DE3544504A1 - Combustion chamber/pipe arrangement in forced-through steam generators - Google Patents

Abstract

In this pipe arrangement, the wall pipes stand vertically (2a) in the burner region and, in the upper part of the combustion chamber, they run obliquely in the form of a helical (spiral) or stepped coil (2b), the ratios being selected in such a manner that the beginning of evaporation lies to as great an extent as is possible in the vertical pipes at all loads. In the convection duct, the spiral pipes become vertical pipes once more (Fig. 2). <IMAGE>

Description

Combustion chamber tube arrangement in once-through steam generators

For the production of the combustion chamber and the convection duct of Nowadays, gas-tight membrane tube walls come into question for larger steam generators. They are used as evaporators and consist of tubes that are held by sheet metal webs are welded together. They are suspended in a shoring so that they can expand downwards unhindered when heated.

For once-through boilers, two main types are known, which are shown schematically in Fig. 1 are shown: - Variant a: pure forced flow according to the Benson-Sulzer principle - Variant b: forced throughfeed with superimposed circulation In variant a, the Pipes (2) in the combustion chamber area (1) of the membrane tube walls at an angle in the form of a spiral or screw winding (spiral, screw and stair winding is to understand according to Fig. 3). In variant b, the tubes (2) are vertical. In The membrane tube walls of the convection channel (5) are the same in both variants Way executed with vertical tubes.

The supercooled feed water (6) flows to the distributor header (7) on Combustion chamber floor. There are collectors at the upper edge of the convection channel (5) (8), from which the water-steam mixture reaches the water separator (9).

For cooling the combustion chamber tube walls and for film evaporation in To prevent the pipes from entering, a minimum mass flow density is required. These with variant a during start-up and low-load operation (under 40 %) by stirring or overfeeding. With variant b, at full load, water is recirculated via a circulation pump (10).

Disadvantages of the known types: With variant a, the Tube bends for burner openings, especially with coal firing, constructive difficulties in addition to pipe length differences. Furthermore, the flow resistances or the power losses due to the large pipe lengths and mass flows of the spiral pipes great. In terms of strength, pipes arranged at an angle are due to their own weight loaded in the tangential direction and poorly used. You therefore need mechanical support elements in the vertical direction.

In variant b, the individual vertical tubes are given because of the Heating profiles across the wall width uneven heat absorption, which large Can cause temperature differences in the combustion chamber wall tubes. To reduce Due to the temperature differences, a greater pressure drop in the equalization throttling is necessary. Because of the shorter pipes and the correspondingly low flow rates, there is one Circulation of the water over the entire load range is necessary for film evaporation to prevent.

The object of the invention is to achieve advantageous properties and to overcome the disadvantages. The essential features of the invention result from the claim. The following is an embodiment of the invention described with reference to FIG.

In that area of the combustion chamber above the funnel (3), which is in the is essentially determined by the burner, the tubes are the membrane tube walls vertically (2a) and in the upper part of the combustion chamber area after the start of evaporation they run obliquely in the form of a helical or step winding (2b).

In the convection channel (5) the tubes of the membrane walls are the same as in the known variants a and b vertically. In the area of the funnel (3) you can the pipes can be made in any number and arrangement.

The supercooled feed water (6) is in the area of the funnel (3) and the vertical tubes (2a) of the combustion chamber wall are preheated to the saturation temperature. The point of the start of evaporation should be subcritical for all Operating areas if possible in the vertical tubes, but close to the transition into the screw winding, lie.

This steam generator variant can be used for different types of firing be used and is particularly suitable for lignite firing, which with large burner openings and relatively low heat load.

The new variant can be used for steam generators with subcritical or supercritical Pressure, in tower or two-pass construction and as a forced flow system with overfeed can be used for start-up and low load or with superimposed circulation.

Advantageous properties can be achieved with the new steam generator variant achieve, which are only partially found in variant a or b. These are explained in detail below: 1. Burner openings During the tube bends for large burners with the spiral tubes of variant a, constructive difficulties cause, the production of large openings happens in the new variant exclusively or mainly in an area with vertical tubes of the combustion chamber problem-free and with little effort.

2. Pipe length differences By avoiding pipe bends for the burner openings, as they are necessary in the case of variant a Pipe length differences are practically avoided, especially with large burner openings will.

3. Prevention of film evaporation in the upper part of the combustion chamber, i.e. in the area of actual evaporation and the greater heat flow density, film evaporation can occur, which severely impedes heat transfer.

Because of the transition to the inclined position of the pipes, only a small one can Number of pipes to be arranged t e.g. two or more vertical pipes go into a spiral tube). The flow cross-section of the is accordingly reduced entire evaporator tubes and there is a larger mass flow in a spiral tube, whereby the film evaporation is substantially prevented in the load area.

The advantage of the new variant compared to the known variant b lies in the recirculation of the water to prevent film evaporation is not necessary over the entire load range.

During start-up and low-load operation, this new variant requires as with variant a, the use of a combined system with overfeed or Circulation.

in extremely unfavorable cases such as large steam generators, Oil firing, low subcooling of the preheated feed water, can result in overfeeding or overturning are necessary with low loads, which are also slightly above the 40th % Load lies, so that a possible film evaporation before the transition to the spiral tubes is prevented.

In the area of vertical pipes, internally grooved pipes can also be used in extreme cases Pipes are used.

4. Pressure drop Since the tubes are in the lower part of the combustion chamber area stand vertically, a large number of tubes with a smaller mass flow per tube can be used and the smallest pipe lengths.

With the same design of the wall tubes in the upper part of the combustion chamber and in the convection channel, as in variant a, there is a smaller pressure drop between the inlet and outlet collector of this new variant, which is approx. 30% lower lies than with variant a.

Although the upper part of the combustion chamber in variant b is shorter vertical tubes and in the new variant with longer spiral tubes is, the pressure drop is between the inlet and outlet headers of the new variant also below that of variant b, since this has an overlaid circulation is necessary over the entire load range. In addition, variant b needs one greater pressure drop of the equalizing throttling around the flow of the heating cross profile adapt.

5. Adjustment of the heating transverse profile in the case of an inclined fire position, i.e. if the burners in operation are unevenly around the circumference of the combustion chamber are distributed, there are greater heat loads in certain combustion chamber areas.

But even with a symmetrical fire position it is possible to use a horizontal one Calculate the heating profile across the width of the wall (corners get less heat). Since there is a relatively higher heat load in the upper part of the combustion chamber it is advisable - to make the wall pipes helical in this area. Thus the heating cross profile, or an inclined fire position, is essentially balanced. The heating profiles in the lower combustion chamber area have no effect large differences in the heat flows to the individual vertical tubes. The new Steam generator variant requires a smaller equalizing throttling of the individual Pipe groups and thus avoids a large throttle pressure drop and temperature differences across the wall width and the load range in comparison with variant b.

6. Sensitivities to heating disturbances Are individual pipe groups (especially in the area of the vertical tubes of the combustion chamber) by an oblique Firing position, slag formation or other heating disturbances heated more strongly, like this the starting point of evaporation shifts downwards and the geodetic pressure drop decreases or the lift increases. The negative change in the geodetic Pressure drop compensates or outweighs the positive change in the friction pressure drop. Stronger heated pipes can get more water as a result. The maximum use of this The effect is achieved when the starting point of evaporation is in the area of the vertical Pipes shifts, while this is weakened by the factor (are) in the case of spiral pipes occurs. The new steam generator variant has good flow sensitivity of the single tubes. The larger mean mass flow in the individual tube of the spiral area (2b) allows a weaker enthalpy and temperature sensitivity in the case of single pipe disturbances.

The new steam generator variant has favorable static sensitivities which are not in one of the two known steam generator variants at the same time a or b are concentrated.

With regard to the formation of slag and its self-cleaning, it is therefore it is advisable to place the wall pipes according to the new variant in the lower half the combustion chamber with vertical tubes.

7. Strength ratios The combustion chamber tubes are by means of sheet metal webs welded together lengthways and are less resilient in the tangential direction as pipes without a weld seam. It is also known that the pipe wall tension by Internal pressure in the longitudinal direction is half as great as in the tangential direction (sJt = 2 L = 9t). The welded pipe walls bear their own weight with the water content and the ash residue. These additional loads apply to vertical pipes in the axial direction in which the pipes have the most load-bearing capacity, while they are in the case of spiral pipes fall in the tangential direction in which the pipes are weakest. That's why the spiral tubes require mechanical support elements in the vertical direction. However, such support is not necessary in the area of vertical tubes. at of the new steam generator variant are only in the small area of the spiral pipes Support elements required in the vertical direction.

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Claims (1)

PATENT CLAIM Combustion chamber tube arrangement in once-through steam generators, characterized in that the tubes (2) in the burner area (4) of the combustion chamber (1), above the funnel (3), vertically (2a) and in the upper part of the combustion chamber after the area where evaporation begins, inclined (2b) in the form of a spiral, Helical or stair winding run (Fig. 2).