Abstract:
This invention relates to a water heater comprising a water tank ( 1 ) having a wall ( 2 ) formed from material having heat transfer properties and a cold water inlet ( 5 ) adjacent one end of the tank. A tube ( 8 ), adapted to carry a refrigerant fluid, is secured externally about the tank wall ( 2 ). Heat-conductive material is provided along the length of the tube, with the tube and the heat-conductive material in heat-conductive contact with the external surface of the wall of the tank to transfer heat from condensation of the refrigerant fluid in the tube through the wall to the water contained in the tank ( 1 ). An evaporator ( 15 ) is positioned so as to be exposed to ambient conditions for absorbing heat energy from the ambient conditions. The evaporator is provided with a passage for carrying the refrigerant fluid whereby the fluid may be heated by the ambient conditions. A compressor ( 12 ) is connected to the passage and to the tube ( 8 ) so as to circulate the refrigerant fluid through the tube and to the evaporator ( 15 ).

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to improvements in the design and manufacture of water heaters, and more particularly the design and manufacture of heat pump water heaters.  
         BACKGROUND OF THE INVENTION  
         [0002]    Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.  
           [0003]    Australian Patent No. 603510 discloses a water heater and a method of making a heat exchanger for a water heater wherein a tube adapted to carry a refrigerant fluid is wound around a water tank and is bonded to the wall of the water tank by a heat conductive bonding material. The tube is wound around the tank under tension so as to reduce the likelihood of the heat conductive bonding material breaking during the expansion and contraction of the tube and tank during use. The tank is baked in an oven to enable the bonding material to harden. The bonding material performs two roles; namely to bond the coil onto the tank and to secondly improve the heat transfer performance between the tube and the tank.  
           [0004]    However, a problem associated with this method is that it requires the use of an expensive, specially formulated bonding material that requires baking in an oven or kiln at an elevated temperature of the order of 250° C. for a time period of approximately two hours so as to enable the paste to harden. The need to use an oven to harden the paste also adds to the time and cost of manufacture.  
           [0005]    It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.  
         SUMMARY OF THE INVENTION  
         [0006]    To this end, a first aspect of the present invention provides a water heater comprising:  
           [0007]    a water tank having a wall formed from material having heat transfer properties; a cold water inset adjacent one end of the tank;  
           [0008]    a tube adapted to carry a refrigerant fluid secured externally about said tank wall by mechanical means;  
           [0009]    a non-bonding, heat-conductive material applied in a paste form along the length of said tube; said heat-conductive material injected along the length of the tube at the interface between the tube and said tank wall;  
           [0010]    said tube and heat-conductive material in heat-conductive contact said wall of said tank to transfer heat from condensation of said refrigerant fluid in said tube through said wall to the water contained in the tank;  
           [0011]    an evaporator positioned to be exposed to ambient conditions for absorbing heat energy from said ambient conditions, and having a passage for carrying the refrigerant fluid whereby said fluid may be heated by said ambient conditions; and  
           [0012]    a compressor connected to said passage and to said tube to circulate refrigerant fluid through said tube and to said evaporator.  
           [0013]    Unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.  
           [0014]    Preferably, the tube is wound around said tank.  
           [0015]    Preferably, the heat conductive material is substantially coextensive with the length of the tube.  
           [0016]    Preferably, the tube is formed from copper or a copper-based alloy.  
           [0017]    Preferably, the tube is mechanically fastened at one or more locations to the wall of the tank.  
           [0018]    In one preferred embodiment two or more tubes are wound around the tank and carry the refrigerant fluid.  
           [0019]    A second aspect of the present invention provides a method of making a water heater comprising the steps of: attaching one end of a tube to an external surface of a water tank, winding the tube around the external surface of said tank, and injecting a non-bonding heat conductive paste along the length of said tube at the interface between the tube and said tank wall.  
           [0020]    Preferably, the tube is wound around the external surface of the tank. More preferably, the tube is wound around the external surface of the tank under a predetermined tension.  
           [0021]    In one preferred embodiment, the pitch of the winding of the tube around the tank varies. More particularly, the windings of the tube are mote closely spaced at the bottom of the tank and the spacing between adjacent windings progressively increases towards the top of the tank.  
           [0022]    Advantageously, in comparison to the prior art water heater disclosed in Australian Patent No. 603510, the present invention alleviates the need for the curing of a bonding material at an elevated temperature. In comparison to the water heater disclosed in Australian Patent No. 603510, the heat transfer material acts to enhance the heat transfer between the tube and the tank and, unlike the bonding material disclosed in the prior art, does not act to retain the tube on the wall of the water tank. According to the present invention the tube is mechanically retained on the wall of the tank 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:  
         [0024]    [0024]FIG. 1 is a fragmentary sectional elevation of a water tank incorporating a heat exchanger suitable for use with the water heater embodying the invention;  
         [0025]    [0025]FIG. 1A is an enlarged fragmentary sectional elevation of portion of the tank showing the attachment of the tube thereto;  
         [0026]    [0026]FIG. 2A is a schematic elevation of mechanism for applying the tube to the tank;  
         [0027]    [0027]FIG. 2B is an enlarged schematic elevation of the tube deforming rollers shown in FIG. 2A;  
         [0028]    [0028]FIG. 3 is a schematic diagram showing the layout of a solar boosted heat pump water heating system embodying the invention;  
         [0029]    [0029]FIG. 4 is a schematic diagram showing an arrangement for cooling the compressor used in the system of FIG. 3;  
         [0030]    [0030]FIG. 5 is a schematic diagram showing one layout of the refrigerant passages in the solar collector used in the system of FIG. 3; and  
         [0031]    [0031]FIG. 6 is a sectional side elevation of one of the solar evaporator/collectors used in the system of FIG. 3. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0032]    Referring to the drawings, 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. A tube  8  carrying a refrigerant, such as refrigerant R 22 , is wrapped around the external surface of the tank wall  2 . The tube  8  is manufactured from copper or a copper-based alloy. The tube  8  is preferably flattened as shown in FIG. 1A, so that it is substantially D-shaped in cross-section, with the flattened face of the tube located against the wall of the tank. A heat sealing paste  9  is located between the adjacent surfaces of the tube  8  and the wall of the tank so as to provide heat transfer therebetween. Preferably the heat sealing paste is located along the entire length of the interface between the tube  8  and the tank wall  2 . However, it is to be appreciated that the heat sealing paste may be intermittently located along the interface, although typically this would result in a decrease in-heat transfer efficiency. To ensure the best possible contact during expansion and contraction of the tube and tank wall  2  in use, the tube  8  is wound around the tank  1  under tension and is mechanically secured to the tank  1  while under tension. This may be achieved in the manner shown schematically in FIG. 2A of the drawings.  
         [0033]    Whilst it is preferable for the tube  8  to be helically wound around the external surface of the tank wall  2 , it should be noted that alternative placings of the tube  8  on the external wall of the tank may be possible. For example, the tube may be formed in a concertina manner and placed on the external wall of the tank so that the tube extends up and down the external wall of the tank. However, it is noted that such a configuration may add additional complexity to the manufacture of the system and hence be less desirable than a helical tube winding.  
         [0034]    In accordance with the present invention, a heat transfer material  9  in the form of a paste is inserted or injected between the tube  8  and the tank wall  2 . Preferably the paste does not need to be cured, or if curing is required it may be cured under ambient conditions hence alleviating the need for curing at elevated temperatures in an oven or kiln. A suitable heat transfer paste is “HTSP Silicone Heat Transfer Compound Plus”, marketed in Australia by Electrolube. This is a silicone oil-based heat transfer paste with a thermal conductivity of approximately 3.0 W/mK. Other suitable kinds of heat transfer paste, such as “Bostik  1128  Heat Transfer Sealer” marketed in Australia by Bostik, may also be used. There are several benefits associated with the use of a heat transfer sealer including cost savings for the product, as well as eliminating the need for a curing step which results in a significant reduction in production time. Furthermore, preliminary testing indicates that the heat transfer performance of the system is improved.  
         [0035]    As shown in FIG. 2A, the tank  1  is supported on a rotating table  20 , which is rotatably driven by a motor  21  through a gear box  22  and chain drive  23 . A suitable length of tube  8  is fed by feed rollers  24  and deformed into the D-shape shown in FIG. 1A by deforming rollers  25 . The feed rollers  24  and the deforming rollers  25  form part of an assembly including a nut engaging a feed screw  26  which is rotatably driven by a motor  27  to move the assembly up the tank  1  at the required rate relative to rotation of the table  20  so that the tube  8  is wound around the tank with the required spacing between adjacent turns.  
         [0036]    As shown in greater detail in FIG. 2B, the deforming rollers  25  include a knurled roller  28  and a grooved support roller  29 . The rollers  28  and  29  are driven by gears  30 ,  31  which are driven at the same speed as the pinch rollers  24  to ensure that the tube  8  is fed and deformed at the same rate. The pinch rollers  24  and the rollers  28  and  29  are restrained against free rotation by means of brake pads, such as those shown in FIG. 2B at  32 . The tension applied to the tube may be adjusted by clamping the brake pads  32  into engagement with the roller  29  and gear  30  under the influence of a spring  33  which is adjustably compressed by means of a tension nut  34 .  
         [0037]    In the present embodiment, the part of the tank  1  to which the tube  8  is to be fixed in cleaned and a copper flash is applied in a known manner. The tube  8  is then fixed by spot welding to the bottom of the tank wall  2  and the tube  8  is then wound around the tank using the mechanism shown in FIG. 2A of the drawings. Once the tube has been completely wound over the length of the tank, the upper end thereof is fixed to the wall  2  of the tank  1  by spot welding to maintain the tube  8  under tension.  
         [0038]    The tube  8  extends from the position adjacent the bottom wall  3  to an appropriate height on the tank  1  and defines a heat exchange surface S on the wall  2 . The lowermost turn  8 A of tube  8  is located under the cold water inlet  5 , which is usually cold, and this causes sub-cooling of the refrigerant whereby it is rendered stable enough for transportation. The tube  8  is connected to a solar boosted heat pump (FIG. 3 as described below) of the general type described in Australian Patent No. 509901, although it will be noted that modifications to that system have been made in the system described further below. Other forms of heat pump may also be used and the heat exchange medium carried by the tube  8  may be varied as desired.  
         [0039]    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 least from materials having similar coefficients of thermal expansion.  
         [0040]    In the present case, the tank  1  is fabricated from steel or stainless steel whilst the tube  8  is made from copper or copper-based alloy. Where materials having differing 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 the manner described above. 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.  
         [0041]    Furthermore, the described heat exchange arrangement is eminently suitable for use with a fluid A, such as water which is likely to contain precipitatable contaminants since a major portion of the heat exchanger surface S is vertical whereby the accumulation of precipitates on the heat exchanger surface is discouraged. Still further, the heat exchanger surface S is sufficiently enlarged to allow for the application of coatings, such as vitreous enamel, while still maintaining the product of heat transfer coefficient and surface area at an efficient level and minimising the temperature difference between the fluid to be heated A and the heat exchanger surface S.  
         [0042]    The area of the heat exchange surface S is selected so as to give the best compromise between the following conflicting requirements:  
         [0043]    (a) The requirement that the heat exchanger surface be substantially vertical, downwardly facing or downwardly sloping as discussed above;  
         [0044]    (b) The requirement that the heat flux density within the fluid A to be heated should be low enough to prevent destabilisation of unstable components within the fluid whereby the maximum temperature difference between the heat exchanger surface S and the fluid A is limited so that fluids in contact with the surface are not locally heated to a point where the fluid A reaches some critical destabilisation temperature;  
         [0045]    (c) The requirement that the heat exchanger surface S should be as large and yet as compact as possible.  
         [0046]    Requirement (c) minimises the irreversibility of the system of which the heat exchanger is a part which implies that the temperature at which heat is being transferred between the refrigerant B and the fluid A to be heated is as close as possible to the coldest sink temperature of fluid A. By making the heat exchanger compact the passive volume of the vessel is increased. This volume acts as a stored volume of fluid ready for delivery to the end user of the fluid A.  
         [0047]    To enable the heat exchanger to be both compact and efficient, the spacing between the turns of tubing  8  and the flow rate of the refrigerant or other heat transfer fluid B must be optimised so as to give adequate fluid side surface area S, heat transfer medium side surface area T, heat transfer coefficients, heat exchanger fin efficiency and adequate passive storage if required. The requirements of this optimisation is always a compromise and depends largely on the size of the system and the fluids in use.  
         [0048]    In general terms the design procedure for the above preferred embodiment can be summarised as follows:  
         [0049]    Considering the passive storage volume requirement and the stratification of fluid A, the surface area S is made as large and yet as compact as possible until the benefits outlined above are negated by the increase in heat transfer temperature difference across surface S caused by the reduction in area available to transmit the desired total heat flux.  
         [0050]    The spacing and sizing of the tubes on the outside face of the heat exchanger surface S is then determined by established engineering design procedures involving consideration of internal tube heat transfer, tube to wall bond conduction and fin efficiency if applicable.  
         [0051]    Reconsideration of the surface area S determined may then be necessary if the tube spacing computations reveal that further optimisation of the design is possible through some compromise with surface area S.  
         [0052]    To avoid degradation of the stored volume of the fluid through mixing with fluid which has yet to be fully heated or cooled, stratification of fluids which are at different temperatures should be promoted as far as possible. To this end the entry and exit conduits are arranged so as not to promote mixing by stirring up the fluid A. Also the cold conduit  5  is positioned at the bottom of the tank and the hot conduit  7  at the top of the tank To achieve a reduction in stirring, it will be noted that the axis of the entry conduit  5  is perpendicular to the axis of stratification. In addition a downwardly facing diffuser-deflector  6  is used at the cold water inlet  5  while an upwardly facing diffuser-deflector  6   a  is used at the hot water outlet  7  to further reduce mixing and stirring.  
         [0053]    The heat exchanger should preferably be arranged so that the counterflow principle is embodied as this further improves heat transfer. In the embodiment shown, it will be noted that the refrigerant B flows from the top of the coiled tube  8  to the bottom.  
         [0054]    A preferred solar boosted heat pump water heating system is shown schematically in FIG. 3 of the drawings and will be seen to include a heat exchange system embodying the invention in which the tank  1  and refrigerant carrying tube  8  is enclosed in a housing  10  containing insulating foam  11 . For convenience, the compressor  12  and receiver/filter/drier  13  of the heat pump system are mounted on a refrigeration chassis  14  located on top of the tank housing  10 . This arrangement avoids the need for the housing  10  to be supported at an elevated position to allow location of the compressor and receiver under the tank housing as is usual, thereby reducing construction costs.  
         [0055]    The compressor  12  is preferably a rotary compressor, although other forms of refrigerant compressors may be used without materially detracting from the efficiency of the system. A rotary compressor is preferred because of its relatively smooth quieter operation. Furthermore, a rotary compressor is able to accept slugs of liquid on the suction side of the compressor whereas other types of compressors have greater difficulty accepting such slugs. These slugs may occur in a solar boosted heat pump due to the rapid variations in temperature which may be experienced as a result of changing weather conditions.  
         [0056]    To reduce heat losses from the system to a minimum, the compressor  12  is preferably externally insulated. To compensate for the heat build up partially caused by such external insulation, the compressor is cooled by bleeding refrigerant from the outlet of the condenser or receiver directly into the inlet suction line or into the “suction side” of the cylinder through a bypass line which is preferably controlled by a control valve, capillary tubing or fixed orifice (not shown). One arrangement for achieving this is shown schematically in FIG. 4 of the drawings and will be seen to comprise a cylinder  12 A enclosed within an insulating casing  12 B and containing a rolling piston  12 C and vane  12 D. A liquid injection tube  12 E is connected from the refrigerant liquid line to the suction line  12 G into the compressor  12 . By this arrangement, heat which would normally have been wasted to the air is passed to the water in the tank through the condenser. It has been found that the arrangement shown in FIG. 4 works more reliably than injection directly into the cylinder as used in commercially available compressors.  
         [0057]    The system includes a thermostat control system including a thermostat T.  
         [0058]    More complex thermostat variations are possible, including:  
         [0059]    a) a variable or dual thermostat setting which depends on the level of sunlight, or  
         [0060]    b) sensing evaporation temperature when the unit is running and using this as an indication of the potential performance which in turn increases or decreases the thermostat set point.  
         [0061]    The overall aim of the above described systems is to bias the system towards running during the day by making it raise the water to a higher temperature during daylight hours than during the night. Further sophistications are possible by making the set point of the thermostat a function of solar radiation and ambient air temperature although control “intelligence” is required in this case so that adequate water temperatures are reached during the winter. Similarly, biasing the system to operate primarily during a low tariff (off-peak) period is possible.  
         [0062]    The compressor  12  and the receiver  13  are connected to a series of solar evaporator plates  15  which are located in a position exposed to the sun. Each evaporator plate contains a number of refrigerant passages  16  which are preferably arranged in the configuration shown in FIG. 5 of the drawings. Each evaporator plate  15  is made from two sheets of metal which are bonded together, except in the regions of the passages  16 , by the so-called Roll-Bond™ process, which is well known in the art. Since the evaporator plates are formed from thin sheet metal, each evaporator plate is supported in an outwardly curved profile as shown in the sectional elevation of FIG. 6 of the drawings. The outwardly curved profile is maintained by the positioning of a moulded insulating foam former  17  behind each plate  15 , the assembly being supported by two bearers as shown in FIGS. 3 and 6 of the drawings. Each evaporator plate  15  is further strengthened by the formation of an angle section  18  along each longitudinal edge of the plate  15 . The evaporator plate arrangement described above has been found to perform well in wind tests conducted to assess its ability to withstand wind forces of the type encountered when the evaporator plate is mounted on the roof of a dwelling.  
         [0063]    As will be appreciated from FIG. 5 of the drawings, each evaporator plate is formed with three separate parallel refrigerant passages  16  connected at either end by a manifold  19  to which the refrigerant lines (not shown) are connected. The plates  15  are, as shown in FIG. 3 of the drawings, connected in series so that the outlet manifold of the first plate is connected to the inlet manifold of the second plate and so on. As will be further seen from FIG. 5 of the drawings, the first passage connected to a manifold at one end of the plate is the last passage connected to the manifold at the other end of the plate to assist in equalising the flow of refrigerant in the parallel passage  16 . The cross-sectional area of the manifold is reduced  2  after each junction to further assist in flow equalisation. The arrangement mininises the cross-sectional area of the manifold and its junctions for a given design pressure drop across the manifold. This results in an improved burst pressure for a given refrigerant pressure drop compared with the performance of the typical “waffle” type distributor used in other “Roll Bond” evaporators.  
         [0064]    As a result of these features the evaporator plate can be used with high evaporation pressure refrigerants such as R 22  without incurring a high pressure drop across the evaporator. Furthermore the evaporator may be mounted at downward sloping angles, as is usually desired, as flow equalisation between passages  16  is relatively unaffected by the forces of gravity which usually cause the lowermost passage to be favoured. Refrigerant fluid is delivered to the top of each plate  15  rather than to the bottom, as is usually the case, and this allows the use of less refrigerant since the plates do not need to be flooded as is the case with bottom entry. An additional advantage is that positive oil return is achieved since the oil does not tend to accumulate at the bottom of the plates as it does with bottom entry. Top entry of the refrigerant in combination with high velocity refrigerant circulation causes annular flow of wet fluid in the passages  16  which improves the heat transfer from the plates to the fluid. I this mode of operation refrigerant gas flows within the wet fluid annulus.  
         [0065]    A liquid trap is provided prior to returning the refrigerant to the compressor suction to prevent any accumulation of liquid in the plates  15  while the system is off from flowing by gravity down into the compressor suction. The trap must be sized such that oil is carried over with the refrigerant gas during operation.  
         [0066]    It will be noted from FIG. 3 of the drawings that the TX valve in the liquid line is located inside the refrigeration chassis  14  rather than at the evaporator plates  15  as is usually the case. Although compensation must be allowed for the location of the TX valve in this position, it has been found that valve performs in a superior manner in this location and manufacture of the system is simplified by the location of the valve in this position. The TX valve is biased to give an appropriate superheat by setting to an appropriate superheat setting.  
         [0067]    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 , as shown in broken outline in FIG. 3, 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.  
         [0068]    Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.