Patent Publication Number: US-2009218075-A1

Title: Coiled Heat Exchanger

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This application claims the priority of International Application No. PCT/EP2006/010651, filed Nov. 7, 2006, and European Patent Document No. EP 05025682.5, filed Nov. 24, 2005, the disclosures of which are expressly incorporated by reference herein. 
    
    
     The invention relates to a coiled heat exchanger having a plurality of tubes, which are wound around a core tube, having a shell which delimits an outer space around the tubes. 
     Natural gas is continuously liquefied in large quantities in LNG baseload systems. Most of the time, liquefaction of the natural gas is accomplished by heat exchange with a coolant in coiled heat exchangers. However, other applications of coiled heat exchangers are also known, both at temperatures below the ambient temperature all the way to the lowest temperatures as well as at high temperatures all the way to far above the ambient temperature. Basically, coiled heat exchangers can be used at every temperature for which its material is suited. 
     In a coiled heat exchanger, several layers of tubes are spirally wound on a core tube. A medium is piped through the individual tubes, and this medium exchanges heat with a medium flowing in the space between the tubes and a surrounding shell. The tubes are merged into one or more groups on the upper and lower end of the heat exchanger. 
     These types of coiled heat exchangers and their application, for example for liquefaction of natural gas, are described in each of the following publications:
     Hausen/Linde, Cryogenic Engineering, 2nd ed., 1985, pages 471-475;   W. Scholz, “Coiled Tube Heat Exchangers,” Linde Reports on Science and Technology, No. 33 (1973), pages 34-39;   W. Bach, “Offshore Natural Gas Liquefaction with Nitrogen Cold—Process Design and Comparison of Coiled Tube and Plate Heat Exchangers,” Linde Reports on Science and Technology, No. 64 (1990), pages 31-37;   W. Förg et al., “A New LNG Baseload Process and Manufacturing of the Main Heat Exchanger,” Linde Reports on Science and Technology, No. 78 (1999), pages 3-11 (English version: W. Förg et al., “A New LNG Baseload Process and Manufacturing of the Main Heat Exchanger,” Linde Reports on Science and Technology, No. 61 (1999), pages 3-11);   DE 1501519 A;   DE 1912341 A;   DE 19517114 A;   DE 19707475 A; and   DE 19848280 A.   

     Coiled heat exchangers are known for their great internal elasticity. Because of the tubes that are formed as spiral springs, they are as such very elastic, and especially suited for neutralizing stress induced by thermal expansions or contractions and therefore permit a particularly high level of operating safety. 
     The invention is therefore based on the objective of further increasing the reliability of a coiled heat exchanger. 
     This objective is attained in that at least one elastic component that does not transfer heat is used in the coiled heat exchanger, which component comprises a metallic material and is arranged inside the shell and radially outside the bundle. 
     Until now it was assumed that the intrinsic elasticity of the coiled tubes, if applicable in connection with an elastic bearing of the tubes on their ends, imparted a coiled heat exchanger with such an excellent ability to neutralize thermal stress that additional measures in this direction were unnecessary. However, it emerged within the scope of the invention that even in the case of coiled heat exchangers thermal stress can reduce reliability during operation. The invention does not target the conventional parameters, e.g., an increase in the intrinsic elasticity of the tubes or a change in the bearing of the tubes, but attains the above-mentioned objective by using a component that does not transfer heat radially outside the bundle. This makes it possible not just to neutralize stress, which occurs because of changes in temperature, but to compensate for greater stationary temperature differences and temperature gradients of the entire bundle vis-à-vis the outer or inner space. 
     “Does not transfer heat” should be understood in this case as a component whose surfaces do not come into direct contact during operation with the two fluids, between which the intended heat transfer is taking place. Of course, every component has a heat transfer effect through thermal conduction. This is not ruled out in the case of the term “does not transfer heat.” 
     The “bundle” in this case is understood as a geometric space in the form of a hollow cylinder, which is formed by the cylindrically symmetric section of the tube winding. It includes the helically coiled sections of the tubes and extends in the interior up to the cylinder shell surface, which touches the inside of the tubes of the innermost layer of tubes (without including the core tube itself), and outwardly up to a cylinder shell surface, which touches the outside of the tubes of the outermost layer of tubes. If a shroud is arranged between the shell and the outermost layer of tubes, the “bundle” no longer includes this shroud. 
     A component is arranged “radially outside the bundle” if it is at least partially situated radially outside the hollow cylinder formed by the bundle, i.e., in one of the spaces between the outermost layer of tubes and the shell or between the innermost layer of tubes and core tube, or in the interior of the core tube. Components, which are arranged exclusively above or below the axial ends of the bundle, i.e., for example those that serve to position the ends of the tube, are not arranged “radially outside the bundle.” The elastic component in terms of the invention can be arranged for example between two concentric tube bundles (see German Patent Application 102006033697 and the associated applications), between the outermost layer of tubes and the shroud or between the innermost layer of tubes and the core tube. 
     A component is designated as “elastic” in this case if its spring stiffness (spring constant) is less than that of the heat transferring components, particularly the tubes and the tube bundle. The spring constant of the “the elastic component(s) that does/do not transfer heat” is in particular less than 80%, preferably less than 50%, less than 10% or less than 1% of the tube bundle. Its arrangement and elasticity is embodied in such a way that, during operation of the heat exchanger, the thermal stress in the tubes caused by the temperature expansion remains below the yield point of the tubes, in particular below two thirds of the yield point, preferably below 5% of the yield point of the tubes. 
     The elastic component that does not transfer heat has a metallic material, i.e., it is formed at least partially of one or more metallic materials. 
     A plurality of elastic components that do not transfer heat are preferably used in the invention radially outside the bundle. 
     The elastic component(s) that does/do not transfer heat are preferably connected positively, non-positively or in a sliding manner to a heat transferring component, in particular to at least one of the tubes. The connection can be established directly, e.g., by welding or soldering, or even via one or more intermediate pieces, which have lower elasticity. For example the elastic component can be coupled via one or more connecting pieces to one or more tubes. Alternatively, the elastic component that does not transfer heat can also be a portion of a heat transferring component or be a formed-on part thereof. 
     A “connecting piece” represents an inelastic component and is in direct contact with at least one tube at at least two contact points, which as a rule are arranged at adjacent coils of the same tube. The connection between the connecting piece and the tube is positive at these points and is formed in particular by suitable depressions in the connecting piece, which have approximately the shape of a cylinder shell segment for example. 
     Alternatively, the elastic component(s) that does/do not transfer heat can preferably in addition be connected positively, non-positively or in a sliding manner to a component that does not transfer heat. For example, the resilient elements can be connected non-positively on one side to one or more tubes and on the other side to the shell. This makes it possible, for example, to neutralize the thermal changes in length of the tube bundle, on the one hand, and the shell, on the other. 
     However, a positive or non-positive connection on the one side combined with a sliding connection on the other side is also possible. 
     There is normally a larger annular gap between the outer container shell and the outermost layer of tubes. If a portion of the fluid flowing through the outer space were to flow through this gap instead of through the intermediate spaces between the tubes, this portion would not participate at all or only to a small degree in the heat exchange with the other fluid, which is flowing through the interior of the tubes. As a result, it is common to arrange a shroud between the outermost layer of tubes and the shell. In this case, it is beneficial if the elastic component that does not transfer heat is connected non-positively or in a sliding manner to the shroud. 
     In particular, if the elastic component that does not transfer heat is connected on the other side to one or more tubes, a neutralization of the thermal changes in the diameter of the tubes, on the one hand, and the shroud, on the other, is achieved and thus a reduction in the thermally induced stress. 
     The invention can be used advantageously especially when the heat transferring components, particularly the tubes, have a coefficient of thermal expansion, which is greater than 8·10 −6  1/K, in particular greater than 16.1·10 −6  1/K, in particular greater than 20·10 −6  1/K. The tube material can be comprised for example of stainless steel, particularly V2A, or of aluminum or an aluminum alloy. 
     It is favorable if the elastic component has a resilience in at least two opposing directions. It is elastically embodied for both tensile as well as compressive stress. The elastic component preferably has resilience in all directions. 
     It is advantageous if at least two, in particular at least four, elastic components that do not transfer heat are arranged along the axis of the bundle. As a result, an elastic coupling is possible over the entire height or a large portion of the axial extension of the bundle. The locations of the elastic components with a different axial arrangement can have the same or different location of components in the tangential direction (circumferential direction), i.e., can be arranged directly next to one another or offset from one another in terms of their axial sequence. 
     In addition or as an alternative it is favorable if at least two, in particular at least four, elastic components that do not transfer heat are arranged along the circumference of the bundle. This makes an elastic coupling possible over the entire circumference or a large portion of the circumference of the bundle. The locations of the elastic components with different tangential location components can have the same or different location of components in the axial direction (height), i.e., can be arranged at the same or different heights on the circumference. 
     A plurality of elastic components that do not transfer heat are preferably distributed over an entire cylinder shell surface outside or inside the bundle&#39;s hollow cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The invention as well as addition details of the invention are explained in greater detail in the following on the basis of the exemplary embodiments depicted in the drawings, which each schematically represent a section of an inventive coiled heat exchanger. The drawings show: 
         FIG. 1  is a first exemplary embodiment of an elastic component that does not transfer heat within the scope of the invention; and 
         FIG. 2  is a second exemplary embodiment for the use of the invention in a coiled heat exchanger. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiment in  FIG. 1  depicts two elastic components  1   a ,  1   b  that do not transfer heat in terms of the invention. They can, as depicted, be embodied as spiral springs, but can also assume any other shape that causes its spring constant to be less than that of the heat transferring components (not shown in  FIG. 1 ), in particular less than 80%, preferably less than 50%, less than 10% or less than 1% of the tube bundle of the coiled heat exchanger. 
     The elastic components  1   a ,  1   b  that do not transfer heat are connected non-positively on one side (bottom in  FIG. 1 ) via a first intermediate piece  2  to heat transferring components such as tubes (not shown), e.g., by a joined connection. 
     On the other side (top in  FIG. 1 ), the elastic components  1   a ,  1   b  that do not transfer heat are connected non-positively via a second immediate piece  3  to a component that does not transfer heat such as a shroud, which is arranged between the outermost layer of tubes and the shell of the coiled heat exchanger. 
       FIG. 2  shows a similar exemplary embodiment, which like  FIG. 1  has two elastic components  1   a ,  1   b  that do not transfer heat and a first and a second intermediate piece  2 ,  3 . The first intermediate piece  2  is connected non-positively to the tubes of the outermost layer of tubes  4 , e.g., by a welded joint. The second intermediate piece  3  is connected non-positively to a shroud  5 , e.g., by a welded joint. 
     The design of the components  1   a ,  1   b ,  2 ,  3  is repeated at additional locations (for example  6 ), preferably in regular intervals. 
     In addition,  FIG. 2  shows a further tube layer  7  as well as inelastic connecting pieces  8 , which are arranged between the two layers of tubes  4  and  7 . 
     In principle, the fundamental idea of the invention, the use of an elastic component that does not transfer heat to diminish thermally induced stress, can also be used in all other types of heat exchangers, e.g., U-tube heat exchangers, straight tube heat exchangers or plate heat exchangers.