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The robustness of a thermal process control System has been examined. For a PID (Proportional plus Integrating plus Derivating) controller, ZN (Ziegler-Nichols), AB (A Brambilla) and IMC (Internal Model Control) tuning rules were used and the effects of simultaneous variation in large domains of the process parameters on the closed-loop performance were evaluated. It was proved that one of the difficulties with the popular ZN method is its inability to provide suitable tuning for process with large time delay. Simulation studies showed that if the controller is optimally tuned then large modelling errors or changes in process conditions are required for the closed-loop performance to become unstable, particularly if the tuning rule is AB or IMC method. Also it was been found that, for uncertainty in all process parameters, the sensitivity of the closed-loop performance as measured by ITAE (Integral Time-multiplied Absolute Error) or ISE (Integral Square Error) is low, especially if the controller is tuned according to AB or IMC methods. Finally, a PID controller tuned with AB or IMC techniques has been found to lead to a robust stable loop for temperature control loop of a heat exchanger with large dead time and time constant.