Ohmic heater with multiple operating states, washing appliance and method of heating a liquid

Abstract

An ohmic heater has a structure (20) defining a flow path extending in a downstream direction (D), a first pair of electrodes (34a, 34b) and a second pair of electrodes (36a, 36b). The electrodes of each pair are adjacent to each other in the downstream direction but spaced apart from each other in a direction perpendicular to the downstream direction; the pairs of electrodes are spaced apart from each other in the downstream direction. Circuitry (40, 42, 44, 46, 48, 50) is operable to apply a voltage (i) between the electrodes of the first pair (34a, 34b); or (ii) between the electrodes of the second pair (36a, 36b); or (iii) between at least one electrode (34a) of the first pair and at least one electrode (36b) of the second pair, and can vary the applied voltage. The heater can accommodate varying conditions, such as variations in the electrical conductivity of a liquid flowing through the heater, etc.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 937,877, filed November 20, 2019. Technical Field

[0003] This disclosure relates to an ohmic liquid heating device and a method for heating a liquid. Background Technology

[0004] An ohmic liquid heater includes a structure for containing a liquid to be heated and multiple electrodes spaced apart from each other. The electrodes are in contact with the fluid to be heated, such that the liquid fills the space between adjacent electrodes. A voltage is applied between the electrodes, and a current flows through the liquid between the electrodes, thereby heating the liquid by the power consumed in the resistance of the liquid itself. The heating rate varies with the square of the current and inversely with the resistance of the liquid between the energized electrodes. The current varies with the conductivity of the liquid. For a given conductivity of liquid, the current varies with the spacing between the electrodes. Closely spaced electrodes provide a low-resistance current path, thus providing a high current and a high heating rate. The current and heating rate also vary with the area of ​​the electrodes; larger electrodes provide a higher current. The term "specific resistance," as used in this disclosure to characterize a circuit or part of a circuit having elements electrically connected by a liquid, refers to the ratio of the resistance of the circuit or part of the circuit to the resistivity of the liquid in the circuit. For example, the ohmic heater described in CA1291785 uses multiple pairs of electrodes of different sizes, such that each pair of electrodes defines a different specific resistance. Each pair of electrodes is a plate-like element, positioned opposite each other to define a space between them. A liquid flow path extends sequentially through the space defined by each pair. The desired heating rate is achieved by selecting the electrode pairs and connecting each pair of electrodes to opposite poles of a power source. A heater disclosed in WO 2009 / 100486 uses a generally similar arrangement and further controls the heating rate by rapidly closing and opening a switch that focuses each pair of electrodes to the power source so as to change the applied average voltage over time in a pulse-width modulation scheme. Another ohmic heater utilizing multiple pairs of electrodes is disclosed in US 8,532,474. However, further improvements would be desirable. Summary of the Invention

[0005] One aspect of the invention provides an ohmic heater. The ohmic heater according to this aspect of the invention desirably includes a structure defining a flow path extending in a downstream direction. The heater desirably includes a first pair of electrodes disposed within the flow path, the first pair of electrodes being adjacent one another in the downstream direction but spaced apart from one another in a direction perpendicular to the downstream direction. Desirably, the heater further includes a second pair of electrodes disposed within the flow path downstream of the first pair of electrodes, the second pair of electrodes being adjacent one another in the downstream direction within the flow path but spaced apart from one another in the direction perpendicular to the downstream direction. By way of example only, the structure can include an elongated tube formed of a dielectric material, the first pair of electrodes can face one another at one location along the tube, and the second pair of electrodes can face one another at another location downstream of the first electrodes. In this example, the tubular dielectric structure can define an elongated channel extending between the first electrodes and the second electrodes. The heater also desirably includes a circuit operable in at least three states. The desired states include (i) a first state in which the circuit applies a voltage between the first pair of electrodes; (ii) a second state in which the circuit applies a voltage between the second pair of electrodes; and (iii) a third state in which the circuit applies a voltage between at least one electrode of the first pair and at least one electrode of the second pair. The different states desirably provide different specific resistances. In the third state, current flows between the first pair of electrodes and the second pair of electrodes along a length of the flow path. In the above-discussed example, the current flows through the liquid in the elongated channel along a length of the channel. As discussed further below, this state can provide a specific resistance much higher than the specific resistance in the first or second states. Desirably, the heater can provide a wide range of specific resistances in a compact structure. Desirably, the circuit is operable to vary the average voltage applied to the electrodes. The combined effect of varying between states and varying the voltage to adjust the specific resistance can accommodate a wide range of operating conditions, such as varying the electrical conductivity of the liquid, varying the demand for heat, etc. without exceeding the limits of the circuit.

[0006] Further aspects of the invention provide a warewashing appliance, such as a dishwasher, incorporating a heater as discussed above, and a method of heating a liquid. BRIEF DESCRIPTION OF DRAWINGS

[0007] Figure 1 FIG. 1 is a schematic partial cross-sectional view depicting a heater according to one embodiment of the invention.

[0008] Figure 2 FIG. 2 is a schematic partial cross-sectional view depicting a heater according to another embodiment of the invention.

[0009] Figure 3 FIG. 3 is a schematic partial cross-sectional view depicting a heater according to a further embodiment of the invention.

[0010] Figure 4 is a diagrammatic perspective view depicting a pair of electrodes that can be used in a still further embodiment of the application.

[0011] Figure 5 is a fragmentary view depicting a portion of a heater according to yet another embodiment of the application.

[0012] Figure 6 is a diagrammatic view depicting a dishwasher according to yet another embodiment of the application. DETAILED DESCRIPTION

[0013] A heater according to one embodiment of the application comprises a structure 20 defining a flow path 22 extending from an inlet end 24 to an outlet end 26 in a downstream direction indicated by arrow D in Figure 1 The flow path 22 comprises straight sections adjacent the inlet end 24 and the outlet end 26, and an elongate channel 30 connecting the straight sections to one another. In this embodiment, the elongate channel 30 is curved, but the particular shape shown is entirely arbitrary; the channel 30 can be straight or can comprise a plurality of curves. Also, the structure 20 is depicted as a unitary tubular body, but it can be formed from a plurality of elements connected to one another to define the flow path. As discussed herein, the downstream direction at any point along the flow path should be taken to mean the direction of a centreline 32 of the flow path. Likewise, a direction perpendicular to the downstream direction is a direction perpendicular to the centreline at any point along the flow path. The centreline 32 is a line along the flow path 22 through the centre of the area of the flow path. Of course, in the case of a flow path having a circular cross-section, the area centre is simply the centre of the circle of the cross-section.

[0014] The first pair of electrodes 34a, 34b are disposed in a straight section of the flow path 22 adjacent the inlet end 24, such that the elongate channel 30 is downstream of the first pair of electrodes. The first pair of electrodes 34a, 34b are adjacent one another in the downstream direction. In this embodiment, the electrodes 34a, 34b are of the same size and aligned with one another in the downstream direction, such that the electrodes face one another over their entire upstream-to-downstream extent. The first pair of electrodes 34a, 34b are spaced apart from one another in a direction perpendicular to the downstream direction. The electrodes can be generally plate-like or sheet-like structures. Although in this embodiment the electrodes 34a, 34b are mounted to the wall of the structure 20, this is not essential; the electrodes can be spaced apart from the wall as desired. However, the electrodes should be disposed within the flow path 22 such that the electrodes will contact liquid flowing in the flow path. Desirably, the structure 20 is formed wholly or partially from a dielectric material, such that the structure does not form an electrical connection between the electrodes. The second pair of electrodes 36a, 36b are disposed in a straight section of the flow path adjacent the downstream end 26. Thus, the channel 32 is disposed downstream of the first pair 34 but upstream of the second pair 36. The second pair of electrodes are configured similarly to the first pair, such that the electrodes 36a, 36b are adjacent one another and aligned with one another in the downstream direction, but are spaced apart from one another in a direction perpendicular to the downstream direction. The direction of spacing between the second pair of electrodes can be the same as the direction of spacing between the first pair of electrodes, or can be different. In this particular embodiment, the second electrodes 36a, 36b are larger in area than the first electrodes 34a, 34b and are closer to one another. Thus, the conduction path of liquid in the flow path between the electrodes 36a, 36b will have a lower specific resistance than the conduction path of fluid between the first pair of electrodes 34a, 34b.

[0015] The heater further comprises a variable voltage power supply 40. The power supply 40 has a first pole 42 and a second pole 44. In this case, the first pole 42 is the neutral pole, and the second pole 44 is the "hot" pole. The power supply 40 is arranged to supply electrical power between the poles 42, 44 and to apply a voltage, which can be controlled and varied as desired within an operating range of voltages. Typically, the power supply applies an alternating voltage to the hot pole, while maintaining the neutral pole at a fixed voltage, which can be close to or equal to the ground voltage.

[0016] One electrode 34a of the first pair is permanently connected to the neutral pole 42 of the power supply, while the other electrode 34b of the first pair is connected to the hot pole 44 of the power supply by a switch 46. The electrode 36a of the second pair is connected to the neutral pole 42 of the power supply by a switch 48, while the opposite electrode 36b of the second pair is connected to the hot pole 44 of the power supply by another switch 50. In this embodiment, the switches 46, 48, 50 are relays, but other types of switch can be used. The switches 46, 48, 50 are controlled by a controller 52, which is in turn controlled by a microprocessor 54. The microprocessor 54 is programmed to control the switches 46, 48, 50 to apply a voltage to the electrodes 34a, 34b, 36a, 36b in a predetermined sequence. The microprocessor 54 is also programmed to control the power supply 40 to apply a voltage to the electrodes 34a, 34b, 36a, 36b in a predetermined sequence. The microprocessor 54 is programmed to control the switches 46, 48, 50 and the power supply 40 to apply a voltage to the electrodes 34a, 34b, 36a, 36b in a predetermined sequence. Figure 1The switches 46, 48, 50 are described as conventional mechanical switches, but most typically the switches 46, 48, 50 are semiconductor switches, such as FETs, MOSFETs, etc., that can be electronically controlled.

[0017] The heater further includes an array of sensors arranged to detect one or more conditions of the electrical circuit, the liquid passing through the heater, or both. For example, in this embodiment the sensors include a current sensor 52 arranged to detect the current from the power supply 40 and a voltage sensor 54 arranged to detect the voltage between the poles 42, 44. These sensors also include sensors that can detect one or more conditions of the liquid passing through the heater, for example, an input temperature sensor 56 disposed upstream of the first pair of electrodes 34a, 34b and an output temperature sensor 56 disposed downstream of the second pair of electrodes 36a, 36b, as well as a flow sensor 58 disposed within the flow path and arranged to measure the flow rate of the liquid through the path. It is not necessary to provide Figure 1 all of the sensors depicted in the

[0018] The heater further includes a controller 60. The controller 60 is connected to the switches 46, 48, 50 and to the power supply 40 so that the controller can command each switch to independently enter into a closed state in which the switch is conductive or an open state in which the switch is non-conductive. The controller is also connected to the power supply 40 and arranged to command the power supply to increase or decrease the voltage applied between the poles 42, 44. The controller 60 is also connected to the sensors described above so that the controller can receive signals from the sensors. To show the connections between the controller and the sensors clearly in Figure 1 The controller 60 can include conventional analog and digital circuit elements arranged to perform the operations discussed below. Most typically, the controller includes a digital processor and a memory containing stored instructions instructing the processor to perform the operations. The controller typically also includes appropriate circuitry for interfacing with the sensors and with the switches, for example, like analog to digital conversion circuitry and digital to analog conversion circuitry.

[0019] In Figure 1In the depicted state, with all switches 46, 48, 50 open, the heater is not operating. By closing switch 48 and leaving switches 48 and 50 open, the controller can select the first pair of electrodes 34a, 34b to be connected to the power supply 40 and thus place the circuit in a first state. In this state, the electrodes 34a, 34b are connected to opposite poles of the power supply, such that a voltage difference is applied between the electrodes 34a, 34b. In this condition, current will flow between the electrodes through the liquid present in the flow path 22. Likewise, by opening switch 48 and closing switches 48 and 50, the controller can select the second pair of electrodes 36a, 36b and connect these electrodes to opposite poles 42, 44 of the power supply. In this condition, current flows between the electrodes 36a, 36b through the fluid in the space between these electrodes. Because the first pair of electrodes 34a remains connected to the neutral pole, some current can flow from electrode 36b to electrode 34a through the liquid in the elongated channel 30. However, due to the elongated and relatively narrow current path through the fluid in the channel 30, the specific resistance between the second pair of electrodes 36a, 36b is much lower than the specific resistance between the second pair of electrodes 36b and the first pair of electrodes 34a. Therefore, current will primarily flow between the electrodes 36a, 36b.

[0020] The controller 60 is also operable to place the circuit in a third state in which switches 46 and 48 are open and switch 50 is closed. In this state, the only current path between the poles of the power supply through any electrode extends between the second pair of electrodes 36b and the first pair of electrodes 34a through the channel 30. Alternatively, the controller is operable to place the circuit in a fourth state in which the electrodes 34a, 34b are connected to opposite poles of the power supply and the second pair of electrodes 36a, 36b are also connected to opposite poles of the power supply.

[0021] As discussed above, the two pairs of electrodes are configured such that they define different specific resistances. Thus, the heater as a whole can provide four different specific resistances. These specific resistances can be selected so as to cover a wide range of relatively large steps between the specific resistances. Typically, the power supply 40 has a limited operating range. For example, a voltage source will typically be able to apply a voltage between the poles 42, 44 that is no greater than a predetermined maximum voltage, and will also be able to apply a current through the poles and switches that is no greater than a maximum current without damaging the power supply or the switches. Ideally, the specific resistances provided in the various states are selected such that for any liquid within a predetermined range of electrical conductivities, any heating rate within a predetermined range of heating rate operations can be provided by selecting one of the above states and by adjusting the power supply within its operating range.

[0022] In one embodiment, the controller can use the exit temperature of the fluid from the heater as detected by output temperature sensor 58 as a primary input to perform a simple control scheme. In this control scheme, the controller initially selects the state with the highest specific resistance, in this case the third state in which electrodes 36b, 34a are connected to the poles. With the circuit in this state, the controller actuates the power supply to apply a low voltage between poles 42, 44 and gradually increases this voltage until the output temperature reaches the desired setpoint value or until the applied voltage reaches a predetermined switching threshold voltage, which can be at or just below the maximum operating voltage of the power supply. If the threshold voltage is reached before the output temperature reaches the setpoint value, the controller selects the state with the next lower specific resistance (i.e., the first state discussed above in which the first pair of electrodes 34a, 34b are selected and the voltage applied by the power supply is reduced). The controller then gradually increases the voltage applied by the power supply until the desired exit temperature is achieved or another predetermined switching threshold voltage is reached. If this predetermined threshold switching voltage is reached, the controller again reduces the voltage applied by the power supply and switches to the next lower available specific resistance, in this case selecting the second state with the second pair of electrodes 36a, 36b. If the threshold switching voltage is reached, the controller will switch to the fourth state with the lowest available specific resistance. Of course, if the fluid temperature rises above the desired setpoint temperature, the controller will reverse the same steps, first reducing the voltage provided by the power supply to the selected minimum voltage threshold and then switching to a higher specific resistance state if this minimum voltage threshold is reached. Optionally, the controller can monitor the current flow as detected by current sensor 52 and reduce the voltage, switch to a higher specific resistance state, or both, if the current increases to a maximum threshold. This can occur, for example, if the conductivity of the liquid increases significantly.

[0023] In a more elaborate control scheme, the controller can acquire data representative of the conductivity of the liquid by placing the circuit in any one of the states, immediately actuating the power supply to apply a low voltage between the poles and measuring the current flow using sensor 52. The applied voltage can be measured with sensor 54 or can be known with sufficient accuracy from the voltage commanded by the controller. The known current and voltage, together with the known specific resistance between the poles in each state, can be used to calculate the electrical conductivity. The controller can use data from input temperature sensor 56 and flow sensor 58 to estimate the heating rate that will raise the temperature of the liquid to the desired setpoint, and can select the circuit state and applied voltage to achieve the required heating rate while keeping the circuit within its operating range.

[0024] A heater according to a further embodiment of the present invention Figure 2 ) is described above with reference toFigure 1 The heater under discussion is the same as discussed above, except as discussed below. In the heater of Figure 2 In the heater of Figure 1 the controller 160 is arranged to vary the voltage by pulse width modulation. Thus, the controller 160 is arranged to control the average voltage applied to the selected electrode by repeatedly opening and closing one or more switches associated with the selected electrode. In the first state, with the electrodes 34a, 34b selected, the controller can be arranged to repeatedly open and close the switch 46. In the second state, with the electrodes 36a, 36b used, the controller can repeatedly open and close one or both of the switch 48 and the switch 50. Likewise, in the third state, the controller will open and close the switch 50. In other respects, the system operates as described above. In other words, the controller can control the average voltage applied to the selected electrode by controlling the voltage applied by the power supply (as shown in Figure 2

[0025] A heater according to another embodiment (not shown) is similar to the heater discussed above, except that the heater includes a first pair of electrodes 234a, 234b, a second pair of electrodes 236a, 236b and a third pair of electrodes 237a, 237b. In this embodiment, the structure 220 defines a flow path 222 that includes a first elongate passage 230 between the first pair of electrodes and the second pair of electrodes, and a second elongate passage 231 between the second pair of electrodes 236a, 236b and the third pair of electrodes 237a, 237b. Here again, the electrodes of each pair are disposed adjacent to one another in the downstream direction along the flow path. However, in this particular embodiment, the electrodes 236a, 236b of the second pair are disposed in a partially overlapping configuration, while the electrodes 237a, 237b of the third pair do not overlap one another in the downstream direction D along the flow path. Also in this embodiment, the pairs of electrodes are configured so that each pair of electrodes provides a different specific resistance. Figure 3

[0026] ​​As in this embodiment, the first pair of electrodes 234a is permanently connected to the neutral pole 242 of the power supply 240, while the remaining electrodes are connected to the poles of the power supply through switches 246, 248, 250, 251, 253. Again, the controller is operable to place the circuit in any of the states discussed above when the electrodes 237a, 237b are disconnected from the power supply. The controller is also operable to place the circuit in additional states. For example, the controller can select only the third pair of electrodes so that the electrodes 237a, 237b of the third pair are connected to opposite poles. In yet another state, the electrode 237b of the third pair is connected to the hot pole 244; the electrode 237a of the third pair is disconnected from the neutral pole; the electrode 236a of the second pair is connected to the neutral pole and the electrode 236b of the second pair is disconnected from the hot pole. In this state, the poles 244, 242 are electrically connected to each other through the liquid in the second channel 231. Because the second channel 231 has a different configuration than the first channel 230, the specific resistance between the poles in this state will be different than in the third state discussed above in which current flows through the liquid in the first channel 230. In yet another state, the electrode 237b of the third pair is connected to the hot pole and the electrode 234a of the first pair is connected to the neutral pole via the permanent connection, while the remaining electrodes are disconnected from the poles. In this state, the current path between the poles of the power supply extends through the liquid in the channels 231 and 230 in series. Such a current path provides the highest specific resistance available.

[0027] As discussed above with reference to Figure 1 and Figure 2 , the power supply 240 can be a variable voltage power supply controlled by the controller 260 as discussed above. In another arrangement, the power supply 240 can be a fixed voltage power supply as shown in Figure 2 and the controller 260 can be arranged to repeatedly open and close the switches through which the current path passes through the selected electrodes in order to provide pulse width modulation of the applied voltage. As will be appreciated, still other embodiments using a greater number of pairs of electrodes can be employed.

[0028] In the embodiments discussed above, the electrodes are plate-like structures extending along opposite sides of the flow path. However, other arrangements can be employed. For example, as depicted in Figure 4 , a pair of electrodes can include an elongated rod-like electrode 334b extending in a downstream direction of the flow path and a tubular electrode 334a surrounding the rod-like electrode, the inner diameter of the tubular electrode 334a being greater than the outer diameter of the rod-like electrode 334b so that the electrodes are spaced apart from each other in a radial direction R transverse to the downstream direction. Numerous other electrode configurations can be employed.

[0029] Typically, the heater will include safety features, such as a ground electrode (not shown) disposed in the flow path upstream and downstream of the electrodes that are connectable to a power source, the ground electrode being permanently connected to a ground potential.

[0030] In another variant, one or both electrodes of a pair of electrodes can be formed in segments. As shown, Figure 5 the pair of electrodes includes a first electrode 532a formed as a single unitary element and a second electrode formed as two segments 534b1, 534b2. The two segments of the second electrode are disposed adjacent to the first electrode in the downstream direction. The first electrode 534a is connected to one pole 542 of the power source through a switch 548. The segments 534b1, 534b2 of the second electrode are connected to the opposite pole 544 of the power source through separate switches 548 and 549, respectively, so that each segment can be connected to or disconnected from the pole of the power source independently of the other segment. This arrangement can be used to vary the effective area of the segmented electrode, and thus the specific resistance when selecting the segmented electrode, such as the specific resistance between the first and second electrodes of the pair or the specific resistance between the segmented electrode and an electrode of another pair, etc. This arrangement can be applied in any or all of the pairs of electrodes.

[0031] In the embodiment depicted in Figures 1-3 one electrode is permanently connected to the neutral pole. Optionally, this electrode can be connected to the neutral pole through a further switch operated by the controller.

[0032] The heaters discussed above can provide a variety of conductive paths having different specific resistances with a relatively small number of electrodes and a relatively small number of switches. The heaters as discussed herein can be used in any application in which a liquid is to be heated. However, they are particularly useful when the electrical conductivity of the liquid is expected to vary over a wide range during operation of the heater. For example, a heater for heating water in a washing appliance, such as a washing machine or dishwasher, can vary over a very wide range of electrical conductivity during operation. The water supplied to a washing appliance is typically potable water, and the electrical conductivity of potable water can vary due to factors such as the content of dissolved minerals in the water, etc. Furthermore, as the washing appliance operates, its electrical conductivity will typically increase as electrolytes, such as ionic components of the soap and materials washed from the articles to be washed, etc., are added to the water during the wash cycle. A heater as described above can be configured to provide a wide range of specific resistances so that the circuit components remain within their operating range despite the sharp variations in electrical conductivity. Furthermore, the heater can provide this capability in a very compact structure. The portion of the structure that provides the elongate channel can include a tube having substantially any configuration. In some embodiments, the tube can extend around other components of the appliance. For example, Figure 6The washing appliance depicted in the middle is a washing appliance having a housing 501 defining a washing chamber 501. The housing includes a rack (not shown) adapted to hold articles to be washed (in this case, crockery 505 within the washing chamber). A pump 507 is arranged to circulate washing liquid (such as water or the like) into the washing chamber so that the washing liquid contacts the articles to be washed, for example by forcing the liquid through a spray device 509. The heater as described above includes a structure 520 defining a flow path (not shown) for the washing liquid, the flow path being connected between the outlet of the pump and the spray device 509. A portion of the heater structure 520 extends collectively around the pump so that the structure occupies space within the appliance that would otherwise be wasted.

[0033] In the heaters discussed above, the channels extending between pairs of electrodes are elongate and have a relatively small cross-sectional area. That is, the cross-sectional area of each channel is less than the area of the electrodes, and the length of the channel is greater than the distance between the electrodes of a pair. In the heaters described above, therefore, the electrically conductive path extending through the channels has a higher specific resistance than any electrically conductive path between the electrodes of a pair. However, in the case where the electrodes of a pair are widely spaced apart from one another and the channel between the pair is short, the electrically conductive path through the channel can have a lower specific resistance than the electrically conductive path between the electrodes of a pair.

[0034] Features disclosed in the various embodiments discussed above can be interchanged in different embodiments. For example, in any of the heaters of Figure 1 and Figure 2 the electrode structure shown in Figure 3 , Figure 4 and Figure 5 may be employed. The foregoing description should therefore be taken as illustrative and not as limiting the present invention.

Claims

1. An ohmic heater comprising: a structure defining a flow path extending in a downstream direction; a first pair of electrodes disposed within the flow path, adjacent to each other in the downstream direction but spaced apart from each other in a direction perpendicular to the downstream direction; a second pair of electrodes disposed within the flow path, downstream of the first pair of electrodes, the second pair of electrodes being adjacent to each other in the downstream direction within the flow path but spaced apart from each other in a direction perpendicular to the downstream direction, the structure comprising a dielectric wall defining an elongate channel forming a portion of the flow path between the first pair of electrodes and the second pair of electrodes; and a circuit operable in at least three states, the at least three states comprising: a first state in which the circuit applies a voltage between the first pair of electrodes; a second state in which the circuit applies a voltage between the second pair of electrodes; and a third state in which the circuit applies a voltage between at least one electrode of the first pair and at least one electrode of the second pair, such that in the third state the only current path through any of the electrodes extends between one electrode of the second pair and one electrode of the first pair, through the elongate channel, and current flows through the liquid in the channel, wherein in the third state the only current path through any of the electrodes passes through one electrode of the second pair and one electrode of the first pair and does not pass through any electrodes other than one electrode of the second pair and one electrode of the first pair.

2. The heater of claim 1, wherein, the circuit comprises a controller and one or more sensors arranged to detect one or more conditions of the circuit, one or more conditions of a liquid flowing through the flow path, or both, the controller being operable to set the circuit to one of the first state, the second state and the third state in response to a signal from at least one of the sensors.

3. The heater of claim 2, wherein, the controller is operable to control an average voltage in response to a signal from at least one of the sensors while maintaining the circuit in one of the first state, the second state and the third state.

4. The heater of claim 1, wherein, the specific resistance of the circuit in the third state is higher than the specific resistance of the circuit in the first state and the second state.

5. The heater of claim 1, wherein, the electrodes of the first pair are at least partially aligned with each other in the downstream direction and the electrodes of the second pair are at least partially aligned with each other in the downstream direction.

6. The heater of claim 1, wherein, the circuit comprises a power supply having two poles, and wherein the circuit is operable in a fourth state in which the electrodes of the first pair are connected to opposite poles of the power supply and the electrodes of the second pair are also connected to opposite poles of the power supply.

7. The heater of claim 1, wherein the electrical circuit comprises a power source having two poles, and wherein, in the third state, the only current path between the poles of the power supply through any of the electrodes extends between one electrode of the second pair and one electrode of the first pair through the channel.

8. The heater of claim 1, the dielectric wall defining a tubular elongated passage forming the entire flow path between the first pair of electrodes and the second pair of electrodes, the dielectric wall surrounding the entire flow path about an axis extending in a downstream direction so as to define the tubular elongated passage, and wherein, in the third state, the only current path through any of the electrodes extending between one electrode of the second pair and one electrode of the first pair, through the tubular elongated passage, and current flowing through the liquid in the tubular elongated passage.

9. The heater of claim 8, the electrical circuit comprising a power supply having two poles and configured to vary a voltage applied between the two poles, the electrical circuit operating in at least three states, the at least three states comprising: the first state in which the electrical circuit applies a variable voltage between the electrodes of the first pair; the second state in which the electrical circuit applies a variable voltage between the electrodes of the second pair; and the third state in which the electrical circuit applies a variable voltage between at least one electrode of the first pair and at least one electrode of the second pair, such that in the third state, the only current path through any of the electrodes extends between the at least one electrode of the second pair and the at least one electrode of the first pair, through the tubular elongated passage, and current flows through the liquid in the tubular elongated passage, wherein the electrical circuit has a first specific resistance between the electrodes of the first pair in the first state, the electrical circuit has a second specific resistance between the electrodes of the second pair in the second state, the second specific resistance being different from the first specific resistance, and the electrical circuit has a third specific resistance between the at least one electrode of the second pair and the at least one electrode of the first pair in the third state, the third specific resistance being higher than the first specific resistance and the second specific resistance. the cross-sectional area of the elongated passage being smaller than the area of the electrodes of the first pair and the area of the electrodes of the second pair, and the length of the elongated passage being greater than the distance between the electrodes of the first pair and greater than the distance between the electrodes of the second pair.

10. The heater of claim 1, wherein, 11. A washing appliance comprising a housing defining a washing chamber adapted to hold articles to be washed, a pump arranged to pump a washing liquid through the heater and into the washing chamber so that the washing liquid contacts the articles in the washing chamber, and the heater of any one of claims 1 to 10.

12. A method of heating a liquid, comprising the steps of: ​ (a) passing a liquid in a downstream direction between a first pair of electrodes, then through an elongated channel defined by a dielectric wall, and then through a second pair of electrodes, the first pair of electrodes being arranged adjacent to each other in the downstream direction but spaced apart from each other in a direction perpendicular to the downstream direction, the second pair of electrodes being arranged adjacent to each other in the downstream direction but spaced apart from each other in a direction perpendicular to the downstream direction; (b) actuating a circuit to apply a voltage between the electrodes of the first pair in a first state, between the electrodes of the second pair in a second state, and between at least one electrode of the first pair and at least one electrode of the second pair in a third state, in which the only current path through any of the electrodes extends between one electrode of the second pair and one electrode of the first pair, through the elongated channel, and current flows through the liquid in the elongated channel, wherein, in the third state, the only current path through any of the electrodes passes through one electrode of the second pair and one electrode of the first pair and does not pass through any electrode other than one electrode of the second pair and one electrode of the first pair; and (c) detecting at least one condition in the liquid, the circuit, or both the liquid and the circuit, and (d) controlling the circuit to select the first state, the second state, or the third state in response to the at least one condition detected in step (c).

13. The method of claim 12, further comprising controlling the circuit to change the average voltage applied in response to the at least one condition detected in step (c).

14. The method of claim 12 or 13, wherein, The step of actuating the circuit includes connecting the electrodes to poles of a power source, and wherein, in the third state, the only current path through any of the electrodes between the poles of the power source extends between one electrode of the second pair and one electrode of the first pair, through the channel.

15. The method of claim 12 or 13, wherein, The circuit includes a power source having two poles, and wherein the step of actuating the circuit further includes actuating the circuit in a fourth state in which the electrodes of the first pair are connected to opposite poles of the power source and the electrodes of the second pair are also connected to opposite poles of the power source.

16. The method of claim 12, wherein, The elongated channel has a cross-sectional area that is less than the area of the electrodes of the first pair and the area of the electrodes of the second pair, and the elongated channel has a length that is greater than the distance between electrodes in the first pair and greater than the distance between electrodes in the second pair.

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