Chemical Engineering February 2016 - 45

like a tailor-made dress that fits perfectly.
designs of standard boilers were developed
several decades ago when there were
no emission regulations. As mentioned earlier
The FGr and excess air rates will vary
with type of fuel, combustion temperature,
nOx and CO emission limits, and hence,
two boilers with identical steam parameters
may differ in size depending on the total fluegas
quantity flowing through the unit. Standard
boiler suppliers have fewer options to
choose from and one has to carefully review
the efficiency, fan power consumption and
overall performance.
Convective versus radiant superheaters
In order to understand how boilers with radiant
versus convective superheaters differ in
performance, an example is now presented.
Design and operating parameters. A
boiler is designed to deliver 137,000 lb/h of
steam at 720 psig and 800ºF, with a feedwater
temperature of 230ºF. Fuel is natural
gas (90% methane, 5% ethane and 5% nitrogen).
Tables 1 and 2 show the tube geometry
that is used for both designs. The
radiant superheater is located at the turning
section as shown in Figure 4B, while
the convective superheater is located several
rows downstream of the screen section
(Figure 4C). The superheated steam
temperature is uncontrolled as variations
up to ±20°F are accepted by the plant.
Furnace dimensions are the same for both
options, namely 30-ft long, 8.5-ft high and
7-ft wide, and is a completely water-cooled
furnace. The same economizer is used in
both options. The only difference is in the
tube thickness of the superheater and the
number of rows of screen and evaporator.
Excess air used is 15% and no fluegas recirculation
is used in the basic design. The
same boiler is likely to operate with 20%
fluegas recirculation (FGr) later. The performance
for both designs was calculated
and the results are presented in Table 3
(design and off-design calculation methods
for superheaters and tube-wall temperature
are detailed in ref. 2).
Discussions on the two designs. The following
points may be noted from Table 3.
1.Though the steam temperature is nearly
the same at 100% load for both the radiant
and convective designs, the tube wall
temperature for the radiant design is about
1,085ºF versus 1,020ºF for the convective
design. This is due to the higher gas temperature
(nearly 400ºF) entering the radiant
superheater and with some contribution
Table 3. performanCe of boiler wiTh and wiThouT fGr
item
Excess air,%
Fluegas Recirculation rate,%
radiant
superheater
15
Steam
temp. at 100% load,±10ºF 807
SH tube wall temperature, ±10ºF 1,085
Gas temp to superheater, ±10ºF
Gas temp to evaporator, ±10ºF
2,379
1,897
Gas temp to economizer, ,±10ºF 795
Gas temp leaving eco, ,±10ºF
Efficiency, % HHV
300
Steam temperature@ 50% load
84.27
815
convective
superheater
15
812
1,020
1,980
1478
823
300
84.27
767
from direct radiation from the furnace. By
using a slightly higher tube thickness, the
same superheater material can be used for
both the radiant and convective design.
2.The life of the superheater is significantly
reduced due to an increase in tube wall
temperature. let us estimate the life assuming
the boiler operates at 100% load
all the time. Figure 7, showing the stress
versus larson miller Parameter (lmP), is
used to estimate the life [1].
First estimate the operating stress, which
is given by the relation: Stress = pressure ×
radius/thickness.
For the radiant superheater: Stress = 720
× 1/0.2 = 3,600 psi. From Figure 6, the lmP
corresponding to this value is 39. The absolute
tube-wall temperature is found by T = 1,085 +
460 = 1,545ºr. 39,000/1,545 = 25.24 = (20 +
log t) or t = 174,000 h or about 22 yr.
For the convective superheater: Stress =
720 × 1/0.15 = 4,800 psi. lmP = 38.3. T
= 1,020 + 460=1,480°r. 38,300/1,480 =
25.87 = (20 + log t) or t=743,000 h or more
than 92 yr.
Though this is a simplistic method of estimation
of potential lifespan, it shows that the
life of the convective design is much longer
than that of the radiant design, due to the
lower operating temperatures.
3.The surface area of the convective superheater
is much larger and hence costlier,
but considering the low operating tubewall
temperature, it is considered a better
design. The author has seen several
superheaters with radiant design fail in a
short period due to overheating. Frequent
repairs and shutdowns are expensive and
impacts the cost of business. As mentioned
earlier, there are non-uniformities
in gas temperatures and fluegas flows
entering the superheater, particularly if it
ChemiCal engineering www.Chemengonline.Com february 2016
45
radiant superheater
15
20
777
1,030
2,138
1,762
809
320
83.6
814
convective
superheater
15
20
800
990
1,828
1,420
834
324
83.5
785
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