C:\NRPonbl\DCCWAMX'24367_ .DOC-163/211 -1 COILED HEAT EXCHANGER HAVING DIFFERENT TUBE DIAMETERS Description 5 The invention relates to a coiled heat exchanger having a plurality of tubes which are wound around a core tube. In LNG baseload plants, natural gas is continuously liquefied in large quantities. The liquefaction of natural 10 gas is mainly accomplished by heat exchange with a coolant in coiled heat exchangers. Many other applications of coiled heat exchangers, however, are also known. In a coiled heat exchanger, many layers of tubes are coiled 15 around a core tube in a screw-like manner. A first medium is introduced into the interior of at least a part of the tubes, which exchanges heat with a second medium that flows between the tubes and a surrounding casing. The tubes are collected into several groups at the end of the heat exchanger, and 20 are led out of the outer space in a bundle. Coiled heat exchangers of this type and their application, for example for the liquefaction of natural gas, are described in each of the following publications: 25 - Hausen/Linde, Low Temperature Engineering, 2nd Edition, 1985, pp. 471-475 - W. Scholz, "Gewickelte Rohrw&rmetduscher [Coiled Tube Heat Exchangers]", Linde-Berichte aus Technik und Wissenschaft, [Linde Reports on Science and Technology), No. 33 (1973), 30 pp. 34-39 - W. Bach, "Offshore-Erdgasverflhssigung mit Stickstoffkdlte - Prozessauslegung und Vergleich von Gewickelten Rohr- und C:NRPonblDCC\WAM\3524367_1 DOC-16A)3/2011 -2 Plattenwdrmetiuschern [Offshore Natural Gas Liquefaction with Nitrogen Coolant - Process Design and Comparison of Coiled Tube and Plate Heat Exchangers", Linde-Berichte aus Technik und Wissenschaft, [Linde Reports on Science and 5 Technology, No. 64 (1990), pp. 31-37 - W. F6rg et al., "Ein neuer LNG Baseload Prozess und die Herstellung der Hauptwdrmetduscher", Linde-Berichte aus Technik und Wissenschaft No. 78 (1999), pp3-11 (English edition: W. F6rg et al., "A New LNG Baseload Process and 10 the Manufacturing of the Main Heat Exchanger", Linde Reports on Science and Technology, No. 61 (1999), pp. 3-11) - DE 1501519 A - DE 1912341 A 15 - DE 19517114 A - DE 19707475 A - DE 19848280 A Tubes with uniform cross section are used in the known 20 coiled heat exchangers. One or more embodiments of the invention may further optimize coiled heat exchangers of this type, especially with regard to weight, number of tubes, process conditions, 25 and/or operational reliability. The present invention provides a heat exchanger having a plurality of tubes which are wound around a core tube and having a casing that delimits an outer space for a medium to 30 flow therethrough around the tubes, wherein the tubes of a first tube group have a first inner diameter and a first outer diameter, with a resulting first wall thickness, and C-\NRPorth\DCC\WAM\1524367_1.DOC-16AI3/2011 -3 the tubes of a second tube group have a second inner diameter and a second outer diameter, with a resulting second wall thickness, wherein the second inner diameter is different from the first inner diameter, and/or the second 5 outer diameter is different from the first outer diameter, and wherein the second wall thickness is different from the first wall thickness. A "tube group" consists of at least one, preferably a plurality of tubes. The tubes in a tube group can, but need not, be adjacent to one another in a 10 tangential and/or radial direction. Both tube groups are preferably located in the same tube bundle. A "tube bundle" describes the entirety of an internal component of a coiled heat exchanger, comprising core tube, tube layers wound around it, and any accessory items located between them, 15 such as supports, etc., which is manufactured in a coiling process. A coiled heat exchanger has one or several tube bundles of this type within a casing. In this manner, the tube geometry can be better adapted to 20 the specific technical process requirements. Such specific requirements may, for example, consist of different thermal properties of different process fractions that flow through the associated tube groups, or in the different length of tubes in the different tube layers. A further advantage 25 consists in that the wall thicknesses can be adapted to different process pressures of the media flowing through the tubes, and thus weight can be saved. Within the scope of the invention, the following 30 combinations of geometric parameters for the tubes in the two tube groups are possible: C XNRPonbnDCC WA'AM\'524I67_ II)OC- 61U2,1i -4 Outer diameter Inner diameter Wall thickness Same Different Different Different Same Different Different Different Different "Different" here is understood to mean a deviation in the associated dimension that is significantly larger than the applicable manufacturing tolerance. One parameter is 5 considered especially to be different from another if its value differs by at least 2%, preferably at least 5%. Within the scope of the invention, the inner diameter especially can be varied, preferably while the outer 10 diameter remains the same. By varying the inner diameter, for example, the pressure drop along the tubes can be influenced. Two different tube groups can thus be optimized independently of one another 15 for two different process fractions. This can be done, fundamentally, with the same wall thickness; that is, both tube groups also have different outer diameters. Alternatively, all tubes can have the same outer diameter; then only the wall thickness and inner diameter vary. 20 Within the scope of the invention, it is therefore useful in many cases to provide the same outer diameter for the two tube groups, and to achieve the inner diameter by using different wall thicknesses. Two tube groups with different 25 inner diameters can then be coiled in the same tube layer, and can have two different process fractions flowing through them. Compared to an arrangement with different process fractions in different tube layers, this provides improved C:\RPonb\DCC\WAM\524367_ .DOC. 16A1/20l 1 -5 uniformity of distribution of heat flows in the heat exchanger. A difference in the wall thickness can be implemented with 5 the use of the same material, or also with the use of different materials (for example, aluminium and steel) for the two tube groups. The use of different materials is described in detail in the German patent application 102005036413.6, which was submitted at the same time as this 10 application, and in the corresponding applications. The two tube groups can be arranged in the same or in different tube layers. Of course, more than two tube groups with different dimensions can also be provided. For example, 15 a first and a second tube group can be arranged within a first tube layer, and a third tube group in a second tube layer. It is useful, especially to adapt to process fractions with 20 different pressures for which the two tube groups are intended, if two tube groups have different wall thicknesses. For the tube group with the lower internal design pressure, a smaller wall thickness is used, and thus weight is saved. Depending on the pressure loss desired and technical 25 manufacturing capabilities, either the inner diameter or the outer diameter of the two tube groups can be different; alternatively, both diameters can be different. It is fundamentally also possible to vary the inner and/or 30 outer diameter of the same tube within the heat exchanger, for example in order to achieve better adaptation to the volume of an evaporating or condensing process flow. In this C:\NRPonblWDCCWAM\352417_lDOC-16A)312111 -6 case, the first tube group comprises, for example, a first section of tubes and the second tube group comprises another section of the same tube, for example one that connects to the first section. 5 The invention also relates to the application of a heat exchanger of this type to the implementation of an indirect heat exchange between a hydrocarbon flow and at least one heating or cooling fluid. There is also provided 10 application of a heat exchanger in accordance with the invention for carrying out indirect heat exchange between a hydrocarbon flow and at least one heating or cooling fluid. The hydrocarbon flow comprises, for example, natural gas. 15 The hydrocarbon flow is liquefied, cooled, heated, and/or evaporated in the course of the indirect heat exchange. The heat exchanger is preferably used to liquefy or evaporate natural gas. 20 Various embodiments of the invention are described in more detail in the following, with the application example shown schematically in the drawing. Shown is a coiled heat exchanger 1 according to the invention for the liquefaction 25 of a natural gas flow 2 to produce liquefied natural gas (LNG - liquid natural gas) 3, by means of indirect heat exchange with three coolant flows, a low-pressure coolant 4, a first high-pressure coolant 5, and a second high-pressure coolant 6. 30 The coiled heat exchanger here has a single tube bundle with three tube groups. The tubes in the tube groups are coiled C:\NRPonbADCC\WAMU524367_ DOCIM-lA13/2011 -7 alternately in different layers in a screw pattern around a common core tube. (The tube coil corresponds to the generally known principle of a coiled heat exchanger; the geometric arrangement is thus not shown in the schematic 5 drawing.) The tube groups in this example are divided according to process flows. Natural gas 2 flows through the tube of a first tube group 7; one each of the two high pressure coolants 5, 6 flows through the tubes of a second and third tube group 8, 9 respectively. The high-pressure 10 coolants are thereby fed from bottom to top; that is, in the same direction as the natural gas. The low-pressure coolant 4 flows from top to bottom; that is, in the counterflow direction to the natural gas, in the outer space of the tubes, and thereby evaporates. Evaporated low-pressure 15 coolant 10 is drawn off of the outer space at the bottom end of the heat exchanger. In a concrete example with numbers, the process pressures are: 20 Natural gas 2 ......................... 120 bar Low-pressure coolant 4 ................... 15 bar First high-pressure coolant 5 .......... 60 bar Second high-pressure coolant 6 ........ 60 bar 25 The tubes are made of a light metal material, such as aluminium or an aluminium alloy, and have different wall thicknesses for the different tube groups. The outer diameters of the tubes in all tube groups are the same. 30 In a first variant, which is optimized for weight, the wall thicknesses are: C\NRPOrb\DCXWAM15241M7_ 1 DOC.6M/20 I l -8 Tube group 7 .......................... 1.4 mm Tube groups 8 and 9 ..................... 0.9 mm In a further variant, the wall thicknesses were optimized 5 with regard to thermal and hydraulic design, and with regard to as a homogeneous a tube bundle construction as possible, whereby process-driven parameters (e.g., prescribed maximum pressure drops in individual process flows) needed to be maintained. In this second variant, the wall thicknesses 10 are: Tube group 7 .......................... 1.4 mm Tube groups 8 and 9 ..................... 1.2 mm 15 In the second variant, identical tube lengths were achieved in the individual tube groups, whereby the heat exchanger was optimized both with regard to heat transfer and with regard to cost-effectiveness. 20 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter 25 forms part of the common general knowledge in the field of endeavour to which this specification relates. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", 30 and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step C:\4RPonbl\DCC\WAN\1524167_ .DOC- 16)3f2o1 I -8A or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.