Hydrocarbon Processing - January 2022 - 78

Heat Transfer
assuming zero thickness
fouling-is like the clean one.
b. FIGS. 4 and 5 show the heat flux at
the tube inlet and the tube metal
temperature (TMT), for fouled
and clean conditions, against
the number of working tubes.
The TMT is given for the tube
inlet, averaged across the tube
thickness, and for tube length,
averaged along the length and
across the tube thickness. Trends
in FIGS. 4 and 5 indicate that these
parameters increase by increasing
the number of clogged tubes, and
that the increase becomes steeper.
c. In FIGS. 4 and 5, it is apparent
that fouling is " protecting " the
tube from high gas temperature
and heat flux. In other words:
1. When several tubes are
clogged, the remaining working
tubes are cleaner and the
conditions from a thermalhydraulics
standpoint are more
critical. At the tube inlet where
the heat transfer is the most
critical, the heat flux and the
TMT of the working tubes
increase with growing clogging:
this can have consequences on
boiling mechanism and cooling
efficiency on the water side.
It should be noted that, when
several tubes are clogged, the
working tubes are in overloaded
conditions and are at risk of
overheating and/or corrosion.
d. In FIG. 5, the TMT averaged
along the tube length and across
the thickness increases with the
number of clogged tubes. This has
a consequence from a thermalmechanical
standpoint since tube
buckling is intensified. It must be
noted that clogged tubes are colder
than working tubes, so differential
thermal elongations may give
high local stresses, particularly at
tube-to-tubesheet joints. Referring
to tube buckling, FIG. 5 shows that
again clean conditions (i.e., no
fouling deposited along the tube
length) can be more critical than
fouled conditions since the TMT
averaged along the tube length
is higher for clean conditions.
e. FIG. 6 shows trends of pressure
drop (in ordinates) at different
fouling thicknesses in tubes
(in abscissas) and at a different
percentage of clogged tubes. The
nominal design pressure drop
(6 kPa) is doubled or tripled if
either heavy fouling deposition
along the tube length (several
millimetres of fouling thickness)
or several clogged tubes-or
both-occur. However, FIG. 6 also
indicates that when pressure drop
across the TLE rapidly rises, this
is likely due to clogging of tubes
rather than fouling deposition
along the tube length. This must
be considered because, if the
pressure drop limit for cleaning
operations is set at a high value, the
TLE may work with a significant
portion of clogged tubes, which
means that the remaining working
tubes are in overloaded conditions.
f. FIG. 6 also indicates that when the
gas-side fouling mainly occurs
along the tube length, it is likely
that the outlet gas temperature
limit is reached before the outlet
gas pressure limit. Conversely,
when the gas-side fouling growth
is more promoted at the inlet
tubesheet and tube inlet (i.e., the
fouling tends to clog the tubes),
it is likely that the outlet gas
pressure limit is reached before
the outlet gas temperature limit.
g. The outlet gas temperature
trend in FIG. 3 and previous
point (e) emphasize that:
1. For the theoretical condition
of nearly 50% of tubes clogged
and 50% of tubes working and
clean, the TLE would not reach
the outlet gas temperature
limit; however, the working
tubes would be at 200% of
load (overloaded conditions).
FIGS. 4 and 5. The heat flux at the tube inlet (top) and the tube metal temperature (bottom), for
fouled and clean conditions, against the number of working tubes.
78 JANUARY 2022 | HydrocarbonProcessing.com
2. When fouling growth is more
promoted at the tube inlet

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