Page 50 - Boiler plant and distribution system optimization manual

Energy Management Basics for Boilers and Systems 35

for 0% excess air and 2,300 F [1,260C] for 100% Table 3.5—Excess air flame temperature relationship
excess air. ————————————————————————
The basic equation for radiant heat transfer, Temperature Deg F C
in the flame zone section, is: Excess Natural Propane #2 #6
Air% Gas
Q = ρ ST 4 ————————————————————————
0% 3,400 3,700 3800 4,000 F
Q is Btu/Hr
ρ is the Stefan-Boltzman constant, 1,871 2,038 2,093 2,204 C
–9
1.17 x 10 Btu/sq ft, hr, T 4 25% 2,900 3,100 3,200 3,400 F
S is surface area, sq ft 1,593 1,704 1,760 1,871 C

T is absolute temperature F + 460 50% 2,500 2,600 2,800 2,900 F
1,371, ,424 1,538 1,593 C
It can be seen from the mathematics in-
75% 2,300 2,300 2,400 2,600 F
volved, the higher the temperature of the flame,
the more intense the heat transfer. It becomes ob- 1,260 1,260 1,316 1,427 C
vious that if the temperature of the radiant heat
100% 2,000 2,000 2,200 2,300 F
transfer section of the boiler is lowered by lower
flame temperatures, there will be lower radiation 1,093 1,093 1,204 1,260 C
heat transfer. This will ultimately increases flue ————————————————————————
gas exit temperature and decrease efficiency.
Stack temperatures have been observed to
change over 50 F [28C] due to this phenomena.
Changes in excess air directly affects flame tem-
perature, and radiation heat transfer in the com-
bustion zone.