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					Continuous-Type Jet Heating Furnaces for Preheating and
Heat Treatment of Aluminium Castings and Forgings
Werner Schütt, Dipl.-Ing. - BSN Thermprozesstechnik GmbH, Simmerath/Germany


Introduction
Jet heating is fast evolving into the technology of choice for
aluminium components, having all but fully displaced inductive
heating in several fields. The method of heating by an impingement
flow offers numerous benefits such as compact equipment design,
high temperature uniformity, significantly reduced energy costs, and
the possibility of heating differently sized parts efficiently in the same
furnace.

Text
Until recently, aluminium forging billets and discs were typically preheated
to the forging temperature in induction furnaces where high throughputs
were involved. Such furnaces are of very compact design even when sized
for high-throughput service. On the downside, however, they are associated
with major investment costs since multiple induction coils are needed for
different billet diameters. Moreover, induction heating is unsuitable for
treating pre-forged components and castings exhibiting complex shapes.
Furnaces with conventional air recirculation are likewise of limited use for
high throughputs. It is true that these units offer significant advantages in
terms of temperature accuracy and temperature profile repeatability, apart
from the additional benefit of supporting gas-firing. On the other hand, they
are rarely a cost-efficient alternative to inductive heating due to their
relatively low power density and the resulting large furnace dimensions.
This is where Jet Heating furnaces come in. These systems combine the
high power densities achievable by inductive heating with the uniform,
repeatable and low-cost performance typical of convective heat transfer
technology. Moreover, they allow parts of different geometries to be heated
in the same furnace without requiring a furnace conversion or retrofitting of
additional equipment.
Thanks to their high temperature accuracy, these furnaces are also suitable
for post-metal forming heat treatments such as solution heat treatment and
artificial ageing.




           BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                   1
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de
Delivering operating temperatures of between 170 and 700 °C, Jet Heating
furnaces are used for
   -   preheating blanks and pre-forged components to the forging
       temperature;
   -   solution heat treatment and artificial ageing, e.g., of castings and
       forged suspension system components;
   -   homogenizing and solution heat treatment of blooms, blanks and
       rods.


Uniform and efficient heating
In a Jet Heating furnace, heat is transferred by a hot airflow delivered by an
array of nozzles and directed vertically onto the load at high velocity (Fig.
2). Due to the extremely high heat transfer coefficient α, the power density
achieved by a Jet Heating furnace is higher by a factor of 3 to 4 than that
provided by conventional air recirculation units.
As a result, the time needed for heating the load is reduced substantially.
Forging billets of different diameters are preheated in only 25 to 33% of the
cycle time required in an air recirculation furnace.
Moreover, Jet Heating offers the advantage of heating more uniformly since
all parts of the load will always be exposed to the same temperature at a
constant airflow velocity.
Jet heating also does away with temperature heads, i.e., all parts to be
preheated or heat-treated are subjected to exactly the preset setpoint
temperature regardless of the operating condition. A temperature accuracy
of +/- 3 K ensures a high process quality.
Apart from short heat-up times and uniform heating, Jet Heating furnaces
provide a high level of process reliability. Conveyor disruptions or changes
in throughput or furnace capacity utilization will leave the heating rate,
heating uniformity or metal temperature unaffected. As a result, Jet Heating
delivers very accurately repeatable heat treatment cycles.




           BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                   2
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de
Suitable for different parts
A Jet Heating furnace requires no reconfiguration or conversion even if the
parts to be heat-treated differ greatly in geometry. As an option, these
furnaces are equipped with a nozzle lifting / lowering mechanism so that the
optimum nozzle-to-load distance is maintained even where product
dimensions vary.
Due to their modular design, Jet Heating systems can be easily matched to
given customer needs. BSN Thermprozesstechnik of Simmerath supplies
continuous-type furnaces for throughputs of 500 to 10,000 kg / hour with a
number of nozzle arrays optimized for the specific application.
In addition, these furnaces can be readily adapted to different power
requirements in the heat treatment cycle. The useful furnace length is
divided into several nozzle arrays or temperature control zones; thanks to
this design, the thermal power input, temperature level and flow intensity
can be varied over the furnace length. The basic architecture of a
continuous-type Jet Heating furnace for preheating aluminium billets to a
specified forging temperature is illustrated in Fig. 3.


High efficiency saves costs
Since the heat transfer is several times more intense than with a
conventional system, the furnace can be of more compact design, i.e.,
investment costs will be lower.
But Jet Heating furnaces are also cost efficient in operation. While induction
heating relies on electricity, which implies a fairly low efficiency and hence,
high cost, Jet Heating furnaces run on natural gas at high efficiency rates.
At throughputs of 1,000 to 4,000 kg per hour, the annual energy savings
associated with the use of a Jet Heating furnace instead of an induction-
based system lie in the region of EUR 40,000 - 170,000 (Fig. 4).


Integration into in-line manufacturing processes
Billets, forging blanks, blanks or castings are moved through the furnace on
a chain, belt or roller-type conveyor.
Heat is produced by high-velocity burners, sometimes with integrated
recuperators. The nozzle arrays are arranged to cover the entire load
surface. Each nozzle array is supplied with hot air by a high-performance
recirculation unit (Fig. 5).



           BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                   3
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de
Used in combination with feeding and discharge systems which position the
parts on the furnace conveyor to within a few millimetres accuracy so that a
precision-controlled material flow is obtained, Jet Heating furnaces are
ideally suited for integration in in-line production processes (Fig. 6).
One proven approach is to award projects on a turn-key basis, i.e., to
include the entire periphery (complete with feeding and discharge systems)
in the furnace manufacturer's scope. This will ensure an unambiguous
distribution of responsibilities, especially with regard to the critical
interfacing with the furnace.
Fig. 7 shows the layout of a complete heat-treatment line for aluminium
wheels. The vendor's scope includes the Jet Heating furnace used for
solution heat treatment, the artificial ageing furnace based on conventional
atmosphere recirculation, the quench, and the handling and charging
systems ensuring an automatic material flow. The charging system
comprises linear guide elements, servo motors and a handling fork for
various wheel sizes.
Fig. 8 illustrates the configuration of a forging line with preheating, forging,
solution heat treatment and quench stations for round billets and pre-forged
components. The automatic material flow throughout the processing line,
including transfer to the press and on from the press to the solution heat
treatment furnace via the deburring station, is handled by standard
commercial robots equipped with special grippers to seize the diverse parts
reliably.
A billet discharge system is shown in Fig. 9. Billets distributed over the
useful furnace width in freely definable quantities are picked up by a
transversely displaceable set of gripper pincers which enters the furnace
compartment and puts the billets down on a press feed conveyor arranged
downstream of the furnace.


Practical operating experience
Given its convincing advantages, Jet Heating has evolved into the preferred
heat-treatment technology for aluminium parts throughout the industry.
Many renowned automotive manufacturers and their tiered suppliers are
relying on Jet Heating furnaces by now, and Jet Heating has established
itself as the standard technology for rapid heat treatment in nearly all West
European forges for aluminium automotive components. The fact that BSN
has supplied more than 25 Jet Heating furnaces in the last three years
alone clearly attests to the success of this design.




           BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                   4
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de
More recently, the Jet Heating method has also come to prevail in heat-
treatment applications such as solution heat treatment, artificial ageing and
homogenizing, where the superior treatment quality and good integration
capability of Jet Heating furnaces into overall production lines are a major
criterion along with the high power densities achieved.




About BSN Thermprozesstechnik
BSN Thermprozesstechnik develops and manufactures heat treatment
equipment for aluminium, non-ferrous metals and steel. The focus is on
Jet Heating – the rapid, uniform and cost-efficient heating system of
choice in a variety of heating processes all over the world.


Simmerath/Germany-based BSN also offers the planning and
implementation of turn-key systems including all peripherals such as
feeding and discharge equipment, etc.
All furnace equipment is made within the BSN network relying on
proven manufacturing partners in Germany.


BSN Thermprozesstechnik GmbH
Kammerbruchstraße 64
52152 Simmerath / Germany
Phone: +49 (2473) 9277-0
Fax: +49 (2473) 9277-111
E-mail: werner.schuett@bsn-therm.de




           BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                   5
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de
Captions
Fig. 1:   Jet heating is suitable for complex shaped parts.


Fig. 2:   The Jet Heating principle: a high-velocity airflow
          impinging vertically on the load


Fig. 3:   Principle of Jet Heating illustrated by a lengthwise
          sectional view of the furnace


Fig. 4:   Energy savings achieved with Jet Heating over
          induction-heating solutions


Fig. 5:   View of the furnace compartment of a Jet Heating
          furnace with four-strand chain conveyor and roof-
          mounted nozzle arrays


Fig. 6:   Continuous-type Jet Heating furnace with feeder for
          aluminium alloy wheels


Fig. 7:   Jet heating furnace with feeding and discharge
          systems integrated into a production line


Fig. 8:   Layout of a forging line for aluminium components
          embodying an automatic material flow through the
          pre-heating, pressing, solution heat treatment and
          quench stages


Fig. 9:   Automatic billet gripper covering the entire furnace
          width, shown with transverse belt conveyor toward
          the press.




            BSN Thermprozesstechnik GmbH, Kammerbruchstraße 64, D-52152 Simmerath                  6
   Telefon (02473) 9277-0, Telefax (02473) 9277-111, E-Mail: info@bsn-therm.de, www.bsn-therm.de

				
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