US2000019A - Art of electrical precipitation - Google Patents

Description

y 7, 1935. R. HEINRICH El AL' 2,000,019

ART OF ELECTRI ICAL PRECI PITATION Filed Dec. 5, 1951 A ax, I

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a? INVNTQR51 7 WM fl/ Patented May 7, 1935 ART OF ELECTRICAL PRECIPITATION Richard Heinrich, Bcrlin-Sudende, and Wilhelm Feldmann, Berlin-,Spandau, Germany, assignors to International Precipitation Company, Los Angeles, Calif., a corporation of California Application December a, 1931, Serial No. 578,830 In Germany December 16, 1930 1 Claim. (01. 183-7) This invention relates to an improved method and apparatus for electrical precipitation, and the main object of the invention is to provide for energizing electrical precipitating apparatus in 5 such manner as to increase the efliciency of the precipitating operation and increase the working capacity of the apparatus.

In the usual operation of electrical precipitat'ors, an electrical field is maintained between electrodes adapted to produce electrical discharge into the gases in such manner as to cause precipitation of suspended particles and collection of same on the electrodes. The high potential difference between the electrodes necessary to pro- -duce the required electrical field has been usually produced by supplying to the electrodes a unidirectional current of suitable high tension, such current being obtained by rectifying an alternating current of ordinary commercial frequency. In order to operate the electrical precipitator efiectively, the high potential difference applied to the electrodes in this manner must be suflicient to produce silent electrical discharge into the gases being treated, and the discharging efiect and resulting precipitating action increase as the potential difference is raised, so that in order to obtain maximum efficiency and working capacity of the precipitatdr, it is desirable to maintain the potential difference ashigh as possible. The potential difference that can be maintained between the electrodes is, however, limited by. the .tendency to disruptive discharge between the electrodes, whichhas to be avoided as faras possible. The operating potential difference .that can be maintained in the precipitator is therefore limited to the range between the potential difference necessary to produce ionization and the potential difference resulting in excessive disruptive discharge.

It has been found that the efliciency and working capacity of-electrical precipitating apparatus can be increased by applying to such apparatus intermittent impulses of such high peak voltage a as to result in momentary increased ionization in o the .precipitator, the period during which this high voltage is maintained during each such impulse being, however, of such short duration as toprevent or minimize disruptive discharge between the electrodes. In this manner a sudden electrical potential difierence may be applied to the electrodes whose peak voltage is far in excess of that which can be applied as a steady potential difierence without resulting in excessive disruptive discharge or breakdown of the dielectric T between the electrodes.

In applying such a momentary high potential impulse or shock voltage impulse 'to an electrical precipitator of large operating capacity, such as is necessary for treatment of large volumes of gas, it is found that on account of the large electrostatic capacity of such an apparatus it is diificult to obtain the required sudden or abrupt increase in potential difference or steep wave front between the electrodes and the short period of high potential which is requisite for the effective functioning of the voltage impulses, on account of the tendency of such impulses to be flattened out by the efiect of the large electrical capacity of the precipitating apparatus. An important object of the present invention is to enable such shock potential or voltage impulses to be efficiently applied to an electrical precipitator of large size and to-enable the desired momentary high potential difference tobe produced between, the electrodes of the precipitator irrespective of the size of the precipitator.

According to the present invention, the electrical precipitating apparatus is subdivided into a plurality of sections and the voltage impulses are applied successively to the several sections, each of such sections being of such small size and small electrostatic capacity that the voltage impulses are delivered with full efiect to each section. and the requisite high potential diiierence is momentarily applied thereto to produce the desired increased ionization.

The accompanying drawing illustrates embodiments of our invention and referring thereto:

Fig. 1 is a diagram of the circuit including the electrical precipitating apparatus and the energizing circuit therefor.

Fig. 2 is a diagram showinga modified form of rectifying means used in connection with the apparatus for generating the shock potential impulses.

Fig. 3 is a graph showing the relation between voltage and time, in an electrical precipitator operated in accordance with this invention.

Referring to Fig. l, the electrical'precipitator is shown diagrammatically as comprising a plurality of sections l, each section comprising one or more discharge electrodes 2, and corresponding .opposing or collecting electrodes 3, which are mounted in any usual manner and insulated from one another 'as well known in the art. The discharge electrodes may be formed as wires, rods, chains or screens, of relatively small area, adapted to facilitate electrical discharge. therefrom, and the opposing electrodes may be formed as tubes, plates or screens of relatively large The opposing or collecting electrodes 3, may be grounded as indicated at 4, and the discharge electrode means 2 are connected by conductors -2 to current distributing means 5, whereby said conductors 2' are successively connected to an energizing circuit connection i, leading from the means for generating shock potential or voltage impulses.

The distributing means 5 may comprise a rotating arm 5a making contact with fixed contacts 5b connected to the respective lines 2 leading to the precipitator sections, said arm Ba being rotated at suitable velocity by a motor I which may be an electric motor provided with suitable speed reducing means.

The means for generating shock potential'or voltage impulses may comprise a step-up transformer whose primary winding e is connected to any suitable alternating current "service circuit and whose secondary winding 8 is connected through rectifying means and spark gap means 1 to the connection 6 aforesaid. In the form shown in Fig. l, the rectifying means comprises two thermionic valve tubes in and ii, whose filament terminals are connected to opposite ends of the secondary winding 9 (said winding having a centralground connection l3) and whose plate terminals are connected to wire or conductor I! for delivering rectified current impulses of high potential difference, said wire It being in connection, as hereinafter described, with the wire 6 leading. to the electrical preclpitator sections. Suitable transformers indicated at it and ii are provided for energizing the filaments of the respective thermionic valve tubes.

A spark gap means ll is connected between the rectifying means and the wire 6, so that the electrical potential diiference generated by the stepup transformer and rectified by the rectifying.

means, is by the operation of such spark gap applied intermittently to the connection leading to the precipitating apparatus in the manner hereinafter described. We also prefer to provide suitable capacity means in the connections of the spark gap so as to store electrical energy in such manner as to increase the suddenness and magnitude oi the shock potential difference or voltage impulse applied to the precipitating apparatus. The capacity eifect may be obtained by means 'of a condenser i9 connected between the line i2 and ground 19' or, if desired, the line 32 may be formed in part as a cable having an outer metal casing 28, grounded as indicated at 2|, 9. suitable solid dielectric 22 being provided between said casing and the line t2, the electrostatic capacity of such cable being such as to provide the required electrostatic capacity for supply of sufilcient energy to the spark gap. If desired, both the cable and the condenser may be used to provide the desired capacity.

The spark gap means I I is preferably provided with cooling means, suchas means for applying a stream of cooling gas thereto, in order to prevent undesirable residual conductivity of the spark gap when the energizing impulse ceases.

In case a separate condenser is used, such as indicated at I9, it is placed between the generating means and the spark gap, preferably in the immediate neighborhood of the latter, so that disturbing action due to the self-induction of the line is avoided.

In many cases it is advantageous to interpo'se in the line a non-inductive resistance, indicated at 23, of such magnitude as to prevent the propa- 2,000,010 area, adapted to minimize electrical discharge.

gation of oscillations back through the line. Also, a self-induction coil, such as indicated at 24, may be provided in advance of the connection of the condenser I! so as to suppress high frequency oscillations in the line between the generating means and the spark gap.

The operation is as follows: Alternating current is supplied to the primary winding 8 of the transformer and the alternating high tension current thereby generated in the secondary winding, is

rectified by the action of the valve tubes In and I I in well known manner, so as to produce rectified high tension current waves corresponding to both half waves of the alternating current. The electrical energy thus supplied to line I2 is stored in the capacity means ahead 01' the spark gap H, such capacity means being provided either by condenser l9 or by means of a cable having the necessary capacity, as above described. Such capacity means eventually becomes charged to sufficiently high voltage to cause a spark to pass through the spark gap 0?, the electrical resistance of said gap being thereby momentarily greatly decreased with the result that a high potential impulse or voltage impulse having a steep wave front is momentarily imparted to the line 6 leading to the precipitating apparatus. By the operation of the distributing switch 5, this impulse is communicated to the line 2' leading to the particular electrical precipitator section which is at that moment in connection with the contact arm Ea of said'switch, and in the operation of said switch, this connection is progressively shifted so as to bring the respective precipitator sections successively into connection with the charging line 8 and apply the voltage impulse or impulses to each precipitator section in succession. It will be observed that the electric current is rectified before reaching the condenser, so that the high potential impulses produced by discharge of said condenser across the spark gap are unidirectional impulses, that is, all of said impulses are in the same direction. Consequently,. these impulses serve to create a unidirectional field in the respective precipitator sections.

The connections of the spark gap circuit are preferably such that only enough energy is thus supplied to the precipitating held of each section that the voltage therein exceeds the ionization voltage for only a. short time at each voltage impulse and then falls again to approximately the voltage at which ionization ceases. The development of disruptive discharge in the precipitator is thereby effectively suppressed. By the method and apparatus above described it is possible to apply to the electrical precipitator sections a momentary voltage as much as'80% in excess of that which can be applied by means of the usual rectified alternating current.

By appropriate selection of the voltage, electator line, the rapidity at which the voltage impulses succeed one another can be regulated within wide range so that the conditions best adapted for the operation of. the precipitator can be obtained, for example, by increasing or decreasing the electrostatic capacity the rapidity of sequence of the sparks can be varied between from several hundred impulses per second down to say one impulse per second. Each voltage impulse is initiated when the condenser i9 or other capacity means becomes charged by the generating means to the. sparking potential for the gap, and the energy which has been stored in the line due to its electrostatic capacity is then discharged through the spark gap to theparticuiar precipitator section which for the time being is in connection therewith, the time required for discharge of such energydepending upon the capacity in connection with the spark 83D, and the time required for charging the condenser before another spark occurs also depending on such capacity, so that by proper adjustment of the electrostatic capacity, as stated. any desired rapidity of sequence of voltage impulses may be provided.

It will be understood that the time between successive breakdowns oi the spark gap, and consequently the number of impulses impressed upon the precipitator in a given unit of time, depends entirely upon the characteristics of the circuit and not upon the frequency of the rectified alternate current impulses used for charging the condenser l9.

In Fig. 3, time is plotted horizontally and voltage vertically. The voltage supplied to the elcctrical precipitation apparatus by the above described impulse circuit is indicated by the line AA, while the line O-O represents zero volt-.

age. successive impulses are shown as starting at the times indicated at t1, t2 and ts, while the period between successive impulses is indicated at T.- At the time of each such impulse the voltage rises rapidlyfrom V1 to V2. The latter value represents the maximum or peak voltage attained at the precipitator during each impulse. The voltage across gap l1 ialls almost instantaneously to a sufiiciently low value to break the spark, and the energy stored in the circuit is rapidly dissipated, causing the voltage between the precipitator electrodes to fall rapidly to approximately the value at which ionization in the precipitator ceases to occur, after which the voltage falls more gradually to the value V1, before the start of the next succeeding impulse. It will be understood that the voltage plotted in Fig. 3 representsthevoltage applied to the electrical precipitating apparatus as a whole, and

that one or any desired number of successive impulses of the character shown may be applied, in turn, to the respective sections I, depending upon the relation between the rapidity of sequence of the impulses and the speed of operation of the distributing switch 5.

By the above described means, it is possible to apply the high voltage impulses to a large precipitating apparatus as eflicientlv as to arelatively small precipitating apparatus.

The distributing switch I may be operated at such rate as to put each precipitator section in connection with the spark gap for a sumcient time to receive one or any desired number of impulses therefrom.

Any other suitable means may be .provided for generating the required unidirectional .high potential diiierenoe for energization of the spark gap. For example, as shown in Fi 2, instead of the thermionic valve tubes shown in Fig. 1, we may'use a mechanical rectifier such as indicated at 16 having terminals connected to the secondary winding 0' of the step-up transiormer having a ground connection indicated at I! and a line connection indicated at l2, the latter leading to the spark gap means in the same manner as above described.

This application is a continuation in part or our application. SerialNo. 527,690, flied April 4,

'We claim:

The method of electrical precipitation which consists in storing electric energy in a condenser in connection with a park gap until the spark potential or said gap is reached, thereby delivering a high voltage impulse through said spark gap, repeating said operations so as to produce a succession of such impulses, and applying such high voltage impulses to energize successively, one at a time, a plurality of electrical flelds containing gases carrying suspended material so as to eflect electrical precipitation by the action of said electrical fields.

. RICHARD HEINRICH.

' WILHEIM FEIDMANN.

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