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16
Introduction
Theaboveconstantvaluesforcalculatingtheincreaseofstandardther-
modynamicpotentialareparticularlyusefulintheanalysisofthekineticsof
reactionsoccurringincombustion.Theirusefulnessisevidentinsimplecalcu-
lationsofprocessesoccurring,forinstance,inexhaustgasesbehindthecom-
bustionchamber,i.e.inplaceswhereaone-dimensionalstreamcanbeas-
sumed.
Thestreamofheattransferredfromthefurnacewalltotheenvironment
Q
-
out
willbethesumoftheamountofenergypassedtotheenvironment
throughradiation
Q
-
rad
andoftheconvectiveheatexchangecon
Q
-
between
thewallsofthefurnaceofsurfaceAandtheenvironment.Theequationcanbe
asfollows:
Q
-
out
:
Q
-
con
+
Q
-
rad
(7)
where
Q
-
rad
=
σ
⋅
ε
⋅
(
T
4
s
–
T
sur
4
)
⋅
A
and
σ
=
5
.
67
⋅
10
–
8
[
W
/
m
2
K
]
presentsStefan-Boltzmannconstant.
Finally
Q
-
con
=
C
⋅
A
⋅
(
T
s
–
T
sur
)
1
9
25
(8)
Inaboveequations
ε
presentsstandsforanaveragecoefficientofemissiv-
ityofwalls.
Inequation(8)theconvectivepartoftheheatexchangeisexpressedwith
theformulaoftenusedforindustrialfurnaces.CoefficientCdependsonthe
averageNusseltnumberandtheexponentforthedifferenceoftemperatures
isanapproximationbasedontheexperienceofdesignersofindustrialfur-
naces.While,atpresentitispossibletocalculatewithprecisionthestreamof
heatusingafullanalysisofheatexchangeatspecificsectionsofthefurnace
walls(especiallywithnumericalprogramsCFX),inpractice,thisisnotneces-
saryforfurnacesusedinindustry.Inprinciple,thefurnaceshouldbeinsulated
forthetemperatureofitswallnottoexceed50
oC.Thenthestreamofheat
passedtotheenvironmentisnotbiggerthanonetotwopercentofthetotal
energyemittedinthefurnace.
Theefficiency
η
ofcombustioninanindustrialfurnaceisdefinedasthe
ratiooftheeffect(orenergytransferredtotheheatedmaterial)totheinputor
theenergycontainedinthegasfedtothefurnace:
η
=(effectasenergytransformedintoheatedmaterial)x(energysuppliedinto
furnace)
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