TM 5-810-15
more coarse ROM coal has been purchased for use
the high speed rotating hammers.
(3) Granulators. Granulators crush coal with
in the boiler plant. A separate crusher bypass chute
should be provided to divert coal around the
a slow, positive rolling action which produces a
crusher when properly sized coal is purchased and
granular product with a minimum amount of fines.
no crushing is required. The separate bypass chute
Power plants particularly choose this type of
is also advantageous when maintenance work has
crusher for its high reduction ratio and high
to be performed, requiring the shutdown or re-
capacity. The product size is externally adjusted by
moval of the crusher from the coal handling system.
changing the clearance between the case assembly
In boiler plants that have both pulverized coal and
and the ring hammers.
(4) Impactors. Impactors break the material
stoker fired boilers, all the coal supplied is sized to
suit the stokers, with coal going to pulverizers
by dropping it centrally into the path of the rotating
when required. This simplifies the storage and
hammers. The material then impacts against
handling facilities required for the system. The
breaker plates and rebounds back into the rotating
crusher will be selected to handle the hardest
hammers. A variety of product sizes can be attained
material that may be encountered in the system. A
by adjustment of the breaker plates. Impactors are
stationary or vibrating grizzly screen placed ahead
usually recommended for secondary and tertiary
of the crusher reduces the crusher size. This can be
crushing applications where high reduction ratios,
part of the feed chute to the crusher.
high capacity and a well shaped and graded product
b. Crusher sizing. Generally, crusher drive motor
are required.
sizing is 1/2 hp per tph for roll crushers,
granulators and hammermills and one hp per tph is
5-15.
Vibrating feeders.
used for sizing impactors.
a. General. Vibrating pan feeders consist of a
c. Coal reduction methods. Reducing coal to
pan or trough to which is imparted a vibrating
smaller size can be separated into the two catego-
motion so that material moves in a controlled flow.
ries of breaking and crushing. Breakers reduce raw
Feeders are instantly adjustable for capacity and
mine coal into a manageable size, while crushers
controllable from any near or remote point. Feeders
break the coal down to small manageable particles.
are normally positioned under track hopper
Rotary breakers are used to crush or size run-of-
openings, the bottom of a bin or under a storage
mine coal by gravity impact and are often used to
pile to induce and regulate the flow of material
clean debris from coal which has already been
onto a belt conveyor or other means of moving
sized. This type of breaker is usually used at the
coal. Vibrating feeders are used for handling prac-
mine site and not at the boiler plant location. When
tically all kinds of bulk materials, but will be
used at a boiler plant, a built in hammer mill is
avoided where the material has a tendency to stick
included.
to the pan. The feed rate in tons per hour of the
d. Crusher types.
feeder is a function of bulk density, material size,
(1) Roll crushers. Roll crushers compress
material angle of repose, angle of decline, fre-
the coal between a roll and a breaker plate. Teeth
quency of vibration, trough stroke and feeder
on the roll help to split the coal through impact and
length.
toward the bottom of the breaker plate, the teeth
b. Feeder design. Feeders will be the electro
shear the coal, which minimizes product fines and
mechanical type which employ easily adjustable,
reduces power demand. Adjustable clearance be-
rotating eccentric weights driven by a heavy duty
tween the breaker plate and roll determine the
electric motor which transmits power to the feeder
finished product size. The breaker plate is usually
pan through heavy duty springs, which in turn
spring loaded for protection against uncrushable
induces the material to flow. Electromagnetic type
debris and adjustable from outside the machine.
feeders will be avoided where possible. These type
Roll crushers are well suited for western coal due
of feeders have an extremely high noise level when
to control on minimum size of product.
installed in underground pits and tunnels, and have
(2) Hammermills. Hammermills break the
trouble meeting explosionproof atmosphere
coal by the impact of rotating hammers throwing
requirements.
the coal against breaker bars and then dragging the
(1) Each feeder will be designed and sized
coal against the screen bars. This type of crusher is
for nonflushing operation.
used as a primary reduction of dry or friable
(2) Hopper design and inlet arrangement to
material where uniform product size is required and
the vibrating feeder are very important in obtaining
large amounts of fines are not objectionable.
the required capacity and preventing overloading
Hammermill crushers must be provided with a vent
and choking of the pan. An adjustable depth
arrangement because of the air being displaced by
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