Steam generator

Abstract

The boiler comprises a vessel (10) having vertically extending walls (12, 13) defining a plurality of vertically extending passages (26, 28, 30, 32). The vessel includes an intake opening (44) in communication with a liquid inlet adjacent a lower end of each of the passages, and a steam exhaust opening (38) in communication with a liquid exhaust adjacent an upper end of each of the passages. The boiler also includes a set of electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B) located in the passages, with some of the electrodes in each passage being located at various heights. The boiler power supply (50) has at least two poles (51, 53) and is operable to supply different electrical potentials to the different poles. The circuit (52) is operable to selectively connect and disconnect the electrodes to and from the poles in a plurality of different connection schemes, in each of which a current flows between the connected electrodes along a current path through the liquid in at least one of the passages.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 63 / 311,160, entitled “Steam Generator,” filed February 17, 2022, the disclosure of which is incorporated herein by reference. [Background technology]

[0002] One aspect of the present invention relates to an electrode boiler. In this disclosure, the term "boiler" refers to a structure adapted to boil a liquid in its normal operation to generate steam. An "electrode boiler" is a structure having electrodes connected to a power source and disposed within a structure that holds a conductive liquid, such that an electric current can be passed between the electrodes through the liquid to heat and boil the liquid. Electrode boilers are most commonly used to generate steam for industrial processes. Examples of electrode boilers are shown in GB2196820, GB2072898, DE402950, ​​and CN111322600(A).

[0003] The electrode configuration used in electrode boilers is different from that used in devices intended for use in heating liquids without boiling them. Examples of such devices are shown in US Patent Application Publication No. 2010 / 0322605, US Patent No. 7,817,906, WO2020231386 and WO2018085773. As described, for example, in WO2018085773, such devices may comprise multiple electrodes disposed in contact with the liquid, arranged such that different sets of electrodes define conductive paths through the liquid having different resistivities. In this disclosure, the term "resistivity" is used in reference to a circuit or part of a circuit having elements electrically connected by a fluid. Resistivity is the ratio between the electrical resistance of the circuit or part of a circuit and the resistivity of the fluid in the circuit. A control system operates switches to connect different sets of electrodes to a power source, thereby controlling power losses in the liquid and maintaining a desired liquid outlet temperature despite variations in the liquid's conductivity, liquid flow rate and liquid inlet temperature. Such systems rely on the spacing between the electrodes being completely filled with liquid and have not previously been applied to boilers. Summary of the Invention

[0004] One aspect of the invention provides a boiler. The boiler according to this aspect of the invention preferably comprises a structure having vertically extending walls defining a plurality of vertically extending passages. The structure preferably comprises a liquid supply connection communicating with a liquid inlet adjacent a lower end of each of the passages, and a steam outlet communicating with a steam outlet adjacent an upper end of each of the passages. The boiler also preferably comprises a set of electrodes located within the passages, some of the electrodes in each passage being located at different heights. A power source for the boiler preferably has at least two poles and is operable to provide different electrical potentials to different poles. The boiler also preferably comprises circuitry operable to selectively connect and disconnect the electrodes to and from the pole passages in a plurality of different connection schemes, each of which is associated with an electrical current flowing between the connected electrodes along a current path through the liquid in at least one of the passages.

[0005] The boiler circuit according to the above aspects of the invention may be operable to selectively connect isolated electrodes that are not connected to the poles of the power source and are disposed in different passages, In at least one of the connection modes, a current path may extend through the liquid in the multiple passages and through the isolated electrodes.

[0006] According to some of the above aspects of the invention, the structure may define a headspace in communication with and extending above the passageway outlet, and the steam outlet may be in communication with the passageway outlet through the headspace. The boiler according to some aspects of the invention may further include a level control system operative to maintain a liquid level in the headspace about the passageway outlet. According to at least some aspects of the invention, the boiler may further include a set of baffles located in the headspace. Such baffles may be positioned to define a tortuous path for the steam to rise in the headspace. Alternatively, or in addition, such baffles may be positioned to allow entrained liquid falling from the steam to drain downwardly.

[0007] According to some of the above aspects of the invention, the set of electrodes disposed in each passageway may include a plurality of pairs of electrodes, with each pair of electrodes facing one another.

[0008] According to some of the above aspects of the invention, the boiler may further comprise a control system operative to repeatedly select a connection scheme from a plurality of connection schemes and command the circuit to connect the electrodes together in the selected connection scheme. Such a control system may be operative to detect the presence of steam in the current paths associated with the various connection schemes and to substantially prevent extended operation in the connection scheme in which steam is present. The control system may further be operative to select a connection scheme, connect the electrodes in the selected connection scheme, monitor the actual current passing between the poles of the power source, compare the actual current to an expected current for the selected connection scheme, and select a new connection scheme if the actual current is less than the expected current by more than a current tolerance.

[0009] In accordance with some of the above aspects of the invention, a boiler structure may include a vessel and a plurality of dielectric walls disposed within the vessel and defining a passageway, The dielectric walls may be planar plates extending substantially parallel to one another.

[0010] Another aspect of the invention provides an apparatus for heating an electrically conductive liquid. The apparatus according to this aspect of the invention preferably comprises a structure defining a plurality of adjacently extending passages, with a dielectric wall between each of the passages, and provided with means for directing the liquid through the passages. The boiler also preferably comprises a set of electrodes in each passage, including electrodes located at a plurality of locations along the length of each passage. The power source of the apparatus preferably has at least two poles and is operative to provide different potential differences to the different poles. The apparatus also preferably comprises a circuit operative to selectively connect and disconnect the electrodes to and from the poles in a plurality of different connection schemes, each of which is associated with a current flowing between the connected electrodes along a current path through the liquid in at least one of the passages. The circuit is preferably operable to select two different sets of isolated electrodes not connected to the poles of the power source, and to connect the selected isolated electrodes to each other such that a current path extends between the connected electrodes through the isolated electrodes.

[0011] Another aspect of the invention provides a boiler. The boiler according to this aspect of the invention preferably comprises a vessel having a liquid inlet and a steam outlet. The boiler also preferably comprises a plurality of electrodes located within the vessel and arranged vertically and in at least one horizontally extending array, some electrodes disposed at a higher level than others and some electrodes horizontally offset from others. The boiler's power source preferably has at least two poles and is operable to provide different potentials to different poles. The boiler preferably comprises circuitry operative to selectively connect and disconnect the electrodes to and from the poles in a plurality of different connection schemes, each of which is associated with a current flowing between the connected electrodes along a current path through the liquid in the vessel. The boiler according to this aspect of the invention also preferably comprises a control system operative to repeatedly select a connection scheme and instruct the circuitry to connect the electrodes in the selected connection scheme. The control system desirably operates to detect the presence of vapor in the current paths associated with the various connection types and to substantially prevent extended operation in connection types in which vapor is present.

[0012] Another aspect of the invention provides a method of operating a heater, the method comprising the steps of: (a) contacting a plurality of electrodes with a conductive liquid; (b) selectively connecting electrodes of the plurality of electrodes to establish a conductive path through the liquid between poles of the power source; (c) applying a voltage between the electrodes to heat the liquid and measuring the actual current flowing through the conductive path; (d) comparing the actual current to an expected current for the conductive path; (e) if the comparison made in step (d) indicates that the actual current is less than the expected current by more than a current tolerance, repeating steps (b) through (e) for a different selected pair of electrodes to establish a different conductive path; Preferably,

[0013] According to some of the above aspects of the invention, step (b) may include setting a desired heating rate in response to measurements of one or more parameters indicative of the results achieved by heating the liquid, and selecting electrodes and current paths based on the desired heating rate. According to at least some of the above aspects of the invention, the liquid may boil during steps (a)-(e). [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic vertical cross-sectional view of an electrode boiler for generating steam according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A boiler according to an embodiment of the present invention comprises a vessel 10 having vertically extending side walls 12, 13 on opposite sides, a bottom wall 14 and a top wall 16. The vessel also has front and rear walls (not shown) by which the vessel is closed except for an intake opening 44 in the bottom wall and a steam exhaust opening 38 in the top wall. In this disclosure, terms such as "top", "bottom", "vertical", "above" and "below" are used with respect to the boiler installed in its normal gravitational reference frame of operation. The term "vertically extensive" means that the considered elements are vertically extensive, but does not require a completely vertical orientation, the considered elements may be horizontally extensive.

[0016] Vertically extending inner walls 20, 22, 24 are disposed within the vessel. The inner walls extend into and out of the plane of the drawing in FIG. 1, preferably from the front wall to the rear wall. Thus, the inner walls, together with the side walls 12, 13, divide the vessel's space into four passages 26, 28, 30, 32. The inner walls terminate above the bottom wall 14 and below the top wall 16, such that a lower space 34 exists below the passages and an upper space 36 exists above the passages. Each passage has an inlet opening into the lower space 34 adjacent to its lower end and an outlet opening into the upper space 36 adjacent to its upper end.

[0017] An electrode set is disposed within each of the passages 26, 28, 30, 32. The electrode set disposed in each passage includes a pair of opposing electrodes spaced apart from one another. In this embodiment, the opposing electrodes are attached to opposing walls of the passage. The electrode pairs within each passage are spaced apart from one another along the length of the passage such that they are disposed at different vertical heights. For example, the electrode set disposed within passage 32 includes, in ascending order of height, opposing electrode pair 32-1A, 32-1B, electrode pair 32-2A, 32-2B, pair 32-3A, 32-3B, and pair 32-4A, 32-4B. All of the electrodes designated with the letter "A" are attached to wall 24, while all of the electrodes designated with the letter "B" are attached to side wall 12 of the vessel. In this embodiment, the electrodes of the middle pairs 32-3A, 32-3B and 32-2A, 32-2B are the same size. The electrodes of the bottom pair 32-1A, 32-1B and the top pair 32-4A, 32-4B are the same size, but smaller than the middle pairs. The electrodes disposed in the other passages are identical to the electrodes in passage 32 and are disposed in the same arrangement. Thus, the electrodes attached to each of the interior walls 20, 22, 24 are disposed in back-to-back alignment with one another. For example, electrode 30-4B disposed in passage 30 is in back-to-back alignment with electrode 32-4B in passage 32.

[0018] The vessel side walls 12, 13 and interior walls 20, 22, 24 are each dielectric walls. These walls may be formed entirely of dielectric material or may have a layer of dielectric material on one or more surfaces that carry the electrodes. In either case, the dielectric walls are unable to conduct electrical current from one path to another.

[0019] The vessel has a steam exhaust opening 38 disposed at or near the top of the vessel. A set of baffles 40 are disposed in the headspace 36 below the steam exhaust opening 38. The baffles are positioned to define a tortuous path for the steam to rise in the headspace and to allow entrained liquid falling out of the steam to flow downwardly. A steam delivery conduit 42 is connected to the steam exhaust opening 38 for delivery of the steam to a point of use.

[0020] The vessel has an intake opening inlet 44 in communication with the lower volume 34. A liquid source 46 is connected to the intake opening. The source 46 is controllable to vary the flow of liquid into the vessel. The variation may be stepwise, such as either off with no flow or on with a predetermined flow, or it may be gradual from no flow to a maximum flow rate. For example, the source 46 may comprise a variable flow pump connected to a universal liquid supply and valve, or to a tank of liquid.

[0021] The boiler further comprises a power supply 50 having opposite poles 51, 53. The power supply is arranged to apply different electrical potentials to the two poles. For example, one pole may be a "neutral" pole having a constant potential, and the other pole may be applied with an alternating electrical potential.

[0022] The switch circuit 52 is connected to the poles of the power supply and to a control system 54. The switch circuit typically comprises a number of semiconductor switches (not shown), such as MOSFETs, JFETs, etc., although other forms of switches, such as relays responsive to control signals, may be used. The switch circuit is arranged to selectively connect some electrodes to the poles of the power supply and connect other electrodes to each other, according to a connection scheme selected by the control system. This connection scheme defines a conductive path between the poles of the power supply, through the conductive liquid in the boiler. For example, in a first connection scheme, in which electrodes 26-2A, 26-2B are connected to the poles and all other electrodes are disconnected from the poles, the conductive path extends only through the interval between these electrodes, as shown diagrammatically by line 60. In the second connection scheme, electrodes 26-3A, 32-3B are connected to the poles of the power source, while back-to-back electrode pairs 26-3B and 28-3A, 28-3B and 30-3A, and 30-3B and 32-3A are disconnected from the poles but connected to each other. This connection scheme creates a conductive path 62 through the liquid in all four passages that is longer than the conductive path 60. For a given voltage applied by the power source and a given conductivity of the liquid, the conductive path 62 will have lower current and power losses than the conductive path 60. In other words, the second connection scheme with the path 62 has a higher resistivity than the first connection scheme with the path 60. The second connection scheme with the conductive path 62 is an example of a "straight across" conductive path in that the conductive path extends only between electrodes disposed at the same level, i.e., the electrodes of the third tier. In the third conductive scheme, electrodes 26-1A, 32-1B are connected to the poles of the power supply, while back-to-back electrode pairs 26-4B and 28-4A, 28-1B and 30-1A, and 30-4B and 32-4A are disconnected from the poles but connected to each other. In this connection scheme, the conductive path 64 zigzags between the electrodes of levels 1 and 4, thus extending longitudinally within each passage, which results in a much longer current path through the liquid. Thus, the third connection scheme has a higher specific resistance and a lower current than the second connection scheme. In yet another connection scheme, multiple current paths can be provided between the poles, which can then be electrically connected in parallel to provide a lower specific resistance.Electrodes that are not directly connected to the electrodes are referred to herein as "isolated electrodes." For example, the back-to-back connected electrode pairs included in the connection schemes discussed above are isolated electrodes. Of course, the connection schemes discussed above are merely exemplary, and the electrodes and switches can provide a large number of different connection schemes with numerous and widely differing resistivities. It is not essential to be able to connect every electrode in every possible way. For example, the switch circuit may not be able to connect some back-to-back electrodes to each other. This would reduce the number of available connection schemes, but it would also reduce the cost of the switch circuit. Conversely, for further versatility, the switch circuit can be adapted to connect isolated electrodes that are not back-to-back to each other. For example, the switch circuit may allow electrodes 28-3B, 32-3A to be connected to each other through a shunting bus (not shown). This would modify path 62 so that the current flowing along the path would bypass the liquid in passage 30 and would therefore have a lower resistivity.

[0023] The boiler includes a level sensor 72 operative to measure the level of liquid in the headspace 36. The level sensor may be a float sensor, ultrasonic sensor or other conventional device. The level sensor is operatively coupled to the liquid source 46, whereby the liquid source operates to maintain the liquid level slightly above the interior walls 20, 22, 24 and thus slightly above the outlets of the passages 28-32. The operative connection between the level sensor and the liquid source may be through the control system 54 or through a separate feedback circuit or linkage.

[0024] The temperature sensor 70 is adapted to measure the temperature of the liquid in the interval between the pair of opposing electrodes 26-1A, 26-1B near the inlet of the passage 26. The boiler also includes one or more sensors adapted to measure conditions indicative of steam being discharged from the boiler. These sensors may include a flow sensor 74 coupled to the steam discharge opening 38, a pressure sensor 76 operative to measure the pressure in the headspace, and an outlet temperature sensor 78 arranged to measure the temperature of the steam in the headspace. An inlet flow sensor (not shown) may also be provided to measure the inflow of liquid through the inlet inlet 44. A current sensor 80 is operative to measure the current passing between the poles of the power supply 50. All sensors are operatively connected to the control system 54.

[0025] The control system 54 includes logic circuitry 56 and memory 58. For example, the logic circuitry 56 may be a programmable element such as a microprocessor. The memory 58 stores data, as discussed below, and preferably also stores instructions that direct the logic circuitry to perform the functions, as discussed below. The control circuitry also includes conventional interfacing circuitry (not shown) for receiving signals from the various sensors, and conventional drive circuitry (not shown) for converting outputs from the logic circuitry into control voltages applied to the switches. Although the control system 54 is illustrated as a single unit in FIG. 1, the elements of the control system may be physically separate from one another. Similarly, the memory 58 may include multiple physical elements, and the logic circuitry 54 may include multiple physical elements.

[0026] The memory 58 stores a list of possible connection schemes. The entry in the list for each connection scheme includes the settings required for the switch to enable the connection scheme. The entry for each connection scheme also includes a current parameter that represents the expected current between the poles when the connection scheme is activated and all gaps between the electrodes included in the connection scheme are filled with liquid. For example, the current parameter may be the specific resistance between the poles of the power source when the electrodes are connected according to the connection scheme. For a power source with a given voltage between the poles and a liquid with a given conductivity, the expected power loss and the expected current are inversely proportional to the specific resistance. The current parameter may also be the current that flows between the poles when the electrodes are connected according to a scheme where the conductivity of the liquid is known. The entry for each connection scheme also includes height data that represents the height of some or all of the conductive paths established by the connection scheme. In this embodiment, the height data is a height parameter that represents the height of the highest gap between opposing electrodes included in the conductive path established by the connection scheme. For example, all electrodes in the conductive path 62 are disposed on level 3, so the conductive path passes only through the gaps between the electrodes on level 3. As a result, the height parameter of the connection scheme associated with the conductive path 62 indicates layer 3. The conductive path 64 passes through the electrodes 26-4B and 28-4A disposed on layer 4, and the height parameter of the associated connection scheme indicates layer 4.

[0027] The control system desirably includes a means for monitoring the conductivity of the liquid flowing into the boiler. In this embodiment, the system is arranged to monitor the conductivity of the liquid at the inlet to the passage by momentarily switching to a connection scheme that establishes a conductive path extending between the electrodes 26-1A, 26-1B and monitoring the current flowing along this conductive path. In one variation, the boiler may include a separate pair of monitor electrodes (not shown) disposed adjacent the intake opening 44, and the temperature sensor 70 may be disposed between these electrodes. The control system may intermittently operate a switch to disconnect the other electrode from the power supply and connect the monitor electrode to the opposite pole of the power supply and measure the current passing between the poles. In either arrangement, the conductivity of the liquid may be calculated from the current measured by the sensor 80 and the known resistivity of the selected conductive path. In either arrangement, the conductive path used to determine the conductivity is near the bottom of the boiler, where no boiling occurs during normal operation. It is not necessary to explicitly calculate the conductivity, and the current measured between the monitoring electrodes can be used directly in determining the expected current for any connection scheme. Conductivity measurements are preferably performed at start-up or intermittently thereafter.

[0028] In operation, the liquid level sensor 72 and liquid source 46 cooperate to maintain a liquid level in the vessel above the passageway outlet. The control system selects a connection scheme and activates switches to connect the electrodes according to the connection scheme. If the pressure sensor 76 and temperature sensor 78 indicate that the boiler is not producing any steam after start-up, the control system may select a connection scheme where the expected current is at or near the maximum allowable current and the height parameter is high.

[0029] The control system monitors the current passing between the poles and the discharge of steam from the boiler. The control system calculates the expected current for the selected connection scheme based on the current parameters of the connection scheme and the conductivity of the liquid in the boiler, determined by the conductivity measurement described above. Optionally, the control system can apply a correction factor for the liquid conductivity. The correction factor is based on the estimated temperature of the liquid in each interval included in the conductive path and on a known relationship between the liquid conductivity and temperature. For example, if the head space 36 is filled with steam, the temperature in each interval located near the top of the passage on level 4 can be interpreted as the boiling point of the liquid at the pressure indicated by the pressure sensor 76, while the temperature in each interval on level 1 can be interpreted as the temperature indicated by the temperature sensor 70. The temperatures in the intervals on levels 2 and 3 can be estimated by interpolation between these values. If the actual value of the current is smaller than the expected current by more than the current tolerance, this indicates that one or more intervals between the electrodes included in the connection scheme are at least partially filled with steam.

[0030] In this state, the control system tries to select a new connection scheme with the same expected current but a lower height parameter. If such a connection scheme is found, the control system selects it. Otherwise, the control system selects a new connection scheme with a higher expected current and a lower height level, if the steam discharge is less than the discharge setting and the higher expected current does not exceed the maximum allowed current. If the next available higher expected current exceeds the maximum allowed current or the steam discharge is less than the discharge setting, the control system selects a new connection scheme with a lower expected current and a lower height parameter.

[0031] Regardless of the current monitoring and steam sensing processes discussed above, if the steam discharge from the boiler is greater than the discharge tolerance relative to the discharge setpoint, the control system selects a new connection scheme with a lower expected current. If the steam discharge is less than the discharge tolerance relative to the discharge setpoint, the control system selects a new connection scheme with a higher expected current. When the control system selects a new connection scheme in response to deviations from the steam setpoint, if several connection schemes have a higher or lower current than desired, the control system first selects the one with the highest height parameter.

[0032] Using the control schemes discussed above, the system avoids extended operation with any connection scheme that passes through an interval where significant amounts of steam are present, and if the system were to select such a connection scheme, the system would detect the presence of steam and immediately switch to another connection scheme.

[0033] With the control scheme discussed above, boiling typically begins at or near the top of the passageway, with steam bubbling through the liquid above the passageway outlet. The steam rises through the headspace 36 and flows out of the vessel via the steam delivery line 42. Some small amount of liquid may be entrained by the steam, but this liquid will strike the baffle 40 and flow back down into the vessel. As boiling continues, the gap between the electrodes at the higher elevation fills with a mixture of steam and liquid. The control system responds by switching the current path to the lower elevation. In a steady state situation, the system will tend to settle into a condition where boiling occurs near the midpoint of the passageway, with steam rising through the overlying liquid to the outlet. In other words, the control scheme ensures that large bubbles of paper do not form at or near the bottom of the liquid-filled passageway. This results in substantial prevention of a column of liquid spurting upward into the headspace, which could engulf the baffle 40 and send a large chunk of liquid into the steam delivery line 42.

[0034] In the operation of a boiler, minerals and other components dissolved in the liquid fed to the boiler remain in the boiler. This build-up usually increases the electrical conductivity of the liquid in the boiler. The boiler can be flushed with new liquid to remove the volume of components. This can be done periodically or in response to an increase in electrical conductivity being detected by the control system. For example, the control system can shut off the power supply 50 and the liquid level sensor 72 and then activate the liquid supply to flow new liquid through the boiler and out through the steam discharge opening 38. During this operation, a diverter valve (not shown) isolates the liquid passing through the steam discharge opening from the steam delivery line (conduit) 42 and directs it into a drain (not shown). After flushing with liquid, the liquid level can be restored by stopping or restricting the flow into the boiler and activating the electrodes to boil the liquid in the vessel until the sensor 72 indicates that the liquid in the vessel is at the desired level.

[0035] The vapor sensing and control schemes discussed above can be modified. For example, the system can detect the presence of substantial amounts of vapor at various levels in individual passages. In one such scheme, the control system can detect the presence of vapor in individual spaces between pairs of opposing electrodes. In this scheme, the control system can briefly shut down the connection scheme in use. While the connection scheme is shut down, the control system enters a vapor sensing mode. This activates the switch circuit 52 to connect a single pair of opposing electrodes to opposite polarities, determine the actual current between the electrodes, and compare the actual current to the expected current for each pair. Again, if the actual current is less than the expected current by more than the current tolerance, this indicates that the space between the electrodes contains a significant amount of vapor. By repeating this process for different pairs of electrodes, the system collects data for various spacings between the pairs of electrodes that indicate whether or not a significant amount of vapor is present in each space. The control system can operate the power supply 50 to provide a low voltage during the vapor sensing mode to avoid exceeding the maximum allowable current when testing each pair of electrodes. In this embodiment, the height data may include data identifying the location of each electrode within the current path. During normal operation, when selecting a new connection scheme, the control system avoids selecting any connection scheme that extends through an electrode that borders an interval in which vapor is present.

[0036] The number of passageways, the number of electrodes, and the arrangement of the electrodes can vary.

[0037] The present invention has been described above in terms of a boiler. In this disclosure, the term "liquid heater" refers to a device capable of heating a liquid, whether or not adapted to boil the liquid. Thus, the term "heater" as used herein is intended to encompass boilers and liquid heaters. The features discussed above can also be used in liquid heaters that do not boil the liquid. In particular, the electrode arrangements, dielectric walls and switch circuits discussed above can provide multiple conductive paths with resistivities spanning a wide range of resistivities and can be used in any liquid heater without requiring the vapor sensing capabilities discussed above.

[0038] Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and other arrangements can be devised without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. (a) a vessel (10) having a liquid inlet (44) and a vapor outlet (38); (b) a plurality of electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B) disposed within the container (10), the electrodes being arranged such that some of the electrodes are disposed at a higher level than other electrodes; (c) a power supply (50) having at least two poles (51, 53) and operable to supply different potentials to different poles of said at least two poles (51, 53); (d) In a plurality of various connection methods, the electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 2 8-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32 -3A, 32-3B, 32-4A, 32-4B) to the poles (51, 53) and to selectively disconnect the electrodes from the poles (51, 53), wherein in each of the connection modes at least two of the electrodes are connected to different poles such that current flows between the electrodes connected to different poles along a current path through the liquid in the container (10); (e) Repeatedly selecting one connection method from the plurality of connection methods, and performing the same on the electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B) in the selected connection method. a control system (54) operative to command the circuit (52) to connect various types of connections (32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B), the control system (54) operative to detect the presence of steam in the current paths associated with the various connection types and to substantially prevent prolonged operation with connection types in which steam is present; A boiler equipped with:

2. A boiler as described in claim 1, wherein the vessel (10) has vertically extending walls (12, 13) defining a plurality of vertically extending passages (26, 28, 30, 32) each having a lower end and an upper end, each passage (26, 28, 30, 32) having an inlet adjacent its lower end and an outlet adjacent its upper end, the liquid inlet (44) communicating with the inlet of the passage and the steam outlet (38) communicating with the outlet of the passage.

3. The circuit (52) is configured to include isolated electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-2A, 30-3B, 30-4A, 30 ...3A, 30-3B, 30-4A, 30-4B, 3 , 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B), and in at least one of said connection manners, the current path extends through liquid in a plurality of passages and through the isolated electrodes.

4. A boiler as described in claim 2 or 3, wherein the container (10) defines an upper space (36) that communicates with the outlets of the passages (26, 28, 30, 32) and extends above the outlets of the passages, and the steam outlet (38) communicates with the outlets of the passages via the upper space (36).

5. A boiler as described in claim 4, further comprising a liquid level control system operable to maintain the liquid level in the upper space (36) above the outlet of the passage (26, 28, 30, 32).

6. A boiler as described in claim 4 or 5, further comprising a set of baffles (40) disposed in the upper space (36) and arranged to define a tortuous path for the steam to rise in the upper space (36) and to allow entrained liquid falling from the steam to flow downwardly.

7. 7. The boiler of claim 2, wherein the set of electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B) disposed in each passage (26, 28, 30, 32) includes a plurality of pairs of electrodes, with the electrodes of each pair facing each other.

8. The control system (54) selects one connection method and controls the electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 8. The boiler of claim 7, wherein the boiler is operable to connect the poles (51, 53) of the power source (50) to the poles (51, 53) of the power source (50), monitor the actual current passing between the poles (51, 53) of the power source (50), compare the actual current with an expected current for the selected one of the connection schemes, and select a new connection scheme if the actual current is smaller than the expected current by more than a current tolerance.

9. A boiler as described in Claim 8, wherein the new connection method is selected so that the height of the highest electrode among the electrodes connected along the current path in the new connection method is lower than the height of the highest electrode among the electrodes connected along the current path in the one connection method.

10. A boiler as described in any one of claims 2 to 9, wherein the vessel (10) has a plurality of dielectric walls arranged within the vessel (10) and defining the passages (26, 28, 30, 32).

11. A boiler as described in claim 10, wherein the dielectric walls are flat plates extending substantially parallel to each other.

12. (a) contacting a plurality of electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B) with a conductive liquid; (b) selectively connecting electrodes of the plurality of electrodes to establish a conductive path through the liquid between poles (51, 53) of a power source (50); (c) applying a voltage between the poles (51, 53) to heat the liquid to form vapor and measuring the actual current flowing through the conductive path; (d) comparing the actual current to an expected current for the conductive path; (e) if the comparison performed in step (d) indicates that the actual current flowing through one or more of the conductive paths is less than the expected current by more than a current tolerance, repeating steps (b) through (e) for a different selected set of electrodes to establish a different conductive path; A method of operating a boiler, comprising:

13. The method described in claim 12, wherein step (b) includes setting a desired heating rate in response to measurements of one or more parameters indicative of the results achieved by heating the liquid, and selecting the electrodes and the current path based on the desired heating rate.

14. A method as described in claim 12 or 13, wherein the liquid boils during steps (a) to (e).

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