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Heat recovery steam generator (HRSG) is a critically important subsystem of a combined cycle. The global objective of a HRSG is to heat the stream of water. The HRSG is com-posed of three major sections, including an economizer, an evaporator, and a super-heater. In this study, a water tube HRSG is considered and its main design features are deduced from the minimization of the entropy generation by using the constructal theory. Entropy generation is obtained by considering all irreversibilities associated with the processes. Considering the minimum total entropy generation as the objective function, the optimum parameters in the HRSG unit are derived by using the genetic algorithm method under the fixed total volume condition. In the present work, the number and arrangement of the tubes, the optimal diameters of tubes and spacing between adjacent tubes for three main sections, total length, width, and height of the HRSG unit and the water flow rate are significant features of the flow configuration inducted by the constructal design. Furthermore, the effect of changing in the size of the flow system on the flow archi-tecture is determined. [ABSTRACT FROM AUTHOR]