Page 272 - Mechanical Engineers' Handbook (Volume 4)

10 Burner and Control Equipment 261

10 BURNER AND CONTROL EQUIPMENT
With increasing costs of fuel and power, the fraction of furnace construction and maintenance
costs represented by burner and control equipment can be correspondingly increased. Burner
designs should be selected for better control of flame pattern over a wider range of turndown
and for complete combustion with a minimum excess air ratio over that range.
Furnace functions to be controlled, manually or automatically, include temperature, in-
ternal pressure, fuel/air ratio, and adjustment of firing rate to anticipated load changes. For
intermittent operation, or for a wide variation in required heating capacity, computer control
may be justified to anticipate required changes in temperature setting and firing rates, par-
ticularly in consecutive zones of continuous furnaces.

10.1 Burner Types
Burners for gas fuels will be selected for the desired degree of premixing of air and fuel, to
control flame pattern, and for the type of flame pattern, compact and directional, diffuse or
flat flame coverage of adjacent wall area. Burners for oil fuels, in addition, will need pro-
vision for atomization of fuel oil over the desired range of firing rates.
The simplest type of gas burner comprises an opening in a furnace wall, through which
combustion air is drawn by furnace draft, and a pipe nozzle to introduce fuel gas through
that opening. Flame pattern will be controlled by gas velocity at the nozzle and by excess
air ratio. Fuel/air ratio will be manually controlled for flame appearance by the judgment
of the operator, possibly supplemented by continuous or periodic flue gas analysis. In re-
generative furnaces, with firing ports serving alternately as exhaust flues, the open pipe burner
may be the only practical arrangement.
For one-way fired furnaces, with burner port areas and combustion air velocities subject
to control, fuel/air ratio control can be made automatic over a limited range of turndown
with several systems, including:

Mixing in venturi tube, with energy supplied by gas supply inducing atmospheric air.
Allows simplest piping system with gas available at high pressure, as from some
natural gas supplies.
Venturi mixer with energy from combustion air at intermediate pressure. Requires air
supply piping and distribution piping from mixing to burners.
With both combustion air and fuel gas available at intermediate pressures, pressure drops
through adjustable orifices can be matched or proportioned to hold desired flow ratios. For
more accurate control, operation of flow control valves can be by an external source of
energy.
Proportioning in venturi mixers depends on the conservation of momentum—the product
of flow rate and velocity or of orifice area and pressure drop. With increased back pressure
in the combustion chamber, fuel/air ratio will be increased for the high pressure gas inspi-
rator, or decreased with air pressure as the source of energy, unless the pressure of the
induced fluid is adjusted to the pressure in the combustion chamber.
The arrangement of a high-pressure gas inspirator system is illustrated in Fig. 40. Gas
enters the throat of the venturi mixer through a jet on the axis of the opening. Air is induced
through the surrounding area of the opening, and ratio control can be adjusted by varying
the air inlet opening by a movable shutter disk. A single inspirator can supply a number of
burners in one firing zone, or a single burner.