Modern Engineering Thermodynamics

68 CHAPTER 3: Thermodynamic Properties
Table 3.3 Triple Point Data for Various Materials
Substance T (R) T (K) p (psia) p (kPa)
Ammonia (NH 3 ) 351.7 195.4 0.89 6.16
Carbon dioxide (CO 2 ) 389.9 216.6 75.98 523.8
Helium-4 (λ point) 3.9 2.17 0.74 5.11
Hydrogen (H 2 ) 24.9 13.84 1.03 7.13
Neon (Ne) 44.2 24.57 6.35 43.77
Nitrogen (N 2 ) 113.7 63.18 1.84 12.67
Oxygen (O 2 ) 97.8 54.36 0.02 0.15
Sulfur dioxide (SO 2 ) 355.9 197.7 0.02 0.17
Water (H 2 O) 491.7 273.16 0.09 0.62
Source: Adapted by permission of the publisher from Zemansky, M. W., Abbott, M. M., Van Ness, H. C., 1975. Basic Engineering
Thermodynamics, second ed. McGraw-Hill, New York.
300,000
100,000
Additional solid
ice phases
VI
VII
50,000
30,000
II
IV
III
V
Compressible liquid
Supercritical fluid
10,000
Pressure (psia)
3203.8
1000
100
10
0.089
0.01
Solid
(Ice (I))
Solid−vapor
equilibrium
Liquid
Liqid−vapor
equilibrium
Triple point
Superheated
vapor
Gas
Critical point
0.002
491.69
Temperature (R)
1165.1
FIGURE 3.8
Phase diagram for water (not drawn to scale).
shows a more complete p-T phase diagram for water including 7 of its 15 known solid phases. Each intersection
of three phase transition lines forms a new triple point.
The vaporization curve for all known substances has a peak at a curious point, known as the critical point. This is
the state at which the densities of the liquid and the vapor phases become equal and, consequently, where the
physical interface between the liquid and the vapor disappears. At or above the critical state, there is no longer any
physical difference between a liquid and a vapor. Substances existing under these conditions are called gases. In
this text, we use the term gas to describe the state of any substance whose temperature is greater than its critical
state temperature. A substance in the vapor phase that does not meet the definition of a gas is called a superheated
vapor (sometimes just vapor). These definitions are illustrated in Figures 3.8 and 3.10. Table 3.4 gives the critical
state temperature, pressure, and specific volume for various common pure substances. A larger critical state data
table is given in Table C.12 of Thermodynamic Tables to accompany Modern Engineering Thermodynamics.
Notice in Table 3.4 that, at 14.7 psia and 70.°F (530. R), ammonia is a vapor (T c > 530. R). Also, it should be clear
from Figure 3.10 that, to liquefy any gas whose pressure is initially less than its critical pressure simply by increasing
its pressure alone, the gas must first be made into a vapor by lowering its temperature below its critical temperature.
Vapor-liquid condensation is shown by process A-B in Figure 3.10. Thus, for example, no matter how high the
applied pressure, hydrogen cannot be liquefied unless its temperature is below 59.9 R (see Table 3.4).