MIL-HDBK-1003/6
Section 6:
POLLUTION CONTROL EQUIPMENT AND SYSTEMS
6.1.
Mechanical Cyclone Dust Collectors
6.1.1
Application. Multiple tube cyclone collectors are used to collect
particulate matter emissions from industrial boilers. They are used as
follows:
a) On boilers in low capacity installations where particulate
removal requirements of state and local codes can be met,
b) Protect a fabric filter from fire damage by eliminating glowing
char from the gas stream
c) Act as a prefilter ahead of a wet scrubber or an electrostatic
precipitator to remove large particles inexpensively.
Some pollution control equipment designs do not require upstream
mechanical collectors. Mechanical collectors are the minimum degree of
particulate removal required upstream of an induced draft fan.
Construction. The collector is composed of a number of fixed vane
6.1.2
cyclone tubes, generally 6 to 9 inches (152.4 to 228.6 mm) in diameter; the
tubes are arranged in a tube sheet within a housing having common inlet,
outlet and dust storage hoppers. Valley angle of hopper should be a minimum
of 55 degrees. An access door and poke hole (2-inch coupling with pipe plug)
are required per hopper.
Performance. This type collector can remove particles of 10
6.1.3
microns and larger at efficiencies up to 92 percent. Because efficiency
varies with the flue gas velocity, collection efficiency between 60 and 80
percent is common. Changes in operating conditions such as load swings (with
effects on gas volume, temperature and density) and changes in fuel size and
ash composition (with effects on particulate loading and dust characteristics)
can adversely affect performance.
The vortex in each cyclone tube separates solid particulate matter
such as dust or fly ash from the gas stream. The centrifugal force of the
vortex action causes the particulate to impact the tube wall and fall by
gravity into the hopper. The cleaned gas is discharged into an outlet plenum
duct. The separating operation is carried out without moving parts and
therefore requires only periodic inspection and repair. Sectionalizing the
mechanical collector and the inlet duct can maintain peak efficiencies at
predetermined load levels but corrosion problems can result in the unused
sections under the soot deposits.
Design Requirements. Size hopper(s) for an 8-hour collection
6.1.4
period at full load. Ash must never be allowed to fill hoppers up to the
cyclone tubes. High ash level will cause particulate bypass, erosion and
clogging of tubes. When firing wood chips or bark, hoppers must have ash
removed as fast as it is collected to prevent clinkers and heat damage from
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