EP0581760B1 - 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 through-steam generators with a vertical gas train consisting of essentially vertically arranged and gas-tightly welded tubes, which together form combustion chamber walls and carry burners for fossil fuels, which have an inner tube diameter and on the inside of which a multi-thread ribs with a pitch h and a fin height H are formed have and which are connected in parallel for the flow of a coolant.

Such continuous steam generators with vertical tubing of the combustion chamber walls are cheaper to produce than those with helical tubing and also have a lower water / steam side pressure loss. However, the unavoidable differences in the heat supply to the individual pipes, e.g. As a result of different degrees of slagging before and after soot blowing, temperature differences between individual pipes at the evaporator outlet can reach up to 160 ° C (European patent application 0 217 079), which cause damage due to inadmissible thermal stresses. In addition, such steam generators could previously only be designed for large unit outputs for reasons of pipe cooling. In a publication "Forced-flow boiler for sliding pressure operation with vertical combustion chamber tubing" by H. Juzie et al in VGB KRAFTWERKSTECHNIK 64, Issue 4, from page 292, a lower output limit of 500 MW is specified for steam generators with a combustion chamber with vertical tubing and hard coal tangential combustion .

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 rises when it is heated less.

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

The invention is based on the object of producing and operating continuous steam generators at low cost, thereby economically reducing the temperature differences at the evaporator outlet to permissible values and, in addition, extending the application limit for continuous steam generators with vertical tubing of the combustion chamber walls to unit outputs well below 500 MW.

und

auf der Kurve A, die stetig steigend ist und dabei ist die Gerade B durch Punkte entsprechend den Wertepaaren

und

definiert.According to the invention, this object is achieved for continuous-flow steam generators of the type mentioned at the outset in that the inner pipe diameter is a function of a quotient K and that points, determined by pairs of values from the inner pipe 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

and

on the curve A, which is continuously increasing and the straight line B is through points corresponding to the value pairs

and

Are defined.

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

An advantageous embodiment of the invention consists in that the inner pipe diameter d assigned to a quotient K deviates by at most 30% from the inner pipe diameter d associated with this quotient K on curve A.

Curves A and B are determined in such a way that the continuous steam generator can still be operated with a minimum load of 50% of full load or less in safe continuous operation without the advantages according to the invention being lost.

The design of the continuous flow steam generator according to the invention is very advantageous because it lowers the mass flow density in the pipes through which it flows and the inner pipe diameter d is determined in such a way that the share of the geodetic pressure drop in the total pressure drop forces a change in the characteristics of continuous flow evaporators, according to - starting from the design state - the mass flow rate in the individual pipe is increased when it is heated more strongly and decreases when it is heated less. This novel 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 a further advantage because, with unchanged total mass throughput due to the parallel tube system of the evaporator and while maintaining the same inner tube diameter d, the number of tubes of the combustion chamber walls of the gas flue connected in parallel in flow terms increases compared to previously common designs. This makes it possible to increase the ratio of the combustion chamber circumference to the total mass throughput and to extend the application limit for continuous steam generators with vertically piped combustion chamber walls into a power range far below 500 MW.

However, in order to ensure reliable cooling of the individual pipes, they must be ribbed on the inside. The rib geometry must be such that there is always water on the inner wall of the pipe almost in the entire evaporation area, forced by the swirl of the coolant flow, 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 continuous steam generator with a vertical gas flue 1 is surrounded by combustion chamber walls 2. The combustion chamber walls 2 consist of tubes 3 arranged vertically and next to one another, which are welded together in a gas-tight manner (FIG. 1). The tubes which are welded to one another in a gas-tight manner form, for example in a tube-web-tube 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 form a kind of multi-start thread with a pitch h and a rib height H. The inner tube diameter d of the tubes 3 is defined by the calculated diameter of the circle, which has the same area as the free cross section of the tubes 3 narrowed by the ribs 4. The inner tube diameter d and the pitch h are mutually determined by the function h Z 0, 9: Vd to cause the coolant flow to swirl sufficiently. Both h and d are used in the unit of meter.

The combustion chamber walls 2 of the vertical gas train 1 carry burner fossil fuels, not shown, which burn within the gas train 1 and thereby generate heat. The heat is absorbed by a coolant 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 0.04 times the inner pipe diameter d into the pipe 3 in order to guide the water portion of the flowing coolant on the inside of the pipe, because the swirl presses especially in the area where the water evaporates, the water still present as a liquid to the inside of a tube 3, so that the tube 3 passes the heat it absorbs well to the liquid and is thereby reliably cooled.

In order to ensure this in each case to a sufficient extent, the inner tube diameter d is not selected independently of the quotient K according to the invention. The quotient K is determined by dividing the total mass throughput (kg / s) of all pipes 3 at 100% steam output 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.

und

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

and

given.

Every point in the field between this curve A and a straight line B represents a pair of values in which the proportions of frictional pressure drop and geodetic pressure drop are in such a favorable relationship to one another - in general, the geodetic pressure drop is then greater than the friction pressure drop - that at the additional heating of a single pipe increases the mass throughput through this pipe.

• 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, an inner pipe diameter d assigned to a quotient K should be at most 10% smaller or 30% larger than the inner pipe diameter d assigned to this quotient K on curve A.

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 around 800 and 850 kg / m ² s (curve A). With inner pipe diameters d greater than 25 mm, the mass flow density increases slightly and is then a maximum of 850 and about 950 kg / m ² s (curve A).

The total pressure drop in the pipes 3, i.e. the difference between the pressure in the inlet manifold below and the pressure in the outlet manifold above, is made up of the proportions of friction pressure drop, geodetic pressure drop and acceleration pressure drop. The proportion 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 pipe 3 increases in the case of an additional heating compared to other pipes as a result of the increased volume increase of the water-steam mixture. Since all pipes of an evaporator heating surface of a once-through steam generator connected in parallel are given the same pressure drop by their coupling to a common inlet and outlet manifold, the throughput must decrease to compensate for this pressure drop portion in a more heated pipe. This decreasing throughput in connection with the stronger heating of the pipe consequently leads to greatly increased steam outlet temperatures at the pipe end compared to pipes with average or lower heating.

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

These advantages of the invention will be seen in continuous steam fired with solid fuels such as coal electric generators particularly clearly, since there is a large or small amount of heating of individual pipes due to the different soiling of the combustion chamber walls.

Claims (7)

1. A continuous flow steam generator comprising a vertical gas flue, which is formed of tube welded to one another in a gas-tight manner and on which burners for fossil fuels 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 a coolant flow, characterised in that the internal tube diameter d is a function of a quotient K, 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, the quotient K being formed of a summated mass throughput of all 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 points corresponding to pair of values

and

lying on the curve A having a steady ascending slope, and the points corresponding to pairs of values

and

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 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 minimum load in continuous flow operation is equal to or less than 50% of full load.

5. A continuous flow steam generator according to one of claims 1 to 4, characterised in that the fossil fuel is coal or another solid fuel.

6. A continuous flow steam generator according to one of claims 1 to 4, 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.

7. A continuous flow steam generator according to one of claims 1 to 6, 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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