STEP—A new graphical tool for simultaneous targeting and design of a heat exchanger network

For 40 years, composite curves (CCs) and grid diagram (GD) have been among the most popular graphical tools for designing optimal heat exchanger networks (HEN). However, since CCs represent the temperature versus enthalpy plot of composites rather than individual streams, they have some significant limitations. Among others, CCs cannot completely map individual hot and cold process streams, as well as process and utility streams, and cannot be used for HEN design. In addition, CCs cannot be conveniently and effectively used to predict minimum network area and the optimum Δ T min that should strictly be based on parameters and properties of individual as opposed to composite streams. Grid diagram on the other hand requires designers to provide or calculate stream temperatures as well as enthalpies, to do heat balance and to check temperature feasibility during HEN design as the diagram does not follow any temperature or enthalpy scale. This paper presents STEP (Stream Temperature vs. Enthalpy Plot) as a new graphical tool for simultaneous targeting and design of a HEN that overcomes the key limitations of CCs and the GD. The new STEPs are profiles of continuous individual hot and cold streams being mapped on a shifted temperature versus enthalpy diagram that simultaneously show the pinch points, energy targets and the maximum heat allocation (MHA). The MHA is graphically converted to an MER network and represented on a Heat Allocation and Targeting (HEAT) diagram in terms of STEP temperature and enthalpy. This paper also demonstrates that STEP can provide more realistic solutions for targeting multiple utilities and the minimum network area. STEP application on a palm oil refinery, and finally, its limitations, are also highlighted. With capabilities to overcome the limitations of CCs and GD, STEP can become a vital alternative graphical tool for optimal HEN design.