Abstract:
A heat exchanger ( 10 ) for a water heater. The heat exchanger ( 10 ) has a first part ( 12 ) and second part ( 14 ). A heat exchanging medium flows through the first and second parts. The majority of the length of the first part ( 12 ) is in direct heat exchanging contact with a substance to be heated and the majority of the length of the second part ( 14 ) is encased in an insulative material or substantially thermally isolated from the substance to be heated.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a heat exchanger and a water heater incorporating same. 
         [0002]    The invention has been primarily developed for use with domestic and commercial water heaters and will be described with reference to these applications. However, the invention is not limited to these particular applications, and is also suitable for use with heat exchangers using heat conducting fluids other than water. 
       BACKGROUND OF THE INVENTION 
       [0003]    Heat exchangers for water heaters that have a hollow, generally elongate, U-shaped configuration are known. Heat exchangers of this type have an inlet and an outlet, both near the top of the water heater tank, and a length slightly less than the height of the water heater tank. High temperature heat exchanging fluid flows downwardly from the heat exchanger&#39;s inlet, to near the bottom of the water heater tank. The heat exchanging fluid is then directed in a generally opposite upwardly direction and flows from the bottom of the tank to the top of the tank and so to the heat exchanger outlet. 
         [0004]    As the heat exchanging fluid travels through the heat exchanger, it heats the heat exchanger which then transfers heat to the surrounding water, thereby raising its temperature. As this occurs, the temperature of the heat exchanging fluid is also lowered. During this process it is possible for the falling temperature of the heat exchanging fluid to equal that of the rising temperature of the surrounding water. As a result, from that point onwards in the heat exchanger, heat transfer is reversed and heat is transferred from the (hotter) water to the (cooler) heat transfer medium. This wastes energy and reduces the efficiency of the heat exchanger and thus the water heater overall. 
       OBJECT OF THE INVENTION 
       [0005]    It is the object of the present invention to substantially overcome or at least ameliorate the above disadvantage. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, in a first aspect, the present invention provides a heat exchanger for a water heater, the heat exchanger including: 
         [0007]    a first part through which a heat exchanging medium flows; and 
         [0008]    a second part through which the heat exchanging medium flows, 
         [0009]    wherein the majority of the length of the first part is in direct heat exchanging contact with a substance to be heated and the majority of the length of the second part is encased in an insulative material. 
         [0010]    In a second aspect, the present invention provides a heat exchanger for a water heater, the heat exchanger including: 
         [0011]    a first part through which a heat exchanging medium flows; and 
         [0012]    a second part through which the heat exchanging medium flows, 
         [0013]    wherein the majority of the length of the first part is in direct heat exchanging contact with a substance to be heated and the majority of the length of the second part is substantially thermally isolated from the substance to be heated. 
         [0014]    In a third aspect, the present invention provides a water heater including: 
         [0015]    a tank adapted for containing water; 
         [0016]    a heat exchanger within the tank and including a first part through which a heat exchanging medium generally flows in a first direction and a second part in which the heat exchanging medium generally flows in a second direction, generally opposite to the first direction, 
         [0017]    wherein the majority of the length of the first part is in direct heat exchanging contact with the water in the tank and the majority of the length of the second part is encased in an insulative material. 
         [0018]    In a fourth aspect, the present invention provides a water heater including: 
         [0019]    a tank adapted for containing water; 
         [0020]    a heat exchanger within the tank and including a first part through which a heat exchanging medium generally flows in a first direction and a second part in which the heat exchanging medium generally flows in a second direction, generally opposite to the first direction, 
         [0021]    wherein the majority of the length of the first part is in direct heat exchanging contact with the water in the tank and the majority of the length of the second part is substantially thermally isolated from the substance to be heated. 
         [0022]    Preferably, in the first part, the heat exchanging medium generally flows in a first direction and, in the second part, the heat exchanging medium generally flows in a second direction, generally opposite to the first direction. 
         [0023]    Preferably, substantially all of the length of the second part is encased in an insulative material. Preferably, substantially all of the length of the second part is substantially thermally isolated from the substance to be heated. 
         [0024]    In one form, the heat exchanger first part is a hollow cylinder, most preferably formed from coiled tube, and the second part is generally straight tube, most preferably positioned within the hollow cylinder. 
         [0025]    In another form, the heat exchanger first part is generally straight tube, most preferably having heat exchanging fins externally attached thereto, and the second part is a generally straight tube. 
         [0026]    The heat exchanger first part preferably includes an inlet, most preferably with a length of insulative material substantially adjacent thereto. The heat exchanger second part preferably includes an outlet, most preferably with a length of insulative material substantially adjacent thereto. 
         [0027]    The heat exchanger is preferably generally in the shape of an elongate U. 
         [0028]    The heat exchanger preferably includes a third part joining the downstream end of the first part to the upstream end of the second part. 
         [0029]    The insulative material is preferably an air gap within an external cover, most preferably a cylindrical cover of like material to the heat exchanger material. Alternatively, the insulative material can be a solid or semi solid insulative material applied to the exterior surface of the second part, such as a close coupled polymer that has a low heat transfer coefficient compared to the heat exchanger material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Preferred embodiments of the invention will now be described, by way of examples only, in which: 
           [0031]      FIG. 1  is a perspective view of a first embodiment of a heat exchanger; 
           [0032]      FIG. 2  is an enlarged partial perspective view of the heat exchanger shown in  FIG. 1 ; 
           [0033]      FIG. 3  is a perspective view of a second embodiment of a heat exchanger; and 
           [0034]      FIG. 4  is an enlarged partial view of the heat exchanger shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]      FIG. 1  shows a first embodiment of a hollow heat exchanger  10  suitable for use in a domestic water heater of 25 to 500 litre capacity. The heat exchanger  10  has a generally elongated U-shaped configuration. The heat exchanger  10  has a first part  12  and a second part  14  which are joined by a third part  16 . The heat exchanger  10  also has an inlet  18  and an outlet  20  which are positioned external to the tank of the water heater, near its upper end. 
         [0036]    The majority of the length of the first part  12  has external heat exchanging fins  22  attached thereto, to increase its heat transfer surface area. The majority of the length of the second part  14  is encased in an insulative material, in the form of an air gap between the exterior of the second part  14  and the interior of a copper cylinder  24 . The bottom of the cylinder  24  is sealed with respect to the exterior of the second part  14 , adjacent the bottom thereof. A smaller portion  26  of the first part  12 , adjacent the inlet  18 , is also encased in similar insulative material, to prevent heat loss to atmosphere as this portion of the first part  12  is external to the tank. 
         [0037]    A different metal could be used for the cylinder  24  but like metals reduce corrosion risk. 
         [0038]    The hollow material of the first  12 , second  14  and third  16  parts of the heat exchanger  10  is preferably constructed from double walled copper pipe, a description of which can be found in the Applicant&#39;s International PCT Patent Application No. PCT/AU2006/001086. The double walled copper pipe has an external diameter of 6-8 mm. 
         [0039]    The heat exchanger  10  is typically installed in the water tank of a water heater through an opening in an upper end of the tank and extends through substantially all of the length of the tank. 
         [0040]    In use, relatively high temperature heat exchanger fluid is pumped into the inlet  18  of the heat exchanger  10  and it flows in a generally downward direction through the length of the first part  12 . As the heat exchanging fluid travels through the first part  12 , heating energy is transferred through the walls of the first part  12 , and also through the fins  22 , causing the temperature of the surrounding water to rise. When the heat exchanging fluid reaches the bottom of the first part  12  it travels through the third part  16  and into the second part  14 . The fluid then travels through the second part  14  in the opposite upward direction towards the outlet  20 . During its passage through the first part  12 , the heat exchanger fluid reduces in temperature to a level approximately equal to that of the (now heated) water in the tank. As the heat exchanging fluid flows through the length of the second part  14 , the insulative material  24  prevents (or at least minimises) substantial heat transfer from the heated water to the heat exchanging fluid. This results in an improvement of the heat transfer from the heat exchanging fluid to the water, thereby improving the efficiency of the heat exchanger and thus the water heater overall. 
         [0041]    A second embodiment of heat exchanger  30  is shown in  FIGS. 3 and 4 . The heat exchanger  30  is similar to the heat exchanger  10  shown in  FIGS. 1 and 2  and like features are indicated with like reference numerals. However, in the heat exchanger  30 , the heat exchanging surface area of the first part  12  is increased by forming the majority of the length of the first part  12  into a cylinder of a tightly wound spiral configuration. In addition, in the heat exchanger  30 , the insulated second part  14  is positioned inside the cylindrical first part  12 . The first  12 , second  14  and third  16  parts of the heat exchanger  30  are again formed from the double walled copper pipe described in the aforementioned PCT application. 
         [0042]    The operation of the heat exchanger  30  is also substantially identical to that previously described with reference to the heat exchanger  10 . However, whilst the heat exchanging fluid flowing through the first part  12  still travels overall in a downward direction, it does so whilst simultaneously spiralling. 
         [0043]    To maximise efficiency, the heat exchangers  10  and  30  are configured so that, in use, the refrigerant condensation point is at the bottom of the first part  12 , so that it enters the insulated second part  14  in liquid form. 
         [0044]    Although the invention has been described with reference to preferred embodiments, it would be appreciated by persons skilled in the art that the invention may be embodied in many other forms. For example, the insulative material can be a close coupled polymer that has a relatively low heat transfer coefficient compared to the heat exchanger material. Also, the first, second and third parts of the heat exchanger can be made from single walled pipe.