54    SIDE OUTLET RECYCLE FOR MULTIPLE HEARTH
SEWAGE SLUDGE INCINERATORS
Charles A. Richmond, Incinerator Specialist
BBS Corporation
Columbus, Ohio 43229
CONVENTIONAL MULTIPLE HEARTH FURNACE
Traditionally, Multiple Hearth Furnace manufacturers have written about the virtues of the Multiple Hearth Furnaces as follows (Figure 1).
"The Multiple Hearth Furnace consists of a circular steel shell surrounding a number of vertically
stacked chambers (hearths) and a central rotating shaft to which rabble arms are attached. The rabble
arms have rabble teeth attached to their bottom surfaces. The furnaces are furnished in a number of
diameters 6' OD to 25'-9" OD and number of hearths (from four to sixteen).
In a MHF with a ODD number of hearths, the top hearth is an outfeed hearth, that is the sludge
enters the center and moves outward to a series of peripheral holes where it falls through to the next
hearth which is an infeed hearth.
In a MHF with an EVEN number of hearths, the sludge enters the top hearth near the wall of the
hearth. The rabble arms and teeth move the sludge inward to the center drop hole where it falls to
hearth #2 where it is moved outward to the peripheral drop holes.
The number of rabble arms per hearth, the size, spacing and number of rabble teeth on each arm
move the sludge into an archimedes spiral (Figure 2) creating a number of rows of material. The
surface area of the rows is dependent upon the angle of repose of the sludge, thus area may be more
than 125% of the flat area of the hearth.
Sludge ignited in the center section of the MHF generates hot gases which travel counter-current to
the downward flow of the solids. In so doing, that heat energy contained in the hot gases evaporate
the moisture in the sludge, and in giving up the heat, the gases cool down. Because of the moisture
content, the sludge stays at a temperature between 160-200°F.
The exit temperature of the hot gases at the top of the MHF will generally be 700-900° F. Because
of the moisture content of the sludge, the low solids temperatures of 160-200 will insure that no
odorous materials exit the gas outlet of the MHF.
The MHF has three basic zones:
1. Two or more upper hearths on which a major portion of the free moisture is
evaporated.
2. Two or more intermediate hearths on which sludge burns, producing temperatures in
excess of 1,400°F.
3. A bottom hearth that serves as an ash cooling hearth and in which preheated air from
the air cooling hearth and in which preheated air from the air cooling of the center shaft
is admitted to form a portion of the required combustion air. The hot ash is cooled and
the preheated air is further heated from the hot ash.
The instantaneous position of the zones within the MHF vary with the moisture content of the
sludge and the combustible content, along with the higher heating value of the combustible. As these
factors vary during operation, the zones naturally and automatically adjust themselves for the variation and the fire moves up or down. This inherent ability makes for simple and efficient design.
The center shaft consists of two concentric shafts. Cool air is blown up the center tube out into the
rabble arms to cool them and returned to the annulus between the outer and inner tubes. This provides
cooling to the rabble arms and the heat removed is returned to the MHF as preheated sludge
combustion air.
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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