In a heat exchanger network, typically 20-30 % energy saving, coupled with capital saving, can be realized by state-of-the-art heat exchanger network design. A basic feature of heat exchanger networks is the tradeoff between energy and capital cost. Due to several network alternatives which are usually available, this tradeoff has been regarded as complex. In a network the value of the minimum temperature difference(ΔTmin) determines the maximum heat recovery and thus the minimum heating and cooling loads. As ΔTmin increases demand for utilities increases but total heat exchange area decreases. The total annual cost is minimized for some value of ΔTmin. Confronted with this complex behaviour, designers generally consider several network alternatives and use continuous optimization on each structure. These continuous optimization can be difficult to perform in practice. The result is that there is often no certainty of having obtained the global optimum cost in the final design structure. The problem with this approach is that the minimum capital requirements for the network are not clearly known.
A new methodology for the synthesis of heat exchanger network has been developed. This method is based on the work by Linnhoff and Ahmad which determines the optimum minimum temperature difference (ΔTmin) based on the energy and capital cost target before the synthesis of the heat exchanger network. The proposed method uses the Guthrie's model for capital cost calculation, controls the driving force between hot and cold streams, and handles the match constraints for the intangibles of design such as safety, layout and etc. The software system developed in this study was applied to the Alko process, which is an alcohol production process, for the synthesis of heat exchanger network. It was possible to save about 12% of the total annual cost.
