept below the value which would cause the occurrence of backcorona, as discussed above, yet the corona-producing voltage can be made high enough to effectively charge the dust particles in the gas.
The charged dust particles are driven to the grounded plate electrodes by the electric field in the gas and collect there in a layer. The negative charge acquired by the dust particles is approximately proportional to this field. The force driving the particles toward the grounded electrode is proportional to the particle charge multiplied by the field, so that the velocity of the particles is proportional to the square of the field strength. For this reason, the strength of the field is very important to successful operation of the precipitator, and particularly the field strength in the region near the grounded electrodes. Since the field strength is determined by the corona electrode potential, the critical importance of a high voltage on the corona electrodes is apparent. It is also necessary to provide for maintaining a high precipitating field during the intervals between the pulses of corona current, and the auxiliary electrodes 32 are provided for this purpose but are designed so that the necessary high voltage can be applied to them without causing discharges and with as little disturbance to the distribution of corona current as possible.
To further illustrate the operation of the precipitator disclosed above, the following illustrative example may be given for a precipitator having the configuration and spacings described above. The corona electrodes or wires 24 have a high negative voltage, high enough to produce corona discharges, applied to them in short pulses of duration of the order of 1 to 10 milliseconds. The intervals between pulses should be relatively large compared to the duration of the pulses but must be less than the time constant of the dust layer. This time interval, for example, may be of the order of 80 milliseconds and should not be less than approximately three times the duration of a pulse although it is preferably much longer, within the limit indicated above. With this pulsing of the corona-emitting electrodes, the effect discussed above is obtained of relatively low corona current to prevent the occurrence of back-corona even though the corona voltage itself is high enough to effectively charge the dust particles. With usual spacing between the wires 24, the corona current density in the dust layer cannot be uniform, and it has been found that the current density in the dust layer halfway between the wires is about one half of the current density directly opposite the wires. It is desirable for this current distribution to be maintained and the auxiliary electrodes 32 should be designed to cause as little disturbance in the corona current distribution as possible. For the arrangement described above it has been found that the electrodes 32 may be rods of elliptical cross-section having a major axis of about 1-1/2 inches extending parallel to the grounded plates 12 and 14, and a minor axis of approximately 7/8 inch. Other configurations are, of course, also suitable and rod electrodes of circular cross-section having a diameter of from 1 to 1-1/2 inches may also be utilized. With this arrangement, if the auxiliary electrodes 32 are maintained at a relatively low voltage during the corona pulse, the distribution of current density is not substantially affected. The presence of the auxiliary electrodes causes little disturbance to the corona current; therefore, the voltage of electrodes 32 can be high enough between pulses to maintain a sufficiently high electric field at the grounded electrodes 12 and 14 for efficient dust collection.
The electrodes 24 and 32 of the precipitator may be energized by any desired electrical supply circuit which will operate in the manner described. That is, it must apply a high voltage to the corona-emitting electrodes 24 in a series of short pulses separated by longer intervals at lower voltage, and apply a high voltage to the electrodes 32 during the intervals between pulses with a lower voltage during the pulses. A suitable circuit for this purpose is shown by way of example in FIG. 4. As shown in this figure, two pulse-forming circuits 40 and 42 are provided and connected to the electrode bars 26 and 34, respectively. Referring, first, to the circuit 40 which applies the high voltage pulses to the corona-emitting electrodes 24, this circuit includes two capacitors 44 and 46 arranged in series relation between the bar 26 and ground 48. The capacitor 44 is connected directly to the bar 26 with negative polarity to provide a negative voltage to the wires 24. Capacitor 44 is made large enough to maintain relatively constant voltage; that is, the drop in voltage during a corona pulse should not exceed some permissible value such as 15 percent. The voltage and polarity of the capacitor 44 are maintained by a voltage source 50, indicated as a battery, which charges the capacitor between corona pulses through the diode 52.
The capacitor 46 is controlled to reverse its polarity so as to supply a high voltage when the two capacitors 44 and 46 are in additive relation, and to reduce the voltage on the wires 24 to a low value when the polarities of the capacitors are opposite, the voltage of capacitor 44 always being higher so as to maintain the negative polarity of the wires 24. The capacitor 46 is connected in a circuit consisting of the capacitor itself and an inductance 53 of the proper value to produce an oscillatory circuit in combination with capacitor 46. The circuit is not allowed to oscillate freely, however, as it is controlled by a pair of electric valves, shown as thyratrons 54 and 56, which are oppositely connected in parallel to control the direction of current flow in the oscillatory circuit. A capacitor 58 is connected in the anode circuit of the thyratron 56, and a voltage source 60 is connected across the capacitor 58 through an adjustable resistance 62 to keep the capacitor 58 charged. The thyratron 56 is controlled by a firing circuit 64 actuated by a switch 66, and the thyratron 54 is controlled by a firing circuit 68 actuated by a switch 70. The firing circuits 64 and 68 may be any suitable type of circuit, and each firing circuit is connected between the cathode and grid of the corresponding thyratron to make it conductive when the firing circuit is actuated by closing its switch 66 or 70.
In the operation of the circuit 40, a high voltage pulse on the corona-emitting wires 24 occurs when the capacitors 44 and 46 have the same polarity so that their voltages add. This occurs when thyratron 54 is made conductive, and since it is a unidirectionally conductive or valve device, the oscillatory circuit of the capacitor 46 and inductance 53 cannot reverse and the high voltage pulse is maintained although the thyratron 54 ceases to be conductive. To terminate the voltage pulse, the thyratron 56 is made conductive. This permits the oscillatory circuit to reverse and the polarity of capacitor 46 is thus reversed so that its voltage subtracts from that of capacitor 44. A relatively low voltage is thus applied to the wires 24 which is too low to produce corona discharges. During this period when current flows through thyratron 56, energy is supplied to the oscillatory circuit by the capacitor 58 and stored in the oscillatory circuit. The capacitance of capacitor 58 is made relatively large compared to that of capacitor 46 so that the change in potential of capacitor 58 is small. The voltage source 60 in turn supplies energy to capacitor 58 as determined by its potential and the resistance 62. The power supplied in this way by source 60 through capacitor 58 controls the magnitude of the reversing voltage of capacitor 46 and may be regulated by adjusting the resistance 62 or the potential of source 60. The low voltage interval continues until the thyratron 54 is fired which again reverses the polarity of the capacitor 46 and applies another high voltage pulse to the corona wires. The operation continues in this way under the control of the firing circuits 64 and 68 as actuated by their respective switches 66 and 70.
The pulse-forming circuit 42 which supplies the voltage to the auxiliary electrodes 32 may be the same as the circuit 40 and corresponding elements of circuit 42 are designated by primed reference numerals in FIG. 4. In the circuit 42, the thyratron 54' is controlled by a firing circuit 72 actuated by a switch 74, and the thyratron 56' is controlled by a firing circuit 76 actuated by a switch 78. It will be seen that the operation of the circuit 42 is similar to that of the circuit 40; that is, a high voltage is applied to the electrodes 32 for a period of time determined by actuation of the firing circuits, and a low voltage is maintained on the electrodes in the intervening times between periods of high voltage. The auxiliary electrodes 32 may be operated in either of two somewhat different modes. In one case, the voltage applied to the auxiliary electrodes during the corona pulse on the wires 24 is so low that the electrodes 32 may draw a corona current which is greater than any leakage current through the insulators. In this case, an adjustable resistor 80 is connected from the electrodes 32 to ground, as shown, to allow excess corona current to flow to ground, the resistance 80 being adjusted to maintain the desired average potential. The electrodes 32 might also be operated at a somewhat higher voltage so that any corona current to these electrodes is held to a value less than any leakage current that can occur. In this case, the resistor 80 could be replaced by a voltage source and diode similar to the voltage source 50 and diode 52 in the circuit 40.
It will be understood that while the pulse-forming circuits of FIG. 4 are highly satisfactory and give excellent performance, other types of pulse-forming circuits, such as those using solid-state circuitry, for example, could be utilized.
The firing circuits 64, 68, 72 and 76 may be identical and any suitable or known type of firing circuit may be used. A suitable circuit is shown by way of example in FIG. 5. The firing circuit 82 there shown is connected to the thyratron to be controlled by means of leads 84 and 86 connected to the grid and cathode, respectively, of the thyratron. The grid is normally biased negative with respect to the cathode by means of a capacitor 88 connected as shown across a battery 90 through a suitable resistor 92, the time constant of the circuit being made long compared to the time required to fire the thyratron. The firing circuit is controlled by a switch 94 which connects a capacitor 96 to a primary winding 98 on the transformer core 100. The capacitor 96 is charged by a battery 102 or other suitable voltage source through a resistor 104. It will be seen that when the switch 94 is closed, a current pulse flows through the winding 98 which induces a pulse in a secondary winding 106 on the core 100, and thus applies a pulse to the leads 84 and 86 to make the grid of the thyratron positive and thus render the thyratron conductive. To prevent saturation of the transformer core 100 by the unidirectional currents, an opposing magnetomotive force may be provided in a winding 108 by means of a battery 110 through a suitable inductance 112 and resistor 113. Since the cathode and grid of thyratrons 56 and 56' are at high potential, the transformer 100 is essential in the firing circuits for these two thyratrons to isolate the high voltage circuits so that the control switch can be at ground potential as shown. The transformer is also desirable for the other two thyratrons to isolate the control circuit from possible voltage surges.
The pulse-forming circuits 40 and 42 of FIG. 4 apply the desired voltages to the respective sets of electrodes 24 and 32 in a sequence determined by actuation of the control switches of the several firing circuits. These switches may be actuated and controlled in any desired manner to fire the thyratrons in the necessary sequence. An illustrative control device or switch which may be used for this purpose is shown in FIG. 6. The device 116 there shown is a rotating switch adapted to actuate the respective firing circuits in sequence. It will be noted from FIGS. 4 and 5 that each of the firing circuit switches connects a transformer winding to ground. The rotating switch 116, therefore, may be simply a conductive rotating element 118 mounted on a shaft 120 and connected to ground by a spring contact 122 resting on the shaft 120. The rotating element 118 has a contact member 124 extending radially therefrom in position to engage successive switch contacts. Since it is only necessary to connect the successive firing circuit leads to ground, each switch may consist only of a spring contact member disposed in the path of the rotating contact 124. With the element 118 rotating in the clockwise direction as indicated in FIG. 6 the contact 124 will first engage the switch contact 78 to actuate the firing circuit 76. This fires the thyratron 56' to apply a low voltage to the auxiliary electrodes 32. Immediately thereafter, the contact 124 engages the switch contact 70 to actuate firing circuit 68 to fire thyratron 54 and apply the high corona-producing voltage to the wires 24. After a short time interval corresponding to the desired duration of the high voltage pulse, the contact 124 engages switch contact 66 to actuate firing circuit 64. This fires thyratron 56 to terminate the high voltage pulse, as previously explained, and apply a low voltage to the corona wires 24. Immediately thereafter, contact 124 engages contact 74 to actuate firing circuit 72 and fires thyratron 54'. This applies a high voltage to the auxiliary electrodes 32 which is maintained until the contact 124 completes its revolution and again engages contact 78 and the cycle repeats.
It will be seen that the rotational speed of the element 118 determines the frequency of the corona voltage pulses and the circumferential positions or spacing of the stationary contacts determines the duration of the pulses. In general, the pulse-forming circuits should be controlled so that there is only a moderate drop in voltage through the dust layer in the interval between pulses. For a resistivity of 10 12 ohm-centimeters, for example, a pulse frequency of ten pulses per second is reasonable and the voltage and duration of the corona pulse are made as high as possible without causing back-corona, typical values being given above. It is desirable to increase the frequency of the pulses as the resistivity of the dust decreases and this is easily done by controlling the speed of the member 118. The positions of the stationary contacts are preferably made adjustable so that they can be adjusted when the speed of rotation is changed to maintain the duration of the corona pulses approximately constant. It will be understood, of course, that while the device 116 is a very simple and satisfactory means of controlling the operation of the circuit of FIG. 4, any desired device or control means could be utilized.
It will be noted that the operation of the circuit described above for applying voltage pulses to the corona wires is quite different from that of known pulse circuits which are non-reversible. If a voltage pulse is applied to the corona wires by such a circuit, the stored charge on the wires at the end of the pulse can be dissipated only by the corona discharge. The voltage of the wires thus decays slowly, resulting in a period of weak field strength. High voltage could not be applied to the auxiliary electrodes during this period while corona is still being emitted because the resulting distortion of the electric field would tend to concentrate the corona current in a narrow region immediately opposite the wire and thus tend to produce back corona in this region. In the new circuit disclosed herein, the voltage of capacitor 46 is reversed which has the effect of reducing the potential of the corona wires. That is, at the beginning of a voltage pulse, charge flows to the wires and at the termination of the pulse, charge remaining on the wires flows back to the capacitor because its voltage has reversed. The potential of the wires is thus rapidly reduced, permitting immediate application of high voltage to the auxiliary electrodes.
It should now be apparent that an electrostatic precipitator has been provided which will effectively remove dusts of high electrical resistivity from a gas stream without causing back-corona and with high efficiency. A particular embodiment has been shown and described for the purpose of illustration but it will be understood that the invention is not limited to the specific arrangement shown.