Question

Problem 1 In large steam power plants, the feedwater is frequently heated in closed feedwater heaters (basically heat exchangers) by steam extracted from the turbine at some stage. Steam enters the feedwater heater at 1 MPa and 200°C and leaves as saturated liquid at the same pressure. Feed water enters the heater at 2.5 MPa and 50°C and leaves at 10°C below the exit temperature of the steam. Neglecting any heat losses from the outer surfaces of the heater, determine (a) the ratio of the mass flow rates of the extracted steam and the feedwater heater and (b) the total entropy change for this process per unit mass of the feedwater

Answer 1

- Ques) Ques) IMPa steam 2000 llo water - 2.5 MPa At PiziMPa and Ti=200°c from superheated water table ; h, = 2828.3 KJ/KG 0,= 6-6956 KJ/KGK At P = 2 MPa from saturated water table ; b = hy = 762.51 KJ/KG To = Tsat = 179.88°C S2=* = 2.1381 KJl RgK As saturated temp (at P3=2.5 MPa) is greater than the water inlet temp, At Tz=50c from saturated Liquid Condition) hy = hq = 209. 34 KI/kg Az = $f= 0.7038 kJ/kg Temp. at exit of water = T= T2-10= 179:88-10 = 169.88€

i saturated temp Cat Pq = 2.5 MPa) is greater than the water inlet temp; 80 at T=169.88°c from saturated liquid table ; ha=ht=719.08 kJ/kg 4= f = 2:0417 Kiligk. mi= m = m3 and Mz = mq=mo (mass conservation) mschyrhi) = mw (13-14) mg (762.51-2828.3)= mw 1209.34 -719.08) mon = 0.2468 Entropy generation Sgen = m (b-81) +mw ( 84-83) ms (42-81) +(84-83) - mw +0.2468(2.1381-66956]2.0417-0.7038) I spent = 0.2131 KJ/kg kl of feed water