MET 214 Heat exchanger module-1

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

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.

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

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

RADIATION:RADIATION HEAT TRANSFER BETWEEN TWO SUBSTANCES TAKE

PLACE EVEN WITHOUT ANY MEDIUM THROUGH ELECTROMAGNETIC WAVES

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

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.

Convection: Is the way heat flows through fluids, whether they are

liquids or gases.

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)

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

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.

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.

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.

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