Modern Engineering Thermodynamics

3.8 Quality 73
10 5 300 C
Critical point
10 4
200 C
100 C
50 C
300 C
Super heated
vapor
10 3
Compressed
liquid
200 C
1300 C
1000 C
Pressure, p, kPa
10 2
100 C
Liquid plus vapor
600 C
400 C
300 C
200 C
10
Saturated
liquid
line
50 C
Saturated
vapor
line
100 C
1.0
Saturated
solid
line
Triple point line 0.01 C
Solid plus vapor
10 −1 10 −4 10 −3 10 −2 10 −1 1.0 10 10 2 10 3 10 4
Specific volume, v, m 3 /kg
(a)
FIGURE 3.15a
p-v diagram notation. The actual p-v diagram for water plotted on log-log coordinates. (Source: Wood, B. D., 1982. Applications of Thermodynamics,
second ed. Addison-Wesley Publishing Co., Inc., Reading, MA. Reprinted with permission.)
volume of saturated liquid is written as v f and that of
saturated vapor as v g , and the associated specific internal
energies, enthalpies, and masses are written as u f ,
u g , h f , h g , m f , and m g .
From Figure 3.16, we see that the total volume of a
substance whose state is in the wet (liquid plus vapor)
region is given by
V = mv = m f v f + m g v g
where m is the total mass given by
Pressure, p
Compressed or subcooled
liquid region
Critical point
Saturated
liquid line
x = 0
Liquid plus
vapor (wet)
region, 0 < x < 1
Saturated vapor line, x = 1
Superheated vapor
region
m = m f + m g
Dividing the equation for V by m gives
V/m = v = m f v f /m + m g v g /m (3.21)
Specific volume, v
(b)
FIGURE 3.15b
Schematic p-v diagram for the liquid, mixture, and vapor regions.
WHY USE F AND G SUBSCRIPTS?
The use of f and g as subscripts here is out of tradition. They come from the first letters of the German words flussig (for
liquid) and gas. More appropriate English subscripts might be l for liquid and v for vapor, but they are not used.