Primary Air Fans. Primary air fans may be used on large pulverized coal fired
boilers in lieu of pulverizer blowers or exhausters. Primary air fans usually provide
both hot and cold air which can be tempered before being introduced into the
pulverizers. The cold air is atmospheric air supplied from the fan discharge. Part of
the fan discharge goes through a section of the air heater or separate air heater which
in turn raises the temperature to 500 degrees F (260 degrees C) or 600 degrees F (315
degrees C). The hot air is then ducted and tempered with the cold air to provide the
motive and drying air to the pulverizers at the proper temperature.
Overfire Air Fans. Overfire air fans are used on stoker fed coal fired
boilers to reduce smoke and to improve combustion efficiency by mixing with unburned
gases and smoke. The quantity of overfire air is usually between 5 and 15 percent of
the total air needed for combustion of the coal fuel. The pressure and volume of
overfire air must be sufficient to produce the proper turbulence for efficient burnup of
the unburned gases and suspended fuel particles. Fan size is determined by the boiler
manufacturer and furnished with the boiler.
Cinder Return Fans. Cinder return fans are used on some stoker fed coal fired
boilers for reinjection of fly ash from last pass hoppers and mechanical dust
collectors. Fan size is determined by the boiler manufacturer and furnished with the
For description and sizing of stacks, see MIL-HDBK-1003/6.
Blowdown Equipment. For information relative to boiler blowdown and blowdown
equipment, refer to MIL-HDBK-1003/6, Section 7, "Water Treatment".
Essential Plant Equipment
Steam Drive Auxiliaries. On coal stoker-fired installations, steam driven
boiler feed pumps, with total pumping capacity to suit the ultimate plant capacity, are
required to satisfy the ASME Boiler and Pressure Vessel Code (Section 1, Paragraph PG-
61) requirement of two means of feeding water. These pumps shall be primarily connected
to the boiler feed header from the deaerator and also to the treated water line for an
emergency water source for the boilers.
For design information and requirements needed to design
boiler plants, see Table 6.