Modern Engineering Thermodynamics

CHAPTER 12
Mixtures of Gases and Vapors
CONTENTS
12.1 Wprowadzenie (Introduction). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
12.2 Thermodynamic Properties of Gas Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406
12.3 Mixtures of Ideal Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
12.4 Psychrometrics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
12.5 The Adiabatic Saturator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
12.6 The Sling Psychrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
12.7 Air Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
12.8 Psychrometric Enthalpies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
12.9 Mixtures of Real Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438
12.1 WPROWADZENIE (INTRODUCTION)
In this chapter, we deal with the problem of generating thermodynamic properties for homogenous mixtures of
gases and vapors that are not involved in chemical reactions. Properties of chemically reacting mixtures are
discussed in detail in Chapter 15.
We can define the composition of any mixture based on how we physically create the mixture. For example, we
can create a mixture by combining measured masses (or weights) of things or by combining measured volumes of
things. The items or things that make up the mixture are called the components of the mixture, and knowing
how much of each one is present defines the composition of the mixture.
This all seems very simple, so why do we need to dwell on it here? First of all, there are two ways to measure
mass, the regular mass (lbm or kg) and the molar mass (lbmole or kgmole), and a composition based on the
regular mass is not the same as a composition based on the molar mass. Second, since the conservation of mass
law tells us that the total mass of the mixture is simply the sum of the masses of all the components in the
mixture,isthetotalvolumeofthemixturethesameasthesumofthevolumesofallthecomponentsinthe
mixture? Well, that depends. If the components are immiscible, then the total volume is the sum of the individual
component volumes. But gases and vapors are not immiscible, so how do the component volumes affect
the total volume?
So, whereas the basic definitions of mixture composition for insoluble solids and liquids seems very easy, the
practicality of implementing these definitions for mixtures of soluble gases and vapors is not so easy. However,
the mixture composition is not really our primary goal. Our primary goal is to be able to determine the thermodynamic
properties of a mixture so that we can apply the first and second laws to an engineering system containing a mixture. Are
mixture thermodynamic properties just the sum of the thermodynamic properties of their components? No! With
the exception of mass, the extensive thermodynamic properties (e.g., mixture total volume V m , mixture total
Modern Engineering Thermodynamics. DOI: 10.1016/B978-0-12-374996-3.00012-9
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