Power Plant Engineering

FUELS AND COMBUSTION 225

part of the plant coal-handling system and are usually located in a crusher house at a convenient transfer
point in the coal-conveyor system.
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

Tr a p

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-