Abstract:
A dry-fire protection system and method. The dry-fire protection system includes a tank, a heating element, a powered electrode, a sensor, and a microcircuit. The heating element and the powered electrode are positioned in the tank, the powered electrode being positioned above the heating element. The powered electrode includes an electrode wire and a connector having an electrical connector coupled to the electrode wire, but electrically isolated from the tank, and a fastener for coupling the powered electrode to the tank. The sensor detects an electrical characteristic of a circuit formed by the tank, the powered electrode, and water in the tank. The microcircuit determines if a possible dry-fire condition exists based on a signal received from the sensor.

Description:
RELATED APPLICATION 
       [0001]    This application is a division of co-pending U.S. patent application Ser. No. 10/950,851 filed Sep. 27, 2004, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a water storage device having a powered anode and a method of controlling the water storage device. 
         [0003]    Powered anodes have been used in the water heater industry. To operate properly, a powered anode typically has to resolve two major concerns. First, the powered anode should provide enough protective current to protect exposed steel within the tank. The level of exposed steel will vary from tank to tank and will change during the lifetime of the tank. Second, the protective current resulting from the powered anode should be low enough to reduce the likelihood of excessive hydrogen. 
         [0004]    There are at least two techniques currently available in the water heater industry for using a powered anode to protect a tank. One technique adjusts anode voltage levels based on the conductivity of the water. However, this technique does not measure the protection level of the tank and tanks with excessive exposed steel could be inadequately protected. The second technique periodically shuts off the current to the anode electrode and uses the electrode to “sense” the protection level of the tank. This technique adapts to the changing amount of exposed steel in the tank, but does not adapt to changing water conductivity levels. In addition, this technique can have problems in high conductivity waters since currently produced titanium electrodes with mixed metal oxide films have a tendency to drift in their reference voltage measurements in high conductivity water. It would be beneficial to have another alternative to the just-described techniques. 
       SUMMARY 
       [0005]    In one embodiment, the invention provides a water heater including a tank to hold water, an inlet to introduce cold water into the tank, an outlet to remove hot water from the tank, a heating element (e.g., an electric resistance heating element or a gas burner), an electrode, and a control circuit. The control circuit includes a variable voltage supply, a voltage sensor, and a current sensor. The control circuit is configured to controllably apply a voltage to the electrode, determine a potential of the electrode relative to the tank when the voltage does not power the electrode, determine a current applied to the tank after the voltage powers the electrode, determine a conductivity state of the water in the tank based on the applied voltage and the current, and define the voltage applied to the electrode based on the conductivity state. 
         [0006]    In another embodiment, the invention provides a method of controlling operation of a water storage device. The method includes the acts of applying a voltage to an electrode, ceasing the application of the applied voltage to the electrode, determining the potential of the electrode relative to the tank after the ceasing of the application of the applied voltage, determining a conductivity state of the water, defining a target potential for the electrode based on the conductivity state, and adjusting the applied voltage to have the electrode potential emulate the target potential. 
         [0007]    In another embodiment, the invention provides another method of controlling operation of a water heater including a tank, a heating element, an electrode, and a sensor configured to detect an electrical characteristic of a circuit formed by the tank, the electrode, and water in the tank. The method includes the acts of applying a voltage to an electrode, acquiring a signal from the sensor having a relation to the applied voltage, determining whether the water heater is in a dry-fire state based on the acquired signal, and limiting activation of a heating element when the water heater is in a dry-fire state. 
         [0008]    In another embodiment, the invention provides a method of preventing activation of a heating element when a dry-fire condition exists in a water heater. The water heater includes a tank, a heating element positioned in the tank, a powered electrode positioned in the tank, and a controller. The method includes the steps of applying an analog voltage to the powered electrode, detecting a current having a relationship to the analog voltage, determining if the detected current is below a threshold, and isolating the heating element from a power source when the detected current is below the threshold. 
         [0009]    In another embodiment, the invention provides a dry-fire detection system for a water heater including a tank having a bottom, a heating element positioned in the tank, a powered electrode positioned above the heating element with respect to the bottom, a sensor, and a microcircuit. The powered electrode includes an electrode wire positioned substantially in the tank, a connector assembly having an electrical connector coupled to the electrode wire and electrically isolated from the tank, and a fastener coupling the powered electrode to the tank. The sensor is configured to detect an electrical characteristic of a circuit formed by the tank, the powered electrode, and water in the tank. The microcircuit is coupled to the sensor and receives a signal from the sensor. The microcircuit determines a possible dry-fire condition exists based on the signal. 
         [0010]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]      FIG. 1  is partial-exposed view of a water heater embodying the invention. 
           [0012]      FIG. 2  is a side view of an electrode capable of being used in the water heater of  FIG. 1 . 
           [0013]      FIG. 3  is a electric schematic of a control circuit capable of controlling the electrode of  FIG. 2 . 
           [0014]      FIG. 4  is a flow chart of a subroutine capable of being executed by the control circuit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0015]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limited. The use of “including,” “comprising” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” “supported,” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting, supporting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. 
         [0016]      FIG. 1  illustrates a water heater  100  including an enclosed water tank  105 , a shell  110  surrounding the water tank  105 , and foam insulation  115  filling the annular space between the water tank  105  and the shell  110 . A typical storage tank  105  is made of ferrous metal and lined internally with a glass-like porcelain enamel to protect the metal from corrosion. Nevertheless, the protective lining may have imperfections or, of necessity, may not entirely cover the ferrous metal interior. Under these circumstances, an electrolytic corrosion cell may be established as a result of dissolved solids in the stored water, leading to corrosion of the exposed ferrous metal and to reduction of service life for the water heater  100 . 
         [0017]    A water inlet line or dip tube  120  and a water outlet line  125  enter the top of the water tank  105 . The water inlet line  120  has an inlet opening  130  for adding cold water to the water tank  105 , and the water outlet line  125  has an outlet opening  135  for withdrawing hot water from the water tank  105 . The water heater  100  also includes an electric resistance heating element  140  that is attached to the tank  105  and extends into the tank  105  to heat the water. The heating element  140  typically includes an internal high resistance heating element wire surrounded by a suitable insulating material and enclosed in a metal jacket. Electric power for the heating element  140  is typically supplied from a control circuit. While a water heater  100  having element  140  is shown, the invention can be used with other water heater types, such as a gas water heater, and with other water heater element designs. It is also envisioned that the invention or aspects of the invention can be used in other water storage devices. 
         [0018]    An electrode assembly  145  is attached to the water heater  100  and extends into the tank  105  to provide corrosion protection to the tank. An example electrode assembly  145  capable of being used with the water heater is shown in  FIG. 2 . With reference to  FIG. 2 , the electrode assembly  145  includes an electrode wire  150  and a connector assembly  155 . The electrode wire  150  comprises titanium and has a first portion  160  that is coated with a metal-oxide material and a second portion  165  that is not coated with the metal-oxide material. During manufacturing of the electrode assembly  145 , a shield tube  170 , comprising PEX or polysulfone, is placed over a portion of the electrode wire  150 . The electrode wire  150  is then bent twice (e.g., at two forty-five degree angles) to hold the shield tube in place. A small portion  175  of the electrode wire  150  near the top of the tank is exposed to the tank for allowing hydrogen gas to exit the shield tube. In other constructions, the electrode assembly  145  does not include the shield tube  170 . The connector assembly  155  includes a spud  180  having threads, which secure the electrode rod assembly to the top of the water tank  105  by mating with the threads of opening  190  ( FIG. 1 ). Of course, other connector assemblies known to those skilled in the art can be used to secure the electrode assembly  145  to the tank  105 . The connector assembly also includes a connector  195  for electrically connecting the electrode wire  150  to a control circuit (discussed below). Electrically connecting the electrode assembly  145  to the control circuit results in the electrode assembly  145  becoming a powered anode. As is known to those skilled in the art, the electrode wire  150  is electrically isolated from the tank  105  to allow for a potential to develop across the electrode wire  150  and the tank  105 . 
         [0019]    An electronic schematic for one construction of the control circuit  200  used for controlling the electrode assembly  145  is shown in  FIG. 3 . The control circuit includes a microcontroller U 2 . An example microcontroller U 2  used in one construction of the control circuit  200  is a Silicon Laboratories microcontroller, model no. 8051F310. As will be discussed in more detail below, the microcontroller U 2  receives signals or inputs from a plurality of sensors, analyzes the inputs, and generates outputs to control the electrode assembly  145 . In addition, the microcontroller U 2  can receive other inputs (e.g., inputs from a user) and can generate outputs to control other devices (e.g., the heating element  140 ). As is known in the art, the Silicon Laboratories microcontroller, model no. 8051F310, includes a processor and memory. The memory includes one or more modules having instructions. The processor obtains, interprets, and executes the instructions to control the water heater  100 , including the electrode assembly  145 . Although the microcontroller U 2  is described having a processor and memory, the invention may be implemented with other devices including a variety of integrated circuits (e.g., an application-specific-integrated circuit) and discrete devices, as would be apparent to one of ordinary skill in the art. 
         [0020]    The microcontroller U 2  outputs a pulse-width-modulated (PWM) signal at P 0 . 1 . Generally speaking, the PWM signal controls the voltage applied to the electrode wire  150 . A one hundred percent duty cycle results in full voltage being applied to the electrode wire  150 , a zero percent duty cycle results in no voltage being applied to the electrode wire  150 , and a ratio between zero and one hundred percent will result in a corresponding ratio between no and full voltage being applied to the electrode wire  150 . 
         [0021]    The PWM signal is applied to a low-pass filter and amplifier, which consists of resistors R 2 , R 3 , and R 4 ; capacitor C 3 ; and operational amplifier U 3 -C. The low-pass filter converts the PWM signal into an analog voltage proportional to the PWM signal. The analog voltage is provided to a buffer and current limiter, consisting of operational amplifier U 3 -D, resistors R 12  and R 19 , and transistors Q 1  and Q 3 . The buffer and current limiter provides a buffer between the microcontroller U 2  and the electrode assembly  145  and limits the current applied to the electrode wire  150  to prevent hydrogen buildup. Resistor R 7 , inductor L 1 , and capacitor C 5  act as a filter to prevent transients and oscillations. The result of the filter is a voltage that is applied to the electrode assembly  145 , which is electrically connected to CON 1 . 
         [0022]    As discussed later, the drive voltage is periodically removed from the electrode assembly  145 . The microcontroller deactivates the drive voltage by controlling the signal applied to a driver, which consists of resistor R 5  and transistor Q 2 . More specifically, pulling pin P 0 . 3  of microcontroller U 2  low results in the transistor Q 1  turning OFF, which effectively removes the applied voltage from driving the electrode assembly  145 . Accordingly, the microcontroller U 2 , the low-pass filter and amplifier, the buffer and current limiter, the filter, and the driver act as a variable voltage supply that controllably applies a voltage to the electrode assembly  145 , resulting in the powered anode. Other circuit designs known to those skilled in the art can be used to controllably provide a voltage to the electrode assembly  145 . 
         [0023]    The connection CON 2  provides a connection that allows for an electrode return current measurement. More specifically, resistor R 15  provides a sense resistor that develops a signal having a relation to the current at the tank. Operational amplifier U 3 -B and resistors R 13  and R 14  provide an amplifier that provides an amplified signal to the microcontroller U 2  at pin P 1 . 1 . Accordingly, resistor R 15  and the amplifier form a current sensor  205 . However, other current sensors can be used in place of the sensor just described. 
         [0024]    With the removal of the voltage, the potential at the electrode  145  drops to a potential that is offset from, but proportional to, the open circuit or “natural potential” of the electrode  145  relative to the tank  105 . A voltage proportional to the natural potential is applied to a filter consisting of resistor R 6  and capacitor C 4 . The filtered signal is applied to operational amplifier U 3 -A, which acts as a voltage follower. The output of operational amplifier U 3 -A is applied to a voltage limiter (resistor R 17  and zener diode D 3 ) and a voltage divider (resistor R 18  and R 20 ). The output is a signal having a relation to the natural potential of the electrode assembly  145 , which is applied to microcontroller U 2  at pin P 1 . 0 . Accordingly, the just-described filter, voltage follower, voltage limiter, and voltage divider form a voltage sensor  210 . However, other voltage sensors can be used in place of the disclosed voltage sensor. 
         [0025]    The control circuit  200  controls the voltage applied to the electrode wire  150 . As will be discussed below, the control circuit  200  also measures tank protection levels, adapts to changing water conductivity conditions, and adapts to electrode potential drift in high conductivity water. In addition, when the control circuit  200  for the electrode assembly  145  is combined or in communication with the control circuit for the heating element  140 , the resulting control circuit can take advantage of the interaction to provide additional control of the water heater. 
         [0026]      FIG. 4  provides one method of controlling the electrode assembly  145 . Before proceeding to  FIG. 4 , it should be understood that the order of steps disclosed could vary. Furthermore, additional steps can be added to the control sequence and not all of the steps may be required. During normal operation, voltage is applied from the control circuit  200  to the electrode assembly  145 . Periodically (e.g., every 100 ms), an interrupt occurs and the control circuit enters the control loop shown in  FIG. 4 . 
         [0027]    With reference to  FIG. 4 , the control circuit  200  disables the voltage applied to the electrode assembly  145  (block  220 ). After disabling the voltage, the control circuit  200  performs a delay (block  225 ), such as 250 μs, and determines an electrode potential (block  230 ). The control circuit  200  performs the delay to allow the electrode assembly  145  to relax to its open circuit. The microcontroller U 1  then acquires this potential from the voltage sensor  210 . The control circuit  200  then reapplies the voltage to the electrode assembly  145  (block  240 ). At block  240 , the control circuit  200  determines whether the electrode potential is greater than a target potential. If the electrode potential is greater than the target potential, the control circuit proceeds to block  245 ; otherwise the control proceeds to block  250 . 
         [0028]    At block  245 , the control circuit  200  determines whether the applied voltage is at a minimum value. If the applied voltage is at the minimum, the control circuit  200  proceeds to block  255 ; otherwise the control circuit  200  proceeds to block  260 . At block  260 , the control circuit decreases the applied voltage. 
         [0029]    At block  250 , the control circuit  200  determines whether the applied voltage is at a maximum value. If the applied voltage is at the maximum, the control circuit  200  proceeds to block  255 ; otherwise the control circuit proceeds to block  265 . At block  265 , the control circuit  200  increases the applied voltage. By decreasing or increasing the applied voltage at block  260  or  265 , respectively, the control circuit  200  can indirectly adjust the electrode potential. Increasing the applied voltage will result in an increase in the tank potential measured by the electrode and decreasing the applied voltage will decrease the tank potential measured by the electrode. Therefore, the control circuit  200  can adjust the open circuit potential of the electrode until it reaches the target potential. Furthermore, as the characteristics of the water heater  100  change, the control circuit  200  can adjust the voltage applied to the electrode to have the open circuit potential of the electrode equal the target point potential. 
         [0030]    At block  255 , the control circuit acquires an electrode current. More specifically, the microcontroller U 1  receives a signal that represents a sensed current from the current sensor  205 . At block  270 , the control circuit determines a conductivity state of the water. For example, the conductivity state can be either a high conductivity for the water or a low conductivity for the water. To determine the conductivity state (either high or low), the microcontroller U 1  divides the applied current by an incremental voltage, which is equal to the applied voltage minus the open circuit potential. If the resultant is less than an empirically set value, then the control circuit  200  determines the conductivity state is low and sets the target potential to a first value; otherwise the control circuit sets the target potential to a second value indicating a high conductivity state (block  275 ). The control circuit  200  can repeatedly perform the conductivity test during each interrupt (as shown in  FIG. 4 ), periodically perform the conductivity test at a greater interval than the setting of the electrode voltage, or perform the conductivity test only during a startup sequence. Additionally, while only two set points are shown, it is envisioned that multiple set points can be used. It is also envisioned that other methods can be used to determine the conductivity state of the water. For example, a ratio of the applied current divided by the applied voltage can be used to determine the conductivity state. 
         [0031]    In addition to establishing a set point, the control circuit  200  can use the acquired current to determine whether the water heater  100  is in a dry-fire state. The term “dry fire” refers to the activation of a water heater that is not storing a proper amount of water. Activation of a heating element (e.g., an electric resistance heating element or a gas burner) of a water heater in a dry-fire state may result in damage to the water heater. For example, if water is not properly surrounding the electric resistance heating element  140 , then the electric resistance heating element may burnout in less than a minute when voltage is applied to the heating element  140 . Therefore, it is beneficial to reduce the likelihood of activating the heating element  140  if the water heater  100  is in a dry-fire state. If the acquired current is less than a minimum value (e.g., essentially zero), then it is assumed that the water heater  100  is not storing the proper amount of water and the control circuit  200  prevents the activation of the heating element  140 . It is also envisioned that other methods for determining a dry-fire state can be used. For example, the control circuit  200  can be designed in such a fashion that the electrode potential will be approximately equal to the applied voltage under dry fire conditions. 
         [0032]    Thus, the invention provides, among other things, a new and useful water heater and method of controlling a water heater. Various features and advantages of the invention are set forth in the following claims.