Heat Exchange Assembly and Method
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.
The invention relates to heat exchange assemblies, particularly assemblies used to transfer heat to and from a water tank.
BACKGROUND OF THE INVENTIONHeat exchange assemblies are commonly used to heat water and then extract heat from the water.
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.
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.
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.
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.
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 INVENTIONThe 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.
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.
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.
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.
The mixture of copper and PEX tubing reduces the weight of the heat exchanger, reducing the cost of transportation and simplifying the installation procedure.
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.
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.
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.
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.
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.
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.
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.
Upper coil section 44 extends generally downward from exchanger top 28 to connector 48. As shown in
Lower heat exchanger section 46 extends generally downward from connector 48 to exchanger bottom 30.
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.
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.
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.
The operation of heat exchanger assembly 10 will now be described.
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.
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.
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.
Liquid 58 may be flowed into and out or heat exchanger 12 by use of a conventional water pump.
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.
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.
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.
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.
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.
Claims
1. A heat exchange assembly adapted to flow heat into a heat storage liquid and to extract heat from the heat storage liquid, the heat exchange assembly comprising a tank, a heat storage liquid in the tank; a heat exchanger in the tank, the heat exchanger comprising a vertically coiled tube having a plurality of loops spaced between the top and bottom of the heat exchanger, the tube having a first end connected to a loop at the top of the heat exchanger and a second end connected to a loop at the bottom of the heat exchanger, said ends in flow communication through the coiled length of the tube, the tube loops at the top of the heat exchanger formed from a material having a first coefficient of thermal conductivity and comprising an upper heat exchanger section, the tube loops at the bottom of the heat exchanger formed from a material having a second coefficient of thermal conductivity less than the first coefficient of thermal conductivity and comprising a lower heat exchanger section; and a heat exchange liquid in the tube, wherein the flow of heat between the heat exchange liquid and the heat storage liquid though the upper heat exchanger section of the tube is more efficient than the flow of heat between the heat exchange liquid and the heat storage liquid through the lower heat exchanger section.
2. The heat exchange assembly of claim 1 wherein said upper heat exchanger section is comprised of a metal and said lower heat exchanger section is comprised of a non-metal.
3. The heat exchange assembly of claim 2 wherein said metal is copper.
4. The heat exchange assembly of claim 2 wherein said non-metal is a plastic.
5. The heat exchange assembly of claim 4 wherein said non-metal is cross-linked high-density polyethylene.
6. The heat exchange assembly of claim 2 wherein said heat storage liquid is water.
7. The heat exchange assembly of claim 6 wherein said heat exchange liquid is water.
8. The heat exchange assembly of claim 7 including a tube coupling located between the upper and lower heat exchange sections, said coupling joining said sections to permit flow through the tube.
9. The heat exchange assembly of claim 2 wherein said upper heat exchanger section comprises approximately 20 percent of the vertical height of said coiled tube and said lower heat exchanger section comprises approximately 80 percent of the vertical height of said coiled tube.
10. The heat exchange assembly of claim 2 wherein said coils are circular.
11. The heat exchange assembly of claim 2 wherein said heat exchanger includes an output line extending from the bottom of the coiled tube through the coiled tube and to the second tube end and said second tube end is located proximate the top of the said heat exchanger
12. The heat exchange assembly of claim 2 comprising a frame, said frame comprising a plurality of vertically extending members attached to the coiled tube.
13. The heat exchange assembly of claim 2 comprising a frame proximate said lower heat exchanger section, said frame comprising a plurality of vertically extending members located inside or outside said lower heat exchanger section.
14. The method of heating a body of water using a vertical tube-type heat exchanger located in the body of water where the heat exchanger has an upper tube section having a first coefficient of thermal conductivity and a lower tube section having a second coefficient of thermal conductivity less than said first coefficient of thermal conductivity, comprising the steps of:
- A) flowing a hot liquid through the length of the heat exchanger;
- B) heating the water at the top of the body of water by flowing heat from the hot liquid in the upper tube section through the upper tube section and into the water at the top of the body of water at a first efficiency determined by the first coefficient of thermal conduction; and
- C) heating the water at the bottom of the body of water by flowing heat from the hot liquid in the upper tube section through the lower tube section and into the water at the bottom of the body of water at a second efficiency determined by the second coefficient of thermal conduction, and less than said first efficiency.
17. The method of claim 16 where in the heat exchanger includes a plurality of vertically oriented tubular coils and including the step of:
- D) flowing the hot liquid through tubular coils in the upper and lower tube sections.
18. The method of claim 17 including the step of:
- E) providing an upper tube section made of copper; and
- F) providing a lower tube section made of cross-linked high-density polyethylene.
Type: Application
Filed: Jul 2, 2008
Publication Date: Dec 23, 2010
Applicant: STSS CO., INC. (Mechanicsburg, PA)
Inventor: Sven L. Tjernagel (Mechanicsburg, PA)
Application Number: 12/665,745
International Classification: F28D 7/02 (20060101);
