Patent Publication Number: US-2007119580-A1

Title: Heat exchanger

Description:
BACKGROUND OF THE INVENTION  
      Heat exchangers, for example, refrigerant vaporizers, are used in climate control systems for motor vehicles. These heat exchangers include essentially a block of tubes through which a refrigerant flows and which refrigerant is to be evaporated. Ribs are provided and the air, which is to be cooled, is passed over these ribs and is supplied to the interior of the vehicle. The tubes are connected to collector compartments or distributor compartments via which the refrigerant is supplied, redirected or conducted away.  
      A problem, which occurs with all evaporators, is the uniform distribution of the refrigerant, which is injected into the evaporator, into all tubes. The refrigerant is expanded in an expansion element directly in advance of entry into the evaporator and is present at the injection location of the evaporator as a two-phase mixture comprising a vaporous refrigerant and a liquid refrigerant. If the evaporator tubes are not uniformly charged, then there is a nonuniform evaporation and therefore a nonuniform temperature distribution which effects the air end of the evaporator and leads to a so-called stringiness of the air flow. Furthermore, an increased pressure drop can develop in the individual tubes which is damaging for the capacity of the evaporator. These disadvantages are especially pronounced in the thermal part-load operation which, on an annual average, usually occurs most often, as opposed to the full-load operation. An improvement of the refrigerant distribution not only increases comfort but also reduces the power required and therefore the consumption of fuel.  
      Various solutions are known wherein a uniform distribution of the refrigerant to the individual tubes is strived for. In German patent publication 4,422,178, a distributor element in the form of an apertured body is mounted in the inlet region of the evaporator. The refrigerant exits through the apertures and is intended to then distribute uniformly. Additional suggestions for a distribution of the refrigerant in the inlet region are suggested in the following: German patent publications 197 19 250 and 197 19 257 and U.S. Pat. No. 6,199,401. Separate channels or distributor tubes leading to the tubes and having refrigerant outlet openings are provided. These suggestions are characterized by an increased constructive complexity in the area of the inlet compartments of the evaporator which increases the manufacturing costs.  
      Present-day flat tube evaporators are disclosed, for example, in U.S. Pat. No. 6,449,979 or in international patent publication WO 2005/047800 A1. These present-day flat tube evaporators operate without such distributor means. The known flat tube evaporators are configured in two rows and have respective two double collector compartments wherein the tube end sections are inserted. The injection of the refrigerant takes place from an end face of a collector compartment into an inlet chamber. From there, the refrigerant is distributed to the tubes and is redirected once or several times in the evaporator block (multiple throughflow in individual tube groups). These known flat tubes also lead a nonuniform distribution of the refrigerant and the above-mentioned disadvantages.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide a distribution of the fluid as uniform as possible in a heat exchanger of the above-mentioned type.  
      The heat exchanger of the invention includes: a block including a plurality of tubes for conducting a fluid therethrough; a collector having a tube base; a plurality of tubes having respective end sections; the plurality of tubes being inserted into the tube base so as to cause the end sections to be accommodated in the tube base and to extend into the collector to an insertion depth (x); and, the insertion depth (x) being variable.  
      According to the invention, the insert depth of the tubes in the tube base is variable, that is, the tube end sections project out of the tube base at different elevations. The heat exchanger is advantageously configured as an evaporator and the fluid is advantageously a two-phase fluid, especially, a refrigerant. In a further advantageous embodiment, the evaporator has an inlet chamber with at least an injection opening mounted in an end face. The injection opening or openings can, in principle, also be mounted at other locations, especially, at the longitudinal sides. In an evaporator having an inlet region disposed topside, it is advantageous that the insertion depths of the tube end sections decrease with increasing distance from the location of the injection. In an evaporator having an entry region at the lower end, it is advantageous when the insertion depth increases with increasing distance from the location of injection. The increasing or decreasing insertion depth can advantageously take place as a linear function or a nonlinear function.  
      It has been determined that the refrigerant flow increases greatly with increasing distance from the location of injection up to the tube farthest remote therefrom. More specifically, the refrigerant flow is present in the region of the injection location as an intensely turbulent mixture of refrigerant vapor and liquid droplets. The refrigerant flow passes into a layered flow with increasing distance from the injection location. For a horizontal flow, a layer of liquid refrigerant and a layer of vaporous refrigerant thereabove forms in the geodetically bottom-lying region. The location of the transition from turbulent to layered flow is dependent upon the amount of the refrigerant mass flow. Because of varying insertion depths of the tubes, this phenomenon of the refrigerant flow can be taken into account and an approximately uniform distribution of the refrigerant is achieved to the individual tube cross sections. The solution of the invention is relatively easily realized and therefore cost effective, that is, by simply utilizing different tube lengths and/or insertion depths. It is possible that different insertion depths, at least in a portion of the tubes, can be realized also without different tube lengths when, for example, a compensation at the other end of the corresponding tube is provided. The following advantages are achieved: a uniform temperature distribution on the air side; a uniform, lower pressure drop in the tubes; and, a higher evaporator capacity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described with reference to the drawings wherein:  
       FIG. 1  is a section view of a flat tube evaporator according to the prior art; and,  
       FIG. 2  is a section of an embodiment of the evaporator according to the invention having variable tube insertion depths. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  is a cutaway view of an evaporator  1  as it is known from the prior art, for example, from U.S. Pat. No. 6,449,979 or from international patent publication WO 2005/047800 A1. The evaporator  1  includes a block  2  of flat tubes  3 . Ribs (not shown) are mounted between these tubes over which air flows. The tubes  3  have tube ends  3   a  which are inserted into a tube base  4 . The tube base  4  is part of a collector  5  which has an injection tube  6  mounted at an end face and through which refrigerant is supplied to the evaporator  1 . The axis of the injection tube  6  is identified by reference numeral  6   a  and this axis corresponds to the injection direction of the refrigerant. The collector  5  includes an inlet chamber  7  which is partitioned off relative to a neighboring chamber by a partition wall  8 . The refrigerant reaches the inlet chamber  7  via the injection tube  6  and distributes from there to the tubes  3 . The refrigerant then flows from top to bottom in the drawing, that is, in a direction corresponding to the direction of gravitational force. Thereafter, the refrigerant is redirected in the depth (in air flow direction) and/or in the width (transversely to the air flow direction). In this way, the refrigerant vaporizes in the tubes  3  and thereby effects a cooling of the air to be climatized. In the known evaporator  1 , the tube ends  3   a  have an insertion depth which is characterized by the amount (x). The amount (x), that is, the insertion depth, is the same for all tubes  3 . Because of the changing refrigerant flow, there results a nonuniform charging of the tubes  3  in the known evaporator  1  shown in  FIG. 1 .  
       FIG. 2  shows an evaporator  10  according to an embodiment of the invention. Here, the same parts have the same reference numerals as in  FIG. 1 . The evaporator  10  includes an inlet chamber  7  having an injection tube  6  mounted at an end face and a tube base  4 . The flat tubes  11  have tube end sections  11   a  which are inserted at different insertion depths into the tube base  4 . The upper edges of the tube end sections  11   a  are connected via a straight line (f). The distance of the tubes  11  from the end face of the injection tube  6  is identified by reference character (a). The insertion depth of the tube, which lies closest to the injection tube  6 , is identified by x max  and the insertion depth of the flat tube  11 , which is farthest away from the injection tube  6 , is identified by reference character x min . From  FIG. 2 , it can be seen that the insertion depth (x) follows a linearly decreasing function (f) over the distance (a).  
      The refrigerant flow, which enters horizontally through the injection tube  6  into the inlet chamber  7 , is present first as a two-phase intensely turbulent mixture of refrigerant vapor and refrigerant liquid droplets. With increasing distance (a) from the injection tube  6 , the refrigerant flow changes its state into a layered flow which comprises a lower layer of liquid refrigerant and a layer of vaporous refrigerant thereabove. With the stepped insertion depth (x) of the tube end sections  11   a , the tubes  11  are supplied substantially uniformly with refrigerant, that is, with an approximately equal refrigerant mass flow per tube.  
      The linearly decreasing function (f) shown is only one embodiment of the invention. It does show, not to scale, a schematic representation of the invention. It is likewise possible to have a nonlinear decreasing function.  
      Furthermore, the tube insertion depth (x), which is to be selected, is dependent upon the refrigerant throughput which is applied for the design or configuration of the evaporator. For a maximum refrigerant throughput, the turbulent refrigerant flow extends relatively far (in direction (a)) into the inlet chamber, that is, the transition to a layering of the flow occurs at a greater distance (a) from the injection location. For a weaker refrigerant mass flow, the layered flow adjusts closer to the injection location. The pattern for the variable injection depths of the tube end sections will orientate itself at a design point between these two extremes.  
      Departing from the embodiment shown, the inlet chamber, viewed geodetically, is mounted below so that the refrigerant, which is supplied at the end of the inlet chamber, flows in the refrigerant tubes from below to above, that is, opposite to the direction of the force of gravity. In this case, the function for the insertion depth is preferably an increasing function, that is, the depth of insertion increases with increasing distance (a) from the injection tube  6 .  
      The evaporator  10  shown in  FIG. 2  is preferably configured as a two-row flat tube evaporator and has a multi-flow refrigerant flow, that is, the refrigerant is redirected in the width (in direction +a or −a) as well as in the depth (in airflow direction). In the realization of the invention, it can be provided that the variable insertion depth preferably applies for the first passthrough. In the embodiment shown, these are the flat tubes  11  opening into the inlet chamber  7 . After the first passthrough, it can often be assumed that the refrigerant flow is uniform or homogenized to the extent that a variable insertion depth is no longer necessary or brings with it no significant advantages.  
      The evaporator of the invention is preferably used for climate control systems in motor vehicles.  
      It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.