MIL-HDBK-1003/6
a) Coal Crushing Equipment. Crushers should be used in climates
where the breaking up of sizable frozen lumps is required or where run-of-mine
coal is used. Types of coal crushers commonly used in steam plants are single
roll, double roll, or Bradford breaker (rotating cylinder type). The main
drawback of belt feeders is the space required. A magnet should be installed
upstream of the coal crusher to remove tramp iron from the coal. A screen and
bypass should also be provided to bypass properly sized coal around the
crusher. If the crusher is not to be provided, space should be provided for
future installation.
b) Coal Feeders. Devices that deliver coal at a controlled rate
from a storage area are called feeders. Different types of feeders are used
for different applications in the plant. Simple belt feeders are used from
the unloading hoppers to the crusher or lump breaker, or to other conveyors.
Varying the speed of the driving roller controls the feed rate. The main
drawback of feeders is the space required. A second type of belt feeder,
using either a volumetric or gravimetric measuring system is used to measure
the amount of coal entering the boiler or pulverizer. Multi-blade rotor type
feeders are also used to provide a volumetric measurement of coal flow. Apron
feeders can convey up inclines to 25 degrees and are rugged and long lived.
These feeders, which are expensive and require more maintenance than belt
feeders, should only be used when the service is too rugged for a belt feeder.
c) Bucket Elevators. Bucket elevators are used extensively for
vertical lifting of coal. Those used most often today are the centrifugal-
discharge and continuous-bucket types. Centrifugal discharge elevators are
available to handle 5 to 60 tons (4 535 to 54 420 kg) of coal per hour. This
type elevator has spaced buckets which receive part of their load of coal by
scooping from the bottom of the elevator feed boot. The centrifugal elevator
is generally more economical in the 40 to 50 ton (36 280 to 45 350 kg) per
hour range because it has fewer buckets than the continuous discharge elevator
and a loading leg, with its deeper pit, is not required. Continuous bucket
elevators are available from 15 to 300 tons (13 605 to 272 100 kg) of coal per
hour and can be installed either in a vertical or inclined position. The
power requirements are approximately 10 percent less than required for a
centrifugal discharge elevator because they are loaded by a loading leg rather
than a scooping action. Continuous bucket elevators with double strand roller
chains should be used above 50 tons (45 350 kg) of coal per hour.
d) Belt Conveyors. Inclined belt conveyors can be used in lieu of
bucket elevators for vertical lifting of coal. Belt conveyors have unlimited
possible combinations, low operating cost, low power requirements, are simple,
quiet and reliable but normally require more space than required by an
elevator. The maximum slope of a belt conveyor, which is a function of coal
type and fines, is limited to 15 to 18 degrees for coal (27 degrees for wood
chips). A roughed belt conveyor will convey approximately twice the capacity
of a flat belt. When conveyors are routed across open spaces, galleries
should be incorporated into the bridge structure. See Figures 16, 17, and
Figure 18. Galleries will pay off in system reliability. They prevent ice
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