WO2005068927A1

WO2005068927A1 - Method of manufacturing heat transfer tube

Info

Publication number: WO2005068927A1
Application number: PCT/US2005/000230
Authority: WO
Inventors: William M. Boyle, Jr.; Gary A. Camp; Larry L. Lafitte
Original assignee: Dow Global Technologies Inc.
Priority date: 2004-01-07
Filing date: 2005-01-05
Publication date: 2005-07-28

Abstract

A heat transfer tube for use in a heat exchanger where heat is transferred between a fluid flowing through the tube and a fluid flowing around the exterior of the tube during the heat exchange process. The tube has a plurality of fins extending around a major portion of its external surface. The tube has a heat conductive insert, such as a metal mixing element, inside the tube with the outside diameter of the cylindrical section the insert occupies essentially equal to the inside diameter of the tube after the finning process.

Description

METHOD OF MANUFACTURING HEAT TRANSFER TUBE

CROSS REFERENCSE STATEMENT This application claims the benefit of U.S. Provisional Application No. 60/534,773, filed January 7, 2004.

BACKGROUND OF THE INVENTION The present invention relates generally to heat transfer tubes. In particular, the invention relates to the internal configuration of a heat exchanger tube that is used to increase heat transfer during laminar flow. Many designs have been disclosed in the literature, including mechanical methods for connecting inserts to tube walls, such as U.S. Patent 2,895,508 (Drake), U.S. Patent2,929,408 (Weatherwax), GB 865,983 (Dingley), GB 1,028,000, U.S. Patent3,394,736 (Pearson), U.S. Patent3, 636,982 (Drake), U.S. Patent3,871,407 (Bykov), U.S. Patent4,190,105 (Dankowski), U.S. Patent4,265,275 (Heller), U.S. Patent4,296,539 (Asami), U.S. Patent4,724,899 (Frates), U.S. Patent4,865,689 (Hon), U.S. Patent6,508,983 (McBurney), U.S. Patent6,533,030 (Mitrovic) and WO 02/26370. Brazing methods include those described in U.S. Patent 4,466,567 (Garrison), U.S. Patent 4,688,311 (Saperstein), U.S. Patent 6,206,089 (Uchikawa), and U.S. Patent 6,470,570 (Prater). Mechanical methods for tube wall forming include U.S. Patent 5,781,996 (Spencer), U.S. Patent 5,803,165 (Shikazono) and EP 0 865 838 (Gupte). Examples of heat exchanger tube inserts include U.S. Patent 4,534,409 (Cadars), U.S. Patent 4,700,749 (Cadars) and U.S. Patent 3,800,985. Other systems include those described in U.S. Patent 6,192,583 (Roffelsen), U.S. Patent 6,467,949 (Reeder et al.), U.S. Patent 5,597,236 (Fasano), U.S. Patent 3,775,063 (Grout et al.), U.S. Patent 3,800,985 (Grout et al.), and U.S. Patent 3,806,097 (Devellian et al.). All of the patents in this paragraph are incorporated herein by reference in their entirety. Many types of internal inserts are used to enhance heat transfer rates in shell and tube heat exchangers. The internal inserts are used to modify the flow characteristics of the fluid to enhance the heat transfer rate. Heat transfer is dependant upon the surface area of the tube where the transfer of heat takes place. In laminar flow fluid tends to move slowly at the wall creating a layer that limits the rate of transfer of heat to that approaching a pure conduction mechanism which is a lower rate than convective heat transfer. It is desirable to periodically break up any tendency of the fluid to form a significantly thick layer close to the wall. When a gap exists between the tube wall and the insert (in the region which is designed to modify the flow to prevent the development of a slow moving layer), some of the slow moving or stagnant material is not removed from the wall, thus limiting the transfer of heat. In addition, it is known that such a gap, which is filled with the fluid, prevents the conduction of heat directly from the metal tube wall to the metal insert. The heat transfer rate through the metal tube wall in contact with the surface of the metal insert is much greater than if the heat must pass through fluids. The direct contact of the metal insert and tube wall increases the heat transfer surface area exposed to the moving fluid, thus increasing further the rate of heat transfer. This is effective in both cooling or heating service. To prevent gaps between the tube wall and the insert, methods of brazing the insert to the tube wall have been used for a number of years. This is an expensive method and can cause undesirable changes in the structure of the metal tube which may lead to leaking under some circumstances. If a tube develops a leak it must be plugged, further reducing the total heat exchanger area available for heat transfer. To avoid these problems, a method of manufacturing a tube with an insert with essentially no gap between the tube wall and the desired contact points of the insert has been invented. SUMMARY OF THE INVENTION The present invention is a heat transfer tube with essentially zero clearance between the desired contact points of the insert and the tube wall. During manufacture of the tube it is substantially filled with insert sections. The diameter of the tube and size of the inserts is selected such that the effective diameter of the inserts is only slightly less than the diameter of the tube. Effective diameter refers to the diameter of a cylinder that will contain the insert with contact at multiple points between the insert and the cylinder. The difference in diameter is necessary to enable the inserts to be introduced into the tube easily in a reasonable amount of time and the desired packing density. The tube loaded with inserts is then placed in forming equipment designed to make a fin on the outside of the tube. In the process of forming the fin, the inside diameter of the tube is reduced to substantially match the outside effective diameter of the inserts thus causing direct contact between the tube and inserts. The process of finning the tube to increase the exterior surface area has been used for a number of years and the changes in the properties of the metal are well known. Therefore, standards used for the design of exchangers using finned tubes have been developed by industry organizations. Furthermore, the finning process can be controlled adequately to allow for the finning of the tube where needed, as opposed to methods used to form the tube wall and insert contact through a die. This allows for the ability to fin or not fin the tube at appropriate locations to allow spaces for passage through baffles in the heat exchanger. In one aspect, the present invention is a heat exchanger tube comprising in combination: A) an outer tubular member comprising at least one metal, wherein the tubular member has an inner cylindrical surface and an outer cylindrical surface, B) at least one mixing element disposed within said tubular member, said mixing element comprising at least one heat conductive material, C) a plurality of fins defined on the outer cylindrical surface, preferably concentrically arranged on said outer surface, more preferably comprising at least 80 percent of the outer surface length, preferably having a fin concentration of from 10 fins/inch to 40 fins/inch of such length, and D) said mixing element substantially being in physical contact with said inner cylindrical surface, with said inner cylindrical surface having an interference fit with inner mixing element within said tubular member providing intimate heat-conductive engagement between said mixing element and said tubular member at a plurality of points. Preferably, the mixing element in the tube has a hardness greater than or about the same as the hardness of the tube. Perferably, a plurality of mixing elements comprising metal are used and have a heat transfer coefficient at least that of the tube. A shell and tube heat exchanger is also within the purview of the invention. In another embodiment, the invention is a method of making a heat transfer tube which comprises the steps of: A) fabricating or obtaining an elongate tubular member having an outer surface and an inner surface, each surface being substantially free of surface irregularities, B) fabricating or obtaining a mixing element capable of fluid flow control, especially for a viscous fluid, C) inserting said mixing element into said tubular member such that said mixing element extends within the tubular member for at least a portion of the longitudinal length of said member, and D) finning the outer surface of said tubular member in the direction of the longitudinal axis of said tubular member such that a pressure-fitting relationship is formed between said mixing element and the inner surface of said tubular member, wherein step D) is sequential to step C). Preferably, the heat transfer tube is formed in the absence of flux or brazing. The mixing element in this and other embodiments may comprise a plurality of discrete inserts. In yet another embodiment, the invention is a method of exchanging heat in a system having an outer fluid and an inner fluid, comprising the step of utilizing the tube of the invention. Preferably, the outer fluid comprises a vapor or liquid selected from the group consisting of water, hydrocarbons oils, (poly)glycol, diphenyl-ethane, alkylated aromatics, heat transfer fluids in general, mixtures thereof, and non-reactive gases. The inner fluid can also comprise a hydrocarbon solvent and, optionally a dissolved polymer, especially wherein the inner fluid comprises from 10 to 50 percent, more preferably from 10 to 35 percent (by weight of the total inner fluid) of at least one dissolved polymer. Desirably, the polymer is selected from the group consisting of alpha-olefin homopolymers and copolymers, ethylene/alpha-olefin interpolymers, ethylene homopolymers, propylene homopolymers, and propylene/alpha-olefin inter-polymers. More preferably the polymer is selected from linear low density polyethylene (LLDPE), (metallocene linear low density polyethylene (mLLDPE), high density polyethylene (HDPE), ethylene/propylene diene polymers (EPDM), ethylene/styrene interpolymers (ESI), polystyrene (PS) or polypropylene (PP). The inner fluid and/or the outer fluid can also comprise a food product. The inner fluid preferably has a viscosity of at least 0.2 centipoise and as high as 1,500,000 centipoise, more preferably from 0.2 centipoise to 1,200,000 centipoise, most preferably from 0.2 centipoise to 10,000 centipoise. Viscosity can be measured using any convenient device, typically including Engler, Saybolt, Redwood, Brookfield, and Krebs-Stormer viscometers. In still another embodiment, the invention is a method for making a heat transfer tube having an inner tube wall and an outer surface comprising: A) a first step of fabricating or obtaining an elongate tubular member having an outer surface and an inner surface and a specific length, each surface initially being free of surface irregularities, B) a second step of fabricating or obtaining a mixing element capable of fluid flow control, C) a third step of inserting said mixing element into said tubular member such that said mixing element extends within the tubular member for at least a portion of the longitudinal length of said member, D) A fourth step of forming a plurality of adjacent, radially outwardly extending fins, preferably helically shaped, in the outer surface by the interaction of a finning disk with the outer surface of the tube, whereby circumferential grooves are formed between adjacent fins; such that a pressure-fitting relationship is formed between said mixing element and the inner surface of said tubular member at a plurality of points, wherein step D) is sequential to step C). A preferred method has the heat transfer tube formed in the absence of flux or brazing. In still another embodiment, the invention is a heat exchanger tube comprising in combination: A) an outer tubular member comprising metal, the member having an inner cylindrical surface and an outer cylindrical surface and a specific length, B) an internal mixing element disposed within said tubular member for at least a portion of the length, C) a plurality of fins defined on the outer cylindrical surface, and D) said inner cylindrical surface having an interference fit within said tubular member providing an intimate heat-conductive engagement between said mixing element and said tubular member at a plurality of contact points. The fins are preferably concentrically or helically arranged on said outer surface and also preferably comprise at least 80 percent of the length, but preferably less than about 99 percent of said tube length. More preferably, the fins are present in an amount of from 10 fins/inch to 40 fins/inch of the length. The mixing element disposed within the tube is preferably in contact with at least 50 percent of the tube length and advantageously comprises contact at a plurality of points up to 100 percent of the length of the tube.

Suitable mixing devices include that described and claimed in U.S. Patent 3,286,992, the disclosure of which is incorporated herein by reference in its entirety. By virtue of the features of the invention, a heat exchanger tube is obtained which can be manufactured in a relatively simple and inexpensive manner.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a first embodiment in cross longitudinal section during the process of finning the outer tube surface.

DETAILED DESCRIPTION FIG. 1 shows, in cross longitudinal section, a heat exchanger tube (1) consisting of a mixing element (2) provided therein. The heat exchanger tube (1) has a wall thickness under fin (3), an outer diameter (4), a plain end length (5). An overall length (6) and a defined fins/inch concentration (7). The fins (8) are located on the outer surface of the tube. A heat exchanger tube according to FIG. 1 can be manufactured by sliding the inner mixing element (2) into the outer tube (1). After that, the assembled tubes are deformed in a finning or drawing process through cold deformation so that the inner tube wall of the tube is compressed. Because during the drawing process, the outer diameter (4) of the tube (1 ) is reduced, the assembly is deformed to become a heat exchanger tube containing an integral mixing element in contact with the inner tube wall and the inner tube serving as a one-piece conduit. It is readily understood that within the framework of the invention as laid down in the appended claims still many other modifications and variants are possible. The present invention is a heat transfer tube having one or more fin convolutions formed on its external surface. Notches in the fin may further increase the outer surface area of the tube as compared to a conventional finned tube and the fins need not be continuous. Manufacture of a notched fin tube can be easily and economically be accomplished by adding an additional notching disk to the tool gang of a finning machine of the type that forms fins on the outer surface of a tube by rolling the tube wall between an internal mandrel and external finning disks. Tubes according to the present invention will have nominal outer diameters of from 1/2 inch to 12 inches, preferably from 3/4 inch to 2 inches. The metal used in the tube and the insert can be any metal capable of being machined, and includes tin, lead, gold, silver, zinc, copper, nickel, iron, magnesium, aluminum and alloys of each of these. Various steel alloys are also desirable and include various stainless steel grades. Typical alloys for steel include iron with boron, chromium, nickel, tungsten, molybdenum, manganese, vanadium, cobalt and zirconium, or combinations of these. Typically, there are four groups of stainless steels: (1) austenitic, which contain both chromium (16 percent minimum) and nickel (7 percent minimum) (a stress-corrosion resistant type contains about 2 percent silicon); (2) ferritic, which contains chromium only and cannot be hardened by heat treatment; (3) martensitic, which contain chromium and can be hardened by heat treatment; and (4) duplex, which is a mixture of austenitic and ferritic. For heat transfer from a fluid containing a polymer, the polymer is preferably selected from the group consisting of linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE), (metallocene linear low density polyethylene (mLLDPE) such as that sold by The Dow Chemical Company as AFFINITY* or that sold by ExxonMobil as Exact**, high density polyethylene (HDPE), ethylene/propylene diene polymers (EPDM) such as NORDEL IP sold by DuPont Dow Elastomers, ethylene/styrene interpolymers (ESI), polystyrene (PS), polypropylene (PP) and propylene/alpha-olefin (for example, ethylene) interpolymers, alpha-olefin homopolymers and copolymers, ethylene/alpha-olefin interpolymers, especially ethylene/C3-C20 alpha-olefin interpolymers, and ethylene homopolymers. There are a variety of ways to make these types of polymers, and include slurry, solution and gas phase polymerizations, especially preferred is the solution process. Various patents disclose polymerization techniques, including U.S. Patent 4,076,698 (Andersen et al.), U.S. Patent 5,977,251 (Kao et al.) and WO 97/36942 (Kao et al.), the disclosures of each of which is incorporated herein by reference in their entirety. Use of the heat exchanger tubes of the present invention are particularly preferred in the heat exchanger apparatus and process of U.S. Patent 5,977,251. Polyethylenes present in fluids subject to heat transfer in accordance with this invention fall into two broad categories, those prepared with a free radical initiator at high temperature and high pressure, and those prepared with a coordination catalyst at high temperature and relatively low pressure. The former are generally known as LDPE and are characterized by branched chains of polymerized monomer units pendant from the polymer backbone. LDPE polymers generally have a density between 0.910 and 0.940 g/cm³. Polymer density is measured according to the procedure of ASTM D-792 herein unless otherwise noted. Ethylene polymers and copolymers prepared by the use of a coordination catalyst, such as a Ziegler Natta or Phillips catalyst, are generally known as linear polymers because of the substantial absence of branch chains of polymerized monomer units pendant from the backbone. Linear copolymers of ethylene and at least one α-olefin of 3 to 12 carbon atoms, preferably of 4 to 8 carbon atoms, are also well known and commercially available. As is well known in the art, the density of a linear ethylene/α-olefin copolymer is a function of both the length of the α-olefin and the amount of such monomer in the copolymer relative to the amount of ethylene, the greater the length of the α-olefin and the greater the amount of α-olefin present, the lower the density of the copolymer. LLDPE is typically a copolymer of ethylene and an α-olefin of 3 to 20 carbon atoms, preferably 4 to 8 carbon atoms (for example, 1-butene, 1-octene, etc.), that has sufficient α-olefin content to reduce the density of the copolymer to that of LDPE (for example, 0.910 g/cm³ to 0.940 g/cm³). When the copolymer contains even more α-olefin, the density will drop below about 0.91 g/cm and these copolymers are known interchangeably as ultra low density polyethylene (ULDPE) or very low density polyethylene (VLDPE). The densities of

VLDPE or ULDPE polymers generally range from 0.87 to 0.91 g cm³. Both LLDPE and VLDPE or ULDPE are well known in the art, as are their processes of preparation. For example, heterogeneous LLDPE can be made using Ziegler-Natta catalysts in a slurry, gas phase, solution or high pressure process, such as described in U.S. Pat. 4,076,698 while homogeneous linear ethylene polymers can be made as described in U.S. Pat. 3,645,992. Such linear ethylene polymers are available from, for example, The Dow Chemical Company as DOWLEX™ LLDPE and as ATTANE™ ULDPE resins. mLLDPE, or metallocene catalyzed polyethylene, such as AFFINITY* made by The Dow Chemical Company, can be made in accordance with U.S. Patent 5,272,236 and U.S. Patent 5,278,272. All U.S. patent references in this paragraph are incorporated herein by reference. High density polyethylene (HDPE), generally having a density of 0.941 to 0.965 g/cm³, is typically a homopolymer of ethylene, and it contains few branch chains relative to the various linear copolymers of ethylene and an α-olefin. HDPE is well known, commercially available in various grades, and may be used in this invention. The polypropylene copolymers useful in fluids subject to the heat transfer of this invention are polymers comprising units derived from propylene optionally with ethylene and/or one or more unsaturated comonomers. The term "copolymer" includes terpolymers, tetrapolymers, etc. "Random copolymer" means a copolymer in which the monomer is randomly distributed across the polymer chain. Typically, the polypropylene copolymers comprise units derived from propylene in an amount of at least 60, preferably at least 70 and more preferably at least 80, wt percent of the copolymer. Ethylene and/or the one or more unsaturated comonomers of the copolymer comprise at least 0.1, preferably at least 1 and more preferably at least 3, weight percent, and the typical maximum amount of unsaturated comonomer does not exceed 40, and preferably it does not exceed 30, wt percent of the copolymer. Such random copolymers of polypropylene are commercially available, for example, DOW PolyPropylene RESiNS™ available from The Dow Chemical Company.

Claims

CLAIMS: 1. A heat exchanger tube comprising in combination:

A) an outer tubular member comprising at least one metal, wherein the tubular member has an inner cylindrical surface and an outer cylindrical surface, B) a mixing element disposed within said tubular member, said mixing element comprising at least one heat conducting material,

C) a plurality of fins defined on the outer cylindrical surface, and

D) said mixing element substantially being in contact with said inner surface having an interference fit within said tubular member providing intimate heat-conductive engagement between said mixing element and said inner surface of said tubular member at a plurality of contact points.

2. The tube of claim 1 wherein said fins are concentrically arranged on said outer surface.

3. The tube of claim 1 wherein the fins comprise at least 80 percent of the outer surface length.

4. The tube of claim 1 wherein the outer surface has a fin density of at least 10 fins/inch to 40 fins/inch of the length where fins are present.

5. The tube of claim 1 wherein at least 50 percent of the length of the inner cylindrical surface is in contact with the mixing element at a plurality of points.

6. A shell and tube heat exchanger comprising the tube of claim 1.

7. The tube of claim 1 wherein the mixing element has a hardness greater than or the same as the hardness of the tube.

8. A method of making a heat transfer tube which comprises the steps of: A) fabricating an elongate tubular member having an outer surface and an inner surface, each surface being free of surface irregularities,

B) fabricating a mixing element capable of fluid flow control,

C) inserting said mixing element into said tubular member such that said mixing element extends within the tubular member for at least a portion of the longitudinal length of said member, and

D) finning the outer surface of said tubular member in the direction of the longitudinal axis of said tubular member such that a pressure-fitting relationship is formed between said mixing element and the inner surface of said tubular member at a plurality of contact points, wherein step D) is sequential to step C).

9. The method of claim 8 wherein the heat transfer tube is formed in the absence of flux or brazing.

10. A method of exchanging heat in a system having an outer fluid and an inner fluid, comprising the step of utilizing the tube of claim 1.

11. The method of claim 10 wherein the outer fluid comprises a vapor or liquid selected from the group consisting of water, hydrocarbons oils, (poly)glycol, diphenyl-ethane, alkylated aromatics, heat transfer fluids in general, mixtures thereof, and non-reactive gases.

12. The method of claim 10 wherein the inner fluid comprises a hydrocarbon solvent and, optionally a dissolved polymer.

13. The method of claim 10 wherein the inner fluid comprises from 10 to 50 percent (by weight of the total inner fluid) of at least one dissolved polymer.

14. The method of claim 10 wherein the inner fluid comprises from 10 to 35 percent (by weight of the total inner fluid) of at least one dissolved polymer.

15. The method of claim 13 wherein the polymer is selected from the group consisting of LLDPE, mLLDPE, HDPE, EPDM, ESI, PP, alpha-olefin homopolymers and copolymers, ethylene/alpha-olefin interpolymers, ethylene homopolymers, propylene homopolymers, polystyrene, and propylene/alpha-olefin interpolymers.

16. The method of claim 10 wherein the inner fluid and/or the outer fluid comprise a food product.

17. The method of claim 10 wherein the inner fluid has a viscosity of at least 0.2 centipoise and as high as 1,500,000 centipoise.

18. The method of claim 10 wherein the inner fluid comprises at least one polymer.

19. The method of claim 10 wherein the inner fluid consists essentially of at least one polymer.

20. A method for making a heat transfer tube having a tube wall and an outer surface comprising:

A) a first step of fabricating an elongate tubular member having an outer surface and an inner surface and a specific length, each surface initially being free of surface irregularities,

B) a second step of fabricating a mixing element capable of fluid flow control,

C) a third step of inserting said mixing element into said tubular member such that said mixing element extends within the tubular member for at least a portion of the longitudinal length of said member, and

D) a fourth step of forming a plurality of adjacent, radially outwardly extending fins in the outer surface by the interaction of a finning disk with the outer surface of the tube, whereby circumferential grooves are formed between adjacent fins; such that a pressure-fitting relationship is formed between said mixing element and the inner surface of said tubular member, wherein step D) is sequential to step C).

21. A method for making a heat transfer tube having a tube wall and an outer surface comprising:

A) a first step of obtaining an elongate tubular member having an outer surface and an inner surface and a specific length, each surface initially being free of surface irregularities,

B) a second step of obtaining a mixing element capable of fluid flow control,

C) a third step of inserting said mixing element into said tubular member such that said mixing element extends within the tubular member for at least a portion of the longitudinal length of said member, and D) a fourth step of forming a plurality of adjacent, radially outwardly extending fins in the outer surface by the interaction of a finning disk with the outer surface of the tube, whereby circumferential grooves are formed between adjacent fins; such that a pressure-fitting relationship is formed between said mixing element and the inner surface of said tubular member, wherein step D) is sequential to step C).

22. The method of claim 20 wherein the mixing element comprises a plurality of discrete mixing inserts.

23. The method of claim 20 wherein the heat transfer tube is formed in the absence of flux or brazing.

24. A heat exchanger tube comprising in combination:

A) an outer tubular member of metal having an inner cylindrical surface and an outer cylindrical surface and a specific length,

B) an internal mixing element disposed within said tubular member for at least a portion of the length, C) a plurality of fins defined on the outer cylindrical surface, and D) said inner surface having an interference fit within said tubular member providing an intimate heat-conductive engagement between said mixing element and said tubular member.

25. The tube of claim 24, wherein the fins are concentrically arranged on said outer surface.

26. The tube of claim 24, wherein said fins are present on the outer cylindrical surface comprising at least 80 percent of the length.

27. The tube of claim 24, wherein said fins are present on the outer cylindrical surface comprising less than 99 percent of said tube length.

28. The tube of claim 24, wherein said fins where present are present in the range from 10 fins/inch to 40 fins/inch.