Heat exchange tube having increased heat transfer area

Abstract

The heat exchange tubes of the invention are provided with an inner wall having an increased surface area for contact with a fluid circulating through the tube for the greater exchange of heat energy between the circulating fluid and an external fluid. The outer wall of the heat exchange tube is provided with a series of radially outwardly extending members for increasing the heat exchange surface of the outer wall of the tube.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of provisional application 60/306,279, entitled IMPROVED HEAT EXCHANGE TUBE HAVING INCREASED HEAT TRANSFER AREA, filed Jul. 18, 2001 in the name of Kemel Burkay.

FIELD OF THE INVENTION

[0002] The invention relates to heat exchangers and more particularly to heat exchanger tubes having an increased heat exchange area.

BACKGROUND OF THE INVENTION

[0003] Germane to the transfer of heat energy from one venue to another is the heat exchanger. Heat exchangers conventionally comprise a heat exchange bundle that may comprise plates or tubes through which a heat transfer fluid is circulated. Heat energy is transferred from or to a liquid or gas contacting the members of the heat exchange bundle to the heat transfer fluid circulating through the members of the heat exchange bundle to heat or cool the liquid or gas.

[0004] The efficiency of a heat exchanger, that is the speed and amount of heat energy exchanged, is an important consideration in the design and size of the heat exchanger to be used in a particular situation. It will be understood that improvement in the efficiency of the heat exchanger can result in economics of size, manufacturing cost and cost of operation, i.e lower fuel and energy costs.

SUMMARY OF THE INVENTION

[0005] The invention resides in an improved heat exchange tube for tube type heat exchangers that operates in an improved manner to transfer an increased amount of heat energy more efficiently for a given diameter tube than achieved by conventional heat exchange tubes. The invention further resides in improved heat exchangers employing the improved heat exchange tubes.

[0006] More particularly, the heat exchange tubes of the invention are provided with an inner wall having an increased surface area for contact with a fluid circulating through the tube for the greater exchange of heat energy between the circulating fluid and an external fluid. The outer wall of the heat exchange tube is provided with a series of radially outwardly extending members for increasing the heat exchange surface of the outer wall of the tube. The inner wall surface of the heat exchange tube is formed with a plurality of radially, inwardly extending, spaced apart members that present an increased surface area to the heat transfer fluid circulating through the tube. The members may be in the form of a plurality of spaced apart, inwardly extending ridges or fins that run along the inner wall of the tube parallel to the axis of the tube. The spacing and height of inward extension of the members is not critical and is a matter of design choice although it will be understood that it is preferred to maintain the spacing between members at a minimum so as to have the maximum number of members for the internal diameter of the heat exchange tube. Similarly, the length of the inward extension from the inner wall surface and the configuration of the members can be designed to maximize the surface area that will be presented to the circulating fluid.

BRIEF DESCRIPTION OF THE DRAWING

[0007] FIG. 1 is a perspective view, broken away for compactness of illustration, of a heat exchange tube designed in accordance with the present invention;

[0008] FIG. 2 is an end view of the tube of FIG. 1;

[0009] FIG. 3 is a section of a heat transfer tube in accordance with the invention broken away for compactness of illustration and illustrating the inner wall of the tube and another embodiment of an inwardly extending rib;

[0010] FIG. 4 is a section of a heat transfer tube broken away for compactness of illustration showing another embodiment of an inwardly extending rib;

[0011] FIG. 5 is a section of a heat transfer tube broken away for compactness of illustration showing another embodiment of an inwardly extending rib;

[0012] FIG. 6 is an end view of another embodiment of a heat exchange tube in accordance with the invention; and

[0013] FIG. 7 is an end view of another embodiment of the heat exchange tube of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The invention will be described herein in connection with heat exchangers used in the production of steam such as in a power plant where the steam is used to drive turbines for generation of electrical power. In this connection the conductance of heat energy is from a hot fluid external to the heat exchange tube to a cooler fluid circulating in the heat exchange tube. The circulating fluid in this case is water that receives heat energy to convert the water to steam that is then conveyed to a turbine for driving the turbine and its associated generator.

[0015] Referring to FIG. 1 and FIG. 2 there is illustrated a section of heat exchange tube 12 illustrating the transfer of heat energy from a hot external fluid to cooler circulating water flowing in the tube for the production of steam. The tube 12 defines a bore 14, an exterior wall surface 16 and an interior wall surface 18. A plurality of radially, outwardly extending ridges 20 (FIG. 2) or thin plates 21 (FIG. 7) are disposed on the exterior wall surface 16 of the tube 12 to increase the surface area of the exterior wall of the tube 12. A heat transfer fluid, such as water, is circulated through the tube 12 for the transfer of heat energy between a fluid contacting the exterior wall surface of the tube and the fluid in the interior of the tube. The tubes 12 may be formed of any suitable heat conducting material such as steel, stainless steel or aluminum depending on the application and may be of any cross sectional configuration, such as circular, hexagonal, square or the like. Thus, in the embodiment described herein, the tubes are formed of heat resistant steel capable of withstanding temperatures on the order of 500° C. or higher. Under less severe conditions, such as those encountered in cooling units such as air conditioners, aluminum is preferred for the heat exchange tubes as it has a higher thermal conductivity than steel.

[0016] In the embodiment described the circulating fluid is water that is heated to form steam. For example, in a typical power plant the water is heated to about 500° C. or higher. The source of heat energy is provided by combustion gases produced from the combustion of a suitable fuel, such as coal, and air pumped into the combustion zone of a furnace. The heated air/combustion gas mixture is brought into contact with the heat exchanger tubes 12 of the heat exchanger where heat energy is conducted from the hot gas to the cooler heat exchange fluid circulating through the tubes. The resulting mixture of combustion gases and air reach temperatures of 550° C. or higher and the circulating water under pressure in the heat exchange tubes 12 will reach temperatures of about 500° C. to form superheated steam.

[0017] The efficiency of heat exchangers as determined by the rate of heat transfer across a wall section can be generally represented by the following:

Q=&lgr;/d·(A1+A2) (t1−t2)

[0018] where:

[0019] Q is the rate of heat transfer;

[0020] A1 is the external surface area of the heat exchange pipe;

[0021] A2 is the internal surface area of the heat exchange pipe;

[0022] t1 is the is the temperature of the entering air/combustion gas mixture;

[0023] t2 is the temperature of the entering water;

[0024] &lgr; is the thermal conductivity of the heat exchange barrier;

[0025] d is the distance of the heat exchange barrier.

[0026] It will be understood that the total rate of heat transfer is also a function of the length of the heat exchange tube and its cross sectional configuration. The present invention is directed to increasing the heat exchange area of the heat exchange tube by modifying its inner wall thereby increasing the heat exchange rate for a heat exchange tube of a given length and cross sectional configuration. In one embodiment the inner wall of the heat exchange tube is modified to increase its rate of heat exchange. In another embodiment, the outer wall surface of the heat exchange tube is also modified to further increase the heat exchange rate for the heat exchange tube.

[0027] In accordance with the invention, a plurality of radially inwardly extending members 22 are disposed on the interior wall surface 18 of the tube 12. As illustrated the members 22 extend inwardly towards the axis of the tube 12 (FIG. 1) and run parallel to the axis of the tube (FIG. 2) for increasing the area of interior wall surface that is contacted by the circulating water. The members 22 define side faces 24 that are spaced apart from the side faces of adjacent members. The amount of spacing between the side faces 24 of the adjacent members 22 is not critical as long as they can be contacted by the circulating fluid for the exchange of heat energy. For maximum efficiency it is preferred to dispose as many of the members 24 on the interior wall surfacel8 as can be conveniently and economically formed during manufacture of the tube 12.

[0028] To increase the surface area of the interior wall surface 18 and thus the heat exchange rate for a heat exchange tube, the inner extending free end of the members 22 disposed in the bore of the tube 12 can be formed to define a shape that provides additional surface area within the heat exchange tube. Referring to FIG. 3, the inner end of the member 22 defines a semi-cylindrical shape 24 having an upper surface 24′ and a lower surface 24″ for exposure to the heat transfer fluid circulating through the heat exchange tube 12 thereby to increase the heat exchange area A of the tube. It will be understood that the configuration of the inner end of the member 22 can take any form such as a channel 25 (FIG. 4) defining a bottom surface 26 and inner and outer side surfaces 27 or a planer form such as a flat plate 28 (FIG. 5) defining an upper and lower face, 29 and 29′ respectively, to provide additional surface area for heat transfer.

[0029] An aspect of the rate of heat transfer is the time required for the conductance of a given quantity of heat energy. In addition to being a function of the thermal conductivity of the thermal conductor it has been found that by gradually reducing the cross section of the member 22 from the hot side to the cooler side increases the rate at which heat energy is conducted. Referring to FIG. 6 there is shown a heat exchange tube 30 provided with plurality of radially, outwardly extending ridges 32 on the outer surface of the tube and a plurality of inwardly extending members 34 on the inner surface of the tube. In this embodiment the cross section of each ridge 32 decreases from the outer end, the hot side in first contact with the hot air/combustion gas mixture to the outer surface of the tube 30. Likewise the cross section of each inwardly extending member 34 decreases from its base on the inner surface of the wall of the tube 30 to the extending end, the cool side in contact with the circulating heat transfer fluid. It has been found that this configuration of the thermal conductor conducts heat energy more quickly along its length than if the cross section of the conductor is smaller on the hot side and larger on the cool side of the heat transfer system.

[0030] The members 22 are most conveniently formed on the interior wall surface 18 during production of the tube 12 by extrusion. However, other manufacturing methods may be employed to form the members 22 on the interior wall surface. Thus, it may be desirable to form a solid extrusion of upwardly extending ridges on a thin flat base that is subsequently formed into a cylinder for insertion into a heat exchange tube.

[0031] It will be apparent from the foregoing description that the circulating fluid may be the heat source and that the transfer of heat energy will thus be from the circulating fluid to the fluid in contact with the exterior wall surface 18 of the tube 12. In this configuration the description of the cross section of the outwardly extending ridges 32 and the inwardly extending members 34 will be reversed since the hot side will be the fluid circulating through the tubes 30. Such an arrangement will be useful for cooling or chilling air or other fluid circulating through the heat exchange tube 30.

[0032] As will be understood by those skilled in the art, various arrangements which lie within the spirit and scope of the invention other than those described in detail in the specification will occur to those persons skilled in the art. It is therefor to be understood that the invention is to be limited only by the claims appended hereto.

Claims

1. A heat exchange tube defining a bore for the circulation of a heat exchange fluid, an outer surface and an inner surface wherein an inner surface of a heat conducting tube is provided with radially, inwardly extending, spaced apart members extending along said inner surface parallel to the longitudinal axis of said heat exchange tube thereby to present an increased surface area to a heat transfer fluid circulating through said tube.

2. The heat exchange tube of claim 1 wherein said inwardly extending members comprise a plurality of spaced apart ridges that run along the inner wall of the tube parallel to the axis of the tube.

3. The heat exchange tube of claim 1 wherein said inwardly extending members comprise a plurality of spaced apart fins that run along the inner wall of the tube parallel to the axis of the tube.

4. The heat exchange tube of claim 1 wherein an outer wall of said heat exchange tube is provided with a series of radially outwardly extending members for increasing the heat exchange surface of the outer wall of the tube.

5. The heat exchange tube of claim 2 wherein said ridges define free ends disposed in the bore of said heat exchange tube, said free ends being adapted to provide additional surface area within said heat exchange tube.

6. The heat exchange tube of claim 5 wherein said free end of a ridge defines a semi-cylindrical shape having an upper surface and a lower surface for exposure to the heat transfer fluid circulating through the heat exchange tube.

7. The heat exchange tube of claim 5 wherein said free end of a ridge defines a channel for exposure to the heat transfer fluid circulating through the heat exchange tube.

8. The heat exchange tube of claim 5 wherein said free end of a ridge is planer and defines an upper face and lower faces for exposure to the heat transfer fluid circulating through the heat exchange tube.

9. A heat exchange tube defining a bore for the circulation of a heat exchange fluid, an outer surface and an inner surface wherein an inner surface of a heat conducting tube is provided with radially, inwardly extending, spaced apart members extending along said inner surface parallel to the longitudinal axis of said heat exchange tube and an outer wall of said heat exchange tube is provided with a series of radially outwardly extending members for increasing the heat exchange surface of the outer wall of the tube, said inwardly extending members having free ends disposed in the bore of said heat exchange tube, said inwardly extending members decreasing in cross section from said inner wall surface to said free end, said outwardly extending members having free ends disposed radially outwardly away from said outer surface of said heat exchange tube, said outwardly extending members decreasing in cross section from said free end to said outer surface of said heat exchange tube.

10. A heat exchanger for the exchange of heat energy between a fluid of a higher temperature to a fluid of a lower temperature comprising at least two heat exchange tubes as defined in claim 1 through which a heat exchange fluid is circulating, an outer surface of said heat exchange tubes being contacted by a second fluid having a different temperature than the heat exchange fluid.

11. The heat exchanger of claim 10 wherein the heat exchange fluid is water and and the second fluid is a mixture of a hot combustion gas and air.

12. The heat exchanger of claim 11 for the production of steam.