Optimum thermal design of modular compact heat exchangers structure for heat recovery steam generators

The paper analyses the use of a particular kind of compact heat exchanger (CHE), a plate-fin heat exchanger (PFHE), conceptually based upon the cross-flow heat exchangers. The use of this compact heat exchanger can be proposed to obtain basic modular element for the construction of small size heat recovery steam generators (HRSG), in order to achieve efficiency increase and reduction of size by means of various packaging solutions. With reference to the use of such particular HRSG structure, a method for the optimum thermal design of the HRSG and of each single section is outlined. The optimization is structured in two main levels: the higher level enables to define the main operating parameters of the HRSG, while the second involves the detailed design of the PFHE heat transfer surfaces. The attention is mainly focused on the second optimization level, which is articulated in two steps. The objective of the first step is the minimization of the pressure drop for a given heat flow. At the second step the minimization of the weight, maintaining the imposed HRSG performance in term of heat flow and pressure drop, is performed. The method is tested with reference to a basic 1.5 MW double pressure HRSG structure demonstrating the possibility of obtaining interesting increase of the performance with respect to a usual design method.