Power Plant Engineering

Crushers. Although there are several types of commercially available coal crushers, a few stand
Dut for particular uses. To prepare coal for pulverization, the ring crusher, or granulator (Fig. 7.3) and
the hammer mill (Fig. 7.4) are preferred. The coal is fed at the top and is crushed by the action of rings
that pivot off center on a rotor or by swinging hammers attached to it. Adjustable screen bars determine
the maximum size of the discharged coal. Wood and other foreign material is also crushed, but a trap is
usually provided to collect tramp iron (metal and other hard-to-crush matter.) Ring crushers and hammer
mills are used off or on plant site. They reduce run-of-mine coals down to sizes such as 3/4 × 0 in. Thus
they discharge a large amount of fines suitable for further pulverization, but not for cyclone-furnace
firing. For the latter, a crusher type called the reversible hammer mill is preferred.
Raw
Coal
Loose
Ring
Tramp-
Metal
Trap
Cleanout
Opening
Access
Door
Screen
(Bars)
Adjustable
Plate
Fig. 7.3. Ring Crushers.
Adjustable
Adjustable
Fig. 7.3. Hammer Hill.
A third type, the Bradford breaker (Fig. 7.5), is used for large-capacity work. It is composed of a
large cylinder consisting of perforated steel or screen plates to which lifting shelves are attached on the
inside. The cylinder rotating slowly at about 2f r/min receives feed at one end. The shelves lift the coal,
and the breaking action io accomplished by the repeated dropping of the coal until its size permits it to
be discharged through the perforations, whose size determines the size of the discharge coal. The quan-

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POWER PLANT ENGINEERING
tity of fines is limited because the crushing force, due to gravity is not large. Bradford breakers easily
reject foreign matter and produce relatively uniform, size coals. They are usually used at the mine but
may also be used at the plant.
Other simple devices called roll crushers, which have single or double rolls or rotors equipped
with teeth, have been used but have not proven very satisfactory. because of their inability to produce
coal of uniform size.
Casting
Perforated
Plate
Litter