Chemical Engineering February 2016 - 44

40
35
30
25
20
15
10
T22
32
33
34
Figure 7. This plot of stress
versus LMP for T11,T22 materials
is useful for predicting
the life of a superheater (T is
the temperature in °F, t the life
in h, and LMP is the Larson
Miller Parameter [1])
LMP parameter (T + 460) (20 + log t)/1,000
T11
35
36
37
38
39
40
24 streams are required to meet the velocity
and pressure drop considerations,
then there will be a baffle after the 4th row
in the inlet header (6 × 4 = 24 streams),
after the 8th row in the outlet header, after
the 12th row in the inlet header and so on.
Selection of streams for the superheater
is a very important task and many superheater
failures have been attributed to a
Section
Tube OD, in.
Thickness, in.
Tubes/row
Rows deep
Length, ft
Transverse pitch, in.
Longitudinal pitch
Streams
Material
Flow direction
Fins/in. × height × thickness
Surface area, ft2
113
855
4,072
Section
Tube OD, in
Thickness, in
Tubes/row
Rows deep
Length, ft
Transverse pitch, in
Longitudinal pitch
Streams
Material
Flow direction
Fins/in × height × thickness
Surface area, ft2
44
735
1,065
2,997
Superheater
2.0
0.15
12
24
8.5
4.8
4
24
Sa 178a
T22
Counter
Sa 178a
evaporator
2.0
0.120
12
53
9
5
4
Table 1. Tube GeomeTry of boiler wiTh radianT SuperheaTer
Screen
2.0
Superheater
2.0
0.120
12
2
9
5
4
Sa 178a
0.200
12
16
8.5
4.8
4
24
T22
Counter
Sa 178a
evaporator
2.0
0.120
12
72
9
5
4
0.120
16
12
15
4
4
8
Sa 178a
Counter
5 × 0.75 ×
0.05
16,417
Table 2. Tube GeomeTry of boiler wiTh ConveCTive SuperheaTer
Screen
2.0
0.120
12
13
9
5
4
0.12
16
12
15
4
4
8
Sa 178a
Counter
5 × 0.75 ×
0.05
16,417
economizer
2
economizer
2
wrong selection of this parameter [2].
Horizontal tube. In the horizontal-tube superheater
design, vertical headers are used;
the number of tubes in each row (or a multiple
of it) can be the number of streams.
Central inlet and exit nozzles are preferred
for steam inlet or exit.
A superheater configuration can be parallel-flow
or counter-flow or cross-flow and
with single or multi-stages with intermediate
desuperheating for steam temperature control.
Performance calculations will reveal the
best option for the superheater and are done
on a case-by-case basis, as illustrated in examples
in the references.
Custom designed boilers
The concept of custom-designed boilers
started evolving around the 1980s when
emissions of nOx and CO were regulated
and new boilers were required to adhere to
limits on these pollutants. Fluegas recirculation
was one method used to limit nOx
and excess air and low-nOx burners with
staged combustion were introduced. Even
today, several boiler suppliers offer what
are called " standard, off-the-shelf package
boilers " with low cost. However, a few
boiler suppliers also offer what are called
custom-designed boilers, which consider
the impact of fluegas flow on boiler performance
and optimize the design for lower
operating cost and better efficiency. This
is done by manipulating the boiler bank
spacing, length of evaporator tubes, superheater
location, number of screen
tubes, economizer fin optimization and so
on. Custom-designed units can lower fan
power consumption and also ensure long
superheater life. when the degree of steam
superheat is, say, only 5 to 10ºC, the superheater
may be located between the
evaporator and economizer. A great flexibility
thus exists in custom-designed boilers.
The author is even aware of a case where
a customer wanted only 20ºC of superheat
and the boiler supplier had no idea how to
handle such a low degree of superheat and
suggested to the plant to purchase a separately
fired superheater. This clearly shows
that not many boiler suppliers can customdesign
boilers for a difficult and challenging
boiler application.
Several examples of custom-designed
boiler applications that have created value
for end users may be seen in the references
cited. A standard off-the-shelf boiler is like a
ready-made garment, it may fit you well or
not, whereas a custom-designed boiler is
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Stress, × 1,000 psi
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Chemical Engineering February 2016

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