MIL-HDBK-1003/6
8.10.1
Types of Systems Available.
a)
On-Off controls,
b)
Single point positioning system,
c)
Parallel positioning system,
d)
Parallel metering system,
e)
Parallel metering system with oxygen trim,
f)
Steam flow/air flow metering system.
General Rules. The following general rules apply for determining
8.10.2
control system type to effect maximum energy conservation. Combustion units
larger than 5,000,000 Btu/hr (5 270 000 kJ/hr) input should have metering
combustion controls. Investigate energy savings. Combustion units larger
than 24,000,000 Btu/hr (7 034 kW) input should have oxygen trim to optimize
fuel usage. CO trim should be considered for larger boiler installations as
an adjunct to oxygen trim for increased efficiency, especially for coal
firing.
8.11
Variable Speed Drives
8.11.1
General. Variable speed drives can improve the operational
efficiency of: boiler forced draft fans, boiler induced draft fans and boiler
feedwater pumps at reduced loads. They conserve energy, electricity and
reduce real and reactive demand charges.
8.11.2
Multispeed Motor Drives
8.11.2.1
Two-speed Motor Drives. The dual speed is usually accomplished by
pole-changing motors or two winding motors. The pole-changing motor may be
used at 2:1 speed ratios. For other ratios such as 3:2 or 4:3, the two
winding motor is necessary. The fast speed drives the fan to meet maximum
boiler requirements. The slower speeds are for the reduced load requirements.
Fan control for intermediate loads is accomplished by vanes and dampers.
Wound-Rotor Motors. Wound-rotor motors have AC motors which
8.11.2.2
utilize slip rings and an adjustable impedance rotor to vary motor speed. The
speed of a wound-rotor motor is changed by changing the rotor or secondary
impedance. An increase in the rotor impedance reduces motor speed by
increasing rotor slip. The motor continues to draw full input power; the
excess power draw is lost as heat. Slip energy recovery packages can be
furnished to recover up to about 60 percent of the slip losses. This type of
speed control is generally not used because of its poor efficiencies. The
motors are not well suited for continuous operation at reduced speed.
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