Patent Publication Number: US-7717166-B2

Title: Fin stock for a heat exchanger and a heat exchanger

Description:
RELATED APPLICATION 
   This application claims priority benefits under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/573,646 filed May 21, 2004. 

   FIELD OF THE INVENTION 
   The invention relates to a heat exchanger with stronger fins that facilitate heat transfer. 
   BACKGROUND OF THE INVENTION 
   A heat exchanger typically has a metal tube with fins extending radially away from the metal tube to increase the surface area which facilitates heat transfer. These fins are often called spiral fins and include a narrow aluminum strip that is helically wound to the metal tube, preferably the edge of the strip is secured to the tube. See  FIG. 1  for a heat exchanger  10  in accordance with the prior art.  FIG. 1  shows tube  12 , fin  14 , and edge  16  of fin secured to tube  12 . The strip is usually fixed to the tube either by inserting it into a scored groove or by forming a small L at the base of the fin, which is then secured to the tube. See  FIG. 2 . 
   Because the fin is often bent about the tube to form the fin, particularly a fin having a rectangular cross section, it has commonly been believed that there is a large compressive force at the base of the fin and a large tensile force at the tip of the fin. This may be the rationale for traditionally manufactured fins to utilize a malleable material so that it may be formed and bent about a tube. Moreover, traditionally manufactured fins typically used a thermally conductive material so that it may be able to transfer heat. 
   Further, traditional methods for providing a fin often included the use of a stretchable material, the higher the ductility or elongation percentage the better. Hence, a fully annealed aluminum was normally used to provide fins. Additionally, the fully annealed aluminum often has at least a 30% elongation, a characteristic of the aluminum commonly specified within the spiral fin heat exchanger industry. Generally, aluminum of at least 99.00% purity is used for spiral fins because this offers enhanced thermal conductivity. 99% purity means the alloy has a minimum of at least 99% aluminum. 
   As a result, traditional fins, although having enhanced thermal conductivity, have been extremely soft and prone to damage during manufacture, handling, installation, and maintenance of the finned tubes. During servicing, fins are commonly cleaned using high-pressure air or water and the fins may bend as a result of the cleaning, in which case the fins may need to be repaired. The repair costs and down time often exacerbates the problem, especially when the repairs are needed each time the fins are cleaned. In the alternative, some users do not clean or repair the fins. However, debris on the fins or using distorted fins may reduce the heat-transfer capacity of the heat exchanger, and reduce the life of the fins or heat exchanger. 
   What is desired, therefore, is a fin that maintains the benefits of the traditionally made fins while reducing the disadvantages of the traditionally made fins. Another desire is a fin that resists distortion without a decrease in thermal conductivity. A further desire is a fin that resists damage without sacrificing needed flexibility to be maneuvered about the tube. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the invention to provide a fin with a material that is sufficiently malleable yet strong enough to resist distortion. 
   Another object is to provide a fin that is strong yet flexible enough to bend about a tube of a heat exchanger. 
   These and other objects of the invention are achieved by a fin stock having a material with aluminum, where the material also has a tensile strength of between approximately 14,000 and approximately 26,000 psi, an elongation of less than 30%, and a hardness of between approximately 50 and approximately 70 on a Rockwell 15T scale. 
   In some embodiments, the fin stock has a tensile strength between approximately 17,000 psi and approximately 24,000 psi with a yield strength of between approximately 17,000 psi and approximately 25,000 psi. In other embodiments, the fin stock has a tensile strength between approximately 3000 psi and approximately 6000 psi greater than fully annealed aluminum. 
   In further embodiments, the fin stock has an elongation less than approximately 20%. In some of these embodiments, the elongation is between approximately 1% and approximately 2%. 
   The fin stock is cold worked to achieve the above enhanced strength and hardness. As a result of the cold working, the material has a thickness of between approximately 0.014 gauge and approximately 0.020 gauge. 
   In another aspect of the invention, a method provides the fin stock. The method includes the steps of providing a material having aluminum, increasing a tensile strength of the material to between approximately 14,000 psi and approximately 26,000 psi, reducing an elongation of the material to less than 30%, and hardening the material to a hardness of between approximately 50 and approximately 70 on a Rockwell 15T scale. 
   In some embodiments, the method further includes the step of cold working the material to increase the tensile strength. In some of these embodiments, the method includes increasing the tensile strength of the material to between approximately 17,000 psi and approximately 24,000 psi without annealing the material. 
   As a result of increased tensile strength, the method reduces the elongation of the material to less than 20%. In further embodiments, the elongation is reduced to between approximately 1% and approximately 2%. 
   The method may further include reducing a thickness of the material to between approximately 0.014 gauge and approximately 0.020 gauge and increasing a yield strength of the material to between approximately 17,000 psi and approximately 25,000 psi. 
   In another aspect of the invention, a heat exchanger is provided and includes a tube for transporting a fluid, such as a liquid or gas or other medium or device that transfers heat. The heat exchanger also has a fin stock in contact with the tube for dispersing the temperature of the liquid or gas, where the fin stock contains aluminum. The fin stock has a tensile strength of between approximately 14,000 and approximately 26,000 psi, an elongation of less than 30%, and a hardness of between approximately 50 and approximately 70 on a Rockwell 15T scale. 
   In some embodiments, the fin stock is cold worked without annealing. In further embodiments, the tensile strength is between approximately 17,000 psi and approximately 24,000 psi and the elongation is between approximately 1% and approximately 2%. In other embodiments, the fin stock has a yield strength of between approximately 17,000 psi and approximately 25,000 psi. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a heat exchanger in accordance with the prior art. 
       FIG. 2  depicts another heat exchanger in accordance with the prior art. 
       FIG. 3  depicts the fin stock in accordance with the invention. 
       FIGS. 4   a  and  4   b  depict the fin stock being produced in accordance with the invention shown in  FIG. 3 . 
       FIG. 5  depicts a method for providing the fin stock shown in  FIG. 3 . 
       FIG. 6  depicts the fin stock shown in  FIG. 3  as applied to a heat exchanger. 
       FIG. 7  depicts the fin stock shown in  FIG. 3  being tapered. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 3 , fin stock for use with a heat exchanger includes material  34  of aluminum that is cold worked. Cold working material  34  increases the strength of material  34  more than if material  34  was not cold worked or if material  34  was annealed, which is traditionally done to soften material  34  and, therefore, make material  34  more flexible at the expense of making material  34  weaker. 
   Material  34  is any aluminum alloy used for fin stock in heat exchangers, including 99% aluminum alloys (minimum amount of aluminum is 99%) such as 1100 and 1050.99% purity aluminum alloys are generally desired because of their thermal conductivity capabilities. In other embodiments, material  34  is aluminum alloy with manganese or magnesium, such as  5005  (which contains 0.50-1.1% magnesium). The manganese or magnesium often enhances strength and corrosion resistance. In further embodiments, material  34  is any aluminum alloyed with other elements. All of these aluminum alloys, whether having 99% purity aluminum or not, have higher tensile strengths from being cold worked. Material  34  includes all alloys that might be rolled to temper (full hard) or partially annealed to temper (half hard). Both full hard and half hard materials are described below. 
   Cold working is defined as rolling, or squeezing, material  34  through at least two rollers (see  FIGS. 4   a  and  4   b ) where material  34  that exits the rollers  56 ,  58  are stronger and thinner as a result of being rolled. Cold working is performed on material  34  without raising or negligibly raising the temperature of material  34 . By cold working material  34 , material  34  has a higher tensile strength, yield strength, and lower elongation than material  32  that has not been cold worked, which is located prior to rollers  56 ,  58 . Another term that is interchangeable with cold working is strain hardening. 
   In some embodiments, cold working involves other manners for introducing stress on material  32 , such as bending, striking, or physically altering a shape of material  32 . The introduced stress remains with material  34  and renders material  34  stronger than material  32 . 
   Material  34  exiting rollers  56 ,  58  have a thickness of between approximately 0.014 gauge to approximately 0.020 gauge and preferably between approximately 0.014 gauge to approximately 0.017 gauge. The above thicknesses are for material  34  with an elongation of less than 30%. 
   The grain structure of cold worked material  34  is non-recrystallized and may be coarse or fine, terms having their customary meaning as defined in the industry. 
   The act of annealing releases the resulting stress from cold working and essentially reverses the act of cold working. As a result, annealed materials are less strong but more flexible than materials that are cold worked. Although annealed materials are traditionally preferred for the fin stock shown in  FIGS. 1 and 2 , because the annealed materials were softened and were often believed to be better suited for bending about tube  12 . However, material  34  that is cold worked is sufficiently flexible to be formed about tube  12 . Therefore, material  34  that has been cold worked results in a stronger fin  14  because material  34  is stronger than annealed materials or materials  32  that are neither annealed nor cold worked. 
   Traditional materials for fin stock were usually in a fully annealed condition in order to meet the requirements for a material with high elongation, defined to be generally greater than 30%. Materials with high elongation were believed to bend about tube  12  (see  FIG. 1 ) more easily than harder materials. Therefore, flexibility was often the focus rather than strength. However, the requirement for a material with high elongation was flawed because material  34 , which has a low elongation (less than 30%) and higher tensile strength than the traditional fully annealed material, proved to be both bendable about tube  12  and resist damage due to its higher strength. 
   The tensile strength of fully annealed aluminum alloy is between approximately 11,000 psi and approximately 14,000 psi. The yield strength of fully annealed aluminum alloy is approximately 5,000 psi. 
   Moreover, material  34  may be used to replace fins on heat exchangers that use the traditional fully annealed material because material  34  has many of the same chemical properties and thermal conductivity capabilities as fully annealed materials except material  34  has been cold worked, which does not vary the chemical composition or significantly vary the thermal conductivity of material  34 . 
   As a result, fin stock with a high elongation is not a prerequisite and that cold-worked aluminum, with elongation levels below 20% and as low as 1-2%, can be rolled into a satisfactory helix with minimal adjustments to the manufacturing machine. 
   A range of tensile strengths for material  34  is between approximately 14,000 psi and approximately 26,000 psi. A preferred range of tensile strengths for material  34  is between approximately 17,000 psi and approximately 24,000 psi. A more preferred range of tensile strengths for material  34  is between approximately 18,000 psi and approximately 20,000 psi. 
   The tensile strength of material  34  is between approximately 3000 psi and 6000 psi stronger than fully annealed material, or 0 temper. 
   A range of yield strengths for material  34  is between approximately 10,000 psi and approximately 25,000 psi. A preferred range of yield strengths for material  34  is between approximately 15,000 psi and approximately 22,000 psi. A more preferred range of yield strengths for material  34  is between approximately 17,000 psi and approximately 20,000 psi. 
   As a result of the stronger tensile strength of material  34 , elongation of material  34  is typically less than 30%. In some embodiments, the elongation of material  34  is typically less than approximately 20%. In further embodiments, the elongation of material  34  is typically less than approximately 5%. A preferred elongation of material is between approximately 0% and approximately 25%. A more preferred elongation of material is between approximately 0% and approximately 10%. A most preferred elongation of material is between approximately 1% and approximately 2%. 
   Elongation is defined to be the amount a material will stretch per a 2 inch length of that material until the material yields, or fails to return to the original state before stretching commenced. 
   The hardness of material  34  measured on a Rockwell 15T scale is between approximately 50 and approximately 70. The hardness of material  34  measured on a Rockwell 15T scale is preferably between approximately 55 and approximately 66. The hardness of material  34  measured on a Rockwell 15T scale is more preferably between approximately 60 and approximately 65. 
   In addition to winding, or bending, material  34  about tube  12  to form a helically wound fin, material  34  may be used as a fin in other heat exchangers that utilize any shaped fin, including a flat fin. Moreover, although a spiral fin is shown in the figures, any type of heat exchanger or air conditioner that uses fins can benefit from this invention. 
   It is understood that material  34  is cold worked to meet specific requirements of fin stock by a customer. Some customers require the highest tensile strength possible with fin stock without concern for elongation, in which case material  32  is simply cold worked. Other customers require a tensile strength and elongation somewhere in between a fully annealed material and a fully hard material (described in the immediately preceding sentence), in which case a half hard or partially annealed material would be provided. A partially annealed fin stock material  34  is where material  32  is partially annealed to soften it and then cold worked. Fully annealed materials are too soft to be cold worked. 
   It is understood that the limitations and ranges for tensile strengths, yield strengths, elongations, and other properties of partially annealed materials and materials without any annealing are collectively described above. 
     FIG. 5  shows method  100  for providing the fin stock material  34  described above. Method  100  includes the steps of providing  102  a material having aluminum, increasing  108  a tensile strength of the material to between approximately 14,000 psi and approximately 26,000 psi, reducing  112  an elongation of the material to less than 30%, and hardening  114  the material to a hardness of between approximately 50 and approximately 70 on a Rockwell 15T scale. 
   Method  100  further includes the step of cold working  118  the material to increase the tensile strength and yield strength. In some embodiments, method  100  increases  120  the yield strength to between approximately 17,000 psi and approximately 25,000 psi. In other embodiments, method  100  increases  132  the tensile strength of the material without annealing. 
   In further embodiments, method  100  increases  122  the tensile strength of the material to between approximately 17,000 psi and approximately 24,000 psi and reduces  126  the elongation to less than 20%. In some of these embodiments, method  100  reduces  128  the elongation to between approximately 1% and approximately 2%. 
   In yet other embodiments, method  100  reduces  134  a thickness of the material to between approximately 0.014 gauge and approximately 0.020 gauge. 
     FIG. 6  depicts heat exchanger  150 , where heat exchanger  150  includes tube  152  for transporting a gas or liquid and fin stock  154 , where fin stock  154  includes material  34 . 
     FIG. 7  depicts a top view of cold worked material  34 ′, the material after being compressed by rollers  56 ,  58 , being passed through a pair of tapering rollers  56 ′,  58 ′. 
   Tapered rollers  56 ′,  58 ′ compress outer edge  35  of material  34 ′ and leaves inner edge  37  untouched. By doing this, material  34 ′ exiting tapered rollers  56 ′,  58 ′ form into a natural helix where inner edge  37  is the part of the helix that comes in contact with tube  12  and the compressed outer edge  35  has sufficient material, due to the tapering, to be wound about tube  12  at a distance further away than inner edge  37 .