Siemens LMV 5 Series, Технические инструкции, Страница: 176 / 374

Technical Instructions LMV Series
Document No. LV5 ‐ 1000
Section 6 Page 4 SCC Inc.
For many nozzle mixing burners (traditional boiler burners), the O
2
setpoint will be a higher value at low
fire and a lower value at high fire. This is true since most nozzle mixing burners need more excess air
(higher O
2
) at low fire to achieve complete combustion. For this type of burner an O
2
setpoint of 5% O
2
(wet) at low fire and an O
2
setpoint of 2% O
2
(wet) at high fire is not uncommon.
Pre ‐ control
As mentioned above, all O
2
trim systems with an O
2
sensor in the boiler's exhaust must use "old" O
2
readings due to physical realities of the boiler. This fact, combined with an O
2
setpoint that typically
varies from low fire to high fire means that the O
2
trim system cannot actively trim when the burner
makes a large change in firing rate. This begs the question of what happens to the O
2
trim when the
burner transitions from 20% firing rate to, say, 80% firing rate. The answer is the O
2
trim changes modes
from active trim to what is referred to as "pre ‐ control".
Pre ‐ control is possible because the LMV52 learns the characteristic of the burner at each point on the
curve during the O
2
trim commissioning. The specific characteristic that the LMV52 learns is how much
of a change in airflow (air rate) is necessary to achieve a certain change in the %O
2
. This specific
characteristic is known as the Lambda Factor, and is essentially the burner's signature from an O
2
trim
standpoint. Just like the delay time (Tau time) that was mentioned earlier, it is necessary for the fuel
flow to match the load number so that the Lambda Factor is correct and accurate for each point.
Pre ‐ control uses the Lambda Factor so that the burner can be modulated while keeping the %O
2
close to
the O
2
setpoint, even though the setpoint changes with the firing rate and the O
2
trim is using "old" O
2
readings. Once the burner stops modulating or only modulates a small amount (+/ ‐ 5%), the O
2
trim will
change modes back to "active" and perform active O
2
trim once again.
A properly commissioned O
2
trim system will typically maintain the %O
2
setpoint with a deviation of +/ ‐
0.1% O
2
when the burner is not modulating a significant amount (+/ ‐ 5% load change). With this small
load change, the O
2
trim will stay in the "active" mode. When significant modulation is encountered (+/ ‐
15% load change), the pre ‐ control will engage and the deviation from setpoint will increase. Deviations
of +/ ‐ 0.3% or less are typical during modulation when the pre ‐ control is engaged.
O
2
Trim Terminology
1. %O
2
Wet – In ‐ situ O
2
sensors, like the QGO20 sensor that is used with the LMV52, read the %O
2
on a
wet basis. This is in contrast to the vast majority of portable combustion analyzers that read %O
2
on
a dry basis. %O
2
wet should always be a lower value than %O
2
dry. Figure 6 ‐ 11 (located later in this
section) gives the approximate relationship between %O
2
wet and %O
2
dry.
2. Tau Time ‐ This is the delay time between when an adjustment is made at the burner (moving the
air damper, etc.) and when that adjustment is read at the O
2
sensor. For a given burner / boiler, this
time will be shorter at high fire and longer at low fire.
3. O
2
Ratio Control ‐ This is the measured %O
2
wet when the actuators / VSD are on the Fuel ‐ Air Ratio
Control Curve and are not being trimmed. This is recorded for each point on the curve except Point
1. This is also commonly referred to as the “lean curve”.
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