Question

If a certain amount of steam is produced at a pressure of 8 bar and dryness fraction 0.8. Calculate : 

(i) External work done during evaporation. 

(ii) Internal latent heat of steam.

Answer 1

Pressure of steam, p = 8 bar 

Dryness fraction, x = 0.8 

At 8 bar. From steam tables,

v_g = 0.240 m³/kg, h_fg = 2046.5 kJ/kg

(i) External work done during evaporation

= pxv_g = 8 × 10⁵ × 0.8 × 0.24 N-m

= \(\cfrac{8\times10^5\times0.8\times0.24}{10^3}\) = 153.6 kJ.

(ii) Internal latent heat 

= xh_fg – external work done

= 0.8 × 2046.5 – 153.6 

= 1483.6 kJ.