WO2009024855A2

WO2009024855A2 - Heat exchanger, in particular operating as large- sized steam generator

Info

Publication number: WO2009024855A2
Application number: PCT/IB2008/002171
Authority: WO
Inventors: Luciano Cinotti
Original assignee: Del Nova Vis S.R.L.
Priority date: 2007-08-21
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: WO2009024855A3; ITMI20071683A1

Abstract

The present invention relates to a heat exchanger (1), in particular a large-sized steam generator, having a conveying structure (11) for conveying a first process fluid (F1) and a plurality of heat-exchange tubes (12), in which a second process fluid (F2) circulates; the tubes (12) have respective substantially plane spiral portions (13) set on top of one another in a number of levels to form a substantially annular tube bundle (14). The tube bundle (14) delimits, inside, a substantially cylindrical central space (29) and, outside and together with a casing (2) housing the tube bundle (14), an annular space (30). The internal central space (29) and the annular space (30) constitute respective ducts for passage of the first process fluid (F1) that traverses the tube bundle (14) radially, whilst the second process fluid (F2) circulates within the spiral portions (13) with a radial component of the velocity locally opposite to that of the first process fluid (F1).

Description

"HEAT EXCHANGER, IN PARTICULAR OPERATING AS LARGE-SIZED STEAM GENERATOR"

TECHNICAL FIELD

The present invention relates to a heat exchanger, particularly suitable for operating as large-sized steam generator.

BACKGROUND ART

Heat exchangers and in particular steam generators are components frequently used in numerous industrial applications. In certain plants, there is required the use of high-power (and large-sized) heat exchangers operating at high temperature and high pressure, sometimes also with radioactive fluids or gases for which high reliability is required. In other plants, typically nuclear plants and plants based upon thermodynamic solar energy, there is required the use of steam generators supplied with different process fluids, such as gases, liquid metals, or molten salts. According to known solutions, heat exchangers can be helical-tube exchangers

(i.e., in which the heat-exchange tubes are wound in a helix around an axis, with turns spaced apart along the axis), straight-tube exchangers, U-tube exchangers, all presenting various drawbacks, above all in terms of overall dimensions and fluid-dynamic and heat-exchange effectiveness. In particular, in more burdensome applications there are in general preferred helical-tube heat exchangers/steam generators, which have an excellent capacity for absorbing the thermal gradients linked to their operation. However, helical- tube heat exchangers are relatively complex and costly to produce, and the tubes require a particularly complex supporting system, which can be damaged by fluid- induced vibrations. In addition, to reduce the head losses of the fluid that circulates in transverse flow outside the tubes

(generally, from the top down) , it is necessary to limit the velocity thereof by spacing the tubes out a lot, with consequent increase in the dimensions of the heat exchanger.

DISCLOSURE OF INVENTION

Aim of the present invention is to provide a heat exchanger that overcomes the above highlighted drawbacks of the known solutions and presents advantages in terms of construction and safety.

The present invention hence relates to a heat exchanger, in particular a steam generator, as defined in the annexed Claim 1 and, as regards its auxiliary characteristics and plant configurations, in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in the following non- limiting examples of embodiment, with reference to the figures of the annexed drawings, wherein:

Figure 1 is a schematic view in longitudinal section of a heat exchanger in accordance with the invention, with details represented at an enlarged scale;

Figure 2 is a schematic view in cross section of the heat exchanger of Figure 1, with parts removed for reasons of clarity;

- Figure 3 shows a variant of the heat exchanger illustrated in Figure 2, with details at an enlarged scale,-

Figures 4a to 4e show details at an enlarged scale of the heat exchanger in Figure 1;

Figure 5 shows a further detail at an enlarged scale of the heat exchanger in Figure 1; and

Figures 6-8 are schematic views in longitudinal cross section of respective alternative embodiments of the heat exchanger in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figures 1 and 2, a heat exchanger 1 comprises a conveying structure 11 for conveying a first process fluid Fl, and a plurality of heat-exchange tubes 12, in which a second process fluid F2 circulates.

The conveying structure 11 comprises : a substantially cylindrical casing 2, which extends along an axis A and has closing elements 3, 4 set at respective longitudinal ends of the casing 2; at least one inlet duct 5 and at least one outlet duct 6 for the fluid Fl, arranged through respective openings of the casing 2; and a tubular distributing element 7, which is substantially cylindrical and is housed coaxial within the casing 2 to define, together with the casing 2, an annular chamber 8. The element 7 projects from the closing element 3 along the axis A and is shaped substantially like a glass, presenting a substantially cylindrical side wall 26, provided with radially calibrated through holes 27, and a bottom end wall 28.

The tubes 12 are individually connected, via respective opposite ends, to a delivery manifold 9 and a return manifold 10 for the fluid F2, which, in the example illustrated, are external to the casing 2. In the case where the heat exchanger 1 functions as steam generator, the fluid F2 circulating in the tubes 12 is water brought to boiling point in the tubes 12, and the manifolds 9, 10 constitute, respectively, the manifold for delivery of the water and the manifold for return of the steam. The tubes 12 have respective substantially plane spiral portions 13 wound around the axis A and set on top of one another in a number of levels to form a substantially annular tube bundle 14 set in the chamber 8.

By "spiral portion" is meant a portion having a substantially spiral shape or a shape similar to a spiral, which is wound on itself and about a central axis (axis A) and has a single plane of lie. Each spiral portion 13 is constituted by a plurality of substantially concentric turns 15 set inside one another about the axis A and lying substantially in a common plane perpendicular to the axis A.

The spiral portions 13 are set on top of one another in a number of levels to form the tube bundle 14. On each level there can be set the spiral portion of just one tube, or else on each level there can be set the spiral portions of two or more tubes, the turns of the different tubes of the level being inserted (concatenated) inside one another and lying in a common plane so that the n-th turn of one tube is followed by the n-th turn of the next tube, and so forth.

Each tube 12 comprises, in addition to the spiral portion 13, a delivery branch 16, which connects a radially external end of the portion 13 to the manifold 9, and a return branch 17, which connects a radially internal end of the portion 13 to the manifold 10. The branches 16, 17 are shaped in such a way as to connect the ends of the portions 13 to the manifolds 9, 10 and thus are variously shaped and arranged according to the position of the manifolds 9, 10 with respect to the tube bundle 14. In the example illustrated in Figures 1-2, the branches 16, 17 are substantially orthogonal to the portions 13 and are arranged circumferentially alongside one another around the tube bundle 14, in an annular space 30 delimited between the tube bundle 14 and the casing 2, and, respectively, in a central space 29 inside the tube bundle 14, which is substantially cylindrical and in which is set also the element 7 (the branches 17 being arranged between the tube bundle 14 and the element 7) . The branches 16, 17 are optionally supported by side supports (not illustrated) , which are set spaced circumferentially and axially and project radially from the casing 2 and/or from the element 7 and/or from supporting beams parallel to the axis A.

The heat exchanger 1 comprises a tube bundle supporting system 55 for supporting the tube bundle 14, which supports and/or connects the spiral portions 13 and/or the turns 15. In the example illustrated in Figures 1-2, the spiral portions

13 are rigidly supported in a plurality of circumferential positions via respective supports 18 set radially in the chamber 8 and spaced at angular distances apart from one another .

As illustrated in the enlarged details of Figure 1, each support 18 comprises a plurality of horizontal elements 19, set radially with respect to the axis A and set on top of one another to form a column support 18 that extends vertically substantially throughout the height of the tube bundle 14. Each element 19 is set between the spiral portions 13 set at two consecutive levels, and has two series of saddle- shaped seats 20 set on opposite faces of the element 19 and housing said portions 13 set at consecutive levels.

As illustrated in Figure 2, some supports 18a extend radially throughout the radial extension of the tube bundle 14 and are guided at respective opposite ends by references 24 (just one of which is illustrated in Figure 2) , carried, for example, by the casing 2 and/or by the element 7 or by beams substantially parallel to the axis A and constrained at the top and/or at the bottom to the closing elements 3, 4 so as to maintain the tube bundle 14 centred. Other supports 18b extend radially for less than the radial extension of the tube bundle 14 and are independent of the casing 2 and/or of the element 7 to provide greater flexibility for the tube bundle 14.

Figure 3 illustrates a further example of system 55 for supporting the tubes 12. The terminal turns 15a, 15b, namely the radially innermost turn 15a and the radially outermost turn 15b, of each tube 12 are rigidly connected to the immediately adjacent turns 15c, 15d of the same tube (and precisely to the immediately outer turn 15c and, respectively, to the immediately inner turn 15d) by respective terminal elements 49 for mechanical connection.

The same type of connection can be used also in the case where on each level there are arranged the spiral portions 13 of a number of tubes 12 with concatenated turns: the connection will be made both between inner turns and between outer turns in a number equal to twice the tubes arranged on each level • Each element 49 will engage the terminal turns and the turns adjacent thereto of all the tubes of the level, rigidly- connecting a number of consecutive turns equal to twice the number of tubes present on the level .

The elements 49 can be of various types; illustrated in Figure 3 are two possible alternative embodiments: an element 49 constituted by a pair of opposed jaws 50, gripped to one another and around the tubes 12, for example, via mechanical fixing members 51; and an element 49, which is constituted by a body 52 set between the adjacent turns 15 and has a pair of opposed concave seats, in which the tubes 12 are welded/brazed.

Said arrangement for blocking the tubes 12 prevents the unwinding of the turns 15 of the tubes 12 both during assembly of the heat exchanger 1 and during its operation.

The turns 15 of each tube 12 (or level) are connected by further mechanical-connection elements 54, each of which connects two adjacent turns 15. Each turn 15 is rigidly connected, via elements 54 arranged in circumferential succession, alternately to the inner adjacent turn and to the outer adjacent turn (the turn N is rigidly connected to the turn N-I and to the turn N+l alternately) . The portion of turns comprised between two points of connection can be either constant or variable along the development, as for example in Figure 3, to keep the elements 54 radially aligned. Given the particular configuration of the heat exchanger 1 with tubes 12 set on top of one another, it is possible to identify different arrangements for supporting the tubes 12 (and hence different configurations of the supporting system 55, of the supports 18, and/or of the elements 54) that will prove more or less suitable according to the mechanical and thermal loads acting on the exchanger 1. In general, each supporting arrangement blocks circumferential displacements and allows axial expansions of the tubes 12 (specifically of the spiral portions 13) . As regards the radial displacements, the following modalities can, for example, be envisaged: turns 15 of the individual tubes 12, rendered fixed to one another, and tubes 12, also rendered fixed to one another, for example via supports 18 of the type illustrated in the detail of Figure 1; turns 15 of the individual tubes 12, rendered fixed to one another with possibility of radial sliding of the tubes 12 with respect to one another, for example, via supports 18 of the type illustrated in Figure 4a, in which pairs of elements 19 set on top of one another are coupled by means of guides and projections that enable radial sliding of the elements 19 with respect to one another; turns 15 all radially free, as illustrated in Figure 4b, in which the supports 18 or the elements 54 are constituted by blocks that engage individual turns and are coupled radially slidable one with respect to the others; turns 15 radially free with respect to the horizontal adjacent turns, and constrained, vertically in a column to the turns 15 of the tubes 12 (levels) that are immediately below and above, as illustrated in Figure 4c (which illustrates a solution with aligned tubes 12, i.e., with turns 15 aligned vertically) ;

- turns 15 constrained radially in pairs and constrained in a column, as illustrated in Figure 4d, according to a solution that is applicable, for example, to the interlaced general arrangement of Figure 3 ; - turns 15 constrained by means of crossed tapes 56, as illustrated in Figure 4e, in which the tubes 12 are positioned with respect to one another via spacing tapes 56, set radially and passing alternately above and below the tubes 12 or the turns 15.

With reference also to Figure 5, the exchanger 1 comprises a system 90 for pre-compression of the tube bundle 14, in the case in point an adjustable mechanical system, preferably located in the cold part of the exchanger 1 (top part) , in which the spiral portions 13 of the tubes 12 are set packed tight and compressed by pre-tensioned elastic pusher elements 21, which are set between the tube bundle 14 and fixed contrast elements 22, fixed to the casing 2 of the exchanger 1, and act on the spiral portions 13 and/or on the supports 18 for clamping the tube bundle 14 axially (vertically) .

In the non- limiting example illustrated, each pusher element 21 comprises a spring pack 31 and an adjustment internal- thread/external-thread pair 32.

The spring pack 31 comprises one or more springs 33 carried by a supporting frame 34, which co-operates by contact, via a plate 35, upon a thrust rod 38. The rod 38 transfers the load of the spring 33 to a slidable plate 37, which is arranged in the chamber 8 above the tube bundle 14 and rests directly on the supports 18. The plate 37 is centred on the axis A of the exchanger 1 via guide references 39 fixed with respect to the element 3.

The internal-thread/external-thread pair 32 is installed on a reaction bell 40, which is fixed (for example, via a threaded coupling) to the element 3 and constitutes the fixed contrast element 22, which enables the spring pack 31 to load the supports 18. The internal-thread/external-thread pair 32 comprises a bushing 42, which is fitted on the bell 40 and is provided, inside, with an internal-thread portion, and a threaded rod 41, which is able to slide in the bushing 42 and engages the internal-thread portion. The rod 41 acts via intermediate connection members 43 on the spring pack 31. Preloading of the spring pack 31 is obtained by acting on a top shank of the rod 41 and clamping it with a locknut . A cowling 44 encloses the bell 40 and is coupled in a fluid-tight way to the element 3.

In use, the fluid Fl enters through the inlet duct 5 in the space 29, is distributed radially by the element 7 through the holes 27 (calibrated so that the element 7 will distribute the flow rate of fluid Fl substantially uniformly to the various levels of the tube bundle 14) , traverses radially the tube bundle 14, traverses axially and circumferentially the annular space 30, and finally reaches the outlet duct 6.

The fluid F2 passes from the manifold 9, through the individual tubes 12, to the manifold 10; the fluid F2 traverses each spiral portion 13 of the tubes 12 from the radially external end to the radially internal end so that the radial components of velocity of the two fluids Fl, F2 along one and the same radius of the spiral portions 13 are of opposite sign.

Figure 6, where items that are similar to or the same as the ones already described are designated by the same numbers, illustrates a heat exchanger 1, in particular a steam generator, suitable for the case where the fluid Fl is a liquid metal.

The basic architecture of the exchanger 1 is conceived as follows: the inlet duct 5 for the fluid Fl communicates directly, instead of with the central space 29 as in the example of Figures 1-2, with the annular space 30 delimited between the casing 2 and the tube bundle 14. The conveying structure 11 comprises in this case a tubular element 100 set coaxial in the casing 2 above the tube bundle 14 in a position corresponding to the duct 5. The space 29 communicates, instead, with the outlet duct 6 for the fluid Fl. The fluid Fl enters the casing 2 through the duct 5, flows vertically in the annular space 30 between the casing 2 and the tube bundle 14, traverses the tube bundle 14 radially, and gives out into the space 29 from which it flows vertically towards the duct 6.

The fluid F2 is supplied to the tubes 12 through one or more manifolds 9 (water-delivery manifolds) , which are arranged underneath the tube bundle 14 and are connected to respective sets of tubes 12. There may be envisaged even a number of manifolds 10 (steam-return manifolds) . Figure 6 illustrates, by way of example, two symmetrical manifolds 9 for the water, and a single lateral manifold 10 for the steam.

The element 100 separates a radially external hot header 101 and a radially internal cold header 102, in which the fluid Fl reaches respective pre-set levels. Above the fluid Fl in the headers 101, 102 is a volume of covering gas 103, which enables reduction of the effects of increase in pressure following upon a possible failure of one or more tubes 12.

Especially in the case where the fluids Fl, F2 are incompatible with one another (for example, sodium and water) , failure of a pressurized tube 12 could cause damage to the adjacent tubes triggering a domino effect. In order to prevent, or at least limit, the domino effect, the exchanger 1 is provided with barrier structures 104, 105, 106, which enclose individual tubes 12 or sets of tubes 12 separating them from the other tubes or sets of tubes.

Figure 6 illustrates a constructional solution in which, by way of example, the tubes 12 are divided into sets of four with the insertion every four layers (levels) of shields 104 constituted by plates substantially shaped like an annular disk.

The delivery branches 16, internal to the tube bundle 14, are wrapped, individually or (as illustrated in Figure 6) in sets, by substantially tubular sheathes 105, for example, with a substantially rectangular section, which separate the branches 16 from the internal turns 15 of the spiral portions 13, and from the other branches 16. The sheathes 105 start from progressively decreasing levels of the tube bundle 14 and arrive all at the level of the manifolds 9 (water-delivery manifolds) .

Likewise, the return branches 17, external to the tube bundle 14, are wrapped, individually or in sets, by sheathes 106, which are also substantially tubular with a substantially rectangular section and separate the branches 17 from the external turns 15 of the spiral portions 13 , from the other branches 17, and from the casing 2. The sheathes 106 start from progressively increasing levels of the tube bundle 14 and all arrive at the level of the manifold 10.

Substantially vertical uprights 107 have the function of supporting the sheathes 105, 106 and of positioning the shields 104 and the tube bundle 14 radially. The uprights are arranged so as not to hinder th.e passage of the fluid Fl from the annular space 30 to the central space 29 inside the tube bundle 14. In Figure 6 illustrated by way of example is a constructional solution in which the uprights 107 are fixed to a bottom plate 108, on which the tube bundle 14 rests, and are guided at the top by a top plate 109 arranged on top of the tube bundle 14. The plate 109 is formed by a plurality of concentric rings 110, connected to one another with slight radial spacing by means of respective annular portions set on top of one another. In this way, the plate 109 exerts with its own weight a pre-compression on the underlying tube bundle 14 and enables both sharing-out of the radial thermal gradients and displacement upwards of the outer rings 110 with respect to the inner ones.

Brackets 111, for example with rectangular section, fixed to the casing 2 engage within grooves made on the plates 108, 109, positioning them radially, and also provide a support for the plate 108 and the tube bundle 14. The tube bundle 14 is thus gripped between the fixed bottom plate 108 and the top plate 109, which is free to move vertically (axially) and is formed by the rings 110, which are mobile independently along the axis A, as a function of the axial expansion of the tube bundle 14 that increases from the inner turns towards the outer turns. A gasket 112, with compressible section, provides the tightness between the plate 108 and the casing 2. Insulation elements 113, 114, 115 appropriately arranged reduce the thermal bridges and the thermal gradients on critical parts.

The heat-exchange tubes 12 traverse in a fluid-tight way the casing 2 to converge on the manifolds 9, 10 outside the casing 2.

In Figure 7, where items that are similar to or the same as the ones already described in Figures 1 and 2 are designated by the same numbers, illustrates a heat exchanger 1, specifically a steam generator, particularly suitable in the case where the fluid Fl is a gas .

The tube bundle 14 is always gripped between a pair of end plates 61, 63: the bottom plate 63 is fixed and supported by brackets 64 fixed to the casing 2, and the top plate 61 is mobile in parallel to the axis A and loaded by the pusher elements 21 carried by brackets 65 fixed to the casing 2. The inlet duct 5 and the outlet duct 6 for the fluid Fl are arranged substantially coaxial in a common opening of the casing 2. The duct 5 is connected to a connection structure 58 supported by the casing 2 via a supporting element 59, which is, for example, substantially conical. The connection structure 58 is provided with a slidable seal 60 co-operating with the plate 61, and an insulation element 62 for thermal protection set on an internal surface of the connection structure 58.

The fluid Fl enters the exchanger/steam generator 1 through the duct 5 and is deviated by the connection structure 58 into the space 29 inside the tube bundle 14, and then traverses the tube bundle 14 radially and gives out into the annular space 30 from which it flows vertically towards connection ducts 66, which are connected to a blower 67, which sends the fluid Fl back into a manifold 68 that supplies the outlet duct 6. The fluid F2 coming from the manifold 9 traverses the spiral portions 13 from the radially external ends to the radially internal ends and then reaches the manifold 10.

The manifolds 9, 10 are set inside one another and substantially coaxial to one end of the casing 2. The manifolds 9, 10 are incorporated mechanically in a single manifold body 78, hydraulically divided into a central tubular part (wall) , defining the manifold 10, and an annular part set substantially around the central tubular part (wall) and defining the manifold 9. In particular, the manifold 10 is defined within a tube 69 set substantially along the axis A as prolongation of a steam-outlet piping 70. The tube 69 delimits outside, together with a tubular element 76 set around the tube 69 and through the casing 2, an annular duct 71 for connection of the manifold 9 to a water-supply pipe 72.

The tube 69, provided on the outside with a thermal-insulating element 75, is free to expand axially by means of a slidable fluid-tight coupling 73, provided, for example, on an internal cylindrical surface 74 of the manifold 10.

Figure 8 shows a variant of the exchanger 1 illustrated in Figure 7, in which a number of heat-exchange modules 99 are used, inserted in the same casing 2. Purely by way of example, the exchanger 1 of Figure 8 includes three modules 99.

For a better understanding of the graph of Figure 8 it should be noted that : a) the axes of the three modules 99 are set at the vertices of an equilateral triangle; b) illustrated in the bottom part of Figure 8 is a vertical section of the exchanger 1 drawn along the broken line I -I indicated in the schematic plan view given at the margin of Figure 8 ; c) represented in the top part of Figure 8 are the inlet duct 5 and the outlet duct 6 for the fluid Fl.

The use of a number of modules enables, as compared to a single-module heat exchanger like the one described previously, a more convenient construction on account of the reduced dimension of the individual modules and a smaller overall height of the exchanger.

Located within the common casing 2 are three modules 99, which, from the structural and functional standpoints, are substantially similar to what has been described with reference to Figure 7 and include respective tube bundles 14.

The three modules 99 are arranged circumferentially about the axis A of the heat exchanger 1, spaced at regular intervals from one another. In the example illustrated, the three modules are arranged substantially at 120° from one another.

The inlet duct 5 and the outlet duct 6 for the fluid Fl coincide substantially with those of the basic solution of Figure 7. The duct 5 is, however, connected to a distributor 120 having three outlets, which supply respective modules 99 through respective connection structures 58 connected to a single common plate 121 via a . bellows-type expansion compensator 122. Through the plate 121 the fluid Fl, arriving from the connection structure 58, enters the space 29 inside the tube bundle 14 of each module 99. In each module 99 conveying of the fluid Fl is carried out by means of a tube 123 set within the space 29 and provided with a seal 124 that is free to slide with respect to the plate 121 and with radially calibrated through holes 125 and that terminates at the bottom with an end wall 126. The modules 99 are supported via a bottom plate 127, which is supported in turn by a ferrule 128 welded to diaphragms 129 constrained to the casing 2. A top plate 130, which is slidable and guided by the tubes 123, is rested on the system 55 for supporting the tubes 12 and keeps the tube bundle 14 gripped. Both of the plates 127 and 130 are formed by concentric rings 131, connected to one another by means of respective annular portions set on top of one another with slight radial spacing in order to eliminate the radial thermal gradients. Each ring 131 of the bottom plate 127 supports the radially external adjacent ring. Each ring 131 of the top plate 130 is free to shift upwards with respect to the radially external adjacent ring.

The fluid Fl, after traversing in parallel the tube bundles 14 of the three modules 99, gives out into a plenum chamber 135, which envelopes the modules themselves and is delimited laterally and at the bottom by the casing 2 and at the top by the plate 121. In a way similar to what occurs in the exchanger represented in Figure 7, the fluid Fl flows vertically from the plenum chamber 135 towards the ducts 66 and from here, through the blower 67, reaches the outlet duct 6. From what has been set forth above the advantages of the present invention emerge clearly evident.

The spiral shape of the heat-exchange tubes 12 presents the same advantages as the helical shape as regards absorption of any thermal expansion.

The adoption of a tube bundle 14 with spiral-shaped tubes 12 and with radial flow of the primary fluid leads to a large section of passage for the primary fluid itself, with consequent possibility of limiting the velocity of the primary fluid also with close positioning of the tubes and a significant reduction in the volume of the exchanger (indicatively, for example, half that of known helical-tube solutions) .

The reduction in velocity of the primary fluid and the lower number of arrays of tubes to be traversed (in a radial direction) enables a drastic reduction in the head losses of the primary fluid (indicatively, ten times that of known helical-tube solutions) .

A better uniformity of radial velocity of the primary fluid between the inner part of the exchanger and the outer part can be obtained by varying the radial pitch between the spirals: larger in the inner part and smaller in the outer part .

Both construction and assembly of the tubes with spiral portions are much simpler and less costly than those of helical tubes.

There is no risk of failure of a support 18 with domino effect on all the others because the supports 18 are loaded in compression and exert a constant compression on the tubes 12 by means of an extremely reliable system positioned in the cooler part of the exchanger.

The pre-compression system 90 with elastic elements, which are pre-loaded upon assembly and sized in such a way as to work always in the elastic field in the various possible operating configurations, ensures a predefined compression between spiral portions 13 and supports 18 within a range defined by their elastic characteristic. The system 90 ensures also a limit to the maximum displacements that can take place for example following upon failure of a tube 12.

Loading in compression can be also provided by simply resting a top plate on the system for supporting the tube bundle .

Various configurations of the paths for inlet and outlet of the process fluids are possible, according both to the nature of the process fluids and to the conditions of operation of the exchanger.

The solution is suited for insertion of barrier structures or separation elements between tubes or sets of tubes to reduce the risks of chain propagation of the failure of tubes, which is particularly advantageous in the applications of the exchanger as steam generator operating with liquid metal.

- The solution is suited for adoption of a number of heat- exchange modules, each having a tube bundle with spiral tubes, which is particularly advantageous in the applications of the exchanger as steam generator operating with gas .

Finally, it is understood that numerous modifications and variations can be made to the heat exchanger described and illustrated herein, without thereby departing from the sphere of protection of the annexed claims.

Claims

C L A I M S

1. A heat exchanger (1) , in particular a steam generator, having a conveying structure (11) for conveying a first process fluid (Fl) and a plurality of heat-exchange tubes

(12), in which a second process fluid (F2) circulates; the exchanger being characterized in that the heat-exchange tubes

(12) have respective substantially plane spiral portions (13) set on top of one another in a number of levels to form a substantially annular tube bundle (14) .

2. A heat exchanger according to Claim 1, wherein the exchanger extends along an axis (A) , and the spiral portion (13) of each tube (12) is wound around the axis (A) and is formed by a plurality of substantially concentric turns (15) , which lie in a common plane substantially perpendicular to the axis (A) .

3. A heat exchanger according to Claim 2, wherein set on each level is the spiral portion (13) of just one tube (12), or else set on each level are the spiral portions of two or more tubes, the turns (15) of the various tubes of the level being inserted inside one another so that the n-th turn of one tube is followed by the n-th turn of the next tube, and so forth.

4. A heat exchanger according to any one of the preceding claims, wherein the tube bundle (14) delimits a substantially cylindrical central space (29) that defines a duct for passage of the first process fluid (Fl) .

5. A heat exchanger according to any one of the preceding claims, wherein the conveying structure (11) is shaped so that the first process fluid (Fl) traverses the tube bundle (14) substantially radially, lapping the tubes (12) on the outside.

6. A heat exchanger according to any one of the preceding claims, wherein the conveying structure (11) comprises a distributor element (7) , provided with calibrated holes (27) for distributing the flowrate of the first process fluid (Fl) substantially uniformly to the various levels of the tube bundle (14) .

7. A heat exchanger according to any one of the preceding claims, wherein the radial components of velocity of the first process fluid (Fl) and of the second process fluid (F2) along one and the same radius of the spiral portions (13) are of opposite sign.

8. A heat exchanger according to any one of the preceding claims, comprising a tube bundle supporting system (55) for supporting the tube bundle (14) , which supports and/or connects the spiral portions (13) and/or the turns (15) and is configured so that it blocks circumferential displacements and allows axial expansions of the spiral portions (13) .

9. A heat exchanger according to any one of the preceding claims, wherein the spiral portions (13) are rigidly supported in a plurality of circumferential positions via respective radially arranged supports (18) that are angularly spaced from one another .

10. A heat exchanger according to Claim 9, wherein each support (18) comprises a plurality of elements (19) set on top of one another to form a column that extends substantially throughout the height of the tube bundle (14) .

11. A heat exchanger according to Claim 9 or Claim 10, wherein some supports (18a) extend radially throughout the whole radial extension of the tube bundle (14) and are guided at respective opposite ends by references (24) so as to maintain the tube bundle (14) centred.

12. A heat exchanger according to any one of the preceding claims, wherein the terminal turns (15a, 15b) of each tube (12) are rigidly connected to the immediately adjacent turns (15c, 15d) of the same tube by respective terminal mechanical- connection elements (49) .

13. A heat exchanger according to Claim 12, wherein, in the case where on each level there are arranged the spiral portions (13) of a number of tubes (12) with concatenated turns, the terminal connection elements (49) rigidly connect a number of consecutive turns equal to twice the number of tubes present on the level.

14. A heat exchanger according to any one of the preceding claims, wherein the turns (15) are interlaced via mechanical- connection elements (54) , the turn N being rigidly connected, via said mechanical-connection elements (54) arranged in circumferential succession, to the turn N-I and to the turn N+l alternately.

15. A heat exchanger according to any one of the preceding claims, wherein each turn (15) is radially free.

16. The heat exchanger according to any one of the preceding claims, wherein the turns (15) of a tube (12) are connected fixedly with respect to one another and are free to move radially with respect to the turns (15) of the other tubes (12) .

17. A heat exchanger according to any one of the preceding claims, wherein the turns (15) of a tube (12) are radially free with respect to one another or free in sets, but constrained to the underlying and/or overlying turns .

18. A heat exchanger according to any one of the preceding claims, comprising a system (90) for pre-compression of the tube bundle (14) , which exerts a compression on the tube bundle (14) and/or on a tube bundle supporting system (55) .

19. A heat exchanger according to Claim 18, wherein the tube bundle pre-compression system (90) is an adjustable system.

20. A heat exchanger according to Claim 19, wherein the adjustable pre-compression system (90) is a mechanical system in which the spiral portions (13) are packed tight and compressed by pre-tensioned elastic pusher elements (21) acting on the spiral portions (13) and/or on supports (18) of the spiral portions (13) for clamping the tube bundle (14) axially .

21. A heat exchanger according to any one of the preceding claims, wherein each tube (12) comprises a spiral portion (13) , and a delivery branch (16) and a return branch (17) , which are substantially orthogonal to the spiral portion (13) and are connected to respective radially opposite ends of the spiral portion (13) ; the delivery branches (16) and return branches (17) being arranged circumferentially alongside one another around the tube bundle (14) and on the outside of the tube bundle (14) respectively, or vice versa.

22. A heat exchanger according to any one of the preceding claims, operating as tube-side steam generator, wherein the second process fluid (F2) is water brought to boiling point in the heat-exchange tubes (12) .

23. A heat exchanger according to any one of the preceding claims, wherein the tubes (12) are connected to a delivery manifold (9) and to a return manifold (10) , which are set inside one another and substantially coaxial and incorporated mechanically in a single manifold body (78) hydraulically divided into a central tubular part and an annular part set substantially around the central tubular part and defining said manifolds (9, 10) .

24. A heat exchanger according to any one of the preceding claims, in particular operating as tube-side steam generator, comprising barrier structures (104, 105, 106) that enclose individual tubes (12) or sets of tubes (12) separating them from the other tubes or sets of tubes to prevent a domino effect, triggered by an accidental failure of a first tube, from damaging tubes in sequence .

25. A heat exchanger according to any one of the preceding claims, in particular operating as tube-side steam generator, wherein a number of heat-exchange modules (99) provided with respective tube bundles (14) are installed within a common casing (2) .