At the feedwater heater, heat energy is transferred from extracted turbine steam (state 6) to the condensed liquid. Heat transfer at the feedwater heater can take place by either direct mixing or by a heat exchanger. By "pre-heating" the working fluid, less energy at the boiler is required to superheat the working fluid, therefore increasing overall efficiency of the system.
The construction of a closed feedwater heaters allow for higher operational temperature and pressures as compared to an open feedwater. The disadvantage is the associated high cost of running a high pressure system. Alternatively, cost can be kept low by using an open feedwater heater.
The following explains and analysis both systems.
The closed feedwater heater acts like a heat exchanger. The turbine steam and condensed liquid flow through the heat exchanger in seperate pipes. Heat transfers from the turbine steam to the condensed liquid. A steam trap ensures that the turbine steam leaving the closed FWH is fully condensed before it is passed to the condenser.
The heat transfer from the turbine steam to the condensed liquid can be determined by using 1st law analysis. With the assumptions steady state steady flow, no work energy transfer, and negliable change of kinetic and potiential energy.
The open feedwater heater is analogous to a mixer. High temperature and pressure turbine steam (state 6) is mixed with the low temperature and pressure condensed liquid (state 2). Heat energy is transfered during the blending of the two flow. There the energy of state 3 is higher than the energy state 2.
State 3 can be calculated by using the conservation of mass and energy.
Conservation of Mass
