Patent Publication Number: US-2023140445-A1

Title: Electrostatic separator

Description:
The present invention relates to electrostatic separators or electrostatic precipitators and to a room ventilation unit having such electrostatic separators or precipitators. 
     WO 2017/121 429 A1 describes the use of electrostratic separators in controlled decen-tralised living room ventilation systems with heat recovery. 
     The subject matter of the present invention are electrostatic separators or precipitators which are optimized with regard to efficiency, construction size, power consumption and ozone emissions and which can be used as mobile stand-alone devices or as a compo-nent of air-conditioning and ventilation systems, for example of decentralized or central room ventilation systems. Many industrial applications in the field of air purification and in principle also in the field of cleaning other gases, for example in the area of smoke gas purifications, or also in the automotive sector (e.g. HVAC systems for vehicle interiors) are also imaginable. 
     In recent years, it has been recognized which serious effects air particles having an aer-odynamic diameter of 10 μm or less - the so-called fine dust - or aerosols can have on the human body. Since conventional filters, such as, for example, filter mats, are not very powerful with regard to their filter performance, in particular in the case of smaller parti-Iles, such as fine dusts or aerosols, in particular if greater pressure losses are to be avoided, electrostatic precipitators or separators or electrostatic filters (also known as arrangements for electronic dust separation) operated with high voltages of a few kilovolts have been proposed for use in decentralized residential ventilation systems in the context of the above-mentioned publication. Hereby the particles present in the air are separated by virtue of electrical charges. 
     In particular, two-stage electrostatic precipitator working according to the so-called Penney principle are used for room air purification. 
     In this case, the particles to be deposited are first electrically charged by means of a so-called ionizer. This is done by so-called corona discharges, in the prior art mostly by means of a corona wire positively charged with a high voltage of a few kV or by means of a plurality of correspondingly positively charged corona wires which are arranged at a distance from negatively charged electrodes, so that in the immediate vicinity of the wire or the wires, when a sufficiently high voltage is applied, a high field strength and accordingly a corona discharge is evoked by local field ionization. 
     This corona discharge initially produces positively ionized gas molecules around the ionizer in a spatially narrowly limited zone (so-called corona zone), which gas molecules move to the negative electrodes and thereby accumulate on their further path outside the corona zone to the (fine dust) particles or aerosols to be deposited and also charge them. 
     The second stage of such electrostatic precipitator represents a so-called collector unit arranged downstream of the ionization unit in the direction of flow, which normally consists of a sequence of alternately charged plates, through which the air to be purified flows with the particles previously charged by the ionization unit. As a result of the coulomb forces, the positively charged particles drift to the non-positively charged plates, the so-called collector plates, adhere there and can be removed by periodic cleaning or tapping. The opposite plates of the collector unit, which are charged with the collector plates (positive), are referred to as driver plates; they provide the electric field for the electrostatic separation together with the respective opposing collector plates. 
     A disadvantage of electrostatic filters is the ozone formation caused by high-voltage-induced gas ionization processes, which can lead to anything from minor health impair-ments like slight eye irritation up to more serious health problems, such as headaches or respiratory problems, or to overshoots of the permissible MAK values (MAK=maximum workplace concentrations) for ozone, in particular when this type of electrostatic precipitator is used in interiors. 
     For the impact ionization processes, also a certain amount of electrical energy is required, so that, in view of the fact that electrostatic filters are frequently used in continuous operation and the corresponding high-voltage power supply components are subject to electric losses, a high efficiency of ion generation should be ensured with regard to an energy-saving operation. 
     The object of the present invention is to further optimize the above-described electrostatic separator or precipitator with regard to efficiency, compactness, energy utilization and ozone emissions and to provide an electrostatic separator or precipitator which is easy to clean and to service as well as easily scalable for different performance classes and air cross-sectional geometries and which has the above-mentioned properties. 
     The aforementioned object is achieved by means of an electrostatic separator or precipitator having the features of claim  1  and/or by means of a room ventilation unit having such an electric separator or precipitator according to claim  13 . 
     Advantageous embodiments of the invention are explained in the dependent claims. 
     In the context of the invention, an electrostatic precipitator, through which a stream of air to be cleaned flows in a longitudinal direction, comprises at least one spray or electrospray ionization source, which is arranged within the air flow and is applied by a positive electrical ionization potential, respectively, which further comprises a collector unit arranged downstream of the at least one spray ionization source for particle deposition, having a plurality of substantially parallel arranged electrically conductive collector and driver plates through which the air flow flows, which are alternately charged with electrically negative collector or opposite positive driver potentials. 
     According to the invention, such an electrostatic separator or precipitator is characterized by an upstream-electrode element which is arranged in the flow path upstream of the at least one spray ionization source and is applied by an electrically negative potential. 
     This can in particular be an upstream (metal) grid or mesh which is arranged upstream of the spray ionization source and through which the particle-laden air flows. Alternatively, a metal ring surrounding the flow path or a number of metal lamellae or wires arranged substantially in the flow path or wires (for example as wire harp), where all these elements are intended to be characterized in that they are each acted upon by a collector potential (i.e., with a potential which is significantly more negative than the positive ionization potential, which can be mass potential), in the air flow path or at its immediate border, and which is located—as mentioned—upstream of the ionizers. 
     As a result of experiments (based on an electrostatic separator geometry as described further with reference to the exemplary embodiment, i.e. with approximately point-shaped spray ionization sources and so-called border counter-electrodes), it has been shown that such an upstream collector element (in the test implemented as a closely meshed grid) has improved particle separation efficiency typically by up to ca 50%, as a result of which a predetermined deposition power can be achieved with fewer spray ionizers, as a result of which production process efforts for the electrostatic precipitator, power consumption, but - above all - also absolute ozone emissions can be significantly reduced. 
     The fact that an element, which is arranged upstream of the ionizers, so that the air to be purified strikes the latter before it reaches the spray ionizers, can have such a significant effect on the deposition performance, appears at first somewhat surprising. 
     However, it must be considered that typical ion velocities are generally orders of magni-tude higher than typical air flow rates in such electrostatic separators, so that the gas ions can also readily move “against the stream” when electric fields are set accordingly. The gas volume upstream of the spray ionization sources for particle ionization is thus additionally made usable by the upstream collector element. 
     In the case of approximately point-shaped spray ionization sources, which are preferred in the context of the present invention, in this way, the complete “hemisphere” upstream of spray ionization sources is additionally made usable for particle ionization; which re-sults in a significant increase in deposition performance. 
     Basically, this principle can also be used for other spray ionization sources, e.g. “classi-cal” corona wires, however. 
     Such an upstream collector element can further have an additional benefit, since it can also serve as protection of downstream electrical or electronic components from unde-sired electrostatic charging by the ion currents. 
     In a preferred embodiment, the upstream collector element can be designed as a collector grid or mesh. 
     Such a collector grid preferably has a mesh number which is greater than or equal to the number of spray ionization sources. Thus, if four spray ionization sources are present, at least four meshes can preferably also be provided, or more meshes, if necessary. 
     The mesh width of a collector grid can be in the range between approximately 2.5 mm and 25 mm in typical electrostatic depositors. 
     Further preferred embodiments of the upstream collector element have certain resem-blances to the embodiments of the remaining elements, i.e. in particular with regard to the design of the spray ionization sources and the so-called border counter-electrodes. Therefore, preferred embodiments of the remaining elements will first be discussed in the following, before reference is then made again specifically to the upstream collector elements: 
     The at least one spray ionization source should be formed approximately in a point-shaped or punctiform manner; in particular, the at least one spray ionization source can be formed by a metallically conductive fine needle tip or by the free fiber end of a conductive fiber or by a plurality of adjacent free fiber ends of a bundle of conductive fibers, wherein the conductive fibers are preferably graphite filaments and/or wherein the needle tip or the free fiber ends are preferably arranged facing in the direction of flow. 
     In this way, a small volume, which tends to be spherical or dome-shaped, is produced (typically a few mm in diameter), and since the ozonation takes place practically only in this zone, the absolutely generated ozone quantity is reduced. 
     However, it has proven to be important in such approximately point-shaped spray ionization sources to split up or fan out the gas ions as homogeneously as possible into a gas volume which is as large as possible with the particles to be deposited, in order to create optimum effective cross-sections for the ionization of the particles there. 
     In a preferred embodiment, at least one border counter-electrode can be provided in the electrostatic precipitator,
         each of which is assigned to one of the one or more of the spray ionization sources,   which is arranged in the longitudinal direction between the associated spray ionization source and the collector unit and at a longitudinal distance from the associated spray ionization source,   which has electrically conductive walls which extend substantially in the longitudinal direction of flow and which delimit a flow channel assigned to the respective spray ionization source on all circumferential sides or at least in circumferential partial sections,   wherein the electrically conductive walls of the associated border counter-electrode are applied with a negative border counter-electrode potential opposing the ionization potential, and   wherein the spray ionization source is arranged centrally in a transverse plane with respect to the contour of the respective associated border counter-electrode.       

     The term “border counter-electrode” is intended to express that the conductive walls of the border counter-electrode lie on a generally negative potential opposite to the spray ionisator, similar to the collector plate potential of the collector unit (or similar to the upstream collector element), wherein these potentials need not be identical. 
     These border counter-electrodes are intended to ensure the most efficient utilization of the ion fields generated by the corona discharges downstream of the spray ionization sources. 
     Thus, the border counter-electrodes represent in a certain way a counterpart to the upstream collector element proposed according to the invention. However, they can also advantageously be implemented without the upstream collector element. 
     Preferably, the at least one border counter-electrode can each have a curved contour which can be rectangular, in particular square, polygonal, round, elliptical, honeycomb or otherwise. If a rectangular or polygonal contour of the edge contour is selected, the walls are preferably formed at least on two opposite sides. 
     In the case of a round contour of the border counter-electrode, the distances between the spray ionization source and the facing edges of the collector are each constant per se. If a plurality of spray ionization sources arranged in a matrix-like manner are provided, such a round contour of the border counter-electrodes is not optimal, however, because, in the case of a surface having a plurality of adjacent circles, the transversely available surface is not optimally utilized (similar to a perforated plate), wherein many flow channels have a rectangular basic shape, so that a rectangular or square grid would normally represent the optimum arrangement for a plurality of border counter-electrodes, wherein a “lattice bar” would then simultaneously act as an electrode for two border counter-electrodes located on the side. 
     In the case of such a rectangular contour, however, the distance between the spray ionization source and the facing edges of the respective border counter-electrode is no longer automatically constant, so that ion density becomes more or less inhomogeneous. In a preferred embodiment, it can therefore be provided that face edges of the border counter-electrodes, which face edges facing in the direction of the respective spray ionization source, have a recess which is curved in an arc-shaped manner respectively completely or in partial sections in such a way that an imaginary surface running through all or through at least two opposite facing edges of the border counter-electrode facing the spray ionization source has a generally concave or specially spherical surface with respect to the associated spray ionization source. 
     Referring again to the upstream collector element standing in the focus of the present invention, it is preferably distanced from the at least one spray ionization source in the upstream direction by a distance which corresponds to 50% to 300%, preferably 75% to 150% of the longitudinal distance between the at least one spray ionization source and the associated border counter-electrode, in order to achieve an ion density as uniform as possible in both directions. 
     If no border counter-electrodes are to be provided, then the preferred distance of the upstream collector element from the spray ionization source can preferably be carried out at the distance of the spray ionization source collector unit. 
     In a further preferred embodiment, the upstream collector element has one or more cup-shaped curvatures or protuberances which are concave with respect to the at least one spray ionization source and which are intended to ensure a distance as constant a as possible from all points of the collector element to the at least one spray ionization source. 
     Preferably, it can be provided that the electrodes of the upstream collector element are at least partially aligned in a longitudinal projection direction with be preferably all or at least a part of the border counter-electrodes. Border counter-electrodes and upstream collector element thus correspond, in a preferred embodiment, entirely or partially geo-metrically in such a way that the elements are arranged in a line. 
     The present invention enables a very simple and effective scaling of the electrostatic filter for virtually any desired air passage area, in that a plurality of spray ionization sources are arranged in a matrix-like manner approximately in the centers of the rectangular meshes of an imaginary grid which is composed of a grid row and a plurality of grid columns or of a plurality of grid rows and a grid column or of a plurality of grid rows and a plurality of grid columns, wherein all the row heights and all column widths are no more than ±50%, preferably no more than ±25%, of a uniform basic dimension or size. 
     That is to say, virtually any number of spray ionization sources can be arranged in a matrix-like manner in rows and columns, and thus overall an air flow can be optimally cleaned by means of a rectangular cross-section of any desired size, where even non-rectangular cross-sections (e.g. round cross-sections) can be approximated by a usable number of columns per row (or vice versa). Each cell is preferably based on a specific basic measure (e.g. 5 mm to 25 mm), which is optimized with regard to the electrostatic filter effect, and the individual cell dimensions then deviate only to a limited extent from this basic dimension. 
     In one embodiment, the electrostatic precipitator preferably also has a fan unit for forced air flow conveying through the electrostatic precipitator, which is arranged in the flow path upstream of the collector element and is protected from electrostatic charging by the fan element. 
     For this purpose, various solutions, including efficiency and noise emission aspects, are known in the prior art, such as radial or axial fans or the use of a plurality of fans arranged in series. 
     Of course, an electrostatic precipitator furthermore preferably also has further necessary or optional components, such as a housing which defines the main air channel, with corresponding inlets and outlets. In addition to the actual electrostatic precipitator, it is also possible to provide further filters, for example activated carbon filters, sound insula-tion elements, etc., as it is well known in the art. 
     Finally, within the scope of the invention, a room ventilation unit is proposed which has an electrostatic precipitator as described above. 
     With the electrostatic precipitator according to the invention, very good deposition rates, of, for example, &gt;90% (PM 2.5 or less, PM=particulate matter) can be achieved in a compact construction with a test device corresponding to the exemplary embodiment described further below with four spray ionization sources and with border counter-electrodes and upstream collector grids, wherein the ozone concentration measured at the device was below a limit value of 10 ppb and predominantly also a limit value of 5 ppb has not been exceeded. In this way, measured ozone emissions range in the concentration range of natural background concentrations for ozone for internal/indoor spaces. 
     The invention is explained in more detail below with reference to the exemplary but nor limiting embodiments illustrated in the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a perspective oblique view of an electrostatic separator according to the invention with four spray ionization sources and corresponding border counter-electrode/collector units and an upstream-collector grid; 
         FIG.  2    shows a further embodiment of the invention with upstream collector electrodes; 
         FIG.  3    shows a schematic illustration of a decentralized room ventilation unit with an integrated electrostatic precipitator according to the invention, and 
         FIGS.  4   a - c    show examples of different border counter-electrode configurations. According to the illustration in  FIG.  1   , an electrostatic precipitator  10  according to the invention has as essential components a collector unit  12  which, in principle, consists of a number of collector plates  38  arranged in parallel to driving plates  40  (whereas, in the exemplary embodiment, collector plates are somewhat shorter than driver plates) (see  FIG.  2   ). 
     
    
    
     This collector unit  12  is flowed through in a main flow direction (arrow  28 ) by an air flow to be cleaned of particles, this air flow generally being ensured by one or more fans (not shown in  FIG.  1   ), for which purpose the electrostatic precipitator  10  is located within an air flow channel (also not shown in  FIG.  1   ). 
     The collector unit  12  with its collector plates  38  at a negative potential electrostatically attracts particles (e.g., fine dust, pollutant particles, aerosols, etc.) from the air stream, which have been previously positively ionized, as a result of which the particles adhere to the collector plates  38  and thus can be removed from the air stream. The adhering particles can be removed, for example, by regular cleaning, mechanical knock-off or periodic air impact. Germs, bacteria or viruses adhering to deposited aerosol particles can also be deactivated - if they do not dry out anyway - by additional measures, such as UV light. 
     The collector plates  38  are connected to a negative voltage source - which can also be the ground potential - via a contact strip  26  and via corresponding counter-contact elements provided in a flow channel wall (voltage source and counter-contact elements are not shown). 
     Analogously, the driver plates  40  are connected to a positive potential of a corresponding voltage source via a contact strip  24  located above the contact strip  26 . This is in general an average positive potential which lies between the negative potential of the collector plates  38  and the (high) positive potential of the spray ionization sources  18   a - d  (explained further below). The spray ionization potential would typically be too high for the driver plates  40 , i.e., flashovers could occur between the plates  38 ,  40 . 
     The positively ionized particles are produced according to the so-called Penney principle by positive corona discharges, for which one or more, in the exemplary, not limiting embodiment, four, approximately point-shaped spray ionization sources  18   a - d  are provided according to the invention. 
     In the exemplary, not limiting embodiment shown, it is a bundle or shell of thin, electrically conductive graphite fibers, which are acted upon by a high-resistance high-voltage source (not shown) with a positive high voltage of some kV. Alternatively, individual conductive fibers or needle-like metal tips or the like can also be used as spray ionization sources. 
     The spray ionization sources  18   a - d  (the totality of the spray ionization sources are also designated by the reference numeral  16 ) are mounted on a non-conductive support grid  20  in which high-voltage lines (not shown) for connecting the ionization sources  18   a - d  to one or more high-voltage sources are guided. The ionization current established by the corona discharge can be electronically controlled in one embodiment. 
     Within the scope of the invention it is advantageous if the ions are emitted as far as possible from one point, respectively, whereby it is understood that the term “point-shaped” is of course an idealized specification. At the thin tips of the spray ionization sources  18   a - d -in cooperation with counter-electrodes as described further below - high electric field strengths are generated, which generate ions in a spatially narrowly limited are of the ionization sources  18   a - d , typically of a few mm, of the so-called corona zone; whereby the boundary layer is also known as a so-called corona skin, ionizing gas molecules from the air flowing through. These ionized gas molecules then collide with the particles to be deposited, provided, that they have sufficient opportunity to interact with them, and ionize them, so that they can be correspondingly electrostatically deposited in the collector unit  12 . 
     This secondary ionization of the particles to be deposited by the gas ions is carried out in a substantially larger volume than the corona discharge itself. 
     Within the scope of the present invention, it has been recognized that it is important for optimum deposition performance with minimal ozone emissions to initially generate the gas ions in a volume that is as small as possible (because ozone is only present there due to high ozone generating field strengths) and then distributed as uniformly and homogeneously as possible through the gas volume flow in order to achieve optimum flow cross-sections between gas ions and particles, wherein a focus of the present invention lies at the upstream air volume. 
     For this purpose, on the one hand, to one (preferably each) spray ionization source a border counter-electrode  14   a - d  is assigned, which has walls  30 ,  32  located at a negative potential with respect to the spray ionization source  18   a - d , which walls  30 ,  32 , for the respective spray ionization source  18   a - d , delimit an (section-by-section) flow channel and which are spaced apart from this spray ionization source. 
     When the border counter-electrodes  14   a - d  are rectangular shaped as shown in  FIG.  1   , the opposite walls of the wider side are denoted by  30  and the ones of the narrower side are denoted by  32 . 
     Due to the in totality overall rectangular grid-like structure of the border counter-electrodes  16 , the narrow side walls  32  of the individual border counter-electrodes  18   a - d  are each formed by a common narrow side wall  32  for adjoining border counter-electrodes (i.e., except for the outermost narrow sides). 
     The positive gas ions are drawn through these border counter-electrodes  14   a - d  and are ideally fanned out over the entire flow channel available, so that optimum effective cross-sections between gas ions and particles are achieved. 
     Furthermore, it has been recognized that the homogenization of the gas ion current is then almost optimal if the distances between the spray ionization source  18   a - d  and the associated border counter-electrode  14   a - d  are as constant as possible. Therefore, the spray ionization sources  18   a - d  are arranged approximately centrally or centered with respect to the contour of the border counter-electrodes  14   a - d  or with respect to the flow channels defined by these definite flow channels. “Approximately” is intended to be within the scope of the invention that design-related deviations of typically a few percent to about 10% or 20% are tolerable. 
     In case of a rectangular grid, as it is appropriate for an aerodynamically ideal covering of the overall flow cross-section (in particular of a rectangular flow cross-section) when a plurality of spray ionization sources  18   a - d  are provided, the distances between the idealized point-shaped or punctiform spraying source  18   a - d  and the facing edges of the walls  30  that are relevant to the electric field of the border counter-electrodes  14   a - d  vary in a relatively strong way in the case of an assumed “straight” design of these facing edges, despite the central arrangement of the spray ionization sources  18   a - d . That is to say, in case of a “straight” design of the facing edges, the distance to the respective center of the front edges would be different than to the corners. 
     Therefore, in the embodiment according to  FIG.  1   , it is provided that the walls  30 ,  32  of the border counter-electrodes have rounded incisions  34 ,  36 , which are defined by the intersection curves of a virtual sphere (or a spherical segment, if only one side is considered)  22   a - d  with the respective spray ionization source  18   a - d  in the imaginary center point. As a result of this, a uniform spacing of the spray ionization sources  18   a - d  to the border counter-electrodes  14   a - d  is realized and thus, an ion distribution which is as uniform or homogeneous as possible is achieved, wherein in practice some field distortions are unavoidable, for example by the superposition of the electric fields emanating from the collector electrode and driver plates. 
     In a more general embodiment, the contour of the incisions  34 ,  36  can be formed in the border counter-electrode walls  30 ,  32  such that the virtual surface is concave at least from the perspective of the spray ionization source. 
     In the context of the present invention, as an alternative or in addition to the above-described measures taken downstream of the spray ionization sources, a further upstream collector element  44  is seen upstream, which in  FIG.  1    is represented only schematically as a grid  44  with eight meshes, but which can also have other shapes. 
     The aforementioned upstream collector element or grid  44  is also placed to or applied by a negative potential, e.g., the ground potential, and thus provides for a fanning and homogenization of the gas ion currents upstream of the spray ionization sources  18   a - d    
     The longitudinal distance of the upstream collector element  44  from the spray ionization sources  18   a - d  is preferably selected to be approximately as long as the distance of the border counter-electrodes  18   a - d  from the spray ionization sources  18   a - d  in order to achieve a uniform current distribution in both directions. The ideal distance is not neces-sarily the same in both directions, since the field properties of the border counter-electrodes and of the upstream collector element  44  are generally different. If necessary, the ideal distance may be determined experimentally. The spacing of the mesh or grid  44  shown in  FIG.  1    can therefore be varied; in particular, it is conceivable to arrange the grid  44  approximately equidistant to the border counter-electrodes  14   a - d , i.e. as far as the upstream part of the imaginary spherical surfaces  22   a - d.    
     In principle, other types of upstream collector elements, such as, for example, an only edge-side ring or a wire harness, are also conceivable. 
     Furthermore, in accordance with the preferred embodiment of the border counter-electrodes, a non-planar configuration of the upstream collector element, e.g. with cup-like bulges, is also conceivable in order to keep the distance between the collector element and the spray ionization source as constant as possible. 
     In  FIG.  1   , two variants of the upstream collector element are indicated by dashed lines (and only in partial regions), namely on the one hand a first curved variant  44   a , in which the wire-like electrodes of the collector element are curved approximately corresponding to the border counter-electrodes and thus a more uniform distance from the spray ionization source is ensured (in this embodiment, the elongated central strut of the collector element is then preferably waived). Alternatively, in a second variant  44   b  the upstream collector element can be designed in a manner similar to the border counter-electrodes, namely as a metallic strip with a certain longitudinal extent, in each of which an arc-shaped cut-out is embedded. 
       FIG.  2    shows an embodiment of an electrostatic precipitator  10  which can be realized in a technically particularly simple manner, in which a border counter-electrode  30  is formed by means of two opposite walls from an extension of the outermost collector plates  38  of the collector unit  12 , wherein these walls  30  can be formed either straight or with the above-described round incisions (not shown). In this further embodiment, a collector grid  44  as an upstream collector element is also provided upstream of the spray ionization source  18   a.    
       FIG.  3    schematically illustrates how an electrostatic precipitator  10  according to the invention could be integrated into a decentralized room ventilation unit  46 , whereby—as initially mentioned—alternative uses, e.g. for “stand-alone” air cleaners for residential spaces or for a wide variety of further applications in a similar configuration, are of course also possible and intended. 
     In this case, an ionization/counter-electrode unit is shown schematically as a “black box”  54  adjacent to the collector unit  12 , an upstream collector grid  44  according to the invention being provided upstream and again upstream of which an axial fan  52  is arranged. 
     The elements mentioned are arranged sequentially in a tube in a wall outlet  56 , which extends between a room-side air inlet/air outlet  48  and an outside air inlet air outlet  50 . 
     In this configuration, the upstream collector grid  44  additionally also protects the electron-ics of the fan  52 , or the control thereof, from excess voltage damage by eventual discharges of ion-current-induced electrostatic charges. 
     In  FIGS.  4   a - c   , various variants of border counter-electrode configuration on the example of an electrostatic filter having, for example, a total of six spray ionization sources  18  in a 2×3 (or depending on the designation 3×2) matrix are shown schematically (in contrast, the example of  FIG.  1    has a 1×4 or 4×1 configuration). The respectively thicker lines are intended to indicate guide plates with border counter-electrodes  14 , which have thinner lines. 
     In  FIG.  4   a   , all cell boundaries are provided with border counter-electrodes, whereas in  FIG.  4   b    this is the case only in the case of the longitudinal side electrodes, and in the case of  FIG.  4   c    only in the case of the outer boundaries. 
     It can also be seen from the schematic representation of  FIGS.  4   a - c    that the electrostatic filter  10  can be scaled or matched very easily to different cross-sectional areas, since this is assembled modularly from individual cells, wherein the individual cell is optimized in each case with respect to the electrostatic deposition properties, and virtually any cross-sectional areas can be covered with these cells, without the performance of the individual cells having to be re-engineered. 
     LIST OF REFERENCE SIGNS 
     
         
           10  Electrostatic precipitator 
           12  Collector unit 
           14   a - d  Border counter-electrodes 
           16  Totality of border counter-electrodes (grid) 
           18   a - d  Spray ionization sources 
           20  Support grid 
           22   a - d  Imaginary spherical surfaces 
           24  Driver plate contact row 
           26  Collector plate contact row 
           28  Arrow indicating direction of flow 
           30  Longitudinal side walls border counter-electrodes 
           32  Narrow-side walls of border counter-electrodes 
           34  Cut-in border-counter-electrode walls (longitudinal side) 
           36  Cut-in border-counter-electrode walls (narrow side) 
           38  Collector plates 
           40  Driver plates 
           44  Upstream collector grid 
           44   a  First option for upstream collector grid 
           44   b  Second option for upstream collector grid 
           46  Decentralized room ventilation unit with electrostatic precipitator 
           48  Room-side air inlet/air outlet 
           50  Outer air inlet/air outlet 
           52  Axial fan 
           54  Ionization/border counter-electrode unit 
           56  Wall passage