Question

What amount of heat would be required to produce 4.4 kg of steam at a pressure of 6 bar and temperature of 250°C from water at 30°C ? Take specific heat for superheated steam as 2.2 kJ/kg K.

Answer 1

Mass of steam to be produced, m = 4.4 kg 

Pressure of steam, p = 6 bar 

Temperature of steam, t_sup = 250°C 

Temperature of water = 30°C 

Specific heat of steam, c_ps = 2.2 kJ/kg 

At 6 bar, 250°C : From steam tables, 

t_s = 158.8°C, h_f = 670.4 kJ/kg, h_fg = 2085 kJ/kg 

Enthalpy of 1 kg superheated steam reckoned from 0°C, 

h_sup = h_f + h_fg + cps (T_sup – T_s ) 

= 670.4 + 2085 + 2.2(250 – 158.8) 

= 2956 kJ 

Amount of heat already with 1 kg of water 

= 1 × 4.18 × (30 – 0) = 125.4 kJ 

Net amount of heat required to be supplied per kg 

= 2956 – 125.4 = 2830.6 kJ 

Total amount of heat required 

= 4.4 × 2830.6 = 12454.6 kJ.