Electroseparator with at least an approximately point-shaped spray electrode and spray ionisation source

Abstract

The invention relates to an electroseparator (10), through which an airflow to be purified of particles flows, comprising an ioniser unit (26), which has one or more approximately point-shaped spray ionisation sources (40) which consist of conductive fibre filament clusters, formed in particular of carbon filaments, and comprising a collector unit (12), arranged downstream of the ioniser unit, for particle separation. The spray ionisation sources are operated with relatively high ioniser potentials ≥ 8 kV, preferably ≥ 10 kV or more. In order to reduce or eliminate wear that occurs in the medium to long term on account of discharge-induced streaming effects, the filaments of the spray ionisation sources are at least partially metal-coated, in particular nickel-coated, whereby streaming effects can be significantly reduced / or even avoided entirely.

Description

[0001] Electrostatic precipitator with at least one approximately point-shaped spray electrode and spray ionization source

[0002] The present invention relates to an electrostatic precipitator with approximately point-shaped discharge electrodes. In particular, the invention relates to an electrostatic precipitator with reduced discharge-induced degeneration phenomena through modification measures on the discharge electrodes.

[0003] Furthermore, the invention relates to a spray ionization source for use in such an electrostatic precipitator.

[0004] WO 2021 / 185418 A1 discloses a generic electrostatic precipitator optimized in terms of efficiency, size, power consumption, and ozone emissions. It can be used as a mobile stand-alone device or as a component of air conditioning and ventilation systems, e.g., decentralized or centralized room ventilation systems. A wide range of industrial applications are also conceivable in the field of air purification and, in principle, also in the purification of other gases, e.g., in flue gas purification, or even in the automotive sector (e.g., HVAC systems for vehicle interiors).

[0005] In recent years, it has been recognized what serious impact airborne particles with an aerodynamic diameter of 10 pm or less – so-called fine dust – or aerosols – especially bioaerosols, one thinks of the spread of the coronavirus through aerosols – can have on human health.

[0006] Since conventional filters, such as filter mats, are not very effective in terms of their filtering performance, especially for smaller particles such as fine dust or aerosols, at least not when major pressure losses are to be avoided, high-voltage electrostatic precipitators or electrostatic filters (also known as electrical dust separation systems) were proposed in the above-mentioned publication to separate the particles present in the air using electrical charges.

[0007] Compared to conventional filters based on adsorption and desorption effects and filter mats, these also have the advantage that many aerosol-bound viruses, especially the coronavirus, are immediately biologically inactivated at high electric field strengths, so that they no longer pose a danger, even if the filter is cleaned.

[0008] For indoor air purification—e.g., for living or working spaces—two-stage electrostatic precipitators based on the so-called Penney principle are used, although the present invention is not intended to be limited to discharges based on the Penney principle with positive ionizer potential; the use of negative ionizer potentials would also be conceivable.

[0009] In an electrostatic precipitator, the particles to be separated are generally first electrically charged using a so-called ionizer. When a sufficiently high voltage is applied to the ionizer, a high field strength is generated, resulting in a corona discharge through local field ionization. This corona discharge initially generates positively (or negatively) ionized gas molecules in a spatially confined zone around the ionizer (the so-called corona plasma region). These molecules move toward the negative electrodes and, on their further path outside the corona zone, collide with the particles to be separated (fine dust, ionizing particles, or aerosols), thus also charging them.

[0010] The second stage of such electrostatic precipitators is a so-called collector unit, arranged downstream of the ionizer unit in the direction of flow. This unit typically consists of a sequence of polarized plates or plates with alternating high voltages, through which the air to be cleaned flows, containing the particles pre-charged by the ionizer unit. Due to Coulomb forces, the positively charged particles drift to the negatively charged plates or plates at a lower voltage potential (e.g., even at ground potential), the so-called collector plates, where they adhere and can be removed by periodic cleaning or tapping. When negatively charged particles are separated, the potential relationships between the plates are, of course, reversed.The plates of the collector unit that are oppositely charged to the collector plates are called driver plates; these, together with the opposite collector plates, provide the electric field for electrostatic deposition.

[0011] A disadvantage of electrostatic precipitators is the ozone formation caused by the high-voltage-induced gas ionization processes, which can lead to anything from minor health impairments, such as mild eye irritation, to more serious health problems, such as headaches or respiratory problems, or to exceeding the permissible limit values ​​for ozone, particularly when such electrostatic precipitators are used indoors.

[0012] Furthermore, a certain amount of electrical energy is required for the impact ionization processes, so that, given the fact that electrostatic precipitators are often used in continuous operation and the corresponding high-voltage power supplies are, in principle, subject to considerable losses, it is important to ensure high efficiency of ion generation in order to achieve the most energy-efficient operation possible.

[0013] The electrostatic precipitator known from WO 2021 / 185418 A1 mentioned at the beginning operates by means of approximately point-shaped spray ionization sources, preferably by means of conductive fiber bundles, in particular containing graphite-containing fiber filaments.

[0014] The use of such fiber tufts in the field of electrostatic precipitators is already known in the prior art, see US 10 384 517 B2.

[0015] Due to the tip effect of the thin filament ends, high local field strengths are achieved with small radii of curvature, which is favorable with regard to the ratio of deposition rate to ozone production rate, since small radii of curvature lead to high local field strengths and these in turn lead to a small corona plasma region, which, as mentioned, results in lower ozone production.

[0016] The known electrostatic precipitator uses a relatively large number of spray ionization sources arranged in a matrix, with each spray ionization source generating an electric field opposite associated hole edges of a perforated plate having an electrically opposite potential.

[0017] The ionization voltages used in the prior art, including US 10 384 517 B2, are typically well below 8 kV, which may be sufficient for relatively small distances between the ionization electrode and the counter electrode.

[0018] However, WO 2021 / 185418 A1, mentioned above, proposes an arrangement with larger distances between spray ionization sources and counter electrodes. In this context, it was recognized that such arrangements can perform significantly better than other known arrangements in terms of separation efficiency, power consumption, and ozone generation due to the improved particle ionization cross sections.

[0019] However, it has also been found that the larger electrode distances make it necessary for optimal performance to operate the spray ionization sources with significantly higher voltages than is customary in the state of the art, ie with voltages > 8 kV, typically even with more than 10 kV, or even with approximately 12 kV or more (e.g. > 15 kV or > 20 kV).

[0020] If such a high voltage of > 8 kV is applied to a very pointed, almost point-shaped spray ionization electrode, the field strength increases sharply at the edges of the electrode. In the context of the invention, surprisingly rapid and severe wear of the fiber bundles was observed under such conditions, leading to reduced separation efficiency, increased ozone production, and undesirable noise generation.

[0021] This wear was surprising because the electrostatic precipitators in question are not exposed, or only marginally exposed, to any abrasive particles or particularly reactive gases during room air filtration, and the flow velocities are so low that mechanical wear of the spray ionization sources by the medium to be cleaned—i.e., the particle-laden room air—was not really a concern. Furthermore, in such room air cleaners, electrical arcing, which could lead to damage to the spray ionization sources due to sparks, is avoided wherever possible, simply to minimize ozone. Therefore, this source of wear is also practically eliminated when used as intended.

[0022] It has been found that the wear is primarily due to so-called streaming effects, whereby such streaming effects cause unwanted noise, lead to increased ozone production and electromagnetic interference and greatly accelerate ionizer wear.

[0023] The streaming effects are theoretically primarily attributed to the fact that high local field strengths cause electrons and negative gas ions, which cannot be transported away quickly enough, to collide with the surface of the discharge electrode (anode) with high kinetic energy, tearing out material and thus creating microscopic "craters" with sharp edges. These sharp edges, in turn, can generate such high electric field strengths that the aforementioned streamers appear. Furthermore, it is assumed that the reactive species emitted during a corona discharge, such as O + , O 2+ , NO +, which further promote electrode erosion due to their reactivity. These streaming effects are undesirable, and there is a need to eliminate or at least minimize them to prevent premature wear of the spray ionization sources.

[0024] Although spray ionization sources are generally designed to be wear parts and replaceable, the targeted service life of spray ionization sources, typically one year or more, is often impossible to achieve without additional measures at the desired higher ionization voltages due to streaming effects. Without specific measures, spray ionization sources can wear out after just a few weeks of continuous operation.

[0025] An electrostatic precipitator of the type mentioned above using conductive fiber bundles as ionization sources with a typical operating voltage of approximately 10 kV with a downstream collector unit is also known from EP 3 932 563 A1.

[0026] US 2008 / 0 190 296 A1 discloses an industrial electrostatic precipitator, particularly for flue gas purification, which features parallel ionizer electrode and collector plates. These ionizer electrode plates can be subjected to high voltages of 20 kV to 35 kV. The ionizer electrode plates can be made of a composite material with conductive fibers protruding from the surface. The protruding fiber ends can be coated with a metal to prevent damage due to electrical arcing or wear caused by the abrasive flue gas medium.

[0027] The object of the present invention is to provide measures by means of which undesirable streaming effects can be minimized or eliminated even at ionization voltages of more than approximately 8 kV.

[0028] The aforementioned object is achieved by means of an electrostatic precipitator or a spray ionization source for such an electrostatic precipitator according to the features of the independent patent claims. Advantageous embodiments of the invention are explained in the dependent patent claims.

[0029] To achieve the above-mentioned object, in an electrostatic precipitator through which an air flow to be cleaned of particles, in particular a room air flow to be cleaned, flows, with an ionizer unit which has one or more ionizer rows arranged within the air flow, each ionizer row having at least one approximately point-shaped spray ionization source to which an electrical ionizer potential is applied, at least one of the spray ionization sources being formed essentially from a bundle of electrically conductive fibers, and with a collector unit for particle separation arranged downstream of the ionizer unit, with a plurality of essentially parallel, electrically conductive collector and driver plates through which the air flow flows, which are alternately provided with electrical collector and driver plates.opposite driver potentials, it is proposed that the electrically conductive fibers of the spray ionization source are at least partially provided with a metallic coating.

[0030] These modification measures on the spray ionization sources themselves can also be supported by so-called field-modifying measures, which reduce the maximum field strengths in the immediate vicinity of the spray ionization source and / or improve the ion transport from the corona zone in order to further reduce or eliminate the streaming effects overall.

[0031] The (at least partial) metallic coating is intended to achieve a significant reduction in wear and an extension of the service life of the spray ionization sources due to streaming effects, which can mean that due to the at least partial metallic coating of the electrically conductive fibers, a typical service life of a spray ionization source in an electrostatic precipitator operated with relatively high ionization voltages of > 8 kV is achieved of at least one year (preferably at least approx. 3 years), whereas the spray ionization source without the metallic coating would be worn out after only a few weeks of continuous operation due to streaming effects in such an electrostatic precipitator.

[0032] Without the streaming effects mentioned above, the wear of the fiber bundles - regardless of a metallic coating - would be only slight, since the mechanical stress on the fibers, e.g. due to abrasive particles, is low when used in such electrostatic precipitators and arcing generally does not occur.

[0033] Preferably, the coating consists essentially of nickel or at least a nickel alloy, preferably with a nickel content of more than 50%. For example, a nickel / chromium alloy with nickel as the main component can be used as the nickel alloy.

[0034] Chromium can be present in such a nickel / chromium alloy either in a relatively high proportion (between 10% and less than 50%) or in a proportion less than 10%. Such a chromium addition tends to increase oxidation resistance.

[0035] As is well known in metallurgy, a nickel / chromium alloy can also contain other components, such as aluminum, silicon, manganese, cobalt, hafnium, rhenium, or other rare earth metals such as yttrium, which can be added in small amounts to improve strength, oxidation, and corrosion properties. Nickel and, to a lesser extent, chromium remain the primary components. The term "nickel / chromium alloy" is not intended to exclude other alloying additions, although pure nickel / chromium alloys can also be used.

[0036] However, it is also conceivable that other conductive metals or metal alloys are used for such a coating, e.g. precious metals such as gold, silver, platinum, iridium, palladium, or semi-precious metals such as copper or subgroup elements such as chromium, molybdenum or tungsten or alloys of these elements or other elements.

[0037] However, the coating preferably consists of pure nickel, which is preferably applied to the fibers by chemical vapor deposition.

[0038] The electrically conductive fibers are preferably carbon or graphite filaments or, if appropriate, fibers spun from such filaments, with bundles or tufts of single filaments (as opposed to multifilament fibers) being used.

[0039] Carbon or graphite filaments - these terms should be understood synonymously here - are fibers made from carbon-containing starting materials that are converted into graphite-like carbon in fiber form by chemical processes, whereby the so-called anisotropic types preferred here are known to achieve very favorable mechanical properties combined with very good conductivity.

[0040] In a preferred embodiment, the bundles of electrically conductive fibers of the spray ionization source additionally have one or more of the following properties: the electrically conductive fibers of the fiber bundle (40) are formed as graphite or carbon filaments or as fibers spun from such filaments; and / or the electrically conductive fibers are formed as synthetic fibers made of a conductive polymer or of a polymer with conductivity-increasing additives; and / or the individual fibers each have a fiber thickness of less than 20 pm; and / or the fiber bundle consists of 16 individual fibers or more, preferably up to 96,000 fibers, very particularly preferably between 3,000 and 48.000 fibers; and / or the free fiber length between the exit from a holder holding the bundle together and the front end of the fibers is, for the majority of the fibers of the fiber bundle, between 2 mm and 25 mm, preferably between 5 mm and 12 mm.

[0041] Thus, in addition to or instead of pure carbon filaments or fibers, other conductive fibers can also be used, e.g. electrically so-called self-conducting polymers or, for example, graphite-reinforced plastics (Carbon Fiber Reinforced Plastics, CFRP).

[0042] Overall, the fibers of the fiber bundles are preferably quite filigree due to a preferred diameter of less than 20 pm and the relatively large free fiber length of several millimeters compared to the diameter.

[0043] The term "fiber bundle" or "fiber tuft" further implies that the individual fibers of the bundle - starting from a common socket - diverge in a tuft-like manner with increasing distance from the socket, so that the fiber ends do not form a continuous surface.

[0044] The intrinsic conductivity of the fibers does not necessarily have to be very high - although it is indeed quite high in the case of the carbon fibers that are preferred - because the metallic coating can also contribute to the conductivity of the fibers.

[0045] Particularly preferably, the fibers of the fiber bundle have a diameter of 5 pm to 20 pm, preferably between 5 pm and 10 pm.

[0046] The (average) metal coating thickness may preferably be between 0.05 pm and 1.0 pm, preferably between 0.2 pm and 0.5 pm.

[0047] Particularly preferably, the fibers of the fiber bundle are metallically coated substantially along the fiber circumferential sides, at least in the spatial vicinity of the free ends, whereby the ends of the free ends are preferably uncoated, so that the coating can be carried out before the fibers are assembled.

[0048] Preferably, at least one spray ionization source or preferably all of the spray ionization sources are subjected to a relatively high ionizer potential of at least 8 kV each, preferably more than 10 kV, and most preferably more than 12 kV.

[0049] In one embodiment of the invention, the ionization current per spray ionization source can be limited to less than 100 pA.

[0050] Furthermore, the spatial distance of each spray ionization source from the corresponding counter electrode of the collector unit or from another electrode located at the most opposite potential (e.g. the conductive outer walls of the flow channel located at ground potential) is preferably at least approximately 75 mm, whereby this value is to be understood as an example and depends on many design parameters.

[0051] Furthermore, within the scope of the invention, a spray ionization source comprising a bundle of electrically conductive fibers with a metallic coating as described above is proposed, which is accordingly intended - possibly as a replaceable part - for use in the electrostatic precipitator described above.

[0052] In addition to the bundle of coated conductive fibers, this spray ionization source can of course also comprise other elements, such as an electrically conductive holder for holding the fiber bundle or fiber tuft and, if necessary, an insulated electrical supply line.

[0053] The invention is explained in more detail below with reference to the exemplary embodiments illustrated in the drawings. Figure 1 shows a schematic view of a two-stage electrostatic precipitator using a spray ionization source;

[0054] Figures 2a, b show a schematic isometric view and a plan view of an example of a spray ionization source according to the invention;

[0055] Figure 3 is a schematic perspective view of an electrostatic precipitator with multiple spray ionization sources;

[0056] Figures 4a, b show schematic representations of the wear behavior (before - after) of an uncoated carbon filament;

[0057] Figures 5a, b show schematic representations of the wear behavior (before - after) of a nickel-coated carbon filament;

[0058] Figures 6a, b show microscopic images of the wear behavior (before - after) of an uncoated carbon filament bundle; and

[0059] Figures 7a, b microscopic images of the wear behavior (before - after) of a nickel-coated carbon filament bundle.

[0060] The two-stage electrostatic precipitator according to the Penney principle, shown schematically in Figure 1, is based on the aforementioned WO 2021 / 185418 A1, which document is incorporated herein by reference into the subject matter of the present application with regard to further details of the corresponding electrostatic precipitator.

[0061] The electrostatic precipitator 10 according to Figure 1 is located in an air duct not shown in Figure 1. This can be, for example, an air duct in a separate air purifier or, for example, a duct in a centralized or decentralized residential ventilation system, or a duct in the HVAC system of a motor vehicle. A forced air flow in the direction of arrow 22 is generated in the air duct by suitable means, in particular by one or more fans.

[0062] The core element of the electrostatic precipitator is the approximately point-shaped spray electrode 40, which in the example is subjected to a positive high-voltage potential (the high-voltage source and supply lines are not shown), which is the end of a bundle of thin conductive fibers, usually graphite filaments, which are also referred to as carbon fibers.

[0063] This spray electrode 40 is the main component of an ionizer unit, which in other embodiments may also have several spray electrodes as well as mechanical support and power supply structures.

[0064] The term "approximately point-like" is intended to express that the spray ionization emanates from a tip of a fiber-like element with a very small radius of curvature (compared to the other dimensions of the device), so that due to the electrical tip effect, the corresponding electric field and the ionization effect can be regarded as approximately primarily emanating from a point, although the point here naturally represents a mathematical idealization.

[0065] In the example of Figure 1, only a single spray ionization source 40 is shown. However, several spray ionization sources, e.g., arranged in a matrix, can be provided in different configurations, as is also explained in more detail in WO 2021 / 185418 A1.

[0066] The spray ionization source 40 generates - as already described in the introduction - a corona discharge with the formation of a local corona zone, which leads to an ionization of the air molecules and, through accumulation and interaction processes in a wider volume area, to the positive (or possibly also negative) charging of the particles flowing through to be separated.

[0067] These particles are then deposited in a collector unit 12, which is located at a high absolute voltage potential (positive or negative) relative to the ionization source. For this purpose, an electrostatic field is generated between positively or negatively charged driver electrode plates 16 and collector electrode plates 14, so that the positively ionized particles are deposited by electrostatic attraction on the collector plates, which are subjected to a lower voltage potential or ground potential. The absolute driver potential is generally chosen to be lower than the ionizer potential to avoid arcing between the preferably closely spaced plate pairs.

[0068] Within the scope of the invention described in WO 2021 / 185418 A1, negatively charged additional electrodes are also proposed to improve the separation efficiency; among other things, so-called edge counter electrodes 18, see also the illustration in Figure 3. These additional electrodes can advantageously be used in conjunction with the present invention; however, this expressly does not have to be the case, i.e., the present invention is not limited to application in the electrostatic precipitator with additional collector electrodes described in WO 2021 / 185418 A1.

[0069] Rather, the present invention can generally be used in electrostatic precipitators with approximately point-shaped spray ionizers, independently of the specific design of the collector unit 12 as well as independently of the presence of further electrodes and also independently of the type and arrangement of fans.

[0070] Figure 3 schematically shows an embodiment of an electrostatic precipitator with several, in the example four, evenly spaced spray ionization sources 40, which are each designed as fiber bundles of conductive fibers, and which are arranged on a support structure 20, in which the high-voltage supply lines are also integrated, in a matrix-like manner, each approximately centrally with respect to four main flow sections, spaced from the collector unit 12, which has edge counter electrodes 18 surrounding the main flow sections, each with arc-shaped recesses 24. The spray ionization sources 40 are preferably operated with relatively high voltages of generally significantly more than 10 kV (e.g.12 kV, 14 kV or more) and are geometrically spaced relatively far from the respective elements at counterpotential, so that high ionization performance can be achieved with minimal ozone generation while avoiding sparkovers. However, this can lead to the increased wear caused by streaming effects mentioned above, which is why the metal-coated fiber bundles according to the invention can be advantageously used in such separators (but not only there). For further details of the electrostatic precipitator according to Figure 3, reference is made to WO 2021 / 185418 A1.

[0071] Figures 2a and b schematically depict a spray ionization source 40 with a graphite fiber filament tuft or fiber bundle in an isometric view and a plan view. The individual fibers are designated 44, with the number and diameter of the filaments not to scale.

[0072] The spray ionization source 40 is held by a holder or a socket 42, which is also electrically conductive and applies the ionizer potential to the individual fibers 44 via a high-voltage source not shown here.

[0073] At relatively high ionizer potentials starting from approximately 8 kV, but preferably approximately 10 kV or approximately 12 kV or more, the streaming or streamer effects mentioned above occur over time when using uncoated graphite filaments, so that the service life of the spray ionization sources is unsatisfactory.

[0074] Based on the micrographs in Figures 6a and 6b, which show graphite filaments in their original state (Figure 6a) compared to fibers affected by streaming effects (Figure 6b), it is evident – ​​especially evident in the bottommost fiber in Figure 6b – that the originally "round" fiber tip is now asymmetrical. Such a "pointed" fiber end no longer exhibits the required mechanical stability and is prone to the development of streaming effects, especially since the even stronger electrical tip effect increases the wear intensity.

[0075] Against this background, a metallic coating of the fibers, preferably with a nickel layer or with a nickel alloy layer, is proposed within the scope of the invention, wherein in the example of the microscopic representations (according to Figures 7a and 7b) a pure nickel coating was used.

[0076] The corresponding before-and-after comparison is shown in Figures 7a and 7b, with the corresponding fibers in Figures 6a, b and 7a, b being exposed to comparable parameters (discharge geometry, ionization voltage, service life, etc.). The coated fibers visible in Figure 7b still exhibit a "round" tip shape even after extended service life, thus preventing self-reinforcing streaming effects.

[0077] The inventors suspect that the better stability is largely due to the different wear behavior schematically illustrated in Figures 5a, b and 5a, b:

[0078] In the uncoated fiber end 44 shown in Figures 4a (before) and 4b (after), wear due to the discharge processes begins at the upper edge marked in black and leads to a "tapering" of the geometry, which is symmetrical here, but in practice, as can be seen from Figure 6b, can also be asymmetrical and leads to the streamer effects mentioned above.

[0079] With a nickel-coated fiber end 44 according to Figures 5a (before) and 5b (after) – the nickel coating is not shown to scale in dashed lines, designated 46 – the end edge is protected from wear, resulting in more even wear overall without "tapering," as indicated in Figure 5b. Against this background, a metallic coating on the end face of the fiber is not absolutely necessary (this would also be quite complex in terms of production technology, because the filaments would then have to be coated in prefabricated form), because edge protection is essential for achieving the inventive effect.

[0080] Furthermore, it is conceivable that the wear-protective effect of a nickel coating (or a coating with a nickel alloy) is related to the formation of a protective oxide layer on the metal ("passivation"). According to current theories, the formation of a metal oxide occurs either at the inner (metal-facing) or outer (ambient air-facing) interface of the oxide layer. If the mobility of the metal cations in the metal oxide is much greater than the mobility of the oxygen anions—which is the case with nickel at room temperature conditions and natural oxygen partial pressures—then, according to theories, oxidation occurs primarily at the outer interface.

[0081] Under living and indoor conditions, a stable, passive and outwardly growing oxide layer is formed on a nickel-coated electrode surface.

[0082] It is believed that this provides a dielectric barrier (insulation) for the current and protection against further oxidation.

[0083] Two main mechanisms are assumed for the erosion of metal-coated fibers, as used in the present electrostatic precipitators:

[0084] On the one hand, erosion occurs through so-called ion- and electron-induced sputtering, i.e. collision of electrons or ions with the surface, whereby this process exhibits a significant temperature dependence due to the required activation energies.

[0085] On the other hand, erosion effects caused by reactive species emitted during a corona discharge, such as O + , O 2+ or NO + The latter effect shows only a slight temperature dependence and tends to depend more on the generation rate of the reactive species, i.e. primarily on the ionization current.

[0086] Therefore, both effects can be fundamentally differentiated experimentally by varying temperature and ionization current. Such experiments suggest that nickel passivation is particularly effective in mitigating or preventing the erosion effects caused by the reactive species emitted by a corona discharge.

[0087] Furthermore, experiments have suggested that in the case of materials such as nickel that form oxide layers, those alloys - for example nickel / chromium alloys - whose oxide layers grow slowly in the initial stage, passivate quickly and which form a good adhesion to the metal surface provide particularly high wear resistance.

[0088] In contrast to nickel, precious metals—such as platinum—form only very thin oxide layers or none at all at technically relevant temperatures and may therefore be less effective within the scope of the invention than base metals or their alloys, which form a passivating oxide layer, especially an outwardly growing passivating oxide layer. Nevertheless, more noble metals or their alloys can certainly also be used within the scope of the invention.

[0089] An example of such a coated carbon fiber filament suitable for the purposes of the invention would be a carbon fiber with a filament diameter of approximately 7 pm, coated on the outer surfaces, but not on the end faces, with a nickel coating with a thickness of approximately 0.25 pm. This coating can be applied, in particular, by means of chemical vapor deposition.

Claims

Patent claims 1. Electrostatic precipitator (10) through which an airflow to be cleaned of particles, in particular a room airflow to be cleaned, flows, comprising: i) an ionizer unit (26) comprising one or more ionizer rows arranged within the airflow, each ionizer row comprising at least one approximately point-shaped spray ionization source (40) supplied with an electrical ionizer potential, wherein at least one of the spray ionization sources (40) is formed substantially from a bundle of electrically conductive fibers, and ii) a collector unit (12) for particle separation arranged downstream of the ionizer unit, comprising a plurality of substantially parallel, electrically conductive collector and driver plates (14, 16) through which the airflow flows, alternating with electrical collector and driver plates.are subjected to opposing driver potentials, characterized in that the electrically conductive fibers of the spray ionization source (40) are at least partially provided with a metallic coating.

2. Electrostatic separator (10) according to claim 1, characterized in that, due to the at least partial metallic coating of the electrically conductive fibers, a typical service life of the spray ionization source (40) in an electrostatic separator operated with relatively high ionization voltages of > 8 kV is achieved of at least one year, whereas without the metallic coating, the spray ionization source (40) would be worn out after only a few weeks of continuous operation in such an electrostatic separator due to streaming effects.

3. Electrostatic separator (10) according to claim 1 or 2, characterized in that the coating consists essentially of nickel or of a nickel alloy, in particular of a nickel-chromium alloy, preferably of a nickel alloy with more than 50% nickel content.

4. Electrostatic separator (10) according to claim 3, characterized in that the coating consists of pure nickel, which is preferably applied to the fibers by means of chemical vapor deposition.

5. Electrostatic separator (10) according to one of claims 1 to 4, characterized in that the electrically conductive fibers are carbon or graphite filaments or are formed as fibers spun from such filaments.

6. Electrostatic separator (10) according to any one of claims 1 to 5, characterized in that the bundle of electrically conductive fibers of the spray ionization source has one or more of the following properties: the electrically conductive fibers of the fiber bundle (40) are formed as graphite or carbon filaments or as fibers spun from such filaments; and / or the electrically conductive fibers are formed as synthetic fibers made of a conductive polymer or of a polymer with conductivity-enhancing additives; and / or the individual fibers have a fiber thickness of less than 20 pm each; and / or the fiber bundle (40) consists of 16 individual fibers or more, preferably up to 96,000 fibers, most preferably between 3,000 and 48,000 fibers; and / or The free fiber length between the exit from a suffix holding the bundle together and the end face of the fibers is between 2 mm and 25 mm for the majority of the fibers of the fiber bundle (40), preferably between 5 mm and 12 mm.

7. Electrostatic separator (10) according to one of claims 1 to 6, characterized in that the fibers of the fiber bundle have a diameter of 5 pm to 20 pm, preferably between 5 pm and 10 pm, and / or that the metal coating thickness is between 0.05 pm and 1.0 pm, preferably between 0.2 pm and 0.5 pm.

8. Electrostatic separator (10) according to one of claims 1 to 7, characterized in that the fibers of the fiber bundle are metallically coated substantially along the fiber circumferential sides, at least in spatial proximity to the free end faces, wherein the end faces of the free end faces are preferably uncoated.

9. Electrostatic separator (10) according to one of claims 1 to 8, characterized in that the ionizer potential is at least 8 kV, preferably more than 10 kV, and most preferably more than 12 kV, and / or that the ionization current per spray ionization source (40) is limited to less than 100 pA.

10. Spray ionization source (40) for an electrostatic separator (10), characterized in that the spray ionization source (40) is formed from a bundle of electrically conductive fibers with a metallic coating according to one of the preceding claims.

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