Electric resistance heating devices – Heating devices – Continuous flow type fluid heater
Reexamination Certificate
1999-01-22
2001-01-23
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Continuous flow type fluid heater
C392S480000, C392S485000
Reexamination Certificate
active
06178291
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to fluid heaters, and more particularly to a demand anticipation control system for a high-efficiency, ultra-pure deionized (UPDI) water heater.
Larger wafer sizes, smaller device geometry, and greater circuit density have driven the need for very accurate temperature control of fluids used to produce semiconductors. Heated UPDI water is one such fluid used in the manufacture of semiconductor devices. However, UPDI water is a corrosive liquid. Thus, equipment used for heating UPDI water must be capable of withstanding the corrosive effects of the UPDI water that flows therethrough.
In addition, it is critical that the equipment used to manufacture semiconductor devices be capable of performing specific tasks while not introducing contaminates into the manufacturing process. One such fluid heater that can withstand the corrosive effects of UPDI water and not introduce contaminants into the manufacturing process is described and claimed in U.S. patent application No. 09/006,112 filed Jan. 13, 1998 and titled “High Efficiency Ultra-Pure Fluid Heater”. The referenced U.S. patent application has been assigned to the same Corporation who is to be the Assignee of the present invention, and is hereby incorporated by reference in its entirety.
Fluid heaters conventionally utilize a temperature control system to maintain the desired operating fluid temperature. A commonly available Proportional Integral Derivative (PID) controller is good at maintaining an accurate fluid temperature as long as the load (e.g. fluid flow through the heater) is steady state. To achieve a steady-state fluid flow, a high-flow bypass has been commonly used to allow a steady state flow of UPDI water through a heating system. In this control scheme, UPDI flows at a constant rate, and either is used at the process, or is dumped for possible reclaim.
Alternatively, when fluid demand is low, a fluid heater can be operated in a reduced or low-flow mode (to maintain water purity), and when a high-flow is required, the output flow bypasses the process until the output temperature stabilizes. With the increase in chemical costs, largely due to purity levels and the cost of disposal, these methods are no longer acceptable in the industry. Flow rate changes, and temperature set-point changes for specific process “recipes” are becoming the standard rather than the exception.
Accordingly, it has been considered desirable to develop a new and improved control system for fluid heater which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.
SUMMARY OF THE INVENTION
The Demand Anticipation Control (DAC) system of the present invention provides accurate temperature control over a broad range of loads. This is accomplished by determining the variables that affect the output temperature of the loads, determining the level of power required to achieve the set point temperature, and applying the necessary power to a heater element associated with the fluid heater. In a standard PID-based system, the only variables measured are temperature, and rate of temperature change of the load. The DAC system determines: 1) inlet fluid temperature, 2) outlet fluid temperature, 3) flow rate of fluid heated, 4) power applied to the heater system, and 5) rate of fluid temperature change per unit of time.
In operation, the user of the system inputs the desired operating temperature into a controller via a keyboard or other user interface. The power level required to bring the load to the set point temperature is determined based on input values measured by a plurality of sensors. The power applied to the heater element is continually adjusted in real time.
Inefficiencies of the fluid heater, and inherent errors in measurements taken, can all lead to temperature instability. To overcome these problems, the DAC system has been designed to minimize these effects. Power applied to the heater element is the single largest source of error in an electrically heated system. This is due to the variations in heater resistance with respect to the operating temperature of the heater element. The DAC system measures the power applied to the heater element in real time to remove these errors. Power can be measured/determined Ln a number of ways. One method is to measure the heater element temperature, compare it to a known curve for the type of resistance element used, and calculate the power applied at that temperature. Another method is to measure both voltage and current applied to the heater element and calculate power therefrom. The second method allows for universal heater application, without need for defining the temperature/resistance curve for each heater controlled. Thermal losses also can lead to errors. The DAC system corrects for these losses by evaluating the temperature response of the load to power applied and offsetting power accordingly.
Measuring or calculating all the required variables, and determining power needed provides instant correction to maintain accurate temperature control over any load within the capabilities of the heater system. Even though the system can respond to these changes, thermal lags in the system result in temperature droops when going from a small load to a large load, and conversely temperature over-shoots when going from a large load to a small load. To reduce this effect, the DAC system applies more or less power to the heater element when the rate of change in the fluid temperature up or down exceeds a predetermined range. Additionally, the DAC system boosts, or reduces power to the heater element as a function of fluid temperature difference from the set point.
Thus, in accordance with one aspect of the present invention a control system is provided for a fluid heater. The fluid heater includes a housing having an inlet, an outlet, at least one fluid pathway through the housing, and a heating element proximate the fluid pathway for heating an operating fluid flowing through the pathway. The control system includes an inlet temperature sensor for determining the temperature of the fluid at the inlet, a flow sensor for determining the rate of fluid flow through the fluid pathway, and a switching device connected between a source of electrical power and the heating element. A controller is in communicatton with the temperature sensor, the flow sensor, and the switching device. The controller includes a first mechanism for determining a first power value based on inputs from the inlet temperature sensor and the flow sensor, the first power value being indicative of a power level required to heat the operating fluid to a setpoint temperature. A second mechanism determines a second power value indicative of an actual power level being applied to the heating element, and a third mechanism controls the switching device based on a difference between the first power value and the second power value.
In accordance with a second aspect of the present invention, a method for controlling a fluid heater is disclosed. The fluid heater includes a housing having an inlet and an outlet, at least one fluid pathway through the housing, a heating element proximate the fluid pathway for heating an operating fluid flowing through the pathway, and a switching device connected between a source of electrical power and the heating element. The method includes the steps of a) determining the temperature of the operating fluid at the inlet; b) determining the rate of operating fluid flowing through the fluid pathway; c) determining a first power value based on the temperature of the fluid at the inlet and the rate of operating fluid flowing through the inlet, the first power value being indicative of a power level required to heat the operating fluid to a first temperature setpoint; d) determining a second power value indicative of an actual power level being applied to the heating element; and e) controlling the switching device based on a difference between the first power value and the
Base Howard J.
Zenios Theofani
Dahbour Fadi H.
Fay Sharpe Fagan Minnich & McKee LLP
Lufran Incorporated
Walberg Teresa
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