WO2008083971A2

WO2008083971A2 - Heat exchanger tubes, and method for producing heat exchanger tubes

Info

Publication number: WO2008083971A2
Application number: PCT/EP2008/000166
Authority: WO
Inventors: Simon Scheidegger; Frank Mäckle; Christoph Tönnes
Original assignee: Innospin Ag
Priority date: 2007-01-12
Filing date: 2008-01-11
Publication date: 2008-07-17
Also published as: WO2008083971A3; CN101631998A; EP1944563A1; US20100012308A1

Abstract

The invention relates to a method for producing heat exchanger tubes comprising a structural steel tube (8) through which a medium flows and which is provided with a number of cooling ribs (2) arranged on an outer wall (10). According to said method, the tube (8) is provided with a surface coating (4, 5) on the inner wall (9) and on the outer wall (10), said surface coating consisting of copper, nickel, cobalt, chrome, a nickel alloy, a chrome alloy, a copper alloy, a cobalt alloy or stainless steel, for directly soldering the number of cooling ribs (2) to the outer wall (10) and for resistance of corrosion of the inner wall (9) by a fluid medium in the tube (8).

Description

Heat exchanger tubes and method for the production of heat exchanger tubes

Technical area

The present invention relates to the field of thermodynamics. It relates to heat exchanger tubes, comprising a flow-through tube with a number of arranged on an outer wall cooling fins and a method for producing such heat exchanger tubes.

Technological background and state of the art

Heat exchanger tubes are used, inter alia, in air-cooled condensers in power plants, waste incineration plants, cogeneration plants and industrial plants with energy recovery. Such known, air-cooled capacitors - hereinafter also referred to as capacitors only - perform a similar function as water-cooled capacitors, that is, they liquefy the exhaust steam of a steam turbine, which can no longer be energetically used, and lead the resulting condensate in the closed water vapor Cycle back. In contrast to cooling towers, the thermal energy is removed from the exhaust steam in the case of condensers by means of air cooling (fans). Capacitors thus do without any cooling water. Today there is an increasing demand for so-called "dry cooling condensers" due to the increasing shortage of water and higher requirements or official requirements for the approval of power plants or industrial plants, where the ecological consideration of water consumption and warming of flowing waters plays an essential role ,

It is known to create heat exchanger tubes for capacitors in an A-shaped configuration. DE 690 33 556 T2 shows heat exchanger tubes manufactured in this way and a production method for this purpose. For example, ribbed round tubes, finned oval tubes or finned flat tubes are used. When ribbing steel pipes and any indicated geometry, the pipes are first grooved helically and then flat aluminum sheets are drawn in. Another method is to wind pipes with aluminum sheets mechanically helical.

In particular, when ribbing oval steel pipes made of steel, flat steel ribs provided with spacers are plugged on and then galvanized and joined together with the oval pipe on the outside and on the surface.

Today's production of ribbed flat tubes is accomplished by first forming an aluminum clad flat steel tube, welding and post-coating aluminum at the weld, then forming a rib of a solder-plated aluminum alloy, applying a flux, and forming the rib is attached to both sides of the flat tube and this rib is then brazed in a controlled atmosphere oven at about 600 ⁰ C to the flat tube. Flat tubes enjoy the advantage over ribbed round tubes or oval tubes that they are more resistant to freezing and have a lower air-side pressure loss.

Another important aspect of air-cooled capacitors is the long life. The capacitors must have a lifetime of more than 30 and sometimes even more than 40 years or more. Since such capacitors are exposed to the environmental influences, they must be very resistant to corrosion. In addition, the capacitors along their inner wall are also exposed to contamination, which are caused by contamination of the fluid flowing through them. These impurities are already present during commissioning or they arise due to oxidation or corrosion in the entire water-steam cycle of a power plant.

The inner wall of a flat tube for capacitors is known to consist of unprotected structural steel without oxidation and corrosion resistance. The inner surface of an air-cooled condenser is, however, together with the boiler in the water-steam Circuit of a power plant by far the largest surface which is exposed to the process-side fluid. This inevitably leads to a conditionally correspondingly high conditioning of the water-steam chemistry, inter alia, to a basic pH in order to protect the heat exchanger tubes against massive oxidation, corrosion or even rusting.

Before start-up, this also means that the condenser must be cleaned and rinsed with a time-consuming and expensive process and the deionized water used must be discarded. A similar procedure must also be used after repairs and before recommissioning with the same rejection of "used up" water, with operational and ecological aspects calling for improvement.

Furthermore, during the ongoing operation of the power plant, a filtering out requires the formation of rust particles and a complex polishing plant. At normal operational standstill there is a risk of pitting corrosion and rust through. Even during transport to the installation site and during installation of the condenser, the unprotected surfaces of the heat exchanger tubes require protection by means of covers, protective gas and / or drying devices.

In the production of sheet metal coated with aluminum on one side, after forming and welding to the flat tube along the weld seam, aluminum must be recoated. For this purpose, aluminum is typically applied by means of flame spraying as a 30mm wide strip. This additional production step can have the result that contaminants or material defects occur in a brittle iron-aluminum intermediate layer that has been formed by welding. The sprayed aluminum layer also has a roughness, which can lead to loss of quality of the solder joint during subsequent brazing the tube with cooling fins, which in turn the Thermal efficiency of the total plant affected. Qualitatively limited solder joints may also be a cause of chipping of cooling fins.

It is also known that compounds between aluminum and iron are comparatively brittle and thus set an increased sensitivity to operational thermal / mechanical stresses; this also applies to impact and / or Torsionsbeaufschlagung during transport or installation. There is a low fracture toughness and ductility of the iron-aluminum intermediate layer associated with a high defect sensitivity in terms of pores, sandblasting agents, oxides and inclusions. This deep defect tolerance of the iron-aluminum intermediate layer requires a correspondingly high expenditure in production and quality control.

BRIEF SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide heat exchanger tubes and a method for the production of heat exchanger tubes, whereby the disadvantages of the known heat exchanger tubes and the process for their preparation are overcome.

In particular, it is the object of the present invention to provide heat exchanger tubes and a method for their production, whereby with respect to the prior art significantly improved defect tolerance and corrosion resistance of the outer wall and the inner wall of heat exchanger tubes can be achieved and also a simpler and more cost-effective production is possible.

The problem underlying the invention for the method is solved by the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims 2 to 6. The essence of the invention is to be seen in that a pipe made of structural steel of a heat exchanger tube is provided on an inner wall and on the outer wall with a surface finish of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy or stainless steel, for direct soldering of a number of cooling fins to the pipe and for corrosion resistance of the inner wall to fluid media.

It is particularly advantageous here to emphasize that, due to the surface coating - consisting of the materials mentioned, on the one hand a qualitatively excellent solderability of the pipe with the cooling fins on the outer wall is given, and at the same time the corrosion resistance of the inside of the pipe when exposed to the water. Steam chemistry of a power plant can be significantly increased. Thus, for the first time by a coating or plating process, a tube for a heat exchanger tube can be provided which, with regard to the processability and the wear / aging, exhibits a considerably improved design compared with the cited prior art.

Simplifications can be pointed out not only with regard to production, but also with regard to the ongoing operation of the power plant and the associated commissioning or recommissioning after standstill, the invention shows an outstanding ecological compatibility with respect to what is known.

The brazed joint between the cooling fins and the tube is particularly ductile and tough, so that operational or assembly-related stresses are harmless to the heat exchanger tube according to the invention; Service life can thus be increased, maintenance and monitoring work can be minimized.

A particularly advantageous embodiment of the invention provides that the coating or the plating of the tube on the inner wall and the outer wall with one and the same Material takes place, which only one operation is necessary to provide the pipe essential features of the invention. Advantageously, a tube made of a sheet metal coated or plated on both sides according to the invention, for example of simple structural steel, is shaped into a flat tube and then welded directly along the abutting surface. The resulting weld is free of any iron-aluminum interlayer and the flat tube is directly brazeable with appropriately sized cooling fins. These cooling fins may be made of or coated with aluminum or made of an aluminum alloy for good thermal efficiency; In any case, during brazing, there is no continuous iron-aluminum intermediate layer, so that the heat exchanger tube according to the invention substantially continues to have the ductility and toughness of the tube base material.

In an embodiment of the invention, it is advantageously provided that an intermediate layer forming during the brazing process is ductilised by adding boron.

The problem underlying the invention for the heat exchanger tube is solved by the features of claim 7. A further advantageous embodiment of the invention is the subject of the dependent claim 8.

Since all the advantages that are valid for the heat exchanger tube have already received their appreciation in the process for producing such a heat exchanger tube according to the invention, a repetition in favor of the text economy is dispensed with at this point.

Short description of the drawing

In the drawing, an embodiment of the invention is shown in simplified form, and that shows Fig. 1 is a sectional view through an inventive heat exchanger tube before a brazing process, and the

Fig. 2 shows the sectional view of Fig. 1 after the brazing process.

1 shows a sectional view of the inventive heat exchanger tube 8 before a soldering process, which connects a tube wall 1 with a cooling fin 2 brazing. The pipe wall 1 on an inner wall 9 and an outer wall 10 each have an outer surface coating 4 and an inner surface coating 5, which is arranged adjacent to the pipe wall 1. The cooling rib 2 is provided with a ribbed coating 3 and has a physical contact with the outer surface coating 4 of the tube wall 1.

2, the heat exchanger tube 8 is shown after the brazing process, wherein between the cooling fin 2 and the tube wall 1 is now a solid connection by means of an intermediate layer 6 and brazing 7.

The manufacturing method for the heat exchanger tubes 8 according to the invention provides the following steps:

1. A core band, the steel grade DCOl (EN 10130), for example, is provided on both sides by roller presses with support bands. The core band later represents the tube wall 1 shown in FIGS. 1 and 2 and the support bands the outer and inner surface coating 4, and 5, respectively.

2. The core tape with the support belts is recrystallized for several minutes and the Cefüge is returned to its original state. 3. A subsequent finish rolling puts the core band with the support bands in the desired final state before

4. The three strips are joined together in a metallurgical diffusion annealing process. In this case, directly to the core band towards a solid and ductile diffusion layer, which in Figs. 1 and 2 as an inner surface finish

5 is shown.

5. The present, plated ribbon of core band and support bands is bent into a flat tube and closed by welding along a longitudinal tube seam. This weld is smoothed, no further treatment is necessary.

6. For the production of the cooling fin 2 is a plated band ready, consisting of a core band of an aluminum alloy, which is provided at least on one side with an aluminum-braze alloy. The solder-plated cold strip is cold-formed into a three-dimensional cooling rib form.

7. After brushing the flat tube and the cooling fin with a commercially available aluminum flux both are connected to one another by a continuous brazing process to a heat exchanger tube 8, whereby a solid, gap-free and pore-free solder joint between the cooling fin 2 and the tube wall 1 is formed.

Between the tube wall 1 and the cooling fin 2 there is now a layer with brazing material 7 and an intermediate layer 6, which is also called a reaction zone. This reaction zone consists of an ordered phase and is characterized by good strength and very high oxidation and corrosion resistance. According to the invention, the outstanding properties in terms of solderability and corrosion resistance to fluid media (not shown in the figures) in the heat exchanger tube 8, when the core band has a plating on both sides by means of the support bands and if these support strips made of copper, nickel, cobalt, Chromium, a nickel alloy, a chromium alloy, a copper alloy, a cobalt alloy or stainless steel.

The following material distribution for the production of the heat exchanger tubes 8 is shown by way of example in the form of a nickel alloy Inconel Alloy 825 (IN 825). After the diffusion annealing treatment of the core band with the double-sided coated support strips made of Inconel Alloy, there is the diffusion layer - the inner surface compensation 5 of the heat exchanger tube 8 - from a crystalline disordered mixing structure, in which adjust especially the elements iron, nickel and chromium equal mass of each clad alloy. The outer surface coating layer 4 further consists of Inconel Alloy.

When brazing the cooling fin 2 with the tube wall 1, the reaction zone is formed with an ordered phase of Al-Ni-Fe-Si-Cr. This reaction zone Al-Ni-Fe-Si-Cr is under a slight compressive stress after brazing and during operation due to the deliberately different coefficients of expansion of the alloys involved, which increases the mechanical integrity of the entire solder joint.

Without departing from the spirit of the invention, all other materials shown for plating by means of support strips show corresponding properties to be exhibited with respect to the good solderability between the fin 2 and the tube wall 1 and the corrosion resistance on the inside of the heat exchanger tube 8 to a fluid medium. LIST OF REFERENCE NUMBERS

1 pipe wall

2 cooling rib

3 rib coating

4 outer surface finish

5 inner surface coating

6 interlayer

7 brazing clay

8 heat exchanger tube

9 inner wall 10 outer wall

Claims

claims

1. A method for producing heat exchanger tubes, comprising a flow-through tube (8) with a number on an outer wall (10) arranged cooling fins (2), characterized in that the existing of mild steel tube (8) on an inner wall (9) and on the outer wall (10) is provided with a surface coating (4, 5) of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a cobalt alloy or stainless steel, for direct soldering of the number of cooling fins (2) with the Outer wall (10) and for the purpose of corrosion resistance of the inner wall (9) against a fluid medium in the tube (8).

2. The method according to claim 1, characterized in that the surface coating (4, 5) is carried out as a coating or plating.

3. The method according to claim 1 or 2, characterized in that the surface coating (4, 5) on the inside (9) and the outside (10) - consisting of the same material al - applied in a single operation.

4. The method according to claim 3, characterized in that for the production of the tube (8) a sheet metal strip of structural steel in a first process step on both sides with the surface finish (4, 5) is provided, is formed into a flat tube and is welded, wherein a emerging weld remains free of an iron-aluminum interlayer.

5. The method according to any one of the preceding claims, characterized in that the number of cooling fins (2) made of aluminum, an aluminum alloy, steel, coated steel or alloy steel and the soldering process is feasible so that a connection of the cooling fins ( 2) with the surface treatment Teten tube (8) is free of a continuous iron-aluminum intermediate layer to the pipe wall (1) out.

6. The method according to any one of the preceding claims, characterized in that an intermediate layer (6) forming during the soldering process is ductilised by the addition of boron.

7. heat exchanger tube, comprising a flow-through tube (8) with a number of cooling fins (2), characterized in that the existing structural steel tube (8) on an inner wall (9) and on an outer wall (10) with a surface finish (4, 5) is made of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy or stainless steel, for direct solderability

Number of cooling fins (2) with the outer wall (10) and for the purpose of corrosion resistance of the inner wall (9) against a fluid medium in the tube (8).

8. Heat exchanger tube according to claim 7, characterized in that the surface coating as a coating or plating on the inside (9) and the outer side (10) consists of the same material.