Extraction type turbines are common in all applications. In some applications, when required, steam can be extracted from turbine before steam flowing through the last stage, named extraction turbine. As in back-pressure turbines, extracted steam can be used for many industrial processes or it can be used to improve the efficiency of thermodynamic cycle. The second case is usually known as the heat regeneration.
Almost all large steam turbines use the heat regeneration (i.e. they are extraction turbines), since it reduces the amount of fuel that must be added in the boiler. The reduction in the heat added can be done by transferring heat (partially expanded steam) from certain sections of the steam turbine, which is normally well above the ambient temperature, to the feedwater. Note that, most of energy contained in the steam is in the form of latent heat of vaporization. Extraction flows may be controlled with a valve, or left uncontrolled.
For example, most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine with 3 or 4 self-regulating extraction lines and three parallel multi-stage LP turbines with 3 or 4 self-regulating extraction lines.
The high pressure feedwater heaters are usually heated by extraction steam from the high pressure turbine, HP, whereas the low-pressure feedwater heaters are usually heated by extraction steam from the low pressure turbine, LP. Both are usually self-regulating. It means that the greater the flow of feedwater the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
Schema of a steam turbine of a typical 3000MWth PWR.