Siemens LMV52.240B1 - Page 175

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page3 Section6
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Figure6‐1:PhysicalArrangementofBurner,O
2
Sensor,andGasPath
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SincetheO
2
sensorislocatedintheboiler'sexhaust(stack),therealwaysexistsadelaytime(Tautime)
betweenwhenairflowisadjustedattheburnerandwhenthechangeisdetectedbytheO
2
sensor.This
physicalrealityforcestheO
2
trimtouse"old"O
2
readings,whichwillbediscussedlater.Thedelaytime
dependsonthelengthofthegaspaththroughtheboilerandthevelocityofthegas.Thelengthofthe
gaspathisfixedforagivenboiler,butthegasvelocitydependsuponthefiringrate.Higherfiringrates
equatetohighergasvelocitiesandsmallerdelaytimes(Tautimes).
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Ifthefuelflowincreaseslinearlywiththefiringrate(theloadnumberaccuratelyreflectsthefuelflow),
theTautimewilldecreaseinapredictablemannerasthefiringrateincreases.Likewise,givenan
accurateloadnumber,theTautimewillincreaseinapredictablemannerasthefiringratedecreases.
ThisenablestheTautimetobeautomaticallycalculatedateverypointfromPoint2tohi
ghfire.Once
theTautimeiscalculatedforeverypointfromPoint2tohighfire,thePI(proportional+integral)
responsefortheO
2
trimcanbeautomaticallycalcu latedforeverypointbetweenPoint2andhighfire.
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AnotherplacewheretheTautimemustbeaccountedforisatstartup.Immediatelybeforemainflame
ignition,thegaspathoftheboilerisfullofair.TogetarepresentativeO
2
readingthatcanbeusedfor
trimming,thegaspathoftheboilermustbefullofproductsofcombustion(exhaustgases).Thus,the
mainflameoftheburnermustbeignitedandtheburnermustberunningforaperiodoftimetoflush
outalloftheair.Thetimeittakestoflushoutthisair isestimatedbytakingtheTautime(atPoint2)
timesamultipli
er.Forexample,iftheTautimeatPoint2ismeasuredtobe6seconds,thenthetimeit
takestogetarepresentativeO
2
readingafterlightoffmightbe6x6=36seconds.Inthisexample,
closedloopO
2
trimcannotoccuruntil36secondsafterlightoff.Thisispreciselywherethetemperature
compensationfunctionalityisusedforsomeburners(LMV52.440only).
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