Page 343 - Wastewater Solids Incineration Systems

304 Wastewater Solids Incineration Systems

blades that provide the pressure and shape of the curve. Blowers with more back-
wardly inclined blades have a flatter blower discharge pressure and flow curve. The
turndown on a blower is controlled by inlet valves or vanes and is typically approxi-
mately 60% of the rated blower capacity but may be limited by the fluidizing require-
ment of the fluid bed. The incinerator supplier may recommend a turndown of no
greater than 90% to ensure adequate fluidization.
Some fluid bed systems use the fluidizing blower as the only motive force in the
system and are sized for the pressure drop across the incinerator, heat recovery, air
pollution control, and stack. If a waste heat boiler is used, an induced draft fan is typ-
ically added to keep a negative pressure in the waste heat boiler.

3.1.2 Purge Air Blower
Purge air blower systems consist of positive-displacement blowers, intake filter
silencers, and a control system. Purge air blowers are capable of producing pressures
greater than 70 kPa (10 psig). The advantage of using a positive-displacement blower
for purge air is that they are able to produce constant airflow rates at varying oper-
ating pressures. Flowrates are changed by adjusting the speed of the blower, typically
through an adjustable-frequency drive.
Purge air blowers provide protection of equipment and are independent of the
fluidizing air system to ensure airflow in the event the blower is stopped. The air is
fed to the annular space in the fuel guns, high-pressure spray nozzles, sight glasses,
and at instrument locations to prevent thermal damage.

3.2 Fans
Fans serve the same function as blowers but develop less differential pressure (fans
are typically rated in terms of inches per water column and blowers in pounds per
square inch).

3.2.1 Combustion Air Fan
Combustion air fans are used to supply air to burners or to combustion air ports on
MHFs. Centershaft air fans supply air to cool the centershaft, which is subject to hot
temperatures and is often used as a source for combustion air in MHFs. Outside or
building air is often used as the air supply.

3.2.2 Induced Draft Fans
Induced draft fans are used to keep a negative pressure in the incinerator. The fans
are installed after the air pollution control system and before the stack. As a result,