Question

Determine the amount of heat, which should be supplied to 2 kg of water at 25°C to convert it into steam at 5 bar and 0.9 dry.

Answer 1

Mass of water to be converted to steam, m_w = 2 kg

Temperature of water, t_w = 25°C

Pressure and dryness fraction of steam = 5 bar, 0.9 dry

At 5 bar : From steam tables

h_f = 640.1 kJ/kg ; h_fg = 2107.4 kJ/kg

Enthalpy of 1 kg of steam (above 0°C)

h = h_f + xh_fg 

= 640.1 + 0.9 × 2107.4 = 2536.76 kJ/kg

Sensible heat associated with 1 kg of water

m_w × c_pw × (t_w – 0) 

= 1 × 4.18 × (25 – 0) = 104.5 kJ

Net quantity of heat to be supplied per kg of water 

= 2536.76 – 104.5 = 2432.26 kJ

Total amount of heat to be supplied 

= 2 × 2432.26 = 4864.52 kJ.