Patent Application
20090194266
The invention relates to a heat exchanger with a straight tube bundle, as described in the preamble of claim 1. The invention, in general, is described in terms of a heat exchanger with a straight tube bundle, as it is used in an apparatus for the production of synthesis gas. However, the invention is not limited to use in a synthesis gas apparatus. The heat exchanger in accordance with the invention can be used, in principle, for heat exchange between any two media, and each medium, independently of the other medium, can be present either in liquid or gaseous form.
In a synthesis gas apparatus, in most cases synthesis gas is produced from a fossil fuel by means of thermal cracking. The resultant synthesis gas produced in this case is present at a higher temperature than is required for most applications. To cool the hot synthesis gas, mainly heat exchangers with a longitudinally extended, straight tube bundle are used, according to the prior art. In these so-called straight tube heat exchangers, for example, feed water, which is to be preheated for other applications, enters into heat exchange with the hot synthesis gas. In such a heat exchanger with a longitudinally extended, straight tube bundle, one medium is conveyed into the tube space for the heat exchange, while the second medium is conveyed into the shell space, which encloses the tube space. The tube space of such a heat exchanger essentially consists of two manifolds with mounted supports, which are suitable for feeding and removing the first heat exchange medium, and a longitudinally extended, straight tube bundle, having at least two tubes. The shell space encloses the tube space and has at least one feed inlet and at least one discharge outlet for the heat exchange second medium. The shell space is sealed relative to the tube space.
The synthesis gas can be conveyed either into the tube space and into the shell space in co-current flow or countercurrent flow to the feed water. Usually, the hot synthesis gas is conveyed into the tube space and water is conveyed into the shell space in a heat exchanger of the synthesis gas apparatus. In the case of a synthesis gas apparatus, the two media involved in the heat exchange typically have a very high temperature difference. The heat exchanger is manufactured at room temperature. In a preheater in a synthesis gas apparatus, hot synthesis gas is introduced into the heat exchanger at a temperature of, for example, between 300° C. and 450° C. and conveyed in co-current or counter-current flow to water or feed gas at room temperature. The different heat expansions between tube and shell space caused by this temperature difference result in mechanical stresses. According to the prior art, such mechanical stresses can be absorbed by an expansion joint. An expansion joint is generally designed as a corrugated structure having at least one wave in one or more layers. Due its corrugated structure, the expansion joint is able to readily take up different mechanical expansions at fixed ends. Similar to an accordion, the expansion joint can be pressed together or stretched out. According to the prior art, such an expansion joint can be either incorporated into the shell, or a straight tube heat exchanger with a floating head is used, as in the German patent application “Wärmetauscher mit geradem Rohrbündel und Schwimmkopf [Heat Exchanger with Straight Tube Bundle and Floating Head]” (file number 102007017227.5) assigned to Linde AG.
As materials for the production of a straight tube heat exchanger, many various types of steel or aluminum alloys are suitable depending on the planned use, the pressures and temperatures that are produced therefrom, and the media that take an active part in the heat exchange. When employed as a preheater in a synthesis gas apparatus, the heat exchanger is generally made of heat-resistant, creep-resistant steel, preferably a chromium-molybdenum alloy or chromium-nickel steel. Chromium-nickel steel is considerably more expensive than heat-resistant, creep-resistant steel and therefore is not to be preferred for economic reasons. On the other hand, in the production of a heat exchanger made from creep-resistant steel, the parts are produced in a heat-shaping process or in the welded manner, which requires a subsequent annealing to reduce production of corresponding mechanical stresses. Such an annealing process, however, cannot be performed with an expansion joint in the shell because the expansion joint would lose elasticity due to the annealing process. Therefore, an expansion joint has to be produced from chromium-nickel steel. According to the prior art, the entire heat exchanger is thus produced from chromium-nickel steel. A similar problem also arises in sealing the manifolds from the shell space.
One aspect of the invention is to configure a heat exchanger of the type such that the economic efficiency of its production is improved without it resulting in an increase of the thermal stresses during use and thus in a reduction of the service life.
In accordance with the invention, the shell and expansion joint are made from different materials and are connected by means of an overlay welding, and/or the manifolds are sealed, relative to the shell space, by means of a welding-ring seal, wherein the welding-ring seal is made of a material that is different from that of the manifolds and/or that of the shell, and is connected via an overlay welding to the respective parts.
The heat exchanger according to the invention is made from various parts of different materials, and the materials used to make the various parts can be optimized according to standpoints of technical use and economy. Different parts of a heat exchanger have to meet different mechanical or thermal requirements. An expansion joint in the shell, for example, has to be heat-resistant and elastic enough to accommodate mechanical deformations. The remaining part of the shell, however, only has to be heat-resistant, since it transfers its mechanical stresses to the expansion joint. In terms of the invention, the materials of the different parts of a heat exchanger are matched to these different requirements and conditions of use. The parts made from different materials are connected to one another according to the invention by means of an overlay welding. An overlay welding is a means, known to and tested by one skilled in the art, for connecting metal parts of components made of different materials.
According to a preferred configuration of the invention, the shell and/or the manifolds are made from a heat-resistant, creep-resistant steel, preferably a chromium-molybdenum alloy, and the expansion joint and/or the welding-ring seal are made from chromium-nickel steel. Heat-resistant, creep-resistant steels, in particular chromium-molybdenum alloys, have proven advantageous for use in high temperature heat exchangers. Chromium-nickel steel also has high heat resistance, as well as very good elastic properties. In this configuration of the invention, the majority of the shell can be advantageously made of heat-resistant, creep-resistant steel, while only those parts in the heat exchanger having higher requirements of elastic behavior are made from the more expensive chromium-nickel steel. An optimum matching of the parts of the heat exchanger to the different requirements is thus provided.
Advantageously, the overlay welding is made from a nickel- and/or molybdenum-based alloy, preferably Incoloy 825 (a nickel-iron-chromium alloy containing molybdenum and copper). An overlay welding made from a nickel- and/or molybdenum-based alloy, preferably Incoloy 825, is a suitable means for connecting different metal materials, especially a heat-resistant, creep-resistant steel and chromium-nickel steel.
The invention also relates to a process for the production of a straight tube heat exchanger according to the invention. According to the invention, two parts of different materials are to be connected by means of an overlay welding. At least one part is provided with the overlay welding. The part with the overlay welding is conveyed into an annealing process and then connected to the other part via the overlay welding.
According to an especially preferred configuration of the invention, the shell pieces and/or manifolds made from heat-resistant, creep-resistant steel are provided with an overlay welding, conveyed into an annealing process, and then welded via the overlay welding with parts made from chromium-nickel steel, such as an expansion joint and/or welding-ring seal. By the annealing process, mechanical stresses, which are necessarily created in the production of parts from heat-resistant, creep-resistant steel, are reduced in these parts, thereby considerably increasing their service life. The overlay welding is not influenced by the annealing process and makes possible a reliable connection between parts made from heat-resistant, creep-resistant steel and parts made from chromium-nickel steel.
Advantageously, a straight tube heat exchanger in accordance with the invention is used in a synthesis gas apparatus or hydrogen production plant, preferably as preheaters for cooling hot synthesis gas while simultaneously heating water.
With this invention, it is possible in particular to optimally match different parts of a heat exchanger to their different mechanical and thermal requirements. The economic efficiency of the production of such a heat exchanger is considerably increased, without accompanying losses of mechanical or thermal stability.
Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 102008006559.5, filed Jan. 29, 2008.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102008006559.5 | Jan 2008 | DE | national |
