Very-high-efficiency filtration system comprising a prefilter and an electrostatic precipitator / collector downstream of the prefilter

Abstract

The invention relates to a filtration system comprising: - at least one ionizer (2) for ionizing the particles that are likely to be present in the air, the ionizer comprising electrically conductive fibers (21); - at least one prefilter (40), arranged downstream of the ionizer (2); - at least one electrostatic precipitator / collector (3), arranged downstream of the prefilter and comprising: • at least one channel plate (5) made of substantially electrically insulating material, extending along a longitudinal axis (X), the channels (50) being fluidically separated from one another, each channel extending along the X axis and being open, at both of its opposing ends, to the exterior of the plate; • at least two electrically conductive sheets (6, 7), plates (8, 9), or layers, forming electrodes, at least a portion of which is arranged on each of the main faces of the plate, the two electrodes being capable of generating an electric field (E) in a direction orthogonal to the X axis in the channels.

Description

[0001] Description

[0002] Title: Very high efficiency filtration system comprising a prefilter and an electrostatic precipitator / collector downstream of the prefilter.

[0003] Technical field

[0004] The present invention relates to the field of air purification and purification of aerosols, likely to contain suspended particles.

[0005] The present invention aims to improve the performance and efficiency of existing air purifiers and aerosol purifiers and more generally to propose a new innovative and efficient solution for high efficiency or very high efficiency filtration of channeled flows, in particular in order to reduce manufacturing costs, size, mass and operating expenses from the point of view of energy consumption.

[0006] Although described with reference to an air purification application, the invention applies to any aerosol purification, more specifically the filtration of aerosol particles in low to moderately dusty environments.

[0007] Typically, a preferred application is the treatment of indoor air in premises or confined spaces, including underground spaces, or the treatment of air in ventilation systems in residential buildings, offices, spaces open to the public, or the treatment of air in the passenger compartment of vehicles, or the treatment of channelled emissions from industrial processes which are generally not very concentrated or in series with an upstream mechanical filtration stage.

[0008] Another preferred application is the filtration of outdoor air.

[0009] “Aerosol” means any liquid or solid particle with a diameter between 0.001 pm and 100 pm that is capable of remaining suspended in the air,

[0010] In practice, the target range corresponds to that which is most accessible to measurement and which is also that sought for filtration typically from 0.01 to 10 pm.

[0011] This is particularly the area where, in atmospheric pollution, we find so-called "respirable" particles referenced PM 10, PM2.5 and "fine" particles referenced PMI, i.e. particles whose size is less than 10 pm, 2.5 pm and 1 pm respectively, as well as "ultrafine particles", also called "nanoparticles", or PMO,1 whose size is less than 0.1 pm.

[0012] The nature of these particles can be extremely variable depending on their mode of formation, including particles resulting from human activity or not:

[0013] - combustion fumes, particulate pollution linked to motor vehicle traffic (including exhaust gases, particles from brake pads, tires)

[0014] - domestic fireplaces

[0015] - industrial processes

[0016] - building materials

[0017] - pollen, bacteria, viruses, animal allergens, etc.).

[0018] Prior art

[0019] Most of the places where individuals live are enclosed spaces, whether they are places open to the public (halls, metro stations and transfer corridors, administrations, schools, hospitals, sports halls and cinemas, etc.), professional buildings (offices and businesses), private spaces (individual or collective housing), or mobile means of transport when individuals or groups travel (cabins of airplanes, boats, trains, buses, coaches, automobiles).

[0020] In addition to the external pollution that enters buildings through openings and ventilation, there are sources of internal emissions (tobacco smoke, combustion residues, asbestos, radon, emissions due to materials: particles and volatile organic compounds, allergens of biological origin, etc.).

[0021] This has become a major concern. The World Health Organization (WHO) warns: "Air pollution, both indoors and outdoors, is a major environmental health problem affecting both developed and developing countries."

[0022] For example, air pollution due to fine particles is responsible for 48,000 deaths each year in France. The health burden of this pollution linked to human activities (transport, industry, heating with fossil fuels, etc.) corresponds to 9% of mortality in continental France: [1]. Fine particles, and particularly the PM2.5 fraction, are those for which the socio-economic cost is the highest among the pollutants encountered in the indoor atmosphere: [2].

[0023] Thus, the marketing of air purifiers promising to filter most of these indoor air pollutants has surged.

[0024] Among the known purification methods for filtering aerosol particles, two main categories are usually distinguished:

[0025] - mechanical filtration which consists of passing the air loaded with aerosol particles through fibrous media THE (acronym for Very High Efficiency Filters, or HEPA in English for "High Efficiency Particulate Air Filter"): the particles are deposited on the fibers by impaction, interception and diffusion mechanisms in particular. The main advantage of HEPA filtration is the very high level of efficiency (99.97% for particles of 0.3 pm), and close to 100% for particles finer or larger than this size. However, HEPA filtration has two major drawbacks: on the one hand, a high resistance to flow, in other words a high pressure drop in a ventilation network, resulting in high energy consumption, on the other hand, a relatively short lifespan because the filter cannot be cleaned and must be disposed of when it is clogged;

[0026] - electrostatic filtration, which involves electrically charging particles and precipitating them onto collection plates under the action of an intense electric field. Charging is generally achieved by diffusion and ion bombardment, with the ions being created by the corona effect using high-voltage wires or spikes.

[0027] Compared to mechanical filtration, electrostatic filtration has the dual advantage of having little resistance to flow (low pressure drop in a ventilation network) and of having a long service life because an electrostatic filter can be unclogged and therefore reused. On the other hand, electrostatic filtration is generally less effective and more expensive to purchase.

[0028] In reality, this distinction between mechanical filtration and electrostatic filtration is not always clear because natural electrical effects are also present during conventional mechanical filtration. It is also possible to artificially accentuate these electrical effects as indicated below.

[0029] For example, in so-called "electret" filters, fibers are artificially electrically precharged at the time of their manufacture, which increases the filter's efficiency, at least when it is new. This effect occurs even if the incident particles are electrically neutral. But this increased efficiency decreases over time of use due to the progressive neutralization of the fibers.

[0030] In other cases, the fibers are neutral, but the particles are artificially charged. The filter efficiency is thus significantly improved. First highlighted in the 1950s: [3], this effect is particularly used in hybrid mechanical filter systems with electrostatic filter, mentioned in the literature, for example [4], or in the automotive industry, for example in vehicles marketed by the VOLVO company, where an increase in the efficiency of cabin filters is sought.

[0031] But if these filtration systems are not efficient enough, then charged particles can escape.

[0032] This major drawback is found in negative ion generators that aim to improve the air quality in the passenger compartments of motor vehicles or more generally in an enclosed space. These ion generators electrically charge the particles suspended in a passenger compartment, and the particles thus charged with the same sign, by electrostatic repulsion, are then deposited more quickly against the interior walls of the vehicle, which helps to clean a passenger compartment.

[0033] That being said, as highlighted in the report [5], there appears to be a health risk because electrically charged fine particles such as those which would come from a cabin filter system downstream of an ionizer whose efficiency would be significantly less than 100% could pass through and be inhaled.

[0034] There are different types of electrostatic filtration devices.

[0035] In the publication [4] mentioned above, the filter is said to be "single-stage", the ionization of the particles and their collection being carried out concomitantly between plate electrodes. There are also "double-stage" devices, i.e. comprising a stage forming an ionizer upstream of a precipitation or collection stage strictly speaking.

[0036] Electrostatic precipitator / collector ionizer purifiers, also known as electrostatic filters and electrostatic precipitators (ESPs), appear to be promising solutions.

[0037] Such a device generally designated by the reference 1 is shown in Figure 1 A: it comprises an ionizer 2 upstream of an electrostatic precipitator / collector 3. The aerosol particles are first electrically charged by corona effect by passing in the vicinity of wires 20 carried at high voltage Uf, which are stretched between plates 4 connected to ground, then they are collected on parallel, electrically conductive flat plates 30, 31 between which an electric field is established under the action of a high polarization voltage Up. This geometry is practically always presented in flows of rectangular section, the charging then collection operations being carried out between flat and parallel plates 4, 30, 31.

[0038] The incident particles are pre-charged with a positive or negative polarity and are collected on the plates 30, 31 of opposite polarity.

[0039] A variant of ionizer 2 is shown in Figure 1B: instead of the wires 20, carbon fibers 21 are arranged in the same place, preferably in the center of the aerosol flow channel.

[0040] For a given flow rate and state of charge, it appears that the particle collection efficiency is all the higher as the surface area of ​​the collection plates is large on the one hand, and as the electric field between the plates is strong on the other hand (Deutsch-Anderson equation).

[0041] In practice, an electrostatic precipitator / collector consists of a stack of metal plates 30, 31, 4 in large numbers to have a large collection surface. As shown in figures 1A and 1B, one plate 30, 31 out of two is supplied with high voltage, the other 4 are grounded. They must be equidistant in such a way that a uniform electric field, as high as possible, of the order of 10 kV / cm, can be maintained between the plates without there being any disruptive breakdown. Many air purifiers already on the market operate on this electrostatic principle.

[0042] For example, we can cite the one marketed under the name “FEI” by the company France Air, whose nominal flow rate is equal to 2200 m3 / h.

[0043] We can also mention the one recently marketed under the name “OneLife X” by the company OneLife, whose nominal flow rate is equal to 100 m3 / h.

[0044] Generally, electrostatic air purifiers are of great mechanical and electrical complexity, hence a high cost. Indeed, the interlocking with alternating insulating and electrically conductive parts and the mechanical maintenance of the plates is complex to guarantee their positioning at equal distance from each other while ensuring their electrical connections under high voltage or to ground without disruptive breakdown. In other words, the major technical disadvantages, more particularly those related to the manufacture of the electrostatic collector are:

[0045] - precise production of a large number of parts (insulating and conductive);

[0046] - meticulous assembly;

[0047] - a large mass of metallic material. For example, to treat an air flow equal to 2200 m 3 / h, the entire electrostatic precipitator of the FEI commercial purifier from France-Air weighs 44 kg.

[0048] - a filtration efficiency which, although quite high, remains significantly lower than the efficiency achieved with a HEPA type H12 or H13 filter.

[0049] To overcome some of these drawbacks, while seeking to achieve the level of HEPA filtration, solutions have already been proposed in patent applications / patents US 6749669B 1, EP1262239A2, WO 2006 / 012520A2, CN103301943A, CN104368444A, CN1126576782, CN106179751B and EP3338893A1.

[0050] Rather than establishing an air flow through a stack of electrified parallel metal plates, these solutions consist of establishing the flow through a stack of alveolar plates made of electrically insulating material, each plate being coated with an electrically conductive layer brought to a certain electrical potential in order to establish an electric field in the alveoli. The incident particles, which are electrically charged, are then deposited in the alveoli according to the classical laws of electrostatic precipitation.

[0051] There is still a need to find a filtration system that can achieve HEPA filter efficiency levels that are stable over time, without the drawbacks of a HEPA filter.

[0052] The general aim of the invention is then to meet this need.

[0053] Statement of the invention

[0054] To this end, the invention relates to a filtration system, intended for the filtration of air, in particular of a passenger compartment of a means of transport, such as a motor vehicle, comprising:

[0055] - at least one ionizer, called the main ionizer, of the particles likely to be present in the air, comprising, as a corona ionization device, electrically conductive fibers,

[0056] - at least one mechanical filter or foam, as a pre-filter, arranged downstream of the ionizer,

[0057] - at least one electrostatic precipitator / collector, arranged downstream of the prefilter and comprising:

[0058] • at least one channel plate made of substantially electrically insulating material, extending along a longitudinal axis, the channels of the plate being fluidically separated from one another, each channel extending along the longitudinal axis and opening at its two opposite ends, towards the outside of the plate;

[0059] • at least two electrically conductive sheets or plates or layers, forming electrodes of which at least one portion is arranged on each of the main faces of the channel plate, the two electrodes being adapted to generate an electric field (E) in a direction orthogonal to the X axis in the channels of the plate.

[0060] The system according to the invention is intended for the filtration of air, whether indoors, outdoors and more generally of all types of atmospheres with little or low dust and whose flow is preferably channeled. The system may comprise a fan which may be placed anywhere in the channeled flow, upstream or downstream of the elements described above, operating by blowing or suction, provided that the air flow is ensured in the direction going from the ionizer to the collector via the pre-filter.

[0061] The ionizer is advantageously provided with a device ensuring the emission of negative ions, preferably in very high concentration, by passive destruction of the ammonium nitrate produced on the surface of the electrodes, where the electric field is most intense.

[0062] The destruction of ammonium nitrate is carried out thermally because this compound, solid at room temperature, evaporates from 210°C. This destruction can be carried out continuously or, preferably, discontinuously.

[0063] The heating of the ionizing electrodes is advantageously carried out by passive means, for example by Joule effect heating or by induction.

[0064] Cleaning these ionizing electrodes can be done by mechanical action or by blowing with a pulse of air.

[0065] Preferably, the ionizer is made of carbon fibers. More preferably, the carbon fibers have a diameter of 10 to 20 μm, advantageously grouped to form a brush of approximately 300 fibers over 5 mm in length.

[0066] The prefilter can be standard, and preferably made of pleated and porous fibrous elements to present a low pressure drop, typically a few tens of Pascals, but sufficiently effective against electrically charged incident particles. A prefilter classified G4, M6 or even type F7 to F9 made for example of synthetic fibers in polypropylene, modacrylic and polyester can advantageously be considered [6].

[0067] Compared to the state of the art, the collector according to the invention comprises:

[0068] - at least one channel plate made of substantially electrically insulating material, extending along a longitudinal axis (X), the channels of the plate being fluidically separated from one another, each channel extending along the longitudinal axis (X) and opening at its two opposite ends, towards the outside of the plate;

[0069] - at least two electrically conductive sheets or plates or layers, forming electrodes of which at least one portion is arranged on each of the main faces of the channel plate, the two electrodes being adapted to generate an electric field in a direction orthogonal to the X axis in the channels of the plate.

[0070] According to an advantageous embodiment, the system comprises means for regulating the electrical voltage to the main ionizer, and preferably simultaneously, to the electrodes of the electrostatic precipitator(s) / collector(s). For an application where the variation in the air flow rates to be treated is significant, such as in a motor vehicle passenger compartment, modulation of the high electrical ionization voltage, just like that applied to the cells of the collector plates, makes it possible to obtain a compromise between filtration efficiency and comfortable air flow rate for people. For example, for filtration of a motor vehicle passenger compartment, it is possible to envisage a crescendo increase in this voltage for a high air flow rate, typically equal to 200 m 3 / h, 300 m 3 / h, 400 m 3 / h), followed by a decrease in this voltage while maintaining HEP A level filtration efficiency for a comfort flow rate of 50 to 150 m 3 / h.

[0071] According to another advantageous embodiment, the system comprises an additional ionizer, as an electrical pre-charger, arranged upstream of the main ionizer. The main ionizer is therefore in series with the additional ionizer, at a distance that can be adjusted, depending on the applications. This series connection of an additional ionizer makes it possible to significantly boost the performance of the electrostatic collector. In other words, an efficiency gain is obtained.

[0072] According to another advantageous embodiment, the system comprises an electrical discharge device by grounding the electrode(s) of the electrostatic precipitator. Carrying out grounding allows intervention and maintenance on the alveolar plates of the electrostatic precipitator, without risk for an operator.

[0073] Advantageously, the collector comprises a stack of channel plates, an intermediate electrode being interposed between two adjacent plates in the stack, all the intermediate electrodes of the same polarity being connected together to an output electrode forming an output terminal.

[0074] According to a first advantageous embodiment, the intermediate electrodes of the same polarity each also have a flexible portion which fits the lateral edge of a plate while the output terminals each have a general L shape, one branch of which fits the lateral edge of the stack of plates while being in contact with the portions of the electrodes and the other branch fits one of the main faces of the plate at one of the ends of the stack.

[0075] According to a second advantageous embodiment, the intermediate electrodes of the same polarity each comprise a flexible tab or other electrical contact point which fits the lateral edge of a plate, the tabs of the same lateral edge overlapping one another, so as to form one of the output terminals.

[0076] According to this second mode, the collector preferably comprises two end plates made of electrically insulating material, arranged on either side of the stack.

[0077] For small-scale production, the intermediate electrodes are in the form of metal sheets (such as food-grade aluminum foil) each covering at least part of one main face of a plate. Each metal sheet preferably has a thickness of between 10 and 50 μm. Advantageously, for large-scale production, the intermediate electrodes are in the form of metallized deposits: metallization by a surface deposition process is thus preferable.

[0078] According to the first or second method, each intermediate electrode is preferably sealed between two channel plates so as to constitute a fluid-tight block between plates with respect to fluids and electrically insulating between two intermediate electrodes. The sealed and insulating block can be completed by the electrically insulating end plates. The sealing of the block conferred by the sealing is with respect to the gases to be purified and the washing liquids during the unclogging or cleaning phase for example. This makes it possible to envisage cleaning the collector directly in a dishwasher for example. The electrical insulation conferred by the sealing prevents any disruptive breakdown by surface conduction between adjacent electrodes.

[0079] Preferably, the material of the plate(s) has a transverse electrical conductivity greater than 10 14 S / m at room temperature (volume resistivity less than 10 14Qm and a dielectric strength greater than 15 kV / mm).

[0080] Thus, the material of the plate(s) can be chosen from a wide range of thermoplastic polymers, in particular those that can be easily extruded, such as polyvinyl chloride (PVC) or polycarbonate (PC), whose dielectric strength is 25 kV / mm. Any type of thermoplastic polymer that can be extruded is therefore suitable. Polypropylene, associated with an optimal ionization voltage and a smaller inter-plate distance, is a possible alternative.

[0081] Each plate preferably has a thickness between 1 and 20 mm.

[0082] According to an advantageous characteristic, the channels of the same plate are identical.

[0083] According to another advantageous characteristic, the channels of the same plate can be rectilinear, preferably of square or straight rectangular cross-section, with a side length of less than 10 mm, preferably between 2 and 4 mm.

[0084] As stated before, the system includes at least one fan arranged upstream or downstream of the main ionizer or downstream of the electrostatic precipitator / collector.

[0085] According to one installation configuration, the channel plate(s) of the electrostatic precipitator / collector is / are arranged vertically, the fan(s) being arranged on top of the channel plate(s), the system further comprising a mesh below the ionizer as a prefilter.

[0086] The system may include a battery and a case housing the components to form a self-contained device.

[0087] The invention also relates to a method for operating a filtration system as described above, comprising the following steps: a / automatic detection or voluntary action of at least one individual in a passenger compartment of a means of transport; b / in the event of automatic detection or voluntary action of an individual according to step a / , application of an electrical voltage to the main ionizer, and preferably simultaneously, to the electrodes of the electrostatic precipitator(s) / collector(s).

[0088] Advantageously, the voltage applied according to step b / to the main ionizer and / or to the electrostatic precipitator(s) / collector(s) is between 5 and 15 kV or -5 and -15 kV.

[0089] Preferably, the electric field generated according to step b / in the plate channels of the electrostatic precipitator(s) / collector(s) is greater than or equal to 20 kV / cm.

[0090] According to an advantageous variant, the method comprises, during step b / , a step bl / , of reversing the polarities of the electrodes of the electrostatic precipitator(s) / collector(s) such that those initially electrically grounded are brought to the potential of the electric field to be generated and vice versa. The Reynolds number (Re) of the air flow in the plate channels of the electrostatic precipitator(s) / collector(s), during step b / , is preferably less than or equal to 2000.

[0091] According to another advantageous variant, the method comprises, during step b / , a step b2 / of supplying clean air or other neutral gas, for example N2, to the main ionizer and / or occasional heating of the latter. This makes it possible to maintain the ionization performance over time.

[0092] According to another advantageous variant, the method comprises, even in the event of non-automatic detection or voluntary action by an individual according to step a / , and in the absence of step b / , a step c / of electrically grounding the electrodes of the electrostatic precipitator(s) / collector(s).

[0093] According to another advantageous variant, the method comprises, during step b / , a step b3 / of regulating the electrical voltage at the ionizer, and preferably simultaneously, at the electrodes of the electrostatic precipitator(s) / collector(s) as a function of the air flow rate to be treated.

[0094] The invention also relates to a means of transport, in particular a motor vehicle, comprising:

[0095] - one or more air distribution ducts in the passenger compartment of the means of transport;

[0096] - at least one filtration system as described above with at least one electrostatic precipitator / collector arranged in the section of one or more of the air distribution ducts.

[0097] In the filtration system according to the invention, the advantageous use of a honeycomb electrostatic collector compared to a conventional electrostatic plate filter appears clearly by examining the laminar or turbulent flow regime which passes through them.

[0098] It is known that the collection efficiency Ec of electrostatic filters generally follows the Deutsch-Anderson equation which is written, in turbulent flow regime:

[0099] [Equation 1]: where w is the drift velocity of charged particles between the electrodes in the direction perpendicular to the flow,

[0100] Ac is the collection surface,

[0101] Q is the flow rate.

[0102] In Figure 2, representing the efficiency E as a function of wAc / Q, the Deutsch-Anderson equation is given by curve A. On the other hand, in laminar flow regime, the efficiency E reaches 100% when the term w Ac / Q > 1 (curve B).

[0103] Now, a channel plate can easily operate in this laminar regime. This results from the small dimension that can be given to the channels in which the Reynolds number of the flow can easily be limited to 2000 or less, which provides a new advantage compared to conventional electrostatic precipitators which operate in the turbulent regime.

[0104] In summary, compared to a system with a classic electrostatic collector and for a fixed purification flow rate, a filtration system with an ionizer and alveolar electrostatic collector provides the following advantages:

[0105] - collection efficiency of up to 100% thanks to a laminar flow regime in the plate channels;

[0106] - due to this 100% efficiency, absence of exposure of people to ions (negative or positive);

[0107] - possibility of using significant electric collection fields, without risk of electrical breakdown, thanks to the dielectric strength of the insulation of the plate(s) greater than that of air;

[0108] - absence of ozone production;

[0109] - drastic reduction in the number of components to be assembled;

[0110] - drastic reduction in the mass of materials used;

[0111] - extreme ease of manufacturing and assembly;

[0112] - consequently, drastic reduction in the cost of production; - possibility of installing a collector according to the invention in structures of various shapes, or various devices, including miniaturized ones, which is not easy, or even impossible, to do with electrostatic precipitators according to the state of the art.

[0113] Thus, for the same air flow to be treated, the invention makes it possible to achieve a significant gain in pressure loss, size, mass, lifespan, manufacturing, maintenance and operating costs. As a corollary, the invention makes it possible to achieve significant savings on the overall energy bill compared to systems according to the state of the art.

[0114] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description, given for illustrative and non-limiting purposes, with reference to the following figures.

[0115] Brief description of the drawings

[0116] [Fig IA] Figure IA is a schematic longitudinal sectional view of a double-stage electrostatic precipitator, i.e. with one stage forming an ionizer and one stage forming a particle collector, in continuity with the ionizer.

[0117] [Fig IB] Figure IB is a schematic longitudinal sectional view of a double-stage electrostatic precipitator, according to a carbon fiber ionizer variant

[0118] [Fig 2] Figure 2 illustrates in the form of curves the collection efficiency of electrostatic filters in turbulent and laminar regimes.

[0119] [Fig 3] Figure 3 is a schematic longitudinal sectional view of a filtration system according to the invention.

[0120] [Fig 4] Figure 4 is a schematic perspective view of a first example of an electrostatic collector according to the invention.

[0121] [Fig 4A] Figure 4A is an exploded schematic view of the electrostatic collector according to Figure 4.

[0122] [Fig 5] Figure 5 is a schematic perspective view of a second example of an electrostatic collector according to the invention. [Fig 5A] Figure 5A is an exploded schematic view of the electrostatic collector according to Figure 5.

[0123] [Fig 6] Figure 6 is a schematic perspective view of an experimental prototype incorporating a filtration system in accordance with the invention for testing purposes.

[0124] [Fig 7] Figure 7 is a schematic longitudinal sectional view of a portion of the channel plate stack of an electrostatic precipitator / collector according to the invention, Figure 7 showing the trajectory of the electrically charged particles as they collect within the channels.

[0125] [Fig 8] Figure 8 is a schematic longitudinal cross-sectional view of a portion of the channel plate stack of an electrostatic precipitator / collector according to the invention, Figure 8 showing particles collected within the channels.

[0126] [Fig 9] Figure 9 is a schematic perspective view and partially cut away of a second example of a filtration system as an air purifier, integrating a carbon fiber ionizer and an electrostatic collector according to the invention.

[0127] [Fig 10] Figure 10 graphically shows the filtration efficiency of an experimental prototype made as in Figure 6 as a function of particle size for different fan flow rates.

[0128] [Fig 1 IA] Figure 1 IA graphically shows the efficiency curve of a honeycomb collector as a function of particle size when the collector is at its optimum operating point.

[0129] [Fig 11B] Figure 11B shows graphically the typical efficiency curve of a medium efficiency fiber prefilter as a function of particle size, without or with particle ionization.

[0130] [Fig 11C] Figure 11C shows in graphic form the efficiency curve of the filtration system according to the invention.

[0131] [Fig 11D] Figure 11D summarizes in the form of curves the implementation of the invention to achieve HEP A efficiency. [Fig 12] Figure 12 is a schematic perspective and exploded view of a fresh air distribution network in an apartment or office space, inside which at least one electrostatic collector according to the invention is positioned.

[0132] [Fig 13] Figure 13 is a schematic perspective view of an electrostatic collector according to the invention, integrated into a ventilation duct with an elongated profile.

[0133] [Fig 14] Figure 14 is a schematic longitudinal sectional view of an experimental setup for testing the electrostatic collector according to the invention as illustrated in Figure 13.

[0134] [Fig 15] Figure 15 gives the experimental result of long-term tests with the setup of Figure 14, obtained in the form of a curve giving the particle collection efficiency as a function of time.

[0135] [Fig 16] Figure 16 gives further experimental results of long-term tests with the setup of Figure 6, obtained in the form of a curve giving the collection efficiency over time, interrupted by cleaning steps of the ionizer and the collector.

[0136] [Fig 17A] Figure 14A illustrates in perspective an induction heating device of a carbon fiber ionizer.

[0137] [Fig 147] Figure 17B illustrates in perspective a Joule heating device of a carbon fiber ionizer.

[0138] [Fig 18A] Figure 18A is a schematic longitudinal sectional view of a filtration system according to the invention, aimed at optimizing the location of the ionizers.

[0139] [Fig 18B] Figure 18B shows the different filtration efficiencies obtained when the ionizer is positioned in the duct, upstream of the pre-filter.

[0140] [Fig 18C] Figure 18C shows the different filtration efficiencies obtained when the ionizer is positioned outside the duct, upstream of the suction.

[0141] [Fig 19A] Figure 19A gives the experimental results of the efficiency of a thin electrostatic collector, as a function of time, to highlight the influence of off-periods. [Fig 19B] Figure 19B gives the experimental results of the efficiency of a thin electrostatic collector, as a function of time, to highlight the influence of changing the polarity of the electrodes.

[0142] [Fig 20] Figure 20 is a schematic perspective and exploded view of an air distribution network in the passenger compartment of a motor vehicle, inside which electrostatic collectors according to the invention are positioned.

[0143] [Fig 21] Figure 21 is a schematic longitudinal sectional view of electrostatic collectors according to the invention integrated into an air distribution network in the passenger compartment of a vehicle.

[0144] [Fig 22] Figure 22 is a schematic perspective view of a filtration system designed to treat large air flows by modular assemblies of standardized collectors.

[0145] [Fig 23] Figure 23 is a schematic longitudinal sectional view of the filtration system according to Figure 22, arranged in a protected box in a space receiving the public such as a metro platform.

[0146] [Fig 24] Figure 24 is a schematic perspective view of alveolar electrostatic collectors according to the invention, intended to filter the air at the level of railway tracks in an underground environment to reduce the concentration of particles resulting from the braking of trains.

[0147] Detailed description

[0148] Throughout the present application, the terms "inlet", "outlet", "upstream" and "downstream" are to be understood by reference to the direction of the suction flow through a filtration system according to the invention. Thus, the inlet orifice designates the orifice of the device through which the air to be purified is sucked in while the outlet orifice designates the orifice through which the air flow exits.

[0149] Figures IA, IB and 2 have already been commented on in the preamble. They are not detailed below.

[0150] The same element according to the state of the art and according to the invention is designated by the same numerical reference. Figure 3 illustrates an example of an air filtration system 1 according to the invention, for example that of a vehicle passenger compartment. This system 1' comprises, from upstream to downstream, in the direction of flow:

[0151] - an ionizer 2 adapted to electrically charge the incident particles suspended in the air channeled in a conduit 100,

[0152] - a 40 fiber pre-filter, of the automobile cabin filter type which can be standard,

[0153] - a 3-plate collector forming channels as described below,

[0154] - an extraction fan 14 which induces the flow within the conduit 100.

[0155] Figures 4 and 4A show an example of an electrostatic collector 3 according to the invention.

[0156] It firstly comprises a stack of plates 5, preferably identical, with opening channels 50. Each plate 5 is made of substantially electrically insulating material and extends along a longitudinal axis X. The channels 50, preferably identical, of the same plate are fluidically separated from each other. Each channel extends along the longitudinal axis X and opens at its two opposite ends. In the example illustrated, the channels 50 have a rectangular cross-section.

[0157] The following properties and parameters can be considered for a plate 5:

[0158] - a thickness between 1 and 20 mm;

[0159] - a constituent electrical insulating material chosen from poly(vinyl chloride) (PVC), polycarbonate (PC), polypropylene (PP) or other materials whose electrical properties are consistent with the operation of the invention;

[0160] - 50 channels with rectangular or square section between 2 and 40 mm.

[0161] An intermediate electrode 6, 7 is interposed between two adjacent plates 5 in the stack, all the intermediate electrodes of the same polarity being connected together to an output electrode forming an output terminal 8, 9.

[0162] The intermediate electrodes of the same polarity each also have a portion 60, 70 which fits the lateral edge of a plate 5.

[0163] The output terminals 8, 9 each have a general L shape, one branch 80, 90 of which fits the lateral edge of the stack of plates while being in contact with the portions of the electrodes and the other branch 81, 91 fits one of the main faces of the plate at one of the ends of the stack.

[0164] As illustrated in Figures 4 and 4A, the intermediate electrodes 6, 7 are preferably in the form of sheets or thin metal plates, a portion 61, 71 of which each covers a main face of a plate 5 and a portion 60, 70 of which is folded against a lateral edge of the plate or of an adjacent plate 5.

[0165] The output terminals 8, 9 are in the form of metal plates bent into an L shape.

[0166] Thus, the metal sheets or thin plates 6 are in direct electrical contact with the output terminal 8 to which a certain potential is applied. The metal sheets 7 are in electrical contact with the output terminal 9 to which another potential is applied. This potential difference creates an electric field within the channels.

[0167] Another example of collector 3 is shown in Figures 5 and 5A. The electrodes 6, 7 are also constituted by metal sheets, the main portion 60, 70 of which is applied against one of the main faces of a plate 5.

[0168] The surface dimensions of the main portion correspond substantially to those of a plate 5 to within a setback distance “d” around the entire circumference of the portion 60, 70. This setback d, for example 5 mm, allows a gain in collection surface. This setback “d” can be adjusted. It is advantageously coated with an electrically insulating varnish before assembly of the stack. This varnish is for example a varnish of the type sold under the name RS 199-1480 with a dielectric strength equal to 60 kV / mm.

[0169] Each electrode 6, 7 further comprises a flexible tab 62, 72 which emerges from the main portion 60, 70 and which is bent to fit the lateral edge of a plate 5. As shown in FIG. 8A, the tabs 62, 72 of the same lateral edge overlap one another so as to form one of the output terminals 8, 9.

[0170] Whatever the embodiment, each intermediate electrode 6, 7 is sealed between two plates 5 with channels 50 so as to constitute a fluid-tight block between plates with respect to the fluids (gas to be purified, cleaning liquid) and electrically insulating between two intermediate electrodes. The electrical insulation of the block provided by the sealing prevents any disruptive breakdown by surface conduction between adjacent electrodes 6, 7. In the example of FIGS. 5 and 5A, two solid plates 16, made of electrically insulating material, are arranged on either side at the ends of the stack. Thus, once the stack is assembled with the sealing of all the electrodes 6, 7 and the intercalated plates 5 with channels 50, and the end plates 16, the collector 3 is in the form of the sealed and electrically insulating block.Such a block can be easily unclogged / cleaned, particularly in a dishwasher if its dimensions allow it, or in any other washing device.

[0171] To demonstrate the performance of a honeycomb electrostatic collector 3 of the type described in Figure 4 and 4A, which is in the form of a cube with an edge of 12 cm, the inventors produced a filtration system 1 with a double-stage geometry, i.e. an ionizer 2 and a honeycomb electrostatic collector 3 downstream of the ionizer. Such a double-stage system is shown in Figure 6.

[0172] The collector 3 is therefore arranged in a sealed channel of rectangular section 10.

[0173] A high voltage box 11 supplies one of the output terminals 8.

[0174] The other output terminal 9 is grounded, at zero potential.

[0175] The high voltage (HV) generator 11, powered by 12 volts DC, delivers a negative high voltage of 6500 volts or more depending on the requirement. In the example illustrated, a single HV generator is used, for reasons of simplicity. However, two separate generators are possible, in particular to supply the collector 3 at a very high voltage.

[0176] The HT generator 11 also powers a carbon fiber ionizer 2 whose principle and ion diffusion charging performance are widely described in the literature: see in particular in publication [7]. The high voltage (HT) powering the carbon fiber ionizer 2 must remain fairly limited to avoid the formation of ozone in too high a concentration, especially if the air thus filtered must be recycled in occupied interior spaces.

[0177] Such an ionizer 2 of great simplicity and whose cleaning will be discussed later, makes it possible to efficiently charge particles by diffusion of unipolar ions while producing extremely little or no ozone. Any other type of charger can also be used, in particular with conventional wires. The carbon fiber ionizer 2 is arranged in the center of a volume which is delimited by two metal grids 12 on the one hand, the four sides of the parallelepiped 13, portion of the channel 10, on the other hand, the whole being connected to ground. The ionizer 2 can be oriented in the opposite direction of the flow, as illustrated in Figure 5, or preferably in the opposite direction to avoid impaction or interception of particles at the carbon fibers.

[0178] A fan 14 blows air from the experimental room into system 1, the inventors of which want to measure the filtration efficiency with respect to particles.

[0179] The inventors carried out the tests with aluminum sheets of thickness equal to 12 pm as electrodes 6, 7 and plates 5 marketed to date with the following characteristics:

[0180] - thickness x length x width: 5 mm x 2.6 m x 35 cm;

[0181] - material: PVC;

[0182] - cross-section of a channel 50 extending over the entire length of the plate: 7.2 mm x 4.3 mm;

[0183] - wall thickness separating two channels 0.35 mm.

[0184] By measuring the concentration C of particles larger than 0.3 pm using a suitable counter, upstream of Camont and downstream of Caval, the inventors deduced the filtration efficiency of precipitator 3, i.e., in %:

[0185] Ec = (Camont - Caval / Camont) x 100

[0186] The inventors have achieved an efficiency that can reach 100% and can be adjusted according to the application, which is maintained over time. Indeed, it takes several hundred hours of operation to begin to see a drop in efficiency, initially linked to fouling of the ionizer.

[0187] Furthermore, the inventors found that a high DC voltage of up to 20 kV can be established between electrodes without disruptive breakdown, while the thickness between electrodes is only 5 mm, which clearly shows the advantage of this alveolar collector compared to electrostatic precipitators with metal plates which would not withstand such an intense electric field. Figures 7 and 8 illustrate the transfer of electric charges as well as the collection which occur within channels 50.

[0188] As seen in Figures 7 and 8, electrode 6 is negative and electrode 7 is positive. The potential difference applied between these electrodes creates an electric field E within the channels 50. In the upper channel 50, the electric field E is directed from top to bottom while in the lower channel 50, the field is directed from bottom to top.

[0189] The incident particles P which are previously negatively charged are deposited on one of the longitudinal walls of a channel 50 along the latter according to their electrical mobility, which depends in particular on the number of charges they carry and their size. The electrical charge of each particle P is dissipated by passing through the thickness of one of the plates 5 due to its sufficient electrical conductivity, although very low, and is then collected by the positive electrode 7.

[0190] Another example of an air filtration system 1 incorporating a carbon fiber ionizer 2 and a downstream collector 3 is shown in Figure 9.

[0191] Ionizer 2 and collector 3 are here arranged vertically with collector 3 on top of ionizer 2.

[0192] The filtration system 1 is crossed by a flow of air sucked in by several fans 14 in fluid parallel.

[0193] As symbolized by the arrows, the air flow is introduced below the housing 15 through a grid in the lower part, acting as a pre-filter, the purified air being extracted vertically on top of the housing 15.

[0194] With a number of three 14 fans in fluidic parallel, the air flow rate (CADR, English acronym for "Clean Air Delivery Rate") purified of any particle greater than 0.3 pm is 150 m 3 / h.

[0195] A panel 17 of the box 15 groups together various interfaces, including:

[0196] - an on-off button,

[0197] - a particle concentration display,

[0198] - buttons to switch to pg / m counting 3 (PM10, PM2.5 and PMI) or particle size spectrum in the range of 0.3 to 10 pm, - a button to implement internal disinfection by UVC radiation and / or ozone generation.

[0199] 18 LED strips light up according to the standardized color code for PM2.5 concentrations.

[0200] The flow rate of the fans 14 is preferably controlled by the measured concentrations, which may even lead to the fans being stopped if the air is particularly well purified.

[0201] The purifier 1 can be connected in particular by a wireless communication protocol, Wi-Fi local network or mobile network, on the one hand for remote preventive action in order to subsequently enter a pre-purified space, on the other hand to collect data and monitor the purification efficiency over time, in particular to plan cleaning if a drop in efficiency appears.

[0202] At full power, the noise, including that of the fan, is very low, typically less than 30 dB. Consumption is also very low, 4.5 W (375 mA at 12 V). This low consumption allows a battery power supply, here integrated under the panel 16. Such a purifier is therefore entirely autonomous, easily moved around a room without the inconvenience of being connected to a power cable to the mains. An autonomy of 6 to 8 hours is easily envisaged. The battery is recharged by a suitable connector 19.

[0203] The filtration system 1 in Figure 9 is rather dedicated to purifying the air in larger volumes than vehicle interiors, for example in living rooms.

[0204] A filtration system 1 can be used in many applications in indoor atmospheres, i.e. enclosed spaces in the broad sense, whether they are places open to the public (transport, administrations, schools, hospitals, sports halls and cinemas, etc.), professional buildings (offices and shops) or private spaces (individual or collective housing).

[0205] In addition to these applications cited in the field of air filtration at ambient temperature in indoor spaces, other applications can be envisaged, such as the purification of aerosols in industrial process gases, the treatment of air at lower flow rates (personal protective equipment (PPE)) or at higher flow rates (ventilation ducts), the filtration of air in ventilated road or railway tunnels, or in underground stations for example.

[0206] It is also possible to consider implementing electrostatic collectors according to the invention within equipment or living spaces in environments that are very restrictive due to their atmosphere. For example, life bases in desert environments or NBC (acronym for Nuclear, Biological, Chemical) personal protection systems can be considered as applications.

[0207] In terms of filtration efficiency, an optimized alveolar electrostatic collector alone offers performance close to that of a HEPA filter.

[0208] Thus, at the nominal suction flow rate of approximately 60 m 3 / h for a 12x12x12 cm cube 3 consistent with the collector shown in Figures 4 and 4A, such a honeycomb electrostatic collector has a filtration efficiency close to 100% for particles of sizes between 50 nanometers (0.05 pm) and 10 micrometers (10 pm), as illustrated in Figure 10.

[0209] Measurements for fine and ultrafine particles down to 10 nm were carried out using an electrical mobility spectrometer (or "Scanning Mobility Particle Sizer" in English with the acronym SMPS) while those for larger particles (notably greater than 0.2 pm) were carried out using an Optical Particle Counter (OPC).

[0210] From this figure 10, a decreasing filtration efficiency also emerges for flow rates greater than 60 m 3 / h on the one hand, for particles of size < 0.05 pm on the other hand. Figures 11 A to 11D schematically represent the efficiency of each of the main filtration elements of a filtration system as a function of the particle size.

[0211] With an alveolar electrostatic collector with an ionizer arranged upstream as presented in this filtration system according to the invention, it will always be possible to determine optimal operating conditions which will make it possible to obtain collection efficiency curves of the type shown schematically in Figure 11 A, namely: collection efficiency close to 100% for particles > 0.05 pm, decreasing collection efficiency for particles < 0.05 pm but generally > 90%

[0212] The charging efficiency of particles smaller than 0.05 pm by unipolar ion diffusion is less efficient: in fact, only a fraction of these particles is correctly charged. At this small size, the probability that the particles acquire an electric charge is not 100%. In other words, some of these particles are electrically neutral and, therefore, cannot be trapped by the electric field in the alveoli.

[0213] Curve A in Figure 11B shows the efficiency of a conventional fiber prefilter. This Figure 11B is taken from the work [8]. A minimum efficiency is observed for particles of size close to 0.1 to 0.3 pm, where the diffusion and interception deposition mechanisms are the weakest. This is explained by the particle deposition mechanisms on the fibers, where there is competition between the particle deposition mechanisms by Brownian diffusion for the finest particles on the one hand, and deposition by impaction and interception for the larger particles on the other hand.

[0214] Curve B in Figure 11B shows the efficiency of the same fiber prefilter when the particles are electrically charged by ionizer 2, the charged particles being more easily captured on the fibers of the prefilter under the effect of electrostatic forces. The efficiency of the prefilter is thus increased. In other words, this curve B, with ionization, schematically shows that the thermophoresis deposition mechanism significantly increases the collection efficiency of the prefilter as already reported in the state of the art [4],

[0215] Figure 11C illustrates the result of the combination of an ionizer, a prefilter and a honeycomb electrostatic collector, the efficiency of this entire filtration system reaching almost 100% efficiency over the entire particle size range studied, i.e. between 0.01 pm and 10 pm. Thus, this curve C gives the efficiency of the honeycomb collector, without prefilter, when the particles are electrically charged. It is visible that the collector efficiency tends to drop for ultra-fine particles (see experimental results in Figure 10). This comes from the fact that ionizer 2 is not capable of electrically charging very fine particles, a well-known phenomenon elsewhere. A proportion of them remains neutral and therefore cannot be trapped by any electrostatic technique.

[0216] Curve D in Figure 11D is the characteristic curve of a filtration system according to the invention with ionizer, prefilter consisting of fiber filter downstream of the ionizer and alveolar collector downstream of the prefilter. We see that the efficiency of such a system is close to 100% over the entire particle size range.

[0217] According to the invention, such a system therefore makes it possible to obtain very high efficiency but with a pressure drop much lower than that of a HEPA filter. For the same treated flow rate, this pressure drop typically remains less than 100 Pa with a system according to the invention, whereas it is typically between 250 Pa for a HEPA filter according to the state of the art, in new condition and is worth 500 Pa or more with a HEPA filter according to the state of the art, in a clogged or close to clogged state.

[0218] Consequently, by implementing a filtration system according to the invention, operating savings are possible because the depressurizing device (the blower or extractor fan) consumes much less energy than with a HEPA filter according to the state of the art.

[0219] Other variations and improvements may be made without departing from the scope of the invention.

[0220] For example, a cellular electrostatic collector 3 can be made with a small inlet section S and a long length L.

[0221] Indeed, the collector according to the invention has the advantage of being easily integrated into structures of various shapes, whether cylindrical or, preferably, rectangular.

[0222] Furthermore, for a given flow rate, the flow resistance or pressure drop is very low through the alveolar medium, unlike that encountered in fibrous filter media according to the state of the art. In certain cases, it is therefore particularly advantageous to integrate or even completely produce an electrostatic collector according to the invention inside an already existing air circulation duct to benefit from its volume, which is most often already available.

[0223] Figure 12 shows an example of the integration of an electrostatic collector 3 in a HVAC (Heating, Ventilation, Air Conditioning) duct of premises used for air filtration purposes. More precisely, here the electrostatic collector 3 is integrated into a network R for distributing fresh air in an office space. As illustrated, it is known that to save space in attics or between partitions, extra-flat PVC ducts 100 are increasingly used to convey the various air flows.

[0224] In practice, the inventors carried out a demonstration experiment using a conduit 100 in which an electrostatic collector 3 is arranged as illustrated in figure 13. The rectangular section of the conduit 100 has a standard length x thickness section equal to 220 mm x 55 mm.

[0225] To integrate an electrostatic collector 3 inside this standard conduit 100, a number of six channel plates 5, made of PVC, 8 mm thick and with a length L equal to 1 m, were inserted inside the conduit 100. Thus, the device comprising the electrostatic collector 3 delimited externally by the wall of the standard conduit 100 comprises a total of 84 channels, electrically connected according to the invention.

[0226] The experiment was carried out to determine the evolution of the efficiency of this device with respect to atmospheric particles of size mainly between 0.2 pm and 10 pm.

[0227] The schematic of the experimental setup is shown in Figure 14.

[0228] An atmospheric aerosol is pulsed into a volume 10 by a fan 14. The aerosol particles are electrically charged by an ionizer 2 brought to a voltage Ui of -6500 volts. A part of the aerosol flow is introduced with a flow rate Q2 = 80 m 3 / h in device 3, 100 whose electrodes are subjected to a collection voltage Uc = Ui = - 6500 volts.

[0229] The collection efficiency E is determined twice a day by measuring the upstream Cl and downstream C2 concentration at device 3, 100: E = 1 - C2 / C1.

[0230] The results of the experiment are shown in Figure 15. It was found that an efficiency greater than 99% was maintained for several hundred hours.

[0231] It is observed that over time a progressive fouling appears, particularly at the level of F ionizer. The collection efficiency E remains higher than approximately 95% for continuous operation for more than 900 hours.

[0232] Considering that in a truly commercialized installation, the fresh air is first conditioned and filtered in an air handling unit (AHU) before being blown into the premises, the fouling of the device 3, 100 will be all the less if the quality of the pre-filters mounted upstream in the AHU is good.

[0233] Thus, as regards the application to the filtration of air in the HVAC ducts of premises, it can be considered that a section of a duct 100 equipped with a filtration system 1 according to the invention (ionizer 2 followed by a pre-filter 40 and a collector 3) could very effectively filter particles of all sizes, as seen previously, and this for a long period before cleaning. The same would apply after replacement by a new equipped section or by another section having been cleaned beforehand.

[0234] The integration of system 1 in ventilation ducts, in addition to the action of an AHU, is therefore particularly advantageous for improving the air quality in premises with regard to fine particles down to nanometric sizes, which no filter in a conventional AHU is capable of ensuring, except by using HEPA filters as in hospitals or certain cutting-edge industries, to be disposed of when they are clogged, and whose purchase and operating costs are also high.

[0235] Concerning the fouling of the device as a function of time, in addition to the experimental results shown in Figure 15, the curve in Figure 16 allows us to understand how to optimize the cleaning of the ionizer and / or the collector. The experiment consists of sampling atmospheric air with a new alveolar collector 3 preceded by a new ionizer 2, in the configuration described in Figure 6. The power supply of the ionizer and the collector is carried out using a high negative voltage box providing -6500V. These are very long-term tests to observe certain phenomena. It should be noted here that there is no pre-filter in the test setup, neither upstream of the ionizer nor upstream of the collector, in order to accelerate the fouling of the devices to be studied.

[0236] In this figure 16, a first zone corresponds to the operation of the collector without intervention (Zone 1). In A, the experiment is stopped and the carbon fibers of the ionizer are cleaned with a brush. Then the tests continue (Zone 2). In B, the experiment is stopped again and the collector channels are cleaned using small swabs. Then, the tests continue (Zone 3) and are stopped after a thousand hours of operation.

[0237] Initially, the efficiency is above 98% for about 150 hours, which gradually drops, reaching 50% around 600 hours of operation. After cleaning the ionizer's carbon fibers, the efficiency temporarily rises to around 75%.

[0238] After swabbing the inside of the collector channels, the efficiency rises to around 95%, which suggests that cleaning in a liquid medium, for example in a dishwasher for small collectors, would be even more effective.

[0239] It is recalled that when a negative tip corona ionizer is supplied with high negative voltage, a deposit of ammonium nitrate (NH4NO3) forms on the surface of the tip and degrades its profile, which leads to a drop in ionization intensity. In the experiment reported here, the carbon fibers are particularly whitened by the ammonium nitrate deposit, which very probably explains the drop in efficiency observed in Figure 16 in zone 1 and the increase in efficiency at the beginning of zone 2.

[0240] Knowing that ammonium nitrate is a crystalline solid at room temperature, that it melts at 170°C and that it decomposes from 210°C, a technique for cleaning the tips of negative electrodes would consist of heating them to a temperature above 210°C, for example 230 to 250°C. This operation could be carried out sequentially, at a rate to be defined according to the need, and for certain applications requiring continuous and stable operation for very long periods.

[0241] To do this, the inventors have envisaged a heating device 22 for the ionization tips 21, which operates passively, and which is arranged around them. Such a device 22 can operate for example by induction (figure 17A) or by Joule effect (figure 17B).

[0242] Other techniques for cleaning ionization tips can be proposed. They have already been tested and are effective (dusting by mechanical means, cleaning by compressed air pulse) and can be automated.

[0243] As shown in Figure 18A, an additional ionizer positioned at different 2', 2” relative to the reference position, in other words relative to the position in the ventilation duct 100, can be added to the main ionizer 2: it is therefore possible to have an additional ionizer upstream or downstream of the prefilter 40.

[0244] The following experiments were carried out in a ventilation duct where a flow rate of 300 m circulates 3 / h. They relate more specifically to the application of system 1 according to the invention to the filtration of air in the passenger compartment of vehicles. These results can be extended to any other application.

[0245] An experimental 40 cabin filter has a cross-section of 21 cm x 20 cm and a thickness of 3 cm.

[0246] The alveolar collector 3 has the same section and a length of 15 cm. It comprises a stack of alveolar plates 5, i.e. with channels, made of polypropylene, 4 mm thick, between which are placed the electrodes 6, which are 12 μm thick aluminum sheets.

[0247] Concerning the reference position, Figure 18B gives the filtration efficiency E at different points of a filtration system according to the state of the art and according to the invention, for different possible component integrations. The efficiency measurements, with respect to the atmospheric aerosol for particles larger than 0.18 pm, were carried out with an optical particle size counter marketed under the name Fidas Frog by the company Palas Gmbh. The ionization is carried out by a brush 2 of carbon fibers 20, powered at -6500 V.

[0248] As illustrated in this figure 18B, the efficiency measures (E in %) are as follows:

[0249] - Cabin filter, without ionizer, according to the state of the art: E = 30%

[0250] - Cabin filter, with upstream ionizer: E = 60%

[0251] - Alveolar collector, with upstream ionizer: E = 99.6%

[0252] - System according to the invention, i.e. with a cabin filter and a plate-channel alveolar collector, and ionizer arranged between them: E = 99.85%.

[0253] An arrangement of an additional ionizer 2' in a ventilation duct 100, between the filter 40 and the collector 3, makes it possible to further increase the efficiency of the collector by additionally recharging the particles, while ensuring a longer service life for the main ionizer 2 because the air circulating in its vicinity has already been pre-filtered by the pre-filter 40.

[0254] An arrangement 2' of an additional 2” ionizer upstream of the ventilation duct 100, it makes it possible to further increase the performance of a system as illustrated by figure 18C:

[0255] - Cabin filter, without ionizer, according to the state of the art: E = 30% - Cabin filter, with ionizer: E = 90%

[0256] - Alveolar collector, with ionizers 2, 2't: E = 100%

[0257] - System according to the invention, i.e. with a cabin filter and a plate-channel alveolar collector, and 2.2” ionizers: E = 100%.

[0258] In view of these results, it appears that a 3 alveolar collector without an upstream cabin filter 40, with ionizers, could achieve the desired objective, in other words achieve the efficiency of a HEPA filter according to the state of the art.

[0259] According to the invention, to preserve the life of the alveolar collector 3, the pre-filter 40 is provided upstream of the collector, in particular a pre-filter consisting of a passenger compartment filter for automobiles currently on the market, with a main ionizer 2 upstream, and preferably an additional ionizer 2” arranged between the pre-filter 40 and the collector 3.

[0260] Thanks to this, we can consider having a 3-cell manifold that does not require maintenance and therefore could have the lifespan of a motor vehicle. Only the cabin filters, as pre-filters, would need to be changed periodically as recommended today by car manufacturers and equipment suppliers.

[0261] Other experiments were carried out in the same configuration as that of figure 18 A, but with alveolar collectors 3 of the same section and of lesser thickness to seek to understand the mechanisms of deposition of charged particles in the first centimeters of penetration within the alveoli delimited by the channels 50 of the plates 5.

[0262] Two very flat collectors were tested, one with a thickness of 3 cm (identical thickness to the aforementioned cabin filter), the other with a thickness of 4.7 cm. A selection of efficiency results for the 4.7 cm thick collector is shown in Figures 19A and 19B. It should be noted that in these tested collectors, the internal electrodes in the direction of flow are only 3 cm long, to avoid disruptive breakdowns between electrodes on the inlet and outlet faces, and that the flow rate in the 21 cm x 20 cm section is 150 m 3 / h.

[0263] Figure 19A shows the evolution of the efficiency of this very flat collector as a function of time. While the efficiency is continually decreasing, we note that periods of inactivity (21 h, then 12 h) allow significant efficiencies to be temporarily recovered after these periods of inactivity. Figure 19B shows the evolution of the efficiency of the same collector, as a function of time, by instantaneously permuting the polarity of the electrodes. For a fixed polarity, as in Figure 19A, the efficiency is decreasing as a function of time. But, in all cases, an immediate and very significant increase in efficiency is observed when the polarity of the electrodes is reversed.

[0264] . The implementation of these very flat collectors makes it possible to further increase their intrinsic collection efficiency by controlling the electrical voltage and polarity, as shown in Figure 19B, i.e. by introducing a shutdown period followed by an operating period, and / or a change in polarity of the electrodes. With such control of the electrodes, for certain applications, such as the filtration of a motor vehicle passenger compartment which, by definition, does not operate continuously, it is possible to envisage automatic management of the optimal operation of the filtration system according to the invention.One or more particle concentration sensors can provide the data to ensure this optimal operation, which should lead, for example, to a filtration efficiency of a HEPA level for minimal energy expenditure at the level of the "HVAC" block of a motor vehicle (acronym for Heating, Ventilation and Air-Conditioning), also called the "CVC" block (acronym for Chauffage, Ventilation, Climatisation).

[0265] In the automotive sector, or transport in general, another advantageous example of integration of a filtration system 1 according to the invention is located in the air distribution ducts downstream of the HVAC unit. This integration can, if necessary, be combined with advantageous control as described above. This integration can be envisaged in particular if the space available in the HVAC unit itself is too limited to position the filtration system described above.

[0266] In a modern motor vehicle, there is a fairly dense network of ducts ensuring the distribution of air in the passenger compartment. This network is particularly discreet visually in the passenger compartment of a vehicle, since it is only detectable by its ends, materialized by the air diffusion vents, judiciously distributed, as illustrated as an example in figure 20.

[0267] Even more advantageously, the inventors propose to use the air distribution ducts 101, elongated and very flat in shape, by adding a filtration function by integrating one or more collectors 3 according to the invention. In other words, the ducts 101 could become ducts with a filtration function as desired, that is to say only in the case of applied electrical voltage. The inventors believe that a filtration efficiency greater than 99% for particles of sizes included in a very wide range, between 0.05 and 10 pm, is expected.

[0268] Concretely, an active mode of operation in filtration could advantageously be activated for conduits 101 during episodes of high atmospheric pollution, for example in the event of exposure to high concentrations of fine particles in a traffic jam in an urban environment. This active mode, in passenger compartment recycling or not, could also be activated automatically if a predetermined threshold is exceeded.

[0269] It is possible to envisage integrating one or more electrostatic collectors according to the invention into all types of modern vehicles, regardless of their comfort range. In particular, they can advantageously take any form to adapt to the constraints imposed by the surfaces and volumes available in a vehicle passenger compartment.

[0270] Furthermore, if the cabin filter itself is highly efficient, in particular 97% to 98% efficient for particles larger than 0.5 pm, the lifetime of conduits 101 incorporating one or more electrostatic collectors according to the invention could correspond to the lifetime of the vehicle because the fouling which would be due to the ultrafine particles collected, even in very large numbers, could not be problematic.

[0271] It is also possible to consider applying the invention to means of collective transport (buses, metros, trains, planes, etc.), and this would be even more applicable since integration into ducts with an elongated profile would be particularly suitable.

[0272] In addition to the above, it is recalled that a laminar flow in an electrostatic filter is preferable to a turbulent flow in terms of collection efficiency (Figure 2). Consequently, a passenger compartment duct 101 according to the state of the art, with an empty interior volume traversed by a turbulent flow, collects fewer particles than a duct 101 integrating cell plates 5 of a collector 3 according to the invention traversed by a laminar flow. Figure 20 schematically represents the assembly that could be used in a vehicle passenger compartment for air treatment: the air to be purified is first pulsed at the flow rate Q and the aerosol particles are electrically charged in an ionizer 2. This air is then pre-filtered by a known passenger compartment filter 40, then distributed in a network of ducts passing through electrostatic collectors 3 according to the invention, at flow rates Qi.

[0273] Another application of the filtration system 1 can be considered, for the reduction of the concentration of fine particles in underground environments where people are likely to breathe very dusty air.

[0274] Figure 22 shows the arrangement of an air filtration unit 1000 intended to be installed on a metro platform. The illustrated unit 1000 comprises a number of four filtration systems 1, preferably identical, in the form of modules, mounted in fluid parallel. The inlet section of a module 1 is 50 cm x 50 cm. Its overall length is 90 cm.

[0275] Each module 1 includes, in the direction of flow:

[0276] - a large mesh screen 41 allowing the trapping of relatively large objects, which are suspended in the air, such as leaves, long fibers, etc.,

[0277] - a carbon fiber ionizer 2,

[0278] - a mechanical pre-filter 40 with average filtration efficiency, downstream of the ionizer 2

[0279] - two alveolar collectors 3 with channel plates, preferably identical, joined to each other downstream of the prefilter 40. An alveolar collector 3 has a unitary section equal to 25 cm x 50 cm, a depth of 25 cm and has a weight of 7 kg,

[0280] - a fan 14 capable of sucking, when empty, a flow rate of 4500 m 3 / h.

[0281] Without prefilter 40, block 1000 can treat 15000 m 3 / h of air with an efficiency of 98% against particles larger than 0.18 pm. These measurements were carried out using an optical particle size counter marketed under the name Fidas Frog (by the company Palas Gmbh.

[0282] With prefilter 40, depending on its pressure drop and therefore its intrinsic efficiency, a 1000 block can treat approximately 10,000 m 3 / h with HEPA efficiency and a pressure drop that does not exceed 100 Pa. Figure 23 represents the filtration system 1 as described in Figure 21, arranged in a soundproof box 2000 intended to be installed in a space receiving the public, in particular a metro platform in an underground environment. The box 2000 housing the block 1000 is protected by the grids 41 ensuring physical protection. As shown in this figure 23, an additional ionizer 2” can be arranged, making it possible to significantly increase the particle collection efficiency.

[0283] Figure 24 shows 3-plate alveolar collectors, in accordance with the invention, positioned near the tracks V where the trains of a metro pass, therefore in the immediate vicinity of the emission of particles due to braking, which is a major source of pollution for users leaving the train or for those present on the platform when the train arrives at the station.

[0284] These 3 collectors can operate in passive mode (without fan), because the air flow rates in the collectors are created by the air movements brought by the train traffic and the associated piston effect.

[0285] In this underground application, several embodiments can be considered.

[0286] In a first embodiment, no ionizer(s) are provided. Indeed, the particles emitted by friction which come from the brakes of a metro are naturally electrically charged, which may prove sufficient to allow the collection of particles in the cells of the channel plates of the collectors 3.

[0287] In a second embodiment, an ionizer 2 is arranged upstream of a collector 3, i.e. upstream in the direction of travel of a metro train. An ionizer 2 thus guarantees that all particles are artificially charged, regardless of their natural initial charge, which may be insufficient.

[0288] In a third embodiment, an ionizer 2 is arranged on either side of a collector 3. This makes it possible to adapt the filtration system to the possible double direction of air flows, i.e. in the direction of circulation of a train or in the opposite direction.

[0289] In a fourth embodiment, a collector filtration system 3, operating passively (without fans) can be advantageously deployed on a very large scale to treat very high flow rates, for example at the ventilation outlet of underground galleries to purify the air before releasing it into the atmosphere. The flow rates to be treated are expressed, in this case, in hundreds of thousands of m 3 / h.

[0290] In all the advantageous embodiments illustrated, the ionizers 2, 2', 2” comprise carbon fibers 21. Any small electrically conductive fibers or any element making it possible to produce ions without generating ozone, typically from 10 to 100 pm, can be considered.

[0291] Some high-voltage boxes used typically deliver -6500 volts in the examples cited. It has been shown that the results are even better for higher voltages, for the power supply of the ionizer and / or the collector. A practical limit of around 10 to 15 kV seems reasonable to place oneself well below the risks of disruptive breakdowns.

[0292] The high voltage boxes used in the examples cited deliver a negative polarity, which offers the important advantage of producing a higher concentration of ions than boxes with positive polarity. Alternatively, boxes delivering a high voltage with positive polarity can be considered, which has the advantage of less ozone creation and no creation of ammonium nitrate at the ion emissive peaks. The use of carbon fiber ionizers produces practically no ozone due to the fineness of the fibers: [9].

[0293] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants.

[0294] A filtration system according to the invention may be suitable for any air filtration, whether indoors, outdoors and more generally for all types of atmospheres with little or low dust content such as the passenger compartment of a means of transport, such as a motor vehicle.

[0295] List of cited documents

[0296] [1]: 2016 Public Health France Study.

[0297] [2]: Exploratory study of the socio-economic cost of indoor air pollutants, April 2014, ANS ES -CS TB study report. [3]: Goyer GG, Gruen R., LaMer VK, “Filtration of Monodisperse Electrically charged Aerosols”, Journal of Physical Chemistry, Vol 58, No. 2, pp 137-142 (1954).

[0298] [4] : Feng, Z., Long, Z., Yu, T., “ Filtration characteristics of fibrous filter following an electrostatic precipitator” , J. Electros., 83, 52-62 (2016). [5] : "Residential Air Cleaners - A technical Summary” , 3rd edition, EPA 402-F-09-002

[0299] (2018).

[0300] [6]: Shi, B., Ekberg. L.,” Ionizer assisted air filtration for collection of submicron and ultrafine particles”, Environ. Sci. Technol., 49, 6891-6898 (2015).

[0301] [7] : Han B., Kim Y.J., Sioutas C., "Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers”, Aerosol Sci. Technol, 42 : 793-800 (2008).

[0302] [8] : HINDS, Aerosol Technology, Second Edition, page 199, (1999).

[0303] [9] : Kim, H.J., Han, B., Woo, C.G., Kim Y.J., “Ozone emission and electrical characteristics of ionizers with different electrode materials, numbers, and diameters” , IEE Transaction on Industry Applications, 53, 1, January / February (2017)

Claims

Claims 1. Filtration system, intended for the filtration of the air, in particular of a passenger compartment of a means of transport, such as a motor vehicle, comprising: at least one ionizer (2), called the main ionizer, of the particles likely to be present in the air, comprising, as a device for ionization by corona effect of the electrically conductive fibers (21), at least one mechanical filter or a foam, as a prefilter, arranged downstream of the ionizer (2), at least one electrostatic precipitator / collector (3), arranged downstream of the prefilter and comprising: • at least one channel plate (5) made of substantially electrically insulating material, extending along a longitudinal axis (X), the channels (50) of the plate being fluidically separated from one another, each channel extending along the longitudinal axis (X) and opening at its two opposite ends, towards the outside of the plate; • at least two electrically conductive sheets (6, 7) or plates (8, 9) or layers, forming electrodes of which at least one portion is arranged on each of the main faces of the channel plate, the two electrodes being adapted to generate an electric field (E) in a direction orthogonal to the X axis in the channels of the plate.

2. Filtration system according to claim 1, comprising means for regulating the electrical voltage to the main ionizer, and preferably simultaneously, to the electrodes of the electrostatic precipitator(s) / collector(s).

3. Filtration system according to claim 1 or 2, comprising an additional ionizer, as an electrical pre-charger, arranged upstream of the main ionizer.

4. Filtration system according to one of the preceding claims, comprising an electrical discharge device by grounding the electrode(s) of the electrostatic precipitator.

5. Filtration system according to one of the preceding claims, the electrostatic precipitator / collector (3) comprising a stack of channel plates, an intermediate electrode (6, 7) being interposed between two adjacent plates in the stack, all the intermediate electrodes of the same polarity being connected together to an output electrode forming an output terminal (8, 9).

6. Filtration system according to claim 5, the intermediate electrodes of the same polarity each further having a portion (60, 70) which fits the lateral edge of a plate while the output terminals each have a general L shape of which one branch (80, 90) fits the lateral edge of the stack of plates while being in contact with the portions of the electrodes and the other branch (81, 91) fits one of the main faces of the plate at one of the ends of the stack.

7. Filtration system according to claim 5, the intermediate electrodes of the same polarity each comprising a tab or other flush contact point (62, 72) which fits the lateral edge of a plate, the tabs of the same lateral edge overlapping one another so as to form one of the output terminals (8, 9).

8. Filtration system according to claim 7, comprising two end plates (16) made of electrically insulating material, arranged on either side of the stack.

9. Filtration system according to one of claims 5 to 8, the intermediate electrodes being in the form of metal sheets or metallized deposits each covering at least in part a main face of a plate.

10. Filtration system according to one of claims 5 to 8, each intermediate electrode being sealed between two channel plates so as to constitute a fluid-tight block between plates with respect to the fluids and electrically insulating between two intermediate electrodes.

11. Filtration system according to one of the preceding claims, the material of the plate(s) having a transverse electrical conductivity greater than 10 14 S / m at room temperature.

12. Filtration system according to one of the preceding claims, the material of the plate(s) being a thermoplastic polymer which can be extruded, in particular poly(vinyl chloride) (PVC), polycarbonate (PC), polypropylene (PP) or others.

13.

14. Filtration system according to one of the preceding claims, the channels (5) being of square or rectangular cross-section with a side length of less than 10 mm, preferably between 2 and 4 mm.

15. Filtration system according to one of the preceding claims, the ionizer fibers being carbon fibers.

16. Filtration system according to one of the preceding claims, comprising at least one fan arranged upstream or downstream of the main ionizer or downstream of the electrostatic precipitator / collector.

17. A filtration system according to claim 16, the channel plate(s) of the electrostatic precipitator / collector being arranged vertically, the fan(s) being arranged on top of the channel plate(s), the system further comprising a mesh below the ionizer as a prefilter.

17. Filtration system according to one of the preceding claims, comprising a battery and a housing housing the components constituting a self-contained device.

18. Method of operating a filtration system according to one of the preceding claims comprising the following steps: a / automatic detection or voluntary action of at least one individual in a passenger compartment of a means of transport; b / in the event of automatic detection or voluntary action of an individual according to step a / , application of an electrical voltage to the main ionizer, and preferably simultaneously, to the electrodes of the electrostatic precipitator(s) / collector(s).

19. Operating method according to the preceding claim, the voltage applied according to step b / to the main ionizer and / or to the electrostatic precipitator(s) / collector(s) being between 5 and 15 kV. 20 Operating method according to claim 18 or 19, the electric field generated according to step b / in the plate channels of the electrostatic precipitator(s) / collector(s) being greater than or equal to 20 kV / cm.

21. Operating method according to one of claims 18 to 20, comprising, during step b / , a step bl / , of reversing the polarities of the electrodes of the electrostatic precipitator(s) / collector(s) such that those initially placed at electrical ground are placed at the potential of the electric field to be generated and vice versa.

22. Operating method according to one of claims 18 to 21, the Reynolds number (Re) of air flow in the plate channels of the electrostatic precipitator(s) / collector(s), during step b / , being less than or equal to 23. Operating method according to one of claims 18 to 22, comprising, during step b / , a step b2 / of supplying clean air or other neutral gas, for example N2, to the main ionizer and / or occasional heating of the latter.

24. Operating method according to one of claims 18 to 23, comprising, even in the event of non-automatic detection or voluntary action by an individual according to step a / , and in the absence of step b / , a step c / of electrically grounding the electrodes of the electrostatic precipitator(s) / collector(s).

25. Operating method according to one of claims 18 to 24, comprising, during step b / , a step b3 / of regulating the electrical voltage at the ionizer, and preferably simultaneously, at the electrodes of the electrostatic precipitator(s) / collector(s) as a function of the air flow rate to be treated.

26. Means of transport, in particular motor vehicle, comprising: - one or more air distribution ducts in the passenger compartment of the means of transport; - at least one filtration system according to one of claims 1 to 17 with at least one electrostatic precipitator / collector arranged in the section of one or more of the air distribution ducts.

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