Universal Decimal Classification - English full edition - Heat engines in general - Steam power - Steam engines - Boilers - Water power - Hydraulic energy
Under 621.1 is classed information about the generation and technical applications of steam, and also about heat engines in general, matters common to 621.1, 621.4 and 621.5 being classed under 621.1.
Under 621.22 is classed information about the technical applications of hydraulic energy and water power.