Abstract:
A method of operating a beverage preparation machine, the beverage preparation machine being of the type comprising a brewer comprising: a reservoir containing water; a delivery head for receiving in use a cartridge containing one or more beverage ingredients; a pump for pumping water from said reservoir to said delivery head; a primary heater for heating the water contained in said reservoir; a secondary heater in between the reservoir and the delivery head; a controller for controlling energization of the primary heater and the secondary heater; and wherein the controller operates to prevent energization of the primary heater simultaneous with energization of the secondary heater. An apparatus for carrying out the method is also disclosed and methods and apparatus utilizing first and second brewers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase application filed under 35 U.S.C. §371 of International Application PCT/US2008/063833, filed on May 16, 2008, designating the United States, which claims benefit to U.S. Application No. 60/940,118, filed on May 25, 2007, and Great Britain application GB 0709588.8, filed on May 18, 2007, all of which are hereby incorporated by reference herein. 
    
    
     FIELD 
     The present invention relates to beverage preparation machines and methods of operating beverage preparation machines. In particular, it relates to improvements in heating systems and the power management of such systems. 
     BACKGROUND 
     Beverage preparation machines such as coffee or tea brewing machines are well known. It is known to provide beverage preparation machines which dispense individual servings of beverage directly into a receptacle such as a cup. Such machines may derive the beverage from a bulk supply of beverage ingredients or from packages of beverage ingredients such as pods, pads or cartridges. An example of one type of such packages is shown in EP1440903. In the following specification such packages will be referenced by the general term cartridges. However, the invention is not limited to use with one particular type of pod, pad or cartridge. The beverages are formed from brewing, mixing, dissolving or suspending the beverage ingredients in water. For example, for coffee beverages, heated water is passed through the cartridges to form the extracted solution. 
     It is known to provide machines with a reservoir in which to store water. The reservoir may be manually refillable or, alternatively, it is known for machines to be plumbed into a mains supply of water which allows for semi-automatic or automatic refilling of the reservoir. 
     It is also known to provide beverage preparation machines comprising first and second brewers coupled together within a single housing or installation. 
     A problem with beverage preparation machines is that high demand on the machine—that is the need to dispense a number of beverages in quick succession—can lead to delays since the water in the reservoir must be heated to a particular temperature in order to allow for good quality beverages to be produced. One solution that has been suggested is to provide the machine with a more powerful heater. However, this solution can cause problems where the machine is to be used where there is a limited power supply available. In addition, limited power supply has been found to be a particular problem for beverage preparation machines comprising first and second brewers in a coupled arrangement. 
     It is therefore an object of the present invention to provide a beverage preparation machine and method which helps to overcome at least some of these problems. 
     SUMMARY 
     Accordingly, the present invention provides a method of operating a beverage preparation machine, the beverage preparation machine being of the type comprising a brewer comprising: 
     a reservoir containing water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a pump for pumping water from said reservoir to said delivery head; 
     a primary heater for heating the water contained in said reservoir; 
     a secondary heater in between the reservoir and the delivery head; 
     a controller for controlling energisation of the primary heater and the secondary heater; and 
     wherein the controller operates to prevent energisation of the primary heater simultaneous with energisation of the secondary heater. 
     In this embodiment the machine comprises a single brewer with primary and secondary heaters. The controller ensures that the primary and secondary heaters are not energised at the same time thereby the total energy requirement of the machine can be reduced. In this way it is possible to use a more powerful primary heater and secondary heater than would be the case if both heaters were required to be energised simultaneously. 
     Preferably the beverage preparation machine is operated to dispense a beverage during a dispense cycle from the brewer, wherein the controller operates to energise the primary heater during a portion of the dispense cycle when the pump of said brewer is inactive. 
     Preferably the dispense cycle comprises one or more pauses where water is not pumped to said delivery head and wherein the controller operates to energise the primary heater during said one or more pauses. 
     For example, the one or more pauses may be for steeping the one or more beverage ingredients of a pod or cartridge. 
     During a dispense cycle the primary and secondary heaters may be energised alternately. Alternatively during a dispense cycle the secondary heater may be always energised and the primary heater may be always de-energised. 
     Preferably the water in the reservoir is heated to a temperature of between 70 and 95 degrees Celsius. 
     More preferably the water in the reservoir is heated to a temperature of approximately 85 degrees Celsius. 
     Preferably the temperature of the water pumped to the delivery head is boosted by use of the secondary heater by between 0 and 30 degrees Celsius. 
     Preferably the temperature of the water pumped to the delivery head is boosted by use of the secondary heater such that the temperature of the water on reaching the delivery head is between 85 and 94 degrees Celsius. 
     The method may further comprise passing steam through the delivery head after dispensation of a beverage from the brewer. 
     The steam may be generated by the secondary heater. 
     The present invention also provides a beverage preparation machine comprising a brewer comprising: 
     a reservoir for water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a pump for pumping water from said reservoir to said delivery head; 
     a primary heater for heating water contained in said reservoir; 
     a secondary heater in between the reservoir and the delivery head; 
     a controller for controlling energisation of the primary heater and the secondary heater; and 
     wherein the controller is operable to prevent energisation of the primary heater simultaneous with energisation of the secondary heater. 
     Preferably the controller is operable to prevent energisation of the primary heater during pumping of water by the pump. 
     Preferably the primary heater is located in the reservoir. 
     Preferably the primary heater comprises an electrical power heating source. 
     Preferably the secondary heater is located in the flow path extending from the reservoir to the delivery head. 
     Preferably the secondary heater comprises an instantaneous heater. 
     In another aspect, the present invention further provides a method of operating a beverage preparation machine, the beverage preparation machine being of the type comprising a first brewer and a second brewer, each of the first and second brewers comprising: 
     a reservoir containing water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a primary heater for heating the water contained in said reservoir; 
     wherein the beverage preparation machine further comprises: 
     at least one pump for pumping water from said reservoirs to said delivery heads; 
     a controller for controlling energisation of the primary heater of each of the first brewer and the second brewer, 
     the method of operation comprising operating the controller to prevent simultaneous energisation of the primary heaters of both the first brewer and the second brewer. 
     By controlling energisation of the primary heaters to ensure that they are not both energised at the same time the total energy requirement of the machine can be reduced. In this way it is possible to use a more powerful primary heater in each brewer. 
     Preferably the method further comprises operating the controller to prevent energisation of the primary heater of either of the first brewer or the second brewer during pumping of water by the at least one pump. 
     Preferably the method further comprises operating the controller to prevent energisation of the primary heater of either of the first brewer or the second brewer during delivery of a beverage into a receptacle from either the first brewer or the second brewer. 
     In this way the brewers do not use power for pumping water, or delivering beverages at the same time as either of the primary heaters are energised. Again, this allows for a more powerful primary heater to be utilised in each brewer without the total power requirement of the machine exceeding the available power supply. 
     Preferably the method further comprises energising the primary heater of one of the first brewer and the second brewer on a first demand from the controller and operating the controller to prevent energisation of the primary heater of the other of the first brewer and the second brewer until after the energised primary heater has been de-energised. 
     In this way the controller operates the primary heaters of the two brewers on a first come-first served basis. In other words once the primary heater of one brewer is switched on it remains on until the water in the reservoir of that heater reaches the required temperature or until some other action—such as a demand to dispense a beverage from the other brewer—interrupts heating. This minimises the time delay until at least one of the brewers is available and ready to dispense a beverage. 
     Preferably the method further comprises heating the water in the reservoir of the first and or second brewer to a temperature of between 70 and 95 degrees Celsius. 
     More preferably the water in the reservoir of the first and or second brewer is heated to a temperature of approximately 85 degrees Celsius. 
     Preferably each of the first and second brewers comprises a pump. Using a separate pump in each brewer rather than a single pump for both brewers reduces the complexity of the hydraulic circuit of the brewers and removes the need for complicated valving to divert flow between the brewers. 
     Preferably each of the first brewer and the second brewer further comprises a secondary heater in the flow path between the reservoir and an outlet of the delivery head and wherein the controller is operable to prevent energisation of either of the primary heaters simultaneous with energisation of either of the secondary heaters. 
     The use of secondary heaters is advantageous in order to provide accurate control of the water temperature when it reaches the beverage ingredients and also to speed up the speed of response of the machine when required to dispense successive beverages at different temperatures. By using the controller to prevent energisation of either of the secondary heaters with either of the primary heaters the total power draw of the machine can be limited within the available power supply whilst utilising a high power primary heater in each brewer. 
     Preferably the controller is operable to allow energisation of the secondary heaters of both the first brewer and the second brewer simultaneously. 
     Advantageously, the ability to operate both secondary heaters at the same time allows the machine to dispense beverages from both brewers simultaneously. 
     Preferably the beverage preparation machine is operated to dispense a beverage during a dispense cycle from the first or second brewer, wherein the controller operates to energise the primary heater of said first or second brewer during a portion of the dispense cycle when the at least one pump is inactive. 
     Preferably the beverage preparation machine is operated to dispense one or simultaneously two beverages during a dispense cycle from the first and second brewer, wherein the controller operates to energise the primary heater of the first or second brewer during a portion of the dispense cycle when the at least one pump is inactive. 
     In this way the machine can flexibly dispense one beverage from one or both brewers or two beverages using both brewers in sequential order or simultaneously. In addition, the primary heater of one or other of the brewers can be switched on whenever the secondary heaters of the machine are not being used and or when the pumps are inactive. 
     The dispense cycle may comprise one or more pauses where water is not pumped to said delivery head and wherein the controller operates to energise the primary heater of said first or second brewer during said one or more pauses. 
     For example, the one or more pauses may be for steeping the one or more beverage ingredients of a pod or cartridge or during purging of a pod or cartridge at the end of the dispense cycle. 
     Thus, advantageously even during relatively short periods when the pumps and secondary heaters are inactive one of the primary heaters may be switched on to help to ready the water in the reservoirs to be able to quickly dispense a subsequent beverage. For example, the pauses may be of a duration of a few seconds, such as a 10 second pause for steeping a ground coffee beverage ingredient. 
     Preferably the method further comprises boosting the temperature of the water pumped to the delivery head of the first and or second brewer by use of the secondary heater by between 0 and 30 degrees Celsius. 
     For example, the temperature of the water pumped to the delivery head of the first and or second brewer may be boosted by use of the secondary heater such that the temperature of the water on reaching the delivery head is between 85 and 94 degrees Celsius. 
     Dependant on the actual water temperature in the reservoir and the target dispense temperature of the beverage the temperature of the water may or may not need boosting by the secondary heater. If no temperature boost is required then the water simply passes the heater with the secondary heater switched off. If a temperature boost is required the secondary heater is switched on. 
     The method may further comprise passing steam through the delivery head of the first and or second brewer after dispensation of a beverage from the first and or second brewer. 
     Preferably the steam is generated by the secondary heater of the first and or second brewer. 
     The steam can be used both to clean the delivery head after some or each dispense cycle and can also be used to drive out most or all liquid remaining in the single serve package in the delivery head. This reduces the amount of soiling of the delivery head on ejection of the package and also helps to ensure consistency in the volume of liquid dispensed during each dispense cycle. 
     The method may comprise operating the first brewer and the second brewer simultaneously to dispense a single beverage, the single beverage comprising a first portion dispensed from the delivery head of the first brewer and a second portion dispensed from the delivery head of the second brewer. 
     Alternatively the method can comprise operating the first brewer and the second brewer simultaneously to dispense a first beverage and a second beverage, the first beverage being dispensed from the delivery head of the first brewer and the second beverage being dispensed from the delivery head of the second brewer. 
     The total power drawn by the beverage preparation machine in use may be less than 3120 Watts. This is particularly advantageous for machines to be used in European locations in order to match the available power supply. 
     The total power drawn by the beverage preparation machine in use may be less than 1800 Watts. This is particularly advantageous for machines to be used in the United States in order to match the available power supply. 
     The present invention also provides a beverage preparation machine comprising a first brewer and a second brewer, each of the first and second brewers comprising: 
     a reservoir for water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a primary heater for heating water contained in said reservoir; 
     wherein the beverage preparation machine further comprises: 
     at least one pump for pumping water from said reservoirs to said delivery heads; 
     a controller for controlling energisation of the primary heater of each of the first brewer and the second brewer, wherein the controller is operable to prevent simultaneous energisation of the primary heaters of both the first brewer and the second brewer. 
     Preferably the controller is operable to prevent energisation of the primary heater of either of the first brewer or the second brewer during pumping of water by the pump of either the first brewer or the second brewer. 
     Preferably the controller is operable to prevent energisation of the primary heater of either of the first brewer or the second brewer during delivery of a beverage into a receptacle from either the first brewer or the second brewer. 
     Preferably the primary heater of the first brewer is located in the reservoir of the first brewer and the primary heater of the second brewer is located in the reservoir of the second brewer. 
     Each primary heater may have a power rating of less than or equal to 3120 Watts. 
     Each primary heater may have a power rating of less than or equal to 1800 Watts. 
     Each primary heater preferably comprises an electrical power heating source. 
     The controller may comprise a first controller device for controlling operation of the first brewer and a second controller device for controlling operation of the second brewer. 
     Preferably the first controller device is located in the first brewer and the second controller device is located in the second brewer. 
     The first controller device and the second controller device are preferably operatively interconnected. 
     Preferably each of the first brewer and the second brewer further comprises a secondary heater in between the reservoir and an outlet of the delivery head and wherein the controller is operable to prevent energisation of either of the primary heaters simultaneous with energisation of either of the secondary heaters. 
     Preferably the controller is operable to allow energisation of the secondary heaters of both the first brewer and the second brewer simultaneously. 
     Preferably each secondary heater is located in the flow path extending from the reservoir to the delivery head. 
     Each secondary heater may have a power rating of less than or equal to 1500 Watts. 
     Each secondary heater may have a power rating of less than or equal to 900 Watts. 
     Preferably each secondary heater comprises an instantaneous heater. For example the secondary heaters may be in-line electrical flash heaters. 
     Advantageously, the machine may comprise only a single power inlet connection. This allows the machine to be installed in a wide variety of locations without the need to take up a number of power sockets or to be supplied with a specialised form of power supply. 
     The present invention also provides a beverage preparation system comprising a beverage preparation machine as described above and one or more cartridges containing one or more beverage ingredients. 
     The present invention further provides a beverage preparation machine comprising: 
     a reservoir for water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a pump for pumping water from said reservoir to said delivery head; 
     a primary heater for heating water contained in said reservoir; 
     a temperature sensor for sensing the temperature of water in the reservoir; 
     an inlet valve communicating with the reservoir and connectable to an external source of water and operable to control inflow of water into the reservoir; 
     a controller for controlling operation of the inlet valve, said controller being operatively connected to the temperature sensor to receive temperature signals indicative of the temperature of water in the reservoir and operatively connected to the inlet valve to control opening and closing of the inlet valve; 
     wherein the controller is operable to open the inlet valve to allow inflow of water into the reservoir in response to temperature signals from the temperature sensor; 
     wherein the controller is operable to open the inlet valve to allow inflow of water when the temperature of the water in the reservoir is within a fill differential of a target water temperature; 
     further, wherein the controller is operable to allow actuation of the pump in order to dispense a beverage only when the temperature of the water within the reservoir is within a vend differential of the target water temperature. 
     In this way, filling of the reservoir is controlled in order to limit the impact on the machine&#39;s ability to be ready to dispense beverages on demand. 
     Preferably the target temperature is between 70 and 95 degrees Celsius. 
     Preferably the target temperature is approximately 85 degrees Celsius. 
     Advantageously the fill differential is smaller than the vend differential. Thus, filling of the reservoir is only permitted when there is some capacity for water to be added without dropping the temperature of the reservoir below the point where the machine is able to dispense beverages. 
     Preferably the fill differential is approximately 5 degrees Celsius. 
     Preferably the vend differential is approximately 10 degrees Celsius. 
     Preferably the controller is operable to execute a time delay between closing of the inlet valve and re-opening of the inlet valve. This allows for slopping of the water in the reservoir to subside to ensure accurate readings by any volume sensors installed in the reservoir and also to allow for the cooling effect of the added water to be sensed by the temperature sensor of the reservoir. 
     The beverage preparation machine may comprise a first brewer and a second brewer, each of the first and second brewers comprising a beverage preparation machine as described above. 
     The present invention also provide a method of operating a beverage preparation machine of the type comprising: 
     a reservoir for water; 
     a delivery head for receiving in use a cartridge containing one or more beverage ingredients; 
     a pump for pumping water from said reservoir to said delivery head; 
     a primary heater for heating water contained in said reservoir; 
     a temperature sensor for sensing the temperature of water in the reservoir; 
     an inlet valve communicating with the reservoir and connectable to an external source of water and operable to control inflow of water into the reservoir; 
     a controller for controlling operation of the inlet valve, said controller being operatively connected to the temperature sensor to receive temperature signals indicative of the temperature of water in the reservoir and operatively connected to the inlet valve to control opening and closing of the inlet valve; 
     the method comprising the steps of operating the controller to open the inlet valve to allow inflow of water into the reservoir in response to temperature signals from the temperature sensor; 
     operating the controller to open the inlet valve to allow inflow of water when the temperature of the water in the reservoir is within a fill differential of a target water temperature; 
     further, operating the controller to allow actuation of the pump in order to dispense a beverage only when the temperature of the water within the reservoir is within a vend differential of the target water temperature. 
     Preferably the controller is operated to execute a time delay between closing of the inlet valve and re-opening of the inlet valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is perspective view of a first embodiment of beverage preparation machine according to the present invention which includes first and second brewers; 
         FIG. 2  is a perspective view of a second embodiment of beverage preparation machine according to the present invention which includes a single brewer; 
         FIG. 3  is a schematic diagram of the beverage preparation machine of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a water flow path within the beverage preparation machines of  FIGS. 1 and 2 ; 
         FIG. 5  is a flow diagram illustrating aspects of the operation of a controller of the beverage preparation machine of  FIG. 1 ; and 
         FIG. 6  is a diagram illustrating operation of the beverage preparation machine of  FIG. 1  when first switched on. 
     
    
    
     DETAILED DESCRIPTION 
     The beverage preparation machines  1  of  FIGS. 1 and 2  each comprise a housing  2  containing the internal mechanisms of the machine such as a water reservoir, a pump and a heating means. 
     The machine  1  of  FIG. 2  comprises a single brewer. The machine  1  of  FIG. 1  comprises a first brewer and a second brewer coupled together. 
     Each brewer of the machines  1  comprises a delivery head  3  provided towards an upper part of the housing  2  in which, in use, is received a cartridge containing one or more beverage ingredients. Beverage is dispensed from the brewer through an outlet spout  5  by pumping water from the reservoir of the brewer through the cartridge to form the beverage which is then directed through the outlet spout  5  into a cup  6 . As can be seen in  FIG. 1 , two outlet spouts  5  are provided for a machine with two brewers. 
     As shown in  FIG. 3 , the first and second brewers of the beverage machine of  FIG. 1  are of the same configuration internally and functionally. The components of the first brewer are shown in  FIG. 3  with the suffix ‘a’ and the components of the second brewer are shown with the suffix ‘b’. Each brewer comprises a reservoir  30 , a power supply unit (PSU)  45 , a controller  47 , a pump  50 , a secondary heater  51 , a delivery head (shown in  FIG. 3  by the collective numeral  55 ) and a user interface  61 ,  62 . 
     The machine  1  further comprises a common flow diverter  63  which channels the output from the delivery heads  55  into one or more receptacles  6  positioned on a drip tray  64  or cup stand assembly  4 . The drip tray  64  may be provided with a sensor  66  connected to one or both of the controllers  47  to indicate when the drip tray  64  is full. There is also provided a waste bin  60  for ejected cartridges. The waste bin  60  is provided with a sensor  65  connected to one or both of the controllers  47  to detect when the waste bin  60  is full. The cup stand assembly may comprise a cup stand  6  mounted on a shaft  10 . 
     Each reservoir  30  comprises a primary heater  31  in the form of an immersion heater element, a water temperature sensor  32 , an overheat sensor  33 , water level sensors  34 , a boil sensor  35 , an overflow outlet  36  and a drain point  36 . In addition the reservoir  30  is provided with a filling point where a manual fill  38  may be carried out—for example by accessing the reservoir through a removable lid, or an automatic fill may be carried out using an inlet valve  41  plumbed into a mains supply  40  of water. 
     Each PSU  45  provides electrical power to its respective brewer. The PSUs  45  are connected to an external mains supply  46 . A single external connection to the mains power supply  46  is utilised. For example a single power flex terminating in a two or three pin plug is provided. 
     Each controller  47  comprises a printed circuit board (PCB) having mounted thereon a processor and memory as well as an input/output interface for transmitting and receiving signals from the primary heater  31 , temperature sensor  32 , overheat sensor  33 , water level sensors  34 , boil sensor  35 , inlet valve  41 , PSU  45 , pump  50 , secondary heater  51 , delivery head  55  and user interface  61 ,  62  of its respective brewer. In addition, the controllers  47  of the two brewers transmit and receive signals from each other using interconnect  48  in order to co-ordinate operation of the two brewers as described below. The interconnect  48  may be a collection of wire connects or a dedicated data bus with onboard controller. 
     The memory stores operational code which is used to control the operational behaviour of the machine under various operational scenarios as will be described below. The memory may be a read-only memory or a writable memory such as an EPROM. 
     Each pump  50  has an input connected to its respective reservoir  30  and an output connected to its respective secondary heater  51 . The pumps  50  may be a peristaltic type of pump wherein a known volume of water is throughput on each cycle or revolution of the pumping member. Alternatively, a pulse counter encoder may be utilised connected to the controller for determining volume throughput. The flow rate produced by the pumps  50  may be varied under the control of the controller  47  between dispense cycles and within an individual dispense cycle. Typically flow rates of between 0 and 13 mls −1  are utilised. Advantageously a very slow flow rate is used at the start of the dispense cycle to maximise heat transfer from the secondary heaters  51 . 
     Preferably, an electromechanical check valve is located between the pump  50  and the secondary heater  51 . In addition a 2.5 bar overpressure device is fitted to the flow line. 
     Each secondary heater  51  comprises an instantaneous flash heater having a through-flow tube in which water to be heated passes and an electrical heating element  52  thermally connected to an exterior of the tube. A temperature sensor  53  is located at the exit of the secondary heater  51  to monitor the temperature of the water exiting the secondary heater. This measurement is fed to the controller  47 . 
     The output from the secondary heater  51  is delivered to the delivery head  55 . 
     The delivery head  55  comprises a piercing element  56  and a clamping element  58  which can receive in use a cartridge  70  which is to be dispensed. A barcode reader  57  is provided for reading an identifying barcode located on the cartridge  70 . The delivery head  55  can be moved in between an open configuration in which the cartridge  70  can be inserted into the delivery head  55  and a closed configuration in which the cartridge  70  is clamped by the clamping member  58  and an inlet and outlet are formed by the piercing element  56 . 
     An ejection mechanism  59  may be provided for ejecting the cartridges  70  from the delivery head  55  after dispensation. However, for the purposes of the present invention the ejection mechanism and the detailed operation of the delivery head  55  will not be described in detail. 
     Each user interface comprises a start/stop button  61  and a LCD display panel  62  for displaying information to a user. 
     In use, assuming that the water in one of the reservoirs  30  is at the required temperature the LCD display  62  for that brewer will indicate that a cartridge should be inserted into the delivery head  55 . The cartridge  70  is then inserted by a user and the delivery head  55  closed to pierce the cartridge  55 . The start/stop button  61  is then pressed to commence the dispense cycle. During dispense water follows a flow path as shown schematically in  FIG. 4 . A portion of the water in the reservoir  30  that has been heated to a holding temperature by the primary heater  31  is pumped by the pump  50  through the secondary heater  51  to the delivery head  55 . On passing through the secondary heater  51  the temperature of the water is raised, if required, by energisation of the secondary heater element  52 . On reaching the delivery head  55  the water is directed through the cartridge  70  to form the beverage. The beverage exits the outlet of the cartridge and is directed via the flow diverter  63  out of one of the outlets  5  of the machine  1  into a receptacle  6 . 
     The holding temperature of the reservoir is between 70 and 95 degrees Celsius and preferably is 85 degrees Celsius. 
     The temperature of the water on exiting the secondary heater  53  is set by the controller  47  and a positive feedback control is utilised using the temperature sensor  53  with the controller  47  adjusting the power of the heating element  52  as necessary to achieve the desired temperature for the water at the point the water reaches the delivery head. The temperature of the water reaching the delivery head is desired to be between 85 and 94 degrees Celsius depending on the type of beverage being dispensed. Thus, the desired temperature of the water directly exiting the secondary heater will be greater than this to allow for heat losses during transport of the water from the secondary heater to the delivery head. In practice the required temperature levels at the exit of the secondary heater for the water would be determined by experiment but may, for example, be in the range of 90 to 103 degrees Celsius. 
     According to the present invention, the control of the machine as determined by operation of the controllers  47  is programmed to limit the maximum power requirement of the machine  1 . 
     Firstly, each controller  47  operates to prevent simultaneous energisation of the primary heater  31  and secondary heater  51  of its own brewer. Secondly, the controllers  47  of both brewers communicate using the interconnect  48  to prevent energisation of the primary heater  31  of one brewer at the same time as energisation of the primary heater  31  or secondary heater  51  of the other brewer. In other words, the two primary heaters  31  cannot be energised simultaneously and neither can either primary heater  31  be energised simultaneously with either secondary heater  51 . An advantage of the present machine is that both secondary heaters  51  may be energised simultaneously. This allows both brewers to dispense at the same time. 
     The control logic applied by both controllers  47  is shown schematically in the flow chart of  FIG. 5 . The ‘start’ box represents the start point of the control loop and the machine would initially be in this position on first switching on. The primary heaters  31  of the brewers operate on a first come-first served basis such that the controller  47  that first requests to switch on its primary heater  31  will be successful and this primary heater  31  will remain energised until either the water in the reservoir  30  reaches the holding temperature or a dispense cycle is initiated. 
     Both controllers  47  can set or release a ‘heat inhibit’ condition which prevents energisation of the primary heater  31  of the other brewer. 
     An example of the control logic in operation is shown in  FIG. 6  which illustrates the initial operation of the machine on switching on. At first the reservoirs  30  of the first and second brewers are below the holding temperature therefore on switching on both LCD displays show a ‘Heating Please Wait’ message. On switching on the first brewer, in this example, is first to request energisation of its primary heater  31   a  and thus its primary heater  31   a  switches on. Because of this the primary heater  31   b  of the second brewer remains off. Once the temperature of the reservoir  30   a  is up to the holding temperature a ‘Please Insert Cartridge’ message is displayed on the display  62   a  of the first brewer. At this point the controller  47   a  of the first brewer de-energises its primary heater  31   a  and clears the ‘heat inhibit’ condition allowing the controller  47   b  of the second brewer to energise its primary heater  31   b.    
       FIG. 6  illustrates that if during heating of the water in reservoir  30   b  a user commences a dispense cycle using the first brewer by pressing the start/stop button  61   a  this takes precedence and the primary heater  31   b  of the second brewer is de-energised to allow the secondary heater  51   a  of the first brewer to be switched on. 
       FIG. 6  further illustrates that if dispensing the beverage from the first brewer causes its reservoir  30   a  to fall below the holding temperature (in reality this would require dispensation of a number of beverages) then the primary heater  31   a  will want to switch on at the end of the dispense cycle. However, in this example, using the first come-first served principle, the controller  47   b  of the second brewer is first to request energisation of its primary heater  31   b . Thus the reservoir  30   b  of the second brewer heats up to the holding temperature first at which point the primary heater  31   b  switches off allowing the primary heater  31   a  of the first brewer to switch back on. 
     An advantage of this method of control is that one of the brewers will heat up quickly on initial start up to allow quick dispensation to take place. 
     The intercommunication between the controllers  47  allows for energisation of the primary heater  31  of one of the brewers even during relatively short pauses in use of the pumps or secondary heaters  51  of the brewers. For example, the primary heater  31  of one of the brewers may be energised during a steeping pause in a dispense cycle. 
     Optionally, the brewers may use a steam purge for cleaning the delivery heads  55  and also to help drive out liquid or beverage from the cartridges  70 . The steam is generated by the secondary heater  51 . The steam may be producing from water fed to the secondary heater  51  specifically for this purpose but is preferably generated from residual water remaining in the secondary heater  51  and associated pipework at the end of the beverage dispensation phase of the delivery cycle. The steam may be generated by specific energisation the secondary heater  51  for a period or relying on the residual heat energy contained in the secondary heater  51  and associated pipework from heating the liquid water. 
     Aspects of the present invention also apply to a beverage preparation machine comprising a single brewer as shown in  FIG. 2 . The operation, construction and temperature control of the single brewer is as explained above with regard to the beverage preparation machine comprising first and second brewers mutatis mutandis. In particular operation of the single brewer is controlled by controller  47 . In this case the controller  47  acts to ensure that the primary heater  31  and secondary heater  51  are not energised simultaneously in the same manner as described above. 
     As described in the above embodiments the temperature of the water exiting the secondary heater  51  can be controlled by positive feedback control using the controller  47  and the temperature sensor  53 . It has been found that positive feedback control of the water temperature is somewhat ineffective for the first few seconds of the water flow. In other words, it requires a few seconds for the feedback loop to be established before accurate temperature control is obtained. In order to provide more effective temperature control, especially for the initial period of water flow, feed-forward temperature control is implemented. The controller  47  is pre-programmed with a number of operational scenarios and adjusts the operation of the secondary heater  51  accordingly. The controller  47  selects the operational behaviour based on the time since the last dispense cycle and the temperature of the secondary heater  51 . 
     For example, where a relatively long period has elapsed since the last dispense cycle (e.g. over 10 minutes) a ‘cold start’ program is initiated wherein a small volume (around 10 ml) of water is pumped through the pipework into the secondary heater  51  and then held there whilst the secondary heater  51  is energised to heat the water to the required pre-wet temperature before passing the water to the brew head. The length of this ‘stabilisation time’ will be longer for a ‘cold start’ than for an ‘intermediate start’ where a beverage has been dispensed within, say, the last five minutes. For a ‘hot start’ where a beverage is dispensed immediately or very shortly after a preceding beverage the stabilisation time will be shorter still—or indeed no stabilisation time may be required at all. 
     Feed-forward control of the secondary heater  51  applies equally to all of the embodiments described above. 
     In the above embodiments, a beverage dispense cycle may be operated using energisation of only the secondary heater(s)  51 . In other words, the primary heater(s) may be off during the entire time that a beverage is being dispensed. Thereafter the primary heater(s) may be energised if required to maintain or raise the temperature in the reservoir(s)  30 . 
     The controllers  47  of the machines  1  of  FIGS. 1 and 2  can also be used to control automatic refilling of each reservoir  30 . Operation of the inlet valve  41  is under the control of the controller  47 . The controller  47  operates to limit the temperature drop of the reservoir  30  caused by injecting cold water into the heated water already in the reservoir  30 . This is achieved by monitoring the actual temperature of the water using the temperatures sensor  32  and only allowing the inlet valve  41  to be opened when the actual temperature is within a fill differential of the target holding temperature. For example, when the target holding temperature is 85 degrees Celsius a fill differential of 5 degrees may be used meaning that the inlet valve  41  cannot be opened to admit water to the reservoir until the actual temperature is at least 80 degrees Celsius. In addition, the controller  47  utilises a vend differential parameter to control when the water in the reservoir is sufficiently close to the target holding temperature to allow a dispense cycle to take place. For example, the vend differential may be set at 10 degrees meaning that dispensation can take place as long as the actual water temperature is at least 75 degrees Celsius (in this case the secondary heater  51  is used to make up the initial temperature deficit to ensure the water is at the target delivery temperature when it reaches the delivery head  55 ). 
     As a result a reservoir  30  that is sitting at the target temperature may be refilled when water is pumped out of the reservoir until the temperature in the reservoir falls by 5 degrees. Thus top-up filling of the reservoir in these circumstances does not prevent the immediate use of further water if demanded. Under very heavy usage conditions (or on the first filling of the reservoir after plumbing in or emptying for servicing) the water level in the reservoir  30  may reach the low level sensor  34  at which point the controller  47  will open the inlet valve  41  to allow refilling and dispensation of beverages will not be possible until the reservoir reaches at least 75 degrees Celsius.