Hydrocarbon Processing - March 2021 - 63

Heat Transfer

Estimates of economic impact. The

variability in causes of shutdowns will
impact any economic assessment for
the use of in-tube inserts. If a plant shuts
down often due to tube failures, the value
for preventing those failures is far greater
than incremental increases in run length.
However, the value of using the inserts
for normal operation without special
cause failures can be estimated.
To evaluate the effect on run length,
a model was created for a coker unit furnace. The computational fluid dynamics
(CFD) model of the coil included submodels for phase change of the vacuum
residual oil, as well as the propensity for
coking based on the work of Ebert and
Panchal.3 The predicted coking rate and
daily temperature rise for coils with the
insert in place is three times less than

of that of an empty tube. The predicted
run length of the furnace is increased by
35%. Using validated assumptions for
the coker spread, shutdown days and
current energy costs, the 2-yr net value
addition of inserts is $1.96 MM for a
typical 30,000-bpd coker unit.
Practical considerations and takeaway. The use of tube inserts does raise

the practical concern of how to remove
the inserts when the coil requires pigging. A mechanical solution has been
developed by the author's company
that uses flanged connections outside
the heat-affected zone. This solution
removes the potential for leakage of the
process fluid, which is a problem for traditional plug headers. Physical testing
was performed to ensure that the insert
can be removed from a tube even if coke
or scale has completely seized it in place.
Tube inserts are a well-proven method to increase heat transfer. They also increase process homogeneity. The proper
application of in-tube mixing should
increase coil longevity, run length and
unit profitability. These improvements
should be more pronounced for processes with in-tube phase change. Properly optimized mixing elements provide
both relatively low pressure drop and the
increased mixing required to bring heaters to a new level of performance.

Whitham, J. M., The effect of retarders in fire tubes
of steam boilers, American Society of Mechanical
Engineers (ASME XVII), Philadelphia,
Pennsylvania, 1896.
2
Manglik, R. M. and A. E. Bergles, " Heat transfer
and pressure drop correlations for twisted-tape
inserts in isothermal tubes: Part II, transition and
turbulent flows, " Journal of Heat Transfer, American
Society of Mechanical Engineers, 1992.
3
Ebert, W. and C. B. Panchal, Analysis of Exxon crude
oil slip stream coking data in fouling mitigation of
industrial heat exchange equipment, Begell House,
New York, 1997.
1

MATTHEW MARTIN is the Chief
Research and Development
Scientist for XRG Technologies.
He has 27 yr of experience in the
combustion industry and 19 yr
of experience in computational
fluid dynamics of combustion
equipment. Mr. Martin has 27 patents for burners,
flares, thermal oxidizers and flame detection
technology, and holds a BS degree in computer
science from the University of Tulsa in Oklahoma.

Comparison of area goodness factor
for standard and optimized designs
0.31

Twisted tape

Optimized design

0.29
Area goodness factor, j/f

tion at the inlet and outlet of the tube
for a simulation with 80% liquid and
20% vapor by volume. There are inserts
both before and after the return, but in
this case, the flow requires one straight
section of tube to establish the mixing
motion provided by the insert. With
the tube insert in place, the convection
heat transfer coefficient is more uniform
when compared to the empty tube.
Near the inlet, at Point A, gravity stratifies both flows. At Point B, the return has
temporarily changed the stratified flow to
annular flow, producing a more uniform
heat transfer coefficient at the tube surface for both cases. At Point C, the stratified flow returns in the empty tube, but
the liquid remains adhered to the tube
wall when using the insert.
On the outlet leg of the coil section,
the tube insert increases the area-weighted average heat transfer coefficient by
20% when one compares the empty tube.
More importantly, the minimum heat
transfer coefficient over the same section of the coil using inserts is 50 times
higher than the minimum heat transfer
coefficient of the same section of the
empty tube. Tube failures occur at specific, often initially small, locations. The
use of the insert eliminates the point of
minimum heat transfer coefficient where
this failure would likely occur. By limiting the use of inserts to the tubes that are
prone to failure or that need the most
process improvement, their benefit can
be maximized while reducing the additional pressure drop.

0.27
0.25
30% increase in
goodness factor

0.23
0.21
0.19
0.17

ACKNOWLEDGEMENT
The research results discussed in this publication were made possible in total or in part by funding
through the award for project number AR18-015, from
the Oklahoma Center for the Advancement of Science
and Technology.
LITERATURE CITED

0.15

5
Twist pitch

FIG. 4. A comparison of optimized tube insert
design to a traditional twisted tape shows an
increase in goodness factor of 30%.

FIG. 5. Comparison of convection heat transfer coefficient and liquid volume fraction for a tube
without (left) and with (right) tube inserts.
Hydrocarbon Processing | MARCH 2021

Hydrocarbon Processing - March 2021

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