EP0581760B2 - Continuous flow steam generator with a vertical gas flue of substantially vertically fitted pipes - Google Patents

Abstract

A once-through steam generator includes tubes together forming combustion chamber walls and carrying fossil fuel burners. The tubes are frequently provided on their inner surfaces with fins forming a multiple thread and are connected parallel to one another for conducting a coolant flow. According to the invention, the internal tube diameter is a function of a quotient, and points determined by pairs of values of the internal tube diameter and of the quotient lie in a coordinate system between a curve and a straight line. A summated mass throughput of all of the tubes at 100% steam output divided by the circumference of the gas flue in a horizontal section through the combustion chamber is used to form the quotient, and four defined points then lie on the curve which has a steady ascending slope. Application of this configuration is advantageously possible even for once-through steam generators having nominal outputs down to far below 500 MW.

Description

The invention relates to a once-through steam generator whose minimum load in continuous operation is equal to or less than 50% of the full load, with a vertical throttle cable from essentially vertical arranged and welded together gastight tubes that together form combustion chamber walls and carry burners for fossil fuels which have an inner tube diameter d and on the inside have a multi-thread forming ribs with a pitch h and a rib height H. and which are connected in parallel for the flow of a coolant.

Pass-through steam generators of this type with vertical tubing of the combustion chamber walls are compared to such with helical tubing cheaper to manufacture and also have a lower water / steam side pressure loss. However, the unavoidable differences in heat input can to the individual pipes, e.g. due to different degrees of slagging before and after soot blowing, lead to temperature differences between individual tubes at the evaporator outlet up to 160 ° C (European Patent application 0 217 079), which cause damage due to inadmissible thermal stresses. In addition, such steam generators have so far only been possible for large pipe cooling reasons Unit services are carried out. In a publication "Forced-flow boiler for sliding pressure operation with vertical combustion chamber tubing "by H. Juzie et al in VGB KRAFTWERKSTECHNIK 64, booklet 4, from page 292, is for steam generators with a combustion chamber with vertical tubes and hard coal tangential combustion a lower power limit of 500 MW is specified.

From this publication it also emerges that the mass flow density of the coolant in the tube, in addition to the tube inner diameter, is a determining variable for the fluidic design of the parallel tube system, which acts as an evaporator heating surface. Typical mass flow densities for helical tubing of the combustion chamber with smooth tubes on the inside are between 2000 and 3000 kg / m ² s, for vertical tubing with internally finned tubes between 1500 and 2000 kg / m ² s . With these design parameters, the proportion of the friction pressure drop in the total pressure drop of the once-through evaporator is very high. Evaporators of this type therefore have a typical characteristic, according to which - starting from the design state - the mass flow rate in the individual tube decreases when it is heated more strongly and increases when it is weaker heated.

This characteristic is a cause for larger temperature differences between individual pipes on Evaporator outlet on throttle cables with vertically arranged pipes. To reduce these temperature differences it is known to install throttles at the evaporator inlet and / or in the upper part of the combustion chamber walls to arrange mix collectors outside the throttle cable, into which the pipes open and into which one certain enthalpy compensation takes place by mixing. With unit outputs below 500 MW is with so far continuous steam generators designed for the combustion chamber walls provided a helical tubing in order to maintain the mass flow density in the tubes necessary for cooling the smooth tubes to be able to and to achieve a certain heating compensation with the large pipe length. This However, measure leads to higher production costs of the once-through steam generator and requires proportionately large feed pump capacities due to the high pressure drop that occurs.

The invention has for its object to produce and to pass steam generators inexpensively operate, the temperature differences at the evaporator outlet in an economical manner to allowable Reduce values and also the application limit for continuous steam generators with vertical Extend the combustion chamber walls to unit outputs well below 500 MW.

d₁ = 12,5 mm bei K₁ = 3 kg/s m

d₂ = 20,4 mm bei K₂ = 7 kg/s m,

d₃ = 30,6 mm bei K₃ = 13 kg/s m
und

d₄ = 39,0 mm bei K₄ = 19 kg/s m
auf der Kurve A, die stetig steigend ist und dabei ist die Gerade B durch Punkte entsprechend den Wertepaaren

d₅= 14,3 mm bei K₅ = 1,8 kg/s m
und

d₆ = 38,4 mm bei K₆ = 7,6 kg/s m
definiert.

According to the invention, this object is achieved for continuous steam generators of the type mentioned at the outset in that the inner tube diameter d is a function of a quotient K and that points, determined by pairs of values from the inner tube diameter d and the quotient K, lie in a coordinate system between a curve A and a straight line B. To form the quotient K, the total mass flow rate M of all pipes at 100% steam output is divided by the circumference of the gas flue in a horizontal section, measured on the connecting lines of the pipe centers of neighboring pipes. There are points corresponding to the pairs of values

d ₁ = 12.5 mm at K ₁ = 3 kg / sm

d ₂ = 20.4 mm at K ₂ = 7 kg / sm,

d ₃ = 30.6 mm at K ₃ = 13 kg / sm
and

d ₄ = 39.0 mm at K ₄ = 19 kg / sm
on the curve A, which is continuously increasing and the straight line B is through points corresponding to the value pairs

d ₅ = 14.3 mm at K ₅ = 1.8 kg / sm
and

d ₆ = 38.4 mm at K ₆ = 7.6 kg / sm
Are defined.

According to expedient configurations of the once-through steam generator according to the invention, the slope is h in m of the ribs forming a multi-start thread on the inside of the tubes at most equal 0.9 times the root of the pipe inside diameter d in m and the fin height H is at least 0.04 times the pipe inside diameter d.

An advantageous embodiment of the invention is that the one assigned to a quotient K Pipe inside diameter d by at most 30% of the one belonging to this quotient K on curve A. Inner pipe diameter d deviates.

Curves A and B are determined so that the once-through steam generator still with a minimum load of 50% of the full load or less can be operated in safe continuous operation without the invention Benefits are lost.

The inventive design of the once-through steam generator is very advantageous because of it the mass flow density in the tubes through which flow has been reduced so far and the tube inner diameter d so are determined that the share of the geodetic pressure drop in the total pressure drop is a change in Characteristics of continuous flow evaporators, according to the - based on the design state - the Mass throughput in the single pipe is increased with its stronger heating and with its weaker heating goes back. This new characteristic leads to a significant equalization of the steam and thus the tube wall temperatures at the outlet of the combustion chamber walls forming the evaporator heating surface.

The lowering of the mass flow density in the evaporator tubes has another advantage, because at unchanged total mass flow through the parallel pipe system of the evaporator and while maintaining same tube inner diameter d the number of tubes of the combustion chamber walls connected in parallel in terms of flow of the throttle cable compared to previous designs increased. This makes it possible Increase the ratio of the combustion chamber circumference to the total mass throughput and the application limit for continuous steam generators with vertically tube-shaped combustion chamber walls in a performance range up to far expand below 500 MW.

However, to ensure reliable cooling of the individual pipes, they must be finned on the inside his. The fin geometry must be such that almost the entire evaporation area is forced due to the swirl of the coolant flow, there is always water on the inner pipe wall and thus the risk of film evaporation is eliminated.

FIG 1 einen Ausschnitt aus einem horizontalen Schnitt durch einen vertikalen Gaszug und

FIG 2 einen Längsschnitt durch ein einzelnes Rohr;

FIG 3 ein Koordinatensystem mit Kurven A und B.

The design of continuous steam generators according to the invention is explained in more detail with reference to a drawing. In detail show:

1 shows a section of a horizontal section through a vertical throttle cable and

2 shows a longitudinal section through a single tube;

3 shows a coordinate system with curves A and B.

A once-through steam generator with a vertical throttle cable 1 is surrounded by combustion chamber walls 2 Combustion chamber walls 2 consist of tubes 3 arranged vertically and side by side, which are gas-tight with one another are welded (Figure 1). The tubes, which are welded together gas-tight, form, for example, in one Pipe-web-pipe construction or in a fin tube construction a gas-tight combustion chamber wall 2.

According to FIG. 2, the tubes 3 have ribs 4 on their inside, which have a kind of multi-start thread form a slope h and have a rib height H. The inner tube diameter d of the tubes 3 is defined through the calculated diameter of the circle, which has the same area as that through the ribs 4 restricted free cross-section of the pipes 3. Determine the pipe inside diameter d and the pitch h each other through the function h ≦ 0.9 .√d to reduce the flow of the coolant into a sufficiently large To put a twist. Both h and d are used in the unit of meter

The combustion chamber walls 2 of the vertical gas flue 1 carry burners for fossil fuels, not shown, which burn inside the throttle cable 1 and thereby generate heat. The heat is from a coolant added, which flows through the tubes 3 forming the combustion chamber walls 2 and evaporates in the process Normally, appropriately treated water is used as the coolant. The ribs 4 protrude at least by 0.04 times the inner pipe diameter d into the pipe 3 by the water content of the flowing Lead coolant on the inside of the tube, because the twist squeezes especially in the area in where the water evaporates, the water still present as a liquid on the inside of a pipe 3, so that the tube 3 passes the heat it absorbs well to the liquid and thereby is safely cooled.

In order to ensure this in each case to a sufficient extent, the inner pipe diameter d is according to the Invention not chosen independently of the quotient K. The quotient K is summed by dividing the sum Mass flow rate (kg / s) of all pipes 3 at 100% steam output determined by the circumference (m) of the throttle cable 1. The circumference of the throttle cable 1 is measured along a line 5 shown in broken lines in FIG connects the pipe centers of the individual neighboring pipes 3 with each other.

d₁ = 12,5 mm bei K₁ = 3 kg/s m,

d₂ = 20,4 mm bei K₂ = 7 kg/s m,

d₃ = 30,6 mm bei K₃ = 13 kg/s m
und

d₄ = 39,0 mm bei K₄ = 19 kg/s m
gegeben.

In the coordinate system according to FIG. 3, the inner pipe diameter d can be represented as a function of the quotient K. Four points of a curve A are through the pairs of values

d ₁ = 12.5 mm at K ₁ = 3 kg / sm,

d ₂ = 20.4 mm at K ₂ = 7 kg / sm,

d ₃ = 30.6 mm at K ₃ = 13 kg / sm
and

d ₄ = 39.0 mm at K ₄ = 19 kg / sm
given.

Each point in the field between this curve A and a straight line B represents a pair of values at which the proportions of frictional pressure drop and geodetic pressure drop in such a favorable relationship to each other stand - in general then the geodetic pressure drop is greater than the friction pressure drop - that the mass flow rate through this tube increases when heating a single tube.

d₅ = 14,3 mm bei K₅ = 1,8 kg/s m
und

d₆ = 38,4 mm bei K₆ = 7,6 kg/s m

bestimmt ist. Erfindungsgemäß liegen damit die aus Rohrinnendurchmesser d und Quotienten K gebildeten Wertepaare zwischen den Kurven A und B des Koordinatensystems nach Figur 3.Reliable cooling of the pipes therefore does not allow any choice of the inner pipe diameter d given a quotient K. For this reason, the field is limited to value pairs that usually occur in practice by a straight line B, which is defined by the points corresponding to the value pairs

d ₅ = 14.3 mm at K ₅ = 1.8 kg / sm
and

d ₆ = 38.4 mm at K ₆ = 7.6 kg / sm

is determined. According to the invention, the pairs of values formed from the pipe inside diameter d and quotient K lie between curves A and B of the coordinate system according to FIG. 3.

In the case of particularly unfavorable heating conditions, a pipe inside diameter assigned to a quotient K should be used d at most 10% smaller or 30% larger than the quotient K on curve A assigned pipe inside diameter d.

By determining the size of the pipe inside diameter d in the specified manner, 3 flow conditions are forced in the pipes, in which a portion of the pressure drop caused by friction is in a favorable relationship to the geodetically caused portion of the pressure drop in the total pressure drop, both at full load - as well as in partial load operation, up to a partial load of 50% of full load and below. As a result of the dimensions of the pipes 3 and of the throttle cable 1 which are coordinated with one another in accordance with the invention, these favorable conditions are ensured by a relatively low flow rate of the coolant in the axial direction, based on the mass of the coolant, and at the same time a strong swirl movement thereof. This flow rate, expressed as mass flow density, is at 100% steam output for the pipes up to a pipe inside diameter d of 25 mm at a maximum of about 800 and 850 kg / m ² s (curve A). With inner pipe diameters d greater than 25 mm, the mass flow density increases somewhat and is then at a maximum of 850 and about 950 kg / m ² s (curve A).

The total pressure drop in the tubes 3, i.e. the difference between the pressure in the bottom Inlet collector and the pressure in the outlet collector at the top is made up of the proportions Frictional pressure drop, geodetic pressure drop and acceleration pressure drop. The share of the acceleration pressure drop is 1 to 2% of the total pressure drop and can therefore be neglected here.

The drop in frictional pressure of an individual tube 3 increases in the case of an existing tube compared to other tubes More heating due to the increased volume of the water-steam mixture. Because everyone pipes connected in parallel of an evaporator heating surface of a once-through steam generator through their coupling the same pressure drop is given to a common inlet and outlet manifold is to compensate for this pressure drop in a more heated pipe, the throughput go back. This decrease in throughput leads to the increased heating of the pipe consequently too high steam outlet temperatures at the pipe end compared to average or weaker heated pipes.

The geodetic pressure drop of a single pipe 3, however, decreases when this pipe is heated more compared to other pipes due to increased steam formation, because the water-steam column becomes lighter. The Throughput through the multi-heated pipe therefore increases due to this effect until the sum of increased Frictional pressure drop and decreased geodetic pressure drop due to the coupling via inlet and Outlet collector reaches predetermined pressure drop. This increase in throughput is desirable to keep the steam outlet temperature at the end of the pipe low despite the additional heating. This according to the invention the reason for the change is the comparatively large influence of the geodetically caused pressure drop the characteristic of the once-through steam generator towards a behavior in which larger temperature differences at the tube end of the evaporator are avoided because a stronger heating of an individual Pipe is largely compensated for by a higher throughput of the coolant.

These advantages of the invention will be seen in continuous steam generators fired with solid fuels such as coal particularly clear, because there the different pollution of the combustion chamber walls the over or under heating of individual pipes is very large.

Claims (6)

1. A continuous flow steam generator, the minimum load of which in continuous flow operation is equal to or less than 50% of full load, comprising a vertical gas flue, which is formed of tubes welded to one another in a gas-tight manner and on which burners for fossil fuel are arranged, the tubes of the gas flue being essentially vertically disposed, having an internal tube diameter d, carrying fins on their inner surface forming a multiple thread and being connected in parallel for conducting a coolant flow, characterised

   in that the internal tube diameter d is a function of a quotient K,

   in that points determined by pairs of values of said internal tube diameter d and of said quotient K are located in a coordinate system between a curve A and a straight line B,

   with the quotient K being formed of the summated mass throughput of all the tubes at 100% steam output, divided by the circumference of the gas flue in a horizontal section, measured on lines connecting the tube centres of adjacent tubes, and

   with points corresponding to the pairs of values

   d₁ = 12.5 mm at K₁ = 3 kg/s m,

   d₂ = 20.4 mm at K₂ = 7 kg/s m,

   d₃ = 30.6 mm at K₃ = 13 kg/s m
   and

   d₄ = 39.0 mm at K₄ = 19 kg/s m

   lying on the curve A having a steadily ascending slope, and

   with the points corresponding to the pairs of values

   d₅ = 14.3 mm at K₅ = 1.8 kg/s m
   and

   d₆ = 38.4 mm at K₆ = 7.6 kg/s m

   lying on the straight line B.
2. A continuous flow steam generator according to claim 1, characterised in that a pitch h (given in the measurement unit "metres") of the fins in the tubes is at most equal to 0.9 times the square root of the internal tube diameter d (given in the measurement unit "metres"), and in that a height H of the fins forming the thread is at least equal to 0.04 times said internal tube diameter d.
3. A continuous flow steam generator according to claim 1 or 2, characterised in that the internal tube diameter d associated with a quotient K is at most 10% smaller than and at most 30% greater than the internal tube diameter d associated with said quotient K on the curve A.
4. A continuous flow steam generator according to one of claims 1 to 3, characterised in that the fossil fuel is coal or another solid fuel.
5. A continuous flow steam generator according to one of claims 1 to 3, characterised in that the electric power of the power station block, in which the continuous flow steam generator is incorporated, is significantly less than 500 MW.
6. A continuous flow steam generator according to one of claims 1 to 5, characterised in that a mass flow density in the tubes (3) is at most in a range from about 800 to 850 kg/m²s when the internal tube diameter d is up to 25 mm and is at most in a range from about 850 to about 950 kg/m²s when said internal tube diameter is greater than 25 mm.

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