Process Control: Modeling, Design, and Simulation

12.3. Selective and Override Control

Often a controller needs to select from a set of possible measurements for control. For example, consider the packed-bed reactor shown in Figure 12-2. It is important to maintain the “hot spot” temperature at a certain value to minimize catalyst degradation. In this strategy, the “high selector” (represented as the block with >) chooses the highest of three bed temperatures to send to the temperature controller. In algorithmic form, let T₁, T₂, and T₃ represent the three temperature measurements, and T represent the temperature measurement used by the controller, then

Figure 12-2. High-temperature selector for a packed-bed reactor.

Equation 12.5

When manipulating the fuel gas flow to a fired heater (furnace), ...