Modern Engineering Thermodynamics

11.10 Compressibility Factor and Generalized Charts 391
Nondimensionalizing this equation with T = T c T R and p = p c p R and rearranging it gives
ðh* − hÞ/T c = R
Z pR
0
ðT 2 R /p RÞð∂Z/∂T R Þ pR
dp R
This equation still depends on the substance under consideration, because the value of R is substance dependent.
This dependence can be removed by multiplying both sides of this equation by the molecular mass M (in
lbm/lbmoles or kg/kgmole) and thus converting it into molar units:
h* − h Z pR
/T c = R
0
TR 2 /p R
ð ∂Z/∂TR Þ pR
dp R
where h* = h* M is the ideal gas molar enthalpy, h = hM is the real substance molar enthalpy, and R = RM is
the universal gas constant. Using compressibility factor data, this equation has been integrated and the results
are shown in Figure 11.9.
65
0.50
60
0.55
, kJ/(kgmole . K)
h*–h
T c
Enthalpy correction,
55
50
45
40
35
30
25
20
15
0.75
Saturated liquid
0.80
0.85 0.90
0.92
0.94
0.96
0.98
T R
0.60
0.70
0.65
0.75
0.80
0.85
0.90
0.94 0.92
0.96
0.98
1.00
1.02 1.04
1.06
1.08
1.10
1.15
1.20
1.25
1.30
1.40
1.50
1.60
10
5
0
Saturated gas
0.90
0.95
1.00
1.10 1.20
1.30
1.50
2.00
1.70 1.80
1.90 2.00
2.20 2.40
2.60 2.80
3.00
4.00
−5
0.1 0.2 0.3 0.4 0.5 1.0 2.0 3.0 4.0 5.0 10 20 30
Reduced pressure, p R = p/p c
FIGURE 11.9
Generalized chart for enthalpy correction. Note: To convert this figure to English units, use the factor 1 Btu/(lbmole · R) = 4.1865 kJ/
(kgmole·K). (Source: Van Wylen, G. J., Sonntag, R. E., 1973. Fundamentals of Classical Thermodynamics. Wiley, New York. Copyright ©
John Wiley & Sons. Reprinted by permission of John Wiley & Sons.)