EP1944563A1 - Heat exchanger tube and method for the production thereof - Google Patents

Abstract

Production of a heat exchanger tube comprises forming a surface hardening layer (4, 5) made from copper, zinc, nickel, cobalt, chromium or their alloys on the inner wall (9) of the tube for direct soldering of the cooling ribs (2) to the outer wall (10) and for corrosion resistance of the inner wall towards a fluid medium in the tube. An independent claim is also included for a heat exchanger tube produced by the above method. Preferred Features:.

Description

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 increased demand for so-called "dry cooling capacitors" due to the increasing water shortage and higher requirements or regulatory requirements for the approval of power plants or industrial plants. Here, the ecological consideration of water consumption and warming of flowing waters play an essential role.

It is known to create heat exchanger tubes for capacitors in an A-shaped configuration. The DE 690 33 556 T2 shows such manufactured heat exchanger tubes and a manufacturing method to do so. For example, ribbed round tubes, ribbed oval tubes or finned flat tubes are used. In the ribbing of steel and steel pipes arbitrary, indicated geometry, the tubes are first grooved helically and then flat aluminum sheets are retracted. Another method is to wind pipes with aluminum sheets mechanically helical.

In particular, in the ribbing of oval tubes made of steel flat, provided with spacers steel ribs are plugged and then galvanized and connected together with the oval tube 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, subsequently forming a rib of a solder-plated aluminum alloy, applying a flux, and forming the rib formed on both sides the flat tube is attached and this rib is then brazed to the flat tube in a controlled atmosphere oven at about 600 ° C. 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 an air-cooled condenser but together with the boiler in the water-steam cycle 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 discarding of "used" water. Operational and ecological aspects call for an improvement here.

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 assembly 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 on the subsequent brazing the tube with cooling fins can lead to loss of quality of the solder joint, which in turn affects the thermal efficiency of the entire system. 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 tube of a heat exchanger tube on an inner wall and on the outer wall provided with a surface finish of copper, zinc, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a zinc 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, the quality of the tube with the cooling fins on the outside wall is qualitatively excellent, and at the same time the corrosion resistance of the inside of the tube when exposed to 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 braze joint between the cooling fins and the tube is particularly ductile and tough, so that operational or assembly-related stresses are harmless to the inventive heat exchanger tube; 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, whereby only one operation is necessary to provide the pipe essential to the invention features. 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 essentially 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 ductilized 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

• Fig. 1 eine Schnittdarstellung durch ein erfindungsgemässes Wärmetauscherrohr vor einem Hartlötprozess, und die
• Fig. 2 die Schnittdarstellung aus Fig. 1 nach dem Hartlötprozess.

In the drawing, an embodiment of the invention is shown in simplified form, and that shows

• Fig. 1 a sectional view through an inventive heat exchanger tube before a brazing process, and the
• Fig. 2 the sectional view Fig. 1 after the brazing process.

Ways to carry out the invention

The Fig. 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 fin 2 is provided with a ribbed coating 3 and has with the outer surface coating 4 of the tube wall 1 in touch contact.

In the Fig. 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 firm connection by means of an intermediate layer 6 and brazing 7.

1. 1. Ein Kernband, der Stahlsorte DC01 (EN10130) beispielsweise, wird beidseitig durch Walzenpressen mit Auflagebändern versehen. Das Kernband stellt später die in den Figuren 1 und 2 gezeigte Rohrwand 1 dar und die Auflagenbänder die äussere und innere Oberflächenvergütung 4, bzw. 5.
2. 2. Das Kernband mit den Auflagenbändern wird minutenlang rekristallisationsgeglüht und das Gefüge wieder in den Ausgangszustand versetzt.
3. 3. Ein anschliessendes Endwalzen versetzt das Kernband mit den Auflagebändern in den gewünschten Endzustand, bevor
4. 4. die drei Bänder in einem Diffusionsglühverfahren metallurgische miteinander verbunden werden. Dabei entsteht unmittelbar zum Kernband hin eine feste und duktile Diffusionsschicht, die in den Fig. 1 und 2 als innere Oberflächenvergütung 5 gezeigt ist.
5. 5. Das vorliegende, plattierte Flachband aus Kernband und Auflagebändern wird zu einem Flachrohr gebogen und mittels Schweissen entlang einer Rohrlängsnaht geschlossen. Diese Schweissnaht wird einer Glättung unterzogen, eine weitere Behandlung ist nicht notwendig.
6. 6. Für die Fertigung der Kühlrippe 2 steht ein plattiertes Band bereit, bestehend aus einem Kernband einer Aluminiumlegierung, das zumindest einseitig mit einer A-luminiumlotlegierung versehen ist. Das so lotplattierte Kaltband wird in eine dreidimensionale Kühlrippenform kalt umgeformt.
7. 7. Nach einem Bestreichen des Flachrohres und der Kühlrippe mit einem handelsüblichen Aluminium-Flussmittel werden beide durch einen kontinuierlichen Hartlötprozess zu einem Wärmetauscherrohr 8 miteinander verbunden, womit eine feste, spalt- und porenfreie Lötverbindung zwischen der Kühlrippe 2 und der Rohrwand 1 entsteht.

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

1. 1. A core strip, the steel grade DC01 (EN10130) for example, is provided on both sides by roller presses with support belts. The core band later puts in the Figures 1 and 2 shown pipe wall 1 and the support bands the outer and inner surface coating 4, and 5, respectively.
2. 2. The core tape with the support bands is recrystallization annealed for several minutes and the structure is returned to its original state.
3. 3. A subsequent finish rolling puts the core band with the support bands in the desired final state before
4. 4. The three strips are joined together in a metallurgical diffusion annealing process. This creates a solid and ductile diffusion layer directly to the core band, which leads into the Fig. 1 and 2 is shown as inner surface coating 5.
5. 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. 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 A-aluminum braze alloy. The solder-plated cold strip is cold-formed into a three-dimensional cooling rib form.
7. 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 excellent 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, zinc, nickel, cobalt, Chromium, a nickel alloy, a chromium alloy, a copper alloy, a zinc 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 with the aid of a nickel alloy Inconel Alloy 825 (IN 825) for the support strips. After the diffusion annealing treatment of the core strip with the double-sided plated support strips made of Inconel Alloy, the diffusion layer - the inner surface coating 5 of the heat exchanger tube 8 exists - From a crystalline disordered mixed structure, in which adjust especially the elements iron, nickel and chromium uniformly the respective plated 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 fin

3

ribs coating

4

external surface finish

5

inner surface coating

6

interlayer

7

braze

8th

heat exchanger tube

9

inner wall

10

outer wall

Claims (8)

Method for producing heat exchanger tubes, comprising a through-flowed tube (8) with a number of cooling ribs (2) arranged on an outer wall (10), characterized in that the tube (8) is mounted on an inner wall (9) and on the outer wall (10) with a surface finish (4, 5) of copper, zinc, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a zinc alloy, a cobalt alloy or stainless steel is provided 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). A method according to claim 1, characterized in that the surface treatment (4, 5) is carried out as a coating or plating. A 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 - applied in one operation. A method according to claim 3, characterized in that a sheet metal strip of mild steel is provided on both sides with the surface coating (4, 5) in a first process step, is formed into a flat tube and is welded, with a resulting weld remains free of an iron-aluminum interlayer. Method according to one of the preceding claims, characterized in that the number of cooling ribs (2) consist of aluminum, of an aluminum alloy, of steel, of coated steel or of alloyed steel and the soldering process is feasible such that a connection of the cooling ribs (2) with the surface-tempered Pipe (8) is free of a continuous iron-aluminum intermediate layer to the pipe wall (1) out. Method according to one of the preceding claims, characterized in that an intermediate layer (6) forming during the soldering process is ductilised by adding boron. Heat exchanger tube, comprising a flow-through tube (8) with a number of cooling fins (2), characterized in that the tube (8) on an inner wall (9) and on an outer wall (10) with a surface coating (4, 5) made of copper, Zinc, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a zinc alloy or stainless steel is provided for direct solderability of the number of cooling fins (2) with the outer wall (10) and for corrosion resistance of the inner wall (9) against a fluid medium in the tube (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 outside (10) consists of the same material.

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