Heated water outlet
Fig. 8.23. Feedforward control of a heat-exchange process (Hall et al., 1974).
Fig. 8.21. Cascade control of a heat-exchange process (Hall et al., 1974).
individual set-point and only the secondary controller providing an output to the process. In more complex cascade systems more loops may be included.
When cascade control is used to control water temperature in a heat exchanger process (Fig. 8.21) there is the primary loop (slow or outer loop), consisting of the temperature sensor and the primary temperature controller of the process water temperature, and the secondary loop (fast or inner loop) consisting of the steam flow sensor, the steam flow controller, its process variable (steam flow), the control valve and the process. The addition of the secondary controller, whose measured variable is steam flow, allows steam flow variations to be corrected immediately before they can affect the hot water temperature. This extra control loop should help to minimize temperature cycling.
Feed-forward control (Fig. 8.22) makes it possible to utilize other disturbances besides the measured values of the process that have to be controlled and enable fast control of a process. The cascade control system (Fig. 8.21) which has just been discussed would be adequate provided that the water inlet pressure and temperature do not fluctuate. When fluctuations occur, feed-forward control may be used, but this will require the measurement of both inlet water temperature and flow rate with both variables combined to control the steam flow rate (Fig. 8.23). In the example shown (Hall et al., 1974) a computer is programmed to determine the heat energy required to be added to the cool inlet water to bring it to the appropriate temperature at the outlet and allow the correct amount of steam to enter the heating coils. Inputs for temperature and flow are made from the inlet pipe to the computer. Calculations can then be made to determine the required heat input to raise the water temperature in the heat exchanger. The output control signal to the valve A is computed to allow the correct amount of steam to enter the heating coils.
When one or more of the process variables or characteristics is not known and cannot be measured directly, then adaptive control should be considered. The on-line identification of process characteristics and the subsequent use of this information to improve the process constitute adaptive control (Hall et al., 1974). The sequences and the interactions in the adaptive control loop are outlined in Fig. 8.24. Adaptive control is useful in circumstances where the process dynamics are not well defined or change with time. This may be most useful in controlling a batch fermentation where considerable and often complex changes may occur (Bull, 1985; Dusseljee and Feijen, 1990). Adaptive control strategies have been used in fed-batch yeast cultivation (Montague et al., 1988), amino acid production (Radjai et al., 1984) and penicillin production (Lorenz et al., 1985).
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