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2,600

2,400

2,200

2,000

1

'|

1,400

1,200

1,000

Fig. 1-D¡rect fired heater thermal balance. Fig. 2-Flue gas enthalpy chart.

charts and tables allowchecks on fired heater performance

K, Arora, KTI Ltd., New Delhi, India

THE METHoD PRESENTED gives a quick account of tüebalance of direct fired heaters. A plant engineer or

ootfEooEo6(úÚ)C)ftr

456789Flue gas enthalpy (1,000 Kcal/kg fuel)

conditions, percént excessáir and air preheating temper-res. Based on the analysis of the results these graphs were

ized resulting in the following deviations:tt25"C for flue g:as temperatures for the known.thermal

iÉffñiency of heateri and i 100oC for the adiabatic flameatures.

These graphs are applicable'for fuels hav-ing an average:IV in the ranse of 9,000 to 12,000 Kcal/kg.These graphs áre based on 25"C ambient air temperature65% lelátíve humidity. However, the following points¡uld be noted when using these graphs at different ambi-: air conditions. These changes are very minor.

i,. 1. An increase in the percent absolute humidity of air re-

lsults in a decrease in the adiabatic flame temperature.

1234 5 678

l-B60 i13

Enthalpy basis.2soCBasis of air humidity 650/o RH at 25oC

heck fired heater Performance

n engineer can simply check the performance of-any di-

ired Éeater. The opérating data required is very basic^toa reasonably accuiate estimate of óperating heater effi-

;rThe charts presented as Figs. 2 and 3 are helpful in estab-

shing the following parameters.

lr Thermal balance over the heater, i.e., thermal effi-flue gas outlet temperature

o Énthalpy oÍJl.te gases ai a fúnction of temperature att levéls of excels air, considering üe effect of air pre-

o Aiiabatic flame temperatures at different levels of excess

r considering the effect of air preheating.

description. The charts are derived from a computerm called "COMBN" developed on an HP-85 system.firogram called "COMBN" developed on an HP-85 system.

Flué eas enthalpies (in terms of Kcal/kg of fuel) were ob-tained' for. variol s gi se ou sAiqu.id fuels. at ai,{-e_r¡i1 i*::,

2. A decrease in the percent absolute humidity^resúlts inan increase in the outlet flue gas temperature for fixed heat

absorption in the heater'

Estimating thermal balance. The following information is

required:> LHV of the fuel fired (refer to Table 2/Fíg' + for gaseous/

liquid fuels)> Percent *uifuce heat loss (described later)) Percent excess air-which can be easily established from

the known air/fuel flow rates or from the flue gas samplecomposition (measured at bridge wall) for a particularfuel being fired.

) Thermal"efficiency of the heater/or flue gas outlet temper-ature.

Overall thermal balance is given by (refer to Fig' 1):

Hp+H¿:HP+H¡+H¿+Hgor

Hr-HP+H7+H3or

100 = (HPIHF + H¡lHp + HslHp) 100 (1)

Thermal efficiency of the heater is the fraction of the heatabsá.bed by the p.ot.*t fluid from the. heat generated byfuel.

Percent thermal efficiency : (H7/HF) L00

.É/¡ is the heat absorbed (Kcal/h) by the process fluid' fI¡is thl heat generated (Kcai/h) by the fuel.

Heat abúrbed by ihe Process fluid cante deterrninedfrom the known inlét/outlét temperatures (TtlT2) and flowrates (W Kg/h) of the process ftuia lflo*ing through the

heateicoil iñ a'single phise without any change in the com-position).

Hp : W (') (Tz - T') (2)

Where, s, is the average specific heat of üe-process fluid-'Heat generated by thJfué\, Hr ,:___Vl (!Hy) where W1

is the fue"l burning ráte @g/h) and LHV is the lower heatingvalue at 25"C reference temPerature.

cont¡nued

Hydrocarbon Piocessing, May 1985 85

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2,140 t1,960 3

456

7

II

200250300350400

Enthalpy basis 25oC

Bas¡s of air hum¡dity 650/o RH at 25oC

ooNo

I6'o

ñY

-

odf

EooEoo(úCD

oltr

2,000

1,800

1,600

1,400

1,200

800

600

400

200

00 1 2 3 4. 5 6 7 I I

Flue gas enthalpy (i,000 Kcal/kg tuel)

Fig. 3-Flue gas enthalpy chart for preheated combustion a¡r.

TABLE 1-Surface emissivity

and w¡nd velocity factors

Fig. 4-Lower heating values for liquid f

wind velocity the greater the surface heat loss.Smaller duty heaters have higher percent surface lo

than medium/large duty heaters.Typically, medium dutyheaters (2 to 15 MMKcal/h)

radiant plus convection sections have surface losses of i.S3

percent of the heat release. Ffowever, the following emcal relation2 will give a reasonable estimate of surlace

Mate¡ial

CopperMS

MS aluminumpainted

ct

Factor a

0.92.8

W¡nd condít¡ons

Very still airln factory shop

ln open place

Factor b

2.8

3.0

.-.

TABLE 2-Heating values for gaseous fuels*

losses for some specific set of conditions.

Hí = o (T*o - T.\ 70-8 + b (T¡a, - To¡r'zs

Where:

: Kcal/Wm2: Wall temperature (oK): Ambient temperature (oK)= Surface emisivity factor (given in Táble 1 ): Wind velocity factor (given in Table 1).

Normally, wall temperature is 60 to 70oC higher thanaverage ambient temperature.

HL : (H;)A

Where ,4 is the total outer surface area of üe heater wcontributing to surface heat losses. Then, percent surheat loss is given by:

(HL/HF) 100

The following examples show the use of the methodcussed.

Examptes.

Case A-A direct fired heater (medium duty) is opering at.the_following parameters: Flüe gas tempeiáture atng

-ar-rnerouowmg parameters: .blue gas t(

stack is 450"C and the fuel is naturafgas.

Component

CH*

CzHoCsHoN2

Total

Avg mol wtExcess air

Assuming an 4verage heat loss of 2% (on total heat fiestimate the thermal efficiency of the heater.

Refer to Table 2 for LHV estimation of gaseous fuels.

1.43.4

Component

MethaneEthanePropanen-butanen-pentane

n-hexaneEthylenePropyleneButyleneBenzeneToluenep-xylene

AcetyleneCarbon monoxideHydrogenAmmonia

Heat ofcombustlon

at 250c(Kcal/kg)

HHV LHV

13,256 11,94612,391 11,34212,026 11,07211,829 10,92511,707 10,833

11,627 10,77312,014 11,26411,684 10,93511,574 10,82410,096 9,69210,234 9,77810,339 9,84411,922 11,5192,413 2,413

33,865 28,6515,368 4,435

H;TwTo

a

b

Formula

CH¿

C.HuCsHtCoHro

CsHt,

CoHr¿C"Ho

CsHo

C¿He

CeHu

GzHe

CrHtoC"HzcoHz

NHs

Mol wt

16.0430.0744.0958.1

72.15

86.1728.0542.0856.'l78.1',!

92.13106.16

26.0428.01

2.0217.O3

* Refer to Fig. 4 for LHV of liquid fuels.

We can also deduce:

100 : (% thermal efficiency + 7o strface heat loss

+ 7o heat loss with the outgoing flue gases)Surface heat loss from the outer walls of the heater is

partly by radiation and partlyby.o""".li.". paameters af-lecting the surface losses are:

o Outer wall temperatureo Ambient air temperatureo Wind velocitvo Surface areabf wallso Surface emissivity of walls.

.Typicat heater box r-efractory thicknesses are designed forwall temperatures of 60 to 70oC above the average ámbientarr temperature.

Heaters operating during the winter have relatively moresurface heat losses than in the summer.

Wind velocity also contributes to this loss. The greater the

86 Hydrocarbon Processing, May 19g5

LHV of N.G. = (11,946) (16.04)(30.07) (0.0201)(0.006)

7o rr¡.ole

96.+

2.010.60.99

100

16.5610%

(0.964) + 11,3+ 10,935 (42.

= 194,331.8 Kcat/Kmol

=11,735 Kcal/kg

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@irto the flue gas enthalpy chart (Fig. 2). Enüalpy of-as at 450oC and 10% excess air is 2,100 Kcal/kg.Eq. 1 percent thermal efficiency : (H7/HF) I00

= 100 - (HL/HF) 100

1. Flue gas temperature I15 : 9,600 Kcal/kg at 70% excess

air on Fig. 2 yields an adiabatic flame temperature of1,960"c.

2. Refer to Fig. 3. Fix the ordinate at 1,960"C and move tothe right until line no. 6 (for 250'C) is intersected at A',then move up to intersect the reference line 1 at point B' .

This temperature of 2,740"C is the adiabatic flame tem-perature at preheated conditions.

It is apparent from example C that combustion air pre-heating increases the adiabatic flame temperature. A highertemperature of preheat, coupled with a lower convectiontransfer rate at reduced flue gas flowrates, causes a greaterproportion of the total heat absorption to take place in the

radiant section.

Air preheating may be used for any of the following rea-

sons:

) To increase heater efficiency) To improve combustion) To control heat input to the convection section.

We may finally conclude as follows, based upon the chartspresented here:

¡ For fixed thermal efficiency of the heater, increasing theexcess air decreases the outlet flue gas temperature.

¡ For a fixed outlet flue gas temperature, increasing excess

air decreases the thermalefficiency of the heater.

o For a fixed amount of excess air, increasing the effi-ciency of the heater lowers the outlet flue gas temperature.

¡ For a fixed heat liberation, increasing the excess air tothe burner decreases the heat pick-up in the radiant section

due to a lower adiabatic flame temperature, but tends to in-crease the heat pick-up in the convection section.

. For a fixed heat liberation, increasing combustion airpreheat lowers the equivalent amount of fuel fiied. This inturn, increases üe heater efficiency.

Practically, these charts can be of wide use to the engi-neers/operators in the performance evaluation of operatingheaters.

NOMENCLATURE

A - Outer wall surfacé area (m2)

- (H,/HF) 100

(2,100/11,735) 100glcentefficiency: 100 - 2 -,.:,r. , = B0%

l'Case B-A direct fired heater is operating at the follow-ig.paiameters while using preheatediombustion air from a

,,Assuming 37o stxface loss of heat fired, estimate the flue

$as temperature at the stack.

',.?rocessheat absorbed : W (t) (Tz - Tt)

= 100,000 (0.59) (50)

= 2,959,000 Kcal/h

,LIIV (from Fig. 4 for liquid fuels) :' 9,600 Kcal/kg

lbtal heat release = 361.5 (9,600):-:, = 3,470,400 Kcal/h

Percent thermal efficiency

_ (2,950,000) fio = 85%, (3,470,400)

Flue gas enthalpy : (1 - 0.85 - 0.03) 9,600: : 1,152 Kcal/kg fuel

:From flue gas enthalpy curve (Fig. 2) flue gas temperature',Hs = 1,152 at l0% excess air = 240oC:-

,Now, refer to Fig. 3 and fix flue gas temperature at240"C

on the ordinate and move to the right until line no. 5 (for.200"C) is intersected at point A, then move up until refer-ence line I is intersected at point B. The temperature of

:.390oC at point B is the flue fas temperature at the stack.

, Case C-Estimate the adiabatic flame temperatures forthe following conditions:

- Surface emissivity factor

b - Wind velocity factor

HA - Enthalpy of preheated air (Kcal/h)

HF - Heat generated by fuel (Kcal/h)

H¡ - Heat loss from surface (Kcal/h)

HL - Surface heat flux (Kcal/h/m2)

Hp - Heat absorbed by process fluid (Kcal/h)

Hs - Enthalpy of flue gases (Kcal/h)

HHV - Higher heating value of fuel (Kcal/kg)

LHV - Lower heating value of fuel (Kcal/kg)

r - Average specific heat of process fluid (Kcal/kg/'C)To - Ambient air temperature (oK)

Tq - Outer wall temperature (oK)

Tt - Inlet piocess fluid temperature (oC)T2 - Outlet process fluid temperature (oC)

W - Flow of process fluid through coils (Kg/h)

Wt - Flowrate of fuel (Kg/h)

ACKNOWLEDGMENT

I acknowledge the encouragement extended by Mr. H. M' W¿dhwani (President,M/s Kinetics dchnology Indü Limited, New Délhi) in the publication of üis article .

LITERATURE CITEDI Maxwell, J. 8., Dan book on hydrocarboxr, Ninth Printing, D. Van Nostrand Co',

In., Princeton, NJ., 1950.2 "Design specification üd operation of furnaces," (Report-RP-25), Ingenieures

Bureau 000 & A, ( the Netherlmds).3 Perr¡ H., Chilton, H., Kirkparick, S. D., Chzmical Engireet\ Handbook, 5th ed.,

McGraw-Hill Book Co., New York, N.Y.4 Hougen, Watson and Ragatz, Chemicdl hocxs Principb, (Part I), John Wiley &

Sons, Inc.

source:

fluid (in/out)

air temp€rature

liquid/liquid100,000 Kg/h310/360"C0.59 Kcal/kg "Cliquid (10' API)361.5 Kg/h10%

2000catr

Fuel

Excess aiiliquid (10" API)t0%

1. Combustion air at ambient conditions (i.e., 25"C)2. Combustion air at preheated temperature of 250oC

Refer to the flue gas enthalpy curve (Fig. 2).

The authorV. K. Arora is a seniorp/'oiess eng¡neer with Ki-

netics Technology lndia, Ltd., New Delhi, lndia.(An affiliate of Knet¡cs Technology lnternational,

8.V., the Netherlands). He is specialized in the

design and commissioning of var¡ous types otdirect fired heaters, including steam superheat-erc and refinery/petrochemical heaters. Mr.

Arora is also ¡nvolved in the design of heat ex-

changers, waste'heat recovery syste/ns, car-bon monoxide and hydrogen gas p/anfs and the development of com-puter software for plant design, unit operaüons and especially firedheatet optimizat¡on. Mr. Arora is presently working on an energy con-servation study of refinery lurnaces in lndia. He holds a B. Tech. inchemical engineering from the Indian lnstitute ol Technology, New De-lh¡ (1979), is a member of AlChE, an associate member of llChE and'the author of several papers on heat transfer and enerry conservat¡on.

Hydrocarbon Processing, May 1985 87