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CALCULATING FIRED HEATER EFFICIENCY
James T. Harriz and Edward L. Ritter
Waterland, Viar & Associates, Inc.
Newark, Delaware
ABSTRACT
Tracking utility and process heater efficiency leads to
increased operator awareness and decreased fuel bills. A review of
the Heat Loss and Input/Output methods of efficiency analyses is
presented, along with a discussion of their relative accuracy.
Examples cited include refinery mixed fuel gas, coal, and chemical
recovery fueled applications, using HHV and LHV calculations. The
techniques presented can be applied to improve operator awareness,
moni tor equipment performance, and evaluate capital improvement
opportunities.
INTRODUCTION
What is Fired Heater Efficiency?
Efficiency is the ratio of delivered divided by the work bo i
ler / furnace language, th is delivered to process divided by
heat
useful applied. becomes applied.
work In
heat
Why are efficiency calculations important?
In practically any process, high level heat is provided by the
combustion of fuel. Dollars not spent on fuel can be returned to
the shareholders as profits, and maximizing efficiency at this
point has great leverage in overall process economics. Routine
monitoring of combustion efficiency for various fired equipment can
be used to improve daily operation, and locate opportunities for
maintenance or capital improvements.
A theoretical minimum threshold of high level heat is required
to initiate an endoth'ermic process . The practical minimum is
dictated by
655
design economics, heat transfer technology and safety criteria
which define m1n1mum approach temperatures, engineering materials
and equipment design.
The actual minimum depends on final design, production rate, and
insulation condition. Since fuel pricing is not usually
controllable by the individual user, only fuel savings from heater
efficiency improvements will reduce operating costs.
How Is Efficiency Calculated?
Two methods are available for evaluating fired heater
efficiency. These are commonly referred to as the Input/Output and
Heat Loss methods.
The Input/Output method directly determines the net heat
delivered. This is a very general method, evaluating only the
useful heat delivered to process against fuel supplied. Delivered
heat is the sum of process stream energy increases.
INPUT/OUTPUT HEAT LOSS METHOD METHOiD
HEATTOj PROCESS x100 1- . x100EFF= ~ tLOSSES.....]FUEL TOFUEL TO
HEATERHEATER
The Heat Loss method evaluates flue gas thermal losses,
determining efficiency by difference. Independent calculations
address radiant and convective setting losses and residual
combustibles losses to determine overall heater efficiency.
While neither of these two methods is flawless, the Heat Loss
method is inherently less subject to measurement errors, and the
auxiliary corrections for setting and combustibles losses are not
difficult.
ESTABLISHING THE RULES
Combustion engineers recognize two distinct interpretations of
fuel heating value: Higher Heating Value (HHV) and Lower Heating
Value (LHV~~, The difference between them addresses the
availability of the latent heat of vaporization of water in the
flue gasses.
ESL-IE-86-06-106
Proceedings from the Eighth Annual Industrial Energy Technology
Conference, Houston, TX, June 17-19, 1986
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ESTABLISHING THE RULES
GROSS NET
HIGHER LOWER HEATlNG HEATING VALUE VAWE (HHV) (LHV)
LATENT HE"T LATENT HEAT OF VAPORIZAnON OF VAPORIZATION
(Waler 01 Combustion) (Waler in Fuel)
Higher Heating Value - The primary components of fuel are carbon
and hydrogen. In the chemistry of combustion, hydrogen reacts with
oxygen to form water vapor. The Higher Heating Value is the heat
delivered by cooling all the products of combustion to their
initial temperature, condensing any water vapor in the flue gas.
This definition includes both sensible heat and latent heat of
vaporization as available (recoverable) heat.
Lower Heating Value Lower Heating value represents the sensible
heat recoverable by returning the products of combustion to their
initial temperature. It neglects the latent heat of vaporization as
inherently unavailable, recognizing the difficulty and
inadvisability of condensing water from the product gasses. The
degree of unavailability depends on the phase of any moisture
included in the composition of the fuel supply. The latent heat of
vaporization for included moisture is not considered if the fuel is
in the gaseous 0 phase, since the moisture is already vaporized,
and only sensible heat will be added in combust ion. However, if
the fue 1 is a solid or0 liquid, then the initial moisture will
also be liquid. The heat of vaporization must be added and becomes
unavailable to process streams.
The classical definition of LHV implies that on ly the water
formed as a product of combustion be forgiven as inherent ly unavai
lab Ie. The more prac t ica1 interpretation recognizes all heat
which vaporizes water as unavailable. The distinction is clarified
in the figure above.
TWO CALCULATION METHODS
Input/Output Method This method is straigh t -forward; add up
the produc ts of each heated stream mass flow rate times its
enthalpy increase across the heater, and divide by heat release.
For boilers, the heated streams include steam and blowdolffi flows;
for process heaters, this may include several product passes. The
applicable variables are identified below in bold relief.
INPUTIOUTPUT METHOD
~ PREHEATED FEED WATER
@ ATIEMPERA rlON WATER
/
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Flue gas composition is determined from fuel analysis, and
measured free oxygen in the flue gas. These allow precise
calculation of stoichiometric flue gas composition and excess air
on the per-pound-of-fue I bas is. From compos it ion and ideal gas
properties, the enthalpy increase from air inlet to final stack is
dependent only on the end temperature difference for the N2 and COt
formed, and excess 02 measured. The enthalpy 0 water vapor is
determined from the temperature and calculated partial pressure in
the flue gas.
Residual oxygen measurements for fuel/air mixture controls are
correctly taken at the heater flue gas outlet, ahead of any air
heaters. However, it is crucial for efficiency calculations that
the exit temperature and residual be measured in well-mixed flue
gas, downstream ot the last heat trap in the stack. This will
include the effects of air heater leakage on final temperature and
02 concentration.
Setting losses include radiant and convective losses from the
heater exterior, and are relatively constant (fixed Btu' s/hr)
regard less of firing rate. They are a function of surface area,
interior and exterior temperatures, insulation thickness, type and
condition. These variables are not subject to rapid change, and
typically cons t i tute one to five percent of FULL LOAD heat
release. This means that a two percent loss at full load becomes a
four percent loss when running at half load.
Combustibles losses are highly dependent on fuel type. Natural
gas and No. 2 fuel oil are easily fired and flue gas residual
combustibles are usually negligible. Coal, biomass and trash
burners will usually have measurable unburned carbon in the fly
ash. The losses associated with carbon loss are deducted from the
heating value of the fuel at the heating value of elemental carbon.
If high (-1,000 ppm) carbon-monoxide or methane levels are detected
by flue gas analysis (ahead of the air heater), then attention
should be paid to improving combustion, rather than calculating
efficiency.
PRACTICALITY & SENSITIVITY
The Input/Output and Heat Loss methods provide, in principle,
the same answer. In fact, each method has limitations on overall
accuracy. Efficiency is an intangible value; you can't pick up the
heater and carry it to the Bureau of Standards for precise
calibration, and process environments are not always kind to
sensitive measuring devices. The fa llowing tab Ie summarizes the
measurements required for each method, and the typical error
associated with each.
Where a range of error is ind icated, consider the medium being
measured. Natural gas is more accurately measured than coal for
fuel rate, and enthalpy. for a single-phase process stream is
easier to determine than for a multi-phase stream.
RELATIVE ACCURACY
EQUATION VARIABLE 1/0 HEAT LOSS TYPICAL ERROR TYPICAL ERROR
A"BIENT TEftP. DEGREES F' 1X 12 F'J
FUEL FLOW 2 lOX - HEATING VALUE o 1X a 1X - CO"POSITION o a
PROCESS - FLOW 2X - INLET ENTHALPY 2 - ~x - OUTLET ENTHALPY 2 -
5X
FLUE GAS TEltPERATURE - \02
2)( 2X
(10/500 (0-1.5:
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Refinery Mixed Gas Reviewing the data vaporization for the
formed water and corrected below, the refinery gas sample is mostly
dry setting loss [(HHV/LHV)*(2.5% Btu loss/Btu HHV)]. methane,
propane, and ethane with a carbon/hydrogen mass ratio of 3.3:1.
This is mostly hydrogen, on a Coal - A coal sample has 4.3 percent
moisture molal basis, and we eKpect two pounds of water to in
""t'h'e rough plus a small amount formed by be formed for each
pound of fue 1 fi red. combustion of the 14.16:1 carbon/hydrogen
ratlo Combustibles losses for clean gases are negligible, fuel. The
coal has 5.3 percentage points higher and setting losses are given
at full load. combustion efficiency than plant gas (HHV S&C
losses). This is because the stoichiometric air Using either me
thod, the Higher and Lower for the coal sample is 10.40 Ib./lb.
fuel versus
Heating Value basis efficiencies differ by 7.6 16.23 Ib./lb.
fuel for the gas sample. The lower percent, attributable to the
latent heat of air flow reduces stack heat loss at constant
RIXED REFINERY GAS
COAL COIIPONNT FDRftUU XlT VT. 18Y YOLo REACTAJITS I BY n.
NOLES
p-r .,1. of dr, {Yel FUEL ANA1.YSIS IFlIEL COJIIPOSITIOI
-_---- --- .--- - ---_. - e. __ _.
HYDROGEN 1.76 16.80 CARIOI 74.28 1.000 FORMULA VT. REACTANTS I
IY "'T.R2 COJlPOIIDT IIY IIY VOL. "OLES CARBO.. DIOX CO2 3.20 1.40
KYOROGElI 22.49 3. EIJ1 p-r -.ol_ ETHYLENE C2 Ht 1. 31 0.90 OXYGEI
0.00 0.000 af dry fuel THAiE C2 H6 2O.~9 13.20 SULF'UR 0.03 0.000
IIETHAII. CR4 47.86 ~7.50 RITROGJI 0.00 0.000 C 72.20 CARlON 77. J8
1. ODDC"_I-HEX"IIE C6 H14 0.89 0.20 CARBOI DIDI 3.20 0.012 R2 ~.10
HYDROGDl :5.47 0.842 rRCP,..E C3 H8 14.41 6.30 SULFUR 0101 0.00
0.000 02 9.00 OUGEI
RYOROOEll 9.~ 0.094OXYGS3I 0.006PROPyLJtE C386 3.93 1.80
JIOISnUtE 0.00 0.000 .2 1. 60 SUU'U1t 1.18RIYROllEll lSOBUTAME C4
HID 1.~1 0.50 5 1.10 RITllOGE1I 1.71 0.009SULFUR
10IlmIIlEI-BUTAI C4 HID 1.~1 0.50 Il2 0 4.30 CARBO. 0101 0.00
0.000 lSOPEIiTAII C~ "12 1.50 0.40 6.70 SULFUR PIOX 0.00 0.000ASH
4.61 0.040I-PEIITANE 1.50 0.40 ROISTURE
" H12 SUU'UR 5 0.03 0.02
COIIUSTIOIII COIilSTAlT X- 97.29 HMY- 12.990 LKY-12.436 BTU/U
CO"BUSTIOI COIlS1'AlT Xa 90.23 HNY' 21,6a2 LHV-19.6..3 ITUtU STAeI
TOP- 300.0 De9 F DATUI TEIIP 80.0 o.a r C/N",SS RATIO - 14.16:1
STACk TEltP. 300.0 De9 r DATUII ttIIp 80.0 o.a FeIN IIASS .ATIo
3.3Otl H2O 1111 CoiUnaaUon PRODUCTS 0. ~ U/U lu.l H2O 1ft CoabuaUan
PRODUCTS 2.01 LIlLI :t....l
IIOu:CUUR WEIGHT OF F1IEL' 19.27 .... -_ -- _.- -~~- __e. - _.;
--~. -- --- -. ~~ - - -
.----~~~-~;;-;--_._------~--------~~--------;~--------;--------~~----------
-~~-~~~-~-----------~---------------------_._------------------------------------------------------------------------------------------------..--
12.48 13.~2 14.56 22. 73 UI lEG"D, LIlLI FUELl0.40 11.44 All
REg'D. LI/U F"UL16.23 17.86 19,48 21.10 14.:i2 15.~ COU PROD, LilLI
FUEL17.23 18.86 0.037 0.034
12.44 13.4820.48 22.10 23. 73 COJlI PROD, LIIU F'UEL11."0 0.043
0.0400.107 O,09a 0.091 O.oalI IOI9TUlE, LIlLI PROD 0.047ftOISTURE,
LilLI PROD 0.117 0.070.09 0.04 0.040.00 VOUIIl 1 SO:2 O. 100.00
0.00 0.00'CLUKE I S02 0.00 13.2~ 1:2.34
YOWftE I CO2 10.14 9.30 8,:5. 7.97 6.10 ~...
1~.:l4 14.307.44 VOLU.S: I CO2 17. 02 7.16 6.~9YOLUILI 1 H:20 1.
a414.21 13.26YOLURI I H2O 18.04 U.~ 1~.3O 4.65 ~.77
1.1:5 3.23 4.~ ~.:59 YOLUIlI: I 02 O. 00 1.82 3.34 YOlUIlE I 02
0.00 1~.93 76.1473.70 YOWKE I R2 75. O~ 75.39 75."12.89
73.32YOlUftl I 12 71.77 72.31 3.:18 4.9:5 6.12
VOlUNE I 0:2 (DIY BASE) 0.00 2.10 3.81 0.2:16 0.2~~:!t.24 6.4~
OLUlIE I 02(01' lASE} 0.00 1.96
0.2:18 CP CBTU/LBr heu.t) 0.298 0.293 0.289 89.43 89.000.28~
0.282 CP (ITU/LIF heu.t) 0.261 0.2" 83.71 I COlli IFF' (HM') 90.70
90.27 89.8~ X CO"I ur (HMY) 8~.37 84.9:t 84.~4 84.12 92.50I COJlI
EFF (LHY) 94. :24 93.80 93.37 92.93
ASSURED JIiIERT RESIDUAL I 02 IIlI ,LUE OA5 cORY BASE)---. __ ..
_------.
ASSUJlED INERT 3.~ 5.2:!t 1.00 (DRY lASE) STACK 0.00 ._ -.-_..
-._..RESIDUAL I 02 IN FLUE QAS
-_._- - -- _. -- _. - -- -- --TEIP. ~
MMY 'l EF'FICIENCYSTACK 0.00 3.:.0 :5.2~ 7.00 89.17 8'.32
a8.'"90.70 90.32 TfIP.
300 92.60 LKV I EFFICIENCY
300 a~. :17 8~.03 84.ft2 64.12 83.50 300 94.14 94.~ 300 94.24
93.88 93. 4~ 92.93 92.28 LlY I EFFICIENCY
aHV I EFFICINCY 93.88 93.31
REFINERY KIXED GAS COAL
HEAT LOSS1/0YARIABLE 110 HEAT L.OSS VARIABLE
A"BIENT, 09 F 80 AK8IENT, O.g F' 80
FUEL FLOW, 1000 SCF/HR 128.3 FUEL FLO'll, 1000 LB/HR 12,842 HHY.
BTulscr 1.100 1,100 HHV, BTU/LB 12,1390 1:l.990 12,436LHY, BTU/SCF
997 997 LHY. BTU/L.B 12,436 14. 16J 1C/H "ASS RATIO 3.30.1 CIH "ASS
RATIO 1.1SULFUR CONTENT. WTX 0.03 SULFUR CONTENT, WTX
PROCESS STREA ttS PROCESS STREAKS 8FW FLOW, 1000 L8/HR 103 BFW
FLOW, 1000 LS/KR 103
ENTHALPY, BTUILB 286.0 ENTHALPY, STU/LB 286. a
ATTE"P FLow 1 ATTE"P FLO'll I ENTHALPY 217.8 ENTHALPY 217.8
100STEA" FLOW 100 5TA" FLOW 1438.2ENTHALPY 1438.2 ENTHALPY
BLOW DOWN FLOW SL.OWDOWN FLOW ENTHALPY 580.2 ENTHALPY S80.2
300FLUE OAS TEttP. Ovg F 300 TEKP. 0.0 F'FLUE GAS X02 DRY BASE
1. 7:5 XQ2 DRY BASE 1.7~
2.5SETTl NG LOSSES, X HHY 2. " SETTING LOSSES, X HHY .
0COft8USTIBLES LOSSES. X HHY
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temperature. However, the inherent combustibles loss for
coal-fired boilers reduces their overall efficiency. The difference
between the HHV and LHV is only 3.8 percent since less water is
evolved by burning the low-hydrogen coal than plant gas.
Black LiQour - This "fuel" is fired both for heat and the
liberation of residual chemicals. Black liquor is a by-product of
the Paper Pulp industry. Most of the chemicals used in cooking wood
chips to pulp remain in solution, and are recoverable. The
concentrated solution has a low, but useful, heating value. The
lliV efficiency is similar to coal, but black liquor contains 40
percent water. Little water is formed in burning the low hydrogen
fuel, but the high initial water content brings the HHV efficiency
down to 69 percent, even assuming complete combustion.
BLACK LHlOUR
FUEL AIIALYSISI
CORPONEN! FDRJlUU 18Y WT. IIY VOL. REACTANTS I &Y VT. 1l0LES
peor .01.
at dry lu.l
CARBON 2&.44 1.000 HYDROGEJI H2 2.20 HYDROGEl 2.~ 1.140
SULFUR 5 2.60 oXTGtM 60 0.131 ASH 13.00 SULFUR 2." 0.0'2 DUGEI 02
'.00 NITltOGU 17.47 0.213 UTllOGtM Il2 1~.20 CARBOM D[OlI o.DO
0.000 "DISTURt .2 0 40.00 SULFUR DIDI 0.00 0.000
CARBOJI C 23.00
MOISTURE .~. !II 1.1"
COftBUSTION CONSTANT K- 101.12 HHY- 3.649 LHY 2,", BTU/LI STACK
TDtP. 300.0 ~ r DATU" TEI'Ip. 80.0 n.v' elM ltASS RATIO 10.4',1 H2O
la Co_bueUon PRODUCTS. 0.69 LIlLI fve-l
EXCESS All I o 10 20 30 40
AIR RUI'O, LIlLI FUEL 3.83 4.22 4.60 '.91 ~.31 COll8 PROD, LI/LI
FUEL 4.83 '.22 ~.'O ~. 91 '.31 "DISTURE, LB/LI PROD O. 142 0.131
0.122 0.115 0.101
VOLU"E 'l 502 O.~. O.~ 0.47 0." 0.41 VOlURE I CO2 12.81 11.90
11.10 10.'1 9.7'J YOLU"E I H2O 22.1' 20. ~1 19.20 11. DO 1'.9'
YOLUIlE I 02 0.00 1.~ 2.10 3.94 4.9. YOLU": I 12 '4.49 ~.~3
&&.43 &1.22 &1.91
YOLU"E 1 02fDRT BASE) 0.00 1.19 3.47 ~.9~'.10 CP CBTU/Lar h....t
I 0.309 0.30' 0.300 o.m 0.292 x co"a Err (HHVl 12.00 71.43 10.1~
10. 2~ &9." . ASSU"ED I)(ERT
RESIDUAL X 02 IN FLUE OAS fOI' BASE) STACIC 0.00 3. ~ 5. 2~ 1.00
1[1'1'.
300 12.00 71.48 10.84 10.03 69.04 HHY I EFnCIENCY 300 88.81
88.2' 81.49 8&.'5 e~.40 LHY I EFFICIENCY
VARIABl.E 110 HEAT LOSS HEAT l.OSS (AS fIRED)
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MAINTENANCE PLANNING
>\) z w \) u. u. w
. . . . . . .
AT NOMINAL LOAD
TIME
Efficiequipment preheater
ency can be with
compvery a
arison~ against useful. Replacing
new design, or
upgraan old adding
ded air an
economizer to existing equipment will reduce flue gas losses
predictably, and should be evaluated against the existing
equipment. How about a completely new unit? The old furnace might
be right on spec, just as good as new, but somebody has an idea for
something new; better; more efficient; uses cheaper fuel; etc.
When considering a replacement, be careful of the basis of
comparison. More efficient? Cheaper fuel? Higher Btu's per pound?
Know the basis for claims: Dry fuel or as-delivered; HHV or LHV,
etc. Remember that high-hydrogen fuels have potentially good LHV
but poor HHV efficiency (which is what counts); high-moisture fuels
soak up a lot of
EVALUATING CAPITAL IMPROVEMENTS
NEW TECHNOLOGY
>\) z EXISTING EQUIPMENTw \) u. u. w
LOAD
la ten t heat, too; some fue Is are cheap per pound, but are
harder to burn completely like coal, wood and garbage and have
measurable carbon losses.
SUMMARY
The Heat Loss method is the most reliable and sensitive
indicator of fired heater efficiency. Given the same operating
conditions, di fferent fuels are not capable of equal operating
efficiency (HHV or LHV). Recognizing this, rout ine monitoring of
fired heaters can be used to keep equipment running as efficiently
as possible. This will save money by substantially improving daily
operations, maintenance planning, and capital improvement
decisions. Money not spent on fuel adds to profits, and that's what
it's all about.
660
ESL-IE-86-06-106
Proceedings from the Eighth Annual Industrial Energy Technology
Conference, Houston, TX, June 17-19, 1986
1986 Proceedings Volume II.pdf