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MELTING, RECYCLING & HEAT TREATMENT
The new generation <strong>of</strong> <strong>aluminium</strong>
heat treatment plants – a vanguard concept
Markus Belte and Dan Dragulin, Belte AG
The vigorous discussion in <strong>the</strong> <strong>aluminium</strong>
heat treatment community about <strong>the</strong> development
<strong>of</strong> <strong>the</strong> appropriate technique
becomes within <strong>the</strong> framework <strong>of</strong> <strong>the</strong>
present work a genuine concrete answer.
This paper is completely dedicated to <strong>the</strong>
presentation <strong>of</strong> a new generation <strong>of</strong> heat
treatment plants; it first introduces <strong>the</strong>
fundamentals <strong>of</strong> high speed heat transfer
processes and presents practical results
and <strong>the</strong>irs <strong>the</strong>rmodynamical analysis and
<strong>the</strong>n <strong>of</strong>fers a detailed perspective <strong>of</strong> <strong>the</strong>
vanguard concept <strong>of</strong> <strong>the</strong> new generation
<strong>of</strong> <strong>aluminium</strong> heat treatment plants. The
present work is based on original experiments
and investigations obtained by <strong>the</strong>
authors and will present techniques never
used before in <strong>the</strong> industrial heat treatment
<strong>of</strong> <strong>aluminium</strong>.
Infrared radiation – <strong>the</strong>oretical aspects
The radiation heat transfer process is described
by <strong>the</strong> law <strong>of</strong> Stefan-Boltzmann:
• ∂Q
Q = ⎯⎯ = εσST 4
∂t (1)
• 4 4
Q = εσS [T O - T U ] (2)
•
Where: Q = heat flow rate; ε = emissivity 1 ;
σ = 5.67 x 10 -8 W/m 2 K 4 Stefan-Boltzmannconstant;
S = surface <strong>of</strong> <strong>the</strong> emitting body;
T = temperature <strong>of</strong> <strong>the</strong> emitting body (Kelvin);
T O = surface temperature; T U = environment
temperature 2 .
The heating rate can be calculated using <strong>the</strong>
following formula:
mc 100 TWB Ta
τ = ⎯ ⎯⎯ [ ξ´(⎯⎯)- ξ´´ (⎯⎯)]
α T 3
U
TU TU (⎯⎯)
100
(3) 3
Where: m = mass/surface unity <strong>of</strong> <strong>the</strong> product
to be heated; c =specific heat; TWB = heat
treatment temperature; TU = environment temperature;
ξ = f(TWB/TU) A special case <strong>of</strong> <strong>the</strong> heat transfer process
is <strong>the</strong> infrared heat transfer process.
“Infrared radiation, that portion <strong>of</strong> <strong>the</strong>
electromagnetic spectrum that extends from
1 ε Є (0,1] ; for <strong>the</strong> ideal black body: ε = 1
2 The environment could be <strong>the</strong> interior <strong>of</strong> a furnace.
3 According to [3]
<strong>the</strong> long wavelength, or red, end <strong>of</strong> <strong>the</strong> visiblelight
range to <strong>the</strong> microwave range. Invisible
to <strong>the</strong> eye, it can be detected as a sensation
<strong>of</strong> warmth on <strong>the</strong> skin. The infrared range
is usually divided into three regions: near
infrared (nearest <strong>the</strong> visible spectrum), with
wavelengths 0.78 to about 2.5 micrometres (a
micrometre, or micron, is 10 -6 metre); middle
infrared, with wavelengths 2.5 to about 50 micrometres;
and far infrared, with wavelengths
50 to 1,000 micrometres. Most <strong>of</strong> <strong>the</strong> radiation
emitted by a moderately heated surface is infrared;
it forms a continuous spectrum. Molecular
excitation also produces copious infrared
radiation but in a discrete spectrum <strong>of</strong> lines or
bands.” [Encyclopaedia Britannica [1]]
“Radiant heating has been used in <strong>the</strong> industry
since <strong>the</strong> 1930s, where it was introduced to
bake finishes on cars. It took time for <strong>the</strong> new
technology to gain acceptance but today <strong>the</strong>
technology is used in most industrialised countries
especially for drying and curing <strong>of</strong> paints
and drying <strong>of</strong> textile, pulp and paper. Infrared
energy is unique because it can heat materials
or objects without heating <strong>the</strong> air around <strong>the</strong>m.
That allows infrared heat to be concentrated
exactly where it is wanted without much loss
<strong>of</strong> energy.” [2]
In <strong>the</strong> case <strong>of</strong> an infrared heating <strong>the</strong> maxi-
mum temperature which can be reached depends
on <strong>the</strong> environment temperature:
T - T U = (T max - T U)(1-e -B . τ ) (4) [3]
Where: T = body temperature; τ = duration;
T U = environment temperature; T max = maximum
temperature; B = coefficient which takes
into account <strong>the</strong> process conditions
In <strong>the</strong> case <strong>of</strong> a classic heat transfer process
performed exclusively through air convection,
<strong>the</strong> body temperature can not exceed <strong>the</strong> air
temperature.
Gas radiation
In <strong>the</strong> case <strong>of</strong> flame radiation <strong>the</strong>re are two
types radiation: visible and invisible/infrared
radiation. The last one is <strong>the</strong> invisible infrared
radiation emitted by carbon di<strong>oxide</strong> and
steam 4 .
The quantity <strong>of</strong> heat transferred through
radiation from a <strong>layer</strong> <strong>of</strong> CO 2 respectively H 2O
to a grey surface are:
3.2 3.2
Tg Tw
qCO2 = ε . 10.35(p .
CO2 s) 0.4 [(⎯⎯) - (⎯⎯)
100 100
Tg
0.65
. (⎯) ] Tw 4 After J. H. Brunklaus
Fig. 1: Bilateral (green) and unilateral (red) exposure to radiation (not coated, h = 10 cm)
(5)[[7]→[6]]
60 ALUMINIUM · EAC CONGRESS 2011
Abbildungen: Belte