Hydrocarbon Processing - April 2022 - 42

Heat Transfer
meet the specified duty by operating under
natural convection?
A closer review of the four conditions and the experience
of similar applications revealed that double jeopardy is indeed
a possibility. This will not only make the use of emergency
power for operating the ACHEs under emergency cooling
scenarios unacceptable, but will also result in the emergency
cooling case-under natural draft instead of forced draft with
1 of the 4 fans operating-becoming the controlling case. Instead
of normal operation, the emergency cooling case will
govern the overall size of the ACHE.
Final options-Natural draft with or without chimney.
To size the ACHE for the controlling case (i.e., emergency
cooling scenario) and to arrive at a final configuration of the
ACHE, the following two possible options were reviewed.
The simulation was carried out using standard commercial
heat transfer softwarea
.
Option 1-Natural convection without a chimney: Increase
the heat transfer area to take care of the loss in overall
heat transfer effectiveness-due to natural convection-by increasing
the number of bundles/bays and the number of bays
over and above those required for normal operation case. In
addition, increase the number of tube rows-over and above
those required for the normal operation case-to increase the
stack effect and facilitate natural draft.
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 cases.
The results of the two options, along with the initial option,
are summarized in TABLE 2. Since Option 2 requires less plot
space and lower surface area for all three services, it was preferred
over Option 1.
FIG. 1. Induced-draft ACHE with chimney. Photo courtesy of Spiro-Gills.
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
Operation1
Emergency
cooling2
Normal
operation2
Normal
operation3
Emergency
cooling
Emergency
cooling
Normal
operation4
Emergency
cooling5
Normal
operation5
1 For the emergency cooling case, only 1 of 4 fans will be in operation, and the fan motor will be on emergency power.
2 Emergency cooling, with all fans off and without a chimney, is the controlling case. However, with all fans off and with a 4,000-mm chimney, normal operation is the controlling case.
3 For the emergency cooling case, only 2 of 3 fans will be in operation, and the fan motor will be on emergency power.
4 For the emergency cooling case, only 1 of 6 fans will be in operation, and the fan motor will be on emergency power.
5 Emergency cooling, with all fans off and without a chimney, is the controlling case. However, with a 3,000-mm chimney, normal operation is the controlling one.
42 APRIL 2022 | HydrocarbonProcessing.com
Natural
Natural
2,726
912
6
3
2
2
9
6
11,000
11,000
-
-
27 × 11
14 × 11
Natural
Natural
Forced
5,433
184
851
10
1
3
1
2
2
12
2
6
11,000
11,000
11,000
-
-
2/64
39 × 11
7.8 × 11
12.5 × 11
Natural
Natural
Forced
4,968
555
176
9
2
1
2
2
1
8
4
4
11,000
11,000
11,000
-
-
3/33
54 × 10
12 × 10
3.6 × 11
Type of
draft
Forced
Bare
area, m2
591
No.
of bays
2
Type of draft: Forced or induced. The chimney can be located
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) (FIG. 1).
No. of
bundles
per bay
2
No. of
tube rows
5
Tube
length,
mm
11,000
No. of fans
per bay/
total fans
2/41
Plot area,
m x m
11 × 10
Chimney
height,
mm
Not
applicable
No chimney
4,000
Not
applicable
No chimney
7,000
Not
applicable
No chimney
3,000

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