Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1194137
The invention relates to a device for the dielectric-
heating of a electrically conductive liquid material.
Devices of this type are used, for example, in order to
dry materials in sheet form for instance woodfibre boards, or to
cure the binders contained in such materials. For this purpose,
the capacitor-plates are brought to bear against the material to
be heated, which is then heated to the desired temperature in the
electric field which is produced between the capacitor-plates by
means of a high-frequency generator.
In this process, depending on the nature of the material
to be heated, avoidable losses, of various magnitudes, can occur,
and the supply of energy can, furthermore, become uncontrolled, for
instance when the water content is high, while the matching of
the capacitance to the generator output is, moreover, laborious and
difficult, the intention being that the oscillating circuit of the
generator should, as far as possible, operate in resonance. These
considerations apply most particularly in the case of materials
which are both conductive and liquid, and which also possess a
relatively high dielectric coefficient. This is the case, for
example, when the intention is to manufacture blanks for building
materials, in particular wall-building blocks or bricks, in a mould-
ing apparatus which is designed to receive a pourable raw mix-
ture, composed of a binder which can be activated by heat, for
instance cement with appropriate accelerators, foam, water, sand
and, if appropriate, other additives, such as are used, for
instance, for manufacturing lightweight building blocks based on
calcium silicate or, alternatively, building blocks composed of
aerated concrete or foamed concrete, or composed of heavy clay
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materials. In manufacturing processes of this type, the aim is to
produce a raw mixture which, in order to achieve as low a bulk
density as possible, contains an appropriately large amount of
foam, together with a larye amount of water, it being possible for
the water content to reach 50~ by weight, or more, in order to
achieve bulk densities ranging down to 0.2 g/cm3. Under such
conditions, the confinement of the raw mixture by means of capaci-
tor-plates, which would then form a portion of the moulding
apparatua, would lead to the supply of heat in an uncontrolled
and hence non-uniform manner, as well as to high losses.
The object of the invention is accordingly to provide a
device by which the losses can be minimised, and in which the
energy is supplied uniformly to the material to be heated, even
when this material possesses a xelatively high dielectric coeffic-
ient, due, for instance, to a high water contænt, so that the
material is heated essentially homogeniously and, furthermore,
rapidly, even over a comparatively large volume.
According to a broad aspect of-the present invention
there is provided a dielectric-heating device with two capacitor-
plates which are connected to a high-frequency generator and
between which the material to be heated is arranged, characterised
in that, in order to heat a conductive liquid material possessing
a relatively high dielectric coeffecient, a multi-layer capacitor
is formed by the capacitor-plates and by walls which are made of an
electrically insulating material possessing a relatively low
dielectric coefficient and which, for this purpose, are arranged
adjacent and parallel to the inner surfaces of -the capacitor-pla-tes,
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the material to be heated being accommodated between the walls, in
air-gap being provided, in each case, between the wall and the
capacitor-plate, the width of this air-gap being adjusted in order
to match the capacitance of the multi-layer capacitor to the opti-
mum generator output.
The invention will now be described in greater detail
with reference to the accompanying drawing which is represented in
the attached illustration, showing, partly in section, a heating
device relating to the manufacture of blanks for building materials,
from a pourable raw mixture.
The pourable raw mixture contains in addition to a bind-
er which can be activated by heat, preferably a hydraulic binder,
contains high proportions of water and foam, sand, and other
additives, the process of heating the raw mixture being carried out
in order to initiate strength-developing reac-tions involving the
binder, which may, for instance, be cement containing an acceler-
ator, in the course of an adequate, but short, time, so that the
blanks are of sufficient strength to be removed from the moulds
and, thereafter, to be transported.
In the embodiment represented, a mould lO, composed, for
example, of a plastic, is provided, it being possible for this
mould to be capable of being closed. The mould lO is positioned on
a machine frame ll which can carry, inter alia, devices for filling
the mould lO, for removing the blanks from the moulds, and for
transferring the blanks onto a transport device (these devices
being omitted from the illustration.)
Capacitor-plates 12 are arranged adjacen-t and parallel to
the oppositely-located sidewalls lOa of the mould lO, these
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capacitor-plates 12 extending over the entire area of the sidewalls
lOa. In particular, the capacitor-plates 12 extend, in both
dimensions beyond the adjacent sidewalls lOa, to an extent such
that as uniform a power density as possible is obtained. The two
capacitor-plates 12 are connected to a high-frequency generator 13,
and are held, for example by means of insulators 14, supported on
the machine frame 11 and adjacent to the sidewalls lOa, by means
of post~ 15.
The capacitor-plates 12 are installed at a short
distance from the sidewalls lOa, an air-gap 16 being formed, in
each case, between a capacitor-plate 12 and an adjacent sidewall
lOa, it being possible, by appropriately adjusting the air-gap 16,
to match the capacitance of the multi layer capacitor to the
generator output, in an optimum manner, this capacitor beins
formed by the capacitor-plates 12, the air-gaps 16, the sidewalls
lOa, and the contents 17 of the mould, namely the raw mixture from
which the blank is produced, so that the oscillating circuit of the
high-frequency generator 13 oscillates at the present frequency.
If appropriate, for optimum matching to the high frequency, a
shunt capacitance 21 can be provided, parallel to the multi-layer
capacitor, it being possible to provide this shunt capacitance
by fitting an additional capacitor-plate, spaced clear of one of
the capacitor-plates 12.
Since, under some circumstances, the composition of the
raw mixture can nevertheless vary, for instance when adjustments
are made in order to change the bulk density of the blanks, the
capacitance of the multi-layer capacitor then varies as well. In
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1~94137
order to ensure, even in a case of this kind, that the capacitance
is optimally matched to the generator output, it is possible to
provide a device for adjusting the air-gap 16, or air-gaps 16, this
device comprising, in the illustrative embodiment represented, an
arrangement wherein the post 15, carrying one capacitor-plate 12
via an insulator 14, is mounted in guides 18 and is, for example,
connected to a worm-gear drive 19, which can, possibly, be
actuated by hand, and/or by means of a stepping motor 20, in order
to adjust the width of the air-gap 16.
Under some circumstances, the adjustment of the width
of the air-gap 16 can be effected automatically, by means of the
stepping motor 20, via a control loop (not represented), with
continuous matching to the generator output.
Although, in the illustrative embodiment, a mould 10
is represented which could, for example, be composed of a mould-
box which can be lifted from a baseplate, optionally with a lid,
it would, however, also be possibler instead of such a mould, to
provide a moulding apparatus which comprises a channel in which
a continuous strand, composed of raw mixture, is transported and,
in the zone between the capacitor-plates, is heated and hence
hardened. After emerging from the channel, the strand can then
be divided into the individual blanks. This has the advantage
that the heating can be carried out more slowly, and over a
longer distance, without adversely affecting the cycle-time.
The appliance permits true capacitive heating,
despite the fact that the raw mixture is electroconductive and
possesses a relatively high dielectric coefficient, in conjunction
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with optimum utilisation of the generator output, it being
possible, in addition, to operate at a reduced frequency thereby
simplifying the implementation of this technique under industrial
conditions. The conductive material to be heated is, in addition,
subjected to resistance-heating, corresponding to the imaginary
part of its complex dielectric coefficient. Since the walls lOa
pvssess a relatively lower dielectric coefficient than the material
to be heated and, in particular, a product of the loss angle and
the dielectric coefficient which is lower, the walls lOa remain
essentially cold, despite the heating of the conductive liquid
material. In addition, a high dielectric strength is obtained,
and the choice of the generator voltage becomes more straight-
forward.
The multi-layer capacitor, according to the invention,
can also be used for other purposes than for manufacturing blanks
for building materials, where there is a requirement to heat a
suitable liquid material or, alternatively, a moist material.
