BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a heat exchanger according to the preamble of Claim  1 . 
         [0002]    A weakness in known heat exchangers is the pipe/base connection(s), which are preferably formed as soldered joints between the pipe ends of the pipes and through-holes of the pipe base. The stress on the pipe/base joint is complex: for example, shear and bending forces arise which are transferred from the pipe to the pipe base. In particular, the corner pipes, that is, the outer-positioned pipes, are subjected to an elevated stress, so that cracks can occur in the pipe ends, in the pipe base, or in the solder sites (solder meniscus) in these pipe/base joints. In the case of heat exchangers designed as intercoolers, the pipes and the collection boxes are stressed by the interior pressure of the compressed charge air; likewise shear and bending stresses appear in the pipe/base joints due to varying thermal expansion of the pipes. 
         [0003]    Proposals are already known for reducing or eliminating these elevated stresses: Document DE 103 16 756 A1 by the applicant discloses an intercooler with a pipe base and through-holes into which flat pipes are soldered. Between the longitudinal sides of the pipe base and the narrow sides of the through-holes, sectioned strips are installed and are soldered to the pipe base and to the through-holes, so that a reinforcement of the pipe base and a reduction in stress peaks are achieved. This means can be applied in particular to the bending stress occurring in this region. 
         [0004]    Document DE 103 54 382 by the applicant discloses an intercooler with flat pipes, whose pipe ends are soldered into through-holes of the pipe base. To reinforce the pipe/base joints, an integrated component is set onto the pipe base in the region of the corner pipes, said component extends by prong-like fingers on the narrow sides of the flat pipe into the pipe ends, and said component is soldered to the pipe ends. Thus a stress relief of the pipe/base joint is obtained, in particular in the region of the stress peaks which occur primarily on the narrow sides. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The purpose of the present invention is to stress relieve the pipe base and the pipe/base joints of heat exchangers of the kind specified above in a simple and low-cost manner, in order to ensure the leak tightness of the pipe/base joint, even under stronger stresses. 
         [0006]    This problem is solved by the features of claim  1 . According to the invention, a sheet metal part with insertion openings for the pipe ends is provided that is located on the outside of the pipe base that is, on the side facing the block. The sheet metal part surrounds the circumference of the pipe by its insertion openings and is positively connected thereto. On the other side, the sheet metal part lies upon the underside of the pipe base and is positively connected to the pipe base in this region, preferably by soldering. This yields—at least in part for the highly stressed pipe/base connections—a force transfer plate which evens out and reduces the forces transferred from the pipe into the pipe base. Preferably, the invented force transfer plate is used for the corner pipe, that is, for the outer pipe of the block, since that pipe is exposed to the greatest stress. It is an advantage herein that the invented sheet metal part is easy to manufacture from a plate strip, can be connected in part with the same tools used for the openings, and is easy to connect to the pipe base and the pipes during the soldering process. Thus, the critical pipe/base joints are sufficiently stress relieved. 
         [0007]    In one favorable embodiment of the invention, the pipes are designed as flat pipes and the receiving openings of the pipe base as through-holes. The openings for insertion of the pipe ends into the force transfer plate are likewise designed as through-holes. Favorable soldering conditions are thus obtained both on the base side and on the side of the force transfer plate, since the through-holes of the base and the through-holes of the force transfer plate are oriented in opposite directions. This ensures the advantage of a stable seat for the pipe in the base, in particular with regard to bending and shear stresses. 
         [0008]    According to one additional, preferred embodiment, the pipe base is part of an open box section, like that used, for example, for so-called all-metal or all-aluminum heat exchangers. The additional sheet metal part is designed as a reinforcing or supporting plate such that it encompasses at least a portion of the pipe base, preferably in the region of a separating wall, and also encompasses parts of the two longitudinal sides of the box section, that is, is matched to the box section. The pipe base is slightly domed toward the block side and has inwardly directed through-holes, between said holes there are convex regions curved outward that in a longitudinal cross section through the box section have a crest, and in the transverse cross section have a crest line through the crest. According to one preferred embodiment of the reinforcing plate, said plate rests upon the crest lines and forms a contact zone which is preferably used as solder zone. Thus, a solid, positive connection is created between the reinforcing plate and the pipe base, which results in a significant reinforcement, in particular in the event of interior pressure acting on the box section. The reinforcing plate is cut out in the region of the through-holes and of the flat pipe extending into the through-holes, and thus has a plurality of binding bands located between the pipes or through-holes, said strips lie against the crest lines and thus reinforce the pipe base. Due to the reinforcement and stiffening of the pipe base, the pipe base joints are at the same time stress relieved, so that an improved leak tightness and an increased life of the heat exchanger is achieved. 
         [0009]    The invented force application or reinforcing plate can be used in particular on intercoolers in which hot, compressed charge air for an internal combustion engine, in particular for motor vehicles, is cooled. In intercoolers the stresses caused by the varying thermal expansions of the pipes are particularly critical, that is, the invented force application or load transfer plate has a particularly favorable advantage for intercoolers, since they are subject to an increased interior pressure due to the compressed charge air. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]    One embodiment of the invention is illustrated in the figures and will be explained in greater detail below. We have: 
           [0011]      FIG. 1  a section of an intercooler with the invented load transfer plate 
           [0012]      FIG. 2  the load transfer plate shown as a single part 
           [0013]      FIG. 3  a box section with reinforced pipe base 
           [0014]      FIG. 4  a longitudinal cross section through the box section and 
           [0015]      FIG. 5  a transverse cross section through the box section 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  shows a corner section of an intercooler  1  which has a block  4  consisting of flat pipes  2  and corrugated ribs  3 . This block is sealed off by a side plate  5  at the outermost corrugated rib  3 . A pipe base  6  is connected to the block  4 ; said pipe base has through-holes  7  to hold the flat pipe ends  2   a.  To illustrate the pipe/base connection between flat pipe end  2   a  and through-hole  7 , the pipe base  6  is cut open to show the through-hole  7 , a flat pipe  2  and the side plate  5 . The pipe ends  2   a  are soldered to the upward directed (in the figure) for through-holes  7  and form the pipe/base connection. Furthermore, the corrugated ribs  3  are soldered to the flat sides of the flat pipe  2 , likewise the side plate  5  is soldered to the corrugated rib  3  and the base  6 , which has a strip  6   a  that is surrounded by a claw  5   a  of the side plate  5 . A cover or collector box (not illustrated) sits upon the pipe base  6  and is preferably soldered to the edge region of the pipe base  6 . Alternatively, however, a plastic box could be used which can be joined to the pipe base  6  by a suitable mechanical clamping connection. To this extent the intercooler  1  corresponds to the prior art. 
         [0017]    According to the invention a reinforcing plate part  8 , a so-called load- or force-transfer plate, is located on the underside or outside of the base  6  (the side facing the ribs  3 ). Said plate is connected to the pipe base  6  by soldering. The reinforcing plate  8  has downward pointing (in the drawing) through-holes  9  in the region of the pipe  2 , said holes surround the pipe  2  and are soldered to the pipe ends  2   a.  Thus the pipe end  2   a  is inserted into two oppositely oriented through-holes  7 ,  9  of the pipe base  6  and of the reinforcing plate  8 . In the figure it is evident that due to the opposing orientation of the through-holes, a greater fixing length X for the pipe is obtained, so that the forces and moments transferred from the pipe  2  to the base  6  are on the one hand introduced into the reinforcing plate  8 , and on the other hand are introduced directly into the pipe base  6  via the through-holes  7 . Elevated stresses and stress peaks are thus diminished. The load transfer plate  8  is preferably provided for the outermost three pipes  2 , the so-called corner pipes, because the greatest stress appears there. In another embodiment, in particular for an intercooler, which is exposed to different, in particular more powerful stresses, the reinforcing principle is applied in larger regions, in particular, for example, across the entire length of the block if so required. Otherwise the load transfer plate  8  extends across the entire width of the pipe base  6  and is matched to the contour thereof, so that a full-surface soldering and thus a well-distributed force transfer will occur. 
         [0018]      FIG. 2  shows the load transfer plate  8  as a detailed part with three through-holes  9 , which are located here (in the figure) on the top side  8   a  and which are formed from the plate  8 . The through-holes  9  are produced in a known manner by hole punching. Between the through-holes  9  there are flat bands  10  adapted to the base contour, whereas the outer edges  11 ,  12  running transverse thereto, are slightly curved—corresponding to the contour of the pipe base  6 —in order to entirely surround the rounded area of the pipe base  6 . As is evident from the figure, the through-holes  9  each have an inner, circumferential flange  9   a  which rests against the outer perimeter of the flat pipe ends  2   a  and allows a circumferential, large-area soldering. By means of these soldered areas, the forces and moments initially in the pipe are transferred to the reinforcing plate  8  and then to the pipe base  6 . 
         [0019]    The assembly of the load transfer plate  8  is quite simple: after completion of the block  4  (cassetting of pipes  2  with ribs  3 ), the load transfer plate  8  is set onto the pipe ends  2   a,  preferably at the corner pipe, and specifically in such a manner that the through-holes  9  are directed toward the corrugated ribs  3 . Next, the pipe base  6  is put in place, so that the protruding pipe ends  2   a  are inserted through the through-holes  7  of the pipe base  6 . Next, the pre-assembled unit—perhaps with a solderable collector box—is placed into a soldering furnace (not illustrated) and soldered there, so that a soldered joint is formed between the through-holes  7  and  9  and pipe ends  2   a,  and between the force transfer plate  8  and pipe base  6 . 
         [0020]      FIG. 3  shows an additional embodiment of the invention for an all-metal or all-aluminum heat exchanger, of which only one open box section  20  is illustrated, which has two longitudinal sides  21 ,  22  and one slightly curved pipe base  23  with inward directed through-holes  24 . The box section  20  is sealed by a cover (not illustrated) and is used as a collector or distributor box for the heat exchanger, the flat pipes (not illustrated) of which are held by the through-holes  24 . A heat exchanger of this kind, with an all-aluminum design, is fully soldered and used primarily as an intercooler for motor vehicles. The open box section  20  is divided in the longitudinal direction by a separator wall  25  which is installed in the section and is soldered to it. The pipe base  23  according to the invention is covered by a reinforcing plate  26  matched to the box section  20 ; said reinforcing plate has lateral tabs  26   a ,  26   b  which rest against the longitudinal side  21 . In the region of the through-holes  24 , the reinforcing plate  26 , also called the support plate, has recesses  28  and a plurality of binding bands  26   c  arranged mutually parallel to each other, said strips each being located between the through-holes  24  and contacting and supporting the pipe base  23  at that location. The corresponding contact surfaces, which are not visible in  FIG. 3 , will be described in greater detail below. 
         [0021]      FIG. 4  shows a longitudinal cross section through the box section  20  according to  FIG. 3  in the region of the pipe base  23 , which has convex regions  27  arched outward between the through-holes  24 , with these regions having a crest  27   a.  The through-holes  24  are evidently directed inward. Viewed in longitudinal cross section, the binding bands  26   c  of the reinforcing plate  26  have a straight transverse cross section and contact the convex regions  27  in the region of the crest  27   a  or—when viewed in cross section—along a crest line which runs through the crest  27   a  perpendicular to the plane of the figure. A contact surface is thus obtained which extends across the entire width of the pipe base  23 , that is, from the longitudinal side  21  to longitudinal side  22 , so that the pipe base  23  can be braced against the support plate  26  and/or against the binding bands  26   c.  The reinforcing plate  26  is soldered to the pipe base  23  and to the side walls  21 ,  22 , and very good soldering conditions are obtained between the convex regions  27  and the flat regions of the binding bands  26   c,  so that a corresponding solder meniscus can form. 
         [0022]      FIG. 5  shows the box section  20  in cross section, and the inward pointing through-holes  24  are clearly seen. Underneath the through-holes there are convex regions  27  appearing as curved strips, and specifically with the crest line  27   a ′. Bearing directly against the crest line  27   a  there are binding bands  26   c  of the reinforcing plate  26 , which are under tensile stress when the pipe base  23  is deformed, and said bands are braced laterally against the longitudinal sides  21 ,  22  by means of the tabs  26   a,    26   d.
A heat exchanger is disclosed, comprising a block having pipes and ribs and at least one pipe base provided with receiving openings, wherein the pipes have pipe ends, which can be held in the receiving openings and form pipe/base connections with the pipe base, the connections being at least partially reinforced by an additional sheet metal part located on the side of the pipe base facing the block, wherein the sheet metal part has insertion openings for the pipe ends.