Heat exchanger
FIELD OF THE INVENTION 
     This invention relates to heat exchangers for exchanging heat between a first medium carried in a tube bundle and a second medium flowing over a tube bundle where fins are used to increase the efficiency of heat transfer. 
     BACKGROUND OF THE INVENTION 
     Finned typed heat exchangers are commonly used in a variety of heat exchanger-type apparatus. Finned heat exchangers having a tube bundle of a serpentine arrangement and fins between the tubes are used in condensers and evaporators of refrigeration units, air conditioning units and the like. 
     The standard form of evaporator used in refrigerators involves the use of a serpentine-type tube bundle having a plurality of fins extending perpendicular to the tubes. The fins are planar and usually have a thickness of approximately 0.25 millimeters. Efforts have been made in the past to improve the efficiency of this standard type of evaporator and condensers used in refrigeration devices. U.S. Pat. No. 1,773,249 discloses a condenser arrangement having fins with a plurality of transverse corrugations formed therein. When the fins are assembled and the corrugated portions interconnected, spaces between the fins are defined through which air may flow. Another form of corrugation in thick fins to induce turbulence in the flow of air over the fins of the heat exchanger is disclosed in U.S. Pat. No. 3,645,330. The fins include corrugations which extend transversely of each fin. The fins are all interconnected by collars which are, in turn, welded to the tubes of the coil carrying the medium to be heat exchanged. The corrugations in the fins serve to break up the boundary layer of air flowing over the thick fins. 
     Further improvements in the fin construction involve the use of apertures in conjunction with corrugations in the tubes, such as disclosed in U.S. Pat. Nos. 3,796,258, 4,365,667 and 3,397,741. The openings in the fins, aside from those openings which receive the tubes of the heat exchanger coil, function as louvres to deflect the flow of air through the fins to enhance heat transfer from the fins to the flowing medium. 
     All of these forms of heat exchangers involve the use of plate fins which have a thickness normally in the range of 0.25 millimeters or greater. With the increased costs involved in materials of constructions for heat exchangers, there is a need to reduce the amount of materials in the heat exchanger yet maintain or improve its operating characteristics. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, the heat exchanger for exchanging heat between a first medium carried in a tube bundle of parallel extending, spaced-apart tubes and a second medium flowing over said tube bundles has superior heat transfer characteristics and reduced material and labour costs. The heat exchanger comprises a plurality of essentially parallel spaced-apart fins extending perpendicularly to the tubes and through which the tubes extend. Each of the tubes has a wall of lightweight metal in contact with the plurality of fins to provide for heat conduction between the tubes and the fins. 
     The fins are ultra-thin of a thickness less than 0.20 millimeters formed of a lightweight metal, chemically and thermally compatible with the lightweight metal of the tube walls. Each of the fins has a pattern of embossments extending generally in a length dimension of each fin for increasing structural rigidity of the ultra-thin fin. 
     Each of the fins has an aperture through which a tube of the bundle passes. At least a portion of the ultra-thin fin about the aperture is displaced to form a flange to ensure metal to metal contact between the fin and at least a portion of the corresponding tube wall. The embossment pattern reinforces the fin about the fin flange. The fin is displaced approximately its wall thickness in forming the embossment pattern to induce turbulence in the medium flowing over the plurality of fins when the heat exchanger is in use. 
     According to another aspect of the invention, an ultra-thin fin is provided for use in a tube bundle type heat exchanger. The fin comprises an elongate aluminum alloy sheet having top and bottom edges and opposite side edges. The fin has a thickness of less than 0.20 millimeters and comprises a plurality of apertures through which tubes of a tube bundle extend. An embossment pattern is provided in the fin and extends generally in the length dimension of each fin for increasing structural rigidity of the ultra-thin fin. The embossment pattern curves about a portion of each aperture and is displaced approximately the thickness of the fin to induce turbulence in the medium flowing over the fin when in used on a tube bundle of the exchanger. 
     According to another aspect of the invention, an evaporator for use in a refrigeration device is provided and comprises a serpentine tube bundle arrangement and a plurality of parallel fins through which tubes of the tube bundle extend. The serpentine tube bundle comprises a continuous tube with an inlet and an outlet for a first heat exchange medium carried in the tube. The tube has a series of aligned reverse bends to provide a plurality of parallel tubes. The parallel tubes are arranged in sets of two as determined by the respective reverse bend. Each of the fins has a length and width dimension. Two rows of a plurality of the sets of tubes extend along the length dimension of the fin. The two parallel tubes of each set are offset laterally and slope at an angle relative to the length dimension of the fin to provide two rows of tubes for each set all sloping in the same direction. 
     Each of the fins of the evaporator are identical in shape and comprise an elongate ultra-thin metal body portion having top and bottom edges and opposite side edges. Two rows of spaced-apart slots are provided in the fin which are sloped relative to the length dimension of the fins to be in register with and receive a respective set of tubes. Each slot has a flange at each end which is displaced from the fin material. The parallel tubes of the set are spaced apart slightly greater than the length of the slot between the flanges. Insertion of the set of tubes through the slot provides a press fit to ensure metal to metal contact between the fin flanges and at least a portion of the respective tube wall. 
     Each of the fins has a similar embossment pattern for reinforcing the fin about the fin flange portions contacted by the sets of tubes. The fin is displaced sufficiently in forming the embossment pattern to induce turbulence in a heat exchange medium flowing over the plurality of fins when the evaporator is in use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are shown in the drawings, wherein: 
     FIG. 1 is a section through a refrigerator cabinet showing disposed within the freezer compartment an evaporator according to a preferred embodiment of the invention; 
     FIG. 2 is a perspective view of a portion of the evaporator shown in FIG. 1; 
     FIG. 3 is a side elevation of the evaporator of FIG. 2; 
     FIG. 4 is a section along the lines 4--4 of FIG. 3; 
     FIG. 5 is an edge view of a portion of the evaporator of FIG. 3 showing the edge fin configuration; 
     FIG. 6 is a section of the edge of a fin of FIG. 5; 
     FIG. 7 is a side plan view of a fin having the tubes extending therethrough; 
     FIG. 8 is a section along the lines 8--8 of FIG. 7; 
     FIG. 9 shows a portion of the fin aperture prior to tube insertion; and 
     FIG. 10 shows the fin after a tube is inserted through the fin aperture illustrating the metal to metal contact. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Heat exchangers are used in a variety of environments to exchange heat between a first medium isolated from a second medium. For purposes of demonstrating the principles of the invention, it will be described with respect to the use of a preferred arrangement for the heat exchanger of this invention in a refrigeration device, such as the household refrigerator. Referring to FIG. 1, a refrigerator cabinet 10 has a freezer compartment 12 and refrigeration compartment 14. In accordance with standard refrigeration techniques, cold for the freezer and refrigerator compartments 12 and 14 is provided by a refrigerator evaporator 14. The freezer compartment 12 is sealed closed by door 16 having appropriate perimeter gaskets. At the rear of the compartment 12 are spaced-apart wall portions 18 and 20 with outlet 22 to the freezer compartment. Air is drawn over the evaporator 14 and directed upwardly in the direction of arrows 24 exhausted into the freezer compartment to maintain the cold therein. 
     As frost and ice gather on the evaporator 14, a heater element 26 is actuated to melt ice and frost from the evaporator which is collected by pan 28 and through funnel 30 and tubing (not shown) the melted ice is removed exteriorly of the refrigerator. The evaporator 14, as shown in FIGS. 2 and 3, comprises a serpentine tube bundle 32 which is a single tube having inlet 34 and outlet 36 with a plurality of reverse bends 38 which provide several tube passes for the evaporator. The reverse bends provide sets of two tubes such as 40 and 42 which extend the length of the evaporator and are parallel and spaced apart from one another. 
     As shown more clearly in FIG. 4, tubes 40 and 42 of the tube set generally designated 44 are offset from one another and slope relative to the length of the evaporator fin 46. Each set of tubes of the serpentine tube arrangement 32 are slanted in this manner to provide two rows generally designated 48 and 50 of slanted tubes. The sets of tubes pass through apertures 52 which, according to this embodiment, are elongate slots having rounded end portions 54 and 56. The set of tubes pass through a respective slot to permit mounting of a plurality of fins 46 on the serpentine tube bundle to provide the evaporator arrangement of FIG. 3. The fins, according to this embodiment, are evenly spaced apart along the length of the serpentine tube bundle. It is appreciated, however, that a variety of spacings between the fins and number of fins per length of evaporator may be used. For example, in refrigerators of the residential type approximately seventy-five fins may be used in a tube length of 560 millimeters. In air conditioners, double the number of fins may be used in the evaporator system. 
     The fins 46 have their apertures 52 sized so as to bind with the tube walls by way of a press fit in a manner to be discussed with respect to FIGS. 9 and 10, so that when all the fins are assembled on the serpentine tube bundle 32, the evaporator becomes a rigid structure. This permits the mounting of plates 58 and 60 over the reverse bend portions 38, as shown in FIGS. 2 and 3. The plates 58 are provided with lugs 62 which provide for mounting of the evaporator 14 in the refrigerator cabinet 10. In the bottom of each plate is an opening 64 through which a U-shaped heating element 26 is inserted. The U-shaped heating element 26 has two spaced-apart arms 68 and 70 as shown in FIG. 4 which abut the edges of the opening 64. The heating element has electrical leads 66 as shown in FIG. 3. The leads are connected to a power controller which, in accordance with the program of the refrigerator, is actuated from time to time to melt frost and ice which has collected on the evaporator fins 46. As a result, due to the rigid structure formed by the fins receiving the tubes, a simple mounting means for the evaporator within the refrigerator cabinet is achieved in the form of plates 58 and 60. 
     According to this invention, the fins 46 are ultra-thin compared to fins normally used in heat exchangers. The fins are of a thickness less than 0.20 millimeters and may be as thin as 0.10 millimeters. Referring to FIG. 7, the individual fins 46 have an embossment pattern generally indicated at 70 for structurally reinforcing the fins for handling purposes and reinforcing in the vicinity of the openings 52 through which the tubes extend. With two rows of tubes 48 and 50 and corresponding rows of apertures 52, the embossment pattern 70 consists of two ridges 72 and 74 which extend the length of the fin 46. Each ridge, for example 72, extends parallel to the aperture 52 as designated by ridge portion 72a. At the upper portion of the aperture 52, the ridge curves around the aperture as designated by portion 72b and widens into an enlarged ridge portion at 72c before continuing upwardly parallel to the next aperture 52 in the respective row of apertures. Similarly, ridge 74 extends the length of the fin about the apertures. In each instance, ridges 72 and 74 encircle portions of the apertures which are innermost relative to the fin width. 
     This embossment pattern in the fin considerably increases its structural rigidity, particularly through the central portion of the fin to maintain its planar characteristic when the tubes are inserted through the apertures and when so assembled on the tubes of the tube bundle, to provide a rigid evaporator structure. The embossment pattern 70, as shown in FIG. 8, has the ridge such as 74 which are displaced approximately the wall thickness of the fin 46. This displacement for the ridges is sufficient to provide the necessary structural rigidity in the fins. 
     Each slot 52 is such that for each set of tubes, such as 40 and 42, the portion of the fin about each end 54 and 56 of the aperture is displaced to provide a flange at each end. This displacement of the fin portion is readily accomplished due to the ultra-thin shape of the fin. As shown in FIG. 9, end portion 54 of the aperture 52 is displaced to form flange 54a. Similarly, the bottom portion of the slot is displaced to form flange 56a. Preferably, the flanges 54a and 56a are formed before the fins are assembled on the tube bundle 32; however, it is understood that the flanges may be formed during insertion of the tube sets through aligned apertures of the stack of spaced-apart fins. The formed flanges may crack slightly due to the displacement of the metal. The spacing between tubes 40 and 42 is slightly greater than the spacing between the flanges 54a and 56a. Thus the tube sets are press fitted into the aligned slots of the stack of fins to ensure metal to metal contact between the fin flanges and the tube wall to enhance heat conduction from the tube walls to the fins in exchanging heat with the refrigeration medium as it flows over the fins. Preferably a nominal oversizing of the tube spacing of approximately 0.125 mm provides an adequate press fit of the tube bundle in the stack of fins. 
     The embossment pattern 70, as it encircles the innermost portions of the apertures, provides reinforcing about the fin flanges to prevent the cracks in the flange from travelling further in the fin to maintain the structural integrity of the assembled unit. This pushing of the tubes through the undersized apertures and thereby press fitting against the ultra-thin metal, binds the fins to the tubes to give the necessary rigidity and at the same time, an effective efficient heat transfer between the tubes and fins. 
     According to the arrangement provided in FIG. 1, the refrigeration air travels upwardly over the evaporator 14 and is exhausted through port 22. By slanting the sets of tubes for the evaporator serpentine tube arrangement, more passes of tubes may be provided per predetermined height of the evaporator. Furthermore in slating the two sets of tubes as shown in FIG. 4, the upwardly travelling air in the direction of arrows 76 passes over in essence four aligned rows of tubes. Corresponding tubes of adjacent sets, such as at 78 and 80, are spaced apart twice the distance of a normal evaporator configuration where the sets of tubes are all vertically aligned. This allows the air currents 76 to divide over the first tube 78 and rejoin before being broken up again in travelling over tube 80. Thus tube 80 does not lie in the downward eddy currents of tube 78, so that a more effective heat transfer is realized. To further improve upon the transfer of heat to the air current 76, it has been found that the reinforcing embossment pattern in the fins provides ridges of sufficient height which induces turbulence in the air currents as they travel upwardly along the fins. This avoids boundary layers being formed along the fins so that there is an increased rate of heat transfer from the fins to the moving air. 
     The opposite side edges generally designated 82 and 84 of the fins, as shown in FIG. 7, have an undulating pattern. According to the preferred embodiment of this invention, the undulating pattern is in the form of a wave which may be sinusoidal in shape. In the vicinity of a tube 86, which is adjacent the side edge 82, the fin has an outwardly extending portion 88 which curves around the tube 86. Between tubes 86 and 90 where there is no tube adjacent the side edge 82, the fin has an inwardly extending portion 92. This resultant undulating pattern improves on the efficiencies of heat transfer from the fins to the refrigerant air. This is due to the heat being conducted away from each tube in a radial pattern. In the space between tubes 86 and 90, for example, there is no tube immediately adjacent the side edge so that that portion of the metal does not effectively transfer heat to the refrigerating air. Thus, this portion of the metal may be removed from the fin thereby saving on the amount of metal which goes into the fin. The undulating pattern may be symmetrical, so that when the fins are stamped from a sheet of metal 94, each fin may have the same shape in the form of additional fins 46a and 46b. 
     To further improve the heat transfer efficiencies, the opposite side edges 82 and 84 of each fin are crimped as shown in FIGS. 5 and 6. The crimping may be in the form of a triangular pattern 96 which induces additional turbulence in the air currents 76 as the air travels over the fin edges. 
     The tubing and the fins may be made from various commercial grades of aluminum alloy, such as that commonly used in the making of evaporators for refrigerators. It is appreciated, however, that any other form of suitable metal having appropriate heat conductivities may be used which can be formed into the serpentine tubing and also the ultra-thin fins. By way of the embossment pattern enabling the use of considerably thinner fins, an appreciable reduction in weight of the system may be achieved. A further reduction in weight is also realized by the undulating pattern for the fin edges. Alternatively, because of reduction in weight more fins may be used per evaporator unit to increase the efficiencies of the arrangement. By providing the embossment pattern in the fins, the resultant increase in turbulence over the fin surface greatly improves the heat transfer efficiencies. Comparing the existing evaporator to the normal form of evaporator in a refrigerator, it has been found that per fin there is approximately a 50% reduction in material, resulting in approximately 22% improvement in overall costs of the material for the assembled evaporator. 
     EXAMPLE 1 
     Calorimeter tests were conducted to establish the percent improvement in heat transfer in comparing an existing evaporator design for a refrigerator to that shown in FIG. 1. The following results were obtained as itemized in Table I. 
     
                       TABLE I                                                     
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Air Flow   Heat transfer (B.T.U./Hr. Deg. F)                              
CFM        Current     Design of                                          
                                Percent                                   
(Cu. Ft/Min)                                                              
           Design      FIG. 2   Increase                                  
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20         24.8        28.0     12.9                                      
30         30.5        33.1      8.5                                      
40         36.2        41.2     13.8                                      
50         42.3        49.6     17.3                                      
60         43.4        53.3     22.8                                      
70         54.2        62.0     24.4                                      
80         60.0        75.6     26.0                                      
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     For the increased flows of refrigerant air, increased efficiencies in the range of 25% are obtained with this system. This in combination with the reduced material costs provide an evaporator significantly superior to existing designs. 
     Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.