Patent Publication Number: US-2006016583-A1

Title: Condenser and tube therefor

Description:
The present application is a continuation of U.S. application Ser. No. 09/985,300, filed Nov. 2, 2001, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a condenser and especially to a tube therefor which is particularly suited for use in condensers that operate under operating pressures of approximately 20 bar. One particular condenser according to the invention, is what is known as a flat-tube condenser. In such a flat-tube condenser, tubes of substantially flat cross section extend between header tubes. Cooling fins, supported on the flat surfaces of the tubes, may be disposed between the tubes of substantially flat cross section. With such an arrangement, the intention is that the heat from the refrigerant circulating in the condenser is dissipated to a coolant, usually air, substantially flowing through the condenser.  
      2. Description of Related Art  
      U.S. Pat. No. 5,307,870 describes header tubes for flat-tube condensers having header tubes of arcuate cross section. Parallel tubes run between these header tubes, in a manner such that a parallel flow condenser is formed. In other words, refrigerant vapor is introduced into one of the header tubes, passed through the parallel tubes, then is condensed and passed to the other header tube, and then leaves the condenser. In one embodiment, this printed publication describes tubes with parallel flow channels of round cross section formed therein. According to this United States patent a condenser of this kind is provided for high-pressure condensers.  
      German published application 198 45 336 relates to a heat exchanger which is operated with CO 2  as a refrigerant under high operating pressures of up to 100 bar. A multi-chamber flat tube is used therein and is formed as a rectilinear tube for a parallel flow condenser or as a tube with serpentine curvature for a parallel flow condenser. The channels in the tube are preferably provided with an oval, or alternatively circular, cross section. The circular cross section is disclosed as being suitable for high pressure resistance. In order to achieve a high heat transmission capacity, where the channels are of round cross section, the two wider sides of the flat tubes have an undulating profile.  
      In addition, in parallel flow or serpentine flow heat exchangers, use is made of flat tubes whose flow channels have rectangular or triangular cross sections. Reference is made here by way of example to GB A-2,133,525, JP-A-59-13877, U.S. Pat. No. 3,689,972, U.S. Pat. No. 2,136,641, GB-A-1,601,954, JP-A-57-66389, JP-A-58-221390 or EP-A-583,851. In many cases, the surfaces of the flow channels are enlarged by means of suitable measures, such as fins and grooves, in order to achieve higher heat transmission (cf. JP-A-59-13877, JP-A-57-66389 and JP-A-58-221390). By contrast with these shapes, JP-A-114145 illustrates rhomboidal flow channels which are said to ensure improved contact between the gaseous refrigerant and the walls of the flow channels and an improved outflow of condensate.  
      In addition, what are known as serpentine condensers are known, in which a refrigerant is passed backward and forward a plurality of times through various groups of tubes between two header tubes provided with partitions, cf. EP-A-255 131. The tubes used for this purpose exclusively have flow channels with square or rectangular cross sections.  
     SUMMARY OF THE INVENTION  
      Therefore, one object of the present invention is to provide an improved tube for a condenser operating at pressures of approximately 20 bar and a similarly improved condenser, especially a serpentine condenser.  
      In accomplishing the objects of the invention, there has been provided according to one aspect of the invention a tube for a condenser operating under pressures of approximately 20 bar, comprising: a cross section having a width that is greater than its height; a substantially flat profile along a width of the tube; and a plurality of flow channels arranged side by side along a width of the tube, wherein the flow channels comprise a substantially round cross section and further comprise a hydraulic diameter of 1.10 to 1.30 mm.  
      In accordance with an additional aspect of the invention, there is provided a condenser operating under pressures of approximately 20 bar comprising at least one tube for a condenser, wherein the tube comprises a cross section having a width that is greater than its height; a substantially flat profile along a width of the tube; and a plurality of flow channels arranged side by side along a width of the tube, wherein the flow channels comprise a substantially round cross section and further comprise a hydraulic diameter of 1.10 to 1.30 mm.  
      In accordance with yet another aspect of the invention, there is provided a motor vehicle comprising a condenser operating under pressures of approximately 20 bar comprising at least one tube which comprises a cross section having a width that is greater than its height; a substantially flat profile along a width of the tube; and a plurality of flow channels arranged side by side along a width of the tube, wherein the flow channels comprise a substantially round cross section and further comprise a hydraulic diameter of 1.10 to 1.30 mm.  
      Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows when considered together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is explained in detail below with reference to the accompanying drawings in which:  
       FIG. 1  shows a frontal view of a preferred serpentine condenser according to the invention;  
       FIG. 2  shows a lateral view from the right hand side of the condenser shown in  FIG. 1 ;  
       FIG. 3  shows a view from below of the condenser shown in  FIG. 1 ;  
       FIG. 4  shows a cross section through a preferred tube according to the invention; and  
       FIG. 5  shows a detail of the right-hand end of the cross section shown in  FIG. 4  in a view enlarged  20  fold.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The applicant has found, surprisingly, that a tube of substantially flat cross section and having a plurality of flow channels disposed side by side operates particularly effectively for a condenser operating under pressures of approximately 20 bar if the flow channels are substantially round and have a hydraulic diameter of from 1.10 mm to 1.30 mm. A further advantageous effect is achieved by a tube having flow channels that have a hydraulic diameter of from 1.14 mm to 1.26 mm, and a diameter of from 1.18 mm to 1.22 mm is further preferred. The best results are achieved by a tube having a hydraulic diameter of approximately 1.20 mm.  
      It has also been found that tubes of flat cross section and having circular flow channels disposed in series operate to particularly advantageous effect in serpentine condesners. This is the case to a particularly notable extent in condensers as described subsequently with reference to the preferred embodiments. This is attributed to the fact the pressure drop over the longer flow paths that generally have to be covered by the refrigerant in serpentine condensers is lower as compared to parallel flow condensers. As a result, larger quantities of refrigerant can be passed through the condenser per unit of time with the same overall expenditure of energy. Furthermore, improved heat transfer is evidently achieved as compared to flow paths in conventional serpentine condensers.  
      In addition, the production of such tubes is less elaborate and hence more cost-effective, which is particularly significant in mass production. Production takes place by extrusion, for example. The shape of the flow channels may be created by appropriately formed dies. Round dies have proven advantageous, as the delay on cooling is minimal and relatively uniform, and the dies exhibit much less wear than in the case of angular dies. In conventional angular dies, wear takes place especially at the corners.  
      Turning now to the drawings,  FIG. 1  shows a frontal view of a preferred serpentine condenser  20  in the assembled state. This condenser  20  comprises a first header tube  21  and a second header tube  22  which are preferably disposed parallel to one another. In a further preferred aspect, a refrigerant inlet  24  and a refrigerant outlet  25  are connected to the first header tube  21 . As the refrigerant enters substantially in a gaseous state and leaves in a liquid state, the refrigerant inlet  24  preferably has a larger cross section than the refrigerant outlet  25 . From the refrigerant inlet  24 , a feed pipe leads into the upper part (in the view shown) of the first header tube  21 . A pressure control valve may advantageously be provided shortly upstream of the entry into the first header tube  21 . It is also preferred for an upper part of the header tube  21  to be divided by a partition  27   a  in the first header tube  21 . A plurality of spaced and mutually parallel tubes  10  extend between the first header tube  21  and the second header tube  22 . These tubes  10  are connected in a leakproof manner to the interior of the header tubes  21 ,  22 . Indicated at top left in  FIG. 1  are cooling fins  23 , which may extend between the tubes  10  in a substantially undulating or parallel manner. The tubes have a substantially flat cross section, as will be explained further below. In the view shown in  FIG. 1 , what is shown is merely the relatively low height of the tubes  10 , which have a greater width perpendicularly to the plane of the paper than the height shown (compare also  FIG. 4 ). The cooling fins  23  may each be supported on the flat surfaces of adjacent tubes and are preferably connected thereto, e.g., by brazing. This permits good heat transfer between the tubes  10  and the cooling fins  23  and generally a good structural rigidity of the condenser  20 .  
      The refrigerant located in the upper region of the first header tube  21 , which is substantially in gaseous form, flows through a first set of tubes  10   a  to the second header tube  22 . Due to the partition  27   a , refrigerant can only flow from the refrigerant inlet  24  through the first set of tubes  10   a  which are connected to the upper, separate region of the first header tube  21 . On the path from the first header tube  21  to the second header tube  22 , a first heat exchange takes place between the refrigerant and the cooling medium (e.g., air) flowing perpendicularly to the plane of the paper. Such a condenser is preferably used in automobile air conditioning systems. In this case, air normally flows through the condenser  20 , e.g. between the tubes  10  and the cooling fins  23 , as a coolant. The structure shown is intended to guarantee the best possible heat transfer between the refrigerant and the coolant. In this manner, a first heat exchange takes place and a first condensation of the refrigerant in the first set of pipes  10   a  also takes place.  
      Having arrived in the second header tube  22 , the refrigerant is able to flow as far as the first partition  26   a  in the second header tube  22 . Like the partition  27   a , this partition  26   a  forms a barrier for the refrigerant, so that the refrigerant cannot flow downward, in the view shown, beyond the partition  26   a  in the second header tube  22 . Instead, it is forced to flow back through a second set of tubes  10   b  to the first header tube  21 . When this occurs, a further heat exchange and further condensation take place.  
      A further partition  27   b  is preferably located in the first header tube  21 , which forces the refrigerant through a third set of pipes  10   c  and once again into the second header tube  22 . The refrigerant is then again preferably guided by further partitions  26   b  in the second header tube and  27   c  in the first header tube back to the first header tube  21 , then to the second header tube  22  and back to the first header tube through a fourth set of pipes  10   d , a fifth set of pipes  10   e  and a sixth set of pipes  10   f , respectively. From the bottom region of the first header tube  21 , separated by the third partition  27   c , a tube then leads to the refrigerant outlet  25 .  
      The description above makes it clear why such a condenser is also known as a “serpentine condenser”. This is, of course, because the refrigerant is guided through the condenser through a plurality of loops or meanders. Thus the path covered by the refrigerant in the condenser, in this exemplary embodiment, is quadrupled relative to a parallel flow condenser, depending on the number of sets of tubes.  
      Particularly preferred is the embodiment shown with a total of six meanders, which thus allows the refrigerant to flow six times through the effective width of the condenser. It is further preferred for the number of tubes  10  to decrease, or at least remain the same, between a first set of tubes  10   a  to  10   e  and a further (second) set of tubes  10   b  to  10   f  located adjacently downstream. As a result, a declining circuit or flows path of the tube sets is advantageously achieved.  
      In a particularly preferred embodiment, the first set of tubes  10   a  comprises 17 tubes, the second set of tubes  10   b  10 tubes, the third set of tubes  10   c  7 tubes, the fourth set of tubes  10   d  6 tubes, the fifth set of tubes  10   e  4 tubes and the sixth set of tubes  10   f  likewise  4  tubes. The effect of this is that the refrigerant medium, which initially is still predominantly in gaseous form, is given comparatively more surface and cross section for heat exchange than the refrigerant downstream which is increasingly in liquid form.  
      A heat exchanger according to the invention preferably has a width of from 300 to 1000 mm, particularly preferably from approximately 400 to 700 mm, and even more preferably approximately 560 to 600 mm. The overall height is preferably from 200 to 700 mm, more preferably from 400 to 550 mm and particularly preferably from 460 to 500 mm. An embodiment which is particularly preferred for the abovementioned number of tubes in the individual sets of tubes has an effective end surface of approximately 27.8 dm 2 , giving an effective width of the condenser through which flow takes place of approximately 580 mm and an effective height of approximately 480 mm. A preferred density of fins is 75 fins per dm.  FIG. 1  likewise shows elements for mounting the condenser in the engine compartment of a vehicle. Further details of the elements are not depicted here for the sake of clarity and because these elements are conventional.  
      In a preferred embodiment, the elements of the condenser explained above are brazed to one another, yellow-chromed and powder-coated in black in order to optimize the heat exchange even further.  
      As already discussed in the introduction to the description such a condenser is customarily operated at an operating pressure of 20 bar. A preferred embodiment of a tube or flat tube  10  used in such condensers is shown in an enlarged view in  FIG. 4 . Such a tube preferably has a width of approximately 12 to 20 mm, more preferably 15 to 17 mm and particularly preferably approximately 16 mm. The height h is preferably from 1 to 3 mm, more preferably from 1.5 to 2.1 mm and particularly preferably approximately 1.8 mm. Such external dimensions permit a relatively small end surface of the tube, so that the pressure drop of the air flowing through the condenser does not become too great. On the other hand, the effective surface area is optimized, particularly toward the cooling ribs (the upper and lower outer sides shown in  FIG. 4 ).  
       FIG. 4  illustrates the flat tube cross section with eleven circular flow channels  11 , with webs  12  lying therebetween and with walls  13  formed with the outer flat tube surfaces. A preferred minimum thickness of the webs  12  is S=0.20 mm, and a preferred minimum thickness of the walls  13  is advantageously W=0.30 mm. The flow channels  11  have, according to one preferred embodiment of the invention, a substantially round cross section and a hydraulic diameter of from 1.10 to 1.30 mm. With a circular cross section, the hydraulic diameter corresponds to the diameter of the circle. More preferably, the hydraulic diameter is from 1.14 to 1.26 mm, even more preferably 1.18 mm to 1.22 mm, and most preferably approximately 1.20 mm. It has been found that such a hydraulic diameter permits an optimum, dimensionally conditioned heat transfer, especially when the tube  10  is used in serpentine condensers.  
       FIG. 5  shows a detail from  FIG. 4 , in particular the side of the tube  10  facing the cooling air stream. It has been found that with a slope X from the center of the tube to the upper and lower ends, respectively, of the tube and the stated orders of magnitude of approximately 0.3 mm and a radius R of approximately 0.2 mm guarantee an optimum flow pattern of the air coolant. A particularly preferred heat transfer between the outermost flow channel  11  and its front surface is achieved with an effective distance Y from the flow channel  11  to the front surface which is preferably approximately 0.38 mm.  
      A tube according to the invention is preferably extruded from aluminum or an aluminum alloy. In this case, the round flow channels may be produced by substantially round dies in an extrusion tool. A round form of the flow channels not only permits optimized heat transfer, especially when the tubes are used in serpentine condensers, but also has great advantages in the production of the tubes. The distortion on extrusion is uniform and minimal, and the wear of the circular dies is much less than if dies of an angular contour were to be used, as in the prior art. Thus a plurality of advantages are obtained simultaneously as a result of the shape of the flow channels.  
      The right of priority is claimed based on German Patent Application No. 100 54 185.5, filed Nov. 2, 2000, the disclosure of which is hereby incorporated by reference in its entirety.  
      The foregoing embodiments have been shown for illustrative purposes only and are not intended to limit the scope of the invention which is defined by the claims.