**5. Conclusions**

In this paper, a novel framework for the efficient design of fired equipment has been introduced. The traditional thermal-hydraulic calculation of the radiant and convection section (i.e., the first phase of the calculation procedure) was supplemented by using low-cost modelling systems that take into account the real (non-uniform) distribution of heat flux and process fluids. Designing the combustion chamber was done using the AMPF model [7], the problematic heat exchanger in the convection section was analysed using the heat and fluid flow distribution modelling system [10] presented at the 21s<sup>t</sup> Conference on Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction PRES 2018. The application of these low-cost models (the second phase of calculations in the proposed framework) has been demonstrated in the industrial steam boiler case.

In the context of simplified methods, this framework is a logical outcome of the long-term development that has taken place at the Institute of Process Engineering of the Faculty of Mechanical Engineering, Brno University of Technology. First, the individual simplified models were unified for the analysis of thermal behaviour and flow distribution in combustion chambers and heat exchangers. Second, the presented framework links calculations of radiant and convection sections and offers a systematic approach to designing and fast rating calculations of complex fired equipment. It should be emphasized that the low-cost modelling procedure does not replace CFD analyses neither for purposes of troubleshooting nor for the final detailed design of the equipment. On the contrary, the described procedure enables detailed CFD analyses to concentrate effectively on critical locations identified using low-cost models.

Future work will focus on further refinement of 2D heat distribution calculations in the convection section.

**Author Contributions:** Conceptualization, Z.J.; methodology, Z.J. and D.F.; writing—original draft, D.F.; writing—review and editing, Z.J.; visualization, D.F.; supervision, Z.J.

**Funding:** This research was supported by the EU project Strategic Partnership for Environmental Technologies and Energy Production, funded as project No. CZ.02.1.01/0.0/0.0/16\_026/0008413 by Czech Republic Operational Programme Research, Development and Education, Priority Axis 1: Strengthening capacity for high-quality research.

**Acknowledgments:** Thanks to M. Na' who provided the authors with CDF data. Also, the authors wish to acknowledge the consultations offered by V. Turek.

**Conflicts of Interest:** The authors declare no conflict of interest.
