Design Guidelines for High Performance Industrial Furnaces - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 247

T ABLE 5.3

Calculation Conditions

Furnace length 36 m

Heating time 2.5 h

Fuel M g as

Heating temperature 20-1200˚

C

Slab thickness 220 mm

1500

1400

Conventional furnace

Upper region

1300

1200

Conventional furnace

1100

Regenerative furnace

1000

Lower region

Regenerative furnace

900

0 2 4 6 0 2 4 6

Furnace height m Furnace height m

F IGURE 5.1 Relationship between furnace height and unit fuel consumption: estimation of

optimal furnace height.

of high temperature air comb ustion as compared to a conventional furnace. Further,

the gradient in the vicinity of the lowest value of unit fuel consumption is small,
indicating that high thermal efficiency is available across a wide range of furnace
heights. However, it must be noted here that only the amount of heat transfer by
radiation is taken into consideration in the calculations. It can be estimated that the
optimal furnace height will be much lower when heat transfer due to convection
cannot be ignored.
Figure 5.2 shows the results of the test conducted to examine the total heating
efficiency using the common test furnace II with the furnace height changed from
2.5 to 3.0 m and further to 3.5 m on the basis of the results of the above analysis.
The lower the height of the furnace, the higher the heating efficiency becomes.
However, the efficiency does not increase significantly at a height below 2.5 m. It
can therefore be assumed that the optimal furnace height that maximizes the heating
efficiency lies in this region of furnace height. This result represents a much lower
optimal furnace height than the results shown in Figure 5.1 , presumably because
heat transfer by convection is not considered in the case of Figure 5.1. In short,
lower furnace heights will result in greater amounts of heat transfer due to convection
for the same amount of combustion.