EP2109345B1 - Heating element and heating device containing a heating element - Google Patents

Description

The present invention relates to a heat-generating element of a heater for air heating, comprising at least one PTC element and voltage applied to opposite side surfaces of the PTC element electrical conductor tracks. Such a heat-generating element is, for example, from the date of the present applicant EP 1 061 776 known.

The heat-generating element is used in particular in a heater for a motor vehicle and comprises a plurality of successively arranged in a series PTC elements which are energized via parallel to each other, flat against opposite sides of the PTC elements electrical conductor tracks. The conductor tracks are usually formed by parallel metal strips. The heat-generating elements thus formed are used for example in a heating device for air heating in a motor vehicle, which comprises a plurality of layers of heat-generating elements, abut on the opposite sides of heat-emitting elements. These heat-emitting elements are applied via a holding device in relatively good heat-transfer contact to the heat-generating elements.

In the aforementioned prior art, a holding device of the heating device is formed by a frame in which a plurality of mutually parallel layers of heat-generating and heat-emitting elements are spring-loaded. In an alternative embodiment, which also discloses a generic heater and the spielsweise in the EP 1 467 599 is described, the heat generating element is formed by a plurality of in a row in a plane successively arranged PTC elements, which are also referred to as ceramic elements or PTC thermistors, which are energized on opposite side surfaces by voltage applied to these tracks. One of the tracks is formed by a circumferentially closed profile. The other conductor track through a metal strip, which is supported with the interposition of an electrical insulating layer on the circumferentially shot metallic profile. The heat-emitting elements are formed by lamellae arranged in several parallel layers, which extend at right angles to the metal profile which is closed at the circumference. At the time of the EP 1 467 599 known generic Heater provided a plurality of circumferentially closed metal profiles formed in the manner described above, which are arranged parallel to each other. The lamellae partially extend between the circumferentially closed profiles and partially protrude beyond them.

In the aforementioned heat generating elements, there is a requirement that the electrical traces must be in good electrical contact with the PTC elements. Otherwise, there is the problem of an increased contact resistance, which may result in particular in the use of heat-generating elements in Zuheizem for motor vehicles because of the high currents that a local overheating occurs. By this thermal event, the heat-generating element can be damaged. In addition, the PTC elements are self-regulating resistance heaters that provide lower heat output at elevated temperature, so local overheating can interfere with the self-regulating properties of the PTC elements.

Moreover, at high temperatures in the range of an auxiliary heater vapors or gases can develop, which can lead to an immediate endangerment of persons located in the passenger compartment.

Accordingly problematic is the use of the generic heat-generating elements even at high operating voltages, for example at voltages up to 500 V. Here, on the one hand the problem that the heat-emitting elements flowing air moisture and / or dirt with it, which penetrate into the heater and here an electric flashover, ie can cause a short circuit. On the other hand, there is basically the problem of protecting persons working in the area of the heating device from the live parts of the heating device or the heat-generating element.

From the WO 99/18756 is an immersion heater with PTC heating elements known, which are arranged between electrical conductor tracks, which are each covered with insulating layers for the isolation of the electrical conductor tracks relative to the metallic housing of the immersion heater. In this prior art, the housing sealingly encloses the PTC heating elements. For insulation, a plate made of an insulating ceramic is provided between the housing and the heat-generating element.

The present invention is intended to specify a heat-generating element of a heating device and a corresponding heating device, which offer increased safety. In particular, the present invention aims to increase the safety with regard to a possible electrical flashover.

Specifically, the present invention is also intended to mean a heating device having a plurality of heat-generating elements comprising at least one PTC element and electrical traces adjacent to opposite side surfaces of the PTC element and a plurality of heat-emitting elements disposed in parallel layers and applied to opposite sides of the heat-generating element , which can be safely and effectively operated with high currents.

To solve the problem with respect to the heat-generating element, it is proposed with the present invention to further develop the above-mentioned heat-generating element by covering the outer surfaces of the electrical conductors by an insulating layer comprising at least two interconnected plastic films and the insulating layers fixed to the housing are connected.

It has been shown that a very good dielectric strength, for example, of 4 kV and more can be achieved if a multilayer film is provided directly on the conductor, possibly with the interposition of a ceramic layer. This multilayer film is preferably glued directly to the ceramic layer or the conductor track by lamination. The use of a multilayer film can be achieved with basically the same layer thickness compared to a single-layer film, a better mechanical protection, since the interconnected films better mechanical stresses can absorb without cracking and failure, as a single-layer film. Accordingly, to improve the heat transfer with the same or even better mechanical strength, the layer thickness of the insulating layer can be reduced. The insulating layer may be formed solely by the multi-layered film, which is preferably provided on the outside of the heat-generating element, so that a heat-emitting element, such as a lamella, abuts directly against the film. Alternatively, one or more ceramic layers can be provided as part of the insulating layer between the film and the conductor track.

The insulating layer should preferably rest directly on the electrical conductor tracks, so that the heat transfer from the heat-generating elements to the heat-emitting elements is impaired only to a small extent. The insulating layer should have the best possible thermal conductivity. The aim is a thermal conductivity of more than 4 W / (m K). As appropriate in terms of the best possible protection against short circuit, an insulating layer with an electrical insulation of more than 6 kV / mm has been found. The insulating layer should preferably have an electrical breakdown strength of at least 2000 V, preferably at least 3000 V, in the transverse direction of the layer structure.

In the heat generating element according to the invention, the insulating layers are fixedly connected to the housing, which is an insulating housing. The insulating layers are on the outside of the electrical conductors and cover them. These in turn receive between them the at least one PTC element which is surrounded by the insulating housing. Accordingly, there is a structure in which the top and bottom of the heat-generating element is covered by the insulating layer, while the intermediate extending end face of the heat-generating element is occupied by the insulating housing. Accordingly, the at least one PTC element is captured and encapsulated by the housing and the insulating layers firmly bonded to the housing from the environment. The housing can form a plurality of receiving openings for receiving individual or multiple PTC heating elements. Also, the wall of a receptacle formed by the housing may be contoured for a plurality of PTC heating elements to space individual PTC elements from each other or to form pitches. Thus, for example, an elongated housing receptacle for the arrangement of a plurality of PTC elements in a row can be formed behind one another, wherein the receptacle for individual PTC elements is divided by inwardly projecting webs.

If desired, the insulating layer can be bonded directly to the electrical conductor. To improve the thermal conductivity between the conductor track and the insulating layer of the adhesive should be provided in a thin layer as possible below 20 microns. For the same reasons, the plastic film is preferably laminated to the ceramic plate, if such is provided. The film preferably has on one side a wax layer of between 10 to 15 .mu.m, in particular under the operating conditions of the heat-generating element, ie at higher temperatures of about 80 ° C, and melts when pressing the insulating layer against the conductor and allows efficient heat transfer. In this case, it is beneficial to arrange the heater from parallel extending layers of heat-generating and heat-emitting elements in a frame and to keep this layer structure under spring tension in the frame, as this basically already going back to the applicant EP 0 350 528 is known. An alternative embodiment was, for example, in the EP 1 515 588 described.

The heat-generating element may be formed by a plurality of successively arranged PTC elements, these two-sided conductive tracks, as well as the interconnects each outside surrounding insulating layers. All components of this layer structure can be connected to one another, in particular adhesively bonded. The electrically conductive insulating layer should in this case preferably project beyond the electrical conductor track, so that the electrically conductive and energized components of the heat-generating element are located at a distance behind the outer, insulated edges of the heat-generating element. The electrical conductor can project beyond the insulating layer to form an electrical contacting point.

For accurate positioning of the PTC elements is proposed according to a further preferred embodiment of the present invention to provide the heat generating element a known position frame which forms a frame opening for receiving the at least one PTC element and as an insulating housing in the sense of the present Invention can be considered. This known per se positioning frame is, for example, in the aforementioned EP 0 350 528 described and is usually made of a non-conductive material, in particular a plastic material. The position frame is usually formed as an elongate member, which leaves out a frame openings in the plane of the PTC elements or the heat-generating element for one or more PTC elements. In this frame opening or the PTC elements are positioned. Such a position frame can essentially form the insulating housing and be firmly connected to the top and bottom sides of the insulating layers. For this purpose, the insulating layers can be glued or welded to the position frame. Also, the plastic material of the insulating housing may be reshaped to connect the insulating layers to the housing. Any kind of A compound capable of providing a strong and preferably tight connection between the insulating layer and the housing material is suitable for the realization of the invention.

To further improve the glued together plastic films is proposed according to a preferred embodiment of the present invention that these plastic films are connected to each other including a Fasergewirkes. The plastic films can for example be laminated on both sides of the Fasergewirke. The fiber knitted fabric can, for example, consist only of fiber strands which are provided substantially parallel to one another and which do not overlap or hardly overlap. Preferably, however, a fiber fabric is used, which can better withstand multiaxial stress states within the composite of the at least two plastic films with the fiber fabric arranged therebetween. It is recommended to use fibers whose electrical conductivity is low. Also with regard to the thermal stress of the fibers of the knitted fabric is proposed according to a preferred development to use a fabric of glass fibers. The fibers of the knitted fabric are furthermore preferably silicone-soaked, so that a substantially air-free inclusion of the fiber knitted fabric results between the plastic films. In the case of a complete wetting of all fiber strands of the knitted fabric, moreover, there is also a firm and therefore good bond between the opposing foil layers.

In particular, for the outer insulation of heat-generating elements, which are installed in a heater for air heating, for example, for heating the vehicle interior of a motor vehicle, it has proved advantageous to glue at least two multilayer plastic films together and these on the outside of the heat-generating element, the conductor tracks to cover directly or indirectly. Each of the multilayer films comprises at least two plastic films glued together. A very effective proposal for good heat transfer through the insulating layer to the outside and for reliable and sufficient insulation has been found to be an insulating layer comprising two multi-layered films bonded together, each of the multi-layered films having two plastic films bonded together. which are glued together directly or with the interposition of a Fasergewirkes.

For the specifically envisaged high-voltage applications, such films have proven to be particularly effective in tests for the resistance to flashover of the foils, which have a dielectric strength of at least 1.05 kV. This dielectric strength is provided by each one of the foils bonded together. The thickness of each individual plastic foil should be between 0.05 and 0.09 mm, preferably between 0.06 and 0.08 mm. Suitable materials for forming the plastic film are polyimide, polyamide, silicone or Teflon (PTFE). The layers glued together may be formed identical to the material or formed of different plastic materials. With regard to a good mechanical strength of the interconnected plastic films, these are preferably connected bubble-free, for example by lamination. Suitable for bonding the two plastic films is in particular a silicone-containing adhesive.

For circumferential insulation of the PTC heating element with adjacent conductor tracks according to another preferred embodiment of the present invention, it is proposed to connect the insulating layer in each case by means of encapsulation with a housing. This housing can consist of two housing shells, which are interconnected. Housing shells have proven to be particularly suitable which comprise two housing elements which abut one another with the interposition of a compressible element, the sealing effect of which improves when pressure is applied to the heat-generating element from outside. It is particularly thought of incorporation of the heat-generating elements in a frame of an electric heater in which the at least one heat-generating element and externally applied thereto heat-emitting elements are held under bias of a spring and placed against each other, wherein the spring is supported on the inside of the frame ,

To improve the strength of the housing, it is proposed according to a further preferred embodiment of the present invention to connect the insulating layer, including at least the edges of the conductor track with the housing by means of encapsulation. Accordingly, the plastic material forming the housing encloses at least one edge region of the conductor track, which is usually formed from a sheet-metal strip, so that a relatively rigid housing is provided which is fixed with respect to its contour. The housing is preferably formed of a thermoplastic elastomer, or of silicone.

To solve the problem underlying the invention sibling with respect to the heating device is proposed to further develop the aforementioned heater in that the heat-emitting elements lie with the interposition of at least two interconnected plastic films comprehensive insulating layer on the opposite sides of the heat-generating element: then lie the two Plastic films on the outside of the heat-generating element and form the contact surface for a heat-emitting element, which is formed for example of a meandering bent aluminum or copper strip.

Figur 1

Eine Querschnittsansicht eines ersten Ausführungsbeispiels eines wärmeer- zeugenden Elementes mit daran anliegenden Radiatorelementen eines elektri- schen Zuheizers;

Figur 2

ein zweites Ausführungsbeispiel eines wärmeerzeugenden Elementes;

Figur 3

die bei den Ausführungsbeispielen gemäß den Figuren 1 und 2 verwendete Isolierfolie in einer perspektivischen Seitenansicht der einzelnen Lagen der Iso- lierfolie; und

Figur 4

eine perspektivische Seitenansicht eines Ausführungsbeispiels einer Heizvor- richtung.

Further details and advantages of the present invention will become apparent from the following description of embodiments of the invention in conjunction with the drawings. In this show:

FIG. 1

A cross-sectional view of a first embodiment of a heat-generating element with adjoining radiator elements of an electric auxiliary heater;

FIG. 2

a second embodiment of a heat generating element;

FIG. 3

in the embodiments according to the FIGS. 1 and 2 used insulating film in a perspective side view of the individual layers of Isulierfolie; and

FIG. 4

a perspective side view of an embodiment of a Heizvor- direction.

The FIG. 1 shows a cross-sectional view of a first Ausführbegsbeiette a heat-generating element 1, which comprises two elongated U-shaped housing elements 2.3, which are each made of plastic injection molded parts. The housing shell elements 2, 3 each have on opposite transverse sides a metal strip 10 and an insulating layer applied thereto 7. The edges of the respective metal strips 10 are wrapped by the housing elements 2, 3 substantially forming plastic material. The two metal strips 10 are connected by molding with the respective plastic material of the housing elements 2, 3. On the outside of the respective housing elements 2, 3 and the metal strips 10 overlapping at its longitudinal edge, an insulating film 9 is applied, provided by the present on the outside of the heat-generating element 1 Insulating layer 7 is formed exclusively and will be described in more detail below.

The opposite end faces of the webs of the U-shaped housing elements 2, 3 provided in parallel are joined by a sealing strip 4, which circumferentially seals the interior formed by the two housing elements 2, 3 and accommodating a PTC heating element 5 to the outside. The sealing effect of the sealing strip increases with an externally acting on the housing 2, 3 compressive force.

The thickness of the sealing strip 4 is chosen so that conceivable thickness-wise manufacturing tolerances of at least one PTC element 5 can be compensated by compression of the sealing strip 4 without abutting the two housing elements. In this context, it should be noted that PCT heating elements are subject to certain dimensional variations due to production. If the elastic properties and the dimension of the sealing strip 4 is selected adapted, such thickness tolerances can be compensated by compression of the sealing strip, so that in the conceivable thickness-wise deviations is basically given a circumferential seal the PTC heating element receiving interior.

The compression of the sealing strip made of a compressible plastic which is supported on opposite end faces of the two housing elements 2, 3 leads to a certain mobility of the two housing elements 2, 3 transversely to a plane which extends parallel to the lower or upper metal strip 6, 7. With increasing pressure from the outside on the heat-generating element 1, the sealing effect increases by the compressible plastic.

As usual, the conductor tracks at the front end of the housing elements 2.3 project beyond this, there to possibly overtop the outside of the heat-generating elements surrounding and holding under bias in a layer structure frame and form there electrical frame connections.

In FIG. 2 a cross-sectional view of a second embodiment is shown. Same components are opposite to in FIG. 1 shown embodiment with the same reference numerals.

FIG. 2 shows an alternative embodiment of a heat generating element 1 in cross-sectional view with a housing consisting of a housing shell element 2, and a shell counter-element 3, which are cup-shaped. Both housing elements 2, 3 are produced as injection-molded plastic parts, by means of which both an insulating film 9 and a metal sheet 10 which directly adjoins the inner side and contacts the PTC heating element 5 are attached by means of extrusion coating. On the outer sides of the sheet metal web 10, the multilayer film 9 is provided as part of the insulating layer. This insulating film 9 is applied by laminating directly on the metal strip 10. The plate-shaped element thus formed is connected by encapsulation with the plastic material forming the housing elements, which is preferably silicone. In this direction, the heat-generating element 1 is relatively thin, so that heat generated by the PTC heating element can pass through a conduit to a radiator element 11 almost unhindered. The radiator elements 11 are additionally taken laterally in the embodiment shown by the plastic material of the two housing elements 2.3 and held in position. In particular, the edges of the housing elements 2, 3 produced by encapsulation project beyond the aluminum oxide layer 8 on the outside, as a result of which the radiator elements 11 lying directly against the aluminum oxide layer 8 do not extend transversely to the one in FIG FIG. 2 shown layer structure can be moved.

Like the embodiment in FIG. 1 also has that in FIG. 2 shown embodiment for manufacturing simplification two identically designed housing elements 2, 3. Each one of the end faces formed by the edges of the respective housing elements 2 and 3 has a groove 20; the other end face is surmounted by a spring 21. The spring 21 of one of the housing elements 2, 3 is engaged in the complementarily formed groove 20 of the other housing element 3, 2, so that the interior of the housing 2, 3 is sealed. For this purpose, care should be taken that the width of the groove 20 is only slightly larger than the thickness of the spring 21. The depth of the groove 20 and the length of the spring 21 are selected so that when taken in the housing PTC elements 5, these lie flat against the sheet metal strip 10 and that the housing elements 2.3 in shrinkage and / or Setzbeträgen or due to Manufacturing tolerances in particular on the part of the PTC elements 5 can be at least slightly moved towards each other and in the expected manufacturing tolerances or thermal expansion groove 20 and spring 21 with sufficient overlap to seal the housing are engaged.

The FIG. 3 shows a side perspective view of exploded layers of the insulating sheet 9, which is provided on the outside of the previously described heat generating element. The insulating film 9 has six layers and consists of two each two-ply plastic films 30, 32, 34, 36, which are each formed identically, have a thickness of 0.07 mm and made of silicone. Each of the plastic films 30 to 36 has a dielectric strength of more than 1.05 kV. The outer plastic film 30 is glued to the adjacent plastic film 32 with the interposition of a glass fiber fabric 38. The glass fiber fabric 38 consists of substantially perpendicular to each other arranged glass fiber strands, which are interwoven with each other. The fiberglass strands are saturated with silicone. The space between the plastic films 30 and 32 is filled with silicone as a whole. Through the two films 30, 32 and the enclosed between them glass fiber fabric 38, a two-ply glass fiber reinforced film 40 is formed. An underlying structure has an underlying two-ply glass fiber reinforced film 42. The two-ply glass fiber reinforced films 40, 42 are each connected by themselves with an adhesive layer, whereby a six-ply insulating film 9 comprising two glass fiber fabric 38 and four plastic films 30, 32, 34 and 36 is formed , The adhesive layer provided between the multilayer films 40, 42 consists of silicone adhesive.

The insulating layer is not on the in FIG. 3 shown embodiment limited. Thus, the glass fiber fabric 38 may be provided and other plastic films. At least two films should be bonded together, which has a composite film with a dielectric strength of 2.0 kV and more. Preferably, three of these composite films are used as the insulating layer. This results in a six-layer insulating layer, in which each individual insulating plastic film has a dielectric strength of at least 1.0 kV. The aim is to provide a heat-generating element for use in a heater for the automotive industry, in which the heat-generating element is each secured with a dielectric strength of 300 volts. This protection takes place on the upper and lower sides of the heat-generating element, which usually abut radiator elements, exclusively through the insulating layer 9. At the end faces, ie the regularly perpendicular extending sides of the heat-generating element 1 is a corresponding protection by the plastic material of the housing 2, 3 provided. With a view to the best possible dielectric strength when using the heat-generating element with operating voltages of up to 500 volts, the insulating layers should each by encapsulation and therefore be tightly received in the housing elements 2, 3.

In Fig. 4 an embodiment of a heating device according to the invention is shown. This comprises a holding device in the form of a circumferentially closed frame 52, which is formed by two frame shells 54. Within the frame 52 a plurality of mutually parallel layers of identically formed heat-generating elements (for example Fig. 1 or 2 ). Further, the frame 52 includes a spring, not shown, by which the layer structure is held under pretension in the frame 52. Preferably, all the heat-emitting elements 56 are disposed immediately adjacent to a heat-generating element 60. The in the Fig. 4 shown heat-emitting elements 56 are formed by meandering bent aluminum sheet metal strips -also identical to the radiator elements 11 according to Fig. 1 or 2. The heat-generating elements are located between these individual heat-emitting elements 56 and behind the longitudinal struts 58 of the Luftein- or outlet opening of the frame 52 passing through the grid. One of these longitudinal struts 58 is removed in the middle of the frame 52 for the sake of illustration, so that there is a heat generating element 60 can be seen.

Since the heat-emitting elements 56 abut against the current-carrying parts with the interposition of an insulating layer 8, the heat-emitting elements 56, d. H. the radiator elements, potential-free. The frame 52 is preferably made of plastic, whereby the electrical insulation can be further improved. An additional protection especially against unauthorized contact with the live parts of the heater is additionally provided by the grid, which is also formed of plastic and formed integrally with the frame shells 54.

On a front side of the frame 52 is located in a conventional manner, a plug connection, depart from the power supply and / or control lines through which the heater can be connected in terms of control and Stromversorgungsmäßig in a vehicle. On the front side of the frame 52, a housing is indicated, which in addition to the plug connection may also have control or regulating elements.

LIST OF REFERENCE NUMBERS

1

Heat generating element

2

Housing shell element

3

Shell counter element

4

sealing strips

5

PTC heating element

7

insulating

9

insulation

10

metal strip

11

radiator element

20

groove

21

feather

30

Plastic film.

32

Plastic film

34

Plastic film

36

Plastic film

38

Glass fiber fabrics

40

multilayer film, outside

42

multilayer film, inside

52

frame

54

frame shell

56--

heat-emitting element

58

along longwall

60

heat generating element

Claims (15)

1. A heat-generating element (1), in particular for heating the air in an electric auxiliary heating of a motor vehicle, comprising at least one PTC heating element (5) and an insulating housing (2, 3) enclosing said PTC heating element (5), as well as electric conductors (10) whose inner surfaces are in contact with opposed sides of the PTC heating element (5),
   characterized in that
   the respective outer surfaces of the electric conductors (10) are covered by an insulating layer (7) comprising at least two interconnected plastic sheets (30, 32; 34, 36), and
   that the insulating layers (7) are fixedly connected to the housing (2, 3).
2. A heat-generating element according to claim 1, characterized in that the plastic sheets (30, 32; 34, 36) are connected to one another and enclose between them a knitted fibre fabric (38).
3. A heat-generating element according to claim 2, characterized in that the plastic sheets (30, 32; 34, 36) are connected to one another and include between them a glass fribre fabric (38).
4. A heat-generating element according to claim 3, characterized in that the plastic sheets (30, 32; 34, 36) are connected to one another and include between them a silicone-soaked knitted fribre fabric (38).
5. A heat-generating element according to one of the claims 2 to 4, characterized in that the insulating layer (7) comprises at least two sheets (40) which comprise interconnected plastic sheets (30, 32; 34, 36) and which are adhesively joined.
6. A heat-generating element according to one of the preceding claims, characterized in that the interconnected plastic sheets (30, 32; 34, 36) provide a dielectric strength of at least 2.00 kV.
7. A heat-generating element according to one of the preceding claims, characterized in that the at least two interconnected plastic sheets (30, 32; 34, 36) are in indirect contact with the electric conductors (10), and that the at least two interconnected plastic sheets (30, 32; 34, 36) are provided on the outer side of the heat-generating element (11).
8. A heat-generating element according to one of the preceding claims, characterized in that the insulating layer (7) is connected by means of insert moulding to the insulating housing (2, 3) enclosing the PTC heating element (5).
9. A heat-generating element according to claim 8, characterized in that the insulating layer (7) is connected to the housing (2, 3) such that the end faces of the electric conductor (10) are encompassed.
10. A heat-generating element according to claim 8 or 9, characterized in that the housing (2, 3) is made of silicone.
11. A heat-generating element according to one of the preceding claims, characterized in that the material forming the plastic sheets (30, 32; 34, 36) is selected from the group consisting of: polyimide, polyamide, silicone.
12. A heat-generating element according to one of the preceding claims, characterized in that the plastic sheet (30, 32; 34, 36) has a thickness of from 0.05 mm to 0.09 mm, preferably of from 0.06 mm to 0.08 mm.
13. A heat-generating element according to one of the preceding claims, characterized in that the interconnected plastic sheets (30, 32; 34, 36) are interconnected by a silicone-containing adhesive.
14. A heating device with a plurality of heat-generating elements (60), comprising at least one PTC element (5) and electric conductors (10) which are in contact with opposed lateral surfaces of the PTC element (5), and heat-emitting elements (56) arranged in parallel layers and supported such that they are in contact with opposed sides of the heat-generating element (60),
    characterized in that
    the heat-emitting elements (56) are in contact with the opposed sides of the heat-generating element (60) via an interposed insulating layer (7) comprising at least two interconnected plastic sheets (30, 32; 34, 36).
15. A heating device for heating air, characterized by at least one heat-generating element according to one of the claims 1 to 13.

Images