Page 226 - Mechanical Engineers' Handbook (Volume 4)

3 Furnace Types 215

separators between them, with heat transferred from the inner cover to coil ends by a recir-
culating fan. To start the cooling cycle, the heating cover is removed by an overhead crane,
while atmosphere circulation by the base fan continues. Cooling may be enhanced by air-
blast cooling of the inner cover surface.
For heating heavy loads of other types, such as weldments, castings, or forgings, car
bottom furnaces may be used with some associated door maintenance problems. The furnace
hearth is a movable car, to allow load handling by an overhead traveling crane. In one type
of furnace, the door is suspended from a lifting mechanism. To avoid interference with an
overhead crane, and to achieve some economy in construction, the door may be mounted
on one end of the car and opened as the car is withdrawn. This arrangement may impose
some handicaps in access for loading and unloading.
Loads such as steel ingots can be heated in pit-type furnaces, preferably with units of
load separated to allow radiating heating from all sides except the bottom. Such a furnace
would have a cover displaced by a mechanical carriage and would have a compound metal
and refractory recuperator arrangement. Loads are handled by overhead crane equipped with
suitable gripping tongs.
Continuous-Type Furnaces
The simplest type of continuous furnace is the hearth-type pusher furnace. Pieces of rectan-
gular cross section are loaded side by side on a charge table and pushed through the furnace
by an external mechanism. In the design shown, the furnace is fired from one end, counter-
flow to load travel, and is discharged through a side door by an auxiliary pusher lined up
by the operator.
Furnace length is limited by thickness of the load and alignment of abutting edges, to
avoid buckling up from the hearth. A more complex design would provide multiple zone
firing above and below the hearth, with recuperative air preheating.
Long loads can be conveyed in the direction of their length in a roller-hearth-type
furnace. Loads can be bars, tubes, or plates of limited width, heated by direct firing, by
radiant tubes, or by electric-resistor-controlled atmosphere, and conveyed at uniform speed
or at alternating high and low speeds for quenching in line.
Sequential heat treatment can be accomplished with a series of chain or belt conveyors.
Small parts can be loaded through an atmosphere seal, heated in a controlled atmosphere on
a chain belt conveyor, discharged into an oil quench, and conveyed through a washer and
tempering furnace by a series of mesh belts without intermediate handling.
Except for pusher-type furnaces, continuous furnaces can be self-emptying. To secure
the same advantage in heating slabs or billets for rolling and to avoid scale loss during
interrupted operation, loads can be conveyed by a walking-beam mechanism. Such a walking-
beam-type slab heating furnace would have loads supported on water-cooled rails for over-
and underfiring, and would have an overhead recuperator.
Thin strip materials, joined in continuous strand form, can be conveyed horizontally or
the strands can be conveyed in a series of vertical passes by driven support rolls. Furnaces
of this type can be incorporated in continuous galvanizing lines.
Unit loads can be individually suspended from an overhead conveyor, through a slot in
the furnace roof, and can be quenched in line by lowering a section of the conveyor.
Small parts or bulk materials can be conveyed by a moving hearth, as in the rotary-
hearth-type or tunnel kiln furnace. For roasting or incineration of bulk materials, the shaft-
type furnace provides a simple and efficient system. Loads are charged through the open top
of the shaft and descend by gravity to a discharge feeder at the bottom. Combustion air can
be introduced at the bottom of the furnace and preheated by contact with the descending
load before entering the combustion zone, where fuel is introduced through sidewalls. Com-