Charging Cranes

Two types of cranes are widely used for handling refuse for municipal incinerators. The most versatile is the bridge crane with a clam-shell bucket. The bridge itself travels across the length of the storage pit while a trolley moves the bucket over the length of the bridge. With the bridge crane, the storage pit can be as wide as 30 ft. If the storage pit is wide, the crane has to travel to the far side of the pit to keep refuse from accumulating there. The time required to traverse the pit affects the carrying capacity of the system and wide pits with long bridges are not economical. Figure 10.9.2 shows a layout using a bridge crane. In large furnaces of more than 300 tpd capacity, bridge cranes are used.

The second type of crane, the monorail, can move in one direction only, along the rail at the center line of the pit. The range of the monorail is limited in regard to the pit. The pit width is limited to about 1 m wider than the width of the open bucket. If the storage pit is too wide, the bucket cannot move to the sides of the pit, and the material accumulates because of its tendency to cling together. The monorail system is normally designed to follow a straight path with the pit at one end and to lift MSW into charging hoppers at the other. In a medium 100- to 300-tpd plant, the monorail is often used.

FIG. 10.9.2 Bridge crane installation.

Both crane types have either a clam-shell grapple or an abrasion-resistant steel bucket with a capacity between 30 and 150 cu ft (1-4 cum). An automatic lubrication system for the crane is recommended, and a good preventive maintenance program is essential. Spare buckets are also recommended. Bridges, trolleys, and hoists travel at speeds of 6 ft per sec (100 m per min).

The traveling bridge is also used to mix the MSW. Mixing MSW facilitates combustion particularly if a large amount of one type of waste is discharged into one part of the storage pit. In the past, the crane was operated from an air-conditioned cab mounted on the bridge. With increasing frequency, crane operation is being centralized in a control room, usually located at the charging floor elevation and either over the tipping positions opposite the charging hoppers or close to the charging hoppers.

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