Modern Engineering Thermodynamics

440 CHAPTER 12: Mixtures of Gases and Vapors
where the mixture specific heats are given by
c vm
= ∑ N
i=1
w i^c vi and c pm = ∑ N
6. The study of the special mixture of ideal gases of dry air and water vapor is called psychrometrics, and the two
important composition measures used in the field of meteorology are
a. The relative humidity ϕ of the mixture, defined as
ϕ = p w
p sat
where p w is the actual partial pressure of the water vapor in the mixture and p sat is the saturation pressure
of the water vapor at the temperature of the mixture.
b. The humidity ratio ω of the mixture, defined as
ω = m w
m a
where m w is the mass of water vapor present in the mixture and m a is the mass of dry air present in the
mixture. These two composition measures are related by
ϕp sat
ω = 0:622
p m − ϕp sat
7. The dew point temperature T DP is defined as
T DP = T sat ðevaluated at p w Þ
where p w is the partial pressure of the water vapor present, p w = ϕp sat .
8. The humidity ratio at the inlet to an adiabatic saturator is given by
ω 1 = c paðT 2 − T 1 Þ + ω 3 ðh fg2 Þ
h g1 − h f 2
9. The psychrometric enthalpy h # of the mixture is defined as
i=1
w i^c pi
10. For mixtures of real gases, we can use
a. The Dalton compressibility factor, Z Dm ,
where
h # = H m
m a
= h a + ωh w
p m V m = Z Dm m m R m T m
b. the Amagat compressibility factor, Z Am ,
where
Z Dm = ∑ N
i=1
w i M m
Z Di = ∑ N
χ
M i Z Di
i
i=1
p m V Am = Z Am m m R m T m
c. the Kay compressibility factor, Z Km ,
Z Am = ∑ N
ðw i M m /M i ÞZ Ai = ∑ N
x i Z Ai
i=1
i=1
p m Vm = Z Km m m R m T m
where Z Km is found from the pseudocritical pressure and temperature of the mixture
p cm = ∑ N
x i p ci and T cm = ∑ N
x i T ci
i=1
i=1