Chemical Engineering February 2016 - 42

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1 2
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22
20
18
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3
ST
O O
O O
O O
Gas flow
4
6
8
10
12
14
ST/d
Distribution of external radiation to tubes
Figure 5. This chart shows
the distribution of external radiation
from furnace to heating
surfaces at the furnace
exit (Kn is fraction of direct
radiation absorbed by each
row and ST is the transverse
spacing of tubes)
will be absorbed by the superheater in the
first two to three rows facing the furnace,
thus increasing the tube wall temperature
in these rows. Typically four to five rows of
tubes absorb the complete external radiation,
as seen from the chart in Figure 5. This
adds to the heat flux inside the superheater
tubes. Typically the tube wall temperature
can go up by 20 to 40ºC due to external radiation;
at tube wall operating temperatures
close to 600-650ºC, even a 10ºC increase
can result in tube failure. At partial loads,
the increase in tube wall temperature will
be higher, as shown in some worked out
examples in ref. 1.
3.with uncertainty in the estimation of FEGT,
the direct radiation from furnace to the superheater
is also difficult to predict. while
evaluating the tube wall temperature in the
superheater, one has to add the heat flux
arising out of direct radiation, convection
and non-luminous radiation. The contribution
of direct radiation is a function of the
location of the tube facing the opening and
the tube spacing as shown in Figure 5. It
can be seen that when the tube spacing
to the convection bank is large, a large
amount of energy is sent to the next row
and so on. The radiant superheater also
absorbs more enthalpy at partial loads
compared to full load. At low loads, the
flow distribution on both gas and steam
side will be poor. Hence tube overheating
is likely at all loads [1].
4.The turning section where the superheater
is located adds to non-uniformity in gas
flow, velocity and temperature profiles
across the superheater, making it difficult
to predict its performance. A portion of
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the superheater can have cross flow and a
portion can have parallel flow and the fraction
of each can vary with load. Thus its
performance prediction becomes a guessing
game. One is taking chances with such
a location for the superheater.
5. If the flame shape is not properly set, the
flame can lick the first few rows of superheater
facing the furnace. Since the
heat-transfer coefficient is much lower
inside a superheater tube (about 6 to 10
times lower than that of the evaporator or
furnace tubes), the increase in tube wall
temperature can be high, leading to tube
failures.
6.On the other hand, the convective superheater
is located in a safe region far away
from the furnace outlet. The number of
screen rows (the function of screen tubes
is the same as that of the evaporator) can
be as high as 10 to even 40 rows depending
on steam temperature desired. There
are three advantages of using a screen
section. One, the gas temperature drops
significantly before entering the superheater
say by 250-500ºF (depending on
the number of screen rows) and the heat
flux inside the tubes is significantly lower
compared to the radiant design. The second
is that the fluegas mixes well beyond
the turn and becomes uniform before
entering the superheater due to the gas
pressure drop in the screen section, and
hence the prediction of superheater performance
is far more reliable, unlike the
situation for a radiant superheater. Thirdly,
the direct radiation contribution from the
furnace is also avoided as the screen or
evaporator tubes pick up the direct radiation
and since the screen or evaporator
tubes have a high-boiling heat-transfer
coefficient inside the tubes (exceeding
2,500 Btu/ft2hºF compared to 200-350
Btu/ft2hºF for the superheater), the evaporator
tube wall temperature increase is
not a concern at all.
7. A major advantage for the convective superheater
is its lower tube-wall temperature,
which results in a longer life compared
to the radiant superheater. This is
due to the lower gas-entering temperature
and absence of direct radiation. A simplistic
tube life calculation below illustrates this
point.
8.In heavy-oil-fired boilers, the slag formed
due to the melting of ash in fuel oil can
deposit on a radiant superheater, which
is operating at high temperature and can
cause high-temperature corrosion failures,
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No. of rows deep n
Kn
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Chemical Engineering February 2016

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