TM 5-810-15
surface replaced the refractory setting. Casing,
therefore has less impact energy. Directional or
frequently 10 gage, was used to seal the refractory
straightening vanes should be used at bends in
placed adjacent to the furnace tubes and backed
ductwork to minimize turbulence or draft loss.
h. Desuperheaters. Normally on boilers with an
with block type insulation. This construction is still
in use on some small boilers applicable to this
outlet superheat temperature of no more than 750
manual. However; the products of combustion,
degrees F, desuperheaters will not be used. How-
particularly with coal fired boilers, will cause
ever, if the steam is used for a process at a lower
corrosion to take place and air leaks will develop
pressure and the temperature may be harmful or
when the corrosive (mainly sulfur) substances come
unwanted, a desuperheater can be installed in the
in contact with the relatively cool casing. The first
steam line to control the desired temperature. Wa-
signs of leakage will be the gases condensing and
ter for this device will normally be obtained from
dripping through the casing. This condition led the
the boiler feed pump or a separate pump. The
manufacturers to place the casing behind the
source of the water used by the desuperheater will
refractory and then insulating over the casing and
be such as deaerating heater and will be of the same
protecting the insulation with galvanized or
quality as used in the boiler. If a desuperheater is
aluminum lagging. However, the latest and to date
used and the discharge of the device is into the
best design is the use of welded wall construction.
superheater, the water and entrained impurities will
Welded-wall construction positively contains
be sprayed into the superheater tubes.
i. Fan blades and applications. Table 3-7 pro-
internal flue gas pressure by seal welding metal
plates between the tubes. Insulating materials cover
vides a summary of available fan blade types and
the outside of the welded-wall tubes. Lagging is
their respective applications. Individual fan types
then placed over the insulation.
are more fully described in paragraph 7-12 of this
(2) The advantage of the welded wall
manual. Items that must be identified for the design
construction currently being used by all major
of a particular fan application include: anticipated
boiler manufacturers is that it virtually eliminates
flow of air or combustion gas (pph), temperature of
the flue gas corrosion that has taken place on the
air or gas (degrees F), density of air or gas (pounds
per cubic foot, lb/ft3), fan inlet pressure (inches
boiler casing. Another advantage is that it reduces
air infiltration which in turn reduces exit gas
water gauge, in. wg), fan outlet pressure (in. wg),
temperature and fuel costs as well as the mainte-
and fan curves of applicable fan types.
nance costs that were involved in repair of the
refractory and insulation that previously existed.
Table 3-7. Fan Blades and Applications.
The design of boiler settings will include several
Fan Blade Type
Application
considerations. High temperature air and corrosive
gases will be safely contained. Air leakage will be
held to a minimum. Heat loss is reduced to an
Backware Inclined
Hot Primary Air (HPA)
acceptable level. Differential expansion of the com-
Cold Primary Air (CPA)
ponent parts will be provided. The surface temper-
Backward Curved
FD, ID, CPA, OFA, BF
ature should be such that it would not be a source
Hollow Air Foil
FD, OFA, ID, BF
Radial
HPA, OFA, ID, FTB
of hazard or discomfort to operating personnel. If
Open Radial
Pulverizer Exhauster
located outdoors, should be weatherproofed. The
Radial Tip
CPA, ID
probability of injury or plant damage in the event of
FD - Forced Draft.
an explosion will be reduced. The use of welded
ID - Induced Draft.
wall construction and its inherent strength is
HPA - Hot Primary Air.
probably the most imperative reason for the current
CPA - Cold Primary Air.
design of boiler settings.
OFA - Overfire Air.
g. Flue and ducts. Flues and ducts will be
BF - Booster Fan.
FTB - Fly Ash Transport.
designed to operate at the pressure and tempera-
ture to which they are subjected. As a general rule,
the following velocities will be used in arriving at
j. Fan inlet. The following guidelines apply to
the cross-sectional flow area of boiler flues and
the fan inlet design.
ducts. Cold air ducts--2000 to 2500 fpm. Hot air
(1) Intake areas will be at least 20 percent
ducts--3000 to 3500 fpm. Gas flues upstream of
greater than the fan wheel discharge areas.
particulate collection equipment--2,500 to 3,000.
(2) Fans positioned next to each other will
Gas flues--3500 to 4000 fpm. It should be noted
be separated by at least six fan diameters and a
that velocities can be higher at elevated tempera-
separation baffle.
tures because the air or gas is less dense and
3-24
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