Civil Engineering Reference
In-Depth Information
Thus:
enthalpy of saturated steam
¼
h
i
þ
h
v
If saturation occurs at atmospheric pressure, the total amount of heat added to
convert water at 273.15 K (0
C; 32
F) to 100 % steam is equal to:
6
kJ
1, 149
Btu
lb
enthalpy of saturated steam
¼
418
:
6
þ
2, 252
¼
2, 670
:
kg
¼
When the mixture of liquid and vapor is balanced, the liquid phase is a
saturated liquid and the vapor phase is a saturated vapor.
This mixture can be classified by the quality index
x
which is the ratio between
the mass of vapor present in the mixture and the total mass of the mixture itself.
The value of the quality index
x
ranges from zero to unity: at saturated liquid
state
x
¼
0, and at saturated vapor state
x
¼
1. The quality index is frequently
expressed as a percentage.
For saturated steam, typical values of enthalpy range from 2,670
to 2,770 kJ/kg (from 1,148 to 1,191 Btu/lb).
• Superheating.
When the water is converted into 100 % steam, the temperature will rise if heat is
added. The enthalpy of the steam increases by the amount of added heat, which
can be calculated by multiplying the specific heat of the steam (average value at
typical operating pressures is 2.1 kJ/kg
K, see Table
2.5
) by the temperature
increase above the boiling point.
For a given pressure the temperature depends on the heat added which is
called the enthalpy of superheating (
h
s
). At a given pressure, any temperature
above the saturation value can be reached.
Thus:
enthalpy of superheated steam
¼
h
i
þ
h
v
þ
h
s
• Condensation.
During this phase, which is the opposite of the superheating and vaporization
phases, vapor is transformed into liquid water by the extraction of heat. At a
given pressure, the superheated steam temperature decreases until the saturation
temperature is reached; then, liquid water and steam remain at constant temper-
ature until 100 % liquid water is produced. The temperature depends on the
pressure.
Notice that the heat to extract is equal to the enthalpy of the evaporation phase
at the same pressure and temperature.
Figures
6.1
and
6.2
and Tables
6.3
and
6.4
show the steam properties which
are generally obtainable from a Mollier chart.
