2,600
2,400 2,200
o
2,000
ot f
1
E o
'|
E
o
1,400
6 (ú
1,200
o Ú) C)
1234
l-B60 i
5 678
13
1,000
f
tr Enthalpy basis.2soC Basis of air humidity 650/o RH at 25oC
456789 Flue gas enthalpy (1,000 Kcal/kg fuel)
1-D¡rect fired heater thermal balance.
Fig.
Fig. 2-Flue gas enthalpy chart.
heck fired heater Performance charts and tables allow checks on fired heater performance
2. A decrease in the percent absolute humidity^resúlts in an increase in the outlet flue gas temperature for fixed heat absorption in the heater' Estimating thermal balance. The following information is
K, Arora,
KTI Ltd.,
New Delhi, India
THE METHoD PRESENTED gives
a quick account of tüe
required:
> LHV of the fuel fired (refer to Table 2/Fíg'
balance of direct fired heaters. A plant engineer or n engineer can simply check the performance of-any diired Éeater. The opérating data required is very basic^to a reasonably accuiate estimate of óperating heater effi-
>
;rThe charts presented as Figs. 2 and 3 are helpful in estabshing the following parameters.
)
balance over the heater, i.e., thermal effiflue gas outlet temperature o Énthalpy oÍJl.te gases ai a fúnction of temperature at t levéls of excels air, considering üe effect of air pre-
)
Aiiabatic flame temperatures at different levels of excess r considering the effect of air preheating. description. The charts are derived from a computer m called "COMBN" developed on an HP-85 system. firogram Flué eas enthalpies (in terms of Kcal/kg of fuel) were obtained' for. variol s gi
conditions,
ou
sAiqu.id fuels. at ai,{-e_r¡i1
i*::, percént excessáir and air preheating temperse
res. Based on the analysis of the results these graphs were
ized resulting in the following deviations: g:as temperatures for the known.thermal iÉffñiency of heateri and i 100oC for the adiabatic flame
t
t25"C for flue atures.
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 temperature
65% lelátíve humidity. However, the following points
¡uld be noted when using these graphs at different ambi: air conditions. These changes are very minor. 1. i,. An increase in the percent absolute humidity of air relsults in a decrease in the adiabatic flame temperature.
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 sample
composition (measured at bridge wall) for a particular fuel being fired. Thermal"efficiency of the heater/or flue gas outlet temperature.
Overall thermal balance is given by (refer to Fig' 1):
lr Thermal
o
+ for gaseous/
liquid fuels)
Hp+H¿:HP+H¡+H¿+Hg Hr-HP+H7+H3
or or
100
=
(HPIHF
+ H¡lHp + HslHp) 100
(1)
Thermal efficiency of the heater is the fraction of the heat absá.bed by the p.ot.*t fluid from the. heat generated by fuel. Percent thermal efficiency
:
(H7/HF) L00
the heat absorbed (Kcal/h) by the process fluid' fI¡ heat generated (Kcai/h) by the fuel. Heat abúrbed by ihe Process fluid cante deterrnined from the known inlét/outlét temperatures (TtlT2) and flow .É/¡ is
is
thl
rates (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 heating
value at 25"C reference
temPerature.
Hydrocarbon Piocessing, May
cont¡nued
1985
85
o
2,000
d
1,800
f
2,140 1,960
1,600
E
o o
t
o
3 4 5 6 7
1,400
o
I6'
300 350 400
I I
E o 1,200 o (ú
o N
200 250
o
ñ Y
CD
o l
-
800
tr
J
600 400
Enthalpy basis 25oC Bas¡s of air hum¡dity 650/o RH at 25oC
200 0
0
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
Fig. 4-Lower heating values for liquid fuels.
a¡r.
wind velocity the greater the surface heat loss. Smaller duty heaters have higher percent surface
TABLE
1 emissivity and w¡nd-Surface velocity factors
Mate¡ial Copper
Factor a 0.9 2.8
MS
MS aluminum painted
ct
TABLE
W¡nd condít¡ons Very still air ln factory shop
Factor b 2.8 3.0
Hí =
2-Heating values for gaseous fuels*
Component
Formula
Mol wt
CH¿
16.04
30.07
Propane
C.Hu CsHt
n-butane n-pentane
CoHro
58.1
CsHt,
72.15
n-hexane
CoHr¿
86.17 28.05 42.08
Ethylene Propylene Butylene Benzene Toluene p-xylene
Acetylene Carbon monoxide Hydrogen Ammonia
C"Ho
44.09
CsHo C¿He CeHu GzHe
92.13
CrHto C"Hz
106.16 26.04
co
78.1',!
28.01
2.02
Hz
NHs
* Refer to Fig. 4 for LHV of liquid
56.'l
17.O3
Tw
HHV LHV 13,256 11,946 12,391 11,342 12,026 11,072 11,829 10,925 11,707 10,833 11,627 10,773 12,014 11,264 11,684 10,935 11,574 10,824 10,096 9,692 10,234 9,778 10,339 9,844 11,922 11,519 2,413 2,413 33,865 28,651 5,368 4,435
fuels.
We can also deduce:
is
o Surface areabf walls o Surface emissivity of walls. .Typicat heater box r-efractory thicknesses are designed for wall temperatures of 60 to 70oC above the average ámbient arr temperature. Heaters operating during the winter have relatively more surface heat losses than in the summer. Wind velocity also contributes to this loss. The greater the
86
Hydrocarbon Processing, May 19g5
70-8
+
b (T¡a,
To a
-
To¡r'zs
b
: 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 than
average ambient temperature.
the
HL : (H;)A
Where ,4 is the total outer surface area of
üe heater walls
contributing to surface heat losses. Then, percent sur
heat loss is given by:
(HL/HF) 100
The following examples show the use of the method cussed.
dis-
Examptes. Case A-A direct fired heater (medium duty) is operat. rng at.the_following ing rouowmg parameters: .blue Flüe gas t( tempeiáture at the -ar-rne stack is 450"C and the fuel is naturafgas.
7o rr¡.ole
CH*
partly by radiation and partlyby.o""".li.". paameters aflecting the surface losses are: o Outer wall temperature o Ambient air temperature o Wind velocitv
- T.\
Component
:
(% thermal efficiency + 7o strface heat loss + 7o heat loss with the outgoing flue gases) Surface heat loss from the outer walls of the heater
(T*o
H; : Kcal/Wm2
combustlon at 250c (Kcal/kg)
Methane Ethane
o
Where:
Heat of
100
than medium/large duty heaters. : Typically, medium dutyheaters (2 to 15 MMKcal/h) with radiant plus convection sections have surface losses of i.S to 3 percent of the heat release. Ffowever, the following empiri- ¡ cal relation2 will give a reasonable estimate of surlace Leat i losses for some specific set of conditions. ,
.-.
ln open place
1.4 3.4
.
losses,
96.+ 2.01
CzHo CsHo
0.6 0.99
N2
Total
100
Avg mol wt Excess air
16.56 10%
Assuming an 4verage heat loss of 2% (on total heat fi estimate the thermal efficiency of the heater. Refer to Table 2 for LHV estimation of gaseous fuels.
LHV of N.G.
= = =
(11,946) (16.04) (0.964) + 11,3 (30.07) (0.0201) + 10,935 (42. (0.006) 194,331.8 Kcat/Kmol 11,735 Kcal/kg
the flue gas enthalpy chart (Fig. 2). Enüalpy of @irto -as
at 450oC and 10% excess air is 2,100 Kcal/kg. Eq. 1 percent thermal efficiency : (H7/HF) I00
=
-
100
(HL/HF) 100
)glcentefficiency: 100 ,.:,r.
,
=
B0%
- 2-
-
(H,/HF)
100
l'Case B-A direct fired heater is operating at the followig.paiameters while using preheatediombustion air from a source:
liquid/liquid 100,000 Kg/h 310/360"C 0.59 Kcal/kg "C
Air preheating may be used for any of the following
2000c
,,Assuming 37o stxface loss of heat fired, estimate the flue temperature at the stack.
$as
',.?rocess
heat absorbed
:
= =
!
lbtal heat release
:-:,
= =
:'
9,600 Kcal/kg
361.5 (9,600) 3,470,400 Kcal/h
,
(2,950,000) (3,470,400)
Flue gas
:
enthalpy
: :
(1
-
fio =
0.85
-
To increase heater efficiency To improve combustion To control heat input to the convection section. We may finally conclude as follows, based upon the charts
presented here:
For fixed thermal efficiency of the heater, increasing the air decreases the outlet flue gas temperature. ¡ For a fixed outlet flue gas temperature, increasing excess air decreases the thermal efficiency of the heater. o For a fixed amount of excess air, increasing the efficiency of the heater lowers the outlet flue gas temperature. ¡ For a fixed heat liberation, increasing the excess air to the burner decreases the heat pick-up in the radiant section due to a lower adiabatic flame temperature, but tends to increase the heat pick-up in the convection section. . For a fixed heat liberation, increasing combustion air preheat lowers the equivalent amount of fuel fiied. This in turn, increases üe heater efficiency. Practically, these charts can be of wide use to the engineers/operators in the performance evaluation of operating
excess
Percent thermal efficiency
_
) ) )
¡
W (t) (Tz - Tt) 100,000 (0.59) (50) 2,959,000 Kcal/h
,LIIV (from Fig. 4 for liquid fuels)
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
heaters.
NOMENCLATURE
:-
,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 reference 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 for the following conditions:
liquid (10" API) t0%
Fuel Excess
aii
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 author K. Arora is a seniorp/'oiess eng¡neer with Kinetics Technology lndia, Ltd., New Delhi, lndia.
V.
(An affiliate of Knet¡cs Technology lnternational, 8.V., the Netherlands). He is specialized in the design and commissioning of var¡ous types ot direct fired heaters, including steam superheat-
erc and refinery/petrochemical heaters.
rea-
sons:
10%
air temp€rature
the ordinate at 1,960"C and move to the 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 temperature at preheated conditions. It is apparent from example C that combustion air preheating increases the adiabatic flame temperature. A higher temperature of preheat, coupled with a lower convection transfer rate at reduced flue gas flowrates, causes a greater proportion of the total heat absorption to take place in the
radiant section.
liquid (10' API) 361.5 Kg/h atr
1,960"c.
2. Refer to Fig. 3. Fix
(2,100/11,735) 100
fluid (in/out)
1. Flue gas temperature I15 : 9,600 Kcal/kg at 70% excess air on Fig. 2 yields an adiabatic flame temperature of
Mr.
Arora is also ¡nvolved in the design of heat exchangers, waste'heat recovery syste/ns, carbon monoxide and hydrogen gas p/anfs and the development of computer software for plant design, unit operaüons and especially fired heatet optimizat¡on. Mr. Arora is presently working on an energy conservation study of refinery lurnaces in lndia. He holds a B. Tech. in chemical engineering from the Indian lnstitute ol Technology, New Delh¡ (1979), is a member of AlChE, an associate member of llChE and' the author of several papers on heat transfer and enerry conservat¡on.
A
-
Outer wall surfacé area (m2)
- Surface emissivity factor - Wind velocity factor - Enthalpy of preheated air (Kcal/h) - Heat generated by fuel (Kcal/h) - Heat loss from surface (Kcal/h) - Surface heat flux (Kcal/h/m2) - Heat absorbed by process fluid (Kcal/h) - 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)
b HA HF H¡ HL Hp Hs
ACKNOWLEDGMENT Mr. H. M' W¿dhwani (President, M/s Kinetics dchnology Indü Limited, New Délhi) in the publication of üis article
I
acknowledge the encouragement extended by
.
I Maxwell, J. 8., Dan
In., Princeton, NJ.,
LITERATURE CITED Ninth Printing, D. Van Nostrand Co',
book on hydrocarboxr, 1950.
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.
2 "Design specification
Hydrocarbon Processing, May
1985
87
