EP0720714A1

EP0720714A1 - Steam boiler of once-through type and process for operating the same

Info

Publication number: EP0720714A1
Application number: EP94926796A
Authority: EP
Inventors: Wolfgang Köhler; Eberhard Wittchow
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1993-09-30
Filing date: 1994-09-19
Publication date: 1996-07-10
Anticipated expiration: 2014-09-19
Also published as: KR960705177A; JPH09503284A; CN1132548A; RU2123634C1; DE4333404A1; DE59405540D1; WO1995009325A1; EP0720714B1; US5706766A

Abstract

In order to avoid the inconvenience that results from a faulty heating of individual pipes (10) of a steam generator (2) with vertical pipes arranged in the combustion chamber, the mass flow density m in the pipes (10) is set below a predetermined maximum value (curve B) which depends on the inner diameter d of the used pipes. In order to avoid inadmissibly high pipe wall temperatures even at the proximity of a critical pressure of about 200 to 221 bars, the inner surface of each pipe (10) is further provided with a surface structure that causes a high turbulence and/or a longitudinal vortex in the flowing medium. The components of the flowing medium are thus intimately mixed so that a good heat transmission is achieved.

Description

description

Method for operating a once-through steam generator and a once-through continuous steam generator

The invention relates to a method for operating a once-through steam generator with a gas train consisting of gas-tightly welded, essentially vertically arranged tubes which are flowed through in parallel by a flow medium and through the surface structure of which is provided on the inside, a mixing of the flow medium is produced. The invention is further directed to a once-through steam generator operating according to this method.

A steam generator, the combustion chamber wall of which is constructed from vertically arranged pipes, is more economical to produce than a steam generator having a helical tube. However, the unavoidable differences in the heat supply to the individual tubes can lead to temperature differences between adjacent tubes - in particular at the outlet of an evaporator. These temperature differences can cause damage due to impermissible thermal stresses. The temperature differences can be avoided by drastically reducing the friction pressure loss. The reduction in turn is achieved by a corresponding reduction in the flow rate, i.e. the mass flow density in the pipes. In order to achieve good heat transfer even with a low mass flow density, it is e.g. known from European Patent Application 0 503 116 to use tubes which have ribs forming a multi-start thread on their inside.

In addition, cross-drawn tubes are known from German Offenlegungsschrift 20 32 891, on the inside of which a superimposed second ribbing is superimposed on a first ribbing to form a surface structure. If the combustion chamber wall of a steam generator is piped with internally finned evaporator tubes, a swirl is superimposed on the axial flow, which leads to a phase separation of the flow or heat absorption medium with a water film on the inner wall of the tube, ie on the heating surface. This allows the very good heat transfer from boiling to be maintained almost until the water has completely evaporated. In the pressure range between 200 bar and 221 bar, however, impermissibly high wall temperatures cannot always be avoided with strong heating with a swirl flow alone. In the vicinity of the critical pressure at about 210 bar - where there is only a slight difference in density between the liquid and vapor phases - it is much more difficult to ensure that the heating surface is wetted than in a pressure range below 200 bar. This is due to the fact that a vapor film forming between the tube wall and the liquid phase of the heat absorption medium hinders the heat transfer (film boiling). In this area of vapor conversion, the temperature of the tube wall rises sharply. As described in the article "Vaporizer Concepts for Benson Steam Generators" by J. Franke, W. Köhler and E. Wittchow published in VGB Kraftwerkstechnik 73 (1993), No. 4, pages 352 to 360, range above a pressure of around 210 bar already slight wall overheating in order to get from the boiling state with a wetted heating surface to film boiling with an unwetted heating surface. Even in the case of slight overheating in the superheated boundary layer, vapor bubbles can form in the pressure range mentioned, which combine to form large bubbles and thus hinder heat transfer (homogeneous nucleation).

The heat transfer mechanism described now leads to the fact that in the pipes of steam generators mentioned, which are operated with pressures of about 200 bar and above, the mass flow density and thus the friction pressure loss have to be selected higher than for steam generators which operate with pressures operated below 200 bar. This means that in the case of small internal pipe diameters, the advantage is lost that the throughput also increases when individual pipes are heated. However, since high vapor pressures above 200 bar are required in order to achieve high thermal efficiencies and thus low carbon dioxide emissions, it is necessary to ensure good heat transfer even in this pressure range. For this reason, steam generators with a vertically tube-shaped combustion chamber wall are usually operated with relatively high mass flow densities in the tubes in order to always have a sufficiently high heat transfer from the tube wall to the heat absorption medium, ie to the water / water vapor medium, in the critical pressure range from about 200 to 221 bar. Mixture to achieve. However, this only leads to an unsatisfactory temperature compensation at the outlet of the pipes with different heating.

The invention is therefore based on the object of specifying a method for operating a once-through steam generator with which small temperature differences at the outlet of adjacent steam generator pipes are achieved. This is to be achieved in a continuous steam generator, in the case of which evaporator tubes, particularly near the critical pressure of about 210 bar, a particularly good heat transfer from the tube wall or heating surface to the heat absorption medium is ensured.

With regard to the method, this object is achieved according to the invention in that the mass flow density m - based on full-load operation, ie 100% steam output - in the pipes is set as a function of the inner pipe diameter d, one determined by a pair of values of the mass flow density m and the inner pipe diameter d Operating point lies in a coordinate system between a curve B and the abscissa and where operating points correspond to the value pairs di = 10 mm at __- ^• _ = 1300 kg / m ² -s, d2 = 25 mm at __2 = 1600 kg / m ² -s and d3 = 40 mm at D3 = 1600 kg / m ^a -s lie on curve B.

The invention is based on the consideration that a flow swirl is not sufficient to ensure good heat transfer for operating points between curve B and the abscissa, especially in the vicinity of the critical pressure range above approximately 200 bar. Rather, it is also necessary to mix the flow well. In this way wall overheating can be avoided. A high level of turbulence in the flow can also prevent such large vapor bubbles from forming on the heating surface or in the superheated boundary layer that they can combine to form a vapor film and thus deteriorate the heat transfer.

With regard to the continuous-flow steam generator with a gas train formed from tubes welded to one another in a gas-tight manner, on which burners for a fossil fuel are located, the essentially vertically arranged tubes of the gas train to achieve high flow turbulence and / or

Formation of longitudinal vortices in the flow medium on their inside has a surface structure formed by two superimposed opposing ribs and are connected in parallel for the flow of the flow medium, and the opposing ribs including the same angle with the pipe axis, the object is achieved according to the invention, that the ridges delimited by the ribs are pyramid-shaped. The pyramid-shaped structure leads to a particularly favorable longitudinal vortex formation when overflowing.

Alternatively, the first ribbing of the evaporator tube of such a continuous steam generator includes an acute angle with the tube axis, while the second ribbing runs parallel to the tube axis, a flank angle formed by the first or screw-shaped ribbing with the tube wall being flatter on the inflow side than on the downstream side. The evaporator tube according to this second alternative then has a helical internal ribbing with longitudinal grooves interrupting the ribs in a production-technically simple manner. The longitudinal grooves provide tear-off edges which favor the generation of vertebrae, the formation of longitudinal vertebrae being particularly advantageously promoted by the different flank angles.

The elevations of the inner wall limited by the ribbing are advantageously at least 0.7 mm.

The different design of the surface structure on the inside of the evaporator tubes leads to the fact that the operating points correspond to the pairs of values of mass flow density m and tube inner diameter d in different areas between curve B and the abscissa.

Exemplary embodiments of the invention are explained in more detail with reference to a drawing. In it show:

1 shows a simplified representation of a steam generator with a vertically tube-shaped combustion chamber wall, FIG. 2 shows a section of a horizontal section through a vertical throttle cable,

3 shows a longitudinal section through a small section of a counter-ribbed inner rib

4 shows a section IV from FIG. 3 on a larger scale with an elevation,

Figure 5 shows another embodiment of an opposing

6 shows a detail VI from FIG. 5 on a larger scale with a pyramid-shaped elevation, FIG. 7 shows a further exemplary embodiment of an opposing one

Steam generator tube with internal fins, 8 shows a section AA from FIG. 7 on a larger scale with elevations, and FIG. 9 shows a coordinate system with curves A and B.

1 shows schematically a once-through steam generator 2 with a rectangular cross section, the vertical gas duct of which is formed by a surrounding wall 4 which merges into a funnel-shaped bottom 6 at the bottom.

In a lower region V of the gas flue there are a number of burners for a fossil fuel in each opening 8, of which only two are visible, in the enclosure composed of steam generator tubes 10 according to FIGS. 3, 5 or 7 - or combustion chamber wall 4 attached. The steam generator tubes 10 are arranged in this region V, in which they are welded together gas-tight to form an evaporator heating surface 12 (FIG. 2), and run vertically. The tubes 10 welded to one another in a gastight manner form, for example in a tube-web-tube construction or in a fin tube construction, the gas-tight combustion chamber wall 4.

Convection heating surfaces 14, 16 and 18 are located above this area V of the gas flue. Above this is a flue gas outlet channel 20, via which the flue gas RG generated by combustion of a fossil fuel leaves the vertical gas flue. The flue gas RG serves as a heating medium for the water or water-steam mixture flowing in the damper tubes 10.

The steam generator tubes 10 have on their inside

Surface structure. The steam generator tube 10 according to FIG. 3 is provided on its inside with a first rib - in the direction of the arrow 22 - which is superimposed on an opposing second rib - in the direction of the arrow 24. The opposing ribs 22 and 24, which include acute angles a and b of equal size with the tube axis M, result in a regular pattern on the inside Structure with elevations 26 on diamond-shaped base areas and depressions 28. Such an elevation with a diamond-shaped base area 30 and flattened top side 32 is shown enlarged in FIG.

In the exemplary embodiment according to FIG. 5 too, the superimposed fins 22 ', 24' include equally large, acute angles a ¹ and b ¹ with the tube axis M. The depressions 28 'are wedge-shaped, so that the elevations 26' - as can be seen in the enlarged section VI according to FIG. 6 - are pyramid-shaped. This results in inclined surfaces 33 and 34 both on the inflow side and on the outflow side. As indicated by the arrows 36 'and 38', surfaces 33, 34 overflowed at a certain angle tend to form longitudinal vortices in the wake in the overflow. This leads to thorough mixing of the boundary layer running directly on the inner wall with the core or main flow of the water / steam mixture flowing through the steam generator tube 10.

In the exemplary embodiment according to FIG. 7, the steam generator tube 10 has, in addition to a helical inner rib 22, “longitudinal grooves as depressions 28”. This first ribbing 22 "in turn encloses an acute angle a" with the tube axis M, while the second ribbing 24 "runs parallel to the tube axis M. The longitudinal grooves or depressions 28" define tear-off edges 40 which generate a vortex favor.

As shown in the enlarged section A - A according to FIG. 8, the elevations 26 "of the helical ribbing 22" with the inner tube wall 42 include a flank angle c on the inflow side and a flank angle f on the outflow side. The flank angle c on the inflow side is smaller than or equal to the flank angle f on the outflow side. This in turn favors the training of Longitudinal vortices on the outflow side, as indicated by the arrows 36 ", 38".

The heat generated by the combustion of a fossil fuel in the burners of the combustion chamber wall 4 is absorbed by the water or water-steam mixture (flow or heat absorption medium) which flows through the pipes 10 and thereby evaporates. In this case, the elevations 26, 26 ', 26 "protrude into the pipe 10 by at least H = 0.7 mm in order to ensure thorough mixing and / or swirling of the water component and the steam component of the flow medium and thus high turbulence within the pipe 10 As a result, the pipe 10 transfers the heat it has absorbed from the flue gas RG particularly well to the flow medium and is safely cooled .. In the case of a surface structure on the inside of the pipe 10 according to the exemplary embodiment according to FIG .

In order to ensure low temperature differences at the outlet of adjacent, differently heated steam generator tubes, the mass flow density m is selected according to the invention as a function of the tube inner diameter d. The mass flow density m is the mean throughput per area and time (kg / m ² -s) of all pipes 10 at full load operation, ie 100% steam output.

The mass flow density m can be represented as a function of the inner pipe diameter d in the coordinate system according to FIG. Three points of curve B are due to the value pairs di = 10 mm at __ι = 1300 kg / m ² -s, d2 = 25 mm at __2 = 1600 kg / m ² * s and d = 40 mm at _ 3 = 1600 kg / given m ² -s.

Each point in the field between curve B and the abscissa, along which the inner pipe diameter d is plotted, represents a pair of values (d / m), in which When a single tube 10 is drawn, the mass throughput or mass flow through this tube 10 increases or only drops so little that the temperature difference between adjacent tubes remains small. In order to be able to compensate for an overheating of a single pipe 10, it is necessary that the mass flow in the more heated pipe increases compared to the mass flow in pipes heated on average. This is the case in the parallel pipe system considered here, given by the vertical arrangement of the pipes 10, if the following equation is fulfilled:

This means in other words that the total pressure drop Δpg _it the betrach¬ ended pipe 10 (this is the difference between the pressure in the underlying inlet header and the pressure in the overhead outlet header or in an intermediate collector) decrease in a multi heating .DELTA.Q must if you keep the throughput M constant. M with the unit [kg / s] is the mass flow through the pipe 10. The proportions ΔPR are the friction pressure drop, Δpg the pressure drop due to the geodetic change in altitude and Δpg the pressure drop due to the acceleration of the flow, the latter being Δpg compared to the other two parts Δp ^, Δ P _Q ZU is negligible. In order to obtain an increase in the mass flow in the more heated pipe 10, it is therefore necessary that the increase in the friction pressure drop ΔpR associated with the additional heating is less than the reduction in the geodetic pressure drop Δpg caused by the additional heating caused by the additional heating. Since the friction pressure drop Δp ^ is now proportional to the reciprocal of the inner pipe diameter d, this condition applies to small inner pipe diameters d for a smaller range of mass flow density m in pipes 10 than for pipes 10 with larger inner pipe diameter d. The dashed curve A in FIG. 9 shows this relationship. If the mass flow density m in the tubes 10 lies below the curve A shown in FIG. 9, on the one hand in massively heated tubes 10 the mass flow increases compared to the value in average heated tubes 10. On the other hand there is a reliable cooling of the tubes 10 a minimum flow of electricity in the tubes 10 is necessary. Therefore, if the mass flow in the tubes 10 is selected such that the full load operating point is set above the curve A, the mass flow in the more heated tubes 10 will decrease compared to that in the average heated tubes 10.

If this reduction is small, the temperature differences between adjacent pipes also become small. This is the case when the percentage change in mass flow brought about by heating a pipe 10 is only a fraction of the percentage heating of this pipe 10. Curve B in FIG. 8 reflects the course of the mass flow density m, which is possible from this point of view.

For operating points below curve A, i.e. between the curve A and the abscissa, it is ensured that the mass flow of multi-heated pipes 10 increases. For operating points that are below curve B, i.e. between curve B and the abscissa, the mass flow in multi-heated pipes 10 does not decrease by more than 20% of the percentage of the multi-heating. For example, the additional heating of a tube 10%, the mass flow in this tube will decrease by less than 2% compared to the value of the average heated tubes 10.

Because of the particularly good heat transfer properties of the tubes 10 used, there is no need to increase the mass flow density above m = 1600 kg / m ² * s. Therefore, curve B runs horizontally from an inner tube diameter of d = 25 mm. The mass flow density m in the tubes 10 is therefore expediently selected for a given tube inner diameter d below the associated maximum value lying on the curve B. This will make the adverse consequences of incorrect heating of individual pipes 10 avoided.

The aforementioned limitation of the mass flow density to m = 1600 kg / m ² -s from an inner tube diameter of d = 25 mm is advantageously achieved by using tubes 10 which have a surface structure on the inside according to the exemplary embodiments according to FIGS. 3, 5 or 7 have. Because of this surface structure, the resulting high turbulence in the flow significantly improves the heat transfer compared to the conditions in smooth tubes.

Claims

claims

1. Method for operating a once-through steam generator with a gas flue (4) from tubes (10) which are welded to one another in a gas-tight manner, are arranged essentially vertically and flowed through in parallel by a flow medium, through the surface structure of which is provided on their side, mixing of the flow medium is produced , characterized in that the mass current density m in the tubes (10) as a function of

Inner pipe diameter d is set, an operating point determined by a pair of values of mass flow density m and inner pipe diameter d being in a coordinate system between curve B and the abscissa, and operating points corresponding to the pairs of values d = 10 mm at m-_ = 1300 kg / m ² . s, d2 = 25 mm at _-2 = 1600 kg / m ^{2 •} s, and d = 40 mm at Λ.3 = 1600 kg / m ^{2 •} s lie on curve B.

2 .. continuous steam generator for performing the method according to claim 1 with a vertical gas flue (4) formed from gas-welded tubes (10), on which there are burners for fossil fuels, the tubes (10) of the gas flue (4) are arranged essentially vertically, have a surface structure formed by superimposed opposing ribs (22, 24) on their inside to achieve a high flow turbulence and / or form longitudinal vortices in the flow medium and are connected in parallel for the flow of the flow medium and wherein the opposing ribs (22, 22 ', 24, 24') enclose equal angles (a, b, a ¹ , b ') with the pipe axis (M), characterized in that the ribs (22', 24 ') limited elevations (26') are pyramid-shaped.

3. continuous steam generator for carrying out the method according to claim 1, with a vertical gas flue (4) formed from gas-welded tubes (10), on which there are burners for fossil fuels, the tubes (10) of the gas flue (4) are arranged substantially vertically, have a surface structure formed by superimposed opposing ribs (22, 24) on their inside in order to achieve a high flow turbulence and / or to form longitudinal vortices in the flow medium and are connected in parallel for the flow of the flow medium and the first ribbing (22 ") forms an acute angle (a") with the tube axis (M) and the opposite second ribbing (24 ") runs parallel to the tube axis (M), characterized in that one of the first fluting (22 ") with the tube wall (42) flank angle (c, f) on the inflow side (36") is flatter on the outflow side te (38 ").

4. Steam generator according to claim 2 or 3, characterized in that the by the ribs (22, 22 ', 22 ", 24, 24', 24") limited increases (26, 26 ', 26 ") of the tube wall (42nd ) are at least H = 0.7 mm high.