Project ForgeIn a steam power plant, superheated steam at 10 MPa and 500°C is expanded isentropically in a turbine until it becomes a saturated vapour. It is then reheated at constant pressure to 500°C, and expanded isentropically in a second turbine until it reaches the condenser pressure of 20 kPa. Using standard superheated/saturated steam tables (h1=3373.6, s1=6.5965 kJ/kg.K at 10MPa/500°C; at 1MPa: hg=2778.1 kJ/kg with sg=6.5965 kJ/kg.K, hf=762.9, sf=2.1386; at 1MPa/500°C: h=3478.4 kJ/kg, s=7.7621 kJ/kg.K; at 20kPa: hf=251.38, hg=2609.7, sf=0.8319, sg=7.9085), the work done by both the turbines together is __________ kJ/kg (round off to the nearest integer).
Explanation
Turbine 1: s2=s1=6.5965 kJ/kg.K, which exactly equals sg at 1 MPa, so state 2 is saturated vapor at 1MPa, h2=2778.1 kJ/kg. W_T1=h1−h2=3373.6−2778.1=595.5 kJ/kg. Reheat to 500°C at 1MPa: h3=3478.4, s3=7.7621 kJ/kg.K. Turbine 2 to 20kPa: x4=(7.7621−0.8319)/(7.9085−0.8319)=0.9793; h4=251.38+0.9793×(2609.7−251.38)≈2560.6 kJ/kg. W_T2=h3−h4≈917.8 kJ/kg. Total = 595.5+917.8≈1513.3 kJ/kg.
Source: GATE ME 2020 Session 2 (ME-2) (2020)
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