Heat-exchanger Module

Abstract

A tube bundle for the circulation of a first fluid is disposed within a shell for the circulation of a second fluid. At least one end of the tube bundle has a rectilineal portion followed by expansion-bends for compensating differential expansions. The tubes are subdivided in the vicinity of the rectilineal portion into two half-bundles each disposed on a square lattice with an identical pitch on each side of a principal lattice line along which the two lattices are relatively displaced by one half pitch. At the level of the expansion-bends, the two lattices are in interjacent relation, the tubes have substantially the same cross-section and the tubes of one row located at right angles to the principal line retain the same cross-section as in the rectilineal portion, extend into the other lattice in the plane of said row and pass between the parallel rows of the other lattice.

Description

The invention relates to a heat-exchanger module of tubular type which can be employed alone but is preferably intended to constitute heat exchangers of large size as a result of interassembly of identical modules.

It is known in particular that heat exchangers which are constructed by interassembling standard units having smaller dimensions are employed in particular for heat transfer processes between two fluids which are liable to give rise to violent reactions with each other in the event of leakage. This problem arises in particular in heat exchangers which are employed as steam generators in conjunction with nuclear reactors of the fast-neutron type ; the fluids between which the transfer takes place are in that case water which vaporizes and a liquid metal such as sodium.

In this case, the design concept of unit construction has the effect of facilitating the location of any leakage which may occur and the possibility of isolating each module individually often avoids the need for complete outage of the heat exchanger in the event of leakage ; moreover, a leak which develops in any particular tube is less liable to result in damage to a large number of adjacent tubes.

The present invention proposes a heat-exchanger module which meets these different requirements more effectively than the structures which have been adopted up to the present time. More particularly, the invention is intended to achieve a further increase in operational safety, especially by solving the problems of differential thermal expansions and vibrations, as well as to permit the construction of a module which has a substantially uniform and compact external geometry and which can readily be assembled with other identical modules in order to constitute a heat exchanger having a high output and small overall space requirements while ensuring a high degree of operational safety.

The heat-exchanger module in accordance with the invention comprises a plurality of tubes for the circulation of a first fluid which are disposed in a bundle between two tube-plates within a shell for the circulation of a second fluid and is provided at least at one extremity of said bundle with a rectilineal portion followed by expansion-bends for compensating differential expansions. The heat-exchanger module is characterized in that said tubes are subdivided at least in said rectilineal portion in the vicinity of said tube-bundle extremity into two half-bundles each disposed on a square lattice with an identical pitch on each side of a principal lattice line along which the two lattices are relatively displaced by 1/2 pitch. At the level of the expansion-bends, the two lattices are disposed in interjacent relation and the tubes of each row which is located at right angles to said principal line in each lattice retain substantially the same cross-section as in the rectilineal portion, extend into the other lattice while forming the expansion-bends in the plane of said row and pass between the parallel rows of the tubes of the other lattice.

In accordance with further characteristic features, the heat-exchanger module is characterized in that it comprises a pipe for the discharge or admission of said second fluid which is located at a sufficient distance from said tube-bundle extremity to assist the circulation of said second fluid away from the expansion-bends, that said shell forms in the vicinity of said tube-bundle extremity a chamber which has a larger diameter than the bundle itself and which encloses the expansion-bends and/or the tube bundle is surrounded within said chamber by a sleeve which terminates at the level of the expansion-bends, said sleeve being provided with at least one opening through which the second fluid is permitted to pass.

A particular embodiment which has been chosen by way of example in order to provide an explanatory illustration of the invention will now be described with reference to the accompanying drawings, in which :

FIG. 1 is a vertical sectional view of a heat-exchanger module in accordance with the invention ;

FIG. 2 is a diagrammatic sectional view of the same module taken along line A--A of FIG. 1.

FIG. 3 is a sectional view through the module of FIG. 1 to show the distribution of the tubes therein.

The heat-exchanger module as illustrated in these figures is a unitary module which is intended to be assembled together with other similar modules disposed in parallel so as to constitute a complete heat exchanger.

In the particular case under consideration, said heat exchanger is in fact the steam generator of a nuclear power station in which a breeder reactor is cooled by liquid sodium or another liquid alkali metal. During operation, heat is transferred by means of said heat exchanger between the hot sodium which is cooled and the water which is vaporized.

The heat exchanger is of the tubular type. As shown in FIG. 1, each module is essentially constituted by a shell 1 which is normally vertical and by a bundle of straight tubes 2 which are enclosed within said shell. The ends of said tubes are welded to a bottom tube-plate 4 and a top tube-plate 5, both tube-plates being located within the shell 1 respectively at each end of this latter. The tubes are thus disposed in parallel relation on the flow path of a first heat-transfer fluid which consists in this case of water in the form of either liquid or steam. The second fluid which consists of liquid sodium circulates in contact with said tubes within the shell 1. In order that the circulation should take place in a predetermined flow pattern, an inlet pipe 6 is provided for the admission of sodium into the upper portion of the module and an outlet pipe 7 is provided for the discharge of the sodium at the lower end whilst the water circulates within the tubes in the upward direction from a supply pipe 8 to a discharge pipe 9.

The vaporized water in fact passes out of the module in the state of superheated steam. The module therefore has an upper section forming a superheater above a lower section which constitutes an economizer-evaporator. Moreover, the module is so designed that the superheater section can be placed in parallel with a resuperheater constituted by another module which may be of a different type. To this end, the module as herein described is provided between the superheater section and the economizer-evaporator section with an additional sodium inlet 10 through which the sodium passes from the resuperheater and joins the hot sodium stream from the superheater section at the inlet of the economizer-evaporator section.

At the level of each sodium inlet or outlet, the shell 1 is enclosed by a spaced wall to form chambers 11 having a larger diameter than the cylindrical portions of shell 1 which surrounds the tube bundle. However, the cylindrical portions of shell 1 are extended respectively within the interior of each chamber 11 by means of extensions 12, 13, 14, respectively, which protect the tubes from direct impact of the sodium but are nevertheless provided with one or a number of openings 15 through which the sodium is permitted to pass.

Moreover, the different tubes of the bundle are more closely spaced within the superheater section than within the economizer-evaporator section. In order that the same overall size of the shell 1 should nevertheless be maintained in both sections, provision is made inside said shell and within the superheater section for an inner sheath 16 which surrounds the tube bundle, which is welded to the shell at the upper end of this latter and which limits the zone of sodium circulation.

In order to permit compensation for differential expansions during operation, the configuration of each tube of the bundle forms an expansion-bend 18 near the lower end of the module above the bottom tube-plate 4.

The sleeve 12 which is provided around the tube bundle within the lower sodium-outlet chamber stops short of said expansion-bends and these latter can thus take up the entire available cross-sectional space within said chamber. Moreover, the result thereby achieved is to assist the circulation of sodium away from said extension-bends which are thus located in a calm zone.

In this zone in which the tubes consequently have no further function in the heat transfer process, a compact arrangement is obtained by virtue of an original distribution of the bundle tubes and by virtue of the arrangement of interjacent expansion-bends which is thus made possible.

In the rectilineal portion which precedes the expansion-bends in the vicinity of the lower end of the tube bundle, the different tubes are subdivided into two half-bundles each disposed on a square lattice with an identical pitch on each side of a main lattice line along a diameter of the module. The geometry which is imposed at this end of the tube bundle is not necessarily retained in the remainder of the module.

As shown in FIG. 2, the two lattices conform to the same square configuration and pitch but are relatively displaced by one-half pitch along the principal line of separation. The expansion-bends formed by the tubes of one lattice which form part of a same row at right-angles to said principal line can thus penetrate into the space of the other lattice while remaining in said plane and while passing between two parallel rows of tubes of the other lattice. It should be pointed out that, at the level of the expansion-bends, the tubes still have substantially the same cross-section as in the preceding rectilineal portion.

The module as hereinabove described thus makes it possible to ensure effective compensation for differential expansions by means of the expansion-bends which are provided at the lower extremity but which nevertheless do not call for any increase in overall size of the module. Positioning of said expansion-bends away from the normal sodium flow prevents vibrations.

The invention is naturally not limited in any sense to the particular embodiment which has been described in the foregoing but extends to all alternative forms.

Claims

What we claim is :

1. A heat-exchanger module comprising a plurality of tubes for the circulation of a first fluid which are disposed in a bundle between two tube-plates within a shell for the circulation of a second fluid and provided at least at one extremity of said bundle with a rectilineal portion followed by expansion-bends for compensating differential expansions, wherein said tubes are subdivided at least in said rectilineal portion in the vicinity of said tube-bundle extremity into two half-bundles each disposed on a square lattice with an identical pitch on each side of a principal lattice line along which the two lattices are relatively displaced by one half pitch and wherein said two lattices are disposed in interjacent relation at the level of the expansion-bends and the tubes of each row which is located at right angles to said principal line in each lattice retain substantially the same cross-section as in the rectilineal portion and extend into the other lattice while forming the expansion-bends in the plane of said row and passing between the parallel rows of the tubes of the other lattice.

2. A heat-exchanger module according to claim 1, wherein said module comprises a pipe for the discharge or admission of said second fluid which is located at a sufficient distance from said tube-bundle extremity to assist the circulation of said second fluid away from the expansion-bends.

3. A heat-exchanger module according to claim 1, wherein said shell forms in the vicinity of said tube-bundle extremity a chamber which has a larger diameter than the bundle itself and which encloses the expansion-bends.

4. A heat-exchanger module according to claim 3, wherein the tube bundle is surrounded within said chamber by a sleeve which terminates at the level of the expansion-bends, said sleeve being provided with at least one opening through which the second fluid is permitted to pass.

5. A heat-exchanger module according to claim 1, said module being employed as a steam generator for the vaporization and superheating of said first fluid, wherein the pitch of the tube bundle is smaller in the superheater section than in the evaporator section, said tube bundle being surrounded at the level of the superheater section by an inner sheath which limits the zone of circulation of the second fluid within said shell.