Patent Publication Number: US-7903956-B2

Title: Rapid liquid heating

Description:
RELATED APPLICATIONS AND PRIORITY 
     This patent application is a continuation of PCT application PCT/US09/53798 filed Aug. 13, 2009, entitled “Rapid Liquid Heating,” which claims the benefit of U.S. provisional patent applications 61/088,720 filed Aug. 13, 2008, entitled “Ohmic Liquid Heating,” and 61/178,970 filed May 16, 2009, entitled “Food Steamer Containers with Sequential Ohmic Water Heating,” all of which are incorporated herein by reference. This patent application is related to PCT application PCT/US09/53794, filed Aug. 13, 2009, entitled “Food Steamer Containers with Sequential Ohmic Water Heating,” (“the &#39;794 application”) incorporated herein by reference. 
    
    
     FIELD 
     This patent application generally relates to liquid heating. More particularly it relates to a system for heating a liquid by flowing a current through the liquid. 
     BACKGROUND 
     In standard resistance heating of a liquid, electrical current passes through a resistive heating element that converts electrical energy into heat. The heat conducts from the hot resistive heating element to the liquid, heating the liquid. This scheme is widely used in devices such as residential and commercial water heaters, appliances, such as dishwashers, and industrial processes. In heating water, the scheme has produced problems because the surface of the resistance heating element becomes much hotter than the liquid to be heated. This higher surface temperature causes chemicals and impurities in the liquid to react, to precipitate out of the liquid, and to adhere to the hot surface of the resistance heating element, forming a lime coating on its sheathing. Over time this lime layer builds up, and acts as a thermal insulator. Thus, the now insulated resistance element gets hotter, wasting energy. As it operates at an even hotter temperature the resistance element eventually burns out. In addition, in heating of the liquid with a standard resistance heater the electrical energy dissipated in the resistor has to first heat the resistance heating element, then the resistance element&#39;s sheathing, then any lime buildup on the element&#39;s sheathing surface, and then finally the liquid. Thus, the heating of the liquid comes after some delay. 
     To address these problems, the lime coating on the resistance heater may be periodically removed from the appliance for deliming to prevent burn out and frequent replacement. The maintenance process of removing the mineral surface deposits takes time, adding cost and may use harsh chemicals which are damaging to the environment, costly and potentially dangerous. 
     Thus, better techniques for heating liquids are needed, and these techniques are provided in this patent application. 
     SUMMARY 
     One aspect of the present patent application is a device for heating a liquid. The device includes a tank, electrodes, and a conductive liquid. The tank holds the conductive liquid and the electrodes. The electrodes are connected to provide current flowing in the conductive liquid. The device also includes an electrolytic material supply vessel for holding the electrolytic material. The electrolytic material supply vessel is switchably connected for providing the electrolytic material to the tank. The device also includes an electrical parameter sensor for detecting a parameter of electrical energy dissipated in the conductive liquid. The device also includes a controller connected to automatically add the electrolytic material to the conductive liquid if the electrical parameter sensor detects the electrical parameter differing from a set point. 
     Another aspect of the present patent application is a method of heating a liquid. The method includes providing a tank and electrodes, wherein the electrodes are located in the tank. The method also includes flowing a conductive liquid between the electrodes wherein the conductive liquid has a conductivity. The method also includes providing a system for adjusting the conductivity. The method also includes flowing a current in the liquid between the electrodes, detecting the current flow, and using the system to automatically adjust conductivity of the liquid to achieve a desired current flow. 
     Another aspect of the present patent application is a device for heating a liquid. The device includes a plurality of tank sections, an inflow, an outflow, electrodes, a baffle, and a liquid. The plurality of tank sections holds the liquid and the electrodes. The liquid has a conductivity sufficient to pass current between the electrodes. The baffle is between tanks of the plurality of tanks. 
     Another aspect of the present patent application is a method of heating a liquid. The method includes providing a tank, electrodes, and a liquid, wherein the electrodes are located in the tank. The method includes flowing the liquid between the electrodes. The method includes providing a voltage between the electrodes, flowing a current in the liquid between the electrodes, and without changing the voltage adjusting the current to provide a preset current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following detailed description as illustrated in the accompanying drawings, for clarity not drawn to scale, in which: 
         FIG. 1   a  is cross sectional view of one embodiment of a liquid heating system of the present patent application including tank sections having sets of electrode, a pressurized conductive liquid between electrodes, a source of electrolytic material, and a control system for providing the electrolytic material to increase the conductivity of the conductive liquid; 
         FIG. 1   b  is a three dimensional view of another embodiment of the liquid heating system of  FIG. 1   a  in which liquid in the tank is at atomospheric pressure; 
         FIG. 2   a  is a three dimensional exploded view of the liquid heating system of  FIG. 1   a;    
         FIG. 2   b  is a three dimensional view of the liquid heating system of  FIG. 1   a;    
         FIG. 2   c  is a three dimensional view of the baffles, electrodes, and leads of liquid heating system of  FIG. 1   a ; and 
         FIG. 3  is a block diagram of the electrical supply and control system for the liquid heating system of  FIG. 1   a.    
     
    
    
     DETAILED DESCRIPTION 
     Device  18 ,  18 ′ for heating entering liquid  19  includes tank  20 ,  20 ′ that holds electrode set  22  in tank section  20   a  and electrode set  23  in tank section  20   c , as shown in  FIGS. 1   a - 1   b . In one embodiment, three phase electrode set  22  includes electrode plates  22   a - 22   a ′,  22   b - 22   b ′,  22   c - 22   c ′ and three phase electrode set  23  includes electrode plates  23   a - 23   a ′,  23   b - 23   b ′,  23   c - 23   c ′, as shown in  FIGS. 2   a - 2   c . Tank  20  also holds conductive liquid  24  which is electrically isolated from the outside surface of tank  20 ,  20 ′. Device  18 ,  18 ′ can be used to heat a cold liquid  24 , boost the temperature of a previously heated liquid or to maintain a liquid temperature. 
     Tank  20 ,  20 ′ may be fabricated of a metal, such as steel, that has an inside surface coated with dielectric material  25 , such a fluoropolymer, glass, or porcelain. Tank  20 ,  20 ′ is also insulated and enclosed in a box or container (not shown), further insulating it. For low pressure use, tank  20 ,  20 ′,  20 ′ can be made of a dielectric material, such as plastic. 
     In one application, device  18 ,  18 ′ was used to raise the temperature of entering liquid  19 , such as water from a municipal supply system, that flowed through tank  20 ,  20 ′. Entering liquid  19  may arrive directly from the municipal water supply system or it may have been previously treated, used, or heated, such as in another heating unit. For example entering liquid  19  may have an entry temperature of 150 F and device  18 ,  18 ′ is used to boost its temperature to 200 F. 
     Entering liquid  19  can be water based or have a large component of water, such as sea water, waste water, milk, blood, body fluids, a processed food slurry, an organic waste processing mix, cleaning fluids, beer, or wine. Entering liquid  19  can also be an alcohol, such as ethanol or glycol, or a paraffin based material, such as a heat transfer fluid. If an entering liquid  19  is a liquid other than water-based then electrolytic material  26  includes conductive solutes appropriate for that liquid. Conductive liquid  24 , formed from the mixture of entering liquid  19  and electrolytic material  26 , allows substantially more current to flow between electrodes of electrode sets  22 ,  23  than entering liquid  19  by itself would allow. With substantial current flowing, conductive liquid  24  heats during its residence time in tank  20 ,  20 ′. 
     Electrolytic material  26  is added to entering liquid  19  to provide conductive liquid  24  with enhanced conductivity compared to entering liquid  19 . Electrolytic material  26  can be a solid or liquid material. In one embodiment, electrolytic material  26  is itself a solution that contains electrolytes. For example, electrolytic material  26  can be an aqueous solution containing salt electrolytes, such as sodium chloride or potassium chloride. In addition to salts, aqueous solutions of such additional water soluble electrolytic materials as sodium carbonate, sodium bicarbonate, trisodium citrate, sodium hydroxide, hydrochloric acid, ammonium nitrate, nitric acid, or acetic acid can be used. Aqueous solutions of other salts or other conductive solutes can also be used. Cleansing agents, rinse agents, or metal protecting agents or mixtures of these agents can be added along with electrolytic material  26 . 
     In one embodiment device  18 ,  18 ′ includes electrolytic material supply vessel  28  connected to tank section  20   s  through electrolytic supply inlet pipe  30   a  for supplying electrolytic material  26  to tank section  20   a . Supply can be via a pump or gravity feed. Electrolytic material  26  can be added to entering liquid  19  in inlet pipe  30   b  before entering liquid  19  enters tank section  20   a , as shown in  FIG. 1   a . Alternatively, electrolytic material  26  can be added to tank section  20   a  through its own inlet pipe  30   c , separately from entering liquid  19 , entering through inlet pipe  30   d , as shown in  FIG. 1   b.    
     In another embodiment, the conductive material supply vessel can be separate from device  18 ,  18 ′. For example, conductive material supply vessel  28  can be a water softener (not shown) that provides an aqueous solution containing electrolytic salts. 
     In a prototype, electrolytic material  26  was an aqueous solution of sodium chloride salt having a sodium chloride concentration of 30,000 ppm. Electrolytic material  26  was fabricated by mixing ¼ teaspoon of salt with 7 gallons of water. In an embodiment using municipal water in Winooski, Vt., entering municipal water had a sodium chloride concentration of 90 ppm, and the mixing of this entering municipal water with a carefully metered amount of electrolytic material  26  produced conductive liquid  24  having a sodium chloride concentration in the range of hundreds of ppm. 
     In one embodiment, device  18 ,  18 ′ also includes current sensing switch  36  for detecting current flowing to electrode sets  22 ,  23 . Device  18 ,  18 ′ also includes controlling device  38  connected for using information from current sensing switch  36  to automatically control flow of electrolytic material  26  to tank section  20   a.    
     In the prototype described herein above in which entering liquid  19  was municipal water and electrolytic material  26  was the 30,000 ppm sodium chloride salt solution, current sensing switches were used to implement the current sensing and electrolyte solution controlling functions. The present applicants used the current sensing switches to provide a volume of electrolytic material  26  sufficient to provide the municipal water with a conductivity such that with 208 Volts applied between pairs of electrode plates of electrode sets  22 ,  23 ,  32  amperes of current flowed. The 32 amperes was more than 60% of the maximum current available from the 50 ampere wall outlet circuit that provided the power. The current set point can be higher, for example the current set point can be 70% or up to 80% of the wall outlet circuit breaker rating. This electric current flowing through conductive liquid  24  between each pair of electrode plates in electrode sets  22 ,  23  boosted its temperature from about 150 F at the inlet  39  of tank  20 ,  20 ′ to about 200 F at outflow  40 . The heated water was then used for the sanitizing rinse of dishes in a commercial dishwasher. With entering municipal water flowing through tank  20 ,  20 ′ for this sanitizing rinse at 293 gallons per hour, a small pump was used to mix in the 30,000 ppm sodium chloride salt solution, and sufficient heat was added to conductive liquid  24  by the current flow through conductive liquid  24  during its residence time between electrodes in tank  20 ,  20 ′ to raise its temperature by 50 degrees F. Measuring conductive liquid  24  flowing out of tank  20 ,  20 ′, applicants found that it had a sodium chloride concentration of 450 ppm. 
     Device  18 ,  18 ′ can also be used for residential heating. For this use entering liquid  19  can be water that recirculates between baseboard heaters and device  18 ,  18 ′. In other embodiments, the device receiving the heated water can be a hot water faucet, a shower head, a hot water supply tank, a car wash, a pool heater, or an industrial process that uses hot water. In these embodiments, the municipal water at inlet  39  of tank  20 ,  20 ′ can be at a temperature such as room temperature or below. Device  18 ,  18 ′ provides heated water at outflow  40  is at a preset hot water temperature. The water arriving at inlet  39  can already be conductive if, for example, it includes electrolyte from a water softener or if it is recirculating water that previously had electrolyte added. 
     Device  18 ,  18 ′ allows for increasing or decreasing the concentration of electrolyte in conductive liquid  24  flowing through tank  20 ,  20 ′ to maintain a desired current level. To increase the concentration of electrolyte in conductive liquid  24 , more electrolytic material  26  was injected. In one embodiment, this was accomplished by pumping in solution  26  for longer increments of time while plain municipal water was flowing into tank  20 ,  20 ′. To decrease the concentration of electrolyte in conductive liquid  24 , electrolytic material  26  was pumped into tank  20 ,  20 ′ for shorter amounts of time while plain municipal water was flowing into tank  20 ,  20 ′. Thus, device  18 ,  18 ′ allows for changing the concentration of conductive solutes in liquid  24  in either direction. 
     Increasing the concentration of conductive solutes dissolved in conductive liquid  24  increases the conductivity of conductive liquid  24  which increases current flowing through conductive liquid  24  at a given applied voltage. Increasing the current increases the heating rate proportionally, since the heating rate is the current times the voltage. Similarly, decreasing the concentration of conductive solutes decreases the heating rate. 
     Pump  41  is connected for supplying electrolytic material  26  from conductive material vessel  28  to tank  20 ,  20 ′ through inlet pipe  30 . In one apparatus set up by applicants, model PQM-1/230 AC motor driven gear pump obtained from Greylor Company, Cape Coral, Fla., was used. Other pumps, such as the Mec-o-matic VSP series peristaltic pump modul number VSP20 from Pulsafeeder, Inc., an unit of Idex Corporation, can also be used. 
     In prototypes, the present applicants implemented controlling device  38  with normally open current sensing switch  36  and normally closed current sensing switch  42 , as shown in  FIGS. 4   a ,  4   b . Such current sensing switches are available from Eaton Corporation, Cleveland, Ohio, with part numbers ECSNOASP and ECSNCASP respectively. Normally open current sensing switch  36  closes, turning on pump  41  and opening valve  110  when current falls below a set point of 32 amperes. 
     Normally closed current sensing switch  42  opens when the current reaches a set point, such as 38 amperes. Normally closed current sensing switch  42  operates as a high current safety and is wired in series with current sensing switch  36 , providing this over-amperage safety. Should the current draw exceed the 38 ampere set point, normally closed current switch  42  would open, stopping pump  41  and the flow of electrolytic material  26  into tank  20 ,  20 ′. When the current falls back below 38 amperes normally closed current switch  42  would close, and when the current falls below 32 amperes both switch  42  and switch  36  would then both be closed, pump  41  would resume pumping, and electrolytic material  26  would flow into tank  22   a  to increase current back to the 32 ampere range. 
     In this embodiment, both the pump  41  and valve  110  are energized at the same time to prevent back flow of line pressure into the electrolytic solution container  28  when pump  41  is off. 
     In another embodiment electrolytic material  26  is provided with a gravity feed unit. In this embodiment, when current sensing switch  36  detects current falling below the 32 ampere set point, current sensing switch  36  closes, turning on normally off solenoid valve  110 . Turning on solenoid valve  110  allows gravity flow of electrolytic material  26  into tank  20 ,  20 ′. Thus, in either embodiment, electrolytic material  26  is automatically added to tank  20 ,  20 ′ to achieve a preset current at the line voltage, and this current and voltage provides a preset heating rate. 
     By contrast, allowing entering liquid  19  to enter tank  20 ,  20 ′ without also adding electrolytic material  26  dilutes the concentration of electrolyte in conductive liquid  24  in tank  20 ,  20 ′, lowering conductivity of conductive liquid  24  and lowering the current traveling through conductive liquid  24 . Thus, as heated conductive liquid  24  is drawn from tank  20 ,  20 ′, fresh entering liquid  19  is drawn in, and the conductivity of conductive liquid  24  in tank  20 ,  20 ′ is continuously adjusted to provide and maintain the desired current level. 
     Liquid inlet  39  is electrically isolated with dielectric spacer  51  and liquid outflow  40  is electrically isolated with dielectric spacers  53  to isolate inlet metal pipe  30  and outlet metal pipe  54  from tank  20 ,  20 ′, preventing leakage current from reaching pipes  30 ,  54 . Dielectric spacers  51  and  53  also include grounding cables that connect conductive liquid  24  to ground through an earth leakage protect device. With the earth leakage protect device, if current to ground exceeds a threshold, all current to tank  20 ,  20 ′ is disconnected. Tank  20 ,  20 ′ also has its own separate grounding line. 
     Liquid outflow  40  is connected to the machine or structure (not shown), such as a commercial or domestic dishwasher in which the heated water  56  will be used, for example, for sanitizing dishes. 
     In the embodiment of  FIG. 1   b , in which tank  20 ,  20 ′ is unpressurized, for example, at atmospheric pressure, and a level sensing float switch (not shown) in tank  20 ,  20 ′ controls operation of a solenoid operated fill valve connected to entering liquid inlet  39 . The float switch can be part number M8700 from the Madison Company, Branford Conn. 
     In devices  18 ,  18 ′ built by applicants, tank  20 ,  20 ′ had three baffled sections  20   a ,  20   b ,  20   c . Preheated municipal water entered at water inlet  39  located at bottom  62  of first tank section  20   a , as shown in  FIGS. 1   a ,  1   b . Salt water electrolytic material  26  was added in tank section  20   a  to provide conductive solution  24 . 
     Conductive liquid  24  was heated by current flowing through conductive liquid  24  between electrode plates  22   a - 22   a ′,  22   b - 22   b ′,  22   c - 22   c ′ of first electrode set  22  in first tank section  20   a . Heated conductive liquid  24  rose to top  68  of first tank section  20   a  and flowed out of first tank section  20   a  through holes  74  at top  76  of first baffle wall  78  and entered middle tank b. Conductive liquid  24  then flowed out of middle tank  20   b  through holes  86  at bottom  88  of second baffle wall  90  and into third tank section  20   c  where it was heated by current flowing through conductive liquid  24  between electrode plates  23   a - 23   a ′,  23   b - 23   b ′,  23   c - 23   c ′ in second electrode set  23 . Heated conductive liquid rose to top  102  of third tank section  20   c , and the further heated conductive liquid exited through conductive liquid outflow  40  of third tank section  20   c.    
     The present applicants recognized that dissolved solids normally present in municipal water did not precipitate out of the water and did not form lime deposits on electrode plates  22   a - 22   a ′,  22   b - 22   b ′,  22   c - 22   c ′ and  23   a - 23   a ′,  23   b - 23   b ′,  23   c - 23   c ′ as would ordinarily happen if standard electric resistance heaters were used. Lime deposits were absent because electrode sets  22 ,  23  remain at the temperature of the liquid in which they are immersed, whereas electric resistance heaters ordinarily operate at a much higher temperature. Thus, the present system reduces or eliminates the need for deliming and repairs common to resistance type heaters. 
     In this embodiment, middle tank section  20   b  has no electrodes; middle tank section  20   b  serves to avoid stratification of the water based on temperature, improving operation. In addition, residence time for water in each tank section is enhanced. Thus, if tank  20 ,  20 ′ is initially empty, water completely fills first tank section  20   a  before any spills through holes  76  at the top of baffle wall  78  and enters middle tank section  20   b , maximizing residence time in tank section  20   a . This heated water then enters the bottom of third tank section  20   c  through holes  86  at the bottom of baffle wall  90  and resides in third tank section  20   c  until third tank section  20   c  fills, and it then exits through outflow  40  at the top of third tank section  20   c , maximizing residence time in third section  20   c.    
     In the atmospheric pressure embodiment, as conductive liquid  24  was drawn from third tank section  20   c  by the dishwasher, float switch  55  turns on solenoid valve  110 , and 150 F entering liquid  19 , preheated municipal water, was drawn into enter first tank section  20   a . Float switch  55  was part number M8700 from Madison Company, Branford, Conn. The 150 F preheated municipal water drawn into first tank section  20   a  lowered the concentration of salt in conductive solution  24  in first tank section  20   a , thereby lowering conductivity of water in this first tank section  20   a , and lowering the current flowing between first electrode plates  22   a ,  22   b ,  22   c  in first tank section  20   a . Detecting lowered current below a set point caused current sensitive switch  36  to turn on pump  41  to provide more electrolytic material  26  in first tank section  20   a . This raised conductivity of conductive liquid  24 , raising current flowing between electrodes of electrode sets  22 ,  23 , and caused a higher rate of heating in tank sections  20   a ,  20   c . Pump  41  continued to operate until the current flowing reached the set point of 32 amperes. At that current sensitive switch  36  turned off pump  41  and flow of electrolytic material  26  into tank section  20   a  temporarily stopped while current continued to flow between electrodes of electrode sets  22 ,  23 . Pump  41  turned on and off to maintain the current flowing at the 32 ampere set point until a preset temperature set point for the water reaching outflow  40  was reached. 
     In the pressurized system of  FIG. 1   a , float switch  55  was not needed and tank sections  20   a ,  20   b ,  20   c  were continuously kept filled by water line pressure. 
     K type thermocouples were used as temperature sensors  112 ,  113  to measure temperature of conductive liquid  24  in first tank section  20   a  and in third tank section  20   c  respectively, as shown in  FIGS. 1   a ,  1   b , and  FIG. 3 . Temperature controller  114  connected to temperature sensors  112 ,  113  was of a type that turned off current flow to electrode sets  22 ,  23  if the temperature measured reached a set point, which in the prototype was 200 F. Thus overheating conductive liquid  24  was avoided and a minimum of electrical energy was used to reach the desired temperature. Other temperature sensors can be used, such as thermisters. In the prototype, power was supplied by one power supply to electrode sets in both tank sections. In the prototype, temperature controller  114  was an ECM-40 controller from Athena Controls, Plymouth Meeting, Pa. and type K thermocouples were used. 
     In the temperature circuit, three phase AC power is fed through field wiring terminal block  130 , and distributed in a parallel arrangement to the line and ground sides of electrode sets  22  and  23  through relay sets  138  and  140 . Relay set  138  includes 3 solid state relays  138   a ,  138   b ,  138   c  while relay set  140  includes 3 solid state relays  140   a ,  140   b ,  140   c  to provide one solid state relay for each phase of power in each relay set. The load side of solid state relay set  138  is connected to the individual line electrodes  22   a ,  22   b ,  22   c  of electrode set  22 , and the load side of solid state relay set  140  is connected to the individual line electrodes  23   a ,  23   b , and  23   c  of electrode set  23 . The load side of solid state relay set  138  is connected to individual electrodes  22   a ′,  22   b ′,  22   c ′ of electrode set  22 , and the load side of solid state relay set  140  is connected to the individual electrodes  23   a ′,  23   b ′, and  23   c ′ of electrode set  23 . Relay sets  138 ,  140  can be part number CWD2450 from Crydom, San Diego, Calif. 
     Power is supplied to temperature controller  114  through main power switch  132 , high-limit temperature safety switch  134  and a 208-volt-primary to 18-volt-secondary transformer  136 . Temperature feedback from the two temperature sensors  112 ,  113  is used by temperature controller  114  to determine whether more heat is required in tank section  20   a  or tank section  20   c  to reach a preset temperature set point for each tank section. If more heat is required in one or both tank sections, temperature controller  114  sends a voltage output signal to the coils of solid state relay sets  138  or  140  to close that relay and allow current to flow between electrodes in the respective tank section depending on which requires more heat. In another embodiment, each tank  20   a ,  20   c  has its controller. Temperature controller  114  can be a DCH controller from Antunes Controls, Carol Stream Ill. 
     High-limit temperature safety switch  134  is included to ensure that an over temperature condition does not occur, preventing harm to the operator or to device  18 ,  18 ′. Safety switch  134  is a normally closed bi-metal snap disc style switch that is mounted to the exterior of tank  20 ,  20 ′ and monitors its surface temperature. If the surface temperature rises above the upper set point of switch  134 , switch  134  opens, thereby stopping all current flow to electrode sets  22 ,  23  and stopping any additional heating of conductive liquid  24  by electrode sets  22  and  23 . Switch  134  automatically resets itself to a closed position once temperature declines below a lower threshold. For a system designed to heat water to 200 F, the upper set point of switch  134  could be set at about 250 F and the lower set point at about 220 F. 
     Electrodes  22   a - 22   c  and  22   a ′- 22   c ′ were fabricated of graphite plates, as shown in  FIGS. 2 and 3 . In one experiment, graphite plates had dimensions of 4 inches by 9 inches and were mounted 1.688 inches apart. Each graphite plate was mounted on brackets  121  of type 2 titanium sheet which was connected to leads  120  formed of 0.125 inch diameter type 2 titanium rods extending through dielectrically isolated bushings in tank cover  122 , as shown in  FIG. 2   a - 2   b . Applicants found that the graphite electrodes had longer life than electric resistance heaters. 
     Alternatively, tank  20 ,  20 ′ may just have a single tank section. One, two, or more electrode sets can be used in the single tank section embodiment. A two tank section system can also be used. In the two tank section embodiment, holes may be provided at the top of the dividing wall. Tank  20  can also have more than three tank sections. To heat conductive liquid  24  that is flowing at a higher flow rate, more electrode sets may be provided, and these may be provided in the additional tanks. 
     While a system with a three phase AC voltage supply is shown, a single phase system can also be used. While in the example described a 208 volt system was used, any voltage can be used, such as 480 Volts, 240 Volts or 120 Volts. While a control system with a constant voltage source was described a control system with a constant current source and a voltage that varies with conductivity of the conductive liquid can also be used while the system provides control to provide a high voltage. 
     In one embodiment one power supply is connected in a parallel arrangement to electrode sets  22  and  23  in both tanks  20   a  and  20   c , as shown in  FIG. 3 . In this embodiment whichever tank  20   a ,  20   c  has conductive solution  24  with the highest conductivity at a particular moment gets the most current and the most heating. If conductive solution  24  is flowing, current in tank section  20   c  will generally track the current in tank section  20   a  but with a delay for the time it takes for conductive solution  24  to travel from tank section  20   a  to tank section  20   c . In another embodiment each electrode set is powered by its own power supply. In this embodiment, the two power supplies can provide the same phase and voltage. Alternatively, the two power supplies provide different voltages and/or different phases. 
     In one embodiment, current is supplied to electrodes  22  with alternating current power supply  46 , such as a standard 3 phase 208 Volt supply. Other supply and electrode configurations, such as a single phase power supply with a single pair of electrodes can also be used. 
     In another embodiment with a single power supply connected to temperature controller  114 , current is independently pulsed to each set of electrodes  22 ,  23  as full allowable current. Temperature controller  114  uses pulse width modulation to modulate the current supplied to each electrode set  22 ,  23 , as described in the &#39;794 application, incorporated herein by reference. In this way full current or half-wave current is provided alternatively to each electrode set  22 ,  23  until the set point temperature is reached. In one embodiment, temperature controller  114  is set to provide a square wave output to switches for each electrode set  22 ,  23 , allowing control of the duty cycle of the two electrode sets. With power to each switching on and off a fraction of the energy is delivered to each electrode set. 
     In one embodiment, electric current controller  114  includes a circuit that provides electric current to electrodes  22  for a first period of time while not providing any electric current to electrodes  23  during that same first period of time. Then after this first period of time is complete, the circuit in electric current controller  114  provides electric current to electrodes  23  for a second period of time while not providing any electric current to electrodes  22  during that second period of time. This cycle repeats, supplying current to electrodes  22 , then to electrodes  23  sequentially. Applicants have built and tested apparatus using this scheme that has a frequency of about ¼ second. In that embodiment, each set of electrodes received full power for ⅛ second intervals separated by ⅛ second gaps during which that set of electrodes received no power and during which the other set of electrodes received the full power. In this manner water in a two containers, each heated by one of the sets of electrodes was heated to boiling while electrodes in each container received nearly the full current that could safely be provided by the wall outlet circuit at a voltage that was equal to or close to the full voltage available from the wall outlet. With a duty cycle of 50%, each container received nearly the maximum current available from the wall outlet circuit, and power provided to both tanks was substantially higher than could be achieved in either a standard parallel or a series circuit arrangement. A standard parallel arrangement would divide the current between the tanks and require a substantially lower voltage across the tanks to avoid the combined current exceeding the maximum current available from the wall outlet circuit. A series arrangement would divide the voltage between the tanks, lowering the power to each. The unique parallel arrangement in this embodiment, of providing current sequentially to the sets of electrodes while adjusting conductivity of the liquid to maintain a preset current level, also permits one of the sets of electrodes to be turned off for an extended period while allowing the other set of electrodes to continue to be in use with the full line voltage applied and the current at a preset value near the maximum allowed by the wall outlet circuit, a feature that would not be available with a series arrangement of the electrodes. 
     By providing a system in which conductivity of the liquid being heated was varied the present applicants continually provided maximum Voltage while also providing a desired current level near the limit of the wiring, so maximum power could continually be delivered, providing maximum heating rate to the liquid. They could then adjust the duty cycle to avoid exceeding the desired temperature. They could adjust the duty cycle by by going from a full sine wave to pulsed modulation, for example providing a half wave when conductivity of conductive liquid  24  got too high. They could also modulate the conductivity of conductive liquid  24  to adjust the current level. They could also selectively turn on and turn off power to one or more of electrode sets  22 ,  23  while maintaining conductivity of conductive liquid  24 . 
     Applicants found that the direct heating of water by modulating the conductivity of the water and passing a current passing through the water provided substantially faster heating with better control and efficiency than was available for systems that used a standard resistance heater in the water. They found that using the water itself as the heating element converts electrical energy to heated liquid with less delay. The avoidance of lime buildup, as described herein above also contributes to faster and more efficient heating. 
     They also found that the direct heating of liquid of the present application overcomes an overshoot problem inherent with the delay in the resistance heating of the liquid. The delay in heat transfer with standard resistance heaters means that electrical energy continues to be provided after enough has already been provided to reach the desired temperature and this temperature is often over shot, wasting energy. 
     In one experiment, applicants found that device  18 ,  18 ′ was able to self-adjust to unplanned conditions in order to achieve the results the control system called for. A 200 F temperature was called with the supply water entering inlet  39  at 150 F with a flow rate of 293 gallons per hour. When the supply water temperature fell to only about 90 F device  18 ′ self adjusted to provide the 200 F discharge at the device&#39;s flow rate by increasing the amount of time that electrodes  22 ,  23  were powered in order to provide the additional heat needed to compensate for the lower input water temperatures. 
     In one experiment they found that the salt added to entering water  19  left no residue on dishes. They found that the apparatus was useful for a booster heater as a separate box that heats water and feeds it to the dishwasher. They recognized that such a heater could also be built into the dishwasher. They also recognized that the system can also be used for primary heating of cold water for residential, commercial, and industrial hot water supply. 
     While the disclosed methods and systems have been shown and described in connection with illustrated embodiments, various changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.