Page 227 - Mechanical Engineers' Handbook (Volume 4)

216 Furnaces

bustion gases are then cooled by contact with the descending load, above the combustion
zone, to preheat the charge and reduce flue gas temperature.
With loads that tend to agglomerate under heat and pressure, as in some ore-roasting
operations, the rotary kiln may be preferable to the shaft-type furnace. The load is advanced
by rolling inside an inclined cylinder. Rotary kilns are in general use for sintering ceramic
materials.

Classification by Source of Heat
The classification of furnaces by source of heat is as follows:

Direct-firing with gas or oil fuels
Combustion of material in process, as by incineration with or without supplemental fuel
Internal heating by electrical resistance or induction in conductors, or dielectric heating
of nonconductors
Radiation from electric resistors or radiant tubes, in controlled atmospheres or under
vacuum

4 FURNACE CONSTRUCTION
The modern industrial furnace design has evolved from a rectangular or cylindrical enclosure,
built up of refractory shapes and held together by a structural steel binding. Combustion air
was drawn in through wall openings by furnace draft, and fuel was introduced through the
same openings without control of fuel/air ratios except by the judgment of the furnace
operator. Flue gases were exhausted through an adjacent stack to provide the required furnace
draft.
To reduce air infiltration or outward leakage of combustion gases, steel plate casings
have been added. Fuel economy has been improved by burner designs providing some control
of fuel/air ratios, and automatic controls have been added for furnace temperature and fur-
nace pressure. Completely sealed furnace enclosures may be required for controlled atmos-
phere operation, or where outward leakage of carbon monoxide could be an operating hazard.
With the steadily increasing costs of heat energy, wall structures are being improved to
reduce heat losses or heat demands for cyclic heating. The selection of furnace designs and
materials should be aimed at a minimum overall cost of construction, maintenance, and fuel
or power over a projected service life. Heat losses in existing furnaces can be reduced by
adding external insulation or rebuilding walls with materials of lower thermal conductivity.
To reduce losses from intermittent operation, the existing wall structure can be lined with a
material of low heat storage and low conductivity, to substantially reduce mean wall tem-
peratures for steady operation and cooling rates after interrupted firing.
Thermal expansion of furnace structures must be considered in design. Furnace walls
have been traditionally built up of prefired refractory shapes with bonded mortar joints.
Except for small furnaces, expansion joints will be required to accommodate thermal ex-
pansion. In sprung arches, lateral expansion can be accommodated by vertical displacement,
with longitudinal expansion taken care of by lateral slots at intervals in the length of the
furnace. Where expansion slots in furnace floors could be filled by scale, slag, or other
debris, they can be packed with a ceramic fiber that will remain resilient after repeated
heating.
Differential expansion of hotter and colder wall surfaces can cause an inward-bulging
effect. For stability in self-supporting walls, thickness must not be less than a critical fraction
of height.