CA2243993A1 - Continuous steam generator - Google Patents

Abstract

The invention concerns a continuous steam generator (2) having a combustion chamber (4) with vertically extending pipes (12) which have a surface structure (26) on the interior. A flow medium (S) flows upwards through the pipes (12). According to the invention, a particularly advantageous mass flow density m in the pipes (12), at a load at which critical pressure (pcrit) prevails therein, corresponds, according to the invention, to the relation (7).

Description

~ CA 02243993 1998-07-23 ~ ,~, o~ ., 0, ~,0 ,~,f Description Once-through steam generator and method for designing a once-through steam generator The invention relates to a once-through steam generator with a combustion chamber surrounded by a containment wall composed of tubes connected to one another in a gastight manner, a flow medium being capable of flowing from the bottom upward through the vertically extending tubes which have a surface structure on their inside. It relates, further, to a method for designing a once-through steam generator of this type.
A steam generator of this type is known from the paper "Verdampferkonzepte f~r Benson-Dampferzeuger"
["Evaporator concepts for Benson steam generators"] by J. Franke, W. Kohler and E. Wittchow, published in VGB
Kraftwerkstechnik 73 (1993), No. 4, pages 352 to 360.
In a once-through steam generator of this type, in contrast to a natural circulation or forced circulation steam generator with only partial evaporation of the water/steam mixture, the heating of evaporator tubes forming the combustion chamber leads to the complete 2S evaporation of the flow medium in the evaporator tubes - in a single pass. Whereas, in the natural circulation steam generator, the evaporator tubes are basically arranged vertically, the evaporator tubes of the once-through steam generator may be arranged both vertically a!nd spirally, hence at an inclination.

once-through steam generator, the combustion chamber ~alls of which are composed of vertically arranged evaporator tubes, can be produced more cost-effectively than a once-through steam generator having spiral tubing. Furthermore, once-through steam generators with AMENDED SH~T

- ~ CA 02243993 1998-07-23 vertical tubing have lower water-side/steam-side pressure losses than those with evaporator tubes which are inclined or are arranged so as to ascend spirally.
Furthermore, in contrast to a natural circulation steam generator, a once-through steam generator is not subject to any pressure limitation, so that fresh steam pressures well above the critical pressure of water (PCrit= 221 bar), where there is only a slight density difference .......

_ between the liquid-like and steam-like ~edium. High fresh-steam pressures are necessary in order to achieve high thermal efficiencies and consequently low C0z emissions.
A particular problem, in this case, is to design the combustion-chamber or containing wall of the once-through steam generator with regard to the tube-wall or material temperatures which occur there. In the subcritical pressure range up to about 200 bar, the temperature of the combustion-chamber wall is determined essentially by the value of the water saturation temperature, when wetting of the heating surface in the evaporation zone can be ensured. This is achieved, for example, by the use of internally ribbed tubes. Tubes of this type and their use in steam generators are known, for example, from European Patent ~pplication 0,503,116. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the inner wall to the flow medium.
In the pressure range of about 200 to 221 bar, the heat transmission from the tube inner wall to the flow medium decreases sharply, so that the flow velocity - the mass flow density usually being used as a measure of this - has to be increased correspondingly, in order to ensure that the tubes are cooled sufficiently. Consequently, in the evaporator tubes of once-through steam generators operated at pressures of approximately 200 bar and above, the mass flow density and therefore the pressure loss due to friction must be selected higher than in once-through ;team generators which are operated at pressures of below 200 bar. Particularly in the case of small tube inside diameters, the higher pressure loss due to friction cancels out the advan~ageous property of vertical tubing but, when there is multiple heating of individual tubes, their throughput also rises. However, since high steam pressures of more than 200 bar are required in order to achieve high thermal ef~iciencies and therefore low CO2 emissions, it is necessary, in this pressure range too, to ensure good heat transmission. Consequently, once-through steam generators with a combustion-chamber wall having vertical tubing are conventionally operated with relatively high mass flow densities in the tubes, so as to ensure, in the unfavourable pressure range of about 200 to 221 bar, that there is always suf~iciently high heat transmission from the tube wall to the flow medium, that is to say to the water/steam mixture. In this context, the publication "Thermal Engineering"
I.E. Semenovker, Vol. 41, No. 8, 1994, pages 655 to 661, speci~ies a mass flow density at 100~ load o~
about 2000 kg/m2s consistently both for gas~fired and for coal-fired steam generators.
The object on which the invention is based is to specify, for tubes with a containing wall of a once-through steam generator, a design criterion which is suitable in terms of a particularly favourable mass flow density in the tubes.
This object is achieved, according to the invention, in that the steam generator is designed in such a way that the mass flow density m in the tubes of the containing walls at that load at which critical pressure Pcrit prevails in the tubes conforms to the relation:

m= qi (kg/m2s), c ( TmaX ~ Tcrit--~Tw ) in which qi (kW/M2) is the heat flow density on the inside of the tube, Tm~x (~C) is the maximum permissible material temperature of the tube, <~
CA 02243993 l998-07-23 ~ GR 96 P 3052 P - 4 -l'Cri~ (~C) is the temperature of the flow medium at critical pressure Pcrit~
~ Tw (K) is the temperature difference between the outer wall and inner wall of the tube, and C > 7.3 10-3 kWs/kgK is a constant.
The invention proceeds from the consideration that, in the flow-related design of the internally ribbed tubes, two basically contradictory conditions have to be satisfied with regard to the mass flow density. On the one hand, the mean mass flow density in the tubes must be selected as low as possible. This is to ensure that a higher mass flow flows through individual tubes, to which more heat is supplied than to other tubes on account of unavoidable heating differences, than through tubes which have average heating. This natural-circulation characteristic known ~rom the drum-type boiler leads, at the outlet of the evaporator heating surface, to an equalization of the steam temperature and consequently of the tube-wall temperatures.
On the other hand, the mass flow density in the tubes must be selected high enough that reliable cooling of the tube wall is ensured and permissible material temperatures are not exceeded. High local overheating of the tube material and the consequential damage (tube cracks) are thereby avoided. Essential influencing variables for the material temperature are, in addition to the temperature of the flow medium, the external heating of the tube wall and the heat t:ransmission ~rom the inner tube wall to the flow medium (fluid). There is therefore a connection between the internal heat transmission, which is influenced by the mass flow density, and the external heating of the tube wall.
The invention, then, proceeds from the finding l_hat the connection between the internal minimum heat transmission coefficient ami~ an~ the mass flow density ~ can be described in permissibly simplified form by l_he relation:

CA 02243993 l998-07-23 ~ GR 96 P 3052 P ~ 5 ~
~~min = C ~m (1) in which ~Xmin (kW/m2K) iS the heat transmission coefficient, m (kg/m2s) is the mass flow density in the ribbed tubes, and C is a constant with the mean value of C = 7.3 ~ 10-3 kWs/kgK for commercially available tubes.
Depending on the structure of the inner surface of the tubes, this constant C can also be selected in the 10 :range between 7.3 ~ 10-3 kWs/kgK and 12 ~ 10-3 kWs/kgK.
The said relation gives an optimum mass flow density in the tubes which both results in a favourable throughflow characteristic (natural-circulation characteristic) and also ensures reliable cooling of the tube wall and conse~uently adherence to the permissible material temperatures.
A fundamental consideration in deriving the said relation for the mass flow density in the tubes is that, in the case of predetermined external heating of the tube wall - the so-called heat flow density (kW/M2), that is to say the heating per unit area, being used hereafter for this - the material temperature of the tube wall is only slightly, but definitely, below the ~ermissible value. In this case, it is necessary to bear in mind the physical phenomenon that the heat transmission from the inner tube wall to the flow medium is most unfavourable in the critical pressure range of about 200 to 221 bar.
Comprehensive tests show that the highest material stress is obtained when a relatively low mass flow density is combined with the highest occurring heat flow density in the evaporation zone at about 200 to 221 bar. This is the case, for example, in that region of the combustion chamber in which the burners are arranged. If evaporation is subse~uently terminated and steam superheating commences, the material stress on the tubes of a combustion-chamber wall decreases again. The reason for this is that, in a conventional ~ .

burner arrangement and a conventional combustion cycle, the heat flow density also decreases.
It wa~ found, furthermore, that, in other pressure ranges too, no heat transmission problems arise if, when ribbed tubes are used, sufficient cooling of the tube wall is ensured in the said pressure range of 200 to 221 bar. Thus, at low pressures, that is to say of below approximately 200 bar, the internal ribbing of the tubes causes critical boiling to commence only at the end of the evaporation zone, that is to say in a region having a reduced heat ~low density. Critical boiling no longer occurs in the supercritical pressure range. Heat transmission, then, is so intensive that sufficient cooling of the tube wall is ensured.
To determine the optimum mass flow density m in the tubes of the tube wall, the said optimum mass flow density ensuring an advantageous throughflow characteristic on the one hand and reliable cooling of the tube wall on the other hand, the following procedure can be adopted:

Step 1:
Determination of the heat flow density q~ on the tube outside, based on the thermal calculation of that load at which a pressure of 210 bar prevails in the tubes of the tube wall. This heat flow density determined in this way must be increased by a factor of between 1.1 and 1.5, in order to allow for local irregularities in heat transmission.

Step 2:
Calculation of the maximum permissible material temperature TmaX at the tube apex on the heated side of the tube wall. If it is assumed that the containing or combustion-chamber wall has a mean temperature which corresponds to the mean value of Tm~X and TCrit, the maximum thermal stress is calculated as:

- GR 96 P 3052 P ~ 7 ~ ~s~
T -T
. ~m~= m~2 ~t ~-E~N/n~n2), ~(2) with ~max maximum thermal stress (N/mm2) Tmax maximum material temperature (~C) Tcrit temperature of the fluid at the critical point (~C) ~ coefficient of thermal expansion (1/K) E modulus of elasticity (N/mm2) Since the stresses which are crucial here are thermal stresses, these can be guarded against as secondary stresses according to the ASME Code with triple the value of the permissible stresses ~per~ This results in the temperature TmaX as ~-E

The permissible stress can be taken from the particulars supplied by the tube manufacturers.
Step 3:
Conversion of the predetermined heat flow density q~ (related to the outside of the tube wall) to a heat flow density qi which is related to the inner wall of the tubes:

qi = d ~qa (kWlm ) (4) The determination of the heat redistribution factor K is based on temperature field calculations and can be arrived at with sufficient accuracy as follows:

K = A (d~2 q~) + B (5) with A = 0.45 and B = 0.625 for (d~2 ~ ~) < 0.5 kW
and A = 0.25 and B = 0.725 ~or (d~2 ~ q~) ~ 0.5 and < 1.1 kW

~ .

and A - 0 and B - l for (d.2 ~ q~) > l.l kW, with da = tube outside diameter (m) di = tube inside diameter (m) ~ = heat flow density on the outside (kW/m2) qi = heat flow density on the inside (kW/m2) Step 4:
Determination of the temperature difference ~ Tw between the tube outer wall and the tube inner wall. The temperature difference ~ Tw is determined by means of the heat conduction equation:

~Tw=( 2 ) qn2~ 1n n (K) (6) with ~ = thermal conductivity of the tube material (kW/mK).

Step 5:
Determination of the necessary mass flow density m according to the relation:

C(Tm~--T --~T ~ (kg/m2s) (7~

An exemplary embodiment of the invention is explained in more detail by means of a drawing. In this:

Figure l shows a simplified representation of a once-through steam generator with vertically arranged evaporator tubes, Figure 2 shows an individual evaporator tube in cross-section, Figure 3 shows, in a graphical representation, curves E, F, G and H for the mass flow density in the case of different geometries of an ~ CA 02243993 1998-07-23 ., .
- GR 96 P 3052 P - g -evaporator tube consisting of the material 13 CrMo 44, and Figure 4 sho~s, in a graphical representation, the dependence of the maximum permissible material temperature of 13 CrMo 44 on the permissible stress (N/mm2).

Parts corresponding to one another are provided with the same reference symbols in all the figures.
Figure 1 shows diagrammatically a once-through steam generator 2 of rectangular cross-section, a vertical gas flue of which is formed from a containing wall 4 which merges at the lower end into a funnel-shaped bottom 6. The bottom 6 comprises a discharge orifice 8, not shown in any more detail, for ash.
A number of burners 10, only one of which can be seen, for a fossil fuel are mounted, in the lower region A of the gas flue, in the containing wall or combustion chamber 4 formed from vertically arranged evaporator tubes 12. In this region A, the vertically arranged evaporator tubes 12 are welded to one another via tube fins or tube webs 14 to form gas-tight combustion-chamber or containing walls. The evaporator tubes 12, through which the flow passes from the bottom upwards when the once-through steam generator 2 is in operation, form an evaporator heating surface 16 in this region A.
When the once-through steam generator 2 is in operation, a flame body 17 occurring during the combustion of a fossil fuel is located in the combustion chamber 4, so that this region A of the once-through steam generator 2 is distinguished by a very high heat flow density. The flame body 17 has a temperature profile which, starting from about the middle of the combustion chamber 4, decreases both upwards and downwards in the vertical direction and in the horizontal direction towards the sides, that is to say towards the corners of the combustion chamber 4.
Located above the lower region A of the gas flue is a second flame-distant region B, above which a third upper region C of the gas flue is provided. Convection heating surfaces 18, 20 and 22 are arranged in the regions B and C o~ the gas flue. Located above the region C of the gas flue is a flue-gas outlet duct 24, via which the flue gas RG generated as a result of the combustion of the fossil ~uel leaves the vertical gas flue.
Figure 2 shows an evaporator tube 12 which is provided with ribs 26 on the inside and which, while the once-through steam generator 2 is in operation, is exposed on the outside, within the co'mbustion chamber 4, to heating at the heat flow density qa and through which the flow medium S flows internally. At the critical point, that is to say at the critical pressure ]?crit Of 221 bar, the temperature of the flow medium or Eluid in the tube 12 is designated by TCrit. The maximum permissible material temperature TmaX at the tube apex ~8 on the heated side of the tube wall is used for calculating the maximum thermal stress ~x. The inside diameter and outside diameter of the evaporator tube 12 are designated by di and d~ respectively. In the case of internally ribbed tubes, it is necessary to use the equivalent inside diameter which allows for the influence of the rib heights and rib valleys. The tube-wall thickness is designated by dr~
Figure 3 shows, in a system of coordinates, four curves E, F, G and H for different outside diameters da (mm) and tube-wall thicknesses dr (mm). For this purpose, the heat flow density ~ (kW/m2) on the tube outside is plotted on the abscissa and the preferred or optimum mass flow density m (kg/m2s) is plotted on the ordinate. The curve E shows the trend for a tube outside diameter da of 30 mm in the case of a tube-wall thickness dr of 7 mm. The curve F represents the trend for a tube outside diameter d~ of 40 mm in the case of a tube-wall thickness dr ~f 7 mm The curve G
shows the trend of the mass flow density ~ in dependence on the heat flow density ~ for a tube 12 having an outside diameter dA of 30 mm and a tube-wall thickness dr of 6 mm. The curve H shows the trend o~ a tube 12 with an outside diameter d~ of 40 mm in the case of a tube-wall thickness dr o~ 6 mm. The mass flow densities m are calculated for heat flow densities qa of 250, 300, 350 and 400 kW/m2 at the critical pressure Pcrit Of the flow medium S for the tube material 13 CrMo 44 .
An example of the determination of the optimum mass flow density m is shown below. In this case, the following conditions are presupposed:
q~ = 250 kW/m2; heat ~Clow densïty on the tube outside at a pressure of 210 bar, 1.4 as raising factor for allowing for local irregularities in the heat transmission to the tubes 12, d~ = 40 mm tube outside diameter, dr = 7 mm tube-wall thickness, and tube material: 13 CrMo 44.
It follows from da and dr that: di = 26 mm tube inside diameter.

1st Step: Calculatinq the heat flow density The heat flow density based on thermal calculation is multiplied by the raising factor. This results in:
qa = 350 kW/m2 2nd Step: Determininq the maximum permissible material temperature According to equation (3), this temperature is calculated at TCrit = 374~C (temperature of the fluid at critical pressure Pcrie)~ with ~ = 16.3 ~ 10-6 (1/K) ~coefficient of thermal expansion of 13 CrMo 44), E =
~78 ~ 103 (N/mm2) (modulus of elasticity of 13 CrMo 44) and ~per = 68.5 (N/mm2) (permissible stress of 13 CrMo 44 at the maximum permissible material temperature) as:

TmaX = 515~C.

~ GR 96 P 3052 P - 12 -This determination of TmAX, to be carried out iteratively, shows the dependence of the permissible stress ~per on the material temperature. Figure 4 represents graphically this dependence between the permissible stress ~per on the maximum material temperature TmaX for the material 13 CrMo 44.

3~d Step: Heat flow density on the tube inside By means of the equations (4) and (5), there follows for A = 0.25 and B = 0.725 for the heat flow density qi on the inside of the tubes 12:

qi = 466 kW/m2.

4th Step: Determininq the temPerature dlfference T~, between the tube outer wall and tube inner wall According to equation (6), with the thermal conductivity of 13 CrMo 44 of ~ = 38.5 - 10-3 kW/m K:
~ Tw = 73 K.

5th Ste~: Determininq the necessary mass flow density According to equation (7), with C = 7.3 10-3 kWs/kgK:
m = 939 kg/m2s.

The optimum mass flow density m can thus be determined by means of the available values for the heat flow density qa on the tube outside and the maximum permissible material temperature T~x. This value is represented by broken lines in Figure 3 for the specified conditions. It can be seen that, for the assumed heat flow density qa of the tube outside of 350 kW/m2, optimum mass flow densities m of between 740 and 1060 kg/m2s are obtained in the case of tubes 12 having outside diameters da of between 30 and 40 mm and wall thicknesses dr of between 6 and 7 mm.

' CA 02243993 1998-07-23 ~, .
~ GR 96 P 3052 P - 13 -For the flow-related design of the tubes 12 of the tube wall or containing wall 4, the mass flow density m th~s determined can still be converted to the conditions prevailing under 100~ load. For this purpose, the operating pressure at the inlet of the tubes 12 is calculated at 100~. The abovementioned mass flow densities m are subsequently converted in proportion to the operating pressure under 100~ load.
If, for example, the operating pressure under 100~ load is p~ = 270 bar, the mass flow density m increases from 740 to 951 kg/mZ or from 1060 to 1363 kg/m2s.
It may be expedient to allow for uncertainties in the determination of the heat flow density ~ by raising the mass flow density m from +15~ to +20~ in relation to the calculated value.

-List of reference symbols m mass.flow density ~ thermal stress A, B region ~a outside diameter dr tube-wall thickness E,F,G,H curve Pcr~t pressure ~ heat flow density qi heat flow density RG flue gas S flow medium TmaX maximum permissible material temperature 2 once-through steam generator 4 containing wall 6 bottom 8 discharge orifice burner 12 evaporator tube 14 tube web 16 evaporator heating surface 17 flame body 18,20,22 convection heating surface 24 flue-gas outlet duct 26 ribs 28 tube apex

Claims (16)

Claims

1. Once-through steam generator having a combustion chamber (4) surrounded by a containing wall consisting of tubes (12) connected to one another in a gas-tight manner, a flow medium (S) being capable of flowing from below upwards through the vertically extending tubes (12) which have a surface structure (26) on their inside, characterized in that the mass flow density ~ in the tubes (12) at that load at which critical pressure Pcrit prevails in the tubes (12) conforms to the relation:

, in which qi (kW/m2) is the heat flow density on the inside of the tube (12), Tmax (°C) is the maximum permissible material temperature of the tube (12), TCrit (°C) is the temperature of the flow medium (S) at critical pressure (Pcrit), .DELTA. Tw (K) is the temperature difference between the outer wall and inner wall of the tube (12), and C ~ 7.3 - 10-3 kWs/kgK is a constant.

2. Once-through steam generator according to Claim 1,. characterized in that the heat flow density qi related to the inner wall conforms to the relation:

with K = A (da2 ~ qa) + B, in which: A = 0.45 and B = 0.625 for (da2 ~ qa) ~ 0.5 kW, A = 0.25 and B = 0.725 for (da2 ~ qa) > 0.5 and ~ 1.1 kW, A = 0 and B = 1 for (da2 ~ qa) 1.1 kW, and qa being the heat flow density on the tube outside (kW/m2) and da being the tube outside diameter (m).

3. The once-through steam generator as claimed i.beta.n claim 1 or 2, wherein the maximum admissible material temperature Tmax conforms to the relation:

(°C), .sigma.adm being the admissible thermal stress (N/mm2), .beta. the coefficient of thermal expansion (l/K) and E the modulus of elasticity (N/mm2) of the tube material.

4. The once-through steam generator as claimed in one of claims 1 to 3, wherein the temperature difference .DELTA.Tw between the tube outer wall and the tube inner wall conforms to the relation:
(K) with K = A (da2 ~ qa) + B, in which A=0.45 and B=0.625 for (da2 ~ qa) ~ 0.5 kW, A=0.25 and B=0.725 for (da2~qa) > 0.5 and ~ 1.1 kW, A=0 and B=1 for (da2~qa) > 1.1 kW, and qa is the heat flow density on the tube outside (kW/m2), da the tube outside diameter (m), d1 the tube inside diameter (m) and .lambda. the thermal conductivity of the tube material (kW/mK).

5. The once-through steam generator as claimed in one of claims 1 to 4, wherein, for a tube (12) made from the material 13 CrMo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve E defined for a tube outside diameter da of 30 mm and a tube wall thickness dr of 7 mm and passing through the points determined by the pairs of values:
qa = 250 kW/m2, m = 526 kg/m2s, qa = 300 KW/m2, m = 750 kg/m2s, qa = 350 kW/m2s, m = 1063 kg/m2s, and qa = 400 kW/m2, m = 1526 kg/m2s.

6. The once-through steam generator as claimed in one of claims 1 to 4, wherein, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve F defined for a tube outside diameter da of 40 mm and a tube wall thickness dc of 7 mm and passing through the points determined by the pairs of values:
qa = 250 kW/m2, m = 471 kg/m2s, qa = 300 KW/m2, m = 670 kg/m2s, qa = 350 kW/m2s, m = 940 kg/m2s, and qa = 400 kW/m2, m = 1322 kg/m2s.

7. The once-through steam generator as claimed in one of claims 1 to 4, wherein, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve G defined for a tube outside diameter da of 30 mm and a tube wall thickness dr of 6 mm and passing through the points determined by the pairs of values:
qa = 250 kW/m2, m = 420 kg/m2s, qa = 300 KW/m2, m = 576 kg/m2s, qa = 350 kW/m2s, m = 775 kg/m2s, and qa = 400 kW/m2, m = 1037 kg/m2s.

8. The once-through steam generator as claimed in one of claims 1 to 4, wherein, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve H defined for a tube outside diameter da of 40 mm and a tube wall thickness dr of 6 mm and passing through the points determined by the pairs of values:
qa = 250 kW/m2, m = 399 kg/m2s, qa = 300 KW/m2, m = 549 kg/m2s, qa = 350 kW/m2s, m = 737 kg/m2s, and qa = 400 kW/m2, m = 977 kg/m2s.

9. A method for designing a once-through steam generator with a combustion chamber (4) surrounded by a containment wall composed of tubes (12) connected to one another in a gastight manner, a flow medium being capable of flowing from the bottom upward through the vertically extending tubes (12) which have a surface structure (26) on their inside, wherein the tubes (12) are selected in such a way that, under the load at which a critical pressure Pcrit prevails in the tubes (12), a mass flow density m of:

flows through said tubes, qi (kW/m2) being the heat flow density on the inside of the tube (12), TmaX (°C) the maximum admissible material temperature of the tube (12), TCrit (°C) the temperature of the flow medium (S) at critical pressure (Pcrit), .DELTA. TW(K) the temperature difference between the outer and inner wall of the tube (12), and C ~
7.3 ~ 10-3 kWs/kgK a constant.

10. The method as claimed in claim 9, wherein the tubes are selected in such a way that the heat flow density qi related to the inner wall conforms to the relation:

(kW/m2) with K = A (da2~qa) + B, in which: A=0.45 and B=0.625 for (da2~qa) ~ 0.5 kW, A=0.25 and B=0.725 for (da2~qa) > 0.5 and ~ 1.1 kW, A=0 and B=1 for (da2~qa) > 1.1 kW, and qa is the heat flow density on the tube outside (kW/m2) and da is the tube outside diameter (m).

11. The method as claimed in claim 9 or 10, wherein the tubes (12) are selected in such a way that the maximum admissible material temperature TmaX
conforms to the relation:

(°C), .sigma.adm being the admissible thermal stress (N/mm2), .beta. the coefficient of thermal expansion (l/K) and E the modulus of elasticity (N/mm2) of the tube material.

12. The method as claimed in one of claims 9 to 11, wherein the tubes (12) are selected in such a way that the temperature difference .DELTA.Tw between the tube outer wall and the tube inner wall conforms to the relation:

(K) with K = A (da2~qa) + B, in which: A=0.45 and B=0.625 for (da2~qa) ~ 0.5 kW, A=0.25 and B=0.725 for (da2~qa) > 0.5 and ~ 1.1 kW, A=0 and B=1 for (da2~qa) > 1.1 kW, and qa is the heat flow density on the tube outside (kW/m3), da the tube outside diameter (m), di the tube inside diameter (m) and .lambda. the thermal conductivity of the tube material (kW/mK).

13. The once-through steam generator as claimed in one of claims 9 to 12, wherein the tubes (12) are selected in such a way that, for a tube (12) made from the material 13 CrMo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve E defined for a tube outside diameter da of 30 mm and a tube wall thickness dr of 7 mm and passing through the points determined by the pairs of values:

qa = 250 kW/m2, m = 526 kg/m2s, qa = 300 KW/m2, m = 750 kg/m2s, qa = 350 kW/m2s, m = 1063 kg/m2s, and qa = 400 kW/m2, m = 1526 kg/m2s.

14. The method as claimed in one of claims 9 to 12, wherein the tubes (12) are selected in such a way that, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve F defined for a tube outside diameter da of 40 mm and a tube wall thickness dr of 7 mm and passing through the points determined by the pairs of values:

qa = 250 kW/m2, m = 471 kg/m2s, qa = 300 KW/m2, m = 670 kg/m2s, qa = 350 kW/m2s, m = 940 kg/m2s, and qa = 400 kW/m2, m = 1322 kg/m2s.

15. The method as claimed in one of claims 9 to 12, wherein the tubes (12) are selected in such a way that, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve G defined for a tube outside diameter d a of 30 mm and a tube wall thickness d r of 6 mm and passing through the points determined by the pairs of values:

q a = 250 kW/m2, m = 420 kg/m2s, q a = 300 KW/m2, m = 576 kg/m2s, q a = 350 kW/m2s, m = 775 kg/m2s, and q a = 400 kW/m2, m = 1037 kg/m2s.

16. The method as claimed in one of claims 9 to 12, wherein the tubes (12) are selected in such a way that, for a tube (12) made from the material 13 Cr Mo 44, points in a coordinate system which are determined by pairs of values of the heat flow density m (kg/m2) lie on a curve H defined for a tube outside diameter d a of 40 mm and a tube wall thickness d r of 6 mm and passing through the points determined by the pairs of values:

q a = 250 kW/m2, m = 399 kg/m2s, q a = 300 KW/m2, m = 549 kg/m2s, q a = 350 kW/m2s, m = 737 kg/m2s, and q a = 400 kW/m2, m = 977 kg/m2s.