Page 114 - Wastewater Solids Incineration Systems

Combustion Technology 83

Out-hearths are provided with holes that allow cake solids that are rabbled from
the inside of the hearth to drop to the hearth below and gases to pass from the hearth
below. These holes are constructed with special refractory shapes that form the
opening. The inner ring is constructed with a lip to prevent cake solids from getting
between the hearth and the central shaft and to keep air from short-circuiting
between hearths. The central shaft has a hearth cap that fits over the lip to complete
the seal.
In-hearths are constructed with an annular space between the hearth and the cen-
tral shaft. This configuration allows cake solids that are rabbled inwards to discharge
to the hearth below and for the gases from the hearth below to enter the in-hearth.

9.3.2 Central Shaft and Rabble Arms
The central shaft, also known as the center shaft, supports the rabble arms at each
hearth. It rotates to provide mixing and movement of cake solids across the hearth
and to provide a duct for air that cools the shaft and rabble arms. The central shaft is
driven by a motor and gear reducer through a bevel gear mounted on the bottom of
the shaft and a pinion gear mounted on the driven end of the gear reducer. The vari-
able-speed drive allows central shaft speed to vary from 0.3 to 3 rpm. Several types
of variable drives are available, including electric drives with mechanical variable
sheaves, direct current electric motors, electric motors with variable-frequency
drives, and hydraulic drives.
The central shaft is constructed of a cast alloy metal (heat-resisting cast iron),
suitable for the temperature and environmental conditions within the furnace. It is
constructed in multiple flanged sections joined end-to-end with two annular rings.
The inner ring provides shaft cooling air; the outer provides heated (shaft return) air.
Each central shaft section contains four socket holes for mounting rabble arms. The
rabble arms are mounted in pairs and pinned to the shaft section. The holes are
plugged when there are less than four arms in a hearth. The lower part of the shaft is
provided with air inlet openings for the cooling air. A fixed air housing constructed
of steel with seals to prevent air leakage between the fixed housing and rotating shaft
is mounted over the openings (Figure 5.14). The shaft is supported by a lower thrust
bearing which is mounted on the floor. The top of the shaft is supported by the top
bearing; a fixed steel housing with seals is provided over the heated air outlet.
The rabble arms are constructed of a cast alloy metal (chrome-nickel cast stain-
less steel) suitable for the temperature and environmental conditions within the fur-
nace. Each rabble arm is hollow; an air tube allows shaft cooling air to enter the inner