A New Graphical Approach for Heat Exchanger Network Retrofit Considering Capital and Utility Costs

Abstract

Development of methodologies and tools for heat exchanger network (HEN) retrofit has focused on generating retrofit solutions to address issues of vital importance to industry including energy, economy and sustainability. This work extends the use of the individual stream temperature versus enthalpy plot (STEP) to enable users to select economically feasible retrofit alternative based on capital and utility costs estimation. A new graph known as the heat exchanger area versus enthalpy plot (A vs H plot) is proposed in this work to be used in combination with the STEP diagram to enable visualisation of the area of every heat exchanger in the HEN. Note that, both graphs share the same enthalpy axis. As a result, heat exchange between the stream pairs shown in the STEP diagram can be projected to the A vs H plot. The A vs H plot contains several useful information of the existing HEN which includes the total area and the number of heat exchanger in the HEN. Firstly, retrofit options are generated by using STEP diagram based on the procedure described in a prior work. Users can later use the A vs H plot to select retrofit option which requires the minimum total heat exchanger area or to minimise the total heat exchanger area needed for the retrofit design generated based on the retrofit methodology which is introduced in this work. Capital cost needed for the retrofit options can be estimated based on total area of new heat exchangers added. Application of the combination of A vs H plot with STEP diagram on a literature case study shows that the combined graphical tools are able to provide clear visual insights to assist users identify capital-energy trade off in generating cost-effective retrofit solutions. The retrofit design and area of the individual heat exchangers in the retrofitted HEN are clearly shown in the combined graphical tools to enable users to screen the most economically viable retrofit option prior to performing economic analysis. Application of the new method on a case study shows that the annualised total cost required by the retrofit design after area minimisation is lower than that of the retrofit design before area minimisation by 3.34 %.