Heating cartridge with ceramic casting compound

Abstract

A heating cartridge is described comprising a ceramic core (11), a heating wire which surrounds the ceramic core (11) as a heating coil (12), and a metal sleeve (13) in which the ceramic core (11) and the heating coil (12) are arranged, wherein the heating coil (12) is embedded in a ceramic potting compound which fills a space between the ceramic core (11) and the inside of the metal sleeve (13).

Description

[0001] The invention relates to a heating cartridge, in particular for heating a spray nozzle. Such heating cartridges contain a heating wire arranged as a heating coil in a metal sleeve. Heating cartridges according to the preamble of claim 1 are known from US 2 831 951 A, US 10 728 956 B2, and US 3 970 822 A.

[0002] DE 20 2007 010 865 A1 discloses a heating cartridge having a continuous central bore for accommodating a plastic injection nozzle. This heating cartridge has an annular interior enclosed by a metal casing. Within this interior, a heating coil, wound on a ceramic winding body, is embedded in ceramic powder. The metal casing is composed of two parts, with a gap enclosed by an annular wall. All parts are compressed together by radial pressing. The annular wall forms a bead on the outer metal casing, from which the connections of the heating coil extend. This bead is filled with potting compound or insulating granules.

[0003] Heating cartridges for spray nozzles and other applications must be compact and enable high heating outputs, which requires good thermal coupling between the heating coil and the surrounding metal sleeve. Typically, the interior of the metal sleeve is filled with ceramic powder such as magnesium oxide, and the metal sleeve is then plastically deformed, compacting the ceramic powder. After the plastic deformation of the metal sleeve and subsequent compaction, the exterior of the metal sleeve often requires further processing, particularly grinding. Furthermore, not all metal sleeves are suitable for this process. For example, the metal sleeve may have a shape, material, thickness, or coating that is unsuitable for compaction. Furthermore, the process is very complex.

[0004] The object of the present invention is to show a way how a heating cartridge can be created with less effort, which enables a high power density.

[0005] This object is achieved by a heating cartridge having the features specified in the claim. Advantageous developments of the invention are the subject of subclaims.

[0006] In a heating cartridge according to the invention, the metal sleeve in which the heating coil is arranged is filled with ceramic casting compound. A gap between a ceramic core surrounding the heating coil and the inside of the metal sleeve is thus filled with ceramic casting compound. Since the ceramic casting compound is poured into the metal sleeve as a liquid, subsequent compaction through plastic deformation of the metal sleeve is eliminated. Reworking the outside of the metal sleeve is therefore not necessary.

[0007] Since plastic deformation of the metal sleeve is no longer necessary with a ceramic potting compound, the risk of damage to components inside the heating cartridge is eliminated. The heating coil can therefore be easily wound around a ceramic core containing one or more channels, in which, for example, a supply line for the heating coil is arranged. It is also possible to first insert the heating coil into the metal sleeve without a core and then fill the interior of the metal sleeve with ceramic potting compound. In this way, the ceramic potting compound then forms a ceramic core that surrounds the heating coil and also fills the space between the core and the inside of the metal sleeve. The ceramic core is preferably made in one piece, for example by the potting compound or as a separate component. However, individual segments such as discs that are joined together are also conceivable.

[0008] Furthermore, a temperature sensor can be arranged directly in the very heat-conducting potting compound, since no mechanical pressure is exerted on it. A protective housing is therefore not necessary. This advantageously allows for very precise temperature measurement. The temperature sensor is preferably arranged between a base of the metal sleeve and the ceramic core in order to measure the temperature directly at the spray opening. Its connections can be guided together in one or separately in two channels of the ceramic core. However, it is also possible to arrange at least one temperature sensor itself in a channel, in particular in the center of a channel, of the ceramic core. In an advantageous embodiment, the heating cartridge also has a plurality of temperature sensors, for example to record the temperature at the spray nozzle as well as inside the ceramic core.

[0009] In addition, the metal sleeve can have a helical flow channel on its exterior. Flow channels can extend like a thread around the metal sleeve and—when the heating cartridge is inserted into a spray nozzle—define the flow path of a liquid to be heated, so that the heat generated by the heating cartridge is well absorbed. Alternatively, a metal coil can also be arranged on the exterior of the metal sleeve. This, together with the exterior of the metal sleeve and the interior of a spray nozzle housing, can also define a helical flow path for a liquid to be heated. If a flow channel is present, it can be located, for example, in a cylindrical section of the metal sleeve.

[0010] Such a flow channel can be formed by one or more grooves on the outside of the metal sleeve. Grooves can also be provided on the outside of the metal sleeve to increase the surface area and thus improve heat dissipation. For this purpose, grooves can be provided that run in the longitudinal direction of the metal sleeve and whose side walls are designed as ribs.

[0011] The ceramic potting compound preferably contains aluminum oxide, magnesium oxide, or aluminum nitride as a component, particularly as the main component. The potting compound may also contain a binder, such as a silicone resin, as a further component. Water-soluble silicone resins are preferred as binders. The use of a binder increases moisture resistance. The potting compound can advantageously be mixed with water before use. After the metal sleeve has been potted, the potting compound can be dried and cured by applying heat.

[0012] Potting compounds with a thermal conductivity of at least 15 W / mK, preferably at least 30 W / mK, are preferably used. Potting compounds with a viscosity of at least 11,000 mPas, in particular at least 15,000 mPas, are also preferred. Furthermore, the potting compound advantageously has a volume resistance of at least 10 9 Ohm / cm at RT, preferably at least 10 13 Ohm / cm at RT, to ensure electrical safety. A thermal expansion coefficient of at least 5 10 -6 / K is advantageous to prevent gap formation on the inner metal sleeve wall.

[0013] An advantageous development of the invention provides for a spacer, preferably made of ceramic, to be arranged between the heating coil and the inside of the metal sleeve. The spacer can be ring-shaped or sleeve-shaped, for example. However, partially ring-shaped spacers, i.e., spacers that only contact part of the circumference of the heating coil, are preferred.

[0014] Several partially annular spacers, which are located on different circumferential sections of the heating coil, can together center the heating coil in a coil area.

[0015] Advantageously, the spacers can have a radially inward-facing projection that sits between adjacent turns of the heating coil, for example, is clamped there. Depending on the length of the heating coil, the number of spacers can be varied; for example, two, three, or more spacer areas can be provided, in each of which two or more spacers rest against a circumferential section of the heating coil. Spacers are particularly advantageous when the heating coil is first inserted into the metal sleeve without a core and then the interior of the metal sleeve is filled with ceramic potting compound to form the core. One or more spacers can stabilize the heating coil while the potting compound is being poured in.The axial extension of the spacers is preferably one to two times the coil pitch, i.e. the distance from one turn of the heating coil to the adjacent turn, measured from wire center to wire center.

[0016] Further details and advantages of the invention are explained using exemplary embodiments with reference to the accompanying drawings. Identical and corresponding components are designated by identical reference numerals. They show: Fig. 1 a spray nozzle with a heating cartridge in a sectional view; Fig. 2 the heating cartridge of Fig. 1 ; Fig. 3 an oblique view of the lower end of the installed heating coil; and Fig. 4 an oblique view of the upper end of the heating cartridge; Fig. 5 another embodiment of a heating cartridge in a sectional view; Fig. 6 enlarged partial view of Fig. 5 ; Fig. 7enlarged spacers made of Fig. 5; Fig. 8 shows a further embodiment of a heating cartridge; and Fig. 9 shows a sectional view of Fig. 8 .

[0017] The Fig. 1 The spray nozzle shown can be used, for example, to heat or vaporize reducing agents before they are introduced into the exhaust system of a motor vehicle. The spray nozzle has a housing 1 with a spray opening 2, a connection 3 for the electrical lines, and a connection 4 for liquid. A heating cartridge 10 is arranged in the housing 1, which is inserted into the Figures 2 to 4 is shown and explained in more detail below.

[0018] The heating cartridge 10 has a ceramic core 11, a heating coil 12 consisting of one or more heating wires wound around the ceramic core 11, and a metal sleeve 13 in which the ceramic core 11 and the heating coil 12 are arranged. The interior of the metal sleeve 13 is filled with ceramic casting compound. The space between the heating coil 12 and the inside of the metal sleeve 13 is thus filled with ceramic casting compound, preferably over the entire length of the heating coil 12, resulting in a good thermal coupling between the heating coil 12 and the metal sleeve 13.

[0019] The ceramic potting compound is poured into the metal sleeve 13 as a pasty liquid and consists predominantly of aluminum oxide, magnesium oxide, or aluminum nitride. The ceramic potting compound may also contain silicon oxide and, especially during filling, water and / or a binder, such as a silicone resin. Both water-insoluble and water-soluble silicone resins are suitable. After filling, the ceramic potting compound is dried and cured by heat treatment. Any binder initially present may burn during this process.

[0020] The ceramic core 11 contains several channels 6. A connecting section 14 of the heating wire, which also forms the heating coil 12, runs through one of these channels 6. Connecting leads 9 of a temperature sensor 15, which is arranged between a base 16 of the metal sleeve 13 and the ceramic core 11, run through two other channels 6. The temperature sensor 15 can, for example, be an electrical resistor, such as an NTC resistor. However, the connecting leads 9 can also run together in a common channel 6. The ceramic core 11 can contain one or more further channels 6 in which no leads run. These further channels 6 are filled with ceramic potting compound, but can also be empty. The ceramic core 11 can consist of ceramic discs stacked on top of one another.

[0021] The metal sleeve 13 has a cylindrical section with an external thread. This creates a helical flow channel 17 on the outside of the metal sleeve 13. This helical flow channel ensures that the liquid to be heated is guided along the heating cartridge 10 in a helical path in the spray nozzle, allowing it to absorb heat effectively from the heating cartridge 10.

[0022] The end of the heating coil 12 facing the bottom 16 of the metal sleeve 13 is surrounded by a ceramic ring or a ceramic sleeve 18. Particularly when the bottom 16 of the metal sleeve 13, as in the illustrated embodiment, has a conical shape or forms a dome, thus creating a space between the bottom 16 and the ceramic core 11, the heating coil 12 can be advantageously supported at its bottom end using a ceramic ring or a ceramic sleeve 18. Therefore, the ceramic ring or the ceramic sleeve 18 preferably has a radially inwardly projecting annular shoulder at its bottom end to support the heating coil 12, while a cylindrical side wall of the ceramic ring or the ceramic sleeve 18 centers the heating coil 12. A further function arises with respect to the temperature element 15.By increasing the distance between the ceramic core 11 and the base 16 of the metal sleeve through its annular shoulder, the ceramic sleeve 18 creates space for the arrangement of a temperature sensor 9.

[0023] The ceramic ring or ceramic sleeve 18 thus surrounds an end section of the ceramic core 11 or the heating coil 12 and can be supported on the bottom 16 of the metal sleeve 13. The ceramic ring or ceramic sleeve 18 can be provided with slots or openings to allow the ceramic casting compound to more easily fill all cavities in the heating cartridge 10.

[0024] At the upper end, a ceramic bushing 19 centers the ceramic core 11. Above the ceramic bushing 19, the temperature sensor connections 9 and the heating wire connections 14 are each connected to a contact pin 20, in particular by weld points 23. A metallic head sleeve 21, which, as in the exemplary embodiment, is wider than the metal sleeve (13) and can have a stepped cylindrical side wall, is integrally connected to the upper end of the metal sleeve 13, for example by a circumferential weld seam 22 or by the head sleeve 21 and the metal sleeve 13 being formed as a single piece. The potting compound is preferably dimensioned such that the entire heating wire with heating coil 12 and heating wire connections 14 is embedded in the potting compound. In the present example, at least part of the interior of the head sleeve 21 is thus filled with the potting compound, in particular at least filled to the extent that the heating wire connections 14 are completely embedded therein.

[0025] Fig. 5 shows a further embodiment of a heating cartridge 10 for a spray nozzle according to Fig. 1 In this embodiment, the ceramic core around which the heating coil 12 is arranged is made of potting compound. In contrast to the embodiment of Fig. 2 The ceramic core here is therefore not a separate component that is inserted into the metal sleeve 13 together with the heating coil 12. Instead, the heating coil 12 is first inserted into the metal sleeve 13 without the core, and then the interior of the metal sleeve 13 is filled with ceramic potting compound. In this way, the ceramic potting compound then forms a ceramic core that surrounds the heating coil 12 and also fills the space between the core and the inside of the metal sleeve 13.

[0026] In order to stabilize the heating coil 12 during the filling of the casting compound, in addition to centering aids or spacers in the end area, such as the ceramic sleeve 18 and the ceramic bushing 19, further spacers 24 are provided along the heating coil 12 between the end areas, which surround the heating coil 12. In Fig. 6 the heating coil 12 is shown together with spacers 24. Fig. 7 shows spacer 24 in a detailed view.

[0027] The spacers 24 are made of ceramic and, for example, partially annular. In the illustrated embodiment, two almost semicircular spacers 24 support the heating coil 12 in all directions along one winding. However, other spacers extending completely around the heating coil 12 or around more or less than half the circumference of the heating coil 12 are also conceivable. In the illustrated embodiment, the spacers 24 each have a contact surface 26 that is in contact with the inside of the metal sleeve 13. However, only a linear or multi-point contact is also conceivable.

[0028] The spacers 24 can have one or more radially inwardly directed projections 25, for example in the form of an annular bead, which protrude between adjacent turns of the heating coil 12 and thus prevent or at least impede axial displacement of the spacers 24 when filling with potting compound. In an embodiment not shown, the projections 25 can also be thickened at their ends so that they form a locking function and are fixed between the turns during assembly, which greatly facilitates assembly. If the spacers 24 have projections 25, they usually follow the course of the turns. In this case, the spacers 24 are therefore not exactly aligned with a radial plane, but slightly inclined to it, so that individual sections of the spacers 24 are not directly radially opposite one another. Figs. 6 and 7 show enlarged partial views of Fig. 5, in which only the heating coil 12 with spacer 24, or only the spacers 24, are shown. This inclination is clearly visible there. The contact surface 26 in the example thus exhibits an axial displacement along the circumference corresponding to the coil height. This would also be the case with only linear or even multi-point support between the spacer 24 and the inside of the metal sleeve 13.

[0029] In addition to the Fig. 1 to 4 The version shown in Fig. 5 shown version also has a guide aid 27 for guiding the temperature sensor (in Fig. 5 not shown). In the exemplary embodiment, this has a base part adapted to the ceramic sleeve 18 with a central opening for the temperature sensor and a tubular section for guiding the electrical supply line to the temperature sensor. The guide aid 27 can also be designed as a single piece with the ceramic sleeve 18.

[0030] In the Figures 8 and 9 Another embodiment of a heating cartridge 10 is shown. As the sectional view of the Fig. 9 shows, the internal structure of the heating cartridge 10 essentially corresponds to the embodiment of Fig. 5 . A heating coil 12 is arranged in the metal sleeve 13 and the interior of the metal sleeve is filled with ceramic casting compound. Just as in the embodiment of Fig. 5 The potting compound thus forms a ceramic core around which the heating coil 12 is arranged and fills the space between the core and the inside of the metal sleeve 13. The heating coil 12 is held in place by spacers 24 as shown in the Figures 6 and 7 are shown, as well as by a ceramic sleeve 18 into which one end of the heating coil 12 projects.

[0031] The difference to the embodiments described above essentially lies only in the design of the outside of the metal sleeve 13. As Fig. 8 As shown, the metal sleeve 13 of this heating cartridge 10 has grooves on its outer side running longitudinally, the side walls of which are designed as ribs 28, thus improving heat dissipation. Other configurations of grooves and ribs, e.g., transversely running, are equally conceivable. In this exemplary embodiment, a separate head sleeve has also been omitted; that is, the head sleeve is an integral part of the metal sleeve 13.

[0032] The Figures 8 and 9The heating cartridge 10 shown can be used, for example, for a hydrogen tank in fuel cell technology or hydrogen combustion. Such tanks can withstand pressures of up to 600 bar. Since the heating cartridge 10 is filled with ceramic casting compound and thus contains no cavities, compaction or deformation of the metal sleeve 13 due to static pressure of several hundred bar is practically impossible, so that the heating cartridge 10 can easily withstand such pressure loads. List of reference symbols

[0033] 1 Housing 2 Spray opening 3 Connection 4 Connection 6 Channel 9 Temperature sensor connection cable 10 Heating cartridge 11 Ceramic core 12 Heating coil 13 Metal sleeve 14 Heating wire connection section 15 Temperature sensor 16 Base of the metal sleeve 17 Flow channel 18 Ceramic sleeve 19 Ceramic socket 20 Contact pins 21 Head sleeve 22 Weld seam 23 Weld point 24 Spacer 25 Projection 26 Contact surface 27 Guide aid for temperature sensor 28 Ribs

Claims

1. Heating cartridge, in particular for heating a spray nozzle, comprising a ceramic core (11) a heating wire which is arranged as a heating coil (12) around the ceramic core (11), and a metal sleeve (13) in which the ceramic core (11) and the heating coil (12) are arranged, characterised in that the heating coil (12) is embedded in a ceramic casting compound which fills an intermediate space between the ceramic core (11) and the inside of the metal sleeve (13), the casting compound having been filled into the metal sleeve as a liquid and having been dried and hardened after filling.

2. Heating cartridge according to claim 1, characterised in that the ceramic core (11) contains a channel (6) in which a terminal section (14) of the heating wire runs.

3. Heating cartridge according to any one of the preceding claims, characterised in that a temperature sensor (15) is arranged between a base (16) of the metal sleeve (13) and the ceramic core (11).

4. Heating cartridge according to any one of the preceding claims, characterised in that the heating coil (12) contains a plurality of channels (6), wherein an electrical line (9) of a temperature sensor (15) or a temperature sensor (15) itself is arranged in at least one of these channels (6).

5. Heating cartridge according to any one of the preceding claims, characterised in that the ceramic core (11) has at least one continuous channel (6) which is filled only with ceramic casting compound.

6. Heating cartridge according to any one of the preceding claims, characterised in that grooves are formed in the outside of the metal sleeve (13).

7. Heating cartridge according to any one of the preceding claims, characterised in that at least one helical flow channel (17) is formed in the outside of the metal sleeve (13).

8. Heating cartridge according to any one of the preceding claims, characterised in that the ceramic casting compound contains aluminium oxide, magnesium oxide or aluminium nitride as the main constituent.

9. Heating cartridge according to any one of the preceding claims, characterised in that a bottom end of the heating coil (12) is surrounded by a ceramic ring or a ceramic sleeve (18).

10. Heating cartridge according to any one of the preceding claims, characterised in that the ceramic ring or the ceramic sleeve (18) have slots or openings.

11. Heating cartridge according to any one of the preceding claims, characterised in that the metal sleeve (13) is materially connected to a head sleeve (21) which is wider than the metal sleeve (13) and within which connection sections (14) of the heating wire are connected to contact pins (20).

12. Heating cartridge according to any one of the preceding claims, characterised in that the heating wire including heating coil (12) and connection sections (14) is completely embedded in casting compound.

13. Heating cartridge according to any one of the preceding claims, characterised in that a spacer (24) is arranged between the heating coil (12) and the inside of the metal sleeve (13), preferably several spacers (24).

14. Heating cartridge according to claim 13, characterised in that the spacer (24) has a radially inwardly directed projection (25), which is arranged between adjacent turns of the heating coil (12), preferably clamped between adjacent turns.

15. Heating cartridge according to claim 13 or 14, characterised in that the spacer (24) abuts only part of the circumference of the heating coil (12).

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