2. Out Line
INTRODUCTION TO HEAT TRANSFER
Definition of thermal unit operation
Unit Operations: Classification
Mechanism of heat transfer
Conduction
Convection
Radiation
3. Chapter One
1. Definition Of Thermal Unit Operation
1.1 unit operation
A unit operation is any part of potentially multiple step
process W/C can be considered to have A single function.
In Chemical engineering and related fields, a unit
operation is a basic step in a process.
The basic physical operations of chemical engineering in
a chemical processes plant, that is, distillation, fluid
transport, evaporation, extraction, drying, crystallization,
filtration, mixing, size separation, crushing and grinding,
conveying.
4. Cont…
Unit operations involve a physical change or
chemical transformation such as;
Separation
Crystallization
Evaporation
Filtration
Polymerization
Isomerization, and other reactions which are
connected to create the overall process.
5. Conti…
A process may require many unit operations to
obtain the desired product from the starting
materials or feed stocks.
For example, in milk processing, homogenization
pasteurization, chilling, and packaging are each unit
operations
Large processes are broken into unit operations in
order to make them easier to analyze.
The main thing is that conservation of lows is
applied not only to the process but also to each
individual unit operation.
6. Unit Operations: Classification
Fluid flow processes
fluid transport
solids fluidization
mixing
Heat transfer processes
(Thermal unit operations)
heating/cooling
evaporation/condensation
distillation
Mass transfer processes
absorption
distillation
extraction
8. Cont…..
Transport phenomena:
-Three area of study
1) Fluid mechanics
2) Heat transfer study of transfer something
3) Mass transfer
Fluid mechanics:- deals with the transfer of
momentum in a fluid.
- on a molecular scale that means the molecules
bagging in to each other transfer their momentum
to other molecules.
9. Conti…
Mass transfer is:-deals with the transfer of mass.
• Fore example: taking a glass of water and put one
drop of red dye in it. even if you don’t stir the
water eventually, the water is all pinkish color so,
• The study of how the dye spreads out is mass
transfer.
How are they all related?
• There is a driving force (momentum, T⁰ or
concentration difference or gradient), w/c become
smaller as time progresses & eventually becomes
zero when no more transfer of stuff takes place.
10. Application Areas
The unit operations cut across widely different
processing applications, like,
Manufacture of chemicals,
fuels,
pharmaceuticals,
pulp and paper,
processed foods, and
primary metals etc.
Unit operations approach serves as a very powerful
form of morphological analysis, which
systematizes process design, and greatly reduce
both the number of possibilities that should be
considered in synthesizing a particular process.
11. Heat Transfer
It Is A Science Dealing With The Rate Of Heat Exchange B/n Hot
& Cold Bodies Called The Source & Receiver.
The Energy Change During A Pound Of Water Vaporize Or
Condense Is Similar But The Rate Of Processes In An Independent
Source Or Receiver Is Different As Vaporization Is Very Rapid Than
Condensation.
The phases of a single substance (solid, liquid & gase) associated
with its energy content.
12. Cont…
The closing up of atoms in a solid gives its
rigidity while in a liquid phase rigidity lost b/c of
sufficient thermal energy that extends distance
b/n adjacent molecule.
The presence of additional thermal energy in a
gases phase results the separation of molecules
enabling them to wander anywhere in a confined
space.
13. Cont…..
Thermodynamics:-deals with systems in
equilibrium;
it may be used to predict the amount of energy
required to change a system from one
equilibrium state to another; it may not be used to
predict how fast a change will take place since
the system is not in equilibrium during the
process.
As an example of the different kinds of problems
that are treated by thermodynamics and heat
transfer, consider the cooling of a hot steel bar
that is placed in a pail of water.
14. Cont…
Thermodynamics may be used to predict the final
equilibrium temperature of the steel bar–water
combination.
It will not tell us how long it takes to reach this
equilibrium condition or what the temperature of
the bar will be after a certain length of time
before the equilibrium condition is attained.
Heat transfer may be used to predict the
temperature of both the bar and the water as a
function of time.
15. 1.2 Heat Transfer Mechanisms
Any energy exchange b/n bodies occurs through
one of those modes or a combination of them.
These are ;conduction, convection and radiation
Conduction:- is the transfer of heat through solid
or stationary fluids.
Convection:- uses the movement of fluids to
transfer heat.
Radiation:- doesn’t require a medium for
transferring heat. it uses the electromagnetic
radiation emitted by an object for exchanging
heat.
16. Conduction
In solids atoms are bound to each other by series
of bonds analogous to springs.
When there is a temperature difference in the
solids , the hot side of fluid experiences more
vigorous atomic movements. The vibrations are
transmitted through the springs to the cooler side
of the solid.
Eventually they reach the equilibrium where all
the atoms are vibrating with the same energy.
Electrons in the effectiveness by which heat is
17. Cont…
transferred through a material is measured by
thermal conductivity; a poor conductor or an
insulator, has a low conductivity.
Conduction is the direct path way heat transfer
Conductivity is measured in watts per meter per
Kelvin (w/mk).
The rate of heat transfer by conduction is given
by;
q(conduction)=-KA∆T /∆X ……………..eq(1.1)
Where; A -is the cross-sectional area through which
18. Conti…
the heat is conducting
T-is the temperature difference between the two
surfaces separated by distance ∆X .
Negative sign of q implies heat is transferring in
to the body
Positive sign of q implies heat leaving from the
body
19. Cont….
Thermal energy may be conducted in solids by two
modes: lattice vibration and transport by free
electrons.
In good electrical conductors a rather large
number of free electrons move about in the lattice
structure of the material.
Just as these electrons may transport electric
charge, they may also carry thermal energy from a
high-temperature region to a low-temperature
region, as in the case of gases.
Energy may also be transmitted as vibrational
energy in the lattice structure of the material.
20. Cont…
In general, however, this latter mode of energy
transfer is not as large as the electron transport,
and for this reason good electrical conductors are
almost always good heat conductors, namely,
copper, aluminum, silver and electrical insulators
are usually good heat insulators.
A notable exception is diamond, which is an
electrical insulator, but which can have a thermal
conductivity five times as high as silver or
copper.
It is this fact that enables a jeweler to distinguish
between genuine diamonds and fake stones.
21. Cont…
A small instrument is available that measures the
response of the stones to a thermal heat pulse. A
true diamond will exhibit a far more rapid
response than the non genuine stone.
At high temperatures, the energy transfer through
insulating materials may involve several
modes: conduction through the fibrous or porous
solid material; conduction through the air trapped
in the void spaces; and, at sufficiently high
temperatures, radiation.
22. Cont…
An important technical problem is the storage
and transport of cryogenic liquids like liquid
hydrogen over extended periods of time.
Such applications have led to the development of
super insulations for use at these very low
temperatures (down to about−250◦C). The most
effective of these super insulations consists of
multiple layers of highly reflective materials
separated by insulating spacers.
23. Cont…
The entire system is evacuated to minimize air
conduction, and thermal conductivities as low as
0.3 m W/m ◦C are possible.
convection
Uses the motion of fluids to transfer heat.
Natural convection (free convection) refers to a
case where the fluid movement is created by the
warm fluid itself.
The density of fluid decreases as it is heated &
These hot fluids are lighter than cold fluids.
24. Cont…..
If a heated plate were exposed to ambient room air
without an external source of motion, a movement
of the air would be experienced as a result of the
density gradients near the plate. We call this
natural , or free, convection as opposed to forced
convection, which is experienced in the case of the
fan blowing air over a plate.
Boiling and condensation phenomena are also
grouped under the general subject of convection
heat transfer.
25. Cont…
Warm fluid surrounding a hot object rises, & is
replaced by cooler fluid. the result is the
circulation of air above the warm surface.
Convection coefficient ,h, is the measure of how
effectively a fluid transfers heat by convection.
It is measured in w/m²k,& is determined by
factors such as the fluid density, viscosity &
velocity .
Newton's low of cooling (the rate of heat transfer
from surface by convection) is given by
26. Cont…
q(convection) =-hA(T surface-T∞) ……..eq (1.2)
Where; A–surface area of object exposed to fluid,
TSurface-surface temperature of object &T∞-is the
ambient or fluid temperature.
The convective heat transfer coefficient (h)
Is not a material property
Is a complicated function of many parameters that
influence convection such as fluid velocity, fluid
properties, and surface geometry
Is often determined by experiment rather than
theory
27. Cont..
In the convection gain or loss resulting from a
fluid flowing inside a channel or tube as shown
above the heated wall at Tw loss heat to the
cooler fluid, causes to rise the temperature.
from inlet conditions at Ti to exit conditions at
Te. Using the symbol I to designate enthalpy
Fig,1.1
28. Cont…
(to avoid confusion with h, the convection
coefficient), the energy balance on the fluid is
q=˙m(ie−ii)……….eq (1.3)
where˙ m is the fluid mass flow rate. For many
single-phase liquids and gases operating over
reasonable temperature rangesi=cpT and we have
q=˙mcp(Te−Ti)
which may be equated to a convection relation like
Equation 1.2
29. Cont…
• q=˙mcp(Te−Ti)=hA(Tw,avg−Tfluid,avg)…eq 1.4
The fluid temperatures Te, Ti, and T fluid are called
bulk or energy average temperatures. A is the
surface area of the flow channel in contact with the
fluid.
We must be careful to distinguish between the
surface area for convection that is employed in
convection Equation (1.2) and the cross-sectional
area that is used to calculate the flow rate from˙
m=ρumeanAc
Where Ac=πd2/4 for flow in a circular tube. The
surface area for convection in this case would be
πdL,
30. Cont….
• where L is the tube length. The surface area for
convection is always the area of the heated
surface in contact with the fluid.
31. Types of convection
• Forced convection: flow caused by an external
source such as a fan, pump, or atmospheric
wind
• Free or natural convection: flow induced by
buoyancy force such as that from a heated plate
• Phase change convection: flow and latent heat
exchange associated with boiling or
condensation
32. Radiation
Unlike conduction and convection, Radiative heat
transfer doesn’t require a medium to pass through;
these it is the only form of heat transfer present in
vacuum.
Uses electromagnetic radiation (photons) which
travel at the speed of light and is emitted by any
matter with T⁰ above 0⁰k (-273⁰C).
Radiative heat transfer occurs when the emitted
radiation strikes another body & is absorbed.
Solar radiation; absorbed by our skin, is why we
feel warmer in the sun than in the shade.
33. Cont…
Thermodynamic considerations show that an ideal
thermal radiator, or blackbody, will emit energy at a
rate proportional to the fourth power of the absolute
temperature of the body and directly proportional to
its surface area. Thus
qemitted=σAT4.....................eq (1.5)
Where σ is the proportionality constant and is called
the Stefan-Boltzmann constant with the value of
5.669×10−8W/m2K4. Equation (1.5) is called the
Stefan-Boltzmann law of thermal radiation, and it
applies only to blackbodies.
It is important to note that this equation is valid only
for thermal radiation; other types of electromagnetic
radiation may not be treated so simply.
34. Cont…
The net radiant exchange between two surfaces
will be proportional to the difference in absolute
temperatures to the fourth power; i.e.,
qnet exchange /A∝σ(T4
1−T4
2) ..…eq(1.6)
To take account of the “gray” nature of such
surfaces we introduce another factor into
Equation (1-6), called the emissivity Ɛ, which
relates the radiation of the “gray” surface to that
of an ideal black surface.
In addition, we must take into account the fact
that not all the radiation leaving one surface will
reach the other surface since
35. Cont…
Electromagnetic radiation travels in straight lines and
some will be lost to the surroundings. We therefore
introduce two new factors in Equation (1-5) to take
into account both situations, so that
q=FƐFGσA (T41−T42)…………..eq(1.7)
Where:-FƐ is the emissivity function, and FGis the
geometric “view factor” function.
36. Radiation in an Enclosure
A simple radiation problem is encountered when
we have a heat-transfer surface at temperature T1
completely enclosed by a much larger surface
maintained at T2.
the net radiant exchange in this case can be
calculated with
q= Ɛ1σA1(T4
1−T4
2)………..eq [1.8]
values of Ɛ will be given.
37. Chapter two
2. Heat Exchangers
2.1 definition of heat exchanger
Heat exchangers are devices that facilitate the
exchange of heat between tow fluids that are at
different temperatures while keeping them from
mixing with each other.
Heat exchangers are commonly used in practice
in a wide range of applications, from heating
and air conditioning systems in a household, to
chemical processing and power production in
large plants.
39. Compact heat exchanger
The ratio of heat transfer surface area of a heat
exchanger to its volume is called the area density ᵦ
A heat exchanger with (В=700m²/m³), glass
ceramic gas turbine heat exchangers to
counteract the low heat transfer coefficient
associated with gas flow with increased
surface area.
A gas-to-liquid compact heat exchanger
fore a residential air-conditioning system.
