Cycle Test 2

8/14/2019 Cycle Test 2

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KAMARAJ COLLEGE OF ENGINEERING AND TECHNOLOGYDEPARTMENT OF POLYMER TECHNOLOGY

SECOND INTERNAL TEST 2010

SUB CODE: PT 44 SUB NAME : PRINCIPLES OF CHEMICAL ENGINERING

SUB INCHARGE: K.AGATHIAN

TOTAL MARKS: 50 MARKS TIME: 1 HOURS

PART A 5 X 2 = 10 Marks

1. What are the different modes of heat transfer?

2. Define Fouriers law of heat conduction?

3. Define Ficks law of diffusion?

4. What is parallel & Counter flow in heat exchanger?5. What is LMTD?

PART B 4X 10 = 40 Marks

6. Explain in detail about shell and tube heat exchanger?

7. Derive the equation to find out the rate of heat transfer by conduction for a rectangular block

made of three different materials?

8. Derive the equations for equi molar counter diffusion in gas?

9. Derive the equation to find out the rate of heat transfer by conduction for a hollow cylinder

made of three different materials?

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KEY TO FIRST INTERNAL QUESTION

PART A1. Modes of heat transfer:

CONDUCTION Heat can be conducted between two bodies which are in contact with each other; heat "flows"

from one to the other. Materials which conduct heat well are called conductors of heat. Electrical conductors(such as metals) are good conductors of heat. Materials which do not conduct heat well are called insulators . Electrical insulators(for example, wood or glass) are usually good insulators of heat. Materials with low density,such as air or foamed plastic, are normally also good insulators unless they happen to beelectrical conductors. To prevent heat from moving from one place to another, we usually

place an insulator between.

CONVECTION

This is a different kind of heat transfer than conduction. In conduction, heat itself is moving;in convection, hot portions of a fluid move through the body of the fluid.

The hot fluid mixes with the cold fluid, and heat is transferred more quickly than byconduction.

What we commonly call a "rolling boil" results from convection. Hot fluids rise through surrounding, cooler fluid because they are less dense; cooler fluids

sink through warmer fluids because they are denser. This causes circular motion of the fluid away from a source of heat. Convection in water drives ocean currents; convection in air drives weather patterns; and

convection of molten rock inside the earth is thought to drive plate tectonics.

RADIATION

Radiation is the simplest means of heat transfer. Heat radiation is carried not by moving atoms(as in conduction or convection) but by electromagnetic waves.

Radiation is the only way that heat can move through a vacuum, and is the reason that even aclosed thermos bottle (which has a vacuum between the inner and outer parts) will eventuallycome to the same temperature as its surroundings.

2. Fourier law of heat conduction:

When there exists a temperature gradient within a body, heat energy will flow from the regionof high temperature to the region of low temperature. This phenomenon is known as conduction heattransfer, and is described by Fourier's Law (named after the French physicist Joseph Fourier),

This equation determines the heat flux vector q for a given temperature profile T and thermalconductivity k . The minus sign ensures that heat flows down the temperature gradient.

3. Ficks law of diffusion:2
http://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=heatflux#heatfluxhttp://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=cond#condhttp://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=cond#condhttp://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=heatflux#heatfluxhttp://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=cond#condhttp://www.efunda.com/formulae/heat_transfer/home/glossary.cfm?ref=cond#cond


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,

also,

Where:

J is the flux D is the diffusivity constant of proportionality

is the concentration gradient in the x-direction c is the concentration u1 is the velocity relative to a stationary plane u0 is the bulk fluid velocity

4. Flow arrangement in heat exchanger:

Parallel flow: Both the shell side fluid and tube side fluid are flow in the same direction.

Counter flow: Both the shell side fluid and tube side fluid are flow in the opposite direction.

5. LMTD:

L ogarithmic M ean T emperature Difference

T m = T2 - T1 / ln ( T2 / T1)

PART B

6. Shell and tube heat exchanger: 10 Marks

A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical

processes, and is suited for higher-pressure applications. As its name implies, this type of heatexchanger consists of a shell (a large pressure vessel ) with a bundle of tubes inside it.

One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) totransfer heat between the two fluids.

The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.

THEORY AND APPLICATION

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside

the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to

shell side or vice versa.

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http://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Pressure_vesselhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Pressure_vessel


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The fluids can be either liquids or gases on either the shell or the tube side. In order totransfer heat efficiently, a large heat transfer area should be used, leading to the use of manytubes.

In this way, waste heat can be put to use. This is an efficient way to conserve energy. Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or

single-phase heat exchangers.

Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor),sometimes called boilers , or cool a vapor to condense it into a liquid (called condensers ), withthe phase change usually occurring on the shell side.

Boilers in steam engine locomotives are typically large, usually cylindrically-shaped shell-and-tube heat exchangers.

In large power plants with steam-driven turbines , shell-and-tube surface condensers are usedto condense the exhaust steam exiting the turbine into condensate water which is recycled

back to be turned into steam in the steam generator.

7. Composite wall: 10 Marks

Conduction is the transfer of heat by direct contact of particles of matter. The transfer of energy could be primarily by elastic impact as in fluids or by free electron

diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one

another, or as electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. In liquids (except

liquid metals) and gases, the molecules are usually further apart, giving a lower chance of molecules colliding and passing on thermal energy.

Heat conduction is directly analogous to diffusion of particles into a fluid, in the situationwhere there are no fluid currents.

This type of heat diffusion differs from mass diffusion in behaviour, only in as much as itcan occur in solids, whereas mass diffusion is mostly limited to fluids.

Metals (eg. copper, platinum, gold, iron, etc.) are usually the best conductors of thermalenergy. This is due to the way that metals are chemically bonded: metallic bonds (asopposed to covalent or ionic bonds ) have free-moving electrons which are able to transfer thermal energy rapidly through the metal.

As density decreases so does conduction. Therefore, fluids (and especially gases) are lessconductive. This is due to the large distance between atoms in a gas: fewer collisions

between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that

the density of the gas is such that molecules of the gas may be expected to collide with

each other before they transfer heat from one surface to another. After this point in density, conductivity increases only slightly with increasing pressureand density.

To quantify the ease with which a particular medium conducts, engineers employthe thermal conductivity , also known as the conductivity constant or conductioncoefficient , k .

In thermal conductivity k is defined as "the quantity of heat, Q, transmitted in time (t)through a thickness (L), in a direction normal to a surface of area (A), due to atemperature difference (T) Thermal conductivity is a material property that is primarilydependent on the medium's phase, temperature, density, and molecular bonding.

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http://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Condenserhttp://en.wikipedia.org/wiki/Locomotivehttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Surface_condenserhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/w/index.php?title=Free_electron_diffusion&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Free_electron_diffusion&action=edit&redlink=1http://en.wikipedia.org/wiki/Phononhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Covalent_bondshttp://en.wikipedia.org/wiki/Ionic_bondshttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Physical_propertyhttp://en.wikipedia.org/wiki/List_of_phases_of_matterhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Phase_(matter)http://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Condenserhttp://en.wikipedia.org/wiki/Locomotivehttp://en.wikipedia.org/wiki/Power_planthttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Surface_condenserhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/w/index.php?title=Free_electron_diffusion&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Free_electron_diffusion&action=edit&redlink=1http://en.wikipedia.org/wiki/Phononhttp://en.wikipedia.org/wiki/Heat_conductionhttp://en.wikipedia.org/wiki/Metallic_bondhttp://en.wikipedia.org/wiki/Covalent_bondshttp://en.wikipedia.org/wiki/Ionic_bondshttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Physical_propertyhttp://en.wikipedia.org/wiki/List_of_phases_of_matter


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Cycle Test 2