Page 49 - Boiler plant and distribution system optimization manual
P. 49
34 Boiler Plant and Distribution System Optimization Manual
Figure 3.3—Efficiency change for each 10°F change in stack
temperature for various levels of excess air.
IMPROVING RADIANT HEAT TRANSFER FLAME TEMPERATURE
The radiation section or that section of the Table 3.5 shows that as the excess air is re-
boiler exposed to direct radiation from combus- duced, flame temperatures rise. Combustion
tion, is the most effective heat transfer zone of a flame temperatures are very complex to analyze
boiler. Figure 3.4 shows that 8% of the heat ex- and this chart should be treated only as an ap-
change surface of a boiler is absorbing about 48% proximation. It can be seen in Table 3.5, at 0%
of the energy from combustion. The convection excess air, the flame temperature is about 3,400 F
section, which comprises 34% of the heat ex- [1,871C] for natural gas and at 100% excess air this
change surface only absorbs about 20% of the temperature drops to about 2,000 F [1,093C]. For
heat of combustion. number 6 oil these numbers are 4,000 F [2,204C]
