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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
www.digitalrefining.com/article/1000715 PTQ Q1 2013 3
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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