Abstract:
A header for a heat exchanger includes a substantially planar base portion and a pair of step portions. The step portions are angled from the plane of the base portion. The header is also provided with a plurality of substantially parallel slots spaced apart along the length of the header. Each slot has an elongate section extending across the width of the base portion and end sections extending from the elongate section into the step portions of the header.

Description:
BACKGROUND  
       [0001]     1. Technical Field  
         [0002]     The present invention relates generally to heat exchangers, and more particularly relates to headers for heat exchangers.  
         [0003]     2. Background Information  
         [0004]     Typically, automotive vehicles are provided with an engine cooling system with a heat exchanger, such as a radiator. When the engine is running, heat is transferred from the engine to a coolant that flows through the engine, thereby cooling the engine. The coolant then flows from the engine to the heat exchanger through a series of conduits. At the heat exchanger, heat is transferred from the coolant to cooler air that flows over the outside of the heat exchanger. This process repeats itself in a continuous cycle.  
         [0005]     A typical heat exchanger includes a series of tubes supported by two headers. One type of conventional header is a flat header. When these flat headers are joined to a respective tube, for example, by brazing, the joint between the header and the tube lies in a flat plane. These types of header/tube combinations are prone to failure because of the stress concentrations that occur along the header/tube joint. These stresses are typically attributable to the thermal loading (i.e., stresses induced by the rise and fall of the temperature of the heat exchanger components) on the header and tubes during the operation of the engine.  
         [0006]     From the above, it is seen that there exists a need for an improved heat exchanger header that experiences less thermal loading.  
       BRIEF SUMMARY  
       [0007]     In overcoming the above mention and other drawbacks, the present invention provides a heat exchanger header which when combined with a tube removes the highest stress concentrations in the header/tube joint.  
         [0008]     In one embodiment, a header for a heat exchanger includes a substantially planar base portion and a pair of step portions. The step portions are angled from the plane of the base portion. The step portions are connected by either a straight or a curved section. The header is also provided with a plurality of substantially parallel slots spaced apart along the length of the header. Each slot has an elongate section extending across the width of the base portion and end sections extending from the elongate section into the step portions of the header.  
         [0009]     Various embodiments of the header can have one or more of the following features. The end sections each can have terminal ends spaced apart from the plane of the base portion, defining a separation distance. Each slot can be provided with a tube inserted into the slot. In certain embodiments the tube is brazed to the respective slot. The juncture between each tube and the elongate section of a respective slot defines a transition line of deformation spaced apart from the highest stress concentrations occurring in the brazing joint at or near the location of the juncture between the terminal ends and the tube.  
         [0010]     The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the views. In the drawings:  
         [0012]      FIG. 1  depicts an automotive radiator;  
         [0013]      FIG. 2A  depicts a portion of a conventional heat exchanger header with a flat tube;  
         [0014]      FIG. 2B  is a side view of the conventional header with a portion of the flat tube along the line  2 B- 2 B of  FIG. 2A ;  
         [0015]      FIG. 3A  depicts a portion of a heat exchanger header with several flat tubes in accordance with the invention;  
         [0016]      FIG. 3B  depicts the header of  FIG. 3A  with one of the flat tubes in accordance with the invention;  
         [0017]      FIG. 3C  is a view of the header along the line  3 C- 3 C of  FIG. 3B ;  
         [0018]      FIG. 4  depicts a conventional flat header without tubes;  
         [0019]      FIG. 5  depicts a trapezoidal header without tubes in accordance with the invention;  
         [0020]      FIG. 6  is a cross-sectional view of an alternative header in accordance with the invention; and  
         [0021]      FIG. 7  is a cross-sectional view of yet another alternative header in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  illustrates a typical automotive radiator  2  with a heat exchanger core or matrix  3 . The core  3  includes a number of parallel coolant tubes  4  with heat exchanger fins  5  of concertina form positioned between and in contact with the tubes  4 . The tubes  4  are mounted to a pair of headers  6 . A pair of side walls  7  provide additional structural support to the core  3 . When the radiator  2  is in use, coolant heated by the engine enters an inlet  8  and circulates through the tubes  4  as air moves through the fins  5 . As such, heat in the tubes  4  is exchanged with the air passing through the fins. The cooler coolant exits the radiator  2  through an exit  9  and returns to the engine to repeat the engine cooling process.  
         [0023]     A heat exchanger in an automotive vehicle typical experiences a significant amount of thermal loading, since the heat exchanger is subjected to extreme temperature variations during its lifetime, thereby leading to a failure of the exchanger. For example, referring to  FIG. 2A , in a conventional heat exchanger tube  10 , failure, such as a crack, caused by thermal loading usually occurs on the tube at or near the intersection  12  between a flat tube  14  and a header  16 , in particular, at the location  22  where the externally induced stress (or service stress) from the thermal loading overlaps with the highest stress concentrations of the joint between the header  16  and tube  14 , as described below in greater detail.  
         [0024]     Externally induced service stress typically occurs on the tube at or near the boundary between the tube  14  and the header  16 . On one side of this boundary (i.e. the internal or coolant side), the tube  14  does not deform because of the restriction of the header  16 . On the other side, however, the tube  14  deforms under thermal loading. For purposes of illustration, the intersection of the tube  14  and the header  16  define a plane, which in turn defines a “transition line of deformation”  20 , as shown in  FIG. 2B , when the tube/header combination is viewed along the line  2 B- 2 B of  FIG. 2A .  
         [0025]     The tube  14  and header  16  are in many cases joined together by a suitable process, for example, by brazing. Thus, stresses occur along the brazing between the tube  14  and header  16 . Note that stress concentration is a physical property related to the geometry of the tube-to-header joint configuration. The highest stress concentration generally occurs at or near the narrowest region of the tube  14  that intersects the header  16 , namely, at the locations identified by the reference numerals  22 . When the “transition line of deformation”  20  overlaps the “stress concentration”  22 , as in the case of the tube/header combination of  FIGS. 2A and 2B , the externally induced stress intensifies, leading typically to early failure of the heat exchanger.  
         [0026]     Referring now to  FIG. 3A , there is shown a heat exchanger  30  with flat tubes (now identified as  32 ), cooling fins  5  positioned between the tubes  32 , and a header  34  in accordance with the invention. Referring also to  FIGS. 3B and 3C , the header  34  is configured to separate the externally induced service stress along the aforementioned “transition line of deformation”  20  from the highest stress concentrations occurring at the narrowest regions  36  of the juncture between the tube  32  and the header  34 . This separation (d) effectively reduces the stress intensification at these regions  36  and distributes the stress more evenly over the entire tube-to-header joint, thereby prolonging the tube-to-header joint life. As shown in  FIG. 3B , a header with a trapezoidal cross section can achieve such a separation.  
         [0027]     For the sake of comparison, a conventional flat header  40  shown in  FIG. 4  was compared with that of a trapezoidal header  50  shown in  FIG. 5  in thermal cycling tests. As can be seen in the comparison of  FIGS. 4 and 5 , the conventional header  40  has a series of essentially straight tube slots  42 , while the trapezoidal header  50  has tube slots  52  that are not straight. Instead, each slot  52  has an elongate section  54  extending across a planar portion  56  of the header  50  and two end sections  58  that extend from the elongate section  54  into two stepped portions  60  of the header. The stepped portions  60  and hence the end sections  58  of the slots  52  rise at an angle, following a straight segment (or a curved segment as shown in  FIGS. 6 and 7 ), from the plane of the planar portion  56 , such that the terminal ends  62  of the end sections  58  are separated from the plane of the planar portion  56  by the separation distance (d). Depending upon the application of the header  50 , the separation distance (d) may be the range from about 2 mm to about 20 mm. Surrounding each slot  52  is a raised region  64 . These regions  64  provide added rigidity to the header  50  and a convenient platform along which the tubes are brazed to the header  50 .  
         [0028]     In certain embodiments, the header  50  is made from a metal such as aluminum or steel, or any other suitable material. Depending on the vehicle, the header  50  can be provided with six to two hundred slots. The slots  52  are spaced apart by about 4 mm to 15 mm, and each slot  52  is about 1 mm to 12 mm wide. The elongate section  54  of each slot is about 3 mm to 85 mm long and the end sections  58  are about 2.5 mm to 28 mm long. As mentioned above, each slot  52  is joined to a respective tube by a suitable method such as brazing, soldering, or mechanically assembling.  
         [0029]     An example of the results of the thermal cycling tests is shown below in Table 1. In these tests, the headers were subjected to a cyclic thermal loading with a high-low temperature differential of about 130° C.  
                                                 TABLE 1                                   Crack Initiation   Crack Propagation   Radiator Failed                                    Flat Header   110 Cycles   119 cycles   119 cycles (two                   samples)       Trapezoidal   854 Cycles   No visible crack   Tests for two       Header       propagation   samples were                   suspended after                   1572 cycles                  
 
         [0030]     In Table 1, crack initiation cycle is defined as the cycle count at which there is evidence of coolant at the tube/header joint. Crack propagation cycle is defined as the cycle count at which there are several drops of coolant leakage per cycle. And radiator failure cycle is defined as the cycle count at which the test is terminated because of significant amount of leakage of coolant from the heat exchanger. As can be seen in Table 1, crack initiation occurred in the flat header around 110 cycles, and crack propagation was seen around 119 cycles. Thus, the radiator with the flat header was considered to have failed at 119 cycles. This example used a sample size of two for each configuration.  
         [0031]     As for the trapezoidal header, crack initiation was observed around 854 cycles. However, crack propagation was never observed; that is, the radiator did not fail during the test. The test for the trapezoidal header was eventually terminated at 1572 cycles. In view of the above, it is seen that radiators provided with trapezoidal headers have life spans that vastly exceed that of radiators with flat headers.  
         [0032]     It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention. For example, as shown in  FIGS. 6 and 7 , the header  34  can be provided with convex segments  70  ( FIG. 6 ) or concave segment  72  ( FIG. 7 ) rather than the straight segments shown in  FIG. 3C .