Hydrocarbon Processing - October 2021 - GP-22

SPECIAL FOCUS: LNG TECHNOLOGY
ity (RH) at any location fluctuates within
a day and throughout the year; they also
fluctuate within an LNG regasification
terminal. Varying atmospheric temperature
and RH influence the performance of
an AAH. Therefore, it is crucial for a process
engineer to carefully analyze past meteorological
site data while determining
operating scenarios or cases for designing
the AAH. A minimum approach temperature
(i.e., the difference between the inlet
temperature of ambient air and the outlet
temperature of the intermediate fluid) of
3°C-4°C is recommended to achieve an
economical and efficient AAH design.
The AAH performs at maximum capacity
during higher atmospheric temperatures,
and some of the fans forcing
air into the AAH may be required to
switch off. However, it is recommended
to use variable frequency drives for at
least 50% of the total number of fans and
motors to ensure effective process control
and maximum electrical power savings
in the AAH.
During lower atmospheric temperatures,
water vapor present in ambient air
commonly becomes condensed when
passing through the tube bundles due
to the transfer of heat to the intermediate
fluid flowing inside the tube bundles.
This condensed water vapor tends to slide
down to grade by its gravity.
Condensed water and fan issues.
As shown in FIG. 2, it is recommended to
force the air into the tube bundles from
the top by forced-draft fans and install
the complete drive assembly (including
fans, motors and a belt drive) at the top
of the tube bundles to avoid rust, corrosion
and damage. This recommended arrangement
also helps minimize departed
cold air recirculation, discussed later in
this article. It is important to mention
that a good amount of condensed water is
collected at grade below the AAH, which
must be suitably utilized.
While fans are located at the top of
the tube bundles and force air from top
to bottom, it is recommended to flow the
intermediate fluid from bottom to top inside
the tube bundles. Therefore, the inlet
nozzles for the intermediate fluid should
be located at the bottom of the tube
bundles and the outlet nozzles should
be located at the top of tube bundles, as
shown in FIG. 2. This way, a counter-current
to crossflow arrangement is achieved
between the two fluids, which improves
the logarithmic mean temperature difference
(LMTD) and the heat transfer coefficient,
therefore improving the thermal
performance of the AAH.
Due to the condensation of water
FIG. 3. Parallel installation for the AAH is more
susceptible to cold air recirculation than AAH
installation in series.
vapor present in ambient air, substantial
fog and mist is generated below the
tube bundles and around the AAH. This
phenomenon becomes more significant
when the unit is operating during lower
ambient temperatures, high humidity and
low wind conditions.
With the given AAH layout, site meFIG.
4. Recommended minimum fin spacing to
enable the release of condensed water from
tube and fin surfaces on the AAH.
teorological data (e.g., prevailing wind
direction and wind velocity) and wind
obstructions (e.g., surrounding tall structures,
LNG tanks, buildings, etc.), it is
recommended to carry out a computational
fluid dynamics (CFD) analysis for
the AAH to predict air stream behavior.
Based on this CFD analysis, a suitable
elevation and appropriate layout for the
AAH should be selected so that the surrounding
hot atmospheric air can mix
with the exiting cold air from the AAH
to considerably reduce fogging. However,
a higher elevation can result in the
carryover of condensed water with wind,
22 SEPTEMBER/OCTOBER 2021 | GasProcessingNews.com
which must be prevented by a condensed
water removal system.
Cold air recirculation. Some amount
of cold air exiting the AAH recirculates
back to the fan suction at the top of the
tube bundle (i.e., cold air recirculation).
This further reduces the temperature of
air entering the AAH, which can impact
the overall performance of the AAH. As
previously recommended, a CFD analysis
is essential to accurately estimate the occurrence
of cold air recirculation and its
impact on suction air temperature. Based
on the results of this analysis, necessary
corrections to the design air temperature
or implementation of adequate overdesign
margins must be taken into consideration
for the design of the AAH.
Detailed investigations that have been
carried out at various LNG regasification
terminals reveal that cold air recirculation
cannot be fully avoided, but its effect can
be minimized by selecting a suitable elevation
and proper layout for the AAH.
As shown in FIG. 3, a parallel installation
for the AAH is more susceptible to cold
air recirculation than AAH installation in
series. This is because cold air recirculation
is mainly caused by the confluence of
air streams coming from the parallel units.
Hence, the simplest and most efficient
solution for minimizing cold air recirculation
is to install AAHs in series.
Latent heat considerations. The most
important aspect in the optimization of
the thermal design of AAHs is to consider
the latent heat released by the condensed
water vapor. The amount of condensed
water depends on the site-specific relative
humidity and dewpoint temperature. The
latent heat of the condensation of water is
substantially higher than the heat of air.
Considering a higher relative humidity in
design, the size and cost of the AAH can
be considerably reduced; alternatively, a
low relative humidity can increase the size
and cost of the AAH. In light of this, a process
engineer must carefully determine the
design relative humidity prior to planning.
Note: The boundary layer of condensed
water vapor that forms on the tube
and fin surfaces acts as a resistance to the
heat transfer from air to the intermediate
fluid, which must be taken into consideration
in the thermal design of the AAH.
Fin spacing in tubes. Fin spacing in
the finned tubes of the AAH must be suitably
selected to enable the easy release of
condensed water from the tube and fin
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Hydrocarbon Processing - October 2021

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