Thermodynamics

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WELCOME TO

THERMODYNAMICS

CHEM 222

Zin-Eddine Dadach

2014-2015


What is thermodynamics?

A branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics

http://en.wikipedia.org/wiki/Physics

http://en.wikipedia.org/wiki/Temperature

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Volume

http://en.wikipedia.org/wiki/Physical_system

http://en.wikipedia.org/wiki/Macroscopic

http://en.wikipedia.org/wiki/Statistics


DEFINITION OF

THERMODYNAMICS

Thermodynamics (from the Greekthermos meaning heat and dynamicsmeaning power)

During our course, we will study the effects of heat and work transfer on a specified system.

http://en.wikipedia.org/wiki/Greek_language


LAWS OF

THERMODYNAMICSThe thermodynamics is the lawyer of nature and has different laws to

be applied.


THE IMPORTANCE OF THE

LAWS

The starting point for most thermodynamic considerations are the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work.

They also postulate the existence of a quantity named entropy, which can be defined for any system.

http://en.wikipedia.org/wiki/Laws_of_thermodynamics

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Mechanical_work

http://en.wikipedia.org/wiki/Entropy


THE SYSTEM

The most important point in thermodynamics:

DEFINING THE SYSTEM


SYSTEM VS SURROUNDINGS

In thermodynamics, interactions between large ensembles of objects are studied and categorized.

Central to this are the concepts of system and surroundings.

http://en.wikipedia.org/wiki/System_%28thermodynamics%29

http://en.wikipedia.org/wiki/Surroundings_%28thermodynamics%29


WHAT IS A SYSTEM?

A system is composed of particles, whose average motions define its properties, which in turn are related to one another through equations of state.

In chemical engineering, a system could be a gas or a liquid flowing in a pipe or in a tank or reactor

http://en.wikipedia.org/wiki/Equations_of_state


PROPERTIES OF A SYSTEM

Any system can be defined by its pressure, temperature, composition, or other more complicated properties like SPECIFIC entropy , SPECIFIC enthalpy….


SYSTEM & THERMODYNAMICS

Thermodynamics describes how systems respond to changes in their surroundings.

This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes.

The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, cell biology, biomedical engineering, and materials science to name a few

http://en.wikipedia.org/wiki/Science

http://en.wikipedia.org/wiki/Engineering

http://en.wikipedia.org/wiki/Engines

http://en.wikipedia.org/wiki/Phase_transitions

http://en.wikipedia.org/wiki/Chemical_reactions

http://en.wikipedia.org/wiki/Transport_phenomena

http://en.wikipedia.org/wiki/Black_holes

http://en.wikipedia.org/wiki/Physics

http://en.wikipedia.org/wiki/Chemistry

http://en.wikipedia.org/wiki/Chemical_engineering

http://en.wikipedia.org/wiki/Cell_biology

http://en.wikipedia.org/wiki/Biomedical_engineering

http://en.wikipedia.org/wiki/Materials_science


DEFINING A SYSTEM


WHAT IS A SYSTEM?

A Thermodynamic System is that part of the universe that is under consideration or study.

A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the environment or surroundings(sometimes called a reservoir.)

http://en.wikipedia.org/wiki/Surroundings_%28thermodynamics%29


Exchange between Systems &

Surroundings

A useful classification of thermodynamic systems is based on the nature of the boundary and the flows quantities through it as matter, energy, work, heat, and entropy.

A system can be anything, for example a piston, a fluid in a test tube, a living organism, or a planet

http://en.wikipedia.org/wiki/Piston

http://en.wikipedia.org/wiki/Test_tube

http://en.wikipedia.org/wiki/Organism

http://en.wikipedia.org/wiki/Planet


DIFFERENT KINDS OF

SYSTEMS


WHAT IS AN ISOLATED

SYSTEM?

Isolated systems are completely isolated in every way from their environment.

They do not exchange heat, work or matter with their environment.

An example of an isolated system would be an insulated rigid container, such as an insulated gas cylinder.

http://en.wikipedia.org/wiki/Isolated_system


WHAT IS A CLOSED SYSTEM?

Closed systems are able to exchange energy (heat and work) but not matter with their environment.

A greenhouse is an example of a closed system exchanging heat but not work with its environment.

Whether a system exchanges heat, work or both is usually thought of as a property of its boundary.

Figure 1.16 page 10

http://en.wikipedia.org/wiki/Closed_system


WHAT IS AN OPEN SYSTEM?

open systems: exchanging both energy (heat and work) and matter with their environment.

A boundary allowing matter exchange is called permeable.

The ocean would be an example of an open system.

Figure 1.19 page 11

http://en.wikipedia.org/wiki/Open_systems


PROPERTIES OF A

SYSTEM


WHAT IS A PROPERTY OF A

SYSTEM ?

Any characteristic of a system is called a property.

Some familiar properties are pressure P, temperature T, volume V and mass m.

We can include viscosity, thermal conductivity, modulus of elasticity, thermal expansion, ….

Not all properties are independent however, some are defined in terms of others.

Example : ρ = m/V


WHAT IS AN INTENSIVE

PROPERTY? Thermodynamic properties can be divided into two gen

eral classes, intensive and extensive properties. An intensive property is independent of the amount

of mass. Examples of intensive properties include: temperature viscosity density electrical resistivity melting point boiling point color (in solution) flammability . Figure 1.20 page 12


http://en.wikipedia.org/wiki/Viscosity

http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Electrical_resistivity

http://en.wikipedia.org/wiki/Melting_point

http://en.wikipedia.org/wiki/Boiling_point

http://en.wikipedia.org/wiki/Color

http://en.wikipedia.org/wiki/Solution

http://en.wikipedia.org/wiki/Flammability


WHAT IS AN EXTENSIVE

PROPERTY? An extensive property is a property that

changes when the size of the sample changes.

Examples of extensive properties include: mass volume entropy energy electrical resistance texture Figure 1-20 page 12

http://en.wikipedia.org/wiki/Mass


http://en.wikipedia.org/wiki/Entropy

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Electrical_resistance

http://en.wikipedia.org/wiki/Texture


STATE OF A SYSTEM

DEFINING COMPLETELY THE SYSTEM


STATE OF A SYSTEM

A system not undergoing any change

All the properties can be measured or calculated throughout the entire system

This set of properties describe the state of the system


THERMODYNAMICS &

EQUILIBRIUM

Thermodynamics deal ONLY with equilibrium states, that means there is no driving forces within the system

The word Equilibrium implies a state of balance

Outside the equilibrium, systems can exchange heat and work and matter with the surroundings


ZEROTH LAW OF

THERMODYNAMICS It is observed that a higher temperature

object which is in contact with a lower temperature object will transfer heat to the lower temperature object.

The two objects will approach the same temperature, and in the absence of loss to other objects, they will then maintain a constant temperature.

They are then in thermal equilibrium. Thermal equilibrium is the subject of the

Zeroth Law of Thermodynamics. Also Figure 1.24 page 14

http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/temper.html#c1

http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heatra.html#c1


WHAT IS A THERMODYNAMIC

EQUILIBRIUM? If you put a hot cup of coffee or tea in a cold room, heat

will flow from the cup and its contents into the room. But if you leave it there for two hours and come back, the cup of coffee or tea will be at the same temperature as the room.

The coffee and the room will have achieved a state of thermodynamic equilibrium Thermodynamic Equilibrium or Thermal equilibrium.

Thermodynamic equilibrium is when heat ceases to flow between two systems.

There is no heat transfer when thermodynamic equilibrium is reached.

http://discover.edventures.com/functions/termlib.php?action=&single=&word=heat

http://discover.edventures.com/functions/termlib.php?action=&single=&word=heat+transfer


Class Work

Study examples 1-1 and 1-2 page 8


The different phases of

the system

Remember in our course the system will be a fluid or solid

SOLID-LIQUID-GAS


DIFFERENT PHASES OF THE

MATTER (THE SYSTEM)

Matter can exist in four phases (or states), solid,

liquid,

gas, and

plasma

plus a few other extreme phases, like critical fluids and degenerate gases


SOLID PHASE

A solid is matter in which the

molecules are very close together and cannot move around.

Examples of solids include rocks,

wood, and ice (frozen water).


LIQUID PHASE

A liquid is matter in which the

molecules are close together and move around slowly.

Examples of liquids include

drinking water, alcohol, oil, mercury at room temperature, and lava (molten rock).


GAS PHASE

A gas is matter in which the molecules are widely separated, move around freely, and move at high speeds.

Examples of gases include the gases we breathe (nitrogen, oxygen, and others), the helium in balloons, and steam (water vapor).


This P-T phase diagram of water

shows its phases at various

temperatures and pressures


LET’S ANALYZE THE DIAGRAM

FROM THE SOLID STATE: When a solid is heated (or as pressure decreases), it will change to a liquid form,

and when the liquid is heated it will eventually become a gas.

FOR THE DIAGRAM: ice (frozen water) melts into liquid water when it is heated or pressure decreases. As the water is heated , the water evaporates and becomes water vapor.


WHAT IS A SUBCOOLED LIQUID?

Liquid refrigerant which is cooled below its saturation temperature.

NO READY TO VAPORISE EXAMPLE: Consider a piston cylinder ( Figure 3-6, page 114) device containing liquid water at 200C and 1 atm. Under these conditions, water exist in liquid phase and is called compressed liquid or subcooled liquid


WHAT IS A SATURATED LIQUID?

A Liquid that is about to vaporize

Example:

If we heat the water liquid at 400C , the liquid will expand and its specific volume increases

If we continue to heat the water liquid up to 1000C ( Figure 3.7 page 114)

At this point the water is still liquid BUT any additional heat will cause some liquid to vaporize ( Phase change process from liquid to gas)

WE HAVE A SATURATED LIQUID


WHAT IS A SATURATED VAPOR?

A Vapor that is about to condense

A vapor + one drop of liquid

Once boiling starts, the temperature of the liquid stops rising and the vapor starts to be produced

we have change of phase from

vapor to liquid

Figure 3.9 page 115


WHAT IS A SUPERHEATED

VAPOR?

Once the phase change is completed, we are in a single phase state which is vapor only

When a vapor is far from condensing , it’s called Superheated vapor

Figure 3-10 page 115


WHAT IS WET STEAM?

Mixture of liquid and steam of the same substance in which both are at saturation temperature.

If additional heat is added to the wet steam at constant pressure, the temperature remains constant until all liquid is evaporated (saturated steam);

it is only at this point that the temperature increases above the saturation temperature (superheated steam )


WHAT IS SATURATION

TEMPERATURE?

The term saturation defines a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure.

The temperature at which vaporization ( Boiling) starts to occur for a given pressure is called the saturation temperature or boiling point


WHAT IS A SATURATION

PRESSURE?

The pressure at which the vaporization ( boiling) starts to occur for a given temperature is called the saturation pressure

For water at 2120F, the saturation pressure is 14.7 psia

For water at 14.7 psia, the saturation temperature is 2120F.

The graphical representation of this relationship between temperature and pressure at saturated conditions is the VAPOR PRESSURE CURVE


DRAW A VAPOR PRESSURE

CURVE (Psat vs Tsat) OF WATER

T ( 0C) :

-10, -5, 0, 5,10,15,20,25,30,40,50,100, 150,

Saturation Pressure ( kPa)

0.26,0.40,0.61,0.87,1.23,1.71,2.34,3.17,

4.25,7.38,12.35, 101.3, 475.8,


GROUP- WORK

Draw a T-V diagram for the temperature change from 100 to 3000C.


CHANGES OF PHASE


Latent heat and sensitive heat

when your system is heated up or cooled down without changing phase the temperature changes and we have sensitive heat

When the system is changing phase, the heat is used by the system for the change of phase by breaking down the bridges between molecules.

if we have a pure component, the temperature stays constant

if we have a mixture, the temperature will change during the change of phase


WHAT IS LATENT HEAT?

The amount of energy needed or is released during a phase-change process is called thelatent heat

For a pure substance, the change of phase happens in constatnt temperature


Latent heat of fusion

The amount of heat absorbed during melting


Latent heat of vaporization

A change of state from saturated liquid to saturated vapor at constant temperature also requires the input of energy, called the latent heat of vaporization.


Magnitudes of latent heat

The magnitudes of the latent heats of condensation and vaporization depend on the temperature or pressure at which phase change occurs.

At 1atm , the latent heat of fusion of water is 333.7 kJ/kg and the latent heat of vaporization of water is 2257.1 kJ/kg around 99.80 0C


Wet steam region

Figure 3.34 page 129: relative amount of liquid and vapor

Figure 3.35: Average volume


Specific volume of Sat. Liquid and

Sat. vapor In the wet steam region, we have a mixture of

saturated liquid and saturated vapor

Let’s study saturated steam tablesA-4 & A-5 pages 916-917:

At 450C => PS= 9.5953 kPa ( A-4)At 20kPa =>TS= 60.060C ( A-5)At 1000C => Specific Volume of Sat.Liq= 0.001043

m3/kg=> Specific Volume of Sat.Vap= 1.6720

m3/kg


Quality of wet steam

The quality x of a wet steam is defined as the ratio of the mass of vapor over the Total mass

x= Massvapor/Masstotal

And:

Masstotal= Mass vapor + Massliquid


Example of the use of Quality

For the volume:

Vtotal= Vliquid + Vvapor

In table A-4 ( page 916) for 1000C

Vf ( liquid)= 0.001043 m3/kg

Vg(vapor) = 1.6720 m3/kg

V= (mf .vf) + (mg.vg)

V= ({mt-mg} .vf) + (mg.vg)


Average value of a property of a

mixture

V=m.Vaverage

Diving both sides by mt

Vaverage= (1-x).Vf + x.Vg

Vaverage= Vf-x.Vf+x.Vg

= Vf + x. ( Vg-Vf)

= Vf +x. Vfg

Can be applied to other properties in the tables.


CLASS WORK

Work problems 3-1c to 3-9c page 154

WORK EXAMPLES 3-1 TO 3-9 PAGES 128-135


Class work

Complete the tables 3.23 to 3.28 page 155

Solve : 3-29 to 3-36 pages 155-156


EXCHANGE OF HEAT

AND WORK

SYSTEM AND SURRONDINGS


SYSTEM & SURRONDINGS

DURING ANY PROCESS, A CLOSED SYSTEM AND SURRONDINGS EXCHANGE HEAT AND WORK.

THE LEVEL OF ENERGY OF THE SYSTEM WILL BE CHANGED DURING THE CHANGE OF STATE.

1) Across a pump, the pressure of liquid will increase ( Energy increases)

2) Across a heat exchanger, the temperature of the system will change ( energy changes).


WHAT IS ENERGY?

Energy is defined as: "the ability to do work.“

Our bodies transform the energy stored in the food into energy to do physical work

Work means moving something, lifting something, warming something, lighting something. All these are a few of the various types of work.

But where does energy come from?


SOURCES OF ENERGY?

The forms of energy in nature we will look at include:

Electricity

Biomass Energy - energy from plants

Geothermal Energy

Fossil Fuels - Coal, Oil and Natural Gas

Hydro Power and Ocean Energy

Nuclear Energy

Solar Energy

Wind Energy


Total and specific energy

Energy can exist in different forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical and nuclear

The total energy E is an extensive property

The specific energy is an intensive property defined as the total energy of a system by unit mass:

e=E/m


Energy in thermodynamics

Thermodynamics deals only with the change of the total energy and give no information about the absolute value of the total energy ( we need a reference).

The macroscopic forms of energy are those a system posses as a whole such as kinetic energy and potential energy.

The microscopic energy are related to the molecular structure and the sum of all the microscopic energies is called internal energy (U)


WHAT IS KINETIC ENERGY?

Kinetic energy is energy of motion. The kinetic energy of an object is the energy it possesses because of its motion.

The kinetic energy* of a point massm is given by : ( Figure 2.3 page 53)

http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#coneng

http://hyperphysics.phy-astr.gsu.edu/hbase/mass.html


WHAT IS POTENTIAL ENERGY?

Gravitational potential energy is energy an object possesses because of its position in a gravitational field.

Figure 2.37 page 70

http://hyperphysics.phy-astr.gsu.edu/HBASE/pegrav.html#pe

http://hyperphysics.phy-astr.gsu.edu/HBASE/conser.html#coneng


WHAT IS INTERNAL ENERGY?

Internal energy is defined as the energyassociated with the random, disordered motion of molecules. Figures 2.5 and 2.6 page 55

it refers to the invisible microscopic energy on

the atomic and molecular scale

http://hyperphysics.phy-astr.gsu.edu/hbase/enecon.html


WHAT IS WORK?

Systems possess energy BUT they can not posses work or heat

Energy can be transferred to or from a closed system in two distinct forms : work or heat ( Figure 2-11 page 60)

Work is an energy interaction between a system and its surroundings ( Figures 2.24-2.30 pages 65-67)

Work is an energy transfer associated with a force acting through a distance


WHAT IS HEAT?

Form of energy that is transferred between a system and its surroundings by virtue of a temperature difference ( Figures 2.12 to 2-14 page 60)

Energy is recognized as heat transfer only as it crosses the system boundaries ( Figure 2.13 page 61)


WHAT IS POWER?

Power is defined as work (or energytransfer) per unit time: P = dW / dtThe SI unit of power is watt (w).

1 w= 1 J/s

http://www.taftan.com/thermodynamics/WORK.HTM

http://www.taftan.com/thermodynamics/ENERGY.HTM

http://www.taftan.com/thermodynamics/TIME.HTM

http://www.taftan.com/thermodynamics/SIUNIT.HTM


CLASS WORK

Do problems 2-7 to 2-11 page 98.

Do problems 2-18 to 2-20 page 98.


THERMODYNAMIC

STATES


WHAT IS A THERMODYNAMIC

STATE?

A thermodynamic state (from Latin status = to stand) is the state of a thermodynamic system as defined by its properties

A minimum number of parameters are necessary to specify the state of the system.

http://en.wikipedia.org/wiki/Latin

http://en.wikipedia.org/wiki/Thermodynamic_system


STATE OF A SYSTEM

The state of a system can be thought of as an optimal ensemble of thermodynamic parameters, namely temperature, pressure, density, composition, etc., which characterize the system, but neither by its surroundings nor by its history.



http://en.wikipedia.org/wiki/Density

http://en.wikipedia.org/wiki/Composition


WHAT IS A STATE FUNCTION?

In thermodynamics, a state function, is any property of a system that depends only on the current state of the system, not on the way in which the system got to that state

Some familiar state functions are volume, V, pressure, P, and temperature, T.

http://en.wikipedia.org/wiki/Thermodynamics


STATE FUNCTION

For example, if we take a state from 00C to 1000C, the change in the temperature is +100oC whether we go straight up the temperature scale or we first cool the sytem for a few degrees then take the system to the final temperature.

The change in temperature is independent of the route taken so temperature is a state function ( FIGURE 1-26 page 15 & figure 1.28 page 16)

Conversely, if the change in a function isdependent on the route taken, then the function is known as a path function

PATH FUNCTION

A function that depends on the path

Heat and Work depend on the way the exchange is performed between system and surroundings.

Efficiency depends on the way

Work and Heat are path functions



Internal energy U &

Enthalpy H


WHAT IS INTERNAL ENERGY?

. Internal energy, represented by U , is essentially the thermal energy contained in a system (or particles making up the system)

. A change in internal energy ΔU is due to the transfer of energy into or out of a system, but the volume stays constant.

For example, energy transferred into the system, usually heat (q) and work (w), represents an increase of internal energy, ΔU, of the system. Thus,

ΔU = q + w


Internal Energy

Internal Energy U of a system does not tell how energy was transferred.

It is purely an accounting of energy content of the system.

The internal energy, U, is called a state function.


WHAT IS ENTHALPY?

Enthalpy (symbolized H, also called heat content)

Is the sum of the internal energy of matter and the product of its volume multiplied by the pressure

Enthalpy is defined by the following equation:

Enthalpy is a quantifiable state function, and the total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead.

We need a reference. Example : Enthalpy at 00C.

http://en.wikipedia.org/wiki/Internal_energy

http://en.wikipedia.org/wiki/State_function

http://en.wikipedia.org/wiki/System_%28thermodynamics%29


Enthalpy

The enthalpy of a system includes the internal energy and work ( external energy).

Work is performed only if there is a volume change (Figures 4-1 & 4-2 page 166)

W= P.ΔV


P-V Diagram

In a P-V diagram, Work is the area under the process from point 1 to point 2 ( Figure 4-3 & 4-4 page 167)


Class work

Work examples 4-1, 4-2, 4-3 and 4-6 pages 168-176)

Work problem 4.9 page 202


Sensitive Heat &

Specific heat

Intensive property in heat transfer

Latent & Sensitive heat

Latent heat or hidden is the heat for the change of phase. Heat needed to destroy the intra-molecular forces.

Sensitive heat is visible heat due to the rise of temperature during the heating of a single phase ( solid, liquid or gas).



WHAT IS SPECIFIC HEAT?

The specific heat is an intensive property.

The amount of sensitive heat per unit mass required to raise the temperature by one degree Celsius.

The relationship between sensitive heat and temperature change is usually expressed in the form shown below where c is the specific heat.

unlike the extensive variable heat capacity, which depends on the quantity of material, specific heat is an intensive variable and has units of energy per mass per degree (or energy per number of moles per degree).

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/temper.html#c1

http://scienceworld.wolfram.com/physics/ExtensiveVariable.html

http://scienceworld.wolfram.com/physics/HeatCapacity.html

http://scienceworld.wolfram.com/physics/IntensiveVariable.html


Specific heat a constant pressure

A heat capacity by a capital C with a subscript denoting which variable is held constant during the temperature change.Figures 4-17 & 4-18 page 178

For example, the heat capacity at constant pressure is defined by:

CP= ( ΔQ/ΔT)p= (ΔH/ΔT)

P where (ΔQ/ ΔT) is the change in heat

with temperature. Here we can have work Enthalpy is used

http://scienceworld.wolfram.com/physics/Temperature.html

http://scienceworld.wolfram.com/physics/Pressure.html


Specific heat at constant volume

At constant volume : no work

CV= ( ΔQ/ΔT)V= (ΔU/ΔT)v

where (ΔQ/ ΔT) is the change in heat with temperature.


Class Work

Work examples 4-7, 4-8 and 4-10 pages 183 -185


PROCESSES


WHAT IS REVERSIBLE

PROCESS?

A reversible process for a system is defined as a process that, once having taken place, can be reversed, and in so doing leaves no change in either the system or surroundings

In other words, the system and the surroundings are returned to their original condition


REVERSIBLE PROCESS

In reality, there are no truly reversible process , therefore the reversible process is a starting point for thermodynamics calculations.

One way to make a real process close to a reversible process is to carry out the process in a series of infinitesimal steps


WHAT IS AN IRREVERSIBLE

PROCESS?

An irreversible process is a process that can not return both the system and the surroundings to their original conditions

It means if the system is reversed, the system and the surroundings will not return to their original conditions.


EXAMPLE

For example, an automobile engine does not give back fuel it took to drive up a hill as it coasts back down the hill.

Main causes of Irreversibility: Friction, unrestrained expansion, heat transfer through finite temperature difference and mixing two different substances


WHAT IS A CYCLIC PROCESS?

When a system in a given initial state goes through a number of different states ( go through various processes) and finally returns to its initial values , the system has undergone a cyclic process or cycle.

Therefore, all the properties have the same value they had at the beginning.


EXAMPLE

Steam that circulates through a closed cooling loop undergoes a cycle.


WHAT IS AN ISOTHERMAL

PROCESS?

An isothermal process is a thermodynamic process in which the temperature of the system stays constant: ΔT = 0.

This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and processes occur slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange



http://en.wikipedia.org/wiki/System

http://en.wikipedia.org/wiki/Constant

http://en.wikipedia.org/wiki/Heat_bath

http://en.wikipedia.org/wiki/Process

http://en.wikipedia.org/wiki/Heat


In an isothermal process, therefore, all of the heat absorbed by the system is converted into work.


WHAT IS AN ISOBARIC

PROCESS?

An isobaric process is a thermodynamic process in which the pressure stays constant; ΔP = 0.

The heat transferred to the system does work but also changes the internal energy of the system.

http://en.wikipedia.org/wiki/Thermodynamic


WHAT IS AN ISOCHORIC

PROCESS?

An isochoric process, also called an isometric process, is a thermodynamic process in which the volume stays constant; ΔV = 0.

This implies that the process does no pressure-volume work, since such work is defined by ΔW = PΔV,

http://en.wikipedia.org/wiki/Thermodynamic

http://en.wikipedia.org/wiki/Work_%28thermodynamics%29


WHAT IS AN ADIABATIC

PROCESS? In thermodynamics, an adiabatic process is a

process in which no heat is gained or lost in the working fluid.

For example, there are no chemical processestaking place in the fluid and there is no heat transfer from the environment.

The term "adiabatic" describes things that are impermeable to heat transfer; for example, an adiabatic boundary is a boundary that is impermeable to heat transfer and the system is said to be adiabatically (or thermally) insulated.

An adiabatic process which is also reversible is called an isentropic process.


http://en.wikipedia.org/wiki/Heat

http://en.wikipedia.org/wiki/Fluid

http://en.wikipedia.org/wiki/Chemical_process

http://en.wikipedia.org/wiki/Heat_transfer

http://en.wikipedia.org/wiki/Reversible_process

http://en.wikipedia.org/wiki/Isentropic_process


CLASS WORK # 1

DO EXAMPLES 2-1 to 2-9 in pages 57-69 of the book


THERMODYNAMIC

DIAGRAMS


THE P-V-T SURFACE

The equilibrium states of a simple, compressible substance can be specified in terms of its pressure, volume and temperature.

If any two of these state variables is specified, the third is determined.

This implies that the states of the substance can be represented as a surface in a three dimensional P-V-T space


PVT SURFACE ( FIGURE 2.26)


WHAT ARE THE REGIONS IN A

PVT SURFACE? The solid, liquid and gas (vapor) phases can be

represented by regions on the surface.

Note that there are regions on the surface which represent a single phase, and regions which are combinations of two phases.

A point lying on a line between a single-phase and a two-phase region represents a "saturation state".


Saturation Lines

The line between the liquid and the liquid-vapor regions is called the liquid-saturation line and any point on that line represents a saturated-liquid state.

A point on the boundary between the vapor and the liquid-vapor regions is called a saturated-vapor state.


P-V-T SURFACE FOR

LIQUID-VAPOR REGION


P-V Diagram

A process performed at constant pressure is called an isobaric process. It’s a horizontal line

A process performed at constant volume is called an isochoric process. It’s a vertical line

A process performed at constant temperature is called an isothermal process.

On a p-V diagram, lines of constant temperature curve from the upper left to the lower right.

During an adiabatic process no heat is transferred to the gas, but the temperature, pressure, and volume of the gas change as shown by the dashed line.

http://www.grc.nasa.gov/WWW/K-12/airplane/heat.html


T-V Diagram


In this figure :

The isotherms are ………..

The isochores are ……….

The isobars are …………..


WHAT IS THE CRITICAL POINT?

The two phase region is bounded by the saturated liquid curve on one side and the saturated vapor curve on the other.

Notice that these two curves meet at a point that corresponds to where one (and only one) of the isobars becomes horizontal at a single point.

This point is called the critical point and

it corresponds to the highest temperature and highest pressure for which a vapor and liquid can coexist.

The temperature at this point is called the critical temperature Tc.

http://www.eng.usf.edu/~campbell/ThermoI/Proptut/tut6f3.html




The isobar that passes through this point (the only isobar that is horizontal at a single point) is called the critical pressure Pc.

The specific volume at this point is called the critical volume Vc.


WHERE IS THE SUBCOOLED

LIQUID REGION?

The single phase liquid region, also known as the subcooled liquidregion is to the left of the two phase region.

It is sometimes considered to end at the critical temperature as shown



WHERE IS THE SUPERHEATED

VAPOR REGION?

The single phase vapor region, also known as the superheated vaporregion, is to the right of the two phase region.

This region often is (arbitrarily) considered to end at the critical isobar.



WHAT IS THE SUPERCRITICAL

FLUID REGION?

You may have noticed that there is a region on the diagram that we have not yet discussed.

The supercritical fluid region is so called because states in this region are above both the critical temperature and critical pressure.



Supercritical Fluid

Material in this region has properties somewhat intermediate between what most people would call a gas and what most would call a liquid.

Sometimes this is called the dense gasregion and other times the expanded liquid region.

The word fluid covers both gases and liquids so perhaps supercritical fluid is the best way to describe this region.


WHAT IS THE CRITICAL STATE?

Note the critical state where the saturated-liquid and saturated-vapor lines meet.

The state variables of this unique point are denoted by Pc, vc and Tc.

If a substance is above the critical temperature Tc, it cannot condense into a liquid, no matter how high the pressure.


Supercritical State

This merging of the liquid and vapor states above the critical temperature is a characteristic of all known substances.

While a pure vapor state can exist at a pressure lower than Pc, at pressures above Pc it is constrained to be a vapor.

States with pressures above Pc are described as "supercritical states".


WHERE IS THE SUPERCRITICAL

FLUID REGION?


THE P-T DIAGRAM


The vapor pressure curve which is shown in red.

The vapor pressure curve terminates at the critical point. Remember that this point marks the highest temperature (T = Tc) and highest pressure (P = Pc) for which vapor and liquid can coexist.

The other end of the vapor pressure curve is marked by the triple point





CLASS WORK

DO EXAMPLES 3-1 TO 3-9 PAGE 128 BY STUDYING THE DIFFERENT THERMODYNAMIC CURVES CORRESPONDING TO THE PROBLEMS.


THE GAS PHASE


WHAT IS A GAS?

In a gas phase, the atoms or molecules constituting the matter basically move independently, with no forces keeping them together or pushing them apart.

Their only interactions are rare and random collisions.

http://en.wikipedia.org/wiki/Atoms

http://en.wikipedia.org/wiki/Molecules

http://en.wikipedia.org/wiki/Interaction

http://en.wikipedia.org/wiki/Random


THE GAS ATOMS


VAPOR VS GAS ( FIGURE 3.26

page 125) Although vapor and gas are frequently

(incorrectly) used interchangeably,

vapor refers to a gas phase in a state of equilibrium with identical matter in a liquid or solid state below its boiling point, or at least capable of forming solid or liquid at the temperature of the vapor.

The term gas refers to a compressible fluid phase, as in common usage

GAS EXIST AT TEMPERATURES HIGHER THAN TC

VAPOR EXIST AT TEMPERATURES LOWER THAN TC

http://en.wikipedia.org/wiki/Thermodynamic_equilibrium

http://en.wikipedia.org/wiki/Liquid

http://en.wikipedia.org/wiki/Solid

http://en.wikipedia.org/wiki/Boiling_point


THE GAS PHASE

The thermodynamic state of a gas is characterized by:

its volume, its temperature, which is determined by the average velocity or kinetic energy of the molecules,

and its pressure, which measures the average force exerted by the molecules colliding against a surface.

These variables are related by the fundamental gas laws, which state that the pressure in an ideal gas is proportional to its temperature and number of molecules, but inversely proportional to its volume.




http://en.wikipedia.org/wiki/Gas_laws

http://en.wikipedia.org/wiki/Ideal_gas


IDEAL GAS VS REAL GAS

An ideal gas or perfect gas is a hypothetical gas consisting of identical particles of zero volume, with no intermolecular forces.

Real gases do not exhibit these exact properties, although the approximation is often good enough to describe real gases.

The approximation breaks down at high pressures and low temperatures, where the intermolecular forces play a greater role in determining the properties of the gas

http://en.wikipedia.org/wiki/Gas

http://en.wikipedia.org/wiki/Intermolecular_force


IDEAL GAS


IDEAL-GAS LAW

Any equation that relates the pressure, temperature and specific volume of a substance is called equation of state.

In 1662, Robert Boyle observed, during his experiments with a vacuum chamber that the pressure of gases is inversely proportional to their volume

In 1802, Guy Lussac, experimentally determined that a low pressures the specific volume of a gas is proportional to its temperature

That is Pv = RT -1-

With R being the gas constant, this equation is called And v is the specific volume by unit mass

IDEAL-GAS EQUATION OF STATE


UNIVERSAL GAS CONSTANT

In equation -1-, R is different for each gas ( Figure 3.45 page 137)

ON THE OTHER HAND, The universal gas constant RU is THE SAME FOR ALL GASES AND defined as :

RU= R.M -2-

Where M is the molar mass or molecular weightof the gas.

Values of RU in different units are given in page 1`38 .

For example in SI units RU= 8.314 kJ/kmol.K


IDEAL GAS

Pv = Ru.T with v is the specific volume by unit mole OR PV=NRU T

Pv=RT with v is the specific volume by unit mass OR PV=mRT


EXAMPLES OF IDEAL GASES?

At low pressures and high temperatures, the density of gases decreases and the gas behaves like a ideal gas ( FIGURE 3.49 page 140)

Gases like Air, O2, N2, H2 and also heavy gases like CO2 CAN be treated as ideal gases at low densities

However, dense gases such as water vapor in steam power plant and refrigerant vapors in refrigerators can not be treated as ideal gases.


REAL GASES


WHAT IS A REAL GAS?

If a gas behaves exactly as the ideal gas laws would predict it to behave in terms of volume, pressure, moles, and temperature, then the gas is said to be an ideal gas.

If, on the other hand, the gas deviates from Ideal Gas behavior, then the gas is said to be acting like a "real gas".


What causes deviation from

ideal gas behavior? Intermolecular forces called Van Der Waals forces

. There are three such types of Van Der Waals forces:

London Dispersion Forces which are forces that exist between molecules as a result of positive nuclei of one molecule attracting the electrons of another molecule..

Dipole-Dipole interactions which are forces that exist between polar molecules where the positive end of one molecule attracts the negative end of another molecule.

Hydrogen bonding interactions are forces that exist between molecules that have a hydrogen atom bonded to a highly electronegative atom such as Oxygen, Nitrogen, or Flourine.


What is the compressibility factor?

The term "compressibility" is used in thermodynamics to describe the deviance in the thermodynamic properties of a real gas from those expected from an ideal gas.

The compressibility factor is defined as

•

In the case of an ideal gas, the compressibility factor Z is equal to unity, and the familiar ideal gas law is recovered:•

Z can, in general, be either greater or less than unity for a real gas.


http://en.wikipedia.org/wiki/Thermodynamic_properties

http://en.wikipedia.org/wiki/Real_gas

http://en.wikipedia.org/wiki/Ideal_gas

http://en.wikipedia.org/wiki/Ideal_gas_law


REAL PHASE REGION

The deviation from ideal gas behavior tends to become particularly significant (or, equivalently, the compressibility factor stays far from unity) :

near the critical point,

in the case of high pressures

low temperatures.

In these cases, an alternative equation of state better suited to the problem must be utilized to produce accurate results.

http://en.wikipedia.org/wiki/Critical_point

http://en.wikipedia.org/wiki/Equation_of_state


COMPRESSIBILITY FACTOR

The compressibility factor can be estimated from figure 3-51 page 141 if we know the reduced temperature and the reduced pressure of the gas:

PR= P/PC and TR= T/TC

The compressibility factor Z is almost the same for all the gases at the same reduced pressure and reduced temperature . This is called the principle of corresponding states


CLASS WORK

Work examples 3-10 to 3-12 page 139 143

Work problems 3.73, 3.74 and 3.77 page 158

Work problems : 3-82; 3-85;3-88 and 3.91 page 159

REAL GASES

EQUATIONS OF STATE



EQUATIONS OF STATE FOR

REAL GASES

The ideal-gas equation is very simple, but its range and applicability are limited.

Several equations of state have been proposed in the literature to describe the behavior of real gases

We will discuss :

Van der Waals equation

Beattie- Bridgeman equation


Van der Waals equation of state

Proposed in 1873

It has two constants

a/v2 is a correction of the ideal gas equation related to the intermolecular forces

b is a correction of the ideal gas equation related to the volume occupied by the gas molecules

RT)bv)(v

aP(

2


Determination of a and b

cr

cr

P

TRa

64

27 22

cr

cr

P

RTb

8


Beattie- bridgeman equation

of state Proposed in 1928

It has five experimental constants

Values are given in table 3.4 page 146

232

u

v

A)Bv)(

vT

c1(

v

TRP

)1(....).......1( 00v

bBBand

v

aAA


ENERGY OF GASES


SPECIFIC HEATS

vvT

uC )(

ppT

hC )(


INTERNAL ENERGY OF IDEAL

GAS The internal energy of an ideal gas is a

function of the temperature only, that is

u=u (T)

Figure 2.66 shows the experiment done by Joule.

du= Cv(T) dT

dTTCu v )(

2

1


Enthalpy of ideal gas Definition: h=u+pv and for ideal gas (

Pv=RT), we have the relation h=u+RT

Therefore h depends also only on temperature for an ideal gas

dh = Cp(T)dT

And

2

1

)( dTTCh p


Specific heat relations of ideal

gases

h=u+RT

dh= du +RdT

Replacing dh by CpdT and du by CvdT

WE OBTAIN Cp= Cv +R

When the specific heats are given on a molar basis, R is replaced by Ru


CLASS WORK

Work examples 3-13 and 3-14 pages 147 and 152

Work problems 3.85; 3.88, 3.91 3.98, page 159-160


ENERGY TRANSFER

DURING PROCESSES


ENERGY TRANSFER BY HEAT,

WORK AND MASS ( Chapter 3)

HEAT TRANSFER:

Energy can cross a boundary of a

system as heat or work

Heat is defined as the form of

energy that is transferred by virtue of temperature difference

A process with no heat transfer is

called adiabatic


Heat Transfer

Heat can be transferred by conduction, convection or radiation

Heat is easy to recognize , its driving force between the system and the surroundings being the temperature difference ( Figure 2.12 Page 60)


ENERGY TRANSFER BY WORK

If the energy crossing a boundary is not heat It must be WORK

The work done per unit time is called POWER

Work can be electrical , mechanical work ( moving boudary, shaft work, spring work,….)


CONVENTION SIGNS

Heat and work are directional quantities

A complete description of work and heat need a magnitude and a direction

The generally accepted formal sign convention for heat and work is as follow:

Heat amd work In :POSITIVE

Heat and work OUT : NEGATIVE


Moving Boundary work

( Figure 4.1, page 166) It is associated to an expansion or

compression of a gas in a piston-cylinder device.

It is also called PdV work

In Figure 4.3, the work can be defined as

PdVPAdsFdsWb


Boundary Work

The total boundary work done during the entire process is defined by:

The area under the process is defined by ( Figure 4.3 page 167):

2

1

.dVPWb

2

1

PdVAAREA


A GAS CAN FOLLOW DIFFERENT

PATHS ( FIGURE 4.4)

A gas can follow different paths and since work is a path function

the area under each path is different ( Figure 4.4 )


Class work

1) Work examples 4.1 to 4.3 pages 168-171)

2) Work problems 4.11, 4.12, 4.14 page 202


FIRST LAW OF

THERMODYNAMICS

CONSERVATION OF ENERGY


FIRST LAW OF

THERMODYNAMICS?

Also known as the conservation of energy principle, the first law states that:

energy can never be created not

destroyed, it can only changes form

Example: ( Figure 2.37 page 70) The rock at some elevation possesses some potential energy. As the rock falls, part of its potential energy is converted into kinetic energy.


WHAT IS ENERGY?

Energy can exist in different forms:

Internal ( sensible, latent, chemical and nuclear)

Kinetic

Potential

Electrical

Magnetic

Surface tension effects


FOR A SIMPLE COMPRESSIBLE

SYSTEM

The change of total energy is :

PEKEUE

)(

)(2

1

)(

12

2

1

2

2

12

zzmgPE

vvmKE

uumU


FOR A STATIONARY ( CLOSED)

SYSTEM

0 PEKE

UE


MECHANISMS OF ENERGY

TRANSFER

Energy can be transferred to or from a system in three forms:

Heat transfer: Q

Work : W

Mass Flow : m


HEAT TRANSFER?

Caused by a difference of TEMPERATURE

TO A SYSTEM : Increases the ENERGY of molecules and thus the INTERNAL ENERGY

FROM A SYSTEM : Decreases the INTERNAL ENERGY of the system because the heat comes from the energy of molecules


WORK TRANSFER?

ENERGY interactions not caused by a temperature difference

Examples :

Rising Piston

Rotating Shaft

Electrical wire Work transfer to a system Esystem ↑

Work transfer from a system Esystem↓


MASS FLOW ?

Because mass carries energy with it

Mass enters the system Esystem↑

Mass leaving the system Esystem↓


NET ENERGY TRANSFER?

Energies transferred by heat, work and mass can de added ( FIGURE 2.45 page 74):

Ein-Eout=( Qin-Qout)+( Win-Wout)+(Emass,in-Emass out)

IN RATE FORM ( KW):

ΔË = Ëin-Ëout

outinsystem EEE


ENERGY BALANCE FOR

CLOSED SYSTEM

QNET IN= QIN-QOUT

WNET OUT= WOUT-WIN

outnetinnet WQE ,,

outnetinnet WQU ,,


CLASS WORK

WORK EXAMPLES 4.5, 4.8,4.9 and 4.10 pages 174-187


CLASS WORK#1

Work examples 5.1 page 226; 5.3 page 231; 5.6 page 238 and 5.7 page 239.


HOMEWORK #2

Do problems : 4.5 , 4.6E , 4.18, 4.24, 4.26E, 4.33, 4.36 pages 201-204.


THE USES OF THE FIRST

LAW OF

THERMODYNAMICS

1) ENERGY BALANCES

2) DESIGN OF THE EQUIPMENTS

( FIND THEIR DUTY)


WHAT IS ENERGY ( ENTHALPY

OR INTERNAL ENERGY)?

THE LINK BETWEEN THE SYSTEM AND THE SURRONDINDS

CAN BE DEFINED BY THE PROPERTIES OF THE SYSTEM

FROM TABLES

CAN BE CALCULATED BY THE ENERGY BALANCE

out,netin,net WQE


SOLVING PROBLEM

CASE I

( ENERGY BALANCE)

WE WANT TO DETERMINE ONE OF THE STATES OF THE SYSTEM KNOWING THE DUTIES QNET IN

AND WNET OUT


STATIONARY RIGID TANK

SYSTEM

ΔKE=ΔPE =0

NO MASS EXCHANGE WITH THE SURRONDINDS

NO BOUNDARY WORK

ΔE=ΔU outnet,innet, WQΔU


STATIONARY AND CLOSED

PISTON CYLINDER ΔKE=ΔPE =0

NO MASS EXCHANGE WITH THE SURRONDINDS

THERE IS BOUNDARY WORK

WNET OUT SHOULD NOT INCLUDE BOUNDAY WORK

outnet,innet, WQΔH


STEADY FLOW SYSTEM

WE WILL STUDY NEXT

)gz2

vh(m)gz

2

vh(mWQ i

2

iiie

2

eee


IN CASE I

1) WE SHOULD KNOW THE SPECIFIC ENERGY OF ONE STATE ( ENTHALPY OR INTERNAL ENERGY)

2) CALCULATE THE TOTAL ENERGY KNOWING THE MASS OR MASSFLOW

3) CALCULATE THE ENERGY OF THE SECOND STATE USING ENERGY BALANCE ( KNOWN DUTIES OF THE EQUIPMENTS)


CALCULATE THE SPECIFIC ENERGY OF THE SECOND STATE BY DIVIDING BY THE MASS ORMASS FLOW

USING THE CALCULATED SPECIFIC ENERGY TO DEFINE THE SECOND STATE AND CALCULATE ALL ITS PROPERTIES


HOW TO FIND u OR h FOR

DIFFERENT SYSTEMS

SUBCOOLED OR COMPRESSED LIQUID WE NEED TWO VARIABLES FROM P,T,v

APPROXIMATION FOR COMPRESSED LIQUIDS:

v(P,T)= vsat (T)

u(P,T)=usat (T)

h(P,T)= hsat (T)+ vsat(T) { P-Psat(T)}


SATURATED LIQUID: WE NEED ONE VARIABLE P OR T OR v

WET STEAM: WE NEED TWO VARIABLES ( P OR T) AND OTHER VARIABLE ( EX: QUALITY OR SPECIFIC VOLUME)


SATURATED VAPOR: WE NEED ONE VARIABLE P OR T OR v

SUPERHEATED VAPOR: WE NEED TWO VARIABLES BETWEEN P,T AND v

IDEAL GAS: WE NEED ONLY TEMPERATURE BECAUSE U AND H DEPENS ONLY ON TEMPERATURE


CASE 2

( DESIGN PROBLEM )

WE WILL HAVE THE ENERGIES OF BOTH STATES AND WILL CALCULATE

THE MISSING DUTY OF THE EQUIPMENT (WORK OR HEAT )

EXCHANGE WITH THE SURRONDINGS


WE SHOULD DEFINE THE TWO STATES

FIND THE SPECIFIC ENERGIES AT BOTH STATES

CONVERT TO TOTAL ENERGIES

APPLY THE ENERGY BALANCE

FIND THE DUTY OF THE EQUIPMENT USED TO EXCHANGE WORK ( TURBINE, COMPRESSORE, PUMP) OR HEAT ( HEAT EXCHANGER , COOLER. HEATER, FURNACE)


STEADY FLOW SYSTEMS ?

( FIGURE 5.7 page 225)

During a steady flow process, no intensive or extensive property within the control volume change with time V, m and E of the

control volume remains constant

mcv=CST AND ECV=CST

A large number of engineering devices (

turbines, compressors, nozzles) are classified as steady flow devices


MASS BALANCE FOR STEADY

FLOW SYSTEMS mcv=CST min=mout

With multiple inlets and exits: FIGURES 4.23 and 4.24 page 181

ei mm


ENERGY BALANCE FOR STEADY

FLOW SYSTEMS

ECV=CST ΔECV=0

Rate of net energy transfer by heat, work and mass IN is equal to the rate of net energy transfer by heat, work and mass OUT

).(0 FLOWSTEADYEEE systemoutin

outin EE


ENERGY BALANCE FOR A

GENERAL STEADY FLOW

SYSTEM

)2

()2

(22

ii

iiee

ee gzv

hmgzv

hmWQ


SECOND LAW OF

THERMODYNAMICS

DIRECTION OF A PROCESS AND ENTROPY


THE SECOND LAW OF

THERMODYNAMICS

In the last chapter, we have studied the first law of thermodynamics or the conservation of energy principle.

Any process can take place only if the first law is respected

Is This enough?

The answer belongs to the second law of thermodynamics or law of entropy


EXAMPLE #1

If we put a hot coffee in a cold room the coffee will lose heat to the room ( Fig 6.1 page 284)

If we consider the opposite process: Can the coffee get hotter getting heat from the cold room?

The answer is NO but the inverse process still respect the first law of thermodynamics which is conservation of energy

SO WHAT IS WRONG IN THE REVERSE PROCESS?


RESTRICTION OF THE SECOND

LAW

It is clear, from this example, that the first law of thermodynamics places no restriction on the direction of the process.

The reverse process violates the second law of thermodynamics


EXAMPLE #2

CONSIDERING A REFRIGERATOR: THE INLET IS GETTING COLDER AND THE OUTSIDE AIR IS GETTING HOTER

DOES IT VIOLATE THE SECOND LAW OF THERMODYNAMICS?


EXAMPLE #3

From Figure 6.8 page 286, we can observe that the mechanical energy done by the shaft is first converted into INTERNAL ENERGY of water

This energy may leave the water as heat

However, can we rotate the shaft by heating the water?

ANSWEER: LATER


CAN WE TRANSFORM HEAT

INTO WORK?

To explain this , is often convenient to have a hypothetical body with a relatively large thermal energy capacity that can supply or absorb finite amount of energy without undergoing any changein temperature

Such a body is called THERMAL

ENERGY RESERVOIR ( FIGURE 6.6 page 285)


WHAT IS A THERMAL ENERGY

RESERVOIR?

A thermal energy reservoir that supplies energy in the form of heat is a SOURCE ( FIGURE 6.7)

A thermal energy reservoir that absorbs energy in the form of heat is a SINK (FIGURE 6.7)

Thermal energy reservoirs are known as HEAT RESERVOIRS.


THE SECOND LAW ( FIGURE 6.9)

In figure 6.9 PAGE 286, we can see that only a part of heat received by a heat engine is converted into work while the rest is rejected to a sink

CONCLUSION:

WE CAN NEVER TRANSFORM HEAT COMPLETELY INTO WORK


FINAL ANSWEER

WORK CAN BE CONVERTED TO HEAT DIRECTLY AND COMPLETELY

HOWEVER, TO TRANSFORM HEAT INTO WORK REQUIRES THE USE OF SOME DEVICES CALLED: HEAT ENGINES


HEAT ENGINES


HEAT ENGINES?

Heat engines differ considerably from one another but all can be characterized by the following STEPS:

They receive heat from a high temperature source ( solar, furnace, nuclear reactor

They convert part of this heat into work ( usually in the form of rotating shaft: turbine)

They reject the remaining waste heat to a low temperature sink ( air, rivers, sea,…)

They operate on a cycle


EXAMPLE: STEAM POWER

PLANT ( figure 6.10) page 287

Qin= amount of heat supplied to steam in boiler from a high temperature source ( furnace)

Qout = amount of heat rejected from steam in condenser to a low temperature sink ( atmosphere)

Wout= amount of work delivered by steam as it expands in turbine

Win = amount of work required to compress water to boiler pressure


NET WORK OUT?

WE PRODUCE Wnet.out

Wnet, out= Wout- Win ( kJ)

Since is a cycle ΔU=0

Wnet,out= Qin-Qout


WHAT IS THERMAL

EFFICIENCY? In general, we define efficiency as the

amount of energy you give divided by the amount of energy you spent for:

The fraction of the heat input that is converted into net work is a measure of the performance of the heat engine and is called thermal efficiency


CLASS WORK #1

From figure 6.10 page 287 show that

in

out

in

out,net

thQ

Q1

Q

W


Class work #2

Work examples 6.1 and 6.2 page 290

Work problems 6.20-6.23 page 321

Work problems 6.18 ,6.19,6.24,6.28


REFRIGERATORS &

HEAT PUMPS


REFRIGERATORS ( FIG 6.19

PAGE 292)

In order to transfer heat from a cold space ( inside refrigerator) to hot space ( kitchen) refrigerator must receive work.

As shown in figure 6.20 page 292 , the performance of a refrigerator ( COPR) is the ratio between the desired output over the required input

COPR= QL/Wnet,in


HEAT PUMPS ( FIGURE 6.21

PAGE 293)

Work like the refrigerator but the objective is different

Now we want to heat up an apartment during cold weather

Therefore : form figure 6.21 page 293, we have

COPHP = QH/Wnet in


Class work #4

Work examples 6.3 and 6.4 page 295

Work problems 6.39, 6.42, 6-50 AND 6.52 pages 322-323


CARNOT HEAT

ENGINE

THE CARNOT CYCLE


REVERSIBLE PROCESS

Reversible versus Irreversible process?

Reversible process is defined as a process that can be reversed without leaving any trace on the surroundings.

Both the system and the surroundings are returned to their initial states at the end of the reverse process

This is possible only if the net heat and net work exchange is zero for the combined ( original and reverse process)


IRREVERSIBLE PROCESS

All real processes are Irreversibleand the Reversible process is a theoretical process used as a limit for the corresponding irreversible process

Irreversibility is caused by FRICTION


THE CARNOT CYCLE

WE HAVE ALREADY MENTIONNED THAT HEAT ENGINES ARE CYCLIC DEVICES

REVERSIBLE CYCLES CAN NOT EXIST BECAUSE OF THE FRICTION.

THE CARNOT CYCLE IS A THEORITICAL REVERSIBLE CYCLE PROPOSED IN 1824 BY SADI CARNOT ( A FRENCH ENGINEER)


THE CARNOT CYCLE :

FIGURE 6.37 page 304

THE CARNOT CYCLE IS COMPOSED OF:

Reversible Isothermal Expansion

Reversible Adiabatic Expansion

Reversible Isothermal Compression

Reversible Adiabatic Compression


CARNOT CYCLE


CARNOT CYCLE

FIGURE 6.38 ( PAGE 305) SHOWS THE CARNOT CYCLE

PROCESS 1->2 EXPANSION AT CONSTANT TH:

The gas is in contact with a source at TH.

As the gas expands , TH tends↓ by dT

However, the heat input QH from the source TH, will increase again T to TH in a reversible way

AT THE END OF THE PROCESS , THE GAS HAS ABSORBED QH


CARNOT CYCLE

PROCESS 2->3:ADIABATIC EXPANSION WHERETEMPERATURE DROPS FROM TH

TO TL:

At State 2, the reservoir is removed and replaced by an insulation

The gas continues to expand slowly doing work on the surroundings until T TL


CARNOT CYCLE

PROCESS 3->4: ISOTHERMAL COMPRESSION AT TL

At state 3, the insulation is removed and the gas is in contact with a sink at temperature TL.

The piston is pushed inward by an external force doing work on the gas

As the gas compresses , T ↑by dT but the sink will absorb this elevation of temperature

At the end of the process 3->4, the gas has rejected QL


CARNOT CYCLE

PROCESS 4->1: ADIABATIC COMPRESSION WHERE TEMPERATURE RISES FROM TL TO TH:

The sink is removed and the insulation is put back and the gas is compressed in a reversible manner

The gas returns to its initial state which completes the cycle


EFFICIENCY OF CARNOT HEAT

ENGINE The second law of thermodynamics states that no heat

engine can have efficiency of 100%

The maximum efficiency is for reversible cycle as Carnot heat engine. CARNOT EFFICIENCY IS EQUAL TO:

Temperature in Kelvin.

H

L

H

LREVTHCARNOT

T

T

Q

Q 11,


CLASS WORK

WORK EXAMPLE 6.5 PAGE 310

Work problems 6.80 and 6.81 page 325


THE CARNOT REFRIGERATOR

Operates on the reverse Carnot cycle

The Coefficient of performance of any refrigerator ( reversible or irreversible):

1

1

L

HR

Q

QCOP


THE CARNOT HEAT PUMP Operates as a reverse Carnot cycle

The Coefficient of performance of any heat pump ( reversible or irreversible):

H

LHP

Q

QCOP

1

1


COP FOR REVERSIBLE

PROCESS REVERSIBLE REFRIGERATOR:

REVERSIBLE HEAT PUMP

1

1,

L

HREVR

T

TCOP

H

LREVHP

T

TCOP

1

1,


CLASS WORK

WORK EXAMPLES : 6.6 AND 6.7 page 314

WORK PROBLEMS: 6.94-6.97pages 326-327


ENTROPY?


WHAT IS ENTROPY?

MEASURES DESORDER

AS A SYSTEM BECOMES MORE DESORDERED THE POSITION OF MOLECULES BECOME LESS PREDICTABLE THE ENTROPY INCREASES

USEFUL PROPERTY FOR SECOND LAW

EXTENSIVE PROPERTY


CHANGE OF ENTROPY?

The entropy is a state function

The entropy change of a process from state 1 to state 2 can be determined by the relation:

revT

QSSS int,

2

1

12 )(


SPECIAL CASE

For an internally reversible isothermal heat transfer process, the change of entropy reduces to :

WORK EXAMPLE 6-1 page 305

0T

QS


PROPERTY DIAGRAMS

INVOLVING ENTROPY In the second law, is very useful to use

the T-S diagram or H-S diagrams

From the previous equations , we can write

2

1

int, .dSTQ rev


T-S DIAGRAM

In figure 6-16 page 314, we can calculate Qint,rev as the area of the curve under the states 1 and 2

Isentropic process is a vertical line ( Figure 6-17 page 315)

T-S diagram of water is in appendix in figure A-9


T-S DIAGRAM OF A CARNOT

CYCLE?

WORK EXAMPLE 6-6 PAGE 315


CARNOT VAPOR

CYCLE

Chapter 10 page 565


CARNOT VAPOR CYCLE

CARNOT CYCLE is the most efficient cycle operating between two temperatures

IS CARNOT CYCLE EFFICIENT FOR VAPOR POWER PLANT?


CARNOT VAPOR CYCLE

FIGURES 10.1 a and b PAGE 566 show two Carnot vapor cycles:

Figure 10.1 a:

Process 1-2 : The fluid is heated reversibly and isothermally in a boiler

Process 2-3: The fluid is expanded isentropically in a turbine

Process 3-4 : The fluid is condensed reversibly and isothermally in a condenser

Process 4-1:The fluid is compressed to the initial state


IMPRACTICALITIES OF THE

CARNOT VAPOR CYCLE A) In Figure 10.1 A , Processes 1-2 and 3-4 (

Boiler and condenser) use two phases- region for the water , This can be easily done in practice by maintaining a constant pressure in the boiler and condenser , however the maximum temperature of the source should remain below the critical temperature of water ( 3740C)

This will limit the thermal efficiency

B) To increase the maximum temperature of water above the critical temperature, we need to heat the steam water at constant temperature

not easy in practice


PROCESS 23

B) The turbine will have to handle steam with poor quality ( steam with high moisture)

The droplets of water will

cause erosion in the turbine


PROCESS 4 1

C) In the isentropic compression, the pump will handle 2 phases fluid ???

cavitation of the pump


Figure 10.1 b page 566

This Carnot process involves isentropic compression at very high pressures and isothermal heat transfer at variable pressures

Not possible in practice


RANKINE VAPOR

CYCLE

IDEAL CYCLE FOR STEAM POWER PLANT


RANKINE VAPOR CYCLE The difficulties encountered in the

CARNOT vapor cycle can be eliminated using the RANKINE vapor cycle

This can be done by;

SUPERHEATING the steam in boiler

CONDENSE all the steam in

condenser

FIGURE 10.2 PAGE 567


RANKINE VAPOR CYCLE IS THE IDEAL CYCLE FOR VAPOR POWER

CYCLES: DIAGRAM IN PAGE 567

PROCESSES OF THE CYCLE:

3-4:Isentropic EXPANSION (turbine)

4-1:Isobaric heat rejection (

condenser)

1-2:Isentropic COMPRESSION (pump)

2-3:Isobaric heat addition ( boiler)


WORKS INVOLVED IN RANKINE

CYCLE

Work output of the cycle (Steam turbine) W1 Work input to the cycle (Pump) W2

are respectively: W1 = m (h1-h2)

W2 = m (h4-h3)

where m is the mass flow of the cycle.


HEATS INVOLVED IN RANKINE

CYCLE

Heat supplied to the cycle (boiler),Q1

Heat rejected from the cycle (condenser), Q2 are respectively:

Q1 = m (h1-h4)

Q2 = m (h2-h3)


NET WORK OUTPUT

The net work output of the cycle is:

W = W1 - W2


THERMAL EFFICIENCY

The thermal efficiency of a Rankine cycle is:

IN

OUT

IN

NETRANKINE

Q

Q

Q

W 1

http://www.taftan.com/thermodynamics/EFFIC.HTM

Class Work

Work Example 10-1 page 569



SOLUTION OF EXAMPLE 10-1

A) Draw the process

B) Draw the T-S Diagram of the process

C) Put the data on the process


STATE 1:

P1= 75 kPa and saturated liquid

Table A-5: h1=hf= 384.39 kJ/kg

v1=vf= 0.001037 m3/kg


STATE 2:

P2= 3MPa and s2=s1

wpump = v1 ( P2-P1) = 0.001037 ( 3000-75)

= 3.03 kJ/kg

h2= h1+wpump= 384.39+3.03= 387.42 kJ/kg


STATE 3: ( superheated vapor)

P3= 3Mpa and T3= 3500C

Table A-6 : h3= 3115.3 kJ/kg

s3= 6.7428 kJ/kg.K


STATE 4: ( MIXTURE)

P4= 75 kPa and s4=s3

WHAT IS THE PERCENTAGE OF VAPOR?

x4= ( 6.7428- 1.213)/ 6.2434 = 0.8857

fg

f

s

ssx

4

4


h4?

h4= hf + x4.hfg

= 384.39 + 0.8857x 2278.6

= 2402.6 kJ/kg


Heats in and out ?

Qin= h3 - h2= 3115.3- 387.42= 2727.9 kJ/kg

Qout= h4-h1= 2402.6-384.39= 2018.2 kJ/kg


THERMAL EFFICIENCY

%269.2727

2.201811

in

out

Q

Q


Class work

Solve problems 10.15 , 10.18, 10.19 page 605


RANKINE VAPOR CYCLE

THE IDEAL CYCLE FOR VAPOR POWER CYCLES

PROCESSES OF THE CYCLE:

1-2:Isentropic EXPANSION (turbine)

2-3:Isobaric heat rejection (

condenser)

3-4:Isentropic COMPRESSION (pump)

4-1:Isobaric heat addition ( boiler)


DEVIATION FROM RANKINE

CYCLE FIRST Cause of deviation:

A) Fluid friction causes pressure drop in the boiler, condenser and the piping:

RESULTS:

1)The steam leaves the boiler at some LOWER pressure

2) The pressure at the inlet of turbine is somewhat LOWER

3) The pressure drop in the condenser is usually very small

SEE FIGURE 10-4 A PAGE 572 AND COMPARE ACTUAL CYCLE TO IDEAL RANKINE CYCLE


Solution for pressure drop in the

system

TO COMPENSATE the problem of pressure drop in the system: THE PUMP SHOULD WORK AT HIGHER PRESSURE THAN THE IDEAL CYCLE.


Second cause of deviation from

Rankine cycle

B) The other cause of irreversibility is the heat loss from the steam to the surroundings

As a result : MORE HEAT NEEDS TO BE TRANSFERRED TO THE STEAM


IRREVERSIBILITY OCCURING IN

THE PUMP AND THE TURBINE

AS A RESULT OF IRREVERSIBILITY ON THE IDEAL RANKINE CYCLE: Figure 10-4b page 572

A pump requires a greater work

input and the turbine produces a smaller work output


ISENTROPIC EFFICIENCIES FROM FIGURE 10-4b page 573

A) ISENTROPIC EFFICIENCY OF THE PUMP :

B) ISENTROPIC EFFICIENCY OF THE TURBINE

12

12

hh

hh

w

w

a

s

a

sPUMP

s

a

s

aTURBINE

hh

hh

w

w

43

43


CLASS WORK

WORK EXAMPLE 10-2 PAGE 573

WORK PROBLEM 10.19 PAGE 605


HOW CAN WE INCREASE THE

EFFICIENCY OF RANKINE

CYCLE Steam power plant are responsible

for the production of most electric power in the world

Even small increase in thermal efficiency can mean large savings in fuel consumption


The basic concept

The basic concept is to: η= 1-(TL/TH)

Increase the average temperature at which heat is transferred to the working fluid in the boiler

Decrease the average temperature at which heat is rejected from the working fluid to the condenser

BUT DO NOT FORGET: ONLY FOR CARNOT CYCLE , WE HAVE η= 1-(TL/TH)


The actions

A) Lowering the condenser pressure this will lower the TLOW,av

Figure 10.6 page 574

B) Superheating the steam to higher temperatures Increases Thigh, av

Figure 10.7 page 575

C) Increasing the boiler pressure Increases Thigh,av Figure 10.8 page 575

Increase efficiency but increases the moisture at the turbine


IDEAL REHEAT RANKINE CYCLE

How we can take advantage of increasing the efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stage of the turbine?

TWO POSSIBILITIES


POSSIBILITY #1

SUPERHEAT THE STEAM TO VERY HIGH TEMPERATURES BEFORE IT ENTERS THE TURBINE

COULD BE UNSAFE FOR THE METAL

OF THE TURBINE


POSSIBILITY #2

EXPAND THE STEAM IN THE TURBINE IN TWO STAGES AND REHEAT IT BETWEEN THE TWO STAGES

REHEAT IS THE PRACTICAL SOLUTION FOR THE HIGH MOISTURE IN THE TURBINE T

FIGURE 10-11 PAGE 579

CLASS WORK

Work example 10.4 page 580

Work problems 10.35, 10.39 pages 607-608



HOMEWORK

TO PREPARE YOUR FINAL EXAM, TRY TO SOLVE THE PROBLEMS 4.28, 4.31, 4.38, 4.44, 5.50, 5.52, 5.58, 6.17, 6.18,6.42, 6.44, 6.52, 6.82, 6.83, 6.95,6.96, 6.99, 9.17, 9.18 , 10.15, 10.20,10.22,10.35

GOOD LUCK

Thermodynamics

Education

Transcript of Thermodynamics