Siemens LMV 5 Series, Технические инструкции, Страница: 175 / 374

LMV Series Technical Instructions
Document No. LV5 ‐ 1000
SCC Inc. Page 3 Section 6
Figure 6 ‐ 1: Physical Arrangement of Burner, O
2
Sensor, and Gas Path
Since the O
2
sensor is located in the boiler's exhaust (stack), there always exists a delay time (Tau time)
between when air flow is adjusted at the burner and when the change is detected by the O
2
sensor. This
physical reality forces the O
2
trim to use "old" O
2
readings, which will be discussed later. The delay time
depends on the length of the gas path through the boiler and the velocity of the gas. The length of the
gas path is fixed for a given boiler, but the gas velocity depends upon the firing rate. Higher firing rates
equate to higher gas velocities and smaller delay times (Tau times).
If the fuel flow increases linearly with the firing rate (the load number accurately reflects the fuel flow),
the Tau time will decrease in a predictable manner as the firing rate increases. Likewise, given an
accurate load number, the Tau time will increase in a predictable manner as the firing rate decreases.
This enables the Tau time to be automatically calculated at every point from Point 2 to high fire. Once
the Tau time is calculated for every point from Point 2 to high fire, the PI (proportional + integral)
response for the O
2
trim can be automatically calculated for every point between Point 2 and high fire.
Another place where the Tau time must be accounted for is at startup. Immediately before main flame
ignition, the gas path of the boiler is full of air. To get a representative O
2
reading that can be used for
trimming, the gas path of the boiler must be full of products of combustion (exhaust gases). Thus, the
main flame of the burner must be ignited and the burner must be running for a period of time to flush
out all of the air. The time it takes to flush out this air is estimated by taking the Tau time (at Point 2)
times a multiplier. For example, if the Tau time at Point 2 is measured to be 6 seconds, then the time it
takes to get a representative O
2
reading after light off might be 6 x 6 = 36 seconds. In this example,
closed loop O
2
trim cannot occur until 36 seconds after light off. This is precisely where the temperature
compensation functionality is used for some burners (LMV52.440 only).
HOME