Advanced and Optimizing Controls

Override controls are used to protect against fouling of the heat transfer surfaces, when the water outlet temperature exceeds 50°C (122°F) or to prevent 

In heat exchanger applications, cascade loops are configured so that the master detects the process temperature and the slave detects a variable, such as steam pressure, that may upset the process temperature. The cascade loop, responds immediately and corrects for the effect of the upset before it can influence the process temperature. The cascade master adjusts the set point of the slave controller to assist in achieving this. Therefore, the slave must be much faster than the master. A rule of thumb is that the time constant of the primary controller should be ten times that of the secondary, or the period of oscillation of the primary should be three times that of the secondary. One of the quickest (and therefore best) cascade slaves is the simple and inexpensive pressure regulator. 

The feedback portion of the loop has to do much less work in this configuration, as it only has to correct for minor load variables, such as heat losses to the atmosphere, steam enthalpy variations, and sensor errors. The feedback and feedforward portions of the loop complement each other. The feedforward portion is responsive, fast, and sophisticated, but inaccurate. The feedback portion is slower, but is capable of correcting the upsets caused by unknown or poorly understood load variations, and it is accurate. 

Similarly to cascade control, load upsets or supply disturbances are corrected by feedforward control before they can upset the main variable, and therefore contribute to stable operation and to fast recovery from upsets. In addition to feedforward, more sophisticated model-based strategies are also available, as discussed in Sections 2.6.16 through 2.6.18. 

Post-Oil Energy Technology After the Age of Fossil Fuels 

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