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Steam tables consist of two sets of tables of
the energy transfer properties of water and steam saturated
steam tables and superheated steam tables. Portions of the
tables are shown in Figure A-2. Both sets of tables are tabulations of
pressure (P), temperature (T), specific volume
(n), specific enthalpy (h), and
specific entropy (s). The following notation is used in steam tables. Some tables use v
for n (specific volume) because
there is little possibility of confusing
it with velocity.
The saturated steam tables give the energy
transfer properties of saturated water and saturated steam
for temperatures from 32 to 705.47°F (the critical
temperature) and for the corresponding pressure
from 0.08849 to 3208.2 psi. Normally, the saturated steam
tables are divided into two parts:
temperature tables, which list the properties according to
saturation temperature (Tsat);
and pressure tables,
which list them according to saturation pressure (Psat).
Figure A-2 shows a portion
of a typical saturated steam temperature table and a portion
of a typical saturated steam pressure
table. The values of enthalpy and entropy given in these
tables are measured relative to
the properties of saturated liquid at 32°F. Hence, the
enthalpy (hf) of
saturated liquid and the entropy
(sf) of
saturated liquid have values of approximately zero at 32°F. Most practical applications
using the saturated steam tables involve steam-water
mixtures. The key
property of such mixtures is steam quality (x), defined as
the mass of steam present per unit mass
of steam-water mixture, or steam moisture content (y),
defined as the mass of water present per
unit mass of steam-water mixture. The following relationships
exist between the quality of a
liquid-vapor mixture and the specific volumes, enthalpies, or
entropies of both phases and of the
mixture itself. These relationships are used with the
saturated steam tables.
In order to solve problems in Thermodynamics,
information concerning the "state" of the substance studied must be obtained. Usually,
two properties (for example, v, p, T, h, s) of the substance must be known in order to determine
the other needed properties. These other properties are usually obtained utilizing
either the Mollier diagram (if the substance is steam) or the saturated and superheated steam tables,
as shown in the Figures A-1 and A-2.
The following two examples illustrate the use
of the Mollier diagram and the steam tables.
Example 1: Use of Mollier Chart. Superheated
steam at 700 psia and 680°F is expanded at constant entropy
to 140 psia.
What is the
change in enthalpy?
Solution:
Use the Mollier Chart. Locate point 1 at the
intersection of the 700 psia and the 680°F
line. Read h = 1333 Btu/lbm.
Follow the entropy line downward vertically
to the 140 psia line and read h = 1178
Btu/lbm.
h = 1178 - 1333 = -155 Btu/lbm
If the substance is not water vapor, the
"state" of the substance is usually obtained
through the use of T-s
(temperature-entropy) and h-s (enthalpy-entropy) diagrams,
available in most thermodynamics
texts for common substances. The use of such diagrams is
demonstrated by the following
two examples.
Example 3: Use of the h-s diagram Mercury
is used in a nuclear facility. What is the enthalpy of the
mercury if its pressure is
100 psia and its quality is 70%?
Solution:
From the mercury diagram, Figure A-3 of
Appendix A, locate the pressure of 100 psia. Follow
that line until reaching a quality of 70%. The intersection
of the two lines gives an
enthalpy that is equal to h = 115 Btu/lbm.
Example 4: Use of the T-s diagram
Carbon dioxide is used in a particular
process in which the pressure is 100 psia and the temperature
is 100°F. What is the enthalpy value of the gas?
Solution:
From the carbon dioxide diagram, Figure A-4
of Appendix A, locate the pressure of 100 psia.
Follow that line until reaching a temperature of 100°F. The
intersection of the two lines
gives an enthalpy that is equal to h = 316 Btu/lbm. Once
the various states have been fixed for the particular process
the substance has passed through
(for example, going from a saturated liquid state to a
compressed liquid state across a pump),
energy exchanges may be determined as was shown in Example 1.
The energy exchanges are
never 100 percent efficient, as already discussed. The degree
of efficiency obtained by the system
depends upon the process through which the system has passed.
Generally, the efficiency of
a component depends upon how much friction exists in the flow
of the substance, the pressure drops
within the system, the inlet and outlet temperatures, and
various other factors. The properties
affecting the efficiency of the system are determined by use
of the charts and diagrams mentioned
in this section. When
power cycles are utilized for large systems, the efficiency
of each component should be maximized
in order to have the highest possible overall efficiency for
the system. Each component
affects the system efficiency in a different manner. To
maximize efficiency, the practical
approach to large systems is to have multistage expansion
with reheat between stages and
regenerators in the system where applicable.
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