AU2005204334B2 - Water heater with freeze protected bypass radiator - Google Patents

Description

104 1 Water Heater With Freeze Protected Bypass Radiator FIELD OF THE INVENTION [01] This invention relates to water heaters, and is applicable to water heaters which derive their heat from an unregulated heat source. Such heat sources include solar heaters, geothermal heaters, heat pump systems, and so on. One problem with water heaters having an unregulated heat source is the potential for overheating. [02] Our co pending patent application entitled Overtemperature Protection System discloses an arrangement for dealing with the problem of overheating by providing a bypass valve and a heat bypass radiator whereby the heat transfer fluid is diverted away from a HTF/water heat exchanger and directed to the bypass radiator which dissipates- the excess heat to atmosphere. In another embodiment, the water is diverted to the bypass radiator. [03] However, solar water heaters need to operate in extremes of temperature, and another problem which can arise is the freezing of the HTF or water in the bypass radiator which is usually in a location exposed to ambient conditions. Freezing can damage the plumbing associated with the bypass radiator. [04] It is thus desirable to provide a means of protecting the bypass radiator from damage due to freezing of the contents of the bypass radiator arrangement. BACKGROUND ART [05] The thermal mass/radiation profile refers to the volume of fluid cooled by an adjacent radiation surface in relation to the radiation capacity of the adjacent radiation surface. [06] The applicant is not aware of previous use of freeze protection arrangements with bypass radiators. [07] However, US patent 5413091, in the name of Rheem Australia Limited, discloses an arrangement for protecting a solar collector panel from freeze damage. This patent discusses a tapered fin arrangement for promoting controlled progressive freezing to permit the unfrozen fluid in a tube to be pushed out the end of the tube rather than being trapped in the tube and then freezing. The unfrozen fluid is intended to be pushed to a larger volume of fluid having a sufficiently large thermal mass to prevent freezing in expected overnight conditions. US 5413091 discusses the reasons why such a system does not work in practice.

104 2 DISCLOSURE OF THE INVENTION [08] This invention proposes a method and arrangement for preventing freeze damage to a bypass radiator in a solar water system. [09] The method includes providing the or each fluid path of the bypass radiator with a thermal mass to radiation profile which provides controlled freezing of the fluid in the fluid paths. [010] The arrangement includes a bypass radiator having one or more fluid paths, in which the or each fluid path has a thermal mass to radiation profile to produce a controlled freezing of the fluid in the fluid path. [011] Preferably the freezing is controlled to progressively freeze the fluid towards at least one end of the tube. [012] Freezing maybe controlled to start at the middle of the tube. [013] Freezing may be controlled to start at one end of the tube. [014] Where the bypass radiator is a finned tube, the fins may be modified in a number of ways to produce the thermal mass/radiation profile. [015] The fins may be tapered from one end. [016] The fins may be tapered from the middle to each end. [017] The fins may have their radiation capacity altered by progressively partially coating the fins with an insulating or radiation inhibiting material to produce the desired thermal mass/radiation profile. [018] Alternatively, the fluid passage of the tube may be modified to produce the desired thermal mass/radiation profile. [019] The tube may be tapered from one end to progressively increase the amount of fluid adjacent to each fin along the length of the tube. [020] The tube may be tapered from the middle to progressively increase the amount of fluid. adjacent to each fin in either direction towards each end of the tube. [021] This may be done by providing a tapered tube, a stepped tube, or by providing a tapered insert within the tube. [022] A combination of tapered tube and modified fins is also within the inventive concept.

104 3 BRIEF DESCRIPTION OF THE DRAWINGS [023] Figure 1 shows an embodiment of a solar water heating system with a bypass radiator. [024] Figure 2 shows a solar water heating system with a bypass radiator in which the fins are modified according to an embodiment of the invention. [025] Figure 3 shows a bypass radiator modified according to another embodiment of the invention. [026] Figure 4 shows a bypass radiator modified according to a further embodiment of the invention. [027] Figure 5 shows an embodiment in which the profiled radiation profile is achieved by modifying the radiation characteristics of the fine. [028] Figure 6 shows an embodiment in which the profiled radiation profile is achieved by use of a profiled insert. [029] Figure 7 shows an embodiment in which the radiation profile is modified by varying the diameter of the pipe. [030] Figure 8 shows a further embodiment in which the profiled radiation surface is formed of a partially flattened tube. DESCRIPTION OF THE INVENTION [031] The invention will be described with reference to the accompanying drawings. [032] Figure 1 shows a solar water heating system fitted with a bypass radiator. [033] A hot water tank 14 has a cold water inlet 20 and a hot water outlet 18. The tank is surrounded by a heating jacket 16, which may be formed between a double wall, or by a series of coils wound around the tank. The heating jacket is connected to a pair of solar panels 28 via an inlet pipe 24 and an outlet pipe 22. [034] Alternatively a free-standing heat exchanger may be used to transfer heat between the heat transfer fluid from the solar collectors 28 and the water to be heated. [035] The solar panels are exposed to solar radiation, which is an unregulated heat source. Thus, in hot cloudless weather, the amount of heat absorbed may be sufficient to cause the water to overheat. It is thus necessary to make provision for dealing with such events.

104 4 [036] An overtemperature sensor (not shown in detail) is provided to operate-valve 26 if the temperature of the water exceeds a specified value. The valve 26 opens a path to bypass radiator 10. The bypass radiator may be fed via a pump (not shown) or it may be driven by thermosyphoning if the radiator 10 is located above the tank 14. [037] The bypass radiator 10 shown in this embodiment is a finned tube, but other types of radiator may be used. The water is fed to the bypass radiator which dissipates excess heat to the atmosphere. [038] The valve 26 may be triggered by a thermal expansion trip mechanism such as a phase change wax device or other suitable expansion device such as a polyethylene rod, or it may be triggered by an electronic sensor and control means. The trip mechanism may be arranged to provide a gradual open in of valve 26 over a predetermined temperature range, or it may be arranged to provide a fairly abrupt opening when the temperature reaches a predetermined threshold value. [039] Other parameters indicative of temperature, such as the pressure of the water, the pressure of the heat transfer fluid, or the temperature of the heat transfer fluid, may be used to switch the valve 26. [040] Where the valve 26 is operated in an abrupt switching mode, the control means operating valve 26 is designed to re-close the valve when the temperature returns to a second predetermined value lower than or equal to the initial threshold value. Preferably this temperature is sufficiently below the valve opening temperature to prevent continuous opening and closing of valve 26. [041] The fin and tube radiator 10 is susceptible to freezing damage because the fluid in the tube may at or near the ends while fluid further in the tube may still be unfrozen. When the unfrozen fluid falls below freezing point, it will expand if it is water, or if it contains sufficient water to expand on freezing. If the ends of the tube are already closed by frozen fluid, the expansion may burst the tube or rupture joints in the tube. [042] This invention addresses the problem of avoiding freezing damage in such circumstances. [043] Figure 2 shows detail of a fin and tube radiator 40 modified according to a first embodiment of the invention. the radiator includes a tube 41 and fins 42. The drawing schematically represents a longitudinal cross-section. The tube 41 and fins 42 may have any chosen transverse cross-section, such as circular, square, rectangular, etc. In the longitudinal 104 5 direction, the fins 42 are tapered from the centre to the ends and the tube 41 is of constant cross section. This means that there is a larger cooling surface for the fluid adjacent larger fins 43 than there is for the fluid adjacent the smaller fins 44. Thus the fluid in the tube at the centre of the tube will lose heat more rapidly than the fluid at the ends of the tube. [044] This means that, in freezing conditions, the fluid will start to freeze from the centre of the tube and freezing will progress towards the ends. This reduces the probability of unfrozen fluid being trapped in the pipe and subsequently freezing if the fluid at the ends of the tube freeze before fluid further into the tube. The unfrozen fluid is forced back into the connecting pipes because the ends of the tube freeze after the fluid further up the tube. [045] Preferably, the connecting pipes between the bypass radiator 10 and the water tank 14 are lagged. Additionally or alternatively, these pipes may be of a larger cross-section than the bypass radiator tube 41 so that they form a larger thermal reservoir and will thus not freeze as readily as the contents of tube 41. In addition, the pipes do not have cooling fins, so they will not lose heat as rapidly as tube 41. [046] While the taper of the fins is shown as commencing at the centre of the tube, it could alternatively start from one end and progress to the other. An advantage of starting at the centre is that the overall taper difference is halved for the same result. By way of example, if the centre taper arrangement required the centre fins to be twice the size of the end fins, starting the taper at one end would require the start fins to be four times the size of the small end fins, "size" in this context referring to heat transfer capacity. [047] Starting the taper at the centre (double taper) is also more suited to long lengths of tube than the straight taper from one end (single taper). [048] Thus one means of modifying the thermal mass/radiation profile is to leave the fluid path cross-section constant and vary the radiation surface according to the desired profile. [049] An alternative means of achieving the desired thermal mass/tadiation profile is to leave the radiator surface constant and to vary the fluid path cross-section. [050] This method is illustrated in Figure 3. In Figure 2, the pipe 51 retains a constant cross section and the fins 52 also have a constant cross-section, and a tapered insert 53 is placed in the tube 51. [051] The insert may have a double taper from the centre as shown, or a single taper (not shown) starting from one end.

104 6 [052] The insert 53 may include retaining members 54 to restrict movement of the insert. [053] Alternatively, the insert may be in the form of a tube with a profiled inner circumference and an outer circumference adapted to the inner circumference of tube 51. [054] In the embodiment of Fig 4, the thermal reservoir is stepped along the length of the tube. The centre portion 64 has the smallest diameter, and the diameter progressively widens in steps 65, 66, to the end 67. The low thermal capacity of the smaller tube section 64 which has the same or a larger radiation surface compared with the other sections, means that section 64 will freeze first, and the other stages will progressively freeze from the smaller cross-section portions to the larger cross-section portions. [055] Figure 5 shows a bypass radiator arrangement in which the effective radiation surface of the fins 72 is modified by applying a progressively tapered radiation reducing coating 73 to the fins. [056] The coating may be applied by dipping one end of the assembled fin and tube radiator into a tank of liquid coating material at an angle corresponding to the desired taper so that one half of the assembly is in contact with the coating material, and the other is above the liquid, and rotating the assembly about its longitudinal axis. [057] The liquid coating material will set on the fins. It is selected to have a radiation reducing effect or an insulating effect to reduce the amount of heat lost from the parts of the fins coated with the coating material. [058] The viscosity of the coating material needs to be such as to limit the amount of "run off'. [059] If the viscosity is too low, the assembly is not rotated but raised until the coating is sufficiently "set". When the coating is set, the assembly can be rotated to coat another part of fins. [060] Alternatively sufficient coating may be achieved with a single "dip". It is not essential that the coating be applied to the fins with axial symmetry. What is required is to alter the radiation profile to achieve progressive freezing towards the end(s). [061] Alternatively, the fins may be pre-coated in batches before being assembled onto the tube. [062] In a further embodiment, the radiator may consist of a tube arrangement without radiation fins. In this case, the protection is achieved by progressively changing the fluid path cross section diameter. Embodiments of this arrangement is shown in figures 6& 7. Figure 6 104 7 corresponds to the tube arrangement of Figure 3 without the fins, and Figure 7 corresponds to the tube arrangement of Figure 4 without the fins. In these embodiments, the outer surface of the tubes forms the radiation surface, so that the smaller cross-section portions have a greater radiation surface to volume ratio than the larger cross-section portions. (For circular tubes,. volume is proportional to square of radius, surface is proportional to radius). [063] Figure 8 shows a further embodiment in which the radiation profile is modified by partially flattening the centre portion of a tube. This leaves the surface area substantially unchanged, but reduces the internal cross-section in the region which has been flattened. Figure 8A shows a perspective view of a partially flattened tube which has been partially flattened in the middle. Figure 8B shows a side view of the partially flattened tube. Figure 8C shows a plan view of the partially flattened tube. A radiator can be built of an array of such flattened pipes. The pipes may also interconnect header tanks. [064] It will be understood that the invention disclosed herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the specification. All these different combinations constitute alternative aspects of the invention. [065] The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art, can be made without departing from the scope of the present invention.

Claims (14)

1. A method of protecting a bypass radiator from freezing damage, the radiator including one or more fluid paths, the method including imposing a thermal mass/radiation profile on the fluid path, whereby, in freezing conditions, the fluid starts to freeze in a controlled manner to avoid trapping unfrozen fluid in the fluid path.

2. A method as claimed in claim 1, wherein the thermal mass/radiation ratio is smaller at one end of a path than at the other end of said path.

3. A method as claimed in claim 1, wherein the thermal mass/radiation ratio is smaller at the middle of a path than at either end of said path.

4. A bypass radiator for a solar water heater, the radiator including one or more fluid paths, wherein the or each fluid path is formed with a thermal mass/radiation profile whereby, in freezing conditions, the fluid starts to freeze in a controlled manner to avoid trapping unfrozen fluid in the fluid path.

5. A bypass radiator as claimed in claim 4, wherein the radiator is of the fin and tube type, and wherein the effective radiation surface of the fins is tapered to control the freezing progressively from the fins with the larger effective radiation surface to the fins with the smaller radiation surface.

6. A bypass radiator as claimed in claim 5, wherein the fins are partially coated with an insulating coating to produce the desired thermal mass/radiation profile.

7. A bypass radiator as claimed in any claim 5 or claim 6, wherein the size of the radiation fins is tapered to control the freezing progressively from the larger to the smaller fins.

8. A bypass radiator as claimed in any one of claims 4 to 7, wherein the effective cross section of the fluid path is tapered to progressively alter the thermal mass of fluid to control freezing progressively from the lower thermal mass to the higher thermal mass.

9. A bypass radiator as claimed in claim 4, wherein the radiator is of the fin and tube type, and wherein the fins are coated with a radiation altering coating so as to produce the desired thermal mass/radiation profile.

10. A bypass radiator as claimed in claim 4, wherein the radiator includes a tube, and wherein a tapered insert is placed in the tube to progressively reduce the cross-section of the tube available to carry the fluid. 9

11. A bypass radiator substantially as described herein with reference to the accompanying drawings.

12. A hot water system including a bypass radiator as claimed in any one of claims 4 to 9.

13. A hot water system substantially as herein described with reference to the accompanying drawings.

14. A bypass radiator as claimed in any one of claims 4 to I including one or more pipe sections in which the centre of the pipe section is progressively flattened to produce an altered thermal profile.