CHAPTER 1

INTRODUCTION

Anita Bt. Abu Bakar

Department of Chemical & Petroleum Engineering

anita@ucsiuniversity.edu.my

OUTLINE

Introduction Heat Conduction Forced Convection Natural Convection Application of Conduction &

Convection Thermal Radiation Application of Radiation

• Heat transfer is the science that deals with

the study of rates of exchange of heat

between hot and cold bodies.

• The hot and cold bodies are called source

and receiver respectively.

• In all such cases temperature difference

between a source and the receiver is the

driving force for heat transfer.

INTRODUCTION

HEAT TRANSFER Vs THERMODYNAMICS

Example:

Cooling of hot steel bar placed in a pail water

THERMODYNAMIC predict the final equilibrium

temperature of the steel bar-watercombination

HEAT TRANSFER predict the temperature of both the

bar and the water as a function of

time also the temperature of the bar will

be after a certain length of time

WHAT IS HEAT TRANSFER?

Heat transfer is thermal energy transferthat is induced by a temperature difference(or gradient)

Mechanism of heat transfer

i. Conduction - Surface to surface

ii. Convection - Surface to air

iii. Radiation - Direct exchange across

space

Conduction

Occurs when a temperature gradient exists through a solid or

a stationary fluid (liquid or gas). (Surface to surface)

Transfer of energy from the more energetic to less energetic

particles of a substance by collisions between atoms and/or

molecules.

Conduction in:

Solid

Conduction occur at molecular scale, the hotter the molecules with

greater energy imparting to the adjacent molecules at low energy level.

Thermal energy conducted in solid by 2 modes:

i. lattice vibration

ii. Motion of free electrons (Metallic solid)

Stationary Fluid (Liquid or Gas)

Freedom of movement (at macroscopic level they are stationary but

moving at microscopic level) transfer of energy occur due to collision of

molecules

The transfer of energy from one region to another due to themacroscopic motion in a fluid, added on to the energytransfer by conduction is called heat transfer by convection.

Convection is restricted to the flow of heat in fluids. If thefluid motion is caused by differences in the density resultingfrom temperature difference in a fluid, the heat transfer issaid to be free or natural convection.

If the fluid motion is artificially created by means of anexternal agency like pump, blower or agitator, the heattransfer is termed as forced convection

Convection

The transfer of energy from one region to another due to themacroscopic motion in a fluid, added on to the energy transfer byconduction is called heat transfer by convection.

Convection is restricted to the flow of heat in fluids. If the fluidmotion is caused by differences in the density resulting fromtemperature difference in a fluid, the heat transfer is said to befree or natural convection.

If the fluid motion is artificially created by means of an externalagency like pump, blower or agitator, the heat transfer is termedas forced convection

Convection

Application:

1. Refrigerator & Freezer

2. Air Conditioner

Application(Conduction & Convection)

Ultra Low Temperature Freezer

Simple Refrigeration Unit

P-H diagram of simple refrigeration unit

Application(Conduction & Convection)

Refrigeration evaporators

Fourier’s Law of Heat Conduction

dx

dTk

A

qx

Heat Convection

fw TThAq

Application(Conduction & Convection)

LNG Liquefaction

Liquefied natural gas takes up about 1/600th

the volume of natural gas in the gaseous

state

The natural gas is then condensed into a

liquid at close to atmospheric pressure

(maximum transport pressure set at around

25 kPa/3.6 psi) by cooling it to

approximately −162 °C (−260 °F)..

Application(Conduction & Convection)

LNG LIQUEFACTION

Benefits

Compactness, saving

installation space and

investment costs

Many process streams can

be handled in a single unit,

thus avoiding expensive

interconnecting piping of

different units

Low equipment weight

The vacuum brazed

aluminium plate-fin heat

exchangers are preferred in

small LNG plants

Higher Transfer Area : 10 x

(m2/m3) compared CWHE

Plate Fin Heat Exchanger

LNG LIQUEFACTION

Coil-Wound Heat Exchanger

Benefits

Providing a

large heating

surface per

shell

Tolerant

against thermal

shocks due to

its robust

(kokoh) design

Applicable in

wide range of

cold and warm

Radiation

All physical matter emits thermal radiation in the form

of electromagnetic waves because of vibrational and

rotational movements of the molecules and atoms

which makes up the matter

Ex:- Transfer heat from the sun to the earth

Characteristics of Radiation

1. Rate of emission increases with temperature

level.

2. No material medium required for the energy transfer to

occur

In many of the applications of heat transfer in

chemical engineering, heat will be transferred

by the combination of two or three of the

basic mechanism/modes of heat transfer.

Ex:- solar energy panels

Solar Cell

Which one gives the highest radiation rates? And

why? what fuel that will yield combustion with

blue flame and red flame? Make a comparison on

their application..

Blue flame Vs Red flame

The Laws of Heat Transfer

The foundation of HT based on number of

Fundamental laws

Subsidiary laws.

A fundamental law is the one which rest on the fact that it

has not been proved to be wrong or false in the bound area

of application of the subject under consideration. Therefore

these laws must be satisfied in solving any problem.

Subsidiary laws may be empirical in nature i.e. a statement

based upon the experimental evidences and intuition

Fundamental Laws used in HT are :

1. Law of conservation of mass

2. Newton’s laws of motion

3. Laws of thermodynamics

Subsidiary laws generally are:

1. Fourier’s law of heat conduction

2. Newton’s law of cooling

3. Laws of thermal radiation

4. Equation of state (for describing as gas)

Closed System

1. Fixed amount of matter

2. No mass flows

3. Energy flows may occur

4. Boundaries may change

Generally problem on solids

Control volume/open system

1. Volume of a fixed size containing mater

2. Mass flows across boundaries

3. Energy flow across the boundaries

Generally problems on liquid and gases

Thank you