AU2015100075A4 - Improved Water Heater - Google Patents

Abstract

A gas water heating system is described that utilises most of the heat of combustion of the fuel in an area remote from the tank itself. It also provides a cold lower tank area that stays cooler than the bulk of the volume of the tank through the heating cycle. It also provides a cold lower tank area above the flue outlet of the remote heat exchanger further enhancing the efficiency. It also delivers hotter water to the top of the tank, thereby filling the tank with hot water from the top down. It also utilises the superior insulating properties of plastic foam. It also efficiently uses the pilot flame heat without it causing a high airflow through the unit, with attendant thermal losses. The tank unit is easily replaced in the event of a tank leak or rupture, but retaining the gas equipment and heat exchanger for further use. The central flue has been deleted, and the products of combustion are now directed up the outside of the tank surface in a gap provided between the tank outer wall and a metal sheet wrap to keep the hot gases against the tank wall. Two or more separate fluid paths that are in good mutual contact for efficient heat transfer. The fluid paths are generally tubular and the length of the tubing on one side of the heat exchanger carrying a first fluid may be shorter or longer than that for the second fluid. iPi2 11

Description

1 IMPROVED WATER HEATER Field of the invention [001] The following relates to flammable gas energised storage type hot water generating appliances, and in particular to domestic gas hot water services. The following also relates to heat exchangers, particularly those used in domestic drinking water applications but not limited to this application. Background of the invention [002] The following applies to gas storage water heaters, popularly known as gas hot water services. [003] These generally fit the description of mains pressure storage type domestic water heaters. [004] Some storage water heaters utilise a vented or low pressure storage tank with a coil immersed that can withstand mains pressure, known as indirect mains pressure water heaters. [005] There is also known low pressure or gravity type storage heaters that only use gravity to supply the water to the house taps or outlets. [006] The present invention applies to all the above types of storage water heaters, but is not limited to these. [007] The general format of gas storage water heaters is based on a storage tank, (cylindrical in shape normally), vertically orientated and vertically disposed above a burner of appropriate size housed in a burner box. [008] Appropriate controls to the relevant standards are fitted and due to regulations these generally are fitted in similar areas of all heaters to ensure compliance with performance and other aspects of the standard. [009] The burner box and burner below the tank are generally configured to direct combustion products and heat of combustion firstly to the lowest portion of the storage tank, and then subsequently either up through a vertical flue normally central through the cylinder, or up the outside of the cylinder utilising the higher surface area. It is also known to utilise both the flue and the outer surface of the tank.

2 [010] Generally these designs are more efficient than a simple central flue unit, and depending on the design and performance, much work is done to prevent thermal losses from the unit after heating up has occurred. [011] The tank is generally insulated with a fibreglass or similar blanket trapped between the tank and the outer wrap of the appliance. Higher temperature insulation such as fibreglass is needed because the flue temperatures seen at the tank surface are much higher than the maximum allowable for plastic foam type insulation, (which is superior in insulating properties.). [012] Thermal losses generally are defined as losses through the outer insulation and outer case, and losses caused by convection losses through the open flue way. The flue way is normally constructed to include an inverted "U" shape to act as a partial thermal trap. Most units have a pilot flame operating at all times and this pilot can be a source of high heat loss as the small hot flame induces a relatively big flow of air through the tank/heat exchanger area, causing high heat losses. [013] In general a unit constructed as above will lose more than 3 times as much heat as a plastic foamed insulation tank unit such as an electric storage water unit of the same size. [014] It is also known that conventional gas fired hot water services with burner at the bottom heats the water as a single mass and the water at the top is not much hotter than the water at the bottom during the heat up cycle. [015] In use after the gas has shut off, and a consumer opens a hot tap in the house, cold water is introduced into the bottom of the tank, displacing hot water from the top of the tank to the open tap or outlet. As this amount of cold water rises in the bottom of the tank, it finally reaches the thermostatic probe of the main gas control valve, and the burner again ignites and a heating cycle commences again. The cold water at the bottom of the tank is again brought up to temperature as a single mass, until the probe is again satisfied. It is therefore demonstrated that it is not easy to have a gas heated water tank that has cold water at the bottom, and hotter water above, except for a transient condition after some small use, and prior to the gas cycle starting again. [016] The following applies to heat exchangers, particularly those used in domestic drinking water applications but not limited to this field.

3 [017] It is well known that heat exchangers are used to supply clean hot water for domestic or sanitary purposes from heat sources that are not generally clean, or contain chemicals or compounds that are not desirable (or lawful) to have in drinking water. Central heating boilers with domestic hot water heating coils immersed within, and domestic hot water heat pumps are two examples. [018] There are several types of common heat exchangers used, and the most common of these would be immersed copper coil type, whereby a copper tube is wound into a coil shape and immersed in a reservoir of hot water or other fluid. Clean (potable) cold water is forced through the coil thus heating it up. [019] Another type is described as a "plate" type heat exchanger, which is made up of a stack of basically flat plates, dimpled and deformed in patterns to provide water paths between the plates, the whole assembly normally being brazed together into a compact block of plates. This type of heat exchanger is commonly used in heat pump hot water systems where one side of the heat exchanger has hot refrigerant gas flowing through it which condenses to liquid on the internal walls thus giving up latent heat, and the other side has the domestic hot water to be heated pumped through it to be stored in the hot water cylinder. These types are very efficient but have some problems in that they can have pockets where water does not readily move around and also the construction, by its nature, almost ensures that there are tight crevices and thin water spaces which preclude adequate flow of fresh water into these areas. In areas of water with high dissolved solids such as calcium carbonate, these types of heat exchangers can easily clog up, especially where water temperatures exceed 602C. Where the construction includes plates of stainless steel, the effect of crevices etc where there are chlorides and other chemicals in the water, is to promote corrosion and stress corrosion and cracking of the stainless steel plates. [020] A third type of heat exchanger is designated "tube in tube' which as the name implies, is a tube of certain diameter contained within another tube. [021] It is a common requirement for hot water heat exchangers of all types to be 'double walled" that is, to have 2 separate walls between the potable (drinkable) water, and fluid that is suspect in cleanliness, or may contain chemicals detrimental to humans or animals. There is normally a requirement that the walls have to be independent to a point that if there is a leak in one or other of the pipes or cavities, then the leaked fluid will be detectable in the area between the fluid paths, in the case of tube 4 in tube, generally at the ends. Where the coil is not immersed but is a free standing type, and the need for double walled is defined, this tube in tube design needs 3 tubes, and in the case of plate type, extra plates are required between the fluid paths. [022] In both of the above examples, the addition of the extra wall normally means that there is a gap, generally containing air, which acts as a real barrier to heat transfer from one fluid to the other. [023] The most used partial remedy for this is to use a heat transfer paste which excludes air and contains metal or oxides to enhance the thermal contact between the tubes or plates.. This paste in itself must not contaminate drinking water, and at best is only fair in its performance as a heat transfer medium, compared to metal to metal contact. [024] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. Summary of the Invention [025] The present invention provides a water heater including: a tank for holding water to be heated, the tank having a cold water inlet and a hot water outlet located at a top region of the tank; and a heat exchanger that is located remote to the tank, so that a cold well is maintained in a bottom region of the tank, the heat exchanger having an input end which is adapted to receive water from a bottom region of the tank, and an output end which is adapted to deliver heated water to a top region of the tank, the output end being located higher than the input end during operation of the heat exchanger; a gas burner box including a burner located below the heat exchanger, adapted to deliver energy to the heat exchanger. [026] The heat exchanger can be mounted at an angle of between 10 degrees and 90 degrees from the horizontal. [027] The water heater can further include a temperature probe located inside the tank, and a gas control for controlling gas supply to the burner box, wherein the gas control is adapted to shut off a gas supply to the burner box when a water temperature detected by the temperature probe is above a minimum level.

5 [028] Gas from the burner can be directed to one or more directional baffles, and gas from a pilot flame located beneath the heat exchanger is directed to the same directional baffles. [029] An inlet which delivers water to be heated by the exchanger can include or can be connected to a dip tube that connects to a bottom region of the tank. [030] It is an object of the present invention to provide a gas fired storage type domestic water heater that utilises most of the heat of combustion of the fuel in an area remote from the tank itself. [031] It is a further object of the present invention to provide a cold lower tank area that stays cooler than the bulk of the volume of the tank right through the heating cycle, therefore providing the coldest water to the heat exchanger for maximum efficiency. [032] It is a further object of the present invention to provide a cold lower tank area above the flue outlet of the remote heat exchanger to provide a cool surface on which to condense the water vapour from the exhausted flue gases, further enhancing the efficiency, but without changing the temperature of the water in the bottom of the tank substantially. [033] It is a further object of the present invention to provide a first remote (from the tank) heat exchanger that allows sufficient water flow to perform without the need of a mechanical pump. [034] It is a further object of the present invention to provide a gas hot water service that delivers hotter water to the top of the tank, thereby filling the tank with hot water from the top down. The amount of hot water thus stored can be limited by positioning the temperature probe of the main gas control where the ratio of hot water volume above, and cold water below, is selectable, thus allowing a cold reservoir to be formed in the lower portion of the tank. This can then be utilised by a waste heat recovery system or Solar heater, that is much more efficient when only used to preheat water from cold (102C) up to say 5 02C, rather than having to accept water at 6 5 2C and attempting to heat it above that level. At the moment, except for very complex systems, this can only be attained with a separate pre heater tank. [035] It is a further object of the present invention to provide a gas fired hot water system that utilises the superior insulating properties of plastic foam, by minimising the 6 contact of hot flue gases with the insulation, and where there is contact, by lowering the flue gas temperature first by a remote heat exchanger. [036] It is a further object of the present invention to provide a gas hot water service that efficiently uses the pilot flame heat without it causing a high airflow through the unit, with attendant thermal losses. [037] It is a further object of the present invention to provide a gas hot water services that has the tank unit easily replaced in the event of a tank leak or rupture, but retaining the gas equipment and heat exchanger for further use. In conventional units it is seldom economic to service the tank and gas equipment separately. At end of life, the gas equipment can still be quite usable. [038] It is an object of the precent invention to provide an allowable double walled heat exchanger that provides 2 or more separate fluid paths that are in good mutual contact for efficient heat transfer, and to provide heat enhancing material that is 100% metal and therefore efficient in enhancing the heat transfer from one tube to the other. [039] It is a further object to provide a heat exchanger that is easy to mass produce using common manufacturing techniques and materials, not specialised heat exchanger methods and tooling. [040] It is a further object of the present invention to provide a heat exchanger where the fluid paths are generally tubular thus ensuring adequate scrubbing of the whole of the internal walls, preventing the formation of dead pockets and still areas. [041] It is a further object to provide a heat exchanger where the length of the tubing on one side of the heat exchanger carrying a first fluid, may be shorter or longer than that for the second fluid, depending on required flow rates or other performance for each. [042] It is also an object of the present invention to easily allow for the use of a "parallel" low-pressure-drop tube layout for a first fluid, and a "series" circuit for a second fluid, depending on required performance. This can be very important where 2 different fluids are used such as water and hot refrigerant. [043] It is also an object of the present invention to allow for different layouts depending on space available in the host machine, and to provide a very dense heat 7 exchanger where minimal external surface area, compared to volume, is required to minimise heat losses. [044] It is an object to provide a heat exchanger where the fluid paths are easily made from dissimilar materials, commonly copper for water and steel or aluminium for refrigerant. Brief description of the drawings [045] An embodiment or embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [046] Figure 1 is a cross sectional view of one embodiment of the present invention; [047] Figure 2 shows a view of the preferred heat exchanger type to be utilised in the present invention; [048] Figure 3 shows a general layout of a type of gas hot water service unit; [049] Figure 4 shows the basic totally submerged (surrounded) firebox design; [050] Figure 5 is a cross sectional view of one embodiment of the present invention; [051] Figure 6 is a cross sectional view of one embodiment of the present invention; [052] Figure 7 is a cross sectional view of the present invention as in Figure 6 but the dome is inverted; [053] Figure 8 shows how the basic combustion chamber can be strengthened without extra components; [054] Figure 9 illustrates how the basic combustion chamber can be reinforced with extra components; [055] Figure 10 shows yet another embodiment whereby the combustion chamber wall and/or the wall are formed locally; [056] Figure 11 shows the main tank disposed generally above the generator; [057] Figure 12 shows a tubular heat exchanger made into a serpentine shape; [058] Figure 13 shows a tubular heat exchanger as in Figure 12 but the overall size has been minimised by deforming the heat exchanger but leaving the straight lengths untouched; 8 [059] Figure 14 shows a plan view of a slab into which grooves are formed or machined to accept a serpentine coil, and clearance for the returns; [060] Figure 15 shows a serpentine coil in place in the slab; [061] Figure 16 shows a cross section of simple form of the present invention; [062] Figure 17 shows a slab which has grooves moulded from both sides to better utilise the moulding operation and allowing some size flexibility in doubling the tube area; [063] Figure 18 shows slabs bolted together to form a more dense heat exchanger; [064] Figure 19 shows a parallel heat exchanger tube assembly mounted in slab. Detailed description of the embodiment or embodiments [065] Figures 1 and 2 illustrate an embodiment of a first invention. Figure 1 is a cross sectional view of one embodiment of the present invention. 1.1 is the tank, 1.2 is the outer case, 1.3 is the thermal insulation, 1.4 is the remote heat exchanger mounted on the top of burner box 1.17. The burner 1.16 is mounted in the base of box 1.17 and is connected to gas control 1.11 by gas pipe 1.10.The gas control 1.11 is activated and de-activated by signal from temperature probe 1.12. The main burner 1.16 is ignited when required by a pilot flame 1.20, which is present 100% of the time. The inlet of remote heat exchanger 1.4 is connected to the cool part if the tank 1.1 by pipe 1.14. The hot outlet of heat exchanger 1.4 is connected to the tank 1.1 by pipe 1.9. The position of shown at the top of the tank but may be connected at any other point, but preferably above the level of probe 1.12. A 'dip" tube 1.8 is provided if the format of the tank precludes the pipe 1.14 from connecting at a low point in the tank. The hot water outlet is shown at 1.18. A guide for flue gases is shown at 1.19. A flue outlet to atmosphere is shown at 1.21. [066] In operation, initially the tank and heat exchanger are full of cold water. Gas is provided by control 1.11 to burner 1.16, because probe 1.12 is cooler than the desired setting. The energy liberated is contained in burner box 1.17 and directed through the fins and tubes of remote heat exchanger 1.4. This heat exchanger is preferably a finned tube heat exchanger which has parallel water paths from top to bottom as shown in Figure 1.2, but may be any convenient type of gas to water heat exchanger. A heat exchanger that is a serpentine or has up paths and down paths will not naturally 9 convect. The energy from the flue gas is absorbed into the water inside the heat exchanger 1.4, which is mounted at an angle from the horizontal of 10 degrees minimum, up to 90 degrees. This angle is required to ensure that water is circulated, by convection or "thermo-syphon", through the heat exchanger 1.4 and tank 1.1 sufficiently to prevent localised boiling within the tubes of heat exchanger 1.4. If the angle exceeds 45 degrees and up to 90 degrees it has been found that the burners, which should remain basically horizontal, may require the fitment of turning or guide vanes in the firebox 1.17 to ensure distribution of hot gases evenly over the heat exchanger 1.4. Hot water from the heat exchanger 1.4 is transported via pipe 1.9 to a convenient position on tank 1.1. As this water is heated it will naturally rise to the top of the tank, and in a preferred embodiment the flow through heat exchanger 1.4 can be varied to ensure that water of the desired temperature is delivered to the top of the tank. This can be by restricting the flow in pipe 1.9, changing the gas rate or by addition of other sensors and valves. It is most preferable to supply the tank with water above to the minimum useable temperature, (generally 450C for showers). The tank will gradually "fill" with hot water from the top down. [067] This characterises this unit in that most other gas fired water heaters do not "stratify", and thus cannot deliver a small amount of usable water after say 50% of the normal heat up time. They tend to heat the water mass as a single entity and therefore after 50% of the heat up time will have 100% of the water "half" heated, as opposed to the present invention which allows for 50% of the water to be fully heated after 50% of the heat up time. [068] Eventually the hot water will fill down to reach the probe 1.12 of gas control 1.11. When the hot water level fills down to the probe 1.12, the control 1.11 will shut off the gas supply, leaving a reservoir below the probe 1.12 basically unheated. This factor allows the designer to place the probe at whatever level in the tank is preferred to enable any ratio of gas heated water to Solar heated water to be provided. This control 1.11 and probe 1.12 are shown in Figure 1 at a fairly high position but can be placed vertically anywhere in the tank. The water volume under the probe is available for Solar or the like to use this reservoir as a preheat 'tank" but contained within the main reservoir. This facility is not available to other gas hot water system units because they heat from under the bottom of the cylinder and thus cannot preserve a cold well in the bottom portion of the tank. The water heats as a single mass and will almost always 10 heat the water without any stratification at all, irrespective of where the probe 1.12 is positioned. The deliberate stratifying nature of the present invention is a therefore a distinct advantage. [069] During the heating cycle, approximately 15% of the heat liberated from the gas will escape through the fins of heat exchanger 1.4. This will be of relatively low temperature and normally difficult to recover because of this low temperature. What is provided in the present invention is a cold lower tank surface which will allow usable amounts of energy to be recovered from this low temperature exhaust flue gas. In addition, there is water held in the flue gas as steam, and this will readily condense on the cold lower tank giving up the latent heat of evaporation to the tank bottom, and subsequently to the cold water inside. The amount of energy available as latent heat, plus the small residual energy in the flue gases is insufficient to heat the lower mass of water in the tank to any great degree, so the solar performance is protected by keeping the lower volume in the tank relatively cool. [070] It is also desired to allow the use of superior low temperature foam insulation and to limit standby heat losses when the burner 1.16 is not operating.. The design is shown in Figure 1 and the insulation is shown at 1.3. In a conventional high efficiency gas hot water service, the flue gases are directed almost all over the tank outer surface by a heat shield spaced off the tank. The very hot flue gases are directed between the tank and the heat shielding. The outside of the heat shielding is therefore in contact with the insulation, and far exceeds the insulation maximum allowable temperature. Glass fibre and other types of high temperature insulation are used, but generally these are inferior to foamed plastic in insulation value. The gap between the heat shield and the tank also acts as a thermal flue path. In the off cycle, this gap generates an upward air stream, causing considerable heat loss upward and out of any flue outlet. [071] The present invention as shown has the tank approximately 75%-90% covered with foam insulation as the remote heat exchanger 1.4 contains and converts the high temperature flue gases to low temperature gases, prior to the flue gases contacting any surfaces insulated with foam. The insulation 1.3 thereby blocks that path of any flue gas to the tank 1.1 vertical walls, thereby limiting standby thermal convection paths. [072] The pilot flame 1.20 is placed conventionally in close proximity to the main burner 1.16. It is desired to utilise the pilot flame energy efficiently during the off cycle (standby), further limiting standby losses and maximising overall efficiency.

11 [073] The pilot flue gas rises from the pilot and negotiates the quite restrictive fin spacing of the heat exchanger 1.4. In addition, the heat exchanger 1.4 should always contain the coolest surfaces in the system due to being lowest in the thermal stack. This extracts more heat than allowing the pilot to indiscriminately waft up and over the tank surface, which it does in a conventional high efficiency gas hot water service. Also, any water vapour left in the pilot flue gas should condense on the very cool tank bottom surface. A most important factor in extracting pilot heat is the fact that the pilot flue gas is subjected to the same directional baffles as the main burner 1.16 flue gas, thus limiting the generation of unwanted flue movement in the off or standby mode. It is important that the pilot always has a cool heat exchanger above it to effectively heat exchange pilot flue gas to the water. After the main burner is extinguished, the thermosiphon effect will continue whilst there is a temperature difference between the heat exchanger 1.4 and the tank lower volume. When they approach the same cool temperature the system stops circulating and the pilot is left with a cool target. This feature ensures that the sensible heat stored in the heat exchanger system is effectively scavenged into the water after burner turn off. [074] Reference is made to Figure 2. Some numbers refer to Figure 1. [075] Figure 2 shows a view of the preferred heat exchanger 1.4, (Fig 1) type to be utilised in the present invention. [076] 1.30 is the hot water outlet to the tank 1, Figure 1, 1.31 is the cool water inlet from the same tank . A top header 1.34 is connected to lower header 1.35 by parallel connecting tubes or risers 1.32. The risers are in the flue path of the unit as shown in Figure 1. The risers are connected in good thermal contact with thin copper or other fins 1.33. The risers and fins provide good thermal extraction of energy from any hot gases rising through the fin and tube arrangement. The risers 1.32 are shown as parallel paths which allow good thermal convection from the lower header 1.35 to the top header 1.34 and subsequently into the tank 1. [077] The heat exchanger is further characterised by the provision of service ports 1.36 in the headers, shown in the lower header only but may be fitted into either header as shown and/or at the ends of each header to facilitate cleaning out in service as required. [078] The invention as described is intended to enhance efficiency of gas storage hot water service during the burner- on cycle, and to limit standby losses to a minimum, 12 thus increasing the overall or task efficiency of this gas hot water service over existing products. [079] The optimisation of these parameters is very important in the present day energy rating of appliances to meet minimum standards, especially when applied to hybrid Solar/Gas installations. The present invention allows the use of a single larger cylinder as the standby losses of a standard gas hot water service preclude the use of a single tank, and the provision of a separate tank incurs cost and loss of overall efficiency. [080] Figures 3 to 11 illustrate embodiments of a second invention. [081] Gas hot water services are generally well known and known variously as tank type, storage, cylinder type, as opposed to instantaneous or continuous flow or tankless types, which as the name implies, do not have any substantial storage capability at all. [082] Originally these storage gas hot water services consisted of a simple cylinder (tank) of appropriate volume, say about 125 Litres or 25 gallons, mounted vertically, with a gas burner disposed generally underneath, and a flueway consisting of a steel pipe welded into the cylinder at its vertical axis. Appropriate shielding and insulation ensured that sufficient products of combustion impinged on the tank lower surface and flueway inner surface, to heat the water at an efficiency of approx 65%. These units also generally incorporated a "Pilot" or small starter flame that was always burning, ready to ignite the main burner when required.(e.g. when the tank had cooled or water had been used.) [083] These units suffered poor overall performance because the long warm flueway, especially in conjunction with the pilot flame draft, caused a lot of heat to be lost up the flue when the unit was not running but just waiting for some water to be drawn off. (These are known as standby losses.) [084] In the more modern era, the same basic unit is still used but more emphasis on energy efficiency has meant some changes. The central flue has been deleted, and the products of combustion are now directed up the outside of the tank surface in a gap provided between the tank outer wall and a metal sheet wrap to keep the hot gases against the tank wall. This has the effect of increasing the surface area and hence the thermal efficiency. The pilot light is retained, but it no longer aids in loss of heat by inducing flue draft as before, but in most applications the pilot light actually contributes its heat to the tank and therefore the water. The flue outlet of these types of units is 13 generally quite low down on the outer case to further discourage normal thermal convection, thereby preserving heat and lowering standby losses. [085] A general layout of this type of unit can be seen in Figure 3. [086] The main components are easily identified but particular attention is drawn to the "firebox" region A. This region is the main area of interest in relation to the existing invention. It can be seen that there is a region of firebox disposed below the tank. The problem identified with this area is that the gases, after combustion are at temperatures exceeding 6 00C or 10002F. During the combustion cycle even with good insulation surrounding the firebox, much heat is lost out through the insulation, (a secondary function of which is to protect the outer case.) [087] Surprisingly it has been found that during a typical heat up from cold, that over a 1 hour run time, approximately 1 kW of heat can be lost through this region. This is a substantial amount if the unit is used one heat up cycle per day. Most homes would exceed this depending on tank size which in most cases is approx 150Litres or 40 gallons. [088] One method of trapping and utilising this heat is by a known design, known as "submerged firebox" or semi submerged firebox. Generally these have been confined to central heating boilers and other low pressure applications. [089] Figure 4 shows the basic design of this principle. This is a totally submerged (surrounded) firebox. [090] A burner 2.25 is disposed within the firebox 2.26 which is surrounded by water. The outlet of the firebox generally has a flue 2.27 connecting the firebox 2.26 to atmosphere. It can be seen that any heat generated in the firebox can easily be transferred by radiation or convention/conduction, into the water2.28. It can also be seen that any insulation outside the tank would only be subjected to the water temperature, at all times below 1 OOC. The design depicted in Figure 4 is a well known design in boilers and low pressure applications but suffers from standby losses etc as described in the preceding paragraphs. [091] It is desired to provide a storage pressurised gas hot water service that has enhanced thermal efficiency, low standby losses and ease of manufacture.

14 [092] Reference is made to Figure 5, where now a pressurised tank 2.8 is mounted in a case 2.3, insulated from each other by fibreglass or similar insulation 2.4. [093] A flue passage 2.7 is provided by the addition of an intermediate metal wall 2.6 spaced from the tank 2.8 appropriately to allow flue gases to escape easily but give up most of the heat contained in the flue gas into tank 2.8. [094] The tank 2.8 is characterised in that it has a combustion chamber formed by cylinder 2.13 and domed end 2.14. The domed end 2.14 may be inverted for strength. The dome 2.14 and the cylinder 2.13 may be formed from the one sheet of steel in a press or be a welded assembly. This combustion chamber is connected into the main tank 2.8 by any common means such as welding at the lower edge. [095] A gap 2.21 between the tank cylinder 2.9 and the combustion chamber 2.20 has been found to be ideal at 20-30mm. A cold water inlet 2.12 is provided as low as practicable in the tank 2.8 and placed so that any settled out dirt that is prone to settle in the lowest point in any tank, is agitated whenever cold water flows into the tank. If the cold water inlet is placed higher it has been found that the addition of an auxiliary pipe or similar can achieve the same result. [096] A gas valve 2.5 is fitted through the tank wall and the temperature sensing probe 2.24 is placed at a minimum 6mm distance to the combustion chamber surface, to ensure that the probe 2.24 senses the general water temperature and not water directly affected by the heat of the dome 2.14. [097] The unit is fitted with a gas burner 2.18 which has generally vertical gas flames. The unit is further characterised by the provision of a burner tube 2.20, disposed near centrally on the burner head axis. A preferred material for this is stainless steel due to the operating temperature, but may be of vitreous enamelled steel or similar. Secondary air passage 2.30 is allowed at the perimeter of the burner, and if necessary up through the centre of the burner. [098] A flue gas collector area is provided between cap 2.23 and tank top 2.10 and a flue pipe 2.22 is provided finally resulting in a flue outlet 2.29. [099] In operation, gas is liberated from gas valve 2.5 and flows into burner 2.18. Burner 2.18 mixes air with the gas in the generally known manner, and ignition source is provided, (not shown).

15 [0100] Combustion takes place indicated by flames 2.12. Any other type of burner or combustion system may be used. [0101]The burner tube 2.20 surrounds the flames 2.12 and prevents direct flame or flame radiation from escaping, and the heating effect of the flame quickly heats the burner tube to approximately 300-600 degrees Celsius. This tube is of appropriate size to contain the flames and combustion products, and direct them in a generally vertical direction. The flue products then impinge on the dome 2.14 of the combustion chamber and are cooled somewhat. The hot gases are then directed down the outside of the burner tube and being cooler than the gasses inside the burner tube, are pushed downwards by the rising flue products inside the tube 2.20 cooling further on the inside wall 2.13 of the combustion chamber. At the bottom of this wall 2.13 the gases are forced around the tank lower edge into a vertical rise again, and still being hotter than the surrounding metalwork 2.9 and 2.6, continue to rise up the flue passage 2.7, further giving up heat as they go. At the top they collect and flow down flueway 2.22 and escape from outlet 2.29. [0102] The burner tube 2.20 in the first instance and when the tank is cold, provides separation of the flue gases from where combustion is still taking place within the tube 2.20, and direct them to a separate area outside the tube 2.20 to ensure that they are not able to circle back and be ingested with the burner air. Burner tube 2.20 and secondary air baffle 2.16 also direct secondary air up through the confined air/gas mixture in the tube 2.20 and this ensures that most of the secondary air is exposed to the combustion process. This helps promote complete combustion and minimises excess pure air (excess air) escaping into the flueways and out through the outlet, causing poor thermal efficiency. The secondary air baffle 2.16 also serves as a collector of condensate from the flue cooling process and would have a drain hole or similar (not shown) to ensure that condensate is directed to where it does not have any ill effect. [0103] The burner tube 2.20, being low mass, quickly heats up when the burner is on and promotes upward high velocity convection of the combustion products which provides the extra convection forces to push the hot flue products down outside the burner tube 2.20, against the natural tendency of hot gases to rise. These gases get cooled against the tank surfaces, causing a further density differential between flue products inside the tube 2.20, and those outside the tube 2.20. This ensures good flue push/pull characteristics due to density differences.

16 [0104] The annulus shaped gap 2.21 between the tank wall 2.9 and the combustion chamber wall 2.13 is provided to place a heat absorbing barrier of water between the hot flue gases and the outer walls of the unit. Without this feature it has been found that much energy is lost through the insulated walls in this area. The heat lost to the water in the barrier region is in fact recovered by convection as the water heated in the barrier region is part of the unit volume. [0105] It is desired to provide a unit that provides a barrier and also provides a decreasing flue volume along the flue path because as the flue gases cool they contract in volume. The reduction of flue cross sectional area along the flue path ensures that good scrubbing of the tank surface occurs all along the flue path, enhancing efficiency. [0106] It has also been found that standard designs of existing hot water services as shown in Figure 3 suffer high standby losses when in the burner off condition. It has been found that the pilot flame contributes to this loss, because as well as the natural consumption of gas, in a lot of designs the pilot flame actually creates extra flue draft and losses because the pilot promotes convection when the burner is off. It is common to use a flue down tube as shown at Figure 5 item 2.22. [0107] It is desired to also limit how much effect the pilot flame can have on inducing draft in the flue. In Figure 5 a pilot and flame 2.30 is shown. The unit is characterised in that the pilot is positioned to ensure that the pilot flame flue gases are inboard of the side of the Burner tube. In operation, the pilot flue gases are also directed up onto the relatively cold tank bottom where they either give up their heat to the tank and/or pool in the top area of the burner tube. It has been found that this arrangement limits total airflow through the unit when the burner is off, thus preserving energy and limiting standby losses. The unit is further characterised in that 2 separate inverted heat traps are provided. The initial heat trap just described between burner tube 2.20 and combustion chamber wall 2.13, and as well, the standard flue heat trap formed by the down tube shown at 2.22. [0108] The combustion chamber heat trap is characterised by the relationship of the top of the burner tube 2.20 to the bottom of the combustion chamber wall 2.13. It has been found that they must overlap by at least 150mm minimum to be effective as a heat trap.

17 [0109] It is a preferred embodiment that the burner heat tube 2.20 be made from an insulating material or be insulated to prevent thermal short circuiting by conduction, through the material of the burner tube thus negating some of the heat trap effect. [0110] In a further embodiment an improvement is disclosed to enable the structure of the combustion chamber to withstand the forces generated by the effect of water pressure on the combustion chamber. In practice, the force exerted onto the top of the combustion chamber 2.14 can be in the order of 2.18 tonnes. These forces can deform the dome 2.14 to a point where it will try to deform and turn inside out. As well the combustion chamber walls 2.13 are under extreme "column" forces, and if distorted even slightly will buckle under the forces and collapse. The hydrostatic pressure in the annulus between wall 2.13 and tank wall 2.9 provide forces that are easily taken by wall 2.9 as it becomes stressed in tension, whereas the wall 2.13 becomes stressed in compression. If the wall bows slightly inwards then the column forces provided by the pressure acting on the dome 2.14 will surely collapse the wall. [0111] It is desired to provide re-enforcement of the combustion chamber dome and walls to withstand the present forces, as well as the forces presented by the statutory codes for this type of appliance. [0112] Referring now to Figure 6 one embodiment of the present invention discloses that the walls 2.13 of the combustion chamber are angled outwards from a position at a minimum of 30% of the height of the wall, out to meet the bottom edge of tank wall 2.9. In this arrangement the down ward forces act in the wall 2.13 as column forces and these are taken almost in line with the bottom edge of the tank, thus allowing the tank wall to support the combustion chamber without leverage bending the lower edge 99/13 as would be the case in Figure 3. [0113] Also shown in Figure 6 is a tie bar or flue tube designed to resist the opposing forces applied to the top tank dome and the combustion chamber dome 2.14. This bar or tube will be in pure tension and thus can be relatively small in cross sectional area. [0114] Figure 7 depicts a similar layout but the dome 2.14 is inverted, shown at 2.32. [0115] Figure 8 shows how the basic combustion chamber can be strengthened without extra components by judiciously applying forms pressed or stamped into the combustion chamber walls and dome.

18 [0116] Figure 9 illustrates how the basic combustion chamber can be re enforced with a "cage" of extra components. A preferred method is to have the dome 2.14 re-enforced with crescent or semicircular plates 2.35 as shown, and longitudinal struts 2.36 all welded as required. The plates 2.35 in the dome 2.14 stop deformation inwards and also transmit force to the wall 2.13 and the struts 2.36. The struts 2.36 are also intended to stop hydrostatic pressure in the volume between the walls 2.13 and 2.9 from distorting the wall 2.13 inwards and leading the assembly to collapse. [0117] Figure 10 shows yet another embodiment whereby the combustion chamber wall 2.13 and/or the wall 2.9 are formed locally so as to meet at a point where they can be welded together to again allow the tank wall 2.9 to support the longitudinal forces down the combustion chamber wall 2.13 by way of the welds at the deformations 2.37 and 2.38 being in shear. Also the forces outward on the wall 2.9 would be counter balanced by the forces inwards on the wall 2.13 and in that scenario the weld would be in tension. [0118] What is therefore disclosed is first method to re-enforce the combustion chamber by the addition of extra components, a second method where the combustion chamber is re-enforced by application of form to the plain geometric shape of the chamber, and thirdly to a situation where the chamber and the tank are made to mutually re-enforce each other. Of course any combination of the above could also be adopted. [0119] In a further embodiment as shown in Figure 11, a low volume hot water generator 2.39 is disclosed that is constructed along the lines of the unit shown in Figure 5, but then deployed as depicted in Figure 11. This unit (Figure 11) can embody any or all of the features described in this application. In one application as shown, the generator is mounted directly below the main cylinder 2.41. The generator uses all of the advantages of the present invention but utilises the water heated, in a different manner. The water heated is transferred to the main tank by natural convection in that the generator makes very efficient use of the available gas energy but the water generated is transported by convection into the main holding tank 2.41. Ideally a supply line 2.42 of cooler water from the lower part of the cylinder 2.41 to the generator is utilised. [0120] The return line 2.40 from the generator is directed back into the main tank at any point above the outlet of the cold water. Ideally the return line 2.40 enters the tank 2.41 in the top 30% of the volume, or enters the tank at the bottom as shown in Figure 11 and the hot water is delivered to the top of the tank by convection within the tank 2.40 19 and inertia of the water leaving the generator 2.39 by the more energetic convection generated by high temperatures within the generator 2.39. Several other layouts of this application will work but not depart from the spirit of the present invention. [0121] It can be seen that the use of a separate generator can be used to generate convection and thus transport heated water from the generator 2.39 to the main tank 2.40. It will be obvious that the warmer water can be directed towards the top of the main tank 2.40 if desired. It is also desired to be able to select the desired temperature of the water in the main tank 2.40. [0122] It is disclosed that a restrictor in either the supply line 2.42 Or the return line 2.40 can be utilised to balance the flow of water from the generator 2.39 to the thermal input from the gas energy, at whatever temperature is desired. It is known that the convection force is a relationship between the temperature difference and the relative heights of the supply and return lines. The resultant flow rate in these lines therefore determines the outlet temperature, for a given gas input. [0123] It is desired that a selectable temperature is provided for by the addition of flow adjusters 2.43 in either the supply line 2.42 or the return line 2.40 or a combination. [0124] The outlet temperature is adjustable also by the use of a valve in the gas input line as is usual practice. [0125] Adjustment of these any of these valves described may be manual, automatic or a combination. [0126] It is desired to ensure that the energy used in this gas hot water heater is utilised to maximum efficiency. To this end the unit depicted in Figure 11 shows the main tank 2.41 disposed generally above the generator 2.39, Flue gases leaving the generator may still have some sensible or latent heat energy still available and the present invention gathers some or all of this energy by passing the flue gas from the generator 2.39 over the cool bottom area 2.44 of the main cylinder 2.41, causing heat exchange of the sensible heat, and condensation of the water in the flue products thus liberating latent heat, and sensible heat into the main tank 2.41. It is desirable to enable useable smaller amounts of hot water (above 4 02C) to be quickly collected in the top of the main tank after a relatively short run time, whereas in conventional units, all of the water must be raised to a useable temperature (above 4 02C) before it is useable. [0127] Figures 12 to 19 illustrate embodiments of a third invention.

20 [0128] The heat exchanger is best described as a slab or block of metal, e.g. aluminium, that is cast or machined into a shape to accept metal tubes of e.g. copper, characterised by the fact that the grooves are deliberately narrower than the diameter of the tube which has to fit into the grooves. When the tubes are pressed into the grooves, the round tubes are elastically deformed into a square with round corners, the sides of the grooves withstanding the outward pressure of the tubes during and after the pressing operation. This elastic deformation, which may also include a percentage of plastic deformation, ensures that a line of contact with the tube against the aluminium block, is much wider than a line contact and a considerable contact force is maintained by the elastic deformation of the tube. The groove is designed to accept 1 or more tubes . If more than 1 tube in a single groove they would be forced together for good contact tube to tube but also tube to the carrier metal block. They could be side by side or one after the other and a logical use would be to have a first fluid passing through one tube, and a second fluid passing through the other. If the slab is cast only there will be a draft angle of approx 2 degrees on the walls of the grooves, so if the tube is nominally 10mm in diameter, the 2 tubes would probably be the maximum in any groove. The slab can be double sided however and another 2 tubes can be inserted from the other side, thus utilising the casting operation better. As well, the slabs can be stacked onto each other forming a high energy density block, approaching a cube shape, thus minimising both the cost of insulation and the heat loss of the heat exchanger. It is also important that there are no thermal paths intersecting the joins between adjacent blocks. In the stack up of slabs, there will be tube to tube contact, or tube to carrier block. Any interruption of the thermal paths by machines faces of the blocks will be detrimental to heat transfer. The casting details can provide keys or dowels to ensure accurate line-up with each other. The grooves are preferably only be applied to the straight lengths of tube and not the curved ends or corners as these areas are difficult to replicate in the slab and any dimensional difference will cause severe stress on the tubes and slab, and could cause premature failure of the unit. [0129] The tubes to be pressed into the slab will generally be of copper, especially on the potable water side, and this tube may be in a series (or continuous) format, generally a serpentine shape. (series of "S" shapes.) Depending on use, it can also be a grid or parallel pattern, made up prior to pressing or after pressing into the block. It is important however that the tubes are not heated above annealing temperatures or 21 pressure on the side walls of the grooves will be diminished. The tubes can be spaced or the shape compressed to limit overall size but still produce tubes in a standard manufacturing facility. [0130] The slab acts as a retainer for the tubes as well as a good heat transfer medium itself, as heat paths from tube to tube can be direct or via the area of the slab in intimate contact with both tubes. The slab can be designed with a minimum of material but maximum stiffness by common design techniques. [0131] Covers are provided for the open sides of the slabs and these retain the tubes as well as limiting airflow into and around the details of the slab. Insulating foam may also be applied both externally and to any pockets and voids within the slab provided. These are provided to reduce metal material and cost. The covers may be screwed into place or riveted using posts of slab material deliberately left protruding from the slab as rivets, and secured under a press whilst the tubes are held deformed by the press force. [0132] Figure 12 is a tubular heat exchanger 3.10 made into a serpentine shape which is common in heat exchangers. The returns 3.1 can only be formed at approx 3-4 times the diameter. The straights 3.2 can be any length and have an inlet end 3.3 and outlet end 3.4. Preferably the straights 3.2 are the only parts that contact the slab 3.5 in grooves. [0133] Figure 13 is similar but the overall size has been minimised by deforming the heat exchanger but leaving the straight lengths untouched. [0134] Figure 14 is a plan view of a slab 3.5 into which grooves 3.6 are formed or machined to accept a serpentine coil, and clearance 3.7 for the returns 3.1. The clearance area 3.7 as shown is important to allow mass production of the coils or serpentines without unrealistic tolerances on the overall length of the coil or the form of the bend. Irregularities in these areas would make a perfect fit of the coil into the groove almost not achievable in normal production machinery. The slab has walls 3.8 with notches 3.9 for tube ends 3.3 & 3.4 to exit the heat exchanger slab 3.5. [0135] Figure 15 shows a serpentine coil 3.10 in place in the slab 3.5. Intimate contact between straight tube 3.2 is shown. Clearance around return 3.1 is shown. [0136] Figure 16 shows a cross section of simple form of the present invention. The serpentine coils 3.10 are pressed into groove 3.6 of slab 3.5. It is preferred that the slab 3.5 is cast and the grooves used "as cast". If that is the case the sides of the groove 3.6 22 will be tapered from top to bottom for casting purposes. If the tubes are nominally 10mm, the maximum number of tubes pressed into one groove will be 3.2, as the draft angle will still allow some compression on the top tube and not over-compression on the bottom tube. A cap 3.11 is provided to hold tubes 3.10 in place and is fixed where needed by screws 3.12, or integrated rivets 3.13, preferably fixed whist the unit is under compression whilst in a press. This will aid in keeping the integrity of the surface contact between tubes 3.10 and tubes to slab 3.5. [0137] Figure 17 shows a slab 3.5 which has grooves moulded from both sides to better utilise the moulding operation and allowing some size flexibility in doubling the tube area in a given footprint. In this case 3.2 caps are utilised, and preferably the material shown at 3.15 is retained to keep mechanical integrity if the outer walls 3.8 of slab 3.5 are weakened by inlet outlet notches 3.9. A void 3.14 is shown and its purpose is to minimise material in the slab 3.5 as material in this area does not contribute much to the heat transfer from pipe to pipe. [0138] Figure 18 shows 3.2 slabs 3.5 bolted together to form a more dense heat exchanger. Preferable long through bolts 3.16 are used to ensure elasticity for heat up and cool down (expansion and contraction) cycles. [0139] Figure 19 shows a parallel heat exchanger tube assembly 3.17 mounted in slab 3.5. Heat exchanger 3.17 consists of headers 3.19 and risers 3.18. Preferably only risers 3.18 are pressed into slab 3.5. [0140] The present invention provides for a simple yet effective double wall heat exchanger that can be made in non specialised facilities and at low cost. It provides a dry, easy assembly method and due to elastically deforming the tubes, retains consistent results unit to unit, and long term integrity of the heat exchanger so produced. [0141] The present invention also provides for flexibility in application as the tubes can be configured and cross connected to provide reverse flow or parallel flow or cross flow or many other options depending on end use. [0142] The present invention also provides for different materials for each side of the heat exchanger to minimise costs and ensure correct application of materials for the task.

23 [0143] The present invention provides for greatly differing flows and heat exchange media such as gas and liquid as different diameter tubes can be accommodated within the heat exchanger. Also the present invention provides for one side to be a long series heat exchanger whilst the other side can be a low pressure drop parallel heat exchanger if required. [0144] The common uses would be in potable water heating from a contaminated source such as heat pumps, boilers and solar heaters using an antifreeze heating medium. [0145] Many other uses and modifications will be apparent to those skilled in the art without detracting from the spirit of the invention [0146] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of". A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear. [0147] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention. [0148] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims (5)

1. A water heater including: a tank for holding water to be heated, the tank having a cold water inlet and a hot water outlet located at a top region of the tank; and a heat exchanger that is located remote to the tank, so that a cold well is maintained in a bottom region of the tank, the heat exchanger having an input end which is adapted to receive water from a bottom region of the tank, and an output end which is adapted to deliver heated water to a top region of the tank, the output end being located higher than the input end during operation of the heat exchanger; a gas burner box including a burner located below the heat exchanger, adapted to deliver energy to the heat exchanger.

2. A water heater as claimed in claim 1, wherein the heat exchanger is mounted at an angle of between 10 degrees and 90 degrees from the horizontal.

3. A water heater as claimed in any one of claims 1 to 2, further including a temperature probe located inside the tank, and a gas control for controlling gas supply to the burner box, wherein the gas control is adapted to shut off a gas supply to the burner box when a water temperature detected by the temperature probe is above a minimum level.

4. A water heater as claimed in any one of claims 1 to 3, wherein gas from the burner is directed to one or more directional baffles, and gas from a pilot flame located beneath the heat exchanger is directed to the same directional baffles.

5. A water heater as claimed in any one of claims 1 to 4, wherein an inlet which delivers water to be heated by the exchanger includes or is connected to a dip tube that connects to a bottom region of the tank.