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A comparative analysis of feedback and feedforward controllers for an industrial scale heat exchanger

Kallergis Emmanouil

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Summary

This thesis tackles the problem of dynamic modeling of a shell-and-tube heat exchanger as well as developing different strategies for its control. In essence, this study relates to control systems theory, a field within mathematics and engineering that focuses on analyzing the behavior of dynamic systems through the use of various controllers. It is widely known that most systems follow a non-linear pattern, making them difficult to interpret. Thus, the need of linearization arises. By differentiating the input and output with one differential equation each mathematical representation such as the Taylor Series, the Laplace Transformation and the State-Space model achieve a similar behavior (with minor deviations) . Doing so valuable information are drawn for the system such as its process transfer function. To develop practical designs for a variety of controllers a range of appropriate methodologies is applied. Those are the Ziegler–Nichols, the Tyreus-Luyben and the Morari-Zafiriou for a feedback control layout, while for a feedforward control layout one control system approach was introduced. Simulations and a comparative analysis followed in order to determine the best among these controllers for the heat exchanger. A total of seven controllers were used in the simulation scenarios. In more detail, the three controllers P, PI, PID by Ziegler–Nichols, the two controllers PI, PID by Tyreus-Luyben, the Morari-Zafiriou (model-based) and the feedforward controller were tuned and developed. Their comparison was performed under three different scenarios i) a steady reference signal, ii) a variable reference signal and iii) a steady reference signal with two disturbances (temperature of the inlet hot and cold streams). The results drawn from the simulations conclude that the most suitable controller for the heat exchanger is the one by Morari-Zafiriou methodology (model-based). It is the most robust among the seven, giving near zero error values while reaching the predetermined set point in minimal time. Right after follows the PID of the Ziegler–Nichols methodology as the second most appropriate for the control of the exchanger.

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