FR1449629A - Sampling device for heat exchangers, and method comprising the same - Google Patents

Description

<B> Sampling device for heat exchangers, and method comprising </ B> ap = plication. The present invention relates generally to the recovery of heat produced by a chemical or nuclear reaction for conversion into noble energy.

Heat sources used at the industrial scale, namely combustions and nuclear reactions, generally provide a primary fluid at high temperature, fumes or cooling fluids from the reactor, respectively, whose heat is transferred by exchange. to a vaporizable liquid, most of the water, and then converted into noble energy by expansion of the steam produced in a thermal machine, for example a turbine.

The overall efficiency of energy conversion in such an installation depends essentially on the temperature of the hot source, that is to say, in fact, the maximum temperature of the primary fluid, which is limited by the behavior of the materials constituting the boiler, or the associated reactor and exchanger.

In conventional installations, where heat is produced by burning coal, fuel oil, natural gas, etc., the very high temperature of the combustion products makes it possible to obtain a good overall efficiency, in particular by the high overheating that can be achieved. and by the possibility of placing the reheater (s) in the flue gas circuit, in series with the other stages of the steam generator.

In nuclear installations, on the other hand, the temperature of the primary fluid, for example carbon dioxide in a plant of the natural uranium-graphite-gas type, is limited to a relatively low level, in particular by the strength of the fuel sheathings. This handicap requires, in order to achieve acceptable overall efficiency, to take the utmost care in obtaining an excellent clean performance of the thermodynamic heat cycle and heat exchanges, while limiting as much as possible the losses of heat. their. In these respects, it is advantageous to use the steam, successively or in parallel, at several pressure levels, with overheating and / or reheating beforehand, to substitute for the heat exchange between gases exchanges between liquids or between gas and liquid, and to overheat and / or re-heat the steam in the immediate vicinity of its point of use, i.e. the turbine.

These various advantages can be obtained by the device and the method of the invention, which essentially allow the overheating and / or the re-heating of the steam by means of water sampled at a substantially boiling temperature and under a higher pressure in at least some of the tubes of the steam generator heat exchanger.

According to a first feature of the device of the invention, the sampling is carried out in each of the tubes concerned in an area where, regardless of the operating speed of the generator, the fluid is in the liquid state and substantially at a temperature of boiling, by means of a sampling duct disposed inside said tube and leaving at an accessible point, and in particular at the end.

According to other features of the device of the invention, the aforesaid conduit exits the upstream end, or inlet, of the aforementioned tube, and is advantageously designed to ensure thermal insulation of the fluid taken.

The process according to the invention is characterized in particular by the fact that the liquid fraction taken substantially at boiling temperature is used to overheat and / or reheat steam at lower pressure, for example to reheat superheated steam from said liquid and having undergone at least partial relaxation in a thermal machine, or to overheat saturated steam produced at low pressure.

Other features and advantages of the device and of the method of the invention will appear more clearly on reading the following description of several examples of covering, illustrated by the appended drawings, in which FIG. partial axial of an exchanger tube provided with a sampling device according to the invention; Figure 1A shows on a larger scale a detail of the embodiment of the tube of Figure 1; Figure 2 is constituted by two partial axial sections illustrating an alternative embodiment of the sampling device of Figure 1; Figure 3 illustrates an advantageous arrangement of tube connections in an exchange provided with a sampling device according to the invention; and Figs. 4 and 5 are the respective block diagrams of two power generation plants whose operating cycles implement the method of sampling the invention.

FIG. 1 shows, in axial section, the ends and an intermediate portion of a tube intended to be incorporated in the beams of a steam-generating heat exchanger, said tube being provided with a device for sampling fluid according to the invention.

The exchange tube shown is a tube with internal annular helical circulation, and countercurrent longitudinal external circulation, of the type described in French Patent No. 1,267,796 of January 22, 1960 and in French Patent No. 1,370,564. of 7 June 1961 filed in the name of the plaintiff. In the embodiment described and illustrated, this tube, designated as a whole by 10, is straight and vertical, some ten meters long and 70 mm in average outside diameter. Over most of its length, it is provided with transverse fins 11, formed for example by spinning or cold knurling, designed to increase the exchange surface with the external fluid, and a helical fin rap 12, aiming to create at all points a transverse diffusion of the external fluid. The ends of the tube 10 are smooth and their diameter is gradually narrowed, for example by hammering or cold forging, to an outside diameter of about 48 mm. This results in an increase in the thickness of the wall of the tube which, for example, is 4 mm, for example, to 6 mm, and in a corollary manner, an increased resistance of the connecting elbows 13 and lower 14, by which the tube 10 is connected either to other tubes of the same beam, or to collectors; thus, the connecting bends 13 and 14 alone can support the tube 10, while allowing its expansion due to their shape.

Inside the tube 10 is disposed a tubular core 15, narrowed or pinched at both ends; a calibrated wire 16 helically wound between the tube 10 and the core 15 on the one hand maintains the latter in the axis of the tube and on the other hand compels the internal fluid to follow a helical path; as a result, said internal fluid is subjected to the action of centrifugal forces which press against the inner wall of the tube 10 in the form of a very stretched meniscus film, and which create density segregation which on the one hand This causes the cooler fluid to come into contact with the tube and, on the other hand, considerably accelerates the release of the incipient vapor bubbles.

The tube 10 described and illustrated is provided for an upward internal circulation; that is to say that the connecting elbows 13 and 14 are connected, directly or indirectly, to water and steam collectors respectively. Under these conditions, the core 15 is advantageously open at least at two levels, including its lower end, so that it can accumulate a quasi-stagnant water reserve, substantially in thermal equilibrium with the circulating fluid. between the tube 10 and the core.

The cold inlet water through the lower connection elbow 13 is progressively warmed by its passage in the lower part of the annular circulation space, which thus acts as an economizer; after about two meters of vertical travel, under normal charging conditions, the water is brought to a substantially boiling temperature. In the next portion of the tube, which acts as a vaporizer, the water becomes emulsion, then saturated steam, in about 5 meters of vertical course. In the. In the upper part of the tube 10, the saturated steam is progressively superheated to a temperature a few degrees lower than that of the external fluid.

In parallel, the temperature of the stagnant water column contained in the lower part of the core 15 rises progressively over the height of the economizer, then remains at saturation temperature, and therefore at a substantially constant temperature, over the entire height of the vaporizer, that is to say about 5 meters.

The large relative height of the vaporizer means that, in spite of the sliding of the transition levels accompanying the load variations, there exists inside the core 15 a fixed zone, corresponding to the overlap of the different positions of the vaporizer, in which the water is constantly at a substantially boiling temperature.

Accordingly, and for virtually all steady state or transient operating conditions, substantially boiling water may be withdrawn by means of a fixed conduit leading to an appropriate level in said zone of the core 15.

It is important, however, to avoid that the removal of water thus carried out substantially disturbs the thermal equilibrium of the column of standing water inside the core 15, and more particularly to prevent that the water taken directly from the tube inlet through the lower portion of the core 15, thereby creating a detrimental thermal shunt; accordingly, the lower end of the core 15, necessarily open to preserve the possibilities of vidan ge, must however be relatively tight; the removal of water must necessarily be carried out at the expense of the circulating fluid, and for this purpose, a second communication must be established through the wall of the core 15, preferably in the constant temperature zone common to all the vaporizers , and at a level such that the pressure in the circulation space is at least equal to that at the same level in the core 15.

However, as one goes up in the economizer, the dynamic pressure losses experienced by the circulating water alone make the stagnant water is in increasing relative overpressure; in the vaporizer stage, this light pressure is quickly canceled and gives way to a relative depression, due to the sudden density drop that accompanies the formation of the emulsion; this relative depression is in turn progressively canceled due to friction in the superheater. If, therefore, one or more communication orifices are made in the wall of the core 15 at equilibrium level or better in the zone of relative depression, the circulating fluid tends to penetrate of itself inside the core 15.

However, the fluid thus introduced into the core 15 may be in the emulsion state; therefore, it is advantageous that the water withdrawal in the core 15 is performed at a level sufficiently lower than that of said orifices to avoid any entrainment by water taken from non-condensed bubbles.

For the same purpose, it is advantageous for the above-mentioned communication orifices to be shaped in such a manner as to preferentially pick the circulating water pressed against the inner wall of the tube by the centrifugal force; for this purpose, a simple means consists in deforming the upper and lower edges of each orifice, as shown in Figure 1A, for example by levering by means of a pin.

Of course, in addition to the aforementioned orifices, the core 15 may be open at one or more upper levels, and in particular at its upper end. In the latter case, however, it is desirable that the opening be reduced in order to damp the transient pressure oscillations.

In the exemplary embodiment illustrated in FIG. 1.1, the core 15 is open at its two extremities, and comprises a series of communication orifices 17 distributed over a certain height at the level of the vaporizer; the water intake duct, designated by 18 as a whole, is arranged coaxially with the lower part of the tube 10, into which it enters through a sealed trap arranged in the lower connection elbow 13, and comes to the inside of the core 15, below the communication ports 17. Throughout its entire length inside the tube 10, the sampling duct 18 comprises two concentric walls 19, 20, held apart by a wire 21 wound helically; the upper ends of these two walls, advantageously reinforced by forging, are welded to each other in a sealed manner, and advantageously form a flare 22; at the bottom of the duct, only the outer wall 20, the thickness of which can advantageously be locally reinforced, is welded at 23 to the thick wall of the lower connecting elbow 13. This construction has the advantages of ensuring excellent thermal insulation of the sampled fluid, while allowing the differential expansions of the two walls; in addition, the solder 23 assembles two walls whose respective thicknesses are of the same order of magnitude, and both of which are in contact with the cold water entering the tube 10, a condition which is eminently favorable to keeping in service. Finally, in order to allow the emptying of the core 15 and the balance of the pressures between the circulating water and the stagnant water, the duct 18 advantageously passes through the lower end of the core 15 with a small calibrated clearance. ; this provision, added to the presence of the flange formed by the flare 22, has the advantage of limiting any displacement of the sampling duct 18 with respect to the axis of the exchange tube 10.

FIG. 2 shows in detail the lower end and an intermediate portion of an exchange tube 17 identical to that previously described, but provided with a sampling duct designated as a whole, the embodiment of which differs substantially from that of the duct 18 previously described.

The sampling duct 30, like the con duct 18, is arranged coaxially with the lower part of the exchange tube 10 into which it enters through a tight passage arranged in the wall of the connecting elbow 13. The conduit 30 is essentially consisting of a steel tube 31 of the same grade as that of the exchange tube 10, and whose wall thickness, for example 3 mm, is relatively large, so as to allow easy and safe welding to the thick wall of the connecting elbow 13, by means of a weld bead 32 widely fed.

The portion of the tube 31 located inside the exchange tube 10 is surrounded by an outer sleeve 33 with relatively thin walls, advantageously made of stainless steel; manchon 33 is secured to the tube 31 by sim ple crimping its upper end on the wall of the tube; in the exemplary embodiment illustrated, this crimping is made very simply by forced expansion of the upper end of the sleeve 33 during the formation of the end flare 34 of the tube 31. The manch 33 thus made integral with the tube 31 is kept at constant spacing by any appropriate means, and for example by solder drops 35 deposited from place to place on the outer surface of the tube 31. As shown, the lower end of the sleeve 33 is advantageously cut in whistle and directed that a small and relatively constant distance separates each of its points from the inner surface of the connecting elbow 13.

The tube 31 is further provided with an inner sleeve 36 extending a certain distance on either side of the passage of the wall of the connecting elbow <B> 13; </ B> the inner sleeve 36, like the outer sleeve 33 previously described, is relatively thin-walled and is advantageously made of stainless steel; its upper end 37 is crimped on the inner surface of the tube 31, for example by swaging; its lower end is free, and preferably located at the beginning of a bend of the tube 31, so as to prevent slipping of the sleeve inside the tube; finally, a constant spacing is maintained between the tube 31 and the inner sleeve 36, for example by means of a few solder drops 38 deposited in place on the outer surface of the sleeve.

The embodiment of the sampling duct which has just been described has numerous advantages, both from the point of view of construction and from the point of view of safety; In this connection, it has already been mentioned that the weld 32 contains two walls of the same kind, the thicknesses of which are relatively strong and of the same order of magnitude; these conditions are perfectly favorable to the realization of a welding of very good quality; in addition, in use, the portion of the tube 31 adjacent the weld 32 is exposed in direct contact with the cold water entering the exchange tube 10 by the connecting elbow 13, this thanks to the clearance between the on the inner face of the latter and the whistled end of the outer sleeve <B> 33; </ B> by cons, this same portion of the tube 31 is isolated from the boiling water taken by the inner sleeve 36. Thus, the weld 32 not only concerns two walls of similar thickness and therefore of the same thermal conductivity, but also two walls which, in all circumstances, have substantially the same temperature; these conditions are eminently favorable to the good performance in service of the weld, thus maintained at relatively low temperature and preserved from the harmful influence of thermal shocks and corollary differential expansions. In a similar manner, the upper ends of the sleeves 33 and 36 are constantly brought to the same temperature as the portions of the wall of the tube 31 on which they are respectively crimped; this circumstance, because of the coefficients of expansion different from ordinary steel and stainless steel, has the result of reinforcing the service life of these assemblies.

The embodiment of the withdrawal conduit 30 has the additional advantage of minimizing heat loss; first, it will be recalled that the stainless steel constituting the sleeves 33 and 36 has a relatively low thermal conductivity; the arrangement of the inner sleeve 36 on either side of the weld 32 effectively isolates the hot water taken from the cold part of the tube 31, and strongly limits the thermal gradient, and therefore the flow of heat along said tube which Very gradually passes from the temperature of the cold water to the temperature of the boiling water along the length separating the weld 32 from the ends of the sleeve 36. Likewise, the outer sleeve 33 traps a layer of stagnant water whose The temperature increases very progressively, in equilibrium with that of the wall of the tube 31.

 Finally, it will be noted that the arrangement of the sampling circuit 30, like that of the previously described conundice 18, is particularly suitable for precise assembly, which does not risk weakening the resistance of the elements concerned. In fact, in both cases, the conduit previously provided with its or. of its thermal insulation sleeves can be inserted into the tube 10 and into the core 15 by their upper ends, before the formation of the corresponding necks, so that the opening in the lower connection elbow 13 can be very precisely calibrated; in these conditions, the dimensions of this opening are reduced to the maximum, so that the resistance of the lower connection elbow 13, which carries the weight of the whole of the exchange tube, is not substantially affected; in addition, the correct adjustment of the walls to be welded is particularly conducive to obtaining a weld of good mechanical and thermal quality.

FIG. 3 shows schematically a possible method of connection of the exchange tubes on the one hand and associated connection conduits on the other hand. The exchange tubes, shown in solid lines, being arranged as a multi-branched beam-shaped bundle, in accordance with the arrangement proposed by the applicant in a prior patent, the sampling ducts, shown in phantom, are extended over a certain length beyond the connecting bends of the exchange tubes, and are in turn connected to each other by a multi-branched branch candlestick of similar arrangement. This arrangement has the advantage of postponing the connections of the exchange tubes of the same family, and the connections of the associated sampling ducts, in an area of good accessibility, whatever the density of the tubes in the exchanger .

FIG. 4 shows the block diagram of a power plant whose heat source is for example constituted by a nuclear reactor (not shown); the heat is extracted from the reactor core by a primary fluid F1, for example carbon dioxide under pressure, circulating in a closed circuit. The hot primary fluid exiting the reactor flows up and down the enclosure of a heat exchanger in which are bundles of tubes constituting the successive stages of a steam generator; in the example selected, this generator comprises, in series and from top to bottom, an economizer stage E, a vaporizer stage V and an overheating stage S in the economizer E, the water supplied under suitable pressure by a pump supply PA is brought to substantially boiling temperature; the water thus heated is vaporized in the vaporizer V and then superheated above its saturation temperature in the supercharger S.

The superheated steam from the generator works for the first time in the HP high stage of a turbine driving an alternator A; the partial relaxation and the corollary cooling of the steam having brought its humidity rate to a dangerous value, it must be reheated before working a second time in the LP low pressure stage of the turbine, from where it returns to a condenser C.

For this purpose, and in accordance with the process of the invention, the cold vapor at low pressure from both the HP stage of the turbine passes through a reheater R in which it receives a portion of the heat conveyed by a stream of water under pressure and substantially at boiling temperature taken at the outlet of the economizer stage E of the steam generator by a con duct T, shown in phantom, and a sampling pump PP.

This resuperheating by taking water has many advantages. Firstly, the initial heat exchange between the primary fluid F1 and the water passing through the economizer stage E occurs under the best conditions, because of the very strong coefficient of exchange between gas and liquid pressurized on the one hand, and because of the significant difference in the temperature of the water and the primary fluid, which must return to the reactor at a temperature slightly different from its temperature of exit; likewise, the coefficient of exchange between the water taken and the steam relaxed in the reheater R is very strong.

Furthermore, the fluid being taken in the liquid state can convey a large amount of heat under a low volumetric flow rate; the passage sections, and consequently the weight of the tubes, are therefore reduced accordingly, and in a corollary manner, their thermal insulation possibilities are improved.

On the other hand, the reheater can be placed in the immediate vicinity of the turbine, so that the disadvantages inherent in the return of steam to the boiler are removed.

Finally, the degree of reheating can be easily controlled by adjusting the flow rate of the PP sampling pump.

Some of these benefits are known and used. in the art, as in the case of example French Patent No. 1,144,465 filed October 16, 1954 in the name of Electricité de France; this patent indeed describes a method according to which a preheated water fraction is taken and used to overheat saturated steam obtained by simple expansion of the water fraction not sampled. However, it will be appreciated that the process of the invention differs essentially from that mentioned above in that the expansion of the steam produces work.

FIG. 5 shows the block diagram of a power generation plant which differs essentially from that previously described in that it comprises two steam generators supplying steam at high pressure and other low pressure. The elements specific to the low pressure generator and the associated water and steam circuits are assigned a prime index.

The two generators each comprise an economizer stage E, E 'and a vaporizer stage V, V, arranged in series in the enclosure of an exchanger through which the primary fluid F1 flows from the nuclear reactor (not shown); the water supplied under pressure suitable for the two generators by respective feed pumps PA, PA 'leaves in the saturated vapor state at high and low pressure respectively.

The high-pressure steam passes through an independent booster S, through which, for example, flue gases F2 at high temperature emerge from combustion.

The superheated high pressure steam works for the first time in the high pressure stage HP of the turbine driving the alternator A, then flows - as in the previous example - a reheater R heated by a stream of water substantially at a temperature of boiling, taken at the outlet of the economizer stage E of the high-pressure generator by a duct T, shown in phantom, and a sampling pump PP; the reheated steam works a second time in a first low pressure stage BPl of the turbine, and returns to the condenser C.

The saturated steam supplied by the low pressure generator E ', V' passes through a superheater S 'combined with the reheater R, and works in a second low pressure stage BP2 of the turbine, before returning to the condenser C.

In the latter case, the advantages of overheating and re-heating of steam by a water sample are obviously added to the advantages of using two thermodynamic cycles in combination, and possibly to superheating independent of high pressure steam.

 In order to illustrate the efficiency improvement provided by the process of the invention, an example will be taken of a heat exchange balance ensuring the transfer of 2,700 kg Joule per kilogram of water-steam; As a reference, an exchanger is used whereby the feed water at 105 is converted to superheated steam at 37 bar and 3751. </ B>

The implementation of the sampling method of the invention makes it possible to obtain, all things being equal, and by dividing the tubes by half between a low pressure circuit and a high pressure circuit, a relative increase in the overall yield of exchange of the order of 5%: in fact, the flow rate of feed water can be increased to 130% of the flow-weight in the reference system, of which 50% is transformed in the low pressure circuit into steam overheated at 40 bar and 380, of which 80%, treated in the high pressure circuit, provide on the one hand, 42% pressurized water taken from 295, and on the other hand, 38% superheated steam at 80 bar and 370.

Of course, the invention is in no way limited to the embodiments and implementations which have been described and illustrated. In particular, the arrangement of the sampling ducts could be easily adapted to. exchange tubes or similar to those described but planned-for an internal fluid circulation of asbestos, as is the case for example in nuclear power plants annular exchangers, or to tubes of any other type, examples shaped as a planar or helical coil. Otherwise; it is obvious that the vaporizable fluid is not necessarily water, and that the choice of a nuclear reactor as a hot source is in no way limiting and has only been mentioned as an example particularly suitable for: implementation of the invention, due to the relatively low temperature of the primary fluid supplied.

Claims (1)

SUMMARY The present invention essentially relates to objects I. A device for sampling fluid in at least some of the straight, helical, serpentine or other exchange tubes of a steam generator, and especially a heat exchanger heat exchanger for nuclear power generation plant, this device being remarkable in particular by the following characteristics, considered separately or in combi naison a. The sampling is carried out in each of the tubes concerned in a zone where, whatever the operating speed of the generator, the aforementioned fluid is in the liquid state and substantially at boiling point, by means of a pipe of sampling disposed inside said tube and out at an accessible point, and especially at the end; b. The aforementioned conduit exits the upstream end, or inlet, of the aforementioned tube; c. The aforementioned duct is formed, over all or part of its length, of a material adapted to provide thermal insulation of the fluid taken; d. The above-mentioned duct is provided, over all or part of its length, with a lining or doubling adapted to ensure the thermal insulation of the aforementioned fluid; e. The aforementioned liner is constituted by one or more sleeves or the like disposed coaxially with the aforementioned conduit; f. The aforesaid conduit is provided with an outer sleeve whose ends are welded respectively on the tube and on the pipe mentioned above; g. The aforesaid conduit is welded directly to the aforesaid tube as it passes through said tube and is provided with an outer sleeve extending inside said tube from said bushing; h. The aforesaid conduit is welded directly to the aforesaid tube at its crossing of said tube, and is provided with an inner sleeve extending on either side of said crossing, i. An annular space closed at one end is provided between the aforementioned sleeve or sleeves and the aforementioned conduit; j. An end of the aforementioned sleeve or sleeves is fixed to the aforementioned conduit by crimping and / or swaging; k. The aforementioned conduit is disposed over all or part of its length, substantially along the axis of the aforementioned tube, and conforms if necessary to the sinuosities of the latter; 1. The aforesaid duct is shaped, over all or part of its length, in planar or helical coil, independently of any nuities of the aforementioned tube, to allow its free expansion; -m. The abovementioned tube being of the annular and possibly helical fluid circulation type, and comprising a hollow coaxial core, the aforementioned conduit penetrates inside said core by one of its ends and opens into the fluid circulation space. by a stitching or other crossing arranged in the wall of said core; not. The above-mentioned tube being of the annular and possibly helical fluid circulation type, and comprising a hollow coaxial core, the aforesaid connee penetrates inside said core by one of its ends and opens out inside said core, whose wall is consequently pierced with communication orifices with the fluid circulation space; o. The aforesaid tube being upwardly internal circulation, and constituting at least the economizer and vaporizer stages of the aforementioned generator, the aforementioned orifices are located in the portion of said tube acting as a vaporizer; p. The aforementioned orifices are staggered in height; q. The aforementioned orifices comprise a visor protruding into the annu lar circulation space aforesaid; r. The outlet of the aforementioned conduit is located below the aforementioned orifices; s. The outlet of the aforementioned conduit is flared. II. A process for recovering the heat produced by a chemical or nonnuclear reaction, wherein said heat, exchanged to a vaporizable liquid, is used to heat and then vaporize said liquid, the saturated vapor thus obtained being superheated, either by part of the heat resulting from said reaction, either by a booster of heat from another source, and according to which the energy of the superheated steam is recovered by its expansion in a thermal machine, this process being remarkable in particular by the characteristics, considered separately or in combination with A fraction of said liquid is withdrawn at a point where said liquid is raised substantially to boiling temperature, and is used to overheat and / or reheat the steam at lower pressure; b. The expansion of the above superheated steam being fractionated, the liquid taken is used to reheat said vapor having undergone at least partial expansion in the above-mentioned machine; c. The aforesaid liquid being heated and then vapourized under at least two different pressures, the liquid is taken at high pressure and is used to overheat the saturated steam produced at low pressure; d. The liquid taken is recycled. III. As new industrial products, the steam generators, and in particular the heat exchangers, provided with a sampling device according to I. IV. As new industrial products, the energy conversion plants implementing the process according to II.