Hydrocarbon Processing - April 2022 - 41

Heat Transfer
R. PRAMANIK, Consultant, New Delhi, India
Air-cooled heat exchanger operation
under emergency cooling service
One of the challenges in the operation of air cooled heat
exchangers (ACHEs) is to decide whether or not to operate
under natural convection. There is limited information
available for the simulation of an ACHE with no fans under
operation. Under emergency conditions, such as fan failures
or when bringing the ACHEs onstream from standby conditions
to meet emergency cooling requirements, with none of
the fans operating, the use of natural convection becomes unavoidable.
This article discusses case studies to evaluate the
pros and cons of both options:
1. Operating with fans running on emergency power
2. Operating in natural convection with none of the fans
operating to meet the emergency duty conditions.
ACHE under natural convection. Simulation of ACHE
performance under natural convection has the following limitations:
*
Non-availability of sufficient experimental data:
Most of the available experimental data based on wind
tunnel tests are applicable for the large natural draft
cooling towers with high natural draft velocity and are
not applicable for ACHEs in process industries with
relatively smaller sizes and lower natural draft velocity.
* Impact of structures, heat source, local wind
velocity and ground effect: All these factors affect
the natural draft by impacting the velocity of air at
the exit of the ACHE. None of the available thermal
design software considers these parameters in sizing
an ACHE. Therefore, a detailed computation fluid
dynamic (CFD) analysis, with its associated impact
on project cost and time, is required to properly
simulate the impact of natural convection.
1
1
1
2
2
3
3
Normal operation
Emergency cooling
Emergency blowdown
Normal operation
Emergency cooling
Normal operation
Emergency cooling
Fans in operation
All
1 out of 4
1 out of 4
All
2 out of 3
All
1 out of 6
* Effect of chimney (stack) on the performance:
It has been established that the use of a chimney
(stack) on top of the air cooler bundle (for forced
draft unit) and on top of the fan ring (for induced
draft unit) enhances the effectiveness of heat transfer
due to natural convection. However, it is still a work
in progress as far as the exact relationship of chimney
height with natural draft performance is concerned.
Initial proposition: Avoid natural convection and use
emergency power. In view of the challenges highlighted in
simulating ACHE performance under natural convection,
three services were proposed to operate the ACHEs during
emergency conditions with the fans running on emergency
power (TABLE 1). This operation allows achieving higher airside
velocity, resulting in a higher mean temperature difference
and higher airside heat transfer coefficient. As a result, the capital
cost is reduced by decreasing the number of bays and/or
heat transfer surface area vs. the natural draft option.
How dependable is emergency power? For emergency
cooling scenarios, the likelihood of fan motor failure at startup
was reviewed for the following four conditions:
1. Will the emergency cooling scenario with all fans
off be the governing case?
2. If the above is true, can emergency cooling be met
with the help of emergency power, which is being
provided as backup power for the process unit,
to power the fan motors during startup?
3. Will there be a case of double jeopardy when the
motors may fail to start even with emergency power?
4. If double jeopardy is a possibility, can the air cooler
TABLE 1. Three service cases for ACHE usage during emergency conditions
Service # Case of operation
Cooling mechanism
Forced draft
Forced draft
Forced draft
Forced draft
Forced draft
Forced draft
Forced draft
Remarks
Controlling case: Fans on normal power
Alternate case: Fan on emergency power
Alternate case: Fan on emergency power
Controlling case: Fans on normal power
Alternate case: Fan on emergency power
Controlling case: Fans on normal power
Alternate case: Fan on emergency power
Hydrocarbon Processing | APRIL 2022 41
Hydrocarbon Processing - April 2022

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