Thermodynamics and Statistical Mechanics First Law of Thermodynamics.
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Transcript of Thermodynamics
THERMODYNAMICS
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
CONTENT:1.Laws of Thermodynamics2.Heat Engines3.Applications: Refrigerators, Air Conditioners and Heat Pump.
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.
INTRODUCTION
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
INTRODUCTION
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
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
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
INTRODUCTION
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
CLOSED, OPEN & ISOLATED SYSTEM
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
CLOSED, OPEN & ISOLATED SYSTEM
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
LAWS OF THERMODYNAMIC
ISOTHERMAL PROCESS – process that carried out at constant temperature
PV = constant
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
LAWS OF THERMODYNAMIC
THERMODYNAMIC PROCESSES
PV diagram for an ideal gas undergoing isothermal processes
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
LAWS OF THERMODYNAMIC
THERMODYNAMIC PROCESSES
PV diagram for an ideal gas undergoing isothermal processes
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
LAWS OF THERMODYNAMIS
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
THERMODYNAMIC PROCESSES
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
LAWS OF THERMODYNAMIS
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
Q = mc ΔT = mc (T2 – T1)
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 TL.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
HEAT ENGINE
Schematic diagram of energy transfer for heat engine
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
HEAT ENGINE
Reciprocating type
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
HEAT ENGINE
Turbine
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.
In internal combustion engine, the high temperature is achieved by burning the gasoline-air mixture in the cylinder itself.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
HEAT ENGINE
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.
Since energy is conserved, the heat input QH must equal the work done plus the heat that flows out at the low temperature QL.
** e could be 1.0 (@100%) only if QL were zero – that is only if no heat were exhausted to the environment.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
EFFICIENCY
HQ
We
H
L
H
LH
LH
LH
Q
Qe
Q
QQe
QQW
QWQ
1
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
CARNOT ENGINE
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
Real engine always have an efficiency lower than this because of losses due to friction and the like.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMET
CARNOT ENGINE
H
Lideal
H
LHideal
T
Te
T
TTe
1
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
Compressor
High Pressure Gas
Liquid Refrigerant
Evaporator
Condenser
Capillary tube
Compressor
High Pressure Gas
Liquid Refrigerant
Evaporator
Condenser
Capillary tube
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
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
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
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.
[refrigerator and air conditioner]
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
W
QCOP L
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;
Ideal refrigerator;
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
LH
LL
HL
Q
W
QCOP
QWQ
LH
Lideal TT
TCOP
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
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.
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
W
QCOP H
A refrigerator is removing heat at a rate of 6 kJ. The required power input to the refrigerator is 2kJ.
(a) COP = = = 3
(b) QH = QL + Wnet,in
= 6kJ + 2kJ= 8 kJ
QL
Wnet
6 kJ2 kJ
5℃25℃
QL = 6kJ
Wnet = 2kJ
QH = 8kJ
COP of an electric heater = 1, because the electricity is totally converted to heat
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
Start using an inverter in 1997
Change Refrigerants in 2000R134a(HFC) to R600a(HC)
Improvement of insulator in 2003Poly-urethane to Vacuum
insulator
Brushless DC compressor in 1992
Start using an inverter in 1996
Continual improvement of heat exchanger and Magnetic motor
Technical Breakthrough for energy savings
Refrigerator
0
20
40
60
80
1990 1992 1994 1996 1998 2000 2002 2004
kWh/
Mon
th t
h
Air conditioner
0
200
400
600
800
1000
1200
1990 1992 1994 1996 1998 2000 2002 2004
Wat
t t
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
Table Characteristics of Refrigerants
R-12 (HCFC)
R-134a(HFC)
R-600a(HC)
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
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
Chemical Characteristics
Environmental Characteristics
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP
DEPARTMENT OF APPLIED SCIENCE & ADVANCED TECHNOLOGY
UNIKL MIMETAPPLICATIONS: REFRIGERATOR, AIR CONDITIONERS &
HEAT PUMP