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CLB 20703
Chemical Engineering
Thermodynamics
Chapter 1:
Basic Concepts in Thermodynamics

Objective of Chapter 1
Introduce the students towards some of
the fundamental concepts and definitions
that are used in the study of Engineering
Thermodynamics
...
1 INTRODUCTION
What is Thermodynamics?
 Thermodynamics is the Science that deals
with Heat and Work and those properties of
substances that bear a relation to Heat and
Work
...

Thermodynamics is only concerned with
large scale observation
...
1 INTRODUCTION
Scopes of Thermodynamics
 First and Second Laws of Thermodynamics


To cope with variety of problems especially
in the calculation of Energy Changes, Heat
and Work requirements for processes



Property Values are essential to application
of Thermodynamics



Development of “Generalized Correlations”
to provide property estimates in the
absence of data

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...


Derived dimensions = combination of a few primary dimensions
...
2 DIMENSIONS AND UNITS
UNITS
(= magnitudes assigned
to the dimensions)
FUNDAMENTAL /
PRIMARY UNITS

-accompany primary
dimensions

DERIVED /
SECONDARY
UNITS*
-accompany derived dimensions

2 types of unit systems widely used:
i) English System / United States Customary Systems (USCS)
ii) Metric System, SI (International System)

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...
2 DIMENSIONS AND UNITS
Multiples and Decimal Fractions of SI units
are designated by prefixes
 Standard prefixes in SI units:


Prefix
tera, T
giga, G
mega, M
kilo, k
deci, d

Multiple
1012
109
106
103
10-1

Prefix
centi, c
milli, m
macro, 
nano, n
pico, p

Multiple
10-2
10-3
10-6
10-9
10-12

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 Boundary is the one that separates System
from its surrounding
...

BOUNDARY
 2 types of system:


 Closed

system/control mass
 Open system/control volume

SYSTEM

SURROUNDING

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...

Example of open system: compressor, turbine,
pump, nozzle

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...

 Energy in theform of heat and work can cross
the boundary
 Volume does not have to be fixed
...
3 SYSTEM

Mass cannot cross
the boundaries of a
closed system, but
energy can

An example of closed
system with a moving
boundary  pistoncylinder device

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Intensive
Property

-independent
of
mass of a system
Eg: Temperature T
Pressure P
Density 

Extensive
Property

-depend on the size of
a system
Eg: Mass
m
Volume
V
Total Energy
E

PROPERTY

the

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...

At a given state, all the properties of a
system have fixed values
...


m = 2 kg
T1 = 20oC
V1 = 1
...
5 m3
State 2

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 A System that is in equilibrium
experiences no changes when it is
isolated from its surroundings
...
3 SYSTEM







Thermal equilibrium if the temperature is the
same throughout the entire system
...

Phase equilibrium when the mass of each
phase reaches an equilibrium level and stays
there such as water and ice inequilibrium
...


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 Example of process – A
compression process in
a piston-cylinder device

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“Process that proceeds in the manner that
the System remains infinitesimally/approx
...
”
 Is a slow and Ideal process that allow the
System to adjust itself internally in order
that properties in one part of the system
do not change any faster than those at
other parts
...
3 SYSTEM


Processes in which one thermodynamic
property is kept constant:
Process
Constant property
Isobaric
Pressure
Isothermal
Temperature
Isochoric/isometric
Volume
Isentropic
Entropy

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...



P

2

Process
A
1

Process
B
V

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5 FORCE


From Newton’s second law:

Force = mass x acceleration (F = ma)
 Unit: N or kg/ms2 (SI unit), lbf (ES unit)
 The Newton, N is defined as
a force required to accelerate
A mass of 1 kg at the rate of
1 meter per second
...
6 TEMPERATURE







Temperature - a measure of “hotness” or
“coldness” or the energy content of a body
...

When heat is transferred to a body, E  T
...

Temperature applied in thermodynamic
problems must be in absolute units
...


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8T(oC) + 32
T(K) = T(oC) + 273
...
67
T(R) = 1
...
6 TEMPERATURE

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Other units: bar, standard atmosphere (atm)
...
7 PRESSURE
 Pressure

at any point in a fluid is same in all

directions
...

Pa

Pa=Pb=Pc
P1

P2

Pc

P3
Pb

P1=P2P3

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...
Measured relative to absolute
vacuum ( absolute zero )
...

 Vacuum Pressure – Pressure below
atmospheric pressure
...
7 PRESSURE

Pvac = Patm – Pabs

Pgage = Pabs – Patm

(for P
(for P>Patm)

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 The height of the fluid in the tube represents the
pressure difference between the system and the
surroundings of the manometer which is equal to the
gage pressure:
Patm

Pgage  P  P1  Patm  ρgh
P  P2  Patm  ρgh
1
 Pgas  Patm  ρgh
Patm  atmospheric pressure,
P  gas pressure in the tank,
1
ρ  density of the fluid in the manometer tube,
h  the height of fluid between two points in the U - tube,
g  gravitatio nal acceleration  9
...


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Therefore, if the energy crossing the
boundary of a closed system is not heat, it must be work
...
s

 Work

is also a form of energy transferred like heat and,
therefore, has energy units such as kJ/kNm

 The work done per unit time  power and is denoted W
...


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 Path
functions have inexact differentials
designated by the symbol  - a differential amount
of heat or work is represented by Q or W,
respectively:
2

 W  W
1

 The

12

total work is obtained by adding the
differential amounts of work (W) done along the
way or can represented by the area under the
followed path
...
8 WORK


Properties, however, are point functions - depend
on the state only, and not on how a system reaches
that state, and they have exact differentials
designated by the symbol d
...
a small change
in volume, is represented by dV, and the total
volume change during a process between states 1
and 2:



2

1

dV  V2  V1  V

The volume change is the
same regardless the path
followed
...
8 WORK
Mechanical forms of work
In many thermodynamic problems, mechanical
work is the only form of work involved
...

Some common forms of mechanical work are:
Boundary work
Accelerational work
Shaft work
Spring work
Gravitational work

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...

Expansion work (W=+ve) and compression work (W=-ve) are
categorized as boundary work
...
8 WORK
The area under the process curve on a
P-V diagram is equal, in magnitude, to
the work done during a expansion or
compression process of a closed
system
P

represents the absolute pressure and is
always positive
...


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8 WORK
Common processes
Constant Volume/Isochoric/Isometric
 If

the volume is constant, dV=0, the boundary work :
2

Wb   PdV  0
1

 for constant volume process, the boundary work = 0
...
8 WORK
Constant Pressure/Isobaric
 If

the pressure is constant, the boundary work:
2

2

Wb   PdV  Po  dV  Po (V2  V1 )
1

1

or Wb  mPo (v2  v1 )
 The

magnitude of the work can also be determined
through the area under the curve of P-V diagram
...
8 WORK
Constant temperature/Isothermal
 If

the temperature of an ideal gas is constant, hence the
equation of state for the ideal gas:
P

 And

mRT
V

the boundary work:
2

Wb   PdV  
1

 The

2

1

mRTo
V2
dV  mRTo ln
V
V1

above equation is applicable for ideal gases only
...


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The non-adiabatic expansion
or compression of a fluid is an example of a polytropic process
...
3
A rigid tank contains air at 500kPa and
150OC
...
Determine the work done
during the process
...
4
A piston cylinder device initially contains
0
...

The nitrogen is now expand politropically to a
state of 100kPa and 100OC
...


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Definition: Energy = Force x Distance (Unit = N
...

Specific energy- Total energy based on a unit of mass,
e(kJ/kg)
Total energy can be divided into 2 groups:
1) Macroscopic energies – related to motion and the
influence of some external effects such as gravity,
magnetism, electricity, surface tension, kinetic and
potential energies
...
Eg : chemical, nuclear, latent heat, sensible
heat
...


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- Phase change of a system such as
liquid phase changes to gas phase
...

- Strong bonds within the nucleus of
atoms
...
9 ENERGY
Macroscopic Energy
 2 main forms of macroscopic energies:
1) Kinetic energy – a system possesses as a result of
its motion relative to some reference frame:
mV 2
(kJ)
KE  2
with, V = velocity of the system relative to a fixed
reference frame
...

PE  mgz (kJ)
with, g = gravitational acceleration, z = elevation of
the gravity centre of a system
...


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Hence, the change in total
energy of a stationary system is equal to the
change of its internal energy:
E  U
2

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Heat has energy units, kJ (or Btu) and denoted by
Q
...


The heat transfer rate is denoted Q
...


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There are two ways a process can be
adiabatic: Either the system is well
insulated - only a negligible amount of
heat can pass through the boundary,
or both the system and the
surroundings are at the same
temperature - there is no driving force
(temperature difference) for heat
transfer
...
Isothermal process refers to
the temperature of the system that
are the same at different state

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Title: Basic Concept in Thermodynamics
Description: In this notes, you will learn the basic concept of thermodynamics. This note is specifically to 1st year degree that are taking course of chemistry engineer.

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