Pressure drop considerations with heat transfer enhancement in heat exchanger network retrofit

The success of heat exchanger network (HEN) retrofit not only lies in the amount of energy savings that can be obtained but also being able to comply with any pressure drop constraints imposed on the network. The techniques used for heat transfer enhancement increase the performance of the heat exchanger but at a penalty of increased pressure drop. In order for a retrofit design with heat transfer enhancement to be realistic, pressure drop must be considered. The effects of heat transfer enhancement on pressure drop can be mitigated by applying different techniques. For shell and tube heat exchangers, the decision on the best technique to apply is dependent on the side that is constrained i.e. shell side or tube side. The variety of options available for pressure drop mitigation can make the process more complex. To effectively solve the problem, a step wise approach for the application of heat transfer enhancement with pressure drop consideration is proposed. The first step is the identification of the heat exchangers that bring about pressure drop violation when enhanced. Pressure drop mitigation techniques can then be applied. Considering there might be multiple mitigation techniques that can be used for a given heat exchanger, a ranking criterion has been defined for the identification of the best modification required. After mitigation techniques have been applied to candidate heat exchangers and the network is no longer constrained by pressure drop, a simple nonlinear optimisation based model is used, together with the new geometry of modified heat exchangers to apply heat transfer enhancement. The objective of the non-linear optimisation model is to maximise the retrofit profit i.e. the difference between profit from energy savings and the total cost of retrofit subject to a specified payback operating period. This is achieved by varying the overall heat transfer coefficient of candidate exchangers and fixing the heat transfer area of existing exchangers to maintain the target temperature of all streams in the network. Heat transfer enhancement, even with pressure drop considerations remains an attractive option for HEN retrofit as the costs associated with changing the network topology are eliminated, and it requires reduced time for implementation.