Technical field

The present document relates to a filter element that can be electrified. More particularly, the present disclosure relates to a filter system comprising the filter element, a method of using the filter element and system and the use of the filter element and filter system.

Background

Filtration of air has become increasingly important in environments where the outdoor air may be detrimental to the health. Air filters are therefore installed both in private and official buildings and in different types of vehicles, such as cars, busses airplanes, to clean the air from particles.

Air filters are often combined with the ventilation system of the buildings or vehicles, where the air is forced to pass through the filter.

There are many different types of particle and gas filters available on the market. Mechanical air filters can either act passively through screening, interception or diffusion, or actively, for instance through cyclone separation or through electrostatic charge, where the filter is charged when it is manufactured.

The filters can be designed in different ways the most conventional types are bag filters, flat (so called panel filters) and folded, pleated or creased filters.

There are also many different grades of filters or filter media available according to different classifications and standards. One such standard is the European standard for air filters, EN779:2012. In this standard the air filter is classified by its minimum efficiency (ME) for air purification.

Increasing the particle separation in a filter media, i.e. increasing the effiency is conventionally achieved by decreasing the pore size or the density of the filtermedia, such as for instance in high effiency particulate air (HEPA) filters or microfilters, this usually also leads to an increased pressure drop over the filter, and thus also results in an increased energy consumption, which of course is not desirable from an environmental and cost perspective.

The high density filters can also be combined with for instance electrostatic charging of the filter, but one drawback with the electrostatic filters is that they may quickly be neutralized by the particles entering the filter media.

In EP1638666 B1 a particle filter is disclosed, where the particle filter can be provided with an electrostatic charge during operation, i.e. a voltage can be applied over the filter. This is achieved by using a conductive polymer doped to make it conductive as the particle filter. This filter is directed to attracting and catching particles, not gases, and it requires that the entire filter media is of the same conductively doped material and that a very high voltage in the range of 10 to 100 kV is applied across the filter material.

In WO2007/135232 A1 a gas filter structure is disclosed, where an isolating material has been placed between two layers of a porous conductive layer, that comprises a porous fiber matrix substrate which is treated with an inherently conductive polymer. An electric voltage is applied across the filter to improve the filter efficiency of particles. This filter construction causes a larger pressure loss than current metal plate filters, but it can be modified to give an acceptable pressure loss, and an acceptable filtering efficiency.

A further disadvantage of the filter disclosed in WO2007/135232 is that it requires that the dust particles be pre-charged using ionization in order to achieve efficient separation of the particles from air. Ionization involves the generation of ozone which is a hazardous gas which needs to be removed from the effluent of the filtration.

There is thus a need for a filter media, which does not only effectively reduce the particles in the air, but also reduces the amount of different types of harmful gases, such as ozone (O3), carbon dioxide or carbon monoxide (COx), nitrogen oxide (NOx) and volatile organic compounds (VOC), while still providing a low pressure drop over the filter. Summary

It is an object of the present disclosure, to provide an improved electrified air filter element, which eliminates at least some of the

disadvantages of the prior art air filters.

More specific objects include providing a filter element having a high particle separation, and a low pressure drop and providing a reduction of harmful gases and disturbing odors.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims and in the following description and drawings.

According to a first aspect, there is provided a filter element for filtering a gas, comprising at least one particle capturing layer, wherein said particle capturing layer is provided for capturing, from a flow of gas, particles having a diameter in the range of 0.1 μιτι - 10 μιτι, and wherein said isolation layer has a thickness in the range of 1 - 2 mm, wherein said filter element further comprises at least one gas adsorbing layer comprising activated carbon, and at least one electrically conductive layer, wherein said electrically conducting layer is arranged to face said flow of air or gas; wherein said particle capturing layer is arranged between the gas adsorbing layer and the electrically conductive layer; and wherein said filter element provides, when subjected to a voltage applied across the particle capturing layer, a particle separation of more than 90 % based on the most penetrating particle size (MPPS) and a pressure loss of 45 Pa or less at an airspeed of 5.3

mm/second.

The voltage is thus applied across, or over, the particle capturing layer, which is not in itself conductive, rather, this layer also functions as an isolating layer between the conductive layer facing the flow of air or gas, and the gas adsorbing layer, which in its turn is conductive enough for the voltage to beapplied across the particle capturing layer.

This filter element thus provides for a very efficient separation of particles, comparable to that of a HEPA filter, while not impairing on the pressure loss over the filter, when it is subjected to a voltage applied over the filter element, i.e. when it is being electrified. The filter element of course function as an air filter even when it is not electrified, but the high particle separation in combination with the low pressure drop is achieved by applying voltage over the filter element.

The gas adsorbing layer adsorbs gases such as COx, NOx, VOC etc. and odors. Further to this the gas absorbing layer adsorbs ozone.

The particle capturing layer may comprise a polymer or a cellulosic material or a mixture thereof.

The gas adsorbing layer and the electrically conducting layer may be made from different materials.

The gas adsorbing layer may have a thickness in the range of 1 to 2 mm.

The thickness of the gas adsorbing layer should be such that an optimal, or reduced pressure drop is achived while still allowing for a sufficient amount of harmful gasses to be adsorbed or captured in the gas adsorbing layer.

The gas adsorbing layer may comprise a layered structure wherein a matrix comprising activated carbon is interposed between two polymer layers.

The electrically conductive layer may comprises porous carrier material of any one of a polymer, a cellulose fiber material or a mixture thereof and having a basis weight in the range of 15 to 45 g/m ² .

This means that the conductive layer facing the air flow is relatively thin, also compared to the isolating and gas adsorbing layer. This layer also allows for an efficient flow of air through and into the filter element.

The electrically conductive layer may be treated to become electrically conductive. The porous material may be treated with any one of a metal, a conductive ink, a conducting polymer.

The porous material may for instance be treated with gaseous aluminum, or printed, coated or impregnated with a conductive ink or an electrically conductive polymer. The filter element may be folded. This increases the surface area of the filter element and thus enhances the filtering efficiency. The filter element may also be fitted into a frame, which may either be metallic or plastic.

The filter is folded such that the particle caputring layer has an even thickness troughout the folding. This ensures that the distance between the conductive layer and the gas adsorbing layer, which also acts as a second conductive layer is kept constant.

The even thickness of the particle capturing layer will provide for a reduced risk of short circuiting the filter element when a voltage is applied across the particle capturing layer. The resistivity of the folded particle capturing layer may be in the range of 1 to 4 kV/mm.

According to a second aspect there is provided a filter system comprising a filter element providing a particle separation of at least 90 %, preferably at least 95 %, and a pressure loss of 50 Pa or less, or preferably 45 Pa or less, at an air speed of 5.3 cm/s according to the first aspect, a voltage source connected to the filter element, and wherein said voltage source is arranged to provide a voltage over the filter element or particle capturing layer which is in the range of 2 to 8 kV.

The system may further comprise an ionizing device, wherein said ionizing device is arranged to be in contact with a flow of gas before the flow of gas enters into the filter element.

The ionizing device is thus able to provide particles in the air flowing towards the filter element with a charge, i.e. pre-charge the particles, such that the filtering capacity is even more enhanced. The ionizing device may for instance be an ion emttor such as a corona wire or any other suitable device for charging particles in air.

According to a third aspect there is provided a method comprising the following steps: providing a filter element according to the first aspect;

providing a voltage source; connecting either one of the positive or the negative pole of said voltage source to said filter element and the opposite pole of said voltage source to ground; and applying a voltage over said particle capturing layer in the range of 2 kV to 8 kV such that said filter element provides a particle separation of at least 90 %, preferably at least 95 % and a pressure loss of 50 Pa or less, preferably of 45 Pa or less, at an airspeed of 5.3 cm/second.

Through this method it is possible to achieve a stable conductivity in the filter element, avoiding short circuits in the filter media, and thus achieving the high filtering efficiency and at the same time a low pressure drop.

The voltage source may be connected directly to the filter element or through connectors arranged in contact with the filter element.

The method may further comprise the steps of providing an ionizing device, connecting said ionizing device to a voltage source; and providing said ionizing device with either one of a positive or a negative charge.

This means that the ionizing device may provide the particles in the flow of air with a charge opposite that of the charge applied to the filter element.

According to a fourth aspect there is provided a use of a filter element according to the first aspect, or a filter system according to the second aspect, for purifying air in any one of a vehicle, a building and a machine.

This means that the filter element and filter system may be used for filtering air in a wide variety of applications, such as in cars, busses, trains, airplanes, ships, and in buildings both public buildings such as hospitals, schools, banks, pre-schools, shopping centers, research facilities, and in private buildings. The air filter element and filter system may also be used in different types of machines and engines.

The filter element may further be incorporated in any conventional type of filter arrangement, or be of any conventional type, such as a flat, panel, folded or bag type filter.

The filter system may either be pre-fabricated or assembled on site, for instance into the ventilation system of a car. The filter element may further be disposable or recyleable. Brief Description of the Drawings

Embodiments of the present solution will now be described, by way of example, with reference to the accompanying schematic drawings. Fig. 1 is a schematic side view of a filter element.

Fig. 2 is a schematic perspective view of a filter system according to one embodiment.

Fig. 3 is a graph showing the filtration efficiency in a trial of the inventive filter element.

Detailed Description

As illustrated in Fig. 1 the filter element 1 comprises a layered structure. The layers may be fixedly attached or detachably attached to each other.

The filter element 1 comprises an isolation or particle capturing layer 2, which is interposed between a layer of a conducting material 3 and a layer of a material 4 which is able to adsorb gases, in particular harmful gases, such as O3, NOx and COx, and VOCs. The conducting layer 3 is arranged to face a flow F of air.

When a voltage is applied over the filter element 1 a particle separation of the most penetrating particles (MPPS) of at least 90 % and a pressure loss of 50 Pa or less at an airspeed of 5.3 m/s is provided. In a preferred embodiment a pressure loss of less than 45 Pa is achived.

A particle separation preferably above 98 %, or even more preferred above 99 %, may be achieved when the voltage is applied over the filter element.

The pressure drop is preferably less than 40 Pa, when the voltage is applied.

The particle size of the particles captured in the particle capturing layer may be in the range of 0.1 - 2 μιτι. The filter element 1 may be used in any conventional type of filter arrangement, such as a bag filter, a flat (panel) filter or a pleated or folded filter.

The filter element may be arranged in a frame. The frame may be made of any one of a paper material, a plastic material and a metal or metal alloy, or any combination thereof.

The particle capturing layer 2 may be made from a fibrous material of any one of a polymer and cellulose, or a combination thereof. As an example the isolation layer may be made from a polyester fiber. The layer can further be treated with a water-repellent agent, such as an organo-polysiloxane, a fluororesin, a wax or a similar substance which enhances hydrophobicity of the layer.The isolation layer has the dual function of providing a sufficient isolation between the conductive layer and the gas adsorbing layer, in order to reduce the occurrence of short circuits in the filter element. The dust holding capacity of the particle capturing layer is preferably high. The prticle capturing layer is further preferably soft enough to be folded, or bended, and the thickness is preferably chosen such that the dust holding capacity is not impaired, while the pressure drop is held as low as possible. The thickness of the particle capturing layer may be in the range of 0.1 to 5 mm, and is preferably in the range of 1 to 3 mm, or even more preferred in the range of 1 to 2 mm.

The thickness of the particle capturing layer may be chosen depending on the desired particle separation efficiency and the voltage applied over the layer. For instance a voltage of 1 kV/mm over a 2 mm filter provides an 85 % particle separation for the most penetrating particle size (MPPS). A voltage of 2 kV/mm over a 2 mm thick layer may provide a particle separation of 90 % of the MPPS. Even further a voltage of 3 kV/mm over a 3 mm thick layer may provide a particle separation of 95 % of the MPPS. To achieve a particle separation of 99 % of the MPPS over a 3 mm thick layer a voltage of 3.5 kV/mm is preferably applied. When the filter element 1 is folded the thickness of the particle capturing layer is prefereably kept constant, i.e. even and uniform troughouht the foldd filter element.

This is achieved by a process where parameters such as pressue, and the configuration of the knifes/drums provides for a folded filter element where the distance between the conductive layer and the gas capturing layer is kept constant throughout the filter material.

In one embodiment, the particle capturing layer has a grammage of 50 to 250 g/m ² , in particular 70 to 200 g/m ² .

Typically, the isolating layer exhibits a surface resistivity of more than

10 MOhm/sq or a volume resistivity of more than 5x10 ¹⁰ Ohm-cm.

The conductive layer 3 is arranged such that when the filter element is in use it is substantially perpendicular to the flow of gas or air through the filter element. The conductive layer may be made from any one of a cellulosic fiber and a polymer fiber, or a combination thereof, thus forming a base material. The base material is preferably porous, i.e. allowing for air and particles to enter into the filter element and into the isolation layer. The conductive layer is preferably formed as a thin layer having a thickness of less than 1 mm or even mor preferred less than 0.5 mm. Some particles will be captured also in the conductive layer, however this layer is not intended to be the main particle capturing layer, rather the air and particle permeaibilty allows for gases particles in the air to pass through and enter into the particle capturing or isolation layer 2.

In one example the conductive layer is made from a polymer cellulose material, i.e. a combination between polymer and cellulose fibers.

The polymer can for example be selected from the group of polymeric fibers and inorganic fibers, wherein the polymeric fibers are preferably selected from polyester, polyethylene, polyethylene terephthalate, polyolefin, polybutylene terephthalate and/or polyamide, and wherein the inorganic fibers are preferably glass fiber strands.

The cellulose fibers can be selected from the group of natural or regenerated cellulose fibers, such as cellulose, lyocell, viscose and any other derivatives of cellulosic fibers. Chemical wood pulp can also be used as such or in combination with other natural or regenerated cellulose fibers.

Natural or regenerated natural fibers will give good inherent strength properties as such. Additional binding fibers, for example bicomponent synthetic fibers need not be used.

There are a number of ways to produce such a combination material, one is by adding the polymer fibers to a wet laid web of cellulose fibers, and another is to spin the fibers together.

The porous material may have a basis weight in the range of 15 to 50 g/m ² , in particular 15 to 25 g/m ² .

The conductive layer may be made conductive by treating it with a substance or material that will provide the porous base material with conductive properties.

Examples of such treatments can be spraying, printing or impregnating the material with a conductive ink. The conductive ink may be made from powdered or flaked silver or carbon like materials, such as carbon black, or from conductive polymers.

The porous material may also be treated with a gaseous aluminum.

Alternatively it may be printed or impregnated with an electrically conductive polymer, which is bound to the base material. Examples of conductive polymers is polyaniline and/or polymers that have been doped (i.e. mixed, treated) to become conductive. These electrically conductive polymers typically have both ionic and electronic conductivity, and the conductivity can be altered or adjusted depending on the desired application. Suitable polymers include polyaniline, polypyrrole, polyacetylene, polytiophen and polyparaphenylene and their derivatives and mixtures. Suitable doping agents are selected depending on the polymer; dodecylbenzensulphonacid (DBSA) is a doping agent typically used for polyaniline.

Typically, the conductive ink also contains a binder for the conductive component for bonding it to the substrate or material which is to be rendered conductive. The binder can be a thermoplastic or thermosetting material, for example an acrylate, alkacrylate (methacrylate), epoxide or amine binder. The concentration of the binder is, for example up to 50 wt %, preferably 1 - 35 wt %, more preferably 2 - 25 wt %, based on the total weight of the layer.

The conductive ink mixed with the binder is applied to the porous material such that porosity is at least essentially maintained. Therefore, the coating is applied such that the formation of a continuous film is

avoided. Typically, the conductive layer exhibits a surface resistivity of less than 1 MOhm/sq or a volume resistivity of less than 10 ¹⁰ Ohm-cm.

The gas adsorbing layer 4 may be made from a material having a fibrous matrix interposed between two polymer sheets or layers. The fibrous matrix may be provided with activated carbon in order to adsorb harmful gases from the flow of air or gas.

In the present context the terms "adsorb" will be used for designating the sorption of substances, such as gases, to the activated carbon, and thus the layer containing activated carbon will be referred to as an "adsorption" layer.

The porous gas adsorbing layer may comprise a combination of three overlapping layers, viz. a first layer with first fibers having a first average diameter; a second layer with second fibers having a second average diameter; and a third layer with third fibers having a third average diameter.

Typically, at least a majority of the fibers of three (or more) overlapping layers are independently from each other selected from the group consisting of polymeric fibers and inorganic fibers, wherein the polymeric fibers are preferably selected from polyester, polyethylene, polyethylene terephthalate, polyolefin, polybutylene terephthalate and/or polyamide, and wherein the inorganic fibers are preferably glass fiber strands.

The fibers make up a majority of weight of the various layers. Thus, in one embodiment, the fibers of the first layer are present in an amount of at least 65 wt %, preferably at least 80 wt % based on the total fiber weight of the first layer; the fibers of the second layer are present in an amount of at least 65 wt %, preferably at least 80 wt % based on the total fiber weight of the second layer; and/or the fibers of the third layer are present in an amount of at least 65 wt %, preferably at least 80 wt % based on the total fiber weight of the third layer.

The first, second and third fibers can each be the same fibers or they can be made up of mixtures of fibers having different diameters.

In one embodiment of a multilayered, for example 3-layered structure, the second layer, interposed between a first and a third layer, contains synthetic fibers having an average diameter of less than 10 μηη mixed with synthetic fibers having an average diameter of greater than 30 μηη. It can also contain fibers having an average diameter in the range from 10 to 30 μηη. For example, the second layer may be formed from a mixture of fibers having an average diameter of 7 μητι, 12 μηη and 40 μηη. The fiber mix will provide for increased porosity of the second layer which is advantageous for

incorporation of activated carbon, for example in particulate form (as granules).

Typically, the first and the third layers comprise fibers which have average diameters that are smaller than the average diameter of the fibers of the second layer. For example the first and third layers may be formed from a mixture of fibers having an average diameter of 7 μηη and 12 μηη.

The first, the second and/or the third layer may further comprise up to 50 wt %, preferably from 2 - 35 wt %, more preferably 2 - 25 wt % of binder fibers based on the total weight of each of the first and/or third layers.

Examples of suitable binding fibres include bicomponent synthetic fibers, e.g. bicomponents fibers selected from the group of PET-CoPET, PET-CoPE, PE- PET and combinations thereof.

Bicomponent fibers may be present as binder fibers in the various layers of the gas adsorbing layer have an average diameter in the range of from 17 μηη up to 30 μηη.

The gas adsorbing layer 4 may be - just as the conductive layer 3 and the isolation layer 2 - a porous layer which will allow for low pressure drop during filtration. The gas adsorbing layer 4 exhibits properties of conductivity which are sufficient to enable the use of the gas adsorbing layer 4 as a conductive layer in combination with a conductive layer (for example the conductive layer 3) and a isolating layer (for example the isolating layer 2) in a filter element which, when it is subjected to a voltage applied over the filter element, is being electrified. Typically, the gas adsorbing layer 4 comprises activated carbon in an amount sufficient to render the layer conductive.

The activated carbon may comprise activated carbon powder, in particular a powder formed by particles having an average particle size in the range of 0.1 to 1 .5 mm, in particular 0.3 to 1 .0 mm.

The activated carbon may also comprise activated carbon fibers or activated carbon in the form of pellets having a diameter of 0.1 to 2 mm, in particular 0.5 to 1 .5 mm, and a length of 0.5 to 2 mm.

The activated carbon typically has a large specific surface, amounting to more than 500 m ² /g, in particular more than 750 m ² /g, for example 800 m ² /g or more, typically up to 2000 m ² /g.

The gas adsorbing layer will not only provide for electrical conductivity but it will also adsorb gases, such as nitrogeneous oxides and carbon oxides, and in particular ozone. Thus, the gas adsorbing layer will prevent hazardous ozone generated during ionization of particles contained in gas to be filtered from escaping the filter element. This is of particular relevance with regard to the use of the present filter elements in applications such as ventilation of buildings and vehicles.

The first and/or the third layer may further comprise up to 50 wt %, preferably from 2 - 35 wt %, more preferably 2 - 25 wt %, based on the total weight of each of the first and/or third layer, of a binder. Such a binder will enhance the mechanical properties of the layers. Typically the binder can be selected from the group of acrylate, methacrylate, epoxide and amine binders and combinations thereof. Preferably, the second layer is free from a binder, which is liquid or solid (hot melt binders) at room temperature, to avoid a loss of specific surface are of the activated carbon with would impair gas adsorption properties of the layer 4.

To enhance properties of electrical conductivity, the gas adsorption layer 4 can be coated with a layer of conductive substances, as explained above in relation to conductive layer 3. Thus, on the surface of the gas adsorption layer, a coating comprising a conductive ink can be deposited. The conductive ink may be made from powdered or flaked silver or carbon like materials, such as carbon black, or from conductive polymers.

The porous material may also be treated with a gaseous aluminum. Alternatively it may be printed or impregnated with an electrically conductive polymer, which is bound to the base material. Examples of conductive polymers and doping agents are the same as listed above in relation to the conductive layer 3.

Typically, the conductive ink also contains a binder for the conductive component for bonding it to the substrate or material which is to be rendered conductive. The binder can be a thermoplastic or thermosetting material, for example an acrylate, alkacrylate (methacrylate), epoxide or amine binder. The concentration of the binder is, for example up to 50 wt %, preferably 1 - 35 wt %, more preferably 2 - 25 wt %, based on the total weight of the layer.

The conductive ink mixed with the binder is applied to the porous material such that porosity is at least essentially maintained. Therefore, the coating is applied such that the formation of a continuous film is avoided.

In one embodiment, the gas adsorbing layer has a grammage of 250 to 500 g/m ² , in particular 250 to 450 g/m ² .

Typically, the conductive layer exhibits a surface resistivity of less than

1 MOhm/sq or a volume resistivity of less than 10 ¹⁰ Ohm-cm.

The thickness of the gas adsorbing layer 4 may be in the range of 1 to

2 mm.

As illustrated by Fig. 2 the filter element 1 may be arranged to be connected to a voltage source 1 1 , forming a filter system 10. The voltage source provides a voltage or potential over the filter element in the range of 6 to 8 kV. How the voltage source is connected to the filter element or to ground depends on the desired application. This has been illustrated by the dashed lines from thv voltage source to the different components.

In one example an 8 kV charge is used, and 0.25 - 4 kV may then be connected to either one of the conductive layer and the gas adsorbing layer and preferably 1 - 3 kV to the other layer respectivley. One of the poles of the voltage source, for instance the negative pole, may be connected to the filter element, and the other pole may be connected to the ground.

The filter system 10 may further comprise an ionizing device 12. This device may be an ion emitter, for instance a corona discharger or wire. The ionizing device may be placed in the gas flow F towards the filter

arrangement, and thus charge the particles that enters into the filter element. The ionizing device 12 may be connected to the same voltage source as the filter element or a separate voltage source. In general the voltage applied to the ionizing device is around 20 % of that applied over the filter element.

The volume resistivity of the conductive layers of the filter element may be up to 10 ¹⁰ cm, preferably less. The filter element 1 and/or the filter system 10 can be adapted to be used in various types of applications.

It is for instance possible to fit the filter element into existing filter arrangements in the ventilation of buildings and vehicles. Particular uses of the filter element is for cabin air filtration in passenger cars and other vehicles, for HVAC filtration of indoor air, in portable air purification units and in vacuum cleaners.

The filter arrangement then only needs to be adapted with the voltage source giving a voltage or potential difference over the filter element. Further the conventional filter arrangements may easily be adapted or modified to also include the ionizing device, thus forming a filter system according to the present invention.

In the alternative ready-made systems may be provided, for easy installation in a desired application.

The filter element may of course be disposable, i.e. in the filter system or arrangement the filter element may be regularly changed when necessary. This may be monitored to provide the optimal filtering efficiency, for instance when the pressure drop over the filter reaches above a certain level, a signal may be sent to an operator or to an automated system, such as in a car or any other vehicle. Trial

The filter medium tested was formed by a combination of a conductive layer, a particle capturing layer, a gas adsorption layer with a coating on top of the gas adsorption layer. The material layers were mechanically joined together by placing the layers in overlapping relationship while avoiding the use of adhesives which otherwise might impair porosity and increase pressure drop.

The conductive layer had grammage of about 37 g/m ² and it was formed by a mixture of natural and regenerated cellulose fibers (cellulose, chemical wood pulp and viscose) mixed with PET fibers and an acrylate binder and carbon black. The viscose and the PET fibers had an average diameter of about 15 μηη.

The insulation or particle capturing layer was a uniform layer consiting to 100 % by polyester fibers and it had a grammage of 100 g/m ² .

The gas adsorption layer was formed by a three-layered non-woven structure comprising a layer of a mixture of PET fibers having average diameters of up to 40 μηη containing activated carbon granules (250 g/m ² ) having an average diameter of 0.3 to 1 .0 mm, interposed between two polymer sheets or layers formed by of PET fibers having average diameters of 7 μΐη and 12 μηη. Each layer contained bicomponent binding fibers having an average diameter of 17 to 30 μητι, while the polymer sheets also contained acrylate binder. Furthermore, one surface of the non-woven structure was provided with a coating layer formed by chemical particals of carbon black mixed with an acrylate binder. The grammage of the structure was 350 g/m ² .

Non-woven materials, modified with activated carbon, are supplied by

Ahlstrom, Tampere, Finland, under the tradename Trinitex.

The aim of the trial was to measure the pressure drop and filtration efficency of an electrified fibrous filter. The tests were done by applying the principle of EN779 standard for general ventilation filters.

The efficiency was measured in four cases: no voltages connected, only charger voltage connected, only precharger voltage connected and both charger and precharger voltage connected (at different voltages). The tests were made for clean filter and filter materials in normal room conditions. The filtration properties measured were pressure drop and fractional filtration efficiency.

The pre-charger used was a corona wire charger, and the polarity of the corona wires was positive.

Neutral or charges test particles were mixed with HEPA filtered air which was supplied to the filter in a controlled chamber environment at a test facility.

The efficiency was determined by measuring the particle

concentrations alternatley from upstream and downstream of the filter material. The measurements were made with optical particle size analyzer in particle size range 0.1 - 2 μιτι.

The filter element was installed and sealed between two chambers in a filter measurement system, in a controlled environment at an independent test facility. The air flow was measured with a measurement ring. The pressure drop was determined by measuring the pressure difference between upstream and downstream chambers with a micromanometer.

The pressure drop was measured to less than 50 Pa at an air speed of

5.3 cm/s. As shown in Fig. 3 the filtering efficiency when introducing both pre-charger and charger was remarkably higher then witouth any charging of the filter. At the low pressure drop of less than 50 Pa a filtering efficiency of over 90 % and even over 95 % was achieved for the MPPS, having an optical particle size in the range of about 0.1 to 2 μιτι.