EP0936946A1

EP0936946A1 - Ionizing and polarizing electronic air filter

Info

Publication number: EP0936946A1
Application number: EP96934280A
Authority: EP
Inventors: Constantinos J. Joannou
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-11-08
Filing date: 1996-11-08
Publication date: 1999-08-25
Anticipated expiration: 2016-11-08
Also published as: DE69621163D1; WO1998020978A1; CA2270976C; CA2270976A1; JP2001503326A; EP0936946B1

Abstract

A high efficiency electronic air filter is based on a fibrous pad of dielectric fibres which is sandwiched between an electrically charged, ionizing grid and a grounded screen. The ionizing grid charges the dust particles passing through the filter and at the same time the grid and screen polarize the fibrous pad. In this way, the charged particles are attracted and collected on the polarized fibrous pad with improved efficiency.

Description

Title: IONIZING AND POLARIZING 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.

Background of the Invention

In 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 non- ionizing, 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 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 0.5 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 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 on charged walls or plates which, as with all precipitators, must be cleaned regularly. 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 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 combining features of polarization and ionization in one simple design. Generally, an embodiment of the present invention is based upon use of a fibrous filter pad of dielectric material positioned between a charged, active grid and an oppositely charged, preferably grounded, screen. The active grid is positioned adjacent to and in contact with the pad upstream in the air flow in respect to the pad. 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 through 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.

Ionization to effect charging of dust particles may be achieved either by providing the active grid with fine wires which produce ions because of the high potential gradient formed around the fine diameter and sharp curvature of such wires; or by providing the grid with an array of fine, sharp points carried by conductive elements to produce ions by the same effect.

When the ionizing grid is to be formed by providing an array of very fine wires, if the diameters of such wires is small enough, a high, ionizing field gradient can be formed without the application of exceptionally high external voltages. For example, wires with a diameter of from .03 to .06 millimetres will ionize air with an minimal to moderate production of ozone if charged to a potential of from 5 to 10 kilovolts of positive potential combined with a grounded screen positioned at about 10mm to 30mm from the ionizing wires. Such wires are, however, fragile and expensive to employ.

An alternate convenient way of providing an ionizing grid is to render a cord of composite filaments of short fibres, such as cotton, conductive. Each of the short fibres may be of a small enough diameter to effect ionization. Alternately, or additionally, each fibre may provide an end that is pointed and has around it a higher field gradient than the fibre itself, thereby creating ions in the region of the steep field gradient. 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 similarly be deposited onto a woven 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.

Whether fine wires or an array of pointed ends are employed as the ionizing grid, such grid is preferably carried directly adjacent, and preferably bonded to a dielectric fibrous filter pad. On the opposite side of the grid, a conducting screen held at differing potential will provide an electrical field gradient across the thickness of the dielectric fibrous filter to polarize the fibres. Electrical coupling means are further provided to ensure that voltage is applied to and between the ionizing grid and screen when the pad is installed in a filter support frame.

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.

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 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 further.

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.

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. 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 alternate arrangements for the ionizing grid in the configurations of Figures 1 and 2.

Figure 7 shows an exploded perspective view of a filter assembly wherein the ionizing grid 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 the central ionizing grid is 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 is attached to and carried by one of the fibrous pads. High voltage to the 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 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. Figure 14 is a perspective view of a cartridge filter with an exterior ionizing grid fixed over an exterior screen 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.

Description of the Preferred Embodiment Referring to the drawings, Figure 1 shows one example of an assembly of components constituting a cartridge filter based 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. 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 5.

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 action of ionization and polarization makes for a filter of improved efficiency.

Figure 4 shows an alternate construction of the central ionizing grid 4. 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.

Figure 5 shows another alternate construction 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 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 filter assembly shown in Figures 1 and 2. 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 depending 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 30mm - grid to screen separation from 10mm to 30mm

Figure 8 shows a filter arrangement where 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. 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 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 frame-mounted conducting strip 22 and wire 23. Strip 22 is 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 and grid 4 in the arrangement of Figures 1 and 2. However, 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, similar to but lower in cost than the filter cartridge of Figures 1 and 2.

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 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" 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 a sealed 570 cubic feet room. A ventilator was used to circulate air through the filters and the level of contamination was measured using a CLIMET INNOVATION^(TM> 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 EBTR0N^<TM) 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 it is far more expensive to operate. The precipitator 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 latter 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 "H". 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 40, especially a polarized filter pad 40, than neutral particles. This arrangement produces results even more favourable than those shown 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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY ARE CLAIMED AS FOLLOWS:

1. An electronic filter comprising: (a) a conducting screen 2; (b) a fibrous pad 3 of non-conducting, dielectric material positioned adjacent said screen 2;

(c) an ionizing grid 4 located on the side of the fibrous pad 3 opposite the screen 2 ;

(d) a high voltage source 6, electrically connected between said screen 2 and said grid 4 characterized by said ionizing grid 4 being located adjacent and contacting said pad 3 and spaced from said screen 2 by a distance, said grid 4 having conducting and ionizing elements therein to provide, when connected to the high voltage source 6, a region of ionizing potential to ionize particle components of a gas passing therethrough and to polarize the fibrous, dielectric pads 3 to trap such ionized particulate components.

2. An electronic air filter as in claim 1 characterized in that said ionizing grid 4 comprises fine wires 5 having a diameter of between 0.030 and 0.06 millimetres and said high voltage source 6 provides a potential difference of between 5,000 and 10,000 volts.

3. An electronic air filter as in claim 1 characterized by said ionizing grid 4 comprising lengths of multi-stranded fibrous filaments 5a which are rendered conducting and which provide multiple ionizing fibre ends.

4. An electronic air filter as in claim 3 characterized by said lengths of filament 5a being attached to said fibrous pad 3.

5. An electronic air filter as in claim 1 characterized by said ionizing grid 4 comprising a porous, fibrous fabric 11 with multiple fibre ends upon which conducting elements 12 are formed by a deposit of a conducting material to render said fibre ends ionizing.

6. An electronic air filter as in claim 1 characterized by said ionizing grid 4 comprising a sheet of perforated paper 13 with multiple fibre ends therein upon which conducting elements 12 are formed by a deposit of conducting material to render said fibre ends conducting.

7. A filter pad for use in an ionizing air filter comprising:

(a) a pad 3 of fibrous, non-conducting, dielectric material; and

(b) a conductive path 5a carried by such pad 3 characterized by the conductive path 5a comprising a plurality of conductive filaments that, when charged to an ionizing voltage will form ions in the surrounding air.

8. A pad as in claim 7 characterized in that the filaments 5a provide a plurality of filaments ends that provide the ionizing field gradient.

9. A pad as in claim 8 characterized in that the conductive filaments are provided in the form of lengths of multi-stranded filaments that are attached to the pad 3.

10. A pad as in claim 8 characterized in that the conductive filaments are portions of a porous fibre fabric 11 that have been rendered conductive.

11. An ionizing apparatus comprising an air filter having:

(a) a conducting screen 2;

(b) a fibrous pad 3 of non-conducting, dielectric material having first and second sides and positioned with the first of said sides adjacent said screen 2 ;

(c) an electrical coupling means 7 for receiving an electric potential from a high voltage source 6; and (d) a polarizing and ionizing grid 4 located adjacent to and in contact with the second side of the fibrous pad 3, opposite the screen 2, characterized by said polarizing and ionizing grid 4 comprising an electrical conductor having multiple, exposed ionizing points distributed along its length for providing a high potential gradient to ionize particle components of a gas passing therethrough, said conducting screen 2 and said polarizing and ionizing grid 4 being connected to the electrical coupling means 7 to produce said high potential gradient and to polarize the fibrous pad 3 to trap said ionized particulate components when supplied with charge from a high voltage source through said electrical coupling means 7.

12. An ionizing apparatus as in claim 11 characterized in that said conductor comprises one or more lengths of fibrous string 5a rendered conductive by a conductive coating present thereon.

13. An ionizing apparatus as in claim 12 characterized in that said one or more lengths of string 5a are attached to said fibrous pad 3.

14. An ionizing apparatus as in claim 11 characterized by being in combination with a voltage power supply 6 which provides a potential between the ionizing element and the conducting screen of between 5,000 and 10,000 volts.

15. An ionizing apparatus as in claim 11 characterized in that said pad is a first fibrous pad 3, said filter having a second, non-conducting, fibrous pad 3 of dielectric 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, which is electrically connected to the first conducting screen 2, is positioned adjacent to the second fibrous pad 3 on the side opposite to said polarizing and ionizing grid 4.

16. An ionizing apparatus as in claim 11 characterized in that said electrical conductor is provided by a porous, fibrous fabric 11 comprising a deposit 10 of a conductive material present on the surface of the fibers of said fabric 11 to render the coated portion of the fabric 11 conductive.

17. An ionizing apparatus as in claim 11 in which said electrical conductor is provided by a sheet of porous fibrous, paper 13 upon which conducting portions are formed by a deposit 13 of conductive material thereon.

18. A filter cartridge for use in an ionizing air filter comprising:

(a) first and second pads 3 of fibrous, non- conducting, dielectric material each having inner and outer sides, said inner sides being juxtaposed to each other;

(b) a conductive path provided with the first of said pads 3 on the inner side of said pad 3 ; (c) electrical coupling means 22 attached to said conductive path for receiving an electrical potential from a high voltage source 6; and (d) first and second conductive screens positioned against the outsides of said respective pads, characterized by the conductive path comprising conductive filaments 5a having multiple, exposed filament ends to form, when electrically charged to an ionizing potential, an electrical gradient that produces ionization in air that surrounds said filaments 5a.

19. A filter cartridge as in claim 18 wherein the conductive filaments are in the form of one or more lengths of fibrous string 5a attached to said first pad 3, said string 5a being rendered conductive by a conductive coating present thereon.

20. A filter cartridge as in claim 18 characterized in that the first pad 3 carries a porous, fibrous fabric 11, the conductive filament being provided by portions of said fabric 11 that have been rendered conductive by a conductive coating 10 present thereon.

21. A filter cartridge as in claim 18 characterized in that the first pad 3 carries a sheet of fibrous paper 13 upon which said conductive filament is formed by a deposit of conductive material 12 thereon.

22. A filter cartridge as in claims 18, 19, 20 or 21 comprising a second conducting path positioned on an insulating support carried by one of said conductive screens, said second conductive path providing ionization to air passing therethrough when attached to a high voltage source.