Page 42 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Example 2.2

Find the dryness fraction, specific volume and specific enthalpy of
steam at 8 bar and specific internal energy 2450 kJ/kg.

Solution example 2.2

An extract from the steam tables,
p ts vg uf ug hf hfg hg sf sfg sg

8 170.4 0.2403 720 2577 721 2048 2769 2.046 4.617 6.663

At 8 bar, ug = 2577 kJ/kg, since the actual specific internal energy is given as 2450
kJ/kg, the steam must be in the wet steam state ( u < ug).

From equation 2.5,

u = uf + x(ug -uf)

2450 = 720 + x(2577 - 720)
 x = 0.932

From equation 2.2,

v = xvg
= 0.932 (0.2403)

3
= 0.2240 m /kg

From equation 2.4,

h = hf + xhfg
= 721 + 0.932 (2048)

= 2629.7 kJ/kg

P
bar x = 0.932

8

o
ts = 170.4 C

3
v m /kg
v vg
33 | P a g e