| 摘要 |
A method and apparatus for controlling combustion in a process heater provides for efficient control of the combustion with a minimum of components. The apparatus includes means for generating a signal from a condition of a process, such as temperature, or pressure in the case of a boiler. The signal, which may be a pressure signal, is presented in parallel to a needle valve, to a first fluid-responsive relay, and to a second fluid-responsive relay. A volume tank is connected in series to the needle valve and in parallel to the first relay and to the second relay. The first relay communicates with means for controlling the flow of combustion air. The second relay communicates with means for controlling the flow of fuel. When the signal of the process condition is below the point at which a controller is set, then the signal is an increasing pressure signal. When the signal of the process condition is above the point at which the controller is set, then the signal is a decreasing pressure signal. The first relay responds to a higher of two signals to which it is subjected, and the second relay responds to a lower of two signals to which it is subjected. In either case, an increasing pressure signal or a decreasing pressure signal, the arrangement of the needle valve and volume tank causes a delay in the response of either the first or second relay so that a fuel-lean mixture is maintained during any change in the operating conditions of the process. It is readily seen, however, that opposite responsiveness of the process condition control and fuel and air control devices could be utilized and the same effectiveness of a fuel-lean mixture still be maintained, the important responsiveness being a delay in transmitting flow control signals.
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