Heat transfer notes
Transcript of Heat transfer notes
CHAPTER 1
INTRODUCTION
&
MECHANISM OF HEAT TRANSFER
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
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