1-10 Description – Combustion Control System
FlexFit – Linkageless Control – Revision 1.0
PREFERRED
UTILITIES MFG CORPORATION
DD
EE
SS
CC
RR
II
PP
TT
II
OO
NN
• Oxygen Trim – compares the oxygen levels in the ue gas to a value determined during commissioning and will implement
control actions to correct the error
• Draft Control – maintains a constant furnace or boiler outlet draft pressure (positive or negative) as the boiler ring rate
modulates from low to high re and as outdoor weather conditions change the draft effect
Parallel Positioning
In this system, both the fuel valve(s) and the air damper each have their own servo(s). Each servo is equipped with a position
feedback potentiometer. Unlike a jackshaft position system where every device is mechanically linked, the individual servos can be
electronically characterized for greater fuel/air ratio control accuracy.
The parallel positioning system is also safer as the FlexFit continually monitors the demand to the servo versus the feedback
potentiometer and will trip the burner should discrepancies be detected.
With a parallel positioning system, the adaptation of ue gas oxygen trim becomes an easier task. Instead of the added cost
of a link trim servo, the compensation for oxygen trim is performed by manipulation of the demand signal to the air damper servo.
Other advantages of parallel positioning over jackshaft positioning are that a Variable Speed Drive (VSD) can be used to save
energy and provide another means of oxygen trim.
Parallel positioning is especially suitable for larger boilers as all the control devices no longer must be in close proximity to a
common jackshaft. Fans, VSDs, and dampers can now be located distributed throughout the boiler room.
Figure 1 – 5
FlexFit Parallel Positioning (Hot Water Boiler)
Hot Water Boiler
S
S
S
Oil
Gas
VFD
S
PT
AE
TE
Servos
TE
4-20mA
Plant Master
Demand
Hot Water Supply
Hot Water Return
Water
Temp.
Flue
Temp.
Draft
Oxygen
FlexFit Parallel Positioning Example
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