fig1 shows an embodiment of the invention which can be used to dispense boiling water , on demand into a cup 2 for making hot beverages . the temperature of the water can also be adjusted by turning a knob 4 . the dispense temperature can be varied from 65 ° c . to just boiling and beyond that to ‘ rolling boil ’ where more energy is put into the water to ensure that the whole volume is fully boiling the amount of water to be dispensed is controlled by a second knob ( not shown ). also visible is a steam outlet 6 . on the upper part of the main part of the apparatus is a water tank 8 which must be filled periodically by a user . fig2 shows some of the main internal components of the apparatus with other parts omitted for clarity . from here may be seen the water tank 8 , extending downwardly from which is an outlet pipe 10 connected to the inlet side of a low voltage , centrifugal pump 12 . the outlet side of the pump 12 is connected via a tube 14 to a water distribution plenum block 16 which distributes water entering the block between two parallel flow heater sections 18 , 20 as will be explained in greater detail below with references to fig4 and 5 . at the downstream end of the flow heater portions 18 , 20 is a boiling pool 22 . this is formed by a deep - drawn stainless steel cup 23 fitted to an approximately circular stainless steel element head 54 ( see fig3 and 5 to 7 ). the boiling pool 22 has an outlet spout 24 projecting downwardly from it for dispensing heated water into the user &# 39 ; s cup 2 . at least one of the flow heater portions 18 , 20 has a small hole 70 drilled into the side of its outer casing near to the boiling pool to accommodate a thermistor . alternatively it could be placed on the outside of the casing . the cross - section of fig3 shows the interior of the water tank 8 . from this it can be seen that the base of the water tank 8 has a circular aperture 26 which is designed to receive a water filter , for example the applicant &# 39 ; s aqua optima water filter . this is represented very schematically by the component marked with the reference numeral 28 . the water filter 28 has a restricted outlet aperture ( typically of the order of 4 mm ) which has the additional benefit in the present context that it is too small to allow air to pass into the filter when there is water in the filter ; were this not the case bubbles of air could get into the filter and reservoir so allowing the continuous flow of water . the lower part of the water filter 28 is received inside a further , intermediate holding chamber 30 , in the center of which is an outlet connected to the pipe 10 which connects it to the pump 12 . a vertical tube 32 extends from the upper part of the holding chamber 30 into the main water tank 8 and terminates just inside an indented portion 34 of the top of the water tank 8 . this allows pressure equalization between the holding chamber 30 and the water tank 8 . fig4 shows a horizontal cross section through the distribution plenum block 16 and the two parallel flow heaters 18 , 20 . the outlet side of the pump ( not shown here ) connects via a tube 14 to a vertical inlet channel 36 in the distribution block 16 . this connects within the block to two laterally extending tubes 38 which open out into corresponding larger bore circular section cylindrical chambers 40 , 42 at right angles to the lateral tubes 38 . the cylindrical chambers 40 , 42 receive the ends of the two flow heater sections 18 , 20 respectively . as can now be seen , each of the flow heater sections 18 , 20 comprises an outer jacket 44 , 46 and a length of a sheathed immersion - type heating element 48 which , although not depicted , comprises a stainless steel casing and a coiled resistance wire packed in magnesium oxide insulating powder . the cold tails 50 , 52 of the immersed element emerge through holes provided in the rear of the distribution plenum block 16 . the two flow heater sleeves 44 , 46 are wider in diameter than the corresponding heating element 48 and so define therebetween a corresponding annular channel for each of the flow heater sections 18 , 20 . as may be seen from this cross - section , the sleeves 44 , 46 make a sealing connection with the front end of the circular channels 40 , 42 in the block 16 but stop short in those channels of the point where they meet the lateral channels 38 so that the aforementioned annular channel in each of the flow heaters 18 , 20 is open to the cylindrical chambers 40 , 42 formed within the distribution block 16 whilst the sheathed element 48 extends through the block and is sealed against it . the result of this is that there is a fluid path from the block inlet 36 , via the lateral channels 38 and the cylindrical chambers 40 , 42 to the interior annular channels of the two flow heaters 18 , 20 . fig5 shows the full length of the flow heaters 18 , 20 . from here it can be seen that the respective outer sleeves 44 , 46 thereof are sealed at the other end to the boiling chamber 22 . the heating element 48 extends into the boiling chamber 22 and is bent round to form two elongate arms which respectively form part of the two flow heaters 18 , 20 . this is seen more clearly in fig6 and 8 . as can be seen from fig5 and 6 , the bent portion of the element 48 is brazed to an immersed element head plate 54 which closely resembles the element head plates seen in traditional immersed element kettles . this arrangement is known as a hot return and , as can be seen from fig7 , the other side of the head plate 54 is formed with a semi - circular indentation 56 to receive the snap - acting bimetallic actuator 57 of a standard immersed element control unit 58 . fig7 also shows a copper strip 60 extending from the hot return against which bears a nylon thermal fuse 59 of the control unit 58 for providing secondary level overheat protection . alternatively in nickel - plated copper immersed element heads , no copper strip is required as is also well known in the art . it will be appreciated by those skilled in the art looking at fig7 that the cold tails of the element 50 , 52 do not project through the element head 54 as would be conventional for an immersed kettle element , but rather they project through the distribution plenum block 16 ( omitted for clarity in fig7 ). instead , two dummy cold tail components 62 , 64 project through the element head 54 to make electrical contact with the control unit 58 and are in turn connected by means of flying leads ( not shown ) to the cold tails proper 50 , 52 . this allows a standard production control unit 58 to be employed without modification , which is a substantial cost saving as against having to design and produce a new dedicated control unit . the element head is provided with three mounting studs 66 for the control unit 58 . the interior of the boiling chamber 22 is best seen from the view of fig8 which has the element head 54 removed . from here it can be seen that the chamber 22 is broadly of a squat cylindrical shape although its internal volume is limited by the two dummy cold tails 62 , 64 , the bent portion of the element 48 and by a vertically protruding boss 66 which has a central bore 68 in communication with the outlet spout 24 . the height of the boss 66 is approximately level with , or slightly the level of the top of the element tube 48 . taking into account the meniscus depth , this ensures that the element 48 remains just covered in water , so preventing overheating in normal use . a hole 72 is formed in the top wall section of the cup 23 forming the boiling chamber 22 to provide a steam outlet which communicates with the steam outlet 6 at the top of the appliance . a variant of this embodiment is shown in fig9 . in this embodiment an additional u - shaped baffle 69 is provided above the boss 66 and with its open end facing away from the ends of the annular heating channels 20 . this prevents water emerging the channels from flowing directly through the outlet 68 , thereby ensuring it is properly heated to boiling . operation of the apparatus will now be described . first the user fills the water tank 8 with water by removing it , inverting it , removing the water filter 28 and filling from a tap . the filter 28 is then replaced , the tank re - inverted and then placed back on to the apparatus . the water immediately starts to be passed through the water filter 28 inside it at a rate determined by the restricted outlet from the water filter as is known . as water passes through the filter 28 it begins to fill the connecting pipe 10 and then the lower holding chamber 30 , displacing air through the ventilation tube 32 into the sealed head - space of the water tank 8 . when the water level in the holding chamber 30 reaches the bottom of the ventilation tube 32 , air can no longer be expelled from the chamber and so the flow of water stops . when the user wishes to dispense water he / she sets the required temperature on the first knob 4 and then turns the second knob ( not shown ) round from an ‘ off ’ position to the required volume . initially the controlling circuit ( not shown ) activates the heating element 48 . after a delay of one or two seconds ( depending on the temperature of the water already in the heater ) the pump 12 is operated to pump water from the lower chamber 30 through the pipes 10 and 14 into the distribution plenum block 16 . in other embodiments the pump may be started before the heater . as water passes through the channels 38 in the plenum block , the flow is balanced between the left and right channel . the bore of these channels 38 is chosen so that the pressure drop through this section is greater than for all the rest of the hydraulic system . this is very important in maintaining correct flow through the downstream annular channels 18 , 20 . for example , if a minor restriction arises in one channel 18 , 20 but not the other , there is little effect on the flow rate , as the dominant pressure drop is through the plenum channels 38 . a pressure drop ratio of say 10 : 1 gives the required effect . for example if the pressure drop across the tubular heaters 18 , 20 is equivalent to a 40 mm head of water , the pressure drop across the plenum channels 38 would be equivalent to a 400 mm head . once water has been pumped into the distribution block 16 , it is pumped through this and down the annular channels of each of the two flow heaters 18 , 20 between the heating element 48 and the corresponding stainless steel outer jacket 44 , 46 . this heats the water rapidly as it passes through from ambient temperature ( of the order of 20 ° c .) in the distribution block 16 to approximately 85 ° c . at the downstream ends of the flow heaters 18 , 20 . the temperature of the water is monitored by the thermistor projecting into the holes 70 in the sides of the flow heaters 18 , 20 near the boiling chamber 22 . the temperature can be monitored accurately and reliably here since the water is not boiling and therefore there is no significant amount of steam bubbles within it . the water then passes out of the flow heaters 18 , 20 and into the interior of the boiling chamber 22 where it begins to fill this chamber , thereby covering the curved portion of the element 48 which projects into the boiling chamber . the curved part of the heating element 48 continues to heat the water in the boiling chamber , now producing significant numbers of bubbles which break the surface of the pool of water in the chamber and escape as steam . however , the steam can easily escape the boiling chamber by means of the steam outlet 72 at the top of it . referring particularly to fig8 , it can be seen that as the water level in the boiling chamber 22 rises level with and above the top of the protruding boss 66 , it will start to pour out through the aperture 68 and through the outlet spout 24 and into the user &# 39 ; s cup 2 . the pumped flow rate and the power of the element 48 are matched such that by the time the water leaves the boiling chamber through the aperture 68 and spout 24 it is effectively boiling . this includes an ‘ over - allowance ’ of energy input as compared to the energy that would theoretically be required to raise the water temperature to 100 ° c . to ensure a rolling boil and thus that the water attains a true temperature very close to boiling . the height of the boss 66 is chosen to ensure that the element 48 remains covered in water regardless of the flow rate . boiling water continues to be dispensed until the volume set by the user has been dispensed . at which point the pump 12 is switched off . to increase the energy efficiency of the device , the heating element 48 is turned off about 2 seconds before the pump is turned off . there is sufficient stored energy in the element and other components to ensure that the water continues to be boiled . referring back to fig3 , it will be appreciated that because of the intermediate holding chamber 30 from which the water is drawn , water can be drawn at a greater rate than it passes through the filter 28 . the capacity of the chamber 30 is designed so that , taking into account the extent to which it is replenished while the pump is running , a large ordinary mug can be filled with water before the flow rate becomes too low ( i . e . limited by the flow rate through the filter ) and so the apparatus has to switch off to prevent the element 48 overheating . to give an example , assuming it takes 15 seconds to dispense 250 ml of water ( equivalent to a large mug ) at 65 ° c ., only 100 ml will flow through a typical aqua optima filter in that time . however by providing an intermediate holding chamber of only 150 ml , the large mug can be safely filled without the element overheating . it will also be appreciated that the intermediate holding chamber 30 means that the head of pressure of the water entering the pump 12 at least initially is known , which means that a known flow rate can be achieved even by using a relatively inexpensive and quiet centrifugal pump . of course , once the chamber refills with water from the tank 8 , dispensing can be recommenced . if the water tank 8 should run dry , the heating element 48 will begin to overheat . however , this can be sensed by the temperature sensor projecting through the aperture 70 into the flow heaters 18 , 20 . as a backup the bimetallic actuator on the control unit 58 will sense overheating of the element 48 and therefore snap into its reverse curvature thereby opening a set of contacts in the control unit in the well - known manner . secondary backup protection is provided by the thermal fuse of the control unit 58 , again as is very well known in the art . the element is arranged to ensure that in the case of dry boil or dry switch - on , the hot return portion brazed to the head 53 is the first to become dry . this is achieved by ensuring that the flow in the dual tubes 18 , 20 of the first heater is balanced under all adverse conditions ( as explained earlier ) and also by ensuring that the hot return is slightly higher than the rest of the element 48 by having it and the surrounding tubes 18 , 20 slightly inclined . this has a further benefit , on start - up from dry , of ensuring that the tubes 18 , 20 are free venting , and that the flowing water can easily push the initial volume of air ahead of it and out into the boil chamber 22 without airlock should the user wish to dispense water at a lower temperature , he or she can use the knob 4 at the top of the appliance to set a lower temperature which will cause the pump 12 to operate at a higher speed and therefore give a higher flow rate of water through the apparatus which will mean that it is heated to a lower temperature before it is dispensed . again , having the known head of pressure for the water entering the pump allows the actual flow rate that will be achieved for a given speed to be calculated and hence the temperature of the dispensed water can be calculated . the temperature sensors projecting into the apertures 70 allow the temperature of the water being dispensed through the outlet spout 24 to be predicted from a knowledge of the proportion of the heating element 48 which is upstream of it and the corresponding proportion of the element 48 which is downstream of it — i . e . the curved portion in the boiling chamber . the sensor can also be used to introduce a relative delay between operating the pump and switching on the element 48 depending upon the ambient temperature of the water sitting in the apparatus ( e . g . as a result of previous operation ) taking into account the temperature of water requested by the user . thus it will be seen that the embodiment described above provides the benefit of a flow heater , i . e . being able to dispense a controllable volume of water on demand , but with the significant advantage of being able to provide boiling water . the boiling chamber and the separation of the steam through the steam outlet 72 from the water outlet 68 , 24 is key to allowing this and prevents spitting and localized hot spots on the element . turning to fig1 , this shows two views of an assembly of a sheathed heating element 80 , of conventional construction , disposed inside a thin stainless steel jacket 82 so that water can flow between the outer surface of the element 80 and the inner surface of the jacket 82 . in this respect it is similar to the arrangement described above with reference to the previous embodiment , in particular fig4 and 5 thereof . however , in this embodiment the stainless steel jacket is formed with a series of indentations 84 spaced both longitudinally and circumferentially which produce protrusions on the inner surface that locally reduce the diameter of the jacket 82 sufficiently that it touches the surface of the sheathed element 80 . this maintains the element 80 centrally coaxially disposed in the jacket 82 . the indentations 84 do not have any material impact in the flow of water between the element 80 and the jacket 82 since water can still flow substantially all the way round the element 80 . in a variant of this embodiment , the indentations 84 could be merged longitudinally to form continuous depressions with corresponding continuous ribs on the inner surface which would also serve to locate the element 80 centrally . in this variant , separate channels for water flow would be defined between the respective ribs . of course , the number and distribution of the depressions / protrusions is not critical . moreover protrusions could be provided on the surface of the element as well or instead . these features may be applied to other embodiments of the invention . fig1 shows a variant on the embodiments shown in the previous figures with a slightly different arrangement for the water inlet to the annular channels formed between the respective outer jackets 44 , 46 and the u - shaped immersion element 48 . instead of the distribution plenum block 16 as shown in fig4 , this embodiment has an inlet manifold arrangement comprising a common inlet 86 communicating with two respective branching tubes 88 , 90 which direct the water to respective inlet collars 92 , 94 . as may be seen , the collar 94 provide a sealing connection at one end to the respective outer sleeves 44 , 46 of the flow heater section and at the other end provide a sealing connection against the surface of the sheathed heating element 48 . this can be arranged so that the element cold tails 50 , 52 extend inside the elements 48 far enough that the part of the heater surface in contact with the inlet seal is not heated . it will further be noted that the branching channels 88 , 90 join the respective inlet collars 92 , 94 in a direction offset from the central axis thereof . the effect of this is that as the water enters the inlet collars 92 , 94 and passes down into the annular channels defined by the sleeves 44 , 46 it is given a component of motion around the centrally located element 48 i . e . it has a component of swirling motion as well as a forward velocity component . as this flow pattern continues down the tube the swirling motion will continue which assists in mixing of the water inside the tube and therefore promotes a more even temperature distribution further along the tube which facilitates accurate temperature measurements . fig1 to 14 depict another embodiment of a heater which is designed to encourage swirling of the water inside the annular channels of the flow heater . fig1 shows , in isolated view , a sheathed heating element 48 ′ similar to those of previous embodiments . the difference exhibited in this particular embodiment is that each of the two arms of the element has a thin wire 96 , 98 wrapped tightly around it in a helical fashion . this is carried out during manufacture prior to insertion of the arms of the element 48 ′ into the respective sleeves 44 , 46 ( see e . g . fig4 ) to form the annular channels along which the water flows in use . the wires 96 , 98 are conveniently made from stainless steel having a diameter of for example 0 . 6 mm , although the material , dimensions and pitch of the wire may be chosen to suit the particular application . it will be noted from fig1 , however , that in this particular embodiment at least the wire 96 which is wrapped around the element 48 is not wide enough to fill the annular channel formed between the element 48 and the outer sleeve 44 completely . it may , for example , be less than half the height of the channel e . g . approximately a third of the height of the channel . in use the presence of the wire 96 , 98 has been found to encourage a swirling motion of water inside the respective channels which , as was explained above , gives a more even circumferential temperature distribution and so facilitates temperature measurements . the arrangement shown in the embodiment of fig1 and 13 could be used in conjunction with the inlet manifold arrangement shown in fig1 , although this is not essential . a further embodiment of the invention will now be described with reference to fig1 to 16 . turning initially to fig1 , this embodiment comprises broadly a conventional flow heater section 100 which is approximately s - shaped and a boil pool arrangement 102 at the downstream end of the flow heater section 100 . with additional reference now to fig1 , it will be seen that the flow heater section 100 comprises an approximately rectangular section tube 104 to the underneath of which is brazed a sheathed heating element 106 . the water tube 104 and element 106 conform closely to one another as they bend to take up the approximate s - shape referred to above . an aluminum heat diffuser plate 108 is brazed onto the underside of the heating element 106 along various parts of its length . the heat diffuser plate 108 is provided on its underside with three mounting bosses 110 ( only two of which are visible ) which permit a thermo - mechanical overheat protection control unit 112 to be mounted in good thermal contact with the heat diffuser plate 108 . the control element 112 depicted in these figures is the applicant &# 39 ; s u11 control which comprises a pair of snap - acting bimetallic actuators which are individually operable in the event of an overheat being detected to open respective sets of contacts to interrupt power to the element 106 . of course , many other control arrangements both thermo - mechanical and / or electronic could be employed for this purpose . turning now to fig1 , the details of the boil pool 102 may be seen more clearly from the cross - section shown there . the lid of the boil pool 102 has also been omitted for clarity . fig1 shows a sectional view of the boil pool 102 with its lid removed for clarity . at the upstream end of the boil pool , a sealed entry is provided for the water tube 104 and for the heating element 106 . as may be seen from the figure , the water tube 104 terminates just inside the boil pool 102 whilst the element 106 extends across the length of the boil pool 102 to project from the other end via another sealed aperture . this permits electrical connection to be made to the other cold tail 114 . it will be seen that the boil pool 102 has an overall elongate rectangular shape in which the portion of the element 106 inside it occupies the lower portion , although water can pass all the way around it . an aperture in the upper part of one of the side walls of the boil pool opens out into a steam vent 116 which allows steam to be vented to atmosphere away from the user or to be captured and condensed in a suitable trap , drip tray or the like . near the downstream end of the boil pool 102 is a hot water outlet tube 118 which extends vertically up into the boil pool to a height just above the height of the element 106 . the hot water outlet 118 is offset to the side of the element 106 and is accommodated in a suitable lateral extension of the vertical wall . operation of this embodiment of the invention is similar to those previously described . at first the heating element 106 is energized by applying electrical power across the cold tails 107 , 114 via connections ( not shown ) to the control unit 112 . water is also pumped by means of a pump from a reservoir ( neither of which is shown ) to the upstream end of the water channel tube 104 . as explained in the context of previous embodiments , the water pumping may begin simultaneously with , before or after energization of the element as appropriate . as the water flows through the tube 104 it is heated by the element 106 until it emerges in to the boil pool 102 at a temperature of approximately 85 ° c . the water in the boil pool 102 continues to be heated by the element 106 , during which time localized boiling occurs at the element surface , giving rise to violent movement of the water and large bubbles of steam being generated . however , this steam can easily pass out through the steam vent 116 , although a tortuous steam path downstream of the steam vent 116 may be provided so that the steam pressure inside the boil pool 102 may be allowed to rise a little above atmospheric pressure e . g . by 0 . 25 to 0 . 5 bar . this slightly raises the temperature at which the water boils so that the exit temperature of the water is maximized . when water in the boil pool reaches the level of the top of the hot water outlet tube 118 it can then freely flow down the tube and out of the spout of the appliance ( not shown ) into a user &# 39 ; s receptacle . it will be seen therefore that the vertical wall of the outlet tube 118 which is inside the boil pool acts as a weir to maintain a minimum water level inside the boil pool . since this minimum water level is above the top of the element 106 , it may be ensured that during normal operation the element 106 remains covered with water and cannot therefore overheat . however , in the event of the reservoir running out of water or the appliance being switched on without any water in , the temperature of the element 106 will rise very rapidly . this rapid temperature rise is communicated by means of the aluminum heat diffuser plate 108 to the bimetallic actuators of the control unit 112 which causes them to operate thereby opening their respective contacts and interrupting the power supply to the element 106 . thus dangerous overheating and / or damage is prevented . it will be noted that the section of element 106 inside the boil pool is not in direct thermal contact with the heat diffuser plate 108 . this has a positive advantage in the situation where the appliance runs out of water in the reservoir since in those circumstances the element bonded to the tube 104 will begin to overheat first whilst the portion of the element 106 in the boil pool remains immersed in the minimum pool of water retained by the weir formed by the outlet pipe 118 . accordingly , even in these circumstances the power to the element may be rapidly interrupted to prevent overheating . although this invention has been shown and described with respect to the detailed embodiments thereof , it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention .
a heater for heating liquid to boiling comprises a heating element , a first heating region heated by said heating element for heating liquid flowing therethrough to a temperature below boiling , and a second heating region for heating said liquid to boiling . the second region permits the exit of steam therefrom separately from heated water .