CHAPTER 1:INTRODUCTION AND BASIC CONCEPTS

CHE 433 THERMODYNAMICS

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THERMODYNAMICS AND ENERGY

Thermodynamics: The science of energy.

Energy: The ability to cause changes.

Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant. Energy cannot be created or destroyed.

The first law of thermodynamics: An expression of the conservation of energy principle.

The first law states that energy is a thermodynamic property.

Energy cannot be created or destroyed; it can only change forms (the first law).

Thermodynamics = therme + dynamis (heat) (power)

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The second law of thermodynamics: Energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

E.g. A cup of hot coffee left on a table cools down when exposed to surroundings, but a cup of cool coffee in the same room never gets hot by gaining heat from surroundings i.e. without external heat supply.

Heat flows in the direction of decreasing temperature.

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Application Areas of Thermodynamics

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IMPORTANCE OF DIMENSIONS AND UNITS Dimensions: Any physical quantity. Units: Magnitudes of dimensions. Primary or fundamental dimensions

- mass m, length L, time t, and temperature T

Secondary dimensions, or derived dimensions - velocity V, energy E, and volume V

Unit system: Metric SI system – kg, m, s English system – lbm, ft, s

The SI unit prefixes are used in all branches of engineering.

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Some SI and English Units

The definition of the force units.

Work = Force Distance1 J = 1 N∙m

1 cal = 4.1868 J1 Btu = 1.0551 kJ

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The relative magnitudes of the force

units newton (N), kilogram-force

(kgf), and pound-force (lbf).The weight of a unit mass at sea level.

Weight changes with gravitational acceleration.A body weighing 60 kgf on earth will weigh only 10 kgf on the moon.

W weightm massg gravitational acceleration

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Dimensional homogeneity

All equations must be dimensionally homogeneous.

To be dimensionally homogeneous, all the terms in an equation must have the same unit.

kg.lbm

s

m.ft

mNkPa

mNkJ

sJW

smkgN

sftlbm.lbf

4535901

60min1

304801

10001

10001

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11

17432 1

2

2

2

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SYSTEMS

System: A quantity of matter or a region in space chosen for study.

Surroundings: The mass or region outside the system

Boundary: The real or imaginary surface that separates the system from its surroundings.

The boundary of a system can be fixed or movable.

Systems may be considered to be closed or open.

SYSTEM

SURROUNDINGS

BOUNDARY

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CLOSED SYSTEM

A fixed amount of mass, and no mass can cross its boundary. Also known as CONTROL MASS.

CLOSED system with moving boundary

CLOSED system

m = const.

Mass NO

Energy YES

GAS2 kg1 m3

GAS2 kg3 m3

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OPEN SYSTEM

A properly selected region in space. Also known as CONTROL VOLUME.

Boundary of open system is called CONTROL SURFACE.

E.g. Water heater, car radiator, turbine, compressor.

OPEN system

Mass YES

Energy YES

In Out

Imaginary Boundary

Real Boundary

OPEN system with real and imaginary boundary

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PROPERTIES OF A SYSTEM

PROPERTY: Any characteristic of a system.

Intensive : Independent on mass of system. - e.g. Temperature (T), Pressure (P)

Extensive : Dependent on mass of system. - e.g. Total mass, total volume

Specific : Extensive properties per unit mass.- e.g. Sp. Vol (v=V/m), Sp. Enthalpy (h=H/m)

e.g. Pressure (P), Volume (V), Temperature (T) and mass (m)

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DENSITY AND SPECIFIC GRAVITY

Density is mass per unit volume; specific volume is volume per unit mass.

Specific gravity: The ratio of the density of a

substance to the density of some standard

substance at a specified temperature (usually

water at 4°C).

Density

Specific weight: The weight of a unit volume of a substance.

Specific volume

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STATE AND EQUILIBRIUM

Assume a system NOT undergoing any change. Set of properties to completely describe the condition of the system is known as its STATE.

m = 2 kgT1 = 25 ºCV1 = 3 m3

m = 2 kgT1 = 25 ºCV1 = 1 m3

STATE 1 STATE 2

At a given state, all properties of a system have fixed values.If the value of even one property changes, the state will change.

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EQUILIBRIUM : State of balance

Thermal Equilibrium :- NO Temperature Gradient throughout the system.

Mechanical Equilibrium :- NO Pressure Gradient throughout the system.

Phase Equilibrium :- System having more than 1 phase. - Mass of each phase is in equilibrium.

Chemical Equilibrium :- Chemical composition is constant - NO reaction occurs.

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PATH AND PROCESS

Any change a system undergoes from one equilibrium state to another is known as PROCESS.

Series of states through which system passes during the process is known as its PATH.

Property A

State 1

State 2

Pro

pert

y B

Path State 1State 2

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QUASI-STATIC or QUASI-EQUILIBRIUM : Process proceeds in such a manner that system remains infinitesimally close to equilibrium conditions at all times.

Slow compression Very fast compression

Quasi-StaticNon-Quasi-Static

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State 1 State 2

Pre

ssure

Quasi-Static Process Path

Volume

NOTE : Process Path is a CONTINUOUS line only if it is having Quasi-Static Process.

Non-Quasi-Static Process is denoted by a DASHED line.

State 1 State 2

Pre

ssure

Volume

Non-Quasi-Static Process Path

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Pre

ssure

(P)

Volume (V)

V=ConstIsochoric

P=ConstIsobaric

Tem

pera

ture

(T)

Enthalpy (h)/ Entropy (s)

T=ConstIsothermal

h=ConstIsenthalpic

s=ConstIsentropic

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CYCLE

CYCLE :A system is said to have undergone a cycle if it returns to its ORIGINAL state at the end of the process.

Hence, for a CYCLE, the INITIAL and the FINAL states are identical.

Property A

State 1

State 2

Pro

pert

y B

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Temperature

No EXACT definition.

Broad definition : “Degree of Hotness / Cold”

This definition is based on our physiological sensation.

Hence, may be misleading.

e.g. Metallic chair may feel cold than wooden chair; even at

SAME temperature.

Properties of materials change with temperature, thus this

forms the basis to deduce EXACT level of temperature.

e.g. Mercury in glass thermometer is based on the

expansion of mercury with temperature.

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1. Celsius Scale ( ºC ) – SI System2. Fahrenheit Scale ( ºF ) – English System3. Kelvin Scale ( K ) – SI System4. Rankine Scale ( R ) – English System

Celsius Scale and Fahrenheit Scale – Based on 2 easily reproducible

fixed states,

Freezing and Boiling points

of water.

i.e. Ice Point and Steam Point Thermodynamic Temperature Scale - Independent of properties of any

substance.

- In conjunction with Second Law of

Thermodynamics

Thermodynamic Temperature Scale - Kelvin Scale and Rankine Scale.

Temperature Scales

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Hot End

Regenerator Pulse Tube

T ( K ) = T ( ºC ) +

273.15

T ( R ) = T ( ºF ) +

459.67

T ( ºF ) = 1.8 T ( ºC ) +

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T ( R ) = 1.8 T ( K )

-273.15 0

273.16 0.01

0 -459.67

491.69 32.02

ºC K ºF R

Conversion Factors :

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PressureDefinition : Normal Force exerted by a fluid per unit Area.

SI Units :

1 Pa = 1 N/m2

1 kPa = 103 Pa

1 MPa = 106 Pa = 103 kPa

1 bar = 105 Pa = 0.1 MPa = 100 kPa

1 atm = 101325 Pa = 101.325 kPa = 1.01325 bar

1 kgf/cm2 = 9.81 N/m2 = 9.81 X 104 N/m2 = 0.981 bar = 0.9679 atm

English Units :

psi = Pound per square inch ( lbf/in2)

1 atm = 14.696 psi

1 kgf/cm2 = 14.223 psi

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Absolute Pressure : Actual Pressure at a given position.

Measured relative to absolute vacuum i.e. absolute zero pressure.

Pressure Gauges are generally designed to indicate ZERO at local atmospheric pressure.

Hence, the difference is known as Gauge Pressure.

i.e. P (gauge) = P (abs) – P (atm)

Pressure less than local atmospheric pressure is known as Vacuum Pressure.

i.e. P (vacuum) = P (atm) – P (abs)

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P (gauge) = P (abs) – P (atm)

P (vacuum) = P (atm) – P (abs)

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Variation of Pressure with Depth

Free-body diagram of a rectangularfluid element in equilibrium.

The pressure of a fluid at restincreases with depth (as a result of added weight).

When the variation of density with elevation is knownPressure difference between two

points is proportional to z and

For fluids whose density changes significantly with elevation

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In a room filled with a gas, the

variation of pressure with

height is negligible.

Pressure in a liquid at rest increaseslinearly with distance from the free surface. The pressure is

the same at all points on a horizontal plane in a given fluid regardless of geometry, provided that the points are interconnected by the same fluid.

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Pascal’s law: The pressure applied to a confined fluid increases the pressure throughout by the same amount.

Lifting of a large weight by a small

force by the application of Pascal’s law.

The area ratio A2/A1 is called the ideal mechanical advantage of the hydraulic lift.

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The Manometer

In stacked-up fluid layers, the pressure change across a fluid layer of density and height h is gh.

Measuring the pressure drop across a flow

section or a flow device by a differential

manometer.

The basic manomete

r.

It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil.

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Other Pressure Measurement Devices

Various types of Bourdon tubes used to measure pressure.

• Bourdon tube: Consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.

• Pressure transducers: Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance.

• Pressure transducers are smaller and faster, and they can be more sensitive, reliable, and precise than their mechanical counterparts.

• Strain-gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs.

• Piezoelectric transducers: Also called solid-state pressure transducers, work on the principle that an electric potential is generated in a crystalline substance when it is subjected to mechanical pressure.

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THE BAROMETER AND ATMOSPHERIC PRESSURE• Atmospheric pressure is measured by a device called a barometer;

thus, the atmospheric pressure is often referred to as the barometric pressure.

• A frequently used pressure unit is the standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0°C (Hg = 13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2).

The basic barometer.

The length or the cross-sectional

area of the tube has no effect on

the height of the fluid column of a

barometer, provided that the tube diameter is large enough to

avoid surface tension (capillary)

effects.

PREPARED BY:NORASMAH MOHAMMED MANSHORFACULTY OF CHEMICAL ENGINEERING, UiTM SHAH ALAM.0192368303/0355436333norasmah@salam.uitm.edu.my

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