This paper shows that the architecture of a steam generator for a power plant can be deduced on the basis of the constructal law. According to constructal theory, the flow architecture emerges in time such that it provides progressively greater access to its currents. The circulation of water is driven by buoyancy in one large tube (the downcomer) and many parallel smaller tubes (the riser). The total flow volume is fixed. Two flow models are used: single phase liquid in the downcomer and riser, and liquid-vapor mixture in the riser tubes. Features that result from constructal design are: the tube diameters, the number of riser tubes, the water circulation rate. the rate of steam production, and how the flow architecture should change when the operating pressure and the size of the flow system change. (C) 2008 Elsevier Ltd. All rights reserved.