4.2.2 Feedheating plant stages — wet steam cycle
A PWR station is now under construction in the UK, based upon a standard design for large capacity plant (1200, 1300 MW). This design, designated 'Standard Nuclear Unit Power Plant System' has a feedheating plant involving LP heaters, a de-aerator/heater and HP heaters but with certain variations arising from the wet steam cycle of the PWR.
Steam admitted to the HP cylinder of the turbine is at a dry saturated condition, i.e., without superheat. On expansion through the cylinder, the steam becomes wet. The water fraction is removed in a passive 'moisture separator' and the water and its heat is reclaimed and pumped into the feedheating system at the de-aerator (where separator water temperature and de-aerator temperature coincide) for maximum efficiency (Fig. 1.65). The dried steam is then passed through two-stage reheaters for superheating, steam being the heating medium. Steam bled from the HP cylinder is used in the primary reheater and live steam (i.e., at boiler conditions) is used in the secondary reheater. Condensate formed from the reheating steam is recovered and introduced into the HP feedwater heaters at appropriate temperature points in the cycle.
The PWR steam generators have a continuous 'blowdown' system to limit the concentration of slight impurities in the steam generators. The 'blowdown' water, at a temperature approximating to boiler saturation temperature, is passed through a heat exchanger incorporated in the LP feedheating plant upstream of the de-aerator/heater, where the heat is recovered.
Condensate, formed from bled-steam extracted from the LP turbines and supplied to LP feedwater heaters, is collected in a drains receiver and pumped into the de-aerator/heater tank for recovery of both condensate and heat.
The cycle involves four stages of LP feedheating, the de-aerator/heater and three stages of HP feed-heating, with an ultimate heater utilising condensate from the secondary reheater to preheat the feed to a temperature of 226°C before admission to the steam generators.