Introduction to

Heat Exchanger & Classification Prepared by: Nimesh Gajjar

Device that facilitates the exchange of heat between two fluids that are at different temperatures without mixing each other Heat transfer in a heat exchanger involves

Convection in each fluid Conduction through wall separating each fluid

Overall heat transfer co efficient accounts for the above Conduction and convection effects

HEAT EXCHANGERS

Hot Fluid

Hot Fluid

Cold Fluid Convection

Conduction

Applications of Heat Exchangers

Heat Exchangers prevent car engine

overheating and increase efficiency

Heat exchangers are used in Industry for

heat transfer

Heat exchangers are used in AC and

furnaces

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These are most common heat exchangers in which hot and cold fluid do not come into contact with each other but are separated by a tube wall or a surface which may be flat or curved .

Energy exchange by hot fluid to surface by convection through the wall or plate by conduction and then by convection from the surface to the cold fluid.

They are used where mixing of hot and cold fluid is objectionable. For eg. Oil coolers, intercoolers, air preheaters, economisers, condensers. 2) radiators of automobiles 3) evaporator of an ice plant and milk chiller of a pasteurizing plant

Classification of Heat Exchangers

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These are heat exchangers in which hot and cold fluids flow alternatively ( i.e. periodically) through the same space with no or little physical mixing between the streams.

The heat carried away is accumulated in the walls of the equipment called solid matrix and is then transferred to the cold fluid when it passes the surface next.

Mostly used in gas to gas heat exchangers such as IC engines and gas turbines.

Other applications in glass melting furnace and air heaters of blast furnace.Depending Parameters-1) heat capacity of regenerating material 2)rate of absorption and release of heat

According to Construction :

Tubular heat exchanger (double pipe, shall and tube, coiled tube) TubularHeatEx.swf TripleTubeHe.swf shelltubehex.swf

Plate heat exchanger ( spiral, plate coil, lamella) PlateHeatEx.swf

Extended surface exchangers (tube fin, plate fin)

Regenerators (fixed matrix, rotary)

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Spiral Plate HE Lamella Plate HE

PLATE HEAT EXCHANGER

According to Transfer Process

1) Indirect Contact (double pipe, shall and tube, coiled tube)

2) Direct contact (cooling towers)

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COMPACT HEAT EXCHANGERS

β >700 m2/m3 –CompactCar radiator β =1000 m2/m3

Gas turbine HE β =1000 m2/m3

Human lungs β =20000 m2/m3

Achieve high heat transfer rates between two fluids in a small Volume

The ratio of the heat transfer surface area of a heat exchanger to its volume is called the “area density β’’.

A heat exchanger with area density β is greater than 700 m2/m3 or 200 ft2/ft3 is classified as being compact.

volumeexchangerHeat

areasurfacetransferHeatdensityArea

According to Flow Arrangement

Concurrent – Flow in same directionThermodynamically poorHigh thermal stresses since large temperature difference at inletHeat sensitive materialsTubularHeatEx.swf

Counter current- flow opposite to each otherThermodynamically superiorMinimum thermal stressesMaximum heat recoveryLeast heat transfer area TubularHeatEx.swf

Cross flow- Flow perpendicular to each otherIn between the aboveDesign of headers require less space

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CROSS FLOW HEAT EXCHANGER

Cross-flow(unmixed)

Tube flow(unmixed)

a) Both fluids unmixed

Cross-flow(mixed)

Tube flow(unmixed)

b) One fluids mixed, one fluid unmixed

Double pipe heat exchanger: Parallel Counter flow

Hot Fluid

Cold Fluid

T

Hot Fluid

Cold Fluid

T

Hot fluid in Hot fluid out

Cold fluid in

Cold fluid out

Hot fluid in Hot fluid out

Cold fluid in

Cold fluid out

TubularHeatEx.swf

According to Pass Arrangement

• Single Pass• Two Pass• Multi Pass

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One shell two tube pass

Two shell four tube pass

Shell-side fluid In

Tube-side fluid

Out

In

Out

Tube-side fluid

Shell-side fluid In

In

Out

Out

• These classifications is made according to the phase of the fluid.

1. gas-gas, 2. liquid-liquid, 3. Gas-liquid etc.

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According to Phase of Fluid:

According to Mechanism of Heat Transfer1. Single phase convection, (forced or free).

2. Two phase convection (condensation or evaporation) by force or free convection .

According to Extended Surface:

Overall heat transfer coefficientAi – Inside area of the tubeAo – Outside area of the tubehi – inside heat transfer coefficientho – Outside heat transfer coefficientk – thermal conductivity of tube

oo

io

iiowalliTotal Ah

1kL2

rrln

Ah1

RRRR

oo

io

iiooii Ah1

kL2r

rln

Ah1

AU1

AU1

U is meaningless unless area is specified

Cold fluid

Hot fluid

1i

i i

Rh A

1o

o o

Rh A

wallR

Hot fluid

Cold fluid

WalliAoA

ihoh

Heat transfer

iT

iT

oT