AU2011201690B2

AU2011201690B2 - Distributor Cap for a Falling Film Water Heater

Info

Publication number: AU2011201690B2
Application number: AU2011201690A
Authority: AU
Inventors: Quentin Arthur Carl Adam; Brendan Vincent Bourke; Fred M. Chachaty; Steven Scott Hunt
Original assignee: Rheem Australia Pty Ltd
Current assignee: Rheem Australia Pty Ltd
Priority date: 2004-09-16
Filing date: 2011-04-14
Publication date: 2011-10-06
Anticipated expiration: 2025-09-13
Also published as: AU2005284678A1; WO2006029457A1; NZ553105A; AU2005284678B2; AU2011201690A1

Abstract

A distributor cap 500 for a falling film water heater having a hot water storage tank 100 with an internal heat exchanger 102. The heat exchanger 102 is in the form of a cylinder closed at the top by a cap 104 and also closed at the base. A pump 110 draws off water from the bottom of the heat exchanger 102 circulates the heat transfer fluid back to the top of the heat exchanger at 112 when required. The cap 500 includes a falling film distributor apertures 514 which serve to distribute the heat transfer fluid in a thin film around the inner wall of the heat exchanger 102. The cap is formed of a pair of plates defining a heat transfer fluid cavity therebetween. The heat transfer fluid can be heated in solar water heating panels. When the pump is turned off, the heat transfer fluid drains into the heat exchanger to at least partially fill the heat exchanger. Figure 5 FIGURE 5 514 500504 516 502 A ...... ... ........5 -1 2 ...... ........... A 700 712 708

Description

Distributor Cap for a falling Film Water Heater Field of the invention [001] This invention relates to water heating tanks and water heating systems using such tanks. The invention will be described with reference to solar water heating system. However, the invention is not limited to solar water heating systems and may be applied to other water heating systems which use a heat transfer liquid to transfer heat from a heat source to the water to be heated, usually via some form of heat exchange means. Background of the invention [002] Some gas water heaters include a gas burner located at the bottom of a tank to heat the base of the tank. The tank may have an internal conduit of flue which passes up through the centre of the tank to enable the hot gasses to transfer heat to the water in the tank through the walls of the flue. The base of the tank may be concave to conduct the combustion gas towards the flue. Summary of the invention [003] The present invention also provides a distribution cap for a falling film heat exchanger including a first plate and a second plate, the first and second plates being interfaced and formed to define one or more fluid cavities between the plates to contain heat transfer fluid in the or each fluid cavity, each fluid cavity having one or more outlet apertures to permit the fluid to pass therethrough; wherein the end of at least one of the one or more fluid cavities is shaped to impart an at least partly transverse flow to the fluid. [004] The outlet apertures can be proximate the edge of the plates [005] The one or more fluid cavities can include channels. [006] The channels can be formed by pressing the channels in one or both of the first and second plates. [007] The first and second plates can be interfaced so that the first and second plates are contiguous along the periphery of the fluid channels to confine heat transfer fluid in fluid channels. [008] The distribution cap can include a fluid inlet aperture in the first plate, the fluid aperture being in fluid communication with the or each fluid cavity. [009] The end of at least one of the fluid cavities can be shaped to impart an at least partly transverse flow to the fluid.

2 [010] The outlet of at least one cavity can impart an at least partially tangential flow to the fluid. [0111 The cap can have a substantially circular perimeter and is adapted to fit a substantially cylindrical falling film heat exchanger. [012] A least one of the plates can include peripheral protrusions. [013] The first plate can be formed in the shape of a bowl. [014] The first plate can include a sloping wall. [015] The second plate can engage the first plate. [016] The base of the bowl shape can include a star shape cavity, wherein the star shaped cavity is adapted to serve as a flow path for the heat transfer fluid in use. [017] The points of the star shaped cavity can terminate in deflecting outlets adapted to impart a tangential component and a radial component to the flow of the heat transfer fluid in use. [018] The second plate can include at least one aperture aligned with an aperture in the first plate. [019] The first and second plates can be engaged by welding or brazing. [020] A drain-back water heater system including a tank with a substantially upright internal conduit forming part of a heat transfer liquid circuit and whose contents are isolated from the contents of the tank, and a distribution cap can be proximate the top of the internal conduit. [021] The invention provides a heat transfer fluid distribution cap for a water heater tank, the tank having an internal conduit and a heat transfer liquid delivery means to deliver heat transfer liquid to the conduit. [022] The heat transfer liquid delivery means can distribute the heat transfer liquid around at least part of the inner wall of the conduit. [023] The heat transfer liquid delivery means can deliver a thin film of heat transfer liquid to an upper portion of the inner wall of the conduit. [024] The heat transfer liquid delivery means can be one or more apertures associated with a cap closing a first end of the conduit. [025] The heat transfer liquid delivery means can be a nozzle adapted to deliver the heat transfer liquid to the inner walls of the conduit.

3 [026] The nozzle can be adapted to deliver the heat transfer liquid to the inner walls of the conduit as a thin film. [027] The conduit can be a vertical tube having an upper end proximate to the top of the tank and a lower end proximate to the bottom of the tank, the heat transfer liquid delivery means being located at the upper end of the conduit. [028] The lower end of the conduit can be closed by a closure having a heat transfer liquid outlet therein. [029] The heat transfer liquid outlet can be connected to a pump inlet. [030] The conduit can be made of a material having a high thermal conductivity. [031] The water heater tank can include a first valve connected proximate the bottom of the conduit, and a second valve connected above the fill level of the conduit. [032] The present invention also provides a water heating system which can include a water tank in which the heat transfer liquid delivery means is connected to an external heat exchange means which absorbs heat from a source of heat. [033] The heat transfer liquid outlet can be connected to the external heat exchange means to form a heat transfer liquid circuit. [034] The water heating system can include a pump in the heat transfer liquid circuit and control means to control the rate of circulation of the heat transfer liquid by the pump. [035] The control means can be responsive to one or more temperature sensors to control the rate of circulation of the heat transfer liquid. [036] The control means can be responsive to a flow sensor sensing the flow rate of the heat transfer liquid to control the rate of circulation of the heat transfer liquid. [037] The external heat exchange means can include a solar collector. [038] The water heating system can include an oxygen detector in the heat transfer fluid circuit. [039] The oxygen detector can be connected to a controller which is programmed to indicate a fault condition when the level of oxygen detected by the oxygen detector exceeds a predetermined level for a predetermined period of time after the water heating system is commissioned.

4 [040] The drain-back water heating system can include an oxygen scavenger in the heat transfer fluid circuit. [041] The present invention further provides a method of heating water in a water heating system having a tank with an internal conduit, the method including the steps of: externally heating a heat transfer liquid in a heat transfer liquid heating means; applying the heat transfer liquid to the inner wall of the conduit; and recalculating the heat transfer liquid through the heat transfer liquid heating means to the inner wall of the conduit. [042] The heat transfer liquid can be circulated by a pump, and the rate of circulation of the heat transfer liquid by the pump can be controlled to maintain the temperature of the water and/or the heat transfer liquid within a predetermined range while the heat transfer liquid is being heated externally. [043] The circulation of the heat transfer liquid can be turned off when the heat transfer liquid is not being heated externally. [044] At least some of the heat transfer liquid drains back into the conduit when the heat transfer liquid is not being circulated. [045] The external heat transfer liquid heating means can be a solar collector, and an over temperature sensing means can sense when the temperature of the heat transfer fluid or the water exceeds a threshold temperature and switch off the circulation of the heat transfer liquid, whereupon the heat transfer fluid drains back from the solar collector. [046] The present invention also provides a method of adding heat transfer fluid to a heat transfer fluid circuit of a water heating system in which the drain back is utilized, the heat transfer liquid circuit including an external heat exchange means connected to a conduit by pipes, the method including the following steps: a. determining the capacity of at least the external heat exchanger, b. determining the height of a volume of liquid equivalent to the capacity of the capacity of the external heat exchanger, c. connecting a manometer tube to proximate the bottom of the conduit, and d. filling the conduit to approximately the height determined in step c.

5 [047] The method can also include the steps of: e. determining the minimum fill level of the conduit, and f. filling the conduit to a height equivalent to that determined in step b plus the height determined in step e. [048] The method can include the steps of: g. determining the equivalent height in the conduit of the liquid capacity of the pipes, h. adding the height determined in step g to the fill level. [049] The manometer can be connected to a first valve proximate the bottom of the conduit and to a second valve located at a height above the fill level. [050] The amount of heat transfer liquid added can be such as to result in a drain back head of at least 0.5m. [05 1] The drain back head can be greater than I m. [052] The invention further provides a water heating system having a closed heat transfer fluid circuit and an oxygen sensor in the heat transfer fluid circuit, in which the oxygen sensor is connected to a controller which is programmed to indicate a fault condition when the level of oxygen detected by the oxygen sensor exceeds a predetermined level for a predetermined period of time after the water heating system is commissioned. [053] The present invention also provides a cap for a falling film heat exchanger including a first plate having one or more fluid paths impressed therein and a closure plate applied to form a cover over the or each fluid path, the end of the fluid path being open to permit the fluid to escape therefrom. [054] The cap can include a fluid inlet aperture in the first plate, the fluid aperture being in fluid communication with the or each fluid path. [055] The end of at least one fluid path can be shaped to impart an at least partly transverse flow to the fluid. [056] The cap can have a substantially circular perimeter and is adapted to fit a substantially cylindrical falling film heat exchanger. [057] The outlet of at least one flow path imparts an at least partially tangential flow to the fluid.

6 [058] The second plate can include peripheral protrusions. Brief description of the drawings [059] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [060] Figure I shows a cross-section of a hot water tank embodying the invention. [061] Figure 2 shows detail of a nozzle suitable for use with the tank of Figure 1. [062] Figure 3 shows a solar water heater system embodying the invention. [063] Figures 4A, 4B, & 4C show views of a distributor device suitable for use in an embodiment of the invention. [064] Figure 5 shows a top view of an embodiment of a drain back cap plate. [065] Figure 6 shows a sectional view of the drain back cap plate of Figure 5. [066] Figure 7 shows a closure plate for use with the drain back cap plate of Figure 5. [067] Figure 8 shows a water heater including a falling film heat exchanger using the drain back cap of Figures 5, 6, and 7. Detailed description of the embodiment or embodiments [068] Where an element of an external heat transfer liquid circuit is higher than the level of heat transfer liquid in the conduit, circulation of the heat transfer liquid is aided by the phenomenon of drain back. This means that the heat transfer liquid enclosed in the external heat transfer liquid circuit in the distance between the heat transfer liquid delivery means and the highest point of the external heat transfer liquid circuit is delivered to the heat transfer liquid delivery means under highest feed. Thus, once the pump has initiated the circulation of the heat transfer liquid by pumping the heat transfer liquid through the highest point of the external heat transfer liquid circuit and on to the heat transfer liquid delivery means, the work required of the pump is reduced, as the pump only needs to supply a head equivalent to the distance between the top of the heat transfer liquid collected in the conduit and the heat transfer liquid delivery means. Thus, the pump can be run on reduced power once circulation has commenced. However, the pump can be run on higher power if it is required to increase the circulation rate.

7 [069] Drain back comes into action where the external heat exchanger or the other reservoir of heat transfer liquid in the external heat transfer liquid circuit is located above the top of the conduit. This is of particular use where the system is a solar water system in which the solar panels are at a considerable hight above the tank. However, the same mechanism applies to any case where part of the external heat transfer liquid circuit is above the tank and the heat transfer liquid circuit has a larger capacity than the volume of the heat transfer liquid [070] Figure I shows a cross-section of a tank 100 having an internal conduit 102. The conduit is closed at the top by a cap 104 and at the bottom by a plate 106. A pipe 108 passes through plate 106 and connects to the inlet of pump 110. An inlet aperture 112 is provided in cap 104. A pair of valves 114, 116 are provided at the bottom and top of the conduit. A flow meter 120 and/or temperature sensors 122, 124 are connected to controller 126. [071] The cap 104 includes heat transfer liquid delivery means such as nozzle 118, also shown in detail in Figure 2. [072] When installing the tank, or recharging the heat transfer liquid, the installer can attach a transparent tube between valves 114 and 116 and add heat transfer fluid through inlet 112. This permits the installer to see the level of heat transfer fluid in the conduit. The capacity of the external heat exchanger will be known and the capacity of the connected plumbing readily calculated, so that the installer will know the required volume of heat transfer fluid needed to fill the system to the chosen operating volume. The operating volume can be chosen to leave sufficient free capacity so that, when the heat transfer fluid is not circulating, it can drain back to the conduit. [073] Where the heat source is above the tank, the volume of heat transfer fluid need only be sufficient to fill the external heat transfer fluid circuit and to partly fill the conduit. The pump 1 10 can be used to circulate the heat transfer fluid. [074] In operation, there must be sufficient heat transfer fluid to fill the external heat transfer fluid circuit which can include an external heat exchange means, such as a solar collector 306 shown in Figure 3 or a brazed plate heat exchanger as shown in Figure 4, the interconnecting plumbing and to provide any required head in the conduit. The volume of heat transfer fluid does not need to fill the conduit. However, where the system includes a pump which requires priming, the heat transfer fluid must be provided to a sufficient height above the pump to provide priming as shown at 310 in Figure 3. This can be achieved by providing sufficient heat transfer fluid to 8 provide a head which is adequate to prime the pump. A pump which requires priming can be damaged by cavitation if it is run without priming. Typically, a head of 0.5m is sufficient. Advantageously the pump may be located under the tank or lower than the tank as this will provide at least part of the head. Where such a pump is used, a level for the pump to be switched on. A lift pump can be used to circulate the heat transfer liquid. [075] The temperature sensors 122, 124 can be used to control the circulation rate of the heat transfer fluid. For example, in a solar heating system, the input temperature of the heat transfer fluid can be permitted to have an operating temperature range having a lower and an upper threshold. The operating range can be 65 0 C to 80 0 C. If the inlet temperature of the heat transfer fluid falls below 65 0 C, the circulation rate generated by the pump 110 can b slowed to allow the heat transfer fluid to absorb more solar energy. Conversely, if the temperature exceeds the upper threshold, the circulation rate can be increased to reduce the amount of solar energy absorbed. Additionally, a temperature sensor can sense whether the temperature of the water exceeds a permitted threshold, whereupon the pump can be shut off, permitting the heat transfer fluid to drain back to the conduit and cease heating of the water. [076] Referring to Figure 2, the heat transfer fluid delivery means shown in this embodiment includes a heat transfer fluid receptacle 204 having an inlet 202. The bottom of receptacle 204 includes a plate 206 which is attached to the receptacle by discontinuous welds leaving small apertures between the welds around the periphery of the plate 206 through which the heat transfer fluid can escape. The edges of the plate 206 are sufficiently close to the wall 210 of the conduit so that the apertures are adjacent to the inner wall of the conduit 210 and the escaping heat transfer fluid contacts the wall 210 and runs down the wall. Where the pressure of the heat transfer fluid is sufficient, the apertures may alternatively be spaced from the wall 210 and configured so that the pressure of the heat transfer fluid causes the heat transfer fluid to be sprayed onto wall 210 through the apertures. [077] The apertures are appropriately sized so as to result in a thin film of heat transfer fluid 212 running down the wall 210. This has the benefit that heat in the heat transfer fluid is immediately available for transfer to the wall 210 and hence to the water. The thin film of heat transfer fluid can be of a thinness whereby boundary layer effects within the heat transfer fluid at the contact surface with the wall 210 are overcome. Such a thin film ensures an efficient transfer of heat from the heat transfer fluid to the wall 210. However, we have found that the size of the apertures 9 has an effect on the efficacy of the drain back process, and that the apertures must be sufficiently large to ensure that the surface tension of the heat transfer liquid does not inhibit the drain back effect. In a further embodiment, a surfactant is added to the heat transfer liquid to reduce surface tension. [078] This system results in top down heating of the water, because the heat transfer fluid is at its hottest as it enters the conduit, and it looses its heat rapidly due to the efficient thin film heat transfer. Top down heating results in stratified temperature of the water, with the hot water at the top of the tank and cooler water below. This is advantageous in that it produces a more rapid reheating of the water which is to be drawn off from the top of the tank, as the whole of the contents of the tank do not have to be heated. [079] The inner surface of the conduit may include fins to increase the surface area available for heat transfer. The fins may be in the form of a spiral or they may be, for example, vertical or other suitable configuration. [080] The conduit itself may also be of a shape and configuration other than a vertical tube. For example, the conduit may be a spiral tube with a sufficient internal cross-section to permit drain back action. [081] Where drain back is implemented, the volume of heat transfer fluid id not sufficient to fully fill the external heat transfer fluid circuit and the conduit. The external heat transfer fluid which is located at a greater height than the heat transfer fluid in the conduit can drain back to the conduit when the system is not in operation, for example, where the pump is turned off. This is the case, for example, in a solar water system in which the solar panels are located at a greater height than the tank. [082] Alternative heat transfer fluid delivery means are within the scope of the invention. For example, a rotating spray nozzle driven by the heat transfer fluid pressure can be used. Figure 4A, B, and C illustrate such a rotary spray nozzle generally at 400. The rotary includes a plate 402 adapted to close off the conduit 102, and four spray arms 404 mounted on a central turret 412 which is free to rotate in relation inlet connection 408. As shown in Figure 4C, inlet connection 408 may be fixed to plate 402 on the opposite side to turret 412 and arms 404. The turret 412 may be mounted in inlet connection 408 and have a rotary seal which permits turret 412 and the arms 404 to rotate relative to connection 408. The pressure from the pump should be sufficient so that, as the heat transfer liquid discharges from the sprays arms 404, it causes them to rotate and 10 spray the heat transfer liquid onto the inner wall of conduit 102. In one embodiment, the arms may have mister nozzles 410 fitted to their ends as shown in Figure 4A to produce a spray which impinges on the inner walls of the conduit 102 and acts to transfer heat to the water via the conduit 102. We have found that arms with an inner diameter of 7mm are adequate to prevent the surface tension inhibiting drain back. [083] Alternatively, the arms may have open ends, as shown in Figures 4B & 4C, to permit the heat transfer liquid to discharge and spray against the inner wall of the conduit 102. [084] The plate 402 may also have a hole 406 for mounting the valve 116 (see Figure 1) [085] In an alternative embodiment, the arms and turret may be fixed and the heat transfer liquid sprayed onto the walls from the end of the arms, the spray direction may be such as to produce a circumferential flow when the spray strikes the inner wall of the conduit 102 to provide substantially all round contact. [086] Figure 3 shows a solar water heating system including a tank embodying the invention. [087] A solar panel 300 includes header and footer tanks 302, 304, for the heat transfer fluid leaving and entering the panel, and is connected in the heat transfer fluid circuit of the system by appropriate plumbing. The solar panel 300 can be located at some height above the tank 100, for example, the solar panel 300 can be up to three stories above the tank 100. When the system is installed or after the pump has been turned off, the heat transfer liquid will have drained down at least partly fill the conduit 102, leaving the upper parts of the external heat transfer liquid circuit, including, in this case, the solar panel 300, empty. At start-up, the pump I 10 pumps heat transfer liquid form the conduit up to the solar panel and around to the heat transfer liquid delivery means. The height differential from the solar panel to the heat transfer liquid delivery means provides the pressure head to spray the heat transfer fluid through nozzle 118 onto the walls of the conduit 102. This can be of the order of 10 kPa where the solar panel is three stories above the tank. [088] The heat transfer fluid runs down the walls of the conduit and collects at the bottom of the conduit. The pump 1 10 then recirculates the heat transfer fluid back to the solar panel 300. Once the pump 1 10 has initiated the circulation of the heat transfer liquid, the power required to drive the pump 1 10 is reduced because the pump now needs to produce only sufficient head pressure to raise the heat transfer liquid to the height of the heat transfer liquid delivery means instead of to the height of the solar panel. This equates to the height "h" shown in Figure 3. This is because the 11 drain back effect of the heat transfer liquid in the pipe from the solar panel to the heat transfer liquid delivery means, pipe 123, cancels the corresponding "upstream" path to the solar panel via pipe 125 and the height of the solar panel 300. [089] The height "h" may be greater than half a meter, and is preferably at least one meter. In one embodiment, the height "h" is about 1.2m. This height provides the "head" to drive the drain back as it is the difference between the "flow" and :return" paths minus the pump head in the bottom of the conduit 102, where the "flow" path is the path from the conduit outlet to the top of the external heat transfer liquid circuit, and the return path is the height from the top of the external heat transfer liquid circuit to the heat transfer liquid delivery means. [090] Preferably the pump is designed to permit back flow when it is not running. The pump impeller should be in "freewheel" mode when the pump is not powered. This facilitates drain back through the pump to the conduit 102. [091] The conduit is at least partly empty as shown in Figure 3 when the heat transfer fluid is being circulated by the pump 110. This provides the exposed walls of the conduit 102 on which the film of heat transfer fluid is sprayed to transfer the heat to the water. [092] Where the water is at mains pressure, the heat transfer fluid may be at a lower pressure. This is advantageous as it ensures that, if there is a hole in the wall of the conduit 102, the leakage will be from the water to the heat transfer fluid circuit, and not the other way. This avoids contamination of the water by the heat transfer fluid. [093] The heat transfer fluid may be, for example, water, distilled water, or other suitable fluid, and these may be combined with additives such as glycol and/or rust inhibitor. [094] Even in the case where the heat transfer fluid circuit includes mild steel components, it may not be not necessary to bleed excess air from the heat transfer fluid circuit, because the heat transfer fluid circuit is sealed to atmosphere and the oxygen initially entrained in the heat transfer fluid circuit will be used up without causing excessive rusting. However, the heat transfer fluid circuit, or some of the components of the heat transfer fluid circuit may be selected as corrosion resistant of treated to reduce corrosion. For example, the inner wall of the conduit may be glass coated or enameled. [095] In a further embodiment, also shown in Figure 3, leak detection can be provided by the use of an oxygen sensor 128 in the heat transfer fluid circuit. The output from the oxygen sensor 12 128 can be connected to controller 126 which is programmed to determine whether the level of oxygen exceeds a threshold value for a predetermined time after the water heater has been commissioned. The oxygen sensor 128 can be located so that it is normally covered by heat transfer fluid and detects the level of oxygen in the heat transfer fluid, or the oxygen sensor can be located in the air gap in the conduit 102 and can detect the oxygen in the air gap in the conduit 102. The presence of oxygen above a predetermined level for a predetermined period of time can be used as an indicator that there is a leak in the heat transfer fluid circuit, because it can be empirically determined that, in the absence of a leak, most of the oxygen will be consumed in oxidation reactions within the predetermined time period. [096] In the event that the detected level of oxygen exceeds the predetermined value after the predetermined time period, the controller 126 can initiate an alarm signal and/or turn the water heating system off. [097] Figure 5 shows one embodiment of a cap for a drain back tank made from a pair of plates. A first plate 500 is formed in the shape of a shallow bowl 500 which includes a sloping wall 502. The bowl 500 has a star shaped cavity 506 impressed into the base 504. This cavity, when closed by the second plate 700 shown in Figure 7, serves as a flow path for the heat transfer fluid. The edges of the impressed formation 506 are sloped as shown at 508. The points of the arms of the star shape 506 terminate in deflecting outlets 510, 514 which serve to impart a tangential component as well as a radial component to the flow of the heat transfer fluid draining back into the drain back tank. The heat transfer fluid enters the star shaped cavity via a pipe connected to aperture 5 12. An additional aperture 5 16 can be used to align with a corresponding aperture 708 of the second plate 700 to enable a probe or pressure relief valve to be inserted into the flue. [098] Figure 6 is a cross-sectional view of the drain cap 500 along line AA. The wall of the dish is shown at 602. The cavity formed by the star shaped impression is shown at 606. As shown at 610, the outlet 510 is slightly behind the section line AA at the point at which it opens through the wall 602. The outlet 614 is shown as an opening in the wall 602. The inlet aperture is shown at 612. [099] In the star shaped cavity, the cross section of the arms reduces towards the tips of the arms. The curved tips 510, 514 impart a tangential component to the heat transfer fluid as it contacts the wall. This facilitates the spreading of the heat transfer fluid over the inner wall of the heat exchange tube to form a falling film to provide efficient heat transfer.

13 [0100] Figure 7 shows the closing plate which completes the water path with the upper plate 500. The plate 700 is provided with one or more apertures. Aperture 708 aligns with a corresponding aperture 516 in the upper plate 500. Peripheral protrusions 702 can be provided around the edge of the lower plate 700. These protrusions can be sized to have a small clearance from the inner wall of the drain back tank. The protrusions can facilitate distribution of the heat transfer fluid as a falling film on the inner wall of the drain back tank. A dotted outline 706 illustrates the orientation of the star shaped cavity when the lower plate 700 is attached to the upper plate 500. [010 1] Plates 500 and 700 can be joined by brazing, welding or other suitable means for forming a water tight peripheral join around the edge of the star shaped cavity 506. However, it is not essential that the periphery of the star shaped cavity be sealed to the closure plate as the star shaped cavity forms a lower flow resistance path so most of the heat transfer fluid will pass through the arms of the cavity. Thus the closure plate can be joined to the upper plate by welding around the periphery of one or more of the "blank" holes 704, 710, 712 in the lower plate. As mentioned, the outer edges of the lower plate 700 have a slightly smaller circumference than the outer edge of tapered wall 502, 602 the upper plate 500. [0102] The drain back tank cap formed by joining the plates 500 and 700 can be sealingly attached to the top of the flue by welding, brazing or other suitable means for forming a water tight seal. Preferably, the join is formed around the upper rim of the wall 502, 602. [0103] Alternatively, the cap can be attached by the use of a sealing ring or gasket. In this case provision can be made to bolt the cap to the drain back tank. [0104] A similar sealing cap can be used seal the lower end of the flue. The lower cap may or may not include the second plate 700. The second plate 700 can be included to provide strength and rigidity. Because it is not necessary for the water to be sprayed around the walls of the flue at the lower end of the drain back tank, the formation of the star shaped cavity is not required, and the outlet pipe can be connected through both the dish shaped plate 500 and the closure plate 700. [0105] An oxygen scavenger may be included in the heat transfer fluid circuit to reduce corrosion. This can be in the form of a mild steel pipe which acts as a corrosion target. The mild steel pipe can be suspended in the drain back tank. Preferably, the corrosion target is suspended above the high water mark of the heat transfer fluid when it has drained back into the drain back tank.

14 [0106] A pressure relief valve can pass through the upper and lower plates 500, 700 to relieve excess pressure in the heat transfer fluid circuit. [0107] Figure 8 shows a water tank 802 with an internal drain back tank 804 which is closed at the top by a falling film cap 801 made in accordance with the embodiment of Figures 5, 6 and 7. The sloping wall of the cap is shown at 806, and the star shaped cavity is indicated in dotted outline at 808. The closure plate is shown in dashed outline at 810. [0108] A corrosion target 812 is shown suspended below the cap. [0109] The bottom of the drain back tank 804 is closed by a second plate 814 which corresponds to the upper plate shown in Figures 5 & 6. [0 110] One advantage of the present invention is that it allows the option of sing an existing flued gas heater tank in this new application by closing off the ends with the cap 104 and plate 106. The plate 106 can be integrally formed as part of the dome shaped minus end of the tank 100, with pipe 108 sealing passed therethrough. [0111] In a solar water heating system, drain back has the advantage of providing protection against damage to the heat transfer liquid by providing a gas expansion region above the heat transfer liquid in the conduit 102. Where the solar energy is excessive, the pump is turned off and the heat transfer liquid drains back to the conduit. This prevents the heat transfer fluid heating above its designed operating temperature and possibly boiling or increasing the internal pressure in the heat transfer liquid circuit to the point where elements of the circuit are ruptured or damaged. [0112] Another advantage is that, because the conduit is inside the water tank, it does not require insulation of its own, because the water tank is insulated. This "integrated" arrangement provides an advantage in that the internal heat exchange function provided by the internal conduit provides a "clean" appearance by doing away with the need for a major external component. The footprint of the arrangement is also improved by removing the need for a major external component such as a heat exchanger. This is an advantage in finding a suitable location for the tank installation, particularly in restricted spaces. [0 113] In addition, this system can provide drain back in circumstances where drain back may be difficult to implement, such as where there are extended lengths of horizontal of "up-and-down" sections in the flow/return plumbing. The system can provide a significant vertical distance in 15 height between the level of the working fluid and the heat transfer liquid return fitting inside the integrated drain back vessel. [0 114] It will be understood that the invention disclose and defined herein extends to all practicable alternative combinations of two or more of the individual features mentioned of evident from the text. All these different combinations constitute various aspects of the invention. The foregoing describes embodiments of the invention, and modifications obvious to those skilled in the art can be made thereto without departing from the scope of the present invention.

Claims

1. A distribution cap for a falling film heat exchanger including a first plate and a second plate, the first and second plates being interfaced and formed to define one or more fluid cavities between the plates to contain heat transfer fluid in the or each fluid cavity, each fluid cavity having one or more outlet apertures to permit the fluid to pass therethrough; wherein the end of at least one of the one or more fluid cavities is shaped to impart an at least partly transverse flow to the fluid.

2. A distribution cap for a falling film heat exchanger as claimed in claim 1, wherein the outlet apertures are proximate the edge of the plates

3. A distribution cap for a falling film heat exchanger as claimed in claim I or claim 2, wherein the one or more fluid cavities include channels.

4. A distribution cap for a falling film heat exchanger as claimed in claim 3, wherein the channels are formed by pressing the channels in one or both of the first and second plates.

5. A distribution cap for a falling film heat exchanger as claimed in claim 3 or claim 4, wherein the first and second plates are interfaced so that the first and second plates are contiguous along the periphery of the fluid channels to confine heat transfer fluid in fluid channels.

6. A distribution cap as claimed in any one of the preceding claims, including a fluid inlet aperture in the first plate, the fluid aperture being in fluid communication with the or each fluid cavity.

7. A distribution cap as claimed in any one of the preceding claims, wherein the outlet of at least one cavity imparts an at least partially tangential flow to the fluid.

8. A distribution cap as claimed in any one of the preceding claims, wherein the cap has a substantially circular perimeter and is adapted to fit a substantially cylindrical falling film heat exchanger.

9. A distribution cap as claimed in any one of the preceding claims, wherein at least one of the plates includes peripheral protrusions.

10. A distribution cap as claimed in any one of the preceding claims, wherein the first plate is formed in the shape of a bowl.

I1. A distribution cap as claimed in claim 10, wherein the first plate includes a sloping wall. 17

12. A distribution cap as claimed in claim 11, wherein the second plate engages said first plate.

13. A distribution cap as claimed in claim 12, wherein the base of the bowl shape includes a star shape cavity, wherein the star shaped cavity is adapted to serve as a flow path for the heat transfer fluid in use.

14. A distribution cap as claimed in claim 13, wherein the points of the star shaped cavity terminate in deflecting outlets adapted to impart a tangential component and a radial component to the flow of the heat transfer fluid in use.

15. A distribution cap as claimed in any one of claims 10 to 14, wherein the second plate includes at least one aperture aligned with an aperture in the first plate.

16. A distribution cap as claimed in any one of claims 10 to 15, wherein the first and second plates are engaged by welding or brazing.

17. A drain-back water heater system including a tank with a substantially upright internal conduit forming part of a heat transfer liquid circuit and whose contents are isolated from the contents of the tank, and a distribution cap as claimed in any one of the preceding claims proximate the top of the internal conduit.

18. A distributor cap substantially as herein described with reference to the embodiments of the invention illustrated in the drawings.