Page 276 - Mechanical Engineers' Handbook (Volume 4)

10 Burner and Control Equipment 265

heat transfer from recirculated furnace atmosphere to the end surfaces of coils. For bright
annealing, the furnace atmosphere will be nonoxidizing, and the load will be enclosed in an
inner cover during heating and cooling, with the atmosphere recirculated by a centrifugal
fan in the load support base, to transfer heat from the inner cover to end faces of coils. There
will also be some radiation heat transfer from the inner cover to the cylindrical surface of
the coil stack.
Inner covers are usually constructed of heat-resisting alloy, with permissible operating
temperatures well above the desired final load temperature. A preferred design provides for
initial control of furnace inside wall temperature from a thermocouple inserted through the
furnace wall, with control switched to a couple in the support base, in control with the
bottom of the coil stack, after load temperature reaches a present level below the desired
final temperature.
To avoid leakage of combustion gases outward through furnace walls, with possible
overheating of the steel enclosure, or infiltration of cold air that could cause nonuniform
wall temperatures, control of internal furnace pressure to slightly above ambient is desirable.
This can be accomplished by an automatic damper in the outlet flue, adjusted to hold the
desired pressure at the selected point in the furnace enclosure. In furnaces with door openings
at either end, the point of measurement should be close to hearth level near the discharge
end. A practical furnace pressure will be 0.01–0.05 in. H O.
2
With recuperative or regenerative firing systems, the preferred location of the control
damper will be between the waste-heat recovery system and the stack, to operate at minimum
temperature. In high-temperature furnaces without waste-heat recovery, a water-cooled
damper may be needed.
With combustion air preheated before distribution to several firing zones, the ratio con-
trol system for each zone will need adjustment to entering air temperature. However, if each
firing zone has a separate waste-heat recovery system, the zone air supply can be measured
before preheating to maintain the balance with fuel input.
The diagram of a combustion control system in Fig. 43 shows how these control func-
tions can be interlocked with the required instrumentation.
For automatic furnace pressure control to be effective, it should be used in combination
with proportioning-type temperature control. With on–off control, for example, the control
of furnace pressure at zero firing rate cannot be accomplished by damper adjustment, and
with a continuous variation in firing rate between maximum and minimum limits, or between
maximum and off, the adjustment of damper position to sudden changes in firing rate will
involve a time-lag factor that can make control ineffective.
An important function of a furnace control system is to guard against safety hazards,
such as explosions, fires, and personal injury. Requirements have been well defined in codes
issued by industrial insurers, and include provision for continuous ignition of burners in low-
temperature furnaces, purging of atmosphere furnaces and combustion of hydrogen or carbon
monoxide in effluent atmospheres, and protection of operating personnel from injury by
burning, mechanical contact, electrical shock, poisoning by inhalation of toxic gases, or
asphyxiation. Plants with extensive furnace operation should have a safety engineering staff
to supervise selection, installation, and maintenance of safety hazard controls and to coor-
dinate the instruction of operating personnel in their use.

10.4 Air Pollution Control
A new and increasing responsibility of furnace designers and operators is to provide controls
for toxic, combustible, or particulate materials in furnace flue gases, to meet federal or local