This paper presents an effective quasi one-dimensional (1-D) computational simulation methodology for steady annular/stratified internal condensing flows of pure vapor. In-channel and in-tube flows are considered for a range of gravity component values in the direction of the flow. For these flows, three sets of results are presented and they are obtained from: (i) a full 2-D CFD based approach, (ii) the quasi-1D approach introduced here, and (iii) relevant experimental results for gravity driven condensing flows of FC-72. Besides demonstrating differences between shear and gravity driven annular flows, the paper also presents a map that distinguishes shear driven, gravity driven, and “mixed” driven flows within the non-dimensional parameter space that govern these duct flows. The paper also demonstrates that μm-scale hydraulic diameter ducts typically experience shear/pressure driven flows.