CLB 20703

Chemical Engineering

Thermodynamics

Chapter 1:

Basic Concepts in Thermodynamics

Objective of Chapter 1

Introduce the students towards some of

the fundamental concepts and definitions

that are used in the study of Engineering

Thermodynamics.

Outline

Introduction

Dimensions And Units

System

Measure Of Amount

Force

Temperature

Pressure

Energy

Heat

Work

1.1 INTRODUCTION

What is Thermodynamics?

Thermodynamics is the Science that deals

with Heat and Work and those properties of

substances that bear a relation to Heat and

Work.

Thermodynamics is the study of the effects

of Work, Heat and Energy on a System.

Thermodynamics is only concerned with

large scale observation.

1.1 INTRODUCTION

Scopes of Thermodynamics

First and Second Laws of Thermodynamics

To cope with variety of problems especially in the calculation of Energy Changes, Heat and Work requirements for processes

Property Values are essential to application of Thermodynamics

Generalized Correlationsto provide property estimates in the absence of data

1.2 DIMENSIONS AND UNITS

DIMENSIONS

(= measure of

physical quatity)

FUNDAMENTAL /

PRIMARY

DIMENSIONS

DERIVED /

SECONDARY

DIMENSIONS*

Mass (m), Length (L), Time

(t), Temperature (T), Current

(I) & Amount of matter (mol)

Velocity (v), Energy (E),

Volume (V), Force (F),

Power (P), etc.

Derived dimensions = combination of a few primary dimensions.

Eg: Velocity = Distance/Time = L/t

UNITS

(= magnitudes assigned

to the dimensions)

DERIVED /

SECONDARY

UNITS*

-accompany primary

dimensions

-accompany derived dimensions

2 types of unit systems widely used:

i) English System / United States Customary Systems (USCS)

ii) Metric System, SI (International System)

FUNDAMENTAL /

PRIMARY UNITS

1.2 DIMENSIONS AND UNITS

Differences of Unit Systems

Fundamental / Derived Dimensions SI Unit ES Unit

Mass (m) kg lbm, oz

Length (L) m ft, in

Time (t) s s

Temperature (T) K oC, oF, R

Ammount of matter (mol) kmol lb mol

Velocity (v) ms-1 ft s-1

Energy (E) J (Joule) Btu, cal

Volume (V) m3 gal

Force (F) N (Newton) lbf

Power (P) W (Watt) hp

Pressure N/m 2 (Pascal) psia, psig

1.2 DIMENSIONS AND UNITS

Multiples and Decimal Fractions of SI units

are designated by prefixes

Standard prefixes in SI units:

Prefix Multiple

tera, T 1012

giga, G 109

mega, M 106

kilo, k 103

deci, d 10-1

Prefix Multiple

centi, c 10-2

milli, m 10-3

macro, 10-6

nano, n 10-9

pico, p 10-12

1.2 DIMENSIONS AND UNITS

1.3 SYSTEM

System is a quantity of matter or a region in

space being chosen for study.

Boundary is the one that separates System

from its surrounding. Can be real or

imaginary.

2 types of system:

Closed system/control mass

Open system/control volumeSYSTEM

BOUNDARY

SURROUNDING

Its volume always fixed but its mass not

necessarily fixed.

Example of open system: compressor, turbine,

pump, nozzle

OPEN

SYSTEM

Mass

Energy

Open system

Also known as control volumes

Both mass and energy can cross the boundary

of a control volume

1.3 SYSTEM

Closed System

Also known as control mass

It has fixed amount of mass and no mass can

cross the boundary.

Energy in theform of heat and work can cross

the boundary

Volume does not have to be fixed.

In special case, when energy is not allowed to

cross the boundary -> Isolated system

Example: Rigid tank, piston cylinder device

1.3 SYSTEM

Mass cannot crossthe boundaries of aclosed system, butenergy can

An example of closedsystem with a movingboundary piston-cylinder device

1.3 SYSTEM

Properties of a system

Any characteristic of a system property.

PROPERTY

Intensive

Property

Extensive

Property

- independent of themass of a system

Eg: Temperature TPressure PDensity

- depend on the size ofa system

Eg: Mass mVolume VTotal Energy E

1.3 SYSTEM

State and Equilibrium

For a system not undergoing anychange, at this point all the propertiescan be measured or calculatedthroughout the entire system a setof properties that completely describesthe condition the state of thesystem.At a given state, all the properties of asystem have fixed values. If the valueof even one property changes, thestate will change to a different state.

m = 2 kg

T1 = 20oC

V1 = 1.5 m3

m = 2 kg

T2 = 20oC

V2 = 2.5 m3

State 1

State 2

1.3 SYSTEM

Equilibrium indicate the State Of Balance.

A System that is in equilibrium

experiences no changes when it is

isolated from its surroundings.

Types of Equilibrium:

Thermal

Mechanical

Phase

Chemical

1.3 SYSTEM

Thermal equilibrium if the temperature is the same throughout the entire system.

Mechanical equilibrium if there is no change in pressure at any point of the system with time.

Phase equilibrium when the mass of each phase reaches an equilibrium level and stays there such as water and ice inequilibrium.

Chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur.

1.3 SYSTEM

Process, Path And Cycle

Any change that a system undergoes from

one equilibrium state to another process,

and the series of states through which a

system passes during a process the

process path.

Example of process A

compression process in

a piston-cylinder device

1.3 SYSTEM

Quasi-static/Quasi Equilibrium Process.

the System remains infinitesimally/approx.

close to Equilibrium State at all time

Is a slow and Ideal process that allow the

System to adjust itself internally in order

that properties in one part of the system

do not change any faster than those at

other parts.

1.3 SYSTEM

Processes in which one thermodynamicproperty is kept constant:

Process Constant property

Isobaric Pressure

Isothermal Temperature

Isochoric/isometric Volume

Isentropic Entropy

1.3 SYSTEM

A system is said to have undergone acycle if it returns to its initial state at theend of the processFor a cycle, the initial and final states areidentical.

Process

A

Process

B

1

2P

V

1.3 SYSTEM

1.4 MEASURES OF AMOUNT / SIZE

Three common measures of amount/size:

Mass, m

Number of moles, n = m/M

Total volume, Vt

1.5 FORCE

Force = mass x acceleration (F = ma)

Unit: N or kg/ms2 (SI unit), lbf (ES unit)

The Newton, N is defined as

a force required to accelerate

A mass of 1 kg at the rate of

1 meter per second.F

Acceleration,a

1.6 TEMPERATURE

Temperature - a measure of oror the energy content of a body.

The temperature difference causes the heattransfer from a hot body (with highertemperature) to an another cold body (with alower temperature).

When heat is transferred to a body, E T .

Two bodies are in thermal equilibrium whenboth of the bodies achieve similar temperature.

Temperature applied in thermodynamicproblems must be in absolute units. Absolutetemperature scale in SI unit is Kelvin andRankine in ES unit.

Unit

Property

SI ES

Temperature scale oC oF

Absolute temperature scale K R

Melting point 0oC 32oF

Boiling point 100oC 212oC

Relation between temperature scales:

T(oF) = 1.8T(oC) + 32 (oC to oF)

T(K) = T(oC) + 273.15 (oC to K)

T(R) = T(oF) + 459.67 (oF to R)

T(R) = 1.8T(K) (K to R)

1.6 TEMPERATURE

1.6 TEMPERATURE

1.7 PRESSURE

Pressure is defined as the normal force exerted

by a fluid per unit area of the surface

P = F/A = mg/A

Pressure only deal with gas or liquid

SI unit: Pascal(Pa)/Nm-2

ES unit: psi = lbf/in2 (pound-force per squareinch)

psia = pound-force per square inch absolute

psig = pound-force per square inch gage.

Other units: bar, standard atmosphere (atm).

P1

Pa

Pb

Pc P2

P3P1=P2 P3

Pa=Pb=Pc

Pressure at any point in a fluid is same in all

directions.

Pressure varies in vertical directions due to gravity

effects but does not vary in the horizontal directions.

1.7 PRESSURE

Absolute Pressure - The actual pressure at a

given position. Measured relative to absolute

vacuum ( absolute zero ).

Gage Pressure - The difference between

absolute pressure and local atmospheric

pressure.

Vacuum Pressure Pressure below

atmospheric pressure.

1.7 PRESSURE

Absolute P must be used in

Thermodynamics

calculations

1.7 PRESSURE

Pvac = Patm Pabs

(for PPatm)

Pressure measuring device

Manometer is used to measure small and moderatepressure differences.

The height of the fluid in the tube represents thepressure difference between the system and thesurroundings of the manometer which is equal to thegage pressure:

Patm

ghPPPPgage atm1

.m/s 9.8 onaccelerati nalgravitatiog

tube,- Uin the points obetween tw fluid ofheight the

tube,manometer in the fluid theofdensity

tank,in the pressure gas

pressure, catmospheri

2

1

atm

h

P

P

ghPP

ghPPP

atmgas

atm21

1.7 PRESSURE

1.8 WORK

Energy can cross the boundary of a closed system in the formof heat or work. Therefore, if the energy crossing theboundary of a closed system is not heat, it must be work.

Work is the energy transfer associated with a force actingthrough a distance:

Work is also a form of energy transferred like heat and,therefore, has energy units such as kJ/kNm

The work done per unit time power and is denoted . Theunit of power is kJ/s, or kW.