WO1995009324A1

WO1995009324A1 - Internally ribbed tube for a steam generator, and a steam generator using such tubes

Info

Publication number: WO1995009324A1
Application number: PCT/DE1994/001049
Authority: WO
Inventors: Wolfgang Köhler; Eberhard Wittchow
Original assignee: Siemens Aktiengesellschaft
Priority date: 1993-09-30
Filing date: 1994-09-13
Publication date: 1995-04-06
Also published as: JPH09503046A; EP0720713A1; KR960705176A; CN1132549A

Abstract

In order to avoid film boiling in steam generators, internally ribbed tubes are used in which a first ribbing pattern (2) is superimposed on a second ribbing pattern (4) running in the opposite direction. In order to produce additional turbulence to improve the mixing in the flow, and hence the transfer of heat to the heat-absorbing medium (water, mixed water/steam), the invention proposes that the opposite ribbing patterns (2, 4) have different rib heights (s, s').

Description

description

Steam generator tube with internal fins and steam generator for its use

The invention relates to a steam generator tube with internal ribbing, in the manner of intersecting trains of a first ribbing, an opposing second ribbing is superimposed. It continues to focus on the use of such a steam generator tube in a fossil-fired steam generator.

A steam generator, the combustion chamber wall of which is constructed from vertically arranged tubes, is more cost-effective 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 inadmissible 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 velocity or the mass flow density in the tubes. However, the use of internally finned pipes is necessary for this, since these have particularly good heat transfer properties even at low mass flow densities. Pipes of this type with fins forming a multi-start thread on the inside and their use in steam generators are known, for example, from B. from European patent application 0 503 116 known. A steam generator tube, in which a second ribbing is superimposed on one another in the manner of crossing trains, is known from German Offenlegungsschrift 20 32 891.

When 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 recording medium with a film of water on the inner wall of the pipe, ie on the heating surface. As a result, the very good heat transfer during boiling can be maintained almost until the water evaporates completely. In the pressure range between 200 bar and 221 bar, however, inadmissibly 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 wetting of the heating surface 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 the steam transformation, the temperature of the pipe 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 Kraftwerk¬technik 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 heated 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 aforementioned pipes of steam generators

Pressures of about 200 bar and above are operated, the mass flow density and thus the friction pressure loss must be chosen higher than for steam generators that are operated with pressures below 200 bar. As a result, especially in the case of small inner pipe diameters — the friction pressure loss is inversely proportional to the inner pipe diameter — the advantage is lost that if individual pipes are heated more than once their throughput also increases. However, since high vapor pressures above 200 bar are required 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 internally finned 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 mixture, in the critical pressure range of approximately 200 to 221 bar , to reach. In order to further improve the heat transfer, an additional mixing of the flow in the heat absorption medium can be brought about. This is usually achieved in that the internally finned steam generator tubes have double fins, a first fins being superimposed on opposite fins. These methods, however, can only achieve an unsatisfactory temperature compensation at the outlet of the pipes with different heating of the steam generator.

The invention is therefore based on the object of designing the inside of the steam generator tubes, in particular the evaporator tubes of a steam generator, in such a way that particularly close to the critical pressure of about 210 bar, a particularly good heat transfer from the tube wall or Heating surface on the heat absorption medium, especially at low mass flow densities, is possible.

For such steam generator tubes with internal fins, this object is achieved according to the invention in that the profile depths of the two fins are different.

The resultant surface structure on the inside of the tube favors high turbulence and / or the formation of longitudinal vortices in the medium flowing through the tubes. The additional edges created by the different profile depths of the ribs act when Flow through the heat absorption medium increases turbulence. In other words: as a result of the edge formation, vortex generation is promoted, which in turn brings about a good mixing of the flow, so that overheating of the tube wall is avoided.

The invention is based on the consideration that - especially in the vicinity of the critical pressure range above about 200 bar - a flow swirl is not sufficient to ensure good heat transfer. Rather, in addition to a good mixing of the flow to avoid wall overheating of the boundary layer, high turbulence in the flow is necessary. A high level of turbulence in the flow prevents 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 worsen the heat transfer.

The opposing fins can include the same or different angles with the pipe axis. The first fins advantageously form an acute angle with the pipe axis, while the second fins run parallel to the pipe axis. The steam generator tube then has, in a technically simple manner, a helical ribbing with longitudinal grooves interrupting the ribs.

The longitudinal grooves define tear-off edges that favor the generation of eddies. The formation of longitudinal vortices is advantageously also favored in that a flank angle formed by the first or helical ribbing with the tube wall is flatter on the inflow side than on the outflow side.

To use steam generator pipes designed according to the invention in a fossil-fired steam generator, a large number of similar steam generator pipes are advantageously used to form a gas-tight combustion chamber wall which covers at least a part of the Steam generator forms, in particular its evaporator heating surface, welded to one another, the steam generator tubes advantageously being arranged vertically.

Embodiments of the invention are explained in more detail with reference to a drawing. Show:

FIG. 1 shows a longitudinal section through a small section of a steam generator tube with opposing internal ribs,

FIG. 2 shows a detail II from FIG. 1 on a larger scale with an elevation,

3 shows another exemplary embodiment of a steam generator tube having opposed internal ribs, FIG. 4 shows a section IV-IV from FIG. 3 on a larger scale with elevations, and

Figure 5 in a simplified representation of a steam generator with a vertically tube-shaped combustion chamber wall.

The inside of the steam generator tube is provided with a first rib - in the direction of arrow 2 - which is superimposed on an opposing second rib - in the direction of arrow 4. The opposing ribs 2 and 4, which enclose the angles a and b with the tube axis M, result in a regular structure on the inside with elevations 6 on parallelogram-shaped or rhombic bases and depressions 8, 8 '. Such an elevation 6 with a parallelogram-shaped base area 10 and flattened upper side 12 is shown enlarged in FIG. 2. In the exemplary embodiment, the angle a is smaller than the angle b.

As can be seen from the enlarged section II shown in FIG. 2, the depressions 8 and 8 'are of different sizes due to different profile depths s, s' of the ribs 2, 4. The depressions 8 of the first ribs 2 are thus with the elevations 6 'of the opposing second ribbing 4.

Due to the different profile depths s, s ', additional edges 22, 22' are created, which have a turbulence-increasing effect when flowing over them through a heat absorption medium and thus contribute to improving the heat transfer.

In the exemplary embodiment according to FIG. 3, the steam generator tube has 2 "longitudinal grooves as depressions 8" in addition to a helical internal ribbing. This first ribbing 2 "in turn encloses an acute angle a" with the tube axis M, while the second ribbing 4 "runs parallel to the tube axis M. The longitudinal grooves or depressions 8" in turn provide additional tear-off edges 22 which promote vortex generation.

As shown in the enlarged section IV-IV according to FIG. 4, the elevations 6 "of the helical ribbing 2" with the inner tube wall 24 include a flank angle c on the inflow side and a flank angle d on the outflow side. The flank angle c on the inflow side is smaller than or equal to the flank angle d on the outflow side. This favors the formation of longitudinal vortices on the outflow side, as indicated by the arrows 16 ", 18".

The elevations 6, 6 ", that is to say the profile depths s, ε ', advantageously amount to at least H = 0.7 mm.

FIG. 5 schematically shows a steam generator 30 with a rectangular cross section, the vertical throttle cable of which is formed by a surrounding wall 32 which merges into a funnel-shaped bottom 34 at the lower end.

In a lower region B of the gas train, a number of burners for a fossil fuel are each in an opening. tion 36, of which only two are visible, in the enclosure or combustion chamber wall 32 composed of the steam generator tubes according to FIGS. 1, 3 or 5. The steam generator tubes 38 are arranged in this region B, in which they are welded together gas-tight to form an evaporator heating surface 40, in a vertically extending manner.

Convection heating surfaces 42, 44 and 46 are located above this area B of the gas flue. Above this is a flue gas outlet channel 48, 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 steam mixture flowing in the steam generator tubes 38.

Claims

claims

1. steam generator tube with internal fins, a cross-shaped second fins (4) being superimposed in the manner of intersecting trains of first fins (2), characterized in that the profile depths s, s' of the fins (2 or 4) are different.

2. Steam generator pipe according to claim 1, so that the first fins (2 ") enclose an acute angle a" with the pipe axis M and the opposite second fins (4 ") runs parallel to the pipe axis M.

3. Steam generator tube according to claim 1 or 2, characterized in that one of the first fins (2 ") with the tube inner wall (24) is formed flank angle c, d on the inflow side (16") is flatter than on the outflow side (18 " ).

4. Fossil-heated steam generator with an internally finned steam generator pipes according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that a

A large number of similar steam generator tubes (38) are welded together to form a gas-tight combustion chamber wall (32).

5. Fossil-fired steam generator according to claim 4, d a d u r c h g e k e n n z e i c h n e t that the

Steam generator tubes (38) are arranged at least approximately vertically.

6. Fossil-fired steam generator according to claim 4 or 5, d a d u r c h g e k e n n z e i c h n e t that its

Evaporator heating surface (40) is composed of steam generator tubes (38).