Dynamic modelling and optimal sizing of industrial fire-tube boilers for various demand profiles

In the present paper, a detailed dynamic model of an industrial fire-tube boiler is first developed and five different geometrical configurations, each of which corresponds to a boiler model, are considered. Next, a PID controller is implemented and tuned for each configuration aiming at controlling the steam pressure, while addressing a demand with a variable flow rate. The operation of the developed boiler models, while providing four different steam demand profiles, are next simulated. The resulting cumulative average efficiency along with the cumulative pressure deviations and minimum and maximum pressure levels, which are achieved in each simulation, are then determined. The obtained results provide practical information regarding the trade-off between the size of the boiler and its corresponding performance and controllability. As an instance, the obtained results demonstrated that utilizing a boiler with the heat transfer surface of 36.76 m2 instead of one with the corresponding surface of 56.55 m2, in the worst-case scenario, leads to less than 2% of reduction in the efficiency and a negligible increment in the amplitude of pressure deviations. However, the former boiler is considerably smaller than the latter one and the mentioned choice can result in a significant saving in the required initial investment. Detailed information regarding the resulting pressure deviations has also been provided in order to facilitate verifying the consistency of the variations in each boiler's supplied steam pressure with the corresponding acceptable range specified by the customer. Therefore, the provided results offer useful insights about the possible saving opportunities for small-medium scale industries, specifically in Italy, which are commonly employing oversized boilers with an On/Off control systems.