Feedwater heaters are a shell & tube type heat exchanger designed to preheat the boiler feedwater. The bleed-off steam from the high pressure turbine is on the shell side which heats the feed water on the tube side. The heating process by means of extraction steam is referred to as being regenerative.
The feedwater heaters are an integral portion of the power plant thermodynamic cycle. The low pressure feedwater heaters are placed between the Condenser and deaerator and the high pressure feedwater heaters are placed between the deaerator and steam generator.
The number of low pressure and high pressure feedwater heaters is determined from the cycle optimization, as there are multiple stages of feed water heating. Each stage corresponds to a turbine extraction point. These extraction points occur at various stages of the expansion of steam through the turbines. The presence of the feedwater heaters in the cycle enhances the thermal efficiency of the power plant; the greater the number of extraction stages, the lower the amount of thermal energy required to generate a given amount of electrical energy. A beneficial by-product of the energy extracted by the heaters is the reduced rate of rejection of energy to the environment.