Modern Engineering Thermodynamics

430 CHAPTER 12: Mixtures of Gases and Vapors
EXAMPLE 12.13 (Continued )
From the point where the lines ω = 65.8 grains/(lbm dry air) = constant and h # = 26 Btu/(lbm dry air) = constant intersect
on the psychrometric chart, we can read from this chart that
T DB = 63°F, T WB = 59°F, ϕ = 75%, and T DP = 56°F
Exercises
33. If the 2000. ft 3 /min of air entering station 1 in Example 12.13 is at a dry bulb temperature of 70.0°F instead of 50.0°F
with all the remaining parameters unchanged, determine its new mass flow rate. Answer: _m a1 =147 lbm dry air/min.
34. If the 1000. ft 3 /min of air entering station 2 in Example 12.13 is at a relative humidity of 10.0% rather than 40.0% with
all the remaining parameters unchanged, determine its new mass flow rate. Answer: _m a2 = 70:5 lbm dry air/min.
35. Determine the dry bulb temperature and relative the humidity of the outlet mixture in Example 12.13 if the volume
flow rate at station 2 is increased from 1000. ft 3 /min to 2000. ft 3 /min and all the remaining variables remain
unchanged. Answer: T DB = 74°F and ϕ = 60.%.
9 The examples and problems throughout this text have been written such that at least three significant figures appear in the variables. Unfortunately, small
textbook charts can often be read to only two significant figures; however, psychrometric computer programs can be found on the Internet that provide much
more accuracy and are recommended for student use.
12.9 MIXTURES OF REAL GASES
If the components of an ideal gas mixture interact in any way or one or more of the gases is not ideal, the
resulting mixture is not ideal and does not obey the Gibbs-Dalton and Amagat laws. Then, its partial properties
must be determined from accurate pressure, volume, temperature, and specific heat data by the techniques discussed
in the previous chapter.
Though Amagat’s law, Eqs. (12.16) and (12.19), may not hold for a mixture of real gases, the definition of partial
specific volumes, Eqs. (12.5) and (12.6), is always valid. The difference is that, for real gases,
^v i = ð∂V m /∂m i Þ pm, T m,m j
≠ v i = R i T m /p m
For a binary mixture of gases A and B, ^v A and ^v B can be determined at any composition from experimental data
of v m vs. w A , as shown in Figure 12.11.
v A
Amagat’s law
v A
v m
Mixture of real
gases A and B
v B
Tangent
at ω * A
Composition
of interest
ω * A
v B
0 1
FIGURE 12.11
Determining ^v A and ^v B at w A = w A from real gas data by the method of tangents.
ω A