HEAT EXCHANGER FOR WATER HEATER USING HEAT PUMP

Abstract: A water heater comprises a water tank (21) and a heat pump system (20). The heat exchanger (26) includes an inlet manifold (27) connected to the compressor (22), an outlet manifold (28) connected to the evaporator (25) and a plurality of heat exchange tubes (29, 30) connected in parallel between the inlet and outlet manifolds (27, 28). The heat exchange tubes (29, 30) are wound helically around the water tank (21) and are secured to the tank wall (32) in a heat conductive relation with the tank (21). Heat from condensation of the refrigerant liquid circulating through the heat pump system (20) is transferred to water contained in the tank (21). The provision of two or more heat exchange tubes (29, 30) in parallel provides more rapid and efficient heating of water in the tank (21) compared to a single coiled heat exchanger.

Description: HEAT EXCHANGER FOR WATER HEATER USING HEAT PUMP This invention relates to improvements in water heaters, and more particularly to an improved heat exchange arrangement for a water tank. The use of solar energy to supplement water heating systems is becoming increasingly practical with developments in technology. In Australian Patent No. 509901 there is described a solar boosted heat pump system which converts solar energy particularly efficiently and transfers that energy by the use of a refrigerant heat pump. In European Patent No. 0 336 751 there is disclosed a water heater comprising a water tank having a cold water inlet adjacent one end and a hot water outlet adjacent the other end, a heat exchange tube adapted to carry a refrigerant fluid and wound around the water tank by a heat-conductive bonding, an evaporator for absorbing heat energy from ambient conditions and a compressor for circulating refrigerant fluid through the heat exchange tube and to the evaporator. The heat exchange tube is secured externally to the wall of the water tank by a heat-conductive bonding material and the tube is under applied tension when it is secured to the tank to reduce the likelihood of the heat-conductive bonding material breaking during expansion or contraction of the tube and tank during use. The heat exchange arrangement of European Patent No. 0 336 751 operates quite effectively to heat water in the water tank. For example, it is possible to heat water in a 340 litre tank to about 60°C in approximately 4 hours. It is, however, desirable to provide an improved heat exchange arrangement which can heat water in a water tank more efficiently than the arrangement of European Patent No. 0 336 751. According to one aspect of the invention there is provided a water tank having a tank wall formed from a material having heat transfer properties; and a heat pump system including a heat exchanger for transferring heat to the water tank, an evaporator, and a compressor for circulating refrigerant fluid to the evaporator via the heat exchanger, wherein the heat exchanger includes an inlet manifold connected to the compressor, an outlet manifold and a plurality of heat exchange tubes connected in parallel between the inlet and outlet manifolds, said plurality of heat exchange tubes being wrapped around the water tank and secured to the tank wall in heat-conductive relation with the tank wall to transfer heat from condensation of the refrigerant liquid within the tubes through the tank wall to water contained in the tank. Preferably, the heat exchange tubes are secured to the tank wall by a heat-conductive bonding material and are under applied tension when they are secured to the tank wall. This reduces the likelihood of the heat-conductive bonding material breaking during expansion and contraction of the tube and tank during use. Preferably, a pair of heat exchange tubes are provided which are wound helically around the water tank. The tubes preferably extend from a position adjacent a lower end of the tank to a position not exceeding about 80% of the height of the tank. The upper ends of the heat exchange tubes are conveniently connected to the inlet manifold and the lower ends of the heat exchange tubes are connected to the outlet manifold, preferably by one or more riser tubes. Preferably, the water tank and the heat exchange tubes are made from steel. Tliis helps to reduce the likelihood of heat transferring contact being lost due to different co-efficients of expansion of the tank and tube. A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of a solar boosted heat pump system for heating water; Figure 1 A is an enlarged fragmentary sectional elevation of a portion of a water tank of the system showing the attachment of a heat exchange tube thereof; and Figure 2 is a schematic diagram of a heat pump system incorporating a water tank with an improved heat exchanger in accordance with the invention. Referring to Figure 1, there is shown a solar boosted heat pump system as disclosed in European Patent No. EP 0 336 751, the contents of which are incorporated herein by reference. The heat pump system comprises a water tank 1, a heat exchange tube 8 carrying refrigerant wrapped around the water tank 1, a compressor 12, a receiver/filter/drier 13 and a series of solar evaporator plates 15. Each evaporator plate 15 contains a number of refrigerant passages 16 arranged in a serpentine configuration. The water tank 1 and heat exchange tube 8 are enclosed in a housing 10 containing insulating foam 11, and the compressor 12 and receiver/filter/drier 13 are mounted on top of a refrigeration chassis 14 located on top of the tank housing 10. The system has a thermostat control system including a thermostat T. The system also has a Tx expansion valve in the liquid line between the receiver/filter/drier 13 and the evaporator plates 15. The water tank 1 has a cylindrical wall 2, a bottom wall 3, a convex top wall 4, a cold water inlet 5 adjacent the bottom wall 3, incorporating a diffuser or diverter 6 and a hot water outlet 7 adjacent the top wall 4. While the bottom wall 3 is shown to be concave, it may be convex if desired. The heat exchange tube 8 carrying a refrigerant B, such as refrigerant R12, is wrapped around the external surface of the tank wall 2. The tube 8 is preferably flattened as shown in figure 1A, so that it is D-shaped in cross-section, and the flattened portion is heat conductively bonded to the surface of the wall 2 by means of a thin film of solder 9 or the like between the outer surface of the wall 2 of the tank 1 and the flat face of the tube 8. To ensure the best possible contact during soldering and during expansion and contraction of the tube and tank wall 2 in use, the tube 8 is wound around the tank 1 under an applied tension of the order of 1,286 N, and is secured to the tank 1 while under tension. This may be achieved in the manner described in European Patent No. EP 0336 751. It will be appreciated that by attaching the refrigerant carrying tube 8 to the external surface of the tank 1, a double- wall effect is automatically achieved and the protection required by the relevant water authorities, which stipulates a double walled tube where a refrigerant carrying tube is associated with water, is satisfied. Both the tube 8 and the tank 1 are preferably made from a similar material, or at lest from materials having similar co-efficients of thermal expansion. In the present case, the tube 8 and the tank 1 are fabricated from steel or stainless steel. For example, the tube may be Bundyweld steel tubing while the tank 1 may be made from mild steel. As mentioned above, the thermal bond between the tube 8 and the tank 1 is maintained by the winding of the tube 8 around the tank 1 under tension. Where materials having slightly different coefficients of thermal expansion are used, the different rates of expansion and contraction of the materials may be compensated for by increasing the winding tension of the tube 8. In any event, the winding of the tube under tension ensures that the thermal bond is maintained notwithstanding the flexing of the materials caused by expansion and contraction in use. In use, the compressor 12 supplies refrigerant gas under pressure to the heat exchange tube 8 and as the refrigerant B condenses in the tube 8, heat is transferred through the tank wall 2 to the water contained in the tank 1. The condensed refrigerant then passes through the receiver/filter/drier 13 and Tx expansion valve to the evaporator plates 15 which are preferably mounted in a position exposed to the sun so that the refrigerant in the passages 16 of the plates 15 can absorb heat from the ambient conditions before the refrigerant is returned via supply line 12G to the compressor 12. While the evaporator plates are shown in the above embodiment as being mounted in a position exposed to the sun, the plates may be mounted on the outside of the housing 10, in a wrap around configuration in areas where the ambient temperature is high or where the tank is able to be mounted on a roof or in another position which is at least partly exposed to the sun. In such a situation, the heat pump operates at least partly as an air source heat pump. Referring to Figure 2, there is shown a heat pump system 20 for a water tank 21 similar to that of Figure 1 but incorporating an improved heat exchange arrangement. The heat pump system 20 includes a compressor 22, a receiver/filter/drier 23, a Tx expansion valve 24, an evaporator 25 and a heat exchanger 26 connected between the compressor 22 and the receiver/filter/drier 23. The evaporator 25 preferably comprises at least one solar collector panel 15 as described with reference to Figure 1. The number of solar panels 15 may vary depending upon the size of the compressor and the climatic region for which the heat pump system is designed. The heat exchanger 26 comprises an inlet manifold 27 connected to the compressor 22, an outlet manifold 28 connected to the receiver/filter/drier 23 and a pair of heat exchange tubes 29 and 30 wrapped around the water tank 21 and connected in parallel between the inlet and outlet manifolds 27 and 28. The water tank 21 is similar to the tank of Figure 1 in that it has a cylindrical wall 32, a bottom wall 33, a convex top wall 34, a cold water inlet 35 adjacent the bottom wall 33 incorporating a diffuser or diverter 36 and a hot water outlet 37 adjacent the top wall 34. The heat exchange tubes 29 and 30 are preferably flattened so as to be D-shaped in cross-section similar to the tube 8 of Figure 1A. The flattened portions of the heat exchange tubes 29 and 30 are secured to the external surface of the cylindrical wall 32 of the water tank 21 by a heat-conductive bonding material, such as a thin film of solder. To ensure the best possible contact during soldering and during expansion and contraction of the tube and tank wall 2 in use, the tube 8 is wound around the tank 1 under an applied tension, preferably of the order of 1,286 N, and is secured to the tank 1 while under tension. This may be achieved in the manner described in European Patent No. EP 0336 751. In a preferred embodiment, the heat exchange tubes 29 and 30 are formed from Bundyweld steel tubing having an internal diameter of about 12mm, and the tubes 29 and 30 are wound helically around the tank 21 from a position adjacent the bottom of the tank to a position not exceeding about 80% of the height of the tank. The tank 21 is preferably fabricated from a material having a similar co-efficient of thermal expansion as the tubes 29 and 30, such as mild steel. Where materials having slightly different co-efficients of thermal expansion are used, the different rates of expansion and contraction of the materials may be compensated for by increasing the winding tension of the tube 8. In any event, the winding of the tube under tension ensures that the thermal bond is maintained notwithstanding the flexing of the materials caused by expansion and contraction in use. The upper ends of the heat exchange tubes 29 and 30 are connected directly to the inlet manifold 27, whereas the lower ends of the heat exchange tubes 29 and 30 are connected to the outlet manifold 28 by respective riser tubes 39 and 40. The riser tubes 39, 40 may be conveniently formed from a 3/8 inch (9.5 mm) diameter copper tube. The inlet and outlet manifolds 27 and 28 are preferably also formed from copper, the inlet manifold 27 having an upper tube section 41 connected to the compressor 22 of about 1/2 inch (12.5 mm) diameter and the outlet manifold 28 having an upper tube section 42 connected to the receiver/filter/drier 23 of about 3/8 inch (9.5 mm) diameter. It will, however, be appreciated that the sizes of the heat exchange tubes 29, 30 , the riser tubes 39, 40 and the upper tube sections 41, 42 of the manifolds 27, 28 and the materials from which they are made may vary from different applications. In a particularly preferred embodiment, for a 340 Litre water tank, the total length of each of the heat exchange tubes 29, 30 is about 32 metres, and each of the tubes 29, 30 is wound helically around the water tank 21 for about 18 turns with the pitch of each winding being about 54 mm. It has been found that a heat exchanger arrangement with at least two heat exchange tubes connected in parallel between the inlet and outlet manifolds provides more rapid and efficient heating of water in a water tank as compared with the single tube arrangement of Figure 1. Two parallel heat exchange tubes positioned very close to each other effectively increase the tank surface temperature and maintain an evenly constant temperature across the nearest tubes. This leads to increased heat transfer between the tank and the water. For instance, for a 340 litre water tank, the arrangement of Figure 2 with a pair of tubes 29, 30 as described above can heat up water from ambient temperature to about 60°C in approximately 2 hours compared with approximately 4 hours for a single tube arrangement such as that of Figure 1. There is a limit to the size of the compressor which can be installed with the heat transfer arrangement of Figure 1, and thus a limit to the recovery rate achievable. A design for a higher recovery capacity is often required for larger domestic or commercial applications. For R22 refrigerant, the limit using the previous arrangement of Figure 1 is about 1300W nominal compressor size for a single 12mm tube wrapped around the tank. Having plural coils wrapped around the tank allows a larger gas flow rate enabling a larger compressor to be installed to achieve the high recovery rate required for larger domestic or commercial installations. It will be appreciated that various modifications and/or alterations may be made to the embodiment of the invention described above without departing from the scope and spirit of the present invention. For instance, it is envisaged that more than two heat exchange tubes could be connected in parallel between the inlet and outlet manifolds and wound around the water tank.

Claims: CLAIMS: 1. A water heater comprising: a water tank having a tank wall formed form a material having heat transfer properties; and a heat pump system including a heat exchanger for transferring heat to the water tank, an evaporator, and a compressor for circulating refrigerant fluid to the evaporator via the heat exchanger, wherein the heat exchanger includes an inlet manifold connected to the compressor, an outlet manifold and a plurality of heat exchange tubes connected in parallel between the inlet and outlet manifolds, said plurality of heat exchange tubes being wrapped around the water tank and secured to the tank wall in heat-conductive relation with the tank wall to transfer heat from condensation of the refrigerant liquid within the tubes through the tank wall to water contained in the tank. 2. A water heater according to claim 1 wherein the heat exchange tubes are secured to the tank wall by a heat-conductive bonding material and are under applied tension when they are secured to the tank wall. 3. A water heater according to claim 2 wherein said plurality of heat exchange tubes have a flattened wall portion secured to the tank wall by said heat-conductive bonding material. 4. A water heater according to claim 3 wherein the heat exchange tubes are substantially D-shaped in cross-section. 5. A water heater according to any one of the preceding claims wherein the heat exchange tubes are wound helically around the water tank. 6. A water heater according to claim 5 wherein the heat exchange tubes extend from a position adjacent a lower end of the tank to a position not exceeding about 80% of the length of the tank. 7. A water heater according to claim 5 or claim 6 wherein a pair of heat exchange tubes are wound helically around the water tank. 8. A water heater according to claim 7 wherein the pitch of the winding for each of the heat exchange tubes is about 55 mm. 9. A water heater according to claim 8 wherein the diameter of the heat exchange tubes is about 12mm. 10. A water heater according to any one of claims 5 to 9 wherein the upper ends of the heat exchange tubes are connected to the inlet manifold and the lower ends of the heat exchange tubes are connected to the outlet manifold by one or more riser tubes. 11. A water heater according to any one of the preceding claims wherein the water tank and the heat exchanger tubes are made from materials having the same or similar co-efficients of thermal expansion. 12. A water heater according to claim 10 wherein the water tank and the heat exchanger tubes are made from steel. 13. A water heater according to any one of the preceding claims wherein an expansion valve is connected between the outlet manifold and the evaporator. 14. A water heater according to any one of the preceding claims wherein the heat pump system includes a receiver/filter/drier. 15. A water heater according to any one of the preceding claims wherein the evaporator comprises at least one solar collector panel having passages through which the refrigerant fluid is circulated in heat-conductive relationship with the panel.

Inventors: ISLAM RAFIQUL, SIDDONS MICHAEL JAMES, ISLAM, RAFIQUL, SIDDONS, MICHAEL, JAMES
Applicants: ISLAM RAFIQUL, QUANTUM ENERGY SYSTEMS INTERNA, SIDDONS MICHAEL JAMES, ISLAM, RAFIQUL, QUANTUM ENERGY SYSTEMS (INTERNATIONAL) PTY. LTD., SIDDONS, MICHAEL, JAMES
Application year: 1998
Main classification: F24H9
IPC codes: F24H4, F25B30, F25B39, F28D1