Heat exchanger tube with enhanced heat transfer co-efficient and related method

Abstract

A method of enhancing the heat transfer coefficient of a tube flowing a fluid in the tube in heat exchange relation with a second fluid external to the tube includes forming a patterned array of grooves on the interior surface of the tube. Each tube has a depth-to-width ratio of 0.10 to 0.30, and the grooves may be formed by mechanical pressing, pulsed ECM or electrodischarge processes.

Description

BACKGROUND

The present invention relates to heat exchangers and particularly to air cooled heat exchangers having an increased heat transfer co-efficient between the fluid flowing within the tube and the tube itself.

Heat exchangers providing heat exchange between a fluid within a series of tubes and cooling air flowing about the tubes are well known. Enhancements to these heat exchangers have taken the form of a plurality of fins applied externally about the tubes enhancing the heat exchange between the cooling air flowing about the tubes and fins and the fluid flowing within the tubes. Various methods for increasing the exchange surface and heat transfer co-efficient are well known in other environments, such as the use of brazed micro turbulators on internal surfaces of gas turbine parts, and internal tube dimpling. See, for example, U.S. Pat. Nos. 6,598,781 and 6,644,921. However, these processes have not been applied to air cooled heat exchangers, and do not address the enhancement of heat transfer between the fluid within a tube and the tube itself. Accordingly, there remains a need for increased heat exchange between the fluid inside a tube and the tube wall in an air cooled heat exchanger.

BRIEF SUMMARY

In accordance with an exemplary embodiment, heat exchange between fluid inside the tube and the tube wall is enhanced through the creation of a pattern of small grooves in the internal tube surface in order to create vortices in the fluid flow.

The creation of grooves on the internal surface of the tube and, in one exemplary embodiment, specifically a criss-crossed grooved pattern on the internal surface, can be accomplished by pulsed electrochemical machining (ECM), electrodischarge or simple tool pressing operations. The cross grooving at shallow depth and with spherical cross-sections serves to create low level fluid vortices for mixing fresh fluid to the walls and increasing heat transfer with little or no added pressure losses as compared to a smooth tube.

Accordingly, in one aspect, the invention relates to a method of enhancing the heat exchange coefficient of a tube flowing a fluid in the tube in heat exchange relation with a second fluid external to the tube comprising forming a patterned array of grooves on the interior surface of the tube.

In another aspect, the invention relates to a heat exchanger tube comprising a hollow tube having an interior surface covered substantially by an array of grooves, said grooves having a depth-to-width ratio of 0.10 to 0.30.

In still another aspect, the invention relates to a heat exchanger comprising a plurality of interconnected tubes adapted to carry a first fluid in heat exchange relationship with a second fluid flowing across said plurality of interconnected tubes, each tube having an interior surface covered substantially by an array of grooves in a criss-crossed pattern, said grooves having a depth-to-width diameter ratio of 0.10 to 0.30.

The invention will now be described in connection with the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art heat exchanger;

FIG. 2 is a schematic illustration of a tube with fins forming part of a prior art heat exchanger of FIG. 1;

FIG. 3 is a partial perspective view of a heat exchanger tube having an internal surface with a criss-crossed pattern of grooves formed therein; and

FIG. 4 is a partial perspective view of an unwrapped interior tube surface illustrating the criss-crossed groove pattern formed in the tube of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, particularly to FIG. 1, there is illustrated a conventional heat exchanger generally designated 10. Heat exchanger 10 is comprised of a plurality of interconnected tubes 12 for carrying/circulating a fluid to be cooled. The hot fluid is typically conveyed back and forth in opposite directions (or in one way only) in the tubes 12 that may be arranged in a large grid-like pattern. As illustrated in FIG. 1, the tubes 12 extend from a hot fluid inlet 14, back and forth in the grid pattern and terminate at an outlet port 16. It will be understood that the tubes can be arranged in many different configurations, e.g., one above the other, in layers offset one above the other or in any other well-known and suitable configuration. It will also be appreciated that, in use, the tubes 12 are in heat exchange relation with a cooling fluid e.g., air, flowing across the grid-like pattern. It will also be appreciated that the tubes may carry a fluid to be heated by flowing a heated fluid across the tubes.

To facilitate the heat transfer, and using as an example heat exchange between tubes carrying a hot fluid and cooling air passing over and about the tubes, a fan 18 with fan blades 20 is disposed, for example, below the tubes 12 for driving the cooling air through and across the grid. Thus, the air and the tubes 12 are in heat exchange relation one with the other such that the heated fluid passing through the tubes 12 is cooled and exits the heat exchanger at outlet port 16 at a lower temperature than fluid at the inlet 14. This invention also contemplates situations where only the latent heat is involved, such that the fluid will have energy removed, but will not actually be cooled.

An enlarged schematic illustration of a finned tube 12 is shown in FIG. 2. Thus, each tube 12 in the heat exchanger may carry multiple fins 22 which are attached to the tubes in a conventional manner. It will be appreciated that the fins increase the effective surface area of the interface between the cooling air and hot fluid enabling enhanced thermal cooling of the hot fluid.

Further enhancement of heat transfer in connection with fins is described in commonly owned co-pending application Ser. No. 11/493,022, filed Jul. 26, 2006.

As used herein, the term “fluid” embraces liquids, gases, steam, two phase mixtures, and multi-component fixtures. Also, heat exchanger's incorporating tubes as described herein may be of the type for condensing evaporated fluid.

Referring now to FIGS. 3 and 4, the interior surface 24 of a tube 26 (shown in “unwrapped form” in FIG. 4) may be treated mechanically by a tool pressing operation, or by pulsed ECM or electrodischarge processes to provide shaped surfaces within the tubes. One exemplary pattern shown to enhance heat transfer is a criss-crossed array of grooves 28 of shallow depth that serve to create lower level fluid vortices for mixing fresh fluid to the walls and increasing heat transfer with little or no added pressure losses compared to a smooth tube. In FIG. 4, groove centerlines 30 and 32 illustrate a pair of intersecting grooves 28, noting that regions 34 represent the original internal surface of the tube, raised relative to the grooves 28. The grooves 28 may be pressed simultaneously or formed in two sets of spiral or helical grooves, one in a clockwise direction and the other in a counter-clockwise direction, by different tools moving through the tube. The differently directed grooves may be at the same or differing relative angles.

In one example, a one-inch diameter tube having a wall thickness of 0.1 inch may have a groove depth of greater than 50 microns (about 2 mils or 0.2% of tube ID), and preferably in a range of 10 to 50 mils (or 1% to 5% of tube ID). The depth-to-width ratio may be in a range of 0.10 to 0.30. The grooves are generally semi-spherical in cross-section, with no sharp features to create added stresses, and preferably extend along the entire length of the tube.

Other patterns of intersecting grooves may be employed and remain within the scope of the invention. For example, the grooves when viewed in the unwrapped form illustrated in FIG. 4 may be curved along the length dimension thereof.

The groove geometry may also be varied in any of the following manners:

(a) The spacing between parallel running grooves, i.e., the perpendicular distance between two grooves may be varied without the grooves actually overlapping.

(b) The cross-sectional shape may only approximate a generally semispherical shape, for example, the groove may have a flat bottom with flat angled sidewalls.

(c) Groove density may vary, i.e., there may be a variation in the spacing of the grooves within the same length tube. In other words, constant spacing leads to a constant density (grooves per unit length), while changing the spacing as a function of distance along with tube length will alter the effects along the length. This can be achieved, for example, by varying the rate of draw of the tube during manufacture such that the tube may be tailored to have more heat transfer in one region and less in another region.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A method of enhancing the heat transfer coefficient of a tube flowing a fluid in the tube in heat exchange relation with a second fluid external to the tube comprising forming a patterned array of grooves on the interior surface of the tube.

2. The method of claim 1 wherein said grooves have a depth of greater than about 0.2% of the internal tube diameter.

3. The method of claim 1 wherein said grooves have a depth of between 1% and 5% of the internal tube diameter.

4. The method of claim 1 wherein said grooves have a semi-spherical cross section.

5. The method of claim 1 wherein said grooves have a depth-to-width ratio of 0.10 to 0.30

6. The method of claim 1 wherein said grooves are formed by a mechanical pressing.

7. The method of claim 1 wherein said grooves are formed by pulsed ECM or electro-discharge processes.

8. The method of claim 1 wherein said grooves are criss-crossed.

9. The method of claim 1 comprising forming each grooves with a flat bottom and angled side walls.

10. The method of claim 1 comprising forming at least some of the grooves to run generally parallel to each other but with varying spacing therebetween.

11. The method of claim 1 comprising forming the tube to vary density of the grooves along the length of the tube.

12. A heat exchanger tube comprising a hollow tube having an interior surface covered substantially by an array of grooves, said grooves having a depth-to-width diameter ratio of 0.10 to 0.30.

13. The heat exchanger tube of claim 12 wherein said grooves have a depth of greater than about 0.2% of the internal tube diameter.

14. The heat exchanger tube of claim 13 wherein said grooves have a depth of between 1% and 5% of the internal tube diameter.

15. The heat exchanger tube of claim 12 wherein said grooves have a semi-spherical cross section.

16. The heat exchanger tube of claim 12 wherein said grooves are criss-crossed.

17. A heat exchanger comprising a plurality of interconnected tubes adapted to carry a first fluid in heat exchange relationship with a second fluid flowing across said plurality of interconnected tubes, each tube having an interior surface covered substantially by an array of intersecting grooves in a criss-crossed pattern, said grooves having a depth-to-tube diameter ratio of 0.10 to 0.30.

18. The heat exchanger of claim 17 wherein said grooves have a depth of greater than about 0.2% of the internal tube diameter.

19. The heat exchanger of claim 18 wherein said grooves have a depth of between 1% and 5% of the internal tube diameter.

20. The heat exchanger of claim 17 wherein said grooves have a semi-spherical cross section.