Optimum Design of Heat Exchanger Networks in Retrofit Situations

Doktorsavhandling, 1996

Today, the most important activity in heat exchanger network synthesis is the retrofitting, rather than grass-root design, of networks. There is a vast potential for energy improvement in many processes by increasing the level of internal heat exchange. Other common reasons for heat exchanger network retrofits is that modifications are made within a process whitch affect the structure of its existing network. These changes can be connected with the need for an increased thoughtput, the exploation of electricity production by gas turbines, and integration of heat pumps. Regardless of why the network is modified, it is important that the retrofitted network is based on the best possible design in terms of energy. The aim of this project has been to develop a user-driven metod for optimal retrofitting of heat exchanger networks, with whitch all aspects relevant in a retrofit design situation can be taken into account. No such metod exists today. The following parameters have been considered: heat exchanger area, the types of heat exchangers, material requirements, physical distance between streams, annual pressure drop costs, auxiliary equipment (valves), space requirements, and annual maintenance costs (cleaning). This approach, termed the matrix method for network retrofits, uses pinch thechnology to identify whitch heat exchangers waste energy. Rearrangement of all heat exchangers that waste energy usually results in a retrofitted network with an excessive number of new and extended units. Therefore, an approach has been developed to identify which parts of the existing network to rearrange. It is also possible to find designs whith the fewest necessary modifications to the existing network at a certain level of the recovery. Apart from the above mentioned parameters, optimum design routines have been included. With these the optimum heat transfer area and preassure drop of the heat exchangers, and optimum diameter and drop of the piping, can be found. Various analytical expressions for calculating the optimum design of a single heat exchanger have been derived for different design situations, based on both restricted and unrestricted pressure drops, and taking free available pressure drops into consideration.