This nonprovisional application is a continuation of International Application No. PCT/EP2008/009271, which was filed on Nov. 4, 2008, and which claims priority to German Patent Application No. DE 102007059672.5, which was filed in Germany on Dec. 10, 2007, and to German Patent Application No. DE 102008017485.8, which was filed in Germany on Apr. 3, 2008, and which are all herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heat exchanger. 
     2. Description of the Background Art 
     Heat exchangers, in particular heaters for motor vehicles, have a liquid medium, such as coolant, flowing through them on the primary side, and are exposed on the secondary side to ambient air that is delivered to the passenger compartment. Conventional heaters have a block having tubes and ribs. The air to be heated enters this block and exits it again at its rear. A problem in heating the air in the heater block is that the outlet air temperatures at the air outlet area are not the same everywhere, so that strands of differing air temperature occur. This is a disadvantage for controlled heating of the interior. 
     A variety of flow patterns are known for flow through a heater, which is generally designed with multiple rows or multiple flows, with the simplest form being parallel flow in which flow passes through all tubes in the same direction. Also known is a U-shaped flow through the heater in which a baffle (transverse baffle) is located in a header tank. Since this redirection of the coolant takes place transverse to the direction of air flow, it is referred to as redirection “across the width.” With respect to the two media flows, coolant and air, this is called a cross-flow. The coolant cools off on the way from the coolant inlet to the coolant outlet, so that the air at the half of the heater on the inlet side is heated more than that on the outlet side half, resulting in the aforementioned strand effect. It is also known to direct the coolant in the parallel direction or counterflow direction to the airflow, in other words the coolant is redirected from one row into the adjacent row in a multiple-row heater. This requires a longitudinal baffle, which separates adjacent rows on one side. This is referred to as redirection “over depth.” Depending on whether the redirection takes place in or opposite to the direction of airflow, this is referred to as parallel flow or counterflow. It is known that better efficiencies can be achieved with counterflow. It is a disadvantage, in particular for relatively wide heaters, that the coolant at the inlet side must be distributed over the full width; this can have the result that flow through the outer tubes is slower with a central coolant inlet, which likewise has an unfavorable effect on the outlet air temperature. 
     DE 10 2005 048 227 A1, which is incorporated herein by reference, discloses a heater with flat tubes in which the coolant is directed in cross-counterflow to the airflow, which is to say that a redirection in depth takes place towards the air inlet side. In another variant that is not shown and is not described in detail, a redirection in the width is additionally provided. 
     DE 102 47 609 A1 describes a heater in which the coolant is redirected exclusively in width, and specifically in multiple stages, with multiple coolant flows being connected in parallel. The purpose of this arrangement is to achieve relatively high pressure drops at the redirection points of the water tanks through turbulence of the coolant. 
     DE 44 31 107 C1 discloses a heater for motor vehicles which operates according to the counterflow principle. In this concept, the coolant is redirected from the air outlet side towards the air inlet side in one or more stages. Better heat-transfer performance can be achieved in this way. 
     DE 603 06 291 T2 (corresponding to EP 1 410 929 B1) discloses a heater for motor vehicles with separate control of the right and left sides (driver&#39;s side and passenger&#39;s side) of the passenger compartment. In this concept, the coolant is delivered through two supplies, is redirected to the middle in width, and is removed there through a common return. In a special embodiment ( FIG. 8 ), a redirection in depth is provided in addition to the redirection in width, specifically opposite the direction of airflow. In the so-called left/right control, the airflow exiting the heater is split by a baffle into two partial streams, which are directed toward the left and right sides of the passenger compartment. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to create the most homogeneous possible outlet air temperature profile in a heat exchanger of the initially mentioned type. 
     In an embodiment of the invention, in a cross-counterflow heat exchanger the liquid medium (coolant) enters a first region, the inlet region, and in this row on the air outlet side is redirected into a second region, with both the first and second regions having subregions. In other words, the coolant entering the first row of flow channels can be redirected at least once in width. The coolant is then redirected from the first row into the second row, i.e. the row on the air inlet side, with flow through all flow channels in the second row being in the same direction. The inventive coolant routing by means of redirections in width and depth achieves the advantage that a largely homogeneous temperature profile is produced at the air outlet side. 
     In an embodiment, the coolant can be also redirected at least once in the second row as well, which is to say in the windward row. In all, the coolant flow is thus redirected twice in width and once in depth. As a result of the opposite coolant flow in the two rows of tubes, the outlet air temperature profile can be homogenized still further. 
     According to a first aspect of the invention, the inlet region can be located in the center of the first row, while the second region comprises two subregions that are symmetrically arranged next to the first region. The incoming coolant flow is thus divided after the first pass and redirected in opposite directions in the width of the heat exchanger. Subsequently, the coolant flows exiting the two subregions are redirected in depth and distributed over the second row such that flow passes through all flow channels in the same direction. In this way, a symmetrical outlet air temperature profile is achieved, which is to say that any deviations from a homogenous temperature distribution occur symmetrically. Alternatively, redirection in the second row can also take place in width. 
     According to a second aspect of the invention, the inlet region is located off-center in the first row, preferably in a first half, while the second region is located next to the first region. The coolant here flows into the first half of the row in the heat exchanger, is redirected in width, and the entire coolant flow enters the second region. From there, the redirection in depth and the distribution of the coolant flow over the entire second row take place in turn, wherein it is possible for flow through the latter to take place in the same direction or in different directions. 
     According to a third aspect of the invention, two inlet regions, which can be symmetrically arranged, are provided that communicate with one another through a connecting pipe. As a result, two flow branches are obtained on the inlet side, which are deflected inward in width, and enter the second region. This is followed by the redirection in depth and the distribution of the coolant over all the tubes of the second row. Alternatively, redirection in the second row can also take place in width, with a flow pattern similar to that in the first row. 
     The flow cross-sections in the first and second regions can be identical, which is to say that, in accordance with the known continuity equation, equal flow velocities result in the flow channels of the first and second regions, which is to say viewed across the full width. It is especially preferred, however, for the flow cross-section of the second region to be larger than that of the first region—with the result that a slowing of the flow takes place in the flow channels of the second region. This compensates for the cooling of the liquid medium, so that one obtains a homogeneous outlet air temperature distribution as an advantage. 
     In another embodiment, the flow cross-section in the second row can be matched to the flow cross-section of the second region in the first row, namely in such a manner that the entire flow cross-section of the second row is either identical to or larger than the entire flow cross-section of the second region. An expansion of the flow cross-section takes place due to the continued cooling of the liquid medium. In this way, either the same flow velocities can be achieved in the second row as in the first row, or even a delay in the flow—with the result that more heat can be dissipated to the air and a smaller pressure drop takes place. An expansion of the flow cross-section with resultant flow velocity can take place in the case of redirection in the width in the second row, as well. 
     According to an embodiment, the heat exchanger can be designed as a heater of a heating system for motor vehicles, which is to say the flow channels are designed as tubes, preferably as flat tubes or multichamber tubes through which the coolant flows and between which are arranged, preferably, corrugated fins as secondary surfaces. 
     The flat tube cross-sections of the second row can have an equal, larger, or smaller depth as compared to the flat tubes of the first row, depending on the flow pattern. This results in an increase in the flow cross-section after the redirection in depth, with the result that the flow velocity of the coolant is reduced in the second row. A greater cooling of the coolant, and thus greater heat-transfer performance, is achieved in this way. 
     The heater can have collecting reservoirs or chambers, i.e., an inlet chamber through which the coolant enters, an outlet chamber through which the coolant exits, or a coolant inlet and outlet chamber or a redirecting chamber. 
     In order to implement the above-described flow pattern in a heater, baffles in the form of longitudinal and/or transverse baffles are located in the collecting reservoirs, dividing the collecting reservoirs into individual chambers. Preferably, the inlet region for the flow channels or flat tubes of the first region is divided by a longitudinal baffle and at least one transverse baffle in the inlet chamber. In contrast, the outlet chamber has one longitudinal baffle, so that the first and second rows are divided from one another and a redirection in width can take place in the first row. Furthermore, in the case of “double” redirection in width, transverse and longitudinal baffles can be arranged in an H shape. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  is an exploded view of a heat exchanger with a two-part housing; 
         FIG. 2   a  illustrate example embodiments for shapes of tubes; 
         FIG. 2   b  is a flow model according to  FIG. 1  in a schematic view from above; 
         FIG. 3  is an example embodiment of the invention with off-center inlet region; 
         FIG. 4  an example embodiment of the invention with two inlet regions; 
         FIGS. 5   a ,  5   b  illustrate a heater with flow arrows, closed and in exploded view; 
         FIGS. 6   a ,  6   b  illustrate the heater with flow arrows in an exploded view facing the air inlet side and air outlet side; 
         FIGS. 7   a ,  7   b ,  7   c  show views from above and below of the heater block, and an enlarged depiction of the heater tubes; 
         FIG. 8  illustrates an additional example embodiment of the invention, a heater with “double” redirection in width, i.e., in the first and second row; 
         FIG. 9   a  shows the heater from  FIG. 8  in an exploded view; 
         FIG. 9   b  shows the same heater in cross-section; 
         FIGS. 10   a ,  10   b ,  10   c  are views from above and below of the tube ends of the heater block; 
         FIG. 11  is as an additional example embodiment of the invention, a heater with coolant connection on the side; and 
         FIG. 12  shows an additional example embodiment of the invention, a heater with outer inlet regions. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic representation of a first example embodiment of the invention, namely a flow model for a two-row heater  1 , of which only tubes  2  (without fins) of a first row  3  and a second row  4  are shown. Also partially shown are a longitudinal baffle  5  with two transverse baffles  6 ,  7  in the inlet region of the tubes  2  and another, continuous, longitudinal baffle  8  in the lower region of the block  1 . As indicated by flow arrows, coolant that is diverted from a coolant circuit (not shown) of an internal combustion engine of a motor vehicle passes through the tubes  2 . The heater block  1  serves to heat air that flows through the block  1  as shown by the arrow L, and in so doing flows over ribs (not shown), referred to as secondary surfaces, between the tubes  2 . The heated air is delivered to the passenger compartment of the motor vehicle. The first row  3  of the heater, hereinafter referred to as block  1  for short, is divided by the baffles  5 ,  6 ,  7  into three regions, with a first region  9  being located inside the baffles and  5 ,  6 ,  7  and a second region  10 , encompassing two subregions  10   a ,  10   b , being located on both sides of the transverse baffles  6 ,  7 . In the example embodiment shown, the first region  9 , also called the inlet region, encompasses four tubes  2 , while the two subregions  10   a ,  10   b  each encompass two tubes  2 . The coolant enters the tubes  2  through the inlet region  9  as shown by the arrows E, and flows through them from top to bottom (the terms top and bottom refer to the representation in the drawing). After the coolant exits the first region  9 , the coolant flow is divided, redirected outward in each case within the first row  3 , and then enters the tubes  2  of the subregions  10   a ,  10   b , flowing through them from bottom to top. The redirection of the coolant is indicated by the arrows B, where UB designates redirection in width. After the coolant exits the tubes  2  of the two subregions  10   a ,  10   b , a redirection in depth of both flow branches takes place, indicated by the arrows UT. The two flow branches redirected in depth are distributed over all tubes  2  of the second row  4  (eight in the example embodiment shown), flowing through them from top to bottom. This is followed by the exit of the coolant from the block  1 . As is depicted in  FIG. 6   a ,  6   b  that follow, the redirection of the coolant in width as indicated by the arrows UB is made possible by the continuous longitudinal baffle  8  in conjunction with a header tank that is not shown. The flow pattern described above corresponds to cross-counterflow with regard to the coolant and air flows. The first row  3  is the air outlet side row, hereinafter also called the leeward row for short, while the second row  4  is the air inlet side row, hereinafter also called the windward row. Thus, to summarize briefly, the coolant enters the block  1  in the leeward row  3 , and is redirected first in width and then in depth, with flow passing through all tubes  4  of the windward side  4  in the same direction. This flow through the heater block  1  produces a maximally homogeneous outlet air temperature, which is to say after the air exits the first row  3 . 
       FIG. 2   b  shows a schematic view of the heater block  1  according to  FIG. 1  from above, facing the tubes  2 , which are arranged in the two rows  3  and  4 . The air flow is in turn indicated by an arrow L. The direction of flow of the coolant is indicated by dot symbols  11  and cross symbols  12 , where the dot symbols  11  indicate a flow direction upward (out of the plane of the drawing), and the cross symbols  12  indicate a flow direction downward, i.e., into the plane of the drawing. The tubes  2  of the inlet region  9  are indicated with a brace a, the tubes  2  of the two subregions  10   a ,  10   b  are indicated with braces b 1 , b 2 , and the tubes  2  of the row  4  are indicated with a brace c. In this regard, the letters a, b 1 , b 2 , c represent the applicable number of tubes. The cross-sections of the tubes  2  are designed as flat tube cross-sections, and each have a depth T 1  in the first row  3  and a depth T 2  in the second row  4 . The overall depth of the block  1  is labeled T. According to a preferred embodiment, the relationship a≦(b 1 +b 2 ) applies. For the case in which b 1 +b 2 =a, the result is that the tubes  2  of the outer subregions  10   a ,  10   b  have the same flow velocity for the coolant as in the tubes  2  of the inlet region  9 . However, on account of the cooling of the coolant, the flow cross-section for the second region is enlarged somewhat, so that a slowing of the coolant flow is achieved. This also contributes to a homogenization of the outlet air temperature profile. In the example embodiment shown, the number of tubes in the second row  4  matches the number of tubes in the first row  3 , which is to say that a+b 1 +b 2 =c. If it were the case that T 2 =T 1 , the result would be a reduction of the coolant flow velocity by 50%. If T 2 =½ T 1  were the case, the result would be equal coolant flow velocities in the two rows  3 ,  4 . Depending on the cooling of the coolant, the preferred depth dimension T 2  for the second row  4  lies in the range between 0.5 T 1  and T 1 . The described flow model with redirections in depth and width thus makes it possible to reduce the flow velocity of the coolant in a stepwise manner by changing the flow cross-sections. 
       FIG. 2   a  shows two equivalent example embodiments of the tubes  2  mentioned above and shown, each of which has a flat tube cross-section. In principle, it is possible to use separate tubes  2  in different rows (two-row construction), or a two-chambered tube  2 ′, i.e., a tube with two chambers (single-row construction). 
       FIG. 3  shoes a second example embodiment of the invention, using the same reference symbols for the same parts. The block  1  has two rows  3 ,  4  of flat tubes  2 , with the first row  3  being divided into a first region  13 , the inlet region, and a second region  14 . The inlet region  13  is divided by a longitudinal baffle  15  and a transverse baffle  16 . The coolant enters the tubes  2  of the inlet region  13  as indicated by the arrows E, is subsequently redirected in width, i.e., within the row  3 , as indicated by the arrow UB, and then flows through the tubes  2  of the second region  14  from bottom to top. The coolant is then redirected in depth as indicated by the arrow UT and distributed over all tubes  2  of the second row  4 , through all of which it flows in the same direction from top to bottom. The coolant then exits the block  1 . A homogeneous outlet air temperature distribution is also achieved with this flow pattern. 
       FIG. 4  shows a third example embodiment of the invention, with the same reference symbols again being used for the same parts. In contrast to the foregoing example embodiments, a first region  17  with two outer subregions  17   a ,  17   b  is provided here, as well as a central second region  18 . The subregions  17   a ,  17   b  are each divided by longitudinal baffles  19   a ,  19   b  and transverse baffles  20   a ,  20   b , between which a connecting pipe  21  is located. As indicated by the arrows E, the coolant enters the tubes  2  of the subregions  17   a ,  17   b , in part through the connecting pipe  21 , flows through them from top to bottom, is then redirected in width as indicated by the arrows UB, and flows through the central tubes  2  of the second region  18 . There follows a redirection of the coolant flow in depth and a distribution over all tubes  2  of the second row  4 , with flow passing through all of them in the same direction from top to bottom. This flow pattern guarantees a maximally homogeneous outlet air temperature profile. 
       FIGS. 5   a  and  5   b  show a design embodiment of a heater  22  that corresponds to the first example embodiment from  FIG. 1  and  FIG. 2 . However, there is the difference that the coolant inlet, indicated by an arrow E, is at the bottom, and the coolant outlet, indicated by an arrow A, is at the top. This depiction represents the preferred installation position of the heater  22  in the motor vehicle. The heater  22  comprises a heater block  23 , also called the block for short, a bottom collecting reservoir or header tank  24 , and a top collecting reservoir or header tank  25 . The bottom collecting reservoir  24  has an inlet connection  24   a , and the top collecting reservoir  25 , also called the outlet chamber, has an outlet connection  25   a . As shown and explained for the example embodiment according to  FIG. 1  and  FIG. 2 , the block  23  comprises two rows of tubes, not provided with reference symbols here, through which flow passes as indicated by the arrows. The arrow I symbolizes the incoming coolant flow in the first region, the arrows IIa, IIb symbolize the flow branches redirected in width, and the arrow III symbolizes the coolant flow in the second, i.e. windward, row of tubes. The arrows UB, UT indicate the redirection of the coolant flow I in width and the redirection of the flow branch IIb in depth. The direction of flow of the air is indicated by an arrow L, which is to say that the heater block  23  is viewed from the air outlet side. The installation position of the heater  22  with the coolant outlet  25   a  at the top is chosen on account of better air bleeding of the heater  22 . 
       FIG. 5   b  shows the heater  22  in an exploded view, which is to say that the lower inlet chamber  24 , the upper outlet chamber  25 , and the block  23  are shown separated from one another. As a result, the interior of the inlet chamber  24  is visible, in particular the inlet region  26  separated by one longitudinal baffle and two transverse baffles  26   a ,  26   b ,  26   c . The coolant inlet flow in block  23  is indicated by three upward-pointing arrows. The redirection in width takes place as shown by the arrows UB (a longitudinal baffle that is not visible is located in the upper header tank  25  here). The redirection in depth takes place in the lower header tank  24  as shown by the arrows UT. The flow in the windward row is indicated by five upward-pointing arrows. As shown in  FIG. 5   b , the tubes in the rows are spaced along the width of the core. The tubes in the first row define first and second subsets. Also, the tubes in the first row and the tubes in the second row occupy given proportions of the width of the core, where the first subset of the first row occupies a first proportion of the width of the core, the second subset of the first row occupies a second proportion of the width of the core and the second row occupies a third proportion of the width of the core, such that a sum of the first proportion and the second proportion substantially equals the third proportion. 
     For clarification,  FIG. 6   a  and  FIG. 6   b  again show the heater  22  from  FIGS. 5   a ,  5   b  in exploded views, specifically in  FIG. 6   a  looking towards the air outlet side  23   a  and in  FIG. 6   b  looking towards the air inlet side  23   b . The flow direction of the air is indicated by arrows L in each view. Otherwise, identical reference numbers are used for identical parts. This representation makes clear the different flow on the leeward side  23   a  and on the windward side  23   b  of the heater block  23 . In the first case coolant flow takes place in opposite directions, while it takes place in the same direction in the second case. Visible in  FIG. 6   b  is a longitudinal baffle  27 , which corresponds to the longitudinal baffle  8  in the example embodiments from  FIG. 1  through  FIG. 4 . 
       FIG. 7   a  shows a view from above of the heater block  23  corresponding to  FIG. 5   a  through  FIG. 6   b . The block  23  has two rows  28 ,  29  of two-chambered tubes  30 ,  31 . The flow direction of the coolant is again indicated by dot and cross symbols. The direction of air flow is shown by an arrow L. Indicated between the two tube rows  28 ,  29  is the longitudinal baffle  27 . 
       FIG. 7   b  shows a view of the heater block  23  from below, with the first tube row  28  and second tube row  29 , and with the inlet region  26  (first region) and baffles  26   a ,  26   b ,  26   c . The number of tubes in the individual regions, which is to say in the first and second regions, and in the second row  29 , are indicated by the arrow heads a, b 1 , b 2 , c. The number of tubes shown in the drawing or the dimensional relationships correspond to a preferred example embodiment, in which fifteen tubes  30  are provided in the first region a, and nine tubes are provided in each of the second regions b 1 , b 2 . In this way, after the redirection in width an enlargement of the flow cross-section occurs in the second regions b 1 , b 2 , so that a delay of the coolant flow takes place in the tubes  30  with the dot symbol. This is desirable because of the cooling of the coolant from region a to the regions b 1 , b 2 . The following relationship applies: a≦(b 1 +b 2 ). 
       FIG. 7   c  shows an enlarged view of the tubes  30 ,  31  from the first row  28  and second row  29 , wherein the depth dimensions T 1  apply for the tubes  30 , T 2  for the tubes  31 , and T for the overall block depth. The width of the tubes is labeled B. The drawing is dimensionally accurate for a preferred example embodiment, which is to say that the depth dimension T 2  of the second row  29  is smaller than the depth dimension T 1  of the first row  28 . The number of tubes  30 ,  31  in the two rows  28 ,  29  is identical, just as in  FIGS. 7   a ,  7   b . The entire flow cross-section of the tubes  31  in the second row  29  is dimensioned in such a way that an additional delay in the coolant flow results after the redirection in depth. In this way, an increased temperature difference is achieved on the air inlet side, and thus a gain in performance. According to a preferred example embodiment, the depth dimension T 2  is selected in a range from 0.5 T 1  to 1.0 T 1 . 
     According to a preferred embodiment, the inventive heaters or their flat tubes have the following dimensions: The tube width B is in a range from 0.5 to 4.0 mm, preferably in a range from 0.8 to 2.5 mm. The material thickness (tube wall thickness) s of the flat tubes is in a preferred range of 0.10 to 0.50 mm. The depth T of the block (so-called wetted depth) is in a range from 10 to 100 mm, preferably in a range from 20 to 70 mm. 
     Due to the stepwise expansion of the flow cross-section after each redirection in width and/or redirection in depth, there also results, in conjunction with the delay in the coolant flow, a smaller pressure drop on the coolant side, which reduces the power requirement for the coolant pump. 
       FIG. 8  shows another example embodiment of the invention in the form of a two-row heater  32  in which the coolant is redirected in width in both the first and second row of tubes. The coolant&#39;s entry into the heater  32  is indicated by an arrow E and the coolant&#39;s exit from the heater  32  is identified by an arrow A. The direction of air flow through the heater  32  is indicated by two arrows L, which is to say the air and coolant are directed in cross-counterflow to one another. The heater  32  has a first, leeward-side row of tubes  33  and a second, windward-side row of tubes  34 , as well as an upper header tank  35  and a lower header tank  36  in which the tube ends (not labeled with reference numbers) terminate. The coolant first enters an inlet region, identified by arrows I, in the first row of tubes  33 , is redirected outward in width in the lower header tank  36 , corresponding to the arrows UB, enters the two outer subregions, flows through them from the bottom to the top, corresponding to the arrows IIa, IIb, and is redirected in depth in the upper header tank  35 , corresponding to the arrows UT. In the rear, windward-side row of tubes  34 , flow from the top to the bottom occurs—which is not shown here—followed by another redirection in width, flow from the bottom to the top, and finally the exit of the coolant, indicated by the arrow A. As is shown and explained in greater detail in the subsequent figures, the flow through the regions I, IIa, IIb takes place in opposite directions in the front and back rows  33 ,  34 . 
       FIG. 9   a  shows the heater  32  from  FIG. 8  in an exploded view, with the same reference numbers being used for the same parts. The flow of the coolant is indicated by arrows in the tubes and the header tanks  35 ,  36 . Both rows of tubes  33 ,  34  have a plurality of flat tubes  37 , between which are located corrugated fins that are not labeled with reference numbers. The ends of the flat tubes  37  are joined to tube plates  38 ,  39 , preferably by soldering. The tube plates  38 ,  39  are joined to the header tanks  35 ,  36 , preferably by soldering. Located in the lower header tank  36  is a longitudinal baffle  40 , which separates the first and second rows of tubes  33 ,  34  so that a redirection in width can take place for each of the first and second rows of tubes  33 ,  34  in the lower header tank  36 , as shown by the arrows UB 1 , UB 2 , in the opposite direction in each case. Located in the upper header tank  35  are two transverse baffles  41 ,  42  extending across both rows of tubes, as well as a longitudinal baffle  43  extending between the transverse baffles  41 ,  42 . The flow path of the coolant shown by the arrows is a result of this arrangement of the baffles  40 ,  41 ,  42 ,  43 . In the vertical direction, which is to say within the flat tubes  37 , the coolant flows in the opposite direction in the first and second rows  33 ,  34 , and also in the lower header tank  36 . There, a redirection in width from the inside to the outside takes place in the first row  33 , while a redirection in width from outside to the inside takes place in the second row  34 . 
       FIG. 9   b  shows the heater  32  in a cross-sectional view in which can be seen the two rows of tubes  33 ,  34 , the two header tanks  35 ,  36 , the entry of the coolant indicated by an arrow E, the exit of the coolant indicated by an arrow A, and the direction of flow indicated by an arrow L. The counterflow principle is clearly evident here. 
       FIGS. 10   a ,  10   b , and  10   c  show top views of the tube ends, as well as the numbers and dimensions thereof. Once again, the same reference numbers are used for identical parts.  FIG. 10   a  shows a top view (view from above) of the two rows of tubes  33 ,  34 —here called R 1 , R 2 . Together with the longitudinal baffle  43 , the two transverse baffles  41 ,  42  form an H shape. The direction of the coolant&#39;s flow through the flat tubes  37  is indicated by dot and cross symbols. The number of tubes in the individual sections of the rows of tubes R 1 , R 2  is represented by the subsections a, b 1 , b 2 , c. In order to achieve a delay in the coolant flow after the first redirection in width, the sum of the tubes b 1  and b 2  is larger than the number of tubes a, which is to say that (b 1 +b 2 )&gt;a. With respect to the example embodiment in  FIG. 10   a , the section a has fifteen tubes and the sections b 1  and b 2  each have nine tubes, so that as a result, the flow cross-section increases by three tube cross-sections. This produces a reduction in the flow velocity in the sections b 1  and b 2 . After the redirection of the coolant in the row R 1 , it flows upward in the subregions b 1  and b 2  (dot symbol) and then is redirected in depth—opposite the air flow direction L—which is to say into the row R 2 , where it again flows downward (cross symbol). 
       FIG. 10   b  shows a view from below of the tube ends of the rows of tubes R 1  and R 2 , between which is located the longitudinal baffle  40 . The overall width of the rows of tubes R 1 , R 2  is indicated by c; this region is not subdivided by baffles, so that a redirection in width can take place in both the rows R 1 , R 2 . 
       FIG. 10   c  shows an enlarged section of the two rows of tubes R 1 , R 2 , each with five flat tubes  37   a ,  37   b  whose extent in depth (in the direction of air flow) is labeled with T 1  and T 2 . The overall depth of the two rows of tubes (of the block) is labeled T. In order to achieve an additional delay of the coolant flow in the second row R 2  as well, which is to say after the redirection in depth, the depth T 2  of the flat tubes  37   b  can be chosen larger than the depth T 1  of the flat tubes  37   a —while retaining the same tube width B and same number of tubes. 
     For a preferred example embodiment, the tube width B is in a range from 0.5 to 4.0 mm, preferably 0.8 to 2.5 mm. The material thickness of the flat tubes  37   a ,  37   b  is in the range from 0.10 to 0.50 mm. The installation depth T (wetted or block depth) is 10 to 100 mm, preferably 25 to 70 mm. In the drawing, two rows of flat tubes  37   a ,  37   b  are shown which are designed as two-chambered tubes. However, multi-chambered tubes or even a single-row construction with a continuous flat tube which has a baffle (bead) approximately in the center region are also possible. 
       FIG. 11  shows another example embodiment of the invention with a heater  44 , which corresponds to the example embodiment from  FIG. 10   a ,  10   b  in terms of the flow pattern. A design variant provides for lateral inflow of the coolant through an admission tube  45 , by which means the coolant is brought from the outside to the center flow region  46 . In similar fashion, an outlet tube (not shown) can be provided for the outlet region located behind the flow region  46  in the plane of the drawing. A laterally arranged coolant connection of this nature can be advantageous on account of the installation situation in the motor vehicle. 
       FIG. 12  shows another example embodiment of the invention with a heater  47  which has inflow regions  48 ,  49  (subregions) located on the outside that communicate with one another through a connecting pipe  50 . The coolant entering through the inlet connection  51  is thus distributed to both inflow chambers  48 ,  49 . The situation on the outflow side, which is to say in the second row of tubes, is similar, although it is not shown. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.
A heat exchanger is provided that comprises at least two rows of low channels through which a liquid medium can flow, and secondary surfaces arranged between the flow channels and over which air flows, the liquid medium and the air being circulated in the cross-counterflow and the first row being arranged on the air outlet side and the second row on the air inlet side. According to the invention, the liquid medium enters a first region of the first row, is deflected into a second region inside the first row, and from the second region of the first row into the second row.