ALCOA/Cleveland buys 2nd Preheat
Furnace
by Jack
Mahoney, Sales Applications Engineer
In 1996, ALCOA/Cleveland purchased a slat conveyor preheat
furnace from SECO/WARWICK to produce a variety of
forgings. It is used mainly for heating 14 inch diameter aluminum billets at
a rate of approximately 4,000 pounds/hour. After heating, the billets were forged into truck wheels
by steam-powered hammers.
In 2002, ALCOA/Cleveland purchased another slat conveyor style
furnace, but any similarities between the two furnaces stops
there. This latest furnace order is significantly different from the
prior job to say the least. The work to be processed varies from aluminum slabs that are
90 inches long weighing in excess of 3,000 pounds, to 28 inch diameter
billets, each weighing 3,800 pounds.
The task of heating billets of such a large diameter and mass
was then undertaken. To achieve the desired production rate, the jet heating method was
selected. As shown in photo "A", the work enters the main furnace chamber
on top of the stainless steel slats. As the load travels through the furnace, high pressure
heating air is directed downward, from the supply plenum, onto
the load below, from slotted openings in the plenum. This high pressure and high volume of air drives heat
through the aluminum billet and slab, to reach the final
temperature, ranging between 900 - 1000 degrees F.
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Photo A- Slat Conveyor Preheat
Furnace |
As mentioned, the aluminum is supported by stainless steel slats.
The slats are arranged in three (3) rows, also shown in photo
"A". The slats are attached to six (6) strands of high temperature rated
roller chain that is guided and supported underneath. Fully loaded, the furnace will
hold approximately 50,000 pounds of aluminum at any one time.
At the plant site, the furnace will be centrally located between two (2)
opposing hydraulic
forges so that hot metal can be provided to either forge. To accomplish this,
the drive system has been designed with reversing capabilities. Depending upon
production requirements, the unit may operate continuously in one direction, or
be loaded with various aluminum sections and operate in a reversing manner to supply
both forges.
The slat conveyor extends approximately 3' beyond the furnace doors.
This permits work to be loaded onto the slat conveyor via
overhead crane. After heating is complete, the work is discharged through the door,
onto a powered roller conveyor, located outboard of the slat
conveyor drive. These roller conveyors are positioned at each end of the furnace.
Photo "B" shows one of these conveyors. After exiting the furnace,
the billet will roll off the slats and into position on this conveyor. The roll
drive will then perpendicularly transfer the billet and stop it. After
it is positioned at this location, an operator assisted handling
device will remove the billet from the conveyor and transport it
to the forge.
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Photo B - Powered Roller Conveyor |
One (1) motor and gear reducer will be used to drive the slat
conveyor in the forward and reverse directions.
When reversals are made, pneumatic cylinders are used for
taking up any slack in the conveyor chain. These cylinders are shown in photo "B",
directly under the roller table. The advantage of this design is
two-fold: multiple motors are not required for reversals and
the furnace can be mounted directly onto the level plant floor,
without the need for a pit under the unit. Earlier designs needed the pit for the
slack conveyor chain. With the cylinder take-up design, this has been eliminated.
As mentioned earlier, this furnace incorporates high velocity and
high volume air flow for heating these large billets. Each of the four (4) zones
will be provided with a centrifugal fan, capable of producing approximately 70,000 cubic
feet per minute at 6.5 inches static pressure. The horsepower necessary to obtain this performance is
just under 200 BHP. Variable frequency drives will be provided to permit slow speed starting
and greater control flexibility from zone to zone.
In operation, as the air discharges the fans it is directed
into a plenum. Each zone has its own plenum that supplies air to the nozzles.
The plenums and nozzles have been manufactured entirely from
stainless steel and are supported from the furnace roof and
sidewalls.
Since the furnace is used for a wide range of work, that varies
significantly in size and mass, heating the load quickly is
essential. However, it's not the only area where a potential control issue has been
addressed. Maintaining load temperature uniformity after reaching setpoint must also be
considered. As a result, SECO/WARWICK recommended a separate cooling blower
which has been included.
This blower will provide ambient air to each furnace zone
as required. Independent valve motors will control the volume of cooling air that will be
injected into each furnace zone.
The furnace is heated with direct gas-fired burners mounted on the
roof of the unit. Because the final location in Alcoa's plant has limited available space,
literally everything on the furnace is mounted on the roof.
The burners, fans, fan motors, and control cooling fan
are all located on the furnace roof. Special considerations have been made for piping and
wiring the unit, so that all utility feeds are from overhead.
This will permit maintenance personnel to perform their
duties more easily as all areas are accessible.
For insulation in the furnace, Alcoa specified the unit to be lined
with refractory ceramic fiber modules. The module thickness is 9
inches and the fiber materials
used in manufacturing the modules conforms to Alcoa's corporate
standards for chemical analysis.
Now that we have mentioned the basic design parameters of this
latest furnace, is it really that much different from the slat
conveyor furnace Alcoa bought previously? We said that about the only similarity was with the
name "slat conveyor," and that's certainly the case.
The prior furnace could only process 14 inch diameter
billets that were 8 inches long, at a rate of 4,000 pounds/hour.
The new unit can handle 28 inch diameter billets that
are 65 inches long and heat that size at a rate of 6,000
pounds/hour. Plus it will get the job done in just about
the same floor space. Quite a difference to say the least!
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