Hydrocarbon Processing - April 2022 - 43

Heat Transfer
The induced draft option, with a chimney on top of the fan
rings, has the following advantages over the forced draft option:
* A lower chimney height: The induced draft design
provides sufficient draft to allow the advantage
of using a lower height chimney.
* A lower impact of prevailing wind on ACHE
performance: In the induced draft configuration,
the cross-sectional area of the fan ring is lower vs.
the bundle face area (which is almost 2.5 times the
fan area) in case of a forced draft ACHE. As such,
a relatively higher exit velocity results in a low impact
of prevailing wind on ACHE performance.
* Better protection of the tube bundles: The induced
draft design protects the tube bundles from heavy rain and
weather elements, with the plenum chamber on top of the
tube bundle. Therefore, a removable protective roof for
tube bundles is not required with an induced draft design.
Based on the overall advantage of operation, it was recommended
to select Option 2 with induced draft configuration for
all the three services.
Takeaway. Natural convection, in combination with forced or
induced draft, is preferred over emergency power configuration
for applications where an ACHE must be put into operation
quickly from standby mode due to process consideration. The
following points must be reviewed in detail before finalizing an
ACHE configuration:
* Consider the limited availability of performance
data of an ACHE under natural convection in process
industry applications. The various pros and cons of
the natural draft option with respect to other forms of
operation (e.g., operating the ACHE fan motors under
emergency power mode) must be reviewed during
the project definition phase before finalizing the
optimum mode of operation.
* The type of draft, whether forced or induced, to be
adopted in combination with the natural convection
mode should be decided based on the site condition
(e.g., prevailing wind direction, presence of heat sources
in the vicinity and annual rainfall) and specific customer
requirements.
* The limitations of available software for ACHE
performance simulation must be considered before
deciding whether a detailed CFD analysis will be
required to simulate the performance of the ACHE
under natural draft conditions.
NOTE
a Heat Transfer Research Institute's Xace
software
RITABRATA PRAMANIK has more than 35 yr of experience in
working with Engineers India Ltd., Bechtel, Foster Wheeler and
Fluor Daniel in the design and engineering of heat transfer
equipment in the oil and gas, petrochemical, LNG, fertilizer,
chemical and power sectors. Presently, he is working as
an independent consultant.
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Ron Beck
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Editor-in-Chief/Associate Publisher
Hydrocarbon Processing
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