US3469371A - Apparatus for controlling the removal of particle accumulations from the electrodes of an electric precipitator - Google Patents

Description

ELECTRODES F AN ELECTRIC PRECIPITATOR Peter C. Gelfand, Lebanon, Pa., assignor to Buell Engineering Company, Inc., Lebanon, Pa., a corporation of Delaware Filed May 2, 1967, Ser. No. 635,601 Int. Cl. B03c 3/76 U.S. Cl. 55-110 9 Claims ABSTRACT 0F THE DISCLOSURE A system for the removal of particle accumulations from the emitter and collector electrodes of an electric precipitator by rapping or vibrating them periodically in which the intensity of rapping or vibration of each electrode is individually controlled in accordance with the input power to the electrode pair. The intensity of rapping or vibration of the collector electrode is decreased in accordance with a decrease in input power, and vice versa, whereas the intensity of rapping or vibration of the emitter is increased in accordance with a decrease in input power, and vice Versa. The system makes it possible to keep particle re-entrainment (puing) at a minimum but also provides for optimum etliciency of removal at all times.

BACKGROUND OF THE INVENTION This invention relates to electric precipitators and, more particularly, to a system for removing the particle accumulations from the precipitator electrodes in an efcient and controlled manner.

Electric precipitators, which are used to remove suspended particles from a gas stream, operate with a relatively high D.C. voltage applied between an emitting and a collecting electrode. The electrical field created between the electrodes ionizes the particles in the gas stream, thus causing them to be drawn to the electrodes and to accumulate on them. Periodically, the electrodes are rapped or vibrated to dislodge the accumulated particles and permit them to fall by gravity and to be collected and removed.

A significant problem with the particle (dust) removal systems in presently known electrostatic precipitators is the tendency for dust to be re-entrained into the gas stream during rapping or vibration, such re-entrainment often being referred to as putiing Usually, the napping or vibrating device of a precipitator is designed to provide an intensity of rapping or vibration of the electrodes which is a maximum value for effective dust removal -but below a level at which puliing becomes excessive. Most systems, moreover, include a regulator which can be manually operated to alter the intensity of rapping or vibration in the event that excessive puffing is observed. However, such manual regulators do not take into account the changes that can occur at various periods of operation that may significantly atect the dust removal system.

United States Patent O 'ice More particularly, the amount of dust accumulating on a given electrode in a given amount of time can vary considerably, particularly where the degree of contamination of the gas stream varies, but also under other varying operating conditions such as ambient temperature, humidity and so forth. The amount of dust accumulated on a given electrode is, in turn, a signicant factor, not only in the mechanics of dust removal, but also in the electric field across the electrodes and thus `the input current to the electrode. Accordingly, with a iixed rapping or vibration intensity, the operating conditions of the preciptator may be such that the rapping or vibration of the electrodes is inadequate to effectively remove the dust accumulation, or on the other hand, it may be so high that excessive puing occurs.

SUMMARY OF THE INVENTION There is provided, in accordance with the present invention, a novel and improved dust removal system which embodies apparatus for providing optimum dust removal, in other words, provides for rapping or vibration at an intensity providing maximum effectiveness to remove dust accumulations from the electrode and yet to accomplish elective removal with a minimum of dust loss and reentrainment, i.e., putling. More particularly, it has been found that if the input current to the precipitator drops below its normal level, this is an indication that the emiting electrode has been coated with dust to an excessive degree. Accordingly, a greater intensity of vibration or rapping of the emitting electrode should be applied in order to remove the excessive coating. On the other hand, it has also been found that as the power input to the electrodes is decreased due to higher resistivity or for other reasons, there is a greater tendency for dust to be re-entrained into the gas stream during rapping than at greater input powers. Accordingly, the intensity of rapping 'of the collecting electrodes should -be reduced during periods of low power input. The reverse of the above conditions is also the case; that is, an increase in power input to the electrode pair over the normal input calls for a decrease in emitting electrode rapping or Vibration and an increase in collector electrode rapping or vibration.

The control system of the invention makes use of the above principles by providing for vibration or rapping intensities that are functions of the input power to Arxthe electrodes. The apparatus of the invention includes an input power monitor which detects the input power to the precipitator electrodes and produces a proportionate control signal. The control signal is used to control separately the intensities of rapping or vibration of the emitter and collector electrodes, the intensity of rapping or vibration of the emitter electrodes being increased in response to a decrease in the input current to the electrodes, land vice versa, and the intensity of rapping or vibration of the collector electrode being decreased in response to a decrease in input power, and vice Versa.

Preferably, the control units for each of the emitter and collector electrodes include limit controls for establishing upper and lower values of vibration intensity for the respective electrodes so that the intensity of rapping or vibration does not exceed a predetermined maximum or drop below a predetermined minimum. Moreover, the l control units are arranged to afford an adjustable, predetermined rate of change in rapping or vibration intensity for a given change of precipitator input power.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the accompanying drawing, which is a schematic, block diagram of the embodiment.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT Referring to the drawing, the current from an A C. power source is applied to a step-up transformer and a full wave rectifier, indicated schematically and designated by the reference numeral 12, which delivers a high D.C. operating voltage to the precipitator 14. The precipitator 14 is represented, diagrammatically, as comprising an emitter electrode 16 and a collector electrode 18. Gas containing suspended particles (not shown) is passed between the electrodes 16 and 18 and the particles are precipitated by the electric field established by the relatively high D.C. voltage applied between the electrodes. Generally, a precipitator will include a multiplicity of emitter and collector electrode pairs. In addition, the specific form of emitter and collector electrodes can vary considerably; for example, the collector electrode may be a substantially flat plate or it may be a tubular element surrounding the emitter electrode. These and other variations in the particular design of the precipitator are well known in the art, and the control system of the invention is applicable to a variety of specific precipitator designs.

The particle accumulations on the emitter and collector electrodes of the precipitator are periodically removed by mechanical or electrical rapping or vibrating devices which are suitably coupled to the electrodes to impart relatively sharp or intense motion to them. In the embodiment shown in the drawing, an emitter electrode vibrator 20, shown only schematically, is mechanically coupled, as represented by the dashed line 22 to the emitter electrode, and a collector electrode vibrator 24 is mechanically coupled, as indicated by the dashed line 26, to the collector electrode. A type of rapping or vibrating device particularly suited for the embodiment is a spring-return, solenoidal vibrator.

The intensities of vibration of the emitter vibrator and the collector vibrator 24 are individually controlled by an emitter control unit 28 and a collector control unit 30. The control units 28 and 30 are responsive to a control signal that is indicative of the input power to the electrode pair. More particularly, the A.C. input current from the source 10 is monitored by a current transformer 32 and the monitored signal is conducted through a rectifier 34 and filter 36 (which filters out momentary power uctuations caused by sparking, for example) to provide a D.C. control voltage which is a function of the input power to the precipitator electrodes 16 and 18. The transformer 32 and rectifier 34 may be embodied in a single unit, a suitable unit available commercially being the Westinghouse current Teleductor The emitter control 28 and collector control 30 can be substantially identical with, however, one difference described below, and therefore the elements of each as shown in the drawing are designated by the same reference numeral, except that the sufiix E is added to designate the elements of the emitter control 28 and the suix C is added to designate the elements of the collector control 30. The controls 28, or 30, include a differential input, high gain amplifier 40 which receives, as its input, the rectified and filtered control signal from the current transformer 32 and is powered by a power supply 42. The amplifier 40 includes circuitry constituting a limit control 44 that establishes upper and lower limits of current responses so that minimum and maximum values of vibrator intensity can be established, thus permitting the matching of the control system to a given precipitator operating in the field. Also included is a gain control circuit 46 for varying the amplifier gain to provide a predetermined relationship between the rate of increase or decrease in vibration intensity for a given change in precipitator power input.

The power supply 42 for each control 28 or 30 also provides a bias voltage which can be varied by means of a bias control 48. Accordingly, the output from each control unit is composed of an amplified D.C. control voltage which is a function of the precipitator input power and a.D.C. bias voltageof predetermined value as manually setby the control 48.

The control voltage and bias voltage from each unit are employed as the inputs to a magnetic amplifier 50, the control voltage being applied to one amplifier control winding 52 of the respective magnetic amplifier and the bias voltage being applied to a second control winding 54. The output winding 56 of the magnetic amplifier 50 is coupled to a conventional SCR circuit (silicon controlled rectifier circuit) 60 including parallel connected SCRs for full wave operation, the magnetic amplifier output serving to control the ow of current from a vibrator power source 58 through the SCR circuit to the respective vibrators 20 and 24. The phase of the gating current output from themagnetic amplifiers depends upon the level of the control voltages applied to the control windings 52 and 54 and the power delivered to the respective vibrator 20 or 24 is a full wave rectified, pulsed direct current, the current pulses beingtriggered at a variable point during each rectified half cycle of the load current and thus having an R.M.S. value that Varies according to the time of gating the SCRs in the SCR circuit 60. Accordingly, the intensity of vibration of the electrode is a function of the control and bias voltages.

As mentioned briefly above, the precipitator 14 includes a plurality of emitter and collector electrode pairs (only one pair being shown). The dust removal system is operated in a time sequence so that each electrode is periodically vibrated. To this end, the vibrator power sources 58E and 58C are turned on periodically by timers 62E and 62C, respectively. A complete dust-removal cycle maytake l5 minutes, while each emitter and collector electrode in the precipitator is vibrated for a period of 5 seconds, for example.l

It might also be mentioned that the several emitter and collector electrode pairs'of a precipitator will often be operated by a number of separate power supplies, several pairs being on each supply. Therefore the dust removal canbe controlled for each group separately -in accordance with the input power to that group. The invention thus provides for optimum dust cleaning and takes into account the possibility'of differences between the operating conditions in various parts of the precipitator.

The operation of the system is as follows. The input power to the electrodes 16 and 18 is monitored by the current transformer 32, thus providing an A.C. control signal which is rectified and filtered in the rectifier 34 and filter 36, respectively, to provide a D.C. control signal. The control signal is used separately by the emitter control unit 28 and collector control 30, each control providing an amplified D.C. output control voltage which is irnpressed across the respective control windings 52E and 52C of the magnetic'amplifiers 50E and 50C. Meanwhile, bias control voltages are impressed across the other control windings 54E and 54C of the respective amplifiers.

As mentioned above, a drop in the input current to the electrodes 16 and 18 below a normal value is an indication, inter alia, that the emitter electrodes have been coated with dust to an excessive degree, thus dictating a greater intensity of rapping or vibration of the emitter electrode. An increase in the intensity of vibration of the emitter electrode upon a decrease in the control is accomplished by connecting the bias control winding 54E of the magnetic amplifier 50E in reverse polarity with respect to the connection of the control winding 52E, thus providing a subtracting effect on the flux in the magnetic core between the flux induced by the control voltage and that induced by the bias voltage.

On the other hand, a decrease in input power to the electrodes 1-6 and 18 of the precipitator is indicative of a greater tendency for dust to be re-entrained into the gas stream during rapping or 'vibration and therefore dictates a reduction in intensity of the collector rapping or vibration during periods of low power input. The response of the collector control 30 to provide decreased vibration intensity during periods of low power input (and vice Versa) to the electrodes is accomplished by adding the control signal voltage and the bias voltage, in other words by connecting the control windings 52C and 54C of the amplifier 50C in the same polarity.

Each of the magnetic amplifiers 50E and 50C gates the'respective SCR circuit at a point in each half cycle of' the power source voltage that is a function, as described above, of the control voltage. Thus, the SCR circuits conduct portions of full-wave rectified pulses to the vibrators in accordance with the phase of the gating current passed by its respective magnetic amplifier output winding, and the vibrators are therefore powered by R.M.S. voltages that are functions of the control signal. In this manner, the intensities of vibration of the electrodes are, in turn, functions of the input power to them.

By adjusting the bias control 48E or 48C of the respective controls 28 and 30, the intensity of vibration of the emitter and collector electrodes can be appropriately set to a predetermined value for normal operation. Further, by appropriately setting the gain controls of the respective units, the magnitude of change in vibration intensity for a given change in input power delievered to the electrode pair can also be established to provide optimum response. The limit controls 44E and 44C of the respective emitter and collector cont- rol units 28 and 30 enables the system to be set to provide maximum and minimum vibration intensities for the respective electrodes, regardless of the input power to them.

Thus there is provided, in accordance with the invention, a novel and improved system for controlling the dust removal in a precipitator. In particular, the system enables the intensity of vibration by which dust removal is accomplished to be established at an optimum level in accordance with the degree of dust accumulation on the respective emitter and collector electrodes to provide the maximum effectiveness of removal without puffing. The system makes possible separate, individual control of each electrode in a precipitator, thereby taking into account the different conditions that may obtain within the system. Inasmuch as the effective removal of dust accumulations is an important factor in the efficiency of the precipitator, the control system enables optimum efiiciency of operation to be maintained continuously. It will be noted that the system operates copletely automatically and requires only the initial setting of the controls but otherwise needs no attention from an operator.

I claim:

l. In apparatus for removing the dust accumulations from the emitter and collector electrodes of an electric precipitator by rapping or vibrating them, the combination therewith of a system for controlling the intensity of rapping or vibration comprising means for continuously monitoring the input power to the electrodes and producing a control signal indicative thereof, means responsive to the control signal for increasing the intensity of rapping or vibration of the collector electrode in accordance with and in relation to an increase in the input power and for decreasing to a value other than zero the intensity of rapping or vibration of the collector electrode in accordance with and in relation to a decrease in the input power,

and means responsive to the control signal for decreasing to a value other than zero the intensity of rapping or vibration of the emitter electrode in accordance with and in relation to an increase in the input power and for increasing the intensity of rapping or ivibration of the emitter electrode in accordance with and in relation to a decrease in the input power.

2. A system according to claim 1 wherein each of the means for changing the intensity of rapping or vibration of the precipitator electrodes includes amplifier means for altering the magnitude of the control signal, and gain control means associated with the amplifier means for controlling the magnitude of change in rapping or vibration intensity of the precipitator electrodes for a given change in input power to the electrodes.

3. A system according to claim l wherein each of the means for changing the intensity of rapping or vibration of the precipitator electrode includes limit control means for establishing predetermined upper and lower values other than zero of rapping or vibration intensity.

4. In an electric precipitator having at least one emitter and collector electrode pair, apparatus for removing the dust accumulations from the collector electrode comprising means for rapping or vibrating the collector electrode, means for continuously monitoring the input power to the electrode pair and producing a control signal indicative thereof, and means responsive to the control signal for increasing while the vibrating or rapping means is in operation the intensity of rapping or vibration of the collector electrode from one value olher than zero to another value other than zero in accordance with and in relation to an increase in the input power to the electrode pair and for decreasing while the rapping or vibrating means is in operation the intensity of rapping or vibration of the collector electrode from one value other than zero to another value other than zero in accordance with and in relation to a decrease in the input power to the electrode pair.

5. Apparatus according to claim 4 wherein the means for changing the intensity of rapping or vibration of the collector electrodes includes amplifier means for altering the magnitude of the control signal, and gain control means associated with the amplifier means for controlling the magnitude of change in intensity of rapping or vibration of the collector electrode for given change in input power to the electrodes.

6. Apparatus according to claim 4 further comprising limit control means associated with said means responsive to the control signal for establishing minimum and maximum values other than zero or rapping or vibration intensity.

7. In an electric precipitator having at least one emitter and collector electrode pair, apparatus for removing the dust accumulation from the emitter electrode comprising means for rapping or vibrating the emitter electrode, means for continuously monitoring the input power to the electrode pair and producing a control signal indicative thereof, and means responsive to the control signal for decreasing while the rapping or vibrating means is in operation the intensity of rapping or vibration of the emitter electrode from one value other than zero to another value other than zero in accordance with and Iin relation to an increase in the input power to the electrode pair and for increasing while the rapping or vibrating means is in operation the intensity of rapping or vibration of the emitter electrode from one value other than zero to another value other than zero in accordance with and in relation to a decrease in the input power.

8. Apparatus according to claim 7 wherein the means -for changing the intensity of rapping or vibration of the emitter electrode includes amplifier means :for altering the magnitude of the control signal, and gain control means associated with the amplifier means for controlling the magnitude of change in the intensity of rapping or vibration of the emitter electrode for given change in input power to the electrodes.

9. Apparatus according to claim 7 further comprising limit control means associated with said means responsive to the control signal for establishing minimum and maximum values of intensity of rapping or vibration of the emitter-electrode.

References Cited UNITED STATES PATENTS Drenning et al. 317-139 Berg 55-110 Theodore et al. 330-130 X Thomas et al 55-105 Guldemond et al 55-105 Perrins 323-22 Hauck 321-16 Porges et al 330-52 Taylor 55-105 Glaeser 55-13 Wassenman 55-105 FOREIGN PATENTS 1/1960 Canada.

HARRY B. THORNTON, Primary Examinerl j 10 D. E. TALBERT, IR., Assistant Examiner Us. C1. XR.

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