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Flue Gas Analysis: in practice
Below is an example of the flue gas analysis of a methane-fired boiler (natural gas) that is working correctly:
Flue gas temperature Tf
This should be as low as possible: less heat leaving the stack will leave more
heat available for heating purposes.
Combustion air temperature Ta
This is not always the same as the ambient temperature.
Combustion air may be heated by the flue gas in coaxial pipes, or may be
drawn from outside: in these cases the remote air temperature probe is
necessary.
Oxygen O
2
The percentage of oxygen in air is around 21%: an ideal combustion process
will “burn” all the oxygen present; in truth, however, the residual percentage is
never zero due to the presence of excess air.
Carbon Monoxide CO
This is expressed in parts per million and indicates the concentration of CO
“diluted” in the flue gas.
Excess air λ, n
This is the ratio between the volume of air that actually enters the combustion
chamber and that which is theoretically required.
Carbon Dioxide CO
2
This results from good combustion and should approach the theoretical
threshold value as much as possible.
Stack loss Qs
This is the percentage of heat lost through the stack.
Sensible efficiency ηs
It is the burner efficiency calculated according to the UNI 10389-1 standard,
as the ratio between conventional heating power and the burner heating
power. Among the combustion losses, only the sensible heat lost with flue
gasses is taken into account, thus neglecting the radiation losses and
incomplete combustion losses. This value is referred to the Lower Heating
Value (LHV) of the fuel and cannot exceed 100%.
The sensible efficiency value is to be compared against minimum efficiency
stated for the heating system performances.
Condensation efficiency ηc
Efficiency deriving from the condensation of water vapour contained in flue
gasses, calculated according to the UNI 10389-1 standard.
Total efficiency ηt
Total efficiency. It is the sum of sensible efficiency and condensation
efficiency. It is referred to LHV (Lower Heating Value) and can exceed 100%.
Differential temperature ∆T
This is the difference between the temperature of the flue gas and that of the
combustion air.
Carbon Monoxide CO (referenced to 0% O
2
)
This is expressed in parts per million and indicates the concentration of CO
that the law requires us to keep under control (it should be lower than
1000 ppm).
COMPANY Ltd.
Park Road, 9
Tel.02/12345678
Oper.: .............
Sign: .............
Test according to
UNI 10389-1
L. 10/1991 and s.m.i.
D.Lgs.
192/2005 and s.m.i.
Chemist 400
Serial: 421023
Memory: 01
Analysis: average
DatE: 22/11/10
Time: 10:15
Fuel: Natural gas
MEASURED VALUES
T flue 190.1 °C
T air 15.4 °C
O
2
4.2
٪
CO 146 ppm
NO 40 ppm
Draft: 0.05 hPa
T outdoor: 20 °C
CALCULATED VALUES
λ,n 1.25
CO
2
9.3
٪
QS 8.6
٪
ηs 98.5
٪
ηc 4.9
٪
ηt 103.4
٪
∆T 174.7 °C
NO
X
/NO:
1.03
NO
X
41 ppm
Ref. O
2
:
0.0
٪
CO 182 ppm
Ref. O
2
:
0.0
٪
NO 50 ppm
Ref. O
2
:
0.0
٪
NO
X
51 ppm
Note:
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