precast parabolic hot metal inlet design and installation

September 2012
Volume 6, Issue 3
precast parabolic hot metal inlet design
and installation
Furnace downtime is a key issue for any casthouse and can be a challenge
for production staff responsible for production quotas. A frequent cause
of furnace downtime is directly related to the inlet design of the holding
furnace.
Traditionally, the hot metal inlet of an aluminium holding furnace is
constructed using the “cast in-situ” method. This area of the furnace is
designed as a “hole” through the side wall of the furnace which is on average
300 mm wide by 300 mm high (12 by 12 inch). This opening allows the
passage of metal from the melting furnace via a launder or trough.
A traditional replacement of the hot metal inlet will require furnace downtime
of seven to eight days. This includes furnace cool down, demolition of the
existing refractory lining, installation of the new refractory and a refractory
dry out schedule. It is common to see a fillet or slight radius shape at the
entry point. This is primarily intended to eliminate the sharp edge of the inlet
where metal flows. It also serves to provide a durable edge and a cleaner,
more finished appearance.
Traditional hot metal inlet
Common issues associated with this design at the hot metal inlet are discussed below.
Dross Production
Dross production is common due to the profile of the hot metal inlet and the speed at which the hot metal enters the
furnace. Once the metal reaches a suitable depth in the melting furnace to feed the launder and thus the inlet, hot
metal cascades into the bath of the holding furnace in a waterfall-like flow. Dross formation then begins to occur as
a result of the turbulence created from the drop.
Dross formation will vary in quantity during the molten metal transfer process. The amount of dross created is
influenced by the speed that the metal is flowing and the distance that the metal falls. The more turbulence that is
created the, greater the amount of oxide generated and therefore the greater the quantity of dross that is produced.
Hearth Erosion
This is also a very common issue with the typical hot metal inlet design. Similar to the problem of dross formation,
the impact and erosion effect that the cascading hot metal has on to the hearth refractory can be quite dramatic.
Once the refractory material has started to erode the problem becomes compounded as the wear pattern progresses
deeper and wider into the affected area of the drop.
It is not uncommon in some furnace hearths for erosion alone to wear away 200–250 mm (8–10 inches) of refractory
in less than nine months. Once again, the greater the distance the hot metal has to fall, the faster the speed of hot
metal transfer and the quicker the erosion will occur.
The parabolic curved inlet design successfully addresses these two critical issues providing a breakthrough in furnace
inlet design.
Inlet Design Case Study
Pyrotek’s TAB Refractory division was recently contacted by a casthouse
which was experiencing all of the previously mentioned issues. Of the
nine furnaces in-house, four were for melting and five were sixty-five tonne
capacity tilting holding furnaces.
The excessive dross production and hearth wear issues associated with the
holding furnaces were determined to be directly related to the hot metal
inlet on the holding furnace.
TAB determined that the first objective was to engineer a solution for the
cascading metal and associated turbulence. In doing this, the amount of
dross produced would be significantly reduced. Drawing on over thirty years
of experience designing and building furnaces, a redesigned inlet with a
“parabolic curve” would allow molten metal to flow into the furnace with
the least amount of turbulence. A parabolic curve is the natural path a thrown
object will follow based on speed of projection and gravity.
By replicating this curve in the profile of the newly designed TAB inlet, flowing
metal enters the holding furnace sliding down a natural arch into the bath
rather than falling in a cascade pattern from the abrupt lip of the original inlet.
This new design, in addition to reducing dross production, also substantially
reduced the melt splash and thus the wear on the hearth refractory.
The complete project was undertaken in only four days. This included a twoday furnace cool down, two-day demolition and installation running four
shifts and a 50ºC (90ºF) per hour furnace heat up. This reduced total time for
the project by 50% (four days) over previous experiences.
Results
The precast system has been in place for over six months and a recent
inspection of the furnace during a planned shutdown revealed an impressive
reduction in the erosion of the hearth due to the new parabolic profile hot
metal inlet. Data recorded by the customer revealed a reduction in dross
formation of over 20%. This dross reduction and the associated cost savings
from utilising precast shapes are expected to deliver an eighteen month
payback period for the complete project.
TAB Refractory is a division of Pyrotek Inc.
www.pyrotek.info/tab