Page 275 - Mechanical Engineers' Handbook (Volume 4)

264 Furnaces

tank and pipe heating and for atomization. Otherwise, the tank and supply line can be
electrically heated, with atomization by high-pressure air.

10.2 Burner Ports

A major function of fuel burners is to maintain ignition over a wide range of demand and
in spite of lateral drafts at the burner opening. Ignition can be maintained at low velocities
by recirculation of hot products of combustion at the burner nozzle, as in the bunsen burner,
but stability of ignition is limited to low port velocities for both the entering fuel/air mixture
and for lateral drafts at the point of ignition. Combustion of a fuel/air mixture can be
catalyzed by contact with a hot refractory surface. A primary function of burner ports is to
supply that source of ignition. Where combustion of a completely mixed source of fuel and
air is substantially completed in the burner port, the process is identified as ‘‘surface com-
bustion.’’ Ignition by contact with hot refractory is also effective in flat flame burners, where
the combustion air supply enters the furnace with a spinning motion and maintains contact
with the surrounding wall.
Burner port velocities for various types of gas burners can vary from 3000 to 13,000
2
lb/hr ft , depending on the desired flame pattern and luminosity. Some smaller sizes of
burners are preassembled with refractory port blocks.

10.3 Combustion Control Equipment
Furnace temperature can be measured by a bimetallic thermocouple inserted through the wall
or by an optical sensing of radiation from furnace walls and products of combustion. In
either case, an electrical impulse is translated into a temperature measurement by a suitable
instrument and the result indicated by a visible signal and optionally recorded on a moving
chart. For automatic temperature control, the instrument reading is compared to a preset
target temperature, and the fuel and air supply adjusted to match through a power-operated
valve system.
Control may be on–off, between high and low limits; three position, with high, normal,
and off valve openings; or proportional with input varying with demand over the full range
of control. The complexity and cost of the system will, in general, vary in the same sequence.
Because combustion systems have a lower limit of input for proper burner operation or fuel/
air ratio control, the proportioning temperature control system may cut off fuel input when
it drops to that limit.
Fuel/air ratios may be controlled at individual burners by venturi mixers or in multiple
burner firing zones by similar mixing stations. To avoid back firing in burner manifolds, the
pressures of air and gas supplies can be proportioned to provide the proper ratio of fuel and
air delivered to individual burners through separate piping. Even though the desired fuel/air
ratio can be maintained for the total input to a multiple burner firing zone, errors in distri-
bution can result in excess air or fuel being supplied to individual burners. The design of
distribution piping, downstream from ratio control valves, will control delayed combustion
of excess fuel and air from individual burners.
In batch-type furnaces for interrupted heating cycles, it may be advantageous to transfer
temperature control from furnace temperature to load temperature as load temperature ap-
proaches the desired level, in order to take advantage of higher furnace temperatures in the
earlier part of the heating cycle. An example is a furnace for annealing steel strip coils.
Because heat flow through coil laminations is a fraction of that parallel to the axis of the
coil, coils may be stacked vertically with open coil separators between them, to provide for