Turbomachines are those in which energy is transferred to or from a continuously moving fluid by the motion of a rotating element. Some of the most common and important machines used in engineering and industry fall into this category, including those which absorb power (and thereby increase fluid pressure), such as fans, compressors and pumps. Also included are equally vital varieties of machines which generate power from the expansion of fluids to lower pressures such as hydraulic, steam and gas turbines.