14.4. Directional Sensitivity and Operability

In a number of examples in this textbook, we have noted that the design of a process can limit the ability to control the process. For example, if the “gain” for an input-output pairing is not large enough, then there is a limit to how large a setpoint change can be made before the manipulated input saturates (hits a constraint, such as a valve being fully open or closed). For multivariable systems, the effect of a particular input on a particular output is influenced by the other inputs. For example, consider the mixing tank of Example 14.3. A change in the hot-stream flow rate can significantly affect the temperature if the cold-stream flow rate remains constant. If the cold-stream flow rate changes ...

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