Electrostatic Gas Filters

Abstract

An electrostatic gas filter element for use in a gas filter includes three substantially planar electrodes spaced in substantially parallel planes along the direction of the gas flow. Each electrode includes a metallic frame across which are strung either lengths of braided wire or helical springs. The center electrode receives a first voltage while the two outer electrodes a different voltage.

Description

This invention pertains to electrostatic gas filters and more particularly to improvements in my U.S. Pat. No. 3,040,497.

Although filters made according to the above cited patent perform well they have created a demand for even better filters. In particular, the use of two electrodes, one receiving a D.C. voltage and the other receiving an R.F. voltage, limits the rate of agglomeration of particles and the separation of parasites since there is a limit to how varied one can make the gradients of the electrostatic fields. Further, the electrodes of the filter elements are complicated structures using relatively expensive smooth rods to minimize arcing and similar effects.

It is, therefore, an object of the invention to provide an improved electrostatic gas filter element for generating much more complex electrostatic fields which have more complex voltage gradients to enhance agglomeration of sub-micron particles.

It is another object of the invention to provide such filter elements with electrodes which are not only less expensive than previously used electrodes but which at the same time aid in the generation of those voltage gradients which enhance agglomeration of the sub-micron particles.

These and other objects are accomplished by an electrostatic gas filter element which is to be positioned in a gas stream. The filter element comprises first, second and third electrode means which are aligned in substantially parallel and spaced relationship with the second electrode means located between the first and third electrode means. Means apply a first varying voltage to the first and third electrode means; and other means apply a second and different varying voltage to the second electrode means to create very diverse field gradients to enhance particle agglomeration. The invention further contemplates new and unusual electrode means which both simplify fabrication of the filter element and increase the efficiency operation of the filters.

Other objects, features and advantages of the invention will be apparent from the following detailed description when read with the accompanying drawings which show, by way of example, and not limitation apparatus embodying the invention.

In the drawing:

FIG. 1 is a perspective view of a gas filter according to the invention shown schematically positioned in a duct confining a gas stream;

FIG. 2 is a sectional view through the electrostatic gas filter element of the gas filter of FIG. 1 to show the staggering of the conductors of the electrode means;

FIG. 3 is a front elevation of one embodiment of the electrode means of FIGS. 1 and 2;

FIG. 4 is a front elevation of another embodiment of the electrode means of FIGS. 1 and 2;

FIG. 5 shows a portion of the conductors used in the embodiments of FIGS. 3 and 4;

FIG. 6 is a front elevation of a further embodiment of the electrode means of FIGS. 1 and 2; and

FIGS. 7, 8 and 9 are waveforms of the voltages generated by the voltage sources of FIG. 1.

In FIG. 1 an electrostatic gas filter element 10 is shown positioned in a duct 12 (indicated schematically by dashed lines) through which a gas such as air containing pollutants flows in the direction of arrow 14.

In general, upstream from electrostatic gas filter element 10 there is a collector element (not shown) which mechanically, by a screening action, removes the particle pollutants. However, because of the mesh sizes used in such collector elements, many of the sub-micron sized particles are not trapped. Accordingly, the electrostatic gas filter element 10 is used to agglomerate such particles into larger sized particles so that on the next recirculation of the gas through the duct the agglomerated particles are trapped in the collector element. Alternately, the collector element may be positioned downstream of electrostatic gas filter element 10 so that agglomeration takes place before the initial collecting.

The electrostatic gas filter element 10 comprises first, second and third electrode means 16, 18, 20 aligned in, preferably 2-inch, spaced planes perpendicular to the direction of gas flow. The outer electrode means 16 and 20 are connected to a first voltage source 22, and the central electrode means 18 is connected to a second voltage source 24. Since all the electrode means are the same, only typical electrode means 16 will be described in detail.

Electrode means 16 comprises a rectangular frame 26 of conductive material across which extend a plurality of, preferably 2-inch, spaced parallel conductors 28. As can be seen in FIG. 2, the conductors of each electrode means are offset from those of its neighbor to enhance gas flow.

In one embodiment of the electrode means 16, a portion of which is shown in FIG. 3, the frame includes two spaced and parallel bars 30 and 32 of conductive material. Each bar carries a plurality of regularly spaced conductor engaging means in the form of eyelets 34. The conductors 28 are formed from a single length of wire hereinafter more fully described. The wire is laced through the eyelets in a regular pattern to provide a plurality of parallel conductors which extend between the bars 30 and 32.

In an alternate embodiment of the electrode means 16' shown in FIG. 4, the frame also includes a pair of spaced and parallel bars 40 and 42 of conductive material. Fitted on each bar is a plurality of tubular spacers 44. The spacers 44 are disposed in a regularly spaced arrangement. The conductors 28 are again formed from a single length of wire. The wire is wound around one bar at the interspacer gap then to the other bar where it is wound around the bar at another interspacer gap in a regular pattern to provide a plurality of parallel conductors which extend across the frame.

In both of the embodiments of FIGS. 3 and 4, not only is easy fabrication obtained but also complex electric field gradient properties are obtained by using braided wire such as the shielding of microphone cable as shown in FIG. 5. In fact, it has been found that the irregular surface of such wire and particularly the diamond-shaped braided surface gives an irregular field.

FIG. 6 shows another embodiment of the electrode means 16" which has similar properties, includes a frame with two spaced bars 60 and 62 of conductive material, each provided with a plurality of regularly spaced kinks 64. The conductors are now metal springs 66. The ends of each spring have hooks 68 and 70 for engaging kinks 64 of the bars 60 and 62.

As was stated above, the electrode means receive energizing voltages from sources 22 and 24.

First voltage source 22 generates a high amplitude (tens of kilovolt) low frequency (kilohertz) voltage having a generally sawtooth waveform. Again, to enhance the irregularity of the field gradients, the trailing edge of the waveform has an A.C. ripple. The voltage can swing from ground in the positive direction as shown in FIG. 7, or in the negative direction, or swing above and below ground as shown in FIG. 8. Second voltage source 24 can generate the same voltage as source 22, but of opposite polarity. Or, the voltage can be a low amplitude (hundreds of volts) high frequency (hundreds of kilohertz) voltage having a sinusoidal waveform. Such voltage generators are well known, per se, in the art and accordingly, will not be described. For example, a voltage generator for generating waves of the type shown in FIG. 7 is set forth in U.S. Pat. No. 3,040,497. For example, a voltage generator for generating waves of the type shown in FIG. 9 (the wave form supplied by the second voltage source 24) is set forth in Paragraph 66: "Multitube Transmitter Circuits," C-W and A-M RECEIVERS, Department Of The Army Technical Manual TM 11-665, Sept. 1952, pages 103-108.

By using such voltages and such conductors, completely irregular electric fields are generated in the region of electrostatic gas filter 10. Since the charges and the masses of the pollutants varies over a broad range, such irregular fields increase the likelihood of accelerating the pollutants and enhance agglomeration and the separation of the parasites from their particles making for less odor and smoke.

While the invention has been described in detail with respect to certain now preferred examples and embodiments of the invention, it will be understood by those skilled in the art, after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended, therefore, to cover all such changes and modifications in the appended claims.

Claims

What is claimed is:

1. An electrostatic gas filter element to be positioned upstream of a collector element comprising: a duct; three planar electrode means in said duct, said electrode means being perpendicular to the axis from said duct and spaced from each other, whereby a first electrode means is disposed between the other two electrode means, each of said electrode means comprising a frame including two spaced and parallel bars of conducting material, and a length of braided multi-wire metallic conductor connected to said parallel bars to form an array of parallel conductors extending between said bars, the parallel conductors of said first electrode means being positioned with respect to the parallel conductors of at least one other of the planar electrode means so as to be mutually offset with respect to the axis of said duct; means for applying a first voltage which sinusoidally varies about ground potential to said said first electrode means; and means for applying a second pulsating voltage which is unidirectional with respect to ground potential with a portion of its waveform having an A.C. ripple to both of said other two electrode means.

2. The electrostatic gas filter element of claim 1 wherein each of said two spaced and parallel bars carry a plurality of spacers and said length of multiwire metallic conductor is alternately and regularly wrapped in gaps between said spacers and extending across the space between said bars.

3. The electrostatic gas filter element of claim 1 wherein said second voltage has an amplitude in the kilovolt range.

4. The electrostatic gas filter element of claim 1 wherein the wires of said metallic conductor are braided according to a diamond pattern.

5. An electrostatic gas filter element to be positioned upstream of a collector element comprising: a duct; three planar electrode means in said duct, said electrode means being perpendicular to the axis from said duct and spaced from each other, whereby a first electrode means is disposed between the other two electrode means, each of said electrode means comprising a frame including two spaced and parallel bars of conducting material, a plurality of conductor engaging means fixed at regular intervals along said bars, a length of braided wire conductor laced on said engaging means to form an array of parallel conductors extending between said bars, the engaging means of the frame of said first electrode means being positioned with respect to the engaging means of the frame of at least one other of the planar electrode means so that their respective conductors are mutually offset with respect to the axis of said duct; means for applying a first voltage which sinusoidally varies about ground potential to said said first electrode means; and means for applying a second voltage which is unidirectional with respect to ground potential and has a substantially sawtooth waveform with a portion thereof having an A.C. ripple to both of said other two electrode means.