Environmental Engineering Reference
In-Depth Information
Improvement of Heat Transfer by High Temperature Air Combustion
Characteristics
Heating Control Method
Increase in heating potential
•
Adiabatic flame temperature is increased by
returning recovered heat to the heating
system
The volume of fuel supply is adjusted so that the
furnace temperature maintains at the same level
as in the conventional method. ~1/heating
potential
Averaging/raising furnace temperature
•
Maximum flame temperature is lowered
through averaging
Substantially more recovered heat can be
returned to the heating system until the
same maximum temperature is reached
The volume of fuel supply is adjusted so that the
air temperature is raised without exceeding the
permissible upper limit and the heat transfer rate
is fixed
•
Improved heat transfer rate
•
A high temperature zone and a low
temperature zone are prepared and the
furnace temperature is averaged and
controlled on a zone-by-zone basis
The volume of fuel supply is adjusted on a zone-
by-zone basis so that the high temperature zone
is heated to the permissible upper limit and the
heat transfer velocity is uniform throughout the
furnace
at a uniform level and the second case is for a furnace where the in-furnace space
is divided into zones for zone-based heating control.
If the temperature is uniform throughout the furnace, heat exchange between
gases does not occur and the temperature of the gas discharged from the furnace is
equal to the in-furnace gas temperature. The heat balance of the heating system can
be expressed by the following heating potential scenario.
Now, assuming that the calorific value of fuel is
H
0
, the preheated air sensible
heat for burning the unit volume of fuel is
H
a
, and fuel
v
0
is fed when air preheating
is not executed, the fuel input volume
v
a
required to make the total heat input to the
heating system uniform is calculated by the following equation:
H
0
v
0
= (
H
0
+
H
a
)
v
a
(4.1)
If air preheating is executed,
v
0
>
v
a
is always satisfied to save fuel by
v
0
-
v
a
.
It is an improvement when the in-furnace gas temperature is at room temperature
(atmospheric temperature) and achieves a reduction in the volume of fuel supply by
the preheated air sensible heat. However, during actual heating operations, the heat
transferred from the in-furnace gas to the material to be heated or the furnace wall
must be compensated for and the in-furnace gas temperature must be kept high for
the duration of the operation.
Figure 4.13
shows the relationship between the preheated air temperature and
the heating potential of the combustion gas (maximum heat that can be transferred
to the material to be heated), represented in typical form. The enthalpy region above
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