US2659841A - Means for electrifying pulverulent materials - Google Patents

Description

Nov. 17, 1953 P. HAMPE 2,659,841

MEANS FOR ELECTRIFYING PULV-ERULENT MATERIALS Filed July 22, 1949 3 ploying high voltages which may, for example, carry voltages of the order of 1,500 to 2,000 volts per millimeter of distance between electrodes.

It is a feature of the invention that full advantage may be taken of low voltage operation. In my apparatus, the sparking hazard may be lessened by the provision of a high resistance in the feed circuit positioned just ahead of the discharge electrode. Such a resistance not only serves to prevent sparking but prevents glowing discharges that may either damage the electrode or ignite the powder particles. Furthermore, my apparatus is advantageously provided with several nozzles, each nozzle being provided with a resistance, this arrangement thus serving to divide the discharge of the generator between the various nozzles and to reduce the risk of injury should an operator touch one of the nozzles.

I also advantageously reduce sparking by imparting a high speed to the current of particlecarrying gas conducted through the electrifying zone between the electrodes. Alternately I provide means for operating my apparatus at a superatmospheric gas pressure. For a given voltage, the sparking length is inversely proportional to the pressure. A small size ionizer in accordance with the invention is particularly adapted for taking advantage of the aforementioned conditions which are conducive to efiicient electrification. Furthermore, in an ionizer in which there is a relatively small clearance between the electrodes, the time required for the charging of a powder particle is proportionately small. In fact, the time required for obtaining a given proportion of the maximum charge is inversely proportional to the concentration P of the charges in the ionized gas. Thus, the formula shows that the concentration is proportional to the field E and in inverse proportion to the distance R to the electrode.

In apparatus according to the invention, E may reach a value three times as great as that possible in heretofore known ionizers and R may be ten times less, e. g. two millimeters instead of twenty millimeters. Thus, a charging period sixty times smaller is possible and the length of the ionizer may, accordingly, be reduced. In View of this fact, it has been possible in accordance with the invention to replace the standard fine wire electrode by a tip, an arrangement which has not been satisfactory in known ionizers of larger size. On the other hand. the high value of the charge on the particles enables the ionizer to be fed with gas that is more heavily charged with powder particles without damage of clogging the apparatus. '1 have found that in addition to the above mentioned measures taken for avoiding sparking, certain other precautionary measures should also be taken. In a small ionizer, such as an apparatus made in accordance with the invention, the two electrodes are more symmetrical than in larger ionizers. Consequently, the strong field that prevails between the electrodes has a tendency to cause eddy currents to appear on the electrode of larger surface. These electrical discharges of opposite polarity discharge the powder and they are created if the electrode becomes covered with powder, particularly if the powder is of aninsulating nature. In practice, the strong field which is set up by the ionizer has a tendency to create a strongly attractive deposit that even a rapid currentof gas is not effective to prevent.

4 Accordingly, one of the features of the apparatus of the invention is the prevention of the formation of such deposits on the electrodes. There are several means for advantageously preventing these deposits.

One means comprises the addition to the powder to be electrified of an abrasive such as alumina or quartz. In addition, the nozzle of the ionizer may be shaped to facilitate the cleaning process by insuring that no portion of the surface is sheltered from impacts of the flOWing gas. The above mentioned means are particularly effective for insuring that the smaller electrode will be kept clean. Other means are particularly suitable for cleaning the large surface electrode. Thus, in one embodiment of the invention, the larger surface electrode may be arranged to slide in such a way that its inner surface is cleaned by a fixed surface or the larger electrode may take the form of a revolving member which is cleaned by a fixed brush, constantly in engagement with its surface. The cleaning means is located out of the space between the closest portions of the electrodes so that they do not cause arc-overs.

In order to increase the electrical field in the apparatus and to provide the particles with a higher charge, it is advantageous to cause the charge to take place in the.presence of compressed gas. In accordance with one embodiment of the invention, therefore, means are provided for feeding the apparatus with gas at a pressure up to several atmospheres, the larger surface electrode being advantageously formed in such manner that the pressure is maintained during the movement of the particle carrying gas through the electrical field. For creating high gas pressures, I advantageously provide compressors, diaphragm pumps and the like in place of the fan blades which have been used in prior ionizer apparatus.

The objects, characteristics and advantages of the invention will be more fully understood from the following description and claims in conjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal section of an ionizer in accordance with the invention in which an end portion of the nozzle is slidable relative to a fixed portion to keep the nozzle clean during operation of the apparatus.

Fig. 2 is a side elevation, partially in section, of an ionizer in which the larger surface electrode is formed by rotating members.

Fig. 3 is a front view of the arrangement shown in Fig. 2.

The apparatus illustrated in Fig. 1 comprises a central electrode l 0 which is shown as a pointed wire electrode having a sharp point H. The pointed electrode I0 is connected through a high resistance R to a direct current voltage supply HT the voltage of which is preferably in the range of 3,000 to 6,000 volts. A second electrode l2 surrounds the tip portion I I of the central electrode l0 and comprises a section of tubing that is coaxial with the central electrode. The tubing section I2 is telescopically slidable on the end portion of a fixed tube l3 through which a stream of air or other gas carrying the particles of pulverulent material to be charged is supplied.

The tubing l2, I3 is connected to ground, as indicated at G. Thus, an electric field is produced between the tip portion of the central electrode l0 and the inner surface of the tubing section l2 forming the outer electrode. This inner surface may thus be referred to as the active surface of the electrode I2. The stream of gas carrying the pulverulent material to be har ed ws hrou h he t in 2, '3 n he di ecti n eeted by e a row and hus :fiows t r h he field be we n the electrod s and 1:2, b i th eupon dischar d int th at phere. The discharge end of the tubing 12 is preferably flared outwardly, as indicated at 2.

In order-to clean the active inner surface of the tubular electrode 12 during operation of the apparatus and thereby keep it clear of particles of the material being charged, provision is made for reciprocating the movable tubing section 12 relative :to the :fixed .tubing 1:3. "For example, as illustrated in the drawings, the tubing section 12 is reciprocated .by a :flexiblecable or Bowden 'wire 14. As the tubing section 12 is reciprocated, its inner surface rubs on the outer end 123 .of the fixed tube 1.3 so as-to dislodge any material that may have collected on the inner surface of the tubingsect-ion 12. "It will be seen that the end edge :23 of the tubing .13 which constitutes cleaning-means forthe inner surface of the tubular electrode 12 is disposedoutside of the direct field between the closest portions of the electrode .12 and the .central electrode 10. Hence, the cleaning means does not reduce the gap :between the electrodes or tend to cause any arc-over between the electrodes.

in Figs. '2 and 3, there is shown a preferred form of apparatus 'in accordance with the invention in which the outerelectrode comprises two grooved pulleys .15 and i6 rotatable about spaced parallel axes so that the peripheries of the pulleysare close-to one another. The grooves in the peripheries of the pulleys 15, 16 are approximately semi-circular in cross section so as to provide a space 20 of approximately circular cross section between thetwo-pulleys. A pointed centralelectrode 19 projects into, and is concentric with, this circular-space so as to be at least partially surrounded bythe outer electrode composed of the grooved pulleys 15, 16. As in the foregoing embodiment, the central electrode 18 is connected through a resistance R with a-direct current high voltage source to provide an electric-fieldbetweenthe pulleys 1-5, 1-6and the pointed central electrode 19. A tube 21 blows a powder-loaded air stream through thespace 20 between the pulleys 1-5, 16 and thus through the above mentioned electrical field.

During operationof the apparatus, "the grooved pulleys and 116 are rotated in the directions indicated by the arrows by any suitable driving means. Contact brushes 1.! and 18engage the grooved peripheries of the pulleys and keep the grooves clean as the pulleys are rotated. The pulleys 155, 16, contact brushes 1'1, 18 and supply tube are-grounded as indicated at G.

.As aimodifie iema ng ul maybe e instead .of two, thispulley being cleaned in similar-manner by a grounded contact brush.

-It will beseen that, in the embodiment of-Figs. and 3, a .n that .of Fi .l. the u l trod is movable relative tofixed clean n means to keep the active. surface of said-electrode clean during operationof the apparatus. By thus keeping this surface clean, much closer spacing of the electrodes can be used than would otherwise be possible. Since the cleaning means is located outside the direct field between the closest portions of the inner and outer electrodes, it does not in any way interfere with the desirable small electrode gap.

In addition to keeping the active surface of the outer electrode clean, as described above, it is also important to keep the central electrode clean.

B st sul s have been btain d by her 1 the entral l ctrode p int in a d re t n ,QPPQ e to thatof the flow of t-hegaseous cur-rent. gut U way, the central electrode is kept clean .andthere has also been found to be a more desirable distribution of the ions in gradually charging the powder during the short length of time it takes to pass through the ionizer. Cleaning .of the central electrode is also improved by drawing out the tip to a very fine point and by using a hard noneoxidizable metal such vas tungsten, nickel, chromium, etc.

It is also absolutely necessary to protect the point against sparking between the point and the surface of the .outerelectrode. Such sparking is inhibited by the use of a resistance between the centralelectrode and the voltage supply.

In order to assure the satisfactory working of the apparatus, it is desirable for the air cur;- rent to travel at azhigh speed. This is achieved by supplying the ionizer with air compressed to a suitable pressure. For example, it has been found that good results are obtained with pres-.- sures of 20'to 40 grams per square centimeter. Previous devices-employing a fan or zblower have operated with only 1 to -5 grams per square centimeter pressure. -With a view :to increasing the voltage of the electrical field :in the ionizer and thereby transmit to the powders a "higher charge, devices in accordance with the invention may operate with an air compressed up-to several atmospheres, the discharge opening ornozzlebeing restricted so as to provide superatmospheric pressure at the point where electrification takes place.

Apparatus in accordance with the present invention preferably uses direct current with a voltage of the order of 3,000 to 6,000 volts and an output per unit of about microamperes. The required voltage may be supplied by a small generator. Especially suitable are induction coil or transformer generators provided with electronic, copper oxide or rotating commutator-rectifiers, the primary current being supplied by a. small alternator orby a battery. In this way, dusting equipment is obtained that is easy to carry and is especiall suitable for agricultural purposes. The weight of the electric generator required isof the order of one kilogram andthe electricpower requirement is less than one watt.

"It willibe understood that the apparatus shown in the drawings is merely by way of example use that the invention is not limited to the specific apparatus herein particularly shown and described.

In the claims:

n apparatus o p tin highly el c r fied particles of pulverulent material, an outer electrode comprising a pair of grooved pulleys rotatable about spaced axes and having their peripheries .close to one another, peripheral grooves in said pulleys forming ,a spaceqfapproximately circular cross-section .between said pulleys, an inner electrode comprising a pointed wire disposed between said pulleysand .means for directing a jet or gas carrying said particles around said inner electrode and between said grooved pulleys.

2. In apparatus for projecting highly electrified particles of pulverulent material, an outer electrode Comprising a pair of grooved pulleys rotatable about spaced axes and having their peripheries close to one another, peripheral grooves in said pulleys forming a space of approximately circular cross-section between said pulleys, an inner electrode comprising a pointed wire disposed between said pulleys, means for directing a jet of gas carrying said particles around said inner electrode and between said grooved pulleys and means for cleaning the grooved peripheries of said pulleys as they rotate.

3. Apparatus for projecting highly electrified particles of pulverulent material into the atmosphere, comprising a pointed wire electrode and a second electrode having an active surface of materially large area facing and at least partially surrounding the pointed electrode, the distance between the electrodes being of the order of a few millimeters, means for producing an electric field between the pointed electrode and said active surface of the large-surface electrode, the potential between said electrodes being of the order of 2500 to 8000 volts, means for directing a stream of gas carrying said particles through the electric field between said electrodes and into the atmosphere, and fixed rubbing means engaging said active surface of said large-surface electrode, said large-surface electrode being movable relative to said fixed rubbing means during the electrification and projection of said particles to bring substantially the entire area of said active surface into engagement with said rubbing means and thereby keep said active surface clear of said particles, said rubbing means being disposed out of the direct electric field between the closest portions of said electrodes.

4. Apparatus according to claim 3, in which said pointed wire electrode points in a direction opposite to the direction of flow of said stream of gas.

5. Apparatus for projecting highly electrified particles of pulverulent material into the atmosphere, comprising a pointed wire electrode and a second electrode having an active surface of substantially larger area facing and at least partially surrounding said pointed electrode, said active surface being a surface of revolution and the distance between said electrodes being of the order of a few millimeters, means for producing an electric field between the pointed electrode and said active surface of the large surface electrode, the potential between said electrodes being of the order of 2500 to 8000 volts, means for directing a stream of gas carrying said particles through the electric field between said electrodes and into the atmosphere, and means for cleaning particles of material from said active surface, said large-surface electrode being rotatable about the axis of said surface of revolution during the electrification and projection of said particles to bring substantially the entire area of said active surface progressively into engagement with said cleaning means to keep said surface clear of said particles, said cleaning means being disposed out of the space between the closest portions of said electrodes.

6. Apparatus for projecting highly electrified particles of pulverulent material into the atmosphere, comprising a pointed wire electrode and a second electrode comprising a pair of rotatable members having active surfaces of substantially larger area facing and at least partially surrounding said pointed electrode, said active surfaces being surfaces of revolution and the distance between said surfaces and the pointed electrode being of the order of a few millimeters, means for producing an electric field between the pointed electrode and said active surfaces of said second electrode, the potential between said electrodes being of the order of 2500 to 8000 volts, means for directing a stream of gas carrying said particles through the electric field between said electrodes and into the atmosphere, and means for cleaning particles of material from said active surfaces, said rotatable members being rotated during the electrification and projection of said particles to bring substantially the entire area of said active surfaces progressively into engagement with said cleaning means to keep said surfaces clear of said particles, said cleaning means being disposed out of the space between the closest portions of said electrodes.

7. Apparatus for projecting highly electrified particles of pulverulent material into the atmosphere, comprising a pointed wire electrode and a second electrode having an active surface of substantially larger area facing and at least partially surrounding the pointed electrode, the distance between the electrodes being of the order of a few millimeters, means for producing an electric field between the pointed electrode and said active surface of the large-surface electrode, the potential between said electrodes being of the order of 2500 to 8000 volts, means for directing a stream of gas carrying said particles through the electric field between said electrodes and into the atmosphere, means for cleaning particles of material from said active surface, said second electrode being reciprocable relative to the pointed electrode and said cleaning means, and means for reciprocating said second electrode during the electrification and projection of said particles to bring substantially the entire area of said active surface into engagement with said cleaning means to keep said surface clear of said particles, said cleaning means being spaced from said pointed electrode in the direction of reciprocation of said second electrode so as to be disposed out of the space between the closest portions of said electrodes.

PIERRE HAMPE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,444.092 Anderson Feb. 6, 1923 1,928,963 Chaffee Oct. 3, 1933 2,302,289 Bramston-Cook Nov. 17, 1942 2,313,389 Meston Mar. 9, 1943 2,396,533 Root Mar. 12, 1946 2,477,947 Yadoff Aug. 2, 1949 2,484,202 Wintermute Oct. 11, 1949 2,509,277 Ransburg May 30, 1950 2,526,178 Weber Oct. 17, 1950

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