This paper shows that the main geometric features of a ¯ow component can be deduced from the thermodynamic optimization of the global performance of the largest ¯ow system that incorporates the component. This approach represents a departure from the usual approach, where a ¯ow component is optimized in isolation. The example chosen is the counter¯ow heat exchanger of the environmental control system (ECS) used on modern aircraft. The heat exchanger is ®tted with a diuser and a nozzle for the ram air, and the ECS runs on the boot strap air cycle, employing an additional compressor and turbine. Two heat transfer surface types are considered, ®nned and smooth parallel plates. Numerical results are reported for the external geometric aspect ratios of the heat exchanger, and for the plate-to-plate spacing of the smooth-plates model. It is shown that the optimized geometry for the core with ®nned surfaces is nearly the same as the optimized geometry for the core with smooth plates. Several of the optimized geometric features are robust with respect to changes in external parameters that vary from one application to the next. The method illustrated in this paper ± the thermodynamic (constructal) optimization of ¯ow geometry ± is applicable to any system that runs on the basis of a limited amount of fuel (exergy) installed onboard, e.g., automobiles, ships, portable tools.