US2935155A - Apparatus for controlling electrical precipitators - Google Patents

Description

May 3, 1960 M. P. FOLEY 2,935,155

APPARATUS FOR CONTROLLING ELECTRICAL PRECIPITAToRs Filed July 9, 1954 ,2 .F-c: J. g

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yUnited States Patent APPARATUS FOR CONTROLLING ELECTRICAL PRECIPITATGRS Michael P. Foley, Los Angeles, Calif., assigner, by mesheassignments, to Joy Manufacturing Company, a corporation of Pennsylvania Application Jury 9, 1954, seran No. 442,415

-4 claims. (ci. 18s- 7) p ciency.

VFor most eliicient operation in an electrical precipitator, the discharge current between the electrodes should bel maintained as high as possible inasmuch as it is this discharge current that eiectively changes particles to be collected, and thus causes them to be attracted to the electrodes of the precipitator. The upper limit of the discharge current is established by that Value of the inter-electrode voltage at which arc-over between the electrodes occurs. It is found that just prior to arc-over, a certain amount of sparking between theelectrodes takes place. This sparking is not serious except insofar as it is indicative of possible arc-overs occurring. Actual arcing between the electrodes is serious in that it puts large loads on the rectier and other generating' components and particularly because it drops the voltage between the electrodes to a value where particle collection is very poor.

lIt is highly desirable accordingly, to operate a precipitator in such a manner that the average discharge current is |at an optimum value for maximum particle collection eiciency and yet, below the point at which arcover will occur. Various attempts in this direction have been made. For example, it has been found that the discharge current is close to optimum value when a certain sparking rate is achieved. Suitable voltage regulating apparatus responsive to a predetermined sparking rate for any one precipitator may then be employed for ad-v justing the high voltage across the precipitator electrodes to a value which will maintain the discharge current at the particular value for attaining the given sparking rate. A weakness of this system is that other factors enter into determining the optimum value of discharge current so thatithe relation between sparking rate and current is not always constant. Accordingly arc-overs can occur when very little sparking ispresent. On the other hand, these' other fators can result in a high rate of sparking even` though the discharge ycurrent could be safely increased without fear of arc-over. Probably, as many as twenty variable factors aftectthe optimum value of the discharge current and as a result the design ofsuitable regulating apparatus; that regulates by reference to only one ofthe factors involved is well nigh impossible.

The present invention has as its primary object to provide a novel control means for optimizing the discharge currentbetween the electrodes of Y aprecipitator automaticallyoand maintaining` it at optimum values even thoughgthe/various factors aifecting the current may not allebe.- known' and may be continuously changing in relative valueor importance. i.

More particularly, Vit is'an object .of the invention toA provide a novel' apparatus for attaining' maximum parti-A clefrollection eiciencygiin .asprecipitator whlcg .is `110,11

2,935,155 Patented May 3, 1960 fice dependent for operation on only one of several inde-'J pendent variables all affecting such elliciency.`

Briey, these and further objects and advantages ofi the invention are attained by generating a signal that is a function of the average value of the voltage acrossV the electrodes of the precipitator and passing this signal to control apparatus for actuating a voltage regulating mechanism in accordance with the rate of change of this signal.

substantially no possibility of arc-over occurring.

A better understanding of the principles underlying the apparatus of the invention may be had by referring to the accompanying drawings, in which:

Fig. 1 is a block diagram of an electrical precipitatorf embodying the method and apparatus of the present in-r vention; and

Fig. 2 is a graph of the average discharge current be tween the precipitator electrodes of Fig. 1 plotted as a function of the average value of the voltage across the electrodes.

Referring to Fig. 1, there is shown a precipitator comprising a pair of opposing electrodes 10 and 11. These electrodes are arranged to collect particles from a gas stream passing therebetween, and one of the electrodes, 10

for example, may be in the form of a cylindricalmembery providing an extended surface on which particles collect, with the other electrode 11 comprising a thin conducting member coaxially disposed within the cylinder and of a surface conguration to facilitate production of corona discharge at the electrode. Usually, the outer plate or.

collecting electrode 10 is grounded, as shown.

These electrodes are energized from a high voltag Y source by a pair of high voltage leads 12 and 13 from` oppositely disposed shoes 14 and 15 respectivelyrof a mechanical rectiiier MR. The other two shoes of the rectiiier are connected across the secondary 16 of a high voltage transformer 17` having its primary 18 in turn con` nected to the output of an auto transformer 19, whereby in place of the mechanical rectifier MR if desired. The elements described thus far are entirely conventional.

Voltage from the source 20 is regulated by the auto-'1' transformer 19 and stepped up to several thousand-lvolts by the power transformer 17. This high voltage is then rectified bythe rectifier MR and applied across the electrodes 1t) and 11 ofthe precipitator, the negative high voltage terminal being connected to the center electrode Y 11 and the outer electrode 10 being connected to the grounded shoe of the rectifier.

a discharge current from the electrode 10 towards the electrode 11. The actual electron flow is from the centerA electrode 11 to the outer electrode 10. This discharge current serves to charge particles in a stream of gas passed Y' between the electrodes and causing the particles to drift towards the relatively positive outer electrode 10. Control of this discharge current is effected by regulating the high voltage across the electrodes by means of the auto A transformer 19.

As described earlier, a certain amount of sparking between the electrodes is normal and is not serious. It is `found usually to exist'when the value of the discharge current approaches lthe `desired optimum value for maximum particle collection efliciency. Below this optimum value, the discharge current is caused largely if not enp 'tirelyjby corona discharge atelectrodell, while Aabove The result of this operation is to maintain an". optimum average discharge current between the electrodes resulting in maximum particle collection eciency with The unidirectional elecf tric eld established between the electrodes gives rise to t t-h'e optimum value sparking and eventually arc-over cause lthe major portion of the current ow. The optimum value of this dis-charge current often changes continuously inasmuch as it depends upon many independent variables such as atmospheric conditions, type of gas passing between the electrodes, the natu-re of the particles being collected, etc. For a given installation, it can vary from hour to hour.

It has -been discovered that the average discharge current ilowing between the electrodes is a function of the average value, as distinguished from the peak value, of the high voltage across the electrodes. Fig. 2 shows a plot, in a qualitative way only, of this function, as` found in practice in most precipitators ordinates representing average voltage across the precipitator electrodes and 11 and the abscissae representing average discharge curn rent between the electrodes. Plotting this graph may be accomplished by means ofk a suitable voltrneter, preferably of the electrostatic type so that substantially no current is drawn, connected across the electrodes such as Ishown at 21 in Fig. l. The average current reading between the electrodes is then obtained from a meter circuit 2.2Y and milliameter 23 of the type described and claimed-,in applicants -co-pending application Ser. No. 422,440, led April l2, 1954 and entitled, Analyzer Circuit `for Electrical Precipitators, now abandoned. Prior to development of a circuitas disclosed in the above noted application, it was not .possible to obtain sufliciently accurate readings of the average discharge current to plot a curve such as shown in Fig. 2.

Referring :specifically to Fig. `2, it will be noted that the current-voltage characteristic curve 24 rises as the voltage is increased to a certain point and beyond that falls, doubling back on itself as the current increases. Thus, at certain voltages, and especially within the range of normal operation, there are two possible current values. For example, at a voltage value indica-ted by the horizontal line 25 two discharge current values I1 and I2 are possible as noted at the points P-1 and P-2. On the other hand, at the voltage value indicated by the line 26, thereis but one optimum current value I3 as indicated at the point P-3. The reason for the voltage drop after` passing the point P-3, with increasing discharge current, is that sparking normally increases at a suiicient rate to prevent a further build up of the voltage.

Still further increase in the discharge current eventually results in arc-overs creating conditions represented by the dashed line 27. t

nCurve 24 is merely'typical of many curves actually plotted and shows in a general way the characteristic shape of the current voltage curve. It -generally resembles a 'second-degree or quadratic curve which, within thev range` of normal operating voltages, has two current values for a given applied voltage. Hence the average discharge current may be termed a double-value function of thel average applied voltage. 'Ih-is char-acteristic is retained even though the exact shape of the curve may change from one precipitator to another 6r from time to timefor the same precipitator. Because there are so many variables -actually entering into determination of the uiin-al curve, it may shift with reference to the X and Y axes, but still retain its essential shape. Thus while absolu-te values may change, the shape of the curve giving `double values for the average current as a function of average voltage is retained.

It will be seen from the above, that the optimum current values for maximum particle collection eiiciency lie close to the ideal point P-3, or within the shaded area 29, arc-over invariably occurring if the current is increased too much beyond this point. The area of optimum operation as represented by the shaded area may not remain in' one position but may vary with several inde- Y pendent factors, all in tu-rn variable in themselves, including the sparking rate. The shaded area 29 of `optimum Aoperation is, however, centered about the turning Cil point or peak P-3 in the characteristic curve 24-that is, the point a-t which there is but one discharge current value yfor a given voltage.

The apparatus of the present invention is primarily concerned with a system for maintaining the discharge current wit-hin a range of values as `defined by the shaded area 29. It will be immediately apparent that at the desired optimum point P-3 of operation, the rate of change of the voltage with respect to the current is zero. In accordance with the invention, a control apparatus is employed for actuating the autotransforrner of Fig. l in accordance with this rate of change.

Mechanism lfor accomplishing this regulation is illus trated within the dashed box 30 of Fig. 1. As shown, `the apparatus includes high voltage lead 31 with an isolating current limiting resistance R connected to the center electrode li of the precipitator and terminating in a series resistance 32 grounded at its free end. A signal is generated across this resistance which is proportional to the average voltage between the precipitator electrodes. This signal is passed through a D.C. amplifier circuit 33, of any suitable design. The output of the D.C. amplier 33 is serially connected through polarized relay coil 34 and storage `condenser 35 -to ground at 36. The relay and condenser are shunted by a resistance element 37. Flihe operation of this control portion of the circuit Will be described shortly. Y

A reversible motor 38 is adapted to be connected to the power source 20 through ya lead 39 and switch arm 4t) which is operated by relay coil 34 to engage selectively terminal leads 411 and 42 of motor 38. Switch 40 is normally biased to a neutral center position as shown in the drawing. Current passing throughthe polarized relay coil 34 in one direction causes the switch -arm 4l) to contact the terminal lead 41 and energize Inotor 33 `to operate the same in one direction. On the other hand, current passing through the polarized relay coil 34-in the opposite direction causes the switch arm 49 to contact the other terminal lead 42 and energize the motor 38 to operate in the other or reverse direction. The center terminal of the motor is grounded as shown, and the outside terminals 43 and 44 are connected to opposite ends of the motor field windings. Operation of the motor in the'iirst direction moves the tap of the autotransformer to the right, as viewed in Fig. l to increase the voltage fed to the primary 18 of transformer 17 and thus increase the average voltage across the electrodes of `the precipitator. Operation of the motor in the -reverse direction decreases this voltage.

- The direction of current flowing through the polarized relay coil 34 is controlled by the signal developed across the resistance 32 and the time-constant characteristics of the storage condenser 35 and return path resistance element 37.

4Referring once again to Fig. 2, assume that the precipitator is being started up. As the voltage increases across the electrodes 10 and 11, a signal proportional to the changing value ofY this voltage develops across the resistance 32 and is amplified in the D.C. amplilier 33. As this signal is increasing with time, the current through the relay runs from right to left and the storage condenser 35 commences charging. (hlrrent through the relay in the direction indicated by the solid line arrow energizes the relay coil to throw the switch arm 40 to the right thus connecting power to the terminal lead 41 of the motor. The motor will move the auto-tranformer tap to the right thereby continuing to increase the voltage.

As the increasinggvoltage passes P-1, and approaches point P-3, its rate of change with respect to the discharge current approaches zero. The `value of the current ow from the D.C. amplifier 33 through the polarized relay also approaches zero in view ofthe charging up of the storage condenser 3S. Condenser 35 acts as an integrating circuit since it continuously accumulates charge as the voltage acrossresistance 32 increases.- A back voltassures..

age s thus presented by the condenser tendingto buck the current flow Vthrough the polarized relay until it reduces this current to zero. At this point, the condenser has a voltage charge proportional to the voltage drop across the resistance 32. With no current owing through the polarized relay, switch arm 40 returns to its neutral position thereby turning oli motor 38. The autotransformer tap is thus set automatically to provide an average high voltage across the precipitator electrodes corresponding to the point P-3.

If the average Value of the high voltage across the precipitator electrodes begins to decrease due to any one of several variable factors such as an increased sparking rate tending toward the point P-2, the charge on the condenser 35 will be greater than the signal developed at the output of the D.C. amplifier in view of the decreased voltage drop across resistance 32. Current thus passes through the polarized relay from left to right, as indicated by the dotted arrow and through resistance element 37 to ground. The relay 34 then connects the power source by means of switch arm 40 to the motor terminal lead 43 and terminal 44 to start the motor in the reverse direction to lower the voltage applied lby the autotransforrner. This initial decrease in voltage immediately diminishes the discharge current owing between the electrodes, resulting in an increasing of the actual voltage between the electrodes. As will be seen from inspection of Fig. 2, the decreasing discharge cur-y rent takes place along the curve from the point P-Z towards the point P-S. This actual increase in voltage is reflected across the resistance 32 and sets up a bucking voltage against the accumulated voltage on the condenserl 35. The reverse current llow through the polarized relay as indicated by the dotted arrow is thereby stopped and the switch arm 40 will return to its neutral position turning olf the motor 38.

It will be understood accordingly, that the condenser 35, resistance 37, polarized relay 34 and switch arm 40 act as a control circuit which applies a signal to the motor 3S proportional to the rate of change of the average voltage across the precipitator plates with respect to the average current between the plates. As can be seen from Fig. 2, this rate of change is zero at the point P-3 and therefore, the setting of the autotransformer will not be changed once the point is reached.

It will also be appreciated that a change in the voltage is necessary to operate the polarized relay. In other words, the control circuit tends to cut itself off. In order then, to avoid the possibility of the mechanism back tracking along the initial portion of the curve 24 of Fig. 2, or settling at some intermediate point below the point P-3, should no change in the voltage be present, a time delay circuit 45 is connected to the neutral switch position of the arm 40 through lead 46, and to the terminal v 43 of the motor 38 through leads 47 and 48. This time delay circuit periodically applies power to terminal 43 to move the autotransformer tap to the right and thus increase the voltage,'a predetermined length of time after the switch arm 40 has returned to the neutral position. This period of time may be preset in accordance with the type of installation, and may be any suitable length of time, preferably several minutes.

With this time delay arrangement, return of the switch arm 40 to the neutral position indicating a zero rate of change of the voltage is followed by power being applied to the motor after a predetermined interval of time. The resulting increase in voltage is reected by a current through the polarized relay to move the switch arm 4i? to the motor terminal 43 thereby cutting out the time delay circuit. But power will still be applied to the motor through switch arm 40 to continue increasing the voltage until the point P3 is reached at which time the arm 40 will again return to neutral position. It is to be noted that immediately after application of a signal through the time delay 45, the time delay circuit is cut out by movement of the switch arm 40 away from.

the neutral position. Thus, the control circuit tends to always operate inside the shaded area 29 under the curve of Fig. 2.

In some instances, power interruptions may occur in thelow voltage portion of thesystem or in the power source 20. In order to prevent such an interruption from adversely upsetting the balance of the control circuit, an under-current relay 49 is positioned to throw a switch 50 to apply power through leads 51 and 48 to the terminal 43 of the motor to increase the voltage. When the trouble at the source is cured, the undercurrent relay will open switch 50 removing power from the motor and permitting the control circuit to take over.

The above described method and apparatus for controlling electrical precipitator thus insures than an optimum discharge current will always result, and the possibility of prolonged arc-overs is substantially eliminated. So long as the form of the characteristic curve remains the same, the absolute values, which depend upon several variables, are not important. The control circuit is only concerned with maintaining the voltage at a point where its rate of change with respect to the discharge current is zero.

It is to be understood that different types of control circuits may be employed. The particular apparatus illustrated in Fig. 1 is merely illustrative, and other types responsive to the rate of change of the voltage for controlling the discharge current may occur to those skilled in the art. Thus the scope and spirit of the present invention is not to be thought of as limited to the particular apparatus disclosed.

I claim:

l. In an electrical precipitator, an apparatus for regulating the value of the discharge current between high voltage and grounded electrodes of said precipitator for maximum particle collection etciency comprising, in combination: signal generating means including a resistance connected between the high voltage electrode of said precipitator and ground and a direct current amplitier connected across said resistance for amplifying signals appearing across said resistance, said generating means producing a signal that is a function of the average vol-tage across the electrodes of said precipitator; regulating means in the low voltage portio-n of said precipitator for increasing or decreasing said average voltage; and control means responsive only to the rate of change of said signal for actuating said regulating means, said control means comprising a polarized relay coil and a storage condenser serially connected across the output of said direct current amplifier; a resistance element shunting said serially connected relay coil and condenser; and switch means operable in response to current ow in said relay coil for energizing said regulating means.

2. An apparatus according to claim l, in which said regulating means comprises a reversible motor and an autotransformer, said motor being adapted to increase the output voltage of said autotransformer when operating in one direction, and decrease the output of said autotransformer when operating in a reverse direction; said switch means energizing said motor in one direction when the current flow through said polarized relay is in one direction, and reversing said motorwhen the current liow through said relay is in a reverse direction.

3. An apparatus according to claim 2, including time delay means for applying energy to said motor for increasing the voltage output of said autotransformer a predetermined time after the `current ow through said polarized relay coil ceases, and means forrendering said time delay inoperative when current is owing through said polarized relay.

4. An apparatus according to claim 2, including an undercurrent relay in the low voltage portion of said precipitator operatively coupled to said motor for increasing the voltage of said autotransformer when the current to said autotransformer drps below a predetermined value.

References Cited in the le of this patent UNITED STATES PATENTS 8 Hall 'Apr. 13, 1954 Klemperer Sept. 25, 1956 FOREIGN PATENTS Germany Jan. 14, 1939 Great Britain May 17, 1940 Great Britain Mar. 17, 1954

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