Hydrocarbon Processing - August 2022 - 77

Heat Transfer
C.-C. CHEN, CTCI Corp., Taipei, Taiwan
Use CFD analysis to evaluate the impact of
hot air recirculation on air-cooled heat exchangers
Air-cooled heat exchangers are widely
used in the petroleum, petrochemical,
natural gas and other industries for cooling
purposes. In contrast to a water-cooled
system, which consists of a traditional
shell-and-tube heat exchanger and a cooling
tower, an air-cooled heat exchanger
uses ambient air as the main cooling medium.
The air is blown from the exterior of
the bundle to cool the higher-temperature
process fluid inside the tube to the desired
operating temperature. Since this procedure
does not require a large amount of industrial
water, air-cooled heat exchangers
are suitable in areas that face geographical
restrictions, as well as regions that lack water
resources or are required to save water.
However, as hot air discharged from an air
cooler recirculates, a backflow phenomenon
occurs, causing the temperature
of the air used as the cooling medium to
increase, leading to decreased operational
performance of the air cooler.
Several factors play a role in hot air
recirculation. Among them, ambient air
temperature, external wind direction/
speed and the height/distribution of
the surrounding buildings are the most
important factors. While ambient wind
direction and speed are natural forces beyond
human control, computational fluid
dynamics (CFD) software can be used to
conduct flow field analysis and simulate
air circulation of air coolers under various
environment conditions to avoid adverse
effects from hot air recirculation. The
most suitable configuration and improved
measures can be determined to ensure
the heat-exchange efficiency of the whole
system meets operational requirements,
regardless of wind direction and speed in
different seasons.
CFD model formulation. In an overall
field area, building location, building
height, wall size, direction of entry and
exit access, and relative position of each
building determine the environmental
boundary conditions, which in turn affect
the operational performance of the
air-cooled heat exchanger. Since hot air
moves vertically due to thermal buoyancy
and pressure, air discharged from an aircooled
heat exchanger will flow upward
and be blown away by the crosswind.
Therefore, it is also important to consider
the external flow wind direction in the
overall impact of boundary conditions.
This article will discuss these aspects
based on a refinery engineering, procurement
and construction (EPC) project in
Southeast Asia. As shown in FIG. 1, two
major buildings (Side Building A and
Side Building B) are on the west side of
the main building where an air-cooled
heat exchanger is located, as well as a few
smaller buildings. A preliminary flow field
result suggests that these two major buildings
have a greater impact on the flow
field than the other smaller buildings because
they have solid walls that obstruct
air from flowing freely. Therefore, the
smaller buildings can be ignored due to
their negligible effect on flow field-the
focus should instead be placed on simulating
the flow field of these three buildings.
This also has the benefit of lowering resource
loading during simulation. Moreover,
all irrelevant factors that may affect
rapid convergence of the overall simulation
are deleted from the analysis model.
Reference index: Flow field (velocity
field and temperature field). Once
the boundary condition of the steady flow
field is set and the turbulence module calculation
is complete, the flow field can be
observed through simulation.
To quantify the results of CFD numerical
analysis, the now-known flow field
phenomenon was combined with various
energy condition inputs to produce a
temperature distribution map around the
FIG. 1. Schematic plant layout.
Hydrocarbon Processing | AUGUST 2022 77
Hydrocarbon Processing - August 2022

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