thermodynamics lecture note

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998
Thermodynamics
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Third Edition
1
CHAPTER
Basic
Concepts of
Thermodynamics

Thermodynamics
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Applications of Thermodynamics
1-1
Power plants
The human body
Air-conditioning
systems
Airplanes
Car radiators Refrigeration systems

Thermodynamics
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Third Edition
Crossing Closed-System Boundries
1-2
(Fig. 1-13)
Energy, not mass, crosses closed-system boundries

Thermodynamics
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Third Edition
Closed System with Moving Boundry
1-3

Thermodynamics
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Crossing Control Volume
Boundaries
1-4
Mass and Energy Cross Control Volume Boundaries

Thermodynamics
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System’s Internal Energy
(Fig. 1-19)
1-5
System’s Internal Energy = Sum of Microscopic Energies

Thermodynamics
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Quasi-Equilibrium,
Work-Producing Devices
(Fig. 1-30)
1-6
Quasi-Equilibrium, Work-Producing Devices Deliver the Most Work

Thermodynamics
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Compressed Process P-V Diagram
(Fig. 1-31)
1-7

Thermodynamics
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Absolute, Gage, and Vacuum
Pressures
(Fig. 1-36)
1-8

Thermodynamics
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The Basic Manometer
1-9

Thermodynamics
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Temperature Scales Comparison
(Fig. 1-48)
1-10

Thermodynamics
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Third Edition
Many Ways to Supply the Same
Energy
(Fig. 1-52)
1-11
Ways to supply a room with energy equalling a 300-W electric resistance heater

Thermodynamics
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Third Edition
Bomb Calorimeter Used to
Determine Energy Content of Food
(Fig. 1-53)
1-12

Thermodynamics
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Chapter Summary
• Thermodynamics is the science that primarily
deals with energy.
1-13

Thermodynamics
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Chapter Summary
• The first law of thermodynamics is simply an
expression of the conservation of energy
principle, and it asserts that energy is a
thermodynamic property.
• The second law of thermodynamics asserts
that energy has quality as well as quantity, and
actual processes occur in the direction of
decreasing quality of energy.
1-14

Thermodynamics
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Chapter Summary
• A system of fixed mass is called a closed
system, or control mass, and a system that
involves mass transfer across its boundaries
is called an open system, or control volume.
1-15

Thermodynamics
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Chapter Summary
• The mass-dependent properties of a system
are called extensive properties and the others,
intensive properties. Density is mass per unit
volume, and specific volume is volume per unit
mass.
1-16

Thermodynamics
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Chapter Summary
• The sum of all forms of energy of a system is
called total energy, which is considered to
consist of internal, kinetic, and potential
energies. Internal energy represents the
molecular energy of a system and may exist in
sensible, latent, chemical, and nuclear forms.
1-17

Thermodynamics
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Chapter Summary
• A system is said to be in thermodynamic
equilibrium if it maintains thermal, mechanical,
phase, and chemical equilibrium.
1-18

Thermodynamics
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Chapter Summary
• Any change from one state to another is called
a process.
• A process with identical end states is called a
cycle.
1-19

Thermodynamics
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Chapter Summary
• During a quasi-static or quasi-equilibrium
process, the system remains practically in
equilibrium at all times.
1-20

Thermodynamics
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Chapter Summary
• The state of a simple, compressible system is
completely specified by two independent,
intensive properties.
1-21

Thermodynamics
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Chapter Summary
• Force per unit area is called pressure, and its
unit is the pascal. The absolute, gage, and
vacuum pressures are related by
1-22

Thermodynamics
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Chapter Summary
• Small to moderate pressure differences are
measured by a manometer, and a differential
fluid column of height h corresponds to a
pressure difference of
where  is the fluid density and g is the local
gravitational acceleration.
1-23

Thermodynamics
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Chapter Summary
• The atmospheric pressure is measured by a
barometer and is determined from
where h is the height of the liquid column
above the free surface.
1-24

Thermodynamics
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Chapter Summary
• The zeroth law of thermodynamics states that
two bodies are in thermal equilibrium if both
have the same temperature reading even if
they are not in contact.
1-25

Thermodynamics
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Chapter Summary
• The temperature scales used in the SI and the
English system today are the Celsius scale
and the Fahrenheit scale, respectively.
1-26

Thermodynamics
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Chapter Summary
• The absolute temperature scale in the SI is the
Kelvin scale, which is related to the Celsius
scale by
1-27

Thermodynamics
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Chapter Summary
• In the English system, the absolute
temperature scale is the Rankine scale, which
is related to the Fahrenheit scale by
1-28

Thermodynamics
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Chapter Summary
• The magnitudes of each division of 1 K and
1 0C are identical, and so are the magnitude of
each division of 1 R and 10F. Therefore,
and
1-29

Thermodynamics
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Chapter Summary
• An important application area of
thermodynamics is the biological system.
Most diets are based on the simple energy
balance: the net energy gained by a person in
the form of fat is equal to the difference
between the energy intake from food and the
energy expended by exercise.
1-30

Thermodynamics
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Thermodynamics
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2
CHAPTER
Properties of
Pure Substances

Thermodynamics
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(fig. 2-16)
Constant-Pressure Phase-Change
Process
2-1

Thermodynamics
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T-v Diagram of a Pure Substance
2-2
(Fig. 2-18)
Energy, not mass, crosses closed-system boundaries

Thermodynamics
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P-v Diagram of a Pure Substance
(Fig. 2-19)
2-3
SUPERHEATED

Thermodynamics
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P-v Diagram of Substance that
Contracts on Freezing
(Fig. 2-21)
2-4

Thermodynamics
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P-v Diagram of Substance that Expands on
Freezing
(Fig. 2-22)
2-5

Thermodynamics
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P-T Diagram of Pure Substances
(Fig. 2-25)
2-6

Thermodynamics
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P-v-T Surface of a Substance that
Contracts on Freezing
(Fig. 2-26)
2-7

Thermodynamics
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P-v-T Surface of a Substance that
Expands on Freezing
(Fig. 2-27)
2-8

Thermodynamics
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Partial List of Table A-4
(Fig. 2-35)
2-9

Thermodynamics
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Quality Shown in P-v and T-v Diagrams
(Fig. 2-41)
2-10
Quality is related to the horizontal differences of P-V and T-v diagrams

Thermodynamics
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Partial List of Table A-6
(Fig. 2-45)
2-11

Thermodynamics
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Pure Substances
can Exist as Compressed Liquids
(Fig. 2-49)
2-12
At a given P and T, a pure substance will exist
as a compressed liquid if T<T sat @ P

Thermodynamics
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The Region Where Steam
can be Treated as an Ideal Gas
(Fig. 2-54)
2-13

Thermodynamics
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Comparison of Z Factors
for Various Gases
(Fig. 2-57)
2-14

Thermodynamics
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Percent of Error in Equations for the State
of Nitrogen
(Fig. 2-66)
2-15

Thermodynamics
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Chapter Summary
• A substance that has a fixed chemical
composition throughout is called a pure
substance.
2-16

Thermodynamics
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Chapter Summary
• A pure substance exists in different phases
depending on its energy level. In the liquid phase,
a substance that is not about to vaporize is called
a compressed or subcooled liquid.
2-17

Thermodynamics
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Chapter Summary
• In the gas phase, a substance that is not about to
condense is called a superheated vapor.
2-18

Thermodynamics
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Chapter Summary
• During a phase-change process, the temperature
and pressure of a pure substance are dependent
properties. At a given pressure, a substance
changes phase at a fixed temperature, called the
saturation temperature. At a given temperature,
the pressure at which a substance changes phase
is called the saturation pressure. During a boiling
process, both the liquid and the vapor phases
coexist in equilibrium, and under this condition
the liquid is called saturated liquid and the vapor
saturated vapor.
2-19

Thermodynamics
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Chapter Summary
• In a saturated liquid-vapor mixture, the mass
fraction of the vapor phase is called the quality
and is defined as
The quality may have values between 0 (saturated
liquid) and 1 (saturated vapor). It has no meaning
in the compressed liquid or superheated vapor
regions.
2-20

Thermodynamics
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Chapter Summary
• In the saturated mixture region, the average value
of any intensive property y is determined from
where f stands for saturated liquid and g for
saturated vapor.
2-21

Thermodynamics
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Chapter Summary
• In the absence of compressed liquid data, a
general approximation is to treat a compressed
liquid as a saturated liquid at the given
temperature, that is,
where y stands for v, u, or h.
2-22

Thermodynamics
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Chapter Summary
• The state beyond which there is no distinct
vaporization process is called the critical point. At
supercritical pressures, a substance gradually and
uniformly expands from the liquid to vapor phase.
2-23

Thermodynamics
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Chapter Summary
• All three phases of a substance coexist in
equilibrium at states along the triple line
characterized by triple-line temperature
and pressure.
2-24

Thermodynamics
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Third Edition
Chapter Summary
• Various properties of some pure sub-stances are
listed in the appendix. As can be noticed from
these tables, the compressed liquid has lower v, u,
and h values than the saturated liquid at the same
T or P. Likewise, superheated vapor has higher v,
u, and h values than the saturated vapor at the
same T or P. is a major application area of
thermodynamics.
2-25

Thermodynamics
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Third Edition
Chapter Summary
• Any relation among the pressure, temperature,
and specific volume of a substance is called an
equation of state. The simplest and best-known
equation of state is the ideal-gas equation of state,
given as
where R is the gas constant. Caution should be
exercised in using this relation since an ideal gas
is a fictitious substance. Real gases exhibit ideal-
gas behav-ior at relatively low pressures and high
temperatures.
2-26

Thermodynamics
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Chapter Summary
• The deviation from ideal-gas behavior can be
properly accounted for by using the
compressibility factor Z, defined as
2-27

Thermodynamics
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Chapter Summary
• The Z factor is approximately the same for all
gases at the same reduced temperature and
reduced pressure, which are defined as
where Pcr and Tcr are the critical pressure and
temperature, respectively. This is known as the
principle of corresponding states.
(Continued on next slide)
2-28

Thermodynamics
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Chapter Summary
• When either P or T is unknown, Z can be
determined from the compressibility chart with the
help of the pseudo-reduced specific volume,
defined as
(Continued from previous slide)
2-29

Thermodynamics
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• The P-v-T behavior of substances can be
represented more accurately by the more complex
equations of state. Three of the best known are
van der Waals:
where
Chapter Summary
2-30

Thermodynamics
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Chapter Summary
• Beattie-Bridgeman:
where
2-31

Thermodynamics
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Chapter Summary
• Benedict-Webb-Rubin:
The constants appearing in the Beattie-Bridgeman and Benedict-Webb-
Rubin equations are given in Table A-29 for various substances.
2-32

Thermodynamics
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3
CHAPTER
The First Law of
Thermodynamics:
Closed Systems

Thermodynamics
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Chapter Summary
• The first law of thermodynamics is essentially
an expression of the conservation of energy
principle. Energy can cross the boundaries of
a closed system in the form of heat or work.
• If the energy transfer across the boundaries of
a closed system is due
to a temperature difference, it is heat;
otherwise, it is work.
3-22

Thermodynamics
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Chapter Summary
• Heat is transferred in three ways: conduction,
convection, and radiation.
› Conduction is the transfer of energy from the more
energetic particles of a substance to the adjacent less
energetic ones as a result of interactions between the
particles.
› Convection is the mode of energy transfer between a
solid surface and the adjacent liquid or gas that is in
motion, and it involves the combined effects of
conduction and fluid motion.
› Radiation is the energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the
changes in the electronic configurations of the atoms
or molecules.
3-23

Thermodynamics
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Chapter Summary
• The three modes of heat transfer are expressed
as:
3-24

Thermodynamics
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• Various forms of work are expressed as follows:
› Electrical work: (kJ)
› Boundary work: (kJ)
› Gravitational work (=DPE): (kJ)
› Acceleration work (=DKE): (kJ)
› Shaft work: (kJ)
› Spring work: (kJ)
Chapter Summary
3-25

Thermodynamics
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Chapter Summary
• For the ploytropic process (Pvn = constant) of real
gases, the boundary work can be expressed as:
3-26

Thermodynamics
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Chapter Summary
• The energy balance for any system undergoing
any process can be expressed as:
3-27

Thermodynamics
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Chapter Summary
• The energy balances for any system undergoing
any process can be expressed in the rate form as:
3-28

Thermodynamics
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Chapter Summary
• Taking heat transfer to the system and work done
by the system to be positive quantities, the energy
balance for a closed system can also be
expressed as:
where:
3-29

Thermodynamics
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Chapter Summary
• For a constant-pressure process, .
Thus
3-30

Thermodynamics
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Chapter Summary
• The amount of energy needed to raise the
temperature of a unit of mass of a substance by
one degree is called the specific heat at constant
volume Cv for a constant-volume process and the
specific heat at constant pressure Cp for a
constant pressure process. They are defined as:
3-31

Thermodynamics
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Chapter Summary
• For ideal gases u, h, Cv, and Cp are functions of
temperature alone. The u and h of ideal gases
can be expressed as:
3-32

Thermodynamics
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Chapter Summary
• For ideal gases Cv, and Cp are related by:
3-33

Thermodynamics
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Chapter Summary
• The specific heat ratio k is defined as:
3-34

Thermodynamics
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Chapter Summary
• For incompressible substances (liquids and
solids), both the constant-pressure and constant-
volume specific heats are identical and denoted by
C:
3-35

Thermodynamics
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Third Edition
Chapter Summary
• The u and h of incompressible substances are
given by
3-36

Thermodynamics
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Chapter Summary
• The refrigeration and freezing of foods is a major
application area of thermodynamics.
3-37

Thermodynamics
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Heat Transfer
3-1
(Fig. 3-3)

Thermodynamics
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Adiabatic Process
(Fig. 3-4)
3-2

Thermodynamics
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Convection: Heat Transfer
(Fig. 3-8)
3-3

Thermodynamics
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Convection: Cooling
(Fig. 3-9)
3-4

Thermodynamics
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Radiation
(Fig. 3-10)
3-5

Thermodynamics
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Heat and Work
(Fig. 3-15)
3-6

Thermodynamics
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Path Functions
(Fig. 3-16)
3-7

Thermodynamics
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Boundary Work
(Fig. 3-27)
3-8

Thermodynamics
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Net Work per Cycle
(Fig. 3-29)
3-9

Thermodynamics
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Schematic/Diagram for Ex. 3-8
(Fig. 3-31)
3-10

Thermodynamics
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(Fig. 3-32)
3-11

Thermodynamics
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Schematic/Diagram for the
Polytropic Process
(Fig. 3-33)
3-12

Thermodynamics
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(Fig. 3-43)
3-13

Thermodynamics
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Stretching a Liquid Film
(Fig. 3-45)
3-14

Thermodynamics
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System Energy Change
(Fig. 3-52)
3-15

Thermodynamics
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Energy Change for a Cycle
(Fig. 3-54)
3-16

Thermodynamics
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Closed-Systems, First-Law
(Fig. 3-55)
3-17

Thermodynamics
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Formal Definitions of Cv and Cp
(Fig. 3-72)
3-18

Thermodynamics
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Specific Heats for Some Gases
(Fig. 3-76)
3-19

Thermodynamics
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Three Ways to Calculate ²u
3-20
(Fig. 3-80)

Thermodynamics
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Typical Freezing Curve (food)
(Fig. 3-91)
3-21

Thermodynamics
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6
CHAPTER
Entropy:
A Measure
of Disorder

Thermodynamics
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System Considered in the
Development of Claussius inequity
6-1

Thermodynamics
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The Entropy Change Between
Two Specific States
6-2
(Fig. 6-3)
The entropy change between two specific states is the
same whether the process is reversible or irreversible

Thermodynamics
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The Entropy Change of an Isolated
System
6-3
The entropy change of an isolated system is the sum of the entropy changes
of its components, and is never less than zero

Thermodynamics
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The Entropy Change of a Pure
Substance
6-4
(Fig. 6-10)
The entropy of a pure substance is determined from the tables, just as for
any other property

Thermodynamics
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Schematic of the T-s Diagram for Water
(Fig. 6-11)
6-5

Thermodynamics
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System Entropy Constant During
Reversible, adiabatic (isentropic) Process
(Fig.6-14)
6-6

Thermodynamics
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Level of Molecular Disorder
(Entropy)
(Fig. 6-16)
6-7
The level of molecular disorder (entropy) of a substance
increases as it melts and evaporates

Thermodynamics
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Net Disorder (Entropy) Increases
During Heat Transfer
6-8
During a heat transfer process, the net disorder (entropy) increases (the
increase in the disorder of the cold body more than offsets the decrease in
the disorder in the hot body)

Thermodynamics
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Heat Transfer for Internally Reversible
Processes
6-9
(Fig. 6-23)
On a T-S diagram, the area under the process curve represents the
heat transfer for internally reversible processes
d

Thermodynamics
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h-s Diagram for Adiabatic Steady-
Flow Devices
6-11
For adiabatic steady-flow devices, the vertical distance ²h on an h-s
diagram is a measure of work, and the horizontal distance ²s is a
measure of irreversibilities

Thermodynamics
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Schematic of an h-s Diagram for Water
(Fig. 6-27)
6-10

Thermodynamics
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Entropy of an Ideal Gas
(Fig. 6-33)
6-12
The entropy of an ideal gas depends on both T and P. The function
s° represents only the temperature-dependent part of entropy

Thermodynamics
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The Isentropic Relations of Ideal Gases
(Fig. 6-36)
6-13
The isentropic relations of ideal gases
are valid for the isentropic processes of ideal gases only

Thermodynamics
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Using Pr data to Calculate Final
Temperature During Isentropic Processes
(Fig. 6-37)
6-14
The T-ebow of an ordinary shower serves as the mixing chamber
for hot- and cold-water streams.

Thermodynamics
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Reversible Work Relations for
Steady-Flow and Closed Systems
(Fig. 6-41)
6-15

Thermodynamics
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P-v Diagrams of Isentropic, Polytropic,
and Isothermal Compression Processes
(Fig. 6-45)
6-16
P-v Diagrams of isentropic, polytropic, and isothermal compression
processes between the same pressure limits

Thermodynamics
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P-v andT-s Diagrams for a Two-Stage
Steady-Flow Compression Process
(Fig. 6-46)
6-17

Thermodynamics
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The h-s Diagram for the Actual and
Isentropic Processes of an Adiabatic Turbine
(Fig. 6-59)
6-18

Thermodynamics
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The h-s Diagram of the Actual and Isentropic
Processes of an Adiabatic Compressor
(Fig. 6-61)
6-19

Thermodynamics
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The h-s Diagram of the Actual and
Isentropic Processes of an Adiabatic Nozzle
(Fig. 6-64)
6-20

Thermodynamics
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Mechanisms of Entropy Transfer
for a General System
6-21

Thermodynamics
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A Control Volume’s Entropy Changes
with MassFlow as well as Heat Flow
(Fig. 6-73)
6-22

Thermodynamics
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Entropy Generation During Heat
Transfer
6-23
Graphical representation of entropy generation during a heat
transfer process through a finite temperature difference

Thermodynamics
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Chapter Summary
• The second law of thermodynamics leads to the
definition of a new property called entropy, which
is a quantitative measure of microscopic disorder
for a system.
6-24

Thermodynamics
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Chapter Summary
• The definition of entropy is based on the Clausius
inequality, given by
where the equality holds for internally or totally
reversible processes and the inequality for
irreversible processes.
6-25

Thermodynamics
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Chapter Summary
• Any quantity whose cyclic integral is zero is a
property, and entropy is defined as
6-26

Thermodynamics
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Chapter Summary
• For the special case of an internally reversible,
isothermal process, it gives
6-27

Thermodynamics
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Chapter Summary
• The inequality part of the Clausius inequality
combined with the definition of entropy yields an
inequality known as the increase of entropy
principle, expressed as
where Sgen is the entropy generated during the
process.
6-28

Thermodynamics
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Third Edition
Chapter Summary
• Entropy change is caused by heat transfer, mass
flow, and irreversibilities. Heat transfer to a
system increases the entropy, and heat transfer
from a system decreases it. The effect of
irreversibilities is always to increase the entropy.
6-29

Thermodynamics
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Third Edition
Chapter Summary
• Entropy is a property, and it can be expressed in
terms of more familiar properties through the Tds
relations, expressed as
and
Tds = du +Pdv
Tds = dh - vdP
6-30

Thermodynamics
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Third Edition
Chapter Summary
• These two relations have many uses in
thermodynamics and serve as the starting point in
developing entropy-change relations for
processes. The successful use of Tds relations
depends on the availability of property relations.
Such relations do not exist for a general pure
substance but are available for incompressible
substances (solids, liquids) and ideal gases.
6-31

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The entropy-change and isentropic relations for a
process can be summarized as follows:
1. Pure substances:
Any process: s = s2 - s1 [kJ/(kg-K)]
Isentropic process: s2 = s1
6-32

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
2. Incompressible substances:
Any process: s2 - s1 = Cav 1n [kJ/(kg-K)]
Isentropic process: T2 = T1
T2
T1
6-33

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
3. Ideal gases:
a. Constant specific heats (approximate
treatment):
Any process:
T2
T1
v2
v1
T2
T1
P2
P1
s2 - s1 = Cv,av 1n + R1n [kJ/(kg-K)]
s2 - s1 = Cp,av 1n + R1n [kJ/(kg-K)]
and
6-34

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
3. Ideal gases:
a. Constant specific heats (approximate
treatment):
On a unit-mole basis,
T2
T1
v2
v1
T2
T1
P2
P1
s2 - s1 = Cv,av 1n + Ru1n [kJ/(kmol-K)]
s2 - s1 = Cp,av 1n + Ru1n [kJ/(kmol-K)]
and
6-35

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
Isentropic process:
3. Ideal gases:
a. Constant specific heats (approximate treatment):
6-36

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
s2 - s1 = s2 - s1 - R1n [kJ/(kg-K)]
s2 - s1 = s2 - s1 - Ru1n [kJ/(kmol-K)]
3. Ideal gases:
b. Variable specific heats (exact treatment):
Any process,
P2
P1
or
o o
P2
P1
o o
6-37

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
3. Ideal gases:
b. Variable specific heats (exact treatment):
Isentropic process,
s2 = s1 - R1n [kJ/(kg-K)]
P2
P1
o o
where Pr is the relative pressure and vr is the
relative specific volume. The function so
depends on temperature only.
6-38

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The steady-flow work for a reversible process can
be expressed in terms of the fluid properties as
6-39

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• For incompressible substances (v = constant)
steady-flow work for a reversible process
simplifies to
6-40

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The work done during a steady-flow process is
proportional to the specific volume. Therefore,
v should be kept as small as possible during a
compression process to minimize the work input
and as large as possible during an expansion
process to maximize the work output.
6-41

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The reversible work inputs to a compressor
compressing an ideal gas from T1, P1, to P2 in an
isentropic (Pvk = constant), polytropic (Pvn = con-
stant), or isothermal (Pv = constant) manner, are
determined by integration for each case with the
following results:
6-42

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Isentropic:
(kJ/kg)
6-43

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Polytropic:
(kJ/kg)
6-44

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Isothermal:
(kJ/kg)
6-45

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The work input to a compressor can be reduced
by using multistage compression with
intercooling. For maximum savings from the work
input, the pressure ratio across each stage of the
compressor must be the same.
6-46

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Most steady-flow devices operate under adiabatic
conditions, and the ideal process for these
devices is the isentropic process.
6-47

Thermodynamics
Çengel
Boles
Third Edition
• The parameter that describes how efficiently a device
approximates a corresponding isentropic device is
called isentropic or adiabatic efficiency. It is expressed
for turbines, compressors, and nozzles as follows:
In the relations above, h2a and h2s are the enthalpy
values at the exit state for actual and isentropic
processes, respectively.
Chapter Summary
Actual turbine work wa h1 - h2a
Isentropic turbine work ws h1 - h2s
= = =
~
Isentropic compressor work ws h2s - h1
Actual compressor work wa h2a - h1
= = =
~
Actual KE at nozzle exit V2a h1 - h2a
Isentropic KE at nozzle exit h1 - h2s
2
V2s
= = =
~
2
6-48

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The entropy balance for any system undergoing
any process can be expressed in the general form
as
6-49

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The entropy balance for any system undergoing
any process can be expressed in the general rate
form, as
6-50

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• For a general steady-flow process the entropy
balance simplifies to
6-51

Thermodynamics
Çengel
Boles
Third Edition
7
CHAPTER
Exergy:
A Measure of
Work Potential

Thermodynamics
Çengel
Boles
Third Edition
(fig. 7-9)
© The McGraw-Hill Companies, Inc.,1998
Irreversibility is the Difference Between
Reversible Work and Actual Useful Work
7-1

Thermodynamics
Çengel
Boles
Third Edition
Irreversible Heat Transfer Can be Made
Reversible by a Reversible Heat Engine
7-2
(Fig. 7-12)

Thermodynamics
Çengel
Boles
Third Edition
Second Law of Efficiency
7-3
The second law of efficiency is a measure of the performance of a device
relative to its performance under reversible conditions

Thermodynamics
Çengel
Boles
Third Edition
The Second-Law Efficiency of All
Reversible Devices is 100%
(Fig. 7-16)
7-4

Thermodynamics
Çengel
Boles
Third Edition
The Work Potential or Exergy of Potential
Energy Equals the Potential Energy Itself
(Fig. 7-18)
7-5

Thermodynamics
Çengel
Boles
Third Edition
The Exergy of a Specified Mass
(Fig. 7-19)
7-6
The exergy of a specified mass at a specified state is the useful work that can be
produced as it undergoes a reversible process to the state of the environment

Thermodynamics
Çengel
Boles
Third Edition
The Exergy of a Cold Medium
7-7
(Fig. 7-20)
The exergy of a cold medium is also a positive quantity since work can be
produced by transferring heat to it

Thermodynamics
Çengel
Boles
Third Edition
The Exergy of Flow of Work
(Fig. 7-21)
7-8
The exergy of flow of work is the useful work that would be deliverd by an
imaginary piston in the flow section

Thermodynamics
Çengel
Boles
Third Edition
The Exergy of Enthalpy
(Fig. 7-22)
7-9
The exergy of enthalpy is the sum of the exergies of the internal energy
and flow energy

Thermodynamics
Çengel
Boles
Third Edition
The Energy and Exergy contents of (a)
a Fixed Mass and (b) a Fluid System
(Fig. 7-23)
7-10

Thermodynamics
Çengel
Boles
Third Edition
The Transfer and Destruction of Exergy
During Heat Transfer
(Fig. 7-27)
7-11
The transfer and destruction of exergy during a heat transfer process
through a finite temperature difference

Thermodynamics
Çengel
Boles
Third Edition
Mechanisms of Exergy Transfer for
a General System
(Fig. 7-32)
7-12

Thermodynamics
Çengel
Boles
Third Edition
Exergy Transferrence
(Fig. 7-42)
7-13
Exergy is transferred into or out of a control volume by mass
as well as by heat and work transfer

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The energy content of the universe is constant,
just as its mass content is. Yet at times of crisis
we are bombarded with speeches and articles on
how to "conserve" energy. As engineers, we know
that energy is already conserved. What is not
conserved is exergy, which is the useful work
potential of the energy. Once the exergy is wasted,
it can never be recovered. When we use energy (to
heat our homes for example), we are not
destroying any energy; we are merely converting
it to a less useful form, a form of less exergy.
7-14

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The useful work potential of a system at the
specified state is called exergy. Exergy is a
property and is associated with the state of the
system and the environment. A system that is in
equilibrium with its surroundings has zero exergy
and is said to be at the dead state. The exergy of
the thermal energy of thermal reservoirs is
equivalent to the work output of a Carnot heat
engine operating between the reservoir and the
environment.
7-15

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Reversible work Wrev is defined as the maximum
amount of useful work that can be produced (or
the minimum work that needs to be supplied) as a
system undergoes a process between the
specified initial and final states. This is the useful
work output (or input) obtained when the process
between the initial and final states is executed in a
totally reversible manner.
7-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The difference between the reversible work Wrev
and the useful work Wu is due to the
irreversibilities present during the process and is
called the irreversibility I. It is equivalent to the
exergy destroyed and is expressed as
where Sgen is the entropy generated during the
process. For a totally reversible process, the
useful and reversible work terms are identical and
thus irreversibility is zero.
I = Xdestroyed = ToSgen = Wrev,out - Wu,out = Wu,in - Wrev,in
7-17

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Exergy destroyed represents the lost work
potential and is also called the wasted work or lost
work.
7-18

Thermodynamics
Çengel
Boles
Third Edition
• The second-law efficiency is a measure of the
performance of a device relative to the
performance under reversible conditions for the
same end states and is given by
for heat engines and other work-producing
devices and
for refrigerators, heat pumps, and other work-
consuming devices.
Chapter Summary
7-19

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• In general, the second-law efficiency is expressed
as
Exergy recovered Exergy destroyed
Exergy supplied Exergy supplied
= = 1 -
7-20

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The exergy of various forms of energy are
Exergy of kinetic energy: xke = ke =
Exergy of potential energy: xpe = pe = gz
Exergy of internal energy:
xu = (u - uo) + Po(v - vo) - To(s - so)
Exergy of flow energy: xpv = Pv - Pov = (P - Po)v
Exergy of enthalpy: xh = (h - ho) - To(s - so)
V2
2
7-21

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The exergies of a fixed mass (nonflow exergy) and
of a flow stream are expressed as
Nonflow exergy:
Flow exergy:
7-22

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The exergy change of a fixed mass or fluid stream
as it undergoes a process from state 1 to state 2 is
given by
7-23

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Exergy can be transferred by heat, work, and
mass flow, and exergy transfer accompanied by
heat, work, and mass transfer are given by
Exergy transfer by heat:
Exergy transfer by work:
Xwork =
Exergy transfer by mass:
W - Wsurr (for boundary work)
W (for other forms of work
7-24

Thermodynamics
Çengel
Boles
Third Edition
• The exergy of an isolated system during a process
always decreases or, in the limiting case of a
reversible process, remains constant. This is
known as the decrease of exergy principle and is
expressed as
Chapter Summary
7-25

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
General:
• Exergy balance for any system undergoing any
process can be expressed as
7-26

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
General, rate form:
7-27

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
Xheat = 1 - Q
To
T
(xin - xout) - xdestroyed = xsystem
Xwork = Wuseful
Xmass = m
Xsystem - dXsystem / dt
.
.
.
.
.
. .
General, unit-mass basis:
where
For a reversible process, the exergy destruction term
Xdestroyed drops out.
7-28

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Taking the positive direction of heat transfer to be
to the system and the positive direction of work
transfer to be from the system, the general exergy
balance relations can be expressed more explicitly
as
where the subscripts are i = inlet, e = exit, 1 =
initial state, and 2 = final state of the system.
7-29

Thermodynamics
Çengel
Boles
Third Edition
8
CHAPTER
Gas Power
Cycles

Thermodynamics
Çengel
Boles
Third Edition
(fig. 8-2)
Idealizations Help Manage
Analysis of Complex Processes
8-1
The analysis of many complex processes can be reduced to a
manageable level by utilizing some idealizations

Thermodynamics
Çengel
Boles
Third Edition
P-v and T-s diagrams of a Carnot
Cycle
8-2

Thermodynamics
Çengel
Boles
Third Edition
Nomenclature for Reciprocating Engines
(Fig. 8-10)
8-3

Thermodynamics
Çengel
Boles
Third Edition
Reciprocating Engine Displacement
and Clearance Volumes
(Fig. 8-11)
8-4

Thermodynamics
Çengel
Boles
Third Edition
The Net Work Output of a Cycle
(Fig. 8-12)
8-5
The net work output of a cycle is equivalent to the product of the
mean effect pressure and the displacement volume

Thermodynamics
Çengel
Boles
Third Edition
Actual and Ideal Cycles in Spark-
Ignition Engines and Their P-v Diagram
(Fig. 8-13)
8-6

Thermodynamics
Çengel
Boles
Third Edition
Schematic of a Two-Stroke
Reciprocating Engine
8-7
(Fig. 8-14)

Thermodynamics
Çengel
Boles
Third Edition
T-s Diagram for the Ideal Otto
Cycle
(Fig. 8-15)
8-8

Thermodynamics
Çengel
Boles
Third Edition
The Thermal Efficiency of the Otto
Cycle
(Fig. 8-18)
8-9
The thermal efficiency of the Otto Cycle increases with the specific heat
ratio k of the working fluid

Thermodynamics
Çengel
Boles
Third Edition
T-s and P-v Diagrams for the Ideal Diesel
Cycle
(Fig. 8-21)
8-10

Thermodynamics
Çengel
Boles
Third Edition
Thermal Efficiency of the Ideal
Diesel Cycle
(Fig. 8-22)
8-11
The thermal efficiency of the ideal Diesel cycle as a function of
compression and cutoff rates (k=1.4)

Thermodynamics
Çengel
Boles
Third Edition
P-v Diagram of an Ideal Dual Cycle
(Fig. 8-23)
8-12

Thermodynamics
Çengel
Boles
Third Edition
T-s and P-v Diagrams of Carnot, Stirling,
and Ericsson Cycles
(Fig. 8-26)
8-13

Thermodynamics
Çengel
Boles
Third Edition
An Open-Cycle Gas-Turbine Engine
(Fig. 8-29)
8-14

Thermodynamics
Çengel
Boles
Third Edition
A Closed-Cycle Gas-Turbine Engine
(Fig. 8-30)
8-15

Thermodynamics
Çengel
Boles
Third Edition
T-s and P-v Diagrams for the Ideal Brayton
Cycle
(Fig. 8-31)
8-16

Thermodynamics
Çengel
Boles
Third Edition
Thermal Efficiency of the Ideal Brayton
Cycle as a Function of the Pressure Ratio
(Fig. 8-32)
8-17

Thermodynamics
Çengel
Boles
Third Edition
The Net Work of the Brayton Cycle
8-18
For fixed values of Tmin and Tmax, the net work of the Brayton cycle first
increases with the pressure ratio, then reaches a maximum at
rp=(Tmax/Tmin)k/[2(k-1)], and finally decreases

Thermodynamics
Çengel
Boles
Third Edition
The Back-Work Ratio is the Fraction of
Turbine Work Used to Drive the Compressor
(Fig. 8-34)
8-19

Thermodynamics
Çengel
Boles
Third Edition
Deviation of Actual Gas-Turbine Cycle
From Brayton cycle
(Fig. 8-36)
8-20
The deviation of an actual gas-turbine cycle from the ideal Brayton cycle
as a result of irreversibilities

Thermodynamics
Çengel
Boles
Third Edition
A Gas-Turbine Engine With
Regenerator
(Fig. 8-38)
8-21

Thermodynamics
Çengel
Boles
Third Edition
T-s Diagram of a Brayton Cycle
with Regeneration
(Fig. 8-39)
8-22

Thermodynamics
Çengel
Boles
Third Edition
Thermal Efficiency of the ideal Brayton
cycle with and without regeneration
(Fig. 8-40)
8-23

Thermodynamics
Çengel
Boles
Third Edition
A Gas-Turbine Engine
(Fig. 8-43)
8-24
A gas-turbine engine with two-stage compression with intercooling,
two-stage expansion with reheating, and regeneration

Thermodynamics
Çengel
Boles
Third Edition
T-s Diagram of Ideal Gas-Turbine Cycle with
Intercooling, Reheating, and Regeneration
(Fig. 8-44)
8-25

Thermodynamics
Çengel
Boles
Third Edition
Turbojet Engine Basic Components and
T-s Diagram for Ideal Turbojet Cycle
8-26

Thermodynamics
Çengel
Boles
Third Edition
Schematic of A Turbofan Engine
(Fig. 8-52)
8-27

Thermodynamics
Çengel
Boles
Third Edition
Illustration of A Turbofan Engine
8-28

Thermodynamics
Çengel
Boles
Third Edition
Schematic of a Turboprop Engine
(Fig. 8-54)
8-29

Thermodynamics
Çengel
Boles
Third Edition
Schematic of a Ramjet Engine
(Fig. 8-55)
8-30

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• A cycle during which a net amount of work is
produced is called a power cycle, and a power
cycle during which the working fluid remains a
gas throughout is called a gas power cycle.
8-31

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The most efficient cycle operating between a heat
source at temperature TH and a sink at
temperature TL is the Carnot cycle, and its thermal
efficiency is given by
8-32

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The actual gas cycles are rather complex. The
approximations used to simplify the analysis are
known as the air-standard assumptions. Under
these assumptions, all the processes are assumed
to be internally reversible; the working fluid is
assumed to be air, which behaves as an ideal gas;
and the combustion and exhaust processes are
replaced by heat-addition and heat-rejection
processes, respectively.
8-33

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The air-standard assumptions are called cold-air-
standard assumptions if, in addition, air is
assumed to have constant specific heats at room
temperature.
8-34

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• In reciprocating engines, the compression ratio r
and the mean effective pressure MEP are defined
as
8-35

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Otto cycle is the ideal cycle for the spark-
ignition reciprocating engines, and it consists of
four internally reversible processes: isentropic
compression, constant volume heat addition,
isentropic expansion, and con-stant volume heat
rejection.
8-36

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Under cold-air-standard assumptions, the thermal
efficiency of the ideal Otto cycle is
where r is the compression ratio and k is the
specific heat ratio Cp /Cv.
8-37

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Diesel cycle is the ideal cycle for the
compression-ignition reciprocating engines. It is
very similar to the Otto cycle, except that the
constant volume heat-addition process is replaced
by a constant pressure heat-addition process.
8-38

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Diesel cycle thermal efficiency under cold-air-
standard assumptions is
where rc is the cutoff ratio, defined as the ratio of
the cylinder volumes after and before the
combustion process.
8-39

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Stirling and Ericsson cycles are two totally
reversible cycles that involve an isothermal heat-
addition process at TH and an isothermal heat-
rejection process at TL. They differ from the
Carnot cycle in that the two isentropic processes
are replaced by two constant volume regeneration
processes in the Stirling cycle and by two
constant pressure regeneration processes in the
Ericsson cycle. Both cycles utilize regeneration, a
process during which heat is transferred to a
thermal energy storage device (called a
regenerator) during one part of the cycle that is
then transferred back to the working fluid during
another part of the cycle.
8-40

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The ideal cycle for modern gas-turbine engines is
the Brayton cycle, which is made up of four
internally reversible processes: isentropic
compression, constant pressure heat addition,
isentropic expansion, and constant pressure heat
rejection.
8-41

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Under cold-air-standard assumptions, the Brayton
cycle thermal efficiency is
where rp = Pmax/Pmin is the pressure ratio and k is
the specific heat ratio. The thermal efficiency of
the simple Brayton cycle increases with the
pressure ratio.
8-42

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The deviation of the actual compressor and the
turbine from the idealized isentropic ones can be
accurately accounted for by utilizing their
adiabatic efficiencies, defined as
and
where states 1 and 3 are the inlet states, 2a and 4a
are the actual exit states, and 2s and 4s are the
isentropic exit states.
8-43

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• In gas-turbine engines, the temperature of the
exhaust gas leaving the turbine is often
considerably higher than the temperature of the
air leaving the compressor. Therefore, the high-
pressure air leaving the compressor can be
heated by transferring heat to it from the hot
exhaust gases in a counter-flow heat exchanger,
which is also known as a regenerator.
8-44

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The extent to which a regenerator approaches an
ideal regenerator is called the effectiveness e and
is defined as
8-45

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Under cold-air-standard assumptions, the thermal
efficiency of an ideal Brayton cycle with
regeneration becomes
where T1 and T3 are the minimum and maximum
temperatures, respectively, in the cycle.
8-46

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The thermal efficiency of the Brayton cycle can
also be increased by utilizing multistage
compression with intercooling, regeneration, and
multistage expansion with reheating. The work
input to the compressor is minimized when equal
pressure ratios are maintained across each stage.
This procedure also maximizes the turbine work
output.
8-47

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Gas-turbine engines are widely used to power
aircraft because they are light and compact and
have a high power-to-weight ratio. The ideal jet-
propulsion cycle differs from the simple ideal
Brayton cycle in that the gases are partially
expanded in the turbine. The gases that exit the
turbine at a relatively high pressure are
subsequently accelerated in a nozzle to provide
the thrust needed to propel the aircraft.
8-48

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The net thrust developed by the turbojet engine is
where m is the mass flow rate of gases, Vexit is the
exit velocity of the exhaust gases, and Vinlet is the
inlet velocity of the air, both relative to the aircraft
8-49

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The power developed from the thrust of the engine
is called the propulsive power Wp and it is given
by
.
8-50

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Propulsive efficiency is a measure of how
efficiently the energy released during the
combustion process is converted to propulsive
energy, and it is defined as
8-51

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• For an ideal cycle that involves heat transfer only
with a source at TH and a sink at TL, the
irreversibility or exergy destruction is determined
to be
8-52

Thermodynamics
Çengel
Boles
Third Edition
9
CHAPTER
Vapor and
Combined
Power Cycles

Thermodynamics
Çengel
Boles
Third Edition
The Simple Ideal Rankine Cycle
9-1

Thermodynamics
Çengel
Boles
Third Edition
Rankine Cycle: Actual Vapor Power Deviation
and Pump and Turbine Irreversibilities
9-2
(Fig. 9-4)
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.

Thermodynamics
Çengel
Boles
Third Edition
Effect of Lowering Condenser
Pressure on the Ideal Rankine cycle
(Fig. 9-6)
9-3

Thermodynamics
Çengel
Boles
Third Edition
Effect of Increasing Boiler Pressure
on the Ideal Rankine cycle
(Fig. 9-8)
9-4

Thermodynamics
Çengel
Boles
Third Edition
The Ideal Reheat Rankine Cycle
9-5
(Fig. 9-11)

Thermodynamics
Çengel
Boles
Third Edition
Ideal Regenerative Rankine Cycle
with Open Feedwater Heater
9-6
(Fig. 9-15)

Thermodynamics
Çengel
Boles
Third Edition
Ideal Regenerative Rankine Cycle
with Closed Feedwater Heater
9-7
(Fig. 9-16)

Thermodynamics
Çengel
Boles
Third Edition
A Steam Power Plant With One Open
and Three Closed Feedwater Heaters
9-8
(Fig. 9-17)

Thermodynamics
Çengel
Boles
Third Edition
An Ideal Cogeneration Plant
9-9
(Fig. 9-21)

Thermodynamics
Çengel
Boles
Third Edition
Schematic and T-s Diagram for
Example 9-8
9-10
(Fig. 9-23)

Thermodynamics
Çengel
Boles
Third Edition
Mercury-Water Binary Vapor Cycle
9-11
(Fig. 9-24)

Thermodynamics
Çengel
Boles
Third Edition
Combined Gas-Steam Power Plant
9-12

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Carnot cycle is not a suitable model for vapor
power cycles because it cannot be approximated
in practice.
9-13

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The model cycle for vapor power cycles is the
Rankine cycle which is composed of four
internally reversible processes: constant-pressure
heat addition in a boiler, isentropic expansion in a
turbine, constant-pressure heat rejection in a
condenser, and isentropic compression in a pump.
Steam leaves the condenser as a saturated liquid
at the condenser pressure.
9-14

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The thermal efficiency of the Rankine cycle can be increased
by increasing the average temperature at which heat is
added to the working fluid and/or by decreasing the average
temperature at which heat is rejected to the cooling medium.
The average temperature during heat rejection can be
decreased by lowering the turbine exit pressure.
Consequently, the condenser pressure of most vapor power
plants is well below the atmospheric pressure. The average
temperature during heat addition can be increased by
raising the boiler pressure or by superheating the fluid to
high temperatures. There is a limit to the degree of
superheating, however, since the fluid temperature is not
allowed to exceed a metallurgically safe value.
9-15

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Superheating has the added advantage of
decreasing the moisture content of the steam
at the turbine exit. Lowering the exhaust
pressure or raising the boiler pressure,
however, increases the moisture content. To
take advantage of the improved efficiencies at
higher boiler pressures and lower condenser
pressures, steam is usually reheated after
expanding partially in the high-pressure
turbine. This is done by extracting the steam
after partial extraction in the high-pressure
turbine, sending it back to the boiler where it
is reheated at constant pressure, and
returning it to the low-pressure turbine for
complete expansion to the condenser
pressure. The average temperature during the
reheat process, and thus the thermal
efficiency of the cycle, can be increased by
9-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Another way of increasing the thermal efficiency
of the Rankine cycle is by regeneration. During a
regeneration process, liquid water (feedwater)
leaving the pump is heated by some steam bled
off the turbine at some intermediate pressure in
devices called feedwater heaters. The two streams
are mixed in open feedwater heaters, and the
mixture leaves as a saturated liquid at the heater
pressure. In closed feedwater heaters, heat is
transferred from the steam to the feedwater
without mixing.
9-17

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The production of more than one useful form of
energy (such as process heat and electric power)
from the same energy source is called
cogeneration. Cogeneration plants produce
electric power while meeting the process heat
requirements of certain industrial processes. This
way, more of the energy transferred to the fluid in
the boiler is utilized for a useful purpose. The
faction of energy that is used for either process
heat or power generation is called the utilization
factor of the cogeneration plant.
9-18

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The overall thermal efficiency of a power plant can
be increased by using binary cycles or combined
cycles. A binary cycle is composed of two
separate cycles, one at high temperatures
(topping cycle) and the other at relatively low
temperatures. The most common combined cycle
is the gas-steam combined cycle where a gas-
turbine cycle operates at the high-temperature
range and a steam-turbine cycle at the low-
temperature range. Steam is heated by the high-
temperature exhaust gases leaving the gas
turbine. Combined cycles have a higher thermal
efficiency than the steam- or gas-turbine cycles
operating alone.
9-19

Thermodynamics
Çengel
Boles
Third Edition
10
CHAPTER
Refrigeration
Cycles

Thermodynamics
Çengel
Boles
Third Edition
Refrigerator and Heat Pump
Objectives
10-1
(fig. 10-1)
The objective of a refrigerator is to remove heat (QL) from the cold medium;
the objective of a heat pump is to supply heat (QH) to a warm medium

Thermodynamics
Çengel
Boles
Third Edition
Schmatic and T-s Diagram for Ideal
Vapor-Compression Refrigeration Cycle
10-2
(Fig. 10-3)

Thermodynamics
Çengel
Boles
Third Edition
Ordinary Household Refrigerator
(Fig. 10-4)
10-3

Thermodynamics
Çengel
Boles
Third Edition
P-h Diagram of an Ideal Vapor-Compression
Refrigeration Cycle
(Fig. 10-5)
10-4

Thermodynamics
Çengel
Boles
Third Edition
Schmatic and T-s Diagram for Actual
Vapor-Compression Refrigeration Cycle
10-5
(Fig. 10-7)

Thermodynamics
Çengel
Boles
Third Edition
Heat Pump Heats a House in
Winter and Cools it in Summer
10-6
(Fig. 10-9)

Thermodynamics
Çengel
Boles
Third Edition
Schmatic and T-s Diagram for Refrigerator-
Freezer Unit with One Compressor
10-7
(Fig. 10-14)

Thermodynamics
Çengel
Boles
Third Edition
Linde-Hampson System for
Liquefying Gases
10-8
(Fig. 10-15)

Thermodynamics
Çengel
Boles
Third Edition
Simple Gas Refrigeration Cycle
10-9
(Fig. 10-16)

Thermodynamics
Çengel
Boles
Third Edition
Gas Refrigeration Cycle With
Regeneration
10-10
(Fig. 10-19)
COLD
refrigerated space
WARM
environment

Thermodynamics
Çengel
Boles
Third Edition
Ammonia Absorption Refrigeration
Cycle
10-11
(Fig. 10-21)

Thermodynamics
Çengel
Boles
Third Edition
Schematic of Simple
Thermoelectric Power Generator
10-12

Thermodynamics
Çengel
Boles
Third Edition
A Thermoelectric Refrigerator
(Fig. 10-28)
10-13

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The transfer of heat from lower temperature
regions to higher temperature ones is called
refrigeration. Devices that produce refrigeration
are called refrigerators, and the cycles on which
they operate are called refrigeration cycles. The
working fluids used in refrigerators are called
refrigerants. Refrigerators used for the purpose of
heating a space by transferring heat from a cooler
medium are called heat pumps.
10-14

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The performance of refrigerators and heat pumps
is expressed in terms of coefficient of
performance (COP), defined as
10-15

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The standard of comparison for refrigeration
cycles is the reversed Carnot cycle. A refrigerator
or heat pump that operates on the reversed Carnot
cycle is called a Carnot refrigerator or a Carnot
heat pump, and their COPs are
10-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The most widely used refrigeration cycle is the
vapor-compression refrigeration cycle. In an ideal
vapor-compression refrigeration cycle, the
refrigerant enters the compressor as a saturated
vapor and is cooled to the saturated liquid state in
the condenser. It is then throttled to the
evaporator pressure and vaporizes as it absorbs
heat from the refrigerated space.
10-17

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Very low temperatures can be achieved by
operating two or more vapor-compression
Systems in series, called cascading. The COP of a
refrigeration system also increases as a result of
cascading.
10-18

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Another way of improving the performance of a
vapor-compression refrigeration system is by
using multistage compression with regenerative
cooling. A refrigerator with a single compressor
can provide refrigeration at several temperatures
by throttling the refrigerant in stages. The vapor-
compression refrigeration cycle can also be used
to liquefy gases after some modifications
10-19

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The power cycles can be used as refrigeration
cycles by simply reversing them. Of these, the
reversed Brayton cycle, which is also known as
the gas refrigeration cycle, is used to cool aircraft
and to obtain very low (cryogenic) temperatures
after it is modified with regeneration. The work
output of the turbine can be used to reduce the
work input requirements to the compressor. Thus
the COP of a gas refrigeration cycle is
10-20

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Another form of refrigeration that becomes
economically attractive when there is a source of
inexpensive heat energy at a temperature of 100 to
2000C is absorption refrigeration, where the
refrigerant is absorbed by a transport medium and
compressed in liquid form. The most widely used
absorption refrigeration system is the ammonia-
water system, where ammonia serves as the
refrigerant and water as the transport medium.
The work input to the pump is usually very small,
and the COP of absorption refrigeration systems
is defined as
10-21

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The maximum COP an absorption refrigeration
system can have is determined by assuming
totally reversible conditions, which yields
where T0, TL, and Ts are the absolute temperatures
of the environment, refrigerated space, and heat
source, respectively.
10-22

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• A refrigeration effect can also be achieved without
using any moving parts by simply passing a small
current through a closed circuit made up of two
dissimilar materials. This effect is called the
Peltier effect, and a refrigerator that works on this
principle is called a thermoelectric refrigerator.
10-23

Thermodynamics
Çengel
Boles
Third Edition
11
CHAPTER
Thermodynamic
Property
Relations

Thermodynamics
Çengel
Boles
Third Edition
Geometric Representation of
Partial Derivation ( z/ x)y
11-1

Thermodynamics
Çengel
Boles
Third Edition
Geometric Representation of Total
Derivation dz for a function z(x,y)
11-2
(Fig. 11-4)

Thermodynamics
Çengel
Boles
Third Edition
Maxwell Relations are Extremely
Valuable in Thermodynamic Analysis
(Fig. 11-8)
11-3

Thermodynamics
Çengel
Boles
Third Edition
The Slope of the Saturation Curve on a P-T
Diagram
11-4
The slope of the saturation curve on a P-T diagram is constant
at a constant T or P

Thermodynamics
Çengel
Boles
Third Edition
Volume Expansivity
11-5
(Fig. 11-10)
The volume expansivity (also called the coefficient of volumetric expansion) is a
measure of change in volume with temperature at a constant pressure
o o
o
o

Thermodynamics
Çengel
Boles
Third Edition
Development of an h= Constant
Line on a P-T Diagram
(Fig. 11-13)
11-6

Thermodynamics
Çengel
Boles
Third Edition
Constant-Enthalpy Lines of
Substance on a T-P Diagram
(Fig. 11-14)
11-7

Thermodynamics
Çengel
Boles
Third Edition
Alternative Process Path to Evaluate Entropy
Changes of Real Gases During Process 1-2
(Fig. 11-17)
11-8

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Some thermodynamic properties can be measured
directly, but many others cannot. Therefore, it is
necessary to develop some relations between
these two groups so that the properties that
cannot be measured directly can be evaluated.
The derivations are based on the fact that
properties are point functions, and the state of a
simple, compressible system is completely
specified by any two independent, intensive
properties
11-9

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The equations that relate the partial derivatives of
properties P, v, T, and s of a simple compressible
substance to each other are called the Maxwell
relations. They are obtained from the four Gibbs
equations, expressed as
11-10

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Maxwell relations are
11-11

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The Clapeyron equation enables us to determine
the enthalpy change associated with a phase
change from a knowledge of P, v, and T data
alone. It is expressed a
11-12

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• For liquid-vapor and solid-vapor phase-change
processes at low pressures, the Clapeyron
equation can be approximated as
11-13

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The changes in internal energy, enthalpy, and
entropy of a simple, compressible substance can
be expressed in terms of pressure, specific
volume, temperature, and specific heats alone as
11-14

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• For specific heats, we have the following general
relations:
where is the volume expansivity and is the
isothermal compressibility, defined as
11-15

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
•The difference Cp - Cv is equal to R for ideal gases
and to zero for incompressible substances.
11-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The temperature behavior of a fluid during a
throttling (h = constant) process is described by
the Joule-Thomson coefficient, defined as
The Joule-Thomson coefficient is a measure of the
change in temperature of a substance with
pressure during a constant-enthalpy process, and
it can also be expressed as
11-17

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The enthalpy, internal energy, and entropy
changes of real gases can be determined
accurately by utilizing generalized enthalpy or
entropy departure charts to account for the
deviation from the ideal-gas behavior by using the
following relations:
where the values of Zh and ZS are determined from
the generalized charts.
11-18

Thermodynamics
Çengel
Boles
Third Edition
12
CHAPTER
Gas Mixtures

Thermodynamics
Çengel
Boles
Third Edition
Dalton’s Law of Additive Pressures
for the Mixture of Two Ideal Gases
12-1

Thermodynamics
Çengel
Boles
Third Edition
Amagat’s Law of Additive Volumes
for the Mixture of Two Ideal Gases
12-2

Thermodynamics
Çengel
Boles
Third Edition
Compressibility Factors: One Way of
Predicting Real-Gas Mixture P-v-T
(Fig. 12-8)
12-3

Thermodynamics
Çengel
Boles
Third Edition
Another Way of Predicting the P-v-
T Behavior of a Real-Gas Mixture
(Fig. 12-9)
12-4
Treat a real-gas mixture as a pseudopure substance with critical
properties P´cr and T´cr

Thermodynamics
Çengel
Boles
Third Edition
Use of Partial Pressures for
Entropy Evaluation
(Fig. 12-13)
12-5
Partial pressures (not the mixture pressure) are used in the
evaluation of entropy changes of ideal-gas mixtures

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• A mixture of two or more gases of fixed chemical
composition is called a nonreacting gas mixture.
The composition of a gas mixture is described by
specifying either the mole fraction or the mass
fraction of each component, defined as
where
12-6

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The apparent (or average) molar mass and gas
constant of a mixture are expressed as
and
12-7

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Dalton's law of additive pressures states that the
pressure of a gas mix-ture is equal to the sum of
the pressures each gas would exert if it existed
alone at the mixture temperature and volume.
12-8

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Amagat's law of additive volumes states that the
volume of a gas mixture is equal to the sum of the
volumes each gas would occupy if it existed alone
at the mixture temperature and pressure.
12-9

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Dalton's and Amagat's laws hold exactly for ideal-
gas mixtures, but only approximately for real-gas
mixtures. They can be expressed as
Dalton's law:
Amagat's law:
12-10

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• Here Pi is called the component pressure and Vi is
called the component volume. Also, the ratio Pi/Pm
is called the pressure fraction and the ratio Vi/Vm
is called the volume fraction of component i. For
ideal gases, Pi and Vi can be related to yi by
12-11

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The quantity yiPm is called the partial pressure and
the quantity yiVm, is called the partial volume.
12-12

Thermodynamics
Çengel
Boles
Third Edition
• The P-v-T behavior of real-gas mixtures can be
predicted by using generalized compressibility
charts. The compressibility factor of the mixture
can be expressed in terms of the compressibility
factors of the individual gases as
where Z is determined either at Tm and Vm,
(Dalton's law) or at Tm and Pm (Amagat's law) for
each individual gas.
Chapter Summary
12-13

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The P-v-T behavior of a gas mixture can also be
predicted approximately by Kay's rule, which
involves treating a gas mixture as a pure
substance with pseudocritical properties
determined from
and
12-14

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The extensive properties of a gas mixture, in
general, can be determined by summing the
contributions of each component of the mixture.
The evaluation of intensive properties of a gas
mixture, however, involves averaging in terms of
mass or mole fractions:
12-15

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
and
12-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• These relations are applicable to both ideal- and
real-gas mixtures. The properties or property
changes of individual components can be
determined by using ideal-gas or real-gas
relations developed in earlier chapters.
12-17

Thermodynamics
Çengel
Boles
Third Edition
13
CHAPTER
Gas-Vapor
Mixtures and
Air-Conditioning

Thermodynamics
Çengel
Boles
Third Edition
(fig. 13-1)
The Cp of Air
13-1

Thermodynamics
Çengel
Boles
Third Edition
Heat and Temperature Behavior in
Superheated Vapor Region of Water
13-2
(Fig. 13-2)
At temperatures below 50•C, the h= constant lines coincide with the T=
constant lines in the superheated region of water

Thermodynamics
Çengel
Boles
Third Edition
Determining the hg of Water
(Fig. 13-3)
13-3

Thermodynamics
Çengel
Boles
Third Edition
For Saturated Air, Vapor Pressure is
Equal to the Saturated Pressure of Water
(Fig. 13-4)
13-4

Thermodynamics
Çengel
Boles
Third Edition
Expressing the Enthalpy of Moist Air
13-5
The enthalpy of moist (atmospheric) air is expressed per unit mass of dry
air, not per unit mass of moist air

Thermodynamics
Çengel
Boles
Third Edition
Moist Air Constant-Pressure Cooling and Dew-
Point Temperature on T-s Diagram of Water
(Fig. 13-8)
13-6

Thermodynamics
Çengel
Boles
Third Edition
Adiabatic Saturation Process and its
Representation on a T-s Diagram
13-7
(Fig. 13-11)
Liquid water

Thermodynamics
Çengel
Boles
Third Edition
Sling Psychrometer
(Fig. 13-13)
13-8

Thermodynamics
Çengel
Boles
Third Edition
Schematic for Psychrometric Chart
(Fig. 13-14)
13-9

Thermodynamics
Çengel
Boles
Third Edition
Dry-Bulb, Wet-Bulb, and Dew-Point
Temperatures Identical for Saturated Air
(Fig. 13-15)
13-10
Quality is related to the horizontal differences of P-V and T-v diagrams

Thermodynamics
Çengel
Boles
Third Edition
Various Air-Conditioning Processes
(Fig. 13-20)
13-11

Thermodynamics
Çengel
Boles
Third Edition
Evaporative Cooling
(Fig. 13-27)
13-12
At a given P and T, a pure substance will exist
as a compressed liquid if T<T sat @ P

Thermodynamics
Çengel
Boles
Third Edition
Mixing Airstreams Adiabatically
(Fig. 13-29)
13-13
When two airstreams at states 1 and 2 are mixed adiabatically, the state
of the mixture lies on the straight line connecting the two states

Thermodynamics
Çengel
Boles
Third Edition
Schematic and Psychrometric Chart for
Example 13-8
13-14
(Fig. 13-31)

Thermodynamics
Çengel
Boles
Third Edition
A Natural-Draft Cooling Tower
(fig. 13-32)
13-15

Thermodynamics
Çengel
Boles
Third Edition
A Spray Pond
13-16

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• In this chapter we discussed the air-water-vapor
mixture, which is the most commonly encountered
gas-vapor mixture in practice. The air in the
atmosphere normally contains some water vapor,
and it is referred to as atmospheric air. By
contrast, air that contains no water vapor is called
dry air.
13-17

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• In the temperature range encountered in air-
conditioning applications, both the dry air and the
water vapor can be treated as ideal gases.
13-18

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The enthalpy change of dry air during a process
can be determined from
13-19

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The atmospheric air can be treated as an ideal-gas
mixture whose pressure is the sum of the partial
pressure of dry air Pa and that of the water vapor
Pv,
P = Pa + Pv
13-20

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The enthalpy of water vapor in the air can be taken
to be equal to the enthalpy of the saturated vapor
at the same temperature:
in the temperature range - 10 to 50oC (15 to 120oF).
13-21

Thermodynamics
Çengel
Boles
Third Edition
Chapter Summary
• The mass of water vapor present in 1 unit mass of
dry air is called the specific or absolute humidity
,
where P is the total pressure of air and Pv is the
vapor pressure.
13-22

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