Note: Descriptions are shown in the official language in which they were submitted.
2039389
~iO 90/03715 PCT/CB89/01149
Title: Improved Microwave-Powered Heating Device
Field of the invention
This invention concerns microwave-powered heating devices,
and in particular heating apparatus for heating
foodstuffs, more particularly for defrosting and heating
to a serving temperature frozen pre-cooked foodstuffs.
Background to the invention
With the increasing demand for quick, hot snacks and so-
called "fast food", there is an increasing requirement to
be able to process frozen pre-cooked foodstuffs ready for
consumption. In particular, it is necessary to be able .o
heat up from the frozen condition a pre-cooked ~foodsturr
to a serving temperature in the shortest possible time,
and typically a temperature change from -17C to +74-C
must be achieved. Preferably the time should be as short
as possible and ideally should be well under one minute.
Conventional microwave ovens are not particularly well
suited to this function, and it has been proposed to
concentrate the available energy by providing concave or
other appropriately-shaped reflecting surface within a
chamber into which microwave energy is transmitted so that
the latter is focused generally into the region of the
chamber in which the foodstuff is located. In this way
the energy is concentrated into the foodstuff and the
latter is raised in temperature at the ~aximum ~ossibl~
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rate.
It is an object of the present invention to provide a more
uniform heating of a foodstuff within a chamber in which
microwave energy is intentionally reflected and thereby
focussed onto the foodstuff.
Su _ ary of the Invention
According to the present invention there is provided
heating apparatus comprising a heating chamber in which a
foodstuff is to be heated using microwave energy which is
focussed onto the foodstuff by means of appropriate
reflecting surfaces within the chamber, and at least two
independent microwave sources for supplying microwave
energy to the chamber so as to reduce the risk of standing
waves being set up within the chamber thereby resulting in
a more uniform heating-through of the foodstuff, t~.e
microwave energy being directed through wave guides whose
cross sections increase from relatively small circular or
rectangular cross section inputs to two or more outlets
which cooperate together to describe a complete annulus,
the outer diameter of which annulus corresponds to the
diameter of the reflecting surfaces within the chamber and
the inner diameter of which annulus corresponds to th.e
diamter of (or comprises the walls of) a circular platform
on which the foodstuff is to be located.
Alternatively, a concentration of standing waves may be
achieved, centred around and in the foodstuff being heated
he chambeL.
The design of the focussing elements and the arrangements
within the cavity are preferably matched to the shape and,
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structure and composition of the foodstuff. This matching
may be achieved by empirical means, or by computer or
mathematical modelling of the E-field within the cavity.
According to a preferred feature of the invention, the
apparatus is such that microwave energy entering the
chamber does not impinge directly on the foodstuff but can
only reach the foodstuff after being reflected at least
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2039389
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WO 90/03715 PCT/GB89/1)1149
once.
According to a further preferred feature of the invention,
the interior of the chamber is formed at least in part
with a concave microwave energy reflecting surface,
typically in the form of a dome.~ The micrawave energy
~from the plurality of microwave sources is directed
towards peri~eter region of the concave surface to be
reflected therefrom and generally focused towards the
centre of the chamber where the foodstuff i6 to be
located, so that it is in fact prevented from reaching the
foodstuff other than by first being reflected by the
perimeter regions of the concave reflector either directly
onto the foodstuff or onto a microwave reflecting surface
situated below the foodstuff 80 that microwave energy
incident thereon is reflected up onto the underside of the
foodstuff.
By locating the foodstuff on a microwave transparent
platform, above the microwave reflecting surface, so
microwave energy reflected by the latter can pass through
the platform and through the foodstuff to be reflected by
the concave reflecting surface back again towards the
foodstuff.
In this way virtually all of the microwave energy
introduced into the chamber will be reflected one way or
another into the foodstuff, and this substantially
decreases the time required to heat a standard foodstuff
from its frozen condition to serving temperatures. In a
typical example, an item weighing l~O grams and containing
50% water was heated from -17C to ~74C in 50 seconds
using electrical energy readily available from a standard
domestic 13 amp ring-main socket.
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The microwave sources, eg magnetrons, are conveniently
coupled to wave guides feeding the chamber by "Magic - T"
couplings or the like.
Although a circular array of wave guides is the ideal, an
improvement has also been obtained by merely using two
wave guides directing microwave energy onto two
diametrically opposite regions of the concave reflecting
surface. More magnetrons may be employed, equally
spaced.
The wave guides and magnetrons may be located within a
base unit and the concave reflector may form part of a
hingëd domed lid which when closed seals the chamber
against the egress of RF energy.
Where a food product is to be browned, a microwave
absorbing plate may be located within the chamber which
with the incidence of microwave energy thereon becomes
heated and emits infra-red radiation for heating and
browning the surface of the foodstuff. Such a browning
aid may be introduced into the chamber during the heating
process or may be permanently located within the chamber.
In the latter event, means is preferably provided for
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either shielding the browning-aid from microwave energy
until a particular part of the heating cycle has been
reached, or means is provided in, or associated with, the
wave guides to deflect or otherwise de-tune the emission
of microwave energy for a portion of each heating cycle so
as to largely prevent microwave energy from being incident
on the browning aid until an appropriate time in the
heating cycle.
A platform which is required to be transparent to
microwaves may be formed from a plastics material, glass
or a ceramic material.
The microwave-reflecting surfaces and the wave guides of
the apparatus will typically be formed from metal.
The invention will now be described, by way of example, by
reference to the accompanying drawings in which
Figure 1 is a cross-sectional schematic view of a heating
chamber embodying the invention; and
Figure 2 illustrates how a browning aid may be located
within the chamber to complete a heating and food
preparing cycle.
Detailed description of the Drawing
In Figure 1 two magnetrons 10 and 12 are located at the
lower ends of two flared metal wave guides 14 and 16,
respectively, which terminate in openings in a flat
circular plate at 18 and 20, respectively, the plate
forming the flat circular base of a hemispherical cooking
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2039389
W090/0371~ PCT/GB89/011~9
compartment which is closed by a hemispherical cover 22
the internal surface of which at 24 is formed from
microwave reflecting material such as metal. ~he plate 20
consists of microwave-transparent material.
The plate containing the apertures 18 and 20 is designated
by reference numerial 26 and a surrounding surface 28 may
co~prise a worktop or the upper surface of a housing
within which the magnetrons and wave guides are located.
A hinge 30 joins the domed cover 22 to the surface 28.
Mounted centrally on the plate 26 is a support platform 32
formed from glass, plastics or ceramic material which is
transparent to microwaves, and a foodstuff 34 is shown
located on the platform 32.
Microwave energy eminating from either of the apertures 18
and 20 cannot impinge directly on the foodstuff 34 but
must first be reflected from the domed surface 24 and may
have to be reflected from the planar surface 26 before the
energy can reach the foodstuff.
Where a browning-aid is to be used in the heating process,
the simplest procedure is terminate the heating cycle at
an appropriate point, open the lid 22 and locate within
the chamber the browning-aid. As shown in Figure 2, the
latter comprises a dished plate 36 carried by supports 38
and 40 which are adapted to clip over the corners of the
absorbs microwave energy and becomes hot in the process
and thereby emits infra-red radiation.
The most likely arrangement is a permanently-mounte~ shell
of microwave-absorbing heat-resistant materia' inside the
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domed lid. This shell could be of low mass, thermally-
isolated from the main body of the domed lid.
Although not shown, the outer surface of the plate 36 may
be coated in:such a way as to restrict the amount of
infra_red~radiation given off in a direction away from the
f~oodstuff so that the majority of the radiation is in fact
directed towards the foodstuff to brown the surface
the~reof.
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