**4. Conclusions**

This paper develops an Extended Grid Diagram named SRTGD-STR by considering the shifted temperature range of heat exchangers for HEN retrofit design. It inherits the advantage of SRTGD for HEN information (topology and stream data) illustration and easy retrofit options identification. The extended method also considers the types and capital costs of heat exchangers, which makes the retrofit application more practical. Six types of widely used heat exchangers are considered, and their shifted working temperature ranges are coupled in the diagram.

By using the novel SRTGD-STR, the retrofit plan can be easily determined. It can provide insight into the identification of the Process Pinch and Network Pinch. The combination of the shifted temperature ranges of different types of heat exchangers and the grid diagram can help engineers to identify potential HEN retrofit plans with the consideration of heat exchanger types and illustrate these plans visually. The illustrative example and case study show the advantages of using this tool in the retrofit process. The case study shows that an extra 10.7% of heat can be recovered by adding additional heat exchangers for the studied HEN. In addition, seven solutions can be identified by the proposed SRTGD-STR. The easy identification of these solutions could help the designers compare the retrofit plans and make a wiser decision. The correct selection of heat exchanger types can help to achieve a relatively lower capital cost.

Future research should further consider other issues such as phase change, freezing risk, and material cost in the retrofit design to extend the application field of this tool. Moreover, this approach can be further developed by considering the uncertainty temperature variation margins to make the retrofit plan adapt to a wider range of working conditions. Although the SRTGD-STR could identify retrofit plans according to the insight of thermodynamics, there still exists some potential to minimise the sum of the energy cost and investment cost by a trade-off of these two costs.

**Author Contributions:** Conceptualization, B.W. and J.J.K.; Methodology, B.W., J.J.K. and P.S.V.; Software, B.W.; Investigation, B.W.; Writing-Original Draft Preparation, B.W.; Writing-Review & Editing, J.J.K., P.S.V. and M.Z.; Visualization, B.W.; Supervision, J.J.K.; Project Administration, J.J.K. and M.Z.; Funding Acquisition, J.J.K. and M.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [Czech Ministry of Education, Youth and Sports] grant number [LTACH19033], [National Key Research and Development Program of China] grant number [2018YFE0108900], and [Czech Republic Operational Programme Research and Development, Education] grant number [No. CZ.02.1.01/0.0/0.0/15\_003/0000456]" under collaboration agreement with with Xi'an Jiaotong University, China and by the project LTACH19033 "Transmission Enhancement and Energy Optimised Integration of Heat Exchangers in Petrochemical Industry Waste Heat Utilisation", under the bilateral collaboration of the Czech Republic and the People's Republic of China.

**Acknowledgments:** The project LTACH19033 "Transmission Enhancement and Energy Optimised Integration of Heat Exchangers in Petrochemical Industry Waste Heat Utilisation", under the bilateral collaboration of the Czech Republic and the People's Republic of China (partners Xi'an Jiaotong University and Sinopec Research Institute Shanghai; SPIL VUT, Brno University of Technology and EVECO sro, Brno), programme INTER-EXCELLENCE, INTER-ACTION of the Czech Ministry of Education, Youth and Sports; and by the National Key Research and Development Program of China (2018YFE0108900) and the EU-supported project Sustainable Process Integration Laboratory (SPIL), funded as project No. CZ.02.1.01/0.0/0.0/15\_003/0000456, by Czech Republic Operational Programme Research and Development, Education, Priority 1: Strengthening the capacity for quality research in the collaboration agreement with Xi'an Jiaotong University, China, based on the SPIL project, are gratefully acknowledged.

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