Use of advanced composite curves for assessing cost-effective HEN retrofit II. Case studies

In this paper a graphical method for cost-effective heat exchanger network (HEN) retrofit is applied in two cases. The method uses information about the placement of heaters/coolers in the existing HEN to identify the potential for cost-effective retrofit. In Case 1 detailed matrix calculations are compared with the results from the graphical method, and in Case 2 the graphical method is compared with earlier published results.In Case 1, the retrofit investment cost for a certain heat recovery was more than doubled for a HEN with heaters placed at high temperature, in comparison to a HEN with heaters placed at low temperature, although both HENs have the same stream data and utility requirements. In a HEN with heaters placed at both low and high temperatures, the heaters placed low could be detached as cheaply as the case with all heaters at low temperatures. The heaters at high temperatures were cheaper to detach than in the case with all heaters at high temperatures, since the initial changes to heaters at low temperatures keep the total investment cost down.Case 2 demonstrates how the graphical method is used to identify potential heat recovery. The conclusions from this method were compared with detailed calculations. Results showed that the PBP increased by a factor of four, when heat saving increased from the level suggested by the advanced curves to the maximum possible heat recovery.