decreases on decreased volume output and fan efficiency increases. Dampers are
closed during the startup of the boiler to reduce starting load on the motor.
b) Variable inlet vanes. These are used to change characteristic
curves of forced draft fans by providing swirl to the fan impeller. Variable
inlet vanes are slightly more effective in saving power than parallel blade
inlet box dampers. When furnishing variable inlet vanes for either
centrifugal of fixed-pitch axial fans, fan manufacturers provide a complete
performance envelope showing the effect of vane position on fan performance
and power. With fixed speed motors, power only slightly diminishes as air
volume is reduced. This system operates economically if air volume is 75
percent or above the design volume.
c) Two-speed motor drive. A centrifugal fan maintains its highest
level of efficiency at high loads. As the load drops the dampers close and
fan efficiency drops off rapidly. Two speed motors allow the fan to operate
at overall high efficiency with the dampers open at two running speeds. The
two speed motor is less expensive and easier to control than variable speed
Variable-speed motor drive. A variable speed gives an infinite
series of fan curves from which the points of highest system efficiency can be
chosen. A variable speed system significantly improves fan efficiency during
periods when the boiler is operating at less than its maximum load.
Additional Information. Fans and their usage are discussed in
Combustion Engineering Inc. Combustion/Fossil Power Systems, (1981),
Fan Drives. Fans are generally driven by either electric motor or
steam turbine. A steam turbine may be used when plant heat balance permits
and the exhaust steam is used for feedwater, space heating, or process. Dual
drive may be found necessary in the case of boiler startup.
Selection. Motors shall be selected for the maximum duty required
by the driven machine under most severe anticipated operating conditions.
Consider selection of high efficiency motors if service conditions can be met
and such motors are economically justified. For variable speed drive,
compatible controls must be available.
Power supply shall be designed to meet motor requirements under all
load conditions. Low voltage on A.C. induction motors results in the
following variations in design factors: