Reference Manual

7−2
Figure 7-1. Temperature enthalpy diagram for
water. Note that the greatest amount of thermal
energy input is used to vaporize the water.
Maximum efficiency in heat transfer requires
operation at near saturation temperature to recover
this energy.
E0117
300
200
100
32
0
0 500 1000
Btu added to 1 pound of water
Temperataure, def F
1/2 Btu per degree
Ice heating at about
144 Btu to melt ice
Water heating at 1 Btu per degree
Atmospheric pressure
Evaporation at 14.7 psi
Evaporation at more than 14.7 psi
All data for 1 lb. water
which water cannot
exist as a liquid
These lines curve and meet at
705.4 deg F the critical
temperature, above
Steam superheating at about 0.4 Btu per degree
212 deg F
970 Btu to
boil water
In the lower left portion of the graph, the water is
frozen at atmospheric pressure and below 32°F.
At this point, heat is being rejected from the water
as it maintains its solid state. As heat is gradually
added the ice begins to change. Addition of heat
to the ice raises the temperature and slows the
rate of heat rejection. It requires approximately
1/2 BTU of thermal energy to be added to a pound
of ice to raise its temperature 1°F. Upon reaching
32°F, the addition of more heat does not
immediately result in an increase in temperature.
Additional heat at this point begins to melt the ice
and results in a transformation of state from a
solid to a liquid. A total of 144 BTUs is required to
melt one pound of ice and change it to water at
32°F.
Once the phase change from a solid to a liquid is
complete, the addition of more heat energy to the
water will again raise its temperature. One BTU of
heat is required to raise the temperature of one
pound of water by 1°F. This relationship remains
proportionate until the boiling point (212°F) is
reached. At this point, the further addition of heat
energy will not increase the temperature of the
water. This is called the saturated liquid stage.
Figure 7-2. Temperature enthalpy diagram for
water showing that saturation temperature varies
with pressure. By choosing an appropriate
pressure, both correct system temperature and
thermal efficiency can be accommodated.
Temperataure, def F
T-H DIAGRAM
WATER
LIQUID
800 PSIA
14.7 PSIA
LIQUID-VAPOR
VAPOR
ENTHALPY, BTU/LBM
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The water begins once again to change state, in
this case from water to steam. The complete
evaporation of the water requires the addition of
970 BTUs per pound. This is referred to as the
latent heat of vaporization, and is different at each
individual pressure level. During the vaporization
process the liquid and vapor states co-exist at
constant temperature and pressure. Once all the
water, or liquid phase, has been eliminated we
now have one pound of steam at 212°F. This is
called the saturated vapor stage. The addition of
further thermal energy to the steam will now again
increase the temperature. This process is known
as superheating. To superheat one pound of
steam 1°F requires the addition of approximately
0.4 BTUs of thermal energy.
The potential thermal energy release resulting
from a steam temperature change differs
significantly depending on temperature and
superheat condition. It is much more efficient, on
a mass basis, to cool by addition of ice rather than
by the addition of cold fluids. Similarly, it is more
efficient to heat with steam at temperatures near
the saturation temperature rather than in the
superheated region. In the saturated region much
more heat is liberated per degree of temperature
change than in the superheated range because