Thermodynamics

DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET

CONTENT:
1.Laws of Thermodynamics
2.Heat Engines
3.Applications: Refrigerators, Air
Conditioners and Heat Pump.

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 The study of thermodynamics is concerned with the ways energy is
stored within a body and how energy transformations (involve heat
and work).

 One of the most fundamental laws of nature is the conservation of
energy principle which states that during an energy interaction,
energy can change from one form to another but the total amount
of energy remains constant.

 That is, energy cannot be created or destroyed.

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 Thermodynamics is
 The science that examines the effects of energy transfer on
macroscopic materials systems.

 Thermodynamics predicts
 Whether a process will occur given long enough time
• driving force for the process

 Thermodynamics does not predict
 How fast a process will occur
• mechanism of the process

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 A thermodynamic system, or simply system, is defined as a quantity
of matter or a region in space chosen for study.

 The region outside the system is called the surroundings.

 The real or imaginary surface that separates the system from its
surroundings is called the boundary. The boundary of a system may
be fixed or movable.

 Surroundings are physical space outside
the system boundary.

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 Systems may be considered to be closed or open, depending on
whether a fixed mass or a fixed volume in space is chosen for study.

 A closed system consists of a fixed amount of mass and no mass may
cross the system boundary. The closed system boundary may move.

 Examples of closed systems are sealed tanks
and piston cylinder devices (note the volume
does not have to be fixed). However, energy
in the form of heat and work may cross the
boundaries of a closed system.

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 An open system, or control volume, has mass as well as energy
crossing the boundary, called a control surface. Examples of open
systems are pumps, compressors, turbines, valves, and heat
exchangers.

 An isolated system is a general system of fixed mass where no heat or
work may cross the boundaries.

 An isolated system is a closed system with no energy crossing the
boundaries and is normally a collection of a main system and its
surroundings that are exchanging mass and energy among themselves
and no other system.

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 The change in internal energy of a closed system U, will be equal to
the energy added to the system by heating the work done by the
system on the surroundings.

U=Q–W 1st Law of
Thermodynamics

 Q is the net heat added to the system
W is the net work done by the system
U is the internal energy of a closed system.

**First law of thermodynamics is conservation of energy.

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ISOTHERMAL PROCESS – process that carried out at constant
temperature

PV = constant

PV diagram for an ideal gas undergoing isothermal
processes

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ADIABATIC PROCESS – An adiabatic process is one in which no heat is
gained or lost by the system. The first law of thermodynamics with Q=0
shows that all the change in internal energy is in the form of work.

PV diagram for an ideal gas undergoing isothermal
processes

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ISOBARIC PROCESS – A process is one which the pressure is kept
constant.

ISOVOLUMETRIC PROCESS – A process is one in which the volume
does not change

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 Second Law of Thermodynamics is a statement about which
processes occur in nature and which do not.

Heat can flow spontaneously from a hot object to a cold object;
heat will not flow spontaneously form a cold object to a hot object.

Q = mc ΔT = mc (T2 – T1)

Q = quantity of heat transferred (J)
m = mass of the material (kg)
c = specific heat capacity (J/kg K)
T1= initial temperature (K or °C)
T2= final temperature (K or °C)
ΔT= temperature difference = T2 – T1

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 The idea is that the energy can be obtained
from thermal energy only when heat is allowed
to flow from a high temperature to a low
temperature.

 In each cycle the change in internal energy of
the system is U = 0 because it returns to the
starting state. QH at a high temperature TH is
partly transformed into work W and partly
exhausted as heat QL at a lower temperature
T L.
Schematic diagram of energy transfer
for heat engine

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Heated steam passes through the intake valve and
expand against a piston (forcing it to move)

As the piston returns to its original position, it
forces the gases out the exhaust valve.

Reciprocating type

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 Reciprocating piston is replaced by a rotating turbine
that resembles a paddlewheel with many set of
blades.

 The material that is heated and cooled, (steam) is
called working substance.

 In a steam engine, the high temperature is obtained
by burning coal, oil, or other fuel to heat the steam.

Turbine

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In internal combustion engine, the high temperature is achieved by burning the
gasoline-air mixture in the cylinder itself.

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 The efficiency, e, of any heat engine can be defined as the ratio of the work it
does, W, to the heat input at the high temperature, QH.

W
e
QH

 Since energy is conserved, the heat input QH must equal the work done plus the
heat that flows out at the low temperature QL.
QH W QL
W QH QL
QH QL
e
QH
QL
e 1
QH

** e could be 1.0 (@100%) only if QL were zero – that is only if no heat were exhausted to the environment.

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 Carnot engine consist of four
processes done in a cycle, two of
which are adiabatic and two are
isothermal.

 Each of the processes was done
slowly that the process could be
considered a series of equilibrium
states, and the whole process could
be done in reverse with no change
in the magnitude of work done or
heat exchanged.

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 Carnot showed that for an ideal reversible engine, the heat QH and QL
are proportional to the operating temperatures TH and TL so the
efficiency ca be written as
TH TL
eideal
TH
TL
eideal 1
TH
 Real engine always have an efficiency lower than this because of losses
due to friction and the like.

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Evaporator

Capillary tube

Liquid Refrigerant

Condenser

High Pressure Gas

Compressor

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 Electrical Energy => Kinetic Energy => Heat energy
 When refrigerants change from vapor to liquid, heat is discharged.
 On the contrary, changing from liquid to vapor, heat is absorbed

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 The principle of refrigerators, air conditioners and heat pumps is just the
reverse of a heat engine.

 Refrigerator: no work is required to take heat from the low-temperature
region to high-temperature region [no device is possible whose sole effect
is to transfer heat from one system at a temperature TL into a second
system at a higher temperature TH.]

 The coefficient of performance (COP) of a refrigerator is defined as the
heat QL removed from the low-temperature area divided by the work
done W to remove the heat.

QL
COP [refrigerator and
W air conditioner]

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 More heat , QL, that can be removed from inside the refrigerator for a
given amount of work, the better (more efficient) the refrigerator is.
 Energy is conserved;
QL W QH
QL QL
COP
W QH QL
 Ideal refrigerator;
TL
COPideal
TH TL

 Air conditioner works very much like a refrigerator, it takes heat QL
from inside a room or building at a low temperature, and deposits heat
QH outside the environment at a higher temperature.

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 Heat naturally flows from high to low
temperature, but for refrigerators and
air conditioners do work to
accomplish the opposite to make
heat flow from cold to hot.

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 Heat pump is usually reserved for a device that can heat a house in
winter by using an electric motor that does work W to take heat QL
from the outside at low temperature and delivers heat QH to the
warmer inside of the house.

 The objective of heat pump is to heat pump is to heat rather than to
cool. Thus the COP is defined directly than for an air conditioner
because it is the heat QH delivered to the inside of the house.
QH
COP
W

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A refrigerator is removing heat at a rate of 6 kJ.
The required power input to the refrigerator is 2kJ.
QL 6 kJ = 3
(a) COP = =
Wnet 2 kJ 25℃
(b) QH = QL + Wnet,in 5℃
= 6kJ + 2kJ= 8 kJ QL = QH =
6kJ 8kJ

 COP of an electric heater = 1,
because the electricity is totally Wnet = 2kJ
converted to heat

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R ef gerator
ri A i condi oner
r ti
80 1200
th

1000

t
60
kW h/M onth

800

W att
40 600
400
20
200
0 0
1990 1992 1994 1996 1998 2000 2002 2004 1990 1992 1994 1996 1998 2000 2002 2004

Technical Breakthrough for energy savings
 Start using an inverter in 1997  Brushless DC compressor in 1992
 Change Refrigerants in 2000  Start using an inverter in 1996
R134a(HFC) to R600a(HC)  Continual improvement of heat
 Improvement of insulator in 2003 exchanger and Magnetic motor
Poly-urethane to Vacuum insulator

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Table Characteristics of Refrigerants
R-12 R-134a R-600a
(HCFC) (HFC) (HC)
Chemical Characteristics
Effective displacement (L/kJ) 0.79 0.81 1.52
Evaporator pressure (kPa) 181.9 163.6 89.2
Condenser pressure (kPa) 743.2 770.7 403.6
Flammability no no yes
Environmental Characteristics
Atmospheric life time 130 yrs 16 yrs less than 1yr
Ozone Depletion Potential (R-12 = 1) 1 0 0
Global Warming Potential (CO2 = 1) 8500 1300 3

Thermodynamics

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