Feedwater heaters are used in steam power plants to pre-heat water delivered to boilers. They work by using extracted steam from turbine stages to gradually heat feedwater up to saturation temperature. This improves efficiency by reducing costs and preventing thermal shock to boiler metal. Feedwater heaters come in open and closed designs, with open designs mixing extracted steam directly into feedwater and closed using heat exchangers. Their use recovers some energy from steam and optimizes the balance between extracted steam and turbine power output.