This is first part of shell and tube heat exchanger series. This series will include many parts emphasizing all the necessary parameters of STHE including design, inspection and operation. This is part-1 which will give reader an insight of "When to select STHE" and factors to consider while selecting any kind of heat exchanger.

1. Shell and Tube Heat Exchanger Series.
By PrathameshMDeshpande
We are all aware of wide use of Heat Exchangerinprocessindustry.Hence Iwill notgoto verybasics
of the same assumingthatreaderknowswhata heat exchangerdoes.Iwill trytocoveralmostall
stagesof this staticequipmentnamely,
1. Selection.
2. Heat transferanalysismethods.
3. Thermal Design.
4. DesignStandards.
5. Guidelinestoconsiderfordesignanddetaileddesignprocedure.
6. Fouling.
7. Vibrationsin Shell andTube HeatExchanger(STHE).
8. Mechanical Designof STHE.
9. Material Selectionandsome partof fabrication.
10. QA/QC,InspectionandNDT.
11. Operationandmaintenance.

2. Series Part -1: Heat Exchanger Selection.
Here are some distinctiveselectioncriteriabasedonwhichone should selecttype of heat
exchanger.
1. Materialsof Construction
For reliable and continuous use, the construction materials for pressure vessels and heat exchangers
should have a well-defined corrosion rate in the service environments. Furthermore, the material
should exhibit strength to withstand the operating temperature and pressure. STHEs can be
manufactured in virtually any material that may be required for corrosion resistance, for example,
from non-metals like glass, Teflon, and graphite to exotic metals like titanium, zirconium, tantalum,
etc. Compact heat exchangers with extended surfaces are mostly manufactured from any metal that
has drawability,formability,andmalleability.
2. OperatingPressure and Temperature
The design pressureisimportantto determine the thicknessof the pressure-retainingcomponents.
The higherthe pressure,the greaterwill be the requiredthicknessof the pressure-retainingmem-
branesand the more advantage there isto placingthe high-pressure fluidonthe tubeside.
At lowpressures,the vapor-phase volumetricflow rate ishighandthe low-allowable pressure drops
may require adesignthatmaximizesthe areaavailable forflow,suchascrossflow orsplitflow with
multiple nozzles.
At highpressures,the vapor-phase volumetricflow ratesare lowerandallowable pressure dropsare
greater.These leadtomore compact units.
In general,higherheattransferratesare obtainedbyplacingthe low-pressuregasonthe outside of
tubularsurfaces.
Design temperature:Thisparameterisimportantasitindicateswhetheramaterial at the design
temperature canwithstandthe operatingpressure andvariousloadsimposedonthe component.
Temperature programinbotha single-passandmultipassSTHEdecides the meanmetal
temperaturesof variouscomponentslikeshell,tube bundle,andtubesheet,and possibilityof
temperature cross.
The effective temperature drivingforce isameasure of the actual potential forheattransferthat
existsatthe designconditions.Withacounterflow arrangement,the effective temperature
difference isdefinedbythe logmeantemperaturedifference (LMTD).Forflow arrangementsother
than counterflow arrangement,LMTDmustbe correctedbya correctionfactor, F.
3. FlowRates

3. Flow rate determines the flow area: the higher the flow rate, the higher will be the crossflow area.
Higher flow area is required to limit the flow velocity through the conduits and flow passages, and
the higher velocity is limited by pressure drop, impingement, erosion, and, in the case of shell and
tube exchanger, by shellside flow-induced vibration. Sometimes, a minimum flow velocity is
necessarytoimprove heattransfertoeliminatestagnantareasandto minimize fouling.
4. Performance Parameters—Thermal EffectivenessAndPressure Drops
Thermal effectiveness: For high-performance service requiring high thermal effectiveness, use
brazed plate-fin exchangers (e.g., cryogenic service) and regenerators (e.g., gas turbine applications),
use tube-fin exchangers for slightly less thermal effectiveness in applications, and use shell and tube
unitsforlow-thermal effectivenessservice.
Pressure drop: Pressure drop is an important parameter in heat exchanger design. Limitations may
be imposed either by pumping cost or by process limitations or both. The heat exchanger should be
designedinsuchaway that unproductive pressure dropisavoidedtothe maximumextent.
5. Fouling
Foulingisdefinedasthe formationonheatexchangersurfacesof undesirabledepositsthatimpede
the heat transferandincrease the resistance tofluidflow,resultinginhigherpressuredrop.The
growthof these depositscausesthe thermohydraulicperformance of heatexchangertodeclinewith
time.Foulingaffectsthe energyconsumptionof industrial processes,anditalsodecidesthe amount
of extramaterial requiredtoprovide extraheattransfersurface tocompensate forthe effectsof
fouling.
In a shell andtube unit,the fluidwithmore foulingtendenciesshouldbe putonthe tubeside for
ease of cleaning.Onthe shellside withcrossbaffles,itissometimesdifficulttoachieve agoodflow
distributionif the baffle cutiseithertoohighortoo low.Stagnationinanyregionsof low velocity
behindthe bafflesisdifficulttoavoidif the bafflesare cutmore thanabout 20%–25%.
6. Overall Economy
There are twomajor coststo considerindesigningaheatexchanger:the manufacturingcostand the
operatingcosts,includingmaintenance costs.Ingeneral,the lessthe heattransfersurface areaand
lessthe complexityof the design,the loweristhe manufacturingcost.The operatingcostisthe
pumpingcostdue to pumpingdevicessuchasfans,blowers,andpumps.The maintenance costs
include costsof sparesthat require frequentrenewal due tocorrosion,andcostsdue to
corrosion/foulingpreventionandcontrol.Therefore,the heatexchangerdesignrequiresaproper
balance betweenthermal sizing andpressure drop.
7. Intendedapplications
Let’sfigure outwhenwe selectShell andTube HeatExchanger(STHE).
Application Remarks
Low-viscosityfluids For hightemperature/pressures,use STHE

4. Low-viscosityliquidto
steam
Use STHE in carbon steel.
Foulingliquids Use STHE withremovable tube bundle.
Gas or airunder
pressure
Use STHE withextendedsurface onthe gasside
Vaporcondensation Surface condensersof STHE incarbon steel are preferred.
Air–airor gas–gas
applications
Regeneratorsandplate-finheatexchangers.AlsoconsiderSTHE.
In processindustries,shell andtube heatexchangersare usedingreatnumbers,farmore thanany
othertype of exchanger.More than90% of heatexchangersusedinindustryare of the shell and
tube type. Theyare the firstchoice because of well-establishedproceduresfordesignand
manufacture froma wide varietyof materials,manyyearsof satisfactoryservice,andavailabilityof
codesand standardsfordesignandfabrication.Theyare producedinthe widestvarietyof sizesand
styles.There isvirtuallynolimitonthe operatingtemperature andpressure.
Constructional Featuresof STHE:
The most commonlyusedheatexchanger.Itisthe “workhorse”of industrialprocessheattransfer.
Theyare usedas oil cooler,surface condenser,feedwaterheater, etc.
The major componentsof a shell andtube exchangerare tubes,baffles,shell,fronthead,rearhead,
and nozzles.
Performance Featuresof STHE:
Advantages:Extremelyflexibleandrobustdesign,easytomaintainandrepair.
Disadvantages:
1. Require large site (footprint) areaforinstallationandoftenneedextraspace toremove the
bundle.
2. Constructionisheavy.
So thiswas regardingthe selectionof heatexchangers.Ibelieve readercanget complete insightof
selectionof STHEwhichismost importantinplanningaSTHE for intended process.