Chemical Engineering March 2016 - 66

Table 4. HeaT excHanger geomeTry as Per mds and Vendor drawings
As per MDS
1
2
3
4
5
6
TEMA type
Shell inside diameter
Overdesign
Allowable pessure drop (shellside /
tubeside)
Calculated pressure drop (shellside
/ tubeside)
Tube information:
Tube outside diameter
Tube wall thickness
Tube length
Layout angle
Tube pitch
Tube count
Tube passes
7
Baffle information
Type
Orientation
Number
Central spacing
Inlet spacing
Outlet spacing
Baffle cut
8
9
10
Total tube sheet thickness
Full support plate at floating head
Support plate to head distance
mm
mm
mm
%
mm
mm
* From a fabrication point of view, it
was not possible to accommodate
808 tubes (the number of tubes
proposed in the preliminary pdS)
with the given tube layout and
within a shell of diameter 920 mm.
only 792 tubes were possible. Figure
4 illustrates the sixteen tubes
(black shaded) that could not be
accommodated from the mechanical
design point of view.
* in the preliminary pdS, the tube
bundle-to-shell clearance was
specified as 31.3 mm. the mechanical
engineer recommended a
figure of 44.0 mm or higher.
* the total tubesheet thickness in
the preliminary pdS was 68.0 mm
based on a preliminary assumption.
the mechanical engineer calculated
it as 104.0 mm. tubesheet
thickness affects the effective
tube length. hence the tubesheet
thickness recommended by the
mechanical engineer needs to
be considered for thermal performance
check.
Design step 3 (final PDS). based
on the feedback from the mechanical
design check, the tube count
was reduced to 792. in addition, the
66
mm
mm
m
deg
mm
19.05
2.11
6
90
25.4
848
4
Single segmental
Parallel
16
316
461
588
27
104
Provided
107
Units
mm
%
bar
bar
AES
960
7.8
0.3 / 0.7
0.26 / 0.64
TEMA type
Shell inside diameter
Overdesign
Allowable pressure drop (shellside / tubeside)
Calculated pressure drop (shellside / tube-sid
e)
Tube information:
Tube outside diameter
Tube wall thickness
Tube length
Layout angle
Tube pitch
Tube count
Tube passes
Baffle information
Type
Orientation
Number
Central spacing
Inlet spacing
Outlet spacing
Baffle cut
Total tube sheet thickness
Full support plate at floating head
Support plate to head distance
bundle-to-shell clearance was increased
from 31.3 mm to 44.3 mm.
the tubesheet thickness was also
increased to 104.0 mm.
a thermal calculation was carried
out with the above changes. this resulted
in a heat-exchanger surface
area of 275 m2 and an overdesign
(based on surface area) of 2.8%. this
overdesign of 2.8% is rather low.
in order to arrive at a reasonable
overdesign, the option was to increase
the shell diameter, keeping the
tube length unchanged. the shell diameter
was therefore increased from
920 mm to 960 mm, while keeping
the tubes 6 m in length (a tube length
of 6 m is a standard length and is
also the preferred tube length by
most users). the number of tubes
was now 848. after incorporating
the above mentioned changes, we
arrived at an overdesign of 7.8%.
table 3 illustrates the revised heat
exchanger geometry based on the
final pdS. Figure 5 illustrates the revised
tube layout.
the final pdS, after incorporation
of the above numbers, was then released
for mechanical design.
Design step 4 (final mechanimm
mm
mm
%
mm
mm
mm
mm
m
d
eg
mm
19.05
2.11
6
90
25.4
848
4
Single segmental
Parallel
16
305
550
506
26
104
Provided
259
cal design and MDS). based on
the final pdS along with the corresponding
tube layout drawing
(Figure 5), the mechanical engineer
carried out the final mechanical design.
the results of this mechanical
design were reproduced in the form
of an mdS. most of the revised parameters
in the final pdS were incorporated
in the mdS. the baffle
spacings were, however, slightly
adjusted. the mdS typically consists
of the following information:
* elevation drawing
* tube layout drawing
* tubesheet thickness, girth flange
thickness, thickness of shell and
bonnet
* key plan
* design data
* nozzle data
* material of construction
the mdS was then submitted to
the vendor for generating the vendor
drawings.
Design step 5 (vendor drawings).
the vendor provided a number of
drawings of the heat exchanger.
certain parameters have an impact
on the thermal performance of the
heat exchanger, hence the relevant
ChemiCal engineering www.Chemengonline.Com marCh 2016
As per vendor drawing
Units
mm
%
bar
bar
AES
960
5.9
0.3 / 0.7
0.28 / 0.65
http://www.Chemengonline.Com
Chemical Engineering March 2016

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