Inner diam. of the "church window" condenser tube was optimized by minimization entropy generation during heat transfer. The pressure drop on the cooling water side was taken into account. Calcns. were performed for 2 relations of flow resistance and for 4 different values of the condenser tube roughness. The anal. gave a slightly lower inner diam. of the tube (20 mm) than the actual value (22 mm). The optimum diam. increased with increasing the tube surface roughness.