Page 284 - Mechanical Engineers' Handbook (Volume 4)

15 Representative Heating Rates 273

Systems for collecting operating data from one or more furnaces, and transmitting the
data to a central recording or controlling station, may also be part of the responsibility of
the furnace supplier.

13 FURNACE CAPACITY

Factors limiting the heating capacity of industrial furnaces include building space limitations,
available fuel supplies, limited temperature of heat sources such as electric resistors or metal
radiant tubes, and limits on final load temperature differentials. Other factors under more
direct control by furnace designers are the choice between batch and continuous heating
cycles; time–temperature cycles to reach specified final load temperatures; fuel firing ar-
rangements; and control systems for furnace temperature, furnace pressure, and fuel/air ra-
tios. In addition, the skills and motivation of furnace operating personnel, as the result of
training, experience, and incentive policies, will directly affect furnace efficiency.

14 FURNACE TEMPERATURE PROFILES
Time–temperature patterns can be classified as uniform wall temperature (T ), uniform com-
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bustion gas temperature (T ), or variable T and T designed to secure the best combination
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g
of heating capacity and fuel efficiency.
In a batch-type furnace with fairly massive loads, the temperature control system can
be arranged to allow firing at the maximum burner capacity until a preset wall temperature
limit is reached, adjusting firing rate to hold that wall temperature, until load temperature
approaches the limit for the heated surface, and reducing the wall temperature setting to hold
maximum load temperature T while the minimum T reaches the desired level.
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In continuous furnaces, control systems have evolved from a single firing zone, usually
fired from the discharge end with flue gas vented from the load charge end, to two or three
zone firing arranged for counterflow relation between furnace loads and heating gases.
Progress from single to multiple zone firing has improved heating rates, by raising
furnace temperatures near the charge end, while increasing fuel demand by allowing higher
temperatures in flue gas leaving the preheat zone. Load temperature control has been im-
proved by allowing lower control temperatures in the final zone at the discharge end.
With multiple zone firing, the control system can be adjusted to approach the constant-
gas-temperature model, constant wall temperature, or a modified system in which both T g
and T vary with time and position. Because gas temperatures are difficult to measure di-
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rectly, the constant-gas-temperature pattern can be simulated by an equivalent wall temper-
ature profile. With increasing fuel costs, temperature settings in a three-zone furnace can be
arranged to discharge flue gases at or below the final load temperature, increasing the tem-
perature setting in the main firing zone to a level to provide an equilibrium wall and load
temperature, close to the desired final load temperature, during operating delays, and setting
a temperature in the final or soak zone slightly above the desired final load surface temper-
ature.

15 REPRESENTATIVE HEATING RATES
Heating times for various furnace loads, loading patterns, and time–temperature cycles can
be calculated from data on radiation and non-steady-state conduction. For preliminary esti-
mates, heating times for steel slabs to rolling temperatures, with a furnace temperature profile