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


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INTRODUCTION


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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

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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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.


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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.


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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.


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LAWS OF THERMODYNAMIC

ISOTHERMAL PROCESS – process that carried out at constant temperature

PV = constant


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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.


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PV diagram for an ideal gas undergoing isothermal processes


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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


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.


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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.


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HEAT ENGINE

Schematic diagram of energy transfer for heat engine


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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.


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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.


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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.


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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.


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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.


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CARNOT ENGINE

H

Lideal

H

LHideal

T

Te

T

TTe

1


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



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



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]



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.



HEAT PUMP

LH

LL

HL

QQ

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.



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.



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



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



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



HEAT PUMP



HEAT PUMP

Thermodynamics

Technology

open system

isolated system

closed system boundary

thermodynamic system

main system

closed system u

internal energy ofthe

process thermodynamics