A piston cylinder has 2.5 kg ammonia at -20°C, 50 kPa. It is heated to 50°C at constant pressure from external hot gas at 200°C. Find the total entropy generation.
Answer: A
v1 = 2.4463 m^3/kg, h1 = 1434.6 kJ/kg, s1 = 6.3187 kJ/kg K
v2 = 3.1435 m^3/kg, h2 = 1583.5 kJ/kg, s2 = 6.8379 kJ/kg K
1Q2 = m(h2 – h1) = 2.5 (1583.5 – 1434.6) = 372.25 kJ
1S2(gen) = m(s2 – s1) – 1Q2/T(gas)
= 2.5 (6.8379 – 6.3187) – 372.25/473.15 = 0.511 kJ/K.