Abstract:
A motor vehicle combined heat exchanger has a tube bank linked to two manifolds and divided into an oil cooler having tubes for oil, and a condenser having tubes for a cooling fluid. The two types of tubes are different and possess respective hydraulic diameters related by the following inequality: 
     
       
         0.8 mm 2 ≦DHa×DHb≦3.00 mm 2   
       
     
     where the hydraulic diameter (DH) of a tube is defined by the formula DH=4S/P, in which S designates the area of the internal cross-section of the tube (expressed in mm 2 ) and P the internal perimeter, or “wet perimeter”, of the tube (expressed in mm).

Description:
FIELD OF THE INVENTION 
     The invention relates to a combined heat exchanger, particularly for a motor vehicle, including a bank of tubes linked to manifolds and divided into two parts capable of being traversed by different fluids. 
     BACKGROUND OF THE INVENTION 
     In a heat exchanger of this type, the two fluids are cooled by the same airflow which sweeps through the bank. 
     The invention relates more particularly to a combined heat exchanger in which the bank of tubes is divided into a part forming an oil cooler, the tubes of which are suitable for being traversed by oil, and into a part forming a condenser, the tubes of which are suitable for being traversed by a cooling fluid. 
     In such a heat exchanger, the oil is typically the transmission oil, in particular for an automatic gearbox of a motor vehicle. As for the condenser, it serves to cool the cooling fluid for a motor vehicle air-conditioning installation. 
     At the present time, the cooling of the cooling fluid and the cooling of the transmission oil are carried out by two separate exchangers, usually a parallel-flow condenser and an oil exchanger, of the vane type, placed in proximity to the condenser. 
     It is known, moreover, according to the Japanese Utility Model No. 61-167202 to produce a combined heat exchanger comprising a part forming a condenser and a part forming a heat exchanger. This heat exchanger comprises a common bank of tubes linked to two tubular manifolds. 
     The production of a combined heat exchanger, including a part forming an oil cooler and a part forming a condenser, poses many problems because the two fluids exhibit very different characteristics. Thus, the viscosity of the oil is very much greater than that of the coolant and the loss of pressure head in the oil is therefore very high. 
     Moreover, the two fluids circulate at very different temperatures, that of the oil being very much higher than that of the cooling fluid. These substantial temperature differences are capable of engendering differential-expansion phenomena which may damage the heat exchanger and lead to leakage. 
     Moreover, it may happen that the cooling fluid is heated by the oil, which then leads to a degradation in performance of the condenser part. 
     The invention aims to afford a solution to the above problems. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a combined heat exchanger including a bank of tubes linked to manifolds and divided into a part forming an oil cooler, the tubes of which are suitable for being traversed by oil, and into a part forming a condenser, the tubes of which are suitable for being traversed by a cooling fluid, wherein the tubes of the oil-cooler part and the tubes of the condenser part are different and possess respective hydraulic diameters related by the following inequality: 
     
       
         0.8 mm 2 ≦DHa×DHb≦3.00 mm 2   
       
     
     where the hydraulic diameter DH of a tube is defined by the formula DH=4S/P, in which S designates the area of the cross-section of the tube (expressed in mm 2 ) and P the internal perimeter, or “wet perimeter”, of the tube (expressed in mm). 
     Hence, the combined heat exchanger of the invention comprises different tubes, that is to say that the tubes of the condenser part are adapted to the circulation of the cooling fluid, whereas the tubes of the oil-cooler part are adapted to the circulation of the oil. 
     Moreover, it is essential for the product of the respective hydraulic diameters DHa and DHb to satisfy the foregoing inequality relationship. This is because it has been observed that when the product DHa×DHb is greater than 3.00 mm, the thermal power exchange within each of the two fluids drops off significantly. Moreover, when this product is less than 0.8 mm 2 , the loss of pressure head in the oil circuit increases vary greatly. 
     In the invention, the tubes of the bank are advantageously multi-channel tubes. 
     The hydraulic diameter of the tubes of the oil-cooler part is preferably greater than the hydraulic diameter of the tubes of the condenser part. 
     It is particularly advantageous for the number of channels of the tubes of the oil-cooler part to be less than the number of channels of the tubes of the condenser part. This means, in other words, that the tubes of the oil-cooler part contain fewer partitions than the tubes of the condenser part. This makes it possible to increase the hydraulic diameter and thus significantly to lower the loss of pressure head generated by the circulation of the oil in these tubes. 
     The tubes of the bank are advantageously obtained by extrusion. 
     According to another characteristic of the invention, the tubes of the bank are linked to two manifolds each of which includes a separating partition for isolating the oil circulating in the oil-cooler part and the cooling fluid circulating in the condenser part. 
     Taking into account the differences in temperatures between these two fluids, there is a benefit in using partitions forming thermal insulation. 
     According to yet another characteristic of the invention, the heat exchanger comprises means forming a thermal barrier between the tubes of the oil-cooler part and the tubes of the condenser part. 
     These means make it possible to limit the stresses due to the phenomena of differential expansion and to prevent the cooling fluid being heated by the oil, which is at a very much higher temperature. 
     In one embodiment of the invention, the means forming a thermal barrier comprise a tube of the bank, called “inactive tube” or “dead tube”, which is not traversed by any fluid and which opens out between double partitions of each of the manifolds. 
     In another embodiment of the invention, in which corrugated spacers are provided between the tubes of the bank, the thermal-barrier forming means comprise a region devoid of corrugated spacers, which extends between two adjacent tubes belonging respectively to the oil-cooler part and to the condenser part. 
     According to another characteristic of the invention, the bank and the manifolds are assembled by brazing. 
     Hence, the combined heat exchanger of the invention can be produced according to the well-known technology of brazed exchangers, such as that used, for example, in the production of the condensers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the description which follows, given by way of example, reference will be made to the attached drawings, in which: 
     FIG. 1 is a view in longitudinal section of a combined heat exchanger according to a first embodiment of the invention; 
     FIG. 2 is a view in section, on an enlarged scale, of a tube of the oil-cooler part; 
     FIG. 3 is a view in section, on an enlarged scale, of a tube of the condenser part; 
     FIG. 4 is a partial view in longitudinal section of a combined exchanger according to a second embodiment of the invention; and 
     FIG. 5 is a partial view in longitudinal section of an exchanger of the combined heat exchanger according to a third embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the various figures, like reference numerals refer to like parts. 
     The combined heat exchanger represented in FIG. 1 comprises a bank  10 , also called core, consisting of a multiplicity of tubes  12  extending parallel to each other and between which are arranged corrugated spacers  14  forming cooling fins. The ends of the tubes  12  open out, at one end, into a common manifold  16  and, at the other end, into another common manifold  18 . These two manifolds are of tubular configuration and extend parallel to each other. 
     The various components of the heat exchanger, that is to say the tubes  12 , the fins and the manifolds  16  and  18  are made of metal and assembled together by brazing. 
     The bank is divided into two parts, namely a part A forming an oil cooler and consisting of tubes  12   a  and a part B forming a condenser and consisting of tubes  12   b . The tubes  12   a  are suitable for being traversed by oil H, such as the transmission oil for a motor-vehicle automatic gearbox. The tubes  12   b  are suitable for being traversed by a cooling fluid R of a motor vehicle air-conditioning installation. It will be understood that these two fluids circulate in two different parts of the bank and are intended to be swept by the same airflow which sweeps over the bank  10 . 
     The manifolds  16  and  18  include respective insulating partitions  20  and  22  for insulating the two fluids from one another. 
     The partition  20  divides the manifold  16  into a compartment  24  for the oil (here placed in the upper part) and a compartment  26  for the cooling fluid (here placed in the lower part). Correspondingly, the partition  22  divides the manifold  18  into a compartment  28  for the oil (here placed in the upper part) and a compartment  30  for the cooling fluid (here placed in the lower part). 
     The oil to be cooled enters the compartment  24  through an entry pipe  32 , then flows in the tubes  12   a  by parallel flow so as to reach the compartment  28 . It then leaves the compartment  28  through an outlet pipe  34 . 
     The compartment  26  is itself divided into two parts, namely an upper part  36  and a lower part  38 , by a partition  40 . Likewise, the compartment  30  of the manifold  18  is divided into two parts, namely an upper part  42  and a lower part  44 , by a partition  46 . The cooling fluid R enters the compartment  36  through a entry pipe  48 , flows in a part of the tubes  12   b  so as to reach the compartment  42 , then flows in the opposite direction to reach the compartment  38 . Next, the cooling fluid reaches the compartment  44 , flowing again in the reverse direction, and leaves the heat exchanger through an outlet pipe  50 . Hence, in this example, the cooling fluid R flows alternately according to a three-pass mode. 
     It is important for the separating partitions  20  and  22  to constitute thermal insulation given that the oil H is at a temperature very much higher than that of the cooling fluid R. The tubes  12   a  and  12   b  (FIGS. 2 and 3) are flat, multi-channel tubes, obtained by extrusion from an appropriate metal alloy, generally aluminum based. 
     In the example, each tube  12   a  (FIG. 2) includes two channels  52  separated by a partition  54 , whereas each tube  12   b  (FIG. 3) includes four channels  56  separated by three partitions  58 . 
     However, the tubes  12   a  and  12   b  have the same outer cross-section, which allows standardization of manufacture, in the sense that the ends of the tubes are accommodated in identical holes formed in the manifolds  16  and  18 . 
     The tubes  12   a  and  12   b  have hydraulic diameters DH of DHa and DHb respectively. 
     It will be recalled here that the hydraulic diameter DH of a tube is defined by the formula DH=4S/P, in which S designates the internal cross-section of the tube (expressed here in mm 2 ) and P the internal perimeter, also called “wet perimeter”, of the tube (here expressed in mm). 
     The tubes  12   a  and  12   b  thus have specific characteristics making it possible to adapt them respectively to the cooling of the oil and to the cooling of the cooling fluid. Because the tubes  12   a  have fewer channels (and thus fewer partitions) than the tubes  12   b , the hydraulic diameter of the tubes  12   a  is increased, which makes it possible significantly to lower the loss of pressure head generated by the flowing of the oil in the tubes  12   a.    
     In accordance with the invention, the product DHa×DHb takes a value which falls in an interval defined by the following inequality: 
      0.8 mm 2 ≦DHa×DHb≦3.00 mm 2   
     When this inequality is satisfied, a combined heat exchanger is obtained in which the thermal power exchanged in terms of each of the two fluids is optimal, while limiting the loss of pressure head of the oil circuit. 
     As indicated above, because the tubes  12   a  and  12   b  are traversed by fluids at different temperatures, there is a risk of differential-expansion phenomena appearing and generating stresses, particularly in the region of the brazed joints between the tubes and the manifolds. 
     In the embodiment of FIG. 1, partitions  20  and  22  are provided which are particularly good insulators and which, advantageously, may be double partitions. 
     Referring now to FIG. 4, another embodiment of the invention is shown with means forming a thermal barrier between the tubes  12   a  and the tubes  12   b.    
     In this embodiment, the bank  10  includes an inactive tube  12   i , also called “dead tube”, which is not traversed by any fluid and which opens out between a double partition  20  of the manifold  16  and a double partition  22  of the manifold  18 . 
     The heat exchanger of FIG. 5 includes other means forming a thermal barrier. To that end, the bank is configured in such a way as to include a region  60  devoid of corrugated spacers, which extends between the parts A and B of the bank, that is to say between two adjacent tubes  12   a  and  12   b  belonging to these two parts A and B. 
     In one embodiment, the tubes  12   a  and  12   b  each have a length of 600 mm. The hydraulic diameter DHa of each of the tubes  12   a  is equal to 1.6, while the hydraulic diameter DHb of each of the tubes  12   b  is equal to 1.313, the product DHa×DHb thus being equal to 2.1. 
     Obviously, the invention is not limited to the embodiments described above and extends to other variants.