Injector arrangement for a plasma torch for thermal spraying

Abstract

Injector arrangement (20) for a burner, in particular a plasma burner (10), for carrying out a thermal coating process, comprising: an injector (30) which forms a feed channel (32) for feeding an abrasive filler material (80) which comes into contact with it during feeding to a combustion or plasma flame (100), an adapter (40) for connecting the injector (30) to a powder line (50); wherein the injector (30) and the adapter (40) are connected to each other but form separate parts, wherein the injector (30) is formed from a ceramic material.

Description

Technical Field:The present invention relates to an injector arrangement for a plasma torch for thermal spraying for coating workpieces, said injector arrangement having one or more injectors with feed channels.Technical Background:It is known to coat surfaces of components with metallic and / or nonmetallic materials, such as oxidic or carbidic materials, for example, by carrying out a thermal spraying process for this purpose. During thermal spraying, these materials, referred to as filler materials, are supplied inside or outside a spray burner to an energy carrier, such as a plasma or fuel oxidizer flame, an electric arc, a laser beam or the like, in order to melt them, depending on the configuration of the filler material as powder, wire or rod. The correspondingly melted additives formed into injection particles are accelerated simultaneously or immediately subsequently in a gas stream and spun onto the surface of the component to be coated. There, the injection particles flatten out, solidify, adhere and mechanically clamp themselves to the substrate as well as to the gradually building up layer components. A low porosity with mechanically reliable attachment to the component is the consequence, likewise freedom from cracks and a completely homogeneous microstructure of the coating. Such coatings can fulfil various purposes, for example in order to increase a wear resistance, corrosion protection, heat insulation or electrical conductivity of the component locally or overall.A specific embodiment of thermal spraying, in which a plasma flame is used as energy carrier, is plasma spraying. The device generally referred to as a spray torch, with which the spraying method is carried out in this case, is a plasma torch here. Such a plasma torch generally comprises electrical connections for supplying current and voltage to one or more anodes and to one or more cathodes, a gas supply, a cooling system (generally water), a combustion chamber through which the gas flows, in which the anode(s) and cathode(s) are situated opposite one another, and, close to an orifice of the combustion chamber which generally extends centrally, one or more feed channels which are oriented perpendicularly to a combustion chamber axis and through which the filler material is fed to the hot plasma jet generated by the electrodes.A corresponding plasma torch is disclosed, for example, in DE 199 63 904 C2. A working gas is guided into the combustion chamber via channels, wherein a swirl is obtained between the electrodes when it enters the combustion chamber by a suitable design of the channels. The voltage applied to the anodes arranged on the outside and the cathodes extending centrally is sufficient to ionize the working gas and thus to form a hot plasma which exits at a conically widening orifice of the combustion chamber. The plurality of anodes are arranged in succession in the axial direction with respect to the combustion chamber axis. Viewed in the axial direction, an insulating element surrounding the combustion chamber in the form of a ring is arranged between two of the anodes.According to DE 199 63 904 C2, the insulating element is made of an electrically insulating material such as aluminum oxide, sapphire, magnesia or silicon carbide. Alternatively, such an insulating element can be made of an anodized aluminum hard alloy such as AlMgSi1.5. In one variant, the insulating element is made of a metal material that conducts heat well and is provided with an electrically insulating coating.According to DE 199 63 904 C2, the insulating element is provided with a feed channel (or a plurality of feed channels) which is formed integrally therein and is oriented transversely and in particular perpendicularly to the combustion chamber axis. The feed duct itself has an orifice opening which is oriented such that a fluid can be blown into the combustion chamber transversely to a radial direction and in particular tangentially to an azimuthal direction of the combustion chamber. The feed channel extends through the housing wall in order to be able to jet the fluid, which can absorb the filler material, into the combustion chamber from the outside. The filler material is injected into the combustion chamber via the feed channel with the aid of a transport gas, which is in particular an inert gas such as argon, helium, nitrogen or neon. The orifice opening can be arranged in such a way that the fluid can be blown in at or in the vicinity of a side face of the combustion chamber in order to enable tangential feeding of the fluid via the insulating element which is thus also designed as a feed device.The feed channels configured for feeding the filler material can generally also be referred to as injectors. Injectors are generally formed from a hard metal. These are specialized components which are used in thermal coating systems. Cemented carbides, also known as cemented carbides, are composites made from hard carbide (e.g., tungsten or silicon carbide) and a binder metal (often cobalt or nickel). Due to their high hardness, wear resistance and ability to withstand high temperatures and corrosion, they are ideal for use in plasma torch injectors that can produce a plasma at very high temperatures (e.g., 20,000 K).The main object of these cemented carbide injectors in thermal coating is to inject the coating material (powder, wire, rod, etc.) precisely and uniformly into the spray flame or plasma stream, which then applies the material to the target surface. Because of the extreme conditions to which they are exposed, including high temperatures, abrasion and chemical corrosion, the use of cemented carbide for these components is actually of great importance to maximize their life and performance. In total, injectors made of hard metal thus improve the efficiency, reliability and quality of thermal coating processes, in particular during plasma spraying, in that they ensure a constant and precise supply of the coating material and at the same time withstand the demanding operating conditions.Injectors made of hard metal are also used, for example, in plasma torches in the production of components for high-voltage heaters for use in vehicles, in particular motor vehicles. As filler material or coating powder, for example, ceramic coating powder in the form of Al 2 O 3 and ZrO 2 or the like is used there and injected into the corresponding plasma flame of the plasma torch.Especially in this application, however, the following problems and disadvantages have been found, in contrast to the above-mentioned advantages: the abrasive powder thus grinds out the injectors over time, so that the injection conditions change negatively. Furthermore, the tips of the injectors that are situated close to the plasma flame are affected by wear in particular due to the heat input through the plasma flame and the subsequent softening.The injection conditions changed as a result in turn cause undesired changes in the product properties unless an early exchange of the wear-afflicted injectors takes place. In particular, the changed injection conditions cause a loss of application efficiency and thus an additional consumption of material (powder). If, on the other hand, the injectors are replaced at an early stage, additional interruptions in the series production occur more frequently, and this sometimes occurs well before the intended changeover interval of the plasma torches, apart from the fact that the injectors themselves are cost-intensive.Preparation of the Invention:It is therefore an object to specify an injector and an injector arrangement comprising the latter, which make it possible to reduce wear, avoid premature replacement, improve injection conditions and / or reduce the outlay for replacement and overall the costs.According to aspects of the invention, an injector arrangement for a burner, in particular a plasma burner, is proposed for this purpose, which is suitable for carrying out a thermal coating method. The injector arrangement comprises in particular at least one injector which forms a feed channel for feeding an abrasive additive material coming into contact therewith during the feeding to a burning or plasma flame. It further comprises an adapter for attaching the injector to a powder line, wherein the adapter preferably has little or no contact with the abrasive additive in comparison to the injector when said abrasive additive is supplied by the powder line during operation. In addition, the adapter can thereby be located outside the negatively acting heat influence zone (e.g. the plasma jet), whereby it is not softened and thus maintains its wear resistance. The powder line is not itself part of the injector assembly. It here denotes a device known as such, comprising a powder reservoir, a feed device which actively feeds the powder as filler material and a line through which the powder is fed, which can be connected to the adapter. For this purpose, the adapter can have a receptacle for this line.A slight contact of the adapter compared to that of the injector with the abrasive additive when it is supplied by the powder line during operation can mean that the adapter at least partially also forms the supply channel through which the abrasive additive material is guided to the combustion or plasma flame. A comparatively low contact may for example include 50% or less of the total contact area, preferably 30% or less, more preferably 20% or less, preferably 15% or less. In the detailed exemplary embodiment described below, the proportion of the contact surface of only the adapter in the entire contact surface is, for example, approximately 10%. The total contact surface here means the inner surface of the feed channel, limited to injector and adapter, i.e. without powder line.Furthermore, the injector and the adapter are connected to one another during operation, but form separate, i.e. separate parts from one another. This allows the injector assembly to be assembled and disassembled again. It thereby becomes possible to exchange the parts individually. In particular, the injector arrangement can advantageously be designed such that the injector which is severely affected by abrasive wear can be replaced, while the adapter which is less or not affected at all continues to be used. This saves costs overall by recourse to the replacement of possibly standardized components, and the extent of a replacement can also be reduced.Furthermore, it can be provided in particular that the injector is formed from a ceramic material, in particular from aluminum oxide (Al 2 O 3) that is to say no longer from a hard metal. On the one hand, this has the advantage that the selected aluminum oxide offers a very good price performance ratio: at comparatively low costs, this material ensures excellent or at least sufficient wear resistance that the ceramic material preferably comprises aluminum oxide (Al 2 O 3). More preferably, the ceramic material comprises aluminum oxide (Al 2 O 3) to an extent of at least 50% by weight and even more preferably to an extent of at least 90% by weight, based on the total mass. The ceramic material can also preferably consist entirely of aluminum oxide.Furthermore, the combination of the features, as a result of which the component subjected to the abrasive contact is made of a resistant ceramic and is designed as a geometrically simple and easily replaceable element, makes it possible to avoid increased service lives which ensure uninterrupted series operation between the burner changeover intervals. Furthermore, this ensures constant injection conditions over a long time and thus constant product properties in series operation.In addition, a particular advantage also arises in the case of use in a burner for the thermal coating of components, inter alia in heating devices of vehicles, in particular motor vehicles, ships, boats, aircraft, construction vehicles etc., in that the ceramic substances usually used there can be compatible with the ceramic material of the injector. As a result, no foreign substances are incorporated into the coating even in the case of abrasion which cannot be completely ruled out, in particular if the powder and the injector consist of identical ceramic materials.According to a specific embodiment, in the injector arrangement, the adapter can be formed from a metal. In principle, any materials which sufficiently withstand the comparatively low abrasive, thermal and mechanical loads, preferably hardened steel, are possible here. Specific examples relate to 1,2379 / X153CrMoV12 cured to greater than 55 HRC and annealed 2 x 1h at 400°C or else 1,2436 / X210CrW12 cured to greater than 55 HRC and gas nitrided: 90 min at 580°. This allows cost-effective production and also lowers the material costs, since the adapter has little or no contact with the abrasive powder.According to one exemplary embodiment, the adapter has a first receptacle which is configured to receive a first section of the injector in itself in a frictional and / or positive-locking manner. This can be a press-fit connection or also a connection with a thread or bayonet lock, etc., which can be released without great effort if an exchange becomes necessary after a long time. As a result, the injector is fastened mechanically reliably and permanently (and again releasably) by the adapter.A further exemplary embodiment of aspects of the injector arrangement which can be combined therewith provides that the injector forms a tube having a length L and a continuously constant inner and outer diameter D 1, D 2. Such a tube is a very good example of a simple and cost-effectively producible component geometry. Furthermore, the predominant contact surface with the filler material can be accommodated by the pipe here within the arrangement, while the accommodating adapter forms no or only a very small proportion of the feed duct otherwise formed by the pipe. The constant diameter ensures the smallest possible resistance surfaces with respect to the abrasive filler material or powder flowing through.According to a development, an injector holder is provided which has a second receptacle which is in turn configured to receive a second section of the injector. Preferably, the first and second portions of the injector are different portions. The portions may also partially overlap, for example, when the receptacle of the injector holder receives the adapter with the injector received therein. In particular, the first and second portions of the injector may form opposing end portions of the injector. Preferably, the injector contacts the second receptacle in a first subsection of the second receptacle with its outer surface from the inside. In the case of the injector designed as a tube, this first subsection can have a hollow cylindrical shape, in particular if the tube is likewise at least partially cylindrical. As a result, the second section situated toward the orifice of the injector can be held securely in position.According to a further development, the injector holder furthermore has a first connecting portion, in particular a first thread, preferably an internal thread, which interacts with a second connecting portion, in particular a second thread matching the first thread, preferably an external thread, which is configured on the adapter in order to receive the adapter with the injector attached thereto or received therein in the second receptacle. In other words, the second receptacle has the corresponding internal thread. Alternatively, however, other types of connections can also be present here for the connecting section, e.g. also again press-fit or bayonet connections. These features, alternatives and options make it possible to hold the injector reliably in the second receptacle without the injector made of ceramic material having to have complex connecting structures, which would again significantly increase its production costs. It therefore only needs to fulfil its main function of forming a long feed channel for the filler material which is resistant to abrasion and which extends as far as possible up to the powder line.According to a further development, the second receptacle of the injector holder has a narrowed section at an end facing the plasma flame of the burner or plasma torch. This is open toward the plasma flame in order to allow injection of the additive material from the injector into the plasma flame, the inner diameter D 3 of which, however, is smaller than the outer or an outer diameter D 2 of the injector. In particular in the case of the tubular shape of the injector, this narrowed section consequently forms a type of stop for the injector toward the plasma flame. However, a stop function is not absolutely necessary, since the position of the injector can already be fixed by the above-mentioned first and second connecting sections and a hollow shape (e.g. hollow cylindrical shape) of the second receptacle. The great advantage of the narrowed section is rather that, as a component of the injector holder, it forms an additional thermal protection against thermal radiation for the injector in a cooled embodiment without itself being worn.According to a further development, the injector arrangement (further comprising: an injector holding plate, wherein the injector holding plate has substantially a disk shape, in the middle of which a through opening for the passage of the plasma flame is formed. The direction of the plasma flame and the through-opening (second axis, see below) may be substantially perpendicular to a plane in which the injector support plate extends. The injector holder may be fastened to the injector holder plate or preferably formed integrally therewith.According to specific exemplary embodiments, the injector holder is fastened to the injector holding plate or is formed integrally therewith in such a way that a first axis for the supply of the filler material, which first axis is defined by the injector, lies perpendicular to a second axis defined by the through-opening and intersects it.According to a further development of the injector arrangement, a total of two, three or more of the injector holders are each formed in a corresponding manner on or in the injector holding plate. In the manner described above, these each accommodate a corresponding adapter with an injector accommodated therein, wherein the injector holders are preferably grouped at equidistant intervals in the circumferential direction around the through-opening.One or more of the above-mentioned objects can also be achieved by a burner, in particular a plasma torch, for carrying out a thermal coating method. The burner comprises a combustion chamber (or plasma chamber) and a connection for injecting a working gas into the combustion chamber, an electrical connection for the power supply, at least one anode and at least one cathode which is situated opposite the at least one anode in the combustion chamber and which are connected to the electrical connection in a corresponding manner in each case in order to allow a voltage to be applied between the at least one anode and the at least one cathode, with which a plasma flame is generated in the injected working gas in the combustion chamber. The burner further includes an injector assembly as described above in accordance with aspects, embodiments, and embodiments. The through-opening forms an opening of the combustion chamber, through which the plasma flame exits.An exact relative positioning of the injector and the combustion chamber or plasma chamber on the basis of the injector plate design ensures that the filler material is effectively heated and accelerated before it hits the workpiece to be coated in melted or partly melted particles. Here, the geometric arrangement is designed to achieve uniform coating and high adhesion of the material to the substrate. The advantages are the same as those respectively described above.Brief Description of the Drawings:Aspects and embodiments of the invention will become more fully understood from a detailed description of the preferred embodiments which will be explained below in conjunction with the accompanying drawings. Shown therein are: FIG. 1 shows a schematic diagram of a structure of a burner according to an exemplary embodiment of the present invention; FIG. 2 shows a perspective view with a partial sectional illustration of an injector arrangement according to an exemplary embodiment; FIG. 3 shows a sectional side view through an injector holder with adapter and injector accommodated therein according to the embodiment example from FIG. 2.Detailed Description of Preferred Embodiments:In the following description of the preferred embodiments, it is to be understood that the present disclosure of the various aspects is not limited to the details of the construction and arrangement of the components as set forth in the following description and figures. The embodiments may be implemented or implemented in various ways. It should also be noted that the terms and terms used herein are for the purpose of the specified description only and should not be understood as limiting as such by those skilled in the art. Further, in the following description, identical reference numerals denote the same or similar features or objects in the embodiments or the figures, so that repeated detailed description thereof is omitted in some cases to preserve the compactness and clarity of the description.FIG. 1 shows a schematic illustration of an overview of a burner, in particular a plasma burner 10, according to an exemplary embodiment of the present invention. The plasma torch 10 comprises a connection 11 for working gas 16 to be introduced, an electrical connection 12 (positive pole) for a cathode 18, an electrical connection 13 (negative pole) for an anode 17 and a power supply 14 which supplies the electrical connections 12, 13 and thus the anode 17 and cathode 18 with power, in particular with sufficient voltage, in order to form, in a manner known as such, during operation, a plasma flame 100 in a combustion chamber 15 in which the anode 17 and the cathode 18 are located opposite one another. A plurality of anodes 17 and / or a plurality of cathodes 18 can also be configured. To form the plasma flame 100 therein, the working gas 16 is introduced into the combustion chamber 15 via ducts indicated schematically in FIG. 1 past the cathode 18. The working gas can be, for example, argon, helium, nitrogen, hydrogen or mixtures thereof without limiting the generality of the invention.As a result of the plasma formation (in particular stress-induced ionization of the working gas), the working gas is heated to very high temperatures, for example 15,000 K to 20,000 K), thereby increases the pressure and is accelerated to high speed by the narrowing orifice of the combustion chamber 15 and is spun out of the combustion chamber as a plasma jet along an axis designated as second axis 74.Above the orifice is an injector support plate 70 which is part of an injector assembly 20. The injector holding plate 70 has a through-opening 72, which is cylindrical and whose cylinder axis coincides with the second axis 74. The plasma jet exits the combustion chamber 15 through this orifice 72.As can be seen in greater detail in FIG. 2, injector holders 60 with injectors 30 accommodated therein and held by them are arranged on the injector holding plate 70. The number of injector holders 60 is five in the exemplary embodiment. Other numbers are also possible. Via the injector holding plate 70, the injector holders 60 formed integrally therewith and, associated therewith, the injectors 30 are positioned in fixed spatial relationship with respect to the plasma jet (or the plasma flame 100) emerging from the orifice.The injectors 30 each have a feed channel 32, via which a filler material 80, here a powder, can be supplied to the plasma jet or the plasma flame 100. The powder can be, inter alia, carbides, iron-based substances, MCrAlY-based substances, molybdenum-based substances, nickel-based substances, oxides or pure metals or alloys, etc. In the exemplary embodiment, the filler material is aluminum oxide (Al 2 O 3).The filler material 80 is supplied from a powder line 50. This has, for example, a powder reservoir 52, which supplies the filler material 80 to the injector 30 via a powder line 54. The powder line 50 comprises a conveyor (not shown separately, part of Bz for delivery purposes. 52). Via the injector 30 or the feed channel 32, the filler material 80 is conveyed into the plasma jet, which melts or melts the particles and entrains them. The conveying direction determined by the positioning of the injector 30 corresponds to a first axis 36 which forms an axis of the feed channel 32.The hot working gas with the powder particles 82, now referred to as spray particles, of the original filler material 80 now expands in a spray-like manner and spins these spray particles 82 at high speed onto a workpiece 2, where it forms a coating 6 on its surface. This results in a dense coating (for example 95-98% of a maximum packing density) at deposition rates of purely by way of example 2,000 to 8,000 g / h, with layer thicknesses of for example 0.1 to 2.5 mm.With reference to FIG. 2, it can be seen that the first axis 36 of the conveying direction of the filler material 80 (powder) and the second axis 74 of the plasma jet / plasma flame 100 or of the orifice opening 72 are perpendicular to one another.It can also be seen in FIG. 2 that the injector arrangement 20 has an adapter 40 in addition to the injector holding plate 70, the injector holder 60 and the injector 30. The adapter 40 serves for connecting the injector 30 to the powder line 50.For this purpose, the adapter 40 has-as can be seen in FIG. 3-a bore 46 for connection to the powder line 50 or for receiving the powder line 54, a tube section 48 which is very short in comparison to the feed duct 32 in the injector 30, and a first receptacle 42, into which the injector 30 is fitted (for example by means of a re-detachable press fit). The adapter 40 and the injector 30 thus form a unit connected to one another. This unit is in turn accommodated in a second receptacle 62 of the injector holder 60. For this purpose, the injector holder 60 has a first connecting portion 64 and the adapter has a second connecting portion 44, which are designed as matching threads.The injector 30 is formed as a tube 31 having an outer diameter D 2 and an inner diameter D 1. The tube is formed of alumina (Al 2 O 3). A first section 33 of this tube 31 faces the powder line 50 and is accommodated in the first receptacle 42 of the adapter 40, which is correspondingly cylindrical. A second portion 34 of the tube faces the plasma flame 100, has a corresponding mouth 38 of the feed duct 32 forming the interior of the tube 31, and is fitted into a cylindrical portion of the second receptacle 62 of the injector holder 60. The mouth 38 of the tube 31 abuts a short narrowed portion 63 of the second receptacle 62, which at least partially shields the mouth 38 from the plasma flame 100. However, an inner diameter D 3 of the narrowed portion 63 is equal to or larger than the inner diameter D 1 of the pipe 31, but is also smaller than the outer diameter D 2 of the pipe 31.List of reference characters2 Workpiece 6 Coating 10 Plasma torch 11 Connection for working gas to be introduced 12 Electrical connection (cathode) 13 Electrical connection (anode) 14 Power supply 15 Combustion chamber 16 Working gas 17 Anode 18 Cathode 20 Injector arrangement 30 Injector 31 Pipe 32 Feed duct 33 First section 34 Second section 36 First axis (injector) 38 Orifice of the injector 40 Adapter 42 First receptacle 44 Second connecting section 46 Bore for connection of powder line 48 Pipe section 50 Powder line 52 Powder reservoir 54 Powder line 60 Injector holder 62 Second receptacle 63 Narrowed section 64 First connecting section 70 Injector holder plate 72 Passage opening 74 Second axis (passage opening of the injector holder plate) 80 Filler material 82 Spray particles in the plasma jet 100 Plasma flameReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 199 63 904 C2 [0004, 0005, 0006]

Claims

Injector arrangement (20) for a burner, in particular a plasma torch (10), for carrying out a thermal coating method, comprising: an injector (30) which forms a feed duct (32) for feeding an abrasive additive material (80) which comes into contact therewith during feeding to a combustion or plasma flame (100), an adapter (40) for bonding the injector (30) to a powder line (50); wherein the injector (30) and the adapter (40) are connected to one another but form separate parts, wherein the injector (30) is formed from a ceramic material.Injector arrangement (20) according to Claim 1, wherein the adapter (40) has little or no contact, in comparison with the injector (30), with the abrasive filler material (80) guided through the feed duct (32) and / or is formed from a metal; and / or the injector (30) is formed from aluminium oxide (Al 2 O 3) and / or the material of the filler material.Injector arrangement (20) according to either of Claims 1 and 2, wherein the adapter (40) has a first receptacle (42) which is configured to receive a first section (33) of the injector (30) therein in a frictional and / or positive-locking manner.The injector assembly (20) according to any one of claims 1 to 3, wherein the injector (30) forms a tube (31) having a length (L) and a continuously constant inner and outer diameter (D1, D2).The injector assembly (20) of any of claims 1 to 4, further comprising: an injector mount (60), the injector mount (60) having a second receptacle (62) configured to receive a second portion (34) of the injector (30).The injector arrangement (20) according to claim 5, wherein the injector holder (60) further comprises a first connecting portion (64), in particular a first thread, preferably an internal thread, which interacts with a second connecting portion (44), in particular a second thread matching the first thread, preferably an external thread, which is configured on the adapter (40) in order to accommodate the adapter (40) with the injector (30) attached thereto or received therein.Injector arrangement (20) according to Claim 5 or 6, wherein the second receptacle (62) of the injector holder (60) has, at an end facing the plasma flame (100), a narrowed section (63) which is open towards the plasma flame in order to allow injection of the additive material (80) from an orifice (38) of the injector (30) into the plasma flame (100), wherein an inner diameter (D3) of the narrowed section is smaller than the outer or an outer diameter (D2) of the injector (30) and in particular of the orifice (38).The injector assembly (20) according to any one of claims 5 to 7, further comprising: an injector support plate (70), wherein the injector support plate (70) has a disc shape, in the middle of which a through hole (72) for the passage of the plasma flame (100) is formed, wherein the injector support (60) is fastened to the injector support plate (70) or preferably is formed integrally therewith.The injector assembly (20) of claim 8, wherein the injector mount (60) is attached to or integrally formed with the injector mounting plate (70) such that a first axis (36) defined by the injector (30) for supplying the filler material (80) is perpendicular to and intersects a second axis (74) defined by the through hole (72).The injector assembly (20) according to claim 9, wherein a total of two, three or more of the injector holders (60) are respectively formed in a corresponding manner on or in the injector holding plate (70) and each accommodate an adapter (40) with the injector (30) accommodated therein, wherein the injector holders (60) are grouped at equidistant intervals in the circumferential direction around the through-opening (72).

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