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Heat Transfer Letcure 1

By Satztg1 Feb 27, 2013 1009 Words
HEAT TRANSFER
LECTURE 1

OUTLINE









Introduction
Heat Conduction
Forced Convection
Natural Convection
Application of Conduction & Convection
Thermal Radiation
Application of Radiation

Introduction


What does the subject of heat transfer
deal with? And why it is important.



What does the subject of mass transfer
deal with?



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.

Difference between TD and HT


The former deals with heat and other forms of energy.
Whereas, the latter is concerned with the analysis of the rate of heat transfer.



TD deals with systems in equilibrium so it cannot be used to predict how fast a change will take place since a system is
not in equilibrium during the process, but it may predict the amount of energy required to change the system form one
equilibrium state to another.



Ex:- cooling of hot steel bar place in a jar of water

Why it is important to study HT?


Once we have the LAWS which govern the HT
we will be in position to design an equipment
and to size an equipment in which the HT
process occurs.



Ex-: car Radiator (HT from cylinder wall to water
and water to air) is typical example of a HE.



To design this HE one should have the
knowledge of convective HT occurring at air and
water side and conduction occurring on Fins and
Tube of radiator

Mass Transfer
The study of rates of exchange of Mass
from region of high conc to a region of low
conc.
 EX:- drying of a film
- evaporation of water from a pool to
the atmosphere.


Some Problem of interest in HT


Heat loss through thermal insulation of
steam pipe



Heat transfer to water flowing through a
tube

Modes of Heat Transfer
Conduction
 Convection
 Radiation


Conduction


Conduction is a flow of heat in a substance due to exchange of energy between molecules having high energy and molecules having less energy

Conduction in solids


Conduction occur at molecular scale, the hotter the molecules with greater energy imparting to the adjacent molecules at low energy level

1. Lattice Vibration
2. Motion of free electrons
(Metallic solid)



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

Convection


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



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



If the fluid motion is artificially created by means of an external agency like pump, blower or agitator, the heat transfer is termed as forced convection

Application

Simple Refrigeration Unit

Application:
1. Refrigerator & Freezer
2. Air Conditioner

Ultra Low Temperature Freezer

Application

P-H diagram of simple refrigeration unit

Application

Fourier’s Law of Heat Conduction

qx
dT
 k
A
dx
Heat Convection
Refrigeration evaporators

q  hATw  T f 

Application
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)..

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 platefin heat exchangers are preferred in small LNG plants
– Higher Transfer Area : 10 x (m2/m3)
compared CWHE

Plate Fin Heat Exchanger

LNG LIQUEFACTION
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

Coil-Wound Heat Exchanger

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

Solar Cell
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

Blue flame Vs Red flame
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..

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

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