EP1793164A1 - Steam generator tube, method of manufacturing the same and once-through steam generator - Google Patents

Abstract

The tube (10) has an insert (22) arranged in an inner space of the tube for forming an inner profile. The insert has a set of wires (24) which are spirally wound along a tube inner wall (26) in the form of multi-start windings. A lead angle of the wires opposite to a reference plane oriented perpendicular to a tube axis amounts between 30 and of 70 degrees. The wires lying on the inner wall are connected with one another by radial reinforcement rods and/or with a middle wire that is running along the tube axis. An independent claim is also included for a method for manufacturing steam generator tubes with an inner profile.

Description

The invention relates to a steam generator tube with a spin-producing inner profile. It further relates to a continuous steam generator with such steam generator tubes. The invention further relates to a method for producing a steam generator tube provided with a swirl-producing inner profile.

In the combustion chamber walls of a continuous steam generator, gas-tightly welded steam generator tubes are usually used with one another via webs to form a throttle cable surrounding the combustion chamber, which are connected in parallel for the flow of a flow medium. Instead of tubes with separate, separate flat iron bars in between, it is also possible to use tubes which are already factory-equipped with molded fins. The steam generator tubes can be arranged vertically or obliquely. For a reliable operating behavior of the continuous steam generator, the steam generator tubes are generally designed such that sufficient cooling of the steam generator tubes is ensured even at low mass flow densities of the steam generator tubes flowing medium.

An important design criterion is the heat transfer properties of a steam generator tube. A high heat transfer allows a particularly effective heating of the medium flowing through the steam generator tube with simultaneous reliable cooling of the steam generator tube per se. The heat transfer behavior of a steam generator tube may be affected by the occurrence of so-called boiling crises in conventional steam generators, which are operated at subcritical pressures. The pipe wall is no longer wetted by the liquid flow medium - usually water - and thus insufficiently cooled. As a result of too early drying could then reduce the strength values of the pipe wall.

To improve the heat transfer behavior usually steam generator tubes are used, which have as a result of a deformation process (eg cold drawing) on its inside a surface structure or an inner profile in the form of helically wound ribs. Due to the shape of the ribs, a swirl is impressed on the medium flowing through the steam generator tube, so that the heavier liquid phase collects on the tube inner wall as a result of the acting centrifugal forces and forms a wetting liquid film there. This ensures a reliable heat transfer from the pipe inner wall to the flow medium even at comparatively high heat flux densities and low mass flow densities.

A disadvantage of the known steam generator tubes that they are relatively complicated to produce due to the limited deformability of the pipe material. In particular, in high-temperature steels with high chromium content, the deformability is severely limited. Such materials are playing an increasingly important role in steam generator tubes today, since they permit - at least in principle - a design of a steam generator for particularly high steam parameters, in particular for high live steam temperatures, and consequently particularly high efficiencies. Due to the material-related limitations in processing, however, it is not possible in practice or only with considerable effort to produce internally ribbed tubes with the desired, fluidically advantageous rib profiles in the context of a deformation process of smooth tubes. In particular, sufficiently steep flank angles and sharp-edged transitions in connection with large rib heights are difficult to produce. In addition, the height of the ribs is manufacturable only within a narrow frame. In addition, there is only a slight flexibility with regard to the profile design along the tube.

Alternatively, various types of swirl-producing components have already been proposed for subsequent installation in a steam generator tube. These include, in particular, the so-called "twisted tapes": strips made of a metal strip that are twisted or twisted. However, the tube fittings known hitherto have the disadvantage in common that on the one hand they obstruct the (originally) free cross section in the center of the tube and therefore lead to very high pressure losses, and on the other hand they divert the entire flow extremely strongly and in some cases "overdrive". , A simple twisted tape z. B. leads at higher vapor contents in the two-phase flow to an accumulation of the water phase in the gusset between the pipe wall and the tape with simultaneous drying and thus insufficient cooling of the inner wall regions leeward side of the tape. Steam generator tubes with inserts in the type of twisted tapes are therefore not equally suitable for all operating conditions usually occurring in steam generators.

The invention is therefore based on the object of specifying a steam generator tube of the aforementioned type, which has a particularly favorable heat transfer behavior with simple and inexpensive held production and a wide range of different operating conditions. Furthermore, a production method suitable for the production of such a steam generator tube as well as a continuous steam generator are to be specified, which has a particularly simple design with high operational safety and high efficiency.

With regard to the steam generator tube, the stated object is achieved according to the invention in that at least one insert is arranged in the tube interior to form a swirl-producing inner profile, wherein the insert comprises a plurality of wires which helically wind along the tube inner wall in the manner of a multi-start thread.

The invention is based on the consideration that the multiphase flow should have a twist within a steam generator tube to improve the heat transfer, so that the liquid phase is guided due to the rotation of the pipe inner wall and this wets as evenly as possible. For a specific production and maintenance of such a swirl flow, therefore, suitable flow-guiding elements should be arranged inside the tube. As has been found, the flow guide is particularly favorable when on the one hand neither a "Überdrallen" nor too large pressure losses along the flow path occur, on the other hand, the swirl effect is still intense enough to the liquid phase of the flow medium over the entire pipe circumference to the pipe inner wall conduct.

To avoid high pressure losses, which lead to a high own energy demand for the feedwater pump, and to ensure the removal of steam in the tube interior, the flow-guiding elements should be arranged substantially in the manner of an inner profile on the pipe inner wall and not or only slightly obstruct the pipe cross-section in the center. Moreover, in order to circumvent the manufacturing limitations associated with the finned tubes of conventional design, the swirl-producing internal profile should be realized by tube fittings or inserts which can be made in the desired shape independently of the steam generator tubes and retrofitted into the tube. For this purpose, in the new concept presented here, wires or bands are provided which, after introduction into the steam generator tube, wind helically along the inner wall of the tube so that a substantial part of the tube cross-section remains free (more than 50%) and the steam in the interior of the tube thus remains free accumulate and can flow away.

Furthermore, it was recognized that a simple, ie catchy coil spring usually generates only a weak swirl. The flow can shear over the voltage applied to the pipe inner wall wire. Due to the lower rotation Does it then come to an earlier occurrence of the boiling crisis. Although this effect could for example be compensated by a larger wire diameter (analogous to a larger rib height), but this leads to a wire assembly in the manner of a simple helical spring easily accumulating or damming the water phase in the gusset between the pipe wall and the wire insert while drying the Inner wall portions leeward of the wire, ie to an insufficient cooling of the corresponding wall areas. Such disadvantages are avoided according to the concept presented here in that a plurality of wires in the manner of a multi-start thread respectively bears helically on the pipe inner wall. In this embodiment, a uniform wetting of the pipe inner wall with liquid fluid is achieved even with moderate swirl strength and relatively low pressure loss; on the other hand, overflow of the flow is completely avoided.

It is also particularly advantageous that, in contrast to finned tubes of conventional design, which are produced by a deformation process using considerable deformation forces from smooth tubes, a great flexibility in terms of flow-related parameters, such as profile height, number of gears, pitch angle, flank angle and Scharfkantigkeit exists. Corresponding design specifications can be particularly simple and precise implemented in the execution as insert component, as this usually only wires or metal bands must be provided with the appropriate cross-sectional profile and brought into the desired arrangement, eg. B. by twisting and / or bending.

In steam generator tubes of conventional dimensions and dimensions, an arrangement of the wires in the manner of a two- or three-start thread is particularly useful. But even four- to six-speed versions may be advantageous; For steam generator tubes with a particularly large diameter even eight-speed variants are conceivable. Advantageously, the pitch angle of the respective wire is opposite a plane oriented perpendicular to the tube axis reference plane at least 30 ° and preferably at most 70 °. Especially advantageous is a pitch angle from the interval 40 ° to 55 °.

For a particularly simple and inexpensive manufacturability, the respective wire has a round or a substantially rectangular cross-section. In the latter embodiment, in particular, the edges can be post-processed, so that it is possible to realize comparatively steep flank angles and sharp-edged transitions. The wires can vary in diameter depending on the diameter of the steam generator tube and depending on the intended flow and temperature conditions. In general, a wire diameter or a mean cross-sectional dimension of 5% to 15% of the inner diameter of the smooth tube is advantageous.

Advantageously, the respective wire or the tube insert formed from the wires sits at the intended operating temperature of the steam generator tube due to its residual stress slip-resistant in the tube interior. The wire material and the residual stress are thus matched to the geometric conditions that a creeping or slipping of the individual turns is prevented against each other.

If necessary, the wires resting on the tube inner wall can be connected to each other via radial stiffening struts and / or to a central wire running along the tube axis. By such a support core slipping of the individual spring courses is prevented even in a possible decrease in the wire or spring tension, so that the tube insert permanently retains its original shape and position in the steam generator tube. Additionally or alternatively, a number of extending in the direction of the tube axis holding wires may be provided, which are respectively fixed to the pipe interior facing the side of the helically wound wires to this. In this way, a similar effect results as in the execution with the radial stiffening struts. The stiffening struts and / or the retaining wires and / or the central wire comprehensive support core can be made of a lower compared to the swirl-producing, applied to the pipe inner wall wires inferior material, since it must be protected against corrosion or scaling, but not directly with the very high temperatures of the pipe inner wall is loaded.

Although the pipe insert already sits relatively firmly and safely in the steam generator tube due to the residual stress of its wires, an additional fixation is preferably provided in which the respective profile-forming wire is firmly connected at least at one point, preferably in the vicinity of its two ends, with the tube inner wall. The solid compound is advantageously a high-temperature welded connection. A more elaborate variant to be produced, but which ensures a particularly secure fixation comprises a plurality of distributed over the longitudinal extent of the respective wire spot welds. The welding fixation can be produced particularly well if at least the wires resting on the pipe inner wall of the insert are made of a material having a composition similar to the pipe material.

Furthermore, it is desirable, especially for a comparatively long, over the entire height of the steam boiler extending steam generator tube along its longitudinal extent depending on the location provide different guide profiles in the tube interior, which take into account the spatial development or variation of both the vapor content and the heating profile. Such a concept can be advantageously realized in that a plurality of inserts is introduced into the steam generator tube, which are arranged in separate pipe sections, wherein the respective insert with its geometric parameters to the provided during operation local heating and / or to the local flow conditions is adjusted. As further pointed out has that the swirl is maintained after a single generation even with a two-phase flow over at least a flow path of five pipe diameters, no complete, complete assembly of the tube is necessary. Rather, the inserts can be installed spaced apart from each other in the steam generator tube.

Conveniently, the steam generator tubes described here are used in a fossil-heated continuous steam generator. Due to the swirl-producing inner profile of the tubes and the associated improvements in the heat transfer behavior, a sufficient heat transfer to the flow medium or cooling of the tube walls is ensured even in boiler designs with vertical pipe arrangement ("vertical pipe"). A vertical pipe with a higher number of tubes and with relatively short pipe lengths allows due to the opposite obliquely or spirally arranged pipes lower flow rates and lower mass flow densities operation of the steam generator with reduced pressure loss and reduced minimum flow rate. Thus, the steam generator comprehensive power plant can be designed for a lower minimum load. The separation effects known from inclined steam generator tubes, in which water and steam only flow stratified when falling below a minimum flow velocity or a minimum load so that partial regions of the tube walls are no longer wetted, do not occur in the case of vertical bore. In addition, no costly, associated with extensive and costly welding supporting structures for the steam boiler, since a boiler wall with vertical pipe can be designed self-supporting usually.

Furthermore, the above-mentioned tube installations can also result in convective heating, such as is present in waste heat boilers of combined cycle power plants, due to the improved heat transfer to a reduction of the heat exchanger surface and thus to significant cost savings.

With regard to the manufacturing method, the above-mentioned object is achieved by inserting a plurality of tensioned wires into a smooth tube, the wires being arranged in the form of a multi-thread, and wherein the wires are relaxed to the point of insertion until their turns abut the pipe inner wall. In other words, the multi-start helical spring formed by the pre-aligned wires is biased by being pulled apart or twisted, for example. In this reduced diameter condition, the insert is retracted into the tube. After its partial relaxation, it presses itself against the inner wall of the pipe. The remaining residual stress of the wires is chosen so that no creep can take place at the intended operating temperature of the evaporator tube. In addition, the wires are advantageously welded after their partial relaxation at least at one end to the pipe inner wall.

The advantages achieved by the invention are in particular that with the new tube inserts a flexible, usable for all pipe materials flow guidance in the pipe interior results, which can be adjusted according to the need for heat transfer improvement. Due to the design flexibility brought about by the freely configurable parameters wire diameter, number of turns of the wire arrangement, pitch angle, flank angle and sharp edge, it is possible to set a swirl profile that varies over the length of the evaporator tube and is adapted exactly to the respective local heating. The manufacturing limitations of conventional finned tubes are bypassed. Especially with new power plant developments with higher design values for the steam parameters, the production of finned tubes becomes more and more complicated due to the higher chromium content of the new materials required for higher temperatures and pressures. Here, the new spin-producing internals replace the finned tube or allow such applications in the first place.

FIG 1

einen Durchlaufdampferzeuger in vereinfachter Darstellung mit vertikal berohrter Brennkammerwand,

FIG 2

eine geschnittene Ansicht eines Dampferzeugerrohres mit einem ein drallerzeugendes Innenprofil ausbildenden Einsatz,

FIG 3

eine geschnittene Ansicht und einen Querschnitt durch ein Dampferzeugerrohr gemäß einer alternativen Ausführungsform, und

FIG 4

eine geschnittene Ansicht und einen Querschnitt durch ein Dampferzeugerrohr gemäß einer weiteren Ausführungsform.

Various embodiments of the invention will be explained in more detail with reference to a drawing. Show:

FIG. 1

a continuous steam generator in a simplified representation with vertically annotated combustion chamber wall,

FIG. 2

a sectional view of a steam generator tube with a spin forming inner profile forming insert,

FIG. 3

a sectional view and a cross section through a steam generator tube according to an alternative embodiment, and

FIG. 4

a sectional view and a cross section through a steam generator tube according to another embodiment.

Like parts are given the same reference numerals to all figures.

In Figure 1, a continuous steam generator 2 is shown schematically with a rectangular cross-section, the vertical throttle cable is formed by a Umfassungsungs- or combustion chamber wall 4, which merges at the lower end in a funnel-shaped bottom 6.

In a firing range V of the throttle cable, a number of burners for a fuel in each case an opening 8, only two of which are visible, mounted in the combustion chamber wall 4 composed of steam generator tubes 10. The vertically arranged steam generator tubes 10 are gas-tightly welded together in the firing zone V to form an evaporator heating surface 12.

Above the firing range V of the accelerator cable there are convection heating surfaces 14. Above this there is a flue gas outlet channel 16, via which the flue gas RG produced by combustion of a fossil fuel leaves the vertical throttle cable. The flowing in the steam generator tubes 10 flow medium is due to the radiant heat of the burner flames and heated by convective heat transfer from the flue gas RG and thereby evaporated. As the flow medium water or a water-steam mixture is provided in the embodiment.

In addition to the one-pull boiler shown in FIG 1 (so-called tower boiler) are of course also other boiler configurations, eg. B. in the manner of a two-pass boiler, possible. The steam generator tubes to be described below can be used in all these variants, both in the firing area and in the rest of the flue gas duct. A use in a heat recovery steam generator is conceivable.

FIG. 2 shows in a sectional view a section of a steam generator tube 10 used for the bore of the combustion chamber wall 4 of the continuous steam generator 2. In the pipe interior 18 of a smooth tube 20, an insert 22 is introduced which forms a swirl-producing inner profile to improve the heat transfer behavior. In the exemplary embodiment, the insert 22 comprises three wires 24, which run along the tube inner wall 26 in the manner of a three-start thread with a constant pitch angle (and thus with a constant pitch). As a result of their residual stress, the wires 24 are firmly against the tube inner wall 26. In addition, the wires 24 are each fixed at several points, in particular in the vicinity of their two ends, by spot welding to the tube inner wall 26.

The wires 24 are in the embodiment as well as the tube wall 28 of the female tube 20 receiving them from a highly heat-resistant metallic material with a high chromium content. In addition, of course, there are also other suitable materials which are familiar to the expert, for. Eg 13CrMo44. In addition to the number of wires 24 (number of turns of the coil spring) and the pitch angle, the cross-sectional profile of the wires 24 is an important design criterion. In particular, due to the separate from the smooth tube 20 production of the respective Wire 24 whose height and width and the flank angle relative to the pipe inner wall 26 and the sharpness of the edges are arbitrarily specified. As a first approximation, the geometrical parameters are usually chosen similar to the ribs of conventional finned tubes. In addition, however, also a location-dependent adaptation and optimization can take place, which takes into account the course of the heating profile along the combustion chamber wall 4.

3 shows a development of the known from FIG 2 embodiment of the steam generator tube 10, in which the voltage applied to the pipe inner wall 26 wires 24 are connected via welded radial stiffening struts 30 with a running along the tube axis center wire 32, so that slippage of the individual spring courses or Wire turns against each other is effectively prevented even with decreasing spring action. Since the support core comprising the stiffening struts 30 and the central wire 32 is not exposed to temperatures as high as the swirl-producing wires 24 resting on the pipe inner wall 26, it is made of a less high-quality material.

In the exemplary embodiment according to FIG. 3, in each case three of the thin radial reinforcing struts 30 are combined to form a regular star lying in a common cross-sectional plane through the steam generator tube 10. Several of these stars are arranged at regular intervals in the longitudinal direction of the steam generator tube 10 in a row. As can be seen from the cross-section through the steam generator tube 10 shown in the upper right-hand section of FIG. 3, all the stars are aligned identically, so that the mutually corresponding stiffening struts 30 of consecutively arranged stars come to lie congruently in cross-section. As a result, the swirl flow in the pipe interior 18 is disturbed only insignificantly.

Finally, FIG. 4 shows a further embodiment which is also combined with the variant known from FIG can. In this case, three parallel to the tube axis extending holding wires 34 are provided which prevent slippage of the swirl-producing, helically wound wires 24. The retaining wires 34 are viewed in cross-section evenly distributed over the inner tube circumference and each fixed to the pipe interior 18 facing side of the profiling wires 24 to the same.

Claims (14)

A steam generator tube (10) wherein at least one insert (22) is disposed in the tube interior (18) to form a spin-producing inner profile, the insert (22) comprising a plurality of wires (24) helically shaped in the manner of a multi-thread thread wind along the pipe inner wall (26). Steam generator tube (10) according to claim 1, wherein the pitch angle of the respective wire (24) relative to a reference plane oriented perpendicular to the tube axis is at least 30 ° and preferably at most 70 °. A steam generator tube (10) according to claim 1 or 2, wherein the respective wire (24) has a round cross-section. Steam generator tube (10) according to claim 1 or 2, wherein the respective wire (24) has a substantially rectangular cross-section. Steam generator tube (10) according to any one of claims 1 to 4, wherein the respective wire (24) due to its residual stress at the intended operating temperature non-slip in the pipe interior (18) sits. A steam generator tube (10) according to any one of claims 1 to 5, wherein the wires (24) abutting the tube inner wall (26) are connected to each other via radial stiffening struts (30) and / or to a center wire (32) running along the tube axis. Steam generator tube (10) according to one of claims 1 to 6, wherein a number of extending in the direction of the tube axis holding wires is provided, which are each fixed to the tube interior (18) side facing the wires (24) on the wires (24) , Steam generator tube (10) according to one of claims 1 to 7, wherein the respective wire (24) at least at one point, preferably in the vicinity of its two ends, with the tube inner wall (26) is firmly connected. A steam generator tube (10) according to claim 8, wherein the fixed connection is a welded joint. A steam generator tube (10) according to any one of claims 1 to 9, wherein at least the parts of the insert (22) abutting the tube inner wall (26) are made of a material having a composition similar to the tube material. Steam generator tube (10) according to one of claims 1 to 10, with a plurality of inserts (22), which are arranged in separate pipe sections, wherein the respective insert (22) with its geometric parameters to the provided during operation local heating and / or adapted to the local flow conditions. A continuous flow steam generator (2) comprising a number of steam generator tubes (10) formed according to any one of claims 1 to 11. A method of making a spin tube inner tube (10) having a plurality of live wires (24) inserted into a smooth tube (20), the wires (24) being arranged in a multi-threaded fashion, and wherein the wires (24) are relaxed after insertion until their turns against the tube inner wall (26). The method of claim 13, wherein the wires (24) are welded after their partial relaxation at least at one end with the tube inner wall (26).

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