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TECHNOLOGY
Fig. 5: For enhanced corrosion resistance,
a valve blade made of a ceramic material
should be used where salts or other substances
are used to treat the metal and/or
dross
led by a sensor with a probe tube positioned
inside the furnace chamber in
an area where the furnace atmosphere
is least turbulent, viz far away from
the furnace door and the burner and
close to the roof. The sensor measures
the pressure difference between the
atmosphere inside the furnace and
ambient pressure.
Where salts or other substances are
used to treat the metal and/or dross,
it is recommended that a valve blade
made of a ceramic material be used
because of its much higher resistance
to chemical attack by the combined action
of chlorine and the hot gases. Such
a blade reduces maintenance work and
enables the furnace to be operated for
longer periods without interruption.
Fig. 5 shows such an installation.
Burner system
The burner system is a key feature of
any furnace and needs to be considered
when an existing furnace is upgraded
or in the case of a new installation.
An optimised burner system
should incorporate:
• modern burner heads and nozzles
together with furnace control elements
for the gas and air train
• electronic control of the gas/air
ratio to ensure the most effective
and economical combustion (and
to also maintain a high degree of
operational safety)
• an optimised positioning of the
burners and the inclination of the
flames towards the bath and the
level of the metal to achieve the
best heat transfer to the metal.
Correct burner design, burner location,
flame characteristics and flame
directions are the most important
factors for optimising heat transfer
(rather than having only a large burner
capacity). In addition, one also has
to consider:
• the state of the metal surface as
a result of the melt treatment,
with dross on the bath
• the lower roof temperature
• the bath depth
• the possibility that there is bath
movement e.g. due to the use of
stirring devices such as gas injec-
tors, rotating systems or electro-
magnetic devices.
If the burner capacity is too high, this
often only results in higher energy
consumption rather than in optimum
heat transfer because the size and
physical condition of the metal surface
is continuously changing during
the melting and/or skimming process.
Consequently the firing rate has
to be properly controlled in order to
reduce energy consumption as well as
heat transfer. Furthermore, the heat
requirement and heat absorption capacity
of the different alloys has to be
taken into consideration too.
Regardless of whether it is regenerative,
recuperative or cold air type,
the most effective burner system is
one that operates at
100% capacity for as
long as possible during
a melting cycle.
Consequently, a key
feature of a burner
system is the ability
to be able to reduce
the load to about 80%
of its maximum rating
and to then switch off
one or more burners
when the heat requirement
starts to
drop off during the
melting cycle. The
burners that remain
in operation then
operate at full load
again, until the end
of the melting cycle
is reached. Thus, for
example, it is prefer-
able to distribute the required total
burner capacity over four smaller
burners than two larger ones. If the
installed burner capacity is oversized,
this will result in early – and energy
consuming – down-regulation of the
burners, non-optimal heat transfer
and a longer melting cycle.
Furnace pouring spout system
From a metallurgical point of view,
the furnace spout should not create
turbulence or waterfalls when metal
is poured into the launder system. The
level of the metal exiting the furnace
spout should be constant and the
same as the metal level in the launder
during casting. This can be achieved
by installing a furnace spout consisting
of two cast iron pieces which have
their swivel point on the furnace tilting
axis (see Fig. 6).
Furnace tilting control
Another important aspect is the need
for smooth and gentle movement
when tilting the casting furnace to
avoid cracking the oxide skin as a result
of jerks. In modern casting processes
the metal level in the distribution/casting
launder is controlled by
measuring devices and these should
ensure that the metal level is kept
constant by tilting the casting furnace
Kippachse = tilting axis O. K. Bad = upper edge of bath
O. K. Metallstand = upper edge of molten metal / launder
O. K. Fundament = upper edge of base
Fig. 6: A furnace pouring spout consisting of two cast iron pieces
with their swivel point on the furnace tilting axis helps prevent turbulence
when molten metal is poured into the launder system
62 ALUMINIUM · 6/2007