FI129337B

FI129337B - A particle charging unit, an electrostatic precipitator and a supply air device

Info

Publication number: FI129337B
Application number: FI20185477A
Authority: FI
Inventors: Jari Heinovaara
Original assignee: Alme Solutions Oy
Priority date: 2018-05-24
Filing date: 2018-05-24
Publication date: 2021-12-15
Also published as: EP3801915A1; CN112154032B; KR20210011930A; CN112154032A; FI20185477A1; EP3801915A4; WO2019224427A1

Abstract

The invention relates to a particle charging unit for a non-metallic electrostatic filter, wherein the particle charging unit comprises one or more corona needle chargers arranged in a distance from each other, wherein each of the needle chargers have a tip; a guide surface for surrounding each of the one or more needle chargers; wherein low power is arranged to be used for forming a wide electric field between a tip of the corona needle charger and the surrounding guide surface for ionizing particles of a gas stream passing through the formed wide electric field. The invention also relates to an electrostatic precipitator, wherein the electrostatic precipitator comprises a non-metallic electrostatic filter and the particle charging unit. The invention also relates to a supply air device comprising the electrostatic precipitator.

Description

A PARTICLE CHARGING UNIT, AN ELECTROSTATIC PRECIPITATOR AND

A SUPPLY AIR DEVICE Technical field The present invention relates to a particle charging unit to be used with a non- metallic electrostatic filter structure. Furthermore, the invention relates to an electrostatic precipitator comprising a particle charging unit, and to a supply air device.

Background Different kind of electrostatic precipitators (ESP) are used for separating particles from an air stream. An air purification device comprising such an electrostatic precipitator is commonly marketed to the public. An air purification device can be, for example, arranged in an air duct or it may be a portable device. These electrostatic precipitators usually comprise a high voltage particle charging unit and a particle collector unit. The particle charging unit usually comprises, for example, at least one corona wire charger or several corona needle chargers for ionizing or charging the particles existing in the air stream, with an electronic charge. The high voltage particle charging unit charging the particles of gas stream can similarly produce unwanted ozone. The particle collector unit comprises an electrostatic metallic plate stack that is arranged downstream relative to the particle charging unit. In the particle collector unit plates are charged with an opposite electrical charge so that an = electric field forms between the plates. Charged particles are collected and N removed from the air stream when they pass through the plates of the particle 3 collector unit and they collide with the plates. Plates are usually made of metal N and arranged into frame structures as a stack of plates. Usually, even I 30 significant quantities of ozone may be generated in a metallic particle collector N unit, especially when the frame structure get dirty. 3 © Summary

N — It is an aim of the present invention to provide a low power particle charging unit to be used with a non-metallic electrostatic filter structure. The low power particle charging unit can be used in a non-metallic electrostatic precipitator suitable to be used, for example, in a supply air device or in an air purification device.

The low power particle charging unit is arranged to comprise at least one corona needle charger for ionizing or charging the particles existing in a gas stream with an electric charge provided by using only low power in order to minimize ozone production during the particle charging.

The low power particle charging unit comprises a guide surface surrounding said at least one corona needle charger.

In the case of two or more corona needle chargers, the guide surface around each corona needle charger is configured to separate the corona needle chargers into segments, each segment comprising one corona needle charger.

According to a first aspect, there is provided a particle charging unit for a non- metallic electrostatic filter, wherein the particle charging unit comprises one or more corona needle chargers arranged in a distance from each other, wherein each of the needle chargers have a tip; a guide surface for surrounding each of the one or more needle chargers; wherein low power is arranged to be used for forming a wide electric field between a tip of the corona needle charger and the surrounding guide surface for ionizing particles of a gas stream passing through the formed wide electric field.

According to an embodiment, the guide surface is made of electrically conductive material.

According to an embodiment, the guide surface is made of conductive plastic. 00 N According to an embodiment, the guide surface comprises one or more guide 3 surface walls extending between or next to said one to few corona needle N chargers.

I 30 N According to an embodiment, one or more guide surface walls extend 5 crosswise or adjacent one on the other. © According to an embodiment, the guide surface is made of grid material.

According to an embodiment, the guide surface is connected to a voltage source. According to an embodiment, the potential difference between the guide surface and corona needles is at least 8kV. According to a second aspect, there is provided an electrostatic precipitator, wherein the electrostatic precipitator comprises a non-metallic electrostatic filter and a particle charging unit according to the first aspect and the above embodiments. According to a third aspect, there is provided a supply air device comprising an electrostatic precipitator according to the second aspect. — Brief description of the drawings In the following, various embodiments of the invention will be described in more detail with reference to the appended drawings, in which Fig. 1a-i show a low power particle charging unit according to an example embodiment from the inlet flowing direction of gas stream; Fig. 2 shows a non-metallic electrostatic precipitator according to an example embodiment; = Fig. 3 shows a supply air device comprising a non-metallic electrostatic N precipitator according to an example embodiment in a perspective 3 view; <

N I 30 Fig. 4a shows a part of an electrostatic plate filter suitable to be used with a N a particle charging unit according to an example embodiment in a 5 perspective view; and co N Fig. 4b shows an electrostatic plate filter suitable to be used with a particle charging unit according to an example embodiment in a perspective view.

Detailed description A low power particle charging unit according to various embodiments is a corona needle charger that comprises a guide surface.

The low power particle charging unit is suitable to be used with/in connection with a non-metallic electrostatic filter, for example, in an electrostatic precipitator.

The non-metallic electrostatic filter can be a plate filter, or some other filter structure suitable for the requirement of the present invention.

The electrostatic precipitator according to various embodiments is suitable to be used in an air purification device.

In this connection, the air purification device may be a supply air device, an air cleaning device, an air conditioning device or any other device suitable for using an ionization charger and a filtering structure for separating and removing particles and other impurities from a gas stream.

The gas stream may be, for example, an air stream.

An air purification device comprises an electrostatic precipitator, which in turn comprises a low power particle charging unit according to the various embodiments for ionizing or charging particles existing in a gas stream with an electric charge that is provided by using low power, and at least one non- metallic electrostatic filter.

The particle charging unit may comprise one corona needle charger that is surrounded by a guide surface, or the particle charging unit may comprise more than one corona needle chargers, for example, 2-9, corona needle chargers that are separated from each other and surrounded by a guide surface comprising one or more guide surface walls extending = crosswise and/or adjacent one on the other.

The needle of the corona needle N charger has been manufactured from a material that is so thin that even after 3 being worn out, the sharp tip of the needle stays sharp, i.e. substantially N maintains its diameter.

I 30 N When the particle charging unit is in use, the gas stream flows through the 5 space(s) around the corona needle charger(s) and between the guide surface o or surface wall(s) for ionization.

Then the gas stream comprising ionized particles may pass through the stack of non-metallic plates of the electrostatic plate filter acting as a particle collector.

The guide surface surrounds each needle charger in a longitudinal direction. When the voltage applied by at least one corona needle charger is high enough, an electric field is formed from the tip of the needle charger to the guide surface so that the gas in the electric field is ionized. The formed electric 5 field can be thought to be a three-dimensional field, meaning that it is broad in the direction of the gas flow. Due to this, the particles to be ionized stay in the low power electric field long enough so that the particles can be charged (i.e. ionized) thoroughly. The ionized particles in the gas stream can be removed from the gas stream by means of the non-metallic electrostatic filter following the corona needle chargers as a particle collector. For generating the electric field, the guide surface and the needle charger(s) have to be in different potentials. For example, the guide surface may be grounded, and the tip of the needle charger may be applied with a high voltage, — which sets the tip into electrically high potential. When the voltage is high enough, an electric field is formed between the guide surface and the needle charger(s). At the same time, small amount of current starts to flow from the tip of the needle charger to the electric field. Since the amount of the current flow is small, the power in the needle charger stays low. As a non-limiting example of the magnitudes, the high voltage to be conducted to the tip of the needle charger may be 8 — 15 kV. However, the amount of conducted voltage can vary according to the situation, and thus may be different. As mentioned, the electric field formed from the tip of the needle charger can berealized three-dimensionally in the sense that it is generated around the tip = of the needle towards the guide surface. Furthermore, the guide surface is N arranged in a certain distance (not close) from the needle charger(s) so that 3 the formation of a broad, i.e. wide, electric field is possible. In other words, a N formed long and wide electric field has a stretched shape both in the flow I 30 direction and in the opposite direction to the gas stream. Particles pass the N electric field, and therefore the wider the electric field is, the more likely the 5 particles will be ionized. This kind of electric field is capable for ionizing the o particles effectively even with low power.

N The above-described effective ionization may not be possible, for example, in a situation where there is a plate comprising circular openings and a corona needle charger in the middle of each opening. This is because only a quite flat (i.e. substantially two-dimensional) electric field will be formed between tip of the needles and a planar edge of the opening due to the plate. This may lead to a situation, where a particle to be ionized slips through the flat electric field without being ionized. The slipping through is even more likely, if high enough power is not used for forming the electric field. Therefore, it is likely that high power is used for charging or several chargers are used, which in turn produces ozone. Furthermore, due to the use of a limited number of sharp corona needle chargers in example embodiments, for example one, but in some situations 9 or less chargers, it is possible to use less power compared to a situation where more needle chargers or a wire chargers or wires are used. Use of a limited number of sharp corona needle chargers produces less ozone.

Commonly used high power particle charging unit charging the particles of gas stream may produce unwanted ozone and, in some cases, even a harmful amount of ozone. It is usually necessary to use high power for charging particles, because it is not possible to use high electric fields in the following metallic electrostatic filter. High electric fields are needed for collecting particles that are charger by low power, but low electric field may be used for collecting particles that are charger by high power. If the formed electric field between two adjacent metallic filter plates is too strong, for example, 5kV or over 5kV, short circuits i.e. accidental diversion of the current could happen between adjacent plates. This may cause further produce of ozone and also = undermine the operation of the electrostatic filter.

N 3 However, use of a low power particle charging unit according to example N embodiments especially with an electrostatic filter that comprises plates made I 30 of, for example, electrically conductive plastic or other suitable non-metallic N material may reduce the amount of produced ozone. This is possible, for 5 example, because the structure of the low power particle charging unit o according to example embodiments enables use of smaller ionizing power, N compared to a particle charging unit ionizing particles by the same efficiency relating to the filtering performance of the filter, but without a guide surface and/or with a plurality of needle chargers. This is because a plurality of needle chargers needs higher ionizing power. The experiments have shown that in the low power solution according to present embodiments, the ionization power of the charger can be 10 — 30 times smaller than in the traditional solutions.

Also, ionization of particles by a wire charger(s) usually requires higher ionizing power than the needle chargers. In other words, structures like low power particle charging units according to example embodiments that use smaller ionizing power produce less ozone. In addition, use of non-metallic plates in particle collectors enable forming of stronger electric fields between two adjacent non-metallic plates of the electrostatic filter, because it is unlikely that short circuits are formed between the non-metallic plates. As has been already stated above, it is possible to charge the particles in a lighter manner i.e. with less power when stronger electrical fields are used in the electrostatic filter.

The potential difference between adjacent non-metallic plates may be, for example, 4kV-10kV or even higher. And when no short circuits are formed, further ozone may not be produced, or it is produced only minor amounts inside the electrostatic filter.

The particle charging unit according to an embodiment comprises 1 corona needle charger. Instead of one, the particle charging unit according to another embodiment comprises a plurality of corona needle chargers, for example 2 to 6 corona needle chargers. A particle charging unit according to an embodiment may comprise even more corona needle chargers, for example, 9. Each of the one or more corona needle chargers are situated in their own compartments = i.e. sections arranged/surrounded by a guide surface.

N 3 A particle charging unit according to an embodiment comprises four needle N chargers, which are surrounded by and separated by crosswise extending I 30 guide surface walls of a guide surface into four compartments. Instead of the N guide surface walls extending crosswise, the needle chargers may be 5 arranged adjacently or otherwise close to each other so that there is a guide o surface between two adjacent or closely spaced needle chargers. The N distance between a needle charger and a guide surface surrounding it may be, for example, 5 — 50 centimetres. However, is appreciated that the distance can be — in some other embodiments — below 5 or above 50 centimetres. The distance between the tip of the needle charger and the guide surface can be defined as a function of the used charging power and the air flow rate. The guide surface may be made of, for example, metallic material or electrically conductive plastic or other suitable conductive material. The guide surface can be formed of solid or perforated or gridded surface walls. The guide surface and guide surface walls of a guide surface are relatively thin to allow as large volume as possible around the tip of the needle, between the walls, but also because thin guide surface restrict flow of gas stream as less as possible. For example, if there is a plate perpendicular to the gas flow, the plate comprising circular openings and a corona needle charger in the middle of each opening, the solid parts of the plate between openings will restrict a gas stream.

According to an embodiment, a non-metallic electrostatic filter suitable to be used with a particle charging unit may be a particle collector unit comprising two groups of plate-like elements i.e. electrostatic filter plates arranged essentially parallel between each other at a predetermined distance from each other and essentially parallel to the flow direction of a gas stream arranged to be purified and ionized by the particle charging unit and flowing through the electrostatic filter, past the plates. Plates of both groups are electrically conductive plates that are made of, for example, electrically conductive plastic or other suitable non-metallic conductive material. A first group of plates is arranged into a first electric potential and a second group of plates is arranged into a second electric potential so that there is a potential difference between two adjacent plates and an electric field is formed between two adjacent plates. The formed potential difference may be, for example, 4kV-10kV as stated = above. As an example, if the first electric potential is earth, the second electric N potential may be negative or positive high voltage or vice versa or if the first 3 electric potential is negative high voltage, the second potential may be positive N high voltage, or vice versa. Plates that are arranged into negative or positive I 30 high voltage are connected to a corresponding DC high voltage source, for N example, between -4kV to -10kV or +4kV to +10kV. Plates that are arranged 5 into earth potential are connected to a ground voltage source. Plates of the o second group of plates are electrically separated compared to the first group N of plates. There is a voltage difference between the plates of the first group and the second group and an electric field is formed between plates. The plates are charged and grounded by connecting them to the voltage source.

Plates of the electrostatic filter of an electrostatic precipitator form a stack, wherein every second plate of the stack is in the first electric potential and every second plate is in the second electric potential. By particle charging unit charged particles of the gas stream are attracted to the plates with different charge, also referred to as collection plates. If particles are positively charged, they are collected by negatively charged plates or grounded plates of the filter and if particles are negatively charged, they are collected by positively charged plates or grounded plates. In other words, charged particles are led into the plate filter, in which the separation of the particles from the gas stream is affected primarily by electric forces of an electric field. The distance between overlapping plates arranged in different potentials may be may be for example, 1-15mm, for example, 4mm. Therefore, it is not likely that unintentional coupling happens between the overlapping non-metallic plates being in different potential even if the moisture accumulates or plates get dirty. This kind of non-metallic plate filter structure has an improved ability to cope with moisture and dirt accumulating on the plates, because parts, for example, plates in different potentials will not come into contact despite the accumulated moisture and dirt. The desired DC voltage can be brought to the plates by a voltage supply mean, for example, a voltage connecting structure that is connected to an electric source i.e. DC voltage source providing the desired voltage for plates. The desired potential may be for example the ground or a voltage, for example, between 4kV-10kV or -4kV- -10kV.

According to another embodiment, instead of having plate-like filter elements, = the particle collector unit may have different kind of electrostatic filter structure N that functions as the electrostatic plate filter discussed above. Any suitable 3 current or future filter structure can be utilized as long as the structure N comprises two different potentials for generating an electric field. The material I 30 being used should have low electric conductivity so that it does not produce N ozone. The shape of the filter elements can thus be anything, e.g. plates (as 5 discussed above), or cylinder or honeycomb etc. © Figure 1a shows an arrangement 10 comprising a particle charging unit according to an embodiment of the invention, and a filter structure 13a. The particle charging unit comprises 2 adjacent corona needle chargers, each having a tip 11 and a dividing wall 12, i.e. guide surface, between the corona needle chargers and surrounding the corona needle chargers. An example of a rectangular shaped electrostatic filter 13a for the particle charging unit is shown by dashed lines.

Figure 1b shows an arrangement 14 comprising a particle charging unit according to an embodiment of the invention, and a filter structure 13b. The particle charging unit comprises 4 corona needle chargers, each having a tip 11, which are arranged in corner areas of a square shape. The particle charging unit also comprises a guide surface surrounding the corona needle chargers, and having two crosswise extending guide surface walls 12 between the corona needle chargers. An example of a square shaped electrostatic filter 13b for the particle charging unit is shown by dashed lines. Figure 1c shows an arrangement 15 comprising a particle charging unit according to an example embodiment, and a filter structure 13c. The particle charging unit comprises 3 corona needle chargers, each having a tip 11, wherein the corona needle chargers are arranged in corner areas of a triangular shape. The particle charging unit comprises a guide surface surrounding the corona needle chargers, and guide surface walls 12 between the corona needle chargers. An example of a triangular shaped electrostatic filter 13c for the particle charging unit is shown by dashed lines. Figure 1d shows an arrangement 16 comprising particle charging unit according to an example embodiment, and a filter structure 13d. The particle = charging unit comprises 6 corona needle chargers, each having a tip 11, N wherein the corona needle chargers are arranged in a rectangular shape. The 3 particle charging unit comprises a guide surface 12 surrounding the corona N needle chargers and three crosswise extending guide surface walls 12 I 30 between the corona needle chargers. An example of a rectangular shaped N electrostatic filter 13d for the particle charging unit is shown by dashed lines. 3 © Figure 1e shows an arrangement 17 comprising a particle charging unit according to an example embodiment, and a filter structure 13e. The particle charging unit comprises 9 corona needle chargers, each having a tip 11, wherein the corona needle chargers are arranged in a rectangular shape. The particle charging unit comprises a guide surface surrounding the corona needle chargers and four crosswise extending guide surface walls 12 between the corona needle chargers. An example of a rectangular shaped electrostatic filter 13e for the particle charging unit is shown by dashed lines.

Figure 1f shows an arrangement 18 comprising a particle charging unit according to an example embodiment, and a filter structure 13f. The particle charging unit comprises 2 corona needle chargers, each having a tip 11, wherein the corona needle chargers are arranged next to each other. The particle charging unit comprises a guide surface surrounding the corona needle chargers and a guide surface wall 12 between the corona needle chargers. An example of a planar electrostatic filter 13f for the particle charging unit is shown by dashed lines. However, instead of two there may be a different number adjacent or successive corona needle chargers in the particle charging — unit, for example one or more than two. Figure 1g shows an arrangement 19a comprising a particle charging unit according to an example embodiment, and a filter structure 13g. The particle charging unit comprises 4 corona needle chargers, each having a tip 11. The particle charging unit comprises a guide surface surrounding the corona needle chargers and a crosswise extending guide surface wall 12 between the corona needle chargers. Two of the corona needle chargers are arranged next to each other and the other two corona needle chargers are arranged successively to them. An example of a sharp-angled arc shaped electrostatic filter 13g for the particle charging unit is shown by dashed lines. 00 N Figure 1h shows an arrangement 19b comprising a particle charging unit 3 according to an example embodiment, and a filter structure 13h. The particle N charging unit comprises 2 corona needle chargers, each having a tip 11, I 30 wherein the corona needle chargers are arranged next to each other. The N particle charging unit comprises a guide surface wall surrounding the corona 5 needle chargers and a guide surface wall 12 between the corona needle o chargers 11. An example of a planar arc shaped electrostatic filter 13h for the N particle charging unit is shown by dashed lines. However, there may be one or more adjacent or successive corona needle chargers 11 in the particle charging unit

Figure 1i shows an arrangement 19c comprising a particle charging unit according to an example embodiment, and a filter structure 13i. The particle charging unit comprises one corona needle charger having a tip 11. The particle charging unit comprises a guide surface 12 wall surrounding the corona needle charger. An example of a circular electrostatic filter 13i for the particle charging unit is shown by dashed lines. The embodiments shown in Figures 1a — 1i are examples of possible configurations of the particle charging unit. It is appreciated that the features between different configurations are combinable to provide even further embodiments for the particle charging unit. In addition, the arrangement and the form may vary from what has been presented in Figures 1a — 1i. Instead of rectangular and triangular arrangements of the corona needle chargers, the one or more corona needle chargers can be arranged in a circular form. Also, instead of having 2, 3, 4, 6, 9 corona needle chargers in the particle charging unit, there may be a different number of corona needle chargers, for example 1,5, 7, 8 corona needle chargers or more, arranged into a suitable form. Figure 2 shows an electrostatic precipitator 20 according to an embodiment of the invention. The electrostatic precipitator 20 comprises an electrostatic filter 21 and a particle charging unit comprising 4 corona needle chargers 11 for ionizing and charging particles existing in gas stream and 2 crosswise extending guide surface walls 23 of a guide surface. 4 corona needle chargers 11 are arranged in corner areas of a rectangular shape filter 21 in a distance = from adjacent needles and in addition in a distance from crosswise extending N guide surface walls 23. The inner edge of the electrostatic filter 21 is a part of 3 the guide surface.

N I 30 The particle charging unit is arranged inside the electrostatic filter 21. The N guide surface walls surround the corona needle chargers 22, and divide the 5 particle charging unit into four areas each comprising one corona needle o charger 22. The guide surface 23 may be made of metal or conductive plastic, N or other suitable conductive material, and it may also be a grid made of — previously mentioned materials. The guide surface may act as a ground plane. In some embodiments the guide surface may be arranged into negative or positive potential. In any case there is a large enough potential difference between the guide surface and corona needles 22. The large enough potential difference may be, for example, 8kV — 15kV.

The flowing direction of gas stream into the electrostatic precipitator 20 is indicated by an arrow 24 and flowing direction of purified gas stream after electrostatic plate filter 21 is indicated by arrows 25. The purified gas flows back to the space wherefrom it enters to the electrostatic precipitator 20.

Figure 3 shows a supply air device 30 comprising an electrostatic precipitator according to an example embodiment in a perspective view. The electrostatic precipitator comprises an electrostatic filter 31 and a particle charging unit 32 according to an example embodiment comprising 4 corona needle chargers and a guide surface comprising 2 crosswise extending guide surface walls 23 between corona needle chargers and as an inner edge of the electrostatic filter

31. The electrostatic filter 31 is in this embodiment arranged in the central portion of the casing 33 of the device 30. When the device 30 is in use, there would be a grating in front of the electrostatic precipitator. When the device 30 is fixed to the roof of a room to be ventilated or to the wall of a room to be ventilated, the tips of corona needle chargers and the grating (not shown) are towards the room to be ventilated. Figure 4a shows a part of a non-metallic electrostatic plate filter 40 suitable to be used with a particle charging unit according to an example embodiment as an electrostatic precipitator. The electrostatic plate filter 40 comprises plates = that are arranged into a first electric potential and into a second electric N potential. The filter 40 may be used as such or it may be used as a part of a 3 larger electrostatic filter entity i.e. a combined electrostatic filter comprising N more than one electrostatic filter, for example, filters corresponding to the I 30 electrostatic filter 40 of Figure 4a. The combined electrostatic filter acts still as N one electrostatic filter. The shape of an electrostatic plate filter may also be 5 other than this or the ones shown in Figures 1a-i. © Figure 4b shows a combined electrostatic plate filter suitable to be used with a particle charging unit according to an example embodiment as an electrostatic precipitator. The combined electrostatic plate filter 45 comprises four similar separate electrostatic plate filters 40 corresponding to electrostatic plate filters 40 shown in Figure 4a. The shape of electrostatic plate filters 40 is such that they are suitable to be arranged in a box-like-shape i.e. as a quadrangle, wherein each of the four sides is an electrostatic plate filter 40. Electrostatic plate filters 40 may be connected to each other in different ways so that different combined filter shapes are formed, the electrostatic plate filters 40 may be arranged, for example, one next to each other or one on the other. The combined electrostatic filter 45 comprising more than one separate electrostatic plate filter 40 may also be formed in shape in which it is arranged to be used at the place of use. This offers the advantage that the storage and transportation of combined electrostatic plate filters 45 comprising more than one separate electrostatic plate filters 40 is easier and cheaper. An electrostatic plate filter may be connected to at least one other electrostatic plate filter by any suitable connecting device. The connecting device may be, for example, a clip, glue, tape, Velcro, magnet, pin-hole-device etc. It should be noted that the shape of an electrostatic plate filter or a combined plate electrostatic filter suitable to be used with a particle charging unit as an electrostatic precipitator may be freely selected according to intended use and/or location of an electrostatic precipitator. The shape is no limited any way. It is obvious that the present invention is not limited solely to the above- presented embodiments, but it can be modified within the scope of the appended claims.

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Claims

Claims:

1. An electrostatic precipitator, wherein the electrostatic precipitator comprises an electrostatic filter comprising a stack of electrically conductive non-metallic plates and a particle charging unit comprising one to nine corona needle chargers arranged in a distance from each other, wherein each of the needle chargers has a tip and a guide surface for surrounding each of the needle chargers, and wherein low power is arranged to be used for forming a wide electric field between a tip of the corona needle charger and the surrounding — guide surface for ionizing particles of a gas stream passing through the formed wide electric field, and wherein the gas stream comprising ionized particles continues through the stack of electrically conductive non-metallic plates removing ionized particles from the gas stream.

2. The electrostatic precipitator according to claim 1, wherein the inner edge of the electrostatic filter surrounding or being next to the one or more corona needle chargers is a part of the guide surface.

3. The electrostatic precipitator according to claim 1 or 2, wherein the guide surface is made of electrically conductive material.

4 The electrostatic precipitator according to any of claim 1 to 3, wherein the guide surface is made of conductive plastic.

5. The electrostatic precipitator according to any of claim 1 to 4, wherein the = guide surface comprises one or more guide surface walls extending between N or next to said one to few corona needle chargers. <Q S 6. The electrostatic precipitator according to claim 5, wherein one or more E 30 guide surface walls extend crosswise or adjacent one on the other.

NM 5 7. The electrostatic precipitator according to any of claim 5 or 6, wherein the o guide surface is made of grid material.

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8. The electrostatic precipitator according to any of claim 1 to 7, wherein the guide surface is connected to a voltage source.

9. A supply air device comprising an electrostatic precipitator according to any of claim 1 to 8. oO

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