US20080078535A1

US20080078535A1 - Heat exchanger tube with enhanced heat transfer co-efficient and related method

Info

Publication number: US20080078535A1
Application number: US11/541,682
Authority: US
Inventors: Ronald S. Bunker; Wayne C. Hasz; Mohamed A. Ali; Giuseppe Malcaus
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2006-10-03
Filing date: 2006-10-03
Publication date: 2008-04-03

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 applying metal particles to an interior surface of the tube to thereby create an increased wetted area for interaction with fluid flowing through the tube; wherein the particles are applied by: providing an applicator head including an electric arc and at least one metal wire; displacing the head along the interior of the tube; feeding the wire into the electric arc as the head is displaced along the interior of the tube; melting the wire to form molten metal particles; and applying the molten metal particles about and along the length of the interior wall surface of the tube as the head is displaced along the interior of the tube.

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

This invention increases the wetted area inside a heat exchanger by one of two methods.
In one exemplary embodiment, random or patterned microturbulators in the form of metal particulates are bonded to the internal surface of the tube by a method described further herein. The microturbulated surfaces allow fluid to interact with all portions of the increased wetted area, and also create fluid-transitional or turbulent flows. The preferred method for applying particles to the interior surface of a heat exchanger tube is a wire spray process that ablates two fed wires held at opposite electrical polarity. A single wire spray process is also contemplated. The resultant molten metal particles fuse to the interior of the tube, thus creating a random or patterned array of micro-turbulator particles onto the tube interior surface.
A second method uses a movable/retractable welding tip to apply welding material to the interior surface of the tube in any desired pattern that promotes and enhances heat transfer. For example, weld material may be applied so as to form a continuous spiral along the length of the tube interior. Alternatively, material may be added in discrete amounts to form patterned roughness on the interior surface of the tube.
Each of the methods briefly described above results in bonded internal tube surface microturbulations that enhance thermal performance.
Accordingly, in one aspect, the present invention relates to 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 applying metal particles to an interior surface of the tube to thereby create an increased wetted area for interaction with fluid flowing through the tube. The particles may be applied by: providing an applicator head including an electric arc and at least one metal wire; displacing the head along the interior of the tube; feeding the wire into the electric arc as the head is displaced along the interior of the tube; melting the wire to form molten metal particles; and applying the molten metal particles about and along the length of the interior wall surface of the tube as the head is displaced along the interior of the tube.
In another aspect, the invention relates to a method of enhancing heat transfer characteristics of a heat exchanger tube adapted to carry a fluid there through, the method comprising: locating a MIG welding torch nozzle within the heat exchanger tube; moving the torch nozzle axially relative to the tube while applying welding material to and along the interior surface of the tube to thereby enhance heat transfer between the tube and the fluid flowing through the tube.
In another aspect, the invention relates to a heat exchanger tube comprising a hollow tube having an interior surface substantially covered with metal particles for enhancing heat exchanger properties of the tube.
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 schematic illustration of an electric dual wire spray for spraying particulates onto the interior surfaces of the heat exchanger tube in accordance with another exemplary embodiment;

FIG. 4 is a simplified side elevation of conventional MIG welding apparatus that may be used in another exemplary embodiment of the invention; and

FIG. 5 is a perspective cut-away of a tube formed with an internal spiral groove, not necessarily to scale, using the apparatus of FIG. 4.

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 tubes arranged in a large grid-like pattern. In the illustrated form, 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 be appreciated that, in use, the tubes 12 are in heat exchange relation with a cooling fluid e.g., air flowing across and through 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 air passing over and about the tubes, a fan 18 with fan blades 20 is disposed, for example, below the tubes 12 for driving 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, the tubes 12 in the heat exchanger may carry 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 as a result of the finned configuration.
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 in the description of various embodiments of this invention, the term “fluid” embraces liquids, gases, steam, two phase mixtures, and multi-component mixtures. Also, the heat exchanger may be of the type for condensing or evaporating fluid.
Referring now to FIG. 3, there is illustrated a dual wire spray head in which a random or patterned array of microturbulators may be applied and bonded to the interior surface 24 of a heat exchanger tube 26. Specifically, a moveable/retractable dual wire spraying mechanism 28 is utilized to controllably apply the metallic particles to the interior surface of the tube (of circular cross section). In this instance, the mechanism 28 includes an applicator head 30 having a pair of wires 32, 34 which are fed through electrical contact tubes 36, 38, respectively. The head 30 is also constructed to provide primary atomizing gas through a central aperture 40 and secondary atomizing gas through an annular secondary aperture 42. Any suitable gas may be employed, but air is less preferred since it may oxidize the metal particles preventing them from bonding to the tube. It will be appreciated that an electric arc between the wires 32, 34 vaporizes the wires, causing particulates 44 to mix with the atomizing gas and to be metalurgically bonded to the interior surface 24 of the tube 26, without having to apply an adhesive coat or layer.
Referring now to FIG. 4, a known MIG welding apparatus is illustrated that is available from Bore Repair Systems, Inc. of Alstead, N.H. The apparatus is also shown and described in U.S. Pat. No. 6,137,076. The apparatus includes a weld torch assembly 46 that incorporates a first hollow arm 48 and a second hollow arm 50 with a clutch control mechanism 52 in a housing 54 between the hollow arms. A nozzle 56 at the distal end of arm 50 is arranged to apply weld material to the interior surface of a round tube 58, with clamping means 60, adjustable support bracket 62 and mounting rod 64 enabling positioning of the nozzle. At the same time, a threaded guide 66, spindle 68 and clutch control mechanism 52 control axial movement of the torch through the tube.
In an exemplary but non-limiting embodiment, the weld torch 46 is rotated as it moves axially through an elongated heat exchange tube 58 (FIG. 5) located in the position of tube 58 in FIG. 4.
As the torch rotates and moves axially, the welding ingredients (welding current, welding wire and welding gas) are supplied through the arms 48 and 50 to the nozzle 56, depositing welding material 72 on the inner diameter surface of the heat exchanger tube 58, as a continuous spiral rib 74 (thus creating a spiral rib/groove configuration) extending substantially the entire length of the tube, as best seen in FIG. 5. Note in FIG. 5, a portion 76 of the tube has yet not been altered, or will remain in its original configuration and serve, for example, as a tube-end connector. Alternatively, weld material may be added to roughen the interior surface of the tube, the roughness achieved by depositing the weld material in a random or patterned array.
In each case, the interior surface area of the tube is increased to thereby enhance the heat transfer characteristics of the tube.
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 applying metal particles to an interior surface of the tube to thereby create an increased wetted area for interaction with fluid flowing through the tube.

2. The method of claim 1 wherein the particles are applied by:

providing an applicator head including an electric arc and at least one metal wire;

displacing the head along the interior of the tube;

feeding the wire into the electric arc as the head is displaced along the interior of the tube;

melting the wire to form molten metal particles; and

applying the molten metal particles about and along the length of the interior wall surface of the tube as the head is displaced along the interior of the tube.

3. The method of claim 1 including providing a second wire, melting the second wire together with the first mentioned wire to form the molten metal particles, and applying the metal particles of said first and second wires along the length of the interior wall surface of the tube.

4. The method of claim 1 including mixing the molten particles with atomized gas to facilitate applying the molten metal particles to the interior wall surface of the tube.

5. A heat exchanger tube comprising a hollow tube having an interior surface substantially covered with metal particles for enhancing heat exchanger properties of the tube, said metal particles in molten form when applied to said interior surface and metalurgically bonded thereto.

6. A method of enhancing heat transfer characteristics of a heat exchanger tube adapted to carry a fluid there through, the method comprising:

locating a MIG welding torch nozzle within the heat exchanger tube;

moving the torch nozzle axially relative to the tube while applying welding material to and along the interior surface of the tube to thereby enhance heat transfer between the tube and the fluid flowing through the tube.

7. The method of claim 6 wherein the welding material is applied in discrete amounts to increase surface roughness of said interior surface.

8. The method of claim 6 including rotating the welding torch such that material is added to the interior surface of the tube in the form of a continuous spiral rib.