Hydrocarbon Processing - March 2021 - 61

Heat Transfer
M. MARTIN, XRG Technologies, Tulsa, Oklahoma

The use of tube inserts in fired heaters

typical fired heater. Burners fire into a
" radiant section, " transferring heat from
the flue gas to the process fluid, which
flows through pipes commonly called
" tubes. " The process fluid typically receives 60%-70% of the heat within the
radiant section. The flue gas then flows
into the " convection section, " which
transfers 15%-20% of the remaining heat
to the process fluid. The principal mode
of heat transfer in the radiant section is
thermal irradiation, while the principal
mode of heat transfer in the convection
section is convective.
The process flow in practical fired
heaters is turbulent, with a Reynolds
number on the order of 106. Most of the
heat transferred to the process occurs
within the radiant section. The convection section compensates for small re-

process coil wall and the process flow.
When the combustion heats the process
flow, vapor first forms at the interior wall
of the pipe. Gases have a significantly
lower overall convection heat transfer
coefficient when compared to liquids, so
the process flow transfers less heat away
from the pipe wall. In this way, adverse
feedback ensues wherein the high wall
temperature begets more boiling that, in
turn, reduces the inside convection heat
transfer coefficient, thereby increasing
the wall temperature and the resulting
boiling. In this way, " hotspots " can form
on the heater tubes given an initial slight
difference in heat transfer.
Gravity further exacerbates phase nonuniformity inside horizontal sections of
process coils. Gravity pulls liquid-being
denser than vapor-to the bottom of the

Process inlet
Convection
section

Process coil

Radiant section

Fired heaters, heat transfer and
heat flux uniformity. FIG. 1 shows a

ductions in radiant section heat transfer
due to fouling and non-ideal flames; a
higher radiant section exit temperature
results in more heat transfer within the
convection section. Fired heater designers have had little incentive to increase
heat transfer intensity using turbulators.
The process flow is turbulent, resulting in a high tube-side convection heat
transfer coefficient, and the convection
section design already results in the desired flue gas exit temperature. However,
the benefit of in-tube mixing enhancement goes far beyond the benefits of increased heat transfer.
Reformers and pyrolysis heaters fall
among a class of heaters where intentional and valuable reactions take place
inside the heater tubes. In some reactor
charge heater tubes, undesirable chemical reactions cause in-heater feed conversion, which reduces the ultimate yield.
Other heaters produce unintended reactions that reduce the value of the product. Heaters used in certain services,
such as crude distillation, vacuum distillation and delayed coking, have both
unintended reactions and phase change
within the heater tubes. Similar devices
to heaters, such as once-through steam
generators (OTSGs), do not have chemical reactions within the tubes but do exhibit phase change. In all cases, not only
is the total absorbed heat important, but
also the location of the absorbed heat.
To see why the variation in temperature along the outside coil surface is critical in heaters with multiphase process
flow, consider the idealized graph of heat
transfer coefficient vs. temperature difference between wall and fluid, illustrated in
FIG. 2. Beneath the chart is a representative picture of the liquid/vapor composition within the process coil corresponding to the heat transfer coefficient. The
highest temperature at any point in the
flow occurs at the boundary between the

Stack

In 1896, Whitham1 reported the successful use of twisted tapes (originally
called retarders, and now also called
turbulators) to increase heat transfer in
boiler fire tubes-their effectiveness in
increasing heat transfer is well known.
Engineers usually use twisted tapes to improve heat transfer in laminar flows, but
academics and industry have extensively
studied their use in turbulent flows.2
Refining and petrochemical plants
commonly use fired heaters where the
process requires high-intensity heat.
State-of-the-art fired heaters are some
of the most fuel-efficient devices in use,
with efficiencies over 92%. It is not readily apparent how the use of turbulators
might benefit such highly efficient systems. However, using the knowledge that
the process flow in many fired heaters at
least partially vaporizes, one can show
that properly applied tube inserts that
outwardly resemble turbulators can improve fired heater performance.

Process outlet
Burners

FIG. 1. A typical fired heater. Burners fire into
a radiant section, generating hot flue gas.
Radiation transfers 60%-70% of the heat in this
section, after which the flue gas flows through
the convection section where the process
absorbs 15%-20% of the remaining heat.
Hydrocarbon Processing | MARCH 2021

61



Hydrocarbon Processing - March 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - March 2021

Contents
Hydrocarbon Processing - March 2021 - Intro
Hydrocarbon Processing - March 2021 - Cover1
Hydrocarbon Processing - March 2021 - Cover2
Hydrocarbon Processing - March 2021 - Contents
Hydrocarbon Processing - March 2021 - 4
Hydrocarbon Processing - March 2021 - 5
Hydrocarbon Processing - March 2021 - 6
Hydrocarbon Processing - March 2021 - 7
Hydrocarbon Processing - March 2021 - 8
Hydrocarbon Processing - March 2021 - 9
Hydrocarbon Processing - March 2021 - 10
Hydrocarbon Processing - March 2021 - 10A
Hydrocarbon Processing - March 2021 - 10B
Hydrocarbon Processing - March 2021 - 11
Hydrocarbon Processing - March 2021 - 12
Hydrocarbon Processing - March 2021 - 13
Hydrocarbon Processing - March 2021 - 14
Hydrocarbon Processing - March 2021 - 15
Hydrocarbon Processing - March 2021 - 16
Hydrocarbon Processing - March 2021 - 17
Hydrocarbon Processing - March 2021 - 18
Hydrocarbon Processing - March 2021 - 19
Hydrocarbon Processing - March 2021 - 20
Hydrocarbon Processing - March 2021 - 21
Hydrocarbon Processing - March 2021 - 22
Hydrocarbon Processing - March 2021 - 23
Hydrocarbon Processing - March 2021 - 24
Hydrocarbon Processing - March 2021 - 25
Hydrocarbon Processing - March 2021 - 26
Hydrocarbon Processing - March 2021 - 27
Hydrocarbon Processing - March 2021 - 28
Hydrocarbon Processing - March 2021 - 29
Hydrocarbon Processing - March 2021 - 30
Hydrocarbon Processing - March 2021 - 31
Hydrocarbon Processing - March 2021 - 32
Hydrocarbon Processing - March 2021 - 33
Hydrocarbon Processing - March 2021 - 34
Hydrocarbon Processing - March 2021 - 35
Hydrocarbon Processing - March 2021 - 36
Hydrocarbon Processing - March 2021 - 37
Hydrocarbon Processing - March 2021 - 38
Hydrocarbon Processing - March 2021 - 39
Hydrocarbon Processing - March 2021 - 40
Hydrocarbon Processing - March 2021 - 41
Hydrocarbon Processing - March 2021 - 42
Hydrocarbon Processing - March 2021 - 43
Hydrocarbon Processing - March 2021 - 44
Hydrocarbon Processing - March 2021 - 45
Hydrocarbon Processing - March 2021 - 46
Hydrocarbon Processing - March 2021 - 47
Hydrocarbon Processing - March 2021 - 48
Hydrocarbon Processing - March 2021 - 49
Hydrocarbon Processing - March 2021 - 50
Hydrocarbon Processing - March 2021 - 51
Hydrocarbon Processing - March 2021 - 52
Hydrocarbon Processing - March 2021 - 53
Hydrocarbon Processing - March 2021 - 54
Hydrocarbon Processing - March 2021 - 55
Hydrocarbon Processing - March 2021 - 56
Hydrocarbon Processing - March 2021 - 57
Hydrocarbon Processing - March 2021 - 58
Hydrocarbon Processing - March 2021 - 59
Hydrocarbon Processing - March 2021 - 60
Hydrocarbon Processing - March 2021 - 61
Hydrocarbon Processing - March 2021 - 62
Hydrocarbon Processing - March 2021 - 63
Hydrocarbon Processing - March 2021 - 64
Hydrocarbon Processing - March 2021 - 65
Hydrocarbon Processing - March 2021 - 66
Hydrocarbon Processing - March 2021 - 67
Hydrocarbon Processing - March 2021 - 68
Hydrocarbon Processing - March 2021 - 69
Hydrocarbon Processing - March 2021 - 70
Hydrocarbon Processing - March 2021 - 71
Hydrocarbon Processing - March 2021 - 72
Hydrocarbon Processing - March 2021 - 73
Hydrocarbon Processing - March 2021 - 74
Hydrocarbon Processing - March 2021 - 75
Hydrocarbon Processing - March 2021 - 76
Hydrocarbon Processing - March 2021 - 77
Hydrocarbon Processing - March 2021 - 78
Hydrocarbon Processing - March 2021 - 79
Hydrocarbon Processing - March 2021 - 80
Hydrocarbon Processing - March 2021 - 81
Hydrocarbon Processing - March 2021 - 82
Hydrocarbon Processing - March 2021 - 83
Hydrocarbon Processing - March 2021 - 84
Hydrocarbon Processing - March 2021 - 85
Hydrocarbon Processing - March 2021 - 86
Hydrocarbon Processing - March 2021 - 87
Hydrocarbon Processing - March 2021 - 88
Hydrocarbon Processing - March 2021 - 88A
Hydrocarbon Processing - March 2021 - 88B
Hydrocarbon Processing - March 2021 - 89
Hydrocarbon Processing - March 2021 - 90
Hydrocarbon Processing - March 2021 - Cover3
Hydrocarbon Processing - March 2021 - Cover4
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https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018_v2
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https://www.nxtbook.com/nxtbooks/gulfpub/hp_201812
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