EP0738862A1 - Heat exchanger with brazed plates, and corresponding method for processing a two-phase fluid - Google Patents

Abstract

The exchanger consists of a series of plates (2, 2A, 2B) separated by spacers (3, 14), and a number of flat passages (4, 5) between the plates. Each primary passage (4) for circulating a diphase fluid lies adjacent to at least one secondary passage (5) carrying a heating or cooling fluid. At least one of the primary passages has a zone of increased section covering part of its length and collectors/evacuators (9, 10) for one of the two phases of the fluid. The plates (2A) delimiting the primary and secondary passages extend above the level of intermediate plates (2B) and bars (9) which mark the upper ends of the secondary passages.

Description

The present invention relates to a heat exchanger with brazed plates, of the type comprising a stack of plates separated by spacer waves, and a set of generally flat passages delimited by these plates, namely a series of first passages of circulation of a two-phase fluid, each of which is adjacent to at least one second passage for circulation of another fluid, circulating or refrigerant.

Heat exchangers of this type make it possible to carry out gas condensation and / or vaporization of liquids, by circulating in passages adjacent to the circulation passages of the two-phase fluid an respectively refrigerant and circulating fluid.

To avoid drying out certain areas of the plates, which reduces the performance of the exchanger, the vaporization of a liquid is generally carried out partially. Likewise, the condensation operations are often only partial, for example for cryogenic gas purification.

In these applications, it is necessary, in at least one location the length of the exchanger, to be able to separate the two liquid and vapor phases of the treated fluid.

In the conventional technique, the two phases to be separated are sent into a free volume associated with the exchanger but distinct from the structure thereof.

The object of the invention is to ensure the separation of the phases by the structure of the exchanger itself, in order to simplify the production of the entire apparatus.

To this end, the invention relates to a heat exchanger of the aforementioned type, characterized in that at least one of said first passages comprises, in at least one location of its length, an increase in its passage section as well as means for collecting and discharging one of the two phases of the two-phase fluid.

• l'échangeur de chaleur étant à circulation ascendante du fluide diphasique, une des plaques délimitant ledit premier passage et la plaque opposée d'un second passage, adjacent, de circulation dudit autre fluide, sont prolongées vers le haut au-delà de l'extrémité supérieure de la plaque mitoyenne de ces deux passages, laquelle dépasse au-dessus de la barre de fermeture supérieure dudit second passage adjacent;
• l'échangeur de chaleur étant à circulation ascendante du fluide diphasique et lesdits seconds passages étant délimités vers le haut par des barres de fermeture supérieures, toutes les plaques se prolongent vers le haut au-delà des barres de fermeture supérieures, et certaines au moins des plaques, à l'exception des plaques extrêmes, sont munies d'ouvertures au-dessus du niveau de ces barres de fermeture;
• l'échangeur de chaleur comprend, au-dessus des plaques, un dôme supérieur de recueil de la phase gazeuse dudit fluide diphasique;
• l'échangeur de chaleur comprend à son extrémité supérieure, dans chaque intervalle interplaque, un distributeur de sortie collecteur de vapeur qui débouche sur une fenêtre de sortie, et une boîte collectrice de vapeur qui coiffe toutes ces fenêtres de sortie;
• lesdites barres de fermeture supérieures sont inclinées;
• ledit premier passage comporte, en amont dudit emplacement, une plaque intermédiaire qui en rétrécit la section libre jusqu'audit emplacement et qui forme, avec une demi-barrette de fermeture, une cavité de recueil de l'une des deux phases du fluide diphasique;
• l'échangeur de chaleur étant à circulation descendante du fluide diphasique, ladite cavité est délimitée supérieurement par la demi-barre de fermeture et contient, au-dessous de celle-ci, un distributeur de sortie qui débouche latéralement sur une fenêtre de sortie de vapeur;
• ledit emplacement est voisin de l'extrémité inférieure dudit premier passage, et ce passage est obturé à sa base par une barre de fermeture inférieure, notamment inclinée, qui s'étend latéralement jusqu'à une fenêtre de sortie de liquide;
• ladite cavité est ouverte vers le haut et est délimitée inférieurement par la demi-barre de fermeture, qui est notamment inclinée;
• ledit premier passage comporte plusieurs plaques intermédiaires espacées les unes des autres sur sa longueur.

The heat exchanger according to the invention may include one or more of the following characteristics:

• the heat exchanger being in upward circulation of the two-phase fluid, one of the plates delimiting said first passage and the opposite plate of a second, adjacent passage, of circulation of said other fluid, are extended upwards beyond the end upper of the dividing plate of these two passages, which protrudes above the upper closing bar of said second adjacent passage;
• the heat exchanger being in upward circulation of the two-phase fluid and said second passages being delimited upwards by upper closing bars, all the plates extend upwards beyond the upper closing bars, and at least some of the plates, with the exception of the end plates, are provided with openings above the level of these closing bars;
• the heat exchanger comprises, above the plates, an upper dome for collecting the gaseous phase of said two-phase fluid;
• the heat exchanger comprises at its upper end, in each interplate interval, a vapor collector outlet distributor which opens onto an outlet window, and a steam manifold box which covers all these outlet windows;
• said upper closing bars are inclined;
• said first passage comprises, upstream of said location, an intermediate plate which narrows the free section to said location and which forms, with a half-closing bar, a collection cavity of one of the two phases of the two-phase fluid;
• the heat exchanger being in downward circulation of the two-phase fluid, said cavity is delimited above by the closing half-bar and contains, below the latter, an outlet distributor which opens laterally onto a steam outlet window ;
• said location is close to the lower end of said first passage, and this passage is closed at its base by a lower closing bar, in particular inclined, which extends laterally to a liquid outlet window;
• said cavity is open upwards and is delimited below by the closing half-bar, which is in particular inclined;
• said first passage comprises several intermediate plates spaced from one another along its length.

The invention also relates to a method for treating a two-phase fluid in a brazed plate heat exchanger, characterized in that at least one location along the length of each circulation passage of the two-phase fluid, the quantity is substantially increased the section offered to this fluid and one of its two phases is collected.

• la Figure 1 représente schématiquement un échangeur de chaleur conforme à l'invention;
• la Figure 2 est une vue partielle, à plus grande échelle, prise en coupe suivant la ligne II-II de la Figure 1;
• la Figure 3 est une vue analogue à la Figure 2 d'une variante;
• la Figure 4 est une vue analogue à la Figure 1 d'une autre variante;
• la Figure 5 est une vue partielle en coupe longitudinale d'une autre variante de l'échangeur de chaleur suivant l'invention;
• la Figure 6 est une vue partielle prise en coupe suivant la ligne VI-VI de la Figure 5;
• la Figure 7 illustre schématiquement l'application d'un échangeur de chaleur suivant l'invention à l'épuration cryogénique d'un gaz; et
• la Figure 8 illustre schématiquement la structure de l'échangeur de la Figure 7.

Examples of embodiments of the invention will now be described with reference to the appended drawings, in which:

• Figure 1 schematically shows a heat exchanger according to the invention;
• Figure 2 is a partial view, more large scale, taken in section along line II-II of Figure 1;
• Figure 3 is a view similar to Figure 2 of a variant;
• Figure 4 is a view similar to Figure 1 of another variant;
• Figure 5 is a partial view in longitudinal section of another variant of the heat exchanger according to the invention;
• Figure 6 is a partial view taken in section along line VI-VI of Figure 5;
• Figure 7 schematically illustrates the application of a heat exchanger according to the invention to the cryogenic purification of a gas; and
• Figure 8 schematically illustrates the structure of the exchanger of Figure 7.

The heat exchanger 1 shown in Figures 1 and 2 is of the brazed plate type. It essentially comprises a parallelepipedic body consisting of a stack of rectangular metal plates 2 separated by spacer waves 3. The latter may in particular be perforated corrugated sheets, as shown diagrammatically in Figure 2, or alternatively, corrugated sheets having flat on their flanks, say "waves serrated". The dimensions of the plates 2 can reach for example 6 m × 2 m.

The plates 2 define between them a large number of generally flat passages. These passages, in the example shown, are divided into two alternating groups: first passages 4 for upward circulation of a two-phase fluid F during vaporization, and second passages 5 for downward circulation of a circulating fluid f . Plates 2, including the lower edges are all at the same level, extend alternately up to a high level N1 (plates 2A) and up to a level N2 lower than N1 (plates 2B). The two end plates extend to the high level N1.

As is well known in the art, the passages 4 and 5 are delimited below and on each side by closing bars which leave free rows of fluid inlet / outlet windows, which are capped by inlet boxes / outlet of generally semi-cylindrical shape. Thus, there is provided a lower box 6 for fluid inlet F, a side box 7 for fluid inlet f , and a side box 8 for fluid outlet f . Each passage 5 is delimited upwards by an upper closing bar 9 inclined from the top point of the corresponding inlet window for the fluid f . A free volume 10 is defined above this bar 9, in each passage 5, between the plates 2A and 2B which frame the latter.

Conventionally, there is provided, opposite each box 6 to 8, in each corresponding passage, an inlet / outlet distributor constituted by a set of waves, some of which are oriented obliquely. Thus, there is above each lower closing bar of the exchanger a distributor, which is a fluid inlet distributor F in the passages 4 and a lateral fluid outlet distributor f in the passages 5. In addition , a lateral inlet distributor 11 for the fluid f is located just below each upper bar 9. These distributors have been shown diagrammatically in phantom in Figure 1.

The exchanger is completed by an upper dome 12, conventionally made from four quarter-cylinder sheets welded together at their intersections, and welded by their lower edges along the upper edge of the two extreme plates 2A as well as along the other two upper edges of the exchanger. These two other edges are, as will be understood, made up of the upper end edges of the plates 2A and of the lateral closing bars. A pipe 13 for leaving the vapor phase of the fluid F is welded to the top of the dome 12.

Between the levels N1 and N2, only the plates 2A remain. These are then braced by waves 14 of corresponding height, and not significantly greater than that of current waves, or heat exchange waves, 3.

In operation, the fluid F, supplied at source, in liquid form, in each passage 4 via the box 6, gradually vaporizes. It arrives in the two-phase state at level N2, where the passage section which is offered to it increases suddenly, substantially double. The speed of the fluid therefore decreases suddenly, which causes the separation of the liquid phase, which falls by gravity. However, the rapid two-phase current which rises in the passage 4 has the result that the liquid does not descend in this passage, but collects in the cavity 10 which overcomes the upper bar 9 of the associated passage 5. This bar 9 being inclined, the liquid descends along the bar and, through liquid outlet windows provided just above the lower end of this bar, falls into a manifold 15 which covers these outlet windows .

As for the vapor phase F _V of the fluid F, it collects in the dome 12 and is evacuated via the tubing 13.

In the variant of FIG. 3, the increase in cross section of the passages 4 is obtained in the following manner: all the plates 2 are extended upwards to the high level N1 and, between the levels N1 and N2, all the plates are provided with openings 16, with the exception of course of the two end plates.

From the level N2 approximately, the plates 2 can be braced by means of a wave 17 of the same height as the heat exchange waves 3 but of greater pitch, in order to increase the free section offered to the fluid F.

Thus, from level N2, the stack of plates 2 forms a continuous space, which substantially doubles the section offered to the ascending two-phase fluid. Given the speed of this fluid in the passages 4, the liquid drops only in the regions which surmount the bars 9, and, by trickling, it collects in the cavities 10, before being discharged laterally as before.

FIG. 4 schematically represents a variant of a heat exchanger which applies to the structure of FIG. 2 as well as to that of FIG. 3, since it only relates to the means for evacuating the vapor F _V.

In this Figure 4, the dome 12 is deleted, and the plates 2A (in the case of Figure 2) or 2 (in that of Figure 3) are extended upwards beyond the level N1. Above this level N1, in each phase separation passage, that is to say in each of the passages delimited between the plates 2A (Figure 2) or 2 (Figure 3), there is an outlet distributor. 18 adapted to bring the vapor F _V towards the middle zone of the passage. As can be seen in FIG. 4, the distributor 18 comprises two oblique waves 19, 20 which converge upwards to a median vertical wave 21 of triangular shape, having its point at the bottom. This wave 21 opens onto an upper outlet window, and all of the outlet windows are capped by an outlet box 22 of generally semi-cylindrical shape, from which the tubing 13 starts.

The arrangement of FIG. 4 makes it possible to treat two-phase fluids under high pressures for which the upper dome 12 is no longer acceptable.

Figures 5 and 6 illustrate a possibility of adaptation of the invention to the case of a descending two-phase fluid F, for example during partial vaporization against the flow of an ascending circulating auxiliary fluid F (Figure 6).

Each passage 4 is closed, at its lower end, by a lower closing bar 23 whose upper face is sloping. An intermediate plate 24 divides the passage 4 into two sub-passages 25, 26 from a location spaced from the bar 23. The sub-passage 25 is free, while the sub-passage 26 is closed, at the edge upper of the plate 24, by a half closing bar 27. Just below the latter, the sub-passage 26 contains an outlet distributor 28, consisting (Figure 5) of an oblique wave 29 and a horizontal wave 30, the latter opening into a lateral half-window 31. All of the half-windows are capped by an outlet box 32.

In operation, the two-phase fluid F is accelerated when it arrives in the sub-passage 25, then it suddenly slows down when it exceeds the lower edge of the plate 24. The vapor phase F _V rises in the sub-passage 26, then is evacuated via the distributor 28 and the box 32, while the liquid phase F _L trickles and collects on the lower bar 23, then s' discharged laterally via an outlet box 33.

FIG. 7 illustrates a general diagram for the purification of a gas F under pressure in a heat exchanger 1 according to the invention, by condensation of its heavy impurities. The gas F circulates from bottom to top in the passages 4 of the exchanger. At several levels, a condensed fraction F _L1 , F _L2 , F _L3 is withdrawn, expanded in an expansion valve 34 and returned against the current, that is to say from top to bottom, at about the same level. of the exchanger, in passages 5, to produce cold. At the cold end of the exchanger, which is its upper end, the purified gas F is returned against the current in passages 5 to recover the sensible heat.

In this process, it is necessary to separate the two phases of the fluid F during partial condensation at each level where it is desired to draw off a liquid fraction. This can be achieved by the arrangement of Figure 8, which will now be described.

Each passage 4 is subdivided in its thickness, over most of its height, by intermediate plates 35 spaced from one another. The lower plate 35 starts from two lower closing half-bars 36 and, on one side (the right side in FIG. 8), the half-passage which it defines is closed by an upper closing half-bar 37, located a small distance below the upper edge of the plate 35. Likewise, each intermediate plate 35 comprises, same side, a lower half closing bar 36 and an upper half closing bar 37, arranged in a similar manner.

It can therefore be seen that each passage 4 has an active part 38 (its left part in FIG. 8), for the upward circulation of the fluid F, the thickness of which is half (or, as a variant, a different fraction) of the distance which separates the two plates 2 which delimit this passage, while it is only equal to this distance in the regions 39 which separate the intermediate plates 35.

Thus, in operation, the fluid F, introduced at the base of the passage 4 via a distributor 40, circulates at relatively high speed in the lower zone 38, then decelerates suddenly in the first zone 39, then is accelerated in the second zone 38 , And so on. In this way, the successive liquid phases, corresponding to increasingly light impurities, separate in the zones 39. Due to the ascending speed of the fluid in the zones 38, the liquids flow in the calm part of the zones 39, that is to say in their right half, and gather in the space 41 delimited above the bars 37. These can, as before, be inclined to facilitate the lateral evacuation of liquids.

The spaces 42 between each pair of half-bars 36, 37 can be inactive, or else be used for the circulation of appropriate fluids.

Claims (12)

Brazed plate heat exchanger, of the type comprising a stack of plates (2, 2A, 2B) separated by spacer waves (3, 14; 17; 25, 26), and a set of passages (4, 5) of generally flat shape delimited by these plates, namely a series of first passages (4) for circulation of a two-phase fluid, each of which is adjacent to at least one second passage (5) for circulation of another fluid, circulating or refrigerant , characterized in that at least one of said first passages (4) comprises, in at least one location of its length, an increase in its passage section as well as means (9, 10; 26, 28; 37, 41) for collecting and discharging one of the two phases of the two-phase fluid. Heat exchanger according to claim 1, with ascending circulation of the two-phase fluid, characterized in that one (2A) of the plates (2A, 2B) delimiting said first passage (4) and the opposite plate (2A) of a second passage (5), adjacent, of circulation of said other fluid, are extended upwards beyond the upper end of the dividing plate (2B) of these two passages (4, 5), which protrudes above the bar upper closure (9) of said second adjacent passage (5). Heat exchanger according to claim 1, with upward circulation of the two-phase fluid, and wherein said second passages (5) are delimited upwards by upper closing bars (9), characterized in that all the plates (2) are extend upwards beyond the upper closure bars (9), and in that at least some of the plates, with the exception of the end plates, are provided with openings (16) above the level of these bars closing. Heat exchanger according to claim 2 or 3, characterized in that it comprises, above plates (2; 2A; 2), an upper dome (12) for collecting the gaseous phase of said two-phase fluid. Heat exchanger according to claim 2 or 3, characterized in that it comprises at its upper end, in each interplate interval, an outlet distributor (18) vapor collector which opens onto an outlet window, and a manifold of steam (22) which covers all these outlet windows. Heat exchanger according to any one of claims 2 to 5, characterized in that said upper closing bars (9) are inclined. Heat exchanger according to claim 1, characterized in that said first passage (4) comprises, upstream of said location, an intermediate plate (24; 35) which narrows the free cross-section thereof to said location and which forms, with a half -closing bar (27; 37), a cavity (26; 41) for collecting one of the two phases of the two-phase fluid. Heat exchanger according to claim 7, with downward circulation of the two-phase fluid, characterized in that said cavity (26) is delimited above by the closing half-bar (27) and contains, below the latter, a distributor outlet (28) which opens laterally on a steam outlet window. Heat exchanger according to claim 8, characterized in that said location is close to the lower end of said first passage (4), and in that this passage is closed at its base by a lower closing bar (23), in particular inclined, which extends laterally to an exit window liquid. Heat exchanger according to claim 7, with upward circulation of the two-phase fluid, characterized in that said cavity is open upwards and is delimited below by the half-closing bar (37), which is in particular inclined. Heat exchanger according to claim 10, characterized in that said first passage (4) comprises several intermediate plates (35) spaced apart from one another along its length. Process for treating a two-phase fluid in a heat exchanger with brazed plates, characterized in that at least one location along the length of each passage (4) for circulation of the two-phase fluid, the section offered for this is substantially increased fluid and one of its two phases is collected.

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