Application of Mathematics in Chemical Engineering · 02/01/2014 · Engineering Heat Transfer...
Transcript of Application of Mathematics in Chemical Engineering · 02/01/2014 · Engineering Heat Transfer...
Historical Background
Caloric theory: Heat is fluid like substance called caloric
that is mass-less, colorless, odorless, tasteless substance
that can be poured from one body into another)
James P. Joule: published in
1843 that heat was not a
substance after all
Heat: Perceived to be something that produces in us a
sensation of warmth
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Historical Background
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Thermodynamics?
Heat Transfer?
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Thermodynamics
It is the branch of engineering science which deals with
transformation of energy from one form to another.
The study of the relationship between heat, work and
energy.
It deals with the equilibrium and feasibility of process
and properties of system, direction of change etc.
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Work
The energy required to move an object against a force.
Work = force × distance
Energy
The capacity to do work or transfer heat.
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Energy can exist in various forms such as:
thermal
mechanical
kinetic
potential
electrical
magnetic
chemical
nuclear
Their sum constitutes the total
energy E of a system.
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Internal energy is the energy stored in a body.
It increases when the temperature of the body rises or
when the body changes from solid to liquid or from
liquid to gas.
Internal energy is the sum of kinetic and potential
energy of all particles in the body.
Unit of internal energy: joule (J)
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Heat
Heat is a form of energy.
It is energy in transit form which moves from high to low
temperature level.
Energy associated with the random motion of atoms and
molecules.
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When two bodies of different temperatures touch each
other, energy is transferred from the hot body to the cold
body until they reach the same temperature. (The bodies
are then said to be in thermal equilibrium.)
Heat is the energy transferred from one body to another as
a result of a temperature difference.
Heat is a form of energy which passes from a body at
higher temperature to a body at a lower temperature.
Unit of heat: joule (J)
Note: Heat cannot be measured directly by an instrument
as temperature is by a thermometer.
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• The transfer of energy as heat is always from the
higher-temperature medium to the lower-temperature
one.
• Heat transfer stops when the two mediums reach the
same temperature.
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Heat transfer: Science of thermodynamics and fluid
mechanics
Engineering Heat Transfer consists of the study of thermal
energy movement
Heat transfer is the science which predicts the rate of
energy transfer taking place between material bodies as a
result of temperature difference between them.
Heat transfer cannot be measure directly but the effects
produced by it can be observed and measured.
Heat Transfer
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Heat Temperature
It is a form of energy called
thermal energy.
It is a thermal state of a
body
It is measure of degree of
hotness or coldness of
system
Measured in kcal, BTU etc. Measured in 0C, 0F, K etc.
Colorimeter is used for
measurement.
Thermometer is used for
measurement.
Extensive property Intensive property
Extensive properties (i.e. mass) are dependent upon
the amount of a substance, while intensive
properties (i.e. density) are independent of quantity.
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• Zeroth law of Thermodynamics deals with thermal
equilibrium and establishes a concept of Temperature.
• First law of Thermodynamics throws light on concept of
internal energy.
• Second law of Thermodynamics indicates the limit of
converting heat into work and introduces the principle of
increase of entropy.
OR
It states that "Heat will flow naturally from one reservoir to
another at a lower temperature but not in opposite
direction without assistance.” (This law established the
direction of energy transport).
• Third law of Thermodynamics defines absolute zero of
entropy.
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Thermodynamics Heat Transfer
It is concerned with
equilibrium states and
precludes existence of
temperature gradient
It is concern with non-
equilibrium states.
Temperature gradient must
exist for exchange of heat
It helps to determine quantity
of heat and work interactions
when a system changes from
one equilibrium state to
another but do not provide
information on nature of
interaction and time rate at
which interaction occurs
∆u = ∆Q ± ∆W
It helps to predict rate at
which energy is exchanged
and also predicts temperature
distribution as a function of
coordinates and time within
regions of matter
We can not obtain
temperature profile
We can obtain temperature
profile
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• We are normally interested in how long it takes for the
hot coffee in a thermos bottle to cool to a certain
temperature which can not be determined from a
thermodynamic analysis alone.
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Consider the cooling of a hot steel bar placed in a pail of
water.
Thermodynamic analysis will predict the final equilibrium
temperature of the steel bar-water combination but it will not
predict how long it takes to reach this equilibrium condition or
what the temperature of bar will be after a certain length of
time before the equilibrium condition is attained.
But heat transfer study will predict the temperature of both the
bar and water as a function of time.
Heat transfer studies mainly require the knowledge of
thermodynamics, fluid mechanics, physics and mathematics.
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Energy can be transferred to or from
a given mass by two mechanisms:
heat transfer and work
Heat transfer rate: The amount of
heat transferred per unit time.
Heat flux: The rate of heat transfer
per unit area normal to the direction
of heat transfer.
Power: The work done per unit time.
when is constant:
The sensible and latent forms of internal energy can be transferred as a result of a
temperature difference and they are referred to as heat or thermal energy
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Application Areas of Heat Transfer
Energy production and conversion
For thermal design of boilers, steam turbines,
condensers, gas turbines, cooling towers, feed
heaters, internal combustion engine
Have to make a detailed heat transfer analysis.
Refrigeration and air conditioning:
The thermal design of compressors, evaporators,
incinerators involves an intensive study of heat
transfer.
Electric machines:
Insulations provide on electric wires need a heat
transfer analysis to avoid conditions which will cause
overheating.
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Application Areas of Heat Transfer
Civil Engineering
Design of buildings for the minimization of heat losses
need heat transfer study.
Construction of dams and other heavy structures,
calculation of thermal expansion of suspension bridge
and railway tracks.
Manufacturing process
The casting of metals, extrusion, metal cutting and
heat treatment of metals involve heat transfer study.
Chemical and Petrochemical operations
Environmental Engineering
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Application Areas of Heat Transfer
Earth Sciences
Astronomy
Agriculture and Food processing
Application:
The human body is constantly rejecting heat to its
surroundings and human comfort is closely tied to the rate
of this heat rejection.
We try to control this heat transfer rate by adjusting our
clothing to the environmental conditions.
Computer, TV, VCR, Freeze, Air-conditioning System
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Application Areas of Heat Transfer
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Engineering Heat Transfer
Heat transfer equipment such as heat exchangers, boilers,
condensers, radiators, heaters, furnaces, refrigerators, and
solar collectors are designed primarily on the basis of heat
transfer analysis.
The heat transfer problems encountered in practice can be
considered in two groups: (1) rating and (2) sizing
problems.
The rating problems deal with the determination of the
heat transfer rate for an existing system at a specifiedtemperature difference.
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Engineering Heat Transfer
The sizing problems deal with the determination of the
size of a system in order to transfer heat at a specified
rate for a specified temperature difference.
An engineering device or process can be studied either
experimentally (testing and taking measurements) or
analytically (by analysis or calculations).
The experimental approach has the advantage that we
deal with the actual physical system, and the desired
quantity is determined by measurement, within the limits
of experimental error. However, this approach is
expensive, time consuming, and often impractical.
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Questions1. The capacity to do work is _____________.
2. Heat is measured in _________.
3. The from of energy that produced feeling of hotness is
called________.
4. __________is neither created nor destroyed it can only
change one form to another.
5. What is the driving force for a heat transfer?
6. Heat transfer takes place according to __________law of
thermodynamics.
7. How does the science of heat transfer differ from the
science of thermodynamics?
8. How do rating problems in heat transfer differ from the
sizing problems?