EP0035450A1 - Preheating steam generator - Google Patents

Abstract

A steam generator having cold branch and hot branch zones defined, inside the secondary envelope, by vertical partitions separating the cold limbs from the hot limbs, and outside the secondary envelope by a skirt surrounding a portion of the latter on the cold branch side, forming therewith a space closed on its sides and at its lower portion while leaving a passage to the cold branch zone from the inside of the envelope. The space is open at the top so that recycled water can return to the bundle of U-tubes through both zones, but secondary feed water circulates for the most part in the cold branch zone. Recycled water may be distributed between the cold and hot branch zones, and the pressure in these zones may be balanced at the level of a horizontal, perforated tubular plate.

Description

The invention relates to a steam generator, which can be used for example in a nuclear installation, for the production of electrical energy or for naval propulsion.

A conventional type steam generator has a cylindrical outer enclosure of vertical axis, resistant to pressure and closed at each end by a cap. A horizontal tubular plate is arranged inside the enclosure, integral with the latter and delimiting with the lower cap a chamber separated into two collectors, respectively for admission and evacuation of a primary heat-transfer fluid. A bundle of U-shaped tubes is mounted on the tube plate. Each U-shaped tube has a hot leg which communicates with the intake manifold and a cold leg which communicates with the exhaust manifold, the set of hot legs constituting the hot branch and the set of cold legs constituting the cold branch . A secondary envelope surrounds the bundle without resting on the tube plate and defines an annular space with the outer enclosure. A water intake device is provided for supplying the annular space with secondary water. This secondary water rises along the bundle tubes and vaporizes on contact. In the upper part of the enclosure, there is a set of dividers; the steam produced is discharged through an opening in the upper cap. The water from the separators, called recirculation water, returns to the tubular plate using the annular space between the outer enclosure and the secondary envelope, before rising again along the bundle of U-shaped tubes. Recirculation water mixes a food water which comes from the intake device of which we spoke above. This intake device is placed in the upper part of the steam generator so that the mixture of drinking water and recirculating water is homogeneous before it enters the tube bundle.

The generator of the conventional type which has just been described has the following drawback in particular: the primary-secondary exchange surface is not used optimally; therefore, the thermal efficiency of the entire generator is not maximum.

To remedy this drawback, that is to say to try to increase the efficiency of the generator, provision has been made to preheat the water. food before mixing with recirculating water. This preheating allows better use of the exchange surface and an increase in the thermal efficiency of the entire generator.

From a theoretical point of view, maximum efficiency is reached when the drinking water is brought to the immediate vicinity of the tube plate and then rises along the cold branch of the U-shaped tubes. in contact with which it must heat up over a certain distance, called the preheating zone. The food water is only mixed with the recirculation water coming from the separators when it leaves the preheating zone.

However, this configuration has several drawbacks. In particular, since the drinking water must not be mixed with the recirculating water before it has been preheated, the inlet and guide structures of this water must be relatively tight. In addition, it has the drawback due to the fact that at low load or during certain operating transients (introduction of emergency water), the temperature of the drinking water decreases. This water can then initiate in the structures with which it comes into contact (tubes, tube plate, pressure vessel) unacceptable thermal stresses.

To overcome these drawbacks, several solutions have been proposed, including those which consist of incorporating temperature support for the drinking water or providing a secondary supply at the top of the steam generator.

These two solutions however lead to an increase in the operation and operation of the steam generator.

To reduce the thermal stresses in the tubular plate, the zone where the food water enters the bundle can be moved away from this plate while keeping it at the bottom. This is the subject, for example, of French patent n ⁰ 2 191 704 in the name of SIEMENS AG

This geometry is expressed in relation to the theoretical case considered above by a drop in the saturation pressure delivered by the steam generator and complex structures to be installed in the lower part of the device.

It has also been proposed to create a buffer zone traversed by recirculating water above the tube plate.

May for example refer to French Patent Application No. 2,285,573 ^O on behalf of Westinghouse Electric Corporation. In the buffer zone, the tubular plate, c8té cold branch, is swept by recirculating water. This recirculation water is guided by a very slightly permeable plate, which is in fact constituted by the floor of the preheater, this preheater and the food water intake device being located in the lower part of the steam generator.

However, the preheater of the floor can create contrain _- your bending in the tube (pressure deformation of the tube plate, differential expansion of the tube plate of the preheater and the floor), and further makes difficult the casing operation because its lower permeability than that of the spacer plates of the tube bundle and its proximity to the tube plate.

On the other hand, this floor cannot be perfectly sealed, so that part of the drinking water passes through it and goes towards the hot branch: this reduces the overall performance of the steam generator.

• 1. Accomoder des variations importantes de température , ce qui nécessite des épaisseurs faibles.
• 2. Limiter, en situation accidentelle (rupture de tuyauteries, coup de bélier) de déformations qui pourraient mettre en danger la barrière primaire-secondaire que constituent les 'tubes. Il est donc nécessaire de prévoir un ensemble rigide.

In addition, the introduction of drinking water into the bundle without mixing with recirculating water whatever the load, makes it difficult to dimension the structures of the preheater because it must meet two contradictory requirements:

• 1. Accommodate large variations in temperature, which requires low thicknesses.
• 2. Limit, in an accidental situation (broken pipes, water hammer), deformations which could endanger the primary-secondary barrier constituted by the tubes. It is therefore necessary to provide a rigid assembly.

Finally, the steam generator of the type described in the French application ^No. 2285573 has the disadvantage of comprising a smaller space for the tubes for a given diameter of the in- 'develops under pressure, part of the space being occupied by the preheater.

We also know the French patent application No. 2 387 417 in the name of the French Atomic Energy Commission which describes a steam generator which does not have the aforementioned drawbacks. In this generator, all of the drinking water is mixed with the recirculating water and sent to the cold branch of the tube plate via a central box located between the legs of the U-shaped tubes, the water being channeled towards the box by two flat rings of elliptical shape.

However, this achievement leads to a very complex structure.

The object of the invention is therefore to propose a new steam generator provided with a preheating device but which does not have the drawbacks of the prior art.

The invention applies to any conventional type steam generator, as described above.

According to the invention, the steam generator is separated into two zones, a cold branch zone and a hot branch zone, the separation being carried out on the one hand, inside the secondary envelope, by a vertical partition separating the cold legs hot legs, on the other hand. outside the secondary envelope, by a skirt surrounding a part of the secondary envelope, on the side of the cold branch, and constituting with said envelope a closed space on the removed and at its lower part, while leaving a passage towards the cold branch zone of the interior of the envelope, and open at its upper part; so that the secondary water recirculated after passing through the separators, called recirculation water, can return to the bundle of U-shaped tubes both through the cold branch zone and through the hot branch zone, but as the water secondary food, arriving by an intake device located at the top of the generator circulates most of its flow in the cold branch area.

In addition, the generator according to the invention comprises means for distributing the recirculating water between the two zones, cold branch and hot branch, and for balancing, at the level of the tube plate, the pressures in these two zones. One can use on the one hand a distribution flange placed in the space between the secondary casing and the skirt, perpendicular to the direction of the secondary water current of the cold branch zone; on the other hand, a distribution plate placed inside the secondary envelope in each of the zones, cold branch and hot branch, perpendicular to the direction of the U-shaped tubes, these two plates generally being of different permeability.

In a preferred embodiment of the invention, the skirt is located at a constant distance from the secondary envelope, defining with it a space whose section by a plane parallel to the tabular plate is a sector of a ring whose the angle may vary with the section plane; the skirt extends vertically from the tube plate to the feed water intake device; its sides are folded back towards the secondary envelope and welded to this envelope; its lower part is connected to the tubular plate by a semi-tight connection limiting leaks but allowing movements of the skirt relative to the plate so as to accommodate the relative deformations due to pressure and expansion.

In a particular embodiment of the invention, the skirt is merged with the external enclosure, except for the edges which are constituted by two vertical partitions extending between the external enclosure and the secondary envelope.

Preferably, the vertical partition separating the cold branch zone from the hot branch zone inside the secondary envelope extends from the tubular plate to a height at least equal to the height necessary for preheating the water. food; it is welded on the sides inside the secondary casing and connected to the tube plate by a semi-sealed partition.

To make the semi-sealed connections, one can for example use rails integral with the tube plate, in which the lower parts of the skirt and the vertical partition are engaged. If, at the level of the tubular plate, the section of the skirt is a ring sector of 180 °, the folded sides of the skirt can be engaged in the same rectilinear rail as the vertical partition.

In a preferred embodiment of the semi-sealed connections, the lower part of the skirt can be engaged in a rail secured to the outer enclosure and not to the tubular plate, in order to reduce the stresses in this plate

Preferably, the secondary food water is sent for all of its flow in the cold branch zone.

• La figure 1 représente une vue d'un 1er mode de réalisation du générateur selon l'invention.
• La figure 2 représente une coupe,par un plan perpendiculaire à la plaque tubulaire et à la cloison verticale, du générateur de la figure 1.
• La figure 3 représente une coupe par un plan parallèle à la plaque tubulaire, au niveau de la partie supérieure de la jupe, du géné- raleur de vapeur de la figure 1 (coupe suivant III-III).
• La figure 4 représente un deuxième mode de réalisation de la jupe selon l'invention, dans une vue semblable à celle de la figure 1.
• La figure 5 représente le mode de réalisation de la figure 4, vue dans une coupe analogue à celle de la fugure 2.
• La figure 6 représente le mode de réalisation de la figure 4, vue d'une façon analogue à celle de la figure 3.
• La figure 7 représente un troisième mode de réalisation de la jupe, vue de façon analogue à la figure 3.
• Les figures 8, 9, 10 et 11 représentent divers modes de réalisation des moyens de liaison de la partie inférieure de la jupe et de la cloison verticale avec la plaque tubulaire.
• Les figures 8a, 8b et 8c montrent les parties inférieures engagées dans des rails.
• Les figures 9a et 9b montrent les parties inférieures engagées dans des joints à labyrinthe.
• Les figures 10a et 10b montrent un système de liaison avec couvre-joints.
• La figure 11 représente un système de liaison par joints du type à ressort.

In order to be better understood, the invention will be described in a few preferred embodiments, chosen only by way of examples and represented by the appended drawings.

• FIG. 1 represents a view of a 1st embodiment of the generator according to the invention.
• FIG. 2 represents a section, through a plane perpendicular to the tube plate and to the vertical partition, of the generator of FIG. 1.
• FIG. 3 represents a section through a plane parallel to the tube plate, at the level of the upper part of the skirt, of the steam retarder of figure 1 (section along III-III).
• FIG. 4 represents a second embodiment of the skirt according to the invention, in a view similar to that of FIG. 1.
• FIG. 5 represents the embodiment of FIG. 4, seen in a section similar to that of fugure 2.
• FIG. 6 represents the embodiment of FIG. 4, seen in a manner similar to that of FIG. 3.
• FIG. 7 represents a third embodiment of the skirt, seen in a manner similar to FIG. 3.
• Figures 8, 9, 10 and 11 show various embodiments of the means for connecting the lower part of the skirt and the vertical partition with the tube plate.
• Figures 8a, 8b and 8c show the lower parts engaged in rails.
• Figures 9a and 9b show the lower parts engaged in labyrinth seals.
• Figures 10a and 10b show a connection system with joint covers.
• Figure 11 shows a spring-type joint connection system.

In all the figures, the corresponding elements bear the same references.

We will first refer to Figures 1, 2 and 3.

The steam generator shown in Figures 1, 2 and 3 comprises a cylindrical outer enclosure 1 of vertical axis, resistant to pressure, closed at each end by a cap. The upper cap has not been shown but we can see the lower cap 2. A horizontal tubular plate 3 is arranged inside the enclosure 1, integral with the latter and delimiting with the lower cap 2 a separate chamber at two collectors 4 and 5, the manifold 4 being an intake manifold and the manifold 5 a discharge manifold for a primary heat-transfer fluid; this primary fluid can for example come from a nuclear power plant reactor.

U-shaped tubes, 6, grouped in a bundle, are mounted on the tube plate 3 and each have a hot leg 7 which communicates with the intake manifold 4 and a cold leg 8 which communicates with the exhaust manifold 5; the set of hot legs 7 constitutes the hot branch and the set of cold legs constitutes the cold branch.

A secondary envelope 9 surrounds the bundle of tubes 6 without resting on the tube plate 3 and. Defines an annular space 10 with the external enclosure.

An inlet device 11 for supplying the annular space 10 with secondary water is provided in the upper part of the generator.

This water is intended to descend to the tubular plate 3 then to rise along the tubes 6 and to heat up and then vaporize on contact. In the upper part of the enclosure 1 is provided a set of separator-dryers 12 intended to separate the vapor obtained in the upper part of the tubes; this vapor is then discharged through an opening in the upper cap, not shown.

FIG. 2 shows the spacer plates 13 which hold the tube bundle 6. These plates 13 and the secondary envelope 9 are blocked in translation relative to the external enclosure 1 by blocks 14.

The space between the secondary casing 9 and the tube plate 3 provided for the secondary fluid to enter the bundle has been called.

A skirt 16 surrounds a part of the secondary envelope 9, of the side of the cold branch and constitutes with the envelope 9 a space 17 closed on the sides and at its lower part. The space 17 constitutes the circuit for the return of the recirculation water to the cold branch or the return of the cold branch water, while the annular space 10 between the pressurized enclosure 1 and the secondary envelope 9, d 'on the one hand and the space between the enclosure 1 and the skirt 16, which will be called 10', on the other hand, delimit the return of hot branch water. The sides 18 and 18 ′ of the skirt 16 are folded over the secondary casing 9 and welded to the latter, thus preventing the passage of fluid from the cold branch water return to the hot branch water return.

The lower part of the skirt 16 is connected to the tube plate 3 by a semi-tight connection limiting leaks. This connection is here constituted by the goige 19a of a semi-circular rail 19 welded to the tubular plate in which a flange 19b of the lower part of the skirt 16 is fitted. In the same way, the lower part of the sides 18 and 18 'is engaged in the ends 20a and 20b of a straight rail 20.

Inside the secondary envelope 9, a vertical partition wedge 21 separates the cold legs 8 from the hot legs 7. This vertical partition 21 is engaged in the rectilinear rail 20. The plate 21 is in fact placed in the extension of the partition plate 22 separating the collectors 4 and 5. It is welded to the secondary envelope 9 and extends vertically to a height at least equal to the height necessary for preheating the drinking water. The partition 21 can thus prevent transverse flows in the bundle before all the food water is preheated.

A feed water inlet manifold is fixed in the tubing 11 and is provided with J-shaped tubes, 23, the openings of which open in the upper part of the cold branch water return 17.

A flange 24 provided with orifices is located at the lower part of the space 17 and is intended to create a pressure drop there. This flange 24 also allows a homogeneous supply of the cold branch.

Distribution plates 25a and 25b with different permeabilities are located inside the secondary envelope 9, one on the hot branch side and the other on the cold branch side, perpendicular to the direction of the tubes U. These plates have main function to ensure a good sweeping of the tube plate in order to avoid the creation of areas of low speed of water near the tube plate. They also contribute to the balance of pressures at the bottom of the bundle between cold branch and hot branch and make it possible to avoid the flow of water from one branch to the other.

• le fluide primaire caloporteur circule vers le haut dans les tubes c8té branche chaude et redescend c8té branche froide.

The steam generator according to the invention operates as follows:

• the primary coolant circulates upwards in the tubes on the hot branch and back down on the cold branch.

The secondary water in upward flow is partially vaporized in the bundle of tubes to a vapor titer of 20 to 40%; the emulsion thus produced is directed into a separation-drying assembly ensuring the separation of water and steam and the drying thereof. The separated water, called recirculation water, returns to the base of the bundle according to the arrowed circuits represented in FIG. 2, which include a common area A, a return circuit from the cold branch B and a return circuit from the branch hot C. Part of the secondary recirculation water is mixed with the drinking water coming from the device / diadmissior 11 while 1 remaining part penetrates directly into the hot branch, and the whole is vaporized again. In operation, a single water level is established at the head of the circuit in the vicinity of the base of the separators 12.

Whereas, in the prior art, the recirculation water is distributed uniformly in a space between the external enclosure and the secondary envelope, here, at full load, a large fraction of the recirculation water, greater than 50%, goes towards the hot branch of the bundle by the return of hot branch water, that is to say through spaces 10 and 10 ', while the remaining fraction of the recirculation water is mixed in the cold branch water return 17 to all of the food water coming from the tubing 11, before reaching the tube plate 3 and the preheating zone (that is to say the cold branch zone included in the inside the secondary envelope 9). This fractionation of the recirculation water results from the pressure drop effect created on the one hand by the flange 24 and on the other hand by the difference in permeability of the distribution plates 25a and 25b located one in the branch hot and the other in the cold branch.

Compared to certain solutions of the prior art, where only the drinking water enters the cold branch of the bundle of U-shaped tubes, the solution of our invention causes a drop in the pressure delivered by the steam generator, due to the reduction in the primary-secondary temperature gap. However, this reduction in the temperature difference is partially offset by an increase in the transfer coefficient in the secondary film due to an increase in the total flow rate passing through the cold branch zone of the bundle and a very satisfactory yield is obtained.

The flange 8 and the distribution plates 25a and 25b are adjusted so as to obtain an optimal distribution of the pressure losses, that is to say a distribution for which, at nominal point, therefore at full load, the fraction of recirculation water going towards the hot branch side is close to 100%.

During power reductions, which lead to reductions in the flow rate and temperature of the drinking water, the relative proportion of recirculating water / drinking water naturally increases in the return of cold branch water, which has the effect of compensating for the drop in temperature of drinking water. Thus, the temperature of the mixture arriving on the tube plate remains substantially constant both at full load and at low load.

Reference may now be made to FIGS. 4, 5 and 6 which represent a second embodiment of the skirt 16.

In these figures, we have essentially attempted to represent the skirt 16, the other parts of the generator being similar to those of FIGS. 1, 2 and 3.

In the case of FIGS. 4, 5 and 6, the skirt 16 is such that the section of this skirt by a plane parallel to the tube plate is a ring sector, but this ring sector has an angle which varies with the section plane. From the upper part of the steam generator to an intermediate plane 26, the angle of the ring sector is less than 180 °, while, from the intermediate plane 26 to the level of the tube plate 3, the sector d 'ring increases continuously from the value it has at the intermediate plane 26 up to 180 °.

One could imagine still other possibilities, provided that the section of the skirt is always a ring sector.

In a borderline case, shown in FIG. 7, the skirt 16 is located at a distance from the secondary envelope 9 which is equal to the distance between the external enclosure 1 and the secondary envelope 9; this amounts to saying that the skirt 16 is merged with the external enclosure 1, except for the sides 18 and 18 '. In fact, the skirt is formed only by these sides 18 and 18 ', that is to say by a simple set of two vertical partitions extending between the enclosure 1 and the casing 9 in extension of the vertical partition 21.

FIGS. 8, 9, 10 and 11 show examples of semi-sealed connections, between the lower parts of the skirt and of the vertical partition 21 and the tube plate 3.

Figure 8 shows the lower parts engaged in rails. The vertical partition 21 is engaged in a rail 20. The lower part of the skirt, 19b, is engaged in the groove 19a of a rail 19. The rail 19 can be fixed either directly on the tubular plate or, preferably, on the outer enclosure, as shown in Figure 8c.

In FIG. 9, the semi-sealed connections are made by means of labyrinth seals secured either to the tubular plate (FIG. 9a) or directly to the outer enclosure 1 (FIG. 9b).

FIG. 10 shows a system for connecting the lower parts by means of joint covers. The joint covers 27 are fixed by sectors on the lower parts of the vertical partition 21 or of the skirt 16 by means of bolts 28. The clearance between the covers is canceled during assembly. joints and the tube plate.

As for Figure 11, it shows a connection made by means of a spring-type seal.

The present invention has many advantages.

In particular, it combines, from a design point of view, the advantages of the conventional steam generator with toroidal feed water distribution and recirculation, and, from a performance point of view, the advantages of steam generators with integrated preheater such as those that we described above.

The present invention makes it possible to obtain a vapor pressure higher than that of a conventional steam generator and close to that of steam generators with integrated preheater, while being of a very simple structure.

The invention solves the problems of thermal shock at the level of the tube plate and at the level of the external enclosure. Indeed, the introduction of drinking water is carried out in the upper part of the generator, that is to say away from the tube plate, and this water is mixed on the cold branch with a fraction of hotter recirculating water. ; this fraction of recirculating water increases at low load, that is to say when the temperature and the flow rate of drinking water decrease. The temperature of the secondary water arriving on the secondary plate is therefore practically constant. As for the pressurized enclosure 1, it is maintained at a uniform temperature since it is always in contact with the recirculating water contained in the annular spaces 10. and 10 ′, and not with drinking water. It can be added that the consequences of a rupture of the food piping on the integrity of the U-shaped tubes and the structures of the preheater are less severe than in the case of generators comprising an integrated preheater, as described at the beginning of the description, because the pressure effects are absorbed.

In the steam generator according to our invention, there is no difficulty such as those encountered in generators provided with preheaters or boxes whose dimensions were delicate. In addition, the difficulties associated with the presence of a preheating floor are eliminated.

It can also be added that the sealing requirements in the zone of the tube plate are limited due to the adjustment of the pressures hot branch-cold branch provided by the distribution plates. The transverse flows inside the envelope are condaire at the top of the preheating zone are also minimized.

Another important advantage of the invention resides in the fact of its very smooth structure.

Of course, the invention is not limited to only the embodiments which have been described by way of examples, but it also applies to all the other embodiments which would differ from it only in details, for example. variants or by the use of equivalent means.

Thus, it is not necessary for all the food water to flow through the annular space 17 between the secondary envelope 9 and the skirt 16, but a part of this food water could flow in the annular conduits. 10 and 10 '. Of course, the proportion of drinking water which would thus circulate in the hot branch zone should remain in low proportions.

One could also place the flange 24, not at the bottom of the generator but rather at its top. Thus, this collar would be accessible during stops; it could be fitted with closable orifices and, when stationary, adjust the distribution of the pressure drops. One could, moreover, to effect this adjustment, act on the permeability of the braced plates 13 located in the preheating zone, that is to say below the subject of the vertical partition21.

With regard to the distribution plates 25a and 25b, it can be added that these can be located in different planes.

Claims (12)

1.- Steam generator comprising a cylindrical external enclosure (1) of vertical axis, resistant to pressure, closed at each end by a cap, a horizontal tubular plate (3) disposed inside the enclosure, integral of the latter and delimiting with the lower cap (2) a chamber separated into two manifolds, respectively of intake (4) and evacuation (5) of a primary heat-transfer fluid, a bundle of U-shaped tubes (6) mounted on the tube plate (3), each having a hot leg (7) which communicates with the intake manifold and a cold janbe.f (-) which communicates with the exhaust manifold, the set of hot legs constituting the hot branch and the set of cold legs constituting the cold branch, a secondary envelope (9) surrounding the bundle of tubes (6) without resting on the tubular plate (3) and defining an annular space (10) with the external enclosure (1), an intake device (11) for the food ation of the annular space (10) in secondary water intended to go up along tubes (6) of the bundle and to vaporize on their contact, and, in the upper part of the enclosure, a set of separators (12) and an opening in the upper cap for the evacuation of the steam produced, characterized in that the steam generator is separated into two zones, a cold branch zone and a hot branch zone, the separation being carried out on the one hand , inside the secondary envelope (9), by a vertical partition (21) separating the cold legs (8) from the hot legs (7), on the other hand, outside the secondary envelope, by a skirt (16) surrounding a part of the secondary envelope on the side of the cold branch and constituting with said envelope (9) a space (17) closed on the sides and at its lower part, while leaving a passage towards the cold branch area of the interior of 1% envelope (9), and open at its upper part, so that e the secondary water recirculated after passing through the separator, called recirculation water, can return to the bundle (6) of U-shaped tubes both by the cold branch zone and by the hot branch zone, but as the water secondary food, arriving by an intake device (11) located at the upper part of the generator, circulates, for the main part of its flow, in the cold branch zone. 2.- steam generator according to claim 1, characterized in that it comprises means (25a, 25b) for distributing the recirculating water between the two zones, cold branch and hot branch, and balancing, at the tube plate (3), pressures in these two zones. 3.- Steam generator according to claim 2, characterized in that a distribution flange (24) is placed in the space (17) between the secondary envelope (9) and the skirt (16), perpendicularly to the direction of the secondary water flow in the cold branch zone. 4.- Steam generator according to one of claims 2 or 3, characterized in that, inside the secondary envelope (9), each zone, cold branch or hot branch, compares a distribution plate ( 25b, 25a) placed perpendicular to the direction of the U-shaped tubes, the two plates being of different permeability. 5.- Steam generator according to one of claims 1 to 4, characterized in that the skirt (16) is located at a constant distance from the secondary envelope (9), defining therewith a space (17 ) whose section through a plane parallel to the tube plate is a ring sector whose angle can vary with the section plane, that it extends vertically from the tube plate (3) to the device (11 ) of food water intake, that its sides (18, 18 ') are folded back towards the secondary envelope (9) and welded to this envelope and that its lower part is connected to the tubular plate (3) by a connection (19) is waterproof. 6.- Steam generator according to claim 5, characterized in that the skirt is merged with the outer enclosure (9), except for the sides which are constituted by two vertical partitions (18,18 ') extending between the outer enclosure (1) and the secondary enclosure (9). 7.- Steam generator according to one of the preceding claims, characterized in that the vertical partition (21) separ.la cold branch area of the hot branch area inside the secondary envelope (9) s' extends from the tube plate (3) to a height at least equal to the height necessary for preheating the drinking water and that it is welded on the sides inside the secondary envelope (9) and connected to the tubular plate (3) by a semi-sealed connection (20). 8.- Steam generator according to one of claims 5, 6 or 7, characterized in that the lower parts of the skirt (16) and the vertical partition (21) are engaged in rails (19, 20) integral with the tube plate. 9.- Steam generator according to claim 8, characterized in that the vertical partition (21) and the sides (18, 18 ') folded down from the skirt (16) are engaged in the same straight rail. 10.- Steam generator according to one of claims 5, 6 or 7, characterized in that the lower part of the skirt (16) is engaged in a rail (19) secured to the outer enclosure. 11.- Steam generator according to one of claims 5, 6 or 7, characterized in that the lower parts of the skirt (16) and the vertical partition (21) are connected to the tube plate (3) at using spring type seals. 12.- Steam generator according to any one of the preceding claims, characterized in that the secondary food water is sent for the whole of its flow in the cold branch zone.

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