Coaxial heat exchanger and circuit having a coaxial heat exchanger

Abstract

A coaxial heat exchanger having an inner tube having a tube wall, an outer face and an inner face. Fins are formed on the outer face of the inner tube, and an outer tube has a tube wall, an outer face and an inner face. Projections are formed on the inner face of the outer tube and protrude from the tube wall of the outer tube and extend along the inner face of the outer tube. The projections enclose an angle of at least 60° with the fins and are connected thereto. Adjacently running channels are formed between the tube wall of the outer tube and the tips of the fins of the inner tube. Adjacent channels are separated by a projection and a ratio of a flow-through cross-sectional area of the inner tube to a sum of cross-sectional areas of all channels is at least 0.5.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This claims priority from German Application No. 10 2024 002 556.1, filed Aug. 7, 2024, the disclosure of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The invention relates to a coaxial heat exchanger for reversible operation, a thermodynamic circuit having such a coaxial heat exchanger and a method for operating such a circuit.BACKGROUND AND SUMMARY

[0003] Coaxial heat exchangers are frequently used in heat pumps and in cooling and air-conditioning systems as condensers or as evaporators due to their robustness and insensitivity to contamination. In particular, coaxial heat exchangers are used in applications with reversible operation. “Reversible operation” means that in a first operating period, for example in winter, the heat exchanger is operated as a condenser while in a second operating period, for example in summer, the heat exchanger is operated as an evaporator. The heat exchanger thus has to be able to be used both as an evaporator and as a condenser. The performance in both operating modes has to be approximately at the same level.

[0004] Heat exchangers having a coaxial arrangement of an inner tube and an outer tube are disclosed in U.S. Pat. Nos. 5,409,057 and 5,551,504. One tube is used as the inner tube, the tube wall thereof being deformed such that a profile is formed with a plurality of flow channels. In addition, the tube is twisted so that the flow channels run in a helical manner.

[0005] Coaxial heat exchangers are disclosed in U.S. Pub. No. 2011 / 0214847 in which either the outer tube has axially parallel inner fins or the inner tube has axially parallel outer fins.

[0006] Moreover, a coaxial heat exchanger is disclosed in U.S. Pat. No. 813,918 which has an inner tube with axially parallel running outer fins and an outer tube also with axially parallel running inner fins. The outer fins of the inner tube extend as far as the tube wall of the outer tube, while the inner fins of the outer tube extend as far as the tube wall of the inner tube. The outer fins of the inner tube and the inner fins of the outer tube run parallel to one another so that channels, which are defined in each case by an inner fin of the outer tube and an outer fin of the inner tube, are formed between the outer tube and the inner tube. The boundary surfaces of the channels are smooth. The advantage of a heat exchanger with axially parallel fins is a minimal drop in pressure. A drawback is that axially parallel smooth fins produce only minimal turbulence of the medium flowing in the channels and therefore do not substantially improve the heat transfer.

[0007] One object of the invention is to specify a coaxial heat exchanger for reversible operation which is improved regarding costs and performance. The reversible operation requires that approximately the same thermal performance is achieved in evaporating operating mode as in condensing operating mode. Moreover, the coaxial heat exchanger is intended to be easily scalable regarding the rated performance. A further object of the invention is to specify a thermodynamic circuit for reversible operation and a method for operating such a circuit.

[0008] The invention relates to a coaxial heat exchanger which can be operated in a reversible manner, having an inner tube and having an outer tube which is arranged coaxially to the inner tube. The inner tube has a tube axis, a tube wall, an outer face, an inner face and a flow-through cross-sectional area. Fins are formed on the outer face of the inner tube. The fin height is measured from the tube wall of the inner tube as far as the undeformed tip of the fins. The inner tube is preferably made of copper or a copper alloy. The outer tube has a tube wall, an outer face and an inner face. Projections are formed on the inner face of the outer tube, which projections protrude substantially in the radial direction from the tube wall of the outer tube, i.e. toward the inner tube, and continuously extend axially parallel or helically along the inner face of the outer tube over the entire length of the outer tube or over a part of the length of the outer tube. Preferably, the projections of the outer tube enclose an angle of a maximum of 30° with the axis of the inner tube and, as a result, also with the axis of the outer tube. Particularly preferably, this angle is a maximum of 15°. The projections of the outer tube enclose an angle of at least 60°, preferably of at least 80°, with the fins on the outer face of the inner tube and are connected thereto by at least being in contact with the fins. As a result, a plurality of adjacently running channels which can be flowed through in parallel are formed between the tube wall of the outer tube and the tips of the fins of the inner tube. The channels have in each case a cross-sectional area measured transversely to the direction of flow and a channel height, wherein the channel height is measured from the tube wall of the outer tube as far as the undeformed tips of the fins of the inner tube. Adjacent channels are separated from one another in each case by a projection of the outer tube. The sum of the cross-sectional areas of all of the channels represents the entire flow-through cross-sectional area between the outer tube and the inner tube. The ratio of the flow-through cross-sectional area of the inner tube to the sum of the cross-sectional areas of all of the channels is at least 0.5, preferably at least 0.9, particularly preferably at least 1.4, and at most 5, preferably at most 4, particularly preferably at most 3.3.

[0009] The coaxial heat exchanger can be operated both as an evaporator and as a condenser, wherein the evaporating or the condensing of the medium used takes place in the channels between the outer tube and the inner tube. The medium is generally a synthetic coolant, such as for example R410A, or a natural coolant, in particular a hydrocarbon, such as for example propane. Generally a single-phase fluid, usually water or brine, flows in the inner tube during operation. If the coaxial heat exchanger is operated as an evaporator, the single-phase fluid cools down. If the coaxial heat exchanger is operated as a condenser, the single-phase fluid heats up.

[0010] The invention is based on the consideration as to how the flow chamber located between the inner tube and the outer tube has to be designed so that it is suitable both for evaporating a medium and for condensing a medium and under normal operating conditions produces approximately the same thermal performance in both operating modes. The outer tube has on its inner face projections which extend away from the tube wall of the outer tube in the radial direction toward the inner tube and which are at least in contact with the fins on the outer face of the inner tube. As a result, the flow chamber is divided into a plurality of adjacently running channels which can be flowed through in parallel. Preferably, depending on the performance and diameter, the coaxial heat exchanger has 10 to 25, preferably 12 to 22, such channels. The cross-sectional area and the height of the channels can be selected such that optimal flow conditions are achieved both for evaporating and for condensing. The pressure level of the evaporating or condensed medium plays a significant role in determining the design. The use of an inner tube which has fins on its outer face promotes both the evaporating and the condensing. The channels which are formed by the projections of the outer tube enclose an angle of at least 60°, preferably of at least 80°, with the fins on the outer face of the inner tube. The angle which is no more than 90° is always specified as the angle of intersection. The fins of the inner tube thus run substantially transversely to the direction of flow in the channels. In comparison with fins which run substantially along the direction of flow, the heat transfer is significantly increased by the transversely running fins, both during evaporating and during condensing.

[0011] The flow-through cross-sectional area of the inner tube and the entire flow-through cross-sectional area between the inner tube and outer tube have to be adapted to one another. Depending on the conditions of use of the heat exchanger, the ratio of the flow-through cross-sectional area of the inner tube to the sum of the cross-sectional areas of all of the channels is at least 0.5, preferably at least 0.9, particularly preferably at least 1.4, and at most 5, preferably at most 4, particularly preferably at most 3.3. If this ratio is less than 0.5, then the entire flow-through cross-sectional area between the inner tube and the outer tube is too great, so that the flow rate of the evaporating or condensing medium is too low and the heat transfer is thus impaired. If this ratio is greater than 5, then the entire flow-through cross-sectional area between the inner tube and outer tube is too small, so that the drop in pressure of the evaporating or condensing medium is too great. This is a drawback, in particular, for operation in evaporating mode.

[0012] The above-described coaxial heat exchanger is characterized by a very good performance characteristic. Under normal conditions, the performance in evaporator mode is at the same level as in condenser mode. Moreover, the concept which forms the basis of the construction of the coaxial heat exchanger permits a simple adaptation of the heat exchanger to the predetermined operating conditions and the properties of the medium used, such as for example the pressure level thereof. Starting from a specific rated performance, it is also possible to modify the heat exchanger in a simple manner by varying the design parameters, such that it achieves twice the performance or half the performance. The construction of the coaxial heat exchanger is thus very flexible and easily scalable.

[0013] The inner tube preferably has on its inner face helical inner fins or similar elements which produce a turbulence of the fluid flowing in the inner tube. Moreover, the inner tube can be designed as a double-walled tube in which small channels are present in the double wall of the tube for detecting leakages. Such tubes are known as safety tubes.

[0014] The outer tube can preferably be connected non-positively, in particular by a press fit, to the fins of the inner tube.

[0015] Moreover, the outer tube can be enclosed by a jacket tube made of a different material, such as for example stainless steel or titanium.

[0016] The coaxial heat exchanger can be straight or it can be wound to form a spiral or have a different shape.

[0017] In the context of one embodiment, the ratio of the height of the fins of the inner tube to the channel height can be at least 0.1, preferably at least 0.2 and at most 1.1, preferably at most 0.8, particularly preferably at most 0.5. If the ratio is less than 0.1, the fins of the inner tube have too little effect on the heat transfer. If the ratio is greater than 1.1, then the drop in pressure of the medium flowing in the channels is too great.

[0018] In the context of a further embodiment, the channel height can be at least 1 mm and at most 10 mm, preferably at most 5 mm. In this range, it results in particularly advantageous ratios with respect to the heat transfer and drop in pressure. Moreover, channel heights in this range can be produced in a simple manner.

[0019] In the context of a further embodiment, the cross-sectional area of the channels can be at least 2.5 mm2, preferably at least 5.0 mm2, and at most 23 mm2, preferably at most 13.0 mm2, particularly preferably at most 8.0 mm2. In this range, it results in particularly advantageous ratios with respect to heat transfer, drop in pressure and ease of production.

[0020] In the context of a specific embodiment, the outer tube can be an extruded profile tube, preferably made of aluminum or an aluminum alloy. In the case of an extruded profile tube, the tube wall and the projections on the inner face of the tube are generated at the same time by an extrusion process. The projections are thus connected integrally to the tube wall. If an extruded profile tube is used as the outer tube, the above-described coaxial heat exchanger can be produced particularly simply and inexpensively. In the case of an extruded profile tube, the diameter and height of the projections and the number thereof can be easily varied. This results in a high degree of flexibility in the design of the heat exchanger. Preferably, the projections run axially parallel.

[0021] In the context of a further embodiment, the inner tube can be a rolled finned tube, wherein the fins are connected integrally to the tube wall and circulate in a helical manner at an angle of 80° to 89.5°, preferably of 85° to 88.5°, measured relative to the tube axis. Rolled finned tubes have excellent properties with respect to heat transfer and can be produced in a plurality of sizes and design variants.

[0022] In the context of a specific design of this embodiment, the fins can have a spacing of 0.40 mm to 0.75 mm, preferably of 0.45 mm to 0.65 mm. The spacing of the fins is measured longitudinally to the tube axis. In this range, it results in particularly advantageous conditions both for evaporating and for condensing.

[0023] In the context of a further specific design of this embodiment, the fins can have a height of at least 0.3 mm, preferably at least 0.5 mm and at most 1.1 mm. In this range, it results in particularly advantageous ratios with respect to heat transfer, drop in pressure and ease of production.

[0024] In the context of a further specific design of this embodiment, the fins can have notches. A plurality of convex edges, which are advantageous for the condensing, are produced by notching the fins. Surprisingly, it has been shown that a finned tube with notched fins also has good performance in evaporating mode.

[0025] In the context of a further specific design of this embodiment, the fins can have a structure which comprises undercuts and / or cavities. A finned structure having undercuts and / or cavities is advantageous for the evaporation. Surprisingly, it has been shown that a finned tube with such a structure also has very good performance in condensing mode.

[0026] In the context of a further design of this embodiment, the inner tube can have a coating on its outer face and / or on its inner face.

[0027] Advantageously, the projections of the outer tube can penetrate into the fins on the outer face of the inner tube. As a result, the fins are plastically deformed at least in the region of their tip at the penetration points. As a result of the penetration, a mechanical interlocking of the outer tube and the inner tube is formed, which acts particularly advantageously on the heat transfer of the coaxial heat exchanger. In an extreme case, the projections can penetrate over the entire height of the fins of the inner tube as far as the tube wall thereof. Preferably, the penetration depth of the projections is at least 10% of the height of the fins of the inner tube.

[0028] In the context of a further embodiment of the invention, the projections of the outer tube can be formed only along a part of the length of the inner tube, i.e. only partially. In this embodiment, the projections are only formed in a first portion relative to the overall length of the inner tube, while an outer tube which has no projections on its inner face is used in a second portion. The length of the first portion can be preferably 40% to 70% of the overall length of the inner tube. Since no projections are present in the second portion, the flow chamber between the inner tube and the outer tube is not divided here into a plurality of channels but formed as an annular space. As a result, the drop in pressure of the medium flowing between the inner tube and outer tube is reduced.

[0029] With respect to further technical features and advantages of the coaxial heat exchanger according to the invention, reference is explicitly made hereby to the explanations in connection with the thermodynamic circuit described below, and to the figures, the description of the figures and the exemplary embodiments.

[0030] A further aspect of the invention relates to a thermodynamic circuit having a coaxial heat exchanger as described above. The circuit is designed such that the coaxial heat exchanger can be operated in a first operating period as an evaporator and can be operated in a second operating period as a condenser and the evaporating or the condensing takes place in the channels of the coaxial heat exchanger. Preferably, the condensing takes place in the counterflow to the fluid flowing in the inner tube, while the evaporating preferably takes place in the counterflow to the fluid flowing in the inner tube. The thermodynamic circuit generally also comprises a compressor, an expansion valve and a further heat exchanger which can be operated in the first operating period as a condenser and can be operated in the second operating period as an evaporator.

[0031] A further aspect of the invention relates to a method for operating a thermodynamic circuit as described above. The thermodynamic circuit comprises a coaxial heat exchanger as described above. In a first operating period, a medium is evaporated in the channels of the coaxial heat exchanger, while in a second operating period a medium is condensed in the channels of the coaxial heat exchanger.

[0032] With respect to further technical features and advantages of the method according to the invention, therefore, reference is explicitly made hereby to the explanations in connection with the coaxial heat exchanger according to the invention and to the figures, the description of the figures and the exemplary embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Exemplary embodiments of the invention are explained in more detail by way of the schematic drawings, in which:

[0034] FIG. 1 shows an oblique view of a coaxial heat exchanger;

[0035] FIG. 2 shows a cross-section of a coaxial heat exchanger;

[0036] FIG. 3 shows an oblique view of an inner tube with notches; and

[0037] FIG. 4 shows a longitudinal section of an inner tube with cavities.

[0038] Parts which correspond to one another are provided in all of the figures with the same reference signs.DETAILED DESCRIPTION

[0039] FIG. 1 shows an oblique view of a coaxial heat exchanger 1 with a tube axis 10. The coaxial heat exchanger 1 comprises an inner tube 2 with a tube wall 20, an outer face 21 and an inner face 22. The inner tube 2 is designed as a rolled finned tube which is structured on both sides. Inner fins 29, which run in a helical manner inclined at an angle of approximately 45° relative the tube axis 10, are arranged on the inner face 22 of the inner tube 2. Fins 25, which circulate in a helical manner at a pitch angle of approximately 88° measured relative to the tube axis 10, are arranged on the outer face 21. In the exemplary embodiment shown in FIG. 1, the fins 25 of the inner tube 2 have no additional structure for reasons of simplicity. The coaxial heat exchanger 1 also comprises an outer tube 3, arranged coaxially to the inner tube 2, with a tube wall 30, an outer face 31 and an inner face 32. A part of the outer tube 3 is not shown in FIG. 1, so that the inner tube 2 can be more clearly seen. Projections 35 which run continuously parallel to the tube axis 10 over the entire length of the outer tube 3 are arranged on the inner face 32 of the outer tube 3. The outer tube 3 is designed as an extruded profile tube and the projections 35 are connected integrally to the tube wall 30 of the outer tube 3. The projections 35 protrude in the radial direction from the tube wall 30 and extend as far as the fins 25 of the inner tube 2 so that the projections are at least in contact with the fin tips 26. As a result, flow-through channels 4, which extend continuously parallel to the tube axis 10 over the entire length of the coaxial heat exchanger 1, are formed between the inner face 32 of the outer tube 3 and the fins 25 of the inner tube 2. Adjacent channels 4 in the circumferential direction are separated in each case by a projection 35 of the outer tube 3. The outer tube 3 is preferably made of aluminum or an aluminum alloy. The outer face 31 of the outer tube 3 in the exemplary embodiment shown in FIG. 1 is covered by a jacket tube 5. The jacket tube 5 can serve for protection from corrosion or improve the pressure resistance of the coaxial heat exchanger 1.

[0040] FIG. 2 shows a cross section of a coaxial heat exchanger 1 according to FIG. 1. The cross section is located in a plane which is perpendicular to the tube axis 10. In order to be able to identify the individual components more clearly, the outer tube 3 is shown in hatched lines, while the inner tube 2 and the jacket tube 5 are not shown in hatched lines. The inner fins 29 can be identified on the inner face 22 of the inner tube 2. The flow-through cross-sectional area of the inner tube 2 is denoted by A1. A fin 25 circulating in a helical manner is shown on the outer face 21 of the inner tube 2. The height H1 of the fin 25 is measured from the tube wall 20 as far as the tip 26 of the undeformed fin 25. The outer tube 3 is arranged coaxially to the inner tube 2. Eighteen projections 35, which extend inwardly in the radial direction from the tube wall 30 of the outer tube 3, are formed on the inner face 32 of the outer tube 3. The internal ends of the projections 35 in the radial direction are in contact with the tips 26 of the fins 25 of the inner tube 2 by being at least in contact therewith. The projections 35 can also penetrate into the fins 25 so that the fins 25 are deformed at least in the region of the fin tips 26. In this case, the projections 35 are connected non-positively to the fins 25, in particular by a press fit. As a result, a secure mechanical bond and a good thermal contact is achieved between the outer tube 3 and the inner tube 2. Due to the projections 35, the space between the tube wall 30 of the outer tube 3 and the tips 26 of the fins 25 of the inner tube 2 are divided into eighteen channels 4 arranged in parallel to one another. The channels 4 have in each case a flow-through cross-sectional area which is denoted by A2. The height H2 of the channels 4 is measured from the undeformed tip 26 of the fins 25 of the inner tube 2 as far as the tube wall 30 of the outer tube 3. Preferably, the flow-through cross section of the channels 4 has an approximately square shape. “Approximately square” means in this context that the shape of the cross section can be approximated by a rectangle, the longer side thereof having a length which at most is 1.3 times the length of the shorter side. As also shown in FIG. 1, the outer face 31 of the outer tube 3 is covered by a jacket tube 5.

[0041] As a specific exemplary embodiment of an inner tube 2, FIG. 3 shows an oblique view of the outer face 21 of a fin tube with notched fins 25. A partial view of a fin 25 is shown. The fin 25 has notches 27 which extend from the tip 26 of the fin 25 in the direction of the tube wall 20. Due to the notches 27, additional convex edges are formed in the fin 25. Convex edges improve the process of condensation since the condensate formed during the condensing is particularly rapidly removed from the convex edges due to the surface tension. Preferably, such a finned tube can additionally have a structure (not shown in FIG. 3) on its inner face 22.

[0042] As a further specific exemplary embodiment of an inner tube 2, FIG. 4 shows a longitudinal section of a finned tube with cavities 28. A partial view is shown with T-shaped fins 25 on the outer face 21 of the tube 2. Due to the T-shape of the fins 25, cavities 28 are formed between the tip 26 of the fins 25 and the tube wall 20. Such cavities 28 promote the formation of bubble nucleation sites and thus improve the heat transfer during evaporation. Preferably, such a finned tube can additionally have a structure (not shown in FIG. 4) on its inner face 22.LIST OF REFERENCE SIGNS1 Coaxial heat exchanger

[0044] 10 Tube axis

[0045] 2 Inner tube

[0046] 20 Tube wall

[0047] 21 Outer face

[0048] 22 Inner face

[0049] 25 Fin

[0050] 26 Fin tip

[0051] 27 Notches

[0052] 28 Cavity

[0053] 29 Inner fins

[0054] 3 Outer tube

[0055] 30 Tube wall

[0056] 31 Outer face

[0057] 32 Inner face

[0058] 35 Projection

[0059] 4 Channel

[0060] 5 Jacket tube

[0061] A1 Inner tube cross-sectional area

[0062] A2 Channel cross-sectional area

[0063] H1 Fin height

[0064] H2 Channel height

Examples

Embodiment Construction

[0039]FIG. 1 shows an oblique view of a coaxial heat exchanger 1 with a tube axis 10. The coaxial heat exchanger 1 comprises an inner tube 2 with a tube wall 20, an outer face 21 and an inner face 22. The inner tube 2 is designed as a rolled finned tube which is structured on both sides. Inner fins 29, which run in a helical manner inclined at an angle of approximately 45° relative the tube axis 10, are arranged on the inner face 22 of the inner tube 2. Fins 25, which circulate in a helical manner at a pitch angle of approximately 88° measured relative to the tube axis 10, are arranged on the outer face 21. In the exemplary embodiment shown in FIG. 1, the fins 25 of the inner tube 2 have no additional structure for reasons of simplicity. The coaxial heat exchanger 1 also comprises an outer tube 3, arranged coaxially to the inner tube 2, with a tube wall 30, an outer face 31 and an inner face 32. A part of the outer tube 3 is not shown in FIG. 1, so that the inner tube 2 can be more ...

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This claims priority from German Application No. 10 2024 002 556.1, filed Aug. 7, 2024, the disclosure of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The invention relates to a coaxial heat exchanger for reversible operation, a thermodynamic circuit having such a coaxial heat exchanger and a method for operating such a circuit.BACKGROUND AND SUMMARY

[0003] Coaxial heat exchangers are frequently used in heat pumps and in cooling and air-conditioning systems as condensers or as evaporators due to their robustness and insensitivity to contamination. In particular, coaxial heat exchangers are used in applications with reversible operation. “Reversible operation” means that in a first operating period, for example in winter, the heat exchanger is operated as a condenser while in a second operating period, for example in summer, the heat exchanger is operated as an evaporator. The heat exchanger thus has to be able to be used both as an evaporator and as a condenser. The performance in both operating modes has to be approximately at the same level.

[0004] Heat exchangers having a coaxial arrangement of an inner tube and an outer tube are disclosed in U.S. Pat. Nos. 5,409,057 and 5,551,504. One tube is used as the inner tube, the tube wall thereof being deformed such that a profile is formed with a plurality of flow channels. In addition, the tube is twisted so that the flow channels run in a helical manner.

[0005] Coaxial heat exchangers are disclosed in U.S. Pub. No. 2011 / 0214847 in which either the outer tube has axially parallel inner fins or the inner tube has axially parallel outer fins.

[0006] Moreover, a coaxial heat exchanger is disclosed in U.S. Pat. No. 813,918 which has an inner tube with axially parallel running outer fins and an outer tube also with axially parallel running inner fins. The outer fins of the inner tube extend as far as the tube wall of the outer tube, while the inner fins of the outer tube extend as far as the tube wall of the inner tube. The outer fins of the inner tube and the inner fins of the outer tube run parallel to one another so that channels, which are defined in each case by an inner fin of the outer tube and an outer fin of the inner tube, are formed between the outer tube and the inner tube. The boundary surfaces of the channels are smooth. The advantage of a heat exchanger with axially parallel fins is a minimal drop in pressure. A drawback is that axially parallel smooth fins produce only minimal turbulence of the medium flowing in the channels and therefore do not substantially improve the heat transfer.

[0007] One object of the invention is to specify a coaxial heat exchanger for reversible operation which is improved regarding costs and performance. The reversible operation requires that approximately the same thermal performance is achieved in evaporating operating mode as in condensing operating mode. Moreover, the coaxial heat exchanger is intended to be easily scalable regarding the rated performance. A further object of the invention is to specify a thermodynamic circuit for reversible operation and a method for operating such a circuit.

[0008] The invention relates to a coaxial heat exchanger which can be operated in a reversible manner, having an inner tube and having an outer tube which is arranged coaxially to the inner tube. The inner tube has a tube axis, a tube wall, an outer face, an inner face and a flow-through cross-sectional area. Fins are formed on the outer face of the inner tube. The fin height is measured from the tube wall of the inner tube as far as the undeformed tip of the fins. The inner tube is preferably made of copper or a copper alloy. The outer tube has a tube wall, an outer face and an inner face. Projections are formed on the inner face of the outer tube, which projections protrude substantially in the radial direction from the tube wall of the outer tube, i.e. toward the inner tube, and continuously extend axially parallel or helically along the inner face of the outer tube over the entire length of the outer tube or over a part of the length of the outer tube. Preferably, the projections of the outer tube enclose an angle of a maximum of 30° with the axis of the inner tube and, as a result, also with the axis of the outer tube. Particularly preferably, this angle is a maximum of 15°. The projections of the outer tube enclose an angle of at least 60°, preferably of at least 80°, with the fins on the outer face of the inner tube and are connected thereto by at least being in contact with the fins. As a result, a plurality of adjacently running channels which can be flowed through in parallel are formed between the tube wall of the outer tube and the tips of the fins of the inner tube. The channels have in each case a cross-sectional area measured transversely to the direction of flow and a channel height, wherein the channel height is measured from the tube wall of the outer tube as far as the undeformed tips of the fins of the inner tube. Adjacent channels are separated from one another in each case by a projection of the outer tube. The sum of the cross-sectional areas of all of the channels represents the entire flow-through cross-sectional area between the outer tube and the inner tube. The ratio of the flow-through cross-sectional area of the inner tube to the sum of the cross-sectional areas of all of the channels is at least 0.5, preferably at least 0.9, particularly preferably at least 1.4, and at most 5, preferably at most 4, particularly preferably at most 3.3.

[0009] The coaxial heat exchanger can be operated both as an evaporator and as a condenser, wherein the evaporating or the condensing of the medium used takes place in the channels between the outer tube and the inner tube. The medium is generally a synthetic coolant, such as for example R410A, or a natural coolant, in particular a hydrocarbon, such as for example propane. Generally a single-phase fluid, usually water or brine, flows in the inner tube during operation. If the coaxial heat exchanger is operated as an evaporator, the single-phase fluid cools down. If the coaxial heat exchanger is operated as a condenser, the single-phase fluid heats up.

[0010] The invention is based on the consideration as to how the flow chamber located between the inner tube and the outer tube has to be designed so that it is suitable both for evaporating a medium and for condensing a medium and under normal operating conditions produces approximately the same thermal performance in both operating modes. The outer tube has on its inner face projections which extend away from the tube wall of the outer tube in the radial direction toward the inner tube and which are at least in contact with the fins on the outer face of the inner tube. As a result, the flow chamber is divided into a plurality of adjacently running channels which can be flowed through in parallel. Preferably, depending on the performance and diameter, the coaxial heat exchanger has 10 to 25, preferably 12 to 22, such channels. The cross-sectional area and the height of the channels can be selected such that optimal flow conditions are achieved both for evaporating and for condensing. The pressure level of the evaporating or condensed medium plays a significant role in determining the design. The use of an inner tube which has fins on its outer face promotes both the evaporating and the condensing. The channels which are formed by the projections of the outer tube enclose an angle of at least 60°, preferably of at least 80°, with the fins on the outer face of the inner tube. The angle which is no more than 90° is always specified as the angle of intersection. The fins of the inner tube thus run substantially transversely to the direction of flow in the channels. In comparison with fins which run substantially along the direction of flow, the heat transfer is significantly increased by the transversely running fins, both during evaporating and during condensing.

[0011] The flow-through cross-sectional area of the inner tube and the entire flow-through cross-sectional area between the inner tube and outer tube have to be adapted to one another. Depending on the conditions of use of the heat exchanger, the ratio of the flow-through cross-sectional area of the inner tube to the sum of the cross-sectional areas of all of the channels is at least 0.5, preferably at least 0.9, particularly preferably at least 1.4, and at most 5, preferably at most 4, particularly preferably at most 3.3. If this ratio is less than 0.5, then the entire flow-through cross-sectional area between the inner tube and the outer tube is too great, so that the flow rate of the evaporating or condensing medium is too low and the heat transfer is thus impaired. If this ratio is greater than 5, then the entire flow-through cross-sectional area between the inner tube and outer tube is too small, so that the drop in pressure of the evaporating or condensing medium is too great. This is a drawback, in particular, for operation in evaporating mode.

[0012] The above-described coaxial heat exchanger is characterized by a very good performance characteristic. Under normal conditions, the performance in evaporator mode is at the same level as in condenser mode. Moreover, the concept which forms the basis of the construction of the coaxial heat exchanger permits a simple adaptation of the heat exchanger to the predetermined operating conditions and the properties of the medium used, such as for example the pressure level thereof. Starting from a specific rated performance, it is also possible to modify the heat exchanger in a simple manner by varying the design parameters, such that it achieves twice the performance or half the performance. The construction of the coaxial heat exchanger is thus very flexible and easily scalable.

[0013] The inner tube preferably has on its inner face helical inner fins or similar elements which produce a turbulence of the fluid flowing in the inner tube. Moreover, the inner tube can be designed as a double-walled tube in which small channels are present in the double wall of the tube for detecting leakages. Such tubes are known as safety tubes.

[0014] The outer tube can preferably be connected non-positively, in particular by a press fit, to the fins of the inner tube.

[0015] Moreover, the outer tube can be enclosed by a jacket tube made of a different material, such as for example stainless steel or titanium.

[0016] The coaxial heat exchanger can be straight or it can be wound to form a spiral or have a different shape.

[0017] In the context of one embodiment, the ratio of the height of the fins of the inner tube to the channel height can be at least 0.1, preferably at least 0.2 and at most 1.1, preferably at most 0.8, particularly preferably at most 0.5. If the ratio is less than 0.1, the fins of the inner tube have too little effect on the heat transfer. If the ratio is greater than 1.1, then the drop in pressure of the medium flowing in the channels is too great.

[0018] In the context of a further embodiment, the channel height can be at least 1 mm and at most 10 mm, preferably at most 5 mm. In this range, it results in particularly advantageous ratios with respect to the heat transfer and drop in pressure. Moreover, channel heights in this range can be produced in a simple manner.

[0019] In the context of a further embodiment, the cross-sectional area of the channels can be at least 2.5 mm2, preferably at least 5.0 mm2, and at most 23 mm2, preferably at most 13.0 mm2, particularly preferably at most 8.0 mm2. In this range, it results in particularly advantageous ratios with respect to heat transfer, drop in pressure and ease of production.

[0020] In the context of a specific embodiment, the outer tube can be an extruded profile tube, preferably made of aluminum or an aluminum alloy. In the case of an extruded profile tube, the tube wall and the projections on the inner face of the tube are generated at the same time by an extrusion process. The projections are thus connected integrally to the tube wall. If an extruded profile tube is used as the outer tube, the above-described coaxial heat exchanger can be produced particularly simply and inexpensively. In the case of an extruded profile tube, the diameter and height of the projections and the number thereof can be easily varied. This results in a high degree of flexibility in the design of the heat exchanger. Preferably, the projections run axially parallel.

[0021] In the context of a further embodiment, the inner tube can be a rolled finned tube, wherein the fins are connected integrally to the tube wall and circulate in a helical manner at an angle of 80° to 89.5°, preferably of 85° to 88.5°, measured relative to the tube axis. Rolled finned tubes have excellent properties with respect to heat transfer and can be produced in a plurality of sizes and design variants.

[0022] In the context of a specific design of this embodiment, the fins can have a spacing of 0.40 mm to 0.75 mm, preferably of 0.45 mm to 0.65 mm. The spacing of the fins is measured longitudinally to the tube axis. In this range, it results in particularly advantageous conditions both for evaporating and for condensing.

[0023] In the context of a further specific design of this embodiment, the fins can have a height of at least 0.3 mm, preferably at least 0.5 mm and at most 1.1 mm. In this range, it results in particularly advantageous ratios with respect to heat transfer, drop in pressure and ease of production.

[0024] In the context of a further specific design of this embodiment, the fins can have notches. A plurality of convex edges, which are advantageous for the condensing, are produced by notching the fins. Surprisingly, it has been shown that a finned tube with notched fins also has good performance in evaporating mode.

[0025] In the context of a further specific design of this embodiment, the fins can have a structure which comprises undercuts and / or cavities. A finned structure having undercuts and / or cavities is advantageous for the evaporation. Surprisingly, it has been shown that a finned tube with such a structure also has very good performance in condensing mode.

[0026] In the context of a further design of this embodiment, the inner tube can have a coating on its outer face and / or on its inner face.

[0027] Advantageously, the projections of the outer tube can penetrate into the fins on the outer face of the inner tube. As a result, the fins are plastically deformed at least in the region of their tip at the penetration points. As a result of the penetration, a mechanical interlocking of the outer tube and the inner tube is formed, which acts particularly advantageously on the heat transfer of the coaxial heat exchanger. In an extreme case, the projections can penetrate over the entire height of the fins of the inner tube as far as the tube wall thereof. Preferably, the penetration depth of the projections is at least 10% of the height of the fins of the inner tube.

[0028] In the context of a further embodiment of the invention, the projections of the outer tube can be formed only along a part of the length of the inner tube, i.e. only partially. In this embodiment, the projections are only formed in a first portion relative to the overall length of the inner tube, while an outer tube which has no projections on its inner face is used in a second portion. The length of the first portion can be preferably 40% to 70% of the overall length of the inner tube. Since no projections are present in the second portion, the flow chamber between the inner tube and the outer tube is not divided here into a plurality of channels but formed as an annular space. As a result, the drop in pressure of the medium flowing between the inner tube and outer tube is reduced.

[0029] With respect to further technical features and advantages of the coaxial heat exchanger according to the invention, reference is explicitly made hereby to the explanations in connection with the thermodynamic circuit described below, and to the figures, the description of the figures and the exemplary embodiments.

[0030] A further aspect of the invention relates to a thermodynamic circuit having a coaxial heat exchanger as described above. The circuit is designed such that the coaxial heat exchanger can be operated in a first operating period as an evaporator and can be operated in a second operating period as a condenser and the evaporating or the condensing takes place in the channels of the coaxial heat exchanger. Preferably, the condensing takes place in the counterflow to the fluid flowing in the inner tube, while the evaporating preferably takes place in the counterflow to the fluid flowing in the inner tube. The thermodynamic circuit generally also comprises a compressor, an expansion valve and a further heat exchanger which can be operated in the first operating period as a condenser and can be operated in the second operating period as an evaporator.

[0031] A further aspect of the invention relates to a method for operating a thermodynamic circuit as described above. The thermodynamic circuit comprises a coaxial heat exchanger as described above. In a first operating period, a medium is evaporated in the channels of the coaxial heat exchanger, while in a second operating period a medium is condensed in the channels of the coaxial heat exchanger.

[0032] With respect to further technical features and advantages of the method according to the invention, therefore, reference is explicitly made hereby to the explanations in connection with the coaxial heat exchanger according to the invention and to the figures, the description of the figures and the exemplary embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Exemplary embodiments of the invention are explained in more detail by way of the schematic drawings, in which:

[0034] FIG. 1 shows an oblique view of a coaxial heat exchanger;

[0035] FIG. 2 shows a cross-section of a coaxial heat exchanger;

[0036] FIG. 3 shows an oblique view of an inner tube with notches; and

[0037] FIG. 4 shows a longitudinal section of an inner tube with cavities.

[0038] Parts which correspond to one another are provided in all of the figures with the same reference signs.DETAILED DESCRIPTION

[0039] FIG. 1 shows an oblique view of a coaxial heat exchanger 1 with a tube axis 10. The coaxial heat exchanger 1 comprises an inner tube 2 with a tube wall 20, an outer face 21 and an inner face 22. The inner tube 2 is designed as a rolled finned tube which is structured on both sides. Inner fins 29, which run in a helical manner inclined at an angle of approximately 45° relative the tube axis 10, are arranged on the inner face 22 of the inner tube 2. Fins 25, which circulate in a helical manner at a pitch angle of approximately 88° measured relative to the tube axis 10, are arranged on the outer face 21. In the exemplary embodiment shown in FIG. 1, the fins 25 of the inner tube 2 have no additional structure for reasons of simplicity. The coaxial heat exchanger 1 also comprises an outer tube 3, arranged coaxially to the inner tube 2, with a tube wall 30, an outer face 31 and an inner face 32. A part of the outer tube 3 is not shown in FIG. 1, so that the inner tube 2 can be more clearly seen. Projections 35 which run continuously parallel to the tube axis 10 over the entire length of the outer tube 3 are arranged on the inner face 32 of the outer tube 3. The outer tube 3 is designed as an extruded profile tube and the projections 35 are connected integrally to the tube wall 30 of the outer tube 3. The projections 35 protrude in the radial direction from the tube wall 30 and extend as far as the fins 25 of the inner tube 2 so that the projections are at least in contact with the fin tips 26. As a result, flow-through channels 4, which extend continuously parallel to the tube axis 10 over the entire length of the coaxial heat exchanger 1, are formed between the inner face 32 of the outer tube 3 and the fins 25 of the inner tube 2. Adjacent channels 4 in the circumferential direction are separated in each case by a projection 35 of the outer tube 3. The outer tube 3 is preferably made of aluminum or an aluminum alloy. The outer face 31 of the outer tube 3 in the exemplary embodiment shown in FIG. 1 is covered by a jacket tube 5. The jacket tube 5 can serve for protection from corrosion or improve the pressure resistance of the coaxial heat exchanger 1.

[0040] FIG. 2 shows a cross section of a coaxial heat exchanger 1 according to FIG. 1. The cross section is located in a plane which is perpendicular to the tube axis 10. In order to be able to identify the individual components more clearly, the outer tube 3 is shown in hatched lines, while the inner tube 2 and the jacket tube 5 are not shown in hatched lines. The inner fins 29 can be identified on the inner face 22 of the inner tube 2. The flow-through cross-sectional area of the inner tube 2 is denoted by A1. A fin 25 circulating in a helical manner is shown on the outer face 21 of the inner tube 2. The height H1 of the fin 25 is measured from the tube wall 20 as far as the tip 26 of the undeformed fin 25. The outer tube 3 is arranged coaxially to the inner tube 2. Eighteen projections 35, which extend inwardly in the radial direction from the tube wall 30 of the outer tube 3, are formed on the inner face 32 of the outer tube 3. The internal ends of the projections 35 in the radial direction are in contact with the tips 26 of the fins 25 of the inner tube 2 by being at least in contact therewith. The projections 35 can also penetrate into the fins 25 so that the fins 25 are deformed at least in the region of the fin tips 26. In this case, the projections 35 are connected non-positively to the fins 25, in particular by a press fit. As a result, a secure mechanical bond and a good thermal contact is achieved between the outer tube 3 and the inner tube 2. Due to the projections 35, the space between the tube wall 30 of the outer tube 3 and the tips 26 of the fins 25 of the inner tube 2 are divided into eighteen channels 4 arranged in parallel to one another. The channels 4 have in each case a flow-through cross-sectional area which is denoted by A2. The height H2 of the channels 4 is measured from the undeformed tip 26 of the fins 25 of the inner tube 2 as far as the tube wall 30 of the outer tube 3. Preferably, the flow-through cross section of the channels 4 has an approximately square shape. “Approximately square” means in this context that the shape of the cross section can be approximated by a rectangle, the longer side thereof having a length which at most is 1.3 times the length of the shorter side. As also shown in FIG. 1, the outer face 31 of the outer tube 3 is covered by a jacket tube 5.

[0041] As a specific exemplary embodiment of an inner tube 2, FIG. 3 shows an oblique view of the outer face 21 of a fin tube with notched fins 25. A partial view of a fin 25 is shown. The fin 25 has notches 27 which extend from the tip 26 of the fin 25 in the direction of the tube wall 20. Due to the notches 27, additional convex edges are formed in the fin 25. Convex edges improve the process of condensation since the condensate formed during the condensing is particularly rapidly removed from the convex edges due to the surface tension. Preferably, such a finned tube can additionally have a structure (not shown in FIG. 3) on its inner face 22.

[0042] As a further specific exemplary embodiment of an inner tube 2, FIG. 4 shows a longitudinal section of a finned tube with cavities 28. A partial view is shown with T-shaped fins 25 on the outer face 21 of the tube 2. Due to the T-shape of the fins 25, cavities 28 are formed between the tip 26 of the fins 25 and the tube wall 20. Such cavities 28 promote the formation of bubble nucleation sites and thus improve the heat transfer during evaporation. Preferably, such a finned tube can additionally have a structure (not shown in FIG. 4) on its inner face 22.LIST OF REFERENCE SIGNS1 Coaxial heat exchanger

[0044] 10 Tube axis

[0045] 2 Inner tube

[0046] 20 Tube wall

[0047] 21 Outer face

[0048] 22 Inner face

[0049] 25 Fin

[0050] 26 Fin tip

[0051] 27 Notches

[0052] 28 Cavity

[0053] 29 Inner fins

[0054] 3 Outer tube

[0055] 30 Tube wall

[0056] 31 Outer face

[0057] 32 Inner face

[0058] 35 Projection

[0059] 4 Channel

[0060] 5 Jacket tube

[0061] A1 Inner tube cross-sectional area

[0062] A2 Channel cross-sectional area

[0063] H1 Fin height

[0064] H2 Channel height

Examples

Embodiment Construction

[0039]FIG. 1 shows an oblique view of a coaxial heat exchanger 1 with a tube axis 10. The coaxial heat exchanger 1 comprises an inner tube 2 with a tube wall 20, an outer face 21 and an inner face 22. The inner tube 2 is designed as a rolled finned tube which is structured on both sides. Inner fins 29, which run in a helical manner inclined at an angle of approximately 45° relative the tube axis 10, are arranged on the inner face 22 of the inner tube 2. Fins 25, which circulate in a helical manner at a pitch angle of approximately 88° measured relative to the tube axis 10, are arranged on the outer face 21. In the exemplary embodiment shown in FIG. 1, the fins 25 of the inner tube 2 have no additional structure for reasons of simplicity. The coaxial heat exchanger 1 also comprises an outer tube 3, arranged coaxially to the inner tube 2, with a tube wall 30, an outer face 31 and an inner face 32. A part of the outer tube 3 is not shown in FIG. 1, so that the inner tube 2 can be more ...

Claims

1. A coaxial heat exchanger for reversible operation, having an inner tube with a tube axis, a tube wall, an outer face and an inner face, wherein the inner tube has a flow-through cross-sectional area and wherein fins are formed on the outer face of the inner tube, having a fin height measured from the tube wall of the inner tube as far as an undeformed tip of the fins,and having an outer tube, which is arranged coaxially to the inner tube, having a tube wall, an outer face and an inner face, wherein projections are formed on the inner face of the outer tube, which projections protrude substantially in a radial direction from the tube wall of the outer tube and continuously extend axially parallel or helically along the inner face of the outer tube, wherein the projections of the outer tube enclose an angle of at least 60° with the fins on the outer face of the inner tube and are connected thereto, whereby a plurality of adjacently running channels which can be flowed through in parallel are formed between the tube wall of the outer tube and the tips of the fins of the inner tube, each of which channels has a cross-sectional area measured transversely to a direction of flow and a channel height measured from the tube wall of the outer tube as far as the undeformed tips of the fins of the inner tube, wherein adjacent channels are separated from one another in each case by a projection of the outer tube and wherein a ratio of a flow-through cross-sectional area of the inner tube to a sum of the cross-sectional areas of all of the channels is at least 0.5 and at most 5.

2. The coaxial heat exchanger according to claim 1, wherein a ratio of a height of the fins of the inner tube to a channel height is at least 0.1 and at most 1.1.

3. The coaxial heat exchanger according to claim 1, wherein a channel height is at least 1 mm and at most 10 mm.

4. The coaxial heat exchanger according to claim 1, wherein a cross-sectional area of the channels is at least 2.5 mm2 and at most 23 mm2.

5. The coaxial heat exchanger according to claim 1, wherein the outer tube is an extruded profile tube.

6. The coaxial heat exchanger according to claim 1, wherein the inner tube is a rolled finned tube, and wherein the fins are connected integrally to the tube wall of the inner tuber and circulate in a helical manner at an angle of 80° to 89.5° measured relative to the tube axis.

7. The coaxial heat exchanger according to claim 6, wherein the fins have a spacing of 0.40 mm to 0.75 mm measured along the tube axis.

8. The coaxial heat exchanger according to claim 6, wherein the fins have a height of at least 0.3 mm and at most 1.1 mm.

9. The coaxial heat exchanger according to claim 6,wherein the fins have notches.

10. The coaxial heat exchanger according to claim 6,wherein the fins have a structure comprising undercuts and / or cavities.

11. The coaxial heat exchanger according to claim 1,wherein the projections of the outer tube penetrate into the fins on the outer face of the inner tube, and the fins are plastically deformed at least in a region of their tip at penetration points.

12. The coaxial heat exchanger according to claim 1, wherein the projections of the outer tube are formed only along a part of a length of the inner tube.

13. A thermodynamic circuit comprising:a coaxial heat exchanger, the coaxial heat exchanger comprising:an inner tube with a tube axis, a tube wall, an outer face and an inner face, wherein the inner tube has a flow-through cross-sectional area and wherein fins are formed on the outer face of the inner tube, having a fin height measured from the tube wall of the inner tube as far as an undeformed tip of the fins, and having an outer tube, which is arranged coaxially to the inner tube, having a tube wall, an outer face and an inner face, wherein projections are formed on the inner face of the outer tube, which projections protrude substantially in a radial direction from the tube wall of the outer tube and continuously extend axially parallel or helically along the inner face of the outer tube, wherein the projections of the outer tube enclose an angle of at least 60° with the fins on the outer face of the inner tube and are connected thereto, whereby a plurality of adjacently running channels which can be flowed through in parallel are formed between the tube wall of the outer tube and the tips of the fins of the inner tube, each of which channels has a cross-sectional area measured transversely to a direction of flow and a channel height measured from the tube wall of the outer tube as far as the undeformed tips of the fins of the inner tube, wherein adjacent channels are separated from one another in each case by a projection of the cuter tube and wherein a ratio of a flow-through cross-sectional area of the inner tube to a sum of the cross-sectional areas of all of the channels is at least 0.5 and at most 5;wherein the thermodynamic circuit is configured such that the coaxial heat exchanger can be operated as an evaporator in a first operating period and can be operated as a condenser in a second operating period, and the evaporating or the condensing takes place in the channels of the coaxial heat exchanger.

14. A method for operating a thermodynamic circuit having a coaxial heat exchanger according to claim 1, wherein in a first operating period a medium is evaporated in the channels of the coaxial heat exchanger and in a second operating period a medium is condensed in the channels of the coaxial heat exchanger.

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