POWER February 2011 - 22

5. Tightly integrated. One unique design feature of this package boiler was the elevated
steam drum. The boiler shipping envelope is not constrained by the drum size, as headers are
used for the evaporator module. This figure illustrates the relationship between the external
drum, downcomer, and riser piping. Courtesy: Cleaver-Brooks Engineered Boiler Division
6. More efficient wall design. The
water-cooled D-type boiler furnace uses a
leakproof membrane wall. The arrangement
is shown without insulation during shop assembly.
Courtesy: Cleaver-Brooks Engineered
Boiler Division
7. Increase boiler efficiency. A recirculating glycol system moves heat from the exhaust
gas to heat combustion air, which improves boiler efficiency. Courtesy: Cleaver-Brooks Engineered
Boiler Division
Flue gas recirculation
To stack. Flue
gas temperature
maintained at 100C.
Glycol flow
managed based on
boiler load
Scavenger
Glycol pump
Glycol flow through
air heater #2
managed to optimize
heat recovery
Economizer
gas outlet
Glycol flow through
air heater #1
managed to maintain
the air temperature at
fan inlet between 5C
and 40C
60% glycol water mixture
single unit and was the largest that could
be shipped by land in U.S. and Canada.
Headers were used for the convection bank
instead of steam and mud drums, which
are commonly used in such boilers. The
steam drum was connected to the boiler
bank by a system of external downcomer
and riser piping. The only items requiring
field assembly were the drum and downcomer/riser
piping (Figure 5).
Using this design and assembly approach,
the drum size was not limiting the
shipping dimensions and the additional
space obtained was used to increase the
boiler furnace and convection bank dimensions.
This in turn helped to increase
the furnace effective projected area and
volume and, hence, lower the area and
volumetric heat release rates as well as
reduce the furnace back pressure (Table 1,
22
p. 20). It also was possible to make the
steam drum larger (hold-up time from normal
level to empty exceeds 5.5 minutes)
because there were no shipping limitations
for the drum.
A dual low-NOx
burner with flue gas
recirculation provision along with a completely
water-cooled furnace was used to
meet Canadian NOx
emission standards
(Figure 6). Typical burner front wall design
in these boilers has some refractory behind
the furnace tubes, which reradiates energy
back to the flame. That, in turn, increases
local combustion temperatures and generates
additional NOx
. The membrane-walled
front wall where the burner is mounted is
completely water-cooled and, as a result,
cools the flame better. The completely
water-cooled furnace design has a higher
effective cooling area for a given volume
www.powermag.com
Elevated Steam Drum Design
The first unique design feature on this
package boiler was the elevated steam
drum. The boiler shipping envelope is not
constrained by the drum size, as headers
are used for the evaporator module. Furnace
heat flux evaluation was performed
based on inputs from the burner supplier,
as the flame temperature varies along the
flame path, affecting the local heat flux
and steam generation. Heat flux and cirPOWER
| February 2011
Natcom
burner
FD fan
Air heater
#2
and lowers the heat flux to the steam/water
mixture by about 12%.
In addition to reducing reradiation from
refractory to the flame base, and therefore
reducing NOx
formation, other advantages
of completely water-cooled designs include
these:
Air heater
#1
Ambient air
-40C to 27C
■ There is lower heat flux for a given
volume, about 9% to 12%, due to the
higher effective area for a given furnace
volume.
■ The lower area heat release rate results
in lower NOx
production.
■ Lower excess air or flue gas recirculation
rates may be used due to a lessintense
combustion process compared
with a refractory-lined furnace.
■ The leakproof furnace membrane wall
prevents leakage of gases to the second
pass, which can increase CO formation
and reduce boiler efficiency.
■ No refractory is used, so boiler start-up
rates can be faster and refractory maintenance
is eliminated.
■ Minimum thermal stresses are experienced
between casing and tubes during
start-up or shutdown as the entire
enclosure is at a constant temperature
equal to the saturation temperature of
steam.

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POWER February 2011

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