Draft & O2

Engr. M. Usman Saeed
E-III Visbreaking& Gas Concentration Unit
Email:ing.usmansaeed@gmail.com
Pak ArabRefineryLtd| QasbaGujrat MehmoodKot DistrictMuzaffarGarh
DRAFT AND O2 CONTROL IN FIRED HEATER

PAK ARAB REFINERY LTD Department: Process
Visbreaking& Gas ConcentrationUnit Area: 200 (U – 130 & 411)
Engr. Engr. M. Usman Saeed – 5509
CONTENTS
Draft & Excess O2 Control in Furnaces............................................................................................ 1
1 Introduction....................................................................................................................... 1
2 ExcessAir........................................................................................................................... 1
3 Flue Gas Quantity............................................................................................................... 1
4 Stack Effect........................................................................................................................ 2
5 Combustion Air Preheater...................................................................................................2
6 Air Leakage Through Openings............................................................................................ 3
7 Fuel Saving......................................................................................................................... 4
8 CO2 Emissions..................................................................................................................... 4
9 Typical Values of Draft and Excess Air .................................................................................. 5
10 Draft Adjustments...........................................................................................................5
10.1 Excessive Draft— Positive Pressure Created.................................................................5
10.2 Excessive Draft— Negative Pressure Created............................................................... 5
11 Recommendations..........................................................................................................7
11.1 Design Stage............................................................................................................... 7
11.2 Maintenance .............................................................................................................. 7
11.3 ExcessAir Control .......................................................................................................8

Engr. M. Usman Saeed – 5509 1
DRAFT & EXCESS O2 CONTROL IN FURNACES
1 INTRODUCTION
Processheatersare the largestconsumersof energyinmostplants.A refinery,onaverage,burns
approximately2billionBtu/hrof fuel infiredheaters.The total quantityof fuel burned(heatreleased) isso
highthat any improvementwill resultinsignificantfuelsavings. Highenergycostsandtighteremissions
regulationsrequire increasedunderstandingandcontrol of excessair.Anyreductioninexcessairwill raise
the efficiencyof aheaterand reduce total emissions.NOX emissionsare of the highestconcernina fired
heater,althoughexcessaircontrol will alsoreduce refineryCO2 emissions andboostheaterefficiency.
It isrecommendedthatthe flue gastemperature approach(definedasflue gastemperature leaving
convection,minusprocessinlettemperature) be between50°Fand 100°F, dependingonheatertube
material andthe cost of fuel.However,heaterefficiencymaydeclinewiththe degradationof heater
components.The degree of degradationisdependentonthe qualityof the maintenance program
implementedatthe refinery.
2 EXCESS AIR
Excessair isdefinedasthe amountof air above the
stoichiometricairrequirementthatisneededtocompletethe
combustionprocess.Excessoxygen(O2) isthe amountof O2 inthe
incomingairnot usedduringcombustion.Inanoperatingplant,the
airflowrate can be adjustedata fixedabsorbed-heatduty(constantfeed
flowrate andinlet/outletconditions) untilanoptimumfuel-to-airratiois
achieved.Itisimportanttonote that there isa limitonminimum
possible excessO2.Below thislevel,combustiblescanenterthe flue gas,
whichposesa safetyhazard.Heaterandburnermanufacturersestablish
thisminimumlimitduringthe designstage.Operatorsshouldalsokeepa
safe marginfor upsetconditions.
HigherexcessO2 helpsachieve astable flame inthe firebox.At
the same time,itreducesthe efficiencyof the heater.Asageneral rule,
3% O2 influe gas isequivalentto15% excessair.
3 FLUE GAS QUANTITY
Flue gasquantityincreaseswitharise inexcessair,whichlowers
heatand increasesthe fuel requirement.Figure providesacorrelation
betweenfluegasgeneratedduringcombustion,andexcessair.The
followingequationcanbe usedto calculate approximateflue gasquantity
for natural gas: QF = 1 + 0.167 3 (100 + EA)
Where:EA = excessair,%; O2 = vol% of flue gasoxygen(dry); QF =
flue gasquantityinlb/lbof fuel.Asageneral rule,flue gasquantityis
approximately20 timesthe fuel quantity at15% excessair.

4 STACK EFFECT
The net draftavailable isthe draftcreatedbythe stack effect,minusfrictional andvelocitylosses.
The net draftshouldbe sufficienttoobtaina negative pressure alongthe heaterfluegaspath.
It isimportantto maintainasafe draft level inafiredheatertoachieve the bestpossible efficiency
and operation.The targetdraftof 0.1 inchWC issetat the heaterarch. A highervalue of draftwill resultin
ingressof “tramp air” intothe heater.Trampair takesheat from the
combustionprocessandexitsthe stack,reducingheaterefficiency.The
flue gassample takenfromthe stack doesnotrepresentthe actual
volume of O2available forcombustion.Itisthe sumof unusedO2 from
the firebox (actual excessO2) andO2 fromtramp air.
A positive draftvalue will resultinthe leakage of hotflue gases
throughopeningsinthe heater.Thisisa hazardousoperationthatcan
overheatthe steel structure,refractoryandheatersupports,and,
consequently,shortenheaterlife.
Figure providesthe value of draftgeneratedinthe heaterfor
flue gastemperature andambientairtemperature.Itshouldbe noted
that stack effectdecreaseswithanincrease insite altitude.The
calculateddraftshouldbe amendedusingthe correction factorforsite
altitude.The followingequationcanbe usedtocalculate the draft generatedinaheater:
Where:H = Height,ft;PATM = Atmosphericpressure,psia; TAMB = Ambientairtemperature,°R; TFG =
Flue gastemperature,°R.Asa general rule,forevery10 ft of firebox height,the draftincreasesby0.1
inchWC:Draft at burner(inchWC) ≈ 0.1 + HFB ÷ 100 where:HFB = Firebox height,ft.
5 COMBUSTION AIR PREHEATER
The typical combustionairpreheater(APH) will increase the
heaterefficiencyby approximately10%.Fuel gasgenerallycontains
H2S or sulfur,whichconvertintoSO2 andthenintoSO3.The APH’s
heat-transfersurface issubjecttocold-endcorrosioncausedby
condensationof sulfurtrioxide(SO3),whichresultsinAPHleakage.Air
preheaterleakage isone of the mostcommonAPHoperating
problems,andanysuch leakage resultsinareductioninthe overall
efficiencyof the heater.
In APHoperation,the flue gasisgenerallyatnegativepressure,
and the air isat positive pressure.Therefore,leakageoccursfromair to
the flue gasside.Thisreducesthe quantityof airavailable for
combustion,anditincreasesthe quantityof flue gasleavingthe APH.
Thisleakage canbe detectedbymeasuringthe fluegasO2 contentat the APHinletandoutlet.Any
leakage will resultinhigherfluegasO2 at the APHexit,comparedtothe APH inlet.Generally,the APHisnot

equippedwithaflue gasO2 analyzeratthe inletandthe outlet;however,the inclusionof 2-in.connections
at the APH’sinletandoutletwillenable operatorstomeasure O2 levelsusingaportable analyzer.
(M 3 CP ∆T)FLUE GAS = (M 3 CP ∆T)AIR
For a typical fuel gasat 15% excessair: MFLUE GAS ≈ 1.05 3 MAIR ; PFLUE GAS ≈ 1.15 3 CPAIR (7)
∆TAIR ≈ 1.2 3 ∆TFLUE GAS
Where:M = Flowrate CP = Specificheat∆T = Temperature difference acrossthe APH.
Anyleakage inthe APHwill reduce the ratioof ∆TAIR to ∆TFLUE GAS. For example,fora10% leakage in
the APH,the ratioof temperature differencewillbe around1.1.
Figure indicatesthe percentage of airleakage basedonthe ratioof ∆TAIR to ∆TFLUE GAS fora typical
natural gas firing.
6 AIR LEAKAGE THROUGH OPENINGS
A firedheaterisnota 100% sealedunit;
there are alwaysopeningsthroughwhichairingress
(trampair) can move. The volume of trampair
dependsonthe openingsize andthe draftat the
locationof the opening.Afterthe draftatthe
openinglocationisestimated,the followingequation
can be usedtoestimate the airleakage throughan
opening:
∆P = C 3 0.003 3 ρ 3 V2
Thisequationcanbe simplifiedforthe
leakage calculationpurposebasedonthe following
data:
Molecularweight(MW) of air = 28.96;
Atmosphericpressure(psia)=14.7; Velocityhead(C)
= 1
where:ΔP = Draft at openinglocation,inchWC;ρ =Density of air at ambienttemperature,lb/ft3
;V =
Velocityof airthroughopening,ft/s; C = Velocityhead;QL = Airleakage,lbperft2
/s;T = Ambientair
temperature,°R.
Figure providesthe quantityof airleakage perft2
of openingsize.Thisfigureisbasedonanambient
air temperature of 60°F.Once the openingsize isknown,the amountof airleakage canbe estimated.The
estimatedaircanbe translatedintothe additionalfiringrate required.

7 FUEL SAVING
The absorbedheatdutyof the firedheateris
constant.Anyincrease inthe O2level will reduce the
efficiency,resultinginahigherfiringrate.Thisincrease in
the firingrate will leadtoa rise instack temperature,
whichresultsinanotherreductioninefficiency.This
reduction,inturn,demandsafurtherincrease inthe firing
rate.
The methodof efficiencycalculationforoff-design
operatingconditionspresentedinAPI-560Appendix Gcan
be usedto estimate the stacktemperature whenexcessair
ispresent.Thismethodcanbe simplifiedforexcessairas
follows:
Where:TS = Flue gasstack temperature,°R; EA =
Excessair,%; TF = Feedinlettemperature,°R(TF1 = TF2)
Φ = Excessair correctionfactor
Once the newflue gasstack temperature atexcessairisknown,thenthe heaterefficiencycanbe
estimated. Figure showsthe estimatedfuel savingsforareductioninthe O2 level to3%. Thisgraph isbased
on a fuel price of $6/MMBtu. The designflue gastemperaturelinesindicate the baseline stacktemperature
(i.e.,the flue gasstack temperature at3% O2).
8 CO2 EMISSIONS
The volume of CO2 emissionsgeneratedinafired
heaterisdirectlyproportional tothe firingrate.In
combustionprocesses,fuel carbonconvertsintoCO2.
Therefore,excessairreductionwill lowerCO2emissions.
Figure providesestimateddecreasesinCO2 emissions
througha reductioninthe O2 level to3%.

9 TYPICAL VALUES OF DRAFT AND EXCESS AIR
Draft referstothe flowof gasesthrough the heatgenerating
equipment,beginningwiththe introductionof airat the back of the burner.
Once combustionoccurs,the heatedgasleavesthe combustionchamber,
passesheatexchangersandexitsthe exhauststack.
5 - 10% for natural gas, 5 - 20% for fuel oil, 15- 60% for coal
Carbondioxide - CO2 - is a productof the combustionandthe content
inthe flue gasisan importantindicationof the combustionefficiency.An
optimal contentof carbondioxide - CO2 - aftercombustionis
approximately 10% for natural gas and approximately 13% forlighteroils.
10 DRAFT ADJUSTMENTS
Stack dampersandsecondaryair registersaffectthe draftandboth
adjustmentsare related.The hotgaspushessothat the pressure isalways
greatestat the firewall.The stackdraftpullsandwhencorrectlybalanced
the pressure at the bridgewallshouldbe close tozeroorveryslightly
negative.A processheateroperatingproperlywill alsohave azero,or
slightlynegative draft,atthe shieldsection.The firebox will be slightly
positive (+0.5to +2.0 “watercolumn(wc)) andthe stack will have arange of
-0.5 to -1.0” wc. Excessive draft,eitherpositive pressure ornegative
pressure,canleadto severe problemsinthe convectionsection.
10.1 EXCESSIVE DRAFT — POSITIVE PRESSURE CREATED
In Figure 4, the air registersare wide openandthe dampermostlyclosed.Thisgeneratesapositive
pressure whichforcesflue gasesoutwardthroughleaksinthe convectionsectionleadingtoserious
structure damage,as well asheatloss.
10.2 EXCESSIVE DRAFT — NEGATIVE PRESSURE CREATED
In Figure 5, the air registersare mostlyclosedandthe stackdamperiswide openleadingtoahigh
negative pressure inthe convectionsection.Coldambientairissuckedinthroughleaksinthe convection
sectionleadingtoerroneousoxygenreadings,aswell asheatloss.Inadditionthe excessive draftcausestall
flameswhichcanreach the tubesresultinginseriousdamage.

11 RECOMMENDATIONS
Heaterexcessaircontrol starts at the designstage.Well-designedheatershave low trampair.There
are three stagesof excessaircontrol:
 Designstage
 Maintenance
 Control
11.1 DESIGN STAGE
A heaterhasmany potential leakpointsforairingress:
 Clearance aroundthe bottomcoil guide (spigots)
 Sightdoorsand peepholes
 Headerboxes,manholesandotheropeningsforviewingandaccess
 Modules andduct splice joints
 Terminalsandcrossovertubes
 Weldjointsonthe heatercasing
 Soot-blowersleeves
 The APH.
These leakpointsmustbe designedforthe lowestpossibleleakage.Suggestionsfordesigningalow-
leakage heaterincludethe following:
 Seal the clearance space aroundthe bottom tube guidesbyusingafloorsleeve withanend
cap, or seal boots
 Use sightdoors,withsafetyglass,thatare equippedwithaninterlockcoverorflapper
 Use a self-closingpeepholecoverinthe heaterfloor
 Ensure that headerbox panelsandotheropeningsare airtight,anduse gasketsbetweenthe
gaps
 Seal-weldall splice jointsbetweenmodulesfromthe inside,oruse high-temperature
sealant;also,use closer-boltspacing(6in.fromcenterto center)
 Seal all terminalsandcrossoveropeningswithflexible seals
 Ensure that all headerbox drainpointsare plugged
 Ensure that no leakage isoccurringthroughinstrumentmountings
 Limitleakage throughthe APHduringthe designstage,andperformanair-leakage testin
the shop.
11.2 MAINTENANCE
Routine maintenanceof the heatersisessential,since corrosiveagentscanbe presentinflue gases.
Deteriorationfromsulfuroxidesoccursmostlyoncoldsectionsof the steel casing.Climateconditionscan
alsoleadto rustingon exposedsurfacesof the heatercasing.Suggestedinspectionand
maintenance methodsincludethe following:
 Checkfor heatercasingcorrosion;if anyleaksare discovered,theyshouldbe sealedtostop
air ingress
 Ensure that observationdoors(generally locatedinthe bottomsectionof the radiantbox)
are closedaftertechniciansinspectthe heaterflame

 Checkpeepholes,accessdoors,etc.,forproperclosing
 Checkflue gasO2 contentinthe convectionsectionandonthe APH; if there isany increase
inO2contentacross the flue gaspath,it indicatesleakage
 Use a smoke testduringheatershutdowntodetectleakage
 Use infraredscanning,while the heaterisinoperation,topinpointlocationswithair
leakage;these will have localized,lowerheatercasingtemperatures
 To reduce leakage inburners,keepall burnersinoperation,evenduringloweroperating
loads;and close the airregisterwhenaburneris takenoutof service.
11.3 EXCESS AIR CONTROL
Knowingthe targetflue gasO2 contentisthe firststepin excessaircontrol.Each heaterisunique in
itsdesign.The O2 level requiredtoachieve ideal combustionmaybe anywhere from1%–4% or higher,
dependingonthe designandoperatingcharacteristicsof the heater.
The followingtwoinstrumentsare necessarytocontrol excessair:
 Flue gas O2 analyzer. Thisis the mostimportantinstrumentonthe heater.Itis
recommendedtoinstall anO2 analyzeratthe radiantsectionarch.
 Draft gauge.A draft gauge shouldbe installedatthe heaterarch. The arch isthe pointof the
highestflue gaspressure inthe heater.
HeaterO2 and draftat the radiantarch shouldbe checkedand,if necessary,adjustedatleastonce
pershiftand wheneverthere isachange in processload.All operatorsshouldbe familiarwiththe heater
controls.Often,heaterswithairregistersandstackdampersbecome jammedsimplybecause theyare not
used. Figprovidestacticsforcontrollingexcessairina natural draft heater.Forcontrollingexcessairinother
typesof heaters, Table 1 andTable 2.

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