Abstract:
A water jacket assembly for an instantaneous gas fired water heater, the assembly including pressed profiled plates made of copper or copper coated steel, one plate being the inverted image of the other, said plates being placed together in pairs, the pairs of plates being arranged in a parallel array to form a heat exchanger, the heat exchanger being bordered by a water jacket comprising overlapping side and end panels of copper or copper coated steel attached to the array of plates, the assembly being fused together to define a combustion chamber with discrete combusted gases and water passages within said assembly.

Description:
FIELD OF THE INVENTION 
     This invention relates to a heat exchanger or water jacket assembly suitable for use with water heaters and in particular to instantaneous gas fired water heaters to provide hot water on demand, and a method of manufacturing the same. 
     PRIOR ART 
     Approximately 30% of the world&#39;s hot water heaters manufactured are of the “storage type”, namely a water heater whereby a tank of water is heated by electricity, gas or oil fired burners. This type of water heater can be inefficient, bulky and constantly suffers from the escape of heat to the surroundings. A problem with storage water heaters is the limitation in volume of hot water they can provide at any one time. 
     Approximately 70% of the world&#39;s water heaters manufactured are of the “instantaneous type” where hot water is provided on demand by use of means to instantaneously heat the water as it flows through the heater. This type of water heater has been very pressure dependent with limitations on the volume of hot water it can provide on an ongoing basis. Also problems are commonly derived from difficulties in constructing and assembling cost effective, long life heat exchangers and water jackets which achieve minimum heat loss to the surroundings and maximum fluid-to-gas heat exchange surface areas. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, there is provided a water jacket assembly for an instantaneous gas fired water heater, the assembly including pressed profiled plates made of copper or copper coated steel, one plate being the inverted image of the other, said plates being placed together in pairs, the pairs of plates being arranged in a parallel array to form a heat exchanger, the heat exchanger being bordered by a water jacket comprising overlapping side and end panels of copper or copper coated steel attached to the array of plates, the assembly being fused together to define a combustion chamber with discrete combusted gases and water passages within said assembly. 
     It is preferable that the profiled plates of the heat. exchanger are adapted to cause turbulent flow of water through the water passages, and turbulent flow of combusted gases past the exterior, the exterior also providing escape routes for condensate that in use collects on the external surfaces of the heat exchanger. 
     It is preferable that the water jacket has a cold water inlet and a hot water outlet, at least one gas burner being positioned within the combustion chamber whereby cold water flows through the assembly to exit as hot water. 
     Preferably, the at least one gas burner is positioned above the heat exchanger and the heater includes a fan that mixes gas with air and forces the gas/air mixture to the burner and, as combusted gases past the heat exchanger. 
     According to a further aspect of the present invention there is provided a method of manufacturing a water jacket assembly comprising pressing profiled heat exchanger plates, side panels and end panels out of copper or copper coated steel, placing two plates together, one being the inverted image of the other to form a pair of abutting plates, placing a plurality of pairs of heat exchanger plates together to form a sandwich, attaching the side panels to the sandwich and placing the end plates on each corner so that the side and end panels overlap, holding the assembly with a jig, and placing the assembly in an oven for a predetermined time to fuse the copper surfaces together to provide an integral assembly having a combustion chamber and discrete combusted gases and water passageways within said assembly. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     An embodiment, incorporating all aspects of the invention, will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a front elevational view of a water heater, 
     FIG. 2 is a side elevational view of the heater, 
     FIG. 3 is an opposite side elevation view of the heater, 
     FIG. 4 is an exploded perspective view of components for a water jacket assembly, 
     FIG. 5 is a side elevational view of the water jacket assembly, 
     FIG. 6 is an elevational view of the water jacket assembly, 
     FIG. 7 is a plan view of the water jacket assembly, 
     FIG. 8 is a series of views showing a method of assembly and flow path of a heat exchanger that forms part of the water jacket assembly, and 
     FIG. 9 is a schematic view of a flow control valve that forms part of the water heater. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The domestic water heater illustrated in the accompanying drawings is fuelled by gas and operates to provide an instantaneous flow of hot water thus there is no need for a tank to store a quantity of hot water. 
     As shown in FIGS. 1 to  3 , the water heater  10  is housed in a rectangular enclosure  11  that is designed to be mounted flush against an external wall. The heater needs to be coupled to a supply of gas and it is understood that the heater can be adapted to work on a variety of commercially available gases. The exhaust gases are vented to the atmosphere via a small aperture  12  at the front  13  of the heater. Alternatively, the heater can be installed internally with exhaust gases being vented to the atmosphere via a small flue that would extend either through the wall cavity or up through the ceiling. 
     In summary, the water heater  10  comprises a series of gas burners  20  positioned above a water jacket assembly  50  so that heat from the gas burners  20  passes through a heat exchanger  51  that forms part of the water jacket assembly  50  to heat up a supply of cold water that is arranged to flow through the heat exchanger to exit the heat exchanger as hot water. A control mechanism controls the amount of gas that is burned at the burners  20  dependent on the flow of water and the temperature requested, ie on the demand. The burning capacity of the gas burners is enhanced by the provision of a fan  30  that mixes gas with air before the burners  20  to ensure use of the most efficient air fuel mixture. The fan  30  also operates to force the hot air generated by the burners  20  down in a vertical direction through the heat exchanger  51 . Heat exchangers of this type produce condensation which drips down into a collection tray  71  mounted at the base of the enclosure  11  for discharge  72  into either the sewerage or storm water drains. It is understood that suitable plumbing would be used to facilitate this discharge. 
     The series of burners  20  are positioned across the top of the heater  10  and are fed by an air gas mixture via a mixing chamber  31  which is in turn fed from a modulating gas valve  32  and an electrically driven fan  30  that mixes the gas in the air prior to feeding the gas mixture to the burner. The burners  20  are in the form of a ceramic plate  35  having a series of small apertures (not shown) extending therethrough. The apertures provide a very large number of small flames that project downwardly towards the water jacket assembly  50 . The flames are arranged to terminate at a position that is just above the position of the heat exchanger  51  that is positioned in the lower half of the water jacket assembly. 
     As shown in FIGS. 1 to  3 , the cold water inlet  14  extends into the water jacket base on the left hand side of the unit as viewed in FIG. 2 with the hot water exiting the water jacket assembly  50  from the right hand side of the unit towards the top of the heat exchanger  51  at the hot water outlet  15 . Notwithstanding this arrangement it is understood that the direction of flow may be reversed. A water flow meter  90  monitors flow of water at the cold water inlet  14 . A first temperature sensor T 1  is positioned on the cold water inlet and a second temperature T 2  sensor is positioned on the hot water outlet  15  from the heat exchanger  51 . A third temperature sensor T 3  is positioned on a water flow control valve  60  which is coupled both to the cold water inlet  15  and the hot water outlet  16 . The supply of gas flows up from the base of the unit along one side through the modulating gas valve  32  to the fan  30  as shown in FIGS. 1 to  3 . The hot water outlet  16  from the water valve  60  has a first outlet  17  that is designed to provide water up to a temperature of 80° C. and a second lower temperature outlet  18  that dispenses water up to a temperature of 50° C. via a flow sensor  19 . Thus, it is important that water heaters of this kind have safety controls to prevent scalding. When flow is detected in outlet  18 , the electronic control system automatically limits the maximum available temperature to 50° C. The combustion gases on passing through the heat exchanger  51  exit the unit at the base of the heat exchanger via the rectangular outlet  12  in the front face  13  of the heater. These gases are at a temperature that is lower than the temperature of the hot water, for the main part of the water heaters operation, thus the loss of the heat to the surrounding is kept to a minimum. 
     An electronic controller  80  is mounted near the top of the heater as shown in FIG. 1 to control operation of the heater  10 . To operate, the heater has to be coupled to a source of gas, a source of cold water and a source of electricity. 
     The water jacket assembly  50 , in accordance with preferred embodiments of some aspects of the present invention, is illustrated in detail with reference to FIGS. 4 to  8  and essentially comprises an external water jacket  52  that supports an internally positioned heat exchanger  51  that is in the form of varying numbers of pairs of rectangular plates  53 ,  54  depending on the heat exchange requirements. Each pair is positioned in abutting contact to define a convoluted water path therebetween. As shown in FIG. 8, the plates are mounted as a sandwich S with each pair positioned parallel and spaced apart from the adjacent pair. The gaps  55  between the pairs allow passage of hot air from the gas burners and the plates are interconnected so that the water flows through a convoluted passage along each plate and through adjacent plates as shown in FIG.  8 . The cold water enters one side  56  of the rectangular sandwich S and exits as hot water at the exit  57  on the same side of the sandwich S. Thus the hot water exit  57  is near the hot combustion gases whilst the cold inlet  56  is adjacent the base of the unit that is near the warm flue gases. This design of heat exchanger ensures that there is an even heat distribution laterally of the heat exchanger with a temperature distribution becoming progressively hotter upwards from the base to the top. The uniform lateral temperature distribution ensures uniformity of heat transfer and prolongs the life of the heat exchanger. 
     The water jacket assembly  50  essentially comprises three metal plates, namely a side plate  100  that is inverted to encase opposite sides of the assembly and an end plate  101  that is inverted twice to produce four plates  101   a, b, c, d,  that, as shown in FIG. 4 envelop and overlap the ends to define the water jacket  55 . The third plate defines the rectangular plates  53 ,  54  which when mounted in spaced apart pairs define the sandwiches that constitutes the heat exchanger  51 . As shown in FIGS. 5 to  7 , the sandwiches of heat exchanger plates is located towards the base of the unit with the water jacket having a fluid passageways along the top half of the assembly and down across the ends. The space defined between the water jacket and the top of the heat exchanger  51  is a combustion chamber. The water jacket is positioned externally of the heat exchanger  51  with the gas flames of the burners playing along the centre line of the assembly  50  within the combustion chamber. This feature has the effect of drawing off heat from the gas flames to reduce sideways escape of heat and also reduce the temperature of the hot gases at the heat exchanger  51 . As shown in FIG. 5, the cold water enters the assembly  50  from one side at the base and exits the assembly on the opposite side towards the top of the heat exchanger  51 . Initially, the water moves in two directions around the sides and ends of the water jacket  55  so that water flows through the whole of the water jacket before passing through the heat exchanger. This reduces the likelihood of the heat exchanger  51  being overheated and reduces waste of hot gases. 
     By manufacturing the assembly from three plates that are simply reversed, the whole assembly can be produced from a simple stamping operation. Furthermore, in the preferred embodiment, the assembly is manufactured from stainless steel plates coated in copper and the components are assembled together by use of a jig (not shown) so that the componentry is in abutting contact with all the abutting surfaces being copper to copper. When the assembly is placed in an oven for a predetermined period at a temperature to fuse the copper to provide an integral unit in which all the components are bonded together and the water and air passageways are defined accurately with no leakages. There is thus no need for welding, soldering, or other fasteners and this fusing of the copper coating ensures satisfactory operation over a long life. The design of a convoluted passage for water flow is also specifically designed to encourage turbulent flow ensure that there are no stagnant water pockets or hot spots in the unit. Furthermore the external shape of the plates provides a convenient route for run-off of condensate that forms on the exterior of the assembly. The water jacket assembly  50  has proved extremely efficient and allows maximum transfer of heat from the gas flames to the water without excessive heat being lost to exhaust. 
     A gas pressure sensor  84  is positioned at the gas entry of the modulating gas valve  32  to sense a drop in gas pressure to reduce the output of the unit should there be a shortage of gas pressure. Conventional domestic gas pressures operate to a maximum of 200 megajoules per hour and are limited by the gas pressure so that if too many appliances are used at once there is often a drop in the gas pressure. To ensure that a drop in gas pressure does not reduce the temperature of the hot water, the gas pressure sensor  84  causes the rate of flow of water to be reduced to compensate for a reduction in gas pressure so that the unit operates at the desired temperature albeit at a reduced output in terms of litres per minute. Another feature of the gas valve and controller is the use of an oxygen sensor that detects the amount of oxygen in the flue gases. If the oxygen content of the flue gas is either too high or too low, a signal is fed back to the controller  80  to change the gas flow to ensure an optimum mixture. The computerized controller  80  monitors three temperatures, namely the T 1  which is the temperature at the inlet of the cold water, T 2  the temperature at the heat exchanger outlet, and T 3  which is the outlet hot water temperature of the unit. The third temperature monitor T 3  includes an adjustable dial by which the user can adjust the desired output temperature. The controller  80  on sensing the three temperatures can then control the rate of water flow through the unit and also the gas input through the modulated gas valve  32  and the air input by varying the fan speed. The controller  80  varies the parameters to ensure maximum efficiency. The heater is designed to produce 32 litres per minute of water at 25° C. above start up temperature. A conventional shower uses 7 to 11 litres per minute which means that the heater can operate to run three to four showers at once without loss of temperature or reduced water flow. 
     The flow meter  90  positioned in the cold water inlet  14  comprises a substantially cylindrical casing that includes a deflector that deflects the flow of water and an impeller. The impeller is mounted on a shaft and is constructed of plastics material impregnated with a magnetic material. To reduce corrosion of the impeller, an epoxy coating is positioned on the exterior of the impeller. The magnetic field that is caused by rotation of the impeller ensures that an electrical signal is produced that is responsive to the speed of rotation of the impeller. The speed of rotation is in turn dependent on the rate of flow of water pumped through the meter. The electrical signal that is produced by the flow meter  90  is then sent to the controller  80  to control the operation of the water heater in relation to demand. It is also understood that with this meter  90  a visual indication of the rate of flow through the meter can be displayed at the unit and/or at remote controls. The electrical signal that is produced by the flow meter  90  is then sent to the controller  80  to control operation of the water heater in relation to demand. 
     The flow control valve  60  as shown in FIG. 9 comprises a valve chamber  61 , sub chambers  61   a  and  61   b  separated by a piston or a flexible diaphragm  62  that is coupled on one side  61   b  to a flow constriction member  63  that seats on a valve seat  64  at the outlet  65  of the valve. The sub chambers  61   a  and  61   b  are coupled to the inlet  14  and outlet  15  of the heat exchanger  51 . The sub chamber  61   a  is also coupled to the cold water inlet  14  via a first solenoid cut-off valve  67 . The sub chamber  61   a  is also coupled to the outlet  65  through a by-pass conduit  69  cut-off via a second solenoid valve  68 . There is a pressure drop across the heat exchanger which means that P 1  at the inlet is greater than P 2  at the outlet which is in turn greater than the exit pressure P 3 . 
     In operation of the flow control valve the solenoid valves are generally closed, however, if there is too much demand for hot water and a need to reduce the flow, the first solenoid cut-off valve  67  opens while the second solenoid valve  68  remains closed which has the effect of causing a greater pressure P 1  in the sub chamber  61   a  which forces the diaphragm and the valve  63  to partially close against the seat  64  to reduce output flow. If an increase in flow is required the first solenoid valve  67  closes and the second solenoid valve  68  opens which has the effect of reducing the pressure difference across the diaphragm which causes the diaphragm to resiliently return to open the valve  63  to increase the output flow. In a situation where the heat exchanger  51  has an excess quantity of hot water and there is a danger that the hot water will overshoot the maximum temperature, both solenoid valves  67  and  68  opens which has the effect of causing a proportion of cold water to flow from the inlet  14  past the diaphragm into the outlet via the second solenoid valve  68  and conduit  69 . This in turn cools down the temperature of the outlet water to ensure that it is within the desired parameters. 
     To start up the heater, an electrically operated glowing surface is utilised in the combustion chamber and the control ensures that when the tap is turned on causing flow of water, there is first a pause to purge any combustible gases within the combustion chamber. Then, there is a short pause during which the glow surface ignitor commences to glow or spark ignition activates and then an air gas mixture enters the combustion chamber. If there is no combustion, the water heater shuts down the gas flow and the whole process is repeated. If this fails on two occasions then the unit shuts down and a warning light comes on warning the user of the system that a service call is required. 
     Since modification within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.