HEAT EXCHANGER WITH THICK-FILM RESISTOR

Abstract

 The present invention relates to a heat exchanger (1) for heating a fluid having at least two distanced tube bodies (3) through which a flow path (8) of the fluid leads.
A simplified manufacture and/or a reduced construction size and/or an increased heating efficiency of the heat exchanger (1) are achieved by arranging a separately produced thin-film resistor (11) on the outer surface (12) of at least one of the tube bodies (3) and connecting thin-film resistor (11) to the outer surface (12) of the corresponding tube body (3) and by arranging an outer interface (13) made of a thermal interface material between the thick-film resistor (11) and the next neighbouring tube body (3)

Description

[0001] The present invention relates to a heat exchanger for heating a fluid, in particular for heating a coolant, wherein the heat exchanger comprises tube bodies and at least one thick-film resistor.

[0002] For heating a fluid with a heat exchanger, the fluid usually flows through the heat exchanger. Within the heat exchanger, heat is transferred to the fluid in order to heat the fluid. In generic heat exchangers, heat is generated by the consumption of electric power.

[0003] For this purpose, it is known to use positive temperature coefficient elements, also known to the skilled person as PTC-elements. Corresponding heat exchangers typically comprise tube bodies which delimit a flow path of the fluid to be heated. The PTC-elements are connected to the tube bodies in a heat transferring manner, thereby, in operation, heating the fluid flowing through the tube bodies. In most applications the PTC-elements are arranged outside the tube bodies, in particular for the protection of the PTC-elements and/or for avoiding electric interaction of the PTC-elements with the fluid.

[0004] These heat exchangers are usually manufactured by arranging single PTC-elements, commonly in a successive manner, on a corresponding tube body. This leads to a complicated and expensive manufacture of the heat exchangers.

[0005] Due to the temperature dependency of their electrical resistance PTC-elements are appreciated for their heating behaviour. However, the temperature dependent electrical resistance undergoes a minimal resistance with at a so-called critical temperature. This behaviour leads to a peak of current flowing through the PTC-element at the critical temperature. This peak means a considerable load for the electric components of the heat exchanger and/or neighbouring components and can lead to their failure.

[0006] It is therefore known to use alternative electric heating elements. Thick-film resistors might be used here because of their availability and their cost-effective production.

[0007] A corresponding heat exchanger might comprise a single tube body on which a thick-film resistor is arranged. Such a heat exchanger, however, is expensive and comprises a low efficiency.

[0008] As an alternative, the heat exchanger could comprise several tube bodies. In such a heat exchanger, a corresponding thick-film resistors could be arranged on each tube body. This, however, leads to a complicated and complex manufacture of the heat exchanger. Moreover, the construction size is rather big and/or the heating efficiency is rather low.

[0009] The problem addressed by the present invention is therefore the disclosure of an improved, or at least an alternative, form of embodiment of a heat exchanger for heating a fluid, which is in particular characterized by a simplified manufacture and/or a reduced construction size and/or an increased heating efficiency.

[0010] According to the invention, this problem is solved by the subject matter of the independent claim 1. Advantageous forms of embodiment are the subject matter of the dependent claims.

[0011] The present invention is based on the general idea of, in a heat exchanger for heating a fluid, providing at least two tube bodies through which the fluid flows and of arranging a thick-film resistor produced separate from the tube bodies on the outer surface of at least one of the tube bodies and connecting the thick-film resistor to the next neighbouring tube body in a heat transferring manner. By arranging the thick-film resistor on the surface of the corresponding tube body, a simplified manufacture of the heat exchanger and a reduced construction size are achieved. Moreover, by connecting the thick-film resistor to the next neighbouring tube body in a heat transferring manner, heat is also transferred to the next neighbouring tube body and hence to the fluid flowing through the tube body.

[0012] In accordance with the general idea of the invention the heat exchanger, in operation, heats the fluid. The fluid, in operation, flows through the at least two tube bodies. The heat exchanger thus comprises at least two tube bodies. Each tube body extends in a direction in which the fluid flows through the tube body. This direction is also referred to as extension direction in the following. Each tube body is further circumferentially enclosed and thus in circumferential direction. As a result, each tube body delimits a flow path of the fluid along the extension direction. The tube bodies are distanced to another transverse to the extension direction. That is, the tube bodies are distanced to another in a distance direction transverse to the extension direction. A thick-film resistor produced separately from the tube bodies is arranged on an outer surface of at least one of the tube bodies. The thick-form resistor is connected to the outer surface of the corresponding tube body. The thick-film resistor is further thermally connected to the next neighbouring tube, wherein the thermal connection is achieved by a thermal interface material. That is, an interface made of thermal interface material is, on the side of the thick-film resistor averted from the corresponding tube body, arranged between the thick-film resistor and the outer surface of the next neighbouring tube body and connects the thick-film resistor to the next neighbouring tube body in a heat transferring manner. This interface is hereinafter also referred to as outer interface.

[0013] In operation, the at least one thick-film resistor is electrically supplied and generates heat. The heat exchanger is therefore an electric heat exchanger.

[0014] In operation, the fluid flows along the flow path through the tube bodies, wherein heat is transferred to the fluid from the at least one thick-film resistor via the tube bodies.

[0015] The separate production of the thick-film resistor, in particular, means that the thick-film resistor is a separately produced element/component which is connected to the outer surface of the corresponding tube body.

[0016] The connection of the thick-film resistor on the outer side of corresponding tube body, in particular, means that the thick-film resistor is attached to the outer surface.

[0017] Preferably, at least one of the at least one thick-film resistors is directly connected to the outer surface of the corresponding tube body. This leads to improved heat transfer between the thick-film resistor and the corresponding tube body and thus to increased heating efficiency.

[0018] Directly connected, in particular, means that the thick-film resistor directly contacts the outer surface of the corresponding tube body. In case the tube body is electrically conducting, a thin electrically insulating layer is preferably arranged between the thick-film resistor and the outer surface of the corresponding tube body. The insulting layer can be a dielectric layer that might be produced directly on the outer surface, for instance by means of oxidation of the outer surface.

[0019] As an alternative to the direct connection, at least one of the at least one thick-film resistors can be connected to the outer surface of the corresponding tube body by means of an intermediate layer for increasing heat transfer between the thick-film resistor and the outer surface of the corresponding tube body.

[0020] In advantageous forms of embodiment, the intermediate layer is a thermal interface material. That is, according to advantageous forms of embodiment, an interface made of a thermal interface material is arranged between the thick-film resistor and the outer surface of the corresponding tube body, wherein the interface connects the thick-film resistor to the corresponding tube body in a heat transferring manner. This interface is also referred to an inner interface hereinafter.

[0021] Each interface, that is inner interface and outer interface, allows to adjust the amount of heat transferred from the thick-film resistor to the corresponding outer surface, for instance by means of the thickness and/or heat transfer coefficient of the interface. This allows for a control heat transfer to each tube body and thus a controlled and improved operation of the heat exchanger. The at least one outer interface and at least one inner interface can in particular, by means of their thickness and/or heat transfer coefficient, be such that, in operation, each tube body receives substantially the same amount of heat. This leads to a substantially equal/homogenous heating of fluid flowing through all tube bodies and thus to an increased heating efficiency.

[0022] At least one of the at least two tube bodies is advantageously made of a metal or an alloy. This leads to improved thermal conductivity of the tube body and thus an improved heat transfer from the at least one thick-film resistor to the fluid. Preferably, at least one of the at least two tube bodies is made of aluminium or an aluminium alloy.

[0023] Each tube body can generally have an arbitrary shape. It is for instance possible that at least one of the at least two tube bodies has a circular shape and/or cross section.

[0024] Preferably, at least one of the at least two tube bodies is a flat tube. The tube body thus has two opposing flat sides. This leads to a reduced construction size of the heat exchanger.

[0025] A flat tube further allows a simplified and large-scale connection and contact of the thick-film resistor to the flat side of the tube body. It is therefore preferred, if at least one of the at least one thick-film resistors is arranged on the flat side of the corresponding flat tube. In particular, the thick-film resistor is only arranged on at least one of the flat sides. In particular, the thick-film resistor is only arranged on one of the flat sides.

[0026] Each flat tube can be manufactured in an arbitrary manner. For instance, the flat tube can be extruded, brazed, welded, die-casted or the like.

[0027] Each thick-film resistor can in general be of any kind known by the skilled person.

[0028] Each thick-film resistor advantageously comprises two or more layers. Preferably at least one of the at least one layers is a dielectric layer.

[0029] Each thick-film resistor advantageously has a substantially even thickness along the extension direction.

[0030] The distanced arrangement of the at least two tube bodies in distance direction leads to a gap between the next neighbouring tube bodies.

[0031] Preferably, each gap is filled by the outer interface and one such thick-film resistor, and if so by the inner interface.

[0032] The thermal interface material is in particular known to the skilled person as "TIM". The thermal interface material can generally be of any kind, provided that it improves the thermal conductivity between corresponding outer surface and the thick-film resistor.

[0033] The thermal interface material can be electrically isolating. This is preferably achieved by providing the thick-film resistor with at least one di-electric layer. In a variant, the isolation can be partially or completely achieved by designing the thermal interface material itself isolating.

[0034] Preferably, the interface is arranged between the corresponding thick-film resistor and the corresponding tube body. Preferably, the interface is in contact with the corresponding thick-film resistor and the corresponding tube body.

[0035] The thermal interface material and thus the interface is advantageously insulating, that is an electric insulator.

[0036] Preferably, the thermal interface material comprises silicone and/or is silicone.

[0037] The manufacture of the heat exchanger is simplified and the heat transfer is improved, if the interface is potted between the corresponding thick-film resistor and the corresponding outer surface. As an alternative, the interface can be applied as an isolation foil or the like.

[0038] In general, a thick-film resistor can be applied on each of the at least two tube bodies.

[0039] In favourable forms of embodiment, the heat exchanger comprises three or more tube bodies, wherein one of the tube bodies is free of thick-film resistors. That is, the heat exchanger comprises a total number of N tube bodies, wherein N is equal to or larger than three. On N - 1 of the tube bodies such a thick-film resistor is arranged and one of the tube bodies is free of thick-film resistors.

[0040] In preferred forms of embodiment, the heat exchanger comprises three tube bodies distanced to another in distance direction. The heat exchanger thus comprises two outer tube bodies and a centre tube body arranged between the two outer tube bodies in distance direction. Preferably, on each of the outer tube bodies a corresponding thick-film resistor is arranged and connected thereto, whereas the centre tube body itself is free of thick-film resistors. Appropriately, each thick-film resistor is arranged on the side of the corresponding outer tube body which faces the centre tube body. Moreover, an interface is arranged between each of the thick-film resistors and the centre tube body. In this way, by using two thick-film resistors three tube bodies are heated. Thus the heat exchanger has a size and cost reduced design. At the same time, the heat exchanger comprises an improved efficiency.

[0041] According to advantageous forms of embodiment, the heat exchanger is designed in a manner that, in operation, over 50%, preferably two-thirds (2/3), of total transferred heat of each thick-film resistor is transferred to the corresponding tube body, that is the corresponding outer tube body, and less than 50%, preferably one-third (1/3), is transferred to the centre tube body. As a result, each of the tube bodies receives an equal ratio of the total transferred heat. Hence, the fluid is heated homogenously. The homogenous heating is improved by the provision of equal thick-film resistors. The homogenous heating can be further improved by equal thick-firm resistors and/or equal tube bodies.

[0042] The mentioned different heat transfer rates can be achieved by any measure. In general, the heat transfer ratio can be adjusted by adapting the thickness and/or thermal coefficient of the interfaces.

[0043] Hence, in advantageous forms embodiment, such an inner interface is arranged between each thick-film resistor and the outer surface of the corresponding outer tube body.

[0044] In preferred forms embodiment, a thickness of the outer interfaces and a thickness of the inner interfaces and/or a heat transfer coefficient of the outer interfaces and a heat transfer coefficient of the inner interfaces are in such relation that over 50% of total transferred heat of each thick-film resistor is transferred to the corresponding outer tube body and less than 50% is transferred to the centre tube body. More preferably, the thickness of the outer interfaces and the thickness of the inner interfaces and/or a heat transfer coefficient of the outer interfaces and a heat transfer coefficient of the inner interfaces are in such relation that 2/3 of total transferred heat of each thick-film resistor is transferred to the corresponding outer tube body and 1/3 is transferred to the centre tube body.

[0045] Each of the at least one interfaces can have any thickness along the distance direction.

[0046] Advantageously, the at least one inner interface has smaller thickness than the at least one outer interface.

[0047] In preferred forms of embodiment, at least one of the at least one interfaces has a thickness of between 50 µm and 300 µm, in particular between 100 µm and 250 µm, for instance between 150 µm and 250 µm, preferably between 175 µm and 225 µm, for instance 200 µm. These ranges of thickness in particular lead to the advantageous heat transfer ratios of the thick-film resistor to the corresponding tube body and the next neighbouring tube body.

[0048] The heat transfer ratio can, in an alternative or additional manner, be adjusted by applying a material with a reduced heat transfer coefficient. This allows to use a thinner interface. By doing so, the thickness of the interface can be reduced. As a result, the interface can comprise a smaller thickness, e.g. 50 µm.

[0049] The heat exchanger can be used for heating any fluid. The heat exchanger can for example be used to heat air, wherein the heated air can be supplied to a subsequent application. The heat exchange can for instance be part of an air conditioner.

[0050] The heat exchanger can also be used for heating a coolant, for instance of a vehicle and/or an air conditioner and/or a traction battery.

[0051] Further important characteristics and advantages of the invention proceed from the sub-claims, the drawings and the associated description of the figures, with reference to the drawings.

[0052] It is understood that the above-mentioned characteristics, and those to be described hereinafter, are not only applicable in the respective combination indicated, but also in other combinations, or in isolation, without departing from the scope of the present invention.

[0053] Preferred exemplary embodiments of the invention are represented in the drawings and described in greater detail in the following description, wherein identical reference numbers identify identical, similar or functionally equivalent components.

[0054] In the figures, schematically in each case:

Fig. 1

shows a highly simplified, schematic-type view of a cycle with a heat exchanger,

Fig. 2

shows a section through the heat exchanger,

Fig. 3

shows the section of Figure 2 in another exemplary embodiment.

[0055] A heat exchanger 1, as shown in Figures 1 to 3 by way of example, can be part of a cycle 2 which is shown in Figure 1 by way of example. The heat exchanger 1 serves to heat a fluid. In the exemplary embodiments shown, the heat exchanger 1, in operation, heats a coolant as fluid. The fluid circulates through the cycle 2. The cycle 2 might comprise a pump (not shown) for this purpose. The cycle 2 and/or the heat exchanger 1 can be part of an otherwise not shown vehicle 15.

[0056] As can be seen in Figures 2m and 3, the heat exchanger 1 comprises at least two tube bodies 3 through which the fluid flows. In the exemplary embodiments shown, the heat exchanger 1 comprises a total of three tube bodies 3, wherein the tube bodies 3 are identical. In the exemplary embodiments shown, each tube body 3 is a flat tube 4 with two opposing flat sides 5. Each tube body 3 might be made of metal or an alloy, preferably of aluminium or an aluminium alloy. Each tube body 3 extends in a direction 6, in which the tube body 3 is flown through by the fluid. This direction is also referred to as extension direction 6 in the following. Thus, each tube body 3 extends in extension direction 6 and is enclosed in a circumferential direction 7 and thereby delimits a flow path 8 of the fluid along the extension direction 6. The tube bodies 3 are distanced to another transverse to the extension direction 6 and thus in a direction 9 transverse to the extension direction 6, wherein this direction 9 is also referred to as distance direction 9 in the following. The heat exchanger 1, In the exemplary embodiments shown in Figures 2 and 3, thus comprises, in distance direction 9, two outer tube bodies 3a and one centre tube body 3b arranged between the outer tube bodies 3a. The tube bodies 3 are arranged such that flat sides 5 of the tube bodies 3 follow each other in distance direction 9. Due to the distanced arrangement of the tube bodies 3 a gap 10 is arranged between the neighbouring tube bodies 3.

[0057] According to the invention, a thick-film resistor 11 produced separately from the tube bodies 3 is arranged on an outer surface 12 of at least one of the at least two tube bodies 3 and connected to the outer surface 12 of the corresponding tube body 3. The thick-film resistor 11 is thus an element 18 connected to the outer surface 12 of the corresponding tube body 3. In addition, an interface 13 made of a thermal interface material is, on the side of the thick-film resistor 11 averted from the corresponding tube body 3, arranged between the thick-film resistor 11 and the outer surface 12 of the next neighbouring tube body 3. The interface is also referred to as outer interface 13 hereinafter. The outer interface 13 connects the thick-film resistor 11 to the next neighbouring tube body 3 in a heat transferring manner. Each of the thick-film resistor 11 can comprise two or more layers (not shown), wherein at least one of the layers can be dielectric.

[0058] In the exemplary embodiment shown in Figure 2, the respective thick-film resistor 11 is directly connected to the outer surface 12 of the corresponding tube body 3.

[0059] In the exemplary embodiment shown in Figure 3, an interface 17 made of a thermal interface material is arranged between the respective thick-film resistor 11 and the outer surface 12 of the corresponding tube body 3 and connects the thick-film resistor 11 to the corresponding tube body 3 in a heat transferring manner.

[0060] Each interface material can be electrically insulating and is preferably silicone.

[0061] In the exemplary embodiments shown in Figures 2 and 3, the respective thick-film resistor 11 is arranged on and connected to the outer surface 12 of the outer tube bodies 3a. Thus the number of tube bodies 3 with a corresponding thick-film resistor 11 is one less than the total number of tube bodies 3. That is, in the exemplary embodiments shown the heat exchanger 1 comprises a total number of three tube bodies 3 and two thick-film resistors 11, wherein one of the tube bodies 3, in the exemplary embodiments shown the centre tube body 3b is free of thick-film resistors 11 itself.

[0062] In the exemplary embodiment shown, a thick-film resistor 13 is arranged on and connected to solely the outer surface 12 of the flat side 5 facing the centre tube body 3b. In addition, in the exemplary embodiments shown, such an interface 13 is arranged between each thick-film resistor 11 and the flat side 5 of the centre tube body 3b.

[0063] Each of the outer interfaces 13 can be potted between the corresponding flat side 5 of the centre tube body 3b and the corresponding thick-film resistor 11. In the exemplary embodiment shown in Figure 3, each of the inner interfaces 17 can be potted between the corresponding flat side 5 of the outer tube body 3a and the corresponding thick-film resistor 11. Thus, in the exemplary embodiment shown in Figure 2, each gap 10 is filled with a thick-film resistor 11 and an outer interface 13. Moreover, in the exemplary embodiment shown in Figure 3, each gap 10 is filled with a thick-film resistor 11, an outer interface 13 and an inner interface 17.

[0064] In the exemplary embodiments shown, the outer interfaces 13 and the thick-film resistors 11 are identical, respectively. In the exemplary embodiments shown in Figure 3, the inner interfaces 17 are identical.

[0065] In the exemplary embodiments shown, the tube bodies 3, in extension direction 6, project the outer interfaces 13 and the thick-film resistors 11. In the exemplary embodiments shown in Figure 3, the tube bodies 3, in extension direction 6, project the inner interfaces 17. That is, the tube bodies 3 are, in extension direction 6, longer than the interfaces 13, 17 and the thick-film resistors 11.

[0066] Each interface 13, 17 extends in distance direction 9 and thus comprises a thickness 14. That is, each outer interface 13 has a thickness 14a, also referred to as outer thickness 14a hereinafter, and each inner interface 17 has a thickness 14b, also referred to as inner thickness 14b hereinafter. The thickness 14 of each interface 13, 17 is advantageously between 50 µm and 300 µm, for instance 200 and thus 0,2 mm, wherein the interfaces 13, 17 are shown exaggerated relative to the tube bodies 3 for comprehensive reasons. The thick-film resistors 11 might have thicknesses (not shown) similar to the outer interfaces 13.

[0067] Each of the thick-film resistors 11, in operation and thus when electrically supplied, generates heat. The heat is transferred to the next neighbouring tube body 3 via the corresponding outer interface 13. In the exemplary embodiment shown in Figure 2 the heat is further directly transferred to corresponding tube body 3. In the exemplary embodiment shown in Figure 3 this heat is further transferred to the corresponding tube body 3 via the corresponding inner interface 13.

[0068] As indicated by thick arrows 16 in Figures 2 and 3, in the exemplary embodiments shown and preferably, the heat exchanger 1 is designed in a manner that the ratio of heat transferred from each thick-film resistor 11 to the corresponding tube 3 to the heat transferred to the next neighbouring tube body 3 via the outer interface 13 is substantially more than 2:1 (two to one). Thus, more than 50 % of the total transferred heat of each thick-film resistor 11 is transferred to the corresponding tube body and less than 50 % of the total heat, via the outer interface 13, is transferred to the next neighbouring tube body 3. Preferably and in the exemplary embodiment shown, substantially 2/3 (two-thirds) of the generated heat of each thick-film resistor 11 is transferred to the corresponding outer tube body 3a and substantially 1/3 (one-third) to the centre tube body 3b via the corresponding outer interface 13. This results in a substantially homogenous heat transfer to each tube body 3 and thus a homogeneous heating of fluid flowing through each tube body 3. In the exemplary embodiment shown in Figure 2 this can be achieved by using a heat transfer blocking layer (not shown) between each thick-film resistor and the outer surface 12 of the corresponding tube body.

[0069] In the exemplary embodiment shown in Figure 3 said ratios are achieved by adapting the outer interfaces 13 and the inner interfaces 17 accordingly. That is, the outer thickness 14a of the outer interfaces 13 and the inner thickness 14b of the inner interfaces 17 and/or a heat transfer coefficient of the outer interfaces 13 and a heat transfer coefficient of the inner interfaces 17 are in an according relation. In the exemplary embodiment shown in Figure 3, the same interface material is used for the outer interfaces 13 and the inner interfaces 17. The ratios are thus achieved by the outer thickness 14a and the inner thicknesses 14b being equal, respectively, wherein the ratio of each outer thickness 14a to each inner thickness is substantially three to two, 14a : 14b ≈ 3 : 2.

Claims

1. Heat exchanger (1) for heating a fluid, in particular for heating a coolant,

- with at least two tube bodies (3), wherein each tube body (3) extends in an extension direction (6) and is enclosed in a circumferential direction (7),

- wherein each tube body (3) delimits a flow path (8) of the fluid along the extension direction (6),

- wherein the tube bodies (3) are distanced to another in a distance direction (9) transverse to the extension direction (6),

characterized in that

- a thick-film resistor (11) separately produced from the tube bodies (3) is arranged on an outer surface (12) of at least one of the at least two tube bodies (3) and connected to the outer surface (12) of the corresponding tube body (3),

- an outer interface (13) made of a thermal interface material is arranged between the thick-film resistor (11) and the outer surface (12) of the next neighbouring tube body (3) and connects the thick-film resistor (11) to the next neighbouring tube body (3) in a heat transferring manner.

2. Heat exchanger according to claim 1,
characterized in that
at least one of the at least one thick-film resistors (11) is directly connected to the outer surface (12) of the corresponding tube body (3).

3. Heat exchanger according to claim 1 or 2,
characterized in that
an inner interface (17) made of a thermal interface material is arranged between at least one of the at least one thick-film resistors (11) and the outer surface (12) of the corresponding tube body (3) and connects the thick-film resistor (11) to the corresponding tube body (3) in a heat transferring manner.

4. Heat exchanger according any of claims 1 to 3,
characterized in that

- the heat exchanger (1) comprises three tube bodies (3) such that a centre tube body (3b) is arranged between two outer tube bodies (3a),

- such a thick-film resistor (11) is arranged on the outer surface (12) of each of the outer tube bodies (3a) facing the centre tube body (3b),

- the centre tube body (3b) is free of thick-film resistors (11),

- such an outer interface (13) is arranged between each of the thick-film resistors (11) and the centre tube body (3b).

5. Heat exchanger according to claim 3 or 4,
characterized in that
the heat exchanger (1) is designed in a manner that, in operation, over 50% of total transferred heat of each thick-film resistor (11) is transferred to the corresponding outer tube body (3a) and less than 50% is transferred to the centre tube body (3a).

6. Heat exchanger according to claim 5,
characterized in that
the heat exchanger (1) is designed in a manner that, in operation, 2/3 of total transferred heat of each thick-film resistor (11) is transferred to the corresponding outer tube body (3a) and 1/3 is transferred to the centre tube body (3a).

7. Heat exchanger according to any of claims 4 to 6,
characterized in that
such an inner interface (17) is arranged between each thick-film resistor (11) and the outer surface (12) of the corresponding outer tube body (3a).

8. Heat exchanger according to claim 7 and claim 5 or 6,
characterized in that
an outer thickness (14a) of the outer interfaces (13) and an inner thickness (14b) of the inner interfaces (17) and/or a heat transfer coefficient of the outer interfaces (13) and a heat transfer coefficient of the inner interfaces (17) are in such relation that over 50% of total transferred heat of each thick-film resistor (11) is transferred to the corresponding outer tube body (3a) and less than 50% is transferred to the centre tube body (3a).

9. Heat exchanger according to any of claims 1 to 8,
characterized in that
at least one of the at least two tube bodies (3) is a flat tube (4).

10. Heat exchanger according to claim 8,
characterized in that
at least one of the at least one thick-film resistors (11) is arranged on a flat side (5) of the corresponding tube body (3).

11. Heat exchanger according to any of claims 1 to 10,
characterized in that
at least one of the at least two tube bodies (3) is made a metal or an alloy.

12. Heat exchanger according to any of claims 1 to 11,
characterized in that
at least one of the at least one thick-film resistors (11) has at least two layers including at least one dielectric layer.

13. Heat exchanger according to any of claims 1 to 12,
characterized in that
at least one of the at least one interfaces (13, 17) comprises silicone.

14. Heat exchanger according to any of claims 1 to 13,
characterized in that
at least one of the at least one interfaces (13, 17) is potted between the corresponding thick-film resistor (11) and the next neighbouring tube body (3).

15. Heat exchanger according to any of claims 1 to 14,
characterized in that

- the heat exchanger (1) comprises a total number of N tube bodies (3), wherein N is equal to or larger than three,

- on N - 1 of the tube bodies (3) has a corresponding such thick-film resistor (11) arranged and one of the tube bodies (3) is free of thick-film resistors (11).

Drawing