WO2003027580A1

WO2003027580A1 - Liquid heating apparatus

Info

Publication number: WO2003027580A1
Application number: PCT/GB2002/004321
Authority: WO
Inventors: Steven Moulder
Original assignee: Imi Vision Limited
Priority date: 2001-09-22
Filing date: 2002-09-23
Publication date: 2003-04-03
Also published as: GB0406171D0; GB2395771A; GB0122856D0; US20050041962A1

Abstract

A method and apparatus for preventing build-up of deposits on internal surfaces (6) of a water heater (1). A heat shock treatment is periodically applied to the surfaces (6) resulting in differential thermal expansion and/or contraction between the internal surfaces (6) and deposits (7) adhering thereto that causes the deposits (7) to crack and break away from the surfaces (6). The heat shock treatment may include the step of alternately heating and cooling the internal surfaces (6) in any order and may be repeated one or more times. The internal surfaces (6) may be low friction surfaces to reduce adhesion between the surfaces (6) and any deposits (7) that facilitates removal of the deposits (7) during the heat shock treatment.

Description

LIQUID HEATING APPARATUS

This invention relates to liquid heating apparatus. The invention is especially concerned with a method for removing deposits that form during operation of liquid heating apparatus and to liquid heating apparatus in which the removal of such deposits is facilitated. The invention has particular, but not exclusive, application to water heaters, boilers and similar apparatus.

For convenience, the invention will be described hereinafter with reference to apparatus for heating water but it will be understood that the invention is not limited thereto and has application generally to apparatus for heating liquids where removal of deposits formed during operation of the apparatus is desirable.

Accordingly, as used herein the term "water" is intended to include any liquid from which deposits may be formed when heated and is to be construed accordingly.

A common problem, especially in water heaters, is the accumulation of deposits on the internal walls, in particular, but not exclusively, calcium carbonate deposits. This occurs particularly where the calcium carbonate content of the water is high.

In water heaters, the build-up of such deposits can cause problems by affecting the heating cycle and may eventually result in premature failure of heating elements by collecting around the heating element and causing the element to overheat and/or burn out. The present invention has been made from a consideration of the foregoing problems and disadvantages of the existing water heaters.

Thus, it is an object of the present invention to provide a method of removing at least some of the deposits that form during operation of water heaters and to water heaters in which removal of such deposits is facilitated.

According to a first aspect of the invention, there is provided a method for removing deposits from internal surfaces of water heating apparatus by applying a heat shock treatment to the internal surfaces.

By the invented method, deposits formed on internal surfaces of the water heater are subjected to a heat shock treatment that promotes cracking of the deposits and reduces the adhesion of the deposits to the internal surfaces.

More particularly, the internal surfaces are subject to thermal contraction and/or expansion during the heat shock treatment. In this way, the differential rate of expansion and/or contraction between the surfaces and the deposits results in cracking of the deposits which break away from the surfaces and fall to the bottom of the apparatus. For example, the internal surfaces may be caused to expand by heating and/or contract by cooling

Cooling the surfaces may be effected by flushing the apparatus with cold water. Heating the surfaces may be effected by heating water in the apparatus and/or by heating the surfaces directly e.g. by resistance heater (s) surrounding or embedded in the surfaces. The steps of flushing with cold water and heating the water may be carried out in any order. Preferably, the flushing with cold water follows a cycle in which the water has been heated. In this way, the thermal contraction caused by rapid flushing with cold water is enhanced promoting cracking of the deposits and increasing further the effectiveness of the heat shock treatment.

In a typical application, the apparatus may be operable to provide and maintain a supply of hot water. In this case, the apparatus is rapidly cooled down by flushing with cold water producing thermal contraction of the internal surfaces causing the deposits to crack and begin to break off from the internal surfaces. The apparatus may then be rapidly heated up by heating the water and/or the internal surfaces producing thermal expansion of the internal surfaces causing the deposits to crack and break away even further from the internal surfaces.

In this way, the thermal contraction and expansion of the apparatus produced by the heat shock treatment produces forces acting on the deposits adhered to the internal surfaces that are more extreme than the deposits would otherwise be subjected to under normal operating conditions of the apparatus. As a result, the heat shock treatment enhances cracking and breaking away of the deposits from the internal surfaces. Flushing with cold water and/or expansion of water trapped in the deposits during re-heating may assist in breaking away any deposits that have been loosened by cracking and are no longer tightly adhered to the internal surfaces.

The method may include repeating one or both steps of heating and cooling more than once. For example, we may repeat the step of flushing the tank with cold water after the step of heating the water in the tank. This may further assist removal of deposits that have been cracked and may allow the deposits to be flushed from the tank.

In a preferred arrangement, the method further includes the step of providing a low adhesion surface on the internal surfaces of the apparatus. In this way, breaking away of deposits that have cracked is enhanced promoting removal of the deposits from the internal surfaces and increasing the effectiveness of the heat shock treatment still further. As used herein "a low adhesion surface" is a surface to which deposits from the water adhere less strongly than to a surface formed of stainless steel.

Advantageously, the method includes the step of collecting deposits that have broken away from the internal surfaces and periodically removing the deposits. For example, the deposits may collect at the bottom of a tank in which the water is heated and the tank is adapted to allow removal of the deposits. Thus, the tank may be provided with an openable drain in the bottom through which the deposits can be flushed at intervals. Alternatively, the bottom of the tank may be in the form of a detachable sump that can be disconnected at intervals to dispose of deposits collected therein.

According to a second aspect of the invention, there is provided apparatus for heating water comprising a tank for holding water to be heated and means for heating water in the tank, wherein the tank has low adhesion internal surfaces.

By providing the invented water heater of the present invention with low adhesion internal surfaces, deposits adhere less strongly to the internal surfaces and are more easily broken away from the internal surfaces. For example, the deposits may crack and break away from the internal surfaces when subjected to a heat shock treatment by the method according to the first aspect of the invention. In this way, removal of deposits from the internal surfaces is facilitated.

The tank may be made of metal, for example stainless steel or aluminium, provided with low adhesion internal surfaces by a lining of a material to which deposits adhere less strongly than the metal of the tank. The lining may be applied to all the internal surfaces of the tank or to selected regions where it is desired to facilitate removal of deposits formed during operation of the water heater.

The lining material is preferably inert to water and capable of withstanding temperature changes occurring during operation of the water ^• heater without degrading to any appreciable extent. A suitable lining material is polytetrafluoroethylene (PTFE) but it will be understood that other materials may be employed to suit any particular application.

The lining may be pre-formed for insertion in the tank. More preferably, however, the internal surfaces of the tank are coated with PTFE or other suitable material to form the lining. Any suitable method may be employed to coat the internal surfaces, for example spraying or dipping. The lining is preferably of substantially uniform thickness, for example from a few microns up to about 2mm and the coating may be applied in one or more layers to provide the desired thickness of lining. If the lining is not applied to all the internal surfaces, areas where it is desired not to form the lining may be masked or otherwise treated to prevent adhesion of the lining material. The lining provides the tank with relatively smooth, low adhesion internal surfaces that facilitates removal of deposits formed during operation of the water heater by the above-described heat shock treatment. Thus, the thermal contraction and expansion of the tank produced by alternately flushing the tank with cold water and then heating the water in the tank generates forces that cause the deposits to crack and break away from the lining more readily than from the material forming the surfaces of the tank covered by the lining.

In an alternative arrangement, the tank may be made from a material providing low adhesion of deposits formed on internal surfaces of the tank. For example, the tank may be made of a plastics material such as polyethylene by any suitable method, eg moulding. In this arrangement, a lining of low adhesion material is not required but the material chosen should produce sufficient expansion and contraction of the tank under the conditions of the heat shock treatment to cause cracking of the deposits.

Preferably, the tank has a base wall and side walls upstanding from the base wall of which at least the side walls have low adhesion internal surfaces. In this way, deposits that crack and break away from the side walls during the heat shock treatment fall to the bottom of the tank where they collect. The side walls may extend normal to the base wall and can be of circular cross-section, for example the tank may be of cylindrical shape. More preferably, however, the side walls are inclined relative to the base wall at an angle of less than 90° so as to converge in a direction away from the base wall. The side walls may have an inclination > 70° < 90° relative to the base wall. The inclination of the sidewalls may assist break away of the deposits from the side walls during the heat shock treatment under the force of gravity. The water may be heated in the tank by any suitable means positioned internally or externally of the tank. Preferably, the heating means comprises at least one electrical heating element positioned within the tank. The or each electrical heating element may be of any type conventionally employed in water heaters. The power rating is preferably chosen to enable the water in the tank to be heated in relatively short period of time, especially during the heat shock treatment to enhance cracking of deposits caused by the thermal expansion of the tank.

The tank may have a single heating element of appropriate power rating for both normal operation and for the heat shock treatment. Alternatively, we may provide two or more heating elements allowing selection of different power ratings. For example, at least one heating element may be employed for heating the water during normal operation of the water heater with additional heating element(s) being employed during the heat shock treatment. In this way, the power rating can be increased to heat the water more rapidly during the heat shock treatment to accelerate the thermal expansion of the tank which may further assist cracking of deposits on the side walls.

In a preferred arrangement, the or each electrical heating element located within the tank is positioned in the upper half of the tank. In this way, operation of the heating element(s) is not affected by deposits that break away from the internal surfaces of the tank during the heat shock treatment and fall to the bottom of the tank. As a result, the heating efficiency is maintained and the risk of the heating element(s) overheating or burning out as a result of deposits over-growing or burying the heating element(s) is considerably reduced. This may lead to the useful operating life of the heating element being extended. The tank may be adapted to enable deposits that have broken away from the internal surfaces during the heat shock treatment and collected at the bottom of the tank to be removed. In one arrangement, the tank may be provided with a removable sump at the bottom that can be detached for disposal of deposits collected therein and the sump re-attached for continued use of the water heater. In another arrangement, an openable drain may be provided at the bottom of the container through which deposits settling on the bottom of the tank can be flushed. This arrangement has the advantage that the deposits can be removed without disassembling the water heater.

Preferably, the tank has an inlet for connection to a water supply, eg mains water, and an outlet from which water can be discharged from the tank. In a preferred arrangement, the inlet and/or outlet have low adhesion internal surfaces whereby any deposits formed therein can be removed by the heat shock treatment.

According to a third aspect of the invention, there is provided apparatus for heating water comprising a tank for holding water to be heated, the tank having a base and sidewalls upstanding from the base, at least one electrical heating element positioned in the tank for heating the water, the or each heating element being positioned in an upper half of the tank spaced from the base, and the side walls of the tank being inclined at an angle of less than 90° relative to the base so as to converge in a direction leading away from the base.

According to a fourth aspect of the present invention, there is provided apparatus for heating water comprising a tank for holding water to be heated, the tank having a base and sidewalls upstanding from the base, means for heating water in the tank, the sidewalls having low adhesion internal surfaces for assisting removal of deposits, and the base having means for removing deposits collected therein.

The method and apparatus according to the various aspects of the present invention may be employed in any installation where it is required to heat water. For example, the invented water heater may be used as a water purifier to provide a supply of pure water for use in a beverage dispenser such as a coffee machine, especially in areas where the natural water supply can be of poor quality and/or purity. In such application, the water heater may be operable to purify the incoming water and provide a source of purified water for supply to a dispense outlet, eg a nozzle, for mixing with a concentrate to produce a desired beverage when required.

Deposits that build-up during operation of the water heater may be removed by carrying out the heat shock treatment at regular intervals and periodically removing deposits collected at the bottom of the water heater. The heat shock treatment may be effected automatically by a suitable control system or it may be initiated manually. Similarly, removal of collected deposits may be carried out automatically by opening a drain and flushing the deposits from the tank or it may be done manually by detaching a removable sump.

According to a fifth aspect of the invention, there is provided in or for a beverage dispenser having a water heater for providing a source of hot water for dispense of a beverage and for maintaining the water at an elevated temperature in a stand-by mode between beverage dispenses, a method of removing deposits from internal surfaces of the water heater comprising the steps of switching off heating means for heating the water, and flushing the water heater with cold water. In use, the water heater maintains the water at an elevated temperature and rapidly boiling the water when a beverage is to be dispensed e.g. coffee. When the heating means is switched off and the water heater flushed with cold water, the internal surfaces contract causing deposits to crack and break away from the internal surfaces. The cold flush may be effected periodically to prevent build-up of deposits on the internal surfaces. Removal of the deposits may be further enhanced by the expansion of the internal surfaces when the heating means is switched on again to re-heat the water. The cold flush may be repeated.

Features, benefits and advantages of the invention will be apparent from the following description of exemplary embodiments with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a water heater according to a first embodiment of the invention after a period of use;

Figure 2 is a diagrammatic representation of the water heater shown in Figure 1 after the heat shock treatment;

Figure 3 is a diagrammatic representation of a water heater according to a second embodiment of the invention with inclined side walls;

Figure 4 is a diagrammatic representation of a water heater according to a third embodiment of the invention with a removable sump; and Figure 5 is a diagrammatic representation of a water heater according to a fourth embodiment of the invention with a drain.

Referring first to Figure 1 of the drawings, there is shown a first embodiment of a water heater 1 according to the invention. The water heater 1 has a tank 2 with a stainless steel body comprising a base 3, sidewalls 4 upstanding from the base 3 and a top 5. The tank 2 is cylindrical with the sidewalls 4 extending normal to the base 3 and top 5.

Internal surfaces of the sidewalls 4 are provided with a lining 6 of PTFE that adheres to the side walls 4 and forms a low adhesion surface for deposits of calcium carbonate 7 produced during operation of the water heater 1.

The tank 2 has an inlet (not shown) for connection to a water supply and an outlet (not shown) for delivering water to be used for any desired purpose. The tank 2 is provided with an electrical heating element 9 in the upper half of the tank 2 spaced from the base 3 and located below the water level for normal operation indicated by reference numeral 8.

The heating element 9 is a conventional electrical resistance heater typically having a power rating of 1-2 kilowatts sufficient for heating the water in the tank. In this embodiment, the heating element 9 is in the form of a flat, circular loop connected to a power supply via a control system (not shown) for switching the heating element 9 on and off. It will be understood, however, that the heating element may be of any size or shape. Additionally, more than one heating element may be provided to allow the power input to the water heater to be varied by the use of one or more heating elements in any desired combination according to the required power input and/or heating cycle for any application. In use, incoming water from a supply is heated in the tank 2 by the heating element 9 to boil and purify the water for end use. For example, the purified water may be transferred to a storage reservoir (not use) to provide a supply of pure water for a post-mix beverage dispenser where the water is mixed with a syrup concentrate to produce a desired beverage. Water drawn off from the tank 2 is replaced through the inlet and purified by heating in the same manner.

During such operation of the water heater, deposits 7, eg. calcium carbonate deposits, slowly build up on the lining 6 of the side walls 4. As shown in Figure 1 , these deposits 7 eventually bridge the gap between the lining 6 and the heating element 9 and, if uncontrolled, would continue to grow over the heating element 9. This can have an adverse affect on the heating cycle and may lead to premature failure of the heating element 9 where the formation of the deposits 7 causes the heating element 9 to overheat and burn out.

In accordance with the present invention, a heat shock treatment is carried out to remove the deposits 7 by switching off the heating element 9 and flushing the tank 2 with cold water, and then switching the heating element 4 back on and heating the water in the tank 2.

Flushing the tank 2 with cold water causes the metal body of the tank 2 to contract as it is rapidly cooled by the cold water. Subsequent heating of the water causes the metal body of the tank 2 to expand as it is rapidly heated by the hot water. This rapid thermal contraction and thermal expansion generates forces that cause the deposits 7 on the lining 6 to crack. As a result of the low adhesion of the deposits 7 to the lining 6, the deposits 7 readily break away from the lining 6 and fall to the bottom of the tank 3 where they collect on the base 3 as indicated by reference numeral 7' in Figure 2. In a modification (not shown) , the tank 2 may be provided with heating means surrounding or embedded in the side walls 4 for effecting heating to cause thermal expansion of the tank 2 during the heat shock treatment. This may be employed in addition to or in place of heating the water in the tank 2 following the cold flush.

Removal of deposits 7 in this way may be carried out at pre-determined intervals by initiating the heat shock treatment either manually by the user or service engineer or automatically by means of a suitable control system (not shown) . The deposits 7' collected at the bottom of the tank 3 do not have any significant adverse affect on the heating cycle and, by locating the heating element 9 in the upper half of the tank 2, the heating element 9 is clear of the deposits 7' . As a result, the operating life of the heating element 9 may be usefully extended.

More particularly, the low adhesion of the deposits 7 to the lining 6 coupled with the heat shock treatment has been found to result in the deposits 7 breaking away in relatively small pieces. In this way, a large mass of deposits 7' can be collected before operation of the heating element 9 is adversely affected by build-up of the deposits 7' removed from the lining 6 by the heat shock treatment.

As will now be appreciated, the heat shock treatment produces rapid changes in temperature with the resulting thermal contraction and expansion of the tank body producing forces that exceed those generated during normal operation of the water heater. Thus, thermal changes during the heat shock treatment occur more rapidly and over a wider temperature range than during normal operation of the water heater 1. In particular, the addition of cold water to replace water discharged from the tank 2 during normal operation does not produce the same rapid cooling effected by flushing the tank 2 with cold water and may not be followed immediately by rapid heating of the water again. The extreme conditions created under the heat shock treatment are believed to result in more extensive cracking of the deposits 7 than would otherwise occur and, coupled with the low adhesion of the deposits 7 to the lining 6, is thought to result in the successful removal of the deposits 7.

Referring now to Figure 3, a second embodiment of a water heater according to the present invention is shown in which like reference numerals in the series 100 are used to indicate parts corresponding to the first embodiment.

The second embodiment differs from the first embodiment by having side walls 104 of the tank 102 that are inclined at an angle of about 75° relative to the base 103 and converge in a direction leading away from the base 103. As a result, the tank 102 is wider at the base 103 than the top 105.

The inclination of the side walls 104 is believed to assist deposits 107 that have been cracked by the heat shock treatment to the break off from the lining 106 under the force of gravity for collection at the base 103 of the tank 102. In other respects, the construction and operation of the water heater 101 is similar to the first embodiment.

With reference now to Figure 4 of the drawings, a third embodiment of a water heater according to the present invention is shown in which like reference numerals in the series 200 are used to indicate parts corresponding to the first embodiment. The third embodiment differs from the first embodiment by having a detachable sump 210 at the base 203 of the tank 202. The sump 210 is releasably secured by any suitable means providing a fluid tight connection indicated generally by reference numeral 211. The sump 210 collects deposits 207' that have been removed from the side walls 204 by the heat shock treatment and can be detached at regular intervals to discard the deposits 207' and re-attached to allow continued use of the water heater 201.

In this way, the useful operating life of the water heater 201 may be further extended by preventing build-up of a significant mass of deposits 207 in the base 203 of the water heater 201 sufficient to interfere with the heating cycle and/or operation of the heating element 209. As a result, the heating element 209 may be located at a lower position within the tank 202. The construction and operation of this embodiment is otherwise similar to the first embodiment.

Referring now to Figure 5 of the drawings, there is shown a fourth embodiment of a water heater according to the present invention in which like reference numerals in the series 300 are used to indicate parts corresponding to the first embodiment.

In this embodiment, the tank 302 is provided with a drain 312 in the base 303 of the tank 302. The drain 312 is closed by a valve 313 during normal operation of the water heater 301. The valve can be operated manually or automatically to open the drain 312 when it is desired to flush out of the tank 302 deposits 307' broken off from the lining 306 during the heat shock treatment and collected at the base 303 of the tank 302. This arrangement enables the deposits 307' to be removed by controlling operation of the valve 313 through the control system to effect removal of the deposits 307' at regular intervals. This may be beneficial in allowing longer intervals between servicing the apparatus. The construction and operation of this embodiment is otherwise similar to the first embodiment.

Although, the invention has been described with reference to the exemplary embodiments it will be understood that the invention is not limited thereto and that various modifications can be made within the spirit and scope of the invention.

For example, the water heater may form part of a beverage dispenser for hot beverages, eg tea or coffee, where the water heater delivers hot water for the beverage to an outlet on demand. Alternatively, the water heater may form part of a water sterilisation apparatus where the water heater delivers purified water to a storage reservoir.

The low adhesion surfaces may be provided on all the internal surface of the tank or at least those internal surfaces where deposits are formed. For example, the base and sidewalls of the tank may be provided with low adhesion internal surfaces. The inlet and/or outlet pipe connections may also be provided with low adhesion surfaces.

Alternatively, the low adhesion surfaces may be provided in selected areas or regions only of the tank. For example, the low adhesion surfaces may be provided adjacent to the heating element to allow removal of deposits that could otherwise overgrow the heating element reducing its effectiveness and causing premature failure of the heating element. The low adhesion surfaces may be provided by a lining applied to the internal surfaces of the tank body. Alternatively, the tank body may be made of a material providing low adhesion surfaces on the inside of the tank.

The tank may have a removable lid secured in a fluid tight manner for access to the heating element and inspection of the interior of the tank. This may allow manual flushing of the tank during the heat shock treatment.

The water heater may be provided with an electrical heating element internally or externally of the tank. Alternatively any other suitable heating means may be employed, for example a gas powered heater externally of the tank.

The heat shock treatment may involve carrying out the steps of flushing with cold water and heating the water in the tank in any order and/or repeating one or both steps more than once. For example, we may rapidy heat the water after a quiet period followed by flushing with cold water. In this way, the cold water flushing may assist to break away deposits that have cracked and may remove from the tank some or all of the deposits that have broken away.

Although the heat shock treatment has been described in combination with the provision of low adhesion surfaces in the exemplary embodiments, the heat shock treatment may have application generally to water heaters with or without low adhesion surfaces where the thermal expansion and contraction generated by the treatment are sufficient to cause the deposits to crack and break away from the internal surfaces of the water heater. It will be appreciated that the exemplary embodiments described herein are intended to illustrate the range and application of the invention and that features of the embodiments may be employed separately or in combination with any other features of the same or different embodiments to produce any desired method and/or apparatus embodying the invention in any of the aspects referred to herein.

Moreover, while the specific materials and/or configuration of the apparatus and steps of the method described and illustrated are believed to represent the best means currently known to the applicant for carrying out the invention in each of its aspects, it will be understood that the invention is not limited thereto and other modifications and improvements will be apparent to those skilled in the art and are deemed within the scope of the invention.

Claims

1. A method of removing deposits from internal surfaces of water heating apparatus by applying a heat shock treatment to the internal surfaces .

2. A method according to claim 1 wherein the internal surfaces are subject to thermal expansion and/or contraction during the heat shock treatment.

3. A method according to claim 2 wherein the internal surfaces are caused to expand by heating and/or contract by cooling whereby the differential rate of expansion and/or contraction between the surfaces and the deposits results in cracking of the deposits.

4. A method according to claim 3 wherein cooling the surfaces is effected by flushing the apparatus with cold water.

5. A method according to claim 3 wherein heating the surfaces is effected by heating water in the apparatus.

6. A method according to claim 3 wherein heating the surfaces is effected by heating the surfaces directly.

7. A method according to claim 6 wherein the surfaces are directly heated by resistance heater (s) surrounding or embedded in the surfaces.

8. A method according to any one of claims 3 to 7 wherein the surfaces are alternately heated and cooled in any order.

9. A method according to claim 7 or claim 8 wherein one or both steps of heating and cooling the surfaces is repeated.

10. A method according to any one of the preceding claims wherein deposits that break away from the internal surfaces are collected and periodically removed.

11. A method according to any one of the preceding claims wherein the internal surfaces are low friction surfaces.

12. A method according to any one of the preceding claims wherein the internal surfaces are subjected to a change in temperature during the heat shock treatment producing forces that exceed the forces experienced during normal operation of the apparatus.

13. A method of operating a water heater comprising the steps of heating water in a tank to an elevated temperature, and periodically applying a heat shock treatment to remove deposits adhering to the internal surfaces by switching the water heater off and flushing the tank with cold water to cause thermal contraction of the internal surfaces, and switching the water heater on after the cold flush to heat water in the tank.

14. A method according to claim 13 wherein the water heater is operable to maintain the water at the elevated temperature in a stand-by mode between heat shock treatments.

15. A method according to claim 14 wherein the water heater is operable during the stand-by mode to provide a source of hot water at a temperature higher than the elevated temperature.

16. Apparatus for heating water comprising a tank for holding water to be heated and means for heating water in the tank, wherein the tank has low adhesion internal surfaces by means of which deposits adhering to the internal surfaces can be removed by applying a heat shock treatment to the internal surfaces.

17. Apparatus according to claim 16 wherein the tank is made of metal provided with low adhesion internal surfaces by a lining of a material to which deposits adhere less strongly than the metal of the tank.

18. Apparatus according to claim 17 wherein the lining is inert to water and capable of withstanding temperature changes occurring during operation of the water heater without degrading to any appreciable extent.

19. Apparatus according to claim 18 wherein lining material is polytetrafluoroethylene (PTFE) .

20. Apparatus according to claim 18 or claim 19 wherein the lining is pre-formed for insertion in the tank.

21. Apparatus according to claim 18 or claim 19 wherein the internal surfaces of the tank are coated with material to form the lining.

22. Apparatus according to any one of claims 16 to 21 wherein the tank has a base wall and side walls upstanding from the base wall of which at least the side walls have low adhesion internal surfaces.

23. Apparatus according to claim 22 wherein the side walls are inclined relative to the base wall at an angle of less than 90° so as to converge in a direction away from the base wall.

24. Apparatus according to any one of claims 16 to 23 wherein the heating means is positioned internally or externally of the tank.

25. Apparatus according to claim 24 wherein the heating means comprises at least one electrical heating element positioned within the tank.

26. Apparatus according to claim 25 wherein the or each electrical heating element located within the tank is positioned in the upper half of the tank.

27. Apparatus according to any one of claims 16 to 26 wherein the tank is adapted to enable deposits that have broken away from the internal surfaces to be removed.

28. Apparatus according to claim 27 wherein the tank is provided with a removable sump at the bottom that can be detached for disposal of deposits collected therein and the sump re-attached for continued use of the water heater.

29. Apparatus according to claim 27 wherein the tank is provided with an openable drain at the bottom through which deposits settling on the bottom of the tank can be flushed.

30. Apparatus according to any one of claims 16 to 29 wherein the tank has an inlet for connection to a water supply and an outlet from which water can be discharged from the tank.

31. Apparatus according to claim 30 wherein the inlet and/or outlet have low adhesion internal surfaces.

32. A water purifier comprising apparatus according to any one of claims 16 to 31.

33. A beverage dispenser including apparatus according to any one of claims 16 to 31.

34. A beverage dispenser according to claim 33 wherein the heating means is operable to maintain the water at an elevated temperature in a stand-by mode of operation.