US20130306288A1

US20130306288A1 - Tube structures for heat exchanger

Info

Publication number: US20130306288A1
Application number: US13/981,364
Authority: US
Inventors: Michael F. Taras; Sunil S. Mehendale; Mel Woldesemayat
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2011-01-28
Filing date: 2012-01-26
Publication date: 2013-11-21
Also published as: CN103339460A; EP2668460A1; WO2012103278A1; CN103339460B

Abstract

A fluid-carrying tube for a heat exchanger includes an outer perimeter, an inner perimeter, and a plurality of ridges extending from the inner perimeter inwardly into an interior of the tube. Each ridge includes a ridge height, a base width and a tip width. A ratio of the ridge height to the base width is between about 0.2 and about 4.0, and a ratio of the tip width to the base width is between about 0.015 and about 0.965.

Description

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to heat exchangers. More specifically, the subject disclosure relates to improved tube structures for a heat exchanger.
A simplified typical vapor compression refrigeration cycle includes an evaporator, a compressor, a condenser and an expansion device. Refrigerant flow is such that low pressure refrigerant vapor passes through a suction line to the compressor. The compressed refrigerant vapor is pumped to a discharge line that connects to the condenser. A liquid line receives liquid refrigerant exiting the condenser and directs it to the expansion device. A two-phase refrigerant is returned to the evaporator, thereby completing the cycle.
Two of the main components in a vapor compression cycle are the evaporator and condenser heat exchangers. The most common type of heat exchanger in use is of the round tube plate fin (RTPF) construction type. Historically, the tubes were made of copper while the fins were typically made of aluminum in such heat exchangers. The thermal performance of a heat exchanger, the ability to transfer heat from one medium to another, is inversely proportional to the sum of its thermal resistances. For a typical heating, ventilation, air conditioning and refrigeration (HVAC&R) application using refrigerant inside the tubes and air on the external fin side, the airside thermal resistance contributes 50-70% while refrigerant side thermal resistance is 20-40% and the metal resistance is relatively small and represents only 6-10%. Due to the continuous market pressure and regulatory requirements to make HVAC&R units more compact and cost effective, a lot of effort has been devoted to improving the heat exchanger performance on the refrigerant side as well as the airside.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a fluid-carrying tube for a heat exchanger includes an outer perimeter, an inner perimeter, and a plurality of ridges extending from the inner perimeter inwardly into an interior of the tube. Each ridge includes a ridge height, a base width and a tip width. A ratio of the ridge height to the base width is between about 0.2 and about 4.0, and a ratio of the tip width to the base width is between about 0.015 and about 0.965.
According to another aspect of the invention, a heat exchanger includes a plurality of fins and a plurality of tubes passing a fluid therethrough and extending through the plurality of fins. At least one tube of the plurality of tubes includes an outer perimeter, an inner perimeter, and a plurality of ridges extending from the inner perimeter inwardly into an interior of the at least one tube. Each ridge has a ridge height, a base width, and a tip width. A ratio of the ridge height to the base width is between about 0.2 and about 4.0, and a ratio of the tip width to the base width is between about 0.015 and about 0.965.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of an embodiment of a heat exchanger;

FIG. 2 is a partial cross-sectional view of an embodiment of a heat exchanger tube; and

FIG. 3 is a cross-sectional view of an embodiment of a heat exchanger tube.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1 is an embodiment of a round tube plate fin (RTPF) heat exchanger 10, such as one utilized as an evaporator or condenser. The RTPF heat exchanger 10 includes a plurality of tubes 12 and a plurality of fins 14. The plurality of tubes 12 carry a fluid, for example, a refrigerant. Thermal energy is exchanged between the fluid and air flowing past the plurality of fins 14. In some embodiments, the tubes 12 may be formed of an aluminum or aluminum alloy by, for example, an extrusion process, while in other embodiments, the tubes 12 maybe formed of other materials, for example, copper, Cu—Ni, steel or plastic.
FIG. 2 illustrates a partial cross-sectional view of a tube 12 of a heat exchanger 10. The tube 12 includes a plurality of enhancements, or ridges 16 extending into an interior 18 of the tube 12. As shown in FIG. 3, the tube 12 has an outer perimeter 32 and an inner perimeter 34, with the ridges 16 extending inwardly from the inner perimeter 34 into the interior 18 of the tube 12. The ridges 16 extend along a length 20 of the tube 12. In some embodiments, the ridges 16 extend substantially axially, while in other embodiments, the ridges 16 extend helically along the tube 12 at a helix angle α with respect to a tube axis 24. Ridges 16, such as those described herein, improve the heat transfer characteristics of the tubes 12 while maintaining a balance with pressure drop requirements to achieve a desired refrigerant flow through the tubes 12. Specific geometric configurations of the ridges 16, enhancing both the pre-expansion and post-expansion tube 12 surface geometry, are described below by way of example.
Referring again to FIG. 2, the ridges 16 have a number of characteristics to define their shape and arrangement in the interior 18 of the tube 12. Each ridge 16 has a ridge 16 height h, a base 26 width w, and a tip 28 width b. Sides 30 of the ridge 16 extend from the base 26 to the tip 28 at an apex angle Y. Adjacent ridges 16 are spaced by a ridge 16 pitch P_r. Each tube 12 has a tube diameter D, and a baseline tube 12 wall thickness t_bbetween adjacent ridges 16.
Shape of the ridges 16, as well as ridge 16 pitch P_rand a number of ridges 16 in the tube 12, N_r, are all taken into account when comparing an internal surface area of a tube 12 including the ridges 16 to a typical tube having a smooth wall, and thus an internal diameter as shown in equation (1) of:
D−2*t _b (1)
The increased internal surface area of the tube 12 including ridges 16 compared to the smooth-walled tube increases the effectiveness of thermal energy transfer between fluid in the tube 12 and an external environment. The effect of the increased surface area can be expressed as an enhancement ratio R_xas in equation (2) below:
R _x=(2*h*N _r*((1-sin(Y/2)/(π*(D−2 *(t _b +h))*cos(Y/2)))+1)/cos α (2)
As can be seen from a review of equation (2), the enhancement ratio Rx is a strong linear function of h/(π*(D−2*(t_b+h))/N_r), which is a ratio of the ridge height h, to the ridge pitch P_r.
In some embodiments, the ridges 16 may extend substantially axially along the length 20, or may extend at helix angle a of between about 18 degrees and about 35 degrees. Further, a ratio of the number of ridges Nr to a maximum internal diameter of the tube 12, or N_r/D_imaxmay be between about 5.4 and about 10.1, where D_imaxis specified in millimeters. In some embodiments, a ratio of the ridge height, h, to the ridge pitch, P_r, is between about 0.17 and about 1.36. R_x, as shown in equation 1, is between about 1.28 and about 3.49 in some embodiments, for example, those where the ridges 16 extend substantially axially along the tube 12. In other embodiments, for example where the helix angle α is not zero, R_xis between about 1.34 and about 4.26. In some embodiments, a ratio ridge height h to maximum internal diameter of the tube 12, or MD. , is between about 0.0008 and about 0.0870. For some ridges 16, the apex angle Y is between about 10 degrees and 25 degrees. Further, in some embodiments, the ridge height h and base width w are related such that a ratio of the ridge height to the base width, or h/w is between about 0.2 and about 4.0. Similarly, in other embodiments, the tip width b and the base width w, or b/w, is between about 0.015 and about 0.965.
Such ratios and ranges described above may vary for specific tube 12 outer diameters. For example, for tubes 12 with outer diameters of about 0.5 inches, N_r/D_imaxmay be between about 5.4 and about 9.25. Further, h/P_ris between about 0.17 and about 1.22. R_xis between about 1.28 and about 3.23 in embodiments where the ridges 16 extend substantially axially along the tube 12 and where the helix angle α is not zero, R_xis between about 1.34 and about 3.94. In embodiments of 0.5 inch diameter tube, h/D_imax, is between about 0.0008 and about 0.035.
In other embodiments where the tubes 12 have outer diameters of about 0.375 inches, N_r/D_imax, where D_imaxis expressed in millimeters, may be between about 5.8 and about 10.1. Further, h/P_ris between about 0.19 and about 1.36. R_xis between about 1.30 and about 3.49 in embodiments where the ridges 16 extend substantially axially along the tube 12 and where the helix angle α is not zero, R_xis between about 1.37 and about 4.26. In embodiments of 0.375 inch diameter tube, h/D_imax, is between about 0.0117 and about 0.0488.
In other embodiments where the tubes 12 have outer diameters of about 7 millimeters, N_r/D_imaxmay be between about 5.4 and about 9.5, where D_imaxis specified in millimeters. Further, h/P_ris between about 0.18 and about 1.30. R_xis between about 1.28 and about 3.37 in embodiments where the ridges 16 extend substantially axially along the tube 12 and where the helix angle α is not zero, R_xis between about 1.35 and about 4.12. In embodiments of 7 millimeter diameter tube, h/D_imax, is between about 0.021 and about 0.087.
In still other embodiments where the tubes 12 have outer diameters of about 5 millimeters, N_r/D_imaxmay be between about 5.5 and about 9.4, where D_imaxis specified in millimeters. Further, h/P_ris between about 0.18 and about 1.30. R_xis between about 1.29 and about 3.39 in embodiments where the ridges 16 extend substantially axially along the tube 12 and where the helix angle a is not zero, R_xis between about 1.36 and about 4.14. In embodiments of 5 millimeter diameter tube, h/D_imax, is between about 0.021 and about 0.087.
While the tubes 12 illustrated herein are substantially circular, it is to be appreciated that, in other embodiments, the tubes 12 may be noncircular in cross-section having, for example, an oval, an elliptical, or a race-track cross-section. In such tubes, an equivalent to tube 12 diameter D would be a circular cross-section tube diameter that would have identical mass or material content in the cross-section as the particular non-circular cross-section. All geometrical ratios described hereabove are equally applicable to such non-circular tube configurations allowing achieving substantially improved in-tube thermal and hydraulic performance.
Referring to the geometric ratios described herein, tubes 12 including such ridges 16 that conform to the exemplary ranges of these ratios exhibit substantially improved thermo-hydraulic performance over prior art tubes. The ratios, and described ranges for the ratios, are not obvious and have been developed via extensive simulation and experimentation on the component and sub-component level, while specifically focusing on the two-phase refrigerant flows.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A fluid-carrying tube for a heat exchanger comprising:

an outer perimeter;

an inner perimeter; and

a plurality of ridges extending from the inner perimeter inwardly into an interior of the tube, each ridge having;

a ridge height;

a base width; and

a tip width;

wherein a ratio of the ridge height to the base width is between about 0.2 and about 4.0; and

wherein a ratio of the tip width to the base width is between about 0.015 and about 0.965.

2. The tube of claim 1, wherein the plurality of ridges extend substantially axially along a length of the tube.

3. The tube of claim 1, wherein the plurality of ridges extend helically along a length of the tube.

4. The tube of claim 3, wherein a helix angle of the plurality of ridges is between about 18 degrees and 35 degrees.

5. The tube of claim 1, wherein a ratio of a number of ridges in the plurality of ridges to a maximum internal diameter of the tube expressed in millimeters is between about 5.4 and 10.1.

6. The tube of claim 5, wherein the ratio of a number of ridges in the plurality of ridges to a maximum internal diameter of the tube expressed in millimeters is between about 5.5 and 9.25.

7. The tube of claim 1, wherein a ratio of the ridge height to a ridge pitch between adjacent ridges of the plurality of ridges is between about 0.17 and about 1.36.

8. The tube of claim 7, wherein the ratio of the ridge height to the ridge pitch is between about 0.19 and about 1.22.

9. The tube of claim 1, wherein a ratio of the ridge height to a maximum internal diameter of the tube is between about 0.0008 and about 0.0870.

10. The tube of claim 9, wherein the ratio of the ridge height to the maximum internal diameter of the tube is between about 0.021 and about 0.035.

11. The tube of claim 1, wherein the tube is formed from an aluminum or aluminum alloy.

12. The tube of claim 1, wherein the tube is of a substantially non-circular cross-section, including but not limited to oval, elliptical and race-track cross-sections.

13. The tube of claim 1, wherein the tube has an effective diameter of between about 5 millimeters and about 13 millimeters.

14. A heat exchanger comprising:

a plurality of fins;

a plurality of tubes passing a fluid therethrough and extending through the plurality of fins, a least one tube of the plurality of tubes including:

an outer perimeter;

an inner perimeter; and

a plurality of ridges extending from the inner perimeter inwardly into an interior of the tube, each ridge having:

a ridge height;

a base width; and

a tip width;

15. The heat exchanger of claim 13, wherein the plurality of ridges extend substantially axially along a length of the at least one tube.

16. The heat exchanger of claim 13, wherein the plurality of ridges extend helically along a length of the at least one tube.

17. The heat exchanger of claim 15, wherein a helix angle of the plurality of ridges is between about 18 degrees and 35 degrees.

18. The heat exchanger of claim 13, wherein a ratio of a number of ridges in the plurality of ridges to a maximum internal diameter expressed in millimeters of the at least one tube is between about 5.4 and 10.1.

19. The heat exchanger of claim 17, wherein the ratio of a number of ridges in the plurality of ridges to a maximum internal diameter expressed in millimeters of the at least one tube is between about 5.5 and 9.25.

20. The heat exchanger of claim 13, wherein a ratio of the ridge height to a ridge pitch between adjacent ridges of the plurality of ridges is between about 0.17 and about 1.36.

21. The heat exchanger of claim 19, wherein the ratio of the ridge height to the ridge pitch is between about 0.19 and about 1.22.

22. The heat exchanger of claim 13, wherein a ratio of the ridge height to a maximum internal diameter of at least one tube of the plurality of tubes is between about 0.0008 and about 0.0870.

23. The heat exchanger of claim 13, wherein at least one tube of the plurality of tubes is formed from an aluminum or aluminum alloy.

24. The heat exchanger of claim 13, wherein at least one tube of the plurality of tubes is of a substantially non-circular cross-section, including but not limited to oval, elliptical and race-track cross-sections.

25. The heat exchanger of claim 13, wherein at least one tube of the plurality of tubes has an effective diameter of between about 5 millimeters and about 13 millimeters.