WO2010106348A2

WO2010106348A2 - Heaters

Info

Publication number: WO2010106348A2
Application number: PCT/GB2010/001019
Authority: WO
Inventors: Vincent Joseph Garvey; Colin Moughton
Original assignee: Strix Limited
Priority date: 2009-05-20
Filing date: 2010-05-20
Publication date: 2010-09-23
Also published as: WO2010106348A3

Abstract

A flow heater comprises a heated flow conduit (12) for heating liquid therein to a temperature below boiling and a final heating chamber (18) for heating the liquid to boiling. The heating chamber (18) comprises a space above the liquid surface for allowing the escape of steam from the liquid surface. A liquid outlet (20; 102) may be configured to maintain a substantially constant level of liquid as the liquid inlet flow rate varies between a predetermined maximum and a predetermined minimum.

Description

Heaters

This invention relates to heaters for heating, particularly to boiling liquids, e.g. water.

A number of methods are known to provide hot or boiling water for domestic consumption. Traditionally electric kettles or jugs are used to boil a quantity of water e.g. for making hot beverages.

More recently products have been marketed which promise to deliver small quantities of hot water very quickly. Rather than heat a body of water uniformly, these are based on flow-heaters which heat water as it passes through a narrow passage with a thick film printed element on one side. However such technology has significant drawbacks, the most important of which is that they cannot be used to boil water as is explained below.

During boiling of water in a conventional kettle, the bulk of the water is at substantially the same temperature which gradually rises as heating progresses. Only the boundary layer close to the heated surface is significantly hotter. Heat is transferred from the heated surface to the boundary layer by conduction and initially at least, from the boundary layer to the bulk by convection. In heaters with a high surface temperature, the water in the boundary layer can reach 100⁰C and boil while the bulk water is relatively cool. The bubbles of steam, initially condense and collapse due to contact with the cooler bulk water.

As heating continues, bubbles of steam, being lighter than the surrounding water rise from the heater surface. As the bubbles rise they conduct heat to the cooler surrounding water and the resultant condensation eventually causes the bubble to collapse. However as the bulk of the water approaches boiling temperature, it no longer causes full condensation of the rising bubbles, and these rise to the surface and break free which is generally considered to indicate that the water is boiling. In practice the bulk water temperature will not quite be at 100⁰C at this stage. Conventionally, domestic jugs and kettles will maintain a "rolling boil" for several seconds which enables the bulk water liquid to uniformly reach a temperature very close to 100⁰C, although it never quite gets there and moreover the actual boiling point is dependent on other factors such as atmospheric pressure and the presence of dissolved substances in the water.

A flow heater, by comparison, has the benefit of being able to heat water on demand and to be operated only for as long as necessary to deliver the required quantity of water. However consumers expect a start-up time that appears virtually instantaneous - certainly no longer than a few seconds. In the context of small domestic products the amount of power is fixed by that available from the wall outlet socket (1500W to 3000W typically) and can't be increased. Under steady state conditions, the flow-rate of water will be matched to the heater output power according to the basic laws of thermodynamics (for a 3kW heater, a flow-rate of around 0.5 litres/minute up to 1 litre/minute will provide water with a temperature range from near boiling down to about 65⁰C). The heater type, and heat exchange mechanism have little influence.

When designing a flow heater with very fast start-up it is important to minimise the thermal mass of the heater itself and the temperature to which it needs to be heated. It is also important to maximise the contact area between the water and the heater. These requirements have been addressed in the recent prior art by the use of a thick film heater bonded via an intermediate electrical insulation layer to a stainless steel heat exchanger. The heat exchanger is designed with a complex chamber facing the heater to maximise the contact area. However the Applicant has realised that care must be taken over the distribution of water flow over the heater surface. If any section of water in contact with the surface is allowed to stagnate, it will quickly boil, creating a pocket of steam. A pocket of steam will no longer provide cooling to the element surface. The effect of this is rapid localised heating of the surface, and a failure, usually of the insulation between the heater track and heater substrate surface. To avoid this, the water is therefore constrained to flow in a tortuous narrow channel to avoid stagnant spots.

The Applicant has also appreciated that another problem arises with the use of a narrow water channel. As the water approaches the end of the heater, it will be at its hottest - typically 85 ⁰C. The water channel, although small, nevertheless still consists of a boundary layer and a bulk water channel; the water in the boundary layer will often boil, creating bubbles of steam. In this configuration, a bubble of steam, emerging into the very small channel is unable to transfer heat by conduction and condensation, as it cannot expose its surface area to surrounding water, instead, the expanding bubble will simply push the remaining water ahead of it. It can be seen, that if this bubble occurs, for example 80% of the way along the channel, it will in fact cause all of the water in the last 20% of the channel to be ejected violently. In addition to the undesirable effect of "spitting" from the users perspective, the depletion of water cover at the end sections of the heater can often lead to premature element failure. Despite the appearance to the user, the majority of the water ejected will be significantly below boiling.

The problems of localised hot spots and spitting means that flow heaters can't be used to provide boiling water. In fact the hotter the water temperature aimed for, the greater these problems are. In practice therefore, flow heaters have been restricted to applications requiring water at temperatures below boiling such as shower heaters, and hot water dispensers that do not boil water.

When viewed from a first aspect the invention provides a flow heater comprising a heated flow conduit for heating liquid therein to a temperature below boiling and a final heating chamber for heating said liquid to boiling wherein said heating chamber comprises a space above the liquid surface for allowing the escape of steam from the liquid surface.

Thus it will be seen by those skilled in the art that in accordance with the invention the a final heating chamber is provided which allows steam to escape from the surface of the water without forcing the heated water out - i.e. the phenomenon of spitting is reduced or avoided. Moreover the facility for steam to escape allows the surface of the heater to remain flooded in water and so avoid localised hot spots. Whilst the invention allows boiling water to be produced in its preferred embodiments, only the water in the final heating chamber boils; it is not necessary to heat the whole of the contents of the heater to boiling before boiling water can be produced as would be the case with a kettle or other 'batch' heater. For example a cold water temperature of 20 ⁰C preheated to 90 ⁰C in the first region will have an average temperature of only 55 ⁰C. In some embodiments the heating chamber comprises a heating element formed on or mounted to the underside thereof. The heating element could, for example, comprise a sheathed heating element mounted to a plate, or a thick film element formed on or mounted to a plate.

There are many possible arrangements for how the heated liquid flows out from the final heating chamber in accordance with the invention. One possibility would be a simple valve or tap for allowing water to drain out of the chamber. The problem with such an arrangement is that the outflow through such a valve or tap would have to be precisely co-ordinated with the inflow (e.g. from a pump). For example, if the outflow rate is even slightly greater than the inlet flow rate, (or if it commences to flow out too early) the heater will run dry. If the outflow rate is slightly lower, then the outflow chamber will overflow, or, as the water level increases, the effect of boiling in the chamber will result in water spitting. This will occur because, as the steam bubbles generated at the surface now must travel through a vertical body of water, they will entrain droplets of water and carry them at high velocity to the surface. The pump inflow may start and stop at irregular times, and/or be constantly varying in response to all the input variables - desired outlet temperature, inlet water temperature, voltage fluctuations, and the natural oscillations that can occur in any closed loop control system. The difficulty in controlling the outflow is further exacerbated by the need, on start-up, to prevent outflow until such time as sufficient water has entered to fill the system to its intended working level.

In a set of preferred embodiments therefore means are provided to permit automatic outflow of liquid upon the liquid reaching a predetermined level. This ensures that a certain amount of liquid is retained and can therefore ensure that a heater surface is covered sufficiently to prevent it overheating. Such a function could be achieved electronically or through use of a float but preferably a weir is provided such that liquid escapes over the weir and out of the final heating chamber when the water level in the chamber exceeds a predetermined height (determined by the height of the weir). The Applicant has further appreciated that this arrangement allows even a relatively large heater surface to remain covered with a relatively thin covering of liquid and so avoid overheating.

When viewed from another aspect the invention provides a liquid heating apparatus comprising: a heating chamber having an electric heating element for heating liquid therein to boiling or near boiling, said heating chamber comprising a space above the liquid surface for allowing the escape of steam from the liquid surface; a liquid inlet for allowing liquid into the heating chamber; and a liquid outlet configured to maintain a substantially constant level of liquid as the liquid inlet flow rate varies between a predetermined maximum and a predetermined minimum.

In accordance with this aspect of the invention liquid, e.g. water, can be safely heated to, or close to, boiling (such that steam is produced) but on a continuous flow basis (i.e. contrasted with batch heating). In a preferred set of embodiments of this aspect, the liquid outlet comprises a weir disposed such that liquid escapes over the weir and out of the heating chamber when the water level in the chamber exceeds a predetermined height (determined by the height of the weir). As previously, this ensures that a certain amount of liquid is retained and can therefore ensure that a heater surface is covered sufficiently to prevent it overheating.

Preferably a heated, typically planar, base plate forms the base of the heating chamber. This could for example have a sheathed heating element attached or mounted to the underside or have a thick film printed heater formed on the underside. The outlet preferably comprises a hole in the base plate. Conveniently an outlet tube could project through such a hole, proud of the base plate to form a weir.

In some embodiments the heating chamber is configured, e.g. by means of a wall or baffle, to ensure a minimum residence time for liquid in the chamber - i.e. to prevent liquid leaving the chamber through the outlet before it has been adequately heated. Additionally or alternatively a wall or baffle may be provided to retain liquid preferentially, or exclusively, on a portion of the base plate directly opposite the heating element on the underside. A wall or baffle may of course do both jobs simultaneously.

Some embodiments of the invention which comprise a heater having a substantially constant level of liquid in the heating chamber are operable in a sterilisation mode in which the heating chamber is filled with a volume of liquid which is insufficient to bring the level of liquid in the heating chamber to said substantially constant level. In such arrangements the water does not leave the water outlet, but it is boiled and so produces steam which can act as the sterilising fluid. The steam exits the water outlet and therefore can be used to sterilise other parts of the appliance such as a heat exchanger or other cooling means.

Although, as set out above, this aspect of the invention does not necessarily require pre-heating of the liquid before it enters the boiling chamber, in at least a set of preferred embodiments the liquid is heated before it enters the chamber. This could be in a heated flow conduit, another type of flow heater, or simply a preheated reservoir. More generally, although the invention is described mainly in the context of being supplied from a reservoir, there are many other applications in which the principles of the invention can be employed. For example it could be used in a plumbed-in or other in-line system such as in a garden hose heater - or to form part of a more complex apparatus where the liquid is subjected to other treatment before entering the heater.

Where provided in accordance with either aspect of the invention, the heated flow conduit could be heated in a number of ways. In one set of embodiments it is provided with an electric heating element for heating the liquid therein. The heating element could take any convenient form. In one set of embodiments, the heating element is provided on the outside of the conduit. The element could take the form of a so-called thick film printed element. Such elements are conventionally planar, but can also be produced with non-planar substrates. Alternatively it could comprise a sheathed resistance heating element, with or without an intermediate metallic heat diffuser plate as is commonly found in so-called underfloor heaters for domestic kettles. The advantages of having an element on the outside of the channel are that it is relatively easy to manufacture and it allows overheat protection to be provided in close thermal contact with the element to switch off the element in the event of it being energised without water in the channel.

In another set of embodiments the heated flow conduit is provided by one side of a heat exchanger, through the other side of which a hotter liquid or gas is passed. In one set of embodiments a first side of the heat exchanger provides the heated flow conduit and a second side of the heat exchanger is in fluid communication with the liquid outlet of the final heating chamber. This provides an efficient arrangement allowing liquid to be heated to a high temperature and then cooled again before dispensing. Such an arrangement is particularly suitable for providing a supply of sterilised water - e.g. for mixing with powdered infant formula feed.

However in a more general set of embodiments means are provided downstream of the heating chamber for cooling said liquid. The cooling means need not comprise a heat exchanger as discussed above but could take one of several forms: for example it could comprise a heat sink - e.g. with cooling fins, a fan or other means to produce a forced air flow, a solid state cooling device such as a Peltier device, a closed loop refrigerant system or any combination of the above. In some embodiments the apparatus is adapted to dispense water at a temperature of between 30 and 50⁰C, more preferably between 35 and 45°C. The apparatus may be provided with temperature sensing means at the outlet in order to allow control of the outlet temperature. In one set of embodiments, control over the outlet temperature is exercised by controlling the rate of flow of liquid into the heating chamber. This could be controlled either by a suitable valve or by altering the speed of a pump. In some embodiments the outlet temperature is varied during a dispense cycle. Where the volume of liquid to be dispensed is known, it can still be ensured that the overall average temperature has the desired value.

In accordance with all aspects of the invention, the liquid flow could be driven by hydrostatic pressure achieved by arranging a reservoir of liquid above the outlet and using a valve or tap. Preferably, however, a pump is provided for driving liquid through the heater. In various embodiments of the invention the heated or boiling liquid can exit the heating chamber to be dispensed directly into a user's receptacle, or can be conveyed to another part of an appliance for further treatment - e.g. a heat exchanger.

In accordance with various aspects of the invention, steam is allowed to exit from the heating chamber separately from the heated liquid. The steam could be vented directly to the atmosphere although it is preferably directed to exit from a part of an appliance away from the user in normal use. It could, for example, be vented to the rear of the appliance. In other embodiments the steam could be captured and condensed in a suitable trap, drip tray or the like. This could be a special drip tray or, more conveniently, a drip tray beneath the spout could be used. In all of these cases it is preferred in some embodiments of the invention that the steam path between the heating chamber and the atmosphere is sufficiently restricted to give rise to a pressure difference across it in use of between 0.1 and 1 bar, preferably between 0.2 bar and 0.5 bar. By allowing the heating chamber to become slightly pressurised in use as compared to the atmosphere, the boiling temperature of the water or other liquid is slightly increased which helps to hasten sterilisation or to raise the temperature of liquid actually received in a user's receptacle in embodiments where the liquid is dispensed straight out of the heating chamber.

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a highly schematic diagram illustrating the principle features of an embodiment of the invention;

Fig. 2 is a perspective view of the major components of the heating and cooling system;

Fig. 3 is an exploded, cross-sectional view of the heating chamber shown in Fig. 2;

Fig. 4 is a perspective view from beneath of the upper housing of the heating chamber; Fig. 5 is a perspective view of the upper housing member from above;

Fig. 6 is a sectional view through the heat exchanger;

Fig. 7 is a cross-sectional view through a continuous flow water heater in accordance with a further embodiment of the invention;

Fig. 8 is a view of the baffle plate member of Fig. 7 from the underside; and

Fig. 9 is a perspective view of the heater chamber assembly.

Considering first Fig. 1 , there may be seen a schematic representation of an apparatus for producing warm, sterilised water suitable for preparing infant feed. On the lower part of the diagram is a cold water storage reservoir 2 having an outlet 4 communicating with a pump 6. Although not shown, the reservoir may be provided with a filter cartridge such as one of the Applicant's Aqua Optima (registered trade mark) water cartridges.

On the outlet side of the pump 6, a conduit 8 communicates with the inlet 10 of the cold side of a heat exchanger 12. The heat exchanger is described in greater detail hereinbelow with reference to Fig. 6. The outlet 14 of the cold side of the heat exchanger 12 is connected to an inlet tube 16 of the heating chamber 18. The heating chamber 18 is described in greater detail hereinbelow with reference to Figs. 3 to 5. The heat exchanger 12 and heating chamber 18 may also be seen, in isolation, in Fig. 2.

The heating chamber 18 is provided with a sheathed heating element 19 on the underside thereof as may be seen in further detail in Figs. 3 to 5 as mentioned above. The liquid outlet 20 of the heating chamber is connected to the inlet 22 of the hot side of the heat exchanger 12. A temperature sensor, e.g. a thermistor is disposed in the heating chamber outlet 20, the purpose of which will be described later. A further outlet 26 is provided at the top of the heating chamber which is closed by a pressure valve 28 which could comprise a weight, spring bias, or a combination thereof. Although not shown, a small bleed drain aperture could be provided in the heating chamber to allow the chamber to empty back into the reservoir 2 between uses.

The outlet 30 of the hot side of the heat exchanger 12 is connected to a diverter valve arrangement 32 which is able selectively to communicate the outlet 30 with an external dispensing nozzle 34 or a drain tube 36 which is directed into the cold water reservoir 2. A further temperature sensor, e.g. a thermistor, is provided at the outlet 30 of the hot side of the heat exchanger. The cold water pump 6, the heating element 19, the temperature sensors 24, 38 and the diverter valve arrangement 32 are all connected to a control circuit (not shown).

As mentioned above, Fig. 2 shows in isolation the heat exchanger 12 and heating chamber 18 some of the inlets and outlets of these parts are shown using the same reference numerals as used in Fig. 1 , although the connections between them are not shown.

Fig. 3 is an exploded and sectioned view of the heating chamber 18. From this it may be seen between an upper housing part 40 which is closed off from below by a circular and substantially planar heater plate 42. The heater plate 42 is sealed to the upper housing part 40 by means of a peripheral annular channel 44 which receives a corresponding annular downwardly depending wall portion 48 of the upper housing part 40, with the walls of the channel 44 being clamped or crimped to retain the downwardly depending wall portion 48 securely. An annular seal 50 is provided in the channel 44 between the inner wall of the channel and the downwardly depending wall portion 48. In other words, the heater plate 42 is attached to the upper housing part 40 using the applicant's well-established Sure Seal sealing system. Of course, any other suitable sealing system could be employed instead. A conventional sheathed heating element 19 is attached to an aluminium heat diffuser plate 52 which is, in turn, fixed to the underside of the heater plate 42.

The liquid outlet of the heating chamber comprises a cylindrical tube 20 which projects through the plate 42 so as to be somewhat proud of the heater surface. This effectively forms a weir so that when the liquid level inside the heating chamber reaches the top of the outlet tube 20, it tends to overflow down into it. The upper housing part 40 will now be described with continuing reference to Fig. 3 but also with reference to Figs. 4 and 5. From these Figures it may be seen that the upper housing part 40 comprises a further downwardly depending wall 54 which, as may be seen most clearly in Fig. 4 is approximately Omega shaped. As Fig. 3 shows, the opening formed by the shape of the wall is located in the vicinity of the liquid outlet 20. Diametrically opposite to this, there is an indented portion of the wall 54a which accommodates the water inlet tube 16. The wall 54 thus creates two semi-circular channels on either side of the inlet 16 around which water entering through the inlet 16 must pass before it reaches the outlet 20. The position of the wall 54 means that the two semi-circular channels which are formed are substantially aligned with the heating element 19 provided on the underside of the plate 42. A connector 58 for the inlet tube 16 is provided on the outer face of the upper housing part 40. The connector 58 receives the end of a connecting hose (not shown) connected to the cold side outlet 14 of the heat exchanger.

Fig. 6 shows a cross section through the heat exchanger 12. As may be seen, the heat exchanger comprises a stack of six alternating interleaved plates 66, 68. The top-most plate 66a defines (together with a cover, not shown) a chamber in the cold side of the heat exchanger. This is the first such chamber encountered by the cold water entering the cold water inlet 10, the mouth 70 of which opens into the chamber. Water entering from the mouth of the inlet tube 70 spreads across the surface of the plate 66a to exit the chamber through an outlet 71 in the diagonally opposite corner from where it passes into the next chamber formed between the second and third plates 68a and 66b. The water then flows in the opposite direction in this chamber through an outlet vertically aligned with the inlet 70 above and from there into the final chamber formed between third and fourth plates 68b and 66c. It then flows out of here through the cold side outlet 14.

The chambers formed alternate between those on the heating side of the heat exchanger make up the cooling side of the exchanger. These are arranged so that water flows from the hot water inlet 22, through the three interleaved chambers to the outlet 30. The plates are designed so that the water flows in opposite directions in adjacent chambers and so that water near the inlet end of the cold side of the exchanger is near the water at the outlet end of the hot side of the exchanger and vice versa. The plates 66, 68 are typically made of thin, non-corrosive material e.g. stainless steel.

Operation of the apparatus will now be described with reference to Figures 1 to 6. The apparatus has three distinctive phases of operation. Initially, a user ensures that the cold water reservoir is adequately filled with cold water (or in another embodiment the device could be permanently plumbed in). When the user presses a button to request warm water, a sterilisation phase is commenced. In this phase, the pump 6 is operated for a fixed amount of time in order to deliver a fixed amount of water via the cold side 10-14 of the heat exchanger through the inlet 16 and into the heating chamber 18. The amount of water delivered into the heating chamber 18 is sufficient to cover the portion of the heater plate directly above the element 19 but is not sufficient to overflow into the outlet 20.

Once the desired amount of water has been delivered, the pump 6 is switched off and the heater 19 is energised to heat the water to boiling (these two operations could overlap however - i.e. the heater could be energised before the pump is switched off). As the water approaches boiling, steam is generated in the heating chamber 18, raising the pressure therein and forcing the steam out of the outlet 20 and into the hot side 22, 30 of the heat exchanger and from there via the diverter valve 32 to the outlet nozzle 34. The valve 28 on the steam outlet 26 ensures that the pressure inside the heating chamber 18 is not allowed to rise too far.

The heating element 19 is energised so as to continue to produce steam until either a predetermined temperature is detected by the outlet temperature sensor 38 or after a predefined temperature has been exceeded for a predetermined amount of time. This ensures that the whole of the cooling side of the system is suitably sterilised. For example, the sterilisation may be satisfactorily achieved by ensuring that all parts of the system reach 100⁰C, or that a temperature of 70⁰C is maintained for 30 seconds. The vertical arrangement of the heat exchanger 12 and the configuration of the connecting pipes means that the heat exchanger and the valve arrangement 32 downstream of it are free-draining so that condensate forming within any of these parts can freely exit e.g. drip tray (not shown) beneath the outlet nozzle 34. In an alternative embodiment, the diverter valve 32 is set to divert the steam to the drain pipe 36 and therefore back into the water reservoir 2. This has the advantage of avoiding steam exiting in the vicinity of a user, but means that the outlet nozzle 34 must be separately sterilised.

Once sterilisation is complete, the apparatus enters its second, pre-dispense mode. The pump 6 is once again operated to pump cold water into the heating chamber 18 via the heat exchanger 12. In this phase, however, the pump 6 is operated continuously so that the water in the heating chamber 18 overflows the top of the outlet tube 20 and thus starts to fill the hot side of the heat exchanger 12, via the inlet 22. At the same time, the heater 19 is energised so that the water in the heating chamber 18 is heated to boiling or near boiling by the time it exits the chamber through the outlet 20. The rate at which the water is pumped by the pump 6 and/or the power of the heating element 19 are modulated to maintain boiling at the outlet 20 at all times whilst producing only a modest amount of excess steam.

The outlet valve 32 is closed either fully or partially so that the water backs up in the heat exchanger until it reaches nearly the top of the outlet tube 20 of the heating chamber. This can be detected by an electrical conductivity level detection circuit (not shown) that comprises a conductive electrode formed by the stainless steel sheath of the temperature sensor 24 which is positioned close to the heater outlet 20. The diameter of the outlet 20 and the hose connecting this to the heat exchanger inlet 22 is of sufficient diameter, e.g. greater than 12 mm, to ensure that the heat exchanger is flooded and that any air is allowed to escape back up into the heating chamber. This is advantageous in avoiding air remaining on the surfaces of the heat exchanger 12 which would otherwise compromise the heat transfer efficiency which it could achieve.

Once the temperature sensor 24 has determined that the heat exchanger 12 is flooded, the pre-dispense stage is finished and the dispense stage commenced. In this stage, the outlet valve 32 is arranged to direct the water from the outlet 30 of the hot side of heat exchanger to dispensing nozzle 34. The flow rate of the cold water pump 6 is now controlled by the temperature of the outlet temperature sensor 38 to maintain the optimum dispensed temperature. The power of the heating element can be fixed, in which case it should normally be sufficient to match the flow rate of the water and ensure that it is brought to boiling under the "worst case" circumstances. Alternatively, it can be controlled to boil the water and provide a small safety margin in the form of excess steam production, which can be allowed to exit via the steam outlet 26 with the valve 28 being opened during this phase.

To give some examples of temperatures, the cold water in the reservoir 2 may be at 20⁰C and may be heated via its passage through the heat exchanger 12 to 80⁰C. The heating chamber 18 then raises the temperature of the water from 8O⁰C to boiling or near boiling (e.g. 99°C+), from which it passes through the hot side of the heat exchanger 12 to be reduced in temperature to approximately 40⁰C. This is an ideal temperature for producing baby milk since generally a temperature of 38°C is considered ideal for a baby bottle and a dispensed temperature of 2-4°C above this has been found to be sufficient to allow for warming a bottle and feed formula therein to achieve an optimum final temperature.

In presently preferred arrangements the pump speed is altered to compensate for variability in the performance of the heat exchanger 12. The power of the heater is varied, as a consequence, to maintain boiling under all circumstances. The performance of the heat exchanger is strongly affected by the inlet water temperature. With very cold inlet water (say 10⁰C) the temperature difference across the heat exchanger for a nominal outlet temperature of 40°C is of course 30°C. With warm inlet water however, which could be for example at 3O⁰C, the temperature difference is only 10⁰C. Since the heat energy transferred across the heat exchanger is proportional to the temperature drop, a change from 10 to 30 represents a potential 300% increase. To compensate for this, the flow rate can be increased or decreased to maintain a fixed 40⁰C outlet temperature. The effect of this is that the corresponding outlet power required from the heating element can vary from 200 W to 2000 W.

Once a user signals that dispensing is to be stopped (or once a predetermined volume has been dispensed), the heater and pump are switched off and the divert valve 32 is altered to close the outlet 34 and direct the remaining water back into the reservoir 2 by means of the drain tube 36. Although not shown a small bleed- back drain from the heating chamber to the reservoir may be provided. In this way, the entire system can be drained. Figs. 7 to 9 show the heating chamber assembly of a further embodiment of the invention. The assembly comprises a heated base plate 86 and a cover member 88. The heated base plate 86 is of the same basic construction as those used in water boiling vessels such as kettles. It comprises a stainless steel plate 90 having a planar central region, to the underside of which is brazed an aluminium heat diffuser plate 92. An arcuate sheathed heating element 94 is brazed to the heat diffuser plate 92 in a manner well known in the art. At the periphery of the heater 86 is an upwardly open channel 96 which receives an annular seal 98 and a downwardly depending annular wall 100 of the cover member 88. The edges of the channel 96 are clenched together to clamp the seal 98 and downwardly depending wall portion 100 in the channel, thereby forming a water-tight seal between the heater 86 and the cover member 88. This sealing arrangement is described in further detail in WO 96/18331.

The heater 86 is not entirely conventional since it comprises an outlet pipe 102 which extends through and projects proud of the upper surface of the heater plate 90. It is connected at the lower end by a pipe to a spout (not shown) provided on the outside of the appliance for dispensing heated water into a receptacle placed underneath it by a user.

The cover member 88, as may be seen most clearly in Figs. 11 and 12, comprises an annular upper face 104 with a downwardly depending vertical wall portion 108. The path of the wall 108 is approximately tear-drop shaped with the circular portion extending around the majority of the edge of the circular opening in the upper surface 104, and with the apex portion extending beyond this to encompass within the wall the outlet tube 102 when the cover member 88 is fitted to the heater 86. The wall 108 is continuous except for a gap 110 opposite the apex portion. Just outside the wall 108 at the apex portion is a vertically projecting inlet tube 112 which is connected to a pump (not shown) which is in turn fed by a reservoir (also not shown).

In practice the heating chamber assembly shown in Figs. 7 to 9 is installed inside a suitable housing which also includes a water reservoir and pump as previously described and pipes connecting these to the chamber inlet 112. Also in use a control unit such as one of the Applicant's U17 series of controls is affixed to bosses provided on the underside of the heat diffuser plate 92 in order to protect the heater against overheating in the event of being energised with no water present. Also provided in the appliance but not shown are control electronics which allow control of the pump and which accept user input to control dispensing, temperature etc.

Operation of the embodiment described with reference to Figures 7 to 9 will now be described.

When a user presses a button (not shown) to initiate dispensing of boiling water, the pump is initiated in order to pump water from the reservoir (not shown) through the heating chamber inlet 112 and therefore into the chamber formed between the upper surface 104 of the cover member 88 and the heating plate 90 of the heater. The heating element 94 may be energised either before, after or simultaneously with operation of the pump. This could be made dependent upon the sensed temperature of the water inside the heating chamber. For example, if the water is already hot, pumping could be begun relatively soon after the element 94 is energised. On the other hand, if there is a volume of cold water in the heating chamber, the heating element 94 could be energised for a predetermined amount of time (this amount of time could also be dependent upon the temperature) in order to heat the water in the chamber before any additional water is pumped in.

Water entering the heating chamber through the inlet 112 will pass around the outside of the vertical wall 108. As may be seen particularly from the cross- sectional view in Figure 7, this part of the heating chamber is directly above the heating element 94 and is therefore most directly heated. The heated water flows into the central region of the heating chamber through the gap 110 in the vertical wall 108 where it continues to be heated by heat conducted laterally through the heater plate.

As water approaches boiling, the steam generated may escape through the centre of the annular surface 104 of the cover member. As the level of water in the central region rises, it will start to spill over the edge of the outlet tube 102 and therefore drain out therefrom into the spout. As more water is introduced into the inlet 112, more water is displaced through the outlet 102 after it has been heated by the heating element 94. Thus, a continuous flow of boiling or near-boiling water can be provided by continuously pumping water into the inlet 112.

In this embodiment, it can be seen that there are two distinct heating zones: defined outwardly of and inwardly of the vertical wall 108 respectively; with steam being allowed to escape from the second, inner zone. However, this is not essential and the embodiment could also operate without the cover member 88 with water simply being flowed onto the heating plate 86 where it is heated and then allowed to spill over into the outlet tube 102. There are many variants of this embodiment possible, for example it does not need to be water that is heated but may be any other liquid such as milk, coffee etc. Moreover it is not essential to provide a pump for example a gravity feed reservoir with a suitable tap or valve could be provided instead.

Claims

Claims:

1. A flow heater comprising a heated flow conduit for heating liquid therein to a temperature below boiling and a final heating chamber for heating said liquid to boiling wherein said heating chamber comprises a space above the liquid surface for allowing the escape of steam from the liquid surface.

2. A heater as claimed in claim 1 wherein the heating chamber comprises a heating element formed on or mounted to the underside thereof.

3. A heater as claimed in claim 1 or 2 comprising means to permit automatic outflow of liquid from the heating chamber upon the liquid reaching a predetermined level.

4. A heater as claimed in claim 1 , 2 or 3 wherein comprising a weir arranged such that liquid escapes over the weir and out of the final heating chamber when the water level in the chamber exceeds a predetermined height.

5. A liquid heating apparatus comprising: a heating chamber having an electric heating element for heating liquid therein to boiling or near boiling, said heating chamber comprising a space above the liquid surface for allowing the escape of steam from the liquid surface; a liquid inlet for allowing liquid into the heating chamber; and a liquid outlet configured to maintain a substantially constant level of liquid as the liquid inlet flow rate varies between a predetermined maximum and a predetermined minimum.

6. An apparatus as claimed in claim 5 wherein the liquid outlet comprises a weir disposed such that liquid escapes over the weir and out of the heating chamber when the water level in the chamber exceeds a predetermined height.

7. An apparatus as claimed in claim 5 or 6 wherein a heated base plate forms the base of the heating chamber.

8. An apparatus as claimed in claim 7 comprising a wall or baffle to retain liquid preferentially on a portion of the base plate directly opposite the heating element on the underside.

9. An apparatus as claimed in claim 7 or 8 wherein the outlet comprises a hole in the base plate.

10. An apparatus as claimed in claim 9 comprising an outlet tube projecting through said hole, proud of the base plate, to form a weir.

11. An apparatus as claimed in any of claims 5 to 10 configured to ensure a minimum residence time for liquid in the chamber.

12. An apparatus as claimed in any of claims 5 to 11 arranged to operate in a sterilisation mode in which the heating chamber is filled with a volume of liquid which is insufficient to bring the level of liquid in the heating chamber to said substantially constant level.

13. An apparatus as claimed in any of claims 5 to 12 comprising means for heating the liquid is heated before it enters the chamber.

14. An apparatus as claimed in claim 13 wherein said heating means comprises a heated flow conduit.

15. An apparatus as claimed in any of claims 1 to 4 or 14 wherein said heated flow conduit is provided with an electric heating element for heating the liquid therein.

16. An apparatus as claimed in claim 15 wherein the heating element is provided on the outside of the conduit.

17. An apparatus as claimed in any of claims 1 to 4 or 14 wherein said heated flow conduit is provided by one side of a heat exchanger.

18. An apparatus as claimed in claim 17 wherein a first side of the heat exchanger provides the heated flow conduit and a second side of the heat exchanger is in fluid communication with the liquid outlet of the heating chamber.

19. A flow heater or apparatus as claimed in any preceding claim comprising means for cooling said liquid downstream of the heating chamber.

20. A flow heater or apparatus as claimed in claim 18 or 19 adapted to dispense water at a temperature of between 30 and 50⁰C.

21. A flow heater or apparatus as claimed in any preceding claim comprising temperature sensing means at the outlet in order to allow control of the outlet temperature.

22. A flow heater or apparatus as claimed in any preceding claim arranged so that control over the outlet temperature is exercised by controlling the rate of flow of liquid into the heating chamber.

23. A flow heater or apparatus as claimed in any preceding claim comprising a pump is provided for driving liquid through the heater.

24. An apparatus for providing heated liquid on demand comprising a flow heater as claimed in any of claims 1 to 4 .

25. An apparatus as claimed in any of claims 5 to 24 configured to provide a steam path between the heating chamber and atmosphere, the steam path being sufficiently restricted to give rise to a pressure difference across it in use of between 0.1 and 1 bar, preferably between 0.2 bar and 0.5 bar.