A New Method To Determine The Optimum Heat Exchanger Network Approach Temperature

Abstract

Supertargeting is a widely used procedure to determine the optimum approach temperature (∆T min ) for heat exchanger networks (HEN). It involves plotting the sum of HEN capital and operating costs, and finding the minimum total cost at an optimum ∆T min . The capital and operating costs are determined from the heat transfer area and the minimum utility targets obtained from the temperature (T) vs enthalpy (H) plot of composited process streams (composite curves (CC)). Supertargeting using CC has two key limitations. Firstly, the HEN area calculations are drastically simplified through the assumption that CC segments are pseudo-single hot and cold streams exchanging heat via only one heat exchanger that is governed by a single cost correlation. Secondly, the current Supertargeting approach of considering only one hot and one cold utility levels at highest and lowest temperatures respectively is simplistic at best since a plant typically uses multiple levels of utilities. These limitations may lead to a very crude estimation of the total HEN cost and the corresponding ∆T min . This work presents the Stream Temperature vs. Enthalpy Plot Supertargeting (STEPS) method that overcomes the aforementioned ∆T min targeting limitations. Multiple utility levels, various heat exchanger types and streams individual heat transfer coefficients can be considered using STEPS. Application of STEPS on two case studies shows that the conventional supertargeting method can lead to up to 50% error in the total cost target and poor ∆T min estimation.