EP3009780B2 - Heat exchanger - Google Patents

Description

The present invention relates to a heat exchanger for cooling a heat source of a motor vehicle with coolant channels and coolant channels according to the preamble of claim 1.

From the DE 10 2011 107 281 A1 a generic heat exchanger for cooling a heat source of a motor vehicle is known, comprising a plurality of coolant channels and a plurality of coolant channels. The coolant channels are formed by free spaces provided between the coolant channels, with heat-transferring surfaces being provided between a coolant guided in the coolant channels and a coolant guided in the coolant channels. In the area of the heat transfer surfaces, the coolant channels have a coolant-carrying volume that is larger by a factor of between 4 and 6 than the coolant-carrying volume of the coolant channels in the area of the heat transfer surfaces. This is intended to achieve a so-called chiller with a compact design and high heat exchange efficiency.

From the DE 10 2005 020 499 A1 a heat exchanger for a motor vehicle is known through which refrigerant can flow. The refrigerant stream flowing into the heat exchanger is divided by a valve device into at least two separate strands in such a way that there is no mutual mixing of the incoming refrigerant partial stream. This is intended to ensure that the temperature distribution is as even as possible.

In the ever-increasing segment of hybrid and electric vehicles, particularly effective temperature management of high-voltage batteries is of central importance for extending the range and using electrical energy efficiently. To cool the batteries, so-called “chillers” are used, which are compact. In order to increase the cooling capacity, CO ₂ is increasingly being used as a refrigerant, which means that the systems have to be designed for higher system pressures and temperatures. Previous plate heat exchangers for cooling a low-temperature circuit are not suitable for this.

JP-A-2000356488 , and DE-A-102004024825 disclose a heat exchanger according to the preamble of claim 1.

The present invention is therefore concerned with the problem of specifying an improved or at least an alternative embodiment for a heat exchanger of the generic type, which in particular enables efficient cooling with at the same time low weight and low costs of the heat exchanger.

This problem is solved according to the invention by the subject matter of independent claim 1. Advantageous embodiments are the subject of the dependent claims.

The present invention is based on the general idea of combining the advantages of an indirect evaporator (chiller) with the advantages of CO ₂ as a refrigerant and thereby being able to provide a compact, highly efficient and inexpensive heat exchanger, in particular for battery cooling. The heat exchanger according to the invention is therefore used to cool a heat source, for example a high-voltage battery or an electronic component, in a motor vehicle and has coolant channels and coolant channels in a known manner. The individual refrigerant channels in a flat tube, together with the adjoining refrigerant channels of the other flat tubes, form a refrigerant flow path. In an analogous manner, this also applies to the coolant channels, which, when lined up next to one another, form a coolant flow path. Of course, the coolant flows around the flat tubes. Carbon dioxide (CO ₂ ) is now used as the refrigerant, with the refrigerant flow path being deflected at least once in a U-shape and the refrigerant channels also having a ratio between their wall thickness and their free diameter (inner diameter) of at least 0.4. Through the at least one-time U-shaped deflection, both the running length and the flow speed can be increased and thus the heat transfer rate can be increased. Due to the comparatively high pressure and the associated small volume flows, deflection on the refrigerant side is preferred, with a U-shaped deflection of the coolant flow path also being provided. A U-shaped flow path can be understood as meaning a flow path that first runs in one direction and then, after a 180° turn, in the opposite direction, so that the refrigerant flows in opposite directions in the two flow path sections. Of course, multiple reversals or redirections are also possible. The refrigerant channels are positioned parallel to one another in so-called flat tubes, so that such a flat tube comprises a plurality of refrigerant channels running parallel to one another. The coolant channels are located between the individual flat tubes, so that an outer wall of a respective flat tube also forms a wall of a coolant channel. Between the individual flat tubes, ie in the coolant channels, there can be heat exchanger elements, such as turbulence inserts or corrugated fins, which improve heat transfer. In order to be able to make the refrigerant channels themselves wear-resistant in the long term against the comparatively high pressure, they are dimensioned in such a way that the ratio of their wall thickness to the free diameter or the channel width is at least 0.4. A further requirement for the refrigerant channels according to the invention is that a web present between two refrigerant channels of a flat tube has a width that is at least 40% of the channel width, ie the diameter of the refrigerant channel, preferably even 70 or even 100% of the (inner) diameter of the refrigerant channel amounts. Thanks to such massive webs, it is easily possible to absorb the pressures occurring in the refrigerant channels over the long term. With a heat exchanger designed in this way, not only can a compact heat exchanger with a comparatively high heat transfer rate be achieved, but it can also be manufactured comparatively inexpensively, which is of great advantage, particularly with regard to competition in the automotive supply sector.

In an advantageous development of the solution according to the invention, the refrigerant channels and the coolant channels are arranged in sections, that is to say locally, in cross-flow and as a whole, that is to say globally, in counter-current. A particularly favorable embodiment from a thermodynamic point of view results when both the refrigerant flow path and the coolant flow path are deflected in the same way and thus a countercurrent or cross-flow can be maintained across all paths. Since the refrigerant is expected to overheat by approx. 5 Kelvin in normal operation, it is advantageous if the last section in particular before the refrigerant exits is in countercurrent. The countercurrent principle is used here because the refrigerant has often already evaporated in the last flow path and is only heated up further, i.e. overheated. While there is no temperature change during evaporation, the refrigerant heats up in the superheated area. This is where the power supply is particularly important. This results in useful variants in particular if both the coolant side and the coolant side are deflected and both the coolant and the coolant are in countercurrent in the respective flow path. A 2-, 4- or 6-flow power supply is conceivable here. If both the coolant side and the refrigerant side are deflected, both flow paths can also be in cross-flow, in which case it makes particular sense to place the coolant outlet and the coolant inlet in the same section and thereby form a counterflow characteristic from a global perspective. Here too, the refrigerant side can be designed with 4 or 6 channels.

In an advantageous development of the solution according to the invention, a hydraulic diameter of the refrigerant channels is between 0.3 and 1.0 mm. The hydraulic diameter is a calculated quantity that is used to calculate pressure loss and throughput in pipes and channels if the cross section of the pipe or channel deviates from the circular shape. The hydraulic diameter is therefore to be determined in particular for refrigerant channels whose cross section is, for example, square with rounded corners or elliptical. For such channels, the hydraulic diameter indicates the diameter of the circular channel that would have the same pressure loss as the given channel with the same length and the same average flow velocity. With the empirically determined hydraulic diameter between 0.3 mm and 1.0 mm, both optimal pressure resistance and optimal heat transfer can be achieved. Of course, round or elliptical channels are particularly advantageous.

In a further advantageous embodiment of the solution according to the invention, the inner walls of the coolant channels are smooth, whereas the inner walls of the coolant channels are structured, i.e. H. especially rough, in order to achieve improved heat transfer. The improved heat transfer is generated by the larger surface area. In addition, the edges of the component lead to flow separation and thus to increased turbulence. However, due to the high pressure load and the requirements for internal cleanliness, a structured inside of the refrigerant channel does not make sense. In order to generally rule out any impairment of the circuit, there are purity requirements for all media-carrying parts for the components installed in the circuit.

For example, particles (tinsel, chips, etc.) are only tolerated up to a certain amount and quality. Structuring on the inside makes sense for this (e.g. if the flow channels do not have round but star-shaped cross-sections.

Heat exchanger elements, in particular turbulence inserts or corrugated fins, are expediently arranged in the coolant channels. Such heat exchanger elements increase the surface area available for heat transfer and thereby enable improved heat exchange.

Further important features and advantages of the invention emerge from the subclaims, from the drawings and from the associated description of the figures based on the drawings.

It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or alone, without departing from the scope of the present invention.

Preferred exemplary embodiments of the invention are shown in the drawings and are explained in more detail in the following description, with the same reference numbers referring to the same or similar or functionally the same components.

Fig. 1

einen erfindungsgemäßen Wärmeübertrager mit umgelenkten Kältemittel- und Kühlmittelkanälen,

Fig. 2

eine Schnittdarstellung durch ein Flachrohr mit erfindungsgemäß ausgebildeten Kältemittelkanälen,

Fig. 3

eine Darstellung wie in Figur 2, jedoch mit anderen Kältemittelkanälen,

Fig. 4

einen erfindungsgemäßen 4-flutigen Wärmeübertrager im Kreuzstrom und zusätzlicher Umlenkung in der Tiefe,

Fig. 5

einen 2-flutigen Wärmeübertrager im Kreuzstrom sowohl kühlmittelseitig als auch kältemittelseitig und Umlenkung in der Breite,

Fig. 6

einen erfindungsgemäßen Wärmeübertrager im Gegenstrom einer kältemittelseitigen Umlenkung in der Breite,

Fig. 7

einen Wärmeübertrager im Gegenstrom mit kühl- und kältemittelseitiger Umlenkung,

Fig. 8

einen als Verdampfer ausgebildeten Wärmeübertrager mit vorgeschaltetem Expansionsorgan.

It shows, schematically,

Fig. 1

a heat exchanger according to the invention with diverted refrigerant and coolant channels,

Fig. 2

a sectional view through a flat tube with refrigerant channels designed according to the invention,

Fig. 3

a representation like in Figure 2 , but with different refrigerant channels,

Fig. 4

a 4-flow heat exchanger according to the invention in cross-flow and additional deflection in the depth,

Fig. 5

a 2-flow heat exchanger in cross-flow on both the coolant side and the refrigerant side and deflection across the width,

Fig. 6

a heat exchanger according to the invention in the countercurrent of a refrigerant-side deflection in width,

Fig. 7

a countercurrent heat exchanger with deflection on the coolant and refrigerant sides,

Fig. 8

a heat exchanger designed as an evaporator with an upstream expansion element.

According to the Figures 1 and 4 to 8, a heat exchanger 1 according to the invention for cooling a heat source of a motor vehicle, in particular for cooling a heat pump or a high-voltage battery or an electronic component, has coolant channels 2 and coolant channels 3. According to the invention, carbon dioxide (CO ₂ ) is used as a refrigerant in all heat exchangers 1 and, in addition, a refrigerant flow path 7 is deflected at least once in a U-shape. Due to the at least one-time U-shaped deflection of at least the refrigerant flow path 7, the efficiency and also the performance of the heat exchanger 1 according to the invention can be significantly increased. Since CO ₂ is used as a refrigerant and comparatively high pressures occur, the refrigerant channels 3 according to the invention also have a ratio between their wall thickness w and their diameter d of at least 0.4 (cf. in particular also Figures 2 and 3 ). Due to the high pressure in the refrigerant channels 3 and the associated small volume flows, a deflection on the refrigerant side is preferred.

The individual refrigerant channels 3 are arranged in flat tubes 4 running parallel to one another, with a web 5 present between two refrigerant channels 3 having a width b which is at least 40% of the diameter of the refrigerant channel 3, preferably even 70 or 100% of the diameter of the refrigerant channel 3 (cf. again the Figures 2 and 3 ). Such thick webs 5 ensure the required tensile strength. Of course, it is conceivable that the individual refrigerant channels 3 are connected evenly or progressively. Progressive means that the flow cross-sectional area of the refrigerant side increases from one flow path to the next. This takes into account the increasing volume of the refrigerant flow during evaporation. This does not affect the geometric shape of the individual refrigerant channels 3 in the respective flat tube 4, but is set by the number of flat tubes 4 per flow path 6, 7.

In order to be able to guarantee the required high pressure resistance, the refrigerant channels 3 are preferably round or elliptical (cf. Figure 3 ), but can also have a square cross section with rounded corners, as for example according to Figure 2 is shown.

If you look at the heat exchanger 1 according to the Figure 1 , it can be seen that this works in cross-flow, so that a coolant flow path 6 flows essentially orthogonally to the coolant flow path 7. Of course, the design as a countercurrent cooler is also conceivable. Since an overheating of the refrigerant of approximately 5 Kelvin is to be expected in a heat exchanger 1 (chiller) in normal operation, it is also advantageous if the last section in particular before the refrigerant exits is in countercurrent.

If you look at the individual flow paths 6, 7 in the heat exchanger 1 according to Figure 1 , it can be seen that both the coolant flow path 7 and the coolant flow path 6 are redirected, which results in particularly effective cooling. The refrigerant and the coolant, for example a water-glysantine mixture, are in cross-flow in both flow paths 6, 7, just as in the heat exchanger 1 according to FIG Figures 4 and 5 . It is particularly useful here to place the coolant outlet and the coolant inlet in the same section, although the coolant side can of course also be designed with 4 or 6 channels.

The heat exchanger 1 according to Figure 5 works in cross-flow and has two flows, both on the coolant side and on the coolant side, and each has a deflection of the coolant flow path 6 and the coolant flow path 7 in width. The heat exchanger 1 according to Figure 4 is designed with 4 flutes and has one compared to that according to the Figure 5 Heat exchanger 1 shown has a higher flow velocity, which improves the heat transfer. The 4-flow design also ensures better protection against overheating.

In principle, the refrigerant channels 3 are contained in one or two collectors 8, in which a channel height h in relation to the material thickness w ₁ (wall thickness of the collector 8) is a maximum of 3, preferably even less than 1.5. Such a collector 8 is, for example, in the Figures 6 and 7 shown.

To ensure sufficient heat transfer as well In order to ensure sufficient pressure resistance, a hydraulic diameter d _H of the refrigerant channels 3 is between 0.3 and 1.0 mm. A comparable hydraulic diameter d _H for the coolant channels 2 is preferably between 0.5 and 2.0 mm. This allows an optimal ratio of pressure drop and heat transfer to be achieved on the coolant side. A particularly advantageous ratio between the hydraulic diameter of the coolant channels 2 and the hydraulic diameter of the coolant channels 3 is greater than 1.0, preferably this ratio is between 1.5 and 3. A two-phase mixture is usually heated on the coolant side, which is usually too significantly worse heat transfer coefficients on the refrigerant side than on the coolant side. In order to heat the refrigerant efficiently and thus cool the coolant efficiently, high heat transfer surfaces and small hydraulic diameters must be implemented on the refrigerant side. On the coolant side, however, there is very good heat transfer, although a lower pressure drop should be aimed for on the coolant side.

In order to be able to further improve heat transfer, heat exchanger elements 9, for example turbulence inserts or corrugated fins, are arranged in the coolant channels 2, which increase the surface area available for heat exchange. Of course, the surface available for heat transfer can also be designed to be structured, which in turn increases the surface area. Due to the high pressure load and the requirement for internal cleanliness, a structured surface is required for the refrigerant side, i.e. H. However, it is not specifically suitable for the inner surface of the refrigerant channels 3.

Finally, if you look at the heat exchanger 1 according to the Figure 6 , here too the coolant flow path 7 is deflected at least once in a U-shape, specifically in width, whereby the coolant flow path 6 can be deflected in corresponding coolant collectors 10. The coolant flow path 6 and the coolant flow path 7 run in countercurrent.

In the heat exchanger 1 according to Figure 7 There is both a U-shaped deflection of the coolant flow path 6 and a U-shaped deflection of the coolant flow path 7, each with two flows, with the flow in countercurrent here too. Since losses from the heat-transferring surface have to be accepted with counterflow, cross-flow is generally preferable here.

With counterflow, distribution channels must be installed in the direction of flow, whereas crossflow is structurally simpler, but not particularly good in terms of efficiency and reaction to overheating. For very compact heat exchangers, the best solution is a combination in which the individual sections are operated in cross-flow, but they are arranged one behind the other (at least in part) according to the counter-current principle.

According to the Fig. 8 a heat exchanger 1 designed as an R744 evaporator is shown with an upstream expansion element 11. The expansion element 11 can be designed, for example, as an electronic expansion valve (EXV). This expansion element 11 was usually attached to conventional R134a evaporators. However, for R744 and also for components with electronic expansion valves (EXV), these are usually integrated into the circuit separately from heat exchanger 1. If the expansion element 11 is installed in a structural unit with the evaporator, there are cost advantages, advantages in handling and, if necessary, advantages in terms of the interfaces. The term “unit” is understood to mean that the expansion element 11 (in particular TXV) is mechanically (possibly even cohesively) connected to the evaporator/chiller. Such a unit could be achieved, for example, by integrating the valve housing into the evaporator/chiller flange (+ soldering if necessary).

With the heat exchanger 1 according to the invention, a high performance, ie a high efficiency of the heat exchanger 1 can be achieved, with a small space requirement and a favorable connection situation, in particular if a connection for both the coolant flow path 6 and for the coolant flow path 7 is arranged on the same side of the heat exchanger 1 are. The refrigerant channels 3 designed according to the invention can also ensure high pressure resistance, which enables the use of CO ₂ as a refrigerant.

Claims (9)

1. Heat exchanger (1) for cooling a heat source of a motor vehicle, having coolant channels (2) forming a coolant flow path (6) and refrigerant channels (3) forming a refrigerant flow path (7), wherein

   - the refrigerant is CO₂,

   - the refrigerant flow path (7) is deviated at least once in the shape of a U,

   - the refrigerant channels (3) have a ratio of at least 0.4 between their wall thickness (w) and the diameter (d),

   - a web existing (5) between two refrigerant channels (3) has a width b equal to at least 40% of the diameter of the refrigerant channel (3), preferably even 70%, particularly preferably even 100% of the diameter of the refrigerant channel (3),

   - the refrigerant channels (3) are combined in a manifold (8),

   characterised in that the manifold (8) comprises distribution channels for which h/w₁ < 3.0, in particular h/w₁ < 1.5, where h is the height of the distribution channel/manifold and w₁ is the wall thickness of the manifold and the coolant flow path (6) is deviated at least once in the shape of a U.
2. Heat exchanger according to claim 1,
   characterised in that
   the refrigerant channels (3) have a square cross section with rounded corners.
3. Heat exchanger according to claim 1,
   characterised in that
   the refrigerant channels (3) are configured to be round or elliptical.
4. Heat exchanger according to any one of claims 1 to 3,
   characterised in that
   the refrigerant channels (3) and the cooling channels (2) are arranged in sections in cross-flow and overall in counter-flow.
5. Heat exchanger according to any one of claims 1 to 4,
   characterised in that
   heat exchanger elements (9), in particular turbulence inserts or corrugated fins, are arranged in the refrigerant channels (2).
6. Heat exchanger according to any one of claims 1 to 5,
   characterised in that

   - a hydraulic diameter d_h of the refrigerant channels (3) is 0.3 mm < d_h < 1.0 mm,

   - a hydraulic diameter d_h of the coolant channels (2) is 0.5 mm < d_h < 2.0 mm.
7. Heat exchanger according to any one of claims 1 to 6,
   characterised in that
   a refrigerant flow path (7) is configured progressively.
8. Heat exchanger according to any one of claims 1 to 7,
   characterised in that
   the distance between two flat tubes (4) framing refrigerant channels (3) forms a maximum coolant channel height of 3.5 mm.
9. Heat exchanger according to any one of claims 1 to 8,
   characterised in that
   the heat exchanger (1) is configured as an evaporator and is configured in a constructional unit with an upstream expansion member (11), in particular an electronic expansion valve (EXV).

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