Page 36 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Specific Internal Energy

Similarly, the specific internal energy of a wet steam is given by the internal energy of the

liquid plus the internal energy of the dry steam,

i.e. u = uf(1 – x) + xug
 u = uf + x(ug – uf ) (2.5)

Where,

uf = specific enthalpy of saturated liquid (kJ/kg)
ug = specific enthalpy of saturated steam (kJ/kg)

ug – uf = difference between ug and uf

Equation 8.5 can be expressed in a form similar to equation 8.4. However, equation 8.5

is more convenient since ug and uf are tabulated. The difference is that, ufg is not
tabulated.

Specific Entropy
A person looking at the steam tables carefully will notice two new properties i.e. enthalpy

h and entropy s. Entropy is a property associated with the Second Law of

Thermodynamics, and actually, we will properly define it in Unit 9 . However, it is

appropriate to introduce entropy at this point.

The entropy of wet steam is given by the sum of the entropy of the liquid plus the entropy

of the dry steam,

i.e. s = sf(1 – x) + xsg
s = sf + x(sg – sf )

 s = sf + xsfg (2.6)

Where,

sf = specific enthalpy of saturated liquid (kJ/kg K)
sg = specific enthalpy of saturated steam (kJ/kg K)

sfg = difference between sg and sf (that is, sfg = sg - sf )

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