750-91 (revised 2009)
Model CB-LE Packaged Boiler Manual
6-17
6.17 — Gas Fuel Combustion Adjustment
it’s not practical to attempt to meet this condition in a boiler. Stoichiometric combustion, however, is the reference
point used when setting air/fuel ratios in a boiler.
There must always be excess air in the combustion process to account for changes in boiler room temperature and
atmospheric conditions, and to ensure the combustion is on the proper side of the combustion curve (see Figure 6-
11).
Proper setting of the air/fuel ratios at all rates of firing must be established by the use of a combustion or flue gas
analyzer.
The appearance or color of the gas flame is not an indication of its efficiency, because an efficient gas flame will
vary from transparent blue to translucent yellow.
FIGURE 6-11. Flue Gas Analysis for Natural Gas
Most flue gas analyzers in use today measure the content, by percentage of oxygen (O
2
) and carbon monoxide
(CO) either by percent or parts per million (ppm). Carbon dioxide (CO
2
) is not normally measured with today’s flue
gas analyzers, but may be displayed via a calculation.
The O
2
levels through the entire firing range of the burner, low-fire to high-fire should be tested. The burner man-
ufacturer’s recommendations on turndown should also be followed and the turndown range of the burner should
not be exceeded.
It is important to understand what the readings shown on an instrument refer to when setting combustion in a
boiler. To assist with this understanding Figure 6-11 shows the relationship between O
2
levels (excess air) and the
products of combustion for a typical flue gas analysis (natural gas).
One of the products of combustion is CO
2
(carbon dioxide). This is shown in percentage.
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