Civil Engineering Reference
In-Depth Information
Minimum levels of flue gas temperature are limited by potential corrosion and
sulfuric acid condensation in the cold part of the stack. The minimum temperature
depends on the sulfur content of the fuel and on the amount of moisture in the flue
gas. As a general rule, natural gas allows for lower stack flue gas temperatures than
oil fuel (around 383 K, 110
C or 230
F if recovery equipment is installed) because
of the absence of sulfur content.
The minimum flue gas temperature is higher than that given above if recovery
equipment is not installed. In this case the minimum value depends on the boiler
operating pressure, which determines the saturation steam temperature. A differ-
ence of about 70-80 K or
C (126-144
F) between the two temperatures is
generally suggested. Lower values would require a larger convective surface area
and consequently additional cost.
Table
6.6
shows minimum stack temperatures and minimum losses for different
fuels. If stack gas temperature and excess air are higher than these values, higher
losses will occur.
In condensing boilers the water vapor from the exhaust gases is turned into liquid
condensate and the related latent heat is recovered. The boiler efficiency increases
by 3-4 %, depending on the temperature of the cold fluid to be heated.
Losses due to incomplete combustion are generally low if the right amount of
excess air is maintained. The carbon monoxide content in stack flue gas must be
kept approximately equal to zero.
Boiler firing rate or output considered as a percentage of rated capacity affects
efficiency, particularly at low load. Figure
6.12
shows boiler losses versus boiler
firing rate in two operating conditions: (1) O
2
% is kept constant over the load range
(2) O
2
% increases linearly as load is reduced (a common condition if combustion
control is not installed). In the first situation, efficiency remains constant over a large
range of load changes; in the second, efficiency diminishes as the load decreases.
In conclusion, to improve the efficiency of a boiler, excess air and stack tempera-
ture must be kept as low as possible at each load. Reduction of stack temperature is
generally due to a reduction of excess air, that is, a reduction of excess O
2
.
Improvement of efficiency depends on all the previous operating
conditions; an improvement of about 1-2% is generally possible even if
the boiler is operating near the maximum efficiency; of course, greater
improvement is possible if previous efficiency values were very low. Any
efficiency improvement at constant output power will lead to energy
saving as follows:
P
out
1
η
c
1
η
i
power saving
¼
where
η
c
¼
current efficiency
η
i
¼
improved efficiency
P
out
¼
constant output power
