IL27866A - Heat exchanger - Google Patents

Description

•niun |Π3 TJinrn m PATENT ATTORNEYS ■ D'DID D 'DTIU PATENTS AND DESIGNS ORDINANCE SPECIFICATION Heat exchanger I/We COMMISSARIAT A L'BNERGIB ATOMIQUE, of 29, rue de la F½6ra ion,- Paris 15®, France do hereby declar.e the nature of this inYention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:- This invention relates to a heat exchanger which is designed for the purpose of transferring heat between two fluids which are liable to react dangerously with each other, the primary object of said heat exchanger being to permit the vaporization of water by heat transfer with a liquid metal such as sodium „ In nuclear power plants in particular, when the reactor is of the fast neutron type, the steam generator is usually heated by liquid sodium which constitutes the secondary reactor coolant „ It is known that the transfer of heat between liquid sodium and water presents complex problems of safet 0 Any contact between the two fluids must in fact be prevented since they react violently with each other For the construction of steam generators of this type, use is sometimes made of double-walled exchanger tubes, provision being made if necessary for the circulation of an intermediate fluid which permits the detection of leakage „ These complex arrangements are particularly costly. For this reason, preference is now given to single-walled exchangers which are much more economical to construct and in which a first heat-transfer fluid which is usually sodium flows inside envelope tubes or outer tubes which are connected in parallel to sodium inlet and outlet headers, each of said envelope tubes being designed to contain a plurality of inner tubes for the circulation of the second heat-transfer fluid which usually consists of water, the two ends of said inner tubes being connected in parallel to water inlet and outlet headers In these heat exchan r - ν.-' it must be made possible to control to an adequate extent the quality of tubes employed as well as the quality of the welds which are formed between tubes,, In addition, should leakage of water occur in spite of the precautions taken, it must also be made possible to isolate the faulty tube in order to reduce the heat exchanger outage time to a minimum ; this requirement of rapid re-commissioning is particularly crucial in nuclear power plants since heat exchanger outages entail high capital cost in such cases.

It is also necessary to provide for ready isolation and replacement of each standard unit formed by an outer tube and the corresponding water tubes in the event that the reaction produced as a result of leakage in a water tube were liable to cause damage to adjacent tubes .

The aim of this invention is to satisfy the above-noted requirements to a greater extent than has hitherto been possible in comparable systems of the prior art„ A further object of the invention is to solve the problems raised by differential expansion processes between the different tubes and headers while at the same time permitting the utilization of tubes of substantial length within a small space » To this end, the invention proposes a heat exchanger which is intended especially for the vaporization of water by heat transfer with a liquid metal such as sodium, said heat exchanger being essentially characterized in that it comprises a plurality of outer tubes disposed in parallel planes and each comprising straight sections joined to each other by bends so as to form snaked coils, said outer tubes being connected in parallel to inlet and outlet headers through which a first heat-transfer fluid such as sodium is admitted and discharged and which are disposed substantially in the mid-plane of said straight sections, each of said outer tubes being designed to contain a plurality of inner tubes for the circulation of a second heat-transfer fluid such as water, the ends of said outer tubes being connected in parallel to the inlet and outlet headers for the admission and discharge of said second fluid „ This arrangement of headers in the central zone of the snaked coils makes it possible to provide automatic compensation or differential expansion processes between the inner tubes, outer tubes and headers „ In a preferred embodiment of the heat exchanger according to the invention, whereby the use of tube sheets at the delivery end of the outer tubes is dispensed with and each inner tube for the circulation of the second heat-transfer fluid can readily be isolated, said inner tubes · are each passed individually through the wall of the corresponding outer tube prior to branching on the headers „ In order to facilitate the circulation of the first heat-transfer fluid, it is then of particular interest to provide the outlet of the inner tubes for the circulation of the second heat-transfer fluid in the side wall of the outer tube, the end of which can thus be branched directly on the corresponding header „ One exemplified embodiment of a heat exchanger according to the invention will now be given in the following description which relates more specifically to a steam generator in which the steam is heated by liquid sodium . It will be understood that this, description is * given solely "by way of example without thereby implying any limitation of the invention, reference being made to the accompanying drawings, in which : - Fig, 1 is a longitudinal sectional view of the steam generator herein described ; - Fig. 2 is a side view of the steam generator of Fig. 1 5 - Fig. 3 shows a first mode of assembly of the water tubes inside the outer tube (envelope tube) through which sodium is circulated ; - Fig, illustrates a second mode of assembly which is employed as an alternative to Fig. 3 ; - Fig, 5 shows the detail of the outlets of the water tubes from an outer tube.

The steam generator herein described employs as heating fluid liquid sodium which constitutes the secondary coolant of a fast reactor. Said secondary coolant is in turn heated by heat transfer with the main reactor coolant which also consists of liquid sodium. The steam generator is supplied with water which is successively reheated, vaporized, then superheated by heat transfer with the liquid sodium which is circulated countercurrent to the water, whether liquid or vaporized. The steam generator is made up of a large number of standard units of identical design, each forming either an economizer and evaporator or a superheater. These standard units are mounted together with the corresponding headers in a number of separate blocks as shown in Figs. 1 and 2, each block being fitted with the requisite isolation valves for taking it off line both on the sodium side and on the water side . The steam generator can comprise in .addition either one or a number of resu er- heaters v/hich are also constructed in the form of standard units of similar design to those mentioned above.

The block which is shown in Figs. 1 and 2 comprises a plurality of economizer and evaporator units 1 as shown in the lower half 2 of the figures (there being fourteen in the example shown) and a corresponding number of superheater units 3 as shown in the upper half of the figures. These standard units as well as the associated headers are contained in an enclosure 5? the walls of which are lined with insulating material 6, Bach standard unit 1 or 2 is constituted by an envelope tube or outer tube 7 in which sodium is circulated and v/hich contains the water circulation tubes 8 (as shown in Fig, 5)« Each outer tube 7 is designed in the form of a snaked coil having a double S configuration in a vertical plane. The two ends of the outer tubes are joined to sodium header-pipes which bear the reference numerals 10 and 11 in the case of the economizer and evaporator units and 12 and 13 in the case of the superheater units. These four headers are horizontal and located in a same vertical plane at right angles to the planes defined by the coils in the central zone of these latter.

In other words, each outer tube 7 comprises five substantially horizontal straight runs joined to each other by bends at 180° which are located alternately in the vicinity of two opposite lateral faces of the heat-insulated enclosure 5« The two straight outer-tube sections which are located at the ends are of shorter length and extend only from one lateral face of the enclosure 3 up to the appropriate header on which they are branched laterally.

In order to facilitate the connection of outer tubes to the sodium headers and to obtain a highly compact assembly, adjacent outer tubes are accordingly disposed in top-to-tail relation and their ends are joined to the headers alternately along each of two diametrically opposite generatrices This alternate arrangement of the outer tubes also facilitates 'subsequent withdrawal of any one casing tube for repair purposes.

There are disposed inside each outer tube 7 seven water tubes 8 which are all parallel to the outer tube . Said water tubes are held in position by braces of the type shown in Fig. 3 whereby the water tubes are permitted to expand relatively to the outer tubes while remaining at the same distance from each other and whereby any possibility of vibration of the tubes under the action of the flow of fluids is prevented.

The brace of Fig. 3 is made up of an assembly of flat welded plates 1 and is provided with fins 15 which are intended to mix the sodium which flows within the outer tube in order to avoid any danger of laminar flow. As an alternative form, steel "hairpin" spacers 16 can be employed to join the different tubes together, as shown in Fig. . These hairpins are nevertheless subject to a disadvantage compared with the previous solution in that they must be of relatively substantial thickness, thereby reducing the cross-sectional area which is provided for the flow of sodium.

As is apparent from Figs. 1 and 5, the seven water tubes 8 each pass in leak-tight manner through the surrounding envelope tube 7 in the vicinity of each end of this latter. This arrangement of separate water-tube outlets dispenses with the need of tube-sheets which would have to be of relatively substantial thickness in order to afford adequate resistance to the sodium pressure.

Furthermore, by reason of the fact that the water tubes pass through the side of the envelope or outer tube, said tube can be branched directly on the sodium header, thereby facilitating the circulation of this fluid.

After being passed through the outer tubes 7 » the water tubes 8 open into water or steam sub-headers. At the inlet of the economizer and evaporator units 1 (as shown at the bottom of the enclosure 4), said sub-headers 18 are connected to a feedwater header 19 » At the outlet of said units, each steam sub-header 20 supplies the water tubes 8 of a superheater unit 3 directly, Finally, at the outlet of said superheater unit, the superheated steam sub-header 21 is connected to a header 22 which collects the whole quantity of superheated steam which is produced in the block. The water header 19 and the superheated steam header 22 are located in the mid-plane of the standard units 1 and 2 in the same manner as the sodium headers.

The circulation of the two fluids is carried out in a stage-by-stage process. The hot sodium is distributed to the outer tubes of the superheater units via the feed header 13 , After having passed through these units from the top downwards, the sodium flows out of the upper half of the enclosure 5 via the header 12 which is joined by a bend 23 located outside the enclosure to the header 11 which serves to supply the . economizer and evaporator units 1 , After passing through these latter inside the outer tubes, the sodium is discharged from the block via the cold sodium header 10, The arrangement of the different water or steam headers on the one hand and sodium headers on the other hand which occupy a central position with respect to all the snaked-coil units permits of automatic compensation for differential expansion processes which take place on the one hand between each outer tube and the water tubes contained therein and, on the other hand, between each water or steam sub-header or each outer tube and- the corresponding headers.

The different outer tubes as well as the headers are all suspended from the top wall of the heat-insulated enclosure 5 "b means of tie-rods which are not shown in the drawings. This arrangement provides a solution to the problems presented by differential expansions between headers or outer tubes and the heat-insulated enclosure on the one hand and, on the other hand, between headers at different temperatures „ In addition to providing compensation for differential expansions and thus preventing the development of strains, the heat exchanger as hereinabove described has the further advantage of readily permitting the isolation of a water tube when a leak has been detected,, In fact, it is merely necessary to cut off and isolate said tube to which access can readily be gained between the sub-header and the corresponding outer tube, and the steam generator can be put back into service « The arrangement adopted also makes it possible to eliminate the problems of thermal shock inasmuch as the thicknesses of metal employed always remain of a sufficiently small orde „ When substantial leakage takes place and causes damage to adjacent water tubes, the corresponding standard unit must be wholly replaced by a new unit. In the heat exchanger which has been described in the foregoing, this replacement is very easily performed since the staggered arrangement of the outer tubes makes it possible to withdraw these latter separately from the enclosure without any difficulty » As is apparent, the invention is not limited in any respect to the mode of execution which has been described above by way of example but is intended to include within its scope all alternative forms of execution which remain within the definition of equivalent mechanical means.

Claims (4)

HkVlUQ KOI particularly described nd aeeertained the nature of our eaid invention and in whe taanner e ea^ is to be performed, we declare that what we claim Is:

1. , A heat exchanger comprising a plurality of outer tubes disposed in parallel planes and each comprising straight sections joined to each other by bends so as to form snaked coils,, said outer,,tubes being connected in ( I · ' '' < parallel to inlet and outlet headers through which a first heat-transfer fluid is admitted and discharged, each of said outer tubes being designed to contain a plurality of inner ' tubesjfor the circulation of a second heat-transfer fluid, the ends of said inner tubes being designed to 'pass individually through the lateral wall of the corresponding outer tube at each end of it and being connected in parallel externally of said outer tubes to inlet and outlet headers for the admission and discharge of said second fluid.

2. A heat exchanger in accordance with Claim 1, wherein the headers of the first fluid are disposed substantially in the centre plane normal to the straight sections.

3. A heat exchanger in accordance with Claim i or Claim 2, wherein the outer tubes are disposed in head-to-tail relation, said outer tubes being branched on the inlet and outlet headers for the first heat-transfer fluid at the ends of the outer tubes and alternately along two diametrically opposite generatrices of the header.

4. A heat exchanger in accordance with Claim 1 , substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. Bati!d hie 24th April 196? For the Applicantsί □ ?n9ft . mim mm & CO.