Page 100 - Boiler plant and distribution system optimization manual
P. 100
The Control of Boilers 85
element air heater cold end temperature control desired value, the control system will take no ac-
may have to be used. tion to correct it. For this reason it is highly desir-
able to modify a boiler control system to include
Controls to Improve Efficiency feedback of information on the air to fuel ratio.
To a great extent, the efficiency of a boiler is This is done by adding either an oxygen trim or
dependent on the design of the burner, heat ex- carbon monoxide trim system to the boiler con-
changer and other design parameters which can- trols.
not be easily changed. However, changes in fuel The effect on air/fuel ratio on flue gas anal-
composition, air temperature, pressure and hu- ysis is shown in Figure 8.12, knowing the rela-
midity, boiler load and equipment condition all tionship of measured flue gas oxygen, carbon
introduce changes in boiler performance which dioxide, carbon monoxide, combustibles, special
can be accommodated for with improved control. instruments can be used to measure and control
In general “improved control” means better the boiler for efficient air fuel ratios.
control of the air fuel ratio. The simple parallel
positioning and series positioning systems have
open-loop control of the fuel and airflows. There OxYGEN TRIM
is no feedback into the control system which tells
the system what the actual fuel and airflows are. The idea behind oxygen trim controls is to
Feedback maximize boiler efficiency by operating at the
is provided only by the steam header pressure. point where the combined efficiency losses due to
If the header pressure is at the control set unburned fuel and excess air losses is minimized.
point but the fuel to air ratio is nowhere near the An oxygen trim system measures the excess
Figure 8-12—Composition of boiler flue gases with changes in air/fuel ration for natural gas.
