Electric resistance heating devices – Heating devices – Continuous flow type fluid heater
Reexamination Certificate
1999-06-16
2001-06-12
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Continuous flow type fluid heater
C219S497000, C219S483000, C392S466000
Reexamination Certificate
active
06246831
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the controls for a flow-through heating apparatus for the instantaneous heating of fluids, and more particularly relates to an improved control which may include an optimized power modulation algorithm, a method for low power standby control, a method of extending the operating life of heating circuit electromechanical relays, a redundant fail-safe circuit, a self-diagnostic capability with audible-visual annunciator, and an improved method of sensing water level.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 3,909,588 issued to Walker et al. discloses an electric fluid heater using electrodes immersed in an electrically-insulated flow-through tank with controls sensing both fluid temperature and heating electrode current.
U.S. Pat. No. 4,337,388 issued to July discloses a rapid-response water heating and delivery system incorporating water heating means, water temperature sensing means, and proportional integral derivative (PID) method of closed loop control.
U.S. Pat. No. 4,638,147 issued to Dytch et al. discloses a microprocessor controlled flow-through water heater regulating heating power by switching combinations of heating elements of different wattages.
U.S. Pat. No. 4,829,159 issued to Braun et al. discloses a method of switching electrical heating elements loads to reduce switching transients by energizing all loads neither switched off nor full on in sequence.
U.S. Pat. No. 4,920,252 issued to Yoshino discloses a temperature control method for a plurality of heating elements by allocating a required actuating time within one cycle of a predetermined length of time.
U.S. Pat. No. 5,216,743 issued to Seitz discloses a thermoplastic heat exchanger for a flow-through instantaneous fluid heater including a control system using temperature comparisons.
U.S. Pat. No. 5,479,558 issued to White, Jr. et al. discloses a flow-through tankless water heater with a flow-responsive control means.
U.S. Pat. No. 5,504,306 issued to Russell et al. discloses a tankless water heater system incorporating a microprocessor based control sensing water outlet temperature, accepting an option remote temperature-setting means and providing control of heating elements by applying power in fractions of a power line cycle.
U.S. Pat. No. 5,866,880 issued to Seitz et al. discloses using a plurality of heating elements wherein each of the elements receives a substantially equal amount of power and the delay between each element being powered is no more than 32 half cycles.
Electric flow-through fluid heaters, which are often described as electric tankless fluid heaters, heat fluids as they pass through the heat exchanger. The objective of such heaters is to heat fluid as it enters and passes through the heat exchanger to the desired setpoint by the time it is dispensed at the outlet of the heater. In concept, this process is relatively simple to achieve in closed loop systems in which the operating parameters for flow and temperature can be predetermined.
In this type of application, control of the heater may theoretically be accomplished with standard Proportional Derivative Control (PID) algorithms. Additionally, since many applications are commercial or industrial, one is conventionally not limited by the lack of availability of sufficient electrical service for these applications.
In residential water heating, however, one is presented with an entirely different set of conditions. The process of heating fluids in a flow-through heater can be quite dynamic and requires very responsive and precise control of temperature—not only for the user's comfort but also for safety. Over the years, many efforts have been made to design the “perfect” residential “flow-through” or “tankless” water heater. These efforts have been plagued with a myriad of problems relating to the use of conventional flow detection devices that were unreliable and often failed early due to exposure to highly diverse and aggressive water conditions. A number of other problems arise in fluid heaters as described hereinbelow.
Problem 1. The standard method of temperature control in a fluid heater attempts to regulate the output temperature of the fluid, Tout, based on a reference temperature. In many cases, the reference input is a constant temperature called the setpoint, Tsp. In a single-input (inlet fluid at temperature Tin), single-output (outlet fluid) system, the heater attempts to maintain the temperature of the output fluid equal to the setpoint, or Tout=Tsp. In a heater application, it is assumed that Tin is less than Tout. Flow measurement can provide “feed forward” information to facilitate control. Conventional flow measurement devices such as turbines are expensive and can be adversely affected by water conditions. The classical feedback method of achieving this regulation is to first measure the outlet temperature and compute an error as the difference ERROR=(Tsp−Tout). Using some control scheme, such as PID (Proportional Integral Derivative) the system attempts to reduce the error to zero so that ERROR=0 for steady-state fluid flow.
Several problems plague standard control schemes when they are applied for temperature control of fluid in a tank. First, there is a delay between the application of heat and the sensing of a change in temperature in the fluid. Secondly, such systems are typically very sensitive to changes in the components of the system or the presence of noise in the measurements of temperature, causing errors in the measurement of the temperatures that are used by the system. For example, if accurately calibrated thermistors are used initially to measure temperatures, they can change characteristics with age causing the measurements to be in error. Finally, because of system lag time, these systems are generally not effective in controlling the outlet fluid temperature in the presence of disturbances, such as a rapid change in fluid flow rate.
Problem 2. The Seitz et al U.S. Pat. No. 5,216,743 addressed the use of temperature differential to detect flow
o flow conditions and developed a means for not only detecting these conditions but also monitoring temperature gradients by periodically heating the water to maintain low-energy use in standby conditions. A major drawback of Seitz et al's teaching is in the selection of temperature sensing devices, i.e. thermistors. Seitz et al refer to the use of commodity-type thermistors which inherently vary greatly in their resistance one to another and therefore the resulting temperature measurements vary. The result of this variation impacts the temperature measurements which are used to provide responsive control of shutdown of power at “no flow,” as well as start up “in flow,” and the ability to establish the small temperature gradients necessary for maintaining the standby condition.
Even when the fluid temperature is the same in the heater, the resulting temperature reading obtained from these types of thermistors can differ one to another, for example, by as much as 5 degrees Fahrenheit or more. In most tankless water heaters, differential temperatures measured between two thermistors are used for temperature regulation. Because of the variations in characteristics, such as resistance versus temperature, between one thermistor and another, an inaccurate difference in temperature readings generally exists. In order to compensate for these variations, the control parameters usually include temperature thresholds for temperature measurements. These temperature thresholds are often required to be much greater than desirable. These higher thresholds, coupled with the difference in characteristics between the thermistors, result in widely different performance between one fluid heater and another. In a heater where the deviation in accurate temperature measurement between thermistors is small, the shutdown is more precise and faster, thereby reducing temperature overshoot at shutdown. The response to flow conditions is also shorter, thereby avoiding an obvious dela
Everett Louis J.
Harman Thomas Lamson
Neumann Rodney H.
Seitz David E.
Sharp David Paul
Bushman Browning
Dahbour Fadi H.
David Seitz
Walberg Teresa
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