Abstract:
Thermally induced stress tending to cause breakage or leakage at tube to header joints in a heat exchanger is eliminated by placing lines of weakening in the side plates conventionally employed in such heat exchanger prior to subjecting the heat exchanger to brazing.

Description:
FIELD OF THE INVENTION 
     This invention relates to heat exchangers, and more specifically, to improved side pieces for heat exchangers; as well as methods of making a heat exchanger. 
     BACKGROUND OF THE INVENTION 
     Many heat exchangers in use today, as, for example, vehicular radiators, oil coolers, and charge air coolers, are based on a construction that includes two spaced, generally parallel headers which are interconnected by a plurality of spaced, parallel, flattened tubes. Located between the tubes are thin, serpentine fins. In the usual case, the side most tubes are located just inwardly of side plates on the heat exchanger and serpentine fins are located between those side most tubes and the adjacent side plate. 
     The side plates are typically, but not always, connected to the headers to provide structural integrity. They also play an important role during the manufacturing process, particularly when the heat exchanger is made of aluminum and components are brazed together or when the heat exchanger is made of other materials and some sort of high temperature process is involved in the assembly process. 
     More particularly, conventional assembly techniques involve the use of a fixture which holds a sandwiched construction of alternating tubes and serpentine fins. The outside of the sandwich, that is the outer layers which eventually become the sides of the heat exchanger core, is typically provided with side pieces whose ends are typically connected mechanically to the headers. Pressure is applied against the side pieces to assure good contact between the serpentine fins and the tubes during a joining process such as brazing to assure that the fins are solidly bonded to the tubes to maximize heat transfer at their points of contact. If this is not done, air gaps may be located between some of the crests of the fins and the adjacent tube which adversely affect the rate of heat transfer and durability, such as the ability to resist pressure induced fatigue and to withstand elevated pressures. 
     At the same time, when the heat exchanger is in use, even though the side plates may be of the same material as the tubes, because a heat exchange fluid is not flowing through the side plates but is flowing through the tubes, the tubes will typically be at a higher temperature than the side plates, at least initially during the start up of a heat exchange operation. 
     This in turn results in high thermal stresses in the tubes and headers. Expansion of the tubes due to relatively high temperatures tends to push the headers apart while the side plates, at a lower temperature, tend to hold them together at the sides of the core. All too frequently, this severe thermal stress in the heat exchanger assembly results in fracture or the formation of leakage openings near the tube to header joints which either requires repair or the replacement of the heat exchanger. 
     It has been proposed to avoid this problem, after complete assembly of the heat exchanger, by sawing through the side plates at some location intermediate the ends thereof so that thermal expansion of the tubes is accommodated by the side plates, now in multiple sections, which may move relative to one another at the saw cut. However, this solution adds an additional operation to the fabrication process and consequently is economically undesirable. 
     SUMMARY OF THE INVENTION 
     It is the principal objection of the invention to provide a new and improved heat exchanger, and a method of making the same, that eliminates heat exchanger failure problems due to thermally induced stresses resulting from a difference in thermal expansion between the expansion of the tubes of the heat exchanger and the expansion of the side pieces thereof. It is also an object of the invention to provide a method of making such a heat exchanger. 
     According to one facet of the invention, there is provided a heat exchanger that includes a pair of spaced, generally parallel headers, a plurality of spaced, generally parallel tubes extending between and in fluid communication with the interior of the headers, a pair of elongated side plates, one at each side of the heat exchanger, and extending between and connected to the headers and spaced from the adjacent tube at the corresponding side of the heat exchanger. Serpentine fins are disposed between adjacent tubes as well as between the side plates and the tubes adjacent thereto. The invention contemplates the improvement wherein each side plate includes at least one opening between its edges which has a periphery with a part of the periphery in close proximity to at least one of the edges along with a score line in each side plate that extends from the part of the periphery to the edge. As a consequence, thermally induced stress will cause the side piece to sever at the location of the opening and the score line and thereafter the stress at tube to header joints or the like is relieved on a permanent basis. 
     According to one embodiment of the invention, the opening is elongated and is at an acute angle to the direction of elongation of the corresponding side plate. 
     A preferred embodiment contemplates that the side plate is formed as a channel having a base with at least one leg extending therefrom terminating at the one edge. The opening is formed in the base and in the one leg and the score line is in the one leg. 
     In a highly preferred embodiment, there is provided a heat exchanger generally as stated previously with the invention contemplating the improvement wherein each side plate is channel shaped having abase and two spaced outstanding legs extending from the base and terminating at opposite edges. First and second, elongated openings are disposed in each of the side plates in side by side relation and have respective center lines that intersect in an acute angle. Each of the openings has a periphery that includes a first part in close proximity to a corresponding one of the edges and a second part in close proximity to the other of the openings. The first and second parts are spaced from one another in the direction of elongation of the side piece and a first line of weakening is located at each of the first parts and extends between a corresponding one of the openings and the adjacent one of the edges. A second line of weakening extends between the openings at the second part. 
     As a consequence of this construction, the side plates may sever at the lines of weakening to relieve stresses as before. Moreover, the formation of the side plate as a channel and the effective staggering of the first and second lines of weakening does not materially reduce the bending strength or resistance to bending of the side plate so that a high degree of structural integrity is maintained prior to assembly of the side plates to the core. This accomplishes two purposes: It allows handling of the side pieces prior to assembly to the core without requiring great care on the part of the persons that must handle the side pieces; and it distributes the expansion stress that otherwise would be focused at the location of severance. 
     In a preferred embodiment, the acute angle is on the order of 90°. 
     Preferably, the first and second lines of weakening are defined by V-shaped notches in the legs and in the base of the channel respectively. 
     In a highly preferred embodiment, the lines of weakening have a length of about 4.6 mm or less. 
     The invention also contemplates the provision of a method of making an aluminum heat exchanger which includes the steps of: (a) assembling the components of a heat exchanger core in a fixture to have spaced headers, spaced tubes extending between the headers, side plates extending between the headers at the sides of the core and serpentine fins located between adjacent tubes as well as between the side plates and the adjacent tube at each side of the core; (b) mechanically fixing each end of the side plate to the adjacent header; (c) prior to steps (a) and (b), weakening the side plate at a point intermediate its ends so as to reduce its ability to withstand tension while not materially affecting its ability to withstand bending; and (d) subjecting the assembly resulting from step (b) to brazing temperatures to (i) braze the components together and (ii) allow the severance of each side piece at said point as a result of thermally induced stress. Whether services will in fact occur generally depends on the rate at which the brazed components cool following brazing. 
     Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a somewhat schematic, side elevation of a heat exchanger made according to the invention; 
     FIG. 2 is a fragmentary, plan view of a partially fabricated side plate made according to the invention; 
     FIG. 3 is a plan view of a fully fabricated side plate; 
     FIG. 4 is a perspective view of a fully fabricated side plate; 
     FIG. 5 is an enlarged sectional view taken approximately along the line  5 — 5  in FIG. 3; and 
     FIG. 6 is an enlarged, fragmentary view of part of the side plate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described hereinafter as a vehicular radiator, as, for example, a radiator for a large truck. However, it should be understood that the invention is applicable to radiators used in other contexts, for example, a radiator for any vehicle or for stationary application as an internal combustion engine driven generator. The invention is also useful in any of the many types of heat exchangers that utilize side plates to hold serpentine fins against parallel tubes extending between spaced headers, for example, oil coolers and charge air coolers. Accordingly, no limitation to any particular use is intended except insofar as expressed in the appended claims. 
     Referring to FIG. 1, a typical heat exchanger of the type of concern includes spaced, parallel header plates  10 ,  12 , between which a plurality of flattened tubes  14  extend. The tubes  14  are spaced from one another and their ends are brazed or welded or soldered and extend through slots, not shown, in the headers  10  and  12  so as to be in fluid communication with the interior of a tank  16  fitted to each of the headers  10 ,  12 . In this regard, it is to be noted that as used herein, the term “header” collectively refers to the header plates  10 ,  12 , to the headers  10 ,  12  with the tanks  16  secured thereon, or integral header and tank constructions known in the art as, for example, made by tubes or various laminating procedures. Side pieces  18 ,  20  flank respective sides of the heat exchanger construction and extend between the headers  10 ,  12  and are typically mechanically connected thereto as well as metallurgically bonded thereto. 
     Between the spaced tubes  14 , and between the endmost tube  14  and an adjacent one of the side plates  18 ,  20  are conventional serpentine fins  22 . As is well known, the fins  22  may be formed of a variety of materials. Typical examples are aluminum, copper and brass. However, other materials can be used as well depending upon the desired strength and heat exchange efficiency requirements of a particular application. 
     In a highly preferred embodiment of the invention, all of the just described components, with the possible exception of the tanks  16  which may be formed of plastic, are formed of aluminum or aluminum alloy and are braze clad at appropriate locations so that an entire assembly is illustrated in FIG. 1 may be placed in a brazing oven and the components all brazed together. In the usual case, prior to brazing, an appropriate fixture is employed to build up a sandwich made up of the tubes  14  alternating with the serpentine fins  22  and capped at each end by the side plates  18  and  20 . The headers  10 ,  12  are fitted to the ends of the tubes  14  and in the usual case, the side plates  18  and  20  may be mechanically coupled to the headers  10 ,  12  typically by bending tabs on the side plates  18  over the corresponding ends of the headers  10 ,  12 . 
     FIG. 2 illustrates a partially fabricated preform for making up the side plates  18 ,  20 . As illustrated, it is elongated and has header connecting ends  24 ,  26 . Non-existing bend lines shown only for the purpose of illustrating the position of a bend to be formed in the side plates are illustrated at  28  and  30 . Ultimately, the strips shown in FIG. 2 will be bent along the lines  28  and  30  to form a channel having a base  32  flanked by upstanding legs  34 ,  36  which are spaced from one another and which terminate in respective, opposite edges  38 ,  40 . While a channel cross section is preferred, in some cases, the side plates  18 ,  20  may remain generally planar or flat. 
     The strip is weakened intermediate its ends by the provision of at least one opening  42  at one or more locations between the ends  24 ,  26  of the strip. In a preferred embodiment, an opening  42  is paired with an opening  44 . The openings  42 ,  44  are generally in the form of an oval with each having a center line  46  that coincide with the major axis of the associated opening  42 ,  44 . The center lines of the openings  42 ,  44  intersect one another at an acute angle to form a V-shape. In a highly preferred embodiment, the angle of intersection is 90°, and each center line is at 45° to the longitudinal axis of the side plate. 
     The opening  42  has a periphery  48  that includes a part  50  in close adjacency to the edge  38 . As seen in FIG. 5, the spacing between the part  50  of the periphery  48  and the edge  38 , in a highly preferred embodiment, is 1.6 mm. However, other values may be utilized as will become apparent to those skilled in the art hereinafter. 
     The opening  44  likewise has a periphery  52  with a part  54  in close adjacency to the edge  40 . Again, the spacing between the part  54  will be 1.6 mm in a highly preferred embodiment. 
     The peripheries  48  and  52  of the openings  42  and  44  also have parts  56  that are closely adjacent to the corresponding part of the other of the openings  42 ,  46 . As shown in FIG. 6, the parts  56  of the openings  42  and  44  are spaced from one another a distance of 3.2 mm although other values may be employed. In general, the dimension will be 4.6 mm or less. 
     In the embodiment illustrated, there are two sets of the openings  42 ,  44  at different locations between the ends  24 ,  26  of the side piece. 
     The side piece may be provided, at its ends  24 ,  26  with notches  60 ,  62  which then define tabs  64  which may be bent about the ends of the headers  10 ,  12  or received in slots therein to mechanically secure the side plates  18 ,  20  to the assemblage prior to brazing. 
     One significant feature of the invention is the provision of lines of weakening in each side plate. Thus, at the part  50  of the periphery  48  of the opening  42 , a line of weakening  70  extends to the adjacent edge  38  of the side plates. A similar line of weakening  72  is located at the part  54  of the periphery  52  of the opening  44  and extends to the edge  40  of the side plate. 
     In addition, a line of weakening  74  extends between the openings  42 , 44  at those parts  56  of their peripheries  48 ,  52  which are in close adjacency. It will be observed that the lines of weakening  70 ,  72 ,  74  are spaced from each other along the length of the side plates  18 ,  20 . 
     In a preferred embodiment, the lines of weakening are formed as V-shaped notches as shown by the line of weakening  74  in FIG.  5 . The sides of the V-shaped notches forming the lines of weakening are angularly spaced approximately 90° and the depth of each notch will be approximately half way through the thickness of the side piece. In the embodiment illustrated, the depth of each of the notches forming a line of weakening  70 ,  72 ,  74  is 0.8 mm. 
     Of course, the depth of each notch may be varied depending upon the thickness of the material as well as the distance between the parts  56 , or the parts  50 ,  54  in the respective edges  38  and  40 . The lines of weakening are in fact square lines achieved by the process of coining. However, it is contemplated that the lines of perforation could be used in some instances. Further, any way of weakening metal along a line may be employed if desired. 
     The heat exchanger made according to the invention is fabricated by an inventive method that includes, as a first step, the step of assembling the components of the heat exchanger, namely, the headers  10 ,  12 , the tubes  14 , the side pieces  18 ,  20  and the serpentine fins  22  in a fixture so that the headers are spaced with the tubes spaced and extending between the headers into slots therein and side plates extending between the headers at the sides of the core together with serpentine fins located between adjacent tubes and between the side plates and the adjacent tube at each of the sides of the core. The side plates are typically, but not always, mechanically fixed at each end to the adjacent header. Prior to performing the foregoing steps, however, it will be understood that the side plates are fabricated to have a line of weakening at a point intermediate the ends  24 ,  26 . Preferably, the embodiment disclosed will be employed but as will be apparent to those skilled in the art, other embodiments can be used as well. The point of weakening the side plate is to reduce its ability to withstand tension while not materially affecting its ability to withstand bending; and this is particularly facilitated where the score lines  70 ,  72 ,  74  are offset from one another as shown. 
     The resulting assembly is then subjected to brazing temperatures to both braze the components together and to allow the thermal stresses involved in the brazing process to sever each side piece at the point of weakening as a result of thermally induced stress. Whether severance actually occurs will depend upon the rate the assembly cools following brazing. 
     In some cases, the severing may not occur fully during the brazing process but when the heat exchanger is placed in use, it is sufficiently weakened in the side plates at the lines of weakening that, after a few thermal cycles of operation, the side plates will break in use well before damage to the tube to header joints or elsewhere in the heat exchanger can occur.