Chapter 5 Adjustment Procedures
5-8 750-177
Flow
Since the gas flow rate is based on standard conditions of flow, correction must be made for the supply
pressure through the meter of 3 psig. Determine the flow rate by dividing the Btu content of the gas into the
burner input (Table 6-1) and “correct” this answer by applying the correction factor for 3 psig (Table 6-2).
Btu/hr Input = CFH (Cubic feet/hour) Required
Btu/cu-ft
OR
5,000,000 = 5,000 CFH (At 14.7 Ib-atmospheric base 1,000 pressure)
THEN
5,000 = 4237 CFH
1.18
This is the CFH (at line pressure) that must pass through
the meter so that the equivalent full input
requirement of 5,000 CFH (at base pressure) will be delivered.
Checking Gas Flow
Your gas supplier can generally furnish a gas meter flow chart from which gas flow can be determined. After
a short observation period, the information aids in adjusting the regulator to increase or decrease flow as
required to obtain the rating.
Final adjustment of the gas fuel is carried out by mean
s of Fine tuning the linkage adjustments, while
performing a combustion efficiency analysis. See Section O for details.
Note: The information given in this section is for all practical purposes sufficient to set and adjust
controls for gas input. Your gas supplier can, if necessary, furnish exact correction factors that take
into consideration Btu content, exact base pressure, specific gravity, temperature, etc., of the gas
used.
P. GAS FUEL COMBUSTION ADJUSTMENT
After operating for a sufficient period of time to assure a warm boiler, adjustments should be made to obtain
efficient combustion.
Burner efficiency is measured by the amount or percentage of O
2
present in the flue gas. O
2
readings
determine the total amount or excess air in the combustion process, above the point of stoichiometric
combustion or perfect combustion. Stoichiometric combustion is a term used to describe a condition when
there is the exact amount, molecule for molecule, of air for the fuel attempting to be burned. This can be
accomplished under laboratory conditions, however it’s not practical to attempt to meet this condition in a
boiler. Stoichiometric combustion however, is the reference point used when setting fuel/air ratios in a boiler.
There must always be excess air in the combustion process to acco
unt for changes in boiler room temperature
and atmospheric conditions, and to ensure the combustion is on the proper side of the combustion curve.
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.
Most flue gas analyzers in use today measure the content, by perce
ntage 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 todays
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. Cleaver-Brooks
recommendations on turndown should also be followed and the turndown range of the burner should not be
exceeded.
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