The steam engine is a heat engine converting heat energy to work. The conversion is performed by a cycle of processes.
Heat created externally is transfered to water in a steam generating unit or boiler.
The steam leaves the expander and is condensed back to water rejecting some heat energy. This is known as an external combustion engine. These processes must conform to the First Law of Thermodynamics expressed in the General Energy Equation .This cycle is called the Rankine cycle and is the accepted standard of comparison for steam plants today. You may select one of the power plant components maped out below for further details.