Abstract:
The invention concerns a heat exchanger which includes a stack of plates defining passages, containing corrugated fins which include a transverse section with repeated corrugated pattern extending between two upper and lower end planes. The pattern includes a base corrugated pattern that includes corrugated legs linked to corrugated summits and corrugated bases, this base pattern being modified by a sub-pattern which defines, between at least some corrugated legs, additional leading edges located at an intermediate level between the end planes. The invention is applicable to cryogenic gas—gas heat exchangers.

Description:
BACKGROUND 
   The present invention relates to a brazed-plate heat exchanger, whose passages contain at least one corrugated fin of the type comprising, in cross section, a repeated corrugated pattern which extends between two upper and lower extreme planes defined by the plates of the exchanger. 
   The invention is in particular applicable to gas—gas cryogenic exchangers for air distillation apparatuses, such as the main heat exchange line of these apparatuses, which cools the incoming air by indirect heat exchange with the cold products from the distillation column. 
   The corrugated fins in question are widely used in brazed-plate heat exchangers, which have the advantage of offering a large heat exchange surface area in a relatively small volume, and of being easy to manufacture. In these exchangers, the fluid flows may be cocurrent, countercurrent or crosscurrent flows. 
     FIG. 1  of the appended drawings shows, in perspective, with partial cutaways, an example of such a heat exchanger, of conventional structure, to which the invention is applicable. In particular, it may involve a cryogenic heat exchanger. 
   The heat exchanger  1  shown consists of a stack of parallel rectangular plates  2  which are all identical and which between them define a plurality of passages for fluids to be brought into indirect heat exchange relationships. In the example shown, these passages are, in succession and cyclically, passages  3  for a first fluid,  4  for a second fluid and  5  for a third fluid. 
   Each passage  3  to  5  is bordered by closure bars  6  which define the passage, leaving inlet/outlet windows  7  of the corresponding fluid free. Placed in each passage are spacer waves or corrugated fins  8  acting both as thermal fins, as spacers between the plates, especially during brazing and in order to avoid any deformation of the plates when using pressurized fluids, and for guiding the fluid flows. 
   The stack of plates, closure bars and spacer waves is generally made of aluminum or aluminum alloy and is assembled in a single operation by furnace brazing. 
   Fluid inlet/outlet boxes  9 , of semicylindrical overall shape, are then welded to the exchanger body thus produced so as to sit over the rows of corresponding inlet/outlet windows, these boxes being connected to fluid feed and discharge pipes  10 . 
   There are various types of spacer waves  8 . Thus mention may be made of straight fins, with rectilinear, possibly perforated, generatrices, fins known as “herringbone” fins, with sinuous generatrices, louvered fins, the wave legs of which have rows of recesses, and partially offset or “serrated” fins. 
   In these various fins, the wave may have a square, rectangular, triangular, sinusoidal, etc., cross section. 
   SUMMARY 
   A brazed-plate heat exchanger apparatus comprising:
         (i) a stack of parallel plates wherein said parallel plates define a plurality of generally flat-shaped fluid flow passages;   (ii) closure bars wherein said closure bars define passages; and   (iii) corrugated fins wherein said corrugated fins comprise, in cross section, a repeated corrugated pattern extending between two upper and lower extreme planes defined by two adjacent plates of the exchanger,
 
wherein said pattern comprises a basic corrugated pattern-comprising wave legs connected by wave crests and wave troughs and wherein said pattern are modified by a subpattern which comprises additional exchange surfaces located between at least some pairs of wave legs, wherein said additional exchange surfaces are located at an intermediate level between the two extreme planes.
       

   The aim of the invention is to improve the thermal performance of exchanges with corrugated fins. To this end, the subject of the invention is a brazed-plate heat exchanger, of the type comprising a stack of parallel plates which define a plurality of generally flat-shaped fluid flow passages, closure bars which define these passages, and corrugated fins placed in the passages, at least some of the corrugated fins being of the type comprising, in cross section, a repeated corrugated pattern extending between two upper and lower extreme planes defined by two adjacent plates of the exchanger, characterized in that the pattern comprises a basic corrugated pattern comprising wave legs connected by wave crests and wave troughs, this basic pattern being modified by a subpattern which defines, between at least some pairs of wave legs, additional exchange surfaces located at an intermediate level between the two extreme planes. 
   According to other optional aspects:
         the subpattern defines a subcorrugation which extends only over a portion of the distance which separates the two extreme planes.   the subpattern comprises at least one nonvertical part located at an intermediate level between the two extreme planes.   the subpattern further comprises pairs of limbs which connect the nonvertical parts alternately to a wave crest and to a wave trough.   the limbs are vertical.   the subpattern comprises at least one additional oblique exchange surface.   the subpattern has a V-shaped section.   the subpattern comprises a step adjacent to at least some legs of the main pattern.   the fin is partially offset.   the offset distances ensure that the main pattern is offset both with respect to itself and with respect to the subpattern.   the pattern repeats every N rows of waves, where N ≧3 and in particular, N=4.   at least some parts of at least some troughs and/or subpatterns comprise a notch in at least one leading and/or trailing edge and in at least part of their height or their width.   the wave has a square, rectangular, triangular or sinusoidal cross section.   the basic corrugated pattern is constant over the entire length of the two extreme planes.       

   The following will mainly concern serrated fins, but it will be understood that the invention is also applicable to other types of fins described above. 
   Exemplary embodiments of the invention will now be described with respect to the appended drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein: 
       FIG. 1  illustrates a conventional heat exchanger as know in the art; 
       FIG. 2  shows, in perspective, a serrated fin according to the invention; 
       FIG. 3  is an end view of this fin; 
       FIG. 4  is an end view of a variant; 
       FIG. 5  shows, in perspective, another serrated fin according to the invention; 
       FIG. 6  is a view in exploded perspective of the fin of  FIG. 5 ; 
       FIG. 7  is an end view of the fin of  FIG. 5 ; and 
       FIG. 8  is an end view of another serrated fin according to the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   A brazed-plate heat exchanger apparatus comprising:
         (i) a stack of parallel plates wherein said parallel plates define a plurality of generally flat-shaped fluid flow passages;   (ii) closure bars wherein said closure bars define passages; and   (iii) corrugated fins wherein said corrugated fins comprise, in cross section, a repeated corrugated pattern extending between two upper and lower extreme planes defined by two adjacent plates of the exchanger,
 
wherein said pattern comprises a basic corrugated pattern-comprising wave legs connected by wave crests and wave troughs and wherein said pattern are modified by a subpattern which comprises additional exchange surfaces located between at least some pairs of wave legs, wherein said additional exchange surfaces are located at an intermediate level between the two extreme planes.
       

   The serrated fin  1  shown in  FIGS. 2 and 3  has an overall main corrugation direction Dl and comprises a large number of adjacent wave rows  12 A,  12 B, . . . , which are all identical and are oriented in a direction D 2  perpendicular to the direction Dl. 
   For convenience in the description, it will be assumed that, as shown in  FIG. 2 , the directions D 1  and D 2  are horizontal, similarly with the plates  2  of the exchanger. 
   Each wave row  12  has, in cross section perpendicular to D 1 , a basic pattern M which comprises two vertical wave legs  13 . With respect to an overall sense F of the flow of the fluid along the direction D 1  in the passage in question, each leg comprises a leading edge  14  and a trailing edge  15 . The legs are alternately connected along their upper edge by means of a rectangular, flat and horizontal wave crest  16 , and along their lower edge by means of a wave trough  17  which is also rectangular, flat and horizontal. 
   The basic pattern M is modified by a subpattern M 1  consisting of a rectangular projection extending downward in the middle of each crest  16  and upward in the middle of each trough  17 . 
   Each subpattern M 1  consists of one flat end part  18  located half way between the extreme planes defined by the adjacent plates  2 , and two vertical limbs  19  which connect the edges thereof to the corresponding crest  16  or trough  17 . 
   Thus, each subpattern forms a notch which comes in between the two adjacent legs  13 . This notch defines three additional exchange surfaces, that is a horizontal exchange surface  20  and two vertical exchange surfaces  21 . 
   The rows  12  are offset one with respect to another in the direction D 2 , alternately in one sense and in the other. By using the term “pitch” to refer to the distance p which separates two successive legs  12  (ignoring the thickness e of the thin sheet material forming the wave), the offset is alternately p/6 in one sense and in the other, while the notch width M 1  is p/3. 
   Thus, each row  12  is connected to the following row  12  by means of the crests  16 , along right-handed segments  22  of length p/6, and by means of the troughs  17 , along The serrated fin  1  shown in  FIGS. 2 and 3  has an overall main corrugation direction Dl and comprises a large number of adjacent wave rows  12 A,  12 B, . . . , which are all identical and are oriented in a direction D 2  perpendicular to the direction Dl. right-handed segments  23  of the same length p/6. The offset planes are the vertical planes such as P AB  and the offset lines, seen from the top, are denoted by  24 . 
   Moreover, l is used to denote the length of each row  12  in the direction D 1 , this length being called the “serration length”, and h is used to denote the height of the fin. 
   In practice, the shapes of various wave parts may differ to a greater or lesser degree from the theoretical shapes described above, especially with regard to the flatness and the rectangular shape of the facets  13  and  16  to  19 , and the verticality of the facets  13  and  19 . 
   Seen from the end ( FIG. 3 ), the patterns M are offset sideways with respect to themselves and with respect to the patterns M 1 , that is to say that the legs  13  of a given serration row  12  each appear between a leg  13  of the adjacent rows and a limb  19  of a neighboring subpattern M 1 . Conversely, the limbs  19  of the same row  12  each appear either between two limbs  19 , or between a limb  19  and a leg  13 , of the adjacent rows  12 . 
   Because of the presence of the subpatterns M 1 , the flow separation is increased at each offset line  24 , which increases the temperature difference between the fluid and the fin, thus increasing the heat flux exchanged. The presence of additional leading edges  20  and  21  further generates turbulence within the fluid, which promotes heat transfer by convection toward the core of the flow and not by conduction through the limiting layer, which promotes heat exchange. 
   The variant of  FIG. 4  differs from that of  FIG. 3  by a greater depth of the notches M 1 , this depth changing from about h/2 to 2h/3. In this way, the preferential flow regions, which miss out on the beneficial effect of the notches M 1  described above, are reduced. 
   With the same objective,  FIGS. 5 to 7  show a serrated fin whose pattern M+M 1  repeats not every other row, but one row in N, where N≧3. This makes it possible to increase the symmetry of flow. In the example shown, N=4. Four successive rows  12 A to  12 D will subsequently be described below. 
   As previously, each row has the same rectangular basic pattern M, comprising vertical legs  13  spaced apart by the pitch p and alternately connected by a wave crest  16  of width p and by a wave trough  17  of the same width p. The pattern M is modified by a subpattern M 1 A to M 1 D:
         subpattern M 1 A: in each upwardly open corrugation, the lower part of the right leg  13  is deformed by a step which comprises a horizontal part  24  located half way up the leg and a vertical part  25  located half way between this leg and the other leg of the corrugation. Thus, the lower half of the leg and the right half of the adjacent wave trough are removed, as shown by chain line;   subpattern M 1 B: in each downwardly open corrugation, the upper part of the left leg  13  is deformed by a similar step, that is to say a rectangular step of dimensions p/2 and h/2;   subpattern M 1 C: in each upwardly open corrugation, the lower part of the left leg  13  is deformed by a similar step. This subpattern is therefore symmetrical with respect to the subpattern M 1 A;   subpattern M 1 D: in each downwardly open corrugation, the upper part of the right leg  13  is deformed by a similar step. This subpattern is therefore symmetrical with respect to the subpattern M 1 B;       

   Moreover, in this embodiment, the offset from one row to the next is p/2, alternating in one sense and in the other (?).  FIGS. 5 and 6  indicate two neighboring vertical planes P 1  and P 2 , in order to make it easier to understand the structure of the fin. 
   The embodiment of  FIG. 8  is derived from that of  FIG. 3  in that each subpattern M 1  is triangular and is no longer rectangular or square. Thus two oblique leading edges  25 , which are symmetrical with respect to the vertical plane of symmetry P of the wave, are inserted into each wave. 
   In the example shown, the height of the triangle is h/2, but, as before, it may have a different value, especially a value greater than h/2 in order to reduce the preferential flow regions. 
   In all the above examples, high thermal performance of the exchanger, with highly divided and turbulent flow and with a two-dimensional, or even three-dimensional configuration is obtained. 
   Note that the fins may be manufactured by simple folding of a flat product on a press or using a cogged wheel, as for the conventional corrugated, especially serrated, fins. This is because the surfaces are all developable, such that it is enough to match the profile of the folding tools. 
   The presence of the subpatterns M 1  causes passage restriction at the offset lines, and therefore pressure drops. These pressure drops can possibly be reduced by providing notches carefully placed in at least some leading and/or trailing edges of the patterns M and/or M 1 . These notches will preferably be located facing the leading and/or trailing edges of the subpatterns M 1 , or therewithin, as indicated in chain line by  26  in  FIG. 2 . 
   Whatever the fin type, the latter may be made either from solid sheet metal, or from perforated sheet metal or sheet metal provided otherwise with apertures. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.