Abstract:
A heat exchanger assembly of the type having a cylindrical coil in an insulated tank, the assembly having different heat exchanger sections having different thermal conductivities.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to heat exchange assemblies, particularly assemblies used to transfer heat to and from a water tank. 
       BACKGROUND OF THE INVENTION 
       [0002]    Heat exchange assemblies are commonly used to heat water and then extract heat from the water. 
         [0003]    Solar energy systems include a solar heat exchanger that heats a liquid that is flowed through a heat exchange assembly positioned in a large body of water in an insulated tank. The solar heated water flows through the heat exchange assembly to heat the water in the tank. Heated water stores solar energy that may be extracted from the heated water by the heat exchange assembly to heat liquid flowing through the assembly. The heated liquid may be used to heat domestic hot water, to heat the interior of a building or otherwise. 
         [0004]    In a conventional heat exchange assembly used to heat or cool a body of water, a heat exchange liquid flows though a vertically coiled tube immersed in the water. The coiled tube is vertically positioned in the tank. As the heat exchange liquid in the tube is flowed through the coiled tube, thermal energy is exchanged between the heat exchange liquid in the tube and the water in the tank. 
         [0005]    The water in the tank is thermally stratified so that hotter water rises to the top of the tank and cooler water falls to the bottom of the tank. 
         [0006]    In order for heat energy to be efficiently transferred between the heat exchange liquid in the coil and the heat storage liquid in the water in the tank, the tube must have a high coefficient of thermal conductivity. Tubes in traditional heat exchangers are made entirely of a heat conducting metal tubing, such as copper tubing. This makes the tubes expensive to produce due to raw material and manufacturing costs. Specialized machines are needed to wind a full metal coil and time and skilled workmanship is needed to weld supports to the metal coil to hold its spiral shape. Traditional coils are heavy, increasing transportation costs and installation difficulty. 
         [0007]    It is the object of the present invention to provide a heat exchange assembly with an improved two-part coil that performs comparatively to a traditional, all metal coil but is less expensive to produce. The two-part coil of the invention should weigh less than a conventional heat-exchanger coil to lower transportation costs and simplify installation. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is an improved heat exchange assembly to heat a heat storage liquid maintained in an insulated tank and to extract heat from the heat storage liquid. The assembly includes a heat exchanger made up of an elongate coiled tube having an upper heat exchanger section made from metal which may be copper tubing and a lower heat exchanger section made from a coil of plastic material having a lower coefficient of thermal conductivity that the upper heat exchanger section which may be cross-linked high-density polyethylene (PEX) tubing. 
         [0009]    A heat exchange liquid, conventionally water, is heated by solar energy, combustion or electricity is flowed into the top of the heat exchanger and then down along the length of the copper and the PEX tubing. The heat storage liquid at the top of the tank is heated by conduction through the upper heat exchanger section made of copper tubing. The heat storage liquid at the lower portion of the tank is heated by conduction through the lower coil section of PEX tubing in the lower portion of the tank. A connector joins the upper and lower coil sections together. Heat is extracted from the tank by reversing the flow through the coil. 
         [0010]    PEX tubing is slightly less efficient at conducting heat than copper tubing at the operating temperature of the heat exchanger, but is much cheaper and lighter than copper tubing. PEX tubing can be hand coiled quickly without the need for an expensive winding machine. The smaller amount of copper can be wound in a few minutes. PEX tubing is manufactured as straight pipe and is easily coiled. A frame or cage surrounds the PEX tubing coil to force the PEX tubing coil to maintain its cylindrical, vertical spiral shape. 
         [0011]    The ability to reduce the amount of copper in a heat exchanger greatly reduces the cost of the heat exchanger while providing nearly identical functionality. 
         [0012]    The mixture of copper and PEX tubing reduces the weight of the heat exchanger, reducing the cost of transportation and simplifying the installation procedure. 
         [0013]    Another advantage of the disclosed heat exchanger over an exchanger made entirely of non-metal or PEX tubing is that the combination of metal and non-metal tubing allows the exchanger to better absorb heat spikes that occur when heating cycles begin and end. Such heat spikes would damage a heat exchanger made only of non-metal or PEX tubing. Heat energy that would damage PEX tubing is safely transferred by the copper tubing to the liquid held in the tank without damage. The use of copper and PEX also improves the low temperature transfer characteristics of the heat exchanger. 
         [0014]    Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawing sheets illustrating the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cut-away perspective view of the heat exchange assembly; 
           [0016]      FIG. 2  is a side view of the heat exchanger and frame; 
           [0017]      FIG. 3  is a top view of the heat exchanger and frame; 
           [0018]      FIG. 4  is a vertical sectional view of the  FIG. 2  heat exchanger; 
           [0019]      FIG. 5  is a detail view of the PEX/Copper tubing connector; and 
           [0020]      FIG. 6  is a representational view of an installed heat exchange assembly. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    Heat exchange assembly  10  includes heat exchanger  12  installed in tank  14 . Tank  14  may be a conventional insulated water tank having a lid  16  and a brace  18 . Brace  18  supports lid  16  when the lid is placed on the tank. Brace  18  extends upward from tank bottom  20  to tank top  22 . Drain  24  is located at tank bottom  20  to allow draining tank  14 . 
         [0022]    Heat exchanger  12  is made up of an elongate coiled tube  26  extending generally axially from exchanger top  28  to exchanger bottom  30 . Tube  26  forms a number of circular coiled loops  32  that extend from exchanger top  28  to exchanger bottom  30 . Tube first end  34  is located at exchanger top  28  and includes a fitting  36 . Fitting  36  may be a conventional female pipe threading. The bottom coiled loop  32  of tube  26  is joined to vertical output line  38 . Output line  38  extends up through the center of the exchanger beyond exchanger top  28  to a tube second end  40 . Tube second end  40  includes a fitting  42  which may be conventional female pipe threading. 
         [0023]    Upper heat exchange section  44  is formed from loops  32  at exchanger top  28 . Lower heat exchange section  46  is formed from loops  32  at exchanger bottom  30 . Connector  48  joins upper heat exchange section  44  to lower heat exchange section  46 . Upper heat exchange section  44  is preferably made of a heat conducting metal or metal alloy such as copper and lower heat exchanger section  46  is preferably made of heat conducting non-metal tubing such as coiled cross-linked high-density polyethylene (PEX). The copper and PEX tubing may be ⅞ inch diameter tubing. 
         [0024]    Preferably, the upper heat exchange section  44  extends 20% of the axial height of heat exchanger  12  and the lower heat exchange section  46  extends 80% of the axial height of heat exchanger  12 . The ratio of section  44  metal tubing axial height to section  46  non-metal tubing axial height may be varied to suit particular liquid heating applications. 
         [0025]    Heat exchanger  12  may be constructed so that loops  32  are about 21 inches in diameter and the heat exchanger is about 42 inches in height. 
         [0026]    Upper coil section  44  extends generally downward from exchanger top  28  to connector  48 . As shown in  FIG. 5 , connector  48  is made of a solder connector  50  joined to section  44  and crimp connector  52  joined to section  46 . 
         [0027]    Lower heat exchanger section  46  extends generally downward from connector  48  to exchanger bottom  30 . 
         [0028]    Lines  54  and  56  are joined to ends  34  and  40  to flow quantities of a heat exchange liquid  58  though heat exchanger  12  to transfer heat to and from heat storage liquid  60  in tank  12 . Liquids  58  and  60  may be water or some other liquid capable of holding heat. 
         [0029]    Heat exchanger  12  may include a frame or cage  62 . Frame  62  rests on tank bottom  20  and includes frame base  64 . Outer frame arms  66  extend upwardly from base  64  around the exterior of exchanger  12  and are joined to frame top  68 . Inner frame arms  70  are located within loops  32  and are joined to outer frame arms  66  by fasteners  72 . Outer frame arms  66  and inner frame arms  70  surround lower heat exchanger section  46  and may be formed from heat conducting metal tubing identical to the tubing of upper heat exchanger section  44 . Frame  62  maintains the shape of coils  32  in lower heat exchanger section  46 . 
         [0030]    If desired, fame  62  may only extend around lower heat exchanger section  46 . Frame  62  may include only one set of frame arms  66  or  70  and include stainless steel or Monel brand stainless metal alloy fasteners to secure portions of heat exchanger section  46  to frame  52  to maintain the shape of coils  32 . Frame  62  may be made of metal tubing and incorporated into upper heat exchanger section  44 . 
         [0031]    The operation of heat exchanger assembly  10  will now be described. 
         [0032]    Tank  14  is filled with heat storage liquid  60 . Heat is transferred to and from liquid  60  through heat exchanger  12 . Heat rises in liquid  60  so that liquid at the top of tank  12  is hotter than liquid at he bottom of the tank. 
         [0033]    To transfer heat energy into liquid  60 , heat exchange liquid  58  is heated outside of tank  12  by solar energy, combustion, electricity or other means and flowed through line  54  and into heat exchanger  12  through first end  34 . Heat exchange liquid  58  is flowed into upper heat exchanger section  44 . Liquid  58  proceeds to flow downward though loops  32  to heat exchanger section  46 . Heat storage liquid  60  at the top of tank  14  is heated by conduction through upper heat exchanger section  44  of copper tubing in the upper portion of the tank. Heat storage liquid  60  at the lower portion of tank  14  is heated by conduction though lower heat exchanger section  46  of PEX tubing in the lower portion of the tank. Heat exchange liquid  58  then exits heat exchanger  12  though second end  40  and out of the tank though line  56 . 
         [0034]    To extract heat energy from the liquid  60  held in tank  14 , the flow of liquid  58  is reversed. Unheated liquid  58  is flowed though line  56  and into heat exchanger  14  through second end  40 . Liquid  58  flows into second heat exchanger section  46  and absorbs heat from surrounding liquid  60  in the lower portion of tank  12  though the PEX tubing. Liquid  60  flows upward into upper heat exchanger section  44  and absorbs heat from surrounding liquid  60  in the upper portion of tank  12  though the copper. Liquid  58  then exits heat exchanger  12  though first end  34  and line  54 . 
         [0035]    Liquid  58  may be flowed into and out or heat exchanger  12  by use of a conventional water pump. 
         [0036]      FIG. 6  shows a representational view of an installed heat exchange assembly  10 . Heat exchange liquid  58  is heated by, solar, oil or other conventional heating device  74  and flowed by pump  76  though open valve  78  into tube first end  34  of heat exchanger  12  to transmit heat into heat storage liquid  60  as describe above. Heat exchange liquid  58  is then flowed out of heat exchanger  12  though tube second end  40 . 
         [0037]    If the assembly is only storing heat for later use, heat exchange liquid  58  then flows though open valve  80  and back to heating device  74  for reheating. 
         [0038]    To access heat stored in heat storage liquid  60 , valves  78  and  80  are closed and valves  82  and  84  opened. Pump  86  is activated and heat exchange liquid  58  is flowed though heat exchanger  12  to extract heat from heat storage liquid  60  as describe above. Heat exchange liquid  58  flows though a radiator  88  to transmit heat from heat exchange liquid  58  for a desired application. Heat exchange liquid  58  is then flowed to heat exchanger  12  to regain heat. 
         [0039]    When thermal energy in heat storage liquid  60  is depleted, valves  82  and  84  are closed and valves  78  and  80  are opened to allow heater  74  to flow heat into heat storage liquid  60 . 
         [0040]    Reducing the amount of copper in a heat exchanger reduces the cost of the heat exchanger without compromising its ability to function, and the mixture of copper and PEX reduces the weight of the heat exchanger thereby reducing transportation costs simplifying installation procedures. 
         [0041]    A heat exchanger as described having a height of 41 inches and a diameter of 21 inches weighs 43 pounds. A conventional heat exchanger of the same size made entirely of copper tubing weighs 89 pounds. The improved heat exchanger weights 51.7% less then the all copper heat exchanger.