Question

A steam power plant operates on an ideal reheat Rankine cycle between the pressure limits of 5 MPa and 100 kPa. The temperature of the steam att the turbine is 500 C degree and the mass flow rate of steam through the cycle is 35 kg/s.

a) Determine the thermal efficiency of the cycle.

b) determine the net power output of the power plant ( Assume both the turbine and the pump have isentropic efficiency of 100%).

c) Draw the cycle on the T-S diagram.

Answer 1

From stream table At 50 bar 500 degree c Rightarrow h (enthlopy) = 34335 kg/mg Therefore h_4 = 3433.5 KJ/mg h_5 = (h_g)_1 bar = 2675 Kj/kg h_1 = (h_f)_1 bar = 417.4 KJ/kg workdone by turbane = h_4-h_5 workdone by pump = h_2-h_1 = v_dp [v = specific volume d_p = change in pressure] [pump workdone will be very small as sp volume liquid is very less comp to turbine work so generally we neglect pump work Rightarrow h_2-h_1 = 0 Rightarrow h_2 = h_1] But here well consider pump work = v d_p = (1.286 times 10^-6) (4900 kpa) = 1.286 times 10^-3 times 4900 times = 6.3 kj/kg Therefore h_2-h_1 = 6.3 kj/kg h_2 = 423.7 KJ/kg W_turbane = h_4-h_5 = 3433.5-2675 = 758.5 KJ/kg net power output = W_turbine -W_pump = 7585-6.3 = 752.2 kj/kg thermal efficiency = Net power output/Heat supplied = 752.2/h_4-h_2 = 752.2/3433.5-423.7 eta_thermal = 0.249 almostequalto 25 %