Coaxial heat exchanger and circuit comprising a coaxial heat exchanger

Abstract

The invention relates to a coaxial heat exchanger (1) for reversible operation, comprising an inner tube (2) with a tube axis (10), a tube wall (20), an outer surface (21) and an inner surface (22), wherein the inner tube (2) has a flowable cross-sectional area (A1) and wherein ribs (25) with a rib height (H1) are formed on the outer surface (21) of the inner tube (2), and comprising an outer tube (3) arranged coaxially to the inner tube (2) with a tube wall (30), an outer surface (31) and an inner surface (32), wherein projections (35) are formed on the inner surface (32) of the outer tube (3) which extend substantially radially from the tube wall (30) of the outer tube (3) and extend continuously parallel to the axis or helically along the inner surface (32) of the outer tube (3), wherein the projections (35) of the outer tube (3) are connected to the ribs (25) on the outside (21) of the inner tube (2) enclose an angle of at least 60° and are connected to these,wherein several parallel, flowable channels (4) are formed between the pipe wall (30) of the outer tube (3) and the tips (26) of the ribs (25) of the inner tube (2), each having a cross-sectional area (A2) measured transversely to the flow direction and a channel height (H2), wherein adjacent channels (4) are each separated from one another by a projection (35) of the outer tube (3), and wherein the ratio of the flowable cross-sectional area (A1) of the inner tube (2) to the sum of the cross-sectional areas (A2) of all channels (4) is at least 0.5 and at most 5. The invention further relates to a thermodynamic cycle with such a coaxial heat exchanger and a method for operating such a cycle.

Description

[0001] The invention relates to a coaxial heat exchanger for reversible operation, a thermodynamic cycle with such a coaxial heat exchanger, and a method for operating such a cycle.

[0002] Coaxial heat exchangers are frequently used in heat pumps and refrigeration and air conditioning systems as condensers or evaporators due to their robustness and resistance to fouling. In particular, coaxial heat exchangers are used in applications with reversible operation. Reversible operation means that in one operating period, for example in winter, the heat exchanger operates as a condenser, while in a second operating period, for example in summer, it operates as an evaporator. The heat exchanger must therefore be usable as both an evaporator and a condenser, with performance remaining approximately the same in both operating modes.

[0003] Heat exchangers with a coaxial arrangement of an inner and an outer tube are known from US patents 5,409,057 A and 5,551,504 A. The inner tube is a tube whose wall is deformed to form a profile with multiple flow channels. Additionally, the tube is twisted so that the flow channels are helical.

[0004] Coaxial heat exchangers are known from publication US 2011 / 0 214 847 A1, in which either the outer tube has axially parallel inner fins or the inner tube has axially parallel outer fins.

[0005] Furthermore, a coaxial heat exchanger is known from US patent 813,918 A, which has an inner tube with axially parallel outer fins and an outer tube with axially parallel inner fins. The outer fins of the inner tube extend to the wall of the outer tube, while the inner fins of the outer tube extend to the wall of the inner tube. The outer fins of the inner tube and the inner fins of the outer tube run parallel to each other, so that channels are formed between the outer and inner tubes, each bounded by an inner fin of the outer tube and an outer fin of the inner tube. The boundary surfaces of the channels are smooth. The advantage of a heat exchanger with axially parallel fins is a low pressure drop. The disadvantage is that axially parallel, smooth fins cause only slight turbulence of the medium flowing in the channels and therefore do not significantly improve heat transfer.

[0006] The invention is based on the objective of providing a coaxial heat exchanger for reversible operation that is improved in terms of cost and performance. Reversible operation requires that approximately the same thermal output is achieved in evaporation mode as in condensation mode. Furthermore, the coaxial heat exchanger should be easily scalable with respect to its nominal capacity. The invention also aims to provide a thermodynamic cycle for reversible operation and a method for operating such a cycle.

[0007] The invention is described with respect to a coaxial heat exchanger by the features of claim 1, with respect to a thermodynamic cycle by the features of claim 13, and with respect to a method for operating a thermodynamic cycle by the features of claim 14. The further referenced claims relate to advantageous embodiments and further developments of the invention.

[0008] The invention relates to a reversibly operable coaxial heat exchanger with an inner tube and an outer tube arranged coaxially with the inner tube. The inner tube has a tube axis, a tube wall, an outer surface, an inner surface, and a cross-sectional area through which flow occurs. Ribs are formed on the outer surface of the inner tube. The rib height is measured from the tube wall of the inner tube to the undeformed tip of the ribs. The inner tube is preferably made of copper or a copper alloy. The outer tube has a tube wall, an outer surface, and an inner surface. Projections are formed on the inner surface of the outer tube, extending substantially radially from the tube wall of the outer tube, i.e., towards the inner tube, and extending continuously parallel to the axis or helically along the inner surface of the outer tube over the entire length of the outer tube or over a portion of its length.Preferably, the projections of the outer tube form an angle of at most 30° with the axis of the inner tube and consequently also with the axis of the outer tube. Particularly preferably, this angle is at most 15°. The projections of the outer tube form an angle of at least 60°, preferably at least 80°, with the ribs on the outside of the inner tube and are connected to them by at least touching the ribs. This creates several parallel, flow-through channels between the wall of the outer tube and the tips of the ribs of the inner tube. Each channel has a cross-sectional area measured perpendicular to the flow direction and a channel height, the channel height being measured from the wall of the outer tube to the undeformed tips of the ribs of the inner tube. Adjacent channels are separated from each other by a projection of the outer tube.The sum of the cross-sectional areas of all channels represents the total flowable cross-sectional area between the outer and inner pipes. The ratio of the flowable cross-sectional area of ​​the inner pipe to the sum of the cross-sectional areas of all 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, and particularly preferably at most 3.3.

[0009] The coaxial heat exchanger can be operated as both an evaporator and a condenser, with the evaporation or condensation of the refrigerant taking place in the channels between the outer and inner tubes. The refrigerant is typically a synthetic refrigerant, such as R410A, or a natural refrigerant, especially a hydrocarbon such as propane. During operation, a single-phase fluid, usually water or brine, flows through the inner tube. When the coaxial heat exchanger is operated as an evaporator, the single-phase fluid cools down. When the coaxial heat exchanger is operated as a condenser, the single-phase fluid heats up.

[0010] The invention is based on the consideration of how the flow space between the inner and outer tubes must be designed to be suitable for both the evaporation and condensation of a medium, while delivering approximately the same thermal output in both operating modes under normal operating conditions. The outer tube has projections on its inner surface that extend radially from the outer tube wall towards the inner tube and at least touch the fins on the outer surface of the inner tube. This divides the flow space into several parallel channels. The coaxial heat exchanger preferably has 10 to 25, and more preferably 12 to 22, such channels, depending on the capacity and diameter. The cross-sectional area and height of the channels can be selected to achieve optimal flow conditions for both evaporation and condensation.The pressure of the evaporating or liquefiing medium plays a crucial role in the design. Using an inner tube with fins on its outer surface promotes both evaporation and condensation. The channels formed by the projections of the outer tube form an angle of at least 60°, preferably at least 80°, with the fins on the outer surface of the inner tube. The angle of intersection is always defined as the angle that does not exceed 90°. The fins of the inner tube thus run essentially perpendicular to the flow direction within the channels. Compared to fins that run essentially parallel to the flow direction, transverse fins significantly increase heat transfer during both evaporation and condensation.

[0011] The cross-sectional area of ​​the inner tube and the total cross-sectional area between the inner and outer tubes must be matched. Depending on the operating conditions of the heat exchanger, the ratio of the cross-sectional area of ​​the inner tube to the sum of the cross-sectional areas of all 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, and particularly preferably at most 3.3. If this ratio is less than 0.5, then the total cross-sectional area between the inner and outer tubes is too large, resulting in an insufficient flow velocity of the evaporating or condensing medium and consequently reduced heat transfer.If this ratio is greater than 5, then the total cross-sectional area through which the fluid flows between the inner and outer tubes is too small, resulting in an excessive pressure drop of the evaporating or condensing medium. This is particularly disadvantageous for operation in evaporation mode.

[0012] The coaxial heat exchanger described above is characterized by excellent performance. Under typical conditions, its performance in evaporator mode is on par with that in condenser mode. Furthermore, the underlying design concept of the coaxial heat exchanger allows for easy adaptation to specific operating conditions and the properties of the medium used, such as its pressure. Starting with a given nominal capacity, it is also easily possible to modify the heat exchanger by varying the design parameters to achieve double or halve its performance. The design of the coaxial heat exchanger is therefore highly flexible and scalable.

[0013] The inner tube preferably has helical internal ribs or similar elements on its inner surface, which cause turbulence in the fluid flowing within the inner tube. Furthermore, the inner tube can be designed as a double-walled tube in which small channels are present in the double wall for leak detection. Such tubes are known as safety tubes.

[0014] The outer tube can preferably be connected to the ribs of the inner tube by a force-fit connection, in particular by a press fit.

[0015] Furthermore, the outer tube can be encased in a jacket tube made of a different material, such as stainless steel or titanium.

[0016] The coaxial heat exchanger can be straight, wound in a helix, or have another shape.

[0017] In one embodiment, the ratio of the height of the inner tube fins 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, and 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 heat transfer. If the ratio is greater than 1.1, the pressure drop of the medium flowing in the channels is too great.

[0018] In another embodiment, the channel height can be at least 1 mm and at most 10 mm, preferably at most 5 mm. In this range, particularly advantageous conditions arise with regard to heat transfer and pressure drop. Furthermore, channel heights in this range are readily achievable.

[0019] In 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, and particularly preferably at most 8.0 mm². In this range, particularly advantageous conditions arise with regard to heat transfer, pressure drop, and manufacturability.

[0020] In a specific embodiment, the outer tube can be an extruded profile tube, preferably made of aluminum or an aluminum alloy. With an extruded profile tube, the tube wall and the projections on the inside of the tube are produced simultaneously by an extrusion process. The projections are thus monolithically bonded to the tube wall. When an extruded profile tube is used as the outer tube, the coaxial heat exchanger described above can be manufactured particularly easily and cost-effectively. With an extruded profile tube, the diameter, height, and number of projections can be easily varied. This results in great flexibility in the design of the heat exchanger. Preferably, the projections run parallel to the axis.

[0021] In another embodiment, the inner tube can be a rolled finned tube in which the fins are monolithically bonded to the tube wall and spirally arranged at an angle of 80° to 89.5°, preferably 85° to 88.5°, measured against the tube axis. Rolled finned tubes exhibit excellent heat transfer properties and can be manufactured in many dimensions and design variations.

[0022] In a specific embodiment of this design, the ribs can have a spacing of 0.40 mm to 0.75 mm, preferably 0.45 mm to 0.65 mm. The rib spacing is measured along the length of the tube axis. This range offers particularly favorable conditions for both evaporation and condensation.

[0023] In a further specific embodiment of this design, the ribs 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, particularly advantageous conditions arise with regard to heat transfer, pressure drop, and manufacturability.

[0024] In a further specific embodiment of this design, the fins can have notches. Notching the fins creates many convex edges, which are advantageous for liquefaction. Surprisingly, it has been shown that a finned tube with notched fins also exhibits good performance in evaporation operation.

[0025] In a further specific embodiment of this design, the fins can have a structure that includes undercuts and / or cavities. A fin structure with undercuts and / or cavities is advantageous for evaporation. Surprisingly, it has been shown that a finned tube with such a structure also exhibits very good performance in condensation operation.

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

[0027] Advantageously, the projections of the outer tube can penetrate the fins on the outside of the inner tube. This causes the fins to be plastically deformed at least in the region of their tips at the penetration points. As a result of this penetration, a mechanical interlock between the outer and inner tubes is formed, which has a particularly beneficial effect on the heat transfer of the heat exchanger. In extreme cases, the projections can penetrate the entire height of the inner tube fins, reaching the tube wall. Preferably, the penetration depth of the projections is at least 10% of the height of the inner tube fins.

[0028] In a further embodiment of the invention, the projections of the outer tube can be formed only along a portion of the length of the inner tube, i.e., only partially. In this embodiment, the projections are formed only in a first section relative to the total length of the inner tube, while in a second section an outer tube is used that has no projections on its inner surface. The length of the first section can preferably be 40% to 70% of the total length of the inner tube. Because no projections are present in the second section, the flow space between the inner and outer tubes is not divided into several channels there, but rather forms an annular space. This reduces the pressure drop of the medium flowing between the inner and outer tubes.

[0029] Regarding further technical features and advantages of the coaxial heat exchanger according to the invention, explicit reference is hereby made to the explanations in connection with the thermodynamic cycle described below, as well as to the figures, the figure description and the exemplary embodiments.

[0030] Another aspect of the invention relates to a thermodynamic cycle with a coaxial heat exchanger as described above. The cycle is configured such that the coaxial heat exchanger can be operated as an evaporator in a first operating cycle and as a condenser in a second operating cycle, with evaporation and condensation taking place in the channels of the coaxial heat exchanger. Preferably, condensation occurs counter-currently to the fluid flowing in the inner tube, while evaporation preferably occurs in the same direction as the fluid flowing in the inner tube. The thermodynamic cycle typically also includes a compressor, an expansion valve, and a further heat exchanger, which can be operated as a condenser in the first operating cycle and as an evaporator in the second.

[0031] Another aspect of the invention relates to a method for operating a thermodynamic cycle as described above. The thermodynamic cycle comprises a coaxial heat exchanger as described above. In a first operating phase, a medium evaporates in the channels of the coaxial heat exchanger, while in a second operating phase, a medium liquefies in the channels of the coaxial heat exchanger.

[0032] With regard to further technical features and advantages of the method according to the invention, explicit reference is hereby made to the explanations in connection with the coaxial heat exchanger according to the invention, as well as to the figures, the description of the figures and the exemplary embodiments.

[0033] Exemplary embodiments of the invention are explained in more detail with reference to the schematic drawings. These show: Fig. 1: Oblique view of a coaxial heat exchanger. Fig. 2: Cross-section of a coaxial heat exchanger. Fig. 3: Oblique view of an inner tube with notches. Fig. 4: Longitudinal section of an inner tube with cavities.

[0034] Corresponding parts are marked with the same reference symbols in all figures.

[0035] Fig. 1 Figure 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 surface 21, and an inner surface 22. The inner tube 2 is designed as a rolled, double-sided structured finned tube. On the inner surface 22 of the inner tube 2, internal fins 29 are arranged, which run helically at an angle of approximately 45° to the tube axis 10. On the outer surface 21, fins 25 are arranged, which run helically at an angle of inclination of approximately 88° measured to the tube axis 10. In the case of the Fig. 1 In the illustrated embodiment, the ribs 25 of the inner tube 2 do not have any additional structure for the sake of simplicity. The coaxial heat exchanger 1 further comprises an outer tube 3 arranged coaxially to the inner tube 2, with a tube wall 30, an outer surface 31, and an inner surface 32. A portion of the outer tube 3 is in Fig. 1 The inner tube 2 is not shown to allow for better visibility. On the inner surface 32 of the outer tube 3, projections 35 are arranged, extending continuously parallel to the tube axis 10 over the entire length of the outer tube 3. The outer tube 3 is designed as an extruded profile tube, and the projections 35 are monolithically connected to the tube wall 30 of the outer tube 3. The projections 35 extend radially from the tube wall 30 and reach to the fins 25 of the inner tube 2, so that they at least touch the fin tips 26. This creates flow-through channels 4 between the inner surface 32 of the outer tube 3 and the fins 25 of the inner tube 2, extending continuously parallel to the tube axis 10 over the entire length of the coaxial heat exchanger 1. Circumferentially adjacent channels 4 are each separated by a projection 35 of the outer tube 3. The outer tube 3 is preferably made of aluminum or an aluminum alloy.The outer surface 31 of the outer tube 3 is located in the . Fig. 1 The illustrated embodiment is covered with a jacket tube 5. The jacket tube 5 can serve to protect against corrosion or improve the pressure resistance of the coaxial heat exchanger 1.

[0036] Fig. 2 shows a cross-section of a coaxial heat exchanger 1 according to Fig. 1 The cross-section lies in a plane perpendicular to the pipe axis 10. For better identification of the individual components, the outer pipe 3 is shown hatched, while the inner pipe 2 and the outer casing 5 are shown unhatched. The inner ribs 29 are visible on the inner surface 22 of the inner pipe 2. The cross-sectional area of ​​the inner pipe 2 through which the flow passes is marked A1. A helically circumferential rib 25 is shown on the outer surface 21 of the inner pipe 2. The height H1 of the rib 25 is measured from the pipe wall 20 to the tip 26 of the undeformed rib 25. The outer pipe 3 is arranged coaxially with the inner pipe 2. Eighteen projections 35 are formed on the inner surface 32 of the outer pipe 3, extending radially inwards from the pipe wall 30 of the outer pipe 3.The radially inner ends of the projections 35 are in contact with the tips 26 of the ribs 25 of the inner tube 2, at least touching them. The projections 35 can also penetrate the ribs 25, causing the ribs 25 to be deformed, at least in the region of the rib tips 26. In this case, the projections 35 are positively connected to the ribs 25, in particular by an interference fit. This results in a strong mechanical bond and good thermal contact between the outer tube 3 and the inner tube 2. The projections 35 divide the space between the tube wall 30 of the outer tube 3 and the tips 26 of the ribs 25 of the inner tube 2 into eighteen parallel channels 4. Each channel 4 has a flowable cross-sectional area, which is designated A2. The height H2 of the channels 4 is measured from the undeformed tip 26 of the ribs 25 of the inner tube 2 to the tube wall 30 of the outer tube 3.Preferably, the cross-sectional area through which the flow passes through the channels 4 has an approximately square shape. "Approximately square" in this context means that the shape of the cross-section can be approximated by a rectangle whose longer side has a length that is at most 1.3 times the length of the shorter side. As also in . Fig. 1 As shown, the outer surface 31 of the outer tube 3 is covered with a jacket tube 5.

[0037] As a special embodiment of an inner tube 2, the following is shown Fig. 3 An oblique view of the outer surface 21 of a finned tube with notched fins 25. A partial view of one fin 25 is shown. The fin 25 has notches 27 extending from the tip 26 of the fin 25 towards the tube wall 20. The notches 27 form additional convex edges in the fin 25. Convex edges improve the condensation process because the condensate formed during liquefaction is drawn away from the convex edges particularly quickly by surface tension. Preferably, such a finned tube can additionally have a (in Fig. 3 (not shown) structure on its inner side 22.

[0038] As a further special embodiment of an inner tube 2, the following is shown Fig. 4 A longitudinal section of a finned tube with cavities 28 is shown. A partial view is depicted, showing T-shaped fins 25 on the outer surface 21 of the tube 2. The T-shape of the fins 25 creates cavities 28 between the tips 26 of the fins 25 and the tube wall 20. Such cavities 28 promote the formation of nucleation sites and thus improve heat transfer during evaporation. Preferably, such a finned tube can additionally have a (in Fig. 4 (not shown) structure on its inner side 22. Reference symbol list

[0039] 1 Coaxial heat exchanger 10 Pipe axis 2 Inner pipe 20 Pipe wall 21 Outer side 22 Inner side 25 Rib 26 Rib tip 27 Notches 28 Cavity 29 Inner ribs 3 Outer pipe 30 Pipe wall 31 Outer side 32 Inner side 35 Projection 4 Channel 5 Jacket pipe A1 Cross-sectional area of ​​inner pipe A2 Cross-sectional area of ​​channel H1 Rib height H2 Channel height

Claims

1. Coaxial heat exchanger (1) for reversible operation, comprising an inner tube (2) having a tube axis (10), a tube wall (20), an outer surface (21) and an inner surface (22), wherein the inner tube (2) has a flowable cross-sectional area (A1) and wherein ribs (25) with a rib height (H1) measured from the tube wall (20) of the inner tube (2) to the undeformed tip (26) of the ribs (25) are formed on the outer surface (21) of the inner tube (2), and comprising an outer tube (3) arranged coaxially to the inner tube (2) having a tube wall (30), an outer surface (31) and an inner surface (32), wherein projections (35) are formed on the inner surface (32) of the outer tube (3) which extend substantially radially from the tube wall (30) of the outer tube (3) and are continuous in an axially parallel or helical manner along the inner surface (32) of the outer tube (3) extend,wherein the projections (35) of the outer tube (3) form an angle of at least 60° with the ribs (25) on the outside (21) of the inner tube (2) and are connected to them, thereby forming several adjacent, parallel flowable channels (4) between the tube wall (30) of the outer tube (3) and the tips (26) of the ribs (25) of the inner tube (2), each having a cross-sectional area (A2) measured transversely to the flow direction and a channel height (H2) measured from the tube wall (30) of the outer tube to the undeformed tips (26) of the ribs (25) of the inner tube (2), wherein adjacent channels (4) are each separated from each other by a projection (35) of the outer tube (3), and wherein the ratio of the flowable cross-sectional area (A1) of the inner tube (2) to the sum of the cross-sectional areas (A2) of all channels (4) is at least 0.5 and at most 5 amounts., 2. Coaxial heat exchanger (1) according to claim 1, characterized by the fact thatthe ratio of the height (H1) of the ribs (25) of the inner tube (2) to the channel height (H2) is at least 0.1 and at most 1.

1.

3. Coaxial heat exchanger (1) according to claim 1 or 2, characterized by the fact that the channel height (H2) is at least 1 mm and at most 10 mm.

4. Coaxial heat exchanger (1) according to one of claims 1 to 3, characterized by the fact that the cross-sectional area (A2) of the channels at least 2.5 mm 2 and at most 23 mm 2 amounts.

5. Coaxial heat exchanger (1) according to one of the preceding claims, characterized by the fact that the outer tube (3) is an extruded profile tube.

6. Coaxial heat exchanger (1) according to one of the preceding claims, characterized by the fact that the inner tube (2) is a rolled finned tube in which the fins (25) are monolithically connected to the tube wall (20) and helically run at an angle of 80° to 89.5° measured against the tube axis (10).

7. Coaxial heat exchanger (1) according to claim 6, characterized by the fact that the ribs (25) have a pitch of 0.40 mm to 0.75 mm, measured along the tube axis (10).

8. Coaxial heat exchanger (1) according to claim 6 or 7, characterized by the fact that the ribs (25) have a height (H1) of at least 0.3 mm and at most 1.1 mm.

9. Coaxial heat exchanger (1) according to any one of claims 6 to 8, characterized by the fact that the ribs (25) have notches (27).

10. Coaxial heat exchanger (1) according to any one of claims 6 to 9, characterized by the fact that the ribs (25) have a structure that includes undercuts and / or cavities (28).

11. Coaxial heat exchanger (1) according to one of the preceding claims, characterized by the fact that the projections (35) of the outer tube (3) penetrate into the ribs (25) on the outside (21) of the inner tube (2), causing the ribs (25) to be plastically deformed at the penetration points, at least in the area of ​​their tip (26).

12. Coaxial heat exchanger (1) according to one of the preceding claims, characterized by the fact thatthe projections (35) of the outer tube (3) are formed only along part of the length of the inner tube (2).

13. Thermodynamic cycle with a coaxial heat exchanger (1) according to one of the preceding claims, characterized by the fact that the cycle is set up such that the coaxial heat exchanger (1) can be operated as an evaporator in a first operating period and as a condenser in a second operating period, and that the evaporation or condensation takes place in the channels (4) of the coaxial heat exchanger (1).

14. Method for operating a thermodynamic cycle with a coaxial heat exchanger (1) according to any one of claims 1 to 12, characterized by the fact that in a first operating period a medium evaporates in the channels (4) of the coaxial heat exchanger (1) and in a second operating period a medium liquefies in the channels (4) of the coaxial heat exchanger (1).

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