Chemical Engineering October 2013 - 56

Engineering Practice
Velocity profile
fluegases flowing parallel to the drum
over the boiler bank tubes, the drum
baffling is designed as shown in Figure
2b. This will ensure that all the
risers are in the hot gas section and
downcomers are in the cooler gas section.
Also, this type of baffling ensures
that the tubes in any cross-section are
acting either as risers or downcomers.
In some large boilers, it may be necessary
to have the drum at a separate
location and feed the boiler evaporator
tubes using an external downcomer
system and collect the steam from the
evaporator using an external riser
system as shown in Figure 2c.
Figure 4 shows a waste-heat boiler
with external downcomers and risers.
The layout can vary depending on the
boiler design adopted.
In a typical boiler, the tubes perpendicular
to gas flow direction are symmetrically
baffled. However in this
boiler, by using this non-symmetrical
baffling system, a portion of the evaporator
tubes in the hot gas section in
the same cross-section are forced to be
downcomers, while some tubes in the
same cross-section are under the baffle
acting as risers. This was determined
to be the source of the problem.
The fluegas temperature in the first
row of downcomers is close to 1,400°F
and the intense bubbling inside these
tubes will force these tubes to act as
risers, even though they are baffled
as downcomers, due to the high heat
transfer rate and the lower density of
the steam-water mixture inside the
tubes. Hence, these tubes - though
not baffled - will still function as risers.
Flow will continue through these
tubes and circulation will be good.
In essence, these tubes have not
failed, even though they are in the hot
gas region. However, as the gas cools
in the next nine rows, the gas temperature
drops to about 1,200°F. This is
the region where tubes at the far end
of the furnace have failed.
These failed tubes have difficulty
acting as downcomers due to the
high gas temperatures; vapor formation
at the inlet to the tube will
still be intense and thus will prevent
the flow of water in the downcomers.
Also, the head available in the colder
end of the boiler is not high enough
to force the steam-water mixture
through the tubes, since they have
longer lengths and hence greater resistance
to flow. The thermal head at
the cool downcomer end is nearly the
same as in these hotter tubes, and
hence there is difficulty in forcing
the steam-water mixture through
these tubes.
Also, if you look at the typical gasvelocity
profile in any cross section, it
will be as shown in Figure 7, with the
tubes in the middle receiving higher
energy transfer compared to the end
tubes. Thus, the end tubes that have
failed do not generate sufficient steamwater
mixture for the cold downcomer
tubes at the rear to force the circulation
of the steam-water mixture through
these tubes. Simply put - these failed
tubes are neither acting as good risers
nor as good downcomers, and thus
a stagnant vapor mixture may have
formed in the tubes.
The vapor heat-transfer coefficient
is very small compared to the twophase
boiling coefficient and hence the
tube wall temperature will reach fluegas
temperature, as there is not much
of cooling inside these tubes due to the
stagnant column of vapor in this region.
Measurement of tube-wall temperatures
near the failed region also
confirmed the high temperatures.
The tubes within the same crosssection
in the riser section (under the
baffles and closer to the furnace) face
no problem, as the head available for
circulation is higher in baffled regions.
The normal water level in the drum is
typically at the drum center line and is
at a higher elevation than the baffled
section, so this ensures circulation.
Hence, these tubes near the furnace
act as risers. The mid-section is likely
to have a higher steam formation than
the ends due to the velocity profile and
higher heat flux and hence have lower
mixture density resulting in some circulation
through these tubes.
Solution. The failed tubes have difficulty
acting either as risers or downcomers
due to the skewed baffling inside
the drum. This is mainly due to
the fact that many so-called downcomers
are located in a very hot section.
Stagnation of the flow is suspected in
the failed tubes. If baffling inside the
drum is done as shown in Figure 2a,
the evaporator tubes under the baffles
56 CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2013
Gas flow direction
Maximum velocity is in the middle
and tapers off at the ends
FIGURE 7. Shown here is a typical gasvelocity
profile in a boiler bank crosssection.
Tubes in the mid-section usually
operate at higher heat flux compared to
end section
in the hot gas zone will be forced to act
as risers and the cooler unbaffled section
where the gas temperature drops
to less than 900°F will act as downcomers,
which will ensure good circulation
of the steam-water mixture
through the evaporator tubes. This solution
is currently being implemented
at the facility.
Final thoughts
Before buying a boiler plant, engineers
should review the boiler thermal performance,
including circulation issues.
Many plant engineers review information
such as construction details,
painting, duct thickness, structural
integrity, code documents and so on,
but few review the boiler's thermal
performance calculations, drum-baffling
system details or thermal performance
and circulation issues. However,
engineers should review these
things before buying a new boiler
plant, using either in-house expertise
or third-party consultants. Getting
a second opinion on thermal, process
and performance issues can help the
facility to purchase a better boiler and
thus avoid unnecessary plant shutdowns
and costly modifications later
- an important consideration, given
the fact that a boiler has a life of over
thirty years.
n
Edited by Suzanne Shelley
Author
Viswanathan Ganapathy
is a boiler consultant
from Chennai, India
(v_ganapathy@yahoo.com),
who specializes in thermal
design and performance aspects.
He has over 40 years
of experience in the thermal
design and performance aspects
of steam generators and
waste-heat boilers. He has
authored over 250 articles on
boiler-related subjects that have been published
in a number of U.S., Indian and U.K. magazines.
He has also authored several books and conducts
courses on boilers. He graduated from I.I.T Madras
with a degree in mechanical engineering.
http://WWW.CHE.COM
Chemical Engineering October 2013

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