Page 35 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

The volume of the liquid is usually negligibly small as compared to the volume of dry

saturated steam. Hence, for most practical problems,

v = xvg (2.2)

Where,

3
vf = specific volume of saturated liquid (m /kg)
3
vg = specific volume of saturated steam (m /kg)
x = dryness fraction

Specific enthalpy

In the analysis of certain types of processes, particularly in power generation and
refrigeration, we frequently encounter the combination of properties

U + PV. For the sake of simplicity and convenience, this combination is defined as a new

property, enthalpy, and given the symbol H.

H = U + PV (kJ)
or, per unit mass

h = u + Pv (kJ/kg) (2.3)

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

enthalpy of the dry steam,

i.e. h = hf(1 – x) + xhg

h = hf + x(hg – hf )
 h = hf + xhfg 2.4)

Where,
hf = specific enthalpy of saturated liquid (kJ/kg)

hg = specific enthalpy of saturated steam (kJ/kg)

hfg = difference between hg and hf (that is, hfg = hg - hf )

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