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Building Technologies Basic Documentation LMV5... CC1P7550en
19 O2 trim control with LMV52 and PLL52 22.05.2018
19.2 Functioning principle of O2 trim control
The residual O2 control system reduces the amount of combustion air depending on
the control deviation (O2 setpoint minus actual of O2). The amount of combustion air is
normally influenced by several actuators and, if used, by a VSD. Reduction of the
amount of air is reached by reducing the «air rate» of the air-regulating actuators.
For that purpose, the damper positions of these actuators are calculated from some
other load point on the ratio control curves. Hence, due to the parameterized ratio
control curves, the air-regulating actuators are in a fixed relation to one another.
O2 trim control is supported by precontrol. Precontrol calculates the air rate
reduction such that changes in burner load do not require the O2 trim controller
to interfere. Consideration is given to a number of measured values that are
ascertained when the burner is set. This means that the controller only becomes active
when environmental conditions (temperature, pressure) change, and not when the
burner load changes.
19.2.1 Air rate change
Positioning angle (°)
Load (%)
7
5
5
0
d
3
4
e
/
0
9
0
3
Fuel damper
Air damper
Air rate
Fuel rate
Figure 117: Air rate reduction
Due to the air rate reduction, the O2 trim controller reduces the amount of air. For that
purpose, the air-regulating actuators are driven to a smaller load (air rate) on the ratio
control curve. Fuel rate and air rate on the ratio control curve are the same.
Example: If the air rate is 50% of the fuel rate, half the amount of air is delivered to the
burner compared to the ratio control curve (if κ is the same for all curvepoints).
If Startmode is parameterized to Ign Load TC, IgnPtWithTC or IgnPtWoutTC, the air
rate can also be increased in relation to the fuel rate (refer to chapter Burner start mode
for O2 trim control).
Additionally, the fuel rate is altered if the required difference between the fuel and air
rate cannot be achieved by altering the air rate (e.g. at the highest curvepoint when air
rate > fuel rate).
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