Steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Also, because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator, about 86% of all electric generation in the world is by use of steam turbines.