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Controlling film temperature in firedheaters
Direct-red heaters have
been widely used in theoil rening and chemical
process industries to heat thecrude oil contained in
tubularcoils by the combustion of fuelwithin an internally
insulatedenclosure. A successful redheater design relies on
manyfactors. Film temperaturecontrol is one of the key factorsthat
play a crucial role in redheater design, particularly for
units processing heavy feed-stocks that are thermallyunstable,
such as Canadian oilsands-based feedstocks.
Film temperature determinesthe susceptibility of a processuid
towards coking. Bulk oiltemperature plus a temperaturerise across
the oil lm sets thelm temperature. In mostapplications, it is the
oil lm
temperature, not the bulk oiltemperature, that limits theheater
duty and the oil life.1Film temperature is an impor-tant factor in
red heaterdesign for many reasons.Firstly, oil degradation starts
inthe uid lm, since this is thehottest place for the bulk oil.Fluid
life is shortened becauseof degradation, which can lead
Film temperature control is critical to the sucessful design of
fired heaters,
especially for heaters employed in upgrading heavy
feedstocks
JINYU JIAO, YURIY MORAYKO, MORTEN THEILGAARD and MICHAEL HO
WorleyParsons Canada
www.digitalrefining.com/article/1000715 PTQ Q1 2013 1
to a costly result. Secondly, ifthe lm temperature exceedsthe
limitation, the stationaryuid lm on the inside tubesurfaces is
subject to thermaldecomposition, which resultsin coke deposition at
that loca-tion. Coke deposits increaseresistance to heat transfer
andraise the tube metals tempera-ture. Once the tube
walltemperature reaches the designtemperature, the heater must
be shut down for decokingto avoid coil damage.
Thirdly,overheating of the uid lmaccelerates the fouling rate.
Fouling requires more heatinput and a hotter tube
metaltemperature to maintain thesame heater outlet
temperature.These factors cause heaters toshut down much
morefrequently and eventuallyreduce the whole
plantsprotability.
Due to the importance of thelm temperature, its control has
become a hot topic for redheater designs, especially for
crude heaters, vacuum heatersand coker heaters. In this
article,some feasible methods ofcontrolling the lm temperature
360
380
340
320
300
280
260Filmt
empera
ture,
C
From inlet to outlet
240
Without steam injection
With steam injection
Figure 1 Effect of steam injection on film temperatures
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higher pump head upstream ofthe heater. Since lm tempera-ture
control is basicallyintended to control the peaklm temperature, it
is advisableto reduce tube sizes for tubeswith peak lm
temperature
only, to minimise the increasein pressure drop caused byreducing
tube sizes.
Double fired vs single firedA red heater can be single ordouble
red. The heat ux onthe tubes circumferentialsurface is not uniform
becauseof the shading of radiant heat.The single-red heater
receivesradiant heat on one side of theprocess tubes (directly from
the
burner ame), while the otherside of the tubes, facing theheater
wall, gains radiant heatfrom the refractory. The portionof the tube
facing the burnershas a higher local heat ux,while the side facing
the refrac-tory is much lower. For a givenred heater with nominal
twodiameter tube spacing and a
very uniform longitudinal heatux distribution, the local
peakheat ux (q
m) is approximately
1.8 times the average heat ux(q
a) for single-red heating. In
contrast, the double-red heaterhas radiant heat on both sidesof
the tubes, which greatlyreduces the peak ux to about1.2 times the
average heat ux.3The correlations mentioned
above for single and doublered can be simply representedin the
following equation:
qm
= Xqa
(1)
where X represents the timefactor, which is approximatelyequal
to 1.8 and 1.2 for singleand double red, respectively.
The local lm temperature
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can be calculated by the follow-ing equations:4
Tf
= Tb+ T
f(2)
Tf
=q
m(D
o) (3)
Kf D
i
where Tf and T
b are lm
temperature and oil bulktemperature, respectively. T
f
is the lm temperature rise andKf is a lm heat transfer
coefcient.From the equations, it can be
seen that it is the localised heatux, not the average heat
ux,that directly governs the lmtemperature. For a heater with
agiven average heat ux, a
double-red heater has a lowerlocalised heat ux distributionthan
a single-red heater. Alower localised heat ux reducesthe lm
temperature at thatlocation. Figure 3 shows acomparison of the lm
tempera-ture between single and doublering for the same heater
withthe same average heat ux. Theresults are from the
simulation
of a vacuum heater with adesign duty of 50 MW. It can beseen
that using double red cangreatly reduce the lm tempera-ture of the
radiant coil for theheater.
Lowering the average heat fluxThe rst step in designing ared
heater is to set up theallowable average radiant heat
ux. For a given heater, eithersingle or double red, it ishelpful
to control the lmtemperature by lowering theaverage heat ux.
FromEquation 1, the localised heatux reduces with lower aver-age
heat ux, no matterwhether it is single or doublered. Oil lm
temperaturedepends on the heat ux and
oil mass velocity. Decreasingthe heat ux reduces the oillm
temperature at a xedmass velocity.5
The average radiant sectionheat ux is dened as the totalradiant
section absorbed dutydivided by the total radiantsection tube
surface area. For agiven radiant duty of a redheater, the only way
to lower
320
340
360
380
300
280
260
240
220
Filmt
emp
erature,
C
From inlet to outlet
200
Single fired
Double fired
Figure 3 Comparison of the film temperature between single and
double firing
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the average heat ux is toincrease the radiant sectionssurface
area. It may be reasona-
ble to assume a relatively lowaverage heat ux to design ared
heater with very tight lmtemperature control. However,
it is also noted that oil resi-
dence time increases as thesurface area increases, whichmay
partially counteract the
benet of decreasing the lmtemperature by lowering theaverage
heat ux. A loweraverage heat ux means morecapital cost in the
heater coils,which is another drawback incontrolling the lm
tempera-ture by reducing the averageheat ux. Thus, the effect
onheater design of lowering theaverage heat ux should becarefully
evaluated before adecision is made.
Other design considerationsThere are other design
consid-erations that should not beneglected when designing ared
heater with better lm
temperature control. The radi-ant section heat ux at anypoint in
the heater is controlled
by the temperature differencebetween the hot ue gas andoil in
the tube. The heat trans-fer rate increases with thetemperature
difference betweenthe hot ue gas and the coldoil.6 In a vertical
up-red heater,it is not rare to see that heat
ux is low at the oor andgradually increases along thelength of
the ame. Itis highest at the point wheremaximum combustion
takesplace in the ame, then reducesat the top of the re box.
Thus,real red heaters may encoun-
ter more or less signicant heatux imbalances. This heat
uximbalance can cause high lmtemperatures and high rates offouling
formation. Effortsshould be made to optimise thedesign parameters
to minimise
the heat ux imbalance. Thesedesign parameters include radi-ant
section height to widthratio, burner to tube distance,number of
burners, ame shapeand dimensions, and radiantsection tube
layout.7
Flame impingement cancause extremely high localisedheat ux,
which results in ahigher lm temperature andrapid coke formation.
Flameimpingement occurs when aame actually touches orengulfs the
tubes. Vinayagamhas discussed the causes ofame impingement for a
redheater.8 Some precautions needto be considered in the designof
red heaters to preventame impingement occurring;for instance, an
adequate re
box to contain the ame, more
and equally spaced burners,the correct type of burners,
andimproved distribution ofcombustion air ow.
SummaryFilm temperature control iscritical to the successful
designof red heaters, especially forthose heaters employed
inupgrading heavy feedstocks.
This article has discussedseveral ways to control the
lmtemperature in the design ofred heaters. These methodshave been
discussed in detailand have proved to be effectivemeans of
controlling lmtemperature.
References
1 Pelini R G, Heat flux and film
temperature in fired thermal-fluid
heaters, Chemical Engineering,Dec 2008.
2 Hanson D, Martin M, Low capital
revamp increases vacuum gas oil yield,
March, Oil & Gas Journal,2002.
3 Romero S, Delayed coker fired heater
design and operations, Rio Oil & GasExpo and Conference,
2010.
4 API 530, Calculation of heater-tube
thickness in petroleum refineries, 6th ed,
Sept 2008.
5 Golden S W, Barletta T, Designing
vacuum units,Separations, Apr 2006.
6 Martin G R, Heat-flux imbalance in
fired heaters cause operating problems,
Hydrocarbon Processing,May 1998.
7 Nogay R, Prasad A, Better design
method for fired heaters, Hydrocarbon
Processing, Nov 1985.8 Vinayagam K, Minimizing flame
impingements in fired heaters, Chemical
Engineering, May 2007.
Jinyu Jiao is a Mechanical Engineer
with WorleyParsons in Calgary, Canada.
He holds an MS degree in mechanical
engineering as well as a PhD in chemical
engineering.
Yuriy Moraykois a Mechanical Engineer
with WorleyParsons in Calgary, Canada.
He holds a BSc degree in mechanical
engineering.Morten Theilgaard is a Mechanical
Department Manager for WorleyParsons
in Calgary, Canada. He holds a BSc in
mechanical engineering.
Michael Ho has eight years experience
in the oil and gas industry in an EPCM
environment. He holds masters and
bachelors degrees in mechanical
engineering.
4 PTQ Q1 2013 www.digitalrefining.com/article/1000715
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