Heat-Exchanger Mechanical Engineering Power Engineering

Increased operating temperatures and higher efficiency in gas turbines and jet engines can reduce CO2 emission, thus contributing to the prevention of global warming. To achieve this goal, it is essential to improve the properties of high temperature materials

1. HEAT EXCHANGER
By
Farhan Ahmad
Department of Chemical Engineering,
University of Engineering & Technology Lahore
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2. Criteria for the selection of
heat exchanger
–Suitableonthegroundsofoperatingpressureand
temperature,fluid-materialcompatibility,handling,
extremethermalconditions
–Estimatingthecostofthosewhichmaybesuitable
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3. General considerations
•Tubes and cylinders can withstand higher pressures than
plates
•If exchangers can be built with a variety of materials,
then it is more likely that you can find a metal which
will cope with extreme temperatures or corrosive fluids
•More specialist exchangers have less suppliers, longer
delivery times and must be repaired by experts
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4. Double pipe heat exchanger
•Normal size
Double-pipe heat exchangers are competitive at
duties requiring 100-200 ft
2
•Built of carbon steel where possible
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5. Advantages/disadvantages of double-pipe HE
•Advantages
–Easy to obtain counter-current flow
–Can handle high pressure
–Modular construction
–Easy to maintain and repair
–Many suppliers
•Disadvantage
–Become expensive for large duties (above 1MW)
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6. Scope of double pipe HE
•Maximum pressure
–300 bar(abs) (4500 psia) on shell side
–1400 bar(abs) (21000 psia) on tubeside
•Temperature range
–-100 to 600
o
C (-150 to 1100
o
F)
–possibly wider with special materials
•Fluid limitations
–Few since can be built of many metals
•Maximumε= 0.9
•MinimumΔT= 5 K
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7. Shell and tube heat exchanger
•Size per unit 100 -10000 ft
2
(10 -1000 m
2
)
•Easy to build multiple units
•Made of carbon steel where possible
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8. Advantages/disadvantages of S&T
•Advantages
–Extremely flexible and robust design
–Easy to maintain and repair
–Can be designed to be dismantled for cleaning
–Very many suppliers world-wide
•Disadvantages
–Require large plot (footprint) area -often need
extra space to remove the bundle
–Plate may be cheaper for pressure below 16 bar
(240 psia) and temps. below 200
o
C (400
o
F)
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9. Scope of shell and tube
(Essentially the same as a double pipe)
•Maximum pressure
–300 bar(abs) (4500 psia) on shell side
–1400 bar(abs) (21000 psia) on tubeside
•Temperature range
–-100 to 600
o
C (-150 to 1100
o
F)
–possibly wider with special materials
•Fluid limitations
–Few since can be built of many metals
•Maximumε= 0.9 (less with multipass)
•MinimumΔT= 5 K
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10. Plate and frame heat exchanger
•Plates pressed from stainless steel or
higher grade material
–titanium
–incoloy
–hastalloy
•Gaskets are the weak point.Made of
–nitrile rubber
–hypalon
–viton
–neoprene
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11. Advantages of plate and frame HE
•Highheattransfer-turbulenceonbothsides
•Highthermaleffectiveness-0.9-0.95possible
•LowΔT-downto1K
•Compact-comparedwithaS&T
•Cost-lowbecauseplatesarethin
•Accessibility-caneasilybeopenedupforinspection
andcleaning
•Flexibility-Extraplatescanbeadded
•Shortretentiontimewithlowliquidinventoryhence
goodforheatsensitiveorexpensiveliquids
•Lessfouling-lowrvaluesoftenpossible
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12. Disadvantages of plate & frame HE
•Pressure-maximumvaluelimitedbythesealingof
thegasketsandtheconstructionoftheframe.
•Temperature-limitedbythegasketmaterial.
•Capacity-limitedbythesizeoftheports
•Blockeasilywhensolidsinsuspensionunlessspecial
widegapplatesareused
•Corrosion-Platesgoodbutthegasketsmaynotbe
suitablefororganicsolvents
•Leakage-Gasketsalwaysincreasetherisk
•Fireresistance-Cannotwithstandprolongedfire
(usuallynotconsideredforrefineryduties)
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13. Scope of plate & frame HE
•Maximum pressure
–25 bar (abs) normal (375 psia)
–40 bar (abs) with special designs (600 psia)
•Temperature range
–-25 to +175
0
C normal (-13 to +350
0
F)
–-40 t0 +200
0
C special (-40 to +390
0
F)
•Flow rates
up to 3,500 m3/hour can be accommodated in standard
units
•Fluid limitations
–Mainly limited by gasket
•Maximum ε= 0.95
•MinimumΔT= 1 K
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14. Principal Applications
Gasketedplateandframeheatexchangershavealarge
rangeofapplicationstypicallyclassifiedintermsofthe
natureofthestreamstobeheated/cooledasfollows:
Liquid-liquid.
Condensing duties.
Evaporating duties.
Gasketed units may be used in
refrigeration
heat pump plants and
extensively used in the processing of food and drinks.
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15. Comparison with Shell and Tube Heat
Exchangers
Inquantitativeterms,200m2ofheattransfersurface
requiresaplateandframeheatexchangerapproximately
3 metreslong,
2 metreshigh and
1 meter wide.
For a tubular heat exchanger achieving the same effect, some
600 m2 of surface would be required in a shell
5 metreslong and
1.8 metrein diameter,
plus the extra length
needed for tube bundle removal.
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16. Welded plates heat exchanger
•Widevarietyofproprietarytypeseachwithoneor
twomanufactures
•Overcomesthegasketproblembutthencannotbe
openedup
•Pairsofplatescanbeweldedandstackedin
conventionalframe
•Conventionalplateandframetypeswithall-welded
(usinglasers)constructionhavebeendeveloped
•Manyotherproprietarytypeshavebeendeveloped
•Tendtobeusedinnichemarketsasreplacementto
shell-and-tube
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17. Principal Applications
•As for gasketed plate and frame heat exchanger, but
extended to include more aggressive media.
•Welded plate heat exchangers are used for the
evaporation and condensation of refrigerants such as
ammonia and hydrochlorofluorocarbons (HCFCs), and
for different chemicals.
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18. Comparison with Shell and Tube Heat Exchanger
•As for gasketed plate and frame units.
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19. Plate Fin Exchangers
•Formedbyvacuumbrazing
aluminiumplatesseparated
bysheetsoffinning
•Notedforsmallsizeand
weight.Typically,500m
2
/m
3
ofvolumebutcanbe1800
m
2
/m
3
•Mainuseincryogenic
applications(airliquifaction)
•Alsoinstainlesssteel
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20. Scope of plate-fin exchanger
•Max. Pressure 90 bar (size dependent)
•Temperatures -200 to 150
o
C in Al
Up to 600 with stainless
•Fluids Limited by material
•Duties Single and two phase
•Flow configurationCross flow, Counter flow
•Multistream Up to 12 streams (7 normal)
•Low ΔT Down to 0.1
o
C
•Maximum ΔT 50
o
C typical
•High ε Up to 0.98
use only with clean fluids
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21. Principal Applications
The plate-fin heat exchanger is suitable for use over a wide
range of temperatures and pressures for
•gas-gas,
•gas-liquid and
•multi-phase duties.
Typically, these involve
•Chemical and petrochemical plant:
•Hydrocarbon off-shore applications:
•Miscellaneous applications:
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22. Comparison with Shell and Tube Heat Exchanger
•Aplate-finheatexchangerwith6fins/cmprovides
approximately1,300m2ofsurfaceperm3ofvolume.
Thisheatexchangerwouldbeapproximately10%ofthe
volumeofanequivalentshellandtubeheatexchanger
with19mmtubes.
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23. Spiral heat exchangers
•Theclassicdesignofaspiralheatexchangerissimple
•thebasicspiralelementisconstructedoftwometal
stripsrolledaroundacentralcoreformingtwo
concentricspiralchannels.
•Normallythesechannelsarealternatelywelded,
ensuringthatthehotandcoldfluidscannotintermix
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24. Operating Limits
Maximumdesigntemperatureis400
o
Csetbythelimits
ofthegasketmaterial.
Specialdesignswithoutgasketscanoperatewith
temperaturesupto850
o
C.
Maximumdesignpressureisusually15bar,with
pressuresupto30barattainablewithspecialdesigns.
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25. Applications
•It is ideal for use in the food industry as well as in
brewing and wine making.
•Spiral heat exchangers have many applications in the
chemical industry including TiCl4cooling, PVC slurry
duties, oleum processing and heat recovery from many
industrialeffluents.
•Spiral heat exchangers also provide temperature control
of sewage sludge.
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26. Comparison with Shell and Tube Heat Exchanger
•Spiraldesignshaveanumberofadvantagescomparedto
shellandtubeheatexchangers:
•Optimumflowconditionsonbothsidesoftheexchanger.
•Anevenvelocitydistribution,withnodead-spots.
•Aneventemperaturedistribution,withnohotorcold-
spots.
•Morethermallyefficientwithhigherheattransfer
coefficients.
•Smallholduptimesandvolumes.
•Removalofonecoverexposesthetotalsurfaceareaofone
channelprovidingeasyinspectioncleaningand
maintenance.
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27. PLATE AND SHELL HEAT EXCHANGERS
•Theplateandshellheatexchangercombinesthemerits
ofshellandtubewithplateheatexchangers
•Currentplateandshellheatexchangermodels
accommodateupto600platesinashell2.5mlongwith
a1mdiameter
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28. Operating Limits
•The maximum operating temperature of a plate
and shell heat exchanger is 900
o
C
•maximum working pressure is 100 bar
•handle flow rates of 11 litres per second on the
shell side.
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29. Principal Applications
•The principal applications for plate and shell
heat exchangers are:
•· Heating including district heating.
•· Cooling including cryogenic applications.
•· Heat recovery.
•· Combined exchanger/reactors vessels.
•· Condensation/evaporation
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30. Comparison with Shell and Tube Heat Exchanger
•For heat exchangers of equivalent area and capacity,
plate and shell designs are smaller due to the higher
ratio of heat transfer area and specific volume. It is
claimed that the plate and shell heat exchanger will
occupy only 20 to 30% of the footprint of equivalent
capacity shell and tube types.
•The maximum operating pressure of the plate and shell
unit will also be higher.
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31. Stream Location
(Rules of thumb)
•more corrosive fluid goes tube-side
–saves costs when using alloys, cheaper to construct
tubes from alloys rather than the shell and tubesheet
•higher pressure stream goes tube-side
–small diameter tubes handle stress better than large
diameter shells.
•more severely fouling fluid goes tube-side
–easier to clean tube-side using high pressure water
lance, brushing, chemical cleaning, etc.
•fluid with lower film coefficient goes shell-side
–allows use of finned tubing to increase A
oh
o
•fluid with low ΔP
maxgoes shell side
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