introduction to heat exchanger & classification prepared by: nimesh gajjar
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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
4
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
1
kL2
rrln
Ah
1RRRR
oo
io
iiooii Ah
1
kL2
rrln
Ah
1
AU
1
AU
1
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
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