7 Flue gas measurement
7.1.11 Efficiency (η)
•
Combustion efficiency measures the content of fuel energy that remains after subtracting the flue gas losses
(approximately 5 - 10 %) and the losses due to incomplete burnout (extremely low).
•
Boiler efficiency is the measure of the content of fuel energy that remains after subtracting the flue gas
losses, the losses due to incomplete burnout, and the casing losses of the boiler surface (approximately
0.5 - 2 %).
•
Flue gas losses depend on the flue gas temperature and on the CO2 content of the flue gas.
•
Annual efficiency coefficient (65 - 80 %) is the ratio of thermal energy required per year to the fuel energy
consumed. Here standstill, start-up and distribution losses of the heating system are taken into account.
•
Flue gas temperature increases with
- higher combustion temperature,
- higher levels of excess air or
- a heavier ash coating of the heat exchanger
•
Combustion temperature increases with
- lower levels of fuel moisture or
- lower levels of excess air
7.1.12 Draught
• Draught is the negative pressure in the smoke pipe relative to the ambient pressure.
• Units: 1 hPas (hectopascal) = 100 Pas = 1 mbar
• Optimal value: 0.1 – 0.2 mbar
• Chimney draught can be adjusted via the draught regulating shutter. With the induced draught fan the
conveyor pressure is increased and the draught requirement is decreased, however the chimney draught is
not increased!
• Poor draught at: Lower flue gas temperature (insulate the smoke pipe) narrow smoke pipe/chimney (fly ash)
horizontal or angled smoke pipe/chimney, down draught, or permeable smoke pipe (draught control shutter)
• Excessive draught: Causes high levels of excess air, a lot of fly ash, high standstill losses.
• Insufficient draught: Causes a lack of air with large boilers, smoke escapes in the boiler room, heat
accumulation in the substructure, puff-back into the tank and the fans.
7.2 Incorrect measurements – causes
7.2.1 Causes associated with operation
Suction nozzle is not in the core flow: If the flue gas is suctioned out in a “dead” corner in terms of flow, the
measured values change too slowly. The flue gas temperature is incorrect.
No reliable value: Because the measured values fluctuate, a mean value must be expressed. To do this the measured
values must be observed for at least 5 minutes.
Unstable burner condition: Only take the measurement when the fuel bed is stabilised (does not increase, does
not decrease). This can take up to 30 minutes at low output levels.
MA-Easyfire 10.2008_EN.indd 76MA-Easyfire 10.2008_EN.indd 76 30.03.2009 13:30:12 Uhr30.03.2009 13:30:12 Uhr
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