MIL-HDBK-1003/6
Flue Gas Temperature Control. Automatic flue gas temperature
6.2.9
control with the sensing elements located at the baghouse inlet should be
provided to prevent flue gas temperatures from dropping below the dew point.
To elevate the flue gas temperature entering the baghouse to an acceptable
level, two methods are discussed in para. 6.2.9.1 and para. 6.2.9.2.
Flue Gas Bypass Duct. A flue gas bypass duct can be provided
6.2.9.1
around the economizer which is automatically controlled. The bypass duct
around the economizer should be sized for approximately one-third of the
maximum flue gas flow with the flue gas pressure drop equal to that of the
economizer. This method is capable of handling fairly low boiler turndown
conditions.
Water Bypass System. An automatic water bypass system can be used
6.2.9.2
to bypass water around the economizer to prevent the flue gas leaving the
economizer from dropping below its predetermined limit. This method has
potential problems at low loads. There is a possibility of steaming
conditions developing at low water flows through the economizer which can
cause water hammer damages, thus forcing the boiler out of operation. To
protect against a control system failure, an alarm should be provided to
indicate flue gas temperature drop below a preset value. This will allow time
for the operator to switch to manual operation prior to the flue gas
temperature dropping below the acid dew point.
Fabric Filter (Baghouse) Bypass. To prevent damage to fabric
6.2.9.3
filter bags from excessively high flue gas temperatures or low temperature
moist flue gas at start-up or shut down of boilers, an automatic flue gas
bypass system should be installed. Local environmental regulations may
require a waiver to permit this necessary feature. Dampers are to be rated
for minimum of 550 degrees F (287.5 degrees C) and shall be of the positive
seal type. Dampers are actuated by a signal from the temperature probe
sensing high or low temperature of air entering the baghouse. Dampers can
also be actuated by an oxygen probe sensing low oxygen in flue gas entering
the baghouse as additional protection to prevent combustible gases from
passing through the baghouse. Baghouse should be bypassed under oil firing
conditions.
Bypass duct around the baghouse shall be sized such that the flue
gas pressure drop through the bypass approximately equals the pressure drop
through baghouse under normal operating conditions.
Bypass dampers shall be of the zero leakage multilouver isolation
type (i.e. two sets of dampers with sealed air space within) or zero leakage
poppet type.
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