met 214 heat exchanger module-1
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MODULE-1According to thermodynamic concept ,
the energy that is transferred as a result of a temperature difference is called HEAT
THREE DIFFERENT TYPES OF HEAT TRANSFER:
1.CONDUCTION2.CONVECTION3.RADIATION
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Itmportances of heat transfer:1.electrical engineerg:cooling system for
motors,generators and transformor2.chemical engineering:
evaporation,condensation,heating and cooling of fluid
3. civil engineering: constructions dams,structures,building design.
4.mechanical engineering:heat transfer in internal combustion engine,steam
generations,refrigeration and heating and ventillation.
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Conduction:it is the process by which heat flows from a region of higher temperature to a region of lower
temperature within the substance. Conduction take place by two mechanism:
a)by lattice vibration:molecules at higher temperature imparts energy to adjacent molecules at lower
temperature.b)by free electrons: transfer of heat by free electrons, the free electrons concentration in non-metals is very
low.Hence conduction is found in solids
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Convection:it is possible only in the presence of fluids( liquid and gases).As fluid passes over hot objects,they pick up heat energy and carry it to
colder area.Two types of convection:
1.free convection2.forced convection
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RADIATION:RADIATION HEAT TRANSFER BETWEEN TWO SUBSTANCES TAKE
PLACE EVEN WITHOUT ANY MEDIUM THROUGH ELECTROMAGNETIC WAVES
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Heat flux: it is the amount of heat flow per unit area through a
body=Q/A W/m2
the materials having higher thermal conductivity are conductors, while
material having lower thermal conductivity are called as insulators
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Radiation is the only way that heat can move through a vaccum.Radiant heat can be felt by holding a hand near any hot object,higher the temperature of the object, more energy being released. The denser the solid, the faster heat will be transferred from the hottest regions to coldest.
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Convection: Is the way heat flows through fluids, whether they are
liquids or gases.
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THE LAWS OF HEAT TRANSFER.1.Fourier’s law of heat conduction.
The rate of heat flow is proportional to the product of the area of flow A, and the temperature gradient (-dt/dx),the constant of proportionality being the
thermal conductivity k, which is a property of material.
Unit is j/s or WQx=-kA(dt/dx)
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2.NEWTONS LAW OF HEAT CONVECTION
Q=hA(t1-t2)heat transfer is occuring from a surface area A at temperature t1, to a fluid at a
lower temperature t2.
h is the convection co-efficient in w/m2K
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STEFAN-BOLTZMANN LAW OF RADIATION:Q=EσAT4
T-absolute temperature,
A-surface area,
E-emissivity
σ-stefan boltzmann constant,5.67*10-8w/m2k4
It states that the radiant energy emitted by a black body is proportional to the surface area and fourth power of its absolute temperature.
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Thermal conductivity:thermal conductivity, k is the
property of a material's ability to conduct heat
PURE METALS HAVE HIGHER VALUES OF THERMAL
CONDUCTIVITY WHILE GASES AND VAPORS HAVE LOWEST.
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K for a pure metals decreases with temperature.
K=ko(1+bф+cф2)
where ф=T-Tref.
K for a non homogeneous materials increases both with increasing
temperature and increasing density.
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Thermal conductivity: it is defined as the ability of a substance to conduct heat. It is expressed in
w/m-k or w/moC.Heat transfer co-efficient(h): ability
of the fluid carry away heat from the surfaces which in turn depends upon velocities and other thermal properties.unit w/m2 k or w/m2oC