Hydrocarbon Processing - October 2022 - 56G

Heat Transfer
jeopardy " when the motors
may fail to start, even with
emergency power?
4. If double jeopardy is indeed a
possibility, can the air cooler meet
the specified duty by operating
under natural convection?
A closer review of these conditions
and past experience with similar applications
have revealed that double jeopardy
is indeed a possibility. This not only
makes the use of emergency power for
operating ACHEs under emergency cooling
scenarios unacceptable, but will also
result in the emergency cooling case (under
natural-draft rather than forced-draft
with one of the four fans on) becoming
the control case. So, the emergency cooling
case will govern the overall size of the
ACHE, rather than normal operation.
Options-Natural draft with or without
a chimney. To size the ACHE for the
controlling case (i.e., the emergency cooling
scenario), the following two options
TABLE 2. Evaluation of various options
Service
No.
1
1
1
2
2
2
3
3
3
Option
Initial
1
2
Initial
1
2
Initial
1
2
Controlling
case
Normal
operation
(Note 1)
Emergency
cooling
(Note 2)
Normal
Operation
(Note 2)
Normal
operation
(Note 3)
Emergency
cooling
Emergency
cooling
Normal
operation
(Note 4)
Emergency
cooling
(Note 5)
Normal
operation
(Note 5)
Type of
draft
Forced
Natural
Natural
Forced
Natural
Natural
Forced
Natural
Natural
Bare area,
m2
591
4,968
555
176
5,433
184
851
2,726
912
Number
of bays
2
9
2
1
10
1
3
6
3
Number of
bundles per
bay of bundle
2
2
2
1
1
2
2
2
2
Number
of tube
rows
5
8
4
4
12
2
6
9
6
Tube
length,
m
11
11
11
11
11
11
11
11
11
Number of
fans per bay/
total fans
2 of 4
(Note 1)
-
Plot area,
m × m
11 x 10
54 x 10
-
3 of 3
(Note 3)
-
-
2 of 6
(Note 4)
-
27 x 11
14 x 11
39 x 11
7.8 x 11
12.5 x 11
12 x 10
3.6 x 11
Chimney
height, mm
Not
applicable
No
chimney
4,000
Not
applicable
No
chimney
7,000
Not
applicable
No
chimney
3,000
Note 1: For the emergency cooling case, only one out of four fans will be in operation and the fan motor will be on emergency power.
Note 2: Emergency cooling, with all fans off and without a chimney, is the controlling case. However, with all fans off and with a 4-m chimney, normal operation is the controlling case.
Note 3: For the emergency cooling case, only two out of three fans will be in operation and the fan motor will be on emergency power.
Note 4: For the emergency cooling case, only one out of six fans will be in operation and the fan motor will be on emergency power.
Note 5: Emergency cooling, with all fans off and without a chimney, is the controlling case. However, with a 3-m chimney, normal operation is the controlling case.
Hydrocarbon Processing | OCTOBER 2022 56G
were reviewed to arrive at the final configuration
of the ACHE. The simulation was
carried out using a proprietary softwarea
Option 1: Natural convection with.
out
a chimney. The heat transfer area can
be increased to manage the loss in overall
heat transfer effectiveness due to natural
convection by increasing the number of
bundles/bay and the number of bays over
and above those required for the normal
operation case. To increase the stack effect
and facilitate natural draft, increase the
number of tube rows, over and above those
required for the normal operation case.
Option 2: Natural convection with a
chimney. Increase the velocity of natural
convection over the ACHE bundles by
providing a chimney (stack) on top of the
bundle without increasing the number of
bundles and bays from those required to
cater to the normal operation case.
The results of the above two options, as
well as the initial option, are summarized
in TABLE 2. Since Option 2 requires less
plot space for all three services and a much
FIG. 1. An induced-draft ACHE with chimney.
Courtesy of Spiro-gills.
Type of draft: Forced or induced. The
chimney can be provided either on the top
of the tube bundles when fans (which are
required for normal operation) are below
the bundle (forced-draft), or on top of the
fan ring when fans are above the bundle
(induced-draft), as shown in FIG. 1.
The induced-draft option (with the
chimney on top of the fan rings) has the
lower surface area, Option 2 was preferred
over Option 1.

Hydrocarbon Processing - October 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - October 2022