US2364641A - Composite electrode - Google Patents

Description

' Dec. 4- A. F. MESTON v COMPOSITE ELECTRODE Filed May 5, 1941 a transporting Patented Dec. 12, 1944 Archibald F. Meston, Bo to Research Corpora tion,

and Brooln N. J.. asalgnor I New York; N. Y., a

corporation of New York Application May 3, 1941, Serial No. 391,789

4 Claims.

This invention relates to a system of, electrodes usefulin establishing an electric field or electric fields across a gas-filled space for the purpose of ionizing. the gas and electrically transporting particles suspended in the gas. It relates in particular to a composite electrode comprising one or more attenuated elements from which corona discharge can be made to emanate and an adjacent element of extended surface upon which a strong non-discharging electric field can be made to terminate.

In the following description and exposition, the term collecting electrode nate those electrodes which incommon electrical precipitation practice provide surfaces for the reception or-collection of particles which have been driven from the air stream by an electric field, and upon which a portion, at least, of the electric field terminates. The complementary electrode; usually called a discharge electrode, will be referred to as a compositeelectrode or precipitating electrode? from time to timeherein and will be understood to comprise a non-discharging element; or elements-which is designed to serve as one terminal. of the strong electric field that is established to bring about the migration of any charged particles suspended in air which may flow through the'electric field, and a discharge or ionizing element which is comprised of one or more members having surfaces of very small radii-of curvature and from which discharges capable ofionizing' adjacent gases are caused to emanate.

A principal object of the electric treating means whereby'a composite discharge electrode energized at one potential when opposed by a suitable complementary electrode of the so-called other potential eil'ects the formation of a charging field, comprising an ionizing discharge, and or precipitating field, without discharge, in closely adjacent zones.

This and other objects and advantages are obtained by the provision of a composite electrode comprising an-extended surface portion free from' of curvature small I enough to produce corona discharge under operating conditions and adischarge surface portion surface. elements of radius of very small radius of curvature supported by the extended surface portion, the discharge surface portion projecting from the extended surface to a distance not exceedingthesmallest radius of curvature of the extended surface portion adjacent the dischargesurface portion and having an aggregatelength at least a multiple of such projection distance. said composite electrode being symmetrically positioned between parallel spaced extended surface collecting electrodes defining a gas treating s ace therebetween and inwill be used to desiginvention is to provide collecting type energizedat aneral, include surface curvature substantially less than one-tenth incL strengths adjacent the cluding opposed portions projecting into the gas treating space to provide a constricted zone in said space, at least one of the portions of the composite electrode being positioned in the u a ed zone provided by the projecting portions of the collectin'gelectrode.

While the character of surface which will produce corona discharge will vary with such conditions as potential difference, electrode space and character of the gas, the extended surface member of the discharge electrode should not, in genportions having a radius of while the attenuated discharge members will include surface. portions having radii of curvature of the order of five-thpusandths of an inch or less.

The invention will be more particularly described with reference to the accompanying drawing in which: I

Fig. 1 is a diagrammatic sectional elevation of an electrical treater embodying the principles of the invention;

Fig. 2 is a partial section online 2-2 of Fig. 1;

Fig. 3 is a diagrammatic representation in longitudinal section of a modified form of electrical treater embodying the principles of theinvention; i

.Fig. 4 is an enlarged fragmentary view of the lower end of the composite electrode of Fig. 1;

Fig. 5 is an enlarged end view of the same electrode;

Figs.,-6. 7 and 8 are representations of modified forms of electrodes embodying the principles of the invention;

Fig. 9 is' a sectional view of the electrode of Fig. 8 on line 9-9 of Fig. 8;

The treater diagrammatically shown in Figs. 1 and 2 comprisesa casing for'the flow of gases, including flat plates l0 providing collecting electrode surfaces, a top or cover plate II, and a bottom plate I2. In the construction of Figs. 1 and 2 means is provided for varying the relative field precipitating and the discharging elements of the discharge electrode, by varying the position of the discharge electrode with respect toportions Illa of the collecting rlectrode members, wh ch portions project into the interelectrode space. In the position of the discharge electrode I 9, shown in full lines, the difference between the strengths of the electric fields adjacent the two portions of the electrode The composite discharge electrode l9 cominsulated support 23.

.manner well known in the designated as prises precipitating member 20, typically a smooth, rigid rod or tube of circular, elliptical, ovate or suitably shaped section, and discharge members 2|, 22 consisting of, in the embodiment shown, prongs of very fine I electrode is suspended in the known manner from The collecting electrodes are grounded at 24 and the discharge electrode is connected at 25 to an energizing system in the art of electrical precipitation.

The construction shown in Fig. 3 is a modified embodiment of the construction of Figs. 1 and 2 which permits a very strong precipitating field to be realized under conditions which make for continuity of operation.

Auxiliary electrode members llib are smooth, round rods or tubes, which function as collecting electrode elements. They .arecaused to rotate continuously against cleaning means 2| which prevent the building up of deposits of material which might cause back-corona. As in the construction of Figs. 1 and '2, the relative strengths of the precipitating field and the ionizing field maybe varied by adjusting the position i of the discharge electrode ments lob.

When a voltage difference of say 10 to 30 kilovolts per inch is impressed across the space be- I! with respect to eletween electrodes [9 and III, a strong electric field exists in that space which is strongest where the so-called lines of force are the shortest and most concentrated and most nearly parallel.v

' The conditions realized by energization of the arrangement just described can be summarized as follows: 4

An electric field is established which is most intense, on the average over its entire length, between electrode surfaces from which no corona discharge emanates. This portion of the electric field is designated as a precipitating field. Portions of the electrical'field-of appreciably .less average intensity terminate on attenuated portions of that electrode known as the discharge electrode and cause local corona discharges therefrom? This portion or portions of the field is the ionizing field though it may function to some extent as a precipitating field.

The average strengths of the precipitating and ionizing fields will vary with the distance of the non-ionizing and ionizing portions, respectively, of the discharge electrode from the collecting electrodes. The relative values of precipitating and ionizing fields can thus be made to vary by changing the shape and dimensions of the electrodes, particularly of the discharge electrode.

The flatter and smoother the non-ionizing surfaces of the complementary electrodes the closer they can be positioned at a given voltage differonce without disruptive discharge and the stronger the resulting electric field,

The more attenuated and isolated the discharge elements, the more intense will be the discharge from them, conditions of voltage and electrode spacing remaining constant.

When the discharge elements are positioned in close proximity to a portion of the non-discharging surface, but at a considerably greater distance from the collecting electrode than certain other portions of the non-discharging surface, the discharge elements will be masked or shadowed by the close-by non-discharging surface and a very high voltage drop across theelectrodes will be required if the electric field at the wire. The discharge a discharge elements is to become intense enough to cause corona discharge. Under these conditions, the precipitating field will have become very strong and effective for migrating .charged particles when the ionizing field is still relatively weak and comprises low energy discharges only. Such conditions areespecially desirable when air is to be cleaned for human consumption. The intensity of the discharges and theresulting yield of ozone canbe controlled while the precipitating field is maintained'at as near maximum strength as ispractical. v

A150,, if the discharge elements are of very small radii of curvature and are spaced some 'dis- I tance apart, the electric field-as it leaves such an element expandsto fiIIthe-space iust'beyond it and in averyshortdistance has becomeso weak that the phenomena which are responsible for chemical changes in the air-the .formatio'no'f' ozone and ofoxides ofnitrogen, as examples, are not sustained. It is believed that such changes are limited to every shortperiod withinthe' discharge itself, when use is made of the present invention. I

Electrodes .for providing the above results can be of simple design and inexpensive. As an illustration, electrode 19 of Figs. 1 and :2 of the accompanying drawing, as shown more particularly in Figs. 4 and 5, may advantageously be made of two half-round bars, with a radius of one-half inch which are joined together with solder to form a round member 20, and prongs 2|, 22 of .005" diameter tungsten wire, oneend of eachprong being'inserted between the half rounds (with flat sides solder coated) before the half roundsare sweated together. All surfaces are made smooth, the end of bar 20 is rounded if it is not joined to other members, and any joints are left free from cracks or other openings. The prongs will vary in length and number, depending upon the voltage and spacing to be maintained betweenthe electrodes and the degree of ionization desired in the gases undergoing treatment. Good results have been obtained, for example, when the discharge electrode comprised a bar one inch in diameter and prongs of .005

inch diameter wire, one-eighth inch long, spaced one-quarter inch apart along the bar.

This was placed in the center of a treating.pas-,

sage for air defined by parallel flat plates spaced six inches apart. The fiat plates functioned as collecting electrodes and were maintained at ground potential. The discharge electrode was connected to the negative terminal of ,a source of unidirectional current with a peak voltage above ground of about 65 k. v. The prongs pointed in the opposite direction to the air flow which was at aboutfive feet per second.

For highvelocity air fiow and high efficiency of dust=removal, several electrodes, such as the one just described, are preferably placed in series in 'the air passage. a I

In the discharge electrode of Figs. 4' and 5,

means is shown to prevent undesirable discharge off the rounded end of member 20. Rounding the end of member 20, does relieve the concentrated fields that exist when that member has a squared end or a pointed end, but, even so, it has been found that at very high voltages a single rough spot, or a small lump of foreign matter of particular composition, or a crack in the surface there will become the focus of a locally strong field and cause a discharge of unwanted character. The discharge from such a spot is inclined to be of reddish color and extends well out from the surface of the electrode. The amount of ozone formed by such discharge is considerably greater than that formed by the discharge leaving electrodes.

A valuable characteristic of the discharge obtained with electrode I 9 is the so-called electric wind it produces. This electric wind, causing air that would otherwise be motionless to move in the interelectrode spacein a resultant direction parallel to the collecting electrodes, has been measured and has been found to have an average velocity in excess of three feet per second. Actually, the wind comprises local currents that tend to follow the electric field as represented by lines of force and, as electric lines of force tend to repel one another, the result is a fan-like movement of the ions and charged particles inducing movement of the air generally out from the prongs and over to the collecting electrodes. Of course, air must enter the region about the prongs if it is to be aspirated out of that region. so something of a vortical movement of the air results. This movement of air caused by the electric wind is imposed on the normal movement of the air as it is blown through the interelectrode space, say, by a fan producing an induced draught. with the electric wind opposing the normal flow of air through the precipitator-a result obtained by having the discharge prongs point toward the inlet of the precipitator, a very general and thor-.

ough mixing of the ions produced by the discharges and the passing air is obtained with the desired result that all the suspended solid and liquid particles in the air are electrically charged and are caused to migrate in the electric field. The precipitating fields, previously discussed, which extend from the flat portions of the composite electrode to the collecting electrodes are relatively free from electric wind and are particularly adapted for removing the charged particles from the air stream.

As indicated above, the preferred embodiment of the invention includes prongs .or just points,

providing ionizing discharges adjacent weak electric fields it is to be considered intermediate in this respect between an edge and spaced points.

Fig. 7 shows a composite electrode with prongs 2lb extending out front and back, a construction touch one another the condition pre-' sometimes useful under particular circumstances as when a greater degree of ionization of the gases is desired. l

The composite electrode, of Figs. 8 and 9, provides a wire discharge element 2| c spaced from extended surface element 20c by means of fingers 23, to which the wire is soldered.

With an apparatus of the form and dimensions described, effective precipitation of suspended particles from air, with no spark-over and little or no production ofozone, can be obtained with a voltage of 30 to 35 k. v. (root mean square), maintained across the electrodes by connecting the precipitating electrode to a source of rectified high potential alternating current, such as the transformer and rotary mechanical switch arrangement, commonly used in electrical precipitation circuits and grounding the collecting electrode.

I claim:

1. In an electrical precipitator, substantially parallel spaced extended surface collecting electrodes defining a gas treating space therebetween, and including opposed portions projecting into the gas treating space to provide a constricted zone in said space, and an elongated discharge electrode symmetrically positioned between said collecting electrodes, extending transversely of the gas stream and comprising an extended surface member free from surface portions of radius of curvature small enough to produce corona discharge under operating conditions and a plurality of attenuated discharge members carried by said extended surface member presenting surface portions of very small radius of curvature substantially confined to a single plane parallel to the collecting electrodes, at least one of said members being positioned in the constricted zone provided by the projecting portions of the collecting electrode. I 1

2. In an electrical precipitator, substantially parallel spaced extended surface collecting electrodes defining a gas treating space therebetween,

and including opposed portions projecting into the gas treating space to provide a constricted zone in said space extending transversely of the gas stream, and an elongated discharge electrode symmetrically positioned between said collecting electrodes, extending transversely of the gas stream and comprising an extended surface member free from surface portions of radius of curvature small enough to produce corona discharge under operating conditions and a plurality of attenuated discharge members carried by said extended surface member presenting surface portions of very small radius of curvature substantially confined to a single plane parallel to the collecting electrodes, at least one of said members being positioned in the constricted zone provided by the projecting portions of the collecting electrode.

3. In an electrical precipitator, electrodes as defined in claim 1 in which the extended surface member of the discharge electrode is positioned in the constricted zone provided by the projecting portions of the collecting electrode.

4. In an electrical precipitator, electrodes as defined in claim 2 in which the extended surface member of the discharge electrode is positioned in the constricted zone provided by the projecting portions of the collecting electrode.

ARCHIBALD F. MESTON.

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