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1000 kg of steam at a pressure of 16 bar and 0.9 dry is generated by a boiler per hour. The steam passes through a superheater via boiler stop valve where its temperature is raised to 380°C. If the temperature of feed water is 30°C, determine :(i) The total heat supplied to feed water per hour to produce wet steam. (ii) The total heat absorbed per hour in the superheater. Take specific heat for superheated steam as 2.2 kJ/kg K. |
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Answer» Mass of steam generated, m = 1000 kg/h Pressure of steam, p = 16 bar Dryness fraction, x = 0.9 Temperature of superheated steam, Tsup = 380 + 273 = 653 K Temperature of feed water = 30°C Specific heat of superheated steam, cps = 2.2 kJ/kg K. At 16 bar. From steam tables, ts = 201.4°C (Ts = 201.4 + 273 = 474.4 K) ; hf = 858.6 kJ/kg ; hfg = 1933.2 kJ/kg (i) Heat supplied to feed water per hour to produce wet steam is given by : H = m [(hf + xhfg) – 1 × 4.18 × (30 – 0)] = 1000 [(858.6 + 0.9 × 1933.2) – 4.18 × 30] = 1000(858.6 + 1739.88 – 125.4) = 2473.08 × 103 kJ. (ii) Heat absorbed by superheater per hour = m[(1 – x) hfg + cps (Tsup – Ts)] = 1000[(1 – 0.9) × 1933.2 + 2.2 (653 – 474.4)] = 1000(193.32 + 392.92) = 586.24 × 103 kJ. |
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