Abstract:
A heat exchanger having a pair of manifolds and flat tubes for heat exchange between a first fluidum flowing inside the tubes and a second fluidum flowing outside of the tubes. The manifolds receive ends of the tubes and have an inlet and an outlet for introducing the first fluidum into the flat tubes and for discharging therefrom. Each manifold has at least two adjacent parallel channels with a partition wall therebetween. Each channel is further defined by at least a second wall, with at least part of the second wall having a curved surface. The partition wall defines two parallel substantially flat surfaces facing the channels. Each channel has a perimeter in cross-section defined by a continuous line having curved and straight portions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of International Application No. PCT/EP01/09142 filed Aug. 6, 2001. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to a heat exchanger comprising a plurality of flat tubes for heat exchange between a first fluidum flowing inside said tubes and a second fluidum flowing outside of said tubes, a pair of manifolds connected to the end of the flat tubes and provider with an inlet and an outlet for introducing the first fluidum into the flat tubes and for discharging therefrom, each manifold being provided with at least two parallel channels, at least part of the walls of the channel having a curved surface. 
     (2) Description of the Related Art 
     Such a heat exchanger is known from WO-A-9851983. 
     In this known heat exchanger the manifolds are composed of a number of parallel tubes with circular cross-sections, each pair of adjacent tubes having a common wall portion, in such a way that the tubes of each manifold constitute a flat array of tubes. The circular cross-section of the tubes is selected because the high pressure inside the tubes, such as is common in modern heat exchangers used in cars and based upon CO2. It is common then to use a pressure well above 100 bar and the use of round cross-section channels avoid that stresses are built up in the walls of the manifold. Using round cross-section allows the inner wall to be thinner thereby saving weight and increasing heat transfer. 
     Otherwise a flat tube has to be inserted through holes, in one flat side of the manifold in order to have communication between the tubes and the manifolds. In order to have the least possible flow restriction it is preferred to insert the end portion of the flat tubes up to half way into the diameter of the channels in the manifold, as in this way the part of the end face of the flat tubes blocked by the partition walls in the manifold is minimal. However, in this way half of each channels in the manifold is blocked causing flow restriction in that part of the heat exchanger. As a compromise the end part of the flat tubes is not inserted up to half the diameter of the channels, but to about one third of the diameter. In this way the blockage in the channels of the manifold is substantially reduced, whereas the blockage of the end face of the flat tubes is only slightly increased and kept within acceptable limits. 
     However the disturbance of the fluid flow inside the heat exchanger, and especially inside the manifold channel is still highly disturbed by the inserted end portion of the flat tubes, which especially in high pressure systems can cause substantial pressure drops. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefor an object of the invention to provide a heat exchanger in which this problem is substantially reduced. 
     This object is achieved in that the partition wall between any of two adjacent channels is provided with two parallel substantially flat surfaces facing the channels. 
     In this way it becomes possible to make channels having elongated cross-sections, each of which is only blocked to a minor extent by the inserted flat tubes. The use of flat surfaces in the partition walls is possible without causing exaggerated stresses in the walls, because both surfaces of the wall are subjected to the same, be it high pressure, thereby balancing the forces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic view of a heat exchanger according to the invention, 
         FIG. 2  is a cross-section along line  2 — 2  of the manifold of  FIG. 1 . 
         FIG. 3  is a plan view of a portion of a flat wall of the manifold of  FIGS. 1 and 2 . 
         FIG. 4  is a cross-sectional view of the manifold along line  4 — 4  of  FIG. 3 . 
         FIG. 5  is a perspective view of a part of the manifold of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the illustrated heat exchanger includes a plurality of flat heat transfer tubes  1  stacked in parallel and corrugated fins  2  sandwiched between the flat tubes  1 . The ends  1   a  of the tubes  1  are connected to manifolds  3  and  4 . Each heat transfer tube may be made of extruded aluminium, having a flat configuration. Alternatively, the flat tubes can be multi-bored flat tubes, commonly called multiport tubes or else, electrically seamed tubes can be used. Multiport tubes may be made by extrusion, but otherwise it is possible to make such tubes by rolling from clad sheet, folding and brazing. Furthermore, it is possible to use a welded tube with an inserted baffle. 
     In the embodiment shown each corrugated fin  2  has a width approximately similar to that of the flat tube  1  but other widths may be used as well. The fins  2  and the flat tubes  1  are brazed to each other. The manifolds  3 , 4  are made up of aluminium tubes with holes  5  of the same shape as the cross-section of the heat transfer tubes  1  so as to accept the tube ends  1   a . The holes  5  can also be tailor made, e.g. conical, so as to allow easier access for the flat tubes. 
     The inserted tube ends  1   a  are brazed in the holes  5 . As shown in  FIG. 1 , manifolds  3  and  4  are connected to an inlet manifold  6  and an outlet manifold  7 , respectively. The inlet manifold  6  allows a heat exchanging fluid to enter the manifold  3 , and the outlet manifold  7  allows the heat exchanging fluid to discharge. The manifolds  3  and  4  are closed with caps or plugs  8  and  9 , respectively. The reference numerals  13  and  14  denote side plates attached to the outermost corrugated fins  2 . 
     The manifold  3  has its inner space divided by a baffle  10  into two sections, and the manifold  4  is divided into two sections by a baffle  11 . In this way a medium path is provided starting from manifold  3 , passing through a first set of tubes  1 , through part of the manifold  4 , passing through a second set of tubes  1  to manifold  3  and passing through a third set of tubes  1  to manifold  4  and to leave the heat exchanger unit through outlet  7 . It is clear that these manifolds without baffles are also possible and otherwise manifolds with more than one baffle per manifold can be applied as well. 
     The Heat Exchanging Fluid Flows in Zigzag Patterns Throughout the Heat Exchanger Unit 
     The manifolds  3  and  4  are basically identical and in the  FIGS. 2–4  an example of the manifold  3  is shown in more detail. The manifold  3  consists in fact of a multiple port extruded tube and in the example shown three channels  16 ,  17  and  18  are present. It is however clear that any number of channels may be present. As clearly shown in  FIG. 2  the central channel  17  has an oval cross-section, i.e. it has two parallel side walls  20 ,  21  and two semi-circular end walls  22 ,  23 . In case the manifold has more than three channels each intermediate channel will have that type of shape. Otherwise the two outer channels  16  and  18  have identical cross-sections and are composed of a substantially semi-circular side-wall  24  and  25  respectively and a flat side wall  26  and  27  respectively facing the respective flat side walls of the channel  17 . 
     The outer surface of the manifold is formed by walls which are substantially parallel to the inner walls of the channels  16 ,  17  and  18  facing the outer wall, except for one side wall  30  which is perpendicular to the side walls  26 ,  20 ,  21  and  27  and which is made flat. By shaping the manifold in this way it is possible to withstand high internal pressures without generating excessive stresses in the walls of the manifold  3 . In fact the pressure in channel  17  on the flat side wall  20  and  21  is compensated by the pressure acting on the flat side walls  26  and  27 . Furthermore the remaining side walls are all curved thereby avoiding the building up of excessive stresses and making the manifold suitable for high pressure applications. 
     Moreover the cross-section of the manifold  3  can be easily adapted for different applications without having to increase the width of the manifold, by simply adjusting the length of the flat sidewalls  26 ,  20 , 21  and  27 , whereby the volume of the channels is adjusted accordingly. 
     As shown in  FIGS. 3 and 4 , the flat outer wall  30  is provided with a number of the holes  5  discussed above in reference to  FIG. 1 . The holes  5  are shown as longitudinal and extending perpendicular to the longitudinal direction of the manifold  3 . Each hole  5  is made in the following way, as clearly shown in  FIG. 3 . Up till the line  36 – 37  there is made a groove with rectangular cross-section and a width equal to the width of the flat tube to be inserted in the hole  5 , i.e. the smallest dimension of the flat tube  1 . This groove can be made by sawing, or the like. Subsequently the hole  5  is further shaped by punching, using a die with the right shape, whereby the groove is connected to the channels  16 ,  17  and  18 . The punch die is shaped in such a way that both longitudinal sides of the hole  5  are provided with an edge  38  serving as a stop for the insertion of the flat tube  1  in the hole  5 . Furthermore the wall portion  26 – 20  and  21 – 27  between the channels  16 ,  17  and  18  respectively are pushed back to some extent below the edge  38 , as seen in  FIG. 3 , thereby forming two substantially semi-circular top walls  41  and  42 , so that after insertion of a flat tube up till the edge  38 , an open connection is present between the channels  16 ,  17  and  18 , enabling a cross-flow of the medium in the manifold  3 . In this way a manifold is obtained which makes an easy mounting of the flat tubes possible. Because of the shape of the channels  16 ,  17  and  18 , the end portions of the flat tubes  1  will only slightly penetrate in the flow section of the channels  16 ,  17  and  18 , and thereby only influence to a minor degree the flow of medium through the channels  16 ,  17  and  18 . Because of the lower position of the separation walls  41  and  42  between the channels  16 ,  17  and  18  at the place of the flat tubes, the flow of medium from the manifold  3  to the tubes  1  or reverse will not be hindered by the separation walls  41  and  42 , as there is sufficient space between the walls  41  and  42  and face of the inserted flat tube  1  which will reach up till the line  38 . 
     As shown in  FIG.4  an additional opening  40  is present between two adjacent holes  5 , which opening  40  can be used for the insertion of a baffle  10  or  11  as explained above. The only difference with the holes  5  for the flat tubes  1  is that there is no edge  38  and the wall portions  41  and  42  shown in  FIG. 3  are removed up till halfway the height of the channels  16  and  17 . 
     The flat wall  30  of the manifold  3  is provided with two longitudinal grooves  46  and  47 . These grooves  46  and  47  can be used to clamp a brazing sheet on top of the manifold  3 . After placing a brazing sheet on the surface  30 , and folding the edges of that sheet into the grooves  46  and  47 , the grooves  46  and  47  can be deformed in such a way that the longitudinal edges of the brazing sheet are clamped to the manifold  3 . After insertion of the flat tubes  1  into the manifold  3  and insertion of the baffle  10 , the whole manifold  3  can be heated, e.g. by means of a brazing oven, and during this process the brazing sheet ensures that a reliable connection is obtained between the flat tubes  1  and the manifold  3 . 
     It is clear that the invention is not restricted to the embodiment described above, but that modifications can be applied without departing from the scope of the invention. More especially it is possible to use other systems for connecting the tubes to the manifold. 
     Therefore, while the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.