AU713501B2 - Ionizing apparatus for an electronic air filter - Google Patents

Description

Title: IONIZING APPARATUS FOR AN ELECTRONIC AIR FILTER Field of the Invention This invention relates to gas and air filtration systems. In particular, it relates to the removal of fine particulates like dust from gaseous flows.

Backaround of the Invention Ii the previous art, various combinations of ionizing and dust collecting elements have been used to produce high efficiency electronic air filters. One classic example is the standard precipitator type electronic air filter in which ionizing fine wires of about 0.01 millimetres diameter, charged at about 7 kilovolts, are placed between grounded plates to generate a corona and charge the dust particles passing therethrough. Further down the air flow path, alternating charged and grounded plates collect the charged particles of dust.

Precipitating filters, while highly efficient, produce large numbers of ions and generate ozone. They also consume distinct quantities of current at high voltage, thereby requiring substantial power supplies.

Another type of electronic air filter is the nonionizing, polarized dielectric media type. This is not as efficient as the precipitator type but it is cheaper and easier to maintain. This filter uses filament pads of non-conducting, dielectric material sandwiched between charged and grounded screens which produce electrostatic fields to polarize these pads. Any .particulates passing through the filter also get polarized A.MENDED SHEET WO 98/20978 PCT/CA96/00730 2 and they are attracted and collected by the packed filaments within the pads. This type of system produces very few ions, if any at all, no ozone and consumes virtually no current. The power supply required is thus of a low power type.

Two examples of prior art patents based on the polarization principle are U.S. No. 4,549,833 and No.

4,828,586, the contents of which are adopted herein by reference. The first patent describes a pair of outer hinged screens for enclosing a pair of glass fibre pads with a central grid located therebetween. The central grid, made of coarse wire mesh that is on the order of millimetres in diameter, is charged to around 7000 volts and the outer screens are grounded. This combination does not generate ions significantly. The spacing between the charged screens is between two and five centimetres, producing an electric field gradient. This field gradient polarizes the non-conducting glass fibres rendering them active in trapping dust particles, and more effective than non-polarized pads.

An advantage of this type of filter is that the accumulated dust is readily removed by exchanging the glass fibre pads for fresh pads.

Both of the above designs have disadvantages.

The precipitator type, although it is very efficient when clean, because of the limited surface of the collecting plates, its efficiency drops as the filter loads up with dust. The filter's loading capacity, especially for the WO 98/20978 PCT/CA96/00730 3 larger particles, is very low. Maintenance of the precipitator type filters is very tedious especially in industrial and commercial applications as the plates must be individually wiped to clean them. Also they are expensive both in original investment and operating costs.

This is because they have very elaborate construction and have large, high voltage power supplies that consume anywhere from 80 to 150 watts.

The polarizing filter systems do not have the disadvantages of the precipitator filters but they lack efficiency.

United States Patent No. 5,403,383 to Jaisinghani depicts an "Ionizing Field Electrically Enhanced Filter" wherein air passes through ionizing wires before reaching a separately-spaced pad of dielectric material that has a grounded electrode on its downstream side. To effect increased ionization Jaisinghani provides a further charged "control grid" upstream from the ionizing wires in the air flow to provide field gradients that will create the desired degree of ionization. Filter replacement does not disturb the ionizing wires which are separated from the filters and are permanently connected to the supporting body.

Both United States Patents Nos. 945,917 and 2,593,869 to Cottrell and Fruth respectively describe a precipitator-type air purifier that relies upon an ionizing electrode incorporating multiple, conducting, frayed strands of wire or thread that provide an array of pointed ends. A steep, ionizing potential gradient is formed at the sharp points of the frayed strands. Ionized dust particles are collected of charged walls or plates which, as with all precipitators, must be cleaned regularly.

A further reference that combines ionization and polarization is U.S. patent No. 3,763,633 to Soltis. In this patent wires are used to charge particles by ionization of the air. The use of wires requires higher applied voltages for the production of ions, and this increases the risk of producing ozone.

A major concern in the process of ionizing air is to minimize the production of ozone. Ozone is offensive to some and can be injurious above certain levels. Any system that relies on ionization should also minimize the production of ozone.

In view of the foregoing, it is the object of my present invention to provide an electronic filter which is highly efficient, easy to maintain and inexpensive to install and operate.

The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and Si~U more specific forms will then be further described, and defined, in each of the -individual claims which conclude this Specification.

Summary of the Invention The invention herein is based on ionizing dust particles to enhance their entrapment by a filter pad.

Ionization to effect charging of dust particles is achieved by providing an active ionizing grid in the form of a conductive path having multiple, exposed, ionizing points that, when charged, produce ions because of the high potential gradient formed around such sharp points.

The ionizing grid of the invention is located directly adjacent, and preferably bonded to a fibrous filter pad. Located on the opposite side of the pad from the grid, a conducting screen held at differing potential will provide an electrical field gradient across the thickness of the fibrous filter pad to induce ionization.

Electrical coupling means are further provided to ensure that voltage is applied to and between the ionizing grid andy screen when the pad is installed in a filter support frame.

A preferred way of providing an ionizing grid is to render a cord of multi-stranded filaments of short fibres, such as cotton, conductive. Each fibre provides an end that is pointed and has around it a higher field gradient than the fibre itself, thereby creating ions in F the region of the steep field gradient.

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6 This grid of broken fibre lengths joined in a conductive string may be prepared by applying a conductive material, such as a high carbon ink, to the fibres. A conductive path may similarl' be deposited onto a fabric having similar filaments and fiber ends therein.

It is desirable in such systems to minimize the production of ozone. The provision of ionization with minimal production of ozone can be effected by a selection of the field gradient condition at the ionizing grid. As well, such grid can be positively charged as a positive electrode has a lesser tendency to create ozone.

In this manner, a source of ionization is provided that is substantially less expensive than a system based upon use of a filter support frame that carries a separate, fragile ionizing grid of fine wires. Further, maintenance of the system is facilitated by the ease by which the filter pad and ionizing grid, provided together in cartridge form, may be replaced. In this manner the convenience of a throw-away pad is combined with efficiencies based upon the use of ionization.

The features of both polarization and S ionization may be combined in one simple design by use of a fibrous filter pad of dielectric material positioned between a charged, active ionizing grid and an oppositely charged, preferably grounded, screen. The active ionizing grid is positioned adjacent to and in contact with the pad upstream in the air flow in respect to the pad.

t AMENCED SHEET \,72, By selection of the appropriate level of charge and grid-to-screen spacing, the charged, active grid provides a degree of ionization within the air flow thus charging dust particles passing thrQugh the filter and thereby constituting it an "ionizing grid". The closeness of the ionizing grid and screen on opposite sides of the.

pad form a polarizing field gradient within the dielectric material of the pad that polarizes the dielectric material.

This combination of charged dust particles and a polarized pad of dielectric fibers achieves high efficiency as an air filter, removing such dust particles from a stream of air more effectively than with either feature used separately.

To conceal and protect the ionizing grid, pads may be provided on both sides of such grid. An optional, complementary second screen grid may be placed against the outside surface of this additional pad on the upstream side from which the airflow is originating. The upstream pad in such case is advantageously exposed to rapidly diffusing ions which flow upstream against the air flow, charging dust particles present inside the first, upstream filter pad of a two pad unit. This increases the trapping efficiency of the combined assembly.

As a further alternate embodiment, an additional ionizing grid may be placed on the upstream surface of the upstream, second screen of an air filter that has an upstream screen. This additional ionizing grid is separated from and supported on the upstream screen by an insulating layer, such as a polyester film. By charging A 2Ni 2K.

8 this additional ionizing grid to an ionizing potential, dust particles in the air- flow may be charged before they enter the first polarized air filter pad, increasing filtering efficiency even fur'ther.

The convenience of this invention is that the filter pad material may be separately removable from between the" ionizing grid and screen to further increase the efficiency of this air filter system.- Alternately, the ionizing grid and filter pad material are bonded together for replacement and disposal as a unit.

In yet another variant a replaceable "cartridge" may include: 1) two dielectric fiber pads; 2) an ionizing grid between the pads; 3) two external screens.

An optional second ionizing grid may be fixed on the outside of one screen.

The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.

Several embodiments of the present invention will hereinafter be described by way of example only and with reference to the following drawings herein.

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Summary of the Figures Figure 1 shows an exploded, perspective view of the components of a basic filter assembly with fine wires as the ionizing grid that is not claimed as the invention.

Figure 2 shows the construction of the assembled filter of Figure 1 in cross-sectional view.

Figure 3 shows a central grid composed of fine, ionizing wires for use in the filter assembly of figures 1 and 2.

Figures 4, 5 and 6 show the invention based on an ionizing grid that differs from the configurations of Figures 1 3 by the presence multiple, exposed ionizing points that form ions when charged to an ionizing potential.

Figure 7 shows an exploded perspective view of a filter assembly wherein the ionizing grid of Figure 4 is attached to one of the fibrous pads.

Figure 8 shows a perspective view of a hinged filter arrangement where the two outside screens are hinged together and a central ionizing grid is composed of wires and not claimed as the invention supported with insulating hinges. Power to the central grid is supplied by a high voltage power supply attached to one of the outside screen frames.

Figure 9 is a similar embodiment to that of Figure 8 except that the central ionizing grid incorporates the ionizing points of the invention and is attached to and carried by one of the fibrous pads. High voltage to the r 4, 2- AMENDED SHEET grid is supplied via a conducting strip connected to a high voltage power supply.

Figure 10 is a similar arrangement to Figure 9 except that the ionizing-grid is sandwiched between the fibrous pads that are bonded together enclosing a portion of the ionizing grid. The conducting strip is exposed between portions of the pads that are not bonded together.

Figure 11 is an arrangement where the ionizing grid of the invention and conducting strip are fully contained between the fibrous pads which are bonded together to enclose the grid and strip.

Figure 12 shows how the arrangement of Figure 11 is used in a filter frame.

Figure 13 is a graph showing the removal of particles over time from a room using respectively a prior art polarized filter, and a filter according to the invention relying on an ionizing grid of conductive string with multiple ionizing points.

Figure 14 is a perspective view of a cartridge filter with an exterior ionizing grid of conductive string, according to the invention, fixed over an exterior screen r by being mounted on insulating tape.

Figure 15 is an edge view of Figure 14.

Figure 16 is a graph showing the improved performance used by having a second, upstream ionizing grid.

i AMENDED SHEET Description of the Preferred Embodiment Referring to the drawings, Figure 1 shows one example of an assembly of components not incorporating a cartridge filter based on -the invention, but instead relying upon use of very fine ionizing wires. Two outside perforated retainers 1 form the outside frames of the filter. Two outside conducting polarizing screens 2 are mounted within the frames 1. Two dielectric fibrous pads 3, preferably made of glass fibres, are placed centrally between the polarizing screens 2. Preferably, the screens 2 are grounded.

Located centrally between the pads 3 is the ionizing grid 4. Ionizing grid 4 in this embodiment comprises fine wires 5 which ionize the surrounding air when high voltage is applied to them by virtue of a high potential gradient which is present around the wires. The diameter of wires 5 is preferably between 0.030 and 0.06 millimetres causing ionization when charged to a potential of between 5,000 and 10,000 volts, depending on the spacing of the screens 2. The spacing between such wires is preferably from about 1 to 5 centimetres. The spacing between the grid 4 and screens 2 is between one and two and one half centimetres in order to produce the desired polarizing field gradient. Preferably, the grid 4 charged with positive potential ions as this reduces the production of ozone.

MENED SHEET "AMENDED

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i 12 While Figure 1 depicts a symmetrical cartridge with the ionizing grid 4 -carried between and adjacent to two filter pads 3, only one filter pad 3 need be employed.

Use of a second filter pad'3 helps shield and protect the charged ionizing grid 4 and provides improved filtration efficiency.

Figure 2 shows a cross-sectional view of the cartridge filter shown in Figure 1 when *assembled with a high voltage power supply 6 mounted along one side. This power supply is connected to ionizing grid 4 via a high voltage contacting means in the form of a probe 7. Power supply 6 and probe 7 may be mounted in an air filter support frame (not shown) but are preferably detachably attached to one side of the cartridge frame 1. (See U.S.

Patent No. 4,828,586).

Figure 3 shows a detail of construction of central grid 4 which comprises fine ionizing wires Operation of the filter is as follows: High voltage (about 5 to 10 KV) is applied to central grid 4 which, by virtue of its fine wires, ionizes the air and dust particles in the space between grid 4 and outside screens 2. At the same time, because of the high voltage applied to grid 4, an electrostatic field is also created between grid 4 and grounded screens 2 and thus polarizes the non-conducting, dielectric fibrous pads 3. Dust particles or any particulate matter entering the filter become charged due to ionization and are attracted and collected by the polarized fibrous pads 3. This double a\2 ,Jxr2vDEoD SH-EET 1.01t iLo action of ionization and polarization makes for a filter of improved efficiency.

Figure 4 shows an alternate construction of the central ionizing grid 4' which is the basis of the invention, identified herein. A length of fibrous string 8, such as one made of cotton having broken fibre ends, is treated with a conducting solution, such as colloidal graphite, to render it conducting. String 8 is attached to a conducting frame 9. Fibrous string 8 which has been rendered conducting, because of its composition of fine fibres or filaments with multiple, sharp ends, functions the same way as fine wires in ionizing dust particles but produces more ions and less ozone.

Figure 5 shows another alternate construction of the invention where a fabric-based ionizing grid 10 laid over a pad 3 is formed by depositing conducting paint or colloidal graphite on a sheet of gauze 11. Gauze 11, because of its composition of fine fibres and because it is rendered conducting, provides within the conductive material a grid 10 which functions the same way as fine wires 5 in effecting ionization.

Figure 6 shows another alternate construction for the central ionizing grid 4. In this case, a paper-based grid 12 is formed by painting conducting paint or colloidal graphite on coarse, fibrous paper 13. This paper 13 is perforated with perforations 14 to allow air to pass through. This arrangement also functions the same way as grid 10 in effecting ionization of dust particles because Bi/ AMENDED

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the coarse fibrous paper also has fine fibers which act in the same manner as the fibers in string 8 of Figure 4.

Figure 7 shows an alternate construction for a cartridge filter assembly -which is similar to the mechanical filter assembly shown in Figures 1 and 2 and an ionizing grid as in Figure 4. In this case, the ionizing grid element is based on use of an electrical conductor in the form of a fibrous conductive string 5a composed of fine filaments with multiple filament ends attached to one of the fibrous filter trapping pads 3. The fibrous string with its multiple filament ends is again made conductive by coating it with conductive material like colloidal graphite. Conductive string 5a is connected to a high voltage power supply in a similar manner as shown in Figure 2. Operation of this filter is as described above.

When the ionizing grid 4 is based upon use of a conducting element that provides multiple protruding point ends, it has been found that satisfactory ionization with minimum ozone production can be produced using the following parameters: ionizing grid voltage from 5KV to 10KV kilovolts Sdepending on the space between the ionizing grid on the grounded screens ionizing grid charged with positive potential conducting element separation or spacing from 10mm to grid to screen separation from 10mm to AMENDED

SHEET

Figure 8 shows a filter arrangement not claimed as the invention wherein-two outer, conducting screens 2 within frames 1 are hinged together to form the outside of a filter cartridge. (See also U.S. patent No. 4,549,883).

Replaceable fibrous pads 3 are positioned on either side of central ionizing grid 4 of wires. Grid 4, through its supporting frame, is attached to one of frames 1 by insulating hinges 17. A high voltage power supply 6 is attached to one of the outside frames 1 and connects to grid 4 via electrode probe 7 when the filter assembly is closed. When closed, the grid 4 lies directly adjacent to the fibrous pad 3. A cord 20 is connected to a low voltage power supply for supplying power to high voltage power supply 6. Operation of this filter is the same as described above for the cartridge filter shown in Figures 1 and 2.

Figure 9 shows a similar arrangement as that of Figure 8 except that in this case a conducting ionization grid 4a having multiple ionizing points according to the invention is attached on one side of one of the fibrous pads 3. Again, these fibrous pads 3 are removable for easy replacement. Grid 4a is made by attaching fibrous, conducting elements with multiple ionizing ends directly onto the surface of fibrous pad 3. Thus in Figure 9, the grid 4a is also removable and replaced with the installation of fresh pads.

Grid 4a is connected to power supply 6 via a

I

frame-mounted conducting strip 22 and wire 23. Strip 22 is AMEN,;OED SHEET attached to one of the frames 1 by insulating hinges 24.

Grid 4a functions the same way as the grid formed by the fibrous string 5a in Figure 7. As it is'bonded to a pad 3 and is composed of an inexpensive ionizing structure that provides ionization at multiple pointed ends, it is readily disposable.

Figure 10 shows another filter arrangement similar to that of Figure 9. In this case, the two filter media pads 3 are bonded together over part of their opposed surfaces as by gluing or stitching but portions of each of the pads 3 are left free so that, when placed collectively in the filter frame 1, metal strip 22 may be inserted between pads 3 to make contact with ionizing grid 5a. In this embodiment the two bonded pads 3 with ionizing grid 4a there between make a convenient package for filter replacement.

Figure 11 shows another arrangement where the filter media pads 3 are bonded together over their entire opposed faces. Between the pads 3 ionizing grid 4a is held in place as by stitching or gluing, by friction, or by other suitable means. Metal strip 22 is also held in place between pads 3 to make contact with grid 4a.

Figure 12 shows how the arrangement of Figure 11 is used in a filter frame similar to that of Figures 8, 9 and 10. Here, high voltage from power supply 6 is supplied to strip 22 by insulated electrode 31 which connects to the high voltage power supply 6 through frame 1. Electrode 31 AMENDED

SHEET

17 is thin and narrow, enabling it to pierce one of pads 3 and touch strip 22 to complete the electrical circuit.

Figure 13 shows the results of comparative tests made on a 20" x 20" x 2' r prior art, cartridge-type, polarizing filter and a filter of similar dimensions with conductive fibrous strings to serve as an ionizing grid as ion contemplated by the invention. The high voltage used was 10 KV on both cartridge filters.

The tests were made by generating smoke in asealed 570 cubic feet room. A ventilator was used to circulate air through the filters and the level of contamination was measured using a CLIMET INNOVATIONTMI'500 particle counter. The particle counter is capable of counting different particle sizes in the air as the air is drawn through the tube into the instrument. The counts used were for particles down to a .3 micron size, which is the most difficult particle size to capture, and the most numerous. The instrument was set to count the particles in .2 cubic feet of air every minute. All tests were made with 1000 cubic feet per minute (CFM) of air circulating through the filters as measured by an EBTRONTM) air velocity meter.

The results show that by using ionization as well as polarization, (lower curve) the efficiency of filter improves as compared to using only polarization.

A

precipitator would be more efficient but it uses much more energy to operate. It has much less loading capacity and t is far more expensive to operate. The precipitator

A

AMENDED SIFT requires between 80 to 100 watts of power to operate while both the polarized media and the new polarized media/ionization type filters use only about 1.5 watts to operate. In both of the Iatter cases, the trapping pads, once coated with dust, may be readily removed and exchanged for fresh, clean pads.

Another variant of the invention is shown in Figures 14 and 15. In these Figures a thin insulating strip of plastic such as polyester 37, is applied over and fastened to an outer upstream screen 36 of a cartridge filter assembly 35, in this case in the shape of the letter On top of strip 37 and along its middle line, a fibrous conducting string 38 is attached. A high voltage power supply (not shown in the drawings) is connected between string 38 and grounded screen 36. String 38 is thereby charged to a voltage of between 5 KV and 12KV. A high resistance value limiting resistor (not shown) in the high voltage source ensures that no danger of injurious electric shock can arise from contacting the charged string 38.

Operation of this arrangement is as follows: The conducting string 38 ionizes the air in the vicinity of the string by emitting charges 39 via its fine fibre ends.

These charges ionize (charge) the dust particles in the space in front of the filter 35. The dust particles are then drawn into the filter 35 by the air flow and are collected by.the filter pad 40. The filter's efficiency improves by this arrangement because charged particles of dust are more readily captured by a filter pad especially a polarized- filter pad 40, than neutral particles. This arrangement produces results even more favourable than those showh in the graph of Figure 13. The improved results are shown in the graph of Figure 16.

While two fibrous pads have been shown throughout as embracing the high voltage grid, only one is essentially required. Two pads are preferred to cover the high voltage grid and prevent inadvertent contact. The'symmetrical two pad format also protects the contained ionizing grid when the invention is applied in its preferred, replaceable cartridge with ionizing string format.

Conclusion The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow.

These claims, and the language used therein, are to be understood in terms of the variants-of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.

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Claims (12)

1. A filter for use in an ionizing air filter comprising: a pad 3 of fibrous,-non-conducting material; and a conductive path 5a positioned adjacent pad 3 characterized by the conductive path 5a comprising a.. plurality of conductive filaments having multiple, exposed, ionizing points formed at filament ends that, when.charged to an ionizing voltage will form ions in the surrounding air.

2. A pad as in claim 1 characterized in that the filaments 5a are in the form of lengths of multi-stranded filaments that are attached to the pad 3.

3. A pad as in claim 1 characterized in that the conductive filaments are portions of a porous fibrous fabric 11.

4. An electronic filter comprising: a conducting screen 2; a fibrous pad 3 of non-conducting material positioned adjacent said screen 2; an ionizing grid 4 located on the side of the fibrous pad 3 opposite the screen 2; and an electrical coupling means for receiving an electrical potential from a high voltage source i AMENDED SHEET K -7 7 6 to provide an ionizing electrical potential between said screen 2 and said grid 4 characterized by said ionizing grid '4 being located adjacent said pad 3 and spaced from said screen 2 by a distance, said grid 4 comprising a plurality of conductive filaments 5a having multiple ionizing points formed at.. filament ends to provide, when connected to the high voltage source 6, ions in a gas stream passing therethrough thereby charging particulate components present within said gas stream to enhance trapping such charged particulate componentsby said pad 3. An electronic air filter as in claim 4 wherein the fibrous pad 3 is of dielectric material and the ionizing grid 4 is located adjacent to and in contact with the fibrous pad 3, opposite the screen 2, characterized by said ionizing grid 4 and said conducting screen 2 effecting polarization of said fibrous pad 3 upon connection of the electrical coupling means 7 to the high voltage source 6.

6. An electronic air filter as in claim 4 characterized in that said pad is a first fibrous pad 3, said filter having a second fibrous pad 3 of non-conducting material positioned adjacent to the ionizing grid 4 opposite said first fibrous pad 3, and wherein said conducting screen 2 is a first conducting screen 2 and a second conducting screen 2, which is electrically connected to the first conducting screen 2 and is positioned adjacent DIED SHEET 22 to the second fibrous pad 3 on the side opposite to said ionizing grid 4.

7. An electronic air filter as in claims 4, 5 or 6 characterized by said filaments 5a being attached to said fibrous pad 3.

8. An electronic .air filter as in claims 4, 5 or 6 characterized by said filaments being in the form of multiple strands of filaments.

9. An electronic air filter as in claims 4, 5 or 6 characterized by said filaments 5a being in the form of a porous, fibrous fabric 11. An electronic air filter as in claims 4, 5 or 6 characterized by said filaments 5a being in the form of a sheet of perforated paper 13.

11. An electronic air filter as in claims 4, 5 or 6 characterized by said filaments 5a being provided by a r sheet of porous fibrous, paper 13 upon which conducting portions are formed by a deposit 13 of conductive material thereon.

12. An electronic air filter as in claims 4, 5 or 6 characterized by said filaments 5a being provided by a sheet of fibrous paper 13 upon which said con-ductive 23 filament is formed by a deposit of conductive material 12 thereon.

13. An electronic air filter as in claims 4, 5 or 6 characterized in that said plurality of conductive filaments 5a comprise one or more lengths of fibrous string rendered' conductive by a conductive coating present thereon.

14. An electronic air filter as in claim 6 comprising a further ionizing conductor 38 positioned on an insulating support 37 carried by said second conductive screen 26, said further ionizing conductor 38 providing multiple exposed ionizing points to effect, when attached to a high voltage source, ionization within air arriving at the electronic air filter for passage therethrough,. when attached to a high voltage source. An electronic air filter as in claim 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13 characterized by being in combination with a voltage power supply 6 which provides a potential between the screen 2 and the grid 4 of between 5,000 and 10,'000 volts. E AMENDED SHEET v^IYC