Fuel-powered air heater

Abstract

A fuel-operated air heater (100) for a vehicle comprises a heat exchanger (30), wherein the heat exchanger (30) comprises a tubular housing body with an opening (38) on an end face (61) for receiving a housing (36) of a combustion chamber (35), which forms at least one exhaust-side channel with an inner surface of the housing body, wherein the housing body has a cylindrical outer surface (40) around a longitudinal axis (M) of the housing body, which defines a heating air side. Furthermore, the heat exchanger (30) comprises a first plurality of hot air-side fins (44), each extending parallel to the longitudinal axis (M) and parallel to each other from the outer surface (40) or from a main fin (42) projecting from the outer surface (40), wherein two hot air-side fins (44) of the plurality enclose a fin channel (43) between them, through which hot air is guided during operation to absorb heat.The hot air-side ribs (44) of the first plurality define a first median plane (P1) parallel to them and symmetrical with respect to the housing body, which encloses the longitudinal axis (M). The axial position (X) with respect to the longitudinal axis (M) of the leading edges (50) of the hot air-side ribs (44) bounding the rib channels (43), which are provided on the hot air-side ribs (44) towards the end face (61), is located closer to the rear (62) of the housing body the closer the respective hot air-side rib (44) is positioned to the first median plane (P1).

Description

Technical field:

[0001] The invention relates to a fuel-operated air heater with a heat exchanger, in particular one for a vehicle. Technical background:

[0002] Mobile, fuel-powered air heaters for use in vehicles, which in this document include passenger cars, commercial vehicles, construction machinery, and ships of all types, continue to be in high demand. Conventionally, they are constructed according to a common concept in which heat is released in a combustion chamber using fuel supplied from the vehicle's own tank. The hot combustion gases or exhaust gases are then brought into contact with a heat exchanger, to which they transfer some of the usable heat energy on the exhaust side. On the opposite side of the heat exchanger, a fan supplies heated air, which absorbs the usable heat energy transferred from the heat exchanger on the hot air side.The heated air is generally drawn in from the surrounding environment at one end of the air heater and exits in its heated state at the opposite rear of the unit after passing through the heat exchanger. The heated air inlet and outlet can conventionally be located on the same spatial axis, so that the main flow direction of the heated air remains unchanged along the entire length of the heater; however, alternative designs are also known.

[0003] The heat exchangers used in air heaters for mobile applications are generally very similar in design. Around the outer casing of the generally round burner, fins are arranged on the exhaust gas side of the heat exchanger, forming exhaust gas channels and increasing the contact area with the exhaust gas. Between the exhaust gases and the heated air flowing past the outside of the heat exchanger is a tightly sealed shell formed by the heat exchanger's casing, which has an approximately round or oval cross-section.

[0004] On the other hand, the outer casing of an air heater typically has a rectangular or even square cross-sectional profile, at least in the area of ​​the heat exchanger. This design usually conforms to convention and meets the requirements of vehicle manufacturers for space-saving integration of vehicle components. A geometric transition between the round or oval cross-section of the heat exchanger's base body and the approximately rectangular or square outer casing of the air heater is achieved through the design of a heating air-side finning system that fills the gap and forms flow channels for heat absorption. The heating air-side fins therefore extend from the heat exchanger's base body, and their outer edges define contact surfaces for the outer casing, which seals the intervening channels tightly to the outside.The seal is completely airtight only in relation to the surrounding environment. Flow is possible between the individual channels formed by the fins. For technical reasons, a gap of approximately 1–1.5 mm may exist between the fin tips and the inner wall of the outer casing (this is purely an example). Other configurations are also possible. Therefore, the cross-sectional profile of the heat exchanger's outer contour is typically also essentially rectangular or square.

[0005] Common heat exchangers for fuel-operated air heaters are typically manufactured as die-cast aluminum parts using a multi-part mold. The mold parting lines for the hot-air-side finning are located at the corners of the rectangular or square cross-section. To allow for the removal of the mold sections that form the fins after casting, the fins are designed to be parallel to each other for each mold section, purely for manufacturing reasons. This results in a total of four "main fins" at the corners, which supply heat to the "secondary fins" located at the corners of the cross-section and supported by them, via solid-state heat conduction.Only in the outermost end area of ​​the corners are no secondary ribs formed, since only a limited amount of heat arrives here along the length of the main ribs in the radial direction, so that the result of a marginal heat transfer is in poor proportion to the additional costs of aluminum material used.

[0006] The secondary ribs, as well as the ribs extending centrally from the main body, are arranged parallel to each other and with the smallest possible spacing from a manufacturing perspective to ensure optimal heat transfer. The spacing of the ribs along the outer contour of the cross-sectional profile is therefore, in most cases, constant and the same for all channels formed by the ribs. Furthermore, the lengths of the ribs and channels are conventionally designed to be as long as possible across the available surface area of ​​the main body, in order to maximize the contact area and heat transfer. Exceptions are usually only due to design requirements, for example, to prevent turbulence in the inlet or outlet area of ​​the subsequent collector channels or to create space for fasteners for the outer casing, etc. Description of the invention:

[0007] Against this background, the present invention aims to improve the efficiency of heat transfer. A further objective is to counteract local increases in maximum surface temperatures when heat transfer is locally inhomogeneous. Additionally, it aims to counteract excessive pressure drops during the flow through the hot air channels and / or to improve flow acoustics by ensuring largely laminar flow, thereby preventing disruptive noise during operation.

[0008] This problem is solved by a fuel-operated air heater with the features according to claim 1. Advantageous further developments result from the dependent claims.

[0009] According to one aspect of the invention, a fuel-operated air heater in a vehicle comprises a heat exchanger. The heat exchanger has a tubular housing body with an end-face opening for receiving a combustion chamber housing, which can form at least one exhaust-side channel with an inner surface of the housing body. The housing body has a cylindrical outer surface around a longitudinal axis M of the housing body, which defines a heating air side. For the purposes of this invention, a shape is also defined as having a cross-section that is not circular but oval or similar.

[0010] Furthermore, the heat exchanger comprises a first plurality of hot air-side fins, each extending parallel to the longitudinal axis and parallel to each other from the outer surface or from a main fin projecting from the outer surface, wherein two hot air-side fins of the first plurality enclose a fin channel between them, through which hot air is guided during operation to absorb heat, preferably in a direction starting from the front face towards an opposite rear of the housing body.

[0011] The main fin is considered to be the fin described at the beginning, extending, for example, radially from the outer surface towards a corner of a rectangular or square cross-sectional profile of the heat exchanger. This main fin also carries secondary fins and serves to transfer heat energy to them. The heating air-side fins of the first plurality define a first median plane P1 parallel to them and symmetrical with respect to the housing body, which encloses the longitudinal axis M. This median plane P1 is uniquely defined for the first plurality.

[0012] The first plurality can, for example, denote the group of ribs mentioned at the outset, which are to be assigned to a tool component for manufacturing purposes and are characterized by their parallel alignment. In particular, the first plurality can comprise two or more, preferably three or more, and more preferably four or more, hot-air-side ribs arranged directly adjacent to one another, which consequently form at least one, two, or three rib channels between them. The first plurality can also have two or more groups of correspondingly adjacent hot-air-side ribs. This first plurality of ribs has in common the characterizing feature described below. In the case of two or more groups, the hot-air-side ribs can be separated by one or more intervening ribs that do not fulfill this feature.

[0013] According to this feature, the leading edge of the majority of the ribs on the hot air side, which defines the fin channels, is located axially with respect to the longitudinal axis M. This leading edge is positioned closer to the rear of the housing body and the closer the respective rib is to the first central plane P1. A leading edge of a rib is an edge that defines the rib in a direction opposite to the intended flow direction of the hot air during operation. In other words, the leading edge represents the end face of a given hot air rib. It may be opposite a trailing edge oriented towards the rear.

[0014] In the case of a cylindrical outer surface of the housing body and an essentially square or rectangular outer contour profile of the outer edges of all hot-air-side fins, which defines a corresponding cross-sectional profile of an outer housing of the air heater, the outer surface is closest to the outer housing where the first median plane P1 intersects the outer surface. At this point, the fin heights of the hot-air-side fins extending from the outer surface are consequently at their minimum. As a result, the cross-section relevant for hot-air flow of the relevant fin channel formed by two adjacent fins is also at its minimum at this point among all fin channels, assuming—as in the typical case described—that the mutual spacing of the fins is constant.

[0015] As the distance from the central plane P1 increases, so does the distance between the outer surface and the respective, more or less flat, outer contour of the ribs or from the outer casing. This means that the rib height, and therefore the cross-section, also increases.

[0016] The length of the fin channels can be determined by the shorter rib of each pair of adjacent ribs. This length can be measured from a leading edge to a trailing edge of the corresponding rib. The position of trailing edges along the longitudinal axis M is generally as close as possible to the back or towards the hot air outlet and would ideally be the same for all ribs, but in practice it can vary from rib to rib. Reasons for a rib ending before reaching the back or for the corresponding trailing edge being spaced away from the back can include, for example,to enable the flow of hot air to be guided in terms of fluid mechanics by a hot air outlet that is significantly smaller compared to the cross-sectional profile of the outer housing (this applies especially to ribs located near the four corners), or to enable better demolding from the injection mold (this applies especially to ribs located near the central plane P1).

[0017] The positioning of the entry edges (given a specific position of the exit edges) as provided for in certain aspects of the invention allows for direct influence on the lengths of the rib channels. Conventionally, due to the desired maximum contact area, positioning all entry edges as close as possible to the front face is preferred.

[0018] However, the aspects proposed here deviate from this approach. By positioning the leading edges closer to the rear of the housing body, and thus closer to the respective hot-air-side fin to the first central plane P1, a correlation between the length of the cooling channels and their corresponding cross-section can be configured. In other words, the individual lengths of the fin channels can be varied, for example, by positioning the leading edges depending on the cross-section defined by the fin height and spacing. If, as described, the cross-section of these fin channels increases with increasing distance from the central plane, then the length of these fin channels is increased by shortening their lengths (due to the positioning of the leading edges towards the rear) towards the central plane P1.As a result, differences in flow resistance between the finned channels are compensated for, and finned channels with a small cross-section but the highest temperatures located near the central plane P1 experience increased flow (higher mass flow), while the flow is reduced in finned channels with a comparatively larger cross-section. Consequently, areas with locally high temperatures can experience increased flow and thus dissipate their heat more efficiently. Temperature spikes are avoided.

[0019] Unlike conventional heat exchangers, the present design aims for flexible length adjustment of the fin channels. In cases where, conventionally, maximum fin coverage of the outer surface is achieved, this means, for example, that for a similarly dimensioned heat exchanger according to the presented embodiments, a reduction in the length of individual fin channels—i.e., the fins themselves—is accepted, thus reducing the surface area available for fins. Surprisingly, it was found that this actually improves heat transfer, rather than worsening it.

[0020] Between the fins on the heating air side, the heated air flows within the fin channels, which have varying fin heights and an approximately rectangular cross-section. The pressure drop along a channel between two fins over the length L of the channel can be described as follows: Δpv=λ⋅1 / 2⋅ρ v2⋅(L / Dh)

[0021] In this, L denotes the channel length, λ the coefficient of friction of the rib channel, D h The hydraulic diameter, ρ the density, and v the flow velocity averaged over the channel cross-section. Because the area before the flow enters the channels and the area after the flow exits are identical in all cases, the same pressure drop occurs across each channel of the heat exchanger, regardless of its length (which is conventionally identical anyway). From this, the equation above results in a significant mass flow distribution that follows the distribution of the channel geometry. Consequently, in the conventional case of equal fin lengths in the longitudinal direction (L = const), the mass flow of each channel is distributed proportionally to the fin height of that channel.

[0022] This variable distribution in the duct mass flow conventionally leads to a situation in practice where, in areas with large temperature gradients but small cross-sections, less heated air is available for heat transfer than would actually be physically desirable. This is where the aspects of the invention come into play, by adjusting the length of such finned ducts via the positioning of the inlet edges. This increases the mass flow in the finned ducts of the affected areas and thus enhances the heat transfer in these ducts. Since, according to the formula above, the pressure drop is also proportional to the duct length, the overall pressure loss is advantageously reduced. This, in turn, reduces the occurrence of turbulence, which beneficially improves the flow acoustics.

[0023] Furthermore, as mentioned above, this aspect of the invention makes it possible to avoid local temperature increases at critical points by shortening the length of the corresponding fin channel at the relevant point.

[0024] One underlying concept of the invention is therefore to influence the mass flow of heated air through the finned channels by appropriately selecting their length and positioning their inlet edges. This allows for targeted control of the distribution of the heated air supply around the circumference of the heat exchanger. The required channel length L can be derived from the above equation, assuming an approximately rectangular channel cross-section, using the following relationship between the desired mass flow rate through the respective finned channel: L=Δp⋅4⋅b3⋅h3⋅ρλ⋅(b+h)⋅m˙z.

[0025] In this, b denotes the fin spacing, h the average fin height of both fins, and ṁ the mass flow rate. Therefore, according to the invention, to achieve the best possible utilization of the available heat exchanger surface area with minimal pressure drop, the heating air volume flow rate in areas with high surface temperature can be increased, for example, by shortening the length of the relevant channels.

[0026] Above, aspects of the invention were described using the example of a heat exchanger with, for instance, a rectangular cross-sectional profile of the outer contours formed by the fins. However, the invention is also generally applicable to other cross-sectional profiles, such as pentagonal, hexagonal, or even round or oval outer contours that deviate from the geometry of the outer surface of the housing body. The underlying problem therefore arises particularly when the fins have to create a transition between the inner geometric shape and the outer geometric shape.

[0027] According to a specific embodiment, limited to polygonal outer contours, the hot-air-side fins of the first plurality each have an outer edge facing away from the outer surface or from the main fin, wherein the outer edges together define a contour plane K1 parallel to the longitudinal axis and perpendicular to the first intermediate plane P1, in which a section of an outer housing extends, covering the fin channels outwards. The same advantages as described above are observed.

[0028] Among the ribs that form channels, there may also be those intended to ensure the mechanical integrity and stability of the housing body and are therefore elongated, or those that are interrupted or shortened due to design features on the outer surface. For the reasons mentioned, these ribs cannot be adjusted in length. Therefore, for the purposes of this application, they do not belong to the first plurality. A further specification of the invention provides that the first plurality comprises at least three or at least four, more preferably at least five, and even more preferably at least six ribs on the heating air side for forming corresponding at least three, four, or five rib channels.

[0029] Another embodiment provides that, in a top view of the housing body, all the leading edges of the heating air-side fins of the first plurality assume a parabolic or circular shape along the central plane P1 perpendicular to the longitudinal axis M. The vertex of this parabola or arc lies in the central plane P1, and the arc opens in one direction towards an (entry-side) end face of the housing body. This geometry is visually appealing and, moreover, results almost automatically from the preceding aspect if the leading edges are left in essentially the same axial position. The top view can be defined along a direction that is parallel to, but opposite, an outlet channel (outlet nozzle) for combustion-generated exhaust gases arranged laterally to the air heater.Since the outlet channel / nozzle entails design modifications to the heat exchanger, e.g. a larger recess starting from the front, the parabolic shape may only fully develop on the other sides of the heat exchanger in certain cases.

[0030] Another aspect stipulates that the mutual spacing of adjacent fins is constant or the same for all hot-air-side fins of the first plurality. This also corresponds to the classic case, since, as described, from a manufacturing perspective, the closest possible fin spacing is desirable, which is then the same for all fins. However, aspects of the invention do not fundamentally preclude the possibility of alternatively or additionally varying the channel width from channel to channel to achieve the same effects as described.

[0031] According to exemplary embodiments, the heat exchanger can be formed in one piece from the housing body and the hot air-side fins, preferably as aluminum die casting.

[0032] Furthermore, at least one exhaust-side channel along the inner surface of the housing body can be formed by appropriately designed exhaust-side ribs, which extend, for example, radially inwards from an inner surface of the housing body.

[0033] Furthermore, the at least one main rib can extend inclined with respect to the first median plane P1 or the orientations of the hot-air-side ribs (44), preferably at an angle of 40° to 50°, more preferably 45°. Along the entire length of the main rib in the radial direction, hot-air-side ribs can extend from the main rib at a constant mutual distance from the outer surface (40) to an outer edge.

[0034] This eliminates the fin-free space near the outer edges of the main fins, a feature of the conventional design. As described, this means that more material (valuable aluminum) must be used in manufacturing, which then contributes very little to heat transfer during operation because very little heat energy reaches the outer edges (the outer corners in the cross-sectional profile) of the main fins. However, it was found that adding the outermost fins and extending the next outermost ones increases the flow resistance in these areas, thus increasing the mass flow rate in the fin channels with high temperature gradients, and preventing the heated air from passing unused through the already cool fin channels in the corner areas.

[0035] Since the main rib in this case extends further to the plane defined by the outer edges of the hot air-side ribs, the heights of the rib channels decrease again towards the outer corner.

[0036] According to a further development of the heat exchanger, the housing body has a second intermediate plane P2, which is perpendicular to the first intermediate plane P1, with the longitudinal axis M being the line of intersection between the first intermediate plane P1 and the second intermediate plane P2. Each main fin, projecting from the outer surface, bisects the quadrants formed by the two intermediate planes. This results in four main fins, each carrying secondary fins on the hot air side, extending perpendicular or parallel to each other and offset by 45° from the intermediate planes P1 and P2. This design allows for a symmetrical arrangement of a heat exchanger with a rectangular or square cross-sectional profile.

[0037] In particular, viewed circumferentially around the longitudinal axis (M), the first plurality as well as the corresponding second, third and fourth pluralitys of hot air-side ribs can extend between the main ribs in the same way as the first plurality of hot air-side ribs, each oriented along a relevant central plane P1, P2.

[0038] According to a further aspect of the invention, the fuel-operated air heater further comprises, in addition to the heat exchanger, an outer housing which accommodates the heat exchanger and which has a substantially flat section that covers the finned channels to the outside, wherein the outer housing has a hot air inlet and a hot air outlet which are connected to each other via the finned channels, a hot air blower operated by a motor which, in operation, draws in hot air through the hot air inlet, drives it through the finned channels and discharges it through the hot air outlet, and a combustion chamber housing which is accommodated within the housing body of the heat exchanger and forms at least one exhaust gas-side channel with an inner surface of the housing body of the heat exchanger.

[0039] Further embodiments of the invention are set out in the dependent claims. Brief description of the drawings:

[0040] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show: Fig. 1 a schematic representation of a fuel-operated air heater which can be equipped with the heat exchanger according to an exemplary embodiment; Fig. 2. A side view of a heat exchanger according to a comparative example; Fig. 3 a cross-sectional view of the heat exchanger according to the comparison example from Fig. 2; Fig. 4 a perspective view of a heat exchanger according to an exemplary embodiment; Fig. 5 a side view of the heat exchanger according to the embodiment shown Fig. 4; Fig. 6 a cross-sectional view of the heat exchanger according to the embodiment shown Fig. 4 and Fig. 5; Detailed description of preferred embodiments:

[0041] In the following description of preferred embodiments, it should be noted that the present disclosure of the various aspects is not limited to the details of the construction and arrangement of the components as shown in the following description and in the figures. The embodiment can be implemented or carried out in practice in various ways. Furthermore, it should be noted that the language and terminology used here are solely for the purpose of concrete description and should not be interpreted restrictively by those skilled in the art.

[0042] In the following description of the invention, the same reference numerals are used for identical and identically acting elements, so that in some cases a repeated detailed description of the same is omitted in order to maintain the compactness and clarity of the presentation.

[0043] The Fig. Figure 1 shows a schematic longitudinal sectional view of a fuel-operated air heater 100 for a vehicle (not shown), which can be used with a heat exchanger 30 according to an embodiment of the invention. The air heater 100 has an outer housing G with a heated air inlet 11 and a heated air outlet 12. A combustion chamber housing 36 with a (fuel-operated) combustion chamber 35 is located in a heat exchanger 30 of the air heater 100. A combustion air blower 31 provides the air supply and airflow to the combustion chamber 35. The combustion air blower 31 has an air inlet 33 and an exhaust gas outlet 34. Heated air is drawn in from the environment at the front of the air heater by means of a hot air blower 14 and exits through the hot air outlet 12 at a rear of the air heater opposite the front, after flowing around the heat exchanger 30.The hot air inlet 11 and hot air outlet 12 of the air heater 100 are, purely by way of example, located on a common longitudinal axis M of the heat exchanger 30 and the combustion chamber 35, along which the air flows through the air heater 100. The combustion air blower 31 and the hot air blower 14 are driven by a motor 20, which is controlled by a control device 18 in a known manner.

[0044] On the exhaust gas side of the heat exchanger 30, hot combustion gases are deflected (sharply) by 180° after leaving the combustion chamber and then flow through the heat exchanger 30 in the opposite direction to the heating air towards the exhaust gas outlet 34. Due to the high flow velocities and the sharp deflection of the flow, an impact flow 2 with very high heat transfer is established on the exhaust gas side. The exhaust gases are then guided through flow channels 37, which are formed by fins 38 projecting inwards from an inner surface of the heat exchanger 30 and extending along the longitudinal axis M to improve heat transfer.

[0045] In the Fig. 2 and Fig. Figure 3 shows a side view and a cross-sectional view of an air heater 100 according to Fig. One usable heat exchanger of 30 conventional design is shown. However, this is a comparative example and need not be disclosed as such in the prior art.

[0046] The one in the Fig. 2 and Fig. The heat exchanger 30 shown is manufactured as a single-piece component in an aluminum die-casting. It has an end face 61 and a back face 62. The housing body is tubular and has an opening 38 on the end face 61 for receiving the housing 36 of the combustion chamber 35, which forms at least one exhaust gas-side channel with an inner surface of the housing body. The housing body has a cylindrical outer surface 40 around a longitudinal axis M of the housing body, which defines a hot air side.

[0047] In the cross-sectional view of the Fig. Figure 3 shows that in the interior accessible through opening 38, a multitude of exhaust-side ribs 24 project inwards from a similarly cylindrical inner surface of the housing body, each forming rib channels 23 between them. These rib channels 23 are enclosed inwards by the schematically shown Fig. 3 combustion chamber housing 36, shown as a dashed circle.

[0048] From the outer surface 40 of the housing body, heating air-side fins 44 and four main fins 42 extending diagonally project. Further heating air-side fins 44 extend from these main fins 42. The main fins 42 define a total of four quadrants between them, within which the heating air-side fins 44 extend parallel to each other and at a constant distance from each other in the same direction. The heating air-side fins 44 of each quadrant define a median plane P1 extending parallel to them, which also extends through the longitudinal axis M of the housing body, which is cylindrical in this section.

[0049] In Fig. Figure 3 further shows that the outer edges of the hot-air-side fins 44 define a plane for each of the quadrants bounded by the main fins 42. This is shown purely schematically in the Fig. 3. An outer casing G is shown with a dashed line, which seals the fin channels 43 formed by the hot air-side fins 44 to the outside. The outer casing G can rest partially or completely on the fins 44, or be slightly separated from them (by placeholders or insulating pads) to prevent overheating by heat conduction (risk of injury from external contact). The outer contour of the heat exchanger 30 formed by the aforementioned planes, or the cross-section of the outer casing G, forms an essentially square profile.

[0050] The main ribs 42, which extend to the corners of the square profile, do not form any further hot-air-side ribs 44 at their outer ends. In other words, a hot-air-side rib 44 is missing at each corner. This creates a rib channel 49 towards the corners of each quadrant (see Fig. 3) with a slightly larger cross-section. In the conventional approach, this is intended to somewhat reduce the pressure loss caused by the hot air-side ribs 44.

[0051] As in the Fig. As can be seen in Figure 2, the hot air-side ribs 44 extend, regardless of their position relative to the first central plane P1, from a front leading edge towards the front face 61 to a rear exit edge adjacent to the rear face 62. Alternating hot air-side ribs 44 shortened to the same length are provided, in which the leading edge is offset by a certain distance towards the rear face 62 compared to the respective adjacent ribs 44.

[0052] In the Fig. Figures 4 to 6 show an embodiment of an air heater 100 with a heat exchanger 30 according to the present invention in a perspective view, a side view, and a cross-sectional view. The heat exchanger 30 is shown in the following sections. Fig. The heat exchangers 30 shown in 4 to 6 are located in the... Fig. The air heater shown can be used with 100.

[0053] The basic structure of the heat exchanger 30 of the exemplary embodiment is similar to that of the heat exchanger 30 of the comparative example from the Fig. 2 and Fig. 3. Therefore, only specific differences will be discussed.

[0054] The heat exchanger 30 has a base body manufactured in an aluminum die-casting process and designed as a single-piece component. The base body has a cylindrical outer surface 40 and a cylindrical inner surface (see Fig. 6) from which exhaust-side fins 24 extend into an interior of the heat exchanger 30 accessible via an opening 38. The exhaust-side fins 24 define exhaust-side fin channels 23 between each other, which are closed off inwards in the cross-sectional profile by a combustion chamber housing 36. Inside the combustion chamber housing 36 is the combustion chamber 35, the exhaust gases produced during operation of which are as described in Fig. 1 shown through the rib channels 23 towards an exhaust gas outlet 48.

[0055] In Fig. Figure 4 shows that the opening 38 formed in the heat exchanger 30 defines an end face 61 of the heat exchanger 30, which is opposite a rear face 62. A longitudinal axis M (cf. Fig. 5) extends from the front face 61 to the rear face 62 and is defined by the cylindrical shape of the inner and outer surfaces of the base body (central axis). Mounting means 72 (screw holes) for attaching the heat exchanger 30 to other components of the air heater 100 and / or to the frame parts of the outer housing G are located at the opening 38. The outer housing G may consist of multiple parts. Additional mounting means 71 (here a recess) may be provided for receiving the air intake nozzle contained in the blower housing on the heat exchanger 30.

[0056] The one in Fig. As can be seen in section 6, they extend similarly to those in the Fig. 2. According to the comparative example, the hot-air-side fins 44 extend parallel to and at equal intervals from the outer surface 40 or from the respective main fins 42. This applies to quadrants formed by four main fins 42, the main fins 42 extending diagonally and perpendicularly to each adjacent main fin 42 in the direction of the corners of a cross-sectional profile of the outer casing G. The planes defined by the main fins 42 intersect on the longitudinal axis M of the base body.

[0057] The hot-air side fins 44 of a respective quadrant define a first midplane P1 or a second midplane P2, respectively, by running parallel to the hot-air side fins 44 and extending through the longitudinal axis M. As in the Fig. As can be seen in Figure 6, the rib 45 extending directly within the first median plane P1 (or analogously, the second median plane P2 perpendicular to it) has the smallest rib height among the ribs 44 extending from the outer surface 40. The cross-section of the rib channels 43 extending close to the first median plane P1 or the second median plane P2 is therefore comparatively small. The greater the distance of the hot-air-side ribs 44 or the rib channels 43 from the first median plane P1 or the second median plane P2, the larger the cross-section becomes. Conversely, for hot-air-side ribs 44 extending from the main ribs 42 in the respective quadrant, the cross-section decreases again as the distance to the median planes increases. This is also evident in the comparative example of the Fig. 2 before.

[0058] With regard to the Fig. Section 5 explains in more detail the arrangement of the hot air-side fins 44 according to the exemplary embodiment. The hot air-side fins 44 each have an inlet edge 50 and an outlet edge 51. As described above, the positions of the respective outlet edges 51 are preferably positioned near the rear side 62. However, due to design or manufacturing reasons, in areas near the central plane P1 and also furthest from it, namely at the corners of the cross-sectional profile of the heat exchanger 30, some outlet edges are slightly offset towards the front face 61.

[0059] On the other hand, the positions X along the longitudinal axis M of the leading edges 50 are determined further towards the rear side 62, the closer the respective hot-air-side rib 44 is to the first median plane P1 (or analogously to the second median plane P2 perpendicular to it). The positions X of the leading edges 50 along the longitudinal axis M are chosen such that the length between the leading edge 50 and the corresponding trailing edge 51 of the same hot-air-side rib 44 correlates with the cross-section, which varies with the distance to the median plane. In particular, by choosing the position of the leading edge 50, the length of the respective rib 44 is selected such that a change in flow resistance in the considered rib channel 43 due to an increasing cross-section with distance is counteracted or even compensated for.

[0060] In this example, the top view shows how in Fig. 4 or Fig. As can be seen in Figure 5, the positions of the inlet edges 50 on the outer surface 40 of the heat exchanger 30 have a distinct parabolic shape. Due to the offset of the outlet edges 51 towards the end face 61 described above, various multiples of adjacent finned channels 44 are formed. In the Fig. These are, firstly, those ribs 44 whose trailing edges are essentially adjacent to the rear face 62, and secondly, those ribs 44 that are offset towards the front face 61. Both plurals also form two subgroups opposite each other across the median plane P1, P2 for each quadrant, in each of which there are two or more adjacent ribs 44 facing the hot air side, which form rib channels between them, and for which the following applies: the closer they are to the respective median plane P1, P2, the closer the leading edge is to the rear face.

[0061] Exceptions in the exemplary embodiment are made by the ribs 45 which extend precisely in the respective central planes P1, P2. These simultaneously fulfill a function that reinforces the integrity and stability of the heat exchanger 30 and therefore do not participate in the compensation.

[0062] As also in the Fig. 6 can be seen, are different from in Fig. Two hot-air-side fins 46 are also formed in the corners of the outer contour K of the heat exchanger 30 or the outer casing G. In other words, there is no defect of a hot-air-side fin 44 here. Furthermore, in this embodiment, the above-mentioned condition for the positions of the leading edges 50 is also extended to those as air-side fins 44 that extend from the main fin 42, although, as described, the cross-section decreases again when the distance from the first or second median plane P1, P2 increases. In particular, as in Fig.As can be seen in Figure 5, the parabolic shape of the arrangement of the leading edges 50 continues further outwards in the top view. The reason for this is that an intentionally increasing flow resistance is created in the relevant fin channels 43 up to the tips of the main fins 42 or corners of the outer contour K, because, on the one hand, the temperature gradients are lower here, and on the other hand, a previously excessive mass flow of heated air in comparatively cooler edge regions remains more or less unused for the heating effect. This measure would increase the overall pressure losses. However, because the leading edges 50 are offset towards the rear side 61 near the central plane, the opposite effect of a reduction in pressure losses occurs here.Overall, the effects can balance each other out, or the pressure losses can even be minimized overall, which may be reflected in a possible reduction in the drive power of the blower motor 20. At the same time, a better, more efficient distribution of the mass flow through the finned channels 43 can be achieved.

[0063] It should be noted at this point that all the features of the invention described above, considered individually and in any technically meaningful combination, in particular the details shown in the drawings, belong to the invention. Reference symbol list 2. Impact flow 11 Heated air inlet 12 Heated air outlet 14 hot air blowers 18 Control device 20 Motor (blower) 23 exhaust-side ribbed channels 24 exhaust-side ribs 30 heat exchangers 31 combustion air blowers 33 Air intake 34 Exhaust gas outlet 35 Combustion chamber 36 Combustion chamber housing 38 Opening 40 cylindrical outer surface 42 Main rib 43 ribbed channels on the heating air side 44 hot air side fins 45 Reinforcing rib 46 additional secondary ribs (on the main rib) 49. Marginal rib canal enlarged by the absence of a rib 50 entrance edge 51 Exit edge 61 Front 62 Back 100 Air heater, fuel-operated M Longitudinal axis (central axis of heat exchanger) G Outer casing P1 first intermediate level P2 second middle level X axial position

Claims

[1] Fuel-operated air heater (100) for a vehicle, comprising a heat exchanger (30), wherein the heat exchanger (30) comprises: a tubular housing body with an opening (38) on an end face (61) for receiving a housing (36) of a combustion chamber (35), which forms at least one exhaust-side channel with an inner surface of the housing body, wherein the housing body has a cylindrical outer surface (40) around a longitudinal axis (M) of the housing body, which defines a hot air side; and a first plurality of hot air-side ribs (44), each extending parallel to the longitudinal axis (M) and parallel to each other from the outer surface (40) or from a main rib (42) projecting from the outer surface (40), wherein two hot air-side ribs (44) of the plurality enclose a rib channel (43) between them, through which hot air is guided during operation; wherein the hot air-side fins (44) of the first plurality define a first median plane (P1) parallel to them and symmetrical with respect to the housing body, which encloses the longitudinal axis (M), characterized by , that an axial position (X) with respect to the longitudinal axis (M) of the leading edges (50) of the hot air-side ribs (44) limiting the rib channels (43), which are provided on the hot air-side ribs (44) towards the front face (61), the closer to the rear (62) of the housing body the hot air-side rib (44) is positioned, the closer the hot air-side rib (44) is to the first central plane (P1). [2] Fuel-operated air heater (100) according to claim 1, wherein the heating air-side fins (44) of the first plurality each have an outer edge facing away from the outer surface (40) or from the main fin (42), wherein the outer edges together define a contour plane (K1) parallel to the longitudinal axis (M) and perpendicular to the first median plane (P1), in which a section of an outer housing (G) extends, which covers the fin channels (43) outwards. [3] Fuel-operated air heater (100) according to claim 1 or 2, wherein the first plurality comprises at least four, preferably at least five, further preferably at least six, each adjacent to each other heating air-side fins (44) to form correspondingly at least three, four or five fin channels (43). [4] Fuel-operated air heater (100) according to one of claims 1 to 3, wherein the heating air side ribs (44) of the first plurality have a length measured along the longitudinal axis (M) from the inlet edge (50) to an outlet edge (51), wherein the respective lengths of the heating air side ribs (44) increase in the direction of the longitudinal axis (M) with increasing distance of the relevant rib channel (43) from the first median plane (P1). [5] Fuel-operated air heater (100) according to claim 4, wherein an arrangement of all the leading edges (50) of the heating air-side ribs (44) of the first plurality in a top view of the housing body along the central plane (P1) perpendicular to the longitudinal axis (M) assumes a form similar to a parabola, the vertex of which lies in the central plane (P1) and which opens in a direction towards the end face (61) of the housing body. [6] Fuel-operated air heater (100) according to one of claims 1 to 5, wherein the mutual distance of adjacent ribs is constant or the same for all heating air-side ribs (44) of the first plurality. [7] Fuel-operated air heater (100) according to any one of claims 1 to 6, wherein the heat exchanger (30) is formed in one piece from the housing body and the hot air side fins (44), preferably as aluminum die casting. [8] Fuel-operated air heater (100) according to one of claims 1 to 7, wherein the at least one exhaust gas-side channel is formed along the inner surface of the housing body by correspondingly designed exhaust gas-side ribs (37). [9] Fuel-operated air heater (100) according to any one of claims 1 to 8, wherein the at least one main rib (42) extends inclined with respect to the first intermediate plane (P1), preferably at an angle of 40° to 50°, more preferably 45°; and extending radially along the entire length of the main rib (42), away from the outer surface (40) to an outer edge, at least some of the heating air-side ribs of the majority extending from the main rib (42) at a constant mutual distance from each other. [10] Fuel-operated air heater (100) according to any one of claims 1 to 9, wherein the housing body has a second median plane (P2) that is perpendicular to the first median plane (P1), wherein a line of intersection between the first median plane (P1) and the second median plane (P2) is the longitudinal axis (M); wherein Each main rib (42) projecting from the outer surface (40) bisects one of the quadrants formed by the two median planes, whereby four main ribs (42) each supporting the heating air side (44) extend perpendicularly or parallel to each other. [11] Fuel-operated air heater (100) according to claim 10, wherein, viewed circumferentially around the longitudinal axis (M), the first plurality and corresponding second, third and fourth pluralitys of hot air-side ribs (44) extend between the main ribs (42) in the same or similar manner as the first plurality of hot air-side ribs (44), each oriented along and parallel to a relevant central plane (P1, P2). [12] Fuel-operated hot air appliance (100) according to any one of claims 1 to 11, further comprising: an outer casing (G) which accommodates the heat exchanger (30) and which has a substantially flat section that covers the finned channels (43) to the outside; wherein the outer casing has a hot air inlet (11) and a hot air outlet (12) which are connected to each other via the ribbed channels (43); a hot air blower (14) operated by a motor (20), which during operation draws in hot air through the hot air inlet (11), guides it through the finned channels (43) and discharges it through the hot air outlet (12); a housing (36) of a combustion chamber (35) which is accommodated within the housing body of the heat exchanger (30) and forms at least one exhaust gas-side channel (23) with an inner surface of the housing body of the heat exchanger (30).

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