Environmental Engineering Reference
In-Depth Information
22
Furnace width 10m
1500K
1578K
1.2m
1550K
2.0m
1600K
20
18
16
14
1
1.5
2.0
2.5
Zone height (m)
FIGURE 4.10
Examination of the optimum furnace height. Relationship between the zone
height and the radiant heat flux to steel materials, taking into account the effect of the furnace
wall (numerical analysis solution).
heat-treatment furnace. As for heating furnaces, there have been four types of
recuperators as shown in
Figure 4.11
: the multipipe type, the Hasen type, the
radiating type, and the tile type.
In a recently designed recuperator, a very large heat exchanger, which has a heat
transferring area in the 10,000 m
2
class for applying to three heating furnaces, is set
between the furnace end and the smoke stack. The heat resistance temperature of
this recuperator is about 1050˚C, and the maximum temperature obtained of the
preheated air is 720 to 730˚C. If the temperature of the preheated air is raised to
more than 730˚C, problems occur, such as the increase of NO
x
content, the increase
in equipment investment due to the expanded heat transferring area of the recuper-
ator, and the lowering of the durability of the recuperator itself. Between 720 and
730˚C should be the limit for the maximum temperature of preheated air. The surplus
waste heat is utilized for preheating the gas to about 300˚C, which would be the
final temperature attained assuming that the gas composition is not transformed.
However, with a regeneration-type furnace, a heat reservoir for exchanging heat
is included in the burner itself, so heat exchanging is completed within the paired
burner. Thus, the heat recovery is carried out by the heat-exchange method of a
distributed type, an entirely different system, not used by the conventional heat
exchange of an integrated type. The heat recovery ratio in the case of the conven-
tional-type recuperator is 60% at most, as shown in
Figure 4.12
.
But, in the regen-
eration-type furnace, up to about 80% of heat can be recovered by a direct heat
exchange method of a heat-reserving type. This is almost 100% in terms of temper-
ature efficiency. When the temperature in the heating furnace is 1300˚C, the tem-
perature of the preheated air reaches between 1150 and 1250˚C, which is 50 to
150˚C less than the temperature in the heating furnace. Thus, “realization of the in-
system critical heat recovery” is included.
Regarding the burners, there has been a transition to technologies for lengthening
the flame against a broader width of the heating furnace and for reducing the NO
x
to pass the NO
x
regulation values stably. Various NO
x
reduction technologies have
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