Design Guidelines for High Performance Industrial Furnaces - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 324

Low: Possibly excessive direct flames on the surface of the cold aluminum
and the molten bath led to imperfect combustion to the detriment of heat
transfer by convection. Because the ceiling was low, the radiation effect
from the ceiling was also low and not much could be expected either of
the radiation heat from the gas layer, which probably caused the heat
transfer to be the worst.

The test results stated above indicate that the size reduction should be arranged in
such a way that the distance between burners and material equals the length of the
flame.

5.4.4.3 Method of Improving the Heat Transfer Efficiency

inside the Furnace

By improving the heat transfer efficiency and by reducing the furnace size, economic
benefits in terms of initial costs and fuel cost can be anticipated. This is because of
reduced heat loss from the furnace wall. Below are approaches to improve the heat
transfer efficiency:

• Optimization of the distance between burners and material: The optimum
design is to set the distance between burners and material equal to the
length of the flame expected when the burners are at 100% combustion.
The flame length depends on the burner capacity and the type of fuel
used. However, the distance between burners and material increases as
melting advances. This is the reason an average distance should be
adopted. Care should be taken not to make the distance too short; other-
wise imperfect combustion will take place.
• Adoption of burners with a flame of high momentum: The aluminum
material will receive heat transferred by convection from the burner flames
and radiation heat transferred from the furnace wall and the gas layer. In
the case of aluminum-melting furnaces, the furnace temperature is in the
midrange, and it is understood that heat transfer by convection is more
effective than radiation heat transfer in terms of heat transfer rates, which
is the reason it is necessary to choose high momentum burners whose jet
flow velocity is larger (jet flow velocity of 100 to 120 m/s).
• Optimization of the gas flow inside the furnace: The gas inside the furnace
flows out mostly through regenerative burners, and it is necessary to decide
their location and angle so that the optimum heat exchange with the
material is ensured. It should be designed to prevent the combustion gas
shortcuts and flows to the burner port before it hits the material.
•Tilting of the burners themselves: This involves changing in a certain
frequency the place where the flames hit the material by mechanically
swinging the burners themselves. It was learned from tests that the flame
shift-speed of 3.3 m/min on the surface of the material produced the best
results and showed improvement by 5% both in terms of melting time
and fuel consumption compared with the case without tilting. It was also